Seal Selection by Function

Static sealing is the most forgiving O-ring application. Without motion, the seal only needs to resist compression set, chemical attack and thermal cycling. The primary design consideration is compression rate: 15-25% for most static applications. Too little compression and the seal will leak under low pressure. Too much and the seal will take a permanent set.

NBR is the default choice for oil and hydraulic applications. FKM is preferred for high temperature and aggressive chemicals. EPDM is the standard for water, steam and outdoor exposure. For extreme chemical environments, PTFE or FFKM may be required. Always verify the specific fluid compatibility before specifying. Do not use a plain O-ring in static applications where the extrusion gap exceeds 0.15 mm and pressure exceeds 150 bar without a backup ring, or where the fluid will chemically attack the selected elastomer.

Reciprocating seals are the most demanding O-ring application. The seal must slide against a moving surface while maintaining sealing force, resisting wear and managing frictional heat. Compression rate is typically 10-20% to balance sealing and friction. Too much compression increases friction and heat. Too little and the seal leaks.

Surface finish is critical: Ra 0.4-0.8 µm for the dynamic surface. Harder materials wear better but seal less effectively at low pressure. Softer materials seal better but wear faster. NBR and polyurethane are common for hydraulic cylinders. FKM is used for high-temperature or chemical applications. Consider backup rings when pressure exceeds 1,500 psi. Do not use a plain rubber O-ring for dynamic reciprocating service when surface speed exceeds 0.5 m/s, when pressure exceeds 200 bar without a backup ring, or when the required service life exceeds 1 million cycles — consider PTFE or spring-energized seals instead.

Rotary O-ring seals are challenging because frictional heat builds up at the contact surface. The seal material must have low friction, good thermal conductivity and resistance to wear. Compression rate is typically 8-15% to minimize heat generation. The shaft surface should be hard (HRC 55 minimum) and polished to Ra 0.2-0.4 µm.

NBR is suitable for low-speed rotary applications below 0.5 m/s. FKM is preferred for higher temperatures and speeds. PTFE has the lowest friction but requires a backup spring or energizer to maintain sealing force. For high-speed rotary shafts, consider a dedicated rotary shaft seal rather than an O-ring. Do not use a plain elastomer O-ring for rotary service when surface speed exceeds 0.5 m/s or when continuous operation is required — frictional heat will destroy the seal within hours. Use PTFE with energizer or a dedicated rotary shaft seal instead.

High pressure causes O-ring extrusion into the clearance gap between mating parts. The primary design consideration is the extrusion gap: the larger the gap, the lower the pressure the seal can withstand. Use backup rings (also called anti-extrusion rings) when pressure exceeds 1,500 psi (100 bar) or when the extrusion gap exceeds recommended limits.

Harder materials resist extrusion better. Polyurethane (PU) is the standard for high-pressure hydraulics up to 400 bar. PTFE backup rings are common with elastomeric seals. For extreme pressure above 500 bar, consider metal-to-metal seals or specialized high-pressure O-ring designs with integral backup rings. Always verify groove dimensions with the Compression Calculator. Do not use a plain O-ring without backup rings when pressure exceeds 150 bar or when the extrusion gap exceeds the limits for the selected cross section. Do not use soft elastomers such as VMQ or low-durometer NBR in high-pressure service.

High temperature accelerates chemical degradation and compression set. The seal material must resist thermal oxidation while maintaining elasticity. FKM is the standard for temperatures up to 200°C. For temperatures above 200°C, consider VMQ (silicone) up to 220°C, or FFKM for extreme environments up to 320°C.

Thermal expansion must be considered in groove design. The groove may expand more than the seal at high temperature, reducing compression. Design for the maximum operating temperature, not the average. Allow for thermal cycling: repeated heating and cooling causes stress cracking in some materials. For continuous operation above 150°C, specify a high-fluorine FKM or perfluoroelastomer. Do not use a plain NBR O-ring above 120°C or a standard FKM above 200°C. Do not use plain O-rings in thermal cycling applications where the temperature swing exceeds 150°C per cycle — consider FFKM or metal seals instead.

Food-grade and pharmaceutical seals must meet regulatory requirements for material composition and extractables. FDA 21 CFR 177.2600 and USP Class VI are the most common standards. The seal material must not leach harmful substances into the product, must resist cleaning and sterilization cycles, and must be traceable to a batch certificate.

VMQ (silicone) and EPDM are the most common food-grade materials. Silicone passes FDA and USP Class VI, resists steam sterilization and has a wide temperature range. EPDM is preferred for hot water and steam applications but is not compatible with oils. For pharmaceutical processes with aggressive chemicals, FFKM may be required despite the higher cost. Always request a certificate of conformance and material batch traceability. Do not use a plain NBR or standard FKM in food or pharmaceutical contact without explicit FDA certification and extractables testing per USP Class VI or EU 10/2011.

Standard elastomeric O-rings fail when exposed to strong oxidizers, chlorinated solvents, mixed chemical streams, or temperatures below their TR10. In these environments, the polymer backbone degrades rapidly, causing swelling, cracking, or complete dissolution.

PTFE offers universal chemical resistance and cryogenic capability down to −200°C, but it has no elastic recovery and cold-flows under pressure. PTFE O-rings require an energizer spring or must be used in static applications with sufficient compression. Spring-energized PTFE seals combine chemical resistance with consistent sealing force through a wide temperature range.

FFKM (perfluoroelastomer) resists over 1,800 chemicals and operates up to 320°C, but it is 50–100× the cost of NBR. For extreme chemical exposure at moderate temperatures, FEP encapsulated O-rings provide a fluoropolymer barrier over an elastomer core. For cryogenic LNG and hydrogen applications, specialized low-temperature FFKM compounds or PTFE spring-energized seals are required. Do not use a plain elastomer O-ring in strong oxidizers, chlorinated solvents, or mixed chemical streams that attack the polymer backbone. Do not use standard elastomers below their TR10 temperature — the seal will harden, crack, and leak.