Leonardo da Vinci studied a ball bearing using spheres between moving surfaces to reduce friction.
His late-15th-century sketches show balls placed in a structure so they could move independently rather than rubbing directly against one another. This arrangement anticipates the basic principle of a bearing: rolling elements allow one part to rotate relative to another with less sliding friction.
Leonardo’s design is often described as an early ball-bearing concept, not as the invention of the complete modern bearing used today. Modern bearings also depend on carefully manufactured races, cages, materials, lubrication, and precise tolerances.
The spheres in Leonardo’s sketch were intended to prevent them from touching and locking together. That detail shows that he was thinking about both friction reduction and the practical control of rolling elements.