Leonardo da Vinci studied and helped popularize the ball bearing in a wooden bearing design.
Around 1498, while working in Milan, Leonardo drew a bearing in which balls separated two rings. The rolling balls reduced sliding friction and prevented the rings from rubbing directly against each other. His design used separators to keep the balls apart, an idea closely related to the cages found in many modern ball bearings.
Leonardo did not invent the general idea of reducing friction with rolling elements. Ancient and medieval builders used rollers and other methods to move heavy loads, and earlier bearing concepts existed in mechanical engineering. His importance lies in the clarity of his documented design and in recognizing that freely moving balls could support rotation while limiting contact between surfaces.
The drawing was not a mass-produced modern bearing. Renaissance manufacturing lacked the precision, hardened materials, lubrication systems, and standardized dimensions needed for reliable high-speed bearings. Modern ball bearings became practical much later, especially during industrial development. The safest description is that Leonardo sketched an early ball-bearing design, rather than claiming he created the first complete modern bearing.