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John from PA

Sr. Member
Posts: 963
Reply with quote  #16 
Originally Posted by stace1g
Usually structural resonances cause relatively small negative going peaks in the bode plot that are ignored. They are also associated with small phase changes, not the large changes (180ยบ) associated with rotor flexural modes. The small amplitude and phase changes due to structural resonances cause small loops in the polar plot.

I was under the impression that structural resonances give you the same 90 degree phase shift (at peak resonance)? Nagesh states that he is indeed seeing a 90 degree change. 

BTW I agree with RustyCas regarding checking the structural integrity. Seen this a few times with and without an associated resonance.


Gary, you are correct but perhaps need to re-phrase slightly to "...structural resonances can give you the same 90 degree phase shift (at peak resonance)."  

In the instance of a structural, the damping is low and the structural response can be limited if it traversed quite rapidly, as is often the case.  In those instances both amplitude and phase changes simply may not have time to exhibit the full theoretical changes that could occur.  This can be seen in a rotor kit.  If the speed ramp is rapid you will get a significantly different response than when it is very slow and particularly at the structural resonances.

Sr. Member
Posts: 104
Reply with quote  #17 
Ahh i see. Thanks John. i guess you would still see the full phase change if you conducted an FRF impact test or similar.



Posts: 7
Reply with quote  #18 
So I did a cross channel phase measurement on the bearing housing to the structure since we were observing unidirectional vibration (11 mm/s in horizontal & 1.2 mm/s vertical) to check for symptoms of looseness. But there isn't any. We found the phase difference to be zero (Plummer block bolt to the plummer block mounting, then on the structure and finally the concrete). So I strongly feel that it could be some structural abnormalities causing it to resonate.

But one thing I want to eliminate here. The ID fan runs on flue gas which is around 170 degrees C. Do any of you think a thermal gradient might cause a bow kind of situation here at full speed?

And may be I forgot to mention that we have trended the casing vibration from the past 6 months. It went like this 4 mm/s, 7 mm/s, 9 mm/s and now 11 mm/s (Horizontal). The vertical and axial is same at around 1-1.5 mm/s. Purely 1X. Only the amplitude had increased.

Any new ideas?

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