Screw rotor lobe profile for simplified screw rotor machine capacity control

A female rotor for a screw rotor machine comprising:

• an elongated formed female rotor rotatable about a central longitudinal axis and having a pitch circle centered on said axis, said rotor having an outer diameter;
• a plurality of elongated helical lobes extending longitudinally of said rotor and circumferentially spaced about said pitch circle so as to provide intervening grooves therebetween, forming addendum portions outside the pitch circle and dedendum portions inside the pitch circle;
• a major portion of each of said lobes extending generally radially inwardly of said pitch circle;
• the profile of each of said lobes in a plane perpendicular to said axis, having a tip portion and respective, generally concave groove leading and trailing flank portions extending intermediate said tip portion and a root portion of the respective adjacent grooves;
• said lobes of said female rotor engageable with grooves of a male rotor defined by corresponding helical lobes with contact between the flank portions of respective female and male rotors during rotation of one rotor relative to the other, and in mesh therewith;
• the improvement wherein the profile of the grooves of said female rotor at said female rotor lobe tip portion and said root portion are defined by involute curves which mate with respect to involute curves on the male rotor root portion and male rotor lobe tip portions, respectively, resulting in mating surfaces having low leakage path between high and low pressure volumes in the rotors when in mesh, and said lobe tips of said female rotor, each having a ramp with a high point which acts as a sealing strip between the male and female rotor lobe tips and the screw rotor machine casing thereby simplifying rotor geometry.