Abstract:
A hook member of a vehicle seat lock engages with a striker fixed to a vehicle body. An opening lever is pivotally secured to the first base plate to constitute the first subassembly. The hook member is pivotally secured to the second base plate to constitute the second subassembly. The first base plate is coupled to the second base plate. Between the first and second base plates, a sensing member is pivotally secured in parallel with the hook member to detect invasion of the striker in the hook member.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a vehicle seat lock to hold the seat back of a chair in a standing position, the seat back being capable of standing up or falling down. 
     JP2003-146126A discloses a vehicle seat lock comprising a hook member mounted to the seat back supported to a vehicle body to stand up and fall down, and an opening lever operated by an operating device at a shoulder of the seat back, the hook member and opening lever being pivotally secured on each pivot between a base plate and a cover plate fixed to the seat back. 
     In the vehicle seat back, the hook member, the pivot on which the hook member is secured and a spring for forcing the hook member are assembled in place when the base plate is joined to the cover plate. Thus, several assembling steps are necessary before the final step in which the base plate is joined to the cover plate, which makes assembling more complicated and reduces efficiency. 
     JP2001-130295A discloses a vehicle seat lock comprising a hook member mounted to the seat back and engagable with a striker of a vehicle body, and an opening lever connected to an operating knob at the seat back. By the operating knob, the opening lever is operated to allow the hook member to disengage from the striker thereby enabling the seat back to move from a standing position to a falling position. 
     However, in the vehicle seat lock, a structure that is capable of detecting whether or not the hook member is engaged with the striker is added which makes the structure more complicated and increases its size. It is likely to make it more difficult for the vehicle seat lock to be positioned in a small space of the seat back. 
     SUMMARY OF THE INVENTION 
     In view of the disadvantages in the prior art, it is an object of the present invention to provide a vehicle seat lock enabling assembling efficiency to improve. 
     It is another object of the present invention to provide a vehicle seat lock allowing it to be simplified and reduced in size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will become more apparent from the following description with respect to embodiments as shown in the accompanying drawings wherein: 
         FIG. 1  is a perspective view of a rear seat for a vehicle including the present invention; 
         FIG. 2  is an exploded perspective view of a locking assembly; 
         FIG. 3  is a rear perspective view of the locking assembly provisionally assembled; 
         FIG. 4  is a rear perspective view of the locking assembly; 
         FIG. 5  is a side view of the locking assembly; 
         FIG. 6  is a side view of the locking assembly; 
         FIG. 7  is a horizontal sectional view taken along the line VII-VII in  FIG. 5 ; 
         FIG. 8  is a vertical sectional view taken along the line VIII-VIII in  FIG. 6 ; 
         FIG. 9  is a horizontal sectional view taken along the line IX-IX in  FIG. 6 ; 
         FIG. 10  is an enlarged view of a hook member; 
         FIG. 11  is a view showing an action of main parts when an operating knob is in the second rest position; 
         FIG. 12  is a view showing an action of main parts when the operating knob is in the first rest position; 
         FIG. 13  s a view showing an action of main parts when the operating knob is in the first operating position; 
         FIG. 14  is a view showing an action of main parts when the operating knob is in the second rest position; and 
         FIG. 15  is a view showing an action of main parts when the operating knob is in the first rest position. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in  FIG. 1 , a rear seat  1  comprises a seat cushion  2  mounted on a body floor, and a seat back  3  which is supported by a hinge shaft (not shown) at the rear end of the seat cushion  2  to enable the seat back  3  to fall down on the seat cushion  2   
     In the seat back  3 , a locking assembly  4  is provided below a shoulder part close to a body panel and engages with a metal striker  5  fixed to the body panel to enable the seat back  3  to be held at the first standing position by a solid line and at the second standing position reclined backwards of the first standing position by a dotted line and to enable the seat back  3  to fall down forwards with operation of an operating device  6  in the shoulder part of the seat back  3 . 
     The striker  5  comprises a base  53  fixed to the body panel. facing the side of the seat back  3 , and a U-shaped engaging member  54  comprising the first engagement portion  51  and the second engagement portion  52 . 
     In  FIG. 4 , the locking assembly  4  comprises the first metal base member  7  fixed to a seat frame  31  in  FIG. 4  in the seat back  3  with a bolt (not shown), and the second metal base member  11  fixed to the side of the first metal member  7  facing an outside. The first and second base members  7 , 11  have striker-engaging grooves  71 , 111  in which the first and second engagement portions  51 , 52  of the striker  5  engage forwards when the seat back  3  stands up. 
     Between the first base member  7  and the second base member  11 , there are disposed a hook member  8  containing metal as matrix to engage with the first or second engagement portions  51 , 52  of the striker  5  to allow the seat back  3  to be held at the first or second standing position, a synthetic-resin opening lever  9  operated by the operating device  6 , and a synthetic-resin sensing member  10  movable depending on engagement of the striker  5  and the hook member  8 . 
     To a metal mounting bracket  43  fixed on the first base member  7 , an operating device  6  and a bell crank  23  are mounted. The mounting bracket  43  may be molded together with the first base member  7 . The operating device  6  may be mounted to the second base member  11 . 
     As shown in  FIG. 4 , above the striker-engagement grooves  71 , 111  of the first and second base members  7 , 11 , there are provided the first fixing member  73  and the second fixing member  112  connected to each other with a bolt  18 . 
     As shown in  FIG. 7 , a burring  74  that cylindrically projects rearwards or downwards in  FIG. 7 , from the first fixing portion  73 . On the inner circumferential surface of the burring  74 , a female thread is formed to engage with the bolt  18 . 
     In the second fixing portion  112  on the rear surface of the first fixing portion  73 , there is an engagement hole  113  in which the burring  74  engages. 
     To connect the first base member  7  to the second base member  11 , the burring  74  of the first fixing portion  73  engages in the engagement hole  113  of the second fixing portion  112 . In this condition, the bolt  18  is screwed in the burring  74 . As shown in  FIG. 7 , the second fixing portion  112  is held between a head  181  of the bolt  18  and the first fixing portion  73 . By connecting opening sides of the striker-engaging grooves  71 , 111  of the first and second base members  7 , 11 , deformation of the first and second base members  7 , 11  can be effectively prevented when high load acts to the seat back  3 . As a result, by preventing deformation of the hook member  8 , locking strength of the locking assembly  4  can be improved. 
     The first base member  7  is connected to the second base member  11  after the opening lever  9 , the sensing member  10  and the bell crank  23  are mounted to the first base member  7  and after the hook member  8  is mounted to the second base member  11 . 
       FIG. 8  is a vertical sectional view taken along the line VIII-VIII in  FIG. 6 ;  FIG. 9  is a horizontal sectional view taken along the line IX-IX;  FIG. 10  is an enlarged view of the hook member  8 ; and  FIGS. 11-15  are views for showing operation. 
     In  FIGS. 11-15 , the operating device  6 , the bell crank  23 , the hook member  8 , the opening lever  9  and the sensing member  10  are on the same surface. Thus, in the operating device  6  and the bell crank  23 , the front of the figures is deemed as “rear side”, and the back is deemed as “front side”. In the hook member  8 , the opening lever  9  and sensing member  10 , the left side of the figures is deemed as “rear side” and the right side is deemed as “front side”. 
     The hook member  8  is molded with synthetic resin on the outer circumferential surface of metal. A pivot portion  88  is pivotally secured on the first pivot  12  in the striker-engaging groove  111  of the second base member  11  to enable it to move up and down. The pivot portion  88  is forced by the first spring  13  to go down. The first pivot  12  is positioned on an extension of movement of the striker  5 . 
     As shown in  FIG. 6 , the first spring  13  has one end  131  which engages with the hook member  8  and the other end  132  which engages with a projection  114  of the second base member  11 . As shown in  FIGS. 8 and 9 , the first pivot  12  has a larger-diameter portion  121  between the first base member  7  and the second base member  11 . 
     The hook member  8  has the first engagement groove  81  opening downwards and capable of selectively engaging with the first and second engagement portions  51 , 52 ; and the second engagement groove  82  capable of engaging with the first engagement portion  51 . The first arm  84  having an inclined edge  83  is provided in the hook member  8  and the second arm  85  is provided between the first engagement groove  81  and the second engagement groove  82 . From the side of the hook member  8 , a protrusion  86  projects towards the cabin. 
     When the hook member  8  disengages from the striker  5  or when the seat back  3  folds down, it is held horizontally in a standby position in  FIGS. 6 and 11 . When the first engagement portion  51  or the second engagement portion  52  of the striker  5  engages in the first engagement groove  81 , the hook member  8  is held in an engagement position where it slightly rotates upwards from the standby position in  FIG. 12  or  15 . According to operation of the operating device  6  to the first operating position (C) below, the hook member  8  rotates in an unlocking direction or a clockwise direction in FIGS.  6  and  11 - 15  to shift to the first-step unlocking position in  FIG. 13  in which the first engagement portion  51  is releasable from the first engagement groove  81  only rearwards. 
     Furthermore, according to operation of the operating device  6  to the second operating position (D) below, the hook member  8  rotates upwards from the first-step unlocking position and enables it to shift to the second-step unlocking position in  FIG. 14  in which the first engagement portion  51  can engage in the second engagement groove  82  from the first engagement groove  81 . 
     When the seat back  3  is held in the first standing position, the first engagement portion  51  of the striker  5  engages in the first engagement groove  81  of the hook member  8 , and when it is held in the second standing position, the second engagement portion  52  engages in the first engagement groove  81  and the first engagement portion  51  puts in the second engagement groove  82  with play. 
     The first and second arms  84 , 85  project downwards to put on the inner side surface of the second base member  11 , and the lower end of the second arm  85  is allowed to contact the lower surface of the second base member  11  when the hook member  8  is in the standby position. 
     As shown in  FIG. 10 , the first engagement groove  81  is formed like a wedge to reduce a width upwards gradually. A width (W 1 ) is set to put the striker  5  between the engagement portions  51  and  52  without loosening. Thus, the first engagement groove  81  tightly fits in the first engagement portions  51  and  52  thereby preventing the seat back  3  from moving back and forth. 
     A receiving edge  8   a  of the first engagement groove  81  contacts the first and second engagement portions  51 , 52  of the striker  5  when the seat back  3  is leaned backwards. On the rear edge of the second arm  85  which comprises the receiving edge  8   a  and a contact portion  8   b  continuous with the receiving edge  8   a,  metal matrix is exposed without synthetic resin molding. 
     The front edge of the first arm  84  comprises the inclined edge  83  of the first arm  84  and a coming-out preventing edge  8   c  which faces the receiving edge  8   a  of the first engagement groove  81 . The coming-out-preventing edge  8   e  contacts the first and second engagement portions  51 , 52  when the seat back  3  falls down and folds on the seat. The front edge is molded with synthetic resin. 
     Immediately before the first or second engagement portion  51  or  52  of the striker  5  engages in the first engagement groove  81  of the hook member  8 , metal parts contact each other. Even though the hook member  8  is molded with synthetic resin, deformation of synthetic resin over the parts does not prevent the hook member  8  from moving out. Thus, the first engagement portion  51  does not engage in the second engagement groove  82  beyond the first engagement groove  81 . 
     When the first and second engagement portions  51 , 52  engage in the first engagement groove  81 , the metal-exposing receiving edge  8   a can receive weight of the leaning seat back  3  when a passenger sits down on the rear seat  1 . Even though the hook member  8  is molded with synthetic resin, fast collapse or deformation of the synthetic resin is prevented thereby assuring stable engagement of the hook member  8  with the striker  5  for a long time. 
     When the passenger leans against the seat back  3  or when weight is applied to the seat cushion  2 , one needs to operate an operating knob  21 . Load can be received by the receiving edge  8   a  of the hook member  8 , so that the hook member  8  is unlikely to rotate in an unlocking direction without deformation of synthetic resin. Synthetic resin is not applied to the rear edge or the receiving edge  8   a  and contact portion  8   b  of the second arm  85 . Thus, when the operating knob  21  is operated, synthetic resin is not caught to decrease the change in operation ensuring suitable operation. 
     When the first and second engagement portions  51 , 52  of the striker  5  goes into the striker-engaging grooves  71 , 111  to contact the inclined edge  83  of the hook member  8 , noise is relieved with the synthetic resin part. 
     The second engagement groove  82  is formed between the second arm  85  and the pivot portion  88  pivotally supported by the first pivot  12 . When the first engagement portion  51  of the striker  5  engages in the second engagement groove  82  in the second standing position of the seat back  3 , the width is determined not to allow the first engagement portion  51  to engage with the second engagement groove  82 . 
     A front edge  85   a  of the second arm  85  comprises an arc of a circle around the first pivot  12  as a center. At the rear edge of the pivot portion  88 , there is formed a buffer portion  87  molded with synthetic resin. 
     The front edge  85   a  of the second arm  85  is formed as an arc of a circle around the first pivot  12  as center. When the seat back  3  is held in the second standing position, forward excessive load acts to the seat back  3  via a seat belt connected to the seat back  3 . Even if the second engagement portion  52  is deformed to go out of the first engagement groove  81 , the first engagement portion  51  contacts the front edge  85   a  of the second arm  85  in a substantially perpendicular direction, so that the first engagement portion  51  neither press up the second arm  85  nor rotates the hook member  8  in an unlocking direction. Thus, the present invention affects locking strength even in engagement relationship of the first engagement portion  51  and the second engagement portion  82 , so that the seat back  3  can be held firmly in the second standing position. 
     The buffer portion  87  is positioned out of the movement track L of the first and second engagement portions  51 , 52  of the striker  5  when the hook member  8  is in an engagement position in  FIGS. 12 and 15 . The buffer portion  87  moves into the movement track L when the hook member  8  shifts to the second-step unlocking position in  FIG. 14 . Thus, when the first engagement portion  51  comes into the second engagement groove  82  of the hook member  8 , the first engagement portion  51  contacts the buffer portion  87  to restrict invasion of the striker  5  and to reduce noise when the striker  5  comes in. The hook member  8  rotates from the second-step unlocking position in a direction of engagement or in an anticlockwise direction in FIGS.  6  and  11 - 15  to shift to the engagement position to allow the buffer portion  87  to shift out of the movement track L to go away from the first engagement portion  51 . Without subjecting to variation in distance between the first engagement portion  51  and the second engagement portion  52  of the striker  5  or to errors in a mounted position of the locking assembly  4 , the first engagement groove  81  can surely engage with the second engagement portion  52 . 
     The buffer portion  87  may preferably be molded with the hook member  8  in synthetic resin. Instead, it may be formed separately from the hook member  8 . 
     An opening lever  9  is pivoted with the second pivot  14  above the striker-engaging groove  71  of the first base member  7 , forced in a standby direction or in a clockwise direction in FIGS.  6  and  11 - 15  by the second spring  15  wound on the second pivot  14  and held in the standby position in which a lower acting portion  91  is positioned downwards in  FIGS. 6 ,  11 ,  12  and  15 . 
     The acting portion  91  is between the hook member  8  and sensing member  10  and allowed to contact a projection  86  of the hook member  8  when the opening lever  9  rotates anticlockwise from the standby position. When the opening lever  9  is in the standby position, a certain gap is formed not to contact the acting portion  91  with the projection  86  owing to variation of the mounted positions of the hook member  8 , striker  5  and other parts. 
     As mentioned below, based on the operating knob  21  of the operating device  6 , the opening lever  9  rotates anticlockwise from the standby position to the first operating position in  FIG. 13  to allow the front edge of the acting portion  91  to contact the projection  86  of the hook member  8  to shift the hook member  8  from the engagement position to the first-step unlocking position. The opening lever  9  shifts to the second operating position in  FIG. 14  in which it rotates further anticlockwise from the first operating position to allow the hook member  8  to shift to the second-step unlocking position. 
     The sensing member  10  is pivotally secured on a third pivot  16  above the striker-engaging groove  71  of the first base member  7 ; forced by a third spring  17  wound around the third pivot  16 ; and usually held in the standby position in  FIGS. 6 and 11  in which the upper front portion contacts a stopper  75  of the first base member  7 . 
     The sensing member  10  has at the lower rear end a contact portion  101  which can go on the first and second engagement portions  51 , 52  of the striker  5 . 
     The first engagement portion  51  or second engagement portion  52  of the striker  5  engages in the first engagement groove  81  of the hook member  8  to allow the contact portion  101  to go on the first or second engagement portion  51  or  52  to enable the sensing member  10  to shift to a detecting position to which the standby position rotates upwards in  FIGS. 12-15 . 
     Then, the operating device  6  will be described. As described in  FIG. 4 , the operating device  6  is exposed from the shoulder of the seat back  3  and comprises a rectangular guide member  20  in a mounting hole  431   a  of the mounting bracket  43  and an operating knob  21  which is stored to enable it to slide in the guide member  20  up and down. 
     The operating knob  21  is connected to a connecting shaft  233  fixed to one end of the bell crank  23  and moved with the bell crank  23  between the first rest position A in which the upper surface is substantially coplanar with the upper surface of the guide member  20  in  FIGS. 12 and 15  and the second rest position in which the upper surface is retracted in  FIG. 11 . According to the passenger, the upper surface of the operating knob  21  is shifted to the first operating position C in  FIG. 13  which is retracted further from the second rest position B and the second operating position D in  FIG. 14  which is retracted further from the second operating position C. 
     In the first rest position A, the first or second engagement portion  51  or  52  of the striker  5  engages in the first engagement groove  81  of the hook member and the sensing member  10  is in the detecting position. In the second rest position B, the sensing member  10  is in the standby position and the first and second engagement portions  51 , 52  disengage from the first engagement groove  81  of the hook member  8 . In the first operating position C, the opening lever  9  is moved to the first acting position to allow the hook member  8  to move to the first-step unlocking position. In the second operating position D, the opening lever  9  is moved to the second acting position to allow the hook member  8  to move to the second-step unlocking position. 
     On the upper inner circumferential surface of the guide member  20 , there is an indicator (not shown) comprising a different color from the guide member  20  or a word such as “UNLOCK”. The indicator is hindered when the operating knob  21  is in the first rest position A, while it is exposed when the operating knob  21  is out of the first rest position A, thereby allowing the passenger to be warned that the locking assembly  4  is disengaged from the striker  5  to improve security. 
     The bell crank  23  is pivotally secured on the fourth pivot  22  to the mounting bracket  43  to move up and down, and forced clockwise by a fourth spring  25  wound on the fourth pivot  22  in FIGS.  6  and  11 - 15 . The bell crank  23  is moved together with the operating knob  21  to the first rest position A 1  in  FIGS. 12 and 15 ; the second rest position B 1  in  FIG. 11  to which it rotates anticlockwise from the first rest position AS 1 ; the first operating position C 1  in  FIG. 13  to which it rotates anticlockwise from the second rest position B 1 ; and the second operating position D 1  in  FIG. 14  to which it rotates anticlockwise from the first operating position C 1 . 
     The force of the fourth spring  25  acting to the bell crank  23  is set to be smaller than the force of the third spring  17  acting to the sensing member  10 . 
     The other end of the bell crank  23  has the first elongate opening  231  and second elongate opening  232  extending vertically. 
     In the first elongate opening  231 , the upper end  24   a  of the first connecting rod  24  connected at the lower end to the sensing member  10  engages with play to slide up and down. In the second elongate opening  232 , the upper end  26   a  of the second connecting rod  26  connected at the lower end to the opening lever  9  engages with play to slide up and down. 
     The first elongate opening  231  is set to have a length having a play to the upper end  24   a  of the first connecting rod  24  corresponding to stroke of the bell crank  23  between the first rest position A 1  and the second operating position D 1  with respect to anticlockwise rotation of the bell crank  23  when the operating knob  21  and bell crank  23  are in the first rest position A, A 1 . Thus, motion of the operating knob  21  and bell crank  23  from the first rest position A,A 1  to the second operating position D,D 1  and vice versa is not transmitted to the sensing member  10 . 
     The second elongate opening  232  is set to have a length having a play to the upper end  26 a of the second connecting rod  26  corresponding to stroke of the bell crank  23  between the first rest position A 1  and the second rest position B 1  with respect to anticlockwise rotation of the bell crank  23  when the bell crank  23  is in the first rest position A 1 . Thus, motion of the operating knob  21  and bell crank  23  from the first rest position A,A 1  to the second rest position B,B 1  and vice versa is not transmitted to the opening lever  9 . 
     The operating knob  21  and the bell crank  23  moves from the first rest position A,A 1  to the second rest position B,B 1  below. 
     When the sensing member  10  is in the standby position in  FIG. 11 : 
     While the upper end  24   a  of the first connecting rod  24  contacts the upper end of the first elongate opening  231 , the operating knob  21  and bell crank  23  are held in the second rest position B,B 1 . In this case, force of the third spring  17  acting to the sensing member  10  is set to be greater than force of the fourth spring  25  acting to the bell crank  23 , so that the bell crank  23  does not rotate clockwise or in the first rest position A 1  by the force of the fourth spring  25 . 
     When the sensing member  10  shifts from the standby position to the detecting position from  FIG. 11  to  FIG. 12  or  15 : 
     The sensing member  10  shifts from the standby position to the detecting position, so that the first connecting rod  24  goes down. Following the motion of the sensing member  10 , the bell crank  23  shifts by the force of the fourth spring  25  from the second rest position  81  to the first rest position A 1 . 
     When the sensing member  10  shifts from the detecting position to the standby position in  FIG. 12  or  15  to  FIG. 11   
     The sensing member  10  shifts from the detecting position to the standby position, so that the first connecting rod  25  goes up to allow the upper end  24   a  to contact the upper edge of the first elongate opening  231  to press up the bell crank  23 . Thus, the bell crank  23  shifts to the second rest position B 1  against the force of the fourth spring  25 . 
     Then, it will be described how to assemble the locking assembly  4 . As shown in  FIG. 3 , before the first base member  7  is connected to the second base member  11 , the first subassembly  41  for the first base member  7  and the second subassembly  42  for the second base member  11  are formed. 
     The ends of the pivots  14 , 16  are previously crimped. The opening lever  9  and the sensing member  10  are pivotally connected to the first base member  7  with the pivots  14 , 16  and the springs  15 , 17  are connected, so that the first subassembly  41  is formed. 
     One end of the first pivot  12  or the end which projects from the second base member  11  is crimped and the hook member  8  is pivotally connected to the second base member  11  with the pivot  12 . As shown in  FIGS. 8 and 9 , the hook member  8  is put between the second base member  11  and the larger-diameter portion  121  of the first pivot  12  to enable the hook member  8  to rotate. 
     After the first and second subassemblies  41 , 42  are formed, a positioning projection  76  of the first base member  7  is engaged in a positioning opening  115  of the second base member  11  and positioned. And the other end of the first pivot  12  is put into an axial hole  72  of the first base member  7  and crimped in  FIG. 2 . As mentioned above, the first fixing portion  73  is joined to the second fixing portion  112  with a bolt  18  in a direction perpendicular to the pivots  12 , 14 , 16 . The first subassembly  41  and the second subassembly  42  are previously formed, which results in efficient assembling of the locking assembly  4 . 
     The first base member  7  is connected to the second base member  11  with the bolt  18  at an opening side of the striker-engaging grooves  71 , 111  with the first pivot  12  at the inner part of the striker-engaging grooves  71 , 111  thereby preventing deformation around the striker-engaging grooves  71 , 111  in which load is gathered and improving lock strength. 
     The hook member  8  is pivotally connected to the second base member  11 , and the opening lever  9  and the sensing member  10  are pivotally connected to the first base member  7 . Thus, the hook member  8  and the sensing member  10  are arranged in parallel between the first base member  7  and the second base member  11 , thereby shortening height of the locking assembly  4 . As a result, the locking assembly  4  can be disposed in a narrow space close to the shoulder of the seat back  3 . Thus, the locking assembly  4  can be disposed at a position spaced from a hinge shaft at maximum thereby reducing moment acting to the locking assembly  4  and improving support strength of the seat back  3 . 
     The hook member  8  can be disposed closer to the body panel to which the striker  5  is fixed thereby shortening the length of the engagement portions  51 , 52  and improving locking strength. 
     The function of the present embodiment will be described: 
     (i) When the seat back  3  is held in the first standing position in  FIG. 12 : 
     The first engagement portion  51  of the striker  5  engages in the first engagement groove  81  of the hook member  8 , In this condition, the first engagement portion  51  contacts the receiving edge  8   a  of the hook member  8 . 
     The contact portion  101  of the sensing member  10  rides on the first engagement portion  51  of the striker  5  to move the detecting position. Thus, the operating knob  21  is held in the first rest position. The passenger visually confirm the first rest position in which the upper surface of the operating knob  21  is coplanar with the upper surface of the guide member  20  thereby easily making sure that the first engagement portion  51  of the striker  5  engages in the first engagement groove  81  of the hook member  8  to allow the seat back  3  to be held surely in the first standing position. 
     (ii) When the seat back  3  is held in the second standing position in  FIG. 15 : 
     The second engagement portion  52  of the striker  5  engages in the first engagement groove  81  of the hook member  8  and the first engagement portion  51  engages in the second engagement portion  82  with play. In this situation, against leaning of the seat back  3 , the second engagement portion  52  contacts the receiving edge  8   a  of the first engagement groove  81 . The first engagement portion  51  is spaced apart from the buffer portion  87  in the second engagement groove  82 . 
     The contact portion  101  of the sensing member  10  rides on the second engagement portion  52  to move to the detecting position, thereby holding the operating knob  21  in the first rest position A. Thus, as well as the above (i), it allows the passenger to visually confirm that the second engagement portion  52  engages in the first engagement groove  81  of the hook member  8  so that the seat back  3  may be held in the second standing position. 
     (iii) When the seat back  3  falls down or when the striker  5  disengages from the hook member  8  of the locking assembly  4  in  FIG. 11 : 
     The hook member  8  is held in the standby position by the force of the first spring  13 ; the sensing member  10  is held in the standby position by the force of the third spring  17 ; and the bell crank  23  is held in the second rest position B,B 1  against the force of the fourth spring  25 . 
     The passenger visually confirms that the upper surface of the operating knob  21  is retracted in the guide member  20  to allow the indicator to be exposed thereby making sure that the striker  5  disengages from the hook member  8  of the locking assembly  4 . 
     (iv) When the seat back  3  is moved from the falling position to the first standing position: 
     When the seat back  3  stands up from the falling position, the first engagement portion  51  of the striker  5  engages in each of the striker-engaging groove  71 , 111  to get in touch with the inclined edge  83  of the hook member  8  and the inclined edge  102  at the end of the sensing member  10  to move the hook member  8  upwards from the standby position against the force of the first spring  13 , to jump up the sensing member  10  against the force of the third spring  17  and to allow the contact portion  101  on the first engagement portion  51  to move to the detecting position. 
     When the first engagement portion  51  contacts the contact portion  8   b  of the second arm  85  of the hook member  8  in  FIG. 13 , the seat back  3  rests at the first standing position and the hook member  8  moves to the engagement position by the force of the first spring  13  to allow the first engagement portion  51  of the striker  5  to engage in the first engagement groove  81  in  FIG. 12 . As a result, the seat back  3  is held in the first standing position and the operating knob  21  moves from the second rest position B to the first rest position A. 
     When the operating knob  21  moves from the second rest position B to the first rest position A, the bell crank  23  is connected to the opening lever  9  with play corresponding to a moving distance between the second rest position B 1  and the first rest position Al of the bell crank  23 . So movement of the operating knob  21  from the second rest position B 1  to the first rest position Al is not transmitted to the opening lever  9 . As a result, even though the operating knob  21  moves to the first rest position A, the hook member  8  can be held in the engagement position. 
     (v) When the seat back  3  is moved from the first standing position to the falling position: 
     The operating knob  21  is pushed from the first rest position A to the first operating position C. Thus, the bell crank  23  rotates from the first rest position Al to the first operating position C 1  against the force of the fourth spring  25  and the hook member  8  rotates from the standby position to the first-step unlocking position via the second connecting rod  26  and the opening lever  9  in  FIG. 13 . Thus, the first engagement portion  51  of the striker  5  becomes releasable from the first engagement groove  81 . In this situation, the seat back  3  is rotated forwards to move to the falling position. 
     When the operating knob  21  is pressed in from the first rest position to the first operating position C, the operating knob  21  is joined to the sensing member  10  such that movement of the bell crank  23  between the first rest position A 1  and the first operating position C 1  is transmitted to the sensing member  10 , so that action of the operating knob  21  is not transmitted to the sensing member  10 . 
     When the seat back  3  moves to the falling position, the operating knob  21  is moved to the second rest position and retracted thereby preventing the operating knob  21  from contacting a front sheet (not shown) to damage the surface of the front sheet. 
     (vi) When the seat back  3  is moved from the first standing position to the second standing position: 
     As shown in  FIG. 14 , the operating knob  21  is pressed into the second operating position D to move the bell crank  23  to the second operating position D 1 . Thus, the hook member  8  is moved to the second-step unlocking position via the second connecting rod  26  and opening lever  9  to enable the first engagement portion  51  of the striker  5  to be released from the first engagement groove  81  of the hook member  8 . In this condition, the seat back  3  is pushed rearwards to allow the first engagement portion  51  to engage in the second engagement groove  82  to release the operation of the operating knob  21 . As shown in  FIG. 15 , the hook member  8  is moved to the engagement position by the force of the first spring  13  to allow the second engagement portion  52  to engage in the first engagement groove  81  and allow the first engagement portion  51  to put in the second engagement groove  82  with play. 
     When the first engagement portion  51  of the striker  5  comes into the second engagement groove  82 , the first engagement portion  51  contacts the buffer portion  87  of the hook member  8  moved to second-step unlocking position thereby relieving contacting sound when the seat back  3  stops in the second standing position and determining a position for stopping the seat back  3 . As a result, the first engagement groove  81  of the hook member  8  can surely engage with the second engagement portion  52 . When the hook member  8  moves to the engagement position, the buffer portion  87  goes away from the first engagement portion  51  and does not bind the first engagement portion  51  thereby allowing the first engagement groove  81  to engage the second engagement portion  52  surely without subjecting to irregularity in size between the first engagement portion  51  and the second engagement portion  52  of the striker  5  and errors in a mounting position of the striker  5  and locking assembly  4 . 
     (vii) When the seat back  3  is moved from the second standing position to the falling position: 
     Similar to the above (vi), the operating knob  21  is pressed into the second operating position D to allow the hook member  8  to move to the second-step unlocking position, so that the second engagement portion  52  of the striker  5  becomes releasable from the first engagement groove  81  of the hook member  8  and the first engagement portion  51  becomes releasable from the second engagement groove  82  to allow the seat back to fall forwards. 
     The foregoing merely relate to embodiment of the invention. Various changes and modifications may be made by a person skilled in the art without departing from the scope of claims wherein: