Patent Publication Number: US-2023159090-A1

Title: Steering device

Description:
TECHNICAL FIELD 
     The present invention relates to a steering device. 
     BACKGROUND ART 
     Patent Document 1 below discloses a conventional steering device. 
     The steering device transmits rotational force as steering assistance force from an electric motor to a steering shaft, via a speed reduction mechanism composed of a worm shaft and a worm wheel. Furthermore, the steering device is structured to adjust a state of engagement between the worm shaft and the worm wheel, by biasing a bearing supporting a tip of the worm shaft toward the worm wheel, with use of a spring. 
     PRIOR ART DOCUMENT(S) 
     Patent Document(s) 
     
         
         Patent Document 1: JP 2017-222257 A 
       
    
     SUMMARY OF THE INVENTION 
     Problem(s) to be Solved by the Invention 
     However, the conventional steering device above has a problem of changing an action direction of reaction force generated in the worm gear, depending on whether the worm shaft is in positive rotation or negative rotation, especially in case that the worm shaft and the worm wheel are not right-angled in torsion angle therebetween. This may produce a backlash between the worm shaft and the worm wheel upon switching of the worm shaft between positive rotation and negative rotation, and thereby cause a noise due to the backlash. 
     In view of the foregoing technical problem of the conventional steering device, it is desirable to provide a steering device structured to suppress a backlash between a worm shaft and a worm wheel. 
     Means for Solving the Problem(s) 
     According to one aspect of the present invention, a bearing retained by a center adjuster is biased by a biasing member in a direction inclined with respect to a direction of gear engagement between a worm shaft and a worm wheel. 
     Effect(s) of the Invention 
     The present invention serves to suppress a backlash between a worm shaft and a worm wheel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view showing an exterior of a steering device according to the present invention. 
         FIG.  2    is an enlarged view of a region A of  FIG.  1   . 
         FIG.  3    is a sectional view along a line B-B of  FIG.  2   . 
         FIG.  4    is an exploded perspective view of a center adjuster shown in  FIG.  3   . 
         FIG.  5    is a perspective view of the center adjuster in  FIG.  3    when viewed from a side of a holder. 
         FIG.  6    is a perspective view of the center adjuster in  FIG.  3    when viewed from a side of a lid. 
         FIG.  7    is a side view of the center adjuster in  FIG.  3   . 
         FIG.  8    is a sectional view of the center adjuster in  FIG.  7    along an axial direction thereof. 
         FIG.  9    is a sectional view along a line C-C of  FIG.  7   . 
         FIG.  10    is a sectional view along a line D-D of  FIG.  7   . 
         FIG.  11    is a planar view of a collar shown in  FIG.  4   . 
         FIGS.  12 A and  12 B  show a collar assembly unit formed by integrating the collar and a bearing. 
         FIG.  12 A  is a planar view of the collar assembly unit. 
         FIG.  12 B  is a bottom view of the collar assembly unit. 
         FIG.  13    is a sectional view along a line E-E of  FIG.  12   . 
         FIG.  14    is a view in a direction of an arrow F of  FIG.  13   . 
         FIG.  15    is a planar view of the holder shown in  FIG.  4   . 
         FIG.  16    is a view in a direction of an arrow G of  FIG.  7    where the lid of the center adjuster is removed. 
         FIG.  17    is a sectional view along a line H-H of  FIG.  16   . 
         FIG.  18    is a sectional view along a line I-I of  FIG.  3   . 
         FIG.  19    is a planar view of the lid shown in  FIG.  4   . 
         FIG.  20    is a sectional view along a line J-J of  FIG.  19   . 
         FIG.  21    is a view in a direction of an arrow K of  FIG.  20   . 
         FIG.  22    is an enlarged view of a part of  FIG.  5   . 
         FIG.  23    is a sectional view of the center adjuster corresponding to  FIG.  9   , which illustrates actions and effects of the center adjuster. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     The following details an embodiment of a steering device according to the present invention, with reference to the drawings. The embodiment shows an example of the steering device employed in an automobile as conventional. 
     (Explanation for Steering Device) 
       FIGS.  1  to  3    show the steering device according to the embodiment of the present invention.  FIG.  1    is a perspective view showing an exterior of the steering device.  FIG.  2    is a partial enlarged view of  FIG.  1   , which enlarges a region A of  FIG.  1   .  FIG.  3    is a sectional view along a line B-B of  FIG.  2   , which shows a mounting state of a center adjuster  90  to a center adjuster retainer  122  where a worm gear and a seal member  95  are omitted. 
     The steering device shown in  FIG.  1    is a so-called dual pinion type steering device structured to receive input of steering force via a first pinion gear G 1  and input of steering assistance force for assistance of the steering force via a second pinion gear G 2 . Specifically, the steering device includes a steering mechanism SF and a steering assistance mechanism AF, wherein steering mechanism SF performs steering by transmitting the steering force inputted to a steering wheel not shown to turning wheels, and steering assistance mechanism AF supplies the steering assistance force to steering mechanism SF (in detail, rack shaft  3  described below) depending on the steering force. In addition, the steering device is mounted and fixed to a vehicle body not shown via bushes BM disposed in an outside part of a steering mechanism housing  11  containing steering mechanism SF. 
     Steering mechanism SF includes the steering mechanism housing  11 , a steering shaft  2 , and a rack shaft  3 . Steering mechanism housing  11  includes steering shaft container  111  and a rack shaft container  112  formed integrally. Steering shaft  2  is rotatably contained in steering shaft container  111 . Rack shaft  3  is contained in rack shaft container  112  movably in a direction of an axis X. In addition, steering shaft container  111  includes a known torque sensor TS that measures a steering torque inputted to steering shaft  2 . 
     Steering shaft  2  extends in a direction of an axis Y obliquely to rack shaft  3 , and includes a first end in the axis Y direction (i.e., an upper end in  FIG.  1   ) linked to the steering wheel so as to rotate in response to rotation of the steering wheel. Furthermore, steering shaft  2  includes a second end opposite to the first end in the axis Y direction (i.e., a lower end in  FIG.  1   ), and includes first pinion teeth  20  formed in an outer periphery of the second end. 
     Although the present embodiment shows steering shaft  2  as a single unit directly connecting the steering wheel to rack shaft  3 , the steering shaft according to the present invention is not limited to such configuration. For example, instead of the one directly connecting the steering wheel to rack shaft  3 , the steering shaft according to the present invention may be one structured for a so-called steer-by-wire type steering device in which the steering shaft is dividable and transmits and shutdown rotation of the steering wheel via a clutch etc. 
     Rack shaft  3  substantially horizontally extends in the axis X direction, and includes first rack teeth  31  and second rack teeth  32  respectively within predetermined ranges in the axis X direction. First rack teeth  31  engage with first pinion teeth  20  of steering shaft  2 . Second rack teeth  32  engage with second pinion teeth  70  of a transmission shaft  7  described below. Thus, rack shaft  3  moves in the axis X direction in response to rotation of steering shaft  2  and transmission shaft  7 . Furthermore, rack shaft  3  includes ends in the axis X direction both of which are connected via tie rods not shown to steering knuckle arms not shown linked to the right and left turning wheels not shown. Thus, the movement of rack shaft  3  in the axis X direction pushes and pulls the steering knuckle arms, and changes a direction of the turning wheels. In addition, first pinion teeth  20  and first rack teeth  31  compose first pinion gear G 1 , and second pinion teeth  70  and second rack teeth  32  compose second pinion gear G 2 . 
     Steering assistance mechanism AF includes an electric motor  4 , a control unit  5 , and a speed reduction mechanism RG. Electric motor  4  generates the steering assistance force. Control unit  5  controls driving of electric motor  4 . Speed reduction mechanism RG performs speed reduction of rotational force of the steering assistance force and transmits it to steering mechanism SF (in detail, rack shaft  3 ). Although the present embodiment shows an example of connecting speed reduction mechanism RG to rack shaft  3  and providing the steering assistance force to rack shaft  3 , the present invention is not limited to such configuration. For example, instead of the dual pinion type steering device according to the present embodiment configured to connect speed reduction mechanism RG to rack shaft  3  via transmission shaft  7 , the present invention may be applied to a single pinion type steering device that includes first pinion gear G 1  alone and is configured to connect speed reduction mechanism RG to steering shaft  2  and supply the steering assistance force to steering shaft  2 . 
     Electric motor  4  includes a motor element not shown, a motor housing  40 , and a motor shaft  41 . Motor housing  40  contains the motor element. Motor shaft  41  extends in a direction of an axis P being substantially parallel with axis X, and outputs rotation of the motor element. Furthermore, motor shaft  41  is connected to a worm shaft  61  described below of speed reduction mechanism RG, via a known shaft coupling not shown. 
     Control unit  5  is disposed in a side of electric motor  4  opposite to motor shaft  41 , and is integrated with electric motor  4 . Furthermore, control unit  5  is electrically connected to various sensors such as torque sensor TS via a sensor harness H, and controls driving of electric motor  4  based on vehicle operation information, such as a steering torque signal, outputted from the various sensors. 
     As shown in  FIG.  2    and enlarged in  FIG.  2   , speed reduction mechanism RG includes a worm gear composed of worm shaft  61  and a worm wheel  62  engaging with worm shaft  61 . Worm shaft  61  and worm wheel  62  are contained in a gear housing  12  attached to an outside part of steering mechanism housing  11  (in detail, rack shaft container  112 ). Gear housing  12  includes a worm gear container  121  and a center adjuster retainer  122 . Worm gear container  121  contains the worm gear. Center adjuster retainer  122  is disposed adjacently to worm gear container  121 , and includes in its interior a center adjuster containing space formed to contain a center adjuster  90  described below. Furthermore, worm gear container  121  extends in the direction of axis P being a rotational axis of electric motor  4 , and includes a worm shaft container  123  and a worm wheel container  124 . Worm shaft container  123  contains worm shaft  61 . Worm wheel container  124  contains worm wheel  62 , and is disposed adjacently to worm shaft container  123 . 
     Worm shaft  61  extends in the axis P direction coaxially with motor shaft  41 , and includes a worm first end  611  and a worm second end  612  in the axis P direction. Worm first end  611  is connected to motor shaft  41  via the known shaft coupling not shown. Worm second end  612  is supported by a bearing  8 . Bearing  8  is, for example, a ball bearing. 
     Worm wheel  62  is fixed to a first end of transmission shaft  7  linked to rack shaft  3 , and is structured to rotate integrally with transmission shaft  7 . This allows rotation of worm wheel  62  to be transmitted to rack shaft  3  via transmission shaft  7 . Transmission shaft  7  is inclined with respect to rack shaft  3  similarly to steering shaft  2 , and includes second pinion teeth  70  engaging with second rack teeth  32 , wherein second pinion teeth  70  are disposed in an outer periphery of a second end of transmission shaft  7  opposite to the first end. Thus, second pinion gear G 2  composed of second rack teeth  32  and second pinion teeth  70  is allowed to transmit rotation of worm wheel  62  to rack shaft  3  via transmission shaft  7 , while converting the rotation into movement of rack shaft  3  in the axis X direction. 
     Worm shaft  61  and worm wheel  62  composing speed reduction mechanism RG may have a gap therebetween (i.e., between their teeth) due to wear. In view of this, the steering device includes a center adjustment mechanism  9  structured to bias worm shaft  61  toward worm wheel  62  and thereby adjust a state of engagement between worm shaft  61  and worm wheel  62 . 
     Center adjustment mechanism  9  includes center adjuster  90  retained by center adjuster retainer  122  that is formed in gear housing  12  so as to surround worm second end  612  of worm shaft  61 . Center adjuster retainer  122  includes an open end in the axis P direction, wherein the open end is liquid-tightly closed by a seal member  95  having a substantially disk shape. Seal member  95  is fixed to the open end of center adjuster retainer  122  by press-fitting. 
     Center adjuster  90  is contained and retained in center adjuster retainer  122 , and is biased toward worm wheel  62  by a wire spring  93  being a biasing member disposed in center adjuster  90 . Center adjuster  90  includes a collar  91  and a holder  92 . Collar  91  retains an outer periphery of bearing  8 . Holder  92  has a shape of a bottomed tube, and contains collar  91  so as to allow collar  91  to slide toward worm wheel  62 . This allows center adjuster  90  to bias bearing  8  toward worm wheel  62  via collar  91  with use of biasing force of wire spring  93 , and thereby bias worm shaft  61  toward worm wheel  62 . 
     Holder  92  surrounding an outer periphery of center adjuster  90  has a cross section (i.e., a section orthogonal to axis P) being substantially circular, while center adjuster retainer  122  including the center adjuster retainer space in its interior has a tubular shape and has a cross section (i.e., a section orthogonal to axis P) being substantially circular. Accordingly, center adjuster  90  includes an outer peripheral surface being in contact with an inner peripheral surface of center adjuster retainer  122 , and this determines a position of center adjuster  90  with respect to center adjuster retainer  122 . 
     Holder  92  having the shape of bottomed cylindrical tube includes an open end closed by a lid  94 . Holder  92  and lid  94  have therebetween a space filled with grease not shown. The grease is lubricant that allows collar  91  to slide inside the holder  92  under lubrication. 
     (Explanation for Center Adjuster) 
       FIGS.  4  to  21    show the center adjuster of the steering device according to the embodiment of the present invention. For convenience, the following description refers to a direction along axis P (shown in  FIG.  3   ) as an axial direction, and refers to a direction perpendicular to axis P as a radial direction, and refers to a direction around axis P as a circumferential direction. 
       FIG.  4    is an exploded perspective view of center adjuster  90 .  FIG.  5    is a perspective view of center adjuster  90  when viewed from a side of holder  92 .  FIG.  6    is a perspective view of center adjuster  90  when viewed from a side of lid  94 .  FIG.  7    is a side view of center adjuster  90 .  FIG.  8    is a sectional view of center adjuster  90  along the axial direction.  FIG.  9    is a sectional view along a line C-C of  FIG.  7   .  FIG.  10    is a sectional view along a line D-D of  FIG.  7   . These  FIGS.  4  to  10    are drawings to show general configurations of center adjuster  90 , and are referred to as appropriate in explanation for  FIGS.  11  to  21    described below. 
     As shown in  FIGS.  4  to  10   , center adjuster  90  includes bearing  8 , collar  91 , holder  92 , wire spring  93 , and lid  94  which are assembled and integrated into center adjuster  90 . Collar  91  has a substantially U-shape in planar view, and is fitted to the outer periphery of bearing  8 , as a part of collar  91 . Holder  92  substantially has the shape of the bottomed cylindrical tube, and includes a collar container  920  shaped concave in an inner peripheral side of holder  92 , and contains a collar assembly unit  90 S in collar container  920 . Collar assembly unit  90 S is movable relatively with respect to holder  92  in a direction of engagement between worm shaft  61  and worm wheel  62  (i.e., a direction M shown in  FIG.  23   ) which is simply referred to as “gear engagement direction” in the following. 
     Wire spring  93  has an arc shape (specifically, a C-shape) in planar view, and is interposed between collar  91  and holder  92  (i.e., in a gap in the radial direction), and biases bearing  8  toward worm wheel  62  via collar  91  with binding force of wire spring  93  itself. Wire spring  93  according to the present invention is structured to bias bearing  8  via collar  91  in a direction (i.e., a direction N in  FIG.  23   ) inclined with respect to the gear engagement direction (i.e., direction M in  FIG.  23   ), where a force in the inclined direction (direction N) includes a biasing force in the gear engagement direction (direction M) as a component force. 
     Lid  94  is disposed to cover an axial opening of collar container  920  of holder  92 , and is locked and fixed to holder  92  via a plurality of (e.g., two in the present embodiment) holder retainers, i.e., a first holder retainer  941  and a second holder retainer  942  that are disposed in an outer periphery of lid  94 . As shown in  FIG.  3   , the outer periphery of lid  94  further includes a plurality of (e.g., three in the present embodiment) lid engagement parts, i.e., a first lid engagement part  943 , a second lid engagement part  944 , and a third lid engagement part  945  that are bendable in the radial direction. The first, second, and third lid engagement parts  943 ,  944 , and  945  engage with a center adjuster locking groove  125  formed in center adjuster retainer  122  of gear housing  12 , and fix the lid  94  to center adjuster retainer  122 . Center adjuster locking groove  125  is an concave annular groove extending in the circumferential direction in an inner peripheral surface of center adjuster retainer  122 , where first lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945  respectively include in their outer peripheral edges a first lid engagement projection  943   b , a second lid engagement projection  944   b , and a third lid engagement projection  945   b  that engage with center adjuster locking groove  125  so as to restrict movement of lid  94  in the axial direction inside center adjuster retainer  122 . 
       FIG.  11    is a planar view of collar  91 .  FIGS.  12 A and  12 B  show collar assembly unit  90 S formed by integrating collar  91  and bearing  8 .  FIG.  12 A  is a planar view of collar assembly unit  90 S.  FIG.  12 B  is a bottom view of collar assembly unit  90 S.  FIG.  13    is a sectional view along a line E-E of  FIG.  12   .  FIG.  14    is a view in a direction of an arrow F of  FIG.  13   . 
     As shown in  FIGS.  11 ,  12 A, and  12 B , collar  91  is made of a resin less in friction coefficient than materials of holder  92  and lid  94 , and has a substantially U-shape in planar view, and includes a collar body  910  having the U-shape and an opening  911  being open in the radial direction due to the U-shape of collar body  910 . For convenience of explanation, out of end surfaces of collar  91  in the axial direction, one end surface facing a holder bottom  921  described below is referred to as a collar first end surface  91   a , and the other end surface facing a lid body  940  described below is referred to as a collar second end surface  91   b.    
     Collar body  910  has the arc shape that surrounds bearing  8  to cover a circumferential range greater than a half circumference of an outer periphery of bearing  8 , except a circumferential range overlapping with opening  911 . Inside the arc shape, collar body  910  includes a bearing retainer  912  having an inner diameter slightly smaller than an outer diameter of bearing  8 . In detail, bearing retainer  912  in a natural state before installing bearing  8  to bearing retainer  912  is smaller in inner diameter than bearing retainer  912  after installing bearing  8  to bearing retainer  912 . Accordingly, bearing retainer  912  exerts biasing force (i.e., elastic force) on a contact part with bearing  8  radially inwardly toward a center of bearing  8 , and retains bearing  8  with the biasing force (the elastic force). In other words, bearing retainer  912  has a predetermined interference for the biasing force (the elastic force), and retains bearing  8  with use of the interference. 
     Collar body  910  includes a collar bottom  913  and a collar peripheral wall  914 . Collar bottom  913  forms collar first end surface  91   a , and is in contact with an axial first end of bearing  8 . Collar peripheral wall  914  rises from an outer peripheral edge of collar bottom  913 , and surrounds the outer periphery of bearing  8 . Collar bottom  913  extends continuously in the circumferential direction with a substantially constant radial width, and, as enlarged in  FIG.  13   , includes a slope being in contact with the axial first end of bearing  8 , wherein the slope is inclined such that collar peripheral wall  914  gradually increases in inner diameter with increase in distance from collar bottom  913 . Thus, the slope of collar bottom  913  is in contact with an outer peripheral edge of the axial first end of bearing  8 . This allows biasing force to be exerted on bearing  8  in the axial direction and the radial direction. Collar peripheral wall  914  includes a spring winding part  914   a  and a protrusion part  914   b . Spring winding part  914   a  is positioned oppositely to collar bottom  913  in the axial direction, and serves as a part around which wire spring  93  is wound. Protrusion part  914   b  is positioned nearer to collar bottom  913  than spring winding part  914   a  in the axial direction, and protrudes with respect to spring winding part  914   a  outwardly in the radial direction. 
     As shown in  FIG.  13   , spring winding part  914   a  occupies a region overlapping with bearing  8  in the radial direction, and has an axial width greater at least than a radius of a wire of wire spring  93 . Spring winding part  914   a  includes spring contact parts  914   c  structured to be in contact with wire spring  93 , and shaped flat, and formed at a plurality of (e.g., two in the present embodiment) positions in the circumferential direction, in a section axially adjacent to protrusion part  914   b  in spring winding part  914   a . The number of spring contact parts  914   c  is not limited to two exemplified in the present embodiment, but may be any number equal to or greater than two. 
     As shown in  FIG.  14   , each of spring contact parts  914   c  is formed flat by deleting (i.e., chamfering) a corner of a projection  914   d  projecting outwardly in the radial direction. Each of spring contact parts  914   c  includes an axial first end adjacent to protrusion part  914   b  and an axial second end opposite to protrusion part  914   b  and adjacent to projection  914   d . In other words, spring contact parts  914   c  are structured such that wire spring  93  is interposed by protrusion part  914   b  and projections  914   d  in the axial direction, and thereby locked. 
     As shown in  FIG.  12 B , spring contact parts  914   c  are points of contact with wire spring  93 , and are positioned so as to shift a center O with respect to an axis Z, where: center O is a center of a virtual circle V passing through the points of contact with wire spring  93 ; and axis Z is an axis passing through the rotational axis of worm shaft  61  (i.e., axis P) and extending in the gear engagement direction (i.e., direction M in  FIG.  23   ). This allows collar  91  to be biased in the direction (direction N in  FIG.  23   ) inclined with respect to the gear engagement direction (direction M in  FIG.  23   ) with the biasing force of wire spring  93 , as described above. 
     As shown in  FIG.  13   , protrusion part  914   b  extends continuously in the circumferential direction, and has a constant radial width sufficient to form a step  914   e  with respect to spring winding part  914   a  such that step  914   e  is larger than a diameter of the wire of wire spring  93 . In other words, protrusion part  914   b  and spring contact parts  914   c  interpose therebetween step  914   e  having a radial width greater at least than the wire diameter of wire spring  93 . This prevents wire spring  93  from radially outwardly protruding beyond an outer peripheral surface of protrusion part  914   b , when wire spring  93  is in contact with spring contact parts  914   c . Furthermore, in an outer peripheral edge of an axial end of protrusion part  914   b  adjacent to collar bottom  913 , protrusion part  914   b  includes a tapered surface  914   f  formed by so-called C chamfering to gradually decreases in outer diameter toward collar bottom  913  and extend continuously in the circumferential direction. In addition, tapered surface  914   f  may be formed to be a curved surface by R-chamfering instead of C-chamfering exemplified in the present embodiment. 
     Collar peripheral wall  914  includes a peripheral wall  914   g  and a bearing presser  914   h  in an inner periphery thereof. Peripheral wall  914   g  is in contact with the outer peripheral surface of bearing  8 . Bearing presser  914   h  is disposed in an axial end of collar peripheral wall  914  opposite to collar bottom  913 , and is in contact with the axial second end surface of bearing  8  such that bearing presser  914   h  and collar bottom  913  sandwiches bearing  8  therebetween. Furthermore, peripheral wall  914   g  extends straight in the axial direction, substantially parallel with the outer peripheral surface of bearing  8 . Bearing presser  914   h  is substantially parallels with the axial second end surface of bearing  8  such that the entire part of bearing presser  914   h  is in contact with the axial 
     Collar body  910  includes in its ends in the circumferential direction a pair of collar engagement parts  915  that holds bearing  8  through a circumferential range wider than a half of the outer circumference around bearing  8  and thereby suppresses bearing  8  from dropping off collar body  910  (in detail, bearing retainer  912 ). Each of collar engagement parts  915  has a hook-like shape so as to gradually increase in distance from a corresponding one of a holder first slide surface  923   a  and a holder second slide surface  923   b  described below. Thus, the pair of collar engagement parts  915  catch bearing  8  and thereby prevent bearing  8  from dropping through the opening  911 . 
     As shown in  FIG.  12 A , in radially both sides across bearing retainer  912 , collar body  910  includes a collar first slide surface  916   a  and a collar second slide surface  916   b  being a pair of sliding contact surfaces that allow collar  91  to slide with respect to holder  92 . The pair of collar first slide surface  916   a  and collar second slide surface  916   b  extend to be tangent to bearing retainer  912  being substantially circular in planar view, and thereby define therebetween a width-across-flats region along which collar  91  slides with respect to holder  92 . Specifically, collar first slide surface  916   a  and collar second slide surface  916   b  are respectively in sliding contact with holder first slide surface  923   a  and holder second slide surface  923   b  described below of holder  92  (in detail, holder tubular wall  922  described below). 
     Each of the both circumferential ends of collar body  910  includes a slope  917  tapered toward the each of the both circumferential ends. Slopes  917  are slopes defined such that the outer peripheral surface of collar body  910  gradually goes away from collar first slide surface  916   a  and collar second slide surface  916   b , as approaching the both circumferential ends of collar body  910 . In other words, slopes  917  are inclined so as to gradually go away from holder first slide surface  923   a  and holder second slide surface  923   b , as approaching the both circumferential ends of collar body  910 , in a state in which collar  91  is contained in holder  92 . Thus, slopes  917  are chamfered portions tapered toward the both circumferential ends of collar body  910 , and may be formed flat by C-chamfering or formed curved by R-chamfering. 
     Collar body  910  includes a lubrication groove  918  in its axial end face adjacent to collar bottom  913 . Lubrication groove  918  serves to retain the grease not shown filling the space between holder  92  and lid  94 , and is composed of a circumferential groove  918   a  and a radial groove  918   b . Circumferential groove  918   a  has a shape of an arc extending continuously in the circumferential direction with a constant width, in planar view. Radial groove  918   b  has a straight shape extending continuously in the radial direction across a circumferentially middle part of circumferential groove  918   a . Circumferential groove  918   a  pierces an outer surface of collar bottom  913  in the circumferential direction, and includes circumferential ends that respectively open at collar first slide surface  916   a  and collar second slide surface  916   b . Similarly, radial groove  918   b  pierces the outer surface of collar bottom  913  in the radial direction, and includes a radial first end being open at an outer peripheral edge of collar  91  and a radial second end being open at an inner peripheral edge of collar  91 . 
       FIG.  15    is a planar view of holder  92 .  FIG.  16    is a view in a direction of an arrow G where the lid of center adjuster  90  is removed.  FIG.  17    is a sectional view along a line H-H of  FIG.  16   .  FIG.  18    is a sectional view along a line I-I of  FIG.  3   . 
     As shown in  FIGS.  15  to  17   , holder  92  substantially has the shape of bottomed cylindrical tube, and includes in its interior the collar container  920  containing collar  91  slidably. Collar container  920  is defined by a holder bottom  921  and a holder tubular wall  922 . Holder bottom  921  is in contact with collar bottom  913  of collar  91 . Holder tubular wall  922  extends from holder bottom  921  continuously in the axial direction, and surrounds the outer periphery of collar  91 . For convenience of explanation, out of axial ends of holder  92 , the one end adjacent to holder bottom  921  is referred to as a holder first end  92   a , and the other end opposite to holder bottom  921  is referred to as a holder second end  92   b.    
     Holder bottom  921  includes a collar first sliding-contact surface  921   a . Collar first sliding-contact surface  921   a  is a flat surface substantially corresponding to a projected plane of collar bottom  913 , and is in sliding contact with collar first end surface  91   a . Holder bottom  921  further includes a holder bottom stepped section  921   b  and a holder bottom opening  921   c . Holder bottom opening  921   c  is an axial opening formed in an inner circumferential side with respect to collar first sliding-contact surface  921   a  via holder bottom stepped section  921   b  formed at a lower level than collar first sliding-contact surface  921   a . Worm second end  612  of worm shaft  61  is inserted inside the holder  92  via collar first sliding-contact surface  921   a , and is supported by bearing  8  contained in holder  92 . 
     Holder tubular wall  922  includes a holder first tubular wall  923  and a holder second tubular wall  924 . Holder first tubular wall  923  overlaps with (i.e., faces) protrusion part  914   b  of collar  91  in the radial direction. Holder second tubular wall  924  is positioned nearer to holder second end  92   b  than holder first tubular wall  923  in the axial direction, and overlaps with (i.e., faces) spring winding part  914   a  of collar  91  in the radial direction. 
     Holder first tubular wall  923  includes holder first slide surface  923   a , holder second slide surface  923   b , and a holder-first-tubular-wall concave arc surface  923   c . Holder first slide surface  923   a  and holder second slide surface  923   b  are a pair of slide surfaces disposed to define a width-across-flats region therebetween. Holder-first-tubular-wall concave arc surface  923   c  is interposed between holder first slide surface  923   a  and holder second slide surface  923   b  in the circumferential direction so as to connect holder first slide surface  923   a  and holder second slide surface  923   b . Furthermore, holder first tubular wall  923  includes a tubular part tapered section  923   e  adjacently to an outer peripheral edge of holder first end  92   a . Tubular part tapered section  923   e  has a substantially conical shape with an outer diameter that gradually decreases toward holder first end  92   a.    
     Holder first slide surface  923   a  is parallel with collar first slide surface  916   a , and is structured to be in sliding contact with collar first slide surface  916   a  upon movement of collar  91  in the gear engagement direction. Similarly, holder second slide surface  923   b  is parallel with collar second slide surface  916   b , and is structured to be in sliding contact with collar second slide surface  916   b  upon movement of collar  91  in the gear engagement direction. 
     Holder-first-tubular-wall concave arc surface  923   c  has a concave arc shape corresponding to a collar convex arc surface  914   i  of collar peripheral wall  914  where collar convex arc surface  914   i  has a substantially arc shape in planar view. Holder-first-tubular-wall concave arc surface  923   c  is structured to restrict a maximum amount of retraction of collar  91  in the gear engagement direction (i.e., a direction away from worm wheel  62  in the gear engagement direction), by contacting with collar convex arc surface  914   i.    
     Holder first tubular wall  923  further includes a pair of collar contact parts  923   d ,  923   d  in a side opposite to holder-first-tubular-wall concave arc surface  923   c  across axis P. The pair of collar contact parts  923   d ,  923   d  are structured to contact with the both circumferential ends of collar body  910  when collar  91  has traveled a predetermined slide distance in the gear engagement direction. Thus, the pair of collar contact parts  923   d ,  923   d  are structured to restrict a maximum amount of advance of collar  91  in the gear engagement direction (i.e., a direction to approach worm wheel  62  in the gear engagement direction), by contacting with the both circumferential ends of collar  91 . In other words, the pair of collar contact parts  923   d ,  923   d  and the both circumferential ends of collar  91  have a distance D therebetween in a state in which collar  91  is in contact with holder-first-tubular-wall concave arc surface  923   c , wherein the distance D is a slidable amount of collar  91 , i.e. an engagement adjustable amount of the worm gear, allowed in the center adjustment mechanism. 
     Holder second tubular wall  924  has a substantially arc shape in planar view, and extends in the circumferential direction, and occupies a circumferential range passing through at least two of spring contact parts  914   c  of collar  91 . According to the present embodiment, the circumferential range occupied by holder second tubular wall  924  overlaps with holder-first-tubular-wall concave arc surface  923   c  in the axial direction, and is wider than a circumferential range occupied by holder-first-tubular-wall concave arc surface  923   c . Holder second tubular wall  924  is diameter-expanded to form a step with respect to holder-first-tubular-wall concave arc surface  923   c , and is greater in inner diameter than holder-first-tubular-wall concave arc surface  923   c . This forms a predetermined radial gap C 1  between holder second tubular wall  924  and wire spring  93  wound around spring winding part  914   a  of collar  91 . 
     Holder second end  92   b  includes a spring locking part  925  in a side opposite to holder second tubular wall  924  across a center of holder  92  (i.e., axis P). Spring locking part  925  has a substantially arc shape in planar view, and extends in the circumferential direction, and is structured as a part on which wire spring  93  is locked. In detail, spring locking part  925  occupies a circumferential range facing the two spring contact parts  914   c , in a state in which collar  91  is mounted to holder  92 . 
     Furthermore, holder second end  92   b  includes a spring rotation restricter  926  between collar  91  and holder  92 . Spring rotation restricter  926  is in contact with a pair of circumferential ends of wire spring  93  in a state of winding the wire spring  93 . Spring rotation restricter  926  occupies a circumferential range corresponding to a circumferential gap formed between the pair of circumferential ends of wire spring  93 , and restricts circumferential movement of wire spring  93  in the wound state, by contacting with the pair of circumferential ends of wire spring  93 . 
     Holder  92  includes a plurality of (e.g., two in the present embodiment) retainer engagement grooves: i.e., a pair of first retainer engagement groove  927  and second retainer engagement groove  928  that are a pair of convex grooves extending in the axial direction to engage with a pair of first holder retainer  941  and second holder retainer  942  described below of lid  94 . Each of first retainer engagement groove  927  and second retainer engagement groove  928  extends straight in the axial direction through an outer peripheral part of holder tubular wall  922 . In detail, first retainer engagement groove  927  is positioned to overlap with holder second tubular wall  924  in the axial direction. Second retainer engagement groove  928  is positioned to overlap with spring rotation restricter  926  in the axial direction. 
     Each of first retainer engagement groove  927  and second retainer engagement groove  928  has a depth (i.e., a radial width) greater than a thickness (i.e., a radial width) of each of first holder retainer  941  and second holder retainer  942  of lid  94 . Accordingly, in a state in which first holder retainer  941  and second holder retainer  942  respectively engage with first retainer engagement groove  927  and second retainer engagement groove  928 , each of first holder retainer  941  and second holder retainer  942  is less in outer diameter than parts of an outer peripheral surface of holder tubular wall  922  which are circumferentially adjacent to a corresponding one of first retainer engagement groove  927  and second retainer engagement groove  928 . In other words, in the state in which first holder retainer  941  and second holder retainer  942  respectively engage with first retainer engagement groove  927  and second retainer engagement groove  928 , first holder retainer  941  and second holder retainer  942  are retracted radially inwardly with respect to the parts of the outer peripheral surface of holder tubular wall  922  which are circumferentially adjacent to first retainer engagement groove  927  and second retainer engagement groove  928  (see  FIG.  22   ). 
     First retainer engagement groove  927  and second retainer engagement groove  928  respectively include a first retainer hook engagement groove  927   a  and a second retainer hook engagement groove  928   a  in their axial ends adjacent to holder first end  92   a . First retainer hook engagement groove  927   a  and second retainer hook engagement groove  928   a  extend in the radial direction, and are structured as parts on which a first holder retainer hook  941   a  and a second holder retainer hook  942   a  described below are respectively locked. Each of first retainer hook engagement groove  927   a  and second retainer hook engagement groove  928   a  is formed as a radially inward convex by cutting the holder first end  92   a , in an axial end in holder first end  92   a  of a corresponding one of first retainer engagement groove  927  and second retainer engagement groove  928 . 
     As shown in  FIGS.  17  and  22   , holder bottom  921  of holder  92  includes in its outer side face a pair of holder-positioning projections  929 ,  929  projecting toward holder first end  92   a . As shown in  FIG.  18   , the pair of holder-positioning projections  929 ,  929  engage with a holder-positioning-projection engagement groove  127  (described below) formed in center adjuster retainer  122 , and thereby restrict circumferential movement of holder  92  in center adjuster retainer  122 . Each of the pair of holder-positioning projections  929 ,  929  has a predetermined gap C 2  with a groove bottom of holder-positioning-projection engagement groove  127  in the radial direction. Furthermore, the pair of holder-positioning projections  929 ,  929  respectively include in their tip, tapered sections  929   a ,  929   a  shaped tapered and substantially conical. 
     As shown in  FIGS.  3  and  18   , center adjuster retainer  122  includes a holder-positioning contact part  126  that is a flat step formed by decreasing an inner diameter of center adjuster retainer  122  and thereby structured to receive holder first end  92   a  of holder  92 . As shown in  FIG.  23   , holder-positioning contact part  126  restricts axial movement of holder  92  inside the center adjuster retainer  122 , by being in contact with holder first end  92   a  of holder  92 . Holder-positioning contact part  126  includes in its inner peripheral edge the holder-positioning-projection engagement groove  127  extending through the holder-positioning contact part  126  in the axis P direction and engaging with the pair of holder-positioning projections  929 ,  929 . Holder-positioning-projection engagement groove  127  is positioned to determine a circumferential position of holder  92  at a phase at which holder first slide surface  923   a  and holder second slide surface  923   b  extend in the gear engagement direction (direction M in  FIG.  23   ). 
     Holder-positioning-projection engagement groove  127  is a concave groove having a predetermined groove width and extending along a line tangent to the inner peripheral edge of holder-positioning contact part  126  being substantially circular. Holder-positioning-projection engagement groove  127  includes a pair of curved ends in a direction of the groove wherein each of the ends has an inner R-shape (i.e., a concave arc shape). The pair of ends of holder-positioning-projection engagement groove  127  in the direction of the groove are respectively in contact with the pair of holder-positioning projections  929 ,  929  so as to establish engagement between holder-positioning-projection engagement groove  127  and the pair of holder-positioning projections  929 ,  929 . This restricts circumferential relative rotation of holder  92  with respect to center adjuster retainer  122 . 
       FIG.  19    is a planar view of lid  94 .  FIG.  20    is a sectional view along a line J-J of  FIG.  19   .  FIG.  21    is a view in a direction of an arrow K of  FIG.  20   .  FIG.  22    is an enlarged view of a part of  FIG.  5   . Lid  94  includes lid body  940  including an inward surface facing holder  92  and an outward surface in the opposite side. For convenience of explanation, the following description refers to the inward surface as a lid first end surface  94   a , and refers to the outward surface as a lid second end surface  94   b.    
     As shown in  FIGS.  19  to  21    with reference to  FIG.  8   , lid  94  is made of a resin material smaller in Young&#39;s modulus than a material of collar  91  and smaller in Young&#39;s modulus than a material of holder  92 . Lid  94  includes a lid body  940  having substantially a disk shape, and is structured to close the opening of holder  92  by fitting the lid body  940  in the opening of holder  92 . Lid body  940  includes a lid body base  940   a  and a pair of a first lid body fitting part  940   b  and a second lid body fitting part  940   c . Lid body base  940   a  is structured to be in contact with an end surface of the opening of holder  92 . Each of the pair of first lid body fitting part  940   b  and second lid body fitting part  940   c  is shaped to be less in diameter than lid body base  940   a  so as to form a step with respect to lid body base  940   a . The pair of first lid body fitting part  940   b  and second lid body fitting part  940   c  are fitted in an inner peripheral surface of the opening of holder  92  to form a so-called spigot joint. In addition, the pair of first lid body fitting part  940   b  and second lid body fitting part  940   c  serve also as collar support parts that support the collar second end surface  91   b  of collar  91 . 
     Lid body  940  has a substantially annular shape, and includes a lid through hole  940   d  in its central part through which axis P passes. Lid through hole  940   d  is an axial through hole having a substantially circular shape in planar view. This allows lid body  940  to accept worm second end  612  of worm shaft  61  supported by bearing  8 . As shown in  FIGS.  19  and  21   , lid body  940  further includes a plurality of (e.g., three in the present embodiment) center adjuster rotation grooves  940   e  in a peripheral edge of lid through hole  940   d . Center adjuster rotation grooves  940   e  are arranged at substantially equal intervals in the circumferential direction. Each of center adjuster rotation grooves  940   e  has a concave arc shape, and is structured to engage with a jig not shown. This allows adjustment of a circumferential position of center adjuster  90  inside the center adjuster retainer  122 , in cooperation with first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a  as described below. 
     Lid first end surface  94   a  of lid body  940  includes a collar second sliding-contact surface  940   f  in an outer circumferential region with respect to lid through hole  940   d . Collar second sliding-contact surface  940   f  is a substantially flat so as to be in sliding contact with collar second end surface  91   b  of collar  91  contained inside with respect to lid body  940 . In an inner circumferential region with respect to collar second sliding-contact surface  940   f , lid first end surface  94   a  includes a grease retention depression  940   g  depressed to form a step in a direction away from collar  91  with respect to collar second sliding-contact surface  940   f . Grease retention depression  940   g  retains grease not shown filling an inside of grease retention depression  940   g , and serves for lubrication for collar  91 . 
     First lid body fitting part  940   b  and second lid body fitting part  940   c  are formed in the inward surface (i.e., lid first end surface  94   a ) of lid body  940  facing holder  92  so as to face each other across axis P. Each of first lid body fitting part  940   b  and second lid body fitting part  940   c  has a substantially arc shape extending in the circumferential direction, and is disposed between first holder retainer  941  and second holder retainer  942  in the circumferential direction as described below. First lid body fitting part  940   b  is relatively large in radial width, and is longer in circumferential length than second lid body fitting part  940   c . Second lid body fitting part  940   c  is shorter in circumferential length than first lid body fitting part  940   b , and is smaller in radial width than first lid body fitting part  940   b  through the entire length of second lid body fitting part  940   c . Furthermore, the radial width of second lid body fitting part  940   c  within a predetermined region facing second holder retainer  942  in the circumferential direction decreases gradually as approaching a slidable region of collar  91 , i.e., as approaching axis Z that crosses the rotational axis P of worm shaft  61  and extends in the gear engagement direction. 
     As shown in  FIGS.  6 ,  7 , and  21   , lid first end surface  94   a  of lid body  940  includes a spring restriction projection  940   h  structured to restrict axial movement of wire spring  93 . Spring restriction projection  940   h  is formed in a part of lid body  940  in the circumferential direction, and projects toward holder  92  with respect to a part axially adjacent to spring restriction projection  940   h . Furthermore, spring restriction projection  940   h  is formed such that lid body  940  is a predetermined distance apart in the axial direction from a surface of holder first tubular wall  923  facing lid body  940 , in a state in which spring restriction projection  940   h  is in contact with wire spring  93  (see a gap C 3  shown in  FIG.  7   ). 
     As shown in  FIGS.  20  and  21   , lid first end surface  94   a  of lid body  940  includes a pair of first holder retainer  941  and second holder retainer  942  that retain holder  92 . First holder retainer  941  and second holder retainer  942  extend in the axial direction, and are parallel with each other, and respectively engage with first retainer engagement groove  927  and second retainer engagement groove  928  of holder  92 . Each of first holder retainer  941  and second holder retainer  942  has a substantially arc shape cross section, and has the thickness (i.e., the radial width) smaller than the groove depth (i.e., the radial width) of a corresponding one of first retainer engagement groove  927  and second retainer engagement groove  928 . In addition, engagement combination between first and second holder retainers  941  and  942  and first and second retainer engagement grooves  927  and  928  is limited to one kind. This serves to suppress incorrect mounting of lid  94  to holder  92 . 
     First holder retainer  941  includes in an inward side of a tip thereof the first holder retainer hook  941   a  that has a hooklike shape and is locked on first retainer hook engagement groove  927   a  of holder  92 . Similarly, second holder retainer  942  includes in an inward side of a tip thereof the second holder retainer hook  942   a  that has a hooklike shape and is locked on second retainer hook engagement groove  928   a  of holder  92 . First holder retainer hook  941   a  and second holder retainer hook  942   a  respectively include a first-holder-retainer-hook locking surface  941   b  and a second-holder-retainer-hook locking surface  942   b  that are inclined with respect to the axial direction and are structured to be in contact with first retainer hook engagement groove  927   a  and second retainer hook engagement groove  928   a  respectively. In detail, each of first-holder-retainer-hook locking surface  941   b  and second-holder-retainer-hook locking surface  942   b  is inclined to gradually decrease in distance from axis P as approaching a tip of a corresponding one of first holder retainer hook  941   a  and second holder retainer hook  942   a.    
     First holder retainer hook  941   a  includes in an outward side of the tip thereof a first holder retainer slope  941   c  inclined with respect to the axial direction. Similarly, second holder retainer hook  942   a  includes in an outward side of the tip thereof a second holder retainer slope  942   c  inclined with respect to the axial direction. Each of first holder retainer slope  941   c  and second holder retainer slope  942   c  is inclined to gradually decrease in distance from axis P as approaching a tip of a corresponding one of first holder retainer hook  941   a  and second holder retainer hook  942   a.    
     As shown in  FIGS.  6  and  19   , lid second end surface  94   b  of lid body  940  include the plurality of (e.g., three in the present embodiment) lid engagement parts: i.e., first lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945 . First lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945  are arranged at circumferential positions corresponding to center adjuster rotation grooves  940   e , and extend substantially parallel with each other in the axial direction from an outer peripheral edge of lid second end surface  94   b  of lid body  940 , and engage with the inner peripheral surface of center adjuster retainer  122 . 
     First lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945  respectively include a first lid bendable groove  943   a , a second lid bendable groove  944   a , and a third lid bendable groove  945   a , in their base ends adjacent to lid body  940 . Each of first lid bendable groove  943   a , second lid bendable groove  944   a , and third lid bendable groove  945   a  has a shape of a depression being open in the axial direction. This thins connection sections between lid body  940  and first, second, and third lid engagement parts  943 ,  944 , and  945 , and facilitates bending deformation of first, second, and third lid engagement parts  943 ,  944 , and  945  in the radial direction. 
     As shown in  FIGS.  3 ,  6 , and  19   , first lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945  respectively include in their radially outward side the first lid engagement projection  943   b , the second lid engagement projection  944   b , and the third lid engagement projection  945   b  that engage with center adjuster locking groove  125  having the annular shape and extending continuously in the circumferential direction in the outer peripheral surface of center adjuster retainer  122 . First lid engagement projection  943   b , second lid engagement projection  944   b , and third lid engagement projection  945   b  are respectively greater in outer diameter than parts of holder tubular wall  922  adjacent to first retainer engagement groove  927  and second retainer engagement groove  928 . 
     As shown in  FIG.  20   , first lid engagement projection  943   b , second lid engagement projection  944   b , and third lid engagement projection  945   b  respectively include a first thickness gradual-decrease section  943   c , a second thickness gradual-decrease section  944   c , and a third thickness gradual-decrease section  945   c . Each of first thickness gradual-decrease section  943   c , second thickness gradual-decrease section  944   c , and third thickness gradual-decrease section  945   c  has a tapered shape, and gradually decreases in radial width (i.e., amount of projection) in a direction toward the base end of a corresponding one of first lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945 , i.e., in a direction of insertion of center adjuster  90  into center adjuster retainer  122 . Furthermore, first lid engagement projection  943   b , second lid engagement projection  944   b , and third lid engagement projection  945   b  respectively include in their axially opposite side ends a first lid engagement projection slope  943   d , a second lid engagement projection slope  944   d , and a third lid engagement projection slope  945   d . Each of first lid engagement projection slope  943   d , second lid engagement projection slope  944   d , and third lid engagement projection slope  945   d  has a tapered shape, and gradually decreases in radial width (i.e., amount of projection) in a direction toward a tip of a corresponding one of first lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945 , i.e., in a direction of disengagement of center adjuster  90  from center adjuster retainer  122 . 
     Each of first thickness gradual-decrease section  943   c , second thickness gradual-decrease section  944   c , and third thickness gradual-decrease section  945   c  is a gentle slope relatively small in inclination with respect to axis P. On the other hand, each of first lid engagement projection slope  943   d , second lid engagement projection slope  944   d , and third lid engagement projection slope  945   d  is a sharp slope relatively large in inclination with respect to axis P, in comparison with first thickness gradual-decrease section  943   c , second thickness gradual-decrease section  944   c , and third thickness gradual-decrease section  945   c.    
     First lid engagement part  943 , second lid engagement part  944 , and third lid engagement part  945  respectively include in their tip a first center adjuster detachment projection  943   e , a second center adjuster detachment projection  944   e , and a third center adjuster detachment projection  945   e  that project in the axial direction and serve to release engagement between center adjuster locking groove  125  and first, second, and third lid engagement projections  943   b ,  944   b , and  945   b . Specifically, the engagement between center adjuster locking groove  125  and first, second, and third lid engagement projections  943   b ,  944   b , and  945   b  can be released by tilting first, second, and third lid engagement parts  943 ,  944 , and  945  inwardly in the radial direction via first, second, and third center adjuster detachment projections  943   e ,  944   e , and  945   e.    
     Lid second end surface  94   b  of lid body  940  faces the opening of center adjuster retainer  122  before installation of seal member  95 , and includes a marker  946  that allows visual inspection of a direction of sliding of collar  91 , i.e., a direction toward the worm wheel. Marker  946  shows a triangle pointing the radially outward direction, and thereby indicates the direction in which collar  91  approaches worm wheel  62 . 
     Thus-configured center adjustment mechanism  9  is installed by inserting center adjuster  90  sub-assembled beforehand into center adjuster retainer  122  of gear housing  12  with holder first end  92   a  facing center adjuster retainer  122  such that holder-positioning projections  929 ,  929  meet holder-positioning-projection engagement groove  127 . During this insertion, center adjuster  90  becomes in sliding contact with the inner peripheral surface of center adjuster retainer  122 , which determines a radial position of center adjuster  90  center adjuster  90  inside the center adjuster retainer  122 . After the insertion, pair of holder-positioning projections  929 ,  929  engage with holder-positioning-projection engagement groove  127 , which determines a circumferential position of center adjuster  90  inside the center adjuster retainer  122 . Simultaneously, holder first end  92   a  of center adjuster  90  becomes in contact with holder-positioning contact part  126 , which determines an axial position of center adjuster  90  inside the center adjuster retainer  122 . Furthermore, first lid engagement projection  943   b , second lid engagement projection  944   b , and third lid engagement projection  945   b  engage with center adjuster locking groove  125 , which fixes center adjuster  90  to center adjuster retainer  122 . In center adjuster  90  that has been installed, first lid engagement projection  943   b , second lid engagement projection  944   b , and third lid engagement projection  945   b  are in elastic contact with an end edge of center adjuster locking groove  125  (i.e., an axial end edge facing the opening of center adjuster retainer  122 ), which generates elastic force to press and fix center adjuster  90  onto holder-positioning contact part  126 . 
     Effects of the Present Embodiment 
     As described above, the conventional steering device has a problem of changing an action direction of reaction force generated in the worm gear, depending on whether the worm shaft is in positive rotation or negative rotation, especially in case that the worm shaft and the worm wheel are not right-angled in torsion angle therebetween. This may produce a backlash between the worm shaft and the worm wheel upon switching of the worm shaft between positive rotation and negative rotation, and thereby cause a noise due to the backlash. 
     In view of the foregoing, the steering device according to the present embodiment includes: the steering shaft  2  structured to rotate in response to rotation of the steering wheel; the worm wheel  62  linked to the steering shaft  2 ; the worm shaft  61  engaging with the worm wheel  62  and including the worm first end  611  and the worm second end  612  as the pair of ends; the electric motor  4  that is structured to supply rotational force to the worm shaft  61  and includes the motor shaft  41  connected to the worm first end  611  of the worm shaft  61 ; the gear housing  12  including the worm gear container  121  and the center adjuster retainer  122 , wherein the worm gear container  121  contains the worm wheel  62  and the worm shaft  61 , and wherein the center adjuster retainer  122  surrounds the worm second end  612 ; the bearing  8  supporting the worm second end  612 ; the center adjuster  90  that is disposed in the center adjuster retainer  122  and includes the holder  92  and the collar  91 , wherein the collar  91  is disposed in the holder  92  and retains the bearing  8  so as to allow the bearing  8  to move relatively with respect to the holder  92  in the gear engagement direction that is the direction of engagement between the worm wheel  62  and the worm shaft  61 ; and the biasing member (i.e., the wire spring  93 ) disposed in the center adjuster  90  and structured to bias the bearing  8  in the direction inclined with respect to the gear engagement direction. 
     As shown in  FIG.  23   , wire spring  93  biases bearing  8  in the direction (direction N in  FIG.  23   ) inclined with respect to the gear engagement direction (direction M in  FIG.  23   ). This generates a biasing force F exerted in the inclined direction (direction N), wherein biasing force F includes a component force F 1  exerted in the gear engagement direction (direction M). Component force F 1  moves collar  91 , which supports bearing  8 , in the gear engagement direction (direction M), and thereby serves to appropriately adjust the engagement between worm shaft  61  and worm wheel  62 . 
     Biasing force F in the inclined direction (direction N) further includes a component force F 2  exerted in a direction (i.e., a direction W in  FIG.  23   ) orthogonal to the gear engagement direction. Component force F 2  biases collar  91 , which supports bearing  8 , in the gear engagement orthogonal direction (direction W) orthogonal to the gear engagement direction, and thereby serves to suppress collar  91  from rattling in the gear engagement orthogonal direction (direction W). When representing a reaction force due to positive rotation of worm shaft  61  by reference sign A, and representing a reaction force B due to negative rotation of worm shaft  61  by reference sign B, reaction force A includes a component force A 2  that is in a direction opposite to component force F 2  and is canceled by component force F 2 , while reaction force B includes a component force B 2  that is in a direction same with component force F 2  and is added with component force F 2 . Thus, component force F 2  serves to suppress collar  91  from rattling, and thereby reduce generation of a noise due to the rattling and deterioration in steering feeling. 
     Furthermore, the present embodiment is configured such that: the holder  92  includes the holder slide surfaces composed of the holder first slide surface  923   a  and the holder second slide surface  923   b  that are the pair of surfaces facing each other; the collar  91  is disposed between the holder first slide surface  923   a  and the holder second slide surface  923   b , and retains the bearing  8 ; the collar  91  includes the collar slide surfaces composed of the collar first slide surface  916   a  and the collar second slide surface  916   b ; and the collar first slide surface  916   a  is in sliding contact with the holder first slide surface  923   a , and the collar second slide surface  916   b  is in sliding contact with the holder second slide surface  923   b.    
     Thus, the collar slide surfaces composed of collar first slide surface  916   a  and collar second slide surface  916   b  slide along the holder slide surfaces composed of holder first slide surface  923   a  and holder second slide surface  923   b . This allows collar  91  supporting bearing  8  to move relatively with respect to holder  92  in the gear engagement direction. 
     Furthermore, the present embodiment is configured such that: the collar  91  includes the collar body  910  and the opening  911 ; the collar body ( 910 ) has the arc shape, and retains the bearing ( 8 ) inside the collar body ( 910 ); and the bearing ( 8 ) includes the outer periphery composed of a first part surrounded by the collar body ( 910 ) and a second part facing the opening ( 911 ). 
     Thus, collar  91  includes opening  911  as a part in the circumferential direction. This allows bearing  8  to be mounted to collar  91  via opening  911 , and thereby facilitates installation of collar  91  to bearing  8 . 
     Furthermore, the present embodiment is configured such that collar  91  includes collar engagement parts  915  that suppress bearing  8  from dropping off collar body  910 . 
     Thus, collar engagement parts  915  serves to suppress bearing  8  from dropping off after bearing  8  has been mounted to collar body  910 . 
     Furthermore, the present embodiment is configured such that the collar body  910  exerts biasing force toward the center of the bearing  8 , on the contact part with the outer periphery of the bearing  8 . 
     Thus, collar body  910  exerts elastic force (i.e., the biasing force) on bearing  8  so as to retain bearing  8 , and thereby serves to suppress bearing  8  from dropping off collar body  910 . 
     Furthermore, the present embodiment is configured such that: collar body  910  includes slopes  917  that are respectively disposed in the pair of ends of collar body  910  in the circumferential direction; and slopes  917  are defined such that collar body  910  includes the outer peripheral surface gradually increasing in distance from the holder slide surfaces (i.e., holder first slide surface  923   a  and holder second slide surface  923   b ) with decrease in distance from the pair of ends of collar body  910  in the circumferential direction. 
     Thus, slopes  917  of collar body  910  is inclined such that the outer peripheral surface of collar body  910  gradually goes away from the holder slide surfaces (holder first slide surface  923   a  and holder second slide surface  923   b ), as approaching the pair of ends. This suppresses corners of the pair of ends of collar body  910  from interfering with the holder slide surfaces (holder first slide surface  923   a  and holder second slide surface  923   b ), and facilitates smooth sliding of collar  91  inside the holder  92 . 
     Furthermore, the present embodiment is configured such that collar body  910  in a natural state before mounting bearing  8  to collar body  910  is smaller in inner diameter than collar body  910  in a state after mounting bearing  8  to collar body  910 . 
     Thus, collar body  910  has an interference with respect to bearing  8 . This serves to suppress bearing  8  from dropping off collar body  910 . 
     Furthermore, the present embodiment is configured such that the collar  91  includes the lubrication groove  918  disposed in the end surface of the collar  91  in the direction of the rotational axis P of the worm shaft  61 . 
     Thus, lubrication groove  918  retains lubricant such as the grease described above. This reduces a sliding resistance of collar  91  with respect to holder  92 . 
     Furthermore, the present embodiment is configured such that lubrication groove  918  is connected to the outer peripheral edge of the end surface of collar  91 . 
     This facilitates introduction of lubricant such as the grease retained in an outer peripheral side of collar  91  into lubrication groove  918 , and thereby serves to more effectively reduce the sliding resistance of collar  91  with respect to holder  92 . 
     Furthermore, the present embodiment is configured such that: holder  92  includes collar container  920 ; collar  91  is disposed in collar container  920 , and includes collar bottom  913  and collar peripheral wall  914 ; collar bottom  913  is in contact with one of the pair of end surfaces in the axial direction of bearing  8 ; collar peripheral wall  914  is continuous with collar bottom  913 , and surrounds the outer periphery of bearing  8 ; and collar peripheral wall  914  includes a first end in an opposite side to collar bottom  913  in the axial direction, wherein the first end of collar peripheral wall  914  gradually decreases in inner diameter, with increase in distance from collar bottom  913 . 
     Thus, collar peripheral wall  914  of collar  91  includes an inner peripheral surface that decreases in inner diameter as going away from collar bottom  913 . 
     This allows collar  91  to have interferences in the axial direction and the radial direction in a state in which bearing  8  is mounted to collar  91 . This serves to suppress bearing  8  from rattling in the axial direction and/or the radial direction inside the collar  91 . 
     Furthermore, the present embodiment is configured such that: the biasing member is the spring (the wire spring  93 ) having the arc shape; the collar  91  includes the spring winding part  914   a  and the protrusion part  914   b ; the spring (the wire spring  93 ) is wound around the spring winding part  914   a ; and the protrusion part  914   b  protrudes outwardly in the radial direction in comparison with the spring winding part  914   a.    
     This forms a step between spring winding part  914   a  and protrusion part  914   b , and thereby serves to suppress wire spring  93  from dropping off collar  91 . 
     Furthermore, the present embodiment is configured such that: the spring winding part  914   a  includes a first end in an opposite side to the protrusion part  914   b  in the direction of the rotational axis P of the worm shaft  61 ; and the first end of the spring winding part  914   a  protrudes outwardly in the radial direction. 
     Thus, according to the present embodiment, the protruding part of spring winding part  914   a  and protrusion part  914   b  of collar  91  are respectively disposed in both side with respect to wire spring  93  in the axial direction. This serves to further suppress wire spring  93  from dropping off collar  91 . 
     Furthermore, the present embodiment is configured such that the spring (the wire spring  93 ) is wound in a region of the spring winding part  914   a  wherein the region overlaps with the bearing  8  in the radial direction with respect to the rotational axis P of the worm shaft  61 . 
     This allows wire spring  93  to appropriately exert the biasing force on bearing  8 , and serves to effectively suppress collar  91  from rattling. 
     Furthermore, the present embodiment is configured such that spring winding part  914   a  has a width in the direction of rotational axis P of worm shaft  61  wherein the width is greater than the radius of the wire of the spring (wire spring  93 ). 
     This allows wire spring  93  to be certainly in contact with spring winding part  914   a , and serves to obtain an appropriate biasing effect by wire spring  93 . 
     Furthermore, the present embodiment is configured such that the step between spring winding part  914   a  and protrusion part  914   b  has a width greater than a diameter of the wire of the spring (wire spring  93 ). 
     This serves to suppress wire spring  93  from interfering with other members, and thereby ensure appropriate biasing action by wire spring  93 . 
     Furthermore, the present embodiment is configured such that when viewed from an outer side in the radial direction with respect to rotational axis P of worm shaft  61 , holder  92  includes a first section and a second section greater in inner diameter than the first section, wherein the first section is a section in which the holder slide surfaces (i.e., holder first slide surface  923   a  and holder second slide surface  923   b ) are formed, while the second section is a section not overlapping with the holder slide surfaces but overlapping with spring winding part  914   a.    
     Thus, according to the present embodiment, the outer peripheral surface of holder  92  in a region facing spring winding part  914   a  is formed to avoid interference with wire spring  93 . This serves to suppress holder  92  from interfering with wire spring  93 . 
     Furthermore, the present embodiment is configured such that: the biasing member is the spring (the wire spring  93 ) having the arc shape; the collar  91  includes the spring winding part  914   a  and the protrusion part  914   b ; the spring (the wire spring  93 ) is wound around the spring winding part  914   a ; the protrusion part  914   b  protrudes outwardly in the radial direction in comparison with the spring winding part  914   a ; the spring (the wire spring  93 ) is in contact with the outer periphery of the spring winding part  914   a  via at least two contact parts; and each of the contact parts is positioned on the virtual circle that has the center shifted from the axis crossing the rotational axis P of the worm shaft  61  and extending in the gear engagement direction (the direction M in  FIG.  23   ). 
     Thus, according to the present embodiment, center O of virtual circle V passing through the contact parts of spring winding part  914   a  and wire spring  93  is shifted from axis Z that crosses rotational axis P of worm shaft  61  and extends in the gear engagement direction (direction M in  FIG.  23   ). This allows wire spring  93  to bias bearing  8  in the direction (direction N in  FIG.  23   ) inclined with respect to the gear engagement direction. 
     Furthermore, the present embodiment is configured such that: the holder  92  includes the spring locking part  925  on which the spring (the wire spring  93 ) is locked; the spring locking part  925  is disposed in an opposite side to the contact parts between the spring (the wire spring  93 ) and the spring winding part  914   a , across the rotational axis P of the worm shaft  61 . 
     Thus, according to the present embodiment, spring locking part  925  is disposed in the opposite side to the contact parts between spring winding part  914   a  and wire spring  93 , across rotational axis P of worm shaft  61 . This allows bearing  8  to be biased in the direction (direction N in  FIG.  23   ) inclined with respect to the gear engagement direction. 
     Furthermore, the present embodiment is configured such that: the biasing member is the spring (wire spring  93 ) having the arc shape; and holder  92  includes spring rotation restricter  926  that is in contact with the pair of ends of the spring (wire spring  93 ). 
     Thus, according to the present embodiment, the pair of circumferential ends of wire spring  93  interpose spring rotation restricter  926  of holder  92  therebetween. This allows spring rotation restricter  926  to restrict rotation of wire spring  93 . 
     Furthermore, the present embodiment is configured such that: the center adjuster  90  includes the lid  94 ; the collar  91  of the center adjuster  90  includes the collar first end surface  91   a  and the collar second end surface  91   b  that are the pair of end surfaces in the axial direction; the holder  92  of the center adjuster  90  includes the holder bottom  921  and the holder tubular wall  922 ; the holder bottom  921  is in contact with the collar first end surface  91   a ; the holder tubular wall  922  is continuous with the holder bottom  921 , and surrounds the outer periphery of the collar  91 ; the holder tubular wall  922  includes an end in an opposite side to the holder bottom  921  in the axial direction wherein the end of the holder tubular wall  922  is adjacent to the lid  94  of the center adjuster  90 ; and the lid  94  is in contact with the collar second end surface  91   b.    
     Thus, according to the present embodiment, holder  92  and lid  94  interpose collar  91  from both sides in the axial direction. This serves to ensure smooth sliding action of collar  91 . 
     Furthermore, the present embodiment is configured such that: lid  94  includes the collar sliding-contact surface (i.e., collar second sliding-contact surface  940   f ) and grease retention depression  940   g ; the collar sliding-contact surface (collar second sliding-contact surface  940   f ) is disposed in a region in contact with collar second end surface  91   b ; and grease retention depression  940   g  is positioned inner with respect to the collar sliding-contact surface (collar second sliding-contact surface  940   f ), and is depressed to be apart from collar  91  in the direction of rotational axis P of worm shaft  61 . 
     This allows grease retention depression  940   g  to retain grease therein, and supply the grease to collar second sliding-contact surface  940   f . This serves to reduce a sliding resistance between collar  91  and lid  94 . 
     Furthermore, the present embodiment is configured such that: the center adjuster retainer  122  of the gear housing  12  has the tubular shape defining the center adjuster container space inside the center adjuster retainer  122 , and includes the inner periphery including the center adjuster locking groove  125  shaped annular; and the lid  94  of the center adjuster  90  includes the lid engagement projections (i.e., the first, second, and third first lid engagement projections  943   b ,  944   b , and  945   b ) engaging with the center adjuster locking groove  125 . 
     Thus, first, second, and third first lid engagement projections  943   b ,  944   b , and  945   b  engage with center adjuster locking groove  125 . This serves to suppress center adjuster  90  from dropping off center adjuster retainer  122 . 
     Furthermore, the present embodiment is configured such that: lid  94  is made of a resin material, and includes lid body  940  including the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) and the lid bendable grooves (first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a ) and the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ); the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) project from lid body  940  in the direction away from holder  92  in the direction of rotational axis P of worm shaft  61 ; the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) project outwardly in the radial direction from the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ); and the lid bendable grooves (first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a ) are disposed between lid body  940  and the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) in the radial direction with respect to rotational axis P of worm shaft  61 , so as to open in the direction of rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a  are formed in lid  94 , and thereby facilitate bending deformation of first, second, and third lid engagement parts  943 ,  944 , and  945 . This facilitates mounting of center adjuster  90  to center adjuster retainer  122 , and serves to improve the steering device in productivity. 
     Furthermore, the present embodiment is configured such that: lid  94  includes center adjuster rotation grooves  940   e ; and adjuster rotation grooves  940   e  are disposed in the inner periphery of lid through hole  940   d  formed inner with respect to the lid bendable grooves (first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a ), and are depressed outwardly in the radial direction with respect to rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, the rotational position of center adjuster  90  can be adjusted even after inserting center adjuster  90  into center adjuster retainer  122 , by fitting a jig to both of first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a  and center adjuster rotation groove  940   e  and thereby rotating lid  94 . This facilitates installation of center adjuster  90 . 
     Furthermore, the present embodiment is configured such that: the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) of lid  94  respectively include the center adjuster detachment projections (i.e., first, second and third center adjuster detachment projections  943   e ,  944   e , and  945   e ); and the center adjuster detachment projections (first, second, and third center adjuster detachment projections  943   e ,  944   e , and  945   e ) respectively project from the tips of the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) in the direction of rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, first, second, and third lid engagement parts  943 ,  944 , and  945  can be bendable inwardly in the radial direction via first, second, and third lid bendable grooves  943   a ,  944   a , and  945   a , by pinching first, second, and third center adjuster detachment projections  943   e ,  944   e , and  945   e  with a tool not shown. This allows first, second, and third lid engagement projections  943   b ,  944   b , and  945   b  to be separated from center adjuster locking groove  125 , and allows center adjuster  90  to be detached from center adjuster retainer  122 . This facilitates detachment of center adjuster  90 . 
     Furthermore, the present embodiment is configured such that: the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) of lid  94  respectively include the thickness gradual-decrease sections (i.e., first, second, and third thickness gradual-decrease sections  943   c ,  944   c , and  945   c ); and the thickness gradual-decrease sections (first, second, and third thickness gradual-decrease sections  943   c ,  944   c , and  945   c ) are respectively disposed between the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) and lid body  940 , and respectively increase in cross sectional area perpendicular to rotational axis P of worm shaft  61 , as approaching from lid body  940  to the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) in the direction of rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, the thickness gradual-decrease sections (first, second, and third thickness gradual-decrease sections  943   c ,  944   c , and  945   c ) of the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) are shaped to gradually vary in thickness. This serves to suppress concentration of internal stress upon bending deformation of first, second, and third lid engagement parts  943 ,  944 , and  945 . 
     Furthermore, the present embodiment is configured such that: center adjuster locking groove  125  includes a first surface structured to be in contact with the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) wherein the first surface is perpendicular to rotational axis P of worm shaft  61 ; and each of the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) includes a surface structured to be in contact with center adjuster locking groove  125  and inclined with respect to axis P of worm shaft  61 . 
     This facilitates engagement of the inclined surfaces of first, second, and third lid engagement projections  943   b ,  944   b , and  945   b  with center adjuster locking groove  125 , and thereby serves to suppress center adjuster  90  from rattling inside the center adjuster locking groove  125 . 
     Furthermore, the present embodiment is configured such that: the biasing member is the spring (wire spring  93 ) having the arc shape; lid  94  includes lid body  940  including spring restriction projection  940   h ; spring restriction projection  940   h  is disposed in a part of lid body  940  in the circumferential direction around rotational axis P of worm shaft  61 ; spring restriction projection  940   h  is formed such that lid body  940  is a predetermined distance apart in the direction of rotational axis P of worm shaft  61 , from a surface of holder tubular wall  922  (in detail, holder first tubular wall  923 ) facing lid body  940 ; and spring restriction projection  940   h  projects toward holder  92  with respect to a part adjacent to spring restriction projection  940   h  in the direction of rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, wire spring  93  is interposed between spring restriction projection  940   h  and holder tubular wall  922 , while spring restriction projection  940   h  projects toward holder tubular wall  922  with respect to the part adjacent to spring restriction projection  940   h . This serves to narrow a distance between spring restriction projection  940   h  and holder tubular wall  922 , and thereby restrict free movement of wire spring  93  in the axial direction. 
     Furthermore, the present embodiment is configured such that: holder  92  includes the retainer hook engagement grooves (i.e., first and second retainer hook engagement grooves  927   a  and  928   a ); the retainer hook engagement grooves (first and second retainer hook engagement grooves  927   a  and  928   a ) respectively include the surfaces being perpendicular to rotational axis P of worm shaft  61  and being in contact with the holder retainer hooks (i.e., first and second holder retainer hooks  941   a  and  942   a ); lid  94  includes lid body  940  and the holder retainers (i.e., first and second holder retainers  941  and  942 ) retaining holder  92 ; the holder retainers (first and second holder retainers  941  and  942 ) project from lid body  940  toward holder  92  in the direction of rotational axis P of worm shaft  61 ; the holder retainers (first and second holder retainers  941  and  942 ) respectively include the holder retainer hooks (first and second holder retainer hooks  941   a  and  942   a ); and the holder retainer hooks (first and second holder retainer hook  941   a  and  942   a ) are respectively disposed in the tips of the holder retainers (first and second holder retainers  941  and  942 ), and respectively include the surfaces inclined with respect to rotational axis P of worm shaft  61  and structured to be in contact with the retainer hook engagement grooves (first and second retainer hook engagement grooves  927   a  and  928   a ). 
     This facilitates establishment of contact between first and second retainer hook engagement grooves  927   a  and  928   a  and the inclined surfaces of first and second holder retainer hooks  941   a  and  942   a . Holder  92  and lid  94  assembled serve to suppress collar  91  from rattling. 
     Furthermore, the present embodiment is configured such that: holder  92  includes the retainer hook engagement grooves (first and second retainer hook engagement grooves  927   a  and  928   a ); lid  94  includes lid body  940  and the holder retainers (first and second holder retainers  941  and  942 ) retaining holder  92 ; the holder retainers (first and second holder retainers  941  and  942 ) project from lid body  940  toward holder  92  in the direction of rotational axis P of worm shaft  61 ; the holder retainers (first and second holder retainers  941  and  942 ) respectively include the holder retainer hooks (first and second holder retainer hooks  941   a  and  942   a ) and the holder retainer slopes (first and second holder retainer slopes  941   c  and  942   c ); the holder retainer hooks (first and second holder retainer hooks  941   a  and  942   a ) are respectively disposed in the inward surfaces of the tips of the holder retainers (first and second holder retainers  941  and  942 ) in the radial direction with respect to rotational axis P of worm shaft  61 ; and the holder retainer slopes (first and second holder retainer slopes  941   c  and  942   c ) are respectively disposed in the outward surfaces of the tips of the holder retainers (first holder retainer  941  and second holder retainer  942 ) in the radial direction with respect to rotational axis P of worm shaft  61 , and are inclined with respect to rotational axis P of worm shaft  61 . 
     In case of failing to appropriately fit first and second holder retainer hooks  941   a  and  942   a  into first and second retainer hook engagement grooves  927   a  and  928   a , the tips of first and second holder retainers  941  and  942  may project outside in the radial direction. However, according to the present embodiment, first and second holder retainer hooks  941   a  and  942   a  respectively include first and second holder retainer slopes  941   c  and  942   c . This serves to suppress the tips of first and second holder retainers  941  and  942  from interfering with the inner peripheral surface of center adjuster retainer  122 , even in case of somewhat insufficient engagement between first and second holder retainer hooks  941   a  and  942   a  and first and second retainer hook engagement grooves  927   a  and  928   a.    
     Furthermore, the present embodiment is configured such that: holder  92  includes the retainer engagement grooves (i.e., first and second retainer engagement grooves  927  and  928 ); the retainer engagement grooves (first and second retainer engagement grooves  927  and  928 ) are respectively shaped groovy to extend in the direction of rotational axis P of worm shaft  61 , and are disposed in the outer periphery of holder tubular wall  922 ; lid  94  includes lid body  940  and the holder retainers (first and second holder retainers  941  and  942 ) retaining holder  92 ; the holder retainers (first and second holder retainers  941  and  942 ) project from lid body  940  toward holder  92  in the direction of rotational axis P of worm shaft  61 , and engage with the retainer engagement grooves (first and second retainer engagement grooves  927  and  928 ); and the holder retainers (first and second holder retainers  941  and  942 ) in engagement with the retainer engagement grooves (first and second retainer engagement grooves  927  and  928 ) are respectively less in outer diameter than parts of holder tubular wall  922  adjacent to the retainer engagement grooves (first and second retainer engagement grooves  927  and  928 ). 
     Thus, according to the present embodiment, the outer peripheral surface of each of the holder retainers (first and second holder retainers  941  and  942 ) when being in engagement with the retainer engagement grooves (first and second retainer engagement grooves  927  and  928 ) is retracted with respect to the outer peripheral surface of holder tubular wall  922 . This serves to suppress first and second holder retainers  941  and  942  from interfering with the inner peripheral surface of center adjuster retainer  122  in a state in which center adjuster  90  has been mounted to center adjuster retainer  122 , and thereby suppress first and second holder retainers  941  and  942  from affecting a process of bringing holder tubular wall  922  into contact with the inner peripheral surface of center adjuster retainer  122  and performing positioning of center adjuster  90 . 
     Furthermore, the present embodiment is configured such that: center adjuster retainer  122  has the tubular shape defining the center adjuster container space inside the center adjuster retainer  122 , and includes the inner periphery including center adjuster locking groove  125  shaped annular; lid  94  is made of a resin material, and includes lid body  940  including the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ); the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ) project from lid body  940  in the direction away from holder  92  in the direction of rotational axis P of worm shaft  61 , and respectively include the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) in the tips of the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ); and the lid engagement projections (first, second, and third lid engagement projections  943   b ,  944   b , and  945   b ) project outwardly in the radial direction with respect to rotational axis P of worm shaft  61 , from the lid engagement parts (first, second, and third lid engagement parts  943 ,  944 , and  945 ), and engage with center adjuster locking groove  125 , and are respectively greater in outer diameter in the radial direction than parts of holder tubular wall  922  adjacent to the retainer engagement grooves (first and second retainer engagement grooves  927  and  928 ). 
     According to the present embodiment, center adjuster  90  is inserted into center adjuster retainer  122  with holder  92  directed forward and with lid  94  directed backward. On this occasion, first, second, and third lid engagement projections  943   b ,  944   b , and  945   b  project outwardly in the radial direction in comparison with the outer peripheral surface of holder tubular wall  922 . This facilitates establishment of sufficient engagement between first, second, and third lid engagement projections  943   b ,  944   b , and  945   b  and center adjuster locking groove  125 . 
     Furthermore, the present embodiment is configured such that lid  94  includes marker  946  that allows the rotational position of lid  94  around rotational axis P of worm shaft  61  to be visually inspected from an outside of gear housing  12 . 
     Thus, according to the present embodiment, the rotational position of center adjuster  90  can be checked from the outside of gear housing  12 , upon inserting center adjuster  90  into center adjuster retainer  122 . This facilitates the insertion of center adjuster  90  into center adjuster retainer  122 . 
     Furthermore, the present embodiment is configured such that lid  94  is allowed to engage with holder  92 , at only one rotational position with respect to holder  92  in the circumferential direction around rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, holder  92  and lid  94  accord in rotational position, at the only one rotational position. Accordingly, the rotational position of holder  92  can be checked by checking the rotational position of lid  94  with marker  946 . This facilitates the insertion of center adjuster  90  into center adjuster retainer  122 . 
     Furthermore, the present embodiment is configured such that collar  91  is made of a material less in friction coefficient than a material of holder  92  and a material of lid  94 . 
     Thus, according to the present embodiment, collar  91  is small in friction coefficient sufficiently to smoothly slide with respect to both of holder  92  and lid  94 . 
     Furthermore, the present embodiment is configured such that lid  94  is made of a material less in Young&#39;s modulus than a material of holder  92  and a material of collar  91 . 
     Thus, according to the present embodiment, first and second holder retainers  941  and  942  and first, second, and third lid engagement parts  943 ,  944 , and  945  of lid  94  are easily bendable. This facilitates the mounting of lid  94  to holder  92  and the inserting of center adjuster  90  into center adjuster retainer  122 . 
     Furthermore, the present embodiment is configured such that: lid  94  includes the collar supporters (i.e., first and second lid body fitting parts  940   b  and  940   c ) supporting the outer peripheral surface of collar  91 ; and the collar supporters (first and second lid body fitting parts  940   b  and  940   c ) project toward holder  92  in the direction of rotational axis P of worm shaft  61 , and respectively decrease in thickness in the radial direction with respect to rotational axis P of worm shaft  61 , as approaching the axis crossing rotational axis P of worm shaft  61  and extending in the gear engagement direction (direction M in  FIG.  23   ). 
     Thus, according to the present embodiment, each of first and second lid body fitting parts  940   b  and  940   c  serving as the collar supporters is thinned as going in the sliding direction of collar  91 . This serves to suppress first and second lid body fitting parts  940   b  and  940   c  from interfering with collar  91  in case that collar  91  slides in a direction to deepen the engagement of the worm gear. On the other hand, by thickening first and second lid body fitting parts  940   b  and  940   c  as going in a direction orthogonal to the sliding direction of collar  91 , first and second lid body fitting parts  940   b  and  940   c  can be improved in rigidity, and also lid  94  can be improved in rigidity. 
     Furthermore, the present embodiment is configured such that: the center adjuster retainer  122  of the gear housing  12  has the tubular shape defining the center adjuster container space inside the center adjuster retainer  122 ; the center adjuster container space has a circular shape at a cross section perpendicular to the rotational axis P of the worm shaft  61 ; the center adjuster  90  has a circular outline at a cross section perpendicular to the rotational axis P of the worm shaft  61 ; the center adjuster  90  includes the outer peripheral surface that is in contact with the inner peripheral surface of the center adjuster retainer  122 ; and the center adjuster  90  and the center adjuster retainer  122  are structured to determine the position of the center adjuster  90  with respect to the center adjuster retainer  122 , due to the contact between the outer peripheral surface of the center adjuster  90  and the inner peripheral surface of the center adjuster retainer  122 . 
     Thus, according to the present embodiment, the inner peripheral surface of center adjuster retainer  122  is shaped tubular. This facilitates machining of gear housing  12 . In addition, the inner peripheral surface of center adjuster retainer  122  and the outer peripheral surface of center adjuster  90  are both circular. This facilitates axis alignment of center adjuster retainer  122  and center adjuster  90 . 
     Furthermore, the present embodiment is configured such that: holder  92  includes holder bottom  921 , holder tubular wall  922 , and holder-positioning projections  929 ,  929 ; holder-positioning projections  929 ,  929  project from holder tubular wall  922  in the direction away from collar  91 , in the direction of rotational axis P of worm shaft  61 ; center adjuster retainer  122  includes holder-positioning-projection engagement groove  127 ; holder-positioning-projection engagement groove  127  engages with holder-positioning projections  929 ,  929 , and thereby restricts relative rotation of holder  92  with respect to center adjuster retainer  122  in a rotational direction around rotational axis P of worm shaft  61 . 
     Thus, according to the present embodiment, holder  92  includes holder-positioning projections  929 ,  929  engaging with holder-positioning-projection engagement groove  127  disposed in center adjuster retainer  122 . This serves to determine the position in the circumferential direction of holder  92  inside the center adjuster retainer  122 , and thereby obtain the appropriate biasing effect by wire spring  93  (i.e., the biasing action of wire spring  93  biasing bearing  8  in the direction inclined with respect to the gear engagement direction). 
     Furthermore, the present embodiment is configured such that: holder-positioning-projection engagement groove  127  is the groove that is formed in the inner periphery of center adjuster retainer  122  and is open in the direction of rotational axis P of worm shaft  61 ; and each of holder-positioning projections  929 ,  929  has the gap C 2  with holder-positioning-projection engagement groove  127  in the radial direction with respect to rotational axis P of worm shaft  61 . 
     This prevents holder-positioning-projection engagement groove  127  from exerting reaction force in the radial direction on holder-positioning projections  929 ,  929 , and thereby suppress holder-positioning projections  929 ,  929  from affecting axis aligning of holder  92 , inside the center adjuster retainer  122 . 
     Furthermore, the present embodiment is configured such that holder-positioning projections  929 ,  929  respectively include in their tips the tapered sections  929   a ,  929   a  inclined to be tapered. 
     Thus, holder-positioning projections  929 ,  929  respectively include in their tips the tapered sections  929   a ,  929   a  inclined to be tapered. This facilitates insertion of holder-positioning projections  929 ,  929  into holder-positioning-projection engagement groove  127 , and thereby facilitates the mounting of center adjuster  90  to center adjuster retainer  122 . 
     Furthermore, the present embodiment is configured such that: holder  92  includes holder bottom  921  and holder tubular wall  922 ; and center adjuster retainer  122  includes holder-positioning contact part  126  that is in contact with holder bottom  921  in the direction of rotational axis P of worm shaft  61  and thereby perform the positioning of holder  92  in the direction of rotational axis P of worm shaft  61  with respect to center adjuster retainer  122 . 
     Thus, according to the present embodiment, center adjuster retainer  122  includes holder-positioning contact part  126 . This allows holder-positioning contact part  126  to determine the position in the axial direction of holder  92 , and thereby serves for appropriate positioning of center adjuster  90  with respect to center adjuster retainer  122 . 
     The present invention is not limited to configurations and aspects exemplified in the present embodiment, but may be freely modified depending on specifications, costs, etc. of application targets, as long as presenting the effects of the present invention described above. 
     The following describes exemplary aspects of a steering device according to the embodiment above. 
     According to one aspect, the steering device includes: a steering shaft structured to rotate in response to rotation of a steering wheel; a worm wheel linked to the steering shaft; a worm shaft engaging with the worm wheel and including a worm first end and a worm second end as a pair of ends; an electric motor that is structured to supply rotational force to the worm shaft and includes a motor shaft connected to the worm first end of the worm shaft; a gear housing including a worm gear container and a center adjuster retainer, wherein the worm gear container contains the worm wheel and the worm shaft, and wherein the center adjuster retainer surrounds the worm second end; a bearing supporting the worm second end; a center adjuster that is disposed in the center adjuster retainer and includes a holder and a collar, wherein the collar is disposed in the holder and retains the bearing so as to allow the bearing to move relatively with respect to the holder in a gear engagement direction that is a direction of engagement between the worm wheel and the worm shaft; and a biasing member disposed in the center adjuster and structured to bias the bearing in a direction inclined with respect to the gear engagement direction. 
     In addition to the favorable aspect above, the steering device is configured such that: the holder includes holder slide surfaces composed of a holder first slide surface and a holder second slide surface that are a pair of surfaces facing each other; the collar is disposed between the holder first slide surface and the holder second slide surface, and retains the bearing; the collar includes collar slide surfaces composed of a collar first slide surface and a collar second slide surface; and the collar first slide surface is in sliding contact with the holder first slide surface, and the collar second slide surface is in sliding contact with the holder second slide surface. 
     According to another aspect, in addition to any one of the above aspects, the steering device is configured such that: the collar includes a collar body and an opening; the collar body has an arc shape, and retains the bearing inside the collar body; and the bearing includes an outer periphery composed of a first part surrounded by the collar body and a second part facing the opening. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the collar includes a collar engagement part structured to suppress the bearing from dropping off the collar body. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the collar body exerts biasing force toward a center of the bearing, on a contact part with the outer periphery of the bearing. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the collar body includes a pair of ends in a circumferential direction each of which includes a slope; and the slopes are defined such that the collar body includes an outer peripheral surface gradually increasing in distance from the holder slide surfaces with decrease in distance from the pair of ends of the collar body. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the collar body in a natural state before mounting the bearing to the collar body is smaller in inner diameter than the collar body in a state after mounting the bearing to the collar body. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the collar includes a lubrication groove disposed in an end surface of the collar in a direction of a rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the lubrication groove is connected to an outer peripheral edge of the end surface of the collar. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes a collar container; the collar is disposed in the collar container, and includes a collar bottom and a collar peripheral wall; the collar bottom is in contact with one of a pair of end surfaces in the axial direction of the bearing; the collar peripheral wall is continuous with the collar bottom, and surrounds the outer periphery of bearing  8 ; and the collar peripheral wall includes a first end in an opposite side to the collar bottom in the axial direction, wherein the first end of the collar peripheral wall gradually decreases in inner diameter, with increase in distance from the collar bottom. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the biasing member is a spring having an arc shape; the collar includes a spring winding part and a protrusion part; the spring is wound around the spring winding part; and the protrusion part protrudes outwardly in a radial direction in comparison with the spring winding part. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the spring winding part includes a first end in an opposite side to the protrusion part in the direction of the rotational axis of the worm shaft; and the first end of the spring winding part protrudes outwardly in the radial direction. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the spring is wound in a region of the spring winding part wherein the region overlaps with the bearing in the radial direction with respect to the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the spring winding part has a width in the direction of rotational axis P of worm shaft  61  wherein the width is greater than a radius of a wire of the spring. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the spring winding part and the protrusion part forms therebetween a step having a width greater than a diameter of the wire of the spring. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that when viewed from an outer side in the radial direction with respect to the rotational axis of the worm shaft, the holder includes a first section and a second section greater in inner diameter than the first section, wherein the first section is a section in which the holder slide surfaces are formed, while the second section is a section not overlapping with the holder slide surfaces but overlapping with the spring winding part. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the biasing member is the spring having the arc shape; the collar includes the spring winding part and the protrusion part; the spring is wound around the spring winding part; the protrusion part protrudes outwardly in the radial direction in comparison with the spring winding part; the spring is in contact with an outer periphery of the spring winding part via at least two contact parts; and each of the contact parts is positioned on a virtual circle that has a center shifted from an axis crossing a the rotational axis of the worm shaft and extending in the gear engagement direction. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes a spring locking part on which the spring is locked; the spring locking part is disposed in an opposite side to the contact parts between the spring and the spring winding part, across the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the biasing member is the spring having the arc shape; and the holder includes a spring rotation restricter that is in contact with a pair of ends of the spring. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the center adjuster includes a lid; the collar of the center adjuster includes a collar first end surface and a collar second end surface that are a pair of end surfaces in the axial direction; the holder of the center adjuster includes a holder bottom and a holder tubular wall; the holder bottom is in contact with the collar first end surface; the holder tubular wall is continuous with the holder bottom, and surrounds an outer periphery of the collar; the holder tubular wall includes an end in an opposite side to the holder bottom in the axial direction wherein the end of the holder tubular wall is adjacent to the lid of the center adjuster; and the lid is in contact with the collar second end surface. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the lid includes a collar sliding-contact surface and a grease retention depression; the collar sliding-contact surface is disposed in a region in contact with the collar second end surface; and the grease retention depression is positioned inner with respect to the collar sliding-contact surface, and is depressed to be apart from the collar in the direction of the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the center adjuster retainer of the gear housing has a tubular shape defining a center adjuster container space inside the center adjuster retainer, and includes an inner periphery including a center adjuster locking groove shaped annular; and the lid of the center adjuster includes a lid engagement projection engaging with the center adjuster locking groove. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the lid is made of a resin material, and includes a lid body including a lid engagement part and a lid bendable groove and a lid engagement projection; the lid engagement part projects from the lid body in a direction away from the holder in the direction of the rotational axis of the worm shaft; the lid engagement projection projects outwardly in the radial direction from the lid engagement part; and the lid bendable groove is disposed between the lid body and the lid engagement part in the radial direction with respect to the rotational axis of the worm shaft, so as to open in the direction of the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the lid includes a center adjuster rotation groove; and the adjuster rotation groove is disposed in an inner periphery of a lid through hole of the lid formed inner with respect to the lid bendable groove, and is depressed outwardly in the radial direction with respect to the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the lid engagement part of the lid includes a center adjuster detachment projection; and the center adjuster detachment projection projects from a tip of the lid engagement part in the direction of the rotational axis of worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the lid engagement part of the lid includes a thickness gradual-decrease sections; and the thickness gradual-decrease section is disposed between the lid engagement projection and the lid body, and increases in cross sectional area perpendicular to the rotational axis of the worm shaft, as approaching from the lid body to the lid engagement projection in the direction of the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the center adjuster locking groove includes a first surface structured to be in contact with the lid engagement projection wherein the first surface is perpendicular to the rotational axis of the worm shaft; and the lid engagement projection includes a surface structured to be in contact with the center adjuster locking groove and be inclined with respect to the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the biasing member is the spring having the arc shape; the lid includes the lid body including a spring restriction projection; the spring restriction projection is disposed in a part of the lid body in the circumferential direction around the rotational axis of the worm shaft; the spring restriction projection is formed such that the lid body is a predetermined distance apart in the direction of the rotational axis of the worm shaft, from a surface of the holder tubular wall facing the lid body; and the spring restriction projection projects toward the holder with respect to a part adjacent to the spring restriction projection in the direction of the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes a retainer hook engagement groove; the retainer hook engagement groove includes a surfaces being perpendicular to the rotational axis of the worm shaft and being in contact with the holder retainer hook; the lid includes the lid body and a holder retainer retaining the holder; the holder retainer projects from the lid body toward the holder in the direction of the rotational axis of the worm shaft; the holder retainer includes a holder retainer hook; and the holder retainer hook is disposed in a tip of the holder retainer, and includes a surface inclined with respect to the rotational axis of the worm shaft and structured to be in contact with the retainer hook engagement groove. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes the retainer hook engagement groove; the lid includes the lid body and the holder retainer retaining the holder; the holder retainer projects from the lid body toward the holder in the direction of the rotational axis of the worm shaft; the holder retainer includes the holder retainer hook and a holder retainer slope; the holder retainer hook is disposed in an inward surface of the tip of the holder retainer in the radial direction with respect to the rotational axis of the worm shaft; and the holder retainer slope is disposed in an outward surface of the tip of the holder retainer in the radial direction with respect to the rotational axis of the worm shaft, and is inclined with respect to the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes the retainer engagement groove; the retainer engagement groove is shaped groovy to extend in the direction of the rotational axis of the worm shaft, and is disposed in an outer periphery of the holder tubular wall; the lid includes the lid body and the holder retainer retaining the holder; the holder retainer projects from the lid body toward the holder in the direction of the rotational axis of the worm shaft, and engages with the retainer engagement groove; and the holder retainer in engagement with the retainer engagement groove is less in outer diameter than a part of the holder tubular wall adjacent to the retainer engagement groove. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the center adjuster retainer has the tubular shape defining the center adjuster container space inside the center adjuster retainer, and includes the inner periphery including the center adjuster locking groove shaped annular; the lid is made of a resin material, and includes the lid body including the lid engagement part; the lid engagement part projects from the lid body in the direction away from the holder in the direction of the rotational axis of the worm shaft, and includes a lid engagement projection in the tip of the lid engagement part; and the lid engagement projection projects from the lid engagement part outwardly in the radial direction with respect to the rotational axis of the worm shaft, and engages with the center adjuster locking groove, and is greater in outer diameter in the radial direction than a part of the holder tubular wall adjacent to the retainer engagement groove. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the lid includes a marker that allows a rotational position of the lid around the rotational axis of the worm shaft to be visually inspected from an outside of the gear housing. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the lid is allowed to engage with the holder, at only one rotational position with respect to the holder in the circumferential direction around the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the collar is made of a material less in friction coefficient than a material of the holder and a material of the lid. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the lid is made of a material less in Young&#39;s modulus than a material of the holder and a material of the collar. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the lid includes a collar supporter supporting the outer peripheral surface of the collar; and the collar supporter projects toward the holder in the direction of the rotational axis of the worm shaft, and decreases in thickness in the radial direction with respect to the rotational axis of the worm shaft, as approaching the axis crossing the rotational axis of the worm shaft and extending in the gear engagement direction. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the center adjuster retainer of the gear housing has the tubular shape defining the center adjuster container space inside the center adjuster retainer; the center adjuster container space has a circular shape at a cross section perpendicular to the rotational axis of the worm shaft; the center adjuster has a circular outline at a cross section perpendicular to the rotational axis of the worm shaft; the center adjuster includes an outer peripheral surface that is in contact with an inner peripheral surface of the center adjuster retainer; and the center adjuster and the center adjuster retainer are structured to determine a position of the center adjuster with respect to the center adjuster retainer, due to the contact between the outer peripheral surface of the center adjuster and the inner peripheral surface of the center adjuster retainer. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes the holder bottom, the holder tubular wall, and a holder-positioning projection; the holder-positioning projection projects from the holder tubular wall in a direction away from the collar, in the direction of the rotational axis of the worm shaft; the center adjuster retainer includes a holder-positioning-projection engagement groove; the holder-positioning-projection engagement groove engages with the holder-positioning projection, and thereby restricts relative rotation of the holder with respect to the center adjuster retainer in a rotational direction around the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder-positioning-projection engagement groove is a groove that is formed in an inner periphery of the center adjuster retainer and is open in the direction of the rotational axis of the worm shaft; and the holder-positioning projection has a gap with the holder-positioning-projection engagement groove in the radial direction with respect to the rotational axis of the worm shaft. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that the holder-positioning projection includes in its tip a tapered section inclined to be tapered. 
     According to still another aspect, in addition to any one of the above aspects, the steering device is configured such that: the holder includes the holder bottom and the holder tubular wall; and the center adjuster retainer includes a holder-positioning contact part that is in contact with the holder bottom in the direction of the rotational axis of the worm shaft and thereby performs positioning of the holder in the direction of the rotational axis of the worm shaft with respect to the center adjuster retainer.