Patent Publication Number: US-10760626-B2

Title: Centrifugal clutch

Description:
TECHNICAL FIELD 
     The present invention relates to a centrifugal clutch, which cuts off transmission of a rotational driving force to a driven side until a rotational speed of an engine reaches a predetermined value, and transmits the rotational driving force to the driven side when the rotational speed of the engine reaches the predetermined value. 
     BACKGROUND ART 
     Conventionally, the centrifugal clutch is used for a motorcycle, a brush cutter, and the like. The centrifugal clutch transmits the rotational driving force to the driven side when the rotational speed of the engine reaches the predetermined value. For example, in the centrifugal clutch disclosed in Patent Document 1, as the rotational speed of the engine increases, a clutch weight is gradually displaced to a side of a clutch outer and makes frictional contact. Further, in the centrifugal clutch disclosed in Patent Document 2, a columnar roller rides on a cam-shaped protrusion rotationally driven by a driving force of the engine, so that clutch shoes provided on an outer side of the roller press the clutch outer to bring the clutch into a connected state. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP-A-2006-038124 
     Patent Document 2: JP-UM-B-42-015451 
     However, in the centrifugal clutch described in the above-described Patent Document 1, the clutch weight gradually makes frictional contact with the clutch outer as the rotational speed of the engine increases. Therefore, since time until the clutch enters the connected state becomes longer, there is a problem that fuel economy decreases. On the other hand, in the centrifugal clutch described in Patent Document 2, immediately after the cam-shaped protrusion rotates, the roller rides on the cam-shaped protrusion, and the clutch enters the connected state. Therefore, since the driven side suddenly operates, there has been a problem that it is difficult for an operator to handle. 
     The present invention has been made to address the above problems. An object of the present invention is to provide a centrifugal clutch capable of quickly establishing a connected state while preventing sudden startup on a driven side. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above object, the present invention is a centrifugal clutch including a first drive plate rotationally driven together with a driven pulley by receiving a driving force of an engine, a second drive plate disposed outside the first drive plate and contacting the first drive plate with a frictional force capable of being rotationally driven together with the first drive plate while allowing rotational displacement relative to the first drive plate, a clutch weight movably mounted on the second drive plate, displaced outwardly to the second drive plate by a centrifugal force due to rotation drive of the second drive plate, and having a clutch shoe on a surface outside thereof, a clutch outer having a cylindrical surface in frictional contact with the clutch shoe displaced outwardly to the second drive plate, a bulging body formed to bulge outwardly from an outer circumferential portion of the first drive plate, and a pressing body provided on the clutch weight and pressed against the bulging body, wherein at least one of the bulging body and the pressing body is formed to extend rearwardly in a rotational drive direction of the first drive plate and outwardly to the first drive plate. 
     According to a feature of the present invention configured as described above, in the centrifugal clutch, after the first drive plate starts rotation drive, the clutch shoes contact the clutch outer. Thus, when the second drive plate is displaced rearward in the rotational drive direction with respect to the first drive plate, the pressing body rides on the bulging body. Thus, the clutch shoes rapidly press the clutch outer. That is, time until the clutch shoes contact the clutch outer is secured from when the first drive plate starts rotation drive until when the centrifugal clutch enters the connected state. Further, when the clutch shoes contact the clutch outer, the centrifugal clutch rapidly shifts to the connected state. As a result, the centrifugal clutch can quickly enter the connected state while preventing sudden startup on the driven side. 
     Further, in the centrifugal clutch according to the present invention, since the pressing body rides on the bulging body, the clutch shoes are strongly pressed against the clutch outer. Therefore, the rotational driving force from the engine can be transmitted with a clutch weight lighter than a conventional centrifugal clutch even when the rotational speed is the same as that of the conventional centrifugal clutch. That is, according to the centrifugal clutch of the present invention, it is possible to reduce weight of the clutch weight. Therefore, it is possible to prevent occurrence of judder at the time of connecting the clutch. Further, it is possible to realize simplification, compactification and cost reduction of a structure of the centrifugal clutch. This means that when the weight of the clutch weight is the same as the conventional centrifugal clutch, it is possible to increase the driving force which can be transmitted by the centrifugal clutch. In the centrifugal clutch according to the present invention, it is also possible to appropriately adjust the time until the clutch enters the connected state by adjusting magnitude of the frictional force at the contact surface between the first drive plate and the second drive plate. 
     Another feature of the present invention resides in that, in the centrifugal clutch, the pressing body is constituted by a flat surface, and the bulging body is constituted by a curved surface partially contacting the pressing body. 
     According to another feature of the present invention configured as described above, in the centrifugal clutch, the bulging body is formed in a curved surface shape, and the pressing body is formed in a flat shape. Therefore, as compared with a case where both the bulging body and the pressing body make surface contact, frictional resistance is more suppressed, and the clutch weight is more easily rotationally displaced. Further, as compared with a case where the pressing body is formed in a curved surface shape, it is easier to form the bulging body and the pressing body. 
     Further, another feature of the present invention resides in that, in the centrifugal clutch, the bulging body is made of a roller rotatably provided rearward in the rotational drive direction of the first drive plate. 
     According to another feature of the present invention configured as described above, in the centrifugal clutch, the bulging body is made of the roller rotatably provided rearward in the rotational drive direction of the first drive plate. Therefore, by rotation of the bulging body against which the pressing body is pressed, it is possible to prevent increase in frictional resistance and damage due to wear due to friction sliding between the bulging body and the pressing body. 
     Further, another feature of the present invention resides in that, in the centrifugal clutch, at least one of an outer circumferential portion and an inner circumferential portion of the second drive plate has a bent portion bent with respect to a plate surface. 
     According to another feature of the present invention configured as described above, in the centrifugal clutch, at least one of the outer circumferential portion and the inner circumferential portion of the second drive plate has the bent portion bent with respect to the plate surface. Therefore, it is possible to increase rigidity of the second drive plate, which supports the clutch weight and transmits the driving force. 
     Further, another feature of the present invention resides in that, in the centrifugal clutch, the pressing body is made of a material more easily worn than the bulging body. In this case, the pressing body can be made of, for example, a material having lower hardness than the bulging body. Specifically, for example, the pressing body can be made of zinc material, and the bulging body can be made of carbon steel, iron based sintered material or the like. Further, for example, the pressing body and the bulging body can be made of the same material, and by applying a surface hardening treatment such as heat treatment or coating to the bulging member, higher wear resistance than the pressing body can be obtained. 
     According to another feature of the present invention configured as described above, in the centrifugal clutch, the pressing body is made of a material more easily worn than the bulging body. Therefore, the pressing body wears more than the bulging body. In this case, the pressing body is formed in the clutch weight including the clutch shoe. Therefore, the pressing body is renewed by replacing the clutch weight due to wear of the clutch shoe. That is, in the centrifugal clutch, by exchanging the clutch weigh due to wear of the clutch shoe, the pressing body can also be replaced with a new one at the same time, so that maintenance burden can be reduced. 
     Further, another feature of the present invention resides in that, in the centrifugal clutch, at least one of two contact surfaces where the first drive plate and the second drive plate are in contact with each other is subjected to surface treatment to maintain slidability. In this case, various coating processes which can improve the wear resistance can be employed as the surface treatment, in addition to the heat treatment such as nitriding treatment and carburizing treatment. 
     According to another feature of the present invention configured as described above, in the centrifugal clutch, at least one of two contact surfaces where the first drive plate and the second drive plate are in contact with each other is subjected to surface treatment to maintain slidability. Therefore, it is possible to improve durability so that the maintenance burden can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan sectional view schematically showing a configuration of a power transmission mechanism including a centrifugal clutch according to the present invention; 
         FIG. 2  is a side view of the centrifugal clutch taken along a line  2 - 2  shown in  FIG. 1 ; 
         FIGS. 3A and 3B  show appearance configurations of a first drive plate and a second drive plate in the centrifugal clutches shown in  FIGS. 1 and 2 , wherein  FIG. 3A  is a perspective view showing a state before assembling the first drive plate and the second drive plate, and  FIG. 3B  is a perspective view showing a state where the first drive plate and the second drive plate are assembled; 
         FIGS. 4A to 4D  show appearance configuration of the bulging body in the centrifugal clutch shown in  FIG. 1  and  FIG. 2  and an assembled state of the bulging body on the first drive plate, wherein  FIG. 4A  is a perspective view showing a state before assembling the bulging body from outside the first drive plate,  FIG. 4B  is a perspective view showing the state before assembling the bulging body from inside the first drive plate,  FIG. 4C  is a perspective view showing a state where the bulging body is assembled from outside the first drive plate, and  FIG. 4D  is a perspective view showing the state where the bulging body is assembled from inside the first drive plate; 
         FIG. 5  is a partially enlarged view showing the bulging body and the pressing body in the centrifugal clutch shown in  FIG. 2  in a state where the clutch shoes are not in contact with the clutch outer; 
         FIG. 6  is a partially enlarged view showing a contact state between the bulging body and the pressing body in a connected state in contact with the clutch outer in a state where the clutch shoe in the centrifugal clutch shown in  FIG. 5  is not worn, and showing a state where the bulging body pushes up the pressing body; 
         FIG. 7  is a partially enlarged view showing a state where a pushed-up state of the pressing body by the bulging body in the centrifugal clutch shown in  FIG. 6  is canceled; 
         FIG. 8  is a partially enlarged view showing the contact state between the bulging body and the pressing body in the connected state in contact with the clutch outer in a state where the clutch shoe in the centrifugal clutch shown in  FIG. 5  is worn by about 1 mm; 
         FIG. 9  is a partially enlarged view showing the contact state between the bulging body and the pressing body in the connected state in contact with the clutch outer in a state where the clutch shoe in the centrifugal clutch shown in  FIG. 5  is worn by about 2 mm; 
         FIG. 10  is a plan sectional view schematically showing a configuration of a power transmission mechanism including the centrifugal clutch according to a modification of the present invention; 
         FIGS. 11A to 11D  show appearance configuration of the bulging body in the centrifugal clutch shown in  FIG. 10  and an assembled state of the bulging body on the first drive plate, wherein  FIG. 11A  is a perspective view showing a state before assembling the bulging body from outside the first drive plate,  FIG. 11B  is a perspective view showing the state before assembling the bulging body from inside the first drive plate,  FIG. 11C  is a perspective view showing a state where the bulging body is assembled from outside the first drive plate, and  FIG. 11D  is a perspective view showing the state where the bulging body is assembled from inside the first drive plate; 
         FIG. 12  is a partially enlarged view corresponding to  FIG. 5  taken along a line  2 - 2  in  FIG. 10 , showing the bulging body and the pressing body in the centrifugal clutch in a disconnected state where the clutch shoes do not contact the clutch outer; and 
         FIG. 13  is a partially enlarged view corresponding to  FIG. 6  taken along the line  2 - 2  in  FIG. 10 , showing the contact state between the bulging body and the pressing body in the connected state in contact with the clutch outer in a state where the clutch shoe in the centrifugal clutch is not worn. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of a centrifugal clutch according to the present invention will be described with reference to the drawings below.  FIG. 1  is a plan sectional view schematically showing a configuration of a power transmission mechanism  100  including a centrifugal clutch  200  according to the present invention.  FIG. 2  is a side view of the centrifugal clutch  200  taken along a line  2 - 2  shown in  FIG. 1 . The power transmission mechanism  100  including the centrifugal clutch  200  is a mechanical device provided between an engine and a rear wheel which is a driving wheel, mainly in a motorcycle vehicle such as a scooter, and transmits or cuts off a rotational driving force to the rear wheel while automatically changing a reduction ratio with respect to the number of revolutions of the engine. 
     (Configuration of Centrifugal Clutch  200 ) 
     The power transmission mechanism  100  mainly includes a transmission  101  and the centrifugal clutch  200 . The transmission  101  is a mechanical device which decelerates the rotational driving force from the engine (not shown) in a stepless manner and transmits it to the centrifugal clutch  200 . The transmission  101  mainly includes a drive pulley  110 , a V-belt  120 , and a driven pulley  130 . Among these, the drive pulley  110  is provided on a crankshaft  111  extending from the engine and is a mechanical device directly driven to rotate by the rotational driving force of the engine. The drive pulley  110  mainly includes a fixed drive plate  112  and a movable drive plate  113 . 
     The fixed drive plate  112  is a component which is rotationally driven together with the movable drive plate  113  while holding the V-belt  120  therebetween. The fixed drive plate  112  is configured by forming a metal material into a conical cylindrical shape. The fixed drive plate  112  is fixedly mounted on the crankshaft  111  in a state where a convex side surface thereof faces toward the movable drive plate  113  (engine). That is, the fixed drive plate  112  is always rotationally driven together with the crankshaft  111 . Further, a plurality of radiating fins  112   a  are radially provided on a concave side surface of the fixed drive plate  112 . 
     The movable drive plate  113  is a component which is rotationally driven together with the fixed drive plate  112  while holding the V-belt  120  therebetween. The movable drive plate  113  is configured by forming a metal material into a conical cylindrical shape. The movable drive plate  113  is mounted on the crankshaft  111  in a direction where a convex side surface thereof faces the fixed drive plate  112 . In this case, the movable drive plate  113  is mounted on a sleeve bearing  114  fixedly fitted into the crankshaft  111  via an impregnated bush. The movable drive plate  113  is slidably mounted on the sleeve bearing  114  in an axial direction and a circumferential direction thereof. 
     On the other hand, a plurality of roller weights  115  are provided in a state pressed by a ramp plate  116  on a concave side surface of the movable drive plate  113 . The roller weight  115  is a component for pressing the movable drive plate  113  toward the fixed drive plate  112  in cooperation with the ramp plate  116  by being displaced outwardly in a radial direction in accordance with increase in rotational speed of the movable drive plate  113 . The roller weight  115  is configured by forming a metal material into a cylindrical shape. Further, the ramp plate  116  is a component for pressing the roller weight  115  toward the movable drive plate  113 . The ramp plate  116  is formed by bending a metal plate toward the movable drive plate  113 . 
     The V-belt  120  is a component for transmitting the rotational driving force of the drive pulley  110  to the driven pulley  130 . The V-belt  120  is formed in an endless ring shape made of a core wire covered with a resin material. The V-belt  120  is disposed between the fixed drive plate  112  and the movable drive plate  113  and between a fixed driven plate  131  and a movable driven plate  134  in the driven pulley  130 , and is bridged between the drive pulley  110  and the driven pulley  130 . 
     The driven pulley  130  is a mechanical device rotationally driven by the rotational driving force from the engine, which is transmitted via the drive pulley  110  and the V-belt  120 . The driven pulley  130  mainly includes the fixed driven plate  131  and the movable driven plate  134 . 
     The fixed driven plate  131  is a component which is rotationally driven together with the movable driven plate  134  while holding the V-belt  120  therebetween. The fixed driven plate  131  is configured by forming a metal material into a conical cylindrical shape. The fixed driven plate  131  is fixedly mounted on a driven sleeve  132  in a state where a convex side surface thereof faces toward the movable driven plate  134 . 
     The driven sleeve  132  is a metallic cylindrical component which is rotationally driven together with the fixed driven plate  131 . The driven sleeve  132  is mounted on a drive shaft  133  to be relatively rotatable via a bearing. The drive shaft  133  is a metal rotary shaft body for driving the rear wheel of the motorcycle vehicle on which the power transmission mechanism  100  is mounted via a transmission (not shown). In this case, the rear wheel of the motorcycle vehicle is mounted on one end (right end in the drawing) of the drive shaft  133 . 
     The movable driven plate  134  is a component which is rotationally driven together with the fixed driven plate  131  while holding the V-belt  120  therebetween. The movable driven plate  134  is configured by forming a metal material into a conical cylindrical shape. The movable driven plate  134  is fitted into the driven sleeve  132  to be axially slidable in a direction where a convex side surface thereof faces the fixed driven plate  131 . 
     On the other hand, on a concave side surface of the movable driven plate  134 , a torque spring  135  is provided between the concave side surface and a first drive plate  210  of the centrifugal clutch  200 . The torque spring  135  is a coil spring which elastically presses the movable driven plate  134  toward the fixed driven plate  131 . That is, the transmission  101  shifts a rotational speed of the engine steplessly using a magnitude relation between a diameter sandwiching the V-belt  120  defined by a distance between the fixed drive plate  112  and the movable drive plate  113 , and a diameter sandwiching the V-belt  120  defined by a distance between the fixed driven plate  131  and the movable driven plate  134 . The centrifugal clutch  200  is provided on a distal end side of the driven sleeve  132  and on a distal end side of the drive shaft  133 . 
     The centrifugal clutch  200  is a mechanical device which transmits or cuts off the rotational driving force of the engine, which is transmitted via the transmission  101 , to the drive shaft  133 . The centrifugal clutch  200  mainly includes the first drive plate  210 , a second drive plate  220 , clutch weights  230 , and a clutch outer  240 . 
     The first drive plate  210  is a component rotationally driven together with the driven sleeve  132 , and is configured by forming a metal material into a bottomed cylindrical shape. More specifically, as shown in  FIGS. 3A and 3B , in the first drive plate  210 , a through-hole  212  through which the driven sleeve  132  passes is formed in a central portion of a flat bottom portion  211 . Further, three supporting portions  214  are provided at equal intervals in a circumferential direction at a distal end of a cylinder portion  213  erected around the bottom portion  211 . 
     The supporting portion  214  is a portion hooked on an inner circumferential portion of the second drive plate  220  and is a portion for supporting a bulging body  215 . The supporting portion  214  is formed so as to bulge radially outward from the distal end of the cylinder portion  213 . A through-hole  214   a  is formed in the supporting portion  214 . A holding pin  216  passing through the bulging body  215  is fitted into the through-hole  214   a.    
     The bulging body  215  is a component for pressing the clutch weight  230  described later toward the clutch outer  240 . The bulging body  215  is configured by forming a metal material (for example, carbon steel and iron based sintered material) into a cylindrical shape. More specifically, as shown in  FIGS. 4A to 4D , in the bulging body  215 , a through-hole  215   a  thereof through which the holding pin  216  passes is formed in a central portion of a columnar body. In this case, the through-hole  215   a  is formed to have an inner diameter rotatable and slidable with respect to the holding pin  216 , that is, a dimensional tolerance which is a so-called clearance fit with respect to the holding pin  216 . 
     The holding pin  216  is a rod-shaped member for mounting the bulging body  215  on the supporting portion  214  in a rotatable state. The holding pin  216  is formed in such a shape that a distal end portion of a metal rod body thereof projects like a flange. In this case, the holding pin  216  is formed to have an outer diameter rotatable and slidable with respect to the through-hole  214   a , that is, a dimension tolerance which is the so-called clearance fit with respect to the through-hole  214   a . In  FIG. 3B , illustration of the bulging body  215  and the holding pin  216  is omitted. 
     In the first drive plate  210 , the cylinder portion  213  is fitted into a through-hole  222  of the second drive plate  220  illustration of The second drive plate  220  is a component which is rotationally driven together with the first drive plate  210  while allowing relative rotational displacement. The second drive plate  220  is configured by forming a metal material into a flat ring shape. More specifically, in the second drive plate  220 , the through-hole  222  into which the cylinder portion  213  of the first drive plate  210  is slidably fitted is formed in a central portion of a plate-like ring plate  221 . Further, three fulcrum pins  223  are provided in an upright state at equal intervals in a circumferential direction on the ring plate  221 . The fulcrum pin  223  is a component for rotatably supporting the clutch weight  230 , and is made of a metal rod. Further, a bent portion  224  bent at a right angle to a plate surface of the ring plate  221  is formed in each of the inner circumferential portion and an outer circumferential portion of the second drive plate  220 . 
     The clutch weight  230  is a component for transmitting or cutting off the rotational driving force from the engine to the drive shaft  133  by contacting or separating from the clutch outer  240  via a clutch shoe  233  according to the rotational speed of the second drive plate  220 . The clutch weight  230  is configured by forming a metal material (for example, zinc material) in a curved shape extending in a circumferential direction of the second drive plate  220 . One end of the clutch weight  230  is rotatably supported by the fulcrum pin  223 . In this state, the clutch weights  230  are pulled such that the mutually adjacent clutch weights  230  in the three clutch weights  230  are connected to each other by a connecting spring  231  and are rotated inward of the second drive plate  220 . In  FIG. 2 , for easy understanding of configuration of the clutch weights  230 , a part of the two clutch weights  230  out of the three clutch weights  230  is shown broken in different thickness direction faces. 
     Each driven portion  232  recessed concavely is formed in a portion facing the bulging body  215  of the first drive plate  210  on an inner circumferential surface of each clutch weight  230 , more specifically, on a back surface of a lower surface (back side surface in the drawing) of the connecting spring  231 . The driven portion  232  is a portion for cooperating with the bulging body  215  to displace the clutch weight  230  toward the clutch outer  240 . The driven portion  232  is configured to have a pressing body  232   a  which is a flat inclined surface. More specifically, the pressing body  232   a  is formed to obliquely extend rearwardly and outwardly in a rotational drive direction of the first drive plate  210 . 
     In this driven portion  232 , thickness of the pressing body  232   a  is formed thicker than thickness of the bulging body  215 . Further, the pressing body  232   a  is in contact with an outer circumferential surface of the bulging body  215 . Further, in the driven portion  232 , the pressing body  232   a  is made of a material which is more easily worn than the bulging body  215 . In the present embodiment, the driven portion  232  is made of the same metal material (for example, zinc material) as the clutch weight  230 . Further, in the driven portion  232 , a portion which is deeper than the pressing body  232   a  is cut out in an arc shape. Thus, a space for rotation of the bulging body  215  is secured. 
     On the other hand, the clutch shoes  233  are respectively provided on outer circumferential surfaces of the clutch weights  230 . The clutch shoe  233  is a component for increasing frictional force against an inner circumferential surface of the clutch outer  240 , and is configured by forming friction material in a plate shape extending in an arc shape. 
     The clutch outer  240  is a component which is rotationally driven together with the drive shaft  133 . The clutch outer  240  is configured by forming a metal material into a cup shape covering the outer circumferential surface of the clutch weight  230  from the first drive plate  210 . That is, the clutch outer  240  is configured to have a cylindrical surface  241  which frictionally contacts the clutch shoe  233  of the clutch weight  230 , which is displaced toward an outer circumference of the second drive plate  220 . 
     (Operation of Centrifugal Clutch  200 ) 
     Next, an operation of the centrifugal clutch  200  configured as described above will be described. The centrifugal clutch  200  constitutes a part of the power transmission mechanism  100  disposed between the engine and the rear wheel as the driving wheel in the motorcycle vehicle (for example, a scooter) and functions as the part of the power transmission mechanism  100 . First, as shown in  FIG. 5 , when the engine is in an idling state, the centrifugal clutch  200  cuts off transmission of the driving force between the engine and the drive shaft  133 . Specifically, in the centrifugal clutch  200 , the first drive plate  210  and the second drive plate  220  pressed to the first drive plate  210  via the bulging body  215  and the pressing body  232   a  are rotationally driven together due to the rotational driving force of the engine transmitted via the transmission  101 , and further the clutch weight  230  is rotationally driven. In  FIG. 5 , the connecting spring  231  is omitted in order to clarify a contact state between the bulging body  215  and the driven portion  232 . 
     However, in this case, in the centrifugal clutch  200 , since a centrifugal force acting on the clutch weight  230  is smaller than elastic force (tensile force) of the connecting spring  231 , the clutch shoe  233  does not contact the cylindrical surface  241  of the clutch outer  240 , so that the rotational driving force of the engine is not transmitted to the drive shaft  133 . In this case, the bulging body  215  and the driven portion  232  maintain a state where the pressing body  232   a  is pressed to contact a roller surface of the bulging body  215  by the elastic force (tensile force) of the connecting spring  231 . 
     On the other hand, the centrifugal clutch  200  transmits the rotational driving force of the engine to the drive shaft  133  in accordance with increase in the rotational speed of the engine by a driver&#39;s accelerator operation in the motorcycle vehicle. Specifically, as shown in  FIG. 6 , in the centrifugal clutch  200 , as the rotational speed of the engine increases, the centrifugal force acting on the clutch weight  230  is greater than the elastic force (tensile force) of the connecting spring  231 . Thus, the clutch weight  230  rotates around the fulcrum pin  223  and is displaced radially outwardly. That is, in the centrifugal clutch  200 , as the rotational speed of the engine increases, the clutch weight  230  is rotationally displaced toward the cylindrical surface  241  of the clutch outer  240  while resisting the elastic force (tensile force) of the connecting spring  231 . As a result, the clutch shoe  233  contacts the cylindrical surface  241 . In  FIG. 6 , the connecting spring  231  is omitted in order to clarify the contact state between the bulging body  215  and the driven portion  232 . Further, in  FIGS. 2 and 6 , the rotational drive direction of the first drive plate  210 , the second drive plate  220 , the clutch outer  240 , and the bulging body  215  in the centrifugal clutch  200  is indicated by a dashed arrow. 
     When the clutch shoe  233  contacts the cylindrical surface  241 , the clutch weight  230  receives a reaction force in a direction opposite to the rotational drive direction via the clutch shoe  233 . Therefore, the second drive plate  220  is relatively displaced in a direction opposite to the rotational drive direction of the first drive plate  210  by the reaction force received by the clutch weight  230 . Thus, the clutch weight  230  supported by the second drive plate  220  is relatively displaced in the direction opposite to the rotational drive direction of the first drive plate  210 . Therefore, the pressing body  232   a  of the driven portion  232  is pressed against the bulging body  215 . 
     In this case, since the bulging body  215  is rotatably supported with respect to the supporting portion  214  of the first drive plate  210  and the holding pin  216 , it rotates in a counterclockwise direction as shown by pressing by the pressing body  232   a . Thus, in the clutch weight  230 , the pressing body  232   a  is pressed toward the clutch outer  240  on a radially outer side according to rotational displacement of the bulging body  215 . Further, the clutch shoe  233  is pressed against the cylindrical surface  241 . As a result, after the clutch shoe  233  contacts the cylindrical surface  241  of the clutch outer  240 , the clutch shoe  233  is pressed against the cylindrical surface  241  in an extremely short time (in other words, instantaneously). Thus, the centrifugal clutch  200  enters a connected state where the rotational driving force of the engine is completely transmitted to the drive shaft  133 . 
     In the centrifugal clutch  200  in the connected state, since a state where the pressing body  232   a  of the driven portion  232  is pressed against the bulging body  215  is maintained, the first drive plate  210  and the second drive plate  220  are rotationally driven together. Further, since a state where the clutch shoe  233  is pressed against the cylindrical surface  241  of the clutch outer  240  is maintained, the second drive plate  220  and the clutch outer  240  are rotationally driven together. Thus, the motorcycle vehicle can run since the rear wheel is rotationally driven by the rotational driving force of the engine. In this case, although the second drive plate  220  is formed in a ring shape, the second drive plate  220  has high rigidity due to the bent portions  224  formed on the inner circumferential portion and the outer circumferential portion, so that the rotational driving force can be accurately transmitted. 
     On the other hand, when the rotational speed of the engine decreases, the centrifugal clutch  200  cuts off transmission of the rotational driving force of the engine to the drive shaft  133 . Specifically, in centrifugal clutch  200 , the centrifugal force acting on the clutch weight  230  is smaller than the elastic force (tensile force) of connecting spring  231  as the engine speed decreases. Thus, the clutch weight  230  is rotationally displaced radially inwardly around the fulcrum pin  223 . 
     In this case, as shown in  FIG. 7 , the bulging body  215  against which the pressing body  232   a  is pressed is rotationally displaced clockwise in the drawing due to the elastic force (tensile force) of the connecting spring  231 . Therefore, the second drive plate  220  is rotationally displaced forward in a rotation direction of the first drive plate  210  relatively with respect to the first drive plate  210 . Thus, the second drive plate  220  returns to an original position (a position in the idling state) with respect to the first drive plate  210 . That is, the centrifugal clutch  200  enters a disconnected state, where the clutch shoes  233  are not in contact with the clutch outer  240  and the rotational driving force is not transmitted. In  FIG. 7 , the connecting spring  231  is omitted in order to clarify the contact state between the bulging body  215  and the driven portion  232 . Further, in  FIG. 7 , the rotational drive direction of the first drive plate  210 , the second drive plate  220 , the clutch outer  240  and the bulging body  215  in the centrifugal clutch  200  is indicated by a dashed arrow. 
     Even when thickness of the clutch shoe  233  decreases due to wear, the centrifugal clutch  200  can enter the connected state by quickly pressing the clutch shoe  233  against the cylindrical surface  241  of the clutch outer  240 . That is, as shown in  FIGS. 8 and 9 , in the centrifugal clutch  200 , the bulging body  215  is rotatably mounted on the supporting portion  214  and the holding pin  216 . Therefore, even when the clutch shoe  233  is worn, an amount of rotational displacement of the bulging body  215  increases by an amount corresponding to this wear amount. Therefore, pressing force of the clutch shoe  233  against the cylindrical surface  241  of the clutch outer  240  is maintained. In  FIGS. 8 and 9 , the connecting spring  231  is omitted in order to clarify the contact state between the bulging body  215  and the driven portion  232 . Further, in  FIGS. 8 and 9 , the rotational drive direction of the first drive plate  210 , the second drive plate  220 , the clutch outer  240 , and the bulging body  215  in the centrifugal clutch  200  is indicated by a dashed arrow. 
     As can be understood from the above description of the operation, according to the above embodiment, in the centrifugal clutch  200 , after the first drive plate  210  starts rotation drive, the clutch shoes  233  contact the clutch outer  240 . Thus, when the second drive plate  220  is displaced rearward in the rotational drive direction with respect to the first drive plate  210 , the driven portion  232  rides on the bulging body  215 . Thus, the clutch shoes  233  rapidly press the clutch outer  240 . That is, the time until the clutch shoes  233  contact the clutch outer  240  is secured from when the first drive plate  210  starts rotation drive until when the centrifugal clutch  200  enters the connected state. Further, when the clutch shoes  233  contact the clutch outer  240 , the centrifugal clutch  200  rapidly enters the connected state. As a result, the centrifugal clutch  200  can quickly enter the connected state while preventing sudden startup on the driven side such as the driving wheel. 
     Further, embodiments of the present invention are not limited to the above embodiment. The above embodiment can be variously modified without departing from an object of the present invention. In each of the following modifications, the same reference numerals are denoted to the same constituent parts as in the above embodiment, and the description thereof will be omitted. 
     For example, in the above embodiment, the bulging body  215  is rotatably mounted on the supporting portion  214  of the first drive plate  210 . However, the bulging body  215  may be fixedly mounted on the supporting portion  214  of the first drive plate  210 , or integrally formed of the same material with the first drive plate  210 . When the bulging body  215  is integrally formed of the same material with the first drive plate  210 , the bulging body  215  is preferably formed of a curved surface including a circular arc surface of one, two or more curvatures in a range where the pressing body  232   a  slides. Further, the bulging body  215  and the driven portion  232  respectively provided in the first drive plate  210  and the clutch weight  230  may be provided at least one each, that is, at least one pair. 
     Further, in the above embodiment, the bulging body  215  is made of a roller. However, the bulging body  215  may be formed to bulge outwardly from an outer circumferential portion of the first drive plate  210 , and may be formed in a shape such that the clutch weight  230  is pressed out outwardly via the pressing body  232   a . That is, at least one of the bulging body  215  and the pressing body  232   a  may be formed to extend rearward in the rotational drive direction of the first drive plate  210  and outwardly to the first drive plate  210 . In this case, in the bulging body  215  and the pressing body  232   a , a portion extending rearward in the rotational drive direction of the first drive plate  210  and outwardly to the first drive plate  210  may be formed in the whole or a part of the bulging body  215  and the pressing body  232   a . Therefore, for example, as shown in  FIG. 10 , the bulging body  215  can be formed of a plate-like metal material (for example, carbon steel and iron based sintered material). 
     More specifically, in the bulging body  215 , as shown in  FIGS. 11A to 11D and 12 , a through-hole  217   b  through which the holding pin  216  passes through is formed in a central portion of a base portion  217   a  formed in a plate shape. Further, a bulging inclined surface  217   c  and a circular arc surface  217   d  are formed in a portion, which bulges in a direction perpendicular to the outer circumferential portion and is increased in thickness, of an outer circumferential portion of the base portion  217   a . In this case, the through-hole  217   b  is formed to have an inner diameter slidable with respect to the holding pin  216 , that is, a dimensional tolerance which is a so-called clearance fit with respect to the holding pin  216 . 
     The bulging inclined surface  217   c  is a flat surface which allows the pressing body  232   a  of the clutch weight  230  to slide in a face contact state. The bulging inclined surface  217   c  is formed to obliquely extend rearward in the rotational drive direction of the first drive plate  210  and outwardly to the first drive plate  210 . The circular arc surface  217   d  is formed to extend in a circular arc shape from a rear end of the bulging inclined surface  217   c . The bulging inclined surface  217   c  and the circular arc surface  217   d  bulge beyond a plate thickness of the base portion  217   a  and are formed to be thick. This is for easily and accurately assembling the bulging body  215  to the supporting portion  214 . Therefore, the bulging inclined surface  217   c  and the circular arc surface  217   d  can be formed to have the same thickness as the plate thickness of the base portion  217   a.    
     The holding pin  216  passes through the bulging body  215  and the supporting portion  214 . At this time, the bulging inclined surface  217   c  of the bulging body  215  is disposed on the supporting portion  214  in a direction of obliquely extending rearward in the rotational drive direction of the first drive plate  210  and outwardly to the first drive plate  210 . In this case, the holding pin  216  may be inserted into the bulging body  215  from below the drawing in the same manner as in the above embodiment, or may be inserted from above the bulging body  215  in the drawing. 
     As shown in  FIG. 13 , in the centrifugal clutch  200  configured as described above, as the rotational speed of the engine increases, the clutch weight  230  is rotationally displaced toward the cylindrical surface  241  of the clutch outer  240  while resisting the elastic force (tensile force) of the connecting spring  231 . As a result, the clutch shoe  233  contacts the cylindrical surface  241 . In  FIGS. 12 and 13 , the connecting spring  231  is omitted in order to clarify the contact state between the bulging body  215  and the driven portion  232 . In  FIG. 13 , the rotational drive direction of the first drive plate  210 , the second drive plate  220 , the clutch outer  240  and the bulging body  215  in the centrifugal clutch  200  is indicated by a dashed arrow. 
     When the clutch shoe  233  contacts the cylindrical surface  241 , the clutch weight  230  receives a reaction force in a direction opposite to the rotational drive direction via the clutch shoe  233 . Therefore, the second drive plate  220  is relatively displaced in the direction opposite to the rotational drive direction of the first drive plate  210  by the reaction force received by the clutch weight  230 . Thus, the clutch weight  230  supported by the second drive plate  220  is relatively displaced in the direction opposite to the rotational drive direction of the first drive plate  210 . Therefore, the pressing body  232   a  of the driven portion  232 , which is in surface contact with the bulging inclined surface  217   c  of the bulging body  215 , is displaced along the bulging inclined surface  217   c.    
     In this case, the bulging inclined surface  217   c  of the bulging body  215  is formed to obliquely extend rearward in the rotational drive direction of the first drive plate  210  and outwardly to the first drive plate  210 . Further, the pressing body  232   a  of the driven portion  232  is formed on the same plane as the bulging inclined surface  217   c  and is in surface contact with the bulging inclined surface  217   c . Thus, as the pressing body  232   a  slides on the bulging inclined surface  217   c , the clutch weight  230  is pressed toward the clutch outer  240  on the radially outer side, and the clutch shoe  233  is pressed against the cylindrical surface  241 . As a result, after the clutch shoe  233  contacts the cylindrical surface  241  of the clutch outer  240 , the clutch shoe  233  is pressed against the cylindrical surface  241  in an extremely short time (in other words, instantaneously). Thus, the centrifugal clutch  200  enters the connected state where the rotational driving force of the engine is completely transmitted to the drive shaft  133 . 
     In the centrifugal clutch  200  in the connected state, since the state where the pressing body  232   a  of the driven portion  232  is pressed against the bulging inclined surface  217   c  of the bulging body  215  is maintained, the first drive plate  210  and the second drive plate  220  are rotationally driven together. Further, since the state where the clutch shoe  233  is pressed against the cylindrical surface  241  of the clutch outer  240  is maintained, the second drive plate  220  and the clutch outer  240  are rotationally driven together. Thus, the motorcycle vehicle can run since the rear wheel is rotationally driven by the rotational driving force of the engine. 
     On the other hand, when the rotational speed of the engine decreases, the centrifugal clutch  200  cuts off transmission of the rotational driving force of the engine to the drive shaft  133 . Specifically, in centrifugal clutch  200 , the centrifugal force acting on the clutch weight  230  is smaller than the elastic force (tensile force) of connecting spring  231  as the engine speed decreases. Thus, the clutch weight  230  is rotationally displaced radially inwardly around the fulcrum pin  223 . 
     In this case, the pressing body  232   a  of the driven portion  232  of the clutch weight  230  slides on the bulging inclined surface  217   c  of the bulging body  215  by the elastic force (tensile force) of the connecting spring  231 . Therefore, the second drive plate  220  is rotationally displaced forward in a rotation direction of the first drive plate  210  relatively with respect to the first drive plate  210 . Thus, the second drive plate  220  returns to the original position (position in the idling state) with respect to the first drive plate  210 . That is, the centrifugal clutch  200  enters the disconnected state, where the clutch shoes  233  are not in contact with the clutch outer  240  and the rotational driving force is not transmitted. 
     Further, in the above embodiment, the bulging body  215  is formed in a roller shape, and the pressing body  232   a  is formed in a planar shape. However, it is also possible to form the bulging body  215  into a flat shape, and to form the pressing body  232   a  in a rotatable roller shape or a curved surface shape fixedly provided in a non-rotatable manner. 
     Further, in the above embodiment, the second drive plate  220  is formed with the bent portion  224  bent at a right angle in each of the inner circumferential portion and the outer circumferential portion thereof. However, since the bent portion  224  is formed to increase the rigidity of the second drive plate  220 , when the rigidity is secured by the material, thickness, or the like of the second drive plate  220 , the bent portion  224  is not absolutely necessary and may be omitted. Further, the bent portion  224  may be provided on at least one of the inner circumferential portion and the outer circumferential portion of the second drive plate  220 . 
     Further, in the centrifugal clutch  200  of the above embodiment, the pressing body  232   a  of the driven portion  232  is made of a material more easily worn than the bulging body  215 , specifically, a zinc material. Thus, in the centrifugal clutch  200 , since the pressing body  232   a  is made of a material more easily worn than the bulging body  215 , the pressing body  232   a  is worn more than the bulging body  215 . In this case, since the pressing body  232   a  is formed on the clutch weight  230  including the clutch shoe  233 , it is renewed by replacing the clutch weight  230  due to wear of the clutch shoe  233 . That is, in the centrifugal clutch  200 , by exchanging the clutch weigh  230  due to wear of the clutch shoe  233 , the pressing body  232   a  can also be replaced with a new one at the same time, so that maintenance burden can be reduced. 
     DESCRIPTION OF REFERENCE SIGNS 
     
         
           100 : Power transmission mechanism,  101 : Transmission, 
           110 : Drive pulley,  111 : Crankshaft,  112 : Fixed drive plate,  112   a : Radiating fin,  113 : Movable drive plate,  114 : Sleeve bearing,  115 : Roller weight,  116 : Ramp plate, 
           120 : V-belt, 
           130 : Driven pulley,  131 : Fixed driven plate,  132 : Driven sleeve,  133 : Drive shaft,  134 : Movable driven plate,  135 : Torque spring, 
           200 : Centrifugal clutch, 
           210 : First drive plate,  211 : Bottom portion,  212 : Through-hole,  213 : Cylinder portion, 
           214 : Supporting portion,  214   a : Through-hole,  215 : Bulging body,  215   a : Through-hole, 
           216 : Holding pin,  217   a : Base portion,  217   b : Through-hole,  217   c : Bulging inclined surface,  217   d : Circular arc surface, 
           220 : Second drive plate,  221 : Ring plate,  222 : Through-hole,  223 : Fulcrum pin,  224 : Bent portion, 
           230 : Clutch weight,  231 : Connecting spring,  232 : Driven portion,  232   a : Pressing body, 
           233 : Clutch shoe, 
           240 : Clutch outer,  241 : Cylindrical surface