antiresonances at the critical speed 1

[heitor]

Hello.

I'm working with an horizontal rotor that has three planes of measure, one at one bearing and the other two at the shaft. Each plan has two transducers, one at the horizontal direction and the other at the vertical direction.

When the rotor is driven beyond the first forward critical speed, the two measuring planes at the shaft show anti resonances at the forward critical speed and also at the backward critical speed. The antiresonances are found in the vertical direction only, the horizontal direction shows the traditional peaks.

Please, could you help me to understand that?

Thank you!

 

[sms]

Going to need to know a little more about your setup. What you are trying to measure, and how you are measuring it. I assume you are using non contact proximity probe transducers? If so what you are seeing is very likely due to shaft surface effects that induce error in the signal. What is often refered to as a gull wing resonance where the amplitude is high at low speed, goes almost to zero at the critical, and then is high again above the critical is often due to how the shaft runout error adds vectorially to the actual rotor response.

 

[heitor]

Yes I'm using non contact displacement probes.
I didn't understand the gull wing resonance. When I look the displacement curves, the results are just like you said. How could the runout counteract the resonance peak?

Thank you very much!

 

[heitor]

Sms,

Just to say that I found a paper on the dynamics of a bowed rotor that confirms what you said!!!
The paper is "A Note on Jeffcott Warped Rotor" by J. S. Rao (Mechanism and Machine Theory 36).
Now you see why you got a star

Thank you!!!

 

[sms]

You are welcome. Have you determined for sure that you have a bowed or bent shaft? Also at the Bently Nevada website

http://www.bently.com

you can find articles on runout compensation. This technique uses vector subtraction to adjust the data and eliminate the effect of the shaft bow or runout.

 

[heitor]

I have not yet measured the shaft, but I supose that the static deformation caused by the shaft weight could be the 'guilty', what do you think?

Thank you!!!

 

[sms]

Again, you have not told me anything of your machine configuration, but it is not unusual for turbines or compressors rotors to bow either due to heat, gravity, or due to a rub, or even due to the way the wheels are stacked on the shaft. You might run a dial indicator near the journals to corrolate mechanical runout with what the proximity probes are seeing.

If you are running a rotor kit of the kind Bently Nevada makes, then it is very common to have a bowed rotor due to the way the disks attach to the shaft. In that case, loosen the disks, and reattach being carful to evenly torque the screws on each side of the disk.

 

[heitor]

Sorry

It's a turbine, not a rotor kit.

Thank you for helping me, sms.