All of the belts that were thrown no longer lay flat- they looked like they
were stretched on one side. This, I was told, was due to some of the cords
being broken on one side versus the other.

One episode found the belt actually inside out on the pulleys, but still in
place. Yet, the overall alignment looks good to everyone who has inspected
the system.

The only difference so far is that at the last belt change, we noted that
the lower (non-tensioning) alternator bolt was very loose.

I bought two spares, and one of them is now on the plane. I forget the
source, and didn't save any of the old belts, but clearly they were all
"warped" and didn't lay flat.

Again, this was attributed to "whipping" of the belts by two mechanics, as a
result of the engine being detuned due to a sticking crankshaft
counterweight.

I again questioned why this should occur all of a sudden after changing the
belt at annual, since the previous belt had gone for hundreds of hours
without any indication of a problem. No one had a good answer for this, so
somehow the logic of the crankshaft counterweights being the source of the
problem is a little suspicious.
"Jim Burns" wrote in message
...
I'll throw a few things out there based on my experience with v-belts. We
have literally thousands of v-belts on a wide variety of equipment. Our
mechanics have become very picky about which belts they use. Sometimes we
run into a change that the manufacturer has made that causes the same
brand and part numbered belt to do strange things. We've seen things such
as a change in the composition of the rubber of the belting material
causing additional friction and "pulley climb" and changes in the number
of layers of reinforcing fabric and changes to the hardness of the belts
causing problems because it is either more or less flexible than the
previous belt.

Do you still have the old belt(s)? Are the belts that were being thrown
off worn, chaffed, or stretched unusually? Polished or shiny spots on the
sides of the belt will indicate slipping. Cracks and fraying will
indicate that the belt was either overtight and/or old when installed. Do
they lay flat when you simply lay them on the floor? Twisted belts can
indicate misalignment. I'm just wondering if there was something wrong
with those specific belts. How old where they when installed? Brand and
part number identical to the latest 3 hour belt? Can you measure them to
check them against the original length and what the belt number states?

Could either pulley have slipped on the shafts? Do you have room to run a
straight edge across and between the faces of each pulley to check for
alignment?

Too tight? Too loose? Rule of thumb tension is a deflection mid way
between the pulleys of 1/2 the thickness of the belt, but check your MM.

Jim

"Viperdoc" wrote in message
...
Threw the belt again after twenty minutes in IMC on the way to the
avionics shop. Destination was VMC, so elected to continue rather than
turn around or land, and the other alternator was handling the load
easily.

Avionics guy fixed the radar (took three guys around 30 minutes). Belt
replaced.

Flew 1.5 hours back- no problems. After return I tightened belt again,
and then flew another 1.6 hours doing LNAV/VNAV approaches in IMC (much
smoother than ILS).

Checked belt again, which appeared tight. So, last belt has now gone over
three hours, where the others have all thrown in less than 30 minutes.

What gives- can a new belt stretch so much that it gets thrown that
easily, or should I still suspect that the engine is detuned and has a
sticking counterweight?

The engine shop quoted a cost of over $3,000 as a minimum to change the
counterweight bearings, plus any costs associated with finding something
wrong with the cylinders.