Abstract:
A power transmission device is embedded in an electric vehicle equipped with an electric motor and a transmission. The transmission includes a first reduction mechanism and a second reduction mechanism. The power transmission device also includes a spline hub coupled to an output side of the first reduction mechanism, a clutch plate coupled to an input side of the second reduction mechanism, and a coupling portion. The coupling portion is disposed between the spline hub and the clutch plate, and includes a damper mechanism configured to absorb a vibration from the spline hub and transmit a torque to the clutch plate, and a torque limiter configured to transmit the torque and to limit transmission of the torque when the torque is greater than or equal to a predetermined magnitude.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National stage application of International Application No. PCT/JP2013/061196, filed Apr. 15, 2013, which claims priority to Japanese Patent Application No. 2012-093465, filed in Japan on Apr. 17, 2012, the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to a power transmission device, particularly to a power transmission device that is configured to transmit a driving force and is embedded in an electric vehicle including an electric motor and a transmission into which a rotation from the electric motor is inputted. 
         [0004]    2. Background Information 
         [0005]    Electric vehicles, using an electric motor as a power source, have been produced in recent years. Further, this type of vehicle is also provided with a transmission for obtaining an optimal torque characteristic in accordance with a variety of travelling conditions. 
         [0006]    For example, Japanese Utility Model Application Publication 
         [0007]    No. JP-U-S59-172853 describes a two stage transmission including an input shaft, an output shaft, a planetary gear unit, a cone clutch, a one-way clutch and a control unit. 
         [0008]    On the other hand, Japanese Laid-open Patent Application Publication No. JP-A-H06-249302 describes a gear transmission for an electric vehicle, which includes a gear drive train for starting and a gear drive train for high speed. 
       SUMMARY 
       [0009]    An electric vehicle vibrates less than a vehicle using an engine as a drive source. However, cogging occurs in an electric motor. Cogging is a phenomenon in which a magnetic attraction force, generated between an armature and a rotor, minutely pulsates depending on a rotational angle. Such a phenomenon not only affects comfortableness in riding but also becomes a cause of reducing durability of components. 
         [0010]    Further, in general, unlike an engine-driven vehicle, the electric vehicle does not need a starting clutch due to the characteristic of the electric motor. Hence, components are mechanically direct-coupled from the electric motor to drive wheels. In the structure, the components composing the drive train may be damaged when an excessive torque, generated in the electric motor or the drive wheels, is transmitted thereto. 
         [0011]    It is an object of the present invention to inhibit occurrence of cogging attributed to an electric motor or prevent damage of respective components of a drive train attributed to transmission of an excessive torque in an electric vehicle. 
         [0012]    A power transmission device for an electric vehicle according to a first aspect of the present invention is a device that is configured to transmit a driving force and is embedded in the electric vehicle equipped with an electric motor and a transmission into which a rotation from the electric motor is inputted. The power transmission device includes an input portion coupled to an output shaft of the electric motor, an output portion disposed between the input portion and the transmission, and a coupling portion. The coupling portion is disposed between the input portion and the output portion, and includes at least either of a damper mechanism configured to absorb a vibration from the input portion and transmit a torque to the output portion, and a torque limiter that is configured to transmit the torque and is configured to limit transmission of the torque when the torque is greater than or equal to a predetermined magnitude. 
         [0013]    A power transmission device for an electric vehicle according to a second aspect of the present invention is a device that is configured to transmit a driving force and is embedded in the electric vehicle equipped with an electric motor and a transmission. The transmission includes a first reduction mechanism configured to decelerate a rotation from the electric motor and transmit the decelerated rotation, a second reduction mechanism configured to further decelerate the rotation from the first reduction mechanism and transmit the further decelerated rotation, and an output mechanism configured to transmit the rotation from the second reduction mechanism to a drive wheel. Further, the power transmission device includes an input portion coupled to an output side of the first reduction mechanism, an output portion coupled to an input side of the second reduction mechanism, and a coupling portion. The coupling portion is disposed between the input portion and the output portion, and includes at least either of a damper mechanism configured to absorb a vibration from the input portion and transmit a torque to the output portion, and a torque limiter that is configured to transmit the torque and is configured to limit transmission of the torque when the torque is greater than or equal to a predetermined magnitude. 
         [0014]    A power transmission device for an electric vehicle according to a third aspect of the present invention relates to the device of the second aspect. The transmission includes a first shaft into which the rotation from the electric motor is inputted, an input gear configured to be rotated in synchronization with the first shaft, a second shaft disposed in parallel to the first shaft, a reduction gear that is configured to be rotated in synchronization with the second shaft and is meshed with the input gear, an intermediate gear rotatably disposed on the second shaft, and an output gear that is coupled to the output mechanism and is meshed with the intermediate gear. Further, the input portion is coupled to the second shaft, whereas the output portion is fixed to the intermediate gear. 
         [0015]    A power transmission device for an electric vehicle according to a fourth aspect of the present invention relates to the device of the third aspect. The reduction gear is mounted to one end part of the second shaft. The intermediate gear is disposed adjacently to the reduction gear. The input portion is mounted to the other end part of the second shaft while being disposed on a side away from the reduction gear with respect to the intermediate gear. 
         [0016]    A power transmission device for an electric vehicle according to a fifth aspect of the present invention relates to the device of the third aspect. The reduction gear is mounted to one end part of the second shaft. The intermediate gear is rotatably supported by the other end part of the second shaft. The input portion is disposed adjacently to the reduction gear. 
         [0017]    A power transmission device for an electric vehicle according to a sixth aspect of the present invention is a device that is configured to transmit a driving force and is embedded in the electric vehicle equipped with an electric motor and a transmission. The transmission includes a first reduction mechanism configured to decelerate a rotation from the electric motor and transmit the decelerated rotation, a second reduction mechanism configured to further decelerate the rotation from the first reduction mechanism and transmit the further decelerated rotation, and an output mechanism configured to transmit the rotation from the second reduction mechanism to a drive wheel. Further, the power transmission device includes an input portion into which the rotation from the second reduction mechanism is inputted and that is rotatably supported by the output mechanism, an output portion coupled to the output mechanism, and a coupling portion. The coupling portion is disposed between the input portion and the output portion, and includes at least either of a damper mechanism configured to absorb a vibration from the input portion and transmit a torque to the output portion, and a torque limiter that is configured to transmit the torque and is configured to limit transmission of the torque when the torque is greater than or equal to a predetermined magnitude. 
         [0018]    As described above, in the present invention, at least either of the damper mechanism and the torque limiter is provided in the drive train disposed between the electric motor and the drive wheel. Therefore, where the damper mechanism is provided, the occurrence of cogging can be inhibited. Where the torque limiter is provided, damage of respective components attributed to an excessive torque transmitted thereto can be prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a cross-sectional structural view of a drive system for an electric vehicle including a power transmission device according to a first exemplary embodiment of the present invention. 
           [0020]      FIG. 2  is a partial enlarged view of the first exemplary embodiment. 
           [0021]      FIG. 3  is a cross-sectional structural view of a drive system for an electric vehicle including a power transmission device according to a second exemplary embodiment of the present invention. 
           [0022]      FIG. 4  is a cross-sectional structural view of a drive system for an electric vehicle including a power transmission device according to a third exemplary embodiment of the present invention. 
           [0023]      FIG. 5  is a cross-sectional structural view of a drive system for an electric vehicle including a power transmission device according to a fourth exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Exemplary Embodiment 
       [0024]      FIG. 1  illustrates a drive system for an electric vehicle including a power transmission device according to a first exemplary embodiment of the present invention. The drive system includes an electric motor  1  and a transmission  2 . Further, a power transmission device  3  is disposed between the electric motor  1  and the transmission  2 . In the drive system, a rotation of the electric motor  1  is configured to be decelerated by the transmission  2 , and the decelerated rotation is configured to be transmitted to right and left axles  5  and  4 . Wheels (not illustrated in the drawings) are coupled to the right and left axles  5  and  4 . 
         [0025]    The transmission  2  includes an input shaft  11 , an input gear  12 , an intermediate shaft  13 , a reduction gear  14 , an intermediate gear  15 , an output gear  16  and a differential device  17 . Further, the input gear  12  and the reduction gear  14  compose a first reduction mechanism  21 , whereas the intermediate shaft  13 , the intermediate gear  15  and the output gear  16  compose a second reduction mechanism  22 . 
         [0026]    The input shaft  11  is formed in a tubular shape; and both ends thereof are rotatably supported by a housing  2   a  of the transmission  2  through a pair of bearings. The input shaft  11  has a spline hole formed on a motor-side part of the inner peripheral surface thereof. The input gear  12  is integrally formed with the input shaft  11 . 
         [0027]    The intermediate shaft  13  is formed in a tubular shape, and both ends thereof are rotatably supported by the housing  2   a  of the transmission  2  through a pair of bearings. 
         [0028]    The reduction gear  14  and the intermediate gear  15  are disposed to be rotated in synchronization with the intermediate shaft  13 . Specifically, the reduction gear  14  is spline-coupled to the intermediate shaft  13 , whereas the intermediate gear  15  is disposed on the outer peripheral part of the intermediate shaft  13  while being integrally formed with the intermediate shaft  13 . The reduction gear  14  is meshed with the input gear  12 . The intermediate gear  15  is meshed with the output gear  16 . 
         [0029]    The differential device  17  includes a case  24  and a differential gear mechanism  25  accommodated inside the case  24 . The output gear  16  is fixed to the case  24 . Further, the right and left axles  5  and  4  are coupled to the differential gear mechanism  25 . 
         [0030]      FIG. 2  illustrates an enlarged view of the power transmission device  3 . The power transmission device  3  includes a damper mechanism  31  and a torque limiter  32 . 
         [0031]    The damper mechanism  31  has a heretofore known structure and includes a spline hub  34  as an input portion, a pair of plates  35  disposed on both sides of the flange of the spline hub  34 , and a plurality of torsion springs  36  elastically coupling the spline hub  34  and the pair of plates  35  in a rotational direction. It should be noted that a hysteresis torque generating mechanism  37  for absorbing vibrations is disposed between the spline hub  34  and the pair of plates  35 . 
         [0032]    The torque limiter  32  includes a tubular case  38 , a coupling member  39  as an output portion, and a torque limiting portion  40  disposed between the case  38  and the coupling member  39 . 
         [0033]    The motor-side end of the tubular case  38  is bent to the inner peripheral side, and the bent part is coupled to one of the pair of plates  35  of the damper mechanism  31 . 
         [0034]    The coupling member  39  is rotatably supported by an output shaft la of a motor  1  through a bearing. The coupling member  39  has a shaft part  39   a  and a flange part  39   b  formed on the tip end of the shaft part  39   a.  The shaft part  39   a  has a spline shaft formed on the outer periphery thereof, and the spline shaft is spline-coupled to the spline hole of the input shaft  11  of the transmission  2 . 
         [0035]    The torque limiting portion  40  includes a plurality of clutch plates  42   a  and  42   b,  a backing plate  43 , a pressure plate  44  and a cone spring  45 . Regarding the plural clutch plates  42   a  and  42   b,  the drive-side plates  42   a  are engaged with the case  38 , whereas the driven-side plates  42   b  are engaged with the flange part  39   b  of the coupling member  39 . The cone spring  45  is set in a compressed state between the pressure plate  44  and the bent part of the case  38 . Accordingly, when a torque, which is greater than or equal to a torque set by a pressing load of the cone spring  45  and the clutch plates  42   a  and  42   b,  is inputted into the torque limiting portion  40 , the torque limiting portion  40  is configured to slip and the torque is not transmitted to either the transmission- 2  side or the motor side. 
         [0036]    In the device as described above, the rotation of the motor  1  is configured to be transmitted to the transmission  2  through the damper mechanism  31  and the torque limiter  32 . In the transmission  2 , the rotation of the motor  1  is configured to be decelerated by the first reduction mechanism  21  and the second reduction mechanism  22 , and the decelerated rotation is configured to be inputted into the differential device  17 . In the differential device  17 , a torque is distributed and transmitted to the respective axles  4  and  5  in accordance with loads acting on respective drive wheels. 
         [0037]    In the drive system of the first exemplary embodiment, the power transmission device  3 , including the damper mechanism  31  and the torque limiter  32 , is disposed between the motor  1  and the transmission  2 . 
         [0038]    Hence, occurrence of cogging of the motor  1  can be inhibited, and damage of respective components can be prevented by limiting excessive torque transmission to the respective components. Further, the power transmission device  3  is disposed in the input part of the drive system. Hence, a torque to be transmitted becomes relatively small, and the capacity of the torque limiter  32  can be reduced. Yet further, due to a reason similar to the above, the damper mechanism  31  can be compactly formed. 
       Second Exemplary embodiment 
       [0039]      FIG. 3  illustrates a drive system to which a power transmission device  103  according to a second exemplary embodiment of the present invention is applied. The drive system includes the electric motor  1  and a transmission  102 . Further, the power transmission device  103  is disposed inside the transmission  102 . In the drive system, the rotation of the electric motor  1  is configured to be decelerated by the transmission  102 , and the decelerated rotation is configured to be transmitted to the right and left axles  5  and  4 . In the second exemplary embodiment, the same reference signs are assigned to elements similar to those in the first exemplary embodiment, and explanation will not be made for the elements similar to those in the first exemplary embodiment. 
         [0040]    The transmission  102  includes an input shaft  111 , an input gear  112 , an intermediate shaft  113 , a reduction gear  114 , an intermediate gear  115 , the output gear  16  and the differential device  17 . The input gear  112  and the reduction gear  114  compose a first reduction mechanism  121 , whereas the intermediate shaft  113 , the intermediate gear  115  and the output gear  16  compose a second reduction mechanism  122 . 
         [0041]    The input shaft  111  is formed in a tubular shape, and both ends thereof are rotatably supported by a housing  102   a  of the transmission  102  through a pair of bearings. The inner peripheral part of the input shaft  111  and the output shaft la of the motor  1  are spline-coupled. The input gear  112  is disposed on the outer peripheral part of the input shaft  111 , while being integrally formed with the input shaft  111 . 
         [0042]    The intermediate shaft  113  is formed in a tubular shape, and both ends thereof are rotatably supported by the housing  102   a  of the transmission  102  through a pair of bearings. 
         [0043]    The reduction gear  114  is disposed on one end part of the intermediate shaft  113 , while being integrally formed with the intermediate shaft  113 . The intermediate gear  115  is disposed laterally adjacent to the reduction gear  114 . The intermediate gear  115  is supported by the intermediate shaft  113 , while being rotatable relatively thereto. The reduction gear  114  is meshed with the input gear  112 . The intermediate gear  115  is meshed with the output gear  16 . 
         [0044]    The power transmission device  103  is disposed on the opposite side of the reduction gear  114  with respect to the intermediate gear  115 . The power transmission device  103  has a basic structure similar to that in the first exemplary embodiment, and includes the damper mechanism  31  and the torque limiter  32 . 
         [0045]    The spline hub  34  of the damper mechanism  31  is spline-coupled to the intermediate shaft  113 . 
         [0046]    Further, the output side (the driven-side plates  42   b  included in the plural clutch plates) of the torque limiter  32  is engaged with a flange  130  fixed to the lateral surface of the intermediate gear  15 . The flange  130  has a disc-shaped main body  130   a  having an aperture in the center part thereof, and a tubular part  130   b  formed on an end of the outer periphery of the main body  130   a  to axially extend therefrom. 
         [0047]    The inner peripheral part of the main body  130   a  is fixed to the lateral surface of the intermediate gear  115 . Further, the tubular part  130   b  has a plurality of teeth formed on the outer periphery thereof, and the teeth are engaged with the inner peripheral parts of the driven-side plates  42   b  included in the plural clutch plates. 
         [0048]    In the device as described above, the rotation of the motor  1  is configured to be decelerated by the first reduction mechanism  121  of the transmission  102 , and the decelerated rotation is configured to be inputted into the damper mechanism  31  of the power transmission device  103 . Further, the rotation is transmitted to the second reduction mechanism  122  through the torque limiter  32 , and is further inputted into the differential device  17 . In the differential device  17 , a torque is distributed and transmitted to the respective axles  4  and  5  in accordance with loads acting on the respective drive wheels . 
         [0049]    Similar to the first exemplary embodiment, the drive system of the second exemplary embodiment can inhibit occurrence of cogging of the motor  1 , and can prevent damage of the respective components by limiting excessive torque transmission to the respective components. Further, the power transmission device  103  is mounted onto the intermediate shaft  113  to which the rotation decelerated by the first reduction mechanism  121  is transmitted. Hence, a torque to be transmitted becomes large, but the rotation speed becomes relatively low. Thus, strengths of the respective components can be lowered, and cost reduction and weight reduction are enabled. In an electric vehicle, the rotation speed of the motor  1  tends to be higher than the rotation speed of the engine. Therefore, the second exemplary embodiment is especially effective in that the rotation speed of the power transmission device  103  becomes low. 
       Third Exemplary Embodiment 
       [0050]      FIG. 4  illustrates a drive system to which a power transmission device  203  according to a third exemplary embodiment of the present invention is applied. The drive system includes the electric motor  1  and a transmission  202 . Further, the power transmission device  203  is disposed inside the transmission  202 . In the drive system, the rotation of the electric motor  1  is configured to be decelerated by the transmission  202 , and the decelerated rotation is configured to be transmitted to the right and left axles  5  and  4 . 
         [0051]    In the third exemplary embodiment, the same reference signs are assigned to elements similar to those in the first and second exemplary embodiments, and explanation will not be made for the elements similar to those in the first and second exemplary embodiments. 
         [0052]    The transmission  202  includes an input shaft  211 , an input gear  212 , an intermediate shaft  213 , a reduction gear  214 , an intermediate gear  215 , the output gear  16  and the differential device  17 . The input gear  212  and the reduction gear  214  compose a first reduction mechanism  221 , whereas the intermediate shaft  213 , the intermediate gear  215  and the output gear  16  compose a second reduction mechanism  222 . 
         [0053]    The specific shapes of the respective members in the third exemplary embodiment are different from those of the corresponding members in the second exemplary embodiment. However, the other structures in the third exemplary embodiment are basically the same as those in the second exemplary embodiment, although the arrangement of the power transmission device  203  in the third exemplary embodiment is only different from that of the power transmission device in the second exemplary embodiment. 
         [0054]    In short, in the third exemplary embodiment, the reduction gear  214  and the intermediate gear  215  are disposed on both ends of the intermediate shaft  213 , while the power transmission device  203  is disposed between these gears  214  and  215 . 
         [0055]    The power transmission device  203  has a structure similar to that in the aforementioned respective exemplary embodiments, and includes the damper mechanism  31  and the torque limiter  32 . A path for transmitting power is configured similarly to that in the second exemplary embodiment. Power is inputted from the intermediate shaft  213  to the spline hub of the damper mechanism  31 , and is then outputted from the output portion (the driven-side plates) of the torque limiter  32  to a tubular member  230  fixed to the intermediate gear  215 . The tubular member  230  has a fixation part  230   a  fixed to a reduction gear  214  side lateral surface of the intermediate gear  215 , and a tubular engaging part  230   b  axially extending from the outer periphery of the fixation part  230   a.  Further, the tubular engaging part  230   b  has a plurality of teeth formed on the outer periphery thereof, and the teeth are engaged with the inner peripheries of the driven-side clutch plates of the torque limiter  32 . 
         [0056]    The power transmission path of the aforementioned device is similar to that in the second exemplary embodiment. Specifically, the rotation of the motor  1  is configured to be decelerated by the first reduction mechanism  221  of the transmission  202 , and the decelerated rotation is configured to be inputted into the damper mechanism  31  of the power transmission device  203 . Further, the rotation is configured to be transmitted to the second reduction mechanism  222  through the torque limiter  32 , and is further inputted into the differential device  17 . In the differential device  17 , a torque is distributed and transmitted to the respective axles  4  and  5  in accordance with loads acting on the respective drive wheels. 
         [0057]    The drive system of the third exemplary embodiment can also achieve advantages effects similar to those achieved by the drive system of the second exemplary embodiment. In short, occurrence of cogging of the motor  1  can be inhibited, while damage of the respective components can be prevented by limiting excessive torque transmission to the respective components. Further, the rotation speed of the power transmission device  203  becomes low. Thus, the component strengths of the respective components can be lowered, and cost reduction and weight reduction are enabled. 
       Fourth Exemplary Embodiment 
       [0058]      FIG. 5  illustrates a drive system to which a power transmission device  303  according to a fourth exemplary embodiment of the present invention is applied. The drive system includes the electric motor  1  and a transmission  302 . Further, the power transmission device  303  is disposed inside the transmission  302 . In the drive system, the rotation of the electric motor  1  is configured to be decelerated by the transmission  302 , and the decelerated rotation is configured to be transmitted to the right and left axles  5  and  4 . In the fourth exemplary embodiment, the same reference signs are assigned to elements similar to those in the aforementioned respective exemplary embodiments, and explanation will not be made for the elements similar to those in the aforementioned respective exemplary embodiments. 
         [0059]    The transmission  302  includes an input shaft  311 , an input gear  312 , an intermediate shaft  313 , a reduction gear  314 , an intermediate gear  315 , an output gear  316  and the differential device  17 . The input gear  312  and the reduction gear  314  compose a first reduction mechanism  321 , whereas the intermediate shaft  313 , the intermediate gear  315  and a part of the power transmission device  303  compose a second reduction mechanism  322 . 
         [0060]    The input shaft  311  is formed in a tubular shape, and both ends thereof are rotatably supported by a housing  302   a  of the transmission  302  through a pair of bearings. The inner peripheral part of the input shaft  311  and the output shaft la of the motor  1  are spline-coupled. The input gear  312  and the input shaft  311  are integrally formed. 
         [0061]    The intermediate shaft  313  is formed in a tubular shape, and both ends thereof are rotatably supported by the housing  302   a  of the transmission  302  through a pair of bearings. The reduction gear  314  is disposed on one end part of the intermediate shaft  313 , while being integrally formed with the intermediate shaft  313 . The reduction gear  314  is meshed with the input gear  312 . The intermediate gear  315  is disposed on the other end part of the intermediate shaft  313 . The intermediate gear  315  is spline-coupled to the intermediate shaft  313 . 
         [0062]    The power transmission device  303  includes a damper mechanism  331  and a torque limiter  332 . 
         [0063]    The damper mechanism  331  includes a spline hub  334  as an input portion, a pair of plates  335  disposed on the both sides of the flange of the spline hub  334 , and a plurality of torsion springs  336  elastically coupling the spline hub  334  and the pair of plates  335  in the rotational direction. 
         [0064]    The inner peripheral part of the spline hub  334  is rotatably supported by the case  24  of the differential device  17  through a bearing. Further, the spline hub  334  has a hub gear  334   a  on the outer peripheral part thereof, and the hub gear  334   a  is meshed with the intermediate gear  315 . 
         [0065]    It should be noted that a hysteresis torque generating mechanism for absorbing vibrations is disposed between the spline hub  334  and the pair of the plates  335 . 
         [0066]    The torque limiter  332  has a structure similar to the structures of the torque limiters in the respective exemplary embodiments. The torque limiter  332  includes a tubular case, a torque limiting portion having a plurality of clutch plates, and so forth. Further, the driven-side plates included in the plural clutch plates are meshed with the output gear  316  fixed to the case of the differential device  17 . 
         [0067]    In the device as described above, the rotation of the motor  1  is configured to be decelerated by the first reduction mechanism  321  of the transmission  302 , and the decelerated rotation is configured to be inputted into the damper mechanism  331  of the power transmission device  303  through the intermediate gear  315  and the hub gear  334   a.    
         [0068]    Further, the rotation is configured to be inputted into the output gear  316  and the differential device  17  through the torque limiter  332 . In the differential device  17 , a torque is distributed and transmitted to the respective axles  4  and  5  in accordance with loads acting on the respective drive wheels. 
         [0069]    Similarly to the drive systems of the aforementioned respective exemplary embodiments, the drive system of the fourth exemplary embodiment can inhibit occurrence of cogging of the motor  1 , and can prevent damage of the respective components by limiting excessive torque transmission to the respective components. Further, the power transmission device  303  is herein disposed downstream of the first and second reduction mechanism  321  and  322  in the power transmission flow. Therefore, the rotation speed of the power transmission device  303  becomes low. Thus, the component strengths of the respective components can be lowered, and cost reduction and weight reduction are enabled. Other Exemplary Embodiment 
         [0070]    The present invention is not limited to the exemplary embodiments as described above, and a variety of changes or modifications can be made without departing from the scope of the present invention. 
         [0071]    In the power transmission device of the present invention, at least either of the damper mechanism and the torque limiter is disposed in the drive train disposed between the electric motor and the drive wheels. Therefore, where the damper mechanism is provided, it is possible to inhibit occurrence of cogging. Where the torque limiter is provided, it is possible to prevent damage of respective components attributed to an excessive torque transmitted thereto.