the rotor 1 and with its end ending in the housing 19 is mounted by means of a bearing 6 while the end of the shaft 19 projecting out of the housing 18 is mounted by means of a bearing 20 the width of the rotor 1 is denoted by b r the rotor 1 carries at its outer periphery in known manner vanes 17 for reasons of simplicity only a few vanes are illustrated the free ends of said vanes 17 are in contact with the inner periphery of a cam ring 16 the cam ring 16 is adjustable for setting the displacement in known manner along the transverse axis 21 fig2 furthermore the cam ring 16 may also be adjustable for setting the suction instant in the direction of the axis 22 which in turn extends perpendicularly to the transverse axis 21 also for reasons of simplicity the mounting of the cam ring 16 within the housing 18 is not shown in detail a preferred mounting of the ring is described in german patent application p no 32 47 885 2 express reference is made to the latter application of which for disclosure reasons a copy is enclosed in the example of the embodiment illustrated the.