To give a feeling of the magnitude of these forces, a hub engine with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on each dropout. A torque arm can be a separate piece of metal attached to the axle which can consider this axle torque and transfer it further up the frame, hence relieving the dropout itself from acquiring all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut can be loose, in that case axle can rotate some amount and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by enough time this takes place your dropout may already be damaged.
The tolerances on motor axles may differ from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it may go on flawlessly snug, or occasionally a tiny amount of filing could be necessary for the plate to slide on. In circumstances where the axle flats are a little narrower than 10mm and you feel play, it is not much of an issue, nevertheless, you can “preload” the axle plate in a clockwise direction as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways and prevent the torque plate from seated flat against 
the dropout. If this is actually the case, you will want to be sure to get a washer that matches inside the lip region. We make custom “spacer ‘C’ washer” because of this job, though the lock washer that comes with a large number of hub motors is normally about the right width and diameter.
For the hose-clamp unit, a small length of heat-shrink tubing over the stainless steel band can produce the ultimate installation look more discrete and protect the paint job from getting scratched. We contain several bits of shrink tube with each torque arm deal.
Torque Arm china However, in high vitality systems that generate a lot of torque, or in setups with weak dropouts, the forces present can exceed the material durability and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the motor cables and potentially triggering the wheel to fall right out from the bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key in to the dropout slot and offer some measure of support against rotation. Oftentimes this is sufficient.