Patent Publication Number: US-2019193552-A1

Title: Vehicle drive apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-245761 filed on Dec. 22, 2017, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a vehicle drive apparatus for driving a vehicle by an electric motor. 
     Description of the Related Art 
     Conventionally, there is a known vehicle drive apparatus of this type, in which an electric motor is mounted under a vehicle seat in a state with an axis of rotation of the motor oriented in vehicle height direction and torque of the motor is transmitted to a propeller shaft through a shaft installed in the center of a rotor of the motor and a pair of bevel gears. Such an apparatus is described in Japanese Unexamined Patent Publication No. 2012-029369 (JP2012-029369A), for example. 
     However, when the motor is mounted in the state with the axis of rotation oriented in vehicle height direction like the apparatus described in JP2012-029369A, it is necessary to rotatably support the rotor of the motor around the shaft via a bearing while bearing a weight of the rotor. Therefore, bearing loss is likely to become larger. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is a vehicle drive apparatus including: an electric motor including a rotor rotating about an axial line extending in a vertical direction and a stator disposed around the rotor; a shaft disposed rotatably about the axial line inside the rotor and extended along the axial line; a torque transmission mechanism configured to transmit a torque of the electric motor to the shaft; a case including a side wall and a bottom wall and configured to surround the stator; and a bearing attached to the bottom wall to support a bottom portion of the rotor rotatably about the axial line while bearing a weight of the rotor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, and advantages of the present invention will become clearer from the following description of embodiments in relation to the attached drawings, in which: 
         FIG. 1  is a front view showing schematically main configurations of a vehicle drive apparatus according to an embodiment of the invention; 
         FIG. 2  is a side view showing an example of installation of the vehicle drive apparatus of  FIG. 1  in the vehicle; 
         FIG. 3  is a cross-sectional diagram showing schematically a main configuration of the vehicle drive apparatus of  FIG. 1 ; 
         FIG. 4A  is an enlarged view of region IV of  FIG. 3 ; 
         FIG. 4B  is an enlarged view of region A of  FIG. 4A ; 
         FIG. 5  is an exploded perspective view of a main part of  FIG. 3 ; 
         FIG. 6  is an example for a comparison with  FIG. 3 ; and 
         FIG. 7  is a diagram showing a modification of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention is explained with reference to  FIGS. 1 to 7 .  FIG. 1  is a front view showing schematically main configurations of a vehicle drive apparatus  100  according to an embodiment of the present invention. The vehicle drive apparatus  100  includes an electric motor  1  and is configured to output torque from the electric motor  1  to driving wheels of a vehicle. Therefore, the vehicle drive apparatus  100  is mounted on an electric vehicle, hybrid vehicle or other vehicle having the electric motor  1  as a drive (propulsion) power source. The electric motor  1  is also used as a generator. 
       FIG. 2  is a side view showing an example of installation of the vehicle drive apparatus  100  in the vehicle. In  FIG. 2 , the vehicle drive apparatus  100  is installed between left and right front wheels  103  for use as a front wheel drive unit. The vehicle drive apparatus  100  can be also installed between left and right rear wheels  104  for use as a rear wheel drive unit. 
     As shown in  FIG. 2 , the vehicle drive apparatus  100  is arranged near a bottom surface of the body and at the middle in left-right direction of the vehicle. Therefore, height of the vehicle hood can be lowered to realize enhanced superiority of design and the like. Further, although illustrating is omitted, without arising to raise the floor surface inside the vehicle, i.e., narrowing an inside space of the vehicle, it is possible to easily install the vehicle drive apparatus  100  even below the seat or between left and right rear wheels  104 . As a result, a degree of freedom for arrangement of the vehicle drive apparatus  100  is fine. 
     Front-rear, up-down and left-right directions of the vehicle drive apparatus  100  respectively correspond to front-rear (vehicle length), up-down (vehicle height) and left-right (vehicle width) directions of the vehicle under a condition that the vehicle drive apparatus  100  is mounted on the vehicle, for example. Up-down direction and left-right direction are also called vertical direction and lateral direction. 
     As shown in  FIG. 1 , the electric motor  1  includes a rotor  10  that rotates around an axis CL 1  extending in vertical direction and a stator  20  disposed around the rotor  10 . A first gear shaft  2  is coupled to an output shaft of the motor  1 . The first gear shaft  2  extends along the axis CL 1  to project upward of the motor  1  and is provided at its upper end portion with a first gear  2   a  of smaller diameter than the rotor  10  of the motor  1 . The first gear  2   a  is, for example, configured as a spur gear. 
     A second gear shaft  3  is disposed forward of the motor  1  to rotate around an axis CL 2  extending in vertical direction. The second gear shaft  3  extends vertically and is provided at its upper end portion with a second gear  3   a  that engages the first gear  2   a . The second gear  3   a  is, for example, configured as a spur gear of greater diameter than the first gear  2   a . In addition, outer peripheral surface of the second gear shaft  3  is provided below the second gear  3   a  and forward of the motor  1  with a worm  3   b  configured as a worm gear. 
     The worm  3   b  is engaged by a worm wheel (helical gear)  4   a  rotatable around an axis CL 3  extending in lateral direction. The worm wheel  4   a  is joined to a third gear shaft  4  extending along the axis CL 3 , so that the third gear shaft  4  rotates integrally with the worm wheel  4   a . Rotation of the third gear shaft  4  is transmitted through a differential mechanism or the like to the left and right wheels (front wheels)  103  ( FIG. 2 ). As indicated by thick line T 1  in  FIG. 1 , this configuration enables the vehicle to travel by transmitting torque of the motor  1  to the wheels  103  through the first gear shaft  2 , first gear  2   a , second gear  3   a , second gear shaft  3 , worm  3   b , worm wheel  4   a  and third gear shaft  4 . 
       FIG. 3  is a cross-sectional diagram showing more specifically a configuration of the vehicle drive apparatus  100 , in particular, the motor  1  of  FIG. 1 . As shown in  FIG. 3 , the rotor  10  of the motor  1  includes a rotor hub  11  and a rotor core  15 . The rotor hub  11  includes a substantially cylinder-shaped shaft portion  12  centered on the axis CL 1 , a cylindrical portion  13  of larger diameter than and coaxial with the shaft portion  12 , and a substantially disk-shaped plate portion  14  that extends radially to connect the shaft portion  12  and cylindrical portion  13 . The rotor core  15  is a substantially cylinder-shaped rotor iron core centered on the axis CL 1 . The rotor core  15  is fitted on and fastened to (for example, serration coupling) an outer peripheral surface of the cylindrical portion  13  of the rotor hub  11  so as to rotate integrally with the rotor hub  11 . 
     The motor  1  is an interior permanent magnet synchronous motor, and multiple circumferentially spaced permanent magnets  16  are embedded in the rotor core  15 . A sensor  16  for detecting a rotational position (rotational angle) of the rotor  10  is provided above the rotor core  15 . The configuration of the motor  1  is not limited to the above configuration. Alternatively, it is possible instead to use as the motor  1  one having no magnets, such as a synchronous reluctance motor or switched reluctance motor. 
     The stator  20  of the motor  1  has a stator core  21  formed in substantially cylindrical shape centered on the axis CL 1  and disposed across a gap of predetermined radial length from an outer peripheral surface of the rotor core  15 . The stator core  21  is a fixed iron core whose inner peripheral surface is formed with multiple circumferentially spaced radially outward directed slots. A winding  22  (coil) is formed in the slots as a concentrated winding or distributed winding. Upper and lower ends of the winding  22  protrude upward and downward of upper and lower ends of the stator core  21 . The rotor  10  rotates when a revolving magnetic field is generated by passing three-phase alternating current through the winding  22 . A shaft  6  is disposed along the axis CL 1  inside the rotor  10 . 
     The motor  1  is accommodated in a case  30 . The case  30  includes an upper case  31  and a lower case  32  that are vertically separable. The upper case  31  and the lower case  32  are joined by bolts  32   a  disposed at peripheral portions of the upper case  31  and the lower case  32 . The stator core  21  is fastened to the lower case  32  by through-bolts  32   b . At a middle region of the lower case  32 , a bearing support  33  is provided so as to project upward and is formed in a substantially cylindrical shape centered on the axis CL 1 . 
     Bearings  41  and  42  of small diameter and large diameter are provided at an inner peripheral surface and an outer peripheral surface of the bearing support  33 , respectively. A lower end portion of the shaft  6  is rotatably supported centered on the axis CL 1  via the bearing  41 . A bottom portion of the rotor  10  is rotatably supported centered on the axis CL 1  via the bearing  42 . Configurations of support portions of the shaft  6  and the rotor  10  are described later in detail. 
     An opening  31   a  is provided along the axis CL 1  at a middle region of the upper case  31 . A shaft support  34  formed in a substantially truncated cone shape is provided in the opening  31   a  of the upper case  31  to extend downward and radially inward. A cover  35  is attached to an upper surface of the upper case  31  so as to close the opening  31   a  by bolts  35   a.    
     The first gear shaft  2  formed in a substantially cylindrical shape centered on the axis CL 1  is situated between the shaft support  34  and the cover  35 . Upper and lower end portions of the first gear shaft  2  are respectively rotatably supported through taper roller bearings  43  and  44  by the cover  35  and the shaft support  34 . Inner peripheral surface of the first gear  2   a  between the upper and lower taper roller bearings  43  and  44  is coupled to outer peripheral surface of the first gear shaft  2  through splines, so that the first gear shaft  2  and first gear  2   a  rotate integrally. 
     Splines  61  are formed on outer peripheral surface of upper end portion of the shaft  6 , and splines  63  of greater diameter than the splines  61  are additionally formed thereunder in the manner of sandwiching an intervening step  62 . A protrusion  64  projecting radially outward beyond the splines  63  is provided under the splines  63 . The splines  61  of the upper end portion of the shaft  6  are fitted in splines  2   b  of inner peripheral surface of the first gear shaft  2 , so that the shaft  6  rotates integrally with the first gear shaft  2 . Since the step  62  of the shaft  6  abuts bottom face of the first gear shaft  2 , upward movement of the shaft  6  is prevented during the rotation. 
     A planetary gear mechanism  50  is interposed in the torque transmission path between the rotor  10  and the shaft  6 . The planetary gear mechanism  50  includes a sun gear  51  and a ring gear  52 , formed in cylindrical shapes centered on the axis CL 1 , multiple circumferentially spaced planetary gears  53  disposed between the sun gear  51  and the ring gear  52 , multiple circumferentially spaced planetary shafts  54  extending parallel to axis CL 1  for vertically retaining and rotatably supporting the planetary gears  53 , and a carrier  55  formed in a substantially cylindrical shape centered on the axis CL 1  and connected to upper end portion of the multiple circumferentially spaced planetary shafts  54 , for retaining the multiple circumferentially spaced planetary shafts  54 . 
     The sun gear  51  is formed on outer peripheral surface of the shaft portion  12  of the rotor hub  11 . A ring body  36  formed in a substantially cylindrical shape centered on the axis CL 1  is bolted to lower end surface of the shaft support  34  of the upper case  31 , and the ring gear  52  is formed on inner peripheral surface of the ring body  36 . Splines  56  are formed on inner peripheral surface of the carrier  55 . The splines  63  of the shaft  6  are fitted in the splines  56 , so that the carrier  55  rotates integrally with the shaft  6 . Since the splines  56  are located between bottom face of the first gear shaft  2  and the protrusion  64  of the shaft  6 , the carrier  55  is vertically restrained during the rotation. 
     Owing to the aforesaid configuration, rotation of the rotor  10  is transmitted through the sun gear  51 , planetary gears  53  and carrier  55  to the shaft  6 , whereby rotation of the rotor  10  is changed at a predetermined reduction ratio and the shaft  6  rotates. In addition, rotation of the shaft  6  is output through the first gear shaft  2 , first gear  2   a  and second gear  3   a  and transmitted to the wheels  103 . 
     Configuration of the support portion that rotatably supports the rotor  10  and the shaft  6  is explained in detail below.  FIG. 4A  is an enlarged view of region IV of  FIG. 3  including the bearings  41  and  42 . As shown in  FIG. 4A , a step  331  and a step  332  are provided on inner peripheral surface and outer peripheral surface respectively of the bearing support  33  provided to project from upper surface of the lower case  32 . A cylindrical fitting surface  333  and a cylindrical fitting surface  334 , both centered on axis CL 1 , are formed above the step  331  and the step  332 , respectively. 
     Outer peripheral surface of an outer ring  41   b  of the bearing  41  is fitted on the fitting surface  333  of the bearing support  33 . The bearing  41  is, for example, a deep groove ball bearing including an inner ring  41   a , the outer ring  41   b  and balls (rigid spheres)  41   c . The bearing  41  can bear radial load and thrust load. The inner ring  41   a  is attached to lower end portion of the shaft  6  extending vertically along the axis CL 1  inside the rotor  10 . More specifically, a step  6   a  is provided at lower end portion of the shaft  6 , a fitting surface  6   b  of cylindrical shape centered on the axis CL 1  is formed below the step  6   a , and inner peripheral surface of the inner ring  41   a  is fitted on the fitting surface  6   b . Self-weight of the shaft  6  acts on the bearing  41 . 
     Inner peripheral surface of an inner ring  42   a  of the bearing  42  is fitted on the fitting surface  334  of the bearing support  33 . The bearing  42  is, for example, a deep groove ball bearing including the inner ring  42   a , an outer ring  42   b  and balls (rigid spheres)  42   c . The bearing  42  can bear radial load and thrust load. Upper end surface of the bearing support  33  at upper part of the fitting surface  334  is provided therearound with a tapered portion  335  sloped at a predetermined angle (e.g., 45°) relative to axis CL 1 .  FIG. 4B  is an enlarged view of region A of  FIG. 4A . As shown in  FIG. 4B , a groove  42   d  and a groove  336  are provided to predetermined depths in and completely around inner peripheral surface of the inner ring  42   a  and outer peripheral surface the fitting surface  334 , respectively. The positions of the grooves  42   d  and  336  are the same in the axial direction. 
     In a state with the inner ring  42   a  fitted to a predetermined position in the fitting surface  334 , i.e., in a state with lower end surface of the inner ring  42   a  abutting the step  331 , a ring (snap ring)  37  is fitted in the grooves  42   d  and  336  so as to straddle the grooves  42   d  and  336 . The ring  37  is a snap ring partially cut away circumferentially and formed in a substantially C-shape to be expandable and contractible. Radial length (width W) of the ring  37  is approximately equal to depth of the grooves  42   d  and  336 . Axial direction length of the ring  37  (thickness T) is approximately equal to width of the grooves  42   d  and  336 . A tapered portion  37   a  is provided at a radially inward corner portion of lower end surface of the ring  37  to enable smooth sliding of the ring  37  along the fitting surface  334 . The ring  37  of  FIG. 4B  (solid line) is shown in a condition not under action of an external expanding or contracting force. 
     As the inner ring  42   a  moves along the fitting surface  334  during fitting thereon, the ring  37  expands in diameter while sliding along the tapered portion  335  and comes to be wholly accommodated in the groove  42   d  as indicated by dashed line in  FIG. 4B . Once the inner ring  42   a  is fitted to predetermined position, more specifically, once lower end surface of the inner ring  42   a  abuts on the step  332  of the bearing support  33 , axial positions of the groove  42   d  and groove  336  coincide and the ring  37  contracts elastically. As a result, a radial part of the ring  37  enters the groove  336  and the ring  37  restrains vertical position of the inner ring  42   a  relative to the bearing support  33 . 
     As shown in  FIGS. 3 and 4A , a bearing support  17  formed in a substantially cylindrical shape centered on axis CL 1  is provided to project downward from lower end surface of the plate portion  14  of the rotor hub  11 . As shown in  FIG. 4A , a fitting surface  17   a  of cylindrical shape centered on axis CL 1  is formed on inner peripheral surface of the bearing support  17 , and outer peripheral surface of the outer ring  42   b  of the bearing  42  is fit on the fitting surface  17   a . In this state with outer peripheral surface of the outer ring  42   b  fitted on the fitting surface  17   a , upper end surface of the outer ring  42   b  abuts lower end surface of the plate portion  14 , and self-weight of the bearing  42  acts on the rotor  10 . 
     As shown in  FIG. 3 , a bearing cover  18  is attached by bolts  18   a  to lower end surface of the plate portion  14  of the rotor hub  11  radially outward of the bearing support  17 .  FIG. 5  is an exploded perspective view of a main part of  FIG. 3 . As shown in  FIGS. 3 and 5 , the bearing cover  18  includes a flange  181  fastened to the plate portion  14 , a peripheral wall  182  formed in a substantially cylindrical shape and extending downward from radial inward edge of the flange  181 , and a plate  183  formed in a substantially ring-shape and extending radially inward from lower end portion of the peripheral wall  182 . Upper surface of the plate  183  abuts lower end surface of the outer ring  42   b  of the bearing  42 , whereby upward movement of the rotor  10  relative to the bearing  42  is prevented and axial position of the rotor  10  is restrained. 
     Thus in the present embodiment, the rotor  10  of the motor  1  is rotatably supported from the lower case  32  via the bearing  42 . In other words, the rotor  10  is supported by the lower case  32  in gravity direction through the bearing  42 . Therefore, unlike in the configuration shown in  FIG. 6  as an example for comparison with the present embodiment, provision of a thrust needle bearing for supporting the rotor in gravity direction is unnecessary and bearing loss can be reduced. 
     In the example configuration of  FIG. 6 , a shaft  203  is supported by a lower case  201  through taper roller bearings  202   a  and  202   b , and a planetary gear mechanism  206  is rotatably supported on upper surface of the lower case  201  through thrust needle bearings  204  and  205 . In addition, a shaft  200   a  of a rotor  200  is rotatably supported relative to the shaft  203  through a needle bearing  207  and a thrust needle bearing  208 . Loss of a thrust needle bearing is large because of difference in circumferential velocity arising between inside and outside of the needles. Therefore, in the case of using the thrust needle bearings  204 ,  205  and  208  as in  FIG. 6 , loss is greater than in the case of using deep groove ball bearings (bearings  41  and  42 ) as in the present embodiment, and loss becomes still larger particularly when using multiple thrust needle bearings (bearings  204 ,  205  and  208 ). 
     Moreover, in the configuration of  FIG. 6 , the rotor  200  is supported from the lower case  201  through the taper roller bearings  202   a  and  202   b , the shaft  203  and the needle bearing  207 . This leads to increased production cost because tolerance grade of these multiple components must be upgraded in order to enhance accuracy of clearance between the rotor  200  and a stator. The present embodiment is advantageous regarding this aspect because owing to the fact that the rotor  10  is supported from the lower case  32  (bearing support  33 ) via the bearing  42  as shown in  FIG. 3 , accuracy of clearance between the rotor  10  and stator  20  can be easily improved with minimal increase of production cost. 
     Assembly procedure of the vehicle drive apparatus  100  according to the present embodiment is explained in the following. First, the stator  20  (stator core  21 ) is fixed to the lower case  32  shown in  FIG. 3  by the through-bolts  32   b . Next, the bearing  42  is attached to the bearing support  17  of bottom portion of the rotor  10  by press-fitting. Then the bearing cover  18  is attached to the plate portion  14  of the rotor  10  with the bolts  18   a . Then the bearing  41  is attached to the bearing support  33  of the lower case  32  by press-fitting. Next, the bearing  42  is attached to the bearing support  33  of the lower case  32  together with the rotor  10 . At this time, the ring  37  fitted in the groove  42   d  of the inner ring  42   a  of the bearing  42  ( FIG. 4B ) fits in the groove  336  of the fitting surface  334  of the bearing support  33 , thereby fixing the bearing  42  to the lower case  32  through the ring  37 . 
     Next, the ring body  36  formed with the ring gear  52  is bolted to bottom surface of the shaft support  34  of the upper case  31 . Then lower end portion of the shaft  6  is inserted into the bearing  41 . Then the splines  56  of the carrier  55  integral with the planetary gears  53  of the planetary gear mechanism  50  are fitted along the splines  63  on the outer peripheral surface of the shaft  6 . Next, the lower case  32  and upper case  31  are fastened with the bolts  32   a . Finally, the taper roller bearing  43 , first gear  2   a  and taper roller bearing  44  are successively fitted on the first gear shaft  2 , whereafter the first gear shaft  2  is fitted on the shaft  6  and the cover  35  is attached to top of the upper case  31  with the bolts  35   a.    
     The present embodiment can achieve advantages and effects such as the following: 
     (1) The vehicle drive apparatus  100  includes: the electric motor  1  including the rotor  10  that rotates centered on the axis CL 1  extending in vertical direction and the stator  20  disposed around the rotor  10 ; the shaft  6  disposed inside the rotor  10  to be rotatable centered on the axis CL 1  and to extend along axis CL 1 ; the planetary gear mechanism  50  for transmitting torque of the motor  1  to the shaft  6 ; the upper case  31  and lower case  32  surrounding the stator  20 ; and the bearing  42  attached to the lower case  32  for supporting bottom portion of the rotor  10  to be rotatable centered on axis CL 1  while bearing weight of the rotor  10  ( FIG. 3 ). 
     This configuration can lower loss by bearings during rotor rotation compared to that in, for example, a configuration such as shown in  FIG. 6  that rotatably supports the rotor  200  from the shaft  203  in gravity direction via the thrust needle bearings  204 ,  205  and  208 . Moreover, accuracy of clearance between the rotor  10  and the stator  20  can be easily enhanced because the rotor  10  is supported from the lower case  32  via the bearing  42 . 
     (2) The lower case  32  includes the bearing support  33  having the fitting surface  334  of cylindrical shape centered on the axis CL 1  ( FIG. 4A ). Bottom portion of the rotor  10  includes the bearing support  17  having the cylindrical fitting surface  17   a  facing the fitting surface  334  ( FIG. 4A ). The bearing  42  is configured as a deep groove ball bearing including the inner ring  42   a  fitted on the fitting surface  334  and the outer ring  42   b  fitted on the fitting surface  17   a  ( FIG. 4A ). The bearing  42  can therefore bear radial load and thrust load and reliably support the rotor  10  rotating about the vertical axis CL 1 , along with the self-weight thereof. 
     (3) The vehicle drive apparatus  100  further includes the bearing cover  18  attached to bottom portion of the rotor  10  so as to cover bottom surface of the outer ring  42   b  of the bearing  42  (FIGS. and  5 ). This prevents upward movement of the rotor  10  relative to the case  30 . Upward movement of the shaft  6  relative to the case  30  is prevented by abutment of the step  62  of the shaft  6  onto bottom surface of the first gear shaft  2  ( FIG. 3 ). 
     (4) The groove  336  and the groove  42   d  are provided over whole circumferences on the fitting surface  334  of the bearing support  33  and inner peripheral surface of the inner ring  42   a , respectively, and the ring  37  is fitted in both the groove  336  of the fitting surface  334  and groove  42   d  of the inner ring  42   a  ( FIG. 4A ). The inner ring  42   a  can therefore be easily fixed to the lower case  32 . Moreover, the ring  37  can radially expand and contract elastically, so that when, in a state with the ring  37  fitted in the groove  42   d , the inner ring  42   a  of the bearing  42  is fitted on the bearing support  33 , the ring  37  fits in the groove  336 , whereby assembly of the rotor  10  into the vehicle drive apparatus  100  is facilitated. 
     (5) The vehicle drive apparatus  100  further includes the first gear shaft  2  that rotates integrally with the shaft  6 . The first gear shaft  2  has inner peripheral surface of cylindrical shape centered on axis CL 1  (splines  2   b ) fitted on the splines  61  of shaft  6 , is disposed above the rotor  10 , and has the first gear  2   a  ( FIG. 3 ). Torque of the motor  1  can therefore be readily extracted through the shaft  6  and first gear shaft  2  to outside the motor  1 . 
     (6) The bearing support  33  of the lower case  32  includes, inside the fitting surface  334  in radial direction, the fitting surface  333  centered on the axis CL 1  ( FIG. 4A ). The vehicle drive apparatus  100  further includes the bearing  41  (deep groove ball bearing) having the inner ring  41   a  fitted on lower outer peripheral surface (fitting surface  6   b ) of the shaft  6  and outer ring  41   b  fitted on fitting surface  333 . Therefore, the shaft  6  can be rotatably supported from the lower case  32  in gravity direction. Since the bearings  41  and  42  are respectively disposed radially inward and outward of the bearing support  33 , the pair of bearings  41  and  42  can be compactly installed in a limited space without enlarging overall apparatus size in axial direction. 
     In the aforesaid embodiment, the shaft  6  is supported from the lower case  32  through the bearing  41 . However, a support structure of the shaft  6  is not limited to this configuration.  FIG. 7  is a diagram showing a modification of  FIG. 3 . In  FIG. 7 , grooves  2   c  and  61   a  are provided over whole circumferences on inner peripheral surface (splines  2   b ) of the first gear shaft  2  and outer peripheral surface (splines  61 ) of the shaft  6 , respectively, and a ring (snap ring)  39  similar to the circumferentially partially cut away ring  37  is fitted in the grooves  2   c  and  61   a . The shaft  6  is therefore supported on inner peripheral surface of the first gear shaft  2  through the ring  39 . 
     Owing to the provision of the grooves  2   c  and  61   a  over whole circumferences on outer peripheral surface of the shaft  6  and inner peripheral surface of the first gear shaft  2 , and the fitting of the ring  39  in both of the grooves  2   c  and  61   a  in this manner, need for the bearing  41  ( FIG. 3 ) for supporting the shaft is obviated. As a result, structure around the bearing  42  can be simplified and length of the shaft  6  shortened. 
     The structure according to  FIG. 7  can be assembled, for example, by attaching the ring  39  to the groove  61   a  and thereafter inserting the shaft  6  fitted with the carrier  55  into the first gear shaft  2  while contracting the ring  39 , thereby fitting the ring  39  in the groove  2   c . Alternatively, it can be assembled by attaching the ring  39  to the groove  2   c  of the first gear shaft  2  and thereafter inserting the shaft  6  into the first gear shaft  2 , thereby fitting the ring  39  in the groove  61   a.    
     In the aforesaid embodiment, the case  30  of the motor  1  is configured by the upper case  31  and lower case  32 . However, a case can be of any structure insofar as it has a side wall and a bottom wall surrounding the stator of the motor. The planetary gear mechanism  50  serving as a torque transmission mechanism for transmitting torque of the motor  1  to the shaft  6  is not limited to the configuration described in the foregoing. In the aforesaid embodiment, deep groove ball bearings are used as the bearings  41  and  42 , but other type of bearing capable of bearing radial load and thrust load can be used instead. Bearings can be of any configuration insofar they can be attached to the bottom wall (lower case  32 ) of the case  30  and support bottom portion of the rotor  10  so as to be rotatable centered on the axis CL 1  while bearing a weight of the rotor. 
     Although in the aforesaid embodiment, the bearing support  33  having the fitting surface  334  (first cylindrical surface) is provided on the lower case  32 , a first bearing support is not limited to the aforesaid configuration. Although in the aforesaid embodiment, the bearing support  17  is provided on bottom portion of the rotor  10 , a second bearing support having the fitting surface  17   a  (second cylindrical surface) facing the fitting surface  334  is not limited to the aforesaid configuration. Although in the aforesaid embodiment, the bearing cover  18  is attached to the bottom portion of the rotor  10 , a bearing fixing member is not limited to the aforesaid configuration insofar as attached to bottom portion of the rotor so as to cover bottom surface of the outer ring of the bearing. Although in the aforesaid embodiment, the ring  37  is fitted in the groove  336  of the bearing support  33  and the groove  42   d  of inner peripheral surface of the inner ring  42   a , a groove and a ring member are not limited to the above configurations. 
     In the aforesaid embodiment, the shaft  6  is fitted in the splines  2   b  of inner peripheral surface of the first gear shaft  2 . However, a gear shaft is not limited to the configuration of the aforesaid first gear shaft  2  insofar as it has a cylindrical inner peripheral surface centered on the axis and is disposed above the rotor to rotate integrally with the shaft. In the aforesaid embodiment ( FIG. 3 ), the bearing  42  (first deep groove ball bearing) is supported on the fitting surface  334 , i.e., outer peripheral surface of the bearing support  33 , and the bearing  41  (second deep groove ball bearing) is supported on the fitting surface  333  (third cylindrical surface), i.e., inner peripheral surface of the bearing support  33 . However, a third bearing support for supporting the bearing  41  can be provided separately from a first bearing support for supporting the bearing  42 . In the aforesaid embodiment ( FIG. 7 ), the ring  39  is fitted in the groove  61   a  of outer peripheral surface of the shaft  6  and the groove  2   c  of inner peripheral surface of the first gear shaft  2 , but a groove and a ring member are not limited to this configuration. 
     The above embodiment can be combined as desired with one or more of the above modifications. The modifications can also be combined with one another. 
     According to the present invention, a rotor of an electric motor of a vehicle drive apparatus that rotates about an axial line extending in a vertical direction can be supported in a good manner reducing bearing loss. 
     Above, while the present invention has been described with reference to the preferred embodiments thereof, it will be understood, by those skilled in the art, that various changes and modifications may be made thereto without departing from the scope of the appended claims.