Understanding Frame Resonance

Devices

Portable balancer & Vibration analyzer Balanset-1A

Parts

Optical Sensor (Laser Tachometer)

Complete vibration diagnostics and balancing kit from Vibromera, including four vibration sensors with cables, a signal interface module, laser tachometer with magnetic stand, digital scale, USB cable, and a carrying case. The system is designed for field rotor balancing and condition monitoring on industrial equipment.

Devices

Dynamic balancer “Balanset-1A” OEM

Definition: What is Frame Resonance?

Frame resonance is a specific type of structural resonance where the machine’s own structural frame, housing, casing, or enclosure vibrates at one of its natural frequencies in response to excitation from the rotating components. Unlike foundation or pedestal resonances which involve the support structure, frame resonance involves the machine’s body itself—the cast iron or fabricated steel structure that encloses the rotating elements.

Frame resonance is common in machinery with large, relatively lightweight housings such as fans, blowers, pumps, and motors. It typically manifests as excessive noise, visible vibration of covers or panels, and high vibration readings on the frame that are disproportionate to the actual rotor vibration.

Common Frame Resonance Situations

Motor and Generator Frames

Natural Frequencies: Typically 50-400 Hz depending on size and construction
Excitation: 1× (unbalance), 2× line frequency (120 Hz for 60 Hz motors), electromagnetic forces
Symptoms: Frame vibration much higher than bearing vibration; audible hum or buzz
Severity: Can be 5-10× higher vibration on frame than at bearings

Fan and Blower Housings

Natural Frequencies: 20-200 Hz for typical industrial fans
Excitation: Blade passing frequency (number of blades × RPM)
Symptoms: Housing panels vibrating violently; loud aerodynamic noise
Characteristic: May only occur at specific speeds or flow conditions

Pump Casings

Natural Frequencies: 30-300 Hz depending on casing design
Excitation: Vane passing frequency, hydraulic pulsations
Symptoms: Casing vibration, noise, potential for fatigue cracks
Hydraulic Coupling: Fluid-filled casing can couple rotor and casing vibration

Gearbox Housings

Gear mesh frequency excitation
Frame natural frequencies often overlap with mesh frequencies
Characteristic loud gear whine when resonant

Vibration Signature and Detection

Characteristic Symptoms

Location Dependent: Vibration varies dramatically across frame surface (10× differences common)
Bearing vs. Frame: Frame vibration >> bearing vibration (may be 3-10×)
Frequency Specific: Only at resonant frequency; other frequencies normal
Speed Sensitive: Severe in narrow speed range (±10-20% of resonant speed)
Visual Motion: Frame motion often visible to naked eye

Diagnostic Tests

Impact (Bump) Test

Strike frame with rubber mallet or instrumented hammer
Measure response with accelerometer
Identify frame natural frequencies from peaks in frequency response
Compare to operating frequencies (1×, 2×, blade passing, etc.)

Roving Accelerometer Survey

Measure vibration at many points across frame while operating
Create vibration map showing high and low areas
Pattern reveals mode shape (bending, twisting, panel flexing)
Identifies antinodes (maximum motion) and nodes (minimal motion)

Transfer Function Measurement

Measure coherence between bearing vibration (input) and frame vibration (output)
High coherence at specific frequency confirms resonance
Transfer function shows amplification factor

Solutions and Mitigation

Stiffening Modifications

Add Structural Ribs or Gussets

Increase frame bending stiffness
Raises natural frequencies above excitation range
Relatively economical and effective
Can be retrofitted to existing equipment

Increase Material Thickness

Thicken frame walls or panels
Significantly increases stiffness and frequency
May require design modification and new castings/fabrications

Structural Ties and Bracing

Connect opposite sides of frame to prevent flexing
Cross-bracing increases torsional stiffness
Can be added externally without internal modifications

Mass Addition

Lower Natural Frequency: Add mass to reduce frequency below excitation range
Strategic Placement: Add mass at antinode locations for maximum effect
Tuned Mass: Carefully calculated mass addition to shift specific mode
Trade-off: Increased weight, may not be desirable for all applications

Damping Treatments

Constrained Layer Damping

Viscoelastic material sandwiched between metal layers
Applied to large flat surfaces (panels, covers)
Reduces resonance peak amplitude by 50-80%
Effective in 20-500 Hz range

Free Layer Damping

Damping material bonded directly to vibrating surface
Simpler than constrained layer but less effective
Good for accessibility-limited applications

Operational Changes

Speed Change: Operate at speed where resonance doesn’t occur
Reduce Forcing: Improve balance, alignment to reduce excitation amplitude
Process Changes: Alter flow, pressure, or load to shift excitation frequencies

Prevention in Design

Design Principles

Adequate Stiffness: Design frame with natural frequencies > 2× highest excitation frequency
Mass Distribution: Avoid concentrated masses creating low-frequency modes
Ribbing and Reinforcement: Incorporate stiffening features from start
Modal Analysis: FEA during design to predict and optimize natural frequencies

Design Verification

Prototype testing with impact analysis
Operating deflection shape measurement on first units
Modify design before production if resonances found

Case Example

Situation: 75 HP motor driving centrifugal fan, excessive noise and vibration

Symptoms: Motor frame vibration 12 mm/s; bearing vibration only 2.5 mm/s
Frequency: 120 Hz (2× line frequency for 60 Hz motor)
Impact Test: Revealed frame natural frequency at 118 Hz
Root Cause: Frame resonating at electromagnetic forcing frequency
Solution: Added four angle iron gussets connecting motor feet to end bells
Result: Frame natural frequency shifted to 165 Hz, vibration dropped to 3.2 mm/s
Cost: $200 in materials vs. $8,000 for motor replacement

Frame resonance is a common but often misdiagnosed vibration problem. Recognizing the characteristic symptoms (high frame vibration relative to bearing vibration, frequency-specific, location-dependent) and applying proper diagnostic techniques (impact testing, ODS analysis) enables targeted solutions that can dramatically reduce vibration at modest cost.

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What is Frame Resonance? Machine Structure Vibration

Published by admin on October 31, 2025 October 31, 2025