What mechanical component did Leonardo da Vinci study using small metal spheres?

Leonardo da Vinci studied the ball bearing as a mechanical component using small metal spheres.

Around 1498, Leonardo drew designs showing balls placed between moving surfaces. The purpose was to reduce friction by replacing sliding contact with rolling contact. His notes and sketches are often cited as an early study of the principle behind modern ball bearings.

Leonardo did not invent the complete standardized bearing systems used in modern machines. Earlier civilizations used rollers and related methods, and practical industrial ball bearings emerged much later. Leonardo’s importance lies in clearly examining how rolling bodies could improve mechanical motion.

Modern ball bearings usually contain balls held between inner and outer rings, often with a cage that keeps them spaced apart. They appear in motors, bicycles, tools, wheels, and many other machines because rolling friction is generally lower than sliding friction.