Spectrum Plot of a Raw Time Waveform
As a spectrum plot of a raw time waveform has inherent filtering and smoothing effects in the mathematical transform, you will oftentimes find that the computed value of the overall value of the FFT is less than that computed on the time waveform itself. This actually becomes one more tool for the experienced analyst. Comparing the two values on the two plots can help to identify impacting in the raw vibration signal.
See comparison below:
The overall computed value in the time waveform plot is 0.3181 and the FFT overall value is 0.2239, almost 30% less. A closer examination of the waveform plot shows severe impacting with a true pk-pk excursion of 2.53 g and a Crest Factor (pk/RMS) value of 3.977.
Tip provided by: Dennis Shreve, Customer Success Manager, Commtest Inc.
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Comments (2)
1) Posted 8:37 am, 11 January 2012 by amin
0.166 G´S SPECTRUM = 0.186 G´S WAVEFORM DIFERENT; APROX: 0. 1%
IMPACTS= 0.6G´SPEAK, 0.186 RMS; CREST FACTOR = APROX 3.22 ; 0.52 G´S PK-PK
Equipment BAD CONDITIONS:
0.79 G´S SPECTRUM DIFERENT TO 1.600 G´S WAVEFORM, DIFERENT APROX: 50%
IMPACTS= 6 G´SPEAK, 1.6 RMS; CREST FACTOR = APROX 3.75; 5.1 G´S PK-PK
I THINK THE MOST IMPORTANT IS THE VALUE OF THE IMPACT ON THE FORMS OF TIME, EVEN MORE THAN THE CREST FACTOR (close to 4); VERY GOOD TIP .....
Being alert to these omissions MATHEMATICS
GREETINGS FROM MEXICO, beautiful and beloved .....
Jerard
2) Posted 11:46 pm, 12 January 2012 by jerardo