Abstract:
The chair mechanism is configured as a so-called synchronizing mechanism in order, in the case of an adjustment in inclination of the rear support ( 4 ), to achieve simultaneous, harmonic adjustment of the seat panel ( 8 ). The underframe is positioned on a height-adjustable pneumatic spring ( 13,14 ). For the synchronized movement, use is made of a mechanical torsion spring, preferably a rubber spring, and a further mechanical spring, preferably a helical spring, is arranged parallel thereto, with accumulative effect in relation to the resistance of the torsion spring. The strength of the torsion spring can be adjusted. The positioning of the axes of rotation (A 1 -A 4 ) is of considerable importance for the kinematics. The chair can be adjusted from an assembly position, where the springs can be inserted in a state in which they are relieved of stressing, via the vertical position into the inclined position. The most prominent advantages of the chair mechanism are the large opening angle between the seat panel ( 8 ) and backrest in the inclined position, the adjustable degree of stiffness, and the compliant seat shell.

Description:
FIELD OF THE INVENTION 
     The invention relates to a chair mechanism, in particular for a rotatable office chair with a pneumatic spring which is arranged vertically in the under-frame and is intended for adjusting the height of the seat. The chair mechanism causes the seat panel to be lowered synchronously with the rearward inclination of the backrest, which the user initiates by shifting his/her weight. 
     BACKGROUND OF THE INVENTION 
     Office workers spend most of their working day in a sitting position. Staying in a writing position, in which the body is bent forward to a pronounced extent, over a number of hours at a computer workstation results in considerable straining of the skeleton, the muscles and the tissue of the back and the neck region. As a result, many attempts have been made to improve the geometry of chairs, and the mechanisms thereof, in order, in accordance with the respective sitting posture, to relieve the strain on the user more effectively. A decisive step in development was made by synchronizing mechanisms, where, with the adjustment of the backrest, via lever and articulation connections, the seat follows synchronously and, with each inclined position of the backrest, an ergonomically adapted position of the seat surface is established. Such a synchronizing mechanism is disclosed in CH-A-629 945. This mechanism allows the user to move out of the upright working position, by shifting his/her body weight in the rearward direction and applying pressure against the backrest, which is spring-mounted in a damped manner, into a rearwardly inclined, relaxing position, e.g. during a lengthy telephone call. The seat follows the inclining backrest, with the result that a rearwardly inclined, strain-relieved, resting position is achieved for the user. These chairs allow easy and frequent changeover into a relaxed user position. 
     It is usually the case, however, that office work can only be done in a forwardly bent position, rather than in a relaxed, rearwardly inclined position, in order to be in contact with the documents on the desk. In order to reduce the pressure of the seat against the lower leg and to force the user to straighten his/her spine, mechanisms in which, in the forwardly inclined position, the front seat surface is lowered as a result of the shift in weight are proposed. Moreover, attempts have been made, by specific shaping of the backrest with a lumbar support and at least simultaneous raising of the rear seat surface, to prevent the harmful rearward-rolling movement of the pelvis and a curved back. However, the backrest only provides support if one is seated properly in the chair and has definite contact with the backrest. In reality, the back region is not given support since one is usually seated in the vicinity of the front edge of the chair. EP-B-0 592 369 has thus proposed a synchronizing mechanism where, in the case of forwardly bent and upright sitting positions, the seat is lowered in the forward direction and, in the case of a shift in weight, the user slides into a rearwardly inclined, relaxing position. 
     The springs used are of critical importance for the functioning of a chair mechanism. In order to execute the synchronized movement between the backrest and seat panel, use is frequently made of pneumatic compression springs which, in order to improve the kinematics, are often coupled to helical compression springs (e.g. CH-A-629 945). However, pneumatic compression springs involve a number of disadvantages: they increase the cost for the overall chair to a considerable extent and, in order to achieve a favorable movement sequence, have to be coupled to mechanical springs, which further increases the cost and complexity of the design. Furthermore, the wear on the seals means that pneumatic compression springs only have a limited service life. WO-A-92/03072 has thus proposed using a mechanical spring in the form of a torsion bar for the spring mounting during the synchronized movement. 
     WO-A-93/25121 proposes using a rubber spring with an embedded steel core and an outer steel casing, the inner steel core being subjected to torsion. In the normal position of the seat, the use of such a rubber spring provides too low a level of stiffness, as a result of which this sitting position does not have the full amount of stability desired. This is not improved in any way by the envisaged positioning of the axes of rotation, i.e. an approximately central, rocker-like suspension of the seat panel and the articulation of the seat panel in the rear region. 
     OBJECT OF THE INVENTION 
     With the number of computer workstations constantly increasing, one is increasingly forced into an upright sitting position, with the neck region permanently tensed, in order to view the screen. This means a change in the typical working position for many people and, consequently, different requirements have to be met by the mechanisms of a chair in order to optimize the kinetic behavior of the latter. The prior-art synchronizing mechanisms are advantageous for users whose work allows them to change position frequently, namely between forwardly bent, upright and rearwardly inclined sitting positions. However, the known designs are still not ideal for individuals who have to sit permanently at a computer workstation. 
     The object of the invention is thus to provide a mechanism which provides, as a permanent working position, a rearwardly inclined sitting position in which the body is largely free of strain and which has a wide opening angle between the seat surface and rear surface. In the front sitting position, it is necessary to realize effective support for the lumbar region, while, in the rearwardly inclined sitting position, partial sinking into the chair is intended to provide a relaxed sitting position without the eye-contact angle having to be changed to any significant extent. In the relaxed, rearwardly inclined sifting position, the mechanism and the overall chair construction are intended simultaneously to allow a certain capacity for movement, and to provide noticeable support for the shoulders, for the user in the chair. The seat and the backrest are intended to be height-adjustable. Also desired is the capacity for regulating the seat depth and the intensity of the lumbar support as well as an adjustable neck support. The intention is for the chair to be mass-producible efficiently and cost-effectively. 
     Finally, the design must correspond to current tastes. 
     SUMMARY OF THE INVENTION 
     The chair mechanism of the seat rests on an underframe with a base—usually a standard five-armed star-shaped base—which is positioned on the floor, and a vertically arranged pneumatic spring with a telescopically extendible piston rod for adjusting the height of the chair. The seat is positioned on the piston rod, by way of the seat support arranged at the bottom, as a structural unit. Extending through the seat support is the horizontal main axis of rotation with the torsion spring, preferably a rubber spring, positioned along the same. Acting on the torsion spring is the rear support which can be pivoted about the main axis of rotation, counter to the resistance of the torsion spring. The chair also has a seat panel for receiving a seat cushion, possibly with the interposition of a cushion support. The seat panel, on the one hand, is articulated on the rear support along a horizontal rear axis of rotation and, on the other hand, is connected to an articulation along a horizontal front axis of rotation. The articulation is connected to the seat support along a horizontal fixed axis of rotation. The rear support and seat panel execute synchronous changes in position as they are pivoted between a vertical position and an inclined position. 
     The essence of the invention is that provided parallel to the torsion spring, and with accumulative effect in relation to the resistance of the torsion spring, is a further mechanical spring. The parallel spring is preferably a helical spring which can be separated into two symmetrically arranged parts. Another factor which is critical for the properties of the mechanism is constituted by the positions of the axes of rotation, which are all located beneath the seat panel. The front axis of rotation—as connection between the seat panel and articulation—is provided in the vicinity of the front edge of the seat panel, it being possible for the front axis of rotation to be pivoted radially over part of a circle about the fixed axis of rotation—as connection between the articulation and seat support. The rear axis of rotation—as connection between the seat panel and rear support—is located in the vicinity of the rear edge of the seat panel, it being possible for the rear axis of rotation to be pivoted radially over part of a circle about the main axis of rotation. The main axis of rotation is positioned between and beneath the front and the rear axes of rotation. 
     The fixed axis of rotation should preferably be located above the plane of the main axis of rotation and, in the vertical position of the chair, the front axis of rotation should be more or less vertically above the fixed axis of rotation. In the maximum inclined position, the rear axis of rotation should be capable of being lowered at least into the vicinity of the plane of the main axis of rotation. In the vertical position of the chair, the front axis of rotation is arranged in front of the fixed axis of rotation, in relation to the main axis of rotation. Starting from this chair position, the ratio of the horizontal spacing between the fixed axis of rotation and main axis of rotation and between the main axis of rotation and the rear axis of rotation is in the region of 1:2. In the maximum inclined position, the seat panel assumes a sloping position in the region of 15°. 
     The actually conventional torsion spring comprises the inner steel core, the rubber layer which is firmly applied to the steel core, and the outer steel casing which is firmly applied to the rubber layer. Extending through the steel core, along the main axis of rotation, is a polygonal through-passage into which a polygonal carry-along rod of complementary profile is inserted. The carry-along rod has its outer ends positioned in the free ends of the rear support, the latter being designed in the form of a fork with two support arms and said free ends being provided with a polygonal socket. Fastened on the outer steel casing of the torsion spring is an adjustable drawing element for adjusting the prestressing of said spring. 
     The parallel spring is arranged along the front axis of rotation and is supported, as a helical spring, beneath the seat panel by way of one end, while the other end of the helical spring is fixed on a pretensioning bolt. The pretensioning bolt is located along the front axis of rotation, is firmly inserted into the articulation and is positioned rotatably in front articulation extensions provided on the underside of the seat panel. There are in each case two articulations, pretensioning bolts, helical springs and pairs of articulation extensions, arranged symmetrically in relation to the torsion spring. 
     The rear support, between its two support arms, has a bearing bushing which is arranged beneath the seat panel, through which the rear axis of rotation extends and on which, positioned on an axial rod, rear articulation extensions provided on the underside of the seat panel are articulated. Above the seat panel, the two support arms are combined in a U-shaped guide which serves for receiving a backrest in a height-adjustable manner. The seat support comprises first of all the central sleeve body, with the main axis of rotation running through it, for receiving the torsion spring. The sleeve body is adjoined at the rear by the plug-on flange with its conical bore which runs along the vertical axis and is intended for positioning on the piston rod of the pneumatic spring. The bearing flange adjoins the front of the sleeve body. Said bearing flange has a through-passage bore which runs along the fixed axis of rotation, parallel to the main axis of rotation, and is intended for receiving an axial rod on which the articulations are arranged, on either side of the bearing flange. The sleeve body has a cutout for fitting the drawing element on the outer steel casing of the torsion spring. 
     The backrest comprises first of all the bottom, base member for insertion into the guide on the rear support, and an upwardly extending central member which adjoins the base member. Two lumbar extension arms which extend out horizontally in arcuate form symmetrically between the base member and central member serve for retaining and tensioning a flexible back shell. Two supporting arms which open in the form of a Y extend out from the top of the central member. The free ends of the lumbar extension arms have through-passage bores for fastening a back shell, while the ends of the supporting arms are positioned in pockets provided on the back shell. An inclination-adjustable headrest may be fitted on the backrest. The tensioning of the back shell in the lumbar region can be adjusted in a variable manner by means of eccentrics arranged in the lumbar extension arms. The back shell is spanned by a covering to the full extent on the user side and partially on the rear side, a shaped pocket and quick-action closures being provided in order to fix the covering. The headrest can be adjusted to a desired setting angle and selectable height. 
     The essential advantages of the chair mechanism according to the invention lie in the large opening angle between the seat panel and backrest in the inclined position, the adjustable reaction capability of the chair when the user shifts his/her weight, and the flexible, compliant seat shell, with the result that it is possible to take up the relaxed inclined position as a permanent working position. The intensity-adjustable lumbar support, the selectable height adjustment of the backrest and a headrest which can be adjusted in a variable manner in terms of setting angle provide a high level of sitting comfort, which is particularly important when one is working with a keyboard and at computer workstations. In the inclined position, the front edge of the seat panel is lowered and the entire seat moves in the rearward direction to a considerable extent. The optimum seat level thus provides the user&#39;s body with effective support for his/her back and shoulders without eye contact with his/her screen being lost and the neck muscles being strained. The preferably mesh-like covering of the back shell—and possibly also of the head cushion and armrests—gives pleasant seat-climate conditions and a decorative effect. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     In the drawings: 
     FIG. 1A shows a lateral perspective view of an entire chair according to the invention in the vertical position; 
     FIG. 1B shows the chair according to FIG. 1A in a perspective view from the rear; 
     FIG. 1C shows the chair according to FIG. 1A as an exploded illustration in subassemblies with an armrest; 
     FIG. 2A shows an exploded illustration of a rear support, a backrest and a headrest; 
     FIG. 2B shows the rear support and the backrest from FIG. 2A with a covered back shell; 
     FIG. 3A shows the arrangement beneath the seat panel on the height-adjustment pneumatic spring with rear support; 
     FIG. 3B shows the arrangement according to FIG. 3A as an exploded illustration; 
     FIG. 3C shows, partially in section, the structural unit comprising rear support, seat panel and rubber spring positioned on the height-adjustment pneumatic spring, in the vertical position; 
     FIG. 3D shows a perspective view, partially in section, of the structural unit according to FIG. 3C; 
     FIG. 4 shows an exploded illustration of the rubber spring with housing from FIG. 3A; 
     FIG. 5 shows the structural unit according to FIG. 3C in the movement sequence between the assembly position, via the vertical position, to the inclined position; 
     FIGS. 6A to  6 D show the functioning of the adjustment of the seat level; 
     FIG. 6A shows a section along line A—A from FIG. 3A with the actuating lever secured; 
     FIG. 6B shows the illustration according to FIG. 6A with the actuating lever released; 
     FIG. 6C shows a section along line B—B from FIG. 3A with the cushion support drawn forward to the maximum extent and the actuating lever secured; and 
     FIG. 6D shows the illustration according to FIG. 6C with the cushion support pushed rearward to the maximum extent and the actuating lever released. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The detailed description of an exemplary embodiment of the chair mechanism according to the invention is given hereinbelow with reference to the attached drawings. 
     The following applies to the rest of the description: if, in order to avoid ambiguity in the drawings, a figure contains designations which are not explained in the directly associated text of the description, then you are referred to the point at which they are mentioned in prior or subsequent figure descriptions. For reasons of clarity, components are not usually designated again in subsequent figures, provided that it is clear from the drawings that they are “recurring ” components. 
     FIGS. 1A and 1B 
     The chair as a whole is divided into two levels: the under-frame U, which is known per se, and the seat S, which is positioned on the under-frame U and embodies the invention. The under-frame U comprises a typical five-armed star-shaped base  1  with castors  11  which are attached to the ends of the arms  10  and are positioned on the floor. The center of the star-shaped base  1  is formed by a sleeve part  12  in which a pneumatic spring  13  is inserted vertically. A telescopically extendible piston rod  14  projects out of the pneumatic spring  13 , along the axis A, and the seat support  2 , which constitutes the basic part of the seat S as a whole, is positioned on said piston rod. It is possible for the seat S both to be rotated about the axis A and, with the extension and retraction of the piston rod  14 , to be adjusted in terms of height along the axis A. 
     The seat support  2  comprises a central sleeve body  20 , a rearwardly projecting plug-on flange  21  and a bearing flange  22  which adjoins at the front. Extending through the sleeve body  20  is the horizontal main axis of rotation A 1 , along which a conventional rubber spring  3  is received. The vertical axis A runs through the plug-on flange  21  and an axis of rotation A 2 , which is parallel to the main axis of rotation A 1 , runs through the bearing flange  22 . The chair further comprises a rear support  4 , a backrest  5 , a headrest  6 , a back shell  7 , a seat panel  8  and the optionally attachable armrests (not illustrated). A cushion support  80  is arranged on the seat panel  8  in order to receive the seat cushion  801  directly. The seat support  2  along with the inserted rubber spring  3  are positioned beneath the seat panel  8 , the main axis of rotation A 1  being located more or less parallel to the front edge  810  of the seat panel  8 . 
     Fastened along the main axis of rotation A 1  is the rear support  4 , which has its two support arms  40  adjoining the rubber spring  3  on either side in the form of a fork and extends upward in bent form from beneath the seat panel  8  at a spacing from the rear edge  811  of the seat panel  8 . Above the level of the seat panel  8 , the support arms  40  combine to form a guide  41  in which the height-adjustable backrest  5  is secured by way of its bottom, base member  50 . The adjusted height is fixed by means of a clamping screw  42  which engages through the guide  41  of the rear support  4 . Above the base member  50 , two lumbar extension arms  51  extend out symmetrically, approximately horizontally and in arcuate form, and have their outer ends fastened on the back shell  7 , in the bottom lumbar region  70  of the latter. The lumbar extension arms  51  enclose the lumbar region  70  of the back shell  7 , and it would be possible to provide vertical extension slots in the lumbar region  70  in order to increase the elasticity of the back shell  7 . Above the lumbar extension arms  51 , the backrest  5  is in the form of a Y with a bottom, central member  52  and two supporting arms  53  which are spread out in a wing-like manner and of which the ends are positioned in pockets  710  which are located in the top corners of the rear side of the back shell  7 . The vertical strut  60  of the headrest  6  is attached at the point where the supporting arms  53  branch off from the central member  52 . Secured at the top of the vertical strut  60  is a head cushion  61  which is directed toward the rear of the user&#39;s head, it being possible for the inclination of the vertical strut  60  to be adjusted. 
     Arranged beneath the seat panel  8 , in the vicinity of the front edge  810  and in the vicinity of the rear edge  811 , are pairs of spaced-apart articulation extensions  820 , 821 . Arranged on the bearing flange  22  of the seat support  2 , along the axis of rotation A 2  of the same, are articulations  9 , which engage over the front articulation extensions  820  with the horizontal axis of rotation A 3 . Along a horizontal axis of rotation A 4 , the rear articulation extensions  821 , which are located in the vicinity of the rear edge  811  of the seat panel  8 , are each connected to a support arm  40  of the rear support  4 . If the rear support  4  and backrest  5  are inclined in the rearward direction, the seat panel  8  is synchronously lowered and carried along in the rearward direction. 
     Provided for the purpose of covering over the back shell  7  is a covering  73  which, at the bottom, has a rearwardly directed pocket  730  which is open at the top. The bottom edge  72  of the back shell  7  is positioned in this pocket  730  approximately as far as the lumbar region  70 . The covering  73  spans the front side of the back shell  7  to the full extent and has a top portion  731  for folding over the top edge  71  and two lateral portions  732  for folding over the two side edges of the back shell  7 . Cutouts  733  run between the top portion  731  and the two lateral portions  732 , with the result that, in the folded-over state, the pockets  710 , which are provided on the back shell  7 , remain accessible for positioning the ends  630  of the supporting arms  63 . Provided between the top portion  731  and the respectively adjacent lateral portion  732  is a quick-action connection  734 —e.g. a hook or touch-and-close connection—which engages beneath the supporting arms  53 . It is also the case that the lateral portions  732  do not span those ends of the lumbar extension arms  51  which are fastened on the back shell  7 , as a result of which the central area of the back shell  7  remains exposed at the rear. With the quick-action connections  734  released, the covering  73  can thus be quickly drawn onto the back shell  7  during assembly and likewise quickly removed for the purpose of cleaning or renewal. 
     FIG. 1C 
     The under-frame U with the star-shaped base  1  and the pneumatic spring  13 , which is inserted vertically and centrally therein and is intended for adjusting the height of the seat S, can be seen in a state in which they have been dismantled into relatively large subassemblies. Of the seat S, it is possible to see the seat panel  8 , beneath which the majority of the chair mechanism is located and the upwardly projecting rear support  4  is fastened. Positioned on the seat panel  8  is the cushion panel  80  which can be adjusted with the seat level and bears the seat cushion  801  and a covering  802  which spans the latter. Also depicted are the back shell  7  and the headrest  6  with the head cushion  61  and the downwardly extending vertical strut  60  for fastening on the backrest  5 . It is practical and decorative for the head cushion  61  to be provided with a covering  610  which can be changed quickly, e.g. a mesh covering is effective from a visual point of view. 
     In order to finish off the chair, the latter may be equipped with laterally arranged armrests  65 , of which the supporting arms  66 , which are oriented upward in arcuate form, are preferably likewise fitted beneath the seat panel  8 . At the top, the supporting arm  66  has an actuating device  67  for adjusting the height and the inclination of the armrests  68  positioned therein. The armrest  68  has a vertical support strut  69 , projecting through the actuating device  67 , a supporting plate  680  and the padding  681  arranged thereon. This padding  681  is preferably provided with a covering  682 , e.g. likewise a decorative mesh covering. 
     FIG. 2A 
     Arranged between the two support arms  40 , which adjoin the rear support  4  in the form of a fork beneath the guide  41 , is a box-like bearing bushing  43  which is open at the top and in the two side walls  430 —these butting against the support arms  40 —of which there are provided two aligned bores  431  which are positioned along the axis of rotation A 4 . A front and a rear transverse wall  432 , 433  extend between the side walls  430 . The rear articulation extensions  821 , which extend from the seat panel  8 , project into the bearing bushing  43 . An axial rod  44  is pushed through the bores  431  in the bearing bushing  43  and complementary bores provided in the articulation extensions  821 , with the result that the seat panel  8  is fastened rotatably on the rear support  4  along the axis A 4 . In order to fix the axial rod  44 , use is made, for example, of in each case one lateral screw-connection  440  which is supported against the side walls  430  from the outside. 
     The front transverse wall  432  is directed toward the free ends of the support arms  40 , through which the main axis of rotation A 1  runs, while the rear transverse wall  443  is located opposite the front transverse wall  432  and is directed toward the guide  41  of the rear support  4 . A depression  435  is made in the rear transverse wall  433  from above in order to receive a pretensioning wedge  45  (see FIG.  3 B), of which the function will be explained at a later stage in the text. Along the main axis of rotation A 1 , the free ends of the support arms  40  each have a continuous polygonal socket  401 , a polygonal-profile carry-along rod  46  being positioned in a rotationally stable manner through the two polygonal sockets  401  and the rubber spring  3 . The carry-along rod  46  is fastened, for example, by in each case one screw-connection  460  which is supported laterally against the support arms  40  from the outside. 
     For the through-passage of the clamping screw  42 , the U-profiled guide  41 , which is open in the direction of the seat S, has a through-passage bore  410 , and a slot  500  with a vertical extent is provided, to complement said through-passage bore, in the base member  50  of the backrest  5 . The height adjustment of the backrest  5  can thus take place within the slot  500 . The lumbar extension arms  51 , which extend to both sides of the backrest  5 , have through-passage bores  510  at their free ends. Provided for the interaction with the lumbar extension arms  51  are two eccentrics  55  which comprise a grippable turning knob  550 , a cylinder portion  551 , which can be inserted into the through-passage bores  510 , and a tensioning bolt which projects eccentrically through the cylinder portion  551 . Rotation of the eccentric  55  changes the spacing between the two tensioning bolts, and the tensioning in the lumbar region  70  of the back shell  7  can thus be adjusted. With more flexible tensioning, the lumbar region  70  of the back shell  7  becomes more elastically compliant for the user. The tensioning bolts engage in the back shell  7 , with the result that the backrest  5  and back shell  7  can be firmly connected to one another. 
     Provided on the central member  52  of the backrest  5 , in the vicinity of the point at which the supporting arms  53  branch off, is a screw hole  520  which serves for fastening the headrest  6 . The free ends  530  of the supporting arms  53  could also have through-passage bores in order for it to be possible to insert screws for fastening on the back shell  7 . However, the ends  530  are advantageously positioned in pockets  710  which are located on the rear side of the back shell  7 , in the top corners. 
     FIG. 2B 
     For improved climatic conditions of the back shell  7 , the latter is spanned by a decorative covering  73 , e.g. a mesh, which can be changed quickly. The tensioning of the covering  73  means that, in the non-loaded state, said covering does not rest on the concavely curved back shell  7 , as seen by the user, in the central region. The covering  73  thus lifts off from the back shell  7  as soon as the pressure applied by the user is eliminated. This allows both the back shell  7  and the covering  73  to breathe and give off any moisture received. Furthermore, a decorative visual effect is achieved. The side walls  430  of the bearing bushing  43  are contoured in ledge form, with the result that the supporting arm  66 , of the armrest  65 , positioned there is retained particularly well by way of positive locking and stops. 
     FIGS. 3A and 3B and  6 A to  6 D 
     This pair of figures serves merely for illustrating the construction of the chair mechanism and, in conjunction with the sequence of FIGS. 6A to  6 D, for describing the actuating device for the seat level. Arranged in the seat panel  8  is a lever  803  which is angled twice and of which the grip part  804  projects laterally for actuation by the user and of which the plate-like securing part  805  comes to rest in a cutout  812  of the seat panel  8 . Seated beneath the securing part  805  is a spring tongue  806  which is supported on the seat panel  8  and thus presses the securing part  805  in the upward direction with prestressing. Seated on the securing part  805  are three noses  807  which project upward in a row and are intended for engaging in a complementary grid arrangement of holes  800  beneath the cushion support  80  (see FIGS. 6A to  6 D). With actuation of the lever  803 , the noses  807  and the grid arrangement of holes  800  are disengaged, with the result that the cushion support  80  can be displaced with the seat level in stages from a maximum forward position (see FIG. 6C) to a furthest-rearward position (see FIG.  6 D). Such an adjustment range could cover, for example, 50 mm in 5 stages of 10 mm. 
     The seat panel  8  has laterally projecting edges  814  which are cut out at the bottom and beneath which the cushion support  80 , positioned thereon, grips by way of angled-in claws  808  arranged on its underside, with the result that the cushion support  80  is guided, with the seat level, on the seat panel  8 . In the front region, the cushion support  80  has, on its underside, a stop edge  809  which, in the case of the furthest-rearward position of the cushion support  80 , strikes against the front edge  810  of the seat panel  8 . Two spaced-apart stop protrusions  813  are provided on the top side of the seat panel  8 , in the vicinity of the rear edge  811  thereof, for bounding purposes in the furthest-forward position, the cushion support  80  striking against said stop protrusions when it is drawn forward to the full extent (see FIGS.  3 A and  3 B). 
     FIGS. 3C to  4   
     In relation to FIGS. 3A and 3B, the seat panel  8  according to FIGS. 3C and 3D is a simplified embodiment without the possibility of the seat level being adjusted. The rubber spring  3  is in the form of a roller and is of three-layered construction. An innermost steel core  30  has a polygonal—e.g. hexagonal—through-passage  300  which is located, along the main axis of rotation A 1  and is intended for receiving the carry-along rod  46  in a positively locking manner. The carry-along rod  46  positioned in the rubber spring  3  is seated, on either side, in a rotationally stable manner in the polygonal socket  401  at the free end of the support arms  40 . Applied to the steel core  30  is a rubber layer  31  which is enclosed by an outer seal casing  32 . From the steel casing  32 , carry-along elements  320  project into the rubber layer  31 . Said rubber spring  3  is positioned in the sleeve body  20  of the seat support  2 , it being possible for slide rings  33 —e.g. made of plastic—to be applied to the steel casing in order to reduce the friction with the inner wall of the sleeve body  20 . The plug-on flange  21 , which is oriented in the rearward direction on the seat support  2 , has, along the axis A, a vertical conical bore  210  in which the top portion of the piston rod  14 , which can be extended from the pneumatic spring  13 , is positioned. Projecting axially out of the piston rod  14  is the valve stem  15 , the piston rod  14  retracting or extending upon actuation of the same, depending on the loading of the seat S. The valve stem  15  is actuated in a conventional manner via a switching lever (not illustrated), which is usually provided beneath the seat panel  8 . 
     Arranged opposite the plug-on flange  21 , oriented in the forward direction from the sleeve body  20 , is the bearing flange  22 , through which the through-passage bore  220  extends along the axis of rotation A 2 . Between the plug-on flange  21  and the bearing flange  22 , the sleeve body  20  has a cutout  200  for the insertion of a drawing element  23  which is fastened on the outer steel casing  32 , e.g. by a screw-connection  230 . A drawing bolt  24  is fitted into the drawing element  23 , the drawing bolt  24  projecting through the bearing flange  22 , and a hand wheel  26  which is supported on the bearing flange  22  being screwed onto the drawing bolt  24 . As the hand wheel  26  is screwed on, the drawing element  23  is drawn in the direction of the bearing flange  22  and the rubber spring  3  is thus prestressed further. 
     Arranged on either side of the bearing flange  22 , along the axis of rotation A 2 , are the rotatable articulations  9 , which engage over the axis of rotation A 3  and are connected there in a rotatable manner to the front articulation extensions  820  on the underside of the seat panel  8 . The articulation extensions  821  provided at the rear of the underside of the seat panel  8  are articulated in the bearing bushing  43 , along the axis of rotation A 4 , said bearing bushing being seated between the support arms  40  of the rear support  4 . The carry-along rod  46 , which is positioned in the rubber spring  3 , is seated, on either side, in a rotationally stable manner in the polygonal socket  401  at the free end of the support arms  40 . 
     The seat panel  8  has a depression  83  comprising two sections  830 , 831 . One depression section  830  is made from above and is located in front of the bearing bushing  43 , as seen from the front edge  810  of the seat panel  8 . Directly adjoining the depression section  830  is a depression section  831 , which is made from beneath and is positioned above the bearing bushing  43 , between the articulation extensions  821 . It is thus possible to insert into the depression section  830  a locking slide  84  which, loaded by a spring  86 , is always pushed partially into the depression section  831 , with the result that the locking slide  84  is positioned on the front transverse wall  432  of the bearing bushing  43  such that it engages beneath the seat panel  8  (FIG. 3C shows the unlocked state). This means that the vertical position of the chair is secured. Even if the user applies substantial pressure against the backrest  5  and/or the rear support  4 , the rear support  4  and the backrest  5  attached thereon remain in a prestressed, more or less vertical position. The locking slide  84  is adjoined by a restoring mechanism  85  which can be actuated by a switching lever  87  in order for the locking slide  84 , if required, to be drawn back, counter to the action of the spring  86 , into the unlocked position, which is shown in FIG.  3 C. It is only then that the rear support  4  and backrest  5  can be moved in the rearward direction into the inclined position. The locking slide  84  remains, by passing beyond a dead-center position of the restoring mechanism  85 , in the drawn-back state, i.e. the rear support  4  remains movable. The rear support  4  is prevented from pivoting forward about the main axis of rotation A 1  when the seat S has been completely relieved of loading by the pretensioning wedge  45 , which is placed in the depression  435  on the rear transverse wall  433  of the bearing bushing  43  and presses against the seat panel  8  from beneath. 
     It can also be seen in FIG. 3D that beveled sections are provided on either side in the vicinity of the rear edge  811  of the seat panel  8 ; the armrests are also attached here. Provided in relation to the two front articulation extensions  820  on the underside of the seat panel  8  which have been described hitherto are two further, inner articulation extensions  822 , which are likewise located along the axis of rotation A 3  and are each offset in the direction of the seat support  2 , with the result that the articulation extensions  820  are located on the outside, and the two inner articulation extensions  822  are located opposite one another and are each spaced apart from the associated outer articulation extension  820 . Provided on the outside of the outer articulation extension  820  in each case is an articulation  9  which has its articulation leg  90  oriented in the direction of the axis of rotation A 3  and in which a through-passage bore  900  is located along the axis of rotation A 3 . Bores which are aligned with the through-passage bore  900  are likewise provided in the articulation extensions  820 , 822 , located along the axis A 3 , as a result of which it is possible to insert as a spindle, along the axis of rotation A 3 , a pretensioning bolt  88  which extends from the articulation leg  90 —with its head  880  firmly seated therein—through the two associated articulation extensions  820 , 822 . At the shank end  881 , the pretensioning bolt  88  has a transverse slit  882 . Positioned on the inserted pretensioning bolt  88 , between the pair of articulation extensions  820 , 822 , is a helical spring  89 , which has, at one end, a bent-over section  890  extending over the spring diameter and, at the opposite end, a tangential bent-over section  891 . The tangential bent-over sections  891  of the two helical springs  89  each butt against the underside of the seat panel  8 , while the bent-over sections  890 , which extend over the spring diameter, are positioned in the transverse slit  882  of the pretensioning bolt  88 . 
     This means that the effect of the two helical springs  89  is added accumulatively to the effect of the rubber spring  3 . A user who is sitting on the chair and whose body weight bears against the rear support  4  and/or the backrest  5 —the locking slide  84  being drawn back, that is to say unlocked—is supported simultaneously by the effects of the rubber spring  3  and the helical springs  89 . The rubber spring  3  and the helical springs  89  are arranged in parallel. When the rear support  4  is inclined in the rearward direction, the carry-along rod  46  rotates in the rubber spring  3 , counter to the increasing action thereof, and the seat panel  8  is guided along synchronously at the same time, with the result that the helical springs  89  are increasingly rotated and subjected to stressing in the process. For adaptation to individual requirements, the overall spring resistance of the parallel arrangement comprising the helical springs  89  and the rubber spring  3  can be adjusted via the hand wheel  26 , by means of which the position of the drawing element  23 , and thus the prestressing of the rubber spring  3 , is determined. 
     FIG. 5 
     During the assembly of the chair, the pretensioning wedge  46  is not yet inserted into the bearing bushing  43 , and the rear support  4  can thus be advanced in the forward direction, negatively as it were, about the fixed main axis of rotation A 1 , in the direction of the front edge  810  of the seat panel  8 , into the assembly position P 0 . The seat panel  8  follows this movement synchronously, the seat panel being lowered in the region of the front edge  810  and raised in the region of the rear edge  811 . There is movement about the fixed axis of rotation A 2 , with lowering of the axis of rotation A 3 , and movement about the main axis of rotation A 1 , with the axis of rotation A 4  being raised. In the assembly position P 0 , the rubber spring  3  and the helical springs  89  can be assembled in a state in which they are relieved of stressing. In relation to one another, the fixed axis of rotation A 2  is located above the level of the fixed main axis of rotation A 1 . The maximum negative setting angle is achieved when the depression  435  in the bearing bushing  43  strikes, by way of its top edge, against the underside of the seat panel  8  in the vicinity of the rear edge  811 . 
     With the introduction of the pretensioning wedge  45 , the chair is moved into the vertical position P 1 , i.e. the backrest  5 , which is fitted on the rear support  4 , is located approximately vertically and the seat panel  8  is in an approximately horizontal position. Since they are fixed, the positions of the main axis of rotation A 1  and of the axis of rotation A 2  along the bearing flange  22  of the seat support  2  remain unchanged. In this case, however, the rubber spring  3  and the helical springs  89  are prestressed. By virtue of the backrest  5  being positioned vertically, the rear support  4  pivots about the main axis of rotation A 1 , as result of which the axis of rotation A 4  is lowered. At the same time, the articulations  9  pivot about the axis of rotation A 2  and the articulation legs  90  are positioned approximately vertically, with the result that the axis of rotation A 3  is then located in a position in which it is raised at a slight extent vertically above the axis of rotation A 2 . The articulation legs  90  are preferably inclined to some extent in the vertical position P 1 , with the result that the axis of rotation A 3  is arranged in front of the axis of rotation A 2 , as seen in the direction of the main axis of rotation A 1 . This allows the user to move more comfortably into the inclined position P 2 , by shifting his/her weight, without having to press against the backrest  5 . It has also proven advantageous for the axis of rotation A 4 , which can be changed about the main axis of rotation A 1 , to be arranged in the vertical position P 1  such that the axis of rotation A 4 , which is located in the vicinity of the rear edge  811  of the seat panel  8 , comes to rest above the main axis of rotation A 1  and axis of rotation A 2 , but is located beneath the axis of rotation A 3 . Return into the assembly position P 0  by a negative setting angle is ruled out since the top edge of the introduced pretensioning wedge  45  strikes against the underside of the seat panel  8  in the vicinity of the rear edge  811 . 
     With the user shifting sufficient weight, the drawn-back locking slide  84  allows the seat S to be adjusted into the inclined position P 2 . In this case, the rear support  4  rotates about the main axis of rotation A 1 , and the axis of rotation A 4  is lowered at maximum to level with the main axis of rotation A 1 . At the same time, the articulations  9  pivot about the axis of rotation A 2 , with the result that the articulation legs  90  are inclined in relation to the main axis of rotation A 1 , and the axis of rotation A 3  has advanced both horizontally and vertically toward the main axis of rotation A 1 , but is still located above the two axes of rotation A 1  and A 2 . The maximum inclination of the seat S in the inclined position P 2  is bounded by the seat panel  8  being positioned on the front transverse wall  432  of the bearing bushing  43  in the region of the bottom depression section  831 , and this acts as a stop in the end position.