Sign up Calendar Latest Topics Donate
 
 
 


Reply
  Author   Comment  
Noknroll

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 843
Reply with quote  #1 
Hi
got a fan (overhung) with elevated 1X 
when i did cross channel phase on the fan NDE bearing Ch. A vertical Ch. B horizontal i got only 1.7 deg phase difference, whats that all about, I would of expected 90 deg since the accels are 90 apart.
Any comments, tips or ideas appreciated.
MachDiag

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 130
Reply with quote  #2 

Yes, 90 degrees is what you'd think, but I find it's rarely seen.  Are your concerns regarding the elevated 1x amplitudes such that they are possibly caused by resonance?  For balancing purposes, I don't get to concerned about phase difference between verticals and horizontals.  Look more for a high 1x ratio between the verticals and horizontals.  My general rule of thumb when thinking just how tough it'll be to balance, 4:1 or less (no resonance), 5:1 to 6:1 (some resonance possible), 7:1 to 8:1 (resonance very likely), 9:1 or higher (resonance nightmare).  

John from PA

Sr. Member
Registered:
Posts: 963
Reply with quote  #3 
Quote:
Originally Posted by Noknroll
Hi
got a fan (overhung) with elevated 1X 
when i did cross channel phase on the fan NDE bearing Ch. A vertical Ch. B horizontal i got only 1.7 deg phase difference, whats that all about, I would of expected 90 deg since the accels are 90 apart.
Any comments, tips or ideas appreciated.


In a case like this I always suggest the verification of wiring.  This is especially true if it is a permanent monitoring system.  If you are using temporary transducers and only two then it is hard to get things mixed up.

This is similar to the situation with an X-Y pair of proximity probes and the absolute phase with respect to a Keyphasor. Users expect a 90 deg phase difference between the signals due to the 90 deg angle in the physical probe mounting. A 90 deg phase difference will only occur when the major axis of the filtered orbit is in line with a probe.  If the major axis of the orbit is "tilted" and does not point to a transducer, then the difference between the phase lag angles will not be 90 deg, even though the probes are physically at 90 deg.

RustyCas

Avatar / Picture

Admin
Registered:
Posts: 1,810
Reply with quote  #4 
Nok, what are your 1X and Overall amplitudes?


__________________
"The trend is your friend"
James.

Sr. Member
Registered:
Posts: 113
Reply with quote  #5 
Nok, how many averages had the 2140 done, and what was the coherence value?
Wind74

Member
Registered:
Posts: 19
Reply with quote  #6 
What was the phase difference end to end in each plane?
stace1g

Sr. Member
Registered:
Posts: 104
Reply with quote  #7 
untitled.JPG    

E.g As per what John was saying is the orbit of the machinery component is as above then the difference will not be 90 even though the accels are 90 apart.

You can get this situation if a structural resonance is being excited in one plane i.e. you get your 90 degree phase shift

Gary






Noknroll

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 843
Reply with quote  #8 
Thanks all,
a little bit of history,
variable pitch blades
a step change in velocity from approx 3 to 5mm about a month ago.
very slight increase in consecutive readings since.

replies in no particular order
fan nde horizontal = 5.52mm/sec
fan nde Vertical = 4.28mm/sec
portable/temporary setup
10-30 averages coh 1.00

Fan de - Fan nde Horz 0.48mm- 5.6mm/sec 114 deg 

Fan de - motor de horz 0.48 - 0.97mm/sec 138 deg

Fan de - Fan nde vert 0.27 - 4.25mm/sec -67 deg

Fan deVert - Fan deHorz 0.26 - 0.50mm/sec -35 deg





OLi

Sr. Member
Registered:
Posts: 1,915
Reply with quote  #9 
Would you have stuff sticking on blades and falling off? Olov
__________________
Good Vibrations since early 1950's, first patented vibrometer 1956 in the US.
http://www.vtab.se
Curran919

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 599
Reply with quote  #10 
Quote:
Originally Posted by stace1g
    

E.g As per what John was saying is the orbit of the machinery component is as above then the difference will not be 90 even though the accels are 90 apart.

You can get this situation if a structural resonance is being excited in one plane i.e. you get your 90 degree phase shift



Just to add to that, the phases will be more inline when the ratio between the minor and major axes is great (your orbit looks more like a line than elliptical).

Of course, it sounds like he is measuring housing vibe, not shaft vibe. If he had just forced vibration away from resonance, then he would probably see the the 90 degree lag. However, in a resonant, lowly damped condition, just a slight coupling of a horizontal mode to a primarily vertical mode, such that the coupled horizontal vibration is much greater than the forced horizontal component of the vibration, will push the signals in phase. If you have a non-resonant vibration and a high unbalance, then they will tend to be 90 degrees out of phase.

When comparing measurements on a single-blade waste water pump with major unbalance (mostly hydraulic) vs. a vertical turbine pump with many column modes in the operating range, the XY accel signals are routinely in phase at 1x for the vertical pump and 90 degrees out of phase for the single-blade.
Curran919

Avatar / Picture

Sr. Member / Supporter
Registered:
Posts: 599
Reply with quote  #11 
Quote:
Originally Posted by OLi
Would you have stuff sticking on blades and falling off? Olov


My first boss had a story exactly like this. He loves telling it. Step changes in vibration every 2-3 months. They didn't have visuals on the fan, but when they check the filter and see fan blade sized sleeves of asbestos... they understood what was happening.

Step changes in vibration are never a good sign. The alternative would maybe be a foundation failure. You can confirm if you have bump tests to compare eigendynamics before and after step change. Depending on the foundation, there are some failures you will only be able to detect with the unit in operation and won't be as easy to detect as a broken bolt.
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.