Screw pump

Abstract

A screw pump has a male screw accommodation hole and a female screw accommodation hole formed in a pump housing. A male screw member and a female screw member are respectively accommodated in the male screw accommodation hole and the female screw accommodation hole. A male-side clearance is formed in a radial direction between the male screw member and the male screw accommodation hole and a female-side clearance is formed in the radial direction between the female screw member and the female screw accommodation hole. The female-side clearance is made to be larger than the male-side clearance, so that the female screw member can easily move in a direction away from the male screw member to the female-side clearance when any extraneous material is going to be wedged between the male screw member and the female screw member, to thereby prevent a locked condition of the screw members.

What is claimed is: 
     
         1 . A screw pump comprising;
 a male screw member and a female screw member, one of which forms a driving-side screw member and the other of which forms a driven-side screw member;   a driving-side convex portion of a spiral shape formed at an outer peripheral surface of the driving-side screw member, which is rotatable about a first rotational axis; and   a driven-side convex portion of a spiral shape formed at an outer peripheral surface of the driven-side screw member, which is rotatable about a second rotational axis together with the driving-side screw member,   wherein the driving-side screw member and the driven-side screw member are operatively engaged with each other and rotated together in order to pressurize working fluid from a suction port of a low pressure side and to pump out the working fluid from a discharge port of a high pressure side, and   wherein the screw pump further comprises;   (i) a pump housing;   (ii) a driving-side screw accommodation hole formed in the pump housing and movably accommodating the driving-side screw member;   (iii) a driven-side screw accommodation hole formed in the pump housing and movably accommodating the driven-side screw member; and   (iv) a lock-prevention portion for preventing a locked condition of the driving-side screw member and the driven-side screw member, which is caused by extraneous material entering the driving-side screw accommodation hole and/or the driven-side screw accommodation hole,   wherein the lock-prevention portion is formed at least in one of the following portions,   (iv-a) the driving-side screw accommodation hole,   (iv-b) the driven-side screw accommodation hole, and   (iv-c) a screw engagement portion at which the driving-side screw member and the driven-side screw member are engaged with each other.   
     
     
         2 . The screw pump according to  claim 1 , wherein
 a first clearance is formed in a radial direction of the driving-side screw accommodation hole between an outer periphery of the driving-side convex portion of the driving-side screw member and an inner peripheral wall of the driving-side screw accommodation hole,   a second clearance is formed in a radial direction of the driven-side screw accommodation hole between an outer periphery of the driven-side convex portion of the driven-side screw member and an inner peripheral wall of the driven-side screw accommodation hole, and   one of the first clearance and the second clearance is larger than the other of the first clearance and the second clearance.   
     
     
         3 . The screw pump according to  claim 2 , wherein
 the driving-side convex portions has a first tooth width in an axial direction of the driving-side screw member,   the driven-side convex portions has a second tooth width in an axial direction of the driven-side screw member,   one of the tooth widths is larger than the other tooth width.   
     
     
         4 . The screw pump according to  claim 2 , wherein
 the one of the first clearance and the second clearance, which is larger than the other, extends in a circumferential direction of the corresponding screw member from a forward-side boundary portion to a circumferential middle portion of the inner peripheral wall of the corresponding screw accommodation hole,   the forward-side boundary portion is a portion formed between the inner peripheral wall of the driving-side screw accommodation hole and the inner peripheral wall of the driven-side screw accommodation hole, and the forward-side boundary portion is located at a point of a screw engagement area between the screw members on a forward side of a rotation of the corresponding screw member,   the circumferential middle portion is a portion of the inner peripheral wall of the corresponding screw accommodation hole between the forward-side boundary portion and a backward-side boundary portion, and   the backward-side boundary portion is another portion formed between the inner peripheral wall of the driving-side screw accommodation hole and the inner peripheral wall of the driven-side screw accommodation hole, and the backward-side boundary portion is located at a point of the screw engagement area between the screw members on a backward side of the rotation of the corresponding screw member.   
     
     
         5 . The screw pump according to  claim 2 , wherein
 the one of the first clearance and the second clearance, which is larger than the other, extends in a circumferential direction of the corresponding screw member from a forward-side boundary portion and reduces in its size in a direction of a rotation of the corresponding screw member, and   the forward-side boundary portion is a portion formed between the inner peripheral wall of the driving-side screw accommodation hole and the inner peripheral wall of the driven-side screw accommodation hole, and the forward-side boundary portion is located at a point of a screw engagement area between the screw members on a forward side of a rotation of the corresponding screw member.   
     
     
         6 . The screw pump according to  claim 1 , wherein
 the lock-prevention portion is formed in the screw engagement portion of the driving-side screw member and/or the driven-side screw member, wherein the driving-side and the driven-side screw members are engaged with each other at the screw engagement portion.   
     
     
         7 . The screw pump according to  claim 6 , wherein
 the driving-side screw member is made of a male screw member having a double thread screw structure,   the driven-side screw member is made of a female screw member having a triple thread screw structure, and   a groove of the female screw member, with which the driving-side convex portion is engaged, is recessed in a radial-inward direction of the female screw member to such an extent that a radial-outward end of the driving-side convex portion is not brought into contact with such a radial-inward recessed surface of the groove, so that the groove with the radial-inward recessed surface works as the lock-prevention portion.   
     
     
         8 . The screw pump according to  claim 1 , wherein
 the lock-prevention portion includes a member made of material having rigidity lower than that of the pump housing.   
     
     
         9 . The screw pump according to  claim 6 , wherein
 the driving-side screw member is composed of the male screw member having the driving-side convex portions,   the driven-side screw member is composed of the female screw member having grooves, with each of which the driving-side convex portion is respectively engaged,   wherein each of the grooves has a groove surface recessed in a radial-inward direction of the female screw member from a reference line, so that a radial-outward end of the driving-side convex portion is not brought into contact with a bottom end of the groove surface when the driving-side screw member and the drive-side screw member are engaged with each other.   
     
     
         10 . The screw pump according to  claim 6 , wherein
 the driving-side screw member is composed of the male screw member having the driving-side convex portions,   the driven-side screw member is composed of the female screw member having grooves, with each of which the driving-side convex portion is respectively engaged,   wherein each of the grooves has a groove surface recessed in a radial-inward direction of the female screw member from a reference line in a forward-side groove area of the groove, so that a radial-outward end of the driving-side convex portion is not brought into contact with the groove surface of the forward-side groove area when the driving-side screw member and the drive-side screw member are engaged with each other.