Overview
ISO 8426:2008 - "Hydraulic fluid power - Positive displacement pumps and motors - Determination of derived capacity" specifies standardized test methods for determining the derived capacity (volume displaced per shaft revolution) of positive-displacement hydraulic pumps and motors. Tests are performed under steady-state conditions at defined, continuous shaft rotational frequencies. Units may be tested as pumps (mechanical input, hydraulic output) or as motors (hydraulic input, mechanical output). The standard enables consistent performance comparison and verification of displaced volume metrics across manufacturers and test laboratories.
Key topics and requirements
- Derived capacity definition: Volume of fluid displaced per shaft revolution, calculated from measured flow at varying outlet (pump) or inlet (motor) pressures.
- Test conditions: Steady-state operation, controlled shaft rotational frequency, and temperatures within manufacturer-recommended ranges. Test fluid and viscosity/density must be recorded.
- Test circuits: Open- and closed-circuit arrangements for pumps and controlled-flow circuits for motors; figures and guidance provided for typical layouts. Casing drains and boost pumps are addressed where needed.
- Measurement locations: Pressure and temperature transducers placed adjacent to the unit ports - no less than 2d and no more than 4d from the port face (d = internal tube diameter). Flow measurements are made at the port face or corrected per ISO 4409.
- Flow and pressure measurement regimen: Number and spacing of pressure/flow readings depend on measurement accuracy class:
- Class A: ≥10 pressures in equal increments
- Class B: ≥5 pressures in equal increments
- Class C: 3 pressures at 20%, 50% and 100% of rated pressure
- Accuracy classes and tolerances: Annex A defines measurement accuracy classes (A, B, C) and permissible variations for rotational frequency, flow rate, pressure, and temperature (see Table 2 for limits).
- Calculation methods: Annex B defines calculation of derived capacity (including the zero-pressure intercept method); results and test conditions are recorded in a test report.
Applications and who uses ISO 8426:2008
- Manufacturers of positive displacement pumps and motors for validating product specifications and R&D.
- Independent test laboratories and certification bodies performing performance verification, acceptance testing, or warranty investigations.
- Hydraulic system designers and OEMs who need accurate pump/motor displacement data for system sizing, simulation, and control tuning.
- Procurement and quality assurance teams requiring standardized test evidence for supplier comparison.
Practical benefits include reliable capacity data for component selection, repeatable test procedures for QA, and a common basis for comparing hydraulic displacement performance.
Related standards
Normative references integrated into ISO 8426:2008 include:
- ISO 4409 - Flow correction and performance presentation for pumps/motors
- ISO 5598 - Fluid power vocabulary
- ISO 1219-1 - Fluid power graphic symbols
- ISO 3448 and ISO 6743-4 - Fluid viscosity classification and hydraulic fluids
Keywords: ISO 8426:2008, hydraulic fluid power, positive displacement pumps, derived capacity, pump testing, motor testing, flow rate measurement, test circuits, measurement accuracy.
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