Patent Publication Number: US-2023135364-A1

Title: Power transmission apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power transmission apparatus that is able to freely transmit a rotational force of an input to an output or cut off the rotational force. 
     2. Description of the Related Art 
     Usually, a power transmission apparatus included in a motorcycle freely transmits a driving force of an engine to a transmission and a driving wheel or cuts off the driving force. Such a power transmission apparatus includes an input connected to an engine; an output connected to a transmission and a driving wheel; a clutch member connected to the output; and a pressure member that is able to move close to or away from the clutch member. Bringing the pressure member close to the clutch member presses driving and driven clutch plates against each other so as to transmit power therebetween. Moving the pressure member away from the clutch member releases a pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the power therebetween. 
     As disclosed, for example, in WO 2013/183588, a power transmission apparatus known in the art includes a centrifugal clutch means including a weight member that moves from a radially inner position of a groove to a radially outer position thereof with centrifugal force produced by rotation of a clutch housing and is thus able to press driving and driven clutch plates against each other. The power transmission apparatus known in the art is able to apply centrifugal force to the weight member in accordance with the rotation of the clutch housing caused by driving of an engine and is thus able to press the driving and driven clutch plates against each other so as to transmit a driving force of the engine to a wheel. 
     SUMMARY OF THE INVENTION 
     Unfortunately, the power transmission apparatus known in the art may require increasing the number of driving clutch plates and the number of driven clutch plates in order to achieve a clutch capacity required for a multiple-plate clutch. In this case, if the required clutch capacity is achieved by simply increasing the number of driving clutch plates and the number of driven clutch plates in a stacking direction of the multiple-plate clutch, the size of the apparatus in an axial direction may increase, which may make the apparatus larger than a space in a vehicle where the apparatus is installable. Such a problem may occur not only in apparatuses including weight members but also in general multiple-plate clutch type power transmission apparatuses including driving and driven clutch plates. 
     Preferred embodiments of the present invention provide power transmission apparatuses that each achieve an increased clutch capacity while avoiding being increased in size in an axial direction. 
     A power transmission apparatus according to a preferred embodiment of the present invention includes a clutch housing that rotates together with an input that rotates due to a driving force of an engine of a vehicle, the clutch housing including a plurality of driving clutch plates attached thereto, a clutch including a plurality of driven clutch plates attached thereto, the driven clutch plates being alternately arranged with the driving clutch plates attached to the clutch housing, the clutch being connected to an output that is able to rotate a wheel of the vehicle, and a pressure applicator movable between an operating position where the driving and driven clutch plates are pressed against each other so as to enable transmission of the driving force of the engine to the wheel and a non-operating position where a pressing force exerted on the driving and driven clutch plates is released so as to cut off transmission of the driving force of the engine to the wheel. An auxiliary clutch plate different in diameter from the driving and driven clutch plates is included in the clutch housing. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed so as to enable transmission of the driving force of the engine to the wheel. When the pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released so as to cut off transmission of the driving force of the engine to the wheel. 
     The power transmission apparatus may also include a centrifugal clutch including a mass movable from a radially inner position to a radially outer position due to centrifugal force produced by rotation of the clutch housing. When the mass is located at the radially outer position, the centrifugal clutch presses the auxiliary clutch plate concurrently with pressing the driving and driven clutch plates against each other so as to enable transmission of the driving force of the engine to the wheel. When the mass is located at the radially inner position, the centrifugal clutch releases the pressing force exerted on the auxiliary clutch plate concurrently with releasing the pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the driving force of the engine to the wheel. 
     The centrifugal clutch may includes a holder to hold the mass such that the mass is movable between the radially inner position and the radially outer position, a press to press the driving and driven clutch plates against each other by moving in a stacking direction of the driving and driven clutch plates in response to movement of the mass from the radially inner position to the radially outer position, and a spring to urge the mass from the radially outer position to the radially inner position. When the mass is located at the radially outer position, one of the press and the holder presses the driving and driven clutch plates against each other, and the other one of the press and the holder presses the auxiliary clutch plate. 
     The auxiliary clutch plate may be positioned such that the auxiliary clutch plate overlaps, in an axial direction of the clutch housing, with a region where the driving and driven clutch plates are stacked. 
     The auxiliary clutch plate may include a plurality of clutch plates. 
     According to a preferred embodiment of the present invention, the auxiliary clutch plate different in diameter from the driving and driven clutch plates is provided in the clutch housing. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed so as to enable transmission of the driving force of the engine to the wheel. When the pressing force exerted on the driving and driven clutch plates is released, the pressing force exerted on the auxiliary clutch plate is released so as to cut off transmission of the driving force of the engine to the wheel. Consequently, this preferred embodiment is able to increase a clutch capacity while preventing the apparatus from increasing in size in the axial direction. 
     According to another preferred embodiment of the present invention, the power transmission apparatus includes the centrifugal clutch configured to, when the mass is located at the radially outer position, press the auxiliary clutch plate concurrently with pressing the driving and driven clutch plates against each other so as to enable transmission of the driving force of the engine to the wheel, and configured to, when the mass is located at the radially inner position, release the pressing force exerted on the auxiliary clutch plate concurrently with releasing the pressing force exerted on the driving and driven clutch plates so as to cut off transmission of the driving force of the engine to the wheel. Consequently, with the centrifugal clutch, this preferred embodiment is able to press the auxiliary clutch plate or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates against each other or releasing the pressing force exerted thereon. 
     According to a further preferred embodiment, when the mass is located at the radially outer position, one of the press and the holder presses the driving and driven clutch plates against each other, and the other one of the press and the holder presses the auxiliary clutch plate. Consequently, with the press and the holder included in the centrifugal clutch, this preferred embodiment is able to press the auxiliary clutch plate or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates against each other or releasing the pressing force exerted thereon. 
     According to yet another preferred embodiment of the present invention, the auxiliary clutch plate is positioned such that the auxiliary clutch plate overlaps, in the axial direction of the clutch housing, with the region where the driving and driven clutch plates are stacked. Consequently, this preferred embodiment is able to reliably prevent the apparatus from increasing in size in the axial direction. 
     According to an additional preferred embodiment of the present invention, the auxiliary clutch plate includes a plurality of clutch plates. Consequently, this preferred embodiment is able to increase a clutch capacity of the auxiliary clutch plate to any desired level. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an external view of a power transmission apparatus according to a preferred embodiment of the present invention. 
         FIG.  2    is a cross-sectional view taken along the line II-II in  FIG.  1   . 
         FIG.  3    is a cross-sectional view taken along the line in  FIG.  1   . 
         FIG.  4    is a perspective view of a clutch housing of the power transmission apparatus. 
         FIG.  5    is a three-view drawing of a first clutch member of the power transmission apparatus. 
         FIG.  6    is a perspective view of the first clutch member. 
         FIG.  7    is a three-view drawing of a second clutch member of the power transmission apparatus. 
         FIG.  8    is a perspective view of the second clutch member. 
         FIG.  9    is a three-view drawing of a pressure member of the power transmission apparatus. 
         FIG.  10    is a perspective view of the pressure member. 
         FIG.  11    is a longitudinal cross-sectional view of a centrifugal clutch of the power transmission apparatus. 
         FIG.  12    is a partially cutaway perspective view of the centrifugal clutch. 
         FIG.  13    is a three-view drawing of a holding member included in the centrifugal clutch. 
         FIG.  14    is a three-view drawing of a supporting member included in the centrifugal clutch. 
         FIG.  15    is a three-view drawing of a pressing member included in the centrifugal clutch. 
         FIG.  16    is a four-view drawing of a weight member included in the centrifugal clutch. 
         FIG.  17    is a cross-sectional view taken along the line XVII-XVII in  FIG.  16   . 
         FIG.  18    is a plan view illustrating a state of the centrifugal clutch where each weight member is located at a radially inner position. 
         FIG.  19    is a plan view illustrating a state of the centrifugal clutch where each weight member is located at a radially outer position. 
         FIG.  20 A  is a schematic diagram describing how a pressing assist cam of the power transmission apparatus acts. 
         FIG.  20 B  is a schematic diagram describing how a back torque limiter cam of the power transmission apparatus acts. 
         FIG.  21    is a schematic diagram of a vehicle in which the power transmission apparatus is used. 
         FIG.  22    is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at the radially inner position. 
         FIG.  23    is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at an intermediate position between the radially inner position and the radially outer position. 
         FIG.  24    is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at the radially outer position. 
         FIG.  25    is a cross-sectional view illustrating a state of the power transmission apparatus where each weight member is located at the radially outer position and the pressure member is located at a non-operating position. 
         FIG.  26    is a longitudinal cross-sectional view of a power transmission apparatus according to another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates). 
         FIG.  27    is a longitudinal cross-sectional view of a power transmission apparatus according to still another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates and overlaps with a region where driving and driven clutch plates are stacked). 
         FIG.  28    is a longitudinal cross-sectional view of a power transmission apparatus according to yet another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates, and the apparatus includes no centrifugal clutch). 
         FIG.  29    is a longitudinal cross-sectional view of a power transmission apparatus according to still yet another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates and overlaps with a region where driving and driven clutch plates are stacked, and the apparatus includes no centrifugal clutch). 
         FIG.  30    is a longitudinal cross-sectional view of a power transmission apparatus according to another preferred embodiment of the present invention (in which an auxiliary clutch plate includes a plurality of clutch plates, and the apparatus includes no centrifugal clutch but includes a pressing assist cam and a back torque limiter cam). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in detail with reference to the drawings. As illustrated in  FIG.  21   , a power transmission apparatus K according to the present preferred embodiment is disposed in a vehicle so as to freely transmit a driving force of an engine E to a driving wheel T through a transmission M or cut off the driving force. As illustrated in  FIGS.  1  to  17   , the power transmission apparatus K includes a clutch housing  2  provided with an input gear  1  (which is an input) that rotates with the driving force of the engine E of the vehicle, an output shaft  3  (which is an output) connected to the transmission M, a clutch (which includes a first clutch member  4   a  and a second clutch member  4   b ), a pressure member  5  (which is a pressure applicator), a plurality of driving clutch plates  6 , a plurality of driven clutch plates  7 , a centrifugal clutch  9  including weight members  10  (which are masses), and an auxiliary clutch plate  17 . 
     The input gear  1  is rotatable around the output shaft  3  upon receiving a driving force (or a rotational force) transmitted from the engine E. The input gear  1  is connected to the clutch housing  2  with a fastener, such as a rivet. The clutch housing  2  is a cylindrical member with an opening defined at its right end in  FIGS.  2  and  3    and is connected to the input gear  1 . The clutch housing  2  is rotatable together with rotation of the input gear  1  with the driving force of the engine E. 
     As illustrated in  FIG.  4   , the clutch housing  2  is circumferentially provided with a plurality of cut-outs  2   a.  The driving clutch plates  6  are fitted to the cut-outs  2 A and thus attached to the clutch housing  2 . The driving clutch plates  6  are each made of a substantially annular plate material. The driving clutch plates  6  are rotatable together with rotation of the clutch housing  2 . The driving clutch plates  6  are slidable in an axial direction (which corresponds to a right-left direction in  FIGS.  2    and  3 ). 
     The driven clutch plates  7  are attached to the clutch (which includes the first clutch member  4   a  and the second clutch member  4   b ). The driven clutch plates  7  are arranged alternately with the driving clutch plates  6  attached to the clutch housing  2 . The clutch is connected to the output shaft  3  (i.e., the output) that is able to rotate the driving wheel T through the transmission M of the vehicle. The clutch is provided by assembling two members, i.e., the first clutch member  4   a  and the second clutch member  4   b,  to each other. 
     The output shaft  3  is inserted through an insertion hole (see  FIGS.  5  and  6   ) defined in the center of the first clutch member  4   a.  A gear provided on the first clutch member  4   a  and a gear provided on the output shaft  3  mesh with each other so as to be connected to each other in the direction of rotation. As illustrated in  FIGS.  5  and  6   , the first clutch member  4   a  is provided with inclined surfaces  4   aa  each defining a pressing assist cam, and inclined surfaces  4   ab  each defining a back torque limiter cam. The reference signs “ 4   ac”  in  FIGS.  5  and  6    each indicate a boss provided with an insertion hole for a bolt B for connection between the first clutch member  4   a  and a securing member  8 . 
     As illustrated in  FIGS.  7  and  8   , the second clutch member  4   b  is an annular member provided with a flange  4   bb.  The driven clutch plates  7  are spline-fitted to a spline-fitting portion  4   ba  provided on the outer peripheral surface of the second clutch member  4   b  and are thus attached to the second clutch member  4   b.  As illustrated in  FIGS.  2  and  3   , the pressure member  5  is assembled to the clutch member (which includes the first clutch member  4   a  and the second clutch member  4   b ) such that the driving and driven clutch plates  6  and  7  are alternately stacked and secured between a flange  5   c  of the pressure member  5  and the flange  4   bb  of the second clutch member  4   b.    
     As illustrated in  FIGS.  9  and  10   , the pressure member  5  is a disk member provided with the flange  5   c  extending along the peripheral edge of the pressure member  5 . The pressure member  5  is movable between an operating position where the driving and driven clutch plates  6  and  7  are pressed against each other so as to enable transmission of the driving force of the engine E to the wheel, and a non-operating position where a pressing force exerted on the driving and driven clutch plates  6  and  7  is released so as to cut off transmission of the driving force of the engine E to the wheel. 
     More specifically, as illustrated in  FIGS.  7  and  8   , the spline-fitting portion  4   ba  provided on the second clutch member  4   b  defines integral projections and depressions along substantially the entire outer peripheral side surface of the second clutch member  4   b.  Fitting the driven clutch plates  7  to recessed grooves that define the spline-fitting portion  4   ba  restricts movement of the driven clutch plates  7  in the direction of rotation while allowing movement of the driven clutch plates  7  in the axial direction relative to the second clutch member  4   b.  The driven clutch plates  7  are rotatable together with the second clutch member  4   b.    
     The driven clutch plates  7  are stacked alternately with the driving clutch plates  6 , making it possible to allow the clutch plates  6  and  7  adjacent to each other to be pressed against each other or release the pressing force exerted thereon. In other words, the clutch plates  6  and  7  are allowed to slide in the axial direction of the second clutch member  4   b.  Engaging a clutch by pressing the clutch plates ( 6   a,    6   b,    7   a,    7   b ) against each other enables a rotational force of the clutch housing  2  to be transmitted to the output shaft  3  through the second clutch member  4   b  and the first clutch member  4   a.  Disengaging the clutch by releasing the pressing force exerted on the clutch plates ( 6   a,    6   b,    7   a,    7   b ) causes the first clutch member  4   a  and the second clutch member  4   b  to stop following the rotation of the clutch housing  2  such that no rotational force will be transmitted to the output shaft  3 . 
     Accordingly, a state where the driving and driven clutch plates  6  and  7  are pressed against each other enables transmission of a rotational force (i.e., the driving force of the engine E), which is received by the clutch housing  2 , to the driving wheel (or the transmission M) through the output shaft  3  (i.e., the output), and a state where the driving and driven clutch plates  6  and  7  are not pressed against each other makes it possible to cut off transmission of the rotational force (or the driving force of the engine E), which is received by the clutch housing  2 , to the output shaft  3  (i.e., the output). 
     As illustrated in  FIGS.  9  and  10   , the pressure member  5  includes a plurality of fitting holes  5   d  (i.e., three fitting holes  5   d  in the present preferred embodiment) circumferentially arranged. A clutch spring S is fitted into each of the fitting holes  5   d.  As illustrated in  FIG.  2   , one end of each clutch spring S housed in the associated fitting hole  5   d  is in abutment with the securing member  8 , such that each clutch spring S urges the pressure member  5  in a direction in which the driving and driven clutch plates  6  and  7  are to be pressed against each other. Operating a clutch actuator (not illustrated) makes it possible to press the driving and driven clutch plates  6  and  7  against each other or stop pressing the driving and driven clutch plates  6  and  7  against each other. 
     In the present preferred embodiment, as illustrated in  FIGS.  5 ,  6 ,  9 , and  10   , the first clutch member  4   a  is provided with the inclined surfaces  4   aa  and  4   ab,  and the pressure member  5  is provided with inclined surfaces  5   a  and  5   b  respectively facing the inclined surfaces  4   aa  and  4   ab.  Specifically, each inclined surface  4   aa  comes into abutment with the associated inclined surface  5   a  so as to provide the pressing assist cam, and each inclined surface  4   ab  comes into abutment with the associated inclined surface  5   b  so as to provide the back torque limiter cam. 
     Suppose that the rotation speed of the engine E is increased and the resulting rotational force received by the input gear  1  and the clutch housing  2  is transmittable to the output shaft  3  through the first clutch member  4   a  and the second clutch member  4   b  (which means that the weight members  10  are each located at a radially outer position). In this case, as illustrated in  FIG.  20 A , the rotational force is applied to the pressure member  5  in an a-direction, which exerts a force on the pressure member  5  in a c-direction in  FIG.  20 A  under the action of each pressing assist cam. The pressure member  5  is thus moved in a direction in which its flange  5   c  comes closer to the flange  4   bb  of the second clutch member  4   b  (i.e., leftward in  FIGS.  2  and  3   ) so as to increase the pressing force exerted on the driving and driven clutch plates  6  and  7 . 
     Suppose that the rotation of the output shaft  3  exceeds the rotation speed of the input gear  1  and the clutch housing  2 , resulting in a back torque. In this case, as illustrated in  FIG.  20 B , a rotational force is applied to the clutch member  4  in a b-direction, which moves the pressure member  5  in a d-direction in  FIG.  20 B  under the action of each back torque limiter cam so as to release the pressing force exerted on the driving and driven clutch plates  6  and  7 . This makes it possible to prevent a malfunction in the power transmission apparatus K or a power source (i.e., the engine E) caused by the back torque. 
     As illustrated in  FIGS.  11  to  19   , the centrifugal clutch  9  includes the weight members  10  each movable from a radially inner position (see  FIG.  18   ) to the radially outer position (see  FIG.  19   ) with centrifugal force produced by rotation of the clutch housing  2 . When the weight members  10  are each located at the radially outer position, the centrifugal clutch  9  presses the driving and driven clutch plates  6  and  7  against each other so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the weight members  10  are each located at the radially inner position, the centrifugal clutch  9  releases the pressing force exerted on the driving and driven clutch plates  6  and  7  so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). 
     Specifically, the centrifugal clutch  9  includes the weight members  10 , each of which is a substantially symmetrical polygonal member, a holding member  11  (holder) including a supporting member  13  attached thereto, a pressing member  12  (press), first spherical members  14 , second spherical members  15 , and urging members  16 , each of which is a coil spring. The holding member  11  and the pressing member  12  are each circumferentially provided with a plurality of protrusions. Similarly to the driving clutch plates  6 , the protrusions are fitted to the cut-outs  2   a  of the clutch housing  2  such that the holding member  11  and the pressing member  12  are attached to the clutch housing  2 . Accordingly, the holding member  11  and the pressing member  12  are each movable in the axial direction of the clutch housing  2  and in engagement with the clutch housing  2  in the direction of rotation so as to be rotatable together with the clutch housing  2 . 
     As illustrated in  FIG.  16   , each weight member  10  is a substantially symmetrical polygonal member including a first surface X and a second surface Y. As illustrated in  FIGS.  16  and  17   , each weight member  10  includes through holes  10   a  passing through the weight member  10  from the first surface X to the second surface Y, insertion portions  10   b  defined in the second surface Y, and a groove  10   c  defined in the first surface X. As illustrated in  FIGS.  18  and  19   , the weight members  10  are each housed in an associated one of housing portions  11   a  of the holding member  11 . When no centrifugal force is applied to the weight members  10 , the weight members  10  are each held at the radially inner position (see  FIG.  18   ). Application of centrifugal force to the weight members  10  moves the weight members  10  outward against the urging force of the urging members  16  such that each weight member  10  reaches the radially outer position (see  FIG.  19   ). 
     The holding member  11  holds the weight members  10  such that each weight member  10  is movable between the radially inner position and the radially outer position. As illustrated in  FIG.  13   , the holding member  11  is an annular member. The holding member  11  includes the housing portions  11   a  which are arranged in the circumferential direction of the holding member  11  and in which the weight members  10  are to be housed, groove geometries  11   b  defined in the housing portions  11   a,  and a pushing surface  11   c.  Each housing portion  11   a  has a recessed shape conforming to the shape and moving range of the associated weight member  10 . An inner peripheral wall surface  11   aa  of each housing portion  11   a  is allowed to abut against first ends of the associated urging members  16 . 
     The supporting member  13  is secured to a surface of the holding member  11 , which is provided with the housing portions  11   a.  As illustrated in  FIG.  14   , the supporting member  13  is provided with radially extending holding portions  13   a.  The holding portions  13   a  each conform to the groove  10   c  of the associated weight member  10 , so that the weight members  10  are held by the holding member  11 . Specifically, the first surface X of each weight member  10  is centrally provided with the groove  10   c  extending in a direction from the radially inner position to the radially outer position. Causing each holding portion  13   a  to conform to the associated groove  10   c  allows the holding member  11  to hold the weight members  10  such that each weight member  10  is movable in a radial direction (i.e., in a direction from the radially inner position to the radially outer position). 
     Movement of each weight member  10  from the radially inner position to the radially outer position causes the pressing member  12  to move in a stacking direction of the driving and driven clutch plates  6  and  7  (i.e., rightward in  FIGS.  2  and  3   ) such that the driving and driven clutch plates  6  and  7  are pressed against each other. Specifically, as illustrated in  FIG.  15   , the pressing member  12  is an annular member. The pressing member  12  includes inclined grooves  12   a  arranged in the circumferential direction of the pressing member  12 , groove geometries  12   b  each defined at a position where the associated inclined groove  12   a  is defined, and a pushing surface  12   c.    
     Each inclined groove  12   a  is defined at a position corresponding to the position of the associated weight member  10 . Each inclined groove  12   a  is inclined upward from its inner portion to its outer portion. Thus, when the clutch housing  2  is stationary, each weight member  10  is held at the radially inner position with the urging force of the associated urging members  16 . Rotation of the clutch housing  2  applies centrifugal force to the weight members  10  so as to move the weight members  10  along the inclined grooves  12   a  inclined upward. This moves the pressing member  12  away from the holding member  11  (i.e., in the direction in which the driving and driven clutch plates  6  and  7  are to be pressed against each other). 
     When the holding member  11  and the pressing member  12  are assembled to each other, with the weight members  10  interposed therebetween, each inclined groove  12   a  is located at a position corresponding to the position of the associated weight member  10  as illustrated in  FIGS.  11  and  12   . With centrifugal force, the weight members  10  each move along the inclined grooves  12   a  from the radially inner position to the radially outer position, which moves the pressing member  12  in a direction indicated by the arrow in  FIG.  11    (i.e., rightward in  FIG.  11   ). The pushing surface  12   c  of the pressing member  12  thus pushes the driving and driven clutch plates  6  and  7  such that the driving and driven clutch plates  6  and  7  are pressed against each other. The resulting reaction force moves the holding member  11  in a direction opposite to the direction indicated by the arrow in  FIG.  11    (i.e., leftward in  FIG.  11   ), so that the pushing surface  11   c  of the holding member  11  presses the auxiliary clutch plate  17 . 
     As illustrated in  FIGS.  18  and  19   , the weight members  10  according to the present preferred embodiment are each housed in an associated one of the housing portions  11   a  arranged in the circumferential direction of the holding member  11  such that the weight members  10  are movable radially. More than one urging member  16  (i.e., two urging members  16  in the present preferred embodiment) is disposed in the circumferential direction between the inner peripheral wall surface  11   aa  (see  FIG.  13   ) of each of the housing portions  11   a  and an associated one of the weight members  10  so as to urge the associated weight member  10  from the radially outer position to the radially inner position. In this preferred embodiment, the inner peripheral wall surface  11   aa  of each housing portion  11   a  is a flat surface in abutment with the first ends of the associated urging members  16  such that the urging members  16  are stably securable. 
     The weight members  10  according to the present preferred embodiment are each provided with the tunnel-shaped insertion portions  10   b  which are openings defined in a surface of each weight member  10  facing the holding member  11  (i.e., the second surface Y in  FIG.  17   ) and into which the urging members  16  are inserted such that the urging members  16  are attachable to the associated weight member  10 . The weight members  10 , with the urging members  16  inserted into the insertion portions  10   b,  are housed in the housing portions  11   a  of the holding member  11 . The urging members  16  are thus secured such that each urging member  16  is interposed between the inner peripheral wall surface  11   aa  of the associated housing portion  11   a  and the associated weight member  10 . The urging members  16  are disposed such that the first end of each urging member  16  is in abutment with the associated inner peripheral wall surface  11   aa  and a second end of each urging member  16  is in abutment with an end wall surface  10   ba  of the associated insertion portion  10   b,  making it possible to urge each weight member  10  from the radially outer position to the radially inner position. 
     Each first spherical member  14  is a steel ball attached to the associated weight member  10 . As illustrated in  FIGS.  16  and  17   , each first spherical member  14  is partially protruded from a first opening  10   aa  (i.e., a small-diameter opening in the first surface X) of the through hole  10   a  defined in the associated weight member  10  and is in contact with a rolling contact surface of the pressing member  12  so as to be rollable thereon. Each second spherical member  15  is a steel ball attached to the associated weight member  10 . As illustrated in  FIGS.  16  and  17   , each second spherical member  15  is partially protruded from a second opening  10   ab  (i.e., a large-diameter opening in the second surface Y) of the through hole  10   a  defined in the associated weight member  10  and is in contact with a rolling contact surface of the holding member  11  so as to be rollable thereon. 
     As illustrated in  FIG.  17   , the through holes  10   a  according to the present preferred embodiment are tapered such that the diameter of each through hole  10   a  continuously increases from the first opening  10   aa  (i.e., the small-diameter opening in the first surface X) to the second opening  10   ab  (i.e., the large-diameter opening in the second surface Y). Disconnection of each first spherical member  14  from the associated through hole  10   a  is prevented by the outer peripheral edge of one of the first opening  10   aa  and the second opening  10   ab  that has a smaller diameter (which is, in the present preferred embodiment, the first opening  10   aa  in the first surface X). The first and second spherical members  14  and  15  according to the present preferred embodiment are spherical members having different diameters in accordance with the inner diameters of the through holes  10   a.  The second spherical members  15  are larger in diameter than the first spherical members  14 . The small-diameter first spherical members  14  are each rollable while being in contact with the inner peripheral surface of the associated through hole  10   a,  and at the same time, disconnection of the small-diameter first spherical members  14  is prevented by the small-diameter opening edges of the through holes  10   a.    
     As illustrated in  FIGS.  11  and  12   , disconnection of the second spherical members  15  is prevented by the rolling contact surface of the holding member  11 . Thus, disconnection of the small-diameter first spherical members  14  is prevented by the small-diameter opening edges of the through holes  10   a,  and disconnection of the large-diameter second spherical members  15  is prevented by the rolling contact surface of the holding member  11  while the second spherical members  15  are partially protruded from the large-diameter openings of the through holes  10   a.  In the present preferred embodiment, the large-diameter second spherical members  15  are assembled to the weight members  10  such that the second spherical members  15  face the rolling contact surface of the holding member  11 . Alternatively, the second spherical members  15  may be assembled to the weight members  10  such that the second spherical members  15  face the rolling contact surface of the pressing member  12 . In this case, disconnection of the small-diameter first spherical members  14  is prevented by the small-diameter opening edges of the through holes  10   a,  and disconnection of the large-diameter second spherical members  15  is prevented by the rolling contact surface of the pressing member  12  while the second spherical members  15  are partially protruded from the large-diameter openings of the through holes  10   a.    
     As illustrated in  FIG.  13   , the rolling contact surface of the holding member  11  (which is, in the present preferred embodiment, a surface of the holding member  11  on which the second spherical members  15  roll) includes the groove geometries  11   b  each extending in the direction of movement of the associated weight member  10  (i.e., a direction connecting the radially inner position to the radially outer position). As illustrated in  FIG.  15   , the rolling contact surface of the pressing member  12  (which is, in the present preferred embodiment, a surface of the pressing member  12  on which the first spherical members  14  roll) includes the groove geometries  12   b  each extending in the direction of movement of the associated weight member  10  (i.e., a direction connecting the radially inner position to the radially outer position). 
     As illustrated in  FIGS.  16 ,  18 , and  19   , the first and second spherical members  14  and  15  according to the present preferred embodiment are provided such that more than one first spherical member  14  and more than one second spherical member  15  are arranged in the circumferential direction of the holding member  11  (i.e., such that two first spherical members  14  and two second spherical members  15  are arranged in the width direction of each weight member  10  in the present preferred embodiment). In response to movement of the weight members  10 , the first spherical members  14  are movable along the groove geometries  12   b  while rolling within the through holes  10   a,  and the second spherical members  15  are movable along the groove geometries  11   b  while rolling within the through holes  10   a.    
     The auxiliary clutch plate  17  is disposed in the clutch housing  2 . The auxiliary clutch plate  17  is an annular member different in diameter from the driving and driven clutch plates  6  and  7  (i.e., smaller in diameter than the driving and driven clutch plates  6  and  7  in the present preferred embodiment). As illustrated in  FIGS.  2  and  3   , the output shaft  3  (i.e., the output) is inserted through a central opening  17   a  of the auxiliary clutch plate  17  so as to be fitted thereto. The auxiliary clutch plate  17  includes a pushed surface  17   b  facing the pushing surface  11   c  of the holding member  11 . 
     When the weight members  10  are each located at the radially outer position (i.e., when the driving and driven clutch plates  6  and  7  are pressed against each other), the auxiliary clutch plate  17  is able to transmit the driving force of the engine E to the output shaft  3  upon being pushed by the pushing surface  11   c  of the holding member  11  and pressed against the pushing surface  11   c.  When the weight members  10  are each located at the radially inner position (i.e., when the pressing force exerted on the driving and driven clutch plates  6  and  7  is released), the auxiliary clutch plate  17  is able to cut off transmission of the driving force of the engine E to the output shaft  3  upon being relieved of a pressing force applied thereto, owing to a decrease in pushing force exerted by the pushing surface  11   c  of the holding member  11 . 
     Upon movement of each weight member  10  to the radially outer position, each inclined groove  12   a  functions as a cam so as to cause the holding member  11  and the pressing member  12  to move away from each other. Accordingly, the pushing surface  12   c  of the pressing member  12  presses the driving and driven clutch plates  6  and  7  against each other, and the pushing surface  11   c  of the holding member  11  pushes the pushed surface  17   b  of the auxiliary clutch plate  17  such that the pushed surface  17   b  is pressed against the pushing surface  11   c  so as to transmit the driving force of the engine E to the driving wheel T. 
     As illustrated in  FIGS.  5  and  6   , a portion of a surface of the first clutch member  4   a  according to the present preferred embodiment, which faces the pressure member  5 , defines an abutment surface  4   ad.  As illustrated in  FIGS.  9  and  10   , a portion of a surface of the pressure member  5 , which faces the first clutch member  4   a,  defines an abutment surface  5   e.  With the first clutch member  4   a,  the second clutch member  4   b,  and the pressure member  5  assembled to each other (and with no torque being transmitted from the input gear  1  (i.e., the input) to the output shaft  3  (i.e., the output)), the abutment surface  4   ad  and the abutment surface  5   e  are in abutment with each other as illustrated in  FIGS.  2  and  3   . 
     With the abutment surface  4   ad  and the abutment surface  5   e  in abutment with each other as mentioned above, in the course of movement of each weight member  10  of the centrifugal clutch  9  from the radially inner position (see  FIG.  22   ) to an intermediate position (see  FIG.  23   ) and a resulting increase in torque transmitted from the input gear  1  (i.e., the input) to the output shaft  3  (i.e., the output), the first clutch member  4   a  and the pressure member  5  are not allowed to move relative to each other, which restricts operation of the pressing assist cams. 
     Each weight member  10  of the centrifugal clutch  9  then moves from the intermediate position (see  FIG.  23   ) to the radially outer position (see  FIG.  24   ), and the driving and driven clutch plates  6  and  7  are pushed by the flange  4   bb  of the second clutch member  4   b  and thus pressed against each other. When the pushing force of the flange  4   bb  is equal to or greater than the urging force of the clutch springs S, the second clutch member  4   b  and the pressure member  5  are moved in the axial direction (i.e., rightward in  FIGS.  2  and  3   ) relative to the first clutch member  4   a,  so that the abutment surface  4   ad  of the first clutch member  4   a  and the abutment surface  5   e  of the pressure member  5  are located away from each other.  FIG.  25    illustrates a state in which the weight members  10  are each located at the radially outer position and the pressure member  5  is located at the non-operating position (i.e., a clutch-disengaged state). 
     With the abutment surface  4   ad  and the abutment surface  5   e  located away from each other as mentioned above, in the course of movement of each weight member  10  of the centrifugal clutch  9  from the radially inner position to the radially outer position and a resulting increase in torque transmitted from the input gear  1  (i.e., the input) to the output shaft  3  (i.e., the output), the first clutch member  4   a  and the pressure member  5  are allowed to move relative to each other, which allows operation of the pressing assist cams. 
     In the present preferred embodiment, when the driving and driven clutch plates  6  and  7  are pressed against each other, the auxiliary clutch plate  17  is pressed so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the pressing force exerted on the driving and driven clutch plates  6  and  7  is released, the pressing force exerted on the auxiliary clutch plate  17  is released so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). 
     More specifically, the present preferred embodiment involves providing the centrifugal clutch  9  including the weight members  10 . When the weight members  10  are each located at the radially outer position, the centrifugal clutch  9  presses the auxiliary clutch plate  17  concurrently with pressing the driving and driven clutch plates  6  and  7  against each other so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the weight members  10  are each located at the radially inner position, the centrifugal clutch  9  releases the pressing force exerted on the auxiliary clutch plate  17  concurrently with releasing the pressing force exerted on the driving and driven clutch plates  6  and  7  so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). 
     In the present preferred embodiment, in particular, when the weight members  10  are each located at the radially outer position, the pressing member  12  and the holding member  11  move away from each other, the pressing member  12  presses the driving and driven clutch plates  6  and  7  against each other, and the holding member  11  presses the auxiliary clutch plate  17 . Alternatively, the centrifugal clutch  9  may be disposed such that when the weight members  10  are each located at the radially outer position, the pressing member  12  and the holding member  11  move away from each other, the holding member  11  presses the driving and driven clutch plates  6  and  7  against each other, and the pressing member  12  presses the auxiliary clutch plate  17 . 
     In the present preferred embodiment, the auxiliary clutch plate  17  different in diameter from the driving and driven clutch plates  6  and  7  is disposed in the clutch housing  2 . When the driving and driven clutch plates  6  and  7  are pressed against each other, the auxiliary clutch plate  17  is pressed so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). When the pressing force exerted on the driving and driven clutch plates  6  and  7  is released, the pressing force exerted on the auxiliary clutch plate  17  is released so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). Accordingly, the present preferred embodiment is able to increase a clutch capacity while preventing the apparatus from increasing in size in the axial direction. 
     The auxiliary clutch plate  17  according to the present preferred embodiment, in particular, is disposed radially inward of a region where the driving and driven clutch plates  6  and  7  are stacked (which defines a multiple-plate clutch portion) inside the clutch housing  2 . Accordingly, the present preferred embodiment is able to increase a clutch capacity while preventing the apparatus from increasing in size not only in the axial direction but also in a direction perpendicular to the axial direction, and to effectively utilize a radially inner space inside the clutch housing  2 . 
     The present preferred embodiment involves providing the centrifugal clutch  9  configured to, when the weight members  10  are each located at the radially outer position, press the auxiliary clutch plate  17  concurrently with pressing the driving and driven clutch plates  6  and  7  against each other so as to enable transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T), and configured to, when the weight members  10  are each located at the radially inner position, release the pressing force exerted on the auxiliary clutch plate  17  concurrently with releasing the pressing force exerted on the driving and driven clutch plates  6  and  7  so as to cut off transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). Accordingly, with the centrifugal clutch  9 , the present preferred embodiment is able to press the auxiliary clutch plate  17  or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates  6  and  7  against each other or releasing the pressing force exerted thereon. 
     When the weight members  10  are each located at the radially outer position, one of the pressing member  12  and the holding member  11  presses the driving and driven clutch plates  7  and  12  against each other, and the other one of the pressing member  12  and the holding member  11  presses the auxiliary clutch plate  17 . Accordingly, with the pressing member  12  and the holding member  11  included in the centrifugal clutch  9 , the present preferred embodiment is able to press the auxiliary clutch plate  17  or release the pressing force exerted thereon, in addition to pressing the driving and driven clutch plates  6  and  7  against each other or releasing the pressing force exerted thereon. 
     In the power transmission apparatus K according to the present preferred embodiment, the through holes  10   a  of the weight members  10  included in the centrifugal clutch  9  each have a tapered shape from the first opening  10   aa  to the second opening  10   ab.  Disconnection of each first spherical member  14  is prevented by the outer peripheral edge of one of the associated first opening  10   aa  and the associated second opening  10   ab  that has a smaller diameter. Accordingly, the present preferred embodiment is able to easily and accurately attach the first spherical members  14  to the weight members  10 , resulting in a reduction in manufacturing cost. 
     The first and second spherical members  14  and  15  are spherical members having different diameters in accordance with the inner diameters of the through holes  10   a.  The first and second spherical members  14  and  15  are rollable while being in contact with the inner peripheral surfaces of the through holes  10   a.  Thus, during movement of the weight members  10 , the first and second spherical members  14  and  15  are rollable in a stable manner such that the weight members  10  move smoothly. Disconnection of the second spherical members  15  according to the present preferred embodiment is prevented by the rolling contact surface of the holding member  11  or the pressing member  12 . Consequently, assembling the holding member  11  and the pressing member  12  to each other facilitates preventing disconnection of the first and second spherical members  14  and  15 . 
     The rolling contact surface of the holding member  11  or the pressing member  12  includes the groove geometries ( 11   b,    12   b ) extending in the direction of movement of the weight members  10 . Accordingly, the present preferred embodiment enables smoother movement of the weight members  10  while reliably preventing disconnection of the second spherical members  15  from the large-diameter openings and disconnection of the first spherical members  14  from the small-diameter openings. 
     In addition, the weight members  10  according to the present preferred embodiment are each housed in an associated one of the housing portions  11   a  arranged in the circumferential direction of the holding member  11  and are thus movable radially. More than one urging member  16  is disposed in the circumferential direction between the inner peripheral wall surface  11   aa  of each of the housing portions  11   a  and an associated one of the weight members  10  so as to urge the associated weight member  10  from the radially outer position to the radially inner position. Accordingly, the present preferred embodiment is able to accurately urge each weight member  10  from the radially outer position to the radially inner position, enabling stable movement of the weight members  10  in accordance with centrifugal force. 
     The weight members  10  according to the present preferred embodiment are each provided with the insertion portions  10   b  which are openings defined in the surface of each weight member  10  facing the holding member  11  and into which the urging members  16  are inserted such that the urging members  16  are attachable to the associated weight member  10 . This facilitates assembling the urging members  16  to the weight members  10 . The weight members  10  according to the present preferred embodiment are each provided with the groove  10   c  extending in a direction from the radially inner position to the radially outer position. The holding member  11  (or specifically, the supporting member  13  secured to the holding member  11  so as to be integral therewith) is provided with the holding portions  13   a  each conforming to the associated groove  10   c  and holding the associated weight member  10 . Accordingly, the present preferred embodiment enables stable movement of the weight members  10 . 
     The centrifugal clutch  9  according to the present preferred embodiment includes the first spherical members  14  that are partially protruded from the first openings  10   aa  of the through holes  10   a  defined in the weight members  10  and are in contact with the rolling contact surface (or the groove geometries  11   b ) of the holding member  11  so as to be rollable thereon, and the second spherical members  15  that are partially protruded from the second openings  10   ab  of the through holes  10   a  defined in the weight members  10  and are in contact with the rolling contact surface (or the groove geometries  12   b ) of the pressing member  12  so as to be rollable thereon. Accordingly, the present preferred embodiment enables more stable movement of the weight members  10 . 
     The holding member  11  or the pressing member  12 , in particular, includes the groove geometries ( 11   b,    12   b ) extending in the direction of movement of the weight members  10 . The groove geometries ( 11   b,    12   b ) define the rolling contact surface for the first spherical members  14  or the second spherical members  15  so as to enable smoother movement of the weight members  10 . The first and second spherical members  14  and  15  according to the present preferred embodiment are provided such that more than one first spherical member  14  and more than one second spherical member  15  are arranged in the circumferential direction of the holding member  11  (i.e., the width direction of each weight member  10 ). Accordingly, the present preferred embodiment enables more stable movement of the weight members  10 . 
     Although the present preferred embodiment has been described thus far, the present invention is not limited to this preferred embodiment. As illustrated in  FIG.  26   , for example, the auxiliary clutch plate  17  may include a plurality of clutch plates. A multiple-plate clutch provided in this case includes a first auxiliary clutch plate  17   c  and a second auxiliary clutch plate  17   d  in addition to the auxiliary clutch plate  17  connected to the output shaft  3 . Movement of each weight member  10  of the centrifugal clutch  9  to the radially outer position moves the holding member  11 . This causes the pushing surface  11   c  of the holding member  11  to push the second auxiliary clutch plate  17   d  such that the auxiliary clutch plate  17 , the first auxiliary clutch plate  17   c,  and the second auxiliary clutch plate  17   d  are pressed against each other, enabling transmission of the driving force of the engine E to the wheel (i.e., the driving wheel T). Providing the auxiliary clutch plate  17  including a plurality of clutch plates in this manner makes it possible to increase a clutch capacity of the auxiliary clutch plate  17  (which is a multiple-plate clutch including the first auxiliary clutch plate  17   c  and the second auxiliary clutch plate  17   d ) to any desired level. 
     As illustrated in  FIG.  27   , the auxiliary clutch plate  17  may include a plurality of clutch plates (or may be a multiple-plate clutch including first auxiliary clutch plates  17   c  and second auxiliary clutch plates  17   d ) and may overlap, in an axial direction X of the clutch housing  2 , with the region where the driving and driven clutch plates  6  and  7  are stacked. Disposing the auxiliary clutch plate  17  such that the auxiliary clutch plate  17  overlaps, in the axial direction X of the clutch housing  2 , with the region where the driving and driven clutch plates  6  and  7  are stacked in this manner makes it possible to reliably prevent the apparatus from increasing in size in the axial direction X. 
     As illustrated in  FIG.  28   , the auxiliary clutch plate  17  may include a plurality of clutch plates, and the apparatus may include an interlocking member  18  instead of the centrifugal clutch. As illustrated in  FIG.  29   , the auxiliary clutch plate  17  may include a plurality of clutch plates and overlap with the region where the driving and driven clutch plates  6  and  7  are stacked, and the apparatus may include the interlocking member  18  instead of the centrifugal clutch. As illustrated in  FIG.  30   , the auxiliary clutch plate  17  may include a plurality of clutch plates, and the apparatus may include the interlocking member  18  instead of the centrifugal clutch and may also include a pressing assist cam and a back torque limiter cam. When the pressure member  5  is moved such that the driving and driven clutch plates  6  and  7  are pressed against each other, the resulting pressing force moves the interlocking member  18  toward the auxiliary clutch plate  17  such that the interlocking member  18  presses the auxiliary clutch plate  17 . 
     In the present preferred embodiment, disconnection of the second spherical members  15  is prevented by the rolling contact surface (or the groove geometries  11   a ) of the holding member  11  (or the pressing member  12 ). Alternatively, disconnection of the second spherical members  15  may be prevented by any other disconnection preventing structures and methods, such as swaging. In the present preferred embodiment, with the first clutch member  4   a,  the second clutch member  4   b,  and the pressure member  5  assembled to each other (and with no torque being transmitted from the input gear  1  (i.e., the input) to the output shaft  3  (i.e., the output)), the abutment surface  4   ad  and the abutment surface  5   e  are in abutment with each other. Alternatively, the first clutch member  4   a  may include no abutment surface  4   ad,  the pressure member  5  may include no abutment surface  5   e,  and the first clutch member  4   a  and the pressure member  5  may be spaced away from each other. The power transmission apparatuses according to preferred embodiments of the present invention may find applications as various multiple-plate clutch type power transmission apparatuses for, for example, motorcycles, automobiles, three-wheel or four-wheel buggies, or general purpose machines. 
     A power transmission apparatus includes an auxiliary clutch plate in a clutch housing. The auxiliary clutch plate is different in diameter from driving and driven clutch plates. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed so as to enable transmission of a driving force of an engine to a wheel. When a pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released so as to cut off transmission of the driving force of the engine to the wheel. Such a power transmission apparatus may find applications involving, for example, change(s) in external shape or addition of other function(s). 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.