ISO 4392-3:1993

Overview

ISO 4392-3:1993 - Hydraulic fluid power - Determination of characteristics of motors - Part 3: At constant flow and at constant torque specifies a standardized method to determine the low-speed performance of positive-displacement rotary hydraulic motors under constant flow and constant torque conditions. It covers test setup, measurement requirements, procedural controls and output metrics for fixed- and variable-displacement motors, including bidirectional testing where applicable. The standard is intended to reveal periodic phenomena at slow speeds that can influence steady torque output and affect the connected hydraulic system.

Key topics and requirements

• Test scope: Low-speed characterization of positive-displacement rotary motors at constant flow and constant torque (50% and 100% of nominal torque).
• Test conditions: Fluid inlet temperature at either 50 °C or 80 °C; use manufacturer-approved hydraulic fluids and viscosities (measured per ISO 3448).
• Test rig and instrumentation: Requires a hydraulic circuit with a constant-flow regulator, isolation valve, filtration/temperature control and a constant-torque load (e.g., volumetric pump with feedback or equivalent). Instruments must measure:
  • inlet flow, inlet/outlet pressures, inlet temperature
  • output torque, shaft speed and angular position
• Measurement quality: Measurement accuracy is organized into three classes A, B, C (Annex A). Recording equipment must resolve frequencies >10× the highest fundamental frequency expected; angularly regular sampling with at least 10× the number of displacement pulses per revolution.
• Operational controls: Maintain torque and flow variations within specified limits (example: instantaneous variations kept within ±4% of mean). Minimize rotating inertia and dead-volume between flow regulator and motor inlet.
• Results and calculations: Standard prescribes how to determine mean rotational speed, speed irregularity indices, effective differential pressure, volumetric efficiency, hydromechanical efficiency and peak-to-peak amplitudes for speed and pressure over a complete motor cycle.

Practical applications

• Performance validation and quality control of hydraulic motors during R&D, production testing and acceptance testing.
• Comparative evaluation of motor designs (fixed vs. variable displacement), and verification of low-speed behavior that may produce periodic torque ripple affecting machine dynamics.
• Use by hydraulic test laboratories, motor manufacturers, design engineers, system integrators and OEMs to ensure repeatable, comparable performance data for system matching and troubleshooting.

Related standards

• ISO 3448 - Viscosity classification for industrial lubricants (used for fluid selection and viscosity measurement)
• ISO 8426 - Determination of measured displacement (used for volumetric measurement)
• ISO 4409, ISO 4391, ISO 5598 - vocabulary, symbols and steady-state test methods referenced within ISO 4392-3

Keywords: ISO 4392-3:1993, hydraulic fluid power, hydraulic motor testing, constant flow, constant torque, low-speed characteristics, volumetric efficiency, test rig, measurement classes.