Blower Motor - Recurring Problem

At the end of last winter I had to replace the blower motor in my electric furnace. The motor hadn't burned out. The rotor wasn't rubbing on the stator, or at least didn't seem to be. The motor still seemed to be working fine other than one issue. When the fan was running, I would hear very loud metallic banging noises. It was getting worse. I watched the fan run and spotted the problem. I pulled the whole blower assembly out of my furnace and found that the motor shaft had an excessive amount of end play, over a half inch, and that the fan was hitting the bolts on the front of the motor. I checked carefully to make sure that the fan was not loose and moving on the shaft. It wasn't. I also found that if I loosened the fan and slid the fan slightly away from the motor, the fan would hit the blower housing instead. I replaced the motor.

The new motor had no end play in its shaft when I installed it. It's only been in use for a little over four months, but now has about 1/4" of end play. Not enough end play for the fan to be hitting anything yet, but I assume that's coming. Is this a common problem with these motors or is something else causing this to happen?