(R+M)/2
Research Octane Number plus Motor Octane Number, divided by 2. Used now as general measure of road octanes of gasoline.

R&O
Rust and oxidation-inhibited

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

Reclaimed Oil
A lubricating oil which, after undergoing a period of service is collected, reprocessed, and sold for reuse.

Red Oil
The term is now used to describe any oil of red color, regardless of refining process.

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

Redwood Viscometer
Standard British viscometer. The number of seconds required for 50 ml of an oil to flow out of a standard Redwood viscometer at a definite temperature (IP Method 70). Instrument is available in two sizes: Redwood No. I and No. II. When the flow time exceeds 2,000 sec, the No. II must be used.

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

Reid Vapor Pressure
An important test for gasolines. It is a measure of the vapor pressure of a sample at 100°F, and the test is commonly made in a bomb. The results are reported in pounds (ASTM Method D 323).

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.

Research Method- Research Octane Number (RON)
A test for determining the knock rating, in terms of ASTM Research octane numbers, of fuels for use in spark-ignition engines. The knocking tendency of the fuel is compared with those for blends of reference fuels of known octane number when run in the ASTM-CRF engine at 600 rpm under standard operating conditions (ASTM Methods D908 and D 1656).

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.

Road Octane Number
A numerical value based upon the relative anti-knock performance in an automobile of a test gasoline as compared with specified reference fuels. Road octanes are determined by operating a car over a stretch of road or on a chassis dynamometer under conditions simulating those encountered on the highway.

Roll-off cleanliness
The fluid system contam-ination 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.

RSI
Registrations Systems Inc.

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.


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