The measurement of the relative phase of certain vibration components is a powerful tool in diagnosing machine problems. For instance, in a machine with a coupling between two parallel shafts, it is sometimes difficult to distinguish between imbalance and misalignment from the vibration spectra alone. It is important to do so, however, because it is a waste of time to try to balance a misaligned machine.
However, the relative phase of the 1X axial component at the bearing on one side of the coupling compared to the bearing on the other side will be 180 degrees if either the coupling or shafts are misaligned. After aligning the machine, the measurement should be repeated to see that the opposite-phase 1X axial components are greatly reduced. The vibration measurement is a check and verification of the alignment work, and is a far more sensitive check than conventional alignment methods allow.
These phase measurements are best made with a two-channel analyzer by measuring the two axial signals at the same time. With this technique, no tachometer, or other phase reference trigger is required. Alternatively, the measurement can be made with a single channel analyzer if the analyzer is triggered from a tachometer pulse taken from some place on the shaft. Each measurement is made with reference to the tachometer, and the software calculates the relative phase.
In this way, extraneous vibration sources can be tracked down to evaluate the contamination of desired vibration measurements used in predictive maintenance. The cross spectrum can be used to compute other two-channel measurements, including the Frequency Response Function (FRF) and the Transfer Function (TRF). The most useful of these is the FRF, for it is a measure of the dynamic behavior of a structure independent of the excitation to the structure. The FRF is used in Modal Analysis, where modes of vibration of a mechanical system are measured and displayed, and also in mechanical impedance and mobility testing, as will be discussed in the next section. The TRF is used in such tests as operating deflection shape analysis, to be discussed later.
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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