What friction-reducing device did Leonardo da Vinci draw using wooden balls?

The friction-reducing device Leonardo da Vinci drew using wooden balls was a ball bearing.

Leonardo included a ball-bearing concept in notebooks dating from the late 15th century. The design placed balls between two surfaces so that one part could move with rolling friction rather than sliding friction. This principle is central to modern bearings.

His drawing is important because it clearly shows the basic idea of separating moving surfaces with rolling elements. However, Leonardo did not invent every form of bearing, and similar rolling supports had appeared in earlier mechanical contexts. His work belongs to the long development of methods for reducing friction in machines.

A practical bearing also needs to control the balls and keep them spaced. Leonardo’s design addressed that problem with a cage-like arrangement, though manufacturing precision at the time limited widespread use.