What is Holospectrum? Full Spectrum Analysis • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is Holospectrum? Full Spectrum Analysis • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is Holospectrum? Full Spectrum Analysis

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Understanding Holospectrum

Definition: What is Holospectrum?

Holospectrum (also called full spectrum) is an advanced frequency analysis technique in rotor dynamics that processes simultaneous X and Y (horizontal and vertical) vibration measurements to separate shaft motion into forward precession components (orbiting in same direction as rotation) and backward precession components (orbiting opposite to rotation). Unlike conventional spectra that show only vibration magnitude, holospectrum displays both positive frequencies (forward) and negative frequencies (backward), providing complete information about rotor orbital motion direction critical for diagnosing instabilities, identifying forced vs. self-excited vibration, and characterizing rotor dynamic behavior.

Holospectrum is primarily used with proximity probe measurements (XY pairs) on critical turbomachinery, revealing phenomena invisible in standard single-axis spectra. It’s an expert-level diagnostic tool for rotor dynamics specialists troubleshooting complex vibration issues in turbines, compressors, and generators.

Theoretical Basis

Forward vs. Backward Precession

  • Forward Precession: Shaft center orbits in same direction as shaft rotation (most common)
  • Backward Precession: Shaft orbits opposite to rotation direction (indicates specific problems)
  • Significance: Direction indicates excitation mechanism and fault type

Standard Spectrum Limitation

  • Single-axis FFT cannot distinguish forward from backward
  • Both appear as same frequency component
  • Direction information lost
  • Ambiguity in interpretation

Holospectrum Solution

  • Processes XY measurements together
  • Mathematically separates directional components
  • Forward: positive frequencies
  • Backward: negative frequencies
  • Complete rotor motion characterization

Applications and Diagnostics

Instability Diagnosis

  • Oil Whirl/Whip: Appears at negative frequencies (backward precession initially)
  • Steam Whirl: Sub-synchronous backward component
  • Identification: Holospectrum immediately identifies instability vs. unbalance

Forced vs. Self-Excited Vibration

  • Unbalance (Forced): Strong forward component at 1×, minimal backward
  • Instability (Self-Excited): Significant backward component
  • Distinction: Clear in holospectrum, ambiguous in standard spectrum

Rotor Rub Detection

  • Rubbing often creates backward components
  • Friction forces drive reverse precession
  • Holospectrum reveals rub-related backwards motion

Gyroscopic Effects

  • Forward and backward whirl modes separate at different frequencies
  • Holospectrum shows both modes clearly
  • Validates rotor dynamic models

Data Requirements

XY Measurement Pair

  • Two perpendicular vibration measurements required
  • Typically from XY proximity probe pair
  • Must be 90° apart spatially
  • Synchronized sampling essential

Relative Phase

  • Quadrature relationship between X and Y enables direction determination
  • X leads Y by 90° → forward
  • X lags Y by 90° → backward
  • Phase accuracy critical

Interpretation

Holospectrum Display

  • Horizontal Axis: Frequency (positive for forward, negative for backward)
  • Vertical Axis: Amplitude
  • Zero Center: Zero frequency at center of plot
  • Right Side: Forward precession components (+1×, +2×, etc.)
  • Left Side: Backward precession components (-1×, -2×, etc.)

Typical Patterns

Healthy Rotor

  • Large forward component at +1× (unbalance)
  • Small or no backward components
  • Indicates normal forced vibration

Oil Whirl

  • Significant component at negative sub-synchronous frequency
  • Example: -0.45× (backward at 45% of rotor speed)
  • Diagnostic for bearing-induced instability

Misalignment

  • Strong +2× forward component
  • Minimal backward
  • Confirms forced vibration from misalignment

Advantages

Diagnostic Clarity

  • Immediately distinguishes instability from unbalance
  • Identifies rotor rub conditions
  • Characterizes complex rotor motion
  • Reduces diagnostic ambiguity

Completeness

  • Full information about orbital motion
  • No information lost (vs. single-axis analysis)
  • Complete rotor dynamic picture

Limitations

Requires XY Measurements

  • Not applicable to single-axis data
  • Requires proximity probe pairs or synchronized accelerometers
  • More expensive instrumentation

Complexity

  • More complex than standard spectrum
  • Requires understanding of precession concepts
  • Interpretation needs expertise
  • Not routine analysis technique

Limited Application

  • Primarily for rotor dynamics issues
  • Less useful for bearing defects, gears
  • Specialized tool, not general purpose

When to Use Holospectrum

Appropriate Cases

  • Suspected rotor instability
  • Sub-synchronous vibration investigation
  • Rub diagnosis
  • Critical turbomachinery troubleshooting
  • Rotor dynamics validation

Not Needed For

  • Routine unbalance or misalignment
  • Bearing defect analysis
  • Single-axis measurements
  • General machinery surveys

Holospectrum analysis is an advanced rotor dynamics diagnostic technique providing complete orbital motion characterization by separating forward and backward precession components. While requiring specialized XY measurements and expertise, holospectrum delivers unique diagnostic insights—particularly for instabilities and rubs—that are unobtainable from conventional single-axis spectral analysis, making it an essential tool for specialist analysis of complex rotor dynamic problems in critical turbomachinery.

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