Vacuum pumps

Abstract

A vacuum pump includes a pump body in which is mounted two externally threaded rotors. The thread profile of each rotor is tapered in a substantially radial direction to facilitate the machining and fitting of the rotors with the required close tolerances.

I claim:  
     
         1 . A vacuum pump incorporating a screw mechanism and comprising two externally threaded rotors mounted in a pump body and adapted for counter-rotation in the body with intermeshing of the rotor threads and with close tolerances between the threads and internal pump body surfaces in order to pump gas from a pump inlet to a pump outlet by action of the rotors and wherein the thread profile of each rotor is tapered in a substantially radial direction.  
     
     
         2 . The vacuum pump as claimed in    claim 1   , in which the tapering is effected from close to the pitch circle diameter of each rotor.  
     
     
         3 . A vacuum pump as claimed in    claim 1   , in which the angle of taper is between 1° and 6° on each side of the thread.  
     
     
         4 . The vacuum pump as claimed in    claim 3   , in which the angle of the taper is between 2° to 3° on each side of the thread.  
     
     
         5 . The vacuum pump as claimed in    claim 3   , in which the angle of taper is 2.5°.  
     
     
         6 . The vacuum pump as claimed in    claim 2   , in which the tapering is effected from inside the pitch circle diameter of each rotor.