ISO 3601: O-ring standard, dimensions and tolerances

What is ISO 3601?

At international level, ISO 3601 is by far the most important O-ring standard. It is divided into five parts and forms the basis for the design and assessment of an O-ring.

The first version of the international standard ISO 3601 was published back in 1978. This laid the foundation for standardized O-rings to an international standard.

Initially, the first part of the standard only contained the dimensions for O-rings for general applications. In doing so, ISO 3601 was strongly based on the now outdated German standard DIN 3771. At that time, however, the focus was on the inch O-rings that originate from the American standard AS568.

Only since 2008 has ISO 3601 also reflected the American dimensions. In the spirit of a truly international standard, the American dimensions are converted from the unit inch into millimetres and specified in that form.

The year 2008 marked considerable progress for the standardization of O-rings as a sealing element for another reason as well. Because in that year, Part 2 of the standard was published. This part defines the installation spaces for O-rings in general fluid power applications. For the first time, this provided standard sizes with coordinated reference values for specifying compression and stretch.

In this way, Part 1 and Part 2 provide considerable added value for design engineers all over the world. For a large number of standard applications, the tables provide all the important reference values for designing a high-performance sealing system. Part 3 and Part 5 also make a decisive contribution to this, because quality criteria for manufacturing and formulation create binding standards.

Part 1: How is an O-ring defined correctly?

In principle, an O-ring is fully described by specifying its inner diameter and cord thickness.

If the combination of inner diameter and cord thickness matches a standardized dimension, two major advantages arise at once:

• The item probably has a shorter delivery time or is even in stock.
• Compared with unusual sizes, the price is often significantly better.

ISO 3601-1 also divides O-rings into two different classes:

• Class A: O-rings for sensitive applications such as aerospace.
• Class B: O-rings for standard applications, i.e. standard commercial O-rings.

The key difference between Class A and Class B is the tolerance class. In line with the sensitive applications, O-rings of Class A are held to a considerably higher requirement for dimensional accuracy.

If it is not precisely defined, O-ring tolerance lists always refer to O-rings from Class B. You will find an overview of the standardized sizes in our O-ring tables.

Part 2: How do I design an O-ring optimally?

The correct design of an O-ring is absolutely decisive for a long-term sealing effect. When designing O-rings and installation spaces, you should therefore always rely on the specifications from ISO 3601-2.

In practice, two cases are important here:

• The installation space is given and the matching O-ring is being sought.
• The O-ring is given and the matching installation space is being sought.

There is a corresponding table for both cases. Using this table, the matching O-ring or installation space can be read off very easily. The table is divided into three application cases: flange seal, piston seal and rod seal.

All values are given in mm. Here, NB stands for the groove width and NT for the groove depth. Static means without movement, dynamic means with movement.

You will find specific reference values in our overview of O-ring tolerances.

Part 3: What is decisive for the quality of an O-ring?

The manufacturing quality of an O-ring is assessed on the basis of six characteristics:

• Shape deviation, offset
• Notch
• Flow lines
• Flash
• Deflashing area
• Indentations, sink marks

Based on these six characteristics, ISO 3601-3 specifies the permissible deviations. This has created a standard that serves as a basis for assessing an O-ring.

Which surface deviations are permissible depends on the grade designation of the O-ring. A total of three grade designations are distinguished:

• N: Standard quality
• S: Increased quality requirements
• CS: Critical applications

Almost all O-rings are manufactured in the standard quality. If an O-ring is to be manufactured to grade designation S or CS, this must always be specified. Because the more specialized the application case, the tighter the limits that apply to the defect sizes.

O-rings with grade designation CS are reserved above all for critical applications in aerospace or oxygen equipment. The tolerances can only be achieved with considerable effort in the manufacturing processes. For example, O-rings with grade designation CS must not have any measurable notches at all. Accordingly, the cost of these O-rings is significantly higher than that of standard qualities.

Part 4: What types of back-up rings are there?

The fourth section of the O-ring standard deals with the use of back-up rings. Depending on the cord diameter from Part 1, Part 4 assigns back-up rings with specific profile dimensions to the O-rings. They serve to protect O-rings against the effect of excessively high pressure.

High pressure and, above all, rapidly changing pressure conditions can lead to mechanical damage to the O-ring. It can be pressed into the sealing gap on the side facing away from the pressure. Sharp groove edges can shear off parts of the O-ring in the process. Sealing experts also refer to this damage pattern as gap extrusion.

Back-up rings placed on the side facing away from the pressure serve as a countermeasure. Under load, the hard back-up ring deforms and closes the sealing gap without sealing it. Instead, it ensures that the O-ring cannot flow into the gap.

ISO 3601 defines five types of back-up rings:

The selection of the suitable variant depends above all on the accessibility of the installation groove and the pressure conditions. In addition, it should be considered whether it is a static or dynamic seal. Sensibly, the groove dimensions in ISO 3601-2 are matched to the dimensions of the back-up rings.

Part 5: What must an elastomer material be able to do?

Media attack, temperatures and pressure: these parameters set highly specific requirements for O-ring materials. This is not only about selecting a high-performance base elastomer. The formulation must also be chosen so that the material properties are an ideal match for the operating conditions.

Whether they do so can be tested using numerous key figures on mechanical properties as well as temperature and ageing behaviour.

Since the first version from 2002, ISO 3601-5 has set requirements for the properties of O-ring materials. Initially, the standard was at best suitable as a general selection aid. Depending on the contact media, a suitable elastomer material could be identified.

The 2015 edition, however, brings considerable progress. Because it defines material requirements for common O-ring materials. These include, for example, NBR, HNBR, FKM and EPDM.

In addition to the formulation, the manufacturing process also influences the properties of the finished part. Above all, the critical step of vulcanization affects the mechanical parameters. That is why the specification of the properties on the finished part is decisive. In practice, these parameters are used to check for a sufficient degree of vulcanization.

If the O-ring conforms to Part 5 of the standard, the user is equally protected against gross deviations in formulation and vulcanization.

What must appear on data sheets in accordance with ISO 3601?

The O-ring standard does not set any strict requirements for data sheets. However, it has become established practice that the following information should appear on data sheets.

The five mandatory entries for O-ring data sheets:

• Material designation (ISO designation)
• Shore hardness (Shore A or D)
• Elongation at break (%)
• Tensile strength (MPa)
• Compression set (%)

In addition, this information is also useful:

• TR-10 (°C)
• Density (g/cm³)
• Tear resistance (N/mm)

Which dimensional standards exist besides ISO 3601?

Besides ISO 3601, there are further national and international dimensional standards for O-rings. You will find a detailed comparison in our article on the dimensional standards for O-rings.

Read more about the superseded German standard in the article on the O-ring standard DIN 3771.

Frequently asked questions