Leonardo da Vinci studied the ball bearing as a mechanical element for reducing friction in machines. His drawings show balls placed between moving surfaces so that rolling contact could replace much of the sliding contact that wastes energy.
The principle is straightforward: rolling generally produces less friction than sliding. Leonardo explored ways to keep the balls separated, including cage-like arrangements, because loose balls could bunch together and limit the mechanism’s usefulness.
He did not invent every form of bearing, and ancient devices had already used rollers and related methods. Leonardo’s importance is that he analyzed the problem clearly and documented a design resembling the basic arrangement of a modern ball bearing.
Modern bearings use precisely manufactured steel, ceramic, or other balls, raceways, seals, and lubricants. They appear in motors, bicycles, turbines, and countless other machines. Leonardo’s Renaissance sketches therefore connect an old mechanical principle with a component that became highly effective only after advances in manufacturing and materials.