Bearing

A bearing is a object used to reduce friction in a machine?. They are actually a varation on the wheel. Bearing come in many shapes, from tree trunks laid down under sleds, to the precision bearings in clocks. The first bearing were wood, but ceramic? or glass can be used, and steel is quite common. The path that a bearing travels is called a "race".

Bearings need to be as hard as the race that they are rolling against, or either the bearing or race will wear out prematurely. One of the issues with bearings is that they can rub against each other, which will cause additional friction. The captured bearing was originally described by Galileo in the 1600s as a method to eliminate that additional friction. The mounting of bearings into a set was not accomplished for many years after that.