Bearings are what enable the gears to rotate effortlessly. They are generally produced from bronze or brass, and so they in shape into races (or grooves) inside the gearbox housing. Contaminants can enter your gearbox oil by way of vents or the fill tube in the course of checking/filling. More https://edwinaaxvt.blogdomago.com/32994468/the-definitive-guide-to-filter-gearbox