Which common machine part did Leonardo da Vinci study using low-friction balls?

Leonardo da Vinci studied the ball bearing, a machine part that reduces friction between moving surfaces with rolling balls.

Around 1498, Leonardo made drawings showing balls arranged so they would not slide into a pile. His design is commonly cited as an early study of the principle behind the modern ball bearing: rolling elements separate surfaces and allow smoother motion.

Ball bearings are not solely a Leonardo invention. Similar rolling-element ideas appeared in earlier engineering traditions, and modern bearings developed through later improvements in materials, precision, cages, and manufacturing.

Leonardo’s drawings are important because they show a clear attempt to solve a practical mechanical problem. A common mix-up is to call the sketch the first modern ball bearing; it was an early conceptual design, not a mass-produced industrial component.