Abstract:
An image forming apparatus includes a main assembly; a first openable member openably and closably provided in the main assembly; a second openable member openably and closably provided in the main assembly; a fixing unit for fixing a toner image formed on a recording material, the fixing unit including a pressure applying mechanism for applying a pressure to a fixing nip of the fixing unit, wherein a operation force by an operator when at least one of the first openable member and the second openable member is opened by the operator applies to the pressure applying mechanism to release the pressure applied to the fixing nip; wherein the fixing unit is detachably mountable relative to the main assembly and includes a pressure switching member, engaged to the pressure applying mechanism, to receive the operation force when the first openable member is opened and to move the pressure applying mechanism, wherein an operation force when the second openable member is opened is also transmitted to the pressure switching member.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus having a mechanism for removing pressure from the fixation unit of the apparatus. 
         [0002]    There are image forming apparatuses which are structured so that the pressure applied to the fixation nip can be removed to make it easier to remove a recording medium having jammed in the adjacencies of the fixation unit in the image forming apparatuses. One of the methods for removing the recording medium having jammed in the adjacencies of the fixation unit is to open a door (rear door) which is on the downstream side of the fixation unit in terms of the direction in which the recording medium is conveyed. In order to keep image formation apparatus cost as low as possible, some image forming apparatuses are not provided with a power source for removing the pressure applied to the fixation nip portion. Some of such image forming apparatuses are structured so that the force applied to the rear door to open it by an operator is utilized to remove the pressure applied to the fixation nip portion, in order to minimize the amount of time and work of which the operator is required. 
         [0003]    Further, in the case of some image forming apparatuses structured so that the process cartridges therein, which are expendables, are replaceable, the following situation occurs. That is, as a cartridge or cartridges are removed by opening the cartridge replacement door of the apparatus, the recording medium having jammed in the adjacencies of the fixation nip portion are visible through the opening created by the operating of the door. Thus, it is reasonable to think that the jammed recording medium can be removed by the operator by putting his or her hand into the apparatus through the opening. However, even after the opening of the door, the fixation nip portion remains under a substantial amount of pressure applied for image fixation. Therefore, removing the jammed recording medium is not as easy as it seems. 
         [0004]    Thus, it is reasonable to think of structuring an image forming apparatus so that in order to improve an image forming apparatus in usability while keeping the image forming apparatus cost as low as possible, the pressure being applied to the fixation nip portion is removed not only by the opening of the rear door, but also, opening of the cartridge replacement door. One of the image forming apparatuses structured as described above is disclosed in Japanese Laid-open Patent Application 2007-298691. 
         [0005]    In the case of an image forming apparatus such as the one disclosed in Japanese Laid-open Patent Application 2007-298691, which is structured so that the pressure being applied to the fixation nip portion can be removed by opening either of the aforementioned two doors, there are two points at which contact is made between the mechanism for applying pressure to the fixation nip portion, and the member to be placed in contact with the mechanism to remove the pressure being applied to the fixation nip portion. 
         [0006]    From the standpoint of smooth insertion of a fixation unit into an image forming apparatus, or smooth removal a fixation unit from an image forming apparatus, the greater in the number of the abovementioned points of operational contact, the more disadvantageous is an image forming apparatus structured, in consideration of repair and/or replacement of a fixation unit, so that a fixation unit is removably mountable in the main assembly of the apparatus. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention was made in consideration of the above described problem, and its primary object is to provide an image forming apparatus which allows a jammed recording medium therein to be easily removed, and is superior in operability in terms of an operation for removing the fixation unit from the main assembly of the apparatus. 
         [0008]    According to an aspect of the present invention, there is provided an image forming apparatus comprising a main assembly; a first openable member openably and closably provided in said main assembly; a second openable member openably and closably provided in said main assembly; a fixing unit for fixing a toner image formed on a recording material, said fixing unit including a pressure applying mechanism for applying a pressure to a fixing nip of said fixing unit, wherein a operation force by an operator when at least one of said first openable member and said second openable member is opened by the operator applies to said pressure applying mechanism to release the pressure applied to the fixing nip; wherein said fixing unit is detachably mountable relative to the main assembly and includes a pressure switching member, engaged to said pressure applying mechanism, to receive the operation force when said first openable member is opened and to move said pressure applying mechanism, wherein an operation force when said second openable member is opened is also transmitted to said pressure switching member. 
         [0009]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic sectional view of an image forming apparatus and shows the general structure of the apparatus. 
           [0011]      FIG. 2  is a perspective view of the fixation unit in the first embodiment of the present invention. 
           [0012]      FIG. 3  is a perspective view of the heating unit of the fixation unit in the first embodiment. 
           [0013]      FIG. 4(   a ) is a side view of the fixation unit shown in  FIG. 2 , which is under pressure (both doors  27 F and  27 R are remaining closed), and  FIG. 4(   b ) is a side view of the fixation unit in  FIG. 2 , which is not under pressure (at least one of doors  27 F and  27 R is open). 
           [0014]      FIGS. 5(   a ) and  5 ( b ) are perspective views of the main assembly of the image forming apparatus in the first embodiment, when the doors  27 F and  27 R of which are open and closed, respectively. 
           [0015]      FIGS. 6(   a ) and  6 ( b ) are perspective views of the mechanical linkage which acts on the lever  23  shown in  FIG. 2 , when the fixation unit is under pressure (when doors  27 F and  27 R are remaining closed), and is not under pressure, respectively. 
           [0016]      FIG. 7  is a perspective view of the mechanical linkage  29 , connective member  25 , and pressure switching member  20  of the main assembly of the image forming apparatus, as seen from the inward side of the main assembly. 
           [0017]      FIG. 8  is a plan view of the mechanical linkage shown in  FIG. 6  when the fixation unit is under pressure (both doors  27 F and  27 R are remaining closed). 
           [0018]      FIG. 9  is a plan view of the mechanical linkage shown in  FIG. 6  when the fixation unit is not under pressure (only door  27 F is open). 
           [0019]      FIG. 10  is a plan view of the mechanical linkage shown in  FIG. 6  when the fixation unit is not under pressure (only door  27 R is open). 
           [0020]      FIG. 11  is a plan view of the mechanical linkage shown in  FIG. 6  when the fixation unit is not under pressure (both doors  27 F and  27 R are open). 
           [0021]      FIG. 12  is a perspective view of the fixation unit in the second embodiment of the present invention. 
           [0022]      FIG. 13  is a side view of the fixation unit shown in  FIG. 12 , which is under pressure. 
           [0023]      FIG. 14  is a side view of the fixation unit shown in  FIG. 12 , which is not under pressure. 
           [0024]      FIGS. 15(   a ) and  15 ( b ) are perspective views of the combination of the fixation unit and mechanical linkage shown in  FIG. 12 , when the fixation unit is under pressure (both doors  27 F and  27 R are remaining closed), and not under pressure (only door  27 F is open), respectively. 
           [0025]      FIG. 16  is a perspective view of the connective member of the main assembly of the image forming apparatus, and the pressure switching member of the fixation unit, and shows how the two members engage with each other as the fixation unit shown in  FIG. 12  is mounted into the main assembly of the image forming apparatus. 
           [0026]      FIG. 17  is a perspective view of the cam (pressure switching member), and its adjacencies, of the fixation unit in the third embodiment of the present invention, and describes how driving force is transmitted to the cam. 
           [0027]      FIG. 18  is a perspective view of the mechanical linkage of the main assembly of the image forming apparatus in the third embodiment of the present invention, and describes the structure of the mechanism. 
           [0028]      FIG. 19  is a perspective view of the mechanical linkage of the main assembly of the image forming apparatus in the third embodiment of the present invention, when the door  27 R is in its first position, and describes the structure of the mechanism. 
           [0029]      FIG. 20(   a ) is a perspective view of the combination of the arm  30 R and linkage  160  shown in  FIGS. 19 , and  20 ( b ) is a perspective view of the linkage  160  shown in  FIG. 20(   a ). 
           [0030]      FIG. 21  is a perspective view of the combination of the door  27 R, arm  30 R (which connects door  27 R and linkage  160 ), and linkage  160  shown in  FIG. 19 , when the door  27 R is in its first position, and describes the structure of the combination. 
           [0031]      FIG. 22  is a perspective view of the mechanical linkage, and its adjacencies, when the door  27 R shown in  FIG. 17  is in its second position, and describes the structure of the linkage. 
           [0032]      FIG. 23  is a perspective view of the combination of the arm  30 R, linkage  160 , and their adjacencies, when the door  27 R shown in  FIG. 19  is in its second position, and describes the structure of the combination. 
           [0033]      FIG. 24  is a perspective view of the mechanical linkage when the door  27 R shown in  FIG. 19  is in its third position, and describes the structure of the linkage. 
           [0034]      FIG. 25  is a perspective view of the combination of the arm  30 R, linkage  160 , and their adjacencies, when the door  27 R shown in  FIG. 19  is in its third position, and describes the structure of the combination. 
           [0035]      FIG. 26  is a plan view of the mechanical linkage in the fourth embodiment of the present invention when the fixation unit is under pressure (both doors  27 R and  27 R are remaining closed). 
           [0036]      FIGS. 27(   a ) and  27 ( b ) are plan views of the mechanical linkage shown in  FIG. 26  when the fixation unit is not under pressure (only door  27 R is open, and both door  27 F and  24 R are open, respectively). 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1  
       [0037]    Next, the image forming apparatus in the first embodiment of the present invention will be described with reference to the appended drawings. 
         [0038]      FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, and shows the general structure of the apparatus. Referring to  FIG. 1 , the main assembly  1  of the image forming apparatus has an image forming portion  1 A, a sheet storage cassette  6 , a pair of conveyance rollers  7 , a pair of registration rollers  7 A, a fixing apparatus  8  (fixation unit), a pair of discharge rollers  9 , a delivery tray  10 , a cartridge replacement door  27 F (first member which can be opened or closed: first door), and a door  27 R for removal of jammed recording medium (second member which can be opened or closed: second door). The image forming portion  1 A has a photosensitive drum  2 , a transfer roller  3 , a laser scanner  4 , and a toner cartridge  5 . The door  27 R for removal of jammed recording medium in this embodiment doubles as the door for allowing the fixation unit  8  to be mounted into, or removed from, the main assembly  1 . 
         [0039]    As an image forming operation is started, first, a beam of light is projected upon the peripheral surface of the photosensitive drum  2  by the laser scanner  4  while being modulated with image signals. As a result, a latent image is formed on the peripheral surface of the photosensitive drum  2 . This latent image is developed with the toner in the toner cartridge  5  to form a visible image (image formed of toner, which hereafter will be referred to simply as toner image) on the photosensitive drum  2 . 
         [0040]    Meanwhile, a sheet of recording medium S (which hereafter will be referred to simply as sheet S) is fed into the main assembly  1  from the sheet storage cassette  6 , and conveyed by the pair of conveyance rollers  7  and pair of registration rollers  7 A to a transfer portion formed by the photosensitive drum  2  and transfer roller  3 . As the sheet S is conveyed through the transfer portion, the toner image is transferred onto the sheet S. 
         [0041]    After the transfer of the toner image onto the sheet S, the sheet S is conveyed to a fixing apparatus  8 , and conveyed through the fixing apparatus  8 . As the sheet S is conveyed through the fixing apparatus  8 , the toner image on the sheet S is fixed to the sheet S. Thereafter, the sheet S is discharged by the pair of discharge rollers  9  into the delivery tray  10 , which is a part of the top wall of the main assembly  1 . 
         [0042]    In this embodiment, the doors  27 F and  27 R, that is, the front and rear doors of the main assembly  1 , can be opened in the frontward and rearward direction, respectively, of the main assembly  1 , and can be rotationally closed in the rearward and frontward direction, respectively, of the main assembly  1 . However, the main assembly  1  may be structured so that the doors  27 F and  27 R can be opened or closed in the leftward or rightward direction of the main assembly  1 . 
       (Fixation Unit) 
       [0043]      FIG. 2  is a perspective view of the fixation unit in this embodiment. Referring to  FIG. 2 , the fixing apparatus  8  is a unit which can be mounted into, or removed from, the main assembly  1 . That is, the fixing apparatus  8  can be removed from, or remounted into, the main assembly  1  when it needs to be repaired or in the like situations. The fixing apparatus  8  has a pressure roller  13  (pressure applying member) and a heating unit  14 . The pressure roller  13  is held by the left and right lateral plates  11  and  12 , respectively, of the frame of the fixing apparatus  8 . 
         [0044]    Referring to  FIG. 3 , the heating unit  14  has a heater  15 , a fixation film  16 , and a film guide  17 . The heater  15  is for heating the fixation film  16 . The fixation film  16  is cylindrical, and rotates around the heater  15 . The film guide  17  holds the heater  15 , and also, guides the fixation film  16 . 
         [0045]    The pressure roller  13  and heating unit  14  form the fixation nip, through which the sheet S is conveyed after the transfer of the toner image onto the sheet S. While the sheet S is conveyed through the fixation nip, heat and pressure is applied to the sheet S and the toner image thereon to thermally fix the toner image. 
         [0046]    Next, referring to  FIG. 4 , the fixing apparatus  8  has: a metallic pressure plate  18 , which is a part of a pressure application mechanism; a pressure application spring  19  (pressure applying means); and a cam  20  (pressure switching means), which comes into contact with the pressure application mechanism; and a lever  23 . 
         [0047]    The heating unit  14  is kept pressed upon the pressure roller  13  by the pressure application spring  19 , with the presence of the metallic pressure application plate  18  between the heating unit  14  and pressure spring  19 . Incidentally, all that is required of the pressure application spring  19  is to keep the heating unit  14  and pressure roller  13  pressured upon the other. Thus, the fixing apparatus  8  may be structured so that the pressure application spring  19  keeps the pressure roller  13  pressed upon the heating unit  14 . 
         [0048]    There are two cams  20 . One cam  20  is at the left end of the heating unit  14 , and the other is at the right end of the heating unit  14 . They are rotatable about a cam shaft  21 , being enabled to be set in the first or second position. The first position (pressure application position in  FIG. 4(   a )) is where it causes the pressure application spring  19  to apply pressure, whereas the second position (pressure removal position in  FIG. 4(   b )) is where it prevents the pressure application spring  19  from applying pressure. When the cams  20  are in the second positions, they keep the metallic pressure application plate  18  lifted. 
         [0049]    The metallic pressure application plate  18  is rotatable about its supporting point. As it is lifted by the cams  20 , its center portion, which keeps the unit  14  pressed by being pressed by the pressure application spring  19 , is lifted away from the unit  14 . As a result, pressure is removed from the fixation nip. 
         [0050]    The lever  23  is an integral part of the cam  20 . Thus, as the lever  23  is pushed down, the cam  20  is rotated (lifted) into its second position, whereas as the lever  23  is pushed up, the cam  20  is rotated back into its first position. 
       (Connecting Means  25  (Connecting Member), and Mechanical Linkage  29 ) 
       [0051]      FIGS. 5(   a ) and  5 ( b ) are perspective views of the mechanical linkage  29  when the doors  27 F and  27 R are remaining closed and open, respectively. FIGS.  6 ( a ) and  6 ( b ) are perspective views of the mechanical linkage  29  when the unit  14  is under pressure (doors  27 F and  27 R are remaining closed), and not under pressure (only door  27 F is open), respectively. 
         [0052]    Referring to  FIGS. 5 and 6 , the main assembly  1  of the image forming apparatus has two sets of connecting means  25  and mechanical linkage  29 , which are attached to the left and right lateral plates of the apparatus main assembly  1 , one for one.  FIGS. 5 and 6  show the connecting means  25  and mechanical linkage  29 , which are on the left lateral plate  24 . 
         [0053]    Each mechanical linkage  29  has an arm  30 F (first member), an arm  30 R (second member), a link  32 F (first link), a link  32 R (second link), and a linkage guide  34  (guiding member). 
         [0054]    One end of the arm  30 F is in connection with the door  27 F, which is the door for mounting or dismounting cartridges. The other end is indirectly in connection the link  32 F through a shaft  36 F. One of the arms  30 R is in connection with the door  27 R, which is the door for removing a jammed sheet of recording medium. The other end is indirectly in connection with the link  32 R through a shaft  36 R ( FIG. 7 ).  FIG. 7  is a rear view of the mechanical linkage  29  shown in  FIG. 6 . 
         [0055]    The linkage guide  34  is a stationary member. It is solidly fixed to the frame of the apparatus main assembly  1  so that it does not move relative to the frame. It has two guiding holes  34 Fa and  34 Ra. The guiding hole  34 Fa is elongated in the lengthwise direction of the linkage guide  34 , and guides the link  32 F (shaft  36 F): it allows the link  32 F to slidingly move relative to the linkage guide  34  in the lengthwise direction of the linkage guide  34 . The guiding hole  34 Ra is also elongated in the lengthwise direction of the linkage guide  34 , and guides the link  32 R (shaft  36 R): it allows the link  32 R to slidingly move relative to the linkage guide  34  in the lengthwise direction of the linkage guide  34 . 
         [0056]    The linkage guide  34  holds the links  32 F and  32 R so that the links  32 F and  32 R are allowed to move. The links  32 F and  32 R are connected to each other by the shaft  35  so that they constitute the two sides of a triangle, the bottom side of which is the linkage guide  34 , and also, so that they are allowed to rotationally move relative to each other. As the door  27 F and/or  27 R is opened or closed, the links  32 F and  32 R are made to slidingly move relative to the linkage guide  34  in such a manner that the abovementioned triangle is changed in height in response to the angle of the doors  27 R and/or  27 R relative to the apparatus main assembly  1 , and the change in the height of the triangle causes the connecting means  25  to rotate about its supporting point  26 . The connecting means  25  is a part of the apparatus main assembly  1 , and is on the upstream side of the cam  20  in terms of the operational force transmission direction. It is indirectly in connection with the doors  27 F and  27 L through the mechanical linkage  29 , which is moved by the movement of the door  27 F and  27 R, respectively, as the doors  27 F and  27 R are moved. 
         [0057]    Next, referring to  FIG. 7 , the connecting means  25  has a U-shaped portion  25   a  (connective portion). The lever  23  has a protrusion  23   a  (connective portion), which is at the opposite end of the lever  23  from the cam shaft  21 . As the fixation unit  8  is mounted into the apparatus main assembly  1 , the projection  23   a,  which belongs to the fixation unit, is inserted into the U-shaped portion  25   a  of the connecting means  25 , which belongs to the apparatus main assembly  1 . As a result, the lever  23  becomes connected with the connecting means  25 . 
         [0058]    As an operator opens the door  27 F when the connecting means  25  is in connection with the lever  23 , the cartridge door arm  30 F moves rightward in  FIG. 5  (leftward in  FIG. 6 ). As the arm  30 R moves rightward, the shaft  36 F of the link  32  is guided rightward by the linkage guide  34  while remaining in the guiding hole  34 Fa. As the shaft  36 F is guided rightward, the triangle which the links  32 R and  32 R and linkage guide  34  forms, and the bottom side of which is the linkage guide  34 , increases in height. As the triangle increases in height, it presses the connecting means  25  downward, whereby the lever  23  is pressed downward. Thus, the lever  23 , which has been in the position (pressure applying position) shown in  FIG. 6(   a ), is rotated into the position (pressure removal position) shown in  FIG. 6(   b ). Also in a case where only the door  27 R is opened, that is, the door  27 F is left closed, the connecting means  25  presses down the lever  23  as it does when the door  27 F is opened. 
         [0059]    When the fixing apparatus  8  is mounted into, or dismounted from, the apparatus main assembly  1 , the open side of the U-shaped portion  25   a  is facing the fixing apparatus  8  (lengthwise direction of U-shaped portion  25   a  is parallel to direction in which fixing apparatus  8  is mounted or dismounted). Further, the rotational phase of the cam  20 , and that of the lever  23 , are set so that when the fixing apparatus  8  is mounted into, or dismounted from, the apparatus main assembly  1 , no pressure is applied to the fixation nip (they are as shown in  FIG. 6(   b )). 
         [0060]    On the other hand, when the rotational phase of the cam  20  is such that pressure is applied to the fixation nip (it is as shown in  FIG. 6(   a )), the open side of the U-shaped portion  25   a  is facing the opposite direction from the direction in which the fixing apparatus  8  is mounted or dismounted. Thus, it is impossible to mount or dismount the fixing apparatus  8 . 
         [0061]    As described above, in this embodiment, the apparatus main assembly  1  is structured so that unless the apparatus main assembly  1  is in the state in which the fixation nip is not under pressure, the fixing apparatus  8  in the apparatus main assembly  1  cannot be replaced. Therefore, if the connecting means  25  and lever  23  are not synchronous in rotational phase, they cannot be connected to each other, and therefore, the fixing apparatus  8  cannot be mounted into the apparatus main assembly  1 . In other words, the apparatus main assembly  1  is structured so that when the cam  20  of the fixing apparatus  8 , door  27 F, and/or door  27 R are not synchronous in rotational phase, the fixing apparatus  8  cannot be mounted into the apparatus main assembly  1 . Further, the apparatus main assembly  1  is structured to ensure that as the fixing apparatus  8  is dismounted from the apparatus main assembly  1 , it is placed in the state in which no pressure is applied to the fixation nip. Therefore, even if the fixing apparatus  8  is left unattended for a long time after its removal from the apparatus main assembly  1 , the pressure roller  13  and fixation film  16  which form the fixation nip are not deformed. 
       (Operation of Pressure Switching Member  20 , Connecting Means  25 , and Mechanical Linkage  29 ) 
       [0062]      FIGS. 8-11  are plan views of the mechanical linkage  29 . 
         [0063]    As an operator opens the door  27 F when the mechanical linkage  29  is in the state shown in  FIG. 8 , in which the fixation nip is under pressure (when both first and second door are remaining closed), the link  32 F is made to slide along the linkage guide  34  (shaft  36 F is guided by guiding hole  34 Fa) by opening movement of door) as shown in  FIG. 9 . As a result, the triangle which the links  34 F and  34 R, and the link guide  34  which constitutes the bottom side of the triangle, form, increases in height (shaft  35  is moved). Thus, the connecting means  25  is pressed down, being thereby rotated downward about the supporting point  26  (rotational axis). 
         [0064]    As the connecting means  25  is rotated about its supporting point  26  into a preset position, the lever  23  and cam  20  are rotated about the cam shaft  21  into their preset points (rotational phases). Consequently, the arcuate portion of the cam  20  comes under the metallic pressure plate  18 . Thus, the pressure from the pressure application spring  19  is caught by only the cam  20  and metallic pressure plate  18 . Therefore, even when the fixation nip is under no pressure, the mechanical linkage  29  remains under no load. 
         [0065]    As the door  27 F is closed when the mechanical linkage  29  is in the state shown in  FIG. 9 , the link  32 F slides in the opposite direction from the direction in which it slides when the door  27 F is opened. Thus, the connecting means  25  is lifted. Consequently, the fixation nip comes under pressure (state shown in  FIG. 8 ). 
         [0066]    The movement of the mechanical linkage  29  which occurs as the rear door  27 R is opened is the same as that which occurs as the front door  27 F is opened (except that link  32 F does not move, and link  32 R slides along linkage guide  34 : shaft  36 R is guided by guiding hole  34 Ra). Thus, the fixation nip comes under pressure as shown in  FIG. 8 . 
         [0067]    In a case where both the doors  27 F and  27 R are opened, as the doors  27 F and  27 R are opened, both the links  32 F and  32 R slide as shown in  FIG. 11 . Thus, the connecting means  25  is pressed down farther than when only one of the doors  27 F and  27 R is opened. In this case, therefore, the cam  20  is rotated further, that is, beyond the positions shown in  FIGS. 9 and 10 . However, the cam  20  is rotated while its arcuate edge portion is remaining in contact with the metallic pressure plate  18 . Therefore, the amount of distance by which metallic pressure plate  18  is lifted does not change. 
         [0068]    As one of the doors  27 F and  27 R is closed when both doors are open, the link which is in connection with the door, which is being closed, is made to slide by the movement of the door. As a result, the state of the mechanical linkage  29  changes into the one shown in  FIG. 9  or  10 . Then, as the other door is closed, the other link is made to slide by the movement of the door. As a result, the state of the mechanical linkage  29  goes back to the one shown in  FIG. 8 . 
         [0069]    The apparatus main assembly  1  in this embodiment is structured as described above. Thus, as the doors  27 F and  27 R are opened, the mechanical linkage  29  is operated by the opening movement of the doors  27 F and  27 R in a manner to remove the pressure which is being applied to the fixation nip. Thus, the jammed recording medium or the like can be easily removed. Further, the cam  20  and mechanical linkage  29  can be easily engaged with, or disengaged from, each other by the connecting means  25 . Therefore, the fixing apparatus  8  can be easily replaced. Moreover, sensors, motors, etc., are unnecessary. In other words, the present invention can minimize costs. 
         [0070]    As described above, the fixation unit  8  is removably mountable in the main assembly  1  of the image forming apparatus. Further, it has the pressure switching member  20  (cam) which is in contact with the pressure application mechanisms ( 18 ,  19 ), and to which the force applied to the door  27 F (first member which can be opened or closed) is transmitted to move the pressure application mechanism. Further, the apparatus main assembly  1  is structured so that as the door  27 R (second member which can be opened or closed) is opened, the force applied to the door  27 R to open it is transmitted to the pressure switching member  20 . Further, the apparatus main assembly  1  and fixation unit  8  are structured so that regardless of which of the two doors  27 F and  27 R (members which can be opened or closed) is opened, it is only to the pressure switching member  20  that the force applied to open the doors is transmitted. Thus, the present invention can improve an image forming apparatus in the operational efficiency with which the fixation unit  8  can be mounted into, or dismounted from, the apparatus main assembly  1 . 
       Embodiment 2  
       [0071]    Next, referring to  FIGS. 12-16 , the image forming apparatus in the second embodiment of the present invention will be described. The portions of the image forming apparatus in this embodiment, which are virtually the same as the counterparts in the first embodiment, will be given the same referential codes, and will not be described. 
         [0072]    The image forming apparatus in this embodiment is different from that in the first embodiment, in the structure between the mechanical linkage  29  and pressure switching member  50  (cam). In terms of the operation of the mechanical linkage  29 , which is caused by the opening or closing of the two doors, this embodiment is the same as the first embodiment. In this embodiment, however, the rear door  27 R is dedicated to taking care of the jammed sheets; it is not for mounting or dismounting a fixation unit. In this embodiment, the fixation unit is to be mounted into, or dismounted from, the main assembly  1  of the image forming apparatus, by removing the external cover of the apparatus main assembly  1 . Further, in this embodiment, when the fixation unit is mounted or removed, the fixation nip remains under pressure. 
         [0073]      FIG. 12  is a perspective view of the fixing apparatus  8  in this embodiment.  FIG. 13  is a side view of the fixing apparatus  8  when the fixation nip is under pressure.  FIG. 14  is a perspective view of the fixing apparatus  8  when the fixation nip is not under pressure. 
         [0074]    The metallic pressure plate  18  is rotatable about its supporting point  22 . As the end portion of the metallic pressure plate  18  is lifted by a cam  50  when the metallic pressure plate  18  is in the state shown in  FIG. 13 , that is, when the fixation nip is under pressure, the portion of the metallic pressure plate  18 , which is pressing on the heating unit  14 , moves upward, whereby pressure is removed from the fixation nip. Consequently, the positional relationship between the metallic pressure plate  18  and heating unit  14  becomes as shown in  FIG. 14 ; the fixation nip is under no pressure. The cam  50  has a key portion  50   a,  which is in the form of a parallelepiped projection. As the cam rotating force is transmitted to the key portion  50   a  from an external source (connecting means), the cam  50  is rotated about the cam shaft  21 , whereby the metallic pressure plate  18  is lifted, or allowed to come down. 
         [0075]      FIG. 15(   a ) illustrates the case in which both doors  27 F and  27 R are remaining closed, and the fixation nip is under pressure.  FIG. 15(   b ) illustrates the case in which the door  27 F is open and the fixation nip is under pressure. The operation of the mechanical linkage  29  in this embodiment is roughly the same as that of the first embodiment, which was described above with reference to  FIGS. 8-11 . Thus, the case in which only the door  27 F is open, and the case in which both doors  27 F and  27 R are open, will not be described.  FIG. 15  shows only an arm  30 F which is in connection with the first door  27 F, and a shaft  36 F which is in the elongated hole of the linkage guide  34 , slides along the linkage guide  34  in response to the movement of the arm which is in connection with the first door, while being guided by the elongated hole. A shaft  36 R, which is not shown in  FIG. 15 , is in the elongated hole of the linkage guide  34 , slides along the linkage guide  34  in response to the movement of the arm which is in connection with the second door, while being guided by the elongated hole. Referring to  FIG. 15 , the role of transmitting rotational driving force to the cam  50  of the fixation unit is performed by a coupling gear (connecting means) with which the main assembly of the image forming apparatus is provided. The coupling gear  40  is fitted around a gear shaft  41  attached to the left lateral plate  24  of the apparatus main assembly  1  by crimping. After the mounting of the fixing apparatus  8  into the apparatus main assembly  1 , the rotation axis of the coupling gear  40  coincides with the rotational axis of the shaft on which the cam  50  is. Next, referring to  FIG. 16 , the force applied to the doors to operate the doors is transmitted to the cam  50  through the connection between the key portion  50   a  of the cam  50  and the U-shape portion  40   a  (connective portion) of the coupling gear  40 ; the key portion  50   a  is fitted into the U-shaped portion  40 . 
         [0076]    Referring again to  FIG. 15 , there is a gear  42  between the mechanical linkage  29  and coupling gear  40 . The gear  42  is shaped like a fan, and the operational force is transmitted between the mechanical linkage  29  and coupling gear  40  by way of the fan-shaped gear  42 . One of the lengthwise ends of the gear  42  is in the form of a gear portion  42   b,  and the center portion of the gear  42  is provided with an axis  42   a,  about which the gear  42  is rotatable. The gear portion  42   b  meshes with the coupling gear  40 . The other lengthwise end of the gear  42  holds a shaft  35  which connect links  32 F and  32 R. More specifically, it holds the shaft  35  in such a manner that it allows the shaft  35  to slide. 
         [0077]    As at least one of the doors  27 F and  27 R is opened, the movement of the door changes the state of the mechanical linkage  29  from a state, shown in  FIG. 15(   a ), in which the fixation nip is under pressure, to a state, shown in  FIG. 15(   b ), in which the fixation nip is under no pressure, as it did in the first embodiment. More specifically, as at least one of the doors  27 F and  27 R is opened, the triangle which the links  32 F and  32 R, and link guide  34 , form, and the bottom side of which is the link guide  34 , the opening movement of the door causes the triangle to increase in height, whereby the opposite lengthwise end of the gear  42  from the gear portion  42   a  is pushed up, which in turn causes the gear portion  42   a  of the gear  42  to rotationally move in the direction indicated by an arrow mark in  FIG. 15(   b ). This causes the coupling gear  40  to rotate in the direction indicated by another arrow mark in  FIG. 15(   b ). Consequently, the pressure which has been on the fixation nip is removed. Incidentally, the linkage guide  34  is a stationary member solidly attached to the main frame of the main assembly  1  of the image forming apparatus, as is the linkage guide in the first embodiment. 
         [0078]    In this embodiment, the external cover of the main assembly of the image forming apparatus has to be removed in order for the fixation unit to be mounted into, or dismounted from, the main assembly of the image forming apparatus. Referring to  FIG. 16 , as the fixing apparatus  8  is removed from the apparatus main assembly  1  for such reason as repair or replacing the fixing apparatus  8 , the key portion  50   a,  which is a part of the cam  50  of the fixation unit moves in the opposite direction from the direction indicated by the arrow mark in  FIG. 16 . As a result, the key portion  50   a  comes out of the U-shaped portion  40   a.  In a case where the fixing apparatus  8  is mounted into the apparatus main assembly  1 , the key portion  50   a  fits into the U-shaped portion  40   a.    
         [0079]    When the fixing apparatus  8  is mounted or dismounted, the open side of the U-shaped portion  40   a  is facing the direction in which the fixing apparatus  8  is mounted or dismounted (lengthwise direction of U-shaped portion is parallel to direction in which fixing apparatus  8  is mounted or dismounted). Further, the rotational phase of the cam  50  is such that no pressure is applied to the fixation nip. 
         [0080]    On the other hand, when the rotational phase of the cam  50  is such that pressure is applied to the fixation nip, the open side of the U-shaped portion  25   a  is facing the opposite direction from the direction in which the fixing apparatus  8  is mounted or dismounted. Thus, it is impossible to mount or dismount the fixing apparatus  8 . 
         [0081]    As described above, in this embodiment, the fixing apparatus  8  is structured so that unless it is in the state in which the fixation nip is under no pressure, the fixing apparatus  8  in the apparatus main assembly  1  cannot be replaced. Therefore, the fixation nip of the fixing apparatus  8  remains under no pressure after the dismounting of the fixing apparatus  8  from the apparatus main assembly  1 . Further, in order to prevent deformations such as creep from occurring because force is continuously applied to the cam  50  for a long time, fixing apparatuses ( 8 ) to be used for repair, replacement, and the like purposes are also stored in the state in which the fixing nip is under no pressure. 
         [0082]    Incidentally, when the fixation nip is under pressure, the doors  27 F and  27 R are remaining closed. However, the fixing apparatus  8  is mounted or removed by removing the external cover of the image forming apparatus. Therefore, the application of fixation pressure to the fixation nip, or removal of the fixation pressure from the fixation nip, have nothing to do with the opening or closing of the doors. 
         [0083]    The fan-shaped gear  42  is greater in pitch circle than the coupling gear  40 . Thus, the angle by which the coupling gear  40  and cam  50  are rotated by the rotational movement of the mechanical linkage  29  and fan-shaped gear  42  is substantially greater than the rotational angle of the mechanical linkage  29  and gear  42 . Thus, it is possible to provide the cam  50  with a smooth and curved contact surface for gently lifting the metallic pressure plate  18 . In other words, this structural arrangement can reduce in amount the force to which the cam  50  is subjected when the metallic pressure plate  18  is lifted. Thus, this embodiment is advantageous from the standpoint of the durability of the door  27 F and  27 R in terms of the mechanism involved in the opening or closing of the doors. 
         [0084]    The fixation unit in this embodiment is removably mountable in the apparatus main assembly  1  as described above. It has the pressure application mechanism ( 18  and  19 ) and the pressure switching member  50  (cam). The pressure switching member  50  is in contact with the pressure application mechanism. The force applied to the door  27 F, that is, the first member which can be opened or closed, is transmitted to the pressure switching member  50 , the pressure switching member  50  moves the pressure application mechanism. The apparatus main assembly  1  and fixation unit are structured so that the force applied to the door  27 R, that is, the second member which can be opened or closed, to open the door  27 R, is also transmitted to the pressure switching member  50 , that is, the pressure switching member  50  to which the force applied to the door  27 F to open the door  27 F is applied. In other words, whether the door  27 F is operated or door  27 R, the force applied to the door  27  to open the door is transmitted to the pressure switching member  50 . Thus, this embodiment can improve the operational efficiency with which the fixation unit is mounted into, or dismounted from, the apparatus main assembly  1 . 
       Embodiment 3  
       [0085]    Next, the third embodiment of the present invention will be described. The third embodiment is a modification of the second embodiment. The third embodiment is different from the second embodiment in that in the third embodiment, while the rear door  27 R (door for removing jammed sheet) is moved from its closed position (first position) to an in-between position (second position), the pressure removal cam does not move at all; the cam moves only as the rear door  27 R is opened from the in-between position to the fully open position (third position). In other words, in the third embodiment, the pressure removal cam is moved by the movement of the rear door  27 R through the latter half of its opening movement range, and it is by this movement of the pressure removal cam that the pressure having been applied to the fixation nip is removed. The second position is set as the position for allowing the recording sheet to be discharged, instead of being set as the position for dealing with the jammed sheet. The components, parts, etc., of the main assembly of the image forming apparatus in this embodiment, and the fixation unit therefor, which are the same in function as the counterparts in the second embodiment, are given the same referential codes as those given to the counterparts in the second embodiment, and will not be described. 
         [0086]      FIG. 17  is a drawing for describing the transmission of driving force to the cam  50 . Referring to  FIG. 17 , the cam  50  and coupling gear  40  are coaxial. The coupling gear  40  rotates about the gear shaft  41  ( FIG. 15 ) attached to the apparatus main assembly  1 . 
         [0087]    The cam  50  has a key portion  50   a  by which the cam  50  receives the force transmitted to the cam  50  to rotate the cam  50  about the cam shaft  21 . The coupling gear  40  is in connection with the key portion  50   a  of the cam  50 , and transmits to the cam  50  the force for rotating the cam  50 . As the cam  50  rotates by receiving the cam rotating force by way of the key portion  50   a,  it lifts the metallic pressure plate  18  or allows the metallic pressure plate  18  to come down. 
       (Mechanical Linkage) 
       [0088]    Next, the mechanical linkage in this embodiment will be described.  FIGS. 18 and 19  are drawings for describing the structure of the mechanical linkage. Referring to  FIGS. 18 and 19 , the mechanical linkage has an arm  30 F (connected to door for mounting or dismounting cartridge), an arm  30 R (connected to door for removing jammed sheets), a link  32 F, a link  32 R, a link guide  34 , and a link  160  which connects the arm  30 R and door  27 R. 
         [0089]    One end of the arm  30 F is connected with one end of the link  32 R, with the shaft  35 . The links  32 F and  32 R are rotationally movable relative to each other. The other end of the link  32 F is provided with a shaft  36 F, whereas the other end of the link  32 R is provided with a shaft  36 R. One end of the arm  30 R is in connection with the shaft  36 R of the link  32 R. The shafts  36 F and  36 R are in a guiding groove  34 Ra of the link guide  34 , and are allowed to slide along the link guide  34  while remaining in the guiding groove  34 Ra. 
         [0090]    The above described portions of the structure of the image forming apparatus in this embodiment are the same as the counterparts in the second embodiment. Next, the portions of the structure of the image forming apparatus in this embodiment, which are different from the counterparts in the second embodiment, will be described. 
         [0091]    The other end of the arm  30 R is fitted with the link  160  so that they are allowed to slidingly move relative to each other. The arm  30 R and link  160  constitute a shaft which is allowed to extend or shrink. The link  160  is in connection with the door  27 R, which is rotatable about its supporting point  144 .  FIG. 20(   a ) is a drawing which shows the positional relationship between the link  160  and arm  30 R.  FIG. 20(   b ) is a drawing which shows the structure of the link  160 . Referring to  FIG. 20 , the link  160  has two bosses  161   a  and  161   b  (projections), which are on the mid portion of the link  160  in terms of the lengthwise direction of the link  160 . It has also a claw  162  (regulating portion), which is on the tip of the link  161 . The bosses  161   a  and  161   b  are in the guiding groove  30 Ra of the arm  30 R so that the link  160  and arm  30 R are allowed to slidingly move relative to each other while being guided by the bosses  161   a  and  161   b  and the guiding groove  30 Ra. 
         [0092]    Referring to  FIG. 20(   b ), the claw  162  of the link  160  (portion of link  160 , which has claw  162 ) is bendable in the direction indicated by an arrow mark (as indicated by dotted line in  FIG. 20(   b )). Next, referring to  FIG. 19 , the guiding member  145  guides the link  160  and arm  30 R while they slidingly move relatively each other. Further, the guide  145  prevents the link  160  and arm  30 R from becoming disengaged from each other. 
       (Operation of Mechanical Linkage During Opening of Jammed Recording Medium Removal Door  27 R) 
     (First Position) 
       [0093]      FIGS. 19 and 21  are drawings which show the mechanical linkage, arm  30 R, and link  160  while the door  27 R is remaining closed (door  27 R is in first position). 
         [0094]    Referring to  FIG. 19 , while the door  27 R is remaining closed (in first position), the door  27 R serves as a part of the walls of a recording sheet conveyance passage  127 . The recording sheet is conveyed through its conveyance passage  127 , and is discharged into a delivery portion  10  ( FIG. 1 ) in which recording sheets are layered. The delivery portion  10  is a part of the top wall of the image forming apparatus. 
         [0095]    Next, referring to  FIG. 20 , the link  160  is in the position into which it has slidingly moved relative to the arm  30 R in the direction indicated by the arrow mark B (closing direction of door  27 R). The boss  161   b  is at the lengthwise end  30 Ra 1  (in terms of direction indicated by arrow B) of the guiding groove  30 Ra. 
         [0096]    While the link  160  and arm  30 R are in the state shown in  FIG. 21 , the image forming apparatus can perform the normal printing operation, and the fixation nip is under pressure. 
       (Second Position) 
       [0097]      FIGS. 22 and 23  are drawings which show the states in which the mechanical linkage  29 , arm  30 R, and link  16  are, respectively, when the door  27 R is open (second position). 
         [0098]    Referring to  FIG. 22 , when the door  27 R is open (in second position), it serves as a face-up delivery tray (FU tray) in which sheets S are layered as they are discharged from the apparatus main assembly  1 . When the door  27 R is in the second position, the recording sheet conveyance passage is shorter and less winding than when the door  27 R is in the first position. Thus, when the door  27 R is in the second position, the recording sheet conveyance passage can convey even a sheet of recording medium such as a sheet of cardboard. 
         [0099]    Next, referring to  FIG. 23 , when the door  27 R is in the second position, the link  160  has slid in the direction indicated by an arrow mark A (opening direction of door  27 R): the combination of the link  160  and arm  30 R is in the extended state. The boss  161   a  is at the other end  30 Ra 2  (end in direction indicated by arrow mark A) of the guiding groove  30 Ra. 
         [0000]    (First Position         Second Position) 
         [0100]    Referring again to  FIG. 22 , while the door  27 R is opened from the first position to the second position, the link  160  is slidingly moved in the arrow mark A direction by the rotational movement of the door  27 R. Next, referring to  FIG. 23 , as the link  160  is slidingly moved, the bosses  161   a  and  161   b  slidingly move in the guiding groove  30 Ra in the arrow mark A direction. During this action, the arm  30 R is subjected to no force, and therefore, it does not slide. Thus, the fan-shaped gear  42 , key portion  50   a,  and cam  50  do not move. Consequently, the fixation nip remains under pressure. When the image forming apparatus  1  is in this state, it can perform the normal printing operation. 
         [0101]    Next, referring to  FIG. 23 , as the door  27 R is closed, that is, as the door  27 R is rotationally moved from the second position to the first position, the bosses  161   a  and  161   b  slidingly move in the guiding groove  30 Ra in the arrow B direction. During this action, the arm  30 R does not slide, and the fan-shaped gear  42 , key portion  50   a,  and cam  50  do not move. Thus, the fixation nip remains under pressure. 
       (Third Position) 
       [0102]      FIGS. 24 and 25  are drawings which show the mechanical linkage, arm  30 R, and link  160  which are in the state in which the door  27 R is in its third position (in which door  27 R is open wider than when it is in second position). 
         [0103]    Referring to  FIG. 24 , when the door  27 R is in the third position (in which it is open wider than when it is in second position), the shaft  36 R has slid in the guiding groove  34 Ra in the arrow A direction, and the arm  30 R has moved in the arrow A direction. That is, the door  27 R has been opened as far as where it allows the jammed sheet to be removed: the space for removing the jammed sheet has been increased in size. 
         [0104]    Next, referring to  FIG. 25 , the door  27 R is in the third position, the link  160  has slid in the arrow A direction (opening direction of door  27 R): the combination of the link  160  and arm  30 R is in the extended state. The boss  161   a  is at the other end  30 Ra 2  (end in arrow A direction) of the guiding groove  30 Ra. 
         [0000]    (Second Position         Third Position) 
         [0105]    Referring again to  FIG. 24 , during the period in which the door  27 R is opened from the second position to the third position while the boss  161   a  is remaining in contact with the other end  30 Ra 2  of the guiding groove  30 Ra, the arm R slides in the arrow A direction. As the arm  30 R slides, the shaft  36 R slides in the guiding groove  34 Ra in the arrow A direction. As a result, the shaft  35  is lifted in the direction indicated by an arrow mark G. This upward movement of the shaft  35  causes the fan-shaped gear  42  to rotate about the shaft  42   a,  which in turn causes the coupling gear  40  to rotate. This rotation of the coupling gear  40  causes the cam  50  to rotate by the key portion  50   a.  Thus, the metallic pressure plate  18  is lifted. Consequently, the fixation pressure is removed from the fixation nip. While the image forming apparatus  1  is in this state, it cannot perform the normal printing operation. When the arm  30 R is in the position shown in  FIG. 25 , the claw  162  of the link  106  is the position into which it has been bent as indicated by a dotted line in  FIG. 20 , by coming into contact with the guiding member  145 . Thus, the claw  162  is in the guiding groove  30 Ra of the arm  30 R. 
         [0106]    Next, during the period in which the door  27 R is closed, more specifically, during the period in which the door  27 R is rotationally moved from the third position to the second first position, the claw  162  moves in the arrow B direction while pushing the end  30 Ra 1  of the guiding groove  30 Ra and while the combination of the link  160  and arm  30 R remaining extended (remaining in state in which it cannot be extended or shrunk). This movement of the claw  160  causes the shaft  35  to move downward in the direction indicated by an arrow mark H, which in turn causes the fan-shaped gear  42  to rotate about the shaft  42   a  in the direction indicated by an arrow mark E, causing thereby the coupling gear  40  to rotate. This rotation of the gear  40  causes the cam  50  to rotate, by way of the key portion  50   a,  causing the metallic pressure plate to move downward. Consequently, the fixation unit comes under pressure. Thereafter, the claw  162  separates from the guiding member  145  right before the door  27 R moves into the second position. That is, the claw  162  is free from the state into which it was pressed. Consequently, it becomes possible for the bosses  161   a  and  161   b  to move in the guiding groove  30 Ra. That is, it is possible for the link  160  to move relative to the arm  30 R in the direction to cause the combination of the link  160  and arm  30 R to shrink. 
       Embodiment 4  
       [0107]    Next, the image forming apparatus in fourth embodiment of the present invention will be described with reference to drawings. The portions of the image forming apparatus in this embodiment, which are the same as the counterparts in the above described first and second embodiments will be given the same referential codes as those given to the counterparts, and will not be described.  FIG. 26  is a plan view of the connecting means  60  of the image forming apparatus in the fourth embodiment of the present invention, which is in the state in which the fixation nip is under pressure (first and second doors are remaining closed). 
         [0108]    Referring to  FIG. 26 , the image forming apparatus in this embodiment is similar to the image forming apparatus in the second embodiment, except that the image forming apparatus in this embodiment has the connecting means  60  and a return spring  45 , in place of the gear shaft  41 , fan-shaped gear  42 , links  32 F and  32 R, and link guide  34 . 
         [0109]    The connecting means  60  has a U-shaped portion  60   a  (connecting portion) and a protruding portion  60 F (which hereafter will be referred to simply as protrusion  60 F). In terms of the lengthwise direction of the connecting means  60 , the U-shaped portion  60   a  is roughly at the middle of the connecting means  60 , and the protrusion  60 F is at one end. The connecting means  60  is rotatable about a shaft  60   b,  which also is at the middle of the connecting means  60  in terms of the lengthwise direction of the connecting means  60 , but on the opposite surface of the connecting means  60  from the surface which has the U-shaped portion  60   a.    
         [0110]    The U-shaped portion  60   a  can be engaged with, or disengaged from, the key portion  50   a,  which is a part of the cam with which the fixation unit is provided, so that the fixing apparatus  8  can be mounted into, or dismounted from the apparatus main assembly  1  as it is in the second embodiment. 
         [0111]    The projection  60 F is allowed to slide in the elongated guiding hole  30 Fa, with which the end portion of the arm  30 F is provided. The projection  60 R is allowed to slide in the elongated guiding hole  30 Ra, with which the end portion of the arm  30 R is provided. 
         [0112]    The return spring  45  is between the adjacency of the protrusion  60 F and the apparatus main assembly  1 . 
         [0113]      FIG. 27(   a ) is a plan view of the connecting means  60  when the fixation nip is not under pressure (when first door  27 F is open).  FIG. 27(   b ) is a plan view of the connecting means  60  when the fixation unit is not under pressure (both first and second door  27 F and  27 R are open). 
         [0114]    Referring to  FIG. 27(   a ), as the door  27 F is opened, the arm  30 F is pulled by the opening movement of the door  27 F, whereby the projection  60 F is pulled, causing thereby the connecting means  60  to rotate about the shaft  60   b.  This rotational movement of the connecting means  60  causes the cam  50  to rotate by way of the U-shaped portion  60   a  and key portion  50   a.  During this action, the projection  60 R is allowed to slidingly move in the guiding hole  30 Ra. Thus, even while the connecting means  60  rotates, the arm  30 R is not subjected to any force, and therefore, does not move (it does not cause door  27 F to open). 
         [0115]    As the door  27 F is closed when it is in the state shown in  FIG. 27(   a ), the connecting means  60  is rotated by the return spring  45  into the position shown in  FIG. 26 . Incidentally, even if it is only the door  27 R that is opened or closed, the arm  30 R causes the connecting means  60  to rotate. Therefore, the connecting means  60  moves in the same manner as it does when the door  27 F is opened. 
         [0116]    Next, referring to  FIG. 27(   b ), if the door  27 R is opened when the door  27 F is open, the projection  60 R slides in the guiding hole  30 Ra. Thus, the connecting means  60  is not subjected to any force, and therefore, does not rotate. 
         [0117]    As both the doors  27 F and  27 R are closed, the connecting means  60  is rotated by the return spring  45  into the state shown in  FIG. 26 . 
         [0118]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0119]    This application claims priority from Japanese Patent Applications Nos. 093275/2009 and 136797/2009 filed Apr. 7, 2009 and Jun. 8, 2009, respectively, which are hereby incorporated by reference.