Overhung Fan Field Balancing - Trial Weight

[NewfEng]

I am attempting to calculate a trial weight to use for the in-situ balancing of an overhung fan. I have found a lot of information about in between bearing trial weights but very little if any on overhung.

Can the rule of thumb, twice the unbalance specification, be utilized for overhung fans or should other factors be considered??

Any help would be greatly appreciated.

 

[electricpete]

should other factors be considered??

If there is indication the fan is operating near resonance, err on the side of caution and start with smaller trial weight.


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(2B)+(2B)' ?

 

[FeX32]

Do we balancing rotating machines by trial and error nowadays?
Wouldn't the best way consist of analyzing the unbalance first. Then to conduct a mass center analysis where to goal is to keep the global mass center stationary?

I guess you could determine the placement of extra mass by attaching an accelerometer to the fan. Then start with a low mass and keep adding until the unbalance is near zero.

Correct me if im wrong.


Fe

 

[JJPellin]

FeX32,

We would not normally consider the use of a trial weight to be trial and error. The vibration would be analyzed. A trial weight would be placed and the response measured. This would be used to calculate the location and mass of the balance correction. This would produce the best chance of achieving the required final balance with the fewest corrections. In a field-balance situation, it is desirable to limit the number of times you have to climb inside that fan.


Johnny Pellin

 

[FeX32]

Thanks Johnny.


Fe

 

[electricpete]

Do we balancing rotating machines by trial and error nowadays?
Wouldn't the best way consist of analyzing the unbalance first.

The trial weight run is an input to the analysis. It establishes the vector influence coefficient/vector which relates unbalance magnitude and phase angle to resulting vibration magnitude and phase angle. From that we can estimate the magnitude and location of unbalance correction that needs to be added to compensate for the original vibration vector and ideally (in a single-plane balance of linear machine) accomplish the final correction. Typically there is not enough information available to accomplish a one-shot balance (walk up to machine and add the weight that will give desireed effect without any prior trial weight run).... if you haven't balanced this machine before you don't know the influence coefficient/vector(s).

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(2B)+(2B)' ?

 

[FeX32]

Very understandable.
Also thanks Pete!


Fe

 

[GregLocock]

Yes, the size of the trial weight is a bit of a guess. Your rule of thumb doesn't sound ridiculous.

A safer option would be to start with a very small trial mass and increases its mass and later its position until you've got a phase change of 90 degrees at roughly the same level of vibration, but that is 90% of the whole job, I'd only consider that on an expensive or fragile machine.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Tmoose]

Do you have 1X vibration measurements in 3 directions on each fan bearing as it is running now? There can be hints in that info if resonance is amplifying the running speed vibration.

The late, great Charles Jackson suggested a trial weight that would produce "centifugal force" equal to about 10% of the rotor weight.
IRD (among the pioneers of vibration analysis instrument suppliers) suggest the trial weight should make ~10% change in measured 1X vibration amplitude or phase angle to be useful.
Overhung rotors sometimes are kind of sensitive, so starting with a modest trial weight would be prudent, even if you are limited to just a few starts (and thus balance shots) per day.

As others have said, the big risk comes when the vibrating system is resonant, and trial weight(s) that should make a small change results in VIBRATION TEN TIMES HIGHER THAN EXPECTED.

 

[electricpete]

Since phase angle is often measured against an arbitrary reference, what is meant by 10 percent change in phase angle? (Percent of what?)

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(2B)+(2B)' ?

 

[NewfEng]

I have attached a word doc with screen shots of the velocity spectrums showing the 1X vibrations taken in the Axial, Vertical, and Horizontal Directions for each fan bearing.

This fan by no means would be considered expensive or fragile. We have decided to go with the weights calculated using in between bearing calc (

http://www.pumpcalcs.com/calculators/view/104/?)

as we have some history of performing this successfully on similar overhung fans using these values.

Thanks for your responses so far.

 

[Tmoose]

Hi Pete,

thanks for keeping me honest.

"what is meant by 10 percent change in phase angle? "

Intended was a minimum usefully significant angle change if the amplitude with trial weight just happens to be unchanged. I really should look at the IRD manuals in the barn before guessing that I think I thought it might have possibly said 10 degrees or 10% amplitude was likely OK.

I 'll also check if those degrees are Fahrenheit or Celsium.

Dan T

 

[electricpete]

F or C. LOL. Thanks Dan.

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(2B)+(2B)' ?

 

[MachineryWatch]

Hi guys,

Just a little refresher. Most balancing documents refer to the "30-30" rule. This means that by adding the trial weight you hope to achieve either a 30 degree change in phase angle, OR a 30% change in the 1X RPM amplitude. Having one of these numbers change at least this much tends to make the vector calculations (or graphs, if you are so inclined) more accurate.

The correct trial weight is supposed to achieve an increase in centrifugal force equal to the 10% of the rotor mass per bearing. The trial weight mass calculation that is probably spelled out in NewfEng's document is based on rotor mass, rotor RPM and trial weight radius. The same formula is used in modern data collectors with balance programs to calculate the mass of the trial weight.

The formula is designed to be conservative to prevent damage (especially in the case of the possibility of running in resonance) yet still achieve compliance with the "30-30" rule. However, it should be standard practice to establish the presence or absence of a resonance condition before an attempt at balancing the rotor. If there is resonance, balancing may be difficult.

In my experience the calculation of the trial weight mass to achieve an increase of centrifugal force equal to 10% of the rotor mass per bearing is a little too conservative. Meaning I often find it necessary to use a little more mass than this to achieve the "30-30" rule.

I hope this helps.

http://www.machinerywatch.com

 

[skfboiler]

After I get my reference run, I will get a coast down (Bode Plot)data when they shut the machine down for the trial. With the coast down I can determine if the machine is running above or below a critical speed. If the machine is running below critical, the high spot (the refernece phase reading)is close to the heavy spot. I would then place my weight 180 degrees from my reference phase, the high spot. As far as the amount, I would use something like Charles Jackson's method mention earlier to estimate the safe trial weight.

 

[rob768]

Calculation the weight is fine, but doesn't work at or near resonance speeds. Even the location of the weight may be off in such a case. Especially if resonance is expected: err on the safe side.