Abstract:
A preloaded spring arrangement, in particular for spring loading office chair synchronizing mechanisms, comprises a pair of leg springs, which are coaxially positioned on a common axis, each having a supporting leg and a positioning leg; and a preferably manually operated adjusting unit of the type of an eccentric, which the two positioning legs are coupled with and by which they are displaceable for adjustment of the preload of the spring arrangement; wherein, for stepwise locked spring load adjustment, the adjusting unit of the type of an eccentric comprises a pair of eccentric cams sitting axially side by side, the cam surfaces of which, related to the direction of rotation of the eccentric arrangement, successively comprise plane locking sections, eccentric cam control sections and holding sections that are concentric of the axis of rotation of the eccentric arrangement; and wherein the respective locking, cam control and holding sections of the two eccentric cams are offset in the direction of rotation of the eccentric arrangement.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a preloaded spring arrangement, in particular for spring loading office chair synchronizing mechanisms, and to a synchronizing mechanism that comprises such a spring arrangement. 
     2. Background Art 
     DE 199 22 446 A1 teaches a synchronizing mechanism for correlated seat/backrest motion of an office chair, in which a spring arrangement acts on the synchronizing mechanism in the direction of its non-tilted normal position. This spring arrangement comprises a pair of leg springs, which are located coaxially on a common axis, each having a supporting leg and a positioning leg. The positioning legs support themselves on an adjusting mechanism for modification of the preload of the leg springs. The adjusting mechanism is put into practice by a wedge-type sliding transmission which is adjustable crosswise of the longitudinal axis of the synchronizing mechanism and the driving wedge of which is adjustable by way of a spindle drive from the side of the synchronizing mechanism. The respectively associated positioning leg of the leg spring supports itself on this driving wedge. 
     Problems posed by the prior art spring load adjustment reside in the comparatively complicated construction of the adjusting mechanism on the one hand, which comprises a spindle drive and two wedges resting one upon the other for translation of the transverse displacement of the driving wedge occasioned by the spindle drive into a longitudinal displacement of the driven wedge. Since the supporting force of the positioning legs of the leg springs acts, via the driven wedge, directly on the contact surface between the two wedges, strong friction, in particular static friction, is found within the adjusting mechanism in particular in the case of high preloads. This may lead to the adjusting mechanism being comparatively hard to operate. Furthermore, upon preload regulation, both positioning legs are shifted simultaneously and by the same displacement so that sensitive regulation does not go without problems. Moreover, the restoring force of both leg springs must be countered, which implies an increase in energy requirements. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to embody a preloaded spring arrangement such that constructional simplicity is accompanied with the possibility of spring load regulation with great sensitivity and a decrease in energy requirements. 
     This object is attained by an adjusting unit of the type of an eccentric, which the two positioning legs are coupled with, and by which they are displaceable for regulation of the preload of the spring arrangement. For stepwise locked spring-load regulation, the eccentric adjusting unit comprises a pair of cams sitting axially side by side, the cam surfaces of which, related to the direction of rotation of the eccentric arrangement, successively exhibit plane locking sections, cam control sections and holding sections that are concentric of the axis of rotation of the eccentric arrangement. Finally, the respective locking, cam control and holding sections of the two eccentric cams are offset from one another in the direction of rotation of the eccentric arrangement in such a way that while one of the two positioning legs passes a cam control section that occasions displacement and thus load adjustment, the second positioning leg runs along the concentric holding section without experiencing any adjustment. Once the cam control section has been passed, the corresponding positioning leg applies on the locking section, which leads to a defined position of rotation of the adjusting unit. The other positioning leg has reached the beginning of the holding section so that, upon further rotation of the eccentric unit, it applies on the concentric holding section, occasioning no load counter to the adjusting rotation. 
     The construction according to the invention and in particular the eccentric adjusting unit, which comprises a pair of eccentric cams with functional sections that are displaced one relative to the other, help obtain actuation of the spring arrangement in fine steps of locking and with comparative ease during load adjustment. The construction according to the invention excels by extreme simplicity, there being only the need of joining a rotatable element to a turning handle for instance by way of a shaft. 
    
    
     Details of the invention will become apparent from the ensuing description of an exemplary embodiment taken in conjunction with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagrammatic lateral view of the synchronizing mechanism in a normal position; 
     FIG. 2 is a lateral view by analogy to FIG. 1 in a backwards tilted position of the synchronizing mechanism; 
     FIG. 3 is a diagrammatic plan view of the synchronizing mechanism according to FIG. 2; 
     FIGS. 4 and 5 are perspective illustrations of details of the preloaded spring arrangement as used in the synchronizing mechanism according to FIGS. 1 to  3 ; and 
     FIG. 6 is a lateral view of this spring arrangement seen from the direction of the arrow VI according to FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preceding the detailed description of the spring arrangement, the fundamental structure of the synchronizing mechanism, which is denoted by  1  in its entirety, will be explained in conjunction with FIGS. 1 and 3. It comprises a base carrier  2  that is placed on the upper end of a chair column  4  by means of a cone receptacle  3 . Various constructional elements of the synchronizing mechanism  1  are outside and above the lateral cheeks  5  that run parallel to the longitudinal direction L of the chair. The core pieces thereof are a substantially frame-type seat carrier  6  and a backrest carrier  7  which is forked seen in a plan view. Mounted on the seat carrier  6  is the seat (not shown) with an upholstered seat panel. By way of an elbowed cross arm  18 , the backrest carrier  7  holds a backrest (not shown) which is vertically adjustable in today&#39;s office chairs. 
     In terms of kinematics, the entire synchronizing mechanism  1  is designed in mirror symmetry to the longitudinal center plane M as seen in particular in FIG.  3 . In this regard, the ensuing description regularly proceeds from constructional elements that are available bilaterally in pairs. 
     The backrest carrier  7  is articulated to the base carrier  2  by way of a cam arrangement. This arrangement comprises a first cam  8  which is articulated approximately centrally to a pivot bearing  9  on the base carrier  2 . A second cam  10  is mounted between the front cam  8  and the cone receptacle  3  on a pivot bearing  11  on the base carrier  2 . The free ends of the two cams  8 ,  10  are coupled with the backrest carrier  7  by way of joints  12 ,  13 . The two pivot bearings  9 ,  11  and the joints  12 ,  13  define a four-bar chain in which the backrest carrier  7  itself forms the coupling by its respective forked leg  14 . In the normal position of the backrest carrier  7  seen in FIG. 1, the front cam  8  is positioned approximately vertically upwards, while the rear cam  10  inclines backwards. In this case, the longitudinal axes  15 ,  16  of the cams  8 ,  10  that pass through the points of articulation make an acute angle W (FIG. 1) slightly greater than 30° that opens upwards toward the seat carrier  6 . The ratio that the length of the front cam  8  bears to the length of the rear cam  10  is approximately 2.5:3. Owing to this design and the arrangement of the four-bar chain, the backrest carrier  7  makes a combined rotary pivoting motion downward to the rear, which is roughly outlined by the arrow  17  in FIG.  1 . 
     The seat carrier  6  is coupled with the backrest carrier  7  before its rear end  19 , via a bearing lug  20 , to the axis that forms the front joint  12  as seen in FIG. 1, its rear end thus being linked. Consequently, the joint between the seat carrier  6  and the backrest carrier  7  is integrated into the front joint  12  between the cam  8  and the backrest carrier  7 . At its front end  21 —which is on the left in FIG.  1 —the seat carrier  6  is linked to the base carrier  2  by a turning and sliding joint which is denoted by  22  in its entirety. On the one hand it is comprised of an oblong-hole-type connecting member  25  which is incorporated in the longitudinal legs  23 ,  24  that are located on both sides; on the other hand a bearing journal  26  engages from inside with the connecting member  25 . The bearing journal  26  is molded on a prolongation  27  of the base carrier  2 , standing out therefrom at right angles to the longitudinal center plane M and reaching into the connecting member  25 . 
     The synchronizing mechanism  1  is biased by a spring arrangement F counter to the direction of the arrow  17 —i.e. towards the normal position of the synchronizing mechanism  1 . This spring arrangement F is available in the form of two leg springs  28  (FIG. 3) which are in alignment in the cross direction and positioned around the axis  21  that constitutes the pivot bearing  9  of the front cam  8 . The upward leg  29  supports itself on a projection  30  on the seat carrier  6 , whereas the second forward leg  31  supports itself on an adjusting mechanism  32  in the base carrier  2 . The leg springs  20  exercise spring-loading counter to the backward pivoting motion of the backrest which is variable through the adjusting mechanism  32  by actuation of a turning lever  33 . 
     As can be seen from a comparison of FIGS. 1 and 2, the backrest carrier  7 , upon actuation of the backrest rearwards, makes the pivoting motion downward to the rear that is roughly outlined by the arrow  17 , with the rear cam  10  and the front cam  8  of the four-bar chain also tilting further backwards. In the case of a maximal pivoting angle of the backrest carrier  7 , the angle W between the longitudinal axes  15 ,  16  of the two cams  8 ,  10  is approximately 20° (FIG.  2 ). The four-bar chain folds up, as it were, compared to the spread initial position according to FIG. 1 so that this quite compact arrangement becomes even smaller. 
     Another contribution to the compact arrangement resides in that the distance  a  of the two joints  12 ,  13  which are located between the backrest carrier  7  and the cams  8  and  10 , respectively, is approximately equal to the length L10 of the rear cam  10  and, in the ratio specified above, greater than the length L8 of the front cam  8 . 
     By means of the mentioned pivoting motion of the four-bar chain with the backrest carrier  7 , the seat carrier  6  is pivoted downwards to the rear as well as displaced horizontally to the rear in the vicinity of the turning and sliding joint  22 . As a result, there is no relevant lift of the front end  21  of the seat panel, which helps avoid constrictions or pressure on the lower side of the thighs. 
     The synchronizing mechanism  1  is designed in such a way that, in the final position of backward tilt seen in FIG. 2, the backrest carrier  7  passes through a pivoting angle W7 of approximately 26°, while the pivoting angle W6 of the seat carrier  6  is approximately 15°. The pivoting angle W7 to W6 ratio is approximately 1.8:1 in the position of maximal tilt. 
     As seen in FIGS. 1 and 3, the cams  8 ,  10 , which are attached externally to the seat carrier  6 , have an approximately reniform widened sheet configuration, there being overlap of the cams  8 ,  10  in any position of pivot between the two positions according to FIGS. 1 and 2 and in combination with the bearing cheeks  34  of the forked backrest carrier  7  that apply externally on the cams  8 ,  10 , so that there is no possibility of reach-through between the cams  8 ,  10 , base carrier  2  and backrest carrier  7 . In this way, the fingers of someone who sits on the chair are efficiently protected against getting stuck when the synchronizing mechanism is pivoted. 
     In a manner not shown in detail, the synchronizing mechanism  1  is lockable in various positions between the normal position (FIG. 1) and the position of maximal backward tilt (FIG.  2 ). The figures do not explicitly show the corresponding locking mechanism and there is no need of detailed specification because it is prior art. Attention is only drawn to the fact that locking takes place by means of another operating lever  35  on the side of the turning lever  33 . The operating lever  36  on the other side serves for releasing the vertical adjustment of the chair column  4 . 
     The configuration of the spring arrangement F and its adjusting mechanism  32  is going to be explained in conjunction with FIGS. 4 to  6 . These drawings illustrate the two leg springs  28 . 1 ,  28 . 2 , which are positioned coaxially on the pivot bearing axis  9 , and their upward supporting legs  29 . 1 ,  29 . 2  and the two positioning legs  31 . 1 ,  31 . 2 , which extend towards the adjusting mechanism  32 . The projection  30  that serves as an abutment for the two supporting legs  29 . 1 ,  29 . 2  has been omitted. 
     The gist of the adjusting mechanism  32  is a single-piece double eccentric cam  40  comprised of two eccentric cams  41 . 1 ,  41 . 2  sitting axially side by side, with in each case one of the positioning legs  31 . 1 ,  31 . 2  of the respective leg spring  28 . 1 ,  28 . 2  supporting itself thereon. 
     The outward cam surfaces  42 . 1 ,  42 . 2  of the eccentric cams  41 . 1 ,  41 . 2  are divided into successive sections seen in the direction of rotation of the eccentric arrangement. The cam surface  42 . 2  of the eccentric cam  41 . 2 , which is shown in solid lines in FIG. 6, starts with a first plane locking section  43 . 2  that is followed by a first cam control section  44 . 2  that ascends radially outwards as a coplanar, plane prolongation. The cam surface  42 . 2  is continued by a first holding section  45 . 2 , along which the cam surface passes through an angle of rotation of approximately 60° concentrically of the axis of rotation, formed by the shaft  46 , of the double eccentric cam  40 . Sequential thereto as continuous, coplanar, plane surfaces are a second locking section  47 . 2 , a second cam control section  48 . 2  of eccentrically outward ascent, a second holding section  49 . 2 , which is concentric of the shaft  46 , a third locking section  50 . 2 , a third cam control section  51 . 2 —both sections  50 . 2 ,  51 . 2  are coplanar—and a third concentric holding section  52 . 2 . 
     The cam surface  42 . 1  of the other eccentric cam  41 . 1  is shown in FIG. 6 in a solid line that is thinner as compared to the cam surface  42 . 2 , because it is hidden behind the eccentric cam  41 . 2  in the viewing direction of FIG.  6 . As seen in this drawing, the cam surface  42 . 1  starts radially inwardly by a first holding section  45 . 1 , which is again concentric of the shaft  46  and which is followed by a first locking section  43 . 1  and a first cam control section  44 . 2  in the form of a plane prolongation, which coplanar thereof. The cam control section  44 . 2  passes into a second holding section  49 . 1 , which is continued by a second locking section  47 . 1  with a second cam control section  48 . 1  in the form of a coplanar prolongation. Sequential thereto is a third holding section  52 . 1  (again concentric of the shaft  46 ), which is continued by a third locking section  50 . 1  and a subsequent third cam control section  51 . 1 . This cam control section  51 . 1  passes into a final short fourth locking section  53 . 1 . 
     As seen especially in FIG. 6, the locking sections  43 . 2 ,  47 . 2 ,  50 . 2 , the cam control sections  44 . 2 ,  48 . 2 ,  51 . 2  and the holding sections  45 . 2 ,  49 . 2  and  52 . 2  of the cam surface  42 . 2  and the locking sections  43 . 1 ,  47 . 1 ,  50 . 1 , the cam control sections  44 . 1 ,  48 . 1 ,  51 . 1  and the holding sections  45 . 1 ,  49 . 1  and  52 . 1  of the first cam surface  42 . 1  are offset by an angle V in the range of 40° to 70°. 
     The design of the cam surfaces  42 . 1 ,  42 . 2  of the two eccentric cams  41 . 1 ,  41 . 2  gives rise to the following functional interaction between the two leg springs  28 . 1 ,  28 . 2 : 
     By way of example, the initial position is the position of least deflection of the positioning legs  31 . 1 ,  31 . 2  seen in FIG.  4 . In this position of rotation of the double eccentric cam  40 , the positioning leg  31 . 2  bears against the first locking section  43 . 2 , parallel thereto, of the cam surface  42 . 2 . The load K it exercises on the eccentric cam  41 . 2  extends radially towards the axis of rotation of the eccentric (shaft  46 ) so that the positioning leg  31 . 2  does not exercise any load in terms of rotation of the shaft  46 . Any rotation would only be occasioned by a turning moment on the shaft  46 , whereby the position seen in FIG. 4 is quasi locked. The positioning leg  31 . 1  of the other leg spring  28 . 1  rests by point contact on the first holding section  45 . 1  of the cam surface  42 . 1 , as a result of which no turning moment is exercised on the double eccentric cam  40 . 
     For increased preload of the spring arrangement F, the shaft  46  is rotated counter-clockwise in FIGS. 4 and 5 (clockwise in FIG. 6) by the turning lever  33  for the first cam control section  44 . 2  of the cam surface  42 . 2  to deflect the positioning leg  31 . 2  more strongly, which augments the preload of the leg spring  28 . 2 . Simultaneously, the positioning leg  31 . 1  applies neutrally on the first holding section  45 . 1  of the cam surface  42 . 1  so that only the preload of the leg spring  28 . 2  has to be countered for rotation. After an angle of rotation of for instance 66°, the positioning leg  31 . 2  of the cam surface  42 . 2  bears against the first holding section  45 . 2 , whereas the positioning leg  31 . 1  rests on the first locking section  43 . 1 . In this position of rotation, the eccentric cam is again locked into place, because none of the two positioning legs  31 . 1 ,  31 . 2  exerts a turning moment on the double eccentric cam  40 . Any application of load by the positioning leg  31 . 1  on the first locking section  43 . 1  again takes place in a direction radial of the axis of rotation (shaft  46 ). 
     Upon further rotation, the first cam control section  44 . 1  of the cam surface  42 . 1  takes action so that the leg spring  28 . 1  is more strongly biased by the positioning leg  31 . 1  shifting correspondingly, while the positioning leg  31 . 2  applies neutrally on the first holding section  45 . 2  of the cam surface  42 . 2 . Again, only the spring load of one leg spring  28 . 1  has to be countered during this further rotation. 
     This interaction between the actuation of a positioning leg by the cam control section of the corresponding cam surface and the simultaneous neutral application of the other positioning leg on the respective holding section continues beyond the intermediate position seen in FIG. 5, terminating in the final position of maximal deflection seen in FIG.  6 . In FIG. 5, the positioning leg  31 . 1  of the first cam surface  42 . 1  bears for instance against the second locking section  47 . 1 , while the positioning leg  31 . 2  of the other cam surface  42 . 1  rests on the second holding section  49 . 2 . In the final position, both positioning legs  31 . 1 ,  31 . 2  are maximally deflected, with the positioning leg  31 . 1  resting on the end of the last holding section  52 . 1  and the positioning leg  31 . 2  on the fourth locking section  53 . 2 . All in all, the seven locking sections  43 . 1 ,  43 . 2 ,  47 . 1 ,  47 . 2 ,  50 . 1 ,  50 . 2  and  53 . 2  define seven lock-in stages of spring load adjustment by simple manual operation of the shaft  46  via the turning lever  33 .