O-ring hardness, measured on the Shore A durometer scale, directly controls extrusion resistance in high-pressure service and friction in dynamic applications — yet it is frequently specified by default (70 Shore A) without considering whether the pressure, clearance gap, and duty cycle justify a different choice.
Quick answer: 70 Shore A is the correct baseline for general-purpose sealing up to approximately 100 bar dynamic and 150 bar static. Increase to 80 Shore A for 70–200 bar dynamic; increase to 90 Shore A for > 200 bar dynamic or applications with abrasive contamination. Do not specify 90 Shore A for rotary seals, brittle mating surfaces, or low-pressure applications — it increases friction and reduces conformability without benefit.
Shore A Hardness: Definition and Measurement
Shore A is the indentation hardness scale defined in ASTM D2240 for flexible and semi-rigid materials. A standardized spring-loaded truncated cone indenter (Type A) is pressed into the flat surface of the material under 822 grams force. The penetration depth at 15 seconds determines the reading on a 0–100 scale.
| Shore A Reading | Physical Character | Typical O-Ring Application |
|---|---|---|
| 20–40 Shore A | Very soft — gel-like, easily deformed by finger pressure | Soft VMQ medical/lab grades |
| 40–55 Shore A | Soft — easily deformed; low contact force | Soft food-grade VMQ; low-pressure face seals |
| 55–65 Shore A | Medium-soft — visible deformation with hand squeeze | Low-pressure food/pharma grades; vacuum seals |
| 65–75 Shore A | Standard flexible — industry baseline | General-purpose NBR, FKM, EPDM (most stock sizes) |
| 75–85 Shore A | Firm — resists moderate force | Medium-pressure hydraulics; specific dynamic seals |
| 85–92 Shore A | Hard — barely deformable by hand | High-pressure O-rings; abrasive service |
| > 92 Shore A | Very hard — rubber-to-plastic transition | Specialty grades; rare in standard O-ring service |
Approximate Young's modulus by Shore A (for reference, not for design calculations):
| Shore A | Approximate Young's Modulus (MPa) | Relative Stiffness |
|---|---|---|
| 50 | 0.7–1.0 | Very compliant |
| 60 | 1.0–1.5 | Compliant |
| 70 | 1.5–2.5 | Baseline |
| 80 | 2.5–4.0 | Moderately stiff |
| 90 | 4.5–8.0 | Stiff |
Measurement accuracy on O-ring cross-sections: ASTM D2240 requires a flat test surface of at least 6 mm thickness. A standard O-ring cross-section (1.78–3.53 mm CS) is too small for accurate single-ring measurement — indenter stiffness and cross-section curvature produce falsely low readings. Correct procedure: stack 3–5 rings to achieve adequate depth; press indenter perpendicular to the flat surface; read at 15 seconds; average 5 measurements. Acceptable variation within one lot: no reading deviating more than ±5 Shore A from the compound specification midpoint (ASTM D2240 tolerance on O-ring testing).
Shore A tolerance in practice: O-ring compounds are specified as "70 ± 5 Shore A" — this ±5 point range covers 65–75 Shore A. A 70A-specified compound measured at 75A is within tolerance but behaves meaningfully differently from one measured at 65A under high-pressure service. For pressure-critical applications, request the actual measured Shore A of the supplied compound, not just the nominal specification.
How Hardness Affects Each Sealing Performance Parameter
Extrusion Resistance: The Primary Driver for High Hardness
Extrusion resistance scales approximately with the elastic modulus — which increases nonlinearly with Shore A. At equivalent compression and clearance gap:
| Shore A | Relative Extrusion Force Required | Max Pressure (No Backup Ring, 0.15 mm Radial Clearance, Dynamic) | Max Pressure (Static) |
|---|---|---|---|
| 60 Shore A | 1.0× | < 50 bar | < 70 bar |
| 70 Shore A | 1.8× | < 100 bar | < 150 bar |
| 80 Shore A | 3.0× | < 200 bar | < 250 bar |
| 90 Shore A | 5.0× | < 350 bar* | < 400 bar* |
*Indicative values for well-controlled clearance and ideal surface finish. Many handbook guidelines cap standard O-rings around 200–250 bar dynamic even at 90 Shore A; always verify with the compound supplier and consider PTFE backup rings above 200 bar. These values assume standard diametral clearance 0.10–0.30 mm at ambient temperature. At elevated temperature, hardness decreases — see temperature correction below.
Physical mechanism: Harder elastomers have higher elastic modulus — more force per unit area is required to deform the material into the clearance gap. At equal compression rate, the contact stress at the clearance gap edge is higher for a harder compound, resisting pressure-driven flow into the gap.
Sealing Contact Force vs Conformability Trade-Off
Harder O-rings require more compressive force to achieve equivalent contact width. A 90 Shore A O-ring at 15% compression generates approximately 35–50% higher contact pressure than a 70 Shore A ring at the same 15% compression:
| Shore A | Contact Width (relative, at equal groove depth) | Contact Pressure (relative) | Conformability to Surface Irregularities |
|---|---|---|---|
| 60 Shore A | 1.2× | 0.7× | Excellent — fills Ra 1.6 µm surfaces |
| 70 Shore A | 1.0× (baseline) | 1.0× | Good — fills Ra 0.8 µm surfaces |
| 80 Shore A | 0.85× | 1.3× | Moderate — requires Ra ≤ 0.4 µm |
| 90 Shore A | 0.70× | 1.6× | Limited — requires Ra ≤ 0.2 µm |
Implications:
- Against rigid metal surfaces: higher contact pressure seals better — no disadvantage
- Against brittle materials (polycarbonate, acrylic, glass, soft aluminum ≤ 100 MPa tensile strength): 90 Shore A contact force can crack or deform the mating surface. Use 50–60 Shore A for glass, acrylic, or soft polymeric mating materials
- Against rough surfaces (castings, Ra 1.6 µm): softer O-rings bridge surface irregularities at low clamp loads where hard O-rings may leave leak paths; for rough cast surfaces with modest bolt loads, 60–70 Shore A seals more effectively
Dynamic Friction and Heat Generation
Friction at the O-ring-to-rod (or bore) interface increases with contact pressure, which increases with hardness at equal compression.
| Shore A | Relative Friction (sliding, lubricated) | Heat Generation (0.3 m/s, 100 bar, continuous) | Suitable for Dynamic Service |
|---|---|---|---|
| 60 Shore A | 0.7× | Low | Yes — preferred for low-pressure dynamic |
| 70 Shore A | 1.0× (baseline) | Moderate | Yes — standard dynamic specification |
| 80 Shore A | 1.4× | Higher | Yes, with adequate lubrication; limit to < 0.5 m/s |
| 90 Shore A | 2.0× | High | Only when pressure demands; limit to < 0.3 m/s |
Rotary seals and spiral failure risk: For rotary shaft seals, friction torque from a 90 Shore A O-ring can be 2.5–3× higher than 70 Shore A at the same shaft diameter and compression rate. If the friction torque exceeds the torsional resistance of the O-ring, the ring rotates with the shaft rather than sliding against the groove wall — the resulting twisting creates the diagonal "spiral failure" fracture. For all rotary seals, specify ≤ 70 Shore A; 60–65 Shore A is preferred for shaft speeds > 1 m/s.
Compression Set: Hardness vs Compound Formulation
Compression set (ASTM D395 Method B) is often incorrectly assumed to improve with hardness. Compression set is primarily controlled by polymer type, cure system, and filler content — not by hardness alone:
| Compound | Shore A | Compression Set (100°C / 70h, ASTM D395B) |
|---|---|---|
| Sulfur-cured NBR | 90 | 40–60% |
| Peroxide-cured NBR | 70 | 20–30% |
| Peroxide-cured NBR | 90 | 30–45% |
| Peroxide-cured FKM | 70 | 10–18% |
| Peroxide-cured FKM | 90 | 15–25% |
| Platinum-cured EPDM | 70 | 15–20% |
A peroxide-cured 70 Shore A NBR may have lower compression set at +100°C than a sulfur-cured 90 Shore A NBR of the same material family. If compression set is the primary concern for static sealing life, specify by test result: "compression set < 20% per ASTM D395B at [temperature]/70h" — not by Shore A number.
Wear Resistance in Abrasive Service
For dynamic seals in contact with particulate contamination (drilling mud, abrasive slurries, mining fluids), harder materials resist abrasive wear:
| Shore A | Relative Abrasion Loss (ASTM D5963, DIN 53516) | Oilfield/Mining Dynamic Service |
|---|---|---|
| 60 Shore A | 1.8× (more wear) | Not recommended for abrasive service |
| 70 Shore A | 1.3× | Marginal — only for very clean systems |
| 80 Shore A | 1.0× (baseline) | Acceptable |
| 90 Shore A | 0.65× (less wear) | Preferred for abrasive dynamic service |
90 Shore A HNBR or NBR shows approximately 2–3× longer abrasion life than 70 Shore A in ASTM D5963 abrasion testing against 120-grit abrasive. For oilfield downhole and mining applications with abrasive particles, 90 Shore A HNBR is the standard specification — combining abrasion resistance, extrusion resistance, and H₂S-resistant chemistry.
Material-Specific Hardness Availability
| Material | Available Range | Practical Upper Limit | Notes |
|---|---|---|---|
| NBR | 40–95 Shore A | 90 ShA (standard); 95 ShA (specialty) | Full range; 70A most common |
| HNBR | 55–95 Shore A | 90 Shore A | 70A and 90A most common stocked grades |
| FKM | 50–90 Shore A | 90 Shore A | 70A most common; 75/80/90A available |
| EPDM | 40–90 Shore A | 90 Shore A | 70A standard; 80/90A available |
| VMQ (Silicone) | 25–80 Shore A | 80 Shore A | Difficult to compound above 80A with good properties |
| CR (Neoprene) | 40–75 Shore A | 75 Shore A | Less common above 70A |
| FFKM | 60–90 Shore A | 90 Shore A | 75–80A most available |
| PTFE | N/A (thermoplastic) | 55–65 Shore D | Shore D scale — not elastomeric |
VMQ hardness limitation: Silicone elastomers above 80 Shore A become brittle (elongation at break < 100%) and show high compression set. For applications requiring silicone chemistry with high hardness, consider FEP-encapsulated VMQ (which adds FEP jacket stiffness to a softer VMQ core) rather than high-Shore-A VMQ.
Temperature Effect on Effective Hardness
Elastomer hardness decreases with increasing temperature. This reduces extrusion resistance at elevated service temperature:
| Temperature above Ambient (+23°C) | Approximate Shore A Reduction | Effective Hardness (starting at 70A) | Effective Hardness (starting at 90A) |
|---|---|---|---|
| +25°C (operating at +48°C) | −3 to −5 ShA | 65–67 ShA | 85–87 ShA |
| +50°C (operating at +73°C) | −8 to −12 ShA | 58–62 ShA | 78–82 ShA |
| +80°C (operating at +103°C) | −12 to −18 ShA | 52–58 ShA | 72–78 ShA |
| +120°C (operating at +143°C) | −18 to −25 ShA | 45–52 ShA | 65–72 ShA |
| +150°C (operating at +173°C) | −25 to −35 ShA | 35–45 ShA | 55–65 ShA |
Practical implication: A 90 Shore A O-ring specified for 300 bar dynamic service that runs at +120°C may effectively perform as ~72 Shore A at operating temperature — below the threshold for reliable dynamic sealing at 300 bar without backup rings. For high-temperature high-pressure applications:
- Calculate the required hardness at operating temperature from the extrusion resistance table
- Work backwards through the temperature correction to determine what room-temperature Shore A is needed
- Confirm backup ring requirements for the operating temperature and clearance gap
70 vs 90 Durometer: Quick Decision Guide
| Situation | Choose 70 Shore A | Choose 90 Shore A |
|---|---|---|
| Pressure | < 100 bar dynamic, < 150 bar static | > 150 bar dynamic, > 200 bar static |
| Clearance gap | Standard (0.10–0.30 mm) | Small (≤ 0.08 mm) or with backup rings |
| Dynamic speed | Reciprocating > 0.3 m/s, rotary > 0.5 m/s | Reciprocating < 0.3 m/s |
| Surface finish | Ra 0.4–0.8 µm acceptable | Requires Ra ≤ 0.2 µm |
| Mating material | Steel, aluminum, hard plastics | Steel, hard chrome — not glass or acrylic |
| Temperature | Up to material limit | Must account for temperature softening |
| Cost | Lower | Higher |
When in doubt between 70 and 90 Shore A, 75–80 Shore A is often the best compromise for medium-pressure dynamic service.
O-Ring Hardness Tester and Measurement Procedure
A Shore A durometer is the standard hardness tester for O-rings. Key points:
- Type: Use a Type A durometer per ASTM D2240
- Indenter: Truncated cone, 35° included angle, 1.25 mm diameter tip
- Spring force: 822 grams force at full extension
- Reading time: 15 seconds after contact (ASTM D2240 "Type 1" reading)
Measurement procedure for O-rings
- Stack 3–5 O-rings side-by-side to create a test surface ≥ 6 mm thick
- Place the durometer perpendicular to the flat surface
- Apply gentle, steady pressure until the presser foot contacts the stack
- Read the scale at 15 seconds
- Take 5 readings at different positions and average
- Report as "X Shore A per ASTM D2240, 15-second reading"
Common error: Measuring a single thin O-ring gives falsely low readings because the indenter contacts the underlying hard surface through the elastomer. Always stack rings or use a fixture that provides the required minimum thickness.
Application Selection Matrix
| Application | Recommended Shore A | Material | Rationale |
|---|---|---|---|
| General hydraulics (< 70 bar, static or dynamic) | 70 | NBR | Balanced standard; extrusion not limiting |
| Medium hydraulics (70–150 bar, dynamic) | 70–80 | NBR | 80 ShA adds margin without excessive friction |
| High-pressure hydraulics (150–250 bar, dynamic) | 80–90 | NBR or HNBR | Extrusion resistance primary requirement |
| Ultra-high pressure (> 250 bar, dynamic) | 90 + backup rings | NBR/HNBR | Backup rings handle extrusion; O-ring provides sealing force |
| Reciprocating dynamic (< 150 bar, clean fluid) | 70–75 | NBR or FKM | Lower friction; longer dynamic life |
| Rotary shaft seals (all pressures) | 60–70 | FKM or NBR | Spiral failure risk above 75 ShA at high shaft speeds |
| Vacuum sealing (< 10⁻³ mbar) | 60–70 | EPDM or FKM | Conformability maximizes contact area |
| Glass/polycarbonate/acrylic mating surfaces | 50–60 | VMQ or soft NBR | Low contact force prevents surface cracking |
| Food/pharma static seals | 70 | EPDM (peroxide) or VMQ | Standard FDA-compliant grade |
| Pharmaceutical SIP (121°C steam) | 70 | EPDM (platinum cure) | Standard pharmaceutical specification |
| Oilfield downhole (abrasive + high pressure) | 90 | HNBR (NACE-qualified) | Abrasion + extrusion resistance + H₂S compatibility |
| Low-temperature service (< −40°C) | 60–70 | Low-temp NBR or FKM GLT | Elastomers harden at low temp; start softer to maintain sealing |
| Semiconductor vacuum seals | 70–75 | FFKM (UHP grade) | Hardness secondary to chemical/outgassing qualification |
Common Hardness Specification Mistakes
Mistake 1 — Specifying 70 Shore A for all applications by default: For any dynamic seal above 150 bar or static seal above 200 bar, 70 Shore A at standard clearance will extrude within weeks to months. Cross-check hardness against operating pressure and clearance gap table before finalizing the specification.
Mistake 2 — Using 90 Shore A on brittle mating surfaces: Acrylic, polycarbonate, and glass fittings crack under the compressive force of a 90 Shore A O-ring at standard compression. Use 50–60 Shore A for any mating surface material with tensile strength < 80 MPa or known brittleness.
Mistake 3 — Not accounting for temperature softening: Calculate the effective Shore A at operating temperature, not just room temperature. A 90 Shore A NBR compound at +120°C behaves as approximately 68–75 Shore A — potentially below the extrusion threshold for 200+ bar dynamic service.
Mistake 4 — Confusing hardness with compression set: Specifying high hardness to achieve low compression set does not work. Request compression set data directly: "< 20% per ASTM D395B at [temperature]/70h."
Mistake 5 — Using 90 Shore A for rotary seals without checking shaft speed: At shaft speeds above 0.5–1.0 m/s, 90 Shore A generates enough friction torque to cause the O-ring to rotate with the shaft rather than seal against it — spiral failure within hours. For rotary seals, determine shaft speed first; specify ≤ 70 Shore A if > 0.5 m/s.
FAQ
Q1: What does 70 Shore A mean for an O-ring?
Shore A 70 means a moderately flexible elastomer that resists the ASTM D2240 indenter to a degree corresponding to 70 on the 0–100 scale. In practical terms, 70 Shore A is the standard hardness for general-purpose O-rings — it provides the best balance of sealing contact force, conformability to surface imperfections, and friction for applications up to approximately 100 bar dynamic and 150 bar static. Most stock NBR, FKM, EPDM, and VMQ O-rings are compounded to 70 ± 5 Shore A unless otherwise specified.
Q2: Is 90 Shore A always better than 70 Shore A for sealing?
No. 90 Shore A provides better extrusion resistance under high pressure but generates 2× more friction, requires 35–50% more compression force, and conforms less easily to surface irregularities. For applications below 100 bar and for all rotary seals, 70 Shore A seals more effectively because conformability and low friction matter more than extrusion resistance at those pressure levels. Specify 90 Shore A only when operating pressure and clearance gap analysis specifically indicates extrusion risk.
Q3: Can I substitute a 90 Shore A O-ring for a 70 Shore A in the same groove?
Yes on dimensions — same AS568 or ISO 3601 sizes fit the same groove. But the higher hardness changes functional behavior: the 90 Shore A ring requires more clamp force to achieve the same compression, generates higher friction in dynamic service, and conforms less easily to surface irregularities. Only substitute upward in hardness if extrusion at higher-than-expected pressure is the confirmed issue. Substituting 90 Shore A in a low-pressure or rotary dynamic application designed for 70 Shore A will typically reduce service life.
Q4: What is the softest O-ring I can buy?
VMQ (silicone) is available in grades as soft as 25–30 Shore A for medical and laboratory applications. For NBR and FKM, 50 Shore A is available but less common — most suppliers stock 60 Shore A as the practical lower limit. For applications where 70 Shore A cannot be used (brittle mating surfaces, very low assembly loads), 60 Shore A in the same material is the first alternative to evaluate.
Q5: Does higher hardness improve temperature resistance?
No — temperature resistance is determined by the polymer backbone and cure system, not by hardness. FKM resists +200°C regardless of whether it is 70 or 90 Shore A. NBR degrades above +120°C at any hardness. What temperature does affect is the effective hardness at operating temperature — and this reduction reduces extrusion resistance. Select the polymer first for temperature capability; then select hardness for pressure and clearance gap requirements.
Q6: Which hardness is best for dynamic seals?
70–75 Shore A for reciprocating dynamic seals at ≤ 150 bar — minimizes friction heat while providing adequate extrusion resistance. 65–70 Shore A for rotary shaft seals at any speed — lower contact force reduces spiral failure risk. 80–90 Shore A for reciprocating dynamic seals at > 150 bar — extrusion resistance becomes the dominant design constraint. Never use > 75 Shore A for rotary shaft seals at speeds > 0.5 m/s without explicit friction torque analysis.
Q7: How do I measure Shore A hardness on a small O-ring?
Stack 3–5 O-rings flat side to side to build a test surface ≥ 6 mm thick (minimum for accurate ASTM D2240 readings). Press the durometer indenter perpendicular to the flat surface. Read at 15 seconds after contact. Take 5 measurements at different positions and average. Acceptable lot variation: no single reading deviating more than ±5 Shore A from the compound specification midpoint.
Q8: Can I use 75 Shore A or 80 Shore A — is there a benefit compared to jumping from 70 to 90?
Yes — intermediate hardnesses are often the most engineered choice. 75 Shore A adds approximately 30–40% extrusion resistance over 70 Shore A with only 15–20% friction increase, while 90 Shore A adds 5× extrusion resistance but doubles friction. For systems in the 80–150 bar dynamic range, 75–80 Shore A is often the optimum hardness — enough margin against extrusion at moderately elevated clearances without the friction and conformability penalties of 90 Shore A. Many hydraulic seal programs specify 80 Shore A as standard for dynamic service above 100 bar for exactly this balance. Consult the pressure vs clearance gap table and calculate whether 75 or 80 Shore A is sufficient before jumping to 90 Shore A.
---
For a complete step-by-step selection process, see our O-Ring Selection Checklist. For more design resources, visit the O-Ring Engineering Hub.
Need O-rings in specific hardness grades? Contact our engineering team with your operating pressure, clearance gap, dynamic vs static service, and temperature — we select the correct hardness for your application and supply NBR, FKM, HNBR, EPDM, and VMQ in 60, 70, 75, 80, and 90 Shore A grades from MOQ 1 piece. Standard lead time 7–15 business days; 3–5 day express for stocked compounds.