Machinery's Handbook, 31st Edition
O-RINGS
2667
Table 1. Commonly Referenced O-Ring Standards
Inch-System O-Rings: The inch-basis SAE standard O-ring sizes are assigned three- digit size codes. A dash precedes the size code. The first digit refers to cross-sectional di - ameter, and the next two digits refer to inner diameter; however, the numbers that make up the size code are not derivative of the actual inch measurements. For example, for an SAE standard O-ring with designation -230-8307, the O-ring size code is -230, which speci- fies a cross-sectional diameter of 0.139 in. and inner diameter of 2.484 in. The complete designation number indicates that the O-ring is made from the vendor-specific material designated by material code 8307. Metric General-Purpose O-Rings: According to ISO specifications, O-rings for general use are functionally equivalent to those found in SAE AS568 when used in the same glands. Size codes for these use the same three-digit numbers set by the SAE standard. But the ISO size code is completed by an added tolerance class designation (A or B). Tolerance class A O-rings, which are equivalent to those in the SAE (inch) standard, are suitable for aerospace or general applications requiring tight tolerances. Even in applications where tolerance needs are not as tight, class A O-rings are most often used. Tolerance class B O-rings, which are less tight and considered appropriate for general applications, are not as common in industry. To give an ISO example of an SAE equivalent O-ring: for a part with nominal inside di- ameter of 63.09 mm and nominal cross-sectional diameter of 3.53 mm, the ISO size code is -230A, and the full designation is -230A-63.09x3.53-N. The final letter N is the grade. Grade indicates the inspection standards required per ISO 3601-3: Grade N = general purpose, S = special, and CS = critical service. It is important to note that, as with SAE designations, ISO standard designations do not identify the material. A manufacturer-specific compound identifier must be appended or called out separately. Metric Aerospace O-Rings: Another group of O-rings in the ISO standard is intended specifically for aerospace applications. These are designed for use in metric glands. This is advantageous when close fits are required or when working with metric bores and pistons. Codes for aerospace sizes differ from those for general applications; they are preceded by the letter A followed by a dash. The size code then begins with a cross section size identifier, designated from A to E; the next four digits give the inside diameter in tenths of a millimeter. There is only one tolerance class in this group, so a tolerance class identifier is not included, but the inspection standard per ISO 3601-3 is added to the end of the code. For example, a critical service aerospace O-ring with a 3.55 mm cross section and a 63 mm inside diameter has a size code of C0630 and the full designation is A-C0630-63.00x3.55-CS. In this case, the grade code is CS. This series of O-ring sizes are not covered here, but size codes and dimensional information can be found in ISO standards and supplier catalogs.
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