Leonardo da Vinci sketched a ball bearing using freely rotating balls between surfaces.
A ball bearing reduces friction by allowing one surface to roll over another instead of sliding across it. Leonardo’s notebooks from the late 15th century include a design in which balls are separated so they do not jam together. This is an early example of the principle behind modern rolling-element bearings.
Leonardo was interested in friction because it limited the efficiency of machines. His bearing concept could help a rotating shaft move more easily, although it was not a complete industrial bearing system in the modern sense.
Modern ball bearings use carefully manufactured steel, ceramic, or other balls, races, cages, lubrication, and precise tolerances. Leonardo did not invent every part of that later technology, but his sketch shows that he understood the central mechanical advantage of rolling contact.