Vibration Isolation: Springs, Mounts and Isolation Efficiency is the practice of decoupling a vibrating machine from its supporting structure, or protecting sensitive equipment from vibration already in the floor, using resilient elements between the two. The goal is not to eliminate vibration at the source but to reduce the fraction that crosses the mount. Done correctly, isolation cuts structure-borne noise and fatigue loading on foundations. Done incorrectly, an isolator can amplify the vibration it was meant to control.
Rotating and reciprocating machines generate periodic forces at running speed and its harmonics (gear mesh, blade pass, vane pass, combustion frequency). If bolted rigidly to a floor or skid, those forces transmit into the structure, exciting building vibration or radiating as low-frequency noise. The reverse also happens: optical benches and other sensitive equipment must be isolated from vibration already in the floor, arriving from nearby machinery or traffic. Both cases use the same physics, with source and receiver swapped.
A machine on an isolator behaves as a spring-mass system with its own natural frequency, fn, set by isolator stiffness and supported mass. The disturbing frequency, fd, is the machine's forcing frequency, typically running speed. The ratio r = fd / fn sets how much crosses the mount, expressed as transmissibility, T.
This is why soft, low-frequency isolators (steel springs, air springs) suit low-speed machines, while stiffer elastomer pads work for high-speed equipment whose forcing frequency already sits above the mount's natural frequency.
| Isolator type | Typical static deflection | Typical natural frequency | Best suited for |
|---|---|---|---|
| Elastomer / rubber pads | 2 to 6 mm | 6 to 11 Hz | Fans, pumps, compressors above 1,000 to 1,500 rpm |
| Steel coil springs | 10 to 100 mm | 1.5 to 5 Hz | Low-speed equipment, large chillers, roof-mounted machinery |
| Air springs | Very soft, tunable via air pressure | Below 2 Hz achievable | Metrology, semiconductor tools, low-frequency labs |
| Wire rope isolators | Moderate, nonlinear stiffness | Variable, several Hz | Shock plus vibration, shipboard and seismic sites |
| Cork or fiberglass pads | 0.3 to 1 mm | 15 to 30 Hz | Light equipment, secondary noise damping |
An inertia base is a concrete-filled steel frame placed between the machine and its springs, adding mass without adding stiffness. Since natural frequency falls as mass rises at a fixed spring rate, an inertia base lowers fn and raises r, also lowering the center of gravity and reducing rocking on soft springs, common on large fans, chillers, and pump sets with an offset motor. Natural frequency also relates directly to static deflection, the amount an isolator compresses under load: roughly, fn (Hz) is approximately 15.76 divided by the square root of the deflection in millimeters (or 3.13 divided by the square root of the deflection in inches). A mount deflecting 25 mm has a natural frequency near 3.15 Hz, suitable for machines above roughly 600 to 700 rpm. Larger deflection means better low-speed isolation, but also more sway and a taller, less stable mount.
The most common isolation failure is selecting a mount whose natural frequency lands close to the machine's running speed or a startup or coastdown speed it passes through. Variable-speed drives are particularly exposed: even with good transmissibility at full speed, the machine may spend time near fn during ramp-up or ramp-down, briefly amplifying vibration above the unisolated case. Selection should check the full speed range and include damping to limit amplitude at resonance.
Isolators only work as designed if the machine sits level and evenly loaded across all mounts; an uneven load path defeats the calculated natural frequency and can introduce unwanted soft foot once bolted down. Coupling alignment should be checked on the final isolators under normal load, since soft mounts allow more shaft movement between cold and running conditions than rigid grouting does. Vibration severity should still be trended against ISO 10816-3 criteria on the machine itself, since isolators cut transmission to the structure but do not reduce vibration at the source. Isolator condition (cracked elastomer, corroded springs, deflated air springs) deserves its own inspection task with a defined replacement interval, since a degraded isolator changes stiffness and natural frequency, silently moving the system toward resonance. Fabrico lets reliability teams schedule isolator inspections alongside vibration routes and alignment checks. Book a Fabrico demo to see how these checks fit into one workflow.
Isolation begins once the frequency ratio (disturbing frequency divided by isolator natural frequency) exceeds the square root of 2, about 1.41. Below that, transmissibility is 1 or higher, so the mount gives no benefit or amplifies vibration near ratio 1. Practical designs target a ratio of 3 to 5 or more.
Steel springs achieve much larger static deflection than elastomer pads under the same load, giving a lower natural frequency and reaching the required frequency ratio at a lower running speed. Elastomer mounts on a slow machine would leave the system near or below resonance.
It adds mass without changing spring stiffness, lowering natural frequency and improving the frequency ratio. It also stabilizes the machine against rocking on soft springs and distributes load evenly.
Yes. If the isolator's natural frequency sits close to the machine's running speed, or a speed it passes through during startup, the system operates near resonance and transmissibility rises well above 1, amplifying vibration instead of reducing it.
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