Comprehensive guide to O-ring types including AS568 standards, metric sizes, and material options for industrial applications.
An O-ring is a torus-shaped mechanical gasket designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface. O-rings are one of the most common seals used in machine design because they are inexpensive, easy to make, reliable, and have simple mounting requirements.
The AS568 standard is the most widely used O-ring sizing system in North America. It defines O-ring sizes by a three-digit dash number. The first digit indicates the cross-section group, while the remaining digits indicate the inner diameter within that group.
| Dash Number Range | Cross-Section (inches) | ID Range (inches) |
|---|---|---|
| 001–099 | 0.040 | 0.029–0.987 |
| 100–199 | 0.070 | 0.049–1.720 |
| 200–284 | 0.103 | 0.114–2.337 |
| 300–395 | 0.139 | 0.176–3.234 |
| 400–475 | 0.210 | 4.475–16.955 |
Metric O-rings are specified by inner diameter (ID) and cross-section (CS) in millimeters. Common cross-sections include 1.0mm, 1.5mm, 2.0mm, 2.5mm, 3.0mm, 3.5mm, 4.0mm, 5.0mm, and 6.0mm.
NBR is the most widely used O-ring material. It offers excellent resistance to petroleum-based oils and fuels, good mechanical properties, and a temperature range of -40°C to +120°C. NBR is the default choice for most hydraulic and pneumatic applications.
FKM provides superior chemical resistance and high-temperature performance (-20°C to +200°C). It is ideal for applications involving aggressive chemicals, fuels, and high temperatures. FKM is more expensive than NBR but necessary for demanding environments.
EPDM excels in applications involving water, steam, and brake fluids. It has a temperature range of -50°C to +150°C and offers excellent weather and ozone resistance. However, EPDM is not compatible with petroleum-based oils.
PTFE O-rings offer the broadest chemical resistance of any elastomer. They can handle temperatures from -200°C to +260°C and are used in applications where chemical inertness is critical. PTFE rings are harder and less elastic than rubber O-rings.
When selecting an O-ring for your application, consider the following factors:
1. Media Compatibility: Ensure the O-ring material is compatible with the fluid or gas it will contact. NBR for oils, EPDM for water, FKM for chemicals.
2. Temperature Range: Match the material's temperature capability to your operating conditions. Consider both continuous and peak temperatures.
3. Pressure: For pressures above 1500 PSI (10 MPa), consider adding backup rings to prevent O-ring extrusion. Higher durometer (hardness) materials resist extrusion better.
4. Dynamic vs. Static: Dynamic applications (moving parts) require different groove designs and materials than static applications. Dynamic seals experience more wear and need materials with good abrasion resistance.
5. Size and Tolerance: Select the correct size based on the groove dimensions. O-ring compression should typically be 15-30% of the cross-section for static applications and 10-20% for dynamic applications.
Proper installation is critical for O-ring performance. Use chamfered edges on mating parts to prevent cutting during assembly. Apply compatible lubricant to reduce friction during installation. Avoid twisting the O-ring during assembly, especially with larger sizes. Ensure the groove is clean and free of burrs or sharp edges.
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