Understanding Rotor Eccentricity

Devices

Portable balancer & Vibration analyzer Balanset-1A

$2,347.12 Izvorna cijena bila je: $2,347.12.$1,901.46Trenutna cijena je: $1,901.46.

Parts

Vibration sensor

$106.96 Izvorna cijena bila je: $106.96.$71.30Trenutna cijena je: $71.30.

Parts

Optical Sensor (Laser Tachometer)

$147.36 Izvorna cijena bila je: $147.36.$83.19Trenutna cijena je: $83.19.

Devices

Balanset-4

$8,084.78

Parts

Magnetic Stand Insize-60-kgf

$54.67

Parts

Reflective tape

$11.88

Devices

Dynamic balancer “Balanset-1A” OEM

$2,061.90 Izvorna cijena bila je: $2,061.90.$1,663.78Trenutna cijena je: $1,663.78.

Definition: What is Rotor Eccentricity?

Rotor eccentricity (also called eccentricity or geometric runout) is a condition where the geometric center of a rotor or rotor component does not coincide with the axis of rotation (the centerline defined by the supporting bearings). This offset creates a situation where, even if the mass is perfectly balanced, the rotor’s outer surface runs “off-center,” causing the center of mass to orbit around the rotation axis as the rotor spins, generating vibration identical to mass unbalance.

Eccentricity is particularly common in electric motors (from rotor-to-bore offset), pumps and fans (from impeller mounting offset), and any assembled rotors where stacking of manufacturing tolerances can create geometric runout. It is a significant concern in precision machinery where maintaining tight concentricity is critical.

Types of Rotor Eccentricity

1. Static Eccentricity (Parallel Offset)

• Description: Rotor center offset from rotational axis but parallel to it
• Geometry: Constant radial offset along rotor length
• Effect: Creates mass unbalance (geometric center ≠ rotational center)
• Common In: Single-disc components like impellers, pulleys
• Correction: Often correctable by balancing or remounting

2. Dynamic Eccentricity (Angular Offset)

• Description: Rotor centerline at angle to rotation axis
• Geometry: Runout varies along rotor length
• Effect: Creates couple unbalance and varying runout
• Common In: Long rotors with multiple assembly stages
• Correction: Requires realignment or specialized balancing

3. Compound Eccentricity

• Combination of parallel and angular offset
• Most common real-world condition
• Complex runout pattern
• Requires careful analysis to distinguish from other issues

Common Causes

Manufacturing Tolerances

• Bore Runout: Bearing bore not concentric with outer diameter
• Shaft Runout: Machining inaccuracies in shaft journals
• Stack-up: Multiple components assembled with tolerance accumulation
• Casting Variations: Core shift in castings creating wall thickness variation

Assembly Errors

• Off-Center Mounting: Impeller or rotor component not centered on shaft
• Cocked Installation: Component tilted during press-fitting
• Key/Keyway Issues: Oversized keyway or eccentric key installation
• Thermal Fit Problems: Shrink-fit or expansion-fit assembly creating offset

Operational Causes

• Bearing Wear: Excessive clearance allows shaft to run off-center
• Shaft Bending: Permanent or thermal bow creating effective eccentricity
• Plastic Deformation: Overload causing permanent shaft or component distortion
• Looseness: Component worked loose and shifted position

Effects and Symptoms

Vibration Symptoms

• 1× Synchronous Vibration: Primary symptom, appears identical to mass unbalance
• High Runout: Measurable radial runout even at slow roll speeds
• Constant Phase: Unlike some other faults, phase typically stable
• Speed-Squared Response: Vibration increases with speed² like unbalance

Electrical Effects (Electric Motors/Generators)

• Air Gap Variation: Eccentric rotor creates non-uniform air gap
• Magnetic Unbalance Pull (UMP): Asymmetric magnetic forces
• Current Fluctuations: Varying reluctance affects current draw
• Overheating: Localized heating at minimum air gap
• Electromagnetic Noise: 2× line frequency vibration and noise

Mechanical Stress

• Increased bearing loads from unbalance forces
• Cyclic bending stress in shaft
• Reduced clearances at minimum gap locations
• Potential for rubs at close clearances

Diagnosis and Differentiation

Eccentricity vs. Mass Unbalance

Feature	Mass Unbalance	Eccentricity
Vibration Frequency	1× running speed	1× running speed
Slow Roll Runout	Minimal	High (proportional to eccentricity)
Response to Balancing	Vibration reduced	Limited improvement (adds mass unbalance to compensate)
Electrical Effects	None	Air gap variation, UMP (in motors/generators)
Correction	Add balance weights	Remount component, replace if manufacturing defect

Diagnostic Tests

Runout Measurement

• Measure radial runout with dial indicator or proximity probe
• Rotate shaft slowly (< 100 RPM)
• High runout (> 0.05 mm or 2 mils typically) indicates eccentricity or bent shaft
• Runout present even when not rotating confirms geometric issue

Balancing Response Test

• Attempt balancing with trial weights
• Eccentricity limits achievable balance quality
• May achieve acceptable vibration but high correction weights required
• Weights “chase” the geometric offset rather than correcting mass distribution

Correction Methods

Mechanical Correction

• Remount Component: Remove and reinstall with better concentricity
• Machine Surfaces: Re-bore bearing fits or remachine shaft to improve runout
• Replace Component: If manufacturing defect, replacement may be only option
• Shim Adjustment: For assembled components, adjust positioning

Balancing Compensation

• Add balance weights to create counteracting unbalance
• Reduces vibration but doesn’t fix geometric issue
• Acceptable if eccentricity within tolerance and vibration reduced adequately
• Documented limitation for precision applications

For Electric Motors/Generators

• Reposition rotor to minimize air gap variation
• In severe cases, reboring stator or replacement required
• Electromagnetic compensation sometimes possible with advanced controls

Rotor eccentricity is a geometric imperfection that creates dynamic consequences similar to mass unbalance but with distinct diagnostic features. Recognizing eccentricity through runout measurement and understanding its limitations in balancing enables proper corrective actions—mechanical correction when feasible or acceptance with balance compensation when geometric modification is impractical.

---

← Back to Main Index