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Vibration Information for rotating machinery applications.


Phase and Vibration

Friday, June 03, 2011

Phase measurement is a very powerful tool in vibration analysis. Phase is a relative position of two or more signals. Let’s take an example of pendulums. Two pendulums are not “in phase” if one of them is going the other direction or not in harmony with with the other one. The same scenario applies to rotating machinery.

Machines A and B are “out of phase” if the sine waves (vibrations) of Machines A and B do not reach their peaks at the same time. If Machine A reaches the top of the peak and Machine B reaches the bottom of the peak, machine A is said to be 180o out of phase with machine B. Machine A and B are in phase if they rotate together, i.e. sine waves of the two signals reach their peaks at the same time.

Vibration units: displacement, velocity, and acceleration are actually three different waveforms. The three waveforms are out of phase with each other. Acceleration waveform reaches its maximum 90o before the velocity waveform reaches its maximum. Acceleration waveform reaches its maximum 180o before displacement waveform reaches its maximum peak. Velocity is said to lead displacement by 90o, and acceleration leads velocity by 90o.

Phase is a good diagnostic tool to determine machine conditions such as imbalance or misalignment. Let’s consider an industrial fan application where the fan is suspended in between two bearings. Mass imbalance types can be determined by measuring phase difference on the bearings. Vibration signals are in phase with each other in case of static imbalance. Vibration signals are out of phase with each other in case of couple and dynamic imbalances. In the case of misalignment, angular misalignment is in phase, and parallel misalignment is 180o out of phase when the vibration signals are measured on the bearings close to the coupling. Phase is also useful in determining conditions such as looseness, bent shaft, and resonance. It is also used to determined foundation’s flexibility problems.

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