Patent Publication Number: US-2017350482-A1

Title: Ball Screw Device, Steering System Using Ball Screw Device, and Method for Producing Retainer of Ball Screw Device

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2016-110461 filed on Jun. 1, 2016 including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ball screw device, a steering system using the ball screw device, and a method for producing a retainer of the ball screw device. 
     2. Description of Related Art 
     In related arts, ball screw devices as described in Japanese Patent No. 5120040 (JP 5120040), Japanese Utility Model Application Publication No. 2-5145, Japanese Patent Application Publication No. 6-288458 are known, in each of which a cage (retainer) is disposed between a ball screw shaft and a ball nut. The retainer has a plurality of elongated holes (retainer grooves) formed in a cylindrical portion thereof and configured to retain rolling balls in a rollable manner. These retainer grooves are formed, in the cylindrical portion, with wall portions each of which is positioned between adjacent retainer grooves in a circumferential direction, and the balls are retained by the retainer grooves. By this configuration, in the respective retainer grooves, balls adjacent in a raceway are retained while being reliably separated from each other by the width of each wall portion, which eliminates the possibility that the adjacent rolling balls come into contact with each other, and thus increase of running torque due to sliding resistance can be prevented. 
     In the related art described above, in order to produce the retainer at low cost, it is often the case that a thin metal plate material is stamped out, bending such as forming is performed on the stamped-out workpiece to roll up the workpiece in a cylindrical shape, and both end portions of the workpiece thus rolled up are joined to each other by welding, for example, to form the retainer. However, when the retainer is formed by such a production method, a situation may occur in which temperature around the welded portions of the retainer locally increases due to the influence of heat during welding, whereby the retainer grooves formed in the cylindrical portion of the retainer are distorted, and a desired shape cannot be maintained. In particular, when the shapes of both side surfaces of each retainer groove are accurately formed as described in JP 5120040 such that the retainer groove allows movement of the balls toward the radial outside of the retainer and restricts movement of the balls toward the radial inside of the retainer, if distortion increases, the functions described above cannot be maintained. In other words, the yield of retainers during production may decrease, and accordingly the cost may increase. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a ball screw device including a low-cost retainer that is disposed between a ball screw shaft and a ball nut and has a flange portion, a steering system using the ball screw device, and a method for producing the retainer of the ball screw device. 
     A ball screw device according to one aspect of the present invention includes: a ball screw shaft having an outer-peripheral surface on which an outer-peripheral ball rolling groove is spirally formed; a ball nut having an inner-peripheral surface on which an inner-peripheral ball rolling groove is spirally formed; rolling balls arranged between the outer-peripheral ball rolling groove and the inner-peripheral ball rolling groove in a circulatable manner; and a retainer disposed between an outer periphery of the ball screw shaft and an inner periphery of the ball nut, and having a cylindrical portion with a retainer groove that retains the rolling balls. Both side surfaces of the retainer groove are formed so as to allow movement of the rolling balls toward a radial outside of the retainer and to restrict movement of the rolling balls toward a radial inside of the retainer. The retainer has a weld joint at both of opposite ends of the retainer groove in an axial direction of the retainer. 
     As described above, the retainer has the weld joints at the opposite ends of the retainer groove in the axial direction of the retainer. In other words, the weld joints are formed in small areas at the opposite ends of the retainer groove, instead of being formed along, for example, all the length of the retainer in the axial direction. Thus, the weld joints can be joined to each other with a configuration of butt joint, for example, in a shorter time than in the case that long weld joints are joined to each other. This can suppress temperature increase around the welded portions of the retainer, and thus deformation of the retainer groove caused by the welding can be satisfactorily suppressed. Thus, the yield of retainers during production can be increased, and the cost of each retainer and consequently the cost of the ball screw device using the retainer can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a schematic diagram illustrating an entire configuration of a steering system including a ball screw device according to an embodiment; 
         FIG. 2  is a sectional view illustrating a configuration of the ball screw device in  FIG. 1 ; 
         FIG. 3  is a sectional view of a cylindrical portion of a retainer, taken in a plane orthogonal to the axis of the retainer; 
         FIG. 4  is a diagram illustrating a state of contact between the retainer and a rolling ball; 
         FIG. 5A  is a diagram of the retainer according to a first embodiment when viewed from a flange side in the axial direction; 
         FIG. 5B  is a diagram of the retainer in  FIG. 5A  when viewed from below; 
         FIG. 6  is a sectional view taken along the line VI-VI of  FIG. 5B  as viewed in the direction indicated by the arrows; 
         FIG. 7  is a diagram of a retainer according to another aspect, corresponding to  FIG. 5A ; 
         FIG. 8  is a flowchart of a method for producing the retainer; 
         FIG. 9  is a diagram illustrating a shape of a first workpiece of the retainer according to the first embodiment; 
         FIG. 10  is a diagram illustrating a workpiece flange portion of the first workpiece of the retainer according to the first embodiment when being bent; 
         FIG. 11A  is a diagram for explaining a first die-molding step for forming a second workpiece of the retainer according to the first embodiment; 
         FIG. 11B  is a diagram for explaining a second die-molding step for forming the second workpiece of the retainer according to the first embodiment; 
         FIG. 11C  is a diagram for explaining a third die-molding step for forming the second workpiece of the retainer according to the first embodiment; 
         FIG. 11D  is a diagram for explaining a fourth die-molding step for forming the second workpiece of the retainer according to the first embodiment; 
         FIG. 12A  is a diagram of a retainer according to a second embodiment, corresponding to  FIG. 5A ; 
         FIG. 12B  is a diagram of the retainer in  FIG. 12A  when viewed from below; 
         FIG. 13A  is a diagram of a retainer according to a third embodiment, corresponding to  FIG. 5A ; 
         FIG. 13B  is a diagram of the retainer in  FIG. 13A  when viewed from below; and 
         FIG. 14  is a diagram illustrating a notch according to another aspect formed in a flange portion of the retainer, corresponding to  FIG. 5A . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A first embodiment of the present invention will now be described with reference to the drawings.  FIG. 1  is a diagram of an entire configuration of an electric power steering system, illustrating a mode in which a ball screw device according to the present invention is used in the electric power steering system (corresponding to a steering system) of a vehicle. 
     The electric power steering system is a steering system that assists steering steering operation shaftforce with steering assist force. The ball screw device of the present invention may be used in various systems in which the ball screw device can be used, such as a four-wheel steering system, a rear-wheel steering system, a steer-by-wire steering system in addition to the electric power steering system. 
     The following describes a configuration of this steering system  10 . The electric power steering system  10  (hereinafter simply called “steering system  10 ”) is a system that changes the orientation of steered wheels (not depicted) of a vehicle by moving a steering operation shaft  19  coupled to the steered wheels in a reciprocating manner in an axial direction D (lateral direction in  FIG. 1 ). 
     As depicted in  FIG. 1 , the steering system  10  includes a housing  11 , a steering wheel  12 , a steering shaft  13 , a torque detection device  14 , an electric motor M (hereinafter called “motor M”), the steering operation shaft  19 , a steering assist mechanism  30 , and a ball screw device  40 . 
     The housing  11  is a fixed member that is fixed to the vehicle. The housing  11  is formed in a tubular shape through which the steering operation shaft  19  is disposed so as to be relatively movable in the axial direction D. The housing  11  includes a first housing  11   a  and a second housing  11   b  that is fixed to one end (left side in  FIG. 1 ) of the first housing  11   a  in the axial direction D. 
     The steering wheel  12  is fixed to an end portion of the steering shaft  13 , and is rotatably supported in a passenger compartment. The steering shaft  13  transmits torque applied to the steering wheel  12  by operation of a driver to the steering operation shaft  19 . 
     On an end portion of the steering shaft  13  on the steering operation shaft  19  side, a pinion  13   a  that is a component of a rack-and-pinion mechanism is formed. The torque detection device  14  detects torque applied to the steering shaft  13  on the basis of the amount of torsion in the steering shaft  13 . 
     The steering operation shaft  19  extends in the axial direction D. The steering operation shaft  19  is supported by the housing  11  so as to be linearly movable in a reciprocating manner along the axial direction D. On the steering operation shaft  19 , a rack  22  is formed. The rack  22  meshes with the pinion  13   a  of the steering shaft  13 , and constitutes the rack-and-pinion mechanism together with the pinion  13   a . For the rack-and-pinion mechanism, depending on the application purpose, for example, of the steering system  10 , the maximum axial force that can be transmitted between the steering shaft  13  and the steering operation shaft  19  is set. 
     On the steering operation shaft  19 , a ball screw portion  23  (corresponding to the ball screw shaft  20 ) is formed in a position different from that of the rack  22 . The ball screw portion  23  constitutes the ball screw device  40  together with a ball nut  21  described later, and steering assist force is transmitted thereto by the steering assist mechanism  30 . Both ends of the steering operation shaft  19  are coupled to right and left steered wheels (not depicted) via tie rods and knuckle arms (not depicted), for example. The steered wheels are steered right and left by axial movement of the steering operation shaft  19 . 
     The steering assist mechanism  30  is a mechanism that uses the motor M as a driving source to apply steering assist force to the steering operation shaft  19 . The steering assist mechanism  30  includes the motor M, a control unit ECU for driving the motor M, and a driving-force transmission mechanism  32 . The motor M and the control unit ECU for driving the motor M are housed in a case  31  that is fixed to the first housing  11   a  of the housing  11 . The control unit ECU determines a steering assist torque based on an output signal of the torque detection device  14  to control the output of the motor M. 
     As depicted in  FIG. 2 , the driving-force transmission mechanism  32  includes a drive pulley  36 , a driven pulley  34 , and a toothed belt  35 . The drive pulley  36  is attached to an output shaft  37  of the motor M. The output shaft  37  is disposed parallel to the axis of the steering operation shaft  19 . The driven pulley  34  is disposed on an outer periphery of the ball nut  21  so as to be rotatable integrally with the ball nut  21 . The driven pulley  34  on one end side (left side in  FIG. 2 ) in the axial direction is rotatably supported by an inner peripheral surface  11   b   1  of the second housing  11   b  with a ball bearing (not depicted) interposed therebetween. The toothed belt  35  is wound around the drive pulley  36  and the driven pulley  34 . The driving-force transmission mechanism  32  transmits rotational driving force generated by the motor M with the toothed belt  35  between the drive pulley  36  and the driven pulley  34 . 
     The following describes a configuration of the ball screw device  40 . As depicted in  FIG. 2 , the ball screw device  40  includes the ball screw portion  23  of the steering operation shaft  19 , the ball nut  21 , a plurality of rolling balls  24 , deflectors  25 , a retainer  27 , and a wall member  29 . The ball screw portion  23  of the steering operation shaft  19  has an outer-peripheral ball rolling groove  20   a  spirally formed on its outer-peripheral surface. The ball screw device  40  is housed mainly in the second housing  11   b.    
     The ball nut  21  is disposed on the outer peripheral side of the ball screw portion  23 . The inner peripheral surface of the ball nut  21  has an inner-peripheral ball rolling groove  21   a  that is spirally formed. The rolling balls  24  are arranged so as to roll in a ball track formed between the outer-peripheral ball rolling groove  20   a  of the ball screw portion  23  and the inner-peripheral ball rolling groove  21   a  of the ball nut  21  and be circulatable. The deflectors  25  are members configured to circulate the rolling balls  24  between an adjacent pair of the ball rolling grooves  20   a  and  21   a , and are provided in plurality on the circumference of the ball nut  21 . 
     The wall member  29  is attached to an end surface  21   d  of the ball nut  21 , and has an end surface  29   a  that faces the end surface  21   d  of the ball nut  21  with a clearance therebetween. The size of the clearance between the end surface  21   d  and the end surface  29   a  is a size that allows a flange portion  27   b  of the retainer  27  described later to be inserted therein. The wall member  29  may be attached to any position as long as the wall member has the end surface  29   a  that faces the end surface  21   d  of the ball nut  21  with the clearance therebetween. For example, the wall member  29  may be attached to an end surface of the driven pulley  34 . Alternatively, the wall member  29  may be attached to part of the second housing  11   b . Still alternatively, the wall member may be formed by part of the second housing  11   b.    
     The retainer  27  has a cylindrical portion  27   a  having a thin cylindrical shape, a flange portion  27   b  that is formed on an end surface of one end (left side in  FIG. 2 ) of the cylindrical portion  27   a  and that can be in contact with the end surface  21   d  of the ball nut  21 , and butt joints  27   c   1  and  27   c   2  (corresponding to weld joints). The cylindrical portion  27   a  is disposed between the outer periphery  20   b  of the ball screw shaft  20  and the inner periphery  21   b  of the ball nut  21 . The retainer  27  also has, on the circumference of the cylindrical portion  27   a , a plurality of retainer grooves  26  configured to retain the rolling balls  24 . 
     As depicted in  FIG. 2 , the retainer grooves  26  each have an elongated-hole shape that extends in the axial direction D of the steering operation shaft  19  (ball screw shaft), and are formed so as to be arranged at regular angular intervals (at a constant pitch) on the circumference of the cylindrical portion  27   a . Herein, separating portions  28  each of which separates circumferentially adjacent retainer grooves  26  in the cylindrical portion  27   a  each have a width dimension that is sufficiently smaller than the diameter dimension of the rolling balls  24 . This enables a sufficient number of rolling balls  24  to be arranged in the cylindrical portion  27   a  of the retainer  27  so as to satisfy the load-carrying capacity of the ball screw device  40 . 
     Each retainer groove  26  is inclined at a predetermined angle with respect to the axis of the ball screw shaft  20  (the axis of the retainer  27 ) so as to be orthogonal to the respective ball rolling grooves  20   a  and  21   a  of the ball screw shaft  20  and the ball nut  21 . In other words, each retainer groove  26  is formed so as to be inclined by the lead angle of the ball rolling grooves  20   a  and  21   a  and be orthogonal to the respective ball rolling grooves  20   a  and  21   a . However, the present invention is not limited to this, and the retainer groove  26  may be formed so as to be parallel to the axis of the ball screw shaft  20 . 
     As depicted in a cross-section orthogonal to the axis of the retainer  27  in  FIG. 3 , both side surfaces of each retainer groove  26  are formed by inclined surfaces. Specifically, both side surfaces are formed by inclined surfaces  26   a  and  26   b  each of which is inclined by a predetermined angle θ such that the groove width therebetween increases toward the radial outside of the cylindrical portion  27   a . In other words, the cross-sectional shape of the retainer groove  26  is a downward-widening shape formed by the inclined surfaces  26   a  and  26   b . As depicted in  FIG. 4 , with the inclined surfaces  26   a  and  26   b , the groove width of the retainer groove  26  is formed so as to be smaller than the diameter dimension B of the rolling balls  24  in the inner periphery of the retainer  27  and be greater than the diameter dimension B of the rolling balls  24  in the outer periphery of the retainer  27 . In other words, when the groove width in the inner periphery of the retainer  27  is a groove width A and the groove width in the outer periphery of the retainer  27  is a groove width C, a relationship of A&lt;B&lt;C is satisfied. 
     Thus, with the inclined surfaces  26   a  and  26   b  (both side surfaces) of the retainer grooves  26 , the retainer  27  allows movement of the rolling balls  24  toward the radial outside of the retainer  27  and restricts movement of the rolling balls  24  toward the radial inside of the retainer  27 . Consequently, as depicted in  FIG. 4 , the inclined surfaces  26   a  and  26   b  of the retainer groove  26  positioned at the bottom part in contact with the rolling balls  24  rolling between the ball screw shaft  20  and the ball nut  21 , whereby radial (downward in  FIG. 4 ) movement of the retainer  27  is restricted, and thus the retainer  27  is prevented from coming into contact with the outer periphery  20   b  of the ball screw shaft  20  or the inner periphery  21   b  of the ball nut  21 . 
     Thus, even if the clearance between the outer periphery (surface)  20   b  of the ball screw shaft  20  and the inner periphery (surface)  21   b  of the ball nut  21  is small, contact of the retainer  27  with the ball screw shaft  20  or with the ball nut  21  and resultant wear therebetween caused by the radial movement of the retainer  27  can be prevented, and increase of friction and generation of noises due to the contact of the retainer  27  with the ball screw shaft  20  or with the ball nut  21  can be suppressed. As described above, the retainer grooves  26  have a function of accurately controlling and maintaining the relative position between the rolling balls  24  and the retainer  27 . 
     As depicted in  FIG. 5A  and  FIG. 5B , the flange portion  27   b  is formed in an annular shape, and has a notch  27   d  formed in part thereof in the circumferential direction. As depicted in  FIG. 2 , the flange portion  27   b  is disposed between the end surface  29   a  of the wall member  29  and the end surface  21   d  of the ball nut  21 , whereby axial movement of the retainer  27  is restricted. 
     As depicted in  FIG. 5B , on axially opposite ends of one retainer groove  26  among the retainer grooves  26 , butt joints  27   c   1  and  27   c   2  (weld joints) are formed. The butt joints  27   c   1  and  27   c   2  are formed in the cylindrical portion  27   a  at both ends of this retainer groove  26  in the axial direction of the retainer  27 . The butt joints  27   c   1  and  27   c   2  are formed on a central axis C 1  of the retainer groove  26 . The butt joints  27   c   1  and  27   c   2  are joint portions on the cylindrical portion  27   a  side and on the flange portion  27   b  side, respectively. By this design in which the joint portions are formed in the positions described above, the retainer  27  can be produced accurately at low cost. Detailed reasons for this will be described later. 
     As depicted in  FIG. 6 , the respective joint lengths (weld lengths) L 1  and L 2  of the butt joints  27   c   1  and  27   c   2  at the opposite ends of the retainer groove  26  are the same (L 1 =L 2 ). Herein, the respective lengths of the butt joints  27   c   1  and  27   c   2  are the same as the respective joint lengths (weld lengths). In other words, the respective joint lengths extend along the respective entire lengths of the butt joints  27   c   1  and  27   c   2 . 
     The notch  27   d  is formed at the same position as that of the butt joints  27   c   1  in the circumferential direction of the flange portion  27   b  (see  FIG. 5A ). In other words, the butt joints  27   c   1  on the flange portion  27   b  side in the cylindrical portion  27   a  are interrupted by the notch  27   d  formed in the flange portion  27   b , and are formed only in the cylindrical portion  27   a . In the present embodiment, in order to adjust the length of the butt joints  27   c   1 , the notch  27   d  is formed not only in the flange portion  27   b , but is also partially formed in the cylindrical portion  27   a  (see  FIG. 5B ). 
     The present embodiment has been described in which the flange portion  27   b  does not have a butt joint, and has only the notch  27   d . However, the present invention is not limited to this. According to another aspect, the flange portion  27   b  may have both of butt joints  27   c   3  and the notch  27   d  as depicted in  FIG. 7 . In this case, the length of the length L 1  of the butt joints  27   c   1  of the cylindrical portion  27   a  on the flange portion  27   b  side is preferably set shorter such that the total length (L 1 +L 3 ) of the length L 1  and the length L 3  of the butt joints  27   c   3  of the flange portion  27   b  is equal to the length L 2  of the butt joints  27   c   2  of cylindrical portion  27   a  on the side where the flange portion  27   b  is not positioned (on the opposite side from the flange portion  27   b ). 
     The following describes a method for producing the retainer  27  with reference to the flowchart of  FIG. 8 ,  FIG. 9 ,  FIG. 10 , and  FIG. 13A  to  FIG. 13D . The method for producing the retainer  27  according to the present invention includes a step S 10  to a step S 30  (see  FIG. 8 ). 
     At the step S 10 , as depicted in  FIG. 9 , a thin flat plate made of ferrous material, for example, is stamped out to form a first workpiece  27 A that is a plate member having a plurality of workpiece grooves  26 A that pass therethrough in the plate thickness direction and correspond to the retainer grooves  26 , and having workpiece edges  27 C 1  and  27 C 2  that correspond to the butt joints  27   c   1  and  27   c   2  (corresponding to weld joints) at both ends (side surfaces) of the plate member. The term “workpiece edges” herein is a designation of joints before being welded together in the first workpiece  27 A, corresponding to the butt joints  27   c   1  and  27   c   2  that are welded joints. 
     The first workpiece  27 A includes a workpiece cylindrical portion  27 AA (corresponding to a first plate portion) that corresponds to the cylindrical portion  27   a  of the retainer  27  and a workpiece flange portion  27 BB (corresponding to a second plate portion) that corresponds to the flange portion  27   b  of the retainer  27 . The workpiece grooves  26 A corresponding to the retainer grooves  26  are formed in the workpiece cylindrical portion  27 AA. The workpiece cylindrical portion  27 AA is formed below a bend line F (long dashed double-short dashed line) of the first workpiece  27 A. 
     At this time, both side surfaces of each workpiece groove  26 A are formed so that the shapes of both side surfaces (inclined surfaces  26   a  and  26   b ) of the corresponding retainer groove  26  can be obtained when the first workpiece  27 A is rolled up to form a cylinder having the same shape as that of the cylindrical portion  27   a  at the step S 20  described below. The workpiece flange portion  27 BB is formed above the bend line F of the first workpiece  27 A. 
     In other words, at the step S 10 , the first workpiece  27 A is formed by pressing, which has a shape obtained by cutting the cylindrical portion  27   a  and the flange portion  27   b  of the retainer  27  at one location along the axial direction of the retainer  27  and unrolling the resultant shape in a plane. In the present embodiment, as depicted in  FIG. 5B  and  FIG. 9 , the cutting location includes both ends of the retainer groove  26  (workpiece groove  26 A) in the axial direction of the retainer  27 , and extends on the central axis C 1  of the retainer groove  26  (workpiece groove  26 A). 
     Thus, the respective workpiece edges  27 C 1 ,  27 C 1 ,  27 C 2 , and  27 C 2  are formed that are cut surfaces corresponding to the respective butt joints  27   c   1 ,  27   c   1 ,  27   c   2 , and  27   c   2  on both sides of the plate member thus cut. Because the respective workpiece edges  27 C 1 ,  27 C 1 ,  27 C 2 , and  27 C 2  are formed as described above, the workpiece edges  27 C 1  and  27 C 2  on one end side (left side in  FIG. 9 ) and the workpiece edges  27 C 1  and  27 C 2  on the other side (right side in  FIG. 9 ) are parallel to each other. The length L 1  of the workpiece edges  27 C 1  and  27 C 1  and the length L 2  of the workpiece edges  27 C 2  and  27 C 2  are the same (L 1 =L 2 ). 
     At the step S 20 , the workpiece flange portion  27 BB (second plate portion) of the first workpiece  27 A is bent at the bend line F (see  FIG. 10 ). At this time, both end surfaces (laterally both end surfaces in  FIG. 9 ) of the workpiece flange portion  27 BB are formed so as to be parallel to the axis of the cylindrical portion  27   a  when the cylindrical portion  27   a  is formed at the step S 30 . 
     Subsequently, at the step S 20 , forming is performed on the first workpiece  27 A in which the workpiece flange portion  27 BB has been bent, whereby a second workpiece  27 B is formed in which the workpiece cylindrical portion  27 AA corresponding to the cylindrical portion  27   a  and the workpiece flange portion  27 BB corresponding to the flange portion  27   b  have the same shapes as those of the cylindrical portion  27   a  and the flange portion  27   b , respectively. In other words, at the step S 20 , the workpiece cylindrical portion  27 AA corresponding to the cylindrical portion  27   a  is rolled up in a cylindrical shape, and accordingly the workpiece flange portion  27 BB that has been bent at the bend line F is formed in a flange shape. 
     Herein, a method of forming the second workpiece  27 B with cylinder-forming dies  51  to  56  at step S 20  will be described more specifically. As depicted in  FIG. 11A  to  FIG. 11D , the first workpiece  27 A in which the workpiece flange portion  27 BB has been bent is wound around the outer-peripheral surface of a core member  50  (see  FIG. 11A ) having a shape of cylindrical column with its outside diameter being the same as the bore diameter of the cylindrical portion  27   a  to form the second workpiece  27 B. At this time, when the first workpiece  27 A is rolled up by the six cylinder-forming dies  51  to  56  surrounding the core member  50 , the bent workpiece flange portion  27 BB is moved into grooves formed in the respective cylinder-forming dies  51  to  56 , and is thus supported by the respective cylinder-forming dies  51  to  56 . 
     To begin with, as depicted in  FIG. 11A , the first workpiece  27 A with the bent workpiece flange portion  27 BB is placed upon the core member  50 , and the first workpiece  27 A is pressed from above by the first cylinder-forming die  51  (first die-molding step). Subsequently, as depicted in  FIG. 11B , the second and third cylinder-forming dies  52  and  53  that are respectively positioned on the upper-left side and the upper-right side are moved in the arrow directions, whereby the upper hemispherical shape of the cylindrical portion  27   a  depicted in  FIG. 11B  is formed (second die-molding step). 
     Subsequently, as depicted in  FIG. 11C , the fourth and fifth cylinder-forming dies  54  and  55  that are positioned on the lower-left side and the lower-right side are moved in the arrow directions, whereby the rough shape of the lower hemisphere of the cylindrical portion  27   a  is formed (third die-molding step). Finally, as depicted in  FIG. 11D , the sixth cylinder-forming die  56  is pressed against the contact portions at circumferential ends of the cylindrical portion  27   a , whereby the second workpiece  27 B is formed (fourth die-molding step). 
     At this time, in the second workpiece  27 B, between both end surfaces of the workpiece flange portion  27 BB, the notch  27   d  having a fan shape is formed. This is because the circumference of a radially outer portion of the flange portion  27   b  having an annular shape is longer than the circumference of a radially inner portion thereof. Thus, both end surfaces of the workpiece flange portion  27 BB are not in contact with each other, and thus are insulated from each other. 
     At the step S 30 , at the circumferential ends of the second workpiece  27 B, the workpiece edges  27 C 1  and  27 C 1  are joined to each other and the workpiece edges  27 C 2  and  27 C 2  are joined to each other by resistance welding, and thus the retainer  27  depicted in  FIG. 5A  and  FIG. 5B  is formed. The resistance welding is a known technique in which a current is caused to flow between joints of members the surfaces of which are in contact with each other, and heat depending on the electrical resistance between the joints is generated between the joints, whereby the temperature of the joints is raised, and thus the joints are melted to be welded together. Thus, detailed description thereof is omitted. 
     At the step S 30 , both end surfaces of the workpiece flange portion  27 BB face each other while being insulated from each other with the notch  27   d  therebetween as described above. The length L 1  of the workpiece edges  27 C 1  and  27 C 1  and the length L 2  of the workpiece edges  27 C 2  and  27 C 2  are the same. Thus, when a current is caused to flow between the workpiece edges  27 C 1  and  27 C 1  and between the workpiece edges  27 C 2  and  27 C 2 , the electrical resistances between the respective workpiece edges are the same. 
     Thus, when a current is caused to flow between the workpiece edges  27 C 1  and  27 C 1  and between the workpiece edges  27 C 2  and  27 C 2 , the temperatures rise in the same temperature-rising pattern, and the same melted state can be obtained. This makes it possible to join the workpiece edges  27 C 1  and  27 C 1  and join the workpiece edges  27 C 2  and  27 C 2  while maintaining the contact state therebetween in the same state before the welding. Thus, the completed retainer  27  is less likely to distort. 
     The workpiece edges  27 C 1  and  27 C 1  and the workpiece edges  27 C 2  and  27 C 2  are both end portions of the retainer groove  26  (workpiece groove  26 A) in the axial direction of the retainer  27 , and are formed on the central axis of the retainer groove  26  (workpiece groove  26 A). Thus, the length thereof is sufficiently short. Accordingly, the electrical resistance between the workpiece edges  27 C 1  and  27 C 1  and between the workpiece edges  27 C 2  and  27 C 2  is high, which enables a desired temperature to be reached in a short time. Thus, a situation in which the temperature of the joints gradually increases over a long period of time and consequently the temperature of the entire retainer  27  increases can be prevented, whereby deformation of the retainer  27  and the retainer grooves  26  (workpiece grooves  26 A) can be prevented. Consequently, the yield can be increased, and the retainer can be produced at low cost. 
     The following describes operation of the steering system  10  configured as described above. When the steering wheel  12  is steered, steering torque is transmitted to the steering shaft  13 , whereby the steering operation shaft  19  is moved in the axial direction via the rack-and-pinion mechanism including the pinion  13   a  and the rack  22 . 
     The steering torque transmitted to the steering shaft  13  is detected by the torque detection device  14 . The rotational position of the output shaft  37  of the motor M, for example, is detected by a rotation-angle detection sensor (not depicted). Based on the steering torque and the rotational position of the motor M, for example, the motor M is controlled to generate assist force. The assist force generated by the motor M is converted into axial movement of the steering operation shaft  19  by the ball screw device  40  to reduce steering force that the driver needs to apply to the steering wheel  12 . 
     When the ball nut  21  is rotated together with the output shaft  37  by the motor M, the rolling balls  24  are caused to roll in the circumferential direction while rotating in the same direction between the ball screw shaft  20  and the ball nut  21 . At this time, the rolling balls  24  adjacent in the raceway are moved while being separated from each other by the separating portions  28  of the retainer  27  and also being pressed by the separating portions  28 . Thus, it is possible to smoothly operate the ball screw device  40  while preventing the balls from hitting each other and from staying uncirculated. This can prevent fluctuation of running torque, and can reduce operation noise. 
     In the first embodiment, the notch  27   d  is formed in the flange portion  27   b  of the retainer  27 . However, the present invention is not limited to this. As a second embodiment, in a retainer  127 , the notch  27   d  may be formed in the cylindrical portion  27   a , and butt joints  27   c   3  (corresponding to weld joints) may be formed in the flange portion  27   b  as depicted in  FIG. 12A  and  FIG. 12B . In this case, the butt joints  27   c   3  of the flange portion  27   b  and the butt joints  27   c   2  formed in the cylindrical portion  27   a  on the side where the flange portion  27   b  is not positioned are preferably formed so as to have the same length. By this configuration, advantageous effects similar to those in the first embodiment can be expected. A method for producing the retainer  127  is similar to the method for producing the retainer  27  in the first embodiment. 
     As a third embodiment, the flange portion  27   b  and the cylindrical portion  27   a  of a retainer  227  may have no notch  27   d . In this case, as depicted in  FIG. 13A  and  FIG. 13B , the length L 2  of the butt joints  27   c   2  formed in the cylindrical portion  27   a  on the side where the flange portion  27   b  is not positioned only needs to be changed (increased) such that the length L 2  is the same as the total length (L 1 +L 3 ) of the length L 3  of the butt joints  27   c   3  of the flange portion  27   b  and the length L 1  of the butt joints  27   c   1  of the cylindrical portion  27   a  on the flange portion  27   b  side. In the third embodiment also, a method for producing the retainer  227  is similar to the method for producing the retainer  27  in the first embodiment. 
     The present invention is not limited to the embodiments described above, and the retainers  27 ,  127 , and  227  may each consist of the cylindrical portion  27   a  without the flange portion  27   b . In this case, when the retainer  27 ,  127 , or  227  is assembled to the ball screw device  40 , axial movement of the retainer  27 ,  127 , or  227  only needs to be restricted by another method such as installation of a snap ring on an inner-peripheral surface of the ball nut  21 . Furthermore, each joint only needs to be formed such that the length L 1  of the butt joints  27   c   1  and the length L 2  of the butt joints  27   c   2  are the same. By this configuration, similar advantageous effects can be obtained. 
     In the embodiments, the butt joints  27   c   1  and  27   c   2  in each of the retainers  27 ,  127 , and  227  are formed on the central axis C 1  of the retainer groove  26 . However, the present invention is not limited to this, and the butt joints  27   c   1  and  27   c   2  may be formed so as to be parallel to the axis of the retainer  27 . By this configuration, butt joints having a shorter length having a higher electrical resistance can be formed, which can be joined together in a shorter time in resistance welding. 
     The shape of the notch  27   d  of the flange portion  27   b  is not limited to the shape (fan shape) described in the embodiments. As depicted in  FIG. 14 , the notch  27   d  may be formed in a rectangular shape, for example. Alternatively, the notch may be cut out by a curve (not depicted). 
     In the embodiments, the inclined surfaces  26   a  and  26   b  that are both side surfaces of each retainer groove  26  are not limited to those described in the embodiments. Both side surfaces of the retainer groove  26  may be configured such that only either one surface thereof is inclined as long as movement of the rolling balls  24  toward the radial outside of the retainer  27  is allowed and movement thereof toward the radial inside of the retainer is restricted by both side surfaces thereof. 
     The following describes advantageous effects of the embodiments described above. According to the embodiments, the ball screw device  40  includes: the ball screw shaft  20  (steering operation shaft  19 ) having the outer-peripheral surface on which the outer-peripheral ball rolling groove  20   a  is spirally formed; the ball nut  21  having the inner-peripheral surface on which the inner-peripheral ball rolling groove  21   a  is spirally formed; the rolling balls  24  that are arranged between the outer-peripheral ball rolling groove  20   a  and the inner-peripheral ball rolling groove  21   a  in a circulatable manner; and the retainer  27 ,  127 , or  227  disposed between the outer periphery of the ball screw shaft  20  and the inner periphery of the ball nut  21 , and having the cylindrical portion  27   a  with the retainer grooves  26  that retain the rolling balls  24 . The inclined surfaces  26   a  and  26   b  that are both side surfaces of each retainer groove  26  are formed so as to allow movement of the rolling balls  24  toward a radial outside of the retainer  27 ,  127 , or  227  and to restrict movement of the rolling balls  24  toward a radial inside of the retainer  27 ,  127 , or  227 . The retainer  27 ,  127 , or  227  has the butt joints  27   c   1  and  27   c   2  (weld joints) at the opposite ends of one retainer groove  26  among the retainer grooves  26  in the axial direction of the retainer. 
     The butt joints  27   c   1  and  27   c   2  are formed in small areas at the opposite ends of the retainer groove  26 , instead of being formed along, for example, all the length of the retainer  27 ,  127 , or  227  in the axial direction. Thus, the butt joints  27   c   1  and  27   c   1  can be joined to each other and the butt joints  27   c   2  and  27   c   2  can be joined to each other by welding, for example, in a shorter time than in the case that long butt joints are joined to each other. This can suppress temperature increase in the retainers  27 ,  127 , and  227 , and thus deformation of the retainers  27 ,  127 , and  227  and the retainer groove  26  caused by the temperature increase can be satisfactorily suppressed. Thus, the yield of the retainers  27 ,  127 , and  227  can be increased, and the cost of the retainers  27 ,  127 , and  227  and consequently the cost of the ball screw device  40  including the retainer  27 ,  127 , or  227  can be reduced. 
     According to the embodiments, the joint lengths of the butt joints  27   c   1  and  27   c   2  (weld joints) at the opposite ends of the retainer groove  26  are the same. Thus, when resistance welding is performed on workpiece edges  27 C 1  and  27 C 2  before being welded that correspond to the butt joints  27   c   1  and  27   c   2  (weld joints), respectively, the electrical resistance between the workpiece edges  27 C 1  and  27 C 1  and the electrical resistance between the workpiece edges  27 C 2  and  27 C 2  are the same. 
     Thus, between the respective joints, the temperatures rise in the same temperature-rising pattern, and the same melted state can be obtained. This makes it possible to join the respective joints while maintaining the contact (angle) state therebetween in the same state before the welding. Thus, the completed retainer  27  is less likely to distort. 
     According to the embodiments, the butt joints  27   c   1  and  27   c   2  (weld joints) at the opposite ends of the retainer groove  26  extend on the central axis C 1  of the retainer groove  26 . Thus, the workpiece edges  27 C 1  and  27 C 2  can be easily and accurately formed in a forming process. 
     According to the first to third embodiments, the retainer  27 ,  127 , or  227  has, on one end of the cylindrical portion  27   a , a flange portion  27   b  that is capable of being in contact with an end surface  21   d  of the ball nut  21 . By this low-cost configuration, the retainer  27 ,  127 , or  227  can be fixed to the ball screw device  40 . According to the second and third embodiments, the flange portion  27   b  of the retainer  127  or  227  has butt joints  27   c   3  (weld joints). This can reduce the influence of heat on the cylindrical portion  27   a  during the resistance welding. 
     According to the first and second embodiments, the butt joints ( 27   c   1 ,  27   c   3 ) on the flange portion  27   b  side among the butt joints  27   c   1 ,  27   c   2 , and  27   c   3  at the opposite ends of the retainer groove  26  have a notch  27   d . By this configuration, the length (L 1 , L 3 , or L 1 +L 3 ) of the butt joints on the flange portion  27   b  side and the length L 2  of the butt joints on the side where the flange portion  27   b  is not positioned (on the opposite side from the flange portion  27   b ) can be easily set equal to each other. 
     According to the first embodiment, the notch  27   d  is formed in the flange portion  27   b . Because the flange portion  27   b  is formed in an annular shape, the circumference thereof in a radially outer portion is different from the circumference thereof in a radially inner portion. By utilizing this length difference, the notch  27   d  can be easily formed. 
     According to the embodiments, a steering system  10  includes the ball screw device  40 . Because the ball screw device  40  that can be produced at low cost as described above is used, the cost of the steering system  10  can be reduced. 
     According to the embodiments, the method for producing the retainer  27 ,  127 , or  227  used in the ball screw device  40  includes: the step S 10  of stamping out the flat plate to form the first workpiece  27 A that is the plate member having the workpiece groove  26 A corresponding to the retainer groove  26  and the workpiece edges  27 C 1  and  27 C 2  corresponding to the butt joints  27   c   1  and  27   c   2 , respectively; the step S 20  of rolling up the first workpiece  27 A to form the second workpiece  27 B that has the first plate portion corresponding to the cylindrical portion  27   a  and having the shape identical to that of the cylindrical portion  27   a ; and the step S 30  of joining the workpiece edges  27 C 1  and  27 C 1  at the circumferential ends of the second workpiece  27 B to each other and joining the workpiece edges  27 C 2  and  27 C 2  at the circumferential ends thereof to each other by resistance welding to form the retainer  27 ,  127 , or  227 . By this method, the retainers  27 ,  127 , and  227  according to the embodiments can be obtained. 
     According to the embodiments, in the method for producing the retainer  27 ,  127 , or  227  used in the ball screw device  40 , the first workpiece  27 A has, on one end of the cylindrical portion  27   a , a workpiece flange portion  27 BB (second plate portion) corresponding to the flange portion  27   b  that is capable of being in contact with the end surface  21   d  of the ball nut  21 . The workpiece edges  27 C 1  and  27 C 1  and the workpiece edges  27 C 2  and  27 C 2  that are formed on both ends of the first workpiece  27 A are formed so as to be parallel to each other. In this production method, at the step of forming the second workpiece  27 B, the workpiece flange portion  27 BB of the first workpiece  27 A corresponding to the flange portion  27   b  is bent, and then the first workpiece  27 A is rolled up to form the second workpiece  27 B such that the workpiece cylindrical portion  27 AA (first plate portion) corresponding to the cylindrical portion  27   a  and the workpiece flange portion  27 BB corresponding to the flange portion  27   b  have shapes that are identical to those of the cylindrical portion  27   a  and the flange portion  27   b , respectively. By this method, the retainers  27 ,  127 , and  227  according to the embodiments can be obtained. 
     In the embodiments, each joint is formed such that the butt joints  27   c   1  and the butt joints  27   c   2  have the same length (L 1 =L 2 ). However, the present invention is not limited to this. If both of the butt joints  27   c   1  and  27   c   2  are sufficiently short, these joints do not have to have the same length. When the lengths of the butt joints  27   c   1  and  27   c   2  are sufficiently short, temperatures at both of the joints  27   c   1  and  27   c   2  rise up to the temperature at which these joints can be formed in a short time in resistance welding. Thus, tilt is less likely to occur between the joints  27   c   1  and  27   c   2 . The “sufficiently short length” herein is determined in advance based on tests, for example. Welding for forming the butt joints  27   c   1  and  27   c   2  may be performed by arc welding or laser welding. 
     In the embodiments, examples have been described in which the ball screw device  40  is used for the electric power steering system  10 , for example. However, the present invention may be applied to a ball screw device used in a machine tool, for example, in the same manner. 
     In the embodiments, the steering assist mechanism  30  applies steering assist force to the steering operation shaft  19 , using as a driving source the motor M having a rotary shaft disposed parallel to the ball screw shaft of the steering operation shaft  19 . However, the present invention is not limited to this. The steering assist mechanism may be of a type described in a related art (JP 5120040) in which the rotary shaft of a motor is disposed coaxially with the ball screw shaft of the steering operation shaft  19 . In this case also, similar advantageous effects can be expected.