Screw rotor

Abstract

A screw rotor for screw vacuum pumps, comprising a rotor shaft which bears at least two displacement elements. The displacement element is conical in the conveying direction and the adjoining conveying element has a cylindrical design.

What is claimed is: 
     
         1 . Screw rotor for screw vacuum pumps, comprising a rotor shaft,
 at least two displacement elements connected with the rotor shaft, each having at least one helical recess,   wherein a suction-side displacement element is designed tapering in the conveying direction,   wherein a pressure-side displacement element is designed substantially cylindrically.   
     
     
         2 . Screw rotor of  claim 1 , wherein the suction-side displacement element is designed to constantly decrease in the conveying direction. 
     
     
         3 . Screw rotor of  claim 1 , wherein the interior volume ratio is greater than 8. 
     
     
         4 . Screw rotor of  claim 1 , wherein the displacement element have substantially the same diameter at the end faces directed towards each other. 
     
     
         5 . Screw rotor of  claim 1 , wherein the diameter of the substantially cylindrical displacement element is smaller by 5-35% than the suction-side diameter of the conical displacement element. 
     
     
         6 . Screw rotor of  claim 1 , wherein the tapering displacement element has a volume ratio greater than 4. 
     
     
         7 . Screw rotor of  claim 1 , wherein the at least cylindrical displacement element has a volume ratio of 1 to 3. 
     
     
         8 . Screw rotor of  claim 1 , wherein the diameter of the tapering displacement element is 80 to 300 mm in the region of the pump inlet. 
     
     
         9 . Screw rotor of  claim 1 , wherein the diameter of the tapering displacement element is 65 to 180 mm in the transition region to the cylindrical displacement element. 
     
     
         10 . Screw rotor of  claim 1 , wherein the diameter of the substantially cylindrical displacement element is 65 to 180 mm in the region of the outlet. 
     
     
         11 . Screw rotor of  claim 1 , wherein the number of windings of the cylindrical displacement element is greater than 6. 
     
     
         12 . Screw rotor of  claim 1 , wherein the number of windings of the tapering displacement element is 3 to 6. 
     
     
         13 . Screw rotor of  claim 1 , wherein a further displacement element is provided that is arranged upstream of the tapering displacement element in the flow direction, the further displacement element being preferably substantially cylindrical in shape. 
     
     
         14 . Screw vacuum pump with a housing defining a suction chamber and two screw rotors of  claim 1  arranged in the suction chamber. 
     
     
         15 . Screw rotor of  claim 5 , wherein the diameter of the substantially cylindrical displacement element is smaller by 10-25% than the suction-side diameter of the conical displacement element. 
     
     
         16 . Screw rotor of  claim 6 , wherein the tapering displacement element has a volume ratio greater than 8. 
     
     
         17 . Screw rotor of  claim 11 , wherein the number of windings of the cylindrical displacement element is greater than 10. 
     
     
         18 . Screw rotor of  claim 17 , wherein the number of windings of the cylindrical displacement element is greater than 2. 
     
     
         19 . Screw rotor of  claim 3 , wherein the interior volume ratio is greater than 10. 
     
     
         20 . Screw rotor of  claim 19 , wherein the interior volume ratio is greater than 12.