Leonardo da Vinci sketched a ball bearing using balls inside a cage to reduce friction between moving surfaces.
His design appears in studies associated with the Codex Madrid I, written during the late 15th century. The cage was meant to keep the balls separated so they would not rub against one another and impede motion.
The principle is the same basic idea used in modern rolling-element bearings: rolling contact generally produces less friction than sliding contact. Leonardo’s sketch did not represent a complete industrial bearing made to modern manufacturing standards.
Ball bearings became widely useful only after later engineers developed accurate production methods and suitable materials. Leonardo’s drawing is important because it shows an early understanding of a key mechanical solution.