Volvo Repair: 1990 240 DL Chewing up Alternator Belts, crank pulley, gates belts

Question
Two weeks ago my 240 threw an alternator belt- the belt had been on there for probably 30K+ miles without a problem except it made some noise when it rained. I replaced it and that belt (gates 7365) frayed within 100 miles so I assumed I had gotten a DOA part and replaced it. That replacement was thrown this afternoon, these remains were by far the worse in appearance as it appeared that the belt had lost several "teeth" in a few different locations.

Some people who I have spoken to believe that something is locking up but I feel the alternator must be fine since I haven't noticed my alternator indicator even blinking except for when the belt is completely gone. The fan seems to spin effortlessly when the car has been running well recently. I am becoming frustrated with the situation and just need a little help in what I should be looking for. First I am buying good quality belts, I am putting them on correctly, and everything seems to be aligned well. I didn't pay as much attention because I was convinced the last belt was DOA but there appears to be an oddly place "knob" on the front of the engine between the pulley on the fan and the crank pulley. I will bring the car home in the morning and buy a new belt - is there anything I am missing or some way to ensure fewer issues?

Answer
Matthew, sounds like its out of alignment.  Personally I hate Gates belts.  We use contitech and the size should be 11.9 X 918 or 920.  That knob sounds like the adjustment housing.  There should be a bolt there for adjustment.  Lastly check the bushings.  There are 3 of them and they wear out and would cause it to throw a belt.  Roger