Reaction force adjustment mechanism of the backrest reaction force mechanism of the chair

Abstract

To enable materialization of weight saving and down-sizing of a main frame part, for example, reaction adjustment of a chair.SOLUTION: A chair backrest reaction force mechanism 60 generates, by compression of a reaction force spring 65, a force for pushing back a backrest linked with backward tilting of the backrest. In connection with this mechanism, a reaction force adjustment mechanism adjusts the initial compression amount of the reaction force spring 65, by providing displacement in a stretching direction for the reaction force spring 65 by including a reaction force adjustment operation shaft 7 and operating this shaft 7. The reaction force adjustment operation shaft 7 is composed of synthetic resin, with the left and right restrained by a main frame 2 for supporting a back seat, with one end of the reaction force spring 65 supporting in the intermediate position. In the main frame 2, there is installed an intermediate bearing part 11 which receives and supports, in the expansion direction of the reaction force spring 65, the reaction force adjustment operation shaft 7 on the opposite side of a part that supports one end of the reaction force spring 65 of the reaction force adjustment operation shaft 7.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a reaction force adjustment mechanism for a backrest reaction force mechanism of a chair. More specifically, the present invention relates to a reaction force adjustment mechanism that operates a reaction force adjustment operation shaft to adjust the initial compression amount of a reaction force spring in a backrest reaction force mechanism of a chair that obtains a force (reaction force) to push back the backrest to its initial position by compressing a spring.

Background Art

[0002] Conventionally, a spring (reaction force spring) has been used in a locking mechanism of a chair backrest. The locking function is a function that allows the backrest to tilt backward. When a weight is placed on the backrest, it tilts while having an appropriate repulsive force. When the backrest tilts backward, a force (reaction force) is obtained to push back the backrest to its initial position by compressing a spring.

[0003] Therefore, since a large force is applied to the reaction force spring, rigidity is required for the rotating shaft that receives the reaction force spring, and usually a metal member such as iron is used. Alternatively, the surface that receives the reaction force spring is provided with a wall portion, and has a structure that can withstand the load from the spring due to a rigid structure (for example, Patent Document 1, Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in recent years, from the perspective of weight reduction and downsizing of the main frame part including the reaction force mechanism for rocking the backrest of the chair, the reaction force adjustment mechanism, and the backrest support structure (generally called the mechanism part), it has been required to have a simple configuration.

[0006] For example, using a metal member for the reaction force adjustment operation shaft of the reaction force adjustment mechanism increases the weight and makes it difficult to separate and dispose of. Also, increasing the spring receiving wall for receiving the reaction force spring leads to an increase in the size of the reaction force adjustment mechanism and thus the mechanism part and the main frame part.

[0007] The present invention addresses such demands and aims to provide a reaction force adjustment mechanism for the backrest reaction force mechanism of a chair with a structure that enables weight reduction and downsizing of the main frame part of the chair, for example, in reaction force adjustment.

Means for Solving the Problem

[0008] To achieve such an object, the present invention A reaction spring interposed between a main frame with a backrest supported so as to be swingable and the backrest is In conjunction with the backward tilt of the backrest made and compressed, whereby generates a force to push back the backrest between the backrest and the main frame in a backrest reaction force mechanism of a chair, At one end of the reaction spring, a reaction force adjustment cam that gives a cam displacement for expanding and contracting the reaction spring, and a support for the reaction force adjustment cam and rotatably supported by the main frame including a reaction force adjustment operation shaft and a slider that is held linearly movably on one axis connecting the reaction force adjustment operation shaft and a spring support shaft provided on the backrest and the by operating the reaction force adjustment operation shaft rotation it is possible to the adjust the initial compression amount of the reaction force spring The applied cam displacement amount is switched, and against the reaction force spring at the initial position and is provided with a reaction force adjustment mechanism. the The reaction force adjustment operation shaft is composed of a synthetic resin the force applied to the reaction force adjustment operation shaft via the reaction force adjustment cam and supports is the reaction force spring axis in a receiving part On both sides of the reaction force adjustment cam are rotatably supported by in this way.

[0009] Also, the reaction force adjustment mechanism of the backrest reaction force mechanism of the chair of the present invention The reaction force adjustment cam is integrally formed with the reaction force adjustment operation shaft by synthetic resin is preferably like this.

[0010] Further, it is preferable that the reaction force adjusting mechanism of the backrest reaction force mechanism of the chair of the present invention is The reaction force adjustment cam is a planar cam having a polygonal cam contour in which the cam displacement amount is switched step by step, and the cam receiving surface of the slider in contact with the reaction force adjustment cam is a flat surface preferable.

[0011] Further, in the reaction force adjusting mechanism of the backrest reaction force mechanism of the chair of the present invention, while the reaction force adjusting cams are provided at two positions spaced apart on the reaction force adjusting operation shaft, between the two reaction force adjusting cams to bearing portion and a left - right swing prevention part that prevents the axial movement of the reaction force adjustment operation shaft by engaging with the bearing part in the axial direction is preferably arranged.

[0012] Furthermore, it is preferable that the reaction force adjusting mechanism of the backrest reaction force mechanism of the chair of the present invention is The bearing part is composed of a semi - circular open - type bearing opened toward the reaction spring side, and the reaction force adjustment operation shaft is rotatably clamped between the bearing part and an operation shaft retainer that covers the upper half of the reaction force adjustment operation shaft and is held so as not to come off axially preferable.

[0013] Moreover, it is preferable that the chair of the present invention includes the reaction force adjusting mechanism of the backrest reaction force mechanism of the chair according to any one of claims 1 to 5.

Advantages of the Invention

[0014] According to the reaction force adjusting mechanism of the backrest reaction force mechanism of the chair of the present invention, by forming the reaction force adjusting operation shaft with a synthetic resin, it is possible to reduce the weight of the reaction force adjustment and promote separate disposal, and at the intermediate bearing portion on the opposite side of the portion that supports one end of the reaction force spring of the reaction force adjusting operation shaft, the reaction force adjusting operation shaft is supported in the expansion direction of the reaction force spring, so that a support structure in which the reaction force adjusting operation shaft is difficult to bend can be obtained. Accordingly, it is possible to provide a reaction force adjusting mechanism that realizes weight reduction and miniaturization of the chair.

Brief Description of the Drawings

[0015]

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Embodiments for Carrying Out the Invention

[0016] Hereinafter, the configuration of the present invention will be described in detail based on the embodiments shown in the drawings. In this specification, "front or forward" means the front or forward direction for the chair and the seated person sitting on the seat of the chair in each of the seat and the backrest, and "rear or rearward" similarly means the rear or rearward direction for the chair and the seated person. Further, the direction orthogonal to the front-rear direction in the horizontal plane is the left-right direction or the width direction for the seated person or the chair in each of the seat and the backrest. In addition, in the seat and the backrest, the inner side is the side facing the vertical plane in the front-rear direction passing through the seated person side or the center of the chair, and the outer side is the side away from the seated person or the side away from the vertical plane in the front-rear direction passing through the center of the chair. Furthermore, "up" and "down" are the up and down for the chair and the seated person in each of the armrest, the seat, and the backrest, and are the up and down in the vertical plane.

[0017] Figs. 1 to 2 show an embodiment of a chair to which the present invention is applied. This chair has, for example, legs 1 provided with casters, a main frame (support base) 2 supported by the legs 1, and a seat 3 and a backrest 4 that are swingably supported with respect to the main frame 2 and interlock with each other. On the front side of the main frame 2, there are provided a reaction force adjustment mechanism 6 that enables adjustment of the initial reaction force of the backrest reaction force mechanism 5 and its reaction force adjustment operation shaft 7, and on the rear side, there are provided a locking range adjustment mechanism 8 that regulates the locking range of the backrest 4 and an operation shaft for switching the locking range (hereinafter referred to as the locking range switching operation shaft 9) so as to protrude outward from the side of the main frame 2. In addition, a cover member 10 is provided between the back frame of the backrest 4 and the main frame 2.

[0018] Here, in the case of this embodiment, the main frame 2 is molded from a synthetic resin. As the synthetic resin, in order to ensure the mechanical rigidity as a structure, it is preferably molded from a synthetic resin having rigidity such as reinforced nylon with glass fiber such as PA6-GF (polyamide 6). Of course, it is not particularly limited to this, and the main frame 2 may be configured by molding aluminum die-casting, steel materials, etc. into a box shape or other shapes by welding.

[0019] For example, as shown in FIGS. 15 and 16, the main frame 2 of this embodiment forms a box shape having peripheral wall portions 2A on the front, rear, left, and right, a bottom wall portion 2B, intermediate partition walls 2C arranged in the front-rear direction, and reinforcing ribs 2D connecting them, in order to enhance mechanical rigidity, particularly torsional rigidity, as a structure and accommodate a reaction force adjusting mechanism 6 or the like inside. On the rear side of this main frame 2, there are further provided a rear leg support cylinder portion 2E into which a gas spring of a leg column is fitted, and a recess 19 for attaching a buffer block 41.

[0020] Also, on the front side of the main frame 2, an intermediate bearing portion 11 is provided that supports at least the intermediate shaft portion 52 of the reaction force adjusting operation shaft 7 in the expansion direction of the compression coil spring 65 between the left and right intermediate partition walls 2C. Since a large force is applied to the reaction force spring of the backrest reaction force mechanism 5, that is, the compression coil spring 65, rigidity is required for the shaft that receives the compression coil spring 65, that is, the reaction force adjusting operation shaft 7 of this embodiment. For this reason, usually a metal member such as iron is used, but from the viewpoints of weight reduction of the chair and promotion of separation and disposal, resinification of parts is required. However, even if it is molded with a synthetic resin having rigidity such as glass fiber-reinforced nylon such as PA6-GF (polyamide 6) in order to ensure mechanical rigidity as a structure, there is a problem that it is more likely to bend compared to a metal shaft. Therefore, by providing the intermediate bearing portion 11 that supports at least the intermediate shaft portion 52 of the reaction force adjusting operation shaft 7 in the expansion direction of the compression coil spring 65, it is preferable to make it difficult to bend by three-point supporting the intermediate shaft portion 52 of the reaction force adjusting operation shaft 7 together with the bearing portions at both ends.

[0021] The intermediate bearing portion 11 is configured to support at least the force of the compression coil spring 65. In the case of this embodiment, it is composed of a semi-circular open bearing that opens toward the compression coil spring 65 side. For example, it is composed of left and right vertical walls 11A that also serve as bearing portions and ribs, a front wall 11B, and an overhang portion 11C that protrudes rearward of the main frame. The bearing portion 11D is formed by the left and right vertical walls 11A being shaped like a semi-circle. The overhang portion 11C supports the upward force of the compression coil spring 65 applied to the reaction force adjustment operation shaft 7. In the case of this embodiment, the vertical walls 11A, the front wall 11B, and the overhang portion 11C are integrally formed with the bottom wall portion 2B of the main frame 2, and are provided so as to stand up from the bottom wall portion 2B. Of course, the intermediate bearing portion 11 may be formed separately from the main frame 2 and fixed to the bottom wall portion 2B by screws or welding, etc.

[0022] On the left and right side wall portions 2A at the front of the main frame 2, elongated holes 12 that are inclined rearward when viewed from below are provided, and are provided so as to penetrate a swivel shaft 42 that rotatably connects the outer shell 71 and the main frame 2.

[0023] Also, on one peripheral wall portion 2A of the main frame 2, for example, the right peripheral wall portion 2A, a circular through-hole (referred to as a round hole 13) for inserting the reaction force adjustment operation shaft 7 of the reaction force adjustment mechanism 6 from the side is opened, and on the inner partition wall 2C, a semi-circular open bearing portion 14 with an upper edge cut out is provided. Further, on the opposite partition wall 2C, a semi-circular open bearing portion 14 with an upper edge cut out is also provided. Therefore, the reaction force adjustment operation shaft 7 inserted into the main frame 2 is supported by the round hole 13 and the semi-circular open bearing portions 14 of the partition wall 2C. At the same time, the upper half of the reaction force adjustment operation shaft 7 is supported by the operation shaft retainer 43 shown in FIG. 8, and the reaction force adjustment operation shaft 7 is rotatably clamped between the open bearing portion 14 of the partition wall 2C and is held so as not to come off axially.

[0024] Here, as shown in FIGS. 10 and 11 for example, the operation shaft retainer 43 is provided at both ends with semi-circular bearing portions (open-type bearing portions 44) that support the shaft portions 55 and 59 of the reaction force adjustment operation shaft 7. By pressing the shaft portion 56 and the shaft portion 59 of the reaction force adjustment operation shaft 7 from above, the reaction force adjustment operation shaft 7 is rotatably supported between the semi-circular open-type bearing portion 14 of the intermediate partition wall 2C. Further, on the inner peripheral surface of the operation shaft retainer 43, there is provided a regulating projection (stopper) 49 that abuts against the regulating projection (stopper) 54 of the reaction force adjustment operation shaft 7 to regulate the rotation of the reaction force adjustment operation shaft 7 within a certain range (for example, a range of approximately 270°) (see FIG. 6). Thereby, the reaction force adjustment operation shaft 7 is attached to the main frame 2 so as to be swingable within a certain range.

[0025] Further, in the case of this embodiment, the operation shaft retainer 43 is provided with a slider housing 50 that houses the slider 66 so as to be disposed opposite to the reaction force adjustment cam 51 of the reaction force adjustment operation shaft 7 and guides it in the spring axis direction. This slider housing 50 is, for example, a rectangular cylindrical case, and is disposed so as to straddle between the portions covering the upper sides of the left and right intermediate partition walls 2C and is integrally formed with the operation shaft retainer 43. Of course, the slider housing 50 is not limited to the case of being integrally formed with the operation shaft retainer 43, and it goes without saying that a separately formed one may be assembled to the operation shaft retainer 43, or assembled to the main frame 2 or integrally formed with the main frame 2.

[0026] Further, on the operation shaft retainer 43, a pair of protrusions 45 are provided at positions respectively covering the left and right intermediate partition walls 2C of the main frame 2 with an interval substantially equal to the thickness of the intermediate partition wall 2C, thereby forming a recess 48 that sandwiches the intermediate partition wall 2C. Also, on the base end side (the side of the seat rotation shaft 42) of the operation shaft retainer 43, a U-shaped mounting seat portion 47 is formed so as to fit and surround from above the boss portion provided with the screw hole 15 of the main frame 2. Therefore, by placing the mounting seat portion 47 on the boss portion provided with the screw hole 15 and covering the intermediate partition wall 2C from above between the pair of protrusions 45, that is, in the recess 48, and screwing screws from the four through holes 46 in the front, rear, left, and right directions into the screw holes 15, it can be fixed without rattling.

[0027] Furthermore, as shown in FIG. 16, on one side wall portion at the rear of the main frame 2, for example, the left side wall portion 2A, a round hole 16 is opened through which the locking range switching operation shaft 9 of the locking range adjustment mechanism 8 passes, and on the right side wall portion 2A, which is the other side wall portion, and the intermediate partition wall 2C, a semi-circular bearing portion receiver 17 that supports the tip portion of the locking range switching operation shaft 9 and a round hole 18 through which the tip passes are provided. The recess 19 for accommodating the buffer block 41 (resin damper) is a recess surrounded by the side wall of the main frame 2, and a hole 20 for fitting the pin portion 41a of the buffer block 41 is provided on the bottom surface. The buffer block 41 is fixed to the main frame 2 by being fitted into the recess 19 and fitting the pin portion 41a into the hole 20 at the bottom.

[0028] Inside the main frame 2, specifically, inside the left and right intermediate partition walls 2C, a space 2F is formed in which a reaction force mechanism 60 including a compression coil spring 65 is accommodated. A spring receiving arm 21B of a backrest support member (also called a back support rod) 21 that is swingably supported by the main frame 2, a reaction force adjustment operation shaft 7 that is rotatably supported by the main frame 2, and means such as a compression coil spring 65 that are arranged therebetween and generate a force to push back the backrest 4 in conjunction with the backward tilt of the backrest 4 are accommodatable.

[0029] Here, in the case of the main frame 2 in this embodiment, the rear leg support cylinder portion 2E is connected to a leg post (in other words, a gas spring) and is rotatably supported in the horizontal plane. On the other hand, the front is rotatably connected to the outer shell 71 of the seat 3 via the seat rotation axis 42, and is engaged with the seat support axis 21D of the backrest support member 21 behind the location where it is connected by the seat rotation axis 42 of the seat 3. In this way, the seat 3 and the backrest 4 are provided so as to be supported on the main frame 2 in a swingable manner in conjunction with each other. That is, in relation to the swing of the backrest support member 21 via the backrest rotation axis 25, the seat 3 is supported so as to be pulled obliquely rearward and lifted. As a result, a weight-sensitive (or sensing) reaction force applying mechanism 61 is configured that makes the resistance to back locking proportional to the user's weight by interlocking the lifting of the seat 3 when the backrest 4 is tilted backward.

[0030] (Backrest) In the case of this embodiment, as shown in FIGS. 1 and 2 for example, the backrest 4 is composed of a back frame 4a that forms a rectangular frame in front view with an inner void, and a mesh stretched material (not shown) that is stretched across the back frame 4a to form the backrest surface. It is swingably mounted on the main frame 2 via the backrest support member 21. The back frame 4a is preferably formed of a synthetic resin having rigidity, such as reinforced nylon with glass fiber, such as PA6-GF (polyamide 6), in order to ensure mechanical rigidity as a structure.

[0031] The back frame 4a in this embodiment has, for example, a recess 90 for accommodating the rear portion of the backrest support member 21 on the front side of the bottom edge. In a state where the rear portion of the backrest support member 21 is fitted, it is screwed to the nut 24 of the back plate 21C so as to penetrate the bottom edge of the back frame 4a from behind the backrest support member 21, and thus is connected to the backrest support member 21. As a result, the back frame 4a is firmly fixed to the backrest support member 21 in the front-rear, left-right, up-down directions without rattling.

[0032] The backrest support member 21 is swingably attached to the main frame 2 by a backrest rotation shaft 25 that penetrates the main frame 2 in the lateral direction. The backrest support member 21 has a backrest 4 attached to the rear side of the backrest rotation shaft 25 and is structured to be linked with the seat 3 on the front side of the backrest rotation shaft 25 to lift the seat 3 as the backrest 4 reclines. Specifically, for example, the plate member 21A of the backrest support member 21 has a substantially V-shaped bent shape in which the side on the front side of the through-hole 21F is inclined obliquely upward with respect to the side on the rear side of the through-hole 21F with the through-hole 21F penetrating the backrest rotation shaft 25 as a boundary. In the initial position state (the state where no seated person exists), the seat 3 and the backrest 4 are each maintained in the initial position, and the positional relationship is set such that a force for lifting the seat 3 is applied to the seat 3 when the backrest 4 reclines.

[0033] Specifically, in the case of the present embodiment, as shown in FIG. 17, the backrest support member 21 includes a pair of plate members 21A disposed on both sides of the main frame 2, a seat support shaft portion 21D that connects them in the front, a backrest attachment plate 21C that connects them in the rear, and a spring receiving arm 21B that supports one end of the compression spring mechanism of the reaction force mechanism. In the case of the present embodiment, the spring receiving arm 21B generally forms a triangle (V shape), for example. The seat support shaft portion 21D penetrates the corner portion of the apex, and a spring support shaft 21E for engaging the rear spring mount 63 is provided at the end of one side in the hanging state (the position corresponding to the apex), and a through-hole 21F for penetrating the backrest rotation shaft 25 is provided at the end of the other side in the inclined state obliquely backward and downward (the position corresponding to the apex). That is, the backrest support member 21 and the spring receiving arm 21B are rotatably supported with respect to the main frame 2 and are provided to swing simultaneously about the backrest rotation shaft 25 while maintaining a constant angular relationship with each other.

[0034] The portion (i.e., the back mounting seat) where the back frame 4a of the backrest support member 21 is attached is configured by a horizontal connecting plate 21C disposed so as to cross between the plate members 21A on both sides of the backrest support member 21 as shown in FIG. 17. A screw hole is provided by fixedly welding a nut 24 to the horizontal connecting plate 21C. The rear portions of the plate members 21A on both sides of the backrest support member 21 including the horizontal connecting plate 21C are fitted into a recess (backrest support member mounting seat) 90 opened in front of the bottom edge of the back frame 4a, and fastening screws are screwed in from behind the back frame 4a in a state where the horizontal connecting plate 21C and the backrest support member mounting seat are aligned, thereby integrating the backrest support member 21 and the back frame 4a. Note that a hole (not shown) for inserting the fastening screw that appears on the back surface of the bottom edge portion of the back frame 4a is blocked by fitting a cover plate 91 after tightening the fastening screw so as not to appear on the appearance. The cover plate 91 has, for example, locking claws, and is easily detachably provided by hooking the locking claws on a notch or the like of a recess on the back surface of the bottom edge of the back frame.

[0035] (Seat structure) The seat 3 is not limited to a specific structure and shape. In the case of the present embodiment, as shown in FIG. 2, it is composed of an outer shell 71, an inner shell 72, a urethane foam cushion 73, and an upholstery (not shown), and is mounted on the main frame 2 so as to be swingable about the seat rotation axis 42 by connecting the outer shell 71 and the main frame 2 via the seat rotation axis 42. The seat 3 is mounted on the main frame 2 so as to be swingable while moving in the vertical direction and the front-rear direction within a certain range via the outer shell 71 by making the hole 12 of the main frame 2 that supports the front seat rotation axis 42 an elongated hole inclined rearward when viewed from below.

[0036] As shown in FIG. 18 for example, the outer shell 71 of the present embodiment is provided such that the U-shaped peripheral wall portion 74 surrounding the main frame 2 protrudes downward, and the main frame 2 and the backrest support member 21 are housed in the recess 75 inside the peripheral wall portion 74. A slit 76 cut in the longitudinal direction is provided on the backrest side surface of the peripheral wall portion 74, and consideration is given so that interference between the backrest support member 21 and the outer shell 71 does not occur when the backrest support member 21 passes through the slit 76. Then, by arranging the seat rotation shaft 42 to penetrate across the main frame 2 and the peripheral wall portion 74 housed in the recess 75, the outer shell 71 is rotatably connected and supported with respect to the main frame 2. The outer shell 71 is formed with bearing portions that support both ends of the seat rotation shaft 42 on the peripheral wall portion 74. In the case of the present embodiment, the peripheral wall portion 74 is configured by a double-wall structure of an inner wall portion and an outer wall portion that are continuously formed so as to fold back at the bottom, for example, and bearing portions that support both ends with the seat rotation shaft 42 straddling therebetween are formed. For the bearing portion, for example, a blind round hole 77 is opened in one of the peripheral wall portions, and a through round hole 78 is opened in the other peripheral wall portion.

[0037] On the outer wall portion of the peripheral wall portion 74 at the position of the round hole 78, a window-shaped opening (hereinafter referred to as the opening 80) for inserting the seat rotation shaft 42 is provided. After inserting the seat rotation shaft 42 so as to straddle the main frame 2 and the outer shell 71, the opening 80 is closed by a retaining member 81. The shape and structure of the outer shell 71 are appropriately designed as necessary, but at least the left and right peripheral wall portions 2A of the main frame 2 overlap on the same axis, for example, in the horizontal direction (lateral direction), and a portion constituting a bearing portion that is interconnected via the seat rotation shaft 42, for example, the peripheral wall portion 74 is provided, and an opening 80 for inserting the seat rotation shaft 42 laterally into the bearing portion and a structure for installing a retaining member 81 for preventing the seat rotation shaft 42 from coming off are preferably provided. In the present embodiment, one of the bearing portions is a blind hole, but it is not particularly limited to this, and openings 80 may be provided as through round holes for both, and the retaining member 81 may be fitted and closed.

[0038] Outside the through hole 78 that supports the base rotation shaft 42, that is, between the through hole 78 and the opening 80 in the peripheral wall portion 74 of the outer shell 71, as shown in FIGS. 19 to 20, a retaining member insertion hole 79 into which a retaining member 81 is to be mounted is formed. This retaining member insertion hole 79 is, for example, a vertical hole that is arranged to intersect the base rotation shaft 42 and opens to the upper surface of the outer shell 71 (that is, the surface on which the inner shell and the urethane foam are placed), and a stepped portion (overhang) 83 that catches against upward detachment is formed on the inner surface of the outer wall portion of the peripheral wall portion 74 above the opening 80. On the other hand, the retaining member 81 abuts against the end face of the base rotation shaft 42 inserted so as to straddle the main frame 2 and the outer shell 71 to prevent axial movement. In the case of this embodiment, the retaining member 81 has rigidity and spring elasticity to prevent the base rotation shaft 42 from coming off, and is, for example, made of a resin plate having an L-shaped longitudinal cross-sectional shape with a return 82 as shown in FIG. 21. When the return 82 is inserted into the retaining member insertion hole 79, the portion of the return 82 at the tip bends inward, and when it gets over the stepped portion 83, it restores and fits into the recess below the stepped portion 83 and at the same time catches on the stepped portion 83. In the case of this embodiment, the return 82 is composed of an inclined surface portion 86 that fits into the opening 80 and a locking portion 87 that stands upright in the vertical direction and abuts against the stepped portion 83. Therefore, when the portion of the return (reverse hook) 82 of the retaining member 81 approaches the stepped portion 83, the locking portion 87 of the return 82 enters by spring elasticity and catches on the stepped portion 83, so that the retaining member 81 is engaged so as not to come out of the retaining member insertion hole 79. Further, a catching portion 85 that abuts against the edge of the retaining member insertion hole 79 is provided at the rear end of the retaining member 81 so as not to be immersed in the retaining member insertion hole 79. Note that the planar shape of the retaining member insertion hole 79 is not particularly limited, but in the case of this embodiment, it is a rectangular hole when viewed from above.

[0039] Since the retaining member insertion hole 79 communicates with the opening 80 of the peripheral wall portion 74, when the retaining member 81 is inserted, the inclined surface portion 86 of the return 82 is exposed on the surface. In the case of this embodiment, the area of the inclined surface portion 86 of the return 82 of the retaining member 81 is set to be slightly smaller than the opening area of the opening 80. When the retaining member 81 is pushed into the retaining member insertion hole 79, the locking portion 87 of the return 82 fits under the step portion 83 on the inner surface of the outer wall portion, and at the same time, the inclined surface portion 86 of the return 82 fits into the opening 80 and closes it. Thus, the inclined surface portion 86 of the return 82 exposed in the opening 80 from the outside of the peripheral wall portion 74 of the outer shell 71 can be pushed in with a finger and disengaged from the step portion 83.

[0040] Also, as shown in FIG. 3, the seat 3 is supported by, for example, a hook-shaped engaging portion 84 of a locking member of the outer shell 71 being hooked on the seat support shaft portion 21D of the backrest support member 21 in the middle of the front-rear direction of the seat 3, which is behind the seat rotation axis 42 and in front of the backrest rotation axis 25.

[0041] Here, the rear connection position of the outer shell 71 to the backrest support member 21 is provided at the seat support shaft portion 21D in front of the rotation axis (backrest rotation axis 25) of the backrest support member 21. When the backrest support member 21 tilts backward, the seat 3 is lifted by the seat support shaft portion 21D on the front end side of the backrest support member 21. That is, as the backrest 4 tilts backward, the seat 3 is lifted, and a part of the weight of the seated person acts as a resistance against the backward tilt movement of the backrest 4 (the rocking resistance is weight-sensitive), and it is provided so as to constitute a weight-sensing reaction force applying mechanism. Therefore, when the seated person leans against the backrest 4, the backrest support member 21 rotates around the backrest rotation axis 25 so as to tilt backward, so that the seat 3 is pulled obliquely backward and the rear side of the seat 3 is lifted, and a reaction force proportional to the weight of the seated person is applied to the backrest 4 via the backrest support member 21.

[0042] The outer shell 71 is not limited to a specific material or structure. In this embodiment, for example, it is molded from a synthetic resin with rigidity such as reinforced nylon containing glass fibers, and is reinforced by appropriately adopting a rib structure. Since the connection structure between the outer shell 71 and the inner shell is well-known, it is not shown in detail. For example, by engaging a T-shaped locking claw having locking heads dispersedly arranged at four positions on the upper surface of the outer shell 71 with a T-shaped slit extending in the front-rear direction as viewed from above the inner shell, that is, by sliding the T-shaped locking claw in the front-rear direction after passing the head of the T-shaped locking claw through the T-shaped slit, they are mutually fixed.

[0043] The assembly of the seat 3 and the main frame 2 is carried out by covering the outer shell 71 so as to house the main frame 2 in the recess 75 of the outer shell 71, aligning the long hole 12 of the main frame 2 with the hole for the rotation axis of the outer shell 71 (the opening 80, the through hole 78, and the blind round hole 77), and inserting the seat rotation axis 42 from the outside of the outer shell 71 through the opening 80. Then, the seat rotation axis 42 is pushed in until the tip side abuts against the blind round hole 77 of the outer shell 71. Thereafter, a retaining member 81 is inserted into the retaining member insertion hole 79 from the upper surface of the outer shell 71 and pushed in until the flange 82 is housed under the stepped portion 83 while abutting against the rear end surface of the seat rotation axis 42. When the retaining member 81 is mounted in the retaining member insertion hole 79, the axial end surface of the seat rotation axis 42 abuts against the retaining member 81 and is fixed. Thereafter, the inner shell 72 and the cushion 73 are placed on the outer shell 71 and then wrapped with an upper cover. Next, the hook-shaped bearing portion 84 in the middle of the outer shell 71 is aligned with the seat support axis 21D of the backrest support member 21 and pushed in from above, so that the hook-shaped bearing portion 84 and the seat support axis 21D are rotatably fitted. Thereby, in association with the swinging of the backrest support member 21 about the backrest rotation axis 25, the seat 3 is supported so as to be pulled obliquely rearward and lifted.

[0044] In the case of this embodiment, for example, after the outer shell 71 is rotatably connected to the front of the main frame 2 via the seat rotation shaft 42, the retaining member 81 is inserted, and then the inner shell is placed thereon, and a cushion is placed thereon and then wound and fixed with an outer covering. Therefore, after the seat 3 is assembled to the main frame 2 and the seat 3 is assembled, the seat rotation shaft 42 will not fall off. On the other hand, when it is necessary to separate the seat 3 from the main frame 2, with the outer covering, urethane foam cushion, and inner shell removed, by pushing the return 82 of the retaining member 81 exposed at the opening 80 on the outer peripheral wall of the outer shell 71, the return 82 can be removed from the stepped portion 83, and the retaining member 81 can be removed above the outer shell, so that the seat rotation shaft 42 can be removed and the seat 3 and the main frame 2 can be separated. Incidentally, in this embodiment, the retaining member insertion hole 79 is a vertical hole and the retaining member 81 is inserted from the upper surface of the outer shell 71, but it is not particularly limited to this. For example, the retaining member insertion hole 79 and the stepped portion 83 may be formed horizontally so that the retaining member 81 is inserted from the front of the outer shell 71. The retaining member insertion hole 79 only needs to be provided in a direction intersecting the shaft 42. Also in this case, by pushing the return 82 of the retaining member 81 exposed at the opening 80, the return 82 can be removed from the stepped portion 83, and the retaining member 81 can be removed above the outer shell, so that the seat rotation shaft 42 can be removed and the seat 3 and the main frame 2 can be separated.

[0045] (Backrest reaction mechanism) As the backrest reaction mechanism 5, although not limited to a specific form, in the case of this embodiment, as shown in FIG. 3, when the backrest 4 tilts backward, a reaction force mechanism 60 using a spring that compresses the spring and tries to push the backrest 4 back to the initial position (reaction force), and a weight-sensitive (or sensing type) reaction force applying mechanism 61 that makes the seat 3 lift up when the backrest 4 is tilted backward, so as to make the resistance to back rocking proportional to the user's weight are used in combination. Of course, depending on the embodiment, only one of the reaction force mechanisms may be adopted.

[0046] Specifically, a weight-sensitive reaction force applying mechanism 61 and a reaction force mechanism 60 using a spring are incorporated on the front side of the backrest rotation axis 25 of the backrest support member 21, and a force (reaction force) is provided between the backrest support member 21 and the reaction force adjustment operation shaft 7 of the main frame 2 to push back the backrest 4 in conjunction with the backward tilt of the backrest 4.

[0047] Here, the reaction force mechanism 60 using a spring is provided with a spring-type reaction force mechanism 60 including a compression coil spring 65 by interposing the reaction force spring, for example, a single compression coil spring 65 between the spring support shaft 21E of the backrest support member 21 and the reaction force adjustment operation shaft 7 which is a fixed shaft attached to the main frame 2. The spring-type reaction force mechanism 60 is provided to generate a force to push back the backrest 4 in conjunction with the backward tilt of the backrest 4 between the backrest support member 21 swingably supported by the main frame 2 and a fixed shaft on the front side of the main frame 2, for example, the reaction force adjustment operation shaft 7.

[0048] The compression coil spring 65 is mounted between a fixed shaft, for example, the reaction force adjustment operation shaft 7 and the spring support shaft 21E of the arm portion 21B of the backrest support member 21 via a front spring mount 62 and a rear spring mount 63. A mount pin 64 for preventing buckling of the compression coil spring 65 is inserted between the front and rear spring mounts 62 and 63.

[0049] The front spring mount 62 and the rear spring mount 63 are well-known mechanical elements, so detailed descriptions thereof will be omitted. For example, they have a shaft portion fitted inside the spring and a seat for receiving the end of the spring, and means for engaging with a fixed shaft, for example, a reaction force adjustment operation shaft 7 or a movable shaft, for example, a spring support shaft 21E, such as a concave portion, a convex portion, or a hook portion. In the case of this embodiment, since the initial reaction force adjustment mechanism 6 of the reaction force mechanism is provided, the reaction force adjustment operation shaft 7 as the fixed shaft abuts against the slider 66 via the slider 66. Therefore, on the surfaces of the slider 66 and the front spring mount 62 that contact each other, for example, a convex portion 62a and a concave portion 66b having a semi-circular cross-section with an axis (horizontal axis) parallel to the backrest rotation shaft 25 are formed. By fitting the concave portion 66b and the convex portion 62a, a universal joint is configured to vary the angle (formed by the slider 66 and the front spring mount 62) within a plane perpendicular to the backrest rotation shaft 25, and the front spring mount 62 is swingably supported with respect to the slider 66.

[0050] Further, the rear spring mount 63 is provided with a hook that is hooked on the spring support shaft 21E on the back side and a shaft portion that protrudes laterally of the hook and serves as a spacer between the arm portions 21B. By being hooked so that the lateral (width direction) movement of the arm portion 21B with respect to the spring support shaft 21E is blocked, the rear spring mount 63 is swingably supported.

[0051] Therefore, when the backrest 4 tilts backward and the arm portion 21B rotates about the backrest rotation shaft 25 and is pushed forward, the compression coil spring 65 is compressed via the rear spring mount 63. At the same time, the seat support shaft 21D at the tip of the backrest support member 21 is lifted, lifting the rear portion of the seat 3. Since the front end side of the seat 3 is housed in the oblong hole 12 where the main frame 2 tilts backward via a rotation shaft, the seat 3 is lifted so as to be slightly pulled obliquely backward. For this reason, the weight of the seated person on the seat 3 is applied to the seat support shaft 21D at the tip of the backrest support member 21. That is, a force proportional to the weight of the seated person is applied to the tip of the backrest support member, and a reaction force is applied to the backrest 4 via the arm portion 21B of the backrest support member 21.

[0052] (Initial Reaction Force Adjustment Mechanism of Reaction Force Mechanism) The backrest reaction force applying mechanism 5 is provided with a reaction force adjusting mechanism 6 that can adjust the initial reaction force (the repulsive force of the compression coil spring 65 applied in the initial position state).

[0053] The reaction force adjusting mechanism 6 adjusts the compression amount (i.e., the initial compression amount) of the compression coil spring 65 in the initial position by changing the position where one end of the compression coil spring 65 abuts against a fixed member on the main frame 2 side. As shown in FIG. 8, the reaction force adjusting mechanism 6 of the present embodiment changes the compression amount of the compression coil spring 65 in the initial position by applying displacement in the expansion and contraction direction of the compression coil spring 65 using a cam mechanism, and is composed of a reaction force adjusting cam 51 that rotates by a reaction force adjusting operation shaft 7 and a slider 66 as a follower joint.

[0054] In the case of the present embodiment, the reaction force adjusting cam 51 is integrally formed with the reaction force adjusting operation shaft 7 using a synthetic resin having rigidity, such as reinforced nylon with glass fiber, for example, PA6-GF (polyamide 6). As shown in FIG. 14, the reaction force adjusting cam 51 has a cam base circle 51f having a smaller diameter than the minimum shaft diameter of the reaction force adjusting operation shaft 7, for example, the shaft diameter of the tip shaft portion 55 in the case of the present embodiment, and the position of the cam base circle 51f is used as a starting point or an end point. In the case of the reaction force adjusting cam 51 of the present embodiment, it is configured as a planar cam having a polygonal cam contour in which the cam displacement amount is switched in a plurality of stages, for example, about 4 stages, within the swing range of the reaction force adjusting operation shaft 7 (the swingable range restricted between the regulating protrusion 49 of the operation shaft retainer 43 and the regulating protrusion 54 of the reaction force adjusting operation shaft 7).

[0055] Here, the cam surface 51a serving as the starting or ending point consists of a flat surface that is a tangent to the cam base circle 51f having a diameter smaller than the shaft diameter of the minimum shaft portion of the reaction force adjustment operation shaft 7. The cam surfaces 51b, 51c, and 51d that are switched as the reaction force adjustment operation shaft 7 rotates gradually increase or decrease the displacement imparted to the slider 66 in steps. For this reason, while making the reaction force adjustment cam itself compact, the cam displacement can be set in multiple stages, and the cam can be operated with a light force due to the relationship between the moment of force and the torsional moment. Moreover, when in a state other than during the switching operation, since the force from the compression coil spring 65 acts perpendicular to the surface due to the contact between flat surfaces, there is a sense of stability. When switching the initial compression amount of the compression coil spring, a click feeling is generated by overcoming the switching section 51e while the displacement gradually increases.

[0056] Of course, the cam contour surface of the reaction force adjustment cam 51 is not limited to the intermittent flat end surfaces of a polygon as shown in the figure. As a cam contour that gives a continuously variable displacement consisting of a continuous curved surface, the displacement imparted to the compression coil spring 65, that is, the initial compression amount, may be changed to increase or decrease gradually without steps by operating the reaction force adjustment operation shaft 7. Also in this case, by setting the cam base circle 51f to have a diameter smaller than the shaft diameter of the minimum shaft portion of the reaction force adjustment operation shaft 7, while making the reaction force adjustment cam itself compact, the cam displacement can be set in multiple stages, and the cam can be operated with a light force due to the relationship between the moment of force and the torsional moment.

[0057] In the case of this embodiment, as shown in FIG. 13, a pair of reaction force adjustment cams 51 are provided at intervals on the reaction force adjustment operation shaft 7. In this case, as shown in FIG. 9, since the reaction force adjustment cam 51 operates to push the slider 66 at two points away from each other, the movement of the slider 66 is less likely to be biased, and it becomes easy to translate it without causing kinking.

[0058] As shown in Fig. 13, an intermediate shaft portion 52 supported by an intermediate bearing portion 11 is formed between a pair of reaction force adjusting cams 51. Further, on the outer sides of the reaction force adjusting cams 51 (in the direction away from the central intermediate shaft portion 52), shaft portions 53 having a circular cross-section are respectively provided. The shaft portions 53 and the intermediate shaft portion 52 are axes of a circle (i.e., shaft diameter) that encompasses the apexes of the contour portions that give the maximum displacement amount of the reaction force adjusting cam 51, and are provided so as to prevent the slider 66 in contact with the reaction force adjusting cam 51 from swinging outward or inward (in the left-right direction) and deviating. Further, on the outer sides of each of the shaft portions 53, a regulating projection (stopper) 54 that regulates the rotation of the reaction force adjusting operation shaft 7 and a shaft portion 56 having a diameter that enables relative circumferential movement (rotation) of a regulating projection (stopper) 49 that projects from the inner peripheral surface of the operation shaft retainer 43 toward the shaft center are provided. The rotation of the reaction force adjusting operation shaft 7 is regulated within a certain range (generally within a range of 270°) by the regulating projection 49 of the operation shaft retainer 43 and the regulating projection 54 around the shaft portion 56 (see Fig. 6).

[0059] On the further outer sides of each of the shaft portions 56, a tip shaft portion 55 supported by a semi-circular open bearing portion 14 of the inner partition wall 2C on the back side when viewed from the round hole 13 which is an insertion hole for the reaction force adjusting operation shaft 7, and a front side shaft portion 57 supported by a semi-circular open bearing portion 14 of the front side partition wall 2C are provided. Here, the regulating projection 54 on the back side is set to the same height (i.e., the same diameter) as the shaft portion 53, and the regulating projection 54 on the front side is set to the same height (i.e., the same diameter) as the shaft portion 57. Further, on the outer side of the shaft portion 57 supported by the open bearing portion 14 of the partition wall 2C, a shaft portion 59 having approximately the same diameter as the shaft portion 56 is formed so as to form a groove into which the open bearing portion 44 at the edge of the operation shaft retainer 43 is fitted between the shaft portion 58 supported by the round hole 13 of the peripheral wall portion 2A.

[0060] That is, the reaction force adjustment operation shaft 7 of the present embodiment is formed with the smallest diameter among the shaft portions where the shaft portion at the tip (referred to as the tip shaft portion 55) is inserted into the main frame. The shaft portion 53 and the intermediate shaft portion 52 are circles (i.e., shaft diameters) that enclose the apexes of the contour portions that give the maximum displacement amount of the reaction force adjustment cam 51. Further, the shaft portion 57 and the shaft portion 58 are formed as shaft portions having a larger diameter than the shaft portion 53 and a diameter that can penetrate the round hole 13 with a sufficient gap. That is, the reaction force adjustment operation shaft 7 of the present embodiment is formed such that the shaft diameter increases sequentially from the tip, and it is possible to insert the reaction force adjustment operation shaft 7 straight into the round hole 13 using an assembly robot or the like.

[0061] As shown in FIG. 5, the reaction force adjustment operation shaft 7 is supported at the tip shaft portion 55 and the shaft portion 57 by the semi-circular open-type bearing portions 14 of the intermediate partition walls 2C on the back side and the front side (left and right), and at the same time, the shaft portion 59 and the tip shaft portion 55 are pressed from above by the open-type bearing portion 44 at the edge of the operation shaft retainer 43, so that it is rotatably supported while axial movement is blocked. Further, the shaft portion 58 is supported by the round hole 13 in the peripheral wall portion 2A, and the outer flange 7B abuts against the peripheral wall portion 2A, thereby positioning the insertion of the reaction force adjustment operation shaft 7 into the main frame 2.

[0062] On the other hand, in the case of this embodiment, the slider 66 that contacts the reaction force adjustment cam 51 is housed in a slider housing 50 integrally formed with the operation shaft retainer 43, so that it is held linearly movable on a single axis connecting the reaction force adjustment operation shaft 7 and the spring support shaft 21E at the tip of the spring receiving arm 21B of the backrest support member 21 in the initial position state (when there is no seated person). Therefore, the slider 66 applies the same amount of compression to the compression coil spring 65 as the cam displacement applied by the reaction force adjustment cam 51 via the rear front spring mount 62.

[0063] As shown in Fig. 9, the slider 66 has cam receiving surfaces (contacts) 66a that project forward at both left and right ends and abut against the reaction force adjusting cams 51. Here, the cam receiving surfaces (contacts) 66a of the slider 66 that contact the reaction force adjusting cams 51 are flat surfaces (i.e., flat-ended contacts), and a substantially H-shaped block is formed with the cam receiving surfaces 66a at both left and right ends protruding so as to face a pair of reaction force adjusting cams 51 spaced apart with the intermediate shaft portion 52 interposed therebetween. Further, a slight-height protrusion 66c that functions as a rail is formed on the side surface of the slider 66, and it is fitted into a groove on the inner surface of the slider 66 and supported slidably in the front-rear direction, i.e., the expansion and contraction direction of the spring. Incidentally, the cam receiving surfaces (contacts) 66a are flat-ended in the case of this embodiment, but may be pointed-ended or round-ended depending on the case.

[0064] According to the initial reaction force adjusting mechanism of the above-described reaction force mechanism, by rotating the reaction force adjusting operation shaft 7, the cam surface that abuts against the slider 66 of the reaction force adjusting cam 51 is switched, and the displacement amount applied to the compression coil spring 65 is increased or decreased, whereby the initial compression amount of the compression coil spring 65 is switched. Moreover, since the reaction force adjusting cam 51 has the position of the cam base circle 51f with a smaller diameter than the minimum shaft diameter of the reaction force adjusting operation shaft 7 as the starting point or the ending point, when turning the handle 7A of the reaction force adjusting operation shaft 7, the switching operation can be performed with a lighter force due to the shorter diameter. Moreover, by turning the handle grip 7A of the reaction force mechanism, which can adjust the initial reaction force of the spring, the feeling can be switched to be gradually lighter or heavier. In particular, in the case of a chair that uses a weight-sensitive reaction force applying mechanism as a backrest reaction force mechanism as in this embodiment, since a part of the reaction force of the backrest tilting backward can utilize the load of the seated person, the compression coil spring 65 as the spring for applying the reaction force can be made into a spring with a smaller reaction force. As a result, when the backrest 4 tilts backward, the force applied to the reaction force adjusting operation shaft 7 does not increase compared to a backrest reaction force mechanism composed only of the reaction force mechanism 60 using a spring that compresses the spring to obtain a reaction force, and sufficient durability can be obtained even with a resin-made reaction force adjusting operation shaft 7. That is, by turning the handle 7A of the reaction force adjusting operation shaft 7, the feeling that enables adjustment of the firmness of the reaction force of the weight sensing mechanism can be changed.

[0065] Also, FIGS. 28 to 32 show other embodiments of the initial reaction force adjusting mechanism. This initial reaction force adjusting mechanism rotatably supports the reaction force adjusting operation shaft 70 by utilizing the biasing force of the reaction force spring 65 with an intermediate bearing portion 11 and an outer bearing portion 67 provided on the bottom wall portion 2B of the main frame 2, while restricting rotation between the main frame 2 and the reaction force adjusting operation shaft 70. In other words, the reaction force adjusting operation shaft 70 is rotatably supported at both sides of the reaction force adjusting cam 51 with both ends supported by an outer bearing portion 67 that supports the outer shaft portion of the reaction force adjusting cam 51 and an inner bearing portion (i.e., the intermediate bearing portion 11) that supports the inner shaft portion of the reaction force adjusting cam 51, and the vicinity of each reaction force adjusting cam 51 is rotatably supported. That is, the reaction force adjusting cam 51 is supported at both ends with a short distance. Therefore, even if the shaft around the reaction force adjusting cam 51 of the reaction force adjusting operation shaft 70 is thin, since both sides close to the reaction force adjusting cam 51 are supported, a structure that is difficult to bend can be achieved.

[0066] As shown in FIGS. 29 and 30, the reaction force adjusting operation shaft 70 includes a handle (also called a grip) 70A, a flange portion 70B, a shaft portion 93, a reaction force adjusting cam 51, an intermediate shaft portion 92, left and right runout prevention portions 94, a reaction force adjusting cam 51, and a tip shaft portion 95. The shaft portion 93 and the shaft portion 92 beyond the flange portion 70B are formed with the same shaft diameter, and the tip shaft portion 95 has the minimum shaft diameter. Still, in this embodiment as well, the reaction force adjusting operation shaft 70 and the reaction force adjusting cam 51 are integrally formed of a synthetic resin having rigidity, such as reinforced nylon with glass fiber, for example, PA6-GF (polyamide 6), etc., but it is not particularly limited thereto, and in some cases, it may be made of metal, or may be a two-member structure of a metal shaft and a plastic cam member.

[0067] On the other hand, in the case of this embodiment, the outer bearing portion 67 is integrally formed so as to rise from the bottom wall portion 2B of the main frame 2, similarly to the vertical wall 11A of the intermediate bearing portion 11 for example. A semi-circular open bearing portion 69 is provided concentrically with the semi-circular open bearing portion 11D of the intermediate bearing portion 11. The open bearing portion 69 is open toward the compression coil spring 65 side. Therefore, the first half of the reaction force adjustment operation shaft 70 inserted into the main frame 2 is supported by the semi-circular open bearing portion 11D of the intermediate bearing portion 11 and the semi-circular open bearing portion 69 of the outer bearing portion 67. Further, the force of the reaction force spring of the backrest reaction force mechanism 5, that is, the compression coil spring 65, is constantly applied to the reaction force adjustment operation shaft 70 via the slider 66. For this reason, the reaction force adjustment operation shaft 70 is constantly pressed against the intermediate bearing portion 11 and the outer bearing portion 67 and is supported at four points. For this reason, the reaction force adjustment operation shaft 70 is rotatably held without being clamped between the operation shaft retainer 43 and the semi-circular open bearing portions 14 of the left and right intermediate partition walls 2C as in the embodiments shown in FIGS. 3 to 16. Incidentally, the intermediate bearing portion 11 and the outer bearing portion 67 may be formed separately from the main frame 2 and fixed to the bottom wall portion 2B by screws or welding or the like.

[0068] Further, as shown in FIGS. 29 and 30, the reaction force adjustment operation shaft 7 has its tip shaft portion 95 received by the semi-circular open bearing portion 69 of the outer bearing portion 67 on the rear side and its shaft portion 93 received by the semi-circular open bearing portion 69 of the outer bearing portion 67 on the front side. On the other hand, the flange portion 70B abuts against the edge of the insertion hole 98 (that is, the peripheral wall portion 2A), whereby the reaction force adjustment operation shaft 7 is rotatably supported while its axial movement is blocked, and its insertion position into the main frame 2 is determined.

[0069] Although not shown in the drawings, it is desirable that the slider 66 disposed opposite to the reaction force adjustment cam 51 of the reaction force adjustment operation shaft 70 be held by the main frame 2 so as to be movable in the expansion and contraction direction of the reaction force spring at the initial position of the reaction force applying mechanism. Therefore, for example, although not shown in the drawings, as in the embodiments shown in FIGS. 3 to 16, the slider 66 is housed in a slider housing 50 integrally formed with the operation shaft retainer 43 and fixed to the left and right partition walls 2C. Of course, the slider housing 50 is not limited to being integrally formed with the operation shaft retainer 43, and it goes without saying that a separately formed slider housing 50 may be assembled to the operation shaft retainer 43, or may be assembled to the main frame 2 or integrally formed with the main frame 2.

[0070] In the reaction force adjustment operation shaft 70 of the present embodiment, since there are a shaft portion 93 beyond the flange portion 70B and portions such as a reaction force adjustment cam 51 and left and right runout prevention portions 94 having a diameter larger than the shaft diameters of the shaft portion 92, they cannot be inserted into a circular hole having a shaft diameter slightly larger than the shaft diameters of the shaft portion 93 and the shaft portion 92. Further, if an attempt is made to insert the reaction force adjustment operation shaft 70 concentrically with the bearing portions 11D and 69 of the outer bearing portion 67 and the intermediate bearing portion 11, the reaction force adjustment cam 51 and the left and right runout prevention portions 94 will interfere with the outer bearing portion 67 and the intermediate bearing portion 11. Therefore, in the case of the present embodiment, the through hole (referred to as the insertion hole 98) for inserting the reaction force adjustment operation shaft 70 of the reaction force adjustment mechanism 6 formed in one peripheral wall portion 2A of the main frame 2, for example, the right peripheral wall portion, is formed in an elliptical shape having a major axis disposed generally in the front-rear direction. At the same time, the diameter in the minor axis direction of the insertion hole 98 on the side surface of the main frame is formed larger than the maximum dimension portion of the portion inserted into the main frame 2 (the shaft diameter of the left and right runout prevention portions 94 or the distance from the axis to the maximum contour position of the reaction force adjustment cam 51). Thereby, the reaction force adjustment operation shaft 70 can be inserted once at a position away from the bearing centers (original installation positions) of the outer bearing portion 67 and the intermediate bearing portion 11, and then shifted to the original installation position and set (referred to as shift insertion). Therefore, it is possible to insert the reaction force adjustment cam 51 and the left and right runout prevention portions 94 while avoiding interference with the outer bearing portion 67 and the intermediate bearing portion 11 and install them at a predetermined position.

[0071] Also, in the case of this embodiment, the insertion hole 98 also functions as a means for restricting the swing range of the reaction force adjustment operation shaft 70. On the front end side in the major axis direction of the insertion hole 98, a projection 97 corresponding to the restricting projection 49 of the operation shaft retainer 43 of the above-described embodiment is formed so as to project rearward. On the other hand, a projection 96 corresponding to the restricting projection 54 of the above-described embodiment is formed on the reaction force adjustment operation shaft 70. Thus, the reaction force adjustment operation shaft 70 is swingable until the projection 96 of the reaction force adjustment operation shaft 70 abuts against the projection 97 of the insertion hole 98 of the main frame 2. Note that a flange 70B is provided on the reaction force adjustment operation shaft 70, and the flange 70B is provided to abut against the peripheral edge of the hole 98 (that is, the peripheral wall portion 2A) so as to position the insertion position into the main frame 2. Note that in this embodiment, the reaction force adjustment operation shaft 70 is provided so as to be swingable within a certain range and starts or ends at the cam surface 51A that imparts the minimum cam displacement amount, but is not particularly limited thereto, and may be provided so as to rotate one or more times, that is, the switching of the cam displacement amount circulates as necessary. This also applies to the embodiments shown in FIGS. 3 to 16.

[0072] As described above, the reaction force adjustment cam 51 has a cam base circle 51f having a smaller diameter than the minimum shaft diameter of the reaction force adjustment operation shaft 70, for example, the shaft diameter of the tip shaft portion 95 in the case of this embodiment, and starts or ends at the position of the cam base circle 51f (see FIG. 14).

[0073] Here, the cam surface 51a serving as the starting point or the ending point is formed of a flat surface that is a tangent to the cam base circle 51f having a smaller diameter than the shaft diameter of the minimum shaft portion of the reaction force adjustment operation shaft 70, and is configured as a planar cam having a polygonal cam contour in which the cam displacement amount is switched in a plurality of stages, for example, about four stages, within the swing range of the reaction force adjustment operation shaft 70. Therefore, the cam surfaces 51b, 51c, and 51d that are switched as the reaction force adjustment operation shaft 70 rotates increase or decrease the displacement given to the slider 66 step by step. For this reason, it is possible to set the cam displacement in multiple stages while making the reaction force adjustment cam itself compact, and it is possible to operate the cam with a light force due to the relationship between the moment of force and the torsional moment.

[0074] Of course, the cam contour surface of the reaction force adjusting cam 51 is not limited to the intermittent flat end surface of a polygon as shown in the figure, but can be a cam contour that gives a continuously variable displacement consisting of a continuous curved surface. By operating the reaction force adjusting operation shaft 70, the displacement applied to the compression coil spring 65, that is, the initial compression amount, may be changed so as to increase or decrease gradually without steps. Also in this case, by making the cam base circle 51f have a smaller diameter than the shaft diameter of the smallest shaft portion of the reaction force adjusting operation shaft 7, the reaction force adjusting cam itself can be made compact while setting the cam displacement in multiple stages, and the cam can be operated with a light force due to the relationship between the moment of force and the torsional moment.

[0075] As shown in FIG. 28, a pair of reaction force adjusting cams 51 are provided at intervals on the reaction force adjusting operation shaft 70, and are provided so as to give a cam displacement by pushing at two points separated from the slider 66. As a result, it becomes difficult for the movement of the slider 66 to be biased, and it becomes easy to translate it without causing a kink.

[0076] As shown in FIG. 28, an intermediate shaft portion 92 supported by the intermediate bearing portion 11 is formed between the pair of reaction force adjusting cams 51. Further, inside the region supported by the left and right vertical walls 11A of the intermediate bearing portion 11, for example, at the central portion of the intermediate shaft portion 92, left and right runout preventing portions 94 having a larger diameter than the intermediate shaft portion 92 are formed. The left and right runout preventing portions 94 engage axially with a protrusion 68 that protrudes from the tip edge portion of the overhang portion 11C of the intermediate bearing portion 11, which is a member on the main frame 2 side, onto the intermediate shaft portion 92, thereby making it difficult for the reaction force adjusting operation shaft 70 and thus the reaction force adjusting cam 51 to move in the left and right directions (width direction). That is, a left and right runout preventing structure that prevents displacement between the reaction force adjusting cam 51 and the contact surface 66a of the slider 66, which is a driven joint, is configured.

[0077] In the case of this embodiment, since the shaft diameter of the reaction force adjustment operation shaft can be made smaller, the reaction force adjustment operation can be performed with a smaller force. In particular, in the case of a chair that uses a weight-sensitive reaction force applying mechanism as the backrest reaction force mechanism, since a part of the reaction force of the backrest reclining can utilize the load of the seated person, the compression coil spring 65 as the spring for applying the reaction force can be made into a spring with a small reaction force. As a result, when the backrest 4 reclines, the force applied to the reaction force adjustment operation shaft 7 does not increase compared to the backrest reaction force mechanism 60 composed only of the reaction force mechanism using a spring that compresses the spring to obtain a reaction force, and sufficient durability can be obtained even with the resin reaction force adjustment operation shaft 7.

[0078] (Backrest locking range adjustment mechanism) Between the main frame 2 and the backrest 4, as shown in FIGS. 3 and 22, for example, a locking range adjustment mechanism 8 is provided that can adjust the maximum reclining position at which the backrest 4 can be locked by restricting the movement of a member interlocking with the backrest 4, for example, the backrest support member 21.

[0079] This locking range adjustment mechanism 8 regulates and adjusts the maximum reclining position at which the backrest 4 can be locked by changing the position where a member interlocking with the backrest 4, for example, the backrest support member 21, abuts against the main frame 2. It is composed of a locking block (so-called reclining range regulating member) 26 that moves with the backrest 4 and abuts against the main frame 2 to prevent the backrest 4 from reclining, and a fixing block 34 that fixes the position of the locking block 26 with respect to the backrest 4. Here, at least one of the locking block 26 as a movable member and the fixing block 34 as a fixed member is preferably composed of a resin member, for example, a highly slidable polyacetal resin (POM). In addition, a buffer block 41 for reducing the impact and collision sound when the locking block 26 abuts against the main frame 2 side is provided as necessary.

[0080] In the case of this embodiment, the locking block 26 is fixed to the rocking range switching operation shaft 9 attached to the backrest support member 21 and is provided so as to swing together with the backrest support member 21, and is provided so as to rotate about the same axis by the rotation of the rocking range switching operation shaft 9. This locking block 26 is provided with a plurality of stepped portions, for example, stepped portions 28A to 28D having four lengths, which abut against the buffer block 41 of the main frame 2 at different positions from the center of the rocking range switching operation shaft 9 in order to gradually change the maximum reclining position at which the backrest can be locked. For example, as shown in FIG. 24, the locking block 26 has a semi-circular fan-shaped lever-shaped locking portion 28 having stepped portions 28A to 28D having a plurality of steps, for example, four lengths, from the center of the rocking range switching operation shaft 9, that is, the rotation center.

[0081] Specifically, as shown in FIGS. 23 and 24, the locking block 26 includes a cylindrical portion 27 that penetrates the rocking range switching operation shaft 9, a lever-shaped locking portion 28 that protrudes radially outward from the cylindrical portion 27, a spring accommodating portion 32 for accommodating a compression coil spring (not shown) that constantly biases the locking block 26 to the other end side at one end of the cylindrical portion 27, and a convex portion 30 that fits into a concave portion 36 of a fixed block 34 at the other end of the cylindrical portion 27. It is provided so as to be pressed against and fixed (positioned) to the fixed block 34 fixed to the main frame 2 on the other end side on the rocking range switching operation shaft 9 by the spring force of a compression coil spring (not shown) built in at one end.

[0082] The shapes of the uneven portions 30 and 36 between the fixed block 34 and the locking block 26 are not limited to a specific shape as long as they mesh with each other in the axial direction of the locking range switching operation shaft 9 and can be overcome in the circumferential direction when a rotation of a certain magnitude is applied. In the case of the present embodiment, for example, it consists of a combination of a protruding portion and a recessed portion in the shape of a clam shell (medium height and having a semi-circular cross-section). In the case of the present embodiment, one clam shell-shaped protruding portion 30 is provided on the other end side of the cylindrical portion 27 of the locking block 26. On the other hand, as shown in FIG. 25, the fixed block 34 is provided with a plurality of, for example, four clam shell-shaped recessed portions 36 arranged in a fan shape around the locking range switching operation shaft 9 (strictly speaking, a hole 38 through which the locking range switching operation shaft 9 passes).

[0083] That is, the locking range adjustment mechanism 8 in the present embodiment is provided so as to be switchable in four steps at arbitrary intervals, for example, at 6° intervals from a reference position (0°). Further, at both ends (outside the region where the recessed portions 36 are continuously formed) of a group of recessed portions 36 arranged in a fan shape of the fixed block 34 at the same interval, restriction blocks 35 that serve as stoppers for the swing range are respectively formed. The restriction block 35 abuts against the side surface of the fixed base 29 of the locking block 26 to prevent the movement of the locking block 26. Incidentally, the fixed base 29 and the lever-shaped locking portion 28 are enhanced in rigidity by providing ribs. Incidentally, on the other end surface of the cylindrical portion 27 of the locking block 26, a sliding convex portion 31 that is lower than the convex portion 30 and contacts the peripheral surface 37 of the recessed portion 36 of the fixed block 34 is provided, reducing the contact area between the fixed block 34 and the locking block 26 and reducing the force required for the rotation of the locking block 26.

[0084] The lever-shaped locking portion 28 is, for example, divided into four steps at arbitrary intervals, for example, at 6° intervals from a reference position (0°) around the central axis of the cylindrical portion 27, and is formed stepwise so that the length from the rotation center becomes successively shorter or longer. In the case of this embodiment, a semi-circular lever-shaped locking portion 28 having stepped portions of four lengths is formed by continuously forming levers of four lengths. In the case of this embodiment, the lever-shaped locking portion 28 is provided with a pair at intervals, for example, in the middle portion of the cylindrical portion 27, and is connected to each other by ribs so as to enhance rigidity, and is structured to have high mechanical rigidity and be difficult to twist. Of course, the lever-shaped locking portion 28 may be provided at one location, for example, at the center of the cylindrical portion 27. Further, the lever-shaped locking portion 28 is not particularly limited to a fan-shaped one having stepped portions of four lengths on the same plane, and in some cases, stepped portions of four lengths may be formed on four different planes.

[0085] A shaft hole 33 with a key groove formed therein is provided in the cylindrical portion 27. On the other hand, a key 9A is also provided on the locking range switching operation shaft 9. When the locking range switching operation shaft 9 penetrates the cylindrical portion 27, it is supported so as to be axially movable and is provided so as to rotate together in the rotational direction.

[0086] As shown in Fig. 27, the locking range switching operation shaft 9 is divided into two parts: a short first shaft portion 9G having a cylinder receiving portion 9E and a flange 9F, and a long second shaft portion 9H having a key portion 9A and a flange 9D. The second shaft portion 9H is passed through from the outside of one plate 21A of the backrest support member 21 and inserted to the vicinity of the other plate 21A. The first shaft portion 9G is passed through from the outside of the other plate 21A of the backrest support member 21, and the tip of the second shaft portion 9H is fitted into the cylinder receiving portion 9E in the vicinity of the other plate 21A and connected. Thereby, it is installed in a state where the two flanges 9D and 9F are respectively in contact with the outer surfaces of the plates 21A on both sides of the backrest support member 21. The divided locking range switching operation shaft 9 is integrated by press-fitting or caulking after assembly. An operation lever 9B is fitted and screwed to the second shaft portion 9H protruding outside the backrest support member 21. The operation lever 9B and the locking range switching operation shaft 9 are connected in the rotational direction by fitting of a key 9A and a key groove (not shown). The flange 9D made of a washer is provided so as not to come off by a retaining projection 9C formed by partially crushing the shaft portion.

[0087] Grooves for passing the key 9A are formed in the through holes 22 of the plates 21A on both sides of the backrest support member 21. Thus, the second shaft portion 9H may be inserted from either plate 21A.

[0088] As shown in Fig. 25, on the surface of the fixed block 34 opposite to the surface where the recesses 36 are arranged in a fan shape (so-called surface), there are provided a protruding claw 39 that engages with the hole 23 of the plate 21A, and a plate-shaped claw 40 that is applied to and catches on the lower edge of the plate 21A. The protruding claw 39 is a return claw member that deforms to narrow when passing through the hole 23 and returns to its original state after passing through to engage with the plate 21A. Thus, the fixed block 34 can be easily attached by applying the claw portion 40 to the lower edge of the plate 21A while pushing the claw portion 39 into a pair of holes 23 drilled around the through-hole 22 of the backrest support member 21. At the same time as the fixed block 34 is attached to the backrest support member 21, the hole 38 through which the locking range switching operation shaft 9 passes is aligned with the hole 22 of the plate 21A.

[0089] Since the recesses 36 of the fixed block 34 are formed by arranging fan-shaped continuous recesses in the shape of a katamari with the same pitch as the convex portions 30 of the locking block 26, the boundary portions between the recesses 36 become tops with a small radius of curvature. The convex portions 30 of the locking block 26 pressed by a spring (not shown) cannot stop near the top and will fall into one of the bottoms. Therefore, the switching becomes reliable. However, the recesses 36 of the fixed block 34 and the convex portions 30 of the locking block 26 are not limited to the above-described katamari-shaped unevenness, and may be, for example, hemispherical unevenness or other shapes.

[0090] In addition, the buffer block 41 provided on the main frame 2 side is, in the case of this embodiment, for example, a katamari-shaped resin block exhibiting rubber elasticity, and is housed in a recess 19 surrounded by side walls formed at the rear end portion of the main frame 2. A push pin portion 41a is integrally formed on the bottom surface of the resin block 41, and the resin block 41 is fixed to the main frame 2 by fitting the push pin portion 41a into the hole 20 on the bottom surface of the recess 19. This buffer block 41 alleviates the impact and sound when the lever-shaped locking portion 28 of the locking block 26 hits the main frame 2. In some cases, it may be omitted, or it may be provided on the step portions of the lever-shaped locking portion 28 themselves.

[0091] According to the locking range adjustment mechanism 8 of the present embodiment configured as described above, by rotating the locking range switching operation shaft 9, if the stepped portion of the lever-shaped locking portion 28 that abuts against the buffer block 41 of the main frame 2 is selected from among the plurality of stepped portions 28A to 28D, the convex portion 30 of the locking block 26 is fitted into the concave portion 36 at the corresponding position of the fixed block 34, and the inclination angle of the lever-shaped locking portion 28 is fixed. Therefore, when the backrest 4 is about to tilt backward, the stepped portion (any one of 28A to 28D) of the lever-shaped locking portion 28 at the corresponding angle abuts against the buffer block 41 of the main frame 2, and further locking of the backrest can be prevented.

[0092] Note that the above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the gist of the present invention. For example, in the case of the above-described embodiment, the reaction force adjustment cams 51 are provided in a pair at intervals on the reaction force adjustment operation shafts 7 and 70, and the intermediate shaft portions 52 and 92 and the intermediate bearing portion 11 are provided between the pair of cams 51. However, the present invention is not particularly limited to such a cam support structure. One reaction force adjustment cam 51 may be provided, and a pair of backup portions, that is, the intermediate shaft portion 52 and the intermediate bearing portion 11, may be provided on both sides thereof so as to sandwich the reaction force adjustment cam 51. For example, the reaction force adjustment cam 51 may be disposed between the left and right vertical walls 11A of the intermediate bearing portion 11, or a pair of bearing portions independent of each other, such as the outer bearing portion 97 shown in FIG. 31, may be provided. In this case, a flat end surface that abuts against the reaction force adjustment cam 51, for example, at the center of the front surface of the slider 66 is formed.

[0093] Further, in the above-described embodiment, the reaction force adjustment operation shaft 7, the reaction force adjustment cam portion 51, etc. are integrally formed of synthetic resin, but the present invention is not particularly limited thereto, and they may be constituted by integrally processed products by metal machining or forging, or may be constituted by two or more members depending on the case.

[0094] In the above-described embodiment, the backrest is exemplified as being composed of a frame-shaped back frame and a mesh-shaped stretched material stretched thereon. However, this backrest structure is not particularly limited. For example, a backrest structure in which a perforated shell is fixed to the frame-shaped back frame or a back structure in which a cushion is fixed to the back plate and wrapped with a stretched material may also be used.

[0095] In the above-described embodiment, the backrest reaction force applying mechanism 5 is provided with a reaction force adjusting mechanism 6 that can adjust the initial reaction force (the repulsive force of the compression coil spring 65 applied in the initial position state). However, when it is not necessary to provide the reaction force adjusting mechanism 6, it may be configured between the fixed shaft of the main frame 2.

[0096] In the above-described embodiment, as an example of the means for constantly biasing the locking block 26 toward the fixed block 34, a compression coil spring acting in the axial direction of the cylinder portion 27 is used. However, it is not particularly limited to this. For example, a leaf spring may be incorporated, or other members exhibiting spring elasticity, such as a ball plunger, may be used in the convex portion 30 of the locking block 26 for substitution.

[0097] Furthermore, in the above-described embodiment, as a mechanism for fixing the position between two relatively rotating members, mainly an example is described in which it is applied as a positioning member for the locking range switching operation shaft of the locking range adjusting mechanism 8 that can adjust the maximum reclining position at which the backrest 4 can be locked by restricting the movement of a member interlocking with the backrest 4, such as the backrest support member 21. However, it is not particularly limited to this, and any operating member that can exhibit the functions of the chair can be implemented. For example, it can also be applied to a positioning member between the back frame and the headrest.

[0098] In the above-described embodiment, the retaining member 81 is arranged at one end of the seat rotation shaft 42. However, it is not particularly limited to this. By making the bearing holes that support the seat rotation shaft 42 both through holes, it is also possible to insert the retaining members 81 into both ends of the seat rotation shaft 42 respectively to prevent detachment.

[0099] Also, in the above-described embodiment, as an example of a structure for preventing the shaft connecting two relatively rotating members from coming off, an example applied to the seat rotation shaft 42 that connects between the seat outer shell and the main frame has been mainly described. However, it is not particularly limited to this, and it is also applicable to connection structure examples between other constituent members of the chair, for example, the connection structure between the seat outer shell 71 and the backrest support member 21, the connection structure between the headrest and the back frame, or the structure for preventing the shaft connecting two non-rotating members from coming off.

Explanation of Reference Numerals

[0100] 1 Leg 2 Main Frame 3 Seat 4 Backrest 5 Backrest Reaction Force Mechanism 6 Reaction Force Adjustment Mechanism 7 Reaction Force Adjustment Operation Shaft 8 Locking Range Adjustment Mechanism 9 Locking Range Switching Operation Shaft 10 Cover Member 11 Intermediate Bearing Portion 12 Long Hole 13 Round Hole 14 Open-Type Bearing Portion 15 Threaded Hole 16 Round Hole 17 Bearing Portion 18 Round Hole 19 Dimple 20 Pin Hole 21 Backrest Support Member 22 Hole 23 Hole 24 Nut 25 Backrest Rotation Shaft 26 Locking Block 27 Cylindrical Portion 28 Lever-Like Locking Portion 29 Fixed Base 30 Protrusion 31 Sliding Protrusion 32 Spring Accommodation Portion 33 Operation Shaft Hole with Key Groove 34 Fixed Block 35 Regulation Block 36 Concave part 37 Edge part 38 Hole 39 Claw 40 Claw part 41 Buffer block 42 Seat rotation axis 43 Operating shaft retainer 44 Bearing part 45 Protrusion (clamping the middle partition wall) 46 Through hole 47 Fixing part for passing screws 48 Groove (passing through the middle partition wall) 49 Regulation protrusion (stopper) 50 Slider housing 51 Reaction force adjustment cam 51f Cam basic circle 51a, 51b, 51c, 51d Cam surfaces 52 Intermediate shaft part 53 Shaft part 54 Regulation protrusion (stopper) 55 Tip shaft part 56 Shaft part 57 Shaft part 58 Shaft part 59 Shaft part 60 Spring - type backrest reaction force mechanism 61 Body - weight - sensitive reaction force applying mechanism 62 Front spring mount 63 Rear spring mount 64 Mounting pin 65 Compression coil spring 66 Slider 66a Cam receiving surface (contact) 67 Outer bearing part 68 Protrusion protruding towards the intermediate shaft part 69 Semi - circular open - type bearing part 70 Reaction force adjustment operating shaft 71 Outer shell 72 Inner shell 73 Cushion 74 Peripheral wall part of the bottom surface of the outer shell 75 Concave part 76 Slit 77 Blind round hole (bearing for the seat rotation axis) 78 Through-hole (bearing for the base rotation shaft) 79 Vertical hole 80 Opening (for inserting the base rotation shaft) 81 Retaining member 82 Return 83 Step portion 84 Hook-shaped bearing portion 85 Hooking portion of the retaining member 86 Inclined surface portion (retaining member) 87 Locking portion 90 Recess 91 Cover plate 92 Intermediate shaft portion 93 Shaft portion 94 Lateral runout prevention portion 95 Tip shaft portion 96 Projection of the reaction force adjustment operation shaft 97 Projection of the insertion hole 98 Insertion hole

Claims

In the backrest reaction force mechanism of a chair that generates a force to push back the backrest by compressing a reaction spring interposed between a main frame with the backrest swingably supported and the backrest in conjunction with the backward tilt of the backrest, one end of the reaction spring includes a reaction force adjustment cam that gives a cam displacement for expanding and contracting the reaction spring, a reaction force adjustment operation shaft that supports the reaction force adjustment cam and is rotatably supported by the main frame, and a slider that is held linearly movably on a single axis connecting the reaction force adjustment operation shaft and a spring support shaft provided on the backrest. The reaction force adjustment mechanism adjusts the initial compression amount at the initial position of the reaction spring by switching the amount of cam displacement applied to the reaction spring by rotating the reaction force adjustment operation shaft. The reaction force adjustment operation shaft is made of synthetic resin, and both sides of the reaction force adjustment cam are rotatably supported by a bearing portion that supports the force of the reaction spring applied to the reaction force adjustment operation shaft via the reaction force adjustment cam. A reaction force adjustment mechanism of a backrest reaction force mechanism of a chair, characterized by the above. The reaction force adjustment mechanism of the backrest reaction force mechanism of the chair according to claim 1, characterized in that the reaction force adjustment cam is integrally formed with the reaction force adjustment operation shaft by synthetic resin. The reaction force adjustment mechanism of the backrest reaction force mechanism of the chair according to claim 1, characterized in that the reaction force adjustment cam is a planar cam having a polygonal cam contour with the cam displacement amount switched stepwise, and the cam receiving surface of the slider in contact with the reaction force adjustment cam is a flat surface.

4. The reaction force adjustment cam is provided at two positions spaced apart on the reaction force adjustment operation shaft. Between the two reaction force adjustment cams, there are arranged a bearing portion and a left - right swing prevention portion that engages with the bearing portion in the axial direction to prevent the axial movement of the reaction force adjustment operation shaft. A reaction force adjustment mechanism of the backrest reaction force mechanism of the chair according to claim 1, characterized by the above. The bearing portion is composed of a semi - circular open - type bearing that opens toward the reaction spring side, and the reaction force adjustment operation shaft is rotatably clamped between the bearing portion and an operation shaft retainer that covers the upper half of the reaction force adjustment operation shaft, and is held so as not to come off axially. A reaction force adjustment mechanism of the backrest reaction force mechanism of a chair according to any one of claims 1 to 4, characterized by the above.

6. A chair characterized by comprising a reaction force adjusting mechanism for a backrest reaction force mechanism of the chair according to any one of claims 1 to 5.

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