Abstract: A rotor adapted to rotate about a rotary shaft and intended for displacing fluids characterized in that the rotor comprises at least two ridges arranged helically around a core part so that a sectional area at right angles to the axis of rotation comprises at least two lobes with intermediate pits, the depth of the pit measured between the core part and a tangential line to the two lobes one on each side of the pit concerned being at least equal to one third of the distance between the core part and the circle described by the point of a lobe furthest remote from the axis of rotation.