What mechanical component did Leonardo da Vinci study using rolling spheres?

Leonardo da Vinci studied a ball bearing using rolling spheres between mechanical surfaces.

Leonardo’s notebooks contain a design in which balls move between rings, reducing direct rubbing between the parts. This is the basic principle of a ball bearing: rolling elements allow a shaft or another component to move with less friction than sliding contact would create.

The design is often cited as an early ball-bearing concept, although Leonardo did not invent every form of bearing. Ancient and medieval machines already used various low-friction supports, and later engineers developed bearings that could be manufactured and lubricated reliably.

Leonardo’s arrangement also addressed a problem that affects ball bearings: the balls can bunch together if they are not separated. His sketch is therefore important as an early attempt to control rolling motion in a mechanical assembly. Modern bearings use carefully shaped races, cages, hardened materials, and lubricants that were unavailable in his workshop.