What is the
result of a single broken, cracked or chipped gear tooth ? It is an impact
once per shaft revolution (1200 rpm). What does the FFT look like ? Figure
1 shows the time domain:
Figure 1 - Shows Once Per Revolution
Impact
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Figure 2 shows
the FFT generated from the signal seen in Figure 1.
Figure 2 - The Resulting FFT
Plot
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| The spectrum shown goes out to about
22,000 and the number of teeth is 25. The amplitude at 30,000 cpm (GMF
- which is not shown on the plot) is increased on slightly under this circumstance.
So what is the FFT symptom you can expect with this problem ? The time
plot shows a peak amplitude for for the 1x rpm signal of perhaps 0.8 ips
(20 mm/s). The FFT amplitude is under 0.5 ips (12 mm/s). There may be a
slight increase in GMF. |
| Just as impacts in a bearing excite
the natural frequencies of the bearing structure, there will almost certainly
be some excitation of the gear train's natural frequency due to the impacting.
That is an important clue and typical of excessive wear or impacting between
gears but unfortunately you won't, in all likelihood, know what that natural
frequency is. Therefore . . . |
|
The only
effective way to detect this problem from its early stages is with time
domain analysis.
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