ISO 8426-2:2025

FINAL DRAFT
International
Standard
ISO/FDIS 8426-2
ISO/TC 131/SC 8
Hydraulic fluid power —
Secretariat: BSI
Determination of derived
Voting begins on:
displacement of positive
2025-05-20
displacement pumps and motors —
Voting terminates on:
2025-07-15
Part 2:
Zero-pressure intercept method
Transmissions hydrauliques — Détermination de la cylindrée
calculée des pompes et moteurs volumétriques —
Partie 2: Méthode d'interception à pression nulle
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
ISO/FDIS 8426-2:2025(en) © ISO 2025

FINAL DRAFT
ISO/FDIS 8426-2:2025(en)
International
Standard
ISO/FDIS 8426-2
ISO/TC 131/SC 8
Hydraulic fluid power —
Secretariat: BSI
Determination of derived
Voting begins on:
displacement of positive
displacement pumps and motors —
Voting terminates on:
Part 2:
Zero-pressure intercept method
Transmissions hydrauliques — Détermination de la cylindrée
calculée des pompes et moteurs volumétriques —
Partie 2: Méthode d'interception à pression nulle
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2025
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland Reference number
ISO/FDIS 8426-2:2025(en) © ISO 2025

ii
ISO/FDIS 8426-2:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units. 1
5 Test procedure . 2
5.1 General requirements .2
5.2 Number of steady-state differential pressures .3
5.3 Variable displacement unit .3
5.4 Reverse flow unit .3
5.5 Unit used as pump or motor .3
6 Calculation of derived displacement . 3
7 Test report . 4
Bibliography . 6

iii
ISO/FDIS 8426-2:2025(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is 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 Electrotechnical Commission (IEC) on all matters of electrotechnical 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 was 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 of 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 8,
Product testing.
This first edition of ISO 8426-2 cancels and replaces the second edition (ISO 8426:2008), which has been
technically revised.
A list of all parts in the ISO 8426 series can be found on the ISO website.
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 ww
...

Formatted
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
ISO/FDIS 8426-2:2025(en)
Style Definition
...
ISO /TC 131/SC 08/WG 13 8 Style Definition
...
Style Definition
...
Secretariat: BSI
Style Definition
...
Date: 2025-04-17xx
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Hydraulic fluid power — Determination of derived displacement of
Style Definition
...
positive displacement pumps and motors — —
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Part 2:
Style Definition
...
Zero-pressure intercept method
Style Definition
...
Transmissions hydrauliques — Détermination de la cylindrée calculée des pompes et moteurs volumétriques —
Style Definition
...
Style Definition
Partie 2: Méthode d'interception à pression nulle .
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
FDIS stage
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Style Definition
...
Formatted
...
Formatted
...
Formatted
...
Formatted
...
Formatted
...
Formatted
...
Formatted
...
Formatted
...
ISO /FDIS 8426-2:2024(E2025(en) Formatted: Font: 11 pt, Bold
Formatted: Font: 11 pt, Bold
Formatted: Font: 11 pt, Bold
Formatted: HeaderCentered, Space After: 0 pt, Line
spacing: single
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, Formatted: Default Paragraph Font
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
Formatted: Default Paragraph Font
at the address below or ISO’s member body in the country of the requester.
Formatted: Indent: Left: 0 cm, Right: 0 cm, Space
Before: 0 pt, No page break before, Adjust space
ISO copyright office
between Latin and Asian text, Adjust space between
CP 401 • Ch. de Blandonnet 8
Asian text and numbers
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
Formatted: French (France)
EmailE-mail: copyright@iso.org
Formatted: French (France)
Website: www.iso.orgwww.iso.org
Formatted: French (France)
Published in Switzerland
Formatted: Font: 10 pt
Formatted: Font: 10 pt
Formatted: Font: 11 pt
Formatted: FooterPageRomanNumber, Space After: 0
pt, Line spacing: single
ii © ISO 2024 2025 – All rights reserved
ii
ISO/FDIS 8426-2:2025(en)
Formatted: Font: 11 pt
Formatted: HeaderCentered, Left, Space After: 0 pt,
Line spacing: single, Tab stops: Not at 0.71 cm
Contents
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units . 1
5 Test procedure . 2
5.1 General requirements . 2
5.2 Number of steady-state differential pressures . 3
5.3 Variable displacement unit . 3
5.4 Reverse flow unit . 3
5.5 Unit used as pump or motor . 3
6 Calculation of derived displacement . 3
7 Test report . 4
Bibliography . 6

Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units . 2
5 Test procedure . 3
5.1 General requirements . 3
5.2 Number of steady-state differential pressures . 3
5.3 Variable displacement unit . 3
5.4 Reverse flow unit . 3
5.5 Unit used as pump or motor . 3
6 Calculation of derived displacement . 3
7 Test report . 4
Bibliography . 6
Formatted: Font: 10 pt
Formatted: FooterCentered, Left, Space Before: 0 pt,
Tab stops: Not at 17.2 cm
Formatted: Font: 11 pt
Formatted: FooterPageRomanNumber, Left, Space
After: 0 pt, Tab stops: Not at 17.2 cm
iii
ISO /FDIS 8426-2:2024(E2025(en) Formatted: Font: 11 pt, Bold
Formatted: Font: 11 pt, Bold
Formatted: Font: 11 pt, Bold
Foreword
Formatted: HeaderCentered, Space After: 0 pt, Line
spacing: single
ISO (the International Organization for Standardization) is a worldwide federation of national standards
Formatted: Adjust space between Latin and Asian text,
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
Adjust space between Asian text and numbers
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 Electrotechnical Commission (IEC) on all matters of electrotechnical 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 was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directiveswww.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.www.iso.org/patents. ISO shall not be held responsible for identifying any or all such
Commented [eXtyles1]: The URL
https://www.iso.org/patents has been redirected to
patent rights.
http://www.iso.org/iso-standards-and-patents.html. Please
verify the URL.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of 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.htmlwww.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 8,
Product testing.
This first edition of ISO 8426-2 cancels and replaces the second edition (ISO 8426:2008), which has been
technically revised.
A list of all parts in the ISO 8426 series can be found on the ISO website.
Commented [eXtyles2]: Invalid reference: "ISO 8426
series"
Any feedback or questions on this document should be directed to the user’s national standards body. A
Formatted: Default Paragraph Font
complete listing of these bodies can be found at www.iso.org/members.html.www.iso.org/members.html.
Formatted: Default Paragraph Font
Formatted: Default Paragraph Font
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers
Commented [eXtyles3]: The URL
https://www.iso.org/members.html has been redirected to
http://www.iso.org/about/members. Please verify the URL.
Formatted: Font: 10 pt
Formatted: Font: 10 pt
Formatted: Font: 11 pt
Formatted: FooterPageRomanNumber, Space After: 0
pt, Line spacing: single
iv © ISO 2024 2025 – All rights reserved
iv
ISO/FDIS 8426-2:2025(en)
Formatted: Font: 11 pt
Formatted: HeaderCentered, Left, Space After: 0 pt,
Line spacing: single, Tab stops: Not at 0.71 cm
Introduction
In hydraulic fluid power systems, power is transmitted and controlled through a liquid under pressure within
an enclosed circuit. Two types of components of such systems are the positive displacement pumps and
motors. One of the technical parameters of these components is the derived displacement, also known as
derived capacity. This document is intended to describe the zero-pressure intercept method for determining
the derived displacement of hydraulic fluid power positive displacement pumps and motors. The term derived
displacement is preferred over derived capacity.
Formatted: Font: 10 pt
Formatted: FooterCentered, Left, Space Before: 0 pt,
Tab stops: Not at 17.2 cm
Formatted: Font: 11 pt
Formatted: FooterPageRomanNumber, Left, Space
After: 0 pt, Tab stops: Not at 17.2 cm
v
FINAL DRAFT International Standard ISO/FDIS 8426-2:2025(en)

Formatted: Left
Formatted: Font: Bold
Hydraulic fluid power — Determination of derived displacement of
positive displacement pumps and motors — —
Formatted: Main Title 2, Space After: 0 pt, Adjust
space between Latin and Asian text, Adjust space
between Asian text and numbers
Part 2:
Formatted: Adjust space between Latin and Asian text,
Zero-pressure intercept method
Adjust space between Asian text and numbers
Formatted: Default Paragraph Font
1 Scope
Formatted: Default Paragraph Font
This document specifies the zero-pressure intercept method for the determination of the derived
Formatted: Default Paragraph Font
displacement of hydraulic fluid power positive displacement pumps and motors under steady state conditions.
Formatted: Default Paragraph Font
A single value for the derived displacement is determined from measurements at a single shaft speed.
Formatted: Adjust space between Latin and Asian text,
2 Normative references
Adjust space between Asian text and numbers, Tab
stops: Not at 0.7 cm + 1.4 cm + 2.1 cm + 2.8 cm +
The following documents are referred to in the text in such a way that some or all of their content constitutes
3.5 cm + 4.2 cm + 4.9 cm + 5.6 cm + 6.3 cm + 7 cm
requi
...