Abstract:
The present invention aims at simplifying the structure of the inside of a transmission portion and the inside of a clutch portion. Accordingly, in the present invention, a drive shaft is interlockingly connected to an engine by way of a clutch, a distribution power transmission body is mounted on the drive shaft, and a traveling-system power transmission mechanism and a PTO-system power transmission mechanism are interlockingly connected with the distribution power transmission body in parallel. Accordingly, it is possible to form the drive shaft using a single shaft and a clutch is also formed of the single clutch structure and hence, the structure in the inside of the transmission portion and the structure in the inside of the clutch portion can be simplified and, at the same time, miniaturized whereby a manufacturing cost can be also lowered. Further, by performing a disconnection operation of the clutch, it is possible to simultaneously terminate the power transmission of both transmission systems consisting of the traveling-system power transmission mechanism and the PTO-system power transmission mechanism whereby the manipulation property can be enhanced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a power transmission mechanism of a tractor. 
         [0003]    2. Description of the Related Art 
         [0004]    Conventionally, as one mode of a tractor, there has been known a tractor in which a transmission portion is interlockingly connected with a prime mover portion by way of a clutch portion and, a pair of left and right rear wheels are interlockingly connected with left and right side portions of the transmission portion by way of a rear axle casing and, at the same time, a PTO portion is mounted on a rear end portion of the transmission portion (for example, see JP-A-8-80754). 
         [0005]    Further, while an engine is arranged in the prime mover portion, an inner-and-outer duplicate shaft structure is arranged in the clutch portion, wherein the inner-and-outer duplicate shaft structure is constituted of an inner drive shaft which extends in the longitudinal direction and a cylindrical outer drive shaft which surrounds an outer periphery of the inner drive shaft. The inner drive shaft is interlockingly connected with the above-mentioned engine by way of a PTO clutch, while the outer drive shaft is interlockingly connected with the engine by way of a traveling clutch thus providing the dual clutch structure which includes the PTO clutch and the traveling clutch. 
         [0006]    Further, the above-mentioned inner drive shaft is interlockingly connected with a PTO portion by way of a PTO-system power transmission mechanism which is arranged in the inside of the transmission portion. Still further, the above-mentioned outer drive shaft is interlockingly connected with a pair of left and right rear wheels by way of a traveling-system power transmission mechanism which is arranged in the inside of the transmission portion. 
         [0007]    However, in the above-mentioned tractor, the inner drive shaft which is interlockingly connected with the PTO portion by way of the PTO power transmission mechanism and the outer drive shaft which is interlockingly connected with a pair of left and right rear wheels by way of the traveling-system power transmission mechanism which constitute the inner-and-outer duplicate shaft structure is disposed in the inside of the transmission portion. Accordingly, the tractor has a drawback that the inner structure of the transmission portion becomes complicated and, at the same time, large-sized thus pushing up a manufacturing cost. 
         [0008]    Further, the inner drive shaft is interlockingly connected with the engine by way of the PTO clutch and the outer drive shaft is interlockingly connected with the engine by way of the traveling clutch thus constituting the dual clutch structure which includes the PTO clutch and the traveling clutch. Further, the dual clutch structure is disposed at the clutch portion. Accordingly, the tractor has a drawback that the clutch portion becomes complicated and, at the same time, large-sized thus pushing up a manufacturing cost. 
       SUMMARY OF THE INVENTION 
       [0009]    (1) According to a first aspect of the present invention, a drive shaft is interlockingly connected to an engine by way of a clutch, a distribution power transmission body is mounted on the drive shaft, and a traveling-system power transmission mechanism and a PTO-system power transmission mechanism are interlockingly connected with the distribution power transmission body in a state that the traveling-system power transmission mechanism and the PTO-system power transmission mechanism are arranged in parallel to each other. 
         [0010]    In this manner, the distribution power transmission body is mounted on the drive shaft which is interlockingly connected to an engine, and the traveling-system power transmission mechanism and the PTO-system power transmission mechanism are interlockingly connected with the distribution power transmission body in a state that the traveling-system power transmission mechanism and the PTO-system power transmission mechanism are arranged in parallel to each other and hence, it is possible to form the drive shaft using a single shaft and it is also possible to form the clutch using a single clutch mechanism whereby the structure inside a transmission portion and the structure inside the clutch portion can be simplified and miniaturized compared to the prior art and, at the same time, a manufacturing cost can be reduced. 
         [0011]    Further by performing an ON/OFF operation of the clutch, it is possible to simultaneously turn off the power transmission to both power transmission systems consisting of the traveling-system power transmission mechanism and the PTO-system power transmission mechanism and hence, the manipulation property can be enhanced. 
         [0012]    (2) According to a second aspect of the present invention, an advancing/backing changeover mechanism is provided to an upstream side of the traveling-system power transmission mechanism and the advancing/backing changeover mechanism is interlockingly connected to the distribution power transmission body. 
         [0013]    In this manner, the advancing/backing changeover mechanism which is provided to the upstream side of the traveling-system power transmission mechanism which exhibits a high power transmission rotational speed is interlockingly connected to the distribution power transmission body and hence, it is possible to set a torque necessary for synchronizing a gear provided to the advancing/backing changeover mechanism and another gear which is provided to the traveling-system power transmission mechanism to a small value and hence, the advancing/backing changeover mechanism can be miniaturized. 
         [0014]    (3) According to a third aspect of the present invention, a lid body is detachably mounted on a transmission case which encases the traveling-system power transmission mechanism, a counter gear which constitutes a portion of the advancing/backing changeover mechanism mounted on the traveling-system power transmission mechanism is mounted on the lid body, and the counter gear and the lid body are made integrally detachable. 
         [0015]    In this manner, the counter gear which constitutes a portion of the advancing/backing changeover mechanism is configured to be integrally detachable with the lid body and hence, it is possible to provide a specification in which the tractor includes the advancing/backing changeover mechanism by mounting the counter gear integrally with the lid body and, at the same time, it is possible to provide a specification in which the tractor includes no advancing/backing changeover mechanism by removing the counter gear integrally with the lid body. 
         [0016]    As a result, it is possible to easily change the mode of the tractor between the specification in which tractor includes the advancing/backing changeover mechanism and the specification in which tractor includes no advancing/backing changeover mechanism due to the lid detachable operation while maintaining the transmission case in common. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a side view of a tractor according to the present invention; 
           [0018]      FIG. 2  is an explanatory cross-sectional side view of a clutch portion and a transmission portion; 
           [0019]      FIG. 3  is an explanatory cross-sectional side view of the clutch portion; 
           [0020]      FIG. 4A  to  FIG. 4C  are views showing a advancing/backing changeover mechanism, wherein  FIG. 4A  is an explanatory cross-sectional back view,  FIG. 4B  is a plan view showing an appearance of the advancing/backing changeover mechanism, and  FIG. 4C  is a side view showing the appearance of the advancing/backing changeover mechanism; 
           [0021]      FIG. 5  is an explanatory cross-sectional side view of a PTO transmission portion; 
           [0022]      FIG. 6  is an explanatory cross-sectional back view of a PTO transmission portion; 
           [0023]      FIG. 7  is an explanatory cross-sectional side view of a drive shaft of a second embodiment; 
           [0024]      FIG. 8  is an explanatory cross-sectional side view of a drive shaft of a third embodiment; and 
           [0025]      FIG. 9  is an explanatory cross-sectional side view of a transmission shaft of the second embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Symbol A shown in  FIG. 1  indicates a tractor according to the present invention. The tractor A is configured such that a prime mover portion  2  is mounted on a body frame  1 , a transmission portion  4  is interlockingly connected to the prime mover portion  2  by way of a clutch portion  3 , a driving portion  5  is arranged above the transmission portion  4 , a PTO transmission portion  6  which constitutes a PTO portion having a transmission function is detachably and interlockingly connected to a rear portion of the transmission portion  4 , a pair of left and right front wheels  7 ,  7  are interlockingly connected to front axle casings  10  arranged below the body frame  1  and a pair of left and right rear wheels  9 , 9  are interlockingly connected to the transmission portion  4  by way of rear axle casings  8 , 8 . 
         [0027]    Hereinafter, the constitutions of the above-mentioned [prime mover portion  2 ], [clutch portion  3 ], [transmission portion  4 ], [driving portion  5 ] and [PTO transmission portion  6 ] are explained in a specific manner in this order. 
       [Prime Mover  2 ] 
       [0028]    The prime mover portion  2  is constituted such that, as shown in  FIG. 1 , an engine  15  or the like is mounted on the body frame  1  and the engine  15  is covered with a hood  16  which can be opened and closed. 
       [Clutch Portion  3 ] 
       [0029]    The clutch portion  3  is configured such that, as shown in  FIG. 2  and  FIG. 3 , a drive shaft  18  which is extended in the longitudinal direction is rotatably supported in the inside of a clutch housing  17 . With respect to the drive shaft  18 , while a front end portion  18   a  thereof is pivotally supported on a center portion of a flywheel  19  which is arranged at a front portion in the inside of the clutch housing  17  by way of a front bearing  20 , a rear end portion  18   b  thereof is pivotally supported on a support wall body  21  which is formed along a rear-end peripheral portion of the clutch housing  17  by way of a rear bearing  22 . 
         [0030]    Further, a clutch  23  is arranged around a periphery of the front portion of the drive shaft  18 . The flywheel  19  and the drive shaft  18  are connected with each other by way of the clutch  23  in a connectable and disconnectable manner. 
         [0031]    That is, in the clutch  23 , a clutch main body  24  is connected with the flywheel  19 . A cylindrical operating member  26  is slidably mounted on an outer peripheral surface of a cylindrical support member  25  which is arranged at a front outer periphery of the drive shaft  18  and supports a midst portion of the drive shaft  18  in a longitudinal direction. Further, a clutch pedal (not shown in the drawings) which is disposed at the driving portion  5  described later is interlockingly connected with the operating member  26  by way of a clutch operating arm  27 . 
         [0032]    Due to such a constitution, when the clutch pedal is stepped in, a pushing action which is applied to the clutch body  24  by the operating member  26  by way of the clutch operating arm  27  is released and hence, the clutch  23  is disconnected. 
         [0033]    Here, to a rear-end peripheral portion of the clutch housing  17 , a front peripheral portion of a main transmission casing  37  of the transmission portion  4  described later is detachably connected. Further, in the inside of the main transmission casing  37 , a distribution power transmission gear  29  which constitutes a distribution power transmission body is arranged. The distribution power transmission gear  29  is formed such that a rear end portion  18   b  of the drive shaft  18  is extended rearwardly and the distribution power transmission gear  29  is integrally formed on the extended portion. 
       [Transmission Portion  4 ] 
       [0034]    The transmission portion  4  is, as shown in  FIG. 2  and  FIG. 3 , configured such that, in the inside of a transmission casing  30  which extends in the longitudinal direction and is formed in a cylindrical shape, a traveling-system power transmission mechanism  31  which extends in the longitudinal direction and a PTO-system power transmission mechanism  32  are arranged in parallel to each other. 
         [0035]    Further, in the traveling-system power transmission mechanism  31 , an advancing/backing changeover mechanism  33  which sequentially performs an advancing/backing changeover operation from a front side to a rear side, a main transmission mechanism  34  which performs the main transmission of the power which is changed over by the advancing/backing changeover mechanism  33 , a sub transmission mechanism  35  which performs the sub transmission of the power which is obtained after the main transmission by the main transmission mechanism  34 , and a differential mechanism  36  which transmits the power which is obtained after the sub transmission by the sub transmission mechanism  35  to the left and right rear wheels  9 ,  9  in a distributed manner are arranged. 
         [0036]    Further, the above-mentioned advancing/backing changeover mechanism  33  has a start end portion thereof interlockingly connected with the distribution power transmission gear  29  which is integrally formed on the drive shaft  18 . 
         [0037]    Further, while the PTO-system power transmission mechanism  32  has a start end portion thereof interlockingly connected with the distribution power transmission gear  29  which is integrally formed on the above-mentioned drive shaft  18 , the PTO-system power transmission mechanism  32  has a terminal end portion thereof interlockingly connected with the PTO transmission portion  6  described later. 
         [0038]    Due to such a constitution, the power which is transmitted from the engine  15  to the drive shaft  18  is transmitted to the traveling-system power transmission mechanism  31  and the PTO-system power transmission mechanism  32  in a distributed manner by way of the distribution power transmission gear  29 . 
         [0039]    Further, the transmission casing  30  is, as shown in  FIG. 2 , formed in a three split-constitution consisting of a main transmission casing  37  which incorporates the advancing/backing changeover mechanism  33  and the main transmission mechanism  34  therein, a sub transmission casing  38  which incorporates the sub transmission mechanism  35  therein, and a differential casing  39  which incorporates the differential mechanism  36  therein. A front-end peripheral portion of the main transmission casing  37  is detachably connected to the rear-end peripheral portion of the above-mentioned clutch housing  17  using connecting bolts (not shown in the drawings), a front-end peripheral portion of the sub transmission casing  38  is detachably connected to a rear-end peripheral portion of the main transmission casing  37  using connecting bolts  41 , and a front-end peripheral portion of the differential casing  39  is detachably connected to a rear-end peripheral portion of the sub transmission casing  38  using connecting bolts  42 . 
         [0040]    Further, the main transmission casing  37  mounts, as shown in  FIG. 3 , an inner support wall body  43  on a midst portion thereof. With the use of the inner portion support wall body  43 , the inside of the main transmission casing  37  is split in two thus forming a front chamber  44  and a rear chamber  45 . On the other hand, the above-mentioned advancing/backing changeover mechanism  33  is arranged in the inside of the front chamber  44 , and the main transmission mechanism  34  is arranged in the inside of the rear chamber  45 . 
         [0041]    In the advancing/backing changeover mechanism  33 , as shown in  FIG. 3 , between the support wall body  21  which is formed along the rear-end peripheral portion of the above-mentioned clutch housing  17  and the inner support wall body  43 , an advancing/backing changeover input shaft  46  which extends in the longitudinal direction is extended rotatably by way of front and rear bearings  47 ,  48 . On the other hand, between an upper portion of the above-mentioned support wall body  21  and an upper portion of the above-mentioned inner support wall body  43 , an advancing/backing changeover output shaft  49  which extends in the longitudinal direction is extended rotatably by way of front and rear bearings  50 ,  51 . 
         [0042]    Further, the advancing/backing changeover input shaft  46  is arranged coaxially with the drive shaft  18  and an advancing input gear  52  and a backing input gear  53  are integrally mounted on the above-mentioned advancing/backing changeover input shaft  46 . 
         [0043]    Further, on the advancing/backing changeover output shaft  49 , a distribution input gear  54  which is meshed with the above-mentioned distribution power transmission gear  29  and an advancing/backing changeover body  55  which is capable of changing over the input drive force between the advancing side and the backing side are mounted. 
         [0044]    Further, the advancing/backing changeover body  55  allows an advancing output gear  56  and a backing output gear  57  to be rotatably mounted on the advancing/backing changeover output shaft  49  in a state that the advancing output gear  56  and the backing output gear  57  are arranged close to each other. Further, between both output gears  56 ,  57 , a advancing/backing changeover receiving member  58   a  is integrally fitted on the advancing/backing changeover output shaft  49  and, at the same time, an advancing/backing changeover slide member  58   b  is mounted on an outer peripheral surface of the advancing/backing changeover receiving member  58   a  in a state that the advancing/backing changeover slide member  58   b  is slidable in the axial direction. 
         [0045]    In this manner, the advancing/backing changeover slide member  58   b  can perform the changeover operation to provide an advancing changeover state in which the advancing/backing changeover slide member  58   b  is meshed with and connected with the advancing output gear  56  and a backing changeover state in which the advancing/backing changeover slide member  58   b  is meshed with and connected with the backing output gear  56 , and a neutral position in which the advancing/backing changeover slide member  58   b  is meshed with and connected with neither one of the output gears  56 ,  57 . 
         [0046]    Further, as shown in  FIG. 3 ,  FIG. 4A  and  FIG. 4B , an opening portion  59  is formed in a ceiling portion  37   a  of the main transmission casing  37 . The opening portion  59  is closed by a lid body  60  in a state that the opening portion  59  can be opened and closed. On an inner surface of the lid body  60 , a pair of front and rear shaft support members  61 ,  62  is mounted in a state that the shaft support members  61 ,  62  extend vertically downwardly. A gear support shaft  63  which has an axis thereof directed in the longitudinal direction is extended between both shaft support members  61 ,  62  and a counter gear  65  is rotatably supported on the gear support shaft  63  by way of a bearing  64 . Numeral  74  indicates a lid mounting bolt. 
         [0047]    Further, the above-mentioned counter gear  65  is, as shown in  FIG. 3  and  FIG. 4 , in a state that the lid body  60  is mounted in the opening portion  59  of the main transmission casing  37  in a lid-closed state, meshed with both of the above-mentioned backing input gear  53  and backing output gear  57  simultaneously and hence, the power for backing is transmitted to the backing input gear  53  from the backing output gear  57  by way of the counter gear  65 . On the other hand, in a state that the lid  60  is removed from the opening portion  59 , the connection between the backing output gear  57  and the backing input gear  53  is resolved. 
         [0048]    Here, the advancing output gear  56  is meshed with the advancing input gear  52 . 
         [0049]    Further, as shown in  FIG. 4A  and  FIG. 4C , an opening portion  66  is formed in a right side wall of the main transmission casing  37  and a cap-like case body  67  is detachably mounted in the opening portion  66 . A start end portion of the advancing/backing changeover mechanism  68  is interlockingly connected with an advancing/backing changeover lever (not shown in the drawings) which is provided to the driving portion  5 . On the other hand, a terminal end portion of the advancing/backing changeover mechanism  68  is interlockingly connected with the above-mentioned advancing/backing changeover slide member  58   b  and, at the same time, the terminal end portion is supported on the above-mentioned casing body  67 . Numeral  75  indicates casing body mounting bolts. 
         [0050]    That is, at the terminal end portion of the advancing/backing changeover mechanism  68 , a support shaft  69  which has an axis thereof directed in the lateral direction is pivotally supported on an upper portion of a side wall  67   a  of the casing body  67  by way of a boss portion  76 . A proximal end portion of a manipulation arm  70  which extends vertically is mounted on a right end portion of the support shaft  69  which projects outwardly from the side wall  67   a , while a proximal end portion of operating arm  71  which extends vertically is mounted on a left end portion of the support shaft  69  which projects inwardly from the side wall  67   a.    
         [0051]    Further, a fork support shaft  72  which extends in the longitudinal direction is extended between lower portions of the front and rear walls  67   b ,  67   c  of the casing body  67 . A proximal portion  73   a  of an advancing/backing changeover fork  73  is slidably fitted on the fork support shaft  72 . A distal end portion of the operating arm  71  is interlockingly connected with the proximal portion  73   a . Further, a distal-end fork portion  73   b  of the advancing/backing changeover fork  73  is fitted on an outer peripheral surface of the above-mentioned advancing/backing changeover slide member  58   b.    
         [0052]    In this manner, by manipulating the advancing/backing changeover lever provided to the driving portion  5 , it is possible to allow the advancing/backing changeover slide member  58   b  to assume the advancing changeover state or the backing changeover state by way of the advancing/backing changeover mechanism  68 . 
         [0053]    Further, when the advancing/backing changeover slide member  58   b  is shifted to the advancing changeover state, the power transmitted to the drive shaft  18  from the engine  15  is transmitted to the advancing/backing changeover input shaft  46  by way of the distribution power transmission gear  29 →the distribution input gear  54 →the advancing/backing changeover output shaft  49 →the advancing output gear  56 →the advancing input gear  52 →the advancing/backing changeover input shaft  46  so as to allow the advancing/backing changeover input shaft  46  to perform the advancing-side rotation, that is, the normal rotation. 
         [0054]    On the other hand, when the advancing/backing changeover slide member  58   b  is shifted to the backing changeover state, the power transmitted to the drive shaft  18  from the engine  15  is transmitted to the advancing/backing changeover input shaft  46  by way of the distribution power transmission gear  29 →the distribution input gear  54 →the advancing/backing changeover output shaft  49 →the backing output gear  57 →the counter gear  65 →the backing input gear  53 →the advancing/backing changeover input shaft  46  so as to allow the advancing/backing changeover input shaft  46  to perform the backing-side rotation, that is, the reverse rotation. 
         [0055]    The main transmission mechanism  34  is, as shown in  FIG. 2 , arranged in the inside of a rear chamber  45  of the main transmission casing  37 . Between a rear end portion of the advancing/backing changeover input shaft  46  and a shaft support wall forming body  80  which is formed at a front portion in the inside of the sub transmission casing  38 , a main-transmission main shaft  81  which extends in the longitudinal direction is rotatably extended. On the other hand, between the above-mentioned inner support wall body  43  and the above-mentioned shaft support wall forming body  80 , a main-transmission sub shaft  82  which extends in frontward in the longitudinal direction is rotatably extended in parallel with the above-mentioned main-transmission main shaft  81 . 
         [0056]    Further, the main-transmission main shaft  81  has a distal end portion  81   a  thereof fitted in and supported by a fit/support recessed portion  83  formed in a center portion of a rear end of the advancing/backing changeover input shaft  46  and a rear portion  81   b  thereof supported by the shaft support wall forming body  80  by way of a bearing  84 . That is, the main-transmission main shaft  81  is arranged coaxially with the drive shaft  18  and the advancing/backing changeover input shaft  46 . 
         [0057]    Further, a plurality of main-shaft-side transmission gears are rotatably and concentrically mounted on the main-transmission main shaft  81  thus forming a main-shaft-side transmission gear group  85 . A plurality of transmission bodies are axially slidably fitted on the main-transmission main shaft  81  between respective main-shaft-side gears thus forming a transmission body group  86 . Further, the main-shaft-side transmission gear which is meshed with and connected with any one of the transmission bodies is interlockingly connected with the main-transmission main shaft  81  by way of the transmission body. 
         [0058]    On the other hand, a plurality of sub-shaft-side transmission gears are concentrically mounted on the main-transmission sub shaft  82  thus forming a sub-shaft-side transmission gear group  87 . The respective sub-shaft-side transmission gears are respectively meshed with the opposedly facing main-shaft-side transmission gears, wherein the sub-shaft-side transmission gear  87   a  is meshed with the main-shaft-side transmission gear  46   a  which is integrally formed with a rear end portion of the advancing/backing changeover input shaft  46 . 
         [0059]    In this manner, the power transmitted to the advancing/backing changeover input shaft  46  is transmitted to the respective main-shaft-side transmission gears by way of the main-shaft-side transmission gear  46   a →the sub-shaft-side transmission gear  87   a →the main-transmission sub shaft  82 →the respective sub-shaft-side transmission gears→the respective main-shaft-side transmission gears which are meshed with the respective sub-shaft-side transmission gears, whereby it is possible to transmit the power to the main-transmission main shaft  81  by way of any one of transmission bodies in a gear-changed state. 
         [0060]    Here, the above-mentioned transmission body group  86  is, as shown in  FIG. 2 , interlockingly connected with the main transmission lever  88  mounted on the sub transmission casing  38  by way of a main transmission manipulation mechanism  89 , wherein by manipulating the main transmission lever  88 , the desired transmission body in the inside of the transmission body group  86  is slidably operated by way of the main transmission manipulation mechanism  89  thus performing the main transmission in plural stages. 
         [0061]    The sub transmission mechanism  35  is, as shown in  FIG. 2 , arranged in the inside of the sub transmission casing  38 , wherein a sub transmission shaft  91  is interlockingly connected with a rear-end extending portion  81   c  of the main-transmission main shaft  81  which is extended to a front portion in the inside of the sub transmission casing  38  by way of a planetary gear mechanism  90 . 
         [0062]    Here, a sun gear  92  which constitutes a portion of the planetary gear mechanism  90  is mounted on the rear-end extending portion  81   c  of the main-transmission main shaft  81 . On the other hand, the sub transmission shaft  91  is arranged coaxially with the main-transmission main shaft  81 , has a midst portion thereof supported on a shaft support body  93  disposed in the inside of the sub transmission casing  38  by way of a bearing  94 , and has a rear end portion thereof supported on a shaft support wall  95  formed in the inside of the differential casing  39  described later by way of a bearing  96 . 
         [0063]    Further, between an outer peripheral surface of the sun gear  92  and an outer peripheral surface of the front end portion of the sub transmission shaft  91 , a cylindrical shift gear support body  97  is provided by spline fitting in an axially shiftable manner, a distal end portion of a shift fork  98  is engaged with the shift gear support body  97 , and a sub transmission lever  99  which is mounted on an upper portion of the sub transmission casing  38  is interlockingly connected with a proximal end portion of the shift fork  98 . Due to such a constitution, by manipulating the sub transmission lever  99 , it is possible to perform the sub transmission. 
         [0064]    That is, the above-mentioned constitution allows the sub transmission in which the power is directly transmitted to the sub transmission shaft  91  from the main-transmission main shaft  81  and the sub transmission in which the power is transmitted to the sub transmission shaft  91  from the main-transmission shaft  81  by way of the planetary gear mechanism  90 . 
         [0065]    Further, an opening portion  100  is formed in a bottom portion of the sub transmission casing  38 , a front wheel driving power-take-out portion  101  is mounted by way of the opening portion  100 , and a power-take-out shaft  102  which is mounted on the front wheel driving power-take-out portion  101  and the sub transmission shaft  91  which is provided to the above-mentioned sub transmission mechanism  35  are interlockingly connected with each other by way of a transmission gear group  103 . 
         [0066]    The differential mechanism  36  is, as shown in  FIG. 2 , arranged in the inside of the differential casing  39 , a rear end portion of the above-mentioned sub transmission shaft  91  is extended to a front portion in the inside of the differential casing  39 , an output bevel gear  105  is integrally formed with the rear end portion, and the differential mechanism  36  is meshed with and connected with the output bevel gear  105 . 
         [0067]    Further, rear axle power transmission mechanisms (not shown in the drawings) which are respectively disposed in the inside of the respective rear axle casings  8 ,  8  are interlockingly connected with the differential mechanism  36 , and rear wheels  9 ,  9  are mounted on the respective rear axle power transmission mechanisms by way of the rear axles (not shown in the drawings). 
         [0068]    Further, in the differential casing  39 , an opening portion  106  for maintenance is formed in a ceiling portion thereof, a mounting support frame body  107  is mounted on a peripheral portion of the opening portion  106 , a hydraulic circuit body  108  is mounted on a front portion of the mounting support frame body  107 , and a hydraulic control valve  109  is mounted on the hydraulic circuit body  108 . Further, proximal end portions of a pair of left and right lift arms  111 ,  111  are mounted on a rear portion of the mounting support frame body  107  by way of a lift arm support shaft  110 . Lift cylinders  112 ,  112  which extend vertically are interposed between midst portions of the respective lift arms  111 ,  111  and left and right side portions of the PTO transmission portion  6  described later, and the above-mentioned hydraulic circuit body  108  is connected to the respective lift cylinders  112 ,  112  by way of hydraulic pipes (not shown in the drawings). 
       [Driving Portion  5 ] 
       [0069]    In the driving portion  5 , as shown in  FIG. 1 , a steering column  113  is mounted upright at a position behind the prime mover portion  2  and, at the same time, at a position above the clutch portion  3 , a steering wheel  115  is mounted on an upper end portion of the steering column  113  by way of a wheel support shaft  114 , a driver&#39;s seat  116  is arranged at a position behind the steering wheel  115 , and the above-mentioned main transmission lever  88  and sub transmission lever  99  are arranged in a concentrated manner at a position disposed on a side of the driver&#39;s seat  116 . 
       [PTO Transmission Portion  6 ] 
       [0070]    In the PTO transmission portion  6 , as shown in  FIG. 5  and  FIG. 6 , in an opening portion  120  which is formed in a rear end of the differential casing  39 , a PTO case  121  is detachably mounted, and a PTO transmission mechanism  122  and a PTO clutch mechanism  123  are arranged in the inside of the PTO case  121 . 
         [0071]    Hereinafter, the respective constitutions of [the PTO case  121 ], [the PTO transmission mechanism  122 ] and [the PTO clutch mechanism  123 ] are explained in this order in conjunction with  FIG. 5  and  FIG. 6 . 
       [PTO Case  121 ] 
       [0072]    The PTO case  121  has, as shown in  FIG. 5  and  FIG. 6 , the three-split constitution consisting of a front case forming body  124 , an intermediate case forming body  125  and a rear casing forming body  126 , wherein the respective case forming bodies  124 ,  125 ,  126  are detachably connected with each other, the front case forming body  124  and the intermediate case forming body  125  are arranged in a state that these case forming bodies are housed in the inside of the differential casing  39 , and the rear casing forming body  126  is arranged in a state that the rear casing forming body  126  is bulged rearwardly from the differential casing  39 . 
         [0073]    Further, a flange-like mounting member  127  is integrally formed by molding on a peripheral portion of a front end of the rear casing forming body  126 , and the mounting member  127  is brought into contact with a peripheral portion of a rear end of the differential casing  39  from behind. Further, the mounting member  127  is mounted on the differential casing  39  using mounting bolts  128  which have axes thereof directed in the longitudinal direction. 
         [0074]    In this manner, the PTO case  121  is detachably mounted in the opening portion  120  which is formed in the rear end of the differential casing  39  and hence, in a state that the PTO case  121  is removed from the differential casing  39 , it is possible to easily perform the assembling operation and maintenance operation of the PTO transmission mechanism  122  and the PTO clutch mechanism  123  which are housed in the inside of the PTO case  121 . 
         [0075]    Further, in the PTO case  121 , the front case forming body  124  and the intermediate case forming body  125  are mounted in a state that the front case forming body  124  and the intermediate case forming body  125  are housed in the inside of the differential casing  39  and hence, the transmission casing  30  can be miniaturized (or made compact). 
         [0076]    In the inside of the front case forming body  124 , an input shaft projecting opening portion  131  for receiving an input shaft  130  is formed in a state that the opening portion  131  opens in the longitudinal direction, and a transmission-shaft-front-portion receiving portion  132  is formed at a position above the above-mentioned input shaft projecting opening portion  131 . 
         [0077]    A shaft receiving member  134  which receives a front end portion of the PTO shaft  133  is provided in the inside of the intermediate case forming body  125 , and the shaft receiving member  134  forms a PTO shaft front-portion receiving portion  135  which opens in the longitudinal direction in a midst portion thereof. 
         [0078]    A PTO shaft projecting opening portion  138  is formed in the rear casing forming body  126  in a state that the PTO shaft projecting opening portion  138  is opened in the longitudinal direction, and a transmission-shaft rear-portion receiving portion  139  is formed at a position above the PTO shaft projecting opening portion  138 . 
         [0079]    Further, the input-shaft projecting opening portion  131  which is formed in the front case forming body  124 , the PTO-shaft front-portion receiving portion  135  which is formed in the intermediate case forming body  125  and, the PTO-shaft projecting opening portion  138  which is formed in the rear casing forming body  126  are formed communicably with each other on the same axis which extends in the longitudinal direction. 
         [0080]    Further, the transmission-shaft front-portion receiving portion  132  which is formed in the front case forming body  124  and the transmission-shaft rear-portion receiving portion  139  which is formed in the rear casing forming body  126  are arranged to face each other in an opposed manner in the longitudinal direction. 
         [0081]    Further, on left and right side walls of the rear casing forming body  126 , as shown in  FIG. 2 , lift cylinder support shafts  140 ,  140  which constitute a lift cylinder mounting portion are formed in a state that these lift cylinder support shafts  140 ,  140  project in the outer sideward direction and, a lower end portion of the lift cylinders  112 ,  112  are supported by the respective lift cylinder support shafts  140 ,  140 . 
       [PTO Transmission Mechanism  122 ] 
       [0082]    The PTO transmission mechanism  122  is configured as shown in  FIG. 5  and  FIG. 6 , wherein in the inside of the above-mentioned PTO case  121 , an input shaft  130 , a transmission shaft  141  and the PTO shaft  133  which respectively have axes thereof directed in the longitudinal direction are arranged. 
         [0083]    That is, the input shaft  130  is rotatably supported in the input shaft projecting opening portion  131  formed in the front case forming body  124  of the PTO case  121  by way of bearings  142 ,  143 , while the input shaft  130  has a distal end portion  144  thereof projected forwardly and mounts an output gear  145  on a rear end portion thereof. 
         [0084]    Further, between the transmission-shaft front-portion receiving portion  132  which is formed on the front case forming body  124  and the transmission-shaft rear-portion receiving portion  139  which is formed on the rear casing forming body  126 , the transmission shaft  141  is rotatably supported by way of bearings  146 ,  147 . A large-diameter input gear  148 , and a second transmission gear  149  and a first transmission gear  150  which are integrally formed with each other are coaxially mounted on the transmission shaft  141  in order from a front side to a rear side, wherein the large-diameter input gear  148  is meshed with the output gear  145  mounted on the above-mentioned input shaft  130 . 
         [0085]    Further, between the PTO-shaft front-portion receiving portion  135  which is formed in the intermediate case forming body  125  and the PTO-shaft projecting opening portion  138  which is formed in the rear casing forming body  126 , the PTO shaft  133  is rotatably supported by way of bearings  151 ,  152 . 
         [0086]    Further, a shift gear body  153  is mounted on the intermediate portion of the PTO shaft  133  in spline fitting such that the shift gear body  153  is shifted slidably in the axial direction and, at the same time, a second input gear  154  and a first input gear  155  are rotatably mounted on a front position and a rear position of the shift gear body  153 . While a second side shift gear  156  and a first side shift gear  157  are mounted on the shift gear body  153 , on a rear surface of the second input gear  154 , a second fitting/meshing gear  158  into which the above-mentioned second side shift gear  156  is fitted and with which the second side shift gear  156  is meshed is formed. Further, on a front surface of the first input gear  155 , a first fitting/meshing gear  159  into which the above-mentioned first side shift gear  157  is fitted and with which the first side shift gear  157  is meshed is formed. 
         [0087]    Due to such a constitution, when the first side shift gear  157  is fitted in and meshed with the first fitting/meshing gear  159  of the first input gear  155  by shifting the shift gear body  153  rearwardly, the power which is subjected to the first transmission (low speed step) is transmitted to the PTO shaft  133  by way of the input shaft  130 →the output gear  145 →the large-diameter input gear  148 →the PTO clutch mechanism  123  described later →the second transmission gear  149  and the first transmission gear  150  which are integrally formed →the first input gear  155  the first fitting/meshing gear  159 →the first side shift gear  157 →the shift gear body  153 →and the PTO shaft  133 . 
         [0088]    Further, when the second side shift gear  156  is fitted in and meshed with the second fitting/meshing gear  158  of the second input gear  154  by shifting the shift gear body  153  frontwardly, the power which is subjected to the second transmission (high-speed step) is transmitted to the PTO shaft  133  by way of the input shaft  130 →the output gear  145 →the large-diameter input gear  148 →the PTO clutch mechanism  123  described later→the second transmission gear  149  and the first transmission gear  150  which are integrally formed→the second input gear  154 →the second fitting/meshing gear  158 →the second side shift gear  156 →the shift gear body  153 →the PTO shaft  133 . 
         [0089]    Further, in the shift gear body  153 , as shown in  FIG. 6 , a PTO transmission manipulation mechanism  160  is interlockingly connected. 
         [0090]    That is, in the PTO transmission manipulation mechanism  160 , a transmission manipulation support shaft  163  which has an axis thereof directed in the lateral direction is pivotally supported on a right side wall  161  of the rear casing forming body  126  by way of a boss portion  162 , a proximal end portion of a PTO transmission manipulation arm  164  is mounted on an outer end portion of the transmission manipulation support shaft  163  which is outwardly projected from the right side wall  161 , and a proximal end portion of an operation arm  165  is mounted on an inner end portion of the transmission manipulation support shaft  163  which is inwardly projected from the right side wall  161 . 
         [0091]    Further, a shift fork support shaft  166  which has an axis thereof directed in the longitudinal direction is arranged at a position in the vicinity of the distal end portion of the above-mentioned operation arm  165 , a proximal end portion  168  of a shift fork  167  is slidably mounted on the shift fork support shaft  166  in the lateral direction and, while the distal end portion of the above-mentioned operation arm  165  is connected with the proximal end portion  168 , a proximal end fork portion  170  of the shift fork  167  is engaged with an engaging groove portion  169  which is formed in an outer peripheral surface of the shift gear body  153 . 
         [0092]    Further, a distal end portion of the PTO transmission manipulation arm  164  is interlockingly connected with a PTO transmission lever (not shown in the drawings) which is arranged at a position in the vicinity of the driver&#39;s seat  116  by way of an interlockingly connection mechanism (not shown in the drawings). Due to such a constitution, by manipulating the PTO transmission lever, it is possible to perform the shift operation of the shift gear body  153  thus enabling the PTO transmission. 
       [PTO Clutch Mechanism  123 ] 
       [0093]    A PTO clutch mechanism  123  is, as shown in  FIG. 5  and  FIG. 6 , detachably interposed between a cylindrical front boss body  171  which is rearwardly extended from the rear end surface of the above-mentioned large-diameter input gear  148  along an outer peripheral surface of the transmission shaft  141  and a cylindrical rear boss body  172  which is frontwardly extended from the front end surface of the above-mentioned second transmission gear  149  along an outer peripheral surface of the transmission shaft  141 . The PTO clutch mechanism can perform the connection or disconnection of the large-diameter input gear  148  and the second transmission gear  149  while synchronizing rotational speeds of the gears  148 ,  149 . 
         [0094]    That is, the PTO clutch mechanism  123  is constituted as follows. A ring-shaped support body  173  is fitted on and integrally mounted on the outer peripheral surface of the front side boss body  171 , a slidable support member  174  having a small-width ring shape in the longitudinal direction is mounted on an outer peripheral edge portion of the support body  173 . A front engaging groove  175  which is extended in the longitudinal direction is formed on an outer peripheral surface of the slidable support member  174 , while a ring-shaped slide engaging body  176  is fitted on an outer peripheral surface of the slidable support member  174 . Further, an engaging projecting member  177  which is formed on an inner peripheral surface of the slide engaging body  176  is engaged with the above-mentioned front side engaging groove  175  thus allowing the slide engaging body  176  to be slidable in the longitudinal direction. 
         [0095]    Further, a stepped recessed portion  178  is formed on an outer peripheral surface of the cylindrical rear boss body  172 , a ring-shaped engaging body  179  is fitted on and integrally mounted on an outer peripheral surface of the stepped recessed portion  178 , and a rear engaging groove  180  with which the engaging projecting member  177  of the slide engaging body  176  is engaged is formed on an outer peripheral surface of the ring-shaped engaging body  179 . 
         [0096]    Due to such a constitution, when the slide engaging body  176  is made to slide rearwardly so as to engage the engaging projecting member  177  of the slide engaging body  176  with both of the front side engaging groove  175  and the rear side engaging groove  180  in a state that the engaging projecting member  177  is extended between the front side engaging groove  175  and the rear side engaging groove  180 , it is possible to interlockingly connect the large-diameter input gear  148  and the second transmission gear  149 . 
         [0097]    Accordingly, in such a state, a rotational power of the large-diameter input gear  148  which forms a rotation side is transmitted to the second transmission gear  149  by way of the front side boss body  171 →the front side engaging groove  175  which is formed on the slidable support member  174  of the support body  173 →the engaging projecting member  177  which is formed on the slide engaging body  176 →the rear side engaging groove  180  which is formed on the engaging body  179 →the stepped recessed portion  178  which is formed on the rear boss body  172 →the second transmission gear  149  and hence, the second transmission gear  149  is also integrally rotated. 
         [0098]    Further, outer peripheral portions of a plurality of ring-like movable-side clutch plates  181  are mounted on a rear portion of an inner peripheral surface of the slide engaging body  176  in an axially slidable manner, while a plurality of ring-like fixed-side clutch plates  182  are mounted in a fitting state on an outer peripheral surface of a stepped recessed portion  178  in an axially spaced-apart manner at a given interval, and the movable-side clutch plates  181  are interposed one by one between these neighboring fixed-side clutch plates  182 ,  182 . 
         [0099]    In this manner, when the slide engaging body  176  is made to slide rearwardly, the movable-side clutch plate  181  is gradually pushed to the fixed-side clutch plates  182 , 182  and a rotational force of a large-diameter input gear  148  which constitutes a rotation side is gradually transmitted to a second transmission gear  149  which constitutes a stop side. In a state that a rotational speed of the second transmission gear  149  is synchronized with the rotational speed of the large-diameter input gear  148 , an engaging projecting member  177  of the slide engaging body  176  is engaged with a rear engaging groove  180  of an engaging body  179 . As a result, the engagement between the engaging projecting member  177  and the rear engaging groove  180  is smoothly and surely performed thus preventing the generation of uncomfortable sounds. 
         [0100]    Here, the PTO clutch mechanism  123  is detachable mounted on the PTO transmission portion  6  and hence, it is possible to provide the specification (see  FIG. 5 ) which includes the PTO clutch mechanism  123  and the specification which does not include the PTO clutch mechanism  123  (see  FIG. 9 ) in a state that the PTO transmission portion  6  is used in common. 
         [0101]    Further, the PTO clutch mechanism  123  is detachably mounted on the PTO transmission portion  6  and the PTO transmission portion  6  is detachably mounted on the transmission portion  4  and hence, it is possible to easily perform the assembling operation as well as the disassembling operation thus realizing the easy maintenance operation. 
         [0102]    Further, a PTO clutch manipulation mechanism  185  is, as shown in  FIG. 6 , interlockingly connected with the slide engaging body  176 . 
         [0103]    That is, in the PTO clutch manipulation mechanism  185 , a clutch manipulation support shaft  188  which has an axis thereof directed in the lateral direction is pivotally supported on a left side wall  186  of the rear casing forming body  126  by way of a boss portion  187 , a proximal end portion of a PTO clutch manipulation arm  189  is mounted on an outer end portion of the clutch manipulation support shaft  188  which is projected outwardly from the left side wall  186 , and a proximal end portion of a clutch operation arm  190  is mounted on an inner end portion of the clutch manipulation support shaft  188  which is projected inwardly from the left side wall  186 . 
         [0104]    Further, a slide fork support shaft  191  which has an axis thereof directed in the longitudinal direction is arranged at a position in the vicinity of a distal end portion of the above-mentioned clutch operation arm  190 , and a proximal end portion  193  of a slide fork  192  is mounted on the slide fork support shaft  191  in a state that the proximal end portion  193  is slidable in the longitudinal direction. Further, a distal end portion of the clutch operation arm  190  is connected to the proximal end portion  193 , and a distal-end fork portion  195  of the slide fork  192  is engaged with an engaging projecting member  194  which is formed on an outer peripheral surface of the slide engaging body  176 . 
         [0105]    Further, on the proximal end portion  193  of the slide fork  192 , as shown in  FIG. 6 , a temporary stopper body  196  is mounted, wherein the temporary stopper body  196  includes a temporary stopper ball  197  which is arranged in a reciprocating manner toward the inside of the proximal end portion  193  and a pusher spring  198  which resiliently biases the temporary stopper ball  197  in the advancing direction. 
         [0106]    Further, in an outer peripheral surface of the slide fork support shaft  191  on which the proximal end portion  193  of the slide fork  192  is fitted, a connection holding engaging groove  199  and a disconnection holding engaging groove  200  with which the pusher spring  198  is engaged are formed. 
         [0107]    Further, a distal end portion of the above-mentioned PTO clutch manipulation arm  189  is interlockingly connected with a PTO clutch lever (not shown in the drawing) which is arranged at a position in the vicinity of a driver&#39;s seat  116  by way of an interlockingly connecting mechanism (not shown in the drawing), wherein by manipulating the PTO clutch lever, it is possible to slidably operate the slide engaging body  176  so as to perform the clutch connection/disconnection manipulation. 
         [0108]    In this case, during the clutch connection manipulation, the pushing spring  198  is engaged with the connection holding engaging groove  199  of the slide fork support shaft  191  so as to hold the clutch connection state. 
         [0109]    Further, during the clutch disconnection manipulation, the pushing spring  198  is engaged with the disconnection holding engaging groove  200  of the slide fork support shaft  191  so as to hold the clutch disconnection state. 
         [0110]    Here, the PTO clutch lever is manually manipulated. Due to the manual manipulation of the PTO clutch lever, the movable-side clutch plate  181  and the fixed-side clutch plate  182  are brought into pressure contact with each other and the clutch connection manipulation can be performed while delicately synchronizing the rotational speeds of the large-diameter input gear  148  and the second transmission gear  149  and hence, it is possible to ensure the smooth and reliable connecting manipulation and the prevention of occurrence of uncomfortable sounds or the like. 
         [0111]    Further, the distal end portion  144  of the above-mentioned input shaft  130  is, as shown in  FIG. 2 , interlockingly connected with the drive shaft  18  by way of the PTO-system power transmission shaft  210  thus forming the PTO-system power transmission mechanism  32 , wherein the PTO-system power transmission shaft  210  has a front portion to a rear portion thereof arranged in the inside of the transmission casing  30  in a state that an axis thereof is directed in the longitudinal direction. 
       (PTO-System Power Transmission Shaft  210 ) 
       [0112]    The PTO-system power transmission shaft  210  is, as shown in  FIG. 2 , configured such that a front end portion thereof is pivotally supported on the support wall body  21  of the clutch housing  17  by way of a bearing  211 , while a rear end portion thereof is interlockingly connected with a distal end portion  144  of the above-mentioned input shaft  130  by way of a connection sleeve  212 . 
         [0113]    Further, an input gear  213  is mounted on a front end portion of the PTO-system power transmission shaft  210 , while the input gear  213  is meshed with the distribution power transmission gear  29  which is integrally formed with the drive shaft  18 . 
         [0114]    Here, the power-take-out gear  214  is mounted on a front portion of the PTO-system power transmission shaft  210  and the power can be taken out to the outside from the power-take-out gear  214  through a power-take-out opening portion  215  which is formed in a left side wall of the main transmission casing  37 . 
         [0115]    Further, a one way clutch  216  is mounted on a rear portion of the PTO-system power transmission shaft  210 . 
         [0116]    In this manner, the power transmitted to the drive shaft  18  from the engine  15  is transmitted to the input shaft  130  by way of the distribution power transmission gear  29  which is integrally formed on the drive shaft  18 →the input gear  213 →the PTO-system power transmission shaft  210 →the input shaft  130 . 
         [0117]      FIG. 7  shows a drive shaft  18  of the second embodiment. This drive shaft  18  has the basic structure which is equal to the basic structure of the drive shaft  18  of the above-mentioned first embodiment. However, the drive shaft  18  of this embodiment differs from the drive shaft  18  of the first embodiment with respect to a point that the drive shaft  18  has the inner-outer duplicate shaft structure which is constituted of an inner drive shaft forming body  230  which extends in the longitudinal direction and a cylindrical outer drive shaft forming body  231  which is rotatably fitted on an outer periphery of the inner drive shaft forming body  230 . 
         [0118]    Further, in the inner drive shaft forming body  230 , a front end portion  232  is interlockingly connected with the flywheel  19 , a PTO distribution power transmission gear  234  is integrally formed on a rear end portion  233 , and the input gear  213  which is integrally formed on the front end portion of the PTO-system power transmission shaft  210  is meshed with the PTO distribution power transmission gear  234 . 
         [0119]    Further, in the outer drive shaft forming body  231 , a front end portion  235  is interlockingly connected with the clutch  23 , a traveling distribution power transmission gear  237  is integrally formed on a rear end portion  236 , and the distribution input gear  54  which is mounted on the advancing/backing changeover output shaft  49  is meshed with the traveling distribution power transmission gear  237 . Numeral  238  indicates a bearing. 
         [0120]    Here, the PTO distribution power transmission gear  234  and the traveling distribution power transmission gear  237  are arranged in a compact manner by arranging these power transmission gears  234 ,  237  close to each other coaxially and longitudinally. 
         [0121]      FIG. 8  shows a drive shaft  18  of the third embodiment. This drive shaft  18  has the basic structure which is equal to the basic structure of the drive shaft  18  of the above-mentioned first embodiment. However, the drive shaft  18  of this embodiment differs from the drive shaft  18  of the first embodiment with respect to a point that the drive shaft is split in two, that is, a front-half drive shaft forming body  240  and a rear-half drive shaft forming body  241  and, at the same time, a rear end portion  242  of the front-half drive shaft forming body  240  and front end portion  243  of the rear-half drive shaft forming body  241  are interlockingly connected with each other coaxially by way of a cylindrical connecting body  244 . 
         [0122]    Further, a cylindrical support member  25  which supports amidst portion of the front half drive shaft forming body  240  is allowed to penetrate the support wall body  21  formed along a rear-end peripheral portion of the clutch housing  17  and, at the same time, is extended to the inside of the front chamber  44  of the rear main transmission casing  37  in a cylindrical shape, and a front portion of the rear half drive shaft forming body  241  is rotatably supported on a rear end portion  245  by way of a bearing  246 . 
         [0123]    Further, the distribution power transmission gear  29  is integrally mounted on a midst portion of the rear half drive shaft forming body  241  which is positioned right behind the above-mentioned bearing  246 , while the input gear  213  which is integrally formed on the front portion of the PTO-system power transmission shaft  210  is meshed with the distribution power transmission gear  29 . 
         [0124]    Here, the input gear  213  is arranged in the vicinity of the power-take-out opening portion  215  formed in the left side wall of the main transmission case  37  so as to allow the takeout of the power to the outside from the input gear  213  through the power-take-out opening portion  215 . 
         [0125]    Further, the drive shaft  18  of the second embodiment adopts the specification which is not provided with the advancing/backing changeover mechanism  68  and the opening portion  59  is closed with a closure lid body  247 . 
         [0126]    Accordingly, in the specification which is provided with the advancing/backing changeover mechanism  68 , the drive shaft  18  of the first embodiment or the second embodiment is mounted, while in the specification which is not provided with the advancing/backing changeover mechanism  68 , it is possible to change the specification by mounting the drive shaft  18  of the third embodiment. 
         [0127]      FIG. 9  shows a transmission shaft  141  of the second embodiment. Although the transmission shaft  141  of this embodiment has the basic structure which is equal to the basic structure of the first embodiment, this embodiment differs from the first embodiment with respect to a point that the transmission shaft  141  of this embodiment is not provided with the PTO clutch mechanism  123 . 
         [0128]    That is, in the transmission shaft  141 , a shaft body  248 , the second transmission gear  149  and the first transmission gear  150  are integrally formed and the large-diameter input gear  148  is integrally mounted on a front portion of the shaft body  248 . 
         [0129]    Accordingly, in the specification which includes the PTO clutch mechanism  123 , the transmission shaft  141  of the first embodiment is provided, while in the specification which does not include the PTO clutch mechanism  123 , the transmission shaft  141  is mounted in the second embodiment thus facilitating the change of the specification.