Abstract:
A chair is provided comprising a seating platform having a front edge and a back edge, a seat carrier that carries the seating platform and is connected to one of a central chair column and a plurality of chair legs, a rest carrier that proceeds toward the back under the seating platform and upward behind the seating platform, and a backrest carried on the rest carrier. The seating platform, close to its front edge, is hinged to the seat carrier pivotable around a transversely proceeding, first swivelling axis and, offset therefrom toward the back edge, is hinged to the rest carrier pivotable around a parallel, second swivelling axis. The rest carrier is hinged to the seat carrier pivotable around a third swivelling axis that is located between the first and second swivelling axis and parallel thereto. A spring is provided under the seating platform and supported thereat which exerts a pre-stress force acting upwardly on the seating platform and toward the front on the back rest. The rest carrier comprises an extension that extends forward of the third swivelling axis and forms a lever arm. The spring has its end remote from the seating platform supported on the rest carrier extension and the seating platform comprises a vertical motion latitude relative to at least one of the seat carrier and the rest carrier.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a chair having a seating platform, having a seat carrier that carries the seating platform and is connected to a central chair column or a plurality of chair legs, and having a rest carrier that proceeds toward the back under the seating platform and upward behind the seating platform and carries a backrest, whereby the seating panel, close to its front edge, is hinged to the seat carrier pivotable around a transversely proceeding, first swivelling axis and, offset therefrom toward the back, is hinged to the rest carrier pivotable around a parallel, second swivelling axis, whereby the rest carrier is hinged to the seat carrier pivotable around a third swivelling axis that proceeds between the first and second swivelling axis and parallel thereto, and whereby a compression spring arrangement that exerts a pre-stress force acting upwardly on the seating platform and toward the front on the back rest is provided under the seating platform and is supported thereat. 
     DE 43 13 301 C2 discloses a chair of the type just described. It is thereby specifically provided that supports that are downwardly directed in pairs are rigidly secured to the underside of the seating platform in the front and back region thereof, whereby the lower ends of the front supports are pivotably connected to downwardly directed guide members pivotably secured to the seat carrier, and the lower ends of the rear supports are pivotably connected to the rest carrier, and that the compression spring-arrangement is arranged between the seating platform and the seat carrier. What is referred to as a synchronous mechanism is achieved with this design of the chair, whereby, when the back part of the seating platform is loaded, the platform is lowered toward the back and the backrest is simultaneously pivoted toward the back, whereby the swivel angle of the backrest is usually greater than the swivel angle of the seating platform. The relationship of the swivel angle to one another is defined by the interacting lever lengths. 
     What is considered disadvantageous about this known chair is that the pre-stressing force of the back rest is dependent only on the strength of the compression spring arrangement and the lever effect determined in the design of the chair, so that an adaptation of the pre-stressing force of the backrest to different body weights of different users is not possible at all. Although an influencing of the pre-stressing force of the backrest would be possible by employing an adjustable or biasable compression spring arrangement, the user of the chair must manually implement the adjustment for this purpose, which represents an undesirably high outlay, particularly when a chair is used by different users that differ in weight. Moreover, it is not assured that the user will find the suitable setting, as a result whereof settings of the pre-stressing force of the backrest can derive that are incorrect and that may even be harmful to health under certain circumstances. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to create a chair of said species that avoids the disadvantages that have been presented and whereby, in particular, an automatic adaptation of the pre-stressing force of the backrest to users differing in weight is achieved. 
     The features of the chair that are present in a preferred embodiment of the invention are that the seating platform is moved down given use by a heavy user, this necessarily leading to a tensing of the compression spring arrangement. Since the compression spring arrangement is supported on the extension of the rest carrier, the compression spring arrangement exerts a lever moment on the rest carrier that leads to an increase in the pre-stressing force of the backrest. A chair user having a high body weight thus experiences an increased support of his or her back by the backrest, as is desired and ergonomically meaningful. When a lighter weight user sits on the chair, the seating platform assumes a position that lies further up and wherein the compression spring arrangement is tensed less; correspondingly, a slighter pre-stressing force of the backrest necessarily follows, so that a lighter weight user also experiences an appropriately lesser supporting force of the backrest against his or her back in conformity with the lower body weight. At the same time, however, the synchronous adjustment of seating platform and backrest is preserved to its full extent, so that the seating platform and the backrest are also swivelled in a predetermined relationship relative to one another given changes in the sitting position. Despite the synchronous mechanism and the automatic adaptation of the pre-stressing force of the backrest to different user body weights, the chair has a surprisingly simple design, so that the manufacture of the chair is relatively simple and cost-beneficial. 
     In order to achieve a compact structure that occupies little space, it is preferably provided that the second swivelling axis proceeds under the seating platform close to the longitudinal center thereof. The mechanism required for the chair movements can thus be concentrated under the front half of the seating platform, which simplifies the manufacture of the chair and which yield an attractive appearance of the chair and, in particular, of its motion mechanism arranged under the seating platform. 
     It is also provided that the underside of the seating platform comprises at least one respective front and back bearing support through which the first, front swivelling axis and the second, back swivelling axis proceeds. As a result of these bearing supports, the first and the second swivelling axes are lent a desired spacing from the seating platform. Moreover, different seating platforms can be connected to the bearing supports in the simplest way, so that different embodiments and designs of the chair with a different seating platform are possible with little outlay. 
     A technically simple and, at the same time, functionally dependable solution for achieving the aforementioned vertical motional latitude of the seating platform relative to the seat carrier and/or the rest carrier is comprised therein that the bearing supports each comprise an oblong hole through which a first, front and second, back bearing pin rigidly connected to the seat carrier respectively proceeds, whereby the oblong holes proceed on a radius around the respectively non-appertaining, other bearing pin. As a result of said course of the oblong holes, a swivelling of the seating platform both around the front, first swivelling axis as well as around the back, second swivelling axis is possible without seizing, so that the seating platform can effortlessly adapt to different sitting postures of the chair user. 
     It is further proposed that, for forming a motion detent of the rest carrier, the second, back bearing pin also proceeds through an essentially vertically directed oblong hole in the rest carrier. Separate means for limiting the movement of the rest carrier are eliminated in this way, this being a further contribution to the simplification of the mechanical design of the chair. At the same time, said oblong hole, in interaction with the second, back bearing pin, serves for coupling the seating platform and rest swivel within the synchronous mechanism. 
     One embodiment of the chair, which can be preferably employed as an office swivel chair, provides that the seat carrier, proceeding in longitudinal direction of the seating platform, is arranged thereunder in the transverse center thereof, and has its back end connected to the chair column; that a respective rest carrier having a respective, appertaining compression spring arrangement is provided to the left and right of the seat carrier and symmetrically relative thereto; that two respective, front and back bearing supports are present; and that a respective, through, front and back bearing pin is attached to the seat carrier. 
     An alternative embodiment of the chair, which is essentially suitable as a consultation or conference chair, is characterized in that the seat carrier is implemented bipartite and is connected to a respective chair leg arranged proceeding in longitudinal direction of the seating platform laterally to the left and right thereof and thereunder as well as at its front and back ends; that a respective rest carrier with a respective, appertaining compression spring arrangement is provided at the left and right inside of and parallel to the two seat carrier parts; that respectively two front and back bearing supports are present; and that a respective, through, front and back bearing pin is attached to the two seat carrier parts. 
     Said compression spring arrangement is advantageously a coil spring arrangement because coil springs require little installation space and because they are a standard component part that can be accordingly easily acquired in the greatest variety of embodiments. 
     Alternatively, for example, the compression spring arrangement can also be a gas compression spring arrangement or an elastomer spring arrangement instead of a coil spring arrangement. 
     In a further alternative, the compression spring arrangement is formed by a torsion spring arrangement. 
     It is provided in a further development of the aforementioned embodiment of the chair that the/each torsion spring forming the torsion spring arrangement comprises a spring member that comprises one or more turns surrounding the third swivelling axis as well as two spring legs extending tangentially outward therefrom, whereby a first spring leg is supported close to its free end at the underside of the seating platform and a second spring leg is supported at the rest carrier, and whereby the spring legs are biased such that they exert an upwardly directed force onto the seating platform and exert a force on the rest carrier that pre-stresses the backrest toward the front. What this chair advantageously achieves is that the structural height of the spring arrangement is determined only by the outside diameter of the spring member since the torsion spring has a spring member whose longitudinal axis proceeds in horizontal direction transversely under the seating platform. At the same time, the spring member embraces the third swivelling axis, so that the torsion spring is adequately reliably held under the seating platform without additional structural measures. Further, it is unproblematically possible to set the spring characteristic of the torsion spring within broad ranges as desired, whereby changes in the spring characteristic are possible by modifications of the spring strength and/or the number of turns and/or the turn diameter as well as the length of the spring legs. One advantage, finally, is also comprised therein that torsion springs are simple and commercially available spring elements that are cost-beneficial and keep the overall manufacturing costs of the chair low. 
     It is provided in a preferred development, that both spring legs proceed toward the front away from the spring member approximately parallel to one another, whereby the second spring leg is supported at the upper side of the extension of the rest carrier and exerts a downwardly directed force onto the extension. This alignment of the spring legs yields an especially space-saving arrangement, so that the spring arrangement requires only a slight structural height under the seating platform. This is especially advantageous for the visual appearance of the chair in its side view. 
     In order to keep wear phenomena and noises in the use of the chair as low as possible or even avoid them to the farthest-reaching extent, it is also proposed that at least the first spring leg lies in a glide or roll guidance connected to the seating platform. 
     In order to distribute the forces acting on the parts of the chair in conjunction with the spring-bearing and thereby limit them in terms of their size, it is also proposed that two torsion springs are provided symmetrically relative to the longitudinal center axis of the chair, these being preferably formed of one piece with one another on the basis of a single, correspondingly doubly coiled and bent spring steel wire or rod. Due to the one-piece implementation of the two torsion springs, moreover, a further contribution can be made to a simple mechanical design and cost-beneficial manufacture. 
     In another alternative, the compression spring arrangement in the chair is formed by a torsion bar spring. 
     It is provided in a further development of the aforementioned embodiment of the chair that the/each torsion bar spring forming the torsion bar spring arrangement is formed with at least one spring section that is loaded for torsion and proceeds in the transverse chair direction and is formed with at least two spring levers proceeding in longitudinal chair direction, whereby a first spring lever is supported close to its free end at the underside of the seating platform and a second spring lever is supported at the rest carrier, and whereby the spring levers are biased such that they exert an upwardly directed force on the seating platform and exert a force on the rest carrier that pre-stresses the backrest toward the front. 
     The particular advantage of this chair is comprised therein that the compression spring arrangement requires only an extremely slight installation height, as a result whereof the component parts of the chair arranged under the seating platform, including the compression spring arrangement, can be implemented especially flat. As a result thereof, the side view of the chair is lent an especially elegant and light appearance without the stability and the functionality of the chair being deteriorated. 
     A further simplification of the design of the chair, as preferably provided, is achieved in that the spring section loaded for torsion simultaneously forms the first swivelling axis. A separate bearing pin for forming the swivelling axis is thereby eliminated, which advantageously reduces the number of discrete parts required. 
     It is also proposed that the spring levers of the spring section—as seen in a bottom view of the chair—proceed approximately parallel to one another and—as seen in a side view of the chair—proceed toward the back upon inclusion of an acute angle, whereby the second spring lever is supported at the upper side of the extension of the rest carrier and exerts a downwardly directed force on the extension. Given the fashioning and arrangement of the torsion bar spring described here, this also requires relatively little installation space in horizontal direction of the chair in the longitudinal direction thereof, so that a contribution to a compact structure is also made in this respect. 
     The torsion bar spring is preferably formed of one piece on the basis of a spring steel bar bent U-shaped. Such a torsion bar spring can be manufactured in an especially simple and cost-beneficial way and requires only slight material outlay, this contributing to a desired, low, overall weight of the chair. 
     In order to also achieve a distribution of the forces of the compression spring arrangement occurring upon use given this chair, it is also proposed that two torsion bar springs are provided symmetrically relative to the longitudinal center axis of the chair, these being preferably formed of one piece with one another on the basis as a single spring steel bar that is bent cam shaft-like. 
     The front and the back swivelling axis serve for connecting the seating platform to the seat carrier, to which end the seating platform usually comprises bearing supports at its underside. For all chairs implemented with bearing supports, the invention proposes that respectively two left and two right bearing supports are present, these being respectively implemented combined and of one piece to form a left and a right support component part or even to form a single support component part. This measure simplifies the manufacture of the chair and enhances the stability of the bearing of the seating platform at the seat carrier and at the rest carrier. 
     The invention also provides that the seat carrier and at least that part of the rest carrier proceeding under the seating platform as well as, potentially, the bearing supports are punched and pressed parts of sheet steel. Punched and pressed parts of sheet steel can be especially cost-beneficially fabricated in large unit numbers as mass-produced parts, whereby they exhibit high stability and durability at the same time. A high and also durable quality of the chair is thus assured given low manufacturing costs and given low wear. 
     As explained above, the chair preferably has a combination of synchronous mechanism and weight-dependent pre-stressing of the rest. For users who do not wish the synchronous mechanism, the chair can also be alternatively implemented such that the articulation of the seating platform to the rest carrier is omitted, as a result whereof the seating platform and the rest carrier can be swivelled unsynchronized, i.e., independently of one another. The technical modifications required for this purpose are advantageously limited to the omission of individual parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A number of exemplary embodiments of the invention are explained below with reference to a drawing. 
     FIG. 1 shows a chair in a first embodiment as office swivel chair, in a side view; 
     FIG. 2 shows the chair of FIG. 1 in a bottom view; 
     FIG. 3 shows the chair in a second embodiment as conference chair, likewise in a side view; 
     FIG. 4 shows the chair in a third embodiment in a partial side view, partly in vertical section; 
     FIG. 5 shows the chair of FIG. 4 in a bottom view; 
     FIG. 6 shows the chair in a fourth embodiment in a partial side view; and 
     FIG. 7 shows the chair of FIG. 6 in a bottom view. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As critical parts, the office swivel chair  1  shown in FIG. 1 as a first exemplary embodiment, has a seating platform  2 , a seat carrier  3 , a rest carrier  4  with a backrest  4 ′ and a central chair column  5  with a foot cross  50  at its lower end. 
     The seating platform  2  is a plate that is stable in and of itself and that can be provided with a cushion (not shown here) at its upper side. At its underside, the seating platform  2  is connected to two front bearing supports  21  and two back bearing supports  22 . The bearing supports  21 ,  22  of each support pair are arranged one behind the other in the side view shown in FIG. 1, so that respectively only one of the bearing supports  21 ,  22  is visible. The front bearing supports  21  are arranged close to the front edge of the seating platform  2  and each respectively have an oblong hole  21 ′ through which a first bearing pin  31  proceeds horizontally and transversely. This first bearing pin  31  is rigidly connected to the seat carrier  3 , which is in turn rigidly connected to the upper end of the central chair column  5  by means of a chair column receptacle  30 . 
     The back, second bearing supports  22  likewise have a respective oblong hole  22 ′ through which a second bearing pin  32  proceeds. This second bearing pin  32  is rigidly connected to the seat carrier  3  and proceeds parallel to the first bearing pin  31 . The oblong hole  22 ′ in the second bearing supports  22  thereby proceeds on a radius around the first, front bearing pin  31 ; the oblong hole  21 ′ in the front bearing supports  21  proceeds on a radius around the second bearing pin  32 . In this way, movement of the seating platform  2  is possible in vertical direction relative to the seat carrier  3 , whereby this vertical movement can ensue both approximately on a straight line down as well as in the form of pivots around the first, front bearing pin  31  or around the second, back bearing pin  32 . 
     In the illustrated exemplary embodiment, the rest carrier  4  carrying the backrest  4 ′ is formed by two rest carrier parts  40 ,  40 ′. The rest carrier part  40  proceeds under the seating platform  2 ; at its back end, this is connected to the second rest carrier part  40 ′ in the form of a plug-type connection that enables a height adjustment of the second rest carrier part  40 ′ and of the backrest  4 ′ secured thereto. 
     The first rest carrier part  40  proceeding under the seating platform  2  is pivotably seated with a bore  43  and a bearing pin  33 , which is likewise rigidly connected to the seat carrier  3 . The first rest carrier part  40  also comprises an extension  41  directed forward via the bearing pin  33  that proceeds parallel to the two other bearing pins  31  and  32 . 
     Finally, the chair  1  also comprises a compression spring arrangement that is formed here by two coil compression springs  6 . The springs  6  are arranged one behind the other in the side view, so that only one spring  6  is visible in FIG.  1 . The coil compression spring  6  has its upper end supported at the underside of the seating platform  2 , namely in a region between the front bearing support  21  and the back bearing supports  22 . The coil compression spring  6  has its lower end supported at the forwardly directed extension  41  of the first rest carrier part  40 , as a result whereof the coil compression spring exerts an upwardly directed pre-stressing force, on the one hand, on the seating platform  2  and, on the other hand, exerts a forwardly directed pre-stressing force on the backrest  4 ′ via the first and the second rest carrier part  40 ,  40 ′. 
     The chair  1  is shown in a non-loaded condition in FIG. 1 wherein no forces are exerted on the seating platform  2  and on the backrest  4 ′ by a user of the chair  1 . The swivel position of the rest carrier  4  in this position is limited by a detent that is formed by an oblong hole  42  in the first rest carrier part  40 . The second bearing pin  32  proceeds through this oblong hole  42 , whereby the bearing pin  32  lies against the lower end of the oblong hole  42  here. 
     When the chair  1  is loaded by a user, the seating platform  2  moves down to a greater or lesser extent dependent on the body weight of the user. When the seating platform  2  is loaded by a heavier user, the spring arrangement  6  is more greatly compressed and is thereby lent higher tension. As a result thereof, the spring arrangement  6  exerts a greater force on the extension  41  of the first rest carrier part  40 , which necessarily leads via lever action to the fact that the backrest  4 ′ is lent a greater pre-stressing force toward the front, i.e., supports the user&#39;s back with a higher force. Given a lighter weight user of the chair  1 , the seating platform  2  is pressed down correspondingly less, as a result whereof the spring arrangement is also tensed less and as a result whereof the backrest  4 ′ then also experience a lower pre-stressing force. The pre-stressing force or supporting force of the backrest thus automatically adapts to users differing in weight. 
     The chair  1  also offers a synchronous mechanism that allows the position of the seating platform  2  and backrest  4 ′ to be varied coupled to one another. When a user leans back on the chair  1 , the seating platform  2  is lowered at the back, whereby the backrest  4 ′ is simultaneously pivoted toward the back via the lever effects that thereby occur. 
     As is standard and known in and of itself, the backrest  4 ′ can be additionally seated at the second rest carrier part  40 ′ pivotable around a horizontally transversely proceeding axis, which enables a more improved adaptation of the backrest  4 ′ to various users of the chair  1 . The chair column  5  is likewise a known and standard design and is preferably height-adjustable as well as spring-mounted, so that, overall, the chair  1  offers versatile possibilities of adapting to different users, whereby the setting of the pre-stressing force of the backrest  4 ′ advantageously ensues automatically dependent on the body weight of the user of the chair  1 . 
     The bottom view of the chair  1  of FIG. 1 shown in FIG. 2 of the drawing illustrates the symmetrical arrangement of the mechanism of the chair  1  at both sides of the longitudinal center axis  10  of the chair. The seat carrier  3  lies in the middle, this having the chair column receptacle  30  at its back end (the right end in FIG. 2) for connection to the chair column  5  (not visible here). The three bearing pins  31 ,  32 ,  33  that proceed parallel to one another in transverse direction of the chair  1  under the seating platform  2  thereof are secured to the seat carrier  3  transverse thereto. The ends of the bearing pins  31  and  32  lie in the front and back bearing supports  21 ,  22  and proceed through the oblong holes  21 ′,  22 ′ shown in FIG. 1 that are provided therein. 
     FIG. 2 also shows that the first rest carrier part  40  is implemented bipartite with two parallel sections proceeding parallel to the longitudinal center axis  10 . The two sections of the first rest carrier part  40  are pivotably seated at the third bearing pin  33 ; the extensions  41  of the two sections of the first rest carrier part  40  lie in front of the third bearing pin  33 ; i.e., to the left thereof in FIG.  2 . The two coil compression springs  6  have their lower end, which face toward the observer here, supported on these extensions  41 . The upper ends of the coil compression springs  6  facing away from the observer are supported at the underside of the seating platform  2  facing toward the observer. 
     It can be seen at the far right in FIG. 2 that the second rest carrier part  40 ′ (not shown) can be connected here to the backrest  4 ″ (not shown) by being plugged to the two sections of the first rest carrier part  40 . 
     Overall, FIG. 2 illustrates the extremely compact structure of the mechanism of the chair  1 , as a result whereof a simple manufacturability and a compact structure derive. The various swivelling axes  71 ,  72 ,  73  required for the movements of seating platform  2  and rest carrier  4  with the backrest  4 ′ are formed by the bearing pins  31 ,  32 ,  33 , whereby this design is both stable as well as low in wear. 
     FIG. 3 of the drawing shows a conference or consultation chair  1  as a second exemplary embodiment that comprises four chair legs  5 ′ instead of the central chair column, whereby only the respective chair legs  5 ′ facing toward the viewer can be seen in the side view shown in FIG.  3 . 
     This embodiment of the chair  1  also comprises an inherently stable seating platform  2  with a cushion  20 , whereby the front and the back bearing supports  21 ,  22  with their oblong holes  21 ′,  22 ′ are again present here at the underside of the seating platform  2 . The chair legs  5 ′ are rigidly connected to one another via the seating platform  3 . The three bearing pins  31 ,  32 ,  33  are also present here, these being in turn rigidly connected to the seat carrier  3 . 
     The rest carrier  4  is implemented continuous here and again carries the backrest  4 ′ at its upper part  40 ′. That part  40  of the rest carrier  4  lying under the seating platform  2  is seated at the seat carrier  3  pivotable around the bearing pin  33 . Here, too, the second, back bearing pin  32  proceeds through an oblong hole  42  in the rest carrier part  40  in order to limit its swivel path. 
     Here, too, the rest carrier  4  comprises an extension  41  that proceeds toward the front via the swivelling axis  73 . The compression spring arrangement  6  has its lower end supported on the extension  41 , whereby the upper end thereof also lies against the underside of the seating platform  2  here. 
     With respect to the movements of seating platform  2  and rest carrier  4  with backrest  4 ′, the chair  1  according to FIG. 3 behaves like the chair  1  according to FIGS. 1 and 2; here, too, an automatic adaptation of the pre-stressing force of the backrest  4 ′ to the body weight of the user of the chair  1  thus ensues. Moreover, the synchronous mechanism for the coupled swivelling of seating platform  2  and rest carrier  4  with backrest  4 ′ is also assured given the chair according to FIG.  3 . The chair  1  according to FIG. 3 does not have an overall height adjustment as possible as a result of the chair column  5  given the chair  1  according to FIG. 1. A separate height adjustment of the backrest  4 ′ is also not provided given the chair  1  according to FIG.  3 . Since the individual parts of the chair  1  according to FIG. 3 can be simplified as a result thereof, the chair  1  of this embodiment can be especially cost-beneficially manufactured. At the same time, however, it offers the user great comfort due to the synchronous mechanism and due to the automatic adaptation of the pre-stressing force of the backrest  4 ′ dependent on the body weight of the respective user. Since, moreover, the entire mechanism given the chair  1  according to FIG. 3 is very compact under the seating platform  2 , this chair  1  can also be stacked for storage and transport purposes, as known from traditional, rigid chairs, as a result whereof an extremely space-saving arrangement derives. 
     The chair  1  shown in FIG. 4 as third exemplary embodiment is implemented as office swivel chair and its critical parts are a seating platform  2 , a seat carrier  3 , a rest carrier  4  for a backrest (not shown here) arranged farther up, a central chair column  5  with a foot cross (likewise not shown here) at its lower end, and a spring arrangement  6  between seating platform  2  and rest carrier  4 . 
     The seating platform  2  is an inherently stable plate that can be provided with a cushion (not shown here) at its upper side. At its underside, the seating platform  2  is connected to a support component  23  having a respective left and right, downwardly directed cheek  23 ′. Close to the front edge of the seating platform  2  lying at the left in the drawing, the cheeks  23 ′ of the support component  23  respectively comprise an oblong hole  21 ′ through which a first bearing pin  31  proceeds horizontally and transversely. This first bearing pin  31  is rigidly connected to the seat carrier  3  that is in turn rigidly connected to the upper end of the central chair column  5  by means of a chair column receptacle  30  lying centrally under the seating platform  2 . 
     Farther toward the back, here roughly centrally under the seating platform  2 , the cheeks  23 ′ of the support component  23  as well as the rest carrier  4  comprise further oblong holes  22 ′,  42  through which a second bearing pin  32  proceeds. This second bearing pin  32  is also rigidly connected to the seat carrier  3  and proceeds parallel to the first bearing pin  31  offset down by about a pin thickness. The oblong hole  22 ′ thereby respectively proceeds on a radius around the first bearing pin  31  in an essentially vertical direction, and the oblong hole  21 ′ proceeds on a radius around the second bearing pin. In this way, a movement of the seating platform  2  is possible in vertical direction relative to the seat carrier  3 , whereby this vertical motion can ensue both approximately on a straight line vertically as well as in the form of swivels around the first, front bearing pin  31  or around the second, back bearing pin  32 . 
     In the illustrated exemplary embodiment, the rest carrier  4  is formed by two rest carrier parts  40 ,  40 ′. The rest carrier part  40  proceeds under the seating platform  2 ; at its back end, this is connected to the second rest carrier part  40 ′ be a plug-type connection that enables an adjustment of the second rest carrier part  40 ′ with the backrest secured thereto. 
     The first rest carrier part  40  proceeding under the seating platform  2  is pivotably seated at the seat carrier  3  by a third bearing pin  33  that is likewise rigidly connected to the seat carrier  3 . Further, the first rest carrier part  40  comprises an extension  41  directed forward via the bearing pin  33  that proceeds parallel to the two other bearing pins  31 ,  32 . Moreover, the support component  23  and the rest carrier part  40  are connected to one another in articulated fashion here via a dog  24  in the form of a short lever pivotally attached at each end in order to effect the synchronism of the swivel of seating platform  2  and backrest. 
     Finally, the chair  1  also comprises the spring arrangement  6  that is formed by two torsion springs  60  here. The torsion springs  60  are arranged one behind the other in the side view, so that only the one torsion spring  60  is visible in FIG.  4 . The torsion spring  60  has a coiled spring member  61  that proceeds around the bearing pin  33  forming the third swivelling axis  73  and is held thereon with a holder that has not been numbered. A respective upper spring leg  62  and a lower spring leg  64  proceed tangentially from the spring member  61  toward the front roughly parallel to one another. Close to its free end, the first, upper spring leg  62  of the torsion spring  60  is supported at the underside of the seating platform  2 , namely in a region between the bearing pins  31  and  33 . To reduce wear and noise, a pressure member  63  is provided here as part of the support component  23 , this being attached to the underside of the seating platform  2 . The spring legs  62  lie against the pressure member  63 . The torsion spring  60  has its second, lower spring leg  64  supported at the forwardly directed extension  41  of the first rest carrier part  40 , as a result whereof, on the one hand, the spring leg  60  exerts an upwardly directed pre-stressing force onto the seating platform  2  and, on the other hand, exerts a forwardly directed pre-stressing force on the backrest via the first and the second rest carrier part  40 ,  40 ′. 
     FIG. 4 shows the chair  1  in a non-loaded condition wherein no forces are exerted on the seating platform  2  and on the backrest and its rest carrier parts  40 ,  40 ′ by a user of the chair  1 . The swivelled position of the rest carrier  4  is limited in this position by a detent that is formed by the oblong hole  42  in the first rest carrier part  40 . The second bearing pin  32  proceeds through this oblong hole  42 , whereby the bearing pin  32  lies against the lower end of the oblong hole  42  here. 
     When the chair  1  is loaded by a user, the seating platform  2  moves down to a greater or lesser extent dependent on the body weight of the user. When the seating platform  2  is loaded by a heavier user, the spring legs  62 ,  64  of the torsion springs to are more greatly compressed, and the torsion springs  60  are thereby lent a higher tension. As a result thereof, the springs  60  also exert a greater force onto the extension  41  of the first rest carrier part  40 , which, via the lever effect, necessarily leads thereto that the backrest is lent a higher pre-stressing force toward the front, i.e. supports the user&#39;s back with a higher force. Given a lighter weight user of the chair  1 , the seating platform  2  is pressed down correspondingly less, as a result whereof the springs  60  are also tensed to a lesser extent, and as a result whereof the backrest also experiences less of a pre-stressing force. The pre-stressing or supporting force of the backrest thus automatically adapts to users differing in weight. 
     The chair  1  also offers a synchronous mechanism that sees to it that, when the chair  1  is used, the positions of seating platform  2  and backrest  4 ′ change coupled with one another. When a user leans back on the chair  1 , the seating platform  2  is more highly loaded at the back and lowers there, whereby, via the dog  24 , the backrest is simultaneously swivelled toward the back in a fixed swivel relationship via the lever effects that thereby occur. When the synchronous mechanism is not desired, this can be eliminated simply by omitting the dog  24 . 
     As is known in and of itself and standard, the backrest can be additionally seated at the second rest carrier part  40 ′ pivotable around a horizontally transversely proceeding axis, which enables a further improved adaptation of the backrest to various users of the chair  1 . The chair column  5  likewise has a known and standard design and is preferably height-adjustable as well as spring mounted, so that, overall, the chair  1  offers versatile adaptation possibilities to different users. The setting of the pre-stressing force of the backrest thereby advantageously ensues automatically dependent on the body weight of the user of the chair  1 , so that manual adjustments are not necessary therefor. 
     The bottom view of the chair of FIG. 4 shown in FIG. 5 of the drawing illustrates the symmetrical arrangement of the mechanism of the chair  1  at both sides of the longitudinal center axis  10  of the chair. The seat carrier  3 , which comprises the chair column receptacle  30  for connection to the chair column  5  (not visible here) at its back, right-hand end in FIG. 5, lies in the middle. The three bearing pins  31 ,  32 ,  33  that proceed parallel to one another in transverse direction of the chair  1  under the seating platform  2  thereof (which is not shown here) are secured to the seat carrier  3  transverse thereto. The ends of the bearing pins  31  and  32  lie in the two lateral cheeks  23 ′ of the support component  23  and proceed through the oblong holes  21 ′,  22 ′ provided therein that are shown in FIG.  4 . 
     FIG. 5 also shows that the first back carrier part  40  is implemented of one piece as a flat strip with two parallel, lateral flanges that proceed parallel to the longitudinal center axis  10 . The first rest carrier part  40  is pivotably seated at the third bearing pin  33 ; the extension  41  of the first rest carrier part  40  lies in front of the third bearing pin, i.e. to the left thereof in FIG.  5 . The two torsion springs  60  have their spring legs  64  supported on this extension  41 . The torsion springs  60  have their spring legs  62  supported at the underside (facing toward the viewer) of the seating platform  2  (not shown) that lies in the background here. 
     It can be seen at the far right in FIG. 5 that the second rest carrier part  40 ′ (not shown here) can be connected to the backrest by plugging to the first rest carrier part  40 . 
     Overall, FIG. 5 illustrates the extremely compact structure of the mechanism of the chair  1 , as a result whereof a simple manufacturability and a “light” appearance of the chair  1  derive. The three swivelling axes  71 ,  72 ,  73  required for the movements of seating platform  2  and rest carrier  4  with the backrest are formed by the bearing pins  31 ,  32 ,  33 , whereby this design is both stable as well as low in wear. 
     FIGS. 6 and 7 of the drawing show a further office swivel chair  1  as a fourth exemplary embodiment that likewise comprises a central chair column  5 , whereby only the upper end of the chair column that is connected to that end of the seat carrier  3  lying centrally under the seating platform  2  can be seen in the partial side view shown in FIG.  6 . 
     This embodiment of the chair  1  also comprises an inherently stable seating platform  2 , whereby front bearing supports  21  each having a respective, vertical oblong hole  21 ′ and back bearing supports  22  are present at the underside thereof respectively separated from one another. Differing from the first exemplary embodiment, however, only two bearing pins  32 ,  33  are present here, the pin  33  thereof being rigidly connected to the seat carrier  3 , whereas the pin  32  is fixed in the bearing support  22 . 
     The rest carrier  4  is also implemented bipartite here and carries the backrest (not shown) at its upper part  40 ′. That part  40  of the rest carrier  4  lying under the seating platform  2  is seated at the seat carrier  3  pivotable around the swivelling axis  73  by means of the bearing pin  33 . Here, too, the second, back bearing pin  32  proceeds through an oblong hole  42  in the rest carrier part  40 , whereby the oblong hole  42  here proceeds through the rest carrier part  40  in longitudinal direction thereof in order to enable the force-free swivelling thereof. 
     Here, too, the rest carrier  4  comprises an extension  41  of one piece with the rest carrier part  40  that proceeds toward the front via the swivelling axis  73 . 
     A torsion bar spring  60 ′ is installed here as spring element  6 . This spring  60 ′ has two spring sections  61 ′ stressed for torsion that proceed in transverse chair direction through the oblong holes  21 ′ and simultaneously serve as first swivelling axis  71 . At the outside left and right, a respective spring lever  62 ′ extends toward the back from the spring section  61 ′ and lies against a respective detent  26  that is respectively attached to the underside of the seating platform  2 . Two further spring levers  64 ′ that likewise proceed in the direction toward the back and that are supported on the extension  41  of the rest carrier part  40  in a region connecting their free ends lie in the region between the spring sections  61 ′. The spring  60 ′ shown here is implemented as symmetrical double spring and is manufactured of one piece from a correspondingly bent spring steel bar. The torsion bar spring  60 ′ is pre-stressed such that its spring levers  62 ′,  64 ′ exert the desired, upwardly directed force onto the seating platform  2  and the desired, downwardly directed force onto the extension  41 , as indicated by the arrows at the spring levers  62 ′,  64 ′. 
     With respect to the movements of seating platform  2  and rest carrier  4  with backrest, the chair  1  according to FIGS. 6 and 7 behaves like the chair  1  according to FIG. 4 and 5; here, too, an automatic adaptation of the pre-stressing force of the backrest to the body weight of the user of the chair  1  thus ensues. Moreover, the synchronous mechanism for coupled swivelling of seating platform  2  and rest carrier  4  with backrest is also assured given the chair  1  according to FIGS. 6 and 7. 
     The chair  1  according to FIGS. 6 and 7 can also have an overall height adjustment as possible with the chair column  5  given the chair  1  according to FIGS. 4 and 5. A separate height adjustment of the backrest can also be provided given the chair  1  according to FIGS. 6 and 7. 
     It is especially advantageous that the entire mechanism given the chair  1  according to FIGS. 6 and 7 is very compact under the seating platform  2  and occupies only little structural height and that only few discrete parts are required. 
     The present invention has been described utilizing particular embodiments. As will be evident to those skilled in the art, changes and modifications may be made to the disclosed embodiments and yet fall within the scope of the present invention. The disclosed embodiments are provided only to illustrate aspects of the present invention and not in any way to limit the scope and coverage of the invention. The scope of the invention is therefore only to be limited by the appended claims.