Patent Publication Number: US-10787195-B2

Title: Electric power steering system

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2013-108373 and 2013-108374 and 2013-108375 filed on May 22, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric power steering system. 
     2. Description of Related Art 
     International Publication No. 2011/147824 and Japanese Patent Application Publication No. 2006-224945 (JP 2006-224945 A) describe an electric power steering system (hereinafter referred to as “EPS”) for assisting a steering operation of a driver by giving a power of a motor to a steering mechanism of a vehicle. For example, in the EPS, a rack-and-pinion mechanism is adopted as the steering mechanism. The mechanism converts a rotation of a pinion along with the steering operation into an axial movement of a rack shaft meshing with the pinion. 
     SUMMARY OF THE INVENTION 
     For example, as illustrated in  FIG. 7 , the EPS includes a housing  130 , a rack shaft  121 , a ball screw nut  145 , balls  144 , and a bearing  150 . A threaded portion  121 A is formed on an outer circumference of the rack shaft  121 , and the ball screw nut  145  is engaged threadedly to the threaded portion  121 A via a plurality of balls  144 . The bearing  150  is provided between an inner peripheral surface of the housing  130  and an outer peripheral surface of the ball screw nut  145 . The bearing  150  is sandwiched between both sides of the housing  130  in an axial direction ZA. A double row angular contact ball bearing, for example, is adopted as the bearing  150  (e.g., see Published Japanese Translation of PCT application No. 2006-509979 (JP-A 2006-509979)). 
     In the above configuration, when a steering is operated to rotate, a force in the axial direction ZA is added to the rack shaft  121  via a pinion. Hereby, the rack shaft  121  slightly moves in the axial direction ZA. This movement is not accompanied with a rotation of the ball screw nut  145 . Along with the movement of the rack shaft  121 , the bearing  150  (particularly, its outer ring portion  151 ) receives a reaction force Pa from the housing  130 . In view of this, it is necessary for the outer ring portion  151  to be formed thick in consideration of the reaction force Pa. Accordingly, it is difficult to achieve compactification of a size of the EPS in a radial direction ZB. 
     Further, for example, as illustrated in  FIG. 8 , an EPS includes a housing  130 , a rack shaft  121 , a ball screw nut  145 , balls  144 , a bearing  150 , elastic springs  162 , and plates  163 . 
     A threaded portion is formed on an outer circumference of the rack shaft  121 , and the ball screw nut  145  is engaged threadedly to the threaded portion via a plurality of balls  144 . The bearing  150  is provided between an inner peripheral surface of the housing  130  and an outer peripheral surface of the ball screw nut  145 . The elastic spring  162  and the plate  163  each having a tonic shape are provided in a gap between the bearing  150  and the housing  130  in an axial direction ZA of the rack shaft  121 . 
     The elastic spring  162  is a metal waved washer, coned disc spring, or the like, for example. The plate  163  is provided between the housing  130  and the elastic spring  162 , in order to prevent the elastic spring  162  from making contact with the housing  130 . 
     The elastic spring  162  applies its own elastic force to the plate  163  and the bearing  150  (more precisely, a side surface of its outer ring  150   a ). The bearing  150  is retained to be elastically displaceable along the axial direction ZA of the rack shaft  121  due to biasing forces from the elastic springs  162 . 
     In the above configuration, when a steering is operated to rotate, a force in the axial direction ZA is added to the rack shaft  121  via a pinion. Hereby, the rack shaft  121  slightly moves in the axial direction ZA. This movement is not accompanied with a rotation of the ball screw nut  145 . Along with the movement of the rack shaft  121 , the bearing  150  moves integrally with the ball screw nut  145  in the axial direction ZA against the biasing force from the elastic spring  162 . In this state, when a rotational force is added to the ball screw nut  145  via a motor (not shown), a rotation of the ball screw nut  145  is started smoothly. Hereby, the rack shaft  121  starts to move in the axial direction ZA smoothly. 
     A vibration from a road surface may be added to the EPS via tires. In this case, in the configuration, the elastic spring  162  may fall mainly in a gravitational direction along with the vibration. Hereby, as illustrated by an alternate long and two short dashes line in  FIG. 8 , the elastic spring  162  is displaced with respect to the plate  163 , so that an end portion of the elastic spring  162  interferes the housing  130 , which may abrade the housing  130 . Further, due to the displacement of the elastic spring  162 , it may be difficult to elastically retain the bearing  150  appropriately. 
     Further, for example, as illustrated in  FIG. 9 , an EPS described in Japanese Patent No. 4807655 includes a housing  130 , a steered shaft  121 , a ball screw nut  145 , balls  144 , and a bearing  150 . A threaded portion is formed on an outer circumference of the steered shaft  121 , and the ball screw nut  145  is engaged threadedly to the threaded portion via a plurality of balls  144 . The bearing  150  is provided between an inner peripheral surface of the housing  130  and an outer peripheral surface of the ball screw nut  145 . An elastic spring  162  is provided in a gap between the bearing  150  and the housing  130  in an axial direction of the steered shaft  121 . The bearing  150  is sandwiched between the elastic members  162  in an elastically displaceable manner. A pulley is supported at both ends thereof and a load due to tension of a timing belt is not applied to the bearing  150 . 
     In the above configuration, when a steering is operated to rotate, a force in an axial direction ZA is added to the steered shaft  121  via a pinion. Hereby, the steered shaft  121  slightly moves in the axial direction ZA. This movement is not accompanied with a rotation of the ball screw nut  145 . Along with the movement of the steered shaft  121 , the ball screw nut  145  and the bearing  150  move integrally in the axial direction. Along with the movement of the bearing  150 , the elastic member  162  is elastically deformed. In this state, when a rotational force is added to the ball screw nut  145  via a motor (not shown), a rotation of the ball screw nut  145  is started smoothly. Hereby, the steered shaft  121  starts to move in the axial direction ZA smoothly. 
     However, particularly, in a case where a double row angular contact ball bearing is adopted as the bearing  150 , a contact area between the bearing  150  and the inner peripheral surface of the housing  130  becomes large. Accordingly, due to a frictional resistance between the bearing  150  and the inner peripheral surface of the housing  130 , a smooth movement of the bearing  150  may be disturbed. Because of this, there is a possibility that a smooth movement start of the steered shaft  121  is not realized sufficiently. A similar problem can occur in a single row bearing. 
     The present invention provides an electric power steering system configured to have a more compact size in a radial direction. 
     Further, the present invention provides an electric power steering system in which displacement of an elastic spring to a gravitational direction is restrained. 
     Further, the present invention provides an electric power steering system in which a frictional resistance of a bearing with respect to a housing is reduced. 
     An electric power steering system according to an aspect of the present invention includes: a steered shaft configured to move in an axial direction along with a rotation of a steering; a housing containing the steered shaft therein so that the steered shaft is movable in the axial direction; a ball screw nut engaged threadedly to the steered shaft via a plurality of balls and configured to move the steered shaft in the axial direction when the ball screw nut is rotationally driven via a drive source; a double row angular contact ball bearing including a torte outer ring portion having an outer peripheral surface making contact with an inner peripheral surface of the housing, a toric inner ring portion placed inside the outer ring portion and retaining the ball screw nut thereinside, and balls configured to roll between the outer ring portion and the inner ring portion and arranged in several lines along the axial direction, the double row angular contact ball bearing configured to support the ball screw nut rotatably relative to the housing; a pulley fixed to the screw nut beside the double row angular contact ball bearing, the pulley being driven by the drive source; a wall portion placed at either side of the double row angular contact ball bearing in the axial direction of the steered shaft, the wall portion being formed integrally with the housing; and an elastic member provided between the wall portion and the outer ring portion and configured to retain the double row angular contact ball bearing to be elastically displaceable in the axial direction. 
     According to the above aspect, when the steered shaft slightly moves in the axial direction along with a rotation of a steering, the ball screw nut and the double row angular contact ball bearing integrally move relative to the steered shaft against an elastic force of the elastic member. When the ball screw nut and the bearing are moved as such, it is possible to easily rotate the ball screw nut relative to the steered shaft via a drive source afterward, and eventually to easily move the steered shaft in the axial direction. 
     Further, according to the above aspect, since the elastic member is provided between the double row angular contact ball bearing and the housing (the wall portion), it is possible to reduce a reaction force that the outer ring portion receives from the housing (the wall portion) at the time when the bearing moves. This makes it possible to form the outer ring portion thinly, and eventually to configure the electric power steering system in a further compact manner in terms of a size in the radial direction. 
     In the above aspect a ball retaining hole configured to retain the balls in a rollable manner may be formed on an inner peripheral surface of the outer ring portion, and a raceway bottom thicknesses from a vertex of the ball retaining hole in the outer ring portion to an outer surface of the outer ring portion may be set to 25% to 30% of a ball diameter. 
     According to this configuration, the raceway bottom thickness in the outer ring portion of the double row angular contact ball bearing is set to 25% to 30% of the ball diameter. Thus, even if the raceway bottom thickness is set thinner than a conventionally general outer ring portion, it is possible to reduce the reaction force that the outer ring portion in the bearing receives from the housing (the wall portion), as described above. This makes it possible to maintain durability, and to configure the double row angular contact ball bearing in a further compact manner in terms of a size in the radial direction. 
     In the above aspect, the electric power steering system may further include a plate including a retaining portion configured to support the elastic member from a direction opposed to a gravitational direction, the plate being provided between the elastic member and the wall portion. 
     According to the configuration, the retaining portion of the plate supports the elastic member from the direction opposed to the gravitational direction. Accordingly, even in a case where a vibration is added to the electric power steering system along with running of a vehicle, for example, it is possible to prevent the elastic member from being displaced to the gravitational direction. 
     In the above configuration, the elastic member may be formed in a tonic shape, and the retaining portion may be formed at an inner circumferential side of the elastic member over a whole circumference of the elastic member. 
     According to this configuration, the retaining portion is formed at the inner circumferential side of the elastic member over the whole circumference of the elastic member. Accordingly, regardless of a positional relationship in the circumferential direction between the plate and the elastic member after assembly, part of the retaining portion supports the elastic member from the direction opposed to the gravitational direction. Accordingly, it is possible to improve degrees of freedom of assembly at the time when a component in which the elastic member is fitted into the plate is assembled between the bearing and the wall portion. 
     In the above configuration, the elastic member may be a coned disc spring configured to press a whole circumference of a side surface of the outer ring portion of the double row angular contact ball bearing and a whole circumference of the plate in a direction to be separated from each other. 
     The double row angular contact ball bearing can be configured such that the outer ring portion is made thinner than that in a single row bearing or the like. This is because, generally in the double row angular contact ball bearing, the number of balls between the outer ring portion and the inner ring portion is large, so that a load to be added to the outer ring portion can be distributed via the balls. According to the above configuration, when a coned disc spring is adopted as the elastic member, it is possible to press the side surface of the outer ring portion of the bearing by the coned disc spring over a whole circumference thereof. Accordingly, even in a case of the double row angular contact ball bearing including the thin outer ring portion, it is possible to stably restrain the bearing elastically. 
     In the above configuration, the electric power steering system may further include a recessed portion faulted on a surface where the outer peripheral surface of the bearing makes contact with the inner peripheral surface of the housing. 
     According to the above configuration, the recessed portion is formed on a contact surface between the bearing and the housing. The recessed portion reduces a contact area between the bearing and the housing. In view of this, as described above, it is possible to reduce a sliding friction of the bearing with respect to the housing at the time when the ball screw nut and the bearing move integrally. 
     In the above configuration, the recessed portion may be formed on the outer peripheral surface of the double row angular contact ball bearing. Alternatively, the recessed portion may be formed on the inner peripheral surface of the housing. 
     In the above configuration, the recessed portion may be filled with lubricant. According to this configuration, since the recessed portion is filled with the lubricant, it is possible to further reduce the sliding friction of the bearing with respect to the housing. 
     According to the present invention, it is possible to configure an electric power steering system in a further compact manner in terms of a size in a radial direction. 
     According to the present invention, it is further possible to restrain displacement of an elastic spring to a gravitational direction. 
     According to the present invention, it is further possible to reduce a frictional resistance of a bearing with respect to a housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic diagram illustrating an electric power steering system according to one embodiment of the present invention; 
         FIG. 2  is a local sectional view illustrating the electric power steering system according to one embodiment of the present invention; 
         FIG. 3  is a local sectional view illustrating the electric power steering system according to one embodiment of the present invention; 
         FIG. 4  is a local sectional view illustrating the electric power steering system when a bearing moves, according to one embodiment of the present invention; 
         FIG. 5  is an exploded perspective view of a plate and a metal spring in one embodiment of the present invention; 
         FIG. 6  is a magnified view of  FIG. 2 ; 
         FIG. 7  is a sectional view of an electric power steering system according to a background art; 
         FIG. 8  is a sectional view of an electric power steering system according to a background art; and 
         FIG. 9  is a sectional view of an electric power steering system according to a background art. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     One embodiment of an electric power steering system of the present invention is described below with reference to  FIGS. 1 to 4 . As illustrated in  FIG. 1 , an electric power steering system  10  includes a rack shaft  21 , a housing  30 , and a pinion gear  13 A. 
     The rack shaft  21  is formed into a cylindrical shape. Here, a right-and-left direction in  FIG. 1  is prescribed as an axial direction ZA of the rack shaft  21 , and a direction orthogonal to the axial direction ZA is prescribed as a radial direction ZB of the rack shaft  21 . 
     The housing  30  is formed of aluminum and has a through hole  30   a  penetrating therethrough in the axial direction ZA (the right-and-left direction in the figure) of the rack shaft  21 . The rack shaft  21  is inserted into the through hole  30   a  of the housing  30  in a movable manner in its axial direction ZA. On an outer circumference of the rack shaft  21  on a right side from its center in the figure, a rack gear  21 B is formed over a given range. 
     The pinion gear  13 A is placed so as to engage with the rack gear  21 B in the rack shaft  21 . The pinion gear  13 A rotates via a column shaft  11 , an intermediate shaft  12 , and a pinion shaft  13  according to a rotating operation of the steering  2 . When the pinion gear  13 A rotates, the rack shaft  21  is able to move in the axial direction. 
     Respective ball joints  22  are connected to right and left end faces of the rack shaft  21 , and tie rods  23  are further connected to the respective ball joints  22 . Further, knuckles  4  are connected to tips of the respective tie rods  23 . When the rack shaft  21  moves in the axial direction ZA, a force is transmitted to steered wheels  3  via the tie rods  23  and the knuckles  4 , so as to change a steered angle of the steered wheels  3 . 
     The housing  30  includes a first housing portion  31  and a second housing portion  32 . The housing portions  31 ,  32  are configured to be connectable and disconnectable with each other in the axial direction ZA. The first housing portion  31  is constituted by a body portion  31   a  having a tubular shape following an outer shape of the rack shaft  21 , and a left end portion  31   b  having a tubular shape with a diameter larger than that of the body portion  31   a . That is, the first housing portion  31  is formed into a stepped cylindrical shape in which the body portion  31   a  and the left end portion  31   b  are connected to each other. The second housing portion  32  is fitted into the first housing portion  31  from a left side in the figure. A right end portion  32   a  of the second housing portion  32  is formed into a cylindrical shape having the same diameter same as the left end portion  31   b  of the first housing portion  31 . Further, a left portion  32   b  of the second housing portion  32  is formed into a cylindrical shape having the same diameter same as the body portion  31   a  of the first housing portion  31 . The second housing portion  32  is fitted into the left end portion  31   b  of the first housing portion  31  via the right end portion  32   a  thereof. 
     A motor  41  as a drive source is provided below the body portion  31   a  of the first housing portion  31 . An output shaft  41   a  of the motor  41  extends toward the left side in the figure and is inserted into the right end portion  32   a  of the second housing portion  32 . 
     As illustrated in  FIG. 2 , a drive pulley  46 , a driven pulley  47 , a timing belt  33 , and a ball screw mechanism  43  are received in an internal space formed by the right end portion  32   a  of the second housing portion  32  and the left end portion  31   b  of the first housing portion  31 . The drive pulley  46  is fixed to the output shaft  41   a  of the motor  41 . Accordingly, the drive pulley  46  rotates integrally with the output shaft  41   a.    
     As illustrated in  FIG. 1 , a threaded portion  21 A is formed on an outer circumference of the rack shaft  21  in a given range from its left end. The ball screw mechanism  43  is provided on an outer circumference of the threaded portion  21 A. More specifically, as illustrated in  FIG. 2 , the ball screw mechanism  43  includes a ball screw nut  45 , and many balls  44 . The ball screw nut  45  is engaged threadedly to the rack shaft  21  via the many balls  44  arranged along the threaded portion  21 A. The ball screw nut  45  is placed between the first housing portion  31  and the second housing portion  32  in the housing  30 . 
     The driven pulley  47  is engaged threadedly to an outer circumference of a first-housing-portion- 31  side of the ball screw nut  45 . This allows the ball screw nut  45  and the driven pulley  47  to rotate integrally. 
     The timing belt  33  is provided over the driven pulley  47  and the drive pulley  46 . Accordingly, a rotation of the output shaft  41   a  in the motor  41  is transmitted from the drive pulley  46  to the driven pulley  47  via the timing belt  33 , and eventually transmitted to the ball screw nut  45 . 
     Further, a double row angular contact ball bearing  50  is provided between an outer peripheral surface of the ball screw nut  45  and an inner peripheral surface of the second housing portion  32 . The double row angular contact ball bearing  50  is longer in the axial direction ZA and thinner in the radial direction ZB than those of a single row bearing or the like. 
     More specifically, as illustrated in  FIG. 3 , the double row angular contact ball bearing  50  includes an outer ring portion  51 , an inner ring portion  52 , and balls  53 . The inner ring portion  52  is formed generally in a tonic shape. The ball screw nut  45  is fitted inside the inner ring portion  52 . This accordingly allows the double row angular contact ball bearing  50  to support the ball screw nut  45  relatively with high rigidity even in a cantilever manner, thereby making it possible to restrain an inclination of the ball screw nut  45  caused due to a tension of the timing belt  33 . 
     The outer ring portion  51  is also formed generally in a toric shape. An outer peripheral surface of the outer ring portion  51  makes contact with the inner peripheral surface of the second housing portion  32 . Ball retaining holes  51   a  along a circumferential direction of the outer ring portion  51  are formed on an inner peripheral surface of the outer ring portion  51 . 
     The ball retaining hole  51   a  has a curved surface following a peripheral surface of the ball  53 . Two ball retaining holes  51   a  are formed along the axial direction ZA. A thickness from a vertex of the ball retaining hole  51   a  to a top surface of the outer ring portion  51  is prescribed as a raceway bottom thickness T. The raceway bottom thickness T is set to 25% to 30% of a ball diameter Bd. The raceway bottom thickness T is thinned as much as possible while intensity of the outer ring portion  51  is taken into consideration. The balls  53  are filled in each of the ball retaining holes  51   a . In this state, the balls  53  are rollable according to a relative rotation between the outer ring portion  51  and the inner ring portion  52 . 
     As illustrated in  FIG. 3 , a flange  45   a  projecting in the radial direction ZB is formed on that outer peripheral edge portion of the ball screw nut  45  which is on a side opposite to the driven pulley  47 . The double row angular contact ball bearing  50  (more precisely, the inner ring portion  52 ) is placed between the flange  45   a  and the driven pulley  47 , on the outer peripheral surface of the ball screw nut  45 . 
     A wall portion  31   c  is placed in a tip of the first housing portion  31  on an outer-ring-portion- 51  side thereof. Further, a wall portion  32   c  is placed in that part of the second housing portion  32  which is spaced from the outer ring portion  51  in the axial direction ZA. A plate  61  and a metal spring  62  are disposed in a gap between the outer ring portion  51  and each of the wall portions  31   e ,  32   c  in the axial direction ZA. 
     The metal spring  62  is a metal coned disc spring having a toric shape. Further, the plate  61  is provided for the purpose of preventing the metal spring  62  from making contact with the housing  30 , and is formed of iron in a toric shape so as to have an L-shaped cross section. That is, the plate  61  is constituted by a retaining portion  61   a  extending in a short direction and an abrasion prevention portion  61   b  extending in a longitudinal direction. An outside diameter of the retaining portion  61   a  is set in accordance with an inside diameter of the metal spring  62 . Accordingly, the metal spring  62  is fitted into the plate  61  (the retaining portion  61   a ). In this state, the retaining portion  61   a  is placed inside the metal spring  62  over a whole circumference thereof. 
     As illustrated in  FIG. 3 , the plate  61  is provided on an outer peripheral side of the ball screw nut  45  so as to be placed between the outer ring portion  51  and each of the wall portions  31   c ,  32   c  in a state where the plate  61  retains the metal spring  62  thereinside. That is, the retaining portion  61   a  of the plate  61  is placed at an inner side of the metal spring  62  (a rotation center side of the rack shaft  21 ) and the abrasion prevention portion  61   b  of the plate  61  is placed at that side of the metal spring  62  which faces the each of the wall portions  31   c ,  32   c.    
     Further, a ball-screw-nut- 45  side of the metal spring  62  presses the abrasion prevention portion  61   b  by making contact with the abrasion prevention portion  61   b , and that side of the metal spring  62  which is opposite to the ball screw nut  45  presses the double row angular contact ball bearing  50  by making contact with a side peripheral surface of the outer ring portion  51 . Thus, the double row angular contact ball bearing  50  is retained at that position due to elastic forces from the metal springs  62  on both sides. 
     The following describes an operation of the electric power steering system  10 . As illustrated in  FIG. 1 , when the steering  2  is operated, a force in the axial direction ZA is added to the rack shaft  21  via the pinion gear  13 A, the rack gear  21 B, and the like. Hereby, as illustrated in  FIG. 4 , the rack shaft  21  slightly moves to a left direction. 
     Along with this, the double row angular contact ball bearing  50  (the outer ring portion  51 ) compresses one of the metal springs  62  (the one on the left side in the figure), and stretches the other one of the metal springs  62 , so that the double row angular contact ball bearing  50  moves in the left direction integrally with the ball screw nut  45 . 
     The movements of the double row angular contact ball bearing  50  and the ball screw nut  45  cause the ball screw nut  45  to rotate smoothly afterward along with driving of the motor (the drive source)  41 . The same operation is performed when the rack shaft  21  slightly moves in a right direction. 
     As such, when the metal spring  62  is placed on either side of the double row angular contact ball bearing  50  in an elastically deformable manner, it is possible to reduce a reaction force Pa received from the housing  30  (the wall portions  31   e ,  32   c ) along with the movement of the double row angular contact ball bearing  50 . Accordingly, it is possible to restrain a decrease of durability even if the outer ring portion  51  is thinned as described above. Note that the reaction force Pa is determined according to a rate of spring of the metal spring  62 . 
     Further, even in a case where a vibration is added to the electric power steering system  10  via the steered wheels  3 , displacement of the metal spring  62  to a gravitational direction is regulated by the retaining portion  61   a  of the plate  61 . 
     As illustrated in  FIG. 3 , the bearing  50  includes the outer ring portion  51 , the inner ring portion  52 , and the balls  53 . The inner ring portion  52  is formed generally in a tonic shape. The ball screw nut  45  is fitted inside the inner ring portion  52 . 
     The outer ring portion  51  is also formed generally in a tonic shape. The outer peripheral surface of the outer ring portion  51  makes contact with the inner peripheral surface of the second housing portion  32 . A plurality of balls  53  is placed between the outer ring portion  51  and the inner ring portion  52  in a rollable manner along a circumferential direction of the bearing  50 . In the bearing  50  according to the present embodiment, the plurality of balls  53  arranged along the circumferential direction of the bearing  50  are provided in two lines along the axial direction ZA. Thus, since the balls are provided in two lines along the axial direction ZA, the bearing  50  has a shape elongated in the axial direction ZA. The inner ring portion  52  rotates integrally with the ball screw nut  45 . At this time, the inner ring portion  52  rotates the balls  53  between the inner ring portion  52  and the outer ring portion  51 . 
     The flange  45   a  projecting in the radial direction ZB is formed on that outer peripheral edge portion of the ball screw nut  45  which is on a side opposite to the driven pulley  47 . The bearing  50  (more precisely, the inner ring portion  52 ) is placed between the flange  45   a  and the driven pulley  47 , on the outer peripheral surface of the ball screw nut  45 . 
     As illustrated in  FIG. 2 , the wall portion  31   c  is placed in the tip of the first housing portion  31  on the outer-ring-portion- 51  side. Further, the wall portion  32   c  is placed in that part of the second housing portion  32  which is spaced from the outer ring portion  51  in the axial direction ZA. The plate  61  and the metal spring  62  are disposed in the gap between the outer ring portion  51  and each of the wall portions  31   c ,  32   e  in the axial direction ZA. 
     As illustrated in  FIG. 5 , the metal spring  62  is a metal coned disc spring having a tonic shape. Further, the plate  61  is formed of iron in a tonic shape so as to have an L-shaped cross section. That is, the plate  61  is constituted by the retaining portion  61   a  as a short side and the abrasion prevention portion  61   b  as a longitudinal side. The outside diameter of the retaining portion  61   a  is set in accordance with the inside diameter of the metal spring  62 . Accordingly, the metal spring  62  is fitted into the plate  61  (the retaining portion  61   a ). In this state, the retaining portion  61   a  is placed inside the metal spring  62  over a whole circumference thereof. 
     As illustrated in  FIG. 3 , the plate  61  is provided on the outer peripheral side of the ball screw nut  45  so as to be placed between the outer ring portion  51  and each of the wall portions  31   c ,  32   c  in a state where the plate  61  retains the metal spring  62  thereinside. That is, the retaining portion  61   a  of the plate  61  is placed at the inner side of the metal spring  62  (the rotation center side of the rack shaft  21 ) and the abrasion prevention portion  61   b  of the plate  61  is placed at that side of the metal spring  62  which faces the each of the wall portions  31   e ,  32   c.    
     Further, the ball-screw-nut- 45  side (a lower side in the figure) of the metal spring  62  presses the abrasion prevention portion  61   b  by making contact with the abrasion prevention portion  61   b , and that side (an upper side in the figure) of the metal spring  62  which is opposite to the ball screw nut  45  presses the bearing  50  by making contact with the side peripheral surface of the outer ring portion  51 . The bearing  50  is retained at that position by elastic forces from the metal springs  62  on both sides. 
     The following describes the operation of the electric power steering system  10 . As illustrated in  FIG. 1 , when the steering  2  is operated, a force in the axial direction ZA is added to the rack shaft  21  via the pinion gear  13 A, the rack gear  21 B, and the like. Hereby, the rack shaft  21  slightly moves to a direction according to an operation direction of the steering  2 . This movement of the rack shaft  21  is not accompanied with a rotation of the ball screw nut  45 . Accordingly, the bearing  50  compresses one of the metal springs  62 , and moves integrally with the ball screw nut  45 . Such slight movements of the bearing  50  and the ball screw nut  45  cause the ball screw nut  45  to rotate smoothly afterward along with driving of the motor (the drive source)  41 , and eventually cause the rack shaft  21  to smoothly move in the axial direction. 
     Further, even in a case where a vibration is added to the electric power steering system  10  via the steered wheels  3 , displacement of the metal spring  62  to the gravitational direction is regulated by the retaining portion  61   a  of the plate  61 . 
     As illustrated in  FIG. 6 , the bearing  50  includes the outer ring portion  51 , the inner ring portion  52 , and the balls  53 . The inner ring portion  52  is formed generally in a toric shape. The ball screw nut  45  is fitted inside the inner ring portion  52 . The outer ring portion  51  is also formed generally in a tonic shape. The outer peripheral surface of the outer ring portion  51  makes contact with the inner peripheral surface of the second housing portion  32 . The plurality of balls  53  is placed between the outer ring portion  51  and the inner ring portion  52  in a rollable manner. In the bearing  50  according to the present embodiment, the plurality of balls  53  arranged along the circumferential direction of the bearing  50  are provided in two lines along the axial direction ZA. The inner ring portion  52  rotates integrally with the ball screw nut  45 . At this time, the inner ring portion  52  rotates relative to the outer ring portion  51 , while rotating the balls  53 . 
     A recessed portion  34  is formed on that inner peripheral surface of the second housing portion  32  which makes contact with the bearing  50 . The recessed portion  34  is formed in a toric shape. The recessed portion  34  is filled with grease  55  serving as lubricant. The grease  55  seeps out to those contact surfaces  56  which are formed on both sides in the recessed portion  34  in the axial direction ZA and on which the bearing  50  makes contact with the second housing portion  32 . Due to the recessed portion  34  and the grease  55 , a sliding friction of the bearing  50  with respect to the inner peripheral surface of the second housing portion  32  is reduced. 
     As illustrated in  FIG. 6 , the flange  45   a  projecting in the radial direction ZB of the ball screw nut  45  is formed on that outer peripheral edge portion of the ball screw nut  45  which is on a side opposite to the driven pulley  47 . The bearing  50  (more precisely, the inner ring portion  52 ) is placed between the flange  45   a  and the driven pulley  47 , on the outer peripheral surface of the ball screw nut  45 . 
     The wall portion  31   c  is formed in the tip of the first housing portion  31  in the outer-ring-portion- 51  side. Further, the wall portion  32   c  is formed in that part of the second housing portion  32  which is spaced from the outer ring portion  51  in the axial direction ZA. The plate  61  and the metal spring  62  are disposed in the gap between the outer ring portion  51  and each of the wall portions  31   e ,  32   c  in the axial direction ZA. 
     The metal spring  62  is a torte coned disc spring or a waved washer, and provided at a position where the metal spring  62  make contact with a side surface of the outer ring portion  51 . The metal spring  62  is one example of an elastic retaining portion. The plate  61  is formed of iron in a tone shape so as to have an L-shaped cross section. 
     The plate  61  is placed between the metal spring  62  and each of the wall portions  31   c ,  32   c  in a state where the plate  61  retains the metal spring  62  thereinside. The outer ring portion  51  is retained at that position by elastic forces from the metal springs  62  on both sides. 
     The following describes the operation of the electric power steering system  10 . As illustrated in  FIG. 1 , when the steering  2  is operated, a force in the axial direction ZA is added to the rack shaft  21  via the pinion gear  13 A, the rack gear  21 S, and the like. Hereby, the rack shaft  21  slightly moves to a direction according to an operation direction of the steering  2 . Accordingly, the bearing  50  compresses the metal spring  62 , and moves integrally with the ball screw nut  45  in the axial direction ZA. At this time, due to the recessed portion  34  and the grease  55  filled therein, a sliding friction of the bearing  50  with respect to the inner peripheral surface of the second housing portion  32  is reduced. This allows the bearing  50  and the ball screw nut  45  to move smoothly. When the bearing  50  and the ball screw nut  45  move slightly as such, the ball screw nut  45  smoothly rotates afterward along with driving of the motor (the drive source)  41 . 
     According to the above embodiment, it is possible to yield the following effects. When the rack shaft  21  slightly moves in the axial direction ZA along with a rotation of the steering  2 , the ball screw nut  45  and the double row angular contact ball bearing  50  integrally move relative to the rack shaft  21  against the elastic force of the metal spring  62 . When the double row angular contact ball bearing  50  and the ball screw nut  45  are moved as such, the ball screw nut  45  rotates smoothly afterward via the motor  41  relative to the rack shaft  21 , and eventually the rack shaft  21  moves smoothly in the axial direction ZA. 
     Further, when the metal spring  62  is provided between the double row angular contact ball bearing  50  and the housing  30  (the wall portions  31   c ,  32   c ), it is possible to reduce a reaction force Pa that the outer ring portion  51  receives from the housing  30  (the wall portions  31   c ,  32   c ) at the time when the double row angular contact ball bearing  50  moves. This makes it possible to form the outer ring portion  51  thinly, and eventually to configure the electric power steering system  10  in a compact manner in terms of a size in the radial direction ZB. 
     The raceway bottom thickness T in the outer ring portion  51  of the double row angular contact ball bearing  50  is set to 25% to 30% of the ball diameter Bd. Thus, even if the raceway bottom thickness T is set thinner than a general outer ring portion  51  in a conventional technique, it is possible to reduce the reaction force Pa that the outer ring portion  51  in the double row angular contact ball bearing  50  receives from the housing  30  (the wall portions  31   c ,  32   c ), as described above. This makes it possible to maintain durability, and to configure the double row angular contact ball bearing  50  in a compact manner in terms of a size in the radial direction ZB. 
     When the rate of spring of the metal spring  62  is adjusted, it is possible to adjust the reaction force Pa that the outer ring portion  51  receives from the housing  30  via the metal spring  62 . More specifically, as the rate of spring is smaller, the reaction force Pa that the outer ring portion  51  receives becomes smaller. 
     The retaining portion  61   a  of the plate  61  supports the metal spring  62  from a direction opposed to the gravitational direction. This makes it possible to prevent the metal spring  62  from being displaced to the gravitational direction. In view of this, it is possible to further surely support the bearing  50  via the elastic forces of the metal springs  62  on both sides. This stabilizes the operation of the electric power steering system  10 , and eventually operationability of the steering  2 . 
     Further, it is possible to restrain the housing  30  made of aluminum from being abraded due to the metal spring  62  making contact with the housing  30 . Further, it is also possible to restrain abnormal noise caused by the abrasion. As illustrated in  FIG. 5 , the retaining portion  61   a  is formed on an inner peripheral side of the metal spring  62  throughout a circumferential direction ZC of the metal spring  62 . Accordingly, regardless of a positional relationship in the circumferential direction ZC between the plate  61  and the metal spring  62  after assembly, part of the retaining portion  61   a  supports the metal spring  62  from the direction opposed to the gravitational direction. Accordingly, it is possible to improve degrees of freedom of assembly in the circumferential direction ZC at the time when a component in which the metal spring  62  is fitted into the plate  61  is assembled between the bearing  50  and each of the wall portions  31   c ,  32   e.    
     The double row angular contact ball bearing adopted as the bearing  50  can be configured such that the outer ring portion  51  is made thinner than that in a single row bearing or the like. This is because the number of balls  53  between the outer ring portion  51  and the inner ring portion  52  is large, so that a load to be added to the outer ring portion  51  can be distributed via the balls  53 . According to the above configuration, when a coned disc spring is adopted as the metal spring  62 , it is possible to press the side surface of the outer ring portion  51  of the bearing  50  by the coned disc spring over a whole circumference thereof. Accordingly, even in a case of the double row angular contact ball bearing including the thin outer ring portion  51 , it is possible to stably restrain the bearing  50  elastically. 
     The recessed portion  34  is formed on a contact surface between the bearing  50  and the housing  30 . The recessed portion  34  reduces a contact area between the bearing  50  and the housing  30 . In view of this, as described above, it is possible to reduce a sliding friction of the bearing  50  with respect to the housing  30  at the time when the ball screw nut  45  and the bearing  50  move integrally. 
     When the recessed portion  34  is filled with the grease  55 , it is possible to further reduce the sliding friction of the bearing  50  with respect to the housing  30 . The double row angular contact ball bearing  50  is longer in the axial direction ZA than that of a single row bearing or the like. Accordingly, in the double row angular contact ball bearing  50 , the contact area between the bearing  50  and the housing  30 , eventually, the sliding friction become large naturally. However, according to the above configuration, it is possible to reduce the sliding friction of the bearing  50  with respect to the housing  30 . As a result, even with the configuration that adopts the double row angular contact ball bearing, it is possible to smoothly move the double row angular contact ball bearing  50  in the axial direction ZA. 
     Since the recessed portion  34  is formed in the housing  30  made of aluminum, it is possible to easily form the recessed portion  34 . 
     Note that the above embodiment is performable in the following embodiments in which the above embodiment is modified appropriately. In the above embodiment, the housing  30  is constituted by the first housing portion  31  and the second housing portion  32 , but the housing  30  may be formed integrally. 
     In the above embodiment, the metal spring  62  is a coned disc spring, but may be a waved washer. Alternatively, the metal spring  62  may be an elastic member made of resin, such as rubber. Even in this case, displacement of the elastic member is restrained by the retaining portion  61   a . In the above embodiment, the raceway bottom thickness T is set to 25% to 30% of the ball diameter Bd, but may be out of this range. 
     In the above embodiment, the housing  30  is formed of aluminum, but may be formed of iron. 
     In the above embodiment, the retaining portion  61   a  of the plate  61  is formed over a whole circumference of the abrasion prevention portion  61   b , but the retaining portion  61   a  may be partially formed in the abrasion prevention portion  61   b . In this case, the retaining portion  61   a  is formed in the plate  61  at least in that direction of the metal spring  62  which is opposed to the gravitational direction. Further, a plurality of retaining portions may be fondled at given angle intervals. According these configurations, it is possible to form the plate  61  from fewer materials. 
     In the above embodiment, the sectional shape of the plate  61  is an L shape, but the shape thereof is modifiable appropriately. For example, the retaining portion  61   a  may be provided at an outer peripheral side of the metal spring  62 , not at the inner circumferential side thereof. Even in this case, the retaining portion is placed in the gravitational direction with respect to the metal spring  62  on a lower side in  FIG. 2 . As a result, it is possible to prevent a fall of the metal spring  62 . Further, the retaining portion  61   a  may be formed at both of the inner circumferential side and the outer peripheral side of the metal spring  62 . 
     In the above embodiment, the bearing  50  is a double row angular contact ball bearing, but the bearing  50  is not limited to this and may be a bearing of other types. 
     In the above embodiment, one recessed portion  34  is provided, but a plurality of recessed portions  34  may be provided along the axial direction ZA. In the above embodiment, the recessed portion  34  is filled with the grease  55 , but the grease  55  may not to be filled therein. Even in this case, since the recessed portion  34  is formed in a recessed shape, it is possible to reduce the contact area between the bearing  50  and the second housing portion  32 , and eventually to reduce the sliding friction of the bearing  50  with respect to the second housing portion  32 . 
     In the above embodiment, the recessed portion  34  is formed in the housing  30 , but may be formed in an outer peripheral surface of the bearing  50 . Even in this case, it is possible to obtain an effect similar to the above embodiment. The following describes technical ideas that can be understood from the above embodiments, as well as effects. 
     The electric power steering system is characterized in that the elastic member is a waved washer. The electric power steering system is characterized in that the housing is made of aluminum, the plate is made of iron, and the elastic member is made of metal. 
     The electric power steering system is characterized in that the housing is made of aluminum, and the recessed portion is formed in the housing that makes plane contact with the bearing. 
     The electric power steering system is characterized in that the lubricant is grease. The electric power steering system is characterized in that the elastic retaining portion includes: a wall portion that is part of the housing and placed at either side of the bearing in the axial direction of the steered shaft so as to be distanced from the bearing; and an elastic body provided between the wall portion and the bearing.