How to Choose an O-Ring Material: NBR, FKM, EPDM or Silicone?

Which material suits which media?

The material must match the media, otherwise the O-ring swells, hardens or breaks down. NBR (nitrile rubber) is the standard for mineral oils, greases and hydraulic fluids. FKM (fluoroelastomer, often known as Viton) covers fuels, acids and high temperatures.

EPDM is the first choice for water, steam, brake fluid and many chemicals, but it cannot tolerate mineral oils. Silicone (VMQ) excels at extreme temperatures and in food and medical applications, yet it has low mechanical strength.

How do temperature and pressure affect the seal?

Temperature governs elasticity and compression set. Excessive continuous heat hardens the material so the O-ring loses its recovery force, while cold makes it brittle and leaky. Always respect both the lower and upper limit of the material.

Rising pressure forces the O-ring against the groove flank. From roughly 10 MPa (100 bar) with wide clearance gaps, extrusion can occur, pushing material into the sealing gap. A back-up ring or a higher Shore hardness prevents this.

• Shore A 70 is standard; 90 Shore A for high pressures and tight gaps.
• The lower the compression set, the more durable the sealing performance.
• Above 100 bar, plan a back-up ring on the low-pressure side.
• Short temperature peaks are acceptable; continuous operation drives the design.

How to set cross-section and groove geometry?

The groove must squeeze the O-ring by a defined amount without overloading it. Static sealing typically uses 15 to 30 % squeeze of the cross-section; dynamic applications stay near the lower end to limit friction and wear.

Groove fill should not exceed 75 to 90 %, so the material has room for thermal expansion and swelling. Dimensions and tolerances follow ISO 3601, which standardises cross-sections and inside diameters.

Frequently asked questions