Abstract:
A transmission includes first and second rotating shafts arranged in parallel and multiple gear sets including gears fixed to the first rotating shaft and rotatable gears rotatably supported on the second rotating shaft. Each gear set includes first and second engagement units engaged with first and second engagement-target portions formed on end surfaces of each rotatable gear and transmitting torque in first and second rotational directions; and a shift mechanism axially and independently moving the first and second engagement units. Each one of the first and second engagement units has a slanted portion allowing the engagement unit to withdraw from the rotatable gear when the other one of the engagement units is disengaged and torque transmitted in a corresponding direction is input. Each one of the first and second engagement unit of each gear set moves integrally with the other one of the engagement units of an adjacent gear set.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2011-067706 filed on Mar. 25, 2011, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmission that performs a gear change by selecting one from among multiple transmission gear sets, and particularly to a transmission that reduces a torque interruption period in a gear change and that has a simple structure. 
     2. Description of the Related Art 
     Automatic transmissions for vehicles include one having a combination of multiple planetary gear sets and a continuously variable transmission (CVT) employing a variator such as a chain variator, belt variator, and troidal variator. These automatic transmissions have not yet reached the level of manual transmissions, in which multiple gear sets arranged between a pair of rotating shafts are sequentially switched from one to another, in terms of transmission efficiency, weight reduction, and compactness. 
     To address this situation, an automatic transmission has been proposed in which an actuator performs shifting, selecting, and clutching operations that are performed manually in a manual transmission. This automatic transmission, however, needs to temporarily disengage the clutch before performing a gear change, and thus the torque interruption period is long. 
     Furthermore, a double clutch transmission (DCT) has also been proposed that includes two separate clutches and drive shafts for odd and even gear speeds. The DCT performs gear changes by switching between these clutches and drive shafts. However, the DCT has a complex structure and a large weight. 
     Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-536713 describes the following transmission, which is an example of the conventional technology relating to a highly efficient transmission causing a short driving interruption period. Specifically, the transmission causes substantially no torque interruption and performs instantaneous gear changes by sequentially switching engagement states of first and second gear members included in each of gears, which are supported on a rotating shaft so as to be rotatable relative to the rotating shaft. The first gear member transmits a driving force in one rotational direction and the second gear member transmits a driving force in another rotational direction. 
     The transmission described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-536713 includes a selector mechanism that has gear members between first and second gear ratios, between third and fourth gear ratios, and between fifth and sixth gear ratios. 
     The above transmission can perform instantaneous gear changes between first and second gear ratios, between third and fourth gear ratios, and between fifth and sixth gear ratios. However, as to the gear changes between second and third gear ratios and between fourth and fifth gear ratios, it is only after engagement members of the gear ratios with which the engagement members are engaged before the gear changes are moved to neutral positions that the transmission can start driving the engagement members of the gear ratios with which the engagement members are engaged after the gear changes. Thus, the gear changes between these gear ratios take more time. 
     SUMMARY OF THE INVENTION 
     The present invention aims to provide a transmission that enables speedy gear changes between all the gear ratios and that has a simple structure. 
     The present invention has been made to solve the above problems by the following measures. 
     According to an aspect of the present invention, there is provided a transmission including: a first rotating shaft; a second rotating shaft arranged in parallel with the first rotating shaft; and a plurality of gear sets that transmit power between the first rotating shaft and the second rotating shaft. The gear sets are disposed in order from low to high gear ratios. Each gear set includes a fixed gear that is fixed to the first rotating shaft and a rotatable gear that is supported on the second rotating shaft so as to be rotatable relative to the second rotating shaft, and the rotatable gear meshes with the fixed gear. Each gear set includes a first engagement unit, a second engagement unit, and a shift mechanism. The first engagement unit is engaged with a first engagement-target portion formed on one end surface of the rotatable gear and transmits torque in a first rotational direction. The second engagement unit is engaged with a second engagement-target portion formed on another end surface of the rotatable gear and thus transmits torque in a second rotational direction that is a direction opposite to the first rotational direction. The shift mechanism moves the first engagement unit and the second engagement unit independently of each other and relative to the rotatable gear in the axial direction. The first engagement unit has a slanted portion that is contacted by the first engagement-target portion and that thus allows the first engagement unit to withdraw from the rotatable gear while the second engagement unit is disengaged and when torque in the second rotational direction is input. The second engagement unit has a slanted portion that is contacted by the second engagement-target portion and that thus allows the second engagement unit to withdraw from the rotatable gears while the first engagement unit is disengaged and when torque in the first rotational direction is input. The first engagement unit of each gear set is configured to move integrally with the second engagement unit of an adjacent one of the gear sets, and the second engagement unit of each gear set is configured to move integrally with the first engagement unit of an adjacent one of the gear sets. 
     With the aspect of the present invention, the first engagement unit of one gear set and the second engagement unit of an adjacent gear set move integrally. For this reason, there is no need to provide separate actuators for driving these engagement units and thus a simple transmission structure is achieved. 
     The first engagement unit and the second engagement unit of each gear set are disposed on both sides of the gear. Thus, upshifting and downshifting operations between all the gear ratios can be speedily performed in substantially the same manner. 
     In the transmission, the gear sets corresponding first to N-th (where N is an integer) gear ratios may be arranged in order. Between each pair of adjacent gear sets corresponding to the first to N-th gear ratios, the first engagement unit of one of the gear sets corresponding to a lower one of the gear ratios and the second engagement unit of another one of the gear sets corresponding to a higher one of the gear ratios are arranged so as to move integrally. 
     In the transmission, the first engagement unit may transmit torque while power is off. The second engagement unit may transmit torque while power is on. The shift mechanism may disengage the first engagement unit of one of the gear sets that has been engaged before a gear change and concurrently may engage the second engagement unit of another one of the gear sets that is to be engaged after the gear change, and then, after the second engagement unit of the gear set that has been engaged before the gear change is contacted by the second engagement-target portion and thus disengaged, the shift mechanism may engage the first engagement unit of the gear set that is to be engaged after the gear change. 
     As described above, according to the present invention, it is possible to provide a transmission that enables speedy gear changes between all the gear ratios and that has a simple structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a structure of a transmission according to an embodiment of the present invention; 
         FIGS. 2A ,  2 B, and  2 C illustrate an operation of the transmission illustrated in  FIG. 1 , for upshifting from the second to third gear ratio; 
         FIGS. 3A ,  3 B, and  3 C illustrate an operation of the transmission illustrated in  FIG. 1 , for downshifting from the third to second gear ratio; and 
         FIG. 4  illustrates part of shapes of cam grooves formed in a shift drum of the transmission illustrated in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, an object, that is, to provide a transmission that performs speedy gear changes between all the gear ratios is achieved by the following manner. A driving torque and a driven torque are transmitted by engagement members provided independently of each other on both sides of each gear, and a driving-side engagement member for each gear and a driven-side engagement member for an adjacent gear are integrally arranged in a space between these gears. 
     Embodiment 
     A transmission according to an embodiment of the present invention will be described below. 
     The transmission according to the embodiment is, for example, an automatic transmission that accelerates or decelerates the speed of revolutions output from an engine of a vehicle such as a passenger car. The transmission achieves six forward speeds and one reverse speed. 
     The engine changes torque in cooperation with control of the transmission by using output control means such as an electronically controlled throttle for the purpose of, for example, synchronization of the number of revolutions. 
       FIG. 1  is a schematic diagram illustrating a structure of the transmission according to the embodiment. 
     A transmission  1  includes a drive shaft  10 , a driven shaft  20 , and a clutch  30 . 
     The drive shaft  10  is a rotating shaft that receives an output from an engine, which is not illustrated, and drives the driven shaft  20  via gear sets that are described below. 
     The driven shaft  20  is a rotating shaft driven by the drive shaft  10 . 
     The driven shaft  20  transmits power to driving wheels of a vehicle via a driving system, which is not illustrated, including an all-wheel-drive (AWD) transfer and an axle differential. 
     The clutch  30  is disposed between the engine and the drive shaft  10 . The clutch  30  is, for example, a dry friction clutch that transmits or interrupts power. 
     The clutch  30  is engaged or disengaged by an actuator controlled by a command from a transmission control unit, which is not illustrated. 
     The transmission  1  according to the embodiment can upshift without disengaging the clutch  30 . The transmission  1  can also downshift without disengaging the clutch  30 . However, in order to prevent inconveniences such as an excessive increase in the number of engine revolutions or a gear change shock, the clutch  30  is temporarily disengaged, and the clutch  30  is engaged again after blipping control for increasing the number of engine revolutions is performed. 
     The clutch  30  is also used as a starting device for starting a vehicle that is in a stationary state. 
     A first drive gear  41 , a second drive gear  42 , a third drive gear  43 , a fourth drive gear  44 , a fifth drive gear  45 , and a sixth drive gear  46  are mounted on the drive shaft  10  in this order from a side closer to the clutch  30 . 
     The number of teeth increases in order from the first to sixth drive gears  41  to  46 . 
     The drive gears  41  to  46  are formed integrally with or fixed to the drive shaft  10 . 
     The first drive gear  41  drives a reverse driven gear  62  via an idler gear  61  that is described below. 
     A first driven gear  51 , a second driven gear  52 , a third driven gear  53 , a fourth driven gear  54 , a fifth driven gear  55 , and a sixth driven gear  56  are mounted on the driven shaft  20 . 
     The number of teeth decreases in order from the first to sixth driven gears  51  to  56 . 
     The driven gears  51  to  56  are rotatable about the axis of the driven shaft  20  and relative to the driven shaft  20 . 
     driven gears  51  to  56  respectively mesh with the drive gears  41  to  46  for corresponding gear ratios and thus power is transmitted from the drive shaft  10  to the driven shaft  20 . 
     The driven gears  51  to  56  respectively have dog teeth  51   a  to  56   a  on engine-side end surfaces thereof. The dog teeth  51   a  to  56   a  have, for example, a projecting shape. 
     The driven gears  51  to  56  also have similarly shaped dog teeth  51   b  to  56   b  on end surfaces thereof that are on the sides opposite to the engine-side surfaces. 
     The transmission  1  also includes the idler gear  61  and the reverse driven gear  62 . 
     The idler gear  61  meshes with the first drive gear  41  and the reverse driven gear  62  and thus transmits power from the first drive gear  41  to the reverse driven gear  62 . 
     The reverse driven gear  62  is disposed near a front-end portion of the driven shaft  20  and is rotatable about the axis of the driven shaft  20 . 
     The reverse driven gear  62  has dog teeth  62   a  on a surface that is on the side opposite to an engine-side surface thereof. The dog teeth  62   a  have, for example, a projecting shape. 
     The transmission  1  also includes a transmission mechanism  100  that is described below. 
     The transmission system  100  includes a reverse-first engagement member  110 , a first-second engagement member  120 , a second-third engagement member  130 , a third-fourth engagement member  140 , a fourth-fifth engagement member  150 , a fifth-sixth engagement member  160 , and a sixth engagement member  170 . 
     These engagement members  110  to  170  are engaged with the dog teeth  51   a  to  56   a ,  51   b  to  56   b , and  62   a  of the driven gears  51  to  56  and  62  and thus transmit torque between the driven gears  51  to  56  and  62  and the driven shaft  20 . 
     The engagement members  110  to  170  are restrained from rotating about the axis of the driven shaft  20  and supported on the driven shaft  20  so as to be movable straight substantially in the axial direction. 
     Each of the engagement members  110  to  170  is held by a detent system or the like so as to be detained at any of the following three positions along the axis of the driven shaft  20 , that is, a neutral position; a first position to the front of the neutral position (a position displaced toward the engine); and a second position to the rear of the neutral position (a position displaced in such a direction as to move away from the engine). 
     The reverse-first engagement member  110  is disposed between the reverse driven gear  62  and the first driven gear  51 . 
     When at the first position, the reverse-first engagement member  110  transmits a driving torque (torque from the engine to the wheels) from the dog teeth  62   a  of the reverse driven gear  62  to the driven shaft  20 . When at the second position, the reverse-first engagement member  110  transmits a driving torque from the dog teeth  51   a  of the first driven gear  51  to the driven shaft  20 . 
     The first-second engagement member  120  is disposed between the first driven gear  51  and the second driven gear  52 . 
     When at the first position, the first-second engagement member  120  transmits a driven torque (torque from the wheels to the engine, or, what is called a back torque caused by engine braking or the like) from the driven shaft  20  to the dog teeth  51   b  of the first driven gear  51 . When at the second position, the first-second engagement member  120  transmits a driving torque from the dog teeth  52   a  of the second driven gear  52  to the driven shaft  20 . 
     The second-third engagement member  130  is disposed between the second driven gear  52  and the third driven gear  53 . 
     When at the first position, the second-third engagement member  130  transmits a driven torque from the driven shaft  20  to the dog teeth  52   b  of the second driven gear  52 . When at the second position, the second-third engagement member  130  transmits a driving torque from the dog teeth  53   a  of the third driven gear  53  to the driven shaft  20 . 
     The third-fourth engagement member  140  is disposed between the third driven gear  53  and the fourth driven gear  54 . 
     When at the first position, the third-fourth engagement member  140  transmits a driven torque from the driven shaft  20  to the dog teeth  53   b  of the third driven gear  53 . When at the second position, the third-fourth engagement member  140  transmits a driving torque from the dog teeth  54   a  of the fourth driven gear  54  to the driven shaft  20 . 
     The fourth-fifth engagement member  150  is disposed between the fourth driven gear  54  and the fifth driven gear  55 . 
     When at the first position, the fourth-fifth engagement member  150  transmits a driven torque from the driven shaft  20  to the dog teeth  54   b  of the fourth driven gear  54 . When at the second position, the fourth-fifth engagement member  150  transmits a driving torque from the dog teeth  55   a  of the fifth driven gear  55  to the driven shaft  20 . 
     The fifth-sixth engagement member  160  is disposed between the fifth driven gear  55  and the sixth driven gear  56 . 
     When at the first position, the fifth-sixth engagement member  160  transmits a driven torque from the driven shaft  20  to the dog teeth  55   b  of the fifth driven gear  55 . When at the second position, the fifth-sixth engagement member  160  transmits a driving torque from the dog teeth  56   a  of the sixth driven gear  56  to the driven shaft  20 . 
     The sixth engagement member  170  is disposed on a side of the sixth driven gear  56  that is opposite to a fifth driven gear  55  side. 
     When at the first position, the sixth engagement member  170  transmits a driven torque from the driven shaft  20  to the dog teeth  56   b  of the sixth driven gear  56 . 
     Each of the engagement members  110  to  170  has an engagement portion that is engaged with the dog teeth. The engagement portion has a slanted portion on a surface that is different from surfaces against which the dog teeth are pressed during torque transmission. The slanted portion is formed by cutting each of the engagement members  110  to  170  obliquely with respect to the axial direction of the engagement member. 
     When the slanted portion is contacted and pushed by the corresponding dog tooth during gear changes, the engagement member is moved away from the corresponding driven gear and is driven in such a direction as to be disengaged from the driven gear. 
     Functions of the slanted portion will be described in detail below. 
     The reverse-first engagement member  110 , the first-second engagement member  120 , the second-third engagement member  130 , the third-fourth engagement member  140 , the fourth-fifth engagement member  150 , the fifth-sixth engagement member  160 , and the sixth engagement member  170  are respectively joined to shift forks  111 ,  121 ,  131 ,  141 ,  151 ,  161 , and  171  via link members, which are not illustrated, such as hub sleeves. 
     These shift forks  111  to  171  are joined to the shift drum  200  and driven by the shift drum  200  in the axial direction. 
     The shift forks  111  to  171  are engaged with circumferential grooves formed in the outer peripheral surfaces of the hub sleeves. The shift forks  111  to  171  can drive the hub sleeves in the axial direction while allowing the hub sleeves to rotate about the axis. 
     The shift drum  200  is substantially cylindrical. 
     Cam grooves are formed on the outer peripheral surface of the shift drum  200 . The cam grooves guide cam followers formed at end portions of the shift forks  111  to  171  so that the shift forks  111  to  171  are driven in the axial direction of the transmission  1 . 
     The shift drum  200  is rotated by an actuator, such as a stepping motor, that is controlled by a transmission control unit, which is not illustrated. 
     The shapes of the cam grooves of the shift drum  200  will be described in detail below. 
     Next, upshifting and downshifting operations of the transmission  1  according to the embodiment will be described. 
     The upshifting operation will be described first, by taking, as an example, a case of upshifting from the second third gear ratio while the power is on (in a state where torque is transmitted from the engine to the driving wheels). 
       FIGS. 2A ,  2 B, and  2 C illustrate an operation for upshifting from the second to third gear ratio. 
       FIG. 2A  illustrates a state of the transmission  1  while the vehicle is travelling in the second gear ratio. Here, one dog tooth  52   a  of the second driven gear  52  and the first-second engagement member  120  are in pressure contact with each other with the application of the engine torque. One dog tooth  52   b  of the second driven gear  52  and the second-third engagement member  130  are apart from each other at a certain backlash. 
     As illustrated in  FIG. 2B , firstly, the second-third engagement member  130  is driven in such a direction as to move away from the engine from an engaged-with-second position (first position), at which the second-third engagement member  130  is engaged with one dog tooth  52   b  of the second driven gear  52 , to an engaged-with-third position (second position), at which the second-third engagement member  130  is engaged with one dog tooth  53   a  of the third driven gear  53 , via the neutral position. 
     Thus, torque starts to be transmitted from the drive shaft  10  to the driven shaft  20  via the third drive gear  43  and the third driven gear  53 . 
     As the speed of the driven shaft  20  increases and becomes higher than the rotating speed of the second driven gear  52 , the first-second engagement member  120  and the dog tooth  52   a  of the second driven gear  52  become separated from each other. Further, one of the slanted portions of the first-second engagement member  120  is contacted and pushed by another dog tooth  52   a  as illustrated in a region C indicated by a dashed circle in  FIG. 2B . 
     Thus, as illustrated in  FIG. 2C , the first-second engagement member  120  moves toward the engine to the neutral position from an engaged-with-second position (second) position), at which the first-second engagement member  120  is engaged with one of the dog teeth  52   a.    
     After that, the third-fourth engagement member  140  is driven toward the engine from the neutral position to an engaged-with-third position (first position), at which the third-fourth engagement member  140  is engaged with one of the dog teeth  53   b  of the third driven gear  53 . Thus, the operation for upshifting from the second to third gear ratio is complete. 
     Note that synchronization is not required for driving the third-fourth engagement member  140  because simultaneous engagements of two different gear sets are prevented in such a manner that the first-second engagement member  120  is automatically pushed away once the second-third engagement member  130  is engaged with one of the dog teeth  53   a  of the third driven gear  53 . 
     The downshifting operation will be described now. The number of engine revolutions has to be increased before the downshifting operation. The number of engine revolutions is increased by either one of a method of increasing the number of engine revolutions by downshifting without disengaging a clutch and by using inertia of a vehicle and a method of increasing the number of engine revolutions by disengaging a clutch and by blipping. The downshifting operation by the former method is performed to achieve the engine braking effect and thus is performed while the power is off. The downshifting operation by the latter method is performed while the clutch is disengaged. Thus, these operations are both performed while the power is substantially off. 
     For this reason, the operation of downshifting from the third to second gear ratio, in the power-off state (in a coasting state where the engine torque value is negative and the driven torque is input from the driving wheels to the driven shaft  20 ) will be described as an exemplary downshifting operation. 
       FIGS. 3A ,  3 B, and  3 C illustrate an operation for downshifting from the third to second gear ratio. 
       FIG. 3A  illustrates a state of the transmission  1  while a vehicle is travelling in the third gear ratio. Here, one dog tooth  53   b  of the third driven gear  53  and the third-fourth engagement member  140  are in pressure contact with each other with the application of the driven torque. One dog tooth  53   a  of the third driven gear  53  and the second-third engagement member  130  are apart from each other at a certain backlash. 
     As illustrated in  FIG. 3B , firstly, the second-third engagement member  130  is driven toward the engine from the engaged-with-third position (second position), at which the second-third engagement member  130  is engaged with one dog tooth  53   a  of the third driven gear  53 , to the engaged-with-second position (first position), at which the second-third engagement member  130  is engaged with one dog tooth  52   b  of the second driven gear  52 , via the neutral position. 
     Thus, torque starts to be transmitted from the driven shaft  20  to the drive shaft  10  via the second driven gear  52  and the second drive gear  42 . 
     Here, in order for the transmission  1  and the engine to be controlled synchronously, it is preferable that blipping control be performed, in which the clutch  30  is temporarily disengaged and engaged again after the number of engine revolutions is increased. 
     As the speed of the drive shaft  10  increases and becomes higher than the rotating speed of the third driven gear  53 , the third-fourth engagement member  140  and the dog tooth  53   b  of the third driven gear  53  become separated from each other. Further, one of the slanted portions of the third-fourth engagement member  140  is contacted and pushed by another dog tooth  53   b  as illustrated in a region C indicated by a dashed circle in  FIG. 3B . 
     Then, as illustrated in  FIG. 3C , the third-fourth engagement member  140  moves from the engaged-with-third position (first position), at which the third-fourth engagement member  140  is engaged with one dog tooth  53   b , to the neutral position in such a direction as to move away from the engine. 
     After that, the first-second engagement member  120  is driven from the neutral position to the engaged-with-second position (second position), at which the first-second engagement member  120  is engaged with one dog tooth  52   a  of the second driven gear  52 , in such a direction as to move away from the engine. Thus, the operation for downshifting from the third to second gear ratio is complete. 
     As described above, the downshifting operation is usually performed while the power is off. Even in a kick-down operation where the downshifting operation is performed while the power is on, a blipping operation is performed to increase the number of engine revolutions, in which the clutch  30  is temporarily disengaged. For this reason, the order of moving the engagement members in the downshifting operation is not the reverse of that in the upshifting operation. The engagement members moved earlier in the upshifting operation need to be moved earlier in the downshifting operation. 
     In the embodiment, these engagement members are driven by the shift drum  200  having a structure that is described below. 
       FIG. 4  illustrates part of the shapes of the cam grooves of the shift drum  200 . 
     Cam grooves are formed on the outer peripheral surface of the shift drum  200 . The cam grooves include a reverse-first-engagement-member driving groove  210 , a first-second-engagement-member driving groove  220 , a second-third-engagement-member driving groove  230 , and a third-fourth-engagement-member driving groove  240 . 
     In the following description, the expression “a position closer to the K-th gear ratio (K=1, 2, 3, and so on) in the circumferential direction of the shift drum  200 ” means a position closer to a position at which a cam follower is held after a gear change to the K-th gear ratio is complete. 
     The reverse-first-engagement-member driving groove  210  guides the cam follower of the shift fork  111  and thus the shift drum  200  drives the reverse-first engagement member  110  via the shift fork  111  in the axial direction. 
     The reverse-first-engagement-member driving groove  210  includes a downshift-to-first slanted portion  211  and an upshift-to-second slanted portion  212 . 
     During the downshift from the second to first gear ratio, the downshift-to-first slanted portion  211  is used to move the reverse-first engagement member  110  from the neutral position to an engaged-with-first position, at which the reverse-first engagement member  110  is engaged with one dog tooth  51   a  of the first driven gear  51 . 
     During the upshift from the first to second gear ratio, the upshift-to-second slanted portion  212  is used to move the reverse-first engagement member  110  from the engaged-with-first position to the neutral position. 
     The downshift-to-first slanted portion  211  and the upshift-to-second slanted portion  212  are arranged separately from each other in the circumferential direction of the shift drum  200 . Thus, a portion sandwiched between the downshift-to-first slanted portion  211  and the upshift-to-second slanted portion  212  has a width larger than the other portions of the reverse-first-engagement-member driving groove  210 . With this structure, the path that the cam follower of the shift fork  111  takes for engaging the reverse-first engagement member  110  with the first driven gear  51  is different from that for disengaging the reverse-first engagement member  110  from the first driven gear  51 , as illustrated in  FIG. 4 . 
     The first-second-engagement-member driving groove  220  guides the cam follower of the shift fork  121  and thus the shift drum  200  drives the first-second engagement member  120  via the shift fork  121  in the axial direction. 
     The first-second-engagement-member driving groove  220  includes an upshift-to-second slanted portion  222 , a downshift-to-first slanted portion  221 , a downshift-to-second slanted portion  223 , and an upshift-to-third slanted portion  224 . 
     During the upshift from the first to second gear ratio, the upshift-to-second slanted portion  222  is used to move the first-second engagement member  120  from the engaged-with-first position, at which the second engagement member  120  is engaged with one dog tooth  51   b  of the first driven gear  51 , to engaged-with-second position, at which the second engagement member  120  is engaged with one dog tooth  52   a  of the second driven gear  52 , via the neutral position. 
     During the downshift from the second to first gear ratio, the downshift-to-first slanted portion  221  is used to move the first-second engagement member  120  from the engaged-with-second position to the engaged-with-first position via the neutral position. 
     The upshift-to-second slanted portion  222  and the downshift-to-first slanted portion  221  are arranged in such a manner that the path that the cam follower of the shift fork  121  takes for upshifting to the second gear ratio is substantially the same as that for downshifting to the first gear ratio. 
     The upshift-to-second slanted portion  222  and the downshift-to-first slanted portion  221  are located at positions, in the circumferential direction of the shift drum  200 , closer to the second gear ratio than the downshift-to-first slanted portion  211  of the reverse-first-engagement-member driving groove  210  is, and closer to the first gear ratio than the upshift-to-second slanted portion  212  is. 
     During the downshift from the third to second gear the downshift-to-second slanted portion  223  is used to move the first-second engagement member  120  from the neutral position to the engaged-with-second position. 
     During the upshift from the second to third gear ratio, the upshift-to-third slanted portion  224  is used to move the first-second engagement member  120  from the engaged-with-second position to the neutral position. 
     The downshift-to-second slanted portion  223  and the upshift-to-third slanted portion  224  are arranged separately from each other in the circumferential direction of the shift drum  200 . Thus, a portion sandwiched between the downshift-to-second slanted portion  223  and the upshift-to-third slanted portion  224  has a width larger than the other portions of the first-second-engagement-member driving groove  220 . With this structure, the path that the cam follower of the shift fork  121  takes for engaging the first-second engagement member  120  with the second driven gear  52  is different from that for disengaging the first-second engagement member  120  from the second driven gear  52 , as illustrated in  FIG. 4 . 
     The second-third-engagement-member driving groove  230  guides the cam follower of the shift fork  131  and thus the shift drum  200  drives the second-third engagement member  130  via the shift fork  131  in the axial direction. 
     The second-third-engagement-member driving groove  230  includes a downshift-to-first slanted portion  231 , an upshift-to-second slanted portion  232 , an upshift-to-third slanted portion  234 , and a downshift-to-second slanted portion  233 . 
     During the downshift from the second to first gear ratio, the downshift-to-first slanted portion  231  is used to move the second-third engagement member  130  from the engaged-with-second position, at which the second-third engagement member  130  is engaged with one dog tooth  52   b  of the second driven gear  52 , to the neutral position. 
     During the upshift from the first to second gear ratio, the upshift-to-second slanted portion  232  is used to move the second-third engagement member  130  from the neutral position to the engaged-with-second position. 
     The downshift-to-first slanted portion  231  and the upshift-to-second slanted portion  232  are arranged separately from each other in the circumferential direction of the shift drum  200 . Thus, a portion sandwiched between the downshift-to-first slanted portion  231  and the upshift-to-second slanted portion  232  has a width larger than the other portions of the second-third-engagement-member driving groove  230 . With this structure, the path that the cam follower of the shift fork  131  takes for engaging the second-third engagement member  130  with the second driven gear  52  is different from that for disengaging the second-third engagement member  130  from the second driven gear  52 , as illustrated in  FIG. 4 . 
     The downshift-to-first slanted portion  231  and the upshift-to-second slanted portion  232  are positioned, in the circumferential direction of the shift drum  200 , in substantially the same positions as the downshift-to-first slanted portion  211  and the upshift-to-second slanted portion  212  of the reverse-first-engagement-member driving groove  210 , respectively. 
     During the upshift from the second to third gear ratio, the upshift-to-third slanted portion  234  is used to move the second-third engagement member  130  from the engaged-with-second position to the engaged-with-third position, at which the second-third engagement member  130  is engaged with one dog tooth  53   a  of the third driven gear  53 , via the neutral position. 
     During the downshift from the third to second gear ratio, the downshift-to-second slanted portion  233  is used to move the second-third engagement member  130  from the engaged-with-third position to the engaged-with-second position via the neutral position. 
     The upshift-to-third slanted portion  234  and the downshift-to-second slanted portion  233  are arranged in such a mariner that the path that the cam follower of the shift fork  131  takes for upshifting to the third gear ratio is substantially the same as that for downshifting to the second gear ratio. 
     The upshift-to-third slanted portion  234  and the downshift-to-second slanted portion  233  are positioned, in the circumferential direction of the shift drum  200 , closer to the third gear ratio than the downshift-to-second slanted portion  223  of the first-second-engagement-member driving groove  220  is, and closer to the second gear ratio than the upshift-to-third slanted portion  224  is. 
     The third-fourth-engagement-member driving groove  240  guides the cam follower of the shift fork  141  and thus the shift drum  200  drives the third-fourth engagement member  140  via the shift fork  141  in the axial direction. 
     The third-fourth-engagement-member driving groove  240  includes a downshift-to-second slanted portion  241  and an upshift-to-third slanted portion  242 . 
     During the downshift from the third to second gear ratio, the downshift-to-second slanted portion  241  is used to move the third-fourth engagement member  140  from the engaged-with-third position, at which the third-fourth engagement member  140  is engaged with one dog tooth  53   b  of the third driven gear  53 , to the neutral position. 
     During the upshift from the second to third gear ratio, the upshift-to-third slanted portion  242  is used to move the third-fourth engagement member  140  from the neutral position to the engaged-with-third position. 
     The downshift-to-second slanted portion  241  and the upshift-to-third slanted portion  242  are arranged separately from each other in the circumferential direction of the shift drum  200 . Thus, a portion sandwiched between the downshift-to-second slanted portion  241  and the upshift-to-third slanted portion  242  has a width larger than the other portions of the third-fourth-engagement-member driving groove  240 . With this structure, the path that the cam follower of the shift fork  141  takes for engaging the third-fourth engagement member  140  with the third driven gear  53  is different from that for disengaging the third-fourth engagement member  140  from the third driven gear  53 , as illustrated in  FIG. 4 . 
     The downshift-to-second slanted portion  241  and the upshift-to-third slanted portion  242  are positioned, in the circumferential direction of the shift drum  200 , in substantially the same positions as the downshift-to-second slanted portion  223  and the upshift-to-third slanted portion  224  of the first-second-engagement-member driving groove  220 , respectively. 
     The driving grooves  210  to  240  used for gear changes between first, second, and third gear ratios have only been described thus far. However, driving grooves (cam grooves) used for gear changes between other gear ratios can be also formed in substantially the same manner. 
     With the above-described structure, during a gear change between two adjacent gear ratios, an engagement member arranged in a space between adjacent gears corresponding to these gear ratios is driven first, and then other engagement members positioned on both sides of the engagement member are driven substantially simultaneously. 
     Each engagement member includes a detent system, which is not illustrated, for positioning the cam follower at the first and second positions. The reason for using the detent systems is that the cam followers are unable to be positioned at the first and second positions by simply using the driving grooves  210  to  240  while passing through the wide portions of the driving grooves  210  to  240 . 
     After the gear change to and the selection of a desired gear ratio are complete, the engagement members are locked in the corresponding driving grooves. 
     According to the embodiment described above, the engagement members  120  to  160  that transmit a driven torque to gear sets corresponding to lower gear ratios and a driving torque from gear sets corresponding to higher gear ratios are arranged in spaces between adjacent gear sets. Each of the gear sets includes either one of the first to sixth drive gears  41  to  46  and a corresponding one of the first to sixth driven gears  51  to  56 . Thus, the transmission  1  according to the embodiment can perform speedy gear changes between all the forward speeds (first to sixth gear ratios). In addition, the structure of the transmission  1  is made simpler. 
     Modifications 
     The present invention is not limited to the embodiment described above. The embodiment may be modified or changed in various ways, and the modifications and changes also fall within the technical scope of the present invention. 
     (1) In the transmission according to the embodiment, the fixed gears are mounted on the drive shaft while the rotatable gears and the shift mechanisms are mounted on the driven shaft. However, the rotatable gears and the shift mechanisms may be mounted on the drive shaft and the fixed gears may be mounted on the driven shaft.
 
(2) The transmission according to the embodiment achieves six forward speeds and one reverse speed, for example. However, the number of speeds is not particularly limited.
 
(3) The transmission according to the embodiment has been described as being longitudinally mounted, for example, but may also be mounted transversely.
 
     The present invention is not limited to the transmission mounted to be adjacent to the engine. For example, the transmission may be mounted separately from the engine, i.e., may be a transaxle.