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

What is Torsional Analysis? Twisting Vibration Assessment

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Understanding Torsional Analysis

Definition: What is Torsional Analysis?

Torsional analysis is the measurement, evaluation, and modeling of torsional vibration—twisting oscillations about the shaft axis—in rotating machinery drive trains. Unlike lateral vibration (bending) which is readily measured with standard accelerometers, torsional vibration requires specialized measurement techniques (strain gauges, dual tachometers, laser vibrometry) and analysis to detect angular oscillations, determine torsional natural frequencies, and assess fatigue risk in shafts, couplings, and gears.

Torsional analysis is critical for reciprocating engine drives, long drive shafts, high-power gearboxes, and VFD motor applications where torsional vibration can cause catastrophic shaft or coupling failures despite acceptable lateral vibration levels. It’s a specialized but essential diagnostic capability for preventing sudden, unexpected failures in power transmission systems.

Why Torsional Analysis is Needed

Torsional vs. Lateral Vibration

  • Lateral: Bending, side-to-side motion, measured with standard accelerometers
  • Torsional: Twisting about axis, no lateral displacement, invisible to standard sensors
  • Independence: Can have severe torsional with low lateral (and vice versa)
  • Damage: Torsional can cause shaft/coupling failures without warning from lateral measurements

Failure Modes

  • Shaft fatigue fractures (typically 45° to axis)
  • Coupling element failure (gear teeth, flexible elements)
  • Gear tooth breakage from oscillating loads
  • Key and keyway damage from fretting

Measurement Techniques

1. Strain Gauge Method

Direct torsional stress measurement:

  • Strain gauges bonded at 45° to shaft axis (max shear stress orientation)
  • Measures shear strain from twisting
  • Requires slip rings or wireless telemetry for rotating shafts
  • Most accurate but complex and expensive
  • Research and development primary use

2. Dual Tachometer Method

  • Two optical sensors at different shaft locations
  • Measure phase difference between locations
  • Phase difference = angular twist = torsional vibration
  • Non-contact and practical
  • Limited to low-frequency torsional (< 100 Hz typically)

3. Laser Torsional Vibrometer

  • Specialized laser Doppler system
  • Measures angular velocity fluctuations
  • Non-contact
  • Wide frequency range
  • Expensive but powerful

4. Motor Current Analysis

  • Torsional vibration creates current fluctuations
  • Analyze motor current spectrum
  • Indirect but non-invasive
  • Useful screening tool

Analytical Torsional Analysis

Mathematical Modeling

  • Lumped-mass torsional model of drive train
  • Calculate torsional natural frequencies
  • Predict response to excitation sources
  • Identify critical speeds and resonances

Excitation Sources

  • Reciprocating Engines: Firing pulses create torsional excitation
  • Gear Mesh: Tooth engagement creates oscillating torque
  • VFDs: PWM switching frequency harmonics
  • Electrical: Motor pole-passing and slip frequencies

Campbell Diagram for Torsional

  • Plot torsional natural frequencies vs. speed
  • Overlay excitation order lines
  • Identify torsional critical speeds (interference points)
  • Guide operating speed selection

Critical Applications

Reciprocating Engine Drives

  • Diesel generators
  • Gas engine compressors
  • Marine propulsion
  • Large torque pulsations require analysis

Long Drive Shafts

  • Rolling mill drives
  • Marine propeller shafts
  • Paper machine drives
  • Length creates low torsional stiffness

High-Power Gearboxes

  • Wind turbine gearboxes
  • Industrial gear reducers > 1000 HP
  • Gear mesh excitation of torsional modes

VFD Motor Systems

  • Variable frequency drives create torsional excitation
  • PWM harmonics can excite torsional resonances
  • Increasing concern with VFD proliferation

Analysis Results

Torsional Natural Frequencies

  • Identify from measurement or calculation
  • Compare to excitation frequencies
  • Verify adequate separation

Stress Levels

  • Calculate alternating shear stress from measured vibration
  • Compare to material endurance limits
  • Assess fatigue life consumption
  • Determine if stresses acceptable

Damping

  • Measure from response at torsional resonances
  • Typically very low (< 1% of critical)
  • Low damping means sharp resonances

Mitigation Strategies

Frequency Separation

  • Ensure torsional naturals separated from excitation frequencies
  • Change shaft diameter, length, or coupling stiffness
  • Modify inertias (add flywheel)

Damping Addition

  • Torsional dampers (viscous or friction)
  • High-damping couplings
  • Reduces resonance amplification

Operating Speed Changes

  • Avoid continuous operation at torsional critical speeds
  • Restrict speed ranges
  • VFD tuning to minimize excitation

Torsional analysis is a specialized vibration discipline addressing twisting oscillations that can cause catastrophic failures invisible to standard lateral vibration monitoring. While requiring specialized measurement and analysis techniques, torsional analysis is essential for reciprocating engine drives, long shafts, high-power gearboxes, and VFD systems where torsional vibration poses significant reliability and safety risks.

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