Rate of shear -- the difference between the velocities along the parallel faces of a fluid element divided by the distance between the faces.

Reducer -- a connector having a smaller line size at one end than the other.

Refraction -- the change of direction or speed of light as it passes from one medium to another.

Rerefining -- a process of reclaiming used lubricant oils and restoring them to a condition similar to that of virgin stocks by filtration, clay adsorption or more elaborate methods.

Reservoir -- a container for storage of liquid in a fluid power system.

Reservoir (sump) filter - a filter installed in a reservoir in series with a suction or return line.

Residual dirt capacity -- the dirt capacity remaining in a service loaded filter element after use, but before cleaning, measured under the same conditions as the dirt capacity of a new filter element.

Return line -- a location in a line conducting fluid from working device to reservoir.

Return Line Filtration -- filters located upstream of the reservoir but after fluid has passed through the system's output components (cylinders, motors, etc.).

Ring lubrication -- a system of lubrication in which the lubricant is supplied to the bearing by an oil ring.

Rings -- circular metallic elements that ride in the grooves of a piston and provide compression sealing during combustion. Also used to spread oil for lubrication.

Ring sticking -- freezing of a piston ring in its groove in a piston engine or reciprocating compressor due to heavy deposits in the piston ring zone.

Roll-off cleanliness -- the fluid system contamination level at the time of release from an assembly or overhaul line. Fluid system life can be shortened significantly by full-load operation under a high fluid contamination condition for just a few hours. Contaminant implanted and generated during the break-in period can devastate critical components unless removed under controlled operating and high performance filtering conditions.

Roller bearing -- an antifriction bearing comprising rolling elements in the form of rollers.

Rust – a corrosion product consisting of hydrated oxides of iron.

Rust prevention test -- a test for determining the ability of an oil to aid in preventing the rusting of ferrous parts in the presence of water.