Screw pump
A pump including at least three rotors each being provided with a generally helical screw thread, the rotors being mounted for rotation in a housing such that the screw threads of the rotors mesh and rotation of one rotor causes rotation of the other rotor, wherein the pitch of the threads is less than 1.6 times the outer diameter of the rotors or where one of the rotors has a larger outer diameter than the other rotors the outer diameter of the larger diameter rotor.
1. A pump including three rotors each being provided with exactly two generally helical screw threads, the rotors being arranged such that a central rotor is located between the other two outer rotors, and the screw threads mesh such that the rotation of one rotor causes rotation of the other rotors, wherein the outer diameter of the central rotor is larger than the outer diameter of the two outer rotors, the pitch of the threads is less than 1.2 times the outer diameter of the central rotor and is substantially constant along the entire axial length of the rotors, and the thread depth of the screw threads is less than 0.2 times the outer diameter of the central rotor.
2. A pump according to claim 1 wherein the pitch of the threads is less than the outer diameter of the central rotor.
3. A pump according to claim 2 wherein the pitch of the threads is 0.75 times the outer diameter of the central rotor.
4. A pump according to claim 1 wherein the pitch of the threads is at least 0.5 times the outer diameter of the central rotor.
5. A pump according to claim 1 wherein the thread depth of the screw threads is less than 0.175 times the outer diameter of the central rotor.
6. A pump according to claim 5 wherein the thread depth of the screw threads is less than 0.15 times the outer diameter of the central rotor.
7. A pump according to claim 6 wherein the thread depth of the screw threads is at least 0.1 times the outer diameter of the central rotor.
8. A pump according to claim 1 wherein the two generally helical screw threads on each rotor are interposed.
9. A pump according to claim 1 wherein the threads of the central rotor are generally helical ridges which extend radially outwardly of the central rotor, and the threads of each of the outer rotors are generally helical grooves which extend radially inwardly of the outer rotor.