Leonardo da Vinci sketched a ball-bearing concept in which loose balls reduced friction between moving surfaces.
The basic idea is to place rolling balls between rings or other surfaces so that movement occurs through rolling rather than direct sliding. Rolling resistance is generally lower than sliding friction, making the arrangement useful for rotating machinery.
Leonardo’s drawings include arrangements of balls intended to prevent them from rubbing against one another and interfering with smooth motion. His work shows an early understanding of how rolling elements could improve mechanical efficiency.
Modern ball bearings use precisely manufactured races, cages, materials, and lubrication. Leonardo’s sketches were important conceptual studies, but they were not identical to the standardized industrial bearings developed much later.