Screw pump

Abstract

A first rotor and a second rotor have helical teeth each including a dedendum portion, an addendum portion, and an addendum tip portion. Profiles of the addendum portions of the first and second rotors are formed by cycloidal curves. Profiles of the dedendum portions of the first and second rotors are formed by trochoidal curves. A pitch circle of the second rotor is larger than that of the first rotor. The number of teeth of the second rotor is larger than that of the first rotor. A width angle of the first rotor is not less than a minimum angle at which the addendum tip portions of the first rotor are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is approximately equal to that at the minimum angle.

1 . A screw pump comprising:
 a housing in which an inlet and an outlet are formed;   a first rotor having helical teeth each including a first dedendum portion, a first addendum portion, and a first addendum tip portion in contact with the housing, and housed in a rotatable state in the housing; and   a second rotor having helical teeth each including a second dedendum portion, a second addendum portion, and a second addendum tip portion in contact with the housing and meshing with the teeth of the first rotor, and housed in a synchronously rotatable state with the first rotor in the housing,   wherein profiles of the first and second addendum portions are formed by cycloidal curves and profiles of the first and second dedendum portions are formed by trochoidal curves,   wherein a pitch circle of the second rotor is larger than a pitch circle of the first rotor,   wherein the number of teeth of the second rotor is larger than the number of teeth of the first rotor, and   wherein a width angle of the first rotor is not less than a minimum angle at which the first addendum tip portions are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is substantially equal to that at the minimum angle.   
     
     
         2 . The screw pump according to  claim 1 ,
 wherein on the pitch circles where the teeth of the first rotor and the teeth of the second rotor mesh with each other, the cycloidal curve of the first addendum portion varies to the trochoidal curve of the first dedendum portion and the cycloidal curve of the second addendum portion varies to the trochoidal curve of the second dedendum portion.   
     
     
         3 . The screw pump according to  claim 1 ,
 wherein the width angle of the first rotor is within 4° from an angle at which the discharge volume ratio is a maximum.   
     
     
         4 . The screw pump according to  claim 1 ,
 wherein the width angle of the first rotor is not less than the minimum angle and not more than an angle 9° larger than the minimum angle.   
     
     
         5 . The screw pump according to  claim 1 ,
 wherein a ratio of an interval between a rotation axis of the first rotor and a rotation axis of the second rotor to a diameter of an outer circle of the first rotor is in the range of not less than a minimum of an establishment limit and not more than a value 0.02 larger than the minimum.   
     
     
         6 . The screw pump according to  claim 1 ,
 wherein the first rotor has three teeth, and   wherein the second rotor has four teeth.   
     
     
         7 . A screw pump comprising:
 a housing in which an inlet and an outlet are formed;   a first rotor having helical teeth each including a first dedendum portion, a first addendum portion, and a first addendum tip portion in contact with the housing, and housed in a rotatable state in the housing; and   a second rotor having helical teeth each including a second dedendum portion, a second addendum portion, and a second addendum tip portion in contact with the housing and meshing with the teeth of the first rotor, and housed in a synchronously rotatable state with the first rotor in the housing,   wherein profiles of the first addendum portions are formed by cycloidal curves and profiles of the first dedendum portions are formed by involute curves,   wherein profiles of the second addendum portions are formed by involute curves and profiles of the second dedendum portions are formed by trochoidal curves,   wherein a pitch circle of the second rotor is larger than a pitch circle of the first rotor,   wherein the number of teeth of the second rotor is larger than the number of teeth of the first rotor, and   wherein a width angle of the first rotor is not less than a minimum angle at which the first addendum tip portions are in line contact with an outer circle of the first rotor, and not more than an angle at which a discharge volume ratio is substantially equal to that at the minimum angle.   
     
     
         8 . The screw pump according to  claim 7 ,
 wherein on the pitch circles where the teeth of the first rotor and the teeth of the second rotor mesh with each other, the cycloidal curve of the first addendum portion varies to the involute curve of the first dedendum portion and the involute curve of the second addendum portion varies to the trochoidal curve of the second dedendum portion.   
     
     
         9 . The screw pump according to  claim 7 ,
 wherein the width angle of the first rotor is within 4° from an angle at which the discharge volume ratio is a maximum.   
     
     
         10 . The screw pump according to  claim 7 ,
 wherein the width angle of the first rotor is not less than the minimum angle and not more than an angle 9° larger than the minimum angle.   
     
     
         11 . The screw pump according to  claim 7 ,
 wherein a ratio of an interval between a rotation axis of the first rotor and a rotation axis of the second rotor to a diameter of an outer circle of the first rotor is in the range of not less than a minimum of an establishment limit and not more than a value 0.02 larger than the minimum.   
     
     
         12 . The screw pump according to  claim 7 ,
 wherein the first rotor has four teeth, and   wherein the second rotor has six teeth.