By changing the grease in you trailer axles or replacing wheel bearings or by putting some bearings in a tractor or maybe building an rc car when your a kid or riding and fixing a bicycle maybe replacing a rotor on the mowers deck or working on a atv or motorcycle or anything thats a little simpler than a differential.
A diff has more than just bearings going on. A diff is something someone would normally tackle after they are confident they have simpler items mastered without failure. A diff spins fast and generates lots of heat even when done right.
I cant tell you how many guys I run into that tell me they have years of experience with diffs and they still dont understand bearing preloads and crush sleeves/solid spacers properly/fully.
