Full definition
A circular-cross-section elastomer ring — the most widely used sealing element in the world, specified in millions of applications across every industry. The O-ring seals by being compressed (squeezed) in a machined groove between two mating surfaces; the squeeze deforms the circular cross-section into an oval, generating contact stress that exceeds the fluid pressure to maintain a seal. Standard sizing: AS568B (American standard, dash numbers -001 to -475, inch dimensions) and ISO 3601 (metric). Cross-section diameters: 1.02 mm (AS568 -001) to 6.99 mm (AS568 -475). Materials: NBR (most common — oil/fuel resistance, economical, -30 to +100°C), FKM/Viton (chemical/heat resistance, -20 to +200°C), EPDM (water/steam/weather, -50 to +150°C), silicone VMQ (temperature extremes/food/medical, -60 to +230°C), FFKM (universal chemical resistance, up to +325°C, $50-500 each), and PTFE-encapsulated (chemical resistance of PTFE with elastomer spring-back). Design: squeeze 8-25% of cross-section (higher squeeze for vacuum, lower for dynamic). Surface finish: groove Ra <0.8 μm (dynamic), <1.6 μm (static). Per AS568B, ISO 3601, and Parker O-Ring Handbook (the industry reference). Brands: Parker, Trelleborg, Freudenberg, NOK, Dichtomatik.