Enveloping Spectra Plots:
How Does "Impact Energy" Occur ?
Let's examine how impact energy due to a typical bearing defect occurs:
Figure 1
Figure 2
In Figure 1, as each rolling element passes the defect, an impact occurs. As we began discussing in the Time Domain section, if you strike a bell, the bell will vibrate at its natural frequency. That is true of any structure. The time it vibrates will be determined by the force of the impact, the mass, the damping characteristics of the object and other variables. This is called "free" vibration (as opposed to the "forced" vibration caused by energizing a machine and keeping it rotating and, consequently, vibrating). The bearing impact causes the bearing assembly to "ring" briefly until the free vibration due to the impact dampens out. There are two frequencies occurring here that are specifically related to the bearing defect:
1) The bearing assembly natural, or "resonant", frequency (based on the period of the bearing assembly resonance).
  • Since the impact causes the bearing structure to ring, there is a sinusoid generated briefly related to the bearing assembly's 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 (displacement units are useless at those frequencies). 
  • The difficulty lies in the fact that the FFT will have to mathematically account for the fact that the spike suddenly appears, briefly rings down and then is gone until the next impact occurs. It is not a constant sinusoid, it is transient.
2) The "impact" frequency (based on the period between impacts).
  • The impact frequency itself 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.