Enveloping Spectra Plots What Are They ?
- The name 'enveloping spectrum' is not always a technically correct description of the signal processing but for simplicity sake will be the term we use
- An enveloping spectra is the same in appearance (amplitude vs. frequency) to a conventional spectrum except for the amplitude units used
- An enveloping spectra is not sensitive to sinusoidal motion - unlike the conventional displacement, velocity and acceleration units
- An enveloping spectra is sensitive to impact related energy - again, unlike the conventional units used that tend to 'average out' transient impacts
- The ability to quantify an 'impact frequency' (and its intensity) is very helpful and important to the analyst. Although there are machines that generate impact energy normally (i.e. reciprocating equipment), most machines don't. Impacts are destructive forces and normally indicate some type of problem is developing
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:
- CSI uses Peakvue
- Entek uses gSE (spike energy - the original IRD acronym)
- SKF uses HFD (high frequency domain)
- Diagnostic Instruments uses ESP (envelope signal processing)
Filters are used to help process the signal and focus on any impacts that may be occurring.
The filters come in two classes:
- Envelope filter - this type of filter sets a frequency 'envelope' that includes a high frequency (Fmax) and a low frequency (Fmin). Any vibration occurring outside that range is filtered out
- Hi-Pass filter - this type of filter eliminates the Fmax but still sets an Fmin filter below which all vibration influences are filtered out
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 signal processing focuses on the transient, impact type events that conventional amplitude units miss (due to their signal processing procedures)
- If there is a consistent period between impacts (i.e. the impacts are occurring at a regular interval), that period will be converted into the desired frequency units (Hz or cpm)
- The intensity of the impacts will also be assessed. This is related to the size of the impact spike on the signal versus any background noise occurring
- The results are displayed on a spectrum with amplitude peaks at the frequency(s) they are occurring at
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:
In 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:
- The bearing assembly natural, or 'resonant', frequency (based on the period of the bearing assembly resonance)
- The 'impact' frequency (based on the period between impacts)
How do we detect and analyse these frequencies with spectra?
- Since the impact causes a 'ring down' (just as striking a bell causes it to ring), there is a sinusoid generated briefly related to the bearing assembly resonant frequency. Because there is a sinusoid generated, this frequency is detected by the FFT process and amplitude peaks will be generated initially on the acceleration spectra (since it is more sensitive to high frequency vibration) and eventually the velocity spectra. Since this frequency is typically high, displacement units are useless
- The impact frequency has no sinusoidal motion associated with it. In other words, there is no sine wave that connects the start of one impact to the start of the next impact - they are individual 'events' that occur. These impacts (spikes) are specifically what the enveloping signal processing looks for and measures. It will calculate the intensity of the impact (the size of the spike) and the frequency (based on the period between impacts) while filtering out any sinusoidal motion it finds
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:
- A low frequency sinusoid that shows about 9 cycles across Fig. 1. That is the 1x rpm signal
- Some frequency modulation of that signal (compare the positive going side of the wave to the negative going side of the wave)
- A large number of spikes, or impacts, that occur across the plot and appear to vary in intensity (the size of the spike)
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').
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?
- 1x, 2x and 3x rpm peaks. These are probably due to the frequency modulation present
- A series of peaks at high frequencies that are spaced about 5400 cpm apart
- The absence of a peak at or near 5x rpm - the impact frequency. This is because there is no sinusoidal motion associated with the frequency of the impacts - only the ring down frequency that results from the impacts
- But where do the peaks between 31,000 and 65,000 cpm come from? How does the FFT process come to 'see' them?
Enveloping Spectra Plots What Information Do They Provide?
Figure 1 shows an actual enveloping spectrum collected on a bearing with a defect:
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:
- The impact frequency. This piece of information can be used on the velocity or acceleration spectrum to help with harmonics and pattern identification
- The intensity of the impacts. This can be accomplished by displaying the amplitudes on a 'dB' scale (see Figure 2) and comparing the peak amplitude to the surrounding 'carpet' level (which is affected by lubrication and load, among other things). In Fig. 2 (which is the same plot as Figure 1 except the amplitudes were on a linear scale in Fig. 1 and are on a dB scale in Fig. 2), the amplitude on the peak is about 125 dB. The surrounding carpet level, which is an estimate of the surrounding amplitudes, is in the 100 - 102 dB range. The following guidelines can be used:
- Difference of 12-18 dB is a significant level of impacting and should be watched closely
- Difference of > 18 dB is a severe level - intense impact energy, very destructive
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?
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