Balancing a fan

I bet you can whip one of these up in no time!

--Winston

What comes to mind to me. The fan has to come off the shaft (might be dificult).

Take a ball berring that's larger than the shaft hole in the center. Drill a counter sink on the end of a shaft. Put the ball berring on the end of the shaft, and the fan on the ball berring. Ends up looking like a crude lamp shade.

That should give you a rough idea which is the heavy side. Similar to balancing lawn mower blades.

Something like this, only bigger?

Before you dynamic balance, can you check the static balance? If the static balance is perfect then I'd doubt that the thing is out of dynamic balance.

If those bearings aren't as free as free can be then you've got a problem. How much of a pain is it to take the fan out? Could you use the "knife edge" with the fan shaft on a couple of angle irons, corner up?

Or: can you take the shaft out, and make an adapter so the thing will go on a truck tire balancer?

Remember that any vibration would relate to the rotational speed of the item out of balance. Can you get your hands on a resonant reed tachometer? The RRTs are recommended for isolating vehicle driveline vibrations by identifying the frequency of the vibration so you can correlate it to which driveline components rotate at that frequency.

As for fan balance, if you just take the drive belts off you should be able to turn the fan by hand to identify any issues. Mark the fan and spin it by hand a few times and see if it is stopping near the same orientation each time. Again, I doubt it's the fan since I expect it's spinning a lot faster than the 540 RPM PTO and won't have a 9 Hz period if it has an imbalance. While you have the belts off to hand spin the fan, it would be good to check both drive and driven pulleys for runout with a dial indicator.

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

From the text Mechanical Vibrations written by Den Hartog: If you have an out of balance shaft rotating at less than the "Critical Speed" and you approach that shaft with a pencil or a felt tip pen, the mark will be made on the "heavy" side. Add wts to the opposite side from the mark. However if the shaft is rotating at more than the "Critical Speed", your pencil/felt tip pen will mark the light side and wts must be added to the same side as the mark. If you are unfortunate enough to have the shaft rotating at the "Critical Speed" the mark will be made 90 degrees after the heavy side.

This concept has helped me balance helicopter main and tail rotors down by factors of 4 below the normally acceptable levels. However I do have an electronic balancer that uses velocimeters to measure the magnitude of the vibration and a photo sensor to give me the "pencil" mark. We did have a subscriber to our magazine that balanced his tail rotor using just a dial indicator and a graphic technique developed by the Russians. I think that I could make a copy of the article describing that technique available if you want it.

Stu Fields Experimental Helo magazine.

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 one. Julie's not to keen on that idea, but only because we haven't got the money.

karl

Before you go blowing a big wad of money on a new blower, spend a day checking the things we noted. It should not take that long to check the fan for imbalance, the pulleys for runout, etc. This isn't that complicated, it's just an organized process of isolation and elimination.

Yeah, but you would need almost laboratory conditions, as the rotation magnifies imbalance forces according to V^2/R. small = big!

I suggested dynamic balancing at a good garage, altho come to think of it, I haven't seen this in a while Altho, ackshooly, VP Tires, Yonkers NY does dynamic balancing -- $10 per tire. Puts weights on the inside AND outside of the rim, per the pyooter. Great place.

Vinny at VP said static balancing went out with horses'n'saddles -- direct quote. :)

Whether he could do Karl's squirrel cage is another story. BUT, if Karl were to go to a place like this, and himself make the mounting adapters, I'm sure it would work, altho these machines might be used to measuring more forces from big tires/rims than a squirrel cage might offer.

The Fluke gadget is the real solution, and actually sumpn that might be a worthwhile purchase in a number of shops, as it proly pays for itself in bearings. Even on the flywheel/pulley of compressors, etc, or multi-step pulleys in old drill presses, etc..

Jim Pentagrid recently posted about how he balanced a bench grinder using a small computer speaker with a lump of clay added to turn it into an acceleromete I will try to find the thread for you.

Dan

=A0 =A0 =A0 =A0 =A0 =A0 =A0Dan

Orthoganol balancing - twinwheel grinders July 2009

Dan

How about this for a hypothesis: There is nothing at all wrong with the fan, itself. Something got sucked in and is stuck between a couple of blades.

Pete Stanaitis

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Karl Townsend wrote:

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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.

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.

Have you considered how much power it would take to spin that squirrel cage on the dynamic balancer? You'd have to do something to keep the rotor from pumping air, perhaps a disk (precisely balanced, of course) to cover each end.

On Fri, 14 May 2010 07:13:46 -0700, Winston wrote the following:

That's a static balancer, Pooh. He probably needs dynamic balancing.

Karl, ask around for vibration monitoring companies. When I worked for Palomar Technology (one of them, who was bought out by SKF), they had techs who would check everything up to and including the 30' turbines at SDG&E' steam/electric plant. Or ask the bearing folks directly.

Either that or to pull the bird, apple, or mouse out of the fan blades. ;)

-- Work and struggle and never accept an evil that you can change. -- Andre Gide

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".

Video? Pictures? Audio?

Airplane to spray it !

SW

Very astute guess , Pete, probably a rat or snake ..Jerrry

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That could make things rather exciting. :-)

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.