Most test benches for hydraulic pumps and motors are designed for operating speeds of 500 rpm or higher. Worldwide, there are only a few test benches which allow measurements at near zero operating speeds or startup conditions.
Low-speed testing is, however, of great importance for hydraulic motors, since the torque losses increase exponential at these conditions due to mixed lubrication and stick-slip effects. Low-speed operation is also important for hydraulic pumps which are used in electro-hydraulic actuators, in which the flow rate is controlled by the operating speed of the electric motors. Finally, measurements at low operating speeds can give detailed information about the pump or motor issues, since it is possible to measure the operation at many rotational positions of the rotating group, giving the ability to measure the losses of individual displacement volumes.
This paper describes the design and operation of a new test bench for testing hydraulic pumps and motors at extremely low operating speeds, below 1 rpm.