What object did Leonardo da Vinci use as an early example of a ball bearing?

Leonardo da Vinci used a wooden ball as an early example of a ball bearing.

Leonardo’s studies of friction included a mechanism in which balls separated moving surfaces. The balls reduced sliding contact by allowing parts to roll against one another, expressing the same basic principle used by ball bearings today. His version was made from wood rather than the hardened steel common in modern bearings.

The design appeared in Leonardo’s notebooks during his investigations of motion and mechanical efficiency. It was an important engineering insight because rolling resistance is generally lower than sliding friction. However, wooden balls would wear, deform, and respond poorly to moisture compared with precision-manufactured bearing elements.

Leonardo is sometimes credited with inventing the ball bearing outright. A more precise statement is that he drew an early bearing-like mechanism. Rolling elements and bearing concepts existed in earlier forms, while industrial ball bearings developed much later through advances in metallurgy and manufacturing.