To give a feeling of the magnitude of the forces, a hub 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 is normally another piece of metal mounted on the axle which can have this axle torque and transfer it additional up the frame, therefore 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 normally loose, in that case axle can rotate some amount and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by enough 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 a lttle bit of play, it may go on properly snug, or in some cases a little amount of filing could be necessary for the plate to slide on. In conditions where in fact the axle flats happen to 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 quick release “lawyer lips” which come out sideways preventing the torque plate from sitting smooth against the dropout. If this is the case, you will need to be sure to have a washer that matches inside the lip region. We make customized “spacer ‘C’ washer” for 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 model, a small length of heat-shrink tubing over the stainless steel band can make the ultimate installation look even more discrete and protect the paint job from getting scratched. We involve several bits of shrink tube with each torque arm package.
However, in high vitality systems that generate a lot of torque, or in setups with weak dropouts, the forces present may Torque Arm china exceed the material durability and pry the dropout open. When that happens, the axle will spin freely, 
wrapping and severing off the engine 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 in to the dropout slot and provide some measure of support against rotation. Oftentimes this is sufficient.