| A-basis |
Mechanical property value above which at least 99% of the population of values is expected to fall, with a confidence of 95%. Also called A-allowable. |
| Abelian group |
Property of a group of elements associated with a binary operation. In an Abelian group, the group elements commute under the binary operation. If a and b are any two group elements and if the (+) sign denotes the binary operation, then, for an Abelian group, |
| Ablation cooling |
The cooling of a surface exposed to very high external gas temperature which causes the surface material to sublime, melt or decompose. The chemical process absorbs heat while the mass flow of material away from the surface blocks the heat flux from the hot gas. |
| Ablative materials |
Materials, especially coating materials, designed to provide thermal protection to a body in a fluid stream through the loss of mass. |
| Abradant |
The differently sized grits of hard materials such as emery employed for grinding, polishing, etc. The abrasive grade is determined by the grit size (grit number) originally related to the hole sizes in wire sieves. |
| Abrasion |
The removal of surface material by the scratching action of hard particles, either deliberately (abrasive papers, abrasive cleaning, abrasive machining) or as a consequence of operation (wear). |
| Abrasive wear |
The removal or displacement of material from a surface when hard particles slide or roll across the surface under pressure. The particles may be loose or may be part of another surface in contact with the surface being worn. Contrast with adhesive wear. |
| Abrasivity |
The extent to which a surface, particle, or collection of particles will tend to cause abrasive wear when forced against a solid surface under relative motion and under prescribed conditions. |
| ABS polymer |
A class of thermoplastic co-polymer consisting of the three monomers acrylonitrile, butadiene and styrene that has improved properties (particularly toughness) over the individual constituents. |
| Absolute expansion |
The true volumetric expansion of a liquid with temperature, after account is taken of any expansion of the container in which it is held. |
| Absolute humidity |
The weight of water vapor in a gadwater vapor mixture per unit volume of space occupied, expressed, for example, in grains or pounds per cubic foot. |
| Absolute manometer |
A manometer that measures absolute pressure, i.e. pressure measured relative to a perfect vacuum. Absolute pressure cannot be negative. |
| Absolute pressure |
1. The pressure measured relative to zero pressure (vacuum). 2. Gage pressure plus barometric pressure in the same units. |
| Absolute rating |
A theoretical size designation which is an estimation of the largest particle, by length, that can pass through a filter with a specific |
| Absolute specific gravity |
The ratio of the weight of any volume of a substance to the weight of an equal volume of a reference substance at the same temperature, often water at 4°C, both measured in a vacuum to avoid any effect of buoyancy. |
| Absolute stability |
Condtion of a linear system in which there exists a limiting value of the open-loop gain such that the system is stable for all lower values of that gain, and unstable for all higher values. |
| Absolute temperature (Unit K) |
A temperature T measured relative to absolute zero, 0 K or −273.15°C, the lowest temperature achievable at which molecular motion vanishes so that a body would have zero heat energy. The kelvin is equal in magnitude to the degree Celsius (°C). The kelvin temperature scale (kelvin absolute temperature scale) is an absolute or thermodynamic temperature scale derived from the Celsius scale: T(K) = T(°C) + 273.15. The Rankine absolute scale is derived from the Fahrenheit scale such that T(R) = T(°F) + 459.67, i.e. a scale relative to 0 R or −459.67°F where R is the Rankine degree symbol and °F is the Fahrenheit symbol. |
| Absolute value error |
The magnitude of the error dlsregarding the algebraic sign, or, for a vectorial error, disregarding its function. |
| Absolute viscosity |
A measure of the internal shear properties of fluids, expressed as the tangential force per unit area at either of two horizontal planes separated by one unit thickness of a given fluid, one of the planes being fixed and the other moving with unit velocity. |
| Absolute zero |
Temperature of -273.16″C or 459.69″F or OK at which molecular motion vanishes and a body has no heat energy. |
| Absorber |
1. An auxiliary vibratory system that favourably modifies the vibration characteristics of a main system. 2. Part of an absorption refrigeration system in which the refrigerant is absorbed by a transport medium. 3. The absorber plate is the receiver of a concentrating solar collector where the radiation is absorbed. |
| Absorptance |
(absorptivity, absorption coefficient, α) The fraction of radiant flux incident on a surface that is absorbed by the surface. The term also applies to absorption of radiation by a volume of fluid. For a semi-transparent surface, α + ρ + τ = 1, where ρ is the reflectance and τ is the transmittance. |
| Absorption |
The process whereby a fluid permeates a porous solid, or a gas is dissolved by a liquid. |
| Absorption dynamometer |
A dynamometer in which input work or power is dissipated by mechanical friction, electrical resistance, hydraulic resistance, etc. |
| Absorption hygrometer |
An instrument that determines the content of water vapour in the atmosphere by it being absorbed into a hygroscopic medium. |
| Accelerated testing |
A type of testing, for rates of wear, fatigue, corrosion etc., in which failure times are reduced by employing greater loads, more frequent power cycling, higher vibration levels, higher humidity, higher temperatures, greater potential differences etc. than would be encountered in normal operation. |
| Acceleration due to gravity |
(acceleration of free fall, gravitational acceleration, g) (Unit m/s2) The acceleration of a freely-falling body in a vacuum, with a mean value at sea level of approximately 9.81 m/s2. |
| Acceleration-error constant |
When the reference (demand) input to a control system is parabolic, the output signal will also be parabolic in steady state. The signal that is constant in this situation is the acceleration and thus for a parabolic input the steady-state error, referred to as the acceleration-error constant, is the error in the acceleration. |
| Accommodation |
The ability of a robot to respond to changes in the environment. |
| Achme thread |
A screw thread having an included angle of 29° and largely used for feed screws on machine tools. |
| Acicular ferrite |
A highly substructured non-equiaxed ferrite that forms upon continuous cooling by a mixed diffusion and shear mode of transformation that begins at a temperature slightly higher than the temperature transformation range for upper bainite. It is distinguished from bainite in that it has a limited amount of carbon available; thus, there is only a small amount of carbide present. |
| Acicular ferrite |
Ferrite crystallite growing, apparently, as in the course of bainitic transformation. It has a lath-like shape and an increased dislocation |
| Acid rain |
Any form of precipitation, including rain, snow, sleet, fog, dew, and particulates, which contains higher than normal levels of sulfuric and nitric acids. These acids result from natural and man-made emissions of sulfur dioxide and oxides of nitrogen, respectively. Acid soot consists of unburned carbon particles, typically larger than 10 μm, in the atmosphere contaminated with sulfuric acid. The particles result from poor combustion of fossil fuels. |
| Ackerman linkage |
A steering linkage on a motor vehicle that approximately gives rolling without slipping of both wheels about the turning point. This is achieved by having the inner stub axle (on the inside of the turning curve) move through a greater angle than the outer stub axle. |
| Acoustic droplet ejection |
The ejection of small droplets from the surface of a liquid that results from focusing high-intensity ultrasound near the surface. |
| Acoustic emission |
(stress-wave emission) Sound emitted by some materials when deformed under load. It arises from stress waves emitted by sudden dislocation motion in crystals, slip, crack growth, etc. |
| Acoustic power |
(sound power) (Unit W) The rate of flow of acoustic energy across a specified surface. |
| Acoustic pyrometer |
A non-intrusive pyrometer based on the principle that the sound speed in a gas is proportional to the square root of its absolute temperature. |
| Acoustic separation |
(Unit m) The separation of particles in a fluid using standing acoustic waves, typically ultrasound, to drive them to nodal points (acoustic particle concentration). |
| Acoustics |
The science and engineering of sound; its production, propagation, control, interaction with materials, etc. |
| Activation energy |
(Ea, U) (Unit kJ/mol) The minimum energy for a chemical reaction to occur or for processes such as diffusion to take place in crystals. |
| Active accommodation |
The use of information from sensors, for example, in a vision system, that allows a robot to show accommodation to the environment. |
| Active vibration suppression |
The reduction of undesirable vibration in components by feedback control. |
| Actual cubic feet per minute |
(acfm) An obsolete (i.e. non-SI) measure of volumetric flow rate; the volume of a gas flowing per minute at actual operating pressure and temperature, as opposed to the corresponding volume flow rate at STP. |
| Actual power |
(actual horsepower) (Unit W or hp) The power delivered at the output shaft of an engine, before subsequent transmission through a gearbox etc. |
| Actual value |
The output of a plant that is being controlled, i.e. the controlled variable. Not directly accessible by the control system, as it can only be measured by a sensor which may distort the measurement. |
| Actuating system |
A system in which an electrical, pneumatic or hydraulic input supplied to an actuator produces force, torque, or displacement, usually in a controlled way. |
| Adhesive joint |
The point or area in which two structures are held together by an adhesive. |
| Adhesive strength |
The strength of an adhesively bonded joint, usually measured in tension (perpendicular to the plane of the bonded joint) or in shear |
| Adhesive wear |
The removal or displacement of material from a surface by the welding together and subsequent shearing of minute areas of two surfaces that slide across each other under pressure. In advanced stages, may lead to galling. Contrast with abrasive wear. |
| Adhesive wear |
Wear that develops when two surfaces slide across one another under pressure. |
| Aerated bath nitriding |
A type of liquid nitriding in which air is pumped through the molten bath creating agitation and increased chemical activity. |
| Age hardening |
Hardening by aging, usually after rapid cooling or cold working. |
| age hardening |
Raising the strength and hardness of an alloy by heating a supersaturated solid solution at a relatively low temperature to induce precipitation of a finely dispersed second phase. Also known as aging or precipitation hardening. |
| Age softening |
Spontaneous decrease of strength and hardness that takes place at room temperature in certain strain hardened alloys, especially those of aluminum. |
| Aging treatment |
Heat treatment aimed at age hardening; it comprises solution and precipitation treatments. |
| AISC |
The American Institute of Steel Construction. |
| Air classifier |
(air elutriator) A device in which an airstream, which may be swirling, sorts particles by a combination of size, shape, and mass. |
| Air cleaner |
A device, such as a filter, hydrocyclone, or electrostatic precipitator, that removes particles and aerosols from a flow of air. |
| Air composition |
The sea-level composition (in per cent by volume) of air at a temperature of 15°C and a pressure of 1 atm is mainly 78.084% nitrogen, 20.947% oxygen and 0.934% argon. The remaining 0.035% consists of carbon dioxide, neon, helium, methane, krypton, hydrogen, oxides of nitrogen, xenon, ozone, iodine, carbon monoxide, and ammonia. Different sources give slightly different figures for the composition. Not included are water vapour (typically 0.4%) and pollutants such as sulfur dioxide. |
| Air compressor |
A turbomachine that draws in air and delivers it at higher pressure, temperature, and density. It can be of axial, fan, reciprocating, or rotary design. |
| Air conditioning |
The process of controlling the temperature and humidity in rooms, buildings, aircraft, passenger vehicles, etc. More generally it includes control of dust, levels of radiant heat, etc. |
| Air pump |
A machine for providing a flow of air or for increasing or decreasing the mass and pressure of air in a closed container. The term pump is more usual when the working fluid is a liquid, while compressor is more usual for gases. |
| Air-cooled condenser |
A heat exchanger, in which the cooling medium is air, used to condense the exhaust steam from a steam turbine, the condensate being returned to the boiler. |
| Air-cooled engine |
An internal-combustion engine directly cooled by airflow, rather than by water flowing through the engine block being cooled by a radiator. |
| Air-hardening steel |
A steel containing sufficient carbon and other alloying elements to harden fully during cooling in air or other gaseous mediums from a temperature above its transformation range. The terms should be restricted to steels that are capable of being hardened by cooling in air in fairly large sections, about 2 in. (50 mm) or more in diameter. Same as self-hardening steel. |
| Allotropy |
A near synonym for polymorphism. Allotropy is generally restricted to describing polymorphic behavior in elements, terminal phases, and alloys whose behavior closely parallels that of the predominant constituent element. |
| Allowable stress |
The maximum stress a designer can assume that the parts will stand. It is always less than the minimum strength of the material. For example, the ASME Boiler and Pressure Vessel Code typically specifies an allowable stress that is one-quarter of the service temperature yield strength of the material. This introduces a four-to-one safety factor into the design process and is intended to compensate for uncertainties in estimates of strength, service loads, etc. |
| Allowable stress design |
A design procedure developed for the AISC by the Research Council on Structural Connections. Purposely underestimates the strengths of bolts and joint materials to introduce safety factors into the design of structural steel joints. It is an alternative to the more recently defined load and resistance factor design procedure. |
| Alloy |
1. A metallic material composed of two or more elements, one of which is usually a metal to which the other elements are added, e.g. iron-carbon or aluminium-copper (binary alloy); nickel-chromium-iron (ternary alloy). 2. A plastic produced as a blend of two or more immiscible polymers. This results in material that cannot be achieved by polymerization. |
| Alloy steels |
Steels that contain alloying elements such as chromium, nickel, molybdenum, manganese, silicon, tungsten, and vanadium. These are added to improve properties such as the hardenability, toughness, strength, wear resistance, and corrosion resistance compared with those of plain-carbon steel. Low-alloy steels have a total alloy content (including carbon) of less than about 8%. They are hardenable and have strength up to 30% higher than plain-carbon steels. High-alloy steels contain considerably more alloy content and are developed for specific properties such as superior corrosion and chemical resistance. |
| Alloying element |
Component added deliberately with the aim of improving the properties of an alloy. Alloying elements can affect the existence range of equilibrium phases present in an unalloyed material, or lead to the occurrence of new phases, or both. In addition, alloying elements strongly affect the kinetics of phase transformations and thus the microstructure formation in alloyed materials. |
| Alpha iron |
The body-centered cubic form of pure iron, stable below 910 °C (1670 °F). |
| Alternating stress |
(Unit Pa) Originally, stresses of changing sign (tension-tocompression-to-tension, etc.) in a component produced by alternating forces acting in opposite directions, but now generally used to describe stresses that vary but may keep the same sign, as produced by periodic, out-of–balance, or vibrational loads. |
| Alternative energy |
(alternate energy) Energy sources that are renewable and do not have the undesired consequences of fossil fuels and nuclear energy. |
| Altitude |
(Unit m) Vertical height measured relative to a specified datum such as sea level. |
| Altitude chamber |
(hypobaric chamber) A chamber in which conditions at different altitudes are simulated by a combination of appropriate pressures, temperatures and relative humidity. |
| Amontons friction |
(Coulomb friction) Friction between surfaces where the ratio of the frictional force F to the normal force N is constant and independent of the area in contact. |
| Amplifier |
A device by which the output of a hydraulic, pneumatic, or electrical source is increased. |
| Anelastic |
Literally ‘not elastic’, but in practice used for materials that display timedependent recovery on unloading. |
| Angle of attack (α) |
(Unit °) The angle between a reference line on a lifting body and the vector direction of the relative velocity between the body and the fluid through which it is moving. In the case of an aerofoil, turbine, or compressor blade, the usual reference line is the chord line. |
| Angle of contact |
(Unit °) The angle subtended at the centre of a pulley or sprocket wheel by the circumferential contact of a belt or chain. |
| Angle of friction |
(friction angle, β) (Unit °) For a body in contact with a plane surface, the angle between the normal to the surface and the resultant force between the body and the surface. If the friction force is F and normal force is N, β is given by F/N = μ = tan β where μ is the coefficient of friction. |
| Angle of thread |
The included angle between the sides forming the groove of the screw thread. |
| Angle of torsion |
(angle of twist) (Unit ° or rad) The angle relative to a chosen section, normal to the axis of twist, through which another part of a component rotates when subjected to a torque. |
| Ångström (Å) |
An obsolete (non-SI) unit of length, sometimes still employed for atomic and crystallographic measurements. The conversion to SI is 1 Å = 10−10 m. |
| Angularity |
The underfaces of the nut and the bolt head should be exactly perpendicular to the thread or shank axes. If the angle between the face and the axis is, for example, 868 or 948, the fastener is said to have an angularity of 48 (sometimes called Perpendicularity). |
| Anisotropy |
The characteristic of exhibiting different values of a property in different directions with respect to a fixed reference system in the material. |
| Anneal to temper |
A final partial anneal that softens a cold worked nonferrous alloy to a specified level of hardness or tensile strength. |
| Annealing |
A generic term denoting a treatment, consisting of heating to and holding at a suitable temperature followed by cooling at a suitable rate, used primarily to soften metallic materials, but also to simultaneously produce desired changes in other properties or in microstructure. |
| Annealing carbon |
Fine, apparently amorphous carbon particles formed in white cast iron and certain steels during prolonged annealing. Also called temper carbon. |
| Annealing twin |
A twin form in a crystal during recrystallization. |
| Anode |
That electrode in a battery or corrosion cell which produces electrons. It is the electrode which is destroyed (corrodes). |
| ANSI |
The acronym for American National Standards Institute. |
| Anti-freeze |
A substance such as ethylene glycol added to the cooling system of a watercooled engine to lower the freezing point of the cooling water and also inhibit the formation of rust and other deposits. |
| Apparent area of contact |
In tribology, the area of contact between two solid surfaces defined by the boundaries of their macroscopic interface. |
| Applied thermodynamics |
(engineering thermodynamics, thermodynamics) The science of the relationship between heat, work and the properties of systems and the ways in which heat energy from fuels can be converted into mechanical work. It involves the study of all aspects of energy use and energy transformation, including power generation, refrigeration, the relevant properties of the substances involved and the relationships between them. The principle of conservation of energy is a fundamental law of nature. |
| Arbitration bar |
A test bar, cast with a heat of material, used to determine chemical composition, hardness, tensile strength, and deflection and strength under transverse loading in order to establish the state of acceptability of the casting. |
| Archimedes screw |
A machine which comprises a rotating helical blade inside a closefitting tube, which may be used to pump liquids, slurries (such as sewage), granular materials, etc. If water flows into the top of an inclined or vertical Archimedes screw, the screw will rotate and can be used to drive an electrical generator. |
| Archimedes Wave Swing |
A machine for tidal-power energy generation consisting of two concentric, air-filled submerged cylinders. The inner lower cylinder is tethered to the ocean floor while the upper floater unit, which is closed at the top, moves up and down due to the variations in hydrostatic pressure caused by the wave motion. The relative movement of the two cylinders is used to generate electricity in which linear motion replaces the more common rotary motion of an electromagnetic generator. |
| Artificial aging |
Aging above room temperature. |
| ASME |
The American Society of Mechanical Engineers. |
| Aspect ratio |
The ratio of the longest to the shortest dimension of a shape or object e.g. the chord-to-thickness ratio of an aerofoil or the span-to-chord ratio of a wing. |
| Assembly drawing |
An engineering drawing that shows how parts are assembled to produce a component or a complete machine. It may include sections to show internal features, dimensions that are critical for assembly, manufacturing information, and part numbers. |
| Assembly line |
A system of mass production in which work is moved progressively from one operation to another, ultimately to give the final complete product. |
| Athermal transformation |
A reaction that proceeds without benefit of thermal fluctuations; that is, thermal activation is not required. In contrast, a reaction that occurs at constant temperature is an isothermal transformation; thermal activation is necessary in this case and the reaction proceeds as a function of time. |
| Atmometer |
(atmidometer, evaporimeter) An instrument that measures the rate of evaporation of water from a surface into the atmosphere. |
| Atmosphere |
1. (atm) An obsolete (non-SI) unit of pressure equal to 101 325 Pa or 1.013 25 bar and approximately equal to the atmospheric pressure (barometric pressure) measured at mean sea level. |
| Atmospheric pressure |
The downward force exerted by the atmosphere because of its weight, (gravitational attraction to the Earth), measured by barometers, and usually expressed in units of millibars. Standard atmospheric pressure at sea level is 1013.25 mb.expressed in units of millibars. Standard atmospheric pressure at sea level is 1013.25 mb. |
| Atom |
The smallest unit of matter that has the properties of a chemical element. The simplest description of an atom (Bohr model) is of a compact nucleus surrounded by one or more orbiting electrons. |
| atomic volume |
Volume of unit cell per atom. |
| Atomization |
The production of a spray of fine droplets from a liquid, such as diesel oil or petrol, by injection through a nozzle (an atomizer). |
| Atomizing humidifier |
A humidifier that functions by spraying fine droplets of water into an airstream. |
| Ausforming |
Thermomechanical treatment of steel in the metastable austenitic condition below the recrystallization temperature followed by quenching to obtain martensite and/or bainite. |
| Austempered steels |
Medium to high carbon steels with improved mechanical properties as a result of austempering. After heating to between 800° and 900°C to form the austenite phase, they are quenched to, and held at, just above the martensite transformation range (between 260° and 370°C) to form bainite, before cooling further. Austempering of ductile cast irons results in a microstructure of bainite and spheroidal graphite. Austempered ductile irons have excellent toughness and wear properties. |
| Austempering |
A heat treatment for ferrous alloys in which a part is quenched from the austenitizing temperature at a rate fast enough to avoid formation of ferrite or pearlite and then held at a temperature just above Ms until transformation to bainite is complete. Although designated as bainite in both austempered steel and austempered ductile iron (ADI), austempered steel consists of two phase mixtures containing ferrite and carbide, while austempered ductile iron consists of two phase mixtures containing ferrite and austenite. |
| Austempering |
Heat treatment comprising austenitization of a steel article, cooling it to a bainitic range at a rate higher than the critical cooling rate and holding at a fixed temperature until the completion of bainitic transformation. |
| Austenite |
A solid solution of one or more elements in face-centered cubic iron. Unless otherwise designated (such as nickel austenite), the solute is generally assumed to be carbon. |
| Austenite |
Solid solution of alloying elements and/or carbon in γ-Fe. It is named after British metallurgist W. C. Roberts-Austen. |
| Austenite finish temperature (Af) |
Temperature at which the transformation of martensite into austenite completes upon heating. The same designation is also applied to nonferrous alloys in which martensite transforms into some parent phase. |
| Austenite stabilization |
Decrease, in comparison to a continuous cooling, in the amount of martensite occurring from austenite when cooling is interrupted at a temperature between Ms and Mf. This can be explained by the relaxation of stresses induced in the austenite by martensite crystals occurring before the interruption. The relaxation, in turn, leads to the dislocation rearrangement and their interaction with martensite/austenite interfaces, which makes the interfaces immobile. |
| Austenite-stabilizer |
Alloying element expanding the γ-phase field in the corresponding phase diagram, which manifests itself in a decrease of the A3 temperature and an increase of the A4 temperature in binary alloys Fe–M as well as in a decrease of A1 temperature in ternary alloys Fe–C–M (M is an alloying element). The solubility of austenite-stabilizers in ferrite is much lower than in austenite. Under the influence of austenite-stabilizers, austenite can become thermodynamically stable down to room temperature. |
| Austenitic grain size |
The size attained by the grains of steel when heated to the austenitic region; may be revealed by appropriate etching of cross sections after cooling to room temperature. |
| Austenitic stainless steels |
Austenitic stainless steels are non-magnetic. They contain significant additions of chromium (16–30%), but have the austenite (FCC) crystal structure, stabilized by the addition of nickel in the composition range 6–20%. Carbon content ranges from about 0.03 to 0.15%. The ‘18–8’ stainless steels containing approximately 18% chromium and 8% nickel are typical of the class. Austenitic stainless steels have high strength and corrosion resistance even at elevated temperatures, good ductility, good low-temperature properties, and good weldability. Used for aircraft and transport equipment, cutlery, chemical and food-processing equipment, surgical instruments, and cryogenic vessels. |
| Austenitizing |
Forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). When used without qualification, the term implies complete austenitizing. |
| Autonomous energy system |
(stand-alone energy system) A sole source of electricity, usually small-scale, for applications remote from a grid, especially with energy storage in the system. Hydroelectric, photovoltaic, wind-power and other renewable systems are well suited to stand-alone applications. |
| Auxetic materials |
Those man-made materials for which Poisson’s ratio is negative, so that the cross section expands when subjected to a longitudinal tensile stress and contracts when subjected to a longitudinal compressive stress. |
| Auxiliary power unit (APU) |
In aircraft–gas–turbine applications, a small gas turbine used to provide start-up power, electrical and hydraulic power, and compressed air for cabin ventilation. In other applications, an APU may be a gas turbine or internal-combustion engine used to provide emergency power. |
| Available draught (Unit Pa) |
The reduced pressure of combustion gases in a furnace or boiler, either forced or due to the buoyancy of hot gases, which is used to draw in combustion air and remove products of combustion. |
| Available head (Unit m) |
In a hydroelectric power system, the difference between the vertical height of the water level in the supply reservoir above the turbine inlet less the head loss due to friction and fittings in the duct leading to the turbine. |
| Available resource |
(total resource) (Unit J) The total annual energy theoretically available from a renewable-energy source, such as ocean waves, the wind, or the total incident solar energy. |
| Avogadro constant |
(Avogadro number, NA) A fundamental physical constant with the fixed numerical value 6.022 140 76 × 1023 mol−1. The mole contains exactly this number of elementary entities. A minor adjustment to the value of the Avogadro constant was made effective on 20 May 2019. |
| Axial load (Unit N) |
In general, a tensile or compressive load directed along the axis of a component. Strictly the load should pass through the centroid of the cross section to avoid inducing bending moments and be perpendicular to the plane of the section. |
| Bainitic hardening |
Quench-hardening treatment resulting principally in the formation of bainite. |
| Balance piston |
(balance drum, dummy piston) A disc attached to the shaft of a turbine or compressor, to one side of which high or low pressure is applied to counteract the axial thrust produced by the pressure change across the machine. A form of thrust bearing. |
| Balance weight |
A corrective mass used in the static or dynamic balancing of a rotating object. |
| Balancing machine |
A device that assesses the state of static and dynamic balance of a rotating part, and indicates the magnitude and location of weights to be added to give balance. |
| Ball mill |
A mill for grinding and pulverizing materials, consisting of a horizontal rotating drum containing loose steel or ceramic balls. |
| Ball screw and nut |
A nut and bolt assembly having semi-circular helical grooves, as opposed to threads, in which run ball bearings. On rotation of the nut, the balls move along the helix and carry the axial load. Balls reaching the end of the groove are recirculated back to the beginning. Such devices have low friction and very little backlash, and are used in some steering mechanisms. |
| Ballonet |
An airbag used in a hybrid airship. Helium gas expels air from the ballonet during ascent. Fans draw air in during descent. |
| Band clutch |
A friction clutch in which drive is achieved by a band contracting on to the rim of the clutch. |
| Bánki turbine |
A cross-flow, impulse-type water turbine for very low heads in which a jet of water in the form of a flat sheet passes transversely through the turbine, so going through the runner twice. The thin runner blades which run horizontally across the turbine parallel to the axis of rotation are profiled in cross section. |
| Bar |
A widely used (non-SI) unit of pressure approximately equal to normal atmospheric pressure at sea level. The conversion to SI is 1 bar = 105 Pa. |
| Bar |
Unit of pressure – the pressure created by a column of€mercury 75.006 cm high at 0ºC, or about 33.45 feet of water at 4ºC. It is equal to 105 pascal. Standard atmospheric pressure (at sea level) is 1.01325 bar, or 1013.25 mb. |
| Barcol hardness test |
An indentation test, similar to the Shore hardness test but using a pointed indentor with a flat tip, used to determine the hardness of such materials as rigid plastics and composites. |
| Barometer |
An instrument used to measure atmospheric pressure. |
| Barometer |
An instrument for measuring atmospheric pressure. There are two main types – the mercury barometer, and the aneroid barometer. |
| Barrier protection |
The coating on a fastener is said to provide barrier protection if it merely isolates the fastener from the environment. Paint, for example, provides barrier protection. |
| Base metal hardness |
Hardness closest to the surface (when traversing from core to outside diameter) just before an increase or decrease occurs, denoting, respectively, carburization or decarburization. |
| Batch |
A definite quantity of some product or material produced under conditions that are considered uniform. A batch is usually smaller than a lot. |
| Batch furnace |
A furnace used to heat treat a single load at a time. Batch-type furnaces are necessary for large parts such as heavy forgings and are preferred for complex alloy grades requiring long cycles. |
| B-basis |
Mechanical property value above which at least 90% of the population of values is expected to fall, with a confidence of 95%. |
| Bearing area |
The product of the pin (or hole) diameter and specimen thickness in a bearing test. |
| Bearing area |
(bearing surface) (Unit m2) The projected area of a hole, such as a rivet hole, that carries a transverse load. |
| Bearing failure |
A failure that occurs in a riveted or bolted joint when the transverse load divided by the bearing area results in a stress that leads to permanent plastic deformation. |
| Bearing strain |
The ratio of the deformation of the bearing hole, in the direction of the applied force, to the pin diameter in a bearing test. |
| Bearing strength |
The maximum bearing stress that a material is capable of sustaining. |
| Bearing stress |
The force per unit of bearing area. |
| Bearing test |
A method of determining the response to stress (load) of metal products that are subjected to riveting, bolting, or a similar fastening procedure. The purpose of the test is to determine the bearing strength of the material and to measure the bearing stress versus the deformation of the hole created by a pin or rod of circular cross section that pierces the sheet perpendicular to the surface. |
| Bearing yield strength |
The bearing stress at which a material exhibits a specified limiting deviation from the proportionality of bearing stress to bearing strain in a bearing test. |
| Belt drive |
Transmission of motion from one shaft to another by means of a continuous plain, or toothed, flexible band (belt) passing over pulleys. In contrast to chain drives, belt drives tend to be employed in low-torque applications. Reduction in transmissible power may occur due to stretch of a plain transmission belt which results in slack in the drive (belt creep) or slip of a belt on a driving or driven pulley (belt slip). |
| Belt furnace |
A continuous-type furnace which uses a mesh-type or cast-link belt to carry parts through the furnace. |
| Bend test |
A test for determining the relative ductility of metal that is to be formed (usually sheet, strip, plate, or wire) or for determining soundness and toughness of metal (after welding, for example). The specimen is usually repeatedly bent over a specified diameter through a specified angle and then unbent for a specified number of cycles. There are four general types of bend tests, named according to the manner in which the forces are applied to the specimen to make the bend: free bend, guided bend, semiguided bend, and wrap-around bend. |
| Bernoulli’s law |
For a steadily flowing fluid (liquid or gas), the sum of the pressure, kinetic energy per unit volume and potential energy per unit volume is constant at any point in the fluid. Using this relationship, it is possible to measure the velocity of a fluid by measuring its pressure at two points, as with a manometer or Pitot tube. |
| Beta annealing |
Producing a beta phase by heating certain titanium alloys in the temperature range of which this phase forms followed by cooling at an appropriate rate to prevent its decomposition. |
| Bias pressure (Unit Pa) |
In a fluidic device controlled by pressure difference, the magnitude of that difference. |
| Biaxial stress |
Generalized loading of a body in a single plane with no loading normal to it. |
| Bimetallic strip |
A strip formed by welding, riveting or brazing together two metals having different coefficients of expansion, which causes the strip to curl when its temperature changes. Typical combinations are steel and copper or steel and brass. |
| Binary vapour cycle |
A power cycle which combines a steam cycle at relatively low temperature (the bottoming cycle) with a higher temperature cycle (the topping cycle) in which a working fluid such as mercury, sodium or potassium is used. |
| Biodiesel |
A substitute for diesel fuel derived from the oily seeds of sunflowers, oilseed rape, soya beans, etc. |
| Bioenergy |
1. Energy derived from materials such as purpose-grown energy crops, including sugar cane, maize, wheat, and rice, as well as wood, straw, and animal waste, including sewage, manure, and animal litter. 2. A term sometimes used to cover biomass and biofuels together. |
| Bioengineering |
(biological engineering) The application of engineering principles to biology, medicine, agriculture, etc. |
| Biomechanics |
The application of mechanical-engineering principles, including thermodynamics, fluid mechanics, and solid mechanics together with materials engineering, to biological systems. |
| Biomedical engineering |
The application of engineering principles and methodology to the medical field. Examples involving mechanical engineering include the design and construction of artificial limbs and hearts, heart-lung machines, prosthetic eyes, and orthopaedic implants such as hip joints and pins to stabilize fractured bones. |
| Black annealing |
Box annealing or pot annealing ferrous alloy sheet, strip, wire to impart a black color to the oxidized surface. |
| Black oxide |
A black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions. |
| Blade compressor |
An oil-free, double-acting rotary compressor in which an angled blade attached to the inner surface of a rotating housing passes through a slot in a disk rotating about an axis perpendicular to that of the housing but offset from it. Air enters through apertures in the housing and leaves through a port in the housing. |
| Blank carburizing |
Simulating the carburizing operation without introducing carbon. This is usually accomplished by using an inert material in place of the carburizing agent, or by applying a suitable protective coating to the ferrous alloy. |
| Blank nitriding |
Simulating the nitriding operation without introducing nitrogen. This is usually accomplished by using an inert material in place of the nitriding agent or by applying a suitable protective coating to the ferrous alloy. |
| Blanking shear test |
A method used to determine the through-thickness shear strength of sheet material that involves blanking a disk out of flat strip using a simple punch-and-die method. Alson known as punching shear test. |
| Blind rivet |
A rivet that enables a connexion to be made from one side only of an assembly. |
| Blow moulding |
The manufacture of hollow polymer objects (e.g. bottles) by expanding, with internal air pressure, a tube sealed at one end (parison) against the walls of a cavity mould. |
| Blowing |
The process by which a gas is transferred through a porous surface over which there is a gas flow. The effect is used to decrease skin friction and increase heat transfer, for example in turbine-blade cooling. |
| Blue annealing |
Heating hot-rolled ferrous sheet in an open furnace to a temperature within the transformation range and then cooling in air, in order to soften the metal. The formation of a bluish oxide on the surface is incidental. |
| Blue brittleness |
Anomalous loss of ductility when quenched steels are tempered in the range 250–350°C (temperatures that produce blue tints on the surface of components). |
| Blue brittleness |
Brittleness exhibited by some steels after being heated to some temperature within the range of about 205 to 370 °C (400 to 700 °F), particularly if the steel is worked at the elevated temperature. Killed steels are virtually free of this kind of brittleness. |
| Blueing |
Application of blue dye (‘engineers’ blue’) to identify high spots on surfaces in contact, or for marking out. |
| Boiler and Pressure Vessel Code |
A large and complex document, maintained and published by the American Society of Mechanical Engineers. The code |
| Bolt |
(machine bolt) A fastener in the form of an externally-threaded cylinder with a head on one end that is inserted through holes in assembled parts that are then held together by a nut tightened on to the thread. A bolt has an unthreaded region below the head so that when located in a hole, transverse loads are borne by two plain cylindrical surfaces in contact. In contrast, use of a machine screw, that has threads all along its length, results in threads bearing against the plain surface of a hole. |
| Bolt |
Officially, a threaded fastener designed to be used with a nut. |
| Bolt blank |
A rod on which a head has been formed, but on which no thread has been cut, from which bolts or screws may be made as required. |
| Bolt gage |
An ultrasonic instrument used to measure the stress or strain in bolts. |
| Bolt length |
The bolt length shall be the distance measured parallel to the axis of the product from the bearing surface of the head to the extreme end of the bolt including point. |
| Bolt stress (Unit Pa) |
The axial tensile stress induced in a bolt by tightening. |
| Bond strength (Unit Pa) |
The tensile, compressive or shear stress at which joints fail, e.g. by fracture or excessive deformation. |
| Box annealing |
Annealing a metal or alloy in a sealed container under conditions that minimize oxidation. In box annealing a ferrous alloy, the charge is usually heated slowly to a temperature below the transformation range, but sometimes above or within it, and is then cooled slowly; this process is also called close annealing or pot annealing. |
| Boyle’s law |
The product of the pressure of a gas p and its volume is constant at a given absolute temperature T. Together with Charles law, it yields the ideal gas equation p = Mrt where m is the mass of gas and R is the specific gas constant. |
| Boyle’s law |
The volume of a gas at constant temperature is inversely proportional to the pressure. This means that as pressure increases, the volume of a gas decreases. |
| Brake fluid |
The hydraulic fluid used to transmit force to the pistons in disc brakes or the wheel cylinders in drum brakes. Requirements include a high boiling point and low hygroscopy. |
| Brake lining |
The replaceable friction material that covers a brake shoe in an internally expanding brake. |
| Brayton cycle |
(Joule cycle) An air standard cycle that is the ideal cycle for a gasturbine engine. As shown on the diagram of pressure (p) ʋs specific volume (ʋ), it consists of four internally reversible processes: isentropic compression in a compressor (1–2), isobaric heat addition in a combustor (2–3), isentropic expansion in a turbine (3–4) and isobaric heat rejection. The cycle can be extended to include regeneration, reheating, and intercooling. |
| Breaking load |
The maximum load (or force) applied to a test specimen or structural member loaded to rupture. |
| Breaking load (Unit N) |
The load applied at some point to a component or structure which leads to fracture. The breaking stress (breaking strength) is the average stress at which a member breaks, given by the breaking load divided by the area over which it acts. |
| Breaking stress |
The stress at failure. Also known as rupture stress. |
| Breaks |
Creases or ridges usually in “untempered” or in aged material where the yield point has been exceeded. Depending on the origin of the break, it may be termed a cross break, a coil break, an edge break, or a sticker break. |
| Bright annealing |
Annealing in a protective medium to prevent discoloration of the bright surface. |
| Bright nitriding |
Nitriding in a protective medium to prevent discoloration of the bright surface. |
| Brine quenching |
A quench in which brine (salt water-chlorides, carbonates, and cyanides) is the quenching medium. The salt addition improves the efficiency of water at the vapor phase or hot stage of the quenching process. |
| British Standards Institution (BSI) |
The Institution in the UK responsible for the preparation and publication of standard specifications (British standards) for manufactured goods, their design, manufacture and testing. |
| British thermal unit (BTU, Btu) |
An obsolete (non-SI) imperial unit of energy defined as the energy required to raise the temperature of one pound of pure water at 68°F by 1°F. The conversion to SI is 1 Btu = 1.055 056 × 103 J. |
| Brittle |
A brittle material is one that breaks, often suddenly, with no permanent deformation. Examples of brittle materials are some cast irons, glass, concrete, and some plastics. |
| Brittle |
A bolt is said to be brittle if it will break when stretched only a small amount past its yield point, |
| Brittle fracture |
The fracture of a component or structure in the globally-elastic range of loading, so that the broken pieces may be refitted to regain the original article. |
| Brittle fracture |
Separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture occurs by rapid crack propagation with less expenditure of energy than for ductile fracture. |
| Brittle lacquer coating |
A lacquer that, when painted on to an unloaded body, reveals the direction of maximum tension stresses from the pattern of cracking produced in the lacquer when the body is loaded. |
| Bucket |
1. A cup-shaped vane with a central dividing ridge attached to the periphery of the runner of an impulse water turbine such as a Pelton turbine. 2. A rotor blade in a compressor or turbine. |
| Bulk density (Unit kg/m3) |
The average density of a mass of granular or powdered material at ambient conditions. |
| Bulk velocity |
(average velocity, V) (Unit m/s) For fluid flow in a pipe or duct, the average flow velocity, given by V = ṁ/ρA where ṁ is the mass flow rate, ρ is the fluid density, and A is the duct cross-sectional area. |
| Burning |
(1) Permanently damaging a metal or alloy by heating to cause either incipient melting or intergranular oxidation. See overheating, grain-boundary liquation. (2) In grinding, getting the work hot enough to cause discoloration or to change the microstructure by tempering or hardening. |
| Butt joint |
The end-to-end joining of two plates either by welding or by overlapping plates that are bolted or riveted. |
| Butterfly valve |
A valve in which a disc rotates on a shaft at right angles to the axis of a pipe to regulate flow. When open, the disc is edge-on to the flow and offers limited resistance. When closed, the disc is pressed against a seat in the valve body. |
| Bypass flow meter |
(shunt flow meter) A flow meter installed in a pipework bypass which may itself be part of the flow meter. An orifice plate is used to ensure a fraction of the main flow passes through the bypass. |
| Bypass ratio |
In a turbofan engine, the ratio of the mass flow rate of the bypass stream to the mass flow rate through the core of the engine. |
| Bypass valve |
A valve that directs flow through a bypass. |
| Calorie |
(cal, gram calorie, small calorie) An obsolete (i.e. non-SI) unit of energy equal to 4.186 8 J. It is the amount of energy needed to raise the temperature of 1 gram of pure airfree water from 14.5°C to 15.5°C at standard atmospheric pressure. |
| Calorizing |
Imparting resistance to oxidation to an iron or steel surface by heating in aluminum powder at 800 to 1000 °C (1470 to 1830 °F). |
| Cap |
A cover, often in the form of a short cylinder, one end of which is closed. Typically used to close an orifice or pipe end, on to which it can be pushed, welded, screwed, or attached with fasteners. |
| Cap nut |
A nut with a blind threaded hole, for example a dome nut to cover the end of a bolt. |
| Cap screw |
A bolt where the thread runs right up to the head and engages in a threaded hole or captive nut in an adjoining member. |
| Cap screw |
A finished screw 5mm or larger, used for fastening two pieces together by passing the screw through a clearance hole in one part and screwing in into a tapped hole in the other. |
| Cap screw |
A finished screw, used for fastening two pieces together by passing the screw through a clearance hole in one part and screwing it into a tapped hole in the other. Heads may be hexagon, round, flat, fillister or socket type. |
| Captive nut |
A nut attached loosely or rigidly to a sheet member that is too thin to thread and which engages with a cap screw. |
| Car furnace |
A batch-type furnace using a car on rails to enter and leave the furnace area. Car furnaces are used for lower stress relieving ranges. |
| Carbon potential |
A measure of the ability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon level of the steel. Note: In any particular environment, the carbon level attained will depend on such factors as temperature, time, and steel composition. |
| Carbon restoration |
Replacing the carbon lost in the surface layer from previous processing by carburizing this layer to substantially the original carbon level. Sometimes called recarburizing. |
| Carbon steels |
(plain carbon steels) Steels in which carbon is the principal alloying element, and the amount of manganese does not exceed 1.65% and the copper and silicon contents are less than 0.60%. There are three main types: low-carbon steels (0.08–0.35% carbon); medium-carbon steels (0.35–0.50% carbon); and high-carbon steels (0.50–2.0% carbon). |
| Carbonitriding |
A case hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere of such composition as to cause simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, create a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the workpiece. |
| Carbonization |
Conversion of an organic substance into elemental carbon. |
| Carburizing |
Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching. |
| Carburizing flame |
A gas flame that will introduce carbon into some heated metals, as during a gas welding operation. A carburizing flame is a reducing flame, but a reducing flame is not necessarily a carburizing flame. |
| Case |
That portion of a ferrous alloy, extending inward from the surface, whose composition has been altered so that it can be case hardened. Typically considered to be the portion of the alloy (a) whose composition has been measurably altered from the original composition, (b) that appears dark on an etched cross section, or (c) that has a hardness, after hardening, equal to or greater than a specified value. Contrast with core. |
| Case hardening |
A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or a mixture of the two and, by diffusion, create a concentration gradient. The processes commonly used are carburizing and quench hardening; cyaniding; nitriding; and carbonitriding. The use of the applicable specific process name is preferred. |
| Cast steel |
Steel that is cast into shapes. It has superior properties to most cast irons, but is more expensive to produce. |
| Castellated nut |
A nut having slots across the hexagonal faces, used with a bolt having a drilled hole so that a split (cotter) pin may be inserted through both nut and bolt to prevent unscrewing. |
| Catastrophic failure |
A sudden and total failure of a large engineering structure such as an aeroengine, aircraft, space vehicle, bridge, or dam. |
| Caterpillar |
(crawler vehicle) A vehicle that ‘lays its own road’ by running on endless belts, driven by toothed wheels, on each side. Used on soft ground where spreading of the load reduces contact stress. |
| Cathode |
That electrode in a battery or corrosion cell which attracts electrons. |
| Caustic cracking |
A form of stress-corrosion cracking most frequently encountered in carbon steels or iron-chromiumnickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 200 to 250 °C (400 to 480 °F). Also known as caustic embrittlement. |
| Caustic quenching |
Quenching with aqueous solutions of 5 to 10% sodium hydroxide (NaOH). |
| Cavitation |
The formation of vapour- or gas-filled cavities in a liquid due to reduction of the local pressure, often due to acceleration of the fluid, such as in flow through a convergent nozzle. If there is no dissolved gas in the liquid, vaporous cavitation occurs when the pressure falls below the saturated vapour pressure. If the bubbles are formed due to high temperature, the process is termed boiling. If there is dissolved gas, gaseous cavitation occurs due to pressure reduction, temperature increase, or diffusion (degassing). At the tips of marine propellers and in hydraulic machinery, the collapse of cavitation bubbles can cause noise and vibration and lead to surface damage in the form of pitting. |
| Cavitation damage |
Erosion of a solid surface through the formation and collapse of cavities or bubbles in an adjacent liquid that contains vapor and/or gas. |
| Cavitation tunnel |
A closed-circuit recirculating water tunnel in which the static pressure can be reduced to sufficiently low levels for cavitation studies to be performed. |
| Cavitation-resistance inducer |
An axial-flow pump used upstream of a main pump in order to prevent cavitation in the latter by increasing the inlet head. |
| Cavity radiator |
A heated chamber having a small hole through which radiation, approximating blackbody radiation, passes out. |
| Cellular materials |
There are three broad classes of materials that have a sponge-like structure containing many small closed or open (interlinked) pores or cells. Natural cellular materials include bone, cork, sponge, and wood, and have numerous well-known uses. Wide ranges of cellular plastics are produced using blowing or foaming agents, such as air, ammonium carbonate, sodium bicarbonate, etc., to create pores during the processing of thermoplastic or thermosetting polymers. Rigid foams have moderate compressive strength and can be moulded and machined. |
| Celsius |
The temperature scale based on the freezing point of water (0ºC) and the boiling point of water (100ºC). The interval between these points is divided into 100 degrees. The scale was devised by Anders Celsius. |
| Celsius temperature scale |
(centigrade temperature scale) A relative, non-SI, temperature scale now defined in terms of the Kelvin absolute temperature scale as °C = K − 273.15, where °C is the symbol for degrees Celsius. The scale was previously called the centigrade scale, with two fixed points: the melting point of ice (the ice point) as 0°C, and the boiling point of water (the steam point) as 100°C. |
| Cementation |
The introduction of one or more elements into the outer portion of a metal object by means of diffusion at high temperature. |
| Cemented carbides |
Sintered mixtures of refractory metal carbides (e.g. tungsten carbide) in a metal matrix binder such as cobalt, nickel, or iron. They have high melting point, toughness, compressive strength, and wear resistance. Applications include use in grinding wheels and papers, cutting tools, drill bits, wire-drawing dies, and ball-point pen tips. Sometimes known as hard metals when the application is to machine tools. |
| Cementite |
A compound of iron and carbon, known chemically as iron carbide and having the approximate chemical formula Fe3C. It is characterized by an orthorhombic crystal structure. When it occurs as a phase in steel, the chemical composition will be altered by the presence of manganese and other carbide-forming elements. |
| Centi (c) |
An SI unit prefix indicating a multiplier of 0.01; thus centimetre is a unit of length equal to one one-hundredth of a metre or 10 mm. |
| Centigrade heat unit |
(Celsius heat unit, CHU) An obsolete (i.e. non-SI) unit equal to the energy required to increase the temperature of one pound of pure, air-free water from 14.5°C to 15.5°C at a pressure of one standard atmosphere. |
| Centre line |
1. In an engineering drawing, a line of symmetry. 2. An imaginary line along a pipe, duct, or shaft that defines an axis of symmetry. 3. An imaginary straight line parallel to the intended direction of a surface located such that the areas above and below the line and the real wavy (rough) surface cancel out. |
| Centre of pressure |
1. The location, on an aerofoil or other body that develops lift, of the resultant lift force. 2. The location, on a surface submerged in a liquid, of the resultant force due to the pressure acting on the surface. Because hydrostatic pressure increases with depth, the centre of pressure is generally below the centroid of the surface. |
| Centrifugal |
Acting or moving in a direction away from the axis of rotation of a rotating body. |
| Centrifugal clutch |
A clutch that engages and disengages at a defined speed of rotation of the driving shaft, as when expanding friction shoes act against the inside of a drum. |
| Centrifugal compressor |
A compressor in which kinetic energy is added to a fluid by radial acceleration in an impeller and then converted into a pressure increase by flow though a diffuser. |
| Centrifugal fan |
A machine with a rotor consisting of a number of blades mounted around a hub and used for moving air or other gases. The gas enters the rotor axially and is discharged radially at increased pressure. |
| Centrifugal force (Unit N) |
The inertial reaction force to the centripetal force. It is equal in magnitude but opposite in direction. |
| Centrifugal pump |
A pump into which liquid enters axially through the eye of the casing and is then accelerated through an impeller, thereby increasing both its kinetic energy and pressure before being delivered to a ring diffuser (the volute) that further increases the liquid pressure and from which it leaves. |
| Centrifuge |
A machine incorporating a rapidly spinning drum in which liquids and suspended particles of different densities are separated by centrifugal action. |
| Centripetal |
Acting or moving in a direction towards the axis of rotation. |
| Chafing fatigue |
Fatigue damage initiated in a surface damaged by rubbing against another body. |
| Chain |
A series of connected links, typically of steel. For lifting, pulling, securing, etc., each link is a closed loop, often in the form of a ring. For power transmission, the links are designed to mesh with the teeth of a sprocket wheel. In contrast to belt drives, chain drives tend to be employed in high-torque applications. |
| Chain dimensioning |
On an engineering drawing, where the end point of one dimension is the starting point for the next. Parallel dimensioning is preferred as chain dimensioning can lead to the accumulation of tolerances. |
| Charles law |
(Gay–Lussac law) The volume of a fixed mass of gas at constant pressure is proportional to its absolute temperature. |
| Charles’ law |
The volume of a gas at constant pressure is directly proportional to its absolute temperature. |
| Check valve |
(clack valve, non-return valve) A mechanical device that allows fluid flow in one direction only. The numerous designs include ball, diaphragm, disc, lift, split disc, and swing check valves. |
| Checks |
Numerous, very fine cracks in a coating or at the surface of a metal part. Checks may appear during processing or during service and are most often associated with thermal treatment or thermal cycling. |
| Cheese head |
A cylindrical head on a screw or bolt. For driving, it may be slotted, or hexagonally recessed. |
| Chilled castings |
Iron castings cooled at a rate that results in white iron (slow enough to avoid the formation of martensite, yet fast enough to prevent any silicon present from causing decomposition of cementite into iron and graphite). |
| Circlip |
(snap ring) An external or internal retaining ring that locates parts of circular cross section in an axial direction. It consists of an incomplete ring, with holes on either side of the gap, that may be expanded by a plier-like tool to pass into a groove in a shaft or contracted to pass into a groove in a bore. |
| Circular pitch |
The distance from the center of one gear tooth to the center of the next gear tooth measured on the pitch line. |
| Clamping force |
The equal and opposite forces which exist at the interface between two joint members. The clamping force is created by tightening the bolts, but is not always equal to the combined tension in the bolts. Hole interference problems, for example, can create a difference between clamping force and bolt loads. |
| Clearance |
(Unit m or μm) 1. The distance (if any) between mating components in an assembly. 2. The distance between two moving parts, or a moving part and stationary part, in a machine (e.g. the gap between a piston and a cylinder head). 3. With threads, the major clearance is the distance between the design form at the root of an internal thread and the crest of its mating external thread; the minor clearance is the corresponding dimension between the crest of an internal thread and the root of the external thread. |
| Clearance angle |
(relief angle) (Unit °) The angle between the underneath or flank of a cutting tool and the machined surface. |
| Clearance fit |
A range of clearances ranging from close sliding to loose running, i.e. a fit in which the limits for the mating parts always permit assembly. |
| Clearance hole |
A hole of specified size such that a bolt, stud, etc. of the same nominal size will always pass through. |
| Clearance volume |
(Unit m3) The ‘dead’ volume above a piston, including the recess in the cylinder head, in a reciprocating compressor or engine when the piston is at top dead centre. |
| Cleavage |
The tendency of a material to cleave or split along definite crystallographic planes. |
| Cleavage fracture |
A fracture, usually of a polycrystalline metal, in which most of the grains have failed by cleavage, resulting in bright reflecting facets. It is one type of crystalline fracture and is associated with low energy brittle fracture. Contrast with shear fracture. |
| Cleavage fracture |
A fracture created by splitting (cleavage), as between layers in materials like slate or mica. In brittle metals, and brittle microconstituents in alloys, cleavage occurs along particular crystal planes. |
| Clevis |
A U-shaped hook with holes at the ends through which a retaining bolt or pin (clevis pin) passes. |
| Clevis joint |
A joint formed by two parallel cantilever beams pressed upon a component between the beams. |
| Clip gauge |
A displacement gauge consisting of two thin strain-gauged cantilever arms attached through knife edges to a testpiece to give the load–line displacement in fracture mechanics test pieces, or used as an extensometer in tensile tests. |
| Closed system |
A closed thermodynamic system consists of a fixed amount of mass. No mass can cross its boundary although energy can, in the form of work or heat, and its volume can change. |
| Closed-die forging |
The forming of a workpiece by compression within a pair of dies having the female form of the component to be manufactured (closed dies), superfluous metal being expelled as flash where the dies meet. |
| Clutch |
A device for connecting and disconnecting rotating shafts, for example between an engine and a gearbox. |
| Coalescence |
Growth of grains at the expense of the remainder by absorption or the growth of a phase or particle at the expense of the remainder by absorption or reprecipitation. |
| Coarse threads |
Threads with relatively large separation between corresponding points on the threads. |
| Coarsening |
An increase in the grain size, usually, but not necessarily, by grain growth. |
| Coated abrasive |
An abrasive tool consisting of a flexible backing material, such as a woven cloth, paper or vulcanized fibre, a bond material, such as a glue or synthetic resin, and grit. |
| Coaxial |
A term for components having a common axis such as concentric shafts. |
| Coefficient of cubic expansion |
The fractional increase in volume per unit temperature rise. |
| Coefficient of discharge (CD) |
For flow through a nozzle or orifice plate, the ratio of the actual mass flow rate to the theoretical mass flow rate calculated assuming the flow to be isentropic. The coefficient of velocity (velocity coefficient) is the corresponding ratio of the actual average velocity to the theoretical value. For incompressible flow, the theoretical flow rate and velocity can be calculated using Bernoulli’s equation. |
| Coefficient of friction |
(coefficient of kinetic friction, coefficient of sliding friction, friction coefficient, μ) The ratio of the frictional force F to the normal force N between two surfaces in contact, i.e. μ = F/N. Static friction is when there is no relative sliding; kinetic friction when there is. |
| Coefficient of friction |
For structural components sliding on one another, the coefficient of friction is the ratio of the force causing the sliding to the force perpendicular to the sliding component surfaces. The coefficient of friction is dimensionless with values between zero and one. |
| Coefficient of friction |
The number characterising the force necessary to slide or roll one material along the€surface of another. If an object has a weight N and the coefficient of friction is μ, then the force F necessary to move it without acceleration along a level surface is F = μN. The coefficient of static friction determines the force necessary to initiate movement; the coefficient of kinetic friction determines the force necessary to maintain movement. Kinetic friction is usually smaller than static friction. |
| Coefficient of friction (μ) |
The dimensionless ratio of the force (F) between two bodies to the normal force (N) pressing these bodies together: μ (or f) = (F/N) |
| Coefficient of restitution (e) |
The ratio of the relative velocity of two colliding bodies after collision to that before. In perfectly elastic collisions e = 1; when all the impact energy is dissipated, e = 0. |
| Coefficient of rolling friction |
The ratio of force parallel to a surface, on which an object rolls, to the normal force. Unlike sliding friction, rolling friction depends on the size of the contact patch and the radius of the rolling element, and the behaviour depends on whether the contact is elastic, viscoelastic, or plastic and on hysteresis losses. |
| Coefficient of thermal expansion |
(1) Change in unit of length (or volume) accompanying a unit change of temperature, at a specified temperature. (2) The linear or volume expansion of a given material per degree rise of temperature, expressed at an arbitrary base temperature or as a more complicated equation applicable to a wide range. |
| Coextrusion |
The simultaneous extrusion through the same die of two or more materials in combination. |
| Coherent precipitate |
A crystalline precipitate that forms from solid solution with an orientation that maintains continuity between the crystal lattice of the precipitate and the lattice of the matrix, usually accompanied by some strain in both lattices. Because the lattices fit at the interface between precipitate and matrix, there is no discernible phase boundary. |
| Coherent structure |
A term given to the larger eddies of turbulent shear flow, such as boundary layers, jets, and wakes, that show distinctive correlated patterns of motion. |
| Cohesive strength |
(Unit Pa) A theoretical fracture strength for solids based on interatomic forces, approximately equal to E/10 where E is Young’s modulus. |
| Cohesive zone |
In fracture-mechanics modelling and simulation, the region at the crack tip over which an assumed traction (load-displacement) relation has to be overcome to permit initiation and propagation of a crack. |
| Coil spring |
A spiral (‘clockwork’) or helical (cylindrical) spring. |
| Coil spring |
A spring steel wire wound in a spiral pattern. |
| Coining |
A forging operation, employing a closely-fitting punch and die from which no metal is allowed to escape, in which the surface pattern on the punch and die is imprinted on the blank. |
| Cold die quenching |
A quench utilizing cold, flat, or shaped dies to extract heat from a part. Cold die quenching is slow, expensive, and is limited to smaller parts with large surface areas. |
| Cold extrusion |
In simplest terms, cold extrusion can be defined as the forcing of unheated metal to flow through a shape-forming die. It is a method of shaping metal by plastically deforming it under compression at room temperature while the metal is within a die cavity formed by the tools. The metal issues from the die in at least one direction with the desired cross-sectional contour, as permitted by the orifice created by the tools. Cold extrusion is always performed at a temperature well below the recrystallization temperature of the metal (about 1100 to 1300 degrees F. for steel) so that work-hardening always occurs. In hot extrusion, recrystallization eliminates the effects of work-hardening, unless rapid cooling of the extrusion prevents recrystallization from being completed. |
| Cold treatment |
Treatment carried out after quenching to transform retained austenite into martensite, involving cooling and holding at a temperature below ambient. |
| Cold working |
The plastic deformation of a metal, by rolling (cold rolling), drawing, forging (cold forging), etc. at a temperature well below its recrystallization temperature, which results not only in permanent shape change but also increase in strength and loss of ductility owing to work-hardening. |
| Collapse load (Unit N) |
The applied load at which a structure becomes a mechanism owing to the formation of sufficient plastic hinges for collapse to occur. |
| Collar |
A ring secured to, or integral with, a shaft to give axial location. |
| Columnar structure |
A coarse structure of parallel elongated grains formed by unidirectional growth, most often observed in castings, but sometimes in structures resulting from diffusional growth accompanied by a solid-state transformation. |
| Combined carbon |
The part of the total carbon in steel or cast iron that is present as other than free carbon. |
| Combined dimensioning |
The use of chain dimensioning and parallel dimensioning on the same engineering drawing. |
| Combined heat and power plant |
(CHP plant, COGEN, cogeneration plant, total-energy plant) A plant for the simultaneous production of more than one useful form of energy from the same energy source, such as process heat and electric power. In the arrangement shown, partially expanded steam is extracted from the steam turbine and used to produce process heat. |
| Combined stresses |
The stress state at a point in a component subjected to combination of axial, bending, torsional loadings etc., acting along all reference axes. |
| Combustible |
(inflammable) A term for substances that can be ignited and burned. |
| Combustion |
An exothermic chemical reaction in which a fuel and an oxidant, typically air, react together to release a significant quantity of thermal energy in the presence of a flame. In the case of fluid fuels, ignition may be from an electric spark or compression of the |
| Combustion chamber |
1. In a piston engine, the volume between the head of an individual cylinder and the crown of the piston in which the fuel–air mixture burns during each power stroke of the engine. 2. (combustor) In a gas-turbine engine, a ramjet, an afterburner, or a rocket motor, the component, often cylindrical or annular in shape, in which the fuel–air mixture burns in a continuous-flow process. |
| Combustion deposit |
Ash, carbon, and other incombustible solids, often due to impurities in the fuel, that build up on any surfaces exposed to products of combustion. They can lead to corrosion, reduced heat transfer and so higher flue-gas temperatures and reduced efficiency. |
| Combustion efficiency |
(source efficiency, ηCOMB) A performance measure for combustion equipment defined by ηCOMB = amount of heat released during combustion/(calorific value of the fuel burned). |
| Complete decarburization |
Decarburization with sufficient carbon loss to show only clearly defined ferrite grains under metallographic examination. |
| Composite material |
(composite structure) A general term used of two or more materials or structures acting in combination (e.g. concrete, reinforced concrete, filamentreinforced polymers, laminated materials, particulate-reinforced materials, flitched beams), resulting in values of strength, stiffness, or toughness greater than the base matrix material alone. |
| Composite property |
In thermodynamics, a property defined in terms of the properties of a closed system and its surroundings, such as the non-flow exergy function. |
| Compound pendulum |
A rigid body free to swing about an axis. |
| Compounding |
In a steam engine (compound steam engine) or impulse turbine, the progressive reduction in pressure (expansion) across two or more stages in series. |
| Compressed liquid |
A liquid subjected to a pressure greater than the saturation pressure corresponding to its temperature. |
| Compressibility (Unit 1/Pa) |
A measure of the reduction in volume or increase in density when a substance is subjected to an increase of pressure. It is defined as the reciprocal of the bulk modulus. Liquids and solids are normally considered incompressible, whereas gases are highly compressible. |
| Compressible flow |
A gas flow in which the Mach number M is sufficiently high for the gas density to change significantly. For air, this is when M > 0.3. |
| Compression |
1. Loading, the principal effect of which is to squeeze and shorten a component or testpiece. 2. The reduction in volume and increase in density of a substance as a consequence of increased pressure. |
| Compression crease |
A crease formed during the compression of composites having a high volume fraction of filaments. These composites fail in compression by forming a crease at an angle to the loading axis. |
| Compression failure |
The reduction or removal of a component’s load-bearing capacity in compression, caused by buckling, fracture, crease formation in fibre composites, etc. |
| Compression fitting |
A screwed joint for pipework made resistant to leakage by permanent deformation of a closely-fitting ring, called a ferrule or olive, on tightening. |
| Compression member |
A structural component, the major loading on which is compressive. |
| Compression pressure (Unit Pa) |
The pressure produced in a cylinder of a piston engine by compression of air in the absence of fuel. |
| Compression ratio |
For a piston engine, if the swept volume is SW and the clearance volume is CL, the compression ratio is given by ( SW + CL)/CL, i.e. it is a volume ratio rather than a pressure ratio. |
| Compression ratio |
The volume of a combustion chamber with the piston at the top of its stroke as a proportion of the total volume of the cylinder with the piston at the bottom of its stroke. |
| Compression spring |
A spring that resists compression forces, usually in the form of a helix with separated coils (giving a linear axial stiffness) or a cone with separated coils (giving a non-linear axial stiffness). |
| Compression stroke |
The stroke in a reciprocating compressor or engine during which the working fluid is compressed. |
| Compression test |
A method for assessing the ability of a material to withstand compressive loads. |
| Compression test |
The determination of the stress–strain curve of a material by axial loading of a specimen in compression. In brittle materials failure is in the elastic range; more ductile materials will yield before fracture; very ductile materials will plastically deform extensively before failure. The compression strength (compressive strength), with unit Pa, is the compressive stress that causes failure in a component or structure. |
| Compression wave |
(dilatation wave) In a fluid or a solid, a progressive wave or wavefront that compresses the medium through which it propagates. |
| Compressive strength |
Maximum compressive stress a material is capable of developing. With a brittle material that fails in compression by fracturing, the compressive strength has a definite value. For ductile, malleable, or semiviscous materials (which do not fail in compression by a shattering fracture), the value obtained for compressive strength is an arbitrary value dependent on the degree of distortion that is regarded as effective failure of the material. |
| Compressive stress |
A stress that causes an elastic body to deform (shorten) in the direction of the applied load. Contrast with tensile stress. |
| Compressive stress (Unit Pa) |
The compressive load per unit area at a point in a component. |
| Compressometer |
Instrument for measuring change in length over a given gage length caused by application or removal of a force. Commonly used in compression testing of metal specimens. |
| Compressor |
A turbomachine, of either axial or radial type, that increases the pressure of a gas or vapour. |
| Compressor blades |
The aerofoil-shaped vanes that form the rotor(s) and stator(s) of an axial-flow compressor. The aerodynamic design is more critical than is the case for turbine blades because there is an increase in pressure across each row of blades. The arrows in the diagram indicate the flow direction relative to the blades. |
| Compressor bleed |
The removal of air before the final stage of a multistage compressor operating below design speed, to prevent the final stage from choking. |
| Computational domain |
In CFD, FEM, and other numerical methods, the area or volume within which calculations are performed and on the periphery of which the boundary conditions are specified. |
| Computed path control |
In CNC or robotics, the use of a control program to determine the required path for the tool or end effector. This required path forms the set points for the motor controllers moving the machine tool or robot. |
| Computer control |
The control of an engineering device or system by pre-programmed computer or by feedback control. |
| Computer numerical control (CNC) |
A term relating to machine tools in which the movements of a tool and/or the workpiece are controlled by computer. |
| Computer vision |
The digitization and processing of optical images/patterns by computer in order to recognize parts, orientation, etc. in manufacturing. |
| Computer-aided design (CAD) |
Generally, design and calculations performed by computer; more specifically, the use of computer graphics and models to communicate design concepts. Computer-aided design and computer-aided manufacturing (CAD/CAM) is where component dimensions resulting from CAD are passed by electronic means directly to machines for manufacture. |
| Computer-aided manufacturing |
(CAM, computer-integrated manufacturing, CIM) The use of computers in all branches of manufacturing, not only to control machines and robots for manufacturing and assembly, but also for process planning, and monitoring progress of materials and components during production, etc. |
| Concentrated load |
A load on a component which is distributed over a very small area, idealized as the line load of a wedge or knife edge, and the point load of a cone. |
| Concentrated solar power plant (CSP plant) |
A power plant in which solar radiation is concentrated using mirrors or lenses, typically using parabolic trough mirrors which focus solar radiation on to receiver tubes along the trough’s focal line. In an alternative arrangement molten salt, heated by solar receivers mounted on a tower, is circulated through a steam generator. |
| Concentration ratio |
For a concentrating solar collector, the ratio of the projected area of the concentrator facing the solar beam to the actual area of the receiver. |
| Concurrent engineering |
The integration of the procedures for product design, material selection and manufacturing method to include life-cycle analysis. |
| Condensate strainer |
A filter in a steam plant used to remove particulate matter from condensate before it is added to feedwater. |
| Condensation shock |
For supersonic flow of a moist gas through a divergent nozzle, condensation occurs in the form of spontaneous nucleation at some point downstream of that at which the temperature falls to the saturation temperature. The condensation process proceeds rapidly, and results in a fairly thick discontinuity termed a condensation shock. |
| Condenser |
A heat exchanger in which a substance is changed from its vapour phase to its liquid phase by reducing its temperature to below the saturation temperature. |
| Condenser vacuum |
The sub-atmospheric pressure imposed on the condenser of a steam-power plant which leads to an appreciable increase in overall efficiency. |
| Condensing boiler |
A relatively small industrial or domestic boiler that burns sulfur-free natural gas so that the products of combustion do not contain sulfuric acid and can be allowed to condense on heat-transfer surfaces without danger of corrosion. |
| Conditioning heat treatment |
A preliminary heat treatment used to prepare a material for desired reaction to a subsequent heat treatment. For the term to be meaningful, the exact heat treatment must be specified. |
| Conduit |
A hollow object, of metal, plastic, glass, ceramic, etc., which is relatively long compared with its lateral dimensions and through which there is fluid flow. |
| Cone clutch |
A friction clutch in which an internal cone moves axially in or out of engagement with an external cone. One or both surfaces is lined with high-friction material. |
| Cone pulley |
A stepped pulley having several diameters which, when linked by a laterallymoveable transmission belt to a corresponding pulley, gives a series of speed ratios. |
| Congruent melting point (Unit K) |
congruent melting point (Unit K) The temperature at which a solid substance at a specified pressure changes phase to a liquid of identical composition. |
| Congruent transformation |
An isothermal or isobaric phase change in which both of the phases concerned have the same composition throughout the process. |
| Connecting rod |
(con rod) A link that transmits power from one system to another, often changing linear to rotary motion, as in the rod connecting the piston to the crankshaft in a reciprocating compressor or pump or to the crankshaft in an internal-combustion engine, as in the diagram. The big end (bottom end) is the larger end that connects to the bearing on one of the crankpins of the crankshaft. The little end (small end) is joined by a gudgeon pin to the piston. |
| Constant life diagram |
A plot of experimentally derived fatigue-life data; perhaps the most complex and complete of the popular charts used to represent such data. |
| Constant-force spring |
A spring that has the same restoring force regardless of displacement. The most common type takes the form of a coiled strip that, owing to tight coiling during manufacture, is pre-stressed (a steel measuring tape is an example). The uncoiling force is approximately constant as the change of curvature of the strip is approximately constant. Not to be confused with a clockwork spring from which power can be obtained. |
| Constant-mesh gearbox |
A gearbox in which the pairs of gears giving different speed ratios are constantly in mesh, different ratios being obtained by connecting or disconnecting the relevant gear to the driving shaft. |
| Constant-velocity universal joint |
(CV joint, homokinetic joint) A connexion that transmits constant angular velocity between two shafts that are neither necessarily in line nor whose axial position is necessarily fixed. |
| constitutive equation |
(constitutive relation) 1. In solid mechanics or fluid mechanics, an algebraic or numerical relation for the dependency of stress on deformation, strain, strain rate, temperature, etc. in a material. In solid mechanics such a relation is sometimes called an equation of state. Simple linear examples include Hooke’s law and Newton’s viscosity law. 2. In heat transfer, a relation, such as Fourier’s law of heat conduction, connecting heat flux with temperature gradient. |
| Contact strength (Unit N) |
The maximum allowable load between contacting bodies converted into a stress. |
| Continuous precipitation |
Precipitation from a supersaturated solid solution in which the precipitate particles grow by longrange diffusion without recrystallization of the matrix. Continuous precipitates grow from nuclei distributed more or less uniformly throughout the matrix. They usually are randomly oriented, but may form a Widmanstätten structure. |
| Continuous-type furnace |
A furnace used for heat treating materials that progress continuously through the furnace, entering one door and being discharged from another. See belt furnac, direct-fired tunnel-type furnac, rotary retort furnace, shaker-hearth furnace. |
| Controlled cooling |
Cooling from an elevated temperature in a predetermined manner, to avoid hardening, cracking, or internal damage, or to produce desired microstructure or mechanical properties. |
| Cooling coil |
A simple heat exchanger consisting of a coiled tube, typically of copper or stainless steel, through which is circulated a coolant such as a refrigerant, chilled water, or water mixed with ethylene glycol. Applications include air conditioning, process cooling, and refrigeration. |
| Cooling curve |
A curve showing the relation between time and temperature during the cooling of a material. |
| Cooling stresses |
Residual stresses resulting from nonuniform distribution of temperature during cooling. |
| Core |
In a ferrous alloy prepared for case hardening, that portion of the alloy that is not part of the case . Typically considered to be the portion that (a) appears light on an etched cross section, (b) has an essentially unaltered chemical composition, or (c) has a hardness, after hardening, less than a specified value. |
| Corrosion |
The deterioration of an exposed metal surface due to electrochemical oxidation with its surroundings. In the oxidation reaction, metal atoms give up electrons which are transferred to form another chemical species by a reduction reaction, usually with hydrogen and/or oxygen. A common example is the formation of the iron oxide rust Fe(OH)3 on iron or steel in moist air or water. Rust breaks away easily, exposing fresh iron to the atmosphere, whereas the corrosion product of some metals forms an adherent layer that prevents further oxidation reaction at fresh metal, an effect known as passivation. Examples include aluminium oxide on aluminium, chromium oxide on chromium, chromium oxide on alloys containing chromium such as stainless steels, and the patina formed on copper and zinc. Corrosion protection takes many forms. Good design to avoid crevices and stress is important. In some situations, substances known as inhibitors may be added to the environment to decrease its corrosiveness. The electrochemical nature of corrosion results in some metals having a greater tendency for oxidation than others. Zinc has a stronger tendency than iron and this is made use of in galvanizing. This is one example of cathodic protection, in which one metal is connected electrically to a more reactive metal, called a sacrificial anode, that oxidizes by giving up electrons and so protects the first metal. Coatings may be used to resist corrosion. Paints, plating, and diffusion coatings are the most common and the electrochemical process of anodizing is used to deliberately form a passive layer of oxide on aluminium alloys. Material selection plays a major role. For example, stainless steels with their high chromium content are widely used in corrosive environments. |
| Corrosion cell |
A natural ‘‘battery’’ formed when two metals having different electrical potentials (an Anode and a Cathode) are connected together in the presence of a liquid (the Electrolyte). |
| Corrosion failure |
1. The failure of a component or structure after corrosion has reduced the load-bearing area to an unsupportable level. 2. A situation in which a mechanism cannot function owing to corrosion products preventing free movement at joints. |
| Corrosion fatigue |
Cracking produced by the combined action of repeated or fluctuating stress and a corrosive environment at lower stress levels or fewer cycles than would be required in the absence of a corrosive environment. |
| Corrosive wear |
Wear in which chemical or electrochemical reaction with the environment is significant. |
| Cotter pin |
(cotter) 1. A tapered wedge or pin passing through a tapered slot or hole in one member and bearing against a second member to fix it in location. 2. A split cotter is commonly called a split pin. |
| Cotter pin |
A pin with two legs. With legs together, the pin is placed through the hole in a clevis pin. The legs are then opened outward to prevent the cotter pin from backing out of the hole. The cotter pin, in turn, prevents the load-bearing clevis pin from backing out of its retaining hole. |
| Counter sink |
To cut or shape a depression in an object so that the head of a screw may set flush or below the surface. |
| Countersinking |
The flaring out of the rim of a drilled hole to form a truncated conical depression to receive a screw having a conical head, thus giving a flush fitting. |
| Coupling |
1. Any mechanical fastening connecting two or more shafts, or parts of a mechanism, in order to transmit power. 2. A device for connecting two vehicles. |
| Coupling |
A detachable mechanism forming a joint. A connector of two components of a joint. |
| Crack |
A thin fissure-like defect in a component or structure across which material continuity is lost and which reduces the strength of the body. |
| Crack arrest |
Crack propagation that stops of its own accord when the energy release rate of the loaded component or structure falls below a critical value. If predictable, it can be incorporated into structural-integrity assessments. |
| Crack length (depth) (a) |
In fatigue and stress corrosion cracking, the physical crack size used to determine the crack growth rate and the stress-intensity factor. For the compact-type specimen, crack length is measured from the line connecting the bearing points of load application. For the center-cracked-tension specimen, crack length is measured from the perpendicular bisector of the central crack. |
| Crack size (a) |
A lineal measure of a principal planar dimension of a crack. This measure is commonly used in the calculation of quantities descriptive of the stress and displacement fields. In practice, the value of crack size is obtained from procedures for measurement of physical crack size, original crack size, or effective crack size, as appropriate to the situation under consideration. |
| Crack-extension force (G) |
The elastic energy per unit of new separation area that would be made available at the front of an ideal crack in an elastic solid during a virtual increment of forward crack extension. |
| Crack-extension resistance (KR) |
A measure of the resistance of a material to crack extension, expressed in terms of the stress-intensity factor, the crack-extension force, or values of Jderived using the J-integral concept. |
| Crack-growth rate |
Rate of propagation of a crack through a material due to statically or dynamically applied load. |
| Crank angle (Unit °) |
The angle between the crank of a slider-crank mechanism and a line from the crankshaft centreline to the piston axis. |
| Crank pin |
It is a short shaft parallel to the axis of the crankshaft but radially offset from it, to which is attached the big end of a connecting rod in a bearing. Sometimes the crank pin is supported at one end only (a wrist pin) but in built-up or one-piece forged crankshafts, the crank pin is supported by thick plates (crank arms, crank webs) at either end. |
| Crankshaft |
The main shaft, of which the cranks are a part, of a reciprocating single-or multi-cylinder machine. Crankshafts may be built up in sections or forged as a single component. In an engine, the reciprocating motion of the pistons transmits power to the crankshaft and causes it to rotate, whereas in a pump the crankshaft is driven and its rotation causes the pistons to reciprocate. |
| Creep |
Time-dependent and permanent deformation of material under constant load or stress. The strain increases with time at a rate that increases with increasing temperature. Creep can lead to large strain and eventual failure, and so limit the lifetime of components subjected to load at high temperature. Creep in metals becomes significant at homologous temperatures in excess of about 0.4, e.g. 1200°C for tungsten, 504°C for titanium, 450°C for iron, and 100°C for aluminium. |
| Creep |
The slow, plastic deformation of a body under heavy loads. Time-dependent plasticity. |
| Creep fatigue |
The effects of combined creep and fatigue at high homologous temperatures in metals, and in other materials such as polymers having time-dependent behaviour. |
| Creep limit (Unit Pa) |
The maximum allowable stress under the action of which creep deformation of a material does not exceed a specified limit. |
| Creep modulus (Unit Pa) |
The ratio of stress/strain at a chosen strain level obtained from creep tests, often plotted against time to show changes in stiffness. |
| Creep rupture strength (Unit Pa) |
The fracture stress at the end of a creep test in metals and polymers, often plotted against time to give a stress-rupture curve. |
| Creep–time relations |
Algebraic or numerical relations between creep strain and time at constant stress. |
| Crest |
The highest point of a wave or of a screw thread. |
| Crest clearance (Unit m) |
For screw threads and gearing, the radial clearance between the crest of a thread (or gear) and the root of the engaging thread (or gear). |
| Crest of screw thread |
The top surface joining the two sides of flanks of a thread. |
| Critical compression ratio |
1. The compression ratio for incipient knock of hydrocarbon fuels as determined in a variable-compression single-cylinder piston engine. 2. The compression ratio at which an air–fuel mixture will spontaneously ignite due to the temperature increase produced. |
| Critical cooling rate |
The rate of continuous cooling required to prevent undesirable transformation. For steel, it is the minimum rate at which austenite must be continuously cooled to suppress transformations above the Ms temperature. |
| Critical crack length (Unit m) |
The crack length at which a given stress applied to a body will result in crack propagation and fracture. |
| Critical diameter |
(D) Diameter of the bar that can be fully hardened with 50% martensite at its center. |
| Critical load (Unit N) |
1. The applied load that causes propagation of an existing crack of known length, and hence fracture of a component or structure. 2. The applied load that results in buckling of a column of given end fixity. |
| Critical point |
(1) The temperature or pressure at which a change in crystal structure, phase or physical properties occurs. Same as transformation temperature. (2) In an equilibrium diagram, that specific value of composition, temperature and pressure, or combinations thereof, at which the phases of a heterogeneous system are in equilibrium. |
| Critical strain |
The strain just sufficient to cause recrystallization; because the strain is small, usually only a few percent, recrystallization takes place from only a few nuclei, which produces a recrystallized structure consisting of very large grains. |
| Critical temperature |
(1) Synonymous with critical point if the pressure is constant. (2) The temperature above which the vapor phase cannot be condensed to liquid by an increase in pressure. |
| Critical temperature ranges |
Synonymous with transformation ranges, which is the preferred term. |
| Crossed threads |
When the axis of a nut offered up to a bolt is not aligned with the axis of a bolt (or a screw to a threaded member), it may be possible for the threads to engage incorrectly and even for the nut or screw to advance a turn or more, but ultimately the misaligned threads become locked together. Forcing the nut in such circumstances may irretrievably damage the threads on both. |
| Crosshead |
1. A reciprocating member, sliding between guides, to which the piston rod is firmly attached on one side and to which the connecting rod is pinned on the other for the conversion of reciprocating into rotary motion. 2. The moveable beam in early designs of mechanical testing machines. 3. A screwhead having slots in a + shape that takes a mating screwdriver. |
| Crushing strain |
The supposed single strain that results in comminution of a brittle material. The corresponding stress is termed the crushing strength. Neither quantity can be a material constant as they must obey the laws of fracture mechanics and will depend on the type of testpiece, method of loading etc. |
| Crystalline defects |
The deviations from a perfect three-dimensional atomic packing that are responsible for much of the structure-sensitive properties of the materials. Crystal defects can be point defects (dislocations) or surface defects (vacancies), line defects (dislocations), or surface defects (grain boundaries). |
| Crystalline fracture |
A pattern of brightly reflecting crystal facets on the fracture surface of a polycrystalline metal, resulting from cleavage fracture of many individual crystals. |
| Cup washer |
A dished leather or neoprene washer fitted to the rod end in some piston-style pumps. |
| Curie temperature |
The temperature of magnetic transformation below which a metal or alloy is ferromagnetic and above which it is paramagnetic. |
| Cyaniding |
A case-hardening process in which a ferrous material is heated above the lower transformation range in a molten salt containing cyanide to cause simultaneous absorption of carbon and nitrogen at the surface and, by diffusion, create a concentration gradient. Quench hardening completes the process. |
| Cycle annealing |
An annealing process employing a predetermined and closely controlled time-temperature cycle to produce specific properties or microstructures. |
| Cylinder head |
The machined casting, typically of aluminium alloy or cast iron, that fits above the cylinder block of a piston engine and closes off the cylinders. It normally includes part of the combustion chambers and holes for the valves and spark plugs. |
| Damage |
1. The deterioration of a component or structure in fault or accident conditions, reducing or preventing its ability to perform its intended function. 2. The accumulation of defects or microcracks in the microstructure of a body loaded monotonically or in fatigue, which weakens the body and can lead to crack propagation and failure. |
| Damage mechanics |
The theory of degradation in bodies, particularly fracture by accumulated microstructural damage. Analyses take two approaches: (a) the use of some critical integrated function of stress and strain at which cracking is initiated and propagated; (b) incorporation of damage in the stress–strain curves to reflect weakened material. |
| Damage tolerance |
(defect tolerance) A design philosophy that takes into account initial imperfections, crack-growth rates and conditions at final fracture, and uses fracture mechanics to demonstrate that cracks should not grow to their critical length within the design life (or at least should be capable of ready detection). |
| Dead centre |
The point at which, in a crank mechanism, the piston connecting rod and crank are all in line so that there can be no driving moment. |
| Dead load (Unit N) |
A load on a component or structure that is steady with time, e.g. the self-weight of a bridge. |
| Dead space |
(dead volume) (Unit m3) That volume of a gas-filled thermometer in which the gas is not at the same temperature as that being measured. |
| Decalescence |
A phenomenon, associated with the transformation of alpha iron to gamma iron on the heating (superheating) of iron or steel, revealed by the darkening of the metal surface owing to the sudden decrease in temperature caused by the fast absorption of the latent heat of transformation. |
| Decarburization |
Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. |
| Decarburization |
Loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. |
| Decompression chamber |
A chamber in which ambient-air pressure can be increased to levels found in deep-sea diving. It is used to gradually acclimatize divers back to normal conditions and avoid ‘the bends’. |
| Deflection temperature under load (DTUL) |
The temperature at which a simple cantilever beam deflects a given amount under load. Formerly called heat distortion temperature. |
| Deformation |
A change in the form of a body due to stress, thermal change, change in moisture, or other causes. |
| Deformation (Unit 1/s) |
In solid mechanics, any change, reversible (elastic) or permanent (plastic), in the shape or size of parts of a body, or the whole body, caused by external or internal loading. It includes extension, compression, bending, and twisting. The same state of deformation in a body can appear as different combinations of elongation and shear. To state how much of the total strain is shear, it is split into dilatation and deviatoric components, the former changing volume but not shape, the latter shape but not volume. |
| Degradation |
The reduction with time of the physical properties of a material. |
| Degradation failure |
Failure of a system, component, or structure owing to material degradation. |
| Degradation of energy |
Conversion of energy into forms of lower usefulness due to irreversibilities in energy transfer and conversion processes. The increase in entropy can be regarded as a measure of the degradation of energy. |
| Degree (°) |
A measure of plane angle such that 1° is 1/360 of a complete revolution and equal to π/180 rad. |
| Degrees of freedom |
The number of independent variables (such as temperature, pressure, or concentration within the phases present) that may be altered at will without causing a phase change in an alloy system at equilibrium; or the number of such variables that must be fixed arbitrarily to define the system completely. |
| Delamination |
A mode of failure of composite materials, including radial-ply tyres, in which the layers separate due to repeated cyclic loading, impact, or weak bonding. |
| Densimeter |
An instrument used to determine the density or relative density of a solid or liquid. |
| Density |
The mass per unit volume of a solid material. |
| Density |
(mass density, ρ) (Unit kg/m3) The mass per unit volume of a substance that satisfies the continuum assumption. The reciprocal of specific volume. |
| Deposit gauge |
(deposition gauge) An instrument employed in air pollution studies for measuring the amount of pollutant deposited on a given area in a given time under given conditions. |
| Depth (Unit m) |
1. The vertical distance below a datum surface, especially the sea surface. 2. The distance between the top and bottom of a hole, step in a surface, or a container. |
| Depth gauge |
1. A precision instrument, typically consisting of a machine-divided steel rule passing through a hardened-steel cross head, used to measure the depths of slots, holes, shoulders, projections, etc. 2. A device used by divers to indicate the water depth. |
| Design code |
(design standard) A standard or specification for any aspect of engineering design, issued by national organizations such as ANSI, ASME, BSI, DIN, and ISO. |
| Design heating load (Unit kW) |
The heating requirements based on a specified number of heating degree days, or required to maintain a building or other enclosed space at a specified temperature for a given outside temperature. |
| Design load (Unit N) |
The greatest load that a component or structure is expected to experience under normal operating conditions. |
| Design pressure (Unit Pa) |
The greatest pressure that a closed container is expected to experience under normal operating conditions. |
| Design stress (Unit Pa) |
The greatest allowable stress in a component or structure that will not result in failure under normal operating conditions. |
| Destructive testing |
Measurement of the mechanical properties of a material, component, or structure, by increased loading until the sample fails by fracture, collapse, or buckling. |
| Deviation |
The difference between the actual value and the desired value of a controlled variable. |
| Dew point |
The temperature and pressure at which a gas begins to condense to a liquid. |
| Dew point analyzer |
An atmosphere monitoring device that measures the partial pressure of water vapor in an atmosphere. |
| DIN |
Deutsches Institut für Normung, the German Institute for Standardization. |
| Diagonal pitch (Unit m) |
The distance, when components in an assembly are staggered, between the position of a component in one row or column and the position of the corresponding component in the next row or column. The term is applied to rivets, turbine or compressor blades in a cascade, vortex generators on the surface of a wing, etc. |
| Diaphragm compressor |
A machine in which compression is achieved by the reciprocating motion of a flexible membrane of metal, plastic, or elastomeric material. This arrangement is ideally suited to pumping high-purity, toxic, or explosive gases. A similar machine incorporating check valves is a self-priming positive-displacement pump (diaphragm pump, membrane pump). |
| Diaphragm meter |
A dry flow meter in which there are two or more interconnected chambers, each having a diaphragm in the wall. The chambers empty and fill alternately and the flow rate of gas is determined from the movement of diaphragms. Diaphragm meters are commonly used to monitor domestic and commercial gas supply. |
| Die |
1. A tool having an appropriately-shaped hole through which material may be extruded or drawn. 2. A tool employed in forging. 3. A block having the male or female shape employed in stamping operations. 4. A thick circular disc, driven by a die wrench, with internally-threaded cutting edges for producing a screw thread. A die wrench (die stock) holds a screw-cutting die and has two projecting arms for applying the torque necessary to cut the thread. A die having a hexagonal or square shape (die nut), driven by a spanner, is used in confined spaces where a die wrench cannot be rotated through a full circle. |
| Die casting |
A process in which molten metal, particularly alloys of aluminium, magnesium, copper, and zinc, is forced under pressure (10 to 200 MPa) into a reusable hardened-steel mould machined into a die. |
| Die screw |
Tool used when threading cylindrical pieces beyond the capacity of a die plate. |
| Die threading |
Die threading is a machining process for cutting external threads in cylindrical or tapered surfaces by the use of solid or self-opening dies. Die threading is a slower method of producing external threads than thread rolling, but it is faster than single-point threading in a lathe. |
| Diesel engine |
(compression–ignition engine) A piston engine operating on the Diesel cycle in which the air is compressed to a temperature above the autoignition temperature of the fuel and combustion is initiated as the fuel is injected into the hot air. Diesel engines operate at higher compression ratios (typically in the range 12 to 24) than petrol engines. Although named after Rudolf Diesel, the inventor, Diesel engine is often spelled with a lower case d, contrary to normal practice. |
| Differential heating |
Heating that intentionally produces a temperature gradient within an object such that, after cooling, a desired stress distribution or variation in properties is present within the object. |
| Diffusion |
(1) Spreading of a constituent in a gas, liquid, or solid, tending to make the composition of all parts uniform. (2) The spontaneous movement of atoms or molecules to new sites within a material. |
| Diffusion coating |
Coating on metal for corrosion- and/or oxidation-resistance. Coatings are produced by heating a metal in contact with an alloy in powder, liquid, or gaseous form at elevated temperature such that atoms of the coating diffuse into the substrate. Examples include chromium, aluminium, or silicon on substrates such as nickel-based superalloys, steels, and refractory metals. |
| Diffusion coefficient |
A factor of proportionality representing the amount of substance diffusing across a unit area through a unit concentration gradient in unit time. |
| Dilatation (dilation) |
A change of volume caused by external load, compression, temperature change, chemical action, etc. |
| Dilatometer |
An instrument for measuring length or volume changes in a solid during heating and subsequent cooling or isothermal holding. |
| Dilatometer |
· An instrument for measuring the linear expansion or contraction in a metal resulting from changes in such factors as temperature and allotropy. |
| Dimension line |
A line on an engineering drawing with a numeral above it that shows the length of a feature, usually in millimetres. |
| Dimensional stability |
Ability of a plastic part to retain the precise shape in which it was molded, fabricated, or cast. |
| dimensioning |
The specification on an engineering drawing of the size (e.g. length, radius, angle, or spacing) and the relative location (e.g. angular position) of each feature of a component. The numerical values often include the tolerances. There should be no more dimensions than are necessary to manufacture the component. |
| Direct quenching |
(1) Quenching carburized parts directly from the carburizing operation. (2) Also used for quenching pearlitic malleable parts directly from the malleablizing operation. |
| Dislocation density |
The total length of dislocation lines per unit volume, or the number of dislocation lines that cut through a unit cross-sectional area. |
| Displacement |
The distance that a chosen measurement point on a cracked specimen displaces normal to the crack plane as the crack grows. |
| Distortion |
Any deviation from an original size, shape, or contour that occurs because of the application of stress or the release of residual stress. |
| Double aging |
Employment of two different aging treatments to control the type of precipitate formed from a supersaturated matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate or stabilizing, is usually carried out at higher temperature than the second. |
| Double tempering |
A treatment in which a quench-hardened ferrous metal is subjected to two complete tempering cycles, usually at substantially the same temperature, for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting microstructure. |
| Double-shear test |
A shear test having two stationary, shear blades and one moving one and that uses solid round bars as test specimens. |
| Dowel |
A headless cylindrical pin that fits into corresponding holes in mating components, thus ensuring relative location. |
| Dowel |
A round metal or wooden pin. |
| Dowel |
A pin, usually of circular shape like a cylinder, used to pin or fasten something in position temporarily or permanently. |
| Drift pin |
A round tapered steel pin used to align rivet holes so that the rivet will pass through the holes easily. |
| DTI |
Direct tension indicator. A fastener used primarily in the structural steel industry, designed to indicate that a certain minimum amount of tension has been developed in the fastener during assembly. |
| Ductile |
(ductile material) Describes a material that can be deformed permanently to large strains without fracture. |
| Ductile |
If a bolt can be stretched well past its yield point before breaking, it is said to be ductile. |
| Ductile cast iron |
A cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. Also known as nodular cast iron, spherulitic graphite cast iron and SG iron. |
| Ductile crack propagation |
Slow crack propagation that is accompanied by noticeable plastic deformation and requires energy to be supplied from outside the body. |
| Ductile fracture |
Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. |
| Ductile fracture |
A fracture of a component or structure which is preceded by extensive ductile deformation so that the broken pieces cannot be re-fitted to regain the original size and shape of the component or structure. |
| Ductile fracture |
Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. |
| Ductility |
The ability of a material to deform plastically before fracturing. Measured by elongation or reduction in area in a tensile test, by height of cupping in a cupping test, or by the radius or angle of bend in a bend test. |
| Ductility |
The ability of a material to deform plastically without fracturing, measured by elongation or reduction of area in a tensile test, by height of cupping in an Erichsen test, or by other means. |
| Durometer hardness |
Measure of the indentation hardness of plastics. It is the extent to which a spring-loaded steel indenter protrudes beyond the pressure foot into the material. |
| Dynamic compressor |
A machine, such as a centrifugal or axial compressor, that compresses a gas by rotational rather than reciprocating motion. |
| Dynamic coupling |
In vibrations, the existence of inertial terms depending on mass in the governing equations, so that there is only a force if there is a corresponding acceleration. |
| Dynamic hardness |
(rebound hardness) The resistance of a material to local indentation by a rapidly-moving rigid indenter. In most practical methods the indenter is allowed to fall under gravity on to the surface of the material when the rebound height is a measure of the dynamic hardness. |
| Dynamic load (Unit N) |
1. Loading of a component or structure by a moving object whose point of application changes with time, e.g. the live load of a train passing over a bridge. 2. A load applied to a particular part of a component or structure in a short time interval. |
| Dynamic load rating (Unit N) |
The allowable load on a component or structure when the loading is not static. |
| Dynamic mechanical measurement |
A technique in which either the modulus and/or damping of a substance under oscillatory load or displacement is measured as a function of temperature, frequency, or time, or a combination thereof. |
| Dynamic modulus |
The ratio of stress to strain under cyclic conditions (calculated from data obtained from either free or forced vibration tests, in shear, compression, or tension). |
| Dynamic recovery |
A process occurring in hot working of metals in which a fine subgrain structure forms within the elongated grains due to annihilation of dislocations due to easy cross slip and climb. It results in a lowering of the flow stress. Dynamic recovery, as opposed to dynamic recrystallization that occurs in hot working, occurs in metals of high stacking fault energy such as aluminum, αiron, and most bcc metals. |
| Dynamic strain aging |
A behavior in metals in which solute atoms are sufficiently mobile to move toward and interact with dislocations. This results in strengthening over a specific range of elevated temperature and strain rate. |
| Dynamics |
The branch of mechanics that deals with objects in motion. Its two main branches are kinematics, which studies motion without regards to its cause, and kinetics, which also takes into account forces that cause motion. |
| Eccentric load |
The external load on a fastener or groups of fasteners is said to be eccentric if the resultant of that load does not pass through the centroid of the group of fasteners (eccentric shear load ) or does not coincide with the bolt axis (eccentric tensile load ). |
| Edge distance |
The distance from the edge of a bearing specimen to the center of the hole in the direction of applied force. |
| Edge distance ratio |
The ratio of the edge distance to the pin diameter in a bearing test. |
| Effective crack size (ae) |
The physical crack size augmented for the effects of cracking plastic deformation. Sometimes the effective crack size is calculated from a measured value of a physical crack size plus a calculated value of a plastic-zone adjustment. A preferred method for calculation of effective crack size compares compliance from the secant of a load-deflection trace with the elastic compliance from a calibration for the type of specimen. |
| Effective discharge area (Unit m2) |
A nominal area for flow through a pressure relief valve used to determine the valve’s flow capacity given the pressure difference across it, the fluid density and correction factors to allow for compressibility, the back pressure and the coefficient of discharge. |
| Effective length of a bolt |
The grip length plus some portion of the bolt (often one-half of the thickness of the nuts) which lies within the nut(s) plus some portion (often one-half the thickness) of the head. |
| Effective radius of nut, bolt head, or threads |
Distance between the geometric center of the part and the circle of points through which the resultant contact forces between mating parts passes. Must be determined by integration. |
| Effective yield strength |
An assumed value of uniaxial yield strength that represents the influence of plastic yielding on fracture test parameters. |
| Elastic calibration device |
A device for use in verifying the load readings of a testing machine consisting of an elastic member(s) to which loads may be applied, combined with a mechanism or device for indicating the magnitude (or a quantity proportional to the magnitude) of deformation under load. |
| Elastic constants |
The factors of proportionality that relate elastic displacement of a material to applied forces. |
| Elastic deformation |
A change in dimensions directly proportional to and in phase with an increase or decrease in applied force. |
| Elastic energy |
The amount of energy required to deform a material within its elastic range of behavior, neglecting small heat losses due to internal friction. The energy absorbed by a specimen per unit volume of material contained within the gage length being tested. It is determined by measuring the area under the stress-strain curve up to a specified elastic strain. See also modulus of resilience and strain energy. |
| Elastic interactions |
When a bolt is tightened it partially compresses the joint members ‘‘in its own neighborhood.’’ When nearby bolts are tightened later, they further compress the joint in this region. This allows the first bolt to relax a little (lose a little preload). Tightening bolts on the opposite side of the joint, however, might increase preload in some of the earlier bolts tightened on the near side. These shifts and changes in the elastic energy stored in individual bolts, during assembly, are called elastic interactions. |
| Elastic limit |
The maximum stress which a material is capable of sustaining without any permanent strain (deformation) remaining on complete release of the stress. |
| Elastic limit |
The maximum stress that a material is capable of sustaining without any permanent strain (deformation) remaining upon complete release of the stress. |
| Elastic recovery |
Amount the dimension of a stressed elastic material returns to its original (unstressed) dimension on release of an applied load. In hardness testing, the shortening of the original dimensions of the indentation upon release of the applied load. |
| Elastic resilience |
The amount of energy absorbed in stressing a material up to the elastic limit; or, the amount of energy that can be recovered when stress is released from the elastic limit. |
| Elastic strain energy |
The energy expended by the action of external forces in deforming a body elastically. Essentially, all the work performed during elastic deformation is stored as elastic energy, and this energy is recovered upon release of the applied force. |
| Elasticity |
The property of a material by virtue of which deformation caused by stress disappears on removal of the stress. A perfectly elastic body completely recovers its original shape and dimensions after release of stress. |
| Elbow |
1. A fitting that connects the ends of two pipes at an angle, 45°, 90°, and 180° being the most common. 2. The third joint on an articulated robot corresponding to the human elbow. |
| Electrode |
The two metallic bodies in a battery or Corrosion cell which give up electrons (the Anode) or which attract them (the Cathode). |
| Electrolyte |
The liquid with which the Electrodes of a battery or Corrosion cell are wetted. |
| Electroplating |
Deposition of one metal on to another using electrolysis. The metal to be plated forms the cathode in an electrolytic cell, and the metal to be deposited forms the anode. |
| Elongation |
A term used in mechanical testing to describe the amount of extension of a test piece when stressed. |
| Elongation measurement |
Bolt elongation is directly proportional to axial stress when the applied stress is within the elastic range of the material. If both ends of a bolt are accessible, a micrometer measurement of bolt length made before and after the application of tension will ensure the required axial stress is applied. |
| Embedment |
Localized plastic deformation in heavily loaded fasteners allows one part to sink into, or smooth the surface of, a softer or more heavily loaded second part. Nuts embed themselves in joint surfaces. Bolt threads embed themselves in nut threads, etc. |
| Embrittlement |
The loss of ductility or fracture toughness of materials, either during processing or in service. |
| Embrittlement |
The severe loss of ductility or toughness or both, of a material, usually a metal or alloy. Many forms of embrittlement can lead to brittle fracture. Many forms can occur during thermal treatment or elevated-temperature service (thermally induced embrittlement). Some of these forms of embrittlement, which affect steels, include blue brittleness, 885 °F (475 °C) embrittlement, quench-age embrittlement, sigma-phase embrittlement, strain-age embrittlement, temper embrittlement, tempered martensite embrittlement, and thermal embrittlement. In addition, steels and other metals and alloys can be embrittled by environmental conditions (environmentally assisted embrittlement). The forms of environmental embrittlement include acid embrittlement, caustic embrittlement, corrosion embrittlement, creep-rupture embrittlement, hydrogen embrittlement, liquid metal embrittlement, neutron embrittlement, solder embrittlement, solid metal embrittlement, and stress-corrosion cracking. |
| End-quench hardenability test |
A laboratory procedure for determining the hardenability of a steel or other ferrous alloy. Hardenability is determined by heating a standard specimen above the upper critical temperature, placing the hot specimen in a fixture so that a stream of cold water impinges on one end, and, after cooling to room temperature is completed, measuring the hardness near the surface of the specimen at regularly spaced intervals along its length. The data are normally plotted as hardness versus distance from the quenched end. |
| Endurance |
The capacity of a material to withstand repeated application of stress. |
| Endurance limit |
The maximum stress below which a material can presumably endure an infinite number of stress cycles. The value of the maximum stress and the stress ratio also should be stated. |
| Endurance limit |
That completely reversing stress limit below which a bolt or joint member will have an essentially infinite life under cyclic fatigue loads. Note that the mean stress on the bolts here is zero. |
| Energy losses (Unit kJ) |
A commonly-used misnomer in view of the conservation-ofenergy principle, but used to mean energy converted into forms that are not used in a process, for example thermal energy from a heat engine dissipated to the surroundings, or produced by friction in a machine, or by surface drag. |
| Engine |
A machine that converts energy, including the chemical energy in a fuel and electrical energy, into mechanical energy, usually to produce power delivered through a rotating shaft or thrust. Examples include internal-combustion engines, gas and steam turbines, rocket engines, electric, hydraulic and pneumatic motors. |
| Engine emissions |
(exhaust emissions) The pollutants contained in the exhaust gases from an internal-combustion engine. |
| Engineering strain (e) |
A term sometimes used for average linear strain or conventional strain in order to differentiate it from true strain. In tension testing it is calculated by dividing the change in the gage length by the original gage length. |
| Engineering stress (s) |
A term sometimes used for conventional stress in order to differentiate it from true stress. In tension testing, it is calculated by dividing the breaking load applied to the specimen by the original crosssectional area of the specimen. |
| Equation, long form |
An equation which relates the torque applied to a bolt to the preload created in it, and involves fastener geometry and the coefficient of friction between mating surfaces. A theoretical equation based on rigid body mechanics and the assumption that the geometry of the fastener is perfectly described by blueprint dimensions. |
| Equation, short form |
An empirical equation which relates the torque applied to the bolt to the preload created in it, and which depends mainly on an experimentally derived factor called the Nut factor. |
| Equilibrium diagram |
A graphical representation of the temperature, pressure and composition limits of phase fields in an alloy system as they exist under conditions of complete equilibrium. In metal systems, pressure is usually considered constant. |
| Erosion |
Progressive loss of original material from a solid surface due to mechanical interaction between that surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles. |
| Essential conditions |
Each type of failure to which bolted joints are subject is set up by three or four conditions. The conditions vary, depending on the mode of failure, but never number more than four. Eliminating any one of the essential conditions for a particular type of failure can prevent that type of failure. |
| Eutectic |
(1) An isothermal reversible reaction in which a liquid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectic point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectic reaction. |
| Eutectic carbide |
Carbide formed during freezing as one of the mutually insoluble phases participating in the eutectic reaction of ferrous alloys. |
| Eutectic melting |
Melting of localized microscopic areas whose composition corresponds to that of the eutectic in the system. |
| Eutectoid |
(1) An isothermal reversible reaction in which a solid solution is converted into two or more intimately mixed solids on cooling, the number of solids formed being the same as the number of components in the system. (2) An alloy having the composition indicated by the eutectoid point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectoid reaction. |
| Expansion bolt |
When a through bolt cannot be used for attaching a pipe hanger, bracket, or other part, to a wall or ceiling of brick or concrete, what are known as expansion bolts are often used. The body of an expansion bolt is divided and the arrangement is such that, when the head of the bolt is turned, the sections forming the body of the bolt are forced outward and against the wall of the hole which has been drilled into the brick, concrete, or stone, as the case may be. Bolts of this type are made in quite a variety of designs. The nominal size represents the diameter of the bolt proper and not the diameter of the casing or expansion member. |
| Extensometer |
An instrument for measuring changes in length over a given gage length caused by application or removal of a force. Commonly used in tension testing of metal specimens. |
| Extensometer |
Any instrument which measures the change in length of a part as the part is loaded. |
| External load |
Forces exerted on fastener and=or joint members by such external factors as weight, wind, inertia, vibration, temperature expansion, pressure, etc. Does not equal the Working load in the fastener. |
| Extra hard |
A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength and hardness about one-third of the way from full hard to extra spring temper. |
| Extra spring |
A temper of nonferrous alloys and some ferrous alloys corresponding approximately to a coldworked state above full hard beyond which further cold work will not measurably increase the strength and hardness. |
| Eye bolt |
A bolt threaded at one end and bent to a loop at the other end. |
| Eyebolt (eyescrew) |
A bolt or screw with a closed loop in place of a head. |
| Face |
To machine a flat surface perpendicular to the axis at rotation on a lathe. |
| Fahrenheit |
The temperature scale based on the freezing point of water (32ºF) and the boiling point of water (212ºF). The interval between these points is divided into 180 equal parts. Although replaced by the Celsius scale, the Fahrenheit scale is still sometimes used for non-scientific measurements. |
| Failure |
The result when a body, component, or structure is incapable of performing the task for which it was designed. The term is often used without reference to what causes failure, such as fracture, buckling, excessive deformation, wear, or erosion. Failure criteria (failure theories, theories of strength) are mathematical expressions for the combinations of stress, strain, or strain energy at which materials fail, which are employed in design to dimension components. |
| Failure of the bolt |
Term implying that the bolt has broken or the threads have stripped. There can be many reasons for this. |
| Failure of the joint |
Failure of a bolted joint to behave as intended by the designer. Failure can be caused or accompanied by broken or lost bolts, but can also mean joint slip or leakage from a gasketed joint even if all bolts still remain whole and in place. Common reasons for joint failure include vibration loosening, poor assembly practices, improper design, unexpected service loads or conditions, etc. |
| Fastener |
A fastener is a mechanical device for holding two or more bodies in definite positions with respect to each other. |
| Fastener identification marking |
A stamp, paint, or other permanent identifier that may include manufacturer information and applicable grade markings for certification purposes. |
| Fastener manufacturer |
An organization that fabricates raw steel into a fastener meeting specified standards. |
| Fastener quality |
A fastener’s adherence to its specification for dimensional tolerances, mechanical properties, and other requirements stated under applicable standards. |
| Fastener specification |
A precise statement of set requirements to be satisfied by a fastener, its material, or its processing. It also indicates the procedure used to determine whether the requirements given are satisfied. |
| Fastener standard |
A document which details the attributes of a finished fastener and includes such characteristics as geometry, material or chemistry, heat treatment, finish, testing lot size, and packaging. |
| Fastener testing |
A determination or verification that the fastener meets its specification requirements. |
| Fastener with reduced shank |
Finished fastener with a shank diameter of ds ≈ d2. |
| Fastener with waisted shank |
Finished fastener with a shank diameter of ds < d2. |
| Fatigue |
The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. Fatigue failure generally occurs at loads that, applied statically, would produce little perceptible effect. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress. |
| Fatigue |
A term referring, in components and structures subjected to either random or cyclic periodically-varying loads, to a progressive reduction in strength leading to failure at stresses lower than those that cause failure under monotonic loading. Variable loads arise from outof-balance machinery and other vibration sources, wind gusts, etc., and a large proportion of service failures is caused by fatigue. Fatigue results from the initiation and slow propagation of cracks. In manufactured components, crack initiation usually occurs at a point of stress concentration. After a period, often of millions of stress cycles, the crack reaches a critical length at which the next peak load causes sudden brittle or ductile fracture. Fracture surfaces resulting from fatigue display characteristic striations or progression marks emanating from the crack initiation site during the slow crack growth period, with a different surface appearance for the final fracture. |
| Fatigue crack growth rate (da/dN) |
The rate of crack extension caused by constant-amplitude fatigue loading, expressed in terms of crack extension per cycle of load application. |
| Fatigue failure |
Failure that occurs when a specimen undergoing fatigue completely fractures into two parts or has softened or been otherwise significantly reduced in stiffness by thermal heating or cracking. |
| Fatigue life (N) |
The number of cycles of stress or strain of a specified character that a given specimen sustains before failure occurs. |
| Fatigue life for p % survival |
An estimate of the fatigue life that p% of the population would attain or exceed at a given stress level. The observed value of the median fatigue life estimates the fatigue life for 50% survival. Fatigue life for p% survival values, where p is any number, such as 95, 90, etc., may also be estimated from the individual fatigue life values. |
| Fatigue limit |
The maximum stress that presumably leads to fatigue fracture in a specified number of stress cycles. The value of the maximum stress and the stress ratio also should be stated. Compare with endurance limit. |
| Fatigue limit for p % survival |
The limiting value of fatigue strength for p%survival as the number of stress cycles becomes very large; pmay be any number, such as 95, 90, etc. |
| Fatigue notch factor (Kf) |
The ratio of the fatigue strength of an unnotched specimen to the fatigue strength of a notched specimen of the same material and condition; both strengths are determined at the same number of stress cycles. |
| Fatigue strength |
The maximum stress that can be sustained for a specified number of cycles without failure. The value of the maximum stress and the stress ratio also should be stated. |
| Fatigue strength |
The stress to which a metal can be subjected for a specified number of cyclic changes of stress. |
| Fatigue striations |
Parallel lines frequently observed in electron microscope fractographs or fatigue-fracture surfaces. The lines are transverse to the direction of local crack propagation; the distance between successive lines represents the advance of the crack front during the one cycle of stress variation. |
| Fatigue test |
A method for determining the range of alternating (fluctuating)stresses a material can withstand without failing. |
| Fatigue wear |
Wear of a solid surface caused by fracture arising from material fatigue. |
| Feather key |
A parallel key fastened in either the shaft or in the hub of a member sliding on it. When fastened in the shaft, it must be long enough to hold or drive the sliding member in any of its positions on the shaft. When in the sliding member the key need only be as long as the hub. |
| Ferralium |
Ferralium is a type of Super Duplex Stainless Steel which is optimised for use within fasteners. |
| Ferrite |
A solid solution of one or more elements in body-centered cubic iron. Unless otherwise designated (for instance, as chromium ferrite), the solute is generally assumed to be carbon. On some equilibrium diagrams, there are two ferrite regions separated by an austenite area. The lower area is alpha ferrite; the upper, delta ferrite. If there is no designation, alpha ferrite is assumed. |
| Ferritic decarburization |
Decarburization with sufficient loss of carbon to cause a lighter shade of tempered martensite and a significantly lower hardness than that of the adjacent base metal, with the presence of ferrite grains or grain boundary network under metallographic examination. |
| Ferritic stainless steels |
Ferritic stainless steels have the ferrite (BCC) crystal structure and contain 10–28% chromium and typically 0.1–0.35% carbon (all in wt%). They are magnetic and can be hardened by cold working. They have good to moderate mechanical properties, good oxidation and corrosion resistance at elevated temperatures, and susceptibility to embrittlement at temperatures between 400 and 540°C. Used for turbine parts, hightemperature valves, automotive exhaust components, and nuclear reactor core components. |
| Ferritizing anneal |
A treatment given as-cast gray or ductile (nodular) iron to produce an essentially ferritic matrix. For the term to be meaningful, the final microstructure desired or the time-temperature cycle used must be specified. |
| Fibreglass |
Fibreglass is used in studs and nuts for applications when the fastener should be non-corrosive, low in conductivity, or transparent to electromagnetic waves. |
| Fillet |
Transition region between bolt head and shank, or between other changes in diameter. |
| Fillet radius |
Concave junction at two intersecting surfaces of a fastener. Often used to describe the junction between the head and the shank of a threaded fastener (under head fillet). |
| Fillister head |
Similar to a pan head but with a smaller head diameter and a taller head, with higher vertical sides. |
| Fin |
A thin projecting rib. |
| Fin neck carriage bolt |
A plain, circular, oval head bolt with two oppositely located fins to prevent rotation. |
| Final annealing |
An imprecise term used to denote the last anneal given to a nonferrous alloy prior to shipment. |
| Fine pitch |
Fine pitch threads are often used within the automotive industry. They are more easily tapped into harder materials and have less tendency to loosen. |
| Finish annealing |
A subcritical annealing treatment applied to cold-worked low- or medium-carbon steel. Finish annealing, which is a compromise treatment, lowers residual stresses, thereby minimizing the risk of distortion in machining while retaining most of the benefits to machinability contributed by cold working. Compare with final annealing. |
| Finished fastener |
Fastener for which all manufacturing steps have been completed, with or without any surface coating and with full or reduced loadability, and which has not been machined into a test piece. |
| Finished hexagon bolts |
A washer faced or chamfered bearing surface with a close body tolerance. |
| Finishing temperature |
The temperature at which hot working is completed. |
| Finishing washer |
A washer designed for use with countersunk screws. Used to enhance appearance in some applications. |
| Finishing washer |
A washer designed for use with countersunk screws. Used to enhance appearance in some applications. |
| Fit |
The general term used to signify the range of tightness which may result from the application of a specific combination of allowances and tolerances in the design of mating parts. |
| Fitted bolt |
A bolt with a plain unthreaded portion immediately beneath the head. |
| Fitted bolt |
A bolt with a plain unthreaded portion immediately beneath the head. |
| Flame annealing |
Annealing in which the heat is applied directly by a flame. |
| Flame hardening |
A process for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers above the upper transformation temperature, whereupon the workpiece is immediately quenched. |
| Flame straightening |
Correcting distortion in metal structures by localized heating with a gas flame. |
| Flange |
1. Annular rims at the ends of pipes (flanged pipe) or shafts by which they may be coupled together using bolts that pass through holes in the flanges (flange coupling, flange union), or by toggle clamps around the periphery. 2. An extended rim on a wheel that positions it laterally on a track. Commonly used for railway rolling stock. 3. The top and bottom parts of an I-beam. |
| Flange |
Flange refers to a head style for bolts where there is a circular ‘flange’ under the head that acts like a washer to distribute a load. A flange is a projected flat rim or collar. |
| Flange bolt |
A bolt with a built in washer-like flange just below the head. |
| Flange rotation |
Angular distortion of a flange under the influence of bolt and reaction forces. Measured with respect to the center of the cross section of the flange. |
| Flat head |
Flat top surface and a conical bearing surface. |
| Flattening test |
This term as applied to tubing refers to a method of testing a section of tubing by flattening it until the inside walls are parallel and separated by a given distanceusually equal to three times the wall thickness for seamless tubes and five times the wall thickness for lap-welded tubes. Boiler tubes subjected to this test should show no cracks or flaws. The flattening test applied to rivets, consists in flattening a rivet head while hot to a diameter equal to 21⁄2 times the diameter of the shank or body of the rivet. Good rivet steel must not crack at the edges of the flattened head. |
| Flexural strength |
The maximum stress in the outer fiber at the moment of crack or break. |
| Fluid mechanics |
The study of fluids in motion (fluid dynamics) or fluid statics where there is no relative motion between fluid particles. Fluid statics concerns primarily the variation of pressure with altitude or depth; it includes aerostatics and hydrostatics. Fluid dynamics includes the topics of aerodynamics, gas dynamics, hydraulics, hydrodynamics and many aspects of acoustics, chemical engineering, flight, lubrication, meteorology, non-Newtonian fluid flow, oceanography, power-plant technology, propulsion, and turbomachinery. |
| Fog quenching |
Quenching in a fine vapor or mist. |
| Following flank |
The flank of a thread opposite to the leading flank. |
| Force ratio |
The factor by which a simple machine multiplies an applied force. It is the ratio of the load (output force) to the effort (input force). |
| Forced-air quench |
A quench utilizing blasts of compressed air against relatively small parts such as a gear. |
| Forge |
To shape metal while holt and plastic by a hammering or forcing process. Dies used in process. |
| Forging |
forging 1. A method of component manufacture by hammering metal by hand or by machine (drop forge, press). 2. A part made by the process of forging. |
| Forging |
The process of forming raw steel into specified shapes. Some examples of forged products would be hex bolts, clevises, and barrier pins. |
| Forging cracks |
This may occur during fastener manufacturing at the cutting or forging operations and are located on the top of the head or on the raised periphery of indented head bolts. |
| Form of thread |
The profile of a thread in an axial plane for a length of one pitch. |
| Fracture |
(rupture) The separation of materials, components, or structures into two or more parts by the propagation of one or more cracks. Cracking may be globally elastic (brittle) or accompanied by varying degrees of plasticity (ductile). |
| Fracture mechanics |
The stress analysis of bodies containing cracks. The use of stressconcentration factors and other correction factors of traditional strength of materials is inadequate when flaws are present initially (or develop during loading), since fracture depends not only on stress but also the size of the crack. Traditional strength of materials stress calculations cannot predict either the safe working stress in the presence of a known flaw, or the critical size of flaw just tolerable with a given working stress. |
| Fracture mechanisms |
The microstructural mechanisms that cause fracture, such as void initiation, growth and coalescence, cleavage. |
| Fracture strength |
(fracture stress) (Unit Pa) The stress at which a material breaks. It is not absolute for a given material, as it depends on the laws of fracture mechanics and is size-dependent. |
| Fracture stress |
The true normal stress on the minimum cross-sectional area at the beginning of fracture. This term usually applies to tension tests of unnotched specimens. |
| Fracture surface |
The irregular surface produced when a piece of metal is broken. |
| Fracture test |
A test in which a specimen is broken and its fracture surface is examined with the unaided eye or with a low-power microscope to determine such factors as composition, grain size, case depth, or internal discontinuities. |
| Fracture test |
1. The recording of load-deflexion (or stress–strain) in a specimen loaded monotonically until it breaks in tension, compression, shear, or torsion. 2. In fatigue, determination of the number of cycles to fracture for a specimen under different patterns of repeated loading. 3. Experiments to determine fracture toughness. |
| Fracture toughness |
A generic term for measures of resistance to extension of a crack. The term is sometimes restricted to results of fracture mechanics tests, which are directly applicable in fracture control. However, the term commonly includes results from simple tests of notched or precracked specimens not based on fracture mechanics analysis. Results from tests of the latter type are often useful for fracture control, based on either service experience or empirical correlations with fracture mechanics tests. |
| Free carbon |
The part of the total carbon in steel or cast iron that is present in elemental form as graphite or temper carbon. Contrast with combined carbon. |
| Free ferrite |
Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also proeutectoid ferrite. |
| Free joint |
The joint on a robot arm and wrist that causes the greatest end-effector movement when a specified force is applied to the end effector, i.e. that has the lowest stiffness with regard to the force. |
| Free machining |
The property that makes machining easy because of the forming of small chips, a characteristic imparted to steel by sulfur, etc. |
| Freezing point |
The temperature at which a substance changes phase (or state) from liquid to solid. The freezing point for most substances increases as pressure increases. The reverse process, from solid to liquid, is melting; melting point is the same as freezing point. |
| Freezing range |
That temperature range between liquidus and solidus temperatures in which molten and solid constituents coexist. |
| Frequency |
(temporal frequency, f) (Unit Hz) The number of cycles per second in an oscillation or the repetition rate for a cyclic process. |
| Fretting corrosion |
(1) The accelerated deterioration at the interface between contacting surfaces as the result of corrosion and slight oscillatory movement between the two surfaces. (2) A form of fretting in which chemical reaction predominates. Fretting corrosion is often characterized by the removal of particles and subsequent formation of oxides, which are often abrasive and so increase the wear. Fretting corrosion can involve other chemical reaction products, which may not be abrasive. |
| Fretting fatigue |
Fatigue fracture that initiate at a surface area where fretting has occurred. The progressive damage to a solid surface that arises from fretting. |
| Friction |
The resistance that a body encounters when sliding over another body, or experiences when a viscous fluid flows over its surface. |
| Friction loss (Unit J) |
The conversion of mechanical energy to heat due to friction within a machine, mechanism, linkage, etc. |
| Friction materials |
Materials having a high coefficient of friction which, when coupled with a long life, may be employed as brake linings or the facing of the plates of a clutch. |
| Full annealing |
An imprecise term that denotes an annealing cycle to produce minimum strength and hardness. For the term to be meaningful, the composition and starting condition of the material and the time-temperature cycle used must be stated. |
| Full annealing |
Annealing a ferrous alloy by austenitizing and then cooling slowly through the transformation range. |
| Full hard |
A temper of nonferrous alloys and some ferrous alloys corresponding approximately to a coldworked state beyond which the material can no longer be formed by bending. In specifications, a full hard temper is commonly defined in terms of minimum hardness or minimum tensile strength (or, alternatively, a range of hardness or strength) corresponding to a specific percentage of cold reduction following a full anneal. For aluminum, a full hard temper is equivalent to a reduction of 75% from dead soft ; for austenitic stainless steels, a reduction of about 50 to 55%. |
| Full size body |
The body of a bolt or screw which has a diameter between the minimum and maximum limits of the major diameter of the thread. |
| Full-size fastener |
Finished fastener with a shank diameter of ds ≈ d or ds > d, or screw threaded to the head, or fully threaded stud. |
| Furnace |
1. A type of combustion chamber in which solid, liquid, or gaseous fuels are burned to supply hot gases to a boiler or other process plant. Examples include the firebox, boiler furnace (steam-generating furnace), hot-air furnace, oil-fired furnace, updraught furnace, and water-cooled furnace. 2. A chamber, sometimes having a controlled atmosphere or under vacuum (vacuum furnace), for heating and melting materials. Examples include the blast furnace, direct- and indirect-arc furnaces, the induction furnace (high-frequency furnace), muffle furnace, reverberatory furnace, and solar furnace. |
| Gage length |
The original length of the portion of a specimen over which strain, change of length, or other characteristics are determined. |
| Galilean transformation |
In steady fluid flow, the addition or subtraction of a constant velocity to the entire flowfield, e.g. so that flow can be considered relative to a moving object or flow feature, such as a shock wave. |
| Galling |
An extreme form of adhesive wear, in which large chunks of one part stick to the mating part (during sliding contact). |
| Galvanic protection |
The coating on a fastener is said to provide galvanic protection if it is more anodic than the fastener and will, therefore, be destroyed instead of the fastener. Zinc plate (galvanizing) provides galvanic protection to steel fasteners, for example. |
| Gamma iron |
The face-centered cubic form of pure iron, stable from 910 to 1400 °C (1670 to 2550 °F). |
| Gas compressor |
A machine which draws in a gas at low pressure and delivers it at high pressure, usually accompanied by a temperature increase. Compressor designs include axial, centrifugal, diaphragm, reciprocating, and rotary (including the blade compressor). |
| Gasket factors |
Experimentally derived ‘‘constants’’ used to define the behavior of a gasket or the assembly and in-service conditions required for acceptable behavior. |
| Gasket m-factor |
For a gasketed joint of area A in which the clamping force is F, the factor m is defined by F ≥ mpA where p is the pressure that would separate the two halves of the joint and m > 1. |
| Gasket stress |
The contact stress exerted on the gasket by the joint members. |
| Gauge |
1. Any instrument for measuring or checking dimensions, including adjustable gauges such as a micrometer and non-adjustable gauges such as go/no-go limit gauges that indicate the maximum and minimum dimensions allowable in a component during manufacture. 2. A term often used to mean an instrument, e.g. pressure gauge. 3. The diameter of a wire or thickness of a sheet according to a standard. 4. The distance between the rails of a railway track. |
| Gauge pressure (Unit Pa) |
The level of static pressure above the ambient pressure. |
| Gear motor |
Essentially a gear pump run in reverse, in which high-pressure fluid supplied to one side of the casing causes the gears to rotate. |
| Gear pump |
A positive-displacement pump in which fluid is pumped by meshing gears, one driven and the other an idler gear, on parallel axes within a closed casing. |
| Gear ratio |
For two gears in contact, the ratio of the number of teeth on the driving gear to that on the driven gear. |
| Gear wheel |
Any form of toothed wheel, particularly those having conjugate teeth employed in the transmission of motion and power. |
| Gearbox |
A mechanism consisting of meshing gears which transmit power and torque to an output shaft from an input shaft directly connected to an engine. The mechanism is normally contained within a casing filled with lubricating oil. The diagram shows a simple four-speed sliding-mesh gearbox in which gears with different numbers of teeth are slid along parallel splined shafts within the gearbox to mesh with each other and change the ratio of output to input speed (gear ratio). |
| generator |
1. A machine for converting mechanical energy to electrical energy. 2. (generatrix) A point, line, or surface regarded as moving and so forming a line, a surface, or a volume, respectively. |
| GGas turbine |
An engine in which air, compressed by a multi-stage axial compressor and/or one or more centrifugal compressors, flows into a combustion chamber (or chambers) where fuel is burned and the hot gases then drive an axial turbine which powers the compressor. |
| Gibbs–Dalton law |
An extension of Dalton’s law of additive pressures to include the statement that the internal energy, enthalpy, and entropy of a mixture of gases are equal to the sum of the internal energies, enthalpies, and entropies the individual constituents would have if each existed alone at the same temperature and volume. Gibbs rule is that part of the law pertaining to entropy. |
| Graham’s laws |
The laws stating that the rates of diffusion and effusion of a gas are inversely proportional to the square root of its density. |
| Grain |
1. An individual crystal in a polycrystalline solid. Most engineering materials such as metals and ceramics are used in polycrystalline form. 2. (gr) A non-SI unit of mass. The conversion to SI is 1 gr = 6.479 891 × 10−5 kg. |
| Grain coarsening |
A heat treatment that produces excessively large austenitic grains. |
| Grain growth |
An increase in the average size of the grains in polycrystalline metal, usually as a result of heating at elevated temperature. |
| Grain growth |
An increase in the average size of the grains in polycrystalline metal, usually as a result of heating at elevated temperature. |
| Grain refiner |
A material added to a molten metal to induce a finer-than-normal grain size in the final structure. |
| Grain size |
A measure of the areas or volumes of grains in a polycrystalline metal, usually expressed as an average when the individual sizes are fairly uniform. In metals containing two or more phases, the grain size refers to that of the matrix unless otherwise specified. |
| Grain size |
For metals, a measure of the areas or volumes of grains in a polycrystalline material, usually expressed as an average when the individual sizes are fairly uniform. In metals containing two or more phases, the grain size refers to that of the matrix unless otherwise specified. Grain sizes are reported in terms of number of grains per unit area or volume, average diameter, or as a grainsize number derived from area measurements. |
| Grain-boundary liquation |
An advanced stage of overheating in which material in the region of austenitic grain boundaries melts. |
| Granular fracture |
A type of irregular surface produced when metal is broken that is characterized by a rough, grainlike appearance, rather than a smooth or fibrous one. It can be subclassified as transgranular or intergranular. |
| Graphitic carbon |
Free carbon in steel or cast iron. |
| Graphitization |
Formation of graphite in iron or steel. Where graphite is formed during solidification, the phenomenon is called primary graphitization; where formed later by heat treatment, secondary graphitization. |
| Graphitizing |
Annealing a ferrous alloy in such a way that some or all of the carbon is precipitated as graphite. |
| Gravity |
The attractive force by which bodies are drawn towards the centre of Earth or other celestial object. Its intensity is measured by the acceleration produced. |
| Gravity |
The gravitational force of attraction at the surface of a planet or other celestial body. The Earth’s gravity produces an acceleration of around 9.8m / s2 for any unsupported body. |
| Grinding |
1. A process of high-speed multiple scratching of surfaces by a wheel formed from hard grits and a binder which are progressively exposed as the binder wears away. It is a process of micromachining employed particularly in accurate finishing operations on hard materials. 2. A form of comminution. |
| Grinding stress (Unit Pa) |
The residual stresses left in the surface of a component after grinding, owing to non-uniform cooling from the high surface temperatures produced. |
| Grip length |
Combined thickness of all the things clamped together by the bolt and nut, including washers, gaskets, and joint members. |
| Grub screw |
A short headless screw with a recess at one end to receive a screw driver or key. |
| Guided bend |
The bend obtained by use of a plunger to force the specimen into a die in order to produce the desired contour of the outside and inside surfaces of the specimen. |
| Guided-bend test |
A test in which the specimen is bent to a definite shape by means of a punch (mandrel) and a bottom block. |
| Half hard |
A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength about mid-way between that of dead soft and full hard tempers. |
| Half nut |
A mechanism attached to the apron of a lathe and operated by a lever. The lever opens and closes a nut that has been split lengthwise so that the two halves of the nut can be closed upon the lead screw when threads are being cut. Also called Split Nut. |
| Hard temper |
Same as full hard temper. |
| Hardenability |
The relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surface where the metal exhibits a specific hardness (50 HRC, for example) or a specific percentage of martensite in the microstructure. |
| Hardenability |
Defined as the ability of a ferrous material to acquire hardness after austenitization and quenching. |
| Hardening |
Increasing hardness by suitable treatment, usually involving heating and cooling. When applicable, the following more specific terms should be used: age hardening, flame hardening, induction hardening, laser hardening, precipitation hardening, and quench hardening. |
| Hardness |
A measure of the resistance of a material to surface indentation or abrasion; may be thought of as a function of the stress required to produce some specified type of surface deformation. There is no absolute scale for hardness; therefore, to express hardness quantitatively, each type of test has its own scale of arbitrarily defined hardness. Indentation hardness can be measured by Brinell hardness test, Rockwell hardness test, Vickers hardness test, Knoop hardness test, and the Scleroscope hardness test. |
| Hardness |
The ability of a ductile material to resist permanent deformation. It is a measure of plastic flow properties determined principally by indentation testing, but scratch hardness and rebound hardness are also employed. |
| Hardness |
hardness scales The different measures of hardness given by different tests, such as indentation pressure in the Brinell and in the Vickers hardness tests, and the different Rockwell hardness numbers. |
| Hardness profile |
Hardness as a function of distance from a fixed reference point (usually from the surface). |
| Hardness test |
1. Any of various tests in which different hard indenters are forced into the surface of a solid under different loads to give permanent impressions, the pressure to cause which being called the hardness. The Brinell test employs a spherical indenter; the Vickers test a square-based pyramid. The relation between indentation hardness H and uniaxial yield stress Y is approximately H = (2.5 ~ 3.0)Y. Originally hardnesses were given in kg/mm2 but now are often given in Pa. 2. The tests for scratch and file hardness relate either to one material being able to mark another (Mohs hardness scale for minerals), or to the size of groove produced by a rigid indenter slid under load across a surface. 3. Rebound hardness concerns the height of rebound of a dropped indenter, or an indenter at the end of a pivoted arm, having struck a surface. |
| Head height |
The head height shall be that overall distance measured parallel to the axis of the product from the top of the head to the bearing surface and shall include the thickness of the washer face. Raised grade and manufacturer’s identification are excluded from head height. |
| Heat (q, Q) (Unit J) |
A form of energy that is transferred across the boundary of a system at one temperature to another system (or the surroundings) at a different temperature by virtue of the temperature difference between them. Heat can be identified only as it crosses the boundary. A body can never be said to contain heat which is thus a transient phenomenon. |
| Heat balance (heat budget) |
An energy balance involving only heat transfer across the system boundary and the stored energy. |
| Heat capacity (C) (Unit J/K) |
The energy required to raise the temperature of a body by 1K without change of phase. Heat capacity is an extensive thermodynamic property dependent on temperature and pressure. |
| Heat tinting |
Coloration of a metal surface through oxidation by heating to reveal details of the microstructure. |
| Heat treatment |
(heat treating) Alteration of the mechanical properties of materials, particularly metals, by different sequences of heating, holding at temperature, and cooling at different rates. |
| Heat treatment |
Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties. Heating for the sole purpose of hot working is excluded from the meaning of this definition. |
| Heat-capacity rate (c) (Unit W/K) |
For fluid flow in a duct with mass flow rate ṁ, the product ṁC where C is the specific-heat capacity, at constant pressure (CP) in the case of a gas. |
| Heat-capacity ratio (c*) |
A non-dimensional parameter that arises in the analysis of heat exchangers, defined as the ratio cMIN/cMAX where cMIN is the heat-capacity rate c for the fluid with the smaller value of c and cMAX the value for the fluid with the larger value of c. Different flow rates and specific-heat values give rise to different values for c. |
| Heat-treatable alloy |
An alloy that can be hardened by heat treatment. |
| Helical spring |
A spring manufactured by forming elastic wire into a helix which may be cylindrical, conical, barrel-shaped, or hourglass in overall form. Closed-coiled springs are used in tension; open-coiled in both tension and compression. |
| Hexagon nut |
A nut having six sides and shaped like a hexagon. |
| High strength low alloy steels |
(HSLA steels) Steels with various micro-alloying elements such as copper, nickel, chromium, molybdenum, niobium, titanium, and vanadium in small quantities that give improved strength (as high as 900 MPa) and corrosion properties compared with plain carbon steels. Improvements are due to grain refinement and precipitation hardening, better control of the chemistry during steel making, and accurate rolling temperatures. |
| High-tensile bolt |
(high-tension bolt) A bolt manufactured from an alloy steel that has a high tensile strength of about 1 GPa. |
| Homogeneous carburizing |
Use of a carburizing process to convert a low-carbon ferrous alloy to one of uniform and higher carbon content throughout the section. |
| Homogenizing |
Holding at high temperature to eliminate or decrease chemical segregation by diffusion. |
| Honing |
Honing is a controlled, low-speed sizing and surface-finishing process in which stock is removed by the shearing action to the bonded abrasive grains of a honing stone, or stick. |
| Honing |
Finish polishing of surfaces and cutting edges in which very fine abrasive powders, such as rouge, are employed. Honing is usually applied to cylindrical surfaces and lapping to flat, but the usage is not consistent. |
| Hooke’s law |
Within the limit of proportionality, the extension of a material is proportional to the applied force. Approximately, it is the relationship between stress and strain in an elastic material when it is stretched. The law states that the stress (force per unit area) is proportional to the strain (a change in dimensions). The law, which holds only approximately and over a limited range, was discovered in 1676 by Robert Hooke. |
| Horizontal batch furnace |
A versatile batch-type furnace that can give light or deep case depths, and because the parts are not exposed to air, horizontal batch furnaces can give surfaces almost entirely free of oxides. |
| Horsepower |
Unit indicating the rate at which work is done. The electrical equivalent of one horsepower is 746 watts. |
| Hoseclip |
(hose clamp) A circular metal band, one end of which has a screw mechanism through which passes the other end such that the screw engages with indentations in the band, allowing the clip to be tightened or loosened. Used to hold a hose on to a pipe such as a water outlet. |
| Hot dipping |
(hot-dip galvanizing) A process of coating an iron, steel, or aluminium object with a thin layer of zinc by passing it through a bath of molten zinc at a temperature of about 460°C. |
| Hot hardness |
The hardness of materials at high temperatures, often used to rank the performance of cutting tools. |
| Hot isostatic pressing |
Sintering of powdered materials at high temperature and high hydrostatic pressure in order to minimize porosity. |
| Hot pressing |
1. Sintering of metal or ceramic powder at high temperature. 2. Formation of components from fibre-reinforced composite material in which bonding is ensured through application of pressure and temperature, often against a mould to give the required shape. |
| Hot quenching |
An imprecise term used to cover a variety of quenching procedures in which a quenching medium is maintained at a prescribed temperature above 70 °C (160 °F) |
| Hot shortness |
The reduction of ductility in steels at high temperatures caused by melting of sulfides that wet grain boundaries and spread along them. |
| Hot stamping |
Hot forging of brass and bronze alloys. |
| Hot strength (Unit Pa) |
The yield stress (or sometimes fracture stress) of a material at temperatures above about half its melting point in kelvins. |
| Hot working |
Plastic deformation of a metal, by rolling, drawing, forging, etc., at a temperature above its recrystallization temperature (in commercial alloys above about 40% of their melting points in kelvins) which results in permanent shape change but no increase in strength or loss of ductility. |
| Hot-wire analyzer |
An electrical atmosphere analysis device that is based on the fact that the electrical resistivity of steel is a linear function of carbon content over a range from 0.05% C to saturation. The device measures the carbon potential of furnace atmospheres (typically). This term is not to be confused with the hot-wire test which measures heat extraction rates. |
| Hot-wire test |
Method used to test heat extraction rates of various quenchants. Faster heat-extracting quenchants will permit more electric current to pass through a standard wire because it is cooled more quickly. Compare with hot-wire analyzer. |
| Humidify |
To add water vapour to air or another gas to increase its humidity. |
| Hydraulic air compressor |
1. A compressor powered by a hydraulic motor. 2. A device in which air is compressed as a result of being entrained in water flowing in a downcomer pipe. The compressed air is released in a chamber at the bottom of the pipe. |
| Hydraulics |
The study of water flow in open channels (including canals, rivers, etc.), water-supply, drainage, and irrigation systems. |
| Hydrodynamics |
A term sometimes used to mean fluid dynamics, even when the fluid is a gas or a liquid other than water. |
| Hydrogen damage |
A general term for the embrittlement, cracking, blistering, and hydride formation that can occur when hydrogen is present in some metals. |
| Hydrogen embrittlement |
A common and troublesome form of Stress cracking. Several theories have been proposed to explain hydrogen embrittlement, but, at present, the exact mechanism is still unknown. What is known, however, is the fact that if hydrogen is trapped in a bolt by poor electroplating practices, it can encourage stress cracking. Bolts can fail, suddenly and unexpectedly, under normal loads. |
| Hydrogen embrittlement |
A condition of low toughness, low ductility or cracking in metals resulting from the absorption of hydrogen. |
| Hydrogen embrittlement |
Premature crack growth over time under tensile stress leading to unexpected failure in certain metals, caused by small amounts of hydrogen in the microstructure. Hydrogen may enter steels during melting or heat treating, or during processes such as electroplating. |
| Hydrogen-induced delayed cracking |
A term sometimes used to identify a form of hydrogen embrittlement in which a metal appears to fracture spontaneously under a steady stress less than the yield stress. There is usually a delay between the application of stress (or exposure of the stressed metal to hydrogen) and the onset of cracking. Also referred to as static fatigue. |
| Hypereutectic alloy |
In an alloy system exhibiting a eutectic, any alloy whose composition has an excess of alloying element compared with the eutectic composition, and whose equilibrium microstructure contains some eutectic structure. |
| Hypereutectoid alloy |
In an alloy system exhibiting a eutectoid, any alloy whose composition has an excess of alloying element compared with the eutectoid composition, and whose equilibrium microstructure contains some eutectoid structure. |
| Hypoeutectic alloy |
In an alloy system exhibiting a eutectic, any alloy whose composition has an excess of base metal compared with the eutectic composition, and whose equilibrium microstructure contains some eutectic structure. |
| Hypoeutectoid alloy |
In an alloy system exhibiting a eutectoid, any alloy whose composition has an excess of base metal compared with the eutectoid composition, and whose equilibrium microstructure contains some eutectoid structure. |
| Hysteresis |
The phenomenon of permanently absorbed or lost energy that occurs during any cycle of loading or unloading when a material is subjected to repeated loading. |
| Ideal crack |
A simplified model of a crack used in elastic-stress analysis. In a stress-free body, the crack has two smooth surfaces that are coincident and join within the body along a smooth curve called the crack front; in two-dimensional representations, the crack front is called the crack tip. |
| Ideal critical diameter |
(DI). Under an ideal quench condition, the bar diameter that has 50% martensite at the center of the bar when the surface is cooled at an infinitely rapid rate (that is, when H = ∞, where H is the quench severity factor). |
| Ideal gas laws |
The law relating pressure, temperature and volume of an ideal (perfect) gas pV = mRT, where R is the gas constant. The law implies that at constant temperature T, the product of pressure p and volume V is constant (Boyle’s law), and at constant pressure, the volume is proportional to the temperature (Charles’ law). |
| Impact |
Sudden loading of a body or component, as in a vehicle collision or striking by a hammer. |
| Impact energy |
The amount of energy required to fracture a material, usually measured by means of an Izod test or Charpy test.The type of specimen and test conditions affect the values and therefore should be specified. |
| Impact energy (Unit J) |
1. The sum of the kinetic energies of all the bodies involved at the instant of collision of two or more moving bodies. 2. The energy required to fracture a specimen in a Charpy or Izod impact test. |
| Impact extrusion |
The extrusion of separate components of soft metals whose properties permit the action to be performed quickly. |
| Impact loads |
Especially severe shock loads such as those caused by instantaneous arrest of a falling mass, shock meeting of two parts (in a mechanical hammer, for example), or by explosive impact, in which there can be an exceptionally rapid build-up of stress. |
| Impact strength |
The resiliency or toughness of a solid as measured by impact energy. |
| Impact strength 1. (Unit Pa) |
The stress to cause failure (by yielding or fracture) under conditions of high strain rate. 2. (Unit J) A term sometimes used to describe the energy required to fracture a specimen in a Charpy or Izod impact test even though the unit is not that of strength. |
| Impact test |
A test for determining the energy absorbed in fracturing a test piece at high velocity, as distinct from static test. The test may be carried out in tension, bending, or torsion, and the test bar may be notched or unnotched. |
| Impact testing |
Determination of the mechanical properties of materials under high-rate conditions. Often determined from the behaviour of a testpiece when struck by a pendulum or a weight falling from a known height, or when sandwiched between two Hopkinson bars. |
| Impact wrench |
An air- or electric-powered wrench in which multiple blows from tiny hammers are used to produce output torque to tighten fasteners. |
| Impact wrench |
A pneumatically-or electrically-powered socket wrench used to tighten or loosen nuts through the application of torque in a rapid series of impulses. |
| Imperial system |
The units of measurement developed in the UK. Formerly known as the fps system, which is an abbreviation for the ‘foot-pound-second system of units. |
| Imperial system of units |
British imperial system of units) An obsolete system of units first defined in the British Weights and Measures Act of 1824. Its three base units were second (time), yard (length), and pound avoirdupois (mass). The imperial unit of capacity was the gallon. The many associated units introduced subsequently include cubic inch, cubic foot, cubic yard, pint, and quart for both liquid and dry measures of capacity (i.e. volume); ounce, hundredweight, short and long ton for avoirdupois mass; poundal for force; minute and hour for time; fahrenheit for temperature; and British thermal unit for energy. Closely related to the imperial system of units is the US customary system of units. Each imperial unit is now legally defined in terms of the metric (i.e. SI) equivalent. |
| Inch (in) |
A non-SI unit of length now defined as 25.4 mm. The conversion to SI is 1 in = 2.54 × 10−2 m. |
| Inch-pound force/inch2 |
A non-SI unit for the specific work of fracture (fracture toughness). The conversion to SI is 1 in.lbf/in2 = 1.751 268 × 102 J/m2. |
| Inclusions |
Small pieces of nonmetallic impurities trapped within the base metal of. |
| Indentation hardness |
The resistance of a material to indentation as determined by hardness testing. The indenter, which may be spherical or diamond shaped, is pressed into the surface of a metal under specified load for a given time. |
| Indentation hardness |
(Unit Pa or, non-SI, kgf/mm2) A measurement of hardness taken by means of an indenter, pressed into a material by a load, which makes a permanent impression. The hardness is given by the load divided by the projected (sometimes surface) area of the indentation. Plasticity theory shows that the hardness is about 2.5–3 times the uniaxial yield stress. |
| Induction hardening |
A surface-hardening process in which only the surface layer of a suitable ferrous workpiece is heated by electromagnetic induction to above the upper critical temperature and immediately quenched. |
| Induction heating |
Heating by combined electrical resistance and hysteresis losses induced by subjecting a metal to the varying magnetic field surrounding a coil carrying alternating current. |
| Induction tempering |
Tempering of steel using low-frequency electrical induction heating. |
| Infinite life diagram |
A simple plot experimentally derived fatigue-life data, showing the conditions required for infinite life. |
| Initial preload |
The tension created in a single bolt as it is tightened. Will usually be modified by subsequent assembly operations or by in-service loads and conditions. |
| Initial strain |
The strain in a specimen immediately upon achieving the given loading conditions in a creep test (before creep occurs). Sometimes referred to as instantaneous strain. |
| Initial stress |
The stress produced by strain in a specimen immediately on achieving the given constant-strain conditions in a stress-relaxation test before stress-relaxation occurs. Sometimes referred to as instantaneous stress. |
| Inspection gauge |
Any of various gauges used in manufacturing and quality control to check dimensions, finish, etc. |
| Intense quenching |
Quenching in which the quenching medium is cooling the part at a rate at least two and a half times faster than still water. |
| Intercritical annealing |
Any annealing treatment that involves heating to, and holding at, a temperature between the upper and lower critical temperatures to obtain partial austenitization, followed by either slow cooling or holding at a temperature below the lower critical temperature. |
| Intergranular cracking |
Cracking or fracturing that occurs between the grains or crystals in a polycrystalline aggregate. Also called intercrystalline cracking. |
| Intergranular fracture |
Brittle fracture of a metal in which the fracture is between the grains, or crystals, that form the metal. Also called intercrystalline fracture. |
| Intermediate annealing |
Annealing wrought metals at one or more stages during manufacture and before final treatment. |
| International Bureau of Weights and Measures |
(BIPM, Bureau International des Poids et Mesures) The intergovernmental organization which has the mandate to provide the basis for a single, coherent system of measurements, traceable to the International System of Units. |
| International Practical Temperature Scale (IPTS) |
A close approximation to the thermodynamic temperature scale based upon the triple points of hydrogen (13.8 K), neon (24.6 K), oxygen (54.4 K), argon (83.8 K), mercury (234.3 K), and water (273.2 K); the melting point of gallium (302.9 K); and the freezing points of indium (429.7 K), tin (505.1 K), zinc (692.7 K), aluminium (933.5 K), silver (1 234.9 K), gold (1 337.3 K), and copper (1 |
| International System of Units |
(SI system of units, Le Système international d’unités) The standard system of units now used almost universally in science and engineering except in the United States. The definitive reference on the SI is a booklet published by the International Bureau of Weights and Measures, often referred to as the BIPM SI Brochure. There are seven base units: metre (symbol m) for length; kilogram (kg) for mass; mole (mol) for amount of substance; second (s) for time; kelvin (K) for temperature; ampere (A) for electric current; and candela (cd) for luminous intensity. New SI unit definitions, based upon the values of the universal constants of nature (also termed the fundamental physical constants: the Avogadro constant, the Boltzmann constant, and the Planck constant), came into effect on 20 May 2019. Minor adjustments were also made to the values of these constants. In addition to the base units, there are 22 coherent derived units, including radian (rad) for plane angle, steradian (sr) for solid angle, hertz (Hz) for frequency, newton (N) for force, pascal (Pa) for pressure, joule (J) for energy, work, and amount of heat, and watt (W) for power and radiant flux. Recommended practice is to avoid combinations such as N/mm2, MN/m2 being preferred.(kg) for mass; mole (mol) for amount of substance; second (s) for time; kelvin (K) for temperature; ampere (A) for electric current; and candela (cd) for luminous intensity. New SI unit definitions, based upon the values of the universal constants of nature (also termed the fundamental physical constants: the Avogadro constant, the Boltzmann constant, and the Planck constant), came into effect on 20 May 2019. Minor adjustments were also made to the values of these constants. In addition to the base units, there are 22 coherent derived units, including radian (rad) for plane angle, steradian (sr) for solid angle, hertz (Hz) for frequency, newton (N) for force, pascal (Pa) for pressure, joule (J) for energy, work, and amount of heat, and watt (W) for power and radiant flux. Recommended practice is to avoid combinations such as N/mm2, MN/m2 being preferred. |
| Interrupted aging |
Aging at two or more temperatures, by steps, and cooling to room temperature after each step. |
| Interrupted quenching |
A quenching procedure in which the workpiece is removed from the first quench at a temperature substantially higher than that of the quenchant and is then subjected to a second quenching system having a different cooling rate than the first. |
| Ion carburizing |
A method of surface hardening in which carbon ions are diffused into a workpiece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma carburizing or glowdischarge carburizing. |
| Ion nitriding |
A method of surface hardening in which nitrogen ions are diffused into a workpiece in a vacuum through the use of high-voltage electrical energy. Synonymous with plasma nitriding or glowdischarge nitriding. |
| Isothermal annealing |
Austenitizing a ferrous alloy and then cooling to and holding at a temperature at which austenite transforms to a relatively soft ferrite carbide aggregate. |
| Izod test |
A type of impact test in which a V-notched specimen, mounted vertically, is subjected to a sudden blow delivered by the weight at the end of a pendulum arm. The energy required to break off the free end is a measure of the impact strength or toughness of the material. |
| Jack |
A lifting device that exerts large forces over small displacements, achieved by mechanical gearing or hydraulics. |
| J-integral |
A mathematical expression describing a line or surface integral that encloses the crack front from one crack surface to the other, used to characterize the fracture toughness of a material having appreciable plasticity before fracture. The J-integral eliminates the need to describe the behavior of the material near the crack tip by considering the local stress-strain field around the crack front; JIc is the critical value of the J-integral required to initiate crack extension from a preexisting crack. |
| Joint |
The part of a robot arm permitting either rotational or translational motion. Each joint provides a single degree of freedom, and thus a minimum of six is required if the robot is to be able to position and orientate the end effector anywhere within the workspace. A joint is described by four parameters: the joint angle, joint offset, joint twist, and link length. The joint angle is the angle between an extrapolation of the previous link length and the present link length, measured positive anticlockwise in a plane normal to the joint axis. This is variable in a rotational joint and fixed in a translational joint. The joint offset is the distance between the link length for the previous link and that for the present link, measured along the joint axis. This is variable for a translational joint and fixed for a rotational joint. The joint twist for the nth joint is the angle between the axes of joints Jn and Jn+1, measured positive anticlockwise in a plane normal to the link length and viewed from the position of the Jn+1 joint. The link length is the mutually perpendicular distance between planes passing through the axes of joint Jn and Jn+1. Both the joint twist and link length are fixed in all joints. |
| Joint diagrams |
Mathematical diagrams which illustrate the forces on and deflections of fasteners and joint members. |
| Joint space |
The description of the position and orientation of a robot in terms of the rotational angles and translational positions of the joints. Kinematic analysis, normally using homogeneous transforms, allows conversion between this description and the end‐effector position and orientation in x,y,z space. |
| Joule |
The SI unit of energy. One joule is the work done by a force of one newton acting over a distance of one metre. The symbol is J, where 1 J = 1 N m. |
| Junker machine |
A test machine, first proposed by Gerhard Junker, for testing the vibration resistance of fasteners. |
| Kelvin |
The SI unit of temperature. The Kelvin temperature scale has a zero point at absolute zero and degree intervals (kelvins) the same size as degrees Celsius. The freezing point of water occurs at 273K (0ºC) and the boiling point at 373 K (100ºC). |
| Kelvin (K) |
The base unit of thermodynamic temperature in the SI system. It was previously defined to be the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. From 20 May 2019 it has been defined by taking the fixed numerical value of the Boltzmann constant kB to be 1.380 649 × 10−23 when expressed in the unit J/K, which is equal to kg.m2/(s2K). |
| Kiln |
An industrial oven for burning, baking, or drying. |
| Kilo (k) |
An SI unit prefix indicating a multiplier of 103; thus kilometre (km) is a unit of length equal to one thousand metres. |
| Kilocalorie |
(Cal, kg-cal, kilogram-calorie, large calorie) An obsolete (i.e. non-SI) unit of energy equal to 1 000 cal. |
| Kilogram (kg) |
The base unit of mass in the SI system. It was previously defined as being equal to the mass of the International Prototype Kilogram, a right cylinder of height and diameter 39.17 mm made of an alloy of 90% (by mass) platinum and 10% iridium. From 20 May 2019 it has been defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 150 × 10−34 when expressed in the unit J.s, which is equal to kg.m2.s. |
| Kinetic energy |
Energy that an object possesses because it is in motion. It is the energy given to an object to set it in motion. On impact, it is converted into other forms of energy such as strain, heat, sound and light. |
| Kinetic friction |
(dynamic friction, sliding friction) (Unit N) The sliding resistance to relative motion of two surfaces in contact with each other. |
| Knoop hardness test |
An indentation-hardness test using calibrated machines to force a rhombic-based pyramidal diamond indenter having specified edge angles, under specified conditions, into the surface of the material under test and to measure the long diagonal after removal of the load. |
| Knoop hardness test |
An indentation hardness test in which the indenter is an elongated diamond pyramid that gives an impression in the form of a parallelogram, in which the longer diagonal is about seven times the shorter. Used for studies of anisotropy in solid materials. |
| Knot (kn) |
A non-SI unit defined as one nautical mile per hour. The conversion to SI is 1 kn = 5.144 444 444×10−1 m/s. |
| Lap joint |
A riveted, welded, or bonded connection between two plates made by overlapping one plate above the other. Lap joints having plates above and below a butt joint avoid the bending moment associated with a simple lap joint. |
| Laser hardening |
Hardening of steel by a moving CO2 or Nd:YAG laser beam focussed down to a few mm in diameter. A point in the surface of an object is rapidly heated into the austenite range and then rapidly cooled to form tempered martensite. |
| Laser hardening |
A surface-hardening process which uses a laser to quickly heat a surface. Heat conduction into the interior of the part will quickly cool the surface, leaving a shallow martensitic layer. |
| Laser sintering |
A manufacturing technique by which parts are built layer by layer (each typically 20 μm thick) from plastic or metal (Direct Metal Laser Sintering, DMLS) material in powder form, each layer being sintered by a scanning laser. |
| Latent heat |
Thermal energy absorbed or released when a substance undergoes a phase change. |
| Lathe |
A machine tool in which work, gripped in the headstock, is rotated against the cutting tool to produce turned, bored, faced, or threaded components. |
| Lead angle |
The angle of the helix of a screw thread or worm thread. It is the measure of the inclination of a screw thread from a plane perpendicular to the axis of the screw. |
| Lead of thread |
On a single threaded screw, the distance the screw or nut advances in one complete revolution. |
| Lead screw |
The master screw running along the length of the bed of a lathe from which all screws threads on that machine are cut. |
| Lead screw |
Master screw used for cutting a screw thread in a lathe. |
| Least count |
The smallest value that can be read from an instrument having a graduated scale. Except on instruments provided with a vernier, the least count is that fraction of the smallest division which can be conveniently and reliably estimated; this fraction is ordinarily one-fifth or one-tenth, except where the graduations are very closely spaced. |
| Left hand screw |
One that screws into the mating part or advances when turned to the left or counter clockwise. |
| Length of engagement |
It is the axial measurement within which the external and internal threads have theoretical contact. |
| Linear (tensile or compressive) strain |
The change per unit length due to force in an original linear dimension. An increase in length is considered positive. |
| Linear-elastic fracture mechanics |
A method of fracture analysis that can determine the stress (or load) required to induce fracture instability in a structure containing a cracklike flaw of known size and shape. |
| Liquation temperature |
The lowest temperature at which partial melting can occur in an alloy that exhibits the greatest possible degree of segregation. |
| Liquid carburizing |
Surface hardening of steel by immersion into a molten bath consisting of cyanides and other salts. |
| Liquid nitriding |
A method of surface hardening in which molten nitrogen-bearing, fused-salt baths containing both cyanides and cyanates are exposed to parts at subcritical temperatures. |
| Load |
For testing machines, a force applied to a test piece that is measured in units such as pound-force, newton, or kilogram-force. |
| Load and resistance factor design |
A design procedure developed for the AISC by the Research Council on Structural Connections. Assigns uncertainties in the strength of (i.e., resistance of) and in the service loads to be placed on a shear joint to estimate the probable strength of the joint. It is a recently defined alternative to the Allowable stress design procedure. |
| Load factor (F) |
The ratio between an increase in bolt tension and the external load which has caused the increase. |
| Load factors |
Factors reflecting the probability of an increase in load in a shear joint. Used in load and resistant factor design. |
| Load range, P |
In fatigue, the algebraic difference between the maximum and minimum loads in a fatigue cycle. |
| Load ratio, R |
In fatigue, the algebraic ratio of the minimum to maximum load in a fatigue cycle, that is, R = Pmin/Pmax. Also known as stress ratio. |
| Lobe |
The projecting part such as rotor lobe or the cam lobe. |
| Lock nut |
A nut which provides extra resistance to vibration loosening (beyond that produced by proper Preload ), either by providing some form of Prevailing torque, or, in free-spinning lock nuts, by deforming, cramping, or biting into mating parts when fully tightened. |
| Lock nut |
A type of nut that is prevented from loosening under vibration. The locking action is accomplished by squeezing, gripping or jamming against the bolt threads. |
| Lockbolt |
A fastener which bears a superficial resemblance to a bolt, but which engages a collar (instead of a nut) with annular grooves (instead of threads). The collar is swaged over the grooves on the male fastener to develop preload. |
| Locknut |
1. A thin auxiliary nut tightened against another nut to prevent loosening. 2. A single nut with special features that prevent loosening. |
| Lock-nut pipe thread |
The lock-nut pipe thread is a straight thread of the largest diameter which can be cut on a pipe. Its form is identical with that of the American or Briggs standard taper pipe thread. In general, “Go” gages only are required. These consist of a straight-threaded plug representing the minimum female lock-nut thread, and a straight-threaded ring representing the maximum male lock-nut thread. This thread is used only to hold parts together, or to retain a collar on the pipe. It is never used where a tight threaded joint is required. |
| Longitudinal direction |
The principal direction of flow in a worked metal. See also normal direction and transverse direction. |
| Low-E coating |
(low-emittance coating) A thin metallic or metal-oxide coating on a glass sheet to absorb and reflect infrared radiation. The coating is applied either by a pyrolytic chemical vapour-deposition process (hard coat), or by sputtering (soft coat). |
| Lubricant |
Any substance interposed between two surfaces for the purpose of reducing the friction or wear between them. |
| Lüders lines |
Elongated surface markings or depressions, often visible with the unaided eye, that form along the length of sheet metal or a tension specimen at an angle of approximately 45° to the loading axis. Caused by localized plastic deformation, they result from discontinuous (inhomogeneous) yielding. Also known as Lüders bands, Hartmann lines, Piobert lines, or stretcher strains. |
| Machine |
A mechanism that transmits power in the performance of a useful task. More than one machine in one place, either connected or performing separate functions, is termed machinery. |
| Machine screw |
A relatively small screw, usually less than 20 mm in diameter, with the thread running along the whole length up to the head, intended to be screwed into threaded holes. If inserted through plain holes in assembled parts and held together by a nut, an undesirable contact surface of threads bearing against the surface of a hole results. |
| Machine screw |
A countersunk, slotted screw with machined threads such as are found on bolts. |
| Machine tool |
A powered machine, such as a borer, grinder, lathe, milling machine or planer, used for cutting and shaping metal, plastics, composites, etc. (machining). A machining centre is a CNC machine tool working about several axes, having a stock of tools and automatic tool changing ability, which is capable of diverse machining operations under automated control. |
| Macrodeviation |
Errors from–irregular surface departures from the design profile, often caused by lack of accuracy or stiffness of the machine system. |
| Magnetic materials |
Materials in which the magnetic moment of adjacent atoms can adopt either parallel or antiparallel alignment. Ferromagnetism occurs when the magnetic moments adopt parallel alignment. When adjacent magnetic moments cancel due to antiparallel alignment, the material exhibits antiferromagnetism and has no overall magnetic moment. Examples of the latter include ferrous oxide and manganese oxide. The phenomenon of ferrimagnetism occurs when adjacent magnetic moments adopt antiparallel alignment but have unequal magnitude. The region of ferromagnetic or ferrimagnetic material in which the magnetic moments are aligned is known as a magnetic domain and adjacent regions are separated by domain walls. Diamagnetic and paramagnetic materials exhibit weak forms of magnetism that results from independent (uncoupled) alignment of magnetic atomic dipoles induced by an applied magnetic field. Ferromagnetic and ferrimagnetic materials can acquire high degrees of magnetization in relatively weak magnetic fields. They have different hysteresis characteristics and are classified as either hard magnets or soft magnets. Hard magnets retain large magnetization in the absence of an applied magnetic field and are used as permanent magnets. Conventional materials include magnet steels alloyed with tungsten or cobalt. Rare-earth magnets made from alloys of rare-earth elements produce significantly stronger magnetic fields. The most common of these are samarium-cobalt and neodymiumiron-boron permanent magnets. Their strength allows them to be small and used in numerous applications, including cordless tools, disk drives, headphones, toys, and motors for automotive components such as wipers, fans, washers, and windows. Soft magnets may be magnetized and demagnetized relatively easily. When subjected to an alternating magnetic field the energy loss due to hysteresis is small. Common materials include iron-silicon alloys (known as electrical steels), nickel-iron alloys (permalloy), soft ferrites, and amorphous nanocrystalline alloys, the last of which can be produced in the form of tape by meltspinning. Soft magnets are used to enhance the magnetic flux produced by an electric current. |
| Magnetic test |
Method used to test heat extraction rates of various quenchants. The test works by utilizing the change in magnetic properties of metals at their Curie point–the temperature above which metals lose their magnetism. |
| Major diameter |
On a straight thread, the diameter of the imaginary cylinder that just touches the crest of an external thread or the root of an internal thread. |
| Malleable cast iron |
A cast iron made by prolonged annealing of white cast iron in which decarburization or graphitization, or both, take place to eliminate some or all of the cementite. The graphite is in the form of temper carbon. If decarburization is the predominant reaction, the product will exhibit a light fracture surface, hence, “whiteheart malleable;” otherwise, the fracture surface will be dark, hence, “blackheart malleable.” Ferritic malleable has a predominantly ferritic matrix; pearlitic malleable may contain pearlitic, spheroidite or tempered martensite depending on heat treatment and desired hardness. |
| Manometer |
An instrument used in the measurement of a pressure difference in a fluid (manometry). |
| Maraging steels |
Very high-strength iron alloys named after ‘martensitic’ and ‘aging’. They contain nickel, chromium, cobalt, and molybdenum, and form martensite on air cooling with a strength of about 1 GPa. When reheated to about 500°C and aged for some hours, a room-temperature strength of some 2.4 GPa is reached. |
| Martempering |
(1) A hardening procedure in which an austenitized ferrous workpiece is quenched into an appropriate medium whose temperature is maintained substantially at the Ms of the workpiece, held in the medium until its temperature is uniform throughout–but not long enough to permit bainite to form–and then cooled in air. The treatment is frequently followed by tempering. (2) When the process is applied to carburized material, the controlling Ms temperature is that of the case. This variation of the process is frequently called marquenching. |
| Martensitic stainless steels |
Martensitic stainless steels are hardenable magnetic stainless steels that contain 9–18% chromium, 0.06–1.25% carbon, and typically up to about 2.5% in total of manganese, silicon, nickel, and molybdenum. They can be heat-treated such that martensite is the prime microconstituent, and are usually available in the annealed or quenched-and-tempered condition. They have excellent strength and high hardness, and good corrosion resistance. Uses include cutlery, surgical instruments, rifle barrels, steam turbine tubing and blading, jetengine components, hand tools, machine parts, fasteners, valves, springs, bearings, pump shafts, nozzles, mining equipment, and wear-resistant parts. |
| Mass flow meter |
An instrument, such as a Coriolis flow meter, which measures the mass flow rate of a fluid flowing through a pipe or other duct, rather than its volume flow rate. |
| Mass flow rate (ṁ) (Unit kg/s) |
The mass of a material, usually a fluid or powder, that flows across a surface or through a pipe or other duct per unit time. The corresponding mass flux (ṁ″), with unit kg/s.m2, is the mass flow across a real surface or through a duct, divided by the surface or cross-sectional area A, i.e. ṁ″ = ṁ/A or ṁ″ = ρV where ρ is the material density and V is its velocity normal to the surface. |
| Material velocity |
The velocity of sound in a body (e.g., a bolt). A term used in the ultrasonic measurement of bolt stress or strain. |
| Materials science |
The study of the properties, behaviour, and application of solid substances such as metals, ceramics, glasses, polymers, composites, biomaterials, and semiconductors, at all scales from the atomic to the macroscopic. The topic has its origins in metallurgy. |
| Materials selection |
The process of selecting the most suitable material for a particular application. In the case of solid materials, it involves assessment of candidate materials in terms of their physical properties (density, elastic moduli, fracture toughness, yield strength, hardness, fatigue resistance, coefficient of thermal expansion, thermal conductivity, resistance to thermal shock, creep resistance, corrosion resistance, radiation resistance, etc), and such factors as the function of the final product, its shape, method of manufacture, required tolerances, number to be made, ease of inspection and quality control, cost, and design methodology employed. |
| Maximum allowable operating pressure (Unit Pa) |
The highest pressure at which any pressure system may be operated, usually 10 to 20% below the maximum allowable working pressure. |
| Maximum allowable working pressure (Unit Pa) |
The pressure on which the design of a pressure system is based and the highest pressure at which relief valves should be set. The lowest-rated component in the system typically has a design safety factor of 4. |
| Maximum continuous load |
(maximum continuous rating) (Unit kg/s) The maximum rate of steam output that a boiler can supply for a specified period, usually 24 hours. |
| Maximum load (Pmax) |
(1) The load having the highest algebraic value in the load cycle. Tensile loads are considered positive and compressive loads negative. (2)Used to determine the strength of a structural member; the load that can be borne before failure is apparent. |
| Maximum material condition |
(maximum metal condition) The situation where the volume of a manufactured component corresponds to the upper limit for all toleranced external dimensions, and to the lower limit for all internal dimensions. |
| Maximum stress (Smax) |
The stress having the highest algebraic value in the stress cycle, tensile stress being considered positive and compressive stress negative. The nominal stress is used most commonly. |
| Maximum stress-intensity factor (Kmax) |
The maximum value of the stress-intensity factorin a fatigue cycle. |
| Maximum-and-minimum thermometer |
(minimum-maximum thermometer, Six’s thermometer) A liquid-in-glass instrument designed to register the maximum and minimum temperatures experienced over a given time period. |
| Mean diameter (Unit m) |
The average of the inside and outside diameters for a helical spring or hollow circular cylinder or sphere |
| Mean stress (Unit Pa) |
The average of the maximum and minimum stresses for a material subjected to a stress cycle, as in a fatigue test. |
| Mean value |
The average value of a number of data points. Computed by dividing the sum of all data by the number of data points. |
| Mechanical alloying |
Production of alloys from powders by consolidation, e.g. hotpressing or extrusion, followed by hot and/or cold working and final annealing. Materials such as dispersion-strengthened alloys, high-temperature aluminium alloys, and amorphous alloys can be made in this way. |
| Mechanical efficiency (η) |
1. In general for a machine, the ratio of output work to input work. 2. For a compressor, the ratio of indicated power to shaft power; for a reciprocating engine or an expander, the ratio of shaft power to indicated power. |
| Mechanical energy (Unit J) |
The sum of kinetic energy and potential energy for an object or a mechanical system, including the energy stored in springs, etc. |
| mechanical engineering |
That branch of engineering concerned with energy conversion, stress analysis, vibration, dynamics, and kinematics, especially applied to design (machine design, mechanical-engineering design). |
| Mechanical metallurgy |
The science and technology dealing with the behavior of metals when subjected to applied forces. |
| Mechanical properties of solid materials |
The strength and stiffness properties of solid materials such as fracture toughness, the moduli of elasticity, percent elongation, |
| Mechanical units |
The units of physical quantities, the dimensions of which includemass, length, and time. |
| Mechanical vibration |
The motion of a particle or body which oscillates about a position of equilibrium. |
| Mechatronics |
The integration of mechanical engineering, electrical engineering, electronic engineering, and software engineering. |
| Median fatigue life |
The middle value when all of the observed fatigue life values of the individual specimens in a group tested under identical conditions are arranged in order of magnitude. When an even number of specimens are tested, the average of the two middlemost values is used. Use of the sample median rather than the arithmetic mean (that is, the average) is usually preferred. |
| Median fatigue strength at N cycles |
An estimate of the stress level at which 50% of the population would survive N cycles. The estimate is derived from a particular point of the fatigue life distribution, because there is no test procedure by which a frequency distribution of fatigue strengths at Ncycles can be directly observed. |
| Melting point (Unit K or °C) |
The temperature at which a solid material undergoes the phase change to a liquid at a specified pressure, usually 1 atm. Pure metals and eutectics have single-valued melting points, while alloys with other compositions melt over a range of temperature such that there is a well-defined start and end to the melting process, but there are states in between where solid and liquid are both present. |
| Metallurgy |
The study of the atomic, crystallographic, microstructural, mechanical, and physical properties of metallic elements and alloys and their applications. |
| Metre (m) |
The base unit of length in the SI system. It is equal to the distance travelled by light in vacuum in 1/299 792 458 seconds. |
| Metric ton (tonne, t) |
An SI-accepted, but non-SI, unit of mass defined as 1 t = 103 kg. |
| Meyer hardness (Unit Pa) |
A material hardness similar to the Brinell hardness but based upon the projected area of the impression rather than the surface area. |
| Microhardness |
The hardness of a material as determined by forcing an indenter such as a Vickers or Knoop indenter into the surface of a material under very light load; usually, the indentations are so small that they must be measured with a microscope. Capable of determining hardnesses of different microconstituents within a structure, or of measuring steep hardness gradients such as those encountered in case hardening. |
| Microindentation hardness test |
Hardness test using a calibrated machine to force a diamond indenter of specific geometry, under a test load of 1 to 1000 gramforce, into the surface of the test material and to measure the diagonal or diagonals of the indentation optically. |
| Micrometer |
(micrometer gauge) A mechanical-contact device for the accurate measurement of the length, width, diameter, etc. of an object, the depth of a hole, the height of a step, etc. The usual arrangement is a spindle that is moved by rotation of a thimble, the distance then being read off a vernier scale. |
| Microscopic stresses |
Residual stresses that vary from tension to compression in a distance (presumably approximating the grain size) that is small compared with the gage length in ordinary strain measurements. They are not detectable by dissection methods, but can sometimes be measured from line shift or line broadening in an x-ray diffraction pattern. |
| Microstrain |
The strain over a gage length comparable to interatomic distances. These are the strains being averaged by the macrostrain measurement. Microstrain is not measurable by currently existing techniques. Variance of the microstrain distribution can, however, be measured by x-ray diffraction. |
| Millimetre of mercury |
A non-SI unit of pressure equal to the pressure exerted by a vertical column of mercury 1 mm high. Approximately equal to 133.3 Pa or 1 torr. |
| Millimetre of water |
A non-SI unit of pressure equal to the pressure exerted by a vertical column of water 1 mm high. Approximately equal to 9.81 Pa. |
| Milling |
Milling is a machining process in which metal is removed by a rotating multiple-tooth cutter; each tooth removes a small amount of metal with each revolution of the spindle. Because both workpiece and cutter can be moved in more than one direction at the same time, surfaces having almost any orientation can be machined. |
| Milling |
A machining process, typically for metals and plastics, in which a multi-tooth rotary cutter removes material to produce flat or profiled surfaces, slots, grooves, etc. |
| Minimum load (Pmin) |
In fatigue, the least algebraic value of applied load in a cycle. |
| Minimum material condition |
(minimum metal condition) The situation where the volume of a manufactured component corresponds to the lower limit of all toleranced external dimensions and to the upper limit for all internal dimensions. |
| Minimum stress (Smin) |
In fatigue, the stress having the lowest algebraic value in the cycle, tensile stress being considered positive and compressive stress negative. |
| Minimum stress-intensity factor (Kmin) |
In fatigue, the minimum value of the stress-intensity factor in a cycle. This value corresponds to the minimum loadwhen the load ratio is 0 and is taken to be 0 when the load ratio is ≤0. |
| Minor diameter |
On a straight thread, the diameter of the imaginary cylinder which just touches the root of an external thread or the crest of an internal thread. |
| Minute (min) |
A non-SI unit of time. The conversion to SI is 1 min = 60 s. |
| Mode of failure |
The various ways in which a material can fail when subjected to loading, surface damage, chemical attack, heating, cooling, or radiation, in either isolation or combination. The failure modes include fracture, fatigue, creep, corrosion, erosion, and embrittlement. |
| Modulus of elasticity (E) |
The measure of rigidity or stiffness of a metal; the ratio of stress, below the proportional limit, to the corresponding strain. In terms of the stress-strain diagram, the modulus of elasticity is the slope of the stress-strain curve in the range of linear proportionality of stress to strain. |
| Modulus of resilience |
The amount of energy stored in a material when loaded to its elastic limit. It is determined by measuring the area under the stress-strain curve up to the elastic limit. |
| Modulus of rupture |
Nominal stress at fracture in a bend test or torsion test. In bending, modulus of rupture is the bending moment at fracture divided by the section modulus. In torsion, modulus of rupture is the torque at fracture divided by the polar section modulus. |
| Mohr–Coulomb fracture criterion |
A fracture criterion, primarily for brittle materials, according to which failure occurs when the stress at a point in a material falls outside the envelope created by the Mohr’s circles for uniaxial tensile strength and uniaxial compressive strength. |
| Mohr–Coulomb yield criterion |
A pressure-dependent yield criterion, according to which yielding occurs when the stress at a point in a material falls on the envelope created by the Mohr’s stress circles at yielding for various tests such as tension, shear, and compression that have different components of hydrostatic stress. |
| Mohs hardness test |
A scratch hardness test for determining comparative hardness using 10 standard minerals—from talc (the softest) to diamond (the hardest). |
| Mohs scale |
A scale of scratch hardness originally developed for minerals. |
| Mole (mol) |
The base unit of the amount of substance in the SI system. It was previously defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. From 20 May 2019 it has been defined as the amount of substance containing exactly 6.022 140 76 × 1023 elementary entities. |
| Mole number (N) |
The number of moles in a given mass m of a substance, equal to m divided by the molar mass of the substance. |
| Monotron hardness test |
An obsolete method of determining indentation hardnessby measuring the load required to force a spherical penetrator into a metal to a specified depth. |
| Morse taper |
A self holding, standard taper largely used on drilling tools, drilling machine spindles, and some lathes. |
| Multiple threaded screw |
A screw with two or more threads cut around the periphery of the workpiece to provide an increased lead with a specified pitch. |
| Nano machining |
The machining of silicon chips typically less than 1 nm thick. |
| Nanomanufacturing |
1. The manufacture of nanoscale materials. 2. The manufacture of components or devices that have critical dimensions of order 1 nm. |
| Nanomaterial |
A material defined by the European Commission as ‘A natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1–100 nm. In specific cases and where warranted by concerns for the environment, health, safety or competitiveness the number size distribution threshold of 50% may be replaced by a threshold between 1 and 50%.’ |
| Nanoparticles |
Particles with dimensions of order 1 nm which have either been introduced into microstructures for reinforcement or are present as impurities. |
| Nanotechnology |
The science and engineering of materials that have been structured on length scales of 1–100 nm, resulting in modified physical properties owing to changes in the ratio of surface area to volume (atoms on surfaces having different symmetry from those in the bulk) and because many of the fundamental physical processes that underpin the properties of materials have a characteristic length scale of a few nm, so that alteration of microstructure at the nm level alters the bulk properties. |
| Natural aging |
Spontaneous aging of a supersaturated solid solution at room temperature. |
| Natural unit system (n.u. system) |
A non-SI system of units, based on fundamental constants, used in high-energy and particle physics. The n.u. of speed is the speed of light in a vacuum, c0 = 299 792 458 m/s. |
| Nautical mile (M) |
A special unit of distance employed for marine and aerial navigation. The conversion to SI is 1 M = 1 852 m. |
| Net weight (Unit N) |
The difference between the gross weight of any container including its contents and the tare weight of the empty container. |
| Neutralization number |
An ASTM number given to quenching oils that reflects the oil’s tendency towards oxidation and sludging. |
| Newton (N) |
The basic unit of force in the SI system, defined as the force that results in an acceleration of 1 m/s2 when acting on a 1 kg mass. |
| Newton’s law of gravitation |
(Newton’s law of universal gravitation) Any two bodies exert a gravitational force of attraction F on each other directed along the line joining their centres of mass, in magnitude proportional to the product of their masses and inversely proportional to the square of the distance r between them. |
| Newton’s law of viscosity |
The shear stress between two parallel layers of fluid is proportional to the velocity of one relative to that of the other, and inversely proportional to the distance between them. The constant of proportionality is the dynamic viscosity μ. For a Newtonian fluid μ is independent of pressure gradient, shear stress, and strain, but may change with temperature and pressure. |
| Nipple |
A device containing a non-return valve screwed into a lubrication point through which grease may be introduced, for example into a bearing. |
| Noble metals |
Metallic elements from periods (rows) 5 and 6 of the periodic table, namely ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold. They resist corrosion and oxidation and are used for electrical contacts, thin-film circuits, and electroplating. Gold, silver, and platinum used for jewellery are also known as precious metals. They have face-centred cubic crystal structure and good ductility. |
| Nominal diameter |
The ‘‘catalog diameter’’ of a fastener. Usually roughly equal to the diameter of the body, or the outer diameter of the threads. |
| Nominal power (Unit kW) |
1. For an internal combustion engine, the average power output of a typical production engine under normal working conditions measured according to SAE standard J 1349/ISO 1585. 2. For an electric motor, the rated equivalent mechanical power output. |
| Nominal size |
The intended size of a component. The actual size will depend on manufacturing tolerances. |
| Nominal stress |
The stress at a point calculated on the net cross section by simple elasticity theory without taking into account the effect on the stress produced by stress raisers such as holes, grooves, fillets, etc. |
| Non-ferrous metal |
Strictly, all metals and alloys that do not contain iron, but usually taken to mean aluminium- and copper-based alloys. |
| Nonlinear behavior |
A fastener or joint system is said to exhibit nonlinear behavior when the relationship between the External load on the joint and deformation of the parts is nonlinear, or when the relationship between increasing Preload and deformation is nonlinear. |
| Normal stress |
The stress component perpendicular to a plane on which forces act. Normal stress may be either tensile stress or compressive stress. |
| Normal stress (Unit N) |
A stress in a fluid (such as pressure) or a solid, that is perpendicular to the surface on which it acts. The surface may be a real external surface or an imaginary internal one. |
| Normal temperature and pressure (NTP) |
Reference conditions defined by NIST as 20°C and 1 atm. |
| Normality |
The principle that the vector sum of plastic strain increments is perpendicular to the yield surface. |
| Normalizing |
Heating a ferrous alloy to a suitable temperature above the transformation range and then cooling in air to a temperature substantially below the transformation range. |
| Notch brittleness |
Susceptibility of a material to brittle fractureat points of stress concentration. For example, in a notch tensile test, the material is said to be notch brittle if the notch strengthis less than the tensile strength of an unnotched specimen. Otherwise, it is said to be notch ductile. |
| Notch depth |
The distance from the surface of a notched test specimen to the bottom of the notch. In a cylindrical test specimen, the percentage of the original cross-sectional area removed by machining an annular groove. |
| Notch ductility |
The percentage reduction in area after complete separation of the metal in a tensile test of a notched specimen. |
| Notch strength |
The maximum load on a notched tensile-test specimen divided by the minimum cross-sectional area (the area at the root of the notch). Also known as notch tensile strength. |
| Nut |
An internally-threaded fastener used with externally-threaded bolts. Usually hexagonal in shape but there is a multitude of other geometries, such as square, for special purposes and different gripping tools. |
| Nut |
A metal fastener of square, hexagon or other shape, having an internal thread which screws onto a bolt, stud or arbor. |
| Nut box mechanism |
In a lathe it is used to engage and disengage the lead screw for thread cutting. This comprises a pair of half-nuts capable of sliding in vertical slides in or out of mesh with the lead screw. |
| Nut factor |
An experimental constant used to evaluate or describe the ratio between the torque applied to a fastener and the Preload achieved as a result. |
| Nut splitter |
A tool for removing rusted-on and corroded nuts from bolts. It consists of a stiff steel ring that is placed around the nut. A screw thread passing through the ring bears diametrically on a wedged-shape tip that indents and cuts though a face of the nut. |
| Nut thickness |
The nut thickness shall be the overall distance measured parallel to the axis of nut, from the top of the nut to the bearing surface, and shall include the thickness of the washer face where provided. |
| Observed value |
The particular value of a characteristic determined as a result of a test or measurement. |
| Octane number |
(octane rating) A measure of the anti-knock properties of a fuel taken as the percentage by volume of iso-octane in the fuel. |
| Offset |
The distance along the strain coordinate between the initial portion of a stress-strain curve and a line parallel to the initial portion that intersects the stress-strain curve at a value of stress (commonly 0.2%) that is used as a measure of the yield strength. Used for materials that have no obvious yield point. |
| Offset modulus |
The ratio of the offset yield stress to the extension at the offset point (plastics). |
| Offset yield strength |
The stress at which the strain exceeds by a specified amount (the offset) an extension of the initial proportional portion of the stress-strain curve. Expressed in force per unit area. |
| Oil hardening |
The formation of martensite before tempering by quenching suitable steels in oil at lower cooling rates than given by water quenching, thus reducing the likelihood of component fracture or severe residual stresses. |
| Oil hardening |
Quench-hardening treatment involving cooling in oil. |
| Oil quenching |
Hardening of carbon steel in an oil bath. Oils are categorized as conventional, fast, martempering, or hot quenching. |
| Oil-cooled |
A term for a machine that uses circulating oil to remove heat. |
| Oldham coupling |
(double-slider coupling) A device for connecting a pair of misaligned parallel shafts, on the end of each of which are flanges having diametral tenons (tongues) that engage with matching slots spaced at 90° in an intermediate disc. As the coupling rotates, the disc compensates for shaft offset by sliding along each tenon in turn. Low-friction surfaces are requisite and the disc is often made of a polymer. An alternative arrangement has the slots in the flanges and the tenons on the disc. |
| Olsen cup test |
A cupping test in which a piece of sheet metal, restrained except at the center, is deformed by a standard steel ball until fracture occurs. The height of the cup at time of fracture is a measure of the ductility. |
| Orange peel |
A surface roughening in the form of a pebble-grained pattern where a metal of unusually coarse grain is stressed beyond its elastic limit. Also known as pebbles and alligator skin. |
| O-ring (O-ring seal) |
A toroidal ring of synthetic rubber seated in a groove machined into a flat or cylindrical surface to act as a seal. |
| Outside diameter (Unit m) |
(external diameter, OD) The diameter of a circular cylindrical tube or a sphere measured between opposite points on the external surface. |
| Overaging |
Aging under conditions of time and temperature greater than those required to obtain maximum change in a certain property, so that the property is altered in the direction of the initial value. |
| Overheating |
Heating a metal or alloy to such a high temperature that its properties are impaired. When the original properties cannot be restored by further heat treating, by mechanical working, or by a combination of working and heat treating, the overheating is known as burning. |
| Oxidation |
(1) A reaction in which there is an increase in valence resulting from a loss of electrons. (2) A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air. |
| Oxidative wear |
A type of wear resulting from the sliding action between two metallic components that generates oxide films on the metal surfaces. These oxide films prevent the formation of a metallic bond between the sliding surfaces, resulting in fine wear debris and low wear rates. |
| Oxidized surface (on steel) |
Surface having a thin, tightly adhering, oxidized skin (from straw to blue in color), extending in from the edge of a coil or sheet. Sometimes called annealing border. |
| Oxidizing agent |
A compound that causes oxidation, thereby itself becoming reduced. |
| Oxidizing flame |
A gas flame produced with excess oxygen in the inner flame. |
| Oxygen probe |
An atmosphere-monitoring device that electronically measures the difference between the partial pressure of oxygen in a furnace or furnace supply atmosphere and the external air. |
| Partial annealing |
An imprecise term used to denote a treatment given cold-worked material to reduce the strength to a controlled level or to effect stress relief. To be meaningful, the type of material, the degree of cold work, and the time-temperature schedule must be stated. |
| Pascal (Pa) |
The SI unit of pressure, 1 Pa = 1 N/m2. |
| Pascal’s law |
When there is a change in pressure at any point in a confined fluid at rest, there is an equal change at every other point in the fluid volume. |
| Peel strength (Unit N) |
(peel test) The strength of an adhesive bond between two materials as measured by the force required in a given direction to peel apart adhered strips. This force depends on the dimensions and thickness of the adhered strips and their yield stress, together with the fracture toughness of the bond. |
| Percent error |
For testing machines, the ratio, expressed as a percentage, of the error to the correct value of the applied load. |
| Permanent set |
The deformation or strain remaining in a previously stressed body after release of load. |
| Permissible variation |
For testing machines, the maximum allowable error in the value of the quantity indicated. It is convenient to express permissible variation in terms of the percent error. |
| Physical crack size (ap) |
The distance from a reference plane to the observed crack front. This distance may represent an average of several measurements along the crack front. The reference plane depends on the specimen form, and it is normally taken to be either the boundary or a plane containing either the load line or the centerline of a specimen or plate. |
| Physical properties |
Properties of a material the determination of which does not involve the deformation or destruction of the specimen—for example, density, electrical conductivity, coefficient of thermal expansion, magnetic permeability, and lattice parameter. Does not include chemical reactivity or the properties more appropriately regarded as mechanical properties. |
| Physical testing |
Methods used to determine the entire range of physical properties of a material. In addition to density and thermal, electrical, and magnetic properties, physical testing methods can be used to assess simple fundamental physical properties such as color, crystalline form, and melting point. |
| Pin expansion test |
A test for determining the ability of a tube to be expanded or for revealing the presence of cracks or other longitudinal weaknesses in it, made by forcing a tapered pin into the open end of the tube, similar to flare test. |
| Pin or mandrel |
In bend testing, the plunger or tool used in making semiguided, guided, or wrap-around tests to apply the bending force to the inside surface of the bend. In free bends or semiguided bends to an angle of180°, a shim or block of the proper thickness may be placed between the legs of the specimen as bending is completed. This shim or block is also referred to as a pin or mandrel. |
| Piston pump |
A positive-displacement pump in the basic configuration of which a piston reciprocates in a cylinder. On the suction stroke, fluid is drawn into a chamber through the inlet valve, which is then open while the outlet valve is closed. On the delivery stroke the fluid is forced out of the chamber through the outlet valve, which then opens while the inlet valve is closed. Both axial and radial configurations are common. |
| Pitch |
The nominal distance between two adjacent thread roots or crests. |
| Pitch |
In screw threads, the distance from a point on one thread to a corresponding point on the next thread measured parallel to the axis. In the case of spur gears, indicates the size of the gear teeth and is correctly called diametral pitch. |
| Pitch (Unit m) |
1. In a cascade of identical turbine or compressor blades, the distance between successive blades measured parallel to the leading edges. 2. For a screw thread, the distance between adjacent thread forms measured parallel to the thread axis. 3. For an aircraft, the vertical relationship between the nose and the horizon. The pitch axis is an axis in the plane of the wings of an aircraft, perpendicular to the centreline, about which the aircraft rotates. Pitch motion is the corresponding up or down movement of the aircraft nose. Pitch attitude is the angle between the centreline of an aircraft and the horizontal. The angle is positive when the nose is above its position when the centreline is horizontal. |
| Pitch angle (Unit °) |
For a bevel gear, the angle between the axis and the pitch-cone generator. |
| Pitch diameter |
For screw threads, the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points that would make the width of the groove and width of the land equal to one half the pitch. |
| Pitch-circle diameter (Unit m) |
The diameter of the circle (the pitch circle) centred on a component’s axis, around which holes or bolts are equally spaced. |
| Pitting |
In tribology, a type of wear characterized by the presence of small, sharp surface cavities formed by processes such as fatigue, local adhesion, wear, corrosion, or cavitation. |
| Planar anisotropy |
A variation in physical and/or mechanical properties with respect to direction within the plane of material in sheet form. |
| Plane strain |
The stress condition in linear-elastic fracture mechanics in which there is zero strain in a direction normal to both the axis of applied tensile stress and the direction of crack growth (i.e., parallel to the crack front); most nearly achieved in loading thick plates along a direction parallel to the plate surface. Under plane-strain conditions, the plane of fracture instability is normal to the axis of the principal tensile stress. |
| Plane stress |
The stress condition in linear-elastic fracture mechanics in which the stress in the thickness direction is zero; most nearly achieved in loading very thin sheet along a direction parallel to the surface of the sheet. Under plane-stress conditions, the plane of fracture instability is inclined 45° to the axis of the principal tensile stress. |
| Plane-strain fracture toughness (KIc) |
The crack extension resistance under conditions of crack-tip plane strain. |
| Plane-stress fracture toughness (Kc) |
In linear-elastic fracture mechanics, the value of the crack-extension resistance at the instability condition determined from the tangency between the R-curve and the critical crack-extension force curve of the specimen. |
| Plastic deformation |
The permanent (inelastic) distortion of a material under applied stress that strains the material beyond its elastic limit. |
| Plowing |
In tribology, the formation of grooves by plastic deformation of the softer of two surfaces in relative motion. |
| Plug |
1. An object, usually tapered, used to block a hole to prevent fluid from escaping or flowing. 2. The central moveable part of a valve. 3. In the flow of a yield-stress fluid through a pipe or duct, the central region is often assumed to take the form of a plug with uniform axial velocity. 4. In thermoforming, a shaping tool that forces a heated plastic sheet into a female mould. |
| Plug gauge |
A steel gauge used to determine the dimension or dimensions of a hole. It may be straight or tapered, and is typically circular, but in principle can have any cross section. |
| Pneumatic control valve |
(pneumatic valve) A valve in which the position of the valve stem, which varies the open area, is determined by the net force generated by compressed air acting on a diaphragm operating against the force of a compression spring. Depending upon the arrangement of the spring, in the event of air-supply failure, the valve may open or close. |
| Poisson’s ratio (ν) |
The absolute value of the ratio of transverse (lateral) strain to the corresponding axial strain resulting from uniformly distributed axial stress below the proportional limit of the material. |
| Pop rivet |
A hollow rivet that enables a connexion to be made from one side only of an assembly. |
| Population |
The hypothetical collection of all possible test specimens that could be prepared in the specified way from the material under consideration. Also known as universe. |
| Positive-displacement compressor |
Any type of compressor, including piston and rotary-screw types, that delivers a fixed volume of gas at high pressure per unit time. A positive-displacement pump delivers a fixed volume of fluid, usually a liquid, per unit time. |
| Positive-displacement flow meter |
An instrument of high accuracy that determines volumetric flow rate by dividing the flowing fluid into successive fixed volumes, and measuring their times of passage through the meter. |
| Positive-displacement machine |
A machine that incorporates pistons, valves, etc. to ensure positive admission and delivery of a working fluid and prevent undesired reversal of flow. This class of machine incorporates all reciprocating compressors and expanders and some types of rotary compressor, the Roots blower, agear pump, a lobe pump, and a vane pump. |
| Power |
The rate of doing work or of producing or consuming energy. The unit of power is the watt, W, where 1 W = 1 N m/s. |
| Precipitation hardening |
Hardening caused by the precipitation of a constituent from a supersaturated solid solution. |
| Precipitation heat treatment |
Artificial aging in which a constituent precipitates from a supersaturated solid solution. |
| Precipitation-hardenable stainless steels |
Precipitation-hardenable stainless steels contain typically 11–18% chromium, 3–10% nickel, 0.05–0.2% carbon, and small additions of manganese, silicon, aluminium, molybdenum, niobium and titanium. These steels can be supplied in a solution-treated condition, which is readily fabricated or machined, and then hardened by an ageing treatment that produces a fine dispersion of second-phase precipitates. They are available in austenitic, semi-austenitic, and martensitic grades. Uses include springs, knives, and pressure vessels. |
| Precision |
The closeness of agreement between the results of individual replicated measurements or tests. The standard deviation of the error of measurement may be used as a measure of “imprecision.” |
| Preheating |
Heating before some further thermal or mechanical treatment. For tool steel, heating to an intermediate temperature immediately before final austenitizing. For some nonferrous alloys, heating to a high temperature for a long time, to homogenize the structure before working. In welding and related processes, heating to an intermediate temperature for a short time immediately before welding, brazing, soldering, cutting, or thermal spraying. |
| Preload |
The tension created in a threaded fastener when the nut is first tightened. Often used interchangeably, but incorrectly, with Working load or bolt force or bolt tension |
| Preload accuracy |
A measure of the precision with which a given tool or procedure creates preload in a bolt when the bolt is tightened. A common torque wrench, for example, is said to produce preload with an accuracy of +30%. The mean preload, however, may not be that which the designer intended, or may not be what he should have intended. Accuracy as used here, in other words, is synonymous with Scatter. |
| Preload adjustments |
Preloads may be applied directly by axial loading or indirectly by turning of the nut or bolt. When preload is applied by turning of nuts or bolts, a torsion load component is added to the desired axial bolt load. |
| Preload relaxation |
Local yielding, due to excess bearing stress under nuts and bolt heads (caused by high local spots, rough surface finish, and lack of perfect squareness of bolt and nut bearing surfaces), may result in preload relaxation after preloads are first applied to a bolt. |
| Press quenching |
A quench in which hot dies are pressed and aligned with a part before the quenching process begins. Then the part is placed in contact with a quenching medium in a controlled manner. This process avoids part distortion. |
| Pressure (p) (Unit Pa) |
In thermodynamics and fluid mechanics, the compressive force exerted by the fluid per unit area. The pressure exerted by a fluid on a surface acts normal to the surface. |
| Pressure chamber |
A chamber in which components or devices can be subjected to high or low fluid (liquid or gas) pressure. |
| Pressure control valve |
A valve used to set the pressure level in a pressure vessel or piping system. |
| Pressure difference (Unit Pa) |
(pressure differential, Δp) The difference between two pressures, one of which may be a reference pressure such as barometric pressure. In many flow processes, the pressure difference is more important than the absolute pressure level. |
| Pressure gauge |
An instrument used to measure absolute or gauge pressure. The sensing element may be a tube which deflects when pressurized, as in a Bourdon gauge, a bellows as in an aneroid barometer, a piezoelectric crystal, a piezoresistive element, etc. |
| Pressure loss (Unit Pa) |
The loss in stagnation pressure in internal flow due to wall friction and minor losses in fittings. |
| Pressure rating (Unit Pa or bar) |
The internal pressure at which a pressure vessel, boiler, tank, piping, etc. is designed to operate safely. |
| Pressure ratio |
One pressure divided by another which may be a reference pressure. In many flow and thermodynamic processes, the pressure ratio is more important than the absolute pressure levels. For example, in compressible gas flow the Mach number is determined by the ratio of the stagnation pressure to the static pressure. |
| Pressure recovery (Unit Pa) |
The progressive increase in static pressure for unseparated flow through a diverging nozzle or diffuser. |
| Pressure regulator |
(pressure-regulating valve) A device installed in a pneumatic or gas system to maintain the downstream pressure at the required level. |
| Pressure snubber |
(pressure-pulse snubber) A device used for filtering, dampening, and the protection of transducers, pressure gauges, and switches, from pressure surges, pressure spikes, and water hammer. A typical design uses a fixed mesh or porous metal disc installed in an inline body. In a piston-type pressure-gauge snubber, a piston is forced against an orifice leading to the gauge. |
| Pressure storage tank |
(pressure vessel) A closed container for storing gases or volatile liquids, such as liquefied gases, at pressures significantly above atmospheric pressure. Such tanks are commonly cylindrical with domed ends, spherical, spheroidal, torispherical or hemispherical. |
| Pressure surface |
The high-pressure surface of an aerofoil, turbine, or compressor blade. In normal aircraft applications, this is the lower surface of a wing. For applications, such as to high-performance motor vehicles, where the aerofoil is inverted to generate downforce, it is the upper surface. |
| Pressure tap |
A small hole in the wall of a pipe or pressure vessel to which is attached a tube, the other end of which is connected to one side of a pressure transducer. |
| Pressure-relief valve |
A valve that limits the maximum pressure in a pressure vessel or fluid-power system to a specified level. |
| Pre-stress |
To induce stresses into a component or structure before it is subjected to operating loads. |
| Prevailing torque |
Torque required to run a nut down against the joint when some obstruction, such as a plastic insert in the threads, or a noncircular thread, or other, has been introduced to help the fastener resist vibration loosening. Prevailing torque, unlike normal torque on a nut or bolt, is not proportional to the Preload in the fastener. |
| Principal stress (normal) |
The maximum or minimum value of the normal stressat a point in a plane considered with respect to all possible orientations of the considered plane. On such principal planes the shear stress is zero. There are three principal stresses on three mutually perpendicular planes. The state of stress at a point may be: (1) uniaxial, a state of stress in which two of the three principal stresses are zero; (2) biaxial, a state of stress in which only one of the three principal stresses is zero; or (3) triaxial, a state of stress in which none of the principal stresses is zero. Multiaxial stress refers to either biaxial or triaxial stress. |
| Process annealing |
An imprecise term denoting various treatments used to improve workability. For the term to be meaningful, the condition of the material and the time-temperature cycle used must be stated. |
| Profile |
The shape of a cross section of an object, such as an aerofoil or cam. |
| Profiling machine |
(profile milling machine) A milling machine in which the cutter is guided by the contour of a pattern having the required shape. |
| Profilometer |
An instrument used to quantify the roughness of a surface. For a contact profilometer, a diamond stylus sweeps across the surface along a series of parallel lines. Noncontact profilometers use optical techniques to map the surface irregularities. |
| Progressive aging |
Aging by increasing the temperature in steps or continuously during the aging cycle. |
| Proof load |
The maximum, safe, static, tensile load which can be placed on a fastener without yielding it. Sometimes given as a force (in lb or N) sometimes as a stress (in psi or MPa). |
| Proof load (Unit N) |
The test load in quality control to which different components made of different materials must be subjected without failing in order to perform properly. |
| Proof strength (Unit Pa) |
(proof stress, Rp, Rp,) The yield strength (offset yield strength) at some fixed value of the permanent strain given by the intersection of a line offset from, but parallel to, the elastic loading line and the engineering stress-engineering strain curve. The offset is arbitrary but is usually 0.1, 0.2, or 0.5% permanent strain (the proof strain). Used for materials which do not exhibit a sharply defined yield point. The notation Rp0.2 or Rp0.2 is employed for the 0.2% proof stress. |
| Proof stress |
(1) The stress that will cause a specified small permanent set in a material. (2) A specified stress to be applied to a member or structure to indicate its ability to withstand service loads. |
| Proportional limit |
The greatest stress a material is capable of developing without a deviation from straight-line proportionality between stress and strain. |
| Prying |
The magnification of an External load by a pseudolever action when that load is an Eccentric tensile load. |
| PSI (pounds per square inch) |
Pressure measurement. Psia (pounds per square inch absolute) measures actual pressure with no allowance for atmospheric pressure. Psig (pounds per square inch gauge) measures pressure with the gauge set to zero (0) at atmospheric pressure (14.7 psia). In other words psig = psia – 14.7. |
| Pulley |
A free or driven wheel on a shaft with an appropriately shaped rim to carry a flat belt, vee belt, notched belt, rope, or chain. Used to transmit power or motion. |
| Pulse |
An increase or decrease in the magnitude of a physical quantity, such as pressure, voltage, or force, with a time scale short compared with other time scales in a process, after which there is a return to the original level. |
| Pump |
A machine designed to cause a liquid, gas, vapour, or slurry to flow due to the reciprocating motion of pistons, rotation of vanes, or rotation of an impeller. |
| Punch |
1. A hand tool with a sharp point, used to mark the position of a hole centre in a workpiece prior to drilling. 2. A flat-ended tool with sharp edges, used to shear out a hole in a plate that is supported underneath on a die having a slightly larger same-shaped orifice. |
| Push rod |
A rod which opens and closes a valve via a valve rocker in an overhead-valve piston engine. The rod is actuated by a camshaft located in the crankcase. |
| Q-factor |
(quality factor, sharpness of resonance) A non-dimensional parameter that describes, for a given resonant mode, the sharpness of the peak in the frequency response of a lightly-damped linear oscillator: Q = ωRES/Δω where ωRES is the resonance frequency and Δω is the half-power bandwidth of the resonance. It is inversely related to the damping such that a broad peak corresponds to high damping and a narrow peak to low damping. |
| Quarter hard |
A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength aboutmidway between that of dead soft and half hard tempers. |
| Quench (quenching) |
1. The process of rapid-cooling by plunging an object into a bath of water, oil, salt, molten metal, or other media. It is a method of heat treatment used particularly to form martensite preparatory to tempering steels. The bath temperature is the quench temperature. 2. The suppression of combustion. 3. In a piston engine, the cooling of a fraction of the gases during combustion, typically by reducing the clearance between the piston crown and the cylinder head. |
| Quench aging |
Aging induced by rapid cooling after solution heat treatment. |
| Quench annealing |
Annealing an austenitic ferrous alloy by solution heat treatment followed by rapid quenching. |
| Quench cracking |
Fracture of a metal during quenching from elevated temperature. Most frequently observed in hardened carbon steel, alloy steel, or tool steel parts of high hardness and low toughness. Cracks often emanate from fillets, holes, corners, or other stress raisers and result from high stresses due to the volume changes accompanying transformation to martensite. |
| Quench hardening |
(1) Hardening suitable alpha-beta alloys (most often certain copper to titanium alloys) by solution treating and quenching to develop a martensitic-like structure. (2) In ferrous alloys, hardening by austenitizing and then cooling at a rate such that a substantial amount of austenite transforms to martensite. |
| Quench-age embrittlement |
Embrittlement of low-carbon steels resulting from precipitation of solute carbon at existing dislocations and from precipitation hardening of the steel caused by differences in ferrite at different temperatures. Quenchage embrittlement usually is caused by rapid cooling of the steel from temperatures slightly below Ac1 (the temperature at which austenite begins to form), and can be minimized by quenching from lower temperatures. |
| Quenching |
Rapid cooling. When applicable, the following more specific terms should be used: brine quenching, caustic quenching, cold die quenching, forced-air quenching, intense quenching, oil quenching, press quenching, spray quenching, direct quenching, fog quenching, hot quenching, interrupted quenching, selective quenching, time quenching, and water quenching. |
| Quick coupling |
(quick disconnect, fast coupling) A hose connection allowing rapid assembly or disassembly and comprising a socket and a plug incorporating a springloaded locking mechanism. |
| Quimby screw pump |
A screw pump with two meshing screws, each having a righthand and a left-hand screw. Liquid enters at either end and is discharged from the middle. |
| Radial engine |
A piston engine in which the cylinders are arranged radially around the crankshaft, a design commonly used in early aircraft engines. |
| Radial flow |
radial flow Fluid flow for which the principal direction of flow is either radially inward or radially outward relative to an axial direction. |
| Radial load (Unit N) |
1. Centrifugal loading induced in a rotating body. 2. (through-wallthickness load) The radial component of loading arising from internal or external pressurization of a closed vessel. There will also be axial and hoop loading. |
| Radial marks |
Lines on a fracture surface that radiate from the fracture origin and are visible to the unaided eye or at low magnification. Radial lines result from the intersection and connection of brittle fractures propagating at different levels. Also known as shear ledges. |
| Radial wave equation |
A differential equation describing the transmission of a wave in a system with radial symmetry. |
| Radial-flow compressor |
A compressor in which the working gas enters the machine axially and is compressed as it flows radially out through the impeller. |
| Radian (rad) |
A coherent derived SI unit defined as the plane angle subtended at the centre of a circle by an arc having a length equal to the radius. Thus 2π radians are equivalent to 360° and 1 rad ≈ 57.3°. |
| Radiation pressure (Unit μPa) |
The pressure exerted on a surface exposed to any form of electromagnetic radiation. If the radiation is absorbed, it is equal to the power-flux density divided by the speed of light. |
| Radiator |
A heat exchanger used to transfer thermal energy from one fluid to another for heating or cooling purposes. Despite the name, the principal mode of heat transfer is convection rather than radiation. In motor vehicles, water circulated through the engine block is cooled as it flows through the tubes of an air-cooled heat exchanger. In domestic radiators, hot water from a boiler is circulated through a heat exchanger with a large surface area which transfers heat to the surrounding air. |
| Radioactive heat (Unit J) |
Thermal energy released from the nucleus of an atom such as uranium235 by fission due to the absorption of a neutron. Heat is also produced by radioactive decay. |
| Radiosity (J) (Unit W/m2) |
The total radiation leaving a given surface per unit area, including emitted, reflected, and transmitted radiation. |
| Radius of bend |
The radius of the cylindrical surface of the pin or mandrel that comes in contact with the inside surface of the bend during bending. For free or semiguided bends to 180° in which a shim or block is used, the radius of bend is one-half the thickness of the shim or block. |
| Raised-face flange |
A flange which contacts its mating joint member only in the region in which the gasket is located. The flanges do not contact each other at the bolt circle. |
| Range of stress (Sr) |
The algebraic difference between the maximum and minimum stress in one cycle—that is, Sr = Smax – Smin |
| Ratchet coupling |
A joint employing a ratchet system between two shafts, so that not only does the driven shaft run in one direction only, but also the driven shaft can, if necessary, run more quickly than the driving shaft. |
| Ratchet marks |
Lines on a fatigue fracture surface that result from the intersection and connection of fatigue fractures propagating from multiple origins. Ratchet marks are parallel to the overall direction of crack propagation and are visible to the unaided eye or at low magnification. |
| Rated flow (Unit m3/s or kg/s) |
In a hydraulic system or machine, the volume or mass flow rate a manufacturer specifies for a component as the maximum desirable for it to function as designed. |
| Rated load (Unit N) |
The maximum load that a structure or component has been designed to carry. |
| Rated power (Unit kW or hp) |
(rated capacity, rated horsepower) The maximum power output that can be sustained continuously for any power-producing machine or system, such as an engine or a power plant. |
| R-curve |
In linear-elastic fracture mechanics, a plot of crack-extension resistance as a function of stable crack extension, which is either the difference between the physical crack size, or the effective crack size, and the original crack size. R-curves normally depend on specimen thickness and, for some materials, on temperature and strain rate. |
| Recrystallization annealing |
Annealing cold-worked metal to produce a new grain structure without phase change. |
| Recrystallization temperature |
The approximate minimum temperature at which complete recrystallization of a cold-worked metal occurs within a specified time. |
| Reduction in area (RA) |
The difference between the original cross-sectional area of a tensile specimen and the smallest area at or after fracture as specified for the material undergoing testing. Also known as reduction of area. |
| Refractory materials |
Metals or ceramics that do not melt when exposed to temperature above about 1 500°C or deteriorate rapidly in a continuous temperature above about 550°C. |
| Regenerative pump |
(regenerative turbine pump, peripheral pump) A pump with a double-sided impeller having a large number of radial blades. The pressure of the pumped liquid increases progressively over several revolutions of the impeller. It is particularly suited to producing large heads at small flow rates without cavitation. |
| Relative density |
(specific gravity) The ratio of the density of a substance to that of a reference substance, such as water for liquids and solids, and dry air for gases. |
| Relative humidity (ϕ) |
The ratio or percentage of the actual mass of moisture in a given volume of air at a given temperature to the maximum possible mass of moisture at the same temperature. |
| Relaxation |
The loss of tension, and therefore Clamping force, in a bolt and joint as a result of Embedment, vibration loosening, gasket creep, differential thermal expansion, etc. |
| Relaxation curve |
A plot of either the remaining, or relaxed, stress as a function of time. |
| Relaxation rate |
The absolute value of the slope of a stress-relaxation curve at a given time. |
| Relaxed stress |
The initial stress minus the remaining stress at a given time during a stress-relaxation test. |
| Remaining stress |
The stress remaining at a given time during a stress-relaxation test. See also stress relaxation. |
| Repeatability |
A term used to refer to the test-result variability associated with a limited set of specifically defined sources of variability within a single laboratory. |
| Reproducibility |
A term used to describe test-result variability associated with specifically defined components of variance obtained both from within a single laboratory and between laboratories. |
| Residual preload |
The tension which remains in an unloaded bolted joint after Relaxation. |
| Residual strength (Unit N) |
The strength of a damaged body containing defects induced by microcracking, thermal shock, etc. |
| Residual stress |
Stresses that remain within a body as the result of thermal or mechanical treatment or both. |
| Residual stress (Unit Pa) |
(internal stress) An internal-stress system found in components that have experienced elastic unloading from non-uniform plastic-strain fields during manufacture. |
| Resistance curve |
An increase in fracture toughness with crack propagation shown by rising plots of toughness vs growing crack length. |
| Resistance factor |
Probabilistic factor representing the uncertainties in the designer’s estimate of the strength of a shear joint. Used in Load and resistance factor design. |
| Resistance thermometer |
A thermometer in which the sensing element is basically a metal wire, usually of platinum, for which the variation of resistance with temperature is known accurately. A resistance pyrometer is a resistance thermometer intended for use up to about 1 000°C. |
| Retaining clip |
(R-clip, hair cotter pin) A wire clip roughly with the shape of the letter R. The straight part of the clip passes through a hole drilled into a shaft or rod such that a wheel is held in position on the shaft, or the shaft is prevented from moving axially. |
| Reverse engineering |
The disassembly of a machine, mechanism, system or device, measurement of its component parts, and identification of the materials used so that if required, a functioning replica can be produced. |
| Reverse pitch (Unit °) |
The pitch of a variable-pitch propeller that produces negative thrust. |
| Right hand thread |
A screw thread which advances into the mating part when turned clockwise or to the right. |
| Rivet |
A short rod with a head on one end that is inserted through aligned holes in plates to be joined, after which a second head is made on the protruding shank by hammering or forming. The most common head shapes are flat, domed, and inverse conical. In an array, the rivet pitch is the distance between the centres of adjacent rivets. Failure may occur by different mechanisms or modes. |
| Rivet |
A one piece fastener consisting of a head and a body and used for fastening two or more pieces together by passing the body through a hole in each piece and then forming a second head on the body end. It cannot be removed except by taking off the head. |
| Rivet |
A one piece fastener consisting of a head and a body and used for fastening two or more pieces together by passing the body through a hole in each piece and then forming a second head on the body end. It cannot be removed except by taking off the head. |
| Rockwell hardness number (HR) |
A number derived from the net increase in the depth of impression in a Rockwell hardness test as the load on an indenter is increased from a fixed minor load to a major load and then returned to the minor load. Rockwell hardness numbers are always quoted with a scale symbol representing the penetrator, load, and dial used. |
| Rockwell hardness test |
An indentation-hardness test using a calibrated machine that utilizes the depth of indentation, under constant load, as a measure of hardness. |
| Rockwell hardness test |
A direct-reading hardness test based on depth of indentation, in which a minor load (typically 30 to 100 N) is applied before the major load (typically 120 to 1 500 N). There are different scales using conical or spherical indenters for different ranges of hardness. |
| Rockwell hardness test |
An indentation hardness test based on the depth of penetration of a specified penetrator into the specimen under certain arbitrarily fixed conditions. |
| Rockwell superficial hardness test |
Same as Rockwell hardness test, except that smaller minor and major loads are used. |
| Rolled thread |
A thread formed by plastically deforming the surface of the blank rather than by cutting operations. Increases fatigue life and thread strength, but is not possible (or perhaps economical) on larger sizes. |
| Root diameter (Unit m) |
The diameter of the circle passing through the roots of teeth in gear teeth or screw threads. |
| Roots blower |
(rotary-piston blower) A positive-displacement gas compressor with two meshing lobed rotors that rotate within a close-fitting casing. Commonly used as a supercharger. |
| Rosette |
Strain gages arranged to indicate, at a single position, strain in three different directions. |
| Rotary blower |
(rotary compressor) A positive-displacement gas compressor which may be of the blade type, helical-screw type, meshing-lobe type (the Roots blower), or sliding-vane type. |
| Rotary piston flow meter |
A rotary liquid flow meter in which a cylindrical rotor, eccentrically mounted within a cylindrical casing, the axes of the two cylinders being parallel, is caused to rotate by the liquid flow. The flow rate is determined from the number of rotations of the rotor in a given time. |
| Rotary pump |
A positive-displacement pump that pumps a liquid by rotation of internal components, such as a gear pump, lobe pump (similar to a Roots blower), or progressivecavity pump. |
| Rotary valve (rotating valve) |
A cylindrical or conical plug in which there is a transverse hole through which fluid can flow when the hole is aligned with the adjacent piping. |
| rotational joint (revolute joint) |
In robotics, a single degree-of-freedom joint where the controlled variable is the joint angle. |
| Rotor |
A part of a machine that rotates on a shaft (rotor shaft) about its own axis, such as the blade-carrying discs of a turbine, the blades of a helicopter, or the rotating parts of a Roots blower. |
| Roughness |
The deviations from the wavy surface itself, caused by geometry of the cutting tool and its wear, machining conditions, microstructure of the workpiece, vibrations in the system, and so on. Surface roughness changes as a surface goes through the wearing-in process, but may then stabilize. |
| Rpm (N) |
An abbreviation for revolutions per minute. It is a widely used non-SI unit for rotational speed. The corresponding angular velocity ω in rad/s is given by πN/30. Rps (revolutions per second) is also used, the angular velocity then being 2πN. |
| Rubber hardness |
The indentation hardness of rubber-like materials in which the depth of indentation under load is measured. This is a measure of elastic moduli rather than yield stress as with ductile materials, since there is little, if any, permanent impression remaining on unloading rubbery materials. |
| Rupture stress |
The stress at failure. Also known as breaking stress. |
| Safe working load (Unit N) |
The steady or unsteady load against which a component or structure is designed for normal operation. It is lower than that which would cause failure by buckling, fracture, or yielding, so as to accommodate uncertainty, possible fault or accident conditions. |
| Safety valve (safety-relief valve) |
A mechanical valve, fitted by law to all pressure vessels (e.g. steam boilers), which opens to prevent the internal pressure exceeding the maximum design value for that vessel. The valve closes again once the pressure reduces to a safe level. |
| Salt bath heat treatment |
Heat treatment carried out in a bath of molten salt. |
| Salt-velocity meter |
A volume flow meter based on detecting the transit time for a small quantity of salt or radioactive isotope in a flow by measuring electrical conductivity or radiation level. |
| Sample average |
The sum of all the observed values in a sample divided by the sample size. It is a point estimate of the population mean. Also known as arithmetic mean. |
| Sample median |
The middle value when all observed values in a sample are arranged in order of magnitude. If an even number of samples are tested, the average of the two middlemost values is used. It is a point estimate of the population median, or 50% point. |
| Sample percentage |
The percentage of observed values between two stated values of the variable under consideration. It is a point estimate of the percentage of the population between the same two stated values. |
| Sample standard deviation (s) |
The square root of the sample variance. It is a point estimate of the population standard deviation, a measure of the “spread”of the frequency distribution of a population. This value of sprovides a statistic that is used in computing interval estimates and several test statistics. For small sample sizes, s underestimates the population standard deviation. |
| Sample variance (s2) |
The sum of the squares of the differences between each observed value and the sample average divided by the sample size minus one. It is a point estimate of the population variance. |
| Sampling |
The process of measuring a series of values of a time-varying quantity. |
| Sampling frequency (Unit Hz) |
(sampling rate) The number of measurement samples per unit time taken from a continuous analogue signal to produce a discrete signal. It is the inverse of the time between successive measurements (sampling interval, sampling period, or sampling time). |
| Sawing |
Sawing is the process of cutting a workpiece with power band saws, hacksaws, and circular saws. Each of these methods is used in cutoff operations (cutting pieces to a required length), although band sawing also provides a method of cutting contours. |
| Scale effect |
The change in behaviour with size of a material, component, or structure. For example, cube-square scaling where one element changes with volume and another with area, as in the range of a projectile or ship, droplet formation, and fracture mechanics. |
| Scale height (Unit m) |
The altitude H at which the atmospheric pressure has decreased to a certain fraction of its value at sea level B. It depends on the model adopted for the atmosphere: for an isothermal atmosphere, the pressure decreases exponentially with altitude. If H is taken as the altitude at which the pressure has fallen to B/e, then H = RT/g where R is the gas constant for air, T is the absolute temperature T, and g is the acceleration due to gravity. |
| Scatter |
Data points or calculations are said to be scattered when they are not all the same. A ‘‘lot of scatter in preload’’ means wide variation in the preloads found in individual bolts. |
| Scleroscope hardness number (HSc or HSd) |
A number related to the height of rebound of a diamond-tipped hammer dropped on the material being tested. It is measured on a scale determined by dividing into 100 units the average rebound of the hammer from a quenched(to maximum hardness) and untempered AISI W5 tool steel test block. |
| Scleroscope hardness test |
A dynamic indentation-hardness test using a calibrated instrument that drops a diamond-tipped hammer from a fixed height onto the surface of the material being tested. The height of rebound of the hammer is a measure of the hardness of the material. Also known as Shore hardness test. |
| Scoring |
In tribology, a severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding. |
| Scragging machine |
1. A machine to test springs by impulsive loading 2. A machine to increase the service life of coil springs by compressing them to their minimum solid length before use, thereby inducing favourable residual stresses. |
| Scratch hardness |
The resistance of a material, such as a metal, alloy, plastic, or mineral, to scratching by a much harder indenter, such as a diamond stylus, moved slowly across the surface. The scratch hardness number is computed from the loads and the dimensions of the residual scratch. |
| Scratch hardness test |
A form of hardness test in which a sharp-pointed stylus or corner of a mineral specimen is traversed along a surface so as to determine the resistance of that surface to cutting or abrasion. |
| Scratching |
In tribology, the mechanical removal or displacement, or both, of material from a surface by the action of abrasive particles or protuberances sliding across the surfaces. |
| Screw |
Threaded fastener designed to be used in a tapped or untapped (e.g., wood screw) hole, but not with a nut. |
| Screw |
A fastener with a screw thread cut or rolled into its cylindrical or conical shank, intended either to cut its own thread (as in a wood screw) or engage in a threaded hole. A self-tapping screw (sheet-metal screw, tapping screw) has a sufficiently hard thread that it cuts an internal thread in thin sheet or a component made of soft material when driven into a hole in the sheet or component. The screw head is the part of a screw used to apply torque to the screw. Common screw-head designs are circular in cross section with a diametral slot or cross, or hexagonal and recessed with a cross or hexagon. A screw thread is one or more continuous helical grooves of uniform section along either the exterior surface (male thread) or the interior surface (female thread) of a cylindrical or conical body. The three-dimensional shape that results when the thread cross section is rotated and axially advanced along an axis is called a helicoid, the angle that the thread makes when projected on to the axis being called the angle of inclination. Threads are employed in fasteners such as bolts, nuts, and screws; location and measuring instruments; in power drives; in some electrical fittings (Edison thread); and on the ends of crankshafts to suppress oil leakage (thrower thread). Parallel threads are formed on cylinders; tapered threads on cones, typically with a taper rate of 1:16. A screw with a righthanded thread appears to move away from the observer when turned clockwise. All standard screws, bolts and nuts have right-hand threads, but left-handed threads are sometimes employed. The axial distance between corresponding points on adjacent threads is called the screw pitch or screw rate and, for a single continuous helical groove (a single-start thread), is the same as the change in axial distance (the lead) between a nut and the head of a bolt during one revolution, the number of thread forms per mm then being the reciprocal of the pitch. For the same screw diameter, coarse threads have fewer threads per mm than fine threads. A multiple-start screw thread (usually coarse, see later) consists of two or more identical threads running simultaneously along its axis so as to provide greater bearing area and greater velocity ratio. The starts are separated by 180° (double start), 120° (triple start), 90° (quadruple start) etc., depending upon the number of threads. Thus, in double-start threads, the lead is twice the pitch; and so on. The axial distance between corresponding points on two adjacent threads in a multiple start thread is called the divided pitch. The position on a screw thread where there is equal distance between the flanks on the solid part of the thread and in the space between the threads is the pitch point, the associated diameter of which is termed the pitch diameter. The basic nomenclature for threads, some of which is common with that for toothed gearing, is shown in the diagrams. |
| Screw |
A helix formed or cut on a cylindrical surface which may advance along the axis to the right or left. The helix may be single or multiple. |
| Screw area (Unit m2) |
The area of a circle described by the tips of a propeller. |
| Screw compressor |
A positive-displacement rotary compressor in which gas is progressively compressed by two intermeshing, counter-rotating, helical screws. |
| Screw conveyor |
(auger, spiral conveyor, worm conveyor) A machine for bulk handling of semi-solid materials, consisting of a helical screw which rotates in a trough or casing. |
| Screw displacement |
A rotation of a rigid body about an axis accompanied by a translation of the body along the same axis. |
| Screw extractor |
A device for removing broken-off screws from threaded holes. It resembles a drill with, for broken right-hand threads, a fast (long-pitch) left-hand thread that is driven into a hole drilled in the broken screw, thus untwisting it. |
| Screw feeder |
A mechanism for handling bulk materials in which a rotating helicoidal screw moves the material axially forward. Similar to a screw conveyor, but required to discharge material at a controlled rate very accurately. It operates with the screw completely full. |
| Screw jack |
A lifting device (jack) consisting of a nut and square-threaded shaft at the top of which is a load-bearing pad. Rotation of the nut raises or lowers the pad. |
| Screw machine |
A machine tool for high-volume manufacture of small turned components from rod or bar. |
| Screw pair |
Two links connected together to form a kinematic pair, in which the contacting surfaces are screw threads, so that their relative motion consists of rotation and sliding. |
| Screw pump |
A positive-displacement pump that uses one or more helical rotors rotating within a casing to transfer liquids or slurries. |
| Screw thread |
A ridge of uniform section or shape in the form of a helix on the external or internal surface of a cylinder, or in the form of a conical spiral on the external or internal surface of a cone. |
| Scuffing |
A form of adhesive wear that produces superficial scratches or a high polish on the rubbing surfaces. It is observed most often on inadequately lubricated parts. |
| Seal |
A component which controls or prevents leakage of fluids into or out of parts of a machine. |
| Secant modulus |
The slope of the secant drawn from the origin to any specified point on a stress-strain curve. Compare with chord modulus. |
| Second (s) |
The SI base unit of time equal to 9 192 631 770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. |
| Secondary hardening |
The process whereby some low-alloy steels, quenched to produce martensite, produce fine precipitates when tempered above 550°C, which inhibit dislocation motion and reverse the trend towards lower strength at higher tempering temperatures. |
| Secondary stresses (Unit Pa) |
Stresses different from those induced by the major loading but nevertheless resulting from the major loads; for example, the hoop stresses that occur around the circumference of barrelled compression testpieces and forgings. |
| Selective heating |
Intentionally heating only certain portions of a workpiece. |
| Selective quenching |
Quenching only certain portions of an object. |
| Self-locking nut |
A nut with an inherent locking action which minimizes loosening due to vibration. A self-locking screw locks itself in place without the need for a separate selflocking nut or lock washer. |
| Self-loosening |
The process by which a supposedly tightened fastener becomes loose, as a result of vibration, thermal cycles, shock, or anything else which cause transverse slip between joint members and between male and female threads. Vibration loosening is a common, but special, case of self-loosening. |
| Semiguided bend |
The bend obtained by applying a force directly to the specimen in the portion that is to be bent. The specimen is either held at one end and forced around a pin or rounded edge, or is supported near the ends and bent by a force applied on the side of the specimen opposite the supports and midway between them. In some instances, the bend is started in this manner and finished in the manner of a free bend. |
| Semi-rotary pump |
A form of self-priming pump, often hand-operated, suitable for pumping water and light oils such as diesel oil and petrol. As shown in the diagram, liquid is sucked into one side of the pump through flap valves and simultaneously ejected from the other side on one stroke. The sequence is reversed on each successive stroke. |
| Serrations |
A row of notches or tooth-like projections on an edge or surface. On an engineering drawing, serrations on the surface of a circular component are shown over about 60° of arc. |
| Set pressure (Unit Pa) |
The pressure at which a relief valve or safety valve is set to open, corresponding to the relevant code or standard which applies to the pressure vessel being protected. |
| Set screw |
Usually a hardened steel screw having either no head or a square head and with various degrees of points or ends to lock or tighten adjustable machine parts in position on a shaft. |
| Set screw (grub screw, set bolt) |
A short, headless screw with a recess at one end to receive a screwdriver or key, the other end being pointed, square, or otherwise shaped. Typically used to secure a pulley, gear, or other component on a shaft. |
| Shank |
Portion of a bolt which lies under the head. |
| Shank |
1. The stem of a tool, such as a broach, drill bit, reamer, or tap, which fits into a holder such as a chuck. 2. The stem of a rivet or the unthreaded part of a screw or bolt. 3. The shaft of a tool connecting the tip and the handle. |
| Sharp-notch strength |
The notch tensile strength measured using specimens with very small notch root radii (approaching the limit for machining capability); values of sharp-notch strength usually depend on notch root radius. |
| Shear |
The type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact. |
| Shear fracture |
A ductile fracture in which a crystal (or a polycrystalline mass)has separated by sliding or tearing under the action of shear stresses. Contrast with cleavage fracture. |
| Shear joint |
A joint which is subjected primarily to loads acting more or less perpendicular to the axes of the bolts. |
| Shear lip |
A narrow, slanting ridge along the edge of a fracture surface. The term sometimes also denotes a narrow, often crescent-shaped, fibrous region at the edge of a fracture that is otherwise of the cleavage type, even though this fibrous region is in the same plane as the rest of the fracture surface. |
| Shear modulus (G) |
The ratio of shear stress to the corresponding shear strain for shear stresses below the proportional limit of the material. Values of shear modulus are usually determined by torsion testing. Also known as modulus of rigidity. |
| Shear strain |
The tangent of the angular change, as a response to force, between two lines originally perpendicular to each other through a point in a body. Also known as shearing strain. |
| Shear strain rate(Unit 1/s) |
In the deformation of solids, the rate at which shear strain is applied in a test or changes with time in a loaded component or structure. |
| Shear strength |
The maximum shear stress that a material is capable of sustaining. Shear strength is calculated from the maximum load during a shear or torsion test and is based on the original dimensions of the cross section of the specimen. |
| Shear strength (Unit Pa) |
The maximum shear stress that can be withstood by a material either (a) before plasticity occurs or (b) before rupture. |
| Shear stress |
(1) A stress that exists when parallel planes in metal crystals slide across each other. (2) The stress component tangential to the plane on which the forces act. Also known as tangential stress. |
| Shear stress (Unit Pa) |
(shearing stress, tangential stress, τ) 1. The stress which acts parallel to any plane within a solid material. It can arise due to a bending moment, a shear force, or torque applied to the body. 2. The stress corresponding to velocity gradients within a flowing viscous fluid. |
| Shear test |
Any of several tests to obtain shear strength of a metal. Common tests used on mill products include the double-shear test, single-shear test, the blanking shear test (also known as the punching shear test) and the torsion test. |
| Shell hardening |
A surface-hardening process in which a suitable steel workpiece, when heated through and quench hardened, develops a martensitic layer or shell that closely follows the contour of the piece and surrounds a core of essentially pearlitic transformation product. This result is accomplished by a proper balance among section size, steel hardenability, and severity of quench. |
| Shim |
A thin piece of material, such as metal of accurately-known thickness, placed between two surfaces to ensure they are the correct distance apart. |
| Shim |
A thin piece of material placed between two surfaces to obtain a proper fit, adjustment, or alignment. The piece can also be analyzed to measure furnace carbon potential (that is, because while in the furnace it will quickly carburize to a level equal to the furnace carbon potential). |
| Shock load |
The sudden application of an external force that results in a very rapid build-up of stress. |
| Shore hardness |
A measure of the hardness of materials such as elastomers, plastics, and rubbers determined using a Shore durometer which measures the penetration depth of a hardened steel rod (the indenter) 1.1–1.4 mm in diameter into the test material for a given force applied rapidly with a calibrated spring. There are two main indenter designs: type A (for softer materials) for which the end of the rod is a truncated 35° cone tapering to a diameter of 0.79 mm, and type D (harder materials) for which the end is a 30° cone with a 0.1 mm radius tip. The spring force is 8.05 N for type A and 44.45 N for type D. The hardness value is related to the penetration depth (limited to a maximum of 2.5 mm) such that for zero penetration the hardness is 100 and for maximum penetration the hardness is zero. The relevant standards are ISO 48-1, 48-4, and 48-7. Shore hardness values of selected materials are as follows: ABS 100, cellulose acetate 50–95, high-density polyethylene 60–70, low density polyethylene 40–50, polymethylmethacrylate 90–99, PTFE 50–65, and plasticized PVC 15–70. |
| Shoulder |
The portion of a shaft, stepped component, or flanged component, where a change in diameter or other dimension occurs. Even for a nominal step change, in practice there will be a radius where shoulder and shaft meet in order to avoid too severe a stress concentration. |
| Shoulder bolt |
A bolt for which the unthreaded cylindrical section between the thread and the head is of larger diameter than the threaded section, precisely machined to length and diameter and hardened |
| Shoulder joint |
The second joint on an articulated robot, which has a horizontal axis and is analogous to the human shoulder. |
| Shoulder screw |
A screw having two or more diameters or shoulders and commonly used for supporting levers and other machine parts that have to operate freely. |
| Shrink fit |
A tight interference fit between two components resulting from heating an outer part and/or cooling an inner part to allow easy assembly. The outer component contracts on cooling, while the inner part expands on warming to ambient temperature, thus gripping the two parts together. |
| Shrink ring |
A ring, which is expanded by heating, is placed around an assembly of parts, and then contracts upon cooling to hold the assembly in place. |
| SI system |
SI is an abbreviation of Système International d’Unités, i.e., the international unit measurement system – an internationally established system of measurement units built on older metric systems. |
| Sigma phase |
A hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group P42/mnm, occurring in many binary and ternary alloys of the transition elements. The composition of this phase in the various systems is not the same, and the phase usually exhibits a wide range in homogeneity. Alloying with a third transition element usually enlarges the field of homogeneity and extends it deep into the ternary section. |
| Sigma-phase embrittlement |
Embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 565 and 980 °C (1050 and 1800 °F). Sigmaphase embrittlement results in severe loss in toughness and ductility and can make the embrittled material structure susceptible to intergranular corrosion. |
| Significance level |
The stated probability (risk) that a given test of significance will reject the hypothesis that a specified effect is absent when the hypothesis is true. |
| Silky fracture |
A metal fracture in which the broken metal surface has a fine texture, usually dull in appearance. Characteristic of tough and strong metals. |
| Single thread |
A screw thread cut around a cylinder having a single start in which the lead is equal to the pitch. |
| Single-shear test |
(1) A shear test similar to the double-shear test used for round-bar specimens, but that uses only one stationary shear blade. (2) A shear test that uses a sheet or thin-plate specimen. |
| Sinter (sintering) |
A solid-state diffusion densification process for the production of objects, particularly porous objects, from raw material in powder, granular, mesh, perforatedsheet, or fibre form by heating to a temperature below the melting point until the constituents bond together. It is used for ceramic materials and metals. Applications of sintered materials include filtration, separation, flow control, fluidization, and noise reduction. |
| Sintering |
The bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. Sintering strengthens a powder mass and normally produces densification and, in powdered metals, recrystallization. |
| Size dimension (Unit m) |
The specified value of a diameter, length, width, etc. of a feature required to specify the finished form of a component or assembly. |
| Size effect |
Effect of the dimensions of a piece of metal on its mechanical and other properties and on manufacturing variables such as forging reduction and heat treatment. In general, the mechanical properties are lower for a larger size. |
| Sizing |
1. A finishing operation to ensure the specified dimensions and tolerances for a component are met. 2. (size classification) Separating an aggregate of mixed particles into groups according to size using a series of screens. |
| Slack quenching |
The incomplete hardening of steel due to quenching from the austenitizing temperature at a rate slower than the critical cooling rate for the particular steel, resulting in the formation of one or more transformation products in addition to martensite. |
| Slant fracture |
A type of fracture appearance, typical of plane-stress fractures, in which the plane of metal separation is inclined at an angle (usually about 45°) to the axis of the applied stress. |
| Slenderness ratio |
The effective unsupported length of a uniform column divided by the least radius of gyration of the cross-sectional area. |
| Sliding-vane compressor |
(rotary-vane compressor, vane compressor) A rotary compressor in which gas is compressed as the spaces between spring-loaded sliding vanes held in an offset rotor reduce as the rotor revolves within a cylindrical housing. |
| Slip |
Plastic deformation by the irreversible shear displacement (translation)of one part of a crystal relative to another in a definite crystallographic direction and usually on a specific crystallographic plane. Sometimes called glide. |
| Slip band |
A group of parallel slip lines so closely spaced as to appear as a single line when observed under an optical microscope. |
| Slip joint |
A mechanical connection which allows limited endwise relative movement of two components such as pipes, rods, and ducts. |
| Slip line |
The trace of the slip plane on a viewing surface; the trace is usually observable only if the surface has been polished before deformation. The usual observation on metal crystals (under the light microscope) is of a cluster of slip lines known as a slip band. |
| Slot |
A groove machined into a component, e.g. to allow for thermal expansion, or in which the tongue or tip of another component can fit or slide. |
| Slug |
1. A starting workpiece for forging and similar operations, such as a length of wire or rod to make a bolt blank on which a thread can be rolled or cut. 2. (geepound) An obsolete imperial (non-SI) unit of mass, being the mass which under an acceleration of one foot per second squared gives a force of one pound-force. The conversion to SI is 1 slug = 14.593 902 94 kg. 3. A large-scale flow structure which occurs in the transition from laminar to turbulent flow in a pipe. |
| Slug wrench |
A box wrench with an anvil on the end of the handle. Torque is produced by striking the anvil with a sledge hammer. |
| Snap temper |
A precautionary interim stress-relieving treatment applied to high-hardenability steels immediately after quenching to prevent cracking because of delay in tempering them at the prescribed higher temperature. |
| Socket head |
Screw head having a hexagonal or other form of recessed socket in the head so that the screw can be turned with a wrench or key, as a hexagon key. |
| Socket wrench |
A form of spanner with an internally ridged steel socket to fit a nut or the head of a bolt or screw. |
| Solenoid |
A coil of electrically-conducting wire wrapped around a metal core, typically iron, to produce a magnetic field and hence a force on the core when an electric current passes through the coil. Solenoids are widely used to produce linear movement to actuate valves (solenoid valves) and other devices. |
| Solution heat treatment |
Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution. |
| Sound pressure (Unit μPa) |
The instantaneous deviation of pressure from the ambient pressure caused by the passage of a sound wave, which can be measured by a microphone in air and a hydrophone in water. |
| Spalling |
The separation of macroscopic particles from a surface in the form of flakes or chips. |
| Span (Unit m) |
A dimension measured between the extremities of a body or structure, such as between the tips of a wing or the supports of a bridge. |
| Spanner (wrench) |
A tool for applying torque to tighten or loosen a nut, bolt, or screw. Grip is applied by a serrated ring or an open U-shape at the end of the tool. On imperial-size spanners, the BSW and BSF marking refers to the diameter of the screw thread; similarly for BA spanners. Spanners for unified and metric threads are marked with the distance across the flats (‘A/F’) of the nut or head of the bolt. |
| Specific heat capacity |
The heat necessary to raise the temperature of 1 kg of a substance by 1 K. It is measured in J/(kg K). |
| Specific weight (γ) (Unit N/m3) |
The weight per unit volume of a substance: if W is the weight of a volume of the substance, ρ is its density, and g is the acceleration due to gravity, γ = W/ = ρg. |
| Specimen |
A test object, often of standard dimensions or configuration, that is used for destructive or nondestructive testing. One or more specimens may be cut from each unit of a sample. |
| Speed of rotation (N) |
(Unit rps or rpm) For an object rotating about a fixed axis, the speed of rotation is the number of turns (revolutions) of the object per unit time. It is equal to the angular velocity Ω (or ω) (unit rad/s) divided by 2π. |
| Spherical washer |
A washer whose upper surface is semispherical. Used with a nut whose contact face is also semispherical. Reduces bending stress in a bolt or stud, by allowing some self-alignment and some compensation for nonparallel joint surfaces or Angularity. |
| Spinner |
A co-axial streamlined fairing enclosing the hub of a propeller and rotating with it. |
| Spray quenching |
A quenching process using spray nozzles to spray water or other liquids on a part. The quench rate is controlled by the velocity and volume of liquid per unit area per unit of time of impingement. |
| Spring |
An elastic component which stores mechanical energy and exerts a force when deformed. The slope of the curve of applied force F to the deflexion of a spring x, dF/dx, is termed the spring rate or spring modulus k with unit N/m. For a linear spring, the F(x) curve is proportional and k is called the spring constant. |
| Spring |
An elastic device which yields under stress or pressure but returns to its original state or position when the stress or pressure is removed. |
| Spring clip |
A clip made of a material such as spring steel which grips an inserted part. |
| Spring constant |
The ratio between the forces exerted on a spring (or a bolt) and the deflection thereof. Has the dimensions of force per unit change in length (e.g., lb=in.). Also called Stiffness. |
| Spring load (Unit N) |
A load exerted on a component by bending, compressing, extending, or twisting an attached spring. Mechanical energy stored in the spring can be released if any constraints on the component are removed. |
| Spring materials |
Mainly metals including plain-carbon and corrosion-resisting steels, phosphor bronze, spring brass, beryllium copper and nickel alloys, all of which in their workhardened states permit large reversible elastic strains without permanent deformation. |
| Spring temper |
A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength and hardness about two-thirds of the way from full hard to extra spring temper. |
| Springback |
The extent to which metal tends to return to its original shape or contour after undergoing a forming operation. |
| Square |
1. A regular quadrilateral, i.e. a polygon with four equal sides and four right angles. 2. An instrument to check angles of internal and external surfaces and flatness. |
| Square thread |
A form of screw thread in which the cross–section of the thread forms a square, making the width of the thread equal to the space between the threads. |
| Stability |
1. A term relating to whether a body, structure, system, or flow is in a state of stable, unstable, or neutral equilibrium. 2. In numerical analysis, whether numerical errors, such as roundoff or input-data errors, are damped, propagate, or cause divergence of the solution as the numerical method proceeds. 3. A control system is often described as stable when a step change in input results in a steady output without continuing oscillation. |
| Stabilizing treatment |
(1) Before finishing to final dimensions, repeatedly heating a ferrous or nonferrous part to or slightly above its normal operating temperature and then cooling to room temperature to ensure dimensional stability in service. (2) Transforming retained austenite in quenched hardenable steels, usually by cold treatment. (3) Heating a solution-treated stabilized grade of austenitic stainless steel to 870 to 900 °C (1600 to 1650 °F) to precipitate all carbon as TiC, NbC, or TaC so that sensitization is avoided on subsequent exposure to elevated temperature. |
| Stainless steels |
Iron-base alloys that are highly resistant to corrosion in many environments. The predominant alloying element is chromium, which, like iron at room temperature, has BCC crystal structure. Corrosion resistance by passivation is achieved by the formation of chromium oxide on the steel surface. Nickel, which has FCC crystal structure, is another important addition for some steels. Other additions used to enhance properties include manganese, molybdenum, niobium, and titanium. There are several classes of stainless steel. |
| Standard |
(standard specification) A set of specifications for components, machines, materials, or processes intended to achieve uniformity, efficiency, and a specified quality. In the UK, standards issued by the British Standards Institution (BSI) are now generally those of the International Organization for Standardization (ISO). |
| Standard deviation |
A statistical term used to quantify the Scatter in a set of data points. If the standard deviation is small, most of the data points are ‘‘nearly equal.’’ A large deviation means less agreement. |
| Standard deviation |
The most usual measure of the dispersion of observed values or results expressed as the positive square root of the variance. |
| Standard deviation (σ) |
The square root of variance, a measure of the spread of data about the mean value. |
| Standard fit |
The fit of a component machined or otherwise manufactured to standardized clearances and tolerances. |
| Standard gauge |
A highly-accurate reference gauge against which to check working gauges. |
| Standard hole |
A hole in a workpiece bored to a specified tolerance where clearance with a shaft is accomplished by allowance on the shaft. A standard shaft is machined to a specified tolerance where clearance with a hole is accomplished by allowance on the hole. |
| Standardization |
1. National and international agreements for design, manufacture, materials, performance, practices, requirements, strength, etc. which ensure common results wherever an item is made and wherever used. 2. The manufacture of components so that interchangeability of parts during assembly of new, or repair of old, items is possible without ‘fitting’. |
| Starting friction |
(limiting friction, static friction, stiction) (Unit N) The force required to initiate movement between two bodies in contact. |
| Starting taper |
The taper on the end of a reamer or tap which aids in starting the cut. |
| Starting torque (Unit N.m) |
The torque developed by a motor at zero rotational speed in order to initiate rotation of the applied load. |
| Starting valve (regulator) |
A valve which admits steam from the boiler to the cylinder(s) of a steam engine. |
| Static equilibrium |
A solid body, structure, or physical system (or any subdivided part) will be in static equilibrium, either at rest or moving with constant velocity, if the resultants of all external forces and moments acting on it are zero. |
| Static fatigue |
A term referring to a time-dependent reduction in strength with a static (noncyclic) load. The term may refer to hydrogen-induced delayed cracking or the effect of creep on the strength of plastics. |
| static fatigue |
Delayed fracture caused by environmental effects under dead-weight loading of materials such as glass. The name is unfortunate, as cyclic loads are not involved and glass is not susceptible to fatigue. |
| Statistic |
A summary value calculated from the observed values in a sample. |
| Stator |
A ring of non-rotating blades or nozzles in a compressor, turbine, or other turbomachine, which directs fluid flow into an adjacent rotor. |
| Stead’s brittleness |
A condition of brittleness that causes transcrystalline fracture in the coarse grain structure that results from prolonged annealing of thin sheets of low-carbon steel previously rolled at a temperature below about 705 °C (1300 °F). The fracture usually occurs at about 45° to the direction of rolling. |
| Steady loads |
Loads that do not change in intensity or that change so slowly they may be regarded as steady. |
| Steady pin |
A dowel, key, or pin that prevents a pulley from turning on its shaft. |
| Steam gauge |
A pressure gauge used to measure gauge pressure in a line, boiler, cylinder, or other device operating with steam. |
| Steel-wire rope |
A rope or cable made up of many steel strands wrapped helically about an axis, each strand being made of metal wires twisted together like a helix. |
| Stiffness |
(1) The ability of a metal or shape to resist elastic deflection.(2) The rate of stress increase with respect to the rate of increase in strain induced in the metal or shape; the greater the stress required to produce a given strain, the stiffer the material is said to be. |
| Stove bolt |
This bolt has been so named because of its use in stove building. It is made in a number of different forms, either with a round button, or flat countersunk head, the head having a slot for a screwdriver and the threaded end being provided with a square or hexagon nut. |
| Strain |
The unit of change in the size or shape of a body due to force. Also known as nominal strain. |
| Strain aging |
The changes in ductility, hardness, yield point, and tensile strength that occur when a metal or alloy that has been cold worked is stored for some time. In steel, strain aging is characterized by a loss of ductility and a corresponding increase in hardness, yield point, and tensile strength. |
| Strain energy |
A measure of the energy absorption characteristics of a material determined by measuring the area under the stress-strain diagram. |
| Strain gage |
A device for measuring small amounts of strain produced during tensile and similar tests on metal. A coil of fine wire is mounted on a piece of paper, plastic, or similar carrier matrix (backing material), which is rectangular in shape and usually about 25 mm (1.0 in.) long. This is glued to a portion of metal under test. As the coil extends with the specimen, its electrical resistance increases in direct proportion. This is known as bonded resistance-strain gage. Other types of gages measure the actual deformation. Mechanical, optical, or electronic devices are sometimes used to magnify the strain for easier reading. |
| Strain hardening |
An increase in hardness and strengthcaused by plastic deformation at temperatures below the recrystallization range. Also known as work hardening. |
| Strain hardening |
(workhardening) The increase in stress above the initial yield stress required to deform a metal to greater strains in the plastic range at temperatures below the recrystallization range (cold working). Unloading after some strain or work input gives a harder and stronger material. |
| Strain rate |
The time rate of straining for the usual tensile test. Strain as measured directly on the specimen gage length is used for determining strain rate. Because strain is dimensionless, the units of strain rate are reciprocal time. |
| strain-age embrittlement |
A loss in ductility accompanied by an increase in hardness and strength that occurs when low-carbon steel (especially rimmed or capped steel) is aged following plastic deformation. The degree of embrittlement is a function of aging time and temperature, occurring in a matter of minutes at about 200°C (400 °F), but requiring a few hours to a year at room temperature. |
| Strain-hardening exponent (n value) |
The value n in the relationship σ = Kεn, where σ is the true stress, ε is the true strain, and K, the strength coefficient, is equal to the true stress at a true strain of 1.0. The strain hardening exponent is equal to the slope of the true stress/true strain curve up to maximum load, when plotted on log-log coordinates. The n-value relates to the ability of a sheet of material to be stretched in metalworking operations. The higher the n-value, the better the formability(stretchability). Also known as the strain-hardening coefficient. |
| Strength (Unit Pa) |
The maximum stress, in tension, compression, shear, or combinations thereof, that may be monotonically applied to a material, component, or structure before failure (defined as fracture, yielding, buckling, etc. as appropriate). |
| Strength of bolt |
An ambiguous term which can mean Ultimate strength or Proof load or Endurance limit or Yield strength. |
| Strength of materials |
A confusing name for what is really stress analysis, reflecting older engineering design based upon elasticity and ‘theories of strength’ for different materials, without regard to cracks. |
| Stress |
The intensity of the internally distributed forces or components of forces that resist a change in the volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis of the original dimensions of the cross section of the specimen. Stress can be either direct (tension or compression)or shear. |
| Stress (Unit Pa) |
1. Stress ‘at a point’ (i.e. over a volume of material that is very small compared with that of the component or structure) is the load per unit area for every face of an infinitesimal cube surrounding the point. 2. In fluid flow the stress tensor σij is the sum of an isotropic part −pδij, δij being the Kronecker delta, having the same form as the stress tensor for a fluid at rest, p being the static pressure, and the non-isotropic, deviatoric stress tensor dij which is due entirely to the fluid motion. |
| Stress amplitude |
One-half the algebraic difference between the maximum and minimum stress in one cycle of a repetitively varying stress. |
| Stress area |
The effective cross-sectional area of the threaded section of a fastener. Used to compute average stress levels in that section. Based on the mean of pitch and minor diameters. |
| Stress corrosion |
(stress-corrosion cracking) Corrosion failure accelerated by local stresses (particularly tensile) which enhance cracking in both monotonic and fatigue loading. |
| Stress corrosion cracking (SCC) |
A common form of Stress cracking in which an Electrolyte encourages the growth of a crack in a highly stressed bolt. Only a tiny quantity of electrolyte need be present, at the tip or face of the crack. |
| Stress cracking |
A family of failure modes, each of which involves high stress and chemical action. The family includes Hydrogen embrittlement, Stress corrosion cracking, stress embrittlement, and hydrogen-assisted stress corrosion. |
| Stress cycle |
The smallest segment of the stress-time function that is repeated periodically. |
| Stress cycles endured (N) |
The number of cycles of a specified character (that produce fluctuating stress and strain) that a specimen has endured at any time in its stress history. |
| Stress difference (Unit Pa) |
The algebraic difference between the largest and least principal stresses in a loaded body, equal to twice the greatest shear stress in the system. For σ1 > σ2 > σ3, it is given by (σ1 − σ3). |
| Stress distribution |
The manner in which tensile, compressive, and shear stresses are distributed within a loaded body, indicated by loci of constant stress. |
| Stress equalizing |
A low-temperature heat treatment used to balance stresses in cold-worked material without an appreciable decrease in the mechanical strength produced by cold working. |
| Stress factor |
A calibration constant used in ultrasonic measurement of bolt stress or strain. It is the ratio between the change in ultrasonic transit time caused by the change in length of the fastener, under load, to the total change in transit time. |
| Stress raisers |
Changes in contour or discontinuities in structure that cause local increases in stress. |
| Stress ratio (A or R) |
The algebraic ratio of two specified stress values in a stress cycle. Two commonly used stress ratios are the ratio of the alternating stress amplitude to the mean stress, A = Sa/Sm, and the ratio of the minimum stress to the maximum stress, R = Smin/Smax. |
| Stress relaxation |
The slow decrease in stress level within a part (e.g., a bolt) which is heavily loaded under constant deflection conditions. A ‘‘cousin’’ to creep, which is a slow change in geometry under constant stress conditions. |
| Stress relaxation |
The time-dependent decrease in stress in a solid under constant strain at constant temperature due to creep. The stress-relaxation behavior of a metal is usually shown in a stress-relaxation curve. |
| Stress relieving |
Heating to a suitable temperature, holding long enough to reduce residual stresses, and then cooling slowly enough to minimize the development of new residual stresses. |
| Stress wave |
An unbalanced force, such as an impact, applied at one location in a body sets the material there into vibration that is transmitted to adjacent elements and ultimately to all parts of the body in the form of stress waves or wave packets. |
| Stress-concentration factor (Kt) |
A multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole; Kt equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section. Also known as theoretical stress-concentration factor. |
| Stress-corrosion cracking (SCC) |
A time-dependent process in which a metallurgically susceptible material fractures prematurely under conditions of simultaneous corrosion and sustained loading at lower stress levels than would be required in the absence of a corrosive environment. Tensile stress is required at the metal surface and may be a residual stress resulting from heat treatment or fabrication of the metal or the result of external loading. Cracking may be intergranular or transgranular, depending on the combination of alloy and environment. |
| Stress-intensity calibration |
A mathematical expression, based on empirical or analytical results, that relates the stress-intensity factor to load and crack length for a specific specimen planar geometry. Also known as Kcalibration. |
| Stress-intensity factor |
A scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing a crack, the factor is called the critical stress-intensity factor, or the fracture toughness. |
| Stress-relaxation curve |
A plot of the remaining, or relaxed, stress in a stress-relaxation test as a function of time. The relaxed stress equals the initial stress minus the remaining stress. Also known as a stress-time curve. |
| Stress–strain curve |
(stress–strain diagram) A plot of stress vs strain for a solid material obtained from a test in which a specimen is loaded in tension, compression, shear, or combinations thereof. Diagrams of true stress vs true strain deviate from those of nominal stress vs nominal strain only at large strains where appreciable changes in cross section of testpieces occur. |
| Stress-strain diagram |
A graph in which corresponding values of stress and strain are plotted against each other. Values of stress are usually plotted vertically(ordinates or y-axis) and values of strain horizontally (abscissas or xaxis). Also known as deformation curve and stress-strain curve. |
| Stretch forming |
A manufacturing process in which sheet-metal components are physically stretched over a forming die using a press, punch, or edge clamps. Stretching combined with bending reduces springback. |
| Stretch ratio (extension ratio, λ) |
A measure, used for highly-extensible materials such as rubber, of the extensional or normal strain of a component subjected to tension and defined as the ratio of the final length l, for a given load, to the initial or gauge length l0, where the coordinate axes are those of principal strain. It is related to the engineering strain e by λ = 1 + e. |
| Stretch-bending test |
A simulative test for sheet metal formability in which a strip of sheet metal is clamped at its ends in lock beads and deformed in the center by a punch. Test conditions are chosen so that fracture occurs in the region of punch contact. |
| Striation |
A fatigue fracture feature, often observed in electron micrographs, that indicates the position of the crack front after each succeeding cycle of stress. The distance between striations indicates the advance of the crack front across that crystal during one stress cycle, and a line normal to the striation indicates the direction of local crack propagation. |
| Stroke |
1. (stroke length) (Unit m) The linear distance between top dead centre and bottom dead centre of a piston in a reciprocating engine or mechanism. 2. The movement of a piston or plunger in a reciprocating machine to execute a particular function; for example, the exhaust stroke of an engine in which the exhaust gases are expelled from a cylinder. |
| Structural analysis |
The determination of the forces, displacements, stresses, and strains in a given structure. |
| Structural engineering |
A branch of civil engineering concerned with the design of buildings, dams, bridges, and other large structures. |
| Structural frame |
The underlying load-bearing framework of beams, columns, bracing, etc., of steel, concrete, brick, stone, timber etc., required to support the static and dynamic forces a building or component is subjected to, including its weight and that of its contents. |
| Structural-integrity analysis |
Assessment of components and structures for likelihood of failure by buckling, fracture, yielding, etc. under conditions of normal and accident loading. |
| Structure |
An assembly, usually load bearing, of connected (pinned, riveted, screwed, bolted, welded, etc.) components, such as bars, tubes, beams, and plates. |
| Stud |
A headless threaded fastener, threaded on both ends, with an unthreaded body in the middle section, or threaded from end to end. Used with two nuts, or with one nut and a tapped hole. |
| Stud |
A rod having thread on both ends. |
| Stud |
A plain cylindrical piece having a threaded portion of suitable length at each end or a continuous thread over the entire length. One end is screwed into a machine or workpiece after which a second part is placed over the stud and held in place by a nut. |
| Subcritical annealing |
A process anneal performed on ferrous alloys at a temperature below Ac1. |
| Submerged-electrode furnace |
A furnace used for liquid carburizing of parts by heating molten salt baths with the use of electrodes submerged in the ceramic lining. |
| Supercharging |
The process of increasing the mass flow rate of air (or air/fuel mixture) into the cylinder(s) of a piston engine using a compressor driven from the crankshaft (the supercharger). The power output is increased compared with a naturally-aspirated engine of the same capacity. The process also increases the air pressure and density to greater than ambient. In the past, most superchargers were mechanically driven from the engine’s crankshaft, but these have been largely superseded by turbochargers. |
| Supercooling |
Cooling below the temperature at which an equilibrium phase transformation can take place, without actually obtaining the transformation. |
| Superheating |
Heating above the temperature at which an equilibrium phase transformation should occur without actually obtaining the transformation. |
| Surface hardening |
A generic term covering several processes applicable to a suitable ferrous alloy that produces, by quench hardening only, a surface layer that is harder or more wear resistant than the core. There is no significant alteration of the chemical composition of the surface layer. The processes commonly used are carbonitriding, carburizing, induction hardening, flame hardening, nitriding, and nitrocarburizing. Use of the applicable specific process name is preferred. |
| Surface hardness |
(superficial hardness) Indentation hardness on, and for some distance below, the surface of a component, as opposed to its bulk hardness. Depending on the resolution required, micro- or nano-hardness testing machines are employed. |
| Surface roughness (roughness) |
The small-scale, irregular peaks and troughs in a solid surface which are quantified using a surface analyser. Roughness may be a consequence of wear and corrosion, or of the manufacturing process. It leads to increased friction in solid– solid contact or fluid flow over a surface. Errors of form and waviness are excluded, but surface texture includes roughness and waviness. Surface topography incorporates the small-scale, three-dimensional geometry of a surface including surface roughness, machined, and etched features, typically at sub-mm scales. |
| Surface tension (σ, γ) (Unit N/m) |
At the interface between a liquid and a gas or two immiscible liquids, unbalanced cohesive forces acting on the liquid molecules at the interface lead to the property surface tension which causes tensile forces to develop as if it were a skin or membrane. |
| Surface treatment |
Any process, including chemical, electrochemical, mechanical, and thermal, designed to protect a surface against corrosion and wear or to alter its mechanical properties. |
| Swept volume (Unit m3) |
In the cylinder of a piston engine or other device, the volume between top and bottom dead centre positions of a piston; equal to πR2S where R is the piston radius and S is the stroke. |
| Swift cup test |
A simulative cupping test in which circular blanks of various diameters are clamped in a die ring and deep drawn into cups by a flat-bottomed cylindrical punch. |
| System engineering |
A methodology which integrates all disciplines and specialty groups into a team effort, forming a structured development process that proceeds from concept to production to operation. |
| Tab washer (lock washer) |
A washer with one or more protruding tabs which can be bent in such a way that a nut or bolt head is prevented from becoming loose. |
| Tangent modulus |
The slope of the stress-strain curve at any specified stress or strain. See also modulus of elasticity. |
| Tangent screw |
A worm screw used to adjust measuring instruments such as sextants and vernier callipers. |
| Tangential load (Unit N) |
The component of load applied to an object, such as a gear, that tends to cause or resist rotation. |
| Tap |
1. A threaded hard metal plug for cutting screw threads in holes. It has longitudinal grooves for the clearance of chips while cutting. Progressive cutting is achieved by using three taps in succession: taper, second, and plug. First and second taps have tapers to help start the thread along the axis of the hole; the plug tap has no taper and is used on the final cut in a blind hole. 2. A valve in, or at the end of, a pipeline. |
| Tap wrench |
A lever with an adjustable square hole at its centre into which is fitted the shank of a tap so that torque can be applied to cut a thread. |
| Taper |
A gradual, often linear, reduction in cross section or shape. |
| Taper |
A shaft or hole that gets gradually smaller toward one end. |
| Taper key |
A key with parallel sides but tapering cross section along its length. |
| Taper pin |
A pin or peg of circular cross section that tapers along its length. |
| Taper pins |
Steel pins used for locating and holding the machine parts in position on a shaft. |
| Tare weight (Unit N) |
The weight of an empty container. |
| T-bolt |
A threaded bolt having a square or rectangular end which fits into the T slot of a machine table for clamping workpieces. |
| Tear strength (Unit N/m) |
The tensile force divided by the sheet thickness required to tear a pre-split sheet material at a specified rate. Particularly used for plastics, rubber, fabrics, and elastomers. Since the units are not those of stress but equivalent to J/m2, the units of toughness, the term ‘strength’ is strictly incorrect. |
| Teeth |
The projecting elements on gears, cutting tools, etc. |
| Temper |
(1) In heat treatment, reheating hardened steel or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness. The process also is sometimes applied to normalized steel. (2) In tool steels, temper is sometimes used, but inadvisedly, to denote the carbon content. (3) In nonferrous alloys and in some ferrous alloys (steels that cannot be hardened by heat treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized by a certain structure, mechanical properties, or reduction in area during cold working. |
| Temper color |
A thin, tightly adhering oxide skin that forms when steel is tempered at a low temperature, or for a short time, in air or a mildly oxidizing atmosphere. The color, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature. |
| Temper embrittlement |
Brittleness that results when certain steels are held within, or are cooled slowly through, a certain range of temperature below the transformation range. The brittleness is manifested as an upward shift in ductile-to-brittle transition temperature but only rarely produces a low value of reduction in area in a smooth-bar tension test of the embrittled material. |
| Temper embrittlement |
The anomalous reduction in toughness of martensitic stainless steels when tempered in the range 370–600°C. |
| Temper embrittlement |
Embrittlement of alloy steels caused by holding within or cooling slowly through a temperature range just below the transformation range. Embrittlement is the result of the segregation at grain boundaries of impurities such as arsenic, antimony, phosphorus, and tin; it is usually manifested as an upward shift in ductile-to-brittle transition temperature. Temper embrittlement can be reversed by retempering above the critical temperature range, then cooling rapidly. |
| Temperature (Unit K or °C) |
A quantitative measure of the molecular kinetic energy of a substance and so how hot or cold it is. |
| Temperature factor |
A calibration constant used in ultrasonic measurement of bolt stress or strain. Accounts for the effects of thermal expansion and the temperature-induced change in the velocity of sound. |
| Tempered martensite embrittlement |
Embrittlement of ultrahigh-strength steels caused by tempering in the temperature range of 205 to 400 °C (400 to 750 °F); also called 350 °C or 500 °F embrittlement. Tempered martensite embrittlement is thought to result from the combined effects of cementite precipitation on prioraustenite grain boundaries or interlath boundaries and the segregation of impurities at prioraustenite grain boundaries. |
| Temporal decomposition |
The splitting of a control problem into parts with relatively short and long time constant so that different controllers may be used for the different parts. |
| Tensile strength |
In tension testing, the ratio of maximum load to original cross-sectional area. Also known as ultimate strength. |
| Tensile strength |
The resistance that a material offers to tensile stress. It is defined as the smallest tensile stress required to break the body. |
| Tensile strength (Unit Pa) |
(ultimate strength, ultimate tensile strength, UTS, Rm, Rm) The nominal or engineering stress given by the maximum load in a tension test divided by the original cross-sectional area of the specimen. The maximum load could be the fracture load for a brittle material, but for a ductile material it is usually taken as the load at which necking begins, beyond which the load falls. |
| Tensile stress |
A stress that causes two parts of an elastic body, on either side of a typical stress plane, to pull apart. |
| Tensile stress (Unit Pa) |
A stress that tends to stretch a component, or local region of a component, when under load. In simple tension it results from the applied axial load, but tensile stresses occur in bending, torsion and other forms of loading. |
| Tensile test (tension test) |
A test in which a precisely-machined tensile specimen (tensile testpiece), typically circular or rectangular in cross section with large end sections that are gripped in the testing machine, is subjected to an increasing tensile load, usually to the point of fracture, to produce a stress–strain curve from which such material properties as modulus of elasticity, limit of proportionality, proof stress, yield point, and ultimate tensile stress can be determined. The relevant standard is ISO 6892-1. |
| Tensiometer |
An instrument used to measure surface tension. |
| Tension |
The force or load that produces elongation. |
| Tension |
1. The condition in a bar, belt, cable, spring, string, wire, etc. that is being pulled from either end. 2. (tensile force) (unit N) The force associated with tension as in (1). It is measured by a tension meter. |
| Tension joint |
A joint which is primarily subjected to loads acting more or less parallel to the axes of the bolts. |
| Tension testing |
A method of determining the behavior of materials subjected to uniaxial loading, which tends to stretch the metal. A longitudinal specimen of known length and diameter is gripped at both ends and stretched at a slow, controlled rate until rupture occurs. Also known as tensile testing. |
| Tension, bolt |
Tension (tensile stress) created in the bolt by assembly preloads and=or such things as thermal expansion, service loads, etc. |
| Tensioner |
A hydraulic tool used to tighten a fastener by stretching it rather than by applying a substantial torque to the nut. After the tension has stretched the bolt or stud, the nut is run down against the joint with a modest torque, and the tensioner is disengaged from the fastener. The nut holds the stretch produced by the tensioner. |
| Tensometer |
A bench-top device, used to perform tension and compression tests. |
| Tera (T) |
An SI unit prefix indicating a multiplier of 1012; thus terawatt (TW) is a unit of power equal to one trillion watts. |
| Terminal temperature difference (Unit K) |
The temperature difference between the two fluid streams at either inlet or outlet of a heat exchanger. |
| Terminal velocity (Unit m/s) |
The velocity reached by an object in free fall through a fluid when the upward drag and buoyancy forces just balance the object’s weight. For an object with a density lower than that of the fluid, the terminal velocity is vertically upwards. |
| Testing machine |
A machine used to apply either a steady or oscillatory or impact load to a testpiece. The load may be tensile, compressive, shear, bending, or torsional. |
| Testing machine (load-measuring type) |
A mechanical device for applying a load (force) to a specimen. |
| Thermal analysis |
A method for determining transformations in a metal by noting the temperatures at which thermal arrests occur. These arrests are manifested by changes in slope of the plotted or mechanically traced heating and cooling curves. When such data are secured under nearly equilibrium conditions of heating and cooling, the method is commonly used for determining certain critical temperatures required for the construction of equilibrium diagrams. |
| Thermal compressor |
1. (thermocompressor) A jet compressor used to boost the pressure of low-pressure waste or exhaust steam. 2. A device consisting of an absorber, a generator, a pump, and a throttling device, used instead of a mechanical vapour compressor in an absorption-cooling refrigeration system. |
| Thermal efficiency |
The ratio, for a heat engine or thermodynamic cycle, of the net work output to the net heat input into the system. |
| Thermal electromotive force |
The electromotive force generated in a circuit containing two dissimilar metals when one junction is at a temperature different from that of the other. |
| Thermal energy (heat energy) (Unit J) |
The sensible and latent forms of internal energy. |
| Thermal fatigue |
Fatigue resulting from the presence of temperature gradients that vary with time in such a manner as to produce cyclic stresses in a structure. |
| Thermal fatigue |
Fracture resulting from the presence of temperature gradients that vary with time in such a manner as to produce cyclic stresses in a structure. |
| Thermal insulation (insulation) |
A material that may be a low-conductivity solid, or a porous solid or fabric in which there are voids containing air or another gas, resulting in overall low thermal conductivity. When applied to the surface of a hot or cold object, surface heat transfer is generally reduced. |
| Thermal reactor |
An enlarged exhaust manifold bolted directly to the cylinder head of a piston engine, in which the oxidation of carbon monoxide and unburned hydrocarbons in the exhaust gas is enhanced. |
| Thermal shock |
The development of a steep temperature gradient and accompanying high stresses within a structure. |
| Thermal stresses |
Stresses in metal resulting from nonuniform temperature distribution. |
| Thermochemical treatment |
Heat treatment carried out in a medium suitably chosen to produce a change in the chemical composition of the object by exchange with the medium. |
| Thermocouple |
A device for measuring temperature, consisting of lengths of two dissimilar metals or alloys that are electrically joined at one end and connected to a voltage-measuring instrument at the other end. When one junction is hotter than the other, a thermal electromotive force is produced that is roughly proportional to the difference in temperature between the hot and cold junctions. |
| Thermodynamic pressure (Unit Pa) |
The pressure within a fluid given by an equation of state such as p(ρ,T) where ρ is the fluid density and T is the absolute temperature of the fluid. |
| Thermodynamic process |
A change in equilibrium state undergone by a quantity of matter or a system. |
| Thermodynamic properties |
Temperature, pressure, and the intensive properties that define the state of a working fluid: specific internal energy, specific enthalpy and specific entropy. |
| Thermodynamic state |
The condition of a system or working fluid according to its properties. |
| Thermodynamics laws |
The zeroth, first, second, and third laws of thermodynamics. |
| Thermoelastic effect |
The change in temperature of material under adiabatic elastic deformation, e.g. when loading is so rapid that the material is not in thermal equilibrium with its surroundings. For most materials, elastic tension lowers the temperature and compression raises it. The reverse occurs in elastomers such as rubber that exhibit entropic elasticity because stretching reduces the number of possible molecular configurations and so reduces the entropy. The thermoelastic effect gives rise to a stress–strain hysteresis loop in cyclic elastic loading and is one mechanism by which vibrational energy is dissipated internally in materials. |
| Thermoelectric converter |
(thermoelectric generator) A device consisting of series-connected alternate n- and p-type semiconductor elements sandwiched between two ceramic plates. Due to the Seebeck effect, electrical power is generated when a temperature difference is maintained across the plates. Due to the Peltier effect, a thermoelectric converter can act as either a thermoelectric heater or a thermoelectric cooler (thermoelectric refrigerator) by passing an electrical current through it. The thermoelectric figure of merit (Z), with unit 1/K, is a dimensional parameter on which the efficiency of a thermoelectric device is primarily dependent, defined by Z = σα2/k where α is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. The product of Z and a temperature is non-dimensional. |
| Thermoelectric junction |
A welded, soldered, or twisted connection between the ends of two wires of dissimilar metals, as in a thermocouple. |
| Thermoelectric material |
A material in which any of the thermoelectric phenomena, such as the Peltier, Seebeck, and Thomson effects, are especially strong. |
| Thermomechanical working |
A general term covering a variety of processes combining controlled thermal and deformation treatments to obtain specific properties. Same as thermal-mechanical treatment. |
| Thread form |
The cross-sectional shape of the threads, defining thread angle, root, and crest profiles, etc. Thread length Length of that portion of the fastener which contains threads cut or rolled to full depth. |
| Thread grinding |
A method for producing or finishing very precise screw threads using a profiled grinding wheel. |
| Thread insert |
(threaded bushing) Either a thin cylinder with an internal thread (and sometimes also an external thread) or a helical coil of wire, pressed or screwed into a hole to accept a bolt or screw. Used in material too soft or a component too thin to be threaded, to change one form of thread to another or to repair a damaged thread. |
| Thread milling |
Thread milling is a method of cutting screw threads with a milling cutter in a thread mill. Thread milling cutters are either single form or multiple form and are used in either conventional or planetary thread mills. |
| Thread rolling |
Thread rolling (also known as roll threading) is a cold forming process for producing threads or other helical or annular forms by rolling the impression of hardened steel dies into the surface of a cylindrical or conical blank. Polygonal blanks are also thread rolled for the purpose of fabricating thread-forming and self-locking screws. The preferred polygonal shape is trilobular and is produced in flat die machines. |
| Thread run-out |
That portion of the threads which are not cut or rolled full depth, but which provide the transition between full-depth threads and the body or head. Officially called thread washout or vanish, although the term run-out is more popular. (Run-out is officially reserved for rotational eccentricity, as defined by total indicator readings or the like.) |
| Threaded fastener |
Studs, bolts, and screws of all sorts, with associated nuts. One of the most interesting, complex, useful—and frustrating—components yet devised. |
| Threading machine |
A machine used to cut an external thread on a rod, tube, bolt blank, etc. or an internal thread in a hole, tube, nut blank, etc. |
| Three-quarters hard |
A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength and hardness about midway between those of half hard and full hard tempers. |
| Threshold stress for stress-corrosion cracking (σth) |
An experimentally determined critical gross-section stress below which stress-corrosion cracking will not occur under specified test conditions. |
| Thumb screw |
A type of screw having a winged or knurled head for turning by hand when a quick and light clamping effect is desired. |
| Tightness |
A measure of the mass leak rate from a gasketed joint. |
| Tightness parameter |
A dimensionless parameter which defines the mass leakage of a gasket as a functionof contained pressure and a contained fluid constant. |
| Tightness, acceptable |
Wholly leak-free joints are impossible, at least if the contained fluid is a gas, so it has been proposed that the design of a gasketed joint should start with the selection of an ‘‘acceptable’’ leak rate. The designer would dimension bolts and joint members so that the actual leak rate would never exceed this. Three standard levels of tightness have been proposed as well. |
| Time quenching |
A term used to describe a quench in which the cooling rate of the part being quenched must be changed abruptly at some time during the cooling cycle. |
| Toggle (toggle clamp) |
A device consisting of two pinned levers that are almost in line between two end points in the same plane. Bringing the levers into line generates large forces between the ends. Used to obtain a large mechanical advantage or as a locking mechanism. |
| Tolerance |
A fractional allowance for variation from the specifications. |
| Tolerance limits |
The extreme values (upper and lower) that define the range of permissible variation in size or other quality characteristic of a part. |
| Tolerances |
In mass production, using automatic machines, parts are intended to be completely or partially interchangeable, and in batch production parts should ideally fit without the need for selection. In practice it is impossible for mass-produced parts to have exactly the same size, owing to tool wear, variations in raw materials, thermal effects, etc. In consequence, variations from exactness, termed tolerances, are assigned to what is permissible on the basic size. |
| Tooth lock washers |
These washers serve to lock fasteners, such as bolts and nuts, to the component parts of an assembly, or increase the friction between the fasteners and the assembly. They are designated in a manner similar to helical spring lock washers, and are available in carbon steel. |
| Toothed belt |
A flat belt, typically of a reinforced-rubber material, with transverse teeth that engage with teeth on a wheel or pulley. |
| Toothed gearing |
In principle, friction between circular discs in tangential contact could be used to transmit rotation and power between shafts, but the power level would be very limited and the velocity ratio affected by slip. To make the drive positive, gear wheels are used instead in which teeth extend above and below the diameters (the pitch-circle diameters) of discs in ideal rolling contact. The diagram shows the nomenclature for spur gears, the simplest type of gearing between parallel shafts, which are cylindrical in shape with straight teeth parallel to the axis of rotation. |
| Torque |
The twisting moment, product of force and wrench length, applied to a nut or bolt (for example). |
| Torque (T) (Unit N.m) |
The twisting moment of a force or couple about an axis which results in torsion. |
| Torque arm |
A bar fixed at one end used to resist torque applied at the other, for example by an electric drill. |
| Torque coefficient |
1. (KT) For a propeller of diameter D, a non-dimensional parameter defined by KT = T/ρΩ2D5 where Ω is the rotational speed (in rad/s) and ρ is the fluid density. 2. (CT) An alternative to (1) for a wind turbine, defined by CT = 2T/ρV2AR where V is the wind speed, A is the swept area of the blades, and R is the blade radius. 3. (K) An empirical coefficient used to calculate the torque T required to achieve an axial load F in a bolt according to T = KFd where d is the nominal bolt diameter. |
| Torque control |
A system of optimizing rotor speed for a variable-speed wind turbine by controlling the torque demands of the generator. |
| Torque converter |
A turbomachine used for torque amplification consisting of an impeller, a turbine, and a reaction member. Applications include motor-vehicle transmissions. |
| Torque meter |
An instrument for measuring torque using spring-loaded, piezoelectric, or strain-gauged devices. |
| Torque monitor |
A torque tool control system which monitors the amount of torque being developed by the tool during use, but does not control the tool or the torque produced. |
| Torque motor |
A type of induction motor that can apply a steady torque even when prevented from rotating. |
| Torque multiplier |
A gearbox used to multiply the torque produced by a small hand wrench (usually a Torque wrench). The output of the multiplier drives the nut or bolt with a torque that is higher, and a speed that is lower, than input torque and speed. There is no torque gage or readout on the multiplier. |
| Torque pack |
A geared wrench which multiplies input torque and provides a read-out of output torque. In effect, a combination of a Torque wrench and a Torque multiplier. |
| Torque reaction (Unit N.m) |
The torque needed to counteract an applied torque. For example, in a helicopter with a single main rotor, the tendency of the fuselage to rotate in the opposite direction to the rotor. |
| Torque wrench |
A manual wrench which incorporates a gage or measuring apparatus of some sort to measure and display the amount of torque being delivered to the nut or bolt. All wrenches produce torque. Only a torque wrench tells how much torque. |
| Torque wrench |
A socket wrench or ring spanner that can be set to allow a specific torque to be applied to a nut or bolt head. |
| Torr |
A non-SI unit of pressure commonly used in vacuum systems; defined by 760 torr = 1 atm so that 1 torr = 133.322 368 4 Pa, and approximately equivalent to the pressure corresponding to 1 mm of mercury. |
| Torricellean barometer |
A vertical glass tube with its upper end sealed and the lower end submerged in a pool of mercury. The space above the mercury is under vacuum. The height to which the mercury rises up the tube is a measure of the barometric pressure. |
| Torsion |
A twisting deformation of a solid body about an axis in which lines that were initially parallel to the axis become helices. |
| Torsion |
The twisting of an object about an axis due to an applied couple (torque). |
| Torsion bar |
A metal bar designed to act as an elastic spring when torque is applied. |
| Torsion spring |
1. A spring in the form of a torsion bar. 2. A helical spring to which torque can be applied at the ends. |
| Torsion test |
A test designed to provide data for the calculation of the shear modulus, modulus of rupture in torsion, and yield strength in shear. |
| Torsional angle (ϕ) |
(Unit ° or rad) The angular deflexion between two locations on a straight bar subjected to a torque. |
| Torsional shaft vibration |
The rapid back-and-forth angular twisting along a rotating unbalanced shaft which reaches maxima at critical rotation speeds. Shaft eccentricity, unbalanced mass distribution, oscillatory torque, misalignment, etc. all influence torsional vibration. |
| Torsional stress |
The shear stress on a transverse cross section resulting from a twisting action. |
| Torsion-bar suspension |
A type of motor-vehicle suspension in which one end of a torsion bar is rigidly fixed to the chassis while the other carries a lever to which are attached the components that carry a wheel. In a torsion-beam suspension, a beam connects the wheels on either side of the vehicle. |
| Total carbon |
The sum of the free and combined carbon (including carbon in solution) in a ferrous alloy. |
| Total combustion air (Unit kg/s) |
1. The combination of the stoichiometric flow of air required for combustion together with any excess air. 2. The flow of fresh air into a boiler plus any flue gas recirculated. |
| Total elongation |
A total amount of permanent extension of a test piece broken in a tensile test. |
| Total energy (Unit J) |
The sum of all forms of energy associated with a system, including kinetic, potential, internal, magnetic, chemical, and electrical energy. |
| Total head (hT) (Unit m) |
Total pressure pT expressed in terms of the vertical height of a column of liquid, typically water or mercury, i.e. hT = pT/ρg where g is the acceleration due to gravity and ρ is the liquid density. |
| Total heat (Unit kJ) |
An alternative term for enthalpy. It is inappropriate as it suggests that it includes kinetic energy in the same way as total enthalpy, but it does not. Also, heat and enthalpy are quite different quantities, as their definitions show. |
| Total-loss lubrication |
A system in which the lubricating oil for an internal-combustion engine is burned together with the fuel. |
| Toughening mechanisms |
Various methods of increasing the resistance to crack initiation and propagation in materials. They include transformation toughening, in which the microstructure around the crack tip alters so as to slow down or arrest cracks; deflexion of cracks; various ways of de-sharpening crack tips; fibre bridging of cracks; and fibre pull-out. |
| Toughness |
The ability of a metal to absorb energy and deform plastically before fracturing. |
| Toughness (Unit J/m2) |
1. The ability of a material to resist crack initiation and propagation. 2. The ability of a material to absorb strain energy without fracturing. |
| Tow |
1. To cause a vehicle to move by pulling with a rope, chain, bar, etc. 2. A bundle of fibres employed in reinforcement. |
| Track rod |
A bar connecting the ends of the steering arms in an automotive vehicle. |
| Tracking problem |
A control problem where the set point changes with time and thus the plant output must follow the changing set point. The opposite of a regulator problem. |
| Transducer |
A device which converts one form of energy into another. An ultrasonic transducer, for example, converts electrical energy into acoustic energy (at ultrasonic frequencies) and vice versa. |
| Transfer machines |
These specialized machine tools are used to perform various machining operations on parts or parts in fixtures as the parts are moved along on an automatic conveyor which is part of the machine tool set-up. In a set-up, the parts can move in a straight line from their entry point to their exit point, or the setup may be constructed in a U-shape so that the parts are expelled near where they start. |
| Transfer moulding |
(resin-transfer moulding, RTM) A method of compression moulding polymers in which the dies are closed before the operation starts. |
| Transfer ratio |
(transfer constant) A complex variable representing the ratio between the output of a transducer and the input causing that output. |
| Transformation temperature |
The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for iron and steels. |
| Transformation toughening |
The improvement of fracture toughness of a material by stress-induced transformation of the microstructure. |
| Transgranular |
Through or across crystals or grains. Also known as intracrystalline or transcrystalline. |
| Transgranular fracture |
Fracture in crystalline materials where the path of cracking is predominantly across grains. |
| Transition metals |
Metallic elements from groups (columns) 3 to 12 of the periodic table. They include important metals such as titanium, vanadium, chromium, manganese, iron, cobalt, zirconium, niobium, molybdenum, tantalum, tungsten, and the noble metals. They have either body-centred cubic, face-centred cubic, or hexagonal close-packed crystal structure at room temperature and have a wide range of mechanical and physical properties. |
| Transition temperature |
(1) An arbitrarily defined temperature that lies within the temperature range in which metal fracture characteristics (as usually determined by tests of notched specimens) change rapidly, such as from primarily fibrous (shear)to primarily crystalline (cleavage) fracture. (2) Sometimes used to denote an arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature. |
| Transition temperature |
(1) An arbitrarily defined temperature that lies within the temperature range in which metal fracture characteristics (as usually determined by tests of notched specimens) change rapidly, such as from primarily fibrous (shear) to primarily crystalline (cleavage) fracture. Commonly used definitions are “transition temperature for 50% cleavage fracture,” “10 ft · lbf transition temperature,” and “transition temperature for half maximum energy.” (2) Sometimes used to denote an arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature. |
| Transition temperature (Unit K) |
(transition point) 1. The temperature at which the mechanism of fracture in metal alloys having a face-centred cubic crystal structure changes from ductile void growth to brittle cleavage. 2. The temperature at which a material changes from one crystal state to another. |
| Translational joint |
A robot joint where the controlled variable is straight line movement. |
| Transmissibility |
The ratio of the transmitted force to the disturbing force for a system subjected to a vibratory disturbance. The ratio may also be defined in terms of displacements, velocities, or accelerations. |
| Transmission |
The system that transmits power and torque from a power source; for example a shaft, belts and pulleys, or a gear train. In the case of a motor vehicle, it includes the gearbox, clutch, propeller shaft, differential and final drive shafts. |
| Transmission dynamometer |
A dynamometer in which the power transmitted by a shaft is obtained from the product of its rotation speed and the torque determined from measured shear strains along the shaft. |
| Transverse direction |
Literally, the “across” direction, usually signifying a direction or plane perpendicular to the direction of working. In rolled plate or sheet, the direction across the width is often called long transverse, and the direction through the thickness, short transverse. |
| Transverse strain |
Linear strain in a plane perpendicular to the loading axis of a specimen. |
| Trepanning |
Trepanning is a machining process for producing a circular hole or groove in solid stock, or for producing a disk, cylinder, or tube from solid stock, by the action of a tool containing one or more cutters (usually single-point) revolving around a center. |
| Tribology |
The science and technology concerned with interacting surfaces in relative motion. |
| True strain |
(1) The ratio of the change in dimension, resulting from a given load increment, to the magnitude of the dimension immediately prior to applying the load increment. (2) In a body subjected to axial force, the natural logarithm of the ratio of the gage length at the moment of observation to the original gage length. Also known as natural strain. Compare with engineering strain. |
| True stress |
The value obtained by dividing the load applied to a member at a given instant by the cross-sectional area over which it acts. Compare with engineering stress. |
| True stress (σ) (Unit Pa) |
The applied load on a test specimen divided by the current cross-section area over which it acts. σ = s expε where s is the engineering stress given by the applied load divided by the original cross-section area over which it acts, and ε is the true strain. |
| T-slot |
A recessed or undercut slot made with a special T shaped cutter in the tables of machine tools to receive the square head of a T bolt for clamping workpiece. |
| Turbine |
A turbomachine in which a rotor (turbine wheel) or impeller is caused to rotate and convert flow energy into shaft power or thrust. |
| Turbine blades (turbine buckets) |
1. The aerofoil-shaped vanes that form the rotor and stator of a gas, steam or hydraulic turbine. 2. The cup-shaped vanes of a Pelton wheel. |
| Turbine flow meter |
(axial flow meter, propeller meter) An in-line flow meter in which the rotation speed N of a propeller or rotor is a measure of the mass flowrate ṁ. Calibration is always necessary but with appropriate design ṁ is closely proportional to N over a wide range. |
| Turbine pump (turbopump) |
An axial or centrifugal pump driven by a turbine. Typically used to supply fuel to the combustion chamber of a rocket engine such as on the space shuttle. |
| Turbo generator |
(turboset) The combination of a steam or gas turbine and an electrical generator with a single shaft or connected coaxial shafts. |
| Turboblower |
A centrifugal or axial compressor or fan. |
| Turbocharging |
A method of supercharging in which the hot exhaust gas from a piston engine is used to drive a turbine which powers the supercharging compressor. A turbocharger is the turbine/compressor combination. The compressor is usually of radial outflow design while radial, axial, and mixed-flow turbines are all in use. The wastegate is a valve that reduces the flow of exhaust gas into the turbine to limit the boost produced or overspeeding. Not shown in the diagram are bearings, oil passages, etc. |
| Turbomachine |
(rotodynamic machine) A machine in which there is a transfer of energy between a continuous stream of fluid and a component, called a rotor, rotating about a fixed axis. Fans and turbines (gas, hydraulic, steam, or wind) are turbomachines in which energy is transferred to the rotor, causing it to rotate. Pumps and compressors are turbomachines in which energy is transferred from the externally driven rotor to the fluid. |
| Turbomolecular pump |
A high-vacuum (pressure down to about 10−8 Pa) pump in which momentum is transferred to the gas molecules by a rapidly rotating bladed disc. |
| Turbulent flow (turbulence) |
Fluid motion characterized by disorderly, rotational (i.e. vortical) three-dimensional velocity fluctuations covering a wide range of frequency and length scales. The pressure, temperature, and other fluid properties also fluctuate and the diffusion of heat, mass, and momentum is greatly enhanced. As are laminar and transitional flow, turbulent flow of a Newtonian fluid is governed by the Navier–Stokes equations which can in principle be solved by direct numerical simulation (DNS) in which all time and length scales of the fluctuating motion are resolved. |
| Turning |
Turning is a machining process for generating external surfaces of revolution by the action of a cutting tool on a rotating workpiece, usually in a lathe. |
| Turning |
The rotation of a workpiece held against a cutting tool in a lathe to produce components with a circular cross section. |
| Turning angle (θ) |
The change in direction experienced by a gas flow passing through an oblique shock wave or Prandtl–Meyer expansion fan. |
| Turn-of-nut |
Sometimes used to describe the general rotation of the nut (or bolt head) as the fastener is tightened. More often used to define a particular tightening procedure in which a fastener is first tightened with a preselected torque, and is then tightened further by giving the nut an additional, measured, turn such as ‘‘three flats’’ (180°). |
| Turn-of-nut method |
It applies preload by turning a nut through an angle that corresponds to a given elongation. |
| Turret lathe |
A capstan lathe in which the capstan is driven mechanically or hydraulically to provide assigned tool paths to perform a specified sequence of operations in the repetitive production of parts. |
| Twist |
The helix produced in a cylindrical component, such as a shaft, wire, tensioned cable, or rope, when one end is rotated relative to the other. Measured either as the number of turns per unit length, or by the helix angle (twist angle). |
| Twist drill |
A hardened-steel drill bit having one or more helical flutes running from a conical tip to the smooth part of the shank. |
| Two-dimensional flow |
A fluid flow in which the velocity at any time depends upon two spatial coordinates, such as the radial and axial locations in developing pipe flow. |
| Two-phase flow |
A flow in which two phases are present, for example gas bubbles in a liquid, liquid bubbles in another liquid with which it is immiscible, solid particles in a liquid or gas. |
| Two-stage compressor |
A machine in which gas is compressed from low pressure toan intermediate pressure in a low-pressure cylinder, and then to final pressure in a highpressure cylinder. Efficiency is improved if the two cylinders are separated by an intercooler. |
| Two-stroke engine |
(two-cycle engine) A petrol or diesel engine in which an air/fuel charge is introduced through an induction port, compressed and burned, expanded and then exhausted through an exhaust port. The two ports in the cylinder wall are opened and closed by the piston. There are two strokes in each revolution of the crankshaft. In total-loss lubrication petrol engines, the lubricating oil is mixed with the fuel. |
| Typical basis |
The typical property value is an average value. No statistical assurance is associated with this basis. |
| Tyre |
A flexible ring-shaped cover mounted on a wheel rim that supports the weight of a vehicle, such as a motor vehicle, aircraft, or bicycle, and transmits power and torque to a road surface. Some pneumatic tyres are inflated directly, others have a separate inner tube that is inflated. For certain applications, solid tyres are used. |
| U-bend |
U-bend A 180° bend in a pipe. |
| U-bolt |
A rod threaded at both ends and bent into a U-shape. It is used for clamping. |
| U-bolt |
An externally threaded fastener bent in the shape of the letter U and with both ends threaded. |
| Ultimate strength |
The maximum tensile strength a bolt or material can support prior to rupture. Always found in the plastic region of the stress–strain or force–elongation curve, and so is not a design strength. Also called Tensile strength and ultimate tensile strength. |
| Ultimate strength |
The maximum stress (tensile, compressive, or shear) a material can sustain without fracture, determined by dividing maximum load by the original cross-sectional area of the specimen. Also known as nominal strength or maximum strength. |
| Ultrasonic extensometer |
An electronic instrument which measures the change in length of a fastener ultrasonically as, or before and after, the fastener is tightened. |
| Ultrasonic method |
Measuring elongation uses a sound pulse, generated at one end of a bolt, that travels the length of a bolt, bounces off the far end, and returns to the sound generator in a measured period of time. |
| Uniaxial strain |
Increase (or decrease) in length resulting from a stress acting parallel to the longitudinal axis of the specimen. |
| Uniform elongation |
The elongation at maximum load and immediately preceding the onset of necking in a tension test. |
| Uniform strain |
The strain occurring prior to the beginning of localization of strain (necking); the strain to maximum load in the tension test. |
| Union (union joint) |
A threaded pipe fitting that allows two pipes to be connected anddetached without the need for either to be rotated, and without damaging the pipe ends. |
| Unison ring |
In a turbomachine with adjustable guide or stator vanes, the ring to which a set of vanes is linked such that when the ring is rotated, all vane angles are adjusted. |
| Unit |
A numerical indication of the magnitude of a physical quantity. The basic units of mass, length, time, etc. are relative to agreed standards. |
| Universal joint (Hooke’s joint) |
A double-pivoted connection that allows power and torque to be transmitted between two shafts at an angle to each other. For constant driving angular velocity of the input yoke, the angular velocity of the output yoke fluctuates by amounts depending on the angle of intersection of the shafts. Speeds of the driving and driven shafts may be made identical (giving a constant-velocity joint) when an intermediate shaft, at each end of which there is a universal joint, is interposed. The driving and driven shafts must be equally inclined to the intermediate shaft, the two forks of which must lie in the same plane. |
| U-tube |
A device consisting of two vertical tubes connected at either the top or bottom by a length of tubing such that a U-shape is formed. |
| Vacuum annealing |
Annealing carried out at subatmospheric pressure. |
| Vacuum carburizing |
A high-temperature gas carburizing process using furnace pressures between 7 and 55 kPa during the carburizing portion of the cycle. |
| Vacuum pump |
A pump for exhausting air and non-condensable gases from a vessel to be maintained at sub-atmospheric pressure. The pirani and thermal-conductivity gauges are vacuum gauges used to measure the absolute pressure within a vacuum system (vacuum level), usually expressed in torr or Pa. |
| Vacuum relief valve |
A valve that admits gas to a system under vacuum, should the degree of vacuum become excessive. |
| Vacuum servo |
A vacuum-operated servomotor which is used in a motor vehicle to provide a brake effort greater than the capability of the driver. |
| Valve |
Any of various manual or automatic devices that are able to initiate, regulate, or stop the flow of a fluid through a conduit or from a closed container. |
| Valve plug |
A conical or cylindrical plug, in which there are transverse holes, in a valve, such as a plug valve. Flow through the holes occurs when the plug is rotated. |
| Valve spring |
The spring that restores a valve to its closed position after having been opened, and is also intended to prevent valve bounce. |
| Vane engine (vane motor) |
A rotary engine in which high-pressure hydraulic fluid in the spaces between spring-loaded sliding vanes held in an offset rotor acts on the vanes, causing the rotor to revolve within a cylinder. The design is much like a sliding-vane compressor, as is that of a vane pump, used to pump liquids. |
| Vapour |
The gas-like phase of a substance at a temperature below its critical point. A vapour can be condensed to a liquid or a solid by increasing its pressure or reducing its temperature. |
| Vapour static pressure |
(Unit Pa or bar) A pressure analogous to hydrostatic pressure, where the fluid is a vapour such as steam. A term used in geothermal applications. |
| Vapour static pressure |
(Unit Pa or bar) A pressure analogous to hydrostatic pressure, where the fluid is a vapour such as steam. A term used in geothermal applications. |
| Vapour–pressure curve |
For a pure substance, the curve of saturation pressure plotted vs saturation temperature. |
| Vapour-pressure thermometer |
(vapour-filled thermometer) A type of fluidexpansion thermometer in which the working fluid is a volatile liquid. |
| Variance |
A measure of the squared dispersion of observed values or measurements expressed as a function of the sum of the squared deviations from the population mean or sample average. |
| V-belt |
A drive belt having a trapezoidal cross section which runs in pulleys with V-shaped grooves. Higher torques can be transmitted than with a flat belt. |
| V-block |
A block having a 90° V-shaped recess; used in a workshop to hold round workpieces. |
| Vector |
Any physical quantity, such as velocity, acceleration, force, or momentum, that is specified in terms of both its magnitude and its direction. The convention of printing vector quantities in boldface was introduced by the physicist Josiah Willard Gibbs. |
| Vehicle |
A means of conveyance for transporting goods and people, generally with wheels powered by a petrol, diesel, or steam engine, or by an electric motor, or by a hybrid combination of an electric motor and a diesel or petrol engine. |
| Velocity defect (Unit m/s) |
In a viscous flow, such as a boundary layer or wake, the reduction in velocity, due to friction, compared with that of the free stream. |
| Velocity ratio |
1. The ratio between the input velocity to a machine, train of gears, etc. and the output velocity. 2. The ratio between the displacement of an applied force at one part of a mechanism and the movement of the load at a different part. |
| Vent |
A small valve that allows the release of pressurized fluid from a pipe, pressure vessel, etc., often to the atmosphere. |
| Ventilation |
A system for circulating fresh air in a room, building, passenger vehicle, orother enclosed space. |
| Venturi |
A convergent–divergent flow nozzle, usually circular in cross section, with a relatively short convergent section (the confuser) upstream of a throat followed by a gradually diverging section (the diffuser). Such nozzles usually have flanges at either end for installation in a pipeline. Applications include flow meters and ejectors. When a fluid flows through a convergent duct at subsonic speed, there is an increase in velocity accompanied by a decrease in pressure (Venturi effect). |
| Verification |
Checking or testing an instrument to ensure conformance with a specification. |
| Verified loading range |
For testing machines, the range of indicated loads for which the testing machine gives results within the permissible variation specified. |
| Vernier |
A short auxiliary scale that slides along the main instrument scale to permit more accurate fractional reading of the least main division of the main scale. |
| Vertical engine |
A piston engine having the cylinders above the crankshaft. |
| Vibration |
1. A periodic change with time of the displacements of elements making up a component or structure. 2. The study of the oscillatory motion of bodies and systems and the frequencies, amplitudes, and forces associated with them. |
| Vibration isolation |
The prevention of transmission of vibration from one component of a system to another part of the same system, such as a building or other structure. Isolation may be achieved using dampers (vibration damping) or by active feedback-control methods. Mechanical vibration is often attenuated by means of components immersed in oil such as in dashpots (viscous damping). Vibration suppression can be achieved (a) using dampers and absorbers tuned to a particular frequency to suppress vibratory forces in structures and other systems (passive suppression) or (b) by the measurement of vibration at key locations in a structure and the application of cancellation forces (active suppression). |
| Vibration-testing machine (vibrator) |
Any machine that subjects components or systems to vibration at known amplitudes and frequencies in order to determine the response. vibratory equipment Vibrating process equipment, such as shakers, used to separate small and large particles, or feeders to supply particulate or granular material that may otherwise clog. |
| Vibrograph |
An instrument that records vibrations in a system over time. |
| Vibrometer |
A device used to measure the motion of a vibrating surface, typically using a contactless laser-based technique. |
| Vice |
A workshop tool used to hold a workpiece and consisting of two jaws, one fixed and the other moved by turning a screw. |
| Vickers hardness number |
(diamond hardness number, DHN, VHN, VPN) (Unit kg/mm2 originally, sometimes now Pa) Indentation hardness given by load divided by the surface area of the permanent impression obtained when the indenter is in the form of a square pyramid whose opposite faces make an angle of 136° with one another. The mean length d of the diagonals of the indentation is determined, from which VPN = 0.927(2W/d2) where W is the load, since the base of the pyramid has an area equal to 0.927 times the surface area. |
| Vickers hardness test |
An indentation hardness test employing a 136° diamond pyramid indenter (Vickers) and variable loads, enabling the use of one hardness scale for all ranges of hardness—from very soft lead to tungsten carbide. Also know as diamond pyramid hardness test. |
| Viscoelasticity |
A property involving a combination of elastic and viscous behavior. A material having this property is considered to combine the features of a perfectly elastic solid and a perfect fluid. A phenomenon of time-dependent, in addition to elastic, deformation (or recovery) in response to load. |
| Volume (Unit m3) |
The amount of space occupied by a specified mass of substance or by an object. |
| Volume flow rate |
(volumetric flow rate, , ) (Unit m3/s) The volume of a material, usually a fluid or powder, that flows across a surface or through a pipe or other duct per unit time. |
| Volumetric efficiency |
For a piston engine, the ratio of the volume of the induced charge per induction stroke, determined at a reference pressure and temperature, to the swept volume. |
| Waist |
The joint in a robot corresponding to the human waist, i.e. providing rotation about a vertical axis. In an articulated robot, the first joint mounted at the base frame. |
| Wallner lines |
A distinct pattern of intersecting sets of parallel lines, usually producing a set of V-shaped lines, sometimes observed when viewing brittle fracture surfaces at high magnification in an electron microscope. Wallner lines are attributed to interaction between a shock wave and a brittle crack front propagating at high velocity. Sometimes Wallner lines are misinterpreted as fatigue striations. |
| Washer |
An annular disc of metal, rubber, plastic, ceramic, etc., placed between two surfaces in contact either to spread the load (for example, between a surface and a tightened nut or a bolt head), to provide a seal, or to separate or align components. |
| Waste heat |
1. Heat generated by internal-combustion engines, gas turbines, electrical generators, electrical equipment, and industrial processes that is not used directly but is expelled to the environment, often in hot flue or exhaust gases. 2. Heat generated from waste |
| Waste-heat recovery |
The recovery of thermal energy from flue and exhaust gases, or from liquids heated in industrial processes. Recovery devices include pre-heaters, recuperators, regenerators, and waste-heat boilers. |
| Water column |
Water in a tube, which may be vertical or inclined. If the tube is open to the atmosphere, the vertical height h from a datum level to the water surface is a measure of the water static pressure p at the datum level given by p − B = ρgh where B is the barometric pressure, ρ denotes the water density, and g is the acceleration due to gravity. |
| Water hammer |
The reflected pressure surge that occurs in a liquid flowing through a pipe, usually as a consequence of sudden closure of a valve. The surge may cause the pipe to vibrate and a hammering noise to be heard. Key factors affecting the surge amplitude are the compressibility of the liquid (especially if it contains undissolved gas) and the elasticity of the pipe wall. |
| Water jacket |
A casing, typically of sheet metal, surrounding a machine, such as an engine, that requires cooling. Water is circulated through the jacket and a heat exchanger where the heat is removed. |
| Water quenching |
A quench in which water is the quenching medium. The major disadvantage of water quenching is its poor efficiency at the beginning or hot stage of the quenching process. |
| Water-jet cutting |
Cutting of materials such as rock by means of a high-speed jet of water containing abrasive particles. |
| Wave spring |
A type of compression spring, similar to a coil spring, but made of strip shaped into waves around the circumference. |
| Waviness |
Waviness is periodic deviations from geometric surface, often sinusoidal in form and often determined by low-level oscillations of the machine-tool-workpiece system during machining. Typically, wavelengths range from 1 to 10 mm (0.04 to 0.4 in.) and wave heights from a few to several hundred micrometers. |
| Wear |
Damage to a solid surface, generally involving progressive loss of material, due to relative motion between that surface and a contacting surface or substance. |
| Wear |
The deterioration of a component or structure with time and usage, often impairing the function for which it was designed, owing to abrasion, corrosion, fatigue, friction, etc. during relative motion of parts. The wear factor (K), with unit mm3/N.m, is an empirical dimensional factor that quantifies surface wear due to mechanical frictional contact, and defined by K = /Fs where is the worn volume (in mm3), F is the contact load (in N), and s is the sliding distance (in m). |
| Wear rate |
The rate of material removal or dimensional change due to wear per unit of exposure parameter—for example, quantity of material removed(mass, volume, thickness) in unit distance of sliding or unit time. |
| Wedge |
A short triangular prism whose major surfaces subtend an acute angle, that can be driven between two objects or parts of an object to split, tighten, or secure them, or to widen an opening or raise a heavy object. The acute angle gives a high mechanical advantage. |
| Weight |
The force of attraction on a body due to gravity. A body’s weight is the product of its mass and the gravitational field strength at that point. Mass remains constant, but weight depends on the object’s position on the Earth’s surface, decreasing with increasing altitude. |
| Weight (W) (Unit N) |
If g is the acceleration due to gravity, the weight of a mass m is given by W = mg. On Earth, the acceleration due to gravity is approximately 9.81 m/s2, so that the weight of a 1 kg mass is 9.81 N. More generally, the weight is the force exerted on the mass of a body when in the gravitational field of another body. |
| Weld penetration |
It is the depth below the surfaces, as revealed in microstructural sections of joined metals, which is melted during welding. |
| Welding |
In tribology, the bonding between metallic surfaces in direct contact, at any temperature. |
| Welding |
A joining process in which the mating surfaces are at least softened, or more usually melted, unlike soldering and brazing. In solid-state (non-melting) welding, similar or dissimilar metals or thermoplastics may be joined by applying pressure to hot interfaces. In forge welding, joining is achieved by compression across the join between pre-heated workpieces. In friction welding (spin welding) one of the contacting surfaces is rotated rapidly before being loaded against the other. On contact, heat generated by friction softens the materials and permits severe plastic deformation that seals the joint. In friction stir welding, a wear-resistant tool is rotated rapidly, indents the surfaces and is traversed to form a join line. Resistance welding involves simultaneous pressure and electrically-generated heat, as in spot welding where a heavy electric current is passed for a short time through metal sheets which are pressed together between electrodes. |
| Wheel |
A solid disc, or a circular ring with spokes radiating from a central hub, either attached to an axle around which it revolves or which revolves with a rotating axle. |
| Widmanstätten structure |
A structure characterized by a geometrical pattern resulting from the formation of a new phase along certain crystallographic planes of the parent solid solution. The orientation of the lattice in the new phase is related crystallographically to the orientation of the lattice in the parent phase. The structure is readily produced in many alloys by appropriate heat treatment. |
| Width across flats |
A principal dimension of nuts, or of bolt heads. Work hardening The slight increase in hardness and strength produced when a body is loaded past its yield point. Also called strain hardening. |
| Wind energy (Unit kJ) |
The kinetic energy associated with wind that can be converted by a wind energy conversion system (WECS) into electrical or mechanical power by a rotor, such as a multi-bladed propeller, exposed to the wind. If the air density is ρ and the wind speed is V, then the kinetic energy flux is ρV3/2. The actual wind power that can be extracted by a wind turbine intercepting a cross section of wind A is CPρAV3/2 where CP is an empirical efficiency factor termed the power coefficient. The wind-energy distribution is a histogram of the calculated wind power that can be generated annually from the windspeed frequency distribution at a given location. A windmill generates mechanical power whereas a wind turbine (wind generator) generates electrical power. The largest wind turbine, manufactured by MHI Vestas, has a power rating of 9.5 MW, a rotor diameter of 164 m, and a hub height of 105 m (the highest wind turbines have heights in the region of 190 m). A wind farm is an array of wind turbines, typically ten to several hundred, at a single location, either onshore or offshore. Energy derived from wind is a major contributor to sustainable energy generation. The largest operational offshore wind farm, located in the Irish Sea, has 87 turbines and a total generating capacity of 659 MW. The largest onshore wind farm is in Kern County, California with some 586 turbines and a total capacity of 1 550 MW. |
| Wind pressure (Unit Pa) |
The dynamic pressure associated with the wind. The static pressure of the wind is the atmospheric pressure. |
| Wind pump |
A pump driven directly by a windmill on a tower. Widely used in remote locations. |
| Wind tunnel |
A duct in which a controlled flow of air is used for testing and research. Types include blow down, closed circuit, open return, and open section. Closed-circuit tunnels may be pressurized or evacuated. The working section is usually situated immediately downstream of the flow-conditioning elements, which may include a plenum chamber, honeycomb, screens, turbulence grids, and a contraction. Immediately downstream there is usually a diffuser. For basic research the usual requirement is for uniform steady flow in the working section with low swirl and turbulence intensity. For some applications the flow may be density stratified, sheared, unsteady, or of high turbulence intensity. |
| Wind velocity (Unit m/s) |
1. In the Earth’s atmosphere, a vector quantity that quantifies both the magnitude of the wind speed and its direction at a given altitude and location, conveniently represented in the hodograph plane. For convenience, the unit kph is often used. It could in principle also include information about large- and small-scale unsteadiness. Spatial variation in wind velocity, either with altitude or in a horizontal plane, is termed wind shear. 2. The airspeed in the working section of a wind tunnel. |
| Wing |
A relatively long and thin body with a cross section designed to produce lift when there is motion relative to a fluid. The principal applications are to aircraft and highperformance motor vehicles. |
| Wing nut |
A nut having two opposite protruding wings to permit hand tightening. |
| Wire rope |
A type of cable formed from individual strands of wire with a helical twist. |
| Work ratio |
The ratio of the net work out to the actual work out for a thermodynamic cycle. |
| Working fluid |
In thermodynamics, the fluid contained within the boundary of a system that experiences changes in its properties during any fluid dynamic or thermodynamic process. |
| Working load |
The tension in a bolt in use; tension produced by a combination of Residual preload and a portion (usually) of any External load. The Joint diagram is usually used to predict the approximate working load a fastener will see in service. |
| Working load (Unit N) |
The load under which a component or structure is designed to operate under normal conditions. |
| Working pressure (Unit Pa) |
The pressure at which a pressure vessel, such as a boiler, is designed to operate under normal conditions. |
| Working section |
That part of a wind tunnel in which models are placed for testing, or where detailed measurements are performed. |
| Working stress (Unit Pa) |
The stress under which a component or structure is designed to operate under normal conditions. |
| Workspace (Unit m3) |
(working-space volume) The volume around the base frame of a robot, defined by the reach of the robot. The robot can thus only perform tasks within this volume. |
| Wrap-around bend |
The bend obtained when a specimen is wrapped in a closed helix around a cylindrical mandrel. This term is sometimes applied to a semiguided bend of 180° or less. |
| Wrinkling |
A wavy condition obtained in deep drawing of sheet metal, in the area of the metal between the edge of the flange and the draw radius. Wrinkling may also occur in other forming operations when unbalanced compressive forces are set up. |
| Wrist |
The final three joints on a robot which simulate rotations provided by the human wrist and thus allow orientation of the end effector to which it is attached by the wrist socket. |
| Wrist pin |
A stud projecting from a crank as an attachment for a connecting rod. wrought alloys Metal alloys that are initially cast and then shaped into final form by processes such as extrusion, forging, or rolling. |
| Wrought iron |
A highly ductile (but anisotropic) type of iron containing elongated slag fibres that resulted from the method of manufacture in which excess carbon in pig iron was burnt and worked out. The yield strength is some 200 MPa, tensile strength 320 MPa, and reduction of area on a 50-mm gauge length up to 35%. Now replaced by steel. |
| X-ray thickness gauge |
A device comprising an X-ray source and a detector used to determine the thickness of material in sheet or plate form, including metals, paper, plastics, rubber, and ceramics. |
| X–Y recorder |
A device that plots a graph of a variable y against a variable x, both in the form of voltages. |
| Yield point |
The first stress in a material, usually less than the maximum attainable stress, at which an increase in strain occurs without an increase in stress. Only certain metals—those which exhibit a localized, heterogeneous type of transition from elastic to plastic deformation—produce a yield point. If there is a decrease in stress after yielding, a distinction may be made between upper and lower yield points. The load at which a sudden drop in the flow curve occurs is called the upper yield point. The constant load shown on the flow curve is the lower yield point. |
| Yield point (Unit Pa) |
(yield strength, yield stress, Rel, Rel) The stress at which the onset of permanent (plastic) deformation (yielding) occurs in a body under increasing loading. In some materials, a yield-point phenomenon occurs in which stress falls on initial yielding and plastic deformation continues without increase in stress as strain increases further. The yield criterion is the combination of normal and shear stresses which produces yielding under multiaxial loading. |
| Yield strength |
That stress level which will create a permanent deformation of 0.2% or 0.5% or some other small, preselected, amount in a body. Approximately equal to the elastic and proportional limits of the material; a little higher than the proof strength of a bolt. |
| Yield strength |
The stress at which a material exhibits a specified deviation from proportionality of stress and strain. An offset of 0.2% is used for many metals. |
| Yield stress |
The stress level of highly ductile materials, such as structural steels, at which large strains take place without further increase in stress. |
| Yielding |
Evidence of plastic deformation in structural materials. Also known as plastic flow or creep. |
| Yield-point elongation |
During discontinuous yielding, the amount of strain measured from the onset of yielding to the beginning of strain hardening. |
| Young’s modulus |
A term used synonymously with modulus of elasticity. The ratio of tensile or compressive stresses to the resulting strain. |
| Zero initial conditions |
The assumption normally made in control engineering that up to a start time defined as t = 0, all variables have zero value. |
| Zero time |
The time when the given loading or constraint conditions are initially obtained in creep or stressrelaxation tests, respectively. |
| Zero-gravity |
The situation in which there is no force of gravity (i.e. weightlessness). |
| Zirconia-toughened alumina composites |
(ZTA composites) Ceramic composites based on aluminium oxide to which zirconium oxide is added, resulting in improved fracture toughness. |
| Zone melting (zone refining) |
A process of purification of materials in which a narrow molten zone is moved along the length of the material, resulting in impurities being segregated at one end. |
