zookeeper

What am I doing wrong? This is my first loco build and I fitted a DJH GB1C gearbox with an AM1 motor. With only the centre axle fitted (no coupling rods fitted yet) which was very free running before the drive gear locked to the axle. I ran the motor for some second and the worm gear stripped the teeth on the coupling gear. This was replaced by DJH at no cost to me some years ago and fitting it again to the chassis only to find that the same has happened again. I have reamed all the bearings with 1/8" reamer all things seem to run very freely but for some reason the plastic gears don't last very long. Any guidance would be helpful.