Enveloping Spectra Plots
What Information Do They Provide ?
Figure 1 shows an actual enveloping spectrum collected on a bearing with a defect:
Figure 1 - Enveloping Spectrum
The bearing defect frequency identified 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 determine the condition of the bearing (how bad is it ?).
  • After identifying the defect frequency from Figure 1, inspect your velocity or acceleration plot and place your cursor on that same impact frequency and turn on your harmonics.
  • If you are able to relate, through the harmonics, the high frequency peaks to this impact frequency, you have confirmed the presence of a bearing defect.
  • You can then make an assessment of condition based on the amplitudes present, noise level, etc.
The intensity of the impacts:
  • This piece of information can be used to help determine how quickly a bearing can be expected to deteriorate since the impacts are so destructive. 
  • You can compare this to hitting a small pothole in your car or hitting a huge, sharp edged pothole - the first is annoying, the second can destroy your wheel. 
  • The assessment can be made 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). 
Figure 2 - Enveloping Spectrum From 
Figure 1 on dB Scale
  • 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.