Abstract:
A mechanism for an office chair having a backrest support which may be pivoted rearwardly. Adjustment of the backrest pivotal motion is accomplished by the mechanism having a base support which may be positioned on a chair column, a seat support, the backrest support which may be pivoted to the rear, and a pre-tensioned spring arrangement for enabling the mechanism to counter or oppose the movement of the backrest support. The seat support and the base support form a movable unit, which may be moved relative to the chair column depending on the weight of a user applying a load to the seat support. Movement of the movable unit causes an adjustment of the pre-tension of the spring arrangement and/or an adjustment of the spring constant of the spring arrangement.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a mechanism for an office chair, in particular an office chair comprising a backrest support which may be pivoted to the rear. 
     Various solutions are known from the prior art, by which the pivoting of the backrest of an office chair, in particular the “pivoting resistance”, may be adjusted. To this end, complicated adjusting mechanisms are frequently used, which generally take up a considerable portion of the available constructional space, and which are relatively restrictive as regards the design of the office chair. Moreover, the adjustment of the pivoting of the backrest always has to be manually undertaken by the user of the office chair, for example by actuating an adjusting element or, however, by means of an external drive, for example an electric motor. It is a further drawback that the adjustment always takes place “instinctively”, without a decision about any setting being necessarily based on ergonomic considerations. 
     An object of the present invention is to provide a solution which is particularly simple in terms of structure, for adjusting the pivoting of the backrest of an office chair. 
     This object is achieved by the mechanism set forth in the claims. Accordingly, the mechanism is provided with a base support which may be positioned on a chair column, a seat support, a backrest support which may be pivoted to the rear and a spring arrangement for acting on the mechanism counter to the movement of the backrest support. According to the invention, the mechanism is characterized in that the seat support and the base support form a moving unit which may be moved relative to the chair column depending on the weight of a user applying a load to the seat support, a movement of the moving unit resulting in an adjustment of the pretensioning of the spring arrangement and/or an adjustment of the spring constant of the spring arrangement. 
     A fundamental idea of the invention is to provide the adjustment of the pivoting of the backrest automatically, i.e. without the user of the office chair having to carry out additional steps therefor in any form, whether manually or by means of an external drive. Instead, according to the invention the adjustment of the pivoting of the backrest takes place fully automatically, solely by the user sitting on the office chair. The mechanism is automatically adjusted depending on the weight of the user—and thus adjustable from the ergonomic perspective for optimum pivoting features—by the spring arrangement defining the “pivoting resistance” of the backrest, being more or less pretensioned and/or the spring rate (also known as spring rigidity, spring hardness or spring constant) of the spring arrangement being altered. 
     BRIEF SUMMARY OF THE INVENTION 
     With a heavyweight user, this adjustment preferably takes place such that a high “pivoting resistance” opposes a pivoting of the backrest of the office chair whilst, in contrast thereto, a pivoting of the backrest with a lightweight user may be implemented considerably more easily. In this connection, it is left to the actual embodiment of the invention whether, when adjusting the pretensioning of the spring arrangement, one or more spring elements may be tensioned or alternatively relaxed. In other words, on the one hand, a spring element which is fully or partially relaxed in the unloaded state of the office chair may be tensioned when loaded, for example, or alternatively, a spring element which is pretensioned to a maximum extent or partially pretensioned in the resting state, is relaxed when a load is applied to the office chair. Preferably, however, even without a load being applied to the seat support by a user, an active impingement of the mechanism counter to the movement of the backrest support is carried out by a number of already pretensioned spring elements. It is left to the actual design of the invention whether the spring rate of one or more spring elements is altered and how the alteration of the spring rate is implemented. 
     In order to achieve an operation which is as simple and robust as possible with, at the same time, a simple structural design of the office chair, it is provided that the seat support and base support when applying a load to the seat support are moved by a user together as a moving unit relative to the fixed chair column, this relative movement being dependent on the weight of the user. The type of movement is initially unimportant for implementing the invention. Preferably, however, the movement is a linear movement in the vertical direction, i.e. in the direction in which the user also sits on the chair. In this manner, a direct and particularly easy transfer of the weight is possible for acting on the spring arrangement. 
     Advantageous embodiments of the invention are set forth in the sub-claims. 
     The pretensioning of the spring arrangement and the spring rate of the spring arrangement may be fundamentally adjusted according to the invention by two different methods. Firstly, it is possible to alter the position of at least one spring end of a spring element of the spring arrangement, with the overall position of the spring element remaining the same or being altered. This may, for example, take place by pulling apart or pressing together the spring ends of a helical spring or by twisting and/or deflecting one spring leg of a leg spring around the longitudinal axis of the spring extending through the spring center point against the other spring leg or relative to the other spring leg. 
     Secondly, it is possible to alter the position of the spring element itself relative to its fixed spring ends or, in a similar manner, at least the partially movable spring ends. This may be carried out by a leg spring, for example, by displacing the spring center point of the leg spring, when the bearing points are fixed. Both variants of the spring adjustment may be implemented according to the invention, depending on which requirements are set for the design of the seat mechanism. 
     A structurally simple solution and a compact design is achieved according to a particularly preferred embodiment of the invention, in particular when the seat support and the base support are arranged such that they carry out together the movement relative to the seat column, without therefore altering their relative position to one another. In other words, a direct and immediate common movement of the seat support and the base support is performed relative to the seat column. Force deflecting arrangements for moving the base support, such as for example levers between the seat support and the base support, are not provided. The seat support and the base support are moved instead on a single common path of motion. To this end, the base support and seat support may be connected to one another via corresponding connecting elements. As an alternative, however, it is also possible that the base support and the seat support are configured as a common component. 
     In addition, for a design which is as compact as possible a further embodiment is advantageous, according to which the base support and/or the seat support comprises a guide, in particular a linear guide for transmitting the relative movement to the spring arrangement. Preferably, the seat column in this case is guided directly or via a guide element in the base support and/or in the seat support, so that no additional components are required for implementing the invention. It is particularly advantageous if the linear guide is arranged vertically. By the design of the receiver as a linear guide, and the vertical arrangement thereof, a particularly simple and thus economical option is provided to convert the weight of the user of the office chair into a relative movement of the base support in the sense of a vertical deflection. 
     Moreover, the embodiment of the solution according to the invention may be implemented both by means of direct and by means of indirect influence on the spring arrangement, in particular by means of a direct and/or indirect impingement of a spring element of the spring arrangement. A direct impingement of a spring element is understood in this case as an alteration to the spring tension and/or the spring rate by a force acting directly on the spring element itself, whilst an indirect impingement is understood as the alteration of the spring tension and/or the spring rate by an indirect force acting on the spring element—i.e. for example via an auxiliary element. 
     A direct impingement of a spring element of the spring arrangement by the base support moving relative to the chair support allows a particularly simple, robust and reliable adjustment of the “pivoting resistance” of the backrest. In this case, the spring element is preferably arranged in the base support or in the direct vicinity of the base support, so that a direct deflection of the spring element may be implemented in a simple manner. 
     In such a case, a fixed leg spring is preferably used, the one leg thereof being driven by the base support which moves when a load is applied to the seat support or by the fixed chair column, whilst the other leg is supported on the mechanism such that a pivoting of the backrest is only possible counter to the spring element provided with a greater pretensioning, in other words the “pivoting resistance” increases by the coupling of one leg. 
     In one embodiment of the invention, the other leg may in this connection act directly on the backrest support itself. Such an embodiment is able to be applied particularly advantageously in an asynchronous mechanism in which only the backrest pivots, whilst the seat support is fixed. The invention may, however, in a further embodiment also be designed such that the other spring leg is supported on the seat support which is pivotably connected to the backrest support. This design is able to be used particularly advantageously in a synchronous mechanism, in which the seat support may be pivoted to the rear in synchronism with the backrest support, and the spring arrangement is configured for acting on the synchronous mechanism counter to the synchronous movement thereof by the seat support and backrest support. Naturally, however, even in the case of a synchronous mechanism, the other leg may be supported on the backrest support. 
     With an indirect impingement of the spring element of the spring arrangement by the base support which moves relative to the chair column, preferably a transmission means may be used, cooperating with the moving unit, in particular the base support, by which the weight of the user is transmitted to the spring element. Transmission means in the form of tractive means, such as a control cable or Bowden cable, levers and belts, in particular toothed belts, have proved particularly advantageous. Thus the displacement of the moving unit caused by the weight of the user may also be used to influence spring elements arranged remotely from the base support. 
     It is, for example, quite particularly advantageous if the pretensioning of a helical compression spring is increased via a control cable fastened to the fixed chair column and driven by the moving base support, which is supported on the seat support of a synchronous mechanism counter to the pivoting movement of the backrest. Preferably, the control cable is guided in this case from the base support to the spring element via corresponding rollers, which prevent mechanical wear of the control cable and at the same time allow an adjustment of the spring pretensioning in a particularly friction-free manner. At the same time, the control cable is arranged such that it preferably extends entirely in the housing of the base support and/or seat support and thus is not visible from the outside. Thus, not only soiling of the control cable is avoided thereby. Also, a concealed control cable guide is recommendable for reasons of safety. Moreover, the concealed arrangement of the control cable is also advantageous from an aesthetic perspective. 
     In a further particularly advantageous embodiment of the invention, the spring rate is adjusted by means of a transmission means in the form of a slotted guide, in which a pin is guided and/or held, which in turn is connected to spring elements, such as for example tension springs. 
     In summary, the invention relates to the automatic adjustment of the “pivoting resistance” of the backrest, which—by the user himself or herself applying a load to the office chair—may take place either by adjusting the pretensioning or by adjusting the spring rate of the spring arrangement of the mechanism or by a combination of both adjustment options. The structural and functional details disclosed here relative to the adjustment of the pretensioning of the spring arrangement may, in other words, also be associated with the structural and functional details disclosed here relative to the adjustment of the spring rate of the spring arrangement, so that an automatic adjustment of the “pivoting resistance” of the backrest may also be carried out by a combination of both adjusting options. By means of such a combination, the advantages of both techniques may be combined in a simple manner and possible drawbacks of one and/or the other technique may be avoided. 
     Moreover, the invention also discloses a safety device for an office chair, by which an inadvertent adjustment of a spring arrangement adjusted by the weight of the user by pivoting the backrest support, is effectively avoided. 
     Embodiments of the invention are described in more detail hereinafter with reference to the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a perspective view of a first synchronous mechanism, 
         FIG. 2  shows a first synchronous mechanism in plan view, 
         FIG. 3  shows a sectional view of a first synchronous mechanism in the partially loaded state (along the line AA in  FIG. 2 ), 
         FIG. 4  shows a sectional view of a first synchronous mechanism in the loaded state (along the line AA in  FIG. 2 ), 
         FIG. 5  shows a perspective view of a second synchronous mechanism, 
         FIG. 6  shows a second synchronous mechanism in the unloaded state in plan view, 
         FIG. 7  shows a sectional view of a second synchronous mechanism in the unloaded state (along a line displaced from the inside to the outside within the mechanism), 
         FIG. 8  shows a second synchronous mechanism in the loaded state in plan view, 
         FIG. 9  shows a sectional view of a second synchronous mechanism in the loaded state (along a line displaced from the inside to the outside within the mechanism), 
         FIG. 10  shows a perspective view of a third synchronous mechanism, 
         FIG. 11  shows a third synchronous mechanism in plan view, 
         FIG. 12  shows a sectional view of a third synchronous mechanism in the unloaded state (along the line BB in  FIG. 11 ), 
         FIG. 13  shows a sectional view of a third synchronous mechanism in the loaded state (along the line BB in  FIG. 11 ), 
         FIG. 14  shows a fourth synchronous mechanism in plan view, 
         FIG. 15  shows a sectional view of a fourth synchronous mechanism in the unloaded and unpivoted state (along the line AA in  FIG. 14 ), 
         FIG. 16  shows a sectional view of a fourth synchronous mechanism in the unloaded and pivoted state (along the line AA in  FIG. 14 ), 
         FIG. 17  shows a sectional view of a fourth synchronous mechanism in the loaded and unpivoted state (along the line AA in  FIG. 14 ), 
         FIG. 18  shows a sectional view of a fourth synchronous mechanism in the loaded and pivoted state (along the line AA in  FIG. 14 ), 
         FIG. 19  shows a partial sectional view of a fourth synchronous mechanism in the loaded and unpivoted state (along the line AA in  FIG. 14 ), 
         FIG. 20  shows spring characteristics of the first three embodiments, 
         FIG. 21  shows spring characteristics of the fourth embodiment (variant with a 90° arrangement between the longitudinal axis of the tension spring and the first slotted guide), and 
         FIG. 22  shows spring characteristics of the fourth embodiment (variant with an arrangement of the first slotted guide deviating from the vertical). 
     
    
    
     All the figures show the invention merely schematically and with the essential components thereof. 
     A first embodiment of the invention which shows the adjustment of the pretensioning of a spring arrangement, is shown in  FIGS. 1 to 4 . A synchronous mechanism is substantially used as a basis for the mechanism described below, as is disclosed in the German patent DE 10 2005 003 383. The contents of this printed patent specification are hereby fully incorporated in the present description. 
     DESCRIPTION OF THE INVENTION 
     The synchronous mechanism  1  has a base support  2  which, in a manner described in detail below, is connected to the upper end of a chair column (not illustrated). The synchronous mechanism comprises a substantially frame-shaped seat support  4  and a backrest support  5  which is fork-shaped in plan view, the cheeks  6 ,  7  thereof being arranged on both sides of the base support  2 . Moreover, the synchronous mechanism comprises a spring arrangement described in detail further below, for acting on the mechanism counter to the movement of the backrest support  5 . 
     The seat (not shown) provided with an upholstered seating surface is mounted on the seat support  4 . On the lateral frame elements  10  of the seat support  4 , a number of latching lugs  11  are provided, arranged in succession in the longitudinal direction of the chair L, which in a manner known per se and not described in more detail are used for positioning and fastening the seat to the seat support  4 . 
     A backrest which is not shown in more detail is attached to the backrest support  5 , and which is height-adjustable in modern office chairs. The backrest may also be integrally connected to the backrest support  5 . 
     The entire synchronous mechanism  1 , as regards the actual kinematics, is of mirror-symmetrical construction relative to the central longitudinal plane M (see  FIG. 2 ). In this respect, the following description is always based on structural elements of the actual pivoting mechanism which are present in pairs on both sides. 
     The backrest support  5  is, on the one hand, directly connected to the base support  2  in an articulated manner by the lower end  12  of the cheek  6  which is oriented to the front, namely mounted on a pivot pin  13  on the base support  2  such that the backrest support  5  is approximately centrally articulated via the pivot pin  13  directly on the base support  2 . As a result, the backrest support  5  may be pivoted with the backrest in the pivoting direction S, about the central longitudinal axis  14  extending through the pivot pin  13 . On the other hand, the backrest support  5  is connected by the upper end  15  of the cheek  6  via a joint  16  to the seat support  4  at the rear end region  17  thereof. By pivoting the backrest, therefore, the seat support  4  is also driven and lowered in the pivoting direction S. In other words a pivoting takes place about the joint axis  19  of the joint  16 . The geometry of the pivoting mechanism used has the advantage that a high down-tilt angle of the seat support  4  may be achieved, without the pivoting angle of the backrest having to be too great, which might lead to a position similar to a reclining position. Thus the so-called “riding-up effect” of clothes is effectively avoided. 
     The seat support  4  is at its front end region  21  connected to the base support  2  via a turning-and-sliding joint (not shown in detail). For designing the turning-and-sliding joint—and for further structural details of the mechanism—reference is made to the contents of the printed patent specification DE 10 2005 003 383. 
     Due to the shape of the backrest support  5  and the arrangement thereof on the base support  2  and the seat support  4 , when loading the backrest, on the one hand, the backrest support  5  carries out a pivoting motion S downward to the rear. As a result of the pivoting motion, however, the seat support  4  is also pivoted downward to the rear and also horizontally displaced to the rear in the region of the turning-and-sliding joint. As a result, no significant lifting motion of the front end of the seating surface is produced, whereby the underside of the thigh is prevented from being trapped. 
     The synchronous mechanism  1  is pretensioned by a spring arrangement counter to the pivoting direction S—i.e. towards the initial position of the synchronous mechanism. This spring arrangement  50  is provided in the form of two leg springs  41 ,  41 ′ aligned with one another in the transverse direction. The leg springs  41 ,  41 ′ are positioned around the pivot pin  13 . The leg facing upwards  42  is supported on a prismatic guide  55 , which is arranged on the underside  31  of the seat support  4 , whilst the second leg  43  extending to the rear, is supported in an adjusting mechanism according to the invention in the base support  2 . The leg springs  41 ,  41 ′ exert a spring force counter to the pivoting motion S of the backrest oriented to the rear, which may be varied by the adjusting mechanism. 
     The adjusting mechanism is substantially formed by a vertical linear guide  30 , which is designed as part of the base support  2 . The linear guide  30  comprises a square guide opening  22  arranged in the base support  2  as well as a correspondingly formed guide element  23  located in the guide opening  22 . The guide opening  22  is formed in this connection by suitable sub-elements  24  of the base support  2 . The guide opening  22  and/or guide element  23  may also have different cross sections in other embodiments of the invention. On the underside  25  of the guide element  23 , a conical receiver  3  is provided for fastening the upper end of the chair column. In other words, the chair column and the guide element  23  in the assembled state form a sub-assembly, which is fixedly located in the guide opening  22  of the base support  2 . 
     The diameter of the guide opening  22  is enlarged on its side facing in the direction of the seat support  4 , so that a stop  26  is formed in the guide opening. In the unloaded state of the seat support  4 , the guide element  23  bears against the stop  26  with its upper end  27  which is provided with an enlarged diameter, see  FIG. 3 . 
     The guide element  23  has a horizontally extending transverse opening  28  in which the leg  43  extending to the rear of the leg spring  41  is located, and is mounted and supported there on guide elements/mounting elements  29  provided in the transverse opening  28 . The pivot pin  13  and the linear guide  30  are in this case positioned in the vicinity of one another such that the leg  43  in the assembled state may easily pass through a through-opening  32  provided accordingly in the sub-element  24  of the base support  2 , and may be located in the transverse opening  28 . 
     In the unloaded state of the seat support  4 , the leg  43  extends slightly inclined downward from the horizontal through the through-opening and transverse opening  28 ,  32  (not illustrated). In a partially loaded state of the seat support  4 , as illustrated in  FIG. 3 , the leg  43  extends substantially horizontally and thus approximately parallel to the underside  33  of the transverse opening  28  of the guide element  23 , without mechanical contact with the through-opening or transverse opening  28 ,  32 . If a full load is applied to the seat support  4  by a user having sat down on the office chair, the moving unit formed from the seat support  4  and base support  2  is moved downwards as a whole in the direction of movement, namely on a common path of motion, namely a vertically extending straight line  18 , relative to the fixed sub-assembly made up of the chair column and the guide element  23 . The relative motion of the moving unit to the chair column takes place, in this case, without the position of the seat support  4  and base support  2  being altered relative to one another. The friction occurring with the relative movement between the guide element  23  and the guide opening  22 , is in this case reduced by the use of ball bearings, guide rings, slide bushes or the like (not illustrated). 
     The spring leg  43 , extending to the rear, of the leg spring  41  is driven from the underside  33  of the transverse opening  28  and forced upward, i.e. deflected upward from the horizontal, which leads to an increase in the pretensioning of the leg spring  41 . This has the result that the pivoting motion of the seat support  4  and the backrest support  5  takes place in the pivoting direction S against a greater resistance. 
     Due to the loading of the seat support  4  by the user, therefore, initially an adjustment of the pivoting resistance takes place independently of a pivoting motion of the backrest. However, in the present mechanism it is also provided that the pivoting resistance is altered by the pivoting of the backrest itself. 
     As the leg spring  41  is floatingly mounted on both sides, when the seat support  4  is pivoted downward to the rear, i.e. in the pivoting direction S, the point of articulation of the upper spring leg  42  is displaced. The position of the point of articulation is thus altered when a load is applied to the backrest such that the point of articulation is displaced in the direction of the spring center point  56 . As a result, an automatic alteration to the spring behavior of the leg spring  41  additionally takes place with a movement in the pivoting direction S. In other words, when the seat is pivoted, the leg spring  41  and thus the seat as a whole automatically become more rigid. 
     The backrest support  5  is fastened with fastening screws  57  to the central pivot pin  13 . In other words, during the pivoting motion, the pivot pin  13  rotates with the backrest support  5 . The diameter of the pivot pin  13  is selected such that the leg springs  41 ,  41 ′ in the clamped position do not bear against the pivot pin  13 . The internal diameter of the leg springs  41 ,  41 ′ is always greater than the diameter of the pivot pin  13 . As a result, an unhindered rotation of the pivot pin  13  is ensured when the seat is pivoted. Additionally, unpleasant contact noise, such as for example squeaking, is avoided. As the two leg springs  41 ,  41 ′ fitted onto the pivot pin  13  are located with their periphery in a spring support (not illustrated) formed in the manner of a prism, the positioning of the leg springs  41 ,  41 ′ in their operating position is, nonetheless, reliably ensured. 
     A second embodiment of the invention, which shows the adjustment of the pretensioning of a spring arrangement, is shown in  FIGS. 5 to 9 . As a basis for the subsequently disclosed mechanism, a synchronous mechanism is substantially used as is disclosed in the European patent EP 1 396 213. The contents of this printed patent specification are thus entirely incorporated in the present description. 
     As a supporting part of the synchronous mechanism  1 ′, a base support  2 ′ is provided which in the region of its rear end  34 , in a manner disclosed below in detail, is connected to the upper end of a chair column (not illustrated). 
     Further basic components of the synchronous mechanism  1 ′ are the backrest support  5 ′ and the seat support  4 ′. The backrest support  5 ′ in the region of the rear end  34  of the base support  2 ′ is pivotably mounted via a transverse shaft  35  on the base support  2 ′. The backrest support  5 ′ consists of two side struts  36 ,  37  extending obliquely upward to the rear, which form the connection to the actual backrest (not shown). In the front end region  58  of the base support  2 ′, two upwardly projecting bearing posts  38 ,  39  are formed on both sides of the central longitudinal plane M, in which a transverse shaft  40 , not illustrated in detail, is rotatably mounted. 
     The substantially plate-shaped seat support  4 ′, in the region of its front end has a slot (not shown), by which the seat support  4  rests on the transverse shaft  40 . As a result, a turning-and-sliding joint is formed between the base support  2 ′ and seat support  4 ′, i.e. the seat support  4 ′ may pivot about the transverse shaft  40 , and at the same time move relative thereto in the direction of the slots. For the design of the turning-and-sliding joint, —and for further structural details of the mechanism—reference is made to the contents of the printed patent specification EP 1 396 213. 
     In the region of its rear-facing end, the seat support  4  forms together with a corresponding upwardly projecting bearing projection  8 ,  9 , on the two side struts  36 ,  37  of the backrest support  3 , a pivot bearing about a transverse shaft  41 . 
     For acting on the synchronous mechanism  1 ′ counter to the synchronous adjusting motion, from the initial position shown in the figures, a spring arrangement  43  is provided which has four helical compression springs  44  arranged parallel to one another, in a common horizontal plane on both sides of the central longitudinal plane M. In this connection, for each helical compression spring  44 , one respective abutment extension arm  45  is provided, the front end thereof being pivotably articulated relative to the base support  1  via a bearing head  46 . The rod-shaped shaft of the abutment extension arm  45  projects freely to the rear. The rear end of the helical compression springs  44  is supported on an adjusting strip  59  described in more detail further below. 
     The front end of the helical compression springs  44  is located on a supporting strip  48 , which extends transversely to the seat longitudinal direction L and horizontally and which is semi-circular in cross section, in the manner of abutments which are supported with their front-facing semi-circular peripheral surface in corresponding internal cylindrical bearing recesses  49  on the seat support  4 ′. 
     The compressive force of the helical compression springs  44  clamped and pretensioned between the adjusting strip  59  and the supporting strip  48 , urges the seat support  4 ′ forward relative to the base support  2 ′ into the initial position shown. The backrest support  5 ′ is in this case in its maximum upright position. 
     If the pretensioning of the helical compression springs  44  is to be altered, the adjusting mechanism which has already been disclosed in connection with the first embodiment, is used again. Also in this case, said adjusting mechanism is substantially formed from a vertical linear guide  30 ′ as part of the base support  2 ′, said linear guide comprising a cylindrical guide opening  22 ′ arranged in the base support  2 ′, as well as a guide element  23 ′ located in the guide opening  22 ′. The guide opening  22 ′ is in this case again formed by suitable corresponding sub-elements  24 ′ of the base support  2 ′. On the underside  25 ′ of the guide element  23 ′ a conical receiver  3 ′ is provided for fastening the upper end of a chair column, so that the chair column and guide element  23 ′ in the assembled state form a sub-assembly, which is fixedly located in the guide opening  22 ′ of the base support  2 ′. 
     As shown in  FIG. 7 , in the unloaded state of the seat support  4 ′ the guide element  23 ′ bears with its upper end  27 ′, provided with an enlarged diameter, against a stop  26 ′ formed in the guide opening  22 ′ which is formed by the diameter of the guide opening  22 ′ being enlarged on its side facing in the direction of the seat support  4 ′. 
     The adjusting mechanism thus comprises two control cables  60 ,  60 ′ serving as transmission means for transmitting the weight of the user to the helical compression springs  44 . The control cables  60 ,  60 ′ are fastened with one end thereof to the guide element  23 ′ which is fixed to the chair column and with the other end thereof to the adjusting strip  59 . The adjusting strip  59  serves for supporting the rear ends of the helical compression springs  44  and is provided with four openings  61 , through which the shafts of the abutment extension arms  45  pass. In other words, the adjusting strip  59  is displaceably attached to the shafts. The control cables  60 ,  60 ′ extend spaced apart from one another and in the region of the spring arrangement  43  parallel to the helical compression springs  44 , in order to achieve a displacement of the adjusting strip  59  which is as uniform as possible. 
     Each control cable  60 ,  60 ′ is arranged such that it leaves the guide opening  23 ′ downward in the direction of the relative motion of the moving unit, and subsequently partially encompasses at least one sub-element of the base support  2 ′ arranged adjacent to the guide opening  22 ′, in other words, extends beyond a sub-element of the base support  2 ′. This sub-element is preferably a guide pulley  62 , so that the mechanical friction and thus the wear of the control cables  60 ,  60 ′ is only very slight. The control cable  60 ,  60 ′ extends from the guide pulley  62  arranged on the guide opening  22 ′ and then to a further guide pulley  63  in the front end region  58  of the base support  2 ′, the guide pulley  63  in the embodiment shown being fastened to the transverse shaft  11 . From there the control cable  60 ,  60 ′ extends directly to the adjusting strip  59 , to which it is connected. 
     If a load is applied to the seat support  4 ′ by a user sitting down on the office chair, as is indicated in  FIG. 9  by the arrow  47 , the moving unit formed from the seat support  4 ′ and base support  2 ′, is moved as a whole downward in the direction of movement, and namely on a common path of motion, namely a vertically extending straight line  47  relative to the fixed sub-assembly made up of the chair column and guide element  23 ′. The relative motion of the moving unit to the chair column takes place, therefore, without the position of the seat support  4 ′ and the base support  2 ′ being altered relative to one another. 
     As, together with the base support  2 ′, the guide pulley  62  fastened to the base support  2 ′ is also moved downward in the vicinity of the guide opening  23 ′, the control cable  60 ,  60 ′ is driven so that the cable length between the guide pulley  62  and the adjusting strip  59  is reduced. As a result, the adjusting strip  59  is displaced on the shafts of the abutment extension arms  45  in the direction of the supporting strip  48 , whereby the helical compression springs  44  are compressed to a greater degree and a greater compressive force is exerted on the supporting strip  48  and thus on the seat support  4 ′. As a result of the increased pretensioning of the helical compression springs  44 , when a load is applied to the backrest, the pivoting motion S of the seat support  4 ′ and backrest support  5 ′ takes place in the pivoting direction S against a greater “pivoting resistance”. 
     As a result of the loading of the seat support  4 ′ by the user, therefore, in this embodiment initially an adjustment of the pivoting resistance also takes place independently of a pivoting motion of the backrest. However, it is also provided in the present mechanism that the pivoting resistance is altered by the pivoting of the backrest itself. 
     When pushing back the backrest, the backrest support  5 ′ is namely pivoted to the rear. Said backrest support therefore pivots the seat support  4 ′ downward to the rear around the turning-and-sliding joint in the front region of the seat support  4 ′. At the same time, the supporting strip  48  is displaced closer to the end of the abutment extension arms  45 , so that the helical compression springs  44  are compressed to a greater degree and thus create a greater counter force. If the backrest is unloaded, the seat support  4 ′ is again pivoted upward to the front by the helical compression springs  44 , the backrest support  5 ′ being pivoted at the same time. 
     A third embodiment of the invention, which shows the adjustment of the pretensioning of a spring arrangement, is shown in  FIGS. 10 to 13 . The synchronous mechanism already shown in  FIGS. 1 to 4  is substantially used as a basis. 
     The synchronous mechanism  1 ″ has a base support  2 ″ which, in a manner disclosed below in detail, is connected to the upper end of a chair column (not illustrated). The synchronous mechanism comprises a substantially frame-shaped seat support  4 ″ (not illustrated in  FIG. 10  for reasons of clarity) and a backrest support  5 ″ which is fork-shaped in plan view, the cheeks  6 ″,  7 ″ thereof again being arranged on both sides of the base support  2 ″. Moreover, the synchronous mechanism comprises a spring arrangement described below in more detail for acting on the mechanism, counter to the movement of the backrest support  5 ″. The seat (not shown) provided with an upholstered seating surface is mounted on the seat support  4 ″. 
     A backrest, not shown in more detail, which in modern office chairs is height-adjustable, is attached to the backrest support  5 ″. The backrest may be also integrally connected to the backrest support  5 ″. 
     The entire synchronous mechanism  1 ″, as regards the actual kinematics, is of mirror-symmetrical construction, relative to the central longitudinal plane M (see  FIG. 11 ). In this regard, the following description is always based on structural elements which are present in pairs on both sides of the actual pivoting mechanism. 
     The backrest support  5 ″ is firstly directly connected in an articulated manner by the lower end  12 ″ of the cheek  7 ″ oriented to the front, to the base support  2 ″, namely mounted on separate pivot elements  13 ″ on the base support  2 ″, such that the backrest support  5 ″ is approximately centrally articulated via the pivot elements  13 ″ directly on the base support  2 ″. As a result, the backrest support  5 ″ may be pivoted with the backrest about the central longitudinal axis  14 ″ in the pivoting direction S extending through the pivot elements  13 ″. Secondly, the backrest support  5 ″ is connected by the upper end  15 ″ of the cheek  7 ″ via a joint  16 ″ to the seat support  4 ″ at the rear end region  17 ″ thereof. By pivoting the backrest, therefore, the seat support  4 ″ is also driven and lowered in the pivoting direction S. In other words, the joint  16 ″ pivots about the joint axis  19 ″. The geometry of the pivoting mechanism used has the advantage that a high down-tilt angle of the seat support  4 ″ may be achieved, without the pivot angle of the backrest having to be too great which might lead to a position similar to a reclining position. Thus the so-called “riding-up effect” of clothes is effectively avoided. 
     The seat support  4 ″ is at its front end region  21 ″ connected to the base support  2 ″ via a turning-and-sliding joint (not shown in detail). For the design of the turning-and-sliding joint—and for further structural details of the mechanism—reference is made to the contents of the printed patent specification DE 10 2005 003 383 as has already been made with reference to the first embodiment. 
     Due to the shape of the backrest support  5 ″ and the arrangement thereof on the base support  2 ″ and the seat support  4 ″, when a load is applied to the backrest, the backrest support  5 ″ carries out, on the one hand, a pivoting motion in the pivoting direction S downward to the rear. As a result of the pivoting motion, however, the seat support  4 ″ is both pivoted downward to the rear and also displaced horizontally to the rear in the region of the turning-and-sliding joint. As a result, no significant lifting motion of the front end of the seating surface results, whereby the underside of the thigh is prevented from being trapped. 
     The synchronous mechanism  1 ″ is pretensioned by a spring arrangement counter to the pivoting direction S—i.e. to the initial position of the synchronous mechanism. This spring arrangement  50 ″ is provided in the form of two leg springs  41 ″,  41 ′″ aligned with one another in the transverse direction. The leg springs  41 ″,  41 ′″ exert a spring force counter to the pivoting motion S of the backrest oriented to the rear, which may be varied by the adjusting mechanism. 
     The leg  42 ″ of the leg spring  41 ″ facing to the rear, thus extends through a receiver opening  51  in the backrest support  5 ″ and is supported on a prismatic guide (not illustrated) on the backrest support  5 ″, whilst the second leg  43 ″ extending to the front, is supported on a prismatic guide  52 . The position of the two prismatic guides  52  may be vertically adjusted by means of a common adjusting mechanism  20 , not explained in more detail, via a hand wheel or the like, whereby the pretensioning of the leg springs  41 ″,  41 ′″ may also be manually adjusted, by altering the position of the spring leg  43 ″. 
     In this third embodiment, therefore, the leg springs  41 ″,  41 ′″ are not positioned about a pivot pin. Instead, they are located in a vertically displaceable holding tray  53 , which forms part of the adjusting mechanism of this embodiment. The holding tray  53  forms in this case a receiver for the leg springs  41 ″,  41 ′″ configured in the manner of a prism. 
     The adjusting mechanism is substantially formed by a vertical linear guide  30 ″, which is designed as part of the base support  2 ″. The linear guide  30 ″ comprises a square guide opening  22 ″ arranged in the base support  2 ″, as well as a guide element  23 ″ located in the guide opening  22 ″. The guide opening  22 ″ is in this case formed by suitable corresponding sub-elements  24 ″ (in this case housing parts) of the base support  2 ″. On the underside  25 ″ of the guide element  23 ″ a conical receiver  3 ″ is provided for fastening the upper end of the chair column. In other words, the chair column and guide element  23 ″ in the assembled state form a sub-assembly which is fixedly located in the guide opening  22 ″ of the base support  2 ″. For secure guidance of the guide element  23 ″ in the guide opening  22 ″, eight vertically extending guide strips  54  are provided in the guide opening  22 ″ which correspond to corresponding guide grooves (not shown) of the guide element  23 ″ see  FIG. 11 . 
     The guide element  23 ″ has an arm  64  extending forward out of the guide opening  22 ″ in the direction of the leg springs  41 ″,  41 ′″, in the upper face  65  thereof the holding tray  53 , which extends in the transverse direction, being provided for receiving the leg springs  41 ″,  41 ″′. The front sub-element  24 ″ of the base support  2 ″ has to this end a corresponding vertical opening  66 . 
     In an unloaded state of the seat support  4 ″, as illustrated in  FIG. 12 , the legs  42 ″,  43 ″ extend substantially in a linear manner, preferably from below at the front to above at the rear, i.e. the prismatic guide  52  is arranged lower down in the synchronous mechanism  1 ″ than the prismatic guide in the backrest support  5 ″ arranged behind the receiver opening  51  (and concealed in the figures). If the seat support  4 ″ is fully loaded as a user has sat down on the office chair, the moving unit, formed from the seat support  4 ″ and base support  2 ″, is as a whole moved downward in the direction of movement, and namely on a common path of motion, namely a vertically extending straight line  18 ″, relative to the fixed sub-assembly consisting of the chair column and the guide element  23 ″, see  FIG. 13 . The relative motion of the moving unit to the chair column takes place, therefore, without the position of the seat support  4 ″ and the base support  2 ″ being altered relative to one another. 
     By the relative movement of the guide element  23 ″ to the base support  2 ″ the arm  64  of the holding tray  53  is lifted relative to the remaining mechanism. As a result, the vertical position of the spring center point  56 ″ of the leg springs  41 ″,  41 ′″ is altered both relative to the fixed prismatic guide  52  in the base support  2 ″ and to the fixed prismatic guide in the backrest support  5 ″ arranged behind the receiver opening  51 . In other words, a simultaneous pretensioning takes place of both spring legs  42 ″,  43 ″ of the leg springs  41 ″,  41 ′″. This has the result that the resistance against a pivoting motion of the backrest support  5 ″ is markedly increased in the pivoting direction S. 
     Due to the loading of the seat support  4 ″ by the user, therefore, initially an adjustment of the pivoting resistance takes place independently of a pivoting motion of the backrest. However, in the present mechanism, it is also provided that the pivoting resistance is altered by the pivoting of the backrest itself. 
     As the leg spring  41 ″ is floatingly mounted on both sides, when pivoting the seat support  4 ″ downward to the rear, i.e. in the pivoting direction S, the point of articulation of the rear spring leg  42 ″ is displaced. The position of the point of articulation thus alters when a load is applied to the backrest such that the point of articulation is displaced in the direction of the spring center point  56 ″. As a result, an automatic alteration to the spring behavior of the leg spring  41 ″ additionally takes place when moving in the pivoting direction S. In other words, when pivoting the seat, the leg spring  41 ″ and thus the seat as a whole automatically become more rigid. 
     A fourth embodiment of the invention which shows the adjustment of the spring rate of a spring arrangement  75 , is shown in  FIGS. 14 to 19 . 
     The entire synchronous mechanism  1 ″′ as regards the actual kinematics is again of mirror-symmetrical construction relative to the central longitudinal plane M. In this respect, the following description is always based on structural elements of the mechanism present in pairs on both sides. 
     As a supporting part of the synchronous mechanism  1 ″, again a base support  2 ′″ is provided which in the region of its rear end  34 ′ in a manner described in detail below is connected to the upper end of a chair column (not illustrated). Further basic components of the synchronous mechanism  1 ′″ are the backrest support  5 ′″ and the seat support  4 ′″. 
     The backrest support  5 ′″ consists of two side struts  36 ′,  37 ′ extending to the rear, which form the connection to the actual backrest (not shown). 
     In the region of its rear end the seat support  4 ′″ forms, together with a corresponding upwardly projecting bearing projection  8 ′,  9 ′ on the two side struts  36 ′,  37 ′ of the backrest support  3 ′″, a pivot bearing about a transverse shaft  41 ′. The pivot bearing is in this case arranged behind the connection with the chair column. 
     Two upwardly projecting bearing posts  39 ′ are formed just in front of the front end region  58 ′ of the base support  2 ′″ on both sides of the central longitudinal plane M. The bearing posts  39 ′ form with the front regions of the substantially plate-shaped seat support  4 ′″ a turning-and-sliding joint (not shown in detail) whereby a movement of the seat support  4 ′″ is possible to the rear. For the design of the turning-and-sliding joint, reference is made to the contents of the printed patent specification DE 10 2005 003 383. Via the transverse shaft  41 ′, on the one hand, and the turning-and-sliding joint, on the other hand, the seat support  4 ′″ when pivoting the backrest support  5 ′″ is pivoted therewith to the rear. 
     The two side struts  36 ′,  37 ′ of the backrest support  5 ′″ are extended to the front beyond the bearing projections  8 ′,  9 ′ and pivotably mounted on the base support  2 ′″ in the region in front of the conical receiver  3 ′″ via a transverse shaft  35 ′. 
     For urging the synchronous mechanism  1 ′″ out of the initial position into a pivoted position, a spring arrangement  75  is provided which has two tension springs  67  parallel to one another and arranged on both sides of the central longitudinal plane M in a common horizontal plane (symbolized in  FIGS. 17 and 19 ). The tension springs  67  are in this case suspended with their ends  42 ′″,  43 ′″ respectively on transverse axes  68 ,  69  and connected thereto. The one transverse shaft  68  is located fixedly in the front end region  58 ′ of the base support  2 ′″. The other transverse shaft  69  is movable and is held by the cooperation of two linear guides in a working position. The linear guides are slot-like slotted guides  70 ,  71 . The tension springs  67  are in this case pretensioned toward the initial position of the synchronous mechanism. The position of the tension springs  67  does not play a crucial role for implementing the invention. However, the angle between the tension spring longitudinal axes  72 , on the one hand, and the first slotted guides  70 , on the other hand, is important, see  FIG. 19 . 
     These first slotted guides  70  are arranged in the side struts  36 ′,  37 ′ of the backrest support  5 ′″ extending to the front. In an unpivoted state, the first slotted guides  70  extend approximately vertically, the upper end of the slotted guides  70  relative to the lower end being slightly displaced to the rear, see  FIGS. 15 and 17 . The side struts  36 ′,  37 ′ extend in this case sufficiently far to the front that the transverse shaft  69  in the unpivoted state is located in any case in front of the transverse shaft  35 ′. In a maximum pivoted state to the rear, when it is located in the lower end position of the first slotted guides  70 , the transverse shaft  69  is positioned approximately above the transverse shaft  35 ′, see  FIG. 16 . If the transverse shaft  69  is located in this state in the upper end position of the first slotted guides  70 , the transverse shaft  69  is located behind the transverse shaft  35 ′, see  FIG. 18 . The position of the transverse shaft  69  in the first slotted guides  70  and thus the distance between the transverse shaft  69  and the transverse shaft  35 ′, as disclosed further below, is dependent on the weight and is adjusted by the user himself or herself, by sitting on the office chair. 
     The second slotted guide  71  extends horizontally and is arranged in a drive element  73  located in the central longitudinal plane M, which—similar to the arm  64  in FIGS.  12  and  13 —is integrally connected to the guide element  23 ′″ and vertically movable therewith, see  FIG. 17 . The drive element  73  thus extends from the guide element  23 ′″ out of the guide opening  22 ′″ to such an extent to the front that the first slotted guides  70  permit a displacement of the transverse shaft  69  from a first position in which the transverse shaft  69  is located in front of the transverse shaft  35 ′, into a second position, in which the transverse shaft  69  is located behind the transverse shaft  35 ′. If the transverse shaft  69  is located in a central position in the slotted guide  71 , it is positioned approximately above the transverse shaft  35 ′. The second slotted guide  71  serves, amongst other things, to allow a pivoting of the backrest support  5 ′″ and thus to allow a synchronous movement, generally in the structure according to the invention. 
     The tensile force of the tension springs  67  pretensioned between the transverse axes  68 ,  69 , urges the seat support  4 ′″ relative to the base support  2 ′″ forward into the initial position shown. The backrest support  5 ′″ is in this case in its maximum upright position. 
     If the spring rate of the tension springs  67  is to be altered, an adjusting mechanism is used. This is formed substantially by a vertical linear guide  30 ′″, which is designed as part of the base support  2 ′″. The linear guide  30 ′″ comprises a square guide opening  22 ′″ arranged in the base support  2 ′″ as well as a guide element  23 ′″ located in the guide opening  22 ′″. The guide opening  22 ′″ is in this case formed by suitable corresponding sub-elements  24 ′″ (in this case housing parts) of the base support  2 ′″. On the underside  25 ′″ of the guide element  23 ′″ a conical receiver  3 ′″ is provided for fastening the upper end of the chair column. In other words, the chair column and the guide element  23 ′″ in the assembled state form a sub-assembly which is fixedly located in the guide opening  22 ′″ of the base support  2 ′″. For secure guidance of the guide element  23 ′″ in the guide opening  22 ′″, a number of rollers are provided in the guide opening  22 ′″ for forming roller bearings  74 . Similar to the third embodiment, the front sub-element  24 ′″ of the base support  2 ′″ again has a vertical opening  66 ′ for the drive element  73 , see  FIG. 14 . 
     As shown in  FIG. 17 , in the loaded state of the seat support  4 ′″ the guide element  23 ′″ with its upper end  27 ′″ bears against a stop  26 ′″ formed by the base support. 
     The transverse shaft  69  held movably in the slotted guides  70 ,  71  is used, therefore, for transmitting the weight of the user to the tension springs  67 . The position thereof in the slotted guides—and thus the spring rate of the tension springs  67 —is, according to the invention, determined by the weight of the user. 
     If a load is applied to the seat support  4 ′″ by a user sitting down on the office chair, as is indicated in  FIG. 17  by the arrow  47 ′, the moving unit formed by the seat support  4 ′″ and base support  2 ′″, is moved downward as a whole in the direction of movement and namely on a common path of motion, namely a vertically extending straight line  47 ′, relative to the fixed sub-assembly consisting of the chair column and guide element  23 ′″. The relative movement of the moving unit to the chair column takes place in this connection without the position of the seat support  4 ′″ and base support  2 ′″ being altered relative to one another. 
     When a load is applied to the office chair by a user, the transverse shaft  69  is driven by the second slotted guide  71  and moved in the first slotted guides  70  into a working position, whereby the spring rate is adjusted. If the user is relatively lightweight, the transverse shaft  69  thus remains in a lower position of the slotted guides  70 , see  FIGS. 15 and 16 . The spring stroke is relatively short. With a heavier user, the transverse shaft  69  is displaced into an upper position in the first slotted guides  70 , see  FIGS. 17 and 18 . The points of articulation of the tension springs  67  are moved apart from one another. The spring stroke is lengthened according to the weight of the user. As a result, by the altered spring rate of the tension springs  67  when a load is applied to the backrest, the pivoting motion S of the seat support  4 ′″ and the backrest support  5 ′″ takes place in the pivoting direction S against a greater “pivoting resistance”. In other words, by means of this structural solution, the “initial force” required for pivoting the backrest support  5 ′″ is automatically adjusted depending on the weight of the user. A “sudden drop” when a heavyweight user leans against the backrest, when the office chair has been previously used by a lighter user, is eliminated. 
     As a result of a load applied to the seat support  4 ′″ by the user, in this embodiment an adjustment of the pivoting resistance takes place independently of a pivoting motion of the backrest. Moreover, it is again provided that the pivoting resistance itself is altered by the pivoting of the backrest. When pushing back the backrest, the backrest support  5 ′″ is namely pivoted to the rear, whereby—much more markedly when a load is applied by a heavyweight user than with a lightweight user—the position of the transverse shaft  69  is again altered, and namely such that the pivoting resistance increases with increasing pivoting. 
     Whilst, therefore, in the first three embodiments the weight of the user is used to alter the pretensioning of the spring arrangement  75 , in the last-described embodiment, the spring rate is adapted to the spring arrangement  75 . For an explanation, reference is made to  FIGS. 20 to 22 , in which a schematic force-path diagram is provided for the first three embodiments. The lower characteristic curve KU represents the unloaded state, the upper characteristic curve KB the loaded state. The intervals F 1 , F 2  of the starting points from the base line correspond to the pretensioning of the spring arrangement. The increases Z 1 , Z 2  produced by the gradient corresponding to the spring stroke, after pivoting about a pivot angle, alpha, of for example 20°. 
     In  FIG. 21 , characteristic curves are provided as might be implemented by the fourth embodiment, when the first slotted guides  70  might be arranged exactly perpendicular to the longitudinal axes  72  of the tension springs  67 . In this case, a load applied by a user might exclusively lead to an alteration of the spring rate. This variant could also be implemented in an office chair. Both the loaded and the unloaded characteristic curves KB, KU start at a common starting point F 0 , which is irrespective of the weight of the user. Depending on the weight, however, with a pivoting angle of, for example, 20° a widely varying spring stroke Z 1 , Z 2  results. 
     In practice, however, the above-described oblique arrangement of the first slotted guides  70  has proved advantageous, whereby the characteristic curves KU, KB are produced as illustrated in  FIG. 22 . In addition to the alteration of the spring rate, which takes precedence and which is visible in the different increases Z 1 , Z 2 , with a pivoting angle of, for example, 20°, a slight alteration V to the pretensioning of the spring arrangement takes place at the same time, which is reflected in the different starting points of the characteristic curves. 
     With reference to the fourth embodiment, a safety device is described hereinafter by which an inadvertent adjustment of the spring arrangement  75 , adjusted by the weight, may be effectively avoided when pivoting the backrest support  5 ′″. The use of the safety device is not restricted to the fourth embodiment. The main principle of the safety device is, instead, easily able to be adapted to all embodiments of the invention as well as to other chair mechanisms. 
     When pivoting the backrest support  5 ′″, restoring forces act on the transverse shaft  69  in the direction of the spring longitudinal axis  72  on the transverse shaft  68 , on the one hand, and in the direction of the first slotted guides  70  on the transverse shaft  35 ′, on the other hand. In order to prevent the transverse shaft  69  when pivoting the backrest support  5 ′″ from moving in the first slotted guides  70 , a movable latching element  76  is provided which, during pivoting, automatically engages in a fixed retaining element  77  and locks the current setting of the spring rate and/or pretensioning, see  FIG. 19 . 
     As a latching element  76 , in the example disclosed here, a sleeve  78  is used which is mounted freely rotatably on the transverse shaft  69  which on one side thereof comprises a locking pawl  79  in the manner of a latching edge. The sleeve  78  is in this case fixed on the transverse shaft  69  in the region of one of the first slotted guides  70 , such that the locking pawl  79  points forward in the direction of the front inner wall  80  of the first slotted guide  70  and engages in one of the toothlike latching grooves  81  provided there, extending horizontally and used as retaining elements, which are distributed substantially over the entire length of the slotted guide  70 . In order to assist this engagement, a spring element is provided, urging the locking pawl  79  in the direction of the latching grooves  81 , for example in the form of a small leaf spring  82  or the like. As the disclosed latching is self-locking due to the weight loading by a user, a high spring force is not required, however, in order to bring the locking pawl  79  into a latching position. This takes place almost automatically, as soon as a pivoting of the backrest support  5 ′″ begins. 
     A release of the locking pawl  79  takes place automatically when pivoting forward the backrest support  5 ′″ into its initial position. In this connection, the latched locking pawl  79  projecting laterally over the slotted guide strikes to this end against a release block  83  arranged adjacent to the relevant inner wall  80  of the first slotted guide  70  and projecting thereover to the rear, so that the latching is released, see  FIG. 14 . Latching occurs again when the backrest support  5 ′″ is subsequently pivoted. 
     The latching element  76  does not have to be attached on one side; a separate latching element  76  may also be provided for every first slotted guide  70 . 
     The four embodiments described above merely represent preferred embodiments. The invention may also be used with further synchronous mechanisms as well as with asynchronous mechanisms. 
     The invention may also be implemented by other transmission means. With the use of a toothed belt as transmission means instead of the guide pulleys, as are used in the second embodiment, gearwheels are preferably used, which prevent the toothed belt from slipping through. Instead of the slotted guide  71  in the drive element  73  in the fourth embodiment, other guides, in particular open and/or partially open guides may also be used. The guide also does not have to be a linear guide. By using non-linear guides, further advantageous adjusting features of the mechanism may be produced. In particular, non-linear characteristic curves of the spring rate may be achieved as a result. 
     Instead of the above disclosed spring elements, other spring elements may also be used with the invention. Thus, for example, helical springs may be designed as compression springs or tension springs. Also other spring elements, such as for example elastomers or the like, may be used. Also the present invention may be combined with the most varied arrangements of the spring elements. Thus the spring elements may, for example, at positions other than those shown, be arranged in the seat support or base support or even be arranged in the backrest support. For example, the leg spring shown in  FIGS. 1 to 4  does not necessarily have to be positioned in front of the linear guide. In further embodiments, it may also be arranged to the rear, above or below the linear guide. 
     The path of motion of the moving unit formed from the seat support  4  and base support  2 , also does not necessarily have to extend vertically downward, see  FIG. 4 . In a further embodiment (not illustrated) of the invention it is provided that the linear guide comprising a guide opening  22  and guide element  23  is not arranged vertically, i.e. parallel to the vertical, but obliquely, i.e. at a specific angle relative to the vertical, in the base support  2  and/or the base support/seat support sub-assembly. In this case, the user sits on the office chair as shown in  FIG. 4 . The moving unit formed from the seat support  4  and base support  2 , however, does not move downward vertically in the direction of the straight line  18  but on a path of motion extending obliquely to the vertical, which is predetermined by the position of the linear guide. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Synchronous mechanism 
           2  Base support 
           3  Conical receiver 
           4  Seat support 
           5  Backrest support 
           6  Cheek 
           7  Cheek 
           8  Bearing projection 
           9  Bearing projection 
           10  Frame element 
           11  Latching lug 
           12  Lower cheek end 
           13  Pivot pin 
           14  Central longitudinal axis 
           15  Upper cheek end 
           16  Joint 
           17  Rear end region 
           18  Movement of the moving unit when loaded 
           19  Joint axis 
           20  Adjusting mechanism 
           21  Front end region 
           22  Guide opening 
           23  Guide element 
           24  Sub-element 
           25  Underside 
           26  Stop 
           27  Upper end 
           28  Transverse opening 
           29  Guide element 
           30  Linear guide 
           31  Underside 
           32  Through-opening 
           33  Underside 
           34  Rear end 
           35  Transverse shaft 
           36  Side strut 
           37  Side strut 
           38  Bearing post 
           39  Bearing post 
           40  Transverse shaft 
           41  Transverse shaft 
           42  Upper leg 
           43  Spring arrangement 
           44  Helical compression spring 
           45  Abutment extension arm 
           46  Bearing head 
           47  Movement of the moving unit when loaded 
           48  Supporting strip 
           49  Bearing recess 
           50  Spring arrangement 
           51  Receiver opening 
           52  Prismatic guide 
           53  Holding tray 
           54  Guide strip 
           55  Prismatic guide 
           56  Spring center point 
           57  Fastening screw 
           58  Front end region 
           59  Adjusting strip 
           60  Control cable 
           61  Opening 
           62  Guide pulley 
           63  Guide pulley 
           64  Arm 
           65  Upper face 
           66  Vertical opening 
           67  Tension spring 
           68  Transverse shaft 
           69  Transverse shaft 
           70  First slotted guide 
           71  Second slotted guide 
           72  Tension spring longitudinal axis 
           73  Drive element 
           74  Roller bearing 
           75  Latching element 
           76  Retaining element 
           77  Sleeve 
           78  Locking pawl 
           79  Inner wall 
           80  Latching groove 
           81  Leaf spring 
           82  Release block