Bearings are what enable the gears to rotate smoothly. They are usually manufactured outside of bronze or brass, and they in shape into races (or grooves) in the gearbox housing. On the other hand, interior filters are costlier than externals and it is way more challenging to interchange interior filters, https://filtergearbox84715.pointblog.net/an-unbiased-view-of-filter-gearbox-77922676