RAC Horsepower Rating - An obsolete engine horsepower rating system that was used as a designation as to the power of
Racking - The distortion of a rectangular shape to a skewed parallelogram.
Radial - A direction perpendicular to the centreline of a shaft.
Radial Engine - An engine with a number of cylinder arranged in a circle around the crankshaft centre line. A design often
used for aircraft engines.
Radial Force Variation - Radial Force Variation is a dynamic force generated at the wheel due to tyre non-uniformity.
Radiant Energy - The electromagnetic energy emitted by an object due to its temperature.
Radiated Noise - The dissemination of sound energy as acoustic waves from a source (e.g. a vibrating panel, running engine
Radioactivity - Spontaneous decay of some isotopes in nuclei.
Radiography - Nondestructive method of internal examination in which metal objects are exposed to a beam of X-ray or
Rainflow Analysis - A method of representing a random signal by various numbers of sinusoids that would produce the same
amount of fatigue damage.
Ramjet - A jet engine with no mechanical compressor, consisting of specially shaped tubes or ducts open at both ends.
Random Vibration - A vibration whose instantaneous magnitude is not specified for any given instant of time.
Rankine (R) - The absolute temperature scale in degrees Fahrenheit
Rankine Cycle - The thermodynamic cycle that is an ideal standard for comparing performance of heat-engines, steam power
plants, steam turbines, and heat pump systems that use a condensible vapor as the working fluid; efficiency is measured as
work done divided by sensible heat supplied.
Rankine Vortex - An idealized vortex in unbounded fluid with uniform vorticity inside a circular patch and zero vorticity outside.
Rankine-Hugoniot Shock Relations - These are solutions for a steady, normal shock in the frame moving with the shock front.
Rasp - A sort of blowing noise emitted often from the exhaust pipes of internal combustion engines, usually, it is dominated
by a mixture of 4E, 6E and 8E engine orders on 4 cylinder engines.
Rayleigh′s Method - A method used for calculating approximate natural frequencies for a vibrating system assuming a
deflected shape and balancing kinetic and strain energies.
Reaction - A force exerted by a support on an object: sometimes called support reaction.
Reactive Absorber - A sound absorber, such as the Helmholtz resonator which involves the effects of mass and compliance as
well as resistance.
Reaeration - The dissolving of molecular oxygen from the atmosphere into the water
Real Modes - In a real mode, all points on the structure reach a maximum or a minimum value at the same time and all pass
through equilibrium at the same time.
Receptance - Displacement per unit force.
Reciprocating Compressor - Machine in which the compressing element is a piston having a reciprocating motion in a cylinder.
Recrystallization - The formation of a new set of strain-free grains within a previously cold-worked material; normally an
annealing heat treatment is necessary.
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.
Refractive Index - The ratio of the velocity of light in a vacuum to the velocity of light in some medium.
Refractory - A metal or ceramic that may be exposed to extremely high temperatures without deteriorating rapidly or without
Relay - An electromagnetic switching device consisting of an armature which is moved by an electromagnet to operate one or
more switch contacts.
Relief Valve - A valve held closed by a spring and forced open when the pressure in the system rises above the desired limit.
Renewable Energy - Energy obtained from sources that are essentially inexhaustible, unlike fossil fuels.
Repeatability - The consistency (or variation) of readings and results between consecutive sets of measurements.
Resistance - Opposition to the flow of electric current, measured in ohms.
Resistance Brazing - Bonding is produced by the heat obtained from resistance to the flow of electric current in a circuit of
which the workpiece is a part.
Resistance Temperature - The characteristic change in a sensor's resistance when exposed to a change in temperature.
Resistance Temperature Detector (RTD) -- A sensor that uses the resistance temperature characteristic to measure
Resistivity - The parameter of a material that determines the relationship between the current through it and the current
Reverse Osmosis - A technique used in desalination and water treatment; pressure is applied to the surface of a saline (or
waste) solution, forcing pure water to pass from the solution through a membrane (hollow fibers of cellulose acetate or
nylon) that will not pass sodium or chloride ions.
Revolutions per Minute - Unit to describe the rate of rotation of a body.
Revolutions per Second - Unit to describe the rate of rotation of a body.
Reyn - British unit of dynamic viscosity.
Reynolds Number - Non-dimensional number (consistent units) which describes whether fluid flow is laminar, turbulent, or
transitional between the two.
RFI - Request for Information, Contractor-initiated question concerning plans and/or specifications.
RFP - Request for Proposal, Announcement of a willingness to consider proposals for the performance of a project or program
RFQ - Request for Quotations or Request for Qualifications
Right Hand Thread - A screw thread that is screwed in by rotating clockwise. Most threads are right handed.
Rigid - An idealized concept meaning something which does not deform under loading.
Rigid Connection - In two dimensions, a fixed connection between two members restrains all three degrees of freedom of the
connected member with respect to one another.
Ring Grooves - Grooves cut in the piston barrel to hold the piston rings.
Ring Spanner - Each end of the spanner fits around the bolt head or nut.
Ripping Hammer - A hammer which looks like a claw hammer, but the claw is straight.
Riveting Hammer - A hammer with a special head for securing rivets.
Rivets - A permanent fixing used to join plates. The unheaded end is forged or flattened.
Rocker Arm - A lever that transmits the action of the cam, usually by means of a push rod, to the stem of the intake or
exhaust valve, sometimes also to the starting-air valve and fuel valve.
Rockwell Hardness - Hardness determined from size of indentation using a diamond or steel ball and given load.
Roentgen - A unit of gamma radiation measured by the amount of ionization in air.
Rolled Thread - A thread formed by plastically deforming a blank rather than by cutting.
Roller Support - In two dimensions, a roller support restrains one translation degree of freedom.
Rotary Engine - The crankshaft is fixed and the crankcase, pistons and cylinders rotate with the propeller.
Rotation - Motion of an object where the path of every point is a circle or circular arc.
rpm - Revolutions per minute.
rps - Abbreviation of revolutions per second.
Rubber Mallet - A hammer with a rubber head.
Rupture - Failure that is accompanied by significant plastic deformation.
Engineering Glossary, Engineering Dictionary R
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