Leonardo da Vinci drew a ball bearing using continuously rolling spheres to reduce friction.
His bearing concept appears in studies from the late fifteenth century, including sketches associated with the Codex Madrid I. The design placed balls between two rings so that one surface could rotate relative to the other. Because the balls roll instead of allowing the surfaces to slide directly against each other, the arrangement can reduce friction.
Leonardo did not invent every principle behind rolling motion, and his drawing was not a modern standardized bearing. The balls in the sketch could crowd together and rub against one another, limiting efficiency. Later bearing designs added cages and improved materials to keep the rolling elements separated and aligned.
The invention is often credited to Leonardo because his notebooks contain an especially clear early design. Practical ball bearings became important much later in clocks, machinery, bicycles, and engines. The key distinction is between a Renaissance sketch of the principle and the precision-manufactured bearings used in modern machines.