Bearing, device to minimize friction and provide support and guidance for the moving parts of a machine. There are 2 main types of bearings—plain or journal bearings, and ball or roller bearings. In the plain bearing, a sheath lined with a special metal is clamped around a turning or sliding axle or journal. Plain bearings are used in engines and industrial machines. In roller or ball bearings, small round balls or rollers are placed between the journal and the housing of the bearing case. Contact is made only at points (ball bearings) or along thin lines (roller bearings), thus reducing friction to a minimum. Ball bearings are used mainly with revolving axles such as those of cars. Roller bearings are used to carry heavy loads at relatively slow speeds. Bearings were first used in Egypt to move blocks of stone from quarries to build palaces and pyramids.