Abstract: A hydraulic device which is primarily intended for use as a motor, includes an annular stator having an internal cam surface consisting of a plurality of outwardly and inwardly extending reaction surfaces, and a rotor disposed within said stator having a plurality of open-ended fluid pressure chambers with a rolling element therein engaging the outwardly and inwardly extending reaction surfaces and extending the width of the rotor. The rotor is mounted on a shaft and when operating as a motor is rotated by the rolling elements reacting on the outwardly extending reaction surfaces to rotate the shaft. Fluid inlet and outlet passages are disposed in a port plate disposed along the side of the rotor and communicate with the fluid pressure chambers through one of their open ends.