For low-frequency vibration, start with the lowest significant excitation frequency, then work backward to the required natural frequency and static deflection.

As a basic rule, isolation begins when the frequency ratio exceeds √2. For machinery where space and allowable movement permit, a ratio around 3–4 gives a more useful starting point for selection.

Start With the Operating Frequency

The frequency ratio is:

r = f / fn

where f is the excitation frequency and fn is the loaded natural frequency of the isolation system.

For rotating machinery, RPM provides a quick first estimate:

f = RPM / 60

A machine running at 1,200 RPM therefore has a fundamental rotational frequency of:

1,200 / 60 = 20 Hz

This is only a starting point. Pumps, compressors, generators and mobile equipment can also produce harmonics, structural resonances and vibration from connected components. If measured vibration data is available, use the dominant frequencies from that data rather than RPM alone.

Near the system's natural frequency, transmitted vibration can increase. Effective isolation starts only after the system moves beyond the resonance region.

How Much Static Deflection Is Required?

For an idealized vertical linear isolation system, natural frequency and static deflection are related by:

d = g / (4π²fn²)

For quick engineering estimates:

d (mm) ≈ 248 / fn²

This table explains one of the practical limits of low-frequency isolation.

Dropping the target natural frequency from 5 Hz to 3 Hz does not require slightly more movement. Static deflection increases from approximately 10 mm to 28 mm.

That extra travel has to exist somewhere in the installation.

Before choosing a softer mount, check available clearance, cable movement, flexible pipe connections and allowable equipment displacement.

Example: 400 kg Generator at 1,200 RPM

Consider a 400 kg generator set supported by four isolation points and operating at 1,200 RPM.

Its fundamental rotational frequency is:

1,200 / 60 = 20 Hz

Suppose the initial design target is a natural frequency of 5 Hz:

r = 20 / 5 = 4

The corresponding static deflection is:

d ≈ 248 / 5² ≈ 9.9 mm

If the center of gravity is approximately centered, the nominal static load is:

400 / 4 = 100 kg per mount

The first screening requirement is therefore more specific than simply finding a vibration isolator rated above 100 kg.

A candidate isolator should reach approximately the required working deflection around the actual 100 kg load point, while retaining enough travel for dynamic movement.

If the engine is concentrated toward one end of the skid, the four mounting loads may not be equal. Corner loads should then be calculated from the actual center of gravity before selecting mount stiffness.

This matters because an underloaded and an overloaded isolator can operate at different effective natural frequencies even when they are the same model.

Check Dynamic Stiffness, Damping and Creep

Three datasheet values deserve extra attention in low-frequency applications.

Dynamic-to-static stiffness ratio: Static stiffness may not accurately represent the stiffness seen during vibration. A higher dynamic stiffness shifts the actual natural frequency upward. When available, use dynamic stiffness data measured near the expected working load and operating frequency.

Damping: More damping reduces amplification near resonance, which is useful when variable-speed machinery passes through resonance during startup or shutdown. However, higher damping does not automatically mean better isolation at every frequency.

Creep: Elastomeric mounts can change deflection under sustained load and temperature exposure. This matters when equipment alignment and long-term natural frequency must remain stable.

These parameters become more important as the excitation frequency approaches the lower practical range of a passive isolation system.

Rubber, Steel Spring or Wire Rope?

The lowest natural frequency is not the only selection criterion.

A steel spring is often the more practical choice when very low natural frequency alone is the main requirement.

A Wire Rope Vibration Isolator is more relevant when vibration is combined with shock, transportation loads, large transient movement or demanding environmental conditions.

The distinction is important because selecting by product type before calculating the required natural frequency can lead to the wrong mount.

When Going Softer Stops Working

A quick calculation can also show when conventional passive isolation is unrealistic.

Assume the dominant excitation frequency is only 2 Hz and the design target is:

r = 4

The required natural frequency becomes:

fn = 2 / 4 = 0.5 Hz

Using the same static-deflection relationship:

d ≈ 248 / 0.5² ≈ 992 mm

Nearly one meter of static deflection is not practical for ordinary industrial machinery.

At this point, testing progressively softer compact mounts is unlikely to fix the problem. The engineering investigation should move upstream: check the vibration source, foundation, structural transmission path, inertia base or overall isolation architecture.

This calculation should be done before selecting a product.

A Practical Low-Frequency Selection Sequence

For preliminary selection, use this order:

1. Identify the lowest significant excitation frequency.

Use measured vibration data when possible; otherwise start with RPM and known forcing frequencies.

2. Set a realistic target natural frequency.

Check the resulting frequency ratio rather than assuming that any soft mount will isolate the machine.

3. Calculate the required static deflection.

Confirm that the equipment and installation can physically accommodate the movement.

4. Calculate the load at each mounting point.

Use actual center-of-gravity information when load distribution is uneven.

5. Compare the required load and deflection with isolator performance data.

Then check dynamic stiffness, damping, shock requirements and environmental conditions.

This sequence eliminates many unsuitable mounts before prototype testing begins.

Need a Preliminary Isolation Check?

For an initial engineering check, provide the equipment weight, mounting-point layout, operating RPM or measured vibration frequency, allowable movement and shock condition.

From these values, the load per mounting point, target natural frequency and required static deflection can be estimated before a specific isolator model is evaluated.