Understanding Rubbing in Rotating Machinery

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Definition: What is Rubbing?

Rubbing is the friction contact and relative sliding motion between rotating and stationary components in machinery. This term emphasizes the continuous friction aspect of rotor-to-stator contact, distinguishing it from light intermittent contact or impacts. Rubbing generates friction forces, produces significant heat through friction work, and creates distinctive vibration patterns characterized by backward whirl, sub-synchronous components, and thermal effects.

The term “rubbing” is often used interchangeably with “rotor rub,” though rubbing sometimes emphasizes the friction and thermal aspects of the contact, while rotor rub may include all forms of contact including light scraping or impacts.

Friction Mechanics of Rubbing

Coulomb Friction Model

Rubbing follows dry friction (Coulomb friction) principles:

• Friction Force: F = µ × N, where µ is coefficient of friction and N is normal force
• Direction: Always opposes relative motion between surfaces
• Typical Coefficients: Steel on steel µ ≈ 0.3-0.5; steel on seal material µ ≈ 0.2-0.4
• Heat Generation: All friction work converted to heat

Tangential and Normal Forces

During rubbing:

• Normal Force: Pushes radially inward on rotor
• Friction Force: Acts tangentially, opposing rotation
• Resultant Force: Combination tends to slow rotor and deflect it backward
• Torque Increase: Friction dissipates power, increasing drive torque requirement

Characteristic Vibration Patterns

Backward Whirl

The most distinctive feature of rubbing is backward (reverse) whirl:

• Friction force creates tangential component that drives backward orbital motion
• Shaft orbit rotates opposite to shaft rotation direction
• Frequency typically sub-synchronous (less than 1× speed)
• Common frequencies: 0.5×, 0.33×, 0.25× (fractional orders)
• Orbit shape often irregular or distorted

Spectrum Characteristics

• Sub-Synchronous Peaks: Multiple peaks below 1×, often at fractional harmonics
• Synchronous Component: 1× may increase from rub forces
• Higher Harmonics: 2×, 3×, 4× from non-linear friction
• Broadband Noise: Elevated noise floor across spectrum
• Unstable Spectrum: Peaks appear, disappear, or shift frequency

Time Waveform Features

• Impulsive events or spikes when contact initiates
• Clipping or flattening at peak deflections
• Irregular, non-sinusoidal waveform
• Beat patterns from multiple frequencies present

Thermal Effects of Rubbing

Heat Generation

Friction converts mechanical energy to heat:

• Rate: Power dissipated = Friction Force × Sliding Velocity
• Magnitude: Light rub: 10-100 watts; heavy rub: kilowatts
• Concentration: Heat concentrated at small contact area
• Temperature Rise: Local temperatures can exceed 500°C in severe cases

Thermal Bow Development

The heat-vibration feedback loop:

1. Initial rub generates heat on one side of shaft
2. Asymmetric heating creates thermal bow
3. Thermal bow increases shaft deflection
4. Increased deflection causes more severe rubbing
5. More rubbing generates more heat
6. Positive feedback can lead to rapid failure

Secondary Thermal Effects

• Bearing Heating: Heat conducted through shaft to bearings
• Oil Degradation: Excessive temperatures break down lubricant
• Material Changes: Phase transformations or metallurgical changes in heat-affected zones
• Thermal Stress: Can initiate cracks in thermally-stressed regions

Detection Methods

Vibration Monitoring

• Sub-Synchronous Alarms: Alert on peaks at 0.3-0.5× running speed
• Orbit Monitoring: Automated orbit analysis detecting backward whirl
• Spectral Changes: Algorithms detecting sudden appearance of multiple harmonics
• Waveform Clipping: Detection of non-sinusoidal distortion

Temperature Monitoring

• Bearing temperature sensors with rapid-rise alarms
• Infrared temperature monitoring of exposed shaft sections
• Temperature differential monitoring (top vs. bottom bearing)
• Rate-of-change alarms (e.g., > 5°C/minute)

Additional Indicators

• Torque Increase: Power consumption rises due to friction
• Speed Fluctuation: Small speed variations from varying friction torque
• Acoustic Emission: High-frequency sound from contact
• Visual Inspection: Wear debris, discoloration, visible damage

Response Actions

Immediate Actions

1. Reduce Severity: Decrease speed or load if safe to do so
2. Monitor Closely: Continuous observation of vibration and temperature
3. Prepare for Shutdown: Have emergency shutdown ready
4. Emergency Stop: If vibration or temperature escalating
5. Allow Cooldown: Operate turning gear or allow natural cooling before inspection

Investigation

• Inspect for physical evidence of contact
• Measure clearances at suspected rub locations
• Check for thermal bow or permanent shaft bow
• Identify root cause (excessive vibration, insufficient clearance, etc.)

Corrective Actions

• Increase Clearances: Machine out damaged areas or replace components
• Address Root Cause: Balance rotor, correct alignment, fix bearing issues
• Replace Damaged Parts: Seals, bearing components, shaft sections as needed
• Verify Clearances: Confirm adequate clearances at all locations before restart

Rubbing is one of the most serious vibration-related faults in rotating machinery. Its potential for rapid escalation through thermal feedback demands immediate recognition, prompt response, and thorough correction to prevent catastrophic failures in critical equipment.

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