Abstract:
An image heating apparatus includes: a belt member; an inclinable steering roller configured to support the belt member; a heating member configured to contact the belt member and heat an image on a recording material; a detecting member configured to detect a position of a belt end in a widthwise direction of the belt member; a correcting device configured to correct a position of the belt member in the widthwise direction by inclining the steering roller in accordance with a detected position of the belt end by the detecting member; and a supporting device configured to support the detecting member. The supporting device is provided with a contact portion for press-contacting an end surface of one end portion of a pressing roller configured to press the belt member against the heating member.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus suitably usable for a fixing device of an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer and a belt feeding device suitably usable for such an image heating apparatus, for example. 
     As the fixing device for the image forming apparatus such as a copying machine and a printer of an electrophoto graphic type, a belt nipping type is known (a Japanese Laid-open Patent Application 2004-341346) which is provided with an endless heating belt, and, an endless pressing belt for contacting the belt to form a nip. A recording material carrying an unfixed toner image is nipped and fed by the nip, by which the toner image is heat-fixed on the recording material. In the fixing device of a belt nipping type, which uses an endless belt for the fixing belt or the pressing belt, the prevention of an offset (snaking movement) of the belt is an important technical problem. During the rotation of the belt, when the one longitudinal end portion and the other end portion of the roller offsets in the widthwise direction, there is a possibility that the belt disengages from a predetermined belt rotation range and an end of the belt is damaged. For the offset correction of the belt, a belt offset correcting control is used ordinarily, wherein an end position of one of the two rollers supporting the belt is changed, by which the belt produces the offsetting force to maintain the belt in the predetermined range (Japanese Laid-open Patent Application Hei 4-104180). In the fixing device which employs the belt offset correcting control, the temperature of the roller supporting the belt rises because the fixing member which forms the nip by contacting the roller or the belt is heated. Therefore, it is usual that in order to position the roller which does not carry out the offset correction in the longitudinal direction of the roller, one-end portion and the other end portion of the roller shaft are supported with respect to the longitudinal direction of the roller with a predetermined play to a roller supporting member, in consideration of the thermal expansion in the longitudinal direction of the roller. For this reason, at the time of a rotation of the belt, the roller is rotated in the state that it is urged to remove the play in one longitudinal end portion or the other end portion. However, when the urged side changes to the opposite side, the position of the belt end relative to the roller end is unstable, and there is a risk that the position of the belt member relative to the roller end cannot be accurately detected. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image heating apparatus, wherein the sensing accuracy of a position of a belt end relative to a roller end is enhanced. 
     According to an aspect of the present invention and there is provided an image heating apparatus comprising: a belt member; an inclinable steering roller for supporting the belt member; a heating member for contacting to the belt member and heating an image on a recording material; a detecting member for detecting a position of a belt end in a widthwise direction of the belt member; correcting means for correcting a position of the belt member in the widthwise direction by inclining the steering roller in accordance with a detected position of the belt end by the detecting member; supporting means for supporting the detecting member; and the supporting means being provided with a contact portion for press-contacting the end surface of one end portion of a pressing roller configured to press the belt member against the heating 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 DRAWINGS 
         FIG. 1  is a schematic view of a general arrangement of an example of an image forming apparatus. 
         FIG. 2  is a cross-sectional view of a general arrangement of a fixing device according to Embodiment 1. 
       Part (a) of  FIG. 3  is a side view of the fixing device of Embodiment 1, as seen from the other side in a longitudinal direction thereof, and (b) is a side view of the fixing device of Embodiment 1, as seen from the one side in a longitudinal direction thereof. 
         FIG. 4  is a schematic view, as seen from a sheet discharging side of a pressing unit, of the fixing device according to Embodiment 1, and it is an illustration of a positioning structure and a play-removing-urging regulating structure for a pressing roller. 
         FIG. 5  is a schematic view, as seen from the sheet discharging side, of the pressing unit of the fixing device according to Embodiment 2, and it is an illustration of the play-removing-urging regulating structure for the pressing roller. 
       Part (a) of  FIG. 6  is an illustration of a displacement state of a pressing arm of the play-removing-urging regulating structure for the one longitudinal end portion side of the pressing roller in the pressing unit of the fixing device of Embodiment 2, (b) is an illustration of the displacement state of the pressing arm of the play-removing-urging regulating structure for the other longitudinal end portion side of the pressing roller. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiment 1 
     (1) Image Forming Apparatus: 
       FIG. 1  is a schematic view of the general arrangement of an example of the image forming apparatus which employs an image heating apparatus according to the present invention as the fixing device (fixing device). The image forming apparatus is a laser beam printer of an electrophotographic type. An image forming apparatus  100  according to this Embodiment 1 can be divided into an image forming station  101  for forming a toner image on a sheet S functioning as a recording material, and a fixing device  111  for fixing the toner image by a heated member pressing on the sheet S (image heating apparatus). The image forming station  101  is provided with devices described below. A charger  103  functioning as charging means is provided to the circumference of an outer surface (surface) an electrophotographic photosensitive member (photosensitive drum)  102  of a drum configuration functioning as an image bearing member. The surface of a photosensitive drum  102  is uniformly charged by the charger  103 . A laser beam L corresponding to an image is projected on the charged surface of the surface of the photosensitive drum  102  from an exposure device  104  functioning as exposure means, by which an electrostatic latent image is formed on the charged surface. The electrostatic latent image is developed with the toner by a developing device  105  functioning as developing means. By this, the toner image is formed on the surface of the photosensitive drum  102 . On the other hand, the sheets S stacked on a feeding cassette  107  below the image forming apparatus  100  are fed one by one by a feeding roller  108  and reach a registration roller pair  109  functioning as feeding means. Each sheet S is fed to a transfer nip portion between the photosensitive drum  102 , and a transfer roller  110  functioning as transferring means, by the registration roller pair  109  in synchronism with the toner image on the surface of the photosensitive drum  102 . The sheet is nipped and fed between the photosensitive drum  102  surface and the outer surface of the transfer roller  110  (surface) in the transfer nip portion. In the feeding process, the toner image on the surface of the photosensitive drum  102  is transferred electrostatically onto the sheet S by the transfer roller  110 . By this, the sheet S carries an unfixed toner image on the surface of the sheet S. The sheet S carrying the toner image is fed to a fixing device  111 . The toner image is heat-fixed on the sheet S in the fixing device  111  by imparting heat and the pressure to the unfixed toner image. The sheet S that has been heat-fixed is fed to a discharging roller pair  112  by the fixing device  111 . The sheet is discharged to a discharging tray  113  in an upper portion of the image forming apparatus  100  by the discharging roller pair  112 . Untransferred toner which remains on the photosensitive drum  102  surface after the transferring of the toner image is removed by a cleaning device  106 , which functions as a cleaning means. 
     (2) Fixing Device: 
     In the following descriptions, relating to the fixing device, and a member which constitutes at least a part of the fixing device, a longitudinal direction is the direction perpendicular to a sheet feeding direction (recording material feeding direction) in the plane of the sheet. A widthwise direction is the direction parallel with the sheet feeding direction (recording material feeding direction) in the surface of the sheet. The length is a dimension in the longitudinal direction. The width is the dimension in the widthwise direction.  FIG. 2  is a cross-sectional view of the general arrangement of the fixing device according to this Embodiment 1. The fixing device according to this Embodiment 1 is the fixing device of a belt nipping type, which is provided with a belt feeding device, wherein a fixing roller contacts a belt member to form a nip therebetween. The fixing device  111  is provided with a fixing unit  10 , and a pressing unit  20 . The fixing unit  10  is provided with the fixing roller  11  functioning as a heating member. For example, the fixing roller  11  is provided with a metal core  11   a  comprising an aluminum cylindrical tube of an outer diameter of 56 mm and an inner diameter of 50 mm. One longitudinal end portion of the metal core  11   a  and the other longitudinal end portion thereof are rotatably supported by a side plate (unshown) of the fixing unit  10 . The surface of the metal core  11   a  is coated with an elastic layer  11   b  of a silicone rubber which has the thickness of 2 mm and a hardness (Asker C) of 45 degree, for example, and the elastic layer  11   b  is further coated with a PF57 or PTFE heat resistive parting layer  11   c  functioning as a surface layer. A halogen heater  12  functioning as a heating source is provided in the metal core  11   a  of the fixing roller  11 . In this halogen heater  12  mouthpieces in the one longitudinal end portion of the halogen heater  12  and the other longitudinal end portion are supported by the side plate (unshown) of the fixing unit  10 . The pressing unit  20  is provided with an endless pressing belt  21 , functioning as a belt member. The pressing belt  21  is circulatably trained around a pressing roller  22  functioning as the rotatable member for the positioning, and a steering roller  23  with a predetermined tension (100 Ns, for example). The pressing roller  22  is made of a solid stainless steel, for example having an outer diameter of 20 mm. The steering roller  23  has a belt steering function and a function of applying a belt tension, and it is made of hollow stainless steel having an outer diameter of 20 mm, and an inner diameter of approx. 18 mm, for example. The steering roller  23  functions, as a steering member for adjusting the offsetting (snaking movement), in the longitudinal direction, of a pressing belt  21  and functions also as a belt stretching member for adjusting the tension of the pressing belt  21 . In the pressing unit  20 , the pressing belt  21  is provided, so that the outer surface (surface) of the pressing belt  21  opposes to the outer surface (surface) of the fixing roller  11  below the fixing roller  11 . It is trained around the pressing roller  22  and the steering roller  23  so as to incline angularly downwardly relative to the fixing roller  11 . The pressing roller  22  opposes the fixing roller  11  with the pressing belt  21  therebetween. The pressing springs  42 F,  42 R as will be described hereinafter, urge the pressing roller  22  toward the fixing roller  11  to contact a pressing belt  21  surface and a fixing roller  11  surface to each other. The urging forces of the pressing springs  42 F,  42 R are applied on the elastic layer  11   b  of the fixing roller  11  through the pressing belt  21  to elastically deform the elastic layer  11   b  by the predetermined amount, by which a part of a fixing nip is formed. The inside of the pressing belt  21  is provided with a pressing pad  24  made of the silicone rubber (pressing member), for example, which is extended along the longitudinal direction of the pressing belt  21 . A pressing pad  24  is pressed to an inner surface (inner surface) of the pressing belt  21  a pressing spring (unshown) with a predetermined pressure, for example 400 Ns and by pressing the pressing pad  24  against the inner surface of the pressing belt  21 , a contact area between the fixing roller  11  surface and the pressing belt  21  surface is increased with respect to the sheet feeding direction. By this, correspondingly to the contact area, a wide fixing nip N can be formed by the fixing roller  11  surface and the pressing belt  21  surface. By this, the nipping feeding time in which the sheet S carrying the unfixed toner image T is within the fixing nip N can be lengthened, so that the glossiness of the toner image T is increased, and the image formation speed is increased. 
     In the fixing device  111  according to this Embodiment 1, when a driving input gear  70  (part (a) of  FIG. 3 ) provided on an end surface of one longitudinal end portion of the fixing roller  11  is rotated by a fixing motor (unshown), the fixing roller  11  is rotated in the direction indicated by an arrow ( FIG. 2 ). A rotational force of this fixing roller  11  is transmitted to the pressing belt  21  through the fixing nip N to rotate the pressing belt  21  in the direction of the arrow. Electric power is applied from a power supplying circuit (unshown) to the halogen heater  12  to light up the halogen heater  12  to heat the fixing roller  11 . A temperature of the fixing roller  11  surface is detected by a temperature detecting member (unshown) such as a thermistor provided adjacent to a fixing roller  11  surface. A controller (unshown) comprising a CPU and a memory such as RAM and ROM receives an output signal of the temperature detecting member to control the feeder circuit based on the output signal, so that a surface temperature of the fixing roller  11  is maintained at a predetermined fixing temperature (target temperature). In the present Embodiment 1, the fixing temperature on the surface of the fixing roller  11  is maintained at approx. 190 degree C. In the state that the fixing roller  11  surface is maintained at the predetermined fixing temperature, the sheet S carrying the unfixed toner image T is introduced into the fixing nip N of the fixing device  111  with a toner image carrying surface facing up. The sheet S is nipped and fed by the fixing roller  11  surface and the pressing belt  21  surface in the fixing nip N. In the feeding process, by applying the heat of the fixing roller  11  and the pressure of the fixing nip N to the toner image T, the toner image T is heat-fixed on the sheet S surface. 
     (3) Pressing Belt Offset Controlling Mechanism: 
     Part (a) of  FIG. 3  is a side view of the fixing device according to this Embodiment 1, as seen from the other side in the longitudinal direction. Part (b) is a side view of the fixing device according to this Embodiment 1, as seen from the one side in the longitudinal direction. The pressing belt offset controlling mechanism (correcting means)  30  will be described. In the pressing belt offset controlling mechanism  30 , a metal core  22   a R of the pressing roller  22  and a metal core  23   a R of the steering roller  23  are supported by the steering roller supporting arm  31 R adjacent to the side plate ( 41 R) of the pressing unit  20 . Of the metal core  22   a R of the pressing roller  22  and the metal core  23  aR of the steering roller  23 , the metal core  23   a R of a pressing steering roller  23  is movable in the direction perpendicular to the longitudinal direction of the pressing belt  21  more particularly in an up-down direction shown in arrow B ( FIG. 2 ). A metal core  22   a F of the one longitudinal end portion of the pressing roller  22  is rotatably supported by the side plate  41 F in one longitudinal end portion of the fixing device  111  (Part (b) of  FIG. 3 ). The metal core  22   a R of the other longitudinal end portion of the pressing roller  22  is rotatably supported by the side plate  41 R in the other longitudinal end portion of the fixing device  111  (Part (a) of  FIG. 3 ). A metal core  23   a F provided in the one longitudinal end portion side of the pressing steering roller  23  is rotatably supported in the one longitudinal end portion of the fixing device  111  by the side plate  41 F and a steering roller supporting arm  31 F (Part (b) of  FIG. 3 ). The metal core  23   a R provided in the other longitudinal end portion side of the pressing steering roller is rotatably supported in the other longitudinal end portion of the fixing device  111  by the side plate  41 R and a steering roller supporting arm  31 R (Part (a) of  FIG. 3 ). The side plate ( 41 R) steering roller supporting arm  31 R supports a bearing  32 R slidably in a direction of the belt tension (Part (a) of  FIG. 3 ). The bearing  32 R is provided in a side plate  41 R, and rotatably supports the metal core  23   a R of the pressing steering roller  23 , so that it is movable in the up-down direction shown by the arrow B. The steering roller supporting arm  31 R holds a tension spring  33 R for urging the bearing  32 R in the direction of the belt tension to apply a constant tension thereto. The steering roller supporting arm  31 R is provided on an outer surface of the side plate  41 R, and a widthwise end portion opposite from the end which supports the bearing  32 R is rotatably supported on the side plate  41 R by an arm supporting shaft  34 R. To an outer periphery of the steering roller supporting arm  31 R, a sector gear  34  is fixed. The sector gear  34  engages a worm gear  36  provided on an output shaft of a belt offset control motor  35  supported by the side plate  41 R. The steering roller supporting arm  31 F in the side plate ( 41 F) supports a bearing  32 F (Part (b) of  FIG. 3 ) slidably in the direction of the belt tension. The bearing  32 F is provided in a side plate  41 F, and rotatably supports the metal core  23   a F of the pressing steering roller  23 , so that it cannot move in the up-down direction shown by the arrow B. The steering roller supporting arm  31 F holds a tension spring  33 F for urging the bearing  32 F in the direction of the belt tension to apply the predetermined tension to it. The steering roller supporting arm  31 F is fixed on the outer surface of the side plate  41 F. Therefore, of the steering roller supporting arms  31 F,  31 R, the steering roller supporting arm  31 R is swingable about the arm supporting shaft  34 R in the up-down direction shown in arrow B ( FIG. 2 ). For this reason, the pressing steering roller  23  can make a predetermined amount of the steering movement about the metal core  23   a F supported by the steering roller supporting arm  31 F, in the steering roller supporting arm ( 31 R) side in the up-down direction shown by the arrows B 1  and B 2  (Part (a) of  FIG. 3 ). In other words, the pressing steering roller  23  is inclinable in accordance with a swinging of the steering roller supporting arm  31 R about the metal core  23   a F, so that the metal core ( 23   a R) side moves to the up-down direction shown by the arrows B 1  and B 2  perpendicular to the sheet feeding direction through the predetermined amount. 
     The side plate  41 F of the pressing belt offset controlling mechanism  30  and the side plate of the fixing unit  10  corresponding to the side plate  41 F are connected with each other by the pressing spring  42 F (part (b) of  FIG. 3 ). The side plate  41 R of the pressing belt offset controlling mechanism  30  and the side plate of the fixing unit  10  corresponding to the side plate  41 R are connected with each other by a pressing spring  42 R (part (a) of  FIG. 3 ). As has been described in the foregoing, by the pressing springs  42 F,  42 R, the pressing roller  22  is urged and approaches the surface of the fixing roller  12 , by which the pressing belt  21  surface and the fixing roller  11  surface contact to each other to form a part of fixing nip N. 
     (4) Belt Offset Correcting Control for Pressing Belt Offset Controlling Mechanism: 
     Referring to  FIG. 3 , the belt offset correcting control of the pressing belt offset controlling mechanism  30  executed by the controller will be described. In part (a) of  FIG. 3 , the pressing belt  21  offsets toward the side plate ( 41 F) side, for example until the end position at one longitudinal end portion side of the pressing belt  21  is detected by the sensor (detection member,  FIG. 4 )  45 F as will be described hereinafter, and at this time the controller captures the output signal from a sensor  45 F. The output shaft of the belt offset control motor  35  is rotationally driven in a CW direction to move the sector gear  34  through the predetermined amount in the downward direction shown by an arrow B 2  (part (a) of  FIG. 3 ). The worm gear  36  rotates in accordance with a rotation of the output shaft of the belt offset control motor  35 , by which the steering roller supporting arm  31 R moves in the direction of the arrow B 2  with the sector gear  34 . In interrelation with a movement, with respect to the a direction of the arrow B 2 , of the steering roller supporting arm  31 R, a metal core ( 23   a F) side of the pressing steering roller  23  inclines in the direction of the arrow B 2 . With an inclination, in the direction of the arrow B 2 , of the pressing steering roller  23 , the pressing belt  21  starts the offset movement toward the side plate ( 41 R). When the pressing belt  21  offsets toward the side plate ( 41 R), so that the other longitudinal end portion of the pressing belt  21  is detected by the sensor (detection member,  FIG. 4 )  45 R as will be described hereinafter, the controller captures the output signal from a sensor  45 R. The output shaft of the belt offset control motor  35  is rotationally driven in a CCW direction to move the sector gear  34  in the upward direction shown by the arrow B 1  (part (a) of  FIG. 3 ) by the predetermined amount. The worm gear  36  rotates, in accordance with the rotation of the output shaft of the belt offset control motor  35 , by which the steering roller supporting arm  31 R moves in a direction of the arrow B 1  with the sector gear  34 . In interrelation with the movement, in the direction of the arrow B 1 , of the steering roller supporting arm  31 R, the metal core ( 23   a R) side inclines the pressing steering roller  23  in the direction of the arrow B 1 . It follows on the inclination, in the direction of the arrow B 1 , of the pressing steering roller  23 , the pressing belt  21  starts the offset movement to the side plate ( 41 F) side. The pressing belt  21  offsets and moves toward the side plate ( 41 F), and the one longitudinal end portion position of the pressing belt  21  is again detected by the sensor  45 F. Then, by the rotation, in the direction of the output shaft CW, of the belt offset control motor  35 , the pressing steering roller  23  is again inclined in the direction of the arrow B 2 . The pressing belt  21  offsets and moves toward the side plate ( 41 R), and the other longitudinal end portion position of the pressing belt  21  is again detected by the sensor  45 R. Then, by the rotation, in the direction CCW of the output shaft, of the belt offset control motor  35 , the pressing steering roller  23  is again inclined in the direction of the arrow B 1 . By repeatedly carrying out the above-described process, so that the belt does not protrude from the end surface of the one longitudinal end portion of the pressing steering roller  23 , and the other longitudinal end portion, the belt offset correcting control for correcting a position of the pressing belt  21  in the longitudinal direction is carried out. 
     (5) Positioning Structure and Play-Removing-Urging Regulating Structure of Pressing Roller: 
       FIG. 4  is the schematic view, as seen from the sheet discharging side, of the pressing unit of the fixing device according to this Embodiment 1, and it is an illustration of positioning structure for the pressing roller and the play-removing-urging regulating structure. First, the positioning structure for the pressing roller  22  will be described. The metal core  22   a R in the other longitudinal end portion of the pressing roller  22  is rotatably supported through a bearing  22 R by the side plate  41 R, and the metal core  22   a F in one longitudinal end portion of the pressing roller  22  is rotatably supported through a bearing  22 F by the side plate  41 F. The temperature of the pressing roller  22  rises up to about 150 degrees C. by the fixing roller  11  being heated. For this reason, as for the positioning, with respect to the longitudinal direction, of the pressing roller  22 , the metal cores  22   a F,  22   a R are supported by the side plates  41 F,  41 R, so that a play of 0.6 mm is provided between the side plates  41 F,  41 R and the metal cores  22   a F,  22   a R, in consideration of the thermal expansion, with respect to the longitudinal direction, of the pressing roller  22 . More particularly, the metal core  22   a F is provided with large diameter portions  22   a F 1 ,  22   a R 1  and small diameter portions  22   a F 2 ,  22   a R 2 , and the small diameter portion  22   a F 2  is supported by the bearing  22 F, so that a gap of 0.6 mm is provided between the large diameter portion  22   a F 1  and the bearing  22 F. Similarly, the metal core  22   a R is provided with a large diameter portion  22   a R 1  and a small diameter portion  22   a R 2 , and the small diameter portion  22   a R 2  is supported by the bearing  22 R, so that the play of 0.6 mm is provided between the large diameter portion  22   a R 1  and the bearing  22 R. By this, at the time of a rotation of the pressing belt  21 , the pressing roller  22  is rotated in the state that there is no play in one side of the metal cores  22   a F,  22   a R. During the rotation of the pressing roller for example, when the play-removing urging of the pressing roller is carried out on the metal core ( 22   a F) side, until the large diameter portion  22   a F 1  of the metal core  22   a F contacts to the bearing  22 F, the metal core ( 22   a F) side edge  22   b F of the pressing roller deviates from the position shown in  FIG. 4  by 0.6 mm toward the bearing ( 42 F). On the contrary, with this, during the rotation of the pressing roller, when the pressing roller is moved to remove the play with respect to the metal core ( 22   a R), until the large diameter portion  22   a R 1  of the metal core  22   a R contacts the bearing  22 R, a side edge  22   b R of the metal core ( 22   a R) of the pressing roller deviates from the position shown in  FIG. 4  by 0.6 mm toward the bearing ( 42 R). As for a roller length of the pressing roller  22  between the roller end surfaces  22   b F,  22   b R, a manufacturing error of ±0.5 mm arises. Therefore, in order to make the pressing belt  21  reciprocate, without protruding from the roller end surfaces  22   b F,  22   b R in the longitudinal direction of the pressing roller  22 , an end detection position X, with respect to the longitudinal direction, of the pressing belt  21  by sensors (detection members)  45 R and  45 F is as follows. The belt end detection position X satisfies following from the roller end surfaces  22   b F,  22   b R of a pressing roller  21 .
 
 X&gt; 0.6+0.5=1.1 (mm)
 
     In other words, with respect to the roller length of the roller portion of the pressing roller  22 , the pressing belt  22  reciprocates in the range which is the inside by 1.1 mm from the roller end surfaces  22   b F,  22   b R of the pressing roller  22  at the respective sides. Distances to the roller end surfaces  22   b F,  22   b R from the belt end are approx. 5 mm in the state that the pressing belt  21  is maintained in the predetermined position in the longitudinal direction of the pressing steering roller  23 , that is, at the time of a longitudinally central portion of the pressing belt  21  being placed in the longitudinally central portion of the pressing roller  22 . 
     A play-removing-urging regulating structure for the pressing roller  22  will be described. The inner surface of the side plate  41 F is provided with a supporting shaft  43 F which projects toward the side plate ( 41 R) along an axial direction of the pressing roller  22 . The supporting shaft  43 F supports a sensor base (supporting member)  44 F displaceably in the longitudinal direction of the pressing roller  22 . A sensor base  44 F contacts a roller end surface  22   a F of the pressing roller  22  at the side plate  41 F side. The inner surface of the sensor base  44 F is provided with the sensor  45 F for detecting the one longitudinal end portion position of the pressing belt  21 . Between a sensor base  44 F and bearing  22 F, a coil spring (urging member)  46 F is compressed. The coil spring  46 F urges the sensor base  44 F to the roller end surface  22   b F of the pressing roller  22 , by which a large diameter portion  22   a F of the metal core  22   a F of the pressing roller  22  is spaced by an amount of the play from the bearing  22 F. Similarly, the inner surface of the side plate  41 R is provided with a supporting shaft  43 R which projects toward the side plate ( 41 F) along the axial direction which is the pressing roller  22 . The supporting shaft  43 R supports the sensor base (supporting member)  44 R displaceably in the longitudinal direction of the pressing roller  22 . A sensor base  44 R contacts a roller end surface  22   a R of the pressing roller  22  of the side plate  41 R. The inner surface of the sensor base  44 R is provided with the sensor  45 R for detecting the other longitudinal end portion widthwise end position disposition of the pressing belt  21 . Between a sensor base  44 R and the bearing  22 R, the coil spring (urging member)  46 R is compressed. The coil spring  46 R urges the sensor base  44 R to a roller end surface  22   b R of the pressing roller  22 , by which the large diameter portion  22   a R 1  of the metal core  22   a R of the pressing roller  22  is spaced by the amount of the play from the bearing  22 R. By this, the roller end surfaces  22   a F,  22   a R of the pressing roller  22  rotate to the state of contacting sensor bases  44 F,  44 R, and therefore, the pressing roller  22  is not moved to remove the play to the metal core ( 22   a F) side or the metal core ( 22   a R) side. More particularly, the play-removing urging of the pressing roller  22  can be regulated. For this reason, irrespective of the play of the pressing roller and the error in the manufacturing of the roller length of the pressing roller, distances of 1.1 mm from an end surface position, in the longitudinal direction, of the pressing roller, i.e., the positions of the roller end surfaces  22   a F,  22   a R, to the belt end detection position X, X by the sensors  45 F,  45 R can be maintained constant. Therefore, even if the offset correcting control of the pressing belt  21  is carried out, by inclining the pressing steering roller  23 , the distance between the belt end detection positions X, X by the sensors  45 F and  45 R and the positions of the roller end surfaces  22   a F,  22   a R of the pressing roller  22  are always maintained at the constant 1.1 mm. In other words, the distance between the belt end detection positions X, X by the sensors  45 F,  45 R and the positions of the roller end surfaces  22   a F,  22   a R of the pressing roller  22  is always maintained at the constant 1.1 mm, irrespective of the inclination of the pressing steering roller  23 . By this, in the case where a reciprocation range of the pressing belt  21  for carrying out the offset correcting control of the pressing belt  21  is set, it is not necessary to consider the play of the pressing roller and the error on the manufacturing of the roller length of the pressing roller, and therefore, the large reciprocation range of the pressing belt  21  can be selected. By this, it becomes unnecessary that the roller length of the roller portion of the pressing roller  22  is long, and therefore downsizing of the fixing device  111  is accomplished. 
     Embodiment 2 
     Other examples of the fixing device will be described. The fixing device shown in the present Embodiment 2 is the same as the fixing device  111  of Embodiment 1 except for the play-removing-urging regulating structure for the pressing roller  22 . In the present Embodiment 2, the like reference numerals as in the foregoing embodiment are assigned to the elements having the corresponding functions. The play-removing-urging regulating structure will be described for the pressing roller  22  of the fixing device  111  according to this Embodiment 2.  FIG. 5  is the schematic view, as seen from the sheet discharging side, of the pressing unit of the fixing device according to this Embodiment 2, and, and it is an illustration of the play-removing-urging regulating structure of the pressing roller. Part (a) of  FIG. 6  is an illustration showing a displacement state of a pressing arm of the play-removing-urging regulating structure for the one longitudinal end portion side of the pressing roller, and part (b) of  FIG. 6  is an illustration showing a displacement state of a pressing arm of the play-removing-urging regulating structure for the other longitudinal end portion side of the pressing roller. There is provided a sensor base  51 F adjacent to the outer surface of the one longitudinal end portion side metal core  22   a F of the pressing roller  22 . The sensor base  51 F supports the pressing arm (member-to-be-detected)  52 F which is an elongated member rotatable about a rotation axis  53 F in the substantial center portion of a pressing arm  52 F. A coil spring (urging member)  54 F is hooked between an engaging portion  52 Fa provided in the pressing arm  52 F and an engaging portion  51 Fa provided in the sensor base  51 F. By the coil spring  54 F a pressing portion  52 Fb provided in the pressing arm  52 F is contacted to the roller end surface  22   b F of the one longitudinal end portion side of the pressing roller  22  to urge the roller end surface  22   b F to the other longitudinal end portion side of the pressing roller  22 . By this, the large diameter portion  22   a F 1  of the metal core  22   a F of the pressing roller  22  is spaced by the amount of play of 0.6 mm described above from the bearing  22 F. The sensor base  51 F is provided with a sensor (detection member)  55 F for detecting the position of the one longitudinal end portion of the pressing belt  21  through the pressing arm  52 F. The sensor  55 F is disposed opposed to the pressing arm  52 F in the longitudinal direction of the pressing belt  21  and is provided at a position spaced by the predetermined distance a from the pressing arm  52 F. Similarly, there is provided a sensor base  51 R adjacent to the outer surface of the metal core  22   a R by the other longitudinal end portion side of the pressing roller  22 . The sensor base  51 R supports the pressing arm (member-to-be-detected)  52 R which is an elongated member rotatable about the rotation axis  53 R in the substantial center portion of a pressing arm  52 R. A coil spring (urging member)  54 R is hooked between an engaging portion  52 Ra provided in the pressing arm  52 R and an engaging portion  51 Ra provided in the sensor base  51 R. A pressing portion  52 Rb provided in the pressing arm  52 R is contacted to the other longitudinal end portion side edge  22   b R of the pressing roller  22  by the coil spring  54 R to urge the roller end surface  22   b R to the one longitudinal end portion side of the pressing roller  22 . By this, a large diameter portion  22   a R 1  of the metal core  22   a R of the pressing roller  22  is spaced by the amount of play of 0.6 mm described above from the bearing  22 R. The sensor base  51 R is provided with the sensor for detecting the one longitudinal end portion position of the pressing belt  21  through the pressing arm  52 R (detection member)  55 R. The sensor  55 R opposes to the pressing arm  52 R in the longitudinal direction of the pressing belt  21 . 
     In the fixing device according to this Embodiment 2, when the pressing belt  21  offsets to the side plate ( 41 F) side, for example to contact the pressing portion  52 Fb of the pressing arm  52 F (Part (a) of  FIG. 6 ), the pressing arm  52 F is rotated to be displaced in the direction of the arrow about a rotation axis  53 F against the urging force of the coil spring  54 F. By this, the distance between the pressing arm  52 F and the sensor  55 F is increased to β from α. The sensor  55 F detects a rotation amount of the pressing arm  52 F to output a signal corresponding to the rotation amount to the controller as a detection signal of the end position of the one longitudinal end portion side of the pressing belt  21 . The controller inclines the metal core ( 23   a F) side of the pressing steering roller  23  in the predetermined direction (the direction of the arrow B 2  shown in part (a) of  FIG. 3 ), on the basis of the output signal of the sensor  55 F, by which the pressing belt  21  begins to offset and move to the side plate ( 41 R) side. When the pressing belt  21  offsets and contacts the side plate ( 41 R) side to contact to the pressing portion  52 Rb of the pressing arm  52 R (Part (b) of  FIG. 6 ), the pressing arm  52 R is rotated in the direction of the arrow about a rotation axis  53 R against the urging force of the coil spring  54 R (displacement). By this, the distance between the pressing arm  52 R and the sensor  55 R is increased to β from α. The sensor  55 R detects the rotation amount of the pressing arm  52 R to output the signal corresponding to the rotation amount to the controller as the detection signal of the other longitudinal end portion side end position of the pressing belt  21 . The controller inclines the metal core ( 23   a R) side of the pressing steering roller  23  in the predetermined direction (the direction of the arrow B 1  shown in part (a) of  FIG. 3 ), on the basis of the output signal from the sensor  55 R, by which the pressing belt  21  begins to offset and move toward the side plate ( 41 F). Similarly to the fixing device  111  of Embodiment 1, by repeatedly carrying out the above-described process, the belt offset correcting control for correcting the position of the pressing belt  21  in the longitudinal direction is carried out, so that the pressing belt  21  does not protrude from the end surface of the one longitudinal end portion or the other longitudinal end portion of the pressing steering roller  23 . 
     In the fixing device according to this Embodiment 2, the pressing roller  22  is rotated to the state that the roller end surfaces  22   a F,  22   a R of the pressing roller  22  contact the pressing portions  52 Fb,  52 Rb of the pressing arms  52 F,  52 R. For this reason, during the rotation of the pressing roller  22 , it does not move to remove the play to the metal core ( 22   a F) or metal core ( 22   a R) side. More particularly, the play-removing urging of the pressing roller  22  can be regulated. By this, the distances a between the positions of the pressing arms  52 F,  52 R contacted to the roller end surfaces and the sensors  55 F,  55 R can be maintained constant irrespective of the play of the pressing roller or the manufacturing error in the roller length of the pressing roller. Therefore, the distance between the positions of the pressing arms  52 F,  52 R contacted to the roller end surface and the sensors  55 F,  55 R is always maintained at the constant amount a even if the offset correcting control of the pressing belt  21  is carried out by inclining the pressing steering roller  23 . In other words, the distance between the positions of the pressing arms  52 F,  52 R contacted to the roller end surface and the sensors  55 F,  55 R is always maintained at the constant amount a independently of the inclination of the pressing steering roller  23 . By this, in the case where the reciprocation range of the pressing belt  21  for carrying out the offset correcting control of the pressing belt  21  is set, it is not necessary that the play of the pressing roller and the error on the manufacturing in the roller length of the pressing roller are taken into account, and therefore, the large reciprocation range of the pressing belt  21  can be selected. By this, it becomes unnecessary that the roller length of the roller portion of the pressing roller  22  is long, and therefore, the downsizing of the fixing device  111  is accomplished. 
     Other Embodiments 
     1) In the fixing devices of Embodiment 1 and Embodiment 2, the pressing belt is trained around the pressing roller and the pressing steering roller, but the pressing belt may be trained around the three or more rollers. 
     2) In place of the fixing roller, a fixing unit may be used, which is provided with a fixing belt (belt member), a plurality of rollers (fixing roller and fixing steering roller, for example) supporting the fixing belt, and the heating source for heating the fixing belt. The present invention can provide the image heating apparatus and the belt feeding device used for the image heating apparatus even if a steering roller is inclined, and the end of the belt member can be detected at the constant position relative to the position of the rotatable member for the positioning, and therefore, the downsizing of an apparatus itself is accomplished. 
     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 modification 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. 175201/2009 filed Jul. 28, 2009 which is hereby incorporated by reference.