【正文】 building the dynamic model and making the interfaces to connect other models, the real kinetic relationships and motion parameters of the necessary parts should be analyzed. There are several hypotheses as follows.(1) In order to simply the simulation, only necessary parts are considered. Some accessories models, such as mechanism of variable displacement and slipper holddown, are ignored.(2) The rotation of middle shaft is stable and the speed is defined as constant.(3) The angle of swash plate changes in a defined work range by rotation drive.(4) The oil film between piston and cylinder, swash plate and slipper, cylinder and valve plate is stable, and its friction coefficient is constant.As shown in Fig. 1, the middle shaft of swash plate type piston pump rotates around its axis and drives the cylinder, pistons and correspond slippers rotating at a same speed[10?11].The coordinates of point B′ of intersection between the center line of piston and the surface of swash plate is described as follows: (1)From Eq. (1), it is shown that piston moves along zaxis and rotates around the middle shaft. The track of point B can describe the motion of the piston. Based on Eq. (1), the speed and accelerate of point B′ are as follows: (2)The slipper is connected with piston by spherical joint. The track of point B in the spherical joint can describe the motion of slipper. The coordinates of the point B are (3)The motion track of the slipper is ellipse, the vector diameter ρ is (4)The angle θ between ρ and the Langaxis of ellipse is (5)The rotation speed of the point B around the point O is (6)The velocity of the point B is (7)Based on the equations above, the motion of basic parts can be defined. Structural and dynamic modelAs for swash plate type piston pump, showed in Fig. 2(a), the 3D structural model (Fig. 2(b)) was made in a erical CAD software. To simplify the analysis, only necessary parts model were made. The joints and constraints between connecting parts were added. According to the real dynamic relationship between different parts, the proper joints and motion parameters are shown in Table 1 and Table 2. In the dynamic software, all these joints and motions were added to corresponding parts. Then the basic dynamic model was finishied with a 3D structural model adding dynamic relationships.Besides, the ration speed of the middle shaft should be added, then the basic model of piston pump can be drove and all parts can move just like a real pump without oil. Hydraulic model of piston pumpAs there is no fluid force and motion in the basic dynamic model, so the dynamic model can only simulate the motion of piston pump and don’t have the function of sucking oil and charging oil to drive load. In order to build the hydraulic model, there are some hypotheses as follows, (1) The piston pump works stably and the rotation speed of middle shaft is defined constant.(2) The oil film in the gap between piston and cylinder, slipper and swash plate, cylinder and valve plate is stable. And there is only leakage of laminar flow.(3) There is hydrostatic balance in the gap between cylinder and valve plate, slipper and swash plate, and the pressure ratio λ is constant (λ=).(4) The viscidity of the fluid oil is invariable.Based on the motion relationship between the pump parts and the flow influence on the pump, the hydraulic model of the piston pump starts with the basic flow model.As shown in Fig. 3, the low pressure oil is sucked to the cylinder bore when the piston moves to the right. While the piston moves to the left, the piston charges the oil out to drive loads.The model shown in Fig. 3 is the basic unit of one piston. And this unit including three important leakages, qv1 between piston and cylinder, qv2 between slipper and swash plate and qv3 between cylinder and valve plate, which are described in Eqs. (8)?(10)[11]:, (8), (9), (10)where dr —Diameter of piston,hr1—Gap height between piston and cylinder,lr —Contact length between cylinder and piston,η —Dynamic viscidity of the fluid,ε —Eccentricity ratio of piston,p1 —Pressure inside piston chamber,p0 —Environment pressure inside pump,λ —Pressure ratio,hr2—Gap height between the swash plate and slipper,r1, r2 —Structural parameters of slipper,hr3—Gap height between valve plate and cylinder,φ1, φ2, R1,R2, R3, R4 —Structural parameters of valve plate.The input oil and output oil in the cylinder bore are supplied through the valve plate, so the open area of the kidney bore in valve plate is used for controlling oil sucking a