Proper Method for Balancing a 2 Stage Fan Rotor

[aeroUB]

Hi

I would like to get some input from some experienced vibration/balancing people. There has been some debate (between me and some others), about the proper way to balance a 2 stage axial fan rotor. I believe that it should be treated like a 2 plane imbalance and can use a vibration analyzer/balancing computer to 2 plane balance the rotor. Other 'old timers' think it should be treated like a single plane most time and just split the weight between the two hubs of the rotor. Some think the weight split ratio should be determined by the ratio of vibration amplitudes. Some think you should just try to balance the end that is highest with a single plane balance. I want to know the best, fool proof way of balancing these rotors if the issue at hand is in fact a mass imbalance in the rotor, somewhere. This is a main shaft assembly supported by two sleeve bearings (sometimes a anti-friction bearing assembly, but most times two large sleeve bearings). Each 'hub' is it's own assembly that is mounted to the main shaft. It is very possible that one 'hub' had a mass imbalance and the other one doesn't. Treating the whole rotor (both hubs and the main shaft) at a single plane and splitting the weights seems ridiculous to me. I could possibly see where it could be addressed as two separate single planes? I suppose the test would be, does putting a weight on one 'hub', change the vibration in the second plane? But, the bearings are so close to each other (between the two stages of the rotor) that of course they are.

I would think you should do an initial run and collect your data from both planes (IB and OB bearings) along with the phase, then place a trail weight on plane (hub) #1 and collect data, then remove the trail weight from plane #1 and place it on plane #2, then calc. the balance for both planes. Is there a reason this wont work or might not be the best approach!?!

I am trying to attach a quick sketch of the basic rotor configuration. Let me know if it's not showing up.

Thank you very much for any input you might have!

aero

 

[GregLocock]

Two plane balancing will work even if there is a degree of cross coupling between the two measurement locations. The best practice would be to do a 2 plane balance. That is by no means the worst case scenario, flex shafts are much worse. If the owd codgers are right your solution will approximate theirs for the cost of a few more runs.

Cheers

Greg Locock


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[JJPellin]

In a perfect world, I would want to balance each hub separately using either single plane or two plane depending on the L/D ratio. Then I would mount both hubs on their shaft and perform a two plane trim balance with each hub serving as one correction plane. If this is too much trouble, then I would go with your plan. I assume you are performing the balance in the field, so mounting and dismounting a hub is probably not practical. If that is the case, I would absolutely use a two plane method, as you describe.

Johnny Pellin

 

[spciesla]

Two plane balancing as you have described should work well in this situation. You can do the static-couple calculations, and if one is dominant, you may pursue that path. The two plane method should get you there as well, and will address the cross-coupling effects.

At a previous job, we field balanced a number of vertical, 2 stage, vane axial fans. Although symmetrical, the lower fan wheels were more sensitive to balance weights than the upper.

Good luck.

 

[Tmoose]

Is this fan moounted in a duct?

http://patentimages.storage.googleapis.com/US20120128494A1/US20120128494A1-20120524-D00000.png

If the proportions in your image are nearly right the overhung rotors are going to boss the bearings around pretty bad.
I'd be ready for trouble.

Typical fan support coNstRuCtioN is pretty wimpy sometimes, narrowly missing resonances at best, and frequently quite flexible.
If either rotors is "tipped" at all large amounts of couple imbalance result.

 

[spciesla]

Just looked at the OP's image, those are some hefty fan wheels and quite the span. What is the speed of your fan? Where/how are balancing weights added?