Leonardo da Vinci drew an early design for a ball bearing using rolling balls between surfaces.
The purpose of a ball bearing is to reduce friction by replacing sliding contact with rolling contact. Leonardo's sketches from the late fifteenth century show balls arranged in a way that would keep them separated while allowing a shaft or ring to move. This arrangement anticipates the basic principle of modern rolling-element bearings.
Leonardo's design was not a complete modern bearing in every engineering detail. Practical bearings require accurate manufacturing, suitable materials, lubrication, and ways to keep the rolling elements aligned. Those capabilities developed gradually over later centuries as machinery became more precise.
The idea is sometimes credited to Leonardo as though he invented the modern industrial ball bearing. A more precise statement is that he recorded an early bearing concept. The design demonstrates his interest in reducing friction in machines, alongside his work on gears, screws, and mechanical transmissions.