Understanding Phase Angle in Vibration

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Optical Sensor (Laser Tachometer)

Complete vibration diagnostics and balancing kit from Vibromera, including four vibration sensors with cables, a signal interface module, laser tachometer with magnetic stand, digital scale, USB cable, and a carrying case. The system is designed for field rotor balancing and condition monitoring on industrial equipment.

Devices

Dynamic balancer “Balanset-1A” OEM

Definition: What is Phase Angle?

Phase angle (often simply called phase) is the angular position, measured in degrees (0-360°), of the peak vibration relative to a once-per-revolution reference mark on the rotating shaft (from a tachometer or keyphasor). Alternatively, it can represent the timing relationship between two vibration signals at the same frequency. Phase angle provides the “when” information that complements amplitude (the “how much”), together forming a complete vibration vector with both magnitude and direction.

Phase angle is absolutely critical for rotor balancing (determines where to place correction weights), critical speed identification (180° phase shift confirms resonance), and fault diagnosis (phase patterns distinguish different fault types). Without phase information, many diagnostic and corrective procedures would be impossible.

Phase Measurement Relative to Keyphasor

The Reference System

Reference Mark: Reflective tape or notch on shaft
Sensor: Optical or magnetic tachometer detects mark passage
Once-Per-Revolution Pulse: Defines 0° reference
Vibration Timing: When does peak vibration occur relative to mark?
Angular Measurement: Expressed in degrees (0-360°)

Convention

0°: Reference mark position
Direction: Typically increasing in direction of rotation
Example: Phase = 90° means peak vibration occurs 90° (quarter revolution) after reference mark passes sensor

Critical Applications

1. Balancing (Most Important)

Phase determines correction weight angular position:

Measure phase of unbalance-induced vibration
Phase indicates angular location of heavy spot
Correction weight placed 180° from heavy spot
Phase accuracy ±5-10° needed for effective balancing
Without phase, balancing impossible

2. Critical Speed Identification

Phase shift confirms resonance:

Below critical speed: phase relatively constant
Passing through critical: characteristic 180° phase shift
Above critical: phase shifted 180° from below-critical value
Phase change on Bode plot definitive resonance indicator
Amplitude peak alone insufficient—must have phase shift

3. Fault Diagnosis

Unbalance

Phase stable and repeatable
Same phase at all speeds (below critical)
Phase marks heavy spot location

Misalignment

Characteristic phase relationships between bearings
Axial measurements often 180° different at drive and non-drive ends
Radial phase patterns diagnostic for misalignment type

Shaft Crack

Phase of 1× and 2× change during startup/shutdown
Different behavior than normal unbalance
Phase variations indicate crack breathing

Looseness

Erratic, unstable phase readings
Phase varies ±30-90° between measurements
Non-repeatability diagnostic for looseness

Phase Between Two Measurement Points

In-Phase (0° Difference)

Both points vibrate together
Move in same direction simultaneously
Indicates rigid connection or below-resonance mode
Common for bearings on same rotor below critical speed

Out-of-Phase (180° Difference)

Points vibrate oppositely
One up while other down
Indicates mode shape node between points or above-resonance
Diagnostic for coupled unbalance, certain misalignment patterns

90° Difference (Quadrature)

Points vibrate with 90° time lag
One reaches peak while other at zero
Can indicate circular or elliptical motion
Common at resonances or in certain geometries

Measurement Challenges

Phase Accuracy Requirements

Balancing: ±5-10° accuracy needed
Critical Speed: ±10-20° acceptable
Fault Diagnosis: ±15-30° often sufficient

Factors Affecting Accuracy

Tachometer Quality: Clean once-per-rev pulse essential
Reference Mark Position: Must be secure and visible
Signal Quality: Good signal-to-noise ratio needed
Filtering: Filters can introduce phase shifts
Speed Stability: Speed variations affect phase measurement

Common Errors

Reference mark shifted (tape peeling, mark moved)
Tachometer misaligned or intermittent
Low signal amplitude (noise affects phase)
Wrong frequency component selected for phase

Phase in Vector Analysis

Polar Representation

Vibration vector has magnitude and phase
Magnitude = amplitude
Phase = angle
Plotted on polar plot for balancing

Vector Addition

Vector addition requires both amplitude and phase
Phase determines how vectors combine
0° phase: vectors add arithmetically
180° phase: vectors subtract
Other phases: use vector mathematics

Documentation and Communication

Standard Format

Report as: “Amplitude @ Phase”
Example: “5.2 mm/s @ 47°”
Include frequency: “5.2 mm/s @ 47° at 1×”
Specify reference (keyphasor position)

Phase Plots

Phase vs. speed (Bode plot lower trace)
Phase vs. frequency
Polar plots for balancing
Phase maps for ODS analysis

Phase angle is the essential timing dimension of vibration analysis that transforms amplitude measurements into complete vibration vectors. Understanding phase measurement, interpretation, and application in balancing, resonance identification, and fault diagnosis is fundamental to advanced vibration analysis and essential for effective rotor dynamics assessment and machinery troubleshooting.

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What is Phase Angle? Timing Relationship in Vibration

Published by admin on October 31, 2025 October 31, 2025