Leonardo da Vinci studied an early ball-bearing design using small spheres between moving surfaces.
His sketches show balls placed in a frame so that they could roll rather than slide. This arrangement reduces friction between rotating parts and is the basic principle behind modern rolling-element bearings.
The design is often dated to the late fifteenth or early sixteenth century, but Leonardo did not invent every form of bearing. Ancient and medieval machines already used various low-friction supports, and later engineers developed bearings that could be manufactured accurately and lubricated reliably.
Leonardo’s innovation was his clear attempt to keep the balls separated so they would not rub together and block one another. Modern ball bearings use carefully made races, cages, lubrication, and hardened materials, making them far more durable than a Renaissance prototype.