Patent Publication Number: US-2018051794-A1

Title: Power transmission device and work vehicle provided with power transmission device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2015/066521, filed on Jun. 8, 2015. 
    
    
     BACKGROUND 
     Field of the Invention 
     The present description discloses a power transmission device for a work vehicle. 
     Description of the Related Art 
     A power transmission device has a function for distributing the driving power generated by one power generation source such as an engine, to a plurality of driven devices (for example, a speed reduction gear or a hydraulic pump and the like), and for integrating the driving power generated by the plurality of power generation sources. A spline fitting in a portion for coupling each of the rotating members of the plurality of power generation sources and the plurality of driven devices is often used in this type of power transmission device. A lubrication structure is used in order to prevent wear and damage to the spline fitting section. 
     Japanese Laid-open Patent Publication No. 2013-241792 describes an example of a lubrication structure for a coupling section that has a spline groove on the internal circumferential surface of a motor shaft and in which the pump shaft of a hydraulic pump acts as the spline shaft that fits to the spline groove. An annular oil sump chamber is formed around the pump shaft and the motor shaft and a spline fitting section for the pump shaft and the motor shaft is lubricated by lubricating oil that fills the oil sump chamber in the invention as in Japanese Laid-open Patent Publication No. 2013-241792. 
     SUMMARY 
     However, because the structure in Japanese Laid-open Patent Publication No. 2013-241792 does not take into account the circulation of the lubricating oil that fills the oil sump chamber, degradation of the filled lubricating oil occurs which leads to a drop in lubricity. 
     The present description discloses a power transmission device having a structure in which a reduction in the lubricity of a spline fitting section can be suppressed, and a work vehicle provided with the power transmission device. 
     A power transmission device according to one aspect of the present invention includes a boss section, a spline shaft, a rotational force transmission section, and a housing. The boss section has a spline groove on the internal circumferential surface thereof. The spline shaft fits to the spline groove. The rotational force transmission section is coupled with the boss section or the spline shaft. The housing includes: an oil sump section for retaining lubricating oil; an oil inlet path for introducing the lubricating oil into the oil sump section; and an oil drain path for draining the lubricating oil retained in the oil sump section. The oil sump section is in communication with a fitting section where the spline shaft and the spline groove are fitted to each other. The oil drain path is positioned higher than the lower end of the spline groove. 
     The housing or the boss section may include a protruding section positioned on the axial inward side of the oil sump section. 
     The power transmission device may further include a bearing for rotatably supporting the boss section. The protruding section may overlap the region between an inner ring and an outer ring of the bearing as seen in the direction of the rotational axis of the bearing. 
     The oil drain path may further be positioned higher than the upper end of the spline groove. 
     The boss section may include an oil drain hole that extends from a lubricating oil intake port positioned on the internal circumferential surface further to the axial depth side than the spline groove, to a lubricating oil drainage port positioned on the external circumferential surface of the boss section. 
     The protruding section may be provided on the housing. In this case, a first gap may be provided between the protruding section and the boss section. At least a portion of the first gap may be positioned further to the axial inward side than an oil discharge port of the oil inlet path and a drain port of the oil drain path. The oil drain path may pass through the protruding section in the axial direction. Alternatively, the oil drain path may be provided in the housing, may extend to the outside in the radial direction from the discharge port positioned further in the radial outward direction than the protruding section, and may pass through the housing. The power transmission device may further include a first seal that seals the first gap. 
     The power transmission device may be provided with a second gap between the protruding section and the inner ring of the bearing. At least a portion of the second gap may be positioned further to the axial inward side than the oil discharge port of the oil inlet path and the drain port of the oil drain path. The oil drain path may pass through the protruding section in the axial direction. Alternatively, the oil drain path may be provided in the housing, may extend in the radial outward direction from the drain port positioned further to the axial outward side than the protruding section, and may pass through the housing. The power transmission device may further include a second seal that seals the second gap. 
     The protruding section may be provided on the boss section. In this case, a third gap may be provided between the protruding section and the housing. At least a portion of the third gap may be positioned further to the axial inward side than the oil discharge port of the oil inlet path and the drain port of the oil drain path. The oil drain path may be provided in the housing, may extend in the radial outward direction from the drain port positioned further to axial outward side than the protruding section, and may pass through the housing. The power transmission device may further include a third seal that seals the third gap. 
     The power transmission device may be provided with a fourth gap between the protruding section and the outer ring of the bearing. At least a portion of the fourth gap may be positioned further to the axial inward side than the oil discharge port of the oil inlet path and the drain port of the oil drain path. The oil drain path may be provided in the housing, may extend in the radial outward direction from the drain port positioned further in the axial outward direction than the protruding section, and may pass through the housing. The power transmission device may further include a fourth seal that seals the fourth gap. 
     The protruding section may be formed as a separate part from the boss section and from the housing. The protruding section may be fixed to the boss section or the housing. 
     The amount per unit of time of the lubricating oil flowing into the oil sump section from the oil inlet path may be equal to or greater than the amount per unit of time of the lubricating oil flowing out from a location other than the oil drain path of the oil sump section. The amount per unit of time of the lubricating oil flowing into the oil sump section from the oil inlet path may be equal to the total amount of the amount per unit of time of the lubricating oil flowing out from the oil drain path and the amount per unit of time of the lubricating oil flowing out from the location other than the oil drain path. 
     The lubricating oil flowing out from the locations other than the oil drain path may include the lubricating oil flowing out from the fitting section where the spline shaft and the spline groove are fitted to each other. When the first gap is provided, the lubricating oil flowing out from the location other than the oil drain path may include the lubricating oil flowing out from the first gap. When the second gap is provided, the lubricating oil flowing out from the location other than the oil drain path may include the lubricating oil flowing out from the second gap. When the third gap is provided, the lubricating oil flowing out from the location other than the oil drain path may include the lubricating oil flowing out from the third gap. When the fourth gap is provided, the lubricating oil flowing out from the location other than the oil drain path may include the lubricating oil flowing out from the fourth gap. 
     A work vehicle according to one aspect of the present invention may include the aforementioned power transmission device and a first external connection apparatus. The first external connection apparatus may be fixed to the housing. The first external connection apparatus may include a spline shaft. The oil sump section may be formed within a region surrounded by the first external connection apparatus, the protruding section, the boss section, and the spline shaft. 
     The work vehicle may further include a second external connection apparatus fixed to a side surface of the housing which is the side opposite the side surface of the housing where the first external connection apparatus is fixed, the second external connection apparatus including a drive shaft inserted into a hole section of the boss section. 
     The work vehicle may further include a driving source, a transmission, and a power train. The driving source may include an output shaft that rotates around a rotating shaft different from the spline shaft. The transmission may be coupled to the output shaft. The power train may be coupled to the output shaft and may transmitting a portion of the driving power from the driving source to the rotational force transmission section. 
     Effects of Invention 
     In the above power transmission device, at least a portion of a spline fitting section where the spline groove and the spline shaft are fitted to each other is reliably covered in the lubricating oil and reliably lubricated. Moreover, when the lubricating oil supplied from the oil inlet path in the oil sump section is accommodated up to the height of the oil drain path, a sufficient amount of the lubricating oil is drained by the oil drain path. In this way, the lubricating oil of the spline fitting section circulates without being retained, and consequently a favorable lubricity can be maintained. Therefore, wear and damage to the spline fitting section due to poor lubrication, or more specifically, wear and damage to the spline teeth surfaces, is prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a work vehicle according to an embodiment of the present invention. 
         FIG. 2  is a schematic view of a configuration of the work vehicle. 
         FIG. 3  is a schematic view of an example of a lubrication system for a power transmission device. 
         FIGS. 4A and 4B  are cross-sectional views of a lubrication structure of the power transmission device according to a first embodiment. 
         FIG. 5  is a view of a housing seen from the axial outward side according to the first embodiment. 
         FIG. 6  is a cross-sectional view as seen from cross-sectional plane II-II′ in  FIG. 5 . 
         FIG. 7  is a cross-sectional view of a lubrication structure of the power transmission device according to a second embodiment. 
         FIGS. 8A and 8B  are cross-sectional views of a lubrication structure of the power transmission device according to a third embodiment. 
         FIG. 9  is a view of a housing seen from axial outward side according to the third embodiment. 
         FIG. 10  is a cross-sectional view of a lubrication structure of the power transmission device according to a fourth embodiment. 
         FIGS. 11A and 11B  are cross-sectional views of a lubrication structure of the power transmission device according to a fifth embodiment. 
         FIG. 12  is a view of a housing seen from the axial outward side according to the fifth embodiment. 
         FIG. 13  is a cross-sectional view as seen from cross-sectional plane V-V′ in  FIG. 12 . 
         FIGS. 14A and 14B  are cross-sectional views of a lubrication structure of the power transmission device according to a sixth embodiment. 
         FIG. 15  illustrates a first modified example. 
         FIG. 16  illustrates a second modified example. 
         FIG. 17  illustrates a third modified example. 
         FIG. 18  illustrates a fourth modified example. 
         FIG. 19  illustrates a fifth modified example. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     (Overall Configuration of Work Vehicle) 
       FIG. 1  illustrates an overall configuration of a work vehicle  1  which includes a lubrication structure according to a first embodiment of the present invention. The work vehicle  1  is, for example, a wheel loader. As illustrated in  FIG. 1 , the work vehicle  1  includes a vehicle body frame  2 , a work implement  3 , traveling wheels  4  and  5 , and an operator&#39;s cab  6 . The work vehicle  1  travels by rotating and driving the traveling wheels  4  and  5 . The work vehicle  1  is able to carry out work such as excavation by using the work implement  3 . 
     The vehicle body frame  2  includes a front frame  16  and a rear frame  17 . The front frame  16  and the rear frame  17  are attached to each other in a manner that allows tilting in the left-right direction. The work implement  3  and the traveling wheels  4  are attached to the front frame  16 . The work implement  3  is driven by hydraulic fluid from a belowmentioned work implement pump  23  (see  FIG. 2 ). The work implement  3  includes a boom  11  and a bucket  12 . The boom  11  is mounted onto the front frame  16 . 
     The work implement  3  has a lift cylinder  13  and a bucket cylinder  14 . The lift cylinder  13  and the bucket cylinder  14  are hydraulic cylinders. One end of the lift cylinder  13  is attached to the front frame  16 . The other end of the lift cylinder  13  is attached to the boom  11 . The boom  11  rotates up and down due to the extension and contraction of the lift cylinder  13  with hydraulic fluid from the work implement pump  23 . 
     The bucket  12  is attached to the tip of the boom  11 . One end of the bucket cylinder  14  is attached to the front frame  16 . The other end of the bucket cylinder  14  is attached to the bucket  12  via a bell crank  15 . The bucket  12  rotates up and down due to the extension and contraction of the bucket cylinder  14  with hydraulic fluid from the work implement pump  23 . 
     The operator&#39;s cab  6  and the traveling wheels  5  are attached to the rear frame  17 . A seat on which an operator sits, a monitor for displaying various states inside the work vehicle  1  to the operator, and an operating device and the like are disposed inside the operator&#39;s cab  6 . 
     The work vehicle  1  includes a steering cylinder  18 . The steering cylinder  18  is attached to the front frame  16  and the rear frame  17 . The steering cylinder  18  is a hydraulic cylinder. The front frame  16  and the rear frame  17  of the work vehicle  1  are tilted in the left-right direction due to the extension and contraction of the steering cylinder  18  due to hydraulic fluid from a belowmentioned steering pump  28 . Consequently, the traveling direction of the work vehicle  1  is changed to the left or right. 
       FIG. 2  is a schematic view of a configuration of the work vehicle  1  according to an embodiment of the present invention. As illustrated in  FIG. 2 , the work vehicle  1  includes an engine  21 , a PTO  22 , a transmission  24 , and travel devices  26   a  and  26   b . The PTO  22  and the transmission  24  are referred to collectively as a power transmission device  25  in the present embodiment. 
     The engine  21  is a driving source for the work vehicle  1  and is, for example, a diesel engine. The output of the engine  21  is controlled by adjusting the amount of air and the amount of fuel injected into the cylinders of the engine  21 . 
     The work vehicle  1  includes, for example, a work implement pump  23 , the steering pump  28 , a transmission pump  29 , and a lubricating oil pump  56 . The work implement pump  23 , the steering pump  28 , the transmission pump  29 , and the lubricating oil pump  56  may be referred to as external connection apparatuses in the present embodiment. The work implement pump  23 , the steering pump  28 , the transmission pump  29 , and the lubricating oil pump  56  are hydraulic pumps. The PTO  22  transmits a portion of the driving power from the engine  21  to the hydraulic pumps  23 ,  28 ,  29  and  56 . That is, the PTO  22  distributes the driving power from the engine  21  to the hydraulic pumps  23 ,  28 ,  29  and  56  and to the transmission  24 . 
     The work implement pump  23  is driven by driving power from the engine  21 . Oil discharged from the work implement pump  23  is supplied to the abovementioned lift cylinder  13  and the bucket cylinder  14  through a work implement control valve  41 . The work implement pump  23  is, for example, a variable displacement hydraulic pump. The discharge displacement of the work implement pump  23  is changed by changing the tilt angle of a swash plate or an inclined shaft of the work implement pump  23 . 
     The steering pump  28  is driven by driving power from the engine  21 . Oil discharged from the steering pump  28  is supplied to the abovementioned steering cylinder  18  through a steering control valve  43 . The steering pump  28  is, for example, a variable displacement hydraulic pump. The discharge displacement of the steering pump  28  is changed by changing the tilt angle of a swash plate or an inclined shaft of the steering pump  28 . 
     The transmission pump  29  is driven by driving power from the engine  21 . The transmission pump  29  is, for example, a fixed displacement hydraulic pump. Oil discharged from the transmission pump  29  is supplied to a clutch  24   a  of the transmission  24  via a clutch control valve  44 . The oil discharged from the transmission pump  29  is supplied to the power transmission device  25  and may be used for lubricating the various gears, bearings and sliding parts of the power transmission device  25 . 
     The lubricating oil pump  56  is driven by driving power from the engine  21 . The lubricating oil pump  56  is, for example, a fixed displacement hydraulic pump. The oil discharged from the lubricating oil pump  56  is supplied to the power transmission device  25  and may be used for lubricating the various gears, bearings and sliding parts of the power transmission device  25 . As explained below, the oil supplied to the power transmission device  25  for lubricating the various gears, bearings and sliding parts of the power transmission device  25  among the oil discharged from the abovementioned pumps, is a lubricating oil. If the lubricating oil is supplied to the power transmission device  25  by the transmission pump  29 , the lubricating oil pump  56  may be omitted. 
     The PTO  22  distributes the driving power from the engine  21  to the transmission  24  and the belowmentioned external connection apparatuses. In  FIG. 2 , the work vehicle  1  is depicted as having one PTO  22 , but the work vehicle  1  may have a plurality of PTOs  22 . The PTO  22  includes a drive gear  62  and a PTO gear  70 . The drive gear  62  is coupled to an output shaft  61  of the engine  21  that rotates around a rotating shaft different from a first drive shaft  80  and a second drive shaft  81 . The PTO gear  70  meshes with the drive gear  62 . The drive gear  62  couples the first drive shaft  80  and the second drive shaft  81  through the PTO gear  70 . The drive gear  62  transmits a portion of the driving power from the engine  21  to the PTO gear  70 . In the following explanation, the drive gear  62  may be referred to as a power train. 
     The first drive shaft  80  is a rotating drive shaft of a first external connection apparatus (for example, the work implement pump  23 , the steering pump  28 ). The second drive shaft  81  is a rotating drive shaft of a second external connection apparatus (for example, the transmission pump  29 , the lubricating oil pump  56 ). The first drive shaft  80  and the second drive shaft  81  are coupled to the PTO gear  70 . The first drive shaft  80  and the second drive shaft  81  are coupled to the PTO gear  70 , for example, with a spline fit. A portion of the driving power from the engine  21  is transmitted to the first drive shaft  80  and the second drive shaft  81 , and the first drive shaft  80  and the second drive shaft  81  respectively drive the first external connection apparatus and the second external connection apparatus. 
     The PTO gear  70  may mesh with the drive gear  62  via an idler gear that is not illustrated.  FIG. 2  illustrates a case in which the first external connection apparatus is the work implement pump  23  and the steering pump  28 , and the second external connection apparatus is the transmission pump  29  and the lubricating oil pump  56 . However, the first external connection apparatus may include at least one of the transmission pump  29  and the lubricating oil pump  56 , and the second external connection apparatus may include at least one of the work implement pump  23  and the steering pump  28 . Moreover, the first external connection apparatus or the second external connection apparatus may also be a fan pump or another device. Furthermore, the second external connection apparatus may be omitted. 
     The transmission  24  is coupled to the output shaft  61  of the engine  21 . The transmission  24  transmits the driving power from the engine  21  to the travel device  26   a  of the front wheel shaft and the travel device  26   b  of the rear wheel shaft. The transmission  24  changes the speed and outputs the driving power from the engine  21  by switching a transmission gear ratio by engaging or disengaging the clutch  24   a.    
     The travel device  26   a  of the front wheel shaft includes an axle  45   a  of the front wheel shaft and traveling wheels  4  of the front wheel shaft. The travel device  26   b  of the rear wheel shaft has an axle  45   b  of the rear wheel shaft and traveling wheels  5  of the rear wheel shaft. The travel devices  26   a  and  26   b  are driven by the engine  21  via the transmission  24 . The axles  45   a  and  45   b  transmit the driving power from the transmission  24  to the traveling wheels  4  and  5 . As a result, the traveling wheels  4  and  5  are rotated and driven. 
     The following is an explanation of a lubrication system  63  of the power transmission device  25 .  FIG. 3  is a schematic view of an example of a configuration of the lubrication system  63  of the power transmission device  24 . The lubrication system  63  causes lubricating oil for lubricating the power transmission device  25  to circulate. As illustrated in  FIG. 3 , the lubrication system  63  includes the transmission pump  29 , the lubricating oil pump  56 , an oil cooler  57 , a pipe  58 , and a filter  59 . The transmission pump  29 , the lubricating oil pump  56 , the oil cooler  57 , and the filter  59  are connected to a housing  90  via the pipe  58 . 
     While  FIG. 3  illustrates an example in which two hydraulic pumps including the transmission pump  29  and the lubricating oil pump  56  are included in the lubrication system  63 , only the transmission pump  29  may be included in the lubrication system  63 . Moreover, the lubrication system  63  may include a plurality of the transmission pumps  29  and lubricating oil pumps  56 . 
     The housing  90  is a case for accommodating the configuration elements of the power transmission device  25  such as the various gears (including the drive gear  62  and the PTO gear  70  of the PTO  22 ) of the power transmission device  25  or the clutch  24   a  of the transmission  24 . The first external connection apparatus  64  and the second external connection apparatus  66 , which respectively include at least one of the work implement pump  23 , the steering pump  28 , the transmission pump  29 , and the lubricating oil pump  56 , for example, are attached to the housing  90 . That is, the power transmission device  25  is connected to the first external connection apparatus  64  and the second external connection apparatus  66 . The filter  59  is provided in the pipe  58  between the transmission pump  29  and the clutch control valve  44 . 
     Oil inside the housing  90  is stored inside a tank section  65  at the lower end of the housing  90 . The lubricating oil pump  56  sends the oil inside the tank section  65  to the housing  90 . The lubricating oil sent by the lubricating oil pump  56  is returned to the tank section  65  after lubricating the various gears, bearings and sliding parts inside the housing  90 . The transmission pump  29  sends the oil to the filter  59 . The filter  59  removes impurities inside the oil. Thereafter, the oil is cooled by the oil cooler  57 , supplied to the transmission clutch  24   a  and the inside of the housing  90 , and is used for the operations of the clutch  24   a  and for lubrication inside the housing  90 . 
     Lubrication Structure (First Embodiment) 
       FIGS. 4A and 4B  are cross-sectional views of the lubrication structure of the power transmission device  25  according to the first embodiment.  FIG. 4B  is an enlargement of the portion outlined by the dotted line in  FIG. 4A . The illustrations of the drive gear  62  and the idler gear are omitted in  FIG. 4A . When referring to  FIG. 4A , the power transmission device  25  includes the PTO gear  70 , a first bearing  82 , a second bearing  83 , and the housing  90 . 
     The first external connection apparatus  64  includes the first drive shaft  80 . The surface of the first drive shaft  80  is fabricated with teeth for fitting with the spline groove. That is, the first drive shaft  80  is a spline shaft. The second external connection apparatus  66  includes the second drive shaft  81 . One end section of the PTO gear  70  is coupled to the first drive shaft  80  and the other end section is coupled to the second drive shaft  81 . That is, the first external connection apparatus  64  and the second external connection apparatus  66  are also driven by the PTO gear  70 . As a result, the first external connection apparatus  64  and the second external connection apparatus  66  are disposed on opposite sides of the housing  90  with the PTO gear  70  interposed therebetween. That is, the second external connection apparatus  66  is fixed to the side surface of the housing  90  that is on the opposite side of the side surface of the housing  90  to which the first external connection apparatus  64  is fixed. The rotational center axis of the first drive shaft  80  of the first external connection apparatus  64  and the rotational center axis of the second drive shaft  81  of the second external connection apparatus  66  are disposed coaxially with the rotational center axis of the PTO gear  70 . 
     The PTO gear  70  includes a rotational force transmission section  71  and a boss section  73 . The boss section  73  includes spline groove  76  on the internal circumferential surface at the side coupled with the first external connection apparatus  64 . The spline groove  76  meshes with the first drive shaft  80 . That is, the first drive shaft  80  is a spline shaft that is fitted with the spline groove  76  of the boss section  73 . The boss section  73  further includes a first gear side bearing support section  74  and a second gear side bearing support section  75 . The first gear side bearing support section  74  is supported by the first bearing  82 . The second gear side bearing support section  75  is supported by the second bearing  83 . 
     The first gear side bearing support section  74  includes a hole section  74   i . A continuous hole  73   i  is provided between the hole section  74   i  and the spline groove  76 . The second drive shaft  81  is inserted into the hole section  74   i . The second drive shaft  81  may be, for example, a spline shaft and may be fixed by fitting with a spline groove provided in the hole section  74   i . Moreover, the second drive shaft  81  may be fixed to the hole section  74   i  with another fixing means such as a key or a screw and the like. 
     The housing  90  includes a first case side bearing support section  91  and a second case side bearing support section  92 . The first bearing  82  is supported by the first case side bearing support section  91 . The second bearing  83  is supported by the second case side bearing support section  92 . Therefore, the PTO gear  70  is rotatably supported by the housing  90  via the first bearing  82  and the second bearing  83 . The first bearing  82  and the second bearing  83  also rotatably support the boss section  73 . 
     The rotational center axis of the PTO gear  70  is established by the line C-C′. In the following explanation, the explanations of direction are made based on the central axis C-C′. For example, the axial direction refers to the direction along the central axis C-C′, and the radial direction is a direction perpendicular to the central axis C-C′. Therefore, the radial direction in  FIG. 4A  signifies the up-down direction in the drawing. 
     Furthermore, the axial outward direction is the direction from the axial center (illustrated by center axis D-D′ in  FIG. 4A ) of the PTO gear  70  toward the first external connection apparatus  64  or the second external connection apparatus  66 . Furthermore, the axial inward direction is the direction from the first external connection apparatus  64  or the second external connection apparatus  66  toward the axial center D-D′ of the PTO gear  70 . Therefore, the phrase “the member A is positioned further to the axial outward side than the member B” signifies that the member A is further away from the axial center D-D′ than the member B in the axial direction. In contrast, the phrase “the member A is positioned further to the axial inward side than the member B” signifies that the member A is closer to the axial center D-D′ than the member B in the axial direction. 
     In the present embodiment, the term the “axial depth side” may be used as another meaning for the axial inward side. The axial depth side signifies the continuous hole  73   i  side between the hole section  74   i  and the spline groove  76 . 
     The radial outward direction signifies the direction away from the central axis C-C′ and the radial inward direction signifies the direction approaching the central axis C-C′. Therefore, the phrase “the member A is positioned further to the radial outward side than the member B” signifies that the member A is further away from the central axis C-C′ than the member B. In contrast, the phrase “the member A is positioned further to the radial inward side than the member B” signifies that the member A is closer to the central axis C-C′ than the member B. 
     The first gear side bearing support section  74  includes a first gear step section  74   a  that abuts the inner ring of the first bearing  82  and that restricts the first bearing  82  from moving in the axial inward direction. The first case side bearing support section  91  includes a first case step section  91   a  that abuts the outer ring of the first bearing  82  and restricts the first bearing  82  from moving in the axial outward direction. As illustrated in  FIG. 4B , the second gear side bearing support section  75  includes a second gear step section  75   a  that abuts an inner ring  83   a  of the second bearing  83  and that restricts the second bearing  83  from moving in the axial inward direction. The second case side bearing support section  92  includes a second case step section  92   a  that abuts an outer ring  83   b  of the second bearing  83  and that restricts the second bearing  83  from moving in the axial outward direction. 
     However as illustrated in  FIG. 4A , normally a very small gap is provided between the outer ring of the first bearing  82  and the first case step section  91   a , or between the outer ring  83   b  of the second bearing  83  and the second case step section  92   a  in consideration of the dimensional tolerances and thermal expansion and the like of the parts when supporting with two deep groove ball bearings. 
     The rotational force transmission section  71  includes teeth for fitting with, for example, the drive gear  62  or the idler gear. The rotational force transmission section  71  is connected to the boss section  73 . Specifically, the rotational force transmission section  71  is formed integrally with the boss section  73 . The rotational force transmission section  71  extends in the radial outward direction from the boss section  73 . 
     The housing  90  includes a protruding section  98  on the axial outward side of the second case side bearing support section  92 . The protruding section  98  protrudes in the radial inward direction from the second case side bearing support section  92 . As a result, as illustrated in  FIG. 4B , the protruding section  98  overlaps a region Zm (region between the outer periphery of the inner ring  83   a  and the inner periphery of the outer ring  83   b ) between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as seen in the rotational axis C-C′ direction of the second bearing  83 . More specifically, the protruding section  98  overlaps a total region Za (region between the inner periphery of the inner ring  83   a  and the outer periphery of the outer ring  83   b ) of the second bearing  83  as seen in the rotational axis C-C′ direction of the second bearing  83 . A first gap  110  is provided between the protruding section  98  and the boss section  73 . 
     The housing  90  includes an extending section  95  that extends from the protruding section  98  in the axial outward direction of the protruding section  98 . The extending section  95  is a portion of the side wall of the housing  90 . The housing  90  includes an attachment surface  95   a  for attaching the first external connection apparatus  64  to the axial direction outer end of the extending section  95 . Moreover, the housing  90  includes an outer step section  96  formed on the axial direction outer end and the radial direction inner end of the extending section  95 . The outer step section  96  is an indentation for inserting a belowmentioned O-ring  101 . Screw holes (not illustrated) for fixing the external connection apparatuses are formed in the housing  90 , and the first external connection apparatus  64  abuts the attachment surface  95   a  and is fixed to the housing  90  with a bolt (not illustrated). An oil sump section  97  is formed inside a region surrounded by the first external connection apparatus  64 , the extending section  95 , the protruding section  98 , the boss section  73 , and the first drive shaft  80 . The oil sump section  97  retains lubricating oil for lubricating the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft). The oil sump section  97  is in communication with the fitting section of the first drive shaft  80  (spline shaft) and the spline groove  76 . 
     The housing  90  further includes an oil inlet path  93  for introducing the lubricating oil into the oil sump section  97 . The oil inlet path  93  is connected to the pipe  58 . Therefore, the lubricating oil is supplied to the oil inlet path  93  by the lubricating oil pump  56  or the transmission pump  29 .  FIG. 5  is a view of the housing  90  seen from the axial outward side according to the first embodiment. The illustration of the housing  90  further to the radial outward side than the outer step section  96  is omitted in  FIG. 5 . Moreover,  FIG. 4A  illustrates a cross-sectional plane cut along the cross-sectional plane I-P depicted in  FIG. 5  excluding the first external connection apparatus  64  and the second external connection apparatus  66 . 
     When referring to  FIG. 4A  and  FIG. 5 , the oil inlet path  93  includes a first bottomed hole  93   a  and a second bottomed hole  93   b . The first bottomed hole  93   a  extends downward from the upper surface of the housing  90 . That is, the first bottomed hole  93   a  extends in the radial inward direction from the outer surface of the housing  90 . The lower end of the first bottomed hole  93   a  is formed inside the protruding section  98 . That is, the inner end of the first bottomed hole  93   a  is formed inside the protruding section  98 . The second bottomed hole  93   b  is formed inside the protruding section  98 . The second bottomed hole  93   b  extends from the inner end of the first bottomed hole  93   a  in the axial outward direction and reaches the oil sump section  97 . Therefore, the lubricating oil is introduced into the oil sump section  97  by the oil inlet path  93 . As illustrated in  FIG. 4B , at least a portion of the first gap  110  is positioned on the axial inward side of an oil discharge port EX 1  of the oil inlet path  93 . 
     The O-ring  101  is provided in the outer step section  96 . The O-ring  101  seals the gap between the attachment surface  95   a  and the first external connection apparatus  64  that abuts the attachment surface  95   a , and prevents lubricating oil from flowing to the outside. 
     In the embodiment illustrated in  FIG. 4A , a first seal  102  is provided in the first gap  110  in order to make the lubrication of the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft) more reliable. The first seal  102  is a seal ring, for example, but may be another type of seal as long as sealing can be achieved in the radial direction. As illustrated in  FIG. 4B , a first fixing groove  77  is provided in the boss section  73  so as to restrict the movement of the first seal  102  in the axial direction. That is, the boss section  73  includes the first fixing groove  77 . The first seal  102  seals the first gap  110 . Consequently, the lubricating oil retained in the oil sump section  97  is suppressed from flowing out through the first gap  110  and between rolling elements  83   c  of the second bearing  83 . 
     A portion of the lubricating oil retained in the oil sump section  97  flows out of the oil sump section  97  from a backlash of the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft). Therefore, lubricating oil is desirably supplied from the oil inlet path  93  so that the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93 . 
     Moreover, if the amount of the lubricating oil that leaks out from the first seal  102  cannot be ignored even if the first seal  102  is provided, the lubricating oil is desirably supplied from the oil inlet path  93  so that the total amount of the amount per unit of time of the lubricating oil leaking out from the first seal  102  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section, is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93 . 
     The surface area S 1  of the first gap  110  as seen from the axial direction is desirably smaller than the surface area Sb as seen from the axial direction of portions Zo that are open within the region Zm between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as illustrated by the dot pattern in  FIG. 5 . Moreover, the surface area S 1  of the first gap  110  is desirably smaller than a surface area SEX 1  of the oil discharge port EX 1  (illustrated as the second bottomed hole  93   b  in  FIG. 5 ) of the oil inlet path  93 . 
     Furthermore, the surface area SEX 1  of the oil discharge port EX 1  of the oil inlet path  93  is desirably greater than the sum (S 1 +Sbc) of the surface area S 1  of the first gap  110  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction (SEX 1 &gt;S 1 +Sbc). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 1  and the surface area Sbc are surface area S 1 ′ and surface area Sbc′ respectively, the relationship SEX 1 &gt;S 1 ′+Sbc′ is desired. 
     When the first seal  102  is provided, the surface area of the gap that is the cause of the lubricating oil leakage and that is formed between the first seal  102  and the surrounding members thereof, is set as Sseal. Normally, as long as Sseal is extremely narrow and the leakage from the gap is small enough to be ignored, the relationship SEX 1 &gt;Sbc or SEX 1 &gt;Sbc′ is sufficient. However, if the leakage from Sseal cannot be ignored, the relationship SEX 1 &gt;Sseal+Sbc is desirably established. More specifically, when the surface area of the portion covered in the lubricating oil of the Sseal is set as Sseal′, the relationship SEX 1 &gt;Sseal′+Sbc′ is desirably established. 
     In order to reduce the surface area S 1  of the first gap  110 , it is effective to reduce the distance d 1  (d 1  is set as the minimum gap distance of the first gap  110 ) illustrated in  FIG. 4B , and reduce the diameter ds 1  illustrated in  FIG. 4A . When the first seal  102  is provided, reducing ds 1  is effective in reducing the size of the first seal  102 . 
       FIG. 6  illustrates a portion of the power transmission device  25  cut along the cross-sectional plane II-II′ in  FIG. 5 . When referring to  FIGS. 5 and 6 , the housing  90  further includes an oil drain path  99 . The oil drain path  99  drains the lubricating oil retained in the oil sump section  97 . 
     The lubricating oil is desirably supplied from the oil inlet path  93  so that the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93  is equal to or greater than the amount per unit of time of the lubricating oil flowing out of locations other than the oil drain path  99  of the oil sump section  97 . The amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93  is equal to the total oil amount of the amount per unit of time of the lubricating oil flowing out from the oil drain path  99  of the oil sump section  97  and the amount per unit of time of the lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97 . The lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97  includes the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section and the lubricating oil leaking out from the first seal  102 . 
     The oil drain path  99  is formed inside the protruding section  98 . The oil drain path  99  extends in the axial direction and passes through the protruding section  98 . At least a portion of the first gap  110  is positioned on the axial inward side of an oil drain port EX 5  of the oil drain path  99 . 
     When referring to  FIG. 5 , the oil drain path  99  is positioned higher than the lower end B of the spline groove  76 . Moreover, the oil drain path  99  may be positioned higher than the upper end U of the spline groove  76 . Therefore, the lubricating oil introduced into the oil sump section  97  is retained at least to the height of the lower end of the oil drain path  99 , reliably lubricates at least a portion of the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft) by covering the portion in oil, and the excess lubricating oil is drained into the tank section  65  through the oil drain path  99 . 
     As illustrated in  FIG. 4A , the boss section  73  includes an oil drain hole  79  that extends from a lubricating oil intake port I positioned on the internal circumferential surface further in the axial depth side than the spline groove  76 , to a lubricating oil drainage port  0  positioned on the external circumferential surface of the boss section  73 . As a result, the lubricating oil that passes through the gap (backlash of spline fitting section) between the spline groove  76  and the spline teeth of the first drive shaft  80  is recovered in the tank section  65 . If the hole section  74   i  is opened without the second drive shaft  81  being attached to the hole section  74   i , the oil drain hole  79  may be omitted. 
     Due to the above configuration, the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft) is reliably lubricated. Moreover, the lubricating oil that lubricates the spline fitting section of the spline groove  76  and the first drive shaft  80  circulates. Therefore, wear and damage to the spline fitting section, or more specifically, to the spline teeth surfaces, is prevented. Furthermore, because the lubricating oil is not retained in the spline fitting section, lubrication defects due to degraded lubricating oil are prevented. 
     Lubrication Structure (Second Embodiment) 
     The second embodiment is a structure in which the first seal  102  is omitted from the first embodiment. Because other structures are the same those of the first embodiment, the present embodiment will be explained by disclosing only the structure corresponding to  FIG. 4B .  FIG. 7  is a cross-sectional view of a lubrication structure of a power transmission device  25   a  according to the second embodiment. According to  FIG. 7 , a boss section  73   a  as in the second embodiment has the same structure as the boss section  73  as in the first embodiment except that the boss section  73   a  does not include the first fixing groove  77 . 
     In the present embodiment, the first gap  110  includes a first sub-gap  120  that extends in the axial direction and a second sub-gap  121  that extends in the radial direction. In this case, the gap having the smallest surface area among the first sub-gap  120  that extends in the axial direction within the first gap  110 , the second sub-gap  121  that extends in the radial direction within the first gap  110 , and a second gap  112  that extends in the radial direction between the protruding section  98  and the inner ring  83   a  of the second bearing  83 , mainly performs the role of suppressing the outflow of the lubricating oil retained in the oil sump section  97 . 
     The surface areas of the second sub-gap  121  and the second gap  112  are derived as explained below. The surface area S 1   r  of the second sub-gap  121  is derived as the minimum value of the surface area of the cylindrical surface sandwiched between the protruding section  98  and the boss section  73  along the distance ds 1   r  (ds 1   r  is any radius position between the inner radius of the protruding section  98  and the inner radius of the second bearing  83 ) in the radial direction from the center axis C-C′. 
     The surface area S 2  of the second gap  112  is derived as the minimum value of the surface area of the cylindrical surface sandwiched between the protruding section  98  and the inner ring  83   a  of the second bearing  83  along a distance ds 2  (ds 2  is any radius position between the inner radius and the outer radius of the inner ring  83   a  of the second bearing  83 ) in the radial direction from the center axis C-C′. 
     When the surface area S 1   a  of the first sub-gap  120  or the surface area Sir of the second sub-gap  121  is less than the surface area S 2  of the second gap  112 , the first gap  110  performs the role of suppressing the outflow of the lubricating oil retained in the oil sump section  97 . If the lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97  include the lubricating oil flowing out from the first gap  110  and the lubricating oil flowing out from the backlash of the spline fitting section, the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93  is desirably adjusted so that the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93 . Consequently, a sufficient amount of the lubricating oil, that is, an amount to reach the lower end of the oil drain path  99  can be retained in the oil sump section  97 . 
     The surface area S 1   a  of the first sub-gap  120  is derived according to (formula 1). 
         S 1 a =π×{( ds 1+ d 1) 2 −( ds 1) 2 }  (formula 1)
 
     In this case as well, the surface area S 1  of the first gap  110  that is the smallest surface area among the surface area S 1   a  and the surface area S 1   r  is desirably smaller than the surface area Sb as seen from the axial direction of the portions Zo that are open within the region Zm between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as illustrated by the dot pattern in  FIG. 5  in the same way as the first embodiment. Consequently, the first gap  110  suppresses the lubricating oil retained in the oil sump section  97  from flowing out between the rolling elements  83   c  of the second bearing  83 . 
     Moreover, the surface area S 1  of the first gap  110  is desirably smaller than the surface area SEX 1  of the oil discharge port EX 1  (illustrated as the second bottomed hole  93   b  in  FIG. 5 ) of the oil inlet path  93 . The sum (S 1 +Sbc) of the surface area S 1  of the first gap  110  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction is desirably less than the surface area SEX 1  of the oil discharge port EX 1  of the oil inlet path  93 . More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 1  and the surface area Sbc are set as surface area S 1 ′ and surface area Sbc′ respectively, the relationship S 1 ′+Sbc′&lt;SEX 1  is desired. As a result, it is easy to make the amount per unit of time of the lubricating oil flowing out from the first gap  110  equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93 . 
     Conversely, when S 2  is less than S 1   a  or S 1   r , the second gap  112  performs the role of suppressing the outflow of the lubricating oil retained in the oil sump section  97 . At this time, at least a portion of the second gap  112  is positioned on the axial inward side of the oil discharge port EX 1  of the oil inlet path  93  and of a drain port EX 5  of the oil drain path  99 . If the lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97  include the lubricating oil flowing out from the second gap  112  and the lubricating oil flowing out from the backlash of the spline fitting section, the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93  is desirably adjusted so that the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93 . 
     In this case, the surface area S 2  of the second gap  112  is desirably smaller than the abovementioned surface area Sb. Consequently, the second gap  112  suppresses the lubricating oil retained in the oil sump section  97  from flowing out between the rolling elements  83   c  of the second bearing  83 . 
     The surface area S 2  of the second gap  112  is desirably smaller than the abovementioned surface area SEX 1 . Moreover, the sum (S 2 +Sbc) of the surface area S 2  of the second gap  112  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction is desirably less than the surface area SEX 1  of the oil discharge port EX 1  of the oil inlet path  93  (S 2 +Sbc&lt;SEX 1 ). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 2  and the surface area Sbc are set as surface area S 2 ′ and surface area Sbc′ respectively, the relationship S 2 ′+Sbc′&lt;SEX 1  is desired. As a result, it is easy to make the amount per unit of time of the lubricating oil flowing out from the second gap  112  equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93 . 
     Lubrication Structure (Third Embodiment) 
       FIGS. 8A and 8B  are cross-sectional views of the lubrication structure of a power transmission device  25   b  according to the third embodiment.  FIG. 8B  is an enlargement of the portion outlined by the dotted line in  FIG. 8A .  FIG. 9  is a view of a housing  90   b  seen from the axial outward side according to the third embodiment. Moreover,  FIG. 8A  illustrates a cross-sectional plane cut along the cross-sectional plane III-III′ depicted in  FIG. 9  excluding the first external connection apparatus  64  and the second external connection apparatus  66 . In the third embodiment, the feature of providing a protruding section  98   b  further to the axial inward side than the second bearing  83  is the main difference with the first embodiment. Explanations of configurations having the same reference numeral as in the first embodiment are omitted in the third embodiment. 
     When referring to  FIG. 8A , the protruding section  98   b  is positioned further to the axial inward side than the second bearing  83 . As a result, the second case side bearing support section  92  includes a third case step section  92   b  that abuts the outer ring  83   b  of the second bearing  83  and that restricts the second bearing  83  from moving in the axial inward direction. In place of the second gear step section  75   a , a retainer ring  175  for restricting the second bearing  83  from moving in the axial outward direction is inserted into a boss section  73   b  according to the third embodiment. Therefore, the boss section  73   b  includes a second fixing groove  78  for inserting the retainer ring  175 . 
     An oil inlet path  93   x  according to the third embodiment extends in the downward direction from the upper surface of the housing  90   b  and passes through the housing  90   b . An oil discharge port EX 2  of the oil inlet path  93   x  is positioned further to the axial outward side than the protruding section  98   b . Therefore, at least a portion of a belowmentioned first gap  110   b  is positioned further to the axial inward side than the oil discharge port EX 2 . 
     When referring to  FIG. 8A , an oil drain path  99   b  extends in the axial direction and passes through the protruding section  98   b . At least a portion of the first gap  110   b  is positioned on the axial inward side of a drain port EX 6  of the oil drain path  99   b . When referring to  FIG. 9 , the oil drain path  99   b  is positioned higher than the lower end B of the spline groove  76 . Moreover, the oil drain path  99   b  may be positioned higher than the upper end U of the spline groove  76 . The oil drain path  99   b  is disposed in a position so that the center of rotation C of the first drive shaft  80 , the oil drain path  99   b , and the oil inlet path  93   x  are aligned along a straight line as seen in the axial direction. However, the positional relationship of the oil drain path  99   b  and the oil inlet path  93   x  is not limited to this example. 
     When referring to  FIG. 8A , an oil drain hole  79   b  as in the third embodiment is provided at a position closer to the first gear side bearing support section  74  in comparison to the oil drain hole  79  according to the first embodiment. Specifically, the oil drain hole  79   b  is provided at a position closer to the first gear side bearing support section  74  than the axial center D-D′ of the PTO gear  70  in the axial direction. In this case as well, the lubricating oil intake port I of the oil drain hole  79   b  is positioned on the internal circumferential surface further to the axial depth side than the spline groove  76 . However, the positional relationship of the oil drain hole  79   b  and the axial center D-D′ is not limited to this example. 
     When referring to  FIG. 8B , the first gap  110   b  is provided between the protruding section  98   b  and the boss section  73   b , and a second gap  112   b  is provided between the protruding section  98   b  and the inner ring  83   a  of the second bearing  83 . At least a portion of the second gap  112   b  is positioned on the axial inward side of an oil discharge port EX 2  of the oil inlet path  93   x . The protruding section  98   b  overlaps the region Zm between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as seen in the rotational axis C-C′ direction of the second bearing  83 . 
     The power transmission device  25   b  includes the first seal  102  for sealing the first gap  110   b . Consequently, the lubricating oil retained in the oil sump section  97  is suppressed from flowing between the rolling elements  83   c  of the second bearing  83  and flowing out from the first gap  110   b.    
     The lubricating oil is desirably supplied from the oil inlet path  93   x  so that the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   x.    
     Moreover, if the amount of the lubricating oil that leaks out from the first seal  102  cannot be ignored even if the first seal  102  is provided, the lubricating oil is desirably supplied from the oil inlet path  93   x  so that the total amount of the amount per unit of time of the lubricating oil leaking out from the first seal  102  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section, is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     Therefore, the lubricating oil is desirably supplied from the oil inlet path  93   x  so that the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   x  is equal to or greater than the amount per unit of time of the lubricating oil flowing out of locations other than the oil drain path  99  of the oil sump section  97 . The amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   x  is equal to the total oil amount of the amount per unit of time of the lubricating oil flowing out from the oil drain path  99  of the oil sump section  97  and the amount per unit of time of the lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97 . The lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97  includes the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section and the lubricating oil leaking out from the first seal  102 . 
     Further, the surface area Sb 1  of the first gap  110   b  is desirably smaller than the surface area Sb of the opening portions Zo of the second bearing  83  illustrated in  FIG. 9  by the dot patterns in the same way as the first embodiment. The surface area Sb 1  of the first gap  110   b  is desirably smaller than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x.    
     Furthermore, the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x  is desirably greater than the sum (S 1 +Sbc) of the surface area S 1   b  of the first gap  11013  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction (SEX 2 &gt;S 1 +Sbc). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 1   b  and the surface area Sbc are set as surface area S 1   b ′ and surface area Sbc′ respectively, the relationship SEX 2 &gt;S 1 +Sbc′ is desired. 
     When the first seal  102  is provided, the surface area of the gap that is the cause of the lubricating oil leakage and that is formed between the first seal  102  and the surrounding members thereof, is set as Sseal. Normally, as long as Sseal is extremely narrow and the leakage from the gap is small enough to be ignored, the relationship SEX 2 &gt;Sbc or SEX 2 &gt;Sbc′ is sufficient. However, if the leakage from Sseal cannot be ignored, the relationship SEX 2 &gt;Sseal+Sbc is desirably established. More specifically, when the surface area of the portion covered in the lubricating oil of the Sseal is set as Sseal′, the relationship SEX 2 &gt;Sseal′+Sbc′ is desirably established. 
     The first seal  102  may be omitted in the third embodiment in the same way as the in the second embodiment. If the lubricating oil flowing out from locations other than the oil drain path  99  of the oil sump section  97  includes the lubricating oil flowing out from the first gap  110   b  and the lubricating oil flowing out from the backlash of the spline fitting section to the outside of the oil sump section  97 , the oil amount of the lubricating oil flowing in from the oil inlet path  93   x  is desirably adjusted so that the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     Moreover, the surface area S 1   b  of the first gap  110   b  or the surface area S 2   b  of the second gap  112   b  as seen in the radial direction may be designed as explained in the second embodiment. Consequently, the lubricating oil retained in the oil sump section  97  is suppressed from flowing between the rolling elements  83   c  of the second bearing  83  and flowing out from the first gap  110   b.    
     Lubrication Structure (Fourth Embodiment) 
       FIG. 10  is a cross-sectional view of a lubrication structure of a power transmission device  25   c  according to a fourth embodiment. In the fourth embodiment, the feature of providing the oil inlet path  93   x  of the third embodiment further to the axial inward side than the second bearing  83  is the main difference with the third embodiment. Explanations of configurations having the same reference numeral as in the third embodiment are omitted in the fourth embodiment. 
     An oil inlet path  93   y  according to the fourth embodiment has a shape that is the same as the oil inlet path  93   x  according to the third embodiment. However, an oil discharge port EX 3  is provided further to the axial inward side than the second bearing  83 . However, the oil discharge port EX 3  is positioned further to the axial outward side than the protruding section  98   b . At least a portion of the first gap  110   b  is positioned further to the axial inward side than the oil discharge port EX 3  and the drain port EX 6  of the oil drain path  99   b.    
     While a portion of the lubricating oil discharged from the oil discharge port EX 3  is drained from the oil drain path  99   b , the remaining lubricating oil flows between the rolling elements  83   c  of the second bearing  83  and reaches the oil sump section  97 . Therefore, due to the above lubrication structure, at least a portion of the fitting section of the spline groove  76  and the spline teeth of the first drive shaft  80  is covered in oil and reliably lubricated. However in the present embodiment, the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   y  is designed by taking into account the fact that a portion of the lubricating oil discharged from the oil discharge port EX 3  flows out from the oil drain path  99   b.    
     The surface area Sb 1  of the first gap  110   b  is desirably made as small as possible. By reducing the outflow amount of the lubricating oil from the first gap  110   b , the amount of the lubricating oil that flows between the rolling elements  83   c  of the second bearing  83  and reaches the oil sump section  97  can be increased. When the first seal  102  is provided, the gap that is the cause of the lubricating oil leakage and that is formed between the first seal  102  and the surrounding members thereof, is set as Sseal, and the Sseal is smaller than the Sb 1  and the flow amount of the lubricating oil can be reduced further. As a result, the amount of the lubricating oil that flows between the rolling elements  83   c  of the second bearing  83  and reaches the oil sump section  97  can be further increased. 
     The lubricating oil is desirably supplied from the oil inlet path  93   y  so that the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing out of the oil inlet path  93   y , between the rolling elements  83   c  of the second bearing  83 , and into the oil sump section  97 , in the same way as the first embodiment and the third embodiment. Moreover, if the amount of the lubricating oil that leaks out from the first seal  102  cannot be ignored even if the first seal  102  is provided, the lubricating oil is desirably supplied from the oil inlet path  93   y  so that the total amount of the amount per unit of time of the lubricating oil leaking out from the first seal  102  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section, is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   y , between the rolling elements  83   c  of the second bearing  83 , and into the oil sump section  97 . 
     Therefore, the lubricating oil is desirably supplied from the oil inlet path  93   y  so that the amount per unit of time of the lubricating oil flowing out from the oil inlet path  93   y , between the rolling elements  83   c  of the second bearing  83 , and into the oil sump section  97  is equal to or greater than the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99   b  of the oil sump section  97 . The amount per unit of time of the lubricating oil flowing from the oil inlet path  93   y , between the rolling elements  83   c  of the second bearing  83 , and into the oil sump section  97  is equal to the total amount of the amount per unit of time of the lubricating oil flowing out from the oil drain path  99   b  of the oil sump section  97  and the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99   b  of the oil sump section  97 . The lubricating oil flowing out from locations other than the oil drain path  99   b  of the oil sump section  97  includes the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section and the lubricating oil leaking out from the first seal  102 . 
     Lubrication Structure (Fifth Embodiment) 
     The fifth embodiment is a structure in which the protruding section is provided in the PTO gear  70  of the fourth embodiment.  FIGS. 11A and 11B  are cross-sectional views of the lubrication structure of a power transmission device  25   d  according to the fifth embodiment.  FIG. 11B  is an enlargement of the portion outlined by the dotted line in  FIG. 11A . Because some of the structures in the fifth embodiment are the same structures as in the first embodiment and the fourth embodiment, such structures use the same reference numerals used in the first embodiment and the fourth embodiment. Explanations pertaining to structures that are the same as the first embodiment and the fourth embodiment are omitted. 
     A PTO gear  70   d  according to the fifth embodiment further includes a protruding section  72  in addition to the rotational force transmission section  71  and a boss section  73   d . The protruding section  72  is a separate part from the rotational force transmission section  71  and the boss section  73   d , and is, for example, fixed to the boss section  73   d  by press-fitting. Therefore, the protruding section  72  is provided on the boss section  73   d . The boss section  73   d  includes the second gear step section  75   a  that restricts the protruding section  72  from moving in the axial inward direction. The boss section  73   d  is substantially the same as the boss section  73   b  of the fourth embodiment except for the feature of not including the first fixing groove  77  and the second fixing groove  78 , and the feature of including the oil drain hole  79  of the first embodiment. 
     The protruding section  72  extends in the radial outward direction from the boss section  73   d  further to the axial inward side than the second bearing  83 . When referring to  FIG. 11B , the protruding section  72  overlaps the region Zm between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as seen in the rotational axis C-C′ direction of the second bearing  83 . 
     The protruding section  72  is spaced away from a housing  90   d  according to the fifth embodiment. A third gap  114  is provided between the protruding section  72  and the housing  90   d . At least a portion of the third gap  114  is positioned on the axial inward side of the oil discharge port EX 3 . Moreover, the power transmission device  25   d  according to the present embodiment includes a third seal  106  inside the third gap  114  and further to the axial inward side than the oil discharge port EX 3 . The third seal  106  is a seal ring, for example, but may be another type of seal so long as sealing can be achieved in the radial direction. As illustrated in  FIG. 11B , a third fixing groove  72   b  is provided in the protruding section  72  so as to restrict the movement of the third seal  106  in the axial direction. That is, the protruding section  72  includes the third fixing groove  72   b . The third seal  106  seals the third gap  114 . Therefore, the lubricating oil introduced into the oil inlet path  93   y  and the lubricating oil retained in the oil sump section  97  are suppressed from flowing out from the third gap  114 . 
     The lubricating oil is desirably supplied from the oil inlet path  93   y  so that the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing out of the oil inlet path  93   y , between the rolling elements  83   c  of the second bearing  83 , and into the oil sump section  97 , in the same way as the first embodiment, the third embodiment, and the fourth embodiment. Moreover, if the amount of the lubricating oil that leaks out from the third seal  106  cannot be ignored even if the third seal  106  is provided, the lubricating oil is desirably supplied from the oil inlet path  93   y  so that the total amount of the amount per unit of time of the lubricating oil leaking out from the third seal  106  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section, is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   y , between the rolling elements  83   c  of the second bearing  83 , and into the oil sump section  97 . 
     A third gear step section  72   a  for restricting the movement of the second bearing  83  in the axial inward direction is provided at the outer end in the axial direction of the protruding section  72 . Specifically, the protruding section  72  includes the third gear step section  72   a . The protruding section  72  is in contact with only the inner ring  83   a  of the second bearing  83 . The protruding section  72  is spaced away from the outer ring  83   b  of the second bearing  83 . A fourth gap  116  is provided between the protruding section  72  and the outer ring  83   b . The lubricating oil introduced from the oil inlet path  93   y  reaches the oil sump section  97  after passing through the fourth gap  116  and between the rolling elements  83   c  of the second bearing  83 . The fourth gap  116  is desirably designed so that the inflow of the lubricating oil from the oil inlet path  93   y  is not hindered more than necessary. 
       FIG. 12  is a view of the housing  90  seen from the axial outward side according to the fifth embodiment. The illustration of the housing  90  further to the radial outward side than the outer step section  96  is omitted in  FIG. 12 . Moreover,  FIG. 11A  illustrates a cross-sectional plane cut along the cross-sectional plane IV-IV′ in  FIG. 12  excluding the first external connection apparatus  64  and the second external connection apparatus  66 .  FIG. 13  illustrates a portion of the power transmission device  25   d  cut along the cross-sectional plane V-V′ in  FIG. 12 . 
     As illustrated in  FIG. 12 , the surface area S 3  of the third gap  114  as seen from the axial direction is desirably smaller than the surface area Sb as seen from the axial direction of the portions Zo that are open within the region Zm between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as illustrated by the dot pattern in  FIG. 12 . 
     Furthermore, the surface area SEX 3  of the oil discharge port EX 3  of the oil inlet path  93   y  is desirably greater than the sum (S 3 +Sbc) of the surface area S 3  of the third gap  114  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction (SEX 3 &gt;S 3 +Sbc). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 3  and the surface area Sbc are surface area S 3 ′ and surface area Sbc′ respectively, the relationship SEX 3 &gt;S 3 ′+Sbc′ is desired. 
     The surface area S 3  of the third gap  114  is desirably made as small as possible. By reducing the outflow amount of the lubricating oil from the third gap  114 , the amount of the lubricating oil that flows between the rolling elements  83   c  of the second bearing  83  and reaches the oil sump section  97  can be increased. When the third seal  106  is provided, the gap that is the cause of the lubricating oil leakage and that is formed between the third seal  106  and the surrounding members thereof, is set as Sseal, and the Sseal is much smaller than the S 3  and the flow amount of the lubricating oil can be reduced further. As a result, the amount of the lubricating oil that flows between the rolling elements  83   c  of the second bearing  83  and reaches the oil sump section  97  can be further increased. 
     When the third seal  106  is provided, the surface area of the gap that is the cause of the lubricating oil leakage and that is formed between the third seal  106  and the surrounding members thereof, is set as Sseal. Normally, as long as Sseal is extremely narrow and the leakage from the gap is small enough to be ignored, the relationship SEX 3 &gt;Sbc or SEX 3 &gt;Sbc′ is sufficient for the surface area SEX 3  of the oil discharge port EX 3  of the oil inlet path  93   y . However, if the leakage from Sseal cannot be ignored, the relationship SEX 3 &gt;Sseal+Sbc is desirably established. More specifically, when the surface area of the portion covered in the lubricating oil of the Sseal is set as Sseal′, the relationship SEX 3 &gt;Sseal′+Sbc′ is desirably established. 
     In order to reduce the surface area S 3  of the third gap  114 , it is effective to reduce the distance d 3  (d 3  is established as the minimum gap distance of the third gap  114 ) illustrated in  FIG. 11B , and reduce the diameter ds 3  illustrated in  FIG. 11A . When the third seal  106  is provided, reducing ds 3  is effective in reducing the size of the third seal  106 . 
     When referring to  FIG. 12  and  FIG. 13 , the housing  90   d  includes an oil drain path  99   d  in place of not including the protruding section  98   b  and the oil drain path  99   b  in the housing  90   b  according to the fourth embodiment. When referring to  FIG. 12 , the oil drain path  99   d  is positioned higher than the lower end B of the spline groove  76 . Moreover, the oil drain path  99   d  may be positioned higher than the upper end U of the spline groove  76 . Therefore, the lubricating oil introduced into the oil sump section  97  reliably lubricates at least a portion of the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft) by covering the portion in oil, and the excess lubricating oil is drained into the tank section  65  through the oil drain path  99   d.    
     The lubricating oil is desirably supplied from the oil inlet path  93   y  so that the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   y  is equal to or greater than the amount per unit of time of the lubricating oil flowing out of locations other than the oil drain path  99   d  of the oil sump section  97 . The amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   y  is equal to the total oil amount of the amount per unit of time of the lubricating oil flowing out from the oil drain path  99   d  of the oil sump section  97  and the amount per unit of time of the lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97 . The lubricating oil flowing out from locations other than the oil drain path  99   d  includes the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section and the lubricating oil leaking out from the third seal  106 . 
     When referring to  FIG. 13 , the oil drain path  99   d  is formed inside the extending section  95  and the second case side bearing support section  92 . The oil drain path  99   d  extends in the radial outward direction from a drain port EX 4  and passes through the housing  90   d . The drain port EX 4  is positioned further to the axial outward side than the second bearing  83  and further to the axial outward side than the protruding section  72 . Therefore, at least a portion of the third gap  114  is positioned further to the axial inward side than the drain port EX 4 . The drain port EX 4  is formed between the extending section  95  and the second case side bearing support section  92 . 
     Due to the above configuration, the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft) is reliably lubricated. Moreover, the lubricating oil that lubricates the fitting section of the spline groove  76  and the first drive shaft  80  circulates. 
     The third seal  106  may be omitted in the fifth embodiment in the same way as in the second embodiment. In this case, if the lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97  includes the lubricating oil flowing out from the third gap  114  and the lubricating oil flowing out from the backlash of the spline fitting section to the outside of the oil sump section  97 , the oil amount of the lubricating oil flowing in from the oil inlet path  93   y  is desirably adjusted so that the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99   d  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   y.    
     In this case, the surface area S 3  of the third gap  114  is desirably smaller than the surface area Sb as seen from the axial direction of the opening portions Zo of the second bearing  83  in the same way as the second embodiment. As a result, the lubricating oil introduced from the oil inlet path  93   y  via the protruding section  72  and the lubricating oil retained in the oil sump section  97  is suppressed from flowing out from the third gap  114 . 
     Moreover, the surface area S 3  of the third gap  114  is desirably smaller than the surface area SEX 3  of the oil discharge port EX 3  of the oil inlet path  93   y . The sum (S 3 +Sbc) of the surface area S 3  of the third gap  114  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction is desirably less than the surface area SEX 3  of the oil discharge port EX 3  of the oil inlet path  93   y  (S 3 +Sbc&lt;SEX 3 ). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 3  and the surface area Sbc are surface area S 3 ′ and surface area Sbc′ respectively, the relationship S 3 ′+Sbc′&lt;SEX 3  is desired. As a result, it is easy to make the amount per unit of time of the lubricating oil flowing out from the third gap  114  equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   y.    
     Lubrication Structure (Sixth Embodiment) 
     The sixth embodiment is a structure in which the protruding section  72  of the fifth embodiment is provided further to the axial outward side than the second bearing  83 . When the protruding section  72  is provided on the gear side and provided further to the axial outward side than the second bearing  83 , it is required that the protruding section  72  or the rotational force transmission section  71  is made removable in order to attach the second bearing  83 . The sixth embodiment illustrates an example in which the protruding section  72  is integrated with the boss section and the rotational force transmission section  71  can be removed from the boss section. 
       FIGS. 14A and 14B  are cross-sectional views of the lubrication structure of a power transmission device  25   e  according to the sixth embodiment.  FIG. 14B  is an enlargement of the portion outlined by the dotted line in  FIG. 14A . Because some of the structures in the sixth embodiment are the same structure as in the first embodiment, third embodiment and the fifth embodiment, such structures use the same reference numerals used in the first, third and fifth embodiments. Explanations pertaining to structures that are the same as the first, third and the fifth embodiments are omitted. 
     A PTO gear  70   e  according to the sixth embodiment includes a rotational force transmission section  71   e  and a boss section  73   e . The rotational force transmission section  71   e  and the boss section  73   e  are separate parts and are coupled by a fixing means such as a spline fit. The boss section  73   e  includes a protruding section  72   e . The protruding section  72   e  is formed integrally with the boss section  73   e . The protruding section  72   e  protrudes in the radial outward direction from the second gear side bearing support section  75  on the axial outward side of the second bearing  83 . The protruding section  72   e  overlaps the region Zm between the inner ring  83   a  and the outer ring  83   b  of the second bearing  83  as seen in the rotational axis C-C′ direction of the second bearing  83 . 
     A housing  90   e  according to the present embodiment is substantially different from the housing  90   d  according to the fifth embodiment only due to the position of the oil inlet path. The housing  90   e  includes the oil inlet path  93   x  according to the third embodiment. The oil inlet path  93   x  is positioned further to the axial outward side than the protruding section  72   e . The protruding section  72   e  is spaced away from the housing  90   e . A third gap  114   e  is provided between the protruding section  72   e  and the housing  90   e . At least a portion of the third gap  114   e  is positioned further to the axial inward side than the oil discharge port EX 2  of the oil inlet path  93   x  and the drain port EX 4  of the oil drain path  99   d.    
     The power transmission device  25   e  according to the present embodiment includes the third seal  106  provided in the third gap  114   e . As illustrated in  FIG. 14B , the third fixing groove  72   b  is provided in the protruding section  72   e  so as to restrict the movement of the third seal  106  in the axial direction. The third seal  106  seals the third gap  114   e . Therefore, the lubricating oil introduced from the oil inlet path  93   x  and the lubricating oil retained in the oil sump section  97  are suppressed from flowing out from the third gap  114   e.    
     The lubricating oil is desirably supplied from the oil inlet path  93   x  so that the amount per unit of time of the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   x  in the present embodiment. Moreover, if the amount of the lubricating oil that leaks out from the third seal  106  cannot be ignored even if the third seal  106  is provided, the lubricating oil is desirably supplied from the oil inlet path  93   x  so that the total amount of the amount per unit of time of the lubricating oil leaking out from the third seal  106  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section, is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     Therefore, the lubricating oil is desirably supplied from the oil inlet path  93   x  so that the amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   x  is equal to or greater than the amount per unit of time of the lubricating oil flowing out of locations other than the oil drain path  99   d  of the oil sump section  97 . The amount per unit of time of the lubricating oil flowing into the oil sump section  97  from the oil inlet path  93   x  is equal to the total oil amount of the amount per unit of time of the lubricating oil flowing out from the oil drain path  99   d  of the oil sump section  97  and the amount per unit of time of the lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97 . The lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97  includes the lubricating oil flowing out of the oil sump section  97  from the backlash of the spline fitting section and the lubricating oil leaking out from the third seal  106 . 
     In the present embodiment, the surface area S 3   e  of the third gap  114   e  is desirably smaller than the surface area Sb as seen from the axial direction of the opening portions Zo of the second bearing  83 . The surface area S 3   e  of the third gap  114   e  is desirably smaller than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x . The sum (S 3   e +Sbc) of the surface area S 3   e  of the third gap  114   e  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction is desirably less than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x  (S 3   e +Sbc&lt;SEX 2 ). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 3   e  and the surface area Sbc are surface area S 3   e ′ and surface area Sbc′ respectively, the relationship S 3   e ′+Sbc′&lt;SEX 2  is desired. 
     The surface area S 3   e  of the third gap  114   e  is derived according to (formula 2). 
         S 3 e =π×{( ds 3+ d 3) 2 −( ds 3) 2 }  (formula 2)
 
     When the third seal  106  is provided, the surface area of the gap that is the cause of the lubricating oil leakage and that is formed between the third seal  106  and the surrounding members thereof, is set as Sseal. Normally, as long as Sseal is extremely narrow and the leakage from the gap is small enough to be ignored, the relationship SEX 2 &gt;Sbc or SEX 2 &gt;Sbc′ is sufficient. However, if the leakage from Sseal cannot be ignored, the relationship SEX 2 &gt;Sseal+Sbc is desirably established. More specifically, when the surface area of the portion covered in the lubricating oil of the Sseal is set as Sseal′, the relationship SEX 2 &gt;Sseal′+Sbc′ is desirably established. 
     Due to the above configuration, the fitting section of the spline groove  76  and the first drive shaft  80  (spline shaft) is reliably lubricated. Moreover, the lubricating oil that lubricates the fitting section of the spline groove  76  and the first drive shaft  80  circulates. 
     The third seal  106  may be omitted in the sixth embodiment in the same way as in the second embodiment. In this case, if the surface area S 3   e  of the third gap  114   e  that extends in the axial direction is less than the surface S 4   e  of a fourth gap  116   e  that extends in the radial direction between the protruding section  98  and the outer ring  83   b  of the second bearing  83 , the third gap  114   e  performs the role of suppressing the outflow of the lubricating oil retained in the oil sump section  97 . The surface area S 4   e  can be calculated in the same way as the surface area S 2  of the second gap  112  in the second embodiment. That is, the surface area S 4   e  is derived as the minimum value of the surface area of the cylindrical surface sandwiched between the protruding section  72   e  and the outer ring  83   b  of the second bearing  83  along the distance ds 4   r  (ds 4   r  is any radial position between the outer radius of the protruding section  72   e  and the inner radius of the outer ring  83   b  of the second bearing  83 ) in the radial direction from the center axis C-C′. 
     At this time, if the lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97  includes the lubricating oil flowing out from the third gap  114  and the lubricating oil flowing out from the backlash of the spline fitting section to the outside of the oil sump section  97 , the oil amount of the lubricating oil flowing in from the oil inlet path  93   x  is desirably adjusted so that the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99   d  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     In this case, the surface area S 3   e  of the third gap  114   e  is desirably smaller than the surface area Sb as seen from the axial direction of the opening portions Zo of the second bearing  83 . As a result, the lubricating oil introduced from the oil inlet path  93   x  and the lubricating oil retained in the oil sump section  97  are suppressed from flowing out from the third gap  114   e.    
     The surface area S 3   e  of the third gap  114   e  is desirably smaller than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x . The sum (S 3   e +Sbc) of the surface area S 3   e  of the third gap  114   e  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction is desirably less than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x  (S 3   e +Sbc&lt;SEX 2 ). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 3   e  and the surface area Sbc are surface area S 3   e ′ and surface area Sbc′ respectively, the relationship S 3   e ′+Sbc′&lt;SEX 2  is desired. As a result, it is easy to make the amount per unit of time of the lubricating oil flowing out from the third gap  114   e  equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     Conversely, if the surface area S 4   e  of the fourth gap  116   e  is less than the surface area S 3   e  of the third gap  114   e , the fourth gap  116   e  performs the role of suppressing the outflow of the lubricating oil introduced from the oil inlet path  93   x  and the lubricating oil retained in the oil sump section  97 . At least a portion of the fourth gap  116   e  is positioned further to the axial inward side than the oil discharge port EX 2  of the oil inlet path  93   x  and the drain port EX 4  of the oil drain path  99   d . At this time, if the lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97  includes the lubricating oil flowing out from the fourth gap  116   e  and the lubricating oil flowing out from the backlash of the spline fitting section to the outside of the oil sump section  97 , the oil amount of the lubricating oil flowing in from the oil inlet path  93   x  is desirably adjusted so that the total of the amount per unit of time of the lubricating oil flowing out from the locations other than the oil drain path  99   d  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     In this case, the surface area S 4  of the fourth gap  116   e  is desirably smaller than the surface area Sb as seen from the axial direction of the opening portions Zo of the second bearing  83 . As a result, the lubricating oil introduced from the oil inlet path  93   x  via the protruding section  72  and the lubricating oil retained in the oil sump section  97  are suppressed from flowing out from the fourth gap  116   e.    
     Moreover, the surface area S 4  of the fourth gap  116   e  is desirably smaller than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x . The sum (S 4 +Sbc) of the surface area S 4  of the fourth gap  116   e  and the surface area Sbc of the backlash of the spline fitting section as seen from the axial direction is desirably less than the surface area SEX 2  of the oil discharge port EX 2  of the oil inlet path  93   x  (S 4 +Sbc&lt;SEX 2 ). More specifically, when the surface areas of the portions covered in the lubricating oil retained inside the oil sump section  97  among the surface area S 4  and the surface area Sbc are surface area S 4 ′ and surface area Sbc′ respectively, the relationship S 4 ′+Sbc′&lt;SEX 2  is desired. As a result, it is easy to make the amount per unit of time of the lubricating oil flowing out from the fourth gap  116   e  equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x.    
     (Characteristics) 
     The abovementioned power transmission devices  25  and  25   a  to  25   e  have the following characteristics. 
     The power transmission devices  25 ,  25   a  to  25   e  include the oil sump section  97  that is in communication with the spline fitting section of the spline groove  76  and the first drive shaft  80 , the oil inlet paths  93 ,  9   x ,  93   y  for introducing lubricating oil into the oil sump section  97 , and the oil drain paths  99 ,  99   b ,  99   d  for draining the lubricating oil retained in the oil sump section  97 . The oil drain paths  99 ,  99   b ,  99   d  are positioned higher than the lower end B of the spline groove  76  of the boss sections  73 ,  73   b ,  73   d ,  73   e.    
     In these power transmission devices, the lower end of the spline groove  76 , namely, at least a portion of a spline fitting section is reliably covered by the lubricating oil and reliably lubricated. Moreover, when the lubricating oil is retained up to the height of the oil drain paths  99 ,  99   b ,  99   d  in the oil sump section  97 , excess lubricating oil is drained by passing through the oil drain paths  99 ,  99   b ,  99   d  and the lubricating oil circulates. Therefore, lubrication defects can be suppressed even when the lubrication oil is used for a long time. Therefore, wear and damage to the spline fitting section, or more specifically, wear and damage to the spline teeth surfaces, is prevented. 
     (2) The protruding sections  98 ,  98   b ,  72 ,  72   e  are positioned on the axial inward side of the oil sump section  97 . The protruding sections  98 ,  98   b ,  72 ,  72   e  overlap the region Zm between the inner ring  83   a  and the outer ring  83   b  as seen in the rotational axis C-C′ direction of the second bearing  83 . Consequently, the lubricating oil retained in the oil sump section  97  is suppressed from flowing out between the rolling elements  83   c  of the second bearing  83 . 
     (3) The oil drain paths  99 ,  99   b ,  99   d  are positioned higher than the upper end U of the spline groove  76 . Consequently, all of the spline fitting section can be covered reliably by the lubricating oil. 
     (4) The boss sections  73 ,  73   b ,  73   d ,  73   e  include the oil drain holes  79 ,  79   b ,  79   e  that extend from the lubricating oil intake port I positioned further to the axial depth side than the spline groove  76 , to the lubricating oil drainage port  0  positioned on the external circumferential surface of the boss sections  73 ,  73   b ,  73   d ,  73   e . Consequently, even if the hole section  74   i  is covered by the second drive shaft  81 , the lubricating oil that covers the spline fitting section flows through the backlash of the spline fitting section and drains out from the oil drain holes  79 ,  79   b ,  79   e . Therefore, the lubricating oil is not retained in the spline fitting section and circulates. As a result, lubrication defects can be further suppressed. 
     (5) At least a portion of each of the first to fourth gaps is positioned further to the axial inward side than the oil discharge ports EX 1  to EX 3  of the oil inlet paths  93 ,  93   x ,  93   y  and the drain ports EX 4  to EX 6  of the oil drain paths  99 ,  99   b ,  99   d . As a result, the lubricating oil introduced from the oil inlet paths  93 ,  93   x ,  93   y  is suppressed from flowing to locations other than the oil sump section  97 . 
     (6) When the protruding sections  98  and  98   b  are provided in the housing  90 ,  90   b , or  90   c , the amount of oil per unit of time of the lubricating oil flowing out from locations other than the oil drain paths  99 ,  99   b  of the oil sump section  97  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet paths  93 ,  93   x ,  93   y . More specifically, the total of the amount per unit of time of the lubricating oil flowing out from the first gaps  110 ,  110   b  or the second gaps  112 ,  112   b  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet paths  93 ,  93   x ,  93   y . Consequently, the lubricating oil can be reliably retained in the oil sump section  97  up to the level where the oil drain paths  99 ,  99   b  are provided. 
     (7) The power transmission devices  25 ,  25   b ,  25   c  further include the first seal  102  for sealing the first gaps  110 ,  110   b  in the case of characteristic (6). As a result, the lubricating oil retained in the oil sump section  97  can be further suppressed from flowing out from the first gaps  110 ,  110   b.    
     (8) When the protruding sections  72 ,  72   e  are provided in the boss sections  73   d ,  73   e , the amount per unit of time of the lubricating oil flowing out from locations other than the oil drain path  99   d  of the oil sump section  97  is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet path  93   x ,  93   y . More specifically, the total of the amount per unit of time of the lubricating oil flowing out from the third gaps  114 ,  114   e  or the fourth gap  116   e  and the amount per unit of time of the lubricating oil flowing out from the backlash of the spline fitting section is equal to or less than the amount per unit of time of the lubricating oil flowing in from the oil inlet paths  93   x ,  93   y . Consequently, the lubricating oil can be reliably retained in the oil sump section  97  up to the level where the oil drain path  99   d  is provided. 
     (9) The power transmission devices  25   d ,  25   e  further include the third seal  106  for sealing the third gaps  114 ,  114   e  in the case of characteristic (8). As a result, the lubricating oil retained in the oil sump section  97  can be further suppressed from flowing out from the third gaps  114 ,  114   e.    
     MODIFIED EXAMPLES 
     The present invention is not limited to the above embodiment and various changes and modifications may be made without departing from the spirit of the invention. 
     While the work vehicle  1  is exemplified as a wheel loader in the above embodiments, the power transmission device according to the above embodiments can be widely applied to another construction machine such as a hydraulic excavator, a bulldozer, a dump truck, or a motor grader and the like or another work vehicle other than the wheel loader. 
     (b) While an example of lubricating a spline structure of the inside of the PTO gear is described in the above embodiments, the same lubrication structure may be applied to a portion coupling another pump or motor inside the power transmission device  25 . For example, the first external connection apparatus  64  and the second external connection apparatus  66  may be hydraulic motors. Furthermore, the first external connection apparatus  64  and the second external connection apparatus  66  may be electric motors or generators. Moreover, the lubrication structure may be applied to another spline fitting section inside the power transmission device  25 . That is, the lubrication structure of the above embodiments may be applied to a spline fitting section other than the portion coupling the external connection apparatuses. 
     (c) While an example is described in which the rotational force transmission sections  71 ,  71   e  are each the tooth of a gear in the above embodiments, the rotational force transmission sections  71 ,  71   e  may be formed by another rotational force transmission mechanism such as a sprocket fitted to a chain or a pulley having a belt wound thereto. 
     (d) While an example is described in which the spline fitting section in which the spline shaft (first drive shaft  80 ) and the spline groove  76  are fitted together in the above embodiments, the spline fitting section may be lubricated by splash lubrication. In this case, the oil inlet paths  93 ,  93   x ,  93   y  are provided above the oil sump section  97  and a receiving port for receiving the lubricating oil splashed up by the drive gear  62 , PTO gear  70 , and the like is provided on at least a portion of the oil inlet paths  93 ,  93   x ,  93   y.    
     (e) The third gaps  114 ,  114   e  in the fifth and sixth embodiments may each include a sub-gap that extends the axial direction and a sub-gap that extends in the radial direction in the same way as the second embodiment. The size of the gaps may be determined so that the outflow of the lubricating oil in the gap with the smaller surface area among the sub-gaps is suppressed. 
     (f) Instead of providing the first seal  102  in each of the first gaps  110 ,  110   b  as in the first, third and fourth embodiments, a seal may be provided in each of the second gaps  112 ,  112   b .  FIG. 15  illustrates a first modified example in which a seal is provided in the second gap  112  in the second embodiment. A second seal  104  for sealing the second gap  112  is provided in place of the first seal  102  in the first modified example. That is, a power transmission device  25   y  according to the present modified example includes the second seal  104 . While a V-shaped end surface seal is depicted as the second seal  104  in  FIG. 15 , any type of seal may be used so long as sealing in the axial direction is achieved. The second seal  104  is disposed on the axial inward side surface of the protruding section  98 . While the second seal  104  abuts the end surface of the inner ring  83   a  of the second bearing  83  in  FIG. 15 , the second seal  104  may abut the end surface of a fourth gear step section  75   b  of the second gear side bearing support section  75 . The same effect achieved by providing the first seal  102  is achieved in the present modified example. 
     Furthermore, a seal may be provided on the gear side on the opposite side in a second modified example.  FIG. 16  illustrates the second modified example in which a first seal  103  for sealing the second sub-gap  121  is provided on the gear side in the second modified example. While a V-shaped end surface seal is depicted as the first seal  103  in  FIG. 16 , any type of seal may be used as long as sealing in the axial direction is achieved. The first seal  103  is disposed on the axial outward side of the second bearing  83 . The same effect achieved by providing the first seal  102  of the first embodiment is achieved in the present modified example. 
     The same configuration as the first modified example may be the same for the fifth and sixth embodiments. That is, a fourth seal that is the same as the second seal may be provided in the fourth gaps  116 ,  116   e .  FIG. 17  illustrates a third modified example in which a seal is provided in the fourth gap  116   e  in the sixth embodiment. A fourth seal  108  for sealing the fourth gap  116   e  may be provided in place of the third seal  106  in the third modified example. That is, a power transmission device  25   u  according to the present modified example includes the fourth seal  108 . While a V-shaped end surface seal is depicted as the fourth seal  108  in  FIG. 17 , any type of seal may be used so long as sealing in the axial direction is achieved. The fourth seal  108  is disposed in a fourth fixing groove  72   c  provided on the axial inward side surface of a protruding section  72   u . The same effect can be achieved in the present modified example as when the third seal  106  is provided. 
     While the fourth seal  108  abuts the end surface of the outer ring  83   b  of the second bearing  83  in  FIG. 17 , the fourth seal  108  may abut the end surface of the housing  90 .  FIG. 18  illustrates a fourth modified example in which the fourth seal  108  abuts the end surface of the housing  90 . In a power transmission device  25   v  according to the present modified example, the third gap  114   e  includes a third sub-gap  124  that extends in the axial direction and a fourth sub-gap  125  that extends in the radial direction. In the present modified example, the fourth sub-gap  125  is provided between the fourth seal  108  and an end surface  90   v  of the housing  90 . The fourth seal  108  seals the fourth sub-gap  125 . The same effect can be achieved in the present modified example as when the third seal  106  is provided. 
     (g) While examples in which the protruding sections  98 ,  98   b  are formed integrally with another portion of the housings  90  to  90   c  are described in the first to fourth embodiments, the protruding sections  98 ,  98   b  may be separate parts. In this case, the protruding sections  98 ,  98   b  may be fixed by press-fitting to the housings  90  to  90   c . Alternatively, the protruding sections  98 ,  98   b  may be fixed to the housings  90  to  90   c  by another fixing method such as a bolt and nut and the like. 
     The protruding section  72  may be formed integrally with the boss section  73   d  in the fifth embodiment. In this case, an oil drain hole may be provided in the boss section  73   d  according to the fifth embodiment at the position of the oil drain hole  79   b  according to the third embodiment. Moreover, the protruding section  72   e  of the sixth embodiment may be formed as a separate part from the boss section  73   e , and the protruding section  72   e  may be fixed by press-fitting to the boss section  73   e . In this case, the rotational force transmission section  71   e  and the boss section  73   e  may be formed integrally. When the protruding section and the boss section are separate parts in the fourth and fifth embodiments, the protruding section may be fixed to the boss section with a fixing method other than press-fitting. 
     (h) An oil drain path may be provided in the same position as the oil drain path  99  of the first and second embodiments in place of the oil drain path  99   b  in the third and fourth embodiments. Moreover, the oil drain path  99   d  may be provided in place of the oil drain paths  99 ,  99   b  in the first to fourth embodiments. The shapes of the oil drain holes  79 ,  79   b ,  79   e  are optional and the positions thereof are similarly not limited to the positions of the above embodiments as long as the position is further to the axial inward side than the spline groove  76 . 
     The first bearing  82 , the first gear side bearing support section  74 , and the first case side bearing support section  91  may be omitted in the above embodiments. Similarly, the second bearing  83 , the second gear side bearing support section  75 , and the second case side bearing support section  92  may be omitted in the above embodiments. However in this case, the use of a double-row angular bearing as the support bearing for the PTO gear, or the use of back-to-back tapered roller bearings is required for sufficiently taking into account the support structure for the PTO gear. 
     (j) The positional relationship of the rotational force transmission sections  71 ,  71   e  and the boss section  73  is not limited to the above examples. The rotational force transmission sections  71 ,  71   e  may be formed at a position shifted away from the axial center of the boss section  73 . The rotational force transmission section  71  may also be formed on the surface of the boss section  73 . 
     (k) While the first external connection apparatus  64  and the second external connection apparatus  66  in the above embodiments are respectively described as configurations including the first drive shaft  80  and the second drive shaft  81 , the first external connection apparatus  64  and the second external connection apparatus  66  may include the boss section and the rotational force transmission sections  71 ,  71   e  may be coupled to a spline shaft that fits with the boss section.  FIG. 19  illustrates a fifth modified example in which the first external connection apparatus  64  includes a boss section. The fifth modified example is an example in which a portion of the fourth embodiment is modified. The same reference numerals are attached to the configurations that are the same as the fourth embodiment in  FIG. 19 , and explanations of such configurations are omitted. A break line is used in  FIG. 19  to illustrate the internal structure of a first external connection apparatus  64   z.    
     In the fifth embodiment, a PTO gear  70   z  includes the rotational force transmission section  71  and a shaft section  73   z . The shaft section  73   z  includes the first gear side bearing support section  74  (not illustrated), the second gear side bearing support section  75 , and a spline shaft  80   z . The first external connection apparatus  64   z  includes a boss section  85 . The boss section  85  includes a spline groove  86  on the internal circumferential surface on the side for coupling with the spline shaft  80   z . The spline groove  86  meshes with the spline shaft  80   z . The boss section  85  includes a hole section  85   i  on the axial outward side of the spline groove  86 . An oil drain hole  87  is provided that extends from the internal circumferential surface of the hole section  85   i  in the radial outward direction and passes through the boss section  85 . The housing  90   z  includes an extending section  95   z  that is longer than the extending section  95  of the fourth embodiment. 
     Lubricating oil supplied from the oil inlet path  93   y  passes between the rolling elements  83   c  of the second bearing  83  and reaches an oil sump section  97   z . The lubricating oil that is retained in the oil sump section  97   z  passes through the spline fitting section of the spline shaft  80   z  and the spline groove  86  and flows into the hole section  85   i . The lubricating oil that has flowed into the hole section  85   i  passes through the oil drain hole  86  and returns to the oil sump section  97   z . When a predetermined amount or more of the lubricating oil is retained in the oil sump section  97   z , the lubricating oil passes through the oil drain path  99   b  and flows out. According to the above configuration, the lubricating oil that lubricates the spline fitting section of the spline shaft  80   z  and the spline groove  86  circulates. 
     INDUSTRIAL APPLICABILITY 
     Provided is a power transmission device having a structure in which a reduction in the lubricity of a spline fitting section can be suppressed, and a work vehicle provided with the power transmission device.