What simple machine did Leonardo da Vinci sketch as an early ball bearing?

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

Around 1498, Leonardo studied arrangements of balls that could roll between rings or tracks. The rolling elements separated surfaces that would otherwise slide directly against one another, reducing rubbing friction. His drawings show an important mechanical principle used widely in later machinery.

Leonardo's design was not a finished industrial bearing in the modern sense. Renaissance materials and manufacturing methods made precise, durable bearings difficult to produce, and the surviving sketches represent an experimental concept rather than a mass-produced invention.

Modern ball bearings use carefully shaped steel, ceramic, or other balls held in races and often separated by a cage. The basic idea Leonardo explored—replacing sliding contact with rolling contact—remains central to these machines.