What device did Leonardo da Vinci sketch using five balls between two rings?

Leonardo da Vinci sketched a ball bearing using five balls between two rings.

Around 1498, Leonardo drew a bearing arrangement in which balls sat between an inner and outer ring. The balls were intended to reduce rubbing contact and allow one part to rotate more freely against another. His design also tried to prevent the balls from touching each other, an important issue in bearing operation.

Leonardo did not invent the entire modern bearing industry, and his sketch was not a mass-produced standardized component. Nevertheless, it is an early documented design for a rolling-element bearing and demonstrates his interest in reducing friction in machines.

Modern ball bearings use carefully shaped races, multiple balls, cages, seals, and lubricants. The familiar bearing used in bicycles, motors, and industrial machinery developed through later engineering advances, especially during the nineteenth and twentieth centuries.