O-rings are closed O-rings. Various types of rubber are used for their production. O-rings are used in many applications in a variety of industries around the world. O-rings come in a wide range of sizes. In connection with the above, special standards for the sizes of the most commonly used O-rings have been developed.
American Standard AS568
AS 568 (Aerospace Standard 568) is an American standard that describes 379 O-rings. The AS 568 standard specifies inside diameters, sections, tolerances and size designation for O-rings. All sizes are listed in ascending inside diameter (ID) in fractional and decimal sizes.
According to the AS 569 classification system, all dimensional tolerances comply with the ISO 3601-1A standard.
British Standard BS 1806 / BS 4518 / ISO 3601
BS 1806 was created by the British Standards Institute and lists all imperial standard sizes. The above standard contains the 5 major section groups in AS568 and several sizes that fall within the 1/4 “nominal section group. BS 1806 has been superseded by BS ISO 3601.
ISO 3601 is a standard developed by the International Organization for Standardization. The above standard is divided into two classes:
- Grade A O-rings – These have the same tolerances as those used in AS568, which are narrower than those of Grade B O-rings. Type A O-rings are inch sizes that are suitable for industrial applications. The ISO 3601-1 size code is the same as the AS 568 dash number.
- Grade B O-rings –have slightly wider tolerances and are suitable for general applications. These are metric sizes intended for aerospace applications. The first digit of the size code indicates the section group (A-E) and the last four indicate a diameter rounded to 0.1 mm inside the O-ring.
BS 4518 is a British standard that specifies British Standard metric sizes. The O-ring size code is defined by a four-digit number indicating the O-ring ID in tenths of a millimeter, followed by a dash and two numbers indicating the cross-section of the O-ring, also in tenths of a millimeter.
French Standard NF T 47-501
The French Standards Institute has developed the NF T 47-501 standard, which is very similar to ISO 3601-1, both in terms of size and reference to size. The reference codes in the NF 47-501 standard are identified by a letter corresponding to each of the 5 section groups (AE), 4 numbers indicating the ID (rounded to 0.1 mm), a second letter indicating the accuracy class and a third letter indicating the visual (control) class. Precision class marked with:
- the letter A is suitable for aviation applications
- the letter G is suitable for general use
The visual classes are N (1.0 AQL) and S (0.65 AQL)
German Standard DIN 3771
The German Institute of Standards (Duetsches Institut fur Normung) has developed the DIN 3771 standard, according to which the sizes of O-rings are identified by ID x C / S followed by a letter indicating the quality level and a code indicating the rubber polymer and IRHD hardness.
The DIN 3771 standard defines two quality levels:
- N – normal quality (1.0 AQL)
- S – special quality (0.65 AQL)
Swedish Standard SMS 1586
SMS 1586 refers to the Swedish mechanical standard for the sizes of sealing rings. The SMS 1586 classification system describes 140 O-rings in two groups:
- D – This group describes 65 O-rings intended for dynamic and static installations. Each size is marked with its own dimension: ID x section.
- S – This group describes 75 O-rings intended primarily for static applications. Each size is marked with its dimension: ID x section.
The dimensions of the O-rings in the metric system are exactly matched, that is, a 1 mm O-ring will have a cross section of 1 mm. To ensure a proper sealing of the ring, the filled space must be approx. 10-15% smaller than its cross-section.
Japanese Standard JIS B 2401
JIS B 2401 is the Japanese industry standard for O-ring sizes. The JIS B2401 grading system has two groups of O-ring sizes. The first is for O-rings for general and space applications, while the second is divided into four sections.
The above standard is divided into four sections depending on the application:
- P series – mobile (dynamic)
- G series – fixed (static)
- S series – special sizes
- V series – vacuum flange
Each section contains ascending numeric codes. This solution is intended to help you refer to the size of the O-ring.
The JIS B 2401 standard describes a total of 1,006 O-rings in three groups:
- Lotnicza – this group describes 367 O-rings and corresponds to the standard in the ISO 3601-1 metric system (with a few additional sizes). In the aviation group, each size is marked with its dimension: ID x section. The letter “A” is added after the dimension to indicate aeronautical use.
- General Purpose – This group describes 445 O-rings and conforms to the ISO 3601-1 metric standard (with some additional sizes). In the general purpose group, each size is marked with its dimension: ID x section. A “G” is added after the dimension to indicate general purpose use.
- Traditional – 194 O-rings are featured in this group. Each size is identified by a letter followed by a face number. The letter indicates one of the sections depending on the application (P – dynamic, G – constant, V – vacuum flange).
The dimensions of the O-rings in the metric system are exactly matched, that is, a 1 mm O-ring will have a cross section of 1 mm. To guarantee a proper sealing of the ring, the filled space must be approx. 10-15% smaller than its cross-section or up to 10% larger depending on the use: internal or external.
Useful articles and guides:
- How to dimension rubber O-rings?
- Rubber o-rings. Guide
- Buy rubber O-rings online here