Understanding Structural 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 Structural Resonance?

Structural resonance is a condition where the vibration frequency from rotating machinery (such as 1× running speed, 2× from misalignment, or blade passing frequency) matches a natural frequency of the non-rotating support structure—including the machine frame, baseplate, pedestals, foundation, or even nearby structures. When this frequency matching occurs, resonance amplifies the structural vibration to levels far exceeding what the rotating components themselves experience.

Structural resonance is particularly problematic because it can make a well-balanced, properly aligned machine appear to have severe vibration problems. The high vibration is in the structure, not necessarily indicating rotor problems, but the structural motion can feed back to affect rotor behavior and cause real mechanical damage over time.

How Structural Resonance Occurs

The Resonance Mechanism

Excitation Source: Rotating machinery generates periodic forces (from unbalance, misalignment, etc.)
Force Transmission: These forces transmit through bearings to support structure
Frequency Match: If excitation frequency ≈ structural natural frequency
Energy Accumulation: Structure absorbs energy over multiple cycles
Amplification: Vibration amplitude builds up, limited only by structural damping
Observed Effect: Structure vibrates at 5-50× higher amplitude than input force would normally produce

Typical Frequency Ranges

Foundation Modes: Usually 5-30 Hz for typical industrial foundations
Baseplate Modes: 20-100 Hz depending on size and construction
Pedestal Modes: 30-200 Hz for typical bearing supports
Frame/Cover Modes: 50-500 Hz for sheet metal panels and covers

Common Resonance Scenarios

1X Running Speed Resonance

Example: Machine running at 1800 RPM (30 Hz), foundation natural frequency at 28-32 Hz
Symptom: Very high vibration despite good balance
Effect: Even small residual unbalance creates large structural motion
Solution: Change foundation stiffness, add damping, or change operating speed

2X Resonance (Misalignment Frequency)

Misalignment generates 2× frequency excitation
If 2× matches structural mode, amplification occurs
High vibration may be misdiagnosed as severe misalignment
Alignment improvement helps but doesn’t eliminate resonance

Blade/Vane Passing Frequency Resonance

Fans, pumps, turbines generate blade passing frequency (N × RPM, where N = number of blades)
Often in 50-500 Hz range
Can excite structural modes in this frequency range
High-frequency rattling or buzzing

Diagnostic Identification

Symptoms of Structural Resonance

Disproportionate Vibration: Structure vibration much higher than bearing vibration
Narrow Speed Range: High vibration only at specific speed (±5-10%)
Directional Dependence: Severe in one direction, minimal in perpendicular direction (matching mode shape)
Location Dependence: Vibration varies greatly over structure surface (antinodes vs. nodes)
Minimal Bearing Effect: Bearings and rotor may show acceptable vibration while structure severe

Diagnostic Tests

1. Impact Testing (Bump Test)

Strike structure with hammer, measure response
Identifies all structural natural frequencies
Compare to machine operating frequencies
Most definitive test for structural resonance

2. Measurement Location Comparison

Measure vibration at bearing housing (close to source)
Measure at pedestal base, baseplate, foundation
If structural vibration >> bearing vibration, indicates structural resonance
Transmissibility > 2-3 suggests resonance amplification

3. Operating Deflection Shape (ODS)

Measure vibration at multiple points on structure simultaneously
Create animated visualization of structural motion
Reveals which structural mode is active
Identifies nodes and antinodes

Solutions and Mitigation

Frequency Separation

Change Operating Speed

If variable speed equipment, operate away from resonance
Change motor sheave sizes to adjust speed
Use VFD to select non-resonant speed
May not be practical if speed determined by process requirements

Modify Structural Natural Frequency

Add Mass: Lowers natural frequency (f ∝ 1/√m)
Add Stiffness: Raises natural frequency (f ∝ √k)
Remove Material: In some cases, reducing mass can shift resonance
Structural Modification: Add bracing, gussets, or reinforcement

Damping Addition

Constrained Layer Damping

Viscoelastic damping material bonded to structure
Effective for sheet metal panels and frames
Reduces resonance peak amplitude
Commercially available damping treatments

Tuned Mass Dampers

Add secondary mass-spring system tuned to problematic frequency
Absorbs energy, reduces main structure vibration
Effective but requires careful design and tuning

Structural Damping Materials

Rubber pads or isolators at strategic locations
Damping compounds applied to surfaces
Friction dampers at joints

Isolation

Install vibration isolators between machine and foundation
Decouples machine vibration from structure
Effective if isolator natural frequency < 0.5× excitation frequency
Requires careful design to avoid creating new resonance problems

Reduce Excitation

Improve balance quality to reduce 1× excitation
Precision alignment to reduce 2× excitation
Fix mechanical problems reducing forcing amplitudes
Reduces symptom but doesn’t eliminate resonance potential

Prevention in Design

Foundation Design Criteria

Foundation natural frequency > 2× maximum operating frequency (avoid resonance above)
Or < 0.5× minimum operating frequency (isolated foundation)
Avoid 0.5-2.0 range where resonance likely
Include dynamic analysis in design phase

Structural Design

Design for adequate stiffness relative to forcing frequencies
Avoid lightly-loaded structures prone to resonance
Use ribbing and gussets to increase frequency
Consider adding inherent damping (composite materials, joints with friction)

Structural resonance can transform minor vibration sources into major problems through amplification effects. Identifying structural resonances through impact testing and operational measurements, combined with proper mitigation strategies, is essential for achieving acceptable vibration levels in installations where structural dynamics significantly influence overall machine vibration behavior.

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What is Structural Resonance? Support System Vibration

Published by admin on October 28, 2025 October 28, 2025