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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hinge device suitable for opening and closing a document press-contacting plate of a copying machine, a printing machine, and the like. 
   2. Description of the Related Art 
   In the conventional copying machines, printing machines, and the like, a document press-contacting plate with a built-in automatic document feeder, which is relatively heavy in weight has been widely used, and for opening and closing such document press-contacting plates, such a hinge device as shown in the Japanese Patent Laid-open No. Hei11-174597 is publicly known. 
   The conventionally structured hinge device (a device for opening and closing a document press-contacting plate) comprises: an attachment member attached on an apparatus body side; a support member supporting a document press-contacting plate which is pivotally attached to the attachment member through a hinge pin; a pair of spring cases also serving as spring reception members pivotally supported by the supporting member side and the attachment member in a manner to be swingable and mutually compressible; compression springs elastically provided between the pair of spring cases; and a damper set inside the compression springs and fixably provided in either one of the spring cases. 
   Further, as a means to adjust rotation torque of the supporting member by changing the elastic force of the compression springs, the one shown in the Japanese Utility-model Publication No. Hei5-8599, for example, in which a spring reception seat is provided on the base portion of one of the spring cases by using an adjusting screw in a manner to be axially movable is well known. 
   When the two publicly known arts described above are combined, a structure in which the base portion of the damper is fixed on the spring reception seat is conceived. However, with such a structure, a torque adjustment requires the distance between the confronting spring cases to be changed by moving the spring reception seat in the axial direction. This causes a problem where, upon a closing of a document press-contacting plate, the closure angle of the document press-contacting plate changes at and from which the damper arranged in the compression springs begins to operate. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a hinge device which allows torque adjustment in a manner that the closure angle of the document press-contacting plate at which the damper operates remains constant. 
   In order to attain this object, the present invention comprises: an attachment member attached on an apparatus body side; a supporting member pivotally supporting the document press-contacting plate through a hinge pin; compression springs elastically provided between the supporting member side and the attachment member; and a damper housed in the compression spring, with the base portion thereof being swingably attached on both or either of the attachment member and/or the free side of the supporting member; an adjustment plate being attached on the damper movably in the axial direction, and the compression springs being elastically provided between the adjustment plate and either of the supporting member side or the attachment member side. 
   The damper referred above can be a fluid damper with a piston rod, and the base portion thereof on the cylinder side can be swingably attached to the attachment member side, or the base portion thereof can be swingably attached to the supporting member side. 
   Also in the present invention, the damper can be a telescope-type friction damper in which telescope parts are compressibly fitted to one another, and the base portion of telescope legs are swingably attached respectively on the attachment member side and the free end side of the supporting member. 
   Further, in the present invention, a pair of spring cases which are compressibly structured can be swingably attached between the attachment member and the supporting member, with the base portion of the damper being fixed on either of the pair of spring cases. 
   Furthermore, in the present invention, a lifting member can be rotatably fixed on the free end side of the supporting member, one of the spring cases can be pivotally supported in a manner to be swingable on a position different from the position on which the lifting member is fixed, and the other spring case can be pivotally supported on a position on the attachment side and different from the position on which the supporting member is pivotally fixed. 
   Moreover, in the present invention, the base portion of the damper can be fixed on one spring case side, which is pivotally supported on the attachment member side, the adjustment plate being attached movably in the axial direction is attached on the base portion of the damper, and the compression springs, with the damper being housed therein, can be elastically provided between the adjustment plate and the other spring case. 
   The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevational view of a copying machine using a hinge device of the present invention; 
       FIG. 2  is a vertical sectional view of an internal structure of the hinge device in a state shown in  FIG. 1 ; 
       FIG. 3  is an explanatory view of an operation of the hinge device of the present invention; 
       FIG. 4  is a front view of the hinge device of the present invention; 
       FIG. 5  is a vertical sectional view of a state shown in  FIG. 4 ; 
       FIG. 6  is a sectional view taken along the A—A line in  FIG. 4 ; 
       FIG. 7  is a vertical sectional view of an operation of an adjustment plate of the hinge device in a state of  FIG. 2 ; and 
       FIG. 8  is a vertical sectional view of another embodiment of the hinge device of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention applied to a copying machine will be explained in detail below with reference to the drawings. 
   In the drawings, the reference numeral  1  denotes an attachment member which is attached to the rear end on top of an apparatus body  2  of a copying machine, printer, or the like. A supporting member  4  is pivotally supported of its both side plates  4   a  and  4   a  through a first hinge pin  3  on both side plates  1   b  and  1   b  which are vertically positioned from the both sides of an attachment plate  1   a  of the attachment member  1 . 
   On the free end sides of the both side plates  4   a  and  4   a  of the supporting member  4 , a lifting member  6  is pivotally supported through a second hinge pin  5  so that its both side plates  6   a  and  6   a  are pivotable. This causes the pivoting directions of the supporting member  4  and the lifting member  6  to be opposite so that the attachment member  1 , the supporting member  4 , and the lifting member  6  are pivotally supported in a manner that they are folded to form a switchback line. To a back plate  6   b  of the lifting member  6  is fitted an adjusting screw  6   d , and the tip portion thereof is abutted on a back plate  4   b  of the supporting member  4 . Further, on attachment plates  6   c  and  6   c  of the lifting member  6 , a document press-contacting plate  7  with an automatic document feeder is attached at the rear end thereof (not shown). 
   On a side of the lifting member  6  where is pivotally supported by the second hinge pin  5 , a first spring case  8  which also serves as a spring reception member is pivotally supported by a third hinge pin  9  while changing its supporting position so that attachment plates  8   a  and  8   a  protruding on the top portion of the first spring case  8  can swing. 
   The first spring case  8  supports a second spring case  10  also serving as a spring reception member with support piece  8   c  and  8   c , in a manner to be compressible. The second spring case  10  is swingably supported at a distance from the first hinge pin  3  on the both side plates  1   b  and  1   b  of the attachment member  1 , through a pair of fourth hinge pins  11  and  11 . The detachable support structure described above is formed by providing, on both sides of the bottom part of the second spring case  10 , bearing grooves  10   a  and which receive fourth hinge pins  11  and  11 . Incidentally, this structure is intended to facilitate an assembly of a hinge device of the present invention. Therefore it is possible that one piece of the fourth hinge pin may use instead of a pair of the fourth hinge pins  11  and  11 , or that the bearing grooves  10   a  and  10   a  are communicative to each other to form one piece thereof. It is also possible that the bearing grooves  10   a  and  10   a  serve as a through-hole. 
   In the second spring case  10 , a fluid damper  12  composed of a piston rod  12   a  and a cylinder  12   b  is fitted, in a manner that a base portion  12   c  of the cylinder  12   b  is fitted into a fit hole  10   b  provided at the center of the bottom of the second spring case  10 . On a male screw portion  12   d  provided on the outer periphery of the base portion  12   c , an adjustment plate  13  is rotatably fitted. The internal structure of the fluid damper  12  will be omitted to explain here, which is similar to the one described in the Japanese Patent Laid-open No. Hei9-311389. Further, the piston rod  12   a  protruding from the free end side of the fluid damper  12  confronts an inside top portion  8   b  inside the first spring case  8 , and between the adjustment plate  13  and the inside top portion  8   b  inside the first spring case  8 , a compression spring pair composed of a large compression spring  15   a  and a small compression spring  15   b  are elastically arranged through which the fluid damper  12  is inserted in the axial direction. Reference Numeral  16  is a washer of the fluid damper  12 . Incidentally, while the description hereinbefore refers to an embodiment in which the fist spring case  8  and the second spring case  10  are used, such first spring case  8  and the second spring case  10  may be omitted, wherein the fluid damper  12  is arranged between the attachment member  1  and the supporting member  4 . 
   According to the description hereinbefore, when the document press-contacting plate  7  is closed as shown in FIG.  1  and  FIG. 2 , the supporting member  4  and the lifting member  6  are upfolded to each other, with the compression springs  15   a  and  15   b  being compressed within the first spring case  8  and the second spring case  10  which are forming one set of spring reception member. The compression springs  15   a  and  15   b  generate rotation torque around the first hinge pin  3  which pivotally supports the supporting member  4  which further supports the document press-contacting plate  7 , in a direction so as to open the document press-contacting plate  7 . By changing the elastic force of the compression springs  15   a  and  15   b , the rotation torque generated around the first hinge pin  3  can be adjusted. Further, the piston rod  12   a  of the fluid damper  12  is compressed within the cylinder  12   b.    
   When the document press-contacting plate  7  is opened from the state shown in FIG.  1  and  FIG. 2 , the supporting member  4  and the lifting member  6  together rotate with the first hinge pin  3  as fulcrum, so as to allow an opening operation of the document press-contacting plate  7  up to approximately 90 degrees. At that time, the rotation torque generated by the compression springs  15   a  and  15   b  causes the document press-contacting plate  7  to lightly open without feeling heaviness thereof. Additionally, the maximum degree of the opening angle is adjustable up to around 70 degrees by stopper means (not shown). As the document press-contacting plate  7  opens further, since the space between the first spring case  8  and the second spring case  10  is widened, the piston rod  12  is pressed out of the cylinder  12   b , until being halted at a predetermined position. 
   When the opened document press-contacting plate  7  is closed, it is closed by pressing force by hand and the weight of the pressure-contact plate itself conquering the elastic force of the compression springs  15   a  and  15   b . At that time, the document press-contacting plate  7  for document attempts to reverse, using the second hinge pin  5  of the lifting member  6  for the support member  4  as fulcrum. However, the first spring case  8  attached on the lifting member  6  through the third hinge pin  9  presses in the direction so that the lifting member  6  and the supporting member  4  are upfolded by the pressing force of the compression springs  15   a  and  15   b . This prevents the document press-contacting plate  7  from reversing with the second hinge pin  5  as fulcrum upon normal closing operation. This reverse-prevention system of the lifting member  6  also functions when the document press-contacting plate  7  is unhanded from the opened state. 
   The closing operation of the document press-contacting plate  7  causes an accelerating force. Therefore, the document press-contacting plate  7  is closed at increased closing speed. At that time since the compression springs  15   a  and  15   b  are compressed to increase their elastic force, the lifting member  6  is pressed in the direction to be upfolded with the supporting member  4 . 
   Since the document press-contacting plate  7 , particularly with an automatic document feeder is heavy in weight, an abrupt closure may cause a breakage of a contact glass  19  or a shut-in of a hand. In this embodiment, when the document press-contacting plate  7  is closed to incline to the predetermined angle, the tip of the piston rod  12   a  of the fluid damper  12  is abutted to the inside top portion  8   b  of the first spring case  8 , which exerts a buffer action, and thereby causing the document press-contacting plate  7  to be quietly closed from the predetermined closure angle (normally at 15 degrees), instead of being abruptly shut. 
   Further, when the document press-contacting plate  7  is unhanded at a predetermined closure angle where the elastic force of the compression springs  15   a ,  15   b  and the weight of the document press-contacting plate  7  balance, the document press-contacting plate  7  is supported as it is without free-falling, thereby allowing documents to be set on the contact glass  19  by using both hands. At that time, the document press-contacting plate  7  does not reverse with the second hinge pin  5  as fulcrum, as aforementioned. 
   When copying a thick-volume document such as a book, particularly as shown in  FIG. 3  where a thick-volume document  17  is laid on the contact glass  19  and the document press-contacting plate  7  is closed down, such document press-contacting plate  7  together with the lifting member  6  reverses with the second hinge pin  5  as fulcrum, in conquering the elastic force of the compression spring  15   a  and  15   b , thereby horizontally covering the thick-volume document  17 , which prevents an exposure from leaking-out as much as possible. 
   Furthermore, for an adjustment of rotation torque according to the weight of the document press-contacting plate  7 , as shown in  FIG. 7 , the attachment position of the adjustment plate  13  for the male screw portion  12   d  of the base portion  12   c  of the fluid damper  12  is shifted so as to change the elastic force of the compression springs  15   a  and  15   b . This causes the rotation torque being generated around the first hinge pin  3  by the supporting member  4  to be adjustable. However, at that time, the distance between the tip of the piston rod  12   a  of the fluid damper  12  and the inside top portion  8   b  of the first spring case  8  remains constant. 
   Such a shift of the adjustment plate  13  is implemented by rotating the fluid damper  12  itself together with the cylinder  12   b  through a rotation of the piston rod  12   a  over the compression springs  15   a  and  15   b , since the adjustment plate  13  itself is non-rotationally fixed within the second spring case  10 . Not to mention, there are many other methods for adjusting rotation torque than described herein. 
   The present invention also can be applied to a hinge device in which the document press-contacting plate  7  is directly attached to the supporting member  4  without using the lifting member  6 . As with the aforementioned embodiment, the adjusting screw  6   d  attached on the lifting member  6  is intended for adjusting the document press-contacting plate  7  to be pressed against the contact glass  19  by a uniform pressing force, whereby a misalignment of a thin-volume document  18  upon closure of the document press-contacting plate  7  or a skewing of a document upon an operation of the automatic document feeder can be prevented. 
   It should be added that the structure of the fluid damper  12  is not limited to the one described above, but as shown in  FIG. 8 , for example, a compressible and combined telescope damper  21  is adopted to be a friction-type damper in which a friction force abruptly increases at and from the predetermined compressing position. In this structure, base portions  22   a  and  23   a  respectively of telescope legs  22  and  23  are swingably fixed to the supporting member side and the attachment member side, through the third hinge pin  9  and the fourth hinge pin  11 . Further, the adjustment plate  25  is fitted into the outer side of the base portion of one telescope leg  23  in a manner that it can move in the axial direction. This adjustment plate may be attached on the base portion of the other telescope leg  22 . 
   While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

Summary:
A hinge device comprises: an attachment member attached on an apparatus body side; a supporting member pivotally supported by the attachment member through hinge pins; a document press-contacting plate attached to the supporting member; compression springs elastically provided between the supporting member side and the attachment member in such a manner that the supporting member is enforced to pivot in a direction that the document press-contacting plate is opened; a damper set inside the compression springs and provided between the attachment member side and the free end side of the supporting member so that an abrupt pivoting at and from a predetermined angle measured from the supporting member is buffered. This hinge device allows an adjustment of the elastic force of the compression springs without changing the operation angle of the damper, by attaching an adjustment plate on the damper movably in an axial direction, and by elastically providing the compression springs between the adjustment plate and either of the supporting member side or the attachment member side.