Understanding Mounting 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 Mounting Resonance?

Mounting resonance is a resonance condition where the mounting system—including vibration isolators, mounting rails, brackets, skids, or the complete equipment assembly on its supports—vibrates at one of its natural frequencies in response to excitation from the rotating machinery. This creates a situation where the entire mounted machine bounces, rocks, or oscillates as a rigid body on its mounts, with amplitudes far exceeding what would occur with rigid mounting.

Mounting resonance is particularly common in equipment using vibration isolation systems but can also occur in conventional rigid-mounted installations if the mounting structure has insufficient stiffness. The phenomenon is a key consideration in vibration isolation design and must be carefully avoided or managed through proper isolator selection and mounting system design.

Mounting System Natural Frequencies

Rigid Body Modes on Isolators

Equipment on vibration isolators has six rigid-body natural frequencies:

Translational Modes (3)

Vertical Bounce: Up-and-down motion, typically lowest frequency (5-15 Hz for typical isolation)
Horizontal Translations (X and Y): Side-to-side motions, usually 1.5-2× vertical frequency

Rotational Modes (3)

Roll: Rotation about longitudinal axis
Pitch: Rotation about transverse axis
Yaw: Rotation about vertical axis
Frequencies: Typically 10-30 Hz, depend on equipment dimensions and center of gravity location

Coupled Modes

If isolators not symmetric or CG not centered, modes couple
Translation and rotation occur simultaneously
Creates complex motion patterns
More difficult to analyze and correct

When Mounting Resonance Occurs

Isolation System Resonance

The most common mounting resonance scenario:

Design Intent: Isolators selected to have natural frequency at 1/3 to 1/5 of machine running speed
Problem: If machine operates below design speed or passes through isolator frequency during startup
Symptom: Severe vibration at speeds near isolator natural frequency
Duration: Occurs only in specific speed range

Rail or Skid Resonance

Mounting rails or equipment skids have own bending modes
Typical frequencies: 15-50 Hz depending on span and stiffness
Entire equipment assembly rocks on rails
Common in modular equipment packages

Bracket or Support Resonance

Wall-mounted or ceiling-mounted equipment on brackets
Bracket or support arm has natural frequency
Equipment motion amplified when frequency matches
Can transmit vibration into building structure

Diagnostic Identification

Key Indicators

Amplification: Vibration on mount >> vibration at equipment
Rocking/Bouncing: Visible whole-machine motion
Speed Sensitive: Severe only in narrow speed range
Low Frequency: Typically 5-30 Hz range for isolated systems
Phase Relationships: All mounting points move in-phase (bounce mode) or out-of-phase (rocking mode)

Diagnostic Procedure

Identify Resonant Frequency: From vibration spectrum peak
Impact Test Mounts: Bump test to determine mount natural frequency
Compare: If resonant frequency ≈ mount natural frequency → mounting resonance confirmed
Measure Multiple Locations: Check phase relationships between mount points
Assess Mode Shape: Determine if bounce, rock, or coupled mode

Solutions

For Isolation System Resonance

Change Isolator Stiffness

Stiffer Isolators: Raises natural frequency above operating speed
Softer Isolators: Lowers frequency below startup range (if equipment can tolerate)
Selection: Isolator frequency should be < 1/3 minimum operating speed

Add Damping

Use isolators with built-in damping (elastomeric vs. steel spring)
Add viscous dampers or friction dampers
Reduces resonance peak amplitude even if frequency match remains

Improve Isolator Installation

Ensure all isolators properly loaded (not cocked or binding)
Verify isolators appropriate for actual equipment weight
Check for seized or degraded isolators
Ensure symmetric placement relative to center of gravity

For Structural Mounting Resonance

Stiffen Mounting Structure

Add bracing to rails or skids
Increase bracket thickness or add gussets
Shorten unsupported spans
Tie mounting points together

Change Mounting Configuration

Add intermediate supports to reduce spans
Relocate mounting points to stiffer locations
Use more robust mounting hardware

Operational Solutions

Speed Restriction: Avoid operating at resonant speeds
Rapid Acceleration: Pass through resonance quickly during startup
Reduce Excitation: Improve balance to minimize forcing at resonant frequency

Special Considerations

Vibration Isolation Design

Proper isolation system design prevents mounting resonance:

Frequency Ratio: f_isolator < 0.3 × f_{minimum operating}
Transmissibility: At resonance, transmissibility can be > 10 (amplification, not isolation)
Operating Range: Ensure all operating frequencies > 2-3× isolator frequency for effective isolation
Startup Consideration: Accept high vibration passing through resonance if brief

Coupled Equipment

Motor-driven equipment on common baseplate:

Entire assembly has rigid-body modes on mounts
Motor and driven equipment vibration couple through baseplate
Resonance can be excited by either machine
Requires treating as complete system, not individual machines

Measurement and Analysis Tools

Modal Analysis

Complete characterization of all mounting system modes
Identifies frequency, damping, and mode shape for each mode
Provides data for design modifications
Can be done experimentally or through FEA

Operating Deflection Shape (ODS)

Visualize actual motion pattern during operation
Distinguishes mounting resonance from rotor resonance
Shows which mode is active (bounce, rock, etc.)
Guides placement of stiffening modifications

Mounting resonance can create severe vibration even in well-maintained, properly balanced machinery. Understanding the natural frequencies of mounting systems, particularly vibration isolation systems, and ensuring adequate frequency separation from operating speeds is essential for successful vibration control in rotating equipment installations.

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What is Mounting Resonance? Support System Dynamics

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