O-rings dimensions AS568 metric
The O-ring, also known as a toroidal seal, is a mechanical seal of circular cross-section with 367 sizes standardized in the AS568 standard. It is manufactured from elastomer and seats in a machined housing, compressing between two or more surfaces to achieve sealing in static and dynamic applications. The most common materials are NBR (nitrile), FKM (Viton) and VMQ (silicone), capable of operating from -54 °C / -65 °F up to 260 °C / 500 °F depending on the compound. The standard dimensions are divided into the American series AS568 — based on inches — and the metric series ISO 3601-1, both used worldwide.
AS568 Standard Dimensions
Section titled “AS568 Standard Dimensions”The SAE AS568C standard defines 367 references grouped into five standard cross-sections: 1/16“, 3/32“, 1/8“, 3/16“ and 1/4“ (1.59, 2.38, 3.18, 4.76 and 6.36 mm). The following table lists the most frequent references in industrial equipment design, with inner diameter (ID) and cross-section (CS) expressed in metric and imperial units.
| AS568 Code | ID | CS |
|---|---|---|
| -001 | 0.74 mm / 0.029 in | 1.02 mm / 0.040 in |
| -002 | 1.07 mm / 0.042 in | 1.27 mm / 0.050 in |
| -003 | 1.42 mm / 0.056 in | 1.52 mm / 0.060 in |
| -004 | 1.78 mm / 0.070 in | 1.78 mm / 0.070 in |
| -005 | 2.57 mm / 0.101 in | 1.78 mm / 0.070 in |
| -006 | 2.90 mm / 0.114 in | 1.78 mm / 0.070 in |
| -007 | 3.68 mm / 0.145 in | 1.78 mm / 0.070 in |
| -008 | 4.47 mm / 0.176 in | 1.78 mm / 0.070 in |
| -009 | 5.28 mm / 0.208 in | 1.78 mm / 0.070 in |
| -010 | 6.07 mm / 0.239 in | 1.78 mm / 0.070 in |
| -011 | 7.65 mm / 0.301 in | 1.78 mm / 0.070 in |
| -012 | 9.25 mm / 0.364 in | 1.78 mm / 0.070 in |
| -013 | 10.82 mm / 0.426 in | 1.78 mm / 0.070 in |
| -014 | 12.42 mm / 0.489 in | 1.78 mm / 0.070 in |
| -015 | 14.00 mm / 0.551 in | 1.78 mm / 0.070 in |
| -016 | 15.60 mm / 0.614 in | 1.78 mm / 0.070 in |
| -017 | 17.17 mm / 0.676 in | 1.78 mm / 0.070 in |
| -018 | 18.77 mm / 0.739 in | 1.78 mm / 0.070 in |
| -019 | 20.35 mm / 0.801 in | 1.78 mm / 0.070 in |
| -020 | 21.95 mm / 0.864 in | 1.78 mm / 0.070 in |
| -021 | 23.52 mm / 0.926 in | 1.78 mm / 0.070 in |
| -022 | 25.12 mm / 0.989 in | 1.78 mm / 0.070 in |
| -023 | 26.70 mm / 1.051 in | 1.78 mm / 0.070 in |
| -024 | 28.30 mm / 1.114 in | 1.78 mm / 0.070 in |
| -025 | 29.87 mm / 1.176 in | 1.78 mm / 0.070 in |
| -026 | 31.47 mm / 1.239 in | 1.78 mm / 0.070 in |
| -027 | 33.05 mm / 1.301 in | 1.78 mm / 0.070 in |
| -028 | 34.65 mm / 1.364 in | 1.78 mm / 0.070 in |
| -029 | 37.82 mm / 1.489 in | 1.78 mm / 0.070 in |
| -030 | 41.00 mm / 1.614 in | 1.78 mm / 0.070 in |
Additional series with cross-section 2.62 mm / 0.103 in (CS 3/32“), 3.53 mm / 0.139 in (1/8“), 5.33 mm / 0.210 in (3/16“) and 6.99 mm / 0.275 in (1/4“) are used for larger diameters or requirements for higher compression.
Metric Standard Dimensions (ISO 3601)
Section titled “Metric Standard Dimensions (ISO 3601)”The ISO 3601-1:2012 standard includes 445 metric sizes with cross-sections from 0.8 mm to 8.4 mm, with the sections of 1.0 mm / 0.039 in, 1.5 mm / 0.059 in, 2.0 mm / 0.079 in, 2.5 mm / 0.098 in, 3.0 mm / 0.118 in and 5.0 mm / 0.197 in being the most common in industrial design. The following table shows a selection of inner diameters with a cross-section of 2.0 mm / 0.079 in.
| ID (mm) | ID (in) | CS (mm) | CS (in) |
|---|---|---|---|
| 1.0 | 0.039 in | 2.0 | 0.079 in |
| 2.0 | 0.079 in | 2.0 | 0.079 in |
| 3.0 | 0.118 in | 2.0 | 0.079 in |
| 4.0 | 0.157 in | 2.0 | 0.079 in |
| 5.0 | 0.197 in | 2.0 | 0.079 in |
| 6.0 | 0.236 in | 2.0 | 0.079 in |
| 7.0 | 0.276 in | 2.0 | 0.079 in |
| 8.0 | 0.315 in | 2.0 | 0.079 in |
| 10.0 | 0.394 in | 2.0 | 0.079 in |
| 12.0 | 0.472 in | 2.0 | 0.079 in |
| 15.0 | 0.591 in | 2.0 | 0.079 in |
| 18.0 | 0.709 in | 2.0 | 0.079 in |
| 20.0 | 0.787 in | 2.0 | 0.079 in |
| 25.0 | 0.984 in | 2.0 | 0.079 in |
| 30.0 | 1.181 in | 2.0 | 0.079 in |
| 35.0 | 1.378 in | 2.0 | 0.079 in |
| 40.0 | 1.575 in | 2.0 | 0.079 in |
| 45.0 | 1.772 in | 2.0 | 0.079 in |
| 50.0 | 1.969 in | 2.0 | 0.079 in |
Sealing Capacity and Maximum Pressure
Section titled “Sealing Capacity and Maximum Pressure”A correctly sized O-ring can seal pressures up to 5000 psi (34 MPa) in static service, provided the hardness, material and extrusion gap are within recommended limits. The maximum allowable pressure is reduced in dynamic applications, typically not exceeding 1500 psi (10 MPa) in reciprocating hydraulic cylinders. The ideal compression of the cross-section in a static seal is 15% to 25%, while in dynamic applications it is between 8% and 14%. The surface roughness of the housing must be maintained at Ra 0.8 µm / 32 µin for contact faces and Ra 1.6 µm / 63 µin for the groove bottom.
| Shore A Hardness | Max pressure without anti-extrusion ring (psi) | Max pressure with anti-extrusion ring (psi) |
|---|---|---|
| 70 | 500 psi / 3.4 MPa | 2500 psi / 17.2 MPa |
| 80 | 800 psi / 5.5 MPa | 3000 psi / 20.7 MPa |
| 90 | 1500 psi / 10.3 MPa | 5000 psi / 34.5 MPa |
Selection Criteria
Section titled “Selection Criteria”The choice of O-ring depends on the fluid to be sealed, operating temperature, type of movement and pressure. For hydraulic oils and fuels, nitrile (NBR) offers excellent resistance up to 100 °C / 212 °F. FKM covers ranges of -20 °C to 200 °C / -4 °F to 392 °F and is suitable for acids and aggressive fuels. Silicone (VMQ) is reserved for extreme temperatures, from -54 °C to 230 °C / -65 °F to 446 °F, but with low wear resistance in dynamic applications. The most common hardness is 70 Shore A, increasing to 90 Shore A when pressure exceeds 2000 psi / 13.8 MPa.
Installation Considerations
Section titled “Installation Considerations”The housing must provide for axial or radial compression of the O-ring; in radial installations the inner diameter of the O-ring should be stretched between 1% and 5% over the shaft diameter to prevent it from rolling during movement. The groove width must be at least 1.15 times the O-ring cross-section to allow for thermal expansion. Transitions between surfaces should include chamfers of 15° to 20° to prevent cuts during installation. When pressure exceeds the extrusion limit (indicated in the capacity table), a back-up ring of PTFE or polyurethane is required, placed on the low-pressure side.
Application Selection Table
Section titled “Application Selection Table”| Application | Recommended Cross-Section | Suggested Material | Shore A Hardness |
|---|---|---|---|
| Hydraulic cylinder (rod) | 3/16“ (4.76 mm) | NBR | 70 |
| Pneumatic piston | 1/8“ (3.18 mm) | NBR | 70 |
| Static flange (oil) | 1/8“ (3.18 mm) | NBR | 70 |
| Chemical process equipment | 2.0 mm (metric) | FKM | 75 |
| Hot water system | 2.5 mm (metric) | EPDM | 70 |
| Vacuum < 10⁻³ mbar | 2.0 mm | FKM | 70 |
| High pressure > 3000 psi | 3/16“ + anti-extrusion ring | NBR | 90 |
| Slow rotation | 1/8“ | Carbon NBR | 80 |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What pressure can an O-ring seal without an anti-extrusion ring?
Section titled “What pressure can an O-ring seal without an anti-extrusion ring?”An O-ring of 70 Shore A hardness can seal up to 800 psi (5.5 MPa) without an anti-extrusion ring if the radial gap is below 0.05 mm / 0.002 in. With 90 Shore A hardness and gap controlled to 0.03 mm / 0.001 in, the allowable pressure rises to 1500 psi (10.3 MPa).
What compression percentage should be applied to a static O-ring?
Section titled “What compression percentage should be applied to a static O-ring?”The recommended compression for a static O-ring ranges between 15% and 25% of the cross-section, which corresponds to a deformation of approximately 0.30 mm / 0.012 in for an O-ring with a 2.0 mm cross-section. Values below 10% can cause leakage due to insufficient contact.
How is an AS568 code converted to metric dimensions?
Section titled “How is an AS568 code converted to metric dimensions?”There is no single conversion factor; each reference is looked up in the AS568 table. For example, code -010 corresponds to an ID of 6.07 mm / 0.239 in and a CS of 1.78 mm / 0.070 in. The ISO 3601 metric series uses the designation ID × CS, such as 6 × 2 mm / 0.236 × 0.079 in.
How much stretch can an O-ring withstand before damage?
Section titled “How much stretch can an O-ring withstand before damage?”The maximum recommended stretch during installation is 5% of the inner diameter; i.e., an O-ring with ID 20 mm can be installed in a housing up to 21 mm / 0.827 in. Higher stretches reduce the effective cross-section and promote suction ingress in dynamic movements.
What continuous temperature does an FKM O-ring withstand?
Section titled “What continuous temperature does an FKM O-ring withstand?”Fluoroelastomer FKM maintains its sealing properties between -20 °C and 200 °C / -4 °F and 392 °F continuously, with limited peaks of 230 °C / 446 °F during short cycles. Above these values the hardness drops and the material loses resilience.
What maximum gap does an O-ring with cross-section 1.78 mm allow in static service?
Section titled “What maximum gap does an O-ring with cross-section 1.78 mm allow in static service?”For a pressure of 1500 psi / 10.3 MPa, the maximum recommended radial gap is 0.05 mm / 0.002 in. If the pressure reaches 3000 psi / 20.7 MPa, the gap must be reduced to 0.025 mm / 0.001 in or a PTFE anti-extrusion ring must be used.
Sources
Section titled “Sources”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/metric-threads-d_777.html
- mcmaster.com: https://www.mcmaster.com/products/as568-o-rings/
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/metric-threads-d_777.html
- mcmaster.com: https://www.mcmaster.com/products/as568-o-rings/