O-Rings – Key Parameters and Properties of Technical Seals
Simple, reliable, and cost-effective — the O-ring remains the cornerstone of sealing technology in hydraulics, pneumatics, and general mechanics. Its round shape and elastic behavior make it one of the most widely used sealing elements in the world.
What Is an O-Ring?
An O-ring is an elastic ring with a circular cross-section that seals between two surfaces when compressed in a groove. Its effectiveness comes from material elasticity and the sealing pressure of the medium itself.
- Radial sealing – for pistons and rods.
- Axial sealing – between two flat surfaces.
- Bidirectional action – seals in both pressure directions.
- Self-energizing effect – pressure increases contact force on the sealing surfaces.
Main Technical Parameters
1. Inside Diameter (d₁)
Defines the fit of the O-ring within its groove. Even small deviations can affect sealing performance.
2. Cross-Section (d₂)
Determines compression ratio and contact area. A larger cross-section provides greater sealing safety but also higher friction.
3. Hardness (Shore A)
- 70 ShA – general-purpose standard, most common hardness.
- 80–90 ShA – for higher pressures or when used with backup rings.
4. Material Type
- NBR – nitrile rubber, classic for oils and fuels.
- FPM (Viton) – resistant to high temperatures and aggressive chemicals.
- EPDM – excellent for water, steam, and brake fluids.
- VMQ (Silicone) – wide temperature range, low mechanical load.
- PU (Polyurethane) – superior abrasion resistance for dynamic motion.
5. Operating Temperature Range
- NBR: −30…+100 °C
- FPM: −20…+200 °C
- EPDM: −50…+150 °C
- VMQ: −60…+200 °C
- PU: −20…+90 °C
6. Chemical Resistance
Choose the compound according to the medium:
- NBR / FPM – oils, fuels, greases.
- EPDM – water, steam, glycols.
- FPM – solvents, aromatic hydrocarbons.
- EPDM / VMQ – ozone and outdoor exposure.
7. Working Pressure
- Static applications: up to ~40 MPa (400 bar)
- Dynamic motion: approx. 10–15 MPa
At higher pressures, always use backup rings to prevent extrusion.
8. Compression (Squeeze)
- Static seals: 15–30% of cross-section.
- Dynamic seals: 8–20% of cross-section.
9. Surface Finish
Contact surfaces should be smooth but not polished — typically Ra 0.2–0.8 µm — and free of burrs or sharp edges.
O-Rings Made of NBR – The Hydraulic Standard
- Hardness: 70–90 ShA
- Temperature range: −30…+100 °C
- Excellent resistance to mineral oils, fuels, greases
- Limited resistance to ozone, UV, glycol-based brake fluids
- Commercial names: Buna-N, Perbunan, Hycar, Krynac
Material Comparison
| Material | Oil/Fuel Resistance | Ozone/UV | Temperature | Abrasion | Application |
|---|---|---|---|---|---|
| NBR | Good | Poor | −30…+100 °C | Good | Hydraulic standard |
| PU | Good | Good | −20…+90 °C | Excellent | Dynamic seals |
| FPM (Viton) | Excellent | Good | −20…+200 °C | Good | High-temp, aggressive media |
| EPDM | Poor | Excellent | −50…+150 °C | Medium | Water, steam, brake fluids |
| VMQ (Silicone) | Poor | Excellent | −60…+200 °C | Poor | Extreme temp., clean environments |
Selection and Assembly Guidelines
- Follow standard dimensions: ISO 3601 / DIN 3771 / AS568.
- Select material and hardness according to medium, temperature, and motion type.
- Lubricate before installation — use a compatible oil or grease.
- Avoid twisting and protect edges during assembly.
- Use backup rings for high-pressure applications.
Summary
The O-ring remains the most reliable and economical sealing element available. Proper selection of diameter, hardness, compression, and material ensures a long-lasting, leak-free system. In hydraulic systems, NBR O-rings continue to be the benchmark — proven, affordable, and always available.