Abstract:
An image heating apparatus includes a heating unit including a rotatable image heating member; an induction heating unit for externally induction heating the image heating member; a nip forming member press-contacting an outer surface of the image heating member to form a nip therewith; a pressing member for pressing the heating unit to the nip forming member; a press releasing member for releasing a pressure of the pressing member to the heating unit; fastening means for fastening the heating unit and the induction heating unit together; and a force receiving portion, provided on the induction heating unit, for contacting the pressing member to move the heating unit away from the nip forming member when the press releasing member releases the heating unit from the pressure of the pressing member.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image heating apparatus mountable in an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer, and the like. 
         [0002]    There have been known various fixing apparatuses (fixing devices) mountable in an electrophotographic copying machine, an electrophotographic printer, and the like. One of them has been known as a fixing apparatus of the belt heating type, that is, a fixing apparatus which heats an image with the use of a fixation belt. A fixing apparatus of this type is small in thermal capacity, and also, high in heat transmission efficiency. Thus, it starts up very quickly. Therefore, it has been known as a fixing apparatus of the on-demand type. 
         [0003]    One of the fixing devices which use a fixation belt is disclosed in Japanese Laid-open Patent Application 2002-268414. This fixing device has: a heat generating member, for example, a ceramic heater (which hereafter may be referred to simply as heater) solidly attached to its supporting member; and a heat transferring member, for example, a belt formed of heat resistant resin (which hereafter may be referred to simply as fixation belt) which is moved in contact with the heater. It has also an elastic pressure roller, as a pressure applying member, which is kept pressed against the heater, with the presence of the fixation belt between itself and the heater, forming thereby a fixation nip, in which a toner image is thermally fixed, between itself and the fixation belt. As for its operation, while a sheet of recording medium, on which an unfixed toner is present, is conveyed through its fixation nip, that is, the nip between the pressure roller and fixation belt, remaining pinched between the pressure roller and fixation belt, the unfixed toner image is melted by the heat transmitted to the unfixed image from the heater through the fixation belt, and becomes fixed to the sheet as it cools down. 
         [0004]    Another fixing device which uses a fixation belt has been proposed in Japanese Laid-open Patent Application 2000-181258. This fixing device is structured so that its fixation belt is heated by electromagnetic induction. More specifically, it has a rotational heating member and a magnetic field generating means. The rotational heating member is provided with an electrically conductive layer, and is heated by electromagnetic induction. That is, it is electromagnetically heated by the electric field generated by the magnetic field generating means positioned outside the loop which the rotational heating member forms. The rotational heating member is a thin belt, being therefore small in thermal capacity. Therefore, this fixing device is advantageous in terms of thermal response. 
         [0005]    Generally, a fixing apparatus of the so-called belt heating type is provided with a fixation belt (rotational heating member), a pressure roller (pressure applying rotational member), and a pressure applying auxiliary member. The pressure applying auxiliary member is placed within the loop which the fixation belt forms, and is pressed against the pressure roller, with the presence of the fixation belt between itself and the pressure roller, forming thereby a fixation nip between the pressure roller and fixation roller. 
         [0006]    Even a fixing device of the so-called belt heating type, such as the one described above, sometimes suffers from the problem that it is jammed by a sheet or sheets of recording medium. If an attempt is made to remove the jammed sheet or sheets of recording medium from a fixing apparatus of this type while its fixation belt is still kept pressed upon the pressure roller, the sheet removing operation becomes very troublesome and irritating. Further, an attempt to forcefully pull out the jammed sheet is likely to scar and/or seriously damage the fixation belt and/or pressure roller. Thus, if a fixing apparatus of this type becomes jammed, it is necessary to remove the pressure between the fixation belt and pressure roller (hereafter, this operation for removing pressure between fixation belt and pressure roller may be referred to simply as “pressure removal”). 
         [0007]    Referring to  FIG. 16 , an example of the conventional method for keeping a preset amount of pressure between the fixation belt and pressure roller of a fixing device of the belt heating type is disclosed in Japanese Laid-open Patent Application 2002-268414. In the case of this method, the fixing device is provided with a pair of pressure application springs  617  and a pair of pressure application levers  619 , and the preset amount of pressure is kept between the fixation belt and pressure roller by applying the force generated by the resiliency of the pressure application springs is applied by way of the pressure application levers  619 . More specifically, the fixing device is provided with the pair of pressure application plates  619  (pressure application levers), a pair of pressure transmitting members  605 , a pair of pressure roller supporting members  631 , and a pair of spring retaining members  618 . The pressure application plate  619  is placed on the top surface of the pressure catching portion  605   a  of the pressure transmitting member  605 . One end of the pressure application plate  619  is fitted in the groove  631 B of the pressure roller supporting member  631  so that the pressure application plate  619  can be rotationally moved about the point of contact between the pressure application plate  619  and the pressure roller supporting member  631 , whereas the other end of the pressure application plate  619  is placed in contact with the bottom end of the spring retaining member  618 . 
         [0008]    That is, the pressure application plate  619  is made to function as a pressure applying lever. More specifically, the point of engagement between the pressure application plate  619  and pressure plate engaging groove  631 B is made to function as the fulcrum  619   a  of the pressure application plate  631  (pressure applying lever), and the point  619   b  of the pressure application plate  619 , which is in contact with the bottom end of the spring retaining member  618 , is made to function as the point of pressure input. Further, the point  619   c  of contact between the pressure application plate  619  and the pressure catching portion  605   a  of the pressure transmitting member  615  is made to function as the point of pressure application. That is, the pressure application plate  619  is pressed by the compression spring  617 , whereby the pressure catching portion  605  of the pressure transmitting member  605  is pressed. Thus, the heating member  602  is pressed on the pressure roller  606  by a combination of a metallic stay  604  and a heating member holding member  603 , which is between the pressure transmitting member  605  and heating member  602 . Incidentally, designated by referential codes  601 ,  603 , and  632  in  FIG. 16  are a heat resistant belt, a heating member holding member, and a pressure roller bearing, respectively. 
         [0009]    When it is necessary to remove pressure from the fixation nip of a fixing apparatus (structured like the one described above) in order to deal with a paper jam, the pressure application lever is to be operated in the direction for preventing the force from the pressure application spring from being applied to the pressure roller. Even with the removal of the pressure (from the spring) from the fixation nip, the fixation nip remains under the pressure resulting from weight of the fixation belt unit itself. However, the weight of the fixation belt unit has virtually no effect upon the operation for removing the jammed paper. 
         [0010]    The heat generating member of a fixing apparatus which uses a conventional fixation belt is within the loop which the fixation belt forms. In comparison, a fixing apparatus which uses a fixation belt in which heat is electromagnetically induced has a magnetic field generating means as a heat generating means. The magnetic field generating means is outside the fixation belt loop (on opposite side of fixation belt from pressure roller). It has a coil for generating a magnetic field, and a coil supporting frame. The weight of the magnetic field generating means cannot be ignored in terms of its effect upon the fixation nip. That is, in the case of a fixing apparatus which employs a fixation belt in which heat is generated by electromagnetic induction, removing the pressure from the pressure application coil alone leaves the fixation nip portion of the fixing apparatus under the weigh of the magnetic field generating means (electromagnetic induction heating unit). Thus, even after the removal of the pressure from the pressure application spring, the job for removing a jammed sheet of recording medium is still irritating and time consuming. 
       SUMMARY OF THE INVENTION 
       [0011]    Thus, the primary object of the present invention is to provide an image heating apparatus, which has a mechanism for preventing the weight of the member for heating the rotational heating member by electromagnetic induction, and the weight of the rotational heating member itself, from resting on the fixation nip of the apparatus, after the removal of the pressure from the fixation pressure application spring from the fixation nip, being therefore far superior to an image heating apparatus in accordance with the prior art, which employs a rotational fixing member heatable by electromagnetic induction, in terms of the easiness and efficiency with which a jammed sheet can be removed. 
         [0012]    According to an aspect of the present invention, there is provided an image heating apparatus comprising a heating unit including a rotatable image heating member; an induction heating unit for externally induction heating said image heating member; a nip forming member press-contacting an outer surface of said image heating member to form a nip therewith; a pressing member for pressing said heating unit to said nip forming member; a press releasing member for releasing a pressure of said pressing member to said heating unit; fastening means for fastening said heating unit and said induction heating unit together; and a force receiving portion, provided on said induction heating unit, for contacting said pressing member to move said heating unit away from said nip forming member when said press releasing member releases said heating unit from the pressure of said pressing member. 
         [0013]    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 
         [0014]      FIG. 1  is a perspective view of a fixing apparatus in accordance with the present invention. 
           [0015]      FIG. 2  is a schematic sectional view of the fixing apparatus in accordance with the present invention, shown in  FIG. 1 , at a plane parallel to the direction in which recording medium is conveyed. 
           [0016]      FIG. 3  is a schematic sectional view of an image forming apparatus in which a fixing apparatus in accordance with the present invention is mountable, at a plane parallel to the recording medium conveyance direction. 
           [0017]      FIG. 4  is a schematic sectional view of a combination of the induction heating unit and fixation belt unit of the fixing apparatus in accordance with the present invention, at a plane parallel to the recording medium conveyance direction, and shows how the induction heating unit and fixation belt unit are fastened to each other. 
           [0018]    In  FIGS. 5 , ( a ) and ( b ) are schematic sectional views of the fixing apparatus in accordance with the present invention, at a plane parallel to the recording medium conveyance direction, and show the states of fixing apparatus when the fixation nip of the apparatus is under and free from, respectively, the pressure from the fixation pressure applying means. 
           [0019]      FIG. 6  is a schematic sectional view of a part of the fixation belt in accordance with the present invention, at a plane which is vertical to the surface of the fixation belt and parallel to the recording medium conveyance direction, and shows the structure of the fixation belt. 
           [0020]      FIG. 7  is a schematic sectional view, at a plane parallel to the recording medium conveyance direction, of the fixing apparatus in the second preferred embodiment of the present invention, which is provided with a toggle for fastening or unfastening the induction heating unit and fixation belt unit relative to each other. 
           [0021]      FIG. 8  is an exploded perspective view of one of the lengthwise ends of the induction heating unit, and shows how the toggle, shown in  FIG. 7 , is attached to the toggle holder of the induction heating unit. 
           [0022]      FIG. 9  is a perspective view of one of the lengthwise ends of the fixing device after the attachment of the toggle. 
           [0023]    In  FIGS. 10 , ( a ) and ( b ) are schematic sectional views of the fixing apparatus in accordance with the present invention, which has a toggle, at a plane parallel to the recording medium conveyance direction, when the pressure from the pressure applying means is on, and off, respectively. 
           [0024]      FIG. 11  relates to the rotational movement of the toggle,  FIGS. 11(   a ) and  11 ( b ) showing the state of the fixing apparatus in accordance with the present invention, when the fixation belt unit is fastened to the induction heating unit, and is being fastened to the induction heating unit, respectively, and  FIG. 11(   c ) showing how the fixation belt unit is to be attached to the induction heating unit. 
           [0025]      FIG. 12  is a schematic sectional view, at a plane parallel to the recording medium conveyance direction, of the fixing apparatus in the third preferred embodiment of the present invention, which is provided with a separation guide. 
           [0026]      FIG. 13  is a drawing for showing how the separation guide is to be attached to the flange of the fixing device in the third embodiment. 
           [0027]      FIG. 14  is a perspective view of the fixing device in the third embodiment, which has the separation guide. 
           [0028]      FIG. 15  is a drawing for showing how the fixation belt unit in the fixing device in the third embodiment is to be replaced,  FIGS. 15(   a ),  15 ( b ), and  15 ( c ) showing the state of the fixing device when the fixing device is in operation, after its induction heating unit has been rotationally moved into the fixation belt unit replacement position, and when the fixation belt unit is being replaced. 
           [0029]      FIG. 16  is a perspective view of one of the lengthwise ends of a typical conventional fixing device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Hereinafter, the preferred embodiments of the present invention are described with reference to the appended drawings. If a component, a portion thereof, etc., of the image heating apparatus in one of the preferred embodiments of the present invention, are the same in function to the counterparts of the image forming apparatus in another preferred embodiment, they are given the same referential codes, one for one. 
       Embodiment 1  
     (Image Forming Apparatus) 
       [0031]      FIG. 3  is a schematic sectional view of the image forming apparatus in the first preferred embodiment of the present invention, which employs an image heating apparatus in accordance with the present invention, as its fixing device, at a plane parallel to the recording medium conveyance direction of the apparatus. More concretely, the image forming apparatus in this embodiment is a color printer, which uses an electrophotographic process. It has multiple (four) optical scanning means and multiple (four) photosensitive drums. The four drums are aligned in tandem and are in parallel to each other. It has also an image reader  200  which obtains from a full-color original image, the information necessary to form a copy of the original. That is, it separates an original full-color image into four monochromatic images which are different in color, with the use of its photo-electric conversion element, such as a CCD. The image reader is on top of the main assembly of the image forming apparatus. Designated by a referential code  300  is an apparatus for automatically feeding an original onto the original holding glass platen of the original reader  200 , or a plate for pressing an original against the original holding glass platen. Designated by a referential code  400  is a scanner having multiple optical scanning means which project beams La, Lb, Lc, and Ld of laser light while modulating the beams with the image information, which is in the form of electrical signals, and obtained through the original reader  200 . 
         [0032]    Designated by referential codes Pa, Pb, Pc, and Pd are four image forming stations which form magenta, cyan, yellow, and black monochromatic images, respectively. Each image forming station P has a photosensitive drum (which hereafter may be referred to simply as drum), which is rotatable in the clockwise direction. Each image forming station P has also a charging device, a developing device, and a cleaner, which are in the adjacencies of the peripheral surface of the photosensitive drum and are in the listed order. The image forming station P has also a transferring device  105 , which is under the four drums. The transferring device  105  has a transfer belt  106  and a transfer charger. The transfer belt  106  is a recording medium conveying means shared by the four image forming stations P. It is an endless belt, and is suspended and kept stretched by three rollers. 
         [0033]    The peripheral surface of the drum of each image forming station P is uniformly charged, and then, is scanned (exposed) by a beam La, Lb, Lc, or Ld of laser light projected from the laser scanner  400  while being modulated with the information (in the form of electrical signal) of the original, obtained by the reader  200 . As a result, a latent image, which reflects the information of the original (image to be formed) is effected on the uniformly charged peripheral surface of the drum. This latent image is developed by the developing device. Thus, magenta, cyan, yellow, and black toner images are formed in the four image forming stations Pa, Pb, Pc, and Pd, respectively. 
         [0034]    Referring to  FIG. 3 , designated by referential codes  111  and  112  are first and second sheet feeder cassettes, which are inside the main assembly  100  of the printer. One of the sheets P of recording medium (transfer paper, OHP sheet, etc.) in the sheet feeder cassettes  111  or  112  is moved out of the cassette, while being separated from the rest in the cassette. Then, the sheet P is supported by the transfer belt  106 , and is conveyed by the belt  106  so that it is moved sequentially through the image forming stations Pa, Pb, Pc, and Pd. As the sheet P is conveyed through the image forming stations Pa, Pb, Pc, and Pd, magenta, cyan, yellow, and black images formed on the four drums, one for one, are sequentially transferred in layers onto the transfer belt  106 . 
         [0035]    After the completion of the transfer of the four monochromatic toner images, different in color, onto the sheet P on the transfer belt  106 , the sheet P is separated from the transfer belt  106 , and is conveyed to the fixing device Q of the printer. The toner image on the sheet P of recording medium is fixed to the sheet P by the heat and pressure in the fixing device Q. Then, the sheet P is conveyed, as a full-color print, to a print processing apparatus  500 , which discharges the sheet P onto its delivery tray  502  with the use of its pair of sheet conveyance rollers. The print processing apparatus  500  is structured to allow its delivery tray  502  to downwardly move so that a substantial number of prints can be discharged in layers onto the delivery tray  502 . Further, the apparatus  500  is enabled to staple a bundle of prints. 
         [0036]    When the image forming apparatus is in the monochromatic mode, only the image forming station Pd, that is, an image forming station for forming a black image, is activated. When the image forming apparatus is in the two-sided mode, after the formation of an image on one side of a sheet P of recording medium, the sheet P is directed to the mechanism  113  for turning the sheet P over and sending the sheet P back into the main assembly of the image forming apparatus, in order to form an image on the other side (second surface) of the sheet P. Then, the sheet P is turned over by the sheet turning-and-conveying mechanism  113 , and then, is delivered to the transfer belt  106  for the second time. As a result, a toner image is transferred onto the other surface of the sheet P. Then, the sheet P is reintroduced into the fixing device Q. Then, a two-sided print is conveyed to the print processing apparatus  500 . 
       (Image Heating Apparatus) 
       [0037]      FIG. 2  shows the fixing device Q in this embodiment. The fixing device Q employs a fixation belt which is heated by electromagnetic induction. That is, the image heating member of this fixing device is heated by electromagnetic induction. More specifically, as a magnetic field is generated so that this image heating member, in which heat can be generated by electromagnetic induction, falls within the magnetic field, eddy current is generated in the image heating member. This eddy current generates heat in the image heating member (Joule Effect). The fixing device Q applies this heat generated by the eddy current, to a sheet P of recording medium, that is, an object to be heated. Consequently, the unfixed toner image on the sheet P is thermally fixed to the surface of the sheet P. 
         [0038]    Regarding the direction of the fixing device Q, the lengthwise direction is the direction perpendicular to the recording medium conveyance direction, on the surface of the recording medium. The widthwise direction is the direction parallel to the recording medium conveyance direction, on the surface of the recording medium. Further, the length of the fixing device Q is the measurement of the device Q in terms of its lengthwise direction, and the width of the fixing device Q is the measurement of the device Q in terms of the widthwise direction. 
         [0039]    The fixation belt unit  20  in this embodiment has a fixation belt  21 , which is cylindrical (endless) and flexible. The fixation belt  21  has an electrically conductive layer  21   b  ( FIG. 6 ). The fixation belt unit  20  has also a pair of flanges  22  ( FIG. 1 ) as a fixation belt holding means. The fixing device Q has also an induction heating unit  23  (magnetic field generating means ( FIGS. 1 and 2 ). The flanges  22  are for holding the fixation belt unit  20 . The induction heating unit  23  is a means for heating the image heating member by electromagnetic induction, from the outward side of the loop which the image heating member (fixation belt  21 ) forms. 
         [0040]    Referring to  FIG. 2 , the fixing device Q has also a pressure applying member  24   a,  a stay  24   b,  and an elastic pressure roller  25 . The stay  24   b  is a pressure applying auxiliary member. The elastic pressure roller  25  (which hereafter will be referred to simply as pressure roller), is a pressure applying rotatable member. That is, the fixing device Q is structured so that the fixation belt  21  is heated by electromagnetic induction with the use of the induction heating unit  23 , from the outward side of the fixation belt loop. Incidentally, the pressure roller  25  may be replaced with a circularly movable endless belt which is suspended by multiple rollers, or a stationary pressure pad. 
       (1-1) Induction Heating Unit 
       [0041]    Referring to  FIG. 2 , the fixing device Q is structured so that the induction heating unit  23  is on the outward side of the fixation belt loop, and also, a preset amount of gap is maintained between the induction heating unit  23  and the outward surface of the fixation belt  21 , that is, the flexible image heating member which is heated by electromagnetic induction with the use of the induction heating unit  23 . The induction heating unit  23  has an excitation coil  23   a  (which hereafter may be referred to simply as coil), a magnetic core (which hereafter may be referred to simply as core), and a holder  23   c  for holding the coil  23   a  and core  23   b.    
         [0042]    The holder  23   c  is roughly in the form of a long rectangle, the lengthwise direction of which is parallel to the lengthwise direction of the fixation belt  21 . It is held by the pair of flanges  22  at its lengthwise ends. The bottom surface of the holder  23   c , that is, the surface which faces the outward surface of the fixation belt  21 , is concaved in the form of a semi-cylindrical dome, the curvature of which matches that of the cross section of the fixation belt  21 , providing the aforementioned preset amount of gap between the holder  23   c  and the outward surface of the fixation belt  21 . 
         [0043]    The coil  23   a  is in the form of an oval dome, the lengthwise direction of which is parallel to the lengthwise direction of the fixation belt  21 . It is in the hollow of the holder  23   c  in such a manner that there is a preset amount of gap between the outward surface of the fixation belt  21  and the inward surface of the holder  23   c.  The wire of which the coil  23   a  is formed is Litz wire composed of roughly 80-160 fine insulated strands which are 0.1-0.3 mm in diameter and woven together. The coil  23   a  is wound 8-12 times in a manner to follow the contour of the inward surface of the core  23   b.  It is in connection to the excitation circuit (unshown) so that it can be supplied with AC current from the excitation circuit. 
         [0044]    The core  23   b  is made of a highly magnetically permeable substance, and has a portion which fits in the center of the coil  23   a,  and a portion which surrounds the coil  23   a.  It plays a role of efficiently guiding the alternating magnetic flux generated by the coil  23   a,  to the electrically conductive layer  21   b  of the fixation belt  21 . That is, it is used to contain the magnetic flux to increase in efficiency the magnetic circuit formed by the coil  23   a  and electrically conductive layer  21   b.    
         [0045]    As for the material for the core  23   b,  a substance such as ferrite which is high in permeability and low in residual magnetic flux density is preferable. Since the alternating magnetic flux generated by the coil  23   b  has to be efficiently given to the electrically conductive layer  21   b  of the fixation belt  21 , a core  23   b  formed of a highly magnetic substance is positioned on the inward side of the loop which the fixation belt  21 , that is, the opposite side of the fixation belt  21  from the core  23   b.  That is, the core  23   b  is between the stay  24   b  and the inward surface of the fixation belt  21 . 
       (1-2) Pressure Roller (Pressure Applying Rotational Member) 
       [0046]    The pressure roller  25 , which is a heat resistant rotational member for pressure application, is kept pressed upon the outward surface of the fixation belt  21  (image heating member), forming thereby a nip between itself and the fixation belt  21 . It has: a metallic core  25   a  which is in the form of a piece of round rod; and an elastic layer  25   b  which covers virtually the entirety of the peripheral surface of the metallic core  25   a  ( FIG. 2 ). The material of the elastic layer  25   b  is heat resistant rubber such as silicone rubber and fluorinated rubber, or foamed silicone rubber. The pressure roller  25  is on the opposite side of the fixation belt  21  from the induction heating unit  23 , and is positioned so that its rotational axis is parallel to the widthwise direction of the fixation belt  21 . It is rotatably supported at its lengthwise ends, by the bottom plates  19  of the fixing device frame, with the presence of a pair of bearings between its lengthwise ends and bottom plates  19 , one for one. 
       (1-3) Fixation Belt Unit 
       [0047]    The fixation belt unit  20 , which is a heating unit, has: the fixation belt  21  which is a circularly movable image heating member; a pressure applying auxiliary member  24 ; the magnetic core  23   b;  and flanges  22 . 
       (1-3-1) Fixation Belt 
       [0048]    The fixation belt  21  is a heat resistant, flexible, and cylindrical (endless) member. It is a multilayer belt, comprising an inward layer  21   a,  an electrically conductive layer  21   b,  an elastic layer  21   c,  and a parting layer  21   d,  listing from the inward side of the belt loop ( FIG. 6 ). 
         [0049]    The electrically conductive layer  21   b  is the layer in which heat is generated by electromagnetic induction, that is, by the magnetic field (magnetic flux) generated by the induction heating unit  23 . It is a cylindrical and flexible metallic layer (which hereafter may be referred to simply as metallic layer) formed of metallic substance such as iron, cobalt, nickel, copper, and chrome. It is roughly 1-50 μm in thickness. The elastic layer  21   c  is on the outward surface of the electrically conductive layer  21   b,  and is formed of a substance pre-selected as a suitable material for the elastic layer of the fixation belt  21 . 
         [0050]    The parting layer  21   d,  or the surface layer, directly contacts an unfixed toner image t on the sheet P of recording medium. Thus, the material for the parting layer  21   d  needs to be a substance which is excellent in parting. The substances which may be listed as the material for the parting layer  21   d  are tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), polytetrafluoroethylene (PTFE), silicon copolymer, or combination of these substances, for example. The parting layer  21   d  is formed of one of these substances, selected for a specific purpose, on the outward surface of the elastic layer  21   c.  It is roughly 1-50 μm in thickness. 
         [0051]    The parting layer  21   d  rubs against recording medium. Thus, from the standpoint of its durability related to frictional wear, it is desired not to be too thin. That is, if the parting layer  21   d  is too thin, it reduces the service life of the fixation belt  21 . On the other hand, if the parting layer  21   d  is too thick, it adds to the thermal capacity of the fixation belt  21 , increasing the fixation belt  21  in the length of warm-up time. Thus, the parting layer  21   d  is desired not to be too thick. 
         [0052]    In this embodiment, therefore, the parting layer  21   d  (surface layer) of the fixation belt  21  is formed of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), and its thickness is 30 μm. 
       (1-3-2) Pressure Applying Auxiliary Member 
       [0053]    The pressure applying auxiliary member  24  is heat resistant, and is positioned within the loop which the fixation belt  21  forms ( FIG. 2 ). It has a flat plate  24   a  and a stay  24   b.  The flat plate  24   a  is on the inward side of the belt loop, that is, on the opposite side of the fixation belt  21  from the induction heating unit  23 , and is in contact with the inward surface of the fixation belt  21 . The stay  24   b  is U-shaped in cross section, and is on the flat plate  24   b , with its open side facing the flat plate  24   b.  The flat plate  24   a  is parallel to the recording medium conveyance direction. The positional relationship between the stay  24   b  and plate  24   a  is such that a vertical plane which coincides with the widthwise centerline of the flat plate  24   a  coincides with the vertical plane which coincides with the widthwise centerline of the stay  24   b.    
       (1-3-3) Flange 
       [0054]    The fixation belt unit  20  is provided with a pair of flanges  22  ( FIG. 1 ), which are at the edges of the fixation belt  21 , one for one. The flanges  22  are held by the top plates  18  of the fixation device frame. Each flange  22  has a “flange portion”, which faces the corresponding edge of the fixation belt  21 . It is provided with a groove (unshown), in which one of the lengthwise end portions of the pressure applying auxiliary member  24  is fitted to support the member  24  with the flange  22 . 
         [0055]    Further, the flange  22  has a belt supporting portion (unshown) which is perpendicular to the fixation belt  21 . It has also a pressure bearing portion  22   b  which projects in the opposite direction from the pressure roller  25 . The belt supporting portion is loosely fitted in the belt loop at the edge of the fixation belt so that the fixation belt  21  is allowed to rotate about the belt supporting portion of the flange  22 . That is, the pair of flanges  22  support the fixation belt  21  from within the belt loop, at the edges of the fixation belt  21 , one for one, and guide the cylindrical (endless) fixation belt  21 . As for the “flange portion” of the flange  22 , as the fixation belt  21  shifts in its widthwise direction, it comes into contact with the “flange portion”, being thereby prevented from being shifting further. In other words, the “flange portion” regulates the lateral movement of the fixation belt  21 . 
         [0056]    The pressure bearing portion  22   b  of the flange  22  is under the pressure from the pressure application lever  33  ( FIG. 5 ), which is described later. The pressure applied to the pressure bearing portion  22   b  by the pressure application lever  33  presses the flat plate  24   a  through the stay  24   b.  Thus, the flat plate  24   a  presses the outward surface of the fixation belt  21  on the peripheral surface of the pressure roller  25 . Thus, not only is the fixation belt  21  made to deform in a manner to conform to the surface of the flat plate  24   a,  but also, the elastic layer  25   b  of the pressure roller  25  is made to elastically deform in a manner to conform to the surface of the flat plate  24   a . Therefore, a nip N (fixation nip) with a preset width is formed between the outward surface of the fixation belt  21  and the peripheral surface of the pressure roller  25 . 
       (2) Means for Fastening Induction Heating Unit to Fixation Belt Unit 
       [0057]    Next, referring to  FIG. 4 , the means for fastening the induction heating unit  23  to fixation belt unit  20  is described. In order to keep the fixation belt  21  stable in heat generation efficiency, a preset amount of gap has to be kept between the induction heating unit  23  and fixation belt unit  20 . 
         [0058]    In this embodiment, therefore, the induction heating unit  23  is provided with a portion  23   e  for holding a pressure applying member. In terms of the recording medium conveyance direction, the pressure applying member holding portion  23   e  is on the downstream side of the induction heating unit  23  (outlet side of fixing device). In terms of the lengthwise direction of the fixing device Q, the pressure applying member holding portion  23   e  is at each of the lengthwise ends of the induction heating unit  23 . It is to this pressure generating member holding portion  23   e  (which hereafter will be referred to simply as spring holding portion  23   e ) that one end of a wire spring  36 , as a pressure generating member, is attached. The wire spring  36  is extended downward from the spring holding portion  23   e,  bent upstream at the bottom of the flange  22 , extended further along the bottom of the flange  22 , and bent diagonally upward, away from the flange  22 , at the upstream end of the bottom of the flange  22 . The other end of the wire spring  36  is hooked around a spring holding portion  23   f  of the induction heating unit  23 , which is at each of the lengthwise ends of the induction heating unit  23 . In terms of the recording medium conveyance direction, the spring holding portion  23   f  is on the upstream side of the induction heating unit  23 . 
         [0059]    That is, the means for keeping the units  20  and  23  fastened relative to each other is the wire spring  36 , which is attached to the induction heating unit  23  so that one end of the wiring spring  36  is anchored to the downstream end (first position) of the induction heating unit  23 ; the other end is anchored to the upstream end of the induction heating unit  23 ; and the center portion of the wire spring  36  wraps around the flange  22 . It is by this wire spring  36  that the preset amount of gap is kept between the induction heating unit  23  and fixation belt unit  20  to make it unlikely for the fixation nip N from fluctuating in temperature. 
         [0000]    (3) Operation of Induction Heating Unit and Fixation Belt Unit during Fixing Operation, and Removal of Jammed Recording Medium 
       (Thermal Fixing Operation of Fixing Device) 
       [0060]    Next, referring to  FIG. 2 , the thermal fixing operation of the fixing device Q is described. The fixing device Q in this embodiment has: a motor (unshown) as a mechanical power source; and a driver gear (unshown) which is attached to one of the lengthwise ends of the pressure roller  25 . The fixing device Q rotates the driver gear in a preset direction by rotating its motor, in response to a print start signal. Thus, the pressure roller  25  rotates in the direction indicated by an arrow mark at a preset peripheral velocity. The rotation of the pressure roller  25  is transmitted to the surface of the fixation belt  21  by the friction between the peripheral surface of the pressure roller  25  and the surface of the fixation belt  21 , in the nip N. 
         [0061]    Thus, the fixation belt  21  is rotated by the rotation of the pressure roller  25 , with its inward surface sliding on the flat plate  24   a  of the pressure applying auxiliary member  24 . The surface of the flat plate  24   a,  which is facing the fixation belt  21 , and the inward surface of the fixation belt  21 , are coated with lubricant such as grease to reduce the friction between the two surfaces. 
         [0062]    The excitation circuit begins to supply the coil  23   a  of the induction heating unit  23  with AC current, in response to the print start signal. Thus, the coil  23   a  begins to generate alternating magnetic flux, which is guided to the fixation belt  21  by the core  23   b,  inducing eddy current in the fixation belt  21 . This eddy current generates heat in the fixation belt  21  by an amount related to the specific resistance of the electrically conductive layer  21   b  of the fixation belt  21  (Joule Effect). 
         [0063]    That is, as the coil  23   a  is supplied with AC current, heat is generated in the fixation belt  21  by electromagnetic induction. The temperature of the fixation belt  21  is detected by a temperature detecting means (unshown) such as a thermistor, and the output signals (electrical signals which represent fixation belt temperature) of the thermistor are picked up by an electric power control circuit (unshown), which turns on or off (controls) excitation circuit so that the temperature of the fixation belt  21  remains at a preset level (target temperature). 
         [0064]    As soon as the pressure roller  25  and fixation belt  21  begin to rotate at a preset speed, and the temperature of the fixation belt  21  begins to be maintained at a preset level (fixation temperature), a sheet P of recording medium on which an unfixed toner image t is present, is introduced into the nip N, and conveyed through the nip N, remaining pinched between the outward surface of the fixation belt  21  and the peripheral surface of the pressure roller  25 . While the sheet P is conveyed through the nip N as described above, the unfixed toner image t on the sheet P is subjected to the heat from the fixation belt  21  and the internal pressure of the nip N. Thus, the unfixed toner image t becomes fixed to the surface of the sheet P. As the sheet P is conveyed out of the nip N, it is separated from the surface of the fixation belt  21 . 
         [0000]    (Pressure Application for Fixation, and Operation for Removing Fixation Pressure to Deal with Paper Jam) 
         [0065]      FIG. 1  is a perspective view of the fixing device in this embodiment. The fixing device in this embodiment has a mechanism for moving the fixation belt unit  20  relative to the pressure roller  25  (pressing fixation belt unit against pressure roller  25 , or separating fixing belt unit from pressure roller  25 ). The pressuring applying/removing mechanism has: a cam shaft  31 , which is a rotational member; a pair of pressure application levers  33 ; and a pair of vises  34  with a spring, as a pressure generating means. The cam shaft  31  is rotatably supported by a pair of upper lateral plates  18  which are at the lengthwise ends of the fixing device frame. The cam shaft  31  has a pair of eccentric cams  32 , which are at the lengthwise ends of the cam shaft  31 . The eccentric cams  32  are members for freeing the induction heating unit from the pressure applied to the heat unit by the pressing generating member. The cam shaft  31  has also an unshown pressure removal gear, which is attached to one of the lengthwise end of the cam shaft  31 . 
         [0066]    The motor of the fixing device is rotated in response to a signal for initiating an operation for dealing with paper jam. As the motor is rotated, the unshown pressure removal gear is rotated in a preset direction by a preset amount by an unshown driving force transmitting gear. As the pressure removal gear is rotated, the camshaft  31  rotates, and therefore, the eccentric cams  32  rotate. 
         [0067]    The pressure application levers  33  are rotatably supported at one of its lengthwise ends, by a shaft  17 , which is supported by the aforementioned top lateral plates  18 . The other end of the pressure application lever  33  is kept pressured toward the flange  22  by the compression springs  34   a  with which the vises  34  are fitted. That is, the pressure application lever  33  is rotatable about the shaft  17  (fulcrum D) in the direction to directly press the pressure catching portion  22   b  of the flange  22 , or in the direction to move away from the pressure catching portion  22   b  of the flange  22 . 
         [0068]      FIG. 5  is a drawing for describing the state of the fixing device when the flange  22  (induction heating unit) is under the pressure from the compression spring  34   a,  and that when the flange  22  is not. As the camshaft  31  is rotated, the eccentric cam  32  rotates the pressure application lever  33  about the fulcrum D against the pressure from the compression spring  34   a  of the vise  34 , in the direction to separate from the flange  22 . That is, a force F 2  is applied to the pressure application lever  33  by the eccentric cam  32 . As the amount by which the pressure application lever  33  is being rotated reaches at preset value, the pressure application lever  33  (pressure applying member) comes into contact with a projection  23   d  (pressure catching portion) of the induction heating unit  23 . 
         [0069]    As the pressure application lever  33  comes into contact with the projection  23   d  of the induction heating unit  23 , the pressure application lever  33  catches the combination of the weight of the fixation belt unit  20  and the weight of the induction heating unit  23 , reducing thereby the nip pressure. That is, a force F 2  is applied to the induction heating unit  23  by the pressure application lever  33 . Thus, the weight of the induction heating unit  23  is supported by the pressure application lever  33 . Further, the preset amount of gap is maintained between the fixation belt unit  20  and induction heating unit  23  by the wire spring  36  (resilient member). Therefore, the weight of the fixation belt unit  20  also is supported by the pressure application lever  33 . Thus, the fixation belt unit  20  and pressure roller  25  are kept separated by a gap of ΔY. That is, the combination of the weight of the induction heating unit  23  and the weight of the fixation belt unit  20  is kept away from the fixation nip N. 
         [0070]    That is, as the pressure applied to the induction heating unit  23  by the combination of the pressure application lever  33  and the pressure generating member is removed by the pressure removing member, the pressure applying member (pressure application lever  33 ) comes into contact with the pressure catching portion  23   d  of the induction heating unit  23 , causing thereby the pressure catching portion  23   d  to catch the force (pressure) for removing the pressure applied to the nip forming member by the combination of the weight of the induction heating unit  23  and that of the fixation belt unit  20  of the induction heating unit  23 . 
       Embodiment 2 
       [0071]      FIGS. 7-11  are for describing the fixing device in the second preferred embodiment. The fixing device in this embodiment employs a toggle as a means for keeping the induction heating unit  23  and fixation belt unit  20  fastened to each other. 
       (Structure of Toggle) 
       [0072]    A toggle is a mechanism made up of a pair of jointed arms (links) and a slider. The force to be inputted into the mechanism is transmitted through a linkage. As the force is inputted, the pair of jointed arms moved toward the direction from which the force is inputted, until the jointed arms lock themselves with the objects, such as a wall, around them. Once the jointed arms lock themselves with the surrounding objects, they do not move backward, anchoring the mechanism against the external force which is opposite in direction from the input force. 
         [0073]    The toggle in this embodiment has a lever  37  and a spring  38 . The lever  37  corresponds to the pressure application lever  33  in the first embodiment. The toggle is at each of the lengthwise ends of the induction heating unit  23 . Each lever  37  holds the corresponding flange  22  which is at the lengthwise end of the fixation belt unit  20 . The spring  38  keeps a preset distance between the fixation belt unit  20  and induction heating unit  23 . 
         [0074]    In this embodiment, the means for keeping the induction heating unit  23  and fixation belt unit  20  fastened to each other includes a pressure generating means. The means is enabled to take two positions, that is, the first and second positions. When the fastening means is in the first position, it keeps the fixation belt unit  20  and induction heating unit  23  fastened to each other, whereas when it is in the second position, it keeps the fixation belt unit  20  separated from the induction heating unit  23 . 
         [0075]    Next, the method for attaching the toggle to the induction heating unit  23  is described. Referring to  FIG. 8  which is an exploded perspective view of one of the lengthwise ends of the induction heating unit  23 , the shafts  37   a  of the lever  37  fit in the hole  23   g  of the holder  23   c,  enabling the lever  37  to rotationally move about the axial lines of the shaft  37   a.  One end of a spring  38  is attached to the spring holding portion  37   b  of the lever  37 , which is between the two arms of the lever  37 . 
         [0076]    Next, the other end of the spring  38  is attached to the spring holding portion of the holder  23   c  of the induction heating unit  23 . The lever  37  is positioned so that its bottom end portion supports the flange  22  by the bottom side of the flange  22  while remaining pressed upward by the resiliency of the spring  38 . Thus, when the fixation belt unit  20  is in the normal condition, the lever  37 , the surface  22   a  ( FIG. 7 ) of the fixation belt unit  20  is kept in contact with the surface  23   g  ( FIG. 7 ) of the induction heating unit  23  by the lever  37 , maintaining thereby a preset distance between the fixation belt unit  20  and induction heating unit  23 .  FIG. 9  is a perspective view of one of the lengthwise ends of the fixing device in this embodiment after the attachment of the toggle. 
       (Operation for Applying, or Removing, Pressure) 
       [0077]      FIG. 10(   a ) is a schematic plan view of one of the lengthwise ends of the fixing device in this embodiment when the fixation belt unit  20  is kept pressed against the pressure roller  25 , and  FIG. 10(   b ) is a schematic plan view of the same lengthwise end of the fixing device in this embodiment as the one shown in  FIG. 10(   a ), after the separation of the fixation belt unit  20  from the pressure roller  25 . When the fixation belt unit  20  is kept pressed upon the pressure roller  25 , the fixation belt unit  20  is kept fastened to the induction heating unit  23  by the toggle as the fixation belt unit  20  is by the lever  33  in the first embodiment. Further, since the preset amount of gap is maintained between the fixation belt unit  20  and induction heating unit  23  by the toggle. 
         [0078]    Therefore, the weight of the fixation belt unit  20  is supported by the pressure application lever  33 . Thus, the fixation belt unit  20  is kept separated from the pressure roller  25  by a distance of ΔY, and the combination of the weight of the induction heating unit  23  and the weight of the fixation belt unit  20  is prevented from resting on the peripheral surface of the pressure roller  25 . In other words, the fixing device in this embodiment is superior to any fixing device in accordance with the prior art, in terms of the efficiency with which the paper jam or the like problem can be dealt. 
       (Method for Attaching Toggle) 
       [0079]      FIG. 11  shows how the fixation belt unit  20  is fastened to induction heating unit  23  by the toggle.  FIGS. 11(   a ),  11 ( b ) and  11 ( c ) shows the fixing device after, during, and before the fastening of the fixation belt unit  20  to induction heating unit  23  by the toggle, respectively. 
         [0080]    First, the procedure for unfastening the fixation belt unit  20  from the induction heating unit  23  is described. As described above, one of the lengthwise ends of the spring  38  is attached to the spring holding portion  37   b  of the lever  37 , and the other is attached to the spring holding portion  23  of the induction heating unit  23 . The spring  38  is enabled to be rotated about the spring holding portion  23   e,  and the lever  37  is enabled to be rotated about the shaft  37   a.    
         [0081]      FIG. 11(   a ) shows the fixing device in this embodiment when the fixation belt unit  20  and induction heating unit  23  is kept fastened to (in contact with) each other by the combination of the lever  37  and spring  38 . When the fixing device is in the state shown in  FIG. 11(   a ), the fixation belt unit  20  is kept pulled toward the induction heating unit  23  by the resiliency of the spring  38 . 
         [0082]    If it is necessary to separate the fixation belt unit  20  from the induction heating unit  23 , the fixation belt unit  20  is to be moved in the direction indicated by an arrow mark A. As the fixation belt unit  20  is moved in the direction of the arrow mark A, the bottom portion of the flange  22  pushes the lever  37 , causing thereby the lever  37  to rotate counterclockwise about the shaft  37   a.  As the lever  37  is rotated counterclockwise, the state of the fixing device changes from the one shown in  FIG. 11(   a ) to the one shown in  FIG. 11(   b ). 
         [0083]    Referring to  FIG. 11(   b ), a line K-K′ is a hypothetical line between the shaft  37   a  and spring supporting portion  23   e.  As the lever  37  is rotated counterclockwise beyond the line K-K′, the resiliency of the spring  38  acts to rotate the lever  37  counterclockwise, rotating thereby the lever  37  into the position shown in  FIG. 11(   c ). 
         [0084]    Next, the procedure for fastening the fixation belt unit  20  to the induction heating unit  23  is described. When the fixing device is in the state shown in  FIG. 11(   c ), the fixation belt unit  20  is to be moved in the direction indicated by an arrow mark B. As the fixation belt unit  20  is moved in the direction of the arrow mark B, the flange  22  of the fixation belt unit  20  pushes the lever  37 , causing thereby the lever  37  to rotate clockwise about the shaft  37   a.  As the rotation of the lever  37  continues, the position of the lever  37  changes from the one shown in FIG.  11 ( c ) to the one shown in  FIG. 11(   b ). Then, as the lever  37  is rotated enough for its shaft  37   b  to move beyond the line K-K′, the resiliency of the spring  38  acts to cause the lever  37  to rotate clockwise, changing thereby the position of the lever  37  from the one shown in  FIG. 11(   b ) to the one shown in  FIG. 11(   a ). Consequently, the fixation belt unit  20  is held against the induction heating unit  23  by the lever  37 . 
       Embodiment 3 
       [0085]      FIG. 12  is a schematic sectional view of the fixing device in the third preferred embodiment of the present invention, at a plane parallel to the recording medium conveyance direction. This embodiment is different from the first embodiment in that the fixing device in this embodiment has a recording medium guide  27  for separating a sheet P of recording medium from the fixation belt  21 . Hereafter, this guide  27  is referred to as a separation guide  27 . Otherwise, this embodiment is the same as the first embodiment. That is, the fixation belt unit and induction heating of the fixing device themselves in this embodiment are the same in structure as the counterparts in the first embodiment, and the fixing operation of the fixing device in this embodiment is the same as that in the first embodiment. Therefore, they are not going to be described here. 
       (Separation Guide) 
       [0086]    The fixing device in this embodiment is provided with at least one separation guide  27  for separating a sheet of recording medium (paper) from the fixation belt  21 . The separation guide  27  is not in contact with the fixation belt  21 , and is on the fixation belt side relative to the hypothetical extension of the fixation nip N. When a toner image on a sheet of recording paper is thermally fixed, the fixation belt  21  is on the toner image side of the sheet. Thus, the sheet tends to stay adhered to the fixation belt  21  after the fixation of the toner image. This is why the separation guide  27  is positioned on the fixation belt side. 
         [0087]    The separation guide  27  is positioned close to the fixation belt  21 . More specifically, it is positioned so that a gap G (1 mm, for example) is maintained between the surface of the fixation belt  21  and the sheet separating edge of the separation guide  27 . Next, the method for fastening the separation guide  27  to flange  22  so that the gap G is maintained is described. 
         [0088]    The fixation belt  21  in this embodiment also is supported by the pair of flanges  22  which are at the edges of the fixation belt  21  as in the first embodiment described above. That is, the fixation belt  21  in this embodiment also is kept properly positioned by the flanges  22  in terms of its widthwise direction. In other words, the flanges  22  function as guiding members which hold the fixation belt  21  (as image heating belt) at the edges (widthwise ends) of the fixation belt  21 . 
         [0089]    As a means for controlling the gap G between the outward surface of the fixation belt  21  and the sheet separating edge of the separation guide  27 , it is effective to attach the separation guide  27  to the flanges  22 . In this embodiment, therefore, the separation guide  27  is provided with a pair of projections, which are at the lengthwise ends of the separation guide  17 , whereas each flange  22  is provided with a groove  22   a.  Thus, the separation guide  27  is attached to the flanges  22  by inserting each of the projections  27   a  of the separation guide  27  into the corresponding groove  22   a  in the direction indicated by an arrow mark A, as shown in  FIG. 13 . The separation guide  27  has to be kept properly positioned. Therefore, it has to be kept pressed against the flange  22  in the direction of the arrow mark A. As the means for keeping the separation guide  27  pressed in the direction of the arrow mark A, the wire spring  36 , that is, the means for keeping the preset amount of gap between the induction heating unit  23  and fixation belt unit  20  is utilized as shown in  FIG. 14  which is a schematic perspective view of the fixing device in this embodiment. Since the wire spring  36  is resilient, it keeps the separation guide  27  pressed against the flanges  22  in the direction of the arrow mark A so that the separation guide  27  remains fastened to the flanges  22 . 
         [0090]    As is evident from the description of this embodiment given above, the fixing device in this embodiment can reliably control the gap G ( FIG. 12 ) between the outward surface of the fixation belt  21  and the sheet separating edge of the separation guide  27 , even though is very simple in structure in that the projections of the separation guide for separating a sheet of recording medium from the heating belt (fixation belt) are inserted into the groove of the corresponding fixation belt guiding member, and the separation guide is kept pressed against the fixation belt guiding members by the pair of springs. 
       (Reason for Replacing Fixation Belt Unit) 
       [0091]    Referring to  FIG. 2 , as the fixation belt  21  circularly rotates, its inward surface continuously rubs against the pressure applying auxiliary member  24   a,  in the nip N of the fixing device Q. Thus, in order to minimize the friction between the inward surface of the fixation belt  21  and the pressure applying auxiliary member  24   a,  the inward surface of the fixation belt  21  is coated with lubricant such as grease. However, as the lubricant coated on the inward surface of the fixation belt  21  is repeatedly subjected to heat and friction, it gradually deteriorates, allowing the friction between the fixation belt  21  and pressure applying auxiliary member  24   a  to increase. If the amount of frictional resistance between the fixation belt  21  and pressure applying auxiliary member  24   a  exceeds a certain value, the fixation belt  21  begins to slip and stop and/or continuously vibrates, which is likely to cause the fixing device Q to output a print which is lower in image quality. Thus, as the number of times the fixation belt  21  has been circularly moved exceeds a preset value, it is necessary for the fixation belt unit  20  to be replaced by a user. 
       (Method for Replacing Fixation Belt Unit) 
       [0092]    Next, the method for replacing the fixation belt unit  20  (which is expendable) of the fixing device Q is described.  FIG. 15  is a drawing for describing the method for replacing the fixation belt unit  20 . The fixation belt unit  20  is structured so that the outward end of each of its pair of flanges  22  fits in the corresponding groove  23   a  with which the induction heating unit  23  is provided, and also, so that the induction heating unit  23  and fixation belt unit  20  are fastened to each other by the pair of wire springs  36 , with the provision of the preset gap between the fixation belt  20  and induction heating unit  23 . Further, the bottom side (lateral) plate  19  of the fixation device frame and the induction heating unit  23  are supported by the shaft  29 . Thus, the induction heating unit  23  is rotatable upward or downward relative to the bottom side plate  19  about the shaft  29  in a manner to expose or cover the fixation belt unit  20 . 
         [0093]    The operational sequence for replacing the fixation belt unit  20  is as follows. Referring to  FIG. 15(   b ), first, the induction heating unit  23  is to be rotated about the shaft  29  by a preset angle in a manner to be moved away from the bottom side plate  19 . Incidentally, the fixing device Q is structured so that after the induction heating unit  23  is rotated by the preset angle away from the bottom side plate  19 , it remains upright on its own. Then, the hook portion  36   a,  that is, one of the lengthwise end portions, of the wire spring  36  is to be disengaged from the projection  23   f  of the induction heating unit  23 , in order to remove the force applied between the induction heating unit  23  and fixation belt unit  20  by the wiring spring  36 . Here, in order to prevent that problem that the wire spring  36  falls and/or gets lost after being disengaged, the fixing device Q is structured so that after the disengagement of the wire spring  36 , the wire spring  36  remains held by a pair of projections  23   e  and  23   g  with which the induction heating unit  23  is provided. 
         [0094]    Next, the separation guide  27  is to be disengaged in the direction of the arrow mark A as shown in  FIG. 15(   c ). Then, the fixation belt unit  20  is to be pulled out in the direction of the arrow mark B. Then, a brand-new fixation belt unit  20  is to be mounted. This is how a worn fixation belt unit  20 , that is, a fixation belt unit  20 , the fixation belt  21  of which has been circularly moved a greater number of times than a preset threshold value, is to be replaced with a brand-new one. Incidentally, the brand-new fixation belt unit  20  is placed in the fixing device after the separation guide  27  is attached to the brand-new fixation belt unit  20 . After the placement of the brand-new fixation belt unit  20  in the fixation device, the fixation belt unit  20  is to be fastened to the induction heating unit  23  by the wire spring  36  to put the fixing device back into the state shown in  FIG. 15(   a ). 
         [0095]    In the third embodiment, the wire spring  36  for keeping the induction heating unit  23  and fixation belt unit  20  fastened to each other in such a manner that the preset amount of gap is maintained between the induction heating unit  23  and fixation belt unit  20  is made to double as the means for keeping the preset distance between the fixation belt  21  and separation guide  27 . The fastening toggle in the second embodiment may be made to double as the means for keeping the preset distance between the fixation belt  21  and separation guide  27 . 
         [0096]    In the case of this embodiment, all that is necessary to replace the fixation belt unit  20  in the fixing device is to disengage and re-engage the wire springs  36 . That is, the fixing device in this embodiment is simple in the operation for replacing the fixation belt unit  20  in the fixing device. Further, the fixing device is simpler in structure than any fixing device in accordance with the prior art, and yet, can ensure that the preset amount of gap is maintained between the fixation belt  21  and separation guide  27 . Further, since the above describe method is used for keeping the induction heating unit  23  and fixation belt unit  20  fastened to each other, the combination of the weight of the induction heating member unit and the weight of the rotational heating member does not rest on the nip (pressure roller), after the pressure from the pressure applying means is removed. In other words, the present invention can provide an image heating apparatus which is superior to any image heating device in accordance with the prior art, in terms of the efficiency with which paper jam or the like can be dealt. 
       Modification of Preferred Embodiments 
       [0097]    In the preceding embodiments of the present invention, the pressure applying members ( 33  and  34 ), fastening means ( 36 ,  37  and  38 ), and pressure catching portion ( 23   d ) are positioned at the ends of the image heating member ( 21 ) in terms of the direction perpendicular to the rotational direction of the image heating member. However, these preferred embodiments are not intended to limit the present invention in scope. For example, in place of the above described pressure applying member and/or fastening means, one of the known vacuum generating mechanisms may be positioned in the center portion of the image heating member, in terms of the direction perpendicular to the rotational direction of the image heating member. As is evident from the description of the preferred embodiment of the present invention, the present invention can provide an image heating apparatus which does not suffer from the problem that the combination of the weight of its induction heating member and the weight of its rotational heating member rests on the nip (pressure roller) after the removal of the pressure from the pressure applying member. 
         [0098]    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. 
         [0099]    This application claims priority from Japanese Patent Application No. 278188/2010 filed Dec. 14, 2010, which is hereby incorporated by reference.