bearing

Three types of bearing. The roller and the ball bearing are similar, differing only in the shape of the parts that roll when the middle shaft turns. The simpler journal bearing consists of a sleeve, or journal, lining the surface of the rotating shaft. The bearing is lubricated to reduce friction and wear.

Device used in a machine to allow free movement between two parts, typically the rotation of a shaft in a housing. Ball bearings consist of two rings, one fixed to a housing, one to the rotating shaft. Between them is a set, or race, of steel balls. They are widely used to support shafts, as in the spindle in the hub of a bicycle wheel.

The sleeve, or journal bearing, is the simplest bearing. It is a hollow cylinder, split into two halves. It is used for the big-end and main bearings on a car crankshaft.

In some machinery the balls of ball bearings are replaced by cylindrical rollers or thinner needle bearings.

In precision equipment such as watches and aircraft instruments, bearings may be made from material such as ruby and are known as jewel bearings.

For some applications bearings made from nylon and other plastics are used. They need no lubrication because their surfaces are naturally waxy.