"DoN. Nichols" wrote in message
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On 2010-05-15, Stu Fields wrote:

"DoN. Nichols" wrote in message
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[ ... ]

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

[ ... ]

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

[ ... ]

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 could make things rather exciting. :-)

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".

That is better than "It shouldn't work" at least. :-)

O.K. But the rotor is similar to a standard room fan blade,
rather than the squirrel cage blade, so the dynamic balance is not that
far from the static balance. With a squirrel cage blower, whose length
is similar to its diameter, the dynamic balance points can be very
different from static balance, depending on luck.

Enjoy,
DoN.

Given the geometry you mention, it sounds like a two plane balancing job
similar to that used on the auto crankshafts or in the case of the Helicycle
turbine engine the compressor and turbine wheels. (63,000 rpm) I watched
several of those things being balanced and it does seem to require
simultaneous balancing of both ends.
It would seem that the Squirrel cage might require a two plane static
balance by both balancing in the plane perpindicular to the rotation axis as
well as the plane containing the rotation axis. That might be a first step
in taking care of the rotational balance as well as the "rocking couple".
I've seen a helicopter rotor carefully statically balanced span wise that
needed no more span wise attention when run with the dynamic balancer.

I certainly agree that balancing a spinning mass can often involve requiring
the balancer to have the proper DNA and family history.