Leonardo da Vinci’s ball-bearing design was intended to reduce friction.
His notebooks contain an early drawing of balls arranged so they could move between surfaces without locking together. Rolling elements reduce sliding contact, allowing one part to move more easily against another. This is the basic principle behind ball bearings used in machines today.
Leonardo appears to have studied the design while investigating mechanical motion. The drawing is often cited as an early contribution to bearing technology, although it was not a complete modern bearing assembly and was not mass-produced in the Renaissance.
The design is sometimes credited to Leonardo as the inventor of all ball bearings, but that claim is too broad. Earlier cultures used rolling objects in practical ways, and later engineers developed reliable bearing cages and manufacturing methods. Leonardo’s importance lies in his clear mechanical concept for separating moving balls and reducing friction.