| 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. |
