Patent Publication Number: US-9846448-B2

Title: Acceleration device for vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2013-266909 filed on Dec. 25, 2013, the disclosure of which is incorporated herein by reference. 
     FIELD OF TECHNOLOGY 
     The present disclosure relates to an acceleration device for an automotive vehicle. 
     BACKGROUND 
     An acceleration device is known in the art, according to which an accelerating condition of an automotive vehicle is controlled depending on a stepping amount of an acceleration pedal operated by a vehicle driver. In the known acceleration device, the stepping amount of the acceleration pedal is detected by a rotational angle of a pedal shaft, which is connected to the acceleration pedal. For example, an acceleration device is disclosed in Japanese Patent Application No. 2012-222056 (corresponding to U.S. patent application Ser. No. 14/045,374), which is not published before the filing date (Dec. 25, 2013) of the present application in Japanese Patent Office. The acceleration device of the above patent application has a pedal rotating member, which is fixed to a pedal shaft rotatably supported by a supporting body. A forward end of the pedal rotating member is brought into contact with an inner wall surface of the supporting body in order to limit a fully closed position and/or a fully opened position of the acceleration pedal. 
     In the acceleration device of the above patent application, the forward end of the pedal rotating member may be broken away from a boss portion of the pedal rotating member, when the forward end of the pedal rotating member is strongly brought into contact with the inner wall surface in a direction to the fully closed position of the acceleration pedal and thereby an excess force is applied to the forward end of the pedal rotating member. When a broken piece of the pedal rotating member (including the forward end thereof), which is a part of the pedal rotating member broken away from the boss portion, is moved inside of the supporting body of the acceleration device and brought into contact with the pedal shaft, a normal rotation of the pedal shaft may be adversely affected. As a result, the acceleration pedal (including the pedal shaft) may not return to the fully closed position thereof. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure is made in view of the above problem. It is an object of the present disclosure to provide an acceleration device, according to which an acceleration pedal as well as a pedal shaft can be surely rotated depending on a stepping amount of the acceleration pedal by a vehicle driver even when a part of a rotating member connected to the pedal shaft and rotatably accommodated in a supporting body is broken away from the rotating member. 
     According to a feature of the present disclosure, an acceleration device for an automotive vehicle has a supporting body to be fixed to a vehicle body; a pedal shaft rotatably supported by the supporting body; a rotating member provided at a radial-outer side of the pedal shaft and rotatable in accordance with rotation of the pedal shaft; a biasing member for biasing the rotating member in a pedal closing direction; an acceleration pedal to be operated by a vehicle driver; a pedal arm connected at its one end to the acceleration pedal and converting a stepping movement of the acceleration pedal by the vehicle driver into a rotational torque of the pedal shaft; and a rotational angle detecting unit for detecting a rotational angle of the pedal shaft with respect to the supporting body. 
     The rotating member of the acceleration device is comprised of; 
     a boss portion formed at the radial-outer side of the pedal shaft; 
     a biasing-member holding portion extending from the boss portion in a radial-outward direction of the pedal shaft for holding one end of the biasing member; and 
     a mechanically-weaker portion formed between the boss portion and the biasing-member holding portion. 
     The boss portion, the biasing-member holding portion and the mechanically-weaker portion are integrally formed as one unit. The biasing-member holding portion is so configured to be broken away from the boss portion at the mechanically-weaker portion, when an acting force larger than a predetermined value in the pedal closing direction is applied to the rotating member. 
     A broken piece, which includes the biasing-member holding portion broken away from the boss portion, is pushed by the biasing member to an inner wall surface of the supporting body. Since the broken piece is prevented by the biasing member from freely moving in an inside of the supporting body, it is possible to avoid a situation that the broken piece is brought into contact with the pedal shaft and thereby the rotation of the pedal shaft is adversely affected. As a result, it is possible to surely convert the stepping movement of the vehicle driver into the rotational torque of the pedal shaft, so that the pedal shaft is surely rotated depending on the stepping movement of the vehicle driver. It is further possible to surely return the acceleration pedal to a fully-closed position thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a schematic side view showing an acceleration device for a vehicle according to a first embodiment of the present disclosure; 
         FIG. 2  is a schematic cross sectional view of the acceleration device taken along a line II-II in  FIG. 3 ; 
         FIG. 3  is a schematic cross sectional view taken along a line in  FIG. 2 ; 
         FIGS. 4A to 4C  are schematically enlarged cross sectional views, each showing a portion IV of  FIG. 2 , wherein  FIG. 4C  shows a modification of the first embodiment; 
         FIGS. 5A to 5C  are schematically enlarged cross sectional views, each showing a relevant portion of an acceleration device according to a second embodiment of the present disclosure, wherein  FIG. 5C  shows a modification of the second embodiment; 
         FIGS. 6A to 6C  are schematically enlarged cross sectional views, each showing a relevant portion of an acceleration device according to a third embodiment of the present disclosure, wherein  FIG. 6C  shows a modification of the third embodiment; 
         FIGS. 7A to 7C  are schematically enlarged cross sectional views, each showing a relevant portion of an acceleration device according to a fourth embodiment of the present disclosure, wherein  FIG. 7C  shows a modification of the fourth embodiment; 
         FIGS. 8A and 8B  are schematically enlarged cross sectional views, each showing a relevant portion of an acceleration device according to a fifth embodiment of the present disclosure; and 
         FIGS. 9A and 9B  are schematically enlarged cross sectional views, each showing a relevant portion of an acceleration device according to a sixth embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present disclosure will be explained hereinafter by way of multiple embodiments with reference to the drawings. The same reference numerals are given to the same or similar parts or portions throughout the multiple embodiments for the purpose of avoiding repeated explanation. 
     First Embodiment 
     An acceleration device  1  for an automotive vehicle according to a first embodiment of the present disclosure is shown in  FIGS. 1 to 4 . The acceleration device  1  is an input device, which is operated by a vehicle driver in order to decide a valve opening degree of a throttle valve (not shown) for an internal combustion engine of the automotive vehicle. The acceleration device  1  is of an electronically operated type and outputs an electrical signal representing a stepping stroke amount of an acceleration pedal  37 . The electrical signal is transmitted to an outside electronic control unit (not shown). The electronic control unit drives the throttle valve by a throttle actuator (not shown) based on the stepping stroke amount and other vehicle information. 
     The acceleration device  1  is composed of a supporting body  10 , a pedal shaft  20 , an operation member  30 , the acceleration pedal  37 , a return spring  39  (a pedal-side biasing member  39 ), a rotational angle sensor  25 , a hysteresis mechanism  40  and so on. In  FIGS. 1 to 4 , “UP” is an upper side in a vertical direction, while “DOWN” is a lower side in the vertical direction. 
     The supporting body  10  is composed of a housing  12 , a first cover member  16  and a second cover member  18 . The supporting body  10  forms an inner space  11  for accommodating the pedal shaft  20 , the return spring  39 , the rotational angle sensor  25 , the hysteresis mechanism  40  and so on. An opening  111  is formed at a lower portion of the supporting body  10  for communicating the inner space  11  to an outside of the supporting body  10 . The opening  111  corresponds to a movable range of the operation member  30 , as explained below. 
     The housing  12  is made of resin and composed of a shaft supporting portion  13  for rotatably supporting one axial end  201  of the pedal shaft  20  (hereinafter, a first axial end  201 ), a front-side wall portion  17  formed at a front side of the acceleration device  1  and connected to the shaft supporting portion  13 , a back-side wall portion  15  formed at a back side of the acceleration device  1 , an upper-side wall portion  14  formed at an upper side of the acceleration device  1  and connecting the shaft supporting portion  13  and the front-side wall portion  17  to the back-side wall portion  15 , and so on. Outer wall surfaces of the shaft supporting portion  13 , the front-side wall portion  17 , the back-side wall portion  15  and the upper-side wall portion  14  are formed with patterned indented surfaces. In other words, the outer wall surfaces are formed with net-like concavities and convexities, in order to increase rigidity against external forces applied to the housing  12 . 
     A circular opening, into which the first axial end  201  of the pedal shaft  20  is movably inserted, is formed in the shaft supporting portion  13 , so that the pedal shaft  20  is rotatable in the circular opening. In other words, an inner peripheral surface of the circular opening corresponds to a bearing portion  130  for rotatably supporting the first axial end  201  of the pedal shaft  20 . 
     As shown in  FIG. 1 , multiple fixing portions  131 ,  132  and  133  are formed in the housing  12 . A bolt-hole is formed in each of the fixing portions  131 ,  132  and  133 . The acceleration device  1  is fixed to a vehicle body  8  by multiple bolts (not shown), each of which is inserted through the respective bolt-hole formed in each fixing portion  131 ,  132  and  133 . 
     A full-open side stopper surface  19  of a recessed shape (hereinafter, a stopper surface  19 ) is formed at a lower side of the back-side wall portion  15 . A full-open side stopper pin  31  of a convex shape (hereinafter, a stopper pin  31 ) is formed in the operation member  30 . When the stopper pin  31  is brought into contact with the stopper surface  19 , a rotational movement of the operation member  30  is stopped in an acceleration opening direction (that is, an anti-clockwise direction in  FIG. 1 or 2 ). In other words, when the stopper pin  31  is in contact with the stopper surface  19 , the operation member  30  is held at its fully-opened pedal position, which corresponds to an acceleration fully-opened position. The acceleration fully-opened position corresponds to a pedal position, in which opening degree of the acceleration pedal  37  (that is, the stepping stroke amount of the acceleration pedal  37 ) is 100%. 
     Each of the first cover member  16  and the second cover member  18  is fixed to the housing  12  so as to be parallel to the shaft supporting portion  13 . The first cover member  16  is formed in an almost rectangular flat plate shape and connected to each axial end of the upper-side wall portion  14 , the back-side wall portion  15  and the front-side wall portion  17 . In other words, as shown in  FIG. 3 , the first cover member  16  is connected to each right-hand end of the wall portions  14 ,  15  and  17 , which is located on an opposite side to the shaft supporting portion  13 . The first cover member  16  is also connected to the second cover member  18 . The first cover member  16  prevents extraneous material from going into the inner space  11  of the acceleration device  1 . 
     The second cover member  18  is formed in a triangular flat plate shape and connected to the housing  12  by multiple bolts  181  at each axial end of the back-side wall portion  15  and the front-side wall portion  17 , which is located on the opposite side to the shaft supporting portion  13 . A circular recessed portion  180  is formed in an inner wall of the second cover member  18  in order to movably support another axial end  202  of the pedal shaft  20  (hereinafter, a second axial end  202 ). In other words, an inner peripheral surface of the circular recessed portion corresponds to a bearing portion  180  for rotatably supporting the second axial end  202  of the pedal shaft  20 . 
     An outer wall surface of the second cover member  18  is formed with net-like concavities and convexities, in order to increase rigidity against external forces applied to the second cover member  18 . The second cover member  18  also prevents extraneous material from going into the inner space  11  of the acceleration device  1 . 
     The pedal shaft  20  is horizontally arranged in the acceleration device  1 , as best shown in  FIG. 3 . A sensor accommodating space  22  is formed in the first axial end  201  of the pedal shaft  20  for accommodating a detecting portion of the rotational angle sensor  25 . 
     The pedal shaft  20  is rotated depending on a torque inputted from the acceleration pedal  37 , which is operated by the vehicle driver. The pedal shaft  20  is rotatable within a predetermined angular range from an acceleration fully-closed position to the acceleration fully-opened position. The acceleration fully-closed position corresponds to a pedal position, in which the opening degree of the acceleration pedal  37  (the stepping stroke amount of the acceleration pedal  37 ) is 0 (zero) %. 
     A direction of the rotational movement of the pedal shaft  20  (that is, the rotational movement of the operation member  30 ) from the acceleration fully-closed position to the acceleration fully-opened position (that is, the rotation in the anti-clockwise direction in  FIG. 1 or 2 ) is referred to as the acceleration opening direction (or a pedal opening direction). On the other hand, a direction of the rotational movement of the pedal shaft  20  from the acceleration fully-opened position to the acceleration fully-closed position (that is, the rotation in a clockwise direction in  FIG. 1 or 2 ) is referred to as an acceleration closing direction (or a pedal closing direction). 
     The operation member  30  is composed of a pedal-side rotating member  38 , the acceleration pedal  37  and a pedal arm  33 . The pedal-side rotating member  38  has a pedal-side boss portion  32 , an arm connecting portion  34 , a pedal-side spring holding portion  35  (a pedal-biasing-member holding portion  35 ), a full-close side stopper portion  36  and so on, wherein the pedal-side boss portion  32 , the arm connecting portion  34 , the pedal-side spring holding portion  35  and the full-close side stopper portion  36  are integrally formed as one unit. The full-close side stopper portion  36  is hereinafter referred to as a stopper arm. 
     The pedal-side boss portion  32  is formed in a tubular shape having a circular cross section and provided between the shaft supporting portion  13  and the second cover member  18 . The pedal-side boss portion  32  is fixed to an outer peripheral surface of the pedal shaft  20  by, for example, a press-fit process, so that the pedal shaft  20  is rotated together with the pedal-side rotating member  38 . 
     Multiple helical gear teeth  321  (first gear teeth  321 ) are integrally formed with the pedal-side boss portion  32  at an axial end surface of the pedal-side boss portion  32  on a side to the second cover member  18  (that is, an axial end surface of the pedal-side boss portion  32  on a right-hand side in  FIG. 3  and hereinafter referred to as a second axial end surface). The multiple first gear teeth  321  are formed at equal intervals in a circumferential direction of the pedal-side boss portion  32  of the tubular shape. 
     Each of the first gear teeth  321  protrudes in an axial direction of the pedal shaft  20  toward a hysteresis-side rotating member  48  of the hysteresis mechanism  40  and a height of each gear tooth  321  in an axial direction of the pedal shaft  20  is increased in the circumferential direction from a pedal-opening-direction side to a pedal-closing-direction side. In other words, each of the gear teeth  321  has an inclined surface, which becomes closer to the hysteresis-side rotating member  48  in the pedal closing direction. 
     A first friction member  323  is provided at another axial end surface of the pedal-side boss portion  32  on a side to the shaft supporting portion  13  (that is, an axial end surface of the pedal-side boss portion  32  on a left-hand side in  FIG. 3  and hereinafter referred to as a first axial end surface). The first friction member  323  is formed in an annular shape. The first friction member  323  is provided between the pedal-side boss portion  32  and an inner wall surface of the housing  12  at a radial-outside position of the pedal shaft  20 . When the pedal-side boss portion  32  is pushed in a direction away from the hysteresis-side rotating member  48 , that is, in a direction to the shaft supporting portion  13 , the pedal-side boss portion  32  is coupled to the first friction member  323  in a friction coupling manner. A frictional force between the pedal-side boss portion  32  and the first friction member  323  works as a rotational resistance of the pedal-side boss portion  32 . 
     One end of the arm connecting portion  34  is connected to a radial-outward peripheral portion of the pedal-side boss portion  32 , while the other end of the arm connecting portion  34  outwardly projects to the outside of the supporting body  10  through the opening  111 . 
     The pedal-side spring holding portion  35  is arranged in the inner space  11  and extends from the pedal-side boss portion  32  in a radial-upward direction (that is, a radial-outward direction). The pedal-side spring holding portion  35  holds one end of the return spring  39 . 
     The stopper arm  36  is also arranged in the inner space  11  and further extends from the pedal-side spring holding portion  35  in the radial-upward direction. The stopper arm  36  has a forward end working as a contacting portion. When the contacting portion of the stopper arm  36  is brought into contact with a stopper surface formed by an inner wall surface  150  of the back-side wall portion  15 , the rotational movement of the operation member  30  in the pedal closing direction is stopped. Accordingly, the rotational movement of the operation member  30  is limited at the acceleration fully-closed position. 
     As shown in  FIG. 1 , one end (an upper end) of the pedal arm  33  is connected to the arm connecting portion  34  of the operation member  30 , while the other end (a lower end) extends in a downward direction. In the present embodiment, the pedal arm  33  downwardly extends and is connected to the acceleration pedal  37 . A stepping movement of the acceleration pedal  37  by the vehicle driver is converted into the rotational movement of the pedal shaft  20  (the rotation at a center axis φ 1 ) via the pedal-side rotating member  38  of the operation member  30 . 
     When the acceleration pedal  37  is rotated in the pedal opening direction, a rotational angle of the pedal shaft  20  with respect to the acceleration fully-closed position (an initial position for the rotational movement of the pedal shaft  20 ) is increased in the pedal opening direction. The opening degree of the acceleration pedal  37  is increased in accordance with the increase of the rotational angle of the pedal shaft  20 . On the other hand, when the acceleration pedal  37  is rotated in the pedal closing direction, the rotational angle of the pedal shaft  20  with respect to the initial position is decreased and the opening degree of the acceleration pedal  37  is decreased in accordance with the decrease of the rotational angle of the pedal shaft  20 . 
     The return spring  39  is composed of a coil spring, one end of which is in contact with an inner wall surface  171  of the front-side wall portion  17 . The return spring  39  (also referred to as the pedal-side biasing member) biases the operation member  30  in the pedal closing direction. A biasing force applied to the operation member  30  by the return spring  39  becomes larger as the rotational angle of the operation member  30 , that is, the rotational angle of the pedal shaft  20 , becomes larger in the pedal opening direction. The biasing force is so set that the operation member  30  as well as the pedal shaft  20  is returned to the acceleration fully-closed position (the initial position) independently of a rotational position of the operation member  30 . 
     The rotational angle sensor  25  is composed of a yoke  26 , a pair of permanent magnets  271  and  272  magnetized in different directions to each other, a hall element  28  and so on. The yoke  26  is made of magnetic material and formed in a cylindrical shape. The yoke  26  is attached to an inner peripheral wall of the sensor accommodating space  22  of the pedal shaft  20 . The magnets  271  and  272  are arranged in an inside of the yoke  26  so as to oppose to each other in a radial direction of the pedal shaft  20  across the center axis φ 1  of the pedal shaft  20 . Each of the magnets  271  and  272  is fixed to an inner peripheral wall of the yoke  26 . The hall element  28  is arranged at a position between the magnets  271  and  272  in the radial direction of the pedal shaft  20 . The rotational angle sensor  25  is also referred to as a rotational angle detecting unit. 
     Voltage is generated at the hall element  28  when magnetic field is applied to the hall element  28 , in which electric current flows. Density of magnetic flux passing through the hall element  28  is changed when the magnets  271  and  272  are rotated together with the pedal shaft  20  around the center axis φ 1  of the pedal shaft  20 . Amplitude of the generated voltage is in proportion to the density of the magnetic flux passing through the hall element  28 . The rotational angle sensor  25  detects the voltage generated at the hall element  28  in order to detect a relative rotational angle between the hall elements  28  and the magnets  271  and  272 , namely the rotational angle of the pedal shaft  20  with respect to the supporting body  10 . The rotational angle sensor  25  outputs an electric signal, which represents the detected rotational angle. The electric signal is transmitted to the outside electronic control unit (not shown), which is provided above the acceleration device  1 , via an outside connector  29 . 
     The hysteresis mechanism  40  is composed of the hysteresis-side rotating member  48 , a second friction member  423 , a hysteresis spring  49  and so on. The hysteresis-side rotating member  48  has a hysteresis-side boss portion  42 , a spring holding portion  45  (a hysteresis-side spring holding portion  45 ) and so on, wherein the hysteresis-side boss portion  42  and the hysteresis-side spring holding portion  45  are integrally formed as one unit. 
     The hysteresis-side boss portion  42  is provided between the pedal-side boss portion  32  and an inner wall surface of the second cover member  18  and at the radial-outside position of the pedal shaft  20 . The hysteresis-side boss portion  42  is formed in an annular shape and rotatable relative to the pedal shaft  20  and the pedal-side boss portion  32 . In addition, the hysteresis-side boss portion  42  is movable in the axial direction of the pedal shaft  20  with respect to the pedal-side boss portion  32 , so that the hysteresis-side boss portion  42  is moved closer to or more separated from the pedal-side boss portion  32 . Multiple second helical gear teeth  421  are integrally formed with the hysteresis-side boss portion  42  on an axial side surface thereof facing to the pedal-side boss portion  32 . The multiple second gear teeth  421  are formed at equal intervals in the circumferential direction of the hysteresis-side boss portion  42  of the annular shape. 
     Each of the second gear teeth  421  protrudes in the axial direction of the pedal shaft  20  toward the pedal-side boss portion  32  and a height of each gear tooth  421  in the axial direction of the pedal shaft  20  is increased in the circumferential direction from a pedal-closing-direction side to a pedal-opening-direction side. In other words, each of the gear teeth  421  has an inclined surface, which becomes closer to the pedal-side boss portion  32  in the pedal opening direction. 
     The first helical gear teeth  321  and the second helical gear teeth  421  are engaged with each other in the circumferential direction of the pedal shaft  20 . In other words, since the inclined surfaces of the first helical gear teeth  321  and the inclined surfaces of the second helical gear teeth  421  are brought into contact with each other, the rotational force can be transmitted from the pedal-side boss portion  32  to the hysteresis-side boss portion  42 , or vice versa. More exactly, the rotation of the pedal-side boss portion  32  in the pedal opening direction is transmitted to the hysteresis-side boss portion  42  via the first helical gear teeth  321  and the second helical gear teeth  421 . On the other hand, the rotation of the hysteresis-side boss portion  42  in the pedal closing direction is transmitted to the pedal-side boss portion  32  via the second helical gear teeth  421  and the first helical gear teeth  321 . 
     When the pedal-side boss portion  32  is located not in the acceleration fully-closed position (that is, not in the initial position) but at such a rotational position, which is on a side toward the acceleration fully-opened position, the inclined surfaces of the first and second helical gear teeth  321  and  421  are engaged with each other and the pedal-side boss portion  32  and the hysteresis-side boss portion  42  are pushed by each other in the axial direction of the pedal shaft  20  away from each other. A pushing force of the first helical gear teeth  321  for pushing the pedal-side boss portion  32  toward the shaft supporting portion  13  becomes larger, when the rotational angle of the pedal-side boss portion  32  is increased in the acceleration opening direction from the acceleration fully-closed position. In a similar manner, a pushing force of the second helical gear teeth  421  for pushing the hysteresis-side boss portion  42  toward the second cover member  18  becomes larger, when the rotational angle of the hysteresis-side boss portion  42  is increased in the acceleration opening direction from the acceleration fully-closed position. 
     The hysteresis-side spring holding portion  45  is arranged in the inner space  11  and extends from the hysteresis-side boss portion  42  in the radial-upward direction. A spring supporting member  455  for supporting one end of the hysteresis spring  49  is provided at a forward end of the hysteresis-side spring holding portion  45 . The forward end of the hysteresis-side spring holding portion  45  is formed in a semi-spherical shape and a recessed portion of the spring supporting member  455  is also formed by a semi-spherical surface. Since the spring supporting member  455  is in contact with the forward end of the hysteresis-side spring holding portion  45  via the semi-spherical surfaces, a biasing force of the hysteresis spring  49  is transmitted to the hysteresis-side spring holding portion  45  without being influenced by an angular position of the hysteresis spring  49  with respect to the hysteresis-side spring holding portion  45 . 
     The second friction member  423  is formed in an annular shape and provided between the hysteresis-side rotating member  48  and the inner wall surface of the second cover member  18  at a radial-outside position of the pedal shaft  20 . When the hysteresis-side rotating member  48  is pushed in the axial direction away from the pedal-side boss portion  32 , that is, in the direction to the second cover member  18 , the hysteresis-side rotating member  48  is coupled to the second friction member  423  in the friction coupling manner. A frictional force, which is generated between the hysteresis-side rotating member  48  and the second friction member  423 , works as a rotational resistance of the hysteresis-side rotating member  48 . 
     The hysteresis spring  49  is composed of a coil spring, one end of which is supported by the spring supporting member  455  and the other end of which is in contact with the inner wall surface  171  of the front-side wall portion  17 . The hysteresis spring  49  biases the hysteresis-side boss portion  42  in the pedal closing direction. A biasing force of the hysteresis spring  49  becomes larger as the rotational angle of the hysteresis-side boss portion  42  becomes larger in the pedal opening direction. A torque applied to the hysteresis-side boss portion  42  by the biasing force of the hysteresis spring  49  is transmitted to the pedal-side boss portion  32  via the second helical gear teeth  421  and the first helical gear teeth  321 . 
     In the acceleration device  1  of the present embodiment, the pedal-side spring holding portion  35  is connected to the pedal-side boss portion  32  via a mechanically-weaker portion  300 . A structure and/or configuration of the pedal-side rotating member  38  will be explained more in detail with reference to  FIGS. 4A and 4B . Each of  FIGS. 4A and 4B  is a schematically enlarged cross sectional view showing a portion IV of  FIG. 2 . Namely, each of  FIGS. 4A and 4B  shows a cross sectional view of a relevant portion of the pedal-side rotating member  38 . 
     The mechanically-weaker portion  300  is formed between the pedal-side boss portion  32  and the stopper arm  36  as a portion, which is mechanically weaker than other portions of the pedal-side rotating member  38 . 
     More exactly, the mechanically-weaker portion  300 , which is indicated by a two-dot-chain line in  FIG. 4A , is so made as to satisfy the following expression 1:
 
 Z 1 &lt;Z 2×( L 1 /L 2)  (expression 1)
 
     In the above expression 1; 
     Z 1  is a modulus of section of the mechanically-weaker portion  300 ; 
     Z 2  is a modulus of section of any arbitrary portion  380 , which is any portion of the pedal-side rotating member  38  between the mechanically-weaker portion  300  and the stopper arm  36 , for example, a portion indicated by another two-dot-chain line in  FIG. 4A ; 
     L 1  is a distance between a contacting point P 36  and a mechanically weak point P 300 , wherein the contacting point P 36  corresponds to a point of the stopper arm  36  to be in contact with the inner wall surface  150  of the back-side wall portion  15  and the mechanically weak point P 300  corresponds to a point of the mechanically-weaker portion  300  facing to the inner wall surface  150 ; and 
     L 2  is a distance between the contacting point P 36  and an arbitrary point P 380 , wherein the arbitrary point P 380  corresponds to a point of the arbitrary portion  380  facing to the inner wall surface  150  of the back-side wall portion  15 . 
     A recessed portion  381  is formed in the pedal-side spring holding portion  35  at its back-side surface  351  facing to the inner wall surface  150  of the back-side wall portion  15 . A projection  151 , which is configured to be engaged with the recessed portion  381 , is formed in the back-side wall portion  15  at the inner wall surface  150  thereof facing to the recessed portion  381 . The recessed portion  381  is also referred to as a second recessed portion and the projection  151  is also referred to as a first projection. 
     An operation of the acceleration device  1  will be explained. When the acceleration pedal  37  is stepped on, the operation member  30  is rotated together with the pedal shaft  20  around the center axis φ 1  of the pedal shaft  20  in the pedal opening direction, depending on a stepping force applied to the acceleration pedal  37 . In this operation, the stepping force is necessary to generate such a torque, which is larger than a sum of a torque of the biasing force of the return spring  39 , a torque of the biasing force of the hysteresis spring  49 , and a torque of the resistance force generated by the friction force of the first friction member  323  and the second friction member  423 . 
     When the acceleration pedal  37  is maintained at any pedal position after the acceleration pedal  37  is stepped forward, it is necessary that the stepping force generates such a torque which is larger than a difference between the torque of the biasing force of the return spring  39  and the hysteresis spring  49  and the torque of resistance force generated by the friction force of the first friction member  323  and the second friction member  423 . In other words, the vehicle driver may decrease the stepping force by a certain amount after the acceleration pedal  37  has been stepped forward, when the vehicle driver maintains such a stepped-forward pedal position of the acceleration pedal  37 . 
     When the torque generated by the stepping force becomes such a value, which is smaller than the difference between the torque of the biasing force of the return spring  39  and the hysteresis spring  49  and the torque of resistance force generated by the friction force of the first friction member  323  and the second friction member  423 , the rotational position of the acceleration pedal  37  is moved in a direction to its acceleration fully-closed position (the initial position). In this operation, it is sufficient for the vehicle driver to stop his stepping-forward motion in a case of quickly returning the acceleration pedal  37  to the acceleration fully-closed position. Therefore, it does not place an additional burden on the vehicle driver. On the other hand, in a case that the acceleration pedal  37  is gradually returned to the acceleration fully-closed position, it is necessary for the vehicle driver to continuously apply his stepping force of a certain amount to the acceleration pedal  37  and to gradually decrease the stepping force. 
     In the operation of the acceleration device  1 , the stopper arm  36  of the pedal-side rotating member  38  may be rapidly and/or strongly brought into contact with the inner wall surface  150  of the back-side wall portion  15 , when the pedal arm  33  is pulled up or when the stepping force to the acceleration pedal  37  by the vehicle driver is rapidly released. When the stopper arm  36  is strongly pushed to the inner wall surface  150 , an acting force F 1  (as shown in  FIG. 4A ), which is larger than a predetermined value, is applied to the stopper arm  36 . 
     In the acceleration device  1  of the present embodiment, the pedal-side spring holding portion  35  is so configured to be broken away from the pedal-side boss portion  32  at the mechanically-weaker portion  300 , when the acting force F 1  larger than the predetermined value is applied to the stopper arm  36 . A broken piece  350 , which includes the pedal-side spring holding portion  35  broken away from the pedal-side boss portion  32 , is pushed to the inner wall surface  150  of the back-side wall portion  15  by the biasing force of the pedal spring  39 , as shown in  FIG. 4B . Accordingly, the broken piece  350  is held at a predetermined position inside of the supporting body  10 , if a part of the pedal-side rotating member  38  (that is, the pedal-side spring holding portion  35  or the like) is broken away at the mechanically-weaker portion  300 . It is, therefore, possible to prevent the broken piece  350  from moving in the inner space  11  of the supporting body  10 . 
     As a result, it is possible to prevent the broken piece  350  (including the pedal-side spring holding portion  35 ) from being brought into contact with the pedal shaft  20  and to prevent the rotational movement thereof from being adversely affected. Therefore, the pedal shaft  20  can be surely rotated depending on the stepping force of the vehicle driver, even after the part of the pedal-side rotating member  38  is broken away. In addition, the acceleration pedal  37  can be surely moved to its acceleration full-close position (the initial position). 
     Furthermore, in the acceleration device  1  of the present embodiment, the first projection  151  formed at the inner wall surface  150  of the back-side wall portion  15  will be inserted into the second recessed portion  381  formed in the back-side surface of the pedal-side rotating member  38 , when the broken piece  350  (including the pedal-side spring holding portion  35 ) is pushed toward the inner wall surface  150 . Accordingly, the broken piece  350  is attached to and held at a position of the first projection  151  not only by the biasing force of the pedal spring  39  but also by the engagement between the first projection  151  and the second recessed portion  381 . As above, it is possible to surely prevent the broken piece  350  from adversely affecting the rotation of the pedal shaft  20 . Therefore, the pedal shaft  20  can be surely rotated depending on the stepping force of the vehicle driver. 
     In the present embodiment, the first projection  151  is formed in the inner wall surface  150  of the back-side wall portion  15 , while the second recessed portion  381  is formed in the back-side surface of the pedal-side rotating member  38 . 
     However, the first embodiment may be modified in such a manner as shown in  FIG. 4C , wherein a second projection  381   a  is formed in the back-side surface of the pedal-side rotating member  38 , while a first recessed portion  151   a  is formed in the inner wall surface  150  of the back-side wall portion  15 , so that the second projection  381   a  will be engaged with the first recessed portion  151   a  in a case that the pedal-side spring holding portion  35  is broken away from the pedal-side boss portion  32 . 
     In the present specification, the first projection  151  and the first recessed portion  151   a  (each of which is formed in the inner wall surface  150 ) are collectively referred to as a first engaging portion, while the second recessed portion  381  and the second projection  381   a  (each of which is formed in the back-side surface of the pedal-side spring holding portion  35 ) are collectively referred to as a second engaging portion. 
     Second Embodiment 
     An acceleration device  2  according to a second embodiment of the present disclosure will be explained with reference to  FIGS. 5A and 5B . A structure of a hysteresis-side rotating member  58  of the second embodiment differs from that of the first embodiment. Each of  FIGS. 5A and 5B  is a schematically enlarged cross sectional view showing a relevant portion of the second embodiment, which corresponds to the portion IV of  FIG. 2 . 
     In the acceleration device  2  of the present embodiment, the hysteresis-side rotating member  58  is composed of a hysteresis-side boss portion  52  and a hysteresis-side spring holding portion  55  (also referred to as a hysteresis-biasing-member holding portion  55 ), wherein the hysteresis-side boss portion  52  and the hysteresis-side spring holding portion  55  are integrally formed as one unit. 
     The hysteresis-side boss portion  52  is arranged between the pedal-side boss portion  32  and the inner wall of the second cover member  18  and at the radial-outside position of the pedal shaft  20 . The hysteresis-side boss portion  52  is formed in an annular shape and rotatable relative to the pedal shaft  20  and the pedal-side boss portion  32 . In addition, the hysteresis-side boss portion  52  is movable in the axial direction of the pedal shaft  20  with respect to the pedal-side boss portion  32 , so that the hysteresis-side boss portion  52  is moved closer to or more separated from the pedal-side boss portion  32 . 
     The hysteresis-side spring holding portion  55  is arranged in the inner space  11  and extends from the hysteresis-side boss portion  52  in the radial-upward direction. The hysteresis-side spring holding portion  55  holds one end of the hysteresis spring  49  via the spring supporting member  455 . 
     The hysteresis-side spring holding portion  55  is connected to the hysteresis-side boss portion  52  via a mechanically-weaker portion  500 . The mechanically-weaker portion  500  is formed between the hysteresis-side boss portion  52  and the hysteresis-side spring holding portion  55  as a portion, which is mechanically weaker than other portions of the hysteresis-side rotating member  58 . 
     More exactly, the mechanically-weaker portion  500 , which is indicated by a two-dot-chain line in  FIG. 5A , is so made as to satisfy the following expression 2:
 
 Z 3 &lt;Z 4×( L 3 /L 4)  (expression 2)
 
     In the above expression 2; 
     Z 3  is a modulus of section of the mechanically-weaker portion  500 ; 
     Z 4  is a modulus of section of any arbitrary portion  580 , which is any portion of the hysteresis-side rotating member  58  between the hysteresis-side boss portion  52  and the hysteresis-side spring holding portion  55 , for example, a portion indicated by another two-dot-chain line in  FIG. 5A ; 
     L 3  is a distance between an acting point P 55  and a mechanically weak point P 500 , wherein the acting point P 55  corresponds to a point of the hysteresis-side spring holding portion  55  to which the biasing force of the hysteresis spring  49  is applied, and the mechanically weak point P 500  corresponds to a point of the mechanically-weaker portion  500  on a side closer to the hysteresis spring  49  (that is, a front-side surface of the hysteresis-side rotating member  58 , which is equal to a left-hand side of  FIG. 5A ); and 
     L 4  is a distance between the acting point P 55  and an arbitrary point P 580 , wherein the arbitrary point P 580  corresponds to a point of the arbitrary portion  580  on the side closer to the hysteresis spring  49  (that is, on the left-hand side of  FIG. 5A ). 
     A recessed portion  581  is formed in the hysteresis-side spring holding portion  55  at its back-side surface  551  facing to the inner wall surface  150  of the back-side wall portion  15 . A projection  152 , which is operatively engaged with the recessed portion  581 , is formed in the back-side wall portion  15  at the inner wall surface  150  thereof facing to the recessed portion  581 . The recessed portion  581  is also referred to as a fourth recessed portion and the projection  152  is also referred to as a third projection. 
     In the acceleration device  2 , the hysteresis-side rotating member  58  is repeatedly rotated in the pedal opening direction and in the pedal closing direction in accordance with the rotation of the pedal-side rotating member  38 . The hysteresis-side rotating member  58  may be mechanically damaged (for example, may be broken) as a result of the repeated rotation thereof in the pedal closing direction, due to possible fatigue or deterioration of material for the related parts. 
     In the acceleration device  2  of the present embodiment, a forward end of the hysteresis-side rotating member  58  (that is, the hysteresis-side spring holding portion  55 ) is so configured as to be broken away from the remaining portion of the hysteresis-side rotating member  58  (that is, the hysteresis-side boss portion  52 ) at the mechanically-weaker portion  500 , when an acting force F 2  larger than a predetermined value is applied to the hysteresis-side spring holding portion  55 . 
     A broken piece  550 , which includes the hysteresis-side spring holding portion  55  broken away from the hysteresis-side boss portion  52 , is pushed to the inner wall surface  150  by the biasing force of the hysteresis spring  49 , as shown in  FIG. 5B . The fourth recessed portion  581  of the broken piece  550  (including the hysteresis-side spring holding portion  55 ) is engaged with the third projection  152  formed in the inner wall surface  150  of the back-side wall portion  15 . Accordingly, the same advantages to those of the first embodiment can be obtained in the acceleration device  2  of the second embodiment. 
     The second embodiment may be also modified in such a way as shown in  FIG. 5C , wherein a third recessed portion  152   a  is formed in the inner wall surface  150  of the back-side wall portion  15 , while a fourth projection  581   a  is formed in the back-side surface  551  of the hysteresis-side rotating member  58 , so that the fourth projection  581   a  is operatively engaged with the third recessed portion  152   a.    
     In the present specification, the third projection  152  and the third recessed portion  152   a  (each of which is formed in the inner wall surface  150 ) are collectively referred to as a third engaging portion, while the fourth recessed portion  581  and the fourth projection  581   a  (each of which is formed in the back-side surface  551  of the hysteresis-side spring holding portion  55 ) are collectively referred to as a fourth engaging portion. 
     Third Embodiment 
     An acceleration device  3  according to a third embodiment of the present disclosure will be explained with reference to  FIGS. 6A and 6B . A shape of a recessed portion and a shape of a projection of the third embodiment are different from those of the first embodiment. Each of  FIGS. 6A and 6B  is likewise a schematically enlarged cross sectional view showing a relevant portion of the third embodiment, which corresponds to the portion IV of  FIG. 2 . 
     In the acceleration device  3  of the present embodiment, a pedal-side rotating member  68  is composed of a pedal-side boss portion  62 , a pedal-side spring holding portion  65  (also referred to as a pedal-biasing-member holding portion  65 ), a full-close side stopper portion  66  (also referred to as the stopper arm  66 ), a mechanically-weaker portion  600  and so on, wherein the pedal-side boss portion  62 , the pedal-side spring holding portion  65 , the stopper arm  66  and the mechanically-weaker portion  600  are integrally formed as one unit. 
     The pedal-side boss portion  62  is arranged between the shaft supporting portion  13  and the second cover member  18 . The pedal-side boss portion  62  is fixed to the outer peripheral surface of the pedal shaft  20 . 
     The pedal-side spring holding portion  65  is arranged in the inner space  11  and extends from the pedal-side boss portion  62  in the radial-upward direction. The pedal-side spring holding portion  65  holds one end of the pedal spring  39 . 
     The stopper arm  66  further extends in the inner space  11  from the pedal-side spring holding portion  65  in the radial-upward direction. When the stopper arm  66  is brought into contact with the inner wall surface  150  of the back-side wall portion  15 , the rotation of the pedal-side rotating member  68  in the pedal closing direction is limited and the pedal-side rotating member  68  is maintained at its acceleration fully-closed position. 
     The pedal-side spring holding portion  65  is connected to the pedal-side boss portion  62  by the mechanically-weaker portion  600 , which is indicated by a two-dot-chain line in  FIG. 6A . 
     A recessed portion  681  is formed in the pedal-side spring holding portion  65  at its back-side surface  651  facing to the inner wall surface  150  of the back-side wall portion  15 , with which the stopper arm  66  is operatively brought into contact. As shown in  FIG. 6A , an inside contacting surface  682  of an upper side of the recessed portion  681  is inclined in a direction from an open side  683  to a bottom side  684  of the recessed portion  681 , so that a depth of the recessed portion  681  is gradually increased in a direction from the pedal-side spring holding portion  65  to the pedal-side boss portion  62  (in a radial-inward direction). The recessed portion  681  is also referred to as the second recessed portion. 
     A projection  153 , which is operatively engaged with the recessed portion  681 , is formed in the inner wall surface  150  of the back-side wall portion  15 . The projection  153  is formed at such a position, which is more separated from the pedal-side boss portion  62  in the radial-upward direction (a radial-outward direction) when compared with a position of the recessed portion  681 . More exactly, a center point of the projection  153  in the radial-outward direction (or a top point of the projection  153 ) is located at a position, which is more separated from a center point of the recessed portion  681  (or a deepest bottom point of the recessed portion  681 ) in the radial-outward direction, when compared the positions of both center points with each other in the radial-outward direction. 
     As shown in  FIG. 6A , an outside contacting surface  154  of an upper side of the projection  153  is inclined in a direction from the top point to a root point of the projection  153 , so that a height of the projection  153  is gradually increased in a direction from the root point to the top point of the projection  153  (in the radial-inward direction). The projection  153  is also referred to as the first projection. 
     In the acceleration device  3  of the present embodiment, the pedal-side spring holding portion  65  is so configured as to be broken away from the pedal-side boss portion  62  at the mechanically-weaker portion  600 , when the stopper arm  66  of the pedal-side rotating member  68  is strongly brought into contact with the inner wall surface  150  of the back-side wall portion  15 . 
     When a broken piece  650  including the pedal-side spring holding portion  65  is pushed by the biasing force of the pedal spring  39  to the inner wall surface  150  of the back-side wall portion  15 , the inside contacting surface  682  of the recessed portion  681  is brought into contact with the outside contacting surface  154  of the projection  153 . Then, the broken piece  650  including the pedal-side spring holding portion  65  is moved in the radial-outward direction along the outside contacting surface  154  of the projection  153 , because the projection  153  is formed at the position more separated from the pedal-side boss portion  62  in the radial-outward direction when compared with the position of the recessed portion  681 . 
     As a result, a relatively large gap  601  is formed between the broken piece  650  including the pedal-side spring holding portion  65  and the pedal-side boss portion  62 , as shown in  FIG. 6B . 
     In the acceleration device  3  of the present embodiment, the pedal-side boss portion  62  is rotated together with the rotation of the pedal shaft  20 , even after the pedal-side spring holding portion  65  is broken away from the pedal-side boss portion  62 . Since the broken piece  650  is held at such a position of the inner wall surface  150  with the gap  601 , the pedal-side boss portion  62  can be continuously rotated without being adversely affected. As above, it is possible to surely rotate the pedal-side boss portion  62  in the third embodiment, in addition to the advantages obtained in the first embodiment. 
     The third embodiment may be also modified in such a way as shown in  FIG. 6C , wherein a first recessed portion  153   a  is formed in the inner wall surface  150  of the back-side wall portion  15 , while a second projection  681   a  is formed in the back-side surface  651  of the pedal-side rotating member  68 , so that the second projection  681   a  will be engaged with the first recessed portion  153   a  when the pedal-side spring holding portion  65  is broken away from the pedal-side boss portion  62 . As is further shown in  FIG. 6C , an inside contacting surface  153   b  of a lower side of the first recessed portion  153   a  is inclined in a direction from an open side to a bottom side of the first recessed portion  153   a , so that a depth of the first recessed portion  153   a  is gradually increased in the radial-outward direction. In addition, an outside contacting surface  681   b  of a lower side of the second projection  681   a  is inclined in a direction from a root point to a top point of the second projection  681   a , so that a height of the second projection  681   a  is gradually increased in the radial-outward direction. 
     In the present specification, the first projection  153  and the first recessed portion  153   a  (each of which is formed in the inner wall surface  150 ) are also collectively referred to as the first engaging portion, while the second recessed portion  681  and the second projection  681   a  (each of which is formed in the back-side surface of the pedal-side spring holding portion  65 ) are also collectively referred to as the second engaging portion. 
     Fourth Embodiment 
     An acceleration device  4  according to a fourth embodiment of the present disclosure will be explained with reference to  FIGS. 7A and 7B . A shape of a recessed portion formed in a hysteresis-side rotating member and a shape of a projection formed in a supporting body of the fourth embodiment are different from those of the second embodiment. Each of  FIGS. 7A and 7B  is likewise a schematically enlarged cross sectional view showing a relevant portion of a hysteresis-side rotating member  78  of the fourth embodiment, which corresponds to the portion IV of  FIG. 2 . 
     In the acceleration device  4 , the hysteresis-side rotating member  78  is composed of a hysteresis-side boss portion  72 , hysteresis-side spring holding portion  75  (also referred to as the hysteresis-biasing-member holding portion), a mechanically-weaker portion  700 , wherein the hysteresis-side boss portion  72 , the hysteresis-side spring holding portion  75  and the mechanically-weaker portion  700  are integrally formed as one unit. 
     The hysteresis-side boss portion  72  is arranged between the pedal-side boss portion  32  and the inner wall of the second cover member  18  and at the radial-outside position of the pedal shaft  20 . The hysteresis-side boss portion  72  is formed in an annular shape and rotatable relative to the pedal shaft  20  and the pedal-side boss portion  32 . In addition, the hysteresis-side boss portion  72  is movable in the axial direction of the pedal shaft  20  with respect to the pedal-side boss portion  32 , so that the hysteresis-side boss portion  72  is moved closer to or more separated from the pedal-side boss portion  32 . 
     The hysteresis-side spring holding portion  75  is arranged in the inner space  11  and extends from the hysteresis-side boss portion  72  in the radial-upward direction. The hysteresis-side spring holding portion  75  holds one end of the hysteresis spring  49  via the spring supporting member  455 . 
     The mechanically-weaker portion  700  corresponds to a portion of the hysteresis-side rotating member  78 , which is indicated by a two-dot-chain line in  FIG. 7A . The mechanically-weaker portion  700  connects the hysteresis-side spring holding portion  75  to the hysteresis-side boss portion  72 . 
     A recessed portion  781  is formed in the hysteresis-side spring holding portion  75  at its back-side surface  751  facing to the inner wall surface  150  of the back-side wall portion  15 . As shown in  FIG. 7A , an inside contacting surface  782  of an upper side of the recessed portion  781  is inclined in a direction from an open side  783  to a bottom side  784  of the recessed portion  781 , so that a depth of the recessed portion  781  is gradually increased in a direction from the hysteresis-side spring holding portion  75  to the hysteresis-side boss portion  72  (in the radial-inward direction). The recessed portion  781  is also referred to as the fourth recessed portion. 
     A projection  155 , which is operatively engaged with the recessed portion  781 , is formed in the inner wall surface  150  of the back-side wall portion  15 . The projection  155  is formed at such a position, which is more separated from the hysteresis-side boss portion  72  in the radial-upward direction (the radial-outward direction) when compared with a position of the recessed portion  781 . More exactly, a center point of the projection  155  in the radial-outward direction (or a top point of the projection  155 ) is located at a position, which is more separated from a center point of the recessed portion  781  (or a deepest bottom point of the recessed portion  781 ) in the radial-outward direction, when compared the positions of both center points with each other in the radial-outward direction. 
     As shown in  FIG. 7A , an outside contacting surface  156  of an upper side of the projection  155  is inclined in a direction from the top point to a root point of the projection  155 , so that a height of the projection  155  is gradually increased in a direction from the root point to the top point of the projection  155  (in the radial-inward direction). In other words, the height of the projection  155  is decreased in the radial-outward direction from the top point to the root point of the projection  155  (that is, a direction away from the hysteresis-side boss portion  72 ). The projection  155  is also referred to as the third projection. 
     In the acceleration device  4  of the present embodiment, the hysteresis-side spring holding portion  75  is so configured as to be broken away from the hysteresis-side boss portion  72  at the mechanically-weaker portion  700 , when the hysteresis-side rotating member  78  will be broken due to an excess outside force applied thereto. 
     When a broken piece  750  including the hysteresis-side spring holding portion  75  is pushed by the biasing force of the hysteresis spring  49  to the inner wall surface  150  of the back-side wall portion  15 , the inside contacting surface  782  of the recessed portion  781  is brought into contact with the outside contacting surface  156  of the projection  155 . Then, the broken piece  750  including the hysteresis-side spring holding portion  75  is moved in the radial-outward direction along the outside contacting surface  156  of the projection  155 , because the projection  155  is formed at the position more separated from the hysteresis-side boss portion  72  in the radial-outward direction when compared with the position of the recessed portion  781 . 
     As a result, a relatively large gap  701  is formed between the broken piece  750  including the hysteresis-side spring holding portion  75  and the hysteresis-side boss portion  72 , as shown in  FIG. 7B . 
     In the acceleration device  4  of the present embodiment, the hysteresis-side boss portion  72  is rotatable without being adversely affected by the broken piece  750  (which is held at such a position of the inner wall surface  150  with the gap  701 ), even after the hysteresis-side spring holding portion  75  is broken away from the hysteresis-side boss portion  72 . As above, it is possible to surely rotate the hysteresis-side boss portion  72  in the fourth embodiment, in addition to the advantages obtained in the second embodiment. 
     The fourth embodiment may be also modified in such a way as shown in  FIG. 7C , wherein a third recessed portion  155   a  is formed in the inner wall surface  150  of the back-side wall portion  15 , while a fourth projection  781   a  is formed in the back-side surface  751  of the hysteresis-side rotating member  78 , so that the fourth projection  781   a  is operatively engaged with the third recessed portion  155   a . As is further shown in  FIG. 7C , an outside contacting surface  155   b  of a lower side of the third recessed portion  155   a  is inclined in a direction from an open side to a bottom side of the third recessed portion  155   a , so that a depth of the third recessed portion  155   a  is gradually increased in the radial-outward direction. In addition, an inside contacting surface  781   b  of a lower side of the fourth projection  781   a  is inclined in a direction from a root point to a top point of the fourth projection  781   a , so that a height of the fourth projection  781   a  is gradually increased in the radial-outward direction. 
     In the present specification, the third projection  155  and the third recessed portion  155   a  (each of which is formed in the inner wall surface  150 ) are also collectively referred to as the third engaging portion, while the fourth recessed portion  781  and the fourth projection  781   a  (each of which is formed in the back-side surface  751  of the hysteresis-side spring holding portion  75 ) are also collectively referred to as the fourth engaging portion. 
     Fifth Embodiment 
     An acceleration device  5  according to a fifth embodiment of the present disclosure will be explained with reference to  FIGS. 8A and 8B . The fifth embodiment is different from the first embodiment in that the supporting body  10  of the fifth embodiment has a relatively large recessed portion in the inner wall surface of the back-side wall portion for accommodating a part of a broken piece. Each of  FIGS. 8A and 8B  is likewise a schematically enlarged cross sectional view showing a relevant portion of a pedal-side rotating member  88  of the fifth embodiment, which corresponds to the portion IV of  FIG. 2 . 
     In the acceleration device  5  of the present embodiment, the pedal-side rotating member  88  is composed of a pedal-side boss portion  82 , a pedal-side spring holding portion  85  (also referred to as a pedal-biasing-member holding portion  85 ), a full-close side stopper portion  86  (also referred to as the stopper arm  86 ), a mechanically-weaker portion  800  and so on, wherein the pedal-side boss portion  82 , the pedal-side spring holding portion  85 , the stopper arm  86  and the mechanically-weaker portion  800  are integrally formed as one unit. 
     The pedal-side boss portion  82  is arranged between the shaft supporting portion  13  and the second cover member  18 . The pedal-side boss portion  82  is fixed to the outer peripheral surface of the pedal shaft  20 . 
     The pedal-side spring holding portion  85  is arranged in the inner space  11  and extends from the pedal-side boss portion  82  in the radial-upward direction. The pedal-side spring holding portion  85  holds one end of the pedal spring  39 . 
     The stopper arm  86  further extends in the inner space  11  from the pedal-side spring holding portion  85  in the radial-upward direction. When the stopper arm  86  is brought into contact with the inner wall surface  150  of the back-side wall portion  15 , the rotation of the pedal-side rotating member  88  in the pedal closing direction is limited and the pedal-side rotating member  88  is maintained at the acceleration fully-closed position. 
     The pedal-side spring holding portion  85  is connected to the pedal-side boss portion  82  by the mechanically-weaker portion  800 , which is indicated by a two-dot-chain line in  FIG. 8A . 
     In the acceleration device  5  of the present embodiment, a relatively large recessed portion  831  is formed in an inner wall surface  830  of a back-side wall portion  83  of the supporting body  10 , instead of a recessed portion formed in the pedal-side rotating member to be engaged with a projection formed in the back-side wall portion. The recessed portion  831  is also referred to as a first accommodating space. As shown in  FIG. 8A , an inside contacting surface  832  of a lower side of the recessed portion  831  is inclined in such a manner that a depth thereof between an open side  833  to a bottom side is gradually increased in the radial-outward direction (that is, a direction away from the pedal-side boss portion  82  to the stopper arm  86 ). 
     In the acceleration device  5  of the present embodiment, the pedal-side spring holding portion  85  is so configured as to be broken away from the pedal-side boss portion  82  at the mechanically-weaker portion  800 , when the stopper arm  86  of the pedal-side rotating member  88  is strongly brought into contact with the inner wall surface  830  of the back-side wall portion  83  and thereby the pedal-side rotating member  88  is broken due to the excess outside force applied thereto. 
     When a broken piece  850  including the pedal-side spring holding portion  85  is pushed by the biasing force of the pedal spring  39  to the inner wall surface  830  of the back-side wall portion  83 , the broken piece  850  is brought into contact with the inside contacting surface  832  of the recessed portion  831 . Then, the broken piece  850  including the pedal-side spring holding portion  85  is moved in the radial-outward direction along the inside contacting surface  832  of the recessed portion  831 , because the inside contacting surface  832  is inclined in the radial-outward direction. 
     As a result, a part of the broken piece  850  is accommodated in the recessed portion  831  (the first accommodating space  831 ) and a relatively large gap  801  is formed between the broken piece  850  including the pedal-side spring holding portion  85  and the pedal-side boss portion  82 , as shown in  FIG. 8B . 
     In the acceleration device  5  of the present embodiment, since the broken piece  850  is held at such a position of the inner wall surface  830  with the gap  801 , the pedal-side boss portion  82  can be surely rotated without being adversely affected by the broken piece  850 . 
     Sixth Embodiment 
     An acceleration device  6  according to a sixth embodiment of the present disclosure will be explained with reference to  FIGS. 9A and 9B . The sixth embodiment is different from the second embodiment in that the supporting body  10  of the sixth embodiment has a relatively large recessed portion in the inner wall surface of the back-side wall portion for accommodating a part of a broken piece. Each of  FIGS. 9A and 9B  is likewise a schematically enlarged cross sectional view showing a relevant portion of a hysteresis-side rotating member  98  of the sixth embodiment, which corresponds to the portion IV of  FIG. 2 . 
     In the acceleration device  6 , the hysteresis-side rotating member  98  is composed of a hysteresis-side boss portion  92 , a hysteresis-side spring holding portion  95  (also referred to as the hysteresis-biasing-member holding portion), a mechanically-weaker portion  900 , wherein the hysteresis-side boss portion  92 , the hysteresis-side spring holding portion  95  and the mechanically-weaker portion  900  are integrally formed as one unit. 
     The hysteresis-side boss portion  92  is arranged between the pedal-side boss portion  32  and the inner wall of the second cover member  18  and at the radial-outside position of the pedal shaft  20 . The hysteresis-side boss portion  92  is formed in an annular shape and rotatable relative to the pedal shaft  20  and the pedal-side boss portion  32 . In addition, the hysteresis-side boss portion  92  is movable in the axial direction of the pedal shaft  20  with respect to the pedal-side boss portion  32 , so that the hysteresis-side boss portion  92  is moved closer to or more separated from the pedal-side boss portion  32 . 
     The hysteresis-side spring holding portion  95  is arranged in the inner space  11  and extends from the hysteresis-side boss portion  92  in the radial-outward direction. The hysteresis-side spring holding portion  95  holds one end of the hysteresis spring  49  via the spring supporting member  455 . 
     The mechanically-weaker portion  900  corresponds to a portion of the hysteresis-side rotating member  98 , which is indicated by a two-dot-chain line in  FIG. 9A . The mechanically-weaker portion  900  connects the hysteresis-side spring holding portion  95  to the hysteresis-side boss portion  92 . 
     In the acceleration device  6 , a relatively large recessed portion  931  is formed at an inner wall surface  930  of a back-side wall portion  93  of the supporting body  10 , instead of a recessed portion formed in the hysteresis-side rotating member to be engaged with a projection formed in the back-side wall portion. As shown in  FIG. 9A , an inside contacting surface  932  of a lower side of the recessed portion  931  is inclined in such a manner that a depth thereof between an open side  933  to a bottom side is gradually increased in the radial-outward direction (that is, a direction away from the hysteresis-side boss portion  92  to the hysteresis-side spring holding portion  95 ). 
     In the acceleration device  6  of the present embodiment, the hysteresis-side spring holding portion  95  is so configured as to be broken away from the hysteresis-side boss portion  92  at the mechanically-weaker portion  900 , when the hysteresis-side rotating member  98  is broken. 
     When a broken piece  950  including the hysteresis-side spring holding portion  95  is pushed by the biasing force of the hysteresis spring  49  to the inner wall surface  930  of the back-side wall portion  93 , the broken piece  950  is brought into contact with the inside contacting surface  932  of the recessed portion  931 . Then, the broken piece  950  including the hysteresis-side spring holding portion  95  is moved in the radial-outward direction along the inside contacting surface  932  of the recessed portion  931 , because the inside contacting surface  932  is inclined in the radial-outward direction. 
     As a result, a part of the broken piece  950  is accommodated in the recessed portion  931  (a second accommodating space) and a relatively large gap  901  is formed between the broken piece  950  including the hysteresis-side spring holding portion  95  and the hysteresis-side boss portion  92 , as shown in  FIG. 9B . 
     In the acceleration device  6  of the present embodiment, since the broken piece  950  is held at such a position of the inner wall surface  930  with the gap  901 , the hysteresis-side boss portion  92  can be surely rotated without being adversely affected by the broken piece  950 , in addition to the advantages of the second embodiment. 
     Further Embodiments and/or Modifications 
     (1) In the above embodiments, the mechanically-weaker portion ( 300 ,  500 ,  600 ,  700 ,  800  or  900 ) is formed in either the pedal-side rotating member ( 38 ,  68 ,  88 ) or the hysteresis-side rotating member ( 58 ,  78 ,  98 ). However, the mechanically-weaker portions may be formed in both of the pedal-side rotating member and the hysteresis-side rotating member. 
     (2) In the above first and the third embodiments, the pedal-side rotating member ( 38 ,  68 ) has the second recessed portion ( 381 ,  681 ), while the supporting body ( 10 ) has the first projection ( 151 ,  153 ). On the other hand, in the above second and the fourth embodiments, the hysteresis-side rotating member ( 58 ,  78 ) has the fourth recessed portion ( 581 ,  781 ), while the supporting body ( 10 ) has the third projection ( 152 ,  155 ). However, as already explained with reference to  FIGS. 4C, 5C, 6C and 7C , a projection may be formed in the pedal-side or the hysteresis-side rotating member and a recessed portion (with which the projection is engaged) may be formed in the supporting body. In this case, the recessed portion may be formed at such a position of the supporting body, which is more separated from a pedal-side or a hysteresis-side boss portion than a position of the projection formed in the pedal-side or the hysteresis-side rotating member. 
     (3) In the above third embodiment, the inside contacting surface ( 682 ) of the upper side in the second recessed portion ( 681 ), which is formed in the pedal-side rotating member ( 68 ), is inclined from the open side ( 683 ) toward the bottom side ( 684 ) in the direction from the pedal-side spring holding portion ( 65 ) to the pedal-side boss portion ( 62 ) (in the radial-inward direction). In addition, the outside contacting surface ( 154 ) of the upper side in the first projection ( 153 ), which is formed in the supporting body ( 10 ), is inclined from the top point toward the root point in the direction away from the pedal-side boss portion ( 62 ) (in the radial-outward direction). In the above fourth embodiment, the inside contacting surface ( 782 ) of the upper side in the fourth recessed portion ( 781 ), which is formed in the hysteresis-side rotating member ( 78 ), is inclined from the open side ( 783 ) toward the bottom side ( 784 ) in the direction from the hysteresis-side spring holding portion ( 75 ) to the hysteresis-side boss portion ( 72 ) (in the radial-inward direction). In addition, the outside contacting surface ( 156 ) of the upper side in the third projection ( 155 ), which is formed in the supporting body ( 10 ), is inclined from the top point toward the root point in the radial-outward direction away from the hysteresis-side boss portion ( 72 ). 
     However, a relation between the recessed portion and the projection is not limited to the relations of the above embodiments. For example, an inclined surface may be formed either in the recessed portion or in the projection, so that the broken piece ( 650 ,  750 ) including the pedal-side or the hysteresis-side spring holding portion ( 65 ,  75 ) is moved in the radial-outward direction. 
     (4) In the first and the third embodiments, the pedal-side spring holding portion ( 35 ,  65 ) has the second recessed portion ( 381 ,  681 ) to be engaged with the first projection ( 151 ,  153 ) formed in the back-side wall portion ( 15 ). A position at which the second recessed portion is formed is not limited to the position of the above embodiments. The second recessed portion may be formed at any location of the pedal-side spring holding portion (the broken piece), which is so configured as to be broken away from the pedal-side boss portion, on a side of the broken piece opposing to the inner wall surface of the back-side wall portion. 
     (5) In the above first, the third or the fifth embodiment, the hysteresis mechanism ( 40 ) is provided. The present disclosure, however, can be applied to such an acceleration device having no hysteresis mechanism. 
     The present disclosure should not be limited to the above embodiments and/or modifications, but can be further modified in various manners without departing from a spirit of the present disclosure.