Plant & Works Engineering
O-ring material specification made easier
Published:  04 June, 2015

The new edition of ISO 3601-5 introduces minimum material property data to help ensure O-ring reliability. In this article, the British Fluid Power Association outlines the key points.

The new edition of ISO 3601-5 has been introduced to standardise on minimum material physical property requirements. This is a radical development in standards concerning fluid power sealing and is a fundamental change in the content of this particular standard. ISO 3601-5 was developed by ISO/TC 131/SC 7, the ISO Sealing devices sub-committee of the Fluid power systems Technical Committee. There has also been input from the BFPA Technical Committee TC 7 ‘Fluid seals and their housings’. The five parts of ISO 3601 now comprises:

• ISO 3601-1 Fluid Power Systems – O-rings – Part 1: Inside diameters, cross-sections, tolerances and designation codes.

• ISO 3601-2 Fluid power Systems - O-rings – Part 2: Housing dimensions for general applications.

• ISO 3601-3 Fluid Power Systems – O-rings – Part 3: Quality acceptance criteria.

• ISO 3601-4 Fluid power Systems – O-rings – Part 4: Anti-extrusion rings (back-up rings).

• ISO 3601-5 Fluid power Systems – O-rings – Part 5: Specification of elastomeric materials for industrial applications.

All are available from BSI (endorsed and implemented as BS ISO standards). It should be pointed out that Parts 3 and 5 can be applied to any O-ring, such as the metric sizes not covered by ISO 3601-1 and special sizes used in some connectors.

The new ISO 3601-5

The new Part 5 replaces ISO 3601-5:2002 – Fluid Power Systems – O-rings Part 5: Suitability of elastomeric materials for industrial applications, which, in fact, was a short standard based upon a useful but limited compatibility table of rubber compounds against typical fluids found in fluid power applications. This Table is no longer part of the new document.

The standard now covers physical property specifications for a variety of rubber compounds, which are commonly used in fluid power applications, including nitrile (NBR), hydrogenated nitrile (HNBR), fluoroelastomer (FKM) and ethylene-propylene-diene (EPDM) compounds. Several hardness grades are considered for each material.

Two important physical properties of an O-ring seal are its material hardness and its resistance to permanent set, which is measured as compression set. The hardness of the material is related to its resistance to extrusion through gaps, or clearances, in the housing under pressure. In general, harder materials are more extrusion resistant. The compression set is a measure of the longer term sealability of the material. Better sealing materials are indicated by a lower compression set percentage. Table 2 in the standard gives the expected tolerance range of hardness and maximum compression set values when measured directly on an O-ring.

Further Tables give the detailed requirements of the O-ring materials when measured using samples from standard test sheets or buttons, or using the standard test size of O-ring

ISO 3601-1-214 (d1 = 24.99 mm; d2 = 3.53 mm). The properties specified for each material include:

1. Hardness.

2. Tensile strength.

3. Elongation at break.

4. Compression set.

5. The effects of heat ageing on:

i) change of hardness.

ii) change of tensile strength.

iii) change of elongation at break.

6. Cold flexibility temperature (TR10).

7. The effects of ISO Oils number 1 and number 3 (where appropriate) on:

i) volume change.

ii) hardness change.

These Tables thus give a comprehensive specification of the properties required for O-ring materials.

Often low quality materials will have inferior tensile strength so the guidance provided on this property is also a particularly important factor. It is possible to manufacture seals at a much lower cost by using a large proportion of low grade filler. Although they may meet the hardness requirement they can then have very poor tensile properties.

An example is shown in Table 1. Material B has been compounded for low cost and has a very low tensile strength, it may even break during fitting. Such materials can be offered by stockists as a very low cost option when no material properties have been specified. This potential hazard can be avoided by specifying O-rings to meet the new ISO 3601-5 and purchasing from a reputable source.


The new edition of ISO 3601-5 was developed in order to reduce the number of sealing problems caused by the use of O-rings manufactured from poor quality or unsatisfactory materials. It is hoped that it will be rapidly adopted by the fluid power industry in the specification of O-rings. To ensure optimum reliability we strongly recommend that when purchasing O-rings for typical fluid power applications that the specification should include, not only the size, material type and hardness, but also; material that complies with this new ISO 3601-5 and O-rings that have achieved Quality Acceptance to Grade N of ISO 3601-3.

The above information has been compiled by:

Bob Flitney, C.Eng., M.I.Mech.E. Sealing technology consultant. Member of BFPA/TC 7 ‘Fluid seals and their housings’. Author; Seals and Sealing Handbook.

Dr Nick Peppiatt, C.Eng., M.I.Mech.E. Consultant to Hallite Seals International Ltd. (formerly technical manager). Chairman of BFPA/TC 7 ‘Fluid seals and their housings’.