Patent Publication Number: US-8109833-B2

Title: Vehicle provided with torque damper

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2008-039404 filed on Feb. 20, 2008 the entire contents of which are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a power unit that is provided with a torque damper for absorbing excessive torque occurring in a torque transmission path between a drive rotating shaft and a driven rotating shaft. The power unit is a vehicle power unit that includes e.g. an internal combustion engine and a transmission adapted to receive power produced by the engine. 
     DESCRIPTION OF BACKGROUND ART 
     A power unit is known that is provided with a torque damper for absorbing excessive torque occurring in a power transmission path between a drive rotating shaft and a driven rotating shaft. The torque damper includes an input member adapted to receive torque from the drive rotating shaft and an output member transmitting torque transmitted from the input member to the driven rotating shaft and absorbing excessive torque by relative rotation between the input member and output member. See, for example, Japanese Patent Laid-open No. Sho 58-191680. 
     In addition, a vehicle power unit is known that is provided with an internal combustion engine and with a transmission that is composed of a swash plate type hydrostatic continuously variable transmission. See, for example, Japanese Patent Laid-open No. 2005-248838 and Japanese Patent Laid-open No. 2005-263143. 
     In a power unit including a torque damper mounted to a drive rotating shaft (a driven rotating shaft) rotatably supported by a pair of bearings, the torque damper and an output rotating body (or an input rotating body rotatably driven by a drive rotating shaft) rotatably driving a driven rotating shaft may be disposed so as to put one of the pair of bearings therebetween in the axial direction of the driving rotating shaft (or the driven rotating shaft). In such a case, since the whole of the torque damper is disposed between the pair of bearings, the length of the driving rotating shaft (or the driven rotating shaft) between the pair of bearings is increased. This needs to increase the length of the rotating shaft and the rigidity of the rotating shaft to suppress the occurrence of bending deformation of the rotating shaft due to the provision of the torque damper. Consequently, the weight of the rotating shaft may increase. 
     If, to position one of the pair of bearings, the driving rotating shaft (or the driven rotating shaft) is integrally formed with a positioning portion or a dedicated positioning member is used, the costs are increased. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of an embodiment of the present invention to axially downsize a rotating shaft to which a torque damper is mounted and reduce the weight of the rotating shaft in a power unit provided with the torque damper. In addition, it is an object of an embodiment of the present invention to reduce costs by positioning bearings by use of a torque damper and an input rotating body or an output rotating body. Further, it is an object of an embodiment of the present invention to increase durability of bearings by allowing the bearings to support a rotating shaft to which a torque damper is mounted, via an input member or an output member of the torque member. 
     According to an embodiment of the present invention, a power unit (P) is provided with a torque damper ( 100 ) for absorbing excessive torque occurring in a torque transmission path between a drive rotating shaft ( 72 ) and a driven rotating shaft ( 90 ). The torque damper ( 100 ) includes an input member ( 101 ) adapted to receive torque of the drive rotating shaft ( 72 ) and an output member ( 102 ) adapted to transmit torque from the input member ( 101 ) to the driven rotating member ( 90 ) and for absorbing the excessive torque by relative rotation between the input member ( 101 ) and the output member ( 102 ). The driven rotating shaft ( 90 ) is rotatably supported by a pair of bearings ( 94 ,  95 ). An input rotating body ( 93 ) applying the torque of the drive rotating shaft ( 72 ) to the input member ( 101 ) is provided on the input member ( 101 ) rotatably supported by the driven rotating shaft ( 90 ). The output member ( 102 ) is rotated integrally with the driven rotating shaft ( 90 ). In the torque damper ( 100 ), a torque-absorbing portion ( 100   a ) for absorbing the excessive torque is composed of an input damper portion (projection) ( 101   a ) of the input member ( 101 ) and an output damper portion (projection) ( 102   a ) of the output member ( 102 ) and is disposed between the pair of bearings ( 94 ,  95 ) in an axial direction of the driven rotating shaft ( 90 ). One ( 94 ) of the pair of bearings ( 94 ,  95 ) is disposed between the input damper portion (projection) ( 101   a ) and the input rotating body ( 93 ) in the axial direction. 
     According to an embodiment of the present invention, a power unit is provided with a torque damper for absorbing excessive torque occurring in a torque transmission path between a drive rotating shaft and a driven rotating shaft, the torque damper including an input member adapted to receive torque of the drive rotating shaft and an output member adapted to transmit torque from the input member to the driven rotating member and for absorbing the excessive torque by the relative rotation between the input member and the output member. The drive rotating shaft is rotatably supported by a pair of bearings. The input member can be rotated integrally with the drive rotating shaft. An output rotating body for outputting torque of the output member to the driven rotating shaft is provided on the output member rotatably supported by the drive rotating shaft. In the torque damper, a torque-absorbing portion for absorbing the excessive torque is composed of an input damper portion of the input member and an output damper portion of the output member and is disposed between the pair of bearings in an axial direction of the drive rotating shaft. One of the pair of bearings is disposed between the output damper portion and the output rotating body in the axial direction. 
     According to an embodiment of the present invention, in the power unit (P), the input member ( 101 ) is an input cam member ( 101 ). The output member ( 102 ) is an output cam member ( 102 ) supported by the driven rotating shaft ( 90 ) so as to be movable in the axial direction. The input damper portion (projection) ( 101   a ) is the input cam portion (projection) ( 101   a ) forming an input cam surface ( 101   b ). The output damper portion (projection) ( 102   a ) is an output cam portion (projection) ( 102   a ) forming an output cam surface ( 102   b ) in abutment against the input cam surface ( 101   b ). The one ( 94 ) of the bearings is positioned in the axial direction by the input cam portion ( 101 ) and by the input rotating body ( 93 ). 
     According to an embodiment of the present invention, the input member is an input cam member supported by the drive rotating shaft so as to be movable in the axial direction. The output member is an output cam member. The input damper portion is an input cam portion (projection) forming an input cam surface. The output damper portion is an output cam portion (projection) forming an output cam surface in abutment against the input cam surface. The one of the bearings is positioned in the axial direction by the output cam portion (projection) and by the output rotating body. 
     According to an embodiment of the present invention, the one ( 94 ) of the bearings rotatably supports the driven rotating shaft ( 90 ) via the input member ( 101 ). 
     According to an embodiment of the present invention, the one of the bearings rotatably supports the drive rotating shaft via the output member. 
     According to an embodiment of the present invention, in the torque damper provided on the driven rotating shaft rotatably supported by the pair of bearings, the torque absorbing portion for absorbing excessive torque and the input rotating body are dividedly arranged on both the sides of one of the pair of bearings in the axial direction. Since the length of the driven rotating shaft between the pair of bearings is reduced, the bending deformation of the driven rotating shaft can be suppressed and also the entire length of the driven rotating shaft can be reduced compared with the case where the torque damper is disposed outside of the pair of bearings in the axial direction. Consequently, the driven rotating shaft onto which the torque damper is mounted can axially be downsized and the driven rotating shaft can be reduced in weight, in turn which can downsize the power unit in the axial direction of the driven rotating shaft. 
     According to an embodiment of the present invention, in the torque damper mounted to the drive rotating shaft rotatably supported by the pair of bearings, the torque absorbing portion for absorbing excessive torque and the output rotating body are dividedly arranged on both the sides of one of the pair of bearings in the axial direction. Since the length of the drive rotating shaft between the pair of bearings is reduced, the bending deformation of the drive rotating shaft can be suppressed and also the entire length of the drive rotating shaft can be reduced compared with the case where the torque damper is disposed outside of the pair of bearings in the axial direction. Consequently, the drive rotating shaft onto which the torque damper is mounted can axially be downsized and the drive rotating shaft can be reduced in weight, in turn which can downsize the power unit in the axial direction of the drive rotating shaft. 
     According to an embodiment of the present invention, the one of the pair of bearings rotatably supporting the driven rotating shaft or the drive rotating shaft is positioned by use of the input cam portion and the input rotating body, or of the output cam portion and the output rotating body. Thus, a dedicated portion or member for positioning the one of the bearings becomes unnecessary, which reduces costs. 
     According to an embodiment of the present invention, one of the bearings rotatably supporting the driven rotating shaft or the drive rotating shaft rotatably supports the driven rotating shaft or the drive rotating shaft via the input member or output member of the torque damper. Thus, the diameter of one of the bearings can be increased. Consequently, the capacity of one of the bearings, i.e., the load capability of the bearing can be increased, which can increase the durability thereof. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a left lateral view illustrating a major portion of a motorcycle provided with a sound insulation structure embodying the present invention; 
         FIG. 2  is a lateral view illustrating a major portion of a power unit provided for the motorcycle of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of  FIG. 2  taken along line of  FIG. 2 ; and 
         FIG. 4  is an enlarged view illustrating the vicinity of an output side torque damper of the power unit in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will hereinafter be described with reference to  FIGS. 1 through 4 . 
     Referring to  FIG. 1 , in an embodiment, a power unit P embodying the present invention is mounted on a motorcycle V as a vehicle. 
     It is to be noted that in the embodiment a left and right direction and a back and forth direction correspond to the left and right direction and back and forth direction, respectively, of the motorcycle V mounted with the power unit P thereon and an up-and-down direction is a vertical direction. 
     An axial direction is a direction of a rotational centerline with respect to each of rotational axes described later. An axial direction of a crankshaft  33  (see  FIG. 3 ) included in an internal combustion engine E corresponds to the left and right direction in the embodiment. If one of the right and left is one direction of the axial directions of the crankshaft  33 , the other of the right and left is the other direction of the axial directions of the crankshaft  33 . 
     The motorcycle V includes a body frame F including a head pipe  1 , a main frame  2  and a down tube  3 ; the power unit P carried by the body frame F; a front wheel  6  rotatably supported by a front fork  4  steerably supported by the head pipe  1 ; a rear wheel  7  rotatably supported by a swing arm  5  swingably supported by the main frame  2 ; and a fuel tank  8  and an occupant seat  9  carried by the body frame F. The power unit P is disposed below the occupant seat  9  in the motorcycle V. 
     With additional reference to  FIGS. 2 and 3 , the power unit P includes an internal combustion engine E, which is a water-cooled multi-cylinder 4-stroke internal combustion engine, and a transmission apparatus M including a transmission  60  (see  FIG. 3 ) adapted to receive power from the crankshaft  33 . In addition, the power unit P is such that the internal combustion engine E is integrally joined to the transmission apparatus M and outputs power used to drive the rear wheel  7  as a drive target. 
     The internal combustion engine E carried by the body frame F in a transverse-mounted arrangement where the crankshaft  33  is oriented in a vehicle-width direction is a V-type internal combustion engine having a front bank B 1  and a rear bank B 2 . The transmission  60  is a hydrostatic continuously variable transmission including a hydraulic pump  61  and a hydraulic motor  62 . 
     The power produced by the internal combustion engine E is received by the transmission apparatus M and then transmitted through a power takeout shaft  91  of the transmission apparatus M, and through a terminal transmission mechanism  140  to the rear wheel  7  as the drive wheel. The terminal transmission mechanism  140  is connected to the power takeout shaft  91  via a universal joint  141  and includes a drive shaft  142  housed in a swing arm  5 . 
     The internal combustion engine E includes an engine main body configured to include a cylinder block  10  having a plurality of, two in the embodiment, cylinders  10   a  arranged to form a pair of V-shaped banks B 1 , B 2 . In addition, the engine main body includes a pair of cylinder heads  11  joined to the corresponding upper ends of cylinders  10   a  included in the respective banks B 1 , B 2 . Further, the engine main body includes a pair of cylinder head covers  12  joined to the corresponding upper ends of the cylinder heads  11 ; and a crankcase  13  joined to the lower end of the cylinder block  10 . 
     The respective structures of the cylinders  10   a , the cylinder heads  11  and the cylinder head covers  12  in the banks B 1 , B 2  are basically the same in both the banks B 1 , B 2 . Therefore, such structures are hereinafter described with main reference to the structure of the rear bank B 2 . 
     With reference to  FIGS. 2 and 3 , the cylinder head  11  includes a combustion chamber  21 , an intake port  24 , an exhaust port  25 , an ignition plug  26 , an intake valve  27  and an exhaust valve  28 . The combustion chamber  21  faces a piston  20  in a cylinder axial direction. The intake port  24  is adapted to lead an air-fuel mixture into the combustion chamber  21 . The air-fuel mixture is composed of intake air from an air intake device  22  and fuel from a fuel injector (not shown). The air intake device  22  has a throttle body  22   a  connected to the cylinder head  11 . The exhaust port  25  is adapted to lead exhaust gas from the combustion chamber  21  to the exhaust device  23  having an exhaust pipe  23   a  connected to the cylinder head  11 . The ignition plug  26  faces the combustion chamber  21 . The intake valve  27  and the exhaust valve  28  are adapted to open and close the intake port  24  and the exhaust port  25 , respectively. 
     A valve train  30  for opening and closing the intake valve  27  and the exhaust valve  28  includes a cam shaft  30   a  with a valve-operating cam  30   b ; and rocker arms  30   c  and  30   d  being in contact with the intake valve  27  and the exhaust valve  28 , respectively, and driven by the valve-operating cam  30   b  for swing. The valve-operating cam  30   b  is adapted to open and close the intake valve  27  and the exhaust valve  28  via the rocker arms  30   c  and  30   d  swingably supported by the rocker shafts  30   e  and  30   f , respectively. 
     A valve-operating transmission mechanism  31  in which the cam shaft  30   a  is rotatably driven by torque of the crankshaft  33  includes a drive sprocket  31   a  attached to both shaft-end portions  33   a ,  33   b  of the crankshaft  33 ; a cam sprocket  31  attached to the camshaft  30   a ; and a chain  31   c  wound around both the sprockets  31   a ,  31   b.    
     The crankcase  13  is a left-right-split crankcase formed by joining together a pair of case half-bodies  13   a ,  13   b  which are a plurality of vehicle-widthwise (also of left-right directionally) split case portions. The crankshaft  33  connected to each piston  20  via a connecting rod  32  is housed in a crank chamber  34  defined by the crankcase  13  and rotatably supported by both the case half-bodies  13   a ,  13   b  via a pair of respective main bearings  35 . The crankcase  13  partially forms a transmission case Mc of the transmission apparatus M. 
     The internal combustion engine E includes a pair of covers  14  and  15  fastened respectively to the left and right case half bodies  13   a  and  13   b  with a large number of bolts. 
     One shaft-end portion  33   a  of the crankshaft  33  extending to the left from the inside of the crank chamber  34  extends into a transmission chamber  36  which is a left chamber defined by the left case half body  13   a  and the left cover  14 . The other shaft-end portion  33   b  projecting to the right from the inside of the crankshaft  34  extends into an auxiliary machinery chamber  37  which is a right chamber defined by the right case half body  13   b  and the right cover  15 . An input side transmission mechanism  50  of the transmission apparatus M and a drive sprocket  41   a  are mounted to the shaft-end portion  33   a . A chain  41   c  of the transmission mechanism  41  for driving an oil pump  40  is wound around the drive sprocket  41   a . In addition, an alternator  42  is mounted to the shaft-end portion  33   b . The transmission apparatus M, the oil pump  40  and the alternator  42  are driven units each driven by torque of the crankshaft  33 . 
     The transmission apparatus M includes the transmission  60  rotatably driven by torque of the crankshaft  33 ; the input side transmission mechanism  50  adapted to transfer torque of the crankshaft  33  to the transmission  60 ; an output side transmission mechanism T which receives the torque outputted from the transmission  60 ; and the transmission case Mc defining a transmission chamber  38  for housing the transmission  60  and both the transmission mechanisms  50 , T. 
     Torque (or power) produced by the internal combustion engine E is transmitted from the crankshaft  33  or an engine output shaft to the rear wheel  7  (see  FIG. 1 ) via a torque transmission path formed by the input side transmission mechanism  50 , the transmission  60 , the output side transmission mechanism T and the terminal transmission mechanism  140  (see  FIG. 1 ). This torque transmission path is composed of an input side torque transmission path between the crankshaft  33  and the transmission  60 , the transmission  60  itself, and the output side torque transmission path between the transmission  60  and the rear wheel  7 , with the transmission  60  interposed therebetween. In this way, the input side torque transmission path is formed of the input side transmission mechanism  50  and the output side torque transmission path is formed of the output side transmission mechanism T and the terminal transmission mechanism  140  adapted to receive torque from the output side transmission mechanism T. 
     The transmission case Mc includes the pair of case half bodies  13   a ,  13   b ; a left cover  14  also serving as a left transmission cover; a right cover  16 , which is a right transmission cover joined to the right case half body  13   b ; and a gear cover  17 , which is a cover joined to the rear portion of the left case half body  13   a  along with a bearing housing  96 . Of the first and second transmission chambers  36 ,  37  forming the transmission chamber  38 , the first transmission chamber  36  in which the transmission  60 , the input side transmission mechanism  50  and a major portion of the output side transmission mechanism T excluding a portion such as a running clutch  70  and the like are arranged is formed of both the case half bodies  13   a ,  13   b  and the left cover  14 . The second transmission chamber  37  as a clutch chamber in which the running clutch  70  is arranged is formed of the right case half body  13   b  and the right cover  16 . 
     The crankcase  13 , the left cover  14 , both the right covers  15 ,  16  and the gear cover  17  constitute a power case of the power unit P. 
     The input side transmission mechanism  50  includes a gear mechanism  51 ,  52  adapted to transmit torque of the crankshaft  33  to the transmission  60 ; and an input side torque damper  54  for absorbing excessive torque produced in the input side torque transmission path. 
     The gear mechanism  51 ,  52  includes a drive gear  51  and a driven gear  52 . The drive gear  51  is joined to the crankshaft  33  via a transmission mechanism including a collar  53  spline-fitted to the shaft-end portion  33   a  and an input cam member  55  spline-fitted to the collar  53 . The driven gear  52  is provided to rotate integrally with a pump housing  61   a  as an input rotating body of the transmission  60 . The driving gear  51  is rotatably carried on the collar  53  and is mounted to the shaft-end portion  33   a  via the collar  53 . 
     The torque damper  54  of cam type includes an input cam member  55  as an input member; a drive gear  51  which is an output cam member as an output member; and a damper spring  56  as a biasing member. The input cam member  55  is axially movable with respect to the shaft-end portion  33   a . The drive gear (output member)  51  is engaged with the input cam member  55  so as to receive torque of the crankshaft  33  via the input cam member  55 . The damper spring  56  axially biases the input cam member  55  to bring it into abutment against the drive gear  51 . The damper spring  56  composed of a plurality of disc springs is disposed between a spring receiver  57  held by the collar  53  and the input cam member  55 . The input cam member  55  has an input cam portion (projection)  55   a  and the drive gear (output member)  51  has an output cam portion (projection)  51   a . The input cam portion (projection)  55   a  and the output cam portion (projection)  51   a  are brought into axial and circumferential abutment against each other by the biasing force of the damper spring  56  and can circumferentially slide relatively to each other. 
     The torque damper  54  integrally rotates the input cam member  55  and the drive gear (output member)  51  when torque not greater than a predetermined first set torque is applied between the input cam member  55  and the drive gear  51 . During deceleration of the motorcycle (see  FIG. 1 ) or of the internal combustion engine E, or during engine-braking of the motorcycle V, application of excessive torque exceeding the first set torque causes circumferential slip between the input cam member  55  and the drive gear  51 . Thus, the input cam member  55  and the drive gear (output member)  51  are relatively rotated while the input cam member  55  driven by the output cam portion (projection)  51   a  is axially moved against the biasing force of the damper spring  56 , thereby absorbing the excessive torque. 
     The transmission  60  is the same as that disclosed in Japanese Patent Laid-open No. 2005-248838 and Japanese Patent Laid-open No. 2005-263143 and includes a swash plate type hydraulic pump  61 ; a swash plate type hydraulic motor  62 ; a valve mechanism  63  for controlling flow of hydraulic oil between the hydraulic pump  61  and the hydraulic motor  62 ; a transmission output shaft  64  as an output rotating body; and an input side clutch  65  as a start clutch for switching stoppage and rotation of the transmission output shaft  64 . 
     The hydraulic pump  61  includes a pump housing  61   a  rotatably carried by the left cover  14  via a bearing  66 ; a pump swash plate  61   b  housed in the pump housing  61   a ; a pump body  61   c  disposed to axially face the pump swash plate  61   b ; and a plurality of pump plungers  61   d  fitted to the pump body  61   c  so as to be able to reciprocate and driven by the pump swash plate  61   b  to take in and discharge hydraulic oil. 
     The hydraulic motor  62  includes a motor housing  62   a  secured to the right case half body  13   b ; a support member  62   e  swingably supported by a spherical support surface formed on the motor housing  62   a ; and a motor swash plate  62   b  rotatably supported by the support member  62   e . The hydraulic motor  62  further includes a motor body  62   c  disposed to axially face the motor swash plate  62   b ; a motor plunger  62   d  fitted to the motor body  62   c  to be able to reciprocate and driven by hydraulic oil discharged from the hydraulic pump  61 ; and an electric motor  67  as an actuator for driving the support member  62   e . The support member  62   e  is driven and swung by the electric motor  67  to change the inclination angle of the motor swash plate  62   b . This changes the rotational speed of the transmission output shaft  64  relative to the rotational speed of the crankshaft  33  to thereby change the rotational speed of the crankshaft  33 . 
     The valve mechanism  63  axially interposed between the hydraulic pump  61  and the hydraulic motor  62  includes a plurality of spools  63   a  and control rings  63   b . The spools  63   a  are adapted to control the flow of hydraulic fluid which is oil discharged by the oil pump  40  and supplied from an oil passage formed in the right cover  16  through an oil passage  68  formed in the transmission output shaft  64  and the flow of hydraulic fluid between the hydraulic pump  61  and the hydraulic motor  62 . The control rings  63   b  control the positions of the spools  63   a  in accordance with the rotational positions of the spools  61   a.    
     The transmission output shaft  64  spline-fitted to the motor body  62   c  is rotatably journaled by the pump housing  61   a , the motor housing  62   a , and the right cover  16  via bearings  69   a ,  69   b , and  69   c , respectively. In addition, the transmission output shaft  64  has a rotational centerline L 2  parallel to a rotational centerline L 1  of the crankshaft. Respective rotational centerlines L 3 , L 4  of an output shaft (drive rotating shaft)  72  and of an intermediate shaft (driven rotating shaft)  90  are parallel to both the centerlines L 1 , L 2 . 
     The clutch  65  disposed at an end of the transmission  60  axially close to the left cover  14  transmits and interrupts torque from the driven gear  52  to the transmission output shaft  64 . The clutch  65  includes an input member  65   a , a centrifugal weight  65   b , an output member  65   c , and a clutch spring  65   e . The input member  65   a  is rotated integrally with the pump housing  61   a . The centrifugal weight  65   b  is supported by the input member  65   a  and guided by the input member  65   a  so as to be radially movable by centrifugal force produced according to the rotational speed of the input member  65   a . The output member  65   c  is axially movable depending on the position of the centrifugal weight  65   b  and is rotated integrally with the input member  65   a . The clutch spring  65   e  is disposed between the input member  65   a  and the output member  65   c  to press the centrifugal weight  65   b  to the input member  65   a  via the output member  65   c.    
     The output member  65   c  has a spool  65   d  constituting a spool valve using the transmission output shaft  64  as a sleeve. 
     When the engine speed of the internal combustion engine E is not higher than idling speed, the clutch  65  is located at a torque interruption position shown in  FIG. 3 . In this case, the spool  65   d  allows the hydraulic fluid discharged from the hydraulic pump  61  to return to the hydraulic pump  61  without rotating the hydraulic motor  62 . When the engine speed exceeds the idling speed, the spool  65   d  is driven by the centrifugal weight  65   b  radially outwardly moved by centrifugal force to move to the right, which allows the clutch  65  to take a torque transmission position. At this transmission position, the hydraulic fluid discharged from the hydraulic pump  61  flows in and rotatably drives the hydraulic motor  62 . Thus, the torque of the crankshaft  33  is transmitted to the transmission output shaft  64 . In an area where the engine speed exceeds the idling speed, the transmission output shaft  64  is rotated at a rotational speed varied depending on the inclination angle of the motor swash plate  62   b.    
     The output side transmission mechanism T includes an input mechanism Ti, the running clutch  70 , and an output mechanism To. The input mechanism Ti is composed of an output gear  64   o  provided on the transmission output shaft  64  so as to rotate integrally therewith. The running clutch  70  serves as an output side clutch to switch between a drive position and a neutral position by transmitting and interrupting torque from the transmission  60  to the rear wheel  7 . The output mechanism To is a transmission mechanism which transmits torque transmitted from the transmission  60  via the running clutch  70  to the terminal transmission mechanism  140  (see  FIG. 1 ). 
     The output gear  64   o  as an output rotating body is spline-fitted to a shaft-end portion  64   a  of the transmission output shaft  64  extending to the right from the bearing  69   b  into the second transmission chamber  37  and is engaged with an input gear  71  of the running clutch  70 . 
     Referring to  FIG. 3 , the running clutch  70  which is a hydraulic multiple-disc friction clutch and includes an output shaft (drive rotating shaft)  72  as an output member; an input gear  71  as an input member; a plurality of clutch plates  74 ; a housing as an intermediate member; a pressing piston  76 ; and a clutch spring  77 . The output shaft (drive rotating shaft)  72  is rotatably journaled by the right case half body  13   b  and by the right cover  16  via bearings  72   b   1 ,  72   b   2 . The input gear  71  is rotatably carried by the output shaft (drive rotating shaft)  72  so as to receive torque from the transmission output shaft  64  as an input shaft via the output gear  64   o . The clutch plates  74  are alternately stacked. The housing  75  is spline-fitted to the output shaft (drive rotating shaft)  72  for integral rotation. The pressing piston  76  is fitted to the housing  75  so as to be able to reciprocate and can press the clutch plates  74  and bring them into contact with each other. The housing  75  can transmit torque to the output shaft (drive rotating shaft)  72  from the input gear  71  via the clutch  74  in an engagement state. The clutch spring  77  biases the pressing piston  76  to separate the clutch plates  74  from each other. 
     The running clutch  70  as a hydraulically-operated device operated by hydraulic pressure is formed with a hydraulic chamber  78  defined between the housing  75  and the pressing piston  76  for receiving hydraulic fluid led thereinto with the hydraulic fluid driving the pressing piston  76 . The hydraulic fluid is part of oil discharged from the oil pump  40  (see  FIG. 2 ) driven by the transmission mechanism  41 . 
     The hydraulic pressure in the hydraulic chamber  78  is controlled by a hydraulic control device for controlling the supply and discharge of oil to and from the hydraulic chamber  78 . The hydraulic control device includes a hydraulic control valve  79  provided on the right cover  16  and controlled by a control device in response to the operation of a shift position control member; and an oil passage line where oil controlled by the hydraulic valve  79  flows. 
     This oil passage line is a passage adapted to supply and discharge oil to and from the hydraulic chamber  78 . The oil passage line includes an oil passage  81  formed in a connection member  80  connected to the hydraulic control valve  79  having part of the right cover  16  as a valve body; an oil passage  82  merging with the oil passage  81  and formed in the right cover  16 ; and an oil passage  83  adapted to allow the oil passage  82  to communicate with the hydraulic chamber  78 . 
     A hydraulic sensor  88  which detects the hydraulic pressure of the oil passage line is attached to the right cover  16  in order to monitor the operating state of the running clutch  70  by detecting the hydraulic state in the running clutch  70 . The hydraulic sensor  88  detects the hydraulic pressure of the oil passage  82 . 
     In the running clutch  70  constructed as above, if oil with high hydraulic pressure is supplied to the hydraulic chamber  78  to increase the hydraulic pressure in the hydraulic chamber  78  to a high level, the pressing piston  76  presses the first and second clutch plates  73 ,  74 . Thus, friction between the first and second clutch plates  73 ,  74  provides an engagement state where the input gear  71  and the output gear  75  are rotated integrally with each other. Consequently, the torque of the transmission output shaft  64  is transmitted to the output shaft (drive rotating shaft)  72  via both the gears  64   o ,  71 . On the other hand, if the hydraulic fluid in the hydraulic chamber  78  is discharged to reduce the hydraulic pressure in the hydraulic chamber  78  to a low level, the elastic force of the clutch spring  77  separates both the clutch plates  73 ,  74  from each other. Thus, a disengagement state is provided where transmission of torque between the input gear  71  and the housing  75  is interrupted. Consequently, the transmission of torque of the transmission output shaft  64  to the output shaft  72  is interrupted. In this way, control is exercised to supply and discharge oil to and from the hydraulic chamber  78  via the oil passages  81 ,  82 , and  83  to control the disengagement and engagement of the running clutch  70 . 
     Referring to  FIGS. 3 and 4 , the output mechanism To includes an intermediate shaft  90  which is a driven rotation shaft as a first power takeout shaft; a second power takeout shaft  91 ; a first transmission mechanism; a second transmission mechanism; and an output side torque damper  100 . The intermediate shaft (driven rotating shaft)  90  is rotatably driven by the output shaft (drive rotating shaft)  72  as a drive rotating shaft. The second power takeout shaft  91  is rotatably driven by the intermediate shaft  90  to rotatably drive the drive shaft  142  (see  FIG. 1 ). The first transmission mechanism is interposed between the output shaft (drive rotating shaft)  72  and the intermediate shaft (driven rotating shaft)  90  in the output side torque path to apply the torque of the output shaft (drive rotating shaft)  72  to the intermediate shaft (driven rotating shaft)  90 . The second transmission mechanism is interposed between the intermediate shaft  90  and the power takeout shaft  91  in the output side torque transmission path to apply the torque of the intermediate shaft (driven rotating shaft)  90  to the power takeout shaft  91 . The output side torque damper  100  is interposed between the transmission  60  and the power takeout shaft  91  in the output side torque transmission path to absorb excessive torque occurring in the output side torque transmission path. 
     The first transmission mechanism includes a drive gear  92  as a drive rotating body rotating integrally with the output shaft (drive rotating shaft)  72 ; and a driven gear  93  as an input rotating body meshed with the drive gear  92  to apply the torque of the output shaft (drive rotating shaft)  72  to the intermediate shaft (driven rotating shaft)  90 . The driven gear  93  is spline-fitted to an input cam member  101  of a torque member  100  for integral rotation and connected to the intermediate shaft  90  via the torque damper  100 . 
     The intermediate shaft (driven rotating shaft)  90  has a right shaft portion  90   b  that is rotatably journaled by the right case half body  13   b  via the input cam member  101  and via a bearing  94  and a left shaft portion  90   a  supported by the left case half body  13   a  via a bearing  95  and a bearing housing  96  joined to the left case half body  13   a.    
     In the shaft portion  90   b , the driven gear  93  is disposed via the input cam member  101  at a shaft-end portion  90   b   1  projecting rightward from the bearing  94 . In the shaft portion  90   a , a drive gear  97  is integrally formed with and disposed at a shaft-eng portion  90   a   1  projecting leftward from the bearing  95 . 
     The cam type torque damper  100  includes an input cam member  101  as an input member; an output cam member  102  as an output member; and a damper spring  103  as a biasing member. The input cam member  101  is rotatably carried by shaft portion  90   b  and is adapted to receive torque of the output shaft (drive rotating shaft)  72 . The output cam member  102  is engaged with the input cam member  101  to transmit torque from the input cam member  101  to the intermediate shaft (driven rotating shaft)  90 . The damper spring  103  axially biases the output cam member  102  to bring it into abutment against the input cam member  101 . The damper spring  103  composed of a coil spring is interposed between a spring receiver  95   a  also serving as a member for fixing the bearing  95 , and the output cam member  102 . 
     Since the input cam member  101  is rotatably carried by the intermediate shaft (driven rotating shaft)  90 , also the driven gear  93  is rotatably carried by the intermediate shaft  90 . 
     In this way, the output shaft  72  as a first rotating shaft adapted to receive torque from the transmission output shaft  64  and the intermediate shaft  90  as a second rotating shaft adapted to receive torque from the output shaft  72  are arranged between the transmission output shaft  64  and the torque damper  100  in the output side transmission path. In the output side torque transmission path, a rotating shaft disposed between the transmission output shaft  64  and the intermediate shaft (driven rotating shaft)  90  provided with the torque damper  100  thereon is only the single output shaft (drive rotating shaft)  72 . 
     The input cam member  101  of the torque damper  100  includes an input cam portion  101   a  formed with an input cam face  101   b ; and a cylindrical support portion  101   c  axially extending along and slidably carried by both the bearing  94  and the driven gear  93 . The support portion  101   c  includes a holding portion  101   d  located axially adjacently to the input cam portion  101   a  to hold the bearing  94 ; and an attachment portion  101   e  which is located axially adjacently to the holding portion  101   d  and to which the driven gear  93  is attached by spline-fitting. 
     The bearing  94 , one of a pair of the bearings  94 ,  95  rotatably supporting the intermediate shaft  90 , is axially interposed between the input cam member  101  and the driven gear  93 . The input cam portion (projection)  101   a  of the input cam member  101  is such that an abutment surface  101   f  axially opposite to the input cam surface  101   b  is abutted against one side of an inner race  94   a  of the bearing  94 . In addition, the driven gear  93  is abutted against the other side of the inner race  94   a . In this way, the bearing  94  rotatably supporting the input cam member  101  and the intermediate shaft  90  via the input cam member  101  is axially positioned by the input cam member  101  and by the driven gear  93 . 
     On the other hand, the output cam member  102  rotating integrally with the intermediate shaft  90  has an output cam portion (projection)  102   a  forming an output cam surface  102   b  is mounted to the intermediate shaft  90  through spline-fitting so as to be axially movable. 
     The input cam portion (projection)  101   a , the output cam member  102  and the damper spring  103  are axially disposed between the pair of bearings  94 ,  95 . 
     The torque damper  100  has a torque-absorbing portion  100   a . The torque-absorbing portion  100   a  includes, between the pair of bearings, an input cam portion  101   a  which is an input damper portion (projection)  101   a  of the input cam member  101 ; an output cam portion  102   a  which is an output damper portion (projection)  102   a  of the output cam member  102 ; and a damper spring  103 . In the event that excessive torque exceeding a previously set second set torque is applied between the input cam member  101  and the output cam member  102 , the torque absorbing-portion  100   a  absorbs the excessive torque. 
     The second set torque is set to a value greater than the first set torque of the input side torque damper  54 . Therefore, the capacity of the torque damper  100  is greater than that of the torque damper  54 . 
     More specifically, the input cam portion (projection)  101   a  and the output cam portion (projection)  102   a  are abutted at the cam surfaces  101   b ,  102   b  against each other by the biasing force of the damper spring  103  and can be circumferentially relatively slidable. When torque not greater than the second set torque is applied between both the cam members  101 ,  102 , the torque damper  100  integrally rotates the input cam member  101  and the output cam member  102 . During deceleration of the motorcycle V (see  FIG. 1 ) or of the internal combustion engine E, e.g., during application of engine-braking to the motorcycle (see  FIG. 1 ), when excessive torque exceeding the second set torque is applied, a rotation-directional slip occurs between the input cam member  101  and the output cam member  102 . The output cam member  102  is driven by the input cam member  101  and axially moved against the biasing force of the damper spring  103 . At the same time, the input cam member  101  and the output cam member  102  are relatively rotated to absorb the excessive torque. 
     The second transmission mechanism includes the drive gear  97  composed of a bevel gear; and the driven gear  98  composed of a bevel gear meshed with the drive gear  97  and molded integrally with the power takeout shaft  91 . 
     The power takeout shaft  91  is disposed inside the gear cover  17  joined to the left case half body  13   a  and rotatably supported via the pair of bearings  99   a ,  99   b . A rotational position sensor  19  for detecting the rotational position of the driven gear  98  is provided on the gear cover  17  and detects vehicle speed based on the detection signal of the sensor  19 . 
     With reference to  FIGS. 2 and 3 , the left case half body  13   a , the left cover  14 , and the gear cover  17  (the whole of “the left case half body  13   a , the left cover  14 , and the gear cover  17 ” is hereinafter called “the side cover” as needed) constitute one lateral wall of the power unit P in the vehicle-width direction (also the axial direction of the crankshaft  33 ). A large portion of the side cover is covered by an exterior cover C 1  from the left side as a predetermined direction and on the outside of the power unit P. In addition, a large portion of the right case half body  13   b  and the right cover  16  constituting another lateral wall of the power unit P in the vehicle-width direction is covered by an exterior cover C 2  from the right side which is the outside of the power unit P. Both the exterior covers C 1 , C 2  formed of a synthetic resin are sound insulation covers as cover members which improve the external appearance of the power unit P and reduce radiated sound produced by the power unit P. 
     The exterior cover C 1  is secured to attachment seats  112  (an attachment seat  112  provided on the left case half body  13   a  is shown in  FIG. 3 ), provided on the left case half body  13   a , the left cover  14 , and the gear cover  17 , in a plurality of, three, respective attachment portions  111  with bolts  110 . The exterior cover C 1  covers from left the general whole of the left cover  14  excluding the lower edge portion of the left cover  14 , and the whole of the gear cover  17 . 
     A space S is defined between the side cover and the exterior cover C 1  disposed to form an interval in a left to right direction. 
     A sound-absorbing material  115  made of a foam material (e.g. urethane foam) is arranged along the internal surface, facing the space S, of the exterior cover C 1  so as to reduce radiated sound. 
     A resonator  120  resonating with the specific frequency of radiated sound is disposed between the sound-absorbing material  115  and the side cover in the space S. The resonator  120  communicates with the space S through an opening  127  defined by a neck  126 . 
     A description will next be given of the operation and effects of the embodiment described as above. 
     In the output side transmission mechanism T of the power unit P, the torque damper  100  for absorbing excessive torque occurring in the output side transmission path between the output shaft  72  of the running clutch  70  and the intermediate shaft  90  includes the input cam member  101  adapted to receive the torque of the output shaft  72  and the output cam member  102  adapted to transmit torque from the input cam member  101  to the intermediate shaft  90 . In addition, the input shaft member  101  and the output shaft member  102  are relatively rotated to absorb the excessive torque. The intermediate shaft  90  is rotatably supported by the pair of bearings  94 ,  95 . The driven gear  93  is mounted to the input cam member  101  rotatably supported by the intermediate shaft  90  to apply the torque of the output shaft  72  to the input cam member  101 . The output cam member  102  is rotated integrally with the intermediate shaft  90 . In the torque damper  100 , the torque-absorbing portion  100   a  for absorbing excessive torque includes the input cam portion (projection)  101   a  of the input cam member  101  and the output cam portion (projection)  102   a  of the output cam member  102  and is disposed between the pair of bearings  94 ,  95  in the axial direction of the intermediate shaft  90 . The bearing  94  is disposed between the input cam portion (projection)  101   a  and the driven gear  93  in the axial direction. 
     With this configuration, in the torque damper  100  mounted to the intermediate shaft  90  rotatably supported by the pair of bearings  94 ,  95 , the torque-absorbing portion  100   a  for absorbing excessive torque and the driven gear  93  are dividedly arranged on both sides of the bearing  94  in the axial direction. The length of the intermediate shaft  90  is reduced between the pair of the bearings  94 ,  95 . Thus, the bending deformation of the intermediate shaft  90  can be suppressed. In addition, the total length of the intermediate shaft  90  can be reduced compared with the case where a torque damper is disposed outside of the pair of bearings  94 ,  95  in the axial direction. As a result, the intermediate shaft  90  to which the torque damper  100  is mounted can be downsized in the axial direction and reduced in weight, and in turn the power unit P can be downsized in the axial direction of the intermediate shaft  90 . 
     The output cam member  102  is axially movably supported by the intermediate shaft  90 . The input cam portion (projection)  101   a  has the input cam surface  101   b  and the output cam portion (projection)  102   a  has the output cam surface  102   b  abutted against the input cam surface  101   b . The bearing  94  is axially positioned by the input cam portion (projection)  101   a  and by the driven gear  93 . The bearing  94  of the pair of bearings  94 ,  95  rotatably supporting the intermediate shaft  90  is positioned by use of the input cam portion (projection)  101   a  and the driven gear  93 . Thus, a dedicated portion or member used to position the bearing  94  becomes unnecessary, thereby reducing costs. 
     The bearing  94  rotatably supports the intermediate shaft  90  via the input cam member  101  of the torque damper  100 . Therefore, the diameter of the bearing  94  can be increased. Consequently, the capacity of the bearing  94 , i.e., the load capability of the bearing  94 , can be increased to improve durability of the bearing  94 . 
     The motorcycle V includes the internal combustion engine E provided with the crankshaft  33 ; the hydrostatic continuously variable transmission  60  rotatably driven by torque of the crankshaft  33 ; the rear wheel  7  rotatably driven by torque from the transmission  60 ; the torque dampers  54 ,  100  adapted to absorb excessive torque occurring in the torque transmission path from the crankshaft  33  to the rear wheel  7 . In the motorcycle V, the torque transmission path is composed of the input side torque transmission path between the crankshaft  33  and the transmission  60  and the output side torque transmission path between the transmission  60  and the rear wheel  7 , with the transmission  60  interposed therebetween. The torque dampers  54 ,  100  include the input side torque damper  54  disposed in the input side torque transmission path and the output side torque damper  100  disposed in the output side torque transmission path. With such a configuration, excessive torque occurring in the torque transmission path from the crankshaft  33  through the transmission  60  to the rear wheel  7  is absorbed by the output side torque damper  100  and then applied to the transmission  60 . Thus, excessive torque applied to the transmission  60 , excessive torque occurring in the input side torque transmission path, and vibrations caused when the transmission  60  and the crankshaft  33  resonate with each other are reduced. Consequently, a torque shock caused by rotary inertia of the hydrostatic continuously variable transmission  60  with rotary inertia large because of the provision of the clutch  65 , the hydraulic pump  61  having the rotating pump housing  61   a , and the hydraulic motor  62 , and vibrations caused by the resonance between the transmission  60  and the crankshaft  33  are reduced to improve ride quality of the motorcycle V. 
     The output side torque transmission path includes the output side transmission mechanism T including the output shaft  72  of the running clutch  70  adapted to receive torque from the transmission output shaft  64  of the transmission  60  and the intermediate shaft  90  adapted to receive torque from the output shaft  72 ; and the terminal transmission mechanism  140  adapted to receive torque from the output side transmission mechanism T and drive the rear wheel  7 . In the output side torque transmission mechanism, the rotating shaft disposed between the transmission output shaft  64  and the intermediate shaft  90  is only the single output shaft  72  and the output side torque damper  100  is mounted to the intermediate shaft  90 . The rotating shaft to which excessive torque reduced by the output side torque damper  100  is applied between the transmission  60  and the intermediate shaft  90  is only the single output shaft  72  in the output side torque transmission path. Thus, the rotary inertia of the rotating shaft disposed between the output side torque damper  100  and the transmission  60  can be reduced. Consequently, a torque shock caused by the excessive torque in the input side torque transmission path can be reduced and additionally the input side torque damper  54  can be reduced in capacity and thus reduced in size. 
     The capacity of the output side torque damper  100  is greater than that of the input side torque damper  54 . The excessive torque in the output side transmission path is largely absorbed by the output side torque damper  100 . Thus, the torque shock caused by the rotary inertia of the transmission  60  can significantly be reduced. As a result, the torque shock can further be reduced. In addition, since the capacity of the input side torque damper  54  is small, the input side torque damper  54  can be downsized. 
     A description is hereinafter given of a partially modified mode of the embodiment described above focusing on the modified portion. 
     A torque damper corresponding to the output side torque damper  100 , i.e., a torque damper for absorbing excessive torque occurring in the torque transmission path between the drive rotating shaft and the driven rotating shaft may be mounted to the drive rotating shaft. In this case, the output shaft  72 , the intermediate shaft  90 , the driven gear  93 , the input cam member  101 , and the output cam member  102  in the embodiment correspond to a driven rotating shaft, a drive rotating shaft, a drive gear as an output rotating body, an output cam member  102 , and an input cam member  101 , respectively, in this alternative torque damper. In addition, the alternative torque damper provides the same operation and effects as those of the embodiment. 
     The torque damper may be a spring type torque damper in which a damper spring is circumferentially disposed between an input member and an output member. 
     The power unit P for driving a drive target is not necessarily a unit integrally composed of the internal combustion engine E and the transmission apparatus like the embodiment described above. The power unit P may be composed of only the internal combustion engine or the transmission apparatus or of an engine other than an internal combustion engine. 
     The internal combustion engine may be a multi-cylinder engine other than a V-type 2-cylinder engine, or may be a single-cylinder internal combustion engine. In addition, the transmission may be a hydrostatic continuously variable transmission other than a swash plate type transmission, a continuously variable transmission other than a hydraulic type transmission, or a transmission other than a continuously variable transmission. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.