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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 7, Misalignment

Section 23, Unclear Phase Symptoms Regarding Unbalance or Misalignment Caused by Machines with Unequal Flexibilities in Two Directions

Usually the symptoms of 180° out-of-phase in one direction and in-phase readings in the other direction are the result of shaft/coupling misalignment. Unfortunately, the same symptoms can also result from rotor unbalance. Although this occurs in all types of machines, it especially shows up with machines of relatively unequal directional flexibility and with overhung rotors. (however, this can also occur with rotors mounted between bearings.)

There is no extensive statistical evidence, but Update's experience suggests that rotor unbalance can produce the same phase-related symptoms as shaft/coupling misalignment in approximately 10 to a maximum of 20 percent of these situations. Therefore, when such phase-related symptoms appear, use the many other tests for misalignment given in the vibration course. For example, use all the other phase-related tests as well as tests based on frequencies, evaluation of harmonics, discussions with the operator, mechanic, etc.

The conclusion will then be based on an accumulation of many symptoms and not just one set. If the other symptoms do not point to misalignment, then the chances are about 80 percent that the source is unbalance.

In order to better understand how such different vibration modes can result from unbalance, focus your attention on the diagrams that show possible pivot points for the axes of rotation.

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