Bearing Lubrication

Nov. 15, 2002
Instrument bearings can be lubricated with oil, grease, or dry films.

Instrument bearings can be lubricated with oil, grease, or dry films.

Oils: Synthetics are used most widely for instrument bearings. Petroleums also are used for their excellent lubrication properties under heavy load and high-speed conditions.

Silicones are not true oils, but are listed as such for convenience. Principal advantages are excellent viscosity-temperature characteristics, good resistance to oxidation, and wide operating-temperature range.

Standard method for applying oil lubrication is to dip the bearing in the oil, remove, and centrifuge excess oil from the bearing.

For extremely critical torque applications, oil quantity can be reduced to a minimum by centrifuging at high levels (200 to 500 g) for extended periods of time (2 to 10 min). However, reduction in oil quantity can have substantial effects on bearing life and its use should be carefully considered. For instrument bearings with phenolic, sintered nylon, or porous polyimide retainers, oil is impregnated into the retainer to provide a constant source of lubrication during operation.

Greases: Grease lubrication is usually chosen for moderate to high speeds and light to heavy loads. Often the high-speed torque of a grease-lubricated bearing will be lower than for an oil-lubricated bearing. However, the slow-speed running torque for grease-lubricated bearings is about 15 to 40 times that of oil-lubricated bearings. Specific torque values depend upon the type and quantity of grease.

Standard method of grease application is the grease pack. The bearing is loaded to about one-third full. Other fill quantities can be supplied if required.

A unique process of applying a thin film of grease to all bearing surfaces is by grease plating. The bearing is coated with a mixture of grease and solvents. The solvent is removed by heating, leaving a thin film of grease on all surfaces. Although there are limitations to this method, plating gives lower torque than a grease pack and retains the lubricant on bearing surfaces better than oil lubrication.

Dry-film lubricants: Lubrication by dry film has not been successful in instrument bearings and should be attempted only where conditions prohibit any other type of lubricant. Principal problems encountered are proper treatment of the surface material, control of film thickness, and wear products which are normally generated during bearing operation. These wear products contaminate the bearing. Nominal film thickness normally required for instrument bearings is on the order of 0.0001 to 0.0003 in., with variations not exceeding 5 to 10 <\#181>in.

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