Patent Publication Number: US-2021190201-A1

Title: Lubrication device for vehicle drive-force transmitting apparatus

Description:
This application claims priority from Japanese Patent Application No. 2019-229725 filed on Dec. 19, 2019, the disclosure of which is herein incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a lubrication device for a vehicle drive-force transmitting apparatus including a casing that defines a motor room and a gear room. 
     BACKGROUND OF THE INVENTION 
     There is known a vehicle drive-force transmitting apparatus including a casing that defines a motor room and a gear room such that a motor is disposed in the motor room while a gear mechanism is disposed in the gear room. As an example of such a vehicle drive-force transmitting apparatus, Patent Document 1 discloses a motor unit. In the motor unit disclosed in the Patent Document 1, a motor room and a gear room are defined in a housing, such that a motor is disposed in the motor room while a deceleration device and a differential device are disposed in the gear room. Further, a partition wall, by which an interior of the housing is sectioned into the motor room and the gear room, is provided with a partition-wall hole that communicates between the motor room and the gear room. Owing to the construction described above, an oil supplied to the motor room is moved to the gear room through the partition-wall hole, so as to be stored in an oil reservoir in the gear room. The oil stored in the oil reservoir is pumped up by a pump, and is supplied to the motor room such that the motor is cooled by the oil. Thus, the oil is circulated within the housing in the motor unit disclosed in the Patent Document 1. Further, Patent Document 2 discloses a motor in which an insulation sheet including a foam resin layer is interposed between a stator core and each of a plurality of coils wound on the stator core. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: WO/2018/030372 
     Patent Document 2: JP-2017-127063A 
     Patent Document 3: JP-2001-190042A 
     Patent Document 4: JP-2019-168064A 
     SUMMARY OF THE INVENTION 
     By the way, in the construction disclosed in the Patent Document 1 in which the oil supplied to the motor room is moved to the gear room through a communication hole (partition-wall hole in the Patent Document 1), if foreign substances generated in the motor room are mixed into the oil, the oil containing the foreign substances is stored in the oil reservoir. Consequently, when the oil is pumped by the pump, a filter of a strainer is likely to be clogged with the foreign substances so that there is a risk that a pressure loss could be caused due to the clogging of the filter. For example, in a case in which the foam resin layer made of a foam resin is interposed between the stator core and each of the stator coils, as in the Patent Document 2, the filter of the strainer is more likely to be clogged with the foam resin that is mixed into the oil. 
     The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a lubrication device for a vehicle drive-force transmitting apparatus that includes a casing and a partition wall, such that an interior of the casing is sectioned by the partition wall into a motor room and a gear room, and such that a gear mechanism is disposed in the gear room while a motor is disposed in the gear room, wherein the lubricating device is capable of suppressing problem caused by foreign substances contained in an oil. This object is achieved according to the following aspects of the present invention. 
     According to a first aspect of the invention, there is provided a lubrication device for a vehicle drive-force transmitting apparatus that includes a casing, such that an interior of the casing is sectioned by a partition wall into a gear room and a motor room, and such that a gear mechanism is disposed in the gear room while a motor is disposed in the motor room, wherein the lubrication device comprises a communication hole provided through the partition wall and communicating between the gear room and the motor room, the communication hole having a gear-room-side opening and a motor-room-side opening, such that the communication hole opens at the gear-room-side opening in the gear room and opens at the motor-room-side opening in the motor room, wherein the motor-room-side opening is located on an upper side of a bearing which supports the gear mechanism and which is held in a bearing holding wall of the casing, and wherein the gear-room-side opening is provided in the bearing holding wall, and is in held in communication with a gap defined between an inner race and an outer race of the bearing. It is noted that “the upper side of the bearing” may be interpreted to mean the upper side of the bearing in a vertical direction of a vehicle in an installed state in which the drive-force transmitting apparatus including the lubrication device is installed in the vehicle. 
     According to a second aspect of the invention, in the lubrication device according to the first aspect of the invention, the motor includes a stator core, a plurality of stator coils wound on the stator core and a plurality of insulation sheets each including a foam resin layer, wherein each of the insulation sheets is interposed between the stator core and a corresponding one of the stator coils. It is possible to interpret that the motor constitutes a part of the vehicle drive-force transmitting apparatus. 
     According to a third aspect of the invention, in the lubrication device according to the first or second aspect of the invention, the bearing supporting the gear mechanism is a tapered roller bearing. 
     According to a fourth aspect of the invention, in the lubrication device according to the third aspect of the invention, the tapered roller bearing includes a plurality of tapered rollers disposed between the inner and outer races, wherein each of the tapered rollers has a small-diameter axial end face and a large-diameter axial end face that has a diameter larger than a diameter of the small-diameter axial end face, wherein the tapered roller bearing has a roller-small-diameter-side end face and a roller-large-diameter-side end face, such that the small-diameter axial end face of each of the tapered rollers constitutes a part of the roller-small-diameter-side end face of the tapered roller bearing while the large-diameter axial end face of each of the tapered rollers constitutes a part of the roller-large-diameter-side end face of the tapered roller bearing, and wherein the gear-room-side opening of the communication hole is located in a position closer to the roller-small-diameter-side end face of the tapered roller bearing than to the roller-large-diameter-side end face of the tapered roller bearing. 
     According to a fifth aspect of the invention, in the lubrication device according to the first through fourth aspects of the invention, the gear mechanism is a differential device. 
     According to a sixth aspect of the invention, the lubrication device according to the first through fifth aspects of the invention comprises: an oil reservoir provided in a lower portion of the gear room, so as to store therein an oil; and an oil pump by which the oil is to be pumped up through a strainer and is to be supplied to the motor disposed in the motor room. 
     In the lubrication device according to the first aspect of the invention, the gear-room-side opening of the communication hole is provided in the bearing holding wall in which the bearing is held, and is in held in communication with the gap defined between the inner race and the outer race of the bearing. Owing to this construction, an oil, which has been moved from the motor room to the gear room through the communication hole, is caused to flow into the gap of the bearing. In this instance, when the oil is passing through the gap of the bearing, foreign substances mixed or contained in the oil is crushed by the bearing into smaller pieces. Therefore, for example, in an arrangement in which the oil stored in the casing is to be pumped up by an oil pump through a strainer, it is possible to suppress clogging of a filter of the strainer. 
     In the lubrication device according to the second aspect of the invention, when foam resin is mixed into the oil in process of cooling the motor by the oil, it is possible to cause the foam resin to be crushed by the bearing. 
     In the lubrication device according to the third aspect of the invention in which the bearing is constituted by the tapered roller bearing, it is possible to increase areas of surfaces by which the foreign substances are to be crushed, as compared with an arrangement in which the bearing is constituted by another type of bearing such as a ball bearing, whereby the foreign substances can be efficiently crushed. 
     In the lubrication device according to the fourth aspect of the invention, the gear-room-side opening of the communication hole is located in the position closer to the roller-small-diameter-side end face of the tapered roller bearing than to the roller-large-diameter-side end face of the tapered roller bearing. Owing to this construction, much of the oil flowing out through the gear-room-side opening can be efficiently introduced into the bearing, by utilizing flow of the oil, which is forced to be drawn in a direction away from the small-diameter axial end face of the tapered roller bearing toward the large-diameter axial end face of the tapered roller bearing, when the tapered roller bearing is rotated. 
     In the lubrication device according to the fifth aspect of the invention, since the gear mechanism is the differential device, a large load is to be applied to the bearing supporting the differential device as the gear mechanism, so that the foreign substances contained in the oil can be efficiently crushed by the bearing in process of lubricating the bearing by the oil. 
     In the lubrication device according to the sixth aspect of the invention, the foreign substances mixed or contained in the oil are crushed by the bearing, so that it is possible to suppress clogging of the filter of the strainer, which could be caused in process of pumping up the oil stored in the oil reservoir the oil pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view schematically showing, by way of example, construction of a vehicle drive-force transmitting apparatus to which the present invention is applied; 
         FIG. 2  is a cross sectional view taken in a plane perpendicular to a second axis CL 2  in  FIG. 1  and showing a stator core and stator coils of a second electric motor MG 2  shown in  FIG. 1 ; 
         FIG. 3  is a lubrication circuit diagram for explaining construction of a lubrication device for lubricating and cooling gears, bearings and first and second electric motors disposed in the vehicle drive-force transmitting apparatus of  FIG. 1 ; 
         FIG. 4  is a view of an axle case constituting a part of a casing of the vehicle drive-force transmitting apparatus of  FIG. 1 , as seen from a side of a gear room of the casing; 
         FIG. 5  is a view of the axle case, as seen from a side of a motor room of the casing; and 
         FIG. 6  is a cross sectional view taken in a plane containing an axis of a communication hole which is provided in the axle case and which communicates a motor room and a gear room of the casing. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment of the invention will be described in detail with reference to the accompanying drawings. The figures of the drawings are simplified or deformed as needed, and each portion is not necessarily precisely depicted in terms of dimension ratio, shape, etc. 
     EMBODIMENT 
       FIG. 1  is a view schematically showing, by way of example, construction of a vehicle drive-force transmitting apparatus  10  to which the present invention is applied. The drive-force transmitting apparatus  10  is provided between an engine  12  and right and left drive wheels  14   r ,  141  (hereinafter simply referred to as “drive wheels  14 ” unless they are to be distinguished from each other). The drive-force transmitting apparatus  10  is to be used advantageously in a hybrid vehicle of FF (front engine/front drive) system. The drive-force transmitting apparatus  10  is a drive-force transmitting apparatus of hybrid type, and is configured to transmit a drive force outputted from the engine  12  as a drive-force source and a drive force outputted from a second electric motor MG 2  as another drive-force source, to the right and left drive wheels  14   r ,  141 , via, for example, a differential device  28  and right and left axles  30   r ,  301 . It is noted that the term “drive force” used in the present specification is synonymous with a drive torque or a drive power. 
     As shown in  FIG. 1 , the drive-force transmitting apparatus  10  includes an input shaft  22 , a drive-force transmission shaft  24 , a counter shaft  26 , the above-described differential device  28 , the above-described axles  30 , a planetary gear device  32 , an output gear  34 , a reduction gear  36 , a counter gear  38 , a differential drive gear  42 , a casing  44 , a first electric motor MG 1  and the above-described second electric motor MG 2 . The input shaft  22  is disposed to be rotatable about a first axis CL 1 . The drive-force transmission shaft  24  is disposed to be rotatable about a second axis CL 2 . The counter shaft  26  is disposed to be rotatable about a third axis CL 3 . The differential device  28  and the axles  30  are disposed to be rotatable about a fourth axis CL 4 . The planetary gear device  32 , first electric motor MG 1  and output gear  34  are disposed to be coaxial with the input shaft  22  and are located radially outside the input shaft  22 . The reduction gear  36  is disposed on the drive-force transmission shaft  24 . The second electric motor MG 2  is connected to an axial end portion of the drive-force transmission shaft  24  in a drive-force transmittable manner. The counter gear  38  meshing with the output gear  34  and the differential drive gear  42  meshing with a differential ring gear  40  of the differential device  28  are disposed on the counter shaft  26 . All of these rotary members are disposed in the casing  44  as a non-rotary member. It is noted that all of the first through fourth axes CL 1 -CL 4  are rotary axes that are parallel to a width direction of the vehicle  8 . 
     Each of the first and second electric motors MG 1 , MG 2  may be an electric motor having a function of a motor operable to convert an electric energy into a mechanical drive force and/or a function of a generator operable to convert a mechanical drive force into an electric energy, and preferably, is a motor generator having both of the functions so as to be operated as a selected one of the motor and the generator. In the present embodiment, the first electric motor MG 1  has a function of an electric generator operable to receive a reaction force from the engine  12 , and a function of an electric motor operable to start the engine  12  while the engine  12  is at rest. Meanwhile, the second electric motor MG 2  has a function of an electric motor functioning as a vehicle drive power source to generate a vehicle driving force, and a function of an electric generator operable to perform a regenerative operation for converting a reverse driving force received from the drive wheels  14 , into an electric energy. 
     The input shaft  22  is disposed to be rotatable about the first axis CL 1 . The input shaft  22  is connected to a crank shaft  12   a  of the engine  12  through members such as a damper (not shown), in a drive-force transmittable manner. The input shaft  22  is held, through members such as a bearing  46 , by the casing  44  as the non-rotary member, such that the input shaft  22  is rotatable about the first axis CL 1 . The planetary gear device  32 , first electric motor MG 1  and output gear  34  are disposed to be coaxial with the input shaft  22  and are located radially outside the input shaft  22 . 
     The planetary gear device  32 , which is disposed to be coaxial with the first axis CL 1 , is a single-pinion type planetary gear device (differential mechanism) that includes a sun gear S, a carrier CA and a ring gear R. The planetary gear device  32  serves as a drive-force distributing mechanism configured to distribute the drive force of the engine  12  to the first electric motor MG 1  and the output gear  34 . The sun gear S of the planetary gear device  32  is connected to the first electric motor MG 1  in a drive-force transmittable manner. The carrier CA of the planetary gear device  32  is connected to the engine  12  through, for example, the input shaft  22  in a drive-force transmittable manner. The ring gear R of the planetary gear device  32  is connected to the output gear  34  in a drive-force transmittable manner. 
     The first electric motor MG 1  is disposed in a position adjacent to the planetary gear device  32 , with a partition wall  48  (that is a part of the casing  44 ) being located between the first electric motor MG 1  and the planetary gear device  32  in a direction of the first axis CL 1 . The first electric motor MG 1  includes an annular-shaped stator core  50  that is unrotatably fixed to the casing  44 , an annular-shaped rotor  52  disposed on an inner peripheral side of the stator core  50 , and a rotor shaft  54  connected to an inner periphery of the rotor  52 . The stator core  50  is provided with stator coils  56  wound thereon. The rotor shaft  54  is held at its axially opposite end portions by the casing  44  through a pair of bearings  58   a ,  58   b , such that the rotor shaft  54  is rotatable about the first axis CL 1 . 
     The output gear  34  is connected to the ring gear R of the planetary gear device  32 , and meshes with the counter gear  38  that is disposed on the counter shaft  26 . It is noted that the ring gear R and the output gear  34  are formed integrally with each other, namely, are constituted by a compound gear. 
     The drive-force transmission shaft  24  is disposed to be rotatable about the second axis CL 2 , and is rotatably held at its axially opposite end portions by the casing  44  through a pair of bearings  70   a ,  70   b . The reduction gear  36  is disposed on the drive-force transmission shaft  24 . The second electric motor MG 2  is connected to an axial end portion of the drive-force transmission shaft  24  in a drive-force transmittable manner. The partition wall  48  is located between the second electric motor MG 2  and the reduction gear  36  in a direction of the second axis CL 2 . 
     The second electric motor MG 2  includes an annular-shaped stator core  60  that is unrotatably fixed to the casing  44 , an annular-shaped rotor  62  disposed on an inner peripheral side of the stator core  60 , and a rotor shaft  64  connected to an inner periphery of the rotor  62 . The stator core  60  is provided with stator coils  66  wound thereon. The rotor shaft  64  is held at its axially opposite end portions by the casing  44  through a pair of bearings  68   a ,  68   b , such that the rotor shaft  64  is rotatable about the second axis CL 2 . 
       FIG. 2  is a cross sectional view taken in a plane perpendicular to the second axis CL 2  and showing the stator core  60  and the stator coils  66  of the second electric motor MG 2 . In  FIG. 2 , an upward direction in the drawing sheet corresponds to a direction away from an outer periphery of the stator core  60  toward an inner periphery of the stator core  60 , while a downward direction in the drawing sheet corresponds to a direction away from the inner periphery of the stator core  60  toward the outer periphery of the stator core  60 . It is noted that only a part, in its circumferential direction, of the stator core  60  is shown in the cross sectional view of  FIG. 2 . As shown in  FIG. 2 , the stator core  60  has a plurality of radially extending slots  60   s  each extending radially outwardly from the inner periphery of the stator core  60 , such that the radially extending slots  60   s  are arranged equi-angularly in a circumferential direction of the stator core  60 . The stator coils  66  are provided in the respective radially extending slots  60   s . The second electric motor MG 2  includes, in addition to the stator core  60  and the stator coils  66 , a plurality of insulation sheets  69  each of which is interposed between the stator core  60  and a winding  66   w  of a corresponding one of the stator coils  66 . Each of the insulation sheets  69  is constituted by, for example, an insulation paper of single-layer or multi-layer, and extends along a wall surface of a corresponding one of the radially extending slots  66  of the stator core  60 . Each of the insulation sheets  69  includes at least one foam resin layer in the form of two foam resin layers  71  that are provided on respective surfaces of each insulation sheet  69  that are opposed to the stator core  60  and a corresponding one of the stator coils  66 . It is noted that the second electric motor MG 2  corresponds to “motor” recited in the appended claims. 
     Referring to  FIG. 1 , the reduction gear  36  is formed integrally with the drive-force transmission shaft  24 , and meshes with the counter gear  38  disposed on the counter shaft  26 . The number of teeth of the reduction gear  36  is smaller than the number of teeth of the counter gear  38 . Thus, rotation of the second electric motor MG 2  is transmitted to the counter shaft  26  through the reduction gear  36  and the counter gear  38 , such that a rotational speed of the counter shaft  26  is made lower than a rotational speed of the second electric motor MG 2 . 
     The counter shaft  26  is provided to be rotatable about the third axis CL 3 . The counter shaft  26  is held at its axially opposite end portions by the casing  44  through a pair of bearings  72   a ,  72   b , such that the counter shaft  26  is rotatable about the third axis CL 3 . The counter gear  38  and the differential drive gear  42  are disposed on the counter shaft  26 , unrotatably relative to the counter shaft  26  that is to be rotated about the third axis CL 3 . 
     The counter gear  38  meshes with the output gear  34  and the reduction gear  36  so that the drive force outputted from the engine  12  and/or the drive force outputted from the second electric motor MG 2  are to be transmitted. The differential drive gear  42  meshes with a differential ring gear  40  of the differential device  28 . Thus, when the drive force drive force outputted from the engine  12  and/or the drive force outputted from the second electric motor MG 2  are transmitted to the counter gear  38  through the output gear  34  and/or the reduction gear  36 , the drive force or forces are transmitted to the differential device  28  through the counter shaft  26  and the differential drive gear  42 . 
     The differential device  28  and the axles  30   r ,  301  are disposed to be rotatable about the fourth axis CL 4 . The differential device  28  is constituted by a well-known differential mechanism, and is configured to transmit the drive force to the right and left axles  30   r ,  301  while allowing rotation of each of the axles  30   r ,  301  relative to the other of the axles  30   r ,  301 . It is noted that detailed description of the differential device  28 , which is a known device, is not provided. The differential device  28  is held, at its opposite end portions that are opposite to each other in a direction of the fourth axis CL 4 , by the casing  44  through a pair of bearings  74   a ,  74   b , such that the differential device  28  is rotatable about the fourth axis CL 4 . In the present embodiment, each of the bearings  74   a ,  74   b  is constituted by a tapered roller bearing. 
     In the drive-force transmitting apparatus  10  constructed as described above, the drive force of the engine  12  is transmitted to the right and left drive wheels  14   r ,  141  through the planetary gear device  32 , output gear  34 , counter gear  38 , counter shaft  26 , differential drive gear  42 , differential device  28  and axles  30   r ,  301  in this order of description. Further, the drive force of the second electric motor MG 2  is transmitted to the right and left drive wheels  14   r ,  141  through the rotor shaft  64 , drive-force transmission shaft  24 , reduction gear  36 , counter gear  38 , counter shaft  26 , differential drive gear  42 , differential device  28  and axles  30   r ,  301  in this order of description. 
     The casing  44  of the drive-force transmitting apparatus  10  is constituted by a housing  44   a , an axle case  44   b  and a case cover  44   c  that are fixed to one another. The axle case  44   b  opens in its opposite ends that are opposite to each other in the direction of the first axis CL 1 . The axle case  44   b  is fixedly connected at one of the openings to the housing  44   a  through screw bolts, such that the one of the openings is enclosed by the housing  44   a . The axle case  44   b  is fixedly connected at the other of the openings to the case cover  44   c  through screw bolts, such that the other of the openings is enclosed by the case cover  44   c.    
     The axle case  40   b  is provided with the partition wall  48  that extends in a direction perpendicular to the first axis CL 1 . By the partition wall  48 , an interior of the casing  44  is sectioned into a gear room  78  and a motor room  80 , such that various gears or devices such as the planetary gear device  32 , output gear  34 , counter gear  38 , reduction gear  36  and differential device  28  are disposed in the gear room  78  and such that the first and second electric motors MG 1 , MG 2  are disposed in the motor room  80 . 
     The differential ring gear  40  of the differential device  28  meshes with a pump driving gear  76  that is provided to rotate or drive a first oil pump P 1 . The first oil pump P 1  is a mechanical oil pump that is connected to the differential ring gear  40  of the differential device  28  through the pump driving gear  76  in a drive-force transmittable manner. When the differential ring gear  40  of the differential device  28  is rotated, the first oil pump P 1  is mechanically rotated or driven together with rotation of the differential ring gear  40 . 
     The drive-force transmitting apparatus  10  includes, in addition to the first oil pump P 1 , another mechanical pump in the form of a second oil pump P 2  that is be rotated or driven by the engine  12 . The oil pump P 2  is disposed on one of opposite end portions of the input shaft  22  which is remote from the engine  12  in an axial direction of the input shaft  22 . The second oil pump P 2  includes a drive gear (not shown) that is disposed on the above-described one of the opposite end portions of the input shaft  22 , so that the second oil pump P 2  is rotated or driven together with rotation of the engine  12 . Therefore, with the engine  12  being rotated, the second oil pump P 2  is driven whereby the oil is discharged from the second oil pump P 2 . 
       FIG. 3  is a lubrication circuit diagram for explaining construction of a lubrication device  81  for lubricating and cooling the gears, the bearings and the first and second electric motors MG 1 , MG 2  disposed in the vehicle drive-force transmitting apparatus  10 . An oil reservoir  45  is provided in a lower portion of the gear room  78  in a direction of a vertical line, i.e., in a vertical direction, so as to store therein an oil. The lubrication device  81  is constructed such that the oil stored in the oil reservoir  45  is to be supplied to various gears (such as the gears  28 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ) and various bearings (such as the bearings  46 ,  70   a ,  70   b ,  72   a ,  72   b ,  74   b ) that are disposed in the gear room  78  and also to the first and second electric motors MG 1 , MG 2  that are disposed in the motor room  80 . 
     The differential ring gear  40  of the differential device  28  is partially immersed in the oil stored in the oil reservoir  45 , so that the stored oil is scooped up by the differential ring gear  40  when the differential ring gear  40  is rotated. A part of the scooped oil is to be supplied the gears and bearings disposed in the gear room  78 , while the other of the scooped oil is to be supplied to the first and second electric motors MG 1 , MG 2  and the bearings  58   a ,  58   b ,  68   a ,  68   b  supporting the first and second electric motors MG 1 , MG 2  disposed in the motor room  80  through an oil hole (through-hole)  96  (see  FIG. 4 ) that is provided through the partition wall  48 . 
     The first oil pump P 1  is to be rotated or driven together with rotation of the differential ring gear  40 . With the first oil pump P 1  being driven, the oil stored in the oil reservoir  45  is pumped up through a first strainer  88  and is them delivered to a first delivery passage  82 . A part of the oil delivered to the first delivery passage  82  is supplied to the gears and bearings disposed in the gear room  78  while the other of the oil delivered to the first delivery passage  82  is supplied to the first and second electric motors MG 1 , MG 2  disposed in the motor room  80  through the oil hole  96  (see  FIG. 4 ). The second oil pump P 2  is to be rotated or driven by the engine  12 . With the second oil pump P 2  being driven, the oil stored in the oil reservoir  45  is pumped up through a second strainer  90  and is them delivered to a second delivery passage  84 . The oil delivered to the second delivery passage  84  is supplied to the first and second electric motors MG 1 , MG 2  disposed in the motor room  80 . Further, the oil supplied to the second electric motor MG 2  is supplied through a communication hole (through-hole)  98  (see  FIG. 6 ) to the bearing  74   a  by which the differential device  28  is rotatably supported. It is noted that either one of the first and second strainers  88 ,  90  corresponds to “strainer” recited in the appended claims and that either one of the first and second oil pumps P 1 , P 2  corresponds to “oil pump” recited in the appended claim. 
       FIG. 4  is a view of the axle case  44   b  constituting a part of the casing  44 , as seen from a side of the gear room  78 .  FIG. 5  is a view of the axle case  44   b , as seen from a side of the motor room  80 . In  FIG. 4 , an upward direction in the drawing sheet corresponds to an upward direction (in the direction of the vertical line) of the vehicle in an installed state in which the casing  44 , i.e., the drive-force transmitting apparatus  10 , is installed in the vehicle, and a rightward direction in the drawing sheet corresponds to a forward direction of the vehicle in the installed state. It is noted that a vertical direction in the drawing sheet of  FIG. 4  corresponds to a vertical direction of the drive-force transmitting apparatus  10 , and that a lower portion of the casing  44  constitutes a bottom wall  44   d  of the casing  44  which defines the above-described oil reservoir  45 . In  FIG. 5 , an upward direction in the drawing sheet corresponds to the upward direction of the vehicle in the installed state, and a leftward direction in the drawing sheet corresponds to the forward direction of the vehicle in the installed state. 
     In  FIG. 4 , a circle having a center lying on the first axis CL 1  and indicated by one-dot chain line represents a pitch circle of the output gear  34  when the output gear  34  is placed in a predetermined position lying on the first axis CL 1 . Further, a circle having a center lying on the second axis CL 2  and indicated by one-dot chain line represents a pitch circle of the reduction gear  36  when the reduction gear  36  is placed in a predetermined position lying on the second axis CL 2 . Larger and smaller circles having respective centers lying on the third axis CL 3  and indicated by one-dot chain lines represent pitch circles of the counter gear  38  and the differential drive gear  42 , respectively, when the counter gear  38  and the differential drive gear  42  are placed in respective predetermined position lying on the third axis CL 3 . A circle having a center lying on the fourth axis CL 4  and indicated by one-dot chain line represents a pitch circle of the differential ring gear  40  when the differential ring gear  40  is placed in a predetermined position lying on the fourth axis CL 4 . The gear room  78  is a space in which the gears such as the output gear  34 , reduction gear  36 , counter gear  38 , differential ring gear  40  and differential drive gear  42  are stored or disposed. Further,  FIG. 4  shows bearings  70   a ,  72   a ,  74   a  that are fixedly held in the axle case  44   b , wherein the bearing  70   a  supports the drive-force transmission shaft  24 , the bearing  72   a  supports the counter shaft  26  and the bearing  74   a  supports the differential device  28 . It is noted that a circle having a center lying on the second axis CL 2  and indicated by broken line represents a position of the second electric motor MG 2  that is disposed in the motor room  80 . As shown in  FIG. 4 , the bearing  74   a  supporting the differential device  28  is located on a lower side of the second electric motor MG 2 , and overlaps with the second electric motor MG 2  in the direction of the vertical line, i.e., a vertical direction of the vehicle. 
     The oil reservoir  45  is provided in the lower portion of the gear room  78  in the direction of the vertical line, so that the oil is stored in the oil reservoir  45 . The differential ring gear  40  is partially immersed in the oil stored in the oil reservoir  45 . Therefore, during running of the vehicle, the stored oil is scooped up by the differential ring gear  40 , and the scooped oil is supplied to the gears and bearings that are disposed in the gear room  78 . The axle case  44   b  includes an upper portion in the direction of the vertical line. In the upper portion of the axle case  44   b , there are a rib  92  and a catch tank  94  that extend from the partition wall  48  in a direction perpendicular to the partition wall  48 , so that a part of the oil scooped up by the differential ring gear  40  is supplied through the rib  92  to the catch tank  94  whereby the part of the scooped oil is stored in the catch tank  94 . The oil stored in the catch tank  94  is caused to pass through the oil hole  96  provided though the partition wall  48 , so as to be supplied to the motor room  80 . 
     In the lower portion of the gear room  78  (in the direction of the vertical line) in which the oil reservoir  45  is provided, the first and second strainers  88 ,  90  are also provided. Each of the first and second strainers  88 ,  90  has an inlet port that is immersed in the oil stored in the oil reservoir  45  of the gear room  78 , so that the oil stored in the oil reservoir  45  can be sucked through the inlet port. 
     The first strainer  88  is connected to the inlet port of the first oil pump P 1 , so that, with the first oil pump P 1  being driven, the oil stored in the oil reservoir  45  of the gear room  78  is sucked into the first oil pump P 1  through the first strainer  88 , and is delivered to the first delivery passage  82  (that is not shown in  FIG. 4 ). A filter  88   a  (see  FIG. 3 ) is provided inside the first strainer  88 , so that foreign substances mixed or contained in the oil is caused to stick to the filter  88   a , so as to be removed from the oil while the oil is passing through the filter  88   a.    
     The second strainer  90  is connected to the inlet port of the second oil pump P 2 , so that, with the second oil pump P 2  being driven, the oil stored in the oil reservoir  45  of the gear room  78  is sucked into the second oil pump P 2  through the second strainer  90 , and is delivered to the second delivery passage  84  (that is not shown in  FIG. 4 ). A filter  90   a  (see  FIG. 3 ) is provided inside the second strainer  90 , so that the foreign substances mixed or contained in the oil is caused to stick to the filter  90   a , so as to be removed from the oil while the oil is passing through the filter  90   a.    
     In  FIG. 5 , a circle having a center lying on the first axis CL 1  and indicated by one-dot chain line represents an outside diameter of the rotor  52  of the first electric motor MG 1  when the first electric motor MG 1  is placed in a predetermined position lying on the first axis CL 1 . Further, a circle having a center lying on the second axis CL 2  and indicated by one-dot chain line represents an outside diameter of the rotor  62  of the second electric motor MG 2  when the second electric motor MG 2  is placed in a predetermined position lying on the second axis CL 2 . Further,  FIG. 5  shows the bearings  58   b ,  68   b  that are fixedly held in the axle case  44   b , wherein the bearing  58   b  rotatably supports the rotor shaft  54  of the first electric motor MG 1  while the bearing  68   b  rotatably supports the rotor shaft  64  of the second electric motor MG 2 . Between the first and second electric motors MG 1 , MG 2 , a rib  100  is provided to extend in a space between the first and second electric motors MG 1 , MG 2 . With provision of the rib  100 , the oil is stored on a lower side of the second electric motor MG 2 . 
     As described above, the oil stored in the oil reservoir  45  provided in the lower portion of the gear room  78  is scooped up by the differential ring gear  40 , and the gears and bearings disposed in the gear room  78  are lubricated by the scooped oil. However, a loss is caused by the scooping of the oil by the differential ring gear  40  whereby an efficiency is problematically reduced. Further, since the gear room  78  is not provided with a device for increasing temperature of the oil, a drag resistance generated in each of the bearings in the gear room  78  could be problematically increased when the temperature of the oil supplied to the bearings is lowered. 
     For solving these problems, the communication hole  98  is provided through the partition wall  48  by which the interior of the axle case  44   b  is sectioned into the gear room  78  and the motor room  80 , such that the gear room  78  and the motor room  80  are held in communication through the communication hole  98 . The communication hole  98  is provided such that the oil having cooled the second electric motor MG 2  is supplied to the bearing  74   a  by which the differential device  28  is rotatably supported. Specifically described, as shown in  FIG. 4 , the communication hole  98  has a gear-room-side opening  98   a  that is located in a position in which the bearing  74   a  supporting the differential device  28  is located. Further, as shown in  FIG. 5 , the communication hole  98  has a motor-room-side opening  98   b  that is located in a position which is lower than the second electric motor MG 2  and which overlaps with the second electric motor MG 2  in the direction of the vertical line. The motor-room-side opening  98   b  is located on an upper side of the gear-room-side opening  98   a  in the direction of the vertical line. Therefore, when the oil having cooled the second electric motor MG 2  is dropped by its own weight, the dropped oil is caused to flow into the communication hole  98  through the motor-room-side opening  98   b  and to flow out of the communication hole  98  through the gear-room-side opening  98   a . In this instance, owing to provision of the rib  100 , the oil dropped from the second electric motor MG 2  is guided to the motor-room-side opening  98   b  so as to be caused to efficiently flow into the communication hole  98 . Further, with the oil being caused to flow into the communication hole  98  through the motor-room-side opening  98   b , an upper surface or level of the oil stored on a lower side of the electric motor MG 2  is not excessively increased so that the rotor  62  of the second electric motor MG 2  is avoided from being immersed in the oil. Consequently, it is possible to avoid a loss that could be caused if the rotor  62  is caused to stir the oil, and to avoid a so-called breather blow that could be caused also if the rotor  62  is caused to stir the oil. It is noted that the gear-room-side opening  98   a  and the motor-room-side opening  98   b  correspond to “gear-room-side opening” and “motor-room-side opening” that are recited in the appended claims, and the differential device  28  correspond to “gear mechanism” recited in the appended claims. 
       FIG. 6  is a cross sectional view taken in a plane containing an axis of the communication hole  98  provided in the axle case  44   b . It is noted that  FIG. 6  shows the bearing  74   a  fitted in the axle case  44   b  and also a part (i.e., cylindrical portion) of a differential case  28   a  of the differential device  28 , which is rotatably supported by the bearing  74   a.    
     As shown in  FIG. 6 , in the motor room  80 , the communication hole  98  has the gear-room-side opening  98   a  and the motor-room-side opening  98   b , such that the communication hole  98  opens at the gear-room-side opening  98   a  in the gear room  78  and opens at the motor-room-side opening  98   b  in the motor room  80 . The gear-room-side opening  98   a  and the motor-room-side opening  98   b  open in respective walls that are parallel to the fourth axis CL 4 . One of the walls in which the gear-room-side opening  98   a  of the communication hole  98  opens in the gear room  78 , is an inner circumferential wall  108  in which an outer race  102  of the bearing  74   a  is fitted. The bearing  74   a  is a tapered roller bearing including the outer race  102  that is fitted at its outer circumferential surface with the inner circumferential wall  108 , an inner race  104  that is fitted at its inner circumferential surface with the cylindrical portion of the differential device  28 , and a plurality of tapered rollers  106  that are interposed between the outer and inner races  102 ,  104 . Each of the tapered rollers  106  has a small-diameter axial end face and a large-diameter axial end face that has a diameter larger than a diameter of the small-diameter axial end face. 
     The motor-room-side opening  98   b  is located on an upper side of the bearing  74   a  supporting the differential device  28  in a vertical direction (i.e., direction of the vertical line). Further, the motor-room-side opening  98   b  is located on an upper side of the gear-room-side opening  98   a . Therefore, as indicated by arrows in  FIG. 6 , the oil caused to flow into the communication hole  98  through the motor-room-side opening  98   b , passes through the communication hole  98  and flows out of the communication hole  98  through the gear-room-side opening  98   a . Further, the gear-room-side opening  98   a  is provided in the inner circumferential wall  108  of the axle case  44   b  in which the outer race  102  of the bearing  74   a  is fixedly held, and is held in communication with a gap  110  defined between the inner race  104  and the outer race  102  of the bearing  74   a . Thus, the oil flowing out of the communication hole  98  through the gear-room-side opening  98   a  can be supplied to the bearing  74   a . It is noted that the inner circumferential wall  108  corresponds to “bearing holding wall” recited in the appended claims. 
     As shown in  FIG. 6 , the gear-room-side opening  98   a  is located in a position that is held in communication with a roller-small-diameter-side end face of the tapered roller bearing  74   a  that is disposed on the fourth axis CL 4 , more specifically, in a position that is closer to the roller-small-diameter-side end face of the tapered roller bearing  74   a  than to a roller-large-diameter-side end face of the tapered roller bearing  74   a . The roller-small-diameter-side end face of the tapered roller bearing  74   a  is defined as one of axially opposite end faces of the tapered roller bearing  74   a , wherein the one of the axially opposite end faces includes a part constituted by one of axially opposite faces of the inner race  104  having a larger outside diameter where the inner race  104  has a tapered outer circumferential surface, or includes a part constituted by one of axially opposite faces of the outer race  102  having a smaller inside diameter where the outer race  102  has a tapered inner circumferential surface, as shown in  FIG. 6 . The roller-small-diameter-side end face of the tapered roller bearing  74   a  may be defined as one of axially opposite end faces of the tapered roller bearing  74   a , wherein the one of the axially opposite end faces includes a part constituted by the above-described small-diameter axial end face of each of the tapered rollers  106 . With the gear-room-side opening  98   a  being located in the position that is held in communication with the small-diameter axial end face of the tapered roller bearing  74   a , owing to a pump effect caused by rotation of the inner race  104  of the bearing  74   a , the oil flowing into the gap  110  is drawn in a direction away from the roller-small-diameter-side end face of the tapered roller bearing  74   a  toward the roller-large-diameter-side end face of the tapered roller bearing  74   a , as indicated by arrow in  FIG. 6 . Therefore, the oil flowing out of the communication hole  98  through the gear-room-side opening  98   a  is initiatively guided into the gap  110  and caused to flow in the direction away from the roller-small-diameter-side end face of the tapered roller bearing  74   a  toward the roller-large-diameter-side end face of the tapered roller bearing  74   a , whereby the bearing  74   a  is efficiently lubricated. 
     The oil having flown out of the communication hole  98  through the gear-room-side opening  98   a  has a temperature that has been increased when the oil cooled the second electric motor MG 2 , so that the drag resistance due to viscosity of the oil is reduced when the bearing  74   a  is lubricated by the oil. The oil having lubricated the bearing  74   a  is returned to the oil reservoir  45  of the gear room  78 . In this instance, the oil stored in the oil reservoir  45  is warmed by the oil, which has been warmed when having cooled the second electric motor MG 2  and which has then flown into the oil reservoir  45 , whereby a temperature of the oil stored in the oil reservoir  45  is increased. As a result of the increase of the temperature of the oil stored in the oil reservoir  45 , a resistance to stirring of the stored oil by the differential ring gear  40  is reduced whereby the loss caused by the scooping of the stored oil by the differential ring gear  40  is reduced. 
     By the way, as described above, in the second electric motor MG 2 , the insulation sheet  69  including the foam resin layer  71  is interposed between the stator core  60  and each of the stator cores  66  wound on the stator core  60 , so that the foam resin could be mixed into the oil when the oil cools the second electric motor MG 2 . If the oil containing the foam resin is returned to the oil reservoir  45  of the gear room  78 , the foam resin could be caused to adhere to the filters  88   a ,  90   a  of the respective first and second strainers  88 ,  90  when the oil is sucked through the filters  88 ,  90 . Consequently, a pressure loss could be caused by clogging of the filters  88   a ,  90   a  of the respective strainers  88 ,  90 , thereby causing a risk of reduction of an energy loss. 
     However, in the present embodiment, the oil having cooled the second electric motor MG 2  is caused to pass through the communication hole  98  and is supplied to the bearing  74   a  supporting the differential device  28 , and the foreign substances such as the foam resin are crushed between the tapered rollers  106  and the outer and inner races  102 ,  104  of the bearing  74   a  when the oil is passing through the gap  110  of the bearing  74   a . In this instance, since the bearing  74   a  is constituted by the tapered roller bearing, the bearing  74   a  has surfaces effective to crush the foreign substances, wherein areas of the effective surfaces are larger than in a case in which the bearing  74   a  is constituted by another type of bearing such as a deep groove ball bearing, so that the foreign substances can be efficiently crushed. Further, a larger load is applied to the bearing  74   a  supporting the differential device  28  as compared with other bearings supporting gears in the drive-force transmitting apparatus  10 , so that the foreign substances can be crushed by a larger load. Therefore, the foreign substances such as the foam resin can be crushed into smaller pieces or powders, whereby the clogging of the filters  88   a ,  90   a  of the respective strainers  88 ,  90  can be more reliably suppressed. 
     Further, the gear-room-side opening  98   a  is provided in a position close to a region of the bearing  74   a  in a circumferential direction of the outer race  102  of the bearing  74   a  in which a load applied to the bearing  74   a  during running of the vehicle is small, so that the oil is moved from the region of the bearing  74   a  to another region of the bearing  74   a  to which a larger load is applied to the bearing  74   a . Therefore, with rotation of the bearing  74   a , the oil is caused to flow from the above-described region of the bearing  74   a  to which the smaller load is applied, to the above-described another region of the bearing  74   a  to which the larger load is applied. 
     In  FIG. 4 , arrow F 1  radially extending from the fourth axis CL 4  corresponds to a power running load F 1  that is to be applied to the bearing  74   a  during forward running of the vehicle, and arrow F 2  radially extending from the fourth axis C 4  corresponds to a regenerative load F 2  that is to be applied to the bearing  74   s  during regenerative running of the vehicle. As shown in  FIG. 4 , the position of the gear-room-side opening  98   a  is distant from these arrows F 1 , F 2  in the circumferential direction of the outer race  102  of the bearing  74   a . That is, the position of the gear-room-side opening  98   a  is close to the region of the bearing  74  in which the load applied to the bearing  74  is small. Therefore, the oil is caused to flow into the gap  110  of the bearing  74   a  from the region in which the load applied to the bearing  74   a  is small, and is then moved to the another region in which the load applied to the bearing  74   a  is large, by rotation of the bearing  74   a , whereby the foreign substances contained in the oil can be efficiently crushed. 
     As described above, in the present embodiment, the gear-room-side opening  98   a  of the communication hole  98  is provided in the inner circumferential wall  108  of the axle case  44   b  in which the bearing  74   a  is held, and is in held in communication with the gap  110  defined between the inner race  104  and the outer race  102  of the bearing  74   a . Owing to this construction, the oil, which has been moved from the motor room  80  to the gear room  78  through the communication hole  98 , is caused to flow into the gap  110  of the bearing  74   a . In this instance, when the oil is passing through the gap  110  of the bearing  74   a , the foreign substances mixed or contained in the oil are crushed by the bearing  74   a  into smaller pieces. Therefore, for example, when the oil stored in the casing  44  is pumped up by the oil pumps P 1 , P 2  through the respective strainers  88 ,  90 , it is possible to suppress clogging of each of the filters  88   a ,  90   a  of the respective strainers  88 ,  90 . 
     In the above-described embodiment, when the foam resin is mixed into the oil in process of cooling the second electric motor MG 2  by the oil, it is possible to cause the foam resin to be crushed by the bearing  74   a . Further, the bearing  74   a  is constituted by the tapered roller bearing, it is possible to increase the areas of the surfaces by which the foreign substances are to be crushed, as compared with an arrangement in which the bearing  74   a  is constituted by another type of bearing such as a ball bearing, whereby the foreign substances can be efficiently crushed. Still further, the gear-room-side opening  98   a  of the communication hole  98  is located in the position closer to the roller-small-diameter-side end face of the tapered roller bearing  74   a  than to the roller-large-diameter-side end face of the tapered roller bearing  74   a , so that much of the oil flowing out through the gear-room-side opening  98   a  can be efficiently introduced into the bearing  74   a , by utilizing flow of the oil, which is forced to be drawn in the direction away from the small-diameter axial end face of the tapered roller bearing  74   a  toward the large-diameter axial end face of the tapered roller bearing  74   a , when the tapered roller bearing  74   a  is rotated. Moreover, since a large load is to be applied to the bearing  74   a  supporting the differential device  28 , the foreign substances contained in the oil can be efficiently crushed by the bearing  74   a  in process of lubricating the bearing  74   a  by the oil. 
     While the preferred embodiment of this invention has been described in detail by reference to the drawings, it is to be understood that the invention may be otherwise embodied. 
     For example, in the above-described embodiment, in the above-described embodiment, the lubrication device  81  is constructed such that the oil having cooled the second electric motor MG 2  is to be supplied to the bearing  74   a  through the communication hole  98 . However, the lubrication device  81  may be constructed such that the oil is to be supplied to the bearing  74   a  through another oil passage or passages in addition to the communication hole  98 . For example, it is possible to provide, in addition to the communication hole  98 , another communication hole or holes held in communication with the bearing  74   a  in the gear room  78 , such that a part of the oil scooped up by the differential ring gear  40  and/or a part of the oil pumped up by the first oil pump P 1  can be supplied to the another communication hole or holes. In this arrangement with the another communication hole or holes, the part of the oil scooped up by the differential ring gear  40  and/or the part of the oil pumped up by the first oil pump P 1  are additionally supplied to the bearing  74   a  whereby an amount of the oil supplied to the bearing  74   a  can be increased. Consequently, a larger amount of the foreign substances can be crushed by the bearing  74   a  so that it is possible to more effectively suppress the clogging of each of the filters  88   a ,  90   a  of the respective strainers  88 ,  90 . 
     In the above-described embodiment, the bearing  74   a  is constituted by the tapered roller bearing. However, the bearing  74   a  may be constituted by any other type bearing such as a cylindrical roller bearing. 
     In the above-described embodiment, the lubrication device  81  is constructed such that the oil flowing out of the communication hole  98  through the gear-room-side opening  98   a  is to be supplied to the bearing  74   a  supporting the differential device  28 . However, the lubrication device  81  may be modified such that the oil flowing out of the communication hole  98  through the gear-room-side opening  98   a  is to be supplied to another bearing such as the bearings  70   a ,  72   a  supporting another rotary member. 
     It is to be understood that the embodiment described above is given for illustrative purpose only, and that the present invention may be embodied with various modifications and improvements which may occur to those skilled in the art. 
     NOMENCLATURE OF ELEMENTS 
     
         
           10 : vehicle drive-force transmitting apparatus 
           28 : differential device (gear mechanism) 
           44 : casing 
           45 : oil reservoir 
           48 : partition wall 
           60 : stator core 
           66 : stator coil 
           69 : insulation sheet 
           71 : foam resin layer 
           74   a : bearing (tapered roller bearing) 
           78 : gear room 
           80 : motor room 
           81 : lubrication device 
           88 : first strainer (strainer) 
           90 : second strainer (strainer) 
           98 : communication hole 
           98   a : gear-room-side opening 
           98   b : motor-room-side opening 
           102 : outer race 
           104 : inner race 
           108 : inner circumferential wall (bearing holding wall) 
           110 : gap 
         MG 2 : second electric motor (motor) 
         P 1 :first oil pump (oil pump) 
         P 2 : second oil pump (oil pump)