Patent Publication Number: US-11384801-B2

Title: Power transmission device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/JP2019/047406, filed Dec. 4, 2019, which claims priority to Japanese Application No. 2018-227993, filed Dec. 5, 2018. The disclosures of the above applications are incorporating herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a power transmission device that can appropriately transmit or stop transmitting rotational power of an input member to an output member. 
     BACKGROUND 
     In general, a motorcycle has a power transmission device that is used to appropriately transmit or stop transmitting the engine driving power to a transmission and a driving wheel. The power transmission device includes an input member coupled to the engine side, an output member coupled to the transmission and driving wheel side, a clutch member coupled to an output member, and a pressure member capable of moving toward or away from the clutch member. The power transmission device is configured to transmit power by moving the pressure member toward the clutch member to press a drive-side clutch plate and a driven-side clutch plate against each other. It is also configured to stop transmitting the power by moving the pressure member away from the clutch member to release a press-contact force between the drive-side clutch plate and the driven-side clutch plate. 
     Among existing power transmission devices, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2017-155884, the power transmission device includes a weight member. The weight member can press a drive-side clutch plate and a driven-side clutch plate against each other by moving from a radially-inner position to a radially-outer position, in a groove portion, due to a centrifugal force generated by rotation of a clutch housing. With the existing power transmission device, because the clutch housing rotates as the engine is driven, it is possible to apply a centrifugal force to the weight member and to transmit the engine driving power to a wheel by pressing the drive-side clutch plate and the driven-side clutch plate against each other. 
     The existing power transmission device further includes an activation member that can move the pressure member to an inactive position in response to a shifting operation when the pressure member is at an active position. A bearing holding member, coupled to the activation member, holds a bearing interposed between the activation member and the pressure member. When a shifting operation is performed by hand or by an actuator, the activation member moves to press the pressure member via the bearing holding member, and can stop transmitting driving power by releasing a press-contact force between the clutch plates. 
     SUMMARY 
     However, the existing power transmission device has the following problem. 
     It is difficult to assemble the bearing holding member and the projecting portion together since the bearing holding member is attached to a projecting portion extending from the clutch member. It releases the press-contact force between the clutch plates by moving the pressure member by sliding along the projecting portion in accordance with the shifting operation. Thus, sliding of the bearing holding member relative to the projecting portion may become unstable when the shifting operation is performed. 
     The present disclosure has been made in consideration of such circumstances. Thus, it is an object of a power transmission device to enable a bearing holding member to be easily assembled and to move stably when a shifting operation is performed. 
     According to the disclosure, a power transmission device comprise a clutch housing that rotates together with an input member rotated by a vehicle engine driving power. A plurality of drive-side clutch plates are attached to the clutch housing. A clutch member has a plurality of driven-side clutch plates that are alternately formed with the drive-side clutch plates of the clutch housing. The clutch member is coupled to an output member that rotates a wheel of the vehicle. A pressure member is movable between an active position and an inactive position. In the active position, the pressure member presses the drive-side clutch plates and the driven-side clutch plates against each other to be able to transmit the engine driving power to the wheel. In the inactive position, the pressure member releases a press-contact force between the drive-side clutch plates and the driven-side clutch plates to stop transmitting of the engine driving power to the wheel. A weight member is disposed in a groove portion, extending in a radial direction, in the clutch housing. The weight member is movable from a radially-inner position to a radially-outer position in the groove portion due to a centrifugal force generated by rotation of the clutch housing. An interlocking member moves the pressure member from the inactive position to the active position as the weight member moves from the radially-inner position to the radially-outer position. An activation member moves the pressure member to the inactive position in response to a shifting operation when the pressure member is at the active position. A bearing holding member, coupled to the activation member, holds a bearing interposed between the activation member and the pressure member. The bearing holding member includes a cylindrical member with one open end. The open end portion is fit into and attached to a recessed portion formed in the clutch member. 
     According to another object, the power transmission device includes a release spring that holds the pressure member at the inactive position. The release spring applies an urging force to the pressure member while enabling movements of the interlocking member and the pressure member until the drive-side clutch plates and the driven-side clutch plates reach an engaged state before the drive-side clutch plates and the driven-side clutch plates are pressed against each other. The release spring is attached to both the bearing holding member and the pressure member. It applies the urging force to the pressure member. Also, it applies an urging force to the bearing holding member to transmit the urging force to the activation member. 
     According to the disclosure, the power transmission device release spring includes a coned-disc spring. The release spring generates an urging force due to a displacement between a middle part and a peripheral edge part. The middle part is attached to the bearing holding member. The peripheral edge part is attached to the pressure member. 
     According to the disclosure, in the power transmission device, the clutch member includes a first clutch member, coupled to the output member, a second clutch member, attached to the driven-side clutch plates, and a back-torque transmitting cam. The back-torque transmitting cam presses the drive-side clutch plates and the driven-side clutch plates against each other by moving the second clutch member when a rotational force is input to the first clutch member, via the output member. The recessed portion is formed in the first clutch member. 
     According to the disclosure, the power transmission device bearing holding member has a communication hole formed in a side wall part. This enables oil supplied to the inside to flow to the outside, via the communication hole. 
     According to an disclosure, the power transmission device communication hole is open toward a gap between the clutch member and the pressure member. 
     According to the disclosure, it is easy to assemble the bearing holding member. Also, it is possible to stably operate the bearing holding member when a shifting operation is performed. This is due to the bearing holding member including the cylindrical member with one open end. The open end portion is fit into and attached to the recessed portion formed in the clutch member. 
     According to the disclosure, it is also possible to use the release spring as a spring to prevent play of a shift operation mechanism. The power transmission device includes a release spring applying an urging force to the pressure member while allowing movements of the interlocking member and the pressure member until the drive-side clutch plates and the driven-side clutch plates reach an engaged state before the drive-side clutch plates and the driven-side clutch plates are pressed against each other. The release spring is attached to both the bearing holding member and the pressure member. The spring applies the urging force to the pressure member. Also, it applies an urging force to the bearing holding member to transmit the urging force to the activation member. 
     According to the disclosure, it is possible to stably apply urging forces of the release spring to both the bearing holding member and the pressure member. The release spring includes a coned-disc spring that generates an urging force due to a displacement between a middle part and a peripheral edge part. The middle part is attached to the bearing holding member. The peripheral edge part is attached to the pressure member. 
     According to the disclosure, it is possible to avoid the bearing holding member from interfering with movement of the second clutch member due to the back-torque transmitting cam. Thus, motion of the bearing holding member and the movement of the second clutch member, by the back-torque transmitting cam, are performed smoothly. The clutch member includes a first clutch member coupled to the output member. A second clutch member is attached to the driven-side clutch plates. A back-torque transmitting cam presses the drive-side clutch plates and the driven-side clutch plates against each other. This occurs by moving the second clutch member when a rotational force is input to the first clutch member, via the output member. The recessed portion is formed in the first clutch member. 
     According to the disclosure, it is possible to smoothly supply oil into the clutch housing. The bearing holding member has a communication hole formed in a side wall part. This enables oil supplied to the inside to flow to the outside, via the communication hole. 
     According to the disclosure, it is possible to feed oil that has flowed out from the communication hole to substantially the entire region in the clutch housing. The communication hole is open toward a gap between the clutch member and the pressure member. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a perspective view of a power transmission device. 
         FIG. 2  is a longitudinal sectional view of  FIG. 1 . 
         FIG. 3  is an enlarged cross-section view of drive-side clutch plates, driven-side clutch plates, a back-torque transmitting cam of  FIG. 2 . 
         FIG. 4  is a perspective view of a housing portion of a clutch housing. 
         FIG. 5  is a perspective view of a cover portion of the clutch housing. 
         FIG. 6  is a three-side plan and elevation view of a first clutch member. 
         FIG. 7  is a three-side plan and elevation view of a second clutch member. 
         FIG. 8  is a three-side view of a pressure member of the power transmission device. 
         FIG. 9  is an exploded perspective view of the first clutch member, the second clutch member, the pressure member, and a bearing holding member before being assembled. 
         FIG. 10  is an exploded perspective view of the first clutch member, the second clutch member, the pressure member, and the bearing holding member. 
         FIG. 11  is a perspective view of the first clutch member, the second clutch member, the pressure member, and the bearing holding member after having been assembled. 
         FIG. 12  is a three-side plan and elevation view of the bearing holding member. 
         FIG. 13  is a cross-section view of the function of a press-contact assisting cam. 
         FIG. 14  is a cross-section view of the function of a back-torque limiter cam. 
         FIG. 15  is a plan view of the first clutch member and the second clutch member assembled together, illustrating a state where one side surface of a protruding portion and a first contact surface are in contact with each other. 
         FIG. 16  is a plan view of the first clutch member and the second clutch member assembled together, illustrating a state where the other side surface of the protruding portion and a second contact surface (movement-amount limiting portion) are in contact with each other. 
         FIG. 17  is a cross-section view of the function of the back-torque transmitting cam in a state before the back-torque transmitting cam starts working. 
         FIG. 18  is a cross-section view of the function of the back-torque transmitting cam in a state after the back-torque transmitting cam has started working. 
         FIG. 19  is a cross-section view of a cushioning member accommodated in an accommodation recessed portion. 
         FIG. 20  is a cross-section view of a state where a release spring applies urging forces to both the bearing holding member and the pressure member. 
         FIG. 21  is a three-side plan and elevation view of the release spring. 
         FIG. 22  is a graph representing the movement amount and the pressing load of an interlocking member. 
         FIG. 23  is a graph representing the movement amount and the pressing load of an interlocking member according to another embodiment of the present disclosure. 
         FIG. 24  is a longitudinal sectional view of a power transmission device according to another embodiment. 
         FIG. 25  is a perspective view of a bearing holding member according to another embodiment. 
         FIG. 26  is a longitudinal sectional view of a power transmission device where the bearing holding member is attached. 
         FIG. 27  is a longitudinal sectional view of a power transmission device where the bearing holding member is attached and an activation member is engaged from the opposite side. 
         FIG. 28  is a longitudinal sectional view of a power transmission device where the bearing holding member is attached and an activation member is engaged from the opposite side and that does not have a cushioning member. 
         FIG. 29  is a longitudinal sectional view of a power transmission device where the bearing holding member is attached and an activation member is engaged from the opposite side and a clutch member is integrated). 
         FIG. 30  is a graph representing the movement amount and the pressing load of an interlocking member of a power transmission device that does not have a cushioning member. 
     
    
    
     DETAILED DESCRIPTION 
     Hereafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. 
     A power transmission device according to the present embodiment is a device that is disposed in a vehicle, such as a motorcycle, and is used to appropriately transmit or to stop transmitting the driving power of an engine to a transmission or toward the driving wheel side. As illustrated in  FIGS. 1 to 12 , the power transmission device mainly includes: a clutch housing  2 , with an input gear  1  (input member) rotated by the vehicle engine driving power; a clutch member (a first clutch member  4   a  and a second clutch member  4   b ); a pressure member  5  attached on the right side in  FIG. 2  of the clutch member (the first clutch member  4   a  and the second clutch member  4   b ); a plurality of drive-side clutch plates  6 ; a plurality of driven-side clutch plates  7 ; a weight member  8  including steel ball members that can move (roll) in the radial direction in the clutch housing  2 ; an interlocking member  9 ; and an activation member  10  that can be activated by hand or by an actuator (not shown). The symbol S in the figure shows a spring damper. The symbol B 1  shows a roller bearing and the symbols B 2  and B 3 , respectively, show thrust bearings. 
     The input gear  1  can rotate around an output shaft  3  when the driving power (rotational force) is transmitted from the engine. The input gear  1  is coupled to the clutch housing  2  via a rivet R or the like. The clutch housing  2  includes a housing portion  2   a  that includes a cylindrical member whose right side, in  FIG. 2 , is open and that is coupled to the input gear  1 . A cover portion  2   b  is attached to close the opening of the housing portion  2   a . The clutch housing  2  can rotate together with the rotation of the input gear  1  due to the driving power of the engine. 
     As illustrated in  FIG. 4 , a plurality of cutouts  2   aa  are formed in the housing portion  2   a  of the clutch housing  2  and are arranged in the circumferential direction. The plurality of drive-side clutch plates  6  are attached and fit to the cutouts  2   aa . Each of the drive-side clutch plates  6  is a substantially annular plate material, configured to rotate together with the rotation of the clutch housing  2  and to slide in the axial direction (the left-right direction in  FIG. 2 ). 
     Moreover, as illustrated in  FIG. 5 , a plurality of groove portions  2   ba , extending in the radial direction of the cover portion  2   b , are formed in the bottom surface of the cover portion  2   b  of the clutch housing  2 . The weight member  8  is disposed in each of the groove portions  2   ba . In a state where the clutch housing  2  is stopped (an engine stop or idling state) and in a state where the clutch housing  2  is rotating at a low speed, the weight member  8  is located at a radially-inner position (the position shown in  FIG. 2 ). In a state where the clutch housing  2  is rotating at a high speed, the weight member  8  is located at a radially-outer position. 
     The plurality of driven-side clutch plates  7 , formed alternately with the drive-side clutch plates  6  of the clutch housing  2 , are attached to the clutch member, the first clutch member  4   a  and the second clutch member  4   b . The clutch member is coupled to the output shaft  3  (output member) and can rotate the wheel of the vehicle. The clutch member is configured by assembling two members, the first clutch member  4   a  and the second clutch member  4   b.    
     As illustrated in  FIG. 6 , the first clutch member  4   a  includes a disc-shaped member with a flange surface  4   ac  formed along the peripheral edge part. The first clutch member  4   a  is configured so that the output shaft  3  is inserted into an insertion hole  4   ad  (see  FIGS. 2 and 6 ) formed at its center. The first clutch member  4   a  and the output shaft  3  are coupled to each other in the rotational direction as gears, respectively, formed therein mesh with each other. As illustrated in  FIGS. 6, 9, and 10 , the first clutch member  4   a  includes an inclined surface  4   aa , that constitutes a press-contact assisting cam, and an inclined surface  4   ab , that constitutes a back-torque limiter cam. 
     As illustrated in  FIG. 7 , the second clutch member  4   b  includes an annular member configured so that the driven-side clutch plates  7  are attached by spline fitting to a spline-fitting portion  4   ba  (see  FIGS. 2 and 7 ), formed in an outer peripheral surface. As illustrated in  FIGS. 9 to 11 , the pressure member  5  is assembled with the clutch member, the first clutch member  4   a  and the second clutch member  4   b . The plurality of drive-side clutch plates  6  and the driven-side clutch plates  7 , in an alternately stacked state, are attached between a flange surface  5   c  of the pressure member  5  (see  FIGS. 2 and 8 ) and the flange surface  4   ac  of the first clutch member  4   a  (see  FIGS. 2 and 6 ). 
     As illustrated in  FIG. 8 , the pressure member  5  includes a disc-shaped member with the flange surface  5   c  formed along the peripheral edge part. The pressure member  5  is movable between an active position and an inactive position. In the active position, the pressure member  5  presses the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other to transmit the engine driving power to the wheel. In the inactive position (see  FIG. 2 ), the pressure member  5  releases the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7  to stop transmitting the engine driving power to the wheel. 
     To be more specific, as illustrated in  FIGS. 7, 9 and 10 , the spline-fitting portion  4   ba , formed in the second clutch member  4   b , has a protruding/recessed shape that is integrally formed along substantially the entire circumference of the outer peripheral side surface of the second clutch member  4   b . When the driven-side clutch plates  7  are fit into recessed grooves of the spline-fitting portion  4   ba , movement of the driven-side clutch plates  7  in the rotational direction is restricted while movement of the driven-side clutch plates  7 , relative to the second clutch member  4   b , in the axial direction is allowed. The driven-side clutch plates  7  rotate together with the second clutch member  4   b.    
     The driven-side clutch plates  7  are stacked alternately with the drive-side clutch plates  6 . The clutch plates  6  and  7  can be pressed against each other or the press-contact force therebetween can be released. That is, both clutch plates  6  and  7  are allowed to slide in the axial direction of the second clutch member  4   b . When the pressure member  5  moves leftward in  FIG. 2  and the flange surface  5   c  moves toward the flange surface  4   ac  of the first clutch member  4   a , both clutch plates  6  and  7  are pressed against each other to transmit a rotational force of the clutch housing  2  to the output shaft  3 , via the second clutch member  4   b  and the first clutch member  4   a . When the pressure member  5  moves rightward in  FIG. 2  and the flange surface  5   c  and the flange surface  4   ac  of the first clutch member  4   a  become separated from each other, the press-contact force between the clutch plates  6  and  7  is released. Thus, the first clutch member  4   a  and the second clutch member  4   b  stop following the rotation of the clutch housing  2  and transmission of a rotational force to the output shaft  3  is stopped. 
     In the state where the drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other, a rotational force (driving power of the engine) input to the clutch housing  2  is transmitted to the wheel side via the output shaft  3  (output member). In the state where the press contact between the drive-side clutch plates  6  and the driven-side clutch plates  7  is released, a rotational force (driving power of the engine) input to the clutch housing  2  can stop being transmitted to the output shaft  3  (output member). 
     Moreover, as illustrated in  FIGS. 6, 8, 9, and 10 , in the present embodiment, the inclined surfaces  4   aa  and  4   ab  are formed in the first clutch member  4   a . The inclined surfaces  5   a  and  5   b , that face the inclined surfaces  4   aa  and  4   ab , are formed in the pressure member  5 . That is, the inclined surface  4   aa  and the inclined surface  5   a  contact each other to provide a press-contact assisting cam. The inclined surface  4   ab  and the inclined surface  5   b  contact each other to provide a back-torque limiter cam. 
     As illustrated in  FIG. 13 , a rotational force in the direction (a) is applied to the pressure member  5  when the rotational speed of the engine increases and a rotational force input to the input gear  1  and the clutch housing  2  becomes capable of being transmitted to the output shaft  3 , via the first clutch member  4   a  and the second clutch member  4   b . The weight member  8  is at the radially-outer position. Therefore, a force in the direction (c) in the figure is generated in the pressure member  5  due to the function of the press-contact assisting cam. Thus, the flange surface  5   c  of the pressure member  5  moves in a direction (leftward in  FIG. 2 ) further toward the flange surface  4   ac  of the first clutch member  4   a . The pressure member  5  increases the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7 . 
     On the other hand, while the vehicle is traveling, when the rotational speed of the output shaft  3  exceeds the rotational speed of the input gear  1  and the clutch housing  2 , and a back torque in the direction (b) in  FIG. 14  is generated. Thus, due to the function of the back-torque limiter cam, the pressure member  5  is moved in the direction (d) in the figure to release the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7 . Thus, it is possible to avoid damage to the power transmission device and the power source (engine side) due to the back torque. 
     The weight member  8  is disposed in the groove portion  2   ba  extending in the radial direction of the clutch housing  2  (in the present embodiment, the cover portion  2   b ). The weight member  8  moves from a radially-inner position (see  FIG. 2 ) to a radially-outer position in the groove portion  2   ba  due to a centrifugal force. The centrifugal force is generated as the clutch housing  2  rotates to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other. That is, the rolling surface (bottom surface) of the groove portions  2   ba  where the weight member  8  rolls has an upward inclination from the radially-inner position toward the radially-outer position. When the clutch housing  2  is stopped, the weight member  8  is held at the radially-inner position due to an urging force of a release spring m. When the clutch housing  2  rotates, a centrifugal force is applied to the weight member  8  and moves the weight member  8  along the upward inclination to the radially-outer position as the clutch housing  2  reaches a predetermined rotational speed. 
     The interlocking member  9  is composed of an annular member disposed in the clutch housing  2  (the cover portion  2   b ). The interlocking member  9  is fit and coupled to a groove portion formed in an inner peripheral surface of the cover portion  2   b . It is rotatable together with the clutch housing  2  and is movable in the left-right direction in  FIG. 2 . As the weight member  8  moves from the radially-inner position to the radially-outer position, the interlocking member  9  moves leftward in  FIG. 2  against an urging force of the release spring m. It can press the pressure member  5  to move the pressure member  5  from the inactive position to the active position. 
     The activation member  10  includes a member that can be operated by hand or by an actuator (see  FIG. 2 ). It moves the pressure member  5  in a direction (rightward in  FIG. 2 ) such that the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7  can be released. When a shifting operation is performed, for example, by an operation on a clutch pedal, a clutch lever, or the like of the vehicle, or by working of an actuator, the activation member  10  moves rightward in  FIG. 2  to contact the pressure member  5  via a bearing holding member C and moves the pressure member  5  from the active position to the inactive position. Thus, the activation member  10  can disengage the clutch (stop transmitting the power) by releasing the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7 . 
     As illustrated in  FIG. 2 , the bearing holding member C, coupled to the activation member  10 , holds a bearing B 1  interposed between the activation member  10  and the pressure member  5 . As illustrated in  FIG. 12 , the bearing holding member C includes a cylindrical member with one end open. Also, the open end portion Ca and a top portion Cb on the opposite side of the open end portion Ca. The bearing B 1  according to the present embodiment is attached to the top portion Cb side in the bearing holding member C. A cylindrical part extends from an increased-diameter part to the open end portion Ca. Although a ball bearing is used as the bearing B 1  according to the present embodiment, for example, another bearing such as a needle bearing may be used. 
     Moreover, as illustrated in  FIGS. 2 and 20 , the open end portion Ca of the bearing holding member C, according to the present embodiment, is fit into and attached to a recessed portion  4   d  formed in the clutch member (the first clutch member  4   a ), and is assembled with an inner peripheral wall surface  4   da  of the recessed portion  4   d  by socket-and-spigot joint (fitting joint). The recessed portion  4   d  includes a circular depression having a shape similar to and a size substantially the same as the outer shape of the open end portion Ca (to be strict, a size slightly larger than the open end portion Ca). By fitting the bearing holding member C into the recessed portion  4   d , positioning and centering relative to the power transmission device are performed. 
     In a shifting operation, when the activation member  10  moves rightward in  FIG. 2  by, for example, an operation on a clutch pedal, a clutch lever, or the like of the vehicle or by working of an actuator, the bearing holding member C moves together and contacts the pressure member  5  to move the pressure member  5  from the active position to the inactive position. Thus, the clutch is disengaged (transmission of power is stopped) by releasing the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7 . 
     The release spring m can hold the pressure member  5  at the inactive position. The spring is compressed as the interlocking member  9  moves and the pressure member  5  moves from the inactive position toward the active position. The release spring m can apply an urging force while allowing movements of the interlocking member  9  and the pressure member  5  until the drive-side clutch plates  6  and the driven-side clutch plates  7  reach an engaged state before being pressed against each other. A state where the distance between the drive-side clutch plates  6  and the driven-side clutch plates  7  is zero and immediately before power transmission due to pressed contact is performed. 
     Moreover, as illustrated in  FIG. 21 , the release spring m according to the present embodiment, includes a coned-disc spring that can generate an urging force due to a displacement between a middle part (ma) and a peripheral edge part (mb). As illustrated in  FIGS. 2 and 20 , the middle part (ma) is attached to the top portion Cb of the bearing holding member C. The peripheral edge part (mb) is attached to the pressure member  5 . The pressure member  5  has a projecting portion  5   d  that projects annularly. The peripheral edge part (mb) of the release spring m engages and is attached by a ring-shaped member g (for example, a circlip or the like) attached to the projecting portion  5   d . Thus, the release spring m, according to the present embodiment, is attached between the bearing holding member C and the pressure member  5 . It applies an urging force (an urging force in an orientation indicated by a numeral a 2  in  FIG. 20 ) to the pressure member  5 . Also, it can apply an urging force (an urging force in an orientation indicated by a symbol a 1  in the figure) to the bearing holding member C to transmit the urging force to the activation member  10 . 
     A clutch spring  11  includes a coil spring interposed between the interlocking member  9  and the pressure member  5 . As the interlocking member  9  moves, the clutch spring  11  can press the pressure member  5  to move the pressure member  5  in a direction such that the drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other. Moreover, when the activation member  10  is working, the clutch spring  11  can absorb the pressing force that the pressure member  5  applies to the interlocking member  9 . 
     The clutch spring  11 , according to the present embodiment, moves together with the pressure member  5  without being compressed (deformed), until the drive-side clutch plates  6  and the driven-side clutch plates  7  reach the engaged state described above. After the drive-side clutch plates  6  and the driven-side clutch plates  7  have reached the engaged state, the clutch spring  11  is compressed in the process where the interlocking member  9  moves and the pressure member  5  moves further. The spring can apply a press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7  while allowing movement of the interlocking member  9 . 
     That is, when the weight member  8  moves from the radially-inner position to the radially-outer position and the interlocking member  9  is pressed by the weight member  8  as the clutch housing  2  rotates. The pressing force is transmitted to the pressure member  5  via the clutch spring  11 , moving the pressure member  5  leftward in  FIG. 2 , and presses the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other. When the activation member  10  is activated in this state, although the pressure member  5  moves rightward in the figure due to the pressing force of the activation member  10 , the pressing force applied to the interlocking member  9  is absorbed by the clutch spring  11 . The position of the interlocking member, the position of the weight member  8 , is maintained. 
     Here, the power transmission device according to the present embodiment includes a back-torque transmitting cam (cam surfaces K 1  and T 1 ) that can move the second clutch member  4   b  to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other, when a rotational force is input to the first clutch member  4   a , via the output shaft  3  (output member). As illustrated in  FIGS. 6, 7, 9, and 10 , the back-torque transmitting cam includes cam surfaces (K 1 , T 1 ) that are respectively integrally formed in mating surfaces of the first clutch member  4   a  and the second clutch member  4   b  (mating surfaces when combined). 
     As illustrated in  FIGS. 6 and 9 , the cam surface K 1  includes a plurality of inclined surfaces formed along the entire circumference of the radially-inner side of the flange surface  4   ac  of the first clutch member  4   a  (the mating surface with the second clutch member  4   b ). The cam surface K 1  is formed in one end surface of each of a plurality of groove portions K that are annularly formed along the peripheral edge part of the first clutch member  4   a . That is, the plurality of groove portions K are formed in the first clutch member  4   a  so as to be arranged in the circumferential direction. One end surface of each groove portion K is an inclined surface that constitutes the cam surface K 1  of the back-torque transmitting cam. The other end surface of each groove portion K is a wall surface K 2  extending in the axial direction of the first clutch member  4   a.    
     As illustrated in  FIGS. 7 and 10 , the cam surface T 1  includes a plurality of inclined surfaces formed along the entire circumference of the bottom surface of the second clutch member  4   b  (the mating surface with the first clutch member  4   a ). The cam surface T 1  is formed in one end surface of each of a plurality of projecting portions T that are annularly formed along the bottom surface of the second clutch member  4   b . That is, the plurality of projecting portions T are formed in the second clutch member  4   b  arranged in the circumferential direction. One end surface of each projecting portion T is an inclined surface that constitutes the cam surface T 1  of the back-torque transmitting cam. The other end surface of each projecting portion T is a wall surface T 2 , extending in the axial direction of the second clutch member  4   b.    
     As illustrated in  FIG. 17 , when the projecting portion T is fit into the groove portion K to combine the first clutch member  4   a  and the second clutch member  4   b  with each other, the cam surface K 1  and the cam surface T 1  face each other to constitute the back-torque transmitting cam. The wall surface K 2  and the wall surface T 2  face each other with a predetermined distance therebetween. When a rotational force is input to the first clutch member  4   a , via the output shaft  3 , because the first clutch member  4   a  rotates relative to the second clutch member  4   b , as illustrated in  FIG. 18 , the second clutch member  4   b  is moved rightward in  FIGS. 2 and 18  relative to the first clutch member  4   a  due to the cam function of the cam surface K 1  and the cam surface T 1 . 
     As illustrated in  FIG. 7 , a pressing portion  4   bb  is formed in the second clutch member  4   b  on an extension line of the spline-fitting portion  4   ba . When the second clutch member  4   b  moves rightward in  FIG. 2 , the pressing portion  4   bb  presses in the same direction the driven-side clutch plate  7  on the leftmost side in the figure, which is one of the drive-side clutch plates  6  and the driven-side clutch plates  7  that are attached in a stacked state. Thus, even when the pressure member  5  is at the inactive position, it is possible to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other. When a rotational force is input from the output shaft  3  (output member), it is possible to transmit the rotational force to the engine side to apply an engine brake. 
     In particular, the back-torque transmitting cam according to the present embodiment can move the second clutch member  4   b  in a direction toward the interlocking member  9  (rightward in  FIG. 2 ) to maintain contact between the interlocking member  9  and the weight member  8 . That is, when the back-torque transmitting cam starts working and moves the second clutch member  4   b  rightward in  FIG. 2 , the back-torque transmitting cam presses the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other and presses the pressure member  5  in the same direction. Therefore, the pressing force is transmitted to the interlocking member  9 , via the clutch spring  11 , contact between the interlocking member  9  and the weight member  8  is maintained. 
     If the interlocking member  9  and the weight member  8  are separated from each other when the back-torque transmitting cam is working, even when the weight member  8  subsequently moves between the radially-inner position and the radially-outer position as the clutch housing  2  rotates, it may not be possible for the interlocking member  9  to follow the movement. In contrast, with the present embodiment, it is possible to maintain contact between the interlocking member  9  and the weight member  8  even when the back-torque transmitting cam is working. Thus, the interlocking member  9  can stably follow the movement of the weight member  8 . 
     Moreover, the plurality of the cam surfaces K 1  and T 1  of the back-torque transmitting cam according to the present embodiment are formed along the annular shape of the driven-side clutch plates  7  attached to the second clutch member  4   b . That is, the cam surfaces K 1  and T 1  are formed along the projected-image shape (annular shape) of the driven-side clutch plates  7  that are pressed by the pressing portion  4   bb  when the back-torque transmitting cam works. Thus, due to the cam function of the back-torque transmitting cam, the pressing portion  4   bb  can apply to the driven-side clutch plates  7  a substantially uniform pressing force. Thus, it is possible to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other more efficiency. 
     Furthermore, the back-torque transmitting cam, according to the present embodiment, a cam constituted by the cam surface K 1  and the cam surface T 1  can start working before the back-torque limiter cam (a cam constituted by the inclined surface  4   ab  and the inclined surface  5   b ) starts working. That is, the clearance, the dimension of the gap, between the cam surface K 1  and the cam surface T 1  is smaller than the clearance, the dimension of the gap, between the inclined surface  4   ab  and the inclined surface  5   b . Thus, the back-torque transmitting cam can start working before the back-torque limiter cam starts working. 
     Moreover, the power transmission device, according to the present embodiment, includes a torque transmitting portion that is formed in each of the first clutch member  4   a  and the second clutch member  4   b . It can transmit a rotational force that has been transmitted to the second clutch member  4   b  to the first clutch member  4   a  without using the back-torque transmitting cam (the cam surfaces K 1  and the cam surfaces T 1 ). A movement-amount limiting portion is formed in each of the first clutch member  4   a  and the second clutch member  4   b . It limits the movement amount of the second clutch member  4   b  due to the back-torque transmitting cam (the cam surfaces K 1  and the cam surfaces T 1 ). 
     That is, as illustrated in  FIGS. 6 and 9 , a plurality of (in the present embodiment, three) protruding portions F are integrally formed in the first clutch member  4   a  so as to be arranged at regular intervals in the circumferential direction. As illustrated in  FIGS. 7 and 9 , projecting portions G, extending inward, are integrally formed in the second clutch member  4   b . As illustrated in  FIGS. 15 and 16 , when the first clutch member  4   a  and the second clutch member  4   b  are assembled together, one protruding portion F is interposed between two projecting portions G. One side surface F 1  of the protruding portion F and the contact surface (first contact surface G 1 ) of one of the projecting portions G face each other. The other side surface F 2  of the protruding portion F and the contact surface (second contact surface G 2 ) of the other projecting portion G face each other. 
     One side surface F 1  of the protruding portion F, formed in the first clutch member  4   a , and the first contact surface G 1  of the projecting portion G, formed in the second clutch member  4   b , constitute the torque transmitting portion according to the present embodiment. That is, when the pressure member  5  moves to the active position to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other and the clutch is engaged (driving power is transmitted), while the wall surface K 2  of the groove portion K and the wall surface T 2  of the projecting portion T of the back-torque transmitting cam are maintained in a separated state (see  FIG. 17 ), as illustrated in  FIG. 15 , one side surface F 1  of the protruding portion F and the first contact surface G 1  of the projecting portion G contact each other. Thus, a rotational force of the second clutch member  4   b  can be received and transmitted to the first clutch member  4   a.    
     The other side surface F 2  of the protruding portion F, formed in the first clutch member  4   a , and the second contact surface G 2  of the other projecting portion G, formed in the second clutch member  4   b , constitute the movement-amount limiting portion according to the present embodiment. That is, when a rotational force is input to the first clutch member  4   a , via the output shaft  3 , the first clutch member  4   a  and the second clutch member  4   b  rotate relative to each other. Therefore, the second clutch member  4   b  moves (see  FIG. 18 ) due to the cam function of the cam surface K 1  of the groove portion K and the cam surface T 1  of the projecting portions T of the back-torque transmitting cam. When the movement amount reaches a set value, as illustrated in  FIG. 16 , the other side surface F 2  of the protruding portions F and the second contact surface G 2  of the projecting portion G contact each other. Thus, rotation of the second clutch member  4   b  relative to the first clutch member  4   a  is restricted. Therefore, it is possible to limit the movement amount of the second clutch member  4   b  when the back-torque transmitting cam is working. 
     In the present embodiment, the protruding portion F is formed in the first clutch member  4   a  and the projecting portion G is formed in the second clutch member  4   b . Alternatively, the projecting portion G may be formed in the first clutch member  4   a  and the protruding portion F may be formed in the second clutch member  4   b . In this case, one side surface F 1  of the protruding portion F, formed in the second clutch member  4   b , and the first contact surface G 1  of one projecting portion G, formed in the first clutch member  4   a , constitute the torque transmitting portion according to the present embodiment. The other side surface F 2  of the protruding portion F, formed in the second clutch member  4   b , and the second contact surface G 2  of the other projecting portion G, formed in the first clutch member  4   a , constitute the movement-amount limiting portion according to the present embodiment. 
     Next, the function of the back-torque transmitting cam in the present embodiment will be described. 
     As illustrated in  FIG. 2 , when the engine is stopped or idling, the weight member  8  is at the radially-inner position and the pressure member  5  is at the inactive position. Thus, the driving power of the engine is not transmitted to the input gear  1  or the rotational speed of the input gear  1  is low. At this time, when a rotational force is input to the first clutch member  4   a  via the output shaft  3  (output member), due to the cam function of the back-torque transmitting cam, the second clutch member  4   b  moves rightward in the figure. The drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other to transmit a rotational force to the engine side. 
     When the vehicle starts to move after having been stopped or idling, the rotational speed of the input gear  1  changes from a low rotational speed to a high rotational speed (middle rotational speed range). The weight member  8  is located between the radially-inner position and the radially-outer position and the pressure member  5  is located at the active position. At this time, when a rotational force is input to the first clutch member  4   a , via the output shaft  3  (output member) as, for example, acceleration pedal is released on a downward slope, due to the cam function of the back-torque transmitting cam, the second clutch member  4   b  moves rightward in the figure. The drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other to transmit a rotational force to the engine side. 
     After the vehicle has started, when the vehicle accelerates and travels in a high speed range, the weight member  8  is located at the radially-outer position and the pressure member  5  is located at the active position. Thus, the rotational speed of the input gear  1  is a high rotational speed. At this time, when a rotational force is input to the first clutch member  4   a , via the output shaft  3  (output member) in response to shift down or the like, due to the cam function of the back-torque transmitting cam, the second clutch member  4   b  moves rightward in the figure. The drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other to transmit a rotational force to the engine side. 
     Here, the present embodiment includes a cushioning member  12  that is interposed between the first clutch member  4   a  and the second clutch member  4   b . The cushioning member  4   a , by being compressed (spring is deformed), can apply an urging force while allowing movements of the interlocking member  9  and the pressure member  5  in the process where the interlocking member  9  moves and the pressure member  5  moves from the inactive position toward the active position. The cushioning member  12  includes a spring that is set to a load such that the spring is compressed before the clutch spring  11  starts to be compressed. As illustrated in  FIGS. 2, 3, and 21 , the cushioning member  12  is assembled by being accommodated in an accommodation recessed portion  4   c  formed in a surface where the first clutch member  4   a  and the second clutch member  4   b  face each other (to be specific, a surface of the first clutch member  4   a  facing the second clutch member  4   b ). 
     To be more specific, as illustrated in  FIG. 6 , the accommodation recessed portion  4   c  includes an annular groove. The cushioning member  12  is a spring, such as a coned-disc spring, a wave spring, or the like with an annular shape conformed to the shape of the groove. As illustrated in  FIG. 19 , the accommodation recessed portion  4   c  includes a groove having a wall surface  4   ca  on the radially-inner side and a wall surface  4   cb  on the radially-outer side. The cushioning member  12 , an annular spring, is conformed to the shape of the groove and fit into the accommodation recessed portion  4   c.    
     As described above, the back-torque transmitting cam, according to the present embodiment, is formed in a plurality of annular shapes in a surface where the first clutch member  4   a  and the second clutch member  4   b  face each other. As illustrated in  FIG. 6 , the accommodation recessed portion  4   c  is formed in a concentric circular shape at a position that is adjacent to the back-torque transmitting cam, in the present embodiment, on the radially-inner side of the position where the back-torque transmitting cam is formed. In the present embodiment, the accommodation recessed portion  4   c  is formed in a concentric circular shape at a position on the radially-inner side of the position of the back-torque transmitting cam. However, the accommodation recessed portion  4   c  may be formed in a concentric circular shape at a position on the radially-outer side of the position of the back-torque transmitting cam. In this case, as illustrated in  FIG. 24 , the cushioning member  12  may be configured to apply an urging force to a part (disc pack) where the drive-side clutch plates  6  and the driven-side clutch plates  7  are stacked, in a direction such that the drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other. 
     Next, the function the cushioning member  12  according to the present embodiment will be described in comparison with an existing device that does not have the cushioning member  12 . 
     First, the function of a case where the cushioning member  12  is not provided, in contrast to the present embodiment, will be described with reference to the graph of  FIG. 30 . The graph has the movement amount (mm) of the interlocking member  9  along the horizontal axis and the pressing load (N) generated in the interlocking member  9  along the vertical axis. In the graph of  FIG. 30 , P 1  indicates the pressing load of the interlocking member  9  at a time when the deformation amount (compression amount) of the release spring m is the maximum at a time of reaching the maximum load of the release spring m. P 2  indicates the pressing load of the interlocking member  9  at a time when the clutch spring  11  starts to be deformed, at a time of reaching a set load of the clutch spring  11 . In  FIGS. 22 and 23 , P 3  indicates the pressing load of the interlocking member  9  at a time when the cushioning member  12  starts to be deformed, at a time of reaching a set load of the cushioning member  12 . 
     In the process where the rotational speed of the engine increases and the weight member  8  moves from the radially-inner position to the radially-outer position to move the interlocking member  9 , the release spring m is deformed and the clutch spring  11  is not deformed. That is, the interlocking member  9  and the pressure member  5  move together until the movement amount of the interlocking member  9  reaches α. When the movement amount of the interlocking member  9  reaches α, although the pressing load (N) increases from P 1  to P 2 , the interlocking member  9  stops moving, and becomes a dead zone. 
     When the pressing load (N) reaches P 2 , the set load of the clutch spring  11 , from such a state, the clutch spring  11  starts to deform. The pressing load (N) increases in accordance with the movement of the interlocking member  9 . Thus, the interlocking member  9  and the pressure member  5  stop until the pressing load (N) reaches from P 1  to P 2 . As soon as the pressing load (N) reaches P 2 , the clutch spring  11  starts to be compressed, the clutch plates (the drive-side clutch plates  6  and the driven-side clutch plates  7 ) are pressed against each other to transmit power, and thus a feeling of surprise occurs when power is transmitted. 
     In contrast, the present embodiment includes the cushioning member  12  includes a spring that is set to a load such that the spring is compressed before the clutch spring  11  starts to be compressed in the process where the interlocking member  9  moves, a spring whose set load is set to P 3 . Therefore, as illustrated in  FIG. 22 , the cushioning member  12  starts to be compressed, starts to be deformed, from the time when the movement amount of the interlocking member  9  reaches β. Compression is continued to the time when the pressing load reaches P 2 . 
     That is, according to this graph, in the process where the interlocking member  9  moves as the rotational speed of the engine increases and the weight member  8  moves from the radially-inner position to the radially-outer position, the cushioning member  12  is compressed from the time when the movement amount of the interlocking member  9  becomes β and the pressing load (N) reaches P 3 . Subsequently, the clutch spring  11  starts to be deformed, starts to be compressed, as the movement amount of the interlocking member  9  reaches α and the pressing load (N) becomes P 2 . 
     Thus, when the interlocking member  9  starts moving, the pressing load (N) becomes the set load of the release spring m and the release spring m starts to be deformed, starts to be compressed. When the movement amount of the interlocking member  9  reaches β, the cushioning member  12  starts to be deformed, starts to be compressed. Subsequently, when the movement amount of the interlocking member  9  reaches α, the release spring (m) reaches the maximum load and the pressing load (N) reaches the set load (P 2 ) of the clutch spring  11 . Thus, the clutch spring  11  starts to be deformed, starts to be compressed. Until the clutch spring  11  reaches the maximum load, the upper limit of the working load, the clutch spring  11  continues to be compressed, continues to be deformed, due to the movement of the interlocking member  9 . 
     Accordingly, when the movement amount of the interlocking member  9  is between β and α, the cushioning member  12  is continuously compressed (deformed) to allow the movement of the interlocking member  9  in the process where the pressing load increases from P 3  to P 2 . Therefore, it is possible to reduce the dead zone of the existing device and to smoothly and continuously move the weight member  8  and the interlocking member  9 . Thus, it is possible to suppress shock when the clutch is engaged and to suppress a feeling of surprise when power is transmitted. 
     As illustrated in  FIG. 22 , the cushioning member  12  is configured to be continuously compressed, continues to be deformed, when the pressing load (N) is between P 3  and P 2 . However, as illustrated in  FIG. 23 , the cushioning member  12  may be set so that the cushioning member  12  starts to be compressed, starts to be deformed, from the time when the movement amount of the interlocking member  9  becomes β and the pressing load (N) reaches P 3 . The cushioning member  12  is continuously compressed, continued to be deformed, until the time when the pressing load (N) becomes P 4  that is lower than P 2 . Even in this case, because it is possible to reduce the dead zone that has occurred in the existing device, it is possible to move the weight member  8  and the interlocking member  9  smoothly and continuously, to suppress shock when the clutch is engaged, and to suppress a feeling of surprise when power is transmitted. 
     The present embodiment includes the cushioning member  12  interposed between the first clutch member  4   a  and the second clutch member  4   b . The release spring, being compressed, can apply an urging force while allowing movements of the interlocking member  9  and the pressure member  5  in the process where the interlocking member  9  moves and the pressure member  5  moves from the inactive position toward the active position. Therefore, with the present embodiment, it is possible to suppress a feeling of surprise when power is transmitted and to improve operability while efficiently utilizing a space that is obtained by the arrangement of the first clutch member  4   a  and the second clutch member  4   b.    
     The cushioning member  12  according to the present embodiment includes a spring that is set to a load such that the spring is compressed before the clutch spring  11  starts to be compressed. Therefore, it is possible to more reliably suppress a feeling of surprise when power is transmitted. In particular, it is possible to set in various ways the feeling (driving-start feeling) when power is transmitted in accordance with the type of a vehicle where the present device is applied by appropriately setting the set load and the working load of the cushioning member  12 . 
     Moreover, the cushioning member  12 , according to the present embodiment, is accommodated in the accommodation recessed portion  4   c  formed in a surface where the first clutch member  4   a  and the second clutch member  4   b  face each other. Therefore, it is possible to avoid positional displacement of the cushioning member  12  that may occur if the cushioning member  12  is involved when the first clutch member  4   a  moves relative to the second clutch member  4   b . The accommodation recessed portion  4   c , according to the present embodiment, is formed in a surface of the first clutch member  4   a  facing the second clutch member  4   b . However, the accommodation recessed portion  4   c  may be formed in a surface of the second clutch member  4   b  facing the first clutch member  4   a.    
     Furthermore, the accommodation recessed portion  4   c , according to the present embodiment, includes an annular groove, and a spring having an annular shape conformed to the shape of the groove. Therefore, it is possible to apply an urging force, generated by the cushioning member  12 , to the second clutch member  4   b  or the like substantially uniformly. Thus, it is possible to apply the urging force stably. The back-torque transmitting cam, according to the present embodiment, is formed in a plurality of annular shapes in a surface where the first clutch member  4   a  and the second clutch member  4   b  face each other. The accommodation recessed portion  4   c  is formed in a concentric circular shape at a position that is adjacent to the back-torque transmitting cam. Therefore, it is possible to enable the back-torque transmitting cam to reliably and stably move the second clutch member  4   b . This enables the cushioning member  12  to reliably and stably apply an urging force. 
     In addition, the bearing holding member C, according to the present embodiment, includes a cylindrical member with one open end. The open end portion Ca is fit into and attached to the recessed portion  4   d  formed in the clutch member (the first clutch member  4   a ) and attached in a socket-and-spigot joint state. Therefore, it is easy to assemble the bearing holding member C. Also, it is possible to stably operate the bearing holding member C when a shifting operation is performed. 
     The power transmission device includes the release spring (m) that applies an urging force to the pressure member  5  while allowing movements of the interlocking member  9  and the pressure member  5  until the drive-side clutch plates  6  and the driven-side clutch plates  7  reach an engaged state before the drive-side clutch plates  6  and the driven-side clutch plates  7  are pressed against each other. The release spring (m) is attached between the bearing holding member C and the pressure member  5 . The release spring applies an urging force to the pressure member  5  and applies an urging force to the bearing holding member C to transmit the urging force to the activation member  10 . Therefore, it is possible to use the release spring (m) as a spring to prevent play of a shift operation mechanism, and to reduce the number of components. 
     Moreover, the release spring (m), according to the present embodiment, includes a coned-disc spring that can generate an urging force due to a displacement between the middle part (ma) and the peripheral edge part (mb). The middle part (ma) is attached to the bearing holding member C. The peripheral edge part (mb) is attached to the pressure member  5 . Therefore, it is possible to stably apply urging forces of the release spring (m) to both the bearing holding member C and the pressure member  5 . 
     Furthermore, the clutch member, according to the present embodiment, includes the first clutch member  4   a , coupled to the output shaft  3  (output member), the second clutch member  4   b , attached to the driven-side clutch plates  7 , and a back-torque transmitting cam. The back-torque transmitting cam presses the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other by moving the second clutch member  4   b  when a rotational force is input to the first clutch member  4   a  via the output shaft  3  (output member). The recessed portion  4   d  is formed in the first clutch member  4   a . Therefore, it is possible to avoid the bearing holding member C from interfering with movement of the second clutch member  4   b  due to the back-torque transmitting cam. Also, it is possible to enable the back-torque transmitting cam to smoothly move each of the bearing holding member C and the second clutch member  4   b.    
     With the embodiment described above, the back-torque transmitting cam can move the second clutch member  4   b  in a direction toward the interlocking member  9  to maintain contact between the interlocking member  9  and the weight member  8 . Therefore, it is possible to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other to transmit a rotational force on the wheel side to the engine side to apply an engine brake. Also, it is possible to stably perform activation by the weight member  8  when an engine brake is applied. 
     The back-torque transmitting cam, according to the present embodiment, includes the cam surfaces (K 1 , T 1 ), respectively, integrally formed in the first clutch member  4   a  and the second clutch member  4   b . The cam surface (K 1 , T 1 ) are, respectively, formed in the mating surfaces of the first clutch member  4   a  and the second clutch member  4   b . Therefore, it is possible to enable the back-torque transmitting cam to reliably and smoothly move the second clutch member  2   b.    
     Moreover, the power transmission device includes a press-contact assisting cam that included the inclined surface  4   aa  of the first clutch member  4   a  and the inclined surface  5   a  of the pressure member  5  facing each other. The assisting cam increases the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7  when a rotational force input to the input gear  1  (input member) becomes capable of being transmitted to the output shaft  3  (output member). Therefore, it is possible to apply a press-contact force, due to the press-contact assisting cam, in addition to a press-contact force due to the movement of the weight member  8  by a centrifugal force. Thus, it is possible to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other more smoothly and more reliably. 
     Furthermore, the power transmission device includes a back-torque limiter cam that includes the inclined surface  4   ab  of the first clutch member  4   a  and the inclined surface  5   b  of the pressure member  5  facing each other. The back-torque limiter cam can release the press-contact force between the drive-side clutch plates  6  and the driven-side clutch plates  7  when the rotational speed of the output shaft  3  (output member) exceeds the rotational speed of the input gear  1  (input member) and the clutch member (the first clutch member  4   a ) and the pressure member  5  rotate relative to each other. Therefore, it is possible to avoid excessive driving power from being transmitted to the engine side via the input gear  1  when the weight member  8  is at the radially-outer position. Also, it is possible to reliably activate the back-torque transmitting cam because the back-torque transmitting cam is activated before the back-torque limiter cam starts working. 
     In addition, the present embodiment includes a back-torque transmitting cam that can move the second clutch member  4   b  to press the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other when a rotational force is input to the first clutch member  4   a , via the output shaft  3  (output member). A torque transmitting portion, formed in each of the first clutch member  4   a  and the second clutch member  4   b , can transmit a rotational force that has been transmitted to the second clutch member  4   b , to the first clutch member  4   a  without using the back-torque transmitting cam (the cam surface K 1  and the cam surface T 1 ). Therefore, it is possible to apply an engine brake by pressing the drive-side clutch plates  6  and the driven-side clutch plates  7  against each other to transmit a rotational force on the wheel side to the engine side. Also, it is possible to stably perform power transmission when the weight member  8  moves to the radially-outer position and the pressure member  5  moves to the active position. 
     Further, the power transmission device includes the movement-amount limiting portion, formed in each of the first clutch member  4   a  and the second clutch member  4   b , that limits the movement amount of the second clutch member  4   b  due to the back-torque transmitting cam. Therefore, it is possible to enable the back-torque transmitting cam to move the second clutch member  4   b  in a set range. 
     Moreover, the protruding portion F is formed in either one of the first clutch member  4   a  and the second clutch member  4   b . The torque transmitting portion includes the one side surface F 1  of protruding portion F and the first contact surface G 1  that receives a rotational force by contacting the one side surface F 1 . The movement-amount limiting portion includes the other side surface F 2  of the protruding portion F and the second contact surface G 2  that can limit the movement amount by contacting the other side surface F 2 . Therefore, the protruding portion F can function as both of the torque transmitting portion and the movement-amount limiting portion. 
     Heretofore, the present embodiment has been described. However, the present disclosure is not limited to these. For example, as illustrated in  FIGS. 25 and 26 , the bearing holding member C may have a plurality of (in the present embodiment, three) communication holes Cc formed in a side wall part. The holes allow oil supplied to the inside to flow to the outside via the communication holes Cc. As with the embodiment described above, the bearing holding member C has the plurality of communication holes Cc. The open end portion Ca is fit into and attached to the recessed portion  4   d  formed in the clutch member (the first clutch member  4   a ). 
     Thus, it is possible to smoothly supply oil into the clutch housing  2  by forming the communication holes Cc in a side wall part of the bearing holding member. This allows oil supplied to the inside to flow out to the outside via the communication holes Cc. The positions of the communication holes Cc and the number of the communication holes Cc may be set in any appropriate manner. Preferably, as in the present embodiment, the communication holes Cc are open toward a gap between the clutch member  4  and the pressure member  5 . In this case, it is possible to feed oil that has flowed out from the communication hole Cc to substantially the entire region in the clutch housing  2 . 
     For example, as illustrated in  FIG. 27 , the bearing holding member C may have a top portion Cb with an opening where the activation member  10  is inserted from a side opposite to that of the embodiment described above (the right side in the figure). The bearing holding member C is engaged with a roller bearing B 1 . The bearing holding member C may move in the left-right direction due to an operation by a driver or by working of an actuator. Thus, it may be able to move the pressure member  5  between the active position and the inactive position. In this case, it is possible to satisfactorily form an oil flow path r. 
     For example, as illustrated in  FIG. 28 , the bearing holding member C may be disposed in a power transmission device where the cushioning member  12  is not formed in the facing surfaces of the first clutch member  4   a  and the second clutch member  4   b . Alternatively, as illustrated in  FIG. 29 , the bearing holding member C may be disposed in a power transmission device that has an integrated clutch member  4  (a clutch member  4  that cannot be divided into the first clutch member  4   a  and the second clutch member  4   b ). 
     Moreover, in the present embodiment, the cushioning member  12  is a coned-disc spring, a wave spring, or the like. However, the cushioning member  12  may be replaced with another elastic member. Note that the power transmission device, according to the present disclosure can be applied to various multiple-disc clutch power transmission devices for, in addition to a motorcycle, an automobile, a three-wheeled or four-wheeled ATV, a general-purpose machine, and the like. 
     The bearing holding member can be applied to a power transmission device with a different outer shape or another function, as long as the bearing holding member includes a cylindrical member with one open end. An open end portion is fit into and attached to a recessed portion formed in a clutch member. 
     The present disclosure has been described with reference to the preferred embodiment. Obviously, modifications and alternations will occur to those of ordinary skill in the art upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed to include all such alternations and modifications insofar as they come within the scope of the appended claims or their equivalents.