Bearings are what allow the gears to rotate effortlessly. They are usually manufactured out of bronze or brass, they usually in good shape into races (or grooves) while in the gearbox housing. That's why companies came up with inside filters. These filters can be found on numerous contemporary automatic transmissions. https://filter-gearbox29494.activablog.com/33235720/5-easy-facts-about-filter-gearbox-described