Abstract:
The seat recliner includes a first seat member. The seat recliner includes a second seat member rotatable relative to the first seat member. The seat recliner includes an input cam member for rotating relative to the first seat member. The seat recliner includes a pivot on the first seat member. The seat recliner includes a follower member for being actuated by the input cam member to lock the second seat member relative to the first seat member. The follower member includes a first side. The first side includes a tooth for meshing with the second seat member. The first side includes an engagement side pivotably engaging on the pivot. The follower member includes a second side in proximity to the engagement side. The seat recliner includes a pair of guides on the first seat member. Guides have the pivot interposed therebetween for cooperating with the pivot to pivotably support the follower member on the pivot. A guide has smaller rigidity than the pivot for retaining the second side. 
     Preferably, the seat recliner includes a reinforcement member interconnecting the pivot and the first seat member. The reinforcement member includes a first support face for supporting the follower member. 
     Preferably, the guide is spaced from the second side for retaining the second side under a force.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a seat recliner which allows a seat back to be rotated relative to a seat cushion, and, more specifically, to one suitable for a vehicle such as an automobile. 
     DESCRIPTION OF RELEVANT ART 
     A conventional kind of a seat recliner has been known. The seat recliner includes a frame mounted to the base plate of a seat cushion. The recliner includes a lid mounted to an arm plate of a seat back. The frame and the lid are rotatable relative to each other. The frame includes a lock tooth with an external gear. The lid includes an internal gear formed on the inner peripheral face. The external and internal gears are meshed to prevent the relative rotation between the frame and the lid. 
     The frame includes a pivot for supporting the lock tooth. The lock teeth allow the external and internal gears to be meshed with each other. 
     The frame includes a guide for guiding the lock tooth to be pivoted on the pivot. 
     SUMMARY OF THE INVENTION 
     The pivot has a base, or a connecting portion with the frame, where maximum load occurs. The greater connecting area of the pivot and the frame is superior in strength. However, a greater connecting area causes the projecting length to be lowered. The contact area of the pivot with the lock tooth is reduced, lowering the pivot in strength. 
     Rotational force is converted into straight external force to be exerted on the lock tooth via the internal and external gears. The pivot and lid retain the lock tooth between them to lock with each other without rotating. 
     The greater the rigidity of the guide is to the external force, that is, the more difficulty with which the guide is resiliently deformed against the external force, the more securely the pivot is backed up, thus improving strength. However, when guide has greater rigidity than the pivot, the pivot starts to be deformed, while the guide is not deformed. Internal force is exerted on the pivot to be deformed. The exertion causes the pivot to be damaged. Thus, the providing of the guide to improve strength obtains insufficient effect. 
     The thickening of the lock tooth or the use of high quality material may improve the lock tooth in rigidity against the internal force. However, this deteriorates formability and causes high productive cost. 
     The invention is directed to a seat recliner, which improves a pivot in strength and reduces in productive cost. 
     The first aspect of the invention provides a seat recliner. The seat recliner includes a first seat member. The seat recliner includes a second seat member rotatable relative to the first seat member. The seat recliner includes an input cam member for rotating relative to the first seat member. The seat recliner includes a pivot on the first seat member. The seat recliner includes a follower member for being actuated by the input cam member to lock the second seat member relative to the first seat member. The follower member includes a first side. The first side includes a tooth for meshing with the second seat member. The first side includes an engagement side pivotably engaging on the pivot. The follower member includes a second side in proximity to the engagement side. The seat recliner includes a pair of guides on the first seat member. Guides have the pivot interposed therebetween for cooperating with the pivot to pivotably support the follower member on the pivot. A guide has smaller rigidity than the pivot for retaining the second side. 
     Preferably, the seat recliner further includes a reinforcement member interconnecting the pivot and the first seat member. The reinforcement member includes a first support face for supporting the follower member. 
     Preferably, the pivot includes a second support face extending from the reinforcement member for supporting the follower member, the second support face disposed rearward of the first support face. 
     Preferably, the follower member includes a first engagement face for engaging the first support face. The follower member includes a second engagement face supported for engaging the second support face. The second engagement face is disposed forward of the first engagement face. 
     Preferably, the first engagement face contacts the first support face. The second engagement face is spaced from the second support face for being brought into contact with the second support face under a force. 
     Preferably, the guide includes a back-up face to contact with the follower member. The back-up face includes a recess. 
     Preferably, the back-up face is reduced in contact area with the follower member, so that the guide has smaller rigidity than the pivot. 
     Preferably, the guide includes a depression positioned differently from the back-up face. 
     Preferably, the back-up face is spaced away from the follower member. 
     The second aspect of the invention provides a seat recliner. The seat recliner includes a first seat member. The seat recliner includes a second seat member rotatable relative to the first seat member. The seat recliner includes an input cam member for rotating relative to the first seat member. The reclining mechanism includes a pivot on the first seat member. The seat recliner includes a follower member for actuated by the input cam member to lock the second seat member relative to the first seat member. The follower member includes a first side. The first includes a tooth for meshing with the second seat member. The first side includes an engagement side pivotably engaging on the pivot. The follower member includes a second side in proximity to the engagement side. The seat recliner includes a pair of guides on the first seat member. Guides have the pivot interposed therebetween for cooperating with the pivot to pivotably support the follower member on the pivot. A guide has smaller rigidity than the pivot for retaining the second side. The guide is spaced from the second side for retaining the second side under a force. 
     Preferably, the follower member includes a third side angularly separated from the first side around the pivot and concentric with the second side for sliding on the guide. 
    
    
     BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
     The above and further objects and novel features of the present invention will more fully appear from the following detailed description when the same is read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an illustrative primary front view showing the seat recliner according to the first embodiment of the invention; 
     FIG. 2 is an illustrative sectional view taken along the II—II line in FIG. 1; 
     FIG. 3 is an illustrative sectional view taken along the III—III line in FIG. 1; 
     FIG. 4 is an illustrative enlarged front view showing the lock tooth in FIG. 1; 
     FIG. 5 is an enlarged view showing the portion circled by the dotted V line in FIG. 4; 
     FIG. 6 is an illustrative front view showing the pivot in FIG. 1; 
     FIG. 7 is an illustrative sectional view of a pivot taken along the VII—VII line in FIG. 6; 
     FIG. 8 is an illustrative primary exploded perspective view of the seat recliner in FIG. 1; 
     FIG. 9 is an illustrative front view of the seat recliner mounted to a seat; 
     FIG. 10 is an illustrative sectional view taken along the X—X line in FIG. 9; 
     FIG. 11 is an illustrative view directed by the arrows of XI in FIG. 10; 
     FIG. 12 is an illustrative primary front view showing the seat recliner according to the second embodiment of the invention; 
     FIG. 13A is an illustrative primary front view of the first guide projection according to the second embodiment of the invention; 
     FIG. 13B is an illustrative sectional view taken along the XIIB—XIIB in FIG. 12A; 
     FIG. 14A is a front view showing the first projection of another embodiment which aims to reduce rigidity; and 
     FIG. 14B is an illustrative sectional view taken along XIIIB—XIIIB line in FIG.  13 A; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Like members are designated by like reference characters. 
     First Embodiment 
     Seat recliner E 1  for a vehicle, as the first embodiment, has frame  10  formed with circular recess  14  having the inner side fitted with recliner E. Recliner E includes lid  60  coaxially rotatable with frame  10 . Lid  60  is disposed along inner peripheral face  14   a  and on the bottom face (standard face) of recess  14 . Recliner E includes pivotable lock tooth  20  and rotary cam plate  40  disposed axially between frame  10  and lid  60 . 
     Lid  60  includes two internal gears  61  facing each other along the inner peripheral face. Two lock teeth  20  are mounted on bottom face  14   b  of frame  10  for pivoting on pivots (support parts)  16 . Lock teeth  20  include respective external gears (tooth)  21  meshable with and facing respective internal gears  61 . The rotation of cam plates  40  about the rotational center hole  42  in one direction (counterclockwise direction) allows cam plates  40  to push respective two lock teeth  20  in a radially outward direction. The pushing allows external and internal gears to be meshed with each other. The rotation in another direction (clockwise direction) allows the meshing to be disengaged. 
     One of frame  10  and lid  60  is linked to seat cushion  150 . The other is linked to seat back  160 . External gear  21  is meshed with internal gear  61 . The meshing retains seat back  160  at a predetermined angle relative to seat cushion  150 . In the embodiment, frames  10  to be fixed are disposed at respective portions on the both left and right sides in the width-direction of seat cushion  150 . Lids  60  are fixed to respective portion on both left and right sides in the width direction of seat back  160 . Operation shafts  30  on the both left and right sides are linked to cylindrical shaft  50  by serration (joints)  32  as described later. 
     Pressed into hole  42  of cam plate  40  is operation shaft  30  to be fixed. Operation shaft  30  includes serrations  32 ,  34  for transmission of rotational force from outside. Serrations  32 ,  34  each have a predetermined position (portion) in a rotational direction in correspondence with the position (portion) of frame  10 . In the state, operation shaft  30  is pressed into hole  42  of cam plate  40 . Serration  32  meshes with serration (joints)  51  formed on the inner side of shaft  50 . Serration  34  meshes with serration  35  formed to linkage hole of operation lever  31 . Serrations  32 ,  34  are identical in position, structure of projection and recess (unevenness) in the peripheral direction. 
     The rotation of cam plate  40  achieves the meshing of external gears  21  with internal gears  61 . With predetermined angular positions of serrations  32 ,  34  corresponding to the positions of frame  10 , insertion shaft portion  30   a  of shaft  30  is pressed into hole  42 . Formed to hole  42  is serration  42   a . Serration  42   a  reduces force for the pressing of insertion portion  30   a . Serration  42   a  is effective to securely retain insertion portion  30   a  after the insertion. 
     Serrations  32 ,  51  are linked to each other by the meshing with an angular looseness in a rotational direction. The looseness causes a large force to be exerted from, for example, a belt anchor on one side such as the left side of seat cushion  150 . At the time, the deformation of strength members of seat cushion  150  allows shaft  30  at one side to be rotated. The rotation is transmitted to operation shaft  30  at the other side by shaft  50 . The looseness is set at a minimum angle to prevent the release of the meshing of external gear  21  and internal gear  61 . 
     Lock teeth  20  are arranged at respective positions angularly apart by 180 degrees in a peripheral direction of frame  10 . Lock cam faces  41  are formed at the positions angularly apart by 180 degrees around hole (rotational axis)  42  of cam plate  40 . Cam plate  40  includes outer peripheral restriction faces  45 , which are positioned on L 2  line substantially perpendicular to L 1  line connecting respective lock cam face  41  and angularly apart by 180 degrees around center hole  42 . Frame  10  includes later-described first guide projections (guide part)  11 A, which are positioned on L 4  line substantially perpendicular to L 3  line connecting respective locked cam face  25  of respective lock teeth  20  and angularly apart by 180 degrees around center hole  42 . First guide projections  11 A include sliding faces  11 C to slide against outer peripheral faces  45 . Respective outer peripheral faces  45  are defined by arc-shaped curved faces about center hole  42 . Outer faces  45  sliding faces  11   c  of projections  11 A within the rotational range of cam plate  40 . Sliding faces  11   c  are formed as a plane substantially parallel to the line connecting respective cam faces  25 . Sliding faces  11   c  allow cam plate  40  to be moved toward cam faces  25 . 
     Frame  10  includes two semi-cylindrical pivots  16  in the vicinity of internal gears  61  thereon, which pivotably support lock teeth  20 . Respective pivots  16  include arced curved parts directed to the rotational center (axis). Lock teeth  20  include semi-circular recessed bearings (portion to be supported)  23 , which pivotably engage on arced curved faces of pivots  16 . Formed in opposition to external gears  21  relative to bearings  23  are arced retaining faces  24  for sliding, which are coaxial with bearings  23 . 
     Frame  10  includes the first projections  11 A, which abut against retaining faces  24  under external force P for extra retention. Relative rotational (tangential) force between frame  10  and lid  60  is exerted on pivots  16  by the meshing of internal gears  61  and external gears  21 , and lock teeth  20 , thus occurring external force (load) P. First projections  11 A have a rigidity against external force P not greater (less) than pivots  16 . When a displacement by external force P is transmitted in identical amounts from lock teeth  20  to first projections  11 A and to pivots  16 , first projection  11 A facilitates flex equal to or more than pivots  16 . 
     In the embodiment, Recesses  11   e  are formed on back-up faces  11   d  for retaining faces  24 . Recesses  11   e  act to lower first projections  11 A in rigidity against external force P. The formation of recesses  11   e  decreases the area of contact of back-up faces  11   d  with retaining faces  24 , thus lowering first projections  11 A in rigidity. Back-up faces  11   d  are spaced from retaining faces  24  without the exertion of external force P. External force P occurs when the force is exerted on seat back  160  to be moved rearward. 
     In FIGS. 6 and 7, the pivots  16  each include central part  16 A and reinforcement part  16 B around the central part  16 A. The curved faces of pivots  16 , or reinforcement parts  16 B, include first support faces  16   a  protruding from bottom face  14   a  of frame  10 . Reinforcement parts  16 B include stepped parts  16   b  formed from the top end of first support faces  16   a  in parallel with bottom face  14   a . The curved faces, or central parts  16 A, further include second support faces  16   c , which protrude from stepped parts  16   b  at a right angle in proximity to lid  60 . Central parts  16 A include top faces  16   d  formed from the top ends of second support faces  16   c  in parallel with bottom face  14   b . The faces  16   a ,  16   b ,  16   c , and  16   d  are formed in a stepped structure. Bearings  23  include first supported faces  23   a , which are pivotably supported on first support faces  16   a . Bearings  23  include second supported faces  23   c , which are pivotably supported on second support faces  16   c . The faces  23   a ,  23   c  are formed in a stepped structure. Bearings  23  include engagement faces  23   b , which are interposed between the faces  23   a ,  23   c  and which are engaged with faces  16   b . The faces  16   a ,  16   c  and faces  23   a ,  23   c  are formed in concentric arced curved faces, thus rotatably engaging with each other. 
     When pivots  16  receive external force from bearings  23 , first support faces  16   a  and first supported faces  23   a  are brought in contact with each other under a small load. Under a load greater than a predetermined one, in addition to the contact of faces  16   a ,  23   a  with each other, second support and supported faces  16   c ,  23   c  are established in dimensional relationship to be brought in contact with each other. During small external force P, first faces  16   a ,  23   a  contact each other, while second faces  16   c ,  23   c  are spaced away from each other. 
     The following will describe the above-constitution in further detail. 
     In FIGS. 9 and 11, frame  10  includes an outer side, which is fixed to base plate  110  by welding or bolts. Base plate  110  constitutes one of the strength members of seat cushion  150 . Lid  60  includes outer side, which is fixed to arm plate  120  by welding or bolt. Arm plate  120  constitutes one of the strength members of seat back  160 . At a coaxial position with frame  10  and lid  60 , spiral spring  130  is mounted. Spring  130  biases arm plate  120  in a forward direction (F) to fall seat back  160  down forwardly. 
     Disc-shaped frame  10  is formed coaxially with circular inner peripheral face  14   a  in proximity to the outer peripheral face. Frame  10  includes circular recess  14 , which has flat circular bottom face  14   b  as a standard face. Frame  10  is formed with rotational center hole  17 , through which operation shaft  30  is inserted at the axial position. 
     Protruding from the bottom face of recess  14 , are two of first guide projections  11 A, second guide projections  11 B, projections  13 , pivots  16 . Pivots  16  include the semicircular outer peripheral faces, which engage semicircular bearings  23  of lock teeth  20 . The engagements allow lock teeth  20  to be pivotably supported. First guide projections  11 A include guide faces  11   a , which slidably guide first outer peripheral faces  27  of lock teeth  20  to be pivotably supported on pivots  16 . Second guide projections  11 B include guide faces  11   b , which slidably guide second outer peripheral faces  28  of lock teeth  20  to be pivotably supported on pivots  16 . First faces  27  are positioned closer to the center hole  30  than pivots  16 . At these positions, the above external force P is not exerted on guide faces  11   a . First faces  27  and guide faces  11   a  guide lock teeth  20  are pivoted on pivots  16 . The guide prevents the displacement of lock teeth  20  from pivots  16 . 
     First faces  27 , second faces  28 , and guide faces  11   a ,  11   b  are formed in an arced shape, which are coaxial with bearings  23  about the rotational centers of lock teeth  20  in pivots  16 . First guide projections  11 A include faces directed toward the rotational center, on which the sliding faces  11   c  are formed. Projections  11 A include faces directed toward faces  24 , on which the back-up faces  11   d  are formed. Retaining faces  24  and back-up faces  11   d  are formed in an arced shape coaxial with bearing  23 . 
     Projections  13  are disposed between one first and the other second guide projections  11 A,  11 B and between the other first and one second guide projections  11 A,  11 B. Projections  13  retain bases  70   a  of lock springs  70 . 
     Lock springs  70  are formed as a spiral spring. Springs  70  bias cam plate  40  to rotate counter-clockwise in FIG.  1 . The biasing of lock spring  70  allows cam plate  40  to be rotated counter-clockwise, causing the normal meshing of external gears  21  with internal gears  61 . 
     First projections  11 A, second projections  11 B, projections  13 , pivots  16  are formed integrally with frame  10  by emboss processing with a press. 
     Lid  60 , as well as frame  10 , is formed as a disc. Along the inner peripheral face of rim  60   a  rotatably fitted into inner peripheral face  14   a  of frame  10 , the above internal gear  61  is formed. At the axis of lid  60 , rotational center hole  62  is formed for the insertion of shaft  30 . 
     Lid  60  and frame  10  have outer peripheries, which are held and covered by ring-shaped holder  80 . This allows them  60 ,  10  to be rotatably retained relative to each other without axial separation. 
     Lock teeth  20  include external gears  21 , each of which face internal gears  61  on one side relative to pivot  16 . External gears  21  include rear sides with locked cam faces  25  which receive force from lock cam faces  41  of lock teeth  20 . Teeth  20  pivot clockwise on pivots  16  to mesh external gears  21  with internal gears  61 . 
     External gears  21  are positioned in proximity to pivots  16 . Gears  21 ,  61  have teeth set at a pressure angle of 60 to 90 degrees. External gears  21  have teeth close to pivots  16  lower in height than the other teeth. The teeth close to pivots  16  have a tip circle larger in radius than the other teeth. 
     Outer peripheral faces  22 , in opposition to external gears  21  relative to pivots  16 , are cut flat so as not to contact with internal gears  61 . The opposite sides to outer peripheral faces  22  include lock-release cam faces  26  to receive force from lock-release cam faces  44 . Teeth  20  receive force from cam faces  44  on cam faces  26 . The force allows teeth  20  to be pivoted counterclockwise, thus separating external gears  21  from internal gears  61 . 
     Operation shaft  30  is loosely fitted into respective rotational center holes  17 ,  62  of frame  10  and lid  60 . Shaft  30  is inserted through support holes  112 ,  122  by loose fitting. Support holes  112 ,  122  are formed to base and arm plates  110 ,  120 , respectively. Shaft  30  includes serration  35  projecting outward from base plate  110 . Serration  32  projects outward from arm plate  120 . Serration  35  has operation lever  31  mounted to it, to which operation knob  33  is further mounted. 
     The pressing of pressing-into pivots  30   a  of shaft  30  into center holes  42  allows cam plates  40  to be fixed. Cam plates  40  have outer peripheral faces formed with two cam faces  41 ,  44 , respectively. The outer peripheral faces are formed with locking parts  43 , which lock with outer end sides  70   b  of springs  70 . 
     Lock springs  70  each include a spiral spring. Springs  70  include bases  70   a  retained by projections  13 . Springs  70  include outer ends  70   b  latched with locking parts  43  of cam plates  40 . Outer ends  70   b  bias against cam plate  40  to be rotated counterclockwise in FIG.  1 . Cam plate  40  includes two axially symmetrical locking parts  43  which effectively receive biasing force tangentially from two springs  70 . 
     Arm plate  120  includes mounting part  121  to seat back  160 . Arm plate  120  includes retaining member  140  for retaining inner part  131  of spring  130  in FIG.  11 . 
     Retaining member  140  is provided in the lower part relative to rotational center hole  122  of arm plate  120 . Arm plate  120  is cut and raised along a locus with a radius of curvature about center hole  122 , to form retaining member  140 . Cut and raised part  141  includes an end with a recess (not shown in Figs.), which is locked with inner end  132  of spiral spring  130 . 
     Spring  130  includes outer end  133 , which is latched with locking pin  111  provided to base plate  110 . Spring  130  biases against arm plate  120  to normally rotate in a forward direction (F). 
     In FIG. 11, when arm plate  120 , or seat back  160 , rotates in the forward direction of F, stopper  123  for the prevention of falling forward abuts against pin  111 . The abutment restrains the forward (F) rotation of seat back  160 . Without being limited to the embodiment, arm plate  120  may be mounted to frame  10  and lid  60  may be mounted to seat cushion  150  in contrast with the above structure. 
     Next, the operation and benefit of the seat recliner E for a vehicle will be described. 
     At the assembly of seat recliner E, mounted to frame  10  are cam plate  40  and springs  70 . In this state, the biasing force of springs  70  allows outer ends  70   b  to be engaged with sliding faces  11   c  of projections  11 A, not with locking parts  43 . 
     By use of a shaft-shaped holding fixture with a serration (not shown in Figs.), which is meshed with serration  42   a  of center hole  42 , cam plate  40  is rotated clockwise. The rotation allows locking parts  43  to be engaged with outer ends  70   b  of springs  70 . In this state, lock teeth  20  are installed to frame  10 , and lid  60  is fitted into recess  14 . With the serration of holding fixture inserted into center hole  42  being rotated clockwise, cam faces  44 ,  26  rotate teeth  20  counterclockwise. The rotation allows external gears  21  to be moved radially inward relative to (imaginary) internal gear  61  of lid  60 . The insertion of rim  60   a  between inner peripheral face  14   a  of frame  10  and external gears  21  of lock teeth  20 , allows lid to be fitted into recess  14 . By the removal of force from the holding fixture, the biasing force of springs  70  allows lock teeth  20  to be pivoted clockwise via cam plate  40 . The pivotal motion allows external gears  61  to be meshed with internal gears  21 . Lock teeth  20  pivot clockwise until the external and internal gears  21 ,  61  finish mating with each other. After the mounting of lid, the holding fixture is removed. 
     Next, the outer peripheral faces of lid  10  and lid  60  are covered with holder  80 . The cover allows frame  10  and lid  60  to be rotatably supported relative to each other, without axial separation. 
     After the assembly with all of frame  10 , lock teeth  20 , cam plate  40 , lid  60 , spring  70 , and holder  80 , serrations  32 ,  34  are positioned on frame  10  in a rotational direction. In this state, portion  30   a  of shaft  30  is pressed into center hole  42 . 
     Assembled recliner E is mounted to both left and right sides of a seat in position. In mounting, respective serrations  32  of shafts  30  are mated with serrations  51  of shafts  50 . 
     In a driver&#39;s seat, serration  34  of shaft  30  located on the right side of the seat (door side) is mated with serration  35  of operation lever  31 . A bolt (not shown) fixes lever  31  to shaft  30 . In an assistant driver&#39;s seat, mounted to shaft  30  located on the left side of the seat (door side) is operation lever  31 . 
     In recliner E assembled to a seat, in FIG. 1, cam plate  40  normally is rotated counterclockwise by springs  70  under biasing force. Lock cam faces  41  press against cam faces  25  of teeth  20 . This pressing allows teeth  20  to be pivoted clockwise on pivots  16 . The pivotal motion allows external gears  21  to be meshed with internal gears  61 , thus preventing the rotation of seat back  160 . In this state, lines L 1  and L 2  or lines L 3  and L 4  overlap each other substantially in FIG. 1, respectively. 
     The knob  33  of lever  31  operatively rotates clockwise shaft  30 . The cam faces  41 ,  25  are disengaged. Cam faces  44  press against cam faces  26 . 
     Thus, teeth  20  pivot counterclockwise on pivots  16 . The pivotal motion releases the meshing of gears  21 ,  61  for disengagement. Arm plate  120  or seat back  160  is rotated forward (F) by spring  130  under biasing force. 
     To return a locking state returns from a lock disengagement state, knob  33 , gripped at an angular position to incline seat back  160 , is operatively released. Springs  70  rotate cam plate  40  counterclockwise under a biasing force. The rotation allows teeth  20  to be rotated clockwise. The rotation allows gears  21 ,  61  to be meshed with each other for a locking state. 
     According to recliner E, the maximum load due to the external force P is exerted on the connecting portion of first support face  16   a  and bottom face  14   b . The connecting portion corresponds to the base of pivot  16 . The formation of pivot  16  in a stepped shape including first and second support faces  16   a ,  16   c  allows for a large contact area between first support face  16   a  and frame  10 . The minimizing of second support faces  16   c  to less than first support face  16   a  reduces the contact area of pivot  16  with tooth  20 . Thus, the raising of contact pressure to greater than external force P reduces the strength. Second face  23   c  projects from first support face  23   a  toward pivot  16 . In this state, first support face  23   a  is structured as a rib. In this structure, fist faces  16   a ,  23  enlarge in radius to enlarge the contact face, thus restraining the lowering in strength to a minimum. The lowing of strength due to the reduction of the contact face with tooth  20  is restrained to a minimum, thus improving pivot  16  in strength. 
     Pivot  16  is formed as a projection by embossing processing with a press. When the formation method is employed for straight shaped pivot  16 , in relation to energy supplied to a press, the enlargement of the connection area between pivot  16  and frame  10 , or the area of the portion of pivot  16  projecting from bottom face  14   b , disadvantageously reduces pivot  16  in height. If pivot  16  is established at a height identical to the thickness of tooth  20 , the upper limit of the connection area between pivot  16  and frame  10  may be automatically determined. The determination does not further increase the connection area. 
     In the embodiment, stepped pivot  16  enlarges the connection area allows the connection area between first support face  16   a  and frame  10  to be enlarged without thickening frame  10 . Thus, when an embossing processing forms pivot  16 , the pressure of a press is identical to conventional one. Frame  10  employs one of identical thickness, and the costs of die and manufacturing is cheapened to allow forming at productive costs identical to conventional ones. 
     In the embodiment, in accordance with external force P, firstly, first faces  23   a  are brought into contact with first faces  16   a . Secondly, second faces  23   c  are brought in contact with second faces  16   c . With the fitting allowance between first faces  16   a ,  23   a  or between second faces  16   c ,  23   c , first faces  16   a  previously contact with first faces  23   a . Excessive load is not exerted on second faces  16   c  of a smaller strength. This ensures safety. In addition, if external force is further increased, second faces  16   c  support lock teeth  20 , and thus pivots  16  advantageously bear the high load. 
     The embodiment has recliners E mounted to both left and right sides of the seat. Recliner E may be provided on one of the sides of the seat. 
     The embodiment has stepped pivots  16 . The guide faces  11   b  of second guide projections  11 B and the second peripheral faces of lock teeth  20  may be formed in a step structure, respectively. In such a case, locking strength against the external force in a reverse direction relative to the external force P is improved. 
     Second Embodiment 
     In FIG. 12, recliner E 2  has frame  10 . Frame  10  includes two semi-cylindrical pivots  16  in proximity to internal gears  61  thereon, which pivotably support lock teeth  20 . Lock teeth  20  include semi-circular recessed bearings  23 . Bearings  23  pivotably engage on the arced curved faces of pivots  16 . Teeth  20  include arced retaining faces  24  for sliding coaxially with bearings  23 . Retaining faces  24  are positioned opposite to external gears  21  relative to bearings  23 . 
     Frame  10  is formed in a disc-shape. Circular inner peripheral face  14   a  is formed coaxially with and close to the outer periphery, which forms circular recess  14 . Frame  10  includes rotational center hole  17  on the axis for the insertion of shaft  30 . 
     According to the recliner E 2 , first projection  11 A has less rigidity against external force P than pivot  16 . When the external force P is exerted on first projection  11 A and pivot  16  from lock tooth  20 , the repulsive force on first projection  11 A from lock tooth  20  is smaller than one on lock tooth  20  from pivot  16 . Thus, internal stress occurring on lock tooth  20  becomes small. External force P to be exerted on external gear  21  from internal gear  61  prevents the damage on bearing  23  of lock tooth  20 . This optimizes the balance of rigidity between first projection  11 A, pivot  16  and tooth  20 . Pivot  16  and first projection  11 A retain tooth  20  at a predetermined strength, thus improving whole strength. The reduction of force on tooth  20  allows thinning of tooth  20  or employing a general-purpose material at lower cost, thus reducing productive cost. 
     The recess  11   e  of back-up face  11   d  lowers the rigidity of first projection  11 A against external force P. In the embodiment, recess  11   e  reduces the area of back-up face  11   d  to abut against retaining face  24 . The reduction of the area of back-up face  11   d  effectively reduces the rigidity of first projection  11 A. In addition, back-up face  11   d  is separated away from retaining face  24  without external force P. In this state, when increment of external force P needs back-up, back-up face  11   d  abuts against retaining face  24  to back up pivot  16 . Thus, in a small external force P, retaining face  24  does not abut against back-up face  11   d . In the case of large external force P, after the abutment of back-up face  11   d  against retaining face  24 , the deformation of first projection  11 A is reduced. At the maximum external force P, the force, exerted on first projection  11 A from tooth  20  is reduced. Thus, the internal stress, exerted on tooth  20  by first projection  11 A, is reduced remarkably, thus reducing productive cost. 
     Third Embodiment 
     The third embodiment will be described in reference with FIGS. 13A,  13 B,  14 A, and  14 B. Other parts except for first guides  11 A 2 ,  11 A 3  are common with the second embodiment. 
     First guide projection  11 A 2  includes recess  11   f  behind back-up face  11   d   2  for retaining face  24 . Back-up face  11   d  has no recess  11   e  of the first embodiment, and is formed as an arced curved face identical to retaining face  24 . First projection  11 A may include sliding face  11 C as described in the first embodiment. 
     In the recliner, recess  11   f  reduces the rigidity of first projection  11 A against external force P. Back-up face  11   d   2  tends to be easily resiliently deformed backward by the external force that is received from retaining face  24  (refer to FIG.  12 ). The lowering of the repulsive force to be exerted on retaining face  24  from back-up face  11   d  allows for improvement in strength as in the first embodiment. 
     According to the embodiments, the formation of recess  11   e  on back-up face  11   d , the reduction of the area of back-up face  11   d , and the providing of recess  11   f  behind back-up face  11   d   2 , reduce the rigidity of first projection  11 A 2  against external force P. As shown in FIGS. 14A and 14B, first projection  11 A 3  may be lowered in height H to reduce in rigidity. 
     The entire contents of Japanese Patent Applications P2001-155659 and P2001-155749 (all filed on May 24, 2001) are incorporated herein by reference. 
     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.