Bent-axis hydraulic pump motor

A bent-axis hydraulic pump motor comprising:

• a cylinder block having a center shaft at a position to be an axial center on one end surface, and also having an odd number of cylinder bores which are open along a circumference around the axial center on the one end surface;
• a retainer plate inclinably supporting the center shaft to an end surface of a drive shaft;
• and
• a constant velocity joint connecting the center shaft to the retainer plate,
• wherein, when the drive shaft and the cylinder block are rotated together via the constant velocity joint, a piston rod disposed in each of the cylinder bores is moved at a stroke in accordance with an inclination angle around an inclined rotation center between the drive shaft and the cylinder block,
• the constant velocity joint includes:
• an inner joint element formed at a support end portion of the center shaft, and having an outer surface on which a number of block-side ball grooves corresponding to a number of the cylinder bores are arranged together along a circumferential direction around an axial center of the center shaft;
• an outer joint element formed on the retainer plate at a region facing the support end portion of the center shaft, and having an inner surface on which shaft-side ball grooves respectively corresponding to the block-side ball grooves are arranged in parallel in a circumferential direction centering an axial center of the drive shaft;
• a plurality of balls interposed between the block-side ball grooves and the shaft-side ball grooves corresponding to each other, and configured to transmit torque between the cylinder block and the retainer plate;
• and
• a cage disposed between the inner joint element and the outer joint element and having ball holes configured to house the respective balls in a state in which movement of the balls in the axial direction of the center shaft is regulated, and
• all block-side groove planes, which include respective extending axial lines of the block-side ball grooves and the inclined rotation center, are inclined with respect to the axial center of the center shaft and all drive shaft-side groove planes, which include respective extending axial lines of the shaft-side ball grooves and the inclined rotation center, are inclined with respect to the axial center of the drive shaft, such that rotational moments caused by loads applied to the cage from the balls, namely, rotational moments around two axes orthogonal to an axial center of the cage and passing the inclined rotation center are balanced.