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Vibration Isolation Calculator

Reference: ASHRAE / ACI 351

〰️ MECH

Free vibration isolation calculator. Find the required isolator static deflection and load per mount from equipment speed, weight and target isolation efficiency (ASHRAE / ACI 351).

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📋 ASSUMPTIONS & BASIS

ASHRAE / ACI 351

Design criteria, governing codes and assumptions.

ℹ About This Calculator

Vibrating equipment is mounted on springs or rubber isolators so its motion is not transmitted into the structure as vibration and noise. Good isolation needs the mount natural frequency well below the equipment disturbing frequency, which fixes the required static deflection. This calculator finds the needed isolator static deflection and the load per mount from the equipment running speed, weight, target isolation efficiency and number of isolators (ASHRAE / ACI 351 practice).

Isolation improves as the ratio of disturbing to natural frequency rises; a ratio around 3 gives roughly 90% efficiency, and higher ratios (larger static deflection) give more. Low-speed equipment needs large deflection (spring mounts), while high-speed equipment can use stiffer rubber mounts. Avoid operating near resonance, especially during run-up. Distribute mounts for even loading, allow for an inertia base where torque or stability demands it, and confirm with the isolator manufacturer selection tables.

Isolator Deflection Method

ASHRAE / ACI 351

Step 1 - Disturbing frequency f = running speed (RPM) / 60 (Hz).
Step 2 - Choose target isolation efficiency; this sets the required frequency ratio (disturbing / natural), typically >= 3.
Step 3 - Required natural frequency fn = f / ratio (Hz).
Step 4 - Static deflection delta = g / (4 x pi² x fn²) - about 250 / fn² in mm.
Step 5 - Load per mount = equipment weight / number of isolators; select springs for that load and deflection.

Worked Example

A 1,450 RPM pump has a disturbing frequency of 1,450 / 60 = 24.2 Hz. For a frequency ratio of 3 the mount natural frequency is 8.1 Hz, needing a static deflection of about 250 / 8.1² ≈ 3.8 mm. A 400 kg unit on 4 isolators loads each mount to 100 kg, and springs are selected for 100 kg at ~4 mm deflection.

Vibration Isolation — Guidance

Frequency ratio is everything

Isolation depends on how far the mount natural frequency sits below the disturbing frequency. A ratio of about 3 gives roughly 90% isolation; more deflection (softer mounts) gives more. This is why low-speed machines need big spring deflections.

Match the mount to the duty

Springs for low speed and high efficiency, restrained springs where uplift or seismic is a concern, and rubber or neoprene for higher-speed lighter duties. Add an inertia base where torque, thrust or stability demands it.

Install for even loading

Distribute and adjust the mounts so the equipment sits level and each isolator carries its design load; uneven loading changes deflection and hurts isolation. Confirm selections with the manufacturer tables.

Frequently Asked Questions

What static deflection do I need for vibration isolation? +

Enough to put the mount natural frequency well below the equipment running frequency. Lower-speed equipment needs larger deflection; the deflection follows from the target isolation efficiency and the disturbing frequency.

What isolation efficiency should I aim for? +

Commonly 90% or more for equipment over occupied or sensitive spaces, which needs a frequency ratio of about 3 or higher. Critical areas may need 95% or more.

Springs or rubber isolators? +

Springs give the large deflections needed for low-speed equipment and high efficiency; rubber or neoprene mounts suit higher-speed, lighter or less demanding duties with smaller deflection.

Why avoid resonance? +

If the disturbing frequency equals the natural frequency the system resonates and amplifies vibration instead of isolating it. Equipment must also pass through resonance quickly on start-up and shutdown.

How is the load per mount found? +

Divide the equipment operating weight (plus any inertia base) by the number of isolators, keeping the mounts evenly loaded, then select each spring for that load and the required deflection.