The rotor (18) of the motor or pump has a body (19) with a cylindrical outer face and a number of rotating vanes (21) or blades installed in slots and which are movable in the axial direction relative to the primary axis (22). The chamber (17) inside the pump casing has a wall which is in engagement with the outer ends of the vanes. The chamber wall has a first section (26) with the same radius of curvature as the cylindrical outer face of the rotor and is separated from the primary axis so that there is a seal between the inlet and outlet openings. The chamber has a second wall section (27) which corresponds to the power phase of a rotational cycle of the rotor, and the radius curvature of this second section is larger than that of the first wall section, and is similarly separated from the primary axis.
