Patent ID: 7320579

Claim:
A roots type fluid machine comprising: a housing; a pair of parallel rotary shafts rotatably supported by the housing; a two-lobe rotor disposed on each rotary shaft in a rotor chamber of the housing so that the rotors engage with each other, each of the rotors comprising: two lobe portions each having a profile of convex arc with a radius R; and two well portions each having a profile of concave arc which is an envelope of the convex arc of the lobe portion, wherein the rotor has a configuration defined by a curve which includes the convex arc, the concave arc and further an involute curve with a base radius r between the convex and concave arcs, wherein the base radius r is set in a range L/(2√{square root over ( )}2)<r<0.3(√{square root over ( )}2)L where distance between axes of the rotary shafts is L, and the radius R is set in a range {(√{square root over ( )}2)/16}πL<R<{(27−5√{square root over ( )}2)/56}L.