Shore A Hardness (Durometer)
Also known as: durometer · IRHD · Shore A · rubber hardness
Shore A hardness (durometer) measures rubber stiffness on a 0–100 scale — standard O-ring compound is 70 Shore A; softer grades (40–60) seal at lower clamping loads, harder grades (80–90) resist extrusion at high pressure.
Technical Detail
Shore A is measured by a standardised indentation instrument (durometer) per ASTM D2240 or ISO 868. The indenter is a truncated cone; 100 = no penetration (rigid), 0 = full penetration (liquid). Rubber hardness is also expressed as IRHD (International Rubber Hardness Degrees, ISO 48) — numerically close to Shore A but measured differently. Practical guide for O-ring selection: 40–50 Shore A: low-pressure face seals, delicate mating surfaces, FDA/pharma applications where low compression force needed; 60–70 Shore A: standard static and slow-dynamic seals (most industrial O-rings are 70 Shore A); 75–80 Shore A: medium-high pressure (3–14 MPa) and back-up ring situations; 80–90 Shore A: high-pressure hydraulic seals, extrusion-resistant applications. Hardness increases with lower temperature — an NBR O-ring rated 70 Shore A at 23°C may be 90+ Shore A at –30°C, affecting sealing performance.
Reference Standards
- ASTM D2240
- ISO 868
- ISO 48-4 (IRHD)
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