Why perform Frequency Analysis?
In the figure below, note that the individual frequency components are separate and distinct in the spectrum, and that their levels are easily identified. It would be difficult to extract this information from the time domain waveform.
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It has been argued that the primary reason for the widespread use of frequency analysis is the wide availability of the inexpensive FFT analyzer!
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In the next figure, we see that events that are overlapped and confused in the time domain are separated into individual components in the frequency domain. The vibration waveform contains a great deal of information that is not apparent to the eye. Some of the information is in very low-level components whose magnitude may be less than the width of the line of the waveform plot. Nevertheless, such very low-level components may be important if they indicate a developing problem such as a bearing fault. The essence of predictive maintenance is the early detection of incipient faults, so we must be sensitive to very small values of vibration signals, as we will see shortly. |
In the next figure, a very low-level component represents a small developing fault in a bearing, and it would have been unnoticed in the time domain or in the overall vibration level. Remember that the overall level is simply the RMS level of the vibration waveform over a broad frequency range, and that a small disturbance such as the bearing tone shown here could double or quadruple in level before the overall RMS would be affected.
On the other hand, there are circumstances where the waveform provides more information to the analyst than does the spectrum.
Azima DLI provides products and services for Predictive Maintenance including vibration analysis instruments, monitoring and diagnostic software, and consulting for CBM programs.
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