Phase in Vibration Analysis: Theory, Applications, and Examples

Understanding Phase in Machinery Diagnostics

This article is intended to help the reader develop an understanding of Phase in Vibration Analysis — the concept of phase as it applies to vibration and signal analysis. Simple examples are presented to demonstrate the basics of phase relationships; then machinery diagnostics and troubleshooting examples are presented.

Why Consider Phase?

Often when conducting routine vibration testing of machinery, the overall signal strength (a broad-band reading) is measured as a first step. Actual machinery diagnostics, however, requires narrow-band signature analysis to identify specific peaks in vibration spectra. In the same way that narrow-band signature analysis reveals another layer of information, phase information can provide even more clues when diagnosing machinery and structural problems.

What Is Phase?

A simple example is a pair of pendulums. When they are set so that they swing in unison, they are said to be "in phase." If they are set to swing in opposition, they are "out of phase." Of course, there are infinite conditions between in phase and out of phase, described using angles such as "90 degrees phase difference." These conditions provide critical diagnostic information about the nature of a machinery fault.

Real-World Applications

Phase difference between two points on a machine reveals useful diagnostic information. For example, if a machine with an overhung rotor is vibrating excessively at its rotational rate, the vibration could have several different causes: imbalance, wobble, or misalignment. Observing the phase at two points on opposite sides of the coupling can help differentiate the actual problem — imbalance and wobble result in "in phase" vibration, while misalignment causes out-of-phase vibration.

Complex Spectra and Phase Displays

Narrow-band vibration analysis involves close inspection of vibration spectra. The usual representation puts everything in two dimensions: amplitude versus frequency. Behind the scenes, however, there are phases associated with every frequency line. These phases are retained in a "complex" spectrum, which adds a third dimension. By displaying magnitude and phase plots together, analysts gain a richer understanding of machinery condition and fault type.

Use of Phase for Balancing

When doing dynamic balancing, the vibration of a rotating machine is recorded along with a tachometer signal. The phase difference relative to the tachometer signal is noted, and by adding trial weights at known angles, a computer programme calculates where a balance weight should be placed to minimise imbalance vibration. Phase is the enabling measurement that makes precision balancing possible.