Plastics are not that rigid and misalignment of the shaft during operation due to deflection would shorten the bearing life and could also cause pulsing of the rotor due to the air gap variations between the rotor and stator. The tension on the belt will cause a radial load on the bearing that could over time cause a permanent deformation of the housing that aluminum or steel would be able to withstand. Don’t underestimate how hot it can get inside the motor. The outside temperature is significantly cooler than the winding temperature, so depending on which plastic you choose, there could be deformation at a much lower outside temperature than you expect. This article explains what happens as temperature increases.
It sounds like you got a motor with a B5 style flange, that will be difficult to use as is. I think your best bet is to machine one out of aluminum or steel. Or hopefully you get lucky and can swap the existing drive end housing to the new motor.
The Effects of Temperature
The polymers we work with follow the same principles as the body: the hotter the environment becomes, the less performance we can expect.
www.ptonline.com
It sounds like you got a motor with a B5 style flange, that will be difficult to use as is. I think your best bet is to machine one out of aluminum or steel. Or hopefully you get lucky and can swap the existing drive end housing to the new motor.