Patent Publication Number: US-2016238109-A1

Title: Dual-clutch transmission

Description:
TECHNICAL FIELD 
     The present invention relates to a dual-clutch transmission. 
     BACKGROUND ART 
     To date, a dual-clutch transmission is known that includes a first input shaft provided with a first clutch that transmits or interrupts power from a driving source, a second input shaft provided with a second clutch that transmits or interrupts the power from the driving source, and a plurality of speed-change gear pairs (transmission gear pairs). Such a dual-clutch transmission shifts gears by switching between the first clutch and the second clutch in an alternating manner. 
     For example, PATENT LITERATURE DOCUMENT 1 discloses a dual-clutch transmission in which a primary gear pair is reused as a 6th-speed gear pair, and thus the number of gear pairs is reduced. 
     LISTING OF REFERENCES 
     Patent Literature Documents 
     PATENT LITERATURE DOCUMENT 1: Japanese Patent Application Laid-Open Publication (Kokai) No. 2010-531417 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the above-described conventional technique in which the primary gear pair is reused, as illustrated in  FIG. 10(A) , the gearshift between the 1st speed and the 2nd speed is achieved by connecting a 1st/2nd-speed shared gear pair  320  to an input shaft  140  and by selectively switching between a first primary gear pair  200  and a second primary gear pair  210 . As illustrated in  FIG. 10(B) , the gearshift to the 5th speed is achieved by directly connecting a first input shaft  110  to the output shaft  140 , and the gearshift to the 6th speed is achieved by reusing the first primary gear pair  200  as a gearshift gear pair. 
     In the conventional dual-clutch transmission, the gear ratio of the 1st speed to the 2nd speed equals the ratio of the gear ratio of the first primary gear pair  200  to the gear ratio of the second primary gear pair  210 , and the gear ratio of the 5th speed to the 6th speed equals the inverse of the gear ratio of the second primary gear pair  210  to the gear ratio of the first primary gear pair  200 . Thus, the gear ratio of the 1st speed to the 2nd speed is equal to the gear ratio of the 5th speed to the 6th speed. 
     In general, in order to ensure good feeling of connectedness at the time of shifting gears (hereinafter, referred to as “gearshift feeling”), it is preferred that a greater gear ratio be set to a ratio of one speed to a next speed in lower gears and a smaller gear ratio be set to a gear ratio of one speed to a next speed in higher gears. However, in the above-described dual-clutch transmission, the gear ratio of the 1st speed to the 2nd speed is equal to the gear ratio of the 5th speed to the 6th speed, and these gear ratios cannot be set independently to optimal values. Thus, there is a problem, i.e., the gearshift feeling deteriorates. 
     An object of the dual-clutch transmission disclosed herein is to effectively prevent the deterioration of the gearshift feeling. 
     Solution to Overcome the Problems 
     A dual-clutch transmission disclosed herein includes a first input shaft provided with a first clutch that transmits or interrupts power from a driving source; a second input shaft provided with a second clutch that transmits or interrupts the power from the driving source; an output shaft disposed coaxially with the first input shaft; a countershaft disposed in parallel to the first input shaft, the second input shaft, and the output shaft; a first input gear pair including a first input primary gear fixed to the first input shaft, and a first input counter gear (auxiliary gear) that is provided on the countershaft and rotatable relative to the countershaft and that meshes with the first input primary gear; a second input gear pair including a second input primary gear fixed to the second input shaft, and a second input counter gear that is fixed to the countershaft and that meshes with the second input primary gear; a first speed-change gear pair including a first output primary gear provided on the output shaft and rotatable relative to the output shaft, and a first output counter gear that is fixed to the countershaft and that meshes with the first output primary gear; a second speed-change gear pair including a second output primary gear provided on the output shaft and rotatable relative to the output shaft, the second output primary gear being closer to an input end than the first output primary gear is, and a second output counter gear that is configured to be integrally rotatable with the first input counter gear and that meshes with the second output primary gear; a third speed-change gear pair including a third output primary gear fixed to the output shaft at a position between the second output primary gear and the first output primary gear, and a third output counter gear that is provided on the countershaft and rotatable relative to the countershaft and that meshes with the third output primary gear; a first connecting unit capable of selectively connecting the first input primary gear or the second output primary gear to the output shaft; a second connecting unit capable of selectively connecting the second output counter gear or the third output counter gear to the countershaft; and a third connecting unit capable of selectively connecting at least the first output primary gear to the output shaft. 
     Advantages of the Invention 
     The dual-clutch transmission disclosed herein can effectively prevent the deterioration of the gearshift feeling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a skeleton diagram of a dual-clutch transmission according to an embodiment of the present invention. 
         FIG. 2  illustrates a 1st-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 3  illustrates a 2nd-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 4  illustrates a 3rd-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 5  illustrates a 4th-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 6  illustrates a 5th-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 7  illustrates a 6th-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 8  illustrates a 7th-speed power transmitting path in the dual-clutch transmission illustrated in  FIG. 1 . 
         FIG. 9  is a graph useful to describe an example of a step ratio (ratio of one speed gear ratio to another speed gear ratio) in the embodiment of the present invention. 
         FIG. 10  is a set of views useful to describe power transmitting paths in a conventional dual-clutch transmission. 
     
    
    
     Mode for Carrying out the Invention 
     Hereinafter, a dual-clutch transmission according to an embodiment of the present invention will be described with reference to the accompanying drawings. Identical components are given identical reference characters, and their names and functions are identical as well. Therefore, detailed descriptions of such components will not be repeated. 
     As illustrated in  FIG. 1 , a first clutch  10  is provided at an input end of a first input shaft  11 . A second clutch  12  is provided at an input end of a second input shaft  13 . The second input shaft  13  has a hollow shaft that penetrates in the axial direction, and the first input shaft extends through this hollow shaft such that the first input shaft is rotatable relative to the hollow shaft. 
     An output shaft  14  is disposed coaxially with the first input shaft  11 , and is spaced from the first input shaft  11 . A countershaft  15  is disposed in parallel with the input shafts  11  and  13  and the output shaft  14 . 
     The first clutch  10  includes a first pressure plate  10 A fixed to a crankshaft  3  of an engine  2  and a first clutch disc  10 B fixed to the input end of the first input shaft  11 . As the first pressure plate  10 A moves to make pressure-contact with the first clutch disc  10 B, the power of the engine  2  is transmitted to the first input shaft  11  via the first clutch  10 . 
     The second clutch  12  includes a second pressure plate  12 A fixed to the crankshaft  3  of the engine  2  and a second clutch disc  12 B fixed to the input end of the second input shaft  13 . As the second pressure plate  12 A moves to make pressure-contact with the second clutch disc  12 B, the power of the engine  2  is transmitted to the second input shaft  13  via the second clutch  12 . 
     A first primary gear pair  20 , which is an example of a first input gear pair according to the present invention, includes a first input gear  20 A and a first counter gear (auxiliary gear)  20 B. The first input gear  20 A meshes with the first counter gear  20 B. The first input gear  20 A is fixed to an output end of the first input shaft  11 . The first counter gear  20 B is provided on the countershaft  15  such that the first counter gear  20 B is rotatable relative to the countershaft  15 . The first counter gear  20 B is integrated with a 3rd-speed counter gear  30 B, which will be described later. 
     A second primary gear pair  21 , which is an example of a second input gear pair according to the present invention, includes a second input gear  21 A and a second counter gear  21 B. The second input gear  21 A meshes with the second counter gear  21 B. The second input gear  21 A is fixed to an output end of the second input shaft  13 . The second counter gear  21 B is fixed to an input end of the countershaft  15 . The power of the engine  2  transmitted to the second input shaft  13  via the second clutch  12  is directly transmitted to the countershaft  15  via the second primary gear pair  21 . 
     A 3rd-speed gear pair  30 , which is an example of a second speed-change gear pair according to the present invention, includes a 3rd-speed output gear  30 A and the 3rd-speed counter gear  30 B. The 3rd-speed output gear  30 A meshes with the 3rd-speed counter gear  30 B. The 3rd-speed output gear  30 A is provided on the output shaft  14  such that the 3rd-speed output gear  30 A is rotatable relative to the output shaft  14 . The 3rd-speed counter gear  30 B is formed so as to be integrally rotatable with the first counter gear  20 B. Specifically, a hollow shaft is formed in the first counter gear  20 B and the 3rd-speed counter gear  30 B, which are integrated, so as to penetrate therethrough in the axial direction. The countershaft  15  extends through the hollow shaft such that the countershaft  15  is freely rotatable. The number of teeth in the 3rd-speed output gear  30 A is greater than the number of teeth in the 3rd-speed counter gear  30 B. 
     A 6th-speed gear pair  31 , which is an example of a third speed-change gear pair according to the present invention, includes a 6th-speed output gear  31 A and a 6th-speed counter gear  31 B. The 6th-speed output gear  31 A meshes with the 6th-speed counter gear  31 B. The 6th-speed output gear  31 A is fixed to the output shaft  14 , and the 6th-speed counter gear  31 B is provided on the countershaft  15  such that 6th-speed counter gear  31 B is rotatable relative to the countershaft  15 . The number of teeth in the 6th-speed output gear  31 A is smaller than the number of teeth in the 6th-speed counter gear  31 B. 
     A 1st/2nd-speed shared gear pair  32 , which is an example of a first speed-change gear pair according to the present invention, includes a 1st/2nd-speed output gear  32 A and a 1st/2nd-speed counter gear  32 B. The 1st/2nd-speed output gear  32 A meshes with the 1st/2nd-speed counter gear  32 B. The 1st/2nd-speed output gear  32 A is provided on the output shaft  14  such that the 1st/2nd-speed output gear  32 A is rotatable relative to the output shaft  14 , and the 1st/2nd-speed counter gear  32 B is fixed to the countershaft  15 . The number of teeth in the 1st/2nd-speed output gear  32 A is greater than the number of teeth in the 1st/2nd-speed counter gear  32 B. 
     A reverse gear set  33  includes a reverse output gear  33 A, a reverse counter gear  33 B, and an idler gear  33 C, which mesh with one another. The reverse output gear  33 A is provided on the output shaft  14  such that the reverse output gear  33 A is rotatable relative to the output shaft  14 , and the reverse counter gear  33 B is fixed to the countershaft  15 . 
     A 4th-speed gear pair  34 , which is an example of a fourth speed-change gear pair according to the present invention, includes a 4th-speed output gear  34 A and a 4th-speed counter gear  34 B. The 4th-speed output gear  34 A meshes with the 4th-speed counter gear  34 B. The 4th-speed output gear  34 A is provided on the output shaft  14  such that the 4th-speed output gear  34 A is rotatable relative to the output shaft  14 , and the 4th-speed counter gear  34 B is fixed to the countershaft  15 . The number of teeth in the 4th-speed output gear  34 A is greater than the number of teeth in the 4th-speed counter gear  34 B. 
     A first synchronizer mechanism  40 , which is an example of a first connecting unit according to the present invention, includes a first sleeve  40 A movable in the axial direction in accordance with a shift operation of a shift lever device (not illustrated), a spline  40 B fixed to an input end of the output shaft  14 , a spline  40 C fixed to the first input gear  20 A, and a spline  40 D fixed to the 3rd-speed output gear  30 A. 
     As the first sleeve  40 A moves toward the first input gear  20 A and engages with the spline  40 C, the first input gear  20 A is connected to the output shaft  14  (the first input shaft  11  is directly connected to the output shaft  14 ). On the other hand, as the first sleeve  40 A moves toward the 3rd-speed output gear  30 A and engages with the spline  40 D, the 3rd-speed output gear  30 A is connected to the output shaft  14 . In other words, the first synchronizer mechanism  40  can selectively connect the first input gear  20 A or the 3rd-speed output gear  30 A to the output shaft  14 . 
     A second synchronizer mechanism  41 , which is an example of a second connecting unit according to the present invention, includes a second sleeve  41 A movable in the axial direction, a spline  41 B fixed to the countershaft  15  at a position between the 3rd-speed counter gear  30 B and the 6th-speed counter gear  31 B, a spline  41 C fixed to the 3rd-speed counter gear  30 B, and a spline  41 D fixed to the 6th-speed counter gear  31 B. 
     As the second sleeve  41 A moves toward the 3rd-speed counter gear  30 B and engages with the spline  41 C, the 3rd-speed counter gear  30 B becomes connected to the countershaft  15 . On the other hand, as the second sleeve  41 A moves toward the 6th-speed counter gear  31 B and engages with the spline  42 D, the 6th-speed counter gear  31 B becomes connected to the countershaft  15 . In other words, the second synchronizer mechanism  41  can selectively connect the 3rd-speed counter gear  30 B or the 6th-speed counter gear  31 B to the countershaft  15 . 
     A third synchronizer mechanism  42 , which is an example of a third connecting unit according to the present invention, includes a third sleeve  42 A movable in the axial direction, a spline  42 B fixed to the output shaft  14  at a position between the 1st/2nd-speed output gear  32 A and the reverse output gear  33 A, a spline  42 C fixed to the 1st/2nd-speed output gear  32 A, and a spline  42 D fixed to the reverse output gear  33 A. 
     As the third sleeve  42 A moves toward the 1st/2nd-speed output gear  32 A and engages with the spline  42 C, the 1st/2nd-speed output gear  32 A becomes connected to the output shaft  14 . On the other hand, as the third sleeve  42 A moves toward the reverse output gear  33 A and engages with the spline  42 D, the reverse output gear  33 A becomes connected to the output shaft  14 . In other words, the third synchronizer mechanism  42  can selectively connect the 1st/2nd-speed output gear  32 A or the reverse output gear  33 A to the output shaft  14 . 
     A fourth synchronizer mechanism  43 , which is an example of a fourth connecting unit according to the present invention, includes a fourth sleeve  43 A movable in the axial direction, a spline  43 B fixed to the output shaft  14  at a position where the spline  43 B is closer to the output end than the 4th-speed output gear  34 A is, and a spline  43 C fixed to the 4th-speed output gear  34 A. 
     As the fourth sleeve  43 A moves toward the 4th-speed output gear  34 A and engages with the spline  43 C, the 4th-speed output gear  34 A becomes connected to the output shaft  14 . In other words, the fourth synchronizer mechanism  43  can selectively connect the 4th-speed output gear  34 A to the output shaft  14 . 
     A transmission control unit (TCU)  80  executes gear-shifting control to actuate the first clutch  10 , the second clutch  12 , and the synchronizer mechanisms  40  to  43  in accordance with a shift operation of a shift device (not illustrated). In the following description, the power transmitting paths in the forward-moving positions in the gear-shifting control of the TCU  80  will be described with reference to  FIGS. 2 to 8 . 
       FIG. 2  illustrates a 1st-speed power transmitting path. In the 1st speed, the first clutch  10  is selected, the second synchronizer mechanism  41  connects the 3rd-speed counter gear  30 B to the countershaft  15 , and the third synchronizer mechanism  42  connects the 1st/2nd-speed output gear  32 A to the output shaft  14 . 
     In other words, the power of the engine  2  is transmitted to the output shaft  14  via the first clutch  10 , the first input shaft  11 , the first primary gear pair  20 , the 3rd-speed counter gear  30 B, the countershaft  15 , and the 1st/2nd-speed shared gear pair  32 . 
       FIG. 3  illustrates a 2nd-speed power transmitting path. The 2nd speed is achieved by switching, from the state of the 1st-speed, the first clutch  10  to the second clutch  12 . In other words, the power of the engine  2  is transmitted to the output shaft  14  via the second clutch  12 , the second input shaft  13 , the second primary gear pair  21 , the countershaft  15 , and the 1st/2nd-speed shared gear pair  32 . 
       FIG. 4  illustrates a 3rd-speed power transmitting path. To achieve the 3rd speed, the first synchronizer mechanism  40  connects the 3rd-speed output gear  30 A to the output shaft  14  in the state of the 2nd speed, and a standby state is entered. Then, the second clutch  12  is switched to the first clutch  10 . 
     In other words, the power of the engine  2  is transmitted to the output shaft  14  via the first clutch  10 , the first input shaft  11 , the first primary gear pair  20 , and the 3rd-speed gear pair  30 . 
       FIG. 5  illustrates a 4th-speed power transmitting path. To achieve the 4th speed, the fourth synchronizer mechanism  43  connects the 4th-speed output gear  34 A to the output shaft  14  in the state of the 3rd speed, and a standby state is entered. Then, the first clutch  10  is switched to the second clutch  12 . 
     In other words, the power of the engine  2  is transmitted to the output shaft  14  via the second clutch  12 , the second input shaft  13 , the second primary gear pair  21 , the countershaft  15 , and the 4th-speed gear pair  34 . 
       FIG. 6  illustrates a 5th-speed power transmitting path. To achieve the 5th speed, the first synchronizer mechanism  40  (directly) connects the first input gear  20 A to the output shaft  14  in the state of the 4th speed, and a standby state is entered. Then, the second clutch  12  is switched to the first clutch  10 . 
     In other words, the power of the engine  2  is transmitted to the output shaft  14  via the first clutch  10 , the first input shaft  11 , and the first input gear  20 A. 
       FIG. 7  illustrates a 6th-speed power transmitting path. To achieve the 6th speed, the second synchronizer mechanism  41  connects the 6th-speed counter gear  31 B to the countershaft  15  in the state of the 5th speed, and a standby state is entered. Then, the first clutch  10  is switched to the second clutch  12 . 
     In other words, the power of the engine  2  is transmitted to the output shaft  14  via the second clutch  12 , the second input shaft  13 , the second primary gear pair  21 , the countershaft  15 , and the 6th-speed gear pair  31 . 
       FIG. 8  illustrates a 7th-speed power transmitting path. To achieve the 7th speed, the second clutch  12  is temporarily disconnected in the state of the 6th speed. Then, the first synchronizer mechanism  40  connects the first input gear  20 A to the output shaft  14 , the second synchronizer mechanism  41  connects the 3rd-speed counter gear  30 B to the countershaft  15 , and the second clutch  12  is reconnected. 
     In other words, the power of the engine  10  is transmitted to the output shaft  14  via the second clutch  12 , the second input shaft  13 , the second primary gear pair  21 , the countershaft  15 , the 3rd-speed counter gear  30 B, and the first primary gear pair  20 . Thus, the first primary gear pair  20  can be reused as a speed-change gear pair (transmission gear pair). It should be noted that when shifting from the 6th speed to the 7th speed, the second clutch  12  is disconnected and connected, and thus a so-called torque loss occurs. However, its influence is small in the high-order speeds. 
     The gear ratios and the step ratios in the dual-clutch transmission according to the present embodiment will now be described. The numerical values used in the following description are merely examples of the present embodiment, and any optimal values can be set as appropriate within the scope that does not depart from the spirit of the present invention. 
     Examples of the number of teeth and the gear ratio in each gear pair are shown in Table 1, and the gear ratios in the gear positions  1  to  7  (1st speed to 7th speed) are shown in Table 2. In Table 1, the number of teeth Z 1  in the upper row indicates the number of teeth in a gear provided on the countershaft  15 , and the number of teeth Z 2  in the middle row indicates the number of teeth in a gear provided on the input shaft  11  or  13  or the output shaft  14 . 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Pri-1 
                 Pri-2 
                 3rd 
                 6 th   
                 1st/2nd 
                 4th 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Number of 
                 28 
                 40 
                 30 
                 37 
                 13 
                 27 
               
               
                 teeth Z1 
               
               
                 Number of 
                 45 
                 39 
                 39 
                 29 
                 43 
                 39 
               
               
                 teeth Z2 
               
               
                 Gear ratio 
                 1.61 
                 0.98 
                 1.30 
                 0.78 
                 3.31 
                 1.44 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 1st 
                 2nd 
                 3rd 
                 4th 
                 5th 
                 6th 
                 7th 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Primary gear 
                 1.61 
                 0.98 
                 1.61 
                 0.98 
                 1.00 
                 0.98 
                 0.98 
               
               
                 ratio 
               
               
                 Speed-change 
                 3.31 
                 3.31 
                 1.30 
                 1.44 
                 1.00 
                 0.78 
                 0.62 
               
               
                 gear ratio 
               
               
                 Total gear ratio 
                 5.316 
                 3.225 
                 2.089 
                 1.408 
                 1.000 
                 0.764 
                 0.607 
               
               
                   
               
            
           
         
       
     
     The step ratio of the 1st speed to the 2nd speed, and the step ratio of the 5th speed to the 7th speed are calculated from Table 2. The step ratio of the 1st speed to the 2nd speed is approximately 1.648 (=1st speed/2nd speed), and the step ratio of the 5th speed to the 7th speed is also approximately 1.648 (=5th speed/7th speed). These step ratios are substantially equal to each other. In order to retain favorable gearshift feeling, it is preferred that a greater step ratio be set in lower gears (lower speeds) and a smaller step ratio be set in higher gears (higher speeds). In the present embodiment, even when the step ratio of the 1st speed to the 2nd speed is extended and the step ratio of the 5th speed to the 7th speed is correspondingly extended, a small step ratio can be set in high gears, as illustrated in  FIG. 10  for example, since the 6th-speed gear pair  31  of which the gear ratio can be set independently is provided. 
     In addition, the output gears of the 3rd-speed gear pair  30 , the 1st/2nd-speed shared gear pair  32 , and the 4th-speed gear pair  34 , which are reduction gear pairs, are all supported on the output shaft  14  so as to be idle, and only the output gear  31 A of the 6th-speed gear pair  31 , which is an acceleration gear pair, is fixed to the output shaft  14 . Therefore, there is no counter gear that is continuously accelerated relative to the output shaft  14 , and heat production, wears, and so on to be caused by high-speed rotations are effectively suppressed. 
     Advantageous effects of the dual-clutch transmission according to the present embodiment will now be described. 
     In a conventional dual-clutch transmission, the step ratio of the 1st speed to 2nd speed is equal to the step ratio of the 5th speed to 6th speed, and these step ratios cannot be set independently to optimal values. Thus, an issue remains in that the gearshift feeling deteriorates. 
     On the contrary, in the dual-clutch transmission according to the present embodiment, although the step ratio of the 1st speed to the 2nd speed is equal to the step ratio of the 5th speed to the 7th speed, the 6th-speed gear pair  31  of which the gear ratio can be set independently is provided between the 5th speed and the 7th speed. In other words, even when the step ratio of the 1st speed to the 2nd speed is extended to improve the gearshift feeling and the step ratio of the 5th speed to the 7th speed is correspondingly extended, the step ratio in higher gears can be set small because the 6th-speed gear pair  31  is provided between the 5th speed and the 7th speed. Accordingly, the dual-clutch transmission of this embodiment can set favorable step ratios that are smaller in higher gears and to effectively improve the gearshift feeling. 
     In addition, a conventional dual-clutch transmission suffers from another problem. An output gear of a reduction gear pair (e.g., 4th-speed gear pair) is fixed to an output shaft. Therefore, high-speed rotations of the output shaft cause the counter gear to rotate at even higher speed than the output shaft. This increases heat generation, wears, losses, and so on. 
     On the contrary, in the dual-clutch transmission according to this embodiment, only the 6th-speed output gear  31 A is fixed to the output shaft  14 , and the other output gears are supported on the output shaft  14  so as to be idle. The 6th-speed gear pair  31  is an acceleration gear pair, and thus the rotations of the output shaft  14  are continuously decelerated and transmitted to the 6th-speed counter gear  31 B. Accordingly, the dual-clutch transmission of this embodiment can effectively reduce heat generation, wears, losses, and so on in the counter gears, since there is no counter gear that is continuously accelerated relative to the output shaft  14 . 
     It should be noted that the present invention is not limited to the above-described embodiments and can be implemented with appropriate changes and modifications within the scope that does not depart from the spirit of the present invention. 
     For example, the 3rd-speed counter gear  30 B and the first counter gear  20 B do not have to be integrated and may be provided as separate gears. In this configuration, the 3rd-speed counter gear  30 B and the first counter gear  20 B may be configured to be selectively connectable to the countershaft  15  with a synchronizer mechanism or the like. 
     REFERENCE NUMERALS AND SYMBOLS 
       2 : Engine 
       10 : First clutch 
       11 : First input shaft 
       12 : Second clutch 
       13 : Second input shaft 
       14 : Output shaft 
       15 : Countershaft 
       20 : First primary gear pair 
       21 : Second primary gear pair 
       30 : 3rd-speed gear pair 
       31 : 6th-speed gear pair 
       32 : 1st/2nd-speed shared gear pair 
       34 : 4th-speed gear pair 
       40 : First synchronizer mechanism 
       41 : Second synchronizer mechanism 
       42 : Third synchronizer mechanism 
       43 : Fourth synchronizer mechanism