Patent Publication Number: US-8985688-B2

Title: Office chair mechanism provided with a device for adjusting the swivel force

Description:
FIELD OF THE INVENTION 
     The present invention relates to an office chair mechanism provided with a device for adjusting the swivel force. 
     BACKGROUND OF THE INVENTION 
     To modify the swivel force for the different body weights and preferences of different office chair users, various mechanism adjustment devices have already been proposed, however these mostly act on the preloading of springs. 
     If compression springs are used, an adjustment knob coaxial to the spring is operated. However, if torsion springs are used, the preload is varied by using elements which pull or push the free end of the spring. 
     The main drawback of this system is the fact that the preload adjustment cannot be too large otherwise insurmountable problems are introduced relating to spring reliability and chair design compactness. In addition to overcome the spring force during adjustment, an always critical compromise has to be reached between the physical force and the down-gearing (large number of screw turns) required for the adjustment. However the main drawback from the ergonomic viewpoint is that because the force increase during swiveling is always constant and if a “light” user encounters excessive hardness during swiveling, this for a “heavy” user could possibly be insufficient. 
     To obviate these drawbacks various solutions have already been proposed, such as those described in EP 1874161 or in U.S. Pat. No. 6,238,000, but these are costly and bulky. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to eliminate these drawbacks by providing an adjustment device which while being compact, sufficiently economical and of simple and very wide adjustment, is also ergonomically suitable for both very light and very heavy users. 
     This and other objects which will be apparent from the ensuing description are attained according to the invention by an office chair mechanism as described in hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further clarified hereinafter with reference to the accompanying drawing, in which: 
         FIG. 1  is a lateral schematic view of a chair provided with the device of the invention in its non-stressed condition, 
         FIG. 2  shows it in the same view as  FIG. 1  but in the stressed condition, 
         FIG. 3  is an exploded perspective view of the device of the invention showing the adjustment system in detail, 
         FIGS. 4 and 5  are schematic views of the device of the invention shown respectively in the chair empty and chair stressed configuration, 
         FIGS. 6 and 7  show the device in the same views as  FIGS. 4 and 5  but with the fulcrum in a different position, 
         FIGS. 8 and 9  are schematic views of two different embodiments of the device of the invention, 
         FIGS. 10 ,  11  and  12  show a detail of the device of the invention for locking chair swiveling. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     As can be seen from the figures, the adjustment device according to the invention is applied to an office chair mechanism consisting of a fixed box structure  2  provided lowerly with a frusto-conical bush for inserting the upper end of the shaft  4  of a traditional gas spring (not represented in the drawings), the function of which is to sustain the support as it swivels relative to a resting base  6 . 
     The fixed box structure  2  is hinged about a pin  8  slidable within a horizontal slotted hole  10  provided in a support plate  12  for the chair seating portion  14 . 
     A bar  18  rigid with a support arm  20  for the back rest  22  is pivoted about a pin  16  on the plate  12 . 
     The beam is finally pivoted to the box structure  2  about a pin  24 . 
     About the same pin  16  there is pivoted one end  26  of a lever shaped bar  28  with its fulcrum on a contact element  30 , in the illustrated example a pin, selectively positionable along two resting surfaces  32 ,  32 ′ provided on the box structure  2  and on the lever  28 . 
     The other end  36  of the lever  28  acts on a helical spring  38  which acts against the surface of the box structure (see  FIGS. 4-7 ) or against the chair support plate  12  (see  FIG. 8 ) or against the support bar  18  for the back rest  22  (see  FIG. 9 ). 
     The adjustment of the position of the contact element  30  takes place along said parallel resting surfaces  32 ,  32 ′, by which any other movement of the helical spring  38  and of the lever  28  is avoided. The element which undergoes movement is not subjected to compression when the surfaces are parallel, i.e. when the chair is in the rest position, whereas it becomes the fulcrum of the lever  28  during swiveling. The elements involved in the adjustment are therefore reduced to a minimum. This characteristic therefore enables easy and quick adjustment while using reliable low-cost components, for example by using a simple cam device  34  which moves the contact element (pin)  30 . 
     The lever has a shape such that the contact element  30 , following its adjustment movement, maximizes the difference between the two lever arms. 
     This is obtained preferably by positioning the three points:
           36  (spring resting end/lever)     30  contact element     16  articulation axis lever/movable parts
 
by aligning them along a single axis preferably perpendicular to the direction of the forces and by moving the contact element along said axis.
       

     The cam device is mounted on a single control rod  40  which is provided at one end with a knob  42  for rotating the cam  34  and with a knob  44  for operating a gas spring, and at the other end with a knob  46  for locking the rotation of the back rest. 
     To facilitate sliding of the element  30  which forms the lever fulcrum along the surfaces  32 ,  32 ′, slide shoes  48  of self-lubricating plastic material are provided (wrapped about the pin). 
     In the rear part of the box structure a catch is provided formed from two plates  50  operable axially by an elastic puller  52  connected to the knob  46  controlling the locking operation. 
     The plates  50  are selectively insertable into two of a plurality of facing seats  54  (six in the illustrated example) provided in the bar  18  such as to lock its rotation and hence lock the chair. 
     The operating principle of the device of the invention is as follows. 
       FIGS. 4 and 5  show the spring compression as the chair passes from a configuration in which the chair is not stressed to a configuration in which a user is seated on the chair and has inclined the back rest rearwards to hence stress the spring. 
     Operating the knob  42  of the force adjustment device causes the contact element  30  to move, this constituting the fulcrum of a first order lever  28  having the elastic means  38  at one end  36  and, at the other end  26 , the pin  16  with one of the movable parts  18  of the geometry of which the mechanism is composed. By shifting the position of this fulcrum relative to the power and resistance points positioned at the two ends  26 ,  36  of the lever, the lever effect can be adjusted by inversely varying the length of the arm of the spring relative to that of the arm connected to the movable part  16 . 
       FIGS. 6 and 7  show the chair in the same configurations as in  FIGS. 4 and 5 , but with the fulcrum  30  shifted along the surfaces  32 ,  32 ′ in the sense of causing it to approach the axis of the pivot pin  16 . 
     From a comparison of these figures it is clear that the spring  38 , for the same back rest inclination (h 1 -h 4 ), is more greatly compressed in the configuration shown in  FIG. 7  than in the configuration shown in  FIG. 5 , so that it offers greater swivel resistance. 
     Again comparing  FIGS. 5 and 7 , as the lever  28  is in equilibrium on the fulcrum  30 , the moment of rotation R due to the reaction of the pin  16  about the fulcrum  30  is equal to the moment of rotation due to the thrust P of the spring about the same fulcrum  30 . 
     Hence in  FIG. 5 :
 
 Rb   1   =Pb   2  
 
from which
 
 R=Pb   2   /b   1  
 
     Then on shifting the fulcrum, in  FIG. 7  we have
 
 R′=Pb   2   ′·b   1 ′
 
Now as
 
 b   2   ′&gt;b   2  
 
and
 
 b   1   ′&lt;b   1 ,
 
then
 
 R′&gt;&gt;R  
 
with values of R′ double or even triple the values of R.
 
     In this manner, although using the same spring, persons of 40 kg and persons of 120 kg can both be supported by shifting the fulcrum  30 . 
     From the aforegoing it is apparent that the device of the invention presents numerous advantages, and in particular: 
     chair versatility in that a mechanism is obtained with very wide force adjustment, 
     very valid ergonomics in that the adjustment control is easy and rapid, 
     less constructional complexity than similar devices and hence lesser costs, 
     a compact mechanism design enabling the chair appearance to be improved.