JP7240230B2 - back frame - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a back frame, and more particularly to a back frame that can sufficiently protect a seated occupant in the event of a vehicle collision even if the weight of the back frame is reduced.

2. Description of the Related Art There is a back frame of a seat mounted in a vehicle such as an automobile in which a plate-shaped lower frame is connected to the lower portion of a pair of left and right side frames (Patent Document 1). In the technique disclosed in Patent Document 1, the lower frame is formed in a U shape by projecting both lateral end portions forward with respect to the lateral central portion of the lower frame, thereby increasing the rigidity of the lower frame. The entire back frame, which connects the lower frame to the side frame, is highly rigid.

Japanese Patent No. 5666279

However, in contrast to the conventional technique for increasing the rigidity of the back frame, it is desired to reduce the weight by thinning each part of the back frame. Even when the back frame is thinned to have low rigidity, it is necessary to sufficiently protect the seated occupant in the event of a vehicle collision (particularly in the event of a rear-end collision).

SUMMARY OF THE INVENTION It is an object of the present invention to provide a back frame that can sufficiently protect a seated occupant in the event of a vehicle collision even if the weight of the back frame is reduced.

In order to achieve this object, the back frame of the present invention is a back frame provided in the backrest portion of a seat mounted in a vehicle, comprising a pair of left and right side frames extending in the vertical direction, and upper portions of the pair of side frames. and a plate-like lower frame extending in the left-right direction by connecting the lower portions of the pair of side frames to each other in the up-down direction. A central portion forming a central portion in the direction, and fragile portions located on both left and right sides of the central portion. A fragile lower part that extends to the lower edge of the fragile upper part and is bent forward with respect to the fragile upper part, and a fragile lower part that extends forward from the upper edge of the fragile upper part to the upper edge of the fragile part. and a bent portion, wherein in a cross section of the lower frame perpendicular to the left-right direction, the moment of inertia of area of the weakened portion in the front-rear direction is smaller than the moment of inertia of area of the center portion in the front-rear direction.

According to the back frame of claim 1, in the cross section of the lower frame perpendicular to the left-right direction, the cross section 2 of the weakened portion in the front-rear direction is greater than the geometrical moment of inertia of the central portion in the front-rear direction (neutral axis extending in the vertical direction). Second moment is small. Therefore, when the lower frame receives a load in the front-rear direction from the occupant at the time of a vehicle collision, the load from the occupant is received by the central portion, and the impact at the time of collision is absorbed by the deformation of the fragile portion, resulting in the lower frame. It is possible to make it difficult to destroy the connection part between the side frame and the side frame. As a result, even if the back frame is made lighter and less rigid, it is possible to sufficiently protect the seated occupant in the event of a vehicle collision.

A fragile upper portion of the fragile portion extends vertically. A lower weakened portion bent forwardly relative to the upper weakened portion extends upwardly from the lower edge of the weakened portion to the lower edge of the upper weakened portion. A bent portion extending forward from the upper edge of the fragile upper portion is formed to the upper edge of the fragile portion. In this way, the fragile portion extends upward from the lower end edge to the bent portion, and the lower portion is bent backward in a convex shape. A secondary moment can be reduced. As a result , the fragile portion can be easily deformed at the time of vehicle collision, and the impact at the time of collision can be more easily absorbed.

According to the back frame of claim 2 , since the bent portion is provided over the entire length of the lower frame in the left-right direction including the central portion, the lower frame can be made difficult to deform except at the time of collision. The central portion has a first central portion extending downward from a rear edge of the bent portion, a second central portion bent forward from a lower edge of the first central portion, and a third central portion extending from a front edge of the second central portion to a third central portion. It is formed by bending downward. As a result, the central portion has a cross-sectional shape in which the first central portion and the third central portion are flanges, and the second central portion is a web. Moment can be made larger. As a result, in addition to the effect of claim 1 , it is possible to more reliably receive the load from the seated occupant with the central portion, and the deformation of the fragile portion makes it easier to absorb the impact at the time of collision.

According to the back frame of claim 3 , since the bending angle between the fragile upper portion and the fragile lower portion is larger than the bending angle between the first central portion and the second central portion, the geometrical moment of inertia of the central portion The secondary moment of area of the fragile portion can be made smaller. As a result, in addition to the effect of claim 2 , the fragile portion can be more easily deformed at the time of vehicle collision, and the impact at the time of collision can be more easily absorbed.

According to the back frame of claim 4 , a tether anchor for fixing a child seat is attached to the first central portion. Since the first central portion is made highly rigid by connecting the bent portion and the second central portion that face each other vertically, the child seat can be reliably fixed to the back frame. Furthermore, since the third central portion is formed with a through hole penetrating in the plate thickness direction, the weight of the lower frame can be reduced. Since there is no need to leave a portion for attaching the tether anchor in the third central portion, a large through hole can be formed. As a result, in addition to the effect of claim 2 or 3 , it is possible to easily form the through-holes at the same time as the lower frame is cut out or press-formed, thereby reducing the lower frame forming process.

According to the back frame of claim 5 , the vertical dimension of the central portion is shorter than the vertical dimension of the fragile portion. Thus, in addition to the effects of any one of claims 1 to 4 , the vertical dimension of the central portion of the cross-sectional shape that is difficult to deform can be shortened to reduce the weight of the lower frame, and the vertical dimension of the weak portion of the cross-sectional shape that is easily deformable. can be lengthened to make the fragile portion less likely to deform under a small load other than at the time of collision.

FIG. 4 is a perspective view of a back frame in one embodiment; FIG. 4 is a front view of the back frame showing an enlarged lower frame; FIG. 3 is a cross-sectional view of the back frame taken along line III-III of FIG. 2; FIG. 4 is an enlarged cross-sectional view of the back frame in which the lower frame portion of FIG. 3 is enlarged; FIG. 3 is a cross-sectional view of the back frame taken along line VV of FIG. 2;

Preferred embodiments are described below with reference to the accompanying drawings. First, the back frame 1 in the first embodiment will be described with reference to FIG. As shown in FIG. 1, a back frame 1 is a frame provided in a seat back, which is a backrest of a seat mounted in a vehicle such as an automobile.

The back frame 1 includes a pair of left and right side frames 2, 2 extending in the vertical direction, an upper frame 4 extending in the left and right direction by connecting upper portions of the pair of side frames 2, 2 to each other, and a pair of side frames 2, 2. A lower frame 10 extending in the left-right direction with its lower parts connected to each other and a plurality of wires 6 extending between the pair of side frames 2 and 2 and between the upper frame 4 and the lower frame 10 are provided. The back frame 1 is formed in a substantially square frame shape by side frames 2 , 2 , an upper frame 4 and a lower frame 10 .

The back frame 1 (side frame 2) is fixed to a seating portion (not shown) of the seat via a reclining device (not shown) so that the angle can be adjusted. A rotation axis A for adjusting the angle of the back frame 1 (side frame 2 ) with respect to the seating portion passes through the center of a hole 2 e formed in the lower portion of the side frame 2 .

In this specification, as shown in FIG. 3, in the cross section perpendicular to the left-right direction of the back frame 1, the axial direction of the first virtual axis B connecting the center of gravity G of the upper frame 4 and the rotation axis A is the back. The vertical direction of the frame 1 is assumed. The axial direction of a second imaginary axis C passing through the rotation axis A and perpendicular to the first imaginary axis B is defined as the front-rear direction of the back frame 1 . The vertical and longitudinal directions of the back frame 1 do not change even if the angle of the back frame 1 with respect to the seating portion changes.

As shown in FIGS. 1 and 3, the side frame 2 includes a tubular portion 2a made of steel whose upper portion is connected to the upper frame 4, and a plate portion 2b welded to the lower portion of the tubular portion 2a. . The plate portion 2b is a member formed by bending a steel plate. The plate portion 2b includes a rear flange 2c projecting inward in the left-right direction from the rear edges of the pair of side frames 2, 2, and a front flange 2d projecting inward in the left-right direction from the front edges. . As a result, the rigidity of the side frame 2 can be secured by the rear flange 2c and the front flange 2d while reducing the weight of the lower portion of the side frame 2 by the plate portion 2b.

The rear flange 2c and the front flange 2d are also provided on the lower edge of the plate portion 2b below the second imaginary axis C. As shown in FIG. A lower edge of the plate portion 2b is formed in a substantially U shape when viewed in the left-right direction. At the lower edge of the plate portion 2b, the rear side of the first imaginary axis B is the rear flange 2c, and the front side of the first imaginary axis B is the front flange 2d.

The upper frame 4 is a tubular body made of steel, and one end of the upper frame 4 is formed integrally with the tubular portion 2a of the left side frame 2 . A fixture 8 for fixing a headrest (not shown) is attached to the upper frame 4 . The wire 6 is for supporting a back pad (not shown) attached to the back frame 1 as a cushion material.

The lower frame 10 is a plate material made of steel that connects the pair of side frames 2, 2 to each other along the vertical direction. Both ends (weak portions 13) in the left-right direction of the lower frame 10 are superimposed on the inside (front side) of the rear flange 2c, and the both ends and the rear flange 2c are welded at a plurality of points in the vertical direction. . The lower frame 10 is arranged on the rear side of the first imaginary axis B. As shown in FIG.

As shown in FIG. 2, the lower frame 10 includes a central portion 12 forming a central portion in the left-right direction, fragile portions 13, 13 positioned on both left and right sides of the central portion 12, fragile portions 13, 13 located on both left and right sides of the central portion 12, and fragile portions on both sides of the central portion 12. Connection portions 14 and 14 that connect the portions 13 and 13 are provided. A central portion 12 is a predetermined section in the center of the lower frame 10 in the left-right direction, and predetermined sections on both sides in the left-right direction of the lower frame 10 are weakened portions 13 .

In the cross section perpendicular to the left-right direction of the lower frame 10, the cross-sectional shape of the central portion 12 and the cross-sectional shape of the fragile portion 13 are significantly different, and the cross-sectional shape of the connecting portion 14 that smoothly connects these cross sections gradually changes in the left-right direction. do. In this specification, unless otherwise specified, a section perpendicular to the left-right direction of each section will be omitted from the description as a section of each section.

As shown in FIG. 4, the central portion 12 includes a first central portion 12a extending in the vertical direction, a second central portion 12b bent forward from the lower edge of the first central portion 12a, and a second central portion 12b. A third central portion 12c bent downward from the front edge, and a bent portion 11 extending forward from the upper edge of the first central portion 12a to the upper edge of the central portion 12 are provided.

The first central portion 12a has a substantially flat plate shape extending downward from the rear edge of the bent portion 11, and is formed along the rear flange 2c. The first central portion 12a and the bent portion 11 are provided substantially perpendicularly. The second central portion 12b extends forward from the lower edge of the first central portion 12a and moves away from the rear flange 2c. The second center portion 12b has a substantially flat plate-like center portion in the front-rear direction, and both end portions in the front-rear direction are curved toward the first center portion 12a and the third center portion 12c. The third central portion 12c is formed in a substantially flat plate shape extending downward from the front edge of the second central portion 12b.

In this manner, the central portion 12 has a cross-sectional shape in which the first central portion 12a and the third central portion 12c are flanges, and the second central portion 12b is a web. The moment of inertia of the area of the central portion 12 at this time, that is, the moment of inertia of the area of the central portion 12 in the longitudinal direction (neutral axis extending in the vertical direction) can be increased. Furthermore, the bending portion 11 projecting forward from the upper edge of the first central portion 12a can further increase the moment of inertia of area of the central portion 12 in the front-rear direction. In this specification, unless otherwise specified, the geometrical moment of inertia of each part of the lower frame 10 in the front-rear direction is abbreviated as the moment of inertia of area of each part.

A bending angle θ1 between the first central portion 12a and the second central portion 12b and a bending angle θ2 between the second central portion 12b and the third central portion 12c are both 90° or more. As these bending angles θ1 and θ2 approach 90°, the moment of inertia of area of the central portion 12 increases. The bending angle θ1 is the angle between the substantially flat first central portion 12a and the flat second central portion 12b, and the bending angle θ2 is the angle between the flat third central portion 12c and the second flat central portion 12c. This is the angle between the central portion 12b and the flat portion.

In addition, in this embodiment in which the bending angles θ1 and θ2 are 90° or more, the vertical dimension L1 (see FIG. 2) of the central portion 12 can be made larger than in the case where the bending angles θ1 and θ2 are less than 90°. Thereby, the torsional rigidity of the central portion 12 can be increased. Furthermore, the bending angle θ2 is set larger than the bending angle θ1. Thereby, the rigidity of the first central portion 12a that connects the bent portion 11 and the second central portion 12b that face each other vertically can be made higher.

As shown in FIG. 1 , the bent portion 11 is provided over the entire length of the lower frame 10 in the left-right direction. That is, they are provided not only in the central portion 12 but also in the fragile portion 13 and the connecting portion 14 . A tether anchor 15 for fixing a child seat is attached to the rear surface of the first central portion 12a. The first central portion 12a is made highly rigid by connecting the bent portion 11 and the second central portion 12b, which face each other vertically, so that the child seat can be securely fixed to the back frame 1. As shown in FIG.

The tether anchor 15 is formed into a substantially U shape by bending a round bar, and is welded to a mounting portion 16 projecting from the rear surface by partially recessing the first central portion 12a. The attachment portion 16 makes it easy to weld the round-bar-shaped tether anchor 15 to the first central portion 12a, and prevents variations in the fixed position of the tether anchor 15 to the first central portion 12a.

The first central portion 12a is provided with through holes 18 penetrating in the plate thickness direction on both lateral end sides (connecting portion 14 side) of the mounting portion 16. As shown in FIG. The wires 6 are welded to both ends in the left-right direction of the third central portion 12c, and two through holes 17 are provided in the center in the left-right direction. The through holes 17 and 18 are hollow holes for reducing the weight of the lower frame 10 .

Since the load applied from the wire 6 to the third central portion 12c is smaller than the load applied from the tether anchor 15 to the first central portion 12a at the time of a vehicle collision, the third central portion 12c is more sensitive than the through hole 18 of the first central portion 12a. through-hole 17 can be enlarged. That is, since the tether anchor 15 is not attached to the third central portion 12c, a large through hole 17 can be formed. As a result, since the through-holes 17 can be formed at the same time when the lower frame 10 is cut or pressed, the steps for forming the lower frame 10 can be reduced.

As shown in FIG. 5, the fragile portion 13 includes a vertically extending fragile upper portion 13a, a fragile lower portion 13b bent forward from the lower edge of the fragile upper portion 13a, and a fragile portion extending forward from the upper edge of the fragile upper portion 13a. and a bent portion 11 formed up to the upper edge of the portion 13 . The fragile upper portion 13a has a substantially flat plate shape extending downward from the rear edge of the bent portion 11 and is formed along the rear flange 2c. The fragile upper portion 13a and the bent portion 11 are provided substantially perpendicularly.

The fragile lower portion 13b is a portion that extends upward from the lower end edge of the fragile portion 13 and is formed to the lower edge of the fragile upper portion 13a. The fragile lower portion 13b is curved in a convex shape toward the rear at the connection portion with the fragile upper portion 13a, and the rest of the fragile lower portion 13b is formed in a substantially flat plate shape and formed along the rear flange 2c.

In this way, since the weakened portion 13 extends upward from the lower edge to the bent portion 11 and a part of the lower side is bent rearward in a convex shape, the moment of inertia of the area of the weakened portion 13 in the front-rear direction can be reduced. . More specifically, the geometrical moment of inertia of the fragile portion 13 shown in FIG. 5 can be made smaller than the geometrical moment of inertia of the central portion 12 shown in FIG.

As a result, when the lower frame 10 receives a load in the front-rear direction from the seated occupant during a vehicle collision (particularly at the time of a rear-end collision), the load from the seated occupant is received by the central portion 12, and the deformation of the fragile portion 13 causes the lower frame 10 to be deformed. It can absorb the impact at the time of collision. As a result, even if the rigidity of the back frame 1 is reduced by thinning and reducing the weight of each part of the back frame 1, it is possible to make it difficult to break the connecting part (welded part) between the lower frame 10 and the side frame 2, and to ensure the reliability. Since the seated occupant can be supported by the lower frame 10, the seated occupant can be sufficiently protected in the event of a vehicle collision.

In particular, except for the bent portion 11 common to both, substantially the entire fragile portion 13 is formed along the rear flange 2c that extends in the vertical direction and is partially bent backward in a convex shape. While the first central portion 12a of the central portion 12 follows the rear flange 2c, the second central portion 12b separates from the rear flange 2c. Thereby, the geometrical moment of inertia of the central portion 12 can be made larger than the geometrical moment of inertia of the fragile portion 13 . As a result, while the load from the seated person is reliably received by the central portion 12, the deformation of the fragile portion 13 facilitates absorption of the impact at the time of collision.

Furthermore, when the bent portion 11 is removed, the central portion 12 having a cross-sectional shape with the first central portion 12a and the third central portion 12c as flanges and the second central portion 12b as a web, and a part of the lower side extends backward. The difference in the moment of inertia of area between the fragile portion 13 and the weak portion 13 that is only bent in a convex shape can be further increased. As a result, while the central portion 12 more reliably receives the load from the seated person, the deformation of the fragile portion 13 makes it easier to absorb the impact at the time of collision.

Since the bent portion 11 is formed on the upper side of the fragile portion 13 so as to bend forward with respect to the fragile upper portion 13a, it is possible to prevent the moment of inertia of the area of the fragile portion 13 from becoming too small. Then, the fragile portion 13 can be made difficult to deform. Further, since the bent portion 11 is provided over the entire length of the lower frame 10 in the left-right direction, the lower frame 10 as a whole is less likely to be deformed under a small load except during a collision.

Since the lower edge of the fragile portion 13 is located below the second imaginary axis C, the vertical dimension L2 (see FIG. 2) of the fragile portion 13 can be increased. Therefore, the moment of inertia of area of the fragile portion 13 can be ensured, and the fragile portion 13 can be made more difficult to deform except at the time of a vehicle collision. Furthermore, since the lower edge of the fragile portion 13 is located behind the first imaginary axis B, it is possible to prevent the moment of inertia of the area of the fragile portion 13 from becoming excessively large, and the deformation of the fragile portion 13 can prevent the collision of the vehicle. It can easily absorb shock.

Further, the lower frame 10 (weak portion 13) is connected to the side frame 2 below the second imaginary axis C. As shown in FIG. As a result, when lateral vibration is input to the back frame 1 from a reclining device (not shown), lateral vibration of the side frames 2 with respect to the lower frame 10 with the vicinity of the rotation axis A as a fulcrum can be suppressed.

A bending angle θ3 between the fragile upper portion 13a and the fragile lower portion 13b is 90° or more. In addition, the bending angle θ3 is the angle between the substantially flat fragile upper portion 13a and the flat portion of the fragile lower portion 13b. This bending angle θ3 is larger than the bending angle θ1 between the first central portion 12a and the second central portion 12b and the bending angle θ2 between the second central portion 12b and the third central portion 12c. As a result, the moment of inertia of area of the fragile portion 13 can be made smaller than the moment of inertia of area of the central portion 12 . As a result, the fragile portion 13 can be deformed more easily in the event of a collision of the vehicle, and the impact at the time of the collision can be more easily absorbed.

2, the vertical dimension L1 of the central portion 12 is shorter than the vertical dimension L2 of the fragile portion 13. As shown in FIG. As a result, the weight of the lower frame 10 can be reduced by shortening the vertical dimension L1 of the central portion 12 having a cross-sectional shape that is difficult to deform, and the vertical dimension L2 of the fragile portion 13 having a cross-sectional shape that is easily deformed can be lengthened to avoid collisions. The fragile portion 13 can be made difficult to deform with a small load.

A vertical dimension L2 of the fragile portion 13 connected to the side frame 2 is 100 mm or more and 150 mm or less. Since the vertical dimension L2 of the fragile portion 13 is 100 mm or more, the side frame 2 is less likely to vibrate in the horizontal direction with respect to the lower frame 10.例文帳に追加Furthermore, since the vertical dimension L2 of the fragile portion 13 is 150 mm or less, the upper edge of the fragile portion 13 is less likely to make the seated person feel a foreign object, such as hitting the back of the seated person.

In addition, the vertical dimension L2 of the fragile portion 13 and the cross-sectional shape of the central portion 12 ensure the rigidity of the lower frame 10 except in the event of a collision of the vehicle. It is possible to eliminate the need for a shaft-shaped member for As a result, the weight of the back frame 1 can be reduced compared to the case where the rigidity of the entire back frame is ensured by the shaft-like member.

The connecting portion 14 smoothly connects the central portion 12 and the fragile portion 13 having different cross-sectional shapes and vertical dimensions L1 and L2. As a result, the geometrical moment of inertia of the connecting portion 14 is smaller than the geometrical moment of inertia of the central portion 12 , becomes larger than the geometrical moment of inertia of the fragile portion 13 , and decreases toward the fragile portion 13 . Therefore, even if the deformation of the fragile portion 13 alone cannot absorb the shock at the collision of the vehicle, the connecting portion 14 gradually deforms from the side of the fragile portion 13 to absorb the shock at the time of the collision.

Furthermore, the vertical dimension L3 of the flat plate-shaped third center portion 12c gradually decreases from the center in the left-right direction toward both ends. That is, since the third central portion 12c, which serves as a flange, is shortened, the geometrical moment of inertia of the central portion 12 gradually decreases from the center in the left-right direction toward both ends. As a result, even if the deformation of the fragile portion 13 and the connecting portion 14 alone cannot absorb the impact at the time of collision of the vehicle, the central portion 12 gradually deforms from the side of the fragile portion 13 to absorb the impact at the time of collision.

At the center of the first center portion 12a in the left-right direction, two mounting portions 16 recessed rearward for mounting the tether anchor 15 are provided separately in the left-right direction. As a result, even if the central portion 12 is gradually deformed from the fragile portion 13 side, the central portion 12 closer to the central side in the left-right direction than the mounting portion 16 is less likely to be deformed. As a result, the load from the seated person can be reliably received by the central portion 12 of the portion that is difficult to deform.

The weakened portion 13 is connected to the side frame 2, and a weakened portion 13 is provided with a weakened hole 19 penetrating in the plate thickness direction inside the connecting portion in the left-right direction. The fragile hole 19 is a vertically elongated hole positioned substantially in the vertical center of the fragile portion 13 . As a result, the weakened portion 13 on the left-right direction inner side of the connection portion with the side frame 2 can be easily deformed, and the deformation of the weakened portion 13 on the left-right direction inner side of the weakened hole 19 is shifted to the outer side of the weakened hole 19. Since it can be made difficult to convey, the connecting portion can be made more difficult to break.

The present invention has been described above based on the embodiments, but the present invention is not limited to the above-described embodiments, and it is easily understood that various modifications and improvements are possible without departing from the scope of the present invention. It can be inferred. The shapes of the side frames 2 and the upper frame 4 may be changed as appropriate. Further, if the geometrical moment of inertia of the fragile portion 13 is smaller than the geometrical moment of inertia of the central portion 12, the cross-sectional shapes of the central portion 12 and the fragile portion 13 may be appropriately changed. In addition, it is not limited to the case where the weakened portion 13 is connected to the side frame 2 , and a portion provided on the left-right direction outer side of the weakened portion 13 may be connected to the side frame 2 .

In the above embodiment, the case where the second central portion 12b has a substantially flat portion has been described, but the present invention is not necessarily limited to this. The second central portion 12b may be composed only of a portion that is contiguous with the first central portion 12a and curves rearward and a portion that is contiguous and curved forward with the third central portion 12c. In this case, in the cross section of the central portion 12, the angles between the tangent to the inflection point of the second central portion 12b and the substantially flat plate-like first central portion 12a and third central portion 12c are bending angles θ1 and θ2, respectively. . Also, the entire fragile lower portion 13b may be curved rearward in a convex shape without being limited to the case where the fragile lower portion 13b has a substantially flat portion. In this case, in the cross section of the fragile portion 13, the angle between the tangent line at the lower edge of the fragile lower portion 13b and the substantially flat fragile upper portion 13a is the bending angle θ3. Also, a part or the whole of the first central portion 12a, the third central portion 12c, and the fragile upper portion 13a may be curved.

In the above embodiment, the back frame 1 having no shaft-like member connecting the lower portions of the pair of side frames 2, 2 has been described, but it is not necessarily limited to this. The lower portions of the pair of side frames 2 , 2 may be connected by a shaft-like member having lower rigidity and lower strength than the fragile portion 13 . Thus, in the event of a vehicle collision, the lower frame 10 can easily receive the load from the seated person, and the deformation of the fragile portion 13 can easily absorb the impact of the collision.

In the above embodiment, the case where two through-holes 18 are provided in the first central portion 12a, two through-holes 17 are provided in the third central portion 12c, and one fragile hole 19 is provided in each of the left and right fragile portions 13 has been described. It is not limited to this. The numbers of through holes 17 and 18 and fragile holes 19 may be changed as appropriate. Also, the shapes of the through holes 17 and 18 and the fragile hole 19 may be changed as appropriate.

In the above embodiment, the case where each part of the back frame 1 is mainly made of steel has been described, but the present invention is not necessarily limited to this, and each part of the back frame 1 may be made of metal other than steel.

1 back frame 2 side frame 4 upper frame 10 lower frame 11 bent portion 12 central portion 12a first central portion 12b second central portion 12c third central portion 13 fragile portion 13a fragile upper portion 13b fragile lower portion 15 tether anchor 17 through hole

Claims (5)

A back frame provided in the backrest portion of a seat mounted in a vehicle,
a pair of left and right side frames extending in the vertical direction;
an upper frame connecting the upper parts of the pair of side frames to each other and extending in the left-right direction;
a plate-like lower frame extending in the left-right direction by connecting the lower portions of the pair of side frames to each other in the up-down direction;
The lower frame has a center portion forming a center portion in the left-right direction;
fragile portions located on both left and right sides of the central portion,
The fragile portion includes a vertically extending fragile upper portion;
a fragile lower part extending upward from a lower edge of the fragile part to a lower edge of the fragile upper part and bent forward with respect to the fragile upper part;
a bent portion extending forward from the upper edge of the fragile upper portion to the upper edge of the fragile portion;
A back frame, wherein in a cross section of the lower frame perpendicular to the left-right direction, the moment of inertia of area of the weakened portion in the front-rear direction is smaller than the moment of inertia of area of the center portion in the front-rear direction. The bending portion is provided over the entire length in the left-right direction of the lower frame including the central portion,
the central portion includes a first central portion extending downward from a trailing edge of the bend;
a second central portion bent forward from a lower edge of the first central portion;
2. The back frame of claim 1 , further comprising a third central portion bent downwardly from the leading edge of said second central portion. 3. The back frame according to claim 2 , wherein the bending angle between the fragile upper portion and the fragile lower portion is larger than the bending angle between the first central portion and the second central portion. A tether anchor for fixing a child seat is attached to the first central portion,
4. The back frame according to claim 2 , wherein the third central portion is formed with a through hole penetrating in the plate thickness direction. The back frame according to any one of claims 1 to 4 , wherein the vertical dimension of the central portion is shorter than the vertical dimension of the fragile portion.

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