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

What is Torsional Vibration? Causes and Effects

Published by admin on

Understanding Torsional Vibration in Rotating Machinery

Definition: What is Torsional Vibration?

Torsional vibration is the angular oscillation of a rotating shaft about its axis of rotation—essentially a twisting and untwisting motion where different sections of the shaft rotate at slightly different speeds at any given instant. Unlike lateral vibration (side-to-side motion) or axial vibration (back-and-forth motion), torsional vibration involves no linear displacement; instead, the shaft experiences alternating positive and negative angular acceleration.

While torsional vibration typically has much smaller amplitudes than lateral vibration and is often difficult to detect, it can create enormous alternating stresses in shafts, couplings, and gears, potentially leading to catastrophic fatigue failures without warning.

Physical Mechanism

How Torsional Vibration Occurs

Torsional vibration can be visualized as follows:

  • Imagine a long shaft connecting a motor to a driven load
  • The shaft acts like a torsional spring, storing and releasing energy as it twists
  • When disturbed by varying torques, the shaft oscillates, with sections rotating faster and slower than the average speed
  • These oscillations can build up if excitation frequency matches torsional natural frequency

Torsional Natural Frequencies

Every shaft system has torsional natural frequencies determined by:

  • Shaft Torsional Stiffness: Depends on shaft diameter, length, and material shear modulus
  • System Inertia: Moments of inertia of connected rotating components (motor rotor, couplings, gears, loads)
  • Multiple Modes: Complex systems have several torsional natural frequencies
  • Coupling Effects: Flexible couplings add torsional compliance, lowering natural frequencies

Primary Causes of Torsional Vibration

1. Variable Torque from Reciprocating Engines

The most common source in many applications:

  • Diesel and Gasoline Engines: Combustion events create pulsating torque
  • Firing Order: Creates harmonics of engine speed
  • Cylinder Count: Fewer cylinders produce more torque variation
  • Resonance Risk: Engine operating speed may coincide with torsional critical speeds

2. Gear Mesh Forces

Gear systems generate torsional excitation:

  • Gear mesh frequency (number of teeth × RPM) creates oscillating torque
  • Tooth spacing errors and profile inaccuracies contribute
  • Gear backlash can cause impact loading
  • Multiple gear stages create complex torsional systems

3. Electrical Motor Issues

Electric motors can produce torsional disturbances:

  • Pole Passing Frequency: Interaction between rotor and stator creates pulsating torque
  • Broken Rotor Bars: Creates torque pulses at slip frequency
  • Variable Frequency Drives (VFDs): PWM switching can excite torsional modes
  • Starting Transients: Large torque oscillations during motor start-up

4. Process Load Variations

Variable loading on driven equipment:

  • Compressor surge events
  • Pump cavitation creating torque spikes
  • Cyclical loads in crushers, mills, and presses
  • Blade passing forces in fans and turbines

5. Coupling and Drivetrain Issues

  • Worn or damaged couplings with play or backlash
  • Universal joints operating at angles creating 2× torsional excitation
  • Belt drive slip and chatter
  • Chain drive polygon action

Detection and Measurement Challenges

Why Torsional Vibration is Difficult to Detect

Unlike lateral vibration, torsional vibration presents unique measurement challenges:

  • No Radial Displacement: Standard accelerometers on bearing housings don’t detect purely torsional motion
  • Small Angular Amplitudes: Typical amplitudes are fractions of a degree
  • Specialized Equipment Required: Requires torsional vibration sensors or sophisticated analysis
  • Often Overlooked: Not included in routine vibration monitoring programs

Measurement Methods

1. Strain Gauges

  • Mounted at 45° to shaft axis to measure shear strain
  • Requires telemetry system to transmit signal from rotating shaft
  • Direct measurement of torsional stress
  • Most accurate method but complex and expensive

2. Dual-Probe Torsional Vibration Sensors

  • Two optical or magnetic sensors measure speed at different shaft locations
  • Phase difference between signals indicates torsional vibration
  • Non-contact measurement
  • Can be installed temporarily or permanently

3. Laser Torsional Vibrometers

  • Optical measurement of shaft angular velocity variations
  • Non-contact, no shaft preparation required
  • Expensive but powerful for troubleshooting

4. Indirect Indicators

  • Motor current signature analysis (MCSA) can reveal torsional issues
  • Coupling and gear tooth wear patterns
  • Shaft fatigue crack locations and orientations
  • Unusual lateral vibration patterns that may be coupling with torsional modes

Consequences and Damage Mechanisms

Fatigue Failures

The primary danger of torsional vibration:

  • Shaft Failures: Fatigue cracks typically at 45° to shaft axis (maximum shear stress planes)
  • Coupling Failures: Gear coupling teeth wear, flexible element fatigue
  • Gear Tooth Breakage: Accelerated by torsional oscillations
  • Key and Keyway Damage: Fretting and wear from oscillating torque

Characteristics of Torsional Failures

  • Often sudden and catastrophic without warning
  • Fracture surfaces at approximately 45° angle to shaft axis
  • Beach marks on fracture surface indicating fatigue progression
  • May occur even when lateral vibration levels are acceptable

Performance Issues

  • Speed control problems in precision drives
  • Excessive wear in gearboxes and couplings
  • Noise from gear rattle and coupling impacts
  • Power transmission inefficiency

Analysis and Modeling

Torsional Analysis During Design

Proper design requires torsional analysis:

  • Natural Frequency Calculation: Determine all torsional critical speeds
  • Forced Response Analysis: Predict torsional amplitudes at operating conditions
  • Campbell Diagram: Show torsional natural frequencies vs. operating speed
  • Stress Analysis: Calculate alternating shear stresses in critical components
  • Fatigue Life Prediction: Estimate component life under torsional loading

Software Tools

Specialized software performs torsional analysis:

  • Multi-inertia lumped-mass models
  • Finite element torsional analysis
  • Time-domain simulation of transient events
  • Frequency-domain harmonic analysis

Mitigation and Control Methods

Design Solutions

  • Separation Margins: Ensure torsional natural frequencies are ±20% away from excitation frequencies
  • Damping: Incorporate torsional dampers (viscous dampers, friction dampers)
  • Flexible Couplings: Add torsional compliance to lower natural frequencies below excitation range
  • Mass Tuning: Add flywheels or modify inertias to shift natural frequencies
  • Stiffness Changes: Modify shaft diameters or coupling stiffness

Operational Solutions

  • Speed Restrictions: Avoid continuous operation at torsional critical speeds
  • Rapid Acceleration: Pass through critical speeds quickly during startup
  • Load Management: Avoid conditions that excite torsional modes
  • VFD Tuning: Adjust drive parameters to minimize torsional excitation

Component Selection

  • High-Damping Couplings: Elastomeric or hydraulic couplings that dissipate torsional energy
  • Torsional Dampers: Specialized devices for reciprocating engine drives
  • Gear Quality: Precision gears with tight tolerances reduce excitation
  • Shaft Material: High fatigue-strength materials for torsionally critical shafts

Industry Applications and Standards

Critical Applications

Torsional analysis is especially important for:

  • Reciprocating Engine Drives: Diesel generators, gas engine compressors
  • Long Drive Shafts: Marine propulsion, rolling mills
  • High-Power Gearboxes: Wind turbines, industrial gear drives
  • Variable Speed Drives: VFD motor applications, servo systems
  • Multi-Body Systems: Complex drive trains with multiple connected machines

Relevant Standards

  • API 684: Rotor dynamics including torsional analysis procedures
  • API 617: Centrifugal compressor torsional requirements
  • API 672: Packaged reciprocating compressor torsional analysis
  • ISO 22266: Torsional vibration of rotating machinery
  • VDI 2060: Torsional vibrations in drive systems

Relationship to Other Vibration Types

While distinct from lateral and axial vibration, torsional vibration can couple with them:

  • Lateral-Torsional Coupling: In certain geometries, torsional and lateral modes interact
  • Gear Mesh: Torsional vibration creates varying tooth loads that can excite lateral vibration
  • Universal Joints: Angular misalignment couples torsional input to lateral output
  • Diagnostic Challenge: Complex vibration signatures may have contributions from multiple vibration types

Understanding and managing torsional vibration is essential for the reliable operation of power transmission systems. While it receives less attention than lateral vibration in routine monitoring, torsional vibration analysis is critical during design and troubleshooting of high-power or precision drive systems where torsional failures can have catastrophic consequences.

← Back to Main Index

Categories: GlossaryVibration Diagnostics
WhatsApp