Abstract:
A multi-stage backrest assembly includes a support frame, a back plate, a head plate, a waist plate, a first elastic swivel device, and two second elastic swivel devices. Thus, the multi-stage backrest assembly has a pivotable function so as to fit the curve of the user&#39;s body ergonomically, thereby providing a comfortable effect to the user.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a multi-stage backrest assembly, and more particularly to a multi-stage backrest assembly for a chair. 
   2. Description of the Related Art 
   A conventional backrest  8  for a chair in accordance with the prior art shown in  FIG. 7  comprises a main body  80  made of foam material and formed with a fixing plate  82  secured on a seat (not shown) for mounting the main body  80  of the backrest  8  to the seat. However, the conventional backrest  8  has a fixed structure and cannot be adjusted so as to fit the curve of the user&#39;s body ergonomically, so that the user easily feels uncomfortable during a long period of time. 
   SUMMARY OF THE INVENTION 
   The primary objective of the present invention is to provide a multi-stage backrest assembly having a pivotable function so as to fit the curve of the user&#39;s body ergonomically, thereby providing a comfortable effect to the user. 
   Another objective of the present invention is to provide a multi-stage backrest assembly, the waist plate is pivotable relative to the back plate so as to fit the user&#39;s waist and back ergonomically, thereby providing a comfortable effect to the user. 
   A flier objective of the present invention is to provide a multi-stage backrest assembly, wherein the crossbar of the head plate is pivoted relative to the support bar of the head plate by one of the two second elastic swivel devices, so that the head plate has a pivotable function. 
   A further objective of the present invention is to provide a multi-stage backrest assembly, wherein the crossbar of the waist plate is pivoted relative to the support bar of the waist plate by the other one of the two second elastic swivel devices, so that the waist plate has a pivotable function. 
   In accordance with the present invention, there is provided a multi-stage backrest assembly, comprising a support frame, a back plate, a head plate, a waist plate, a first elastic swivel device, and two second elastic swivel devices, wherein:
         the back plate is mounted on the support frame;   the head plate is mounted on the support frame and includes a support bar, and a head frame pivotally mounted on an upper end of the support bar by one of the two second elastic swivel devices; and   the waist plate is attached to the back plate and includes a support bar having a bent first end pivotally mounted on the back plate by the first elastic swivel device, and a waist frame pivotally mounted on a second end of the support bar by the other one of the two second elastic swivel devices.       

   Further benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is perspective view of a multi-stage backrest assembly in accordance with the preferred embodiment of the present invention; 
       FIG. 2  is a partially exploded perspective view of the multi-stage backrest assembly in accordance with the preferred embodiment of the present invention; 
       FIG. 3  is a partially exploded perspective view of the multi-stage backrest assembly in accordance with the preferred embodiment of the present invention; 
       FIG. 4  is a partially exploded perspective view of the multi-stage backrest assembly in accordance with the preferred embodiment of the present invention; 
       FIG. 5  is a perspective view of a pivot seat of the multi-stage backrest assembly in accordance with the preferred embodiment of the present invention; 
       FIG. 6  is a schematic operational view of the multi-stage backrest assembly as shown in  FIG. 1  in use; and 
       FIG. 7  is a perspective view of a conventional backrest in accordance with the prior art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings and initially to  FIGS. 1-5 , a multi-stage backrest assembly  10  for a chair in accordance with the preferred embodiment of the present invention comprises a support frame  1 , a back plate  2 , a head plate  3 , a waist plate  4 , a first elastic swivel device  5 , and two second elastic swivel devices  6 . 
   The support frame  1  is substantially U-shaped and has two substantially L-shaped sections each having a distal end formed with a support tube  12  formed with a plurality of locking holes  13 . The support frame  1  has a mediate portion formed with a fixing plate  11  mounted on a seat  7  (see FIG.  6 ), so that the multi-stage backrest assembly  10  is mounted on the seat  7 . 
   The back plate  2  is mounted on the support frame  1  and includes a backrest frame  21 , a net  22  made of foam material or the like mounted on the backrest frame  21 , two locking tubes  23  mounted on two sides of the backrest frame  21  and each mounted on the respective support tube  12  of the support frame  1 , two crossbars  26  each mounted on and extended from a respective one of the two locking tubes  23 , a mounting shaft  24  mounted between the two crossbars  26 , and a torsion spring  25  mounted on the mounting shaft  24  and having a first end secured on the mounting shaft  24 . 
   The mounting shaft  24  of the back plate  2  has an end formed with an arch-shaped positioning flange  241  formed with a locking hole  242 , and the first end of the torsion spring  25  is secured in the locking hole  242  of the positioning flange  241  of the mounting shaft  24 . 
   The head plate  3  is mounted on the support frame  1  and includes a substantially inverted T-shaped support rack  35  having a lower end formed with two combination studs  33  each inserted into the respective support tube  12  of the support frame  1 , a substantially inverted L-shaped support bar  30  having a lower end mounted on an upper end of the support rack  35 , a head frame  31  mounted on an upper end of the support bar  30  by one of the two second elastic swivel devices  6 , and a net  32  made of foam material or the like mounted on the head frame  31 . Preferably, the head plate  3  further includes a crossbar  34  mounted on the head frame  31  and combined with one of the two second elastic swivel devices  6 . 
   The waist plate  4  is attached to the back plate  2  and includes a substantially inverted V-shaped support bar  43  having a bent first end mounted on the first elastic swivel device  5 , a waist frame  41  mounted on a second end of the support bar  43  by the other one of the two second elastic swivel devices  6 , and a net  42  made of foam material or the like mounted on the waist frame  41 . Preferably, the back plate  2  further includes a crossbar  46  mounted on the waist frame  41  and combined with the other one of the two second elastic swivel devices  6 . The first end of the support bar  43  of the waist plate  4  is formed with two screw bores  44 . 
   The first elastic swivel device  5  is mounted on the back plate  2 , and includes a casing  51  pivotally mounted on the mounting shaft  24  of the back plate  2 , and an end cap  55  mounted on the casing  51  to seal the casing  51  and the mounting shaft  24  of the back plate  2 . 
   The casing  51  has an inside formed with a receiving chamber  52  for mounting the mounting shaft  24  and the torsion spring  25  of the back plate  2 . The casing  51  has an end formed with a locking groove  53  for locking a second end of the torsion spring  25  of the back plate  2  by a seal plate  58 . The casing  51  has a periphery formed with a plurality of threaded posts  511 , the end cap  55  has a periphery formed with a plurality of through holes  551 , and the first elastic swivel device  5  further includes a plurality of locking screws  57  each extended through a respective one of the through holes  551  of the end cap  55  and each screwed into a respective one of the threaded posts  511  of the casing  51 , so that the end cap  55  is fixed on the casing  51 . 
   The casing  51  has a side formed with a receiving recess  54  for mounting the first end of the support bar  43  of the waist plate  4 . The receiving recess  54  of the casing  51  is formed with two screw bores  56  aligning with the two screw bores  44  of the support bar  43  of the waist plate  4 , and the first elastic swivel device  5  further includes two locking screws  58  (see  FIG. 2 ) each extended through a respective one of the two screw bores  56  of the casing  51  and each screwed into a respective one of the two screw bores  44  of the support bar  43  of the waist plate  4 , so that the waist plate  4  is fixed on the first elastic swivel device  5 . 
   Each of the two second elastic swivel devices  6  includes a substantially semi-cylindrical pivot seat  64  secured on the support bar  30  of the head plate  3  or the support bar  43  of the waist plate  4 , an support ring  641  mounted on a mediate portion of the pivot seat  64 , two rotation members  61  each secured on the crossbar  34  of the head plate  3  or the crossbar  46  of the waist plate  4  as shown in FIG.  2  and each pivotally mounted on the pivot seat  64 , and two torsion springs  67  each mounted between the pivot seat  64  and a respective one of the two rotation members  61  and each having a first end secured on the support ring  641  and a second end secured on a respective one of the two rotation members  61 . 
   The pivot seat  64  is formed with a receiving recess  640  for receiving the two torsion springs  67 , and each of the two rotation members  61  is formed with a receiving recess  60  for receiving a respective one of the two torsion springs  67 . The pivot seat  64  has a periphery formed with an arc-shaped resting plate  642  rested on the two rotation members  61  when the two rotation members  61  are moved to a limit position of the pivot seat  64 . 
   The support ring  641  is formed with a shaft hole  646 , each of the two rotation members  61  is formed with a shaft hole  62 , and each of the two second elastic swivel devices  6  further includes a threaded rod  65  extended through the shaft hole  62  of each of the two rotation members  61 , the two torsion springs  67  and the shaft hole  646  of the support ring  641 , and a nut  66  screwed on the threaded rod  65 , so that the two rotation members  61  are pivotally mounted on the pivot seat  64 . 
   Each of the two rotation members  61  has a periphery formed with a fixing groove  63 , and the second end of each of the two torsion springs  67  is secured in the fixing groove  63  of a respective one of the two rotation members  61 , so that the second end of each of the two torsion springs  67  is secured on a respective one of the two rotation members  61 . 
   The support ring  641  is located between the two rotation members  61  and has two sides each formed with a guide slot  643  and a through hole  644  located in the guide slot  643 , and the first end of each of the two torsion springs  67  is guided through the respective guide slot  643  and locked in the respective through hole  644  of the support ring  641 , so that the first end of each of the two torsion springs  67  is secured on the support ring  641 . 
   In operation, referring to  FIG. 6  with reference to  FIGS. 1-5 , the crossbar  34  of the head plate  3  is pivoted relative to the support bar  30  of the head plate  3  by one of the two second elastic swivel devices  6 , so that the head plate  3  has a pivotable function. In addition, the crossbar  46  of the waist plate  4  is pivoted relative to the support bar  43  of the waist plate  4  by the other one of the two second elastic swivel devices  6 , so that the waist plate  4  has a pivotable function. In addition, the support bar  43  of the waist plate  4  is pivoted relative to the mounting shaft  24  of the back plate  2  by the first elastic swivel device  5 , so that the waist plate  4  is pivotable relative to the back plate  2 . 
   Although the invention has been explained in relation to its preferred embodiment(s) as mentioned above, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the present invention. It is, therefore, contemplated that the appended claim or claims will cover such modifications and variations that fall within the true scope of the invention.