Selecting an O-Ring Material NBR EPDM FKM or FFKM

An O-ring that fails was usually the right size. Cross-section and inside diameter are easy to get right, and the AS568 series standardises them internationally. What fails is the polymer, attacked by a fluid it was never suitable for, and no dimensional accuracy compensates for that.

NBR for Oils and Fuels

Nitrile resists oils, fuels and other hydrocarbons, which is why it is the default in automotive, aerospace, wire and cable and home appliance applications. It is also the cheapest of the engineered elastomers by a wide margin. Where the fluid is mineral oil and the temperature is moderate, specifying anything more expensive is money spent for nothing.

Its weaknesses are ozone, weather and brake fluid. An NBR seal outdoors cracks at the surface, and an NBR seal in a brake system swells.

EPDM for Water Steam and Brake Fluid

EPDM has excellent heat, ozone and weather resistance, and it handles water, steam, glycol, refrigerants and brake fluid. That makes it the correct material for heating systems, water systems, refrigeration units and automotive brakes, and food-grade EPDM extends it into pharmaceutical and food processing.

Its weakness is the mirror image of nitrile's: EPDM must never see petroleum oils. Confusing the two is the single most common sealing error in mixed plants.

HNBR for Cold Media

Hydrogenated nitrile is used predominantly for cold medium sealing, where standard NBR stiffens and loses contact force. In automotive work it appears on power steering pipes and oil seals, and industrially in packer and screw pumps.

FKM for Acids Hydraulic Fluids and Heat

Fluoroelastomer resists a wide range of chemicals and temperatures, including petroleum products, hydraulic fluids and acidic compounds. For most aggressive industrial fluids FKM is sufficient, and it is where you should go when nitrile is failing on chemistry or temperature.

FFKM When FKM Is Not Enough

Perfluoroelastomer offers almost universal chemical resistance and holds sealing integrity through extreme temperatures and thermal cycling. It is specified in semiconductor manufacturing, pharmaceutical production, chemical processing, petroleum, laser plating and battery manufacturing, and it costs considerably more per piece than FKM.

It is bought for risk rather than for price. In a semiconductor or pharmaceutical process, a failed seal contaminates a batch worth far more than any quantity of seals. Applying it across a whole machine, rather than to the joints that need it, is simply expensive.

Dynamic Sealing Is a Different Question

O-rings are excellent static seals and wear quickly in reciprocating service. For rods and pistons, a Y seal uses system pressure to energise its lips, so it seals better as pressure rises. The specification then includes lip ratio, cross-section, lip angle and height, and a hole type is not interchangeable with a shaft type.

What to Send

The fluid, including cleaning chemicals, the maximum and minimum temperature, the pressure, and whether the seal is static or dynamic. With those four, the material follows. The AS568 dash number or the groove dimensions then settle the size.