Axial piston pumps act as the power source in hydraulic systems and their volumetric losses mainly arise from leakage of tribological interfaces. The leakage of tribological interfaces was often modeled as a simple mixture of pressure-induced flow and velocity-induced flow in previous studies. However, few studies have clarified the contribution of discharge pressure and rotational speed to the leakage flow. This work considers both pressure and velocity factors and establishes new analytical models for the leakage of tribological interfaces from Navier-Stokes equations. Experiments were performed on an actual axial piston pump to measure its leakage flow rates at different discharge pressures and rotational speeds. Results show that the total pump leakage excludes Couette flow and only includes Poiseuille and centrifugal flows. The Poiseuille flow predominates the pump leakage while the centrifugal flow contributes little to the pump leakage.

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