Which rotating machine part did Leonardo da Vinci draw an early design for?

Leonardo da Vinci drew an early design for a ball bearing.

His notebooks include a mechanism in which balls move between rings, reducing friction between rotating parts. The basic principle is that rolling contact creates less resistance than sliding contact. Leonardo appears to have explored the idea while studying machines and friction, although his sketch was not a mass-produced bearing in the modern industrial sense.

Ball bearings became important because they allow shafts, wheels, and other components to rotate more efficiently. Modern bearings use carefully manufactured balls, races, cages, seals, and lubricants. Leonardo’s design included the essential insight that the balls should be separated so they do not crowd one another during rotation.

A common mistake is to say that Leonardo invented the complete modern ball-bearing industry. He did not manufacture standardized steel bearings or establish their later industrial applications. His significance is that his drawings show an early systematic treatment of rolling elements in a bearing assembly. Similar concepts were refined by later engineers as metalworking, precision measurement, and machine production improved.