Leonardo da Vinci studied a ball bearing in a design that used rolling balls to reduce friction.
Around 1498, Leonardo drew a mechanism in which balls moved between rings. The arrangement was intended to prevent the balls from touching one another while allowing the inner and outer parts to rotate more freely. This addressed a central problem in bearing design: sliding contact creates more friction than rolling contact.
Leonardo did not invent the general idea of reducing friction with rolling elements, and his drawing was not a modern standardized bearing. It is significant because it clearly shows a cage-like arrangement related to the principle used in later ball bearings. Practical mass-produced bearings required advances in metallurgy, precision manufacturing, lubrication, and interchangeable parts.
The design is often cited as an example of Leonardo anticipating later engineering. That claim should be stated carefully: he produced an influential early drawing, but modern bearing technology emerged through many later inventions and industrial developments.