Leonardo da Vinci drew an early ball-bearing concept to reduce friction between moving surfaces.
His sketches show balls placed between rings or tracks so that motion would involve rolling rather than direct sliding. This arrangement addresses a basic engineering problem: sliding contact creates more resistance and wear, while rolling contact can make movement smoother. Leonardo included the idea in notebooks associated with his mechanical studies.
The design is often credited as an early ball-bearing concept, not necessarily as the first ball bearing ever conceived or as a complete modern bearing ready for manufacture. Leonardo also explored separators that would keep the balls from touching one another, an important practical feature in bearing design.
Modern ball bearings use carefully shaped races, hardened balls, cages, lubrication, and precise tolerances. Leonardo's drawing predates those manufacturing methods, but it clearly demonstrates the central principle of using rolling elements to reduce friction. It is therefore an important example of Renaissance mechanical engineering.