Ball Screw and Manufacturing Method of Nut for Ball Screw

Abstract

A ball screw is provided in which an outer circumferential formation is formed at a portion facing a ball circulating passage out of outer circumferential surface of a nut. The ball screw includes: a screw shaft having on its outer circumferential surface a screw groove; a nut having on its inner circumferential surface a screw groove facing the screw groove; a plurality of balls rollably loaded in a spiral ball rolling passage formed by both screw grooves; and a ball circulating passage to return the balls from a start point to an end point of the ball rolling passage for recirculation. The ball circulating passage is a concaved groove formed by concaving a groove on a part of the cylindrical inner circumferential surface of the nut by plastic working. Then, a flange is integrally provided at a portion facing the ball circulating passage and the screw groove.

1 . A ball screw comprising: a screw shaft having on an outer circumferential surface a spiral screw groove; a nut having on its inner circumferential surface a screw groove facing the screw groove of the screw shaft; a plurality of balls rollably loaded in a spiral ball rolling passage formed by the both screw grooves; and a ball circulating passage to return the balls from an end point to a start point of the ball rolling passage for circulating the balls,
 wherein the ball circulating passage is formed of a concaved groove formed by concaving a groove on a part of the cylindrical inner circumferential surface of the nut, and an outer circumferential formation is integrally formed on a part facing the concaved groove on the outer circumferential surface of the nut,   wherein the outer circumference formation is at least one of an encoder, a teeth train for a gear, a rotation stopper, a key, a key seat, a motor rotor, an inner ring of a one-way clutch, a raceway groove for a bearing, a screw, a circumferential groove, and another shape of the outer circumferential surface.   
     
     
         2 . A ball screw comprising: a screw shaft having on an outer circumferential surface a spiral screw groove; a nut having on its inner circumferential surface a screw groove facing the screw groove of the screw shaft; a plurality of balls rollably loaded in a spiral ball rolling passage formed by the both screw grooves; and a ball circulating passage to return the balls from an end point to a start point of the ball rolling passage for circulating the balls,
 wherein the ball circulating passage is formed of a concaved groove formed by concaving a groove on a part of the cylindrical inner circumferential surface of the nut, and an outer circumferential formation is integrally formed on a part facing the concaved groove on the outer circumferential surface of the nut,   wherein the concaved groove is formed on the inner circumferential surface of the nut blank by pressing the inner circumferential surface of the nut blank with a convex, by a press method using a die of the cam mechanism,   the cam mechanism comprising:   a cam driver internally inserted into a nut blank and moving in an axial direction;   a cam slider disposed between the nut blank and the cam driver, and having the convex corresponding to the concaved groove, the convex moving in a radial direction of the nut with a movement of the cam drive.   
     
     
         3 . The ball screw according to  claim 1 , wherein the nut is produced by machining a cylindrical metal blank to produce a roughly formed nut having a substantially identical shape with the nut, concaving a groove on the part of the cylindrical inner circumferential surface of the roughly formed nut, forming the concaved groove constituting the ball circulating passage, and forming the outer circumferential formation on the outer circumferential surface of the roughly formed nut. 
     
     
         4 . The ball screw according to  claim 1 , wherein the nut is produced by machining a cylindrical metal blank to produce a roughly formed nut having a substantially identical shape with the nut, by concaving a groove a part of the cylindrical inner circumferential surface of the roughly formed nut by plastic working, forming the concaved groove constituting the ball circulating passage, and forming the outer circumferential formation on the outer circumferential surface of the roughly formed nut. 
     
     
         5 . The ball screw according to  claim 1 , wherein a protrusion is provided integrally with the nut on an end face in an axial direction of the nut. 
     
     
         6 . The ball screw according to  claim 1 , further comprising a nut fitted member to be fitted by press fitting onto the outer circumferential surface of the nut. 
     
     
         7 . The ball screw according to  claim 6 , wherein an interference of the nut fitted member and the nut by press fitting exceeds 0.02 percent of an external dimension of the outer circumferential surface of the nut onto which the nut fitted member is fitted. 
     
     
         8 . The ball screw according to  claim 6 , wherein an interference of the nut fitted member and the nut by press fitting exceeds 0.02 percent and less than 0.16 percent of an external dimension of the outer circumferential surface of the nut onto which the nut fitted member is fitted. 
     
     
         9 . The ball screw according to  claim 6 , wherein when the nut fitted member is press fitted into the nut, shrink fitting is used which heats the nut fitted member to fit onto the nut. 
     
     
         10 . The ball screw according to  claim 6 , wherein the nut fitted member is any of a sleeve a bearing, a gear, each having a cylindrical inner circumferential surface, and a combination thereof. 
     
     
         11 . The ball screw according to  claim 1 , further comprising a nut fitted member provided by insert molding on the outer circumferential surface of the nut. 
     
     
         12 . A manufacturing method of ball screw nut comprising: a nut having on an inner circumferential surface a spiral groove and on an outer circumferential surface a protrusion; a screw shaft having on an outer circumferential surface a spiral groove; a plurality of balls rollably loaded between a raceway formed by the both screw grooves; and a ball returning passage formed as a concave on an inner circumferential surface of the nut, to return the balls from an end point to a start point of the raceway, the plurality of balls rolling in the raceway to cause the nut to relatively move with respect to the screw shaft,
 wherein formation of the concave on the inner circumferential surface of the nut, and formation of the protrusion on the outer circumferential surface of the nut are simultaneously performed by cold forging.   
     
     
         13 . The ball screw according to  claim 1 , wherein the concaved groove is formed on the inner circumferential surface of the nut blank by pressing the inner circumferential surface of the nut blank with a convex, by a press method using a die of the cam mechanism,
 the cam mechanism comprising:   a cam driver internally inserted into a cylindrical nut blank and moving in an axial direction;   a cam slider disposed between the nut blank and the cam driver, and having the convex corresponding to the concaved groove, the convex moving in a radial direction of the nut with a movement of the cam driver; and   a restraining member for restraining both end faces in the axial direction and the outer circumferential surface of the nut blank, and having a recess on an inner circumferential surface that receives the outer circumferential surface of the nut blank,   wherein the outer circumferential formation is formed by projecting a periphery of the nut blank into the recess of the restraining member.   
     
     
         14 . The ball screw according to  claim 13 , wherein a working in a subsequent process is performed after a process in the press method using the die, by using the outer circumferential formation, a recess or a tapered surface formed on the inner circumferential surface of the outer circumferential formation, as a reference surface or a holding part. 
     
     
         15 . The ball screw according to  claim 13 , wherein the outer circumferential formation is a torque transmission part, a positing part, or a mounting part. 
     
     
         16 . A ball screw comprising: a screw shaft having on an outer circumferential surface a spiral screw groove; a nut having on an inner circumferential surface a screw groove facing the screw groove of the screw shaft; a plurality of balls rollably loaded in a spiral ball rolling passage formed by the both screw grooves; and a ball circulating passage to return the ball from an end point to a start point of the ball rolling passage for circulating the balls,
 wherein the ball circulating passage is formed of a concaved groove on a part of the cylindrical inner circumferential surface of the nut, and a protrusion is provided on an end face in an axial direction of the nut.   
     
     
         17 . The ball screw according to  claim 16 , wherein the concaved groove is formed on the inner circumferential surface of the nut blank by pressing the inner circumferential surface of the nut blank with a convex, by a press method using a die of a cam mechanism,
 the cam mechanism comprising:   a cam driver internally inserted into a cylindrical nut blank and moving in an axial direction;   a cam slider disposed between the nut blank and the cam driver, and having the convex corresponding to the concaved groove, the convex moving in a radial direction of the nut with a movement of the cam driver; and   a restraining member for restraining both end faces in the axial direction and the outer circumferential surface of the nut blank, and having a recess on an end face to receive the both end faces in the axial direction of the nut blank,   wherein the protrusion is formed by projecting an excess material, produced with the formation of the concaved groove, of the nut blank into a recess of the restraining member.   
     
     
         18 . The ball screw according to  claim 13 , further comprising a nut fitted member provided by insert molding on the outer circumferential surface of the nut, the nut fitted member being formed to cover the outer circumferential formation on the outer circumferential surface of the nut produced with the formation of the concaved groove. 
     
     
         19 . The ball screw according to  claim 11 , wherein the nut fitted member is either of or both of the sleeve or a gear having a cylindrical inner circumferential surface. 
     
     
         20 . The ball screw according to  claim 1 , wherein the screw groove of the nut is provided only at a part where the balls rotate on the inner circumferential surface of the nut, and is not provided at a part where the balls do not rotate. 
     
     
         21 . The ball screw according to  claim 5 , wherein the protrusion is provided at a part in the circumferential direction of the end face in an axial direction of the nut. 
     
     
         22 . The ball screw according to  claim 2 , further comprising a nut fitted member provided by insert molding on the outer circumferential surface of the nut. 
     
     
         23 . The ball screw according to  claim 2 , further comprising a nut fitted member to be fitted by press fitting onto the outer circumferential surface of the nut. 
     
     
         24 . The ball screw according to  claim 23 , wherein an interference of the nut fitted member and the nut by press fitting exceeds 0.02 percent of an external dimension of the outer circumferential surface of the nut onto which the nut fitted member is fitted. 
     
     
         25 . The ball screw according to  claim 23 , wherein an interference of the nut fitted member and the nut by press fitting exceeds 0.02 percent and less than 0.16 percent of an external dimension of the outer circumferential surface of the nut onto which the nut fitted member is fitted. 
     
     
         26 . The ball screw according to  claim 23 , wherein when the nut fitted member is press fitted into the nut, shrink fitting is used which heats the nut fitted member to fit onto the nut. 
     
     
         27 . The ball screw according to  claim 23 , wherein the nut fitted member is any of a sleeve a bearing, a gear, each having a cylindrical inner circumferential surface, and a combination thereof.