ISO 8426-2:2025

International
Standard
ISO 8426-2
First edition
Hydraulic fluid power —
2025-08
Determination of derived
displacement of positive
displacement pumps and motors —
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
Reference number
© ISO 2025
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Published in Switzerland
ii
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
Foreword
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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.
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iv
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.
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