What rolling machine component did Leonardo da Vinci sketch to reduce friction?

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

Around 1498, Leonardo drew mechanisms containing balls arranged between rings. The balls were intended to roll rather than slide, reducing friction when one part turned against another. This principle is the foundation of modern ball bearings.

Leonardo’s sketch is often cited as an early bearing design, but he did not create the standardized, sealed bearings used in modern machinery. His notes also recognized a practical problem: the balls could crowd together and touch, preventing smooth operation. He explored cage-like arrangements to keep them separated.

The invention is sometimes described as Leonardo’s “ball bearing,” although similar rolling-bearing ideas existed before him. His importance lies in clearly documenting a workable mechanical concept and identifying problems that later engineers would solve.