What early machine part did Leonardo da Vinci study using freely rotating balls?

Leonardo da Vinci studied an early ball bearing using freely rotating balls to reduce friction.

A ball bearing places rolling balls between surfaces so that motion occurs mainly through rolling rather than sliding. In the late 15th century, Leonardo sketched a bearing arrangement in which balls were separated so they would not jam against one another. The idea addressed a practical engineering problem: how to support rotating shafts while limiting friction and wear.

Leonardo did not invent the modern standardized bearing industry, and his design was not identical to later enclosed bearings. Historical sources credit him with an early bearing concept, while practical mass production came much later with advances in steelmaking and precision manufacturing. The design is therefore best understood as an important early study of rolling-element mechanics.