Radial piston motors are known for their high efficiency, often above 90%. The efficiency depends on motor speed and fluid pressure. Corresponding efficiency maps are usually provided by equipment manufacturers (see e.g. Figure 7).
motor components contributing to mechanical losses help improve efficiency and reduce torque ripple. However, as with all complex systems, the greatest performance gain is achieved when a whole system approach is applied.
Figure 7: The efficiency map of the radial hydraulic motor (MSE02 by Poclain Hydraulics, 22kW, 330 cc/rev, max. pressure 400 bar, max. speed 200 rpm, max. torque 2000 Nm) used in hydraulic fluid evaluation trials.
At low speed, the total efficiency takes a hit due to fluid leakage (the leak volume per revolution at low speed is higher than at high speed) and friction losses (friction increases due to mixed lubrication at low Sommerfeld numbers (see e.g. Dahlen and Olsson 2002)). At high speed, the total efficiency is mostly affected by viscous dissipation and related head losses. In general, picking a higher viscosity grade helps improve volumetric efficiency at low speed but compromises mechanical efficiency at high speed. Picking a lower viscosity grade has the opposite effect. Staying closer to the upper permissible viscosity is usually considered a safer bet as it averts the risk of lubricant film collapse and component seizure due to heat generated at high power output (Nilsson 2011).
In as much as the ratio of the motor displacement volume to the total friction surface area (bearings and pistons) for large motors is greater than for small motors with a similar design, larger motors will usually have higher efficiency. For example, the peak efficiency of large Hägglunds Quantum motors approaches 97% (Eriksson 2023).
As already mentioned, UHVI synthetic hydraulic fluids bring significant performance advantages over conventional mineral-based counterparts (see Table 1 and Figure 8). Nonetheless, such products are still a niche market because of their high price tag and limited supply.
Both the deployment of friction-modified UHVI synthetic fluids and Triboconditioning®
treatment of
Table 1: Basic properties of hydraulic fluids used in the efficiency trials.
Figure 8: The effect of fluid type on the average efficiency of MSE02 motor at 200 bar and 60o
C fluid temperature.
References W. Bock, Hydraulic Oils, in Lubricants and Lubrication (T. Mang and W. Dresel, Eds.), Wiley, 2017. M.K. Miller,
et.al. Tribology Trans. 57 (2014) 622. A. Holzer,
et.al. Chem. Eng. Tech. 46 (2023) 110. B. Zhmud,
et.al. Gear Technology, May 2025. P. Isaksson,
et.al. J. Eng. Tribology 225 (2011) 975. B. Brodmann and H. Bodschwinna, Proc. Int. Conf. on Gears, VDI Verlag, 2019, p. 1567.
L. Dahlen and H. Ohlsson, 5th JFPS Int. Symp. 2002, 2C22, p. 537.
D. Nilsson, Investigations of Enhanced Tribological Performance of a Hydraulic motor, PhD thesis, LUT, Sweden, 2011.
J. Eriksson, Efficiency of Radial Piston Hydraulic Motors, MSc thesis, UU, Sweden, 2023.
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