Patent Abstract:
An image heating apparatus includes a rotatable heater heating an image on a recording material; a rotatable pressor contacting the heater to form a nip therebetween; a driving source; first and second helical gears rotatable by a driving force supplied from the driving source and having different twisting directions; first and second switching members for switching between rotation transmission and non-transmission from a driving gear to the first and second helical gears, respectively; third and fourth helical gears provided on the heater in meshing engagement with the first and second helical gears, respectively; and a switch for switching at least between drive transmission by first switching member and drive transmission by the second switching member to change a position of the heating rotatable member along a rotational axis direction of the heater.

Full Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus mountable in an image forming apparatus, such as an electrophotographic copy machine, an electrophotographic printer, or the like, to heat the image on a recording medium. 
     As one of the image heating apparatuses mountable in an image forming apparatus, such as an electrophotographic copy machine, an electrophotographic printer, or the like, a fixing apparatus of the heat-roller type has been known. A fixing apparatus of the heat-roller type has: a fixation roller; a heater for heating the fixation roller; a pressure roller which is pressed upon the peripheral surface of the fixation roller to form a nip; etc. A recording medium on which an unfixed toner image is present is heated while being conveyed through the abovementioned nip, remaining pinched by the fixation roller and the pressure roller, whereby the image on the recording medium is thermally fixed to the recording medium. A fixing apparatus of the heat-roller type has also: a thermistor for detecting the temperature of the fixation roller; a parting claw for separating the recording medium from the peripheral surface of the fixation roller after the thermal fixation of the toner image; etc. These components are in contact with the peripheral surface of the fixation roller. Thus, a fixing apparatus of the heat-roller type suffers from the problem that as the fixation roller is rotated, the areas of the peripheral surface of the fixation roller, which are in contact with the abovementioned components, are likely to be scratched and/or frictionally worn, because the thermistor, the parting claw, etc., are in contact with the peripheral surface of the fixation roller as described above. Further, a sheet of a recording medium has a certain amount of burr along its edges. This creates the problem that as multiple sheets of recording media, which are the same in size, are repeatedly and continuously introduced into (and put through) the nip, the portions of the peripheral surface of the fixation roller, which correspond in position to the edges of the sheet of the recording medium, which are parallel to the recording-medium conveyance direction, are likely to be damaged and/or frictionally worn. Japanese Laid-open Patent Application 2000-194216 discloses one of the technologies for solving these problems. According to this technology, the front and rear plates of the fixing apparatus, with which the fixation roller and the pressure roller are rotatably supported are reciprocally moved in the direction perpendicular to the recording-medium conveyance direction by a combination of a motor and a cam, which is dedicated to the reciprocal movement. Further, Japanese Laid-open Patent application 2000-194216 discloses another technology for preventing the occurrence of the above-described problems. According to this technology, one of the fixation roller and the pressure roller is reciprocally moved in the direction perpendicular to the recording-medium conveyance direction. Japanese Laid-open Patent Application H05-173445 discloses another technology for preventing the occurrence of the above-described problems. According to this technology, only the fixation roller is reciprocally moved in the direction perpendicular to the recording-medium conveyance direction by placing a lever, the movement of which is caused by the cam movement, in contact with one of the end surfaces of the fixation roller. Japanese Laid-open Patent Application 2007-148336 discloses yet another technology for preventing the occurrence of the above-described problems. 
     According to this technology, only the fixation roller is reciprocally moved in the direction perpendicular to the recording-medium conveyance direction by placing a fixation roller, end-pressing member, which is connected to a solenoid switch, in contact with one of the lengthwise end surfaces of the fixation roller, with the use of a solenoid switch which is connected to the fixation-roller, end-pressing member. 
     The fixing apparatuses described above are complicated in the structure of their mechanism for reciprocally moving the fixation roller in its lengthwise direction while rotating the fixation roller in order to minimize the friction wear of the portions of the peripheral surface of the fixation roller, which are in contact with a thermistor, a parting claw, etc., and also, the frictional wear of the portions of the peripheral surface of the fixation roller, which come into contact with the edges of recording medium, which are parallel to the recording-medium conveyance direction. Therefore, it has been desired to simplify in structure the mechanism of a fixing apparatus, which is for reciprocally moving the fixation roller in its lengthwise direction while rotating the fixation roller. Thus, the primary object of the present invention is to provide an image heating apparatus which is simple in the structural arrangement for reciprocally moving its rotational heating member in the direction perpendicular to the lengthwise direction of the rotational heating member in order to reduce the rotational heating member in the frictional wear of its peripheral surface. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide an image heating apparatus which is simple in its structural arrangement for reciprocally moving its rotational heating member in the lengthwise direction of the rotational heating member. 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising a heating rotatable member for heating an image on a recording material, a pressing rotatable member for contacting the heating rotatable member to form a nip therebetween; a driving source; a first helical gear rotatable by a driving force supplied from the driving source; a second helical gear rotatable by a driving force supplied from the driving source, the second helical gear has a twisting direction which is different from that of the first helical gear; a first switching member for switching between rotation transmission and non-transmission from the driving gear to the first helical gear; a second switching member for switching between rotation transmission and non-transmission from the driving gear to said second helical gear; a third helical gear provided on the heating rotatable member in meshing engagement with said first helical gear; a fourth helical gear provided on the heating rotatable member in meshing engagement with the second helical gear; and switching means for switching at least between drive transmission by the first switching member and drive transmission by the second switching member to change a position of the heating rotatable member with respect to a rotational axis direction of the heating rotatable member. 
     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 
         FIG. 1  is a schematic sectional view of a typical image forming apparatus, and shows the structure of the apparatus. 
         FIG. 2(   a ) is a schematic sectional view of the fixing apparatus in the first preferred embodiment of the present invention, and shows the structure of the apparatus.  FIG. 2(   b ) is a schematic drawing of the fixing apparatus in the first embodiment, as seen from the side from which a sheet of a recording medium is introduced into the fixing apparatus. 
         FIG. 3(   a ) is a schematic drawing of the fixation roller reciprocating mechanism of the fixing apparatus in the first embodiment, and depicts the operation of the mechanism, which is for moving the fixation roller from the front side of the fixing apparatus to the rear side.  FIG. 3(   b ) is a schematic drawing of the fixation-roller reciprocating mechanism of the fixing apparatus in the first embodiment, and depicts the operation of the mechanism, which is for moving the fixation roller from the rear side of the fixing apparatus to the front side. 
         FIG. 4(   a ) is a block diagram of the hardware for controlling the operation of the fixation-roller reciprocation mechanism in the first embodiment. 
         FIG. 4(   b ) is a schematic drawing for describing the load (thrust) which is generated in the direction parallel to the axial line of the fixation roller by the combination of the helical gears  31  and  33 , and the amount of which is proportional to the helix angle of the helical gears. 
         FIG. 5  is a flowchart of an example sequence for controlling the fixation-roller reciprocation mechanism of the fixing apparatus in the first embodiment. 
         FIG. 6(   a ) is a schematic sectional view of the fixing apparatus in the second preferred embodiment of the present invention, and shows the structure of the apparatus.  FIG. 6(   b ) is a block diagram of the hardware for controlling the operation of the fixation roller reciprocation mechanism of the fixing apparatus in the second embodiment of the present invention. 
         FIG. 7  is a flowchart of an example sequence for controlling the fixation roller reciprocation mechanism of the fixing apparatus in the second embodiment. 
         FIG. 8  is a schematic drawing of the fixing apparatus in the third embodiment of the present invention, and depicts the structure of another example of the fixation roller reciprocation mechanism. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     General Structure of Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of a typical image forming apparatus in which an image heating apparatus in accordance with the present invention is mountable as a fixing apparatus (fixing device). It depicts the structure of the image forming apparatus. The image forming apparatus depicted in  FIG. 1  is an electrophotographic full-color laser printer. The image forming apparatus A in this embodiment has the first, second, third, and fourth image forming portions Py, Pm, Pc, and Pb, which are in the main assembly of the apparatus and are parallel with each other. The image forming apparatus A forms monochromatic toner images, different in color, by carrying out the processes of charging, exposing, developing, and transferring. Designated by a reference numeral  19  is a control portion as a controlling means, which is made up of a CPU, ROMs, RAM, etc. As a print command signal outputted from an external apparatus (unshown) such as a host computer, is inputted into the control portion  19 , the control portion  19  makes the image forming portions Py, Pm, Pc, and Pb sequentially operate, following the image-formation control sequence stored in the memories. In each of the image forming portions Py, Pm, Pc, and Pb, a photosensitive drum  1 , which is an image bearing member, is rotated in the direction indicated by an arrow mark at a preset peripheral velocity (process speed). The image forming apparatus A is also provided with an intermediary transfer belt  7 , which is supported, and kept stretched, by a driver roller  6   a , a follower roller  6   b , and a tension roller  6   c . The intermediary transfer belt  7  is in contact with the photosensitive drum  1  of each of the image forming portions Py, Pm, Pc, and Pb. The intermediary transfer belt  7  is circularly moved by the driver roller  6   a  in the direction indicated by another arrow mark at a speed which corresponds to the peripheral velocity of each of the photosensitive drums  1 . In the image forming portion Py for forming an image of the first color, which is yellow, the peripheral surface of the photosensitive drum  1  is uniformly charged by a charging device  2  to a preset polarity and a preset potential level. Then, an exposing apparatus  3  scans (exposes) the charged portion of the peripheral surface of the photosensitive drum  1  with a beam of laser light while modulating the beam of laser light according to the information of the image to be formed, which is sent from an external apparatus. Thus, an electrostatic latent image which reflects the information of the image to be formed, is effected on the charged portion of the peripheral surface of the photosensitive drum  1 . This latent image is developed by a developing apparatus  4  into a visible image formed of yellow toner (developer); a visible image is formed of yellow toner on the peripheral surface of the photosensitive drum  1  (hereafter, visible image formed of toner will be referred to as yellow toner image). The charging, exposing, and developing processes similar to those carried out in the image forming portion Py are carried out in the image forming portions Pm, Pc, and Pb for forming magenta (second color), cyan (third color), and black (fourth “color”) images, one for one. The monochromatic toner images, different in color, formed on the photosensitive drums  1  in the image forming portions Py, Pm, Pc, and Pb, one for one, are sequentially transferred in layers onto the outward surface of the intermediary transfer belt  7  by first transfer rollers (transferring members) which oppose the photosensitive drums  1 , one for one, with the presence of the intermediary transfer belt  7  between each of the first transfer rollers  8  and the corresponding photosensitive drum  1 . Consequently, a full-color toner image is formed on the outward surface of the intermediary transfer belt  7 . After the transfer of the toner image from the photosensitive drum  1 , the toner remaining on the peripheral surface of the photosensitive drum  1  is removed by a drum cleaner  5 , and is used for the following image formation operation. Meanwhile, sheets of a recording medium P are sent one by one by a recording-medium, feeding-and-conveying roller  11  to a pair of registration rollers  13  through a recording-medium conveyance passage  12   a . Then, each sheet of the recording medium P (which hereafter will be referred to simply as recording medium P) is conveyed by the pair of registration rollers  13  to a second transfer nip Tn, which is between the intermediary transfer belt  7  and a second transfer roller  14 , and then, is conveyed through the second transfer nip Tn while remaining pinched by the intermediary transfer belt  7  and the second transfer roller  14 . During this conveyance of the recording medium P through the second transfer nip Tn, the full-color toner image (four monochromatic toner images) on the intermediary transfer belt  7  is transferred onto the recording medium P by the second transfer roller  14 . After the transfer of the full-color toner image, the toner remaining on the intermediary transfer belt  7  is removed by a belt cleaner  9 . Then, the recording medium P on which the unfixed full-color toner image is present, is introduced into the nip of a fixing apparatus  15 , with the toner image bearing surface of the recording medium P facing upward. Then, the recording medium P is conveyed through the nip while remaining pinched by the aforementioned fixation roller and pressure roller. During the conveyance of the recording medium P through the nip of the fixing apparatus  15 , the full-color toner image on the recording medium P is thermally fixed to the surface of the recording medium P. In an image forming operation in which an image is to be formed on only one surface of the recording medium P, after the recording medium P is discharged from the fixing apparatus  15 , it is guided by a recording-medium-passage switching flapper  16  to a pair of discharge roller  17 . Then, it is discharged by the discharge rollers  17  into a delivery tray  18 , which extends from the side of the image forming apparatus A. In an image forming operation in which an image is to be formed on both surfaces of the recording medium P, after the recording medium P is discharged from the fixing apparatus  15 , it is guided by the recording-medium-passage switching flapper  16  into a recording-medium turning passage  18   b , which is below the passage  13   c . In the recording-medium turning passage  18   b , as soon as the trailing edge of the recording medium P reaches a turning point Rp, it is reversed in direction, and guided into a two-sided printing passage  13   c , being positioned so that its surface having no image faces upward. Then, the recording medium P is sent to the pair of registration rollers  13  through the recording-medium conveyance passage  12   a  from the two-sided printing passage  13   c . Then, the recording medium P is sent by the pair of registration rollers  13  to the second transfer nip Tn. Then, it is conveyed through the second transfer nip Tn while remaining pinched by the second transfer roller  14  and the intermediary transfer belt  7 . During this conveyance of the recording medium P through the second transfer nip Nn, the full-color image (four monochromatic toner images) on the intermediary transfer belt  7  is transferred onto the recording medium P. Then, the recording medium P on which the unfixed full-color toner image is present is introduced into the nip of the fixing apparatus  15 , with the unfixed toner-image bearing surface of the recording medium P facing upward. Then, the recording medium P is conveyed through the nip while remaining pinched by the aforementioned fixation roller and the pressure roller. During the conveyance of the recording medium P through the nip of the fixing apparatus  15 , the unfixed full-color toner image on the recording medium P is thermally fixed to the surface of the recording medium P. Then, the recording medium P is discharged from the fixing apparatus  15 . Then, it is guided by a recording-medium-passage switching flapper  16  to a pair of discharge roller  17 . Then, it is discharged by the discharge rollers  17  into the delivery tray  18 . 
     (Structure of Fixing Apparatus) 
     In the following description of the image heating apparatus as a fixing apparatus and its structural members, the “lengthwise direction” means the direction perpendicular to the recording-medium conveyance direction, whereas the “widthwise direction” means the direction parallel to the recording-medium conveyance direction. The “length” of a given member means the measurement of the member in the “lengthwise direction”, whereas the “width” of a given member means the measurement of the member in the “widthwise direction”. The widthwise direction of the recording medium means the direction perpendicular to the recording-medium conveyance direction on the surface of the recording medium. The lengthwise direction of the recording medium means the direction parallel to the recording-medium conveyance direction on the surface of the recording medium. The width of the recording medium means the measurement of the recording medium in terms of the widthwise direction of the recording medium. The length of recording medium means the measurement of the recording medium in terms of the lengthwise direction of the recording medium.  FIG. 2(   a ) is a schematic sectional view of the fixing apparatus in this embodiment, at a plane perpendicular to the lengthwise direction, and shows the general structure of the apparatus, and  FIG. 2(   b ) is a schematic drawing of the fixing apparatus in this embodiment, as seen from the recording-medium entrance side of the fixing apparatus, and shows the structure of the apparatus. The fixing apparatus  15  in this embodiment is of the heat-roller type. It has a fixation roller  21 , a pressure roller  22 , etc. The fixation roller  21  is a rotatable heating member, and is cylindrical. The pressure roller  22  is a pressure applying member, which also is cylindrical and rotatable. There are halogen heaters  23  and  24 , which are heating members, in the hollow of the fixation roller  21  and the hollow of the pressure roller  22 , respectively. The fixation roller  21 , the pressure roller  22 , and the halogen heaters  23  and  24  are greater in measurement in terms of the lengthwise direction than the long edges of the longest recording medium P usable with the image forming apparatus A. Referring to  FIGS. 2(   a ) and  2 ( b ), the fixation roller  21  is made up of a metallic core  21   a , an elastic layer  21   b , and a parting layer  21   c . The metallic core  21   a  is cylindrical and is formed of aluminum, iron, or the like. The elastic layer  21   b  is wrapped around the peripheral surface of the metallic core  21   a , except for the lengthwise end portions of the metallic core  21   a . The parting layer  21   c  covers the entirety of the outward surface of the elastic layer  21   b . The material for the elastic layer  21   b  is a foamed substance such as foamed silicon rubber. The material for the parting layer  21   c  is a fluorinated resin, for example, PFA (copolymer of tetrafluoroethyline and perfluoroalkylvinyl) or PTFE (poly-tetra perfluoroethylene). The parting layer  21   c  is formed by coating the outward surface of the elastic layer  21   b  with one of the above-mentioned fluorinated resins, or covering the elastic layer  21   b  with a piece of tube made of fluorinated resin. The fixation roller  21  is supported by the first (left side in  FIG. 3)  and second (right side in  FIG. 3 ) frames  25   f  and  25   r , respectively, of the fixing apparatus  15  in such a manner that not only is the fixation roller  21  rotatable, but also, it can be moved in the lengthwise direction. More specifically, the fixation roller  21  is provided with a pair of cylindrical adiabatic bushings  27  and  27 , which are solidly fitted around the lengthwise end portions of metallic core  21   a . Further, the first and second frames  25   f  and  25   r  are fitted with a pair of bearings  26  and  26 , one for one, by which the bushings  27  and  27  are supported at the front and rear ends, respectively. The fixing apparatus  15  is structured so that a preset amount of gap, in terms of the lengthwise direction, is provided between the large diameter portion of the bushing  27  and corresponding bearing  26 , to allow the fixation roller  21  to be moved in the lengthwise direction. That is, the pair of adiabatic bushings  27  and  27  are supported by the pair of bearings  26  and  26 , one for one, so that the adiabatic bushings  27  and  27  are allowed to rotate about their axial lines. The halogen heater  23  is in the hollow of the metallic core  21   a  of the fixation roller  21 , being supported at its metallic connectors which make up the lengthwise end portions of the heater  23 , by a pair of lamp supporting members (unshown) which are at the lengthwise ends, one for one, of the hollow of the metallic core  21   a . Like the fixation roller  21 , the pressure roller  22  is made up of a metallic core  22   a , an elastic layer  22   b , and a parting layer  22   c . The metallic core  22   a  is cylindrical and is formed of aluminum, iron, or the like. The elastic layer  22   b  is wrapped around the peripheral surface of the metallic core  22   a , except for the lengthwise end portions of the metallic core  22   a . The parting layer  22   c  covers the entirety of the outward surface of the elastic layer  22   b . The material for the elastic layer  22   b  is the same as that of the elastic layer  21   b  of the fixation roller  21 , and the material for the parting layer  22   c  is the same as that for the parting layer  21   c  of the fixation roller  21 . The pressure roller  22  is on the bottom side of the fixation roller  21 , and is parallel to the fixation roller  21 . The pressure roller  22  is rotatably supported by a pair of bottom frames  28   f  and  28   r , which are at the front and rear end portions, respectively, of the fixing apparatus  15 . More specifically, the pressure roller  22  is provided with a pair of cylindrical adiabatic bushings  30  and  30 , which are solidly fitted around the lengthwise end portions of metallic core  22   a . Further, the pair of bottom frames  28   f  and  28   r  are fitted with a pair of bearings  29  and  29 , one for one, by which the bushings  30  and  30  are supported at the front and rear ends, respectively. The fixing apparatus  15  is structured so that virtually no gap, in terms of the lengthwise direction, is provided between the large diameter portion of the bushing  30  and corresponding bearing  29 , so that the pressure roller  22  cannot be moved in the lengthwise direction. That is, the pair of adiabatic bushings  30  and  30  are supported by the pair of bearings  29  and  29  so that the diabetic bushings  30  and  30  are allowed to rotate about their axial lines. The halogen heater  24  is in the hollow of the metallic core  22   a  of the pressure roller  22 , being supported at its metallic connectors which make up the lengthwise end portions of the halogen heater  24 , by a pair of lamp supporting members (unshown) which are at the lengthwise ends, one for one, of the hollow of the metallic core  22   a . The bearings  29  and  29  of the pressure roller  22  are kept pressured toward the fixation roller  21  by a pair of compression springs (unshown) which are between the bearings  29 , and the bottom frames  28   f  and  28   r , one for one. Thus, the peripheral surface of the pressure roller  22  is kept pressed upon the peripheral surface of the fixation roller  21  by these springs, so that a preset amount of pressure is provided between the peripheral surface of the fixation roller  21  and peripheral surface of the pressure roller  22  to keep the elastic layer  22   b  of the pressure roller  22  and elastic layer  21   b  of the fixation roller  21  elastically deformed, creating thereby a nip N having a preset width, between the peripheral surface of the pressure roller  22  and the peripheral surface of the fixation roller  21 . The fixing apparatus  15  is provided with a thermistor  51  and a parting claw  53 , which are a temperature detecting means and a recording medium separating member, respectively. The thermistor  51  and parting claw  53  are in contact with the peripheral surface of the fixation roller  21 . The fixing apparatus  15  is also provided with a thermistor  52 , which is also a temperature detecting means. It is in contact with the peripheral surface of the pressure roller  22 . The thermistor  51  is for detecting the surface temperature of the fixation roller  21 , and the thermistor  52  is for detecting the surface temperature of the pressure roller  22 . The parting claw  53  is for separating the recording medium P from the peripheral surface of the fixation roller  21  after the thermal fixation of the toner image(s). 
     (Structure of Fixation Roller Reciprocation Mechanism) 
       FIG. 3  is a schematic drawing of a fixation roller reciprocating mechanism of the fixing apparatus in this embodiment.  FIG. 3(   a ) is a schematic drawing of the fixation-roller reciprocating mechanism (which hereafter will be referred to simply as the reciprocation mechanism), and depicts the operation of the mechanism, which is for moving the fixation roller  21  from the front side of the fixing apparatus  15  to the rear side.  FIG. 3(   b ) is a schematic drawing of the fixation-roller reciprocating mechanism of the fixing apparatus in the first embodiment, and depicts the operation of the mechanism, which is for moving the fixation roller  21  from the rear side of the fixing apparatus  15  to the front side. The reciprocation mechanism Rm is structured not only to rotate the fixation roller  21 , but also to reciprocally moving the fixation roller  21  in the lengthwise direction. The metallic core  21   a  of the fixation roller  21  is solidly fitted with the third and fourth helical gears  33  and  34 , respectively. The third and fourth helical gears  33  and  34  are on one of the lengthwise end portions of the metallic core  21   a , more specifically, the lengthwise end portion of the metallic core  21   a , which is on the front side of the first frame  25   f . The two helical gears  33  and  34  are the same in tooth count and pitch, but, are different in the helix direction. The first and second helical gears  31  and  32 , which mesh with the third and fourth helical gears  33  and  34 , respectively, are supported by the first frame  25   f , which is the front frame. The first helical gear  31  is solidly attached to a gear shaft  31   s , which is rotatably supported by the first frame  25   f , with the presence of a bearing  35  between the first frame  25   f  and gear shaft  31   s . The second helical gear  32  is solidly attached to a gear shaft  32   s , which is rotatably supported by the first frame  25   f , with the placement of a bearing  36  between the first frame  25   f  and gear shaft  32   s . The first and second helical gears  31  and  32  are the same in tooth count and pitch circle, but are different in the helix direction. The reciprocation mechanism Rm is provided with a motor M as a mechanical power source, and a first spur gear  38  as a first driving force transmitting gear, and a second spur gear  39  as the driving force transmitting second member. The motor M, the first spur gear  38 , and the second spur gear  39  are attached to the second top frame  37   f , which is on the outward side of the first top frame  25   f  in terms of the lengthwise direction of the fixing apparatus  15 . 
     The motor M attached to the second top frame  37   f  is solidly fitted with a spur gear  40  as a primary driver gear. The first spur gear  38  is solidly attached to a gear shaft  38   s , which is rotatably supported by the second top frame  37   f  with the placement of a bearing  41  between the gear shaft  38   s  and second top frame  37   f . The first spur gear  38  meshes with the spur gear  40 . The second spur gear  39  is solidly attached to a gear shaft  39   s , which is rotatably supported by the second top frame  37   f , with the placement of a bearing  42  between the first top frame  37   f  and gear shaft  39   s . The second spur gear  39  also meshes with the spur gear  40 . The first and second spur gears  38  and  39  are the same in tooth count and pitch circle. The free end of the gear shaft  38   s  of the first spur gear  38 , and the free end of the gear shaft  31   s  of the first helical gear  31 , are connected to the first electromagnetic clutch CL 1  as a first switching member. The first electromagnetic clutch CL 1  is structured to electromagnetically connect or disconnect the gear shaft  38   s  of the first spur gear  38  and the gear shaft  31   s  of the first helical gear  31 , with the use of a clutch plate, and the like. When the first electromagnetic clutch CL 1  is on standby, the gear shaft  38   s  of the first spur gear  38  and the gear shaft  31   s  of the first helical gear  31 , are kept separated from each other. The free end of the gear shaft  39   s  of the second spur gear  39 , and the free end of the gear shaft  32   s  of the second helical gear  32 , are connected to the second electromagnetic clutch CL 2  as the second switching member. The second electromagnetic clutch CL 2  is structured to electromagnetically connect or disconnect the gear shaft  39   s  of the second spur gear  39  and the gear shaft  32   s  of the second helical gear  32 , with the use of a clutch plate, and the like. The second electromagnetic clutch CL 2  is virtually the same as the first electromagnetic clutch CL 2 . When the second electromagnetic clutch CL 2  is on standby, the gear shaft  39   s  of the second spur gear  39  and the gear shaft  32   s  of the second helical gear  32  are kept separated from each other. The lengthwise end of the metallic core  21   a  of the fixation roller  21 , which is on the second top front frame side, is provided with a flag F 1 , whereas the lengthwise end of the metallic core  21   a  of the fixation roller  21 , which is on the second top rear frame side is provided with a flag F 2 . A sensor bracket  43   f , which is between the first top frame  25   f  and the second top frame  37   f , is provided with a photosensor PS 1  as the first detecting member. 
     As the photosensor PS 1  detects the flag F 1 , it outputs a flag detection signal. A sensor bracket  43   r  which is on the outward side of the first top frame  25   r  is provided with a photosensor PS 2  as the second detecting member. As the photosensor PS 2  detects the flag F 2 , it outputs a flag detection signal. As for the positional relationship between the photosensors PS 1  and PS 2 , the two photosensors PS 1  and PS 2  are positioned so that when one of them can detect the corresponding flag, the other one cannot detect the corresponding flag. 
     (Fixing Operation of Thermal Fixing Apparatus) 
     As soon as a print command is inputted into the control portion  19 , the control portion  19  begins to control the fixing apparatus  15 . First, it controls a temperature control circuit (unshown), following the temperature control sequence stored in the memory, to turn on an electric power source to supply the halogen heaters  23  and  24  with electric power to cause them to generate heat. As the halogen heater  23  generates heat, the metallic core  21   a , the elastic layer  21   b , and the parting layer  21   c  of the fixation roller  21  are heated, causing thereby the peripheral surface of the fixation roller  21  to increase in temperature. As the halogen heater  24  generates heat, the metallic core  22   a , the elastic layer  22   b , and the parting layer  22   c  of the pressure roller  22  are heated, causing thereby the peripheral surface of the pressure roller  22  to increase in temperature. The surface temperature of the fixation roller  21  is detected by the thermistor  51 . The output signal (temperature signal) from the thermistor  51  is taken in by the control portion  19 . 
     It is based on this output signal from the thermistor  51  that the control portion  19  controls the temperature control circuit to keep the surface temperature of the fixation roller  21  at a preset level (target level). The surface temperature of the pressure roller  22  is detected by the thermistor  52 . The output signal (temperature signal) from the thermistor  52  is taken in by the control portion  19 . It is based on this output signal from the thermistor  52  that the control portion  19  controls the temperature control circuit to keep the surface temperature of the pressure roller  22  at a preset level (target level). Further, as soon as the control portion  19  receives a print command Sp ( FIG. 4(   a )), it begins to carry out the reciprocation-mechanism control sequence stored in the memory.  FIG. 4(   a ) is a block diagram of the hardware of the reciprocation mechanism, and  FIG. 4(   b ) is a schematic drawing of the first and third helical gears  31  and  33 , and is for describing the load (thrust) generated in the direction parallel to the axial line of the fixation roller  21  as the first helical gear  31  is rotated. The amount of the load is proportional to the helix angle of the first and third helical gears  31  and  33 .  FIG. 5  is a flowchart of an example of the reciprocation mechanism control sequence. 
     Referring to  FIG. 5 , in Step S 1 , it is determined whether or not the photosensor PS 1  has detected the flag F 1  (whether PS 1  is on or not), and whether or not the photosensor PS 2  has detected the flag F 2  (whether PS 2  is on or not). If the control portion  19  receives the detection signal from the photosensor PS 1 , it proceeds to Step S 2 , whereas if it receives the detection signal from the photosensor PS 2 , it proceeds to Step S 11 . 
     (When Photosensor PS 1  is On) 
     In Step S 2 , the control portion  19  supplies the first electromagnetic clutch CL 1  with electric power to engage the first electromagnetic clutch CL 1  to connect the gear shaft  38   s  of the first spur gear  38  with the gear shaft  31   s  of the first helical gear  31 . That is, in Step S 2 , the first electromagnetic clutch CL 1  is switched in state from the state in which the rotational force cannot be transmitted, to the state in which the rotational force can be transmitted. In Step S 3 , the motor M is turned on to rotate the gear  40  in the direction indicated by an arrow mark. Thus, the first and second spur gears  38  and  39  are rotated in the directions indicated by corresponding arrow marks by the rotation of the gear  40 . The gear shaft  39   s  of the second spur gear  39  and the gear shaft  32   s  of the second helical gear  32 , are kept separated by the second electromagnetic clutch CL 2 . Therefore, the rotation of the second helical gear  32  is not transmitted to the fourth helical gear  34 . On the other hand, the gear shaft  38   s  of the first spur gear  38  and the gear shaft  31   s  of the first helical gear  31  are connected with each other through the first electromagnetic clutch CL 1 . Thus, the rotation of the first spur gear  38  is transmitted to the first helical gear  31  through the first electromagnetic clutch CL 1 . Therefore, the first helical gear  31  rotates in the direction indicated by an arrow mark in response to the rotation of the gear  40 . That is, the first electromagnetic clutch CL 1  transmits the rotation of the gear  40  to the first helical gear  31 . The first helical gear  31  rotates the third helical gear  33 , along with the fixation roller  21 , in the direction indicated by an arrow mark ( FIG. 2(   a )). The rotation of the fixation roller  21  is transmitted to the peripheral surface of the pressure roller  22  through the nip N, whereby the pressure roller  22  is rotated in the direction indicated by an arrow mark ( FIG. 2(   a )) by the rotation of the fixation roller  21 . As the first helical gear  31  is rotated, it moves the third helical gear  33 , along with the fixation roller  21 , rearward in the direction parallel to the axial line of the fixation roller  21 , by the load (thrust) which is generated in the direction parallel to the axial line of the fixation roller  21 , in coordination with the third helical gear  33 , and the amount of which is related to the helix angle of the two helical gears  31  and  33 . That is, as the first helical gear  31  is rotated, it moves the third helical gear  33 , along with the fixation roller  21 , from one lengthwise end (front side) of the fixing apparatus  15  to the other lengthwise end (rear side), by the above-described thrust F. Referring to  FIG. 4(   b ), it is assumed that the torque of the fixation roller  21  is T; the radius of the pitch circle of the third helical gear  33  is R; and the helix angle of the third helical gear  33  is O. 
     The amount of the force F, which moves the third helical gear  33  in the direction parallel to the axial line of the gear shaft  31   s  as the first helical gear  31  is rotated can be obtained by the following equation:
 
 F=T ×tan θ /R.  
 
With this thrust F, the first helical gear  31  moves the third helical gear  33 , along with the fixation roller  21 , from the front side of the fixing apparatus  15  to the rear side in the lengthwise direction. The greater the helix angle θ, the greater the thrust F, and therefore, the faster the speed with which the fixation roller  21  is moved in the lengthwise direction. The speed with which the fixation roller  21  is moved in the lengthwise direction should be set according to the structure of the fixing apparatus  15 . However, in consideration of the fact that the fixation roller  21  is moved in the lengthwise direction while an image forming operation is going on, it is desired that the speed with which the fixation roller  21  is moved in the lengthwise direction is set to be no more than roughly 0.5 mm/sec. If the speed with which the fixation roller  21  is moved in the lengthwise direction is set to faster than the abovementioned one, it is possible that the recording medium P will be wrinkled, and/or the monochromatic images will become misaligned, in the nip N, although it depends on the recording-medium type and/or image type. This wrinkling and/or image misalignment did not occur when the speed with which the fixation roller  21  is moved in the lengthwise direction was set to no more than roughly 0.5 mm/sec. In this embodiment, the first helical gear  31  to the fourth helical gear  34  form a 10° helix angle, for example. The external diameter of the fixation roller  21  was 50 mm, the roller  21  rotates at 191 rpm, and moves roughly 0.25 mm/sec in the speed with which it is moved in the lengthwise direction. The helix angle for the helical gears are desired to be in a range of 5-20°, although it depends on the structure of a fixing apparatus. With the above-described structural arrangement, even when the fixation roller  21  was moved in the lengthwise direction while the recording medium P was introduced into (moved through) the nip N, the recording medium P was not subjected to any kind of stress in the nip N; neither was the recording medium P wrinkled, nor did the monochromatic images become misaligned. Incidentally, the above-mentioned figures for the external diameter and speed of revolution of the fixation roller  21  are nothing but examples, and not intended to limit the present invention in scope. In Step S 4 , it is determined whether or not the photosensor PS 2  is on. If the photosensor PS 2  detects the flag F 2  ( FIG. 3(   b )), and the control portion  19  receives the flag detection signal from the photosensor PS 2 , the control portion  19  determines that the photosensor PS 2  is on (YES), and proceeds to Step S 5 . In Step S 5 , the control portion  19  turns on the second electromagnetic clutch CL 2  by supplying the second electromagnetic clutch CL 2  with electric power. Consequently, the gear shaft  39   s  of the second spur gear  39  and the gear shaft  32   s  of the second helical gear  32 , which were kept separated from each other, are electromagnetically connected to each other through the second electromagnetic clutch CL 2 . In other words, in Step S 5 , the second electromagnetic clutch CL 2  is switched in state from the OFF-state into ON-state. Thus, the rotation of the second spur gear  39  is transmitted to the second helical gear  32 , whereby the second helical gear  32  is rotated by the rotation of the gear  40  in the direction indicated by an arrow mark. That is, the second electromagnetic clutch CL 2  transmits the rotation of the gear  40  to the second helical gear  32 . Then, the second helical gear  32  rotates the fourth helical gear  34  and the fixation roller  21  in the direction indicated by an arrow mark ( FIG. 2(   a )). As the second helical gear  32  rotates, not only does it rotate the fourth helical gear  34  and the fixation roller  21 , but also, moves them frontward in the direction parallel to the axial line of the fixation roller  21  by the load F (thrust) ( FIG. 4(   b )) which it generates in coordination with the fourth helical gear  34 , and the amount of which is proportional to the helix angle of the two helical gears  32  and  34 . That is, as the second helical gear  32  is rotated, it moves the fourth helical gear  34  and the fixation roller  21  from the rear side of the fixing apparatus  15  to the front side in the lengthwise direction, with the abovementioned thrust F. In Step S 6 , the control portion  19  turns off the first electromagnetic clutch CL 1  by stopping the supply of the first electromagnetic clutch CL 1  with electric power, whereby the first electromagnetic clutch CL 1  is switched in state from the state in which it keeps electromagnetically connected the gear shaft  38   s  of the first spur gear  38  and the gear shaft  31   s  of the first helical gear  31 , to the state in which it keeps the gear shaft  38   s  electromagnetically separated from the gear shaft  31   s . That is, in Step S 6 , the first electromagnetic clutch CL 1  is switched in state from the state in which it transmits the rotational force to the state in which it does not transmit the rotational force, while the second electromagnetic clutch CL 2  is on. The timing with which the first electromagnetic clutch CL 1  is to be turned off is set in consideration of the length of time necessary to connect the gear shaft  39   s  of the second spur gear  39  with the gear shaft  32   s  of the second helical gear  32 . Therefore, it will be such a timing that there will be a sufficient amount of time (several tens of seconds) for fully connecting the gear shaft  39   s  of the second spur gear  39  with the gear shaft  32   s  of the second helical gear  32  in Step S 5 . Further, the first electromagnetic clutch CL 1  is turned off while the second electromagnetic clutch CL 2  is on. Therefore, even if the first electromagnetic clutch CL 1  is turned off, it is ensured that the fixation roller  21  is rotated by the rotation of gear  40  which is transmitted to the fourth helical gear  34  through the rotational force transmission passage which does not include the first electromagnetic clutch CL 1 , and therefore, the fixation roller  21  does not change in rotational speed. In Step S 7 , it is determined whether or not the photosensor PS 1  is on. If the photosensor PS 1  detects the flag F 1  ( FIG. 3(   a )), and the control portion  19  receives the flag detection signal from the photosensor PS 1 , the control portion  19  determines that the photosensor PS 1  is on (Yes), and proceeds to Step S 8 . In Step S 8 , the control portion  19  turns on the first electromagnetic clutch CL 1  by supplying the first electromagnetic clutch CL 1  with electric power, thereby causing the first electromagnetic clutch CL 1  to electromagnetically connect the gear shaft  38   s  of the first spur gear  38  with the gear shaft  31   s  of the first helical gear  31 . Thus, the rotation of the first spur gear  38  is transmitted to the first helical gear  31 . Thus, the first helical gear  31  is rotated in the direction indicated by an arrow mark by the rotation of the gear  40 . Therefore, the first helical gear  31  moves the third helical gear  33  and the fixation roller  21  in the lengthwise direction of the fixation roller  21  from the front side of the fixing apparatus  15  to the rear side, as it does in Step S 5 . In Step S 9 , the control portion  19  turns off the second electromagnetic clutch CL 2  by stopping the supply of the second electromagnetic clutch CL 2  with electric power, whereby the second electromagnetic clutch CL 2  is switched in state from the state in which it keeps connected the gear shaft  39   s  of the second spur gear  39  with the gear shaft  32   s  of the second helical gear  32 , to the state in which the gear shafts  39   s  and  32   s  are kept not connected to each other. That is, in Step S 9 , the timing with which the second electromagnetic clutch CL 2  is turned off is set in consideration of the length of time necessary to completely connect the gear shaft  38   s  of the first spur gear  38  with the gear shaft  31   s  of the first helical gear  31 , and therefore, is such a timing that there will be a sufficient amount of time (several tens of seconds) for fully connecting the two shafts  38   s  and  31   s . Further, the second electromagnetic clutch CL 2  is turned off while the first electromagnetic clutch CL 1  is kept turned on. Therefore, even if the second electromagnetic clutch CL 2  is turned off, the fixation roller  21  is rotated by the rotational force transmitted from the gear  40  to the third helical gear  33  through the rotation force transmission route which does not include the second electromagnetic clutch CL 2 . Thus, it is ensured that the fixation roller  21  does not change in rotational speed. In Step S 10 , it is determined whether or not another print is to be outputted. If it is determined that another print is to be outputted (YES), the control portion  19  repeats the processes carried out in Step S 4  to Step S 9 . That is, in a case where another print is to be outputted, the first electromagnetic clutch CL 1  and second electromagnetic clutch CL 2  are turned on or off in Steps S 4 -S 9 , while the recording medium P is conveyed through the nip N. As for the method, in this embodiment, for switching between the two electromagnetic clutches, one of the two electromagnetic clutches CL 1  and CL 2 , which was kept turned off is turned on to turn on both electromagnetic clutches, and then, the other electromagnetic clutch, which was kept turned on is turned off. If it is determined that no print is to be outputted (NO), the operational sequence Steps S 1 -S 9  is ended.
 
(When Photosensor PS 2  is On)
 
     In Step S 11 , the control portion  19  turns on the second electromagnetic clutch CL 2  by supplying the second electromagnetic clutch CL 2  with electric power, to cause the second electromagnetic clutch CL 2  to electromagnetically connect the gear shaft  39   s  of the second spur gear  39  with the gear shaft  32   s  of the second helical gear  32 , as it does in Step S 5 . That is, in Step S 11 , the second electromagnetic clutch CL 2  is switched in state from the state in which it is off, to the state in which it is on. Thus, rotation of the second spur gear  39  is transmitted to the second helical gear  32 . Thus, the second helical gear  32  is rotated in the direction indicated an arrow mark by the rotation of the gear  40 . Therefore, the second helical gear  32  moves the fourth helical gear  34  and fixation roller  21  in the lengthwise direction of the fixation roller  21  from the rear side of the fixing apparatus  15  toward the front side. In Step S 12 , the motor M is started, whereby the gear  40  is rotated in the direction indicated by the arrow mark. Thus, the first spur gear  38  and the second spur gear  39  are rotated in the direction indicated by the arrow mark by the rotation of the gear  40 . The first electromagnetic clutch CL 1  is in the state in which it keeps the gear shaft  38   s  of the first spur gear  38  separated from the gear shaft  31   s  of the first helical gear  31 . Therefore, the rotation of the first spur gear  38  is not transmitted to the first helical gear  31 . On the other hand, the second electromagnetic clutch CL 2  is in the state in which it keeps the gear shaft  39   s  of the second spur gear  39  connected to the gear shaft  32   s  of the second helical gear  32 . Therefore, the rotation of the second spur gear  39  is transmitted to the second helical gear  32  through the second electromagnetic clutch CL 2 . In the case of the fixing apparatus  15  in this embodiment, the driving force transmission route to the third helical gear  33 , and the driving force transmission route to the fourth helical gear  34 , are structured so that the direction in which the fixation roller  21  is rotated when the first electromagnetic clutch is in action is the same as the direction in which the fixation roller  21  is rotated when the second electromagnetic clutch is in action. It is through the above-described driving force transmission routes that the second helical gear  32  is rotated in the direction indicated by the arrow mark by the rotation of the gear  40 . That is, the second electromagnetic clutch CL 2  transmits rotation of the gear  40  to the second helical gear  32 . The second helical gear  32  rotates the fourth helical gear  34  and the fixation roller  21  in the direction indicated by the arrow mark ( FIG. 2(   a )). The rotation of the fixation roller  21  is transmitted to the peripheral surface of the pressure roller  22  by way of the nip N, whereby the pressure roller  22  is rotated in the direction indicated by the arrow mark by the rotation of the fixation roller  21  ( FIG. 2(   a )). As the second helical gear  32  rotates, it moves the fourth helical gear  34  and the fixation roller  21  in the lengthwise direction of the fixation roller  21  from the rear side of the fixing apparatus  15  to the front side, with the load F (thrust) which it generates in the axial direction of the fixation roller  21  in coordination with the fourth helical gear  34 , and the amount of which is proportional to the helix angle of the two helical gears  32  and  34 . That is, as the second helical gear  32  is rotated, the fourth helical gear  34  and the fixation roller  21  are moved frontward from the rear side of the fixing apparatus  15  by the abovementioned thrust F. In Step S 13 , it is determined whether or not the photosensor PS 1  is on. If the photosensor PS 1  detects the flag F 1  ( FIG. 3(   a )), and the control portion  19  receives the flag detection signal from the photosensor PS 1 , and therefore, determines that the photosensor PS 1  is on (YES), the control portion  19  proceeds to Step S 14 . In Step S 14 , the control portion  19  turns on the first electromagnetic clutch CL 1  by supplying the first electromagnetic clutch CL 1  with electric power, causing thereby the first electromagnetic clutch CL 1  to electromagnetically connect the gear shaft  38   s  of the first spur gear  38  with the gear shaft  31   s  of the first helical gear  31 . That is, in Step S 14 , the first electromagnetic clutch CL 1  is switched in state from the state in which it is off, to the state in which it is on. Therefore, the rotation of the first spur gear  38  is transmitted to the first helical gear  31 . Thus, the first helical gear  31  is rotated by the rotation of the gear  40  in the direction indicated by the arrow mark. As the first helical gear  31  is rotated, it moves the third helical gear  33  and the fixation roller  21  in the lengthwise direction of the fixation roller  21  from the front side of the fixing apparatus  15  toward the rear side. In Step S 15 , the control portion  19  turns off the second electromagnetic clutch CL 2  by stopping the supply of the second electromagnetic clutch CL 2  with electric power. Thus, the second electromagnetic clutch CL 2  disengages the gear shaft  39   s  of the second spur gear  39  from the gear shaft  32   s  of the second helical gear  32 , as it does in Step S 9 . That is, in Step S 9 , the second electromagnetic clutch CL 2  is switched in state from the state in which it can transmit the rotational force, to the state in which it cannot transmit the rotational force, while the first electromagnetic clutch CL 1  is on. In Step S 15 , the timing with which the second electromagnetic clutch CL 2  is turned off is set in consideration of the length of time necessary to fully connect the gear shaft  38   s  of the first spur gear  38  with the gear shaft  31   s  of the first helical gear  31 , and therefore, is such a timing that there will be a sufficient amount of time (several tens of seconds) for fully connecting the two shafts  38   s  and  31   s . Further, the second electromagnetic clutch CL 2  is turned off while the first electromagnetic clutch CL 1  is kept turned on. Therefore, even if the second electromagnetic clutch CL 2  is turned off, the rotational force is transmitted from the gear  40  to the third helical gear  33  through the rotation force transmission route which does not include the second electromagnetic clutch CL 2 . Therefore, it is ensured that the fixation roller  21  is continuously rotated, being prevented from changing in rotational speed. In Step S 16 , it is determined whether or not the photosensor PS 2  is on. If the photosensor PS 2  detects the flag F 1  ( FIG. 3(   b )), and the control portion  19  receives the flag detection signal from the photosensor PS 2 , the control portion  19  determines that the photosensor PS 2  is on (Yes), and proceeds to Step S 17 . In Step S 17 , the control portion  19  turns on the second electromagnetic clutch CL 2  by supplying the second electromagnetic clutch CL 2  with electric power, thereby causing the second electromagnetic clutch CL 2  to electromagnetically connect the gear shaft  39   s  of the second spur gear  39  with the gear shaft  32   s  of the second helical gear  32 , as it does in Step  11 . Thus, the rotation of the second spur gear  39  is transmitted to the second helical gear  32 . Thus, the second helical gear  32  is rotated in the direction indicated by the arrow mark by the rotation of the gear  40 . Therefore, the second helical gear  32  moves the fourth helical gear  34  and the fixation roller  21  in the lengthwise direction of the fixation roller  21  from the rear side of the fixing apparatus  15  to the front side. In Step S 18 , the control portion  19  turns off the first electromagnetic clutch CL 1  by stopping the supply of the first electromagnetic clutch CL 1  with electric power, thereby causing the first electromagnetic clutch CL 1  to disconnect the gear shaft  38   s  of the first spur gear  38  from the gear shaft  31   s  of the first helical gear  31  as it does in Step S 6 . That is, in Step S 18 , the first electromagnetic clutch CL 1  is switched in state from the state in which it can transmit the rotational force, to the state in which it cannot transmit the rotational force, while the second electromagnetic clutch CL 2  is kept on. The timing with which the first electromagnetic clutch CL 1  is turned off in Step S 18  is set in consideration of the length of time necessary to completely connect the gear shaft  39   s  of the second spur gear  39  with the gear shaft  32   s  of the second helical gear  32  in S 17 , and therefore, is such a timing that there will be a sufficient amount of time (several tens of seconds) for fully connecting the two shafts  39   s  and  32   s . Further, the first electromagnetic clutch CL 1  is turned off while the second electromagnetic clutch CL 2  is kept turned on. Therefore, even if the first electromagnetic clutch CL 1  is turned off, the rotational force is transmitted from the gear  40  to the fourth helical gear  34  through the rotation force transmission route which does not include the first electromagnetic clutch CL 1 , ensuring that the fixation roller  21  is rotated with no interruption, thereby preventing the fixation roller  21  from changing in rotational speed. In Step S 19 , it is determined whether or not another print is to be outputted. If it is determined that another print is to be outputted (YES), the control portion  19  repeats the processes carried out in Step S 13  to Step S 18 . In a case where another print does not need to be outputted, the operational sequence Steps S 11 -S 18  is ended. In a case where another print is to be outputted, the same processes as those carried out in Step S 4  to S 9 , are carried out in Steps S 13 -S 18 . The method for switching the electromagnetic clutches in operational state is the same as the method used while prints are outputted in Step S 1 -S 9 . During the continuation of the sequential processes carried out in Steps S 1 -S 9 , the recording medium P on which the unfixed toner image t is present is introduced into the nip N while the fixation roller  21  and the pressure roller  22  are kept constant in temperature at a preset level, or during continuation of the sequential processes carried out in Steps S 11 -S 18 , the recording medium P, on which an unfixed toner image t is present, is introduced into the nip N while the fixation roller  21  and the pressure roller  22  are kept stable in surface temperature at a preset level. As the recording medium P is introduced into the nip N, it is conveyed through the nip N while remaining pinched between the peripheral surface of the fixation roller  21  and the peripheral surface of the pressure roller  22 . While the recording medium P is conveyed through the nip N, the recording medium P and the unfixed toner image t are subjected to the heat and pressure from the fixation roller  21  and the pressure roller  22 , whereby the unfixed toner image t is thermally fixed to the surface of the recording medium P. 
     The fixing apparatus  15  in this embodiment can rotate the fixation roller  21 , and also, reciprocally move the fixation roller  21  in the lengthwise direction, with use of only a single motor (motor M). In other words, it is simple in structure, and yet, is substantially smaller in the amount by which certain portions of the peripheral surface of the fixation roller  21  are substantially more worn due to their contact with the thermistor and the recording medium separating claw, and also, the edges of the recording medium P, than the other portions. In other words, the present invention can simplify a fixing apparatus in structure, reduce a fixing apparatus in size, and also, in cost, as well as reduce electric power consumption. Further, in the case of the fixing apparatus  15  in this embodiment, the meshing of helical gears which are different in helix angle is utilized not only to rotate the fixation roller  21 , but also, to reciprocally move the fixation roller  21  in the lengthwise direction. Therefore, it is possible to reciprocally and very gradually move the fixation roller  21  in the lengthwise direction while rotating the fixation roller  21 . Therefore, the fixing apparatus  15  in this embodiment is significantly smaller than any of conventional fixing apparatuses of the same time, in the amount of the deformation which occurs to the peripheral surface of the fixation roller  21  in the nip N as the fixation roller  21  is reciprocally moved in the lengthwise direction while being rotated. Thus, even though the fixation roller  21  is reciprocally moved in the lengthwise direction while the recording medium P is continuously conveyed through the nip N, remaining pinched by the fixation roller  21  and the pressure roller  22 , by switching the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2  in operational state between the state in which the rotation of the motor M can be transmitted to the fixation roller  21 , and the state in which the rotation of the motor M cannot be transmitted to the fixation roller  21 , in such a manner that the rotation of the motor M is transmitted to the fixation roller  21  without interruption, the stress to which the recording medium P is subjected in the nip N is minuscule. Therefore, it is unlikely to occur that the recording medium P is wrinkled, and/or that the toner image t on the recording medium P is made to deviate in position. 
     Embodiment 2 
       FIG. 6(   a ) is a schematic sectional view of the fixing apparatus in the second preferred embodiment of the present invention, and shows the structure of the apparatus. The members, portions, etc., of the fixing apparatus in the second embodiment, which are the same as their counterparts of the fixing apparatus  15  in the first embodiment, are given the same reference numerals and characters as those given to their counterparts and will not be described. In the case of the fixing apparatus  15  in the first embodiment, the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2  are turned on or off while the recording medium P is conveyed through the nip N during a printing operation. Therefore, the fixation roller  21  sometimes slightly changes in rotation speed as the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2  are turned on or off. The changes in the rotational speed of the fixation roller  21  sometimes results in the outputting of images which are nonuniform in appearance, in particular, when a highly precise multicolor image is formed on recording media of certain types. In the second embodiment, therefore, the fixing apparatus  15  is structured so that the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2  are turned on or off while the recording medium P is not conveyed through the nip N. The fixing apparatus  15  in this embodiment is provided with a pair of photosensors PS 3  and PS 4 , which are the third and fourth detecting members, respectively. The photosensor PS 3  is on the recording medium entrance side of the nip N, which is between the peripheral surface of the fixation roller  21  and the peripheral surface of the pressure roller  22 . The photosensor PS 4  is on the recording medium exit side of the nip N. The fixing apparatus  15  in this embodiment is the same in structure as the fixing apparatus  15  in the first embodiment, except that in the case of the fixing apparatus  15  in this embodiment, the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2  are turned on or off while the recording medium P is not where it can be detected by the photosensors PS 3  and PS 4 . The photosensor PS 3  is to detect the recording medium P on the upstream side of the nip N in terms of the recording-medium conveyance direction, and to output a recording-medium detection signal as it detects the recording medium P. The photosensor PS 4  is to detect the recording medium P on the downstream side of the nip N in terms of the recording-medium conveyance direction, and to output a recording-medium detection signal as it detects the recording medium P.  FIG. 6(   b ) is a block diagram of the hardware for controlling the operation of the fixation roller reciprocation mechanism of the fixing apparatus in the second embodiment of the present invention, and shows the structure of the hardware. 
       FIG. 7  is a flowchart of an example of the operation of the fixation-roller reciprocation mechanism of the fixing apparatus in the second embodiment. First, referring to  FIG. 7 , in Step S 21 , the same process as that carried out in Step S 1  in  FIG. 5  is carried out. That is, if the control portion  19  receives a recording-medium detection signal from the photosensor PS 1 , the control portion  19  proceeds to Step S 22 , whereas if the control portion  19  receives a recording-medium detection signal from the photosensor PS 2 , it proceeds to Step S 35 . 
     (Description of Fixing Apparatus Operation when Photosensor PS 1  is On) 
     In Steps S 22 -S 24 , the control portion  19  carries out the same processes as those it carries out in Steps S 1 -S 4  shown in  FIG. 5 . In Step S 25 , it determines whether or not the entrance photosensor PS 3 , which is on the entrance side of the nip N, is off. If it does not receives a recording-medium detection signal from the entrance photosensor PS 3 , it determines that the entrance photosensor PS 3  is off (YES), and proceeds to Step S 26 , in which it determines whether or not the exit photosensor PS 4 , which is on the exit side of the nip N, is off. If it does not receive a recording-medium detection signal from the exit photosensor PS 4 , it determines that the exit photosensor PS 4  is off (YES), and proceeds to Step S 27 . That is, if it does not receives a recording-medium detection signal from the entrance photosensor sensor PS 3 , and does not receive a recording medium detection signal from the exit photosensor PS 4 , it determines that there is no recording medium P in the nip N. In Steps S 27 -S 29 , it carries out the same processes as those it carries out in Step S 5 -S 7  in  FIG. 5 . In Step S 30 , it determines whether or not the entrance photosensor PS 3  is off. If it does not receives a recording-medium detection signal from the entrance photosensor PS 3 , it determines that the entrance photosensor PS 3  is off (YES), and proceeds to S 31 , in which it determines whether or not the exit photosensor PS 4  is off. If it does not receive a recording-medium detection signal from the exit photosensor PS 4 , it determines that the entrance photosensor PS 4  is off (YES), and proceeds to Step S 32 . That is, only if it does not receives a recording-medium detection signal from entrance photosensor PS 3  in Step S 30 , and a recording-medium detection signal from the exit photosensor PS 4  in Step S 31 , it determines that there is no recording medium P in the nip N. In Steps S 32 -S 33 , it carries out the same processes as those which it does in Steps S 8  and S 9  shown in  FIG. 5 . In Step S 34 , it carries out the same process as the process it carries out in Step S 10  shown in  FIG. 5 . That is, if it is necessary to output another print (YES), it repeats the same processes as those it carries out in Steps S 24 -S 33 . If it is unnecessary to output another print (NO), it ends the series of processes it carries out in Steps S 21 -S 33 . In a case where another print is necessary, the control portion  19  moves the fixation roller  21  in the lengthwise direction toward the rear side (or front side) of the fixing apparatus  15  in Steps S 24 -S 33 . Then, if it receives a flag detection signal from the photosensor PS 1  (or PS 2 ), it confirms whether or not the entrance photosensor PS 3  is off. In a case where the photosensor PS 1  (or PS 2 ) is on, that is, while the recording medium P is conveyed through the nip N, the control portion  19  waits until the next recording medium P is introduced into the nip N, and then, it turns on or off the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2 . The length (several 100 ms) of time it takes for the recording medium P to be conveyed from the location of the entrance photosensor PS 3  to the nip N is very small compared to the length of time it takes to turn on or off the first electromagnetic clutch CL 1  (or second electromagnetic clutch CL 2 ). Therefore, even if the fixing apparatus  15  is designed so that if the recording medium P is being conveyed through the nip N, the first and the second electromagnetic clutches CL 1  and CL 2  are not turned on or off until the next recording medium P is introduced into the nip N, there is no problem. Incidentally, if either the entrance photosensor PS 3  or the exit photosensor PS 4  is on, the control portion  19  determines that the recording medium P is being conveyed through the nip N. Further, the fixation roller  21  is moved from where it is when the photosensor PS 1  (or PS 2 ) is turned on to where it is when either entrance photosensor PS 3  or exit photosensor PS 4  is turned on. However, the lengthwise movement of the fixation roller  21  is no more than 0.5 mm/sec. In other words, the amount of the movement of the fixation roller  21  in the widthwise direction of the recording medium P per recording medium P is minute. Therefore, the problem that images which are nonuniform in appearance are outputted does not occur. Next, this subject will be described in detail. It is when the leading edge of the longest recording medium P in terms of the direction parallel to the recording-medium conveyance direction turns on the entrance photosensor PS 3  that the length of time the control portion  19  has to wait before it turns on (or turns off) the electromagnetic clutches becomes longest. For example, if the longest recording medium P is 19 inches (=482.6 cm) in length; the distance between the entrance photosensor sensor PS 3  and the exit photosensor PS 4  is 100 mm, and the recording medium conveyance speed in the nip N is 500 mm/sec, the longest time the control portion  19  has to wait before it turns on (or off) the electromagnetic clutches is 1.16 seconds. If the maximum speed at which the fixation roller  21  is moved in the lengthwise direction is 0.5 mm/sec, the distance by which the fixation roller  21  overruns is 0.58 mm. This is only one example of the specification of a fixing apparatus. In any case, however, the amount of the overrun of the fixation roller  21  is minute. Thus, even if the conditions related to the operation of the image forming apparatus changes, there is no problem as long as the change is small. As for the switching between the first electromagnetic clutch CL 1  and the second electromagnetic clutch CL 2 , the one which was off is turned on. Then, both electromagnetic clutches are turned on when the both the entrance photosensor PS 3  and exit photosensor PS 4  are off. Then, the electromagnetic clutch which was kept on is turned off. 
     (Operation of Fixing Apparatus when Photosensor PS 2  is On) 
     In Step S 35 -S 37 , the control portion  19  carries out the same processes as those which it carries out in Step S 11 -S 13  shown in  FIG. 5 . In Step S 38 , it determines whether or not the recording medium P has been detected by the entrance photosensor PS 3  (whether the entrance photosensor PS 3  is on or off). If the control portion  19  does not receive an indication that recording medium P has been detected by photosensor PS 3  (YES), it proceeds to Step S 39 , in which it determines whether or not the exit photosensor PS 4  has detected the recording medium P (whether the exit photosensor PS 4  is on or off). If it determines that the exit photosensor PS 4  has not detected the recording medium P (YES), it proceeds to Step S 40 . That is, if the control portion  19  does not receive a recording-medium detection signal from the entrance photosensor PS 3  in Step  38 , and also, from the exit photosensor PS 4  in Step S 39 , the control portion  19  determines that there is no recording medium P in the nip N. In Steps S 40 -S 42 , the control portion  19  carries out the same processes as those it carries out in Step S 14 -S 16  shown in  FIG. 5 . In Step S 43 , the control portion  19  determines whether or not the entrance photosensor PS 3  has detected the recording medium P (whether or not the entrance photosensor PS 3  is on or off). If the control portion  19  does not receives the recording medium detection signal from the entrance photosensor PS 3  (YES), it proceeds to Step S 44 , in which it determines whether or not the exit photosensor PS 4  has detected the recording medium P. If it does not receives a recording-medium detection signal from the exit photosensor PS 4  (YES), it proceeds to Step S 45 . That is, if the control portion  19  does not receive a recording-medium detection signal from the entrance photosensor PS 3  in Steps S 43 , and from the exit photosensor PS 4  in Step S 44 , it determines that there is no recording medium P in the nip N. In Steps S 45  and S 46 , it carries out the same processes as those it carries out in Steps S 17  and S 18  shown in  FIG. 5 . In Step S 47 , it carries out the same process as the process it carries out in Step  19  shown in  FIG. 5 . That is, if it is necessary to output another print (YES), it carries out the same processes as those it carries out in Steps S 37 -S 46 . If it is unnecessary to output another print (NO), it ends the sequence of processes carried out in Steps S 35 -S 46 . If another print is to be outputted (YES), the control portion  19  carries out the same processes as those it carries out in Steps S 24 -S 33 . As for the method in which the electromagnetic clutches are sequentially turned on or off, or in combination, it is the same as that in which they are in Steps S 24 - 33 . 
     Also in the case of the fixing apparatus  15  in the second embodiment, only a single motor (motor M) is required to reciprocally move the fixation roller  21  in the lengthwise direction while rotating the fixation roller  21 . Thus, the same effects as those achieved by the fixing apparatus  15  in the first embodiment were achieved. In this embodiment, however, the first electromagnetic clutch and the second electromagnetic clutch are turned on or off when both the entrance photosensor and exit photosensor are off, that is, while no recording medium P is conveyed through the nip N. Therefore, it does not occur that images which are nonuniform in appearance, and the nonuniformity of which is attributable to the change in the rotational speed of the fixation roller, are outputted. In other words, the fixing apparatus  15  in this embodiment can also prevent an image forming apparatus from outputting images which are nonuniform in appearance, including images, the nonuniformity of which is ordinarily nonproblematic. Further, in the case of the fixing apparatus  15  in this embodiment, the first electromagnetic clutch is turned on or off with a different timing from the timing with which the second electromagnetic clutch is turned on or off. However, the first electromagnetic clutch and the second electromagnetic clutch may be turned on or off at the same time. 
     Embodiment 3 
       FIG. 8  is a schematic drawing of the fixing apparatus in the third embodiment of the present invention, and depicts the structure of another example of the fixation-roller reciprocation mechanism. Also in the case of the fixing apparatus  15  in this embodiment, the members, portions, etc., of the fixing apparatus  15 , which are the same as their counterparts of the fixing apparatus  15  in the first embodiment, are given the same reference numerals and characters as those given to their counterparts, and will not be described. In the case of the fixing apparatus  15  in the first embodiment, the third and fourth helical gears  33  and  34 , respectively, are solidly attached to the lengthwise end portion of the metallic core  21   a  of the fixation roller  21 , which is on the front side of the first top frame  25   f . In the case of the fixing apparatus  15  in this (third) embodiment, the third helical gear  33  is solidly attached to one of lengthwise end portions, more specifically, the front end portion, of the metallic core  21   a  of the fixation roller  21 , and the fourth helical gear  34  is solidly attached to the other lengthwise end portion, more specifically, the rear end portion, of the metallic core  21   a  of the fixation roller  21 . Otherwise, the fixing apparatus  15  in this embodiment is the same in structure as the fixing apparatus  15  in the first embodiment. That is, the reciprocation mechanism of the fixing apparatus  15  in this embodiment is different from that of the fixing apparatus  15  in the first embodiment, only in the positioning of the third and fourth helical gears  33  and  34 , respectively. Thus, the effects of the fixing apparatus  15  in this embodiment are the same as those of the fixing apparatus  15  in the first embodiment. 
     As will be evident from the description of the preferred embodiments of the present invention, the present invention makes it possible to provide an image heating apparatus which is simple in the structural arrangement for reciprocating the rotational heating member of the apparatus in the lengthwise direction of the rotational heating member while rotating the rotational heating member, in order to minimize the amount by which the peripheral surface of the rotational heating member is frictionally worn. 
     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. 
     This application claims priority from Japanese Patent Application No. 205782/2009 filed Sep. 7, 2009 which is hereby incorporated by reference.

Technology Classification (CPC): 6