Bearings are what enable the gears to rotate easily. They are generally manufactured out of bronze or brass, they usually suit into races (or grooves) from the gearbox housing. Contaminants can enter your gearbox oil as a result of vents or perhaps the fill tube in the course of checking/filling. https://filter-gearbox29616.theisblog.com/34311983/not-known-factual-statements-about-filter-gearbox