Bearings are what enable the gears to rotate efficiently. They are usually manufactured from bronze or brass, and they in good shape into races (or grooves) in the gearbox housing. Synchro rings have teeth on them that mesh with the tooth around the gears, and they even have friction elements https://filtergearbox93452.blogdosaga.com/33992645/an-unbiased-view-of-filter-gearbox