rotor parts the guidance 28 is covered by a cover 37 and provided with a not shown drainage channel outside the piston 29 the screw cam means of the cam rotor part 19 comprises in the shown example a single thread screw which is circularly profiled with the radius r the radius r is suitably chosen such in relation to the distance a between the rotor axes 15 and 16 that the relation a r amounts to a value between 0 2 and 0 4 the cam rotor part 19 is given a wrap angle of approximately 720 u00b0 and a lead which is defined by the condition that the length of the rotor should be 3 8 times the radius r the cam rotor part 19 forms a right hand thread if it is desired to drive the toothed wheel 24 counter clockwise the hollow rotor part 8 of the screw thread rotor 8 9 fig2 has two for glidable cooperation with the cam rotor part 19 identically formed opposite screw threads 52 53 which are half circular and joined mutually through straight flanks 54 to a hollow profile enclosed by the contour line 38 the cooperation with the cam rotor part 19 requires a play of about 0 1 mm or less between the hollow profile.