In an article from Richard Nailen, answers to questions you may not have thought to ask, such as:
Q: How much endwise force is involved when a motor is restrained against finding its magnetic center?
A: The value is typically quite small. Measurement is difficult because holding a spring scale against the end of the rotating shaft introduces its own axial force. But motor bearings aren’t designed to withstand even these low values. Bearings with thrust capability could be provided-at a price.
Q: Is thrust the only issue?
A: No. Figure 2 shows another effect, largely resulting from the greatly increased magnetizing current associated with the distorted magnetic field. But it is of little importance, because no properly constructed sleeve bearing motor could allow such extreme displacements; they result only from gross manufacturing errors.
Please read the whole thing.