Bearings are mechanical devices, which are designed to reduce friction between two objects allowing one to move while the other is stationary or moving independently of the other object. Bearings are useful to maintain motion, reduce friction, and relieve mechanical forces otherwise present at the interface of two moving parts.

Bearings are designed to support a rotational movement around a fixed axis, however bearing is not only confined to this particular application although it is very common. A popular type of bearing used is called a ball bearing, where two metal rings are placed together, one with a larger diameter than the other. Where the space exists between the two rings, small round balls, usually made out of a hard metal, are inserted into that free space.  This allows the inner ring to spin or rotate about its axis freely while the outer ring can either maintain a fixed location or even rotate a different rate or even in a different direction than that of the inner ring.  By inserting the balls in between the two rings, the balls themselves rotate about a non-fixed axis — allowing the second ring to act independently from the first ring.