Leonardo da Vinci illustrated the ball-bearing principle with sketches of balls placed between moving surfaces.
A ball bearing reduces sliding friction by allowing balls to roll between a rotating part and a stationary support. Leonardo's notes and drawings from the late 15th century show arrangements in which spherical elements separate surfaces, an important step toward the modern bearing concept.
Leonardo was responding to a practical engineering problem: friction limits the efficiency and durability of machines. Rolling elements can reduce resistance, but they also require careful spacing and control. His sketches include ideas for keeping balls from bunching together, although they do not constitute a modern standardized bearing.
The claim that Leonardo “invented the ball bearing” needs nuance. Similar rolling-element ideas existed before him, and modern bearings depend on later manufacturing advances. He is best credited with an early clear study of the principle in a mechanical-design context.