Patent Publication Number: US-2021162894-A1

Title: Seat reclining device for vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a seat reclining device for vehicle that adjusts an angle of a seat back. 
     BACKGROUND ART 
     As a seat reclining device for vehicle used in a seat of a vehicle, for example, a technique described in PTL 1 is known. The seat reclining device for vehicle of PTL 1 includes two brackets configured to rotate relative to each other, a plurality of pawls arranged on a first bracket, and a cam configured to move the pawls in a radial direction. Each pawl is biased by the cam to move toward a radially outer side and moves in the radial direction in response to the rotation of the cam. The pawls move toward the radially outer side, outer teeth of the pawls and inner teeth of a second bracket are meshed with each other, and thus, the second bracket is fixed to the first bracket due to the rotation of the cam. That is, rotation of the second bracket is limited. Here, a ball cam is accommodated between the cam and a predetermined pawl among the plurality of pawls. The ball cam presses the pawl to prevent rattling due to rotation of the cam. 
     CITATION LIST 
     Patent Literature 
     PTL 1: WO 2016/129423 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the seat reclining device for vehicle disclosed in PTL 1, the ball cam is a small component and assembly operation thereof is difficult, which can still be improved. 
     The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a seat reclining device for vehicle that prevents rattling of pawls and improves assembly without increasing the number of components. 
     Solution to Problem 
     A seat reclining device for vehicle that solves the above problems includes: a first bracket; a second bracket configured to rotate in positive and negative directions relative to the first bracket; a cam configured to rotate or move relative to the first bracket; and a plurality of pawls configured to move in a radial direction of the first bracket based on the rotation or the movement of the cam to engage with the second bracket, in which one of the first bracket and at least one of the plurality of pawls has a groove portion extending in a direction corresponding to the radial movement of the plurality of pawls, and another one has an engagement projection slidably inserted into the groove portion, and in which the engagement projection comes into contact with an inner surface of the groove portion when the second bracket and the plurality of pawls are engaged. 
     According to this configuration, the one of the first bracket and at least one of the plurality of pawls has the groove portion extending in the direction corresponding to the radial movement of the plurality of pawls, and the other one has the engagement projection slidably inserted into the groove portion. Also, the engagement projection come into contact with the inner surface of the groove portion when the second bracket and the plurality of pawls are engaged. Thereby, the engagement projection is biased in a direction intersecting the radial movement of the plurality of pawls. Such bias can prevent rattling of the pawls. Further, since the engagement projection and the groove portion are provided to the first bracket and the pawl, which constitute the seat reclining device for vehicle, the number of components can be prevented from increasing. In addition, since the engagement projection is inserted into the groove portion when the pawl is assembled to the first bracket, the assembly can be improved. Therefore, it is possible to provide a seat reclining device for vehicle that prevents rattling of pawls and improves assembly without increasing the number of components. 
     In the seat reclining device for vehicle, the engagement projection is preferably provided to the first bracket, and the groove portion is preferably provided to the at least one of the plurality of pawls. According to this configuration, the groove portion is formed on the pawl, which is an internal component of the seat reclining device for vehicle, and thus is easy to from and is effective. 
     In the seat reclining device for vehicle, the groove portion preferably penetrates in a thickness direction. According to this configuration, when the pawl is assembled to the first bracket, it is easy to confirm whether the engagement projection is inserted into the groove portion, which improves visibility. 
     In the seat reclining device for vehicle, the inner surface of the groove portion preferably has a first gradually changing portion and a second gradually changing portion provided, which are provided as inclined surfaces intersecting the radial movement. The first gradually changing portion and the second gradually changing portion preferably have different inclination angles with respect to the radial movement of the pawls. According to this configuration, by including the first gradually changing portion and the second gradually changing portion which have different inclination angles with respect to the radial movement of the pawls, rattling of the pawl can be further prevented. 
     In the seat reclining device for vehicle, the inclination angle of the second gradually changing portion of the groove portion with respect to the radial movement of the pawls is preferably set larger than the inclination angle of the first gradually changing portion, and the engagement projection preferably comes into contact with the second gradually changing portion when the second bracket and the plurality of pawls are engaged. According to this configuration, when the second bracket and the plurality of pawls are engaged, rattling of the pawl can be further prevented. 
     Advantageous Effects of Invention 
     Due to the means of above inventions, to the present invention is capable of providing a seat reclining device for vehicle that prevents rattling of pawls and improves assembly without increasing the number of components. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view showing a seat including a vehicle seat reclining device. 
         FIG. 2  is a cross-sectional view taken along line II-II of  FIG. 1 . 
         FIG. 3  is an exploded perspective view of the vehicle seat reclining device. 
         FIG. 4( a )  is a plan view showing a first bracket, and  FIG. 4( b )  is a side view showing the first bracket. 
         FIG. 5  is a plan view showing a cam. 
         FIG. 6( a )  is a perspective view showing a first pawl, and  FIG. 6( b )  is a perspective view showing a second pawl. 
         FIG. 7( a )  is a plan view showing a second bracket, and  FIG. 7( b )  is a cross-sectional view taken along line VIIb-VIIb in  FIG. 7( a ) . 
         FIG. 8  is an exploded view showing a step of the second bracket. 
         FIG. 9  is a schematic view showing a positional relationship between a first limitation portion to a third limitation portion. 
         FIG. 10  is a cross-sectional view taken along line X-X of  FIG. 2 . 
         FIG. 11  is a cross-sectional view taken along line XI-XI of  FIG. 2 . 
         FIG. 12  is a cross-sectional view taken along line XII-XII in  FIG. 2 . 
         FIGS. 13( a ) to 13( b )  are cross-sectional views taken along line XII-XII in  FIG. 2  showing changes in the positional relationship of the limitation portions of the pawls and pawl restriction portions relative to rotation of the second bracket  31  in the vehicle seat reclining device of the present embodiment. 
         FIGS. 14( a ) to 14( b )  are exploded views showing changes of the vehicle seat reclining device according to  FIGS. 13( a ) to 13( b ) . 
         FIG. 15  is an exploded view showing the positional relationship of pawl restriction portions and limitation portions of pawls in a second bracket of a vehicle seat reclining device in related art. 
         FIG. 16  is an exploded view showing the positional relationship of the pawl restriction portions and the limitation portions of the pawls in the second bracket of the vehicle seat reclining device of the present embodiment. 
         FIG. 17  is a schematic view showing another embodiment according to the present embodiment. 
         FIG. 18  is a schematic view showing another embodiment according to the present embodiment. 
         FIG. 19  is a schematic view showing another embodiment according to the present embodiment. 
         FIG. 20  is a schematic view showing another embodiment according to the present embodiment. 
         FIG. 21  is a schematic view showing another embodiment according to the present embodiment. 
         FIG. 22  is a schematic view showing another embodiment according to the present embodiment. 
         FIG. 23  is a schematic view showing another embodiment according to the present embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A first embodiment of a vehicle seat reclining device  10  will now be described with reference to  FIGS. 1 to 23 . 
     As shown in  FIG. 1 , the vehicle seat reclining device  10  is applied to, for example, a seat  2  arranged on a vehicle floor  1  or the like. The seat  2  includes a seat cushion  3  that forms a seating surface and a seat back  4  that forms a backrest. The seat back  4  is configured to be rotatable and maintainable at a predetermined angle relative to the seat cushion  3 . 
     The seat back  4  is mounted to the seat cushion  3  by the vehicle seat reclining device  10 . The vehicle seat reclining device  10  maintains the seat back  4  relative to the seat cushion  3  at the predetermined angle. 
     As shown in  FIG. 2 , a rotor of one of a first bracket  21  and a second bracket  31  (described below) is fixed to a plate  3   a  attached to the seat cushion  3 . A rotor of another one of the first bracket  21  and the second bracket  31  is fixed to a plate  4   a  attached to the seat back  4 . In the present embodiment, the first bracket  21  is fixed to the plate  3   a,  and the second bracket  31  is fixed to the plate  4   a.    
     A shaft  5  extends through a central portion of the vehicle seat reclining device  10 . The shaft  5  operates a cam mechanism arranged in the vehicle seat reclining device  10 . An operation lever  5   a  for rotating the shaft  5  is attached to an end of the shaft  5 . 
     In a state where the vehicle seat reclining device  10  is attached to the seat  2 , a rotation axis C 1  of the shaft  5  corresponds to a rotation axis C 2  of the vehicle seat reclining device  10 , namely, a rotation axis of the first bracket  21  and the second bracket  31 . 
     In the following description, the direction extending along a circumference about the rotation axis C 2  of the vehicle seat reclining device  10  is referred to as the circumferential direction, and the direction perpendicular to the rotation axis C 2  (normal direction) is referred to as the radial direction. Further, a direction, in which the second bracket  31  rotates when the seat back  4  is reclined to the rear, is referred to as a rear rotation direction RX (or “rotation in positive direction F”), and a direction opposite to the rear rotation direction RX is referred to as “rotation in negative direction N”. 
     As shown in  FIGS. 2 and 3 , the vehicle seat reclining device  10  includes the first bracket  21 , the second bracket  31 , a first pawl  40 A, a second pawl  40 B, a third pawl  40 C, a cam  50 , a spiral spring  60  that biases the cam  50 , and a holding member  80  that holds the first bracket  21  and the second bracket  31 . The cam mechanism is configured by the cam  50 , the spiral spring  60 , the first pawl  40 A, the second pawl  40 B, and the third pawl  40 C. 
     The holding member  80  includes an annular body  81  and a flange  82  extending from a rim of the body  81  toward the center. The flange  82  includes a projection  83  that projects toward the inner side (toward the second bracket  31 ). The projection  83  adjusts play for axial movement of the second bracket  31 . 
     The body  81  covers an outer circumferential surface  22   c  of the first bracket  21  and an outer circumferential surface  33   b  of the second bracket  31 . The body  81  is laser-welded to the outer circumferential surface  22   c  of the first bracket  21 . The flange  82  covers an outer surface  33   c  (see  FIG. 7 ) of an outer circumferential wall  33  of the second bracket  31 . Accordingly, the holding member  80  holds the first bracket  21  and the second bracket  31  such that a distance between the first bracket  21  and the second bracket  31  in the axial direction is maintained at a predetermined distance. 
     As shown in  FIGS. 4( a ) and 4( b ) , the first bracket  21  includes a circular body  22 , three guides  23  that respectively guide movement of the first pawl  40 A, the second pawl  40 B, and the third pawl  40 C, and projections  27  that attach the first bracket  21  to the plate  3   a,  which serves as a fixing member. 
     Each guide  23  projects from an inner surface  22   a  of the body  22 . Each guide  23  includes guide surfaces  24  extending toward the outside and an inner side surface  23   a  extending in the circumferential direction. The cam  50  is accommodated in a region surrounded by the inner side surfaces  23   a  of the three guides  23 . 
     The three opposing guide surfaces  24  of two adjacent guides  23  are parallel to each other. The guide surfaces  24  form guide grooves  26  in cooperation with the inner surface  22   a  of the body  22 . The guide grooves  26  guide the first pawl  40 A, the second pawl  40 B, and the third pawl  40 C to move in the radial direction. 
     The three guides  23  have the same shape and are arranged in the circumferential direction at equal angles. Thus, the three guide grooves  26  are arranged in the circumferential direction at equal angular intervals. The first bracket  21  includes an outer surface  22   b  recessed at portions located at the opposite side of the guides  23 . 
     The projections  27  project from portions of the outer surface  22   b  of the body  22  located at the opposite side of the guide grooves  26 . The inner surface  22   a  of the first bracket  21  is recessed at the opposite side of the projections  27 . 
     When the first bracket  21  is attached to the plate  3   a  serving as a fixing member, the projections  27  of the first bracket  21  are fitted into holes or cutouts arranged in the plate  3   a  and welded to the plate  3   a.    
     The central portion of the body  22  includes an accommodation portion  25  that accommodates a spiral portion  61  of the spiral spring  60 . The accommodation portion  25  is provided with a communication groove  25   a  cut out toward the radially outer side. An outer engagement portion  62  and an end  62   a  of the spiral spring  60  shown in  FIG. 3  are engaged with the communication groove  25   a.    
     In the guide grooves  26  corresponding to the second pawl  40 B and the third pawl  40 C of the body  22 , which will be described later, an engagement projection  110 B and an engagement projection  110 C project from the inner surface  22   a.  The engagement projection  110 B is slidably inserted into a sliding groove  112 B of the second pawl  40 B, which will be described later. Similarly, the engagement projection  110 C is slidably inserted into a sliding groove  112 C of the third pawl  40 C, which will be described later. 
     As shown in  FIGS. 2, 3, and 5 , the cam  50  is located between the first bracket  21  and the second bracket  31 . The cam  50  is accommodated in the region surrounded by the inner side surfaces  23   a  of the three guides  23  of the first bracket  21 . 
     The cam  50  includes a cam body  51 , three pawl engagement portions  52 , and two spring engagement portions  53  that engage with an inner engagement portion  63  of the spiral spring  60  shown in  FIGS. 3 and 5 . The three pawl engagement portions  52  engage the first pawl  40 A, the second pawl  40 B, and the third pawl  40 C, respectively. The pawl engagement portions  52  are arranged on a first surface  50   a  of the cam  50 , and the spring engagement portions  53  are arranged on a second surface  50   b,  which is shown in  FIG. 2 , of the cam  50 . 
     A fitting hole  54  into which the shaft  5  is fitted extends through a central portion of the cam body  51 . The cam  50  moves in cooperation with the rotation of the shaft  5 . More specifically, operation of the operation lever  5   a  attached to the shaft  5  rotates the cam  50 . 
     The circumferential surface of the cam body  51  is provided with three cam portions, namely, a first cam portion  55 , a second cam portion  56 , and a third cam portion  57 , which respectively come into contact with cam surfaces of the first pawl  40 A, the second pawl  40 B, and the third pawl  40 C and which are arranged at equal angular intervals. The first cam portion  55  includes a first pushing portion  55   a  and a second pushing portion  55   b  that push a first cam surface  44 A of the first pawl  40 A. The second cam portion  56  includes a first pushing portion  56   a  and a second pushing portion  56   b  that push a second cam surface  44 B of the second pawl  40 B. The third cam portion  57  includes a first pushing portion  57   a  and a second pushing portion  57   b  that push a third cam surface  44 C of the third pawl  40 C (see  FIG. 10 ). 
     The cam  50  is biased by the spiral spring  60  in a predetermined rotation direction relative to the first bracket  21  (hereinafter referred to as “biasing direction RB”). That is, the spiral spring  60  applies a biasing force that rotates the cam  50  in the biasing direction RB to the cam  50 . 
     As shown in  FIGS. 2, 5, 6, and 10 , the first pawl  40 A includes a first block  41 A and a second block  42 A which are arranged in different steps. More specifically, the first block  41 A is located at the radially outer side of the first pawl  40 A, and the second block  42 A is located at the radially inner side of the first pawl  40 A. The first block  41 A is shifted from the second block  42 A in the axial direction of the rotation axis C 2 . 
     The first block  41 A is attached to the guide grooves  26  of the first bracket  21 . The first block  41 A and the cam  50  are located at the same position in the axial direction of the rotation axis C 2 . The first block  41 A has an arcuate outer end surface opposing inner teeth  37  of the second bracket  31 . The outer end surface of the first block  41 A has outer teeth  43 A that mesh with the inner teeth  37  of the second bracket  31 . 
     The end surface of the first block  41 A located at a side opposite to the outer end surface has the first cam surface  44 A against which the first cam portion  55  of the cam  50  abuts. The first cam surface  44 A has a portion against which the first pushing portion  55   a  of the first cam portion  55  abuts and a portion against which the second pushing portion  55   b  of the first cam portion  55  abuts. 
     The second block  42 A is arranged on the first surface  50   a  of the cam  50 . That is, the second block  42 A is located between the cam  50  and the second bracket  31 . The second block  42 A has an outer end surface opposing the inner surface of a step  34  of the second bracket  31 . 
     A first limitation portion  46 A is provided between the outer end surface of the second block  42 A and the outer end surface (the outer teeth  43 A) of the first block  41 A. The first limitation portion  46 A projects from the surface of the first block  41 A toward the second bracket  31 . The first limitation portion  46 A abuts against the first pawl restriction portion  34   a  of the second bracket  31  shown in  FIG. 7( a )  to limit the first pawl  40 A to move toward the radially outer side. The first limitation portion  46 A has an abutment surface  48 A that abuts against a first inner surface  35   a  of the first pawl restriction portion  34   a.  The distance in the radial direction between the abutment surface  48 A of the first limitation portion  46 A and the outer teeth  43 A is restricted by a predetermined distance in relation to the step structure of the second bracket  31 . 
     A cam hole  47 A that penetrates through the second block  42 A in the thickness direction is provided at the central portion of the second block  42 A. The cam hole  47 A extends in the circumferential direction toward the inner side in the biasing direction RB shown in  FIG. 3 . The corresponding pawl engagement portion  52  of the cam  50  is inserted through the cam hole  47 A (see  FIG. 6 ). 
     The second pawl  40 B has the same structure as the first pawl  40 A except for the positional structure of the second limitation portion  46 B, namely, the positional relationship of an abutment surface  48 B of the second limitation portion  46 B and outer teeth  43 B, and except for the structure of a first clearance portion  100 B of the second limitation portion  46 B and a sliding groove  112 B. 
     A negative side end  102 B of the second limitation portion  46 B is provided with the first clearance portion  100 B having a corner portion removed and having an inclined surface  104 B, of which a height is changed in the radial direction. The inclined surface  104 B is a smooth surface whose radial height gradually increases from the negative side end  102 B toward the abutment surface  48 B. The inclined surface  104 B may be straight or curved when viewed in a cross-sectional view. The first clearance portion  100 B may have a shape having a surface whose radial height does not increase halfway without being limited to the inclined surface  104 B. The first clearance portion  100 B may be provided at a positive side end  106 B of a second pawl restriction portion  34   b  without being limited to the negative side end  102 B of the second limitation portion  46 B. The surface shape at this time may be a smooth surface whose radial height gradually decreases, or may be a shape partially having a surface whose radial height does not increase. 
     The second pawl  40 B is provided with the sliding groove  112 B corresponding to the engagement projection  110 B. The sliding groove  112 B in the present embodiment is provided as a hole penetrating in the thickness direction of the second pawl  40 B from the viewpoint of improving visibility when assembling to the first bracket  21 . The sliding groove  112 E extends in a direction along the guide groove  26  of the first bracket  21 . That is, the sliding groove  112 B extends along the movement of the second pawl  40 B in the radial direction. The outer teeth  43 B side of the sliding grooves  112 B has a groove shape which is larger than a diameter of the engagement projection  110 B. The sliding groove  112 B has a groove shape that is tapered toward the radially inner side inward from the outer teeth  43 B. A tip end of the groove shape is smaller than the diameter of the engagement projection  110 B. The sliding groove  112 B is biased in a direction intersecting the radial movement of the second pawl  40 B in a state where the outer teeth  43 B mesh with the inner teeth  37 . 
     The tapered shape of the sliding groove  112 B has a first gradually changing portion  114 B and a second gradually changing portion  116 B toward the radially inner side from the outer teeth  43 B side. Each of the first gradually changing portion  114 B and the second gradually changing portion  116 B is provided as an inclined surface intersecting the radial movement of the second pawl  40 B. Here, among the inclination angles with respect to the radial movement of the second pawl  40 B of the first gradually changing portion  114 B and the second gradually changing portion  116 B, the inclination angle of the second gradually changing portion  116 B is set larger than that of the first gradually changing portion  114 B. When the engagement projection  110 B enters the first gradually changing portion  114 B or the second gradually changing portion  116 B, a so-called wedge effect gradually acts to prevent rattling of the second pawl  40 B. 
     Looking into the details, the radius of curvature of the outer teeth  43 B of the second pawl  40 B is smaller than the radius of curvature of the inner teeth  37  of the second bracket  31 . Therefore, in a state where the outer teeth  43 B mesh with the inner teeth  37 , the vicinity of the center in the circumferential direction of the outer teeth  43 B mesh deeply, and both ends in the circumferential direction mesh shallowly. This is because a tolerance is required to smoothly mesh the outer teeth  43 B with the inner teeth  37 . On the other hand, due to such tolerance, rattling may occur when the outer teeth  43 B mesh with the inner teeth  37 . Therefore, the sliding groove  112 B and the engagement projection  110 B contribute to prevention of rattling of the second pawl  40 B. In addition, in the structure of the related art, since a separate component is required in order to prevent rattling of the pawl, there is a concern about the complexity of assembly. However, by providing the engagement projection  110 B on the body  22  of the first bracket  21  and by providing the sliding groove  112 B on the second pawl  40 B, rattling of the second pawl  40 B can be prevented without increasing the number of components. 
     The present embodiment shows an aspect in which the engagement projection  110 B is provided on the first bracket  21  and the sliding groove  112 B is provided on the second pawl  40 B. This is because that if a penetrating hole is provided on the first bracket  21  side, there is a concern that a foreign object or the like enters the vehicle seat reclining device  10 . Further, the sliding groove  112 B can be easily formed on the second pawl  40 B, which is an internal component. However, the relationship between the engagement projection  110 B and the sliding groove  112 B may be reversed. That is, as shown in  FIG. 17 , the engagement projection  110 B may be provided on the second pawl  40 B, and the sliding groove  112 B may be provided on the first bracket  21 . The sliding groove  112 B may have a groove shape that does not penetrate in the thickness direction of the second pawl  40 B. The sliding groove  112 B may be formed into a groove shape tapered in a straight shape as shown in  FIG. 18 , instead of the tapered groove shape as shown in  FIG. 10  whose middle portion in the radial direction is bent. As shown in  FIG. 19 , a surface of the sliding groove  112 B that abuts against the engaged protrusion  110 B in a rattling tightening state may be formed of arcuate surfaces  112 Ba that are warped in a reverse direction so as to be in point contact (linear contact) with the circular outer peripheral surface of the engagement projection  110 B. As shown in  FIG. 20 , the engagement projection  110 B may protrude into a pin shape having a wide cross-sectional width in the radial direction such as an ellipse instead of a pin shape protruding into a circular shape. The rattling tightening structure formed by the sliding groove  112 B and the engagement projection  110 B may be set not in the end region on the one side in the circumferential direction of the second pawl  40 B as shown in  FIG. 10 , but in an end region on the other side in the circumferential direction as shown in  FIG. 21  or the intermediate region in the circumferential direction as shown in  FIG. 22 . 
     The third pawl  40 C has the same structure as the first pawl  40 A except for the positional structure of the third limitation portion  46 C, namely, the positional relationship of an abutment surface  48 C of the third limitation portion  46 C and outer teeth  43 C, and except for the structure of a second clearance portion  100 C and a sliding groove  112 C on the third limitation portion  46 C. 
     A negative side end  102 C of the third limitation portion  46 C is provided with the second clearance portion  100 C having a corner portion removed and having an inclined surface  104 C, of which a height is changed in the radial direction. The inclined surface  104 C is a smooth surface whose radial height gradually increases from the negative side end  102 C toward the abutment surface  48 C. The inclined surface  104 C may be straight or curved when viewed in a cross-sectional view. The second clearance portion  100 C may have a shape having a surface whose radial height does not increase halfway without being limited to the third limitation portion  46 C. The second clearance portion  100 C may be provided at a positive side end  106 C of a third pawl restriction portion  34   c  without being limited to the negative side end  102 C of the third limitation portion  46 C. The surface shape at this time may be a smooth surface whose radial height gradually decreases, or may be a shape partially having a surface whose radial height does not increase. 
     The third pawl  40 C is provided with the sliding groove  112 C corresponding to the engagement projection  110 C. The sliding groove  112 C in the present embodiment is provided as a hole penetrating in the thickness direction of the third pawl  40 C from the viewpoint of improving visibility when assembling to the first bracket  21 . The sliding groove  112 C extends in a direction along the guide groove  26  of the first bracket  21 . That is, the sliding groove  112 C extends along the movement of the third pawl  40 C in the radial direction. The outer teeth  43 C side of the sliding grooves  112 C has a groove shape which is larger than a diameter of the engagement projection  110 C. The sliding groove  112 C has a groove shape that is tapered toward the radially inner side from the outer teeth  43 C. A tip end of the groove shape is smaller than the diameter of the engagement projection  110 C. The sliding groove  112 C is biased in a direction intersecting the radial movement of the third pawl  40 C in a state where the outer teeth  43 C mesh with the inner teeth  37 . 
     The tapered shape of the sliding groove  112 C has a first gradually changing portion  114 C and a second gradually changing portion  116 C toward the radially inner side from the outer teeth  43 C side. Each of the first gradually changing portion  114 C and the second gradually changing portion  116 C is provided as an inclined surface intersecting the radial movement of the third pawl  40 C. Here, among the inclination angles with respect to the radial movement of the third pawl  40 C of the first gradually changing portion  114 C and the second gradually changing portion  116 C, the inclination angle of the second gradually changing portion  116 C is set larger than that of the first gradually changing portion  114 C. When the engagement projection  110 C enters the first gradually changing portion  114 C or the second gradually changing portion  116 C, a so-called wedge effect gradually acts to prevent rattling of the third pawl  40 C. 
     Looking into the details, the radius of curvature of the outer teeth  43 C of the third pawl  40 C is smaller than the radius of curvature of the inner teeth  37  of the second bracket  31 . Therefore, in a state where the outer teeth  43 C mesh with the inner teeth  37 , the vicinity of the center in the circumferential direction of the outer teeth  43 C mesh deeply, and both ends in the circumferential direction mesh shallowly. This is because a tolerance is required to smoothly mesh the outer teeth  43 C with the inner teeth  37 . On the other hand, due to such tolerance, rattling may occur when the outer teeth  43 B mesh with the inner teeth  37 . Therefore, the sliding groove  112 C and the engagement projection  110 C contribute to prevention of rattling of the third pawl  40 C. In addition, in the structure of the related art, since a separate component is required in order to prevent rattling of the pawl, there is a concern about the complexity of assembly. However, by providing the engagement projection  110 C on the body  22  of the first bracket  21  and by providing the sliding groove  112 C on the third pawl  40 C, rattling of the third pawl  40 C can be prevented without increasing the number of components. 
     The present embodiment shows an aspect in which the engagement projection  110 C is provided on the first bracket  21  and the sliding groove  112 C is provided on the third pawl  40 C. This is because that if a penetrating hole is provided on the first bracket  21  side, there is a concern that a foreign object or the like enters the vehicle seat reclining device  10 . Further, the sliding groove  112 C can be easily formed on the third pawl  40 C, which is an internal component. However, the relationship between the engagement projection  110 C and the sliding groove  112 C may be reversed. That is, as shown in  FIG. 17 , the engagement projection  110 C may be provided on the third pawl  40 C, and the sliding groove  112 C may be provided on the first bracket  21 . The sliding groove  112 C may have a groove shape that does not penetrate in the thickness direction of the third pawl  40 C. The sliding groove  112 C may be formed into a groove shape tapered in a straight shape as shown in  FIG. 18 , instead of the tapered groove shape as shown in  FIG. 10  whose middle portion in the radial direction is bent. As shown in  FIG. 19 , a surface of the sliding groove  112 C that abuts against the engaged protrusion  110 C in a rattling tightening state may be formed of arcuate surfaces  112 Ca that are warped in a reverse direction so as to be in point contact (linear contact) with the circular outer peripheral surface of the engagement projection  110 C. As shown in  FIG. 20 , the engagement projection  110 C may protrude into a pin shape having a wide cross-sectional width in the radial direction such as an ellipse instead of a pin shape protruding into a circular shape. The rattling tightening structure formed by the sliding groove  112 C and the engagement projection  110 C may be set not in the end region on the one side in the circumferential direction of the third pawl  40 C as shown in  FIG. 10 , but in an end region on the other side in the circumferential direction as shown in  FIG. 21  or the intermediate region in the circumferential direction as shown in  FIG. 22 . 
     The present embodiment shows an aspect in which both the engagement projection  110 B, the sliding groove  112 B and the engagement projection  110 C, the sliding groove  112 C are formed, but may also be an aspect having one combination of the engagement projection  110 B, the sliding groove  112 B or the engagement projection  110 C, the sliding groove  112 C. As shown in  FIG. 23 , the rattling tightening structure formed by the engagement projection and the sliding groove may be configured such that an engagement projection  110 A and a sliding groove  112 A are provided between the first pawl  40 A and the first bracket  21 . In such case, as a configuration in which the second pawl  40 B and the third pawl  40 C are also provided with the engagement projections  110 B,  110 C and the sliding grooves  112 B,  112 C with the first bracket  21 , the engagement projections  110 B,  110 C and the sliding grooves  112 B,  112 C may be fitted to one another loosely so that a slight gap is formed in the circumferential direction when the engagement projection  110 A is fitted into the sliding groove  112 A and enters the rattling tightening state, which provides an auxiliary rattling tightening function. 
     The second bracket  31  will now be described with reference to  FIGS. 7( a )  to  9 . As shown in  FIG. 7( a )  and  FIG. 7( b ) , the second bracket  31  includes a circular body  32  and an outer circumferential wall  33  arranged along an outer edge of the body  32 . An insertion hole  32   a  through which the shaft  5  is inserted is formed in the body  32 . 
     The inner teeth  37  are arranged over the entire circumference of an inner circumferential surface  33   a  of the outer circumferential wall  33 . The inner teeth  37  mesh with the outer teeth  43 A of the first pawl  40 A, the outer teeth  43 B of the second pawl  40 B, and the outer teeth  43 C of the third pawl  40 C. The outer circumferential surface  33   b  of the outer circumferential wall  33  comes into sliding contact with the holding member  80 . The outer surface  33   c  of the outer circumferential wall  33  comes into sliding contact with the projection  83  of the holding member  80  shown in  FIG. 2 . The inner surface of the body  32  includes an annular step  34  about the rotation axis C 2 . 
     In  FIG. 8 , the upper side is shown as the radially inner side of the second bracket  31 , the lower side is shown as the radially outer side of the second bracket  31 , and the left-right direction is shown as the circumferential direction of the second bracket  31 .  FIG. 8  corresponds to a schematic view in which the cross section along line XII-XII in  FIG. 2  is spread out, and the step  34  and the first limitation portion  46 A to third limitation portion  46 C of the first pawl  40 A to the third pawl  40 C are cut out. 
     The step  34  includes, sequentially in the positive direction F, a first pawl allowance portion  34   x,  the first pawl restriction portion  34   a,  the second pawl restriction portion  34   b,  a second pawl allowance portion  34   y,  and the third pawl restriction portion  34   c.    
     The first pawl allowance portion  34   x  allows the first pawl  40 A to move toward the radially outer side. The first pawl allowance portion  34   x  includes an inner surface  35   x  having a predetermined radius, that is, a radius extending about the rotation axis C 2 . 
     The inner surface  35   x  of the first pawl allowance portion  34   x  is configured such that, when the first pawl  40 A moves toward the radially outer side with the first limitation portion  46 A being located at the first pawl allowance portion  34   x,  the first limitation portion  46 A does not comes into contact with the inner surface  35   x  until the outer teeth  43 A of the first pawl  40 A mesh with the inner teeth  37 . That is, the distance LRa between the inner teeth  37  of the second bracket  31  and the inner surface  35   x  of the first pawl allowance portion  34   x  shown in  FIG. 7( a )  is slightly less than or equal to the distance LP 1  between the outer teeth  43 A of the first pawl  40 A and the abutment surface  48 A of the first limitation portion  46 A shown in  FIG. 6( a ) . 
     The first pawl restriction portion  34   a  restricts the first pawl  40 A to move toward the radially outer side. The first pawl restriction portion  34   a  includes the first inner surface  35   a  having a smaller radius than the inner surface  35   x  of the first pawl allowance portion  34   x.  The first pawl restriction portion  34   a  extends in the positive direction F from the first pawl allowance portion  34   x  and is longer in the circumferential direction than the second pawl restriction portion  34   b.  More specifically, the first pawl restriction portion  34   a  extends to a negative side end  36   b  of the second pawl restriction portion  34   b.    
     The first inner surface  35   a  of the first pawl restriction portion  34   a  is configured such that, when the first pawl  40 A moves toward the radially outer side with the first limitation portion  46 A being located on the first pawl restriction portion  34   a,  the outer teeth  43 A of the first pawl  40 A are separated from the inner teeth  37  of the second bracket  31  in a state where the first limitation portion  46 A abuts against the first inner surface  35   a.    
     The first inner surface  35   a  of the first pawl restriction portion  34   a  is configured such that, when the second pawl  40 B moves toward the radially outer side with the second limitation portion  46 B being located on the first pawl restriction portion  34   a,  the second limitation portion  46 B does not come into contact with the first inner surface  35   a  of the first pawl restriction portion  34   a  until the outer teeth  43 B of the second pawl  40 B mesh with the inner teeth  37 . 
     More specifically, the distance LR 1  between the inner teeth  37  of the second bracket  31  and the first inner surface  35   a  of the first pawl restriction portion  34   a  is longer than the distance LP 1  between the outer teeth  43 A of the first pawl  40 A and the abutment surface  48 A of the first limitation portion  46 A, and slightly less than or equal to the distance LP 2  between the outer teeth  43 B of the second pawl  40 B and the abutment surface  48 B of the second limitation portion  46 B shown in  FIGS. 3 and 6 ( a ). 
     The second pawl restriction portion  34   b  restricts the second pawl  40 B to move toward the radially outer side. The second pawl restriction portion  34   b  has a second inner surface  35   b  having a smaller radius than the first inner surface  35   a  of the first pawl restriction portion  34   a.    
     The second inner surface  35   b  of the second pawl restriction portion  34   b  is configured such that, when the second pawl  40 B moves toward the radially outer side with the second limitation portion  46 B being located at the second pawl restriction portion  34   b,  the outer teeth  43 B of the second pawl  40 B are separated from the inner teeth  37  of the second bracket  31  in a state where the second limitation portion  46 B abuts against the second inner surface  35   b.  That is, the distance LR 2  between the inner teeth  37  of the second bracket  31  and the second inner surface  35   b  of the second pawl restriction portion  34   b  shown in  FIG. 7( a )  is longer than the distance LP 2  between the outer teeth  43 B of the second pawl  40 B and the second limitation portion  46 B. 
     The second pawl allowance portion  34   y  allows the third pawl  40 C to move toward the radially outer side. The second pawl allowance portion  34   y  has an inner surface  35   y  having a larger radius than the second inner surface  35   b  of the second pawl restriction portion  34   b.    
     The inner surface  35   y  of the second pawl allowance portion  34   y  is configured such that, when the third pawl  40 C moves toward the radially outer side with the third limitation portion  46 C being located at the second pawl allowance portion  34   y,  the third limitation portion  46 C does not come into contact with the inner surface  35   y  of the second pawl allowance portion  34   y  until the outer teeth  43 C of the third pawl  40 C mesh with the inner teeth  37 . That is, the distance LRb between the inner teeth  37  of the second bracket  31  and the inner surface  35   y  of the second pawl allowance portion  34   y  shown in  FIG. 7( a )  is slightly less than or equal to the distance LP 3  between the outer teeth  43 C of the third pawl  40 C and the abutment surface  48 C of the third limitation portion  46 C shown in  FIG. 3 . 
     The third pawl restriction portion  34   c  restricts the third pawl  40 C to move toward the radially outer side. The third pawl restriction portion  34   c  includes a third inner surface  35   c  having a smaller radius than the first inner surface  35   a  of the first pawl restriction portion  34   a.    
     The third inner surface  35   c  of the third pawl restriction portion  34   c  is configured such that, when the third pawl  40 C moves toward the radially outer side with the third limitation portion  46 C being located at the third pawl restriction portion  34   c,  the outer teeth  43 C of the third pawl  40 C are separated from the inner teeth  37  of the second bracket  31  in a state where the third limitation portion  46 C abuts against the third inner surface  35   c.  That is, the distance LR 3  between the inner teeth  37  of the second bracket  31  and the third inner surface  35   c  of the third pawl restriction portion  34   c  shown in  FIG. 7( a )  is longer than the distance LP 3  between the third pawl  40 C and the third limitation portion  46 C. 
     The positional relationship of the first to third limitation portions  46 A to  46 C and the first to third pawl restriction portions  34   a  to  34   c  in the circumferential direction will now be described with reference to  FIG. 8 . As shown in  FIG. 8 , the distance LT 12  between a side surface of the first limitation portion  46 A of the first pawl  40 A facing the positive direction F and a side surface of the second limitation portion  46 B of the second pawl  40 B facing the positive direction F is equal to the distance LU 12  from an end surface of the first pawl restriction portion  34   a  facing the negative direction N. 
     The distance LT 23  between a side surface of the second limitation portion  46 B of the second pawl  40 B facing the positive direction F and a side surface of the third limitation portion  46 C of the third pawl  40 C facing the positive direction F is equal to the distance LU 23  between an end surface of the second pawl restriction portion  34   b  facing the negative direction N and an end surface of the third pawl restriction portion  34   c  facing the negative direction N. 
     The distance LT 31  between a side surface of the third limitation portion  46 C of the third pawl  40 C facing the positive direction F and a side surface of the first limitation portion  46 A of the first pawl  40 A facing the positive direction F is equal to the distance LU 31  between an end surface of the third pawl restriction portion  34   c  facing the negative direction N and an end surface of the first pawl restriction portion  34   a  facing the negative direction N. The distance relationship of the first to third limitation portions  46 A to  46 C and the first to third pawl restriction portions  34   a  to  34   c  in the circumferential direction are hereinafter referred to as the “circumferential structure of pawl movement limitation”. 
     The positional relationship of the first to third limitation portions  46 A to  46 C in the radial direction will now be described with reference to  FIG. 9 .  FIG. 9  only shows the first to third limitation portions  46 A to  46 C, and shows the positional relationship of the first to third limitation portions  46 A to  46 C which is set by engagement with the cam  50  without force being applied to any of the first to third limitation portions  46 A to  46 C. 
     As shown in  FIG. 9 , the abutment surface  48 A of the first limitation portion  46 A is located at the radially outer side from the abutment surface  48 B of the second limitation portion  46 B. The separation distance LH 12  between the abutment surface  48 A of the first limitation portion  46 A and the second limitation portion  46 B is larger than the step distance LZ 12  between the first inner surface  35   a  of the first pawl restriction portion  34   a  and the second inner surface  35   b  of the second pawl restriction portion  34   b.  The difference between the separation distance LH 12  and the step distance LZ 12  is set to a slight distance that can be canceled by pushing force of external factors (described below). 
     The abutment surface  48 A of the first limitation portion  46 A is located at the radially outer side from the abutment surface  48 C of the third limitation portion  46 C. The separation distance LH 13  between the abutment surface  48 A of the first limitation portion  46 A and the abutment surface  48 C of the third limitation portion  46 C is larger than the step distance LZ 13  between the first inner surface  35   a  of the first pawl restriction portion  34   a  and the third inner surface  35   c  of the third pawl restriction portion  34   c.  The difference between the separation distance LH 13  and the step distance LZ 13  is set to a slight distance that can be canceled by pushing force of external factors (described below). 
     The distance relationship of the first to third limitation portions  46 A to  46 C and the first to third pawl restriction portions  34   a  to  34   c  in the radial direction is hereinafter referred to as the “radial structure of pawl movement limitation”. In the “circumferential structure of pawl movement limitation” and the “radial structure of pawl movement limitation,” when the rotation angle of the second bracket  31  relative to the first bracket  21  is a predetermined value (hereinafter referred to as specified angle), positive side corners  49 A to  49 C of the first to third limitation portions  46 A to  46 C may abut against negative side ends  36   a  to  36   c  of the first to third pawl restriction portions  34   a  to  34   c,  respectively. 
     The operation of the vehicle seat reclining device  10  will now be described with reference to  FIGS. 10 to 12 .  FIG. 10  shows a locked state in which the outer teeth  43 A to  43 C of the pawls  40 A to  40 C are meshed with the inner teeth  37  of the second bracket  31 .  FIG. 11  shows an unlocked state in which the outer teeth  43 A to  43 C of the pawls  40 A to  40 C are not meshed with the inner teeth  37  of the second bracket  31  when each of the pawls  40 A to  40 C is kept at the radially inner side.  FIG. 12  shows a lock limitation state in which the outer teeth  43 A to  43 C of the pawls  40 A to  40 C do not mesh with the inner teeth  37  of the second bracket  31  when the first pawl  40 A is limited to move toward the radially outer side. 
     The vehicle seat reclining device  10  performs the two basic operations described below. The first basic operation is a movement of each of the pawls  40 A to  40 C when the operation lever  5   a  is operated to rotate the cam  50 . The second basic operation limits the movement of each of the pawls  40 A to  40 C controlled by the rotation angle of the second bracket  31 . In the first basic operation, the first to third pawls  40 A to  40 C operate in the same manner. The first pawl  40 A will now be described as an example. 
     The cam  50  is biased to rotate in the biasing direction RB. When the cam  50  rotates in the biasing direction RB, the first cam portion  55  pushes the first cam surface  44 A of the first pawl  40 A, thus, the first pawl  40 A moves toward the radially outer side. As shown in  FIG. 10 , when the first cam portion  55  pushes the first pawl  40 A toward the radially outer side, the outer teeth  43 A of the first pawl  40 A mesh with the inner teeth  37  of the second bracket  31 . Accordingly, the second bracket  31  is fixed to the first bracket  21  and the vehicle seat reclining device  10  is shifted to the locked state. 
     When the operation lever  5   a  is operated to rotate the cam  50  in the direction opposite to the biasing direction RB, the pawl engagement portion  52  of the cam  50  pushes the inner surface of the cam hole  47 A of the first pawl  40 A, and thus, the first pawl  40 A moves toward the radially inner side. After that, the outer teeth  43 A of the first pawl  40 A are separated from the inner teeth  37  of the second bracket  31  as shown in  FIG. 11 . As a result, the second bracket  31  becomes non-rotatable relative to the first bracket  21  and the vehicle seat reclining device  10  is shifted to the unlocked state. 
     The second basic operation will now be described with reference to  FIG. 12 . As shown in  FIG. 12 , when the first pawl restriction portion  34   a  of the second bracket  31  is located at the first limitation portion  46 A of the first pawl  40 A, cancellation of the operation of the operation lever  5   a  rotates the cam  50  in the biasing direction RB. The rotation of the cam  50  moves the first pawl  40 A toward the radially outer side so that the first limitation portion  46 A of the first pawl  40 A abuts against the first pawl restriction portion  34   a.  Accordingly, the first pawl  40 A is prevented from being moved before the first pawl  40 A moves toward the radially outermost side. That is, when the first pawl  40 A is limited to move in the radial direction, the outer teeth  43 A of the first pawl  40 A remain separated from the inner teeth  37  of the second bracket  31 . Further, abutment of the first limitation portion  46 A of the first pawl  40 A against the first pawl restriction portion  34   a  of the second bracket  31  stops rotation of the cam  50  and restricts the second pawl  40 B and the third pawl  40 C to move toward the outer side. As a result, the outer teeth  43 B of the second pawl  40 B and the outer teeth  43 C of the third pawl  40 C are separated from the inner teeth  37  of the second bracket  31 . In such a manner, when the first pawl restriction portion  34   a  of the second bracket  31  is located at the first limitation portion  46 A of the first pawl  40 A, the vehicle seat reclining device  10  becomes into the lock limitation state in which the vehicle seat reclining device  10  is limited to become the locked state, that is, a state in which the second bracket  31  is allowed to rotate. 
     The changes in the positional relationship of the first pawl  40 A to the third pawl  40 C and the first pawl restriction portion  34   a  to the third pawl restriction portion  34   c  relative to rotation of the second bracket  31  will now be described with reference to  FIGS. 13( a ) to 13( d )  and  FIGS. 14( a ) to 14( d ) . 
       FIGS. 13( a ) and 14( a )  show the positional relationship of the first pawl  40 A to the third pawl  40 C and the first pawl restriction portion  34   a  to the third pawl restriction portion  34   c  when the second bracket  31  is slightly rotated from a specified angle relative to the first bracket  21  in the positive direction F. The first limitation portion  46 A of the first pawl  40 A is located at the first pawl allowance portion  34   x,  the second limitation portion  46 B of the second pawl  40 B is located at the first pawl restriction portion  34   a,  and the third limitation portion  46 C of the third pawl  40 C is located at the second pawl allowance portion  34   y.  The first pawl allowance portion  34   x  allows the first pawl  40 A to move toward the radially outer side, the first pawl restriction portion  34   a  allows the second pawl  40 B to move toward the radially outer side, and the second pawl allowance portion  34   y  allows the third pawl  40 C to move toward the radially outer side. Thus, the vehicle seat reclining device  10  may shift to the locked state. 
       FIGS. 13( b ) and 14( b )  show the positional relationship of the first limitation portion  46 A to the third limitation portion  46 C of the first pawl  40 A to the third pawl  40 C when the second bracket  31  is rotated by a predetermined angle from the specified angle relative to the first bracket  21  in the positive direction F. The third limitation portion  46 C of the third pawl  40 C approaches an end of the second pawl restriction portion  34   b  facing the positive direction F. The first limitation portion  46 A to the third limitation portion  46 C of the first pawl  40 A to the third pawl  40 C and the step  34  of the second bracket  31  is substantially equal to the relationship shown in  FIG. 14( a ) . Thus, the vehicle seat reclining device  10  may shift to the locked state. An angle range in which the first limitation portion  46 A of the first pawl  40 A is disposed at the position of the first pawl allowance portion  34   x  is defined as a “second bracket rotation limitation range” (in other words, referred to as a locked zone). 
       FIGS. 13( c ) and 14( c )  show the positional relationship of the first limitation portion  46 A to the third limitation portion  46 C of the first pawl  40 A to the third pawl  40 C when the second bracket  31  is rotated slightly from the specified angle relative to the first bracket  21  in the negative direction N. The first limitation portion  46 A of the first pawl  40 A is located at the first pawl restriction portion  34   a  and is in contact with the first pawl restriction portion  34   a.  Thus, the vehicle seat reclining device  10  may shift to the lock limitation state. The second limitation portion  46 B of the second pawl  40 B is located at the second pawl restriction portion  34   b,  and the third limitation portion  46 C of the third pawl  40 C is located at the third pawl restriction portion  34   c.  The first pawl restriction portion  34   a  restricts the first pawl  40 A to move toward the radially outer side. The second limitation portion  46 B of the second pawl  40 B is not in contact with the second pawl restriction portion  34   b.  However, when an external factor (described below) applies a pushing force, the second limitation portion  46 B of the second pawl  40 B may be restricted to move toward the radially outer side by the second pawl restriction portion  34   b.  The third limitation portion  46 C of the third pawl  40 C is not in contact with the third pawl restriction portion  34   c.  However, when the external factor applies the pushing force, the third limitation portion  46 C of the third pawl  40 C may be restricted to move toward the radially outer side by the third pawl restriction portion  34   c.  To facilitate illustration,  FIG. 14( c )  shows that the second limitation portion  46 B is in contact with the second pawl restriction portion  34   b  and the third limitation portion  46 C is in contact with the third pawl restriction portion  34   c.  Nevertheless, as described above, each limitation portion is normally not in contact with the corresponding pawl restriction portion when the external factor does not apply the pushing force. 
       FIGS. 13( d ) and 14( d )  show the positional relationship of the first limitation portion  46 A to the third limitation portion  46 C of the first pawl  40 A to the third pawl  40 C when the second bracket  31  is rotated by the predetermined angle from the specified angle relative to the first bracket  21  in the negative direction N. The first limitation portion  46 A of the first pawl  40 A is located at the first pawl restriction portion  34   a,  and the side surface of the first limitation portion  46 A of the first pawl  40 A facing the positive direction approaches the second pawl restriction portion  34   b.  Accordingly, the first pawl  40 A is restricted to move toward the radially outer side, and the second bracket  31  is restricted to move in the negative direction N. Further, when the restriction of the movement of the first pawl  40 A toward the radially outer side stops rotation of the cam  50 , the second pawl  40 B and the third pawl  40 C are restricted to move toward the radially outer side. In this manner, the vehicle seat reclining device  10  shifts to the lock limitation state. An angle range in which the first limitation portion  46 A of the first pawl  40 A is disposed at the position of the first pawl restriction portion  34   a  is defined as a “second bracket rotation allowance range” (in other words, referred to as a free zone). 
     Here, the range of  FIGS. 13( c ) and 13( d )  is the “second bracket rotation allowance range” in which rotation of the second bracket  31  with respect to the first bracket  21  is allowed. The range of  FIGS. 13( a ) and 13( b )  is the “second bracket rotation limitation range” in which rotation of the second bracket  31  with respect to the first bracket  21  is limited. 
     The second limitation portion  46 B and the third limitation portion  46 C when the first limitation portion  46 A transitions from the angle at which the first limitation portion  46 A engages with the first pawl restriction portion  34   a  ( FIGS. 13( c ) and 13( d ) : the “second bracket rotation allowance range”) to the angle at which the first limitation portion  46 A engages with the first pawl allowance portion  34   x  ( FIGS. 13( a ) and 13( b ) : the “second bracket rotation limitation range”) among the rotation angles of the second bracket  31  with respect to the first bracket  21  will be described. 
     When the second bracket  31  rotates from the “second bracket rotation allowance range” toward the “second bracket rotation limitation range”, the second limitation portion  46 B passes above the second pawl restriction portion  34   b,  and the third limitation portion  46 C passes above the third pawl restriction portion  34   c.  At this time, the negative side end  102 B of the second limitation portion  46 B and the positive side end  106 B of the second pawl restriction portion  34   b  come into contact with each other in a direction inclined with respect to the rotation direction of the second bracket  31  with respect to the first bracket  21  due to the first clearance portion  100 B, and thus catching is prevented. Similarly, the negative side end  102 C of the third limitation portion  46 C and the positive side end  106 C of the third pawl restriction portion  34   c  come into contact with each other in a direction inclined with respect to the rotation direction of the second bracket  31  with respect to the first bracket  21  due to the first clearance portion  100 C, and thus catching is prevented. That is, the inclined surface  104 B of the first clearance portion  100 B has a surface shape that prevents catching when returning from the free zone to the locked zone. Similarly, the inclined surface  104 C of the second clearance portion  100 C has a surface shape that prevents catching when returning from the free zone to the locked zone. 
     The step structure of the second bracket  31  will now be described with reference to  FIG. 14( c ) . In the lock limitation state shown in  FIG. 14( c ) , the first limitation portion  46 A of the first pawl  40 A is in contact with the first pawl restriction portion  34   a.  The second limitation portion  46 B of the second pawl  40 B may be separated from or come into sliding contact with the second pawl restriction portion  34   b.  The third limitation portion  46 C of the third pawl  40 C may be separated from or come into sliding contact with the third pawl restriction portion  34   c.  When the second bracket  31  rotates in the positive direction F from the lock limitation state shown in  FIG. 14( c ) , the first limitation portion  46 A of the first pawl  40 A comes into sliding contact with the first pawl restriction portion  34   a.  Continuous rotation of the second bracket  31  gradually decreases the contact area of the first limitation portion  46 A of the first pawl  40 A and the first pawl restriction portion  34   a,  the contactable area of the second limitation portion  46 B of the second pawl  40 B and the second pawl restriction portion  34   b,  and the contactable area of the third limitation portion  46 C of the third pawl  40 C and the third pawl restriction portion  34   c  in the same manner. The above-described the “circumferential structure of pawl movement limitation” and the “radial structure of pawl movement limitation” maintain a contact state of the first pawl  40 A and the first pawl restriction portion  34   a,  a contactable state of the second pawl  40 B and the second pawl restriction portion  34   b,  and a contactable state of the third pawl  40 C and the third pawl restriction portion  34   c  until the second bracket  31  reaches the specified angle. When the rotation angle of the second bracket  31  becomes the specified angle, the positive side corner  49 A of the first limitation portion  46 A may abut against the negative side end  36   a  of the first pawl restriction portion  34   a,  the positive side corner  49 B of the second limitation portion  46 B may abut against the negative side end  36   b  of the second pawl restriction portion  34   b,  and the positive side corner  49 C of the third limitation portion  46 C may abut against the negative side end  36   c  of the third pawl restriction portion  34   c.    
     In the contactable state of the second limitation portion  46 B of the second pawl  40 B, movement of the second pawl  40 B toward the radially outer side allows the second limitation portion  46 B of the second pawl  40 B to come into contact with the second pawl restriction portion  34   b.  The same applies to the contactable state of the third limitation portion  46 C of the third pawl  40 C. The contactable area of the second pawl  40 B and the second pawl restriction portion  34   b  represents a contactable area when the second pawl  40 B moves toward the radially outer side so that the second limitation portion  46 B of the second pawl  40 B comes into contact with the second pawl restriction portion  34   b.  The same applies to the contactable area of the third pawl  40 C and the third pawl restriction portion  34   c.    
     In a case where there is only one limitation portion like in the related art structure, the pushing force of the cam  50  caused by external factors that cause the cam  50  to rotate concentrates in a positive side corner of the limitation portion when the second bracket  31  reaches the specified angle to reduce the contact area. Further, the cam  50  is biased by the spiral spring  60  in the biasing direction RB. Thus, when the second bracket  31  reaches the specified angle to reduce the contact area, the pushing force of the cam  50  based on the force of the spiral spring  60  concentrates in the positive side corner of the limitation portion. In this regard, the above-described structure disperses the pushing force of the cam  50  to the first limitation portion  46 A to the third limitation portion  46 C. Accordingly, deformation of the first pawl  40 A to the third pawl  40 C, in particular, the first pawl  40 A to the third pawl  40 C (in particular, the first limitation portion  46 A to the third limitation portion  46 C) is prevented. 
     The cam  50  is rotated by an external factor in the following case. The vehicle seat reclining devices  10  are arranged at the left and right sides of the seat  2 . The cams  50  of the left and right seat reclining devices  10  move in cooperation. Thus, the vehicle seat reclining devices  10  normally shift from the lock limitation state to the locked state at the same timing. However, on rare occasions, only one of the vehicle seat reclining devices  10  shifts to the locked state, and the other vehicle seat reclining device  10  is maintained in the lock limitation state. In this state, one of the vehicle seat reclining devices  10  fixes the seat back  4  to the seat cushion  3  to restrict rotation of the second bracket  31 , and the other vehicle seat reclining device  10  is maintained in the lock limitation state. In the lock limitation state, the first bracket  21  and the second bracket  31  may rotate relatively, and the cams  50  rotate easily. Thus, swinging of the seat  2  in the front-to-rear direction, vibration of the vehicle, and the like slightly swing the operation lever  5   a.  The vehicle occupant pushes the operation lever  5   a  to limit swinging of the operation lever  5   a  and performs procedures in an order reversed from unlocking. This rotates the cam  50  in the biasing direction RB, which corresponds to the case in which the cam  50  is rotated by external factors. When such a pushing operation is performed, the pushing force that rotates the operation lever  5   a  is applied to the pawls through the cam  50 . A biasing force of the spiral spring  60 , that is, a larger force than the force normally applied to the cam  50 , is applied to the pawls as the pushing force of external factors. 
     The step structure of the second bracket will now be described with reference to  FIGS. 15 and 16 .  FIG. 15  shows the step structure of the second bracket  31  in the related art. As shown in  FIG. 15 , in the structure of the related art, pawl restriction portions  134   a  and pawl allowance portions  134   b  are arranged in correspondence with first to third limitation portions  146 . The pawl restriction portions  134   a  and the pawl allowance portions  134   b  are arranged alternately in the circumferential direction. In this case, the rim of the second bracket  31  is divided into three regions, and the pawl restriction portion  134   a  and the pawl allowance portion  134   b  are arranged at each region. Thus, the same effect as the present embodiment described above is obtained. However, the following problem occurs. 
     The range in which the first pawl  40 A to the third pawl  40 C are allowed to move toward the radially outer side at the step  34  of the second bracket  31  is defined as the “pawl movement allowance range AQ”. The range in which the first pawl  40 A to the third pawl  40 C are restricted to move toward the radially outer side at the step  34  of the second bracket  31  is defined as the “pawl movement restriction range AP”. The range that is the sum of the pawl movement allowance range AQ and the pawl movement restriction range AP is defined as the “control range AR”. In the structure of the related art, the control ranges AR of the first pawl  40 A to the third pawl  40 C do not overlap. Thus, the rotation angle of the second bracket  31  corresponding to the control range AR (hereinafter referred to as control angle range) is virtually less than or equal to 120 degrees, and the control angle range cannot be greater than or equal to 120 degrees. 
     The control angle range that is greater than 120 degrees is effective for forming the vehicle seat reclining device  10  in the manner described below. For example, when a reference position is set at a position where the seat back  4  is arranged perpendicular to the seat cushion  3 , the vehicle seat reclining device  10  can be in the lock limitation state in the range from the reference position to 50 degrees in front of the seat back  4  and in the locked state in the range from the reference position to 80 degrees behind the seat back  4 . In this case, the control angle range is 130 degrees, and the control angle range can be greater than or equal to 120 degrees. The vehicle seat reclining device having the structure of the related art shown in  FIG. 15  cannot be used for a vehicle that requires such a specification. 
       FIG. 16  shows the step structure of the second bracket  31  of the first embodiment. The first pawl restriction portion  34   a  and the second pawl restriction portion  34   b  are located at different positions in the radial direction. Further, the first pawl restriction portion  34   a  of the second bracket  31  extends to the negative side end  36   b  of the second pawl restriction portion  34   b,  which differs from the structure of the related art. In this case, the pawl movement restriction range AP of the first pawl  40 A overlaps the pawl movement allowance range AQ of the second pawl  40 B (this structure is hereinafter referred to as overlapping structure of control range AR). This structure expands the control range AR and the control angle range of the second bracket  31  as compared with the structure of the related art in which the pawl movement restriction range AP of the first pawl  40 A does not overlap the pawl movement allowance range AQ of the second pawl  40 B. 
     In the vehicle seat reclining device  10  (see  FIG. 1 ), as shown in  FIG. 13( a ) , when the vehicle seat reclining device  10  is in the locked state, the first limitation portion  46 A to the third limitation portion  46 C are respectively in contact with the first pawl allowance portion  34 x, the first pawl restriction portion  34   a,  and the second pawl restriction portion  34   b,  or may be in a contactable state, namely, a state of coming into contact when force is applied from the cam  50 . 
     As shown in  FIG. 14( c ) , when the vehicle seat reclining device  10  is in the lock limitation state, only the first limitation portion  46 A is in contact with the first pawl restriction portion  34   a.  When the first limitation portion  46 A is in contact with the first pawl restriction portion  34   a,  the second pawl  40 B and the third pawl  40 C are limited to move in the radial direction by the cam  50 . Thus, the second limitation portion  46 B of the second pawl  40 B and the third limitation portion  46 C of the third pawl  40 C are not in contact with the step  34 . 
     More specifically, as shown in  FIG. 14( a ) , when the vehicle seat reclining device  10  is in the locked state and excessive load may be applied to the first pawl  40 A to the third pawl  40 C, the load is dispersed by the first limitation portion  46 A of the first pawl  40 A, the second limitation portion  46 B of the second pawl  40 B, and the third limitation portion  46 C of the third pawl  40 C being in contact with the step  34 . As shown in  FIG. 14( c )  or  14 ( d ), when the vehicle seat reclining device  10  is in the lock limitation state and excessive load is not applied to the first pawl  40 A to the third pawl  40 C, the positions of the second pawl  40 B and the third pawl  40 C in the radial direction are controlled by the cam  50  by the first pawl  40 A being in contact with the step  34 . In such a manner, in the first embodiment, the number of the first limitation portion  46 A to the third limitation portion  46 C that are in contact with the step  34  changes in accordance with whether the vehicle seat reclining device  10  is in the locked state or the lock limitation state (hereinafter referred to as limitation portion engagement structure). This structure expands the control angle and obtains strength for the locked state. 
     As described above, according to the vehicle seat reclining device  10  of the present embodiment, the negative side end  102 B of the second limitation portion  46 B is provided with the first clearance portion  100 B having a corner portion removed and having the inclined surface  104 B of which a height is changed in the radial direction. When the first limitation portion  46 A transitions from the angle at which the first limitation portion  46 A engages with the first pawl restriction portion  34   a  to the angle at which the first limitation portion  46 A engages with the first pawl allowance portion  34   x  among the rotation angles of the second bracket  31  with respect to the first bracket  21 , the negative side end  102 B of the second limitation portion  46 B and the positive side end  106 B of the second pawl restriction portion  34   b  come into contact with each other in a direction inclined with respect to the rotation direction of the second bracket  31  with respect to the first bracket  21  due to the first clearance portion  100 B. Therefore, it is possible to provide a vehicle seat reclining device  10  capable of preventing catching when the second bracket  31  rotates from the “second bracket rotation allowance range” toward the “second bracket rotation limitation range”. 
     The first clearance portion  100 B is preferably provided at the negative side end  102 B of the second limitation portion  46 B. According to this configuration, it is assumed that the second bracket  31  has setting ranges and shapes of the pawl restriction portions set finely as the number of pawls increases. Therefore, when the first clearance portion  100 B is provided at the pawl restriction portion side, there is a concern of being affected by the normal movement of allowance and restriction of the radial movement of the pawls. However, by providing the first clearance portion  100 B at the second limitation portion  46 B on the second pawl  40 B side as in the above configuration, it is possible to prevent catching of the rotation of the second bracket  31  while obtaining the normal movement during the radial movement of the second pawl  40 B without being affected by allowance and restriction of the radial movement of the second pawl  40 B. 
     The third pawl  40 C of the plurality of pawls preferably includes the third limitation portion  46 C that restricts the movement in the radial direction. The second bracket  31  preferably includes the third pawl restriction portion  34   c  provided in correspondence with the third pawl  40 C to restrict the third pawl  40 C to move in the radial direction by engaging with the third limitation portion  46 C of the third pawl  40 C. The third pawl restriction portion  34   c  is preferably disposed on the positive side from the second pawl restriction portion  34   b,  and on the radially inner side, at the same position in the radial direction, or on the radially outer side of the second pawl restriction portion  34   b.  Either the negative side end  102 C of the third limitation portion  46 C is preferably provided with the second clearance portion  100 C having an inclined surface  104 C of which a height is changed in the radial direction. When the first limitation portion transitions from the angle at which the first limitation portion engages with the first pawl restriction portion to the angle at which the first limitation portion engages with the first pawl allowance portion among the rotation angles of the second bracket  21  with respect to the first bracket  31 , the negative side end  102 C of the third limitation portion  46 C and the positive side end  106 C of the third pawl restriction portion  34   c  preferably come into contact with each other in a direction inclined with respect to the rotation direction of the second bracket  31  with respect to the first bracket  21  due to the second clearance portion  100 C. According to this configuration, by setting the third pawl  40 C similarly to the second pawl  40 B, the forces applied to each pawl can be dispersed. Further, similarly as the second pawl  40 B, by providing the second clearance portion  100 C on the third limitation portion  46 C of the third pawl  40 C, it is possible to further prevent catching of the rotation of the second bracket  31 . 
     The first clearance portion  100 B and the second clearance portion  100 C are preferably inclined surfaces of which heights are gradually changed in the radial direction. According to this configuration, the limitation portions of the pawls and the pawl restriction portions pass easily and smoothly, which further prevents catching. 
     The second pawl  40 B and the third pawl  40 C have sliding grooves  112 B,  112 C extending in the direction corresponding to the radial movement, and the first bracket  21  has engagement projections  110 B,  110 C slidably inserted into the sliding grooves  112 B,  112 C, respectively. The engagement projections  110 B,  110 C come into contact with the inner surfaces of the sliding grooves  112 B,  112 C when the second bracket  31 , the second pawl  40 B, and the third pawl  40 C are engaged. Thereby, the engagement projections  110 B,  110 C are biased in the direction intersecting the radial movement of the second pawl  40 B and the third pawl  40 C. Such bias can prevent rattling of the second pawl  40 B and the third pawl  40 C. Further, since the engagement projections  110 B,  110 C and the sliding grooves  112 B,  112 C are provided to the first bracket  21 , the second pawl  40 B, and the third pawl  40 C, which constitute the vehicle seat reclining device  10 , the number of components can be prevented from increasing. In addition, since the engagement projections  110 B,  110 C are inserted into the sliding grooves  112 B,  112 C when the second pawl  40 B and the third pawl  40 C are assembled to the first bracket  21 , the assembly can be improved. Therefore, it is possible to provide a vehicle seat reclining device  10  that prevents rattling of pawls and improves assembly without increasing the number of components. 
     In the vehicle seat reclining device  10 , the engagement projections  110 B,  110 C are preferably provided to the first bracket  21 , and the sliding grooves  112 B,  112 C are preferably provided to the at least one of the plurality of pawls. Thereby, the sliding grooves  112 B,  112 C are formed on the second pawl  40 B and the third pawl  40 C, which are internal components of the vehicle seat reclining device  10 , and thus are easy to from and are effective. 
     In the vehicle seat reclining device  10 , the sliding grooves  112 B,  112 C preferably penetrate in the thickness direction. Thereby, when the second pawl  40 B and the third pawl  40 C are assembled to the first bracket  21 , it is easy to confirm whether the engagement projections  110 B,  110  C are inserted into the sliding grooves  112 B,  112 C, which improves visibility. 
     In the vehicle seat reclining device  10 , the inner surfaces of the sliding grooves  112 B,  112 C preferably include first gradually changing portions  114 B,  114 C and second gradually changing portions  116 B,  116 C which are provided as inclined surfaces intersecting the radial movement. The first gradually changing portions  114 B,  114 C and the second gradually changing portions  116 B,  116 C preferably have different inclination angles with respect to the radial movement of the second pawl  40 B and the third pawl  40 C. According to this configuration, by including the first gradually changing portions  114 B,  114 C and the second gradually changing portions  116 B,  116 C, which have different inclination angles with respect to the, radial movement of the second pawl  40 B and the third pawl  40 C, rattling of the second pawl  40 B and the third pawl  40 C can be further prevented. 
     In the vehicle seat reclining device  10 , the inclination angle of the second gradually changing portions  116 B,  116 C in the sliding grooves  112 B,  112 C with respect to the radial movement of the second pawl  40 B and the third pawl  40 C is preferably set larger than the inclination angle of the first gradually changing portions  114 B,  114 C, and the engagement projections  110 B,  110 C preferably comes into contact with the second gradually changing portions  116 B,  116 C when the second bracket  31 , the second pawl  40 B, and the third pawl  40 C are engaged. According to this configuration, when the second bracket  31 , the second pawl  40 B, and the third pawl  40 C are engaged, rattling of the second pawl  40 B and the third pawl  40 C can be further prevented. 
     An embodiment for carrying out the present invention has been described, and the present invention may be carried out in various modes other than the above embodiment. For example, the vehicle seat is exemplified as a wheeled vehicle seat, but is not limited to a wheeled vehicle, and may be applied to various vehicles such as a ship and an aircraft. 
     This application is based on Japanese Patent Application 2017-103576 filed on May 25, 2017, and Japanese Patent Application 2018-031916 filed on Feb. 26, 2018, contents of which are incorporated by reference herein. 
     INDUSTRIAL APPLICABILITY 
     According to the seat reclining device for vehicle of the present invention, it is possible to provide a vehicle seat reclining device that prevents rattling of pawls and improves assembly without increasing the number of components. The present invention having this effect is useful for, for example, a seat reclining device for vehicle. 
     REFERENCE SIGNS LIST 
       1  Vehicle floor 
       2  Seat 
       3  Seat cushion 
       3   a  Plate 
       4  Seat back 
       4   a  Plate 
       5  Shaft 
       5   a  Operation lever 
       10  Vehicle seat reclining device 
       21  First bracket 
       22  Body 
       22   a  Inner surface 
       22   b  Outer surface 
       22   c  Outer peripheral surface 
       23  Guide 
       23   a  Inner side surface 
       24  Guide surface 
       25  Accommodation portion 
       25   a  Communication groove 
       26  Guide groove 
       27  Projection 
       31  Second bracket 
       32  Body 
       32   a  Insertion hole 
       33  Outer peripheral wall 
       33   a  Inner peripheral surface 
       33   b  Outer peripheral surface 
       33   c  Outer surface 
       34  Step 
       34   a  First pawl restriction portion 
       34   b  Second pawl restriction portion 
       34   c  Third pawl restriction portion 
       34   x  First pawl allowance portion 
       34   y  Second pawl allowance portion 
       35   a  First inner surface 
       35   b  Second inner surface 
       35   c  Third inner surface 
       35   x  Inner surface 
       35   y  Inner surface 
       36   a  Negative side end 
       36   b  Negative side end 
       36   c  Negative side end 
       37  Inner teeth 
       40 A First pawl 
       41 A First block 
       42 A Second block 
       43 A Outer teeth 
       44 A First cam surface 
       46 A First limitation portion 
       47 A Cam hole 
       48 A Abutment surface 
       49 A Positive side corner 
       40 B Second pawl 
       43 B Outer teeth 
       44 B Second cam surface 
       46 B Second limitation portion 
       48 B Abutment surface 
       49 B Positive side corner 
       40 C Third pawl 
       43 C Outer teeth 
       44 C Third cam surface 
       46 C Third limitation portion 
       48 C Abutment surface 
       49 C Positive side corner 
       50  Cam 
       50   a  First surface 
       50   b  Second surface 
       51  Cam body 
       52  Pawl engagement portion 
       53  Spring engagement portion 
       54  Fitting hole 
       55  First cam portion 
       55   a  First pushing portion 
       55   b  Second pushing portion 
       56  Second cam portion 
       56   a  First pushing portion 
       56   b  Second pushing portion 
       57  Third cam portion 
       57   a  First pushing portion 
       57   b  Second pushing portion 
       60  Spiral spring 
       61  Spiral portion 
       62  Outer engagement portion 
       62   a  End 
       63  Inner engagement portion 
       80  Holding member 
       81  Body 
       82  Flange 
       83  Projection 
       134   a  Pawl restriction portion 
       134   b  Pawl allowance portion 
       146  First limitation portion to third limitation portion 
     AP Pawl movement restriction range 
     AQ Pawl movement allowance range 
     AR Control range 
     C 1  Rotation axis 
     C 2  Rotation axis 
     F Positive direction 
     N Negative direction 
     LRa Distance 
     LRb Distance 
     LR 1  Distance 
     LR 2  Distance 
     LR 3  Distance 
     LP 1  Distance 
     LP 2  Distance 
     LP 3  Distance 
     LH 12  Separation distance 
     LH 13  Separation distance 
     LT 12  Distance 
     LT 23  Distance 
     LT 31  Distance 
     LU 12  Distance 
     LU 23  Distance 
     LU 31  Distance 
     LZ 12  Step distance 
     LZ 13  Step distance 
     RB Biasing direction 
     RX Rear rotation direction 
       100 B First clearance portion 
       110 A Engagement projection 
       112 A Sliding groove 
       102 B Negative side end 
       104 B Inclined surface 
       106 B Positive side end 
       110 B Engagement projection 
       112 B Sliding groove 
       112 Ba Arcuate surface 
       114 B First gradually changing portion 
       116 B Second gradually changing portion 
       100 C Second clearance portion 
       102 C Negative side end 
       104 C Inclined surface 
       106 C Positive side end 
       110 C Engagement projection 
       112 C Sliding groove 
       112 Ca Arcuate surface 
       114 C First gradually changing portion 
       116 C Second gradually changing portion