Abstract: Taking axial variable piston pump with close-type swash plate and a mechanism of cross porting valve plate as example, a compressible flow distribution model of single displacement chamber and computation formula of swash plate moment are established, the variation curves of the area of flow passage section of oil suction and discharge of the valve plate and the precompression and preexpansion curve of single displacement chamber are found, and the variation regularity of swash plate moment is studied under different inlet pressure. It is found out by its result that with increase of inlet pressure, the average value of the component of moment of swash plate on three coordinate axes will decrease to some extent, where the ripple amplitude of the variable moment will decreases obviously.Therefore, properly increasing inlet pressure will be of advantage for decreasing variable moment of pump and weakening the mechanical vibration of swash plate.

Key words: axial variable piston pump, valve plate with cross porting, swash plate moment, inlet pressure