Quick answer: At 18 kg per foot, 60A case rubber feet compress 2.1 mm and 80A only 1.2 mm, a 17.5% strain against 10%, so the case sits 0.9 mm lower on the softer grade.
Table of contents
- Compression at 18 kg per Foot
- Hardness Mapped to Load Capacity
- Grip and Vibration Isolation
- Oil, Cold Hardening and Adhesive Failure
- FAQ
1. Compression at 18 kg per Foot
One case, four feet, 9 kg empty and 72 kg loaded: 18 kg per foot. Fit four 60A pads and four 80A pads of the same 24 mm diameter and measure under a 3 mm steel plate. The 60A pad drops from 12.0 mm to 9.9 mm, 2.1 mm of compression and 17.5% strain; the 80A pad drops to 10.8 mm, 1.2 mm and 10%. Same compound, different crosslink density.
Compression is not a defect. Grip comes from area: 2.1 mm of squash opens the contact circle from 24 mm to 27 mm and lifts area from 452 mm² to 573 mm², so contact pressure falls from 0.39 MPa to 0.31 MPa. With a 0.2 mm water film on the bench, that difference decides whether the case grips or slides.
Widening a pad from 24 mm to 32 mm raises contact area 78% and lets a 60A pad compress 1.5 mm instead of 2.1 mm, at the cost of a 4 mm step in the base.
The bill arrives in height: at 9.9 mm of clearance a 3 mm stone catches the base. Stiffness rises exponentially with hardness, 4.4 MPa at 60A to 7.2 MPa at 80A, predicting 1.75 times the deflection.
2. Hardness Mapped to Load Capacity
Allowable load follows a 25% strain ceiling. Past that line, fatigue life falls from 100,000 cycles to under 20,000 and the set becomes permanent.
| Hardness | Modulus MPa | Compression at 18 kg | Allowable per foot | Set, 22 h at 70 °C | Case weight |
|---|---|---|---|---|---|
| 50A | 3.4 | 2.9 mm | 14 kg | 21% | under 40 kg |
| 60A | 4.4 | 2.1 mm | 25 kg | 18% | 40–100 kg |
| 70A | 5.8 | 1.6 mm | 36 kg | 14% | 100–150 kg |
| 80A | 7.2 | 1.2 mm | 48 kg | 11% | above 150 kg |
The 25 kg figure at 60A comes from 2.1 mm at 18 kg scaled to a 3 mm ceiling, about 25.7 kg, rounded down; the same line gives 48 kg at 80A, 92% higher.
Recovery matters alongside capacity. Held 24 hours at 18 kg, a 60A pad returns to 11.6 mm with 0.4 mm of set; an 80A pad returns to 11.9 mm with 0.1 mm. Twenty moves a month accumulates over 6 mm of set in four years.
A dual-durometer pad, 60A face on an 80A skin at 14 mm total, compresses 1.6 mm and grips at 0.78, but loses 30% peel strength after 50 cycles between −20 °C and 60 °C.
3. Grip and Vibration Isolation
Static friction on dry tile reads 0.85 at 60A and 0.62 at 80A, both above the 0.5 threshold. Wet floors separate them: under a 0.2 mm film the figures fall to 0.55 and 0.38, and 80A is below the 0.4 safety line, creeping on a 2° slope.
A four-foot case resonates near 18 Hz on 60A and 26 Hz on 80A, outside the 4–12 Hz band of truck floor excitation, but transmissibility peaks at 3.2 and 4.1, so a second-order diesel harmonic near 18 Hz is amplified more by the soft pad.
Four hours at −20 °C pushes 60A to 71A and 80A to 88A, cutting friction 22% and 15%, so cold stores lean to 80A.
Floor finish decides the rest: epoxy and tile favour 60A for area, concrete and pallets favour 80A against grit. A 400 mm case tipping back 2° unloads the rear feet by 80% and raises the front pair to 29 kg, where a 60A pad compresses 3.4 mm, past the ceiling.
4. Oil, Cold Hardening and Adhesive Failure
Oil is the usual starting point. After 72 hours in mineral oil a nitrile 60A pad swells 8%, loses 6A of hardness and lifts set from 18% to 27%; an 80A pad swells 5% and loses 4A. The bulge of 0.3 mm lifts the case off three feet.
Cold hardening reverses, swelling does not. Warmth restores the first; the second needs a different compound, and a hydrogenated nitrile swells only 3% in the same oil.
Peel-off pads fail at the bond. Adhesive holds 1.8 MPa in shear when new, 1.1 MPa after 30 days at 60 °C and 0.6 MPa after 50 cycles between −20 °C and 60 °C. An 18 kg case puts 44 N on each foot over 452 mm², about 0.1 MPa, a sixfold margin that means nothing once an edge lifts and the tear crosses the pad within 200 handling cycles.
Mechanical retention answers it: a 3 mm spigot in a counterbore plus one M4 screw lifts peel load from 44 N to 320 N.
| Failure | Trigger | Visible sign | Fix |
|---|---|---|---|
| Oil swelling | 72 h in mineral oil | 8% swell, 0.3 mm bulge | HNBR compound, oil lip |
| Cold hardening | 4 h at −20 °C | 11A harder, 22% less friction | 80A, low-temp compound |
| Adhesive peel | 30 days at 60 °C | Edge lifting, 1.1 MPa shear | Spigot plus screw |
| Permanent set | Strain above 25% | 0.4 mm residual in 10 min | Lower load or raise hardness |
Two of the four are irreversible, oil swelling and permanent set; the rest yield to a grade change or a moulding change.
Conclusion
Hardness on case rubber feet is a division problem, not a feel test. Take the empty case weight, add the payload, divide by four, then multiply by 1.6 for off-centre loading. A design value up to 14 kg takes 50A, 14 to 25 kg takes 60A, 25 to 36 kg takes 70A and 36 to 48 kg takes 80A; beyond 48 kg, grow the pad from 24 mm to 32 mm instead of raising hardness. Then check two numbers before releasing the drawing: strain at or under 25%, and residual set after ten minutes at or under 0.4 mm. Both pass, and the feet hold the case's stance for 100,000 handling cycles without the base rocking on a loading dock or in a cold store.
This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.
Related Reading
- Anti-slip feet matched to floor friction
- Foot load sharing with wheels and castors
- Four-foot load sharing and base rib spans
- Case load capacity versus foot pressure
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