Abstract:
A belt driving apparatus includes a movable belt member, a stretching member configured to stretch the belt member, and a steering unit configured to stretch the belt member and to be inclined to steer the belt member in a widthwise direction substantially perpendicular to a direction of moving of the belt member. The steering unit includes a rotatable member contacting an inner surface of the belt member and configured to be rotatable with movement of the belt member around a rotational axis of the rotatable member, with the rotational axis extending in the widthwise direction, and non-rotatable members contacting an inner surface of the belt member, provided at each opposite axial end of the rotatable member and configured not to be rotatable with movement of the belt member. In addition, a supporting member supports the rotatable member and the non-rotatable members, and is inclined by frictional force from sliding between the belt member and each of the non-rotatable members, around an axis perpendicular to the rotational axis, and urging members urge the belt member in contact with an outer surface of the non-rotatable members, with each of the urging members disposed against one of the non-rotatable members.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a belt driving apparatus for driving a belt member relating to image formation. Specifically, the present invention relates to a belt unit for driving an intermediary transfer belt, a transfer belt, a photosensitive belt, etc., and also relates to an image forming apparatus such as a copying machine, a printer, a printing machine, etc., which includes the belt unit. 
     In recent years, with speed-up of the image forming apparatus, a constitution in which a plurality of image forming portions are arranged corresponding to a belt member and image forming processes for respective colors are concurrently performed goes mainstream. For example, as a representative belt member in a full-color image forming apparatus of an electrophotographic type, the intermediary transfer belt is used. Onto a belt steering of the intermediary transfer belt, respective color toner images are successively transferred superposedly, and then the color toner images are collectively transferred onto a recording material. This intermediary transfer belt is stretched by stretching rollers, including a driving roller, which are a plurality of stretching member, so that the intermediary transfer belt is rotatable. It has been generally known that such a belt member stretched by the plurality of stretching rollers is accompanied with a problem that the belt member is laterally moved in either one of roller end portions depending on roller outer diameter accuracy or alignment accuracy among the rollers. 
     This problem is not limited to the intermediary transfer belt but also occurs in the belt driving apparatus for stretching the belt by the plurality of stretching members to drive the belt. 
     As a countermeasure against this problem, a method in which a steering roller which is a steering member automatically effect belt center alignment by a balance of a frictional force (hereinafter referred to as automatic belt center alignment) has been proposed as a simple and inexpensive method using less number of parts (Japanese Laid-Open Patent Application (Tokuhyo) 2001-520611). 
     Specifically, this method employs a constitution in which a sliding portion is provided at each of end portions of the steering roller. Further, when the belt member is laterally moved to one end side, a frictional force between the one end-side frictional portion and the belt member is increased. By using a difference between the force generated at one end side and the force generated at the other end side, a swing torque of the steering roller is obtained. 
     However, the end portion of the belt member contacted to the frictional portion is a free end and therefore the contact belt the belt member end portion and the frictional force is liable to become unstable depending on a shape of the belt member end portion. 
     When the contact between the belt member end portion and the frictional portion becomes unstable, an amount of the contact between these portions is decreased. As a result, the frictional force generated from the belt per unit width is lowered. Further, when the swing torque necessary to steer the steering roller is intended to be obtained, in order to increase the contact amount, a contact width of the belt member with a sliding portion (frictional portion) is required to be increased. As a result, behavior of the belt member to be conveyed in such that a width of meandering is large and responsiveness during the center alignment is also lowered. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide a belt driving apparatus capable of improving responsiveness to lateral deviation (movement) of a belt member by enhancing contact stability between a belt member free end and a frictional portion. 
     According to an aspect of the present invention, there is provided a belt driving apparatus comprising: 
     a rotatable belt member; 
     a stretching member for stretching the belt member; 
     a steering device for stretching and steering the belt member, wherein the steering device includes a rotatable portion which is rotatable with rotation of the belt member, a frictional portion provided at each of longitudinal outsides of the rotatable portion with respect to a widthwise direction and slidable relative to the belt member by being prevented from rotating, supporting means for supporting the rotatable portion and the frictional portion, and a rotation shaft for rotatably supporting the supporting means, and wherein the steering device is capable of moving the belt member in the widthwise direction by rotating the supporting means by a force produced by sliding between the belt member and the frictional portion; and 
     an urging member, provided at each of longitudinal end portion sides of the rotatable portion with respect to the widthwise direction, for urging the belt member against the frictional portion in contact with an outer peripheral surface of the belt member. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view for illustrating an image forming apparatus of an intermediary transfer type. 
         FIG. 2  is a perspective view for illustrating Embodiment 1 of the present invention. 
         FIG. 3  is a perspective view for illustrating a width of an urging member in the present invention. 
         FIG. 4  is a perspective view for illustrating Embodiment 2 of the present invention. 
         FIG. 5  is a perspective view for illustrating Embodiment 3 of the present invention. 
         FIG. 6  is a perspective view showing a mounted state of a transfer cleaning device in Embodiment 3 of the present invention. 
       Parts (a) and (b) of  FIG. 7  are perspective views for illustrating an intermediary transfer belt unit. 
       Parts (a) and (b) of  FIG. 8  are perspective views for illustrating automatic center alignment. 
       Parts (a) and (b) of  FIG. 9  are schematic views for illustrating a contact width of a belt. 
         FIG. 10  is a sectional view of an intermediary transfer belt wound about a sliding ring portion. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (Embodiment 1) 
     &lt;Image Forming Apparatus&gt; 
     An image forming apparatus in this embodiment of the present invention will be described. 
     First, referring to  FIG. 1 , an operation of the image forming apparatus will be described. Types of the image forming apparatus may include a plurality of types such as an electrophotographic type, an offset printing type and an ink jet type. The image forming apparatus  60  shown in  FIG. 1  is a color image forming apparatus of the electrophotographic type. The image forming apparatus  60  is of a so-called intermediary transfer tandem type in which four image forming portions for four colors are arranged side by side on an intermediary transfer belt.  FIG. 1  is a sectional view of the image forming apparatus  60  of this type, which goes mainstream from the viewpoints of excellent compatibility with thick paper and excellent productivity. 
     &lt;Conveyance Process of Transfer Material&gt; 
     Sheets of recording material S are stacked on a lift-up device  62  in a recording material accommodating portion  61 . The recording material S is fed by sheet feeding device  63  in synchronism with image forming timing. As one of sheet feeding methods, a method using air separation and attraction may be used. In  FIG. 1 , the method using the air separation and attraction is used. It is also possible to employ other sheet feeding methods. The recording material S fed by the sheet feeding device  63  is passed through a conveying path  64   a  of a conveying unit  64 , and then, is conveyed to a registration device  65 . After oblique movement correction and timing correction by the registration device  65 , the recording material S is sent to a secondary transfer portion. The secondary transfer portion is a transfer nip formed by opposing rollers consisting of an inner secondary transfer roller  603  as a first secondary transfer member and an outer secondary transfer roller  66  as a second secondary transfer member. Then, to the recording material S, predetermined pressure and a predetermined electrostatic load bias are applied, and toner images on an intermediary transfer belt  606  are transferred onto the recording material S. 
     &lt;Image Formation Process&gt; 
     An image formation process which is carried out in synchronism with the above-described conveyance process of the recording material S to the secondary transfer portion will be described. 
     The image forming apparatus  60  in this embodiment includes an image forming portion  613 Y which forms an image with yellow (Y) toner, an image forming portion  613 M which forms an image with magenta (M) toner, an image forming portion  613 C which forms an image with cyan (C) toner, and an image forming portion  613 BK which forms an image with black (BK) toner. The image forming portions  613 Y,  613 M,  613 C and  613 BK are the same in structure except that they are different in the color of the toner they use. Thus, the image forming process will be described with reference to the image forming portion  613 Y as a representative portion. 
     The image forming portion  613 Y, which is a toner image forming means, is constituted by a photosensitive member  608  which is an image bearing member, a charging device  612  for charging the photosensitive member  608 , an exposure device  611   a , a developing device  610 , a primary transferring device (member)  607 , and a photosensitive member cleaner  609 . The steering of the photosensitive member  608  rotating in the direction indicated by an arrow m is uniformly charged by the charging device  612 . The photosensitive member  608  is exposed to light by driving the exposure device  611   a , via a diffraction member  611   b , on the basis of an inputted signal of image information, so that an electrostatic latent image is formed. Then, the electrostatic latent image formed on the photosensitive member  608  is developed by the developing device  610 , so that a toner image is formed on the photosensitive member  608 . Then, the yellow toner image is transferred onto the intermediary transfer belt  606 , which is a belt member, by applying predetermined pressure and by applying a predetermined electrostatic load bias to the primary transferring member  607 . Thereafter, transfer residual toner remaining on the photosensitive member  608  is collected by the photosensitive member cleaner  609 , so that the photosensitive member  608  prepares for the next image formation. 
     As the image forming portion  613 , in the case of  FIG. 1 , four image forming portions for forming yellow (Y), magenta (M), cyan (C), and black (Bk) are present. Therefore, a magenta toner image formed at the image forming portion M is transferred onto the yellow toner image on the intermediary transfer belt  606 . The cyan toner image formed at the image forming portion C is transferred onto the transferred magenta toner image on the intermediary transfer belt  606 . Further, the black toner image formed at the image forming portion BK is transferred onto the transferred cyan toner image on the intermediary transfer belt  606 . Thus, the different color toner images are superposed formed (transferred) on the intermediary transfer belt  606 , so that a full-color image is formed on the intermediary transfer belt  606 . Incidentally, the number of the colors in this embodiment is four but is not limited to four. Further, the order of the superposed color toner images is also not limited to the above-described order. 
     Next, the intermediary transfer belt  606  will be described. The intermediary transfer belt  606  is stretched by a driver roller  604  which is a driving member, a steering roller  80  which is a steering member, a stretching roller  617  which is a stretching member, and the inner secondary transfer roller  603  which is an inner secondary transfer (stretching member). The intermediary transfer belt  606  is a belt member which is conveyed and is driven in the direction indicated by an arrow V in the figure. 
     Further, the steering roller  80  functions also as a tension roller, which provide the intermediary transfer belt  606  with a predetermined tension. The above-described image forming processes successively processed at the image forming portions  613 Y,  613 M,  613 C and  613 BK are performed with such timings that the toner image is superposed on the upstream-side color toner image which is primary-transferred onto the intermediary transfer belt  606 . Consequently, a full-color toner image finally formed on the intermediary transfer belt  606 , and then is conveyed to the secondary transfer portion. Incidentally, the number of the rollers for stretching the intermediary transfer belt  606  is not limited to that in the constitution shown in  FIG. 1 . 
     &lt;Secondary Transfer and Subsequent Processes&gt; 
     As described above, the full-color toner image formed, through the above-described recording material S conveyance process and image forming process, on the intermediary transfer belt  606  is secondary-transferred onto the recording material S at the second transfer portion. Then, the recording material S is conveyed to a fixing device  68  by a front conveying portion  67  for fixing. Although there are various constitutions and types for the fixing device  68 , in  FIG. 1 , the fixing device  68  is of the type in which the toner image is melt-fixed on the recording material S by applying predetermined amounts of pressure and heat thereto in a fixing nip formed between a fixing roller  615  and a pressing belt  614 . Here, the fixing roller  615  is internally provided with a heater as a heat source. The pressing belt  614  is provided with a pressing pad  616  urged by a plurality of stretching rollers and the inner peripheral surface of the belt. The recording material S having passed through the fixing device  68  is, by a branching feeding device  69 , subjected to choice of the path as the whether it is discharged onto a sheet discharge tray  600  as it is or is conveyed to a reverse conveying device  601  in the case where both-side image formation is required. In the case where the both-side image formation is required, the recording material S conveyed to the reverse conveying device  601  is changed in direction between its leading end and trailing end by performing a switch back operation to be conveyed into a both-side conveying device  602 . Thereafter, the recording material S enters again the sheet conveying path from a refeeding path  64   b  of a conveying unit  64  while being timed to a recording material, for a subsequent job, fed from a sheet feeding device  61 , and then is sent to the secondary transfer portion in a similar manner. With respect to the image forming process on the back (second) surface, the process is the same as that in the case of the above-described front (first) surface and will be omitted from description. 
     Incidentally, the deposited matter such as the toner remaining on the intermediary transfer belt  606  after the secondary transfer is removed from the intermediary transfer belt  606  by a cleaning unit including a cleaning blade  618 . Thus, the image forming apparatus  60  prepares for the next image formation. The toner removed from the surface of the intermediary transfer belt  606  is finally collected in an unshown collecting container or the like by a feeding screw  619 . 
     &lt;Steering Structure of Intermediary Transfer Belt&gt; 
     Parts (a) and (b) of  FIG. 7  are perspective views of an intermediary transfer belt unit  700  provided in the image forming apparatus  60 , in which (a) shows a state in which the intermediary transfer belt  606  is stretched and (b) shows a state in which the intermediary transfer belt  606  is removed. 
     With respect to the intermediary transfer belt  606  conveyed in the arrow Z direction by a conveying force of the driving roller  604  into which a driving force is inputted from a driving gear  702 , in this embodiment, the steering roller  80  is provided with an automatic belt center alignment mechanism using a balance of the frictional force. 
     Part (a) of  FIG. 8  is a perspective view of the automatic belt center alignment mechanism which is the steering device in the present invention. The steering roller  80  which is a steering member is constituted in the form such that a follower roller portion  81  which is a rotatable portion constituting central portion and a sliding ring portion  82  which is a frictional portion provided at each of longitudinal end side (end portions) with respect to the rotational axis direction of the rotatable portion are co-axially connected. In this embodiment, the follower roller portion  81  has a straight shape. Further, a sliding bearing  83  engaged with a side supporting member  85  at a sliding groove portion (not shown) is slidably urged in a direction indicated by an arrow P T  in the figure. Therefore, the steering roller  80  is also a tension roller for applying tension to the inner peripheral surface of the intermediary transfer belt  606  in an arrow K′ direction. Further, the side supporting member  85  constitutes, together with a rotational movement plate  86 , a supporting table (supporting means) for supporting the follower roller portion  81  and the sliding ring portion  82  and is rotatably supported, by a steering shaft which is a rotation shaft, in a direction indicated by an arrow S in the figure. Here, a frame stay  87  is a member constituting a casing of the intermediary transfer belt unit  500  and is disposed between a unit front-side plate  701 F and a unit rear-side plate  701 R. The frame stay  87  is provided with sliding rollers  88  at side surface portions to perform the function of reducing a rotational movement resistance of the rotational movement plate  86 . 
     &lt;Detailed Constitution of Automatic Center Alignment Portion&gt; 
     A detailed view of the neighborhood of an end portion of the automatic belt center alignment mechanism in the present invention is shown in (b) of  FIG. 8 . The sliding ring portion  82  in this embodiment has a tapered shape such that a diameter thereof is continuously increased toward the outside of a roller shaft  89  with respect to a longitudinal direction (rotational axis direction). In this embodiment, a taper angle φ is set at 8 degrees ((b) of  FIG. 9 ). Incidentally, in this embodiment, the tapered shape is used but the sliding ring portion  82  may also have a straight shape. 
     Relative to the roller shaft  89 , the follower roller portion  81  is rotatably supported by bearings or the like incorporated therein, and the sliding ring portion  82  provided at each of the end portions are non-rotatably supported by using parallel pins or the like. Incidentally, in this embodiment, the sliding ring portion  82  has a constitution in which it is fixed so as not to rotate in the rotational direction of the follower roller portion  81 , but is not limited thereto. The sliding ring portion  82  may also have a constitution in which it is rotatable. However, in this case, when a constitution in which a torque necessary to rotate the sliding ring portion  82  in the rotational direction of the intermediary transfer belt  606  is larger than that necessary to rotate the follower roller portion  81  in the same direction is employed, the intermediary transfer belt  606  is steerable. 
     Here, the end portion of the roller shaft  89  has a D-cut shape or the like and thus is non-rotatably supported by the sliding bearing  83 . Therefore, when the stretched intermediary transfer belt  606  is conveyed, the follower roller portion  81  of the steering roller  80  does not slide relative to the inner peripheral surface of the belt but the sliding ring portion at each end portion slides relative to the belt. The principle on which the automatic belt center alignment can be effected by such a constitution will be described below in detail. 
     &lt;Operation Principle of Automatic Center Alignment&gt; 
       FIG. 10  is a schematic view showing a cross-section of the intermediary transfer belt  606  wound about the sliding ring portion  82 . As already described above, the sliding ring portion at each end portion is supported so that it cannot be rotated by the roller shaft  89  and therefore always receives frictional resistance from the inner peripheral surface during the belt conveyance. In  FIG. 10 , the intermediary transfer belt  606  conveyed and driven in the arrow V direction is wound about the sliding ring portion  82  at a winding angle θ S . Here, a width (with respect to an axial direction of the steering roller) is considered as a unit width. When a belt length corresponding to a minute winding angle portion dθ at a certain winding angle θ is considered, an upstream side is a loosening and thus a tension T acts in a tangential direction, and a downstream side is a stretching side and thus a tension T+dT acts in the tangential direction. Therefore, with respect to the small belt width, a force of the belt exerted on the sliding ring portion  82  in a centripetal direction is approximated as Tdθ and a frictional force dF is, when the sliding ring portion  82  has a friction coefficient μ s , represented by:
 
dF=μ s Tdθ  (1).
 
     Here, the tension T is dominated by an unshown driving roller and when the driving roller has a friction coefficient K r , the following equation is satisfied.
 
 dT=−μ   r   Tdθ   (2)., i.e.,
 
     
       
         
           
             
               
                 
                   
                     
                       ⅆ 
                       T 
                     
                     T 
                   
                   = 
                   
                     
                       - 
                       
                         μ 
                         r 
                       
                     
                     ⁢ 
                     
                       ⅆ 
                       θ 
                     
                   
                 
               
               
                 
                   ( 
                   
                     2 
                     ′ 
                   
                   ) 
                 
               
             
           
         
       
     
     When the formula (2′) is integrated with respect to the above-described winding angle θ s , the tension T is represented by:
 
T=T 1 e −μ     r     θ   (3)
 
     Incidentally, T 1  represents a tension at θ=0. 
     From the formulas (1) and (3), the following equation is satisfied.
 
dF=μ s T 1   e   −μ     r     θ dθ  (4)
 
     As shown in  FIG. 10 , in the case where the rotational movement direction of the supporting table with respect to the steering shaft described above is an arrow S direction, a position of winding start (θ=0) provides an argument (angle of deviation) α. Therefore, of the force represented by the formula (4), a downward component of the S direction is represented by:
 
 dF   s =μ s   T   1   e   −μ     r     θ  sin(θ+α) dθ   (5).
 
     Further, when the formula (5) is integrated with respect to the winding angle θ S  described above, a downward force (per unit width), with respect to the arrow S direction, exerted from the intermediary transfer belt  606  on the sliding ring portion  82  during the belt conveyance is obtained as represented by:
 
 F   s =μ s   T   1 ∫ 0   θ     s     e   −μ     r     θ  sin(θ+α) dθ   (6).
 
     Parts (a) and (b) of  FIG. 9  are schematic views corresponding to top views of the intermediary transfer belt and the steering roller as seen in an arrow TV direction indicated in  FIG. 10 , in which (a) shows the case where a belt winding position is located at a nominal (center) position in a balanced steady state by the automatic center alignment, and (b) shows the case where lateral belt deviation toward the left side occurs when the belt is conveyed in the arrow V direction. 
     As shown in (a) of  FIG. 9 , in this embodiment, a width L B  of the intermediary transfer belt  606  is longer than a length L R  of the follower roller portion  81  and is shorter than a full length L R +2L F  (the follower roller portion+the sliding ring portions (end portions)) of the steering roller. Further, in the nominal stage, the belt always slides relative to the sliding ring portions with a winding width. That is, in the state in which the steering operation can be normally performed, the belt slides on the sliding ring portion with the winding width. For that reason, in this embodiment, in the case where the belt contacts only one sliding ring portion the state is judge as an abnormal state. 
     On the other hand, in (b) of  FIG. 9  in which the lateral belt deviation occurs, a relationship in winding width between the intermediary transfer belt  606  and the sliding ring portions is assumed such that the belt is in a laterally deviated state with a winding width w only at the left side. That is, the left-side sliding ring portion  82  receives a force F S w in the downward S direction and the right-side sliding ring portion  82  receives a force of 0 (zero) in the downward S direction. Further, it is possible to explain that a difference in frictional force between the end portions is motive power for generating moment F S wL with respect to the steering shaft (with respect to a direction in which the left side which is the laterally deviated side in the assumption of (b) of  FIG. 9  is descended. Hereinafter, the moment with respect to the stretch shaft is referred to as steering torque. 
     The steering roller based on the principle described above is inclined so that the intermediary transfer belt  606  is moved in a direction in which the lateral deviation is eliminated (moved toward the central side), so that the center alignment can be effected. Incidentally, in this embodiment, the sliding ring portion  82  is provided with a taper angle, so that a system which depends on only the friction coefficient &amp;u&amp; S  is created. By setting the friction coefficient μ S  at a relatively low value, the sliding ring portion  82  is resistant to fluctuation with time during endurance use and it is possible to avoid an abrupt steering operation. Particularly, in the case of the belt member, relating to the image formation, such as the intermediary transfer belt, a change in belt conveyance direction caused by the abrupt steering operation causes color misregistration with respect to a main scan direction and thus the setting of the friction coefficient μ S  is a very important factor. Specifically, as a material for the sliding ring portion  82  used in this embodiment, a resin material such as POM (polyoxyacetal) is used and the sliding ring portion  82  is set to have approximately μ S =0.3 and taper angle φ=8 degrees. Further, in consideration of an electrostatic harmful influence due to frictional charging with the intermediary transfer belt  606 , the sliding ring portion is also provided with electroconductivity. Further, a dimensional relationship, between the intermediary transfer belt  606  and the sliding ring portions  82  with respect to the widthwise direction, which have already described with reference to (a) of  FIG. 9  is also intended to avoid the abrupt steering operation constituting the factor of the color misregistration. This is because the dimensional relationship shown in (a) of  FIG. 9  permits a fine center alignment operation since a difference in balance of the frictional force can be always detected. 
     &lt;Urging Member Constitution&gt; 
       FIG. 2  is an enlarged view of the end portion of the steering roller for the intermediary transfer belt in this embodiment. Specifically, the steering roller has the same constitution as that of the automatic center alignment mechanism described with reference to  FIG. 8  and a difference portion is a portion where an urging (pressing) member  2  which is a feature of the present invention is provided. 
     The steering roller consists of the follower roller portion  81  rotatably shaft-supported by the roller shaft  89  and the sliding ring portions  82  which are provided at longitudinal ends of the follower roller portion  81  (only one end thereof is shown in  FIG. 2 ) and are non-rotatable by the roller shaft  89 . In this embodiment, the sliding ring portion  82  has the tapered shape such that the outer diameter is gradually increased toward the outside. The end portion of the roller shaft  89  has a rotation-stopping shape such as the D-cut shape and is supported non-rotatably by the sliding bearing  1 . The sliding bearing  1  in this embodiment includes a boss portion  1   a , a holder portion  1   b  and a slide groove  1   c . The boss portion  1   a  is engaged with an inner diameter portion of a tension spring  84  and the entire steering roller is urged against the inner peripheral surface of the intermediary transfer belt  606  with predetermined tension. That is, the steering roller also functions as the tension roller. To the holder portion  1   b , the urging member  2  formed with an elastic member is applied at its inside, and the urging member  2  has a deformation amount which follows the tapered shape of the sliding member  82 . The slide groove  1   c  is engaged with the side supporting member  85  shown in  FIG. 8  and guides the sliding bearing  1  so that the sliding bearing  1  can move in the urging direction of the tension spring described above. 
     The holder portion  1   b  and the urging member  2  will be described more specifically with reference to  FIG. 3 .  FIG. 3  is a sectional view when the steering roller is cut along a plane Pc (including the inside end surface of the urging member  2 ) shown in  FIG. 2  and shows a state in which the intermediary transfer belt  606  is stretched. As is understood from  FIG. 3 , the urging member  2  contacts the outer surface of the intermediary transfer belt  606  and urges the intermediary transfer belt  606  toward the sliding member  82 . The holder portion  1   b  has an arcuate shape so as to cover the belt at the winding angle θ S , and the urging member  2  at the inner peripheral surface of the holder portion  1   c  has a width Wb including a full width of the sliding ring portion  82  and a width of a part of the follower roller portion  81  and is provided on the basis of a large diameter-side tapered shape portion of the sliding ring portion  82 . 
     Here, the width Wb is set so as to satisfy a relationship, with the respective lengths described with reference to  FIG. 10 , of: Wb≧L R +2L F −L B . That is, the Wv of the urging member  2  is set at a value which is not less than an amount in which the intermediary transfer belt  6  can physically meander. As a result, it is possible to create a state in which the urging member  2  always treads on the belt end portion, i.e., a state in which the urging member  2  covers the belt end portion. Consequently, there is no possibility that a belt edge is turned up when the belt edge enters the urging member  2 , so that the intermediary transfer belt  606  can smoothly move in a thrust direction during the automatic center alignment operation. Further, in this embodiment, a constitution in which the urging member  2  has an elastic layer formed in a uniform thickness of a foamed material or the like is employed, so that the deformation amount is increased with the position of the sliding ring portion  82  toward the large diameter side. As a result, an urging force by the urging member  2  can be increased with an increase in lateral belt deviation amount, so that it becomes possible to generate the frictional force more reliably and efficiently even when waving due to elongation of the belt end portion occurs. That is, the urging force with which the urging member urges an end portion-side area (outside first area) of the sliding ring portion  82  with respect to the widthwise direction of the sliding ring portion  82  is made larger than that with which the urging member urges a follower roller portion  81 -side area (inside second area) with respect to the widthwise direction of the sliding ring portion  82 . By this constitution, a contact pressure between the belt and the sliding ring portion  82  is increased with the position of the sliding ring portion  82  toward the outside, so that it is possible to reduce beforehand a phenomenon such that the belt end portion is protruded from the sliding ring portion  82  and thus is completely deviated laterally. 
     Here, the intermediary transfer belt  606  is formed with the resin belt having a base layer of polyimide to have a tensile elastic modulus E of about 18000 N/cm 2 . Thus, the intermediary transfer belt  606  has such a characteristic that the intermediary transfer belt  606  causes substantially no elongation within a practical range and therefore a factor of a change in circumferential length by the automatic center alignment operation is absorbed by expansion and contraction of the tension spring  84 . That is, the axis of the steering roller is changed in its indication with the automatic center alignment. On the other hand, in the constitution in this embodiment, the urging member  2  is integrally held with the sliding bearing  1  described above and therefore, the urging member  2  can follow the inclination change. As a result, even when the automatic center alignment is effected, the deformation amount of the urging member  2  can be kept in a stable state. 
     Thus, according to this embodiment, when the state of the belt end portion which is the free end is not preferable, i.e., even in the case where the waving or the like occurs, it becomes possible to obtain a desired frictional force between the belt member and the frictional portion. As a result, it is possible to enhance responsiveness to the lateral deviation of the belt member. 
     Incidentally, in this embodiment, the color image forming apparatus including the intermediary transfer belt is described as an example but another belt driving apparatus and an image forming apparatus including the belt driving apparatus may also be employed. Specifically, a direct transfer belt unit for successively superposing the respective images on the transfer material by attracting the transfer material to a transfer belt as the belt member and an image forming apparatus including the transfer belt unit may also be used. Further, the present invention is also applicable to a photosensitive member belt unit for directly performing processes of charging exposure and developing with respect to a photosensitive member belt as the belt member and then by successively superposing the respective images on the photosensitive member belt and an image forming apparatus including the photosensitive belt unit. Further, the present invention is also effective with respect to a fixing belt of the fixing device. 
     Incidentally, parameter setting values of the sliding ring portion  82  described in this embodiment are merely an example, so that values of the friction coefficient μ and the taper angle φ are not uniquely limited. 
     (Embodiment 2) 
     In this embodiment, the same constitutions as those of the intermediary transfer belt unit  700  and the image forming apparatus  60  including the intermediary transfer belt unit  700  are basically employed. Therefore, the constitution of the image forming apparatus  60  and the operation principle will be omitted from the description and a different portion will be principally explained. Further, in the following, the same portions (members) are represented by the same reference numerals or symbols and will be omitted from the description. 
     &lt;Urging Member Constitution&gt; 
       FIG. 4  is an enlarged view of the end portion of the steering roller for the intermediary transfer belt in this embodiment of the present invention. Specifically, the steering roller has the same constitution as that of the automatic center alignment mechanism described with reference to  FIG. 8  and a difference portion is a supporting constitution for supporting an urging (pressing) member  2  which is a feature of the present invention is provided. 
     The steering roller consists of the follower roller portion  81  rotatably shaft-supported by the roller shaft  89  and the sliding ring portions  82  which are provided at longitudinal ends of the follower roller portion  81  (only one end thereof is shown in  FIG. 4 ) and are non-rotatable by the roller shaft  89 . The sliding ring portion  82  has the tapered shape such that the outer diameter is gradually increased toward the outside. The end portion of the roller shaft  89  has a rotation-stopping shape such as the D-cut shape and is supported non-rotatably by the sliding bearing  83 . The sliding bearing  83  in this embodiment includes a boss portion (not shown) and a slide groove (not shown). The boss portion is engaged with an inner diameter portion of a tension spring  84  and the entire steering roller is urged against the inner peripheral surface of the intermediary transfer belt  606  with predetermined tension. That is, the steering roller also functions as the tension roller. Further, the slide groove is engaged with the side supporting member  85  and guides the supporting member  85  so that the supporting member  85  can move depending on an expansion and construction operation of a tension spring  84 . 
     Here, in this embodiment, from a side surface of the sliding ring portion  82 , a holder member  3  is connected by a screw  35 . To the holder member  3 , the urging member  2  formed with an elastic member is applied at its inside, and the urging member  2  has a deformation amount which follows the tapered shape of the sliding member  82 . 
     The holder member  3  and the urging member  2  basically have the same constitution as the holder portion and the urging member described in Embodiment 1 with reference to  FIG. 3  and have an arcuate shape so as to cover the belt at the winding angle θ S . Further, the urging member  2  at the inner peripheral surface of the holder member  3  has a width Wb including a full width of the sliding ring portion  82  and a width of a part of the follower roller portion  81  and is provided on the basis of a large diameter-side tapered shape portion of the sliding ring portion  82 . Also with respect to the width Wb, similarly as in Embodiment 1, the width Wb is set so as to satisfy a relationship, of: Wb≧L R +2L F −L B , so that the urging member  2  is always in the state in which it treads on the belt end portion. Consequently, there is no possibility that a belt edge is turned up when the belt edge enters the urging member  2 , so that the intermediary transfer belt  606  can smoothly move in a thrust direction during the automatic center alignment operation. Further, the urging member  2  is of a foamed material or the like, so that the deformation amount is increased with the position of the sliding ring portion  82  toward the large diameter side. As a result, an urging force by the urging member  2  can be increased with an increase in lateral belt deviation amount, so that it becomes possible to generate the frictional force more reliably and efficiently even when waving due to elongation of the belt end portion occurs. 
     As described above, in Embodiment 2, the holder member  3  and the urging member  2  are integrally formed with the sliding ring portion  82 , so that these members are caused to follow the inclination change of the steering roller by the surface center alignment operation, so that the deformation amount of the urging member  2  can be kept in the stable state. 
     Thus, according to this embodiment, when the state of the belt end portion which is the free end is not preferable, i.e., even in the case where the waving or the like occurs, it becomes possible to obtain a desired frictional force between the belt member and the frictional portion. As a result, it is possible to enhance responsiveness to the lateral deviation of the belt member. 
     Incidentally, also in Embodiment 2, similarly as in Embodiment 1, the present invention is applicable to not only the color image forming apparatus including the intermediary transfer belt but also another belt driving apparatus and an image forming apparatus including the belt driving apparatus. 
     (Embodiment 3) 
     In Embodiment 3 of the present invention, the image forming apparatus  60  including the intermediary transfer belt unit  700 , the arrangement of the steering device is changed from that in Embodiment 1. The arrangement of the driving roller  604  and the steering roller  80  is interchanged. That is, the cleaning blade  618  urges the intermediary transfer belt against the steering roller  80 , and the driving roller  604  is disposed between the stretching roller  617  and the inner transfer roller  603 . Therefore, the constitution of the image forming apparatus  60  and the operation principle will be omitted from the description and a different portion will be principally explained. Further, in the following, the same portions (members) are represented by the same reference numerals or symbols and will be omitted from the description. 
     &lt;Urging Member Constitution&gt; 
       FIG. 5  is a perspective view for illustrating a relationship between a steering device  800  including the steering roller  80  and a cleaning unit  43  to be mounted on the steering device  800 . The cleaning unit  43  includes the cleaning blade  618 , for removing the transfer residual toner from the intermediary transfer belt  606 , provided on the transfer cleaner device  620  described with reference to  FIG. 1  and includes a cleaning blade supporting portion for supporting the cleaning blade  618 . 
     The steering roller  80  is provided on a frame stay  87  which is a part of the casing of the intermediary transfer belt unit and a swing operation in an arrow S direction with an axis J as a rotational movement center can be performed. The steering roller  80  is constituted by the follower roller portion  81  and sliding portions at its longitudinal ends, and a roller shaft (not shown) is non-rotatably supported by two sliding bearings  40 . Each sliding bearing  40  is engaged with the side supporting member  85  so as to permit the sliding operation and receives the urging force by the tension spring  84 . That is, the steering roller  80  also functions as the tension roller for imparting the belt tension. Here, the sliding bearing  40  and the side supporting member  85  include a fixing boss  40   a , positioning pins  40   b  and  42  and a tap  41  which are used for positioning and fixing the cleaning unit  43 . 
     Next, the constitution of the cleaning unit  43  will be described. A blade supporting plate  45  integrally holds the cleaning blade  618  constituted by an elastic member such as rubber and an end portion seal holder  46  at each of longitudinal ends of the cleaning blade  618 . Onto the end portion seal holder  46 , an urging member  47  is applied. The blade supporting plate  45  is further attached to a blade pressing plate  48 . The blade pressing plate  48  is swingably supported relative to a cleaning unit stay  49  by a swing center shaft  401 . At this time, the plate blade pressing plate  48  and the cleaning unit stay  49  are connected to each other with a blade spring  400 , so that an end of the cleaning blade  618  contacts the follower roller portion  81  at a predetermined angle and a predetermined pressure. Here, each of a front side plate portion  49 F and rear side plate portion  49 R of the cleaning unit stay  49  includes a positioning hole and an elongated hole which are used when the cleaning unit stay  49  is mounted on the steering roller  80 . Specifically, the positioning pin  40   b  of the sliding bearing  40  is engaged in the positioning hole of the rear side plate portion  49 R, and the positioning pin  42  of the side supporting member  85  is engaged in the elongated hole of the rear side plate portion  49 R. The fixing boss  40   a  of the sliding bearing  40  corresponds to the positioning hole of the front side plate portion  49 F, and the tap  41  of the side supporting member corresponds to the elongated hole of the front side plate portion  49 F. However, the fixing boss  40   a  has a stepped end and includes a tap at its end surface and therefore is fixed after the engagement. With respect to the tap  41 , a stepped fixing bias  50  ( FIG. 6 ) is used and a shaft diameter of the stepped fixing bias is configured and positioned to be engaged in an elongated circular hole of the front side plate portion  49 F. Portions which easily illustrate the above fixing are axes K 1  and K 2  shown in  FIG. 5 . Along the respective axes, when the portions of the steering roller  80  and cleaning unit  43  are positioned and connected to each other, a mounted state as shown in  FIG. 6  is created. Incidentally, the intermediary transfer belt  606  to be stretched is hypothetically shown in  FIG. 6  in an easy-to-understand manner. Incidentally, the cleaning unit  43  constitutes the transfer cleaner device in the form in which the cleaning unit  43  is covered with a cleaner case including a feeding screw. 
     As is also understood from  FIG. 6 , a width of the cleaning blade  618  in Embodiment 3 is narrower than that of the follower roller portion  81  and is necessarily contacted to only the follower roller portion  81 . Correspondingly, the end portion seal holder  46  and the urging member  47  contacts with a width Wb including a full width of the sliding ring portion  82  and a width of a part of the follower roller portion  81  and is provided on the basis of a large diameter-side end surface of the sliding ring portion  82 . Here, with respect to the width Wb, similarly as in Embodiment 1, the width Wb is set so as to satisfy a relationship, of: Wb≧L R +2L F −L B , so that the urging member  47  is always kept in the state in which it treads on the belt end portion. Consequently, there is no possibility that a belt edge is turned up when the belt edge enters the urging member  47 , so that the intermediary transfer belt  606  can smoothly move in a thrust direction during the automatic center alignment operation. Further, the end portion seal holder  46  has an arcuate shape which covers the belt at a winding angle θ S , and at an inner peripheral surface of the belt, the urging member  47  is formed of a foamed material or the like in a uniform thickness, so that the deformation amount is increased with the position of the sliding ring portion  82  toward the large diameter side. As a result, an urging force by the urging member  47  can be increased with an increase in lateral belt deviation amount, so that it becomes possible to generate the frictional force more reliably and efficiently even when waving due to elongation of the belt end portion occurs. 
     As described above, in Embodiment 3, the cleaning unit  43  is positioned relative to the sliding bearing  40  and therefore the cleaning blade  618  and the end portion seal holder  46  can follow the inclination change of the steering roller in the automatic center alignment operation, so that both the blade contact pressure and the deformation amount of the urging member  47  can be kept in a stable state. 
     Thus, according to this embodiment, the constitution in which the steering member and the cleaning blade were opposed to each other via the belt member was employed. However, even in such a constitution, when the state of the belt end portion which is the free end is not preferable, i.e., even in the case where the waving or the like occurs, it becomes possible to obtain a desired frictional force between the belt member and the frictional portion. As a result, it is possible to enhance responsiveness to the lateral deviation of the belt member. 
     Incidentally, in this embodiment, the color image forming apparatus including the intermediary transfer belt and the cleaning blade for cleaning the intermediary transfer belt is described as an example. However, the present invention is also applicable to an apparatus having a constitution in which a cleaning blade for cleaning the belt member; which is not limited to the intermediary transfer belt, and the cleaning blade and the steering member are opposed to each other via the belt member. Specifically, a direct transfer belt unit for successively superposing the respective images on the transfer material by attracting the transfer material to a transfer belt as the belt member and an image forming apparatus including the transfer belt unit may also be used. Further, the present invention is also applicable to a photosensitive member belt unit for directly performing processes of charging, exposure and developing with respect to a photosensitive member belt as the belt member and then by successively superposing the respective images on the photosensitive member belt and an image forming apparatus including the photosensitive member belt unit. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 148201/2010 filed Jun. 29, 2010, which is hereby incorporated by reference.