Patent Publication Number: US-7896439-B2

Title: Chair

Description:
TECHNICAL FIELD 
     The present invention relates to a chair with a backrest supported by a frame. 
     BACKGROUND ART 
     As a chair having a relatively simple structure, there is a chair in which a backrest is attached to a frame device including bars or pipes. The chair of this type is allowed to have a rocking function so that the backrest is tilted backward by the leaning of a person sitting on a seat. An example thereof is described in Patent Document 1. 
     That is, in Patent Document 1, a frame device of a chair includes right and left lower horizontal portions which elongates in back and forth directions and placed on a floor, right and left front legs which rise toward the upper side from front ends of the lower horizontal portions, upper horizontal portions which extend to the rear side from upper ends of the right and left front legs, and right and left rear legs which rise from rear ends of the lower horizontal portions, and a seat is attached to the right and left upper horizontal portions. 
     Rear ends of the right and left upper horizontal portions rise toward the upper side and the right and left upper horizontal portions are integrally connected to each other at a horizontally long lower portion. The horizontally long portion is connected to a lower back face of a backrest. Further, the upper ends of the right and left rear legs are connected to each other by a horizontally long upper portion and the horizontally long upper portion is connected to a back face of a mid-height portion of the backrest. The backward tilting of the backrest of the chair in Patent Documents 1 is realized because the rear legs resist elasticity and are thus tilted backward, mainly. 
     Meanwhile, Patent Document 2 describes an example of a rotating chair which has a leg column formed of a gas cylinder and in which a seat and a backrest are supported by a frame device. Further, in a chair disclosed in Patent Document 3, a frame device has a shape opened toward the rear side in a side view and is fixed to a base, and a backrest is attached to a rear portion of the frame device. 
     Patent Document 1: Japanese Patent Publication No. 8-15446 
     Patent Document 2: Japanese Translation of PCT Application No. 9-502631 
     Patent Document 3: Japanese Registered Design Publication No. 390617 
     DISCLOSURE OF THE INVENTION 
     Problem that the Invention is to Solve 
     In order to improve comfort in a rocking state, it is effective that a backrest is tilted backward as much as possible. Accordingly, in the case of a chair in which a backrest is rocked using elastic deformation of a frame, it is required to largely deform the frame as much as possible. However, in the case of the chair in Patent document 1, since only the rear legs of the frame are elastically deformed, a backward tilting amount of the backrest is small and a problem occurs in that there is a limit on improvement of comfort in a rocking state. 
     In addition, it can be said that, in the chair in Patent Document 2, a rocking function using bending of the frame device is rarely expected because the seat and the backrest only slide in back and forth directions as a whole. Further, it is not known whether the chair in Patent Document 3 has a rocking function. 
     The present invention is contrived in view of the problems and a main object of the invention is to provide a chair which can ensure an improved rocking function together with a simple structure. 
     A chair of the present invention has a seat which is supported by a base, a backrest which is tiltable backward, and back frame devices which have main portions positioned outside the right and left sides of the seat and support the backward tilting of the backrest. The backward tilting of the backrest is allowed by elastic deformation of the back frame devices. 
     As a character of the present invention, the main portions of the right and left back frame devices have a shape opened toward the rear side in a side view. In addition, as another character of the present invention, an upper connecting portion to which the backrest is attached is provided at an upper rear end of the main portion of the back frame device, a lower connecting portion going under the seat is formed integrally with a lower rear end of the main portion of the back frame device, the lower connecting portion has an end portion extending to the front side, and the end portion of the lower connecting portion is fixed to the base or the seat. 
     In the present invention, the main portion of the back frame device can be replaced with the word “side frame” to adopt a representation method that the side frame is provided with an upper connecting portion and a lower connecting portion. 
     In the present invention, the whole back frame device can be elastically tilted. Further, since the main portion of the back frame device has a shape opened toward the rear side in a side view, a total length of the back frame device is long. A deformation amount of the back frame device can be increased because of the shape of the main portion. This is a first functional characteristic of the present invention. 
     In addition, a screwing action and a bending action are applied to the lower connecting portion forming a lower portion of the back frame device upon rocking and thus the lower connecting portion is easily elastically deformed. This is a second functional characteristic of the present invention. 
     In the present invention, the back frame device can be largely elastically deformed with the combination of the characteristics. As a result, it is possible to ensure an improved rocking function together with a simple structure. 
     The “base” in the present invention represents a member supporting the seat and thus a leg is also in a category of the base. When the leg includes a gas cylinder, it is thought that the base is fixed to an upper end of the gas cylinder and the leg and the base are different members. However, when the leg is formed of a pipe or a plate, the seat and the back frame device can be attached to the leg, and thus in this case, the leg and the base are identifiable. 
     The present invention has various variations and these will be described as follows. 
     (1). Variation in Relationship between Backrest and Seat 
     The present invention includes two configurations. One is that the seat and the backrest are integrally connected to each other, and the other is that the seat and the backrest are not connected. When the seat and the backrest are connected to each other, the backrest is supported by the seat and thus a burden on the back frame device is reduced. Accordingly, it can be said that the present invention is particularly valuable in a chair of a type in which the seat and the backrest are integrally connected to each other. 
     Regardless of the connection between the seat and the backrest, there is a high tendency to improve a fitting property to a body when the seat is tilted backward upon rocking. Moreover, the present invention also includes a configuration in which the seat and the backrest are tilted backward together in a state of being connected to each other. The backrest and the seat are integrally tilted backward in this configuration and thus a comfortable rocking state can be provided to a user. It is preferable that a degree of the backward tilting of the seat is smaller than a degree of the backward tilting of the backrest. 
     In one configuration of the present invention, a rising portion supporting a waist of a person sitting on the seat is formed at a rear end of the seat and the backrest is connected to the rising portion to be tiltable backward. Under this configuration, the waist of the person is supported by the rising portion of the seat even in a rocking state and thus a so-called lumbar support function can be ensured. 
     (2). Variation in Structure of Back Frame Device 
     The structure of the backrest attachment to the back frame device can employ various modes. For example, a bracket may be provided at right and left ends of the backrest to be connected to the upper rear end of the main portion of the back frame device by pins. 
     In one configuration of the present invention, upper inward portions which extend along a back face of the backrest are formed to be bent at the upper rear ends of the main portions of the right and left back frame devices, respectively, and the upper inward portions serve as the upper connecting portion. This invention has an advantage in that the structure is simple and the backrest can be strongly supported. 
     The right and left back frame devices each can be formed of one member. However, in a preferable mode of the present invention, the right and left back frame devices each includes a plurality of unit frames extending substantially parallel to each other. The back frame device of the present invention is bent complicatedly as a whole. Accordingly, when the right and left back frame devices each includes a plurality of unit frames, a phenomenon in which the unit frames are deformed restrictively each other occurs and thus it is possible to prevent excessive deformation even when employing thin members (bar-shaped members or pipes). This is an advantage of the present invention. 
     When the right and left back frame devices each includes a plurality of unit frames, it is preferable that each back frame device is composed of two unit frames in view of appearance and economy. Further, it is preferable to employ metal bars or pipes as a material for the unit frame, and particularly, it is preferable to employ steel or stainless round bars in view of strength. 
     In the present invention, an armrest can be attached to an upper portion of the main portion of the back frame device. That is, the back frame device also serves as an armrest support member and this is economical. In one configuration of the present invention, under the assumption that the right and left back frame devices each includes two unit frames, the two unit frames extend substantially parallel to each other in the main portion in a top view and in a front view, and the armrest is attached to an upper portion of the main portion. Thanks to this configuration, the armrest can be stably attached. 
     Further, in another configuration of the present invention, in the attachment of the armrest to the two unit frames of the back frame device, the two unit frames are disposed at different heights so that the unit frame close to the seat is lower than the unit frame distant from the seat. Thus, an upper face of the armrest has a posture obliquely tilted downward toward the seat in a front view. The upper face of the armrest can have a horizontal posture. However, when the upper face of the armrest is tilted downward toward the seat while a person sitting on the seat outstretches his right and left arms (that is, the arms are tilted outside in a front view) and puts each elbow on the armrest, the elbow is put on the upper face of the armrest at nearly 90 degrees and thus stability of the elbows becomes good. This is also an advantage of the present invention. 
     Furthermore, in another configuration of the present invention, the main portion of the back frame device has a substantially horizontally directed V-shape, in a side view, in which a backward tilting portion and a forward tilting portion are continuously connected to each other. In this configuration, excessive deformation of the main portion is suppressed and thus a proper elastic characteristic can be ensured. In addition, since a space opened toward the front side in a side view is generated between the main portion of the back frame device and the seat, there is an advantage in that, for example, the back frame device does not interfere with even a person who sits on the seat and stretches out his legs. Moreover, the main portion of the back frame device is a most appealing portion. When the main portion is formed in a horizontal V-shape, it gives a sharp impression to people and thus it can be said that this is excellent in view of design. 
     (3). Variation in Cross-Section Structures of Backrest and Seat 
     In the present invention, the backrest and the seat include both of a single layer structure and a multilayer structure. In one configuration of the present invention, the backrest has a back inner shell of which a front face is covered with a cushion and a back outer shell which is disposed on the back side of the back inner shell, and the upper inward portion of the back frame device is disposed between the back inner shell and the back outer shell. The chair of the invention has a good appearance because the upper inward portion of the back frame device is not exposed. 
     Further, in another configuration of the present invention, the back outer shell of the backrest is formed of a wooden plate, and in the back outer shell, a backward protruding portion for avoiding the interference with the upper inward portion of the back frame device is formed to be bent and extends over the entire horizontal length. The seat has a seat outer shell which is formed of a wooden plate and a seat inner shell which is disposed on an upper face thereof and covered with a cushion, and the seat outer shell and the back outer shell have substantially the same thickness and extend to be continuous each other in a side view. 
     The reason why the back outer shell and the seat outer shell are formed of a wooden plate is mainly requirement in design and cost. Using wood which is a recyclable resource is helpful in view of global environment. Since the outer shell of the backrest is provided with the backward protruding portion, the outer shell and the inner shell of the backrest are brought in contact with each other as closely as possible and the interference of the back frame device with the outer shell is avoided. Thus, an excellent appearance is obtained. 
     The present invention will be better understood through the following specific examples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a chair of a first embodiment as viewed from the front and  FIG. 1B  is a perspective view of the chair as viewed from the back; 
         FIG. 2A  is a top view of the chair,  FIG. 2B  is a front view of the chair, and  FIG. 2C  is a right side view of the chair; 
         FIG. 3  is a top view illustrating the framework of the chair; 
         FIG. 4  is a sectional view of a base; 
         FIG. 5A  is a longitudinal sectional side view and  FIG. 5B  is a sectional view taken along the line B-B of the longitudinal sectional side view in  FIG. 5A ; 
         FIG. 6A  is a sectional view taken along the line VIA-VIA in  FIG. 5A  and  FIG. 6B  is a sectional view taken along the line VIB-VIB in  FIG. 5(A) ; 
         FIG. 7A  is a sectional view taken along the line VIIA-VIIA in  FIG. 2B , FIG.  7 B is a side view of a cap,  FIG. 7C  is a view taken along the line C-C in  FIG. 7B , and  FIG. 7D  is a sectional view taken along the line VIIB-VIIB in  FIG. 2B ; 
         FIG. 8  is a top sectional view taken along the line VIII-VIII in  FIG. 4 ; 
         FIG. 9  is a sectional view taken along the line IX-IX in  FIG. 8 ; 
         FIG. 10A  is a top view of a holding member for a lifting and dropping operation lever and  FIG. 10B  is a sectional view taken along the line B-B in  FIG. 10A ; 
         FIG. 11  is a sectional view taken along the line XI-XI in  FIG. 4 ; 
         FIG. 12A  is a sectional view taken along the line XII-XII in  FIG. 3  and  FIG. 12B  is a perspective view of a cover; 
         FIG. 13  is a perspective view of a chair of a second embodiment; 
         FIG. 14  is a perspective view of a chair of a third embodiment; 
         FIG. 15  is a front view of the chair; 
         FIG. 16  is a top view illustrating the framework of the chair; 
         FIG. 17  is a view illustrating the movement of the chair; 
         FIG. 18  is a side view of a chair of a fourth embodiment; and 
         FIG. 19A  is a front view of the fourth embodiment and  FIG. 19B  is a top view taken along the line XIXB-XIXB in  FIG. 18 . 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS AND SIGNS 
       1 : SEAT 
       2 : BACKREST 
       3 : LEG 
       4 : BACK FRAME DEVICE 
       4   a : MAIN PORTION OF BACK FRAME DEVICE 
       7 : BASE 
       8 : SEAT OUTER SHELL 
       9 : SEAT INNER SHELL 
       10 : SEAT CUSHION 
       11 : BACK OUTER SHELL 
       12 : BACK INNER SHELL 
       13 : BACK CUSHION 
       17 : HINGE 
       43 : FIRST UNIT FRAME 
       44 : SECOND UNIT FRAME 
       43   a ,  44   a : UPPER INWARD PORTION AS ONE EXAMPLE OF UPPER CONNECTING PORTION 
       43   b ,  44   b : LOWER INWARD PORTION CONSTITUTING LOWER CONNECTING PORTION 
       43   c ,  44   c : FORWARD END PORTION CONSTITUTING LOWER CONNECTING PORTION 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     A first embodiment is applied to a so-called rotating chair. The chair has a seat  1  and a backrest  2  which are integrally connected to each other, a leg  3  which supports the seat and the backrest, and right and left back frame devices  4  for holding the backrest  2  to be rocked. The leg  3  has a plurality of branch feet  3   a  radially extending in a top view and a leg column  5  formed of a gas cylinder, and a caster  6  is provided at a front end of the branch foot  3   a . A base  7  having a circular shape when viewed in a downward direction is fixed to an upper end of the leg column  5 . The shape of the base  7  is not limited to the circular shape and any shape can be selected. 
     For example, as illustrated in  FIG. 5A , the seat  1  has a seat outer shell  8  made of plywood (wooden board), a resin seat inner shell  9  overlapping with an upper face of the seat outer shell  8  and fixed thereto, and a seat cushion  10  covering an upper face of the seat inner shell  9 . 
     The backrest  2  has a back outer shell  11  made of plywood (wooden board), a back inner shell  12  disposed in front of the back outer shell, and a back cushion  13  covering a front face of the back inner shell  12 . Needless to say, the cushions  10  and  13  are covered with cloth (outer cover material). Any of resin and metal can be employed as a material for the back inner shell  12 . In the case where it is required to ensure strength, it is preferable to employ metal (for example, sheet metal or aluminum die-casting product). 
     The seat inner shell  9  and the back inner shell  12  are provided with ribs  14  and  15  overlapping with the outer shells  8  and  11 , respectively. The seat outer shell  8  and the back outer shell  11 ; and the seat inner shell  9  and the back inner shell  12  are manufactured as different members, respectively. As illustrated in  FIGS. 2A ,  2 B and  7 D, a reinforcing plate  16  made of a thin metal plate is fixed to a rear lower face of the seat outer shell  8  and the reinforcing plate  16  and the back inner shell  12  are connected by a hinge  17  to be rotatable relative to each other. A front end portion of the reinforcing plate  16  is fixed to the seat outer shell  8  by a presser plate  18  made of a metal plate. 
     The seat outer shell  8  and the back inner shell  12  may be directly connected by the hinge  17  without the use of the reinforcing plate  16 . In addition, the seat inner shell  9  and the back inner shell  12  may be connected by the hinge and the seat inner shell  9  and the back outer shell  11  may be also connected by the hinge. Moreover, the seat inner shell  9  and the back inner shell  12  may be also manufactured to be integral with each other (in this case, it is preferable to form a flexible hinge portion). 
     The seat inner shell  9  and the seat outer shell  8 ; and the back inner shell  12  and the back outer shell  11  are held to be integral with each other, respectively, by a pair of male and female snap members fitted to each other by deformation occurring against elasticity. Needless to say, they may be fixed to each other by a fastening tool such as a machine screw. 
     A rear portion of the seat inner shell  9  and a rear portion of the seat outer shell  8  slightly rise while gently curved in a side view (in  FIG. 5A , reference sign  1   a  is the rising portion of the seat  1 ). Accordingly, it can be said that a lower portion of the backrest  2  is configured by the seat inner shell  9  and the seat outer shell  8  or the seat  1  is provided integrally with a pelvis supporting portion. However, in this description, the seat  1  includes the rising portions of the rear portion of the seat outer shell  8  and the seat inner shell  9  (this is for convenience in description). 
     In general, the seat  1 , the backrest  2  and the connecting portion (that is, support shaft of hinge  17 ) are disposed at a height near a lower portion of a lumbar area of a person sitting on the seat. 
     (1)-2. Specifications of Base and Attachment Structure of Seat 
     As illustrated in  FIGS. 4 ,  5 A,  8  and  9 , the base  7  has a cylindrical body  21 , and an upper plate  22 , an intermediate plate  23  and a lower plate  24  are disposed in the cylindrical body  21 . The plates  22 ,  23  and  24  are fixed to the cylindrical body  21  by welding and a bush  25  is fixed to central portions of the plates  22 ,  23  and  24  by welding. An upper end portion of the leg column  5  is fitted to the bush  25 . 
     For example, as illustrated in  FIGS. 3 and 4 , a pair of right and left stopper rods  26 , which are vertically long and each of which is partially fitted into the base  7 , are disposed under the seat outer shell  8 . A horizontal support plate  28  is fixed to the upper end of the stopper rod  26  by welding and the support plate  28  is fixed to the seat outer shell  8  by a machine screw. 
     For example, as illustrated in  FIGS. 4 ,  8  and  11 , a seat support  27  surrounding the bush  25  from the right, left and rear sides is fixed to the lower plate  24  of the base  7  by welding (In  FIG. 4 , an upper face of the seat support  27  overlaps with the intermediate plate  23 , but actually, as illustrated in  FIG. 9 , a gap exists between the seat support  27  and the intermediate plate  23 ). 
     The seat support  27  is provided with column portions  27   a  which have a forwardly tilted posture in a side view and rise while extending horizontally in a front view, and the front end of the support portion  27   a  is formed integrally with a horizontal portion  27   b  which faces right and left directions. The horizontal portion  27   b  of the seat support  27  is pressed and held in the seat outer shell  8  by holding pieces  29 . The holding piece  29  is covered with a cover  30 . 
     The cover  30  has a cantilever beam-shaped overhanging portion  30   a  fitted between the seat outer shell  8  and the horizontal portion  27   b  and a rib  30   b  overlapping with a lower face of the holding piece  29 . As shown by the arrow A in  FIG. 3 , the cover is attached to the horizontal portion  27   b  by being inserted thereto from the outside and thus a fastening tool such as a machine screw is not required. 
     In addition, the overhanging portion  30   a  of the cover  30  is firmly inserted between the horizontal portion  27   b  of the seat support  27  and the seat ouster shell  8 , and a lower face of the horizontal portion  27   a  of the seat support  27  is brought into contact with a bottom face of the holding piece  29 . Accordingly, the seat support  27  is held in the seat outer shell  8  without rattling. 
     An auxiliary stay  31 , which is curved in a forward convex shape in a top view, is fixed to the base of the right and left horizontal portions  27   b  of the seat support  27  by welding. Accordingly, the seat  1  is supported only by the auxiliary stay  31 . 
     The lower end of the rod  26  is a free end. In a non-sitting state, the lower end of the stopper rod  26  is positioned near an upper face of the intermediate plate  23  of the base  7 . In addition, the seat  1  resists elastic deformation of the seat support  27  and can be thus tilted backward. When the seat  1  is tilted backward, the stopper rod  26  is dropped, and a maximum backward tilting angle of the seat  1  is restricted by the reach of the stopper rod  26  to the lower plate  24  of the base  7 . The upper plate  22  of the base  7  has a long hole  22   a  (see  FIG. 4 ) for allowing the stopper rod  26  to be rotated. The intermediate plate  23  of the base  7  is notched so as not to interfere with the stopper rod  26 . 
     A lever  33  for lifting and dropping the seat  1  is disposed under a front portion of the seat  1 . The lever  33  is manufactured by bending one bar (or pipe) and has a shape curved in a forward concave shape (or arched shape) in a top view as a whole. Accordingly, the lever  33  employs a form pulled upward under right and left corner portions of the seat  1 . The main reason for employing such a form is that, when lower legs of a person enter under the seat  1 , it is avoided that the lower legs reach to the operation lever  33 . 
     (1)-3. Attachment Structure of Lifting and Dropping Operation Lever 
     Next, an attachment structure of the operation lever  33  will be described based on  FIGS. 8 to 10 . The base of the operation lever  33  has a shape almost the same as a U-shape in a top view and has right and left front-rear long portions  33   a  which extend parallel to each other with a gap interposed therebetween and horizontally long horizontal portions  33   b  which are formed so as to be opposed to the right and left front-rear long portions  33   a , respectively. An operation piece  35  for pushing a push rod  34  of the operating piece leg column  5  is fixed to the right and left horizontally long horizontal portions  33   b  by welding, and the right and left horizontally long horizontal portions  33   b  other than the operation piece  35  are pressed and held in the upper face of the intermediate plate  23  of the base  7  by holding members  36 , respectively. 
     In the holding member  36 , right and left middle portions are notched so as not to interfere with the operation piece  35  and a portion pressing the horizontally long horizontal portion  33   b  is bifurcated. A rear end portion of the holding member  36  is provided with a hook  38  which is fitted to a long hole  37  formed on the intermediate plate  23  of the base  7  and hung so as not to move upward. By putting the hook  38  on the edge of the long hole  37  from the lower side, the rear end portion is fixed by one machine screw  39  so as not to be shifted. The holding member  36  is interposed between the front-rear long portions  33   a  of the operation lever  33  from the right and left sides and thus the operation lever  33  is not horizontally shifted. 
     When a finger is caught on right and left end portions of the operation lever  33  to pull the portions upward, the operation lever  33  is rotated around the horizontally long horizontal portion  33   b , and thus a bush valve  34  of the leg column  5  is pushed down by the operation piece  35  and the lock of the leg column (gas cylinder)  5  is released. In a non-operation state, the front-rear long portion  33   a  of the operation lever  33  overlaps with the upper face of the intermediate plate  23  of the base  7 . Accordingly, the operation lever  33  is held so as not to fall down in the non-operation state. 
     (1)-4. Frame Device for Back 
     Next, the back frame devices  4  will be described. The back frame devices  4  have main portions  4   a  which are positioned outside the right and left sides of the seat  1 . The main portion  4   a  has a backward tilting portion  4   b  constituting an upper portion with respect to a front end thereof as a boundary and a forward tilting portion  4   c  constituting a lower portion with respect to the front end thereof as a boundary. The whole main portion has a V-shape opened toward the rear side. 
     A continuous portion (that is, front end portion of main portion  4   a ) of the forward tilting portion  4   c  and the backward tilting portion  4   b  of the main portion  4   a  is curved in an arc shape in a side view. Further, a degree of the tilting of the backward tilting portion  4   b  is smaller than a degree of the tilting of the forward tilting portion  4   c . The reason of this is that an armrest  42  is attached to the backward tilting portion  4   b . In addition, the backward tilting portion  4   b  and the forward tilting portion  4   c  are gently curved in a side view so as to form a concave shape toward an opposite direction. 
     The right and left back frame devices  4  each are composed of unit frames  43  and  44  formed of two round bars, and both of the whole unit frames  43  and  44  extend substantially parallel to each other. In the following description, when it is required to distinguish the two unit frames  43  and  44  from each other, the unit frame close to the seat  1  is referred to as the first unit frame  43  and the unit frame distant from the seat  1  is referred to as the second unit frame  44  with reference to the postures in the main portion  4   a  for convenience. A bar made of metal such as steel, stainless or aluminum can be used as a material for both of the unit frames  43  and  44 . 
     The first unit frame  43  and the second unit frame  44  have upper inward portions  43   a  and  44   a  bent and going behind a back face of the back inner shell  12  from the rear end of the main portion  4   a , respectively. In this case, the upper inward portion  44   a  of the second unit frame  44  is positioned higher than the upper inward portion  43   a  of the first unit frame  43 , and the upper inward portions  44   a  of the right and left second unit frames  44  each are pressed and held in the back inner shell  12  by presser portions (bearing members)  45 . The presser portion  45  is fixed to the back inner shell  12  by a machine screw. The upper inward portion  44   a  of the second unit frame  44  can be slightly rotated relative to the presser member  45  and the back inner shell  12 . 
     In this embodiment, the upper inward portions  44   a  of the right and left second unit frames  44  are continuously connected to each other. Thus, the right and left second unit frames  44  are manufactured using one round bar as a material. In this embodiment, since the right and left upper inward portions  44   a  extend in series, there is an advantage in that the tilting of the backrest  2  becomes smooth. 
     The upper inward portion  43   a  of the first unit frame  43  has a set length so as to abut on or be brought into close contact with the presser member  45 . Since the upper inward portion  43   a  is set to have such a length, right-left positioning of the back frame device  4  and the backrest  2  is automatically performed. 
     It is preferable that the upper inward portion  43   a  of the first unit frame  43  and the upper inward portion  44   a  of the second unit frame  44  are integrally connected to each other by welding or the like. The back outer shell  11  overlaps with the back face of the back inner shell  12 . Accordingly, in order to prevent the upper inward portions  43   a  and  44   a  of the unit frames  43  and  44  to interfere with the presser member  45 , a backward protruding portion  11   a  curved in a mountain shape is formed to be bent in the back outer shell  11 . A tunnel-shaped space is generated between the back outer shell  11  and the back inner shell  12  by forming the backward protruding portion  11   a  and this space can be sealed with a cap  46  illustrated in  FIGS. 7C  and D. 
     For example, as illustrated in  FIG. 3 , the first unit frame  43  and the second unit frame  44  are provided with lower inward portions  43   b  and  44   b , which are horizontally long and go under the seat  1  while being continuously connected to the rear end of the front tilting portion  4   c  of the main portion  4   a , respectively. In back of the base  7 , the lower inward portions  43   b  and  44   b  go under the seat  1  and the front ends of the lower inward portions  43   b  and  44   b  are integrally connected to forward end portions  43   c  and  44   c . The forward end portions  43   c  and  44   c  are fixed to the intermediate plate  23  of the base  7  by welding. In this embodiment, the lower connecting portion described in the claims includes the lower inward portions  43   b  and  44   b  which are horizontally long and the forward end portions  43   c  and  44   c.    
     Needless to say, a cylindrical body  21  of the base  7  is notched so as not to interfere with the forward end portions  43   c  and  44   c . As illustrated in  FIG. 8 , long holes  47  and notched portions  48  overlapping with the forward end portions  43   c  and  44   c  in a top view are formed in the intermediate plate  23  of the base  7  and this is a measure for facilitating overlaying upon welding of the forward end portions  43   c  and  44   c.    
     In this embodiment, the lower inward portion  43   b  and the forward end portion  43   c  of the first unit frame  43  are surrounded by the lower inward portion  44   b  and the forward end portion  44   c  of the second unit frame  44  from the outside. This is because the forward end portions  43   c  and  44   c  are disposed at the same height. By changing the heights of the forward end portions  43   c  and  44   c , the forward end portions  43   c  and  44   c  can achieve a posture overlapping with each other in a top view and can be fixed to a side face of the base  7 . 
     The first unit frame  43  and the second unit frame  44  are set to be at different heights so that the first unit frame  43  is lower than the second unit frame  44  at a position in the backward tilting portion  4   b  of the main portion  4   a . In addition, as illustrated in  FIG. 5B , a plate member  49  having a posture tilted in a front view is fixed to the first unit frame  43  and the second unit frame  44  of the backward tilting portion  4   b  by welding and the armrest  42  is fixed to the plate member  49 . 
     Since the plate member  49  is tilted in a front view, an upper face of the armrest  42  is tilted downward toward the seat  1  in a front view. Accordingly, when a person sitting on the seat outstretches his arms and puts each elbow on the armrest  42 , the arms are put from a direction substantially perpendicular to the upper face of the armrest  42  as shown by the arrow B in  FIG. 5B  and thus the elbows can be stably held. The armrest  42  is separated into two upper and lower members  42   a  and  42   b  and the upper and lower members  42   a  and  42   b  are fastened by screws (not shown) passing through the plate member  49 . 
     (1)-5. Conclusion 
     When a person sitting on the seat leans on the backrest  2 , a large moment is applied to an upper portion of the backrest  2  and thus the backrest  2  is largely tilted backward around the hinge  17  by elastic deformation of the back frame device  4 . 
     Further, states, specifically, a state “a” in which since the back frame device  4  has a long length and a substantially V-shape in a side view, the back frame device is easily deformed, a state “b” in which since the lower inward portions  43   b  and  44   b  are present under the back frame device  4 , an action (torsion action) is applied to the lower inward portions  43   b  and  44   b  to screw them around axes thereof, and thus the back frame device  4  is largely and easily deformed in a direction so that the backward tilting angle of the backrest  2  increases, and a state “c” in which the screwing action and the bending action are also applied to the forward end portions  43   c  and  44   c  and thus they are allowed to be deformed, are given, and the back frame device  4  is largely elastically deformed with the combination of the states a to c. As a result, the backrest  2  can be largely tilted backward. 
     The back frame device  4  is composed of two inner and outer unit frames  43  and  44 . Since both of the unit frames  43  and  44  have different lengths and are disposed at different heights, the unit frames  43  and  44  are deformed in different manners and deformation occurs with mutual restriction. That is, a phenomenon in which one of the unit frames  43  and  44  is deformed while the other acts as resistance occurs. 
     In this case, the sum of the elastic deformation of the two unit frames  43  and  44  becomes resistance to the backward tilting of the backrest  2  and the forward advance of the seat  1 . The unit frames  43  and  44  restrict each other as the degree of the deformation increases, and thus an increasing rate of the elastic restoring force of the unit frames  43  and  44  as a whole is higher than an increasing rate of the backward tilting angle of the backrest  2 . As a result, the backrest  2  is not steeply tilted backward. The backrest is tilted backward with an appropriate resistance force. In this manner, the chair of this embodiment can provides a comfortable feeling upon rocking. 
     In this embodiment, the seat  1  is tilted backward and either one of the following two modes, specifically, a mode in which the seat is initially tilted backward since a person sitting on the seat leans on the backrest  2  and a mode in which the seat is slightly tilted backward since a person sits on the seat, can be selected as the backward tilting mode of the seat  1 . The mode is determined depending on the setting of the strength (elastic restoring force) of the seat support  27 . 
     Second Embodiment 
       FIG. 13  illustrates a second embodiment. In this embodiment, a leg  3  has a four leg-type frame structure. The portions disposed above the base  7  are essentially the same as them of the first embodiment. However, the lifting and dropping operation lever  33  of the first embodiment is not provided (in this second embodiment, the lifting and dropping operation lever is not required). 
     In the case where the leg  3  has a frame structure composed of pipes or bars, various types are conventionally proposed. The present invention can be applied to any conventional leg and can be also applied to newly manufactured legs. 
     Third Embodiment 
       FIGS. 14 to 17  illustrate a third embodiment. This embodiment has the same basic configuration as that of the first embodiment and there are differences in details. Hereinafter, a description will be given. Descriptions for the same parts as those of the first embodiment will be omitted if not particularly necessary. 
     In this embodiment, a seat  1  and a backrest  2  have a structure in which substrates  1   a  and  2   a  such as synthetic resin or plywood are covered with cushion members  1   b  and  2   b , respectively, and the substrate  1   a  and  2   a  of the seat  1  and the backrest  2  are integrally connected to each other (the substrates  1   a  and  2   a  correspond to the outer shells  8  and  11  of the first embodiment, respectively). The backrest  2  is bent in a side view around a bending point  17 ′ (fold) positioned at a height near a waist of a person sitting on the seat. 
     As a method of bending the backrest  2 , as illustrated in  FIG. 17 , the substrate  2   a  may be separated into upper and lower portions to connect both of the portions to each other by a hinge  17  or a pin, or the substrate  2   a  may be made of a flexible material such as resin to form a thin hinge portion. 
     As illustrated in  FIGS. 15 to 17 , the seat  1  is attached to seat frames  51  attached to a base  7 . The seat frames  51  are formed in a substantially crank shape with lower horizontal portions  51   a  which are horizontally long in a front view, vertically long portions  51   b  which are formed to be bent at opposite ends of the lower horizontal portions, and right and left upper horizontal portions  51   c  which are formed at opposite ends of the vertically long portions. The lower horizontal portion  51   a  is fixed to the base  7  or attached to be rotatable relative to the base  7  via a lower bearing member  52 . The upper horizontal portion  51   c  of the seat frame  51  is attached to be rotatable relative to a lower face of the seat  1  by an upper bearing member  53 . Accordingly, when the seat frame  51  is rotated around the upper horizontal portion  51   a , the seat  1  can slide forward and backward. 
     First and second unit frames  43  and  44  are provided with column portions  4   d  rising in a vertical direction in a front view on the right and left sides of the seat  1 , upper portions  4   e  extending backward while being continuously connected to the upper ends of the right and left column portions  4   d , and lower portions  4   f  which are horizontal in a side view and extend backward while being continuously connected to the lower ends of the right and left column portions  4   d . By the column portion  4   d , the upper portion  4   e  and the lower portion  4   f , a main portion  4   a  opened toward the rear side is configured. 
     Lower inward portions  43   b  and  44   b  which are horizontally long extend continuously from the rear ends of the lower portions  4   f  and forward end portions  43   c  and  44   c  are formed integrally with the front ends of the lower inward portions  43   b  and  44   b . The forward end portions  43   c  and  44   c  are fixed to the base  7  by welding. 
     Moreover, the rear end of an upper portion  4   e  of the second unit frame  44  is integrally connected to one upper inward portion  44   a  extending along a back face of a portion near the upper end of the backrest  2  and the upper inward portion  44   a  is connected to the back face of the backrest  2  by presser members  45  to be relatively rotated. The upper portion  4   e  of the first unit frame  43  is fixed to the upper inward portion  44   a  of the second unit frame  44  by welding. 
     The upper inward portion disposed inside the backrest  2  may be made of a different member from that for both of the unit frames  43  and  44  and the upper portions  4   e  of the unit frames  43  and  44  may be fixed to the upper inward portion by welding or the like. Furthermore, the first unit frame  43  may be provided with an upper inward portion  43   a  which is formed to be bent. 
     As illustrated in  FIGS. 15 and 16 , the second unit frame  44  and the first unit frame  43  extend substantially parallel to each other in a top view and in a front view, except the upper inward portion  44   a.    
     In this embodiment, when the seat frame  51  is fixed to the base  7 , the seat  2  only moves forward upon rocking. On the other hand, when the seat frame  51  is connected to the base  7  to be tiltable, the seat  1  moves forward upon rocking. In addition, the front end of the seat  1  may be dropped by forward movement (that is, forward tilting). 
     A function of the back frame device  4  is essentially the same as that of the first embodiment. However, it can be said that since a length of the back frame device  4  is longer than that of the first embodiment and the first unit frame  43  and the second unit frame  44  are not connected to each other in the main portion  4   a , deformation occurs more easily in this embodiment than in the first embodiment. 
     Fourth Embodiment 
       FIGS. 18 and 19  illustrate a fourth embodiment. In this embodiment, a back frame device is formed of one member. Since a shape of a back frame device  4  is almost the same as in the third embodiment, the same reference signs as in the third embodiment are given and details will be omitted. An upper inward portion is represented by reference sign  4   g , a lower inward portion is represented by reference sign  4   h , and a forward end portion is represented by reference sign  4   i . In the relationship with the above-described embodiments, the reference sign  4   g  corresponds to the reference signs  43   a  and  44   a , the reference sign  4   h  corresponds to the reference signs  43   b  and  44   b , and the reference sign  4   i  corresponds to the reference signs  43   c  and  44   c.    
     In this embodiment, a pair of front and rear seat frames  51  is disposed. The seat frame  51  may be fixed to a base  7  and may be connected to the base  7  to be tiltable. The forward end portion  4   i  of the back frame device  4  is fixed to a lower face of the base  7 . 
     (5). Others 
     The present invention can be embodied in various forms other than the above embodiments. For example, a forward end portion of a lower connecting portion of a back frame device is tiltable in a top view. That is, for example, tilting can be achieved so that right and left spaces of right and left forward end portions become wider as being close to the rear side. In addition, the lower connecting portion may be formed in an arc shape in a top view. 
     Furthermore, when a main portion of a back frame device is formed in a substantially V-shape in a side view, an upper face or a lower face thereof may be formed in a horizontal shape. The main portion of the back frame device may be also formed in a U-shape opened toward the rear side in a side view. 
     In this description, it is disclosed that the back frame device is composed of a plurality of the unit frames and the connecting portion of the back support frame is provided with the end portion extending in substantially front and rear directions, and these can be embodied as independent inventions.