Gear box damage

[ejw]

Evening all

Having got a half decent set up on my 3.0 last night (with a lot of help) my tlr laydown gearbox decided to implode just before my final making a hideous grinding sound. Stripped it down today and found a lot of dust sized metal shavings and the gear the pinion attaches to ( don't know official term) has worn really badly as per attaches pics. Anyone have any idea why this might have happened? Everything seems to be seated properly and don't want to just buy a new gear and end up in same boat in a few weeks!

Any suggestions welcome as I'm not sure what to look for.