Abstract: A positive displacement device utilizing at least two rotors which roll sealingly on one another at a common pitch surface speed. Outwardly extending lobes on one rotor sealing engage the cylindrical bore of the housing of the device, fluid traversing the device circumferentially in the annulus between such bore and the pitch surface of the rotor. Speed of rotation of the two rotors is controlled by a timing chain running between sprockets mounted on the rotor shafts.When the lobe passes the line of sealing contact, a cavity is provided in the meshing rotor, and the lobe makes sealing contact with the wall of this cavity during at least that interval when there is no sealing contact between the pitch surfaces, i.e., the interval when the mouth of the cavity is passing through dead center.