Patent Publication Number: US-9402478-B2

Title: Rocking chair and spring unit used therein

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT application No. PCT/JP2012/068095, which was filed on Jul. 17, 2012 based on Japanese Patent Application Nos. 2011-157063 filed on Jul. 15, 2011, 2011-157065 filed on Jul. 15, 2011, 2011-157067 filed on Jul. 15, 2011, and 2011-250621 filed on Nov. 16, 2011, the contents of which are incorporated herein by reference. Also, all the references cited herein are incorporated as a whole. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a rocking chair in which a backrest tilts rearwards against a spring portion and, more particularly, to a rocking chair in which the magnitude of resistance of a spring portion to a rearward tilting of the backrest can be adjusted. Furthermore, the present invention includes a spring unit used in the rocking chair. 
     2. Background Art 
     A rocking chair includes a spring means that imparts resistance to a rearward tilting of a backrest. However, generally, a resilience adjustment means is provided for changing the magnitude (i.e., magnitude of a reaction force of the backrest acting on a body during rocking) of resistance of the spring means to the rearward tilting of the backrest. A compression coil spring is often used as a spring means. Meanwhile, the resilience adjustment device includes a stepless type using a rotary screw and a step type using a cam or lever. 
     A mechanism for changing the magnitude of resistance of a spring means is roughly divided into a type of changing the magnitude of initial pressing to the spring means and a type of changing the magnitude of moment acting on the spring means. The former type is disclosed in PTL 1 and PTL 2. In PTL 1 and PTL 2, a compression coil spring is supported by a movable spring mount, the movable spring mount is supported by a peripheral surface cam and an initial elastic force of the compression coil spring is changed by rotating the peripheral surface cam. 
     On the other hand, PTL 3 discloses a configuration in which a compression coil spring is fitted into two extendable spring mounts and pivotably coupled to one spring mount using the one spring mount as a base and the other spring mount receives the load of rocking. The compression coil spring is pivoted by moving the other spring mount by an adjustment screw having a knob. 
     Load is applied to a locking spring means even when a person is not leaning against a backrest. Specifically, a pretension (preload) is applied to the spring means. The reason is that the backrest is suddenly largely inclined rearward without resistance by the leaning of a person and this is dangerous when a pretension is not applied to the spring means. Further, in the case of a synchronous type chair in which a seat is tilted rearward in conjunction with the rearward tilting of the backrest, the pretension of the locking spring also serves to hold the seat so that the seat does not tilt rearward just by seating.
     PTL 1: JP-A-10-179312   PTL 2: JP-A-11-169254   PTL 3: Japanese Patent Publication No. 2519167   

     SUMMARY OF THE INVENTION 
     When a peripheral surface cam as disclosed in PTL 1 and PTL 2 is used as a resilience adjustment means (reaction force adjustment means) of a spring means, there is an advantage that it is possible to adjust the resilience at the time of rocking by one-touch manner. However, in rotating the peripheral surface cam, it is necessary to temporarily compress the coil spring in order to shift the point of action of load to an adjacent cam surface. Therefore, it is essential to enlarge the knob in order to lightly rotate the peripheral surface cam. 
     On the other hand, in the case of employing a method in which moment is changed by pivoting the spring as in PTL 3, a force required for operating the resilience adjustment is reduced but it is impossible to change the posture of the coil spring unless the adjustment screw is rotated several times, as compared to a case of supporting the coil spring by a cam. Accordingly, there is a problem that an adjustment operation is cumbersome. 
     The present invention has been made in consideration of such a situation and an object thereof is to provide an improved resilience adjustment mechanism. Further, the present application discloses many improvements and it is also an object of the invention to provide such improvements. 
     A chair of the present invention includes, as a basic configuration, a seat, a backrest that is tiltable rearward, a locking spring portion that imparts resistance to the rearward tilting of the backrest and a resilience adjustment member that changes the degree of resistance of the spring portion to the rearward tilting of the backrest. Further, the present invention can be variously deployed using the basic configuration as a base. First, a first invention is intended to form a broader concept. In the present invention, the resilience adjustment member is a cam that can be rotationally operated by a person seated and the position in which rocking load due to the rearward tilting of the backrest is applied to the spring portion is changed by the cam, so that moment applied to the spring portion changes and the degree of resistance of the spring portion is adjusted. 
     The first invention can be variously deployed. In a second invention as a deployment example of the first invention, the rocking chair includes a base that is provided at an upper end of a leg and a back frame that is connected to the base so as to be tiltable rearward. A pushing part is provided at a front end portion of the back frame that is located across a tilting center and on the opposite side of the backrest, the pushing part being brought into contact with the spring portion. The spring portion is a compression coil spring that is long in a longitudinal direction and wound around an axis thereof and attached to the base so as to be pivotable vertically about a front portion thereof and a rear end thereof is configured as a load support part that is pressed by a pressing part of the back frame. The pushing part of the back frame has a circular arc shape that is concave forward, as seen from the side, so as to allow the pivoting of the spring portion. 
     A third invention is a deployment example of the second invention. In the third invention, the compression coil spring is incorporated in a spring holder that is stretched in a longitudinal direction, the cam is a peripheral surface cam, a plurality of cam surfaces are formed at an outer peripheral surface of the cam and distances of the cam surfaces from an axis are different from each other and, the spring holder is provided with cam mount parts with which a plurality of cam surfaces of the peripheral surface cam is selectively brought into contact. 
     A fourth invention is a preferred deployment example of the third invention. In the fourth invention, the spring holder comprises two spring mounts that are fitted to each other so as to be slidable in a longitudinal direction and support the spring from one end and the other end and the two spring mounts are inseparably retained in a state where the compression coil spring is pre-compressed. The second invention can be deployed as a fifth invention. In the fifth invention, the rocking chair includes a separation prevention portion that holds the spring portion in a state of being close contact with the cam and the separation prevention portion includes an elastic part that allows the rotation of the cam. 
     The present invention also includes a spring unit. A sixth invention pertaining to the spring unit includes a compression coil spring and two spring mounts that support the compression coil spring from one end and the other end thereof. The two spring mounts are fitted to each other so as to be stretchable and inseparably held in a state where the compression coil spring is pre-compressed. One mount of the two spring mounts is provided with a load support part to which a rocking load of the chair is applied and the other mount thereof is provided with a connection part that is pivotably connected to a constituent member of the chair. 
     The present invention is intended to perform the resilience adjustment of rocking by changing moment acting on a locking spring portion but does not change an initial load (pretension) applied to the locking spring. Accordingly, it is possible to prevent or significantly suppress that an elastic restoring force of the spring portion serves as resistance to the rotation of a cam. Therefore, the cam can be operated to lightly rotate even in the case of a compact operation member. In other words, it is possible to lightly perform the resilience adjustment of the locking spring portion by a compact operation member. 
     However, in the case of PTL 3, when the coil spring is pivoted so that a load supporting point thereof is away from a tilt supporting point of a backrest, the moment acting on the coil spring is decreased and therefore the spring becomes a “rigid” state at the time of rocking. On the contrary, when the coil spring is pivoted so that the load supporting point thereof is close to the tilt supporting point of the backrest, the moment acting on the coil spring is increased and therefore the spring becomes a “soft” state at the time of rocking. When the resilience adjustment is performed by changing the moment in such a way, it is preferable that the coil spring has a constant elastic restoring force, irrespective of the posture thereof. 
     However, in PTL 3, a surface (working surface of load) which applies moment of rocking to a compression coil spring is in a straight posture, as seen from the side. Therefore, as the compression coil spring is pivoted, the entire length of the compression coil spring changes and an initial elastic force changes. To be described accurately, when the compression coil spring is pivoted so that the point of action of load thereof is away from the tilt supporting point of the backrest, the compression coil spring is stretched and an initial elastic force thereof is decreased. On the contrary, when the compression coil spring is pivoted so that the point of action of load thereof is close to the tilt supporting point of the backrest, the compression coil spring is shrunk and an initial elastic force thereof is increased. Accordingly, expansion and contraction of the compression coil spring acts to cancel the intensity change of moment. 
     On the other hand, in the second invention of the present application, since the pushing part of the back frame has a circular arc shape that is concave forward, as seen from the side, it is possible to pivot the compression coil spring without changing the length thereof. Therefore, it is possible to change the resilience of rocking to a proper level and also it is possible to more accurately prevent or suppress that an elastic force of the compression coil spring is applied to the cam. In the case of the second invention, it is preferable that the shape of the cam mount part is a circular arc shape of radius of curvature around a pivot support point of the compression coil spring, as seen from the side. 
     When the compression coil spring is incorporated in an extendable spring holder, as in the third invention, it is possible to more simply realize the posture change of the compression coil spring. In this case, when the constituent members of the spring holder are inseparably held, as in the fourth invention and the sixth invention, not only efforts of managing the members can be reduced but also assembly of the chair becomes easy. Further, since it is possible to prevent or suppress that an elastic force of the compression coil spring is applied to the cam mount part, operability of resilience adjustment can be more improved. 
     To be described further, although a pretension is applied to the compression coil spring even in a non-rocking state, as described above, members such as the compression coil spring and the spring mount are individually produced as a separate member and then assembled, in a prior art. Therefore, mounting or the like of a movable spring mount is performed in a state where the compression coil spring is shrunk. Accordingly, efforts of managing the components are caused and assembly of the chair is also troublesome. However, since, in the fourth invention and the sixth invention of the present application, the compression coil spring is incorporated in the spring holder in a state where a pretension is applied to the compression coil spring in advance, effort of managing the components can be reduced and assembly of the chair can be performed in an extremely simple manner. 
     When moment is changed by changing the posture or the like of the spring portion, the spring portion or the like is pressed by the cam surface of the cam and pivoted. However, since, in the case of a simple peripheral surface cam, the spring portion or the like can be pressed but cannot be pulled, it is necessary to maintain, by any portion, reversibility that the spring portion or the like is moved, irrespective of the direction of rotation of the cam. 
     With regard to this, in the case of providing the separation prevention portion by employing the fifth invention, it is possible to secure the reversibility and therefore it is possible to guarantee the function of the cam. Further, in the case of providing the elastic member in the separation prevention portion, as described in claim  5 , the elastic member is temporarily deformed at the time of being shifted to an adjacent cam surface and therefore it is possible to secure a rattling click felling at the time of shifting the cam surface. This is preferable because a person can accurately grasp the adjusting state of resilience. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A to 1C  are views showing an appearance of a chair according to a first embodiment,  FIG. 1A  is a perspective view thereof as seen from the front,  FIG. 1B  is a perspective view thereof as seen from the rear and  FIG. 1C  is a side view thereof. 
         FIG. 2A  is an exploded perspective view of the entire chair and  FIG. 2B  is a longitudinal sectional side view of a back frame. 
         FIG. 3  is an exploded perspective view of the entire chair. 
         FIG. 4A  is a perspective view of a supporting mechanism part as seen from the lower front and  FIG. 4B  is a perspective view of the supporting mechanism part as seen from the lower side. 
         FIG. 5  is an exploded perspective view of the supporting mechanism part. 
         FIG. 6A  is an exploded perspective view in a state where a seat part is turned over,  FIG. 6B  is an exploded perspective view of a seat outer shell and an intermediate bracket and  FIG. 6C  is a partial enlarged perspective view of the seat outer shell. 
         FIG. 7  is a plan view of the supporting mechanism part that centers a base. 
         FIG. 8A  is a partial exploded perspective view of the supporting mechanism part that centers the base,  FIG. 8B  is a sectional view taken along a line VIIIA-VIIIA shown in  FIG. 7  and  FIG. 8C  is a sectional view taken along a line VIIIB-VIIIB shown in  FIG. 7 . 
         FIG. 9A  is a longitudinal sectional side view of the supporting mechanism part and 
         FIG. 9B  is an exploded perspective view of a pushing shaft and a spring contact portion. 
         FIG. 10A  is an explanatory sectional view taken along a line XI-XI shown in  FIG. 7  and  FIG. 10B  and  FIG. 10C  are partial enlarged views of  FIG. 10A . 
         FIG. 11A  is an exploded perspective view of a resilience adjustment unit and the base and  FIG. 11B  is a partial exploded perspective view of the resilience adjustment member. 
         FIG. 12A  is an exploded perspective view of the resilience adjustment member,  FIG. 12B  is an exploded perspective view of a spring holder and  FIG. 12C  is a perspective view of a spring unit. 
         FIG. 13A  and  FIG. 13B  are exploded perspective view of the resilience adjustment member. 
         FIG. 14A  is an exploded perspective view of the intermediate bracket and a lock device and  FIG. 14B  is an exploded perspective view of the supporting mechanism part. 
         FIG. 15A  is a bottom perspective view showing a mounted state of the lock device,  FIG. 15B  is a perspective view of the lock device and  FIG. 15C  is a partially cutaway perspective view showing a retaining structure of the lock device. 
         FIG. 16A  and  FIG. 16B  are exploded perspective views of a resilience adjustment unit according to a second embodiment. 
         FIG. 17A  is a perspective view of the resilience adjustment unit according to the second embodiment and  FIG. 17B  is an exploded side view showing a relationship between the resilience adjustment member and a base. 
         FIGS. 18A to 18C  are schematic views showing third to fifth embodiments. 
         FIG. 19  is an exploded perspective view of a seat. 
         FIG. 20A  is a plan view of essential parts and  FIG. 20B  is a partial sectional perspective view as seen from the side of  FIG. 20A . 
         FIG. 21A  is a sectional view taken along a line IA-IA shown in  FIG. 20A ,  FIG. 21B  is a sectional perspective view taken along a line B-B shown in  FIG. 20A ,  FIG. 21C  is a sectional perspective view taken along a line C-C shown in  FIG. 20A  and  FIG. 21D  is a perspective view of an outer shell  9 , a center engaging piece  132  and a center mount part  133  shown in  FIG. 21A . 
         FIG. 22A  is a sectional view taken along a line IIA-IIA shown in  FIG. 20A  and  FIG. 22B  is a sectional view taken along a line IIB-IIB shown in  FIG. 20A . 
         FIG. 23A  is a sectional perspective view taken along a line A′-A′ shown in  FIG. 20A ,  FIG. 23B  is a sectional perspective view taken along a line B′-B′ shown in  FIG. 20A  and  FIG. 23C  is a sectional perspective view taken along a line C′-C′ shown in  FIG. 20A . 
         FIG. 24A  is a perspective view of a portion to which a seat adjustment operating lever is mounted, as seen from the above, and  FIG. 24B  is an exploded perspective view of the seat adjustment operating lever and a seat outer shell. 
         FIG. 25A  is an exploded perspective view of the seat adjustment operating lever and a slide outer shell,  FIG. 25B  is an exploded perspective view of the seat adjustment operating lever and  FIG. 25C  is a perspective view of a portion to which the seat adjustment operating lever is mounted, as seen from the above. 
         FIG. 26A  and  FIG. 26B  are exploded perspective views of a backrest and a second back frame. 
         FIG. 27A  and  FIG. 27B  are exploded perspective views for explaining an initial angle adjustment device. 
         FIG. 28A  is an exploded perspective view of an operation tool and the backrest,  FIG. 28B  is a perspective view of the initial angle adjustment device and  FIG. 28C  is a perspective view of a lower end portion of a back inner shell. 
         FIG. 29  is a sectional view taken along a line XXIX-XXIX shown in  FIG. 1A . 
         FIG. 30A  is a longitudinal sectional side view of essential parts and  FIG. 30B  is a sectional view taken along a line B-B shown in  FIG. 30A . 
         FIG. 31A  is a sectional perspective view showing the initial angle adjustment device and  FIG. 31B  is a sectional view taken along a line XXXIB-XXXIB shown in  FIG. 29 . 
         FIG. 32  is an exploded perspective view of other backrest initial angle adjustment device. 
         FIG. 33A  and  FIG. 33B  are exploded perspective views. 
         FIG. 34A  is a perspective view showing a fitting state of a peripheral surface cam and  FIG. 34B  is an exploded perspective view of the operation tool and the peripheral surface cam. 
         FIG. 35A  is a longitudinal sectional front view of essential parts and  FIG. 35B  is a sectional view taken along a line B-B shown in  FIG. 35A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, an illustrative embodiment of the present invention will be described with reference to the drawings. First, a first embodiment shown in  FIG. 1  to  FIG. 14  is described. While phrases of “front and rear” and “left and right” are used for specifying the direction in the following description, the phrases of the front and rear and the left and right are referenced to a seated person. The direction as seen from the front is a direction facing the seated person and therefore the left and right as seen from the front is contrary to the left and right as seen from the seated person. 
     (1). Outline of Chair 
     First, an outline of a chair will be described mainly with reference to  FIG. 1  to  FIG. 5 . The present embodiment is applied to a swivel chair that is widely used in an office or the like. As shown in  FIG. 1 , the chair includes a leg device of which only a leg strut  1  is shown, a base  2  fixed to an upper end of the leg strut  1 , a seat  3  disposed on the base  2  and a backrest  4  against which the seated person can lean. For example, as shown in  FIG. 2  and  FIG. 3 , an intermediate bracket (seat-mount bracket)  5  made of metal plate is disposed on the base  2  and a resin seat outer shell  6  is mounted to the intermediate bracket  2 . 
     As shown in  FIG. 2 , the seat  3  includes a resin seat inner shell (seat plate)  7  and a seat cushion material  8  arranged to overlap with an upper surface of the seat inner shell. The seat cushion material  8  is covered with a skin material such as a cloth from the above. In the present embodiment, the seat outer shell  6  includes a fixed outer shell  9  fixed to the intermediate bracket  5  and a slide outer shell  10  protruded forward from the fixed outer shell. The slide outer shell  10  is mounted to the fixed outer shell  9  so as to be slidable in a longitudinal direction. 
     Further, as shown in  FIG. 2 , a certain range on the front side of the seat inner shell  7  is configured as a deformation allowing part  7   c  that can be easily bent and deformed downward, as seen from the side. A front end portion of the deformation allowing part  7   c  is connected to a front end portion of the slide outer shell  10 . Therefore, upon sliding the slide outer shell  10  in a longitudinal direction, the deformation allowing part  7   c  of the seat inner shell  7  is stretched forward or wound downward. As a result, it is possible to adjust the length of the seat  3  in a longitudinal direction. It should be noted that the seat outer shell  6  can be considered as a part of the seat and a seat part is configured by the seat  3  and the seat outer shell  6 . The deformation allowing part  7   c  is formed with a plurality of slits that is oblong in left and right directions. Details of a structure of the seat  3  will be described later. 
     As shown in  FIG. 1  and  FIG. 2 , the backrest  4  includes a resin back inner shell (back plate)  12  and a cushion material  13  arranged to overlap with an entire surface of the back inner shell. The cushion material  13  and the back inner shell  12  are completely covered by a bag-like skin material. The backrest  4  forms a lumbar support part that comes into contact with a lumbar spine of a seated person. In other words, as seen in the longitudinal sectional side view, the backrest  4  has a form curved in a forward convex shape so that the part to come into contact with the lumbar spine of the seated person is positioned at the foremost place. It goes without saying that the backrest  4  and the seat  3  can take an arbitrary form/structure. 
     As shown in  FIG. 1C  and  FIG. 2 , a first back frame  14  is connected to the base  2  so as to be tiltable rearward. A second back frame  15  is positioned behind the first back frame  14  and fixed to the first back frame  14 . The backrest  4  is mounted to the second back frame  15 . The first back frame  14  is made of a resin or aluminum die-cast. As shown in  FIG. 3  or  FIG. 5 , the first back frame  14  includes a base part  14   a  that extends laterally in the rear of the base  2  and arm parts  14   b  that extend forward from both left and right sides of the base part  14   a  at the outside positions of the base  2 . Front end portions of the left and right arm parts  14   b  are connected to the base  2  by a first shaft  16  that is oblong in left and right directions. Accordingly, the backrest  4  tilts about an axis of the first shaft  16 . It goes without saying that the base  2  is provided with a bearing hole  17  into which the first shaft  16  is fitted (see  FIG. 3  and  FIG. 5 ). 
     As shown in  FIG. 3  and  FIG. 4 , front portions of the left and right arm parts  14  of the first back frame  14  are configured as crank parts  14   c  that are positioned laterally inward therefrom. The first shaft  16  extends through base end portions of the crank parts  14   c . Further, front ends of the left and right crank parts  14   c  are integrally connected to each other by a pushing shaft  18  that is oblong in left and right directions. The pushing shaft  18  is covered by a lower cover  19  (see  FIG. 5 ) from the below. At left and right side plates of the lower cover  19 , elongate holes  20  that allow the turning of the pushing shaft  18  are opened upward. 
     As shown in  FIG. 2B , a rear portion of the first back frame  14  and a front portion of the second back frame  15  are overlapped with each other from the above and below and fixed to each other by a screw  21 . The second back frame  15  is made of a resin or aluminum die-cast and includes a base part  15   a  that extends laterally and two left and right back struts  15   b  that have a square shape and are provided in a rear end of the base part  15   a . The backrest  4  is connected to head parts  15   c  of the back struts  15   b  in such a way that the backrest is pivotable back and forth around the height position of the lumbar support part. Pivoting postures of the backrest  4  can be changed in a plurality of steps by an initial angle adjustment mechanism, which will be described later. 
     The back frames  14 ,  14  and the backrest  4  tilt rearward about the first shaft  16 . Thus, as shown in  FIG. 2B ,  FIG. 3  and  FIG. 5 , a resilience adjustment unit  23  is provided in the inside of the base  2  and imparts resistance to the rearward tilting of the first back frame  14 . 
     The chair of the present embodiment is a synchronous type chair in which the seat  3  is tilted rearward while retreating in conjunction with the rearward tilting of the backrest  4 . Thus, as can be inferred from  FIG. 3 , a front portion of the intermediate bracket  5  is connected to a front portion of the resilience adjustment unit  23  (or a front portion of the base  2 ) so as to be movable rearward and a rear portion of the intermediate bracket  5  is connected to a bracket part  24  by a second shaft  25  that is oblong in left and right directions. The bracket part  24  is projected upward at the first back frame  14 . Further, in the present embodiment, a locking gas cylinder  26  is provided as a lock device for holding the backrest  4  at any rearward tilting angle. The locking gas cylinder  26  is arranged over the resilience adjustment unit  23  in a posture of extending in a longitudinal direction. 
     (2). Base/Seat Outer Shell 
     Hereinafter, details of each part will be described with reference to  FIG. 6  and later, in addition to the previous figures. First, the base  2  and a relationship between the base  2  and the seat outer shell  6  will be described. For example, as shown in  FIG. 3  and  FIG. 5 , the base  2  is a box-like form that opens upward and a depth thereof becomes shallower toward the front. An outward flange  29  is formed over an entire periphery of an upper end edge of the base  2 . 
     For example, as can be seen from  FIG. 3 , a bottom of the rear half of the base  2  is uplifted to have a higher portion. A grooved base bracket  30  having a bottom plate and left and right side plates  30   a  is fixed to the higher portion by welding. A bushing  31  that opens vertically is welded to the bottom plate of the base bracket  30  and the bottom plate of the base  2 . An upper end of the leg strut (gas cylinder)  1  is fitted into the bushing  31  from the below. Further, a third shaft  32  that is oblong in left and right directions extends through the left and right side plates  30   a  of the base bracket  30 . The locking gas cylinder  26  is supported by the third shaft  32  in such a way that the locking gas cylinder  26  cannot move forward. The third shaft  32  is mounted to the left and right side plates  30   a  of the base bracket  30 . 
     For example, as shown in  FIG. 3  and  FIG. 6B , the intermediate bracket  5  has a shape close to a generally rectangular shape, as seen in a plan view and includes an upper plate  5   a  and left and right side plates  5   b . The second shaft  25  extends through the side plates  5   b . As shown in  FIG. 2 , the side plates  5   b  of the intermediate bracket  5  are located inside of the bracket part  24  of the first back frame  14 . 
     As shown in  FIG. 6B , the fixed outer shell  9  is formed with a recessed part  33  that is completely fitted into the intermediate bracket  5 . While a pair of left and right forward stoppers  34  is provided on a front end of an upper surface of the intermediate bracket  5 , tunnel-like receiving parts  35  are integrally formed on a front end portion of the recessed part  33  of the fixed outer shell  9 . The forward stoppers  34  are fitted into the receiving parts  35  from the rear. 
     Further, while square-like lock holes  36  that are oblong in left and right directions are provided on a rear end portion of the upper plate  5   a  of the intermediate bracket  5 , lock claws  37  are projected downward at a rear end portion of the recessed part  33  of the fixed outer shell  9  and fitted into the lock holes  36 . The lock claws  37  are fitted into the lock holes  36  after being elastically deformed. In this way, the fixed outer shell  9  is mounted to the intermediate bracket  5  in such a way that the fixed outer shell  9  cannot be separated. 
     For example, as shown in  FIG. 5 , the resilience adjustment unit  23  includes a pair of left and right support brackets  38  that are fitted into a front portion of the base  2 . The support brackets  38  are made of plate material and disposed inside of an inner surface of the base  2 . At a front portion of the support bracket  38 , wing parts  39  are projected transversely and overlapped with the outward flange  29  of the base  2  from the above. The outward flange  29  of the base  2  is formed with protruding portions  29   a  that are overlapped with the wing parts  39  of the support brackets  38 . The protruding portions  29   a  are provided with stopper pieces  40  that protrude upward. The stopper pieces  40  hold the wing part  39  in such a way that the wing part is not shifted laterally. 
     Resin slider mounts  41  are fitted into the protruding portions  29   a  of the base  2  and the wing parts  39  of the support bracket  38  from the left and right outside. As shown in  FIG. 8B , the slider mount  41 , the wing part  39  and the protruding portion  29   a  are fastened together by a screw  42 . For example, as shown in  FIG. 3 , the intermediate bracket  5  is provided with transverse protruding portions  43  that are overlapped with the slider mounts  41  from the above. As shown in  FIG. 8C , a resin slider  44  is mounted to a lower surface of an outer end portion of the transverse protruding portion  43  and brought into contact with the slider mount  41  from the above (see also  FIG. 4A ). 
     As shown in  FIG. 8C , an upper surface of an outer end portion  41   a  of the slider mount  41  that supports the slider  44  has a form curved in an upward convex shape, as seen from the side. Therefore, the intermediate bracket  5  (the seat  3 ) is smoothly moved during rocking and retreats while being tilted rearward. As shown in  FIG. 4A , the transverse protruding portions  43  of the intermediate bracket  5  are provided with wall portions  43   a  that protrude downward. The wall portion  43   a  surround the slider mount  41  from the left and right outside and from the rear. 
     Meanwhile, as shown in  FIG. 8A , an upper region of the outer end portion of the slider mount  41  is configured as protruding portions  41   a  that protrude outward in the left and right directions. As shown in  FIG. 4A , stopper pieces  45  (see also  FIG. 15A ) are bent at the wall portions  43   a  of the intermediate bracket  5  and positioned below the protruding portions  41   a  of the slider mount  41 . Accordingly, a front portion of the intermediate bracket  5  is retained so as not to be movable upward. Therefore, the intermediate bracket  5  is not separated from the base  2  even when the front portion of the seat  3  is lifted upward. While the separation preventing function of the intermediate bracket  5  and the base  2  is also performed by other members, this is not associated with the present invention and therefore a description thereof is omitted. 
     (3). Resilience Adjustment Mechanism 
     Next, a resilience adjustment mechanism will be described with a focus on the resilience adjustment unit  23 . For example, as shown in  FIG. 5 , the resilience adjustment unit  23  includes a pair of left and right support brackets  38  described above, a spring unit  50  disposed between the left and right support brackets  38 , an operation shaft  51  that is rotatably mounted to the left and right support brackets  38  and posture holding members  52  that are mounted to both left and right sides of the spring unit  50 . The posture holding member  52  has a substantially L shape, as seen from the side. The posture holding member  52  is an example of a separation preventing portion described in claims. 
     As shown in  FIG. 12 , the spring unit  50  includes a cylindrical first spring mount  53  that has a substantially square shape and opens rearward, a compression coil spring  54  that is disposed inside of the first spring mount  53  and a second spring mount  55  that is slidably fitted into the first spring mount  53 . A supporting member described in claim is configured by the first spring mount  53  and the second spring mount  55 . Since the supporting member of the present embodiment is configured in a hollow case structure, it can be also said that a spring case is configured by both spring mounts  53 ,  54 . Naturally, the spring holder configured by both spring mounts  53 ,  55  is extendable. 
     Since the second spring mount  55  has a substantially square shape, an interior of the first spring mount  53  also has a shape close to a substantially square shape. Further, while guide ridges  56  are provided on left and right sides of the second spring mount  55 , guide grooves  57  are formed on an inner surface of the first spring mount  53 . The guide ridges  56  are fitted into the guide grooves  57 . On the basis of the expansion and contraction direction, the first spring mount  53  is not moved and the second spring mount  55  is moved. Accordingly, it is also possible that the first spring mount  53  is referred to as a fixed spring mount and the second spring mount  55  is referred to as a movable spring mount. 
     As an example of the separation preventing portion, a pin  58  that is oblong in left and right directions penetrates the first spring mount  53  and the second spring mount  55 . By forming a pin insertion hole  59  of the first spring mount  53  as an elongated hole that is longitudinally long, the longitudinal sliding of the second spring mount  55  (expansion and contraction of the spring unit  50 ) is allowed. A front end portion of the first spring mount  53  is provided with a support shaft  60  that protrudes outward in the left and right directions. The support shaft  60  is fitted into a hole  61  provided in the support bracket  38  via a bushing. Accordingly, in the present embodiment, the support shaft  60  is configured as a connection part described in claim. Since the support bracket  38  is fixed to the base  2 , the spring unit  50  is vertically pivoted about the support shaft  60 . 
     Further, for example, as shown in  FIG. 9  and  FIG. 12A , a pusher  62  is mounted to the pushing shaft  18  that is provided on a front end of the first back frame  14 . The pusher  62  is adapted to push the second spring mount  55 . Accordingly, in the present embodiment, a leading end of the second spring mount is configured as a load receiving part described in claim. The pusher  62  is an example of a pushing part described in claim. While a rear end portion of the second spring mount  55  has a mountain shape that is convex rearward, as seen from the side, a front surface of the pusher  62  is formed as a circular arc surface  62   a  that has a radius of curvature about the support shaft  60 , as seen from the side. Further, a leading end of the second spring mount  55  has a mountain shape (wedge shape), as seen from the side, so that the leading end comes into contact with the pusher  62 . 
     As can be easily appreciated from  FIG. 9B , a positioning member  63  having a front plate  63   a , a bottom plate  63   b  and left and right side plates  63   c  is fixed to the pushing shaft  18 . The pusher  62  is formed with a recessed part  64  that is fitted into the positioning member  63  from the above. Therefore, the pusher  62  is held so as not to be laterally shifted and rotated. A longitudinal groove is formed on the front surface of the pusher  62  in order to prevent one-side hitting with the second spring mount  55 . Lining material having excellent wear resistance may be mounted to the front surface of the pusher  62 . 
     Further, as shown in  FIG. 9B , while an engaging hole  65  is provided in the bottom plate  63   b  of the positioning member  63 , an engaging claw  66  is formed in the pusher  62 . The engaging claw  66  is fitted into the engaging hole  65 . As the engaging claw  66  is caught by the engaging hole  65 , the positioning member  63  is held so as not to be detached from the pushing shaft  18 . At a rear surface of the pusher  62 , an auxiliary groove  62   b  that opens rearward is formed over an entire lateral length thereof. Upon mounting the first back frame  14 , the auxiliary groove  62   b  is intended to temporarily hold the first back frame  14  by being fitted into a forward support piece  2   a  formed in the base  2 . The positioning member  63  may be formed integrally with the pushing shaft  18  or the pushing shaft  18  may be produced by a molding (die-cast or casting) and then a pushing part may be provided integrally with the pushing shaft. 
     For example, as shown in  FIG. 13 , at a region of the first spring mount  53  which is located behind the support shaft  60 , a pair of left and right guide shafts  67  is projected outward in the left and right directions. On the other hand, circular arc-shaped guide holes  68  are formed at the support brackets  38 . The guide shafts  67  are fitted into the guide holes  68  so as to be movable. In this way, a pivoting stroke of the spring unit  50  is restricted. 
     For example, as can be seen from  FIG. 12 , a pair of left and right peripheral surface cams  70  is fitted into the operation shaft  51 . On the other hand, cam mount parts  71  are projected at both left and right sides of the first spring mount  53  configuring the spring unit  50 . An outer peripheral surface of the peripheral surface cam  70  comes into contact with the cam mount part  71 . As clearly shown in  FIG. 10 , in the present embodiment, first to fifth cam surfaces  70   a  to  70   e  are formed at the peripheral surface cam  70  in the order where distances e 1  to e 5  from an axis of rotation are short. Therefore, as the peripheral surface cam  70  is rotated by the operation shaft  51 , a posture of the spring unit  50  changes among five postures and a spacing (span) from the first shaft  16  to the second spring mount  55  changes. As a result, it is possible to adjust the magnitude of resistance against rocking in five steps. 
     The left and right peripheral surface cams  70  are connected to each other via a cylindrical part and formed integrally with one cam member  73 . The operation shaft  51  and the peripheral surface cams  70  are adapted to rotate integrally by inserting the square-like operation shaft  51  into the cam member  73 . For example, as shown in  FIG. 12  or  FIG. 14 , the operation shaft  51  is rotatably supported by the left and right support brackets  38 . Further, one end of the operation shaft  51  protrudes outward of the base  2  and a knob  74  is mounted to the one end. Further, a retaining clip  75  is mounted to the other end of the operation shaft  51 . For example, as shown in  FIG. 12A , the base  2  is provided with recessed parts  76  into which the operation shaft  51  is fitted. Therefore, it is possible to decrease the height of the operation shaft  51  as low as possible. 
     For example, as shown in  FIG. 13 , a posture holding cam part  77  is formed integrally with the inside of the peripheral surface cam  70 . Cam surfaces  77   a  to  77   e  are formed at an outer periphery of the posture holding cam part  77  in such a way that distances from an axis have a relationship contrary to the cam surfaces  70   a  to  70   e  of the peripheral surface cam  70 . The posture holding peripheral surface cam part  77  is set to a size slightly smaller than the peripheral surface cam  70 . 
     On the other hand, the posture holding member  52  is made of metal plate and is pivotably fitted into the support shaft  60  of the first spring mount  53 . The posture holding member  52  includes an upper contact portion  52   a  that comes into contact with the posture holding cam part  77  obliquely from the above and a lower support portion  52   b  that extends rearward so as to be located below the cam mount part  71  of the posture holding member  52 . A support piece  52   c  is projected outward at a lower end of the lower support portion  52   b  and three rubbers  78  are supported by the support piece  52   c . The rubber  78  is an example of an elastic part described in claim and holes  79  for positioning the rubbers  78  is formed in the cam mount part  71 . Here, the number of the rubber  78  may be one or more. Instead of the rubber  78 , a coil spring may be used. Alternatively, an elastic part may be integrally provided as a part of the posture holding member  52 . 
     The peripheral surface cam  70  and the cam mount part  71  are in a state of being sandwiched by the posture holding member  52  from the above and below. Therefore, the spring unit  50  and the peripheral surface cam  70  are retained in such a way that the spring unit  50  and the peripheral surface cam  70  cannot be separated from each other. Accordingly, the spring unit  50  is pivoted, irrespective of the rotation direction of the peripheral surface cam  70 . 
     Although the peripheral surface cams  70  are shown in  FIG. 13B , a spacing E 1  from an axis of the operation shaft  51  to an upper surface of the cam mount part  71  changes and a spacing E 2  from an axis of the operation shaft  51  to the posture holding peripheral surface cam part  77  also changes when the peripheral surface cams  70  are rotated. Further, the shape of the posture holding peripheral surface cam part  77  is set so that the dimension (E 1 +E 2 ) is substantially constant even when the operation shaft  51  is rotated in any manner. As a result, a spacing dimension E 3  between a lower surface of the cam mount part  71  and the support piece  52   c  of the posture holding member  52  is held substantially constant. In other words, the posture holding member  52  is also rotated in the pivot direction of the spring unit  50  when the spring unit  50  is pivoted by the rotation of the operation shaft  51 . Therefore, the dimension E 3  is held substantially constant, irrespective of the posture of the spring unit  50 . 
     Then, in a state where any one of the cam surfaces  70   a  to  70   e  of the peripheral surface cam  70  is in contact with the cam mount part  71 , the rubber  78  is slightly compressed or not compressed at all. As the peripheral surface cam  70  is rotated, there occurs a phenomenon that the cam mount part  71  is pushed to compress and deform the rubber  78  and then the cam mount part  71  is pivoted to return by an elastic restoring force of the rubber  78  when a corner portion that is an intersection between adjacent cam surfaces passes over the cam mount part  71 . In this way, a user can recognize by feel the fact that the cam surface in contact with the cam mount part  71  is switched and thus the magnitude of the resistance to the rocking changes. In other words, a user&#39;s hand can feel a click feeling owing to the changes in the rotation resistance when elasticity changes. 
     Then, upon rotation of the peripheral surface cam  70 , the cam mount part  71  is pushed downward and therefore a spacing dimension between a lower surface of the cam mount part  71  and the support piece  52   c  of the posture holding member  52  is reduced to E 4 . However, since (E 1 +E 2 ) is substantially the same at each stage, E 4  is held substantially the same at any stage. Therefore, an amount of compressive deformation of the rubber  78  is substantially constant even in switching the elasticity to any stage. Accordingly, rotation resistance (or click feeling) is held substantially constant when the operation shaft  51  is rotationally operated. 
     (4). Lock Device 
     Next, a lock device for controlling the rocking of the backrest  4  will be described mainly with reference to  FIG. 14  and  FIG. 15 . As described above, the lock device includes the locking gas cylinder  26 . The locking gas cylinder  26  is commercially available and includes a cylindrical body  26   a  and a rod  26   b  that is slidably fitted into the cylindrical body. In the present embodiment, the rod  26   b  protrudes rearward and a support member  80  made of resin or the like is mounted to a leading end (rear end) of the rod  26   b . The support member  80  is fitted into the third shaft  32 . 
     In the support member  80 , a mounting groove  81  into which the third shaft  32  is fitted is formed in a state of being opened substantially upward. Accordingly, as shown in  FIG. 15B , the locking gas cylinder  26  can be mounted to the third shaft  32  in one-touch type by causing the mounting groove  81  to have a posture of being opened substantially downward, fitting the support member  80  to the third shaft  32  and then changing the posture of the cylindrical body  26   a  to face forward. The mounting of the locking gas cylinder  26  is performed in a state where the intermediate bracket  5  is mounted to the base  2 . For this reason, the base  2  is provided with a hole  82  into which the locking gas cylinder  26  can be fitted from the above. 
     As shown in  FIG. 9A , in a state where the locking gas cylinder  26  is set to a predetermined posture, the support member  80  is held by the base bracket  30  in such a way that the support member  80  cannot be dropped down. Accordingly, the locking gas cylinder  26  is inseparably held. As shown in  FIG. 15B , a lever piece  84  for operating a push valve  83  of the locking gas cylinder  26  is fitted into the support member  80  from the below. A support shaft  85  is formed at a lower end of the lever piece  84 . The support shaft  85  is fitted into a bearing groove  86  that is provided in the support member  80 . As shown in  FIG. 9A , one end of a cable conduit  87  is fixed to a front upper end portion of the support member  80  and a sphere  89  fixed to one end of a wire  88  inserted through the cable conduit  87  is latched to an upper end of the lever piece  84 . As shown in  FIG. 16B , the lever piece  84  is formed with an engaging groove  90  into which the sphere  89  is fitted. The sphere  89  can be moved upward along the engaging groove  90 . 
     The other end of the cable conduit  87  is connected to a left portion or right portion of the fixed outer shell  9  and the other end of the wire  88  is connected to a manual operation lever (not shown). The operation lever is adapted to be selectively retained in one of a lock posture and a free posture. In  FIG. 9A , the operation lever is in the lock posture. In this state, the backrest  4  is held so as not to be tiltable. When the operation lever is pivoted to the free posture from the state shown in  FIG. 9 , the lever piece  84  is pivoted so that an upper end thereof moves forward. Thereby, the push valve  83  is pushed and therefore the locking gas cylinder  26  is in an extendable free state. Accordingly, the backrest  4  can be tilted. 
     A fourth shaft  91  that is oblong in left and right directions is mounted to a front end portion of the locking gas cylinder  26 . The fourth shaft  91  is fitted and held to a pin receiving member  92  from the above. The pin receiving member  92  includes left and right bottom plates, left and right side plates  92   a  and flap pieces  92   b . In other words, the flap pieces  92   b  of the pin receiving member  92  protrude outward from an upper end of the side plates  92   a . From the above, the fourth shaft  91  is fitted into pin receiving grooves  93  which are cut and formed in the flap pieces  92   b  and the side plates  92   a . The flap pieces  92   b  of the pin receiving member  92  are fixed to a lower surface of the intermediate bracket  5  by a screw  94 . 
     The fourth shaft  91  is held by a stopper  95  in such a way that the fourth shaft  91  cannot be shifted upward and shifted laterally. The stopper  95  is inserted and mounted to the intermediate bracket  5  from the front. The stopper  95  is made of resin and includes left and right foot members  95   a  extending rearward from the front side plate. As shown in  FIG. 15A  and  FIG. 15B , the left and right foot members  95   a  have L-shapes (as seen in the rear view) so as to surround left and right end portions of the fourth shaft  91  from the outside and above. The fourth shaft  91  is held by the left and right foot members  95   a  in such a way that the fourth shaft  91  cannot be shifted upward and shifted laterally. 
     A front plate  5   c  is formed at the front end of the intermediate bracket  5 . The front plate  5   c  is provided with mounting holes  97  through which the foot members  95   a  of the stopper  95  extend. The foot members  95   a  are overlapped with a lower surface of the intermediate bracket  5 . Therefore, bending deformation does not occur even when an upward external force is applied to the fourth shaft  91 . As can be clearly understood from  FIG. 15C , two left and right engaging claws  98  are projected rearward at a lower end of the front plate of the stopper  95 . A leading end (rear end) of the engaging claw  98  has an upward hook shape. The engaging claw  98  is hooked to the front plate  5   c  of the intermediate bracket  5  from the below. It goes without saying that the engaging claw  98  is deformed against elasticity thereof and then hooked to the front plate  5   c  of the intermediate bracket  5 . Therefore, the stopper  95  is inseparably held. 
     Meanwhile, a technique using a gas cylinder as a control portion of a rocking posture is widely used conventionally (for example, Japanese Utility Model Registration No. 2555498). The gas cylinder includes a cylindrical body and a piston rod (plunger) that is fitted into the cylindrical body. Locking is released by pushing and operating a push valve protruding at one end of the piston rod. Generally, while a base end of the cylindrical body is pivotably connected to a base or the like by a pin, a leading end of the piston rod is fixed to a backrest side or the like by a nut. However, an operation such as insertion of the pin or fastening by the nut is troublesome. 
     The lock device of the present embodiment is intended to solve the above problem and the locking gas cylinder  26  can be simply mounted in one-touch operation by fitting the support member  80  to the third shaft  32  and then fitting the fourth shaft  91  to the pin receiving groove  93  of the flap piece  92   b  and the side plate  92   a.    
     (5). Summary 
     Upon rocking, the pusher  62  moves forward and the second spring mount  55  is pushed so that resistance to the rocking is imparted. Further, the degree of resistance to the rocking can be switched in multiple steps (five steps) by rotationally operating the operation shaft  51  and thus rotating the peripheral surface cam  70 . It goes without saying that the switching stage of resilience adjustment is not limited to five steps but can be set to any number of steps. 
     The pusher  62  may be molded integrally with the pushing shaft  18 . However, in a case where the pusher  62  is configured separately from the pushing shaft  18 , as in the present embodiment, there is an advantage of being able to improve the function of the pusher  62  by forming the pusher  62  with the material that is different from the first back frame  14  or there is an advantage of being able to simply exchange the pusher  62  when being worn, etc. More specifically, as a preferred aspect, the first back frame  14  and the pushing shaft  18  are configured as an integral molded product made of an aluminum die-cast, for example, and the pusher  62  is made of resin (for example, nylon-based resin or polyacetal, etc.) having excellent strength and wear resistance. Further, it is also possible to eliminate the need for mounting the lining material by forming the pusher  62  with the material having excellent strength and wear resistance. 
     In the present embodiment, as can be appreciated from  FIG. 10A , the third shaft  32  that is a support shaft of the locking gas cylinder  26  is disposed at a height position between the first shaft  16  and the second shaft  25  and the height of the fourth shaft  91  does not change significantly even in the case of rocking. Therefore, line of action of the load acting on the locking gas cylinder  26  due to the rocking is in a posture close to an axis of the locking gas cylinder  26 . Therefore, it is possible to fully exhibit the ability (elastic restoring force) of the locking gas cylinder  26  as resistance to the rearward tiling of the backrest  4 . Further, it is also possible to improve durability of the locking gas cylinder  26 . 
     In the present embodiment, since the spring unit  50  or the support bracket  38  and the operation shaft  51  having the cam member  73  are configured as the resilience adjustment unit  23  of one mass, there is an advantage that it is possible to suppress the efforts of assembly or storage and it is possible to contribute to accuracy up by eliminating unevenness of assembly errors. 
     The assembly of the support mechanism part is performed in the following procedures. Specifically, the assembly is performed in the order of a step of fitting and mounting the pusher  62  to the pushing shaft  18  in advance and setting the first back frame  14  in the base  2 , a step of setting the resilience adjustment unit  23  in the base  2 , a step of mounting and fixing the slider  44 , a step of inserting the front portion of the intermediate bracket  5  to the slider  44  from the rear, a step of connecting the intermediate bracket  5  and the first back frame  14  to each other by the second shaft  25 , a step of setting the locking gas cylinder  26  and a step of setting the stopper  95  to the intermediate bracket  5 . The pin receiving member  92  is fixed to the intermediate bracket  5 , in advance. Since the number of the screw fastening sites is small in the present embodiment, there is an advantage that it is possible to perform the assembly of the chair more efficiently and more accurately. 
     (6). Second Embodiment 
     Next, a second embodiment shown in  FIG. 16  and  FIG. 17  is described. The present embodiment is a modification of the resilience adjustment unit  23  and is mainly different from the first embodiment in that a configuration of a posture holding portion for constantly holding the peripheral surface cam  70  and the spring unit  50  in an overlapped state is different from each other. 
     In the second embodiment, the posture holding members  52  are made of resin. The left and right posture holding members  52  are connected to each other by a joint  100 . A spring part  101  as an example of an elastic part described in claim is provided integrally with the left and right posture holding members  52 . From the below, the spring part  101  comes into contact with a guide ridge  71   a  that are projected at the cam mount part  71  of the cylindrical member  51 . The spring part  101  is a thin linear form and has a mountain shape that is convex upward, as seen from the side. Accordingly, rotation of the peripheral surface cam  70  is allowed by the flexing deformation of the spring part  101 . Since the spring part  101  is molded integrally with the posture holding member  52  in the present embodiment, it is possible to contribute to the assembly operability up or cost saving by reducing the number of parts. 
     The joint  100  is intended for integrally connecting the left and right posture holding members  52  and has a mountain shape that is convex forward, as seen in a plan view. Therefore, the spacing between the left and right posture holding members  52  can be widened by deforming the joint  100  so as to extend in the left and right directions. As a result, the left and right posture holding members  52  are integrally molded and can be fitted into the support shaft  60  of the first spring mount  53 . 
     It goes without saying that the left and right posture holding members  52  may be separated from each other or may be connected to each other by a separate joint, instead of being integrally molded. The support bracket  38  is provided with a corner portion  38   a  protruding forward. The corner portion  38   a  is provided with a bearing hole  102  into which the first shaft  16  is fitted. 
     The spring unit  50  often exhibits a tendency to pivot upward when being pressed by the pusher  62 . Therefore, the support bracket  38  also exhibits a tendency that a rear portion thereof is floated. However, when the corner portion  38   a  is fitted into the first shaft  16  as in the present embodiment, it is possible to securely prevent the floating of the support bracket  38  with a simple structure. As shown in  FIG. 18B , a hook piece  38   a  is formed at a rear lower end of the support bracket  38  and fitted into an engaging hole (not shown) that is provided at the base  2 . The floating of the support bracket  38  is also prevented by the hook piece  38   a . As can be appreciated from  FIG. 17B , the pin  58  connecting the second spring mount  55  and the first spring mount  53  is held by the posture holding members  52  in such a way that the pin  58  cannot be separated laterally. 
     In the first embodiment, as a pivot stroke restricting portion of the spring unit  50 , the circular arc-shaped guide holes  68  are formed at the support brackets  38 . However, in the present embodiment, the pivot stroke is restricted by the peripheral surface cam  70 . Specifically, by referring also to  FIG. 13B , when a corner portion between the first cam surface  70   a  and the fifth cam surface  70   e  is conveniently referred to as an end corner portion, the difference between the height (dimension from an axis of rotation) of the first cam surface  70   a  and the height of the end corner portion is set to a dimension larger than the dimension E 3  and the difference between the height of the fifth cam surface  70   e  and the height of the end corner portion is set to a dimension larger than the dimension E 3 . By doing so, the end corner portion of the peripheral surface cam  70  is not able to rotate by being blocked by the cam mount part  71  even when transition from the first cam surface  70   a  to the fifth cam surface  70   e  or transition from the fifth cam surface  70   e  to the first cam surface  70   a  is performed. Therefore, the structure is simplified. Of course, a stroke restricting portion such as an elongated hole may be separately provided, similar to the first embodiment. 
     (7). Other Embodiments/Others 
     Other embodiments are schematically shown in  FIG. 18 . In a third embodiment shown in  FIG. 18A , the moment from the pushing part  103  is changed by sliding the spring unit  50  in a direction perpendicular to an axis thereof. That is, in the present embodiment, the resilience adjustment is performed by transversely sliding the whole spring unit  50  by the peripheral surface cam  70 . 
     In a fourth embodiment shown in  FIG. 18B , the spring unit  50  has a fixed position and posture whereby the spring unit can be just stretchable. Further, an intermediate pivoting member  104  and a sliding member  105  are disposed between the second spring mount  55  and the pushing part  103 . The intermediate pivoting member  104  comes into contact with the second spring mount  55  and the sliding member  105  transmits the load of the pushing part  103  to the intermediate pivoting member  104 . Moment is changed by sliding the sliding member  105  in a direction perpendicular to an axis of the spring unit  50 . Although not shown, the sliding member  105  is moved by the peripheral surface cam. The present invention may be applied to these types. In the case of this embodiment, a leaf spring, a torsion bar or the like may be used as the locking spring portion. 
     A fifth embodiment shown in  FIG. 18C  illustrates another example of a holding structure of a spring. In the present embodiment, while a cylindrical body  107  is provided in a fixed spring mount  106  which is pivotably connected to the base  2 , an inner shaft  109  is provided in a movable spring mount  108  which receives the load of a pushing part (not shown). The inner shaft  109  is slidably fitted into the cylindrical body  107 . A compression coil spring  110  is fitted into the cylindrical body  107  and the inner shaft  109  from the outside. Accordingly, the compression coil spring  110  is exposed. Further, in the present embodiment, the posture of the spring unit  50  is changed by causing the peripheral surface cam to push the movable spring mount  108 . The fixed spring mount  106  and the movable spring mount  108  are held by a retaining portion such as a bolt so as not to be separated from each other. 
     The present invention can be embodiment in various ways, in addition to the above embodiments. For example, the present invention is not limited to a movable swivel chair but can be applied to a fixed chair such as a theater chair. Components such as the base can take various forms, as necessary. The cam is not necessarily limited to the peripheral surface cam but can employ an end surface cam or grooved cam, etc. As a portion for holding the cam and the spring portion so as not to be separated from each other, a method of pulling the cam and the spring portion just by a spring may be employed. 
     As a connection part of the spring unit, a pin hole may be provided in either of the first spring mount or the second spring mount. In the spring unit, it is essential that the first spring mount and the second spring mount are stretched. However, the spring may be exposed to the outside. 
     (8). Background Art of Seat and Features of Present Example 
     Next, details of a seat and a support mechanism part therefor will be described. Meanwhile, as a technique for adjusting a longitudinal length (depth of front end) of a seat in a chair, a method of winding a front portion of the seat downward has been suggested (for example, Japanese Examined Patent Publication No. Hei 07-77567). In this prior art, the front portion of the seat is configured as a deformation allowing part, a front end of the deformation allowing part is fixed to a front bar that is oblong in left and right directions and the front bar is longitudinally moved whereby the deformation allowing part is wound or stretched. A side bar extending rearward is fixed to both left and right end portions of the front bar. The side bar is supported by a mount member so as to be slidable longitudinally. 
     As an operation method for adjusting the longitudinal length of the seat, a person grabs a front end portion of the seat by hand and pulls or pushes the grabbed portion longitudinally or the side bar is longitudinally moved by an operation tool that is provided separately. However, since a body pressure of a person is applied to the side bar in a state where a person is seated on the chair, it is difficult to adjust the longitudinal length of the seat by a person seated. Therefore, in some cases, an operation of changing the longitudinal length of the seat should be performed in a state where a seated person lifts his waist. Accordingly, there is a problem that the longitudinal length adjustment (depth adjustment) of the seat is troublesome. The present application provides a chair having improved such a situation. 
     A basic configuration of the chair disclosed herein is as follows. The chair includes a seat part and a chair. The seat part includes a seat inner shell having a cushion function and a seat outer shell that supports the seat inner shell from the below. A front portion of the seat inner shell is configured as a deformation allowing part that can be wound downward whereby the longitudinal length of the seat can be adjusted. 
     In the above basic configuration, the seat outer shell includes a fixed outer shell that configures at least a rear half of the seat outer shell and a slide outer shell that has a portion protruding to the front of the fixed outer shell. The slide outer shell is mounted to the fixed outer shell in such a way that the slide outer shell is movable longitudinally. The front portion of the seat inner shell can be wound downward by connecting the front end portion of the seat inner shell to the front end portion of the slide outer shell. 
     Furthermore, the slide outer shell has a wide spread surface so as to support the seat inner shell over a wide range, a body pressure of a seated person is mainly supported by the fixed outer shell and a downward pressing force of the seat inner shell is hardly applied to the slide outer shell. 
     In the present invention disclosed herein, the longitudinal length of the seat is changed by moving the slide outer in a longitudinal direction. However, since the fixed outer shell configures at least the rear half of the seat outer shell, most of the body pressure of a seated person can be supported by the fixed outer shell in a normal seating state where a back of a person abuts against the backrest, for example. Further, since the slide outer shell is not pressed by the seat inner shell from the above in a state where the body pressure of the seated person is mainly supported by the fixed outer shell, little or no load is applied to the slide outer shell in a normal seating state. As a result, it is possible to adjust the longitudinal length of the seat by a person seated. Therefore, the chair has excellent operability and is user-friendly. 
     Further, in the present invention, since the slide outer shell has a wide spread surface, the seat inner shell is not excessively deformed and securely supported by the outer shell even when the body pressure of the seated person is applied to the front portion of the seat. Therefore, the support strength is excellent. Further, since the seat inner shell is supported by the slide outer shell over a wide area when the seat inner shell sinks or comes into contact with the slide outer shell by the body pressure of the seated person, there is no problem that the seat inner shell is largely deformed over a local range and thus gives a push-up feeling to the thigh of the person. Accordingly, the comfortable feeling is excellent. In other words, it is possible to adjust the longitudinal length of the seat without sacrificing the comfortable feeling or strength. 
     (9). Seat Inner Shell 
     The seat inner shell  7  is a molded product made of resin such as PP. For example, as clearly shown in  FIG. 20  and  FIG. 21 , the seat inner shell  7  includes a main support part  7   a  to which the body pressure of the seated person is strongly applied, a rear support part  7   b  which is located behind the main support part  7   a  and the deformation allowing part  7   c  which is located in front of the main support part  7   a , as described above. Schematically, the main support part  7   a  occupies a range slightly smaller than a half of the longitudinal length and the rear support part  7   b  and the deformation allowing part  7   c  occupy a range slightly greater than a quarter of the longitudinal length. 
     A plurality of intermediate slits  112  is formed at the main support part  7   a  of the seat inner shell  7 . Further, the main support part  7   a  and the rear support part  7   b  are connected only at left and right sides. A releasing groove  113  that is oblong in left and right directions is formed between the main support part  7   a  and the rear support part  7   b . Therefore, the downward stretching deformation of the main support part  7   a  by the body pressure of the seated person is allowed. 
     Horizontally long front slits  114  are formed at the deformation allowing part  7   c  of the seat inner shell  7  by three rows in a lateral direction and multiple rows (multiple steps) in a longitudinal direction. With the presence of the group of these front slits  114 , the deformation allowing part  7   c  is allowed to be wound downward in a posture of being extended linearly, as seen from the side. 
     Bridge portions  115  are formed at an intermediate portion and left and right ends in a lateral direction of the deformation allowing part  7   c . The bridge portion  115  has an inverted U shape, as seen from the side. Strip-like portions that are longitudinally divided across the front slit  114  are connected to each other by the bridge portions  115 . With the presence of the inverted U-shaped bridge portions  115 , the deformation allowing part  7   c  can be largely stretched in a longitudinal direction and therefore the wound deformation can be securely performed without resistance. 
     (10). Seat Outer Shell 
     The fixed outer shell  9  and the slide outer shell  10  to configure the seat outer shell  6  are molded products made of resin such as PP. For example, as can be appreciated from  FIG. 21A , a front end of the fixed outer shell  9  is extended to a rear portion of the deformation allowing part  7   c  of the seat inner shell  7  (here, the slits are omitted in  FIG. 21A ). A plurality of ribs is formed at an upper surface of the fixed outer shell  9  in order to increase the rigidity. A through hole  116  is formed at the fixed outer shell  9  and allows the main support part  7   a  of the seat inner shell  7  to be largely sunk. 
     For example, as shown in  FIG. 6 , the slide outer shell  10  includes a base part  10   a  having a wide spread surface whose lateral width is substantially the same as the fixed outer shell  9  and arm parts  10   b  which are projected rearward from both left and right sides of the base part  10   a . The base part  10   a  is configured in such a way that a rear portion thereof is always overlapped with the fixed outer shell  9  from the above even in a state of being fully advanced. When the base part  10   a  fully retreats, almost the whole of the base  10   a  is overlapped with the fixed outer shell  9 . The arm part  10   b  is always overlapped with the fixed outer shell  9  from the above. 
     Basically, the base part  10   a  of the slide outer shell  10  is plate-shaped (may be grid-shaped). Reinforcing ribs are formed on an upper surface of the base part  10   a  so as to extend vertically and horizontally. As shown in  FIG. 19  or  FIG. 22A , etc., while a plurality of first guide protrusions  117  including a head portion is projected at front-side regions of the fixed outer shell  9 , elongated guide holes  118  that are long longitudinally are formed at the base part  10   a  of the slide outer shell  10 . The guide protrusion  117  has a T shape, as seen from the front. The first guide protrusion  117  is fitted into the elongated guide hole  118  in such a way that the first guide protrusion cannot be separated but can be moved longitudinally. A first guide portion is configured by the first guide protrusion  117  and the elongated guide hole  118 . Although the first guide protrusion  117  and the elongated guide hole  118  are formed in four-by-four at intervals in a lateral direction, the number or position thereof can be selected arbitrarily. 
     As can be appreciated from  FIG. 19 , a rear end portion of each elongated guide hole  118  is configured as a wide portion  118   a  to or from which the head portion of the first guide protrusion  117  is fitted or separated. Therefore, the fixed outer shell is inseparably held in the base part  10   a  of the slide outer shell  10  by fitting the elongated guide hole  118  to the first guide protrusion  117  from the sites of the wide portion  118   a  and then sliding the slide outer shell  10  to the rear. 
     As shown in  FIG. 22B  (see also  FIG. 6 ), while a guide groove  119  that is long in a longitudinal direction is formed at the arm part  10   b  of the slide outer shell  10 , a second guide protrusion  120  is formed integrally with the fixed outer shell  9 . The guide groove  119  has a crank shape that opens downward and upward, as seen in a front section view. The guide protrusion  120  has a substantially inverted L shape and is inseparably fitted into the guide groove  119 . The second guide protrusion  120  and the guide groove  119  configure an example of a second guide portion. While the guide groove  119  is formed with a support portion  119   a  that is projected outward in a lateral direction by laterally shifting a downward opening position and an upward opening position, an inward hooked portion  120   a  is formed at the second guide protrusion  120  and positioned over the support portion  119   a . As shown in  FIG. 25 , a rear end portion of the guide groove  119  is configured as a wide portion  119   b  for allowing the fitting of the second guide protrusion  120 . 
     As shown in  FIG. 22 , while a convex rail part  121  having a thin width is formed at a lower end of the arm part  10   b , a concave rail part  122  is formed at the fixed outer shell  9 . The convex rail part  121  is slidably fitted into the concave rail part  122 . The left and right positions of the slide outer shell  10  are restricted by fitting these rail parts  121 ,  122  to each other. The convex rail part  121  is extended to the vicinity of the front end of the fixed outer shell  9  and the concave rail part  122  is extended to the vicinity of the front end of the slide outer shell  10 . 
     (11). Relationship Between Seat Inner Shell and Seat Outer Shell 
     The seat inner shell  7  is connected to both the fixed outer shell  9  and the slide outer shell  10  of the seat outer shell  6 . This point will be described below. 
     As shown in  FIG. 19  or  FIG. 23B , rail-like side support portions (side ridges)  123  that are long longitudinally are formed at sites of the fixed outer shell  9  that are offset to both left and right side edges and located at a substantially rear haft. Support ribs  124  of the seat inner shell  7  are placed on the side support portions  123 . Further, as shown in  FIG. 23B , horizontal stepped parts  125  are formed at sites of the main support part  7   a  of the seat inner shell  7  that is located outside the support rib  124 . The arm part  10   b  of the slide outer shell  10  is disposed below the stepped part  125 . An inward stopper piece  126  is formed at a site of an inner wall of the stepped part  125 . The stopper piece  126  is surrounded, from three directions, by a through hole  127  that is opened vertically. 
     Further, a deterrence piece  128  is provided integrally with the side support portion  123  of the fixed outer shell  9  and adapted to surround the stopper piece  126  from the above. Specifically, both left and right ends of the main support part  7   a  of the seat inner shell  7  are held so as not to be movable inward. Therefore, the main support part  7   a  is deformed to sink downward by the load of the seated person. Further, since both left and right sides of the main support part  7   a  are held by the side support portion  123  so as not to sink, the arm part  10   b  of the slide outer shell  10  is not pushed downward by the main support part  7   a  of the seat inner shell  7  even when a person seats. Accordingly, it is possible to easily slide the arm part  10   b  of the slide outer shell  10  in a longitudinal direction even in a state where a person remains seated. 
     The through hole  127  protrudes rearward of the stopper piece  126 . Therefore, the deterrence piece  128  can be fitted into a rear portion of the through hole  127  by positioning the seat inner shell  7  slightly in front of a predetermined position and then overlapping the seat inner shell  7  with the fixed outer shell  9 . When the seat inner shell  7  is shifted rearward in that state, the deterrence piece  128  is positioned on the stopper piece  126 . 
     As shown in  FIG. 22C , a plurality of support pieces  129  is formed at both left and right end portions of the main support part  7   a  of the seat inner shell  7 . The support pieces  129  are projected inwardly to face the arm part  10   b  of the slide outer shell  10  and provided at intervals in a longitudinal direction. The support pieces  129  are intended for attaching a skin material thereto. 
     As shown in  FIG. 23A , while hook-like rear engaging claws  130  are provided at left and right rear portions of the fixed outer shell  9 , rear engaging holes  131  are formed at the rear support part  7   b  of the seat inner shell  7 . An upper portion of the rear engaging claw  130  is bifurcated. The rear engaging holes  131  are fitted and engaged with the rear engaging claws  130  from the above. The rear engaging holes  131  are also engaged with the rear engaging claws  130  by overlapping the seat inner shell  7  with the fixed outer shell  9  and then shifting the seat inner shell  7  from the fixed outer shell  9 . 
     As shown in  FIG. 21D , while a center engaging piece  132  is provided at an intermediate portion in a lateral direction of the rear support part  7   b  of the seat inner shell  7 , a center mount part  133  is concavely formed at the fixed outer shell  9 . A front end of the center engaging piece  132  is a free end and projected downward. The center engaging piece  132  is fitted into the center mount part  133  so as not to be movable forward. Since three directions of the center engaging piece  132  except for a direction of a base are surrounded by slits, the center engaging piece  132  can be pivoted vertically about the base. 
     As shown in  FIG. 20 , a side engaging portion  134  is formed at sites of the fixed outer shell  9  that is located outside the center mount part  133  in a lateral direction. The side engaging portion  134  has a rectangular shape, as seen in a plan view. A side engaging hole  135  is provided at the rear support part  7   b  of the seat inner shell  7  and fitted into the side engaging portion  134 . In this way, the rearward movement of the seat inner shell  7  is restricted. 
     A connection structure of the front end portion of the seat inner shell  7  is shown in  FIG. 21B . Specifically, while support shafts  137  are formed integrally, via left and right downward bracket pieces  136 , with two sites of the deformation allowing part  7   c  of the seat inner shell  7  that are located at both left and right sides across a center line, a bearing part  138  is provided integrally with the front end of the slide outer shell  10 . The support shaft  137  is fitted into the bearing part  1038  from the above. The support shaft  137  and the bearing part  138  can be rotated relative to each other. 
     Accordingly, since the front end of the deformation allowing part  7   c  is pulled rearward when the slide outer shell  10  retreats, the deformation allowing part  7   c  is wound downward in a state of being folded back. In this way, it is possible to adjust the longitudinal length (depth of the front end) by changing the front end position of the seat  3 . Arrangement positions of the pair of the support shaft  137  and the bearing part  138  are not limited to two left and right sites but the number and arrangement position thereof can be selected arbitrarily. For example, the pair can be provided at three sites of an intermediated portion and both left and right sides. Further, the support shaft  138  may be provided at the slide outer shell  10  and the bearing part  138  may be provided at the deformation allowing part of the seat inner shell  6 . Further, another connection portion may be employed. 
     (12). Depth Operating Mechanism of Seat 
     Next, an operation device for adjusting a longitudinal length of the seat  3  will be described mainly with reference to  FIG. 24  and  FIG. 25 . For example, as shown in  FIG. 25B , the operation device includes a finger contact lever  140  that is mounted to a right portion of the slide outer shell  10  so as to be movable longitudinally, a lock member  141  that slides laterally by the forward and rearward movements of the finger contact lever  140  and a spring (compression coil spring)  142  that urges the lock member  141  to a lock posture. 
     As clearly shown in  FIG. 25A  and  FIG. 25C , a lower surface of left and right side portions of the slide outer shell  10  is inclined to be lower toward the inside. A concave region  143  having a flat bottom surface is formed at the slanted lower surface. The finger contact lever  140  is disposed at the concave region  143  so as to be movable longitudinally. The finger contact lever  140  includes an upper plate  140   a  that is overlapped with the bottom surface of the concave region  143  and a grip part  140   b  that is projected downward from the upper plate  140   a . An engaging protrusion  144  is formed integrally with the upper plate  140   a . The engaging protrusion  144  is exposed on the slide outer shell  10  and has a T shape, as seen from the front. The slide outer shell  10  is formed with a mounting hole  145  that is long in a longitudinal direction. The engaging protrusion  144  is inseparably fitted into the mounting hole  145 . A wide portion  145   a  is formed at a front end of the mounting hole  145 . The engaging protrusion  144  can be fitted into or detached from the wide portion  145   a.    
     The lock member  141  has a rod shape that is long in a lateral direction. The lock member  141  is slidably fitted into a guide groove  146  that is provided at the lower surface of the slide outer shell  10 . The guide groove  146  is long in a lateral direction. The guide groove  146  is continuous integrally with the concave region  143 . Further, the guide groove  146  is formed in a state of dividing the convex rail part  121 . 
     As shown in  FIG. 25B , while an outer end of the lock member  141  that faces the finger contact lever  140  is configured as a contact portion  141   a  that has a U shape, as seen from the bottom, a concave region  147  is formed at the finger contact lever  140  and covers the contact portion  141   a  of the lock member  141  from the below. An inner surface of the concave region  147  is configured as two front and rear slant surfaces  147   a . The slant surfaces  147   a  are inclined in such a way that a spacing therebetween becomes wider. 
     A groove  168  is formed at an upper surface of the lock member  41 . The spring  142  is fitted into the groove  168 . One end of the spring  142  is in contact with an inner surface  146   a  of the guide groove  146 . Accordingly, the lock member  141  is urged outward (in a direction toward the finger contact lever  140 ). Since the concave region  147  of the finger contact lever  140  has a V shape, the lock member  141  retreats outward and the contact portion  141   a  is fitted into a deep position of the concave region  147  when an external force is not applied to the finger contact lever  140 . As a result, a stable state is held. On the other hand, upon sliding the finger contact lever  140  in one of the front and rear directions, the lock member  141  is advanced inwardly by the guide action of the slant surface  147   a.    
     A stopper protrusion  149  is provided integrally with an inner end portion of the lock member  141  and projected downward toward the fixed outer shell  9 . On the other hand, the fixed outer shell  9  is provided with a stopper mount part  150 . The stopper protrusion  149  is fitted into or detached from the stopper mount part  150  when the lock member  141  moves laterally. The stopper mount part  150  is configured by providing inward ribs at intervals to a vertical rib  151  that is long in a longitudinal direction. In the present embodiment, five stopper mount parts  150  are formed side by side in a longitudinal direction. Accordingly, the longitudinal length of the seat  3  can be adjusted in five steps. 
     For example, as shown in  FIG. 6 , a finger contact protrusion  152  is formed integrally with a site of the slide outer shell  10  that is bilaterally symmetric with the slide-type finger contact lever  140 . The finger contact protrusion  152  has the same shape as the slide-type finger contact lever  140 . Therefore, it is possible to perform longitudinal adjustment of the seat  3  smoothly without being twisted. The slide-type finger contact levers  140  may be provided at both left and right sides of the slide outer shell  10 . When slide-type finger contact levers  140  are provided at the left and right sides, the lock members  141  may be provided at the left and right sides. Alternatively, the lock member  141  may be provided only at one side and the other slide-type finger contact levers  140  may be configured as a dummy. 
     In the present embodiment, both left and right sides of the main support part  7   a  of the seat inner shell  7  are held in a state of being placed on the side support portions  123  and therefore the body pressure of the seated person is not or little applied to the arm part  10   a  of the slide outer shell  10 . Accordingly, it is possible to perform the longitudinal adjustment of the seat  3  lightly in a state where a person remains seated. The thigh of the seated person may be brought into contact with the deformation allowing part  7   c  of the seat inner shell  7  from the above. However, since the deformation allowing part  7   c  is just stretched, wound or deformed but does not move in a longitudinal direction, the contact of the thigh of the seated person with the seat inner shell  7  does not affect the longitudinal adjustment of the seat  3 . 
     Further, in the present embodiment, even though the finger contact lever  140  is moved in any direction of the longitudinal direction, the lock member  141  is detached from the stopper mount part  150  and unlocked, so that the slide outer shell  10  can be directly moved in a longitudinal direction. Accordingly, upon adjusting the depth of the seat  3 , the locking is released by moving the finger contact lever  58  in a desired moving manner of the slide outer shell  10 . As a result, the unlocking and the movement of the slide outer shell  10  can be performed in one-action. Accordingly, operability is good. 
     Since the slide outer shell  10  is overlapped with the fixed outer shell  9  from the above in the present embodiment, the slide outer shell  10  is operated to be close contact with the fixed outer shell  9  even when a body pressure is strongly applied to the front portion of the seat  3 , for example. In other words, as the body pressure is strongly applied, there is a tendency that the integration of the slide outer shell  10  and the seat outer shell  9  becomes stronger. As a result, there is no case that the slide outer shell  10  is detached from the fixed outer shell due to the body pressure of a person. Accordingly, support strength/support stability is excellent. 
     Since the arm part  10   b  of the slide outer shell  10  is disposed on the outside of the side support portion  123  in the present embodiment, it is possible to accurately prevent the arm part  10   b  from being pressed by the seat inner shell  7 . In other words, since the main support part  7   a  of the seat inner shell  7  is placed on the side support portion  123 , the portion of the seat inner shell  7  that is located outside the side support portion  123  has a tendency to float upward by the body pressure of the seated person. Thereby, the slide outer shell  10  is prevented from being pressed by the seat inner shell  7  and therefore it is possible to lightly move the slide outer shell  10  in a longitudinal direction. 
     When the convex rail part  121  provided at a lower end of the arm part  10  and the concave rail part  122  provided at the fixed outer shell  9  are fitted into each other, as in the present embodiment, this is desirable since the backlash of the slide outer shell  10  is eliminated. 
     (13). Background Art of Backrest/Features of Present Example 
     In the chair disclosed herein, a mechanism for changing an initial posture (basic posture, initial angle) of a backrest is also improved. This point will be described below. 
     A chair has been suggested in which an initial posture of a backrest in a state of being non-tilted rearward can be changed (for example, Japanese Examined Patent Publication No. S47-49543, JP-A-2002-142897, Japanese Patent Application Publication No. 2010-516433, Japanese Utility Model Publication No. S46-8447, Japanese Patent Publication No. 4185754 and Japanese Patent Publication No. 4220191). 
     Meanwhile, importance of supporting a lumbar region (in particular, around the third lumbar vertebra) of a seated person is pointed out in a chair and therefore a chair has been widely spread, which is provided with a lumbar support part protruding forward, as seen in a side sectional view. In other words, when a person sits on a chair and works in the office or the like, a person can take a proper erecting posture by supporting a waist with the lumbar support part. In this way, it is possible to reduce the burden on the body. 
     On the other hand, although an initial posture of a backrest is changed in order to match the preference of a user, it is not preferable that hitting on the body is changed due to the changes in the initial posture. Specifically, it is not preferable that the body support position is shifted in a longitudinal direction or the hitting position on the body is shifted in a vertical direction, due to the changes in the initial posture of the backrest. In a chair including the lumbar support part, it is preferable that the initial posture of the backrest can be changed in a state of accurately holding a lumbar support function. 
     However, in Japanese Examined Patent Publication No. S47-49543, JP-A-2002-142897 and JP-A-2010-516433, an initial posture of a backrest is changed by tilting the backrest about a pivot supporting point of a back frame. Accordingly, the backrest generally moves back and forth in accordance with the adjustment of the initial posture. As a result, there is a possibility that the push-up feeling occurs in the body or the body support function is decreased. 
     Further, in Japanese Patent Publication No. 4185754 and Japanese Patent Publication No. 4220191, the lumbar support part is moved back and forth. Accordingly, it is difficult to meet the requirements of changing an initial posture of a backrest without changing the lumbar support function. 
     Since the backrest disclosed in Japanese Utility Model Publication No. S46-8447 is pivoted about a vertically middle portion thereof, it can be said that fitting property to the body is high. However, since the backrest does not include a lumbar support part, there is a possibility that a lower end protrudes forward and a push-up feeling is imparted to a person when the backrest is in a posture lying rearward. Further, since the posture is changed by operating a pin provided on an upper end of a bracket, there is also a problem that it is difficult to change the posture in a state where a person remains seated and therefore operability is poor. 
     The adjustment mechanism disclosed herein is intended to improve such a situation. In the adjustment mechanism, an initial posture of a backrest can be changed in a state of properly maintaining the body support function and the operability or the like of the posture change is taken into consideration. 
     The chair disclosed herein includes a seat and a backrest that is disposed behind the seat. The backrest is mounted to a back frame extending rearwardly from below the seat. The backrest includes a lumbar support part to support a lumbar portion of a seated person from the rear. The lumbar support part is projected forward, as seen in a side sectional view. The backrest is connected to the back frame in such a way that the backrest is pivoted about a site of a height position of the lumbar support part, as seen from the side. Further, an initial angle adjustment device is provided at the back frame or the backrest and changes the posture of the backrest, as seen from the side. The initial angle adjustment device can be operated by a person seated. The phrase, “the back frame or the backrest” portion that the initial angle adjustment device may be provided at either or both of the back frame and the backrest. 
     In the invention disclosed herein, the initial posture (initial angle) of the backrest is changed by pivoting the backrest about the lumbar support part and therefore the position of the lumbar support part is not largely changed by the changing of the initial posture. As a result, there is no problem that a push-up feeling occurs in the body or the support function is decreased, due to the changing of the initial posture. In other words, it is possible to adjust the initial posture of the backrest in accordance with the preference of a user while properly securing the lumbar support function. Further, since the initial angle adjustment device can be operated by a person seated, it is possible to adjust the initial posture while testing the feeling on the body. Accordingly, the chair is user-friendly. 
     (14). Specific Configuration of Backrest 
     Next, a specific configuration of the backrest will be described mainly with reference to  FIG. 26  and later. For example, as shown in  FIG. 1 ,  FIG. 3  and  FIG. 30A , the backrest  4  includes a lumbar support part  156  with which a lumbar portion of a seated person comes into contact. For this reason, the lumbar support part  156  has a shape protruding forward, as seen in a side view and a longitudinal sectional view. To be described accurately, the back inner shell  12  is curved in such a way that the lumbar support part  156  is positioned at the foremost place, as seen in a longitudinal sectional view. Further, although the back inner shell  12  is smoothly curved in a forward concave shape, as seen in a plan view, the degree of curvature is largest in the lumbar support part  156 , is made smaller toward the upper end and is substantially flat in the upper end. 
     As shown in  FIG. 1  and  FIG. 3 , the backrest  4  is set in such a way that a lateral width thereof is largest in a site of the lumbar support part  156  and made smaller as being vertically away from the lumbar support part  156 . Accordingly, the backrest  4  (back inner shell  12 ) has a shape close to a substantially hexagonal shape, as seen from the front. Since the lumbar support part  156  is offset to a lower side of the back inner shell  12 , the backrest  4  has a hexagonal shape whose lower side is convex, accurately. Left and right ends of the lumbar support part  156  have a mountain shape that is projected laterally, as seen from the front. 
     Further, side connection parts  157  are provided at both left and right ends of the lumbar support part  156  of the back inner shell  12  and intended to connect the lumbar support part  156  to the head parts  15   c  of the back struts  15   b . The side connection parts  157  are projected forward from the surroundings thereof. 
     For example, as shown in  FIG. 26 , fist-like head parts  15   c  are formed at an upper end of the back struts  15   b  of the second back frame  15  and projected forward. Bearing parts  158  are formed integrally with the head part  15   c  and projected forward. On the other hand, boss parts  159  are formed integrally with the side connection parts  157  of the back inner shell  12  and fitted into the bearing parts  158 . By fitting the bearing parts  158  and the boss parts  159  to each other, the side connection parts  157  of the back inner shell  12  are inseparably connected to the head parts  15   c  of the back strut  15   b . The boss part  159  is connected integrally with a rib  160  that is provided on the outside of the boss part and a restriction plate  161  that is provided on the inside of the boss part. 
     As clearly shown in  FIG. 31B , while the bearing part  158  includes a necking portion, the boss part  159  has a cross-sectional shape of a crushed circle. In a state where the backrest  4  is set to a predetermined posture, the boss part  159  is set to a substantially horizontal posture and an opening direction of the bearing part  158  is set so as to be opened obliquely upward relative to the horizontal. Therefore, by causing the backrest  4  to have a posture inclined rearward relative to a predetermined posture, the boss part  159  can be fitted into the bearing part  158 . Further, upon connecting a lower end of the back inner shell  12  to the second back frame  15 , the boss part  159  is held so as not to be detached from the bearing part  158  even when a forward external force is applied to the backrest  4 . Accordingly, it is possible to simply perform the mounting of the backrest  4 . 
     As shown in  FIG. 26  and  FIG. 29 , a load receiving part  162  is formed at a site of the head part  15   c  of the back strut  15   b  that is located inside the bearing part  158 . The load receiving part  162  has a circular arc-shaped surface whose curvature is much larger than the radius of the boss part  159 . On the other hand, a circular arc-shaped load support part  163  is provided integrally with the side connection part  157  of the back inner shell  12  and fitted into the load receiving part  162 . Since the load support part  163  is brought into contact (surface contact) with the load receiving part  162  over a wide area, the back inner shell  12  is stably supported by the left and right back struts  15   b  while longitudinal pivoting thereof is not inhibited. 
     Further, a groove  164  is provided between the load support part  162  and the bearing part  158  of the head part  15   c  of the back strut  15   b . The restriction plate  161  of the back inner shell  12  is closely fitted into the groove  164  so as not to be shifted laterally. In this way, since the back inner shell  12  is held by the fitting of the groove  164  and the restriction plate  161  so as not to be shifted laterally, the side connection part  157  is not displaced inwardly even when a rearward load is applied to the lumbar support part  156  and therefore the side connection part  157  is pulled inward. Therefore, the mounting strength is high. A bottom surface of the groove  164  and an outer periphery of the restriction plate  161  are formed in a circular arc shape of curvature around an axis of the boss part  159 , as seen from the side. Accordingly, the pivoting of the back inner shell  12  around an axis of the boss part  159  is allowed. 
     As described above, in the mounting structure of an upper end of the back strut  15   b  and the back inner shell  12 , longitudinal retention, support of load applied to the back and lateral shifting prevention are respectively performed at a separate specific site. Specifically, the longitudinal retention is performed by the boss part  159  and the bearing part  158 . In order that the boss part  159  and the bearing part  158  do not have the other functions (the support of load applied to the back and the lateral shifting prevention), the boss part  159  is loosely fitted into the bearing part  158  in a state where a slight clearance is provided between an outer periphery of the boss part  159  and an inner periphery of the bearing part  158 . Further, by setting the lateral width dimension of the boss part  159  to be slightly wider than that of the bearing part  158 , the rib plate  160  does not come into contact with the side surface of the bearing part  158 . Further, a rear end surface of the restriction plate  161  is set so as not to come into contact with the bottom surface of the groove  164  of the back strut  15   b.    
     (15). Initial Angle Adjustment Device 
     A lower end portion of the back inner shell  12  is such that an intermediate portion in a lateral direction is connected to the second back frame  15  via an initial angle adjustment device  155 . This point will be described below. 
     For example, as shown in  FIG. 28A  and  FIG. 28C , a lower connection part  166  is provided at the intermediate portion in a lateral direction of the lower end portion of the back inner shell  12  and projected forward. The lower connection part  166  is formed with a center groove  167  that is projected downward. Further, the lower connection part  166  is formed with a pin hole  168  that traverses the center groove  167 . Furthermore, upper and lower ribs  169 ,  170  are provided at a right region of the lower connection part  166  of the lower end portion of the back inner shell  12  and connected to the lower connection part  166 . An operation tool (slide type lever)  171  is mounted between the upper and lower ribs  169 ,  170  so as to be slidable laterally. The lower connection part  166  and the operation tool  171  are constituent components of the initial angle adjustment device  155 . The operation tool  171  is provided with a finger grip  171   a  that can be grasped by a hand. 
     A pin-shaped locking pin  172  is formed integrally with the operation tool  171  and fitted into the pin hole  168  of the lower connection part  166 . Accordingly, as the operation tool  171  is laterally slid, the locking pin  172  can appear and disappear in the center groove  166  of the lower connection part  166 . The locking pin  172  is an example of the stopper. Claws  173  are provided at the upper rib  169  and intended to hold the operation tool  171  so as not to be detached from the back inner shell  12 . 
     Further, the operation tool  171  is provided with a spring arrangement void  174  that is opened toward the back inner shell  12  and a stroke restriction void  175 . The spring arrangement void  174  is located on the side of the locking pin  172  and the stroke restriction void  175  is located on the side of the finger grip  171 . On the other hand, the back inner shell  12  is provided with a spring mount protrusion  176  that enters the spring arrangement void  174  and a stroke restriction protrusion  177  that enters the stroke restriction void  175 . A compression coil spring  178  is disposed between the spring mount protrusion  176  and an inner surface  174   ee  of the spring arrangement void  174 . Therefore, the operation tool  171  is urged so that the locking pin  172  can be easily fitted into the lower connection part  166  (locking position is held). The spring  178  is placed in the spring arrangement void  174  by being pushed through a window hole  179  provided at a front surface of the operation tool  171 . 
     Restriction of the retreating position when the operation tool  171  is pulled outward by hand is performed by bringing an inner surface of the stroke restriction void  175  into contact with the stroke restriction protrusion  180 . Further, restriction of the advancing position when the operation tool  171  is pressed by the spring may be performed by bringing an inner surface of the spring arrangement void  174  into contact with the stroke restriction protrusion  180  or by bringing an end of the operation tool  171  into contact with a side surface of the lower connection part  166 . 
     As shown in  FIG. 28B , a rear wall  15   d  is formed at a rear end of the base part  15   a  of the second back frame  15 . A block-shaped lock body  181  is provided at the rear wall  15   d  and fitted into the center groove  167  of the lower connection part  166  of the seat inner shell  12  from the below. The lock body  181  is provided with three lock holes  182  into which the locking pin of the operation tool  171  is fitted. Accordingly, the lock body  181  is also a component of the initial angle adjustment device  155 . The three lock holes  182  are present on a circular arc around a pivot supporting point of the back inner shell  12 . The initial posture of the backrest  4  can be changed in three steps by selectively fitting the locking pin into any one of the lock holes  182 . It goes without saying that the number of the lock holes  182  is not limited to three but may be two or four or more. The lock hole  182  is an example of the lock part. 
     The lock body  181  is made of resin, separately from the second back frame  15  and fitted and mounted to the base part  15   a  of the second back frame  15  from the below. For this purpose, for example, as shown in  FIG. 27 , the base part  15   a  of the second back frame  15  is provided with a hole  183  into which the lock body  181  is fitted from the below and a center guard part  184  supporting the lock body  181  from the rear. The center guard part  184  is provided with a forward opening groove  185  into which the lock body  181  is fitted. The lock body  181  is provided with a front wall  181   a  regulating that a lower end of the back inner shell  12  largely advances forward. 
     As shown in  FIG. 30 , a concave region  188  in which the lock body  181  enters is formed at a lower surface of the second back frame  15  and a lower flange  181   b  provided at the lock body  181  is overlapped with a bottom surface of the concave region  188 . Further, support claws  189  are provided at an inner surface of the concave region  188 . As the lower flange  181   b  rides over the support claws  189  from the rear, the lock body  181  is held so as not to be dropped down. The lock body  181  may be provided integrally with the second back frame  15 . 
     Depending on the user, an initial posture adjustment function of the backrest  4  may not be required. Accordingly, for example, as shown in  FIG. 7 , the back inner shell  12  is non-pivotably held by mounting a restricting member  15   f  to a mount bracket  15   e  provided at a rear end portion of the second back frame  15  and fitting a restricting shaft (not shown) provided at a lower end of the back inner shell  12  into the restricting member  15   f . Details are omitted. 
     For example, as is apparent from  FIG. 1C , a lower end of the backrest  4  is located below a seat surface and therefore the initial angle adjustment device  155  is also located below the seat surface. For this reason, the initial angle adjustment device  155  does not come into contact with the body. Further, since the seated person can manipulate the operation tool  171  by extending his hand rearward, adjustment of the initial angle can be performed very simply. 
     (16). Other Initial Angle Adjustment Device 
     Next, another example of an initial angle adjustment device shown in  FIG. 32  to  FIG. 35  will be described. In this example, as shown in  FIG. 32 , a pair of left and right bearing ribs  190  is provided and a peripheral surface cam  191  is rotatably held by the left and right bearing ribs  190  via the operation tool  171 . On the other hand, a box-shaped lower connection part  192  surrounding the peripheral surface cam  191  is provided integrally with a lower end portion of the back inner shell  12  and projected downward. Here, when the peripheral surface cam  191  is rotated by the operation tool  171 , a support position in a longitudinal direction of the lower connection part  192  by the peripheral surface cam  191  changes and therefore the initial angle of the backrest  4  changes. Accordingly, the operation tool  171  is a rotary type. 
     As shown in  FIG. 32 , in this example, while cylindrical bearing parts  193  are formed integrally with both left and right ends of the peripheral surface cam  191 , a groove  194  that is long in a vertical direction is formed at an inner surface of the bearing rib  190 . The left and right cylindrical parts  193  provided at the peripheral surface cam  191  are fitted into the groove  194 . A square-like part  171   a  of the operation tool  171  penetrates the cylindrical part  194  of the peripheral surface cam  191 . However, as clearly shown in  FIG. 34B , while a pair of protrusions  195  is provided at a base portion of the square-like part  171  of the operation tool  171 , a pair of cutout portions  196  is formed at one of the cylindrical parts  193  of the peripheral surface cam  191 . The protrusion  195  is fitted into the cutout portion  196 . Therefore, integrity of the peripheral surface cam  191  and the operation tool  171  is increased. 
     A small-diameter portion  197  is formed at a leading end of the operation tool  171 . The small-diameter portion  197  is rotatably fitted into a bearing hole  198  that is provided at the other bearing rib  190 . While a pair of stopper pieces  199  is provided at a leading end of the small-diameter portion  197  of the operation tool  171  and projected in a direction perpendicular to an axis, a stepped part  200  is formed at an outer surface of the other bearing rib  190 . The stopper piece  199  is inseparably fitted into the stepped part  200 . The bearing hole  198  of the other bearing rib  190  is formed as an elongated hole that is long in a longitudinal direction. Accordingly, the stopper piece  179  is formed transversely and fitted into the bearing hole  198 . When the stopper piece  199  is fitted into the bearing hole  198  and then the operation tool  171  is rotated, the operation tool  171  is inseparably held. 
     Insertion of the operation tool  171  is performed after the peripheral surface cam  191  is set between the left and right bearing ribs  190 . As shown in  FIG. 34B , a projection direction  83  of the protrusion  77  is perpendicular to a projection direction  84  of the stopper piece  81 . As shown in  FIG. 19 , a rib  171   b  for displaying a posture of the backrest  5  is provided at a base end of a grip of the operation tool  171 . 
     As shown in  FIG. 34  and  FIG. 35 , fitting of the peripheral surface cam  191  is performed in order of fitting the peripheral surface cam  191  between the left and right bearing ribs  190  in a posture where an axis of rotation is inclined to the horizontal and then returning the peripheral surface cam  191  to the horizontal posture. Since the elongated groove  194  is long in a vertical direction, the left and right cylindrical parts  75  can be fitted into the left and right elongated grooves  76  by tilting the peripheral surface cam  191 , as seen from the front. As the posture of the peripheral surface cam  191  returns to a posture in which an axis of rotation is horizontal. the peripheral surface cam  191  is held so as not to be movable in a longitudinal direction. 
     For example, as clearly shown in  FIG. 33B , an elongated hole  201  for insertion of the operation tool is opened downward in the lower connection part  192  of the back inner shell  12 . 
     A leading end portion of the operation tool  171  that is located between the left and right bearing ribs  190  is configured as a rectangular column portion  171   b . The peripheral surface cam  191  is fitted into the rectangular column portion  171   b  so as not to rotate relative to each other (i.e., to rotate together with the operation tool  171 ). The peripheral surface cam  191  includes one center cam part  204  and a pair of left and right side cam parts  205  that are located at both left and right sides of the center cam part  204 . 
     Outer peripheral surfaces of the center cam part  204  and the side cam parts  205  are configured as a cam surface, a front inner surface of the lower connection part  192  is configured as a front restricting surface  192   a  with which the center cam part  204  comes into contact, a rear inner surface of the lower connection part  192  is configured as a rear restricting surface  192   b  with which the side cam parts  205  come into contact. First to third cam surfaces  204   a  to  63   c  are formed at an outer periphery of the center cam part  204  and the heights of the first to third cam surfaces from an axis are different. Further, first to third cam surfaces  205   a  to  205   c  are formed at an outer periphery of the side cam parts  205  and the heights of the first to third cam surfaces from an axis are different. The lower connection part  192  is opened downward, as described above. However, a longitudinal spacing between the front restricting surface  192   a  and the rear restricting surface  192   b  is set to be larger toward the lower. 
     Both cam parts  204 ,  205  are configured as follows. The first cam surface  205   a  of the side cam part  205  comes into contact with the rear restricting surface  192   b  when the first cam surface  204   a  of the center cam part  204  is in contact with the front restricting surface  192   a . The second cam surface  205   b  of the side cam part  205  comes into contact with the rear restricting surface  192   b  when the second cam surface  204   b  of the center cam part  204  is in contact with the front restricting surface  192   a . The third cam surface  205   c  of the side cam part  205  comes into contact with the rear restricting surface  192   b  when the third cam surface  204   c  of the center cam part  204  is in contact with the front restricting surface  192   a.    
     In the present example, the initial angle of the backrest  4  can be adjusted in three steps by rotationally operating the operation tool  171  and the backrest  4  is held so as not to be rattled in a longitudinal direction. Although resistance to the rotation of the peripheral surface cam  191  occurs, it is possible to rotate the peripheral surface cam  191  by elastically deforming the lower connection part  192 . In order to allow the lower connection part  192  to be pivoted, elongated holes  201  are provided at left and right side plates of the lower connection part  192 . The operation tool  171  is loosely fitted into the elongated hole  201 . 
     For example, as shown in  FIG. 35  (see also  FIG. 32 ), an elastic piece  202  having an opening shape is formed integrally with a front surface portion of the bearing rib  190 . A lower end of the elastic piece  202  is a free end. While an engaging hole  203  is provided at the elastic piece  202 , a protrusion  206  is respectively provided at each of the cam surfaces  204   a ,  204   b ,  204   c  of the center cam part  204  of the peripheral surface cam  191  and fitted into the engaging hole  203 . As a result, since any one of the protrusions  88  is fitted into the engaging hole  88 , a user can grasp, from the spacing, that the peripheral surface cam  191  is rotated to a predetermined state. In other words, the operation tool  171  can accurately rotate the peripheral surface cam  191  with a click feeling. 
     The initial angle adjustment device can be variously embodied. The operation tool is not limited to a slide type or a rotary type but may be a pivot type. The initial angle adjustment device can be configured in such a way that the backrest is pivoted by providing an operation tool including a push button at a lower end of the backrest and moving the operation tool in a longitudinal direction while maintaining a state where the push button is pushed down and thus the locking is released. In a case where a cam is used in the initial angle adjustment device, various cams such as an end surface cam can be employed. The initial angle of the backrest may be steplessly adjusted by providing a screwed handle in the initial angle adjustment device. A lock mechanism other than a pin or cam may be employed. 
     INDUSTRIAL APPLICABILITY 
     The present invention and each invention disclosed in the present application can be embodied in the chair. Accordingly, the present invention has an industrial applicability. 
     REFERENCE NUMERALS LIST 
     
         
         
           
               1  Leg strut (Gas cylinder) 
               2  Base 
               3  Seat 
               4  Backrest 
               5  Intermediate bracket 
               14 ,  15  Back frame 
               16  First shaft (Shaft serving as a tilting support of a backrest) 
               18  Pushing shaft 
               23  Resilience adjustment unit 
               25  Second shaft 
               26  Locking gas cylinder 
               32  Third shaft 
               38  Support bracket 
               50  Spring unit 
               51  Operation shaft 
               52  Posture holding member 
               53  Cylindrical member (Fixed spring mount) to configure a spring holder 
               54  Compression coil spring as an example of a locking spring portion 
               55  Movable spring mount to configure a spring holder 
               60  Support shaft (Connection part) 
               62  Pusher as an example of a pushing part 
               70  Peripheral surface cam as an example of a cam 
               71  Cam mount part 
               72  Cam surface 
               73  Cam member 
               78  Rubber as an example of an elastic part