Patent Publication Number: US-10322893-B2

Title: Image forming apparatus

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus including a belt feeding device for feeding an endless belt stretched by a plurality of stretching rollers. 
     Conventionally, in the image forming apparatus of an electrophotographic type or an electrostatic recording type, the belt feeding device including the endless belt stretched by the plurality of stretching rollers is used. The belt is used as a feeding member for carrying and feeding a toner image or carrying and feeding a recording material on which the toner image is formed. As the feeding member for carrying and feeding the toner image, a belt-shaped electrophotographic photosensitive member (photosensitive belt), an intermediary transfer member (intermediary transfer belt) for carrying and feeding the toner image in order to transfer the toner image from the photosensitive member onto the recording material, and the like member are used. Further, as the feeding member for carrying and feeding the recording material on which the toner image is formed, a recording material feeding member (recording material feeding belt) for carrying and feeding the recording material onto which the toner image is transferred from the photosensitive member is used. 
     In such a belt feeding device, a problem of “belt shift” such that the belt shifts toward an end portion side of either of the stretching rollers with respect to a rotational axis direction during feeding (traveling) of the belt due to accuracy of outer diameters of the stretching roller, accuracy of relative alignment between the respective stretching rollers, and the like has been known. 
     Of means for solving the problem of the belt shift, a center alignment mechanism constituted so as to realize steering of a steering roller with a relatively inexpensive constitution in which an electrical part such as a sensor or an actuator is not needed has been proposed (Japanese Laid-Open Patent Application 2014-130181). In this mechanism, a force for swinging (tilting) the steering belt by a sliding member slidable with an inner peripheral surface of a moving belt at each of end portions with respect to a widthwise direction of the belt is imparted to the steering roller, so that center alignment (adjustment of a belt feeding position with respect to a widthwise direction) of the belt is carried out. 
     However, in the case where lifetime extension or cost reduction of the belt are promoted, the following problem occurs in some cases. 
     For example, in the case of a constitution in which center alignment of the belt is carried out by sliding between the widthwise end portion of the belt and the sliding member as described above, a load is exerted on the widthwise end portion of the belt. In addition, it turned out that depending on an arrangement of the end portion of the stretching roller with respect to the rotational axis direction, the load concentrates at the widthwise end portion of the belt in some cases (stress concentration). This is particularly conspicuous in the case where end (portion) positions of the respective stretching rollers are aligned. 
     This stress concentration loads to fatigue failure (crack) of the belt in some cases when an amount of repetitive use of the belt increases, and is a problem when the lifetime extension is promoted. Further, in the case where a thickness of the belt is made thin for reducing the cost of the belt or the like, rigidity of the belt lowers, and therefore, loads to the above-described fatigue failure earlier in some cases. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide an image forming apparatus capable of alleviating concentration of a load at a widthwise end portion of an endless belt in a constitution including the endless belt stretched by a plurality of states. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising: an image forming portion configured to form a toner image; an endless belt which is provided opposed to the image forming portion and onto which the toner image formed on the image forming portion is transferred; and a plurality of stretching rollers configured to stretch the belt; the stretching rollers including, a tiltable steering roller configured to adjust a position of the belt with respect to a widthwise direction; a sliding member provided non-rotatably at a position adjacent to each of ends of the stretching roller with respect to a rotational axis direction, the sliding member being tiltable together with the steering roller and slidable with an inner surface of the belt; a first stretching roller having a smallest diameter of diameters of the stretching rollers excluding the steering roller, and a second stretching roller having a largest diameter of the diameters of the stretching rollers excluding the steering roller, wherein with respect to rotational axis directions of the stretching rollers, lengths of contact portions of the stretching rollers with the belt are shorter than a width of the belt, the length of the contact portion of the first stretching roller with the belt is a first length, the length of the contact portion of the second stretching roller with the belt is a second length shorter than the first length, and end positions of the contact portions of the first and second stretching rollers with respect to the rotational axis direction are different from each other. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus. 
     
    
    
     In  FIG. 2 , (a) and (b) are perspective views of an intermediary transfer belt unit. 
       FIG. 3  is a perspective view of an automatic belt center alignment mechanism portion. 
       FIG. 4  is an enlarged perspective view of the automatic belt center alignment mechanism portion at an end portion. 
     In  FIG. 5 , (a) and (b) are schematic views for illustrating an operating principle of the automatic belt center alignment mechanism portion. 
       FIG. 6  is a schematic view for illustrating width and diameter relationships among stretching rollers. 
       FIG. 7  is a schematic sectional view showing an intermediary transfer belt unit in another embodiment. 
     DESCRIPTION OF THE EMBODIMENTS 
     An image forming apparatus according to the present invention will be described with reference to the drawings. 
     Embodiment 1 
     1. General Structure and Operation of Image Forming Apparatus 
       FIG. 1  is a schematic sectional view of an image forming apparatus  100  in this embodiment. 
     The image forming apparatus  100  in this embodiment is a tandem-type multi-function machine which is capable of forming a full-color image using an electrophotographic type and which employs an intermediary transfer type. The multi-function machine has functions of a copying machine, a printer and a facsimile machine. 
     The image forming apparatus  100  includes, as a plurality of image forming portions (stations), four image forming portions (stations) PY, PM, PC and PK for forming images of color of yellow (Y), magenta (M), cyan (C) and black (K), respectively. These four image forming portions PY, PM, PC and PK are provided and arranged along a movement direction of an intermediary transfer belt  1  described later. Incidentally, as regards elements having the same or corresponding constitutions in the respective image forming portions PY, PM, PC and PK, suffixes Y, M, C and K for representing the elements for associated colors, respectively, are omitted, and the elements will be collectively described in some cases. In this embodiment, each image forming portion P is constituted by including a photosensitive drum  11 , a charging roller  12 , an exposure device  13 , a developing device  14 , a primary transfer roller  15  and a drum cleaning device  16 , which are described below. 
     The image forming apparatus  100  includes, as a first image bearing member, the photosensitive drum  11  which is a drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member). The photosensitive drum  11  is rotationally driven in an arrow R 1  direction (clockwise direction) in  FIG. 1 . A surface of the rotating photosensitive drum  11  is electrically charged uniformly to a predetermined polarity (negative in this embodiment) and a predetermined potential by the charging roller  12  which is a roller-shaped charging member as a charging means. The uniformly charged surface of the photosensitive drum  11  is subjected to scanning exposure to laser light modulated depending on an image signal by the exposure device (laser beam scanner)  13  as an exposure means. As a result, an electrostatic latent image (electrostatic image) is formed on the surface of the photosensitive drum  11 . 
     The electrostatic latent image formed on the photosensitive drum  11  is developed (visualized) with toner by the developing device  14  as a developing means. As a result, a visualized image with the toner, i.e., a toner image is formed on the surface of the photosensitive drum  11 . In this embodiment, on an exposed portion of the photosensitive drum  11  where an absolute value of the potential is lowered by the exposure to light after the uniform charging of the photosensitive drum surface, the toner charged to the same polarity as the charge polarity (negative in this embodiment) of the photosensitive drum  11  is deposited (reverse development). In the developing device  14 , a two-component developer in which toner (non-magnetic toner particles) and a carrier (magnetic carrier particles) are mixed is carried on a developing sleeve as a developer carrying member and is fed to an opposing portion (developing portion) to the photosensitive drum  11 . Then, depending on the electrostatic latent image on the photosensitive drum  11 , the developing sleeve supplies the toner of the two-component developer to the photosensitive drum  11 . 
     An intermediary transfer belt unit  20  as a belt feeding device is provided so as to oppose the respective photosensitive drums  11  of the respective image forming portions. The intermediary transfer belt unit  20  includes the intermediary transfer belt  1  which is an intermediary transfer member constituted by an endless belt as a second image bearing member. The intermediary transfer belt  1  is extended around, as a plurality of stretching rollers, a driving roller  2 , a steering roller  3 , an idler roller  4  and an upstream secondary transfer roller (hereinafter referred to as an upstream roller)  5 , and is stretched by these stretching rollers with a predetermined tension. The intermediary transfer belt  1  is rotated (circulated and moved) in an arrow R 2  direction (counterclockwise direction) in  FIG. 1  is rotational drive of the driving roller  2  in an arrow R 3  direction (counterclockwise direction) in  FIG. 1 . In an inner peripheral surface side of the intermediary transfer belt  1 , a primary transfer rollers  15  which are roller-shaped primary transfer members as primary transfer means are provided correspondingly to the photosensitive drums  11 . Each of the primary transfer rollers  15  is urged (pressed) against the intermediary transfer belt  1  toward the associated photosensitive drum  11 . As a result, a primary transfer portion T 1  where the photosensitive drum  11  and the intermediary transfer belt  1  are in contact with each other. 
     The toner images formed on the photosensitive drums  11  as described above are transferred (primary-transferred) onto the intermediary transfer belt  1  at the primary transfer portions T 1  under application of a predetermined pressure and an electrostatic load bias by the primary transfer rollers  15 . For example, during full-color image formation, the toner images of four colors of yellow, magenta, cyan and black formed on the respective photosensitive drums  11  are successively transferred superposedly onto the intermediary transfer belt  1 . 
     In an outer peripheral surface side of the intermediary transfer belt  1 , at a position opposing the driving roller (inner secondary transfer roller)  2 , a secondary transfer roller (outer secondary transfer roller)  17  which is a roller-shaped secondary transfer member as a secondary transfer means is provided. The secondary transfer roller  17  is urged (pressed) against the intermediary transfer belt  1  toward the driving roller  2 , and forms a secondary transfer portion T 2  where the intermediary transfer belt  1  and the secondary transfer roller  17  are in contact with each other. 
     The toner images formed on the intermediary transfer belt  1  as described above are transferred (secondary-transferred) at the secondary transfer portion T 2  onto a recording material S such as paper fed to the secondary transfer portion T 2  under application of a predetermined pressure and an electrostatic load bias by the secondary transfer roller  17 . The recording material S is stacked and accommodated in an accommodating portion  41 , and is fed in synchronism with image formation timing by a feeding roller pair  42  employing a friction separation type. The recording material S fed by the feeding roller pair  42  passes through a feeding pass and is fed to a registration roller pair  43 . The recording material S is subjected to oblique movement correction and timing correction by the registration roller pair  43  and thereafter is sent to the secondary transfer portion T 2 . At the secondary transfer portion T 2 , the toner images are transferred as described above onto the recording material S nipped and fed by the intermediary transfer belt  1  and the secondary transfer roller  17 . 
     The recording material S on which the toner images are transferred is fed to a fixing device  18  as a fixing means, in which the toner images are heated and pressed and thus is fixed (melt-fixed) on the recording material S, and thereafter, the recording material S is discharged (outputted) onto a discharge tray  19  provided at an outer portion of an apparatus main assembly  110  of the image forming apparatus  100 . 
     Further, toner (primary transfer residual toner) remaining on the surface of each of the photosensitive drums  11  after the primary transfer is removed and collected from the surface of the photosensitive drum  11  by the drum cleaning device  16  as a photosensitive member cleaning means. Further, toner (secondary transfer residual toner) remaining on the of the intermediary transfer belt  1  after the secondary transfer is removed and collected from the surface of the intermediary transfer belt  1  by a belt cleaning device  29  as an intermediary transfer member cleaning means. 
     In this embodiment, each of the image forming portions P constitutes a toner image forming means for forming the toner image on the intermediary transfer belt  1 . 
     2. Intermediary Transfer Belt Unit 
     The intermediary transfer belt unit  20  as the belt feeding device in this embodiment will be further described. Incidentally, with regard to the image forming apparatus  100  and elements thereof, a front side on the drawing sheet of  FIG. 1  is referred to as a “front” side (surface), and a rear side on the drawing sheet f  FIG. 1  is referred to as a “rear” side (surface). This front-rear direction is substantially parallel to a rotational axis direction (longitudinal direction) of the photosensitive drum  1 . 
     In  FIG. 2 , (a) and (b) are perspective views of the intermediary transfer belt unit  20 , in which a rear-side side surface is shown so as to be the front side on the drawing sheet. In  FIG. 2 , (a) shows a toner in which the intermediary transfer belt  1  is stretched, and (b) shows a state in which the intermediary transfer belt  1  is demounted from the tiltable unit  20 . 
     As shown in (a) and (b) of  FIG. 2 , each of the driving roller  2 , the upstream roller  5  and the idler roller  4  is rotatably shaft-supported at end portions with respect to the rotational axis direction in the form such that the roller is sandwiched between a front frame  21 F and a rear frame  21 R. The steering roller  3  is, as described specifically later, supported by a frame stay  28  via a swingable plate  26  and is rotatable. The frame stay  28  is extended between the front frame  21 F and the rear frame  21 R and is fixed at one longitudinal end portion of each of the front frame  21 F and the rear frame  21 R. As described specifically later, the steering roller  3  is urged by an urging means from the inner peripheral surface side toward the outer peripheral surface side of the intermediary transfer belt  1  and also functions as a tension roller for imparting tension to the intermediary transfer belt  1 . 
     At one end portion of the driving roller  2  with respect to the rotational axis direction, a driving coupling  22  is mounted. To the driving coupling  22 , a driving force is transmitted by connecting the driving coupling  22  with an output shaft of a belt driving unit (not shown). The driving roller  2  has a surface constituted by a material, such as a rubber, relatively high in friction coefficient, and the surface thereof friction-engages with the inner peripheral surface of the intermediary transfer belt  1 . Then, the driving force is transmitted from the belt driving unit to the driving roller  2 , so that the driving roller  2  feeds (rotates) the intermediary transfer belt  1  in the arrow R 2  direction in (a) of  FIG. 2 . In this embodiment, the driving coupling  22  is used as a drive transmitting means, but the driving roller  2  may also be connected with the belt driving unit by using a gear. 
     In this embodiment, the intermediary transfer belt unit  20  includes an automatic belt center alignment mechanism portion  10  for carrying out center alignment (adjustment of a belt feeding position with respect to a widthwise direction) of the intermediary transfer belt  1  fed as described above. In this embodiment, roughly, the automatic belt center alignment mechanism portion  10  carries out the center alignment of the intermediary transfer belt  1  by self-maintaining a balance of a frictional force between end portions of the steering roller  3  with respect to the rotational axis direction. 
     3. Automatic Belt Center Alignment Mechanism Portion 
     The automatic belt center alignment mechanism portion  10  will be described with reference to  FIGS. 3 and 4 .  FIG. 3  is a perspective view of the automatic belt center alignment mechanism portion  10  in this embodiment, and  FIG. 4  is an enlarged perspective view of the automatic belt center alignment mechanism portion  10  in the neighborhood of a rear-side end portion. 
     As shown in  FIGS. 3 and 4 , the steering roller  3  includes a roller shaft  3   a  projecting from each of ends with respect to the rotational axis direction thereof. At positions opposing the end portions of the steering roller  3  with respect to the rotational axis direction, sliding members  23  are provided. The roller shafts  3   a  are rotatably shaft-supported by the sliding members  23  at the end portions in the form of being engaged and inserted in supporting holes  23   a  provided in the sliding members  23 . These pair of sliding members  23  are mounted to the swingable plate  26  so as to support the end portions of the steering roller  3  with respect to the rotational axis direction. That is, each of the sliding members  23  disposed adjacently to the end portions of the steering roller  3  with respect to the rotational axis direction is supported by a slide guide  24  provided to the swingable plate  26  at an associated one of longitudinal end portions. Between the sliding member  23  and the slide guide  24 , a tension spring  25  which is a compression spring is provided in a compressed state. 
     As shown in  FIG. 4 , the slide guide  24  includes an engaging groove  24   a  for guiding the associated sliding member  23  only along a pressing (urging) direction (an arrow K direction) of the tension spring  25 . That is, the slide guides  24  constitute a guiding portion for guiding the pair of sliding members  23  in the urging direction of the tension spring  25 . Further, the sliding guide  24  includes a stopper (not shown) with respect to the pressing direction of the tension spring  25  so that the sliding member  23  is not disengaged in an assembly state ( FIG. 3 ) of the automatic belt center alignment mechanism portion  10  alone. By these constitutions, the urging force of the tension springs  25  disposed at the longitudinal end portions of the swingable plate  26  can be effectively transmitted to the corresponding sliding members  23 . 
     In a state in which the intermediary transfer belt  1  is stretched by the driving roller  2 , the steering roller  3 , the upstream roller  5  and the idler roller  4  ((a) of  FIG. 2 ), the sliding member  23  moves from a position, where the sliding member  23  is regulated by the stopper, in a direction of compressing the tension spring  25  along an arrow K direction in  FIG. 4 . Thus, the tension spring  25  urges the steering roller  3  via the sliding member  23 , and imparts predetermined tension to the intermediary transfer belt  1 . In this embodiment, by this constitution, as described above, the steering roller  3  also functions as the tension roller. 
     The swingable plate  26  constitutes a swingable member (tiltable member) for swingably (tiltably) supporting the steering roller  3  so as to be capable of changing relative alignment with the driving roller  2 . Further, the tension spring  25  constitutes an urging member which is an urging means for applying a tension force, actable on the inner peripheral surface of the intermediary transfer belt  1 , to the steering roller  3 . In this embodiment, the tension springs  25  comprise a pair of compression springs (spring members) for applying the tension force to the pair of sliding members  23 , respectively, at the longitudinal end portions of the swingable plate  26 . 
     As shown in  FIG. 3 , at a longitudinal central portion of the swingable plate  26 , a rotation shaft member  27  is fixed in a state in which the rotation shaft member  27  projects in a side opposite from a side where the steering roller  3  is provided. Further, at the longitudinal end portions of the swingable plate  26 , the slide guides  24  are fixed, respectively. The rotation shaft member  27  is engaged with an engaging portion (not shown) provided on the above-described frame stay  28  ((b) of  FIG. 2  and  FIG. 4 ), so that the rotation shaft member  27  rotatably (swingably) supports the swingable plate  26  supporting the steering roller  3 . That is, the automatic belt center alignment mechanism portion  10  is supported by the frame stay  28  so as to be rotatable in an arrow Ro direction in  FIG. 3  with respect to a steering axis J passing through the rotation shaft member  27  provided at the longitudinal central portion of the swingable plate  26 . Further, the automatic belt center alignment mechanism portion  10  is constituted as a steering roller supporting unit for supporting the steering roller  3 . 
     As shown in  FIG. 4 , the pair of sliding members  23  includes sliding surfaces  23   b  slidable with the inner peripheral surface of the moving intermediary transfer belt  1 . As described specifically later, by sliding between the inner peripheral surface of the intermediary transfer belt  1  and the sliding surfaces  23   b , a force for changing relative alignment between the steering roller  3  and the driving roller  2  for the purpose of carrying cut the center alignment of the intermediary transfer belt  1  is applied to the steering roller  3 . Each of the sliding surfaces  23   b  is formed in a tapered shape such that a distance thereof from the outer peripheral surface of the steering roller  3  with respect to a radial direction gradually increases from a central portion side toward an end portion side of the steering roller  3  with respect to the rotational axis direction. As a result, a function of automatically carrying out the center alignment of the intermediary transfer belt  1  can be further enhanced. 
     In this embodiment, an outer diameter of the steering roller  3  is set at φ 16  (16 mm), for example. The sliding surface  23   b  of the sliding member  23  has a curved surface portion having a curved shape of φ 16  in outer diameter equal to the outer diameter of the steering roller  3 , at the end portion in a side where the sliding surface  23   b  is adjacent to the end portion of the steering roller  3  with respect to the rotational axis direction. Further, the sliding surface  23   b  has a shape such that a diameter thereof gradually increases from the side adjacent to the end portion of the steering roller  3  toward the end portion thereof with respect to the rotational axis direction with a ratio of 10° in taper angle Ψ increasing from the curved surface portion of φ 16  ((b) of  FIG. 5 ). 
     In this embodiment, a width (length with respect to a direction substantially perpendicular to the feeding direction shown by the arrow R 2  in  FIG. 5 ) of the intermediary transfer belt  1  is set so as to partly extend to a region of the sliding surface  23   b  having the taper angle Ψ. The sliding member  23  has the degree of freedom through the sliding guide  24  only with respect to the arrow K direction in  FIG. 4  as described above. For that reason, when the intermediary transfer belt  1  is fed (rotated), the sliding member  23  simply slides with the inner peripheral surface of the intermediary transfer belt  1  without being rotated by the rotation of the intermediary transfer belt  1 . Incidentally, the sliding member  23  may also have play with respect to the belt rotational direction. 
     An operational principle of the automatic belt center alignment mechanism portion  10  will be described with reference to  FIGS. 4 and 5 . In  FIG. 5 , (a) and (b) are plan views (top views) showing a part of the intermediary transfer belt unit in a state as seen in an arrow TV direction in  FIG. 2 , in which (a) shows a steady state of balance by the automatic center alignment that a winding position of the intermediary transfer belt  1  is a nominal (center) position, and (b) shows a state in which when the intermediary transfer belt  1  is fed, the intermediary transfer belt  1  caused belt shift toward a left side on the drawing sheet. 
     As described above, the sliding members  23  are supported so as not to be rotated by the intermediary transfer belt  1 . Further, the sliding members  23  is always subjected to frictional resistance from the inner peripheral surface of the intermediary transfer belt  1  when the intermediary transfer belt  1  is fed. That is, in this embodiment, a dimensional relationship among the intermediary transfer belt  1 , the steering roller and the sliding members  23  is as follows. As shown in (a) of  FIG. 5 , a width of the intermediary transfer belt  1  is Lb. Further, a length (width) of a contact portion of the steering roller  3  with the intermediary transfer belt  1  with respect to the rotational axis direction is Lr. Further, a width of the sliding surface  23   b  of the sliding member  23  with respect to the rotational axis direction of the steering roller  3  is Lf. At this time, in this embodiment, Lb is set so as to be longer than Lr and so as to shorter than Lr+2Lf (width between outer ends of the sliding surfaces  23   b  of the sliding members  23  with respect to the rotational axis direction of the steering roller  23 ). 
     In the state in which the intermediary transfer belt  1  is in the nominal (center) position shown in (a) of  FIG. 5 , the intermediary transfer belt  1  always slides with both of the sliding members  23  with a predetermined winding width (e.g., 2 mm in this embodiment). Thus, the intermediary transfer belt  1  and the pair of sliding members  23  provide the following positional relationship in a state in which the intermediary transfer belt  1  is uniformly positioned on the basis of the center of the steering roller  3  with respect to the rotational axis direction. That is, the positional relationship is such that both of the end portions of the intermediary transfer belt  1  with respect to a widthwise direction partly cover the associated sliding surfaces  23   b  of the pair of sliding members  23 . Accordingly, at least one of the widthwise end portions of the intermediary transfer belt  1  always contacts the sliding member(s)  23  irrespective of a widthwise feeding position of the intermediary transfer belt  1 . 
     On the other hand, in a state in which the belt shift generated as shown in (b) of  FIG. 5 , a winding width relationship between the intermediary transfer belt  1  and the sliding member  23  in a state in which the winding width is localized as being a winding width D only in a left side on the drawing sheet. In this state, the sliding members  23  receive a force of F(ST)×D in the left side and a force of 0 in the right side in a downward direction (toward the rear side on the drawing sheet of  FIG. 5 ) along an arrow ST direction in  FIG. 4 . A difference in frictional forces of the pair of sliding members  23  in both sides is a motive force for generating moment (steering torque) F(ST)×D about the steering axis J. That is, in the state of (b) of  FIG. 5 , the moment F(ST)×D about the steering axis J generates in a direction in which the left side where the belt shift generated lowers along the arrow ST direction in  FIG. 4 . Also the case where the belt generated in the right side on the drawing sheet contrary to the case of  8   b ) of  FIG. 5  is similar to the case of (b) of  FIG. 5 . In that case, the moment F(ST)×D about the steering axis J generates in a direction in which the right side where the belt shift generated lowers along the arrow ST direction in  FIG. 4 . 
     A direction of the steering angle of the steering roller  3  generated on the basis of the above-described principle coincides with a direction in which the belt shift is eliminated (i.e., the belt position is returned to the original position), and therefore, an automatic center alignment effect can be obtained. 
     In the case of the endless belt, as in the case of the intermediary transfer belt  1 , relating to the image forming process, a change in widthwise feeding position of the intermediary transfer belt  1  caused by an abrupt steering operation invites a positional deviation of the images with respect to a main scan direction (rotational axis direction of the photosensitive drums  1 ). That is, in the image forming apparatus  1  in this embodiment, relative positional deviation of the respective color images during the color image formation leads to color misregistration. Therefore, in this embodiment, the taper angle Ψ is provided to the sliding member  23  and a friction coefficient μS between the intermediary transfer belt  1  and the sliding member  23  is set at a relative low value, so that the abrupt steering operation is suppressed. Specifically, a good result can be obtained by using a resin material, such as POM (polyacetal) having a good sliding property, as a material of the sliding member  23  and by setting the friction coefficient μS at about 0.3 and the taper angle Ψ at about 5°-10°. Further, in consideration of an electrostatically adverse effect due to triboelectric charge with the intermediary transfer belt  1 , also electroconductivity is imparted to the sliding members  23 . 
     Further, also the dimensional relationship among the intermediary transfer belt  1 , the smallest roller  3  and the sliding members  23  in this embodiment as shown in (a) of  FIG. 5  is advantageous in suppressing the abrupt steering operation causing the above-described color misregistration. This is because in the dimensional relationship of (a) of  FIG. 5 , a difference in balance between the frictional forces can be always detected and therefore it becomes possible to perform a frequent center alignment operation. 
     4. Width/Diameter Relationship Among Stretching Rollers 
     With reference to  FIG. 6 , a relationship between end portion positions of the plurality of stretching rollers with respect to the rotational axis direction and diameters of the stretching rollers in this embodiment will be described. 
     Here, the diameter of the idler roller  4  is D 1 , and a length of a contact portion of the idler roller  4  with the intermediary transfer belt  1  with respect to the rotational axis direction (hereinafter referred to as a “contact portion width”) is L 1 . Further, the diameter of the upstream roller  5  is D 2 , and the contact portion width of the upstream roller  5  is L 2 . Further, the diameter of the driving roller  2  is D 3 , and the contact portion width of the driving roller  2  is L 3 . Incidentally, each of the driving roller  2 , the upstream roller  5  and the idler roller  4  has a line-symmetrical shape with respect to the steering axis J (nominal center line of the widthwise feeding position of the intermediary transfer belt  1 ). Further, the diameter of each of the stretching rollers is a diameter of the contact portion of the stretching roller with the intermediary transfer belt  1 . 
     As described above, in this embodiment, the center alignment of the intermediary transfer belt  1  is carried out by generating friction at each of the widthwise end portions of the intermediary transfer belt  1 . For that reason, on overlapping portions (hatched portions in  FIGS. 5 and 6 ) which are regions where the sliding members  23  provided at the widthwise end portions of the intermediary transfer belt  1  slide with the intermediary transfer belt  1 , a load due to the friction is exerted. Further, in this embodiment, the above-described contact portion widths L 1 , L 2  and L 3  are shorter than a width Lb of the intermediary transfer belt  1 , and therefore on the widthwise end portions of the intermediary transfer belt  1 , also loads by the end portions of the idler roller  4 , the upstream roller  5  and the driving roller  2  with respect to the rotational axis direction are exerted. 
     Further, in the case where the end portion positions of the idler roller  4 , the upstream roller  5  and the driving roller  2  are aligned with each other (L 1 =L 2 =L 3 ), when the end portion positions and the above-described overlapping portions overlap with each other, the loads concentrate at the widthwise end portions of the intermediary transfer belt  1 . That is, in this case, at the widthwise end portions of the intermediary transfer belt  1 , in addition to the load by the friction with the sliding members  23 , the loads due to stress concentration at the end portions of the stretching rollers generate with feeding of the intermediary transfer belt  1 . Further, when an amount of repetitive use of the intermediary transfer belt  1  increases, it leads to fatigue failure (breakdown) of the intermediary transfer belt  1  in some cases. 
     Therefore, in this embodiment, the contact portion widths L 1 , L 2  and L 3  of the idler roller  4 , the upstream roller  5  and the driving roller  2 , respectively, are changed depending on the diameters of these stretching rollers, respectively. 
     That is, when the intermediary transfer belt  1  is stretched with predetermined tension, at positions where the intermediary transfer belt  1  contacts the associated end portions of the stretching rollers, a value of stress increases with a decreasing diameter of the stretching rollers. On the other hand, line pressure (force per unit width) exerted on the intermediary transfer belt  1  by the stretching roller decreases when an increasing contact portion width of the stretching roller. Accordingly, with the decrease in line pressure by increasing the contact portion width of the stretching roller, it is possible to reduce the value of the stress exerted on the position where the intermediary transfer belt  1  contacts the end portion of the stretching roller. 
     Thus, in this embodiment, not only the end portion position of the stretching rollers are simply shifted from each other, but also the contact portion widths of the stretching rollers are increased with a decreasing diameter of the stretching rollers. As a result, at the position where the stress is liable to concentrate due to the small diameter of the stretching roller, stress relaxation is preferentially performed, so that it is possible to realize lifetime extension of the intermediary transfer belt  1 . 
     Specifically, in this embodiment, the diameters D 2 , D 2  and D 1  of the driving roller  2 , the upstream roller  5  and the idler roller  4 , respectively, provide a relationship of D 3 &gt;D 2 &gt;D 1 . Accordingly, in this embodiment, the contact portion widths L 3 , L 2  and L 1  of the driving roller  2 , the upstream roller  5  and the idler roller  4 , respectively, are constituted so as to provide a relationship of L 3 &lt;L 2 &lt;L 1 . As a result, a smaller diameter stretching roller liable to cause the stress concentration can be preferentially subjected to the stress relaxation while shifting the end portion positions (i.e., positions where the stress concentration generate) of the driving roller  2 , the upstream roller  5  and the idler roller  4 . 
     In this embodiment, the reason why the diameters D 3 , D 2  and D 1  of the driving roller  2 , the upstream roller  5  and the idler roller  4  provide the relationship of D 3 &gt;D 2 &gt;D 1  is as follows. 
     First, in this embodiment, the driving roller  2  is a drive input means to the intermediary transfer belt  1 . In order to suppress overlapping failure (color misregistration) of the toner images at the secondary transfer portion T 2 , a pitch (rotation center distance) Ld between adjacent photosensitive drums  11  may preferably be an integer multiple of a circumferential length (D 3 ×π) of the driving roller  2  having the diameter D 3 . For example, in the case where the pitch Ld between the photosensitive drums  11  is 100 mm, the diameter D 3  of the driving roller  2  is constituted to satisfy D 3 =31.84 mm or D 3 =15.92 mm. Accordingly, for example, in the intermediary transfer belt unit  20  used in the image forming apparatus  100  having the above-described pitch Ld, of the stretching rollers including the driving roller  2 , the upstream roller  5  and the idler roller  4 , the driving roller  2  is liable to have a largest diameter. Further, in order to form the secondary transfer portion T 2  by the pressing force (pressure) of the secondary transfer roller  17  opposing the driving roller  2 , the pressing force of, e.g., about 5-10 kgf acts on the driving roller  2 . In order to suppress deformation by this pressing force, of the stretching rollers including the driving roller  2 , the upstream roller  5  and the idler roller  4 , the driving roller  2  is made largest in diameter. 
     Next, the upstream roller  5  forms a primary transfer surface between itself and the idler roller  4  disposed upstream thereof with respect to the feeding direction of the intermediary transfer belt  1 , and forms a secondary transfer surface between itself and the driving roller  2  disposed downstream thereof with respect to the feeding direction of the intermediary transfer belt  1 . The primary transfer surface is a stretched surface (flat surface portion) of the intermediary transfer belt  1  where the toner images are transferred from the respective photosensitive drums  11  onto the intermediary transfer belt  1 , and the secondary transfer surface is a stretched surface (flat surface portion) of the intermediary transfer belt  1  immediately before the toner images are transferred from the intermediary transfer belt  1  onto the recording material S. For that reason, on the upstream roller  5 , the pressing force or the like as in the case of the driving roller  2  does not act, but in the case where the upstream roller  5  is deformed, the influence on the image is large, and therefore, the diameter of the upstream roller  5  is made large subsequently to the driving roller  2 . 
     Finally, the idler roller  4  forms the primary transfer surface between itself and the upstream roller  5  disposed downstream thereof with respect to the feeding direction of the intermediary transfer belt  1 , but forms only one stretched surface having the influence on the image, and therefore a degree of the influence on the image is small compared with the upstream roller  5 . 
     For that reason, a degree of necessity for the idler roller  4  to have a large diameter is low relative to the driving roller  2  and the upstream roller  5 , so that the diameter of the idler roller  4  is smaller from the viewpoint of cost reduction or the like. 
     When a part cost or the like is taken into consideration, it is desirable that all of the stretching rollers are made small in diameter, but the stretching rollers are made large in diameter depending on function in some cases. In this embodiment, for the above-described reason, the diameters of the driving roller  2 , the upstream roller  5  and the idler roller  4  provide the relationship of D 3 &gt;D 2 &gt;D 1 . 
     Incidentally, the stretching rollers are rotatably supported with play in general. For this reason, in order to prevent overlapping between the end portion positions of the stretching rollers with movement of the stretching rollers due to the play with respect to the rotational axis direction, it is desired that differences (L 1 −L 2  and L 2 −L 3 ) between the contact portion widths L 1  and L 2  are between the contact portion widths L 2  and L 3  are made sufficiently large. In this embodiment, the difference between L 1  and L 2  and between L 2  and L 3  may preferably be 2 mm or more, for example. 
     Thus, in this embodiment, the image forming apparatus  100  includes the intermediary transfer belt  1  and the plurality of stretching rollers, for stretching the intermediary transfer belt  1 , including the steering roller  3  tiltable for adjusting the widthwise feeding position of the intermediary transfer belt  1 . 
     Further, of the plurality of stretching rollers, with respect to n stretching rollers (n: integer of two or more) other than the steering roller and shorter in length of the contact portion thereof with the belt  1  with respect to the rotational axis direction than the width of the belt  1  with respect to the rotational axis direction, the following constitution is employed. When an n-th stretching roller is Dn in diameter and Ln in length of the contact portion of the n-th stretching roller, in a case that Dn is larger with an increasing n, Ln is smaller with the increasing n. Further, end positions of the contact portions of the n stretching rollers with respect to the rotational axis direction are different from each other. In this embodiment, of the n stretching rollers, the stretching roller having the largest diameter is the stretching roller for transmitting drive (driving force) to the belt  1 . Particularly, in this embodiment, the n stretching rollers are constituted by at least three stretching rollers. Further, of the n stretching rollers, the stretching roller having the largest diameter is the stretching roller for forming the transfer portion T 2  where the toner images are transferred from the belt  1  onto the recording material S. Further, of the n stretching rollers, the stretching roller having the second largest diameter is the stretching roller for forming the stretched surface (secondary transfer surface) of the belt  1  between itself and the stretching roller having the largest diameter. Further, of the n stretching rollers, the stretching roller having the third largest diameter is the stretching roller for forming the stretched surface (primary transfer surface) of the belt  1 , where the toner images are formed by the toner image forming means, between itself and the stretching roller having the second largest diameter. Further, in this embodiment, the belt feeding device  20  includes the sliding members  23 , disposed at the end portions of the steering roller  3  with respect to the rotational axis direction, for imparting the force for tilting the steering roller  3  to the steering roller  3  while sliding with the moving belt  1 . 
     Incidentally, when the n stretching rollers are arbitrarily selected from the plurality of stretching rollers and satisfy the above-described relationship, a corresponding effect of relaxing the concentration of the load at the belt  1  can be obtained. Accordingly, the plurality of stretching rollers for stretching the belt  1  may also include the stretching roller not satisfying the above-described relationship. For example, in the case where as the n stretching rollers, the driving roller  2  of D 3  in diameter and L 3  in contact portion in width and the idler roller  4  of D 1  in diameter and L 1  in contact portion width are selected, a relationship of L 3 &lt;L 1  is satisfied when D 3 &gt;L 1  is satisfied. Further, the diameter D 2  of the upstream roller  5  satisfies a relationship of D 3 &gt;D 2 &gt;D 1 , for example. In this case, even when the contact portion width L 2  of the upstream roller  5  satisfies a relationship of L 2 =L 1  and L 2 =L 3 , for example, end portion positions of the driving roller  2  and the idler roller  4  are shifted depending on the diameters, so that a relaxing effect of the load on the belt  1  can be obtained correspondingly. In addition to the constitution of this embodiment, this is true for also a constitution or the like in which a stretching roller having an arbitrary contact portion width is further added. In the present invention, the stretching roller refers to a roller having a winding angle ( FIG. 7 ), about the intermediary transfer belt  1 , of 10° or more. 
     As described above, according to this embodiment, a degree of the stress concentration due to the stretching rollers is relaxed (alleviated) and thus lifetime extension of the intermediary transfer belt  1  can be realized while realizing stable traveling (movement) of the intermediary transfer belt  1  by the automatic belt center alignment mechanism portion  10 . Further, even in the case where a thickness of the intermediary transfer belt  1  is made thin for the purpose of cost reduction or the like, it becomes possible to suppress fatigue failure due to a lowering in rigidity of the intermediary transfer belt  1 . 
     Embodiment 2 
     Another embodiment of the present invention will be described. Basic constitutions and operations of a belt feeding device and an image forming apparatus in this embodiment are the same as those in Embodiment 1. Accordingly, in this embodiment, elements having the same or corresponding functions and constitutions as those in Embodiment 1 are represented by the same reference numerals or symbols, and will be omitted from detailed description. 
       FIG. 7  is a schematic sectional view of an intermediary transfer belt unit  20  in this embodiment. In this embodiment, a winding angle of the intermediary transfer belt  1  about the idler roller  4  is θ 1 . Further, a winding angle of the intermediary transfer belt  1  about the upstream roller  5  is θ 2 . Further, a winding angle of the intermediary transfer belt  1  about the driving roller  2  is θ 3 . The winding angle refers to an angle (in a side corresponding to a region where the belt is wound about the associated roller) formed between lines drawing from a contact start position and a contact end position of the intermediary transfer belt  1  with the surface of the associated stretching roller with respect to the feeding direction of the intermediary transfer belt  1  to a rotation center of the associated stretching roller. In this embodiment, similarly as in Embodiment 1, the diameters of the idler roller  4 , the upstream roller  5  and the driving roller  2  are D 1 , D 2  and D 3 , respectively, and the contact portion widths of the idler roller  4 , the upstream roller  5  and the driving roller  2  are L 1 , L 2  and L 3 , respectively. 
     As described in Embodiment 1, in consideration of the part cost or the like, it would be considered that the relationship of D 3 =D 2 =D 1  is satisfied. In this case, an effect similar to that in Embodiment 1 can be obtained by changing the contact portion widths L 1 , L 2  and L 3  of the idler roller  4 , the upstream roller  5  and the driving roller  2  depending on the winding angles of the intermediary transfer belt  1  about the respective stretching rollers. This is because when the winding angle is large, the intermediary transfer belt  1  is subjected to large stress concentration by the end portions of the stretching rollers. 
     As shown in  FIG. 7 , in this embodiment, a constitution providing relationships of D 3 =D 2 =D 1  and θ 3 &gt;θ 2 &gt;θ 1  is employed. Accordingly, in this embodiment, a relationship of L 3 &gt;L 2 &gt;L 1  is satisfied. As a result, a stretching roller, having a larger winding angle, liable to cause the stress concentration can be preferentially subjected to the stress relaxation while shifting the end portion positions (i.e., positions where the stress concentration generate) of the driving roller  2 , the upstream roller  5  and the idler roller  4 . 
     Incidentally, the winding angles can be set by an arrangement of the respective stretching rollers. The winding angles θ 3 , θ 2  and θ 1  of the intermediary transfer belt  1  about the driving roller  2 , the upstream roller  5  and the idler roller  4 , respectively, satisfy the relationship of θ 3 &gt;θ 2 &gt;θ 1  for the same reason as that the diameters are changed in Embodiment 1, for example. That is, the driving roller  2  is a drive input means to the intermediary transfer belt  1 , and therefore provides a largest winding angle in some cases in order to satisfactorily ensure the transmission of the driving force. Further, the upstream roller  5  is configured to provide the second largest winding angle after the driving roller  2  in some cases in order to stabilize the first transfer surface and the second transfer surface. Further, the idler roller  4  has a small influence on the image compared with the driving roller  2  and the upstream roller, and therefore is made small in winding angle compared with the driving roller  2  and the upstream roller  5 . 
     Thus, in this embodiment, of the plurality of stretching rollers, with respect to n stretching rollers shorter in length of the contact portion thereof with the belt  1  with respect to the rotational axis direction than the width of the belt  1  with respect to the rotational axis direction, the following constitution is employed. When an n-th stretching roller is θn in winding angle of the belt  1  about the roller and Ln in length of the contact portion of the n-th stretching roller, in a case that θn is larger when the n stretching rollers have the substantially same diameter and n is larger, Ln is larger with the increasing n. Further, end positions of the contact portions of the n stretching rollers with respect to the rotational axis direction are different from each other. In this embodiment, of the n stretching rollers, the stretching roller having the largest winding angle is the stretching roller for transmitting drive (driving force) to the belt  1 . Particularly, in this embodiment, the n stretching rollers are constituted by at least three stretching rollers. Further, of the n stretching rollers, the stretching roller having the largest winding angle is the stretching roller for forming the transfer portion T 2  where the toner images are transferred from the belt  1  onto the recording material S. Further, of the n stretching rollers, the stretching roller having the second largest winding angle is the stretching roller for forming the stretched surface (secondary transfer surface) of the belt  1  between itself and the stretching roller having the largest winding angle. Further, of the n stretching rollers, the stretching roller having the third largest winding angle is the stretching roller for forming the stretched surface (primary transfer surface) of the belt  1 , where the toner images are formed by the toner image forming means, between itself and the stretching roller having the second largest winding angle. 
     As described above, even in the case where the diameters of the stretching rollers are substantially the same, an effect similar to that in Embodiment 1 can be obtained by changing the contact portion widths depending on the winding angles. Incidentally, the substantially same diameter refers to diameters providing a diameter ratio therebetween of 10% or less. 
     OTHER EMBODIMENTS 
     The present invention was described above based on the specific embodiments, but is not limited to the above-described embodiments. 
     In the above-described embodiments, the case where the belt is the intermediary transfer member was described, but the present invention is not limited thereto. For example, an image forming apparatus of a direct transfer type including a recording material carrying belt (feeding belt), for carrying and feeding the recording material, used in place of the intermediary transfer belt in the above-described embodiment has been well known in the field of the present invention. In the image forming apparatus of the direct transfer type, for example, toner images formed at a plurality of image forming portions are successively transferred onto the recording material carried and fed by the recording material carrying belt as a recording material carrying member constituted by an endless belt. Also as regards the recording material carrying belt, it is possible to provide the sliding members in order to carry out the center alignment, and in that case, a problem similar to that, in the above-described embodiments, relating to the intermediary transfer belt can generate. Accordingly, also in the case where the belt is the recording material carrying belt, by applying the present invention thereto, it is possible to achieve an effect similar to those in the above-described embodiments. Similarly, the belt may also be a photosensitive (member) belt, an electrostatic recording dielectric (member) belt, or the like. 
     Further, in the above-described embodiments, the sliding members were disposed adjacently to the end portions of the steering roller. However, it is also possible to employ other constitutions, in which the sliding member is provided in only one end portion side of the steering roller with respect to the rotational axis direction, such as the case where it has been known that there is a tendency that the belt shifts toward one widthwise end portion side or the case where the belt is intentionally constituted so as to have the tendency that the belt shifts toward one end portion side. 
     According to the present invention, in the constitution including the endless belt stretched by the plurality of stretching rollers, the concentration of the load at the widthwise end portions of the belt can be alleviated. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-062831 filed on Mar. 25, 2016, which is hereby incorporated by reference herein in its entirety.