Leonardo da Vinci studied the ball bearing as a way to reduce friction between moving machine parts.
His notebooks include sketches of balls arranged between rings or tracks so that one surface could move more easily against another. The principle is that rolling friction is generally lower than sliding friction. Leonardo’s sketches show that he recognized the value of keeping individual balls separated so they would not interfere with one another.
The drawings do not prove that Leonardo built a complete, modern ball-bearing assembly or introduced the industrial bearing. Similar rolling elements had appeared in earlier devices, and the precision manufacturing needed for durable bearings developed much later. His importance is that he clearly explored the concept in a systematic study of mechanical motion.
Modern ball bearings are standardized components used in machines, vehicles, electric motors, and tools. They normally include inner and outer races, rolling balls, and a cage. Leonardo’s Renaissance sketches were conceptual ancestors rather than finished industrial products.