Abstract:
A belt feeding unit includes a rotatable belt; a first stretching roller; a second stretching roller including a stretching portion; a supporting portion for supporting the stretching portion while permitting movement of opposite end portions thereof up and down; a rotatable portion coaxial with the second stretching roller; wherein when the belt shifts in the longitudinal direction, the belt contacts the rotatable portion to rotate the rotatable portion, thus tilting the second stretching roller about the supporting portion relative to a rotational axis of the first stretching roller.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a belt driving unit which circularly moves an endless belt while suspending the belt with multiple belt suspending members. It relates also to an image forming apparatus such as an electrophotographic print, an electrophotographic copy machine, and the like, which is equipped with a belt driving unit in accordance with the present invention. 
       BACKGROUND ART 
       [0002]    Some electrophotographic image forming apparatuses, for example, electrophotographic printers, electrophotographic copy machines, and the like are equipped with a belt driving unit which circularly moves an endless belt while suspending the belt with multiple belt suspending rollers. A belt driving unit such as the one described above is problematic in that as the belt is circularly driven, it tends to shift in the widthwise direction of the belt (direction roughly perpendicular to moving direction of belt). 
         [0003]    There are various methods for controlling this lateral movement of the belt. One of them is disclosed in Japanese Laid-open Patent Application H05-26314. According to this patent application, the belt driving unit is provided with multiple belt suspending rollers, and one of the multiple belt suspending rollers is utilized as a belt position adjustment roller, that is, a roller for correcting the belt in terms of lateral shift. Further, the belt driving unit is provided with a belt position detection roller which is independently rotatable from the roller for correcting the belt in terms of lateral shift. The belt position detection roller is positioned next to one of the lengthwise ends of the belt suspending roller in order to correct the belt in terms of lateral shift. Further, the belt driving unit is provided with a string with a preset length. One end of the string is attached to the peripheral surface of the belt position detection roller. As the belt laterally shifts, it comes into contact with the belt position detection roller. Thus, the friction between the belt and belt position detection roller causes the belt position detection roller to rotate. Thus, the string is wound around the peripheral surface of the belt position detection roller, causing the rotational axis of the belt position adjustment roller to tilt in such a manner that the belt is made to shift toward the other end of the belt position adjustment roller. 
         [0004]    However, the structural arrangement disclosed in Japanese Laid-open Patent Application H05-26314 makes it necessary for the other end (opposite end from belt position detection roller) of the belt position adjustment roller to be supported. Thus, the distance by which the belt position adjustment roller has to move to cancel the unwanted lateral shift of the belt is substantial. In other words, it is slow in response. Thus, it needs to be improved in responsiveness, and also, requires a large space in order to allow the belt position adjustment roller to sufficiently tilt for the belt to be put back into its proper position. 
       SUMMARY OF THE INVENTION 
       [0005]    Thus, the primary object of the present invention is to provide a belt driving unit which is substantially smaller in the amount by which one of the belt suspension rollers is moved to keep the belt properly positioned in terms of the widthwise direction of the belt, than any belt unit driving unit in accordance with the prior art, and an image forming apparatus equipped with the belt driving unit in accordance with the present invention. 
         [0006]    According to an aspect of the present invention, there is provided a belt feeding unit comprising a rotatable belt; a first stretching roller for stretching said belt; a second stretching roller including a stretching portion for stretching said belt, said stretching portion being rotated by rotation of said belt; a supporting portion for supporting said stretching portion while permitting movement of opposite end portions of said stretching portion with respect to a longitudinal direction perpendicular to a moving direction said belt; and a rotatable portion substantially coaxial with said second stretching roller and which is rotatable codirectionally with a moving direction of said belt relative to said stretching portion, wherein when said belt shifts in the longitudinal direction, said belt contacts said rotatable portion to rotate said rotatable portion, thus tilting said second stretching roller about said supporting portion relative to a rotational axis of said first stretching roller. 
         [0000]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic sectional view of a typical image forming apparatus equipped with a belt driving unit, as an intermediary transfer unit, in accordance with the present invention, and describes the general structure of the apparatus. 
           [0008]      FIG. 2  is a perspective view of the belt driving unit in the first embodiment of the present invention, and shows the structure of the unit. 
           [0009]      FIG. 3  is a perspective view of the belt in accordance with the present invention, and shows the structure of the belt. 
           [0010]      FIG. 4  is a schematic sectional view of the belt in accordance with the present invention, at a plane which is perpendicular to the belt movement direction and parallel to the widthwise direction of the belt, and shows the structure of the belt. 
           [0011]      FIG. 5  is a perspective view of the belt suspending second member of the belt driving unit in the first embodiment, and shows the structure of the member. 
           [0012]      FIG. 6  is a schematic sectional view of the belt suspending second member of the belt driving unit in the first embodiment, at a vertical plane which coincides with the axial line of the belt suspending second member, and shows the structure of the member. 
           [0013]      FIG. 7  is a side view of the lengthwise end portions of the belt suspending second member of the belt driving unit in the first embodiment, and shows the structure of the lengthwise end portions. 
           [0014]      FIG. 8  is an enlarged perspective view of one of the lengthwise end portions of the belt suspending second member in the first embodiment, and shows the structure of the member. 
           [0015]      FIG. 9  is a perspective view of the belt driving unit in the first embodiment of the present invention, and shows the structure of the unit. 
           [0016]      FIG. 10  also is a perspective view of the belt driving unit in the first embodiment of the present invention, and shows the structure of the unit. 
           [0017]      FIG. 11  is a plan view of one of the lengthwise end portions of the belt supporting second member in the first embodiment, as seen from the lengthwise direction of the member, and is for describing the movement of the belt position adjustment automatic mechanism in the first embodiment. 
           [0018]      FIG. 12  is a schematic sectional view of the belt supporting second member in the first embodiment, at a vertical plane which coincides with the axial line of the member, and is for describing the movement of the belt position adjustment automatic mechanism. 
           [0019]      FIG. 13  is an enlarged perspective view of one of the lengthwise end portions of the belt suspending second member in the second embodiment, and shows the structure of the member. 
           [0020]      FIG. 14  is a perspective view of the belt suspending second member, and its adjacencies, in the second embodiment of the present invention, and shows the structure of the belt driving unit. 
           [0021]      FIG. 15  is a side view of the lengthwise end portions of the belt suspending second member of the belt driving unit in the third embodiment, and shows the structure of the lengthwise end portions. 
           [0022]      FIG. 16  is an enlarged perspective view of one of the lengthwise end portions of the belt suspending second member in the fourth embodiment, and shows the structure of the member. 
           [0023]      FIG. 17  is a plan view of one of the lengthwise end portions of the belt supporting second member in the first embodiment, as seen from the lengthwise direction of the member, and is for describing the movement of the belt position adjustment automatic mechanism in the fourth embodiment. 
           [0024]      FIG. 18  is an enlarged perspective view of one of the lengthwise end portions, and its adjacencies, of the belt suspending second member in the fifth embodiment, and shows the structure of the member. 
           [0025]      FIG. 19  is a perspective view of the belt suspending second member of the belt driving unit, and its adjacencies, in the fifth embodiment of the present invention, and shows the structure of the belt driving unit. 
           [0026]      FIG. 20  is a perspective view of the belt suspending second member of the belt driving unit, and its adjacencies, in the sixth embodiment of the present invention, and shows the structure of the belt driving unit. 
           [0027]      FIG. 21  is a schematic sectional view of a typical image forming apparatus equipped with a belt driving unit, as a recording medium conveyance unit, in accordance with the present invention, and describes the general structure of the apparatus. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    Hereinafter, image forming apparatuses in accordance with the present invention will be described in detail with reference to the appended drawings. 
       Embodiment 1 
       [0029]    &lt;Overall Structure of Image Forming Apparatus&gt; 
         [0030]      FIG. 1  is a schematic sectional view of a typical color image forming apparatus equipped with a belt driving unit, as an intermediary transfer unit, in accordance with the present invention, and shows the general structure of the apparatus. To begin with, the structure of the image forming apparatus  100  in this embodiment is described referring to  FIG. 1 . The image forming apparatus  100  can electrophotographically form an image on a sheet P of recording medium such as ordinary recording paper, OHP film, etc., in response to electrical signals sent from an external devices such as a personal computer connected to the image forming apparatus  100  in such a manner that information can be exchanged between the image forming apparatus and external devices. 
         [0031]    The image forming apparatus  100  has multiple (four in this embodiment) image formation units  110 Y,  110 M,  110 C and  110 K, which form yellow, magenta, cyan and black monochromatic toner images, respectively. The image formation units  11 Y,  110 M,  110 C and  110 K are roughly horizontally aligned. The image forming apparatus  100  has also an intermediary transfer unit  120 , which comprises a belt driving unit. The intermediary transfer unit  120  is positioned so that it opposes each of the image formation units  1100 Y,  110 M,  110 C and  110 K. 
         [0032]    The intermediary transfer unit  120  has an intermediary transfer belt  1 , which is a circularly movable endless belt. It is structured so that the intermediary transfer belt  1  can be circularly moved while being in contact with each of the image formation units  1100 Y,  110 M,  1100 C and  110 K. 
         [0033]    In the case of the image forming apparatus  100  in this embodiment, as the belt  1  is circularly moved, monochromatic toner images formed in the image formation units  11 Y,  110 M,  110 C and  110 K, one for one, are sequentially transferred onto the intermediary transfer belt  1 , and then, are transferred together from the intermediary transfer belt  1  onto a sheet  20  of recording medium to effect a full-color image on the sheet  20  of recording medium. 
         [0034]    The image formation units  1100 Y,  110 M,  1100 C and  110 K are the same in structure and function, although they are different in the color of the toner image they form. Thus, unless they need to be individually described, they are going to be described as the same image formation units  110 , that is, without suffixes Y, M, C and K which indicate the color of the monochromatic toner image they form. This rule applies also to the description of the image formation process of the image formation units  1100 Y,  110 M,  1100 C and  110 K. 
         [0035]    The image formation unit  110  forms a monochromatic toner image with use of one of known electrostatic image formation processes. More specifically, the image forming apparatus  100  is provided with a photosensitive drum  111 , that is, an electrophotographic photosensitive member, which is a cylindrical image bearing member on which an electrostatic latent image is formed. The photosensitive drum  111  is rotatable in the direction indicated by an arrow mark in  FIG. 1 . The image forming operation of the image forming apparatus  100  is as follows: First, the peripheral surface of the rotating photosensitive drum  111  is uniformly charged by a charge roller  112  which functions as a charging unit. 
         [0036]    Next, the uniformly charged area of the peripheral surface of the photosensitive drum  111  is scanned by (exposed to) a beam of laser light emitted by a laser scanner, as an exposing unit, while being modulated by signals sent from a computer. Consequently, an electrostatic latent image is effected on the peripheral surface of the photosensitive drum  111 . To the electrostatic latent image on the photosensitive drum  111  (image bearing member), toner is supplied as developer by a development unit  113 , as a developing means, whereby the electrostatic latent image is developed into a visible image, that is, an image formed of toner (which hereafter will be referred to simply as “toner image”). 
         [0037]    The toner image formed on the photosensitive drum  111  through the above described processes is electrostatically transferred onto the belt  1  by the function of a primary transfer roller  121 , which functions as a primary transfer member. The primary transfer roller  121  is positioned on the opposite side of the belt  1  of the intermediary transfer unit  120  from the photosensitive drum  111 . More specifically, the intermediary transfer unit  120  is made up of the belt  1 , the belt driving unit for circularly moving the belt  1 , and primary transfer rollers  121 . The primary transfer roller  121  is kept pressed against the peripheral surface of the photosensitive drum  111  with the presence of the belt  1  between the photosensitive drum  1  and primary transfer roller  121 , forming thereby the primary transfer nip T 1 , between the belt  1  and photosensitive drum  111 . 
         [0038]    The monochromatic toner images formed on the photosensitive drums  111  of image formation units  110  in synchronism with the movement of the belt  1  through the above described image formation processes are sequentially transferred in layers onto the belt  1 . Meanwhile, sheets  20  of recording medium in the recording medium storage  140  are pulled out one by one from the storage  140  by a pickup roller  141  or the like, and are sequentially sent to a pair of registration rollers  142 . Then, each sheet  20  of recording medium is released by the registration rollers  142  with a preset timing so that the sheet  20  is conveyed to the secondary transfer nip T 2 , that is, the area of contact between the secondary transfer roller  143  and belt  1 . In the secondary transfer nip T 2 , the toner images on the belt  1  are electrostatically transferred onto the sheet  20  of recording medium by the function of the secondary transfer roller  143 . 
         [0039]    Then, the sheet  20  of recording medium is separated from the belt  1 , and is conveyed to the fixation unit  150 . Then, the sheet  20  is conveyed through the fixation unit  150 . While the sheet  20  is conveyed through the fixation unit  150 , the sheet  20  and the toner images thereon are subjected to heat and pressure. Consequently, the toner images on the sheet  20  become solidly fixed to the sheet  20 . Then, the sheet P is further conveyed, and discharged by a pair of discharge rollers  160  onto a delivery tray  170 . 
         [0040]    In the case of the image forming apparatus  100  in this embodiment, the photosensitive drum  111 , charge roller  112 , and development unit  113  of each image formation unit  110  are integrally placed in a shell (cartridge), forming a process cartridge, which is removably installable in the main assembly of the image forming apparatus  100 . Further, the intermediary transfer unit  120  also is removably installable in the main assembly of the image forming apparatus  100 . 
       (Intermediary Transfer Unit) 
       [0041]    Next, referring to  FIG. 2 , the belt driving portion  200  of intermediary transfer unit  120 , which is for circularly moving the belt  1  of the intermediary transfer unit  120  is described about its overall structure.  FIG. 2  is a perspective view of the belt driving portion  200  of intermediary transfer unit  120  in this embodiment, and illustrates the external appearance of the portion  200 . 
         [0042]    The intermediary transfer unit  120  has the belt driving portion  200 , the belt  1 , and a belt driving roller  2 . The belt driving roller  2  is one of the multiple belt suspending members (rollers) for suspending the belt while providing the belt with a preset amount of tension. It drives the belt  1 . Further, the intermediary transfer unit  120  is provided with a slave roller  3  and a tension roller  4 . The slave roller  4  is rotated by the circular movement of the belt  1 . The tension roller  4  is for providing the belt  1  with a preset amount of tension. That is, the belt  1  is held by three rollers so that it can be circularly moved. 
         [0043]    The belt driving roller  2  (which hereafter will be referred to simply as driving roller  2 ) and slave roller  3  are rotatably supported by a pair of bearings  5   a  and  5   b  and a pair of bearings  6   a  and  6   b , respectively, by their lengthwise end portions. Here, the “widthwise direction” of the belt  1  means the direction roughly perpendicular to the direction of the belt movement. 
         [0044]    The intermediary transfer unit  120  is provided with a pair of side plate  7   a  and  7   b , which are solidly attached to the main assembly of the image forming apparatus  100 . The aforementioned bearings  5   a ,  5   b ,  6   a , and  6   b , which support two rollers  2  and  4  are held by the side plates  7   a  and  7   b , respectively. The driving roller  2  is rotated by the mechanical force transmitted thereto by an unshown mechanical power source such as a motor with which the image forming apparatus  100  is provided. As the driving roller  2  is rotated, it circularly moves the belt  1  in the direction indicated by an arrow mark B in  FIG. 2 . The slave roller  3  is rotated by the movement of the belt  1 , which is caused by the driving roller  2 . The slave roller  3  is supported roughly in parallel to the driving roller  2 . 
         [0045]    The tension roller  4  is supported in such a manner that it is movable in the direction indicated by an arrow mark T in  FIG. 2 . More specifically, the lengthwise end portions of the axle  8  of the tension roller  4  are fitted with a pair of bearings  9   a  and  9   b , one for one, which are fitted in the long and narrow holes  10   a  and  10   b  with which the aforementioned side plates  7   a  and  7   b , respectively, being allowed to slide in the moving direction of the belt  1 . The bearings  9   a  and  9   b  are under the pressure generated by a pair of springs  11   a  and  11   b  as pressure applying elastic members. Thus, the belt  1  is provided with a preset amount of tension. As the belt  1  is driven by the driving roller  2 , the tension roller  4  is rotated by the circular movement of the belt  1 . 
         [0046]    The belt driving portion  200  has a belt deviation control portion, which changes the angle between the axle  2   a  of the driving roller  2  and the axle  8  of the tension roller  4 , in response to the positional deviation of the belt  1  in the direction roughly perpendicular to the moving direction of the belt  1 . 
         [0047]    In this embodiment, the driving roller  1  may be referred to simply as the first suspension roller, whereas the tension roller  4  which makes up a part of the belt shift control portion may be referred to simply as the second suspension roller. 
         [0048]      FIG. 3  is a perspective view of the belt  1 , and shows the structure of the belt  1 .  FIG. 4  is a schematic sectional view of the belt  1  at a plane which is perpendicular to the belt movement direction and parallel to the widthwise direction of the belt. Referring to  FIGS. 3 and 4 , the belt  1  is provided with a pair of ribs  12   a  and  12   b , which are attached to the inward surface of the belt  1 , being positioned slightly inward of the corresponding lateral edges of the belt  1 , one for one, and also, being position so that the distance between the ribs  12   a  and  12   b  in terms of the widthwise direction of the belt is La. The ribs  12   a  and  12   b  project roughly perpendicular to the inward surface of the belt  1 . They extend across the entire inward circumference of the belt  1 . 
         [0049]    Next, referring to  FIGS. 5-9 , the structure of the tension roller  4  is described. 
         [0050]      FIG. 5  is a perspective view of the tension roller  4 , and  FIG. 6  is a schematic sectional view of the tension roller  4 , at a vertical plane which coincides with the axial line of the tension roller  4 . Referring to  FIGS. 5 and 6 , the tension roller  4  is made up of a cylindrical hollow sleeve  13 , and the axle  8 , as a referential member, put through the sleeve  13 . The belt driving unit is structured so that the belt  1  wraps around the peripheral surface of the sleeve  13 . The sleeve  13  is the portion of the tension roller  4 , that suspends and keeps tensioned the belt  1 . 
         [0051]    The roughly center portion of the axle  8  in terms of the widthwise direction of the belt  1  is fitted with a bearing  14 , which supports the axle  8  in such a manner that the sleeve  13  is allowed to tilt relative to the axle  8 . That is, the axle  8  applies to the sleeve  13 , the pressure applied to the axle  8  by a pair springs  11   a  and  11   b , by holding the sleeve  13  by the roughly center portion of the sleeve  13  in such a manner that the sleeve  13  is allowed to rotationally move about the bearing  14 . In other words, the sleeve  13  is supported by the bearing  14 , only at its roughly center portion. 
         [0052]    Further, the belt driving portion  200  is structured so that the external diameter of the bearing  14  is smaller than the internal diameter of the sleeve  13 , and also, so that the external diameter peripheral surface of the axle  8  smaller than the internal diameter of a pair of flanges  15   a  and  15   b . That is a gap  21  is provided between the axle  3  and each of the flanges  15   a  and  15   b . The presence of this gap  21  enables the sleeve  13  to rotationally move about the bearing  14 . In other words, the rotational axis of the sleeve of the tension roller  4  can be tilted relative to the axle  8 . Incidentally, instead of the bearing  14  in this embodiment, one of the known automatic centering bearings structured so that its outward surface is allowed to tilt relative to its inward surface may be used. 
         [0053]    Next referring to  FIG. 6 , the lengthwise ends of the sleeve  13  are fitted with flanges  15   a  and  15   b  in such a manner that the sleeve  13  and each of the flanges  15   a  and  15   b  are not movable relative to each other. The flanges  15   a  and  15   b  have cylindrical portions  15   a   1  and  15   a   2 , respectively, which are inserted into the hollow of the sleeve  13  in order to keep the flanges  15   a  and  15   b  solidly attached to the sleeve  13 . The flanges  15   a  and  15   b  have also collar portions  15   a   2  and  15   b   2 , which come into contact with the lengthwise ends of the sleeve  13 , one for one, as the flanges  15   a  and  15   b  are inserted into the sleeve  13 . Further, the flanges  15   a  and  15   b  have bosses  15   a   3  and  15   b   3 , which rotatably support a pair of rollers  16   a  and  16   b , which rotate around the bosses  15   a   3  and  15   b   3 , respectively. 
         [0054]    The flanges  15   a  and  15   b  rotate with the sleeve  13 . The internal diameter of each of the flanges  15   a  and  15   b  is greater than the external diameter of the axle  8 . Thus, the sleeve  13  is allowed to rotationally move about the bearing  14 . Further, the flanges  15   a  and  15   b  regulates the movement of the sleeve  13  in the direction of the rotational axis of the sleeve  13 , by coming into contact, by their outward edges, with the bearings  9   a  and  9   b  fitted in the long and narrow holes of the side plates  7   a  and  7   b  solidly attached to the frame of the main assembly of the image forming apparatus  100 . 
         [0055]    The rollers  16   a  and  16   b  are cylindrical and hollow. They are rotatably fitted around the bosses  15   a   3  and  15   b   3  of the flanges  15   a  and  15   b , respectively. More specifically, the rollers  16   a  and  16   b  are fitted around the bosses  15   a   3  and  15   b   3  of the flanges  15   a  and  15   b  in such a manner that their rotational axis coincide with those of flanges  15   a  and  15   b , which coincide with the rotational axis of the tension roller  4 , or the second suspension roller. They are at the lengthwise ends of the tension roller  4 , and are allowed to rotate independently from the sleeve  13  and flanges  15   a  and  15   b , respectively, while remaining coaxial with the sleeve  13  and flanges  15   a  and  15   b.    
         [0056]      FIG. 7  is an enlarged side view of the lengthwise end portions of the tension roller  4 .  FIG. 8  is a perspective view of one of the lengthwise end portions of the tension roller  4 . Referring to  FIGS. 7 and 8 , the rollers  16   a  and  16   b , which are fitted around the bosses  15   a   3  and  15   b   3  of the flanges  15   a  and  15   b , respectively, have a belt contacting portion  16   c , which is roughly the same in external diameter as the sleeve  13 . Further, the rollers  16   a  and  16   b  have a rib separating portion  16   d , the external diameter of which is large enough to create a step between itself and the ribs  12   a  and  12   b . Further, the rollers  16   a  and  16   b  have a rib contacting portion  16   e , which is the slant surface which connects the belt contacting portion  16   c  with the rib separating portion  16   d . Incidentally, the rollers  16   a  and  16   b  may be structured so that the rib contacting portion  16   e  is perpendicular to the axial lines of the rollers  16   a  and  16   b.    
         [0057]    Referring to  FIG. 7 , the belt  1  and ribs  12   a  and  12   b  do not come into contact with the rib separating portion  16   d  of the roller  16   a , nor the rib separating portion of the roller  16   b . As the belt  1  shifts in the left or right direction in  FIG. 7 , either the inward side of the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a , or the inward side of the rib  12   b  comes into contact with the rib contacting portion  16   e  of the roller  16   b . In this embodiment, the angle of the rib contacting portion  16   e  relative to the axial line of the roller  16   a  and  16   b  is 80 degrees. 
         [0058]    Referring again to  FIG. 7 , the distance La between the ribs  12   a  and  12   b , which are on the widthwise end portions of the belt  1 , one for one, is set to be greater than a distance Lb between the point of contact between the rib contacting portion  16   e  of the roller  16   a , and the point of contact between the rib contacting portion  16   e  of the roller  16   b . The rollers  16   a  and  16   b  are positioned at the lengthwise ends of the tension roller  4 , one for one. Therefore, as the belt  1  shifts in its widthwise direction, either the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a , or the rib  12   b  comes into contact with the rib contacting portion  16   e  of the roller  16   b.    
         [0059]    Each of the rollers  16   a  and  16   b  has a cam portion  16   f , which is on the outward side of the rib separating portion  16   d , and also, on the outward side of the edge of the belt  1 , in terms of the widthwise direction of the belt  1 . 
         [0060]      FIG. 9  is a perspective view of the belt driving portion  200 , that is, the intermediary transfer unit  120  minus the belt  1 . The side plates  7   a  and  7   b  solidly attached to the frame of the image forming apparatus  100  are provided with stopper portions  17   a  and  17   b , respectively, which are positioned so that after the assembly of the belt driving portion  200 , the stopper portions  17   a  and  17   b  are a preset distance away from the rotational axis of the tension roller  4 . On the other hand, each of the rollers  16   a  and  16   b  is provided with the aforementioned cam portion  16   f , which comes into contact with the stopper portion  17   a  or  17   b , respectively. 
         [0061]    As the belt  1  shifts in its widthwise direction relative to the tension roller  4  (second suspension roller), either the rib  12   a  which is on the inward surface of the belt  1  and on the slightly inward side of the corresponding edge of the belt  1 , comes into contact with the rib contact portion  16   e  of the roller  16   a , or the rib  12   b  which is on the inward surface of the belt  1  and on the slightly inward side of the corresponding edge of the belt  1  comes into contact with the rib contact portion  16   e  of the roller  16   b . Thus, one of the rollers  16   a  and  16   b  is rotated by the rotational force transmitted thereto by the friction between the roller  16  and rib  12 , causing the cam portion  16   f  of the roller  16  to come into contact with the stopper portion  17  ( 17   a  or  17   b ). Consequently, the roller  16   a  or  16   b  is prevented from rotating further. 
         [0062]    Next, referring to  FIG. 12 , even after the cam portion  16   f  of the roller  16  came into contact with the stopper portion  17 , the rotational force continues to be transmitted to the roller  16 . Thus, the gap  21  (clearance) between the peripheral surface of the axle  8  and the inward surface of the flange  15  ( 15   a  or  15   b ) allows the flanges  15  ( 15   a  or  15   b ), the rotational axis of which coincides with that of the roller  16  ( 16   a  or  16   b ), to displace upward. Consequently, the sleeve  13  of the tension roller  4  is tilted in such a manner that one of its lengthwise ends moves upward, causing the rotational axis of the sleeve  13  of the tension roller  4  to tilt relative to the axle  8 . That is, the rotational axis of the sleeve  13  of the tension roller  4  becomes tilted relative to the rotational axis  2   a  of the driving roller  2 . 
         [0063]    Even after the tilting of the sleeve  13 , the rollers  16   a  and  16   b  are allowed to freely rotate relative to the flanges  15   a  and  15   b  and sleeve  13 . Therefore, the flanges  15   a  and  15   b , by which the rollers  16   a  and  16   b  are held, and the sleeve  13 , are allowed to continue to be rotated by the movement of the belt  1 . 
         [0064]    Next, referring to  FIGS. 10-12 , the mechanism for correcting the belt in position in terms of the widthwise direction of the belt  1 , by utilizing the tilting of the sleeve  13  of the tension roller  4  is described about its operation. Hereafter, this mechanism may be referred to simply as “belt centering mechanism”. 
         [0065]      FIG. 10  is a drawing of the belt driving portion  200 , that is, the intermediary transfer unit  120  minus the belt  1 . As the belt  1  circularly moves in the direction indicated by the arrow mark B in  FIG. 10  by being driven by the driving roller  2 , the belt  1  shifts in the direction indicated by an arrow mark M or N in  FIG. 10 , because of the above-described structure of the belt driving portion  200 . Thus, the rib  12   a  or  12   b , with which the belt  1  is provided, comes into contact with the rib contacting portion  16   e  of the rollers  16   a  or  16   b , respectively, giving rotational force to the rollers  16   a  or  16   b , respectively. 
         [0066]    In terms of the widthwise direction of the belt  1 , the ribs  12   a  and  12   b  are on the outward side of the rib contacting portion  16   e  of the rollers  16   a  and  16   b , respectively. Thus, as the belt  1  shifts rightward (direction indicated by arrow mark M) in  FIG. 10 , the roller  16   a , or the left roller  16  in  FIG. 10 , is given the rotational force, whereas as the belt  1  shifts leftward (direction indicated by arrow mark N) in  FIG. 10 , the roller  16   b , or the right roller  16   b  in  FIG. 10 , is given the rotational force. 
         [0067]    That is, in this embodiment, in a case where the belt  1  shifts in the direction of the rib  12   b  (second protrusion) which is on the roller  16   a  as the first rotational member receives rotational force from the belt  1  through the rib  12   a , which is on the inward side of the rib  12 . In a case where the belt  1  shifts toward the rib  12   a  (first protrusion), the roller  16   b  as the second rotational member rotates by receiving the rotational force from the belt  1  through the rib  12   b.    
         [0068]      FIG. 11  is a drawing for describing what occurs as the cam portions  16   f  of the roller  16   a  or  16   b  comes into contact with the stopper  17   a  or  17   b , respectively. Referring to  FIG. 11 , as the contact occurs, the roller  16  ( 16   a  or  16   b ) comes under the rotational force which it receives from the rib  12  ( 12   a  or  12   b ), and which acts in the direction to rotate the roller  16  ( 16   a  or  16   b ) in the direction indicated by an arrow mark E in  FIG. 11 . However, the roller  16   a  is prevented by the contact between the cam portion  16  and stopper  17  ( 17   a  or  17   b ), from rotationally moving. Thus, the reaction force resulting from the rotational force, which the roller  16  receives from the belt  1  through the rib  12  ( 12   a  or  12   b ), causes the roller  16  ( 16   a  or  16   b ) to move upward from the position indicated by a broken line in  FIG. 11  to the position indicated by a solid line. Consequently, the flange  15  ( 15   a  or  15   b ), around the boss  15  ( 15   a  or  15   b ) of which the roller  16  ( 16   a  or  16   b ) is fitted, and which is coaxial with the roller  16  ( 16   a  or  16   b ) is pushed upward. Thus, the sleeve  13 , in the lengthwise end portion of which the cylindrical portion of the flange  15  ( 15   a  or  15   b ) is fitted, is tilted in such a direction that the corresponding lengthwise end of the sleeve  13  is positioned higher than the opposite end of the sleeve  13 . 
         [0069]      FIG. 12  is a schematic sectional view of the tension roller  4 , at a vertical plane which coincides with the axial line of the tension roller  4 , under the condition in which the sleeve  13  of the tension roller  4  has become tilted relative to the axle  8 . If the roller  16   a , or the left roller  16  in  FIG. 12 , is made to react as shown in  FIG. 11 , the sleeve  13  rotationally moves about the bearing  14  in the direction indicated by an arrow mark F in  FIG. 12 , from the position indicated by the broken line in  FIG. 12 , and in which it is horizontal, to the position indicated by the solid line in  FIG. 12 . 
         [0070]    First, a case in which the belt  1  shifts in the direction indicated by the arrow mark M in  FIG. 10  is described. In this case, the left rib  12   a  in  FIG. 10  comes into contact with the rib contacting portion  16   e  of the roller  16   a . Thus, the roller  16   a  is given rotational force by the friction between the rib  12   a  and the rib contacting portion  16   e  of the roller  16 , being thereby rotated until its cam portion  16   f  comes into contact with the stopper portion  7   a . The contact between the cam portion  16   f  of the roller  16   a  and the stopper portion  7   a  prevents the roller  16   a  from rotating further. Thus, the flange  15   a  is moved (upward) in the direction indicated by an arrow mark G in  FIG. 12 . Consequently, the sleeve  13  is rotationally moved about the bearing  14  in such a direction that its lengthwise end, in which the flange  15   a  is fitted, moves upward. 
         [0071]    As the lengthwise end of the sleeve  13 , in which the flange  15   a  is fitted, moves upward, the opposite (right) end of the sleeve  13  moves in the direction (downward) indicated by an arrow mark H in  FIG. 12 . However, the rib contacting portion  16   e  of the roller  16   b  has been moved by the shifting of the belt  1 , into a position in which it cannot contact the rib  12   b . Therefore, the roller  16   b  is not subjected to the rotational force from the belt  1 . Therefore, it does not interfere with the tilting of the sleeve  13  in the direction indicated by the arrow mark H in  FIG. 12 . 
         [0072]    As described above, as the sleeve  13  of the tension roller  4  tilts relative to the driving roller  2  and slave roller  3 , the force which has been affecting the belt  1  in such a manner that the belt  1  is shifted in the direction indicated by the arrow mark M reduces, being overcome by the force which affects the belt  1  in such a manner that the belt  1  is shifted in the direction indicated by the arrow mark N. Thus, the belt  1  begins to shift in the direction indicated by the arrow N, causing the rib  12   a  to separate from the rib contacting portion  16   e  of the roller  16   a.    
         [0073]    That is, if the belt  1  shifts in the direction indicated by the arrow mark M in  FIG. 10 , the sleeve  13  is tilted in the direction to cancel the force which causes the belt  1  to shift, and is kept tilted until the rib  12   a  disengages from the rib contacting portion  16   e  of the roller  16   a.    
         [0074]    Similarly, as the belt  1  shifts in the direction indicated by the arrow mark N (opposite direction from direction indicated by arrow mark M) in  FIG. 10 , the rib contacting portion  16   e  of the roller  16   b  (opposite roller from roller  16   a ) comes into contact with the side wall of the rib  12   b . Consequently, the sleeve  13  is tilted in the opposite direction from the direction in which it is tilted as the belt  1  shifts in the direction indicated by the arrow mark M. Thus, the belt  1  is shifted back in the direction indicated by the arrow mark M. 
         [0075]    As described above, as the belt  1  deviates in position in its widthwise direction, either the rib  12   a  or  12   b  attached to the belt  1  comes into contact with the roller  16   a  or  16   b , respectively, giving rotational force to the roller  16   a  or  16   b , respectively, whereby the sleeve  13  is directly tilted by the roller  16   a  or  16   b . That is, the force which affects the belt  1  in such a manner that the belt shifts in position in its widthwise direction is directly converted into such force that works in the direction to tilt the sleeve  13  of the tension roller  4 . As a result, it is possible to tilt the sleeve  13  of the tension roller  4  relative to the driving roller  2  and slave roller  3 . 
         [0076]    In this embodiment, the belt driving portion  200  has three rollers, that is, the driving roller  2 , slave roller  3 , and tension roller  4 , by which the belt  1  is suspended so that the belt  1  can be circularly moved. However, the present invention is applicable to a belt driving mechanism having only the driving roller  2  and tension roller  4 . Further, the present invention is also applicable to a belt driving mechanism having four or more rollers, as long as at least one of the roller is provided with a roller tilting mechanism such as the one with which the tension roller  4  in this embodiment is provided. 
         [0077]    As described above, according to this embodiment, the present invention makes it unnecessary that the rollers for rotatably suspending the belt  1  are extremely precise in dimension and parallelism, and also, that the belt  1  and ribs  12   a  and  12   b  are extremely precise in dimension. Further, the present invention can provide a belt driving mechanism which is simple in structure, and yet can automatically prevent in realtime the problem that the belt  1  is made to laterally deviate in position by the distortion of the belt driving mechanism which occurs during its setup, and/or its wear resulting from usage. 
       Embodiment 2 
       [0078]    Next, referring to  FIGS. 13 and 14 , the invention is described with reference to another belt driving unit in accordance with the present invention, and an image forming apparatus equipped with this belt driving unit. In the following description of the second embodiment, the components, portions, etc., of the belt driving unit and image forming apparatus, which are the same in structure as the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, and are not going to be described here. In the first embodiment, both the widthwise edge portions of the belt  1  were provided with ribs  12   a  and  12   b , one for one. Further, both the lengthwise end portions of the tension roller  4  were provided with rollers  16   a  and  16   b , one for one, having the rib contacting portion  16   e , which corresponds to the ribs  12   a  or  12   b , respectively. 
         [0079]    In this embodiment, the belt driving unit is structured so that the belt  1  is made to tend to laterally shift in only one direction in terms of its widthwise direction. Further, the belt  1  is provided with only one rib  12  ( 12   a ), which is attached to the adjacencies of one of the edges of the belt  1 . Moreover, only the end of the tension roller  4 , which corresponds in position to the belt edge having the rib  12  ( 12   a ) is provided with the roller  16  ( 16   a ) having the rib contacting portion  16   e . Further, the belt driving unit is structured so that as the belt  1  shifts in the preset direction, the roller  16   a  is rotated by the rotational force it receives from the belt  1  through the rib  12   a.    
         [0080]      FIG. 13  is an enlarged perspective view of one of the lengthwise end portions of the tension roller  4  of the belt driving portion  200  in this second embodiment. It does not show the belt  1 . Referring to  FIG. 13 , the belt driving portion  200  in this embodiment is provided with a bearing  18 , which is independent from the roller  16   a . The bearing  18  is rotatably fitted around the end portion of the boss  15   a   3  of the flange  15   a , and is kept under the downward force (in  FIG. 13 ) generated by a spring  19  engaged with a part of the bearing  18 . Thus, the sleeve  13  is always under such pressure that works in the direction to tilt the sleeve  13  in one direction. In other words, the belt driving portion  200  in this embodiment is structured so that the belt  1  tends to laterally shift in only one direction (leftward in  FIG. 14 ). 
         [0081]    This embodiment is described with reference to a case in which the flange  15   a , that is, the flange  15 , with which the left end portion of the tension roller  4  in  FIG. 14  is provided, is provided with the above-described pressure applying mechanism. The pressure generated by the spring  19  keeps the sleeve  13  tilted so that the left side of the sleeve  13  is positioned lower (indicated by arrow mark P) than the right side of the sleeve  13 , that is, the right side of the sleeve  13  is positioned higher (indicated by arrow mark Q) than the left side of the sleeve  13 . Thus, as the belt  1  is circularly moved in the direction indicated by an arrow mark B in  FIG. 14  by the belt driving portion  200  in this embodiment, it tends to shift in the direction indicated by an arrow mark M in  FIG. 14 . 
         [0082]    As the belt  1  shifts in the direction indicated by the arrow mark M, the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a , giving thereby the roller  16   a  the rotational force from the belt  1 . Thus, the cam portion  16   f  of the roller  16   a  comes into contact with the stopper portion  17   a , and prevents the roller  16   a  from rotating further. Thus, such force that works in the direction to push the left end portion of the sleeve  13  upward (indicated by arrow mark Q in  FIG. 14 ) is generated. 
         [0083]    As long as the belt driving portion  200  is designed so that the force generated by the spring  19  is substantially smaller than the force generated in the direction to push the left end portion of the sleeve  13  upward, by the interaction between the roller  16   a  and the lateral shift of the belt  1 , the sleeve  13  is tilted (direction indicated by arrow mark Q) against the force generated by the spring  19  in the direction to position the left side of the sleeve  13  higher than the right side of the sleeve  13 ; sleeve  13  is tilted in the direction (indicated by arrow mark S) to position the right side of the sleeve  13  lower than the left side. Thus, the belt  1  begins to shifts in the direction indicated by an arrow mark N in  FIG. 14 , that is, the opposite direction from the direction in which it was shifting. 
         [0084]    As the belt  1  shifts in the direction indicated by the arrow mark N, the rib  12   a  separates from the rib contacting portion  16   e  of the roller  16   a , stopping thereby transferring the rotational force to the roller  16   a . Consequently, the force which was working in the direction to push the left end portion of the sleeve  13  upward in  FIG. 14  vanishes. Thus, the sleeve  13  is tilted by the force generated by the spring  19 , in the direction (indicated by arrow mark P) to position the left side of the sleeve  13  lower than the right side of the sleeve  13 , causing the belt  1  to begin to shift again in the direction indicated by the arrow mark M in  FIG. 14 . 
         [0085]    As described above, if the belt  1  shifts in the direction indicated by the arrow mark M in  FIG. 14 , the sleeve  13  is adjusted in angle by the function of the roller  16   a  so that the belt  1  shifts in the direction indicated by the arrow mark N in  FIG. 14 , whereas if the belt  1  shifts in the direction indicated by the arrow mark N in  FIG. 14 , the sleeve  13  is adjusted in angle by the function of the spring  19  so that the belt  1  shifts in the direction indicated by the arrow mark M in  FIG. 14 . These adjustments are alternately repeated to prevent the belt  1  from continuing to shift in only one direction. In this embodiment, therefore, the lengthwise right end of the tension roller  4  does not need to be provided with the rib  12  ( 12   b ) and roller  16  ( 16   b ), with which the lengthwise right end of the tension roller  4  was provided in the first embodiment. In other words, the lateral deviation of the belt  1  can be simply prevented by providing only one of the lengthwise ends of the tension roller  4  with the belt centering mechanism in this embodiment. That is, the belt driving portion  200  in this embodiment is simpler in structure and lower in cost, than the one in the first embodiment. 
         [0086]    In this embodiment, the belt driving portion  200  was structured to keep the sleeve  13  pressured in a preset direction by the spring  19 . However, the belt driving portion  200  may be provided with a mechanism which makes the sleeve  13  tend to tilt in one direction with the use of a means other than the spring  19 , while being otherwise structured the same as the belt driving portion  200  in the first embodiment. The effects of such modification are the same as those of the first and second embodiments. 
       Embodiment 3 
       [0087]    Next, referring to  FIG. 15 , the third embodiment of the present invention is described with reference to another belt driving unit in accordance with the present invention, and an image forming apparatus equipped with the belt driving unit. The components, portions, etc., of the belt driving unit and image forming apparatus, which are similar in structure as the counterparts in the first embodiment are given the same referential codes as those given to the counterparts, and are not going to be described here. In the first embodiment, the belt  1  was provided with the ribs  12   a  and  12   b , which were attached next to the edges of the belt  1 , one for one, and the lengthwise ends of tension roller  4  was provided with the rollers  16   a  and  16   b , one for one, having the rib contacting portion  16   e  which corresponds in position to the rib  12   a  or  12   b , respectively. 
         [0088]    Further, the belt driving portion  200  was structured so that as the belt  1  shifts in its widthwise direction, only the rib  12  ( 12   a  or  12   b ) which is on the side toward which the belt is shifting comes into contact with the corresponding roller  16  ( 16   a  or  16   b ). In comparison, in this embodiment, the belt driving portion  200  is structured so that both the ribs  12   a  and  12   b  remain in contact with the rib contacting portion  16   e  of the roller  16   a , and the rib contacting portion  16   e  of the roller  16   b , respectively, regardless of the direction of the lateral shift of the belt  1 . 
         [0089]      FIG. 15  is an enlarged side view of the lengthwise end portions of the tension roller  4  in this embodiment. Referring to  FIG. 15 , a referential code La stands for the distance between the ribs  12   a  and  12   b  which are on the inward surface of the belt  1 , and which are in the adjacencies of the edges of the belt  1 , one for one, and a referential code Lb stands for the distance between the rollers  16   a  and  16   b , which are at the lengthwise ends of the tension roller  4 . The belt driving portion  200  is structured so that the distances La and Lb satisfy an inequality (Lb&gt;La), although it may be structured so that Lb=La. 
         [0090]    With the belt driving portion  200  being structured as described above, both the rib contacting portion  16   e  of the roller  16   a , and the rib contacting portion  16   e  of the roller  16   b , always remain in contact with the ribs  12   a  and  12   b , respectively, being therefore always given rotational force from the belt  1 . Thus, as the belt  1  begins to laterally shift, the area of contact between the rib  12   a  and the rib contacting portion  16   e  of the roller  16   a  becomes different in the amount of contact pressure from the area of contact between the rib  12   b  and the rib contacting portion  16   e  of the roller  16   b.    
         [0091]    Thus, the roller  16  ( 16   a  or  16   b ) which is on the opposite side of the belt  1  from the side to which the belt  1  is shifting overwhelms, in terms of the amount of rotational force it receives from the belt  1 , the roller  16  ( 16   b  or  16   a ) on the side of the belt  1  from which the belt  1  is shifting away. However, the roller  16  ( 16   a  or  16   b ) which overwhelmed the other roller  16  ( 16   b  or  16   a ) in terms of the amount of the rotational force is prevented by the stopper portion  17  ( 17   a  or  17   b ) from rotating, because its cam portion  16   f  comes into contact with the stopper portion  17  ( 17   a  or  17   b ). Thus, the flange  15  ( 15   a  or  15   b ) which is on the opposite side of the belt  1  from the side to which the belt  1  is shifting is moved upward in  FIG. 15 , causing thereby the sleeve  13  to rotationally move about the bearing  14 , being tilted in such a manner that the lengthwise end of the sleeve  13 , which is on the opposite side of the belt  1  from the side to which the belt is shifting, is positioned higher than the other lengthwise end of the sleeve  13 , as in the first embodiment. 
         [0092]    More concretely, for example, if the belt  1  shifts in the direction indicated by the arrow mark M in  FIG. 10 , the amount of rotational force which the roller  16   a  in  FIG. 15  is given by the belt  1  through the rib  12   a  becomes greater than the amount of rotational force which the roller  16   b  in  FIG. 15  is given by the belt  1  through the rib  12   b . Therefore, the roller  16   a  overwhelms the roller  16   b  in terms of rotational force, tending to tilting the sleeve  13  in the direction indicated by the arrow mark G in  FIG. 10 . Consequently, the belt  1  stops shifting in the direction indicated by the arrow mark M in  FIG. 10 , and begins to shift in the direction indicated by the arrow mark N in  FIG. 10 . In other words, the same effects as those obtainable by the belt driving portion  200  in the first embodiment can also be obtained by the belt driving portion  200  in this embodiment. In addition, the belt driving portion  200  in this embodiment is shorter in the length of time it takes for the belt  1  to be properly positioned (centered) after it begins to laterally shift. 
         [0093]    By the way, in a case where the ribs  12   a  and  12   b  are made of an elastic material, if the belt  1  shifts in the direction indicated by the arrow mark M, for example, in  FIG. 10 , and therefore, the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a , the rib  12   a  is elastically deformed by the contact pressure between itself and the rib contacting portion  16   e . Thus, the belt  1  is made to shift in the direction indicated by the arrow mark N in  FIG. 10 , by the resiliency of the rib  12   a , the amount of which corresponds to the amount of the deformation of the rib  12   a.    
         [0094]    Thus, in a case where the ribs  12   a  and  12   b  are made of an elastic material, the belt driving portion  200  may be structured so that the as the belt  1  shifts in the direction indicated by the arrow mark M in  FIG. 10 , the rib  12   b  separates from the rib contacting portion  16   e  of the roller  16   b . As the rib  12   b  separates from the rib contacting portion  16   e  of the roller  16   b , the roller  16   b  becomes free from the rotational force from the belt  1 , making it unnecessary for the roller  16   a  to overwhelm the roller  16   b  in terms of the amount of rotational force when it makes the sleeve  13  to tilt in the direction indicated by the arrow mark G in  FIG. 10 . Otherwise, the belt driving portion  200  in this embodiment is the same in structure and effects as the one in the first embodiment. 
       Embodiment 4 
       [0095]    Next, referring to  FIGS. 16 and 17 , the fourth embodiment of the present invention is described with reference to another belt driving unit in accordance with the present invention, and an image forming apparatus equipped with the belt driving unit. The components, portions, etc., of the belt driving unit in this embodiment, and those of the image forming apparatus equipped with the belt driving unit, which are the same in structure as the counterparts in the preceding embodiments are given the same referential codes as those given to the counterparts, and are not going to be described here. In each of the preceding embodiments of the present invention, the belt driving portion  200  was structured so that the ribs  12   a  and  12   b  are attached to the inward surface of the belt  11 , and on the inward side of the belt edges, one for one, in terms of the widthwise direction of the belt  1 , and also, so that the rib contacting portion  16   e  of the roller  16   a , and the rib contacting portion  16   e  of the roller  16   b , are positioned on the inward side of the ribs  12   a  and  12   b , respectively, in terms of the widthwise direction of the belt  1 . 
         [0096]    Further, in each of the preceding embodiments, the belt driving unit was structured so that the rib contacting portion  16   e  of the roller  16   a  and the rib contacting portion  16   e  of the roller  16   b  are positioned on the inward side of the rib  12   a  and  12   b , respectively, and also, so that as the belt  1  laterally shifts, the opposite rib  12  ( 12   a  or  12   b ) from the direction in which the belt  1  is shifting comes into contact with the corresponding rib contacting portion  16   e , and caused the roller  16  ( 16   a  or  16   b ) to push upward the end of the sleeve  13 , which is on the side on which the rib  12  came into contact with the corresponding rib contacting portion  16   e.    
         [0097]    Referring to  FIG. 16 , in this embodiment, the belt driving unit is structured so that the rib contacting portion  16   b  of the roller  16   a , and the rib contacting portion  16   e  of the roller  16   b , are on the outward side of the ribs  12   a  and  12   b  in terms of the widthwise direction of the belt  1 . Thus, as the belt  1  laterally shifts, the rib  12  ( 12   a  or  12   b ) which is on the side toward with the belt  1  is shifting comes into contact with the rib contacting portion  16   e  of the roller  16  ( 16   a  or  16   b ), and causes the roller  16  ( 16   a  or  16   b ) to tilt the sleeve  13  so that the end of the sleeve  13 , which is on the side where the rib  12  ( 12   a  or  12   b ) came into contact with the rib contacting portion  16   e , is positioned higher than the opposite end of the sleeve  13 . 
         [0098]      FIG. 16  is a perspective view of one end of the belt driving unit in this embodiment, in terms of the widthwise direction of the belt  1 . However,  FIG. 16  does not show the belt  1 . Referring to  FIG. 16 , the belt driving unit is structured so that the rib contacting portion  16   e  of the roller  16  is on the outward side of the rib  12  in terms of the widthwise direction of the belt  1 , and also, so that the stopper portion  17   a , with which the cam portion  16   f  of the roller  16   a  comes into contact, is positioned 180 degrees away from the position in which the stopper portion  17   a  was positioned in the first embodiment. 
         [0099]    Thus, as the belt  1 , which is being circularly moved in the direction indicated by an arrow mark B in  FIG. 16 , shifts in the direction indicated by an arrow mark N in  FIG. 16 , for example, the outward side of the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a , transmitting to the roller  16   a  the rotational force from the belt  1 . 
         [0100]    As the roller  16   a  receives the rotational force from the belt  1  through the rib  12   a , the rotational force tends to rotate the roller  16   a  in the direction indicated by an arrow mark U in  FIG. 17 . However, the contact between the cam portion  16   f  of the roller  16   a  and the stopper  17   a  prevents the roller  16   a  from rotating. Consequently, the roller  16   a  is pushed down by the rotational force from the belt  1 , from the position indicated by the broken line in  FIG. 17  to the position indicated by the solid line in  FIG. 17 ; it is pushed down in the opposite direction from the direction in which the roller  16   a  is moved in the first embodiment in terms of the vertical direction. Thus, the sleeve  13  is tilted in such a direction (indicated by arrow mark T) that the lengthwise end of the sleeve  13 , which is on the side where the rib  12   a  came in contact with the rib contacting portion  16   e  of the roller  16   a , is positioned lower than the opposite lengthwise end of the sleeve  13 . Thus, the tension roller  4  (sleeve  13 ) stops tending to shift the belt  1  in the direction indicated by the arrow mark N in  FIG. 16 , and begins to causes the belt  1  to shift in the direction indicated by the arrow mark M in  FIG. 16 . In other words, this embodiment also can provided the same effects as those which can be provided by the first embodiment. 
         [0101]    More concretely, in this embodiment, as the belt  1  shifts in the direction of the rib  12   a  (first rib), that is, the rib attached to the belt  1 , next to one of the edges of the belt  1 , the roller  16   a  (first rotational member) receives rotational force from the belt  1 , and is rotated by this rotational force. On the other hand, as the belt  1  shifts in the direction of the rib  12   b  (second rib), that is, the rib  12  attached to the belt  1 , next to the other edge of the belt  1 , the roller  16   b  (second rotational member) receives rotational force from the belt  1 , and is rotated by this rotational force. Otherwise, the belt driving unit in this embodiment is the same in structure as the one in each of the preceding embodiments. The effects of the embodiment are the same as those in each of the preceding embodiments. 
       Embodiment 5 
       [0102]    Next, referring to  FIGS. 18 and 19 , the present invention is described with reference to the fifth embodiment of the present invention, which is in the form of a belt driving unit, and an image forming apparatus equipped with the belt driving unit. The components, portions, etc., in this embodiment, which are similar in structure to the counterparts in each of the preceding embodiments are given the same referential codes as those given to the counterparts, and are not going to be described here. In each of the preceding embodiments, the belt driving unit was structured so that the sleeve  13  of the tension roller  4  is vertically and perpendicularly tilted relative to the direction in which the belt  1  is driven. In this embodiment, the sleeve  13  of the tension roller  4  was horizontally and perpendicularly tilted relative to the direction in which the belt  1  is driven. 
         [0103]    In each of the preceding embodiments of the present invention, the belt driving unit was structured so that the direction (indicated by arrow mark B in  FIG. 19 ) in which the sleeve  13  of the tension roller  4  tilted is roughly vertical to the axle  8  of the tension roller  14  and. However, the direction in which the rollers  16   a  or  16   b  is pushed by the force given to the roller  16   a  and  16   b  by the belt  1  through the ribs  12   a  and  12   b  as the rollers  16   a  and  10   b  are prevented by the stopper portions  17   a  and  17   b  from rotating does not need to be the direction in which the roller  16   a  or  16   b  is pushed in the preceding embodiments. That is, the unwanted lateral shift of the belt  1  can also be controlled by tilting the sleeve  13  of the tension roller  4  in the direction (indicated by arrow mark V in  FIG. 18 ) which is horizontal and roughly perpendicular to the direction (indicated by arrow mark B in  FIG. 19 ) of the tension given to the belt  1  by the tension roller. 
         [0104]      FIG. 19  is a perspective view of one of the lengthwise end portions of the tension roller  4  of the belt driving unit in this embodiment. It does not show the belt  1 . Referring to  FIG. 18 , in this embodiment, the stopper  17   a  with which the cam portion  16   f  of the roller  16   a  comes into contact is positioned roughly 90 degrees away from the position of the stopper  17   a  in the first embodiment. 
         [0105]    As the belt  1  shifts rightward in  FIG. 19 , that is, the direction indicated by an arrow mark M in  FIG. 19 , while it is being circularly moved in the direction indicated by an arrow mark B in  FIG. 19 , the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a . Thus, the roller  16   a  is given rotational force from the belt  1  through the rib  12   a , being thereby rotated by the rotational force. As the roller  16   a  is rotated, its cam portion  16   e  comes into contact with the stopper portion  17   a , preventing thereby the roller  16   a  from rotating further. Consequently the roller  16   a  is pushed in the direction indicated by an arrow mark V in  FIG. 18 , causing the sleeve  13  to tilt in such a direction that the lengthwise end of the sleeve  13 , which is in contact with the roller  16   a , is pushed in the direction indicated by the arrow mark V in  FIG. 18 . Thus, the sleeve  13  is tilted in the direction (indicated by arrow mark V in  FIG. 18 ) which is parallel to the direction of the tension given to the belt  13  by the tension roller  4 . 
         [0106]    Therefore, as the belt  1  shifts, for example, in the direction indicated by the arrow mark M in  FIG. 19 , the rib  12   a  comes into contact with the rib contacting portion  16   e  of the roller  16   a , transmitting to the roller  16   a  the rotational force from the belt  1 . Consequently, the sleeve  13  is tilted in such a direction that the lengthwise end of the sleeve  13  moves in the direction indicated by the arrow mark V in  FIG. 19 . Therefore, the belt  1  gradually reduces in the speed with which it is shifting in the direction indicated by the arrow mark M in  FIG. 19 , stops shifting, and begins to shift in the opposite direction, that is, the direction indicated by the arrow mark N in  FIG. 19 . In other words, the same effects as those obtainable by the first embodiment can also obtained by this embodiment. Otherwise, the belt driving unit in this embodiment is the same in structure and effects as that in the first embodiment. 
       Embodiment 6 
       [0107]    Next, referring to  FIG. 20 , the present invention is described with reference to the belt driving unit in the sixth embodiment of the present invention, and an image forming apparatus equipped with the belt driving unit. The components, portions, etc., of the belt driving unit in this embodiment, which are similar in structure to the counterparts in each of the preceding embodiments are given the same referential codes as those given to the counterparts, one for one, and are not going to be described here. In each of the preceding embodiments, the belt driving unit was structured so that the sleeve  13  of the tension roller  4  is rotationally movable about the bearing  14  to tilt the sleeve  13  relative to the axle  8 . In comparison, in this embodiment, the belt driving unit is provided with a sub-frame  23  for supporting the tension roller  24  which supports, and keeps tensioned, the belt  1 . Further, the sub-frame  23  is fitted around a pivot  22  with which the frame  120   a  of the intermediary transfer unit  120  is provided. Further, the sub-frame  23  is rotatable in an oscillatory manner about a pivot  22  which is at the center of the sub-frame  23  in terms of the lengthwise direction of the sub-frame  23  of the frame  120   a  of the intermediary transfer unit  120 , in the direction indicated by an arrow mark W in  FIG. 20 . 
         [0108]    In the first embodiment, the belt driving unit was structured so that the bearing  14  was fitted in the hollow of the cylindrical sleeve  13 , at the lengthwise center of the sleeve  13 , and also, so that the sleeve  13  is allowed to pivot in an oscillatory manner about the bearing  14  to be tilted relative to the axle  8 . In comparison, in this embodiment, the belt driving unit is provided with a tension roller supporting sub-frame  23 , which is separate from the main frame of the belt driving unit. Further, the tension roller supporting sub-frame  23  is fitted around the pivot  22  of the frame  120   a  so that it is allowed to rotate about the pivot  22  in an oscillatory manner. This structural arrangement also can allow the sleeve  13  of the tension roller  4  to tilt like the sleeve  13  of the tension roller  4  in the first embodiment. 
         [0109]      FIG. 20  is a perspective view of the tension roller  4  and its adjacencies in this embodiment. It  20  does not show the belt  1 . Referring to  FIG. 20 , the tension roller  24  is supported by its lengthwise end portions, in such manner that it can be rotated, as it was in the first embodiment. The tension roller supporting sub-frame  23  is supported by the frame  120   a  of the belt driving unit in such a manner that roughly the lengthwise center portion of the sub-frame  23  is fitted around the pivot  22  (with which the frame  120   a  is provided) to allow the sub-frame  23  to rotate about the pivot  22  in an oscillatory manner. 
         [0110]    Therefore, the tension roller  24  is allowed to freely rotate about the pivot  22  to be tilted in the direction indicated by an arrow mark W in  FIG. 20 , relative to the main frame  120   a  of the belt driving unit. Like the lengthwise end portions of the tension roller  4  in the first embodiment, the lengthwise end portions of the tension roller  24  are provided with rollers  16   a  and  16   b , one for one, which are rotationally movable relative to the tension roller  24 . With the provision of the above described structural arrangement, the belt driving unit in this embodiment also can keep its belt  1  properly positioned in terms of the widthwise direction of the belt  1 , as can the belt driving unit in the first embodiment. Otherwise, the belt driving unit in this embodiment is the same in structure and effects as the one in the first embodiment. 
       Embodiment 7 
       [0111]    Next, referring to  FIG. 21 , the present invention is described with reference to the seventh belt driving unit in accordance with the present invention, and an image forming apparatus equipped with the belt driving unit. The components, portions, etc., of the belt driving unit and image forming apparatus in this embodiment, which are similar in structure to the counterparts in each of the preceding embodiments are given the same referential codes as those given to the counterparts, one for one, and are not going to be described here. In each of the preceding embodiments, the belt driving unit in accordance with the present invention was a part of the intermediary transfer unit  120 , and the image forming apparatus was provided with the intermediary transfer unit  120  having the intermediary transfer unit which employs the belt driving unit in accordance with present invention. In this embodiment, the belt driving unit in accordance with the present invention is a part of a recording medium conveyance unit of the image forming apparatus. 
         [0112]    In each of the preceding embodiments of the present invention, the belt  1  was an intermediary transfer belt as a means for conveying a sheet of recording medium. However, the present invention is also applicable to a belt  1  used to convey a sheet of recording medium for an image forming apparatus such as an image forming apparatus  101  structured and operated as follows, as has been known to people in this field of business: an electrostatic latent image(s) is formed on one (or more) photosensitive drum  111  as an image bearing member, and is developed into a toner image(s). Then, the toner image is directly transferred onto a sheet  20  of recording medium, which is being conveyed by a belt  1  which is circularly movable while facing the peripheral surface of the photosensitive drum(s). Then, the sheet  20  is separated from the belt  1 , and the unfixed toner image on the sheet P is fixed to the sheet P to obtain a print.  FIG. 21  is a schematic sectional view of a typical image forming apparatus equipped with a belt driving unit, as a recording medium conveyance unit, in accordance with the present invention, and shows the general structure of the apparatus. Referring to  FIG. 21 , the image forming apparatus  101  has multiple image forming means, more specifically, image formation units  11 Y,  110 M,  110 C and  110 K which form yellow, magenta, cyan, and black monochromatic toner image, respectively. Each image formation unit  110  forms an electrostatic latent image on its photosensitive drum  111  (image bearing member), and forms a toner image by developing the electrostatic latent image. 
         [0113]    The description of the processes through which a toner image is formed on the photosensitive drum  111  in each image formation unit  110  are the same as those given in the description of the first embodiment. Thus, the components, portions, etc., of the image forming apparatus  101  in this embodiment, which are the same in function and structure are given the same referential codes as those given to the counterparts of the image forming apparatus  100  in the first embodiment, and are not going to be described here in detail. 
         [0114]    The image forming apparatus  101  has: an unshown recording medium supply unit; a recording medium conveyance unit  116  having a belt  1  and a belt driving unit; and a transferring device  117  as a transferring means, which opposes the peripheral surface of each photosensitive drum  111 , with the presence of the belt  1  between itself and photosensitive drum  111 . The recording medium conveyance unit  116  is positioned so that its belt  1  faces the photosensitive drum  111 . In an image forming operation, a sheet  20  of recording medium is sent out from the recording medium supply unit  116 , in synchronism with the formation of a toner image on the photosensitive drum  111  in each image formation unit  110 . Then, the sheet  20  is delivered to the belt  1  of the recording medium conveyance unit  116 . Then, the toner images, different in color, formed on the photosensitive drums  111 , one for one, are sequentially transferred by the function of the transferring device  117  onto the sheet  20  of recording medium which is being conveyed by the belt  1 . 
         [0115]    After the completion of the transfer process, the sheet  20  of recording medium is separated from the belt  1 , and is conveyed to an unshown fixing device as a fixing means. Then, the unfixed toner images on the sheet  20  are fixed to the sheet P by the fixing device. Then, the sheet  20  is discharged as a finished print from the image forming apparatus  101 . 
         [0116]    In other words, the present invention is also applicable to a belt centering mechanism, as a means for controlling the lateral shift of a belt  1 , for an image forming apparatus having a recording medium conveyance unit. That is, the same effects as those obtainable by each of the preceding embodiments are obtainable by providing the recording medium conveyance unit  116  of the image forming apparatus  101  shown in  FIG. 20 , with a tension roller such as the tension roller  4 , the description of which was given as a part of the description of each of the preceding embodiments. Incidentally, the image forming apparatus  101  and recording medium conveyance unit  116  may be structured so that the latter is removably installable in the former, or the latter is a permanent component of the former. The components, portions, etc., of the image forming apparatus and its recording medium conveyance unit other than those of the recording medium conveyance unit, are the same in structure as the counterparts in each of the preceding embodiment. The effects of this embodiment are the same as those of each of the preceding embodiments. 
         [0117]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
       INDUSTRIAL APPLICABILITY 
       [0118]    According to the present invention, there is provided a belt driving unit which is substantially smaller in the amount by which one of the belt suspension rollers is moved to keep the belt properly positioned in terms of the widthwise direction of the belt.