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Section 13, Using Phase Change to Determine a Resonant Condition when Machine cannot be Shut Down

Typically the running machine cannot be shut down for resonance tests. Yet, the phase change method to determine whether a resonant condition exists can still be used. Remember that almost all troublesome resonances occur in non-rotating parts, such as pipes, bases, pedestals, beams, decks and so on. These parts can usually have their resonant frequencies changed while the machine is still running. Parts may be temporarily braced to add rigidity and, therefore, moved to a higher resonance frequency that is at least 20 to 25 percent away from the source frequency.

Using braces, jacks, added weights, etc., is covered in the chapter "Confirming a Part's Resonance Through Quick and Temporary Means." Through these temporary means, changes in resonance frequency produce either a very small or relatively large amplitude change. A small change indicates that the suspect part is not actually resonant to the source frequency. A large change in amplitude indicates that the source frequency is actually in the resonance range. However, amplitude changes by themselves are limited when it comes to determining whether a part is resonant. Instead, observing phase is considerably more accurate.

Before making a temporary resonance change, observe both amplitude and phase. If the amplitude changes somewhat, but the phase remains approximately the same (within a few degrees), that indicates that the part is not resonant. Appreciable phase change (anywhere from 15°, 90° and up to about 150°) indicates that the part is partially to fully resonant.

Precaution: a small phase change of, for example, only 15°, may not necessarily indicate that the source frequency is only slightly into the part's resonance range. Instead, it may indicate that the final resonance change needs to be greater. For example, the temporary resonance change may have been accomplished by bracing with an angle iron and clamps whereas final detuning may require adding considerably more rigidity.

To review: to determine whether a part was resonant or not, watching amplitude while temporarily changing the part's resonance frequency is very helpful. Watching for phase change or non-change is considerably more helpful.

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