To give a feeling of the magnitude of the forces, a hub engine with a 12mm axle making 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 additional up the frame, hence relieving the dropout itself from taking all of the stresses.
Tighten the 1/4" bolt between the axle plate and the arm as snug as possible. If this nut is definitely loose, after that axle can rotate some volume and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by the time this occurs your dropout may already be damaged.
The tolerances on motor axles may differ from the nominal 10mm. The plate may slide on freely with somewhat of play, it could go on correctly snug, or occasionally a little amount of filing could be essential for the plate to slide on. In situations where the axle flats will be somewhat narrower than 10mm and you are feeling play, it isn't much of a concern, but you can "preload" the axle plate in a clockwise way as you tighten everything up.
Many dropouts have quick release "lawyer lips" that come out sideways preventing the torque plate from resting flat against the dropout. If this is actually the case, you should be sure to have a washer that suits inside the lip place. We make customized "spacer 'C' washer" because of this job, though the lock washer that is included with a large number of hub motors is normally about the Torque Arm china proper width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless band can produce the final installation look even more discrete and protect the paint job from getting scratched. We involve several bits of shrink tube with each torque arm package.
However, in high power systems that generate a lot of torque, or in setups with weak dropouts, the forces present may exceed the material strength and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the electric motor cables and potentially causing the wheel to fall correct 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.