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fredyarevalo

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Reply with quote  #1 
Motor Dc GE 500 hp           rpm 650/1200
I am investigating a fault with this motor. It is connected to gear box with Cardan shaft. driving a Yankee Dryer. Gear box does not show vibration outside normal range.
a month ago joints were replaced because suspected to be bad. how ever the vibration on the motor continue to increase. spectra data shows harmonics of turning speed (750rpm) time waveform show some level of impacting (warble and noise is physically noticeable on motor)  , do not seem to be bearing frequencies and most of energy is synchronous .
I attached spectra images, let me know what you think.
thank you Capture224.png 

Capture22.png   

Capture23.png 
 

Ralph Stewart

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Reply with quote  #2 
fredyarevalo,

Amplitudes appear quite low.
 
Can you give us an in-depth analysis of the data in the picture below.

Like:

(1) the sidebands
(2) spacing between the carriers

  Analysis.jpg 



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jpeg Analysis.jpg (40.38 KB, 4 views)


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Ralph Stewart
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RustyCas

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Reply with quote  #3 
Fredy, as Ralph points out, sideband spacing is very important.  Any time you see clusters of sidebands you should take a high resolution spectra.  In this instance I would use 500 Hz Fmax, 3200 Lines (maybe more).  Can you post a plot with harmonic 'markers' on 1X, and a zoomed spectra with sideband 'markers' around the highest 1X harmonic?  The peak lists are good, but require us to "do the math", which most of us don't have time for.
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Ron Stiemsma

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From what I see the amplitudes are very low.  It looks like the difference is about 78 Hz and the spacing is about 5 Hz.  (hard to be accurate with this data)  So on a 750 RPM shaft that is 6.24 orders and .4 orders.  This very well could be a bearing frequency and cage sidebands.  Should be able to get the motor bearing info from the nameplate to see if they match.
Walt Strong

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What were the events that occurred in 20014 and 2010 when vibrations levels went high and dropped (were corrected?) afterward?

"a month ago joints were replaced because suspected to be bad."
What were the symptoms to suspect bad U-joints?
What was the actual condition of the U-joints after removal?

Walt
Ralph Stewart

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Reply with quote  #6 
Fredy,

I may be wrong, but after printing out the last plot you posted in the original posting, and measuring the sidebands to be approx. 5.15 Hz spacing, right now I am thinking they are either sidebands of FTF off a motor bearing or Yankee running speed sidebands.......... but I do not think a Yankee can be running over 300 rpm,(5.15x60) unless it is very small diameter. But being a tissue machine running 5000 fpm, my guess it is over 15 feet diameter.

And the carrier might be an inner Race at 5.45 orders of motor speed.

Just a S.W.A.G. on my part. [smile]

Like I said, I probably am wrong.

Thanks,
Ralph


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fredyarevalo

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Reply with quote  #7 
thank you all for your input here is a plot showing 1x harmonics CaptureXX.png  CaptureXX1.png  Capture0001.png 
Walt Strong

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Reply with quote  #8 
According to your spectra cursor: 1st Order = 13.28 Hz = 796.8 rpm and this is higher than the default 1xSS listed as 766.02 rpm. I find it helpful to correct the speed using the vibration frequency (using interpolation if available).

The Delta Frequency between the groups of sidebands is labeled 65.3 Hz, but you did not identify the actual sideband spacing that could be more useful.


The time waveform in 1st figure has about 5-periods in 1-second, so the frequency would be about 0.2 Hz (12 cpm). It would help you and us if you identify all drive train components, shaft speeds and bearings. You really need to pull useful data from Trend, Waveform, and Spectrum plots along with documenting all of the machine details. In many cases the diagnosis can become self-evident.

Walt
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