Leonardo da Vinci studied a ball bearing to reduce friction between moving parts.
His notebooks contain drawings of balls placed between rings or surfaces so that rolling replaces much of the sliding friction found in ordinary contact. The idea is mechanically important because rolling resistance is generally lower than sliding resistance, allowing shafts and other components to move more easily.
Leonardo's sketches are often cited as an early recorded design for a modern-style ball bearing. He appears to have been concerned that the balls should not bunch together, and some drawings show separators or arrangements intended to keep them distributed around the bearing.
Leonardo did not create the complete industrial bearing systems used today. Modern bearings require precise materials, manufacturing tolerances, lubrication, seals, and standardized races. The historical significance of his design is that it recognized the basic principle clearly during the Renaissance. It is also a good example of how Leonardo's mechanical studies moved beyond spectacular machines toward small components that could improve many different mechanisms.