Overview
ISO 8426-2:2025 - Hydraulic fluid power - Determination of derived displacement of positive displacement pumps and motors - Part 2: Zero-pressure intercept method - defines a standardized test and calculation method to determine the derived displacement (also called derived capacity) of positive displacement pumps and motors under steady‑state conditions. The standard specifies obtaining a single derived displacement value from measurements at a single shaft speed by extrapolating high‑pressure flow to zero differential pressure using the zero‑pressure intercept method.
Key topics and technical requirements
- Scope: Applies to positive displacement pumps and motors tested under steady‑state conditions; produces one derived displacement value per set of test conditions (shaft speed, fluid temperature, displacement setting).
- Measurements: Record high‑pressure flow rates (qve,i), inlet/outlet/case pressures, differential pressures (Δp), shaft speed (n) and inlet temperature (θ1) in accordance with ISO 4409:2019 test practices.
- Data points: A minimum of five steady‑state differential pressures is recommended, increased in equal increments from minimum to maximum differential pressure.
- Special cases:
- Variable displacement units: follow ISO 4409:2019 subsections for variable units.
- Reverse flow: if displacement depends on flow direction, determine derived displacement for each direction.
- Pump vs motor mode: test in the intended mode; values can differ between modes.
- Calculation:
- Use linear regression (ordinary least squares) on qve versus Δp to determine the zero‑pressure intercept qve,0.
- Convert intercept to derived displacement: V = qve,0 / n.
- Reporting: Test report must include test ID, unit description, shaft speed and pressure ranges, test circuit diagram, fluid properties, measurement accuracy, measured and derived values (including correlation coefficients) and reference to ISO 8426-2:2025.
Applications and who uses it
- Hydraulic component manufacturers use ISO 8426-2:2025 for product characterization, performance data sheets and quality control.
- Test laboratories and certification bodies apply the method to produce repeatable, comparable displacement measurements.
- System designers and engineers use derived displacement figures for sizing, simulation and troubleshooting of hydraulic circuits.
- R&D and validation teams rely on the zero‑pressure intercept method for performance benchmarking and correlation with volumetric efficiency.
Related standards
- ISO 4409:2019 - Methods of testing pumps, motors and transmissions (referenced for measurement procedures)
- ISO 5598 - Fluid power vocabulary (terms and definitions)
- ISO 1219‑1, ISO 4391 - Graphical symbols and parameter symbols (referenced for symbols/units)
Keywords: ISO 8426-2:2025, hydraulic fluid power, derived displacement, zero‑pressure intercept method, positive displacement pump testing, hydraulic testing standards.
Open preview (PDF)Hydraulic fluid power — Determination of derived displacement of positive displacement pumps and motors — Part 2: Zero-pressure intercept method