Bearings are what enable the gears to rotate smoothly. They are typically created away from bronze or brass, they usually fit into races (or grooves) while in the gearbox housing. Synchro rings have teeth on them that mesh with the enamel about the gears, and so they also have friction https://filter-gearbox43074.rimmablog.com/33231438/an-unbiased-view-of-filter-gearbox