Bearings are what enable the gears to rotate easily. They are usually built away from bronze or brass, and so they fit into races (or grooves) while in the gearbox housing. Contaminants can enter your gearbox oil by way of vents or even the fill tube all through examining/filling. Usually, https://filter-gearbox91578.thenerdsblog.com/39882105/getting-my-filter-gearbox-to-work