Direct answer: Shore A hardness describes how resistant an elastomer is to indentation. A softer O-ring can conform more easily to surface variation, while a harder O-ring can resist deformation and extrusion better in some conditions. Harder is not automatically better: the correct choice also depends on material, pressure, extrusion gap, groove, motion, temperature, fluid, and installation.
How common Shore A options compare
| Hardness | Why it may be considered | Main cautions |
|---|---|---|
| 60A | Softer response, lower assembly force, and better conformity in selected low-pressure or irregular-surface applications. | Can be more vulnerable to extrusion, cutting, rolling, and friction-related damage depending on the groove and motion. |
| 70A | A common general-purpose starting point for many static and moderate-duty applications. | Still requires confirmation of material, pressure, gap, motion, and compound-specific performance. |
| 75A | A modest increase in firmness where a 70A compound may be too compliant. | The small numerical difference does not guarantee better sealing or longer life. |
| 80A | Firmer response for selected pressure, extrusion, or handling requirements. | May require more assembly force and may conform less readily to surface imperfections. |
| 90A | High hardness used in selected higher-pressure, extrusion-resistant, or demanding mechanical conditions. | Harder installation, less conformity, and greater risk of damage when the groove, lead-in, or assembly method is unsuitable. |
Hardness does not identify the material
A 70A NBR O-ring and a 70A FKM O-ring do not have identical chemical, temperature, aging, or mechanical performance. Shore A is one property of a specific compound. Confirm the material family and exact compound separately using the Material Guide, Chemical Compatibility Guide, and available product documentation.
Hardness does not prove certification
A hardness rating does not prove food-contact, drinking-water, medical, aerospace, or other regulatory compliance. Treat a certification as confirmed only when documentation is available for the exact product or compound.
Pressure and extrusion gap
Higher hardness can help reduce extrusion in some pressure applications, but hardness alone cannot correct an excessive clearance gap, incorrect groove, poor surface finish, pressure cycling, or rapid gas decompression risk. Backup rings, groove changes, or another sealing design may be required.
Static versus dynamic service
Static, reciprocating, rotary, and oscillating seals have different friction, wear, lubrication, surface-finish, and heat-generation requirements. A harder ring may reduce deformation but can also increase assembly force or friction. Use the equipment drawing or engineering specification for critical dynamic service.
Size and groove still control the seal
The correct hardness cannot compensate for the wrong inner diameter, cross section, stretch, squeeze, or groove geometry. Start with the O-Ring Size Guide and How to Measure an O-Ring. Inspect the groove and mating surfaces before replacing a damaged seal.
Information to collect before selecting hardness
- Exact material or compound requirement
- Fluid, concentration, lubricant, and cleaning process
- Minimum, continuous, and peak temperature
- Pressure, vacuum, cycling, and extrusion gap
- Static, reciprocating, rotary, or oscillating motion
- Groove dimensions, surface finish, and installation path
- Expected service life and failure consequence
- Required tolerance, documentation, or certification
Buying path
- Size, material, hardness, and application confirmed: browse small packs or search by exact size.
- Hardness or material uncertain: review the material, compatibility, temperature, and installation guides.
- Drawing, unusual compound, special documentation, larger quantity, or critical service: use Bulk Quote.
FAQ
Is 90A always better for high pressure?
No. Pressure capability also depends on the compound, groove, clearance gap, temperature, motion, pressure cycling, and sealing design.
Is 70A the standard hardness?
70A is a common general-purpose starting point, but it is not automatically correct for every material or application.
Can I replace a 70A O-ring with 90A in the same groove?
Not without checking assembly force, conformity, squeeze, friction, pressure, and the equipment specification.
Can color identify hardness?
No. Color does not reliably identify hardness, polymer type, compound, or certification.
Technical reference
For additional engineering background, see the Trelleborg Shore Hardness technical reference. See also the OXXRing Technical Information, Sources & Review Policy.
