An axial-type hydraulic pump/motor in which a cylinder block with a plurality of cylinder bores formed around a rotation axis slides with respect to a valve plate that has a valve plate discharge port and a valve plate suction port, and controls the amount of reciprocation of a piston in each cylinder bore depending on the inclination of a swash plate. Based on the rotational direction of the cylinder block, an opening shape of an end portion on the front side in the rotational direction of a cylinder port and an opening shape of an end portion on the rear side in the rotational direction of the valve plate suction port PB1 have the same shape or partially have the same shape.

A fluid pressure rotary machine includes a cylinder block that is fixed to a rotary shaft and includes a plurality of cylinder bores, a piston disposed to be free to slide in each cylinder bore such that a volume chamber is defined thereby, a swash plate that causes the piston to reciprocate such that the volume chamber expands and contracts, and a valve plate that slides against the cylinder block and includes an intake port and a discharge port communicating with the volume chamber. The valve plate includes a sliding surface formed to project in a spherical shape against the cylinder block. The cylinder block includes a sliding surface formed as an indentation corresponding to the shape of the sliding surface of the valve plate. A minute gap is formed between the sliding surface of the valve plate and the sliding surface of the cylinder block in an outer edge position.

Provided is a design method for a self-compensation structure of a cam-lobe hydraulic motor plate distribution system, which comprises that follow steps: firstly, establishing a force balance equation, a pressure distribution equation and a flow balance equation for each balance chamber, setting a boundary condition, and setting an expected nominal clearance; selecting a fit clearance for simultaneous solutions, and obtaining a set of solutions of areas; taking a maximum value in this set of solutions as the median, and setting a set of area values in the optimization design; further solving changing curves of the nominal clearance and the total leakage of the distribution system with the rotation angle of the cylinder block, and calculating the average value of various curves after the operation is smooth, and designing the self-compensation structure based on selection of the optimal combination of the fit clearance and area.