Envelope Spectra

Enveloping Spectra Plots What Are They?Enveloping Spectra Plots What Are They ?

Enveloping Spectra Plots How Are They Processed ?

The measurement is collected as an acceleration signal but the signal is processed differently than a conventional acceleration signal is.

The names for the amplitude unit are manufacturer specific - they each have their own name and/or acronym for the unit. A few of the manufacturers are:

Filters are used to help process the signal and focus on any impacts that may be occurring.

The filters come in two classes:

Each manufacturer sets up its own signal processing and filters. Therefore, although they each provide similar information, they are not directly comparable in the amplitude realm.

The enveloping spectrum provides us with valuable information unavailable on displacement, velocity and acceleration spectra. It provides another useful weapon for the analyst.

Enveloping Spectra Plots How Does 'Impact Energy' Occur?

Let's examine how impact energy due to a typical bearing defect occurs:

Bearing Defect GraphBearing DefectIn Figure 1, as each rolling element passes the defect, an impact occurs. Any time an object is struck (impacted), that object will vibrate at its natural frequency. The time it vibrates will be determined by the mass and damping characteristics of the object. This is an example of 'free' vibration (as opposed to 'forcing' vibration by supplying a machine with energy to keep it rotating and, consequently, vibrating). The bearing impact causes the bearing assembly to 'ring down' briefly as the free vibration due to the impact dampens out. There are two frequencies occurring here that are specifically related to the bearing defect:

How do we detect and analyse these frequencies with spectra?

Enveloping Spectra Plots How Does Impact Energy Affect The FFT ?

Enveloping Spectra Plots How Does Impact Energy Affect The FFT?Let's review how the FFT process works by examining the following computer generated signal:

Enveloping Spectra Plots How Does Impact Energy Affect The FFT?Figure 1 would be a typical looking plot that an analyst might collect - 9 rotations of a shaft. But although the 1x sinusoid is fairly clear, the impacts are not. Let's zoom in a bit.

Cutting the displayed sample to just over 2 cycles (rotations) of the shaft (time sample cut from just over 450 msecs to just over 110 msecs), we can now more clearly see:

The frequency modulation of the 1x rpm signal. The ring down frequency of the impacts. If we simply count the number of impacts in one cycle (from 30 - 80 msecs, for instance), we would find about 5 or so (per shaft revolution, or 'x RPM').

Enveloping Spectra Plots How Does Impact Energy Affect The FFT?It should be clear to us as analysts that this is an impact occurring and investigation of the period involved (time between impacts) should lead us to a diagnosis. But more often than not, the analyst will not be using the time domain - they will be using spectrum analysis. What does the FFT generate from the above signal?

Enveloping Spectra Plots What Information Do They Provide?

Figure 1 shows an actual enveloping spectrum collected on a bearing with a defect:

Enveloping Spectra Plots What Information Do They Provide?

The bearing defect frequency is just over 3x RPM. Notice that there are no significant peaks at 1x, 2x or 3x rpm on Figure 1 (there were on the velocity spectrum). There are, however, extremely significant peaks at 1x, 2x and 3x the impact frequency - in this case a bearing defect frequency (there are other impact sources). The enveloping signal provides the following:

Envelope Spectra

But going back to our bearing defect frequency of 'just over 3x rpm', we have learned that machines generate vibration at exact multiples of running speed - so how can a bearing frequency be 'just over' 3x RPM?

Further Information

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