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 prevention test (turbine oils)

A test for determining the ability of an oil to aid in preventing the rusting of ferrous parts in the presence of water.