"DoN. Nichols" wrote in message
...
On 2010-05-14, Karl Townsend wrote:

"Don Foreman" wrote in message
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[ ... ]

I agree. I also don't see how fan imbalance would generate torque
ripple. I would suspect some aspect of the drive line between PTO and
gearbox: pulley, bent shaft, any couplings or U-joints, etc

Pete and Don, I also agree. I've said it CAN'T be the fan vibration for
years. But I also can't find anything else wrong. And its not for a lack
of
trying. I'm about to pop $11,100 and trade it in for a new used one.
Julie's not to keen on that idea, but only because we haven't got the
money.

I presume that you have tried static balancing the fan? If not,
that would be a lot cheaper to try at least.

Mount the blade on a shaft of proper diameter.

Set up two straightedges as level as you can get them.

Place the shaft on the edges, and observe whether it tends to
roll until one specific point is always down.

If so -- add weight opposite it until you no longer observe
this. Since you have no way to know the dynamic balance, add the weight
in the middle of the vanes (squirrel cage -- right?).

If you then see another point become the one which heads for the
bottom, add more weight opposite that -- unless the heavy spot is where
you just added the weights -- in which case reduce the weight a little.

Dynamic balance is a lot trickier to manage -- you need
expensive tooling for that.

Oh yes -- while you have the blower off its original shaft,
check the bearings for play.

Good Luck,
DoN.

--
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Don: We have an example where a man made a jig to turn his tail rotor, a
simple fan, from his helicopter, by an electric motor. He used a dial
indicator and a graphical technique developed by the Russians, to determine
the amount and location of the correcting weight. In theory it takes 4 spin
ups to determine the amount and location to put the weight. Then a 5th spin
up to verify. I've had some success with this method on a tail rotor of a
helicopter that had a critical speed kind of close to the operating speed.
That is the good news. The bad news is even some Phds who specialize in
vibrations can't explain why some of the steps are done. They revert back
to "Well it does work".