Bearings are what enable the gears to rotate smoothly. They are usually produced away from bronze or brass, and they match into races (or grooves) in the gearbox housing. That is definitely why makers came up with inside filters. These filters can be found on lots of modern-day computerized transmissions. https://filtergearbox50471.webdesign96.com/34582490/helping-the-others-realize-the-advantages-of-filter-gearbox