Whirl mode harmonics
Whirl mode harmonics
(OP)
Is there a such thing as a half harmonic of a whirl mode?
I am chasing a vibration issue that is dominant at a specific speed. It is not a fixed event that I can run through as it appears at a lower magnitude at lower speeds and increases in magnitude up to this specific speed. I created a Campbell diagram and cannot find any driver that is a multiple? It is a axial turbine device with a gearbox output so there are lots of possible drivers.
Comments welcome.
I am chasing a vibration issue that is dominant at a specific speed. It is not a fixed event that I can run through as it appears at a lower magnitude at lower speeds and increases in magnitude up to this specific speed. I created a Campbell diagram and cannot find any driver that is a multiple? It is a axial turbine device with a gearbox output so there are lots of possible drivers.
Comments welcome.





RE: Whirl mode harmonics
There are a number of things that can cause 1/2 running speed vibration. Is that what you are refering to? Is the vibration frequenyc 1/2 of the turning speed of the shaft, if so it could be instability, or a rub...
What kind of bearings on the machine? What is the first critical speed? What is the actual vibration frequency that is bothering you, and what is running speed?
Finally what are the amplitudes? Have you looked at an orbit? If so is it big and round, or is it scrunched up and squashed?
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RE: Whirl mode harmonics
The first two should not occur in the absence of gravity.
http://www.dliengineering.com/vibman/oilwhirl.htm
Cheers
Greg Locock
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RE: Whirl mode harmonics
RE: Whirl mode harmonics
I have a frequency spike being picked up on my case mounted accelerometer that tracks with speed but at a different slope. It hits some sort of resonance at 35,000 rpm turbine speed with a vibration spike at 7k hz. I also picked up a distinct harmonic at 14k hz also. The vibration spike increases in magnitude by almost 10X between 32k rpm and 35k rpm. I can not find some multiple of my 1/rev or anything else that comes close to the 7k hz that I am measuring. I am a little stumped.
RE: Whirl mode harmonics
At a turbine speed of 35000 rpm, you have a vibration spike at 7 KHz and one at 14 KHz. Is this correct?
As you vary rpm the amplitude increases between 32K and 35K rpm, so it sounds very much like a resonance being excited.
Are the 7 KHz and 14 KHz frequencies fixed vs speed, or do they track running speed at all, or do they just appear? In other words at 32000 rpm do you see peaks at 6400 Hz and 12800 Hz (12 and 24 times running speed)?
If so you need to find something that would cause 12 time running speed correct? So what more can you say about the machine configuration?
But I get the feeling that is not what you are trying to tell us. If the frequencies are not directly related to running speed, they may be an acoustic excitation due to the fluid flow through the turibine or turbine piping. Flow induced vibration is not uncommon and is usually a resonant phenomonon.
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RE: Whirl mode harmonics
RE: Whirl mode harmonics
What is the gear box driving?
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RE: Whirl mode harmonics
Do I gather you have 35 blades on the turbine?
Cheers
Greg Locock
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RE: Whirl mode harmonics
RE: Whirl mode harmonics
As sms said, gear tooth combination would be helpful. Maybe simple gear mesh frequency with 12 teeth on the pinion?
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