Bearings are what allow the gears to rotate easily. They are usually designed outside of bronze or brass, and so they suit into races (or grooves) from the gearbox housing. Contaminants can enter your gearbox oil by means of vents or perhaps the fill tube in the course of checking/filling. https://filtergearbox19555.tokka-blog.com/34354336/detailed-notes-on-filter-gearbox