Dynamometer Testing of Hydraulic Fluids in an Axial Piston Pump Under Simulated Backhoe Loader Trenching Conditions

The standard method for determining the efficiency of hydraulic pumps is specified in ISO 4409. This method requires that performance measurements be made under steady state conditions. In this investigation, the efficiency of a variable displacement axial piston pump was evaluated under both steady state and dynamic conditions. The dynamic conditions were derived from an analysis of measurements collected during multiple backhoe loader soil trenching cycles. The trenching cycle was replicated in the dynamometer by controlling pump swashplate motion, outlet pressure, and rotational frequency. In dynamic testing, data was collected at 1000 Hz and the duration of the trenching cycle was 12 seconds. In steady state testing, input values for swashplate position, outlet pressure, and rotational frequency were extracted from the duty cycle at 0.05 s intervals. Fifteen seconds of data was collected for each of the resulting 583 test points. Thus, several hours of testing were required to reproduce the trenching cycle under steady state conditions. Under steady state conditions pump volumetric efficiency was approximately 2% higher than in dynamic testing. The difference in volumetric efficiency was attributed to pump inlet line dynamics. The dynamic response of the pump inlet line was studied using a transmission line model that was based upon well-established methods for the characterization of line resistance, inertance, and capacitance. Non-linear terms for resistance and capacitance were included to investigate the effects of these properties on inlet line pressure ripple. Data from inline viscosity and density sensors was used in a sensitivity analysis. Inlet line pressure ripple was found to increase as the fluid viscosity decreased. This effect was also seen as polymer additives sheared. These findings provide insights into the effects of fluid properties on pump inlet line dynamics.