Leonardo da Vinci illustrated a ball bearing using rolling balls between rings to reduce friction.
The notebook design places spherical elements between an inner and outer track. When one ring turns, the balls roll rather than forcing the surfaces to slide directly against each other. That change from sliding friction to rolling friction is the essential operating principle of a ball bearing.
Leonardo’s design is often credited as an early recorded bearing concept, but it was not the first bearing in human history. Ancient and medieval engineers used rollers, pivots, and other low-friction arrangements before him. Leonardo’s importance is that his drawing clearly explores a cage-like arrangement intended to keep the balls separated.
Modern ball bearings use carefully manufactured steel, ceramic, or other materials, along with races, cages, seals, and lubrication. Leonardo’s sketch was much simpler and was not a mass-produced industrial component. It nevertheless shows a remarkably clear understanding of how geometry can reduce mechanical resistance.