On Fri, 14 May 2010 19:38:48 -0700, "Stu Fields"
wrote:
"DoN. Nichols" wrote in message
...
On 2010-05-14, Karl Townsend wrote:
"Don Foreman" wrote in message
...
[ ... ]
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.
--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---
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".
I think I saw that Russian technique described somewhere along the
line.
The technique may be incomprehensible to some PhD's because a
coherent, linear-thinking approach may fail to derive theory
supporting it. It may be a technique that uses bits of disjoint
theory and perhaps some assumptions that aren't generally supportable,
but works most of the time and/or perhaps all of the time in most
real-world cases -- one of which may be tail rotors.
I led an R&D team that developed a self-balancing washing machine some
years ago. Interesting problem because of severe cost constraints of
the whitegoods market and ill-defined boundary conditions. Example:
consumer washers are never bolted to concrete, and some set on the
flimsy floor of a doublewide. The structure was comprised of stamped
sheetmetal and some plastic, certainly not cast iron like machine
tools or tire balancing machines. There were a ****load of
resonances, and many of them weren't very predictable. In addition,
this washer went much faster than traditional washers in the spin
cycle, because it could self-balance. That reduced energy necessary
for drying. Barely-damp clothes peeled off the drum with pronouced
dimples in them.
The team used a technique using oversensing and SVD (singular value
decomposition) sometimes used in aerospace guidance & control systems.
It worked! At least a dozen patents were spawned.
Then politics and corporate infighting rendered the project
stillborne.