Leonardo da Vinci sketched a ball bearing as a mechanical arrangement using rolling balls to reduce friction. His design placed balls between moving surfaces so that rolling replaced much of the sliding contact. The idea is an early documented form of the principle used by modern rolling-element bearings.
Leonardo studied the design around 1498 while investigating friction and motion. His notes show an effort to keep the balls separated so they would not bunch together and prevent smooth movement. This reflects a practical understanding of a problem that affects machines with rotating shafts.
Leonardo was not the first person to imagine every form of rolling support, and modern bearings use carefully manufactured races, cages, lubricants, and materials. The historical importance of his sketch is that it clearly records a bearing concept centuries before industrial production made precision bearings common.