Patent Publication Number: US-2007099710-A1

Title: Flexible flywheel

Description:
TECHNICAL FIELD  
      The present invention is related to a flexible flywheel, particularly to a flexible flywheel having a flexible plate for flexibly connecting an inertia member with a crankshaft in the bending direction.  
     BACKGROUND ART  
      Conventionally, a flywheel is attached to a crankshaft of an engine for absorbing vibrations caused by combustion variations in the engine. Further, a clutch device is arranged on an axial-direction transmission side with respect to the flywheel. The clutch device usually includes a clutch disc assembly coupled to an input shaft of the transmission and a clutch cover assembly for biasing a frictional coupling portion of the clutch disc assembly against the flywheel. The clutch disc assembly typically has a damper mechanism for absorbing and damping torsional vibrations. The damper mechanism has elastic members such as coil springs arranged for compression in a rotating direction.  
      Furthermore, a structure is known in which a flywheel is connected to a crankshaft via a flexible plate in order to absorb bending vibrations from the engine (refer to Patent Document 1). The flexible plate has a relatively high rigidity in the rotational direction to transmit torque, but has relatively low rigidities in the axial and bending directions. The structure in which the flywheel is connected to the crankshaft via the flexible plate is referred to as a flexible flywheel below.  
      It is noted that the output side of the damper mechanism is fixed to a hub flange which is directly engaged with the transmission input shaft or to a second flywheel onto which a clutch device is attached. In the latter case, torque from the damper mechanism is transmitted to the transmission input shaft through the second flywheel and the clutch disc assembly with the clutch in an engagement state.  
      Patent Document 1  
      Unexamined Patent Publication 2001-12552  
     DISCLOSURE OF INVENTION  
      Some of the known flexible flywheels further have a damper mechanism to which torque from the crankshaft is input. The damper mechanism includes an input member to which torque from the crankshaft is input, an output member rotatably located relative to the input member, and an elastic member to be compressed in the rotational direction when the input member and the output member rotate relative to each other. The damper mechanism is typically engaged with the flywheel so that the flexible plate cannot sufficiently flex in the bending direction when the bending vibrations are transmitted from the crankshaft of the engine to the first flywheel. Therefore, it is difficult to achieve enough bending vibration suppressive (flexible) effects.  
      It is an object of the present invention to achieve an enough of a bending vibration suppressing effect from the crankshaft of the engine in a flexible flywheel having a flexible plate for flexibly connecting an inertia member to the crankshaft in the bending direction.  
      According to a flexible flywheel of claim  1 , to which torque is input from a crankshaft of an engine, the flexible flywheel includes a first flywheel and a damper mechanism. The first flywheel has an inertia member and a flexible plate for connecting the inertia member with the crankshaft. The flexible plate is flexibly deformable in the bending direction and the axial direction. The damper mechanism includes an input member to which torque is input from the crankshaft, an output member located relatively rotatable to the input member, and an elastic member to be compressed in the rotational direction when the input member and the output member rotate relative to each other. The first flywheel can move relative to the damper mechanism in the bending direction within a limited range.  
      In this flexible flywheel, torque from the crankshaft of the engine is transmitted to the first flywheel and the damper mechanism. When torsional vibrations are generated in the damper mechanism, the input member and the output member rotate relative to each other to compress the elastic member therebetween in the rotational direction. As a result, torsional vibrations are absorbed. When bending vibrations are generated in the first flywheel, the flexible plate flexes in the bending direction. As a result, the bending vibrations from the engine are reduced. In this flexible flywheel, since the first flywheel can move relative to the damper mechanism in the bending direction within a limited range, the bending vibration suppressive effects of the flexible plate are sufficiently high.  
      According to a flexible flywheel of claim  2  depending on claim  1 , the flexible flywheel further includes a friction generation mechanism located between the first flywheel and the output member of the damper mechanism to operate in parallel with the elastic member in the rotational direction. The friction generation mechanism includes two members which are engaged with each other such that the two members can transmit torque therebetween but can move relative each other in the bending direction.  
      In this flexible flywheel, when torsional vibrations are generated in the damper mechanism, the input member and the output member rotate relative to each other to compress the elastic member between both the members in the rotational direction. At the same time, the friction generation mechanism operates to generate friction. In this flexible flywheel, since the friction generation mechanism has two members engaged with each other so as to be movable relative to each other in the bending direction, the first flywheel can move relative to the damper mechanism in the bending direction with a limited range, although the first flywheel is engaged with the damper mechanism via the friction generation mechanism. As a result, the bending vibration suppressive effects of the flexible plate are sufficiently high.  
      According to a flexible flywheel of claim  3  depending on claim  2 , the two members are a friction member and an engagement member engaged with the friction member.  
      According to a flexible flywheel of claim  4  depending on claim  3 , the friction member and the engagement member are engaged with each other to maintain a gap therebetween in the rotational direction. That is, both members are not in close contact with each other in the rotational direction so that a large resistance is not generated when both members move relative to each other in the bending direction.  
      According to a flexible flywheel of claim  5  depending on claim  3  or  4 , the engagement member is further engaged with another member so as to be movable in the axial direction. Accordingly, resistance between both the members in the axial direction is unlikely to be generated.  
      According to a flexible flywheel of claim  6  depending on claim  3  or  4 , the friction member can slide against the first flywheel in the rotational direction. The engagement member can rotate integrally with the output member of the damper mechanism.  
      According to a flexible flywheel of claim  7  depending on claim  6 , the engagement member is engaged with the output member of the damper mechanism so as to be movable relative to each other in the axial direction. Accordingly, when the friction member moves together with the first flywheel in the axial direction, resistance between the engagement member and the output member is unlikely to be generated in the axial direction.  
      According to a flexible flywheel of claim  8  depending on any of claims  1  to  7 , the flexible flywheel further includes a second flywheel fixed to the output member of the damper mechanism.  
      According to a flexible flywheel of 9 depending on claim  8 , the second flywheel is formed with a frictional face with which a clutch is frictionally engaged.  
      In a flexible flywheel according to the present invention, bending vibrations from the crankshaft of the engine is sufficiently reduced, in a flexible flywheel having a flexible plate for flexibly fixing the inertia member with the crankshaft in the bending direction. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a schematic cross-sectional view of a dual-mass flywheel in accordance with a preferred embodiment of the present invention.  
       FIG. 2  is an alternate schematic cross-sectional view of the dual-mass flywheel in accordance with a preferred embodiment of the present invention.  
       FIG. 3  is an elevational view of the dual-mass flywheel.  
       FIG. 4  is an enlarged fragmentary cross-sectional view that particularly illustrates a second friction generation mechanism of  FIG. 1 .  
       FIG. 5  is a schematic, plan view to illustrate a structure of the second friction generation mechanism.  
       FIG. 6  is a plan view to illustrate a relationship between a friction washer and an engagement member of the second friction generation mechanism.  
       FIG. 7  is an enlarged fragmentary cross-sectional view that particularly illustrates a first friction generation mechanism of  FIG. 1 .  
       FIG. 8  is an enlarged fragmentary cross-sectional view that particularly illustrates the first friction generation mechanism of  FIG. 1 .  
       FIG. 9  is an enlarged fragmentary cross-sectional view that particularly illustrates the first friction generation mechanism of  FIG. 3 .  
       FIG. 10  is an elevational view of a first friction member.  
       FIG. 11  is an elevational view of an input-side disc-like plate.  
       FIG. 12  is an elevational view of a washer.  
       FIG. 13  is an elevational view of a cone spring.  
       FIG. 14  is an elevational view of a second friction member.  
       FIG. 15  is a view of a mechanical circuit diagram of a damper mechanism and a friction generation mechanism.  
       FIG. 16  is a view of a graph that illustrates torsion characteristics of the damper mechanism.  
       FIG. 17  is a view of a graph that illustrates torsion characteristics of the damper mechanism.  
       FIG. 18  is a view of a graph that illustrates torsion characteristics of the damper mechanism.  
       FIG. 19  is a view of a graph that illustrates torsion characteristics of the damper mechanism.  
       FIG. 20  is a schematic cross-sectional view of a flywheel damper in accordance with a second embodiment of the present invention.  
       FIG. 21  is an alternate schematic cross-sectional view of a flywheel damper in accordance with a third embodiment of the present invention. 
    
    
     EXPLANATIONS OF LETTERS OR NUMERALS  
     
         
           1  dual-mass flywheel  
           2  first flywheel  
           3  second flywheel  
           4  damper mechanism  
           5  first friction generation mechanism  
           6  second friction generation mechanism (friction generation mechanism)  
           11  flexible plate  
           12  second disc-like plate  
           13  inertia member  
           20  input-side disc-like plate (input member)  
           32  output-side disc-like plate (output member)  
           33  output-side disc-like plate (output member)  
           34  first coil spring (elastic member)  
           35  second coil spring (elastic member)  
           36  third coil spring (elastic member)  
           57  friction washer (friction member)  
           60  frictional engagement member (engagement member)  
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     1. First Embodiment  
      (1) Structure  
      1) Overall Structure  
      In  FIG. 1 , a dual-mass flywheel  1  in accordance with a preferred embodiment of the present invention is shown. The dual-mass flywheel  1  transmits torque from a crankshaft  91  on the engine side to an input shaft  92  on the transmission side by way of a clutch (a clutch disc assembly  93  and a clutch cover assembly  94 ). The dual-mass flywheel  1  has a damper function to absorb and to attenuate torsional vibrations. The dual-mass flywheel  1  principally has a first flywheel  2 , a second flywheel  3 , a damper mechanism  4  arranged between both the flywheels  2  and  3 , a first friction generation mechanism  5 , and a second friction generation mechanism  6 .  
      The line O-O in  FIG. 1  is the axial line of rotation of the dual-mass flywheel  1  and the clutch. Further, the engine (not depicted) is disposed on the left-hand side of  FIG. 1 , and the transmission (not depicted) is disposed on the right-hand side. Hereinafter, the left-hand side in  FIG. 1  will be referred to as the axial-direction engine side, and the right-hand side will be referred to as the axial-direction transmission side. The direction in which the arrow R 1  points in  FIG. 3  is the drive side (positive rotational direction), and the direction in which the arrow R 2  points is the reverse drive side (negative rotational direction).  
      The actual numbers in the embodiments described below relate to an example and do not limit the present invention.  
      2) First Flywheel  
      The first flywheel  2  is fixed to a tip of the crankshaft  91 . The first flywheel  2  ensures a large moment of inertia on the crankshaft  91  side. The first flywheel  2  principally has a flexible plate  11  and an inertia member  13 .  
      The flexible plate  11  is a member that absorbs bending vibrations from the crankshaft  91  as well as transmitting the torque from the crankshaft  91  to the inertia member  13 . Therefore, the flexible plate  11  has a higher rigidity in the rotational direction but has a lower rigidity in the axial and bending directions. Specifically, it is preferable that the rigidity in the axial direction of the flexible plate  11  is 3000 kg/mm or less, and is in the range between 600 kg/mm to 2200 kg/mm. The flexible plate  11  is a disc-like member formed with a central hole and made of sheet metal, for example. A radially inner end of the flexible plate  11  is fixed to the tip of the crankshaft  91  by a plurality of bolts  22 . Bolt through-holes are formed in the flexible plate  11  in positions corresponding to the bolts  22 . The bolts  22  are mounted on the crankshaft  91  from the axial-direction transmission side.  
      The inertia member  13  has a relatively thick block shape, and is fixed to the axial-direction transmission side on the radially outer edge of the flexible plate  11 . The radially outermost portion of the flexible plate  11  is fixed to the inertia member  13  by a plurality of rivets  15  aligned in the circumferential direction. A ring gear  14  for engine startup is fixed to the radially outer surface of the inertia member  13 . The first flywheel  2  may also be constructed as an integral member.  
      3) Second Flywheel  
      The second flywheel  3  is an annular, disc-like member located on an axially transmission-side of the first flywheel  2 . The second flywheel  3  is formed with a clutch friction face  3   a  on the axial-direction transmission side. The clutch friction face  3   a  is an annular, flat surface, and is a portion that is engaged by the clutch disc assembly  93  described hereinafter. The second flywheel  3  further has a radially inner cylindrical portion  3   b  extending toward the engine the in the axial direction at the radially inner periphery. In addition, the second flywheel  3  is formed with through holes  3   d  arranged in the circumferential direction at the radially inner portion for allowing the bolts  22  to penetrate therethrough.  
      4) Damper Mechanism  
      The damper mechanism  4  is described below. The damper mechanism  4  elastically engages the second flywheel  3  with the crankshaft  91  in the rotational direction. The second flywheel  3  is connected to the crankshaft  91  via the damper mechanism  4  so as to constitute a flywheel assembly (flywheel damper) with the damper mechanism  4 . The damper mechanism  4  is composed of a plurality of coil springs  34 ,  35 , and  36 , a pair of output-side disk-like plates  32  and  33 , and an input-side disk-like plate  20 . The coil springs  34 ,  35 , and  36  are disposed in parallel with the friction generation mechanisms  5  and  6  in the direction of rotation, as shown in the mechanical circuit diagram in  FIG. 15 .  
      The pair of output-side disk-like plates  32  and  33  is composed of a first plate  32  on the axial-direction engine side, and a second plate  33  on the axial-direction transmission side. Both plates  32  and  33  are disk-like members, and are disposed to maintain a certain distance therebetween in the axial direction. A plurality of windows  46  and  47  aligned in the circumferential direction is respectively formed in each of the plates  32  and  33 . The windows  46  and  47  are structures that support the coil springs  34  and  35  in the axial direction and in the direction of rotation, and have upwardly cut portions that hold the coil springs  34  and  35  in the axial direction and make contact at both ends in the circumferential direction thereof. The number of each of the windows  46  and  47  are two and they are alternately arranged in the circumferential direction (arranged in the same radial position). Each of the plates  32  and  33  is formed with a plurality of third windows  48  arranged in the circumferential direction. The third windows  48  are formed in the positions radially opposing to each other, more specifically radially outward of the first windows  46  to support the third coil springs  36  (later described) in the axial and rotational directions.  
      The first plate  32  and the second plate  33  have radially inner portions maintaining a certain gap in the axial direction and radially outer portions close to each other to be fixed by rivets  41  and  42 . The first rivets  41  are arranged in the circumferential direction. The second rivets  42  couple cut-and-bent abutting portions  43  and  44  formed on the first plate  32  and the second plate  33 . The cut-and-bent abutting portions  43  and  44  are arranged in positions radially opposing to each other, more specifically are arranged radially outward of the second window  47 . As shown in  FIG. 2 , axial positions of the cut-and-bent abutting portions  43  and  44  are the same with that of the input-side disc-like plate  20 .  
      The second plate  33  has a radially outer portion fixed to a radially outer portion of the second flywheel  3  via a plurality of rivets  49 .  
      The input-side disk-like plate  20  is a disk-like member disposed axially between the output-side disk-like plates  32  and  33 . The input-side disc-like plate  20  is formed with first window holes  38  corresponding to the first windows  46 , and second window holes  39  corresponding to the second windows  47 . In addition, each of the first and second window holes  38  and  39  has a straight radially inner periphery whose intermediate portion in the rotational direction has cutouts  38   a  and  39   a  dented radially inward. The input-side disc-like plate  20  is, as shown in  FIG. 1 , further formed with a central hole  20   a  and a plurality of bolt through holes  20   b  around the central hole  20   a.  Circumferentially between each of the window holes  38  and  39  of the radially outer periphery direction are formed projections  20   c  projecting radially outward. The projections  20   c  are positioned apart from the cut-and-bent abutting portions  43  and  44  of the output-side disc-like plates  32  and  33  and the third coil springs  36  in the rotational direction so as to be brought into contact with them when approaching them in the rotation direction. In other words, the projections  20   c  and the cut-and-bent abutting portions  43  and  44  constitute a stopper mechanism for the whole of the damper mechanism  4 . Spaces between the projections  20   c  in the rotational direction serve as third window holes  40  to accommodate the third coil springs  36 . The input-side disc-like plate  20  is formed with holes  20   d  at a plurality of positions (four positions in this embodiment) in the circumferential direction. The hole  20   d  is substantially circular but slightly elongated in the radial direction. A rotational position of the hole  20   d  is between the window holes  38  and  39  in the rotational direction, and a radial position of the hole  20   d  is the same as those of the cutout  38   a  and  39   a.    
      The input-side disc-like plate  20  is fixed to the crankshaft  91  with the flexible plate  11 , a reinforcement member  18  and a support member  19  via the bolts  22 . A radially inner portion of the flexible plate  11  is in contact with an axial-direction transmission side face of an apical surface  91   a  of the crankshaft  91 . The reinforcement member  18  is a disc-like member and is contact with an axial-direction transmission side face of the radially inner portion of the flexible plate  11 . The support member  19  is composed of a cylindrical portion  19   a  and a disc portion  19   b  extending radially from an outer surface of the cylindrical portion  19   a.  The disc portion  19   b  is in contact with an axial-direction transmission side face of the reinforcement member  18 . An inner surface of the cylindrical portion  19   a  is in contact with an outer surface of a cylindrical projection  91   b  formed at the center of the tip of the crankshaft  91  for centering. The inner circumferential surface of the flexible plate  11  and the inner circumferential surface of the reinforcement member  18  are in contact with the outer surface of the cylindrical portion  19   a  on the axial-direction engine side for centering. The inner circumferential surface of the input-side disc-like plate  20  is in contact with the outer surface of the cylindrical portion  19   a  at a base portion on the axial-direction transmission side for centering. To the inner surface of the cylindrical portion  19   a  is attached a bearing  23 , which rotationally supports the tip of the input shaft  92  of the transmission. The members  11 ,  18 ,  19 , and  20  are fastened with each other by screws  21 .  
      As described above, the support member  19  is positioned relative to the crankshaft  91  in the radial direction for fixation, and in turn positions the first flywheel  2  and the second flywheel  3  in the radial direction. Accordingly, since one component has a plurality of functions, the number of the components is decreased and the cost is lowered.  
      An inner surface of the cylindrical portion  3   b  of the second flywheel  3  is supported by the outer surface of the cylindrical portion  19   a  of the support member  19  via a bush  30 . As a result, the second flywheel  3  is centered by the support member  19  relative to the first flywheel  2  and the crankshaft  91 . The bush  30  further has a thrust portion  30   a  located between the radially inner portion of the input-side disc-like plate  20  and the tip of the cylindrical portion  3   b  of the second flywheel  3 . Accordingly, a thrust load from the second flywheel  3  is received via the thrust portion  30   a  by the members  11 ,  18 ,  19 , and  20 , which are arranged in the axial direction. More specifically, the thrust portion  30   a  of the bush  30  is supported by the radially inner portion of the input-side disc-like plate  20  so as to function as a thrust bearing to bear the axial load from the second flywheel  3 . Since the radially inner portion of the input-side disc-like plate  20  is flat so that flatness is improved, load generated in the thrust bearing is stable. Furthermore, since the radially inner portion of the input-side disc-like plate  20  is flat, length of the thrust bearing portion can be longer. As a result, the hysteresis torque is stable. Moreover, since the radially inner portion of the input-side disc-like plate  20  is in contact with the disc portion  19   b  of the support member  19  with no clearance in the axial direction, the radially inner portion of the input-side disc-like plate  20  has a high rigidity.  
      The first coil spring  34  is positioned in the first window hole  38  and the first window  46 . Rotational ends of the first coil spring  34  are in contact with or close to rotational ends of the first window hole  38  and the first window  46 .  
      The second coil spring  35  is disposed in the second window hole  39  and the second window  47 . The second coil spring  35  is made of a spring assembly in which a large and a small spring are combined, and is higher than the first coil spring  34  in rigidity. Rotational ends of the second coil spring  35  are in contact with or close to rotational ends of the second window  47 , but are apart from the rotational ends of the second window hole  39  with a predetermined angle (4 degrees in this embodiment).  
      The third coil spring  36  is disposed in the third window hole  40  and the third window  48 . The third coil spring  36  is smaller than the first coil spring  34  and the second coil spring  35 , and is disposed radially outward of the first coil spring  34  and the second coil spring  35 . The third coil spring  36  is higher than the first coil spring  34  or the second coil spring  35  in rigidity.  
      5) Friction Generation Mechanisms  
      5-1) First Friction Generation Mechanism  5   
      The first friction generation mechanism  5  is a mechanism for operating between the input-side disc-like plate  20  and the output-side disc-like plates  32  and  33  of the damper mechanism  4  in the rotational direction in parallel with the coil springs  34 ,  35 , and  36 , and generates a prescribed frictional resistance (hysteresis torque) when the crankshaft  91  and the second flywheel  3  rotate relative to each other. The first friction generation mechanism  5  generates a constant friction over an entire range of operating angles of the damper mechanism  4 , and is designed to generate a comparatively small friction.  
      The first friction generation mechanism  5  is located radially inward of the damper mechanism  4 , and between the first plate  32  and the second flywheel  3  in the axial direction as well. The first friction generation mechanism  5  is composed of a first friction member  51 , a second friction member  52 , a cone spring  53 , and a washer  54 .  
      The first friction member  51  is a member that rotates integrally with the input-side disc-like plate  20  to slide against the first plate  32  in the rotational direction. As shown in FIGS.  7  to  10 , the first friction member  51  is formed with an annular portion  51   a,  and first and second engagement portions  51   b  and  51   c  extending toward the transmission in the axial direction from the annular portion  51   a.  The annular portion  51   a  is in contact with a radially inner portion of the first plate  32  so as to be slidable in the rotational direction. The first engagement portions  51   b  and the second engagement portions  51   c  are alternately located in the rotational direction. The first engagement portion  51   b  is elongated in the rotational direction and is engaged with the radially inner cutouts  38   a  and  39   a  of the window holes  38  and  39  of the input-side disc-like plate  20 . The second engagement portion  51   c  has a shape of being slightly elongated in the radial direction and is engaged with the hole  20   d  of the input-side disc-like plate  20 . Accordingly, the first friction member  51  can move in the axial direction relative to the input-side disc-like plate  20  but cannot rotate relative to the plate  20 .  
      At a rotational middle position of the axial tip of the first engagement portion  51   b  is formed a first projection  51   d  further extending in the axial direction, thereby forming first axial faces  51   e  on the opposite sides of the first projection  51   d  in the rotational direction. In addition, at a radially inner position of the second engagement portion  51   c  is formed a second projection  51   f  further extending in the axial direction, thereby forming a second axial-side face  51   g  at a radially outer portion of the second projection  51   f.    
      The second friction member  52  is a member that rotates integrally with the input-side disc-like plate  20  to slide against the second flywheel  3  in the rotational direction. The second friction member  52  is, as shown in  FIG. 14 , an annular member which is in contact with a second friction face  3   c  of a radially inner portion of the second flywheel  3  so as to slide in the rotational direction. The second friction face  3   c  is an annular, flat face which is dented toward the transmission in the axial direction compared to the other portion of the second flywheel  3 .  
      The second friction member  52  is formed with a plurality of cutouts  52   a  arranged in the rotational direction at the radially inner periphery. Into the cutouts  52   a,  the first projection  51   d  of the first engagement portion  51   b  and the second projection  51   f  of the second engagement portion  51   c  are fitted. Accordingly, the second friction member  52  can move in the axial direction but cannot rotate relative to the first friction member  51 .  
      The cone spring  53  is a member disposed between the first friction member  51  and the second friction member  52  in the axial direction to urge both members in the axial direction for separation. The cone spring  53  is, as shown in  FIG. 13 , a cone or disc spring which is formed with a plurality of cutout  53   a  at the radially inner periphery. Into the cutouts  53   a,  the first projection  51   d  of the first engagement portion  51   b  and the second projection  51   f  of the second engagement portion  51   c  are fitted. Accordingly, the cone spring  53  can move relative to the first friction member  51  in the axial direction but cannot rotate relative to the member  51 .  
      The washer  54  is a member for reliably transmitting the load of the cone spring  53  to the first friction member  51 . The washer  54  is, as shown in  FIG. 14 , an annular member which is formed with a plurality of cutouts  54   a  arranged in the circumferential direction at the radially inner periphery. Into the cutouts  54   a,  the first projection  51   d  of the first engagement portion  51   b  and the second projection  51   f  of the second engagement portion  51   c  are fitted. As a result, the washer  54  can move in the axial direction relative to the first friction member  51  but cannot rotate relative to the member  51 . The washer  54  is seated on the first axial face  51   e  of the first engagement portion  51   b  and the second axial-side face  51   g  of the second engagement portion  51   c.  The cone spring  53  has a radially inner portion supported by the washer  54  and a radially outer portion supported by the second friction member  52 .  
      5-2) Second Friction Generation Mechanism  6   
      The second friction generation mechanism  6  operates in parallel with the coil springs  34 ,  35 , and  36  between the output-side disk-like plates  32  and  33  and the input-side disk-like plate  20  of the damper mechanism  4  in the rotational direction, and generates a prescribed frictional resistance (hysteresis torque) when the crankshaft  91  rotates in relation to the second flywheel  3 . The second friction generation mechanism  6  generates a constant friction over an entire range of operating angles of the damper mechanism  4 , and is designed to generate comparatively large friction. In this embodiment, hysteresis torque generated by the second friction generation mechanism  6  is five to ten times as much as that generated by the first friction generation mechanism  5 .  
      The second friction generation mechanism  6  is composed of a plurality of washers in contact with each other. The second friction generation mechanism  6  is disposed in the space formed in the axial direction between a second disk-like plate  12  and an annular portion  11   a,  which is a radially outer portion of the flexible plate  11 . The washers in the second friction generation mechanism  6  are disposed adjacent to the radially inner side of the inertia member  13  and the rivets  15 .  
      The second friction generation mechanism  6  has, in order from the flexible plate  11  toward a facing portion  12   a  of the second disc-like plate  12 , a friction washer  57 , an input-side friction plate  58 , and a cone spring  59 , as shown in  FIG. 4 . Thus, the flexible plate  11  has a function of accommodating the second friction generation mechanism  6 , so the number of components is reduced and the structure is simplified compared to conventional structures.  
      The cone spring  59  imparts a load in the axial direction to friction surfaces, and is interposed and compressed between the facing portion  12   a  and the input-side friction plate  58 . Therefore, the cone spring  59  exerts an urging force on both members in the axial direction. Pawls  58   a  formed on a radially outer edge of the input-side friction plate  58  are engaged with axially extending cutaway areas  12   b  of the second disc-like plate  12 . Thus, the input-side friction plate  58  is prevented from rotating relative to the second disc-like plate  12  by this engagement, but is movable in the axial direction.  
      As shown in  FIG. 5 , the friction washers  57  are composed of a plurality of members aligned and disposed in the direction of rotation, and each of these extends in the form of an arc. In this embodiment, there are a total of six friction washers  57 . The friction washers  57  are interposed between the input-side friction plate  58  and the annular portion  11   a  as the radially outer portion of the flexible plate  11 . In other words, an axial-direction engine side surface  57   a  of the friction washers  57  makes contact in a slidable manner with the axial-direction transmission side surface of the flexible plate  11 , and an axial-direction transmission side surface  57   b  of the friction washer  57  makes contact in a slidable manner with the axial-direction engine side surface of the input-side friction plate  58 . A concavity  63  is formed on the radially inner surface of the friction washer  57 , as shown in  FIG. 6 . The concavity  63  is formed roughly in the rotation direction of the friction washer  57 , and more specifically, has a bottom surface  63   a  extending in the direction of rotation, and rotational-direction end faces  63   b  extending from both ends thereof in a roughly radially inward direction (roughly at a right angle from the bottom surface  63   a ). The concavity  63  is formed in the axially middle portion on the radially inner surface of the friction washer  57  so as to have axial-direction end faces  63   c  and  63   d,  thereby forming opposite sides in the axial direction.  
      Frictional engagement members  60  are disposed on the radially inner side of the friction washers  57 , or, more specifically, within the concavities  63 . The radially outer portion of the frictional engagement member  60  is disposed within the concavity  63  of the friction washer  57 . Both the friction washers  57  and the frictional engagement members  60  are made of resin.  
      An engagement portion  64  constituted by the frictional engagement member  60  and the concavity  63  of friction washer  57  is described below. The frictional engagement member  60  has axial-direction end faces  60   a  and  60   b,  and rotational-direction end faces  60   c.  A radially outer surface  60   g  of the frictional engagement member  60  is adjacent to the bottom surface  63   a  in the concavity  63 , and a rotational-direction gap  65  (corresponding to  65 A in  FIG. 6 ) with a certain angle is obtained respectively between the end face  60   c  and the rotational-direction end face  63   b  in each rotational direction. The total of both angles is a prescribed angle whose size allows the friction washer  57  to rotate relative to the frictional engagement member  60 . This angle is preferably within a range that is equal to or slightly exceeds the damper operation angle created by small torsional vibrations caused by combustion fluctuations in the engine. In this embodiment, the frictional engagement members  60  are disposed at the center of the direction of rotation of concavities  63  in the neutral state shown in  FIG. 6 . Therefore, the sizes of the gaps on each side of each frictional engagement member  60  in the direction of rotation are the same.  
      The frictional engagement members  60  are engaged with the first plate  32  to rotate integrally and in a manner that allows movement in the axial direction. More specifically, an annular wall  32   a  extending toward the engine in the axial direction is formed on the radially outer edge of the first plate  32 , and concavities  61  indented on the internal side in the radial direction are formed on the annular wall  32   a  corresponding to the frictional engagement members  60 . In addition, the concavity  61  is formed with a first slit  61   a  penetrating in the radial direction at the center in the rotational direction and second slits  61   b  penetrating in the radial direction at opposite sides thereof. The frictional engagement member  60  has a first leg portion  60   e  that extends inward from the external side in the radial direction in the first slit  61   a,  extends separately outward in the direction of rotation, and makes contact with the radially inner surface of the annular wall  32   a,  and a pair of second leg portions  60   f  that extends inward from the external side in the radial direction in the second slits  61   b,  extends outward in the direction of rotation, and makes contact with the radially inner surface of the annular wall  32   a.  As a result, the frictional engagement member  60  does not move relative to the annular wall  32   a  to the outside in the radial direction. In addition, the frictional engagement member  60  has a convexity  60   d  that extends inward in the radial direction, and is engaged in the direction of rotation with the concavity  61  in the annular wall  32   a.  The frictional engagement members  60  are thereby integrally rotated as convexities with the first plate  32 .  
      In addition, the frictional engagement members  60  are detachably attached to the first plate  32  in the axial direction.  
      The length in the axial direction of the frictional engagement member  60  is less than the length in the axial direction of the concavity  63  (that is to say, the space between the axial-direction end faces  63   c  and  63   d  of the concavity  63  has a greater length than the space between the axial-direction end faces  60   a  and  60   b  of the frictional engagement member  60 ). Thus, the frictional engagement members  60  are capable of moving relative to the friction washers  57  in the axial direction. A gap in the radial direction is provided in the space between the radially outer surface  60   g  of the frictional engagement member  60  and the bottom surface  63   a  of the concavity  63 , so the frictional engagement member  60  is capable of tilting with respect to the friction washer  57  within a prescribed angle.  
      As described above, the friction washers  57  are frictionally engaged with the flexible plate  11  and the input-side friction plate  58 , which are input side members, in the rotational direction, and also are engaged with the frictional engagement members  60  in a manner that allows torque to be transmitted by way of the rotational direction gap  65  of the engagement portion  64 . The frictional engagement members  60  can also integrally rotate with the first plate  32 , and move relative to the first plate  32  in the axial direction.  
      Next, the relationship between the friction washer  57  and the frictional engagement member  60  is described in greater detail. The widths in the direction of rotation (the angles in the direction of rotation) of the frictional engagement members  60  are all the same, but some of the widths in the direction of rotation (the angles in the direction of rotation) of the concavities  63  may be different. That is to say, there are at least two types of friction washers  57  with differing widths in the direction of rotation of the concavities  63 . In this embodiment, these are composed of two first friction washers  57 A that face each other in the up and down directions of  FIG. 5 , and four second friction washers  57 B that face each other in the left and right directions. The first friction washer  57 A and the second friction washer  57 B have roughly the same shape, and are made of the same material. The only point in which these differ is the width in the direction of rotation (the angles in the direction of rotation) of the rotational direction gap of the concavity  63 . More specifically, the width in the direction of rotation of the concavities  63  of the second friction washer  57 B is larger than the width in the direction of rotation of the concavity  63  of the first friction washer  57 A. As a result, the second rotational direction gap  65 B of a second engagement portion  64 B in the second friction washer  57 B is larger than the first rotational direction gap  65 A of a first engagement portion  64 A in the first friction washer  57 A. In this embodiment, the former is preferably 10 degrees and the latter is 8 degrees, and the difference is 2 degrees, for example.  
      Both edges of the first friction washer  57 A and the second friction washer  57 B in the direction of rotation are adjacent to each other. The angle between the edges in the direction of rotation is set to a value that is greater than the difference (2 degrees, for example) between the second rotational direction gap  65 B in the second friction washer  57 B and the first rotational direction gap  65 A in the first friction washer  57 A.  
      6) Clutch Disc Assembly  
      The clutch disc assembly  93  of the clutch has friction facings  93   a  that are disposed adjacent to the clutch friction face  3   a  of the second flywheel  3 , and a hub  93   b  that is spline-engaged with the transmission input shaft  92 .  
      7) Clutch Cover Assembly  
      The clutch cover assembly  94  includes a clutch cover  96 , a diaphragm spring  97 , and a pressure plate  98 . The clutch cover  96  is an annular, disc-like member fixed to the second flywheel  3 . The pressure plate  98  is an annular member having a pressing surface adjacent to the friction facings  93   a  and integrally rotatable with the clutch cover  96 . The diaphragm spring  97  is a member for elastically urging the pressure plate  98  toward the second flywheel as supported on the clutch cover  96 . When a release device (not shown) pushes the radially inner end of the diaphragm spring  97  toward the engine in the axial direction, the diaphragm spring  97  releases its pressure toward the pressure plate  98 .  
      (2) Operation  
      1) Torque Transmission  
      In this dual-mass flywheel  1 , the torque from the engine crankshaft  91  is transmitted to the second flywheel  3  via the damper mechanism  4 . In this damper mechanism  4 , the torque is transmitted in order from the input-side disk-like plate  20 , the coil springs  34  to  36 , and the output-side disk-like plates  32  and  33 . In addition, the torque is transmitted from the dual-mass flywheel  1  to the clutch disc assembly  93  with the clutch in an engagement state, and is finally output to the input shaft  92 .  
      2) Absorption and Attenuation of Torsional Vibrations  
      When combustion fluctuations from the engine are input to the dual-mass flywheel  1 , the output-side disk-like plates  32  and  33  rotate relative to the input-side disk-like plate  20  in the damper mechanism  4 , and the coil springs  34  to  36  are compressed in parallel with each other therebetween. In addition, the first and second friction generation mechanisms  5  and  6  generate the prescribed hysteresis torque. The torsional vibration is absorbed and attenuated by the above-described operation.  
      Next, referring to a torsion characteristics diagram in  FIG. 16 , the operation of the damper mechanism  4  is described. In an area in which the torsional angle is small (near the angle zero), only the first coil springs  34  are compressed to generate relatively low rigidity characteristics. When the torsional angle increases, the first coil springs  34  and the second coil springs  35  are compressed in parallel with each other to generate relatively high rigidity characteristics. When the torsional angle further increases, the first coil springs  34 , the second coil springs  35 , and the third coil springs  36  are compressed in parallel with each other to generate the highest rigidity characteristics at both ends in the torsion characteristics. The first friction generation mechanism  5  operates over the whole range of the torsional angle. It should be noted that the second friction generation mechanism  6  does not operate until the predetermined angle after the orientation of the torsional operation is changed at both the ends of the torsional angle.  
      Next, the operation performed when the friction washers  57  are driven by the frictional engagement members  60  is described. The operation in which the frictional engagement members  60  are twisted from the neutral state in the rotation direction RI in relation to the friction washers  57  is described.  
      When the torsion angle increases, the frictional engagement member  60  in the first friction washer  57 A eventually makes contact with the rotational-direction end face  63   b  on the side in the rotational direction R 1  of the concavity  63  of the first friction washer  57 A. At this time, the frictional engagement member  60  in the second friction washer  57 B have a rotational direction gap (which is one-half of the difference between the second rotation direction gap  65 B of the second friction washers  57 B and the first rotational direction gap  65 A of the first friction washers  57 A, and is 1 degree in this embodiment) in the rotational-direction end face  63   b  of the concavity  63  of the second friction washer  57 B in the rotational direction R 1 .  
      When the torsion angle further increases, the frictional engagement member  60  drives the first friction washers  57 A, and causes them to slide in relation to the flexible plate  11  and the input-side friction plate  58 . At this time, the first friction washer  57 A approaches the second friction washer  57 B in the rotational direction R 1 , but the edge portions of both of these do not make contact.  
      When the torsion angle finally realizes the prescribed magnitude, the frictional engagement member  60  makes contact with the rotational-direction end face  63   b  of the concavity  63  of the second friction washer  57 B. After this, the frictional engagement members  60  drive both the first and second friction washers  57 A and  57 B, causing them to slide in relation to the flexible plate  11  and the input-side friction plate  58 .  
      In summary, driving the friction washer  57  with the aid of the first plate  32  yields an area in which a constant number of plates is driven to generate an intermediate frictional resistance in the torsion characteristics before the start of the large frictional resistance area in which all of the plates are driven.  
      2-1) Small Torsional Vibrations  
      Next, the operation of the damper mechanism  4  when small torsional vibrations caused by combustion fluctuations in the engine are inputted to the dual-mass flywheel  1  is described below with reference to the mechanical circuit diagram in  FIG. 15  and the diagrams of torsional characteristics in FIGS.  16  to  19 .  
      When small torsional vibrations are inputted, the input-side disk-like plate  20  in the second friction generation mechanism  6  rotates relative to the friction washers  57  in the rotational direction gaps  65  between the concavities  63  and the frictional engagement members  60  (the convex portions). In other words, the friction washers  57  are not driven with the first plate  32 , and the friction washers  57  therefore do not rotate in relation to the member on the input side. As a result, high hysteresis torque is not generated for small torsional vibrations. That is, although the coil springs  34  and  35  operate at “DCa”, for example, in the diagram of torsional characteristics in  FIG. 16 , a slippage does not occur in the second friction generation mechanism  6 . That is to say, only a hysteresis torque that is much smaller than normal hysteresis torque can be obtained in a prescribed range of torsion angles. Thus, the vibration and noise level can be considerably reduced because a very narrow rotational direction gap is provided in which the second friction generation mechanism  6  does not operate in the prescribed angle range.  
      As a result, when the operating angle of the torsional vibration is less than the angle (8 degrees, for example) of the first rotational direction gaps  65 A of the first engagement portions  64 A of the first friction washers  57 A, large frictional resistance (high hysteresis torque) is not generated at all and only area A of low frictional resistance is obtained in the second stage of torsion characteristics, as shown in  FIG. 17 . Moreover, when the operating angle of the torsional vibration is equal to or greater than the angle (8 degrees, for example) of the first rotational direction gaps  65 A of the first engagement portions  64 A of the first friction washers  57 A, and is equal to or less than the angle (10 degrees, for example) of the second rotational direction gaps  65 B of the second engagement portions  64 B of the second friction washers  57 B, the areas B of intermediate frictional resistance are generated on the edges of the area A of low frictional resistance, as shown in  FIG. 18 . When the operating angle of the torsional vibration is equal to or less than the angle (10 degrees, for example) of the second rotational direction gaps  65 B of the second engagement portions  64 B of the second friction washers  57 B, the area B of intermediate frictional resistance and the area C in which a certain large frictional resistance is generated are respectively obtained on both edges of the area A of low frictional resistance, as shown in  FIG. 19 .  
      2-2) Operation When the Wide-Angle Torsional Vibrations are Input  
      The operation of the second friction generation mechanism  6  is described below for the case in which large torsional vibrations are inputted. In the second friction generation mechanism  6 , the friction washers  57  integrally rotate with the frictional engagement members  60  and the first plate  32 , and also rotate relative to the flexible plate  11  and the input-side friction plate  58 . As a result, the friction washers  57  slide against the flexible plate  11  and the input-side friction plate  58  to generate frictional resistance. As described previously, when the torsional angle of the torsional vibration is large, the friction washers  57  slide against the flexible plate  11  and the input-side friction plate  58 . As a result, a frictional resistance with a constant magnitude is obtained over the entire range of torsional characteristics.  
      Here, the operation in the edge portion (position in which the direction of the vibration changes) of the torsion angle is described. At the right-hand edge of the torsion characteristic line chart of  FIG. 16 , the friction washers  57  shift toward their most rotational direction R 2  position in relation to the first plate  32 . When the first plate  32  twists from this state toward the rotational direction R 2 , the friction washers  57  rotate in relation to the first plate  32  across the entire angle of the rotational direction gaps  65  of the frictional engagement members (the convexity)  60  and the concavities  63 . In this interval, area A (8 degrees, for example) of low frictional resistance can be obtained because the friction washers  57  do not slide against the member on the input side. Next, when the first rotational direction gaps  65 A of the first engagement portions  64 A of the first friction washers  57 A are no longer present, the first plate  32  drives the first friction washers  57 A. Then, the first friction washers  57 A rotate relative to the flexible plate  11  and the input-side friction plate  58 . As a result, area B of intermediate frictional resistance (2 degrees, for instance) is generated as described above. When the second rotational direction gaps  65 B of the second engagement portions  64 B of the second friction washers  57 B are no longer present, the first plate  32  subsequently drives the second friction washers  57 B. Then, the second friction washers  57 B rotate relative to the flexible plate  11  and the input-side friction plate  58 . Area C of comparatively large frictional resistance is generated because both the first friction washers  57 A and the second friction washers  57 B slide together at this time. It is noted that hysteresis torque generated by the first friction washers  57 A is smaller than that of the second friction washers  57 B, about one-half in this embodiment.  
      As described above, area B of intermediate frictional resistance is provided at an early stage when a large frictional resistance is generated. A barrier of high hysteresis torque does not exist when a large frictional resistance is generated because the buildup of large frictional resistance is graduated in this manner. As a result, the knocking sound of the pawls when high hysteresis torque is generated decreases in a frictional resistance generation mechanism with a very narrow rotational direction gap for absorbing small torsional vibrations.  
      In particular, the number of types of frictional members can be kept low in the present invention because a single type of friction washer  57  is used to generate intermediate frictional resistance. The friction washer  57  is also a simple structure that extends in the form of an arc. Furthermore, through-holes in the axial direction are not formed in the friction washers  57 , and thus, manufacturing costs can be kept low.  
      2-3) Operation When the Small-Angle Torsional Vibrations are Input  
      Next, the operation of the second friction generation mechanism  6  is described for a case in which small torsional vibrations caused by combustion fluctuations in the engine are input to the flywheel damper.  
      When small torsional vibrations are inputted in the second friction generation mechanism  6 , the frictional engagement members  60  rotate relative to the friction washers  57  in the very narrow rotational direction gaps  65 . In other words, the friction washers  57  are not driven by the frictional engagement members  60 , and the friction washers  57  therefore do not rotate in relation to the input side member. As a result, high hysteresis torque is not generated in response to the small torsional vibrations. That is to say, only a hysteresis torque that is much smaller than normal hysteresis torque can be obtained in the prescribed range of torsion angles. Thus, since a very narrow rotational direction gap is provided in which the second generation mechanism  6  does not operate in the prescribed angle range, the vibration and noise level in the torsion characteristics can be considerably reduced.  
      (3) Effects  
      3-1) Effects of the First Friction Generation Mechanism  5   
      Since the first friction generation mechanism  5  makes use of a part of the second flywheel  3  as a frictional face, it is possible to enlarge an area of the sliding face. More specifically, since the second friction member  52  is urged by the cone spring  53  against the second flywheel  3 , it is possible to enlarge the area of the sliding face. As a result, the pressure against the sliding face is lowered, thereby extending a life of the first friction generation mechanism  5 .  
      The radially outer portion of the second friction member  52  and the radially inner portions of the first and second coil springs  34  and  35  are overlapped in the axial direction, and the radial position of the radially outer periphery of the second friction member  52  is radially outward of the radial positions of the radially inner peripheries of the first and second coil springs  34  and  35 . Accordingly, although the second friction member  52  and the first and second coil springs  34  and  35  are very close to each other in the radial direction, the second friction generation mechanism  6  can ensure enough frictional face.  
      The radially outer portion of the annular portion  51   a  of the first friction member  51  and the radially inner portions of the first and second coil springs  34  and  35  are overlapped in the axial direction, and the radial position of the radially outer periphery of the annular portion  51   a  is radially outward of the radial positions of the radially inner peripheries of the first and second coil springs  34  and  35 . As a result, although the annular portion  51   a  and the first and second coil springs  34  and  35  are very close to each other in the radial direction, the second friction generation mechanism  6  can ensure enough frictional face.  
      Only the first friction member  51  is unrotatably engaged with the input-side disc-like plate  20 , and the first friction member  51  and the second friction member  52  are unrotatably engaged with each other. As a result, it is unnecessary to engage the input-side disc-like plate  20  with the second friction member  52 , thereby realizing a simple structure.  
      The first friction member  51  has the annular portion  51   a  slidably in contact with the first plate  32  in the rotational direction, and the engagement portions  51   b  and  51   c  extending from the annular portion  51   a  in the axial direction and engaged with the input-side disc-like plate  20  such that the first friction member  51  can move in the axial direction but cannot rotate relative to the plate  20 . The second friction member  52  is formed with the cutouts  52   a  engaged with the engagement portions  51   b  and  51   c  such that the second friction member  52  can move in the axial direction but cannot rotate relative to the member  51 . Accordingly, since the first friction member  51  is formed with the engagement portions  51   b  and  51   c  extending in the axial direction, it is possible to realize easily a structure in which the annular portion  51   a  of the first friction member  51  and the second friction member  52  are apart from each other in the axial direction.  
      The cone spring  53  is disposed between the second friction member  52  and the engagement portions  51   b  and  51   c  of the first friction member  51  to urge both members in the axial direction, thereby making the structure simple.  
      The washer  54  is seated on the tips of the engagement portions  51   b  and  51   c  of the first friction member  51  to serve as a receiving member that receives the urging force from the cone spring  53 . As a result, the axial load applied to the frictional sliding face becomes stable so that the frictional resistance generated at the sliding face is stabilized.  
      The first friction generation mechanism  5  is disposed radially inward of the clutch friction face  3   a  of the second flywheel  3 , that is, apart from the clutch friction face  3   a  radially inward. As a result, the first friction generation mechanism  5  is unlikely to be affected by the heat from the clutch friction face  3   a,  thereby stabilizing the frictional resistance.  
      The first friction generation mechanism  5  is disposed radially inward of the radially middle portion of the first and second coil springs  34  and  35  of the damper mechanism  4  and radially outward of the radially outermost peripheries of the bolts  22 . As a result, a space-saving structure is realized.  
      3-2) Effects of the Second Friction Generation Mechanism  6   
      Since the second friction generation mechanism  6  is supported by the first flywheel  2  (more specifically, the flexible plate  11 ), the second friction generation mechanism  6  is unlikely to be affected by the heat from the clutch friction face  3   a  of the second flywheel  3 . As a result, the performance of the second friction generation mechanism  6  is stable. Especially, the first flywheel  2  is not connected to the second flywheel  3  via the coil springs  34  to  36 , the heat is unlikely to be transmitted to the first flywheel  2  from the second flywheel  3 .  
      The second friction generation mechanism  6  makes use of the annular portion  11   a  at the radially outer portion of the flexible plate  11  as a frictional face. Since the flexible plate  11  is utilized, the second friction generation mechanism  6  has fewer components in number, thereby simplifying the structure.  
      Since the second friction generation mechanism  6  is disposed radially outward of the clutch friction face  3   a  of the clutch and is apart from the clutch friction face  3   a  in the radial direction, the second friction generation mechanism  6  is unlikely to be affected by the heat from the clutch friction face  3   a.    
      3-3) Effects of the Flexible Flywheel (the First Flywheel  2  and the Damper Mechanism  4 )  
      The first flywheel  2  is a member having the inertia member  13  and the flexible plate  11  for connecting the inertia member  13  with the crankshaft  91 . The flexible plate  11  is flexible in the bending and axial directions. The damper mechanism  4  includes the input-side disc-like plate  20  to which the torque is input from the crankshaft  91 , the output-side disc-like plates  32  and  33  disposed rotatably relative to the input-side disc-like plate  20 , and the coil springs  34 ,  35 , and  36  to be compressed in the rotational direction when the plates are rotated relative each other. The damper mechanism  4  is directly connected to the crankshaft  91 , that is, not via the first flywheel  2 . The first flywheel  2  can move relative to the damper mechanism  4  in the bending direction within the predetermined range. A combination of the first flywheel  2  and the damper mechanism  4  described above is referred to as a flexible flywheel  66 .  
      When the bending vibration is generated in the first flywheel  2 , the flexible plate  11  is bent in the bending direction. As a result, the bending vibration from the engine is dampened. In the flexible flywheel, since the first flywheel  2  can move relative to the damper mechanism  4  in the bending direction within the predetermined range, the dampening effect of the flexible plate  11  on the bending vibration is sufficiently high.  
      The flexible flywheel  66  further includes the second friction generation mechanism  6 , which is disposed between the first flywheel  2  and the output-side disc-like plate  32  of the damper mechanism  4  to operate in parallel with the coil springs  34 ,  35 , and  36  in the rotational direction. The second friction generation mechanism  6  has the friction washers  57  and the frictional engagement members  60 , which are engaged with each other to transmit the torque therebetween but are movable relative to each other in the bending direction. Since two members in the second friction generation mechanism  6  of the flexible flywheel  66  are engaged so as to be movable relative to each other in the bending direction, the first flywheel  2  can move within the predetermined range in the bending direction, although the first flywheel  2  is engaged with the damper mechanism  4  via the second friction generation mechanism  6 . As a result, the dampening effect of the flexible plate  11  on the bending vibration is sufficiently high.  
      The friction washer  57  and the frictional engagement member  60  are engaged with each other with a clearance therebetween in the rotational direction. In other words, since both members are not in direct contact with each other in the rotational direction, relative movement in the bending direction does not generate large resistance.  
      The frictional engagement members  60  are engaged with the first plate  32  of the output-side disc-like plates  32  and  33  so as to move in the axial direction. As a result, when the friction washers  57  move with the first flywheel  2  in the axial direction, axial resistance is unlikely to be generated between the frictional engagement members  60  and the output-side disc-like plates  32  and  33 .  
      3-4) Effects of the Third Coil Springs  36   
      The third coil spring  36  is a member for providing a sufficient stopper torque to the damper mechanism  4  by starting the operation in the torsion characteristics at an area in which the torsional angle becomes the largest. The third coil springs  36  are functionally disposed to operate in parallel with the first and second coil springs  34  and  35  in the rotational direction.  
      The third coil spring  36  has a wire diameter and a coil diameter smaller than those of the first and second coil springs  34  and  35  to a large extent (about a half) so that the third coil springs  36  occupy an less axial space less than the coil springs  34  and  35 . As shown in  FIG. 1 , the third coil springs  36  are disposed radially outward of the first and second coil springs  34  and  35  and at a position corresponding to the clutch friction face  3   a  of the second flywheel  3 . In other words, the radial positions of the third coil springs  36  are within an annular area between the inner diameter and the outer diameter of the clutch friction face  3   a.    
      In this embodiment, presence of the third coil springs  36  heightens the stopper torque sufficiently to improve the performance, and devising sizes and positions of the third coil springs  36  realizes a space-saving structure. Especially, although the third coil spring  36  is located at a position in the second flywheel  3  corresponding to the clutch friction face  3   a,  where an axial thickness is large, an axial size of the portion is sufficiently small and is smaller than the axial size of a portion where the first and second coil springs  34  and  35  are disposed.  
      Furthermore, the third coil springs  36  are positioned at almost the same radial position of the stopper constituted by the projections  20   c  of the input-side disc-like plate  20  and the cut-and-bent abutting portions  43  and  44  of the output-side disc-like plates  32  and  33 . As a result, the diameter of the whole structure becomes smaller compared to a structure in which the components are located at different radial positions.  
     2. Second Embodiment  
       FIG. 20  shows a flexible flywheel  101  as a second embodiment according to the present invention. The flexible flywheel  101  is a device for transmitting torque from a crankshaft  91  of the engine to an input shaft  92  of the transmission. The flexible flywheel  101  is composed of a first flywheel  102  and a damper mechanism  103 . The damper mechanism  103  is directly fixed to the crankshaft  91 , and torque is not input to the damper mechanism  103  from the flywheel  102 .  
      The first flywheel  102  includes an inertia member  113 , and a flexible plate  111  for connecting the inertia member  113  with the crankshaft  91 . The flexible plate  111  is flexibly deformable in the bending direction.  
      The damper mechanism  103  includes input-side disc-like plates  132  and  133  to which torque is input from the crankshaft  91 , an output-side disc-like plate  120  rotatably located relative to the plates  132  and  133 , and coil springs  134  to be compressed in the rotational direction when both the plates rotate relative to each other. The plates  132  and  133  are fixed to each other. The plate  132  has a radially inner portion  132   a  extending radially inward beyond a radially inner portion of the plate  133  and fixed to the crankshaft  91  by crankbolts  122  with a radially inner portion of the flexible plate  111 . The output-side disc-like plate  120  has a radially inner portion  120   a  extending toward an outer circumferential surface of a hub  121  for engagement with each other so as not to rotate relative to each other. The plate  120  and the hub  121  cannot move relative to each other in the axial direction by a structure such as axially abutting surfaces and a snap ring.  
      Accordingly, unlike the first embodiment, the flexible flywheel  101  directly outputs torque to the input shaft  92  of the transmission, not via the second flywheel and the clutch, but via the hub  121 .  
      As clear from  FIG. 20 , the first flywheel  102  has portions other than the radially inner portion apart from the damper mechanism  103  so that the first flywheel  102  can move relative to the damper mechanism  103  in the bending direction within a limited range.  
      When the bending vibrations are generated in the first flywheel  102 , the flexible plate  111  flexes in the bending direction. Accordingly, the bending vibrations from the engine are attenuated. In this flexible flywheel  101 , since the first flywheel  102  can move relative to the damper mechanism  103  in the bending direction within a limited range, the bending vibration suppressive effects by the flexible plate  111  is sufficiently high.  
     3. Third Embodiment  
       FIG. 21  shows a flexible flywheel  101 ′ as a third embodiment according to the present invention. Only different points will be described because the basic structures are the same as those of the second embodiment.  
      The damper mechanism  103 ′ includes an input-side disc-like plate  120 ′ to which torque is input from the crankshaft  91 , output-side disc-like plates  132 ′ and  133 ′ rotatably located relative to the plate  120 ′, and coil springs  134  to be compressed in the rotational direction when the both plates rotate relative to each other. The plates  132 ′ and  133 ′ are fixed to each other. The plate  133  has a radially inner portion  133   a  extending radially inward beyond a radially inner portion of the plate  132  and fixed to a flange  121   a  of the hub  121 ′ by a plurality of rivets  124 . The plate  120 ′ has a radially inner portion  120   a  fixed to the crankshaft  91  by crankbolts  122  with a radially inner portion of the flexible plate  111 .  
      Accordingly, unlike the second embodiment, the plates  132 ′ and  133 ′ function as an output member, and the plate  120 ′ functions as an input member.  
      As it is clear from  FIG. 21 , the first flywheel  102  has portions other than the radially inner portion apart from the damper mechanism  103 ′ so that the first flywheel  102  can move relative to the damper mechanism  103 ′ in the bending direction within a limited range.  
      When the bending vibrations are generated in the first flywheel  102 , the flexible plate  111  flexes in the bending direction. Accordingly, the bending vibrations from the engine are attenuated. In this flexible flywheel  101 ′, since the first flywheel  102  can move relative to the damper mechanism  103 ′ in the bending direction within a limited range, the bending vibration suppressive effects by the flexible plate  111  is sufficiently high.  
     4. Other Embodiment  
      Embodiments of the clutch mechanism in accordance with the present invention were described above, but the present invention is not limited to these embodiments and other variations or modifications that do not depart from the scope of the present invention are possible. More particularly, the present invention is not limited by the specific numerical values of angles and the like described above.  
      In the above-described embodiment, two size types of the rotational direction gap of the engagement portion were used, but it is also possible to use three or more size types. In the case of three size types, the magnitude of the intermediate frictional resistance will have two stages.  
      The coefficients of friction of the first friction member and the second friction member are the same as in the above-described embodiment, but these may also be varied. Thus, the ratio of the intermediate frictional resistance and large frictional resistance can be arbitrarily set by adjusting the frictional resistance generated by the first friction member and the second friction member.  
      In the above-described embodiment, intermediate frictional resistance is generated by providing convexities with an equal size and concavities with different sizes, but the concavities may be set to an equal size and the size of the convexities may be different. Furthermore, combinations of convexities and concavities with different sizes may also be used.  
      In the above-described embodiment, the concavity of the friction washer faces the internal side in the radial direction, but it may also face the external side in the radial direction.  
      In addition, the friction washer in the above-described embodiment has concavities, but the friction washer may also have convexities. In this case, the input-side disk-like plate has concavities, for example.  
      Furthermore, the friction washer in the above-described embodiment has a friction surface that is frictionally engaged with an input side member, but it may also have a friction surface that is frictionally engaged with an output member. In this case, an engagement portion having a rotational direction gap is formed between the friction washer and the input side member.