Abstract:
A more durable rotation transmitting member for a coupling directly connected to shafts has: drive-side rotation transmitting cushion to which an input shaft is directly connected; and a hub to which an output shaft is directly connected. Two parallel flat surfaces are formed on the outer periphery of the front end of the input shaft, and a shaft hole receives the front end of the input shaft and is formed in the rotation transmitting cushion. Surfaces of the inner wall of the shaft hole protrude toward the axis (O) to face input shaft front end flat surfaces. Each protruding surface comprises two flat sloped surfaces extending in the direction of the axis (O). When the input shaft starts to rotate, each front end flat of the front end comes into surface contact with one of the protruding surface sloped surfaces which faces the flat surface.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a coupling that transmits torque applied to one rotating shaft to the other rotating shaft, and particularly to structure of a rotation transmitting member used in a direct-shaft-connection type coupling that is directly connected to rotating shafts. 
       BACKGROUND ART 
       [0002]    There is known a coupling that transmits torque applied to an input shaft to an output shaft. For example, a coupling for an electric power steering is placed between an input shaft connected to an electric motor and an output shaft connected to a steering wheel, to transmit output torque of the electric motor to the steering wheel. 
         [0003]    The below-mentioned Patent Literature 1 discloses a coupling comprising: a first hub (connection base), which is fitted to an end of an input shaft connected to an electric motor or the like; a second hub (connection base), which is fitted to an end of an output shaft connected to a steering wheel or the like; and a spacer (rotation transmitter), which is placed between the first and second hubs. The first and second hubs are connected with each other via the spacer, so that torque is transmitted from the input shaft to the output shaft. Here, as the spacer, is used an insert-molded part obtained by uniting a first rotation transmitting member of metal or hard resin, and a second rotation transmitting member of rubber elastic body to cover the first rotation transmitting member. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Unexamined Patent Application Laid-Open No. 2010-164162 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    To respond to request for miniaturization, necessity for placement in a limited space, and the like, structure is proposed, in which the first hub is omitted from the coupling described in the Patent Literature 1. In a rotation transmitting member used in a coupling of such structure, a shaft hole is formed such that the inner wall of the shaft hole has two flat surfaces parallel to each other. An input shaft has at its end portion two flat surfaces to face the respective flat surfaces of the shaft hole, and this input shaft is inserted into the shaft hole. By this connection between the input shaft having the flat surfaces and the rotation transmitting member provided with the shaft hole having the flat surfaces, torque can be transmitted from the input shaft to the output shaft without using the first hub. Hard resin such as polyacetal resin, polyamide resin or the like, which has high strength among resin materials, is used as a material of the rotation transmitting member used in the direct-shaft-connection type coupling having the structure in which load from the input shaft is directly received by the shaft hole of the rotation transmitting member. The fit between the shaft hole of the rotation transmitting member and the input shaft is a clearance fit in order to prevent transmission of minute vibration and the like of the input shaft to the output shaft. In general, as a fit between a shaft member (such as a pin) and a hole, owing to their dimensional relation, there are a tight fit in which interference arises between the shaft member and the hole, a clearance fit in which clearance arises between the shaft member and the hole, and a transition fit that falls between them. 
         [0006]    However, as shown in  FIG. 6 , in the case where a clearance fit is employed as the fit between flat surfaces  71  of an input shaft  7  and a width-across-flat shaft hole of a rotation transmitting member  8  of a direct-shaft-connection type coupling, a clearance d is formed between the width-across-flat shaft hole  80  of the rotation transmitting member  8  and the flat surfaces  71  of the input shaft  7 , and this causes the following phenomenon. That is to say, when the input shaft  7  rotates about the axis O, mainly the edge portion  73  of each flat surface  71  of the input shaft  7  contacts partially with an inner wall  81  of the width-across-flat shaft hole  8 . When the inner wall  81  of the width-across-flat shaft hole  80  of the rotation transmitting member  8  wears partially because of the above phenomenon, the shaft hole  80  of the rotation transmitting member  8  reduces an area for receiving the load transmitted from the input shaft  7 . This increases the stress applied to the shaft hole  80  of the rotation transmitting member  8 . By this, the durability of the rotation transmitting member  8  and the durability of the direct-shaft-connection type coupling using the rotation transmitting member  8  may be reduced. Further, owing to creep deformation or the like of the shaft hole  80  of the rotation transmitting member  8 , the backlash between the shaft hole  80  and the input shaft  7  becomes larger, and torque transmission efficiency becomes lower. Further, the use of this coupling in a steering device or the like of an automobile makes a driver feel discomfort about handling performance. 
         [0007]    The present invention has been made in view of the above circumstances. An object of the invention is to realize high durability of a rotation transmitting member for a direct-shaft-connection type coupling and to realize high durability of a shaft connecting mechanism using the direct-shaft-connection type coupling. 
       Solution to Problem 
       [0008]    To solve the above problems, a first aspect of the present invention provides a rotation transmitting member for a direct-shaft-connection type coupling, wherein: in an inner wall are of a shaft hole, which is opposed to a flat surface formed on an outer periphery surface of an end portion of a shaft to be inserted into the shaft hole of the rotation transmitting member, there are formed two inclined surfaces inclined inward in a radial direction of the shaft hole from both end portions of the inner wall area to a center of the inner wall area, along a trust direction of the shaft inserted into the shaft hole, so that these inclined surfaces come in surface contact with the flat surface formed on the outer periphery surface of the end portion of the shaft. 
         [0009]    For example, the present invention provides a rotation transmitting member, which is attached to an end of one shaft of an input shaft and an output shaft both rotating about an axis, and which engages with a hub attached to an end of the other shaft of the input shaft and the output shaft, wherein: 
         [0010]    the rotation transmitting member has a shaft hole into which the one shaft having at least one flat surface on an outer periphery surface of an end portion of the one shaft is inserted; and 
         [0011]    in an inner wall area of the shaft hole which is opposed to the flat surface, there are formed two inclined surfaces inclined inward in a radial direction of the shaft hole from both end portions of the inner wall area to a center of the inner wall area, along an axis of the one shaft. 
         [0012]    Further, a second aspect of the present invention provides a shaft connection mechanism using a direct-shaft-connection type coupling, wherein: a flat surface is formed on the inner wall of a shaft hole of a rotation transmitting member used for the direct-shaft-connection type coupling, and in an area of the outer periphery surface of an end portion of a shaft to be inserted into the shaft hole of the rotation transmitting member, the area opposed to the flat surface formed on the inner wall of the shaft hole of the rotation transmitting member, there are formed two inclined surfaces inclined outward in a radial direction of the shaft from both end portions of the area to a center of the area, along an axis of the shaft, so that these inclined surfaces come in surface contact with the flat surface formed on the inner wall of the shaft hole of the rotation transmitting member. 
         [0013]    For example, the present invention provides a shaft connection mechanism comprising: an input shaft and an output shaft both rotating about an axis; and a direct-shaft-connection type coupling for transmitting torque from the input shaft to the output shaft, wherein: 
         [0014]    the direct-shaft-connection type coupling comprises: 
         [0015]    a rotation transmitting member in which a shaft hole is formed for inserting an end portion of one shaft of the input shaft and the output shaft; and 
         [0016]    a hub which is attached to an end of the other shaft of the input shaft and the output shaft and engages with the rotation transmitting member; 
         [0017]    a flat surface is formed on an inner wall of the shaft hole; and 
         [0018]    in an area of an outer periphery surface of the end portion of the one shaft, the area opposed to the flat surface formed on the inner wall of the shaft hole, there are formed two inclined surfaces inclined outward in a radial direction of the one shaft from both end portions of the area to a center of the area, along an axis of the one shaft. 
       Advantageous Effects of Invention 
       [0019]    According to the first aspect of the present invention, the flat surface formed at the end portion of the input shaft or the output shaft each rotating about the axis is received in surface contact by one of the two inclined surface formed on the inner wall of the shaft hole of the rotation transmitting member. According to the second aspect of the present invention, one of the two inclined surfaces formed at the end portion of the input shaft or the output shaft each rotating about the axis is received in surface contact by the flat surface formed on the inner wall of the rotation transmitting member. Therefore, it is possible to prevent local wear of the inner wall of the shaft hole and aging degradation of the torque transmission characteristics. Thus, it is possible to realize high durability of the rotation transmitting member for the direct-shaft-connection type coupling and high durability of the shaft connection mechanism using the direct-shaft-connection type coupling. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1(A)  is a schematic view showing a configuration of a shaft connecting mechanism  5  which uses a direct-shaft-connection type coupling  1  according to one embodiment of the present invention, and  FIG. 1(B)  is a cross-section view of an end portion  22  of an input shaft  2  that is connected to a rotation transmitting cushion  10 ; 
           [0021]      FIGS. 2(A) and 2(B)  are external views showing the rotation transmitting cushion  10 ; 
           [0022]      FIGS. 3(A) and 3(B)  are a front view and a back view showing a first outer plate  1 ,  FIGS. 3(C) and 3(D)  are an A-A cross-section view and a B-B cross-section view of  FIG. 3(A) , and  FIG. 3(E)  is an enlarged view of the part C of  FIG. 3(B) ; 
           [0023]      FIGS. 4(A) and 4(B)  are a front view and a back view showing a second outer plate  12 , and  FIGS. 4(C) and 4(D)  are a D-D cross-section view and an E-E cross-section view of  FIG. 4(A) ; 
           [0024]      FIG. 5(A)  is a view for explaining effects of the shaft connecting mechanism  5 , and  FIG. 5(B)  is a view for explaining a variation  5 ′ of the shaft connecting mechanism  5 ; and 
           [0025]      FIG. 6  is a view for explaining problems of the shaft connecting mechanism using the conventional shaft connecting mechanism. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    In the following, one embodiment of the present invention will be described referring to the drawings. 
         [0027]      FIG. 1(A)  is a schematic view showing a configuration of a shaft connecting mechanism  5  which uses a direct-shaft-connection type coupling  1  according to the present embodiment, and  FIG. 1(B)  is a cross-section view of an end portion  22  of an input shaft  2  which is directly connected to a rotation transmitting cushion  10 . Further,  FIGS. 2(A) and 2(B)  are external views of the rotation transmitting cushion  10 . 
         [0028]    In the shaft connecting mechanism  5  shown in  FIG. 1(A) , the direct-shaft-connection type coupling  1  according to the present embodiment is placed between the input shaft  2  connected to a driving source (not shown) such as an electric motor and an output shaft  3  connected to a driven object (not shown) such as a steering wheel, and transmits output torque of the driving source to the driven object. In detail, this direct-shaft-connection type coupling  1  comprises: the rotation transmitting cushion  10  located at the driving side to which the input shaft  2  is directly connected; and a hub  4  located at the driven side to which the output shaft  3  is directly connected. The direct-shaft-connection type coupling  1  transmits rotation of the input shaft  2  to the output shaft  3  by engagement between the below-described arm portions  105  of the rotation transmitting cushion  10  and the below-described protruding portions  45  of the hub  4 . Here, as shown in  FIG. 1(B) , two flat surfaces  21  opposite to each other at a prescribed width-across-flat h are formed on the outer periphery of the end portion  22  of the input shaft  2 . 
         [0029]    The hub  4  has: a cylindrical portion  42 , in which an end portion of the output shaft  3  is connected on the side of one end surface  43 ; and four protruding portions  45 , which protrude along the axis O toward the input shaft  2  from the other end surface  44  of the cylindrical portion  42 . The four protruding portions  45  are arranged at almost regular angular intervals around the axis O so as to surround a center area  441  of the other end surface  44  of the cylindrical portion  42 . And the four protruding portions  45  grasp the rotation transmitting cushion  10 . Although the present embodiment takes the example of the hub  4  having the four protruding portions  45 , the number of the protruding portions  45  of the hub  4  can be changed suitably. 
         [0030]    On the other hand, the rotation transmitting cushion  10  has: a boss portion  101 , in which the end portion  22  of the input shaft  2  is connected on the side of one end surface  102 ; and four arm portions  105  protruding radially outward from the outer periphery  106  of the boss portion  101 . 
         [0031]    The boss portion  101  has the radius R2 smaller than the distance R1 between the inner periphery surface  46  of the protruding portions  45  of the hub  4  and the axis O. The boss portion  101  is received from the side of the other end surface  103  into the center area  441  of the other end surface  44  of the hub  4 , so as to be grasped by the protruding portions  45  of the hub  4 . Further, in the one end surface  102  of the boss portion  101 , is formed a bottomed shaft hole  104  into which the end portion  22  of the input shaft  2  is fitted. On the inner wall of the bottomed shaft hole  104 , are formed convex surfaces  118  which protrude toward the axis O in a radial direction and face the flat surfaces  21  of the end portion  22  of the input shaft  2 , as described below. 
         [0032]    The arm portions  105  whose number is the same as the protruding portions  45  of the hub  4  are formed on the outer periphery surface  106  of the boss portion  101  at almost regular angular intervals around the axis O. In the state that the boss portion  101  is received in the center area  441  on the side of the other end surface  44  of the hub  4 , these arm portions  105  are each received between adjacent protruding portions  45  of the hub  4 , so as to engage with the protruding portions  45  of the hub  4 . Due to this engagement between the arm portions  105  of the rotation transmitting cushion  10  and the protruding portions  45  of the hub  4 , the hub  45  rotates interlocking with rotation of the rotation transmitting cushion  10 . 
         [0033]    Here, the rotation transmitting cushion  10  has two-layer stacking structure in which two outer plates  11  and  12  formed of materials having different elastic coefficients are superposed in the direction of the axis O. In detail, the rotation transmitting cushion  10  comprises a first outer plate  11  located at the side of the input shaft  2  and a second outer plate  12  located at the side of the output shaft  3 . The first outer plate  11  is formed of hard resin such as polyacetal resin, polyamide resin or the like, and the second outer plate  12  is formed of an elastic member of such as gum, elastmeric resin or the like having a lower elastic coefficient than that of the material of the first outer plate  11 . These two types of outer plates  11  and  12  are joined in a snap-fit manner, so as to make up the rotation transmitting cushion  10  as a unit. 
         [0034]      FIGS. 3(A) and 3(B)  are a front view and a back view of the first outer plate  11 ,  FIGS. 3(C) and 3(D)  are an A-A cross-section view and a B-B cross-section view of  FIG. 3(A) , and  FIG. 3(E)  is an enlarged view of the part C of  FIG. 3(B) . 
         [0035]    Relative to the second outer plate  12 , the first outer plate  11  is positioned at the side of the input shaft  2 . As shown in  FIG. 3 , the first outer plate  11  has: a plate body  110  of a disk shape; and four arm portions  113 , which protrude in a radially-outward direction from the outer periphery  115  of the plate body  110 . When both outer plates  11  and  12  are superposed such that one surface  112  of the first outer plate  11  and one surface  122  of the second outer plate  12  are opposed each other, the plate body  110  is superposed in the direction of axis O on the below-described plate body  120  of the second outer plate  12 , so as to form the boss portion  101  of the rotation transmitting cushion  10 . Further, the four arm portions  113  are provided on the outer periphery surface  115  of the plate body  110  at almost regular angular intervals around the axis O, and are superposed in the direction of axis O on the corresponding below-described arm portions  125  of the second outer plate  12 , so as to form the arm portions  105  of the rotation transmitting cushion  10 . Although, in the present embodiment, the four arm portions  113  are provided on the outer periphery surface  115  of the plate body  110  at regular angular intervals, the number of arm portions  113  provided on the outer periphery surface  115  of the plate body  110  is determined by the number of arm portions  105  which is to be formed for the rotation transmitting cushion  10 . 
         [0036]    In the plate body  110 , is formed a through-hole  114  which passes through from the one end surface  112  to the other end surface  111  such that the axis O of the plate body  110  becomes the axis of the through-hole  114 . This through-hole  114  is connected with the below-described bottomed hole  124  formed in the second outer plate  12 , so as to form the bottomed shaft hole  104  of the rotation transmitting cushion  10 . 
         [0037]    The through-hole  114  is formed such that its dimensions realize a clearance fit for the end portion  22  of the input shaft  2 . Further, on the inner wall  116  of the through-hole  114 , two convex surfaces  118  which protrude toward the axis O are formed. The two convex surfaces  118  are opposed respectively to the two flat surfaces  21  forming the width-across-flat h of the end portion  22  of the input shaft  2  which is to be inserted into the through-hole  114 . Each of these convex surfaces  118  is made up of two inclined surfaces  118 A. The two inclined surfaces  118 A are inclined inward in a radial direction of the through-hole  114 , from both edge portions  150  of the inner wall  116  to the central portion  151  of the inner wall  116 , along the direction of axis O. That is to say, each convex surface  118  is made up of the two inclined surfaces  118 A along the direction of axis O, which form a prescribed angle, so that a ridge line  117  along the direction of axis O is formed. As shown in  FIG. 3(E) , in the present embodiment, when the first outer plate  11  and the second outer plate  12  are superposed, the inclination a of the inclined surfaces  118 A with respect to a flat surface  126  of the bottomed hole  124  of the second outer plate  12  is set to about one degree. Although, in the present embodiment, the boundary between the two adjacent inclined surfaces  118 A is the ridge line  117 , the boundary between the two adjacent inclined surfaces  118 A may be rounded. Or, the boundary may have a shape obtained by cutting the boundary portion including the ridge line  117  between both inclined surfaces  118 A, namely, a shape in which the two inclined surfaces  118 A are adjacent through a flat surface. 
         [0038]    At the end portions of two arm portions  113   a  opposed to each other through the through-hole  114 , are formed respective hook-like snap-fit portions  119  for connecting with the below-described engaging portions  129  of the second outer plate  12 . By providing the snap-fit portions  119  at such two positions, the direction of the through-hole  114  can be conformed with the direction of the below-described bottomed hole  124  of the second outer plate only by fitting the snap-fit portions  119  to the respective engaging portions  129  of the second outer plate  12 . Although, in the present embodiment, the snap-fit portions  119  are formed at the two arm portions  113   a  out of the four arm portions  113 , the number of arm portions  113  at which snap-fit portions are formed can be determined depending on required coupling strength between the first and second outer plates  11  and  12 . However, it is favorable to arrange the snap fit portions  119  so that the through-hole  114  is oriented in a prescribed direction when a plurality of snap fit portions  119  are positioned in a prescribed positional relation to a worker (i.e. so that the direction of the through-hole  114  can be identified by the positions of the snap fit portions). 
         [0039]      FIGS. 4(A) and 4(B)  are a front view and a back view of the second outer plate  12 , and  FIGS. 4(C) and 4(D)  are a D-D cross-section view and an E-E cross-section view of  FIG. 4(A) . 
         [0040]    Relative to the first outer plate  11 , the second outer plate  12  is located at the side of the output shaft  3 . As shown in  FIG. 4 , the second outer plate  12  has: a plate body  120  of a disk shape having the almost same diameter as that of the plate body  110  of the first outer plate  11 ; and arm portions  123 , which protrude in a radial direction from the outer periphery surface  125  of the plate body  120 , and the number of which is the same as the arm portions  113  of the first outer plate  11 . As described above, when both of the outer plates  11  and  12  are superposed such that the one surface  122  of the second outer plate  12  and the one surface  112  of the first outer plate  11  are opposed to each other, the plate body  120  is superposed in the direction of axis O on the plate body  110  of the first outer plate  11 , so as to form the boss portion  101  of the rotation transmitting cushion  10 . Further, the four arm portions  123  are provided at almost regular angular intervals around the axis O so as to correspond to the arm portions  113  of the first outer plate  11 , and are superposed in the direction of axis O on the corresponding arm portions  113  of the first outer plate  11 , so as to form the arm portions  105  of the rotation transmitting cushion  10 . 
         [0041]    In the plate body  120 , is formed the bottomed hole  124  which is open in the one surface  122 , and has, as its axis, the axis O of the plate body  120 . As described above, the bottomed hole  124  is connected with the through-hole  114  of the first outer plate  11 , so as to form the bottomed shaft hole  104  of the rotation transmitting cushion  10 . On the inner wall of the bottomed hole  124 , are formed two flat surfaces  126  which are opposed to each other at a distance less than or equal to the distance between the convex surfaces  118  of the through-hole  114  of the first outer plate  11 . For example, the width-across-flat H of the two flat surfaces  126  is narrower than the width-across-flat h of the end portion  22  of the input shaft  2 . Accordingly, the end portion  22  of the input shaft  2  is fitted into the bottomed hole  124  of the plate body  120  of the second outer plate  12  without a clearance between the flat surfaces  21  of the end portion  22  of the input shaft  2  and the flat surfaces  126  of the bottomed hole  124  of the plate body  120  of the second outer plate  12 , the flat surfaces  126  being opposed to the flat surfaces  21 . 
         [0042]    Among the four arm portions  123 , the arm portions  123   a , which correspond to the arm portions  113   a  having the snap-fit portions  119  in the first outer plate  11 , have the engaging portions  129  formed at the end portions of the arm portions  1231 , so as to engage with the respective snap-fit portions  119 . In each of the other arm portions  123   b , a buffer portion  128  protruding in the direction of axis O from the other surface  121  is formed so as to be abutted against the other end surface  44  of the cylindrical portion  42  of the hub  4 . When the end portion  22  of the input shaft  2  is fitted into the bottomed hole  124  of the second outer plate  12  via the through-hole  114  of the first outer plate  11 , the end portion  22  of the input shaft  2  pushes the bottom  127  of the bottomed hole  124  so that the second outer plate  12  bends about the two buffer portions  128  as fulcrums toward the hub  4 . Thus, by pushing the hub  4  and the rotation transmitting cushion  10  toward the output shaft  3  and the input shaft  2 , strong elastic force acts, so that backlash in the direction of axis O is suppressed and the output shaft  3  and the input shaft  2  can be surely connected by the direct-shaft-connection type coupling  1 . Further, since such bending allows relative movement of the output shaft  3  and the input shaft  2  in the directions of axis O, it is possible to absorb vibration and shock in direction of axis O. 
         [0043]    Here, the width t of the four arm portions  123  may be made slightly larger than the distance L between adjacent protruding portions  45  of the hub  4 . In that case, the rotation transmitting cushion  10  is pressed and fitted into the hub  4 . 
         [0044]    When the one surface  112  of the first outer plate  11  and the one surface  122  of the second outer plate  12  are opposed to each other and the snap-fit portions  119  of the arm portions  113   a  of the first outer plate  11  are joined to the engaging portions  129  of the corresponding arm portions  123   a  of the second outer plate  12 , then the plate body  110  of the first outer plate  11  and the plate body  120  of the second outer plate  12  are superposed in the direction of axis O, so as to form the boss portion  101  of the rotation transmitting cushion  10 . Further, the arm portions  113  of the first outer plate  11  and the corresponding arm portions  123  of the second outer plate  12  are respectively superposed in the direction of axis O, so as to form the arm portions  105  of the rotation transmitting cushion  10 . At that time, the through-hole  114  of the first outer plate  11  and the bottomed hole  122  of the second outer plate  12  are connected with each other, to form the bottomed shaft hole  104  of the rotation transmitting cushion  10 . 
         [0045]    As a result, this bottomed shaft hole  104  has different cross-section shapes and different elastic coefficients on the opening side (on the side of the input shaft  2 ) and on the bottom side (on the side of the hub  4 ). That is to say, on the inner wall on the bottom side (on the side of the hub  4 ), the two flat surfaces  126  opposed to each other are formed of the elastic member (the second outer plate  12 ), and the end portion  22  of the inserted input shaft  2  is elastically held by contact between the flat surfaces  126  and the two flat surfaces  21  of the end portion  22  of the input shaft  2 . On the other hand, on the inner wall on the opening side (on the side of the input shaft  2 ), the convex surfaces  118  protruding toward the axis O are formed of the hard resin (the first outer plate  11 ). Each of these convex surfaces  118  has the ridge line  117  along the axis O at the almost same distance from the axis O to the flat surfaces  126  of the bottom side (the side of the hub  4 ), and is made up of the two flat inclined surfaces  118 A along the axis O, which are symmetrical to each other about the ridge line  117 . 
         [0046]    In the rotation transmitting cushion  10  of the above-described configuration, when the input shaft  2  inserted in the bottomed shaft hole  104  rotates in accordance with output of the driving source, each of the two flat surfaces  21  of the end portion  22  of the input shaft  2  swings around the ridge line  117  of the convex surface  118  of the hard resin (the first outer plate  11 ), while deforming elastically the elastic member (the second outer plate  12 ) which makes the contacting flat surface  126 , so as to come in contact with the whole area of one of the two inclined surfaces  118 A making up the convex surface  118 . 
         [0047]    Thus, in the bottomed shaft hole  104 , the two flat surfaces  21  of the end portion  22  of the input shaft  2  are already in surface contact with the flat surfaces  126  of the elastic member (the second outer plate  12 ) before the two flat surfaces  21  come in contact with the inclined surfaces  118 A of the hard resin (the first outer plate  11 ). Therefore, the torque of the input shaft  2  is first transmitted to the second outer plate  12 , and then transmitted through the arm portions  123  of the second outer plate  12  to the hub  4  that grasps the rotation transmitting cushion  10 . Further, when the flat surfaces  21  having the width-across-flat h of the end portion  22  of the input shaft  2  elastically deform the elastic member (the second outer plate  12 ) which makes the flat surfaces  126 , and come in contact with the inclined surfaces  118 A of the hard resin (the first outer plate  11 ), then the torque of the input shaft  2  is mainly transmitted to the first outer plate  11 . And then the torque of the input shaft  2  is transmitted through the arm portions  113  of the first outer plate  11  to the hub  4  grasping the rotation transmitting cushion  10 . 
         [0048]    Thus, in the bottomed shaft hole  104 , the torque of the input shaft  2  is transmitted to the output shaft  3  through the second outer plate  12  until the flat surfaces  21  of the end portion  22  of the input shaft  2  come in contact with the inclined surfaces  118 A of the first outer plate  11 . After the flat surfaces  21  of the end portion  22  of the input shaft  2  come in contact with the inclined surfaces  118 A of the first outer plate  11 , the torque is transmitted to the output shaft  3  mainly through the first outer plate  11 . 
         [0049]    Hereinabove, an embodiment of the present invention has been described. 
         [0050]    In the present embodiment, on the inner wall  116  of the through-hole  114  of the first outer plate  11  formed of hard resin, there are formed the convex surfaces  108 , each of which is made up of the two inclined surfaces  118 A along the axis O forming a prescribed angle, so that the convex surfaces  108  are opposed to the respective flat surfaces  118 A of the end portion  22  of the input shaft  2 . Therefore, when the input shaft  2  rotates about the axis O, each flat surface  21  of the end portion  22  of the input shaft  2  can be received by the whole surface of one of the two inclined surfaces  118 A which make up the convex surface  108  opposed to the flat surface  21 . For example, in  FIG. 5(A) , when the input shaft  2  rotates in the direction R, the force applied from each flat surface  21  can be received by the whole area of one inclined surface  118   a ,  118   b  of the convex surface  108  opposed to the flat surface  21 . And when the input shaft  2  rotates in the direction L, the force applied from each flat surface  21  can be received by the whole area of the other inclined surface  118   d ,  118   d  of the convex surface  108  opposed to the flat surface  21 . 
         [0051]    Thus, the present embodiment prevents partial contact of the edge portions of the flat surfaces  21  of the end portion  22  of the input shaft  2  with the inner wall  116  of the through-hole  114  of the first outer plate  11 . As a result, it is possible to prevent local wear of the first outer plate  11 , which becomes cause of aging degradation of the torque transmission characteristics. This improves the durability of the direct-shaft-connection type coupling  1 . 
         [0052]    Further, in the present embodiment, the rotation transmitting cushion  10  has the two-layer stacking structure of the first outer plate  1  formed of hard resin and the second outer plate  12  formed of the elastic member, and the width-across-flat H of the bottomed hole  124  of the second outer plate  12  is made narrower than the width-across-flat h of the end portion  22  of the input shaft  2 . Thus, even before the two flat surfaces  21  formed at the end portion  22  of the input shaft  2  come in contact with the inclined surfaces  118 A of the inner wall  116  of the through-hole  114  of the first outer plate  11 , the torque of the input shaft  2  can be transmitted to the output shaft  3  through the second outer plate  12 . This improves the initial motion characteristics of torque transmission. Further, elastic deformation of the second outer plate  12  formed of the elastic member absorbs micro vibration of the input shaft  2  in the range where the flat surfaces  21  are not in contact with any inclined surface  118 A of the facing convex surfaces  108 . Therefore, it is possible to prevent transmission of such vibration to the output shaft  3 . 
         [0053]    Further, after the two flat surfaces  21  formed at the end portion  22  of the input shaft  2  come in contact with some of the inclined surfaces  118 A making up the convex surfaces  118  of the inner wall  116  of the through-hole  114  of the first outer plate  11 , the torque of the input shaft  2  is transmitted to the output shaft  3  through the first outer plate  11 . Therefore, elastic deformation of the second outer plate  12  accompanying rotation of the input shaft  2  can be suppressed less than a prescribed level. This prevents deterioration of the second outer plate  12  formed of the elastic member, and aging degradation of the torque transmission characteristics can be further prevented. Thus, it is possible to provide the direct-shaft-connection type coupling  1  having more superior durability. 
         [0054]    Further, in the present embodiment, the bottomed hole  124  closed on the side of the hub  4  is formed in the second outer plate  12  formed of the elastic member. Thus, the bottom  127  of the bottomed hole  124  can prevent generation of abnormal noise caused by impact of the metal materials i.e. the end portion  22  of the input shaft  2  and the end surface  44  of the cylindrical portion  42  of the hub  4 . 
         [0055]    Further, in the present embodiment, among the plurality of arm portions  123  of the second outer plate  12 , the arm portions  123   b  have the buffer portions  128  protruding relative to the other surface  121  of the plate body  120  toward the hub  4 . In the arm portions  123   b , the engaging portions  129  for engaging with the snap-fit portions  119  of the first outer plate  11  are not formed. Accordingly, when the end portion  22  of the input shaft  2  is pressed into the bottomed shaft hole  104  of the rotation transmitting cushion  10 , the end portion  22  of the input shaft  2  pushes the bottom  12  of the bottomed hole  124  of the second outer plate  12  toward the hub  4 . Therefore, the second outer plate  12  bends about the buffer portions  128  as fulcrums toward the hub  4 , to generate larger reaction forces that press the rotation transmitting cushion  10  toward the input shaft  2  and the hub  4  toward the output shaft  3 . As a result, the direct-shaft-connection type coupling  1  can surely connect the input shaft  2  and the output shaft  3 . 
         [0056]    Although, in the present embodiment, the two flat surfaces  21  are formed at the end portion  22  of the input shaft  2  to have the width-across-flat h, this is not necessary. For example, it is possible to provide the end portion  22  of the input shaft  2  with only one flat surface  21 , and to form, on the inner wall of the shaft hole  104  of the rotation transmitting cushion  10 , one convex surface  118  opposed to the flat surface  21  of the end portion  22  of the input shaft  2 . Or, it is possible to provide the end portion  22  of the input shaft  2  with two flat surfaces forming a prescribed angle, and to form, on the inner wall of the shaft hole of the rotation transmitting cushion  10 , convex surfaces opposed to these flat surfaces respectively. 
         [0057]    Or, it is possible to form flat surfaces in place of the convex surfaces  118  on the inner wall of the shaft hole  104  of the rotation transmitting cushion  10 , and to form, at the end portion  22  of the input shaft  2 , convex surfaces which protrude outward in radial directions of the input shaft  2  and are opposed to the flat surfaces formed on the inner wall of the shaft hole  104 , in place of the flat surfaces  21 . For example, as in the shaft connecting mechanism  5 ′ shown in  FIG. 5(B) , two flat surfaces  118 ′ having a width-across-flat h′ are formed on the inner wall of a shaft hole  104 ′ of a rotation transmitting cushion  10 ′. Further, at the end portion (the portion corresponding to the end portion  22  of the input shaft  2  shown in  FIG. 1(A) ) of an input shaft  2 ′, two inclined surfaces  212   a  and  212   b  inclined outward in a radial direction of an input shaft  22 ′ from both edge portions  210  to the central portion  211  along the axis O of the input shaft  2 ′ are formed in outer peripheral surfaces opposed respectively to the two flat surfaces  118 ′ of the shaft hole  104 ′. Then, one of the two inclined surface  212   a  and  212   b  is made to come in surface contact with the flat surface  118 ′ concerned out of the two flat surfaces  118 ′ formed on the inner wall of the shaft hole  104 ′. At that time, to prevent the ridge line  213  formed at the boundary of the two inclined surfaces  212   a  and  212   b  from coming in contact with the flat surface  118 ′, it is favorable for example that the boundary of the two adjacent inclined surfaces  212   a  and  212   b  is rounded, or the boundary portion of the two inclined surfaces  212   a  and  212   b  including the ridge line  213  is cut out or has a shape in which the two inclined surfaces  212   a  and  212   b  are adjacent through a flat surface  214 . 
         [0058]    Further, in the present embodiment, the two flat surfaces  21  are formed so as to keep the width-across-flat h at the end portion  22  of the input shaft  22 , the end portion  22  of the input shaft  2  is directly connected to the rotation transmitting cushion  10 , and the hub  4  is attached to the end of the output shaft  3 . On the contrary, however, it is possible that the hub  4  is attached to the end portion  22  of the input shaft  2 , two flat surfaces keeping the prescribed width-across-flat h are formed at the end portion of the output shaft  3 , and the end portion of the output shaft  3  is directly connected to the rotation transmitting cushion  10 . In that case, the first outer plate  11  is placed on the side of the output shaft  3 , and the second outer plate  12  is placed on the side of the input shaft  2 . 
         [0059]    Further, in the present embodiment, the first outer plate  11  formed of hard resin is placed on the side of the input shaft  2 , and the second outer plate  12  formed of an elastic member is placed on the side of the output shaft  3 . On the contrary, however, it is possible that the second outer plate formed of an elastic member is placed on the side of the input shaft  2 , and the first outer plate  11  formed of hard resin is placed on the side of the output shaft  3 . In that case, in the second outer plate  12 , a through-hole passing through between the one surface  122  and the other surface  121  is formed instead of the bottomed hole  124 . And in the first outer plate  11 , it is favorable to form, instead of the through-hole  114 , a bottomed hole which has a bottom surface on the side of the hub  4  and has a cross-section shape similar to that of the through-hole  114 . 
         [0060]    Further, in the present embodiment, the first outer plate  11  and the second outer plate  12  are fixed by using the snap-fit portions  119 . The present invention, however, is not limited to this. As a method of fixing both outer plates  11  and  12 , it is possible to use widely other existing fixing methods such as one using an adhesive agent, one using screw clamping, and the like. 
         [0061]    Further, in the present embodiment, the rotation transmitting cushion  10  is made so as to have the two-layer stacking structure consisting of the first outer plate  11  formed of hard resin and the second outer plate  12  formed of an elastic member having a lower elastic coefficient than that of the first outer plate  11 . The present invention, however, is not limited to this. For example, the rotation transmitting cushion  10  may be formed of only the first outer plate  11 . In that case, as a hole to which the end portion  22  of the input shaft  2  is inserted, it is favorable to form, instead of the through-hole  114 , a bottomed hole which has a bottom surface on the side of the hub  4  and has a cross-section shape similar to that of the through-hole  114 . 
       INDUSTRIAL APPLICABILITY 
       [0062]    The present invention can be widely applied to a coupling that transmits torque applied to one rotating shaft to the other rotating shaft, such as a coupling for an electrically-assisted power steering device. 
       REFERENCE SIGNS LIST 
       [0063]      1 : direct-shaft-connection type coupling;  2 : input shaft;  3 : output shaft;  4 : hub;  5 : shaft connecting mechanism;  10 : rotation transmitting cushion;  11 : first outer plate;  12 : second outer plate;  21 : flat surface of the input shaft  2 ;  22 : end portion of the input shaft  2 ;  42 : cylindrical portion of the hub  4 ;  43 ,  44 : end surface of the input shaft  2 ;  45 : protruding portion of the hub  4 ;  46 : inner periphery surface of the protruding portion  45 ;  101 : boss portion of the rotation transmitting cushion  10 ;  102 ,  103 : end surface of the boss portion  101 ;  103 : boss portion of the rotation transmitting cushion  10 ;  104 : bottomed shaft hole of the boss portion  101 ;  105 : arm portion of the rotation transmitting cushion;  106 : outer periphery surface of the boss portion  101 ;  110 : plate body of the first outer plate  11 ;  111 : the other surface of the first outer plate  11 ;  112 : one surface of the first outer plate  11 ;  113 ,  113   a : arm portion of the first outer plate  11 ;  114 : through-hole of the plate body  110 :  115 : side surface of the plate body  110 ;  116 : inner wall of the through-hole  114 ;  117 : ridge line;  118 : convex surface of the inner wall  116  of the through-hole  114 ;  118 A,  118   a - 118   d : inclined surface as a constitutive part of the convex surface  118 :  119 : snap-fit portion;  120 : plate body of the second outer plate  12 ;  121 : one surface of the second outer plate;  122 : one surface of the second outer plate;  123 ,  123   a ,  123   b : arm portion of the second outer plate;  124 : bottomed hole of the plate body  120 ;  125 : outer periphery of the plate body  120 ;  127 : bottom of the bottomed hole  124 :  128 : buffer portion;  129 : engaging portion for engaging with the snap-fit portion  119 ;  150 : edge portion of the inner wall  116  along the axis O; and  151 : central portion of the inner wall along the axis O.