What Leonardo da Vinci machine used balls between surfaces to reduce friction?

Leonardo da Vinci designed a ball bearing that used rolling balls between surfaces to reduce friction.

When two solid surfaces slide directly against each other, friction can waste energy and cause wear. A ball bearing replaces much of that sliding with rolling contact. Leonardo’s design placed balls in a frame so they could move between rings or surfaces while remaining separated from one another.

His sketches are among the earliest known designs for a practical ball-bearing arrangement. They show Leonardo’s understanding that freely moving balls could prevent the rubbing surfaces from locking together. The idea was not immediately developed into modern mass-produced bearings.

A ball bearing is different from a roller bearing, which uses cylindrical rollers, and from a plain bearing, where surfaces slide against a lubricant or low-friction lining. Modern bearings also use carefully engineered cages and materials.