To give a feeling of the magnitude of these forces, a hub electric motor with a 12mm axle creating 40 N-m of torque will exert a spreading force of just under 1000lb on every single dropout. A torque arm can be a separate piece of metal mounted on the axle which can have this axle torque and transfer it even more up the frame, therefore relieving the dropout itself from spending 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 preventing 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 lttle bit of play, it may go on properly snug, or in some cases a small amount of filing could be essential for the plate to slide on. In situations where in fact the axle flats happen to be somewhat narrower than 10mm and you are feeling play, it is not much of an issue, but you can “preload” the axle plate in a clockwise way as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and prevent the torque plate from seated toned against the dropout. If this is the case, you will want to be sure to possess a washer that suits inside the lip location. We make custom “spacer ‘C’ washer” because of this job, though the lock washer that is included with various hub motors is normally about the right width and diameter.
For the hose-clamp version, 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 pieces of shrink tube with each torque arm package.
However, in high electric power devices that generate a whole lot of torque, or in 
setups with weak dropouts, the forces present may exceed the material power and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the engine cables and potentially causing the wheel to fall right from the bike.
In most electric 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.