A New Hydraulic Pump and Motor Test Bench for Extremely Low Operating Speeds

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. INTRODUCTION Hydrostatic pumps and motors have operational limits. Aside from the maximum pressure and the maximum rotational speed, there is often also a certain minimum operating speed: most manufacturers don’t recommend to operate the pump or motor for an extended time below a certain minimum rotational speed. There is an obvious reason for this restriction. Most hydrostatic machines need at least a rotational speed of a few hundred rpm in order to create a sufficient hydrodynamic lubrication and avoid excessive wear due to a lack of bearing capacity. Yet, near-zero operating speeds can not be avoided in many hydrostatic motor operations. A hydrostatic driven fork lifttruck will, for instance, have continuous stop-and-go operation, in which the hydraulic motors often have to breakaway from standstill and are frequently operated at low rotational speeds. At these conditions, the torque capacity of the hydraulic motor is strongly reduced because of mixed lubrication and high friction losses. In addition, the limited number of pistons causes additional torque variations and reductions. Consequently, the motor size needs to be enlarged and sometimes even doubled to overcome these additional torque reductions. In order to measure, analyse and possibly improve the motor capacity at startup and breakaway, measurements at close to zero rotational speed are needed. Another important reason for the study of low-speed behavior is the recent development in the area of electrohydraulic actuators, in which speed controlled electric motors determine the flow output of the pump. These machines often demand near-zero speed operation. However, due to the nonlinear, stick-slip behavior of hydraulic pumps at low rotational speeds, the control of such systems can become unstable [1]. A test bench for low-speed operation is needed to investigate and improve the controllability (and efficiency). Finally, low-speed testing can give valuable information about the leakage and friction of the individual displacement volumes, i.e. pistons and cylinders of the hydrostatic machine [2-9]. Different coatings or other design details can be studied in detail while operating the pump or motor at low rotational speeds [10-12]. The testing of pumps and motors at low rotational speeds has increasingly gained (somewhat) more importance. The test method is however not trivial. Stick-slip effects strongly depend on the operating speed and pressure, as well as on the oil temperature and viscosity, and it is difficult to keep these parameters constant and reproducible. This paper describes a new test bed, which is developed and built by INNAS, for performing measurements at low, and even extremely low operating speeds, as low as 0.01 rpm. The new test bed is integrated in a performance bench for regular, high operating speeds. The new test facility can be used for pumps and motors, and can be operated in both rotational directions. Peter Achten, Jeroen Potma, Sjoerd Eggenkamp INNAS Breda, Netherlands ! Copyright © 20xx by ASME 1 ISO-STANDARDS The International Organization for Standardization (ISO) has three standards for determining the low-speed characteristics or startability of positive displacement rotary fluid power motors, of either fixed or variable displacement types: • ISO 4392-1: At constant low-speed and constant pressure [13] • ISO 4392-2: Startability [14] • ISO 4392-3: At constant flow and at constant torque [15] ISO 4392-1 and 4292-3 are largely overlapping. Both standards describe a method to determine the characteristics of positive-displacement fluid power motors at the minimum allowable rotational speed (ISO 4392-1) or at the lowest possible flow which causes the motor to stop rotating (ISO 4392-3). The advantage of the two methods is that the performance will be measured for all (or at least many) rotational positions of the rotating group of the motor. The disadvantage is that the motor is already rotating: the performance is not measured at real startup (0 rpm) conditions. The measurement of the actual startup torque is described in ISO 4392-2. Since the startup torque strongly depends on the rotational position of the shaft, the measurement has to be repeated for many shaft positions. According to ISO 4392-2: “The number of measurements at different shaft positions shall be greater than the minimum number necessary for the maximum starting pressure over one revolution to be found with a confidence level of 95 %. ”. That makes this procedure extremely time consuming and unpractical. Furthermore, it is almost impossible to create a thermal conditioning or equilibrium of the hydraulic motor and the oil. An alternative is to apply the test-procedure of ISO 4392-1 but now at extremely low, close to zero speed of the shaft. By integrating the low-speed test bench in the general test bench for normal operating speeds, it becomes easily possible to warmup the oil and the motor to the demanded equilibrium conditions. Furthermore, the same hydraulic circuitry, sensors and data-acquisition system can be used as for the general, higher speed test purposes. DEMANDS FOR THE NEW TEST BENCH The new test bench is first of all a general test bench for testing hydrostatic, positive displacement pumps and motors, according to ISO 4409 [16]. The main drive of this test bench is a water cooled, frequency controlled DC electric motor with distributed winding for achieving low torque variations. The motor has 48 stator coils and 16 magnet rotor poles. The motor can be operated between 50 and 5000 rpm. The electric motor has a through drive, enabling the mount of an additional hydraulic motor or pump. With this added hydraulic machine, the hydraulic power of the test object can be recirculated, which results in a strong reduction of the energy consumption and cooling demands. The remaining heat can, if needed, be used to heat the building. The hydraulic test circuit is designed for a rated pressure of 500 bar. The supply-side can be self-priming or pre-charged, up to a pressure level of 20 bar. The oil temperature can be precisely controlled by means of frequency controlled cooling fans and cooling pumps (Figure 1). The maximum oil temperature is 90°C. There is also an additional frequency controller for the electric motor, which drives a hydraulic pump for supplying high pressure oil during the low-speed test. At near-zero operating speeds, this pump foremost needs to compensate the volumetric losses, i.e. leakage of the hydraulic motor to be tested.