Mike
this was on my new machine - haven't cut anything with it yet.
The motor was the Ý'motor, which is one of two which I use on this axis - the other (A) is slaved to the one which sheared off.
No - as far as I could see the motor did not 'grab'or seize prior to the break, and it remained racked square - very solid carriers.

I think Greg is right, and think I had far far too much tension on the belts.
The other interesting thing is that I ran a file across the shaft of one of my steppers and was amazed at the amount of steel it removed. The shafts clearly aren't tool-steel, or anything like it - they don't even appear to be silver-steel. Most probably just mild-steel. If this is the case, then I can understand them shearing if too much lateral pressure is applied.
Just have to wait and see what happens now I have releived the belts.

Cheers

Noel