Patent Publication Number: US-2023133693-A1

Title: Steering apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a steering apparatus. 
     BACKGROUND ART 
     A steering apparatus is applied to a vehicle for steering. As for a technology regarding such a steering apparatus, Patent Document 1 discloses an electrically-assisted power steering device that includes an electric motor, a rack shaft coupled to a steering mechanism, and a power transmission mechanism that transmits power from the electric motor to the rack shaft. The power transmission mechanism includes a screw shaft which is connected to or integrated with the rack shaft, and which is provided with a male-screw thread, a nut which is placed around the screw shaft and which is provided with a female-screw thread, a plurality of rolling elements capable of rolling within a rolling passage formed between the male-screw thread and the female-screw thread, and a sleeve that includes a receiving portion which receives the power transmitted from the electric motor. The nut is internally fitted to the sleeve in such a way that the receiving portion is located outwardly in the radial direction relative to the rolling passage, and is to rotate together with such a sleeve. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Document 1: JP2006-069517A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The electrically-assisted power steering device disclosed in Patent Document 1 has a possibility such that water enters in a casing in which the ball screw, etc., is stored through, for example, matching surfaces of the respective components, and a belt that links the ball screw with a drive source is soaked with the entering water. When the belt is soaked with water, since it becomes difficult for the belt to move under a low-temperature environment at which freezing occurs, there is a technical disadvantage such that the performance of the steering apparatus is likely to decrease. 
     Hence, an objective of the present disclosure is to provide a steering apparatus that can maintain a performance under a freezing-temperature environment. 
     Solution to Problem 
     Upon keen research and development, the inventors of the present disclosure obtained a technical knowledge such that, by providing a retaining portion that traps water entering in a housing in which a ball screw is stored below the lower end surface of the ball screw, a belt is not likely to be soaked with such water. Moreover, by providing a devisal that makes it difficult for water entering in a housing in which a pinion shaft is stored to move to the housing in which the ball screw is stored, the belt is also not likely to be soaked with such water. It is thought that providing such devisal facilitates a steering apparatus to maintain the performance under a freezing-temperature environment. The present disclosure has been made based on such technical knowledges. The present disclosure will be described below. 
     A steering apparatus according to the present disclosure includes: 
     a turning shaft movable in a vehicle widthwise direction; 
     a pinion shaft meshed with the turning shaft; 
     a first housing that retains therein the turning shaft and the pinion shaft; 
     a ball screw that transmits drive force generated by a drive source to the turning shaft; and 
     a second housing that retains therein the ball screw, 
     in which a retaining portion capable of storing water is provided at an inner circumferential surface of the second housing, 
     in which a lower end surface of the retaining portion is located below at least a lower end surface of the ball screw, and 
     in which a first end portion of the retaining portion in an axial direction of the turning shaft is located between a belt member that links the ball screw with the drive source and a rack end stopper at the drive-source side. 
     Advantageous Effects of Invention 
     According to the present disclosure, a steering apparatus can be provided which can maintain a performance under a freezing-temperature environment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram for describing an electrically-assisted power steering device  100 ; 
         FIG.  2    is a diagram illustrating a retaining portion  57 A and the surrounding thereof according to a first embodiment in an enlarged manner; 
         FIG.  3    is a diagram illustrating a part of the electrically-assisted power steering device  100  in an enlarged manner; 
         FIG.  4    is a diagram illustrating a retaining portion  57 B and the surrounding thereof according to a second embodiment in an enlarged manner; 
         FIG.  5    is a diagram illustrating a retaining portion  57 C and the surrounding thereof according to a third embodiment in an enlarged manner; 
         FIG.  6    is a diagram illustrating a part of an electrically-assisted power steering device  200  in an enlarged manner; 
         FIG.  7    is a diagram illustrating a part of the electrically-assisted power steering device  300  in an enlarged manner; 
         FIG.  8    is a diagram for describing an example form of a bottom portion of a retaining portion  57 ; 
         FIG.  9    is a diagram for describing an example form of a bottom portion of the retaining portion  57 ; and 
         FIG.  10    is a diagram for describing an example form of a bottom portion of the retaining portion  57 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An electrically-assisted power steering device that is an example steering apparatus of the present disclosure will be described below with reference to the figures as needed. Note that the embodiments illustrated in the figures are merely examples of the present disclosure, and the present disclosure is not limited to the illustrated embodiments. 
     1. First Embodiment 
     As illustrated in  FIG.  1   , an electrically-assisted power steering device  100  includes tie-rod ends  130  and  130  coupled to right and left wheels  110  and  110 , respectively, through respective knuckles  120  and  120 , and a rack shaft  55  coupled to the tie-rod ends  130  and  130 . According to the electrically-assisted power steering device  100 , steering force from a steering wheel  11  provided for a vehicle is transmitted to the rack shaft  55 . A rack and pinion mechanism is formed by rack teeth  55  formed on the rack shaft  55  and pinion teeth  24  formed on the pinion shaft  21 . The steering wheel  11  and the pinion shaft  21  are connected through an intermediate shaft  12 , and the steering force from the steering wheel  11  is input to the intermediate shaft  12 . Moreover, an unillustrated torsion bar is provided between the pinion shaft  21  and the intermediate shaft  12 . 
     Moreover, the electrically-assisted power steering device  100  includes a motor  41  and a motor shaft  42  that is one of transmitting means that transmits drive force by the motor  41 . The drive force by the motor  41  is transmitted to the rack shaft  55  as steering assist force through the motor shaft  42 . This assists the movement of the rack shaft  55 . 
     As illustrated in  FIG.  1   , the electrically-assisted power steering device  100  includes a first housing  52  and a second housing  53 , and a retaining portion  57  at an inner circumferential surface  531  of the second housing  53  (in the following explanation, the retaining portion  57  of the first embodiment will be also referred to as a “retaining portion  57 A”). The first housing  52  retains therein the rack shaft  55  that extends toward a side apart from the motor  41 , and the second housing  53  retains therein the ball screw  30  and a transmission belt mechanism  70 . As illustrated in  FIG.  1    and  FIG.  2   , the retaining portion  57 A includes a lower end surface  57 AX provided below a lower end surface  30 L of the ball screw  30  and the lower end surface of the transmission belt mechanism  70  (in the following description, the lower end surface of the retaining portion  57 A may be also referred to as a “bottom portion  57 AX”), and water entering in the second housing  53  can be trapped at the bottom portion  57 AX. 
     The ball screw  30  is a kind of conversion mechanisms that converts rotational motion into linear motion, and transmits the drive force generated by the motor  41 , i.e., assist torque, to the rack shaft  55 . The ball screw  30  includes a ball screw portion  51  formed on the rack shaft  55 , a plurality of balls  37 , and a ball housing  36  coupled to the ball screw portion  51  through the balls  37 . The ball housing  36  is supported by the first housing  52  so as to be freely rotatable through a bearing  32 , and the movement is restricted in such a way that the ball housing  36  does not move in the axial direction of the rack shaft  55  independently from the bearing  32 . The bearing  32  is fitted in an inner circumferential surface  521  of the first housing  52 , and the inner race of the bearing  32  is fastened between a screw  31  and the ball housing  36  with such a race being depressed against the ball housing  36  by the screw  31 . As illustrated in  FIG.  2   , an outer race of the bearing  32  has respective end surfaces in the axial direction of the rack shaft  55  held between elastic bodies  38 , and the elastic bodies  38  are supported by the first housing  52  and by an end surface  53 S with such an outer race being held between the first housing  52  and the end surface  53 S of the second housing  53 . 
     As illustrated in  FIG.  1   , the transmission belt mechanism  70  includes a drive pulley  45  provided at the motor shaft  42 , a driven pulley  35  provided at the ball housing  36 , and a belt member  39  tensioned between the drive pulley  45  and the driven pulley  35 . 
     As illustrated in  FIG.  1    and  FIG.  2   , the electrically-assisted power steering device  100  has the retaining portion  57 A at a portion of the second housing  53  at the side of a matching surface with the first housing  52 . The retaining portion  57 A is placed between a rack end stopper  54 R where a first end portion  57 AR of the rack shat  55  in the axial direction is placed at an end of the rack shaft  55  at the motor- 41  side and the belt member  39 . Moreover, a second end portion  57 AL of the rack shaft  55  in the axial direction which is an end portion at the belt-member- 39  side relative to the first end portion  57 AR is located at the pinion-shaft- 21  side relative to both end surfaces of the belt member  39  in the axial direction of the rack shaft  55 , and is an opening opened in the end surface of the second housing  53  facing with the elastic body  38  (i.e., an end surface  53 S of the second housing  53  at the first-housing- 52  side). By employing such a structure, since the elastic body  38  can be placed between the retaining portion  57 A and the bearing  32 , in comparison with a case in which no elastic body  38  is present, water that enters in the second housing  53  is not likely to contact the bearing  32 . Consequently, the bearing  32  becomes not likely to be rusted out. 
     Moreover, since the second end portion  57 AL is opened in the end surface  53 S of the second housing  53 , in a casting process of the second housing  53 , a die for an inner-dimeter portion can be taken out in the opening direction. Consequently, an advantageous effect such that the easiness of casting improves can also be achieved. 
     The lower end surface of the retaining portion  57 A is the bottom portion  57 AX. The bottom portion  57 AX is provided below a lower end surface  30 L of the ball screw  30 . More specifically, the bottom portion  57 AX is placed below the belt member  39  that passes through the lower end surface of the ball housing  36  of the ball screw  30 . 
     By providing the bottom portion  57 AX below the lower end surface  30 L of the ball screw  30  and the lower surface of the belt member  39 , even if water enters in the second housing  53 , such water can be trapped at the bottom portion  57 AX. This prevents the ball screw  30  and the belt member  39  from being soaked with such water, and thus the electrically-assisted power steering device  100  can maintain the performance under a freezing-temperature environment. 
     In the retaining portion  57 A, regarding the positions of the upper surface of the elastic body  38  placed below the rack shaft  55  and of the bottom portion  57 AX in the vertical direction, either one may be located at the upper side. However, from the standpoint such that the electrically-assisted power steering device  100  can maintain the performance under a freezing-temperature environment by causing water entering in the second housing  53  to be easily trapped, it is preferable that the bottom portion  57 AX should be placed below the upper surface of the elastic body  38  placed below the rack shaft  55 . 
     Conversely, as illustrated in  FIG.  1    and  FIG.  3   , the first housing  52  retains therein the rack shaft  55 . The inner circumferential surface  521  of the first housing  52  and the outer circumferential surface of the rack shaft  55  are not in contact with each other, but a clearance is formed therebetween. Moreover, the electrically-assisted power steering device  100  includes a torque sensor  22  that detects the steering torque applied to the steering wheel  11 . 
     In the electrically-assisted power steering device  100 , regarding the distance between the inner circumferential surface  521  of the first housing  52  and the outer circumferential surface of the rack shaft  55 , it is far at a portion that surrounds a meshed portion G where the rack tooth  55 T formed on the rack shaft  55  and the pinion tooth  24  formed on the pinion shaft  21  are meshed with each other, and the above-described distance becomes close as coming close to, from such a portion, the coupled portion between the first housing  52  and the second housing  53 . The first housing  52  includes an inclined portion  52 IT which is formed at the inner circumferential surface  521  and which becomes thin as coming close to the coupled portion between the first housing  52  and the second housing  53 . The inclined portion  52 IT is provided at a part of the inner circumferential surface  521 , and a start point S of the inclined portion  52 IT is located at the motor- 41  side relative to the meshed portion G where the tooth  55 T of the rack shaft  55  and the tooth  24  of the pinion shaft  21  are meshed with each other. The rack end stopper  54 L is provided at the end portion of the rack shaft  55  at the pinion-shaft- 21  side. 
     As illustrated in  FIG.  3   , the electrically-assisted power steering device  100  further includes, in addition to the inclined portion  52 IT, a groove portion  52 IG which is formed at the inner circumferential surface  521  of the first housing  52  facing with the rack shaft  55  and located below the rack shat  55 , and which has an opened upper end and a lower end as a bottom portion. By providing such a groove portion  52 IG, even if water enters in the first housing  52 , such water can be trapped at the groove portion  52 IG. Furthermore, by providing the inclined portion  52 IT, even if water enters in the first housing  52 , it is difficult for the water trapped below the rack shaft  55  to move toward the second-housing- 53  side. Accordingly, since the amount of water that flows in the second housing  53  from the first housing  52  can be reduced, the ball screw  30  and the belt member  39  can be further prevented from being soaked with such water. That is, by providing the retaining portion  57 A, the inclined portion  52 IT, and the groove portion  52 IG, the ball screw  30  and the belt member  39  can be prevented from being soaked with water. Therefore, according to the first embodiment of the present disclosure, the electrically-assisted power steering device  100  can be provided which can maintain the performance under a freezing-temperature environment. 
     In the above description relating to the first embodiment, although a structure has been described in which the second end portion  57 AL in the axial direction of the rack shaft  55  is located at the pinion-shaft- 21  side relative to both end surfaces of the belt member  39  in the axial direction of the rack shaft  55 , and is an opening which is opened in the end surface  53 S of the second housing  53  facing the elastic body  38 , the first embodiment is not limited to this structure. The second end portion of the retaining portion can be in other forms. The other forms thereof will be described below. 
       FIG.  4    is a diagram for describing a retaining portion  57 B that is in a different form from that of the above-described retaining portion  57 A. The retaining portion  57 B illustrated in  FIG.  4    has a first end  57 BR located at the similar position to that of the above-described first end  57 AR, but the form of a second end portion  57 BL differs from that of the second end portion  57 AL. The second end portion  57 BL of the retaining portion  57 B has a position in the axial direction of the rack shaft  55  that is the same as the position of a side face of, at the rack-end-stopper- 54 R side, a convexity  53 B provided at the end of a bottom portion  57 BX in the retaining portion  57 B at the elastic-body- 38  side. In other words, the second end portion  57 BL is located between the position of a side face  39 S of the belt member  39  at the pinion-shaft- 21  side, and an end surface  53 S of the second housing  53  at the pinion-shaft- 21  side. By employing such a form, it becomes easy to trap water in the retaining portion  57 B by what corresponds to the height of the convexity  53 B. Consequently, since it becomes easy to prevent the belt member  39  from being soaked with water, the electrically-assisted power steering device  100  can maintain the performance under a freezing-temperature environment. 
       FIG.  5    is a diagram for describing a retaining portion  57 C that is in a different form from those of the above-described retaining portions  57 A and  57 B. The retaining portion  57 C illustrated in  FIG.  5    has a first end portion  57 CR located at the similar position to that of the above-described first end portion  57 AR, but the form of a second end portion  57 CL differs from those of above-described second end portions  57 AL and  57 BL. The second end portion  57 CL of the retaining portion  57 C has a position in the axial direction of the rack shaft  55  located at the belt-member- 39  side relative to the position of the second end portion  57 BL. In other words, the second end portion  57 CL is located below the side face  39 S of the belt member  39  at the pinion-shaft- 21  side. That is, the position of the end surface  39 S and the position of the second end portion  57 CL in the axial direction of the rack shaft  55  are consistent with each other. In comparison with the retaining portion  57 B illustrated in  FIG.  4   , the retaining portion  57 C illustrated in  FIG.  5    has a width (a length in the axial direction of the rack shaft  55 ) of a convexity  53 C provided at an end of a bottom portion  57 CX in the retaining portion  57  at the elastic-body- 38  side wider than the width of the above-described concavity  53 B. According to such a form, also, it becomes easy to trap water in the retaining portion  57 C by what corresponds to the height of the convexity  53 C. Consequently, since it becomes easy to prevent the belt member  39  from being soaked with water, the electrically-assisted power steering device  100  can maintain the performance under a freezing-temperature environment. 
     2. Second Embodiment 
       FIG.  6    is a diagram illustrating a part of an electrically-assisted power steering device  200  that is an example steering apparatus according to a second embodiment of the present disclosure. The electrically-assisted power steering device  200  employs the same structure as that of the above-described electrically-assisted power steering device  100  except that the groove portion  52 IG is not provided. In  FIG.  6   , the same component as that of the electrically-assisted power steering device  100  will be denoted by the same reference numeral utilized in  FIG.  3   , and the detailed description thereof will be omitted. 
     Instead of the first housing  52  of the electrically-assisted power steering device  100 , the electrically-assisted power steering device  200  includes a first housing  252 . As illustrated in  FIG.  6   , the first housing  252  does not include a structural component corresponding to the above-described groove portion  52 IG, but includes an inclined portion  252 IT corresponding the inclined portion  52 IT, and a start point S′ of the inclined portion  252 IT is present at the second-housing side relative to the meshed portion G. By providing the inclined portion  252 IT, even if water enters in the first housing  252 , the water trapped below the rack shaft  55  is prevented from moving toward the unillustrated second-housing side. This reduces the amount of water that flows in the second housing from the first housing  252 , and thus the ball screw and the belt member retained in the second housing are suppressed to be soaked with such water. Hence, according to the second embodiment of the present disclosure, the electrically-assisted power steering device  200  can be provided which can maintain the performance under a freezing-temperature environment. 
     3. Third Embodiment 
       FIG.  7    is a diagram illustrating a part of an electrically-assisted power steering device  300  that is an example steering apparatus according to a third embodiment of the present disclosure. The electrically-assisted power steering device  300  employs the same structure as that of the above-described electrically-assisted power steering device  200  except that no inclined portion  252 IT is provided. In  FIG.  7   , the same structural component as that of the electrically-assisted power steering device  200  will be denoted by the same reference numeral in  FIG.  6   , and the detailed description thereof will be omitted. 
     The electrically-assisted power steering device  300  includes, instead of the first housing  252  of the electrically-assisted power steering device  200 , a first housing  352 . As illustrated in  FIG.  7   , the first housing  352  does not employ a structure corresponding to above-described inclined portion  252 IT. Hence, unlike the above-described electrically-assisted power steering devices  100  and  200 , according to the electrically-assisted power steering device  300 , the first housing  352  does not include the inclined portion or the groove portion that makes it difficult for water entering in the first housing  352  to move toward the second-housing side. Although such a form is employed, the second housing (unillustrated) of the electrically-assisted power steering device  300  has a retaining portion that can trap water like the electrically-assisted power steering devices  100  and  200 . Hence, the ball screw and the belt member retained in the second housing are suppressed to be soaked with water. Therefore, according to the third embodiment of the present disclosure, the electrically-assisted power steering device  300  can be provided which can maintain the performance under a freezing-temperature environment. 
     In the above description regarding the present disclosure, a form in which the retaining portions  57 A,  57 B, and  57 C (those may be collectively referred to as the “retaining portion  57 ” below, and the bottom portion of the retaining portion  57  may be referred to as the “bottom portion  57 X”) has been described as examples. A form that the bottom portion  57 X can take will be described below with reference to  FIG.  8    to  FIG.  10   . 
       FIG.  8    is a diagram illustrating a cross section taken along a line VIII-VIII in  FIG.  1   . The bottom portion  57 X of the retaining portion  57  may be in, for example, a form of a bottom portion  57 X 1  illustrated in  FIG.  8   . That is, the bottom  57 X has the width in the vehicle back-and-forth direction smaller than the inner diameter of the driven pulley  35  and the outer diameter of the ball housing  36 , thus being in a convex shape protruding toward the lower side of the second housing  53 . By applying such a form, water can be trapped in the retaining portion  57  that protrudes downwardly. Hence, an electrically-assisted power steering device can be provided which can maintain the performance under a freezing-temperature environment by providing such a retaining portion  57 . 
       FIG.  9    is a diagram illustrating a cross section taken along a line IX-IX in  FIG.  1   . The bottom portion  57 X of the retaining portion  57  may be in, for example, a form of a bottom portion  57 X 2  illustrated in  FIG.  9   . That is, the bottom  57 X can have the width in the vehicle back-and-forth direction substantially the same as the width in the vehicle back-and-forth direction at a portion of the second housing  53  where the driven pulley  35  is retained. Such a form can also trap water in the retaining portion  57 . Hence, an electrically-assisted power steering device can be provided which can maintain the performance under a freezing-temperature environment by providing such a retaining portion  57 . 
       FIG.  10    is a diagram illustrating a cross section taken along a line X-X in  FIG.  1   . The bottom portion  57 X of the retaining portion  57  may be in, for example, a form of a bottom portion  57 X 3  illustrated in  FIG.  10   . That is, the bottom portion  57 X can be formed in a curved surface shape protruding downwardly. Such a form can also trap water in the retaining portion  57 . Hence, an electrically-assisted power steering device can be provided which can maintain the performance under a freezing-temperature environment by providing such a retaining portion  57 . 
     The above-described electrically-assisted power steering devices  100 ,  200  and  300  accomplish the following advantageous effects. In the following description, the first end portions  57 AR,  57 BR, and  57 CR may be collectively referred to as the “first end portion  57 R”, and the second end portions  57 AL,  57 BL, and  57 CL may be collectively referred to as the “second end portion  57 L”. 
     Each of the electrically-assisted power steering devices  100 ,  200  and  300  includes the rack shaft  55  movable in the vehicle widthwise direction, the pinion shaft  21  meshed with the rack shaft  55 , the first housing  52  that retains therein the rack shaft  55  and the pinion shaft  21 , the ball screw  30  that transmits drive force generated by the motor  41  to the rack shaft  55 , and the second housing  53  that retains therein the ball screw  30 . The retaining portion  57  capable of storing water is provided at the inner circumferential surface  531  of the second housing  53 . The lower end surface  57 X of the retaining portion  57  is located below at least the lower end surface  30 L of the ball screw  30 . The first end portion  57 R of the retaining portion  57  in the axial direction of the rack shaft  55  is located between the belt member  39  that links the ball screw  30  with the motor  41  and the rack end stopper  54 R at the motor- 41  side. 
     By providing the retaining portion  57  below the lower end surface  30 L of the ball screw  30 , in other words, below the lower end surface of the belt member  39 , water that enters in the second housing  53  can be trapped in the retaining portion  57 . Hence, in comparison with a case in which there is no retaining portion  57 , this makes it difficult for the water entering in the second housing  53  to contact the belt member  39 . Since the water is not likely to contact the belt member  39 , the belt member  39  is facilitated to actuate even under a freezing low-temperature environment. Accordingly, the electrically-assisted power steering devices  100 ,  200  and  300  can be provided which can maintain the performance under a freezing temperature environment. 
     Moreover, in each of the electrically-assisted power steering devices  100 ,  200  and  300 , the second end portion  57 L of the retaining portion  57  which is located at the belt-member- 39  side relative to the first end portion  57 R in the axial direction of the rack shaft  55  may be located at the pinion-shat- 21  side relative to both side faces of the belt member  39  in the axial direction, and may be an opening opened in the end surface  53 S of the second housing  53  at the first-housing- 52  side. 
     According to such a form, since the retaining portion  57  is provided below the lower end surface of the belt member  39 , water entering in the second housing  53  is not likely to contact the belt member  39 . Accordingly, the electrically-assisted power steering devices  100 ,  200  and  300  can be provided which can maintain the performance under a freezing-temperature environment. 
     Moreover, in each of the electrically-assisted power steering devices  100 ,  200  and  300 , the second end portion  57 L of the retaining portion  57  located at the belt-member- 39  side relative to the first end portion  57 R in the axial direction of the rack shaft  55  may have the same position in the axial direction of the rack shaft  55  as the position of the side face  39 S of the belt member  39  at the pinion-shaft- 21  side. 
     According to such a structure, also, since the retaining portion  57  is provided below the lower end surface of the belt member  39 , the water that enters in the second housing  53  is not likely to contact the belt member  39 . Accordingly, the electrically-assisted power steering devices  100 ,  200  and  300  can be provided which can maintain the performance under a freezing-temperature environment. 
     Furthermore, in each of the electrically-assisted power steering devices  100 ,  200  and  300 , the second end portion  57 L of the retaining portion  57  located at the belt-member- 39  side relative to the first end portion  57 R in the axial direction of the rack shaft  55  may have a position in the axial direction of the rack shaft  55  located between the position of the side face  39 S of the belt member  39  at the pinion-shaft- 21  side and the end surface  53 S of the second housing  53  at the pinion-shaft- 21  side. 
     According to such a structure, also, since the retaining portion  57  is provided below the lower end surface of the belt member  39 , the water that enters in the second housing  53  is not likely to contact the belt member  39 . Accordingly, the electrically-assisted power steering devices  100 ,  200  and  300  can be provided which can maintain the performance under a freezing-temperature environment. 
     Still further, in the electrically-assisted power steering devices  100  and  200 , it is preferable that the inner circumferential surface  521  of the first housing  52  facing with the rack shaft  55  should include the inclined portion  52 IT or  252 IT that decreases the internal diameter toward the motor- 41  side from the pinion-shaft- 21  side, and the start point S or S′ of the inclined portion  52 IT or  252 IT should be located at the motor- 41  side relative to the meshed portion G where the tooth  55 T of the rack shaft  55  and the tooth of the pinion shaft  21  are meshed with each other. 
     According to the electrically-assisted power steering devices  100  and  200  with such a structure, in order to move toward the second-housing- 53  side, it is necessary for water that enters in the first housing  52  or  252  to move beyond the inclined portion  52 IT or  252 IT. Since the water that enters in the first housing  52  or  252  is trapped below the rack shaft  55 , the inclined portion  52 IT or  252 IT serves as uphill for such water. It is difficult for such water to go over the uphill. Since it is difficult for the water that enters in the first housing  52  or  252  to move to the second housing  53  because the first housing  52  or  252  includes the inclined portion  52 IT or  252 IT, the amount of water that enters in the second housing  53  can be reduced. This makes it further difficult for such water to contact the belt member  39 , the belt member  39  is not likely to become a state difficult to actuate under a low-temperature environment at which freezing occurs. Hence, by adopting a structure in which the inclined portion  52 IT or  252 IT is provided, the electrically-assisted power steering devices  100  and  200  can be provided which is further facilitated to maintain the performance under a freezing-temperature environment. 
     Yet still further, in the electrically-assisted power steering device  100  that includes the inclined portion  52 IT, it is preferable that the inner circumferential surface  521  of the first housing  52  which faces with the rack shaft  55  and which is located at least below the rack shaft  55  should include the groove portion  52 IG which is located at the pinion-shaft- 21  side relative to the start point S and which has an opened upper surface. 
     According to the electrically-assisted power steering device  100  in such a structure, water that attempts to go over the inclined portion  52 IT to move can be trapped in the groove portion  52 IG. Since it becomes difficult for the water that enters in the first housing  52  to move to the second housing  53  by providing the groove portion  52 IG, the amount of water that enters in the second housing  53  can be reduced. This further makes it difficult for the water to contact the belt member  39 , and thus the belt member  39  is not likely to become a state difficult to actuate under a low-temperature environment at which freezing occurs. Hence, by adopting a structure provided with the groove portion  52 IG, the electrically-assisted power steering device  100  can be provided which is further facilitated to maintain the performance under a freezing-temperature environment. 
     REFERENCE SIGNS LIST 
     
         
         
           
             G Meshed portion 
             S and S′ Start point 
               11  Steering wheel 
               12  Intermediate shaft 
               21  Pinion shaft 
               22  Torque sensor 
               24  Pinion tooth 
               30  Ball screw 
               30 L Lower end surface of ball screw 
               31  Screw 
               32  Bearing 
               35  Driven pulley 
               36  Ball housing 
               37  Ball 
               38  Elastic body 
               39  Belt member 
               39 S Side face of belt member 
               41  Motor (drive source) 
               42  Motor shaft 
               45  Drive pulley 
               51  Ball screw 
               52 ,  252  and  352  First housing 
               521  Inner circumferential surface of first housing 
               52 IG Inner circumferential surface groove portion of first housing 
               52 IT and  252 IT Inner circumferential surface tapered portion (inclined portion) of first housing 
               53  Second housing 
               53 B and  53 C Convexity 
               531  Inner circumferential surface of second housing 
               53 S End surface of second housing 
               54 L Rack end stopper 
               54 R Rack end stopper (rack end stopper at drive-source side) 
               55  Rack shaft (turning shaft) 
               55 T Rack tooth (tooth of turning shaft) 
               57 ,  57 A,  57 B and  57 C Retaining portion 
               57 AR,  57 BR,  57 CR and  57 R First end portion of retaining portion 
               57 AL,  57 BL,  57 CL and  57 L Second end portion of retaining portion 
               57 X,  57 X 1 ,  57 X 2  and  57 X 3  Bottom portion (lower end surface of retaining portion) 
               60  Transmission mechanism 
               70  Transmission belt mechanism 
               100 ,  200  and  300  Electrically-assisted power steering device (steering apparatus) 
               110  Wheel 
               120  Knuckle 
               130  Tie rod end