Bearings are what enable the gears to rotate smoothly. They are typically manufactured out of bronze or brass, and so they in shape into races (or grooves) from the gearbox housing. Contaminants can enter your gearbox oil via vents or even the fill tube for the duration of examining/filling. More https://augustfkhdx.wizzardsblog.com/34038949/5-easy-facts-about-filter-gearbox-described