Home › Blog › Electromechanical & Switchgear › Vibration, Shock & Acoustic Control › Load band, deflection, and creep
Why Isolation Pads Stop Working: Load Band, Deflection Window, Creep, and the Durometer Trap
Isolation pads fail quietly and late. The pads go in, the survey reads clean, the gear energizes, and the hum comes back over months as a pad creeps flat or a bolt carries the vibration straight past the elastomer. The H-O vibration, shock, and acoustic control application page traces these returns to a handful of concepts that are all decided at spec: the load band, the deflection window, the slow mechanics of creep and set, and a units error so common the page names it the durometer trap.
This article walks each one, with the methods named by designation: ASTM D1056 and ASTM D3574 for compression-deflection grades, ASTM D2240 for solid durometer, ASTM D395 for compression set, and ASTM D2632 for resilience where reported.
Request a quote for your project →
Deflection is the mechanism
An isolation pad works by deflecting under its load: the deflection is what de-tunes the mount from the disturbance, and, qualitatively, more working deflection softens the mount and improves isolation of higher-frequency structure-borne vibration, the 100 / 120 Hz hum a transformer feeds into building steel being the canonical case. That is why both extremes fail identically: a pad squeezed nearly flat has no deflection left, and a pad barely touched never engages; both read as "the isolation does not work."
The design habit from the parent page: aim the operating point at the middle of the grade's usable compression range, per its compression-deflection data, with thickness chosen so the working deflection is real, thicker pads where the load or the alignment tolerance demands it.
Load per area picks the family before any catalog opens
Divide the supported weight by the pad footprint available, and the answer sorts you into a material set. Heavy bands (transformer and reactor bases, machine feet) belong to rebonded neoprene, rubberized cork, solid natural rubber, and the Atlas bearing line: dense constructions that develop useful deflection only under serious stress.
Light bands (relays, contactors, boards, displays) belong to the microcellular polyurethanes, PORON and HyPUR-cel, which deflect usefully under grams. The families do not substitute for each other: a soft foam under a motor base crushes solid and transmits; a firm pad under a small relay never deflects and transmits too. Hand feel says nothing about load per area.
Four Quiet Failures, Four Spec-Stage Fixes
| Field failure | Mechanism | Spec-stage fix |
|---|---|---|
| The pad crept flat under sustained load | Creep plus compression set consumed the working deflection; the pad became a shim | Constructions built for sustained load (rebonded, cork, bearing grade); read set lines per ASTM D395 and the D1056 suffix methods; size load per area mid-window; re-survey heavy bases after the first year |
| Firmness specified by feel | Soft foam crushed solid under a motor base, or a firm pad never deflected under a relay | Compute load over footprint; select the family and CD grade so the operating point sits in the working window |
| The bolt short-circuited the isolation | A through-bolt is a rigid bypass; vibration walked down the shank into the floor | Isolate the whole fastening: pad under the foot, sleeve or grommet on the shank, washer of the same family under the head, die-cut as a matched set |
| Wrong chemistry met oil, ozone, or an audit | Natural rubber hardened in oil mist; an outdoor pad cracked from ozone; the TDS had no flame class | Name the exposures and documentation needs on the drawing; neoprene for oil, ozone-tested chemistries outdoors, classed grades where the assembly is classed |
Patterns and fixes as cataloged on the parent application page; values per the TDS on file. Where a bolt must stay rigid for seismic or safety reasons, accept the bypass and treat the remaining paths honestly rather than over-claiming the pad.
Creep and set: the decades problem
A transformer pad is loaded on commissioning day and stays loaded for thirty years. Two slow mechanisms decide whether it is still isolating at the end: creep, the continued deflection under constant load, and compression set, the failure to recover. Materials differ sharply: rebonded constructions and cork composites exist for exactly this duty, and the PORON line's low-set chemistry is why it owns precision cushioning that must not collapse.
Set is tested per ASTM D395 and the ASTM D1056 suffix methods and reported on the TDS; for any pad under sustained load, read the set line before the firmness line.
The durometer trap
Durometer (Shore A per ASTM D2240) measures surface indentation of a solid elastomer; it says nothing useful about how a cellular foam carries load. Specifying "40 durometer foam" is a category error that procurement cannot fill and suppliers fill wrong. The correct callouts: solids (bearing pads, solid natural rubber, solid neoprene) by Shore A; cellular materials (rebonded, vinyl nitrile, PORON, HyPUR-cel, reticulated) by their compression-deflection grade per ASTM D1056 or ASTM D3574, the stress at a standard deflection.
The two metrics do not convert. Write the right metric for the material class on the drawing, and the quotes that come back become comparable.
How H-O Products helps
H-O Products is a precision die-cut converter, not a raw-material producer; we convert sheet, roll, and bun stock and do not mold isolation mounts.
The conversion details are where this article's concepts become parts: base pads cut to the footprint that sets the load band, bolt-isolation washers and sleeves die-cut as matched sets so the fastening does not bypass the pad, thin anti-rattle washers and strips kiss-cut on liner, and acoustic laminations supplied as one part, with material traceability and lot-code TDS records under our ISO 9001:2015 certified quality management system.
Frequently asked questions
How do I size an isolation pad: how much should it compress?
Design the operating point into the middle of the grade's usable compression range per its ASTM D1056 / D3574 compression-deflection data, with thickness chosen so the working deflection is real. A pad squeezed nearly flat has no deflection left to isolate with; a pad barely engaged never starts. State the supported mass and footprint on the drawing so the operating point can be checked against the TDS curve rather than guessed.
Why did my isolation pad stop working after a year or two?
Usually creep and compression set: under constant load every elastomer keeps deflecting slowly and recovers less than it deflected, and when the accumulated loss consumes the working deflection, the pad is a shim and the structure-borne path is back. Specify constructions built for sustained load (rebonded neoprene, rubberized cork, the bearing-grade line), read the set lines per ASTM D395 and the D1056 suffix methods, and re-survey heavy bases after the first year of service.
Can I bolt equipment down through an isolation pad?
Not without isolating the bolt: a through-bolt is a rigid bypass, and the vibration walks straight down the shank into the floor regardless of the pad. Isolate the whole fastening: pad under the foot, sleeve or grommet around the bolt shank, and a washer of the same isolation family under the bolt head or nut, die-cut as a matched set. Where the bolt must stay rigid for seismic or safety reasons, accept the bypass and treat the remaining paths honestly.
Durometer or compression-deflection: which goes on the drawing?
One or the other, by material class, never interchangeably. Solids (bearing pads, solid rubber) are specified by Shore A durometer per ASTM D2240. Cellular materials (rebonded foam, vinyl nitrile, PORON, HyPUR-cel, reticulated foam) are specified by compression-deflection grade per ASTM D1056 or ASTM D3574, the stress at a standard deflection. The two do not convert, and a durometer callout on a foam is the units error the parent page names the durometer trap.
Have the operating point checked before the pads go in
Send the supported mass, the pad footprint, and the mounting method, and engineering will check the operating point against the familys compression-deflection data on the TDS.
Samples typically ship in 3 to 5 business days for common die-cut configurations on materials we keep on hand; standard production runs about two weeks after drawing approval; expedited service is available. MOQ varies by material and part.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
ApplicationVibration, Shock & Acoustic ControlRead →
GuideRebonded Neoprene, Cork, Bearing Pads, or PORON: Choosing Vibration Isolation and Acoustic MaterialsRead →
GuideHow to Spec Vibration Isolation and Acoustic Parts: What to Send H-O for a QuoteRead →
IndustryElectromechanical & Switchgear hubRead →
Request a quote for your vibration, shock & acoustic part
Send H-O your drawing or describe the part—engineering reviews every RFQ and converts samples on your geometry.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- ASTM D1056 (flexible cellular rubber; compression-deflection grades and suffix-B set methods) and ASTM D3574 (flexible cellular polyurethane methods).
- ASTM D2240 (Shore durometer, solids only); ASTM D395 (compression set); ASTM D2632 (resilience by rebound, where reported).
- ASTM D1149 (ozone) and ASTM D2137 (low temperature), as reported on the bearing-pad TDS; UL 94 classes per grade where assemblies are classed.
- Manufacturer technical data sheets for the rebonded, cork, bearing-pad, PORON, and HyPUR-cel grades named on the parent application page.
This article explains isolation-pad sizing and aging concepts qualitatively, following the compression-deflection reasoning and failure patterns cataloged on the parent application page. No numeric isolation prediction, natural frequency, or transmissibility value is stated for any design; operating points are checked against each grade's compression-deflection data on its current technical data sheet.
All parts are custom manufactured to order; minimum production run quantities vary by material and part. H-O is a die-cut converter, not a raw-material producer, and does not mold isolation mounts.