Understanding Mounting Resonance

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Mounting resonance is a resonance condition in which the mounting system — vibration isolators, mounting rails, brackets, skids, or the whole machine assembly sitting on its supports — vibrates at one of its own natural frequencies in response to excitation from the rotating equipment it carries. When that happens, the entire machine bounces, rocks or rolls as a rigid body on its mounts, at amplitudes far larger than the same forcing would produce on a rigid foundation. It is most familiar on machines fitted with vibration isolators, but it can equally afflict a conventionally bolted-down installation whenever the supporting structure lacks stiffness. Either way it is a central concern in isolation design, to be designed out or actively managed rather than discovered in service.

1. Definition: What Is Mounting Resonance?

The key to understanding mounting resonance is to see the machine and its supports as a mass-spring system in their own right. The machine is the mass; the isolators or the flexibility of the support structure are the spring. Like any such system this assembly has natural frequencies, and if the running speed — or one of its harmonics — coincides with one of them, the forced vibration is amplified. What makes mounting resonance distinct from a rotor or shaft resonance is that the whole machine moves more or less as a unit: the vibration measured on the mounts greatly exceeds the vibration of the rotor itself. That signature is the clue that the problem lies in the support, not the rotor. It is closely related to frame resonance and structural resonance, which describe the same amplification arising in the machine frame or surrounding structure.

2. Mounting-System Natural Frequencies

Rigid-Body Modes on Isolators

A machine resting on vibration isolators behaves as a rigid body on springs, and a rigid body in space has six degrees of freedom — so it has six rigid-body natural frequencies.

Translational Modes (three)

• Vertical bounce: up-and-down motion, typically the lowest frequency — around 5–15 Hz for common isolation.
• Horizontal translations (X and Y): side-to-side motions, usually about 1.5–2× the vertical bounce frequency.

Rotational Modes (three)

• Roll: rotation about the longitudinal axis.
• Pitch: rotation about the transverse axis.
• Yaw: rotation about the vertical axis.
• Frequencies: typically 10–30 Hz, depending on the machine’s dimensions and the location of its centre of gravity.

Coupled Modes

• If the isolators are not symmetrically placed, or the centre of gravity is not centred over them, the modes couple.
• Translation and rotation then occur together.
• The result is a complex motion pattern.
• Such coupled modes are harder to analyse and to correct than the clean, uncoupled cases.

3. When Mounting Resonance Occurs

Isolation-System Resonance

The most common scenario, and an ironic one, since it arises from the very isolators meant to reduce vibration:

• Design intent: the isolators are chosen so their natural frequency lies at about one-third to one-fifth of running speed, putting the machine well into the isolation region.
• Problem: if the machine runs below its design speed, or simply passes through the isolator frequency during startup, the forcing meets that natural frequency.
• Symptom: severe vibration at speeds near the isolator natural frequency.
• Duration: confined to a specific, usually narrow, speed band.

Rail or Skid Resonance

• Mounting rails and equipment skids have bending modes of their own.
• Typical frequencies run 15–50 Hz, depending on span and stiffness.
• The whole assembly rocks on the flexing rails.
• This is common in modular, packaged equipment shipped as a unit.

Bracket or Support Resonance

• Wall- or ceiling-mounted equipment carried on brackets is especially exposed.
• The bracket or support arm has its own natural frequency.
• Machine motion is amplified when the running speed matches it.
• The amplified motion can then transmit vibration into the building structure itself.

4. Diagnostic Identification

Key Indicators

• Amplification: vibration measured on the mount is far greater than vibration at the machine — the hallmark of the condition.
• Rocking or bouncing: visible whole-machine motion.
• Speed sensitivity: severe only within a narrow speed range.
• Low frequency: typically 5–30 Hz for isolated systems.
• Phase relationships: all mounting points move in phase for a bounce mode, or out of phase for a rocking mode.

Diagnostic Procedure

1. Identify the resonant frequency from the peak in the vibration spectrum.
2. Impact-test the mounts: a bump test reveals the mount’s natural frequency independent of the running machine.
3. Compare: if the operating resonant frequency coincides with the measured mount natural frequency, mounting resonance is confirmed.
4. Measure several locations to establish the phase relationships between mounting points.
5. Assess the mode shape: decide whether the motion is bounce, rock or a coupled mode.

A crucial early branch in the diagnosis is separating a support problem from a rotor problem. The signature above — large motion on the mounts, modest motion of the rotor, with the peak fixed at a structural frequency rather than tracking speed — points firmly at the mount. Care should also be taken not to confuse mounting resonance with a soft foot, where one support does not sit flat and distorts the frame; the two can coexist and both raise vibration.

5. Solutions

For Isolation-System Resonance

Change Isolator Stiffness

• Stiffer isolators raise the natural frequency above operating speed.
• Softer isolators lower it below the startup range, if the equipment can tolerate the larger static deflection.
• Selection rule: the isolator frequency should sit below one-third of the minimum operating speed.

Add Damping

• Use isolators with built-in damping — elastomeric mounts in place of bare steel springs.
• Add viscous or friction dampers in parallel with the isolators.
• Damping lowers the resonance peak even when the frequency coincidence cannot be removed.

Improve Isolator Installation

• Ensure every isolator is properly loaded — none cocked, binding or carrying no load.
• Verify the isolators suit the actual equipment weight, not an assumed one.
• Check for seized or perished isolators that have lost their designed stiffness.
• Confirm symmetric placement relative to the centre of gravity to avoid coupled modes.

For Structural Mounting Resonance

Stiffen the Mounting Structure

• Add bracing to rails or skids.
• Increase bracket thickness or add gussets.
• Shorten unsupported spans.
• Tie separate mounting points together so they act as one.

Change the Mounting Configuration

• Add intermediate supports to reduce span lengths.
• Relocate mounting points to stiffer parts of the structure.
• Use more robust mounting hardware.

Because all of these moves work by shifting a natural frequency, the supporting structure’s foundation stiffness is the lever they pull; a foundation natural-frequency calculator helps confirm that a stiffening change actually moves the frequency clear of running speed.

Operational Solutions

• Speed restriction: avoid sustained operation at the resonant speed.
• Rapid acceleration: pass through the resonance quickly during startup so little energy builds up.
• Reduce the excitation: improve balance to lower the forcing at the resonant frequency.

6. Isolation Design and Coupled Equipment

Vibration-Isolation Design

Sound isolation design prevents mounting resonance in the first place by keeping operating speeds well clear of the mount’s natural frequencies:

• Frequency ratio: the isolator frequency should satisfy f_isolator < 0.3 × f_{minimum operating}.
• Transmissibility: exactly at resonance the transmissibility can exceed 10 — the mount amplifies rather than isolates, the opposite of its purpose.
• Operating range: all operating frequencies should lie above 2–3× the isolator frequency for effective isolation.
• Startup: a brief, high-amplitude pass through resonance during run-up is acceptable provided it is short.

Choosing isolators that meet these targets is a routine sizing exercise; a vibration-mount selection calculator matches mount stiffness to machine mass and speed, and a machine-vibration-isolation calculator estimates the resulting isolation efficiency.

Coupled Equipment

Motor-driven equipment mounted on a common baseplate adds its own complications:

• The whole assembly possesses rigid-body modes on its mounts.
• The motor and the driven machine couple their vibration through the shared baseplate.
• A resonance can be excited by either machine, regardless of which is the louder source.
• It must therefore be treated as one complete system, not as two independent machines.

7. Measurement and Analysis Tools

Modal Analysis

• Modal analysis fully characterises every mounting-system mode.
• It identifies the frequency, damping and mode shape of each.
• That data feeds directly into design modifications.
• It can be performed experimentally with impact testing or predicted with finite-element analysis.

Operating Deflection Shape (ODS)

• ODS analysis visualises the actual motion pattern while the machine runs.
• It cleanly distinguishes mounting resonance from a rotor resonance.
• It reveals which mode is active — bounce, rock or coupled.
• It guides exactly where stiffening should be added for the greatest effect.

In the field, the same portable instrument used for routine balancing supports much of this work. A two-channel analyser such as the Balanset-1A captures the amplitude and phase at several points on the mounts, and its bump-test capability measures the mount’s natural frequency directly — letting an engineer confirm a suspected mounting resonance, decide whether the cure is a stiffer support or better balance, and verify the fix once it is made.

Mounting resonance can produce severe vibration even on well-maintained, well-balanced machinery, simply because the problem lives in the supports rather than the rotor. Understanding the natural frequencies of mounting systems — vibration isolators above all — and keeping them firmly separated from operating speeds is essential to successful vibration control in any rotating-equipment installation.

---

← Back to Main Index