A practical approach for analysis of leakage flow characteristics in hydraulic pumps

This article presents a practical approach to leakage flow analysing for various types of hydraulic pumps. In this study, the influence of pump-operating conditions such as operating pressures, pump speeds and oil temperature on the leakage flow characteristics is investigated and a mathematical model to calculate the leakage flow is proposed. Currently, an external gear, an internal gear or a vane pump is commonly used for an automatic transmission. These pumps have their own leakage flow characteristics, which depend not only on pump-operating conditions but also on structures and dimensions of the pumps. For various pumps having different constructions, the delivery pressure and the oil temperature differently affect the leakage flow characteristics and especially the delivery pressure also changes the clearances causing the leakage flow. A mathematical model for the leakage flow, which takes into account a change in the thickness of clearance according to the delivery pressure, represents accurately the actual leakage flow characteristics in various pumps. Also, it clarifies the influence of the pump-operating conditions on their leakage flow characteristics. In the external and internal gear pumps, the leakage flow would not approach zero due to an existence of the orifice flow leakage almost independent of the viscosity of oil, even at an extremely low oil temperature. This leakage flow influences significantly the volumetric efficiency at a low oil temperature in the gear pumps.