Leonardo da Vinci studied a ball-bearing arrangement using balls to reduce friction between moving surfaces.
His notebooks include a design in which balls move within a track, allowing one ring or surface to rotate more freely against another. The principle is the same basic idea used by modern rolling-element bearings: rolling contact generally produces less sliding friction than direct rubbing.
Leonardo’s study dates to the late 15th century, but he did not invent the complete modern ball bearing as an industrial product. His design lacked the standardized manufacturing, cages, seals, hardened steel, and precise tolerances later needed for reliable machinery.
The invention is sometimes overstated as proof that Leonardo created modern bearings. A more accurate claim is that he clearly explored an early ball-bearing concept. Similar friction-reducing devices existed in other forms before modern bearing engineering became practical during the Industrial Revolution.