Patent Publication Number: US-2006014602-A1

Title: Motor power train and method of assembling the same

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a division of application Ser. No. 10/241,075 filed on Sep. 11, 2002, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of Invention  
      The present invention relates to a motor power train, particularly to a power train for an electric motor of an electric vehicle or a four-wheel-drive electric vehicle in which the electric motor is used in combination with an internal combustion engine (a fuel engine), and to a method of assembling the same.  
      2. Description of Related Art  
      Japanese Patent Application Laid-Open No. 9(1997)-226394 discloses a drive system for an electric vehicle, which is composed of an electric motor, a reducer and a differential.  
      However, in this drive system, a gear on an intermediate shaft and a large-diameter portion of the differential overlap in axial position. A gear fixed to a flange of a differential case with bolts and a differential gear unit also overlap in axial position. This arrangement requires a large distance between the intermediate shaft and the differential, resulting in a large overall size of the drive system with its degraded mountability.  
      Moreover, in order to assemble this drive system, a gear is set to an output shaft of the electric motor, and then a center distance between the output shaft and the intermediate shaft is adjusted for proper engagement of gears on both shafts. Such assembly is difficult and thereby costly.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a motor power train, which is compact, light-weight, excellent in mountability, easy to assemble and low-cost, and to provide a method of assembling the same.  
      An aspect of the present invention is a motor power train comprising: a reducer for amplifying drive torque of a motor; a differential device for allocating the drive torque transmitted from the reducer to wheels, including a clutch mechanism and a differential gear unit; and a casing for housing the reducer and the differential device, wherein the reducer includes: a first shaft to which the drive torque is inputted; a second shaft to which the drive torque is transmitted form the first shaft; a first reduction gear set comprising a first gear on the first shaft, and a second gear on the second shaft, engaging with the first gear; and a second reduction gear set comprising a third gear on the second shaft, and a fourth gear provided on the clutch mechanism of the differential device, engaging with the third gear, and wherein the fourth gear and a large-diameter portion of the differential gear unit are offset in axial position without overlapping. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described with reference to the accompanying drawings wherein:  
       FIG. 1  is a cross-sectional view of a motor power train according to an embodiment of the present invention.  
       FIG. 2  is a view of the motor power train of  FIG. 1 , which is taken from a direction of an arrow II.  
       FIG. 3  is an explanatory schematic showing a power system of a four-wheel-drive vehicle using the motor power train of  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      An embodiment of the present invention will be explained below with reference to the drawings, wherein like members are designated by like reference characters.  
      In  FIGS. 1 and 3 , the left side shows a driver&#39;s left side (one side in an axial direction) and the right side shows a driver&#39;s right side (the other side in the axial direction).  
      A four-wheel-drive vehicle shown in  FIG. 3  is a hybrid electric vehicle which uses both an engine and an electric motor as drive power sources. A front power system on a front wheel side adopts the engine as the drive power source, and a rear power system on a rear wheel side adopts the electric motor as the drive power source. A power train  1  for the electric motor (hereinafter referred to as the power train  1 ) is applied to the power system on the rear wheel side.  
      The front power system includes a transverse-type engine  3 , a transverse-type transmission  5 , a front differential  7  (a differential for allocating drive torque (drive force) from the engine between right and left front wheels), front wheel shafts  9  and  11 , right and left front wheels  13  and  15 , and the like.  
      The rear power system includes the power train  1 , rear wheel shafts  17  and  19 , right and left rear wheels  21  and  23 , an electric motor  29 , a battery  31 , a sensor  33 , a controller  35 , and the like. The power train  1  includes a reducer  25 , a rear differential device  27  having a clutch function (a differential device for allocating drive torque (drive force) from the electric motor between the right and left rear wheels), and the like.  
      The electric motor  29  is connected to the battery  31  via the controller  35 . The controller  35  performs driving of the electric motor  29 , adjustment of revolutions, discontinuation of driving, and the like based on information from the sensor  33 . In a normal run, the controller  35  discontinues an operation of the rear power system by discontinuing driving of the electric motor  29  and transmission of the drive torque with the rear differential device  27 . In this event, the front wheels  13  and  15  are driven by the engine  3 . Accordingly, the vehicle shifts to two-wheel drive state driven by the front power system.  
      When larger drive torque is required for running, the controller  35  drives the electric motor  29  and starts transmission of the drive torque with the rear differential device  27 , and thereby operates the rear power system. Accordingly, the rear wheels  21  and  23  are driven accessorily and the vehicle shift to four-wheel drive state.  
      As shown in  FIG. 1 , the power train  1  is constituted of the reducer  25 , the rear differential device  27 , and a casing  37  for housing the foregoing elements.  
      The casing  37  is constituted of a casing body  39  and a cover  41 . The cover  41  is fixed to an opening on a left side of the casing body  39  with a bolt  43 . An oil pool is provided on the casing  37 .  
      The reducer  25  includes two stages of a first reduction gear set  45  and a second reduction gear set  47 . The first reduction gear set  45  is constituted of a small-diameter gear  49  (a first gear) and a large-diameter gear  51  (a second gear) which are mutually engaged. The second reduction gear set  47  is constituted of a small-diameter gear  53  (a third gear) and a large-diameter gear  55  (a fourth gear) which are mutually engaged.  
      The small-diameter gear  49  of the first reduction gear set  45  is integrally formed on a left end portion of a first shaft  57  to which a drive torque of the electric motor is transmitted (a torque-transmission shaft on the electric motor side). This first shaft  57  is borne on the casing body  39  with a pair of ball bearings  59  and  61  disposed on the right and the left of the first shaft  57 . The ball bearing  59  on the left side is disposed on a right side of the small-diameter gear  49 . A left portion of an outer ring of the bearing  59  is positioned by use of a snap ring  201  retained on the casing body  39 . Meanwhile, a right end portion of an inner ring thereof is positioned by the first shaft  57 . Moreover, a hub  67  with splines  65  formed inside thereof is welded inside a coaxial hole  63  formed on a right end portion of the first shaft  57 .  
      Moreover, as shown in  FIG. 1 , the ball bearing  59 , the small-diameter gear  53  of a second shaft  71  as an intermediate shaft, and the large-diameter gear  55  of an outer differential case  83  (an outer case) are disposed so as to overlap one another at least partially in axial position. Furthermore, a flange face  207  is provided on the casing body  39  for fitting a motor housing (not shown) of the electric motor  29 . As shown in  FIG. 2 , the electric motor  29  is fixed to the casing body  39  by use of bolts through four screw holes  69  provided on a right side of the casing body  39 . The first shaft  57  is joined to an output shaft of the electric motor  29  with the splines  65  on the hub  67 .  
      An oil seal  70  is disposed between the first shaft  57  and the casing body  39  so as to prevent oil leakage to the outside (to the electric motor  29  side). The large-diameter gear  51  of the first reduction gear set  45  is pushed into the second shaft  71 , and a right end thereof is positioned by a snap ring  205  retained on the second shaft  71 . The large-diameter gear  51  and the large-diameter gear  55  are disposed such that axial projections thereof overlap each other partially.  
      A left end portion of the second shaft  71  is borne on the cover  41  with a ball bearing  73  interposed therebetween and thereby assembled on the cover  41  (sub-assembled). A right side of the ball bearing  73  is positioned by a snap ring  203  retained on the cover  41  and a left side thereof is positioned by the second shaft  71 . An oil seal  75  is provided between the second shaft  71  and the cover  41  so as to prevent oil leakage to the outside. An arrow  157  in  FIG. 2  indicates a position of a center of the second shaft  71 .  
      Moreover, a right end portion  80  of the second shaft  71  is borne by the casing body  39  with a roller bearing  77  interposed therebetween. As will be described later, the roller bearing  77  is fitted to the casing body  39  in advance. A retainer portion  79  thereof is fitted into the casing body  39  and a roller portion  81  thereof supports the right end portion  80  of the second shaft  71 . The right end portion  80  of the second shaft  71  is inserted into the roller bearing  77  in the event of assembly of the power train  1  by fitting the cover  41  to the casing body  39 .  
      The roller bearing  77  is disposed on a right side of the large-diameter gear  55 . Accordingly, the roller bearing  77  and the large-diameter gear  55  are offset without overlapping in axial position.  
      Moreover, the small-diameter gear  53  of the second reduction gear set  47  is integrally formed on the second shaft  71  and is disposed on a right side of the large-diameter gear  51 .  
      The large-diameter gear  51  and the small-diameter gear  53  of the second shaft  71  are disposed between the bearing  73  supporting the left end portion of the second shaft  71  and the roller bearing  77  supporting the right end portion  80  thereof. Here, the small-diameter gear  53  is disposed on the right of the large-diameter gear  51 . In other words, the large-diameter gear  51  and the small-diameter gear  53  of the second shaft  71  and the roller bearing  77  supporting the right end portion  80  are disposed in this enumerating order in a direction from the cover  41  toward the casing body  39 . An outer diameter of the small-diameter gear  53  is smaller than an outer diameter of the large-diameter gear  51 . An outer diameter of the right end portion  80  is smaller than the outer diameter of the small-diameter gear  53 .  
      Moreover, the large-diameter gear  55  of the second reduction gear set  47  constitutes a ring gear, which is welded on a left end portion of the outer differential case  83  of the rear differential device  27 . Here, the large-diameter gear  55  may be integrally formed on the outer differential case  83 .  
      Revolutions of the electric motor  29  are reduced to a range of revolutions of the wheels with drive torque increased and transmitted to the outer differential case  83 . Eventually, the outer differential case  83  is rotated so as to rotate the right and left wheels.  
      The rear differential device  27  includes a clutch mechanism  85  and a differential gear unit  87  of a bevel gear type.  
      The clutch mechanism  85  includes the outer differential case  83  provided with the large-diameter gear  55  which receives the drive torque of the electric motor to rotate, an inner differential case  89  (an inner case), a multiplate main clutch  91 , a ball cam  93 , a pressure plate  95 , a cam ring  97 , multiplate pilot clutch  99 , a return spring  101 , an armature  103 , an electromagnet  105  (an actuator), and the like.  
      The outer differential case  83  is borne as relatively rotatable on the outside of the inner differential case  89  only by ball bearings  107 . In other words, the outer differential case  83  has a floating structure and only performs transmission of the drive torque with the large-diameter gear  55 . A left side of these bearings  107  is positioned by a snap ring  209  retained on a left boss portion  109  of the inner differential case  89  and a right side thereof is positioned by a snap ring  211  retained on the large-diameter gear  55 .  
      The large-diameter gear  55  of the second reduction gear set  47  welded on the outer differential case  83  is disposed to be offset to the left from the differential gear unit  87  so as not to overlap a large-diameter portion of the differential gear unit  87  in axial position.  
      The left boss portion  109  on the left side of the inner differential case  89  is borne on the cover  41  with a ball bearing  111  interposed therebetween, and a right boss portion  113  on the right side is borne on the casing body  39  with a ball bearing  115  and a core  117  of the electromagnet  105  fixed to the casing body  39 , interposed between the right boss portion  113  and the casing body  39 .  
      Moreover, as shown in  FIG. 1 , the small-diameter gear  49  on the first shaft  57 , the large-diameter gear  51  on the second shaft  71 , and the ball bearing  111  supporting a left end portion of the inner differential case  89  are disposed so as to overlap one another at least partially in axial position.  
      Moreover, on an outer periphery of the right boss portion  113  of the inner differential case  89 , provided is a rotor  119  made of a magnetic material. This rotor  119  is positioned in the axial direction by a snap ring  121  retained on the outer periphery of the right boss portion  113  and thereby constitutes a right sidewall member of the outer differential case  83 .  
      The main clutch  91  is disposed between the outer differential case  83  and the inner differential case  89 . Outer plates  123  thereof are joined to splines formed on an inner periphery of the outer differential case  83 , and inner plates  125  thereof are joined to splines formed on an outer periphery of the inner differential case  89 . A left side of a receiving member  215  disposed on a left side of the main clutch  91  is positioned by a snap ring  213  retained on the outer periphery of the left boss portion  109  of the inner differential case  89 , and a right side thereof is positioned by a step portion  153  formed on an end portion of a pinion shaft  143 .  
      The pilot clutch  99  is disposed between the outer differential case  83  and the cam ring  97 . Outer plates  127  thereof are joined to the splines formed on the inner periphery of the outer differential case  83 , and inner plates  129  thereof are joined to the splines formed on an outer periphery of the cam ring  97 .  
      The ball cam  93  is disposed between the pressure plate  95  and the cam ring  97 . The pressure plate  95  is joined to the splines formed on the outer periphery of the inner differential case  89  so as to thrust the main clutch  91  to engage upon receipt of cam thrusting force from the ball cam  93 .  
      Moreover, a thrust bearing  131  is disposed between the cam ring  97  and the rotor  119  for allowing relative revolutions between the cam ring  97  and the rotor  119  while receiving reactive force of the cam thrusting force from the ball cam  93 .  
      The return spring  101  is disposed between the pressure plate  95  and the inner differential case  89  so as to thrust the pressure plate  95  toward the direction for disengaging the main clutch  91 .  
      The armature  103  in a ring shape is disposed as movable in the axial direction between the pressure plate  95  and a rightmost inner plate  129  of the pilot clutch  99 . An inner periphery of the armature  103  is supported as relatively rotatable by an outer periphery of a step portion  133  formed on an outer periphery of the pressure plate  95 , whereby the armature is centered.  
      As shown in  FIG. 2 , a lead wire  135  of the electromagnet  105  is drawn out of the casing body  39  through a grommet  137  and is connected to the battery  31  with a connector  139 .  
      An appropriate gap is provided between the core  117  of the electromagnet  105  and the rotor  119 . When the electromagnet  105  is excited, a magnetic path of the electromagnet  105 , which is constituted of the gap, the rotor  119 , the pilot clutch  99  and the armature  103 , and a magnetic flux loop  141  including the magnetic path are generated.  
      The differential gear unit  87  includes a plurality of pinion shafts  143  and pinion gears  145 , side gears  147  and  149 , and the like.  
      The respective pinion shafts  143  are disposed so as to radiate out from the rotation center of the inner differential case  89 . An outer end portion of each of the pinion shaft is engaged with an engaging hole  151  of the inner differential case  89 . Rotation around the axis thereof is stopped by engagement between the step portion  153  provided on the end portion of the pinion shaft  143  and the receiving member  215  on the left side of the main clutch  91 .  
      The respective pinion gears  145  are borne as rotatable on the respective pinion shafts  143 .  
      The side gears  147  and  149  are respectively engaged with left and right sides of the pinion gears  145 . A thrust washer  150  is disposed between each side gear  147  or  149  and the inner differential case  89  so as to receive reactive force attributable to engagement between the side gear  147  or  149  and the pinion gear  145 .  
      The side gears  147  and  149  are respectively joined with splines to the left and right rear wheel shafts  17  and  19 . The rear wheel shafts  17  and  19  respectively penetrate the left and right boss portions  109  and  113  of the inner differential case  89 , the cover  41  and the casing body  39  and are joined to the left and right rear wheels  21  and  23 .  
      An oil seal  155  is disposed between each of the rear wheel shaft  17  or  19  and the cover  41  as well as the casing body  39 , so as to prevent oil leakage to the outside.  
      Rotation of the inner differential case  89  is allocated between the respective side gears  147  and  149  via the pinion shafts  143  and the pinion gears  145 , and further transmitted to the left and right rear wheels  21  and  23  via the rear wheel shafts  17  and  19 .  
      When the vehicle is running on a rough road or the like, a difference occurs between loads on the rear wheels  21  and  23 . Accordingly, the pinion gears  145  rotate around the axes thereof. In this way, the drive torque of the electric motor is differently allocated between the left and right rear wheels  21  and  23 .  
      The controller  35  controls a current to the electromagnet  105  including excitation or discontinuation of excitation in response to running and steering conditions of the vehicle including road conditions, starting, acceleration, turning, and the like.  
      The excitation of the electromagnet  105  is carried out simultaneously with initiation of rotation of the electric motor  29 , and the discontinuation of excitation of the electromagnet  105  is carried out simultaneously with termination of rotation of the electric motor  29 .  
      The magnetic flux loop  141  is generated when the electromagnet  105  is excited. Accordingly, the outer plates  127  and the inner plates  129  are engaged between the armature  103  pulled by the magnetic flux loop  141  and the rotor  119 , whereby the pilot clutch  99  is engaged. In this way, pilot torque is transmitted to the cam ring  97 , which is joined to the outer differential case  83  via the pilot clutch  99 .  
      The pilot torque transmitted to the cam ring  97  is converted into thrusting force which is amplified by the ball cam  93 . The pressure plate  95  receives this thrusting force and moves leftward to engage the main clutch  91 .  
      When the clutch mechanism  85  is joined accordingly, the drive torque of the electric motor  29  transmitted to the outer differential case  83  via the large-diameter gear  55  is further transmitted to the inner differential case  89 . The drive torque is allocated between the left and right rear wheels  21  and  23  by the differential gear unit  87  as described above. In this event, the vehicle shifts to four-wheel drive state.  
      If the exciting current of the electromagnet  105  is controlled, then a slip ratio of the pilot clutch  99 , i.e. cam-thrusting force of the ball cam  93  is controlled accordingly. In this way, the drive torque to be transmitted to the rear wheels  21  and  23  is controlled.  
      Such control of the drive torque substantially improves, especially when the vehicle is turning, turning characteristics and stability of the vehicle.  
      When the excitation of the electromagnetic  105  is discontinued, then the pilot clutch  99  is disengaged and the cam-thrusting force of the ball cam  93  disappears. Then, the pressure plate  95  is pushed rightward by the return spring  101 , whereby the main clutch  91  is disengaged. In this way, the clutch mechanism  85  is disengaged and the vehicle shifts to front two-wheel drive state.  
      When the clutch mechanism  85  is disengaged simultaneously with discontinuation of the electric motor  29 , components of the reducer  25  including the large-diameter gear  55  of the outer differential case  83  and the electric motor  29  become free from the rotation of the rear wheels  21  and  23 .  
      In other words, when the vehicle is in the two-wheel drive state, the reducer  25  and the electric motor  29  are not forced to rotate by the rear wheels  21  and  23 . Accordingly, durability of the reducer  25  and the electric motor  29  is improved.  
      Rotating large-diameter gear  51  of the first reduction gear set  45  scatters oil in the oil pool formed at the lower portion of the casing  37  (an arrow  156  in  FIG. 2  indicates lower side), whereby respective engaging portions of the gears  49 ,  51 ,  53  and  55 , the bearings  59 ,  61 ,  73  and  77  and the bearings  107 ,  111  and  115  are lubricated and cooled down.  
      The scattered oil further permeates to the inside from gaps on both right and left sides of the outer differential case  83  and the inner differential case  89 , whereby the pilot clutch  99 , a sliding face (face of the step portion  133 ) between the armature  103  and the pressure plate  95 , the ball cam  93 , the thrust bearing  131 , the main clutch  91  and the like are lubricated and cooled down.  
      Furthermore, the scattered oil permeates into the inside from spiral oil grooves formed on the inside of the boss portions  109  and  113  when the inner differential case  89  rotates, whereby respective engaging portions of the gears  145 ,  147  and  149  of the differential gear unit  87  and the like are lubricated and cooled down. Receiving centrifugal force, the oil further moves toward the main clutch  91 . In this way, the main clutch  91 , the call cam  93 , the pilot clutch  99  and the like are lubricated and cooled down.  
      Thereafter, the oil returns to the oil pool.  
      Moreover, an oil passage  159  is provided on the casing body  39  for guiding the oil scattered by the rotating large-diameter gear  55  and thereby supplying the oil from an opening  161  to the roller bearing  77 . Accordingly, the roller bearing  77  and the like are lubricated and cooled down.  
      In this power train  1 , the left end portion of the second shaft  71  is supported by the cover  41 . Accordingly, it is possible to shorten a center distance between the second shaft  71  and the outer differential case  83  of the rear differential device  27 . Therefore, the power train  1  can be compact with improved mountability.  
      Moreover, it is possible to fit the second shaft  71  to the cover  41  in advance before the cover  41  is set on the casing body  39 . Setting of the second shaft  71  is completed upon setting the cover  41  on the casing body  39  with the right end portion  80  of the sub-assembled second shaft  71  inserted into the roller bearing  77  set in the casing body  39  in advance.  
      Moreover, since the roller bearing  77  is used as the bearing for supporting the right end portion  80  of the second shaft  71 , it is easier to insert the right end portion  80  of the second shaft  71  in the event of setting the cover  41  on the casing body  39  together with the second shaft  71 .  
      Moreover, the portions of the second shaft  71 , namely, the large-diameter gear  51 , the small-diameter gear  53 , and the right end portion  80  to be supported by the roller bearing  77  are disposed in the enumerating order in the direction from the cover  41  toward the casing body  39 . In addition, the outer diameter of this right end portion  80  is smaller than the outer diameter of the small-diameter gear  53 . Therefore, the second shaft  71  and the casing body  39  do not interfere with each other upon setting the cover  41  on the casing body  39 .  
      Therefore, assembly of the power train  1  is made easier and assembly costs are thereby reduced.  
      Moreover, since the both end portions of the second shaft  71  are supported by the bearings  73  and  77 , the second shaft  71  is not inclined, obtaining stable engagement of the large-diameter gear  51  and the small-diameter gear  53  disposed between the bearings  73  and  77 .  
      In this power train  1 , the large-diameter gear  55  and the large-diameter portion of the differential gear unit  87  are offset without overlapping in axial position. This arrangement makes it possible to shorten a center distance between the second shaft  71  and the outer differential case  83  of the rear differential device  27 .  
      Moreover, the roller bearing  77  is disposed on the right side of the large-diameter gear  55  and those elements are offset without overlapping in axial position. This arrangement makes it possible to shorten the center distance between the second shaft  71  and the outer differential case  83  of the rear differential device  27 .  
      Moreover, the small-diameter gear  49  of the first shaft  57 , the large-diameter gear  51  of the second shaft  71  and the ball bearing  111  for supporting the left end portion of the inner differential case  89  are disposed so as to mutually overlap in axial position. The ball bearing  59  for supporting the first shaft  57 , the small-diameter gear  53  of the second shaft  71  and the large-diameter gear  55  on the outer differential case  83  are disposed so as to mutually overlap in axial position. Accordingly, center distances among the first shaft  57 , the second shaft  71  and the inner differential case  89  (or the outer differential case  83 ) are respectively shortened.  
      Moreover, the large-diameter gears  51  and  55  are disposed so as to mutually overlap in axial position. Accordingly, a center distance between the second shaft  71  and the outer differential case  83  of the rear differential device  27  is shortened.  
      Moreover, since the large-diameter gear  55  is integrally formed (welded or machined) on the outer differential case  83 , the outer diameter of the large-diameter gear  55  is reduced.  
      Accordingly, the power train  1  is made compact with its improved mountability.  
      In the power train  1 , the first shaft  57 , the second shaft  71  and the inner differential case  89  are accommodated in the casing  37 . Accordingly, assembling of the power train  1  is made only by setting the reducer  25  and the rear differential device  27  in the casing  37  with the electric motor  29  separated. Therefore, assembly is made easier and the assembly costs are thereby reduced.  
      In the power train  1 , the constituents except the electric motor  29  (which include the reducer  25 , the rear differential device  27  and the like) are integrally set inside the casing  37  to form a sub-unit. Such a sub-unit can be used irrespective of a motor housing of the electric motor  29 . Accordingly, freedom of mountability onto the vehicle is improved.  
      Moreover, regarding the rear differential device  27 , the clutch mechanism  85  is built in the outer differential case  83 . Accordingly, the rear differential device  27  is made compact and light-weight, whereby mountability thereof is improved.  
      Moreover, it is not necessary to especially design and fabricate the output shaft of the electric motor  29  or the casing  37  in dependence to the type of the electric motor for use, saving cost attributable thereto.  
      The invention may be practiced or embodied in still other ways without departing from the spirit or essential character thereof.  
      Although the foregoing embodiment has been described as an example of application to an electric vehicle in which the engine is used as a main drive power source and the electric motor is used as an auxiliary drive power source, the power train for an electric motor according to the present invention is also applicable to a vehicle which uses an electric motor as a main drive power source thereof.  
      Moreover, the differential device in the present invention is not limited to the one with the differential of the bevel gear type as described in the embodiment, but also various types of differentials are applicable such as a differential of a planetary gear type, a differential in which a side gear of an output side is joined by a pinion gear housed slidably in a housing hole of a differential case, or a differential using a worm gear.  
      In the present invention, the torque-transmission shaft on the electric motor side is not limited to an input shaft to which the output shaft of the electric motor is directly joined as described above. Instead, the torque-transmission shaft may be also designed as a torque-transmission shaft on a rear stage of a reduction gear set joined to the input shaft.  
      Moreover, the intermediate shaft includes all shafts disposed between the torque-transmission shaft on the electric motor side (or the above-mentioned input shaft) and the differential case of the differential device. In other words, the intermediate shaft should not be limited to only one shaft, but multiple intermediate shafts are also applicable.  
      Therefore, the number of sets of shafts from the input shaft directly joined to the electric motor to the differential case (a shaft at the last stage) of the differential device may be variable.  
      For example, in a model having five stages from the input shaft to the differential case, it is possible to use the input shaft as the torque-transmission shaft on the electric motor side and to use three other shafts between the input shaft and the differential case as the intermediate shafts. Otherwise, it is also possible to use the third shaft from the input shaft as the torque-transmission shaft on the electric motor side and to use one shaft remaining between the torque-transmission shaft and the differential case as the intermediate shaft.  
      As described above, if the number of sets of the shafts (the number of stages of the reduction gear sets) is increased more, then a larger speed reducing function (a torque amplifying function) is obtainable. In addition, loads on the respective reduction gear sets are reduced. Accordingly, durability of the reduction gear sets is improved.  
      The preferred embodiment described herein is therefore illustrative and not restrictive, the scope of the invention being indicated by the claims and all variations which come within the meaning of claims are intended to be embraced therein.  
      The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-280535, filed on Sep. 14, 2001, the disclosure of which is expressly incorporated herein by reference in its entirety.