Leonardo da Vinci studied a ball bearing using paired rotating rings to reduce friction.
His late-fifteenth-century sketches show balls positioned between rings so that one surface could rotate more freely against another. The balls separate the moving surfaces and replace sliding friction with rolling motion. Leonardo appears to have recognized that closely packed balls would rub against one another, so some designs include separators or cages.
The concept was part of his mechanical investigations rather than a documented mass-produced invention. Similar rolling-element ideas existed before Leonardo, and later engineers developed bearings with standardized dimensions, improved materials, lubrication, and manufacturing precision. For that reason, Leonardo is best described as an important early designer and recorder of the concept, not the sole inventor of every modern bearing.
Ball bearings are now used in machines ranging from bicycles to electric motors. Their purpose remains the same basic one Leonardo explored: reducing resistance between moving parts while carrying a load.