When I started reversing my extruder I noticed the motor bracket flexing. Here is a short video showing it in operation: -
It was immediately apparent that I had not made it strong enough.
As the worm gear is about twice the diameter of the threaded pulley the axial force on the motor is about half the force required to push the filament, i.e. a few kilograms. After making a few objects it cracked along the layer where the bearing housing rises out of the flat motor mount.
I designed a new bracket but I was back in a chicken and egg situation with no working extruder to print it. As Erik pointed out you need a Robin Hood / Friar Tuck strategy of having two machines so that one can make replacement parts for the other. I must get my Darwin up and running!
In the meantime I cobbled it back together with some random bits of metal, some tiny G-clamps and tie wraps: -
I made some of the new bracket thicker where I could: 8mm instead of 5mm, which should be ~2.5 times stronger. I also added some ribs and extruded it at 10°C higher temperature.
This one seems solid as a rock, but it did warp a little more. The stronger you make something the more it warps.
Here is a video of it not flexing: -