Leonardo da Vinci studied a ball bearing, using balls between moving surfaces to reduce friction.
In a ball bearing, rolling balls separate an inner ring from an outer ring. Because rolling friction is generally lower than sliding friction, the arrangement allows one part to rotate more easily relative to another. Leonardo’s sketches show an early attempt to prevent the balls from bunching together while they move.
His bearing concept is associated with studies made around 1498, during his investigations of machinery and motion. The principle appears in designs for rotating mechanisms and is often cited as an early example of a caged bearing.
Leonardo did not create the modern standardized bearing industry. Practical bearings were refined over centuries, and modern versions use carefully manufactured races, cages, seals, and materials. The historical point is that Leonardo clearly recognized the value of controlled rolling elements in mechanical systems.