R&O oil: oils with rust and oxidation inhibitors, usually applied to highly refined circulating oils used for long term service, as in compressors, hydraulic systems and turbines.

Rack (gears): a gear with teeth spaced along a straight line and suitable for straight line motion.

Rack and pinion gear (gears): a power-transmitting unit that changes linear or reciprocal movement to rotary motion when the rack gear is the driving member and vice versa when the pinion gear is the driving member.

Radial load bearing: bearing in which the load acts in a radial direction with respect to the axis of rotation.

Rag layer: the layer that forms at the interface when oil and water are separated by gravity. A mixture of solids oil and water, it looks like sludge.

Rancidity: biodegradation of fats or fluids causing an unpleasant odor.

Rapeseed oil: see vegetable oil.

RBOT: see bomb oxidation stability.

RCRA: Resource Conservation and Recovery Act.

Reciprocating pump: device designed to pump with a reciprocating motion, similar to an engine piston.

Refractive index: (ASTM D-1218) number indicating the angle through which a ray of light is deflected as it passes through a solid or fluid medium; the number that expresses the ratio of the sine of the angle of incidence to the sine of the angle of refraction. Typically used for monitoring water-based fluids or solutions.

Relief valve: a hydraulic mechanism designed to limit or control pressure by opening an auxiliary fluid passage at a predetermined or set pressure.

Re-refined oils: spent mineral oils that have been collected, sent to a refinery and reprocessed by distillation, often with hydrofinishing.

Residual oil: oil from the bottom of the distillation column (see bottoms).

Resin: a solid or liquid compounding material, generally a solid or semi-solid, composed of carbon, hydrogen and oxygen; includes polyesters, polystyrenes and acrylics used in the manufacture of varnishes, plastics and elastomers. Some lubricating resins are residual oils.

Reyn: standard unit of absolute viscosity in the English system, expressed in lbs/ in2. (see viscosity).

Rheology: the study of the deformation and/or flow of matter in terms of stress, strain, temperature and time. Apparent viscosity and penetration of grease are examples of rheological properties.

Rheopectic: term applied to greases that thicken and harden when subjected to shear; the opposite of thixotropic.

Ring lubrication: see oil ring lubrication.

RMA: Rubber Manufacturers Association, Inc.

Rolling element/rolling contact/anti-friction bearing: generic terms for all types of rolling bearings (ball, cylindrical roller, tapered roller, spherical convex roller, spherical concave roller and needle roller), all of which roll between rings or races except for needle bearings.

Roller bearing: describes all rolling bearings except for ball bearings (see rolling element et al).

Rolling oil: hot and cold rolling mills, especially cold mills, use formulated coolants.

Rotary bomb oxidation test (RBOT): see bomb oxidation stability.

Rotor: part that rotates in the pumping chamber, sometimes given specific designation such as gear, screw, impeller, etc.

Rubbing oil: low-viscosity mineral oil used with or without an abrasive as a polishing medium.

Rust preventive: a compound containing a rust inhibitor, used to coat metal surfaces to prevent rust and corrosion; base material maybe a petroleum oil, a wax, an asphalt and/or solvent, depending on the environment and the duration of the protection sought.

Rust test (grease/oils): (ASTM D-665 and D-1748) test that measures the effectiveness of a lubricant at preventing the rusting of ferrous parts in the presence of water.