Leonardo da Vinci’s ball-bearing design was intended to reduce friction.
Leonardo drew a mechanism using balls positioned between moving surfaces. The balls were meant to prevent the surfaces from rubbing directly against each other, replacing much of the sliding contact with rolling contact. That change reduces resistance and can make rotary motion more efficient.
His sketches date from the late fifteenth century and are often cited as an early recorded example of the ball-bearing principle. Leonardo was not the first person to use rolling objects in machines, and his drawing was not a modern standardized bearing with cages, seals, and precision-manufactured parts.
A common mix-up is to say that ball bearings eliminate friction. They do not. Rolling bearings reduce friction, but contact, deformation, lubrication, and air resistance still create losses. The same basic principle later became important in clocks, bicycles, machines, and industrial equipment.