Vibration Isolator Calculation Services

Vibration isolator calculations — frequently asked questions

What is included in a vibration isolator calculation?

Operating weight and load per mount from the actual center of gravity, required static deflection, spring rate k = P/δ, isolation-system natural frequency fn = (1/2π)√(k/m), frequency ratio f/fn and transmissibility, isolator selection against the manufacturer's rated capacity and travel, plus support reactions. Where seismic provisions apply we add the ASCE 7-22 §13.3.1 Fp demand, snubber gap and impact forces, and ACI 318-19 Chapter 17 anchor checks on the housekeeping pad.

How is spring rate and static deflection calculated?

Static deflection is the deflection the supported weight produces on the mount, and spring rate is k = P/δ where P is the load carried by that isolator. A 1,200 lb load on a 2 in deflection mount needs k = 600 lb/in. Deflection is selected from the isolation target, not the other way round: 1 in mounts are typical for equipment above roughly 1,200 rpm on grade, 2 in for rooftop or above-occupied-space equipment, and 3–4 in for long-span floors where the structural deflection is a significant part of the system.

How do you calculate natural frequency and transmissibility?

For a single-degree-of-freedom spring-mass system fn (Hz) ≈ 3.13/√δ with δ in inches, which follows from fn = (1/2π)√(k/m). Transmissibility is T = 1/|(f/fn)² − 1| for a lightly damped mount. Isolation only begins above f/fn = √2; below that the mount amplifies. A target of f/fn ≥ 3 gives roughly 85–90% isolation, which is why deflection selection is driven by the lowest disturbing frequency — usually fan or pump shaft speed in rpm divided by 60.

Why can't I just divide equipment weight by the number of isolators?

Pi = W/N is only valid when the center of gravity sits at the centroid of the mounting pattern. Real air handlers, chillers, and generator sets have offset masses — a fan section, a compressor bank, a radiator end — so individual mounts can carry two to three times the average. We resolve reactions from the actual center-of-gravity coordinates and mounting-point layout, then size each mount so every isolator deflects the same amount and the equipment sits level.

Are vibration isolation and seismic restraint the same thing?

No. Isolation deliberately allows movement to reduce transmitted operating vibration; seismic restraint limits that movement during an earthquake. Isolated equipment is more demanding than rigidly mounted equipment because the mass can build velocity across the snubber gap before impact. ASCE 7-22 §13.6.1 requires the design force to be doubled (2Fp) where the nominal clearance between the equipment support frame and the snubber exceeds 1/4 in. Both requirements must be satisfied by one coordinated design.

Do you calculate seismic snubbers and anchorage for isolated equipment?

Yes. The package carries the Fp demand from ASCE 7-22 §13.3.1 through the snubber or restrained-spring housing, into the support frame, and down to the anchors, with concrete breakout, pullout, and combined tension–shear interaction checked per ACI 318-19 §17.6, §17.7, and §17.8, plus the §17.10.6 seismic 0.75 factor in SDC C–F. Overturning, uplift, and sliding at the isolator locations are checked with the operating weight, not the shipping weight.

Can you provide PE/SE stamped vibration isolation calculations?

Yes. Deliverables are sealed by a California-licensed PE/SE and include design criteria, load per mount, deflection and spring-rate schedule, natural frequency and transmissibility, an isolator schedule keyed to the equipment tags, support reactions, and seismic restraint and anchorage calculations where required. For California hospital projects the package is prepared for HCAI (formerly OSHPD) review.

What information do you need to start?

Equipment manufacturer and model, operating (wet) weight, overall dimensions, center-of-gravity coordinates, mounting-point locations, operating speed in rpm of the lowest-speed rotating element, the isolator layout or specification section, structural support type (slab on grade, framed floor, roof curb), project location or site seismic parameters, and any required stamping. Manufacturer drawings matter most — they set the load distribution.

What equipment typically needs isolator calculations?

Air-handling units, chillers, cooling towers, base-mounted and inline pumps, fans and blowers, compressors, emergency generators, transformers, and industrial machinery. Rooftop and above-occupied-space equipment, hospital and laboratory installations, and any equipment covered by a specification section requiring stamped submittals are the most common.

How long does a vibration isolator calculation take?

A single equipment unit with complete manufacturer data: 3–5 business days for a stamped package. A full mechanical-room or rooftop equipment schedule of 10–25 units: 2–3 weeks. Add time when seismic restraint, custom steel support frames, or HCAI submittal coordination are in scope. Send the equipment schedule for a fixed-fee quote within 24 hours.