What Leonardo da Vinci device is often credited as an early ball-bearing design?

Leonardo da Vinci is often credited with an early ball-bearing design.

Around 1498, Leonardo drew mechanisms in which balls moved between surfaces to reduce friction. The basic principle is that rolling contact generally creates less resistance than sliding contact. His sketches show an effort to prevent the balls from crowding together, an important practical issue in a bearing.

The drawings are significant, but the modern ball bearing was not created as a mass-produced machine in Leonardo’s lifetime. Manufacturing accurate, durable bearings required materials, machining and lubrication methods that developed much later. The first patent for a radial ball bearing was granted to Philip Vaughan in 1794.

Leonardo should therefore be described as an early documented designer or observer of the concept, not necessarily the inventor of every later bearing technology. Roller bearings, plain bearings and thrust bearings use related ideas but have different contact arrangements and performance characteristics.