Leonardo da Vinci studied a ball bearing using a cage of loose spheres to reduce friction between moving parts.
His drawings from the late fifteenth century show balls arranged so they would not bunch together while supporting rotational movement. The key idea was to replace sliding contact with rolling contact, reducing friction and making machinery easier to turn. This is the same basic principle used by modern ball bearings.
Leonardo did not invent every form of bearing, and simple rolling supports existed before his work. His importance is that he clearly explored a caged arrangement resembling the separator used in later practical ball bearings. The design appears in manuscripts associated with his engineering studies, including material from around 1498.
Modern ball bearings use carefully manufactured steel, ceramic, or other materials and are made to precise tolerances. Leonardo’s drawing was a conceptual mechanical study rather than evidence of a mass-produced Renaissance bearing industry.