What simple machine principle did Leonardo da Vinci illustrate with a modern-looking ball bearing?

Leonardo da Vinci illustrated the principle of reducing friction with a ball-bearing design.

His notebooks include sketches of balls placed between moving surfaces so that sliding friction could be replaced by rolling friction. This arrangement is recognizably related to the principle used in modern ball bearings, which allow shafts and wheels to rotate more easily.

Leonardo did not invent the complete standardized bearing systems used in modern machines. His drawings show an important mechanical insight, but the practical manufacture of accurately sized balls, cages, seals, and races required later engineering developments.

The key distinction is between a concept and a modern commercial bearing. A rolling element can reduce resistance, but performance also depends on alignment, materials, lubrication, and manufacturing precision. Leonardo’s design is therefore best understood as an early documented study of rolling bearings.