Ball bearing

Ball bearings are small metal balls used in mechanical devices to reduce friction. Often they are used between wheels and axles in motor vehicles to convert sliding friction into rolling friction and greatly improve efficiency and longevity of the system. Some of the old Nikon film cameras had ball bearings in the film advance mechanisms for a smooth feel as the lever was pulled.

Ball bearings are difficult to manufacture and were strategically important during World War II, then the Allies bombed German ball bearing factories.