To give a sense of the magnitude of the forces, a hub motor with a 12mm axle generating 40 N-m of torque will exert a spreading force of just under 1000lb on every dropout. A torque arm is definitely another piece of metal mounted on the axle that may have this axle torque and transfer it even more up the frame, thus relieving the dropout itself from choosing all the stresses.
Tighten the 1/4" bolt between your axle plate and the arm as snug as possible. If this nut is certainly loose, in that case axle can rotate some quantity and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by enough time this occurs your dropout may previously be damaged.
The tolerances on motor axles can vary from the nominal 10mm. The plate may slide on freely with a bit of play, it could go on properly snug, or sometimes a little amount of filing could be necessary for the plate to slide on. In situations where the axle flats happen to be a little narrower than 10mm and you feel play, it is not much of a concern, nevertheless, you can "preload" the axle plate in a clockwise way as you tighten everything up.
Many dropouts have speedy release "lawyer lips" that come out sideways and prevent the torque plate from resting smooth against the dropout. If this is the case, you should be sure to possess a washer that suits inside the lip place. We make customized "spacer 'C' washer" because of this job, although lock washer that comes with many hub motors is normally about the right width and diameter.
For the hose-clamp unit, a small amount of heat-shrink tubing over the stainless steel band can produce the final installation look more discrete and protect the paint job from getting scratched. We involve several bits of shrink tube with each torque arm deal.
However, in high electric power devices that generate a lot of torque, or in setups with weak dropouts, the forces present can exceed the material power and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor cables and potentially triggering the wheel to fall correct out of 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 way of 
measuring support against rotation. Oftentimes this is sufficient.