Abstract:
In the transmissions of the background art, two larger-diameter gears are mounted on a final shaft thereby causing an increase in weight of the transmission. The present invention reduces the weight of a transmission by mounting only one larger-diameter gear on a final shaft. The transmission of the present invention includes an output shaft; a countershaft extending parallel to the output shaft; an intermediate shaft extending parallel to the output shaft; a forward drive gear rotatably supported relative to the countershaft; a reverse drive gear rotatably supported relative to the countershaft; a first intermediate gear rotatably supported relative to the intermediate shaft, the first intermediate gear normally meshing with the reverse drive gear; a second intermediate gear rotatably supported relative to the intermediate shaft, the second intermediate gear being interlocked with the first intermediate gear to rotate therewith; an output shaft driven gear fixed to the output shaft, the output shaft driven gear being normally meshing with the forward drive gear and the second intermediate gear; and a gear selecting and fixing device axially movably mounted on the countershaft for selectively fixing the forward drive gear and the reverse drive gear to the countershaft.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This non-provisional application claims priority under 35 U.S.C. § 119(a) on patent application No. 2002-197503 filed in Japan on Jul. 5, 2002, and priority under 35 U.S.C. § 120 on U.S. patent application Ser. No. 10/609,415 filed on Jul. 1, 2003, the entirety of each which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a transmission, and more particularly to a transmission for a saddle type vehicle (buggy) for operation on rough terrain.  
         [0004]     2. Description of the Background Art  
         [0005]     A conventional transmission of the background art is described in Japanese Patent Publication No. 63-61212, the entirety of which is hereby incorporated by reference. This transmission includes a primary shaft, a main shaft, and a countershaft. Power is transmitted between the primary shaft, main shaft and countershaft respectively, e.g., the countershaft is a final shaft in this transmission.  
         [0006]     In general, gears having diameters larger than the diameters of gears mounted on front-stage shafts are mounted on rear-stage shafts, e.g., so as to sequentially reduce a rotational speed during power transmission. In the above-described transmission of the background art, larger-diameter gears are mounted on the final shaft.  
         [0007]     More specifically, two larger-diameter gears having different diameters are fixed to the final shaft (the countershaft in the above publication). Two smaller-diameter gears normally meshing with the two larger-diameter gears are rotatably supported relative to the shaft (the main shaft in the above publication) provided on the directly front stage of the final shaft. Further, gear selecting and fixing means (a gear selecting mechanism in the above publication) is provided between these smaller-diameter gears. Either of these smaller-diameter gears is selected and fixed to the support shaft by the gear selecting and fixing means, thereby allowing the selection of any one of different operational conditions.  
         [0008]     Applicants have determined that the background art suffers from the following disadvantages. As mentioned above, the two larger-diameter gears are mounted on the final shaft of the transmission in the related art, causing an increase in weight of the transmission.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention overcomes the shortcomings associated with the background art and achieves other advantages not realized by the background art.  
         [0010]     An object of the present invention is to reduce the weight of the transmission by mounting only one larger-diameter gear on the final shaft;  
         [0011]     One or more of these and other objects are accomplished by a transmission comprising an output shaft; a countershaft extending parallel to the output shaft; an intermediate shaft extending parallel to the output shaft; a forward drive gear being rotatably supported relative to the countershaft; a reverse drive gear being rotatably supported relative to the countershaft; a first intermediate gear being rotatably supported relative to the intermediate shaft, the first intermediate gear meshing with the reverse drive gear; a second intermediate gear rotatably supported relative to the intermediate shaft, the second intermediate gear being interlocked with the first intermediate gear to rotate therewith; an output shaft driven gear fixed to the output shaft, the output shaft driven gear meshing with the forward drive gear and the second intermediate gear; and 
        a gear selecting and fixing device axially and movably mounted on the countershaft for selectively fixing the forward drive gear and the reverse drive gear to the countershaft.        
 
         [0013]     One or more of these and other objects are further accomplished by a power unit for a four-wheeled vehicle comprising an internal combustion engine having a crankshaft arranged with respect to a longitudinal direction of the engine; a transmission including a main shaft operatively engaged with the crankshaft through a torque converter and a primary drive gear on the crankshaft and a primary driven gear on the main shaft; an output shaft; a countershaft extending parallel to the output shaft; an intermediate shaft extending parallel to the output shaft; a forward drive gear being rotatably supported relative to the countershaft; a reverse drive gear being rotatably supported relative to the countershaft; a first intermediate gear being rotatably supported relative to the intermediate shaft, the first intermediate gear meshing with the reverse drive gear; a second intermediate gear rotatably supported relative to the intermediate shaft, the second intermediate gear being interlocked with the first intermediate gear to rotate therewith; an output shaft driven gear fixed to the output shaft, the output shaft driven gear meshing with the forward drive gear and the second intermediate gear; and 
        a gear selecting and fixing device axially and movably mounted on the countershaft for selectively engaging the forward drive gear and the reverse drive gear to the countershaft.        
 
         [0015]     With this configuration, only one larger-diameter gear is mounted on the final shaft of the transmission as the output shaft driven gear, thereby allowing a reduction in the overall weight of the transmission.  
         [0016]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0018]      FIG. 1  is a side view of a four-wheeled buggy, e.g., saddle type vehicle for operating in rough terrain, having a transmission according to a preferred embodiment of the present invention;  
         [0019]      FIG. 2  is an elevational view of a power unit in the vehicle shown in  FIG. 1 ;  
         [0020]      FIG. 3  is a rear elevation of a rear crankcase of the power unit;  
         [0021]      FIG. 4  is a longitudinal sectional view of an internal structure of a crankcase, showing a structural relationship between a crankshaft and a main shaft;  
         [0022]      FIG. 5  is a longitudinal sectional view of the internal structure of the crankcase, showing the structural relationship between the main shaft, a countershaft, an intermediate shaft, and an output shaft; and  
         [0023]      FIG. 6  is a longitudinal sectional view showing a driving mechanism for a dog clutch for forward/reverse selection. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The present invention will hereinafter be described with reference to the accompanying drawings.  FIG. 1  is a side view of a four-wheeled buggy, e.g., saddle type vehicle for operating in rough terrain, having a transmission according to a preferred embodiment of the present invention.  FIG. 2  is an elevational view of a power unit in the vehicle shown in  FIG. 1 .  FIG. 3  is a rear elevation of a rear crankcase of the power unit.  FIG. 4  is a longitudinal sectional view of an internal structure of a crankcase, showing a structural relationship between a crankshaft and a main shaft.  FIG. 5  is a longitudinal sectional view of the internal structure of the crankcase, showing the structural relationship between the main shaft, a countershaft, an intermediate shaft, and an output shaft.  FIG. 6  is a longitudinal sectional view showing a driving mechanism for a dog clutch for forward/reverse selection.  
         [0025]     As seen in  FIG. 1 , a buggy includes a body frame  1 , a pair of right and left front wheels  2  provided at a front portion of the body frame  1 , and a pair of right and left rear wheels  3  provided at a rear portion of the body frame  1 . A power unit  6  configured by integrating an internal combustion engine  4  and a transmission  5  is supported by a central portion of the body frame  1 . The power unit  6  is arranged so that a crankshaft  7  extends in the longitudinal direction of the vehicle.  
         [0026]     As will be described in greater detail hereinafter, the rotation of the crankshaft  7  is transmitted through a main shaft  8 , a countershaft  9 , and an intermediate shaft  10  (each being shown in  FIG. 3 ) to an output shaft  11  in the transmission  5 . These shafts  8 ,  9 ,  10 , and  11  also extend parallel to the crankshaft  7  in the longitudinal direction of the vehicle. The front wheels  2  are driven by a front drive shaft  12  connected to the front end of the output shaft  11 , and the rear wheels  3  are driven by a rear drive shaft  13  connected to the rear end of the output shaft  11 . A steering handle  14 , a fuel tank  15 , and a saddle seat  16  are arranged in this order from the front side of the vehicle on an upper portion of the body frame  1 .  
         [0027]      FIG. 2  is an elevational view of the power unit  6  as viewed from the front side of the power unit  6 . The power unit  6  generally includes a cylinder head cover  20 , a cylinder head  21 , a cylinder block  22 , and a crankcase  23  arranged in this order from the upper side of the power unit  6 . The crankcase  23  is divided into four parts along planes perpendicular to the crankshaft  7 . That is, as partially shown in  FIGS. 4 and 5 , the crankcase  23  includes a front crankcase cover  24 , a front crankcase  25 , a rear crankcase  26 , and a rear crankcase cover  27  arranged in this order from the front side of the power unit  6 . In  FIG. 2 , the front crankcase cover  24  is generally shown and the front crankcase  25  is slightly shown around the front crankcase cover  24 . Various devices and pipes are mounted on the front surface of the front crankcase cover  24 .  
         [0028]      FIG. 3  is a rear elevation of the rear crankcase  26 , showing the positions of the crankshaft  7 , the main shaft  8 , the countershaft  9 , the intermediate shaft  10 , and the output shaft  11 .  FIGS. 4 and 5  are longitudinal sectional views showing an internal structure of the crankcase  23  along these shafts  7  to  11 . More specifically,  FIG. 4  shows the relationship between the crankshaft  7  and the main shaft  8 , and  FIG. 5  shows the relationship between the main shaft  8 , the countershaft  9 , the intermediate shaft  10 , and the output shaft  11 . In  FIGS. 4 and 5 , the arrow F indicates the front side of the crankcase  23 .  
         [0029]      FIG. 4  shows a power transmitting mechanism for the crankshaft  7  and the main shaft  8 . The crankshaft  7  is rotatably supported through bearings to the front and rear crankcases  25  and  26 . An extended front end of the crankshaft  7  is supported through a bearing to the front crankcase cover  24 . The crankshaft  7  is divided into front and rear sections in the longitudinal direction. The front and rear sections of the crankshaft  7  are connected at their crank webs  7   a  by a crankpin  7   b.  An alternator  28  for producing alternating current by the rotation of the crankshaft  7  is mounted on a rear end portion of the crankshaft  7  (the rear section). Reference numeral  29  denotes an oil filter provided on the front crankcase cover  24  for cleaning a clutch operating oil.  
         [0030]     A torque converter  30  is mounted on a front portion of the crankshaft  7  (the front section), and a primary drive gear  31  is mounted adjacent to the torque converter  30 . The primary drive gear  31  is rotatably supported through a needle bearing  32  relative to the crankshaft  7 . The torque converter  30  includes a pump impeller  33  fixed to the crankshaft  7 , a turbine runner  34  opposed to the pump impeller  33 , and a stator  35 . The primary drive gear  31  rotatable relative to the crankshaft  7  is connected to the turbine runner  34 , and power from the crankshaft  7  is hydraulically transmitted through the torque converter  30  to the primary drive gear  31 . A primary driven gear  36  normally meshing with the primary drive gear  31  is fixed to a front end portion of the main shaft  8 . The rotation of the crankshaft  7  is transmitted through the torque converter, the primary drive gear  31 , and the primary driven gear  36  to the main shaft  8  with a primary speed reduction obtained by the gears  31  and  36 .  
         [0031]      FIG. 5  shows a power transmitting mechanism and its relationship with the main shaft  8 , the countershaft  9 , the intermediate shaft  10 , and the output shaft  11 . The main shaft  8  is rotatably supported through bearings to the front and rear crankcases  25  and  26 . A first-speed drive gear  40 , a second-speed drive gear  41 , and a third-speed drive gear  42  different in the number of teeth according to gear ratios are mounted on the main shaft  8 . The second-speed drive gear  41  and the third-speed drive gear  42  are fixed to the main shaft  8 , and the first-speed drive gear  40  is relatively rotatably supported through needle bearings  43  to the main shaft  8 . In the following description, a gear relatively rotatably supported through a needle bearing to a rotating shaft will be referred to generally as an idle gear. A first-speed hydraulic multi-plate clutch  50  is interposed between the main shaft  8  and the first-speed drive gear  40 . The first-speed hydraulic multi-plate clutch  50  has an outer member  51  fixed to the main shaft  8  and an inner member  52  connected to the first-speed drive gear  40 . A pressure plate  53  is axially movably engaged in the outer member  51 . The main shaft  8  has a front center hole  55  axially extending from the front end of the main shaft  8  to an intermediate portion and a rear center hole  56  axially extending from the rear end of the main shaft  8  to the intermediate portion. The rear center hole  56  is slightly larger in diameter than the front center hole  55 . Thus, the front center hole  55  and the rear center hole  56  of the main shaft  8  are not in communication with each other at this intermediate portion. The main shaft  8  further has an operating oil supply hole  57  communicating with the front center hole  55  and the first-speed hydraulic multi-plate clutch  50 , and has lubricating oil supply holes  58  communicating with the rear center hole  56  and the needle bearings  43 .  
         [0032]     As shown in  FIG. 5 , an operating oil for the first-speed hydraulic multi-plate clutch  50  is supplied from the front crankcase cover  24  side through an operating oil supply pipe  59  into the front center hole  55 , and is further supplied through the operating oil supply hole  57  into the clutch  50 . The operating oil supplied to the clutch  50  is introduced into a space between the outer member  51  and the pressure plate  53 . When the pressure plate  53  is moved by this oil pressure to engage the clutch  50 , the first-speed drive gear  40  is fixed to the main shaft  8 , so that the rotation of the main shaft  8  is transmitted to the first-speed drive gear  40 . A lubricating oil to the needle bearings  43  for supporting the first-speed drive gear  40  is supplied from the rear center hole  56  through the lubricating oil supply holes  58 .  
         [0033]     The countershaft  9  is composed of a front countershaft  9   a  and a rear countershaft  9   b  integrally connected with each other. The countershaft  9  is rotatably supported through bearings to the front crankcase  25 , the rear crankcase  26 , and the rear crankcase cover  27 . A first-speed driven gear  60 , a second-speed driven gear  61 , and a third-speed driven gear  62  respectively meshing with the first-speed drive gear  40 , the second-speed drive gear  41 , and the third-speed drive gear  42  supported to the main shaft  8  are mounted on the front countershaft  9   a.  The first-speed driven gear  60  is fixed to the front countershaft  9   a.  The second-speed driven gear  61  and the third-speed driven gear  62  are idle gears, which are rotatably supported through needle bearings  63  and  64  relative to the front countershaft  9   a,  respectively.  
         [0034]     A second-speed hydraulic multi-plate clutch  65  is interposed between the front countershaft  9   a  and the second-speed driven gear  61 . A third-speed hydraulic multi-plate clutch  66  is interposed between the front countershaft  9   a  and the third-speed driven gear  62 . The second-speed hydraulic multi-plate clutch  65  has an outer member fixed to the front countershaft  9   a  and an inner member connected to the idle gear  61 , and the third-speed hydraulic multi-plate clutch  66  has an outer member fixed to the front countershaft  9   a  and an inner member connected to the idle gear  62 . These clutches  65  and  66  are similar in configuration and operation to the first-speed hydraulic multi-plate clutch  50  mentioned above. An operating oil for these clutches  65  and  66  is supplied through operating oil supply holes  67  and  68  formed in the front countershaft  9   a,  respectively, thereby stopping idle rotation of the idle gears  61  and  62  to permit power transmission and effect a second-speed or third-speed reduction. A lubricating oil to the needle bearings  63  and  64  for respectively supporting the second-speed driven gear  61  and the third-speed driven gear  62  is supplied through lubricating oil supply holes  69  and  70  formed in the front countershaft  9   a.    
         [0035]     The front countershaft  9   a  has a front center hole  78  axially extending from the front end of the shaft  9   a  to an intermediate portion and a rear center hole  79  axially extending from the rear end of the shaft  9   a  to the intermediate portion. The front center hole  78  has a stepwise diameter, and the rear center hole  79  is larger in diameter than the front center hole  78 . Thus, the front center hole  78  and the rear center hole  79  of the front countershaft  9   a  are not in communication with each other at this intermediate portion. On the other hand, the rear countershaft  9   b  has a through center hole  80  axially extending between the opposite ends of the shaft  9   b.  The front end of the rear countershaft  9   b  is engaged with the rear center hole  79  of the front countershaft  9   a,  thus making an integral rotation of the front and rear countershafts  9   a  and  9   b.  The rear center hole  79  of the front countershaft  9   a  is in communication with the through center hole  80  of the rear countershaft  9   b.    
         [0036]     The supply of the operating oil to the second-speed and third-speed hydraulic multi-plate clutches  65  and  66  is performed through a double pipe  81  inserted in the front center hole  78  of the countershaft  9  from the front crankcase cover  24  side. The double pipe  81  is composed of an outer pipe  81   a  and an inner pipe  81   b  inserted in the outer pipe  81   a.  The operating oil to the second-speed hydraulic multi-plate clutch  65  is supplied through an oil passage defined between the outer pipe  81   a  and the inner pipe  81   b  and through the operating oil supply hole  67 . The operating oil to the third-speed hydraulic multi-plate clutch  66  is supplied through an oil passage defined inside the inner pipe  81   b  and through the operating oil supply hole  68 . The lubricating oil to the needle bearing  63  for supporting the second-speed driven gear  61  is supplied from the front crankcase  25  side through an oil passage defined between the front countershaft  9   a  and the outer pipe  81   a  and through the lubricating oil supply hole  69 . The lubricating oil to the needle bearing  64  for supporting the third-speed driven gear  62  is supplied from the rear crankcase cover  27  side through the through center hole  80 , the rear center hole  79 , and the lubricating oil supply hole  70 .  
         [0037]     A forward drive gear  71  and a reverse drive gear  72  are mounted on the rear countershaft  9   b.  These gears  71  and  72  are idle gears. A manually operated dog clutch  73 , providing a gear selecting and fixing function, is interposed between these gears  71  and  72  so that the dog clutch  73  is engageable with either the gear  71  or  72 . Accordingly, either the gear  71  or  72  engaged with the dog clutch  73  is selectively fixed to the rear countershaft  9   b,  thereby allowing power transmission. The rear countershaft  9   b  is formed with lubricating oil supply holes  76  and  77  for respectively supplying the lubricating oil to needle bearings  74  and  75  for respectively supporting the forward drive gear  71  and the reverse drive gear  72 . The lubricating oil to the needle bearings  74  and  75  is supplied from the rear crankcase cover  27  side through the through center hole  80  and the lubricating oil supply holes  76  and  77  of the rear countershaft  9   b.    
         [0038]     The intermediate shaft  10  is supported by the rear crankcase  26  and the rear crankcase cover  27 . A first intermediate gear  82  normally meshing with the reverse drive gear  72  and a second intermediate gear  83  having a long sleeve portion  83 a connected to the first intermediate gear  82  is rotatably supported relative to the intermediate shaft  10 . These gears  82  and  83  are idle gears. The lubricating oil to sliding portions of the intermediate shaft  10  for sliding the first and second intermediate gears  82  and  83  is supplied from the rear crankcase  26  side through a center hole of the intermediate shaft  10  and lubricating oil supply holes  84  of the intermediate shaft  10 .  
         [0039]     The output shaft  11  is rotatably supported through bearings to the front crankcase cover  24 , the rear crankcase  26 , and the rear crankcase cover  27 . The output shaft  11  extends through the front crankcase  25  in a non-contact relationship therewith. An output shaft driven gear  85  normally meshing with the forward drive gear  71  and the second intermediate gear  83  is fixed to the output shaft  11 . The output shaft driven gear  85  is driven in a forward direction or a reverse direction through either the gear  71  or  72  engaged with the dog clutch  73 , thereby rotating the output shaft  11  in a direction adapted to the forward running or reverse running of the vehicle. The reverse driving of the output shaft driven gear  85  is effected only when the countershaft  9  is being rotated at the first speed.  
         [0040]     All of the gears in this transmission are constant-mesh type gears, and what gear ratio is to be selected is determined by the hydraulic multi-plate clutches  50 ,  65 , and  66  that is engaged. The hydraulic control for these clutches  50 ,  65 , and  66  is performed by a valve body  90  (see  FIG. 2 ) assembled as a hydraulic control unit including a solenoid valve and a pressure switching valve. As shown in  FIG. 2 , the valve body  90  is mounted on the front surface of the front crankcase cover  24 .  
         [0041]     As shown in  FIG. 5 , the operating oil to the first-speed hydraulic multi-plate clutch  50  is supplied from the valve body  90  through an oil passage  91  formed in the front crankcase cover  24  and the operating oil supply pipe  59  inserted in the front center hole  55  of the main shaft  8  into the front center hole  55 , and is further supplied through the operating oil supply hole  57  to the first-speed hydraulic multi-plate clutch  50 .  
         [0042]     The operating oil to the second-speed hydraulic multi-plate clutch  65  or the third-speed hydraulic multi-plate clutch  66  is supplied from the valve body  90  through an oil passage  92  or  93  formed in the front crankcase cover  24  and the outer passage or the inner passage of the double pipe  81  inserted in the front center hole  78  of the countershaft  9  into the front center hole  78 . The operating oil is further supplied through the operating oil supply hole  67  or  68  to the second-speed hydraulic multi-plate clutch  65  or the third-speed hydraulic multi-plate clutch  66 .  
         [0043]     A driving mechanism for the dog clutch  73  for selecting the forward running or the reverse running of the vehicle is shown in  FIGS. 3 and 6 . Referring to  FIG. 6 , the outer surface of the dog clutch  73  is formed with a circumferential groove  73 a, and a shift fork  100  is engaged at its forked portion with the circumferential groove  73 a of the dog clutch  73 . The shift fork  100  is axially slidably engaged with a guide shaft  101 . The guide shaft  101  is a fixed shaft supported to the rear crankcase  26  and the rear crankcase cover  27 . The shift fork  100  is integrally formed with a shifter pin  102  opposite to the forked portion. The head of the shifter pin  102  is slidably engaged with a helical groove  103 a formed on a shift drum  103 .  
         [0044]     The helical groove  103   a  of the shift drum  103  is a short groove extending along a substantially half portion of the outer circumference of the shift drum  103 . Accordingly, an unnecessary portion of the shift drum  103  is cut away for the purpose of weight reduction. The shift drum  103  is supported to a drum shaft  104 . A drum driven gear  105  and a shift cam  106  are also mounted on the drum shaft  104 . The shift drum  103 , the drum driven gear  105 , and the shift cam  106  are joined together by an interlocking pin  107  to restrain their relative rotation, e.g., so that these members  103 ,  105 , and  106  are rotated together.  
         [0045]     A shift spindle  108  is rotatably supported to the rear crankcase  26  and the rear crankcase cover  27 . A sector gear  109  meshing with the drum driven gear  105  is fixed to the shift spindle  108 . When the shift spindle  108  is rotated, the drum driven gear  105 , the shift drum  103 , and the shift cam  106  are rotated together by the sector gear  109 . The shift spindle  108  is connected through an operating cable (not shown) to a shift lever (not shown) provided on the steering handle  14  of the vehicle, and is rotated by manually operating the shift lever.  
         [0046]     As shown in  FIG. 3 , the shift cam  106  is a star plate member, and a roller  111  supported at the upper end of a shift drum stopper  110  is in contact with the outer circumference of the shift cam  106 . The shift drum stopper  110  is pivotably supported to a pin  112 , and a spring  113  is engaged with the shift drum stopper  110  to normally bias the roller  111  against the outer circumference of the shift cam  106 . This mechanism constitutes a rotational position holding device for the shift drum  103  such that the rotational position of the shift drum  103  becomes stable when the roller  111  comes into contact with any one of the valleys formed on the outer circumference of the shift cam  106 . There are three stable positions of the shift drum  103  corresponding to forward, neutral, and reverse conditions.  
         [0047]     When the shift lever provided on the steering handle  14  of the vehicle is rotationally operated from a neutral position to a forward position or a reverse position, the shift spindle  108  and the sector gear  109  are rotated together, thereby rotating the drum driven gear  105  to a stable position given by the shift cam  106 . At the same time, the shift drum  103  is rotated about the drum shaft  104  together with the drum driven gear  105  by the operation of the interlocking pin  107 , so that the shifter pin  102  is pushed by the inner edge of the helical groove  103   a  formed on the outer circumference of the shift drum  103 . As a result, the shift fork  100  supported to the guide shaft  101  is axially slid, and the dog clutch  73  is accordingly moved in the axial direction of the countershaft  9  through the circumferential groove  73   a  of the dog clutch  73 . At this time, one of the projections formed at the opposite ends of the dog clutch  73  comes into engagement with either the forward drive gear  71  or the reverse drive gear  72  to fix the gear  71  or  72  to the countershaft  9 , thus allowing power transmission and effecting forward or reverse running of the vehicle.  
         [0048]     According to this preferred embodiment as described above in detail, the countershaft and the intermediate shaft are provided parallel to the output shaft. The forward drive gear and the reverse drive gear are rotatably supported relative to the countershaft. The first intermediate gear and the second intermediate gear are rotatably supported relative to the intermediate shaft. The first intermediate gear is normally in mesh with the reverse drive gear, and the second intermediate gear is rotatable together with the first intermediate gear. The single output shaft driven gear normally meshing with the forward drive gear and the second intermediate gear is fixed to the output shaft as a final shaft. Further, the dog clutch, performing the gear selecting and fixing function, is provided to selectively fix the forward drive gear and the reverse drive gear to the countershaft, thereby selecting different operational conditions of the vehicle. Thus, only one larger-diameter gear is mounted on the final shaft of the transmission as the output shaft driven gear, thereby allowing a reduction in weight of the transmission.  
         [0049]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.