What mechanical part did Leonardo da Vinci study as a way to reduce friction?

Leonardo da Vinci studied the ball bearing as a mechanical way to reduce friction between moving surfaces.

His sketches from around 1498 show balls arranged between rings so that one part could rotate relative to another. The rolling balls separated the surfaces and reduced sliding friction. This idea is closely related to the principle used in modern rolling-element bearings.

Leonardo’s design is often described as an early ball-bearing concept, but it was not a complete modern bearing assembly. His sketches also show how he tried to prevent the balls from bunching together, an issue known today as friction or contact management within a bearing.

The basic principle had ancient precedents, and Leonardo was not the first person to imagine rolling objects. However, his drawings are historically important because they present a clear mechanical arrangement within a systematic study of machines. Modern bearings use carefully manufactured races, cages, lubrication, and materials that were unavailable to Renaissance engineers.