Patent Publication Number: US-2019176610-A1

Title: In-wheel motor drive device for steered wheel

Description:
TECHNICAL FIELD 
     The present invention relates to in-wheel motor drive devices that are mounted in internal space regions of wheels to be steered and drive the wheels, and more particularly relates to motor casings. 
     BACKGROUND ART 
     For example, an in-wheel motor described in Japanese Unexamined Patent Publication No. 2014-76775 (Patent Literature 1) is conventionally known as an in-wheel motor that is mounted in a wheel to drive the wheel. The in-wheel motor described in Patent Literature 1 includes a hub unit bearing disposed on one side in the axial direction, a reduction gear mechanism disposed in the middle in the axial direction, and an electric motor disposed on the other side in the axial direction. 
     The electric motor has a cylindrical housing, a stator accommodated in the housing, a rotor placed radially inside the stator, and a disc-shaped cover coupled to an end of the housing. The housing has smaller outside and inside diameter dimensions on the one side in the axial direction and larger outside and inside diameter dimensions on the other side in the axial direction. The cover closes the end of the housing which is located on the other side in the axial direction. The outside diameter dimension of the cover is therefore the same as that of the end of the housing which is located on the other side in the axial direction. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2014-76775 
     SUMMARY OF INVENTION 
     Technical Problem 
     The positional relationship between a vehicle body of an electrically powered vehicle and an in-wheel motor will be described below.  FIGS. 9 and 10  are schematic sections of a conventional in-wheel motor  200  as viewed in the vertical direction. An outline of the in-wheel motor  200  will be provided. The in-wheel motor  200  includes a hub unit bearing  201  disposed on the outer side in the lateral direction of a vehicle and an electric motor  202  disposed on the inner side in the lateral direction of the vehicle. The electric motor  202  has a larger diameter than the hub unit bearing  201  and includes a rotor  203 , a stator  204 , a cylindrical housing  205 , and a disc-shaped cover  206 . An end of the housing  205  which is located on the inner side in the lateral direction of the vehicle is closed by the cover  206 . An abutting surface  205   f  of the housing  205  and an abutting surface  206   f  of the cover  206  are located at an end of the in-wheel motor  200  which is located on the inner side in the lateral direction of the vehicle. The abutting surfaces  205   f ,  206   f  are also located on the inner side in the lateral direction of the vehicle with respect to the rotor  203  and the stator  204 . 
     The cover  206  has the largest outside diameter dimension in the in-wheel motor  200 . The reason why the outside diameter of an outer edge  208  of the cover  206  is larger than that of the stator  204  is considered to be as follows. An opening at the end of the housing  205  which is located on the inner side in the lateral direction of the vehicle is made slightly larger than or about the same size as the stator  204  so that, when assembling the in-wheel motor  200 , the stator  204  is inserted into the housing  205  through the opening at the end of the housing  205  which is located on the inner side in the lateral direction of the vehicle and is attached and fixed to an inner peripheral surface of the housing  205 . 
     The in-wheel motor  200  together with a wheel shown by a long dashed double-dotted line is accommodated in a wheel well  210  of the vehicle body. The wheel well  210  is formed on both sides of the vehicle body in the lateral direction of the vehicle. When the electrically powered vehicle moves straight, the cover  206  of the in-wheel motor  200  faces a wall material  211  of the wheel well  210  with predetermined clearance Cl therebetween, as shown in  FIG. 9 . 
     There are cases where such a conventional in-wheel motor is mounted in a steered wheel that is steered in the right-left direction of an electrically powered vehicle. A tire, not shown, is fitted on a road wheel W of the steered wheel. For driving stability, the closer a steering axis K is to a wheel center Wc of the steered wheel, the better. However, the inventors found that the following problems occur in the case where the steering axis K is located at a position on the outer side in the lateral direction of the vehicle with respect to the wheel center Wc of the steered wheel and close to the wheel center Wc and the in-wheel motor  200  is turned about the steering axis K as shown in  FIG. 9 . 
     That is, referring to  FIG. 10 , when the in-wheel motor  200  is steered, the larger a steering angle is, the closer the outer edge  208  of the cover  20  gets to the wall material  211  of the wheel well  210 . The steering angle therefore cannot be increased in order to avoid interference between the in-wheel motor  200  and the wall material  211 . 
     In order to reduce manufacturing cost of electrically powered vehicles, it is desired to apply common vehicle bodies equipped with an internal combustion engine to the electrically powered vehicles. The wheel well  210  of a common vehicle body is not large enough to accommodate the in-wheel motor  200  and also absorb displacement of the in-wheel motor  200 . The wall material  211  of the common vehicle body is therefore located close to the in-wheel motor  200 , resulting in the abovementioned problems of the interference and the steering angle. 
     In view of the above circumstances, it is an object of the present invention to provide an in-wheel motor drive device for a steered wheel which neither sacrifices the steering angle nor requires an increase in size of a wheel well. 
     Solution to Problem 
     In order to achieve the above object, an in-wheel motor drive device for a steered wheel according to the present invention includes: a wheel hub bearing unit that rotatably supports a wheel hub coupled to the steered wheel; and a motor unit that drives the wheel hub. The motor unit has a tubular first casing serving as a part of an outer shell of the motor unit which is located on one side in an axial direction, a tubular second casing serving as a part of the outer shell of the motor unit which is located on the other side in the axial direction and having a smaller outside diameter at its end located on the other side in the axial direction than at its end located on the one side in the axial direction, abutting surfaces formed on an end of the first casing which is located on the other side in the axial direction and the end of the second casing which is located on the one side in the axial direction and abutting on each other, and a stator extending in the axial direction and having its one end placed in the first casing and the other end placed in the second casing. 
     According to the present invention, the one end of the stator extending in the axial direction is accommodated in the first casing, the other end of the stator is accommodated in the second casing, and the abutting surfaces are disposed in a middle region of the stator in the axial direction. When assembling the motor unit, the stator can therefore be inserted into the first casing through an opening of the first casing and the opening of the first casing can be covered by the second casing. Moreover, the outside diameter of the end of the second casing which is located on the other side in the axial direction can be made smaller than that of the end of the second casing which is located on the one side in the axial direction. This can reduce interference between the end of the second casing which is located on the other side in the axial direction and a wall material of a wheel well even when the in-wheel motor drive device is steered, and can increase a maximum steering angle of the in-wheel motor drive device. 
     The first and second casings abut on each other to form a motor casing. It is therefore preferable that the first and second casings have the same wall thickness and be made of the same material. The stator is fixed to the first casing, whereby the first casing supports the stator. The stator may be supported only by the first casing or may be supported by the first and second casings. In one embodiment, the second casing rotatably supports a rotor of the motor unit. Specifically, a motor rotary shaft is placed in the center of the rotor, the second casing supports via a rolling bearing an end of the motor rotary shaft which is located on the other side in the axial direction, and the first casing supports via a rolling bearing an end of the motor rotary shaft which is located on the one side in the axial direction. Alternatively, a cylindrical portion may be formed in a central portion of the second casing substantially perpendicularly to the bottom of the second casing, the motor rotary shaft may be passed through the cylindrical portion, and a plurality of rolling bearings may be disposed in annular clearance between the cylindrical portion and the motor rotary shaft. The plurality of rolling bearings are arranged at intervals in the axial direction, whereby the rotor (motor rotary shaft) is supported only by the second casing. In another embodiment, the rotor (motor rotary shaft) is supported only by the first casing. 
     In one embodiment of the present invention, the stator includes a stator core and a stator coil wound around the stator core, and an end of the stator core which is located on the one side in the axial direction is placed in the first casing, and an end of the stator core which is located on the other side in the axial direction is placed in the second casing. Alternatively, in another embodiment of the present invention, both ends of the stator core in the axial direction and an end of the stator coil which is located on the one side in the axial direction are placed in the first casing, and an end of the stator coil which is located on the other side in the axial direction is placed in the second casing. 
     In a preferred embodiment of the present invention, an outer peripheral surface of the stator is fitted in an inner peripheral surface of the first casing. According to such an embodiment, the stator can be positioned coaxially with the motor unit. Specifically, the inner peripheral surface of the first casing has a constant radius along its length in the axial direction and is fitted on an outer peripheral surface of a region of the stator core which is located on the one side in the axial direction. More preferably, an inner peripheral surface of a region of the second casing which is located on the one side in the axial direction has a constant radius along its length in the axial direction and is fitted on an outer peripheral surface of a region of the stator core which is located on the other side in the axial direction. In another embodiment, an inner peripheral surface of the second casing may be separated from an outer peripheral surface of the stator core. 
     In a more preferred embodiment of the present invention, a stepped portion that faces toward the other side in the axial direction and contacts the stator is formed inside the first casing. According to such an embodiment, the stator can be positioned at a predetermined axial position. It is preferable that the stepped portion be formed integrally with the inner periphery of the first casing. Alternatively, the stepped portion is a separate member that is attached to the inside of the first casing or the motor unit. 
     In one embodiment of the present invention, an end of the first casing which is located on the one side in the axial direction rotatably supports the rotor unit via a bearing. Specifically, it is preferable that the center of the rotor be coupled to the motor rotary shaft and the motor rotary shaft be supported by the first and second casings via rolling bearings. In another embodiment, the rotor is rotatably supported only by the second casing. In a reference example, the rotor is rotatably supported only by the first casing. 
     In one embodiment of the present invention, the wheel hub is disposed so as to extend parallel to an axis of the rotor, and the in-wheel motor drive device further includes: a parallel-shaft gear reducer mechanism that has an input gear coupled to the rotor and an output gear coupled to the wheel hub and that reduces a speed of input rotation from the rotor to output the resultant rotation to the wheel hub. Such an embodiment is advantageous in that interference between the motor unit and the wall material of the wheel well can be avoided in the case where the motor unit is offset in a direction perpendicular to an axis of the wheel hub bearing and the motor unit thus gets closer to the wall material when the in-wheel motor drive device is steered. In another embodiment, the wheel hub and the rotor may be coaxially arranged. 
     Advantageous Effects of Invention 
     As described above, according to the present invention, the maximum steering angle of the in-wheel motor drive device can be increased as compared to conventional examples, and the steering angle of the in-wheel motor drive device and the steered wheel is not sacrificed even when the wheel well accommodating the in-wheel motor drive device and the steered wheel is formed with the same dimensions as a wheel well of a vehicle equipped with an engine. That is, a sufficient maximum steering angle can be provided without making the wheel well accommodating the in-wheel motor drive device and the steered wheel wider than a wheel well of a vehicle equipped with an engine. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic longitudinal section of an in-wheel motor drive device of the present invention. 
         FIG. 2  is a schematic longitudinal section of the in-wheel motor drive device of the present invention. 
         FIG. 3  is a schematic longitudinal section of the in-wheel motor drive device of the present invention which is being assembled. 
         FIG. 4  is a schematic longitudinal section of an in-wheel motor drive device according to a modification of the present invention. 
         FIG. 5  is a schematic longitudinal section of an in-wheel motor drive device according to another modification of the present invention. 
         FIG. 6  is a view showing an in-wheel motor drive device according to a specific embodiment of the present invention as viewed from the outer side in the lateral direction of a vehicle. 
         FIG. 7  is a transverse section of the in-wheel motor drive device of the embodiment. 
         FIG. 8  is a developed section of the in-wheel motor drive device of the embodiment. 
         FIG. 9  is a schematic longitudinal section of a conventional in-wheel motor drive device. 
         FIG. 10  is a schematic longitudinal section of the conventional in-wheel motor drive device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.  FIG. 1  is a schematic longitudinal section showing an in-wheel motor drive device of the present invention at the time a vehicle is moving straight without being steered.  FIG. 2  is a schematic longitudinal section showing the in-wheel motor drive device of the present invention at the time the vehicle is turning at a maximum steering angle.  FIG. 3  is a schematic longitudinal section showing the in-wheel motor drive device of the present invention which is being assembled.  FIGS. 1 to 3  show the in-wheel motor drive device as viewed in the vertical direction. An in-wheel motor drive device  110  includes a wheel hub bearing unit  111 , a motor unit  121 , and a reduction gear unit  131  and is mounted in a wheel well  140  of an electrically powered vehicle. 
     The electrically powered vehicle includes, e.g., wheel wells and wheels in the right front, left front, right rear, and left rear parts of a vehicle body. A pair of right and left front wheels and/or a pair of right and left rear wheels are steered wheels. These wheel wells may have the same dimensions as wheel wells of vehicle bodies equipped with an engine. 
     An axis O of the wheel hub bearing unit  111  corresponds to an axle extending in the lateral direction of the electrically powered vehicle. The wheel hub bearing unit  111  is disposed on one side in the direction of the axis O of the in-wheel motor drive device  110 . The motor unit  121  is disposed on the other side in the direction of the axis O of the in-wheel motor drive device  110 . The reduction gear unit  131  is disposed between the wheel hub bearing unit  111  and the motor unit  121 . The one side in the direction of the axis O is the outer side in the lateral direction of the vehicle (outboard side), and the other side in the direction of the axis O is the inner side in the lateral direction of the vehicle (inboard side). In the following description, the one side in the direction of the axis O and the other side in the direction of the axis O are simply referred to as the one side in the axial direction and the other side in the axial direction. 
     The wheel hub bearing unit  111  includes a wheel hub  112  located on the inner side in the radial direction, a rolling bearing  114 , and a fixing member  115  located on the outer side in the radial direction. The wheel hub  112  extends along the axis O and is passed through a through hole formed in the fixing member  115 . The rolling bearing  114  is placed in annular clearance between the wheel hub  112  and the fixing member  115 . An end of the wheel hub  112  which is located on the one side in the direction of the axis O protrudes to the outside of the in-wheel motor drive device  110  and is coupled to a road wheel W of a steered wheel. The fixing member  115  is connected to an end of a body casing  43  which is located on the one side in the direction of the axis O. The body casing  43  serves as an outer shell of the reduction gear unit  131 . The fixing member  115  serves as an end face of the reduction gear unit  131  which is located on the one side in the direction of the axis O. 
     The axis O extends parallel to the lateral direction of the electrically powered vehicle when the vehicle is moving straight. At this time, the steered wheel (road wheel W) and the in-wheel motor drive device  110  are not steered, as shown in  FIG. 1 . On the other hand, the steered wheel (road wheel W) and the in-wheel motor drive device  110  are steered about a steering axis K when the electrically powered vehicle is turning, as shown in  FIG. 2 . At this time, the axis O extends obliquely with respect to the lateral direction of the vehicle. 
     The motor unit  121  has a rotor  123 , a stator  124 , a first casing  125 , and a second casing  126 . The rotor  123  extends along a motor axis M (also simply referred to as the axis M) and a motor rotary shaft  122  is placed in the center of the rotor  123 . The motor rotary shaft  122  protrudes in the direction of the axis M beyond both end faces of the rotor  123 . The motor axis M may be aligned with the axis O corresponding to the axle. In this case, the motor unit  121  is disposed coaxially with the wheel hub bearing unit  111 . Alternatively, as in another embodiment described below, the motor axis M may extend parallel to the axis O at an interval therebetween. In this case, the motor unit  121  is offset from the wheel hub bearing unit  111  in a direction perpendicular to the axis O. 
     An end of the motor rotary shaft  122  which is located on one side in the direction of the axis M is rotatably supported by a partition wall  125   w  of the first casing  125  via a bearing  129   a . An end of the motor rotary shaft  122  which is located on the other side in the direction of the axis M is rotatably supported by the second casing  126  via a bearing  129   b . That is, the rotor  123  is rotatably supported at its both ends by the first casing  125  and the second casing  126 . Alternatively, in a modification not shown, the rotor  123  may be rotatably supported at its one end by only the first casing  125  or may be rotatably supported at its one end by only the second casing  126  while its other end is not supported by any members. 
     The stator  124  is disposed radially outside the rotor  123  and faces an outer peripheral surface of the rotor  123  with clearance therebetween which opens in the radial direction. The stator  124  includes a stator core  124   b  and a stator coil  124   c . The stator core  124   b  has protrusions and recesses which are repeatedly formed in the circumferential direction, and the stator coil  124   c  is wound around each protrusion. An end of the stator core  124   b  contacts a stepped portion  125   g  formed inside the first casing  125 . The stepped portion  125   g  faces toward the other side in the direction of the axis M, and positions the stator  124  in the direction of the axis M by contacting the stator core  124   b.    
     The first casing  125  and the second casing  126  are two members into which an outer shell of the motor unit  121  is divided and which are located on the one side and the other side in the axial direction. The first casing  125  and the second casing  126  accommodate the stator  124  and the rotor  123 . The first casing  125  is disposed on the one side in the direction of the axis M, and the second casing  126  is disposed on the other side in the direction of the axis M. 
     The first casing  125  has two cylindrical shapes, each cylindrical shape has a constant inside diameter along its length in the direction of the axis M. An end of the first casing  125  which is located on the one side in the direction of the axis M is connected to the body casing  43  of the reduction gear unit  131 . An end of the first casing  125  which is located on the other side in the direction of the axis M has an opening Op 1  as shown in  FIG. 3 . The opening Op 1  is covered by the second casing  126 . The end of the first casing  125  which is located on the other side in the direction of the axis M and an end of the second casing  126  which is located on the one side in the direction of the axis M are abutting surfaces  125   f ,  126   f  abutting on each other. The abutting surfaces  125   f ,  126   f  are strip-like flat surfaces extending along the entire outer peripheries of the first and second casings  125 ,  126 . Alternatively, in a modification, not shown, the abutting surfaces  125   f ,  126   f  may be formed so as to be fitted together like spigot and socket joint. The abutting surfaces  125   f ,  126   f  are flat surfaces perpendicular to the axis M. The abutting surface  125   f  of the first casing  125  defines the opening Op 1  at the end of the first casing  125  which is located on the other side in the direction of the axis M. As shown in  FIG. 3 , the abutting surface  126   f  of the second casing  126  defines an opening Op 2  at the end of the second casing  125  which is located on the one side in the direction of the axis M. 
     The body casing  43 , the first casing  125 , and the second casing  126  are made of the same material. Unlike mere covers made of a resin, these casings have sufficient rigidity. The body casing  43 , the first casing  125 , and the second casing  126  are therefore made of a metal. In order to achieve reduction in weight of the in-wheel motor drive device  110 , the body casing  43 , the first casing  125 , and the second casing  126  are made of a light metal mainly comprised of aluminum. The radial wall thickness of a cylindrical part of the first casing  125  is the same as that of a cylindrical part of the second casing  126 . The circular abutting surfaces  125   f ,  126   f  may have the same radial dimension or different radial dimensions. 
     The first casing  125  has, at its end located on the other side in the direction of the axis M, a plurality of protruding portions  125   p  protruding radially outward. The second casing  126  has, at its end located on the one side in the direction of the axis M, a plurality of protruding portions  126   p  protruding radially outward. Each protruding portion  125   p  has an internally threaded hole extending in the direction of the axis M, and each protruding portion  126   p  has a through hole extending in the direction of the axis M. 
     The plurality of protruding portions  125   p  are formed at intervals in the circumferential direction of the first casing  125  so as to adjoin the abutting surface  125   f . The protruding portions  126   p  are formed similarly and contact the protruding portions  125   p . A bolt  127  is passed through the through hole of each protruding portion  126   p  from the other side in the direction of the axis M, and an externally threaded part of the bolt  127  is screwed into the internally threaded hole of the protruding portion  126   p . The first casing  125  and the second casing  126  are thus connected and fixed to each other. 
     The stator  124  extends in the direction of the axis M. One end of the stator  124  is placed in the first casing  125 , and the other end of the stator  124  protrudes beyond the abutting surface  125   f  of the first casing  125 . With the opening Op 1  of the first casing  125  being open and the stator  124  being inserted and fixed in the first casing  125 , the other end of the stator  124  protrudes through the opening Op 1  of the first casing  125 . The positions of the abutting surfaces  125   f ,  126   f  in the direction of the axis M correspond to a middle part of the stator  124  in the direction of the axis M as shown in  FIG. 1 . 
     The end of the second casing  126  which is located on the one side in the direction of the axis M has a cylindrical shape, and the second casing  126  has a smaller diameter in its end located on the other side in the direction of the axis M than in its end located on the one side in the direction of the axis M. The second casing  126  therefore has a smaller outside diameter in its end located on the other side in the direction of the axis M than in its end located on the one side in the direction of the axis M. The second casing  126  accommodates the other end of the rotor  123  and the other end of the stator  124 . Specifically, the second casing  126  accommodates an end of the stator core  124   b  which is located on the other side in the direction of the axis M and a coil end in an end of the stator coil  124   c  which is located on the other side in the direction of the axis M. 
     The outer periphery of the end of the second casing  126  which is located on the other side in the direction of the axis M has a bowl shape and does not have a cylindrical surface. A radial outer edge  128  of the end of the second casing  126  which is located on the other side in the direction of the axis M is therefore not angular but is rounded as shown in  FIG. 1 . Alternatively, in a modification not shown, the radial outer edge  128  may be chamfered like a conical surface. 
     The first casing  125  is a cylindrical wall and accommodates the end of the stator core  124   b  which is located on the one side in the direction of the axis M and a coil end in an end of the stator coil  124   c  which is located on the one side in the direction of the axis M. An inner peripheral wall surface  125   n  of the first casing  125  is fitted on an outer peripheral surface of the stator core  124   b . The stator  124  is thus positioned coaxially with the axis M. An end of the inner peripheral wall surface  125   n  which is located on the one side in the direction of the axis M is connected to the stepped portion  125   g , and an end of the inner peripheral wall surface  125   n  which is located on the other side in the direction of the axis M is connected to the abutting surface  125   f.    
     The partition wall  125   w  is formed on the one side in the direction of the axis M with respect to the stepped portion  125   g . The partition wall  125   w  is, e.g., a flat circular plate and defines the internal space of the motor unit  121  and the internal space of the reduction gear unit  131 . The partition wall  125   w  has a hole through which the motor rotary shaft  122  extends. One end of the motor rotary shaft  122  extends to the inside of the reduction gear unit  131 , so that the motor rotary shaft  122  applies motor rotation to the reduction gear unit  131 . The bearing  129   a  is interposed between the partition wall  125   w  of the first casing  125  and the motor rotary shaft  122 . 
     An inner peripheral wall surface  126   n  of the second casing  126  which is formed on the one side in the direction of the axis M has a constant inside diameter along its length in the direction of the axis M. The inner peripheral wall surface  126   n  is fitted on the outer peripheral surface of the stator core  124   b . According to the embodiment shown in  FIG. 1 , a region of the stator core  124   b  which is located on the one side in the direction of the axis M is fitted in the first casing  125 , and a region of the stator core  124   b  which is located on the other side in the direction of the axis M is fitted in the second casing  126 , so that the first casing  125  and the first casing  126  support the stator  124 . 
     As shown in a modification of  FIG. 4 , clearance S may be provided between the inner peripheral wall surface  126   n  and the outer peripheral surface of the stator core  124   b  so that only the first casing  125  supports the stator  124 . In this case, the region of the stator core  124   b  which is located on the one side in the direction of the axis M is fitted in the first casing  125 . 
     Alternatively, in another modification shown in  FIG. 5 , a cylindrical wall  125   c  may be extended from the abutting surface  125   f  of the first casing  125  so that an outer peripheral wall surface of the stator core  124   b  is fitted in an inner peripheral wall surface of the cylindrical wall  125   c . In this case, the stator core  124   b  is fitted, from its end located on the one side in the direction of the axis M to its end located on the other side in the direction of the axis M, in the first casing  125 . An outer peripheral surface of the cylindrical wall  125   c  is fitted in the inner peripheral wall surface  126   n  of the second casing  126 . That is, in this modification, the end of the first casing  125  which is located on the other side in the direction of the axis M and the end of the second casing  126  which is located on the one side in the direction of the axis M are fitted together like spigot and socket joint. The second casing  126  is thus positioned coaxially with the axis M. 
     According to the embodiment shown in  FIG. 1 , the motor unit  121  has: the cylindrical first casing  125  serving as a part of an outer shell of the motor unit  121  which is located on the one side in the direction of the axis M; the second casing  126  serving as a part of the outer shell of the motor unit  121  which is located on the other side in the direction of the axis M and having a smaller outside diameter at the radial outer edge  128  of its end located on the other side in the direction of the axis M than at its end located on the one side in the direction of the axis M; the abutting surfaces  125   f ,  126   f  formed on the end of the first casing  125  which is located on the other side in the direction of the axis M and the end of the second casing  126  which is located on the one side in the direction of the axis M and abutting on each other; and the stator  124  extending in the direction of the axis M and having its one end placed in the first casing  125  and the other end placed in the second casing  126 . A maximum steering angle of al can therefore be obtained as shown in  FIG. 2  in the case where there is clearance Cl between the second casing  126  and a wall material  141  of the wheel well at a steering angle of 0 as shown in  FIG. 1 . 
     The effects of the present embodiment will be described in comparison with the conventional example shown in  FIG. 9 . The in-wheel motor  200  of the conventional example shown in  FIG. 9  has: the cylindrical housing  205  serving as a part of an outer shell of the motor unit  202  which is located on the one side in the direction of the axis O; the cover  206  that is a circular plate serving as a part of the outer shell of the motor unit  202  which is located on the other side in the direction of the axis O, having a thickness in the direction of the axis O, and having a constant outside diameter; the abutting surfaces  205   f ,  206   f  formed on an end of the housing  205  which is located on the other side in the direction of the axis O and an end face of the cover  206  which is located on the one side in the direction of the axis O and abutting on each other; and the stator  204  extending in the direction of the axis O and entirely placed in the housing  205 , and having the other end facing the end face of the cover  206  which is located on the one side in the direction of the axis O with predetermined clearance therebetween. The embodiment shown in  FIG. 1  and the conventional example shown in  FIG. 9  have the same axial dimension and the same outside diameter dimension. According to such a conventional example, the outer edge  208  of the cover  206  interferes with the wall material  211  at a steering angle of al as shown in  FIG. 10  in the case where there is clearance Cl between the cover  206  and the wall member  211  of the wheel well at a steering angle of 0 as shown in  FIG. 9 . That is, a maximum steering angle in the conventional example is smaller than the maximum steering angle of al in the embodiment shown in  FIG. 2 . 
     According to the present embodiment, as shown in  FIG. 1 , the stator  124  includes the stator core  124   b  and the stator coil  124   c  wound around the stator core  124   b , and the end of the stator core  124   b  which is located on the one side in the direction of the axis M is placed in the first casing  125 , and the end of the stator core  124   b  which is located on the other side in the direction of the axis M is placed in the second casing  126 . According to the present embodiment, an outer peripheral surface of the stator  124  is fitted in the inner peripheral wall surface  125   n  of the first casing  125 . The stator  124  can thus be positioned coaxially with the axis M. 
     According to the present embodiment, the stepped portion  125   g  facing toward the end of the first casing  125  which is located on the other side in the direction of the axis M and contacting the stator  124  is formed inside the first casing  125 . The stator  124  can thus be positioned at a predetermined position in the direction of the axis M. 
     According to the present embodiment, the first casing  125  has the circular flat partition wall  125   w  at its end located on the one side in the axial direction, and the partition wall  125   w  rotatably supports the rotor  123  via the bearing  129   a.    
     Next, another embodiment of the present invention will be described.  FIG. 6  is a schematic view of an in-wheel motor drive device according to another embodiment of the present invention.  FIG. 7  is a transverse section schematically showing the in-wheel motor drive device of the another embodiment.  FIGS. 6 and 7  show the in-wheel motor drive device as viewed from the outer side in the lateral direction of a vehicle. In  FIG. 7 , each gear in a reduction gear unit is shown by an addendum circle and individual teeth are not shown.  FIG. 8  is a developed section schematically showing the in-wheel motor drive device of the another embodiment. The cutting plane shown in  FIG. 8  is a developed plane connecting a plane including an axis M and an axis Nf shown in  FIG. 7 , a plane including the axis Nf and an axis Nl, and a plane including the axis Nl and an axis O in this order. 
     An in-wheel motor drive device  10  includes a wheel hub bearing unit  11 , a motor unit  21 , and a reduction gear unit  31  that reduces the speed of rotation of the motor unit  21  to transmit the resultant rotation to the wheel hub bearing unit  11 . The in-wheel motor drive device  10  is symmetrically disposed on the right and left sides in the lateral direction of an electrically powered vehicle (not shown). As shown in  FIG. 8 , the wheel hub bearing unit  11  is disposed on the outer side in the lateral direction of the vehicle, and the motor unit  21  is disposed on the inner side in the lateral direction of the vehicle. 
     The in-wheel motor drive device  10  is disposed in an internal space region of a road wheel W shown in phantom in  FIG. 6 , is connected to the center of the road wheel W shown in phantom in  FIG. 8 , and drives the road wheel W of a wheel. 
     Each in-wheel motor drive device  10  is connected to a vehicle body of the electrically powered vehicle via a suspension device, not shown. The in-wheel motor drive devices  10  allow the electrically powered vehicle to move at 0 to 180 km/h on public roads. 
     The motor unit  21  and the reduction gear unit  31  are not disposed coaxially with the axis O of the wheel hub bearing unit  11  as shown in  FIGS. 6 and 7 , but are offset from the axis O of the wheel hub bearing unit  11  as shown in  FIG. 8 . That is, as described in detail later, the in-wheel motor drive device  10  includes a portion facing toward the front of the vehicle, a portion facing toward the rear of the vehicle, a portion disposed in an upper part, and a portion disposed in a lower part. 
     The wheel hub bearing unit  11  has an outer ring  12  that serves as a wheel hub ring coupled to the road wheel W as shown in  FIG. 8 , an inner fixing member  13  passed through a central hole of the outer ring  12 , and a plurality of rolling elements  14  arranged in annular clearance between the outer ring  12  and the inner fixing member  13 , and the wheel hub bearing unit  11  forms an axle. The inner fixing member  13  includes a non-rotary fixed shaft  15 , a pair of inner races  16 , a retaining nut  17 , and a carrier  18 . The fixed shaft  15  has a larger diameter in its root part  15   r  than in its tip end  15   e . The inner races  16  are fitted on the fixed shaft  15  between the root part  15   r  and the tip end  15   e . The retaining nut  17  is screwed on the tip end  15   e  of the fixed shaft  15  to fix the inner races  16  between the retaining nut  17  and the root part  15   r.    
     The fixed shaft  15  extends along the axis O and extends through a body casing  43  that serves as an outer shell of the reduction gear unit  31 . The tip end  15   e  of the fixed shaft  15  extends through an opening  43   p  formed in a front portion  43   f  of the body casing  43  and protrudes outward in the lateral direction of the vehicle beyond the front portion  43   f . The root part  15   r  of the fixed shaft  15  extends from a position located on the inner side in the lateral direction of the vehicle with respect to a back portion  43   b  of the body casing  43  and extends through an opening  43   q  formed in the back portion  43   b . The front portion  43   f  and the back portion  43   b  are casing wall portions that face each other at an interval in the direction of the axis O. The carrier  18  is attached and fixed to the root part  15   r . The carrier  18  is located outside the body casing  43  and connected to the suspension device and a tie rod which are not shown. 
     The rolling elements  14  are arranged in two rows separated in the direction of the axis O. An outer peripheral surface of the inner race  16  located on one side in the direction of the axis O forms an inner raceway surface for the first row of the rolling elements  14  and faces a part of an inner peripheral surface of the outer ring  12  which is located on the one side in the direction of the axis O. An outer peripheral surface of the inner race  16  located on the other side in the direction of the axis O forms an inner raceway surface for the second row of the rolling elements  14  and faces a part of the inner peripheral surface of the outer ring  12  which is located on the other side in the direction of the axis O. In the following description, the outer side in the lateral direction of the vehicle (outboard side) is sometimes referred to as the one side in the direction of the axis O, and the inner side in the lateral direction of the vehicle (inboard side) is sometimes referred to as the other side in the direction of the axis O. The lateral direction in the plane of paper of  FIG. 8  corresponds to the lateral direction of the vehicle. The inner peripheral surface of the outer ring  12  forms an outer raceway surface for the rolling elements  14 . 
     The outer ring  12  has a flange portion  12   f  in its end located on the one side in the direction of the axis O. The flange portion  12   f  forms a coupling seat that is coaxially coupled to a brake disc BD and a spoke portion Ws of the road wheel W. The outer ring  12  is coupled at the flange portion  12   f  to the brake disc BD and the road wheel W and rotates with the road wheel W. In a modification, not shown, the flange portion  12   f  may be protruding portions formed at intervals in the circumferential direction and protruding radially outward. 
     As shown in  FIG. 8 , the motor unit  21  includes a motor rotary shaft  22 , a rotor  23 , a stator  24 , and a motor casing  25   c , which are arranged in this order from the axis M of the motor unit  21  toward the outside in the radial direction. The motor casing  25   c  has a tubular shape and a motor casing cover  25   v  of the motor unit  21  covers an opening of the motor casing  25   c  which is located on the other side in the direction of the axis M. Since the motor unit  21  has a radially inner rotor and a radially outer stator which face each other with a radial gap therebetween, the motor unit  21  is a radial gap motor. However, the motor unit  21  may be of other types. For example, the motor unit  21  may be an axial gap motor, although not shown in the figures. The stator  24  is connected to a power line (not shown) extending from the vehicle body side. The motor unit  21  performs power running with electric power received from the vehicle body side through the power line or performs a regenerative operation in which the motor unit  21  converts rotation of the outer ring  12  to electric power and supplies the electric power to the vehicle body side through the power line. 
     The cylindrical stator  24  includes a stator core  24   b  and a stator coil  24   c . The stator coil  24   c  is provided in at least both ends of the stator core  24   b  in the direction of the axis M. The motor unit  21  of the present embodiment is a three-phase AC rotating electrical machine. 
     The tubular motor casing  25   c  extends about the axis M and has an inner peripheral surface  25   d  having a constant radius from its end located on one side in the direction of the axis M to its end located on the other side in the direction of the axis M, a stepped portion  25   g  formed at the end of the inner peripheral surface  25   d  which is located on the one side in the direction of the axis M, and an abutting surface  25   f  serving as an end face located on the other side in the direction of the axis M. The abutting surface  25   f  is an annular flat surface. An end face of the motor casing cover  25   v  which is located on the one side in the direction of the axis M also forms an abutting surface  25   f  having the same shape. The motor casing  25   c  located on the one side in the direction of the axis M and the motor casing cover  25   v  located on the other side in the direction of the axis M abut on each other at the abutting surfaces  25   f ,  25   f  and are coupled together as shown in  FIG. 8 . A radial outer edge  128  of an end of the motor casing cover  25   v  which is located on the other side in the direction of the axis M is not formed in a cylindrical shape but is formed so as to be narrowed toward the other side in the direction of the axis M. For example, the radial outer edge  128  is chamfered. Alternatively, the radial outer edge  128  is rounded into a bowl shape. 
     When assembling the motor unit  21 , the stator  24  is inserted from the other side in the axial direction into the opening of the motor casing  25   c  which is located on the other side in the direction of the axis M, and the stator core  24   b  contacts the stepped portion  25   g , whereby the stator  24  is positioned in the direction of the axis M. 
     An outer peripheral surface of the stator core  24   b  is fitted in the inner peripheral surface  25   d  of the motor casing  25   c . The stator  24  is positioned coaxially with the axis M. A region of the stator core  24   b  which is located on the one side in the direction of the axis M is fitted in the inner peripheral surface of the motor casing  25   c , and a region of the stator core  24   b  which is located on the other side in the direction of the axis M is fitted in an inner peripheral surface of the motor casing cover  25   v . The motor casing  25   c  corresponds to the first casing  125  shown in  FIGS. 1 to 3 . The motor casing cover  25   v  corresponds to the second casing  126 . The stator core  24   b  corresponds to the stator core  124   b , and the stator coil  24   c  corresponds to the stator coil  124   c.    
     The axis M, which is the center of rotation of the motor rotary shaft  22  and the rotor  23 , extends parallel to the axis O of the wheel hub bearing unit  11 . That is, the motor unit  21  is offset from the axis O of the wheel hub bearing unit  11 . As shown in  FIG. 8 , a large part of the motor unit  21  except a tip end  22   e  of the motor rotary shaft  22  does not overlap the inner fixing member  13  in the axial direction. The cylindrical motor casing  25   c  is coupled at its end located on the one side in the direction of the axis M to the back portion  43   b  of the body casing  43  and is covered by a casing wall portion that extends continuously and flush with the back portion  43   b . Such a casing wall portion has in its center a through hole extending along the axis M. The tip end  22   e  of the motor rotary shaft  22  is passed through the through hole. The cylindrical motor casing  25   c  is sealed at its end located on the other side in the direction of the axis X by the bowl-shaped motor casing cover  25   v . Both ends of the motor rotary shaft  22  are rotatably supported by the motor casing  25   c  and the motor casing cover  25   v  via rolling bearings  27 ,  28 . The motor unit  21  drives the outer ring  12  (i.e., the wheel) and a pump shaft  51  ( FIG. 7 ). The rolling bearings  27 ,  28  correspond to the bearings  129   a ,  129   b  shown in  FIGS. 1 to 3 . The back portion  43   b  corresponds to the partition wall  125   w.    
     The reduction gear unit  31  has an input shaft  32 , an input gear  33 , an intermediate gear  34 , an intermediate shaft  35 , an intermediate gear  36 , an intermediate gear  37 , an intermediate shaft  38 , an intermediate gear  39 , an output gear  40 , an output shaft  41 , and the body casing  43 . The input shaft  32  is a tubular member having a larger diameter than the tip end  22   e  of the motor rotary shaft  22  and extends along the axis M of the motor unit  21 . The tip end  22   e  is placed in a central hole formed in an end of the input shaft  32  which is located on the other side in the direction of the axis M, so that the input shaft  32  is coaxially coupled to the motor rotary shaft  22 . Both ends of the input shaft  32  are supported by the body casing  43  via rolling bearings  42   a ,  42   b . The input gear  33  is an external gear having a smaller diameter than the motor unit  21  and is coaxially coupled to the input shaft  32 . Specifically, the input gear  33  is integrally formed on the outer periphery of a middle part of the input shaft  32  in the direction of the axis M. 
     The output shaft  41  is a tubular member having a larger diameter than a cylindrical portion of the outer ring  12  and extends along the axis O of the wheel hub bearing unit  11 . An end of the outer ring  12  which is located on the other side in the direction of the axis O is placed in a central hole formed in an end of the output shaft  41  which is located on the one side in the direction of the axis O, so that the output shaft  41  is coaxially coupled to the outer ring  12 . The output gear  40  is an external gear and is coaxially coupled to the output shaft  41 . Specifically, the output gear  40  is integrally formed on the outer periphery of an end of the output shaft  41  which is located on the other side in the direction of the axis O. Rolling bearings  44 ,  46  are disposed on both ends of the output shaft  41  in the direction of the axis O. 
     The rolling bearing  44  is disposed on the one side in the direction of the axis O with respect to the output gear  40  and is located between an outer peripheral surface of the output shaft  41  and an inner peripheral surface of the opening  43   p . The rolling bearing  44  is disposed radially outside the outer ring  12  so as to overlap the outer ring  12  in the direction of the axis O. 
     The rolling bearing  46  is disposed on the other side in the direction of the axis O with respect to the outer ring  12  and is located between an inner peripheral surface of the output shaft  41  and an outer peripheral surface of the fixed shaft  15 . The rolling bearing  46  is disposed radially inside the output gear  40  so as to overlap the output gear  40  in the direction of the axis O. 
     Regarding the position in the direction of the axis O, the rolling bearing  44  is disposed so as to overlap a region of the outer ring  12  which is located on the other side in the direction of the axis O, whereas the rolling bearing  46  is disposed on the other side in the direction of the axis O with respect to the outer ring  12  and does not overlap the outer ring  12 . The rolling bearing  46  is disposed radially inside the teeth of the output gear  40  and the rolling bearing  46  overlaps the output gear  40  in the direction of the axis O. 
     The two intermediate shafts  35 ,  38  extend parallel to the input shaft  32  and the output shaft  41 . That is, the reduction gear unit  31  is a four-parallel-shaft gear reducer. The axis O of the output shaft  41 , the axis Nf of the intermediate shaft  35 , the axis Nl of the intermediate shaft  38 , and the axis M of the input shaft  32  extend parallel to each other, namely extend in the lateral direction of the vehicle. 
     The position of each axis in the longitudinal direction of the vehicle will be described. As shown in  FIG. 7 , the axis M of the input shaft  32  is located closer to the front of the vehicle than the axis O of the output shaft  41 . The axis Nf of the intermediate shaft  35  is located closer to the front of the vehicle than the axis M of the input shaft  32 . The axis Nl of the intermediate shaft  38  is located closer to the front of the vehicle than the axis O of the output shaft  41  and closer to the rear of the vehicle than the axis M of the input shaft  32 . In a modification, not shown, the axis M of the input shaft  32 , the axis Nf of the intermediate shaft  35 , the axis Nl of the intermediate shaft  38 , and the axis O of the output shaft  41  may be arranged in this order in the longitudinal direction of the vehicle. This order is also the order in which a driving force is transmitted. 
     The vertical position of each axis will be described. The input shaft  32  is disposed so as to overlap the output shaft  41  in the vertical direction. The axis Nf of the intermediate shaft  35  is located above the axis M of the input shaft  32 . The axis Nl of the intermediate shaft  38  is located above the axis Nf of the intermediate shaft  35 . The plurality of intermediate shafts  35 ,  38  need only be disposed above the input shaft  32  and the output shaft  41 , and in a modification, not shown, the intermediate shaft  35  may be disposed above the intermediate shaft  38 . Alternatively, in a modification, not shown, the output shaft  41  may be disposed above the input shaft  32 . 
     The intermediate gear  34  and the intermediate gear  36  are external gears, and as shown in  FIG. 8 , are coaxially coupled to a middle region of the intermediate shaft  35  in the direction of the axis Nf. Both ends of the intermediate shaft  35  are supported by the body casing  43  via rolling bearings  45   a ,  45   b . The intermediate gear  37  and the intermediate gear  39  are external gears and are coaxially coupled to a middle region of the intermediate shaft  38  in the direction of the axis Nl. Both ends of the intermediate shaft  38  are supported by the body casing  43  via rolling bearings  48   a ,  48   b.    
     The body casing  43  serves as the outer shell of the reduction gear unit  31  and the wheel hub bearing unit  11 , has a tubular shape, and surrounds the axes O, Nf, Nl, M as shown in  FIG. 7 . As shown in  FIG. 8 , the body casing  43  is accommodated in the internal space region of the road wheel W. The internal space region of the road wheel W is defined by the inner peripheral surface of a rim portion Wr and the spoke portion Ws coupled to an end of the rim portion Wr which is located on the one side in the direction of the axis O. The wheel hub bearing unit  11 , the reduction gear unit  31 , and a region of the motor unit  21  which is located on the one side in the axial direction are accommodated in the internal space region of the road wheel W. A region of the motor unit  21  which is located on the other side in the axial direction protrudes beyond the road wheel W toward the other side in the axial direction. The road wheel W thus accommodates a large part of the in-wheel motor drive device  10 . 
     Referring to  FIG. 7 , the body casing  43  has a portion  43   c  located directly below the axis O and a portion located away from the axis O of the output gear  40  in the longitudinal direction of the vehicle, specifically located directly below the axis M of the input gear  33 , and protruding downward. This protruding portion forms an oil tank  47  and is located below the portion  43   c  located directly below the axis O. 
     The body casing  43  has a tubular shape and, as shown in  FIG. 8 , accommodates the input shaft  32 , the input gear  33 , the intermediate gear  34 , the intermediate shaft  35 , the intermediate gear  36 , the intermediate gear  37 , the intermediate shaft  38 , the intermediate gear  39 , the output gear  40 , the output shaft  41 , and a middle part of the wheel hub bearing unit  11  in the direction of the axis O. Lubricating oil is sealed in the body casing  43 , and the reduction gear unit  31  is lubricated. The input gear  33 , the intermediate gear  34 , the intermediate gear  36 , the intermediate gear  37 , the intermediate gear  39 , and the output gear  40  are helical gears. 
     The body casing  43  has a tubular portion including the portion  43   c  located directly below the axis O and the oil tank  47  and surrounding the group of gears  33 ,  34 ,  36 ,  37 ,  39 ,  40  as shown in  FIG. 7 , the substantially flat front portion  43   f  covering the one side in the axial direction of a tubular portion of the reduction gear unit  31  and the substantially flat back portion  43   b  covering the other side in the axial direction of the tubular portion of the reduction gear unit  31  as shown in  FIG. 8 . The back portion  43   b  is coupled to the motor casing  25   c . The back portion  43   b  is also coupled to the fixed shaft  15 . 
     The front portion  43   f  has the opening  43   p  through which the outer ring  12  extends. A sealing material  43   s  is disposed in annular clearance between the opening  43   p  and the output shaft  41 . The sealing material  43   s  is disposed on the one side in the direction of the axis O with respect to the rolling bearing  44  and seals the annular clearance. The outer ring  12 , which is a rotary element, is accommodated, except for its end located on the one side in the direction of the axis O, in the body casing  43 . 
     The input gear  33  having a smaller diameter and the intermediate gear  34  having a larger diameter are disposed in a part of the reduction gear unit  31  which is located on the one side in the axial direction (on the flange portion  12   f  side) and mesh with each other. The intermediate gear  36  having a smaller diameter and the intermediate gear  37  having a larger diameter are disposed in a part of the reduction gear unit  31  which is located on the other side in the axial direction (on the motor unit  21  side) and mesh with each other. The intermediate gear  39  having a smaller diameter and the output gear  40  having a larger diameter are disposed in the part of the reduction gear unit  31  which is located on the one side in the axial direction (on the flange portion  12   f  side) and mesh with each other. The input gear  33 , the plurality of intermediate gears  34 ,  36 ,  37 ,  39 , and the output gear  40  thus mesh with each other and form a drive transmission path from the input gear  33  through the plurality of intermediate gears  34 ,  36 ,  37 ,  39  to the output gear  40 . As the smaller diameter drive gears and the larger diameter driven gears mesh with each other as described above, rotation of the input shaft  32  is reduced in speed by the intermediate shaft  35 , rotation of the intermediate shaft  35  is reduced in speed by the intermediate shaft  38 , and rotation of the intermediate shaft  38  is reduced in speed by the output shaft  41 . The reduction gear unit  31  thus has a sufficient reduction ratio. Of the plurality of intermediate gears, the intermediate gear  34  is the first intermediate gear located on the input side of the drive transmission path. Of the plurality of intermediate gears, the intermediate gear  39  is the last intermediate gear located on the output side of the drive transmission path. 
     As shown in  FIG. 7 , the output shaft  41 , the intermediate shaft  38 , and the input shaft  32  are arranged in this order at intervals in the longitudinal direction of the vehicle. The intermediate shaft  35  and the intermediate shaft  38  are disposed above the input shaft  32  and the output shaft  41 . According to the another embodiment, the intermediate shafts can be disposed above the outer ring  12  that serves as a wheel hub, so that space where the oil tank  47  is disposed can be provided below the outer ring  12  and space that accommodates a ball joint, not shown, of the suspension device can be provided directly below the outer ring  12 . This allows a steering axis passing through the ball joint and extending in the vertical direction to cross the wheel hub bearing unit  11 , whereby the road wheel W and the in-wheel motor drive device  10  can be suitably steered about the steering axis. 
     As shown in  FIG. 7 , the body casing  43  further accommodates the pump shaft  51 . An axis P of the pump shaft  51  extends parallel to the axis O of the output shaft  41 . The pump shaft  51  is separated from the output shaft  41  in the longitudinal direction of the vehicle as shown in  FIG. 7 , is rotatably supported at its both ends in the direction of the axis P via rolling bearings, not shown, and is coaxially coupled to a pump gear  53 . The pump gear  53  is a helical gear and meshes with the output gear  40 . The output gear  40  drives the pump shaft  51 . 
     An oil pump, not shown, is disposed at the end of the pump shaft  51  in the direction of the axis P. The oil pump is connected to a suction oil passage  59   i  and a discharge oil passage  59   o  shown in  FIG. 7 . The suction oil passage  59   i  extends downward from the oil pump into the oil tank  47 , and a suction port  59   j  at the lower end of the suction oil passage  59   i  is located near a bottom wall of the oil tank  47 . The discharge oil passage  59   o  extends upward from the oil pump and a discharge port  59   p  at the upper end of the discharge oil passage  59   o  is located at a higher position than the intermediate gear  37 . 
     As the pump shaft  51  is driven by the output gear  40 , the oil pump, not shown, sucks lubricating oil in the oil tank  47  through the suction port  59   j  and discharges the sucked lubricating oil through the discharge port  59   p . The discharge port  59   p  is located at a higher position than all the gears (the input gear  33 , the intermediate gears  34 ,  36 ,  37 ,  39 , and the output gear  40 ), and the oil pump supplies the lubricating oil to these gears from above through the discharge port  59   p . The lubricating oil is also injected into the motor unit  21  through the discharge oil passage  59   o . The motor unit  21  and the reduction gear unit  31  are thus lubricated and cooled. 
     Referring to  FIG. 7 , the pump shaft  51  of the present embodiment is disposed below the input shaft  32 , and the oil tank  47  is disposed below the pump shaft  51 . For example, the oil pump is a cycloidal pump disposed substantially coaxially with the pump shaft  51  and pumps up the lubricating oil stored in the oil tank  47  to directly above the oil tank  47 . The pump shaft  51  and the oil tank  47  are disposed closer to the front of the vehicle than the output shaft  41 . When the road wheel W is driven by the in-wheel motor drive device  10  to move the vehicle, the oil tank  47  is subjected to running wind from ahead of the vehicle and is thus cooled by air. 
     According to the in-wheel motor drive device  10  of the embodiment shown in  FIGS. 6 to 8 , the motor unit  21  has: the cylindrical motor casing  25   c  serving as a part of an outer shell of the motor unit  21  which is located on the one side in the direction of the axis M; the motor casing cover  25   v  serving as a part of the outer shell of the motor unit  21  which is located on the other side in the direction of the axis M and having a smaller outside diameter at the radial outer edge  128  of its end located on the other side in the direction of the axis M than at its end located on the one side in the direction of the axis M; the abutting surfaces  25   f  formed on an one end of the motor casing cover  25   v  the other end of the motor casing  25   c  and abutting on each other; and the stator  24  extending in the direction of the axis M and having its one end placed in the motor casing  25   c  and the other end placed in the motor casing cover  25   v . A maximum steering angle can therefore be increased as compared to the case where the motor casing cover  25   v  is not chamfered like the radial outer edge  128 . 
     According to the in-wheel motor drive device  10  of the embodiment shown in  FIGS. 6 to 8 , the outer ring  12  serving as a wheel hub is disposed so as to extend parallel to the axis M of the rotor  23 , and the in-wheel motor drive device  10  further includes the reduction gear unit  31  as a parallel-shaft gear reducer mechanism that has the input gear  33  coupled to the rotor  23  and the output gear  40  coupled to the outer ring  12  and that reduces the speed of input rotation from the rotor  23  to output the resultant rotation to the output ring  12 . According to such an embodiment, it is possible not to sacrifice the maximum steering angle in the in-wheel motor drive device in which the motor unit  21  is offset from the center of the wheel (axis O) in the longitudinal direction of the vehicle as shown in  FIG. 7 . 
     Although the embodiments of the present invention are described above with reference to the figures, the present invention is not limited to the illustrated embodiments. Various changes and modifications can be made to the illustrated embodiments without departing from the spirit and scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     The in-wheel motor drive device for a steered wheel according to the present invention is advantageously used in electric vehicles and hybrid vehicles. 
     REFERENCE SIGNS LIST 
       110 : In-Wheel Motor Drive Device,  111 : Wheel Hub Bearing Unit,  112 : Wheel Hub,  121 : Motor Unit,  123 : Rotor,  124 : Stator,  124   b : Stator Core,  124   c : Stator Coil,  125   f ,  126   f . Abutting Surface,  125   g : Stepped Portion,  128 : Radial Outer Edge,  129   a ,  129   b : Bearing,  131 : Reduction Gear Unit,  140 : Wheel Well,  141 : Wall Material