Patent Publication Number: US-2019168799-A1

Title: Steering system

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2017-233378 filed on Dec. 5, 2017 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 steering system. 
     2. Description of the Related Art 
     There has hitherto been a steering system for an automobile, in which operation of a steered shaft (rack shaft) is assisted by generating thrust in the axial direction of the steered shaft by actuating a ball screw device using a motor (see Japanese Patent Application Publication No. 2011-256901 (JP 2011-256901 A) and Japanese Patent Application Publication No. 2014-77459 (JP 2014-77459 A), for example). In the ball screw device according to JP 2011-256901 A and JP 2014-77459 A, a rolling path for rolling elements is formed with an outer peripheral rolling groove, which is formed in the outer peripheral surface of the steered shaft, and an inner peripheral rolling groove, which is formed in the inner peripheral surface of a rolling element nut (ball nut), facing each other. In addition, the ball screw device includes a deflector (circulation member) that connects a passage formed therein to the rolling path to form an endless circulation path to allow endless circulation of the rolling elements. 
     In such a ball screw device, normally, adjacent rolling elements are arranged with a predetermined clearance therebetween in the rolling path. Therefore, when the rolling element nut (inner peripheral rolling groove) is rotated about the axis relative to the steered shaft (outer peripheral rolling groove), the plurality of rolling elements in the rolling path are rolled in the same direction at the same speed while contacting each of the inner peripheral rolling groove surface and the outer peripheral rolling groove surface without contacting adjacent rolling elements, achieving smooth relative rotation with low resistance between the inner peripheral rolling groove and the outer peripheral rolling groove. 
     In the case where the vehicle is in the straight travel state, that is, the steering angle of the steering shaft is in the so-called neutral state, in the steering system described above, however, there may occur a phenomenon called ball clearance reduction in which the clearance between the plurality of rolling elements in the rolling path is reduced and adjacent rolling elements contact each other. 
     When the ball clearance reduction state is caused, the rolling elements which contact each other in the rolling path are rotated in the same direction when the rolling element nut is relatively rotated. Therefore, movement (rotation) in opposite directions is caused at a portion at which the rolling elements contact each other, generating friction. Consequently, a force required for steering is increased, and the driver may feel that the steering torque for a steering wheel has been increased. When the steering system is in the neutral state, the driver often performs a steering operation for a very small steering angle in order to keep the lane or the like, and therefore the driver tends to sense magnitude of steering torque and fluctuations in steering torque. Meanwhile, a load on an electric motor that rotates the rolling element nut may be increased and power consumption or the like may also be increased. 
     In particular, the rack-parallel steering system described in JP 2014-77459 A includes a drive pulley fixed to the distal end of an output shaft of a motor, a driven pulley fixed to the outer peripheral surface of a rolling element nut, and a belt wound between the drive pulley and the driven pulley with predetermined tension. With such a configuration, the rolling element nut (driven pulley) is pulled toward the drive pulley by the tension of the belt. 
     Therefore, the width (clearance) in the radial direction of a space (rolling path) between the inner peripheral rolling groove of the rolling element nut and the outer peripheral rolling groove of the steered shaft becomes non-uniform in the circumferential direction, and the rolling path has a portion with a small clearance and a portion with a large clearance. Consequently, when the rolling element nut is relatively rotated, the plurality of rolling elements in the rolling path tend to be pushed out from the portion of the passage with a small clearance toward the portion of the passage with a large clearance. Then, the plurality of rolling elements which have been pushed out are moved from the portion of the passage with a small clearance to the portion of the passage with a large clearance to be gathered, promoting the ball clearance reduction state in which adjacent rolling elements contact each other. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a rack-parallel steering system that includes a ball screw device in which ball clearance reduction of rolling elements is not likely to occur in the case where the steering angle of a steering wheel is in the neutral state. 
     An aspect of the present invention provides a steering system for a vehicle, including: a steered shaft supported on a housing so as to be movable in an axial direction and moved in the axial direction in accordance with a steering angle of a steering wheel to steer steered wheels; a ball screw device that includes an outer peripheral rolling groove formed in an outer peripheral surface of the steered shaft, a rolling element nut, in an inner peripheral surface of which an inner peripheral rolling groove corresponding to the outer peripheral rolling groove is formed to form a rolling path wound spirally a plurality of times between the inner peripheral rolling groove and the outer peripheral rolling groove, a plurality of rolling elements housed in the rolling path, and a deflector portion in which a deflector passage is formed, the deflector passage being provided in the rolling element nut and communicating with the rolling path such that a first opening and a second opening that form respective ends of the rolling path are connected to form an endless circulation path together with the rolling path, enabling endless circulation of the rolling elements in the circulation path; a motor that is fixed to the housing and that includes an output shaft offset from the steered shaft; and a drive force transfer mechanism that includes a drive pulley provided so as to be rotatable together with the output shaft, a driven pulley provided so as to be rotatable together with the rolling element nut, and a belt wound between the drive pulley and the driven pulley with tension to transfer a rotational drive force of the motor. 
     A position of the housing relative to the steered shaft with the vehicle in a straight travel state is defined as a steering neutral position of the steered shaft. The deflector passage is formed such that respective end portions of the deflector passage, which are connected to the first opening and the second opening of the rolling path, are in a semiperimeter range of the inner peripheral surface of the rolling element nut with the steered shaft at the steering neutral position. The semiperimeter range is a range formed to extend to a phase of 90° on both sides in a circumferential direction of the inner peripheral surface of the rolling element nut from a crossing line that is the farther from the drive pulley, of crossing lines formed with a virtual plane that includes a rotational axis of the driven pulley and a rotational axis of the drive pulley intersecting the inner peripheral surface. 
     In this manner, the clearance between the inner peripheral rolling groove of the rolling element nut and the outer peripheral rolling groove is non-uniform in the circumferential direction with the driven pulley and the rolling element nut pulled toward the drive pulley by the tension of the belt, and both end portions of the deflector passage are disposed in the (semiperimeter) range on the side with a small clearance. In this event, the deflector passage is not affected by the size of the clearance between the inner peripheral rolling groove and the outer peripheral rolling groove, and always has a constant diameter. Therefore, the range in which the clearance is substantially small, in the range on the side with a small clearance, can be reduced by an amount corresponding to the range between both end portions of the deflector passage. 
     Thus, the number of rolling elements between the inner peripheral rolling groove and the outer peripheral rolling groove to be pushed out from the portion on the side with a small clearance toward the portion on the side with a large clearance can be suppressed effectively. In addition, the rolling elements are easily movable from the portion on the side with a large clearance toward the portion on the side with a small clearance. Thus, in the case where the portion on the side with a large clearance is congested with the rolling elements, such congestion can be relaxed effectively. Therefore, occurrence of ball clearance reduction of the rolling elements is suppressed even if the rolling element nut is relatively rotated with the driver operating the steering wheel in the case where the steering angle of the steering wheel is in the neutral state. Consequently, an increase in steering torque required for steering can be suppressed, and there is little possibility that the driver feels that the steering torque has been increased. There is also little possibility that a load on the electric motor which rotates the rolling element nut is increased and power consumption is increased. 
    
    
     
       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 the entire electric power steering system according to the present embodiment; 
         FIG. 2  is an enlarged sectional view of a steering assist mechanism and a ball screw device in  FIG. 1 ; 
         FIG. 3  is a sectional view taken along the line in  FIG. 2 , illustrating a drive force transfer mechanism; 
         FIG. 4  illustrates a rolling element nut in  FIG. 2  as seen from above; 
         FIG. 5  is a sectional view taken along the line V-V in  FIG. 4 ; 
         FIG. 6  is a perspective view of a deflector; 
         FIG. 7  is a schematic diagram of a circulation path; 
         FIG. 8  is a transparent view of a deflector passage as seen in the axial direction; 
         FIG. 9  is a transparent view of the deflector passage in  FIG. 3 , illustrating the deflector passage as being symmetrical in the right-left direction as seen in the axial direction; 
         FIG. 10  illustrates the related art corresponding to  FIG. 3 ; 
         FIG. 11  illustrates an embodiment according to a first modification; and 
         FIG. 12  illustrates an embodiment according to a second modification. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings.  FIG. 1  illustrates the entire electric power steering system (corresponding to the steering system) for a vehicle according to the present invention. The electric power steering system is a steering system that supplements a steering force with a steering assist force. 
     An electric power steering system  10  (hereinafter referred to simply as a “steering system  10 ”) is a device that steers steered wheels  28  and  28  of a vehicle by reciprocally moving a steered shaft  20  coupled to the steered wheels  28  and  28  in the A direction (right-left direction in  FIG. 1 ) which coincides with the axial direction of the steered shaft  20 . The steering state of the steered wheels  28  and  28  in  FIG. 1  corresponds to a state in which the vehicle travels straight, that is, a neutral steering state. 
     As illustrated 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 (corresponding to the “motor”; hereinafter referred to simply as a “motor M”), the steered shaft  20  discussed earlier, a steering assist mechanism  30 , and a ball screw device  40 . 
     The housing  11  is a fixed member fixed to the vehicle. The housing  11  is formed in a tubular shape, and includes a first housing  11   a  and a second housing  11   b  fixed to one end side (left side in  FIG. 1 ) of the first housing  11   a  in the A direction. 
     The steering wheel  12  is fixed to an end portion of the steering shaft  13 , and rotatably supported in a cabin. The steering shaft  13  transfers torque applied to the steering wheel  12  by an operation by a driver to the steered shaft  20 . 
     A pinion  13   a  that constitutes a rack-and-pinion mechanism is formed at an end portion of the steering shaft  13  on the steered shaft  20  side. The torque detection device  14  detects torque applied to the steering shaft  13  on the basis of the amount of torsion of the steering shaft  13 . 
     The steered shaft  20  extends in the A direction. The steered shaft  20  is supported on the housing  11  so as to be reciprocally movable in the axial direction. Rack teeth  22  are formed on a part of the outer peripheral surface of the steered shaft  20 . The rack teeth  22  are meshed with the pinion  13   a  of the steering shaft  13 , and constitute the rack-and-pinion mechanism together with the pinion  13   a.    
     The relative position of meshing between the pinion  13   a  and the rack teeth  22  of the rack-and-pinion mechanism in the neutral steering state (corresponding to the straight travel state of the vehicle) illustrated in  FIG. 1  is defined as a “steering neutral position N” of the rack-and-pinion mechanism. In addition, the position of the steered shaft  20  relative to the housing  11  in the neutral steering state is defined as the steering neutral position N of the steered shaft  20 . The steering neutral position N will be used in the following description. For the rack-and-pinion mechanism, the maximum axial force that can be transferred between the steering shaft  13  and the steered shaft  20  can be set on the basis of usage of the steering system  10  etc. 
     The steered shaft  20  has joints  25  and  25  at both end portions. Tie rods  26  and  26  are coupled to respective end portions of the joints  25  and  25 . The distal ends of the tie rods  26  and  26  are coupled to the right and left steered wheels  28  and  28  via knuckle arms  27  and  27 , respectively. 
     Consequently, when the steering wheel  12  is operated, the steered shaft  20  is linearly reciprocally moved in the A direction via the rack-and-pinion mechanism in accordance with the steering angle of the steering shaft  13  which is coupled to the steering wheel  12 . When this movement along the A direction is transferred to the knuckle arms  27  and  27  via the tie rods  26  and  26 , the steered wheels  28  and  28  which have been in the neutral steering state (see  FIG. 1 ) are steered to change the travel direction of the vehicle by a desired amount. The present invention is also applicable to a steering actuator in a steer-by-wire (SBW) device in which a steering wheel and a steered shaft are not mechanically coupled to each other. The steering actuator has a structure obtained by removing the pinion  13   a  and the rack teeth  22  from the steering system described above, for example. 
     First end portions of boots  29  and  29  are fixed to both ends of the housing  11  in the A direction. The boots  29  and  29  are made of a resin, for example, and each have a tubular bellows portion that mainly covers a joint portion between the joint  25 ,  25  and the tie rod  26 ,  26  and that is expandable in the A direction. Second end portions of the boots  29  and  29  are fixed to the tie rods  26  and  26 . The boots  29  and  29  restrict entry of foreign matter such as dust and water into the housing  11  and the joints  25  and  25 . 
     An outer peripheral rolling groove  23  is formed in the outer peripheral surface of the steered shaft  20  at a position that is different from that of the rack teeth  22 . The outer peripheral rolling groove  23  constitutes the ball screw device  40  together with an inner peripheral rolling groove  21   a  of a rolling element nut  21  to be discussed later. The steering assist mechanism  30  transfers a steering assist force to the outer peripheral rolling groove  23 . 
     The steering assist mechanism  30  is a mechanism that applies a steering assist force to the steered shaft  20  using the motor M as a drive source. The steering assist mechanism  30  includes the motor M, a control unit ECU that drives the motor M, and a drive force transfer mechanism  32 . The motor M and the control unit ECU which drives the motor M are housed in a case  31  fixed to the first housing  11   a  of the housing  11 . The control unit ECU decides steering assist torque and controls an output of the motor M on the basis of a signal output from the torque detection device  14 . 
     As illustrated in  FIGS. 2 and 3 , the drive force transfer mechanism  32  includes a drive pulley  36 , a driven pulley  34 , and a toothed belt  35 . The drive pulley  36  is fixed to the distal end of an output shaft  37  of the motor M so as to be rotatable together therewith. The output shaft  37  is disposed in parallel with the axis of the steered shaft  20 . The driven pulley  34  is fixed to the outer peripheral side of the rolling element nut  21  so as to be rotatable together therewith. 
     As illustrated in  FIG. 2 , the one end side (left side in  FIG. 2 ) of the rolling element nut  21  in the A direction is rotatably supported on an inner peripheral surface  11   b   1  of the second housing  11   b  via a ball bearing  33 . As illustrated in  FIGS. 2 and 3 , the belt  35  is wound between the drive pulley  36  and the driven pulley  34  with predetermined tension T. The drive force transfer mechanism  32  transfers a rotational drive force generated by the motor M between the drive pulley  36  and the driven pulley  34  via the belt  35 . 
     With the configuration described above, the steering assist mechanism  30  drives the motor M in accordance with a turning operation of the steering wheel  12 , and rotates the output shaft  37  of the motor M and the drive pulley  36 . Rotation of the drive pulley  36  is transferred to the driven pulley  34  via the belt  35  to rotate the rolling element nut  21  which is provided integrally with the driven pulley  34 . When the rolling element nut  21  is rotated, a steering assist force (power) in the axial direction of the steered shaft  20  is transferred to the steered shaft  20  via a plurality of rolling balls  24  (corresponding to the rolling elements) of the ball screw device  40 , moving the steered shaft  20  in the axial direction. 
     The ball screw device  40  will be described in detail. As illustrated in  FIG. 2 , the ball screw device  40  is mainly housed in the second housing  11   b . The ball screw device  40  includes the outer peripheral rolling groove  23  which is formed spirally in a part of the outer peripheral surface of the steered shaft  20  discussed earlier, the rolling element nut  21 , the plurality of rolling balls  24 , and a pair of deflectors  51  and  52 . 
     The rolling element nut  21  is formed in a tubular shape. The inner peripheral rolling groove  21   a  in a spiral shape is formed in the inner peripheral surface of the rolling element nut  21 . The inner peripheral rolling groove  21   a  (rolling element nut  21 ) is disposed coaxially with the outer peripheral rolling groove  23  (steered shaft  20 ) and circumferentially outward from the outer peripheral rolling groove  23 . Consequently, the inner peripheral rolling groove  21   a  forms a rolling path R 1  wound spirally a plurality of times together with the corresponding outer peripheral rolling groove  23 . The plurality of rolling balls  24  are disposed in the rolling path R 1 . 
     The rolling element nut  21  includes a pair of (two) attachment holes  41  and  42  that penetrate the rolling element nut  21  between the inner peripheral rolling groove  21   a  and an outer peripheral surface  21   b  at two different locations (B position and C position in  FIGS. 2 and 3 ) including both ends of the rolling path R 1  which is formed to be wound a plurality of times between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  which face each other (see  FIGS. 2 to 5 ). The present invention is not limited to the aspect described above, and the attachment holes  41  and  42  may be formed to penetrate the rolling element nut  21  between the outer peripheral surface  21   b  and the inner peripheral rolling groove  21   a  at positions other than the B position and the C position as long as the attachment holes  41  and  42  are disposed with a plurality of turns of the inner peripheral rolling groove  21   a  provided therebetween. 
     With the attachment holes  41  and  42  penetrating the rolling element nut  21 , two opening portions  41   a  and  42   a  open in the inner peripheral rolling groove  21   a  (see  FIG. 5 ). The pair of attachment holes  41  and  42  have the same configuration, and are disposed in the same arrangement when seen in the opposing axial directions. Thus, only the attachment hole  41  will be described below except when it is necessary to describe the attachment hole  42 . The D direction indicated by the arrow in  FIG. 5  indicates the direction of insertion of the deflector  51  into the attachment hole  41  (hereinafter referred to simply as an “insertion direction D”). 
       FIGS. 2 and 3  illustrate a case where both the relative position of the rack-and-pinion mechanism and the relative position of the steered shaft  20  discussed above are the steering neutral position N. That is,  FIGS. 2 and 3  are each a sectional view illustrating the state of arrangement of the drive pulley  36 , the rolling element nut  21  (driven pulley  34 ), and the deflector  51  (deflector passage  61 ) with the vehicle in the straight travel state. 
     As illustrated in  FIG. 5 , the attachment hole  41  ( 42 ) includes a press-fitting hole portion  411 , a guide hole portion  412 , and a pair of stopping surfaces  44   a  and  44   b . An outer peripheral portion  511  of the deflector  51  to be discussed later is housed in and press-fitted into the press-fitting hole portion  411 . An inner peripheral portion  512  of the deflector  51  to be discussed later is housed in the guide hole portion  412 . The pair of stopping surfaces  44   a  and  44   b  contact a pair of stopped surfaces  54   a  and  54   b  of the deflector  51  to be discussed later to position the deflector  51  in the insertion direction D. 
     The press-fitting hole portion  411  is formed on the outer peripheral surface  21   b  side of the rolling element nut  21  in the radial direction of the rolling element nut  21 . The press-fitting hole portion  411  is formed such that the sectional shape on a plane that is orthogonal to the insertion direction D of the deflector  51  is a generally rectangular hole (not illustrated) with rounded corners. 
     In the present embodiment, the longitudinal direction of the generally rectangular shape in the section of the press-fitting hole portion  411  is not a direction that is parallel to an end surface of the rolling element nut  21 , that is, not a direction that is orthogonal to the axis of the rolling element nut  21 . In the present embodiment, the longitudinal direction of the press-fitting hole portion  411  is a direction that is substantially parallel to the direction of extension of a projected groove formed when the inner peripheral rolling groove  21   a , which is formed in the inner peripheral surface of the rolling element nut  21 , is enlarged and projected outward in the radial direction onto the outer peripheral surface  21   b.    
     The guide hole portion  412  penetrates the rolling element nut  21  to open in the inner peripheral surface (inner peripheral rolling groove  21   a ) thereof. The guide hole portion  412  is formed such that the sectional shape on a plane that is orthogonal to the insertion direction D of the deflector  51  is a generally rectangular hole (not illustrated) with rounded corners. As illustrated in  FIG. 5 , the pair of stopping surfaces  44   a  and  44   b  are formed on different planes. It should be noted, however, that the pair of stopping surfaces  44   a  and  44   b  may be formed on the same plane. 
     As illustrated in  FIG. 4 , a center passage  43  that connects and communicates between the pair of attachment holes  41  and  42  is formed in the outer peripheral surface  21   b  of the rolling element nut  21 . The center passage  43  extends in parallel with the axial direction (A direction) of the rolling element nut  21 , and opens outward in the radial direction of the rolling element nut  21 . The opening width of the center passage  43  is slightly larger than the diameter of the rolling balls  24 . In addition, the bottom surface of the center passage  43  is a curved surface formed with a radius that is slightly larger than the radius of the rolling balls  24 . Consequently, the rolling balls  24  are freely reciprocally rollable in the center passage  43 . 
     Next, the deflectors  51  and  52  will be described.  FIG. 6  is a perspective view of the deflectors  51  and  52  to be housed in the attachment holes  41  and  42 , respectively. As illustrated in  FIGS. 2 to 5 , the deflectors  51  and  52  are fixed as being housed in the attachment holes  41  and  42 , respectively. 
     The deflectors  51  and  52  have a first passage  51   a  and a second passage  52   a , respectively, formed therein as through holes. First end portions of the first passage  51   a  and the second passage  52   a  are connected to respective end portions of the center passage  43  illustrated in  FIG. 4  with the deflectors  51  and  52  housed in the attachment holes  41  and  42 , respectively. Meanwhile, second end portions of the first passage  51   a  and the second passage  52   a  are connected to the rolling path R 1  to open in the rolling path R 1 . The deflector passage  61  is formed from the first passage  51   a , the center passage  43 , and the second passage  52   a.    
     The openings (second end portions corresponding to both end portions of the deflector passage  61 ) of the first passage  51   a  and the second passage  52   a  also serve as openings at both ends of the rolling path R 1 . The openings at both ends of the rolling path R 1  are defined as a first opening  71  and a second opening  72  (see  FIG. 5 ). The first opening  71  and the second opening  72  open in the opening portions  41   a  and  42   a  of the attachment holes  41  and  42 , respectively. 
     A deflector portion  60  (indicated by the long dashed double-short dashed line in  FIG. 4 ) is constituted in the rolling element nut  21  by the pair of deflectors  51  and  52  and a part of the rolling element nut  21 , which are provided with the deflector passage  61 . As illustrated in the schematic diagram in  FIG. 7 , the deflector passage  61  forms an endless circulation path  50  together with the rolling path R 1  by connecting the first opening  71  and the second opening  72  in the rolling path R 1 . Consequently, endless circulation of the rolling balls  24  (rolling elements) in the circulation path  50  is enabled. Endless circulation of the rolling balls  24  through the deflectors  51  and  52  is known, and thus is not described in detail. 
     The first passage  51   a  and the second passage  52   a , which extend from an end portion of the center passage  43  to an opening in the rolling path R 1 , are formed as being connected with a plurality of radii R as illustrated in  FIG. 8  which is a transparent view of the deflector passage  61  as seen in the axial direction of the rolling element nut  21 . It should be noted, however, that the present invention is not limited to this aspect, and the deflector passage  61  may be formed in any shape. 
     Particularly, at the steering neutral position N, the deflector passage  61  is formed such that both end portions  61   a  and  62   a  thereof, which are respectively connected to the first opening  71  and the second opening  72  of the rolling path R 1 , are in a semiperimeter range Ar 1  (see  FIGS. 3 and 8 ) of the inner peripheral surface (inner peripheral rolling groove  21   a ) of the rolling element nut  21 . The semiperimeter range Ar 1  is formed to extend to a phase of 90° on both sides in the circumferential direction of the inner peripheral surface (inner peripheral rolling groove  21   a ) of the rolling element nut  21  from a crossing line L 1  that is the farther from the drive pulley  36 , of crossing lines L 1  and L 2  formed with a virtual plane Q that includes rotational axes C 1  and C 2  of the driven pulley  34  and the drive pulley  36 , intersecting the inner peripheral surface (inner peripheral rolling groove  21   a ). In  FIGS. 3 and 8 , the virtual plane Q is illustrated as being parallel to the up-down direction of the paper surface. However, this is based on the convenience of illustration, and does not mean that the virtual plane Q is parallel to the vertical direction. That is, the virtual plane Q may be tilted by any degree with respect to the vertical plane or the horizontal plane. 
     In other words, the semiperimeter range Ar 1  is a semiperimeter range centered on a portion of the rolling path R 1  with a clearance β 1 , which is smallest of a clearance β (distance) of the rolling path R 1  between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  which is non-uniform. The non-uniform clearance β of the rolling path R 1  is caused with the driven pulley  34  and the rolling element nut  21  pulled toward the drive pulley  36  by the tension T of the belt  35 . Thus, the clearance β of the rolling path R 1  is smallest at the clearance β 1  at the position of the crossing line L 1 . 
     In addition, as illustrated in  FIG. 9 , at the steering neutral position N, both end portions  61   a  and  62   a  of the deflector passage  61  are disposed at positions that are symmetrical in the right-left direction in the circumferential direction with respect to the crossing line L 1  that is the farther from the drive pulley  36 , that is, at positions away from the crossing line L 1  by E degrees on both sides in the circumferential direction, when the rolling element nut  21  is seen in the axial direction. 
     At the steering neutral position N, in order to reliably disposed both end portions  61   a  and  62   a  of the deflector passage  61  at the desired positions described above, the phase of the start point of the inner peripheral rolling groove  21   a , which is formed in the inner peripheral surface of the rolling element nut  21 , in the circumferential direction and the phase of the start point of the outer peripheral rolling groove  23 , which is formed in the outer peripheral surface of the steered shaft  20 , in the circumferential direction may be set in association with the steering neutral position N. 
     For example, assuming that the axial positions of the steered shaft  20  and the rolling element nut  21  with respect to the housing  11  at the steering neutral position N are fixed, the phase of the start point of the inner peripheral rolling groove  21   a  of the rolling element nut  21  can be adjusted by displacing the axial position of the outer peripheral rolling groove  23  with respect to the steered shaft  20 . That is, the phase of the inner peripheral rolling groove  21   a  can be shifted by 360/n degrees by shifting the axial position of the outer peripheral rolling groove  23  by 1/n of a lead L. 
     Next, the effect of the embodiment described above will be described. It is assumed, as a precondition, that the vehicle is traveling in the straight travel state. That is, it is assumed that the driver is driving with the steering angle of the steering wheel  12  (steering shaft  13 ) maintained in the neutral state. Hence, the rack-and-pinion mechanism and the steered shaft  20  are in the steering neutral position N. 
     At the start of operation, as illustrated in  FIGS. 3 and 9 , the plurality of rolling balls  24  are disposed in the rolling path R 1  as arranged with a predetermined clearance therebetween. In this event, the rolling element nut  21  (driven pulley  34 ) is pulled toward the drive pulley  36  by the tension T of the belt  35 . Consequently, the clearance β (distance) between the inner peripheral rolling groove  21   a  of the rolling element nut  21  and the outer peripheral rolling groove  23  of the steered shaft  20  is non-uniform in the circumferential direction. 
     That is, as illustrated in  FIGS. 3 and 9 , the range Ar 1  with a small clearance β (portion on the side starting at (centered on) the crossing line L 1 ) and a range Ar 2  with a large clearance β (portion on the side centered on the crossing line L 2 ) are provided in the rolling path R 1 . In such a state, the driver operates the steering wheel  12  (and the steering shaft  13 ) in order to keep the lane or adapt to the road surface condition or the like while driving the vehicle straight. Accordingly, the rolling balls  24  are rolled in the rolling path R 1 . 
     In this event, when the steering angle is in the neutral state in the normal steering system according to the related art, the positions of the deflector passage  61  and both end portions  61   a  and  62   a  with respect to the steered shaft  20  are determined by circumstances, and are not definite. Therefore, both end portions  61   a  and  62   a  of the deflector passage  61  are occasionally disposed in the range Ar 2  with a large clearance β (see  FIG. 10 ). In this case, many of the plurality of rolling balls  24  (rolling elements) which have been arranged in a portion of the rolling path R 1  with a small clearance β are pushed out toward a portion of the rolling path R 1  in the range Ar 2  with a large clearance β. Then, the plurality of rolling elements may be gathered in a portion of the rolling path R 1  in the range Ar 2  with a large clearance β to be brought into the ball clearance reduction state in which adjacent rolling elements contact each other. 
     In this event, the rolling balls  24  are moved while contacting each other also in the deflector passage  61 , and therefore the ball clearance reduction state in the rolling path R 1  cannot be resolved. In addition, the rolling balls  24  (rolling elements) are supplied from the deflector passage  61  into the rolling path R 1  while contacting each other, and therefore the state in which the rolling balls  24  contact each other is reproduced. Further, the rolling balls  24  are not easily moved and not easily gathered in a portion on the side with a small clearance, and thus congestion of the rolling balls  24  in the ball clearance reduction state in a portion on the side with a large clearance cannot be resolved. 
     In the present embodiment, however, when the steering angle is in the neutral state, both end portions  61   a  and  62   a  of the deflector passage  61  are disposed at positions set in advance by the method discussed above. That is, at the steering neutral position N, the deflector passage  61  is formed such that both end portions  61   a  and  62   a  thereof are in the semiperimeter range Ar 1  (see  FIGS. 3, 8, and 9 ) of the inner peripheral surface (inner peripheral rolling groove  21   a ) of the rolling element nut  21 . As discussed above, the range Ar 1  is a semiperimeter range on the side with a small clearance β (distance) between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  which form the rolling path R 1 . 
     In this case, the deflector passage  61  is not affected by the size of the clearance β between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23 , and always has a constant diameter. Consequently, the rolling balls  24  are moved while being pushed by the following rolling balls  24  without being affected by the clearance between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  in the deflector passage  61 . 
     Thus, a region of the range Ar 1  in which the rolling balls  24  (rolling elements) are affected by the clearance β between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  being small can be reduced by an amount corresponding to the presence of the deflector passage  61 . Consequently, the number of rolling balls  24  (rolling elements) in the rolling path R 1  of the ball screw device  40  to be pushed out from the range Ar 1  with a small clearance β toward the range Ar 2  with a large clearance β can be suppressed. In addition, the rolling balls  24  (rolling elements) flow into the deflector passage  61  while being separated from each other. Therefore, the rolling balls  24  are intermittently supplied from the deflector passage  61  to the rolling path R 1 , forming a clearance between the rolling balls  24  to provide a trigger for resolving the ball clearance reduction state. 
     Further, at the steering neutral position N, as illustrated in  FIG. 9 , both end portions  61   a  and  62   a  of the deflector passage  61  are disposed away from the crossing line L 1  that is the farther from the drive pulley  36  by E degrees in the circumferential direction and symmetrically in the right-left direction when the rolling element nut  21  is seen in the axial direction. Particularly, both end portions  61   a  and  62   a  of the deflector passage  61  are disposed symmetrically in the right-left direction (on both sides in the circumferential direction) with respect to the position of the crossing line L 1  at which the clearance β (of the rolling path R 1 ) between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  is smallest (clearance β 1 ). 
     That is, a part of the range Ar 1  with a smallest clearance is replaced with the deflector passage  61 . Consequently, the number of rolling balls  24 , which have been positioned in the portion of the rolling path R 1  with a small clearance, to be pushed out and moved toward the portion of the rolling path R 1  in the range Ar 2  with a large clearance by relative rotation between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  can be suppressed efficiently even if the ball screw device  40  is actuated at the steering neutral position N. In addition, the rolling balls  24  are easily movable from the portion on the side with a large clearance β toward the portion on the side with a small clearance β. Thus, in the case where the portion on the side with a large clearance β is congested with the rolling balls  24  (rolling elements), such congestion can be relaxed effectively. 
     In the steering system  10  for a vehicle according to the embodiment described above, the position of the housing  11  relative to the steered shaft  20  with the vehicle in the straight travel state is defined as the steering neutral position N of the steered shaft  20 . The deflector passage  61  is formed such that both end portions  61   a  and  62   a  thereof, which are respectively connected to the first opening  71  and the second opening  72  of the rolling path R 1 , are in the semiperimeter range Ar 1  of the inner peripheral surface of the rolling element nut  21  with the steered shaft  20  at the steering neutral position N. The semiperimeter range Ar 1  is a range formed to extend to a phase of 90° on both sides in the circumferential direction of the inner peripheral surface of the rolling element nut  21  from the crossing line L 1  that is the farther from the drive pulley  36 , of the crossing lines L 1  and L 2  formed with the virtual plane Q that includes the respective rotational axes C 1  and C 2  of the driven pulley  34  and the drive pulley  36  intersecting the inner peripheral surface. 
     In this manner, the clearance β between the inner peripheral rolling groove  21   a  of the rolling element nut  21  and the outer peripheral rolling groove  23  is non-uniform in the circumferential direction with the driven pulley  34  and the rolling element nut  21  pulled toward the drive pulley  36  by the tension T of the belt  35 , and both end portions  61   a  and  62   a  of the deflector passage  61  are disposed in the (semiperimeter) range Ar 1  on the side with a small clearance β. In this event, the deflector passage  61  is not affected by the size of the clearance between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23 , and always has a constant diameter. Therefore, the range in which the clearance is substantially small, in the range Ar 1  on the side with a small clearance, can be reduced by an amount corresponding to the range between both end portions  61   a  and  62   a  of the deflector passage  61 . 
     Thus, the number of rolling balls  24  (rolling elements) between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  to be pushed out from the portion on the side with a small clearance toward the portion on the side with a large clearance can be suppressed effectively. Therefore, occurrence of ball clearance reduction of the rolling balls  24  (rolling elements) is suppressed even if the rolling element nut  21  is relatively rotated with the driver operating the steering wheel  12  in the case where the steering angle of the steering wheel  12  is in the neutral state. In addition, the rolling elements are easily movable from the portion on the side with a large clearance toward the portion on the side with a small clearance. Thus, in the case where the portion on the side with a large clearance is congested with the rolling elements, such congestion can be relaxed effectively. Consequently, an increase in steering torque required for steering can be suppressed, and there is little possibility that the driver feels that the steering torque has been increased. There is also little possibility that a load on the motor M which rotates the rolling element nut is increased and power consumption is increased. 
     In the embodiment described above, in addition, the deflector passage  61  includes the first passage  51   a  which is connected to the first opening  71 , the second passage  52   a  which is connected to the second opening  72 , and the center passage  43  which connects the first passage  51   a  and the second passage  52   a  to each other. The first passage  51   a  and the second passage  52   a  are respectively formed in the deflectors  51  and  52  which are respectively housed in the two attachment holes  41  and  42  which are spaced from each other in the axial direction and penetrate the rolling element nut  21  between the outer peripheral surface and the inner peripheral surface. In addition, the center passage  43  is formed in the outer peripheral surface of the rolling element nut  21  to extend in a direction with a component in the axial direction such that the two attachment holes  41  and  42  on the side of the outer peripheral surface of the rolling element nut  21  communicate with each other. In this manner, the deflector passage  61  is formed from the pair of deflectors  51  and  52 , which are formed compactly, and the center passage  43 , which is formed in the rolling element nut  21 , and thus can be manufactured at a low cost. 
     In the embodiment described above, in addition, both end portions  61   a  and  62   a  of the deflector passage  61  are disposed symmetrically in the right-left direction in the circumferential direction with respect to the crossing line L 1 , as the center, that is the farther from the drive pulley  36  when the rolling element nut  21  is seen in the axial direction with the steered shaft  20  at the steering neutral position N. In this manner, the deflector passage  61  is disposed in a well-balanced manner in the circumferential direction for a portion of the rolling path R 1  at which the clearance β is smallest and from which the rolling balls  24  are pushed out most strongly toward a portion with a large clearance. Therefore, even if the driver operates the steering wheel  12  (steering shaft  13 ) in the right-left direction, the number (quantity) of rolling balls  24  in the portion of the rolling path R 1  with a smallest clearance β to be pushed out by the operation and moved toward the portion of the rolling path R 1  with a large clearance β can be suppressed effectively. Thus, ball clearance reduction of the rolling balls  24  (rolling elements) in the portion of the rolling path R 1  with a large clearance β can be suppressed effectively. 
     In the embodiment described above, the deflector portion  60  is formed from the deflectors  51  and  52  and a part of the rolling element nut  21 . However, the present invention is not limited to this aspect. The deflector portion may be formed integrally as with the circulation member  15  disclosed in JP 2011-256901 A. 
     In the embodiment described above, in addition, both end portions  61   a  and  62   a  of the deflector passage  61  are disposed symmetrically in the right-left direction (on both sides in the circumferential direction) with respect to the position of the crossing line L 1  in the inner peripheral rolling groove  21   a , at which the clearance β (of the rolling path R 1 ) between the inner peripheral rolling groove  21   a  and the outer peripheral rolling groove  23  is smallest (clearance β 1 ), when the rolling element nut  21  is seen in the axial direction. However, the present invention is not limited to this aspect. Both end portions  61   a  and  62   a  may be disposed in any manner as long as both end portions  61   a  and  62   a  are disposed in the range Ar 1 . For example, as illustrated in relation to a first modification (see  FIG. 11 ), both end portions  61   a  and  62   a  may be disposed in a region on the left side with respect to the crossing line L 1 . 
     In addition, as illustrated in relation to a second modification (see  FIG. 12 ), both end portions  61   a  and  62   a  may be disposed in a region on the right side with respect to the crossing line L 1 . Also with such a configuration, an equivalent effect can be expected. In addition, although not illustrated, both end portions  61   a  and  62   a  of the deflector passage  61  may be disposed on both sides in the circumferential direction asymmetrically in the right-left direction with respect to the position of the crossing line L 1  at the center.