Correct signal analysis flow (Wind Turbine vibration)? I have vibration data from WT (gearbox) sampled at 25.6 kHz. From a previous post I received a lot of good recommendation. 

What I am trying to do:

Find early stage bearing faults by using an envelope analysis aka. an high frequency resonance technique.
This technique can:


  Magnify the low amplitude, high frequency signal by filtering out low frequency components. .. fault indication frequencies can be determined in the envelope spectrum through a spectrum analyzer 


-Fault Detection Of Rolling Element Bearings by C. Liu 

Current method:


Apply bandpass filter (low: 5000, High: 6000Hz)
Rectify resulting signal (absolute value AND squaring to remove aliasing)
Apply lowpass at 6000 Hz


-> Fault detection: Do this for many other 10 second samples over time to look for changes in kurtosis, FFT plot etc.



Zoomed in images of the signals processed:



Questions:


Should I upsample before or after rectifying?
Does this process look correct?
Should I do another LP filter to "clean" up the signal?
Should I do another LP filter at a lower cutoff frequency to "clean" up the signal?


Thanks in advance.

My approach resembles this one, from -Fault Detection Of Rolling Element Bearings by C. Liu
I think the question as is, is unanswerable, but here I am making an attempt of correct it with these comments as answer.


  Should I upsample before or after rectifying?


For the algorithm, if you are finally assessing against a threshold in magnitude, you should not care about down|upsampling. Do not add that.


  Does this process look correct?


Strictly speaking, you are taking the algorithm from your document. Since your document has 200 pages, this is someway hardly to say if you are taking correctly the approach taken by the author. 


  Should I do another LP filter to "clean" up the signal?


This depend on the specific fault you want to capture for the bearing mechanics. From there, you can decide which frequencies to focus, and how to assess them.


  Should I do another LP filter at a lower cutoff frequency to "clean" up the signal?


Again, please check specifically how your algorithm is built. Once you have it implemented, we could study modifications or alternatives.