Abstract:
A backrest device for a chair, in which a backboard is easily flexible without a reduction in its strength, thus making the chair comfortable to sit on. The lower ends of both right and left sides of the back frame are supported at both sides at the back of a leg body, and the flexible back board is formed on the inside of rear frames. A large number of openings are formed in the backboard so as to be close to each other in the top-bottom and left-right directions. Between openings adjacent to each other in the left-right direction, there is formed a connection section continuing in the top-bottom direction, and the dimension in the front-back direction of the connection section is made larger than the dimension in the left-right direction of the connection section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a backrest device in a chair and particularly relates to a backrest device in a chair comprising a flexible back plate for supporting the back of an occupant in a back frame supported by a leg. 
         [0002]    Such a backrest device in a chair is known from JP2002-125797A and JP2005-160558A. 
         [0003]    The backrest device in the publications disclose that a back frame and a back plate have nearly uniform thickness to make it impossible for an upper part of the back plate onto which the back of an occupant strongly presses to be bent rearward effectively. So the chair would not be comfortable to sit on. 
         [0004]    In order to allow the back of the occupant to fit over the backrest, the whole back plate may preferably be bent to be convex. But it would be very hard to bend a uniform-thickness back plate only by forming a number of opening in the back plate. 
         [0005]    A number of openings close to each other in the back plate would decrease bending rigidity or strength of the back plate to reduce durability. 
         [0006]    In order that side back frames may firmly be mounted to back rods fixed to legs and that the back plate pressed by the back of the occupant in the back frame may effectively be bent rearward to get more comfortable to sit in, JP2002-125797A discloses a backrest device with side back frames having a relatively large circular cross-section to provide higher bending rigidity. 
         [0007]    In contrast to the thinner back plate, the thicker back frame provides poor appearance. 
         [0008]    JP2005-160558A discloses that the side back frames in  FIG. 10  have a circular cross-section for improving bending rigidity, but causes poor appearance. The back frame in  FIG. 12  has a flat cross section and provides lower bending rigidity. When the inner back plate is pressed rearward, the back frame is bent rearward together. So the back plate would be unlikely to bend rearward. 
       SUMMARY OF THE INVENTION 
       [0009]    In view of the foregoing disadvantages, it is an object of the invention to provide a backrest device in a chair in which the back of an occupant is suitably supported by a back plate so that the chair is more comfortable to sit on. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a front elevational view of an embodiment of a chair according to the present invention. 
           [0011]      FIG. 2  is a side elevational view thereof. 
           [0012]      FIG. 3  is a front perspective view thereof. 
           [0013]      FIG. 4  is a front elevational view of the backrest. 
           [0014]      FIG. 5  is an exploded perspective view of a mounting portion of the backrest and armrest. 
           [0015]      FIG. 6  is an enlarged vertical sectional side view taken along the line VI-VI in  FIG. 4 . 
           [0016]      FIG. 7  is an enlarged vertical sectional side view taken along the line VII-VII in  FIG. 4 . 
           [0017]      FIG. 8  is an enlarged horizontal sectional plan view taken along the line VIII-VIII in  FIG. 4 . 
           [0018]      FIG. 9  is an enlarged horizontal sectional plan view taken along the line IX-IX in  FIG. 4 . 
           [0019]      FIG. 10  is an enlarged front view of the part of a back plate. 
           [0020]      FIG. 11  is an enlarged horizontal sectional plan view taken along the line XI-XI in  FIG. 1 . 
           [0021]      FIG. 12  is a front elevational view of another embodiment of a chair according to the present invention. 
           [0022]      FIG. 13  is an enlarged front view of the backrest thereof. 
           [0023]      FIG. 14  is an enlarged vertical sectional side view taken along the line XIV-XIV in  FIG. 13 . 
           [0024]      FIG. 15  is an enlarged horizontal sectional plan view taken along the line XV-XV in  FIG. 13 . 
           [0025]      FIG. 16  is an enlarged front view of the part of a back plate. 
           [0026]      FIG. 17  is an enlarged horizontal sectional plan view taken along the line XVII-XVII in  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    A chair  1  comprises a lower part  4  which consists of five legs  3  each of which has a caster  2  at the end; a post  5  which is retractable in the lower part  4  by a gas spring (not shown); and a support base  6  fixed to the upper end of the post  5 . 
         [0028]    A seat support frame  7  is integrally formed with and extends upward from the front end of the support base  6 . A pair of backrest support rods  8 , 8  extends from the support base  6 . In  FIG. 5 , a shaft  8   a  of each of the support rods  8 , 8  extends through a rubber torsion unit (not shown) in the support base  6  such that the support rods  8 , 8  are urged forward pivotally to move back and forth. 
         [0029]    A cushion  9  is stretched over the upper surface of a seat  10 . The seat  10  is supported by the seat support frame  7  to move back and forth. Each side of the rear end of the seat  10  is pivotally mounted to the upper end of the backrest support rod  8 . Specifically, a pair of support portions  11 , 11  is provided at the upper ends of the rear ends of the seat  10 . A pair of seat support brackets  12  is fixed to the sides of the support portions  11 , 11 . In  FIG. 5 , a rectangular shaft  13   a  projects from an axial support portion  13  fixed to the upper end of the backrest support rod  8 . The rectangular shaft  13   a  engages in the upper part of each of a pair of seat-support brackets  12 , 12  each of which is fixed with a head screw  12   a  to the side of the support portion  11 , so that the seat  10  is supported by the upper ends of the backrest support rods  8 , 8  to allow the seat  10  to tilt rearward. 
         [0030]    With a washer  15  and two bushings  16 , 15 , the rectangular shaft  13   a  engages in a rectangular hole  14   b  of an axial portion  14   a  at the lower part of a back-frame-mounting rod  14  for supporting the backrest  19 . A bolt  17  passes through the axial support portion  13 , the rectangular shaft  13   a  and the seat support bracket  12 , and engages in an internal thread (not shown) in the axial portion  14   a.    
         [0031]    Caps  18 , 18  cover the axial support portion  13  and an axial portion  14   a  of the back-frame mounting rod  14 . So the bolt  17  is not seen from outside. 
         [0032]    The backrest  19  comprises a rectangular back frame  20  and a back plate  21 , and is molded from synthetic resin. 
         [0033]    In  FIG. 4 , at the lower ends, side portions  20   a , 20   a  of the back frame  20  have mounting portions  20   c , 20   c  which are slightly bent outward and attached to the support rods  8 , 8 . 
         [0034]    The side portion  20   a  is integrally formed with the back plate  21 . In  FIGS. 2 and 6 , the side portion  20   a  gradually decreases in thickness upward from the lower end. The backrest  19  has a groove  22  at each side end in  FIGS. 8 and 9 . 
         [0035]    A plurality of ribs  23  extend in the groove  22  in a nearly horizontal direction at regular intervals to improve bending rigidity of the side portion  20   a.    
         [0036]    A lower portion  20   b  of the back frame  20  is curved and its cross section is an inverted U shape in  FIG. 7 , and a plurality of ribs  25  extend in a nearly vertical direction in a groove  24  at the lower end of the lower portion  20   b  to improve bending rigidity of the lower portion  20   b.    
         [0037]    In  FIG. 5 , two stepped bores  26 , 26  are formed in the bottom of the groove  22  at the lower end of the side portion  20   a  to allow a bolt for mounting an armrest  29  to engage therein. 
         [0038]    In  FIG. 7 , the back plate  21  becomes thinner gradually upward from the lower end as well as the side portion  20   a . In  FIG. 8 , the back plate  21  becomes thinner gradually toward the middle from the side ends. 
         [0039]    In the whole surface of the back plate  21 , a plurality of slits  27  are equally spaced vertically and horizontally. The horizontally adjacent slits  27  are staggered by half a length of the slit  27 . 
         [0040]    In  FIG. 10 , a distance of a vertical portion  21   a  provided between the horizontally adjacent slits  27  along a thickness of the back plate is larger than a distance of the vertical portion  21   a  along the width of the backrest  19  in  FIG. 8 . 
         [0041]    In  FIG. 9 , a distance of a horizontal portion  21   b  provided between the vertically adjacent slits  27  along the thickness of the back plate is smaller than the distance of the vertical portion  21   a  along the thickness of the back plate. The horizontal portion  21   b  has a U-like cross section in the rear surface. 
         [0042]    When the back plates  21  are pressed rearward, the U-like rear surfaces of the horizontal portions  21   b  are stretched out to allow the whole back plate  21  to bend rearward with respect to the slits  27  easily, but unlikely to bend rearward with respect to the horizontally adjacent slits  27 . 
         [0043]    The vertical portion  21   a  between the adjacent slits  27  has a rectangular horizontal cross-section in which a distance along a thickness of the back plate  21  is larger than a distance along a width of the back plate  21 , preventing the back plate  21  from decreasing in strength. 
         [0044]    In  FIG. 11 , at the lower end of the side portion  20   a  of the back frame  20 , a rectangular hole  28  vertically extends and engages with the rectangular back-frame mounting rod  14  of the backrest support rod  8 . The rod  14  is fixed together with the armrest  29  to the side portion  20   a  as below. 
         [0045]    In  FIG. 5 , a mounting portion  29   a  of the armrest  29  fits in the groove  22  of the side portion  20   a . Bolts  32 , 32  passes through stepped bores  31 , 31  in a bottom of a groove  29  of the mounting portion  29   a  and through the stepped bores  33 , 33  of the back-frame mounting rod  14  fitted in the groove  28  of the side portion  20   a . The bolts  32 , 32  engages in the stepped bores  26 . 
         [0046]    Thus, the back frame mounting rod  14  of the backrest support rod  8 , the side portion  20   a  of the back frame  20  and the armrest  28  are fastened with the two bolts  32  firmly. Especially, the mounting portion  29   a  of the armrest  29  fits in the groove  22  of the side portion  20   a  and is fixed therein, thereby preventing the back frame from loosening. 
         [0047]    In  FIG. 11 , a projection  34  is provided on the mounting portion  29   a  of the armrest  29  to engage with an outer larger-diameter part of the stepped hole  26  of the side portion  20   a  thereby facilitating positioning to the side portion  20   a.    
         [0048]    At the lower end of the side portion  20   a  of the mounting portion  29   a , a convex portion  35  engages with the upper surface of the axial support portion  13  and with the upper surface of the axial portion  14   a  to prevent loosening and to provide better appearance when the backrest  19  and armrest  29  are mounted to the backrest support rod  8 . 
         [0049]    In  FIG. 11 , the head of the bolt  32  is disposed on the larger-diameter portion of the stepped bore  31  within a groove  30  of the mounting portion  29   a , and is not exposed outside. 
         [0050]    As described above, in the embodiments, the back frame  20  of the backrest  19  is integrally formed with the back plate  21  as a relatively thin plate providing better appearance. 
         [0051]    The side portion  20   a  and lower portion  20   b  of the back frame  20  are curved, and the grooves  22 , 24  in the side ends and lower end are coupled with the flat ribs  23 , 25  respectively. Thus, bending rigidity is high in spite of the plate-like back frame  20  to allow only the back plate  21  to bend rearward, improving sitting comfort when the backrest is pressed rearward by an occupant. 
         [0052]    Furthermore, the side portions  20   a  and back plate  21  becomes thinner upward gradually. Even when the backrest  19  is strongly pressed, the upper part of the back frame  20  can suitably be bent and the upper part of the back plate  21  is likely to be bent rearward, providing more comfortable support for the upper part of the back of the occupant. The back plate  21  gets gradually thinner toward the middle from the side ends. So bending rigidity is lower in the middle pressed by the back of the occupant most strongly to allow the middle of the back plate  21  to bend significantly rearward, providing better comfort not only in the middle of the back but also all over the back of the occupant. 
         [0053]    A number of slits  27  are formed in the surface of the back plate  21  and the middles of the horizontal portions  21   b  between the upper and lower slits  27  are thinner to allow them to be stretched out easily. Accordingly, the whole back plate  21  can be bent rearward with the slits  27  providing complete fitting all over the back of the occupant and making the chair more comfortable to sit on. 
         [0054]    In the vertical portions  21   a  between the adjacent slits  27 , the distance along the thickness of the back plate  21  is larger than the distance along the width thereof, so that the back plate  21  has higher bending rigidity along the thickness of the back plate  21 . The back plate  21  provides sufficient strength in spite of a lot of slits  27 . 
         [0055]      FIG. 12  is a front elevational view of another embodiment of a chair according to the present invention, and  FIG. 13  is a front elevational view of the backrest. In the embodiment, only a back plate  21  is different from the foregoing embodiments. The rest is the same and the same numerals are allotted to the same members and will be omitted in description. 
         [0056]    Like the foregoing embodiment, the back plate  21  becomes gradually thinner upward from the lower end and toward the middle from the side ends in  FIGS. 14 and 15 . 
         [0057]    Hexagonal openings  36  are staggered all over the surface and arranged like a letter X on the back plate  21 . 
         [0058]    In  FIG. 16 , portions  37  which surround each of the openings  36  comprise vertical portions  37   a  between the horizontally adjacent openings  36  and slightly tilted lateral portions  37   b . In  FIG. 17 , a distance L along a thickness of the back plate  21  is larger than a distance t along a width thereof. 
         [0059]    Thus, when the back plate  21  is pressed by the back of an occupant, the whole back plate  21  is bent rearward with suitable bending resistance. The back plate  21  is likely to be bent with the hexagonal openings  36 , but bending strength of the back plate  21  does not become lower. 
         [0060]    The present invention is not limited to the foregoing embodiments. 
         [0061]    The embodiment in  FIGS. 1-11  discloses that the slits  27  in the back plate  21  are staggered. Alternatively, slits may be arranged along the same vertical lines. The lengths of slits may be variable. 
         [0062]    The openings  36  of the back plate  21  in  FIG. 12  may have other shapes such as a rhombus, an eclipse, and polygons other than a hexagon. 
         [0063]    The present invention may apply to a chair in JP2002-125797A. To the upper ends of a pair of rear legs, backrest support rods are fixed. At the upper end of the support rod, the back frame mounting rod  14  in the embodiment may be mounted to the backrest  19  and armrest  29  with bolts. 
         [0064]    The present invention may apply to a chair without an armrest. A shorter bolt  32  may pass through a stepped bore  26  of a side portion  20   a  and directly engage in a stepped bore  33  of a back frame mounting rod  14 . If the shape of the upper end of the backrest support rod engages in a hole of the side portion, the lower end of the back frame can directly be mounted to the upper end of the backrest support rod.