Leonardo da Vinci sketched an early ball bearing as a machine element for reducing friction.
His studies of bearings appear in notebooks from the late 15th century. The concept used balls between moving surfaces so that rolling replaced much of the sliding friction. Leonardo also explored ways to keep the balls separated, an important issue because balls that touch can increase friction and interfere with smooth motion.
The sketches show Leonardo analyzing mechanical motion rather than merely drawing an isolated object. His ideas were not the same as a modern mass-produced bearing, which uses carefully manufactured races, cages, and materials. Industrial bearing technology developed much later as precision manufacturing improved.
Leonardo is often credited with inventing the ball bearing, but that claim needs qualification. Earlier rolling devices existed, and his drawings represent an important early study of the principle rather than proof that he produced the first practical bearing.