What simple machine did Leonardo da Vinci study as a way to reduce friction?

Leonardo da Vinci studied the ball bearing as a way to reduce friction between moving surfaces.

His notebooks contain designs using balls between rings, allowing sliding friction to be replaced by rolling friction. The concept appeared in Leonardo’s mechanical studies during the late 15th and early 16th centuries, long before modern precision manufacturing made reliable bearings practical.

A ball bearing does not eliminate friction completely. Its balls, races, lubrication, alignment, and materials all affect performance. Modern bearings are manufactured to tight tolerances and are used in machines ranging from bicycles to turbines.

Leonardo was not the sole inventor of every bearing idea, and earlier rolling-element concepts existed. His importance lies in clearly exploring the principle and applying it to mechanical design.