Trending of Vibration Data
Trending is the storage of vibration signatures recorded at specific time intervals and plotting the changes in vibration levels at the forcing frequencies vs. time. An upward trend in level indicates a developing problem.
The simplest way to utilize the concept of vibration trending is to establish a representative vibration spectrum of a normally operating machine as a reference, and compare this reference to spectra measured at later times on the same machine. The comparison of the spectra is made possible by order normalization, which will be discussed shortly. When performing the spectral comparison there are several important points that need to be addressed:
The operating conditions of the machine when measuring the new vibration data must match as closely as possible the conditions under which the reference spectrum was recorded. Otherwise, the spectra will not be comparable and gross errors can be made.
The vibration data must be recorded in exactly the same way that the reference data were measured. The transducer must be mounted in exactly the same location, and its calibration must be accurate. If possible, the same transducer should be used for all successive measurements on the machine.
When taking vibration data with an FFT analyzer, or data collector, it is important to average several instantaneous spectra together to reduce random variations and the effects of extraneous noise in the measured signal. The number of spectral averages recorded to produce the spectra must be sufficient to produce a uniform and steady signature. Usually from six to ten averages will do this, but on some machines with a relatively high random noise content in their vibration signature, longer averaging times may be needed. A rule of thumb is to record a spectrum with several averages and then immediately record another one with twice as many averages. If the spectra are significantly different, the number of averages should be doubled again and another spectrum recorded. If the latter two spectra are similar, then the previous number of averages is adequate for this machine.
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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