Leonardo da Vinci developed an early ball-bearing design to reduce friction between moving parts.
His late-fifteenth-century sketches show balls arranged between rings so that one surface could rotate against another with less sliding friction. The basic principle is the same as in a modern ball bearing: rolling elements separate the moving surfaces and make rotation more efficient.
Leonardo was not the first person to use rolling objects in machinery, and his design was not a complete modern bearing assembly. His sketches also addressed a practical problem called packing, in which balls can press against one another and limit smooth movement. He proposed separators so the balls would remain spaced apart.
Industrial ball bearings became widely important much later, after improved manufacturing made accurately shaped steel balls and races possible. Leonardo's contribution is best described as an early documented bearing concept rather than the invention of every modern bearing type.