Leonardo da Vinci developed a ball bearing mechanism with two interlocking rings to reduce friction.
His sketches show balls positioned between inner and outer rings so the surfaces would roll rather than slide against one another. That arrangement is recognizably related to the modern ball bearing, in which rolling elements separate moving races and reduce friction.
Leonardo studied the problem because friction limited the performance of machines. In a conventional bearing, rolling elements can themselves rub together and create resistance. His design included a separator-like arrangement intended to keep the balls apart, an important detail in the history of bearing engineering.
Leonardo's drawings should not be confused with the invention of every modern bearing technology. Ball bearings were refined over centuries, and practical industrial bearings required later advances in materials, precision manufacturing, lubrication, and standardization. His contribution was an early recorded design that clearly addressed friction through rolling contact.