Machines with moving parts must have some way to guide their motions in turning, sliding or rolling. Structures that enable these motions are often called bearings.

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.

Many bearings also facilitate the desired motion as much as possible, such as by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or to the directions of the loads (forces) applied to the parts.

Types of Bearing

Bearings are classified into the following two basic types.

1. Sliding contact bearings
2. Rolling contact bearings