Leonardo da Vinci studied a ball bearing as a low-friction support for rotating parts.
His late-15th-century notes include a bearing concept using balls between moving surfaces. The balls reduce sliding friction by allowing rolling contact, making rotation easier than it would be when two surfaces rub directly against each other. Leonardo also considered ways to keep the balls separated.
The design is often credited as one of the earliest recorded concepts resembling a modern ball bearing. That does not mean Leonardo invented the complete standardized bearing used today. Modern bearings require precise materials, cages, races, lubrication, and manufacturing tolerances that developed much later.
Ball bearings are used in many machines, including wheels, motors, tools, and conveyors. They are one kind of rolling-element bearing; cylindrical rollers and tapered rollers are other types. Leonardo’s sketch is significant because it recognized the mechanical value of replacing sliding contact with rolling contact.