Leonardo da Vinci depicted a ball bearing to reduce friction between moving parts.
In his mechanical studies, Leonardo showed balls positioned between rings so that one surface could move relative to another with less sliding friction. The balls roll, rather than scrape, and their motion helps separate the two main surfaces. This is the basic principle behind modern rolling-element bearings.
Leonardo's design is often cited as an early ball-bearing concept, but it was not a modern standardized bearing. His sketches do not provide the complete manufacturing tolerances, cages, seals, and hardened materials used in later industrial bearings. The idea was valuable because it identified a practical way to reduce friction in machinery.
Ball bearings later became important in clocks, bicycles, engines, and industrial equipment. Leonardo's drawing belongs to his wider investigation of gears, axles, pulleys, and perpetual-motion claims, many of which focused on understanding and controlling friction.