Difference between revisions of "NVGate Envelope analysis"

Using the envelope analysis for roller bearing defect detection.

Abnormal vibrations were detected on a speed reducing gear box. We proceed to the diagnostic of the machine. A default in the roller bearing is suspected. An accelerometer is positioned close to the bearing and connected to the OR3X analyzer.

[[Image:Envelope_01.png|framed|none]]

In this example we are analysing the time signal file recorded with the analyzer. The office mode of NVGate is used.

The first step consists in the traditional FFT spectrum analysis of the signal in the complete frequency bandwith (0-20 kHz).

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In a first step, we determinate the spectrum obtained from the accelerometer signal. The signal is considered on a frequency band 0-20 kHz.

With the cursor we can detect two resonance domains. The first resonance is detected around 4.3 kHz and the second one around 13.3 kHz.

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The traditional approach is to use a zoom analysis around 4.3 and 13.3 kHz in our case. In the NVGate software define the settings of the FFT Zoom by:

* defining the center frequency of the zoom at 4.3 kHz in a first time. We can also use the drag and drop action to place the center frequency in the averaged spectrum window.

Now the Zoomed averaged spectrum window can be displayed (Figure 3).

The zoomed spectrum helps us to have a better resolution around the carrier frequency at which the signal is demodulated. For doing this kind of analysis we can use the side band markers. Actually a typical signal spectrum can be noticed as it usually has a central frequency line with lower amplitude side lines, as shown on the figure below:

If we see such a spectrum shape then it means we can start the envelope analysis.

In the NVGate software, we use the side band markers in the Zoomed averaged spectrum window to identify the modulation frequency (Figure 5).

We can suspect a modulation frequency of 148.4 Hz which exactly corresponds to the value of the inner bearing race defect frequency provided by the manufacturer of the ball bearing.

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The envelope mode is activated by choosing a zoom factor (already done in our example) and checking the box "Envelope" (Figure 6).

The Envelop trigger and the Envelope spectrum windows can now be displayed (number 4 of the Figure 6).

Five shocks per revolution are identified in the Envelope trigger window obtained after the signal demodulation (number 4 of the Figure 6).

[[Image:Envelope_08.png|framed|none]]

From the envelope signal window we can also detect the two characteristic frequencies previously found (Figure 8):