Bearings are what allow the gears to rotate smoothly. They are usually produced out of bronze or brass, and so they fit into races (or grooves) during the gearbox housing. That is certainly why brands came up with internal filters. These filters can be found on several modern-day automated transmissions. https://milozqair.blogozz.com/33223468/an-unbiased-view-of-filter-gearbox