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.