Leonardo da Vinci designed a ball-bearing mechanism using freely rotating spheres to reduce friction.
His studies of friction and motion included balls placed between rings or other surfaces. Instead of allowing two solid surfaces to slide directly against one another, the balls would roll between them. Rolling friction is generally lower than sliding friction, making movement easier and more efficient.
Leonardo's sketches are often cited as an early ball-bearing concept. The idea was part of a broader investigation of machines, gears, force, and mechanical advantage. He did not create the standardized, sealed bearings used in modern engines and machines.
The common claim that Leonardo invented every form of the ball bearing should be treated carefully. Earlier rolling devices existed, and later engineers developed practical manufacturing methods. Leonardo's importance is that he clearly studied and illustrated a recognizable rolling-bearing arrangement.