Averaged Vibration Signatures
Long experience has shown that an excellent way to generate a meaningful reference is to average several spectra together from machines of the same type. If there are a number of similar machines in a plant the statistical average of their reference spectra is a good indication of the overall characteristics of that particular machine. A series of similar machines in good working order will produce vibration spectra that are similar to one another, but will have random variations in level. The spectra of the machines are averaged together and the standard deviations in level at each significant frequency are calculated.
Some types of machines are so individualistic that when averaged together, the standard deviation between the vibration magnitudes is so great that the average is essentially meaningless. In this case, each machine must be used by itself to produce a meaningful reference by averaging a series of measurements over a fairly long time period, and generating a mask from this average reference spectrum.
There are many situations where a large selection of similar machines is not available, and in this case, the averaged reference spectra are taken on the same machine at different times. When averaging spectra from a group of machines to make a reference spectrum, care must be taken to see that the spectra to be averaged are valid and that the machines they come from are not defective. One of the most important jobs of the vibration analyst is to be sure that the average reference spectra are valid and representative of the machines in question. Do not confuse reference spectrum averaging to produce a reference signature with spectrum averaging done at the time of vibration data collection, as described above.
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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