What makes fan belts wear out and break?  Jim replaced the one in his car 2
years ago, and rarely drives, and now it broke (disappeared) again.  Also it
sometimes squeaks but not always.  He hypothesizes that the engine is moving
around and therefore changing the distances involved so that it is sometimes
too loose and sometimes too tight.

6 responses total.

Several things can contribute.

Rubber degrades with age.  It gets stiffer and more brittle, causing it
to crack and eventually fail as it goes over the pulleys.  This happens
whether the car is run or not.  Two years seems like a short life, though.

Misaligned or wobbling pullies will wear belts out prematurely,
sometimes dramatically so.  Bad bearings in the accessory being driven
can contribute to this.  Lay a straightedge against the pullies and make
sure they're perfectly aligned with each other.  If not, find out why. 
Watch the pullies as the engine runs and see if they're wobbling
excessively.

Improper tensioning can contribute too.  Too loose and the belt will
slip, causing it to squeal.  Once a belt starts to squeal, it gets
"glazed" and will slip more easily in the future.  Too tight and the
bearings in the accessory will wear out prematurely.

Generally things that are belt driven are mounted rigidly to the engine.
 If they're moving around seperately as the engine moves, something is
wrong.

(Hmm.  I just realized I used two different plural forms for "pulley' in
that post and I'm not sure which is right.  Oh well.)

"pulleys" is correct, I think.

Oh, another thing that can wear belts out fast is if one of the pulleys
has a rough spot for some reason.  Not likely unless a piece of it broke
off (which has been known to happen with cast pulleys.)

Jim says good idea about the straight edge and he noticed the small pulley
was a bit rough and tried to scrape it.  Alternator.  He thinks an alternator
has slip brings rather than brushes like a generator.  To smooth the pulley
he would need to put it on a lathe.  Can you easily replace these pulleys?
He thinks not because they are pressed on and he would need a new alternator.
It might be cheaper to keep a spare belt in the car ($12).  Last year he had
to replace a pump ($25). Car maintenance is more than for bikes.

Yes, alternators have slip rings.  An alternator is actually a
three-phase AC generator, but it has an internal diode rectifier that
converts the AC to DC.  They're mechanically simpler than generators and
can work over a larger range of speeds.  (Generators generally didn't
charge well at idle.)

Usually the pulley is held on with a nut.  They're a pain to get off
because the alternator wants to turn while you try to loosen the nut.

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