Dooie Pop Pop said:
speaking as a mong iv absolutely no understanding of the shimming process
maybe someone could get a comment from Ian?...
The shafts in the box need to either have a certain amount of freeplay or preload, if the shafts have excessive freeplay the shaft can move back and forth to much, this can lead to gear selection problems and sometimes jumping out of gear, its also possible that if the free play is excessive the tiny momentum that it could muster, could cause the casing to crack due to the shaft basically smashing into the casing under load.
Excessive preload can cause the bearings to wear prematurely, some gearboxes are set up so the shafts run with little preload, so the shafts/bearings are held under constant pressure, this stops the shafts moving. The level of freeplay/preload is generally adjusted by shims placed behind the bearings or bearing races to get the correct setting.