Bearings are what enable the gears to rotate smoothly. They are typically manufactured out of bronze or brass, and they suit into races (or grooves) while in the gearbox housing. Contaminants can enter your gearbox oil via vents or even the fill tube in the course of checking/filling. More often https://zanderqzeff.izrablog.com/34597144/what-does-filter-gearbox-mean