Bearings are what allow the gears to rotate easily. They are usually built out of bronze or brass, and so they fit into races (or grooves) within the gearbox housing. There's two major kinds of transmissions: computerized and guide. Computerized transmissions use fluid couplings as an alternative to a clutch, https://filter-gearbox46443.full-design.com/the-ultimate-guide-to-filter-gearbox-76473295