Leonardo da Vinci’s ball-bearing mechanism is credited as an early design for reducing friction.
A ball bearing places hard balls between moving surfaces so that rolling replaces much of the sliding friction. Leonardo illustrated this principle in his notebooks while studying machines, shafts, and rotating parts. His design used balls arranged so they would not crowd together and stop moving freely.
The idea was important because friction limited the efficiency and durability of Renaissance machinery. A rotating shaft supported by rolling elements could move more smoothly than one rubbing directly against a fixed surface.
Leonardo was not the first person to imagine rolling motion, and modern bearings use carefully manufactured materials, cages, seals, and lubrication. His sketches nevertheless show a clear early understanding of the mechanical advantage of rolling contact.