Predicting the Pump Efficiency of Hydraulic Fluids to Maximize System Performance

Hydraulic system design and component selection should take into account the performance capabilities of the hydraulic fluid. Selecting a hydraulic fluid with the proper viscosity is critical in order to obtain optimum system response and guarantee long-term performance. A fluid with too high a viscosity at low temperature will resist flow and may cause pump cavitation. The use of a fluid with insufficient viscosity at the highest operating temperature will result in poor volumetric efficiency and, in some cases, overheating and pump seizure. The use of the ASTM D 6080 classification and the NFPA recommended practice for viscosity selection criteria can provide improved guidance in selecting the proper hydraulic fluid.

Mobile equipment, with limited cooling capability, must frequently operate under high temperature conditions. If the oil viscosity is too low, excessive internal pump leakage will occur. Work completed in gear and vane pumps has shown that the Poiseuille law can be applied to predict the influence of pressure and viscosity on volumetric efficiency.

This paper demonstrates how an equipment user or designer can estimate the pump volumetric efficiency for a given hydraulic fluid, over a range of pressure and temperature conditions. This approach should simplify the selection of a hydraulic fluid with viscometric properties optimised to meet application performance demands.