Bearings are what allow the gears to rotate effortlessly. They are typically built out of bronze or brass, plus they match into races (or grooves) during the gearbox housing. Which is why producers arrived up with inner filters. These filters are available on a lot of modern day computerized transmissions. https://filter-gearbox54061.thekatyblog.com/32825760/not-known-facts-about-filter-gearbox