Abstract:
A backrest device for a chair, capable of, without a reduction in its strength and rigidity, effectively bending backward so as to follow the shape of the back of a person seated on the chair, providing the person with excellent sitting comfort. In the backrest device, a back board is supported by a backrest frame, and the back board has a flexible back stopper on the inner side of a back frame. The thickness of a vertical intermediate section of the back stopper is gradually increased from both left and right sections of the intermediate section toward the center such that the thickness is less in both left and right sections and is maximum at the center.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a chair backrest device comprising the flexible back support supporting the back of a sitting person in a backrest frame. 
         [0002]    Such a chair backrest device is disclosed, for example, in JP2002-125797A, JP2005-160558A, particularly in FIGS. 10-17, and JP2001-128785A. 
         [0003]    In the backrest shell of the backrest device in JP2002-125797A and JP2005-160558A, a number of openings are formed all over the backrest shell, which is flexed rearward when it is pressed by the back of a person. But the thickness of the backrest shell is almost equal, and the whole backrest shell is not effectively flexed rearward along the shape of the back, so that seating comfort is not obtained. 
         [0004]    To overcome the disadvantage, the backrest shell can be made thinner for easier flexing, but the backrest shell decreases in strength and rigidity, so that durability is decreased and seating comfort is likely to get worse due to too much flexing. 
         [0005]    In the backrest device in JP2001-128785A, in the upper part of the backrest shell, there are two vertically elongate openings which engage with the projections on the upper part of the backrest frame to enable the backrest shell to move up and down. Furthermore, in the lower part of the backrest frame, there are holding portions projecting forward and having grooves. On the backrest shell, there are lobes which engage with the holding portions and which is pressed downward on the grooves, so that the backrest shell is supported by the backrest frame to enable the backrest shell to flex rearward. 
         [0006]    However, in the support structure for the backrest shell, the right and left sides of the backrest shell supported by the backrest frame does not turn around a vertical axis, so that the whole backrest shell does not flex rearward effectively along the shape of the back of a person. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the disadvantages, it is an object of the invention to provide a chair backrest device in which a backrest shell can flex effectively along the shape of the back of a person without decreasing strength and rigidity of the backrest shell, the device providing seating comfort. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a side elevational view of a chair comprising a backrest device according to the present invention. 
           [0009]      FIG. 2  is an enlarged horizontal sectional end view taken along the line II-II in  FIG. 1 . 
           [0010]      FIG. 3  is an enlarged horizontal sectional end view taken along the line III-III in  FIG. 1 . 
           [0011]      FIG. 4  is an enlarged vertical sectional side view taken along the line IV-IV in  FIG. 2 . 
           [0012]      FIG. 5  is an enlarged vertical sectional side view taken along the lien V-V in  FIG. 3 . 
           [0013]      FIG. 6  is an exploded perspective view of a connector between a backrest frame and a backrest shell at the upper end. 
           [0014]      FIG. 7  is an exploded perspective view of a connector at the lower part. 
           [0015]      FIGS. 8A-8C  are views showing how to support the backrest shell over the backrest frame. 
           [0016]      FIG. 9  is a front elevational view of the backrest shell. 
           [0017]      FIG. 10  is a rear elevational view thereof. 
           [0018]      FIG. 11  is an enlarged horizontal sectional end view taken along the line XI-XI in  FIG. 9 . 
           [0019]      FIG. 12  is an enlarged horizontal sectional end view taken along the line VII-XII in  FIG. 9 . 
           [0020]      FIG. 13  is an enlarged front view of a back support. 
           [0021]      FIG. 14  is an enlarged horizontal sectional plan view taken along the line XIV-XIV in  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    One embodiment of the present invention will be described with respect to drawings. 
         [0023]      FIG. 1  is a side elevational view of a chair comprising a chair backrest device according to the present invention.  FIG. 2  is an enlarged horizontal sectional end view taken along the line II-II in  FIG. 1 ; and  FIG. 3  is an enlarged horizontal sectional end view taken along the line III-III in  FIG. 1 . 
         [0024]    A chair  1  comprises a leg unit  4  having five radially-extending legs  3  each of which has a caster  2  at the end; a post  5  standing at the center of the leg unit  4  and retractable with a gas spring (not shown); and a base  6  fixed at the upper end of the post  5 . 
         [0025]    A seat support frame  7  is mounted at the lower end to the front end of the base  6 . To the base  6 , the front ends of side frames  8   a  of a backrest frame  8  are pivotally mounted on a pivot shaft  9 . The backrest frame  8  is always urged forward and downward by urging means (not shown) within the base  6 . 
         [0026]    A seat  10  is supported at the front end by the seat support frame  7  and at the rear end by the side frames  8   a.    
         [0027]    A backrest shell  11  is mounted over the front face of the side frames  8   a  of the backrest frame  8 . 
         [0028]    In  FIGS. 9 and 10 , the backrest shell  11  comprises a rectangular high-rigidity back frame  12  and a flexible back support  13 , and is integrally molded from synthetic resin. 
         [0029]    The backrest frame  8  and the backrest shell  11  are bent such that the upper part projects slightly more than the lower part in  FIG. 1 . 
         [0030]    A distance between the side frames  8   a  and  8   a  of the backrest frame  8  becomes narrower rearward. 
         [0031]    In  FIGS. 1 ,  2 ,  4  and  6 , the first projecting portion  14  is provided on the front face at the upper end of each of the side frame  8   a,  while the first receiving portion  15  is formed on the rear face at the lower end of the back frame  12 . The first projecting portions  14 , 14  are fitted in the first receiving portions  15 , 15  upward, so that the rear face of the back frame  12  is supported by the front faces of the side frames  8 , 8 . 
         [0032]    In  FIG. 6 , the first projecting portion  14  comprises a first shaft  16  and a support  17 , and projects forward on the front face at the upper end of the side frame  8   a.    
         [0033]    The first receiving portion  15  is formed by closing the upper end of a C-like sectioned tube  18  that is open as a slit at the rear end. The first shaft  16  of the first projecting portion  14  is fitted in the tube  18  of the first receiving portion  15  upward and the support  17  is fitted in the tube  18 , so that the first shaft  16  is fitted in the tube  18  to turn around a vertical axis. 
         [0034]    In  FIGS. 1 ,  3 ,  5  and  7 , on the back face at the side of a bent portion  20  of the back frame  12 , a second projecting portion  21  is provided, while a second receiving portion  23  is provided at a bent portion  22  of the side frame  8   a.  The second projecting portions  21 ,  21  engages in the second receiving portions  23 , 23 , so that the back frame  12  of the backrest shell  11  is supported at the bent portions  20 , 22 . 
         [0035]    In  FIG. 7 , the second receiving portion  23  comprises a pair of projections  24 , 24  spaced from each other. Inner surfaces  24   a , 24   a  of the projections  24 , 24  are concavely formed. 
         [0036]    The second shaft  25  of the second projecting portion  21  comprises a shaft body  26  slightly longer than the projections  24 , 24  and a pair of upper and lower horizontal portions  27 , 27  slightly larger than a diameter of the shaft body  26 . 
         [0037]    The shaft body  26  of the second projecting portion  21  is pressed onto an opening  28  between the projections  24  and  24  of the second receiving portion  23  to make the opening  28  open elastically. Thus, the shaft body  26  fits between the opposing inner surfaces  24   a  and  24   a,  and the second shaft  25  of the second projecting portion  21  engages in the second receiving portion  23  to turn around a vertical axis. 
         [0038]      FIGS. 8A-8C  show how the backrest shell  11  is supported by the backrest frame  8 . 
         [0039]    In  FIGS. 8A and 6 , the backrest shell  11  is moved downward. The first shaft  16  of the first projecting portion  14  engages in the tube  18  of the first receiving portion  15 , while the support  17  that fixes the first shaft  16  to the side frame  8   a  fits into the opening  19  of the tube  18 . 
         [0040]    Then, in  FIGS. 8B and 7 , the shaft  26  of the second projecting portion  21  at the bent portion  20  of the back frame  12  is pressed rearward into the opening  28  between the projections  24  and  24  of the second receiving portion  23  at the bent portion  22  of the side frame  8   a , and makes the opening  28  open to engage between the projections  24  and  24 . 
         [0041]    Accordingly, in  FIG. 8C , the backrest shell  11  is supported to turn around the vertical axis with sufficient strength at four points at the upper ends and the bent portions  22  of the side frames  8   a  of the backrest frame  8 . 
         [0042]    In  FIGS. 2 and 3 , after the backrest shell  11  is mounted, there is formed a space  29  where each side portion of the back support  13  can be flexed rearward, between the rear face of the side of the back support  13  and the front face of the side frame  8   a.    
         [0043]    Then, the backrest shell  11  will be described in detail in FIGS.  9 - 14 . 
         [0044]    Two vertical ribs  30 , 30  project on the rear surfaces of the side of the back frame  12  of the backrest shell  11 . 
         [0045]    A plurality of vertical slits  31  are formed at regular intervals all over the surface of the back support  13 . The horizontally-adjacent slits  31  are staggered by a half of the length of the slit  31  horizontally. The slits  31  are in line vertically. 
         [0046]    By the arrangement of the slits  31 , webs between the slits  31  are staggered horizontally and are not in line. A number of slits  31  do not cause the back support  13  to decrease in strength. 
         [0047]    In  FIG. 11 , the back support  13  is the thickest in the middle of the width and gradually decreases toward the side ends horizontally, while the back support  13  is the thickest in the middle of the vertical length and gradually decreases toward the upper and lower ends vertically in  FIG. 12 . 
         [0048]    In  FIGS. 13 and 14 , the back support  13  has rectangular-sectioned vertically-extending ribs  32  on the back surface and thus has U-like horizontal cross section in  FIG. 11 . The slit  31  is formed along thinner portion  33  between the adjacent ribs  32 . 
         [0049]    As described above, in the foregoing embodiments, the back support  13  which is pressed by the back of a person is the thickest in the middle of the width and gradually reduces in thickness toward the right and left side ends, so that flexing rigidity is the highest in the middle to allow the right and left sides to flex more easily than the middle as shown in two-dot-dash lines in  FIG. 11 . Thus, the middle of the back support  13  flexes along the shape of the back of the person, so that the whole back is stably supported providing seating comfort. 
         [0050]    The back support  13  is the thickest in the middle of the length to increase flexing rigidity. The middle onto which the back is pressed strongly is prevented from flexing excessively rearward like a convex as shown by tow-dot-dash lines in  FIG. 12 . The middle of the back is comfortably supported, and the back support  13  is unlikely to decrease in strength. 
         [0051]    A number of rectangular sectioned ribs  32  project on the rear surface of the back support  13  and extend vertically. Thus, the back support  13  increases in bending rigidity along the thickness perpendicular to the width. In spite of a number of slits, the back support  13  improves in strength. The back support  13  has U-like horizontal cross sections, and the slits  31  are formed along the thinner portions  33  between the adjacent ribs  32 . Accordingly, the back support  13  opens rearward in the rear surface around the slits  31  and can be easily flexed rearward. 
         [0052]    The present invention is not limited to the foregoing embodiments. 
         [0053]    In the embodiment, the adjacent slits  31  in the back support  13  are staggered horizontally, but may be formed to be in line with each other. Different-length slits may be formed. 
         [0054]    Without the slits as above, the back support may be formed with different thickness similar to the embodiment.