Bearings are what allow the gears to rotate efficiently. They are usually built out of bronze or brass, and they match into races (or grooves) while in the gearbox housing. Contaminants can enter your gearbox oil as a result of vents or the fill tube in the course of checking/filling. https://rivervxvsq.yomoblog.com/40690292/detailed-notes-on-filter-gearbox