Understanding Phase in Vibration Analysis

Definition: What is Vibration Phase?

Phase is a measurement that describes the timing relationship between two signals or, more commonly, the timing of a vibration signal relative to a specific reference point on a rotating shaft. It is a measure of “where” the vibration is happening in relation to the shaft’s rotation. Phase is typically measured in degrees, from 0° to 360°, representing one full revolution of the shaft.

While amplitude tells us “how much” a machine is vibrating and frequency tells us “how fast,” phase tells us “how it is moving.” This makes phase a powerful tool for distinguishing between different faults that may occur at the same frequency.

How is Phase Measured?

To measure phase, two signals are required:

1. A Vibration Signal: This is the primary signal from an accelerometer or proximity probe measuring the machine’s vibration.
2. A Reference Signal: This is a once-per-revolution timing pulse provided by a tachometer. The tachometer is aimed at a reflective piece of tape or a keyway on the shaft, generating a precise pulse each time that reference mark passes the sensor.

The vibration analyzer then measures the time delay between the tachometer pulse and the first positive peak of the vibration signal at a specific frequency (usually the 1x running speed). It converts this time delay into an angle, which is the phase reading. A phase reading of 90° means the vibration peak occurs one-quarter of a revolution after the reference mark passes the tachometer.

The Diagnostic Power of Phase Analysis

Phase is not just a number; it provides critical information about the character of motion. By taking phase readings at different locations on a machine, an analyst can confirm or rule out specific diagnoses.

Confirming Unbalance

A classic case of simple unbalance will show similar phase readings (typically within ±30°) when measured at the same radial direction (e.g., horizontal) on both bearings of a rotor. This indicates that the entire rotor is moving together, being “pulled” in the same direction at the same time by the heavy spot.

Diagnosing Misalignment

Phase is one of the most definitive ways to diagnose shaft misalignment. When axial phase measurements are taken on either side of a coupling, a 180° phase shift (±30°) is a textbook indicator of angular misalignment. This reading shows that as one shaft is moving in (positive axial), the other shaft is moving out (negative axial), a clear sign of a pivoting motion at the coupling.

Distinguishing Unbalance from a Bent Shaft

Both unbalance and a bent shaft can cause high 1x RPM vibration. Phase analysis can distinguish them. By taking axial phase readings at both ends of the same motor or pump shaft, a 180° phase difference indicates the shaft is bent. The ends are moving in opposite directions as the “bow” rotates.

Identifying Structural Looseness or Cracked Foundations

When phase readings are erratic, unstable, or non-repeatable, it often suggests mechanical looseness. If phase readings change significantly when comparing the machine foot to its baseplate, it indicates a loose anchor bolt or a cracked foundation.

Confirming Resonance

As a machine’s speed passes through a resonance (a critical speed), the phase of the 1x vibration will undergo a characteristic 90° shift exactly at the resonance peak, and a full 180° shift as it passes through the entire resonance region. This is a definitive way to confirm a resonance condition.

A Critical Tool for Balancing

Phase is also indispensable for rotor balancing. The phase reading directly indicates the angular location of the “heavy spot” on the rotor relative to the reference mark. This tells the technician exactly where to place the correction weight to counteract the unbalance.

In summary, without phase measurement, a vibration analyst is working with only part of the picture. Phase analysis provides the crucial context about how a machine is moving, enabling a much higher degree of diagnostic confidence.

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