National foreword

This British Standard is the UK implementation of ISO 7986:2026.

The UK participation in its preparation was entrusted to Technical Committee MCE/18/-/3, Cylinders, fluid seals and their housings.

A list of organizations represented on this committee can be obtained on request to its committee manager.

Contractual and legal considerations

This publication has been prepared in good faith, however no representation, warranty, assurance or undertaking (express or implied) is or will be made, and no responsibility or liability is or will be accepted by BSI in relation to the adequacy, accuracy, completeness or reasonableness of this publication. All and any such responsibility and liability is expressly disclaimed to the full extent permitted by the law.

This publication is provided as is, and is to be used at the recipient’s own risk.

The recipient is advised to consider seeking professional guidance with respect to its use of this publication.

This publication is not intended to constitute a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 October 2026.

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards are normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electro-technical Commission (IEC) on all matters of electro-technical standardization.

The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO documents should be noted. This document is drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

ISO draws attention to the possibility that the implementation of this document may involve the use of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s) which may be required to implement this document. However, implementers are cautioned that this may not represent the latest information, which may be obtained from the patent database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.

Any trade name used in this document is information given for the convenience of users and does not constitute an endorsement.

For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and expressions related to conformity assessment, as well as information about ISO's adherence to the World Trade Organization (WTO) principles in the technical barriers to Trade (TBT), see www.iso.org/iso/foreword.html.

This document was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 7, Sealing devices.

This second edition cancels and replaces the first edition (ISO 7986:1997), which has been technically revised.

The main changes are as follows:

change the reference documents for the surface roughness to the latest edition;
change key names to be clear;
change the minimum pressure at return stroke 3 MPa to 5 % of test pressure;
add the cross-sectional view of Figure 3.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www.iso.org/members.html.

Introduction

0.1 General

It is widely recognised that the results from reciprocating seal testing can be unpredictable. The background research carried out in support of the preparation of document has demonstrated that this unpredictability is primarily a function of lack of control of critical variables affecting seal installation and operation.

In order to carry out direct comparisons of seal performance, it is necessary to control these variables to closer limits than may be normal in industrial practice. The major variables that can affect seal performance, often even within normal manufacturing tolerance ranges, are listed in Clause 3.

0.2 Purpose

The purpose of the test is to provide comparative data on individual reciprocating rod seal designs and to provide a basis for the preliminary selection of seals based upon their relative performance. To achieve this, it is necessary to carry out the tests under strictly defined operating conditions. This document, therefore, defines standard values of speed, pressure, temperature and surface finish. The resulting specified characteristics are applicable to be published in manufacturer’s literature to enable a direct comparison of seal performance.

The seal test is applicable to a single seal or a combination seal package.

To make the test results as useful as possible, a range of operating conditions has been provided so that the most appropriate conditions can be selected as a basis for comparison for initial seal selection.

The pressures are standard working pressures, selected from ISO 2944:

6,3 MPa (63 bar);
16 MPa (160 bar);
31,5 MPa (315 bar).

The speeds have been selected to include slow speed (where stick-slip and high wear may be issues), medium speed for general purpose hydraulics and a representative high speed.

The constraints on installation and operation in this procedure have been proved by research, primarily carried out at BHR Group in the United Kingdom, under sponsorship from European companies and international round robin seal tests to be necessary to achieve repeatable results1. Any departure from the standard operating condition and installation criteria cannot be considered a standard test.

0.3 Factors affecting seal performance

Installation:

sealing system, i.e. design of bearing(s), seal(s) and leakage collection seal;
installation tolerances including seal groove, rod and bearings, extrusion gap;
rod material and hardness;
surface finish of the rod, variations of surface finish outside the range of Ra 0,08 µm to Ra 0,15 µm, or greater than Rz 1,2 µm or greater than Rt 1,5 µm, can significantly affect seal performance. Different seal materials also have significantly varying requirements for the optimum surface finish;
surface finish of the groove surfaces must be less than Ra 0,8 µm, or Rz 3,2 µm, to avoid static leakage and wear of the seal during pressure cycles;
bearing material including its effects on rod texture and boundary layer;
assembly cleanliness;

Operation:

fluid, i.e. viscosity, lubricity, compatibility with seal material including additives and contamination level;
pressure including pressure cycle;
speed, especially speed cycle;
speed/pressure cycle, i.e. stop-start conditions;
stroke, especially short strokes (2 × the seal contact widths or less) which prevent formation of a lubricant film;
temperature, i.e. its effect on viscosity and seal material properties;
external environment.

It is necessary to consider all the above factors and their potential effect on seal performance when comparing potential seal performance in an actual application to results obtained from the standard test

1 Scope

This document defines the test conditions and methods for assessing the performance of seals used in oil hydraulic reciprocating applications.

2 Normative references

3 Terms and definitions

4 Letter symbols

5 Test installation

5.1 General

5.2 Mechanical details

6 Test parameters

6.1 Test environment

6.2 Test fluid

6.3 Test fluid temperature

6.4 Test housing rod bearings

6.5 Test fluid pressure

6.6 Speed of linear actuator

6.7 Pressure and stroke cycle for dynamic test

7 Assembly of seals

8 Methods of measurement and measuring apparatus

8.1 Seal leakage

8.2 Seal friction

8.3 Pressure measurement

8.4 Surface roughness

8.5 Temperature measurement

9 Calibration

10 Test procedure

10.1 Test programme

10.2 Repeat series

11 Presentation of results

12 Identification statement (reference to this document)

Bibliography