Have a 5701D that has been running for years all of a sudden break the drive gear.  Pinion gear on motor shaft looks fine.  Replaced the bearing and gears as a new set and re-installed and without the lower wheel installed worked fine and the instant I put on the NEW lower wheel, BAM drive gear breaks again.  I am at a lost because I don't see the reason behind the breaking.  I have worked on small to ship propulsion drive gears and never had this problem.  Any help would be appreciated!!!