Leonardo da Vinci studied a ball bearing design in which balls reduced friction between moving surfaces.
A drawing in the Codex Atlanticus, dating from the late 15th century, shows balls arranged so that they could roll rather than slide. Rolling contact generally creates less friction than sliding contact, making it useful for rotating mechanisms. Leonardo's arrangement also attempted to prevent the balls from touching and interfering with one another.
The sketch is an early bearing concept, not evidence that Leonardo invented the modern standardized ball bearing. Practical bearing manufacture required accurate dimensions, durable materials, lubrication, and ways to retain the balls in place. Industrial ball bearings developed much later, especially during the 19th century.
Leonardo's interest in bearings formed part of his broader study of machines, gears, axles, and motion. The drawing shows that he recognized friction as a major engineering problem and sought a mechanical solution.