Systems in use
| Standard | Origin | Notes |
|---|---|---|
| AS568 | US (SAE) | Inch sizes, dash numbers such as -210 |
| ISO 3601-1 | International | Metric and inch sizes, tolerances, cross-section series |
| BS 1806 | UK | Inch-based, being replaced by ISO in many areas |
| JIS B2401 | Japan | Metric, series P (fixed), G (static), V |
| DIN 3771 | Germany | Metric, now referenced by ISO 3601 |
Standard sizes within a standard are not exactly the same as in another. A metric 40 x 3 O-ring and a nearby AS568 size may look similar but have different cross sections and tolerance. Do not substitute without checking.
AS568 dash numbers
The dash number defines the cross section by its series.
| Series | Cross section (inch) | Cross section (mm) |
|---|---|---|
| 0xx | 0.070 | 1.78 |
| 1xx | 0.103 | 2.62 |
| 2xx | 0.139 | 3.53 |
| 3xx | 0.210 | 5.33 |
| 4xx | 0.275 | 6.99 |
Dash -210, for example, is a 2xx series ring with a specified inside diameter. Use a size chart.
Tolerance classes (ISO 3601-1)
Class A for precision, class B for general use. Tolerances grow with diameter. Check these when machining tight grooves.
Selecting a size
- Decide the gland dimensions first.
- Choose the cross section for the pressure and the stroke.
- Choose the inside diameter to give 1 to 5 percent stretch (static) or little to no stretch (dynamic piston).
- Find the nearest standard, or specify a custom size.
Custom and cut cord
Large diameters are often made from spliced cord or moulded rings in non-standard sizes. Splice joints are weaker, so avoid them in dynamic or high-pressure service.
Quality grades
ISO 3601-3 defines surface imperfection grades for rings. Grade N is commercial, grade S is special for critical applications. Specify the grade if it matters.
Material marking
Colour coding is not standard. Mark by material code and lot on the bag, and keep the lot traceable.
