Bearings are what enable the gears to rotate efficiently. They are typically built away from bronze or brass, and so they match into races (or grooves) inside the gearbox housing. Contaminants can enter your gearbox oil through vents or maybe the fill tube through checking/filling. Most of the time, on https://martinilife.gynoblog.com/33263734/not-known-facts-about-filter-gearbox