A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.

By far, the most common bearing is the plain bearing, a bearing that uses surfaces in rubbing contact, often with a lubricant such as oil or graphite. A plain bearing may or may not be a discrete device. It may be nothing more than the bearing surface of a hole with a shaft passing through it, or of a planar surface that bears another (in these cases, not a discrete device); or it may be a layer of bearing metal either fused to the substrate (semi-discrete) or in the form of a separable sleeve (discrete). With suitable lubrication, plain bearings often give entirely acceptable accuracy, life, and friction at a minimal cost. Therefore, they are very widely used.

However, there are many applications where a more suitable bearing can improve efficiency, accuracy, service intervals, reliability, speed of operation, size, weight, and costs of purchasing and operating machinery.

Thus, there are many types of bearings, with varying shapes, materials, lubrication, the principle of operation, and so on.

There are at least 6 common types of bearing,^[14] each of which operates on different principles:

• Plain bearing, consisting of a shaft rotating in a hole. There are several specific styles: bushing, journal bearing, sleeve bearing, rifle bearing, composite bearing;

• Rolling-element bearing, in which rolling elements placed between the turning and stationary races prevent sliding friction. There are two main types:

• Ball bearing, in which the rolling elements are spherical balls;

• Roller bearing, in which the rolling elements are cylindrical, taper, or spherical rollers;

• Jewel bearing, a plain bearing in which one of the bearing surfaces is made of an ultrahard glassy jewel material such as sapphire to reduce friction and wear;

• Fluid bearing, a noncontact bearing in which the load is supported by a gas or liquid (i.e. air bearing);

• Magnetic bearing, in which the load is supported by a magnetic field;

• Flexure bearing, in which the motion is supported by a load element that bends.