Custom Mechanical Equipment Isolation Pads
H-O Products die-cuts and converts the load-bearing vibration-isolation pads that sit under building mechanical equipment: ribbed and waffle neoprene, cork-rubber composite, high-density rebonded neoprene, EPDM, and PORON® microcellular urethane pads and washers under rooftop units, air-handling units, chillers, pumps, and standby generators, cut to your drawing.
Built for: RTU and packaged-HVAC curb and rail pads, air-handling-unit and chiller base pads, pump and generator inertia-base pads, and anti-vibration washers, sized so the pad's natural frequency lands below the equipment's forcing frequency (isolation only occurs above a frequency ratio of √2) and so its sustained bearing stress stays inside the pad's working range to limit creep.
To pick an isolation pad for building mechanical equipment, work load first, then tuning. Load: size the bearing area so sustained stress sits in the material’s working range (neoprene isolation pads roughly 15–75 psi) — rebonded neoprene or cork-rubber under the heaviest chillers, gensets, and inertia bases; ribbed or waffle neoprene under lighter RTU and pump feet. Tuning: the pad’s natural frequency must land below the forcing frequency — isolation only occurs above a forcing-to-natural ratio of √2; too stiff amplifies.
The remaining duties are mapped in the When-to-spec list. Values are per the TDS on file; see the material reference below for ordering details.
Material-level, per the maker TDS: ASTM D2240 (durometer / Shore A) · ASTM D575 (rubber in compression) · ASTM D1056 (cellular-rubber classification) · ASTM D3574 (flexible cellular urethane, CFD and compression set) · UL 94 (flammability classes incl. V-0 on the rated grades).
Isolation theory (natural frequency from static deflection, the √2 crossover, transmissibility, creep) cited to named engineering references and ASHRAE selection practice. Assembly-level, by designation (belongs to the installed building): the local noise ordinance (property-line dBA).
- Heavy base / inertia-base pad: rebonded neoprene (25#/27#)
- Machinery mount, high damping: cork-rubber
- RTU / pump / lighter feet: ribbed / waffle neoprene
- Outdoor / rooftop pad or washer: EPDM
- Precision / light AHU-fan isolation: PORON® 4701 series
- Hot room / flame class: silicone foam
- Aux equipment / anti-rattle: vinyl-nitrile
- Bolted-foot washers / shims: solid neoprene
- Indoor / protected only: natural rubber
Where are you in the spec process?
This page serves engineers who already know the isolation pad they want and engineers still deciding it by load, tuning, and exposure. Pick the path that matches where you are; you don't have to read the rest.
Send a drawing, get a quote
A ribbed or waffle neoprene pad, a cork-rubber isolator, a rebonded-neoprene inertia-base pad, an EPDM outdoor washer, or a PORON® anti-vibration washer on your drawing.
Skip to the quote form →Decide the pad by load, tuning, and exposure
Six selection factors (load and bearing stress, natural-frequency tuning, sustained load and creep, outdoor durability, floor rigidity, and the washer at bolted feet), a checklist-driven pad builder, and ten families with TDS-cited data.
Start with selection factors →
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1Send drawingUpload a DXF, STEP, or PDF, or describe the unit, rail, or inertia base and the mounting. A sample pad works too.
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2Material reviewEngineering reviews the equipment mass and bearing area, sizes the pad so its bearing stress sits in the working range, and confirms the natural frequency lands below the forcing frequency, framing the standards honestly: material classes (ASTM D2240, D575, D1056) per the maker TDS; isolation tuning by the cited theory and ASHRAE; the property-line noise result by ordinance at the building.
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3PrototypeSamples typically ship in 3–5 business days for common configurations on materials we keep on hand. Made-to-order; MOQ varies by material and part.
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4ProductionStandard production runs ship about 2 weeks after drawing approval, on flatbed die-cutting, waterjet cutting for thick base pads, and kitting for laminated pad-and-facing sets. Ongoing parts run with material traceability and lot-code TDS records.
Vibration or noise problem → load & tuning → material selection → converted pad → production supply.
- 1Name the equipment & sourceRooftop unit, AHU, chiller, pump, or genset, with its mass and its forcing frequency (fan tone, pump or genset RPM).
- 2Size for loadSet the pad bearing area so the sustained stress sits inside the material's working range, limiting creep.
- 3Tune the natural frequencyConfirm the pad's natural frequency lands below the forcing frequency, keeping it above the √2 crossover.
- 4Choose the materialMatch the family to load, exposure (indoor/outdoor, UV/ozone/oil), and any temperature or flame need.
- 5Die-cut to drawingDie-cut, waterjet-cut, slit, laminate, and kit the pads, strips, and washers to the footprint drawing.
- 6Quote prototype or productionPrototype quantities through full production runs, with the TDS and lot-code traceability behind the part.
This guide is for mechanical, HVAC, and structural engineers, and the contractors and building owners specifying die-cut vibration-isolation pads under rooftop units, air-handling units, chillers, pumps, generators, and cooling towers. If your problem is building acoustics between spaces (partitions, floor/ceiling assemblies, STC/IIC), that is the acoustic isolation & sound control sibling page; this page is the equipment-mount vibration pad.
Which equipment are you isolating?
Application Zones
Six equipment problems define this page: rooftop and packaged HVAC on curbs and rails; air-handling units and fan sections in mechanical rooms; chillers and compressors; base-mounted and inline pumps on housekeeping pads; standby generators on inertia bases; and cooling towers, condensers, and the anti-vibration washers at bolted feet.
In every one, the pad is a load-bearing spring: size it by bearing stress first, then confirm the natural frequency lands below the equipment's forcing frequency. Click a tab to see the duty, the controlling properties, and the families H-O converts for that zone.
Rooftop & packaged HVAC: tune to the fan tone, weather the roof
A rooftop or packaged HVAC unit sits on a curb or rail, and its vibration comes from supply and condenser fans and (on packaged units) a compressor, so the forcing frequency is the fan tone or compressor RPM. That is usually well above 1200 RPM, which on a rigid roof structure is the regime where an elastomeric pad isolates effectively; the pad is a spring sized so its bearing stress sits in the working range and its natural frequency lands below the fan tone.
Because the pad lives outdoors in sun and rain, ribbed or waffle neoprene handles the isolation with good weather and oil resistance, and EPDM is the default where ozone and UV dominate and the pad doubles as a weather seal at the rail. Ribbed and waffle patterns let the pad expand under load without losing stability and keep creep low.
The property-line noise limit is set by the local ordinance and belongs to the installed unit and its curb, not the pad.
Air handlers & fan sections: precision isolation in the mechanical room
An air-handling unit is mostly a fan in a box, and its vibration is the fan's rotational and blade-pass tones coupling into the unit frame and the mechanical-room floor. Two parts do the work. Precision pads and washers of PORON® microcellular urethane isolate the fan section and internal components with a tightly controlled compression-force-deflection window and very low compression set, so the tuning survives years of running; the 4701 industrial series is the workhorse here.
Under the unit itself, a ribbed or waffle neoprene or cork-rubber pad carries the base load and blunts the low-frequency content. Where the mechanical room runs hot near a boiler, silicone foam holds up across the wider temperature range. The pad is sized by bearing stress first, then checked so its natural frequency lands below the fan tone, keeping it above the √2 crossover.
Chillers & compressors: the high-load base pad
A chiller or reciprocating compressor is a heavy machine with a rotating or reciprocating imbalance, and its base pad is doing two jobs at once: carrying a large static dead load and decoupling the vibration from the housekeeping pad and the floor. Sizing is a static-stress calculation first: pick the pad bearing area so the sustained stress sits inside the material's working range (neoprene isolation pads work roughly 15–75 psi), then confirm the natural frequency lands below the machine's dominant frequency.
High-density rebonded neoprene carries the heaviest loads with low creep; cork-rubber adds a high loss factor under continuous rotation. Screw chillers carry very high-frequency content, which an elastomeric pad handles well on a rigid floor; where the floor is flexible or the machine is low-RPM, a larger deflection (spring) is called for, and the elastomeric pad is the layer under that spring or inertia base.
Send the machine mass and the pad bearing area, and the pad is sized to its load.
Pumps: isolate the continuously rotating machine, resist the oil
Base-mounted and inline pumps run continuously for years in mechanical rooms and pump stations, often around oil mist and occasional splash, so the isolation pad has to keep working through humidity, oil, and sustained load.
The logic is the same static-stress-then-natural-frequency calculation, with the material choice leaning on the closed-cell and chemically resistant families: cork-rubber where a high loss factor is wanted under continuous rotation (it is the classic pump-and-compressor mount), ribbed or waffle neoprene for the load-bearing pads with good oil resistance, and vinyl-nitrile for lighter auxiliary pumps and day-tank supports where cost and oil resistance matter.
Solid rubber pads and washers take the tolerance-take-up and mounting duty at bolted feet. A pump that runs continuously for years will find any pad that was loaded past its creep limit, so the pad is sized so its sustained bearing stress stays inside the working range.
Generators & inertia bases: carry the base, keep the firing order out of the slab
A standby generator is a heavy rotating machine with a firing order and an imbalance, and bolting its skid hard to the slab pushes those tones straight into the building. Most gensets ride on an inertia base, a concrete or steel mass that lowers the assembly's center of gravity and evens the loading, and the pad is the resilient layer under that base or under the skid rails.
Base isolation is a static-stress calculation first: size the pad so its bearing stress sits inside its working range, then confirm the natural frequency lands below the genset's dominant frequency. High-density rebonded neoprene carries the heavier base loads with low creep; ribbed or waffle neoprene and vinyl-nitrile cover the lighter auxiliary skids and day-tank supports.
Molded isolation mounts and steel springs, where a larger deflection is required, are coordinated through the partner network, and the elastomeric pad is the layer that breaks the structure-borne path under them.
Cooling towers, condensers & the washers at bolted feet
Cooling towers and outdoor condensers sit on structural steel or dunnage over a roof, wet and weather-exposed, with fan and pump tones to isolate and standing water and ozone to survive. The pads under their feet are the load-bearing, weather-resistant families: EPDM where ozone and UV and standing water dominate, ribbed or waffle neoprene and rebonded neoprene where the load is heavier.
The last of the structure-borne path runs through the bolted feet, and there an anti-vibration washer, a PORON® or elastomer part whose low compression set holds the standoff, decouples the joint without loosening it. Ribbed and waffle patterns keep creep low under sustained load. As always, the pad is sized so its sustained bearing stress stays inside its working range, because equipment that runs for years will find any pad loaded past its creep limit.
Six decisions that drive your equipment isolation-pad spec
An isolation pad is a single-purpose part, and each decision has one controlling number. Miss it and the failure is quiet: a pad amplifies instead of isolating, a pad creeps until the standoff expires, an outdoor pad cracks in the sun, or a pad meant for a spring-mount floor does not have the deflection the job needs.
Size for load first, then tune the frequency. An isolation pad is a spring: pick its bearing area so the sustained stress sits inside the material's working range (which bounds creep), then confirm the natural frequency at that load lands below the equipment's forcing frequency. Isolation only occurs above a forcing-to-natural frequency ratio of √2, so a pad selected by thickness alone can land in the amplification region and make the vibration worse.
Write the equipment mass, the pad bearing area, and the forcing frequency on the drawing; the family and gauge fall out of those.
Below a forcing-to-natural frequency ratio of √2 (≈1.414), an isolator amplifies. A common rule of thumb sizes the pad so its natural frequency is about one third of the frequency of concern, which puts the equipment tone comfortably in the isolation region (at a ratio of 2, roughly 65% of the vibration is isolated; at 3, roughly 85%).
The natural frequency follows from the static deflection under the mounted load (fn ≈ 3.13√(1/δ) with δ in inches, or fn = (1/2π)√(g/δ)), so the load and the pad's stiffness, not its gauge, set the tuning. [9]
Tip: the equipment's running tone is fixed by its RPM; you cannot move it, so you move the pad, tuning its natural frequency far below the tone. On a flexible floor or for low-RPM equipment, an elastomeric pad may not give enough deflection, and a spring mount is called for.
Read the six factors below in order. The first two size the pad for load and tune it; the next two cover sustained-load creep and the outdoor material; the last two check the floor and cover the washer at bolted feet. Every factor names its test method, because in this application the documentation is part of the part.
Show all 6 selection factors tap to expand
Size for load: set the bearing stress, not the footprint
Rule — pick the pad bearing area so the sustained stress sits inside the material's working range. An isolation pad is a spring, and its behavior is set by the stress it carries, not by how big it looks: neoprene isolation pads work in a range of roughly 15–75 psi, and cork-rubber machinery pads are commonly designed near 50 psi maximum base loading.
Size the pad so the equipment mass divided by the total pad bearing area lands in that window; too little area overstresses the pad and it creeps, too much area under-deflects it and it barely isolates. Send the equipment mass and the number and area of the pads; the working stress and the family follow from those two numbers. [5]
Tune the pad by natural frequency, not thickness
Rule — size the pad so its natural frequency sits well below the equipment's forcing frequency. The pad is a spring; its natural frequency follows from the static deflection under the mounted mass (fn ≈ 3.13√(1/δ) with δ in inches, or fn = (1/2π)√(g/δ)). Isolation begins only above a frequency ratio of √2, so a pad tuned too stiff lands in the amplification region and makes the vibration worse.
Get the mounted mass, the bearing area, and the forcing frequency (fan tone, pump or genset RPM) onto the drawing; the stiffness grade and thickness fall out of those. A common rule aims the natural frequency at about a third of the forcing frequency. [9]
Size for sustained load: limit creep so the spec survives
Rule — a multi-ton machine sits on its pad for decades, so creep is a first-class spec input. An elastomer under sustained static load keeps deflecting slowly over time (creep, roughly with the logarithm of load time), and a pad loaded past its working range drifts until the isolation standoff and the tuning are gone.
Keep the sustained bearing stress inside the pad's published working range: a common bearing-pad practice keeps strain modest (on the order of 8–10%) with a compressive-stress limit (about 1500 psi for consistent long-term performance), taking a working figure near 30% creep of the initial deflection.
Cork-rubber and high-density rebonded neoprene resist creep; soft foams drift if overloaded. Send the mass and bearing area so the pad is sized to its load. [3]
Pick the elastomer for the outdoor environment
Rule — most of this equipment lives outdoors on a roof or at grade, so weathering decides the family. EPDM has the standout ozone, UV, and weather resistance and is the outdoor default for isolation-plus-sealing; neoprene (polychloroprene) brings good general weather and oil resistance; silicone covers the widest temperature range and holds up to UV and ozone. Natural rubber is economical but has poor ozone and UV resistance, so it cracks and hardens in prolonged sun and is kept for indoor or protected duty.
Cork-rubber adds load-bearing damping with low creep indoors and in protected outdoor locations. Name the exposure (UV, ozone, oil, standing water) and the temperature range; those route you to the family before cost does. [15]
Check the floor: elastomer pad or spring mount
Rule — the floor and the equipment RPM decide whether a pad is enough. A common selection guide treats equipment above about 1200 RPM on a rigid ground-floor slab as a job an elastomeric pad handles well, and equipment below about 1200 RPM, or on an upper floor where the structure flexes, as a job that needs the larger deflection of a spring mount.
Elastomers practically limit static deflection to about half an inch, so once the tuning needs more deflection than that, a spring is required, and the elastomeric pad becomes the layer under the spring or inertia base that breaks the residual structure-borne path. Tell us the floor (grade slab or upper floor) and the equipment RPM; that decides pad-alone versus pad-under-spring.
The washer at bolted feet: hold the standoff, don't loosen the joint
Rule — the last of the structure-borne path runs through the bolted equipment and rail feet, and there the part is an anti-vibration washer, not a base pad. A PORON® microcellular urethane or elastomer washer whose low compression set holds the standoff decouples the joint without relaxing under the bolt load and losing clamp force. Match the washer's compression-force-deflection to the bolt torque and the foot area, and keep the material's compression set low so the standoff survives; PORON® industrial grades carry that low set per ASTM D3574.
Tell us where the bolted joints are, the foot area, and the bolt pattern; the washer follows. [11]
Specification Tools
Two tools to take you from "we have a vibration problem" to here is the pad checklist for the drawing set: a requirement-driven pad builder that assembles the part list with what to send and its citations, and a side-by-side comparison of every pad family on this page.
1. Mechanical-equipment isolation-pad checklist builder
Check the equipment and duty you have. The builder assembles the corresponding pads into a checklist with the family, what to send with the drawing, and the honest citation language (material classes per TDS; isolation tuning by named reference and ASHRAE; property-line noise by ordinance at the building). The default selection is a typical outdoor rooftop unit with a bolted-foot washer; every part is also printed in the material reference section, so nothing here exists only behind a script.
Pad checklist: 3 parts selected
Each checked item adds its pad below. The list is the starting bill of materials for the engineering review, not a certification: material classes (ASTM D2240, D575, D1056) come from the grade TDS, isolation tuning follows the cited theory and ASHRAE selection practice, and the property-line noise result belongs to the installed building.
- Rooftop / packaged pad: ribbed or waffle neopreneSend: unit mass, per-pad bearing area, fan / compressor RPM. Cite: class per ASTM D1056; working range ~15–75 psi; tune below √2 crossover.
- Outdoor pad / washer: EPDM (UV / ozone)Send: exposure (UV, ozone, standing water), temperature. Cite: EPDM weathering resistance; class per ASTM D1056.
- Bolted-foot washer: PORON® anti-vibration washerSend: foot area, bolt pattern, torque. Cite: low compression set and CFD per ASTM D3574; holds the standoff.
2. Side-by-side: pad material family comparison matrix
Every pad family called out on this page, with construction, the property that drives its selection, the methods its TDS cites, and the job it serves. Click a column header to sort. Click any material name to jump to its accordion entry.
| Material | Construction | Selection driver | Methods on the TDS | Job | |
|---|---|---|---|---|---|
| Load-bearing isolation pads | |||||
| Rebonded Neoprene (25# / 27#)High-density rebond | Dense rebonded foam | Static stress + low creep | ASTM D1056 (14 psi, 27#) | Chiller / genset base pads | |
| Cork & Rubberized CorkCork-rubber composite | Cork + rubber composite | Load-bearing + high loss factor | Load-deflection per TDS | Machinery base pads | |
| Ribbed / Waffle NeoprenePolychloroprene | Closed-cell ribbed / waffle | Weather + oil, ~15-75 psi | ASTM D1056; D2240 | RTU / pump / equipment feet | |
| EPDM Foam & SolidEPDM | Closed-cell / solid EPDM | Ozone / UV / weather | ASTM D1056; D2240 | Outdoor isolation + seal | |
| Vinyl-Nitrile Foam (Ensolite®)PVC/NBR closed-cell | Closed-cell PVC/NBR foam | Cost + oil resistance | ASTM D1056 | Aux equipment / anti-rattle | |
| PORON® Industrial (4701 Series)Microcellular urethane | Microcellular PU foam | CFD window + low set | ASTM D3574; D2240 | Precision AHU / fan pads | |
| Silicone Foam (BISCO® HT)Closed-cell silicone | Closed-cell silicone foam | Temp range + flame class | UL 94 V-0; -55/+200 C (TDS) | Hot room isolation / seal | |
| Washers, shims & indoor | |||||
| Anti-Vibration WashersPORON® / elastomer | Die-cut washer | Low set holds standoff | ASTM D3574 | Bolted equipment feet | |
| Solid Neoprene / RubberSolid elastomer | Solid polychloroprene | Tolerance take-up + shims | ASTM D2240; D575 | Leveling shims / washers | |
| Natural Rubber (foam / solid)NR (indoor only) | Natural-rubber foam / solid | Economical (poor UV/ozone) | ASTM D1056; D2240 | Indoor / protected pads | |
Skip ahead and request your engineering review now
If your drawing set already calls out a ribbed or waffle neoprene pad, a cork-rubber isolator, a rebonded-neoprene inertia-base pad, an EPDM outdoor washer, or a PORON® anti-vibration washer, send it over for engineering review against the TDSs and the isolation tuning.
Equipment isolation-pad failures you can prevent at spec
These pads fail quietly: a pad tuned into amplification, a pad that crept until the standoff was gone, an outdoor pad that cracked in the sun, a pad on a floor that needed a spring, a bolted-foot washer that relaxed. Five patterns cover most of it, and each is a specification decision made before the first pad is cut.
An isolation pad you did not tune can amplify. The most common mistake on this page is treating a base pad as a spacer and selecting it by thickness. If its natural frequency lands within √2 of the equipment's forcing frequency, transmissibility rises above 1 and the pad makes the vibration worse. Size the pad by bearing stress, then tune by natural frequency; state the equipment mass, the pad bearing area, and the forcing frequency.
Show all 5 failure modes tap to expand
1. The base pad that amplified instead of isolating
Fix — size by bearing stress, then tune by natural frequency, and keep the forcing tone above the √2 crossover. A rooftop-unit, chiller, or pump pad was chosen for standoff: the right thickness, the wrong spring. Its natural frequency landed near the equipment's dominant frequency, transmissibility climbed above 1, and the "isolator" pushed more vibration into the slab and the floor below.
Isolation only occurs above a forcing-to-natural frequency ratio of √2, and a common rule sizes the pad's natural frequency at about a third of the frequency of concern.
Get the equipment mass, the pad bearing area, and the forcing frequency (fan tone, pump or genset RPM) onto the drawing, and let the stiffness grade and thickness fall out of the tuning. [9]
2. The pad that crept until the standoff was gone
Fix — size for sustained load, not just footprint, and keep the bearing stress in the working range. A pad under a chiller or inertia base was sized to fit the base, not to carry the load, so it was overstressed and kept deflecting slowly (creep) until the standoff collapsed and the tuning drifted. Elastomers creep under sustained static load roughly with the logarithm of time, and a pad loaded past its working range never stops.
Keep the strain modest (on the order of 8–10%) and the compressive stress inside the pad's published limit (about 1500 psi for consistent long-term performance), take a working figure near 30% creep of the initial deflection, and reach for cork-rubber or high-density rebonded neoprene where creep resistance matters. Send the mass and bearing area with the drawing. [3]
3. The outdoor pad that cracked in the sun
Fix — pick the elastomer for the exposure; natural rubber is an indoor material. A pad or washer chosen for economy was natural rubber, and on a rooftop unit or grade-mounted chiller it hardened and cracked under ozone and UV within a couple of seasons, so it stopped isolating and started leaking at the rail. EPDM is the outdoor default for its ozone, UV, and weather resistance; neoprene brings good weather and oil resistance; silicone covers the widest temperature range with good UV and ozone resistance.
Name the exposure (UV, ozone, oil, standing water) and the temperature range on the drawing, and keep natural rubber for indoor or protected duty. [15]
4. The pad used where the floor needed a spring
Fix — check the floor and the RPM before you assume a pad is enough. An elastomeric pad was specified under low-RPM equipment on a flexible upper floor, where the tuning needed more static deflection than an elastomer can give (elastomers practically limit deflection to about half an inch). The pad landed too stiff, its natural frequency did not clear the forcing frequency, and the isolation never happened.
A common selection guide treats equipment above about 1200 RPM on a rigid ground-floor slab as pad-friendly, and equipment below about 1200 RPM or on an upper floor as spring-mount territory. Tell us the floor and the RPM; where a spring is required, the elastomeric pad is the layer under it that breaks the residual structure-borne path. [4]
5. The bolted-foot washer that relaxed and loosened the joint
Fix — use an anti-vibration washer whose compression set is low, so the standoff holds and the bolt keeps its clamp force. A soft or high-set washer was placed under a bolted equipment or rail foot, and over months it took a compression set, relaxed, and let the bolted joint lose preload, so the joint rattled and transmitted vibration again. The fix is a PORON® microcellular urethane or elastomer washer with low compression set matched to the bolt torque and the foot area; PORON® industrial grades carry that low set per ASTM D3574.
Send the foot area, the bolt pattern, and the torque so the washer is sized to hold, not to relax. [11]
Material reference
Detailed specs for the ten pad families referenced on this page: the load-bearing isolation materials (rebonded neoprene, cork-rubber, ribbed and waffle neoprene, EPDM, vinyl-nitrile, PORON®, and silicone foam), the washers and shims (anti-vibration washers and solid rubber), and the indoor natural-rubber option. Values are per the maker TDS on file for each grade with the method named; isolation theory and floor-selection logic are cited by reference and by designation. H-O die-cuts, kiss-cuts, slits, waterjet-cuts, and kits every family to drawing.
Rebonded Neoprene (25# / 27# Density)Chiller & genset base pads · ASTM D1056 · load-bearing, low creep
Size a base pad by static bearing stress (mass over pad area), then confirm the natural frequency lands below the forcing frequency. Values per the H-O 27# rebonded neoprene TDS on file (ASTM D1056-07).
Cork & Rubberized Cork (Cork-Rubber Composite)Machinery isolation pads · high loss factor · low creep
Cork-rubber is a load-bearing damping material, not a soft spring; size it to its published load-deflection range (near 50 psi maximum base loading is typical). Values per the cork-rubber maker TDS on file.
Ribbed / Waffle Neoprene (Polychloroprene Foam)RTU / pump / equipment feet · ASTM D1056 · ~15-75 psi working range
Ribbed and waffle pads are stackable to multiply load capacity, but confirm the combined deflection still meets the required isolation frequency. Class per ASTM D1056 on the grade TDS.
EPDM Foam & SolidOutdoor / rooftop isolation + seal · ozone / UV / weather · ASTM D1056
EPDM is the outdoor default for isolation combined with sealing; where a hydrocarbon (oil / fuel) exposure is present, neoprene's oil resistance may suit better. Class per ASTM D1056 on the grade TDS.
Vinyl-Nitrile Foam (Ensolite® Family)Aux equipment / anti-rattle · cost + oil resistance · ASTM D1056
Vinyl-nitrile suits lighter loads and oil exposure; for heavier sustained loads step up to rebonded neoprene or cork-rubber. Class per ASTM D1056 on the grade TDS.
PORON® Industrial (4701 Series Microcellular Urethane)Precision AHU / fan pads + washers · low compression set · ASTM D3574
PORON® is a precision low-set material for light, well-controlled loads and washers; for heavy base pads use rebonded neoprene or cork-rubber. Values per the Rogers PORON® 4701 series TDS.
Silicone Foam (BISCO® HT Series)Hot mechanical rooms / flame class · -55 to +200 C · UL 94 V-0 grades
Silicone foam is the choice where temperature or a flame class drives the selection; UL 94 listings are per the individual grade TDS. Values per the Rogers BISCO® HT medium-grade TDS.
Anti-Vibration Washers (PORON® / Elastomer)Bolted equipment & rail feet · low compression set holds the standoff
Match the washer's CFD to the bolt torque and the foot area, and keep the compression set low so the standoff survives. PORON® industrial grades carry low set per ASTM D3574.
Solid Neoprene & Rubber (Shims / Washers)Tolerance take-up & leveling · ASTM D2240 / D575
Solid rubber is for tolerance take-up and leveling, not for spring-like isolation; select the durometer to the duty per ASTM D2240. Values per the grade TDS.
Natural Rubber (Foam / Solid) — Indoor OnlyEconomical indoor pads · poor UV / ozone · ASTM D2240 / D1056
Natural rubber is an indoor material: keep it out of UV and ozone exposure, where it cracks and hardens. Durometer per ASTM D2240 on the grade TDS.
H-O MountWaffle Pads & Point-Support Pads
Mechanical equipment isolation pads: engineer-grade FAQ
Twelve of the questions we hear most from mechanical, HVAC, and structural teams isolating rooftop units, air handlers, chillers, pumps, and generators. If your question isn't here, send a drawing or call, engineering picks up.
Can an isolation pad make the vibration worse instead of better?
Yes, and it is the most common mistake on this page. Isolation only occurs above a forcing-to-natural frequency ratio of √2 (about 1.414). If the pad's natural frequency is too close to the equipment's forcing frequency, the system runs in the amplification region, where transmissibility rises above 1 and the pad transmits more than it blocks, worst at resonance where the ratio is 1.
The fix is to tune the pad so its natural frequency sits well below the forcing tone, often around a third of it. The natural frequency follows from the static deflection under the mounted load, so the equipment mass and the pad's stiffness set the tuning, not the thickness. [9]
How do I size an isolation pad for a rooftop unit, chiller, or pump?
It is a static-stress calculation first, then a frequency check. Size the pad so its bearing stress (the equipment mass over the total pad area) sits inside the pad's working range, roughly 15–75 psi for neoprene isolation pads, so it neither bottoms out nor creeps excessively. Then confirm the natural frequency at that load lands below the forcing frequency (fan tone for an air handler, compressor or pump RPM, genset RPM), keeping the forcing tone above the √2 crossover.
Send the equipment mass, the number and area of the pads, and the forcing frequency, and the family and thickness fall out of those. Rebonded neoprene and cork-rubber carry the heavier loads with low creep. [5]
What is creep, and why does it matter for a pad under a chiller or generator?
Creep is the slow, continued deflection of an elastomer held under sustained static load, and it increases roughly with the logarithm of load time. A pad under a multi-ton chiller or generator inertia base sits loaded for decades, so a pad sized only to fit the footprint (not to carry the load) keeps sinking until the isolation standoff and the tuning are gone.
The prevention is to keep the sustained bearing stress inside the pad's working range: a common bearing-pad practice keeps strain modest (on the order of 8–10%) with a compressive-stress limit (about 1500 psi for consistent long-term performance) and treats about 30% creep of the initial deflection as a working figure. Cork-rubber and high-density rebonded neoprene resist creep best. [3]
When are neoprene pads enough, and when do I need a spring mount?
It depends on the equipment RPM and the floor. A common selection guide treats equipment operating above about 1200 RPM on a rigid ground-floor slab as a job an elastomeric pad handles well, and equipment below about 1200 RPM, or on an upper floor where the structure flexes, as a job that needs the larger static deflection of a spring mount.
Elastomers practically limit static deflection to about half an inch, so once the tuning needs more deflection than that, a spring is required. H-O supplies the elastomeric pad; where a spring or inertia base is used, the pad is the resilient layer under it that breaks the residual structure-borne noise path, since steel springs transmit noise through direct contact. Tell us the floor and the RPM.
Which elastomer should I use for an outdoor rooftop or grade-mounted pad?
Pick for the exposure. EPDM has the standout ozone, UV, and weather resistance and is the outdoor default for isolation combined with sealing at the rail or curb. Neoprene (polychloroprene) brings good general weather and oil resistance where a hydrocarbon exposure is present. Silicone covers the widest temperature range with good UV and ozone resistance for the hottest or coldest spots.
Natural rubber is economical but has poor ozone and UV resistance, so it cracks and hardens in prolonged sun and belongs indoors or in a protected location, not on a rooftop unit. Cork-rubber adds load-bearing damping with low creep. Name the exposure and temperature range on the drawing. [15]
What is the difference between a ribbed pad, a waffle pad, and a cork-rubber pad?
Ribbed and waffle neoprene are closed-cell neoprene pads with a molded surface pattern (ribs one direction, or a two-sided waffle grid) whose purpose is to let the pad expand under load without losing stability, giving resilient high-frequency isolation with minimal creep; they carry lighter loads (roughly 15–75 psi) and resist weather and oil.
Cork-rubber is a composite of cork and rubber, often a cork core between rubber layers, that carries load with a high loss factor (strong damping) and low creep, and is the classic machinery mount under chillers, compressors, and pumps designed near 50 psi base loading.
Ribbed and waffle for lighter equipment feet, cork-rubber for heavy continuously rotating machinery. [5]
What is an inertia base, and where does the pad go?
An inertia base is a heavy concrete or steel mass that a machine (often a pump or generator) is bolted to, which lowers the assembly's center of gravity, evens out the loading on the isolators, and reduces motion. The isolation pad is the resilient layer between the inertia base (or the equipment rails) and the building structure, so it carries the combined equipment-plus-base mass and is sized to that larger static load.
Because the base adds mass, the pad's bearing area is set so the sustained stress still sits inside the working range, then the natural frequency is confirmed below the forcing frequency. High-density rebonded neoprene and cork-rubber are the usual families under an inertia base for their load capacity and low creep. [14]
Do these pads carry a property-line noise rating?
No, and no honest supplier will claim otherwise. A community or property-line noise limit is a dBA figure set by the local ordinance (municipal or county), often stricter at night, and the result is the sound the installed building and its equipment radiate, measured at the boundary. That belongs to the installed asset and its acoustic design, not to a pad.
What the materials carry is their own documentation: material-level classes like UL 94 on the rated grades, the ASTM methods behind their durometer, compression, and cellular-class properties, and lot-code traceability. They support the vibration and noise design; H-O supplies the converted pad layers and the paperwork, and the building owner and acoustician own the system result. [16]
How do I stop a bolted equipment foot from transmitting vibration?
Add an anti-vibration washer under the foot, sized so it holds its standoff. A bolted equipment or rail foot is a direct structure-borne path, and a resilient washer between the foot and the structure breaks it, but only if the washer does not relax under the sustained bolt load and lose clamp force. The material to reach for is one with low compression set: a PORON® microcellular urethane or elastomer washer whose compression-force-deflection is matched to the bolt torque and the foot area.
PORON® industrial grades hold that low set per ASTM D3574. Send the foot area, the bolt pattern, and the torque, and the washer follows; a soft high-set washer that relaxes is worse than none, because it loosens the joint. [11]
Is this the same as building acoustic isolation (partitions and floors)?
No, they are different problems with different parts. This page is equipment-mount vibration isolation: the load-bearing pad that decouples a rooftop unit, chiller, pump, or generator from the structure it sits on, specified by static load and natural frequency. Building acoustic isolation is about controlling airborne and impact sound between occupied spaces, through partitions, floor and ceiling assemblies, and resilient channels, specified by sound-transmission-class and impact-isolation-class ratings at the assembly.
That work lives on the acoustic isolation & sound control sibling page. The two often appear in the same building, and a mechanical room may need both, but the pad under a chiller and the isolation layer under a floated floor are sized by different numbers.
Does H-O mold isolation mounts, or convert the pads?
H-O and converts sheet, roll, and slab stock to drawing: die-cut, kiss-cut, waterjet-cut, slit, laminated, and kitted pads, washers, strips, and shims. That covers the great majority of the isolation and washer parts on this page. Molded isolation mounts and steel spring isolators (used where the tuning needs more deflection than an elastomer can give) are coordinated through a partner network, since H-O does not perform molding or wind springs in-house; the elastomeric pad is the layer that goes under them to break the residual structure-borne path.
Conversion runs in Winsted, Connecticut under an ISO 9001:2015 certified quality management system with material traceability and lot-code TDS records.
What should I send H-O to get a quote?
The equipment type and mass, the pad bearing area (number and size of pads or the footprint), and the forcing frequency (fan tone, pump RPM, chiller or genset RPM), plus the indoor/outdoor exposure and temperature and any oil or standing-water exposure. Add the floor (grade slab or upper floor) so we can flag whether a pad alone is enough or a spring is called for, and for bolted-foot washers the foot area, bolt pattern, and torque.
Plus a footprint or rail drawing, adhesive and liner needs, and quantities for prototype and production. "Recommend the pad" is a valid callout, that is what the engineering review is for; H-O sizes the materials, and the property-line noise compliance result belongs to the installed building.
Glossary: terms used on this page
Quick reference for the vibration-isolation terminology used throughout. Each entry links to the relevant reference or test method where applicable.
Vibration isolation
Reducing the transmission of vibrational energy from one body to another by inserting a resilient (compliant) element, per [9]. A load-bearing pad decouples a rooftop unit, chiller, pump, or generator from the building structure. Isolation only occurs above a frequency ratio of √2 relative to the pad's natural frequency.
Natural frequency (fn)
The frequency at which an isolator naturally oscillates: fn ≈ 3.13√(1/δ) with δ the static deflection in inches, or the equivalent fn = (1/2π)√(g/δ), for the undamped case, per [9]. The tuning target: keep the equipment's forcing frequency well above it.
Forcing (disturbing) frequency (fd)
The dominant tone the equipment produces, from an unbalanced rotating or reciprocating movement; for rotating machinery generally the lowest RPM in the system (RPM ÷ 60 = Hz), per [2]. The pad is tuned so its natural frequency sits below this.
√2 crossover
The frequency ratio (≈1.414) at which transmissibility returns to 1. Below it is the amplification region (transmissibility > 1); above it is the isolation region (transmissibility < 1), per [9].
Transmissibility
The dimensionless ratio of vibration transmitted through an isolator to vibration applied. It rises toward a peak at resonance for a lightly damped system; damping lowers that peak, per [9]. Below 1 means isolation; above 1 means amplification. At a frequency ratio of 2, roughly 65% of the vibration is isolated; at 3, roughly 85%.
Static deflection (δ)
The distance an isolator deflects under the static (dead) weight of the equipment, per [2]. It sets the natural frequency: more deflection gives a lower natural frequency. Elastomers practically limit static deflection to about half an inch; beyond that a spring is used.
Creep (sustained-load drift)
The continued slow deflection of an elastomer under sustained static load, increasing roughly with the logarithm of load time, per [3]. A pad loaded past its working range keeps creeping until the isolation standoff is lost; keeping strain modest and stress inside the published limit bounds it.
Compression set
The permanent thickness a material loses after sustained compression and release, measured per ASTM D3574 [4-alt] / ASTM D1056 [10]. Low set (PORON®, silicone foam, cork-rubber) keeps an isolation tuning or a washer's standoff alive over service life.
Loss factor
A dimensionless measure of a material's damping, its ability to dissipate vibrational energy as low-grade heat. Cork-rubber has a notably high loss factor, which is why it is favored for continuously rotating machinery mounts, per [7].
Inertia base
A heavy concrete or steel mass that equipment is bolted to, lowering the assembly's center of gravity and evening the load on the isolators; the pad is the resilient layer between the base and the structure. Common under pumps and generators.
Durometer (Shore A)
The hardness of a rubber or solid elastomer, measured per ASTM D2240 [8] on the Shore A scale. A harder pad carries more load per unit area but deflects less; durometer is selected to the duty for solid pads, shims, and washers.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
Standards, test methods & technical references
The standards, test methods, engineering references, and maker technical data sheets cited throughout this page. Community / property-line noise limits are cited by designation and belong to the installed building, not a pad; isolation theory is cited to named engineering references and ASHRAE selection practice. Standards editions current as of July 2026; verify against the publishing body before final spec.
H-O materials are aligned to the standards cited through the source manufacturer's TDS, not independently certified by H-O unless explicitly stated on the quote.
[9] Basics of Vibration Isolation (E-A-R / Aearo)
Masterson, P. A., "The Basics of Vibration Isolation Using Elastomeric Materials," E-A-R Specialty Composites / Aearo Company. The named engineering reference for the √2 crossover, the natural-frequency forms (fn = 3.13√(K/W); fn = (1/2π)√(g/δ)), transmissibility, and the one-third rule of thumb cited on this page. vibrationdata.com (Basics of Vibration Isolation, PDF)
[2] Isolation Theory (natural frequency, RPM, floor rigidity)
Vibration Eliminator Co., "Isolation Theory." Explains planned isolation of building mechanical equipment: the three controlling factors (weight, disturbing frequency, structure rigidity), the natural frequency from static deflection, the disturbing frequency taken from the lowest RPM, and the elastomer static-deflection limit near half an inch beyond which springs are used. veco-nyc.com (Isolation Theory)
[3] Creep of elastomeric bearing pads (Mason Industries)
Mason Industries, "Introduction" (bearing-pad design bulletin). Describes creep as additional deflection over time (a percentage of the initial deflection), a working figure near 30% creep, and a compressive-stress limit near 1500 psi with strain kept in the 8–10% range for consistent long-term performance. mason-ind.com (bearing pads)
[4] ASHRAE noise & vibration control selection guide
ASHRAE Handbook — Noise and Vibration Control (selection guide for vibration isolation, incl.
Table 47 notes): isolator selection by equipment type, horsepower, RPM, and floor-span location; equipment above about 1200 RPM on a rigid ground-floor slab is commonly pad-friendly, and lower-RPM or upper-floor equipment leans to spring mounts; the rooftop guidance selects the isolator for about 10 times the additional roof deflection when that deflection is 0.25 in. or less.
Cited by designation as selection context. rlscg.com (ASHRAE noise & vibration control, PDF)
[5] Neoprene & waffle isolation-pad load ranges (Kinetics / VibraSystems)
Neoprene vibration-pad selection data: neoprene isolation pads work in a range of roughly 15–75 psi with maximum loading commonly 60 or 120 psi and rated deflections about 0.04–0.19 in.; the two-sided waffle design lets the material expand without loss of stability, giving resilience with minimal creep, and pads are stackable to multiply load capacity. The basis for the ribbed/waffle-neoprene selection on this page. kineticsnoise.com (neoprene vibration pads)
[7] Cork-rubber vibration isolation data
Cork-rubber isolation-pad references (Amorim AcoustiCork "Vibration Isolation Solutions"; industry cork-rubber pad data): engineered cork-and-rubber compounds with high loss factors that dissipate vibration into low-grade heat, with low creep across a wide frequency range; machinery is commonly designed near 50 psi maximum base loading, and the cork core is sandwiched between rubber layers with opposing ribs for grip. The basis for the cork-rubber isolation-pad selection. amorimcorksolutions.com (cork-rubber isolation)
[11] Rogers PORON® 4701 industrial-grade TDS
Rogers Corporation PORON® 4701 series industrial microcellular polyurethane data sheet (Publication #17-018): CFD@25% about 249/428/643 kPa (36/62/93 psi) across density sub-grades, compression set 10% or less at 70 °C; listed uses include vibration management and shock absorption. The basis for the precision-isolation and anti-vibration-washer selection. rogerscorp.com (PORON industrial 4701)
[13] Rogers BISCO® HT-series (medium) silicone foam TDS
Rogers Corporation BISCO® HT-series Medium Silicone Foam data sheet (Publication #180-070): -55 to +200 °C, density 352 kg/m³, CFD 67 kPa, compression set 2.4% (100 °C), UL 94 V-0 (File E83967). The basis for the hot-room / flame-class silicone-foam selection. rogerscorp.com (BISCO silicones)
[14] H-O Products 27# rebonded neoprene TDS
H-O Products 27# rebonded neoprene foam data: density 27.3 lb/ft³, compression deflection 14 psi (ASTM D1056-07), water absorption 1.8%; stated use includes heavy-duty vibration isolation and load-bearing support in industrial and heavy-machinery environments. h-oproducts.com (27# rebonded neoprene)
[15] Elastomer weathering selection (EPDM / neoprene / natural rubber / silicone)
Elastomer material-selection reference: EPDM's standout ozone, UV, and weather resistance; neoprene's good general weather and oil resistance; natural rubber's poor ozone and UV resistance; and silicone's wide service-temperature range. The basis for the outdoor-material selection on this page. robinsonrubber.com (elastomer selection)
[8] ASTM D2240 (durometer)
ASTM D2240, "Standard Test Method for Rubber Property—Durometer Hardness." The durometer (Shore A hardness) method for the solid rubber and foam pad grades on this page. store.astm.org (ASTM D2240)
[6] ASTM D575 (rubber in compression)
ASTM D575, "Standard Test Methods for Rubber Properties in Compression." The compression / load-deflection method behind the solid rubber pad and shim grades on this page. store.astm.org (ASTM D575)
[10] ASTM D1056 (cellular rubber)
ASTM D1056, "Standard Specification for Flexible Cellular Materials—Sponge or Expanded Rubber." The Type/Class/Grade classification behind the neoprene, EPDM, vinyl-nitrile, and rebonded cellular-rubber pad grades. store.astm.org (ASTM D1056)
[4-alt] ASTM D3574 (flexible cellular urethane)
ASTM D3574, "Standard Test Methods for Flexible Cellular Materials—Slab, Bonded, and Molded Urethane Foams." The CFD and compression-set methods on the PORON® industrial TDSs behind the precision-pad and anti-vibration-washer selection. store.astm.org (ASTM D3574)
[16] Community / property-line noise limits (local ordinance, by designation)
Community and property-line noise limits for building mechanical equipment are set by local ordinance (municipal or county), typically as a dBA limit at the property line and often stricter at night. Cited by designation as design context; the compliance result belongs to the installed building and its equipment, not an isolation pad. Verify the applicable local ordinance for the site.
Updated . Standards editions and links current at publication; verify against the publishing body before final spec. H-O materials are “aligned to” the standards cited; H-O does not certify systems, sound levels, or ordinance compliance. Lot-specific qualification documentation available on request.
To review your equipment isolation-pad, send:
- Equipment type and mass
- Pad bearing area (per pad)
- Forcing frequency / fan tone / RPM
- Floor: grade slab or upper floor
- Indoor / outdoor + UV / ozone / oil
- Temperature & flame requirement
- Bolted-foot area & bolt pattern
- Pad or rail footprint drawing
- Adhesive / liner / lamination needs
- Prototype and annual volume
Get a mechanical equipment isolation-pad quote
Send a drawing, BOM, or a description of the equipment and its vibration problem. We typically respond within one business day with a material and pad recommendation, prototype lead time, and TDS verification against your isolation tuning, sustained load, and outdoor durability requirement.
See also: related H-O application pages
Sibling sub-application
Acoustic isolation & sound control
The building-acoustics side: resilient layers, partitions, and floor and ceiling assemblies specified by sound-transmission and impact-isolation ratings between spaces.
Read the page
Sibling sub-application
Duct & pipe thermal insulation
The insulation side of the same mechanical room: die-cut thermal and acoustic wraps, liners, and gaskets for ductwork, piping, and equipment.
Read the page
Sibling sub-application
Construction surface protection
The protection side: low-tack films, felt, foam, and die-cut edge and corner guards that protect finished surfaces and installed equipment on the construction site.
Read the page
Industry hub
General construction
The full general-construction application family: building-envelope sealing, expansion joints, acoustics, equipment isolation, fire and life safety, insulation, and protection.
Read the page