What common machine part did Leonardo da Vinci sketch in a design dated to 1498?

Leonardo da Vinci sketched an early ball-bearing design in 1498.

The drawing shows balls placed between concentric rings so that one surface could rotate with less sliding friction against another. This arrangement anticipates the basic principle of a modern rolling-element bearing: rolling bodies carry a load while reducing resistance.

Leonardo’s design was part of his broader study of mechanics, gears, machines, and friction. It was not a standardized industrial component, and the surviving sketch does not prove that he manufactured or used a practical bearing based on it.

The bearing is often described as an invention of Leonardo, but the underlying idea of using rolling objects to reduce friction has older precedents. His importance lies in clearly recording a sophisticated bearing arrangement centuries before mass-produced bearings became common.