Bearings are what enable the gears to rotate efficiently. They are generally made outside of bronze or brass, plus they healthy into races (or grooves) from the gearbox housing. Contaminants can enter your gearbox oil as a result of vents or maybe the fill tube in the course of checking/filling. https://zionjqrqn.blogsumer.com/33301833/indicators-on-filter-gearbox-you-should-know