Leonardo da Vinci studied a ball bearing as a mechanical component that uses freely rotating balls to reduce friction.
His notebooks contain drawings of bearings in which balls are arranged so they can roll between surfaces instead of allowing those surfaces to slide directly against each other. Rolling contact generally produces less friction than sliding contact, making the principle useful for rotating shafts and machinery.
Leonardo’s sketches are often cited as an early development in bearing design, but he did not create the complete standardized bearing industry known today. His drawings show an understanding of how separators or arrangements could keep balls from bunching together, a practical issue that remains important in bearing construction.
Modern ball bearings use carefully manufactured races, balls, cages, seals, and lubricants. Those components developed through later engineering and industrial manufacturing. The Renaissance drawings should therefore be described as an early study or concept, not as a modern mass-produced bearing. The key idea is the substitution of rolling motion for sliding friction.