Understanding Cracked Rotors

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Portable balancer & Vibration analyzer Balanset-1A

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Definition: What is a Cracked Rotor?

A cracked rotor is a rotor or rotating shaft that has developed a fatigue crack—a fracture propagating through the material from cyclic stress. This is essentially the same as a shaft crack but emphasizes the complete rotor assembly rather than just the shaft element. Cracked rotors are extremely dangerous because the crack can propagate from a small, undetectable flaw to complete catastrophic fracture in days or weeks once detected through vibration monitoring.

The hallmark vibration signature of a cracked rotor is a prominent 2× (second harmonic) component that grows as the crack propagates, resulting from the twice-per-revolution variation in shaft stiffness as the crack opens and closes during rotation.

How Cracks Develop in Rotors

Crack Initiation Sites

Cracks almost always initiate at stress concentrations:

• Keyways: Sharp corners at keyway ends (most common initiation site)
• Diameter Changes: Shoulders, steps, or transitions
• Threaded Sections: Thread roots creating stress concentration
• Holes and Cross-Drills: For oil passages or mounting
• Press Fit Edges: Interference fits creating residual stress
• Welds: Heat-affected zones and weld toes
• Corrosion Pits: Surface defects from corrosion
• Machining Marks: Tool marks, especially if perpendicular to stress

Crack Growth Process

1. Microcrack Formation: Initiated at stress concentration, typically < 1 mm
2. Slow Propagation: Crack grows incrementally with each stress cycle (may take years)
3. Acceleration: As crack grows, stress intensity increases, growth rate accelerates
4. Detectable Stage: Crack 10-30% through diameter, 2× vibration appears
5. Critical Size: Remaining material insufficient to carry loads
6. Catastrophic Fracture: Sudden, complete shaft failure

The Characteristic 2X Vibration Signature

Why Cracks Produce 2X Vibration

The breathing crack mechanism:

• Crack Closed (Compression): When crack region in compression (bottom of rotation for horizontal shaft), crack faces contact, shaft stiffness higher
• Crack Open (Tension): When crack in tension (top of rotation), crack opens, shaft stiffness lower
• Twice Per Revolution: Stiffness changes twice per revolution (once when crack oriented up, once when oriented down)
• 2× Forcing: Stiffness variation at 2× frequency creates 2× vibration response
• Amplitude Growth: As crack grows, stiffness asymmetry increases, 2× amplitude increases

Vibration Characteristics

• Primary Indicator: 2× component emerging and growing over time
• 1× Changes: 1× vibration may also increase as crack creates residual bow
• Higher Harmonics: 3×, 4× may appear as crack becomes severe
• Phase Behavior: Phase angles may change during startup/coastdown differently than for unbalance
• Temperature Sensitivity: 2× amplitude may vary with shaft temperature (affecting crack opening)

Detection and Diagnosis

Vibration Monitoring

Trending 2X/1X Ratio

• Monitor ratio of 2× amplitude to 1× amplitude
• Normal machinery: 2×/1× < 0.2-0.3
• Suspect crack: 2×/1× > 0.5 and increasing
• Confirmed crack: 2×/1× approaching or exceeding 1.0
• Emergency: 2×/1× > 2.0, immediate shutdown recommended

Transient Testing

• Bode plots during startup/coastdown
• Cracked rotor shows unusual 2× behavior
• May see two peaks at 1/2 of each critical speed
• Phase changes differ from normal unbalance response

Non-Destructive Examination

• Magnetic Particle Inspection (MPI): Detects surface and near-surface cracks
• Dye Penetrant: Visual detection of surface-breaking cracks
• Ultrasonic Testing (UT): Detects internal cracks
• Eddy Current: Surface crack detection without contact
• Radiography: Internal crack detection in critical components

Emergency Response

Upon Detection of Suspected Crack

1. Increase Monitoring: From monthly to daily or continuous
2. Reduce Operating Severity: Lower speed or load if possible
3. Plan Immediate Inspection: Schedule NDT examination at earliest opportunity
4. Prepare for Shutdown: Have replacement shaft on order, plan repair procedures
5. Risk Assessment: Calculate time to potential failure based on growth rate

If Crack Confirmed

• Immediate Shutdown: Unless risk assessment shows safe continued operation for defined period
• No Restart: Until shaft replaced or repaired
• Shaft Replacement: Most reliable solution
• Root Cause Analysis: Determine why crack developed to prevent recurrence

Prevention Strategies

Design

• Eliminate or minimize stress concentrations
• Use generous fillet radii (R > 0.1 × diameter)
• Avoid keyways when possible; use interference fits
• Proper material selection and heat treatment
• Surface treatments (shot peening, nitriding) to improve fatigue resistance

Operation

• Maintain good balance quality (minimize cyclic bending stress)
• Precision alignment (reduce bending moments)
• Avoid operation at critical speeds
• Prevent overspeed events
• Control thermal stresses through proper warm-up/cooldown

Maintenance

• Regular vibration monitoring with 2× trending
• Periodic NDT inspection (annually or per risk assessment)
• Prevent corrosion (protects against pitting initiation)
• Maintain low vibration (reduces cyclic stress)

Cracked rotors represent one of the most critical failure modes in rotating machinery. The combination of vibration monitoring (detecting characteristic 2× signature growth) and periodic non-destructive examination provides essential protection, enabling detection before catastrophic failure and allowing planned shaft replacement that prevents extensive secondary damage and safety hazards.

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