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

What is Axial Vibration in Rotating Machinery?

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Understanding Axial Vibration in Rotating Machinery

Definition: What is Axial Vibration?

Axial vibration (also called longitudinal vibration or thrust vibration) is the back-and-forth motion of a rotor in a direction parallel to its axis of rotation. Unlike lateral vibration which is side-to-side motion perpendicular to the shaft, axial vibration represents the shaft moving in and out along its length, similar to a piston’s motion.

While typically lower in amplitude than lateral vibration, axial vibration is highly diagnostic for certain types of machinery faults, particularly misalignment, thrust bearing problems, and process-related issues in pumps and compressors.

Characteristics and Measurement

Direction and Motion

Axial vibration occurs along the shaft’s centerline axis:

  • Motion is parallel to the shaft rotation axis
  • Rotor moves back and forth like a reciprocating motion
  • Typically measured at bearing housings or shaft ends
  • Amplitude usually smaller than radial vibration but highly significant diagnostically

Measurement Setup

Axial vibration requires specific sensor mounting:

  • Sensor Orientation: Accelerometer or velocity transducer mounted parallel to shaft axis
  • Typical Locations: On bearing housing end caps, motor end bells, or thrust bearing housings
  • Proximity Probes: Can measure axial position directly when mounted facing shaft end surface
  • Importance: Often overlooked but critical for complete machinery diagnosis

Primary Causes of Axial Vibration

1. Misalignment (Most Common Cause)

Shaft misalignment, particularly angular misalignment, is the leading cause of axial vibration:

  • Symptom: High 1X or 2X axial vibration at running speed
  • Mechanism: Angular offset between coupled shafts creates oscillating axial forces transmitted through the coupling
  • Diagnostic Indicator: Axial vibration amplitude > 50% of radial vibration strongly suggests misalignment
  • Phase Relationship: Axial vibration at drive and non-drive ends typically 180° out of phase

2. Thrust Bearing Defects

Problems with thrust bearings that control axial shaft position cause characteristic axial vibration:

  • Thrust bearing wear or damage
  • Insufficient thrust bearing preload
  • Thrust bearing failure allowing excessive axial play
  • Lubrication issues specific to thrust bearings

3. Hydraulic or Aerodynamic Forces

Process forces in pumps, compressors, and turbines create axial forces:

  • Pump Cavitation: Vapor bubble collapse creates axial shock forces
  • Impeller Imbalance: Asymmetric flow creates oscillating axial thrust
  • Axial Flow Turbulence: In axial compressors and turbines
  • Surging: Compressor surge creates violent axial vibration
  • Recirculation: Off-design operation causing flow instabilities

4. Mechanical Looseness

Excessive clearances allow axial movement:

  • Worn thrust bearing surfaces
  • Loose coupling components
  • Inadequate axial restraint in bearing design
  • Worn spacers or shims

5. Coupling Problems

Coupling wear or misinstallation generates axial vibration:

  • Worn gear coupling teeth allowing axial float
  • Improperly installed flexible couplings
  • Coupling spacer length errors
  • Universal joint angles creating axial force components

6. Thermal Growth Issues

Differential thermal expansion can induce axial forces:

  • Piping thermal expansion pushing/pulling on equipment
  • Uneven thermal growth between coupled machines
  • Foundation settling affecting axial alignment

Diagnostic Significance

Misalignment Diagnosis

Axial vibration is the key indicator for diagnosing misalignment:

  • Rule of Thumb: If axial vibration > 50% of radial vibration, suspect misalignment
  • Frequency Content: Predominantly 2X for parallel offset misalignment; 1X and 2X for angular misalignment
  • Phase Analysis: 180° phase difference between axial measurements at opposite ends confirms misalignment
  • Confirmation: High axial vibration that reduces significantly after precision alignment confirms diagnosis

Pump and Compressor Diagnostics

For rotating equipment handling fluids:

  • Cavitation: High-frequency, random axial vibration with broadband characteristics
  • Hydraulic Unbalance: 1X axial vibration from asymmetric impeller loading
  • Surge: Large amplitude, low-frequency axial oscillation
  • Blade Pass Frequency: Axial component at blade passing frequency indicates flow issues

Bearing Condition Assessment

  • Sudden increase in axial vibration may indicate thrust bearing deterioration
  • Axial vibration with thrust bearing defect frequencies confirms bearing problem
  • Excessive axial float measured with proximity probes indicates bearing wear

Acceptable Levels and Standards

General Guidelines

While standards like ISO 20816 primarily address radial vibration, axial vibration limits are typically expressed as:

  • Relative to Radial: Axial should be < 50% of radial vibration under normal conditions
  • Absolute Limits: Typically 25-50% of the radial vibration limits for the machine class
  • Baseline Comparison: Increases of 50-100% from baseline warrant investigation

Equipment-Specific Standards

  • API 610 (Centrifugal Pumps): Specifies both radial and axial vibration limits
  • API 617 (Centrifugal Compressors): Includes axial vibration acceptance criteria
  • Turbomachinery: Often monitored continuously with axial position and vibration sensors

Correction and Mitigation Methods

For Misalignment

  1. Precision Shaft Alignment: Use laser alignment tools to correct angular and parallel misalignment
  2. Soft Foot Correction: Ensure all mounting feet sit flat before alignment
  3. Thermal Growth Consideration: Account for operating temperature expansion in alignment
  4. Pipe Strain Relief: Eliminate piping forces that pull equipment out of alignment

For Thrust Bearing Issues

  • Replace worn thrust bearing components
  • Verify proper thrust bearing preload and clearances
  • Ensure adequate lubrication to thrust bearing surfaces
  • Check for proper thrust bearing installation and shimming

For Process-Related Axial Forces

  • Eliminate Cavitation: Increase inlet pressure, reduce fluid temperature, check for inlet blockages
  • Optimize Operating Point: Operate pumps and compressors within design range
  • Balance Hydraulic Forces: Use balance holes or back vanes on impellers
  • Anti-Surge Control: Implement effective surge prevention for compressors

For Mechanical Issues

  • Replace worn couplings and coupling components
  • Tighten loose mechanical connections
  • Verify correct spacer and shim dimensions
  • Ensure proper coupling installation per manufacturer specifications

Measurement Best Practices

Sensor Installation

  • Firm Mounting: Use studs or adhesive rather than magnets for axial measurements when possible
  • Verify Orientation: Ensure sensor is truly parallel to shaft axis (not at an angle)
  • Both Ends: Measure axial vibration at both drive and non-drive ends for phase comparison
  • Proximity Probes: For critical equipment, install permanent axial position sensors

Data Collection

  • Always collect axial data along with horizontal and vertical radial measurements
  • Record phase relationships between axial measurements at different locations
  • Compare axial to radial amplitude ratios
  • Trend axial vibration over time to detect developing problems

Axial vs. Radial Vibration Comparison

Key Differences

Aspect Radial (Lateral) Vibration Axial Vibration
Direction Perpendicular to shaft axis Parallel to shaft axis
Typical Amplitude Higher Lower (usually < 50% of radial)
Primary Causes Unbalance, bent shaft, bearing defects Misalignment, thrust bearing issues, process forces
Diagnostic Value General machinery condition Specific to misalignment and thrust problems
Monitoring Priority Primary focus Secondary but critical for diagnosis

Industry Applications

Axial vibration monitoring is particularly important for:

  • Centrifugal Pumps: Hydraulic forces and cavitation detection
  • Compressors: Thrust bearing monitoring and surge detection
  • Turbines: Axial turbine blade forces and thrust bearing condition
  • Coupled Equipment: Alignment verification and coupling condition
  • Process Equipment: Flow condition monitoring

While axial vibration is often overshadowed by the more prominent radial vibration, experienced vibration analysts recognize its critical diagnostic value. Many machinery problems that might be missed by examining radial vibration alone are clearly revealed by axial vibration patterns, making it an essential component of comprehensive machinery condition monitoring programs.

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