My Grandparents recently moved into assisted living, so, I got the vacuums when we took their house apart.  I just finished restoring the above mentioned vac.  I replaced the motor housing and the fan chamber - it was making a bad bearing noise and that fixed that problem.  I also replaced the quadraflex agitator brushes and the bearings in the agitator as well as the powerdrive belt and agitator belt.  It runs great now, however, every so often it throws the powerdrive belt.  It seems to happen after you vacuum for a while then stop, to say move stuff around and then restart.  The only thing I can figure is that either the transmission is slightly off alignment or maybe the trans. is heating up and under the restart load it slips the belt.  When you put the belt back on, you can powerdrive vacuum for as long as you want without it coming off, and it runs like a champ.  Any experts here have any ideas?  The U3101 has a smooth drive shaft, or mandrel I believe.  It looks like they may have modified it later to include two "bumps" to keep the belts centered.  While I had it apart I also checked the carbon brushes, the were stll quite long, so either Grandma had them replaced along the way, or, Grandma did not vacuum very much!
 
So today I vacuumed approximatly 15-1800 sq feet of carpet in my home with it.  We have lived here about two years since we built the house new.  I usually vacuum quite frequently 1-2x per week with the central vac with my Lindhaus power head.  The central vacuum dust bin is still collecting carpet fiber.  Today, I filled the bag on the Hoover with carpet fluff to the point it had started to back up in the bag-fill tube.  Is the hoover that much better than my central vac with what is supposed to be the best power head available?  Any ideas?  Please let me know, I'm starting to think the central vac is a waste of time.  It looks like what I really need is a powerhead with the brute force of the Hoover coupled with my current central vac which moves 116cfm and has 125" water lift.   Anybody have any ideas or comments?

Hi Turbo,Carmine pretty much told you what to look for as far as the p.d. belt jumping.
 
 another problem might be this.
 
take the transmission wheels off,you will need snap ring pliers. Chect the phillip head screws that attach the bracket to the trans housing. They over time come loose and back off. Take some thread locker,[loc-tite] and re install. It's very important that the correct belt is used 38528***.  
 
Yes there are bearings inside the transmission, they can grenade also. More than likely the pulley side,  These are  the same bearings used in the brush rollor. You seem to have the ability to do this job.
 
 The lindhaus power head is a very good powerhead ,it should work well behind the central system.
 
good luck.......
 
 O.T.