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Rolling and plain bearings

Rolling bearings (e.g. ball bearings) have rolling elements between the rings and give low friction even at start-up. Plain (sliding) bearings have a lubricant film between the shaft and a fixed bushing; they need speed to build up the film, but can be very quiet and robust at high speeds and heavy loads.

Rolling: low friction, even at rest\text{Rolling: low friction, even at rest}ball or roller bearing
Plain: needs speed for the film\text{Plain: needs speed for the film}hydrodynamic lubrication

Symbols

μ\mufriction coefficient in the bearing

Example

A rolling bearing starts easily from rest,

while a plain bearing can have metal contact and high friction right at start-up.

Choose plain bearings for very high speeds and shock loading, rolling bearings where start-up without a film is common.
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