Patent Publication Number: US-2022219585-A1

Title: Headrest level adjustment mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-004522 filed on Jan. 14, 2021, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a headrest level adjustment mechanism. 
     2. Description of Related Art 
     Japanese Unexamined Patent Application Publication No. 2010-136770 (JP 2010-136770 A) discloses a technology concerning a locking mechanism of a headrest support that is engaged with a headrest stay. When an unlocking knob provided on the headrest support is pressed, to press the headrest downward, the headrest can be lowered until a lower surface of the headrest abuts against an upper surface of a seat back. Namely, according to the related art, the headrest is placed at the lowest-level position when it abuts against the seat back. 
     SUMMARY 
     Generally, the headrest is formed of foam, such as urethane foam, and can be elastically deformed. Thus, a dimension of the headrest measured in the vehicle vertical direction varies due to the elastic deformation. Accordingly, when the position at which the headrest abuts against the seat back is specified as the lowest level of the headrest, the lowest level of the headrest is set in view of the variation in the dimension, and the size of the headrest is increased accordingly. 
     This present disclosure provides a headrest level adjustment mechanism that can make a headrest small-sized, while assuring the level of the headrest that is required at the lowest-level position of the headrest. 
     A headrest level adjustment mechanism for a headrest according to one aspect of the present disclosure includes a main support that is provided on a side of the main headrest stay, and cooperates with the main headrest stay to provide a locking mechanism operable to adjust a level of the headrest provided above a seat back and lock the headrest at a predetermined level, the main headrest stay being held by the seat back and hanging down from a first position of the headrest; and a sub support that is provided on a side of the sub headrest stay, and cooperates with the sub headrest stay to provide a movement restriction mechanism operable to restrict downward movement of the headrest at a lowest-level position at which the headrest is located at a lowest level, in a condition where the main headrest stay is engaged with the main support, and a gap is provided between the headrest and the seat back, and keep the headrest at the lowest-level position, the sub headrest stay being held by the seat back and hanging down from a second position of the headrest that is spaced from the first position in a seat width direction. 
     The headrest level adjustment mechanism according to the above aspect includes the main support and the sub support. The main headrest stay and the sub headrest stay hang down from left and right sections (in the seat width direction) of the headrest provided above the seat back, and the headrest stays are held by the seat back. 
     The main support is provided on the main headrest stay side, and cooperates with the main headrest stay to provide the locking mechanism operable to adjust the level of the headrest and lock the headrest at the predetermined level. On the other hand, the sub support is provided on the sub headrest stay side, and cooperates with the sub headrest stay to provide the movement restriction mechanism operable to restrict the downward movement of the headrest. 
     Here, the movement restriction mechanism restrict the downward movement of the headrest (namely, place the headrest in a so-called locked state) and keep the headrest at the lowest-level position at which the headrest is located at the lowest level, in a condition where the main headrest stay is engaged with the main support, and a gap is provided between the headrest and the seat back. 
     According to the above aspect, the gap is provided between the headrest and the seat back, at the lowest-level position of the headrest, so that the size of the headrest as measured in the height direction can be reduced by the amount corresponding to the gap, where the headrest has the same lowest-level position, as compared with the case where no gap is provided between the headrest and the seat back. 
     The headrest level adjustment mechanism according to the above aspect has an excellent effect that the size of the headrest can be reduced, while assuring the level of the headrest that is required at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism according to the above aspect, the locking mechanism may include a plurality of main notches included in the main headrest stay and arranged in a hanging direction of the main headrest stay, and a main lock part provided in the main support and configured to engage with one of the main notches to restrict upward and downward movement of the main headrest stay. 
     In the headrest level adjustment mechanism as described above, the locking mechanism may include the main notches and the main lock part. The main notches may be included in the main headrest stay, and be arranged in the hanging direction of the main headrest stay (in the height direction of the headrest). On the other hand, the main lock part may be provided in the main support, and the main lock part may be engaged with one of the main notches included in the main headrest stay, so that the upward and downward movement of the main headrest stay (the headrest) is restricted (in the locked state). 
     The headrest level adjustment mechanism as described above has an excellent effect that the upward and downward movement of the main headrest stay can be restricted with a simple arrangement. 
     In the headrest level adjustment mechanism as described above, the main notches may include a first notch that may set the headrest at the lowest-level position, and the movement restriction mechanism may include a sub notch included in the sub headrest stay at the same level as the first notch, and a sub lock part provided in the sub support and configured to engage with the sub notch to restrict downward movement of the sub headrest stay. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include the sub notch and the sub lock part. The sub notch may be included in the sub headrest stay at a position having the same level as the first notch as one of the main notches that may set the headrest at the lowest-level position. On the other hand, the sub lock part may be provided in the sub support, and be engaged with the sub notch to restrict the downward movement of the sub headrest stay (in the locked state). 
     Namely, when the headrest is at the lowest-level position, the main lock part of the locking mechanism may engage with the main notch, to restrict the downward movement of the main headrest stay, and the sub lock part of the movement restriction mechanism may engage with the sub notch, to restrict downward movement of the sub headrest stay (the headrest) (in the locked state). 
     Thus, when the headrest is at the lowest-level position, the downward movement of the headrest is restricted at the sub headrest stay side, as well as the main headrest stay side; therefore, there is no need to set the lowest-level position of the headrest, by bringing the lower surface of the headrest into abutting contact with the upper surface of the seat back, for example. Namely, with the above arrangement, a gap can be provided between the headrest and the seat back, at the lowest-level position of the headrest. 
     The headrest level adjustment mechanism as described above has an excellent effect that the downward movement of the sub headrest stay, as well as that of the main headrest stay, can be restricted by using the sub notch and the sub lock part, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the sub notch may be a recess defined by a part of a surface of the sub headrest stay in a direction perpendicular to a hanging direction of the sub headrest stay, and the sub lock part may be allowed to move into the sub notch. The part of the surface defining the recess may include a horizontal portion that may define an upper side of the recess such that an upper face of the sub lock part received in the recess may be allowed to be in plane contact with the horizontal portion, and an inclined portion that may be connected to the horizontal portion and be inclined toward the surface of the sub headrest stay other than the part of the surface defining the recess, when viewed in a direction toward a lower side of the recess. 
     In the headrest level adjustment mechanism as described above, the sub notch may be a recess defined by the part of the surface of the sub headrest stay in the direction perpendicular to the hanging direction of the sub headrest stay, and the sub lock part may be allowed to move into the recess. 
     Here, the part of the surface defining the recess may include the horizontal portion and the inclined portion, and the horizontal portion may define the upper side of the recess, and be allowed to be in plane contact with the upper face of the sub lock part received in the recess. In the condition where the upper face of the sub lock part is in contact (or plane contact) with the horizontal portion of the recess, the downward movement of the sub headrest stay (the headrest) in which the recess may be included is restricted (in the locked state). 
     On the other hand, the inclined portion may be connected to the horizontal portion, and be inclined toward the surface of the sub headrest stay other than the part of the surface defining the recess, when viewed in the direction toward the lower side of the recess. Thus, the inclined portion functions as a so-called cam face, and the force applied so as to lift the sub headrest stay can be converted into the force that moves the sub lock part outward from the recess. In this manner, the locked state established by the sub lock part is released. 
     Namely, with the above arrangement, the locked state of the sub lock part is maintained, and the headrest cannot be moved downward, at the lowest-level position of the headrest. However, the locked state of the sub lock part is released when the headrest is moved upward. 
     With the above arrangement, the headrest level adjustment mechanism has an excellent effect that the locked state of the sub headrest stay can be released, when the headrest is moved upward while it is in the locked state where the downward movement of the sub headrest stay is restricted. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include a collar that may be provided on the sub headrest stay, and be configured to abut against the headrest and the sub support, to restrict downward movement of the sub headrest stay, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include the color provided on the sub headrest stay. The color may abut against the headrest and the sub support, at the lowest-level position of the headrest, so as to restrict the downward movement of the sub headrest stay (the headrest) (in the locked state). 
     Namely, when the headrest is at the lowest-level position, the main lock part of the locking mechanism may engage with the main notch, to restrict the downward movement of the main headrest stay, and the collar may abut against the headrest and the sub support, to restrict the downward movement of the sub headrest stay. 
     With this arrangement, the headrest level adjustment mechanism has an excellent effect that the downward movement of the sub headrest stay can be restricted by using the collar, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include a bent portion that may be included in the sub headrest stay, and be configured to interfere with the sub support to restrict downward movement of the sub headrest stay, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include the bent portion included in the sub headrest stay. When the headrest is at the lowest-level position, the bent portion may interfere with the sub support, so as to restrict the downward movement of the sub headrest stay (the headrest) (in the locked state). 
     Namely, when the headrest is at the lowest-level position, the main lock part of the locking mechanism may engage with the main notch, to restrict the downward movement of the main headrest stay, and the bent portion included in the sub headrest stay may interfere with the sub support, to restrict the downward movement of the sub headrest stay. 
     With this arrangement, the headrest level adjustment mechanism has an excellent effect that the downward movement of the sub headrest stay can be restricted by the bending portion included in the sub headrest stay, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include an expanded portion that may be included in the sub headrest stay, and be configured to interfere with the sub support to restrict downward movement of the sub headrest stay, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include the expanded portion included in the sub headrest stay. When the headrest is at the lowest-level position, the expanded portion may interfere with the sub support, to restrict the downward movement of the sub headrest stay (the headrest) (in the locked state). 
     Namely, when the headrest is at the lowest-level position, the main lock part of the locking mechanism may engage with the main notch, to restrict the downward movement of the main headrest stay, and the expanded portion included in the sub headrest stay may interfere with the sub support, to restrict the downward movement of the sub headrest stay. 
     With this arrangement, the headrest level adjustment mechanism has an excellent effect that the downward movement of the sub headrest stay can be restricted by the expanded portion included in the sub headrest stay, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include an expanded portion that may be provided on the sub headrest stay, and be configured to interfere with the sub support to restrict downward movement of the sub headrest stay, at the lowest-level position of the headrest. 
     In the headrest level adjustment mechanism as described above, the movement restriction mechanism may include the expanded portion provided on the sub headrest stay. When the headrest is at the lowest-level position, the expanded portion may interfere with the sub support, to restrict the downward movement of the sub headrest stay (the headrest) (in the locked state). 
     Namely, when the headrest is at the lowest-level position, the main lock part of the locking mechanism may engage with the main notch, to restrict the downward movement of the main headrest stay, and the expanded portion provided on the sub headrest stay may interfere with the sub support, to restrict the downward movement of the sub headrest stay. 
     With this arrangement, the headrest level adjustment mechanism has an excellent effect that the downward movement of the sub headrest stay can be restricted by the expanded portion provided on the sub headrest stay, at the lowest-level position of the headrest. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG. 1  is a cross-sectional view as seen from the front side, showing a headrest level adjustment mechanism of one embodiment; 
         FIG. 2  is an exploded perspective view showing the headrest level adjustment mechanism of the embodiment; 
         FIG. 3  is an enlarged cross-sectional view showing a principal part of the headrest level adjustment mechanism of the embodiment shown in  FIG. 1 ; 
         FIG. 4  is an exploded perspective view showing the headrest level adjustment mechanism of the embodiment; 
         FIG. 5A  is a cross-sectional view as seen from the front side, showing a headrest level adjustment mechanism as a comparative example; 
         FIG. 5B  is a cross-sectional view as seen from the front side, showing the headrest level adjustment mechanism of the embodiment; 
         FIG. 6  is a front view useful for describing the operation of the headrest level adjustment mechanism of the embodiment; 
         FIG. 7  is a cross-sectional view as seen from the front side, showing a first modified example of the headrest level adjustment mechanism of the embodiment; 
         FIG. 8A  is a cross-sectional view as seen from the front side, showing a second modified example of the headrest level adjustment mechanism of the embodiment; 
         FIG. 8B  is a cross-sectional view as seen from one side, showing the second modified example of the headrest level adjustment mechanism of the embodiment; and 
         FIG. 9  is a cross-sectional view as seen from one side, showing a third modified example of the headrest level adjustment mechanism of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     A headrest using a headrest level adjustment mechanism according to one embodiment will be described with reference to the drawings. In the drawings, arrow FR indicated as needed indicates the front direction (in which a seated person faces) of a vehicle seat, and arrow UP indicates the upward direction of the vehicle seat, while arrow RH indicates the right-hand direction of the vehicle seat. In the following description, when front and back directions, upward and downward directions, and right-hand and left-hand directions are simply mentioned, they indicate the front and back directions of the vehicle seat, upward and downward directions of the vehicle seat, and right-hand and left-hand directions of the vehicle seat when it faces in the traveling direction, respectively. 
     Configuration of Headrest Level Adjustment Mechanism 
     Initially, the configuration of the headrest level adjustment mechanism according to this embodiment will be described. 
     As shown in  FIG. 1 , a vehicle seat  10  includes a seat cushion (not shown) on which a vehicle occupant is seated, a seat back  12  that is erected from a rear end portion of the seat cushion, and supports the upper body of the occupant seated on the seat cushion, and a headrest  14  that is attached to an upper end portion  12 A of the seat back  12 , and supports the head of the occupant seated on the seat cushion. 
     The headrest  14  is formed of foam, such as urethane foam, for example, such that the headrest  14  can be elastically deformed. Also, the headrest  14  has a trapezoidal shape in front view, and has a generally triangle shape in side view, and a pair of headrest stays  16  (legs) hang down from a lower surface  14 A of the headrest  14 , more specifically, from right and left positions of the headrest  14  that are spaced from each other in the seat width direction. In this embodiment, the headrest stays  16  are two cylindrical members. In this connection, the headrest stays  16  are only required to provide a pair of right and left legs, and thus may be in the form of a single member provided with legs connected to each other at their upper ends. 
     In the following description, one of the headrest stays  16  located on the outer side in the vehicle width direction will be called “main headrest stay  18 ”, and the other located on the inner side in the vehicle width direction will be called “sub headrest stay  20 ”. In this connection, the headrest stay  16  located on the inner side in the vehicle width direction may be “main headrest stay  18 ”, and the headrest stay  16  located on the outer side in the vehicle width direction may be “sub headrest stay  20 ”. 
     The main headrest stay  18  and the sub headrest stay  20  are held by the seat back  12 . Like the headrest  14 , the seat back  12  is formed of foam, such as urethane foam, and can be elastically deformed. 
     Also, insertion holes  22 ,  24  that extend in the vehicle vertical direction are respectively included in the upper portion of the seat back  12 . Each of the insertion holes  22 ,  24  has a rectangular shape in plan view. A main support  26  in the form of a rectangular tube that serves as a support member for supporting the main headrest stay  18  is inserted in the insertion hole  22 , and a sub support  28  in the form of a rectangular tube that serves as a support member for supporting the sub headrest stay  20  is inserted in the insertion hole  24 . 
     In a condition where the main support  26  is inserted in the insertion hole  22 , the main headrest stay  18  is inserted in the main support  26 . In a condition where the sub support  28  is inserted in the insertion hole  24 , the sub headrest stay  20  is inserted in the sub support  28 . 
     In this embodiment, as a level adjustment mechanism  29  for the headrest  14 , a locking mechanism  30  is provided on the main support  26  side, and a movement restriction mechanism  32  is provided on the sub support  28  side. 
     Here, the movement restriction mechanism  32  will be initially described, and the locking mechanism  30  will be then described, for the sake of expediency. 
     Movement Restriction Mechanism 
     As described above, the movement restriction mechanism  32  is provided on the sub support  28  side. 
     As shown in  FIG. 4 , the sub support  28  is provided at its upper end with a flange  34  that extends outward from the sub support  28 . The flange  34  is provided such that its longitudinal direction is in parallel with the seat width direction (indicated by arrow A). A round hole  36  that communicates with the interior of the sub support  28  in the form of the rectangular tube is included in a middle portion of the flange  34 , and the shape of the round hole  36  corresponds to the shape of the cylindrical sub headrest stay  20 . 
     As shown in  FIG. 2 , the sub support  28  has stops  38  respectively provided on generally middle portions (as viewed in the height direction of the sub support  28 ) of a pair of side walls  28 A each extending in a direction perpendicular to the seat width direction (indicated by arrow A). A through-hole  40  having an inverted U shape that is open downward is provided around each of the stops  38 , and the through-hole  40  permits the corresponding stop  38  to be elastically deformed toward the inside of the sub support  28 . 
     On the other hand, engaging holes (not shown) that are recessed in the horizontal direction are provided within the insertion hole  24  in which the sub support  28  is inserted, as shown in  FIG. 1 . When the sub support  28  is inserted in the insertion hole  24 , the stops  38  are elastically deformed inwardly of the sub support  28 , and the sub support  28  can be moved to the deep side of the insertion hole  24 . When the flange  34  of the sub support  28  abuts against an upper surface  12 B of the seat back  12 , and the stops  38  of the sub support  28  reach positions where they face the engagement holes, the stops  38  restore their shapes, and are engaged with hole walls of the engagement holes. 
     In this manner, the sub support  28  is prevented from being pulled out of the insertion hole  24  included in the upper portion of the seat back  12 , and is mounted in the insertion hole  24 . 
     Since the seat back  12  is elastically deformable, as described above, the position of the upper surface  12 B of the seat back  12  differs depending on the pressing force. Also, the insertion hole  24  has a rectangular shape in plan view, and the sub support  28  is in the form of a rectangular tube. Namely, the sub support  28  is set so as not to rotate relative to the insertion hole  24 . 
     As shown in  FIG. 4 , the flange  34  of the sub support  28  is provided with a rectangular stepped portion  42  that is recessed and has the longitudinal direction parallel to the seat width direction. A sub lock plate (sub lock part)  44  in the form of a rectangular frame having the longitudinal direction parallel to the seat width direction is housed in the stepped portion  42 , and a rectangular hole  46  is included in a middle portion of the sub lock plate  44 . 
     The longitudinal dimension of the sub lock plate  44  is set to be shorter than the longitudinal dimension of the stepped portion  42 , and the sub lock plate  44  is movable in the longitudinal direction within the stepped portion  42 . 
     Ribs  48  in the form of rectangular solids are included in the lower surface of the sub lock plate  44  to protrude therefrom, such that the ribs  48  located close to the rectangular hole  46  extend in the width direction perpendicular to the longitudinal direction of the sub lock plate  44 . The ribs  48  are designed so as to reduce the area of contact between the sub lock plate  44  and the stepped portion  42 , and reduce friction that arises when the sub lock plate  44  moves in the stepped portion  42 . 
     Also, a solid cylindrical protrusion  50  protrudes from an outer side face  44 A of the sub lock plate  44  as viewed in the longitudinal direction. One end portion of a coil spring  52  is attached to the protrusion  50 . The other end portion of the coil spring  52  abuts against an inner wall  42 A of the stepped portion  42  that is opposed to the protrusion  50 , and the sub lock plate  44  is biased by the coil spring  52 , toward an inner wall  42 B of the stepped portion  42  that is opposed to the inner wall  42 A. 
     Also, the sub lock plate  44  is designed so as to overlap a part of the round hole  36  included in the flange  34  of the sub support  28 , in a condition where the sub lock plate  44  abuts against the inner wall  42 B of the stepped portion  42 . The flange  34  of the sub support  28  is provided with a support upper  54  that covers the flange  34 . 
     The support upper  54  has substantially the same outside dimensions as the flange  34 , and is provided with a stepped portion  56  and a round hole  58 , like the flange  34 . An upper portion of the sub lock plate  44  is housed in the stepped portion  56 . The support upper  54  is joined by welding, or the like, to the flange  34  of the sub support  28 , in a condition where the sub lock plate  44  and the coil spring  52  are housed in the stepped portion  56 . 
     On the other hand, as shown in  FIG. 1  and  FIG. 3 , the sub headrest stay  20  is provided with one sub notch  60  (a notch for the lowest level that will be described later) as a recessed portion that is recessed in a radial direction (perpendicular to the hanging direction of the sub headrest stay  20 ) of the sub headrest stay  20 . The sub notch  60  is provided on one side on which the coil spring  52  is provided on the sub lock plate  44 , and the sub lock plate  44  can move into (engage with) the sub notch  60 . 
     As shown in  FIG. 3  and  FIG. 4 , in a condition where the sub headrest stay  20  is inserted in the sub support  28 , the sub headrest stay  20  is inserted through the round hole  58  of the support upper  54 , the rectangular hole  46  of the sub lock plate  44 , and the round hole  36  of the flange  34 . 
     In this condition, the sub lock plate  44  is biased by the coil spring  52 , toward the inner wall  42 B of the stepped portion  42 , as described above. The sub lock plate  44  overlaps the round hole  36 , in a condition where the sub lock plate  44  abuts against the inner wall  42 B of the stepped portion  42 . Namely, the sub lock plate  44  partially protrudes over the round hole  36 . 
     Thus, in the condition where the sub headrest stay  20  is inserted in the sub support  28 , the coil spring  52  is contracted by the sub headrest stay  20 , and the inner wall  44 C of the sub lock plate  44  closer to the coil spring  52  abuts against a surface  20 A of the sub headrest stay  20 , under the bias force of the coil spring  52 . 
     The sub notch  60  is recessed in the radial direction of the sub headrest stay  20 , as described above, and includes a horizontal portion  60 A and an inclined portion  60 B. With this arrangement, when the sub notch  60  of the sub headrest stay  20  is opposed to the sub lock plate  44 , the sub lock plate  44  moves into the sub notch  60  under the bias force of the coil spring  52 . Thus, since the sub lock plate  44  is biased by the coil spring  52  toward the sub notch  60 , the sub lock plate  44  is kept in a condition where it is received in the sub notch  60 . 
     The horizontal portion  60 A defines the upper side of the sub notch  60 , and an upper face  44 B of the sub lock plate  44  moved into the sub notch  60  abuts against the horizontal portion  60 A, such that the upper face  44 B and the horizontal portion  60 A can be in plan contact with each other. Namely, in a condition where the sub lock plate  44  is received in the sub notch  60 , and the upper face  44 B of the sub lock plate  44  is in contact or plane contact with the horizontal portion  60 A of the sub notch  60 , downward movement of the sub headrest stay  20  (the headrest  14 ) is restricted, and the sub headrest stay  20  (the headrest  14 ) is placed in a so-called locked state. 
     On the other hand, the inclined portion  60 B is connected to the horizontal portion  60 A, and is inclined toward the surface  20 A of the sub headrest stay  20  other than the part of the surface  20 A defining the recess, when viewed in a direction toward the lower side of the sub notch  60 . Thus, the force applied so as to elevate the sub headrest stay  20  can be converted into the force that moves the sub lock plate  44  from the inside of the sub notch  60  to the outside. In this manner, the locked state established by the sub lock plate  44  is released. 
     At this time, the coil spring  52  is contracted in a direction against the bias force; therefore, when the surface  20 A of the sub headrest stay  20  reaches a position where it is opposed to the inner wall  44 C of the sub lock plate  44 , the inner wall  44 C of the sub lock plate  44  abuts against the surface  20 A of the sub headrest stay  20 , under the bias force of the coil spring  52 . 
     Locking Mechanism 
     Next, the locking mechanism  30  will be described. In the locking mechanism  30 , the same names are given to its constituent members that are substantially identical with those of the movement restriction mechanism  32 , and the constituent members will not be repeatedly described. 
     As described above, the locking mechanism  30  is provided on the main support  26  side. As shown in  FIG. 1  and  FIG. 3 , a flange  62  is provided at the upper end of the main support  26 . Also, stops  66  are respectively provided on a pair of opposite side walls  26 A of the main support  26 , and the stops  66  are engaged with hole walls of engaging holes (not shown) included in the insertion hole  22 . With this arrangement, like the sub support  28 , the main support  26  is prevented from being pulled out of the insertion hole  22  included in the upper portion of the seat back  12 , and is mounted in the insertion hole  22 . 
     Like the sub support  28 , a stepped portion  68  in the form of a recess is included in the flange  62  of the main support  26 , and a main lock plate (a main lock part)  70  in the form of a generally rectangular frame is housed in the stepped portion  68 . The main lock plate  70  is movable in its longitudinal direction within the stepped portion  68 . 
     A coil spring  72  is mounted on a longitudinally outer side face  70 A of the main lock plate  70 , and the main lock plate  70  is biased by the coil spring  72 , away from an inner wall  68 A of the stepped portion  68  that is opposed to the side face  70 A. 
     Like the sub support  28 , the flange  62  of the main support  26  is provided with a support upper  74  that covers the flange  62 , and the main lock plate  70  and the coil spring  72  are housed in space between the support upper  74  and the flange  62 . 
     Also, the support upper  74  is provided with an opening  74 A. The main lock plate  70  is provided with an unlocking knob  76  on the side opposite to the side face  70 A, and the unlocking knob  76  is set to be exposed to the interior of a vehicle cabin  78 , through the opening  74 A. When the unlocking knob  76  is moved in the horizontal direction, the main lock plate  70  is moved in the stepped portion  68 . 
     On the other hand, the main headrest stay  18  is provide with a plurality of (three in this embodiment) main notches  80  that are recessed in a radial direction of the main headrest stay  18 , and are arranged in its hanging direction (in the height direction of the headrest  14 ). The main lock plate  70  can move into any of the main notches  80 . 
     In this embodiment, the main notches  80  are provided as three notches  80 A,  80 B,  80 C. The headrest  14  is located at the lowest level, in a condition where the main lock plate  70  is received in the notch  80 A provided closest to the headrest  14 , in the main headrest stay  18 . Thus, the notch  80 A is denoted as a notch for the lowest level. In the following description, the “notch  80 A” will be called “notch  80 A for lowest level”. 
     Also, the headrest  14  is placed at the highest level, in a condition where the main lock plate  70  is received in the notch  80 C provided at a position (closer to the distal end) remotest from the headrest  14 , in the main headrest stay  18 . Thus, the notch  80 C is denoted as a notch for the highest level. In the following description, the “notch  80 C” will be called “notch  80 C for highest level”. 
     Then, the notch  80 B provided between the notch  80 A for lowest level and the notch  80 C for highest level will be called “notch  80 B for middle level”. In a condition where the main lock plate  70  is received in the notch  80 B for middle level, the headrest  14  is placed at a level (middle-level position of the headrest  14 ) intermediate between the highest-level position of the headrest  14  at which the headrest  14  is at the highest level, and the lowest-level position of the headrest  14  at which the headrest  14  is at the lowest level. 
     In the evaluation of the capability of reducing whiplash injuries, which is carried out according to the European New Car Assessment Programme (E-NCAP), the middle-level position and lowest-level position of the headrest  14  are specified. Thus, the levels of the headrest  14  required at the middle-level position and the lowest-level position P of the headrest  14  need to be ensured. 
     The main notches  80  are recessed in the radial direction of the main headrest stay  18 . Like the sub notch  60  included in the sub headrest stay  20 , when any of the main notches  80  is opposed to the main lock plate  70 , the main lock plate  70  moves into the main notch  80  under the bias force of the coil spring  72 . 
     Thus, since the main lock plate  70  is biased by the coil spring  72  toward one of the main notches  80 , the main lock plate  70  is kept received in the one of the notch  80 A for lowest level, notch  80 B for middle level, and notch  80 C for highest level. With this arrangement, the level of the headrest  14  can be adjusted, and the headrest  14  is kept at the level thus adjusted. 
     In this embodiment, the notch  80 A for lowest level and the notch  80 B for middle level have substantially the same shape as the sub notch  60  included in the sub headrest stay  20 . Namely, each of the notch  80 A and notch  80 B includes a horizontal portion and an inclined portion. Thus, when the main headrest stay  18  (the headrest  14 ) is moved upward, in a condition where the main lock plate  70  is received in the notch  80 A for lowest level or the notch  80 B for middle level, the locked state established by the main lock plate  70  is released. 
     On the other hand, the notch  80 C for the highest level is not provided with any inclined portion. Namely, the notch  80 C has upper and lower horizontal portions that are opposed to each other. With this arrangement, in a condition where the main lock plate  70  is received in the notch  80 C for highest level, the locked state established by the main lock plate  70  cannot be released, even with an attempt to move the main headrest stay  18  upward. 
     Accordingly, in the condition where the main lock plate  70  is received in the notch  80 C for highest level, namely, when the headrest  14  is at the highest-level position, the locked state established by the main lock plate  70  is released by moving the main lock plate  70  by means of the unlocking knob  76 . 
     Operation and Effects of the Headrest Level Adjustment Mechanism 
     Next, the operation and effects of the headrest level adjustment mechanism according to this embodiment will be described. 
     As shown in  FIG. 1  to  FIG. 3 , in this embodiment, the level adjustment mechanism  29  for the headrest  14  includes the main support  26  and the sub support  28 , and the main support  26  is provided on the main headrest stay  18  side, and includes the locking mechanism  30  that can adjust the level of the headrest  14  and lock the headrest  14  at a given level. On the other hand, the sub support  28  is provided on the sub headrest stay  20  side, and includes the movement restriction mechanism  32  that restricts the downward movement of the headrest  14 . 
     More specifically, in this embodiment, the locking mechanism  30  includes the main notches  80  and the main lock plate  70 . The main notches  80  are included in the main headrest stay  18 , and are arranged in the height direction of the headrest  14 . 
     Also, the main lock plate  70  is provided in the main support  26 . When the main lock plate  70  is engaged with one of the main notches  80  included in the main headrest stay  18 , the upward and downward movement of the main headrest stay  18  (the headrest  14 ) is restricted (in the locked state). Namely, in this embodiment, the upward and downward movement of the main headrest stay  18  can be restricted with a simple arrangement. 
     In this embodiment, the main headrest stay  18  is provided with the main notches  80 , and the main lock plate  70  is engaged with one of the main notches  80 , so that the movement of the main headrest stay  18  is restricted at given positions with a simple arrangement. However, the arrangement is not limited to this one, provided that the movement of the main headrest stay  18  can be restricted at given positions. 
     On the other hand, the movement restriction mechanism  32  includes the sub notch  60  and the sub lock plate  44 . The sub notch  60  is included in the sub headrest stay  20  at the same level as the notch  80 A for lowest level located closest to the headrest  14 , as one of the main notches  80 . 
     Also, the sub lock plate  44  is provided in the sub support  28 , and is engaged with the sub notch  60 , so as to restrict the downward movement of the sub headrest stay  20  (the headrest  14 ) (i.e., place it in the locked state). 
     Namely, in this embodiment, at the lowest-level position P of the headrest  14 , the main lock plate  70  on the locking mechanism  30  side engages with one of the main notches  80 , so as to restrict the downward movement of the main headrest stay  18 , and the sub lock plate  44  on the movement restriction mechanism  32  side engages with the sub notch  60 , so as to restrict the downward movement of the sub headrest stay  20  (i.e., place it in the locked state). 
     Thus, at the lowest-level position P of the headrest  14 , the downward movement of the headrest  14  is restricted, not only on the main headrest stay  18  side, but also on the sub headrest stay  20  side. Thus, in this embodiment, the downward movement of the headrest  14  can be more reliably curbed, as compared with the case where the downward movement of the headrest  14  is restricted only on the main headrest stay  18  side. 
     Thus, in this embodiment, it is not necessary to set the lowest-level position P′ of a headrest  100 , by bringing a lower surface  100 A of the headrest  100  into abutting contact with an upper surface  102 A of a seat back  102 , as shown in  FIG. 5A , as a comparative example. As a result, in this embodiment, a gap H can be provided between the headrest  14  and the seat back  12 , as shown in  FIG. 5B . 
     As described above, the movement restriction mechanism  32  maintains the lowest-level position of the headrest  14  by restricting the downward movement of the headrest  14  (namely, placing it in the locked state) in a condition where the gap H is provided between the headrest  14  and the seat back  12 , at the lowest-level position P at which the headrest  14  is at the lowest level in a condition where the main headrest stay  18  is engaged with the main support  26 . 
     Thus, in this embodiment, where the lowest-level position P of the headrest  14  is equal, the size of the headrest  14  can be reduced in the height direction by the amount corresponding to the gap H, as compared with the case where no gap is provided between the headrest  100  and the seat back  102 , as shown in  FIG. 5A , as the comparative example. Namely, in this embodiment, it is possible to reduce the size of the headrest  14 , while assuring the level of the headrest  14  that is required at the lowest-level position P of the headrest  14 . 
     In the meantime, according to the European New Car Assessment Programme (E-NCAP), the middle-level position and lowest-level position of the headrest are specified. The middle-level position of the headrest is set to be within ±10 mm from a middle point of the lowest-level position of the headrest and the highest-level position of the headrest. 
     As described above, the headrest  14  can be elastically deformed; therefore, variations in dimensions appear due to elastic deformation of the headrest  14  itself. Thus, when the lowest-level position P′ of the headrest  100  is set to the position at which the lower surface  100 A of the headrest  100  is in abutting contact with the upper surface  102 A of the seat back  102 , as in the comparative example shown in  FIG. 5A , there arises a need to take account of a variation in dimension due to elastic deformation of the headrest  100 . 
     As a result, as shown in  FIG. 6 , the lowest-level position P′ of the headrest  100  is set to the upper side of a notch  104  for lowest level, in view of a variation X in dimension due to elastic deformation of the headrest  100 , and a middle-level position Q′ of the headrest  100  is set between the lowest-level position P′ of the headrest  100  and the highest-level position R′ of the headrest  100 . 
     On the other hand, in this embodiment, the gap H is provided between the headrest  14  and the seat back  12 ; therefore, there is no need to take account of a variation in dimension due to elastic deformation of the headrest  14  itself. Accordingly, the lowest-level position P of the headrest  14  is set to the position of the notch  80 A for lowest level, and the middle-level position Q of the headrest  14  is set between the notch  80 A for lowest level and the notch  80 C for highest level. 
     Namely, in this embodiment, the level of the headrest  14  at the middle-level position Q is higher than that of the comparative example. Accordingly, in this embodiment, the upper portion of the headrest  14  can be cut, by an amount corresponding to a rise of the level of the headrest  14  from that of the comparative example, and, consequently, the size of the headrest  14  can be reduced. 
     In this embodiment, as the size of the headrest  14  is reduced, a feeling of pressure in the vehicle cabin  78 , which would arise due to a large size of the headrest  14 , is reduced, and the field of front view of an occupant on the rear seat can be broadened as compared with the case where the headrest  14  is large in size. 
     Also, in this embodiment, as shown in  FIG. 1  to  FIG. 3 , the sub notch  60  is a recess where the surface  20 A of the sub headrest stay  20  is recessed in a direction perpendicular to the hanging direction of the sub headrest stay  20 , and the sub lock plate  44  can move into the sub notch  60 . 
     Also, the sub notch  60  includes the horizontal portion  60 A and the inclined portion  60 B, and the horizontal portion  60 A defines the upper side of the sub notch  60 , such that the upper face  44 B of the sub lock plate  44  received in the sub notch  60  abuts against the horizontal portion  60 A for plane contact therewith. In a condition where the upper face  44 B of the sub lock plate  44  is in contact or plane contact with the horizontal portion  60 A of the sub notch  60 , the downward movement of the sub headrest stay  20  in which the sub notch  60  is included is restricted (in the locked state). 
     On the other hand, the inclined portion  60 B is connected to the horizontal portion  60 A, and is inclined toward the surface  20 A of the sub headrest stay  20  other than the part of the surface  20 A defining the recess, when viewed in a direction toward the lower side of the sub notch  60 . Thus, the inclined portion  60 B functions as a so-called cam face, and can convert the force applied so as to lift the sub headrest stay  20  into the force that moves the sub lock plate  44  outward from the inside of the sub notch  60 . In this manner, the locked state established by the sub lock plate  44  is released. 
     Namely, in this embodiment, at the lowest-level position P of the headrest  14 , the locked state established by the sub lock plate  44  is maintained, and the headrest  14  cannot be moved downward. However, when the headrest  14  is moved upward, the locked state of the sub lock plate  44  is released. 
     Modified Examples of the Embodiment 
     In the above embodiment, the movement restriction mechanism  32  has the sub notch  60  included in the surface  20 A of the sub headrest stay  20 , as shown in  FIG. 1 , and is arranged to restrict the downward movement of the sub headrest stay  20  when the sub lock plate  44  moves into (engages with) the sub notch  60 . 
     However, the movement restriction mechanism is not limited to the above arrangement, provided that the downward movement of the headrest  14  is restricted on the sub headrest stay  20  side, as well as the main headrest stay  18  side, in the condition where the gap H is provided between the headrest  14  and the seat back  12 , at the lowest-level position P of the headrest  14 . 
     For example, as a first modified example, a movement restriction mechanism  82  includes a collar  84  provided on the sub headrest stay  20 , as shown in  FIG. 7 . The collar  84  is in the shape of a truncated cone of which the lower face has the smaller diameter, and the sub headrest stay  20  is inserted through the collar  84 . Then, at the lowest-level position P of the headrest  14 , the collar  84  may abut against the headrest  14  and the support upper  54  of the sub support  28 , so as to restrict the downward movement of the sub headrest stay  20  (the headrest  14 ). 
     Also, as a second modified example, a movement restriction mechanism  86  includes a bent portion  90  included in a sub headrest stay  88 , as shown in  FIG. 8A  and  FIG. 8B . The bent portion  90  may interfere with the support upper  54  of the sub support  28 , at the lowest-level position P of the headrest  14 , so as to restrict the downward movement of the sub headrest stay  88  (the headrest  14 ). 
     Further, as a third modified example, a movement restriction mechanism  92  includes an expanded portion  96  included in a sub headrest stay  94 , as shown in  FIG. 9 . The expanded portion  96  may interfere with the sub support  28 , at the lowest-level position P of the headrest  14 , so as to restrict the downward movement of the sub headrest stay  94  (the headrest  14 ). 
     While the expanded portion  96  may be included in a sub headrest stay  94  by altering the shape of the sub headrest stay  94  itself, the expanded portion  96  may also be provided by mounting another member to the sub headrest stay  94 , for example. 
     Although not illustrated in the drawings, a sub support may be provided with a back wall, and a distal end of a sub headrest stay may abut against the back wall, at the lowest-level position P of the headrest  14 , so that the downward movement of the sub headrest stay is restricted. 
     While one embodiment of the present disclosure has been described above, the present disclosure is not limited to the above embodiment, but may be carried out with various modifications other than those as described above, without departing from its principle.