Overhung Fan Field Balancing - Trial Weight
Overhung Fan Field Balancing - Trial Weight
(OP)
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.
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.





RE: Overhung Fan Field Balancing - Trial Weight
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(2B)+(2B)' ?
RE: Overhung Fan Field Balancing - Trial Weight
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
RE: Overhung Fan Field Balancing - Trial Weight
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
RE: Overhung Fan Field Balancing - Trial Weight
Fe
RE: Overhung Fan Field Balancing - Trial Weight
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(2B)+(2B)' ?
RE: Overhung Fan Field Balancing - Trial Weight
Also thanks Pete!
Fe
RE: Overhung Fan Field Balancing - Trial Weight
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
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RE: Overhung Fan Field Balancing - Trial Weight
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.
RE: Overhung Fan Field Balancing - Trial Weight
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(2B)+(2B)' ?
RE: Overhung Fan Field Balancing - Trial Weight
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.
RE: Overhung Fan Field Balancing - Trial Weight
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
RE: Overhung Fan Field Balancing - Trial Weight
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(2B)+(2B)' ?
RE: Overhung Fan Field Balancing - Trial Weight
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
RE: Overhung Fan Field Balancing - Trial Weight
RE: Overhung Fan Field Balancing - Trial Weight