When the axle is put in, the noise generated by the motor reduced, and the axle could not rotate, even if attempted manually. In fact, it's very easy to rotate the axle manually when turned off, but once turned on, it's difficult, if not impossible. If the screws are tight, the whole mechanism just sticks together, perhaps because of the magnetic field and moving the axle would move the whole thing. If the screws are not tight, moving the axle would spin it a bit, but does not start the motor as expected