What simple mechanical device did Leonardo da Vinci study for reducing friction?

Leonardo da Vinci studied a ball bearing as a device for reducing friction.

Around 1498, Leonardo made drawings of bearings using balls between moving surfaces. The balls were intended to prevent the surfaces from rubbing directly against one another, replacing sliding friction with rolling motion. His sketches also addressed ways to keep the balls separated so they would not bunch together.

Bearings were valuable for machines with rotating parts, including shafts and wheels. Leonardo’s studies were part of his broader work on mechanics, in which he investigated friction, motion, gears, and force. The surviving drawings show an important early analysis of a principle later used extensively in engineering.

Leonardo was not the first person to use rolling elements in mechanical devices, and his drawings do not prove that he invented the modern bearing. Modern ball bearings use carefully manufactured races, cages, and hardened materials developed much later.