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.
|