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 just under 1000lb on every single dropout. A torque arm is normally a separate piece of metal mounted on the axle which can consider this axle torque and transfer it further up the frame, therefore relieving the dropout itself from bringing all the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is usually loose, after that axle can rotate some sum and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by enough time this happens 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 lttle bit of play, it could go on correctly snug, or in some cases a little amount of filing may be essential for the plate to slide on. In situations where in fact the axle flats happen to be a bit narrower than 10mm and you feel play, it is not much of a concern, nevertheless, you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and stop the torque plate from seated smooth against the dropout. If this is the case, you will need to be sure to have a washer that matches inside the lip place. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that is included with many hub motors can often be about the proper width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless band can produce the ultimate installation look more discrete and protect the paint job from getting scratched. We incorporate several bits of shrink tube with each torque arm package deal.
However, in high vitality 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 happens, the axle will spin freely, wrapping and severing off the electric motor cables and potentially causing the wheel to fall proper from the bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and offer some way of measuring support against rotation. Oftentimes this is sufficient.