Leonardo da Vinci studied the ball bearing as a way to reduce friction between moving surfaces.
His sketches from the late fifteenth century show balls arranged so that one surface could move over another with rolling contact instead of sliding contact. Rolling generally reduces friction, although real bearings also face problems such as wear, lubrication, deformation, and keeping the balls separated.
Leonardo’s study is often described as an early ball-bearing design, not necessarily the first bearing in history. Ancient and medieval machines used other forms of bearings, and modern ball bearings require carefully made races, hardened materials, cages, and lubrication.
The drawing is significant because it shows Leonardo identifying a practical mechanical problem and separating the rolling elements so they would not bunch together. It does not prove that he produced a standardized industrial bearing. Factory-made precision bearings became common much later, with industrial manufacturing and modern metallurgy.