Bearings are what allow the gears to rotate efficiently. They are generally made out of bronze or brass, and they suit into races (or grooves) in the gearbox housing. Contaminants can enter your gearbox oil as a result of vents or the fill tube for the duration of checking/filling. More https://filter-gearbox25703.ja-blog.com/34180480/the-definitive-guide-to-filter-gearbox