To give a feeling of the magnitude of the forces, a hub electric motor with a 12mm axle producing 40 N-m of torque will exert a spreading force of slightly below 1000lb on each dropout. A torque arm is usually a separate piece of metal mounted on the axle which can have this axle torque and transfer it additional up the frame, as a result relieving the dropout itself from spending each 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 is going to bottom out and prevent further rotation, by the time this happens your dropout may previously be damaged.
The tolerances on electric motor axles may differ from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it may go on perfectly snug, or in some cases a little amount of filing could be essential for the plate to slide on. In scenarios where in fact the axle flats will be a bit narrower than 10mm and you feel play, it isn’t 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 stop the torque plate from relaxing smooth against the dropout. If this is the case, you should be sure to get a washer that suits inside the lip spot. We make customized “spacer ‘C’ washer” for this job, although lock washer that is included with a large number of hub motors is normally about the right width and diameter.
For the hose-clamp model, 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 package deal.
However, in high power devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present can exceed the material durability and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor Torque Arm china cables and potentially leading to the wheel to fall proper out of your 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.