To give a feeling of the magnitude of these forces, a hub engine with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A torque arm is normally another piece of metal attached to the axle which can consider this axle torque and transfer it further up the frame, thus relieving the dropout itself from bringing 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, in that case axle can rotate some sum and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by enough time this takes place your dropout may previously be damaged.
The tolerances on electric motor axles can vary from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it could go on properly snug, or occasionally a little amount of filing may be essential 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 a concern, nevertheless, you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and prevent the torque plate from seated smooth against the dropout. If this is the case, you will need to be sure to possess a washer that matches inside the lip place. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that comes with many hub motors can often be about the proper width and diameter.
For the hose-clamp style, a small length of heat-shrink tubing over the stainless band can produce the ultimate installation look even more discrete and protect the paint job from getting scratched. We contain several bits of shrink tube with each torque arm bundle.
However, in high electricity devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present may exceed the material durability and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor cables and potentially creating the wheel to fall right out of 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 way of measuring support against rotation. Oftentimes this is sufficient.