Patent Publication Number: US-7721858-B2

Title: Clutch actuator, engine unit, and saddle type vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority benefit of Japanese Patent Application No. 2006-114704, which was filed on Apr. 18, 2006 and which is hereby incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a clutch actuator that disengages and engages a clutch. More particularly, the present invention relates to an engine unit that includes such a clutch actuator and a saddle type vehicle, such as a motorcycle, that includes such an engine unit. 
   2. Description of the Related Art 
   Conventionally, a system is known that attaches an actuator to a previously existing manual transmission. The actuator is used to automate certain operations such that the system can automate a series of start, stop and shift change operations (clutch disengagement, gear change, and clutch engagement) based on the rider&#39;s intention or the state of the vehicle. 
   Among the actuators used in the above-described system, one example of a clutch actuator for disengaging and engaging the clutch is the clutch actuator disclosed in JP-A-2005-282784. This clutch actuator is provided with a motor, and a worm shaft that rotates coaxially with a motor shaft of the motor. A tip of the worm shaft is formed with a threaded section. In addition, the clutch actuator includes a worm wheel that meshes with the threaded section; a crank shaft that converts rotational motion of the worm wheel to linear reciprocating motion; and an output rod that is connected to the crank shaft. The clutch actuator finally converts the rotational motion of the motor to an axial reciprocating motion of the output rod, and uses the reciprocating motion of the output rod to disengage and engage the clutch. 
   SUMMARY OF THE INVENTION 
   In the clutch actuator disclosed in JP-A-2005-282784, an I-cut is formed in the tip of the motor shaft of the motor. When the motor shaft and the worm shaft are fitted together, the I-cut section acts as a key, allowing the transmission of driving force from the motor shaft to the worm shaft. When such a clutch actuator is mounted to a motorcycle, clutch disengagement-engagement needs to be highly responsive. However, in the configuration of JP-A-2005-282784, the clutch disengagement and/or engagement operation does not rapidly follow the rotation of the motor. In other words, there may be some slight delay between the rotation and the actuation and there may be some lost motion resulting from the linkage. 
   Thus, one aspect of an embodiment of the present invention seeks to improve the responsiveness of the clutch actuator assembly. Thus, one embodiment features a clutch actuator that comprises a motor having a motor shaft. A worm shaft is formed with a threaded section and the worm shaft preferably rotates generally coaxially with the motor shaft. A worm wheel can mesh with the threaded section of the worm shaft. An output rod can reciprocate along an axial direction to disengage and engage a clutch. A crank shaft can be used to convert rotational motion of the worm wheel to reciprocating motion of the output rod. Preferably, the motor shaft and the worm shaft are splined together to reduce or eliminate backlash. 
   As described above, in the conventional art, the I-cut is formed in the tip of the motor shaft, and this I-cut section transmits the driving force from the motor shaft to the worm shaft. When a motor shaft formed with an I-cut is used, if the shaft center of the motor shaft oscillates during rotation of the motor, the oscillation is not absorbed by the worm shaft. Thus, a reasonably large space has been provided between the I-cut section of the motor shaft and the hole that receives the worm shaft. However, if a space is formed in this manner, the motor shaft rotates idly for a very slight period of time between when the motor shaft rotates and when this rotation is transmitted to the worm shaft. Thus, responsiveness is impaired when the clutch is disengaged and engaged. To address this, a structure can be used in which the space is made smaller in order to improve the responsiveness of the clutch disengagement-engagement operation. However, if the above space is simply made smaller, oscillation of the shaft center of the motor shaft is not able to be absorbed by the worm shaft and the shaft center of the worm shaft would oscillate. 
   Thus, as compared to the structure disclosed in JP-A-2005-282784, in which both side surfaces of the I-cut section transmit driving force, the clutch actuator described herein permits some shaft misalignment between the motor shaft and the worm shaft and improves responsiveness of the clutch disengagement-engagement operation. With the spline fit structure, a plurality of meshing sections are formed in the rotation direction. Accordingly, as compared to the above I-cut, when the same driving force is transmitted, the fitting length in the axial direction can be made shorter. When the fitting length in the axial direction is shorter, as compared to when the fitting length is longer, it is possible to permit greater misalignment of the motor shaft and the worm shaft. More specifically, when the fitting length in the axial direction is short (note that, “short” is only used here to indicate a comparison with when the fitting length is long, and does not indicate any specific reference to the length of the fitting section), oscillation of the shaft center of the motor shaft can be favorably absorbed. Accordingly, the space for absorbing oscillation of the shaft center, namely, the rotation direction space (the space between the spline teeth and a spline groove) between the motor shaft and the worm shaft can be made smaller. As a result, the rotation of the worm shaft rapidly follows the rotation of the motor shaft, thereby improving the responsiveness of the clutch disengagement-engagement operation. 
   Fitting the motor shaft and the worm shaft together by splining in this manner allows a certain degree of misalignment between the motor shaft and the worm shaft. As a result, the space in the rotation direction of both shafts can be made smaller and responsiveness of the clutch disengagement-engagement operation improved. 
   Preferably, therefore, the motor shaft and the worm shaft are engaged by being spline fit together. Although it is necessary that the number of spline teeth formed in the motor shaft or the worm shaft is three or more, there is no specific limitation on the number of teeth. For example, spline fitting can include the use of fine spline teeth that are formed around the shaft, such as when serrations are used for fitting. Spline fitting also can include a configuration in which a plurality of protrusions and depressions are formed around at least a portion of the outer circumference or the inner circumference of one of the shafts and the shafts are fit together such that the protrusions and depressions of both the shafts are engaged. 
   One aspect of an embodiment of the present invention involves a clutch actuator that comprises a motor. The motor comprises a motor shaft. The motor shaft rotates about a first axis. A worm shaft comprises a threaded section. The worm shaft rotates about a second axis. The first and second axes are generally coaxial. A worm wheel is engaged with the threaded section of the worm shaft. An output rod comprises an elongated axis. The axis defines an axial direction. The output rod is adapted to disengage and engage a clutch through movement of the output rod in the axial direction. A crank shaft is driven by the worm wheel and is connected to the output rod such that that crank shaft converts rotational motion of the worm wheel to reciprocating motion of the output rod. The motor shaft and the worm shaft are splined together. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages will now be described with reference to drawings of a preferred embodiment. The drawings comprise the following figures. 
       FIG. 1  is a side view of a motorcycle that is arranged and configured in accordance with certain features, aspects and advantages of an embodiment of the invention. 
       FIG. 2  is an enlarged side view of an automated transmission control device used on the motorcycle of  FIG. 1 . 
       FIG. 3  is another view of the automated transmission control device shown in  FIG. 2 . 
       FIG. 4  is a perspective view of the automated transmission control device. 
       FIG. 5  is a further perspective view of the automated transmission control device. 
       FIG. 6  is a top plant view of the automated transmission control device. 
       FIG. 7  is sectioned view of the internal structure of an engine unit and some portions of the automated transmission control device. 
       FIG. 8  is a schematic view of a shift actuator, a shift rod, and a shift mechanism of the automated transmission control device. 
       FIG. 9  is a side view of the shift actuator, the shift rod, and the shift mechanism. 
       FIG. 10  is a schematic view of a clutch actuator. 
       FIG. 11  is a side view of the clutch actuator. 
       FIG. 12  is a sectioned view taken along line A-A of  FIG. 11 . 
       FIG. 13  is a sectioned view taken along line B-B of  FIG. 11 . 
       FIG. 14  is an enlarged cross sectional view of a coupling portion of a motor shaft and a worm shaft. 
       FIG. 15  is an enlarged view of a portion of the worm shaft in which a threaded section is formed. 
       FIG. 16  is a perspective view of a switch section of a handle grip. 
       FIG. 17  illustrates a control system used with one embodiment of the automated transmission control device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a side view of a motorcycle  1  that is arranged and configured in accordance with certain features, aspects and advantages of an embodiment of the present invention. As can be seen from  FIG. 1 , the motorcycle  1  comprises a head tube  3  and a body frame  2 . The body frame  2  comprises a main frame  4  that extends rearward from the head tube  3  and a rear arm bracket  5  that extends downward from a rear section of the main frame  4 . The main frame  4  has two frame sections  4   a  that extend to the left and right in a rearward direction from the head tube  3  (only one of these is shown in  FIG. 1 ). A rear section of the frame section  4   a  is connected to the downwardly-extending rear arm bracket  5 . 
   A front fork  10  is pivotably supported by the head tube  3 . A steering handlebar  11  is provided at an upper end of the front fork  10  and a front wheel  12  is provided at a lower end of the front fork  10 . In the illustrated configuration, a fuel tank  13  is provided in an upper section of the main frame  4  and a seat  14  is provided to the rear of the fuel tank  13 . The seat  14  is mounted on a seat rail  6  that extends rearward from the main frame  4 . 
   In one configuration, an engine  20  is suspended from the main frame  4  and the rear arm bracket  5 . The engine  20  is supported by an engine attachment section  4   c  and the frame section  4   a  of the main frame  4 , and is also supported by an engine attachment section (not shown) of the rear arm bracket  5 . Note that, the engine  20  is not limited to being an internal combustion engine like a gasoline engine, and may be an electric motor, or any other suitable vehicle powering construction. Further, the engine may combine a gasoline engine and an electric motor such as in a hybrid vehicle, for instance. 
   A front end of a rear arm  21  is supported via a pivot shaft  22  in the rear arm bracket  5  so as to be capable of rocking upward and downward. A rear wheel  23  is supported by a rear end of the rear arm  21 . The rear arm  21  is supported by the body frame  2  via a link mechanism  24  and a rear cushion unit  25 . The illustrated link mechanism  24  has a body side link  24   a  and a rear arm side link  24   b . One end of the body side link  24   a  is rotatably coupled to a link attachment section  5   f  of the rear arm bracket  5 . One end of the rear arm side link  24   b  is rotatably coupled to a link attachment section  21   a  of the rear arm  21 . In addition, a central section of the body side link  24   a  and the other end of the rear arm side link  24   b  are rotatably coupled. A lower section of the rear cushion unit  25  is supported by the other end of the body side link  24   a , and an upper section of the rear cushion unit  25  is supported by a cushion attachment section  5   g . The rear cushion unit  25  is disposed to the rear of the rear arm bracket  5 . 
   Further, a cowling  27  can be provided on the body frame  2 . The cowling  27  preferably comprises an upper cowling  27   a  that covers an area forward of the steering handlebar  11 , and a lower cowling  27   b  that covers forward and to the left and right sides of the main frame  4  and to the left and right sides below the engine  20 . The upper cowling  27   a  can be supported by the main frame  2  via a stay (not shown). The upper cowling  27   a  preferably defines a front surface and both side surfaces in the left and right directions of a body front section. Further, a screen  28  and a head lamp  29 , both of which can made from a transparent material or the like, can be attached to the upper cowling  27   a  and can be positioned at an upper section of the body front. A side cover  30  preferably is disposed on a back stay  7  to cover the region above the rear wheel  23  and to the left and right side of the seat  14 . 
   Although the type of engine used in the invention is not limited, the engine  20  in the illustrated configuration is a liquid cooled, parallel 4-cycle, 4-cylinder engine. The engine  20  preferably is disposed such that the cylinder axis (not shown) lies in the body forward direction and is slightly inclined from horizontal. A crank case  32  that houses a crank shaft  31  can be suspended from and supported by the body frame  2  at both sides in the vehicle width direction. 
   As illustrated, the engine  20  preferably is provided with a transmission  40 . The transmission  40  can comprise a main shaft  41  that is positioned generally parallel to the crank shaft  31 ; a drive shaft  42  that is positioned generally parallel to the main shaft  41 ; and a shift mechanism  43  that comprises multi-speed gears  49 . Preferably, the transmission  40  is integrally assembled with the crank case  32 . The clutch mechanism  44  can disengage and reengage the transmission to the engine when the gears  49  are switched. 
   The drive shaft  42  can be provided with a drive sprocket  48   a  and a chain  47  can be wound around this drive sprocket  48   a  and a driven sprocket  48   b  provided on the rear wheel  23 . Accordingly, engine power is transmitted to the rear wheel  23  via the chain  47 . Other configurations, such as a shaft drive or a belt drive, for example but without limitation, also can be used. 
   The motorcycle  1  preferably comprises an automated transmission control device  50 .  FIG. 2  through  FIG. 6  show one configuration of the automated transmission control device  50 . As is apparent from  FIG. 2 , the automated transmission control device  50  preferably automatically disengages and engages the clutch mechanism  44  and switches the gears of the transmission  40 . The automated transmission control device  50  can comprise a clutch actuator  60  that drives the clutch mechanism  44 ; a shift actuator  70  that switches the gears of the transmission  40 ; and an engine control device  95  (see  FIG. 8 ) that, in one configuration, controls operation of the clutch actuator  60  and the shift actuator  70 . 
   Referring to  FIG. 3 , the clutch actuator  60  is formed from a clutch control unit  63  that can be integrally assembled with an attachment plate  61  on which various structural members are mounted. An engagement hole  62  (refer to  FIG. 4  and  FIG. 5 ) preferably is secured to the attachment plate  61 . As shown in  FIG. 4 , the clutch control unit  63  is attached such that the engagement hole  62  is engaged with a protrusion  20   a  that is fixed to a rear section of the engine  20 , and an attachment section  61   a  of the attachment plate  61  is secured to a member  5   d  of the rear arm bracket  5  using fasteners  64 , such as bolts or the like. Other manners of securing the clutch control unit also can be used. In the illustrated configuration, the clutch control unit  63  is arranged and positioned to the rear of the engine  20  and is surrounded by the rear arm bracket  5  when viewed from the side (refer to  FIG. 1 ). 
   Referring to  FIG. 2  and  FIG. 6 , the shift actuator  70  preferably is integrated with a shift position detection sensor S 2  (refer to  FIG. 6 ), and these members can define a shift control unit  72 . As can be seen from  FIG. 2 , an attachment bracket  73  is fixed to the back stay  7 . In one configuration, the shift control unit  72  is attached by securing the shift actuator  70  to the attachment bracket  73  using fasteners  74 , which can be bolts or the like. In this manner, the shift control unit  72  is arranged and positioned such that, when viewed from the side, the shift control unit  72  is opposite from the transmission  40  with the main frame  4  interposed therebetween while the shift actuator  70  is positioned to the rear of the main frame  4 . 
   The shift mechanism  43  and the shift actuator  70  can be coupled together by a shift power transmission member. In this embodiment, the shift power transmission member comprises a shift rod  75 . The shift rod  75 , when viewed from the side, preferably extends across the body frame  2 . 
   With reference now to  FIG. 7 , the clutch mechanism  44  in the illustrated embodiment comprises, for example, a multi-plate friction clutch, and includes a clutch housing  443 ; a plurality of friction plates  445  provided integrally with the clutch housing  443 ; a clutch boss  447 ; and a plurality of clutch plates  449  provided integrally with the clutch boss  447 . A gear  310  is integrally supported by the crank shaft  31  of the engine  20 , and the main shaft  41  supports a gear  441  that meshes with the gear  310  such that the gear  441  can rotate with respect to the main shaft  41 . The clutch housing  443  is integrally provided with the gear  441 , and torque from the crank shaft  31  is transmitted to the clutch housing  443  through the gear  441 . Torque from the clutch housing  443  is transmitted to the clutch boss  447  by frictional force generated between the plurality of friction plates  445  and the plurality of clutch plates  449 . 
   The gear  441  can be rotatably supported by the main shaft  41  at one end section (the right side in  FIG. 7 ) of the main shaft  41 . The clutch housing  443  preferably is integrated with a boss section of the gear  441 , thus allowing rotation with respect to the main shaft  41  while controlling movement in the axial direction of the main shaft  41 . Further, the clutch boss  447  can be integrated with the main shaft  41  at the side of the one end section of the main shaft  41  (further to the end of the one end section than the gear  441 ). 
   The clutch boss  447  is provided inward of the tubular clutch housing  443 . The gear  441 , the clutch housing  443 , the clutch boss  447 , and the main shaft  41  preferably are coaxial with their centers of rotation aligned. 
   The boss section of the gear  441  is provided with a tubular or cylindrical engagement protrusion  441 A. An engagement member  443 B, formed with an engagement hole  443 A that engages with the engagement protrusion  441 A, can be provided at one end section (on the left side of  FIG. 7 ) of the tubular clutch housing  443 . The engagement protrusion  441 A preferably is received by the engagement hole  443 A to secure the clutch housing  443  to the gear  441 . 
   Each friction plate  445  is a thin ring shaped plate. The external peripheral edge of each friction plate  445  is supported at an inside surface of a tubular section of the clutch housing  443  such that the plate surface of each friction plate  445  is substantially perpendicular with respect to the axial direction of the main shaft  41 . As a result of this support, each friction plate  445  can be capable of very slight relative movement relative to the clutch housing  443  in the axial direction of the main shaft  41 . Further, each friction plate  445  is controlled such that it is not capable of relative rotation in the rotational direction of the main shaft  41  with respect to the clutch housing  443 . In some configurations, the friction plates  445  are secured to the clutch housing  443  with splines, which secure the friction plates  445  to rotate with the clutch housing  443  while allowing the friction plates to move in the axial direction. 
   Note that, there is a predetermined space (with a length that is very slightly larger than the thickness of the clutch plate  449 ) between each of the above described plate surfaces of the friction plates  445 . 
   The clutch boss  447  is tubular, and a circular flange  447 A, which has an external diameter that is substantially the same as the external diameter of the clutch plate  449 , is provided at one end section of the clutch boss  447  (the left side in  FIG. 7 ). The plurality of clutch plates  449  are supported at the external periphery of the tubular section of the clutch boss  447 . As a result of this support, each clutch plate  449  is capable of very slight relative movement in the axial direction of the main shaft  41  with respect to the clutch boss  447 . Further, each clutch plate  449  is controlled such that it is not capable of relative rotation in the rotational direction of the main shaft  41  with respect to the clutch boss  447 . 
   Moreover, the clutch boss  447  is fixed to the one end section side of the main shaft  41  (e.g., the right side in  FIG. 7 ) such that the flange  447 A is positioned at the engagement member  443 B of the clutch housing  443 . 
   Each clutch plate  449  is a thin ring shaped plate. The internal peripheral edge of each clutch plate  449  is supported at the outer surface of the tubular section of the clutch boss  447  such that the plate surface of each clutch plate  449  is substantially perpendicular with respect to the axial direction of the main shaft  41 . 
   Further, there is a predetermined space (with a length that is very slightly larger than the thickness of the friction plate  445 ) between each of the above described plate surfaces of the clutch plates  449 . 
   The external diameter of each clutch plate  449  is slightly smaller than the internal diameter of the tubular section of the clutch housing  443 . The internal diameter of each friction plate  445  is slightly larger than the external diameter of the tubular section of the clutch boss  447 . In addition, the friction plates  445  and the clutch plates  449  are alternately arranged in the axial direction of the main shaft  41 , and a very slight space is formed between each of the friction plates  445  and the clutch plates  449  in the axial direction of the main shaft  441 . 
   A pressing member  447 B, structured by the flange  447 A of the clutch boss  447 , is provided at the engagement member  443 B side (the left side in  FIG. 7 ) of the clutch housing  443  at the external side in the axial direction of the main shaft  1  of the alternately arranged friction plates  445  and the clutch plates  449 . The pressing member  447 B and a pressure plate  451  squeezes the interposed friction plates  445  and the clutch plates  449  in the axial direction of the main shaft  41 , thus generating frictional force between each friction plate  445  and each clutch plate  449 . 
   A plurality of cylindrical guide members  447 C, which extend in the axial direction of the main shaft  41  and which are provided integrally with the clutch boss  447 , are provided at the internal side of the tubular clutch boss  447 . The pressure plate  451  is provided with a plurality of guides  451 A that respectively engage with the guides  447 C. As a result of the guides  447 C and the guides  451 A, the pressure plate  451  is capable of relative movement in the axial direction of the main shaft  41  with respect to the clutch boss  447 , and also rotates generally synchronously with the clutch boss  447 . Note that, the pressure plate  451  is driven by the clutch actuator  60 . The clutch actuator  60  will be described in detail later with reference to the drawings. 
   Further, the pressure plate  451  has a generally flat pressing member  451 B. This pressing member  451 B is substantially parallel to the plate surface of each friction plate  445  and each clutch plate  449 . 
   The clutch mechanism  44  is provided with a plurality of springs  450  that are disposed so as to respectively surround each of the plurality of tubular guides  447 C. Each spring  450  urges the pressure plate  451  toward the left side of  FIG. 7 . In other words, each spring  450  urges the pressure plate  451  in the direction in which the pressing member  451 B of the pressure plate  451  is moved close to the pressing member  447 B of the clutch boss  447 . 
   The pressure plate  451  is engaged at a central section of the pressure plate  451  with one end (the right side of  FIG. 7 ) of a push rod  455  via a bearing like a deep groove ball bearing  457  such that the pressure plate  451  is capable of rotating with respect to the push rod  455 . The other end of the push rod  455  (the left side of  FIG. 7 ) is engaged with the internal side of one end section of the tubular main shaft  41 . A spherical ball  459  that abuts with the end of the push rod  455  (e.g., the left end in  FIG. 7 ) is provided inside of the tubular main shaft  41 . Further, a push rod  461  that abuts with the ball  459  is provided at the right side of the ball  459 . 
   An end section  461 A (e.g., left end section in  FIG. 7 ) of the push rod  461  protrudes outward beyond the other end (e.g., the left end in  FIG. 7 ) of the tubular main shaft  41 . A piston  463  can be integrated with the end section  461 A. The piston  463  can be guided by a cylinder body  465 , and can slide in the axial direction of the main shaft  41 . 
   When hydraulic oil, which acts as a compressed fluid, is supplied to a space  467  enclosed by the piston  463  and the cylinder body  465 , the piston  463  is pushed and moved in the rightward direction in  FIG. 7 . Accordingly, the piston  463  pushes the pressure plate  451  in the rightward direction of  FIG. 7  via the push rod  461 , the ball  459 , the push rod  455 , and the deep groove ball bearing  457 . In this way, the pressure plate  451  is pushed in the rightward direction of  FIG. 7 , and the pressing member  451 B of the pressure plate  451  separates from the friction plates  445 , thereby disengaging the clutch. 
   When the clutch mechanism  44  is to be connected (i.e., engaged), the pressure plate  451  is urged and moves in the direction of the flange  447 A of the clutch boss  447  (e.g., the leftward direction of  FIG. 7 ) by the springs  450 . In this state, the pressing member  447 B of the clutch boss  447  and the pressing member  451 B of the pressure plate  451  cause frictional force to be generated between each friction plate  445  and each clutch plate  449 . Accordingly, driving force can be transmitted from the clutch housing  443  to the clutch boss  447 . 
   On the other hand, when the clutch mechanism  44  is to be disengaged, the push rod  455  moves the pressure plate  451  in the rightward direction in  FIG. 7 . Further, the pressing member  451 B of the pressure plate  451  is separated from the friction plate  445  positioned closest to the pressing member  451 B (e.g., the friction plate  445  on the far right side of  FIG. 7 ). 
   In this state, each friction plate  445  and each clutch plate  449  are not in contact, and thus a very slight space is formed between each friction plate  445  and each clutch plate  449 . Accordingly, frictional force, which enables the transmission of driving force, is not generated between the friction plates  445  and the clutch plates  449 . 
   In this manner, depending on the magnitude of the driving force of the clutch actuator  60  and the urging force of the springs  450 , the pressure plate  451  is moved in one or the other of the axial directions of the main shaft  41 . The clutch is engaged or disengaged in accordance with this movement. 
   With continued reference to  FIG. 7 , an engine rotation speed sensor S 30  can be mounted to the end of the crank shaft  31  of the engine  20 . The crank shaft  31  can be coupled to the main shaft  41  via the multi-plate clutch mechanism  44 . The multi-speed gears  49  are mounted to the main shaft  41 , and a main shaft rotation speed sensor S 31  is also provided on the main shaft  41 . Each gear  49  on the main shaft  41  preferably meshes with a respective gear  420  mounted on the drive shaft  42  that corresponds with each gear  49 . For clarity,  FIG. 7  shows the drive shaft  42  and the main shaft  41  separated from each other with no contact at the gears; it should be understood that these shafts, and in particular the gears on the shafts, are connected together. The main shaft gears  49  and the drive shaft gears  420  are attached such that, apart from the selected pair of gears, either one or both of the gears  49  and gears  420  can rotate freely (in an idle state) with respect to either the main shaft  41  or the drive shaft  42 . Accordingly, transmission of rotation from the main shaft  41  to the drive shaft  42  only occurs via the selected pair of gears. 
   The operation of selecting and changing the gear ratio of the gears  49  and the gears  420  is performed by a shift cam  421  that is mounted to, or defines a portion of, a shift input shaft. The shift cam  421  preferably has a plurality of cam grooves  421   a . A shift fork  422  is mounted in each cam groove  421   a . Each shift fork  422  engages with a dog assigned to each predetermined main shaft gear  49  and/or a predetermined drive shaft gear  420  on the respective main shaft  41  and drive shaft  42 . The dogs couple the gears to the shafts for rotation and are slideable along the shafts. Rotation of the shift cam  421  causes the shift fork  422  to move in an axial direction while being guided by the cam groove  421   a , whereby only the pair of the main shaft gear  49  and the drive shaft gear  420  at the position corresponding to the rotation angle of the shift cam  421  are spline fixed to the respective main shaft  41  and the drive shaft  42 . Accordingly, the gear position is determined, and rotation is transmitted at a predetermined gear ratio between the main shaft  41  and the drive shaft  42 , via the main shaft gear  49  and the drive shaft  420 . 
   The shift mechanism  43  uses movement of the shift actuator  70  to reciprocally move the shift rod  75 , whereby the shift cam  421  is rotated by just a predetermined angle via a shift link mechanism  425 . Accordingly, the shift fork  422  moves just a predetermined amount in the axial direction along the cam groove  421   a . The gear pairs are thus fixed in order to the main shaft  41  and the drive shaft  42 , whereby rotational driving force is transmitted at each reduction gear ratio. 
   The shift actuator  70  may be hydraulic or electric.  FIG. 8  is an outline view of one embodiment of the shift actuator  70 , the shift rod  75 , and the shift mechanism  43 . Referring to  FIG. 8 , in the shift actuator  70  according to the embodiment, a shift motor  70   a  rotates when a signal is output from the engine control device  95 . The rotation of the shift motor  70   a  causes a gear  70   c  of a motor shaft  70   b  to rotate. The rotation of the gear  70   c  causes a coupled reduction gear  70   d  to rotate, whereby a drive shaft  70   g  rotates. 
     FIG. 9  shows a side view of the shift actuator  70 , the shift rod  75 , and the shift mechanism  43 . Referring to  FIG. 9 , a housing  70   h  of the shift actuator  70  is fixed to the attachment bracket  73  fixed to the back stay  7  (refer to  FIG. 2 ) using a fastener  74  (refer to  FIG. 2 ). Any other mounting configuration also can be used. 
   An operation lever  70   j  is provided on the drive shaft  70   g  (refer to  FIG. 8 ). A connecting section of the shift rod  75  on the shift actuator  70  side is connected using a bolt (not shown) to the operation level  70   j . Other connecting constructions also can be used. The connecting section of the shift rod  75  on the shift actuator  70  side is capable of rotating with respect to the operation lever  70   j . Further, the operation lever  70   j  is fastened and fixed to the drive shaft  70   g  by a bolt  70   k , whereby the operation lever  70   j  is generally prevented from moving in the axial direction of the drive shaft  70   g.    
   The shift position detection sensor S 2  preferably is disposed on the drive shaft  70   g  (refer to  FIG. 8 ). This shift position detection sensor S 2  can be disposed at an end (e.g., the end toward the inward direction of the paper of  FIG. 9 ) of the drive shaft  70   g , and can be secured to the housing  70   h  by an attachment bolt (not shown) or in any other suitable manner. The shift position detection sensor S 2  advantageously detects position information based on the rotation of the drive shaft  70   g , and transmits this position information to the engine control device  95 . The engine control device  95  can control the shift motor  70   a  based on the position information. 
   Moreover, a connecting section of the shift rod  75  at the shift mechanism  43  side can be connected to a shift operation lever  43   a  of the shift mechanism  43  by a bolt (not shown). The connecting section of the shift rod  75  at the shift mechanism  43  side preferably is capable of rotating with respect to the shift operation lever  43   a . Further, the shift operation lever  43   a  can be secured to a shift operation shaft  43   b  by a bolt  43   d  or in any other suitable manner, whereby the shift operation lever  43   a  is generally prevented from moving in the axial direction of the shift operation shaft  43   b.    
   When the shift rod  75  moves, the shift operation lever  43   a  also moves. The movement of the shift operation lever  43   a  is a rotational motion centering on the shift operation shaft  43   b  that is spline engaged with the shift operation lever  43   a . Thus, the shift operation shaft  43   b  rotates along with the movement of the shift operation lever  43   a . Preferably, the dimensions of the linkages are chosen such that the relative angular movements are sufficient to generate the resultant gear changes desired. 
   Next, the structure of the clutch actuator  60  will be explained in even more detail.  FIG. 10  shows an outline view of the clutch actuator  60 . Referring to  FIG. 10 , in the clutch actuator  60  according to the embodiment, a clutch motor  60   a  rotates when a signal is output by the engine control unit  95 , and this rotation causes rotation of a worm shaft  103 . Rotation of the worm shaft  103  is transmitted to a worm wheel  105  that is meshed with the worm shaft  103 . The worm wheel  105  is fixed to a crank shaft  110  such that the worm wheel  105  is coaxial with a crank shaft member  111  of the crank shaft  110 . In addition, an output rod  120  is fixed to a crank pin  112  of the crank shaft  110 . Accordingly, rotational motion of the crank shaft  110  is converted to reciprocating motion of the output rod  120  (in terms of the paper of  FIG. 10 , motion from the inward direction to the outward direction). Moreover, a clutch position detection sensor S 3  is provided at an end of the crank shaft member  111  of the crank shaft  110 . Other locations are possible. The clutch position detection sensor S 3  detects the rotation angle of the crank shaft  110  (in the embodiment, the crank shaft member  111 ). The rotation angle of the crank shaft  110  is used to detect the stroke of the output rod  120 , which is then used as basis for detecting the clutch position of the clutch mechanism  44 . The clutch position detection sensor S 3  corresponds to a rotation angle sensor of one embodiment of the invention. 
     FIG. 11  is a cross sectional view of the clutch actuator  60 .  FIG. 12  is a cross sectional view along line A-A of  FIG. 11 , and  FIG. 13  is a cross sectional view along line B-B of  FIG. 12 . Referring to  FIG. 13 , the clutch motor  60   a  is provided with a motor shaft  60   b . The motor shaft  60   b  is disposed to pass through the center of the clutch motor  60   a . A rear side end (the right side end in the figure)  60   c  of the motor shaft  60   b  is supported by a motor bearing  106 . An outer ring of the motor bearing  106  is fixed to a motor case  107  that houses the clutch motor  60   a . In the illustrated configuration, the motor bearing  106  is a ball bearing. 
   A front side end  60   d  (i.e., the left side end in the figure) of the motor shaft  60   b  is formed with a plurality of splines  60   e . The front side end  60   d  is inserted and fitted within a spline hole  103   a  formed in the worm shaft  103 , whereby the motor shaft  60   b  and the worm shaft  103  are spline fitted together. 
     FIG. 14  is an enlarged cross sectional view of the fitting portion of the motor shaft  60   b  and the worm shaft  103 . Referring to  FIG. 14 , the plurality of spine teeth  60   e  are formed in the front side end  60   d  of the motor shaft  60   b . In addition, a spline groove that engages with the spline teeth  60   e  is formed in the spline hole  103   a  of the worm shaft  103 . Further, the front side end  60   d  of the motor shaft  60   b  is inserted and fitted in the spline hole  103   a  of the worm shaft  103 , whereby the members are spline fitted together. 
   In addition, the front side end  60   d  of the motor shaft  60   b  is chamfered to form a rounded section  60   f . The round section  60   f  is formed to spread across an area further to the shaft center side of the motor shaft  60   b  than the bottom of the spline teeth  60   e . As a result of chamfering the front side end  60   d  of the motor shaft  60   b  in this way, even if the shaft center of the motor shaft  60   b  oscillates during rotation of the clutch motor  60   a , this can be absorbed at the worm shaft  103  side. Accordingly, transmission of oscillation to the worm shaft  103  can be greatly reduced or inhibited. Further, in one configuration, the bottom of the spline hole  103   a  of the worm shaft  103  is also chamfered to form a rounded section  103   b.    
   Moreover, a depth “a” of the spline hole  103   a  of the worm hole  103  is smaller than a diameter “b” of the front side end  60   d  of the motor shaft  60   b . In other words, a&lt;b. As a result of making the section of the motor shaft  60   b  fitted into the worm shaft  103  shorter in this way, oscillation of the shaft center of the motor shaft  60   b  during rotation thereof can be favorably absorbed at the worm shaft  103  side. 
   Referring to  FIG. 13 , a threaded section  103   c  (also refer to  FIG. 14 ) is formed in the worm shaft  103 . Further, the worm shaft  103  is supported by bearings  108  that are respectively disposed at the front side (the left side in the figure) and the rear side (the right side in the figure) of the threaded section  103   c . In one configuration, the two bearings  108  are both ball bearings. The outer rings of the bearings  108  can be fixed to a housing  115  of the clutch actuator  60 . In this manner, the front and rear of the threaded section  103   c  can be supported by the bearings  108 , whereby shaft oscillation of the worm shaft  103  during rotation is inhibited or greatly reduced, and rotation of the worm shaft  103  is stabilized. 
     FIG. 15  is an enlarged view of the section of the worm shaft  103  in which the threaded section  103   c  is formed. Referring to  FIG. 15 , the threaded section  103   c  of the worm shaft  103  is formed with a plurality of threads  103   d . Note that,  FIG. 15  is intended to explain the number of threads of the threaded section  103   c , but does not necessarily provide an accurate illustration of the shape of the threads  103   d  formed in the periphery surface of the thread section  103   c . Any suitable thread design can be used. 
   In one configuration, the distance moved in the axial direction when the worm shaft  103  rotates once will be taken to be lead  1 , the distance between adjacent threads  103   d  will be taken to be pitch p, and the number of threads in the threaded section  103   c  will be taken to be n. Given this, the relationship 1=n×p is established. As is apparent from  FIG. 15 , in this embodiment, the number of threads n of the threaded section  103   c  is 4. By setting the number of threads of the threaded section  103   c  formed in the worm shaft  103  to be 2 or more (multiple) in this way, it is possible to make the lead angle of the worm wheel  105  larger. Accordingly, the reduction gear ratio of the worm gear (the reduction gear ratio between the worm shaft  103  and the worm wheel  105 ) can be set smaller, thereby allowing the responsiveness of the output rod  120  to be improved. As a result, responsiveness of the disengagement-engagement operation of the clutch mechanism  44  can be improved. Further, the transmission efficiency of multiple threads is better than that of a single thread, and thus the reciprocating motion of the output rod  120  (refer to  FIG. 10  and  FIG. 11 ) rapidly follows the rotational motion of the clutch motor  60   a . In other words, the rotational motion of the clutch motor  60   a  can be efficiently converted to reciprocating movement of the output rod  120  and output loss of the clutch motor  60   a  can be reduced. As a result, the responsiveness of the clutch disengagement-engagement operation can be improved. 
   Referring to  FIG. 13 , the threaded section  103   c  of the worm shaft  103  meshes with gear teeth  105   a  of the worm wheel  105 . The worm wheel  105  preferably has a substantially ring-like shape, with the gear teeth  105   a  formed in the periphery surface thereof. Torque limiters  105   b  are provided in a shaft center section of the worm wheel  105 . The torque limiters  105   b  may be provided with a transmission plate (not shown), which is spline fitted to the crank shaft member  111  of the crank shaft  110  (refer to  FIG. 12 ) and which rotates along with the crank shaft member  111 , and an inner clutch (not shown) that is provided at the external edge of the transmission plate. When the driving force applied to the worm wheel  105  becomes equal to or greater than a predetermined value, the transmission plate and the inner clutch slip with respect to each other, thereby preventing excessive driving force from being transmitted to the crank shaft member  111 . In some embodiments, the torque limiters  105   b  may be omitted and the worm wheel  105  may be integrally fixed to the crank shaft  110 . 
   Referring to  FIG. 12 , the worm wheel  105  is fixed to an end of the crank shaft member  111  of the crank shaft  110  (the right side end in  FIG. 12 ). The worm wheel  105  is fixed to the crank shaft member  111  such that the worm wheel  105  is coaxial with the crank shaft member  111  and is parallel with a crank arm  114  of the crank shaft  110 . A side surface of the worm wheel  105  is perpendicular with respect to the crank shaft member  111 . The crank arm  114  is defined with a pair of arm sections  113  that are positioned to face each other, and a crank pin  112  that is coupled to the pair of arm sections  113 . Further, the crank shaft member  111  extends from substantially the center of the arm sections  113  toward the side (i.e., in the left-right direction of  FIG. 12 ). 
   The crank shaft member  111  of the crank shaft  110  is rotatably fixed at both ends of the housing  115 . In addition, a Belleville spring  116  is interposed between the worm wheel  105  and the housing  115  on the crank shaft member  111 . 
   The crank shaft member  111  is supported by a pair of bearings  117 ,  118 . The outer rings of these two bearings  117 ,  118  are both fixed to the housing  115 . In one configuration, the pair of bearings  117 ,  118  are both ball bearings. Of the two bearings, the bearing  117  is closer to the worm wheel  105 , and is disposed between the worm wheel  105  and the right-side arm section  113 . The bearing  117  preferably is a double sealed bearing that has seals on both the side of an inner ring  117   a  and an outer ring  117   b . Bearing grease is enclosed in the inside of the bearing  117 . Grease (e.g., molybdenum grease) is used between the above described worm shaft  103  and the worm wheel  105 . However, this grease can potentially have a detrimental impact on the bearing  117  that is adjacent to the worm wheel  105 . More specifically, if the above grease were to adhere to the inside of the bearing  117 , it is possible that performance of the bearing  117  would be impaired. Thus, in one embodiment, a double sealed bearing is used as the bearing  117  and the bearing grease is substantially enclosed within the bearing. As a result, the grease used between the worm shaft  103  and the worm wheel  105  is less likely to enter into the bearing  117  and have a detrimental impact. 
   Further, the bearing  118  can be disposed to the left side of the left-side arm section  113 . Accordingly, the crank arm  114  can be interposed between the bearing  117  and the bearing  118 . By positioning the crank arm  114  between the two bearings  117 ,  118 , it is possible to stabilize rotation of the crank shaft member  111 . In one configuration, the above described bearing  117  corresponds to a first bearing, and the bearing  118  corresponds to a second bearing. 
   Moreover, the bearing  117  disposed in the vicinity of the worm wheel  105  preferably is a larger ball bearing than the bearing  118  that is disposed at a position away from the worm wheel  105 . The bearing  117  disposed closer to the worm wheel  105  is subjected to a comparatively large force acting in the radial direction. However, if a large ball bearing is used for the bearing  117 , the crank shaft member  111  can be supported more stably. Accordingly, it is favorable if a large ball bearing is used in this manner. On the other hand, the bearing  118  disposed at the position away from the worm wheel  105  is subjected to a comparatively small force acting in the radial direction. Accordingly, it is possible to use a smaller bearing (has a smaller diameter) relative to the bearing  117 . 
   Further, the right side end of the crank shaft member  111  of the crank shaft  110  can be provided with the clutch position detection sensor S 3  (refer to  FIG. 10 ). The clutch position detection sensor S 3  detects the rotation angle of the worm wheel  105 , and uses it to detect the stroke of the output rod  120 , which is used as a basis for detecting the clutch position of the clutch mechanism  44 . Other arrangements and/or proxies also can be used. 
   Referring to  FIG. 11 , the illustrated output rod  120  is fixed to the crank pin  112  at the lower side of the illustrated crank shaft  110 . The illustrated output rod  120  includes a base  120   a  formed with a tapped hole and a rod  120   b  formed with a threaded section. The threaded section of the rod  120   b  can be screwed into the tapped hole of the base  120   a . In addition, a lock nut  121  and a nut  122  can be tightened onto the threaded section of the rod  120   b . A section of the lower side crank pin  112  to which the output rod  120  is fixed, as shown in  FIG. 12 , preferably overlaps with the crank shaft member  111  in the diameter direction. More specifically, when the crank shaft member  111  is viewed from the outside in the axial direction (i.e., the right side in  FIG. 12 ), a section of the crank pin  112  overlaps with the crank shaft member  111 . As a result of the overlap with the crank shaft member  111 , the attachment radius of the output rod  120  (i.e., the distance from the center of the crank shaft member  111  to the point where the output rod  120  is attached) can be reduced. As a result, the rotation angle of the worm wheel  105  that is required to move the output rod  120  the same stroke can be increased. Accordingly, the worm wheel  105  can be made smaller, and responsiveness can be improved. Further, size reduction of the worm wheel  105  can also be achieved. 
   The illustrated configuration allows adjustment of the length of the output rod  120 . More specifically, the rod  120   b  can be rotated with respect to the base  120   a  in order to change the length of the output rod  120 . After the length has been changed, the lock nut  121  and the nut  122  can be tightened on the base  120   a  side, thereby fixing the position of the output rod  120 . 
   A tip of the output rod  120  can be provided with, or connected to, a piston  125 . The piston  125  can slide in the axial direction of the output rod  120  within a cylinder  123  (i.e., in the left-right direction in the figure). In the illustrated configuration, a left side section of the piston  125  in the cylinder  123  defines an oil chamber  126  that is filled with hydraulic oil or the like. The oil chamber  126  can be connected with a reservoir (not shown) via a tank connecting member  129 . Other configurations, including integrated reservoirs, also can be used. 
   In addition, one end of an assist spring  130  can be secured to the crank pin  112  at the upper side of the crank arm  114 , or the crank shaft  110 . The other end of the assist spring  130  preferably is secured to the housing  115  in any suitable manner. The assist spring  130  assists rotation of the crank shaft member  111  of the crank shaft  110 , thereby assisting the stroke of the output rod  120 . 
   In the illustrated embodiment, as shown in  FIG. 11 , the assist spring  130  advantageously is attached to the crank arm  112 . Accordingly, the assist spring  130  and the crank arm  114  are substantially positioned in a single plane. Arranging and positioning the assist spring  130  in this manner makes it possible to shorten the axial direction length of the clutch actuator  60  and thus reduce the size of the clutch actuator  60 . 
   When the clutch mechanism  44  (refer to  FIG. 7 ) is to be switched from an engaged state to a disengaged state, the clutch motor  60   a  is driven, thereby causing the coupled worm shaft  103  to rotate. Rotation of the worm shaft  103  is transmitted to the worm wheel  105  that is meshed with the worm shaft  103 , and the worm wheel  105  rotates. When the worm wheel  105  rotates, the crank shaft member  111  of the crank shaft  110  also rotates. Then, the rotational motion of the worm wheel  105  is converted to linear motion of the output rod  120  by the crank shaft  110 , and the output rod  120  moves in the leftward direction of  FIG. 11 . 
   The output rod  120  that moves linearly in the leftward direction of  FIG. 11  pushes the piston  125 , thereby generating hydraulic pressure in the oil chamber  126 . The generated hydraulic pressure is transmitted to the piston  463  (refer to  FIG. 7 ) from a hydraulic fluid outlet  115   a  formed in the housing  115  via an oil hose (not shown) or the like. Then, the hydraulic pressure drives the push rods  461 ,  455  (refer to  FIG. 7 ) to disengage the clutch. Advantageously, the linear motion of the output rod  120  is assisted by the assist spring  130 . 
   With this embodiment, it is possible to automatically disengage the clutch mechanism  44  by driving the clutch motor  60   a , and also to manually disengage the clutch mechanism  44 . Referring to  FIG. 11 , a guide tube  128  for a clutch wire  127  is provided at the lower side of the output rod  120  in the housing  115 . One end of the clutch wire  127  is fixed to the lower side crank pin  112 . As a result of the guide tube  128 , the clutch wire  127  is positioned to run in a direction that extends leftwards and downwards in the figure. When a clutch lever, or the like, (not shown) is manually operated, the clutch wire  127  is pulled in the longitudinal direction of the guide tube  128  (i.e., the leftward-downward direction in  FIG. 11 ), thereby rotating the crank shaft  110  and moving the output rod  120  in the leftward direction of  FIG. 11 . 
   As shown in  FIG. 11 , the clutch motor  60   a , which is connected to the worm shaft  103 , and the output rod  120 , which is fixed to the crank pin  112 , extend in substantially the same direction. However, in order to position the clutch motor  60   a  and the output rod  120  such that there is little or no interference therebetween, the clutch motor  60   a  and the output rod  120  have been positioned apart from each other to a certain extent. In the illustrated embodiment, as shown in  FIG. 12 , the worm wheel  105  is fixed to the end of the crank shaft member  111  of the crank shaft  110  separately from the crank arm  114 . Further, the worm wheel  105  is disposed to be generally parallel to the crank arm  114  and generally coaxial with the crank shaft member  111 . As a result, even if the clutch motor  60   a , which is disposed close to the worm wheel  105 , and the output rod  120 , which is fixed to the crank pin  112 , are positioned so that they do not substantially interfere with each other, the length of the crank shaft member  111  can be shortened. As a result, size and the weight increase of the crank shaft  110  are limited, and any size and weight increase of the clutch actuator  60  itself is also limited. 
   With reference now to  FIG. 16  and  FIG. 17 , one configuration of the automated transmission control device will be explained in more detail. As shown in  FIG. 16 , a shift switch SW 1 , for example, can be provided on a grip on the left side of the steering handlebar  11 . The shift switch SW 1  can comprise an up-shift switch SW 1   a   1  and a down-shift switch SW 1   a   2 . The rider can operate the shift switch SW 1  as necessary or desired to upshift and/or downshift through the gears between a first gear speed and a fastest speed gear (for example, a sixth gear speed). Further, a selection switch SW 2 , an indicator switch SW 3 , a horn switch SW 4  and a light switch SW 5  also can be provided on the left grip. Other configurations are possible. In the illustrated configuration the selection switch SW 2  can be used to select whether the gear shift operation is performed using a semi-automatic mode or a fully-automatic mode. 
   Referring to  FIG. 17 , switching of the shift mechanism  43  and the clutch mechanism  44  preferably are both performed by the automated transmission control device  50 . Further, the motorcycle  1  can be provided with, in addition to the shift position detection device S 2  (refer to  FIG. 6 ) of the shift actuator  70 , the clutch position detection device S 3  (refer to  FIG. 10 ) of the clutch actuator  60 , the engine rotation speed sensor S 30 , and a vehicle speed sensor S 5 , etc. 
   In one preferred configuration, the engine control device  95  controls operation of the clutch actuator  60  and the shift actuator  70  based on data detected by the various sensors and with information provided by the shift switch SW 1 . More specifically, predetermined programs can be pre-stored in the engine control device  95 . In addition to the programs, other operating circuits also can be used to automatically perform a series of shift operations including disengaging the clutch mechanism  44 , switching the gears of the transmission  40 , and reengaging the clutch mechanism  44 . 
   As described above, in one embodiment of the clutch actuator  60 , the motor shaft  60   b  and the worm shaft  103  can be coupled together by a spline fit. Accordingly, as compared to constructions in which both side surfaces of the I-cut section transmit driving force, driving force can be reliably transmitted. Further, the rotation direction space between the motor shaft  60   b  and the worm shaft  103  can be made smaller. As a result, the rotation of the worm shaft  103  rapidly follows the rotation of the clutch motor  60   a , thereby improving the responsiveness of the clutch disengagement-engagement operation. 
   Further, in one embodiment of the clutch actuator  60 , the rear side end  60   c  of the motor shaft  60   b  can be supported by the motor bearing  106 . In addition, the front and rear of the threaded section  103   c  of the worm shaft  103 , which is coupled with the motor shaft  60   b , can be supported by bearings  108 ,  108 . In this manner, the motor shaft  60   b  and the worm shaft  103  can be supported by three bearings (i.e., bearings  106 ,  108 ,  108 ) in the axial direction. As described above, with the illustrated clutch actuator  60 , oscillation of the shaft center of the motor shaft  60   b  can be absorbed by the worm shaft  103 . Accordingly, the motor shaft  60   b  and the worm shaft  103  can be supported by the 3 bearings  106 ,  108 ,  108 . Further, oscillation of the shaft centers of the motor shaft  60   b  and the worm shaft  103  during rotation thereof can be inhibited such that rotation of the threaded section  103   c  is stabilized and the rotation is reliably transmitted to the worm wheel  105 . Moreover, because the threaded section  103   c  of the worm shaft  103  is positioned between the bearings  108 ,  108 , shaft oscillation of the threaded section  103   c  is greatly reduced or eliminated. 
   In one configuration of the clutch actuator  60 , the motor shaft  60   b  is coupled to the worm shaft  103  with a spline fit. In particular, the spline teeth  60   e  can be formed on the motor shaft  60   b  and with a spline hole  103   a  is formed in the worm shaft  103  or vice versa. The shaft formed with the spline teeth has a smaller external diameter than the shaft formed with the spline hole. More specifically, in the case of the shaft formed with the spline teeth, the distance between the shaft center and the external circumference edge of the spline teeth is the external diameter. On the other hand, in the case of the shaft formed with the spline hole, the distance between the shaft center and the internal circumference surface of the spline hole (which is generally the same as the external circumference edge of the spline teeth) is the internal diameter. Thus, the shaft formed with the spline hole has an external diameter that is larger than the shaft formed with the spline teeth. Accordingly, since the spline teeth  60   e  are formed on the motor shaft  60   b , it is possible to make the motor shaft  60   b  thinner and size increase of the clutch motor  60   a  can be limited. 
   In one configuration of the clutch actuator  60 , the depth a of the spline hole  103   a  of the worm shaft  103  is smaller than the diameter b of front end side  60   d  of the motor shaft  60   b . Accordingly, oscillation of the shaft center of the motor shaft  60   b  during rotation can be favorably absorbed at the coupling. 
   In one configuration of the clutch actuator  60 , the front end  60   d  of the motor shaft  60   b  can be chamfered. Thus, when the clutch motor  60   a  is rotating, even if the shaft center of the motor shaft  60   b  oscillates, this oscillation can be absorbed. As a result, transmission of oscillation to the worm shaft  103  can be reduced or inhibited. 
   Moreover, the rounded section  60   f  formed in the front side end  60   d  of the motor shaft  60   b  by chamfering is formed to extend across an area further to the shaft center side of the motor shaft  60   b  than the bottom of the spline teeth  60   e . Accordingly, oscillation of the shaft center of the motor shaft  60   b  during rotation can be favorably absorbed at the worm shaft  103  side. 
   Although the present invention has been described in terms of a certain embodiment, other embodiments apparent to those of ordinary skill in the art also are within the scope of this invention. Thus, various changes and modifications may be made without departing from the spirit and scope of the invention. For instance, various components may be repositioned as desired. Moreover, not all of the features, aspects and advantages are necessarily required to practice the present invention. Accordingly, the scope of the present invention is intended to be defined only by the claims that follow.