Abstract: The cutter head driving system of the tunnel shield boring machine which has large section (>5 m) played an important role in urban construction. The axial piston pump with large displacement (>500 mL/r) and high pressure (>35 MPa) were required in the cutter head driving system. The friction pair of the valve plate was one of the most vulnerable structure to failure inside the axial piston pump, especially under high pressure and large flow conditions. In this case, the design problem of spherical valve pair structure in large displacement axial piston pump was discussed, the bearing characteristics and pressure flow characteristics of different structure valve pairs were studied. Firstly, the flow field model with oil film was established through the method of computational fluid dynamics(CFD).The force and pressure characteristics sampling points within the oil film were solved by the key points within the model. The accuracy of the boundary setting of the simulation model was verified by comparing with the pressure outlet results of the bench experiment. Then, the influence of different kinds of oil film structures were analyzed, the leakage characteristics and bearing characteristics of planar, conical and spherical valve pairs were calculated and analyzed respectively. Finally, the bearing capacity of different spherical curvatures were analyzed, the outlet pressure-flow characteristics and bearing capacity of different valve plate structure were calculated. The results show that the leakage and bearing capacity of the spherical valve plate is much superior to the traditional planar flow pair. Besides, the spherical radius of the spherical valve plate will directly influence its output performance. With the discussion on the optimization analysis, the optimum spherical curvature radius is 455 mm, the pulsation rate is 13.6%, the bearing capacity of the oil film is 76 kN, which has the optimal bearing capacity.
Key words:  positive displacement pump; large displacement axial piston pump; outlet pressure-flow characteristics; oil film bearing characteristics; oil film leakage characteristics; distribution pair structure optimization