Bearings are what enable the gears to rotate efficiently. They are typically made away from bronze or brass, and they in good shape into races (or grooves) during the gearbox housing. Primarily it is made of a metallic plate with mesh-form inserts that reduce contaminants from coming into the transmission https://arthuryjpua.thezenweb.com/5-simple-statements-about-filter-gearbox-explained-72118204