Leonardo da Vinci depicted an early ball-bearing design for reducing friction.
His late-15th-century sketches show balls placed between moving surfaces so that one part could rotate with less sliding friction against another. The balls were intended to roll, replacing much of the rubbing motion that would otherwise resist movement.
Leonardo’s drawing is an important early example of the principle, but it was not a modern standardized bearing. A practical bearing must keep its rolling elements separated and correctly aligned; Leonardo explored that problem with cage-like arrangements in his sketches.
Modern ball bearings became industrially important much later, with hardened materials, precise manufacture, lubrication, and standardized races. Leonardo’s contribution is therefore best described as an early documented design concept rather than the invention of the complete modern bearing industry.