Understanding Rotor Rub in Rotating Machinery

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Portable balancer & Vibration analyzer Balanset-1A

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Balanset-4

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Magnetic Stand Insize-60-kgf

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Reflective tape

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Dynamic balancer “Balanset-1A” OEM

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Definition: What is Rotor Rub?

Rotor rub (also called rubbing or rotor-to-stator contact) is a condition where the rotating components of a machine make intermittent or continuous contact with stationary parts such as seals, bearing housings, or casing walls. This contact creates friction forces, generates heat, and produces distinctive vibration patterns that can rapidly escalate to catastrophic failure if not immediately addressed.

Rotor rub is particularly dangerous because the contact can create a positive feedback loop: vibration causes rub, rub generates heat, heat causes thermal bow, thermal bow increases vibration, which causes more severe rub. This spiral can destroy a machine in minutes if not stopped.

Types of Rotor Rub

1. Light Rub (Intermittent Contact)

• Brief, occasional contact during peak deflection cycles
• May occur only at certain speeds or operating conditions
• Produces erratic, intermittent vibration spikes
• Often at seals or labyrinth clearances
• Can be tolerated briefly but indicates a problem requiring correction

2. Partial Rub (Continuous Light Contact)

• Continuous contact but with light friction force
• Rotor maintains rotation while scraping stationary surface
• Generates continuous sub-synchronous or synchronous vibration
• Produces heat and wear debris
• Can progress to heavy rub if not corrected

3. Heavy Rub (Full Annular Contact)

• Rotor contacts stator around full or large portion of circumference
• Very high friction forces
• Rapid temperature increase (hundreds of degrees in minutes)
• Severe vibration, often chaotic
• Can lead to rotor seizure or catastrophic failure
• Requires immediate emergency shutdown

Common Locations for Rubs

• Labyrinth Seals: Tight clearances make seal rubs common
• Retainer Bearings: Emergency bearings designed to catch shaft during severe events
• Balance Piston Seals: In multi-stage compressors and pumps
• Interstage Diaphragms: In turbines
• Bearing Housing: Severe cases where shaft contacts bearing cap
• Shaft Sleeves: Protective sleeves at seal locations

Causes of Rotor Rub

Excessive Vibration

• Severe unbalance causing large shaft deflection
• Misalignment creating shaft motion
• Operating at critical speed with resonant amplification
• Rotor instability (oil whip, steam whirl)

Insufficient Clearance

• Improper assembly leaving inadequate radial clearance
• Thermal growth reducing clearances during warm-up
• Bearing wear allowing excessive shaft motion
• Foundation settling bringing stationary parts closer to rotor

Transient Events

• Passing through critical speed during startup/coastdown
• Load changes causing sudden shaft deflection
• Trip events or emergency stops
• Overspeed conditions

Vibration Signatures of Rotor Rub

Characteristic Patterns

• Sub-Synchronous Components: Frequencies below 1× (often 1/2×, 1/3×, 1/4×) from backward whirl during rub
• Multiple Harmonics: 1×, 2×, 3×, 4× due to non-linear friction forces
• Erratic Behavior: Sudden changes in amplitude and frequency
• Broadband Noise: Random, high-frequency components from friction and impacts
• Phase Instability: Phase angles vary erratically

Spectrum Characteristics

• Numerous peaks at fractional and multiple orders
• High noise floor from random impacts
• Spectrum changes rapidly and unpredictably
• Waterfall plots show frequency components that appear and disappear

Orbit Analysis

Shaft orbit patterns during rub are highly distinctive:

• Irregular, distorted orbit shapes
• Sharp corners or flat spots where contact occurs
• Orbit shape changes as rub severity varies
• Often shows reverse (backward) precession components

Consequences and Damage

Immediate Effects

• Friction Heating: Contact generates intense local heat (300-600°C possible)
• Thermal Bow: Asymmetric heating causes shaft bending, increasing rub severity
• Wear: Material removed from shaft and stator surfaces
• Debris Generation: Wear particles contaminate bearings and seals

Secondary Damage

• Seal Destruction: Labyrinth seal teeth worn away or broken off
• Bearing Overload: Increased loads and heating from rub forces
• Permanent Shaft Bow: Severe heating can cause plastic deformation
• Shaft Scoring: Grooves worn into shaft surface

Catastrophic Failures

• Shaft Seizure: Complete lockup from heavy rubbing
• Shaft Fracture: Heat-affected zone creates crack initiation
• Rotor Drop: Bearing failure from overheating allows rotor to drop onto retainer bearings or casing
• Fire Hazard: Hot debris or sparks can ignite flammable materials

Detection and Diagnosis

Vibration Analysis Indicators

• Sudden appearance of multiple sub-synchronous components
• Erratic, non-repeatable vibration patterns
• Sharp increases in overall vibration level
• Changes in vibration immediately after speed changes
• Unusual orbit patterns with sharp features

Physical Evidence

• Metallic dust or particles in bearing housings
• Visible wear marks or scoring on exposed shaft surfaces
• Damaged or worn seal components
• Bearing temperature increases
• Audible scraping or grinding sounds

Emergency Response

If rotor rub is suspected during operation:

1. Assess Severity: Light rub may allow controlled shutdown; heavy rub requires immediate emergency stop
2. Reduce Speed: If possible, slowly decrease speed while monitoring vibration
3. Monitor Temperatures: Rising bearing temperatures indicate worsening condition
4. Emergency Shutdown: If vibration continues increasing or temperatures rising rapidly
5. Do Not Restart: Until clearances verified and rub location identified
6. Document Event: Record vibration data, temperatures, speeds for analysis

Prevention and Mitigation

Design Measures

• Adequate radial clearances at all potential rub locations
• Account for thermal growth in clearance design
• Use abradable coatings at seal locations to minimize damage from light rubs
• Install retainer bearings to limit deflection during severe events

Operational Measures

• Maintain good balance to minimize shaft deflection
• Ensure precision alignment
• Follow proper warm-up procedures to manage thermal growth
• Avoid operation at critical speeds
• Monitor vibration continuously on critical equipment

Monitoring and Protection

• Install vibration alarms set below rub threshold levels
• Monitor bearing and seal temperatures
• Use proximity probes to track shaft position and clearances
• Implement automatic shutdown on excessive vibration

Rotor rub represents an emergency condition requiring immediate attention. Understanding its causes, recognizing its distinctive vibration signatures, and implementing proper prevention and protection measures are essential for safe operation of rotating machinery, particularly in high-speed or tightly-clearanced equipment like turbines and compressors.

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