Patent Publication Number: US-8971778-B2

Title: Fixing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2011-205120 filed on Sep. 20, 2011, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a fixing device that heat-fixes a toner image on a recording sheet. 
     BACKGROUND 
     There have been known a fixing device which includes a heating member heating a recording sheet, a pressing roller forming a nip portion between the heating member and the pressing roller and a support frame having a pair of sidewalls rotatably supporting the pressing roller. 
     SUMMARY 
     Illustrative aspects of the invention provide a technique for positioning two frames with good precision in a fixing device including the two frames. 
     According to one illustrative aspect of the invention, there is provided a fixing device configured to heat-fix a developer image on a recording sheet. The fix device comprises: a first fixing member and a second fixing member configured to form a nip portion for heat-fixing the recording sheet; an urging mechanism comprising an urging member and is configured to urge the first fixing member toward the second fixing member by an urging force of the urging member; a first frame configured to support the second fixing member; a second frame that is arranged at an opposite side to the second fixing member with the first fixing member being interposed therebetween; and a switching member configured to apply a pressing force resisting the urging force of the urging member to the first fixing member for switching a width of the nip portion. The switching member comprises: a cam to which the urging force of the urging member is applied; and a shaft configured to support the cam. The first frame and the second frame are connected to each other by the shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic configuration of an image forming apparatus including fixing de cc according to an exemplary embodiment of the invention; 
         FIG. 2  is a sectional view of the fixing device; 
         FIG. 3  is a perspective view of a nip plate, a halogen lamp, a reflection member and a stay; 
         FIG. 4  is an exploded perspective view of the fixing device; 
         FIG. 5A  is a perspective view of an arm member, and  FIG. 5B  is a schematic sectional view showing a relation between the arm member and a recess; 
         FIG. 6A  is an enlarged perspective view of a protrusion, and  FIG. 6B  is a side view thereof; 
         FIG. 7  is a perspective view of the fixing device, which is obliquely seen from the upper-rear; 
         FIG. 8A  is a side view of the fixing device, and  FIG. 8B  is a schematic sectional view of a sidewall, which is taken in the vicinity of a shaft; 
         FIG. 9  is an enlarged sectional view showing a structure in the vicinity of a conveyance roller; 
         FIGS. 10A to 10C  illustrate states of a nip portion, which are switched by a switching member; 
         FIGS. 11A to 11C  illustrate a relation between a cam and an arm member; 
         FIG. 12A  illustrates a state where the arm member contacts a bottom surface of a recess, and  FIG. 12B  illustrates a state where a width of the nip portion becomes a maximum value of a first nip width; and 
         FIG. 13  shows a modified embodiment of a shaft supporting part. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;General Overview&gt; 
     It may be considered a configuration in which an upper frame covering an opposite side of the heating member to the pressing roller is provided so as to prevent heat of the heating member from escaping to an outside, separately from the support frame. However, when the upper frame is fixed so that it is put on the sidewalls of the support frame, the upper frame is inclined relative to the support frame due to manufacturing errors of the respective sidewalls. Thereby, it may be difficult to position the upper frame relative to the support frame with good precision. 
     Therefore, illustrative aspects of the invention provide technique for positioning two frames with good precision a fixing device including the two frames. 
     According to one illustrative aspect of the invention, there is provided a fixing device configured to heat-fix a developer image on a recording sheet. The fixing device comprises: a first fixing member and a second fixing member configured to form a nip portion heat-fixing the recording sheet; an urging mechanism comprising an urging member and is configured to urge the first fixing member toward the second fixing member by an urging force of the urging member; a first frame configured to support the second fixing member; a second frame that is arranged at an opposite side to the second fixing member with the first fixing member being interposed therebetween; and a switching member configured to apply a pressing force resisting the urging force of the urging member to the first fixing member for switching a width of the nip portion. The switching member comprises: a cam to which the urging force of the urging member is applied; and a shaft configured to support the cam. The first frame and the second frame are connected to each other by the shaft. 
     According thereto, since the first frame and the second frame are connected to each other by one shaft, it is possible to position the first frame and the second frame relative to the one shaft. Also, possible to reduce the number of parts, compared to a structure where a positioning shaft is separately provided from the switching member. 
     According to another illustrative aspect of the invention, the shaft is arranged at a first side of the first frame in a conveyance direction of the recording sheet, and a second side, which is opposite to the first side, of the first frame in the conveyance direction is provided with a restraint part configured to suppress the second frame from oscillating relative to the first frame about the shaft. 
     According thereto, it is possible to suppress the oscillation of the second frame about the shaft. 
     According to still another illustrative aspect of the invention, the first frame and the second frame are made of resin. 
     According thereto, it is possible to increase the degree of freedom of the shapes of the first and second frames. 
     According to still another illustrative aspect of the invention, the shaft is made of metal. 
     According thereto, since the two frames made of resin are connected by the metal shaft, it is possible to increase the rigidity of each frame. 
     According to still another illustrative aspect of the invention, the first frame comprises a first opening into which the shaft is inserted. 
     According to still another illustrative aspect of the invention, the first opening is a hole. 
     According to still another illustrative aspect of the invention, the first opening is a notched part in which a part of a hole is opened to an outside. 
     According to still another illustrative aspect of the invention, the cam comprises a cylindrical part made of resin and protruding axially, the shaft is configured to be inserted into the cylindrical part, and the cylindrical part is configured to be inserted into a hole of the first frame and to be rotatably supported by the corresponding hole. 
     According thereto the above configuration, since the cylindrical part of the cam made of resin slides relative to the first frame made of resin, it is possible to reduce the sliding resistance and thus to smoothly rotate the cam. 
     According to still another illustrative aspect of the invention, the second frame comprises a second opening into which the shaft is inserted. 
     According to still another illustrative aspect of the invention, the second opening is a hole. 
     According to still another illustrative aspect of the invention, the second opening is a notched part in which a part of a hole is opened to an outside. 
     According to still another illustrative aspect of the invention, the second opening is arranged at both end portions and a central portion of the second frame in an axial direction of the shaft. 
     According thereto, it is possible to securely suppress the bending of the second frame. Also, since it is possible to minimize a size of the connection part of the second frame with the shaft, it is possible to make the second frame light. 
     According to still another illustrative aspect of the invention, the second frame comprises: a conveyance roller configured to con convey the recording sheet; a bearing part configured to rotatably support the conveyance roller; a guide recess extending from an outer surface of the second frame toward the bearing part and configured to guide the conveyance roller to the bearing part; and an elastic member configured to urge the conveyance roller toward the bearing part. The shaft is disposed on a trajectory of the conveyance roller guided by the guide recess. 
     According thereto, since it is possible to suppress the conveyance roller from deviating from the second frame by the shaft, it is possible to reduce the number of parts, compared to a structure where a member for deviation prevention is separately provided from the shaft. 
     According to the illustrative aspects of the invention, it is possible to position two frames with good precision in a fixing device including the two frames. 
     EXEMPLARY EMBODIMENTS 
     Hereinafter, exemplary embodiments of the invention will be specifically described with reference to the drawings. In the below descriptions, a schematic configuration of an image forming apparatus  1  including a fixing device  100  according to an exemplary embodiment of the invention will be briefly described, and then a specific configuration of the fixing device  100  will be described. Incidentally, a laser printer is one example of the image forming apparatus  1 . 
     Incidentally, in the below descriptions, the directions are described on the basis of a user who uses the image forming apparatus  1 . That is, the right side of  FIG. 1  is referred to as the ‘front’, the left side is referred to as the ‘rear’, the front side is referred to as the ‘left’ and the inner side is referred to as the ‘right.’ Also, the upper-lower direction of  FIG. 1  is referred to as the ‘upper-lower.’ 
     (Schematic Configuration of Image Forming Apparatus) 
     As shown in  FIG. 1 , the image forming apparatus  1  includes, in a body housing  2 , a feeder unit  3  that feeds a sheet S, which is one example of a recording medium, an exposure device  4 , a process cartridge  5  that transfers a toner image (developer image) on the sheet S and a fixing device  100  that heat-fixes the toner image transferred on the sheet S. 
     The feeder unit  3  is provided at a lower part in the body housing  2 . The feeder unit  3  includes a sheet feeding tray  31 , a sheet pressing plate  32  and a sheet feeding mechanism  33 . The sheet S accommodated in the sheet feeding tray  31  is upwardly inclined by the sheet pressing plate  32  and is fed toward the process cartridge  5  (e.g., between a photosensitive drum  61  and a transfer roller  63 ) by the sheet feeding mechanism  33 . 
     The exposure device  4  is arranged at an upper part in the body housing  2 . The exposure device  4  includes a laser emitting unit (not shown), a polygon mirror, a lens, a reflector and the like whose reference numerals are omitted. In the exposure device  4 , a laser light (refer to the dotted-dashed line) based on image data, which is emitted from the laser emitting unit, is scanned on a surface of the photosensitive drum  61  at high speed, thereby exposing the surface of the photosensitive drum  61 . 
     The process cartridge  5  is disposed below the exposure device  4 . The process cartridge  5  is configured to be detachably mounted to the body housing  2  through an opening that is formed when a front cover  21  provided to the body housing  2  is opened. The process cartridge  5  includes a drum unit  6  and a developing unit  7 . 
     The drum unit  6  includes the photosensitive drum  61 , a charger  62  and the transfer roller  63 . Also, the developing unit  7  is configured to be detachably mounted to the drum unit  6 . The developing unit includes a developing roller  71 , a supply roller  72 , a layer thickness regulation blade  73  and a toner accommodation unit  74  that accommodates toner that is one example of developer. 
     In the process cartridge  5 , the surface of the photosensitive drum  61  is uniformly charged by the charger  62  and then exposed by the high-speed scanning of the laser light emitted from the exposure device  4 , so that an electrostatic latent image based on image data is formed on the photosensitive drum  61 . Also, the toner in the toner accommodation unit  74  is supplied to the developing roller  71  via the supply roller  72 , is introduced between the developing roller  71  and the layer thickness regulation blade  73 , and is carried on the developing roller  71  as a thin layer having a predetermined thickness. 
     The toner carried on the developing roller  71  is supplied from the developing roller  71  to the electrostatic latent image formed on the photosensitive drum  61 . Thereby, the electrostatic latent image becomes visible, and a toner image is thus formed on the photosensitive drum  61 . Then, the sheet S is conveyed between the photosensitive drum  61  and the transfer roller  63 , so that the toner image on photosensitive drum  61  is transferred onto the sheet S. 
     The fixing device  100  is arranged at the rear of the process cartridge  5 . The toner image transferred on the sheet S passes through the fixing device  100 , so that the toner image is heat-fixed on the sheet S. Then, the sheet S is discharged on a sheet discharge tray  22  by conveyance rollers  23 ,  24 . 
     (Detailed Configuration of Fixing Device) 
     As shown  FIG. 2 , the fixing device  100  includes a fixing belt  110 , a halogen lamp  120 , a nip plate  130  that is one example of a first fixing member, a pressing roller  140  that is one example of a second fixing member, a reflection member  150  and a stay  160 . 
     The fixing belt  110  is a stainless steel belt of an endless shape (e.g., cylindrical shape) having heat resistance and flexibility. A rotation of the fixing belt  110  is guided by a guide part (e.g., a nip upstream guide  310 , a nip downstream guide  320 , an upper guide  330  and a front part guide  340 ) that is provided to a cover member  200 . Here, the cover member  200  includes a first cover member  210  and a second cover member  220 . 
     The first cover member  210  has a substantially U-shaped section and is elongated to extend in the left-right direction. The first cover member  210  is arranged to cover the stag  160  at an opposite side to the halogen lamp  120  with the stay  160  being interposed therebetween. The first cover member  210  includes a rear wall  211 , a front wall  212 , an upper wall  213  extending to connect upper ends of the rear wall  211  and the front wall  212 , and an extension wall  214  extending rearward from a lower end of the rear wall  211 . 
     A right end of the front wall  212  is formed with a front part guide  340  that guides a front part of the fixing belt  110 , and a lower end of the front wall  212  is formed with a nip upstream guide  310  that guides a front lower part of the fixing belt  110 . Also, a rear end of the extension wall  214  is formed with a nip downstream guide  320  that guides a rear lower part of the fixing belt  110 . 
     The second cover member  220  has a substantially L-shaped section and is elongated to extend in the left-right direction. The second cover member  220  is arranged to cover parts of the rear wall  211  and upper wall  213  of the first cover member  210 . The second cover member  220  includes an upper wall  221 , a rear wall  222  extending downward from a rear end of the upper wall  221 , and an extension wall  223  extending rearward from a lower end of the rear wall  222 . The upper wall  221  is formed with an upper guide  330  that guide an upper part of the fixing belt  110 . 
     The halogen lamp  120  is a member that generates radiation heat to thus heat the nip plate  130  and the fixing belt  110  (e.g., nip portion N), thereby heating the toner on the sheet S. The halogen lamp  120  is arranged at the inside of the fixing belt  110  at a predetermined interval from inner surfaces of the fixing belt  110  and the nip plate  130 . 
     As shown in  FIG. 3 , the halogen lamp  120  is formed by arranging a filament (not shown) in an elongated cylindrical glass tube  121 , closing both longitudinal end portions of the glass tube  121  and enclosing inert gases including halogen element in the glass tube. A pair of electrodes  122  electrically connected to end portions of the filament in the glass tube  121  is provided on both longitudinal end portions of the halogen lamp  120 . 
     Again referring to  FIG. 2 , the nip plate  130  is a plate-shaped member to which the radiation heat from the halogen lamp  120  is applied. A lower surface of the nip plate  130  is arranged to slidingly contact an inner peripheral surface of the fixing belt  110 . In this exemplary embodiment, the nip plate  130  is made of metal, and for example is formed by bending an aluminum plate and the like having thermal conductivity higher than the stay  160  made of steel. Incidentally, when the nip plate  130  is made of aluminum, it is possible to improve the thermal conductivity of the nip plate  130 . 
     As shown in  FIG. 3 , the nip plate  130  includes a base part  131 , a first extension  132  and second extensions  133 . 
     The base part  131  is a part that slidingly contacts an inner peripheral surface of the fixing belt  110  and extends in a conveyance direction of the sheet S to thus form a nip portion N. The base part  131  transfers heat from the halogen lamp  120  to toner on the sheet S via the fixing belt  110 . As shown in  FIG. 2 , a bent portion  131 A that is bent toward an inside (e.g., opposite side to the pressing roller  140 -side) of the fixing belt  110  is formed at an upstream end portion of the base part  131  in the conveyance direction. 
     Thereby, it is possible to suppress the fixing belt  110  from being worn due to the friction with an end edge of the nip plate  130 . 
     A flange portion  131 B extending from the bent portion  131 A toward an upstream side in the conveyance direction (e.g., an opposite side to the base part  131  in the conveyance direction) is formed at an upstream end portion of the bent portion  131 A in the conveyance direction. A corner part between the bent portion  131 A and the flange portion  131 B is provided with lubricant G. Thereby, it is possible to further improve the sliding characteristic of the fixing belt  110  by the lubricant G. 
     As shown in  FIG. 3 , the first extension  132  and the second extension  133  have a flat plate shape, respectively, and are formed to extend rearward from a rear end of the base part  131 . The one first extension  132 , is formed near a center of the rear end of the base part  131  in the left-right direction, and a thermostat  170  (refer to  FIG. 2 ) is arranged to face an upper surface of the first extension. Also, the second extensions  133  are respectively formed near the center and near a right end of the rear end of the base part  131  in the left-right direction, and two thermistors (not shown) are arranged to face upper surfaces of the second extensions. 
     As shown in  FIG. 2 , the pressing roller  140  is a member forming the nip portion N between the fixing belt  110  and the pressing roller by interposing the fixing belt  110  between the nip plate  130  and the pressing roller. The pressing roller  140  is disposed below the nip plate  130 . In this exemplary embodiment, in order to form the nip portion N, one of the nip plate  130  and the pressing roller  140  is urged toward the other. The pressing roller  140  is configured to rotate with the fixing belt  110  being positioned between the nip plate  130  and the pressing roller, thereby conveying the sheet S together with the fixing belt  110 . 
     The pressing roller  110  is configured to rotate as a driving force is transferred thereto from a motor (not shown) provided in the body housing  2 . As the pressing roller rotates, it rotates the fixing belt  110  by a frictional force with the fixing belt  110  (or sheet S). As the sheet S having the toner image transferred thereto is conveyed between the pressing roller  140  and the heated fixing belt  110  at the nip portion N), the toner image is heat-fixed. 
     The reflection member  150  is a member that reflects the radiation heat from the halogen lamp  120  toward the nip plate  130 . The reflection member  150  is arranged at a predetermined interval from the halogen lamp  120  so that the reflection member surrounds (covers) the halogen lamp  120  at the inside of the fixing belt  110 . 
     The reflection member  150  is formed by bending an aluminum plate and the like having high reflectance of the infrared and far-infrared into a substantial U shape, when seen from the section. More specifically, the reflection member  150  includes a reflection part  151  having a bent shape and flange parts  152  extending from front and rear end portions of the reflection part  151  toward the outside in the front-rear direction. 
     The stay  160  is a member that supports the front and rear end portions of the nip plate  130  (e.g., base part  131 ) via the reflection member  150  (e.g., flange parts  152 ) to thus bear load applied from the pressing roller  140 . The stay  160  is arranged to cover the reflection member  150  at the inside of the fixing belt  110 . Incidentally, in the configuration in which the nip plate  130  urges the pressing roller  140 , the load means a reactive force of the force with which the nip plate  130  urges the pressing roller  140 . 
     The stay  160  is formed by bending, for example, a steel plate having relatively high rigidity into a substantially U-shaped section conforming to an outer surface shape of the reflection member  150  (reflection part  151 ). As shown in  FIG. 3 , the stay  160  includes a right fixation part  161  provided at a right side thereof and a left fixation part  162  provided at a left side. The right fixation part  161  and the left fixation part  162  are formed to extend rearward from an upper wall part of the stay  160  and include a penetrated screw hole (reference numeral thereof is omitted), respectively. 
     Also, as shown in  FIG. 4 , the fixing device  100  includes a first frame  400 , an urging mechanism  500 , a second frame  600  and a switching member  700  in addition to the above members. 
     The first frame  400  is a frame made of resin. The first frame  400  includes a lower wall part  410  and a pair of sidewalls  420  protruding upward from both ends of the lower wall part  410  in the left-right direction. 
     The pair of sidewalls  420  rotatably supports the pressing roller  140  at lower parts thereof and slidably supports a heating unit HU in the upper-lower direction at upper parts thereof. Here, the heating unit HU has not only a structure configured by the fixing belt  110 , the halogen lamp  120 , the nip plate  130 , the reflection member  150 , the stay  160  and the cover member  200  but also a side guide (not shown) that supports both left and right ends of the structure (for example, the stay  160 ) and guides both left and right end portions of the fixing belt  110 . 
     The side guide is slidably supported to the pair of side walls  420 , so that the heating unit HU can move in the upper-lower direction. Also, the left sidewall  420  is provided with a driving gear  440  for driving the pressing roller  140 . 
     Specifically, the driving gear  440  is integrally provided to the left end of the pressing roller  140  and is configured to integrally rotate with the pressing roller  140  as a driving force from a motor (not shown) is applied thereto. Also, the pair of sidewalls  420  is provided with the urging mechanism  500 . 
     The urging mechanism  500  is a mechanism for urging the heating unit HU (e.g., nip plate  130 ) toward the pressing roller  140 . The urging mechanism  500  includes a pair of arm members  510  and a pair of tension coil springs  540  that is one example of an urging member. 
     The pair of arm members  510  is arranged at upper parts of both left and right end portions of the heating unit HU and is bilaterally symmetric. As shown in  FIG. 5A , the arm member  510  extends in the front-rear direction and has an L-shaped section by plate-shaped vertical wall part  520  and horizontal wall part  530 . 
     The vertical wall part  520  is a wall orthogonal to the left-right direction. A first extension  521  extending downward is formed at a front end portion of the vertical wall part  520 . The first extension  521  is formed with a rotation center hole  522  in which a shaft (not shown) of the sidewall  420  of the first frame  400  is rotatably supported. Thereby, a rear end portion of the arm member  510  can vertically oscillate about the rotation center hole  522 . 
     Also, the vertical wall part  520  is formed at a rear side thereof with a second extension  523  protruding more downward than the horizontal wall part  530 . As shown in  FIGS. 4 and 5A , the second extension  523  is inserted into a recess  430  of each sidewall  420  of the first frame  400 . The recess  430  is opened upward and includes a bottom surface  431  facing the second extension  523  of the arm member  510  in the upper-lower direction (e.g., urging direction of the tension coil spring  540 , which will be described later). 
     Thereby, when a lower end of the second extension  523  contacts the bottom surface  431  of the recess  430 , the arm member  510  is restrained from moving downward, so that the heating unit HU is not further lowered downward. 
     Also, the second extension  523  is formed at a rear side thereof with a hook part  524  that extends rearward and is then bent upward. The tension coil spring  540  is provided between the hook part  524  and the sidewall  420  of the first frame  400 , so that a rear end part (specifically, an opposite side to a rotational shaft with a pressing part  531  of the arm member  510  being interposed therebetween) of the arm member  510  is urged toward the first frame  400 . 
     The horizontal wall part  530  is a wall orthogonal to the upper-lower direction. The horizontal wall part  530  includes, at its substantially central part, a pressing part  531  for pressing the heating unit HU. The pressing part  531  is arranged at an outer side of the hook part  524  in the left-right direction. In other words, the tension coil spring  540  is arranged at an inner side (e.g., widthwise inner side of the sheet S) of the pressing part  531  in the left-right direction. 
     Thereby, a force that is applied from the urging mechanism  500  to the pair of sidewalk  420  of the first frame  400  is generated toward the inner sides thereof in the left-right direction. 
     Incidentally, the heating unit  11 U is supported to the arm member  510  (pressing part  531 ), and the heating unit HU is moved in the upper-lower direction as the arm member  510  is moved in the upper-lower direction. 
     As shown in  FIG. 4 , the second frame  600  is a frame made of resin and having a long shape extending in the left-right direction. The second frame  600  is arranged at an opposite side to the pressing roller  140  with the heating unit HU being interposed therebetween and is provided to extend over the pair of sidewalls  420  of the first frame  400 . The second frame  600  includes a main body part  610  having a long shape and cover parts  620 . The cover parts  620  protrudes from front-upper parts of left and right side surfaces  611  of the main body part  610  toward the outside in the left-right direction. 
     The main body part  610  is formed to be shorter than a distance between the pair of sidewalk  420  and is thus inserted between the pair of sidewalls  420 . The left and right side surfaces  611  of the main body part  610  are formed at rear-lower parts thereof with protrusions  630  protruding toward the outside in the left-right direction (e.g., widthwise outer side of the conveyance S). 
     The protrusions  630  are arranged at the inside of the sidewalls  420  in the left-right direction. As shown in  FIG. 6A , the protrusions  630  have a leading end face  631 , respectively, which is formed to have a height with being arranged at an interval from each of the sidewalls  420 . Thereby, when the pair of sidewalk  420  is pushed and bent inward in the left-right direction by the urging mechanism  500 , the sidewalls  420  are brought into contact with the leading end faces  631 , thereby suppressing the deformation of the sidewalls  420 . That is, in this exemplary embodiment, the leading end faces  631  of the protrusions  630  serve as restraint surfaces that restrain the deformation of the sidewalls  420 . 
     The deformation of the sidewalls  420  is suppressed as described above, so that a position of the driving gear  440  provided to the sidewall  420  is suppressed from being deviated in the left-right direction. Therefore, it is possible to securely operate the driving gear  440 . Also, since the gaps are formed between the pair of sidewalls  420  and the leading end faces  631 , it is possible to easily assemble the first frame  400  and the second frame  600 . 
     Also, as shown in  FIG. 6B , the protrusion  630  includes a first plate-shaped part  632 , a second plate-shaped part  633  orthogonal to (intersecting with) the first plate-shaped part  632 , and a third plate-shaped part  634  orthogonal to the second plate-shaped part  633  and the protrusion  630  is configured to have an h shape by the plate-shaped parts  632  to  634 . Thereby, since it is possible to improve the rigidity of the protrusion  630 , it is possible to securely suppress the deformation of each sidewall  420  by the protrusion  630 . Also, since the protrusion is formed by the respective plate shaped-parts  632  to  634 , i.e., the protrusion is configured to have a thin structure, it is possible to make the second frame  600  light. 
     As shown in  FIG. 7 , the first frame  400  and the second frame  600  are connected to each other by one shaft  710  of the switching member  700 , which will be described later. Thereby, it is possible to position the first frame  400  and the second frame  600  relative to the one shaft  710  with good precision. Also, it is possible to reduce the number of parts, compared to a structure where a positioning shaft is separately provided from the shaft  710  of the switching member  700 . 
     The shaft  10  extends from one end side to the other end side in the left-right direction of the second frame  600  and penetrates the pair of sidewalk  420  of the first frame  400  and the second frame  600 . Thereby, since the second frame  600  is reinforced by the shaft  710  and the bending of the second frame  600  is thus suppressed, it is possible to securely suppress the deformation of the sidewalls  420  by the leading end faces  631  of the protrusions  630 . Also, since the shaft  710  of the switching member  700  is used for reinforcement of the second frame  600 , it is possible to reduce the number of parts, compared to a structure where a shaft for reinforcement is separately provided. 
     In this exemplary embodiment, the shaft  710  is made of metal. Thereby, while the two frames  400 ,  600  are made of resin and the degree of freedom of the shapes thereof is thus increased, it is possible to improve the rigidity of the frames  400 ,  600  made of resin by the shaft  710  made of metal. 
     The second frame  600  is formed at an upper-rear side thereof with three parts to be supported  641 ,  642 , which are supported by the shaft  710 . Two parts to be supported  641  are provided at both end portions of the second frame  600  in the left-right direction (e.g., axial direction of the shaft  710 ) and have one plate shape, respectively. The two parts to be supported  641  are formed with penetration holes  641 A, respectively, which are one example of a second opening into which the shaft  710  is inserted. 
     One part to be supported  642  is provided at a central portion of the second frame  600  in the left-right direction and is configured by integrally connecting two plate-shaped ribs with a connection part having a diameter larger than the shaft  710 . The part to be supported  642  is formed with a penetration hole  642 A, which is one example of a second opening into which the shaft  710  is inserted. 
     Here, the central portion of the second frame  600  may be an exactly central portion of the second frame  600  in the left-right direction, as shown, or may be deviated leftward or rightward from the central portion. 
     The three parts to be supported  641 ,  642  are arranged as described above, so that it is possible to securely suppress the bending of the second frame  600  by the shaft  710 . Also, since it is possible to minimize a size of the connection part of the second frame  600  with the shaft  710 , it is possible to make the second frame  600  light. 
     Also, the left-right width of the part to be supported  642  provided to the central portion of the second frame  600  is made to be larger than the left-right width of each part be supported  641  provided to both end portions. Accordingly, it is possible to securely suppress the bending of the central portion of the second frame  600 . 
     Also, as shown in  FIGS. 8A and 8B , the upper parts of the rear sides (e.g., one side of the sheet S in the conveyance direction) of the sidewalls  420  of the first frame  400  are formed with through-holes  421 , which are one example of a first opening into which the shaft  710  is inserted. The shaft  710  penetrates the rear part of the first frame  400  and the rear part of the second frame  600 , as described above, so that the front part of the second frame  600  may be able to oscillate about the shaft  710 . However, the oscillation is suppressed by protrusions  422  and engaging recess portions  621  provided to the front part of the first frame  400  and the front part of the second frame  600 . 
     Specifically, the protrusions  422 , which are one example of a restraint part, are formed on the upper parts of the front sides (e.g., the other side of the sheet S in the conveyance direction) of the sidewalls  420  of the first frame  400  so that they protrude outward in the left-right direction. 
     The engaging recess portions  621  are formed on the front sides of the cover parts  620  of the second frame  600 . Specifically, as shown in  FIG. 7 , the cover parts  620  extend outward from both left and right ends of the main body part  610  in the left-right direction, pass over the sidewalls  420 , and are then bent downward to thus face the outer surfaces of the sidewalls  420 , such that the cover parts  620  cover the vicinity of the rotational shaft of the arm members  510 . Also, as shown  FIG. 8A , the engaging recess portions  621  are formed on the front sides of the cover parts  620  facing the outer surfaces of the sidewalls  420 . 
     The engaging recess portion  621  is a recess portion that is opened forward. The engaging recess portion  621  is engaged with the protrusion  422  so that the protrusion  422  is held therein in the upper-lower direction. Incidentally, when mounting the first frame  400  and the second frame  600  configured as described above, the pair of cover parts  620  of the second frame  600  is first slid along the upper surfaces of the pair of the sidewalls  420  of the first frame  400 , so that the pair of engaging recess portions  621  is engaged with the pair of protrusions  422 . 
     Then the shaft  710  is inserted into the respective through-holes  421  of the first frame  400  and the respective through-holes  641 A,  642 A of the second frame  600 . Thereby, the second frame  600  is mounted to the first frame  400 . More specifically, cams  720  that will be described later are attached to both ends of the shaft  710  inserted into the respective through-holes  641 A,  642 A, so that the second frame  600  is mounted to the first frame  400 . 
     As shown in  FIG. 7 , the rear-upper part of the second frame  600  is provided with a plurality of conveyance rollers  650  for conveying the sheet S at an interval iii the left-right direction, and is also provided with a plurality of guide ribs  660  for guiding the sheet S at an interval in the left-right direction so that each conveyance roller  650  is positioned therebetween in the left-right direction. 
     As shown in  FIG. 9 , the guide ribs  660  that are disposed at both left and right sides of the conveyance roller  660  are respectively formed with a recess-shaped bearing part  661  having a substantially U shape and rotatably supporting a rotational shaft part  651  of the conveyance roller  650  and a guide recess  662  for guiding the conveyance roller  650  into the bearing part  661 . The guide recess  662  is a recess extending from the bearing part  661  to the upper surface (e.g., outer surface) of the second frame  600 , and a part adjacent to an upper surface thereof is formed to be wider than the bearing part  661 , so that the guide recess communicates with a space of the upper part of the second frame  600 . 
     Thereby, it is possible to easily insert the rotational shaft part  651  of the conveyance roller  650  into the guide recesses  662  (e.g., wider parts). Also, when the rotational shaft part  651  of the conveyance roller  650  is being inserted along the guide recesses  662 , the rotational shaft part  651  is guided to the bearing parts  661  by the guide recesses  662 , so that the conveyance roller  650  can be mounted to the second frame  600 . 
     When the shaft  710  is inserted into the respective through-holes  421 ,  641 A,  642 A of the respective frames  400 ,  600  after the conveyance roller  650  is mounted to the second frame  600 , the shaft  710  is disposed on a trajectory of the conveyance roller  650  moving along the guide recesses  662 . Thereby, since it is possible to suppress the conveyance roller  650  from deviating from the second frame  600  by the shaft  710 , it is possible to reduce the number of parts, compared to a structure where a member for deviation prevention is separately provided from the shaft  710 . 
     Also, a torsion spring  670 , which is one example of an elastic member, urging the conveyance roller  650  toward the bearing part  660 , specifically, toward a bottom surface of the bearing part  661  having a U shape is provided in the vicinity of the bearing part  661  of the second frame  600 . Thereby, since the conveyance roller  650  is urged toward a driving roller (not shown) disposed below the conveyance roller  650  by the torsion spring  670 , it is possible to enable the conveyance roller  650  to follow the driving roller. 
     As shown in  FIG. 4 , the switching member  700  includes the shaft  710  and a pair of cams  720  that is fixed (supported) to both end portions of the shaft  710 . The shaft  710  is rotatably supported to the first frame  400  and the second frame  600 . Thereby, the pair of cams  720  provided to both end portions of the shaft  710  is rotated relative to the respective frames  400 ,  600  together with the shaft  710 . 
     The cam  720  is a member that is made of resin and can adjust a width of the nip portion by pressing upward the arm member  510  against the urging force of the tension coil spring  540 . The cam  720  is disposed below the arm member  510 . The cam  720  includes a cylindrical part  721 , into which the shaft  710  is inserted, and a plate earn part  722  that extends outward from the cylindrical part  721  in a diametrical direction. 
     The cylindrical part  721  protrudes inward (e.g., inner side in the axial direction) from the plate cam part  722  in the left-right direction, and is inserted into the through-hole  421  of the sidewall  420  of the first frame  400  and thus is rotatably supported therein, as shown in  FIG. 8B . Thereby, since the cylindrical part  721  of the cam  720  made of resin slides relative to the first frame  200  made of resin, it is possible to reduce the sliding resistance and to thus smoothly rotate the cam  720 . 
     Also, as shown in  FIG. 4 , an operation part  730 , which is operated by a user, is integrally provided to an outer side of the right cam  720  in the left-right direction. When a user operates the operation part  730 , it is possible to switch the nip width in three stages, as shown in  FIGS. 10A to 10C . Here,  FIG. 10A  shows a first nip width N 1  at the time of printing a normal sheet and the like,  FIG. 10B  shows a second nip width N 2  smaller than the first nip width N 1 , which is a nip width at the time of printing a cardboard and the like, and  FIG. 10C  shows a state (nip width=0) where the heating unit HU is separated from the pressing roller  140 . Here, in  FIG. 10A to 10C , the cover member  200  and the like are omitted for convenience. 
     Incidentally, the ‘first nip width N 1 ’ and the ‘second nip width N 2 ’ have an allowance (tolerance) regarding a design value, to some extent. 
     Specifically, in the state of the first nip width N 1  shown in  FIG. 10A , the cam  720  is distant from the arm member  510  (refer to  FIG. 11A ), i.e., the cam  720  is not applied with the urging force of the tension coil spring  540 . From this state (hereinafter, referred to as the ‘first direction’), when the cam  720  is unidirectionally rotated into a second direction by the operation of the operation part  730 , as shown in  FIG. 11B , the arm member  510  is pushed up by a predetermined amount. That is, when the pressing force resisting the urging force of the tension coil spring  540  is applied to the heating unit HU, the heating unit HU is moved from the lowest first position to an upper second position. Thereby, as shown in  FIG. 10B , the width of the nip portion is switched from the first nip width N 1  to the second nip width N 2 . 
     Also, when switching the width of the nip portion from the second nip width N 2  to zero, the cam  720  is unidirectionally rotated by a predetermined amount from the second direction to a third direction by the operation of the operation part  730 , as shown in  FIG. 11C , so that the arm member  510  is further pushed up. Thereby, the heating unit HU is moved to the uppermost third position, so that the nip width becomes zero (refer to  FIG. 10C ). 
     Here, when the nip width is the second nip width N 2  or zero, the cam  720  is applied with the urging force of the tension coil spring  540  via the arm member  510 , as shown in  FIGS. 11B and 11C . Incidentally, at each state of the second nip width N 2  and the separated state, the direction of the cam  720  is kept as a first release surface  720 A and a perfect release surface  7203  of the can  720 , which have a planar shape, respectively, are surface-contacted to the arm member  510 . 
     When switching the width of the nip portion from zero to the second nip width N 2 , the cam  720  is rotated in the other direction by a predetermined amount from the third direction to the second direction by the operation of the operation part  730 , so that the arm member  510  is pushed down by a predetermined amount by the urging force of the tension coil spring  540  (refer to  FIG. 11B ). Thereby, the heating unit HU is moved from the uppermost third position to the second position, so that the width of the nip portion is switched from zero to the second nip width N 2 . 
     When switching the width of the nip portion from the second nip width N 2  to the first nip width N 1 , the cam  720  is rotated in the other direction by a predetermined amount from the second direction to the first direction by the operation of the operation part  730 , so that the pressing force being applied to the arm member  510  from the earn  720  is released (refer to  FIG. 11A ). Thereby, the heating unit HU is moved from the second position to the lowest first position, so that the width of the nip portion is switched from the second nip width N 2  to the first nip width N 1 . 
     Here, when the width of the nip portion is the first nip width N 1 , the arm member  510  is not supported by the cam  720 . Thus, when the pressing roller  140  softens due to environment conditions such as temperature and humidity, the heating unit HU is lowered below the first position, so that the first nip width N 1  may exceed a maximum value of an allowed range. Therefore, in this exemplary embodiment, the bottom surface  431  of the recess  430  is formed at a position corresponding to a maximum value of the first nip width N 1 . 
     Thereby, at the state where the pressing force being applied to the arm member  510  from the earn  720  is released, even when the pressing roller  140  softens due to environment conditions such as temperature and humidity and the heating unit HU is thus about to be lowered below the first position, the arm member  510  is brought into contact with the bottom surface  431  of the recess  430 , as shown in  FIG. 12A , so that the moving of the heating unit HU is restrained. Thereby, as shown in  FIG. 12B , it is possible to prevent the width of the nip portion from exceeding the maximum value Nmax of the first nip width N 1  and to thus secure the appropriate fixing performance. 
     Also, the bottom surface  431  of the recess  430  is provided at a position at which a gap is formed between the fixing belt  110  and the flange portion  131 E of the nip plate  130  when the moving of the heating unit HU is restrained by the contact with the arm member  510  (refer to  FIG. 12B ). Thereby, the fixing belt  110  is not contacted to the flange portion  131 B, so that the lubricant G at the corner part between the bent portion  131 A and the flange portion  131 B is suppressed from being moved to the fixing belt  110  beyond necessity. Therefore, it is possible to securely keep the lubricant G at the corner part, so that it is possible to use the lubricant G for a long time. 
     Also, the bottom surface  431  of the recess  430  is provided at a position at which the fixing belt  110  does not contact edges of the extensions  132 ,  133  when the moving of the heating unit HU is restrained by the contact with the arm member  510 . That is, the moving of the arm member  510  and further the heating member HU is stopped by the bottom surface  431  of the recess  430  so that the edges of the extensions  132 ,  133  do not enter the range of the nip portion. Thereby, it is possible to suppress the deterioration of the fixing belt  110  due to the sliding contact of the fixing belt  110  with the edges of the extensions  132 ,  133 . 
     Also, the switching member  700  is configured so that the fixing belt  110  does not contact the bent portion  131 A of the nip plate  130  when the width of the nip portion is the second nip width N 2 . That is, at a state where the arm member  510  is supported by the cam  720  so that the width of the nip portion is the second nip width N 2 , the switching member  700  is configured so that the bent portion  131 A does not enter the range of the nip portion. 
     Thereby, even when the fixing belt  110  contacts the bent portion  131 A (for example, the nip width is the maximum value Nmax of the first nip width N 1 ) at the state where the nip width is the first nip width N 1 , the fixing belt  110  does not contact the bent portion  131 A at the state where the nip width is the second nip width N 2 . Therefore, it is possible to suppress the deterioration of the fixing belt  110  when the nip width is the second nip width N 2 . 
     Modifications to Exemplary Embodiments 
     Although the exemplary embodiment of the invention has been described, it should be understood that the invention is not limited to the exemplary embodiment. The specific configuration can be appropriately changed without departing from the scope of the invention. 
     In the above-described exemplary embodiment, the holes have been exemplified as the first opening and the second opening. However, the invention is not limited thereto. For example, the first opening and the second opening may be a notched part  800  in which a part of the outer periphery of the hole, into which the shaft  710  is inserted, is opened to the outside, as shown in  FIG. 13 . 
     In the above-described exemplary embodiment, the protrusion  422  has been exemplified as the restraint part. However, the invention is not limited thereto. For example, the restraint part may be a recess part. 
     In the above-described exemplary embodiment, the torsion spring  670  has been exemplified as the elastic member. However, the invention is not limited thereto. For example, the elastic member may be a wire spring or plate spring. 
     In the above-described exemplary embodiment, the nip plate  130  is the first fixing member and the pressing roller  140  is the second fixing member. However, the invention is not limited thereto. For example, the pressing roller may be the first fixing member and the nip plate may be the second fixing member. Also, the heating roller may be the first fixing member and the pressing roller may be the second fixing member. 
     In the above-described exemplary embodiment, the urging mechanism  500  is configured by the arm member  510  and the tension coil spring  540 . However, the invention is not limited thereto. For example, the urging mechanism may be configured by the arm member and the torsion spring or may be configured only by the urging member such as tension coil spring or torsion spring. 
     In the above-described exemplary embodiment, the fixing belt  110  (e.g., cylindrical member) is made of stainless steel. However, the invention is not limited thereto. For example, the fixing belt may be formed of other metals, may be formed of resin such as polyimide resin and the like, or may be formed of a material having elasticity such as rubber. When the fixing belt is made of resin, it is possible to reduce the sliding resistance between the fixing belt  110  and the nip plate  130  made of metal and thus to further improve the sliding characteristic of the fixing belt  110 . 
     Also, the cylindrical member may have a multi-layered structure. Specifically, the fixing belt may have a structure where a resin layer and the like for reducing the sliding resistance is provided on a surface of a metal belt, or may have a structure where an elastic layer such as rubber is provided on a surface of a metal belt. 
     In the above-described exemplary embodiment, the upstream end portion of the nip plate  130  in the conveyance direction is bent inward. However, the invention is not limited thereto. For example, the downstream end portion of the nip plate  130  in the conveyance direction may be bent. 
     In the above-described exemplary embodiment, the sheet S such as normal sheet and postcard has been exemplified as the recording sheet. However, the invention is not limited thereto. For example, an OHP sheet and the like may be used. 
     In the above-described exemplary embodiment, the laser printer that forms a black-and-white image has been exemplified as the image forming apparatus having the fixing device of the invention. However, the invention is not limited thereto. For example, a printer that forms a color image may be also possible. Also, the image forming apparatus is not limited to the printer and may be a copier or complex machine having a document reading device such as flat bed scanner.