The simple machine Leonardo da Vinci studied and popularized through designs using rotating balls between surfaces was the ball bearing.
A ball bearing reduces friction by placing small spherical elements between two surfaces that move relative to one another. Instead of sliding directly, the surfaces roll against the balls. Leonardo’s notebooks include designs showing balls arranged so they would not rub together and lock the mechanism.
Leonardo did not invent every earlier form of rolling support. Ancient and medieval engineers had already used rollers and related devices. His importance lies in clearly studying the principle and developing practical arrangements for machine mechanisms.
Modern ball bearings use carefully manufactured races, cages, lubrication, and hardened materials. Leonardo’s sketches were conceptual and did not have the precision or materials of industrial bearings, but they anticipated a key idea in mechanical engineering.