Overview
ISO 4391:1983 - "Hydraulic fluid power - Pumps, motors and integral transmissions - Parameter definitions and letter symbols" standardizes the principal technical characteristics, letter symbols and suffixes used to describe hydraulic pumps, motors and integral variable-speed transmissions. It provides a systematic list of parameter definitions, dimension analysis and practical examples to ensure unambiguous communication in documentation, test reports and catalogues.
Key topics
- Parameter definitions and dimensions: Clear definitions for pressure, flow (theoretical and effective), torque, power (hydraulic, pump, motor), swept volumes (geometric, effective, derived), moments of inertia, losses (volumetric internal/external, compressibility) and temperatures.
- Letter symbols: Alphabetical set of Latin and Greek letters assigned to each characteristic (e.g., P for pressure, qv for volume flow rate, T for torque).
- Suffix system: Standard suffixes (for ambient am, hydraulic h, mechanical m, dry, external ex, rated/norminal conditions, peak P, etc.) and priority rules to combine suffixes and avoid proliferating symbol variants. Suffixes 0, 1, 2 identify inlets/outlets where needed.
- Guidelines and examples: Rules for using symbols in equations, when to prefix unit type (P, M, T), combining multiple suffixes, and worked examples for pumps, motors and integral transmissions.
- Identification statement: Recommended wording for declaring compliance in technical literature and test reports.
- Cross-references: Links to ISO 1219 (graphical symbols), ISO 5598 (vocabulary) and ISO 31 series (quantities and units).
Practical applications
ISO 4391 helps professionals create consistent, machine- and human-readable technical content:
- Designers & engineers use these symbols and definitions in calculations, performance curves and specification sheets to avoid ambiguity when sizing or comparing components.
- Manufacturers & catalogs adopt standard letter symbols and suffixes for product datasheets, labels and marketing materials so customers can compare products reliably.
- Test laboratories & quality teams report measured parameters (flow, pressure, torque, losses) using the standardized notation for unambiguous test reports.
- Procurement & systems integrators reference rated and limiting conditions (inlet/outlet pressures, peak duty, rated power) to specify components for systems reliably.
- Standards writers & technical authors harmonize terminology with ISO 1219 and ISO 5598 for consistent documentation across fluid power standards.
Related standards
- ISO 1219 - Fluid power systems and components - Graphical symbols
- ISO 5598 - Fluid power systems and components - Vocabulary
- ISO 31 series - Quantities, units and dimensions
Using ISO 4391 supports clearer communication of hydraulic parameters, reduces specification errors, and improves interoperability across pumps, motors and integral transmissions in hydraulic fluid power systems. Keywords: ISO 4391, hydraulic fluid power, pumps, motors, integral transmissions, parameter definitions, letter symbols, hydraulic parameters.
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