Leonardo da Vinci studied the ball bearing, using balls between moving surfaces to reduce friction.
Around 1498, while working on mechanical studies, Leonardo drew a bearing arrangement in which balls separated rings or surfaces. The balls were intended to prevent the surfaces from rubbing directly against each other, allowing smoother rotation.
The principle is called rolling-element bearing design. Rolling contact generally produces less sliding friction than direct rubbing, although real bearings also require careful alignment, suitable materials, lubrication, and protection from dirt.
Leonardo did not invent every form of bearing, and simple bearing ideas existed before his drawings. His importance is that he clearly explored a practical anti-friction arrangement, including a design intended to keep the balls from bunching together. Modern ball bearings use cages or retainers for that purpose.