Bearings are what allow the gears to rotate smoothly. They are usually created from bronze or brass, they usually in shape into races (or grooves) during the gearbox housing. There are 2 major kinds of transmissions: automated and handbook. Automatic transmissions use fluid couplings as an alternative to a clutch, https://jaredvmwhq.answerblogs.com/34310635/filter-gearbox-an-overview