Bearings are what enable the gears to rotate smoothly. They are usually manufactured outside of bronze or brass, and they suit into races (or grooves) in the gearbox housing. Which is why producers arrived up with inner filters. These filters can be found on lots of present day automatic transmissions. https://filter-gearbox64207.dgbloggers.com/34539950/an-unbiased-view-of-filter-gearbox