86 coupe a/c belt

[Mark Donohoe]

I need some help identifying  a part of my a/c system.  I cannot seem to get the belt to stop squealing while I'm at idle.   There appears to be a small cylinder shaped part, about an inch long that can be adjusted closer to the a/c belt, maybe acting as a tensioner of some type.  However, the part does not spin like a normal pulley tensioner which leads me to believe my assessment is wrong.  I can't believe it would be a good idea to have the moving belt be in constant contact with a stationary part.  Seems like a sure way to wear your belt out really fast.  Is there supposed to be a pulley there?  I can't find an illustration anywhere in the Bentley.  The belt is the same one that was on the car when I bought it.  It looks small and seats really far down in the crank pulley.  Don't know if that would matter though.  At driving speed, everything works great.  Any thoughts?
Thanks
Mark