Patent Publication Number: US-2015060187-A1

Title: Steering apparatus

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2013-178492 filed on Aug. 29, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a steering apparatus. 
     2. Description of Related Art 
     Some of conventional steering apparatuses for vehicles have been configured as electric power steering apparatuses that convert the rotation of an electric motor into the shaft-direction movement of a steering shaft with a ball screw mechanism to apply an assisting force to a steering system. As such, steering apparatuses that includes an electric motor arranged parallel to a steering shaft and transmit the rotation of the electric motor to a ball screw mechanism via a transmission mechanism composed of a pair of pulleys and a belt have been available (for example, International Patent Application No. 2006/070889 (WO 2006/070889). 
     In the steering apparatus of WO 2006/070889, respective pulleys have outer teeth at the outer peripheries thereof, and a belt has inner teeth at the inner periphery thereof. The belt is wound around the respective pulleys with the inner teeth meshing with the respective outer teeth. Thus, the belt is prevented from being slipped relative to the respective pulleys when the rotation of an electric motor is transmitted. In addition, there has been proposed in the steering apparatus that the respective outer teeth and the inner teeth are configured as helical teeth having tooth traces twisted relative to the axis lines of the respective pulleys (see  FIG. 8  or the like of WO 2006/070889). Thus, vibrations and abnormal noises caused when the outer teeth of the respective pulleys and the inner teeth of the belt mesh with each other are reduced. 
     In recent years, it has been requested that such a steering apparatus be downsized and reduced in weight, and the downsize and lightweight of an electric motor has been discussed to fulfill the request. However, it is generally difficult to ensure the sufficient output performance of the electric motor in the downsized electric motor. Therefore, if higher priority is placed on the downsize and lightweight of the electric motor, the assisting performance of the steering apparatus may not be maintained. 
     SUMMARY OF THE INVENTION 
     The invention provides a steering apparatus that can be downsized and reduced in weight with its assisting performance maintained. 
     An aspect of the invention provides a steering apparatus including: a driving pulley that rotates with driving of an electric motor; a driven pulley arranged coaxially with a steering shaft; a belt wound around the driving pulley and the driven pulley; and a ball screw mechanism that converts a rotation of the driven pulley into a reciprocating movement of the steering shaft, wherein: the ball screw mechanism has a ball screw nut that integrally rotates with the driven pulley, and a plurality of balls provided inside a spiral ball raceway formed by a first thread groove formed at an outer periphery of the steering shaft and a second thread groove formed at an inner periphery of the ball screw nut, the first thread groove and the second thread groove facing each other; the driving pulley and the driven pulley respectively have outer teeth at outer peripheries thereof, and the belt has, at an inner periphery thereof, inner teeth to mesh with the respective outer teeth, and the respective outer teeth and the inner teeth are helical teeth respectively having tooth traces twisted in a direction opposite to the first thread groove and the second thread groove. 
     According to the above configuration, a steering apparatus can be downsized and reduced in weight with its assisting performance maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a partial cross-sectional view showing the schematic configuration of a steering apparatus of an embodiment; 
         FIG. 2  is an enlarged cross-sectional view in the vicinity of a steering force assisting unit of the embodiment; 
         FIG. 3  is a plan view of the ball screw nut of the embodiment; 
         FIG. 4  is a cross-sectional view of the transmission mechanism of the embodiment (cross-sectional view taken along the line IV-IV in  FIG. 2 ); 
         FIG. 5  is a partial cutaway view showing the front face structure of the transmission mechanism of the embodiment; 
         FIG. 6A  is a schematic diagram showing force acting on the ball screw nut of the embodiment; and 
         FIG. 6B  is a schematic diagram showing force acting on the ball screw nut of a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of a steering apparatus will be described with reference to the drawings. As shown in  FIG. 1 , a steering apparatus  1  includes a pinion shaft  2  that rotates with a steering operation and a rack shaft  3  serving as a steering shaft that moves reciprocally in a shaft direction with the rotation of the pinion shaft  2  to change the steering angle of a steering wheel (not shown). In addition, the steering apparatus  1  has a rack housing  5  serving as a housing in which the rack shaft  3  is arranged to be reciprocally movable. 
     The rack housing  5  has a first housing  6  formed in a cylindrical shape and a second housing  7  formed in a cylindrical shape and fixed to an end on one side (left side in  FIG. 1 ) in the shaft direction of the first housing  6 . At an end on the other side (right side in  FIG. 1 ) opposite to the second housing  7  of the first housing  6 , the pinion shaft  2  is rotatably accommodated in a state of obliquely crossing the rack shaft  3 . The rack teeth of the rack shaft  3  and the pinion teeth of the pinion shaft  2  mesh with each other to form a rack and pinion mechanism (not shown). Note that the pinion shaft  2  is coupled to a steering shaft, and a steering wheel is fixed to the tip end of the steering shaft (both the steering shaft and the steering wheel are not shown). Accordingly, in the steering apparatus  1 , the pinion shaft  2  rotates with a steering operation, and the rotation is converted into the shaft-direction movement of the rack shaft  3  with the rack and pinion mechanism to change the steering angle of the steering wheel, i.e., the traveling direction of a vehicle. 
     In addition, the steering apparatus  1  includes a steering force assisting unit  11  that applies an assisting force to a steering system. The steering force assisting unit  11  has an electric motor  12  arranged parallel to the rack shaft  3 . The steering force assisting unit  11  is configured to transmit the rotation of the electric motor  12  to a ball screw mechanism  14  via a transmission mechanism  13  and convert the transmitted rotation into the reciprocating movement of the rack shaft  3  with the ball screw mechanism  14  to apply the assisting force to the steering system. That is, the steering apparatus  1  of the embodiment is configured as so-called a rack parallel electric power steering apparatus. 
     Specifically, as shown in  FIG. 2 , the first housing  6  has a first cylindrical part  21  formed in a cylindrical shape and a first accommodation part  22  formed at an end of the first cylindrical part  21  on the side of the second housing  7  (left side in  FIG. 2 ). The first accommodation part  22  is formed in a cylindrical shape having a diameter greater than that of the first cylindrical part  21 . In addition, the partial peripheral wall of the first accommodation part  22  is formed as a protrusion part  23  having a shape protruding toward a side (lower side in  FIG. 2 ) where the electric motor  12  is arranged. The protrusion part  23  has, at the bottom thereof, an insertion hole  24  penetrating in the shaft direction of the rack shaft  3 . Further, the electric motor  12  is fixed to the external bottom face of the protrusion part  23  by a bolt  25 , and a rotating shaft  12   a  of the electric motor  12  is arranged in the protrusion part  23  via the insertion hole  24 . 
     The second housing  7  has a second cylindrical part  31  formed in a cylindrical shape and a second accommodation part  32  formed at an end of the second cylindrical part  31  on the side of the first housing  6  (right side in  FIG. 2 ). The second accommodation part  32  is formed in a cylindrical shape having a diameter greater than that of the second cylindrical part  31 . In addition, the second accommodation part  32  has a plate-shaped cover  33  extending to the side of the electric motor  12  to cover the protrusion part  23 . 
     The transmission mechanism  13  includes a driving pulley  41  accommodated inside the protrusion part  23  and coupled to be integrally rotatable with the rotating shaft  12   a  of the electric motor  12 , a driven pulley  42  rotatably accommodated inside the first accommodation part  22  and arranged at the outer periphery of the rack shaft  3 , and a belt  43  wound around the driving pulley  41  and the driven pulley  42 . Note that the belt  43  is made of an elastic material such as rubber. The ball screw mechanism  14  includes a ball screw nut  46  provided to be integrally rotatable with the driven pulley  42  and a plurality of balls  47 . The ball screw nut  46  is screwed with the rack shaft  3  via the plurality of balls  47  to form the ball screw mechanism  14 . 
     More specifically, the driving pulley  41  is formed in a cylindrical shape. Further, the driving pulley  41  is fixed to the outer periphery of the rotating shaft  12   a  in an integrally rotatable manner such that it is arranged coaxially with the rotating shaft  12   a  of the electric motor  12 . 
     The driven pulley  42  is formed in a cylindrical shape and has a wound part  51  where the belt  43  is wound an extending part  52  extending from the wound part  51  to the side of the second housing  7 . In addition, the driven pulley  42  has a diameter increasing part  53  at the end thereof on the side of the first housing  6 . The bore diameter of the diameter increasing part  53  is greater than that of the part of the driven pulley  42  on the side of the second housing  7 . 
     The ball screw nut  46  is formed in a cylindrical shape. In addition, the ball screw nut  46  has, at the end thereof on the side of the first housing  6 , an annular flange part  54  extending outward in the radial direction. Note that the outside diameter of the flange part  54  is set to be substantially equal to the bore diameter of the diameter increasing part  53 . On the other hand, the ball screw nut  46  has a male screw part  55  at the end thereof on the side of the second housing  7  of the ball screw nut  46 . 
     At the outer periphery of the ball screw nut  46 , the driven pulley  42  is fitted to allow the flange part  54  to be arranged in the diameter increasing part  53  and a rolling bearing  56  serving as a bearing is fitted to be next to the extending part  52 . 
     Further, a lock nut  57  is screwed to the male screw part  55 , and the driven pulley  42  and the rolling bearing  56  are held between the lock nut  57  and the flange part  54 . Thus, the driven pulley  42  and the inner ring of the rolling bearing  56  are fixed to be integrally rotatable with the ball screw nut  46 . In a state in which the driven pulley  42  is provided to be integrally rotatable with the ball screw nut  46  as described above, the wound part  51  of the embodiment is arranged in a range from the end of the ball screw nut  46  on the side of the first housing  6  to the vicinity of the central part in the shaft direction thereof, and the belt  43  is arranged to overlap with the above range (range where the wound part  51  is arranged) of the ball screw nut  46  in the shaft direction. 
     The rolling bearing  56  provided at the outer periphery of the ball screw nut  46  is fixed to be arranged coaxially with the rack shaft  3  inside the second accommodation part  32  of the second housing  7 . Thus, the driven pulley  42  and the ball screw nut  46  are accommodated to be rotatable coaxially with the rack shaft  3  inside the rack housing  5 . In addition, at the outer periphery of the rolling bearing  56  of the embodiment, an annular elastic member (O-ring)  58  made of an elastic material such as rubber is arranged between the rolling bearing  56  and the second accommodation part  32  in its compressed state. Moreover, on both sides of the rolling bearing  56  in the shaft direction, annular elastic members  59  are arranged between the rolling bearing  56  and the first housing  6  and between the rolling bearing  56  and the second housing  7  in their compressed state. That is, only one end of the ball screw nut  46  is supported by the rolling bearing  56  elastically supported inside the rack housing  5 . 
     Further, at the inner periphery of the ball screw nut  46 , a thread groove  61  is formed. Note that the thread groove  61  of the embodiment is a right hand thread groove. Furthermore, the thread groove  61  is formed in a range from the end of the ball screw nut  46  on the side of the first housing  6  to the part thereof slightly before the male screw part  55 . 
     On the other hand, at the outer periphery of the rack shaft  3 , a right hand thread groove  62  is formed corresponding to the right hand thread groove  61 . Note that the thread groove  62  is formed over a prescribed range substantially equal to the range of the rack shaft  3  where the rack teeth are formed. Further, a spiral ball raceway R1 is formed by the thread grooves  61  and  62 . In the ball raceway R1, the balls  47  are provided in a state of being held between the thread groove  61  of the ball screw nut  46  and the thread groove  62  of the rack shaft  3 . That is, the ball screw nut  46  is screwed to the outer periphery of the rack shaft  3  via the balls  47 . 
     As shown in  FIGS. 2 and 3 , the ball screw nut  46  has a circulating path R2 that takes a shortcut between connecting points P1 and P2 set at two spots inside the thread groove  62 . Specifically, the ball screw nut  46  has an attachment hole  63 . The portions of the attachment hole  63  corresponding to the connecting points P1 and P2 penetrate inward and outward. Further, the circulating path R2 is formed with a circulating member (deflector)  64  attached to the attachment hole  63 . The circulating member  64  has the function of scooping the balls  47  from the ball raceway R1 and the function of ejecting the balls  47  to the ball raceway R1. 
     Thus, the thread groove  62  of the ball screw nut  46  has an area between the connecting points P1 and P2 serving as a rolling area T1 where the balls  47  roll and an area other than the rolling area T1 serving as a non-entering area T2 where the balls  47  do not enter. Note that in  FIG. 3 , only the non-entering area T2 is hatched for the purpose of illustration. In the embodiment, the connecting point P1 on one side is set at a position close to the flange part  54  of the ball screw nut  46 , while the connecting point P2 on the other side is set at a position closer to the male screw part  55  than the central part in the shaft direction of the ball screw nut  46 . Between the connecting points P1 and P2, the thread groove  62  for a several number of turns is included. Further, since the belt  43  is arranged to overlap with the range from the end of the ball screw nut  46  on the side of the first housing  6  (flange part  54 ) to the vicinity of the central part in the shaft direction thereof as described above, part of the belt  43  is arranged to overlap with the rolling area T1 in the shaft direction. 
     In the ball screw mechanism  14  thus formed, when the ball screw nut  46  rotates relative to the rack shaft  3  and the balls  47  receive friction force from the rack shaft  3  and the ball screw nut  46 , the balls  47  roll inside the ball raceway R1 to transmit the torque of the ball screw nut  46  to the rack shaft  3 , thereby moving the rack shaft  3  in the shaft direction relative to the ball screw nut  46 . In addition, the balls  47  having reached one end (connecting point P1 or P2) of the ball raceway R1 after rolling inside the ball raceway R1 pass through the circulating path R2 formed in the ball screw nut  46  to be ejected to the other side (connecting point P2 or P1) of the ball raceway R1. As a result, the balls  47  move from a downstream side to an upstream side in a ball moving direction inside the ball raceway R1. That is, the ball screw mechanism  14  is such that the respective balls  47  rolling inside the ball raceway R1 infinitely circulate via the circulating path R2 to allow the rotation of the ball screw nut  46  to be converted into the shaft-direction movement of the rack shaft  3 . Further, the steering apparatus  1  rotary drives the ball screw nut  46  using the electric motor  12  and transmits the torque to the rack shaft  3  as a pressing force in the shaft direction to apply an assisting force for assisting a steering operation to the steering system. 
     Next, the coupling structure between the driving pulley and the driven pulley and the belt will be described. As shown in  FIGS. 4 and 5 , the driving pulley  41  has outer teeth  41  a protruding outward in the radial direction. In addition, the wound part  51  of the driven pulley  42  has outer teeth  42   a  protruding outward in the radial direction. On the other hand, the belt  43  has inner teeth  43   a  to mesh with the outer teeth  41   a  and  42   a.  Further, the belt  43  is wound around the respective pulleys  41  and  42  with the inner teeth  43   a  meshing with the respective outer teeth  41   a  and  42   a.  Note that since the belt  43  is slightly expanded in a state of being wound around the respective pulleys  41  and  42 , a prescribed tension is generated in the belt  43 . Further, in  FIG. 5 , only the tooth traces of the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are shown for the purpose of illustration. 
     Further, the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are configured as left hand helical teeth having tooth traces twisted in a direction opposite to the respective thread grooves  61  and  62 . More specifically, helix angles θt of the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are set to be equal to lead angles θ1 of the thread grooves  61  and  62 , respectively. Note that the lead angles θ1 of the thread grooves  61  and  62  are the same. In  FIG. 5 , only the lead angle θ1 of the thread groove  61  of the rack shaft  3  is shown for the purpose of illustration. 
     Next, the operation of the embodiment will be described. Since force that the driven pulley  42  receives from the belt  43  acts in a direction (teeth orthogonal direction) orthogonal to the tooth traces of the outer teeth  42   a  as described in  FIG. 6A , the ball screw nut  46  is rotary driven by the force in the teeth orthogonal direction. On the other hand, since the respective balls  47  are arranged in a direction (lead direction) in which the respective thread grooves  61  and  62  extend inside the ball raceway R1 as described above, they roll in the lead direction. 
     Here,  FIG. 6B  shows, as a comparative example, a case in which outer teeth  42   a′  and inner teeth  43   a′  are formed as helical teeth twisted in the same direction as the thread grooves  61  and  62  and thus the teeth orthogonal direction is greatly deviated from the lead direction. In this case, a component of the force in a direction crossing the direction in which the balls  47  roll acts on the ball screw nut  46 . In a state in which the teeth orthogonal direction is greatly deviated from the lead direction as described above, the ball screw nut  46  is likely to be inclined toward the rack shaft  3 , which may hinder the smooth rotation of the ball screw nut  46 . 
     On the contrary, since the outer teeth  42   a  and the inner teeth  43   a  of the embodiment are configured as the helical teeth twisted in the direction opposite to the thread grooves  61  and  62  at the same angle as the lead angles θ1 of the thread grooves  61  and  62  as described above, the teeth orthogonal direction substantially corresponds to the lead direction. Therefore, the component of the force in the direction crossing the direction in which the balls  47  roll hardly acts on the ball screw nut  46 , the ball screw nut  46  is effectively prevented from being inclined toward the rack shaft  3 , and the ball screw nut  46  is smoothly rotated. Thus, the rotation of the ball screw nut  46  is converted into the shaft-direction movement of the rack shaft  3  with a high degree of efficiency. 
     Next, the effects of the embodiment will be described. First, the outer teeth  41   a  and  42   a  of the respective pulleys  41  and  42  and the inner teeth  43   a  of the belt  43  are formed as the helical teeth twisted in the direction opposite to the thread grooves  61  and  62 . According to this configuration, the rotation of the electric motor  12  can be converted into the reciprocating movement of the rack shaft  3  with a high degree of efficiency. Therefore, even if a downsized and lightweight electric motor that produces low torque is used as the electric motor  12 , an adequate amount of assisting force can be applied to the steering system. Accordingly, the steering apparatus  1  can be downsized and reduced in weight with its assisting performance maintained. 
     In addition, the belt  43  is arranged such that part of the belt  43  overlaps, in the shaft direction, with the rolling area T1 of the thread groove  62  of the ball screw nut  4 . According to this configuration, part of the force acting on the ball screw nut  46  from the belt  43  acts on the part of the ball screw nut  46  where the balls  47  exist on the inner periphery side. Here, at the part of the ball screw nut  46  where the balls  47  exist on the inner periphery side, the gap between the ball screw nut  46  and the rack shaft  3  is secured by the balls  47 . Therefore, compared with a case in which the entire force acting on the ball screw nut  46  from the belt  43  via the driven pulley  42  acts on the part of the ball screw nut  46  where the balls  47  do not exist on the inner periphery side, the ball screw nut  46  can be prevented from being inclined. 
     Moreover, only one end of the ball screw nut  46  is rotatably supported by the rolling bearing  56  provided inside the second accommodation part  32  of the rack housing  5 . Therefore, compared with a case in which the both ends of the ball screw nut  46  is rotatably supported, the number of parts can be reduced. However, since this configuration supports the ball screw nut  46  at the one end thereof, the ball screw nut  46  is likely to be inclined toward the rack shaft  3 . Accordingly, the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are formed as the helical teeth twisted in the direction opposite to the thread grooves  61  and  62 , and part of the belt  43  is arranged to overlap with the rolling area T1 of the thread groove  61  in the shaft direction. These configurations produce a great effect to prevent the ball screw nut  46  from being inclined. 
     Furthermore, since the annular elastic member  58  is provided between the rolling bearing  56  and the second accommodation part  32  of the rack housing  5 , the rolling bearing  56  is elastically supported by the elastic member  58  inside the rack housing  5 . 
     Therefore, even if the ball screw nut  46  is decentered relative to the rack shaft  3  due to an assembly error or the like, the smooth rotation of the ball screw nut  46  is possible but the ball screw nut  46  is likely to be inclined toward the rack shaft  3 . Accordingly, the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are formed as the helical teeth twisted in the direction opposite to the thread grooves  61  and  62 , and part of the belt  43  is arranged to overlap with the rolling area T1 of the thread groove  61  in the shaft direction. These configurations produce a great effect to prevent the ball screw nut  46  from being inclined. 
     The above embodiment may be appropriately modified and carried out as follows. In the above embodiment, the elastic member  58  is interposed between the rolling bearing  56  and the second accommodation part  32  to elastically support the rolling bearing  56 . However, instead of this configuration, the rolling bearing  56  may be rigidly supported inside the second accommodation part  32  without the elastic member  58 . 
     In the above embodiment, only one end of the ball screw nut  46  is rotatably supported by the rolling bearing  56 . However, instead of this configuration, both ends of the ball screw nut  46  may be each rotatably supported. 
     In the above embodiment, the belt  43  is arranged such that part of the belt  43  overlaps with the rolling area T1 of the ball screw nut  46  in the shaft direction. However, instead of this configuration, the belt  43  may be arranged such that the entire belt  43  overlaps with the rolling area T1 of the ball screw nut  46  in the shaft direction. 
     Further, the belt  43  may be arranged such that the entire belt  43  does not overlap with the rolling area T1 of the ball screw nut  46  in the shaft direction. 
     In the above embodiment, the helix angles θt of the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are set to be equal to the lead angles θ1 of the thread grooves  61  and  62 , respectively. However, instead of this configuration, as long as the tooth traces of the outer teeth  41   a  and  42   a  and the inner teeth  43   a  are twisted in the direction opposite to the thread grooves  61  and  62 , the helix angles θt of the outer teeth  41   a  and  42   a  and the inner teeth  43   a  may be set to be different from the lead angles θ1 of the thread grooves  61  and  62 , respectively. 
     In the above embodiment, the steering apparatus  1  is configured such that the rack shaft  3  is allowed to move reciprocally with a steering operation (i.e., the steering apparatus  1  serves mainly as a front-wheel steering apparatus). However, instead of this configuration, the steering apparatus  1  may be configured such that the rack shaft  3  is allowed to move reciprocally only with the torque of the electric motor  12  (i.e., the steering apparatus  1  serves as, for example, a rear-wheel steering apparatus or the like).