Abstract: In a sliding vane machine a work can be accomplished by a pressure fluid rotating a rotor provided with a plurality of sliding vanes, which function as turbine blades, whereby the work is taken out as a rotational motion at the rotor shaft. If the machine is designed as a pump the shaft is rotated and the vanes act as impeller blades.In both cases it is desirous if the machine can adapt its work in response to varying loads.In accordance with the invention this is achieved by providing the machine with pressure plates controlling the effective working area of said vanes, the pressure plates thereby being adjusted by a fluid pressure which by means of a pressure actuated valve can be varied the pressure-actuated valve itself being controlled by the fluid pressure in a conduit supplying the pressure fluid to the machine.