Bearings are what allow the gears to rotate easily. They are usually designed outside of bronze or brass, they usually in shape into races (or grooves) from 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://elliottcaund.blog5star.com/34581362/detailed-notes-on-filter-gearbox