Bearings are what enable the gears to rotate smoothly. They are usually designed outside of bronze or brass, they usually fit into races (or grooves) within the gearbox housing. Contaminants can enter your gearbox oil by means of vents or perhaps the fill tube for the duration of examining/filling. More https://filtergearbox32084.mybuzzblog.com/13815808/detailed-notes-on-filter-gearbox