To give a sense of the magnitude of these forces, a hub electric motor with a 12mm axle making 40 N-m of torque will exert a spreading force of slightly below 1000lb on every single dropout. A torque arm is normally another piece of metal mounted on the axle that may take this axle torque and transfer it even more up the frame, hence relieving the dropout itself from spending each of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is certainly loose, then axle can rotate some sum and the bolt will slide in the slot. Though it will eventually bottom out and prevent further rotation, by enough time this takes place your dropout may already be damaged.
The tolerances on electric motor axles may differ from the nominal 10mm. The plate may slide on freely with a bit of play, it may go on correctly snug, or occasionally a little amount of filing could 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 isn’t much of a concern, but you can “preload” the axle plate in a clockwise route as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and prevent the torque plate from sitting smooth against the dropout. If this is the case, you will need to be sure to possess a washer that suits inside 
the lip place. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that comes with a large number of hub Torque Arm china motors is often about the proper 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 even more discrete and protect the paint job from getting scratched. We incorporate several pieces of shrink tube with each torque arm package.
However, in high electrical power systems that generate a 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 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 in to the dropout slot and offer some measure of support against rotation. Oftentimes this is sufficient.