RAC Horsepower Rating - An obsolete engine horsepower rating system that was used as a designation as to the power of
an engine.

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
gamma radiation

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
across it.

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.
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