Patent Publication Number: US-11027616-B2

Title: Vehicle driving device

Description:
BACKGROUND 
     The disclosure relates to a vehicle driving device provided with: a rotary electric machine that serves as a driving force source of a first wheel and a second wheel; a speed reduction device that reduces the speed of the rotation of the rotary electric machine; and a differential gear device that distributes to the first wheel and the second wheel, a drive force from the rotary electric machine that is transmitted via the speed reduction device. 
     An example of the vehicle driving device is disclosed in Japanese Unexamined Patent Application Publication No. 10-287142 (JP 10-287142 A). Hereinafter, symbols indicated with brackets in the description of the background art are the symbols that are used in JP 10-287142 A. A vehicle driving device according to JP 10-287142 A has a motor (M), a differential device (Gd), and a counter gear mechanism (Gc) that transmits the power of the motor (M) to the differential device (Gd). The counter gear mechanism (Gc) configures a speed reduction device that reduces the speed of the rotation of the motor (M) and transmits the rotation to the differential device (Gd). As illustrated in FIGS. 1 and 2 in JP 10-287142 A, the motor (M), the differential device (Gd), and the counter gear mechanism (Gc) are disposed separately on three different axes that are parallel to each other. 
     While taking the on-vehicle mountability of the vehicle driving device into consideration, it is preferable that the size of the entire device be reduced as much as possible. Regarding this point, in the vehicle driving device according to JP 10-287142 A, the three devices that are the rotary electric machine, the speed reduction device (the counter gear mechanism in JP 10-287142 A), and the differential gear device are disposed separately on three different axes that are parallel to each other. Thus, the size of entire device is likely to be increased in a radial direction. In order to decrease the size of the entire device in the radial direction, it is conceivable to dispose the three devices on the same axis. In this case however, the size of the entire device is likely to be increased in the axial direction. 
     SUMMARY 
     An exemplary aspect of the disclosure provides a vehicle driving device in which the size of the entire device in both the radial direction and the axial direction can be decreased. 
     In one aspect, in view of the above, a vehicle driving device includes: a rotary electric machine that is a driving force source of a first wheel and a second wheel; a speed reduction device that reduces a speed of a rotation of the rotary electric machine; and a differential gear device that distributes to the first wheel and the second wheel, a driving force that is transmitted from the rotary electric machine via the speed reduction device, in which the speed reduction device and the differential gear device are disposed coaxially with the rotary electric machine, the speed reduction device has a first planetary gear mechanism and a second planetary gear mechanism, the first planetary gear mechanism is disposed on a rotary electric machine side relative to the second planetary gear mechanism in an order of a power transmission path, and the first planetary gear mechanism is disposed on the rotary electric machine side relative to the second planetary gear mechanism along an axial direction, a helix angle of a helical tooth of the first planetary gear mechanism and a helix angle of a helical tooth of the second planetary gear mechanism are formed so that a thrust force of the first planetary gear mechanism and a thrust force of the second planetary gear mechanism are in mutually opposite directions in the axial direction, and the helix angle of the helical tooth of the first planetary gear mechanism is larger than the helix angle of the helical tooth of the second planetary gear mechanism. 
     In this configuration, the speed reduction device and the differential gear device are disposed coaxially with the rotary electric machine. Thus, compared to when the three devices are disposed separately on three axes parallel to each other, it is possible to suppress the size of the vehicle driving device in a radial direction to be small. Moreover, in this configuration, it is possible to decrease the size of the thrust bearing as described below. Thus, it is possible to suppress the size of the vehicle driving device from being large in the axial direction while disposing the three devices on the same axis. 
     Due to its structure, the helical gear generates a thrust force that is parallel to the rotational axis. When the directions of the thrust forces generated by the first planetary gear mechanism and the second planetary gear mechanism, which are both configured using a helical gear, are mutually opposite directions, it is possible to cause the thrust forces to cancel each other out. Typically, a thrust bearing is provided adjacent to the helical gear in the axial direction in order to receive a load by the thrust force from the helical gear in the axial direction. When the generated thrust force is large, the load applied to the thrust bearing becomes large, thus affecting the durability of the speed reduction device and the vehicle driving device. If the size of the thrust bearing is increased, downsizing of the vehicle driving device is hindered. With the configuration, it is possible to decrease the load applied to the thrust bearings by causing a part of the thrust forces of the two planetary gear mechanisms to cancel each other out. In addition, the size of the thrust bearing can be reduced so that the length of the vehicle driving device in the axial direction can be suppressed to be short. 
     The size of the thrust force is correlated to the transmission torque and the larger the transmission torque is, the larger the thrust force becomes. The second planetary gear mechanism is disposed on the output side of the speed reduction device relative to the first planetary gear mechanism in the order of the power transmission path. The driving force of the rotary electric machine after the speed thereof is reduced in the first planetary gear mechanism, is transmitted to the second planetary gear mechanism. Thus, the transmission torque is larger in the second planetary gear mechanism compared to the first planetary gear mechanism. The thrust force of the second planetary gear mechanism is therefore larger than the thrust force of the first planetary gear mechanism. The size of the thrust force is correlated to the helix angle of the helical tooth of the helical gear, and the larger the helix angle is, the larger the thrust force becomes. When a first helix angle that is a helix angle of the helical tooth of the first planetary gear mechanism is larger than a second helix angle that is a helix angle of the helical tooth of the second planetary gear mechanism, the proportion of the thrust force among forces output from the gear mechanism is larger in the first planetary gear mechanism than in the second planetary gear mechanism. That is, with the present configuration, it is possible to decrease the difference in the thrust force resulting from the difference in the transmission torque, with the helix angle of the helical tooth. This means that it is possible to decrease the load applied to the thrust bearings by causing the thrust forces of the two planetary gear mechanisms to cancel each other out. In addition, the size of the thrust bearing can be reduced so that the length of the vehicle driving device in the axial direction can be suppressed to be short. 
     In this way, with the configuration, it is possible to decrease the size of the entire vehicle driving device in both the radial direction and the axial direction. 
     Further features and advantages of the vehicle driving device will be apparent from the following description of embodiments which is given with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an axial sectional view of a vehicle driving device. 
         FIG. 2  is a skeleton view of the vehicle driving device. 
         FIG. 3  is an axial sectional view of a speed reduction device. 
         FIG. 4  is an explanatory diagram of a configuration of helical teeth of the speed reduction device and thrust forces. 
         FIG. 5  is an axial sectional view of another configuration of the speed reduction device. 
         FIG. 6  is an axial sectional view of an example of a differential mechanism device of a planetary gear mechanism. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of a vehicle driving device will be described based on the drawings.  FIG. 1  is an axial sectional view of a vehicle driving device  100 .  FIG. 2  is a skeleton view of the vehicle driving device  100 . The vehicle driving device  100  is a vehicle driving device that is installed in a hybrid vehicle having an internal combustion engine and a rotary electric machine serving as a driving force source of a first wheel  501  and a second wheel  502 , or an electric vehicle having a rotary electric machine serving as a driving force source of the first wheel  501  and the second wheel  502 . As illustrated in  FIGS. 1 and 2 , the vehicle driving device  100  only has a rotary electric machine  2  serving as a driving force source of the first wheel  501  and the second wheel  502 . In a case of a four-wheel vehicle with a two-wheel drive, an electric vehicle can thus be realized. In a case of a four-wheel vehicle with a four-wheel drive, a hybrid vehicle can be realized by driving the other two wheels with a driving force of an internal combustion engine. As a matter of course, in the case of a four-wheel vehicle with a four-wheel drive, an electric vehicle with a four-wheel drive can be realized by applying the vehicle driving device  100  of the embodiment to the other two wheels as well. 
     In the following description, the term “drivingly coupled” refers to a state in which two rotary elements are coupled to each other in such a manner that allows transmission of a driving force, including a state in which the two rotary elements are coupled to each other, and a state in which the two rotary elements are coupled to each other via one or two or more transmitting members in such a manner than allow transmission of a driving force. Such transmitting members include various members that transmit rotation at the same speed or a shifted speed and include a shaft, a gear mechanism, a belt, a chain, etc. Such transmitting members may include an engagement device that selectively transmits rotation and a driving force, such as, a friction engagement device and a meshing engagement device. When the expression “drivingly coupled” is used for each rotary element of a speed reduction device  3  and a differential gear device  4  described below, this refers to the state where the three or more rotary elements of the devices are drivingly coupled to each other without via other rotary elements. 
     In the following description, the expressions “tubular shape” and “cylindrical shape” mean that the overall general shape is a tube or a cylinder even when there are some deformed parts. Not limited to this, the same applies to other expressions related to shapes that are used by applying the term “shape”. 
     As illustrated in  FIGS. 1 and 2 , the vehicle driving device  100  is provided with a case  1 , the rotary electric machine  2  that has a rotor shaft  27  for outputting a driving force, the speed reduction device  3  including a planetary gear mechanism, and the differential gear device  4 . The speed reduction device  3  reduces the speed of a rotation of the rotary electric machine  2  and transmits a driving force to the differential gear device  4 . The differential gear device  4  distributes the driving force from the rotary electric machine  2  to a first drive shaft  51  and a second drive shaft  52 . In the embodiment, the first drive shaft  51  is drivingly coupled to a distribution output shaft  53 . The differential gear device  4  distributes the driving force to the second drive shaft  52  and the distribution output shaft  53  and the distributed driving force is transmitted to the first drive shaft  51  via the distribution output shaft  53 . 
     In the vehicle driving device  100  of the embodiment, the rotary electric machine  2  the speed reduction device  3 , the differential gear device  4 , the first drive shaft  51 , the second drive shaft  52 , and the distribution output shaft  53  are coaxially disposed with reference to a rotor shaft  27  of the rotary electric machine  2 . A direction along the rotor shaft  27  of the rotary electric machine  2  is equal to a direction along a rotational axis of the vehicle driving device  100 . A direction along a radius of the rotor shaft  27  of the rotary electric machine  2  is equal to a direction along a radius of the vehicle driving device  100 . In the embodiment, the direction along the rotor shaft  27  of the rotary electric machine  2  is referred to as an axial direction L of the vehicle driving device  100 . The direction along the radius of the rotor shaft  27  of the rotary electric machine  2  is referred to as a radial direction R of the vehicle driving device  100 . In the axial direction L, a side in which the rotary electric machine  2  is disposed relative to the speed reduction device  3  is referred to as an axial first side L 1  and a side in which the differential gear device  4  is disposed relative to the speed reduction device  3  is referred to as an axial second side L 2 . In the radial direction R, an outer side on the opposite of the rotor shaft  27  is referred to as a radial outer side R 1  and an inner side of the rotor shaft  27  side is referred to as a radial inner side R 2 . 
     In the vehicle driving device  100  of the embodiment, the rotary electric machine  2 , the speed reduction device  3 , and the differential gear device  4  are disposed so as to be arranged in this order that is the order of a power transmission path. As described below, the speed reduction device  3  has a first planetary gear mechanism  31  and a second planetary gear mechanism  32 . With this in consideration, the rotary electric machine  2 , the first planetary gear mechanism  31 , the second planetary gear mechanism  32 , and the differential gear device  4  are disposed so as to be arranged in this order that is the order of the power transmission path. The rotary electric machine  2 , the speed reduction device  3  (the first planetary gear mechanism  31  and the second planetary gear mechanism  32 ), and the differential gear device  4  are disposed along the axial direction L so as to be arranged in the order of the rotary electric machine  2 , the first planetary gear mechanism  31 , the second planetary gear mechanism  32 , and the differential gear device  4 . That is, in either the order of the power transmission path or the order of disposition along the axial direction L, the first planetary gear mechanism  31  is disposed on the rotary electric machine  2  side relative to the second planetary gear mechanism  32 . 
     The case  1  houses the rotary electric machine  2 , the speed reduction device  3 , and the differential gear device  4  inside. In the embodiment, the case  1  also houses a part of the first drive shaft  51  (an end portion on the axial second side L 2 ), a part of the second drive shaft  52  (an end portion on the axial first side L 1 ), and the distribution output shaft  53  inside. The case  1  is formed so as to have a peripheral wall portion  10  having a tubular shape surrounding the rotary electric machine  2 , the speed reduction device  3 , and radial outer side R 1  of the differential gear device  4 . The case  1  has a case body  11  (first case portion), a body cover  12  (second case portion), and a bottom portion cover  13 . The case body  11  is formed to have a bottomed tubular shape and have a bottom portion  11   a  on an end portion on the axial first side L 1 , and has an opening portion on the opposite side (axial second side L 2 ) of the case body  11  from the bottom portion  11   a . The body cover  12  is disposed so as to abut against the case body  11  on the axial first side L 1  and cover the opening portion, and is formed to have a conical tubular shape in which the diameter is decreased toward the axial second side L 2 . The bottom portion cover  13  is disposed on the axial first side L 1  of the bottom portion  11   a  of the case body  11  so as to cover the bottom portion  11   a . The case body  11  and the body cover  12  are fixed to each other with a fixing member (in the embodiment, a bolt). Similarly, the case body  11  and the bottom portion cover  13  are fixed to each other with a fixing member (in the embodiment, a bolt). 
     The rotary electric machine  2  and a part (first planetary gear mechanism  31 ) of the speed reduction device  3  are disposed in the internal space of the case body  11 . Another part (second planetary gear mechanism  32 ) of the speed reduction device  3 , the differential gear device  4 , and a part (the end portion on the axial first side L 1 ) of the second drive shaft  52  are disposed in the internal space of the body cover  12 . A part (an end portion on the axial second side L 2 ) of the first drive shaft  51  is disposed in the internal space that is formed by the case body  11  and the bottom portion cover  13 . The distribution output shaft  53  is disposed in the internal space that is formed by the case body  11 , the body cover  12 , and the bottom portion cover  13 . 
     As illustrated in  FIG. 3 , the case  1  has a support member  14 . In the embodiment, the support member  14  includes a first support member  141  and a second support member  142 . The first support member  141  is integrally fixed to the case body  11  and the second support member  142  is integrally fixed to the first support member  141 . The first support member  141  is fixed to the peripheral wall portion  10  (here, a part of the peripheral wall portion  10  that is configured of the case body  11 ). That is, in the embodiment, the support member  14  is supported by the peripheral wall portion  10  of the case  1 . A configuration in which the support member  14  is integrally formed with the peripheral wall portion  10  is also possible. The first support member  141  is formed between the rotary electric machine  2  and the speed reduction device  3  (first planetary gear mechanism  31 ) so as to extend along the radial direction R and a circumferential direction. In at least one part of the first support member  141  in the circumferential direction, the end portion of the first support member  141  on the radial outer side R 1  and the case body  11  are fixed by a fixing member (in the embodiment, a bolt). The second support member  142  is formed between the first planetary gear mechanism  31  and the second planetary gear mechanism  32  so as to extend along the radial direction R and the circumferential direction. In at least one part of the second support member  142  in the circumferential direction, the end portion of the second support member  142  on the radial outer side R 1  and the first support member  141  are fixed by a fixing member (in the embodiment, a bolt). The second support member  142  is integrally fixed to the first support member  141  on the axial second side L 2  of the first support member  141 . 
     The rotary electric machine  2  is a permanent magnet rotary electric machine that has a rotor  21  having a permanent magnet  23  in a rotor core  22 , a stator  24  in which a stator coil  26  is wound around the stator core  25 , and the rotor shaft  27  that is coupled to the rotor core  22 . The stator  24  (specifically, the stator core  25 ) is fixed to the case  1 , specifically to the peripheral wall portion  10  of the case  1 . On the radial inner side R 2  of the rotor core  22 , the rotor shaft  27  is coupled to the rotor core  22  and the rotor  21  and the rotor shaft  27  rotate integrally. In the embodiment, the rotary electric machine  2  is a permanent magnet type rotary electric machine, but may be another type of rotary electric machine such as an induction type rotary electric machine. 
     The rotor shaft  27  is formed to have a cylindrical shape. A part of the rotor shaft  27  that protrudes to the axial first side L 1  along the axial direction L more than the rotor core  22  is supported by the case body  11  of the case  1  via a first rotor bearing  61  so as to be rotatable. A part of the rotor shaft  27  that protrudes to the axial second side L 2  along the axial direction L more than the rotor core  22  is supported by the first support member  141  of the support member  14  via a second rotor bearing  62  so as to be rotatable. 
     As described above, in the embodiment, the speed reduction device  3  includes the first planetary gear mechanism  31  and the second planetary gear mechanism  32 . The first planetary gear mechanism  31  is a single-pinion type planetary gear mechanism that has a first sun gear S 31 , a first ring gear R 31 , a first carrier C 31 , and a plurality of first pinion gears P 31 . The first sun gear S 31  is an input element of the first planetary gear mechanism  31  and is coupled so as to rotate integrally with the rotor shaft  27  of the rotary electric machine  2 . The first ring gear R 31  is a fixed element of the first planetary gear mechanism  31  and is supported by the first support member  141  so as not to rotate. That is, the first ring gear R 31  is coupled to the first support member  141  (an example of the fixing member) so that the first ring gear R 31  is stationary. The first carrier C 31  is an output element of the first planetary gear mechanism  31  and is coupled to a second sun gear S 32  of the second planetary gear mechanism  32  as described below. Thus, in the embodiment, the first sun gear S 31  is a rotatable element that is rotatable and the first ring gear R 31  is a fixed element that is stationary. 
     The first pinion gear P 31  is disposed so as to mesh with the first sun gear S 31  and the first ring gear R 31 , and is supported by the first carrier C 31  so as to be rotatable. The first pinion gear P 31  is configured to rotate (spin) around the shaft of the first pinion gear P 31  and rotate (revolve) around the shaft of the first sun gear S 31 . Although not shown, a plurality of the first pinion gears P 31  are provided along a revolution track of the first pinion gears P 31  at intervals. 
     As described above, the second planetary gear mechanism  32  is disposed on the axial second side L 2  relative to the first planetary gear mechanism  31  that is, the second planetary gear mechanism  32  is disposed on the opposite side of the first planetary gear mechanism  31  from the rotary electric machine  2  side. The second planetary gear mechanism  32  is a single-pinion type planetary gear mechanism that has a second sun gear S 32 , a second ring gear R 32 , a second carrier C 32 , and a plurality of second pinion gears P 32 . 
     The second sun gear S 32  is an input element of the second planetary gear mechanism  32 . As described above, the second sun gear S 32  is coupled to the first carrier C 31  that is an output element of the first planetary gear mechanism  31 . In the embodiment, the second sun gear S 32  is coupled to the first carrier C 31  via spline engagement. That is, the embodiment shows a form in which the first planetary gear mechanism  31  and the second planetary gear mechanism  32  are formed independently of each other, and the first planetary gear mechanism  31  and the second planetary gear mechanism  32  are coupled via spline engagement. However, the form is not limited to the form in which the first carrier C 31  and the second sun gear S 32  are configured of different members, and the first carrier C 31  and the second sun gear S 32  may be configured of one component. For example, the first planetary gear mechanism  31  and the second planetary gear mechanism  32  may be configured integrally to form one speed reduction device  3 . Even when the first carrier C 31  and the second sun gear S 32  are configured of different members, the two may be coupled by welding for example, instead of being limited to spline engagement. 
     As illustrated in  FIGS. 1 and 2 , the second drive shaft  52  is disposed adjacent to the differential gear device  4  in the axial direction L and there is the rotary electric machine  2  and the speed reduction device  3  between the first drive shaft  51  and the differential gear device  4  in the axial direction L. Thus, the first drive shaft  51  is coupled to the differential gear device  4  via the distribution output shaft  53  that passes through the rotary electric machine  2  and the speed reduction device  3 . In the embodiment, the first carrier C 31  and the second sun gear S 32  that rotate integrally are supported via a sliding bearing such as a bush so as to be rotatable around the distribution output shaft  53 . 
     The second ring gear R 32  is a fixed element of the second planetary gear mechanism  32  and is supported by the second support member  142  so as not to rotate in the circumferential direction. That is, the second ring gear R 32  is coupled to the second support member  142  (an example of the fixing member) so that the second ring gear R 32  is stationary. The second carrier C 32  is an output element of the second planetary gear mechanism  32 . In the embodiment, the second carrier C 32  is formed integrally with a differential case D 4  of the differential gear device  4 . In addition, the end portion of the second carrier C 32  on the axial first side L 1  is supported, between the first planetary gear mechanism  31  and the second planetary gear mechanism  32  of the speed reduction device  3 , by the second support member  142  via a first differential case bearing  66  such that the second carrier C 32  is rotatable. Thus, in the embodiment, the second sun gear S 32  is a rotatable element that is rotatable and the second ring gear R 32  is a fixed element that is stationary. 
     The second pinion gear P 32  is disposed so as to mesh with the second sun gear S 32  and the second ring gear R 32 , and is supported by the second carrier C 32  so as to be rotatable. The second pinion gear P 32  is configured to rotate (spin) around the shaft of the second pinion gear P 32  and rotate (revolve) around the shaft of the second sun gear S 32 . Although not shown, a plurality of the second pinion gears P 32  are provided along a revolution track of the second pinion gears P 32  at intervals. 
     As illustrated in  FIG. 3 , the support member  14  (an example of the fixing member) has a first support portion  14   a  that supports the first ring gear R 31  and a second support portion  14   b  that supports the second ring gear R 32 . In the embodiment, the first support portion  14   a  is formed on the first support member  141  provided in the support member  14  and the second support portion  14   b  is formed on the second support member  142  provided in the support member  14 . The first ring gear R 31  is supported by the first support portion  14   a  while being at least partially covered by the first support portion  14   a  from the outer peripheral side. The second ring gear R 32  is supported by the second support portion  14   b  while being at least partially covered by the second support portion  14   b  from the outer peripheral side. Specifically, the first support portion  14   a  is coupled to the first ring gear R 31  via spline-fitting from the radial outer side R 1  to support the first ring gear R 31  so that the first ring gear R 31  is stationary in the circumferential direction. Here, the first support portion  14   a  is formed continuously over the entire circumferential region and is disposed so as to cover the first ring gear R 31  over the entire circumferential region. The second support portion  14   b  is coupled to the second ring gear R 32  via spline-fitting from the radial outer side R 1  to support the second ring gear R 32  so that the second ring gear R 32  is stationary in the circumferential direction. Here, the second support portion  14   b  is formed continuously over the entire circumferential region and is disposed so as to cover the second ring gear R 32  over the entire circumferential region. 
     The differential gear device  4  distributes to the first wheel  501  and the second wheel  502 , the driving force that is transmitted from the rotary electric machine  2  via the speed reduction device  3 . Specifically, the differential gear device  4  distributes to the first wheel  501  and the second wheel  502 , the driving force that is transmitted from the rotary electric machine  2  via the speed reduction device  3 , via the first drive shaft  51  that is drivingly coupled to the distribution output shaft  53  and the second drive shaft  52 . In the embodiment, the differential gear device  4  has the differential case D 4  serving as an input element, a pinion shaft F 4  that is supported by the differential case D 4  so as to rotate integrally with the differential case D 4 , a first differential pinion gear P 41  and a second differential pinion gear P 42  that are supported so as to be rotatable with respect to the pinion shaft F 4 , and a first side gear B 41  and a second side gear B 42  serving as distribution output elements. Here, the first differential pinion gear P 41 , the second differential pinion gear P 42 , the first side gear B 41 , and the second side gear B 42  are all bevel gears. That is, the differential gear device  4  is a differential gear device provided with a bevel gear-type gear mechanism. 
     The differential case D 4  is a hollow member and the pinion shaft F 4 , a pair of differential pinion gears P 4  (the first differential pinion gear P 41  and the second differential pinion gear P 42 ), the first side gear B 41 , and the second side gear B 42  are housed in the differential case D 4 . In the embodiment, the differential case D 4  is formed integrally with the second carrier C 32  of the second planetary gear mechanism  32  and the second carrier C 32  is configured as a part of the differential case D 4 . Thus, the end portion of the second carrier C 32  on the axial first side L 1  functions as a first supported portion D 4   a  of the differential case D 4 . The first supported portion D 4   a  is disposed between the first planetary gear mechanism  31  and the second planetary gear mechanism  32  in the axial direction L. The first supported portion D 4   a  is directly supported by the first differential case bearing  66  that is fixed to the case  1  via the support member  14 . As described above, the first support member  141  is fixed integrally to the case body  11  and the first support member  141  and the second support member  142  are fixed integrally to each other. The first supported portion D 4   a  is thus supported by the case body  11  via the first differential case bearing  66 . 
     The differential case D 4  has a second supported portion D 4   b  positioned on the opposite side (the axial second side L 2 ) of the first supported portion D 4   a  in the axial direction L. Here, the second supported portion D 4   b  is formed so as to protrude to the axial second side L 2  along the axial direction L more than the second side gear B 42 . The second supported portion D 4   b  is formed to have a cylindrical shape and to be coaxial with the first side gear B 41  and the second side gear B 42 . The second supported portion D 4   b  is directly supported by a second differential case bearing  67  that is fixed to the body cover  12  of the case  1 . That is, the second supported portion D 4   b  is supported by the body cover  12  of the case  1  via the second differential case bearing  67  so as to be rotatable. 
     The pinion shaft F 4  is inserted in the pair of differential pinion gears P 4  and supports the differential pinion gears P 4  so that the differential pinion gears P 4  are rotatable. The pinion shaft F 4  is inserted in a through hole that is formed along the radial direction R of the differential case D 4  and is locked to the differential case D 4  with a locking member  43 . 
     The differential pinion gears P 4  are attached to the pinion shaft F 4  while facing each other with an interval therebetween along the radial direction R. The differential pinion gears P 4  are configured to rotate in the internal space of the differential case D 4  about the pinion shaft F 4  as the center. 
     The first side gear B 41  and the second side gear B 42  are rotary elements after distribution is performed in the differential gear device  4 . The first side gear B 41  and the second side gear B 42  are provided so as to face each other while sandwiching the pinion shaft F 4  with an interval therebetween along the axial direction L. The first side gear B 41  and the second side gear B 42  are configured to rotate in each circumferential direction in the internal space of the differential case D 4 . The first side gear B 41  and the second side gear B 42  are each meshed to the first differential pinion gear P 41  and the second differential pinion gear P 42 . A spline is formed on an inner peripheral surface of the first side gear B 41  to couple the first side gear B 41  and the distribution output shaft  53 . A spline is formed on an inner peripheral surface of the second side gear B 42  to couple the second side gear B 42  and the second drive shaft  52 . 
     The distribution output shaft  53  is a member that transmits to the first drive shaft  51 , a driving force that is from the rotary electric machine  2  and that is distributed by the differential gear device  4 . The distribution output shaft  53  passes through the rotor shaft  27  of the rotary electric machine  2  on the radial inner side R 2  in the axial direction L. A spline is formed on an outer peripheral surface of the end portion of the distribution output shaft  53  on the axial second side L 2  to couple the distribution output shaft  53  to the first side gear B 41  of the differential gear device  4 . The spline on the outer peripheral surface of the end portion of the distribution output shaft  53  on the axial second side L 2  and the spline on the inner peripheral surface of the first side gear B 41  are engaged so that the distribution output shaft  53  and the first side gear B 41  are coupled so as to rotate integrally. A coupling portion  53   a  to couple the first drive shaft  51  and the distribution output shaft  53  is formed on the end portion of the distribution output shaft  53  on the axial first side L 1 . 
     The coupling portion  53   a  extends from a part on the axial first side L 1  relative to the rotary electric machine  2  in the internal space of the case body  11  into the internal space of the bottom portion cover  13 . The coupling portion  53   a  is formed to have a cylindrical shape and to be coaxial with portions of the distribution output shaft  53  other than the coupling portion  53   a . The coupling portion  53   a  has an outer diameter that is larger than parts of the distribution output shaft  53  other than the coupling portion  53   a . The coupling portion  53   a  is supported by the bottom portion cover  13  of the case  1  via a first output bearing  68  so as to be rotatable and is supported by the bottom portion  11   a  of the case body  11  via a second output bearing  69  so as to be rotatable. A spline is formed on the inner peripheral surface of the coupling portion  53   a  on the axial second side L 2  to couple the coupling portion  53   a  and the first drive shaft  51 . 
     The first drive shaft  51  is drivingly coupled to the first wheel  501  and the second drive shaft  52  is drivingly coupled to the second wheel  502 . In the embodiment, the coupling portion  53   a  is provided on the end portion of the distribution output shaft  53  on the axial first side L 1  and the first drive shaft  51  and the coupling portion  53   a  of the distribution output shaft  53  are coupled by the spline. However, the configuration is not limited to this. For example, a flange yoke may be provided on the end portion of the distribution output shaft  53  on the axial first side L 1  instead of the coupling portion  53   a  and the flange yoke and the first drive shaft  51  may be fastened by a bolt. 
     Hereinafter, the detailed configuration of the speed reduction device  3  having the first planetary gear mechanism  31  and the second planetary gear mechanism  32  will be described. As described above, in the first planetary gear mechanism  31 , the first sun gear S 31  that is the input element is drivingly coupled to the rotary electric machine  2  (specifically, the rotor shaft  27  of the rotary electric machine  2 ), the speed of the rotation of the rotary electric machine  2  is reduced at a first speed reduction ratio, and the driving force is output from the first carrier C 31  that is the output element. In the second planetary gear mechanism  32 , the second sun gear S 32  that is the input element is drivingly coupled to the first carrier C 31  of the first planetary gear mechanism  31 , the speed of the rotation of the rotary electric machine  2  is further reduced at a second speed reduction ratio, and the driving force is output from the second carrier C 32  that is the output element. 
     The second planetary gear mechanism  32  is disposed on the output side of the speed reduction device  3  relative to the first planetary gear mechanism  31  in the order of the power transmission path. The driving force of the rotary electric machine  2  after the speed thereof is reduced in the first planetary gear mechanism  31 , is transmitted to the second planetary gear mechanism  32 . Thus, the transmission torque is larger in the second planetary gear mechanism  32  compared to the first planetary gear mechanism  31 . From the view point of mechanical strength, it is preferable that the larger the transmission torque be, the larger the gear width (face width) along the axial direction L be set. It is possible to configure the speed reduction device  3  having an appropriate structure compatible with the size of the torque by decreasing a first gear width W 1  of the first planetary gear mechanism  31  with a relatively small transmission torque, compared to a second gear width W 2  of the second planetary gear mechanism  32  with a relatively large transmission torque. Thus, it is possible to suppress the length of the speed reduction device  3  in the axial direction L to be small. The embodiment illustrates an example in which the first speed reduction ratio is larger than the second speed reduction ratio. However, regardless of the size relation of the first speed reduction ratio and the second speed reduction ratio, it is preferable that the first gear width W 1  be smaller than the second gear width W 2 , as illustrated in  FIG. 3  (and  FIG. 5  described below). 
     It is preferable that the second gear width W 2  be smaller than the length obtained by multiplying the first gear width W 1  by the first speed reduction ratio. When the ratio of the first gear width W 1  and the second gear width W 2  are made proportional to the respective input torque, the second gear width W 2  is the length obtained by multiplying the first gear width W 1  by the first speed reduction ratio. Although the transmission torque of the second planetary gear mechanism  32  is larger than that of the first planetary gear mechanism  31 , the rotational speed of the second planetary gear mechanism  32  is lower than that of the first planetary gear mechanism  31 . That is, it is possible to ensure a necessary durability even if the second gear width W 2  is smaller than the length obtained by multiplying the first gear width W 1  by the first speed reduction ratio, since the rotational speed is low. When the second gear width W 2  is smaller than the length obtained by multiplying the first gear width W 1  by the first speed reduction ratio, it is possible to shorten the length of the second planetary gear mechanism  32  in the axial direction L, and therefore it is possible to shorten the length of the entire speed reduction device  3  in the axial direction L. 
     In the embodiment illustrated in  FIGS. 1 and 3 , the first speed reduction ratio that is the speed reduction ratio of the first planetary gear mechanism  31  is smaller than the second speed reduction ratio that is the speed reduction ratio of the second planetary gear mechanism  32 . When the first speed reduction ratio is smaller than the second speed reduction ratio, the rotational speed that is input to the second planetary gear mechanism  32  is higher and the input torque to the second planetary gear mechanism  32  is lower, compared to when the first speed reduction ratio is equal to the second speed reduction ratio. Since the input torque is lower, the necessary mechanical strength of the second planetary gear mechanism  32  is decreased and the size of the second planetary gear mechanism  32  can be decreased. In many cases, a face width of the gear mechanism is increased in the axial direction L in order to improve the mechanical strength to be compatible with a large input torque. In this case, the gear widths (the first gear width W 1  and the second gear width W 2 ) of the first planetary gear mechanism  31  and the second planetary gear mechanism  32  in the axial direction L is increased, for example. 
     As described above, it is possible to decrease the second gear width W 2  that is the gear width of the second planetary gear mechanism  32  by decreasing the input torque to the second planetary gear mechanism  32 . Since the rotation speed is higher and the torque is lower in the first planetary gear mechanism  31  than the second planetary gear mechanism  32 , the first gear width W 1  that is the gear width of the first planetary gear mechanism  31  is not increased so much, even if the transmission torque is somewhat increased. That is, as a result of the relationship between the first speed reduction ratio and the second speed reduction ratio, the decrease amount of the second gear width W 2  is larger than the increase amount of the first gear width W 1 . Thus, it is possible to decrease the length of the entire speed reduction device  3  in the axial direction L. In this way, it is possible to decrease the length of the speed reduction device  3  in the axial direction L by appropriately setting the gear ratio of the two planetary gear mechanisms ( 31 ,  32 ) provided in the speed reduction device  3 . 
     In the embodiment illustrated in  FIGS. 1 and 3 , in order to realize the speed reduction device  3  in which the first speed reduction ratio is smaller than the second speed reduction ratio, the second sun gear radius φ 2  that is the radius of the second sun gear S 32  is smaller than the first sun gear radius φ 1  that is the radius of the first sun gear S 31 . As described above, in the first planetary gear mechanism  31  and the second planetary gear mechanism  32 , the ring gear is the fixed element, the sun gear is the input element, and the carrier is the output element. For example, if the ring gears have the same radius, the smaller the radius of the sun gear is, the larger the speed reduction ratio becomes. That is, even if the configuration of each gear such as the radius and the number of teeth is somewhat different, it is easier to increase the speed reduction ratio when the radius of the sun gear is small. As illustrated in  FIG. 3 , in the embodiment, a second sun gear radius φ 2  is smaller than a first sun gear radius φ 1 . Thus, the speed reduction device  3  having the first speed reduction ratio that is smaller than the second speed reduction ratio is realized appropriately. 
     In the embodiment, the first planetary gear mechanism  31  and the second planetary gear mechanism  32  are configured using a helical gear that is suitable for being used at high speed rotation since the strength is higher and the gear noise is lower than a spur gear. However, due to its structure, the helical gear generates a thrust force that is a force in a direction along the rotational axis. Typically, a thrust bearing is provided in a gear unit using a helical gear, in order to receive a load that tends to move the gear unit using the helical gear in the axial direction. As illustrated in  FIGS. 1 and 3 , a first thrust bearing  71  is provided on the axial first side L 1  of the first planetary gear mechanism  31 , a second thrust bearing  72  is provided between the first planetary gear mechanism  31  and the second planetary gear mechanism  32  in the axial direction L, and a third thrust bearing  73  is provided on the axial second side L 2  of the second planetary gear mechanism  32 . In detail, the first thrust bearing  71  is provided on the axial first side L 1  of the first sun gear S 31 . In the embodiment, as illustrated in  FIG. 3 , the support member  14  described above is supported by the peripheral wall portion  10  of the case  1  so that the support member  14  has a portion (target portion  14   c  or target) that is disposed on the opposite side of the first planetary gear mechanism  31  from the side on which the second planetary gear mechanism  32  is disposed in the axial direction L (that is, the axial first side L 1  of the first planetary gear mechanism  31 ). Here, the target portion  14   c  provided in the case  1  is formed on the first support member  141  provided in the support member  14 . The first thrust bearing  71  is disposed between the first sun gear S 31  and the support member  14  (specifically, the target portion  14   c ) in the axial direction L. A configuration in which the target portion  14   c  is formed integrally with the peripheral wall portion  10  is also possible, such as a configuration in which the support member  14  is formed integrally with the peripheral wall portion  10 . The second thrust bearing  72  is provided on the axial second side L 2  of the first sun gear S 31  and on the axial first side L 1  of a coupling portion of the first carrier C 31  and the second sun gear S 32 . The third thrust bearing  73  is provided on the axial second side L 2  of the second sun gear S 32 . In the embodiment, the second thrust bearing  72  functions as a “transmitting member” that transmits the thrust forces of the first planetary gear mechanism  31  and the second planetary gear mechanism  32  that face each other. The “transmitting member” may be a washer. In the embodiment, the second sun gear S 32  is coupled to the first carrier C 31  via spline engagement, as described above. The thrust force is also transmitted in this path and the first carrier C 31  or a member coupling the first carrier C 31  and the second sun gear S 32  also functions as the “transmitting member”. 
       FIG. 4  illustrates a configuration of helical teeth of the speed reduction device  3  and thrust forces. In the first planetary gear mechanism  31  and the second planetary gear mechanism  32 , a helix angle of each helical tooth is formed so that a first thrust force SF 1  that is a thrust force of the first planetary gear mechanism  31  and a second thrust force SF 2  that is a thrust force of the second planetary gear mechanism  32  are in mutually opposite directions in the axial direction L. That is, a helix angle of a helical tooth of a rotatable element (in the embodiment, the first sun gear S 31 ) of the first planetary gear mechanism  31  and a helix angle of a helical tooth of a rotatable element (in the embodiment, the second sun gear S 32 ) of the second planetary gear mechanism  32  are formed so that they are in mutually opposite directions in the axial direction L. When the directions of the thrust forces (SF 1 , SF 2 ) generated by the first planetary gear mechanism  31  and the second planetary gear mechanism  32 , which are both configured using a helical gear, are in mutually opposite directions, it is possible to cause at least a part of the thrust forces (SF 1 , SF 2 ) to cancel each other out. In the embodiment, the helix angles of the helical teeth are set so that when the driving force in a forward direction of the rotary electric machine  2  is transmitted to the first wheel  501  and the second wheel  502 , the direction of the first thrust force SF 1  generated in the first sun gear S 31  and the direction of the second thrust force SF 2  generated in the second sun gear S 32  face each other. It is often that the direction in which the wheels ( 501 ,  502 ) are driven is the forward direction of the vehicle rather than the rearward direction of the vehicle. Thus, it is preferable that when the driving force in the forward direction of the rotary electric machine  2  is transmitted to the first wheel  501  and the second wheel  502 , both thrust forces (SF 1 , SF 2 ) face each other in the axial direction L. In this way, it is possible to decrease the load applied to the first thrust bearing  71  and the third thrust bearing  73  that are disposed on the outer side of the first sun gear S 31  and the second sun gear S 32  in the axial direction L. 
     Thus, in the embodiment, the helix angles of the helical teeth are set so that when the driving force in the forward direction of the rotary electric machine  2  is transmitted to the first wheel  501  and the second wheel  502 , the direction of the first thrust force SF 1  and the direction of the second thrust force SF 2  face each other. However, the helix angles of the helical teeth may be set so that when a regenerative driving force (regenerative torque) of the rotary electric machine  2  is transmitted to the first wheel  501  and the second wheel  502 , the direction of the first thrust force SF 1  and the direction of the second thrust force SF 2  face each other. 
     When the thrust force is large, the load applied to the thrust bearing becomes large, thus affecting the durability of the speed reduction device  3 . However, if the size of the thrust bearing is increased, the length of the speed reduction device  3  in the axial direction L is increased, which hinders downsizing. It is possible to decrease the load applied to the first thrust bearing  71  and the third thrust bearing  73  by causing at least a part of the first thrust force SF 1  generated in the first sun gear S 31  and the second thrust force SF 2  generated in the second sun gear S 32  to cancel each other out. As a result, it is possible to ensure durability of the speed reduction device  3  even if the first thrust bearing  71  and the third thrust bearing  73  are small. That is, the thrust force that is generated in the planetary gear unit can be decreased and the length of the vehicle driving device  100  in the axial direction L can be shortened. 
     As illustrated in  FIG. 4 , when the force generated in a direction orthogonal to the helical tooth are decomposed into a vector component in a direction parallel to the rotational axis (here, coincides with the axial direction) and a vector component in a direction orthogonal to the rotational axis the thrust force corresponds to the vector component that is parallel to the rotational axis. That is, the larger the force generated in the direction orthogonal to the helical tooth is, the larger the thrust force becomes. Thus, the size of the thrust force is correlated to the transmission torque and the larger the transmission torque is, the larger the thrust force becomes. 
     The second planetary gear mechanism  32  is disposed on the output side of the speed reduction device  3  relative to the first planetary gear mechanism  31  in the order of the power transmission path. The driving force of the rotary electric machine  2  after the speed thereof is reduced in the first planetary gear mechanism  31 , is transmitted to the second planetary gear mechanism  32 . Thus, the transmission torque is larger in the second planetary gear mechanism  32  compared to the first planetary gear mechanism  31 . Therefore, the second thrust force SF 2  is larger than the first thrust force SF 1 . 
     The size of the thrust force is correlated to the helix angle of the helical tooth of the helical gear, and the larger the helix angle is, the larger the thrust force becomes. Here, the helix angle is an angle formed by the rotational axis of the helical gear and a tooth trace of the helical gear. When a force is generated in the direction orthogonal to the helical tooth, the larger the helix angle is, the larger the thrust force that is a component along the rotational axis becomes. 
     As illustrated in  FIG. 4 , when a first helix angle θ 1  that is a helix angle of the helical tooth of the first planetary gear mechanism  31  is larger than a second helix angle θ 2  that is a helix angle of the helical tooth of the second planetary gear mechanism  32 , the proportion of the thrust force among forces output from the gear mechanism is larger in the first planetary gear mechanism  31  than in the second planetary gear mechanism  32 . As described above, the thrust force in the second planetary gear mechanism  32  tends to be larger, since the transmission torque of the second planetary gear mechanism  32  is larger than the transmission torque of the first planetary gear mechanism  31 . However, when the first helix angle θ 1  is larger than the second helix angle θ 2 , it is possible to decrease the difference in the thrust force resulting from the difference in the transmission torque. 
     As described above, the second planetary gear mechanism  32  is disposed on the output side of the speed reduction device  3  relative to the first planetary gear mechanism  31  in the order of the power transmission path. Even when the speed reduction ratio (first speed reduction ratio) of the first planetary gear mechanism  31  and the speed reduction ratio (second speed reduction ratio) of the second planetary gear mechanism  32  are equal, the transmission torque of the second planetary gear mechanism  32 , which is transmitted after the speed of the rotation is reduced by the first planetary gear mechanism  31 , is larger than the transmission torque of the first planetary gear mechanism  31 . Thus, even when the first speed reduction ratio and the second speed reduction ratio are equal, the second thrust force SF 2  is larger than the first thrust force SF 1 . As illustrated in  FIGS. 1 and 3 , when the first speed reduction ratio is smaller than the second speed reduction ratio, the transmission torque of the second planetary gear mechanism  32  becomes larger than the transmission torque of the first planetary gear mechanism  31 . The difference between the first thrust force SF 1  and the second thrust force SF 2  is larger than when the first speed reduction ratio and the second speed reduction ratio are equal. However, as described above, the difference between the first thrust force SF 1  and the second thrust force SF 2  can be decreased depending on the setting of the helix angles of the helical teeth. 
     Unlike the embodiment illustrated in  FIGS. 1 and 3 , when the first speed reduction ratio is set to be larger than the second speed reduction ratio, the rotational speed input to the second planetary gear mechanism  32  is decreased and the input torque to the second planetary gear mechanism  32  is increased compared to when the first speed reduction ratio and the second speed reduction ratio are equal, while maintaining the speed reduction ratio of the entire speed reduction device  3 , as illustrated in  FIG. 5 . That is, when the first speed reduction ratio is larger than the second speed reduction ratio, the transmission torque of the second planetary gear mechanism  32  that is relatively large becomes small and the transmission torque of the first planetary gear mechanism  31  that is relatively small becomes large. 
     This means that when the first speed reduction ratio is larger than the second speed reduction ratio, the first thrust force SF 1  can be increased and the second thrust force SF 2  can be decreased, compared to when the first speed reduction ratio and the second speed reduction ratio are equal. As a result, the relatively large second thrust force SF 2  becomes small and the relatively small first thrust force SF 1  becomes large. In this way, the difference between the first thrust force SF 1  and the second thrust force SF 2  is decreased, which allows a larger amount of the thrust forces to cancel each other out. 
     In the embodiment illustrated in  FIG. 5 , in order to realize the speed reduction device  3  in which the first speed reduction ratio is larger than the second speed reduction ratio, the second sun gear radius φ 2  that is the radius of the second sun gear S 32  is larger than the first sun gear radius φ 1  that is the radius of the first sun gear S 31 . As described above, even if the configuration of each gear such as the radius and the number of teeth is somewhat different, it is easier to increase the speed reduction ratio when the radius of the sun gear is small, in the case where the ring gear is the fixed element, the sun gear is the input element, and the carrier is the output element. As illustrated in  FIG. 5 , when the second sun gear radius φ 2  is larger than the first sun gear radius φ 1 , the speed reduction device  3  in which the first speed reduction ratio is larger than the second speed reduction ratio is realized. 
     The embodiment illustrates an example in which the first sun gear S 31  and the second sun gear S 32  are rotatable elements that are rotatable, the first ring gear R 31  and the second ring gear R 32  are fixed elements that are stationary, and the first thrust force SF 1  generated in the first sun gear S 31  and the second thrust force SF 2  generated in the second sun gear S 32  act in directions facing each other and cause at least a part of the forces to cancel each other out. However, the configuration is not limited to this. For example, the configuration may be such that the first ring gear R 31  and the second ring gear R 32  are rotatable elements that are rotatable, the first sun gear S 31  and the second sun gear S 32  are fixed elements that are stationary, and the first ring gear R 31  and the second ring gear R 32  are supported by a thrust bearing in the axial direction L. In such a case, it is preferable that the helix angles of the helical teeth be set so that the first thrust force SF 1  that is generated in the first ring gear R 31  and the second thrust force SF 2  that is generated in the second ring gear R 32  act in directions that face each other. Also in this case, it is possible to decrease the thrust force of the entire speed reduction device  3  by causing at least a part of the forces to cancel each other out. In this case, the first thrust force SF 1  generated in the first sun gear S 31  and the second thrust force SF 2  generated in the second sun gear S 32  act in mutually separating directions. 
     Other Embodiments 
     Hereinafter, other embodiments will be described. The configuration of each embodiment described below may not only be used by itself, but also be combined with any of the configurations of the other embodiments unless inconsistency arises. 
     (1) An example of the bevel gear type differential gear device  4  is described above. However, the differential gear device  4  is not limited to a bevel gear type, and may be a planetary gear type as of a third planetary gear mechanism  9  illustrated in  FIG. 6 . As illustrated in  FIG. 6 , the third planetary gear mechanism  9  is a double-pinion type planetary gear mechanism and has a third sun gear S 9 , a third carrier C 9 , and a third ring gear R 9 . The third ring gear R 9  is an input element of the third planetary gear mechanism  9 , and is coupled to the second carrier C 32  of the second planetary gear mechanism  32  so as to rotate integrally. The third sun gear S 9  and the third carrier C 9  are distribution output elements of the third planetary gear mechanism  9 . Here, the third carrier C 9  is coupled to the distribution output shaft  53  and a coupling portion (spline) for coupling the second drive shaft  52  is formed on the third sun gear S 9 . 
     (2) An example of a configuration is described in which the differential case D 4  of the differential gear device  4  is formed integrally with the second carrier C 32  of the second planetary gear mechanism  32 . However, the configuration is not limited to this, and the configuration may be such that the differential case D 4 , the second carrier, and C 32  can be separated from each other (for example, the differential case D 4  and the second carrier C 32  are coupled to each other with a bolt or a spline etc.). 
     (3) An example of a configuration is described in which the second gear width W 2  is smaller than the length obtained by multiplying the first gear width W 1  by the first speed reduction ratio. However, the configuration is not limited to this, and the second gear width W 2  may be set to be the same as or larger than the length obtained by multiplying the first gear width W 1  by the first speed reduction ratio. 
     (4) The configuration in which the second sun gear radius φ 2  is smaller than the first sun gear radius φ 1  (see  FIGS. 1 and 3 ) and the configuration in which the second sun gear radius φ 2  is larger than the first sun gear radius φ 1  (see  FIG. 5 ) are illustrated above. However, the configuration is not limited to these, and the second sun gear radius φ 2  may be the same as the first sun gear radius φ 1 . Based on such a configuration, the configuration in which the first speed reduction ratio is smaller than the second speed reduction ratio and the configuration in which the first speed reduction ratio is larger than the second speed reduction ratio are illustrated above. However, the configuration is not limited to these, and the first speed reduction ratio and the second speed reduction ratio may be the same. 
     (5) An example of an embodiment is indicated in which the speed reduction device  3  is disposed between the rotary electric machine  2  and the differential gear device  4  in the axial direction L. However, the speed reduction device  3  does not have to be disposed between the rotary electric machine  2  and the differential gear device  4  in the axial direction L, if the speed reduction device  3  and the differential gear device  4  are disposed coaxially with the rotary electric machine  2  and a configuration is satisfied in which the driving force that is transmitted via the speed reduction device  3  from the rotary electric machine  2  is distributed to the two wheels ( 501 ,  502 ) by the differential gear device  4 . 
     Summary of Embodiments 
     Hereinafter, a summary of the vehicle driving device ( 100 ) described above will be described simply. 
     In one aspect, the vehicle driving device ( 100 ) includes: 
     a rotary electric machine ( 2 ) that is a driving force source of a first wheel ( 501 ) and a second wheel ( 502 ); 
     a speed reduction device ( 3 ) that reduces a speed of a rotation of the rotary electric machine ( 2 ); and 
     a differential gear device ( 4 ) that distributes to the first wheel ( 501 ) and the second wheel ( 502 ), a driving force that is transmitted from the rotary electric machine ( 2 ) via the speed reduction device ( 3 ), in which 
     the speed reduction device ( 3 ) and the differential gear device ( 4 ) are disposed coaxially with the rotary electric machine ( 2 ), 
     the speed reduction device ( 3 ) has a first planetary gear mechanism ( 31 ) and a second planetary gear mechanism ( 32 ), the first planetary gear mechanism ( 31 ) is disposed on the rotary electric machine ( 2 ) side relative to the second planetary gear mechanism ( 32 ) in an order of a power transmission path, and the first planetary gear mechanism ( 31 ) is disposed on the rotary electric machine ( 2 ) side relative to the second planetary gear mechanism ( 32 ) along an axial direction (L), 
     a helix angle of a helical tooth of the first planetary gear mechanism ( 31 ) and a helix angle of a helical tooth of the second planetary gear mechanism ( 32 ) are formed so that a thrust force (SF 1 ) of the first planetary gear mechanism ( 31 ) and a thrust force (SF 2 ) of the second planetary gear mechanism ( 32 ) are in mutually opposite directions in the axial direction (L), and 
     a helix angle (θ 1 ) of a helical tooth of the first planetary gear mechanism ( 31 ) is larger than a helix angle (θ 2 ) of a helical tooth of the second planetary gear mechanism ( 32 ). 
     In this configuration, the speed reduction device ( 3 ) and the differential gear device ( 4 ) are disposed coaxially with the rotary electric machine ( 2 ). Thus, compared to when the three devices ( 3 ,  4 ,  5 ) are disposed separately on three axes parallel to each other, it is possible to suppress a size of the vehicle driving device ( 100 ) in a radial direction (R) to be small. Moreover, in this configuration, it is possible to decrease the size of the thrust bearings ( 71 ,  73 ) as described below. Thus, it is possible to suppress the size of the vehicle driving device ( 100 ) from being large in the axial direction (L) while disposing the three devices ( 3 ,  4 ,  5 ) on the same axis. 
     Due to its structure, the helical gear generates a thrust force that is parallel to the rotational axis. When the directions of the thrust forces (SF 1 , SF 2 ) generated by the first planetary gear mechanism ( 31 ) and the second planetary gear mechanism ( 32 ), which are both configured using a helical gear, are in mutually opposite directions, it is possible to cause the thrust forces (SF 1 , SF 2 ) to cancel each other out. Typically, a thrust bearing is provided adjacent to the helical gear in the axial direction (L) in order to receive a load by the thrust force from the helical gear in the axial direction (L). When the generated thrust force is large, the load applied to the thrust bearing becomes large, thus affecting the durability of the speed reduction device ( 3 ) and the vehicle driving device ( 100 ). If the size of the thrust bearing is increased, downsizing of the vehicle driving device ( 100 ) is hindered. With the configuration, it is possible to decrease the load applied to the thrust bearings ( 71 ,  73 ) by causing a part of the thrust forces of the two planetary gear mechanisms ( 31 ,  32 ) to cancel each other out. 
     The size of the thrust force is correlated to the transmission torque and the larger the transmission torque is, the larger the thrust force becomes. The second planetary gear mechanism ( 32 ) is disposed on an output side of the speed reduction device ( 3 ) relative to the first planetary gear mechanism ( 31 ) in the order of the power transmission path. The driving force of the rotary electric machine ( 2 ) after the speed thereof is reduced in the first planetary gear mechanism ( 31 ), is transmitted to the second planetary gear mechanism ( 32 ). Thus, the transmission torque is larger in the second planetary gear mechanism ( 32 ) compared to the first planetary gear mechanism ( 31 ). The thrust force (SF 2 ) of the second planetary gear mechanism ( 32 ) is therefore larger than the thrust force (SF 1 ) of the first planetary gear mechanism ( 31 ). The size of the thrust force is correlated to the helix angle of the helical tooth of the helical gear, and the larger the helix angle is, the larger the thrust force becomes. If a first helix angle (θ 1 ) that is a helix angle of the helical tooth of the first planetary gear mechanism ( 31 ) is larger than a second helix angle (θ 2 ) that is a helix angle of the helical tooth of the second planetary gear mechanism ( 32 ), the proportion of the thrust force among forces output from the gear mechanism is larger in the first planetary gear mechanism ( 31 ) than in the second planetary gear mechanism ( 32 ). That is, with the present configuration, it is possible to decrease the difference in the thrust force resulting from the difference in the transmission torque, with the helix angle of the helical tooth. This means that it is possible to decrease the load applied to the thrust bearings ( 71 ,  73 ) by causing the thrust forces of the two planetary gear mechanisms ( 31 ,  32 ) to cancel each other out. In addition, the size of the thrust bearings ( 71 ,  73 ) can be reduced so that the length of the vehicle driving device ( 100 ) in the axial direction (L) can be suppressed to be short. 
     In this way, with the configuration, it is possible to decrease the size of the entire vehicle driving device ( 100 ) in both the radial direction (R) and the axial direction (L). 
     Here, it is preferable that the thrust forces (SF 1 , SF 2 ) of the first planetary gear mechanism ( 31 ) and the second planetary gear mechanism ( 32 ), the thrust forces (SF 1 , SF 2 ) facing each other, be transmitted via a transmitting member ( 72 ) disposed between the first planetary gear mechanism ( 31 ) and the second planetary gear mechanism ( 32 ) in the axial direction (L). 
     With such a configuration, when the direction of the thrust force (SF 1 ) generated by the first planetary gear mechanism ( 31 ) and the direction of the thrust force (SF 2 ) generated by the second planetary gear mechanism ( 32 ) face each other, the thrust forces are transmitted via the transmitting member ( 72 ). Thus, it is possible appropriately compose the vectors of the thrust forces (SF 1 , SF 2 ) in mutually opposite directions that are generated by the planetary gear mechanisms ( 31 ,  32 ) and cause a part of the thrust forces (SF 1 , SF 2 ) to cancel each other out. 
     It is preferable that the helix angle of the helical tooth of the first planetary gear mechanism ( 31 ) and the helix angle of the helical tooth of the second planetary gear mechanism ( 32 ) be formed so that the thrust forces (SF 2 ) transmitted via the transmitting member ( 72 ) face each other in the axial direction (L) when a driving force in a forward direction of the rotary electric machine ( 2 ) is transmitted to the first wheel ( 501 ) and the second wheel ( 502 ). 
     The directions of the thrust forces differ depending on the directions of the driving forces that are transmitted to each of the planetary gear mechanisms ( 31 ,  32 ). As described above, when the thrust forces (SF 1 , SF 2 ) that are transmitted via the transmitting member ( 72 ) face each other in the axial direction (L), the thrust forces (SF 1 , SF 2 ) act in directions moving away from the thrust bearings ( 71 ,  73 ) that are disposed on both end portions in the axial direction (L). As a result, the load applied to the thrust bearings ( 71 ,  73 ) that are disposed on both end portions in the axial direction (L) can be reduced. In most cases, the direction that the wheels ( 501 ,  502 ) are driven is forward rather than rearward. Thus, for reducing the load applied to the thrust bearings ( 71 ,  73 ) disposed on both sides in the axial direction (L), when the driving force in the forward direction of the first wheel ( 501 ) and the second wheel ( 502 ) is transmitted, it is preferable that the thrust forces (SF 1 , SF 2 ) face each other in the axial direction (L). 
     It is preferable that: the first planetary gear mechanism ( 31 ) and the second planetary gear mechanism ( 32 ) each have a sun gear (S 31 , S 32 ), a carrier (C 31 , C 32 ), and a ring gear (R 31 , R 42 ); in both the first planetary gear mechanism ( 31 ) and the second planetary gear mechanism ( 32 ), one of the sun gear (S 31 , S 32 ) and the ring gear (R 31 , R 32 ) be a rotatable element that is configured to rotate and the other be a fixed element that is stationary; and helix angles of helical teeth of the rotatable elements be formed so as to be in mutually opposite directions in the axial direction (L). 
     With such a configuration, the helix angles of the helical teeth of the rotary elements (input elements) of the two planetary gear mechanisms ( 31 ,  32 ) in the axial direction (L), the rotary elements being adjacent to each other, are formed so as to be in mutually opposite directions. Thus, it is possible to cause a part of the thrust forces (SF 1 , SF 2 ) that are generated in the planetary gear mechanisms ( 31 ,  32 ) to appropriately cancel each other out. 
     It is preferable that: the first planetary gear mechanism ( 31 ) have a first sun gear (S 31 ) that is drivingly coupled to the rotary electric machine ( 2 ), a first carrier (C 31 ), and a first ring gear (R 31 ) that is coupled to a fixing member ( 141 ) so that the first ring gear (R 31 ) is stationary; the second planetary gear mechanism ( 32 ) have a second sun gear (S 32 ) that is drivingly coupled to the first carrier (C 31 ), a second carrier (C 32 ), and a second ring gear (R 32 ) that is coupled to a fixing member ( 142 ) so that the second ring gear (R 32 ) is stationary; and a helix angle of a helical tooth of the first sun gear (S 31 ) and a helix angle of a helical tooth of the second sun gear (S 32 ) be formed so as to be in mutually opposite directions in the axial direction (L). 
     With such a configuration, the helix angles of the helical teeth of the sun gears (S 31 , S 32 ) of the two planetary gear mechanisms ( 31 ,  32 ), the sun gears (S 31 , S 32 ) being adjacent in the axial direction (L), are formed so as to be in mutually opposite directions. Thus, it is possible to cause a part of the thrust forces (SF 1 , SF 2 ) that are generated in the planetary gear mechanisms ( 31 ,  32 ) to appropriately cancel each other out. 
     It is also preferable that a thrust bearing ( 72 ) be disposed between the first sun gear (S 31 ) and the second sun gear (S 32 ) in the axial direction (L). 
     With such a configuration, the thrust force (SF 1 ) generated by the first sun gear (S 31 ) of the first planetary gear mechanism ( 31 ) and the thrust force (SF 2 ) generated by the second sun gear (S 32 ) of the second planetary gear mechanism ( 32 ) are transmitted via the thrust bearing ( 72 ). Thus, it is possible appropriately compose the vectors of the thrust forces (SF 1 , SF 2 ) in mutually opposite directions that are generated by the first sun gear (S 31 ) and the second sun gear (S 32 ) and cause a part of the thrust forces (SF 1 , SF 2 ) to cancel each other out. 
     It is preferable that: a case ( 1 ) housing the rotary electric machine ( 2 ), the speed reduction device ( 3 ), and the differential gear device ( 4 ) be further provided; the case ( 1 ) have a target portion ( 14   c ) disposed on an opposite side of the first planetary gear mechanism ( 31 ) from a side on which the second planetary gear mechanism ( 32 ) is disposed in the axial direction (L); the first planetary gear mechanism ( 31 ) have a first sun gear (S 31 ) drivingly coupled to the rotary electric machine ( 2 ); and a thrust bearing ( 71 ) be disposed between the first sun gear (S 31 ) and the target portion ( 14   c ) in the axial direction (L). 
     With such a configuration, it is possible to restrict, with the thrust bearing ( 71 ), the first sun gear (S 31 ) from moving away from the second planetary gear mechanism ( 32 ) in the axial direction (L). As described above, since it is possible to cause a part of the thrust forces (SF 1 , SF 2 ) of the two planetary gear mechanisms ( 31 ,  32 ) to cancel each other out, it is possible to decrease the load applied to the thrust bearing ( 71 ) and decrease the size of the thrust bearing ( 71 ). 
     It is preferable that the speed reduction device ( 3 ) be disposed between the rotary electric machine ( 2 ) and the differential gear device ( 4 ) in the axial direction (L). 
     With such a configuration, it is possible to configure the vehicle driving device ( 100 ) that is simple, and downsizing of the vehicle driving device ( 100 ) is realized. 
     Here, it is preferable that a first gear width (W 1 ) that is a width of a gear of the first planetary gear mechanism ( 31 ) along the axial direction (L) be smaller than a second gear width (W 2 ) that is a width of a gear of the second planetary gear mechanism ( 32 ) along the axial direction (L). 
     As described above, the transmission torque is larger in the second planetary gear mechanism ( 32 ) compared to the first planetary gear mechanism ( 31 ). From the view point of mechanical strength, it is preferable that the larger the transmission torque be, the larger the width of the gear along the axial direction (L) be set. It is possible to configure the appropriate speed reduction device ( 3 ) having a structure compatible with the size of the torque by decreasing the first gear width (W 1 ) of the first planetary gear mechanism ( 31 ) with a relatively small transmission torque, compared to the second gear width (W 2 ) of the second planetary gear mechanism ( 32 ) with a relatively large transmission torque. Thus, it is possible to suppress the length of the speed reduction device ( 3 ) in the axial direction (L) to be small. 
     As described above, when the first gear width (W 1 ) that is the width of the gear of the first planetary gear mechanism ( 31 ) along the axial direction (L) is smaller than the second gear width (W 2 ) that is the width of the gear of the second planetary gear mechanism ( 32 ) along the axial direction (L), it is preferable that the second gear width (W 2 ) be smaller than a length obtained by multiplying the first gear width (W 1 ) by the first speed reduction ratio. 
     When the ratio of the first gear width (W 1 ) and the second gear width (W 2 ) are made proportional to the input torques respectively, the second gear width (W 2 ) is the length obtained by multiplying the first gear width (W 1 ) by the first speed reduction ratio. Although the transmission torque of the second planetary gear mechanism ( 32 ) is larger than that of the first planetary gear mechanism ( 31 ), the rotational speed of the second planetary gear mechanism ( 32 ) is lower than that of the first planetary gear mechanism ( 31 ). Thus, it is possible to ensure a necessary durability even if the second gear width (W 2 ) is smaller than the length obtained by multiplying the first gear width (W 1 ) by the first speed reduction ratio. Since the second gear width (W 2 ) is smaller than the length obtained by multiplying the first gear width (W 1 ) by the first speed reduction ratio, it is possible to shorten the length of the second planetary gear mechanism ( 32 ) in the axial direction (L), and thus it is possible to shorten the length of the entire speed reduction device ( 3 ) in the axial direction (L).