Leonardo da Vinci designed a ball bearing to create continuous rotation between two moving surfaces.
His notebooks contain sketches of balls placed between rings or tracks. The balls were intended to reduce sliding friction by replacing it with rolling contact. Leonardo also recognized a practical problem that affects bearings: if the balls touch one another, they can interfere with smooth movement, so his designs included ways to keep them separated.
The surviving drawings do not prove that Leonardo manufactured a modern standardized bearing. They do show an early, clear understanding of the mechanical value of rolling elements. Ball bearings later became essential in machinery, vehicles, clocks, electric motors, and many other systems.
Leonardo is sometimes called the inventor of the ball bearing, but rolling devices existed before his notebooks. The most precise claim is that he produced an important early design and analysis of a ball-bearing mechanism, not that he introduced the entire principle to history.