Abstract: This invention provides a power converting device capable of use as a pump and as a motor. The device includes a rotor shaped like a disc and secured to a shaft about which it rotates. Provided in the rotor are a plurality of piston chambers extending from the outer circumferential surface inwardly at an angle to a radiant from the axis. Each of the chambers opens through both side walls of the rotor, and each contains a piston. Each of the pistons is pivoted on either side to an arcuate drive segment adapted to move around an annular recessed channel within structure which defines a stationary enclosure for the rotor. There are two annular recessed channels, one on either side of the rotor, and the channels are aligned with each other and eccentrically located with respect to the axis of the rotor. With this arrangement, rotation of the rotor within the stationary enclosure is simultaneous with reciprocatory movement of the pistons in their respective piston chambers.