What low-friction device did Leonardo da Vinci sketch using freely moving balls?

Leonardo da Vinci sketched a ball bearing using freely moving balls to reduce friction between surfaces.

The drawing appears in Leonardo’s studies of friction and mechanical motion. It shows balls positioned so that one moving component can roll against another rather than sliding directly across it. Rolling contact generally produces less friction than sliding contact, making the arrangement useful for rotating machinery.

Leonardo did not create the modern standardized ball bearing used in engines and machines. His sketch was an early conceptual exploration, and it lacked the cages, precise manufacture, lubrication systems, and standardized races found in later industrial bearings.

The principle remains central to modern engineering. Ball bearings support loads while allowing shafts, wheels, and other parts to rotate efficiently. Leonardo’s study is often cited in histories of bearings because it clearly shows the rolling-element idea, even though practical mass production and widespread use came much later.