Patent Publication Number: US-2021188347-A1

Title: Steering device

Description:
TECHNICAL FIELD 
     The present invention relates to a steering device. 
     BACKGROUND ART 
     Patent Document 1 discloses a steering device having a hydraulic first power steering mechanism and an electric second power steering mechanism. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. JP2016-150645 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In the above conventional steering device, since an electric motor directly rotates a steering shaft, there is a risk that size of the electric motor will be increased. 
     An object of the present invention is therefore to provide a steering device that is capable of suppressing increase in size of the electric motor. 
     Solution to Problem 
     According to one aspect of the present invention, an electric motor rotates a steering shaft through a reduction mechanism, and the reduction mechanism and a torque sensor are accommodated in an integrally-structured housing. 
     Effects of Invention 
     Therefore, increase in size of the electric motor can be suppressed. Also, a structure can be simplified, and increase in size, in a direction of a rotation axis of the steering shaft, can be suppressed. 
     That is, in a case where the housing is formed by two members, a bolt, a nut, a bolt flange, a sealing structure, etc. are needed to connect these two members. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a steering system to which a steering device  1  according to an embodiment 1 of the present invention is applied.  FIG. 2  is a perspective view of a first power steering mechanism A and a second power steering mechanism B of the embodiment 1. 
         FIG. 3  is a local sectional view of the second power steering mechanism B of the embodiment 1. 
         FIG. 4  is a sectional view of the first power steering mechanism A of the embodiment 1. 
         FIG. 5  is an enlarged local sectional view of the first power steering mechanism A of the embodiment 1. 
         FIG. 6A  is a perspective exploded view of a plate  16  of the embodiment 1.  FIG. 6B  is a front view of the plate  16  viewed from opening side, located at a second housing  10  side, of a first housing  9 . 
         FIG. 7A  is a perspective exploded view of a first yoke  123 , a second yoke  124  and a yoke retaining member  126  of the embodiment 1.  FIG. 7B  is a perspective view showing a state in which the first yoke  123 , the second yoke  124  and the yoke retaining member  126  are assembled. 
         FIG. 8A  is a perspective view showing a state in which a magnet  121  and a magnet retaining member  122  of the embodiment 1 are assembled.  FIG. 8B  is a perspective view showing a positional relationship of the magnet  121 , the first yoke  123  and the second yoke  124  of the embodiment 1. 
         FIG. 9  is a sectional view of the first power steering mechanism A according to an embodiment 2 of the present invention. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Embodiments for carrying out the present invention will be explained below with reference to the drawings. 
     Embodiment 1 
       FIG. 1  is a schematic diagram showing a configuration of a steering system to which a steering device  1  according to an embodiment 1 is applied. 
     The steering device  1  has, as a steering mechanism  2 , a steering shaft  3  and a transmission mechanism  4 . The steering shaft  3  has a steering wheel  400  that is operated by a driver. The steering shaft  3  inputs a rotation force (a steering torque) by a steering operation of the steering wheel  400  by the driver. The steering shaft  3  rotates by and according to the steering operation. Further, the steering shaft  3  transmits a steering reaction force to the driver. 
     The rotation force (the steering torque) of the steering shaft  3  is transmitted to a gear mechanism C. 
     The gear mechanism C is a ball-nut type mechanism. The details will be described later. 
     A piston  11  of the gear mechanism C moves in up and down directions in the drawing, then a sector gear  406  rotates. 
     The transmission mechanism  4  has a sector shaft  407 , a pitman arm  408 , a drag link  409  and a tie rod  410 . 
     The sector shaft  407  is fixed to the sector gear  406 . 
     The sector shaft  407  extends coaxially with the sector gear  406 , and rotates integrally with the sector gear  406 . 
     The pitman arm  408  is connected to the sector shaft  407  and the drag link  409 . 
     The drag link  409  is connected to the pitman arm  408  and a knuckle arm  411 R of one steered wheel  412 R. 
     Regarding the tie rod  410 , for instance, in a case of a rigid axle suspension, the tie rod  410  is connected to knuckle arms  411 L and  411 R of both steered wheels  412 L and  412 R. 
     The pitman arm  408  transmits rotation of the sector shaft  407  to the knuckle arm  411 R through the drag link  409 . Movement transmitted to the knuckle arm  411 R is transmitted to the knuckle arm  4111 , of the other steered wheel  412 L through the tie rod  410 . 
     The steering device  1  has a first power steering mechanism A and a second power steering mechanism B. 
     The first power steering mechanism A has a pump device  402 , the piston  11  and a rotary valve  39 . 
     That is, although its detailed configuration will be described later, the first power steering mechanism A is an integral type mechanism in which the piston  11  and the rotary valve  39  are configured integrally with the gear mechanism C. 
     The first power steering mechanism A is a hydraulic power steering mechanism using a hydraulic pressure produced by the pump device  402 . As a working fluid, e.g. a power steering fluid (PSF) is used. 
     The pump device  402  is a hydraulic pressure source that discharges the working fluid. The pump device  402  is provided outside the gear mechanism C. 
     An inlet side of the pump device  402  is connected to a reservoir tank RES provided outside the gear mechanism C through a pipe (a fluid passage)  403 . 
     An outlet side of the pump device  402  is connected to the after-mentioned rotary valve  39  through a pipe (a fluid passage)  404 . 
     The pump device  402  is controlled and driven by a drive source  401  such as an engine or an electric motor. 
     The second power steering mechanism B is an electric power steering mechanism directly using power of an after-mentioned electric motor  7  as a steering assist force. Its detailed configuration will be described later. 
       FIG. 2  is a perspective view of the steering device  1  having the first power steering mechanism A, the second power steering mechanism B and the gear mechanism C according to the embodiment 1. 
     The second power steering mechanism B is configured from a worm shaft  81  of a worm gear  8  as a speed reducer (or a reduction gear) accommodated in a first housing body  91  of a first housing  9 , the electric motor  7  having an electric motor output shaft  71  coaxially connected to the worm shaft  81  and a control unit  13 . 
     The control unit  13  has a control unit body  132  and a power supply connector portion  133 . The control unit body  132  includes a control unit body-housing  132   a  accommodating therein a microprocessor  131 . 
     This microprocessor  131  generates a motor command signal sent to the electric motor  7  on the basis of a steering torque signal sent from a torque sensor  12 . 
     Here, the worm shaft  81 , the electric motor  7 , the control unit body-housing  132   a  and the power supply connector portion  133  are arranged in a direction of a rotation axis Q of the worm shaft  81  in the order of the worm shaft  81 , the electric motor  7 , the control unit body-housing  132   a  and the power supply connector portion  133 . 
     By arranging these components along the direction of the rotation axis Q of the worm shaft  81  in this way, a protruding amount of the second power steering mechanism B in a radial direction of the rotation axis Q of the worm shaft  81  is suppressed, then mountability of the steering device  1  into a vehicle can be improved. 
     A working fluid supply pipe  21  for supplying the working fluid to the after-mentioned rotary valve  39  provided in the first power steering mechanism A is connected to a working fluid supply port  105  provided at a second housing  10 . 
     Further, a working fluid discharge pipe  22  for discharging the working fluid, which is discharged from the rotary valve  39  provided in the first power steering mechanism A, to an outside of the first power steering mechanism A is connected to a working fluid discharge port  106  provided at the second housing  10 . 
     These working fluid supply port  105  and working fluid discharge port  106  are provided so as to protrude to the same side (a right side in the drawing) as the electric motor  7  with respect to an axis S that connects a meshing portion of the worm shaft  81  with an after-mentioned worm wheel  82  and the steering shaft  3  on a plane orthogonal to a rotation axis P of the steering shaft  3 . 
     With this arrangement, it is possible to suppress expansion of an occupied area of the whole steering device with respect to the vehicle. 
     Further, the worm shaft  81  is offset from the working fluid supply port  105  and the working fluid discharge port  106  (in a direction of an arrow R) in a circumferential direction of the rotation axis P of the steering shaft  3  so as not to interfere with the working fluid supply port  105  and the working fluid discharge port  106 . 
     With this arrangement, it is possible to improve workability of connecting work for connecting the working fluid supply pipe  21  and the working fluid discharge pipe  22  to the working fluid supply port  105  and the working fluid discharge port  106  respectively. 
     In addition, the worm shaft  81  can be prevented from interfering with the working fluid supply port  105  and the working fluid discharge port  106 . 
     Furthermore, the worm shaft  81  meshes with the worm wheel  82  obliquely to a plane perpendicular to a rotation axis P of the worm wheel  82  (which is the same as the rotation axis P of the steering shaft  3 ). 
     That is, as shown in  FIG. 2 , the power supply connector portion  133  side of the second power steering mechanism B inclines upward. 
     By this oblique arrangement of the worm shaft  81 , a layout in which the worm shaft  81  can be prevented from interfering with the working fluid supply port  105  and the working fluid discharge port  106  can be easily employed. 
       FIG. 3  is a local sectional view of the second power steering mechanism B of the embodiment 1. 
     The worm shaft  81  coaxially connected to the electric motor output shaft  71  is accommodated in the first housing body  91 , and is supported by a pair of bearings  81   a  and  81   b.    
     The worm shaft  81  meshes with the worm wheel  82  provided at a second shaft  32  of the steering shaft  3 , which forms the worm gear  8  as the speed reducer (or the reduction gear). 
     Here, the first housing body  91  and an electric motor housing  7   a  are fixed together with a plurality of bolts b. 
       FIG. 4  is a sectional view of the first power steering mechanism A of the embodiment 1.  FIG. 5  is an enlarged local sectional view of the first power steering mechanism A of the embodiment 1.  FIG. 6A  is a perspective exploded view of a plate  16  of the embodiment 1.  FIG. 6B  is a front view of the plate  16  viewed from opening side, located at the second housing  10  side, of the first housing  9 .  FIG. 7A  is a perspective exploded view of a first yoke  123 , a second yoke  124  and a yoke retaining member  126  of the embodiment 1.  FIG. 7B  is a perspective view showing a state in which the first yoke  123 , the second yoke  124  and the yoke retaining member  126  are assembled.  FIG. 5A  is a perspective view showing a state in which a magnet  121  and a magnet retaining member  122  of the embodiment 1 are assembled.  FIG. 8B  is a perspective view showing a positional relationship of the magnet  121 , the first yoke  123  and the second yoke  124  of the embodiment 1. 
     First, a configuration will be explained with reference to  FIGS. 4 and 5 . 
     The steering shaft  3  has a first shaft  31 , the second shaft  32 , a third shaft  33 , a fourth shaft  34 , a first torsion bar  35  and a second torsion bar  36 . 
     In  FIG. 4 , at a right direction side, in the direction of the rotation axis P, of the first shaft  31 , the steering wheel  400  is connected. The first shaft  31  inputs the rotation force (the steering torque) by the steering operation of the steering wheel  400  by the driver. The first shaft  31  rotates by and according to the steering operation. Further, the first shaft  31  transmits the steering reaction force to the driver. 
     The first shaft  31  and the second shaft  32  are connected to each other through the first torsion bar  35 . 
     That is, the first shaft  31  has, at a right direction end portion thereof in the drawing, a first shaft-radial direction hole portion  311  formed along a radial direction of the rotation axis P of the steering shaft  3 . The first torsion bar  35  has, at a right direction end portion thereof in the drawing, a first torsion bar radial direction hole portion  351  formed along the radial direction of the rotation axis P of the steering shaft  3 . 
     A fixing pin  18   a  is inserted into both of the first shaft-radial direction hole portion  311  and the first torsion bar radial direction hole portion  351 , then the first shaft  31  and the first torsion bar  35  are connected. 
     Further, the second shaft  32  has, at a left direction end portion thereof in the drawing, a second shaft-radial direction hole portion  325  as a first torsion bar connecting portion formed along the radial direction of the rotation axis P of the steering shaft  3 . The first torsion bar  35  has, at a left direction end portion thereof in the drawing, a first torsion bar radial direction hole portion  352  as a second shaft connecting portion formed along the radial direction of the rotation axis P of the steering shaft  3 . 
     A fixing pin  18   b  is inserted into both of the second shaft-radial direction hole portion  325  and the first torsion bar radial direction hole portion  352 , then the second shaft  32  and the first torsion bar  35  are connected. 
     Between an outer peripheral surface of the first shaft  31  and an inner peripheral surface of the second shaft  32 , a needle bearing  19  as a third bearing and a needle bearing  20  as a fourth bearing are arranged apart from each other in the direction of the rotation axis P of the steering shaft  3 , and rotatably support the first shaft  31  and the second shaft  32 . 
     The third shaft  33  has, at a right direction end portion thereof in the direction of the rotation axis P of the steering shaft  3  in the drawing, a second shaft insertion hole portion  331  opening toward the second shaft  32  in the direction of the rotation axis P of the steering shaft  3  and having an inner peripheral surface where serrations are formed. The left direction end portion, in the direction of the rotation axis P of the steering shaft  3  in the drawing, having an outer peripheral surface where serrations are formed, of the second shaft  32  is inserted into the second shaft insertion hole portion  331 , then the second shaft  32  and the third shaft  33  are connected. 
     Here, a portion (the second shaft-radial direction hole portion  325 , the fixing pin  18   b  and the first torsion bar radial direction hole portion  352 ) where the second shaft  32  and the first tors ion bar  35  are connected overlaps the second shaft insertion hole portion  331  in the direction of the rotation axis P of the steering shaft  3 . With this overlapping arrangement, it is possible to suppress increase in axial direction size of the steering device  1 . 
     The third shaft  33  and the fourth shaft  34  are connected to each other through the second torsion bar  36 . 
     That is, the third shaft  33  has, at a left direction end portion thereof in the drawing, a third shaft-radial direction hole portion  332  formed along the radial direction of the rotation axis P of the steering shaft  3 . The second torsion bar  36  has, at a right direction end portion thereof in the drawing, a second torsion bar radial direction hole portion  361  formed along the radial direction of the rotation axis P of the steering shaft  3 . 
     A fixing pin  18   c  is inserted into both of the third shaft-radial direction hole portion  332  and the second torsion bar radial direction hole portion  361 , then the third shaft  33  and the second torsion bar  36  are connected. 
     Further, as shown in  FIG. 4 , the fourth shaft  34  has, at a left direction end portion thereof, a fourth shaft-radial direction hole portion  341  formed along the radial direction of the rotation axis P of the steering shaft  3 . The second torsion bar  36  has, at a left direction end portion thereof in the drawing, a second torsion bar radial direction hole portion  362  formed along the radial direction of the rotation axis P of the steering shaft  3 . 
     A fixing pin  18   d  is inserted into both of the fourth shaft-radial direction hole portion  341  and the second torsion bar radial direction hole portion  362 , then the fourth shaft  34  and the second tors ion bar  36  are connected. 
     The first housing  9  has the first housing body  91 , a sensor accommodating space  92  and a reduction gear accommodating space  93 . 
     The first housing body  91  is formed as a single-piece member by molding. 
     Therefore, a structure of the first housing  9  can be simplified, and increase in size of the first housing  9  in the direction of the rotation axis P of the steering shaft  3  can be suppressed. That is, in a case where the first housing  9  is formed by two members, a bolt, a nut, a bolt boss (a flange), a sealing structure, etc. are needed to connect these two members, and this consequently causes increase in size. 
     The sensor accommodating space  92  is provided at an inner side of the first housing body  91 , and accommodates the torque sensor  12 . 
     The reduction gear accommodating space  93  is provided at the inner side of the first housing body  91 , and accommodates the worm gear  8  as the speed reducer (or the reduction gear). 
     The second shaft  32  further has a second shaft-torque sensor retaining portion  322  to retain (or support) the torque sensor  12 , a second shaft-first bearing retaining portion  323  to retain (or support) a first ball bearing  14  as a first bearing and a reduction gear fixing portion  321 . 
     An outside diameter, in the radial direction of the rotation axis P of the steering shaft  3 , of the second shaft-first bearing retaining portion  323  is formed to be greater than an outside diameter, in the radial direction of the rotation axis P of the steering shaft  3 , of the second shaft-torque sensor retaining portion  322 . 
     With this, since the first ball bearing  14  has an inside diameter that is greater than the outside diameter of the second shaft-torque sensor retaining portion  322  of the second shaft  32 , it is possible to sufficiently secure a capacity of load which the first ball bearing  14  receives. 
     The reduction gear fixing portion  321  is located between the first ball bearing  14 , as the first bearing, having a first inner race  141 , balls  142  and a first outer race  143  and a second ball bearing  15 , as a second bearing, having a second inner race  151 , balls  152  and a second outer race  153  in the direction of the rotation axis P of the steering shaft  3 . 
     With this, it is possible to suppress fall-down or leaning of the second shaft  32  with respect to the first housing  9 , and also suppress fall-down or leaning of the third shaft  33  that is connected to the second shaft  32 . Further, since the third shaft  33  is provided with a third shaft valve portion  37  that forms the rotary valve  39 , an influence on operating characteristics of the rotary valve  39  due to the fall-down of the third shaft  33  can also be suppressed. 
     The worm wheel  82  of the worm gear  8  is fixed to the reduction gear fixing portion  321  through a key  8   a.    
     The first ball bearing  14  is provided at the first housing body  91  of the first housing  9 . The first ball bearing  14  supports the second shaft  32  with the first outer race  143  being fixed to the first housing body  91  and the first inner race  141  being fixed to the second shaft-first bearing retaining portion  323  of the second shaft  32 . 
     Here, the first ball bearing  14  is located between the two needle bearings  19  and  20 . 
     With this, the first shaft  31  and the second shaft  32  can be supported with respect to the first housing  9  with a good balance. 
     The second ball bearing  15  is provided at the first housing body  91  of the first housing  9 . The second ball bearing  15  is spaced apart from the first ball bearing  14  in the direction of the rotation axis P of the steering shaft  3 , and supports the second shaft  32  with respect to the first housing  9 . 
     Here, the torque sensor  12 , the first ball bearing  14  and the worm gear  8  are arranged in the direction of the rotation axis P of the steering shaft  3  in the order of the torque sensor  12 , the first ball bearing  14  and the worm gear  8 . 
     With this arrangement, the first ball bearing  14  can receive a rotation force inputted to the second shaft  32  from the electric motor  7  through the worm gear  8  at a closer position with respect to the torque sensor  12 . it is therefore possible to suppress fall-down or leaning and flexure of the second shaft  32 , due to transmission of the rotation force from the electric motor  7 , at the second shaft-torque sensor retaining portion  322  where the torque sensor  12  is provided, thereby suppressing decrease in detection accuracy of the torque sensor  12 . 
     Fixing of the second ball bearing  15  as the second bearing to the first housing body  91  will be explained with reference to  FIGS. 4 to 6 . 
     The plate  16  formed from a first plate  16   a  and a second plate  16   b  is accommodated in the first housing body  91  at a closer position to the second housing  10  with respect to the worm gear  8  in the direction of the rotation axis P of the steering shaft  3 , and is fixed to the first housing body  91  with three bolts c. 
     With this, for instance, in a case where the plate  16  is fixed with the plate  16  being sandwiched between the first housing  9  and the second housing  10 , seals are needed between the first housing  9  and the plate  16  and between the plate  16  and the second housing  10 . However, by accommodating the plate  16  in the first housing  9  in the manner of the present embodiment, the seal between the plate  16  and the first housing  9  is not required, thereby suppressing increase in parts count. 
     The first plate  16   a  and the second plate  16   b  have plate second bearing retaining portions  16   a   1  and  16   b   1  to retain (or support) the second ball bearing  15  respectively. 
     These plate second bearing retaining portions  16   a   1  and  16   b   1  sandwich the second outer race  153  of the second ball bearing  15  from both sides in the direction of the rotation axis P of the steering shaft  3 , then retain the second ball bearing  15 . 
     The second shaft  32  further has a second shaft-second bearing retaining portion  324  to retain (or support) the second ball bearing  15 . 
     The second shaft-second bearing retaining portion  324  has a contact surface  324   a  that contacts one of a pair of end surfaces, in the direction of the rotation axis P of the steering shaft  3 , of the second inner race  151 . A C-ring  17  as a second inner race fixing member is fitted into a groove  324   b  formed at the second shaft  32 , and contacts the other of the pair of end surfaces, in the direction of the rotation axis P of the steering shaft  3 , of the second inner race  151 , then fixes the second inner race  151  to the second shaft  32 . 
     In this manner, the second ball bearing  15  is provided between the second shaft  32  and the plate  16  in the radial direction of the rotation axis P of the steering shaft  3 . 
     With this, in a case where the torque sensor  12 , the first ball bearing  14 , the worm gear  8  and the second ball bearing  15  are fixed to the first housing  9  from one direction in this order, since the second ball bearing  15  is fixed to the first housing  9  through the plate  16 , increase in size of the second ball bearing  15  can be suppressed. In other words, in a case where the second ball bearing  15  is fixed to the first housing  9  without through the plate  16 , the second ball bearing  15  requires the same outside diameter as an outside diameter of the plate  16 . 
     Further, both of the second inner race  151  and the second outer race  153  of the second ball bearing  15  are fixed to the second shaft  32  and the first housing  9  respectively from the both sides in the direction of the rotation axis P of the steering shaft  3 . It is therefore possible to retain or support the second shaft  32  with respect to the first housing  9  in the direction of the rotation axis P. 
     A configuration of the second housing  10  will be explained with reference to  FIG. 4 . 
     The second housing  10  has a second housing body  101 , a rotary valve accommodating space  102 , a fourth shaft accommodating space  103  and a fluid chamber  104 . 
     The second housing body  101  is formed by a first member  101   a  and second member  101   b.    
     The first member  101   a  of the second housing body  101  is connected to the first housing  9  with bolts a. 
     The rotary valve accommodating space  102  is provided at an inner side of the first member  101   a  of the second housing body  101 , and accommodates the rotary valve  39 . 
     The fourth shaft accommodating space  103  is provided at inner sides of the first member  101   a  and the second member  101   b  of the second housing body  101 , and accommodates the fourth shaft  34 . 
     The fluid chamber  104  is provided at the inner side of the second member  101   b  of the second housing body  101 . 
     The piston  11  provided inside the second member  101   b  of the second housing body  101  and dividing the fluid chamber  104  into a first fluid chamber  5  and a second fluid chamber  6  provides a steering force to the transmission mechanism  4  according to a pressure difference of the working fluid between the first fluid chamber  5  and the second fluid chamber  6 . 
     The rotary valve  39  is configured by the third shaft valve portion  37  provided on an outer peripheral surface of the third shaft  33  and a fourth shaft valve portion  38  provided on an inner peripheral surface of the fourth shaft  34 . 
     The third shaft valve portion  37  is vertical grooves that extend in the direction of the rotation axis P of the steering shaft  3 , and fluid supply recesses and fluid discharge recesses are provided so as to be alternately arranged in the circumferential direction of the rotation axis P of the steering shaft  3 . 
     The fourth shaft valve portion  38  is vertical grooves that extend in the direction of the rotation axis P of the steering shaft  3 , and right steering recesses and left steering recesses are provided so as to be alternately arranged in the circumferential direction of the rotation axis P of the steering shaft  3 . 
     With this structure, the rotary valve  39  selectively supplies the working fluid, which is supplied from the pump device  402  through the pipe  404 , to the first fluid chamber  5  and the second fluid chamber  6  according to torsion of the second torsion bar  36 . 
     The gear mechanism C is the ball-nut type mechanism, and has a ball-nut mechanism  500 , the fourth shaft  34  having an outer periphery where a screw groove  503  is formed and the sector gear  406 . 
     The ball-nut mechanism  500  has the piston  11  and a plurality of balls  501 . 
     The piston  11  is tubular in shape, and has a screw groove  504  on an inner periphery of the tubular shape. 
     A rack  11   a  is provided on one side surface of an outer periphery of the piston  11 . 
     A ball tube  502  is secured to the other side surface of the outer periphery of the piston  11 . 
     The fourth shaft  34  is inserted into an inner peripheral side of the piston  11 . The piston  11  is fitted onto the fourth shaft  34  so as to be able to move relative to the fourth shaft  34  in the axial direction. 
     The plurality of balls  501  are accommodated between the screw groove  504  of the piston  11  and the screw groove  503  of the fourth shaft  34 . 
     The sector gear  406  meshes with the rack  11   a  of the piston  11 . 
     When the fourth shaft  34  rotates, the balls  501  roll and move in the grooves  503  and  504 , and this moves the piston  11  in the direction of the rotation axis P of the steering shaft  3 . The balls  501  circulate in the grooves  503  and  504  through the ball tube  502 . When the piston  11  moves in the direction of the rotation axis P of the steering shaft  3 , the sector gear  406  rotates, and the movement of the piston  11  is transmitted to the sector shaft  407 . 
     Next, a detailed configuration of the torque sensor  12  will explained with reference to  FIGS. 7A, 7B, 8A and 8B . 
     The torque sensor  12  is configured from the magnet  121 , the magnet retaining member  122 , the first yoke  123 , the second yoke  124 , the yoke retaining member  126  and a magnetic sensor  125  (see  FIG. 5 ). 
     The magnet  121  has an annular shape, and N-pole and S-pole are alternately arranged in the circumferential direction of the rotation axis P of the steering shaft  3 . The N-pole and the S-pole could be provided as a pair or a plurality of pairs. 
     The magnet retaining member  122  is made of metal material, and has a plastic deformation fixing portion  122   a  and a magnet retaining portion  122   b  having a plurality of nail portions  122   c.    
     The plastic deformation fixing portion  122   a  is fixed to the second shaft  32  by plastic deformation (by swaging, pressing or squeezing). The magnet retaining portion  122   b  retains (or supports) the magnet  121  by the plurality of nail portions  122   c.    
     The first yoke  123  is made of magnetic material, and is provided at the first shaft  31 . The first yoke  123  has a first annular portion  123   a  and a plurality of nail portions  123   b.    
     The plurality of nail portions  123   b  are provided at the first annular portion  123   a.  The nail portions  123   b  are arranged in the circumferential direction of the rotation axis P of the steering shaft  3 . Each of the nail portions  123   b  faces the magnet  121  (see  FIG. 8E ) 
     The second yoke  124  is made of magnetic material, and is provided at the first shaft  31 . The second yoke  124  has a second annular portion  124   a  and a plurality of nail portions  124   b.    
     The plurality of nail portions  124   b  are provided at the second annular portion  124   a.  The nail portions  124   b  are arranged such that the nail portion  124   b  and the nail portion  123   b  are alternately arranged in the circumferential direction of the rotation axis P of the steering shaft  3 . Each of the nail portions  124   b  faces the magnet  121  (see  FIG. 8B ). 
     The magnetic sensor  125  is set between the first annular portion  123   a  and the second annular portion  124   a,  and outputs the steering torque signal according to a magnetic field in this setting area of the magnetic sensor  125  (see  FIG. 5 ). 
     Here, the plastic deformation fixing portion  122   a  of the magnet retaining member  122  is located so as to overlap the first annular portion  123   a  and the second annular portion  124   a  in the direction of the rotation axis P of the steering shaft  3 . With this overlapping arrangement, it is possible to suppress increase in axial direction size of the steering device  1 . 
     Next, workings and effects will be explained. The steering device of the embodiment 1 has the following working and effect. 
     (1) The first housing body  91 , having the sensor accommodating space  92  and the reduction gear accommodating space  93  at the inner side thereof, of the first housing  9  has an integral structure formed by a single-piece member by molding. 
     Therefore, a structure of the first housing  9  can be simplified, and increase in size of the first housing  9  in the direction of the rotation axis P of the steering shaft  3  can be suppressed. That is, in a case where the first housing  9  is formed by two members, bolts, nuts, bolt bosses (flanges), sealing structures, etc. are needed to connect these two members, and this consequently causes increase in size. 
     (2) The reduction gear fixing portion  321 , receiving the rotation force transmitted from the electric motor  7  through the worm gear  8  as the speed reducer (or the reduction gear), of the second shaft  32  is supported by the first ball bearing  14  and the second ball bearing  15  at the both side of the reduction gear fixing portion  321  in the direction of the rotation axis P of the steering shaft  3 . 
     Therefore, it is possible to suppress fall-down or leaning of the second shaft  32  with respect to the first housing  9 , and also suppress fall-down or leaning of the third shaft  33  that is connected to the second shaft  32 . Further, since the third shaft  33  is provided with the third shaft valve portion  37  that forms the rotary valve  39 , an influence on operating characteristics of the rotary valve  39  due to the fall-down of the third shaft  33  can also be suppressed. 
     (3) The plate  16  is fixed to the first housing  9  at a closer position to the second housing  10  with respect to the worm gear  8  in the direction of the rotation axis P of the steering shaft  3 . The first plate  16   a  and the second plate  16   b,  which form the plate  16 , have the plate second bearing retaining portions  16   a   1  and  16   b   1  to retain the second ball bearing  15  respectively. The second ball bearing  15  is provided between the second shaft  32  and the plate  16  in the radial direction of the rotation axis P of the steering shaft  3 . 
     Therefore, in a case where the torque sensor  12 , the first ball bearing  14 , the worm gear  8  and the second ball bearing  15  are fixed to the first housing  9  from one direction in this order, since the second ball bearing  15  is fixed to the first housing  9  through the plate  16 , increase in size of the second ball bearing  15  can be suppressed. In other words, in a case where the second ball bearing  15  is fixed to the first housing  9  without through the plate  16 , the second ball bearing  15  requires the same outside diameter as the outside diameter of the plate  16 , and consequently, the second ball bearing  15  becomes large. 
     (4) The plate  16  is fixed to the first housing  9  with the plate  16  being accommodated in the first housing  9 . 
     Therefore, for instance, in a case where the plate  16  is fixed with the plate  16  being sandwiched between the first housing  9  and the second housing  10 , seals are needed between the first housing  9  and the plate  16  and between the plate  16  and the second housing  10 . In contrast, by accommodating the plate  16  in the first housing  9 , the seal between the plate  16  and the first housing  9  is not required, thereby suppressing increase in parts count. 
     (5) The torque sensor  12 , the first ball bearing  14  and the worm gear  8  are arranged in the direction of the rotation axis P of the steering shaft  3  in the order of the torque sensor  12 , the first ball bearing  14  and the worm gear  8 . 
     Therefore, the first ball bearing  14  can receive the rotation force inputted to the second shaft  32  from the electric motor  7  through the worm gear  8  at a closer position with respect to the torque sensor  12 . It is therefore possible to suppress fall-down or leaning and flexure of the second shaft  32 , due to transmission of the rotation force from the electric motor  7 , at the second shaft-torque sensor retaining portion  322  where the torque sensor  12  is provided, thereby suppressing decrease in detection accuracy of the torque sensor  12 . 
     (6) The second shaft  32  has the second shaft-torque sensor retaining portion  322  to retain the torque sensor  12  and the second shaft-first bearing retaining portion  323  to retain the first ball bearing  14 . The outside diameter, in the radial direction of the rotation axis P of the steering shaft  3 , of the second shaft-first bearing retaining portion  323  is formed to be greater than the outside diameter, in the radial direction of the rotation axis P of the steering shaft  3 , of the second shaft-torque sensor retaining portion  322 . 
     Therefore, since the first ball bearing  14  has the inside diameter that is greater than the outside diameter of the second shaft-torque sensor retaining portion  322  of the second shaft  32 , it is possible to sufficiently secure a capacity of load which the first ball bearing  14  receives. 
     (7) The plate  16  and the C-ring  17  as the second inner race fixing member are provided, and the plate  16  is fixed to the first housing  9  at a closer position to the second housing  10  with respect to the worm gear  8  in the direction of the rotation axis P of the steering shaft  3 . The plate  16  has the plate second bearing retaining portions  16   a   1  and  16   b   1  to retain the second ball bearing  15 . The second ball bearing  15  has the second inner race  151 , the balls  152  and the second outer race  153 . The plate second bearing retaining portions  16   a   1  and  16   b   1  sandwich the second outer race  153  from both sides in the direct ion of the rotation axis P of the steering shaft  3 , then retain the second ball bearing  15 . The second shaft  32  has the second shaft-second bearing retaining portion  324  to retain the second ball bearing  15 . The second shaft-second bearing retaining portion  324  has the contact surface  324   a  that contacts one of the pair of end surfaces, in the direction of the rotation axis P of the steering shaft  3 , of the second inner race  151 . The C-ring  17  contacts the other of the pair of end surfaces, in the direction of the rotation axis P of the steering shaft  3 , of the second inner race  151 , then fixes the second inner race  151  to the second shaft  32 . 
     Therefore, both of the second inner race  151  and the second outer race  153  of the second ball bearing  15  are fixed to the second shaft  32  and the first housing  9  respectively from the both sides in the direction of the rotation axis P of the steering shaft  3 . It is thus possible to retain the second shaft  32  with respect to the first housing  9  in the direction of the rotation axis P. 
     (8) The second power steering mechanism B is configured such that the worm shaft  81 , the electric motor  7 , the control unit body-housing  132   a  and the power supply connector portion  133  are arranged in the direction of the rotation axis Q of the worm shaft  81  in the order of the worm shaft  81 , the electric motor  7 , the control unit body-housing  132   a  and the power supply connector portion  133 . 
     Therefore, by arranging these components along the direction of the rotation axis Q of the worm shaft  81  in this way, a protruding amount of the second power steering mechanism B in a radial direction of the rotation axis Q of the worm shaft  81  is suppressed, then mountability of the steering device  1  into a vehicle can be improved. 
     (9) The plastic deformation fixing portion  122   a  of the magnet retaining member  122  is located so as to overlap the first annular portion  123   a  of the first yoke  123  and the second annular portion  124   a  of the second yoke  124  in the direction of the rotation axis P of the steering shaft  3 . 
     Therefore, it is possible to suppress increase in axial direction size of the steering device  1 . 
     (10) The portion (the second shaft-radial direction hole portion  325 , the fixing pin  18   b  and the first torsion bar radial direction hole portion  352 ) where the second shaft  32  and the first torsion bar  35  are connected and the second shaft insertion hole portion  331  overlap each other in the direction of the rotation axis P of the steering shaft  3 . 
     Therefore, it is possible to suppress increase in axial direction size of the steering device  1 . 
     (11) Between the outer peripheral surface of the first shaft  31  and the inner peripheral surface of the second shaft  32 , the needle bearing  19  and the needle bearing  20  are arranged apart from each other in the direction of the rotation axis P of the steering shaft  3 . The first ball bearing  14  is located between the two needle bearings  19  and  20 . 
     Therefore, the first shaft  31  and the second shaft  32  can be supported with respect to the first housing  9  with a good balance. 
     (12) The worm shaft  81  is arranged so as not to interfere with the working fluid supply port  105  and the working fluid discharge port  106 . 
     Therefore, it is possible to improve workability of connecting work for connecting the working fluid supply pipe  21  and the working fluid discharge pipe  22  to the working fluid supply port  105  and the working fluid discharge port  106  respectively. 
     (13) The electric motor  7 , the working fluid supply pipe  21  and the working fluid discharge pipe  22  are located so as to protrude to the same side with respect to the axis S connecting the meshing portion of the worm shaft  81  with the worm wheel  82  and the steering shaft  3  in the circumferential direction of the rotation axis P of the steering shaft  3 . 
     Therefore, it is possible to suppress expansion of an occupied area of the whole steering device with respect to the vehicle. 
     (14) The worm shaft  81  is provided at a position that is offset from the working fluid supply port  105  and the working fluid discharge port  106  in the circumferential direction of the rotation axis P of the steering shaft  3 . 
     Therefore, the worm shaft  81  can be prevented from interfering with the working fluid supply port  105  and the working fluid discharge port  106 . 
     (15) The worm shaft  81  meshes with the worm wheel  82  such that the rotation axis Q of the worm shaft  81  inclines with respect to the plane perpendicular to the rotation axis P of the worm wheel  82  (which is the same as the rotation axis P of the steering shaft  3 ). 
     Therefore, by this oblique arrangement of the worm shaft  81 , a layout in which the worm shaft  81  can be prevented from interfering with the working fluid supply port  105  and the working fluid discharge port  106  can be easily employed. 
     Embodiment 2 
       FIG. 9  is a sectional view of the first power steering mechanism A according to an embodiment 2 
     Unlike the embodiment 1, the first torsion bar  35  and the second shaft  32  are connected by press fitting. 
     That is, as shown in  FIG. 9 , the first torsion bar  35  and the second shaft  32  are connected by press-fitting one end portion  353 , as a second shaft connecting portion formed at one end portion (in a left direction in  FIG. 9 ) in the direction of the rotation axis P of the steering shaft  3 , of the first torsion bar  35  into a hole portion  326 , as a first torsion bar connecting portion formed at one end portion (in the left direction in  FIG. 9 ) in the direction of the rotation axis P of the steering shaft  3 , of the second shaft  32 . 
     The other structures or configurations are the same as those of the embodiment 1. Components, portions, etc. that are common to the embodiment 1 are denoted by the same reference signs, and their explanations are omitted here. 
     Next, workings and effects will be explained. 
     The steering device of the embodiment 2 has the same workings and effects as those of the embodiment 1. 
     Other Embodiment 
     Although the embodiments for carrying out the present invention have been explained above, the present invention is not limited to the above embodiments, but includes all design modifications and equivalents belonging to the technical scope of the present invention. 
     For instance, the gear mechanism in the steering mechanism may be a rack-and-pinion-type steering having a pinion that is provided integrally with the output shaft and a rack that meshes with the pinion. In this case, the transmission mechanism is a rack bar, a pinion shaft, etc. 
     Technical ideas that can be understood from the embodiments described above are the following. 
     As one aspect of the present invention, a steering device comprises: a steering mechanism having a steering shaft and a transmission mechanism, wherein the steering shaft has a first shaft, a second shaft, a third shaft, a fourth shaft, a first torsion bar and a second torsion bar, the first shaft and the second shaft are connected to each other through the first torsion bar, the first shaft can be rotated according to driver&#39;s steering operation, the third shaft is connected to the second shaft, the third shaft and the fourth shaft are connected to each other through the second torsion bar, the third shaft has a third shaft valve portion, the fourth shaft has a fourth shaft valve portion, forms a rotary valve together with the third shaft, and can selectively supply working fluid, which is externally supplied, to a first fluid chamber and a second fluid chamber according to torsion of the second torsion bar, and the transmission mechanism transmits rotation of the fourth shaft to steered wheels; an electric motor having an electric motor output shaft and being able to rotate according to a motor command signal generated on the basis of a steering torque signal; a reduction gear provided between the electric motor output shaft and the second shaft and transmitting a rotation force of the electric motor to the second shaft; a first housing having a first housing body, a sensor accommodating space and a reduction gear accommodating space, wherein the first housing body is formed as a single-piece member by molding, the sensor accommodating space is provided at an inner side of the first housing body, and the reduction gear accommodating space is provided at the inner side of the first housing body, and accommodates the reduction gear; a second housing having a second housing body, a rotary valve accommodating space, a fourth shaft accommodating space and a fluid chamber, wherein the second housing body is connected to the first housing, the rotary valve accommodating space is provided at an inner side of the second housing body, and accommodates the rotary valve, the fourth shaft accommodating space is provided at the inner side of the second housing body, and accommodates the fourth shaft, and the fluid chamber is provided at the inner side of the second housing body; a piston provided inside the second housing, dividing the fluid chamber into the first fluid chamber and the second fluid chamber and providing a steering force to the transmission mechanism according to a pressure difference of the working fluid between the first fluid chamber and the second fluid chamber; a torque sensor provided in the sensor accommodating space, detecting a steering torque generated at the steering mechanism according to a torsion amount of the first torsion bar and outputting the steering torque signal; and a control unit including a microprocessor, wherein the microprocessor is configured to generate the motor command signal on the basis of the steering torque signal. 
     As a preferable steering device, in the above aspect, the steering device further comprises: a first bearing and a second bearing, wherein the first bearing is provided at the first housing, and supports the second shaft with respect to the first housing, the second bearing is provided at the first housing, is spaced apart from the first bearing in a direction of a rotation axis of the steering shaft, and supports the second shaft with respect to the first housing, the second shaft has a reduction gear fixing portion, the reduction gear fixing portion is located between the first bearing and the second bearing in the direction of the rotation axis of the steering shaft, and the reduction gear is fixed to the reduction gear fixing portion. 
     As a preferable steering device, in the above aspect, the steering device further comprises: a plate, wherein the plate is fixed to the first housing at a closer position to the second housing with respect to the reduction gear in the direction of the rotation axis of the steering shaft, and the plate has a plate second bearing retaining portion to retain the second bearing, and the second bearing is provided between the second shaft and the plate in a radial direction of the rotation axis of the steering shaft. 
     As a preferable steering device, in the above aspect, the plate is fixed to the first housing with the plate being accommodated in the first housing. 
     As another preferable steering device, in any aspect of the above, the torque sensor, the first bearing and the reduction gear are arranged in the direction of the rotation axis of the steering shaft in an order of the torque sensor, the first bearing and the reduction gear. 
     As another preferable steering device, in any aspect of the above, the second shaft has a second shaft-torque sensor retaining portion to retain the torque sensor and a second shaft-first bearing retaining portion to retain the first bearing, and an outside diameter, in the direction of the rotation axis of the steering shaft, of the second shaft-first bearing retaining portion is greater than an outside diameter, in the direction of the rotation axis of the steering shaft, of the second shaft-torque sensor retaining portion. 
     As another preferable steering device, in any aspect of the above, the steering device further comprises: a plate and a second inner race fixing member, wherein the plate is fixed to the first housing at a closer position to the second housing with respect to the reduction gear in the direction of the rotation axis of the steering shaft, and the plate has plate second bearing retaining portions to retain the second bearing, the second bearing is a ball bearing having a second inner race, balls and a second outer race, the plate second bearing retaining portions retains the second bearing by sandwiching the second outer race from both sides in the direction of the rotation axis of the steering shaft, the second shaft has a second shaft-second bearing retaining portion to retain the second bearing, the second shaft-second bearing retaining portion has a contact surface that contacts one of a pair of end surfaces, in the direction of the rotation axis of the steering shaft, of the second inner race, and the second inner race fixing member fixes the second inner race to the second shaft by contacting the other of the pair of end surfaces, in the direction of the rotation axis of the steering shaft, of the second inner race. 
     As another preferable steering device, in any aspect of the above, the reduction gear is a worm gear having a worm shaft and a worm wheel, the worm shaft is connected to the electric motor output shaft, the worm wheel is provided at the second shaft, the control unit has a control unit body and a power supply connector portion, the control unit body includes a control unit body-housing accommodating therein the microprocessor, and the worm shaft, the electric motor, the control unit body-housing and the power supply connector portion are arranged in a direction of a rotation axis of the worm shaft in an order of the worm shaft, the electric motor, the control unit body-housing and the power supply connector portion. 
     As another preferable steering device, in any aspect of the above, the torque sensor includes a magnet, a magnet retaining member, a first yoke, a second yoke and a magnetic sensor, the magnet has an annular shape where N-pole and S-pole are alternately arranged in a circumferential direction of the rotation axis of the steering shaft, the magnet retaining member is made of metal material, and has a plastic deformation fixing portion and a magnet retaining portion, the plastic deformation fixing portion is fixed to the second shaft by plastic deformation, the magnet retaining portion retains the magnet, the first yoke is made of magnetic material, is provided at the first shaft, and has a first annular port ion and a plurality of first nail portions, the plurality of first nail portions are provided at the first annular portion, and are arranged in the circumferential direction of the rotation axis of the steering shaft with each first nail portion facing the magnet, the second yoke is made of magnetic material, is provided at the first shaft, and has a second annular portion and a plurality of second nail portions, the plurality of second nail portions are provided at the second annular portion, and are arranged such that the second nail portions and the first nail portion are alternately arranged in the circumferential direction of the rotation axis of the steering shaft with each second nail portion facing the magnet, the magnetic sensor is set between the first annular portion and the second annular portion, and outputs a signal according to a magnetic field in a setting area of the magnetic sensor, and the plastic deformation fixing portion overlaps the first annular portion and the second annular portion in a direction of a rotation axis of the steering shaft. 
     As another preferable steering device, in any aspect of the above, the first torsion bar has, at one end portion thereof in a direction of a rotation axis of the steering shaft, a second shaft connecting portion, the second shaft has, at one end portion thereof in the direction of the rotation axis of the steering shaft, a first torsion bar connecting portion, the first torsion bar and the second shaft are connected by the second shaft connecting portion of the first torsion bar and the first torsion bar connecting portion of the second shaft, the third shaft has a second shaft insertion hole portion opening toward the second shaft in the direction of the rotation axis of the steering shaft, the second shaft is connected to the third shaft with the second shaft being inserted into the second shaft insertion hole portion, and the second shaft connecting portion of the first torsion bar and the first torsion bar connecting portion of the second shaft overlap the second shaft insertion hole portion in the direction of the rotation axis of the steering shaft. 
     As another preferable steering device, in any aspect of the above, the steering device as claimed in claim  1 , further comprises: a first bearing, a second bearing, a third bearing and a fourth bearing, wherein the first bearing and the second bearing are provided at the first housing, support the second shaft with respect to the first housing, and are spaced apart from each other in a direction of a rotation axis of the steering shaft, the third bearing and the fourth bearing are provided between the first shaft and the second shaft in a radial direction of the rotation axis of the steering shaft, and are spaced apart from each other in the direction of the rotation axis of the steering shaft, and the first bearing is located between the third bearing and the fourth bearing in the direction of the rotation axis of the steering shaft. 
     As another preferable steering device, in any aspect of the above, the second housing has a working fluid supply port and a working fluid discharge port, the working fluid supply port is structured such that a working fluid supply pipe for supplying the working fluid to the rotary valve can be connected to the working fluid supply port, the working fluid discharge port is structured such that a working fluid discharge pipe for discharging the working fluid, which is discharged from the rotary valve, to an outside of the second housing can be connected to the working fluid discharge port, the reduction gear is a worm gear having a worm shaft and a worm wheel, the worm shaft is connected to the electric motor output shaft, the worm wheel is provided at the second shaft, and the worm shaft is provided so as not to interfere with the working fluid supply port and the working fluid discharge port. 
     As a preferable steering device, in the above aspect, the working fluid supply port and the working fluid discharge port are provided at the same side as the electric motor with respect to an axis that connects a meshing portion of the worm shaft with the worm wheel and the steering shaft on a plane orthogonal to a rotation axis of the steering shaft. 
     As a preferable steering device, in the above aspect, the worm shaft is provided at a position that is offset from the working fluid supply port and the working fluid discharge port in a circumferential direction of the rotation axis of the steering shaft. 
     As another preferable steering device, in any aspect of the above, the worm shaft meshes with the worm wheel such that a rotation axis of the worm shaft inclines with respect to a plane perpendicular to a rotation axis of the worm wheel. 
     EXPLANATION OF REFERENCE 
       1  . . . steering device,  2  . . . steering mechanism,  3  . . . steering shaft (steering mechanism),  31  . . . first shaft (steering shaft),  32  . . . second shaft (steering shaft),  321  . . . reduction gear fixing portion,  322  . . . second shaft-torque sensor retaining port ion,  323  . . . second shaft-first bearing retaining portion,  324  . . . second shaft-second bearing retaining portion,  324   a  . . . contact surface (second shaft-second bearing retaining portion),  325  . . . second shaft-radial direction hole portion (first torsion bar connecting portion),  326  . . . hole portion (first torsion bar connecting portion),  33  . . . third shaft (steering shaft),  331  . . . second shaft insertion hole portion,  34  . . . fourth shaft (steering shaft),  35  . . . first torsion bar,  352  . . . first torsion bar radial direction hole portion (second shaft connecting portion),  353  . . . one end portion of first torsion bar (second shaft connecting portion),  36  . . . second torsion bar,  37  . . . third shaft valve portion,  38  . . . fourth shaft valve portion,  39  . . . rotary valve,  4  . . . transmission mechanism (steering mechanism),  412  . . . steered wheel,  5  . . . first fluid chamber (fluid chamber),  6  . . . second fluid chamber (fluid chamber),  7  . . . electric motor,  71  . . . electric motor output shaft,  8  . . . worm gear  8  (reduction gear,  81  . . . worm shaft (worm gear),  82  . . . worm wheel (worm gear),  9  . . . first housing,  91  . . . first housing body (first housing),  92  . . . sensor accommodating space (first housing),  93  . . . reduction gear accommodating space (first housing),  10  . . . second housing,  101  . . . second housing body (second housing),  101   a  . . . first member (second housing body),  101   b  . . . second member (second housing body),  102  . . . rotary valve accommodating space (second housing),  103  . . . fourth shaft accommodating space (second housing),  104  . . . fluid chamber  105  . . . working fluid supply port,  106  . . . working fluid discharge port,  11  . . . piston,  12  . . . torque sensor,  121  . . . magnet,  122  . . . magnet retaining member,  122   a  . . . plastic deformation fixing portion,  122   b  . . . magnet retaining portion,  123  . . . first yoke,  123   a  . . . first annular portion,  123   b  . . . first nail portion,  124  . . . second yoke,  124   a  . . . second annular portion,  124   b  . . . second nail portion,  125  . . . magnetic sensor,  13  . . . control unit,  131  . . . microprocessor,  132  . . . control unit body,  132   a  . . . control unit body-housing,  133  . . . power supply connector portion,  14  . . . first ball bearing (first bearing),  141  . . . first inner race,  142  . . . ball  143  . . . first outer race,  15  . . . second ball bearing (second bearing),  151  . . . second inner race,  152  . . . ball  153  . . . second outer race,  16  . . . plate,  16   a  . . . first plate,  16   a   1  . . . plate second bearing retaining portion,  16   b  . . . second plate,  16   b   1  . . . plate second bearing retaining portion,  17  . . . C-ring (second inner race fixing member),  18   a  . . . fixing pin,  18   b  . . . fixing pin,  18   c  . . . fixing pin,  18   d  . . . fixing pin,  19  . . . needle bearing (third bearing),  20  . . . needle bearing (fourth bearing), P . . . rotation axis of steering shaft, Q . . . rotation axis of worm shaft, S . . . axis connecting meshing portion of worm shaft with worm wheel and steering shaft, R . . . circumferential direction of rotation axis of steering shaft