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RHarvey

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Reply with quote  #1 
We are working on making the switch from CSI to SKF and having trouble with the Demod readings. I have attached a comparison of readings on the same bearing, the spectral and waveform data look similar but the SKF amplitudes are about half of the PeakVue amplitudes. Is there something wrong with my setup of SKF?

Thanks in advance,
Ross Harvey

 
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docx skf vs csi.docx (135.82 KB, 76 views)

OLi

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Reply with quote  #2 
Measurements are technically different so it is unlikely they would give same readings.
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Ralph Stewart

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Reply with quote  #3 
I think maybe CSI refers to Peakvue as the "True-Peak" of "stress waves",
where as Demodulation might be more along the lines of the sidebands' amplitudes off a carrier frequency.

"Demodulation" is reverse of modulation.

Demodulation is extracting the original information signal from a modulated carrier signal.

In my experience of using both, one can not be directly compared to or judged by the other.

To me, your data looks correct, provided you are using the same sensitivity of the transducer in both analyzers.[smile]

Just my opinion. [confused]

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MarkL

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Reply with quote  #4 
Hi Robert, always measure gE in peak to peak, never rms.
Other than that setup looks ok.
Walt Strong

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Reply with quote  #5 
I have not used SKF, but I would consider taking several averages to get stable and repeatable data with emphasis on periodic peaks. Do not try to replicate PeakVue, but I think you can do as well or better. I suggest making more comparisons, if possible, to get comfortable with SKF. CSI used a Demodulator attachment to analyzer and still offered Demod setup in the 2120 analyzer. I would get away from the PeakVue marketing hype and make the most of what you have. I also created a Demodulation analysis with DasyLab software that was better than the PeakVue for a low speed hydro turbine gearbox fault.

Walt
OLi

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Reply with quote  #6 
First stage is filtering, they can be similar but doubt they get the same. Second is the transient recorder feature of PV, it run at the full sample rate of the box and catch the highest peak, not the same with standard demod procedure so there are a few reasons why they can't be the same. When do PV patent expire?
There are many ways to skin a cat sometimes PV also come up empty when other show something. I have also used ShockFinder for low speed that filter in another way and did work on Yankee bearings. Normally the standard demod/env. procedure like SKF is good enough IMHO.

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Edwin

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Reply with quote  #7 
I use demod from SKF and find it 'sensitive' enough. Most of the time the overall raises way before I find it time to do something with it. But it is important to set your input filtering right to get the data from where the action is. (almost always Env3, 500 to 10kHz)
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MarkL

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Reply with quote  #8 
@Walt,
Skf advise no averaging of envelope in set up.
Edwin, yes I would agree envelope 3 covers most applications, I normally keep to the 500-1000hz range.
Robert send me your email address on pm and I have a document on envelope set up in skf which may be of interests.


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Walt Strong

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Reply with quote  #9 
"Skf advise no averaging of envelope in set up."

Mark,
Have you seen any technical information why no averaging? Suppose you make 10 single spectrum (1 average!) within a short span of 1 to 5 minutes, then would the amplitudes be identical, within reasonable limits, or significantly differ? As always, it is best to use any tool in a way that gives the best results. I have no argument for the way you use SKF Demodulation.

Walt
MarkL

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Reply with quote  #10 
Walt, Ive been on site all day and just back in my hotel, I am trying to find the document with the explanation, I will post it as soon as I find it.
Ralph Stewart

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Reply with quote  #11 
Markl,

Here is a paper from CSI, just in case you can't find one. On page 29 and 30.

http://www2.emersonprocess.com/siteadmincenter/PM%20Central%20Web%20Documents/PeakVue_IntroductionPaper2001.pdf

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Walt Strong

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Reply with quote  #12 
The discussion and comparison of PeakVue and other forms of Demodulation analysis has been extensive, and sometimes heated, over many years. They look pretty similar for the example that Ross provided. A change from RMS to Peak or Peak-to-Peak units might make the SKF amplitudes closer to PeakVue for that data sample, but other data samples could give different comparison results.

Walt
MarkL

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Reply with quote  #13 
Can I add documents to the forum or can I only send via email? Havent been able to add the doc.
JuddJones

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Reply with quote  #14 
You should be able to hit the attach files link at the bottom (right above the add reply button). If you don't see that option you may try another internet browser, or log in on a computer if your using mobile.
MarkL

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Reply with quote  #15 
Thanks Big J

See Attached.
Walt the comment on averaging is mentioned around page 33-34. .

 
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pdf skfenveloping theory and uses.pdf (750.06 KB, 56 views)

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