Leonardo da Vinci studied a ball-bearing mechanism in a drawing often linked to the modern ball bearing.
Around 1498, while working in Milan, Leonardo explored ways to reduce friction between moving surfaces. One design placed balls between rings so that the balls could roll as the surrounding parts turned. Rolling contact creates less friction than sliding contact, which is the central principle behind ball bearings.
The drawing is commonly credited as an early ball-bearing concept, but it should not be treated as proof that Leonardo invented every feature of the modern bearing. Practical bearings require accurate dimensions, durable materials, lubrication, and methods for keeping the balls correctly spaced. Those manufacturing capabilities developed much later.
Leonardo’s mechanism is also sometimes confused with a roller bearing. A ball bearing uses spherical rolling elements; a roller bearing uses cylindrical or other shaped rollers. The historical importance of the sketch is its clear attempt to solve friction through controlled rolling motion.