Leonardo da Vinci developed a ball-bearing concept using balls to reduce friction between moving surfaces.
His sketches from around 1498 show balls arranged so they could roll instead of allowing two surfaces to slide directly against one another. Rolling contact generally produces less friction than sliding contact, making the idea useful for rotating machinery. Leonardo was studying the problem while designing mechanisms for movement and transmission of force.
He did not invent the complete standardized bearing industry used today. Modern ball bearings use carefully manufactured balls, races, cages, seals, and lubricants. Leonardo's drawings are best understood as an early documented design for the principle.
The concept also illustrates a common feature of his engineering notebooks: he often explored mechanical solutions through several variations rather than presenting one finished commercial product.