Rolling-element bearing

In mechanical engineering, a rolling-element bearing, also called a rolling bearing, is a bearing that carries a load using rolling parts (such as balls, cylinders, or cones) between two grooved rings called races.^[1] The relative motion of the races causes the parts to roll with very little rolling resistance and sliding compared to plain bearings.^[2]

A common kind of rolling-element bearing is the ball bearing. Ball bearings have inner and outer races between which balls roll.^[3] Each race has a groove, usually shaped so the ball fits and is slightly loose. Ideally, the ball touches the races across a very narrow area. However, a load on an very small point would cause very high pressure. In practice, the ball deforms (flattens) slightly where it touches each race, like how a tire flattens where it touches the road, causing friction and drag.

Roller bearings are the earliest known type of rolling-element bearing.^[4] Common roller bearings use cylinders of slightly larger length than their diameter.