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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 20, Examining Phases from One Side of Coupling to the Other

The comparisons of horizontal vibration amplitudes and modes, with vertical amplitudes and modes, give very valuable bases from which to determine whether shafts are misaligned. However, there is another symptom, based on phase and shaking mode, that some analysts consider stronger than comparisons of vertical and horizontal shaking modes. It is determining how one half of the coupling (mounted on one machine's shaft) is shaking compared to the other coupling half (mounted on the other machine's shaft).

Examine the phase/vibration illustration. Start with the horizontal phase reading, measured in the vertical direction "across the coupling," which means one reading at the inboard bearing of one machine and the other reading at the inboard bearing of the other machine to which it is coupled.

For example, assume that the phase measured at the left machine's inboard bearing is approximately at 7:00 o'clock. On the other bearing at the other side of the same coupling, the horizontal phase is at approximately 2:00 o'clock.

This shows that when the motor side of the coupling in the horizontal direction moves one way, the other side of the same coupling is moving approximately in the opposite direction.

This motion, with each end of the coupling moving in approximately opposite directions, can be a simple coincidence whereby one machine's unbalance occurs opposite to the other machine's unbalance. But, most likely, it is the result of shaft misalignment. Review again Fig. 1. The more nearly the phases are 180° opposite to each other, the more certain the symptoms are due to parallel or "offset" misalignment. Across the coupling in the vertical direction, the phases are approximately 30° apart. This is close enough to call them "approximately in phase." This is typical for angular misalignment. Perfectly clear symptoms are not always possible because the 1 x rpm vibration is not always the result of only the primary source for the vibration, but is also vectorially combined with small amounts from other sources (see "Using Understanding of Vectorial Addition for Vibration Analysis.")

Most often, one of the relatively clear symptoms above for misalignment is observed in only one direction or the other. (Horizontal or vertical, but not both.) This can be due to the fact that most misalignment errors occur primarily only in one direction. Another possibility is that the forces from the machine's angular misalignment are added vectorially to the machine's parallel or offset misalignment. The resultant vectors can easily be neither approximately 0° nor 180° out-of-phase even if the main source for the vibration is misalignment.

Compare the phase relationships as measured "across the coupling." If the strong symptoms of approximately 0° or 180° are present, then the symptoms for misalignment are very strong. If these symptoms are not present, but that does not necessarily mean that the source for vibration is not misalignment. To be sure, compare with the other symptoms for misalignment given in this course, such as those in the section "Evaluating Harmonics Due To Shaft-to-Shaft/Coupling Misalignment."

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