To give a feeling of the magnitude of the forces, a hub electric motor with a 12mm axle creating 40 N-m of torque will exert a spreading force of slightly below 1000lb on each dropout. A torque arm can be another piece of metal mounted on the axle which can take this axle torque and transfer it additional up the frame, as a result relieving the dropout itself from choosing 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, in that case axle can rotate some volume and the bolt will slide in the slot. Though it will eventually bottom out and prevent further Torque Arm china rotation, by the time this takes place your dropout may previously be damaged.
The tolerances on motor axles can vary from the nominal 10mm. The plate may slide on freely with somewhat of play, it may go on flawlessly snug, or sometimes a little amount of filing could be necessary for the plate to slide on. In situations where the axle flats are a lttle bit narrower than 10mm and you are feeling play, it isn’t much of a concern, nevertheless, you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways and prevent the torque plate from sitting smooth against the dropout. If this is actually the case, you should be sure to possess a washer that fits inside the lip area. We make custom “spacer ‘C’ washer” for this job, though the lock washer that comes with many hub motors can often be about the right width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless steel band can help to make the ultimate installation look more discrete and protect the paint job from getting scratched. We contain several pieces of shrink tube with each torque arm deal.
However, in high ability 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 motor cables and potentially leading to the wheel to fall correct out from the bike.
In most electrical 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.