Abstract:
A multiple disc clutch including a clutch outer and a clutch inner, for enabling a gentle releasing/engaging operation. The multiple disc clutch includes a clutch inner having a clutch center and a clutch piston, and outer and inner friction discs being alternately arranged between the clutch center and the clutch piston. A clutch spring performs energization to narrow an interval between the clutch center and the clutch piston. A clutch release mechanism reduces friction force between the outer and inner friction discs and releases an engaged state of the clutch by widening the interval between the clutch center and the clutch piston. A release spring is provided between the clutch piston and the clutch release mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present non-provisional application claims priority under 35 USC 119 to Japanese Patent Application No. 2004-060699 filed on Mar. 4, 2004 the entire contents thereof is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a structure of a multiple disc clutch. 
   2. Description of Background Art 
   In general, a multiple disc clutch has a structure including a clutch outer, a clutch center, and an axially movable clutch piston, in which a plurality of axially movable outer friction discs provided on a clutch outer side and a plurality of axially movable inner friction discs provided on a clutch center side are arranged in an alternately stacked manner between the clutch center and the clutch piston. Release/engagement of the clutch is performed by axially moving the clutch piston. 
   In the conventional clutch with the above-described structure, a delicate operation has been required for gently releasing/engaging the clutch in the case of releasing/engaging the clutch. 
   Heretofore, there has been an example were a disc spring is mounted between the clutch piston and the friction discs. For example, see Japanese Utility Model Laid-Open No. S62 (1987)-16834,  FIG. 2 . However, in Japanese Utility Model Laid-Open No. S62 (1987)-16834, the clutch piston and the disc spring are provided in series, and it is difficult to change the respective characteristics thereof. 
   SUMMARY AND OBJECTS OF THE INVENTION 
   The present invention attempts to enable a gentler release/engagement operation in the multiple disc clutch. 
   The present invention has solved the foregoing problem. The invention relates to a structure of a multiple disc clutch including a clutch outer and a clutch inner, wherein the clutch inner is composed of a clutch center and a clutch piston, and outer friction discs and inner friction discs are alternately arranged between both of the members constituting the clutch inner. A clutch spring is provided which performs energization to narrow an interval between both of the members constituting the clutch inner. A clutch release mechanism is provided which reduces friction force between the outer friction discs and the inner friction discs and releases an engaged state of the clutch by widening the interval between both of the members constituting the clutch inner. A release spring is provided between the clutch piston and the clutch release mechanism. 
   In the present invention the release spring is opposite to the clutch spring, and is provided in the inside of a place where the friction discs are arranged. 
   In the present invention the clutch spring has a disc spring shape, and the release spring has a coil spring shape. 
   In the present invention the clutch release mechanism is operated by an actuator motor with electronic control. 
   According to the present invention, the operation of releasing/engaging the clutch can be performed gently, and operability thereof is improved. 
   According to the present invention, both of the springs are arranged compactly, thus enabling a reduction of size in a clutch device. 
   According to the present invention, the thinning of the clutch spring and the ensuring of the size of the release spring are implemented, and spring characteristics thereof can be optimized. 
   According to the present invention, control of the clutch operation is facilitated. 
   Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a longitudinal cross-sectional view of a multiple disc clutch  1  according to a first embodiment of the present invention; 
       FIG. 2  is a configuration view of a shift mechanism S in a power unit of a saddle riding vehicle; 
       FIG. 3  is an enlarged side view of a gear change mechanism  80  of  FIG. 2 ; 
       FIG. 4  is a view showing a relationship between a moving distance (unit: mm) of a coupling plate  34  and a clutch capacity in the multiple disc clutch  1 ; and 
       FIG. 5  is a longitudinal cross-sectional view of a multiple disc clutch  101  according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a longitudinal cross-sectional view of a multiple disc clutch  1  according to a first embodiment of the present invention. This clutch operatively connected to a rotational power transmission drive from a crankshaft (not shown) of a power unit toward a main shaft of a transmission in a saddle riding vehicle, and is engaged and released in response to a gear change operation of a driver. In the drawing, an arrow F indicates the front in the traveling direction of the vehicle. In the above-described saddle riding vehicle, both of the crankshaft and the main shaft of the transmission are placed in parallel to the traveling direction of the vehicle. 
   To the main shaft  2  of the transmission, a clutch outer  3  is freely fitted with a sleeve  4  interposed therebetween so as to be rotatable with respect to the above-described main shaft  2 . To the clutch outer  3 , a body portion of a driven gear  5  is fixed by a rivet  6 . A shock-absorbing member  5   a  is provided between the body portion of the driven gear  5  and a peripheral gear portion. The driven gear  5  meshes with a drive gear (not shown) provided on the crankshaft. 
   Onto the main shaft  2 , a clutch center  7  is attached with a spline  2   a  interposed therebetween so as not to rotate. An annular member  8  is interposed between a boss portion of the clutch outer  3  and a boss portion of the clutch center  7 , and a main shaft end side of the clutch center  7  is fixed by a nut  9  and a washer  10 . More specifically, the clutch center  7  is fixed to the main shaft  2  of the transmission. 
   In the clutch outer  3  and the clutch center  7 , a plurality of outer friction discs  11  and inner friction discs  12  are held, respectively, so as to be axially slidable and not to be rotatable about the main shaft, and are arranged in an alternately stacked state. Onto an outer periphery of the boss portion of the clutch center  7 , a clutch piston  13  is attached so as to be axially slidable. In the clutch piston  13 , a plurality of projecting portions  13   a  are provided, that are inserted into a plurality of through holes provided in the clutch center  7 , and project forward (rightward in  FIG. 1 ). Thus, the clutch center  7  and clutch piston  13  rotate integrally in the rotational direction about the main shaft  2 . In this clutch  1 , a clutch inner  18  is composed of the clutch center  7  and the clutch piston  13 . The above-described outer and inner friction discs  11  and  12  are arranged in the alternately stacked manner and are sandwiched between an outer peripheral portion of the clutch center  7  and an outer peripheral portion of the clutch piston  13 . More specifically, in the clutch inner  18 , the outer friction discs  11  and the inner friction discs  12  are arrayed to be sandwiched by two members constituting the clutch inner  18 . 
   A clutch spring  16  is a disc spring type having an inner periphery side that is engaged by a clip  14  and a retainer  15 , which are attached onto the boss portion of the clutch center  7 , for pushing the clutch piston  13  by an outer periphery side thereof through a spring set ring  17 . The clutch spring  16  is energization in a direction wherein the clutch piston  13  is made to approach the fixed clutch center  7 , and the clutch piston  13  presses the outer and inner friction discs  11  and  12  in a direction of engaging the clutch  1 . 
   An operating unit  20  of the multiple disc clutch is provided in the right portion of the drawing. To a front cover  41  of a power unit casing, an operating unit fixing shaft  21  is fixed by a nut  23  with a washer  22  interposed therebetween. Onto the operating unit fixing shaft  21 , a cam nose member  24  is attached to be held by a cam nose member holding cylinder  25 . One end of an engaging pin  26  is fitted and fixed into a hole in the front cover  41 , the other end of the engaging pin  26  is fitted into a concave portion  24   b  of an engaging portion  24   a  provided on part of the periphery of the cam nose member  24 . Thus, a rotational movement of the cam nose member  24  is restricted. 
   Onto an outer periphery of the cam nose member holding cylinder  25 , an operating member  27  is attached so as to be rotationally movable and axially slidable. Between the cam nose member  24  and the operating member  27 , three steel balls  28  are attached to be held by a steel ball holding member  29 . Part of an outer peripheral portion of the operating member  27  extends outwardly, thereby forming an operating lever  27   a.  In a tip of the operating lever  27   a,  an engaging groove  27   b  is provided, wherein a roller  63  on a tip of a clutch arm  62  to be described later is engaged. To the above-described operating member  27 , a pressing force transmission member  30 , fitted into an inner ring of a ball bearing  32 , is fixed by a rivet  31 . The pressing force transmission member  30  is rotationally movable and axially movable together with the operating member  27 . Onto an outer ring of the ball bearing  32 , a coupling plate  34  is attached with an annular member  33  interposed therebetween. In a peripheral portion of the above-described coupling plate  34 , a plurality of through holes are provided, and bolts  35  penetrating respective the through holes are screwed and fixed into female screw holes provided in the projecting portions  13   a  of the clutch piston  13  while interposing washers  36  abutting on end surfaces of large-diameter portions  35   a  of the bolts  35 . 
   In each large-diameter portion  35   a  of the above-described bolts  35 , the coupling plate  34  is movable in an axial direction of the bolt  35  between the head of the bolt  35  and the washer  36 . Such a movable gap d is 2 mm in this embodiment. Between the coupling plate  34  and the clutch piston  13 , a release spring  37  of a coil spring type is provided which exhibits a helical shape around each projecting portion  13   a  of the clutch piston  13 . In a normal state (clutch engaged state), the energizing force of the release spring  37  is zero or an extremely small energizing force in comparison with energizing force of the clutch spring. When the release spring  37  is pushed and compressed by the coupling plate  34  and an energizing force exceeding the energizing force of the clutch spring  16  occurs, the clutch piston  13  moves by the energizing force of the release spring  37 , and the engagement of the clutch  1  is released. It is desirable that a position where the energizing force of the release spring  37  exceeds the energizing force of the clutch spring  16  be set before the gap between the coupling plate  34  and the washer  36  disappears, that is, before the position where the coupling plate  34  abuts on the washer  36 . Alternatively, the above-described position may be set in the vicinity of a position of the washer  36  when the clutch piston  13  is located at an initial position, position of the clutch piston  13  when the clutch is engaged, which is shown in  FIG. 1 . 
     FIG. 2  is a view of a shift mechanism S in the power unit of the saddle riding vehicle. The shift mechanism S is formed of an actuator mechanism  40 , a clutch operating mechanism  60 , and a gear change mechanism  80 . In the drawing, an arrow F indicates the front of the vehicle. An outer shell of the power unit is composed of the front cover  41 , a front casing  42 , a rear casing  43 , and a rear cover  44  in this order from the front, and these are mutually coupled. 
   On a front end of the main shaft  2  of the transmission, the above-mentioned multiple disc clutch  1  is provided, and between the multiple disc clutch  1  and the front cover  41 , the operating unit  20  of the above-mentioned multiple disc clutch  1  is provided. The operating lever  27   a  extending from the outer peripheral portion of the operating member  27  that forms part of the operating unit  20  extends downwardly in  FIG. 2 . 
   To a front end of the above-described front cover  41 , a gear case  45  is connected. A shift spindle  46  extends in the fore-and-aft direction of the vehicle body while being supported by the gear case  45 , the front cover  41 , the front casing  42 , the rear casing  43  and the rear cover  44 . The shift spindle  46  is supported by the gear case  45  with a ball bearing  47  interposed therebetween and by the front cover  41  with a needle bearing  48  and a seal member  49  interposed therebetween. A front end of the shift spindle  46  is connected to rotational phase detecting means  50  formed of a potentiometer. On the outside of the rotational phase detecting means  50 , a protector  51  is provided that is fixed to the gear case  45 . 
   A gear chamber  52  is formed between the gear case  45  and the front cover  41 . An electronically controlled actuator motor  53  is supported on an outside surface of the gear case  45 . On a steering handle of the vehicle, an upshift switch and a downshift switch (which are not shown) are provided, and are electrically connected to the above-described motor. The above-described motor  53  rotates forward when the above-described upshift switch is pressed, and reverses when the downshift switch is pressed. 
   An output shaft  53   a  of the above-described motor  53  projects in the gear chamber  52 . An output pinion  53   b  is formed on the motor output shaft  53   a.  By the gear case  45  and the front cover  41 , a first reduction shaft  54  and a second reduction shaft  55  are supported so as to be rotatable. On the first reduction shaft  54 , a large-diameter reduction gear  54   a  meshing with the output pinion  53   b  of the motor output shaft and a small-diameter reduction gear  54   b  adjacent thereto are formed integrally with the first reduction shaft  54 . On the second reduction shaft  55 , a large-diameter reduction gear  55   a  meshing with the above-described small-diameter reduction gear  54   b  and a small-diameter reduction gear  55   b  adjacent thereto are formed integrally with the second reduction shaft  55 . On the shift spindle  46 , a sector gear  56  meshing with the above-described small-diameter reduction gear  55   b  is fitted. The gears mesh with one another as described above, thereby configuring a gear train. Accordingly, by driving the motor  53  to rotate forward or to reverse, the shift spindle  45  is driven to rotate forward or to reverse. The actuator mechanism  40  is composed of the motor  53 , the train of the gears  53   b  to  56 , the shift spindle  46  and the like. 
   A sleeve  61  is spline-fitted to the shift spindle  46 , and onto the sleeve  61  concerned, the clutch arm  62  is welded. In such a way, the clutch arm  62  is brought to a state of being fixed to the shift spindle  46 . The roller  63  is provided on the tip of the clutch arm  62 , and the roller  63  concerned is engaged with the engaging groove  27   b,  see  FIG. 1 , on the tip portion of the operating lever  27   a  extending from the operating member  27 . The clutch operating mechanism  60  is composed of members  61  to  63 . 
   By the front casing  42  and the rear casing  43 , a shift drum  70  and a shift fork shaft  71  are supported in the fore-and-aft direction of the vehicle body. On an outer periphery of the shift drum  70 , three cam grooves  70   a,    70   b  and  70   c  are formed, and with these cam grooves, projecting pins  72   a,    73   a  and  74   a  on base end portions of three shift forks  72 ,  73  and  74  are engaged. When the shift drum  70  is rotationally driven, the shift forks  72  to  74  axially move, and a predetermined speed of a gear transmission is established through an axially movable gear provided with a dog, that is, a so-called shifter gear, not shown. By a shift position detecting means  75  formed of a potentiometer connected to a rear end of the shift drum  70 , a rotational drive position of the shift drum  70  is detected. The gear change mechanism  80  is provided between the shift spindle  46  and the shift drum  70 . 
     FIG. 3  is an enlarged side view of the gear change mechanism  80  of  FIG. 2 . The gear change mechanism  80  is configured by including a change arm  81  freely fitted to the shift spindle  46 , a change arm reset spring  83  suspended between a pin  82  fixed to the front casing  42  and the shift spindle  46 , an L-shaped drive arm  84  in which one end is fixed to the shift spindle  46  and the other end is engaged with the change arm  81 , a shifter plate  85  provided on a tip portion of the change arm  81 , a star plate  86  which is attached onto an end portion of the shift drum  70  and rotationally driven by a feed projection  85   a  of the above-described shifter plate  85 , and the like. When the shift spindle  46  is rotationally driven, the L-shaped drive arm  84  is rotationally driven, the change arm  81  moves, the star plate  86  is pushed in the rotational drive direction by the projections  85   a  of the shifter plate  85 . Thus, the shift drum  70  is rotationally driven. 
   Again in  FIG. 2 , in the vicinity of a rear end portion of the shift spindle  46 , a secondary spindle  65  is supported by the rear casing  43  so as to be perpendicular to the shift spindle  46 . An arm  66  fixed to one end of the secondary spindle  65  is engaged with an arm  67  fixed to a rear end of the shift spindle  46 . A hexagonal portion  65   a  is formed on the other end of the secondary spindle  65 . 
   The shift mechanism S of the power unit of the above-described embodiment is configured as described above. The operating mechanism  60  of the multiple disc clutch  1  of the present invention forms part of the above-described shift mechanism S. Next, a function of the above-described shift mechanism is described, and in the process of the description, the function of the multiple disc clutch  1  is described. 
   When the driver of the vehicle presses the upshift switch or the downshift switch, both not shown, which are provided on the steering handle, and thus the motor  53 , shown in  FIG. 2 , is driven to rotate in a forward or a reverse direction, the shift spindle  46  rotationally moves forward or reverse through the train of the gears  53   b  to  56 , and both of the L-shaped drive arm  84  and the clutch arm  62 , which are fixed to the shift spindle  46 , start to be moved in rotationally direction. 
   The L-shaped drive arm  84  rotationally drives the shift drum  70  through the change arm  81 , the shifter plate  85  and the star plate  86 , and attempts to rotationally drive the shift drum  70  from a certain speed position to the next speed position. In the case of this position change, a change of a connection gear by the shifter gear, that is, a gear change is performed. At this time, it is necessary to cut off power transmission from the crankshaft to the main shaft of the transmission. For this purpose, the engagement/release of the multiple disc clutch  1  is performed simultaneously with the gear change. As mentioned above, the clutch arm  62  is rotationally driven simultaneously with the rotational movement of the L-shaped drive arm  84  and the roller  63 , provided on the tip of the clutch arm  62 , pushes the operating lever  27   a  of the multiple disc clutch operating unit  20 . 
   In  FIG. 1 , when the operating lever  27   a  is pushed, and thus the operating member  27  and the pressing force transmission member  30  coupled thereto by the rivet  31  are rotationally driven, the operating member  27  and the pressing force transmission member  30  is move to the left in  FIG. 1  by the reaction force received from the fixed cam nose member  24  through the steel balls  28 . The coupling plate  34  connected to the pressing force transmission member  30  with the ball bearing  32  and the annular member  33  interposed therebetween moves to the left in  FIG. 1  in the large-diameter portions  35   a  of the bolts  35 . The coupling plate  34  presses the release spring  37  while moving, thereby allowing the release spring  37  to generate the energizing force. Before the coupling plate  34  moves in the large-diameter portions  35   a  of the bolts  35 , between the heads of the bolts  35  and the washers  36 , and abuts on the washers  36 , the energizing force of the release spring  37  exceeds the energizing force of the clutch spring  16 . At this time, the clutch piston  13  is pushed to the coupling plate  34  through the release spring  37 , and moves leftward in  FIG. 1 . At this time, the pressing force against the outer and inner friction discs  11  and  12  between the clutch center  7  and the clutch piston  13  is absorbed, and the engagement of the clutch is released through a half clutch state. 
   In  FIG. 2 , while the engagement of the multiple disc clutch  1  is being released, the above-mentioned gear change is completed, and the change arm  81  returns to an original position thereof by function of the change arm reset spring  83 . Thereafter, being driven by the reverse direction rotation of the motor  53 , the shift spindle  46  and the clutch arm  62  return to original positions thereof, the multiple disc clutch  1  returns to an original engaged state thereof, and the rotation of the crankshaft is transmitted to the main shaft  2 . By the above process, the gear change for the power unit is performed. 
   When the motor  53  breaks down, the shift spindle  46  automatically returns to a neutral position by the energizing force of the above-described change arm reset spring  83 . Moreover, at this time, by engaging a tool such as a wrench with the hexagonal portion  65   a  of the secondary spindle  65 , it is possible to manually operate the shift drum  70  and the multiple disc clutch  1 . 
     FIG. 4  is a view showing a relationship between a moving distance (unit: mm) of the coupling plate  34  and a clutch capacity in the above-described multiple disc clutch  1 . In a state where the moving distance of the coupling plate  34  is 0 mm, the state illustrated in  FIG. 1 , the clutch is in an engaged state, and the clutch capacity is displayed as 1. As the coupling plate  34  moves, the clutch capacity is lowered. This shows that the pressing force of the clutch piston  13  against the friction discs is reduced because the energizing force of the release spring  37  increases. Before the coupling plate  34  completes the movement thereof throughout the gap d, that is, before the coupling plate  34  abuts on the washers  36 , the clutch capacity turns to 0. This shows a state where the energizing force of the clutch spring and the energizing force of the release spring are equalized to each other, the pressing force against the friction discs turns to 0, and the engagement of the clutch is released. 
   In the multiple disc clutch  1  of this embodiment, as described above, in the case of moving the clutch piston  13  against the energizing force of the clutch spring  16 , a structure is adopted, in which the clutch piston  13  is not pushed directly and mechanically, but the clutch piston  13  is pushed by the spring force through the release spring  37 . Accordingly, the relationship between the moving amount of the coupling plate and the clutch capacity can be set rather linear. Moreover, the clutch capacity can be controlled by the moving amount of the coupling plate and, accordingly, a smooth gear change with little shock is enabled. In the above-described embodiment, an example of performing the operation of engaging/releasing the multiple disc clutch by an electric motor has been described. However, the operation may also be performed by hand or by foot. Also in this case, a good-feeling in the operation is enabled with little shock. 
   Moreover, when the position where the energizing force of the release spring  37  exceeds the energizing force of the clutch spring  16  is set in the vicinity of the position of the washers  36  when the clutch piston  13  is located at the initial position, position of the clutch piston  13  when the clutch is engaged, which is shown in  FIG. 1 , there is also a case where the coupling plate  34  abuts on and mechanically pushes the washers  36 . Also in such a setting, at the time when the coupling plate  34  abuts on the washers  36 , an energizing force pushing the clutch piston  13 , which is the result of the energizing forces of the clutch spring  16  and the release spring  37 , becomes sufficiently small. Accordingly, a smooth gear change with little shock is enabled, and a sufficient effect is brought about. 
     FIG. 5  is a longitudinal cross-sectional view of a multiple disc clutch  101  according to a second embodiment of the present invention. This clutch is also placed on the rotational power transmission route from the crankshaft, not shown, of the power unit toward the main shaft of the transmission in the saddle riding vehicle, and is engaged and released in response to the gear change operation of the driver. A different point of this embodiment from the first embodiment is that the drive of the coupling plate is performed by hydraulic pressure. 
   To a main shaft  102  of a transmission, a clutch outer  103  is freely fitted with a sleeve  104  interposed therebetween so as to be rotatable with respect to the above-described main shaft  102 . To the clutch outer  103 , a body portion of a driven gear  105  is fixed by a rivet  106 . A shock-absorbing member  105   a  is provided between the body portion of the driven gear  105  and a peripheral gear portion. The driven gear  105  meshes with a drive gear, not shown, of a crankshaft. 
   Onto the main shaft  102 , a clutch center  107  is attached with a spline  102   a  interposed therebetween so as not to rotate. An annular member  108  is interposed between a boss portion of the clutch outer  103  and a boss portion of the clutch center  107 , and a main shaft end side of the clutch center  107  is fixed by a nut  109  and a washer  110 . More specifically, the clutch center  107  is fixed to the main shaft  102  of the transmission. 
   In the clutch outer  103  and the clutch center  107 , a plurality of outer friction discs  111  and inner friction discs  112  are held, respectively, so as to be axially slidable and not to be rotatable about the main shaft, and are arranged in an alternately stacked state. Onto an outer periphery of the boss portion of the clutch center  107 , a clutch piston  113  is attached so as to be axially slidable. In the clutch piston  113 , a plurality of projecting portions  113   a  are provided that are inserted into a plurality of through holes provided in the clutch center  107 , and project forward, to the right in  FIG. 5 . Thus, the clutch center  107  and the clutch piston  113  rotate integrally in the rotational direction about the main shaft  102 . In this clutch  101 , a clutch inner  118  is composed of the clutch center  107  and the clutch piston  113 . The above-described outer and inner friction discs  111  and  112  that are arranged in an alternately stacked manner are sandwiched between an outer peripheral portion of the clutch center  107  and an outer peripheral portion of the clutch piston  113 . More specifically, in the clutch inner  118 , the outer friction discs  111  and the inner friction discs  112  are arrayed to be sandwiched by two members constituting the clutch inner  118 . 
   A clutch spring  116  of a disc spring type, of which an inner periphery side is engaged by a clip  114  and a retainer  115 , which are attached onto the boss portion of the clutch center  107 , pushes the clutch piston  113  by an outer periphery side thereof through a spring set ring  117 . The clutch spring  116  is energization to a direction where the clutch piston  113  is made to approach the fixed clutch center  107 , and the clutch piston  113  presses the outer and inner friction discs  111  and  112  to a direction of engaging the multiple disc clutch  101 . A configuration of the above-described portions, left half of  FIG. 5 , in this embodiment is substantially the same as that of the first embodiment. 
   A cylindrical portion  121  is provided in an extended direction of an axial line of the main shaft  102  on a backside of a front cover  120  of the power unit. Into the above-described cylindrical portion  121 , a pressure-receiving piston  122  is fitted with a seal member  123  and a coil spring  124  interposed therebetween so as to be slidable in an axial direction of a cylinder shaft. A space separated by an inner surface of the cylindrical portion  121  and the pressure-receiving piston  122  is a hydraulic chamber  125 . To the hydraulic chamber  125 , a hydraulic pump, not shown, is connected. 
   In the main shaft  102 , a center hole  102   b  in which one end is made open to a shaft end portion thereof is drilled. A pressing force transmission member  126  is provided between the main shaft  102  and the pressure-receiving piston  122 . One end of the pressing force transmission member  126  is freely fitted to the center hole  102   b  of the above-described main shaft  102  so as to be axially slidable, and the other end is held on the above-described pressure-receiving piston  122  with a steel ball  127  interposed therebetween so as to be rotatable. The pressing force transmission member  126  includes a flange portion  126   a.  In the periphery of the above-described pressing force transmission member  126 , a coupling plate  128  abuts on and is held by the flange portion  126   a  of the member  126 . 
   Bolts  129  penetrating through holes provided in the above-described coupling plate  128  are screwed and fixed into female screw holes provided in the projecting portions  113   a  of a crank piston  113 , while interposing washers  130  abut on end surfaces of large-diameter portions  129   a  of the bolts  129 . A release spring  131  of a coil spring type, which exhibits a helical shape around each projecting portion  113   a  of the clutch piston  113 , is provided with both ends thereof abutting on the clutch piston  113  and the coupling plate  128 . The coil spring  124  interposed between a bottom surface of the above-described cylindrical portion  121  and the pressure-receiving piston  122  lightly pushes an end portion of the release spring  131  through the pressure-receiving piston  122 , the steel ball  127 , the pressing force transmission member  126  and the coupling plate  128 . In a normal state, clutch engaged state, the energizing force of the release spring  131  is zero or an extremely small energizing force in comparison with energizing force of the clutch spring. In the large-diameter portions  129   a  of the above-described bolts  129 , the coupling plate  128  is movable in an axial direction of the bolts  129  between heads of the bolts  129  and the washers  130 . Such a movable gap d is 2 mm in this embodiment. 
   In the multiple disc clutch  101  of this embodiment, when pressurized oil is supplied to the hydraulic chamber  125  by the hydraulic pump, the pressure-receiving piston  122  receiving the hydraulic pressure moves toward the main shaft  102 , and a pressing force thereof pushes the release spring  131  through the steel ball  127 , the pressing force transmission member  126  and the coupling plate  128 . When the release spring  131  is pushed and compressed by the coupling plate  128  and an energizing force exceeding the energizing force of the clutch spring  116  occurs, the clutch piston  113  moves by the energizing force of the release spring  131 , and the engagement of the multiple disc clutch  101  is released. It is desirable that a position where the energizing force of the release spring  131  exceeds the energizing force of the clutch spring  116  be set before the gap between the coupling plate  128  and the washer  130  disappears, that is, before the position where the coupling plate  128  abuts on the washer  130 . Alternatively, the above-described position may be set in the vicinity of a position of the washer  130  when the clutch piston  113  is located at an initial position, position of the clutch piston  113  when the clutch is engaged, which is shown in  FIG. 5 . 
   Also in the multiple disc clutch  101  of this embodiment, in the case of moving the clutch piston  113  against the energizing force of the clutch spring  116 , a structure is adopted, in which the clutch piston  113  is not pushed directly and mechanically, but the clutch piston  113  is pushed by the spring force through the release spring  131 . Accordingly, the relationship between the moving amount of the coupling plate and the clutch capacity can be set rather linear. Moreover, the clutch capacity can be controlled by the moving amount of the coupling plate. Accordingly, a smooth gear change is enabled with little shock. 
   Also, when the position where the energizing force of the release spring  131  exceeds the energizing force of the clutch spring  116  is set in the vicinity of the position of the washers  130  when the clutch piston  113  is located at the initial position, a smooth gear change with little shock is enabled, and a sufficient effect is brought about. 
   As described above in detail, in the multiple disc clutch of the present invention, the operation of releasing/engaging the clutch concerned can be performed gently, and operability thereof is improved. Moreover, the clutch spring and the release spring are arranged compactly, and a size of a clutch device is reduced. Furthermore, the thinning of the clutch spring and the ensuring of the size of the release spring are implemented, and accordingly, the spring characteristics thereof are optimized. While a good-feeling operation with little shock is enabled in the operation by hand or by foot, the multiple disc clutch can also be operated by the actuator motor with electronic control. Accordingly, in this case, the control of the clutch operation is facilitated. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.