A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races. The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. Ball bearings are used to ensure smooth, efficient operation in many machines with rotary motion, from car wheels, engines, and turbines to medical equipment. They carry loads, reduce friction, and facilitate motion. Ball bearings use balls to separate two “races,” or bearing rings, to reduce surface contact and friction across moving planes. The rotation of the balls causes a reduced coefficient of friction when compared with flat surfaces rubbing against each other. Because there is little surface contact between the balls and races, ball bearings typically have a lower load capacity for their size than other rolling-element bearings.

There are several common designs of ball bearings, each offering various performance trade-offs. They can be made from many different materials, including stainless steel, chrome steel, and ceramic (silicon nitride, Si3N4). A hybrid ball bearing is a bearing with ceramic balls and metal races. Some common designs of ball bearings include radial ball bearings, angular contact ball bearings, and self-aligning ball bearings. Ball bearing sizes vary according to their use, and the width of the bearing also depends on the application.

In general, ball bearings are used in most applications that involve moving parts. Some specific applications include skateboards, bicycles, automobiles, and medical equipment.