To give a sense of the magnitude of these forces, a hub engine with a 12mm axle creating 40 N-m of torque will exert a spreading force of just under 1000lb on every dropout. A torque arm is certainly another piece of metal mounted on the axle which can take this axle torque and transfer it further up the frame, thus 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 usually loose, after that axle can rotate some amount and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by the time this happens your dropout may already be damaged.
The tolerances on engine axles can vary from the nominal 10mm. The plate may slide on freely with somewhat of play, it could go on flawlessly snug, or in some cases a little amount of filing may be necessary for the plate to slide on. In circumstances where in fact the axle flats are a little narrower than 10mm and you feel play, it is not much of a concern, nevertheless, you can “preload” the axle plate in a clockwise direction 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 actually the case, you will need to be sure to have a washer that fits inside the lip Torque Arm china location. We make custom “spacer ‘C’ washer” because of this job, although lock washer that is included with various hub motors can often be about the right width and diameter.
For the hose-clamp unit, a small length of heat-shrink tubing over the stainless band can generate the final installation look more discrete and protect the paint job from getting scratched. We include several bits of shrink tube with each torque arm program.

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 strength and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor cables and potentially leading to the wheel to fall correct 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. In many cases this is sufficient.