Understanding Phase Angle in Vibration
Phase angle — closely tied to the broader idea of phase — is the angular position, measured in degrees from 0 to 360, of the peak vibration relative to a once-per-revolution reference mark on the rotating shaft. That reference comes from a tachometer ಅಥವಾ keyphasor. Used another way, phase angle expresses the timing relationship between two vibration signals at the same frequency. Either way it supplies the “when” that complements amplitude — the “how much” — and together the two form a complete vibration vector with both magnitude and direction. Phase angle is indispensable for rotor balancing, where it dictates where to place correction weights; for critical speed identification, where a 180° shift confirms resonance; and for fault diagnosis, where distinctive phase patterns separate one fault from another. Strip away phase and a large part of diagnostic and corrective work simply becomes impossible.
1. Measuring Phase Relative to the Keyphasor
The reference system
- Reference mark: a strip of reflective tape or a notch on the shaft.
- Sensor: an optical or magnetic tachometer that detects the mark each time it passes.
- Once-per-revolution pulse: the event that defines the 0° datum.
- Vibration timing: the question being answered — when does peak vibration occur relative to that mark?
- Angular measurement: the answer, expressed in degrees from 0 to 360.
Sign convention
- 0° corresponds to the reference-mark position.
- Direction typically increases in the direction of rotation.
- Example: a phase of 90° means the vibration peak arrives a quarter-turn after the reference mark passes the sensor.
Because the analyser is timing the delay between the tachometer pulse and the vibration peak, the quality of that pulse train governs everything downstream — a point we return to under measurement challenges.
2. The Critical Applications
Balancing — the most important use
Phase is what points to the heavy spot and therefore to the correction. The procedure is direct:
- Measure the phase of the unbalance-induced 1× vibration.
- The phase indicates the angular location of the heavy spot.
- ದಿ correction weight is placed roughly 180° opposite the heavy spot.
- Phase accuracy of about ±5–10° is needed for effective balancing.
- Without phase, balancing is impossible — there is no way to know which direction to correct.
Critical-speed identification
A phase shift, not an amplitude bump alone, is the definitive signature of resonance:
- Below the critical speed, phase stays relatively constant.
- Passing through the critical speed produces a characteristic 180° phase shift.
- Above it, the phase sits 180° away from its below-critical value.
- That phase change on a Bode plot is the reliable indicator.
- An amplitude peak by itself is not enough; the phase shift must accompany it.
Fault diagnosis
Unbalance: phase is stable and repeatable, holds the same value at all speeds below critical, and marks the heavy-spot location.
Misalignment: shows characteristic phase relationships between bearings — axial readings are often 180° apart at the drive and non-drive ends, and the radial phase pattern helps identify the misalignment type.
Shaft crack: the phase of the 1× and 2× components changes during startup and shutdown, behaving differently from plain unbalance; the variation reflects the crack “breathing” as the shaft turns.
Looseness: produces erratic, unstable phase that can wander ±30–90° between measurements. That very non-repeatability is the diagnostic clue.
3. Phase Between Two Measurement Points
Comparing phase at two locations reveals how a structure or rotor is moving as a whole.
In-phase (0° difference)
- Both points move together, in the same direction at the same instant.
- Indicates a rigid connection or a below-resonance mode.
- Common for two bearings on the same rotor running below critical speed.
Out-of-phase (180° difference)
- The points move oppositely — one rises as the other falls.
- Indicates a mode-shape node between them, or operation above resonance.
- Diagnostic for couple unbalance and for certain misalignment patterns.
90° difference (quadrature)
- The points lag each other by a quarter cycle — one peaks as the other crosses zero.
- Can indicate circular or elliptical motion, visible in a shaft orbit.
- Common at resonances or in particular support geometries.
4. Measurement Challenges
How accurate does phase need to be?
- Balancing: ±5–10°.
- Critical-speed work: ±10–20° is acceptable.
- Fault diagnosis: ±15–30° is often sufficient.
What affects accuracy
- Tachometer quality: a clean once-per-revolution pulse is essential.
- Reference-mark position: the mark must be secure and clearly visible.
- Signal quality: a good signal-to-noise ratio keeps the phase steady.
- Filtering: filters can introduce their own phase shifts that must be accounted for.
- Speed stability: a wandering speed smears the phase reading.
Common errors
- A reference mark that has shifted — tape peeling or a relocated mark.
- A misaligned or intermittent tachometer.
- Low signal amplitude, where noise dominates the phase estimate.
- Reading phase on the wrong frequency component.
5. Phase in Vector Analysis
Polar representation
A vibration measurement is naturally a vector: the magnitude is the amplitude and the angle is the phase. Plotting it on a polar plot is the standard way to visualise and track the response during balancing.
Vector addition
Vector addition — the mathematics behind every trial-weight calculation — needs both amplitude and phase, because phase governs how two vectors combine:
- At 0° they add arithmetically.
- At 180° they subtract.
- At any other angle, full vector mathematics applies.
6. The Practical Field Workflow
On a real machine, capturing phase is the job of a portable two-channel analyser working in the equipment’s own bearings at operating speed. The ಬ್ಯಾಲೆನ್ಸೆಟ್-1ಎ reads the 1× amplitude and phase against the pulse from its laser tachometer, and the software turns that vector into the mass and angle of each trial weight and correction weight before confirming the residual unbalance. If you want to combine or resolve vibration vectors by hand to check a result, the vibration phase angle calculator performs the same vector arithmetic.
7. Documenting and Communicating Phase
Standard format
- Report as “amplitude @ phase” — for example, “5.2 mm/s @ 47°”.
- Include the frequency where relevant: “5.2 mm/s @ 47° at 1×”.
- State the reference, i.e. the keyphasor position the angle is measured from.
Phase plots
- Phase versus speed — the lower trace of a Bode plot.
- Phase versus frequency.
- Polar plots for balancing.
- Phase maps for operating deflection shape analysis.
Phase angle is the timing dimension that turns a raw amplitude into a complete vibration vector. Mastering how it is measured, interpreted, and applied — in balancing, in resonance identification, and in fault diagnosis — is fundamental to advanced vibration analysis and to any sound assessment of rotor dynamics and machinery condition.
