The Determination of Hydraulic Motor Displacement


  • Paul Michael
  • Jose Garcia-Bravo



Hydraulic Components, Efficiency, Test methods


Because the geometrical displacement of a pump or motor is very difficult to measure directly, the derived capacity of motors is used to assess the efficiency of positive displacement machines. The current internationally accepted method for deriving the displacement of hydraulic pumps and motors is ISO 8426:2008. Difficulties in accurately assessing derived displacement via ISO 8426:2008 have been reported by several authors. These inaccuracies can lead to efficiency results that exceed 100% in ISO 4409:2019 performance tests. In the presented work, fixed axial, variable axial, and radial piston motors were evaluated at 50°C and 80°C in dynamometer tests. Linear, orthogonal, and semi-randomized data sets were collected. The Wilson, Toet, and an analytical form of the Toet were compared with ISO 8426:2008. In general, the differences between the various methods for deriving displacement were not statistically significant, except in the instance of the axial piston motor. In the axial piston motor, the ISO 8426:2008 derived displacement was approximately 1% lower than the other methods. Use of this lower ISO 8426:2008 displacement in efficiency calculations produced values exceeding 100%. The error in the ISO 8426:2008 derived displacement determination was attributed to difficulties in detecting speeddependent factors that affect displacement when testing is conducted at a single speed. The ISO 8426:2008 method does not provide instructions for calculating derived displacement when data is collected at more than one speed. It is proposed that the One-Step Toet method be incorporated into ISO 8426 as a method for calculating the derived displacement when users opt to measure performance at multiple speeds. This revision will reduce the potential for speed-dependent errors in the determination of derived displacement.