Leonardo da Vinci illustrated a ball bearing to reduce friction between moving surfaces.
His studies of bearings appear in notes and drawings dating to the late fifteenth century. One design places balls between rings so that one surface can rotate against another with less sliding friction. The balls roll, rather than scrape, between the surfaces.
Leonardo was not the first person to use rolling elements in every possible sense, and simple rollers had been known earlier. His importance lies in clearly recording a practical ball-bearing arrangement and considering how the balls could remain separated while a mechanism moved.
Modern ball bearings use carefully manufactured races, cages, seals, and hardened balls. Leonardo’s sketches were experimental mechanical studies, not a complete industrial bearing specification, but they show his understanding of friction and rotational motion.