Patent ID: 7021904

Claim:
A swash plate type pump or motor, comprising: a cylinder block supported within a pump case so as to freely rotate; a plurality of first cylinder bores and a plurality of second cylinder bores which are formed axially on both sides of the cylinder block, the first cylinder bores and the second cylinder bores communicating with each other; first pistons and second pistons which are inserted into the first cylinder bores and the second cylinder bores from both of the sides of the cylinder block; volume chambers formed in inner portions of the first cylinder bores and the second cylinder bores and defined by the first pistons and the second pistons; a first swash plate and a second swash plate which are disposed axially on both of the sides of the cylinder block and to which the first pistons and the second pistons contact freely to slide, respectively; a first swash plate bearing and a second swash plate bearing which support the first swash plate and the second swash plate so as to be free to tilt, respectively; drive pistons that cause the first swash plate and the second swash plate to tilt; a hydraulic pressure control valve which selectively controls hydraulic pressure acting on the drive pistons; a pair of supply and discharge ports formed in a sliding surface of the first swash plate, the pair of supply and discharge ports being connected to a hydraulic fluid high pressure side and a hydraulic fluid low pressure side, respectively; a port plate disposed in a sliding portion between the first swash plate and the first pistons, the port plate rotating integrally with the cylinder block and guiding the high pressure side hydraulic fluid and the low pressure side hydraulic fluid of the supply and discharge ports to the volume chambers via inner portions of the first pistons, the port plate being formed in a hollow disk shape and comprising a plurality of valve ports whose number is equal to a number of the first pistons, the plurality of valve ports being formed at equal intervals in a circumferential direction of the port plate; piston shoes connected to the first pistons; and shoe ports formed on the piston shoes that communicate with pass through passages in the inner portions of the first pistons; wherein each of the shoe ports communicates with each of the valve ports; wherein a frictional force of the port plate with respect to the first swash plate is set to be smaller than a frictional force of the piston shoes with respect to the port plate; and wherein sizes of pressure receiving surface areas that receive the hydraulic fluid pressure are set to cause a hydraulic pressure reaction force that acts on a contact surface between the port plate and the first swash plate to become larger than a hydraulic pressure reaction force that acts on a contact surface between the piston shoes and the port plate.