Abstract:
An image forming apparatus includes an image bearing member unit including a toner image carrying member; a transfer unit including a movable endless belt for transferring the toner image from the carrying member onto a transfer material, a plurality of stretching members for stretching the endless belt, and a supporting portion, provided at a end portion with respect to a widthwise direction, for supporting the stretching members; and a main assembly frame provided at a position opposing the supporting portion and including a positioning portion for determining a positions of the plurality of stretching members, wherein the supporting portion is movable relative to the main assembly frame.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus such as a copying machine, a printer, etc., which has an endless belt. More specifically, it relates to an electrophotographic or electrostatic image forming apparatus which directly transfers toner image(s) formed on its image bearing member(s) onto a sheet of recording medium electrostatically adhered to its recording medium conveying electrostatic endless belt. It also relates to an electrophotographic or electrostatic image forming apparatus which transfers toner image(s) formed on its image bearing member(s) onto its endless intermediary transfer belt, and then, transfers the toner image(s) from the intermediary transfer belt onto a sheet of recording medium being conveyed by a recording medium conveying means. 
         [0002]    It has been known that some color image forming apparatuses, such as a color copying machine, a color laser printer, or the like, have an intermediary transferring member. A color image forming apparatus having an intermediary transferring member forms a color image through a primary transfer process in which toner image(s) formed on a photosensitive drum, as an image bearing member, are sequentially layered on its intermediary transferring member, and a secondary transfer process in which the layered toner images on the intermediary transferring member are transferred together onto a sheet of recording medium. 
         [0003]    The majority of the intermediary transferring members are in the form of an endless belt. Therefore, in order to form a high quality image, it is necessary to highly precisely position endless belt suspending multiple rollers, relative to the main assembly of an image forming apparatus. This is true in the case of an image forming apparatus which uses an electrostatic endless belt for conveying a sheet of recording medium. It is also true in the case of an image forming apparatus structured so that its endless belt and belt supporting multiple rollers are integrated in the form of a transfer unit, which is removably installable in the main assembly of the image forming apparatus. 
         [0004]    In the case of an image forming apparatus disclosed in Japanese Laid-open Patent Application 2005-195724, the frame of the transfer unit for the image forming apparatus is made up of three sections, and the intermediary frame is given a certain amount of flexibility to enable the transfer unit to adapt to the frame of the main assembly of the image forming apparatus. 
         [0005]    The structural arrangement of the image forming apparatus disclosed in Japanese Laid-open Patent Application 2005-195724, however, was intended to precisely position the top portion of the endless belt, in terms of the loop which the belt forms, relative to the frame of the main assembly of the image forming apparatus, in terms of the moving direction of the top portion of the endless belt. Therefore, it cannot ensure that each of the belt suspending rollers is precisely positioned (aligned) in terms of the moving direction of the top portion of the endless belt, in terms of the belt loop. Further, the belt frame is made up of three sections, allowing thereby the left and right sections of the belt frame to become misaligned, which in turn makes it possible for the transfer unit to be attached to the frame of the main assembly of the image forming apparatus at an unintended angle. 
       SUMMARY OF THE INVENTION 
       [0006]    Thus, the primary object of the present invention is to provide an image forming apparatus which is simple in structure and inexpensive, and yet, can highly precisely position its endless belt relative to the frame of its image assembly. 
         [0007]    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 
         [0008]      FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, and shows the general structure of the apparatus. 
           [0009]      FIG. 2  is a perspective view of the belt unit in the first embodiment made up of an endless belt, belt suspending members, and belt suspending member supporting members. It shows the general structure of the belt unit. 
           [0010]      FIG. 3  is a side view of the belt unit and a part of the main frame of the image forming apparatus, in the first embodiment. It shows the structural arrangement for attaching the belt unit to the main frame of the image forming apparatus. 
           [0011]      FIG. 4  is a schematic sectional view of one of the pair of bearings for the belt driving roller (which is one of belt suspending members), and its adjacent components. It shows the structure of the bearing and adjacent components. 
           [0012]      FIG. 5  is a schematic sectional view of one of a pair of bearings for the tension roller (which also is one of belt suspending members), and its adjacent components. It shows the structure of the bearing and adjacent components. 
           [0013]      FIG. 6  is a schematic sectional view of the image forming apparatus in the second embodiment of the present invention, and shows the general structure of the apparatus. 
           [0014]      FIG. 7  is a perspective view of the belt unit in the second embodiment, which is made up of an endless belt, belt suspending members, and belt suspending member supporting members. It shows the general structure of the belt unit. 
           [0015]      FIG. 8  is a side view of the belt unit and a part of the main frame of the image forming apparatus, in the second embodiment. It shows the structural arrangement for attaching the belt unit to the main frame of the image forming apparatus. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1  
       [0016]    First, referring to  FIGS. 1-5 , the image forming apparatus in the first embodiment of the present invention is described about its general structure. 
       General Structure of Image Forming Apparatus  
       [0017]    Referring to  FIG. 1 , the image forming apparatus  100  in this embodiment is a color image forming apparatus of the so-called intermediary transfer belt type, which forms a toner image (or toner images) on one or more of its photosensitive drums  1  (as image bearing members), transfers the toner image(s) onto its intermediary transfer belt  10  (endless belt) from the photosensitive drum(s), and then, transfers the toner image(s) onto a sheet of recording medium from the intermediary transfer belt  10 . 
         [0018]    Referring to  FIG. 1 , the image forming apparatus  100  has four image formation stations  a , b, c, and d. The four image formation stations are the same in structure, although they are different in the color of the toner they use as developer. Thus, they are going to be described together, with reference to the image formation station  a . 
         [0019]    The image formation station  a  has a photosensitive drum  1   a  as an image bearing member, which is rotated in the direction indicated by an arrow mark in  FIG. 1 , at a preset peripheral velocity (process speed). Hereafter, the image formation station is referred to as a drum unit, that is, an image bearing member unit having an image bearing member. 
         [0020]    While the photosensitive drum  1   a  is rotated as described above, its peripheral surface is uniformly charged to a preset polarity and a preset potential level by a charge roller  2   a  as a charging means. Then, the charged portion of the peripheral surface of the photosensitive drum  1   a  is exposed by an exposure unit  3   a  as an exposing means. Consequently, an electrostatic latent image, which corresponds to yellow component to which a multicolor image to be formed was separated, is formed on the peripheral surface of the photosensitive drum  1   a . Then, the electrostatic latent image is developed by a developing device  4   a  into a visible image, that is, an image formed of toner, in the development station  a . 
         [0021]    The intermediary transfer belt  10  is suspended and kept tensioned by four belt suspending members, more specifically, a belt driving roller  11  (which hereafter will be referred simply as driving roller  11 ), a tension tensioning roller  12  (which hereafter will be referred to simply as tension roller), and an auxiliary roller  18 . It is circularly moved at roughly the same velocity as the peripheral velocity of the photosensitive drum  1   a , in such a direction that in the area of contact between itself and the peripheral surface of the photosensitive drum  1   a , it moves in the same direction as the peripheral surface of the photosensitive drum  1   a.    
         [0022]    After the formation of a toner image on the peripheral surface of the photosensitive drum  1   a , the toner image is moved through the area of contact (which hereafter may be referred to as “primary transfer nip”) between the peripheral surface of the photosensitive drum  1   a  and intermediary transfer belt  10 . While the toner image is moved through the primary transfer nip, it is transferred (primary transfer) onto the intermediary transfer belt  10  by the primary transfer voltage applied to a primary transfer roller  14  as a transferring means. The primary transfer residual toner, which is the toner remaining on the peripheral surface of the photosensitive drum  1  after the primary transfer, is removed by a cleaning device  5 . 
         [0023]    Similarly, a toner image of the second color, a toner image of the third color, and a toner image of the fourth color are sequentially formed by the image formation stations b, c, and d, respectively, and are sequentially transferred in layers onto the yellow toner image on the intermediary transfer belt  10 , synthetically effecting a full-color toner image which reflects the original image. 
         [0024]    The four toner images, different in color, on the intermediary transfer belt  10  (endless belt) are conveyed through the secondary transfer nip, which is the area of contact between the intermediary transfer belt  10 , and a secondary transfer roller  20  (as transferring means) kept pressed against the belt driving roller  11  with the presence of the intermediary transfer belt  10  between itself and driving roller  11 . While the four toner images are conveyed through the secondary transfer nip, they are transferred together (secondary transfer) by the secondary transfer voltage applied to the secondary transfer roller  20  by a secondary transfer voltage power source, onto the surface of a sheet  6  of recording medium conveyed to secondary transfer nip, that is, the area of contact between the intermediary transfer belt  10  and the peripheral surface of the secondary transfer roller  20 , by a sheet feeding/conveying apparatus  50 . 
         [0025]    Thereafter, the sheet  6  of recording medium, on the surface of which the four toner images, different in color, are present, is introduced into a fixing device  30 , and is conveyed through the fixing device  30 . While the sheet  6  is conveyed through the fixing device  30 , the sheet  6  and the toner images thereon are subjected to heat and pressure by the fixing device  30 . Consequently, the four toner images, different in color, melt, mix, and become fixed to the sheet  6 . That is, a full-color print is made through the above described operation. 
         [0026]    As for the secondary transfer residual toner, that is, the toner remaining on the surface of the intermediary transfer belt  10  after the secondary transfer, it is removed by a transfer belt cleaning device  16 . 
       Structure of Transfer Unit  
       [0027]    Next, the transfer unit  25  of the image forming apparatus in this embodiment is described about its structure. The transfer unit  25  is provided with the endless intermediary transfer belt  10 , and three belt suspending members, more specifically, the driving roller  11 , tension roller  12 , and auxiliary roller  18 , by which the intermediary transfer belt  10  is kept suspended and tensioned. Further, the transfer unit  25  has multiple (four in this embodiment) transfer members  14 , as the primary transferring members, which oppose the photosensitive drums  1  with the presence of the intermediary transfer belt  10  between themselves and photosensitive drums  1 , one for one. The transfer unit  25  has also a supporting frame for supporting the belt suspending members, by their lengthwise ends, in terms of the direction perpendicular to the moving direction of the intermediary transfer belt  10 . 
       Structure of Apparatus Main Frame and Structure of Belt Suspending Member Supporting Portion  
       [0028]    Next, the main frame of the image forming apparatus (which hereafter may be referred to as “apparatus main assembly frame”) in this embodiment is described about its structure. 
         [0029]    The apparatus main assembly frame supports the main assembly of the image forming apparatus  100 , and also, precisely positions various units (drum unit, transfer unit, etc.) of the apparatus relative to the apparatus main assembly. The apparatus main assembly frame is made up of a bottom plate  42 , a top plate  43 , and a pair of unshown side plates with which the bottom and top plates  42  and  43  are connected to each other. Referring to  FIG. 3 , the apparatus main assembly frame is also provided with a pair of drum supporting frames  44 , which are above the bottom plate  42 , and which will be on the outward side of the transfer unit  25  (as belt unit) after the installation of the transfer unit  25  into the apparatus main assembly frame. 
         [0030]    Each of the drum supporting frames  44  is provided with a slot (recess, groove)  44   b,  which is semicircular in contour, and an elongated slot  44   c . The semicircular slot  44   b  and elongated slot  44   c  are portions for precisely positioning the driving roller  11  and tension roller  12 , which are two of the belt suspending members. They open upward. The semicircular slot  44   b  and elongated slot  44   c  are where one of the bearings  11   a  which support the lengthwise end portion of the axle of the driving roller  11 , and corresponding bearing  12   a  of the tension roller  12 , are fitted to precisely position the driving roller  11  and tension roller  12  relative to the drum supporting frames  44  as parts of the apparatus main assembly frame. 
         [0031]    Referring to  FIG. 2 , in terms of the widthwise direction of the intermediary transfer belt  10 , the transfer unit  25  is provided with a pair of side plates  41   a  and  41   b  which support the driving roller  11  and tension roller  12  (which are two of belt suspending members). The image forming apparatus  100  is structured so that the side plates  41   a  and  41   b  are allowed to move relative to the drum supporting frame  44  (which is part of apparatus main assembly frame). 
         [0032]    The exposure unit  3  is precisely positioned relative to the apparatus main assembly frame by being attached to the top plate  43 . 
         [0033]    The drum supporting frame  44  shown in  FIG. 3  is one of the pair of drum supporting frames  44  with which the apparatus main assembly frame is provided. The two drum supporting frames  44  are the same in shape and structure, being therefore the same in the shape and structure of the mold therefor. In other words, the two drum supporting frames  44  (left and right frames  44 ) can be made with the use of the same mold. Using the same mold to make the left and right frames  44  can minimize the difference in size and structure between the two frames  44 . The photosensitive drum unit to which the photosensitive drums  1  belong is precisely positioned relative to each of the drum supporting frames  44  by the drum supporting portions  44   a  of the drum supporting frame  44 . 
         [0034]    Each of the drum supporting frames  44  is provided with a semicircular slot (recess, groove)  44   b , as the driving roller positioning portion, into which the bearing  11   a  for supporting the axle of the driving roller  11  (one of belt suspending members) is fitted. It is also provided with an elongated slot  44   c,  as the tension roller positioning portion, into which the bearing  12   a  for supporting the axle of the tension roller  12  (one of belt suspending members) is fitted. It is by these slots  44   b  and  44   c,  as the positioning portions, that the transfer unit  25  is precisely positioned relative to the apparatus main assembly. 
         [0035]    Because the photosensitive drum unit, transfer unit  25 , and drum supporting frame  44  are structured as described above, the drum unit and transfer unit  25  can be highly precisely positioned relative to each other by a single component, that is, the drum supporting frame  44 . 
       Structure of Transfer Unit  
       [0036]    Next, referring to  FIG. 2 , the transfer unit  25  in this embodiment is described about its structure. The intermediary transfer belt  10  is suspended and kept tensioned by the driving roller  11  (which drives intermediary transfer belt  10 ), tension roller  12 , and auxiliary roller  18  (which is positioned next to secondary transfer nip). The tension roller  12  is made by the resiliency of a pair of tension springs  12   b,  to keep the intermediary transfer belt  10  tensioned. The axle of each of the belt suspending members is rotatably supported by the side plates  41   a  and  41   b  of a belt suspending member supporting frame, with the placement of a pair of bearings between the lengthwise ends of the axle of each of the rollers  11  and  12 , and plates  41   a  and  41   b,  one for one, as shown in  FIG. 1 . 
         [0037]    Next, referring to  FIGS. 2 and 4 , the driving roller  11  is provided with a gear  45 , which is fixed to one of the lengthwise ends of the axle  11   b  of the driving roller  11 . Further, the gear  45  is in mesh with an unshown driving gear, with which the apparatus main assembly is provided. Thus, as rotational force is transmitted from the apparatus main assembly to the driving roller  11 , the intermediary transfer belt  10  is circularly moved.  FIG. 4  is a sectional view of one of the lengthwise end portions of the driving roller  11  and side plate  41   a  (which supports driving roller  11 ). 
         [0038]    Referring to  FIG. 4 , the driving roller  11  is attached to the side plate  41   a.  More specifically, a bearing  11   a  for rotatably supporting the driving roller  11  by the lengthwise end portion  11   b  of the driving roller  11  is snap-fitted in a hole with which the side plate  41   a  is provided, and the lengthwise end portion  11   b  is borne by the bearing  11   a.  The lengthwise end portion  11   b   1  is smaller in diameter than the main portion of the axle of the driving roller  11 , and is provided with a groove  11   b   2 , into which a retainer ring  60  is fitted to preventing the driving roller  11  from disengaging from the bearing  11   a  (side plate  41   a ). The structural arrangement for attaching the other lengthwise end portion of the driving roller  11  to the side plate  41   b,  that is, the opposite side plate to the side plate  41   a,  is similar to the above described structural arrangement for attaching the lengthwise end portion  11   b  to the side plate  41   a.    
         [0039]    Because the driving roller  11  is attached to the left and right side plates  41   a  and  41   b  as described above, it is ensured that a preset distance can be maintained between the two side plates  41   a  and  41   b  (which support driving roller  11 ). The transfer unit  25  is structured so that the side plates  41   a  and  41   b  can be pivotally moved about the axle  11   b  of the driving roller  11 , and also, so that the two side plates  41   a  and  41   b  are allowed to move relative to the apparatus main assembly frame, within a range of the play afforded between the axle  11   b  of the driving roller  11 , and the side plates  41   a  and  41   b,  in both the direction parallel to top portion of the intermediary transfer belt  10 , in terms of the loop which the belt  10  forms, and the direction perpendicular to the top portion of the intermediary transfer belt  10  in terms of the loop which the belt  10  forms; the two side plates  41   a  and  41   b  are afforded some latitude in terms of their movement relative to the apparatus main assembly frame in terms of horizontal and vertical direction. 
         [0040]    It is desired that the transfer unit  25  is structured so that the left and right side plates  41   a  and  41   b  are allowed to move horizontally (forward or backward in terms of top portion of belt) and vertically relative to the apparatus main assembly frame, within a range of 0.1 mm-1.0 mm. Structuring the transfer unit  25  as described above allows the transfer unit  25  to be flexible. 
         [0041]    It is also desired that the side plates  41   a  and  41   b  are molded of highly rigid substance such as poly-carbonate. The transfer unit  25  may be provided with an additional member (members), besides the driving roller  11 , for keeping the left and right side plate  41   a  and  41   b  connected. In a case where an additional member (members) is provided, it is desired that the additional member connects the left and right side plates  41   a  and  41   b  in such a manner that two side plates  41   a  and  41   b  are allowed to move relative to each other. 
         [0042]    Next, referring to  FIG. 5 , the tension roller  12  also is supported by the side plates  41   a  and  41   b.  More specifically, each of the side plates  41   a  and  41   b  is provided with an elongated slot  41   c.  The bearing  12   a  which rotatably supports one of the lengthwise ends of the rotational axle  12   c  of the tension roller  12  is fitted in the elongated slot  41   c , being therefore allowed to move within the slot. Thus, the tension roller  12  is supported by the left and right side plates  41   a  and  41   b  in such a manner that it is allowed to move in the direction parallel to the moving direction of the top portion of the intermediary transfer belt  10  in terms of the loop which the intermediary transfer belt  10  forms. 
         [0043]    Referring also to  FIG. 5 , a protruding portion  41   d  of the side plate  41   a,  which provides the elongated slot  41   c  with the top edge, fits in the recess  12   a   1  with which the peripheral surface of the bearing  12   a  for the tension roller  12  is provided. Thus, the bearing  12   a  for the tension roller  12  is allowed to slide along the edge of the elongated slot  41   c;  the tension roller  12  is allowed to move following the edge (contour) of the elongated slot  41   c . Further, the transfer unit  25  is provided with a pair of compression springs  12   b,  which keep the tension roller  12  pressured in such a direction that the intermediary transfer belt  10  is provided with a preset amount of tension. 
         [0044]    Referring back to  FIG. 2 , each of the primary transfer rollers  14  is supported by the side plates  41   a  and  41   b,  with the placement of the bearings  14   a  and  14   b  between the lengthwise end portions of the roller  14  and the bearings  14   a  and  14   b,  one for one. The peripheral surface of each of the bearings  14   a  and  14   b  is provided with a protrusion  14   d,  which is fitted in a groove  41   e  with which the corresponding side plate  41   a  (or  41   b ) is provided. Thus, the primary transfer roller  14  is allowed to vertically ( FIG. 2 ) slide along the groove  41   e.  In terms of the direction parallel to the widthwise direction of the intermediary transfer belt  10 , however, the primary transfer roller  14  is not allowed to move relative to the side plates  41   a  and  41   b.    
         [0045]    Next, referring to  FIGS. 1 and 2 , the transfer unit  25  is provided with a pair of primary transfer springs  14   c,  which are under the bearings  14   a  and  14   b,  one for one, for the primary transfer roller  14 . Thus, the primary transfer roller  14  is kept pressed against the photosensitive drum  1 , with the presence of the intermediary transfer belt  10  between itself and the peripheral surface of the photosensitive drum  1 , by the pressure generated by the primary transfer springs  14   c.    
         [0046]    The primary transfer spring  14   c  is positioned so that its axial line coincides with the straight line (line A-A in  FIG. 2 ) between the axial line of the bearing  11   a  which is between the driving roller  11  and side plates  41   a  or  41   b,  and the axial line of the bearing  12   a  which is between the tension roller  12  and side plates  41   a  or  41   b.  In other words, the transfer unit  25  is structured so that the side plates  41   a  and  41   b  are not subjected to torsional moment, while the transfer unit  25  is allowed to be flexible. 
       Structural Arrangement for Attaching Transfer Unit to Apparatus Main Assembly Frame  
       [0047]    The transfer unit  25  and image forming apparatus  100  are structured so that the former is removably installable in the main assembly of the latter. 
         [0048]    Referring to  FIGS. 2 and 4 , the driving roller  11  is one of the rollers for suspending the intermediary transfer belt  10  of the transfer unit  25 , and its lengthwise end portions are fitted with the pair of bearings  11   a  and  11   b,  one for one, which are put through the side plates  41   a  and  41   b,  and project outward of the side plates  41   a  and  41   b,  respectively. Next, referring to  FIG. 3 , the transfer unit  25  is precisely positioned relative to the drum supporting frame  44  by the bearings  11   a  and  11   b  for the driving roller  11 ; the portion of the bearing  11   a  ( 11   b ) is fitted in the semicircular slot (recess)  44   b,  which opens upward and functions as the driving roller positioning portion of the drum supporting frame  44 . 
         [0049]    A driving roller pressing member  47 , shown in  FIG. 3 , presses on the peripheral surface of the bearing  11   a  for the driving roller  11 , keeping thereby the bearing  11   a  in the slot (groove)  44   b,  that is, the driving roller positioning portion of the drum supporting frame  44 . The driving roller pressing member  47  (which hereafter will be referred to simply as pressing member  47 ) is under the pressure generated by a torsional coil spring  47   a.    
         [0050]    The pressing member  47  is fitted around a boss  7 , being enabled to pivot about the boss  7 . One end  47   a   1  of the torsional coil spring  47   a  fitted around the boss  7  is rested on (attached to) a catch  47   b  with which the pressing member  47  is provided. The other end  47   a   2  of the torsional coil spring  47   a  is rested on (attached to) an unshown catch with which the drum supporting frame  44  is provided. 
         [0051]    Next, referring to  FIG. 3 , the bearing  12   a  by which the axle of the tension roller  12  is borne is fitted in the elongated slot  44   c  with which the drum supporting frame  44  is provided. The elongated slot  44   c  is roughly U-shaped in contour, and opens upward. Thus, the bearing  12   a  is allowed to move along the edge of the elongated slot  44   c.  Therefore, the tension roller  12  is allowed to horizontally slide along the edge of the elongated slot  44   c,  that is, in the lengthwise direction of each of the drum supporting frames  44 . 
         [0052]    The transfer unit  25  is structured to be flexible, and the left and right side plates  41   a  and  41   b  are precisely positioned relative to the drum supporting frame  44  by the bearings  11   a  and  11   b  for the driving roller  11 , and the bearing  12   a  and  12   b  for the tension roller  12 . Therefore, the transfer unit  25  is highly precisely positioned relative to the main assembly of the image forming apparatus  100 . 
         [0053]    As described above, the transfer unit  25  is structured to be flexible, and the bearings  11   a  and  11   b  for the driving roller  11  and the bearings  12   a  and  12   b  for the tension roller  12  are positioned relative to the drum supporting frame  44  by being fitted in the circular slot  44   b  and elongated slot  44   c,  respectively, which function as the driving roller positioning portion and tension roller positioning portion of the drum supporting frame  44 . In other words, the transfer unit  25  and main frame of the image forming apparatus  100  in this embodiment are simple in structure, being therefore inexpensive, and yet, the former is highly precisely positioned relative to the latter. 
         [0054]    Also because the transfer unit  25  and apparatus main assembly are structured as described above, the intermediary transfer belt  10  is stable in performance in terms of recording medium conveyance; it does not suffer from the problem that it deviates in position in its widthwise direction and/or rides onto the side plate. Further, it is ensured that the primary transfer rollers  14  remain precisely positioned relative to the photosensitive drums  1 , one for one. Therefore, it does not occur that the image forming apparatus  100  outputs images suffering from such defects that are attributable to scattered toner, color deviation, and/or the like. 
       Embodiment 2  
       [0055]    Next, referring to  FIGS. 6-8 , the image forming apparatus in the second embodiment of the present invention is described about its structure. The components of the apparatus in this embodiment, 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 in the first embodiment, and are not going to be described here. 
         [0056]    In the above-described first embodiment, it was the driving roller  11  (one of belt suspending members) that was positioned opposite to the secondary transfer roller  20  with reference to the intermediary transfer belt  10 . In this embodiment, the transfer unit  25  is provided with a belt suspending/backing member  40 , which is positioned on the inward side of the intermediary transfer belt  10 , in terms of the loop which the belt  10  forms, and against which the secondary transfer roller  20  is pressed with the presence of the intermediary transfer belt  10  between itself and belt suspending/backing member  40 . That is, the intermediary transfer belt  10  in this embodiment is suspended and kept tensioned by the belt suspending/backing member  40 , driving roller  11 , and tension roller  12 . Thus, as the intermediary transfer belt  10  is circularly moved, its inward surface slides on the belt suspending/backing member  40 . Unlike the driving roller  11  and tension roller (which is rotated by movement of belt  10 ), the belt suspending/backing member  40  does not move with the intermediary transfer belt  10 . That is, it is the same portion of the surface of the belt suspending/backing member  40  that remains in contact with the inward surface of the intermediary transfer belt  10 . 
         [0057]    In this embodiment, it is by the belt suspending/backing member  40 , driving roller  11 , and tension roller  12  that the intermediary transfer belt  10 , which is an endless belt, is suspended and kept tensioned. 
         [0058]    The surface layer of the belt suspending/backing member  40  is formed of polyethylene, which is low in friction and electrically conductive. The secondary transfer roller  20  is kept pressed against the belt suspending/backing member  40  with the presence of the intermediary transfer belt  10  between itself and belt suspending/backing member  40 . Thus, as a sheet  6  of recording medium is conveyed, it is pinched between the secondary transfer roller  20  and intermediary transfer belt  10  backed up by the belt suspending/backing member  40 . 
         [0059]    As the sheet  6  of recording medium is conveyed by the sheet feeding/conveying device  50  through the secondary transfer nip, which is between the intermediary transfer belt  10  and secondary transfer roller  20 , the toner images on the intermediary transfer belt  10  are transferred together onto the sheet  6  by the secondary transfer voltage applied to the secondary transfer roller  20  by a secondary transfer voltage power source. 
         [0060]    Each of the primary transfer nips, which is the area of contact between the photosensitive drum  1  and primary transfer roller  14 , is on the downstream side of the belt suspending/backing member  40  in terms of the moving direction of the intermediary transfer belt  10 . Further, the driving roller  11  is on the downstream side of the belt suspending/backing member  40 . Therefore, it does not occur that the intermediary transfer belt  10  slackens in the secondary transfer nip and primary transfer nip. In other words, it is ensured that the intermediary transfer belt  10  is circularly moved while remaining properly tensioned. Further, the tension roller  12  is on the downstream side of the driving roller  11  in terms of the moving direction of the intermediary transfer belt  10 , and is kept pressed diagonally rightward and downward direction in  FIG. 6  by the force generated by the tension springs  12   b.    
       Structure of Transfer Unit  
       [0061]    Next, referring to  FIG. 7 , the transfer unit  26  in this embodiment is described. Each of the lengthwise ends of the belt suspending/backing member  40  is provided with a positioning boss  40   a,  which functions also as the portion by which the belt suspending/backing member  40  is supported. The belt suspending/backing member  40  is precisely positioned by being supported by the belt backing member supporting portion of the apparatus main assembly frame. More specifically, each of the side plates  41   a  and  41   b  of the apparatus main assembly frame, which support the belt suspending/backing member  40 , is provided with a belt suspending/backing member positioning hole  41   f  (which hereafter will be referred to simply as positioning hole  41   f ). The positioning bosses  40   a  of the belt suspending/backing member  40  are put through the positioning hole  41   f  of the side plate  41   a  and the positioning hole  41   f  of the side frame  41   b.    
         [0062]    The belt suspending/backing member  40  is attached to the side plates  41   a  and  41   b  with the use of a pair of shouldered small screws  62 , one for one. As the belt suspending/backing member  40  is attached to the side plates  41   a  and  41   b,  the three components forms a U-shaped frame. 
         [0063]    The belt suspending/backing member  40  may be connected to the side plates  41   a  and  41   b  with the use of a pair of the small shouldered screws. However, it is desired that they are not extremely rigidly connected. 
         [0064]    The belt suspending/backing member  40  serves three roles, that is, the role of keeping the intermediary transfer belt  10  suspended while providing the intermediary transfer belt  10  with a preset amount of tension, the role of keeping the sheet  6  of recording medium pinched between the intermediary transfer belt  10  and secondary transfer roller  20  by backing the intermediary transfer belt  10  to transfer the toner images on the intermediary transfer belt  10  onto the sheet  6 , and the role of making up a part of the frame of the transfer unit  26 . 
         [0065]    The driving roller  11  is supported by the side plates  41   a  and  41   b;  the bearings  11   a  (and  11   b ) for the driving roller  11  are inserted in the unshown elongated slots with which the side plates  41   a  and  41   b  are provided one for one. The transfer unit  26  is structured so that the driving roller  11  is not allowed to shift in the direction perpendicular to the top portion of the intermediary transfer belt  10 , in terms of the loop which the intermediary transfer belt  10  forms, and also, in the direction (left and right directions in  FIG. 7 ) parallel to the widthwise direction of the intermediary transfer belt  10 , but is allowed to shift (slide) in the direction parallel to the moving direction of the top portion of the intermediary transfer belt  10 , in terms of the belt loop. In this embodiment, the driving roller  11  is not specifically controlled in position in terms of the direction parallel to the top portion of the intermediary transfer belt  10  in terms of the belt loop. 
         [0066]    In terms of the structural arrangement for supporting the primary transfer rollers  14  and tension roller  12 , the embodiment is the same as the first embodiment. 
         [0067]    As described above, the transfer unit  26  also is structured to be flexible. 
       Structural Arrangement for Attaching Transfer Unit to Apparatus Main Assembly Frame  
       [0068]    Next, referring to  FIG. 8 , the structural arrangement for attaching the transfer unit  26  to the drum supporting frame  44 , which is a part of the frame of the main assembly of the image forming apparatus. 
         [0069]    The drum supporting frame  44  is provided with a recess (groove)  44   b,  in which the belt backing member supporting member positioning boss  40   a,  which functions also as the belt backing member supporting portion, is fitted. The recess  44   b  opens upward, and is semicircular in contour. Further, the drum supporting frame  44  is provided with an elongated slot  44   c,  in which the bearing  11   a  for the driving roller  11 , by which the driving roller  11  is supported, is fitted. The elongated slot  44   c  is open upward, and is roughly U-shaped in contour. The positioning boss  40   a  of the belt suspending/backing member  40  is inserted into the slot  44   b  of the drum supporting frame  44 , and is kept pressed by the pressing member  47  in the same manner as the bearing  11   a  for the driving roller  11  was in the first embodiment. 
         [0070]    The pressing member  47  shown in  FIG. 8  is kept pressed upon the top portion of the peripheral surface of the positioning boss  40   a  of the belt suspending/backing member  40 , by the force generated by the torsional coil spring  47   a,  so that the positioning boss  40   a  is kept pressed in the slot  44   b  of the drum supporting frame  44 , which functions as the belt suspending/backing member positioning portion. 
         [0071]    The transfer unit  26  is structured so that the pressing member  47  is pivotally movable about a boss  7 . One end  47   a   1  of the torsional coil spring  47  fitted around the boss  7  is rested on (attached to) a catch  47   b  with which the pressing member  47  is provided. The other end  47   a   2  of the torsional coil spring  47   a  is rested on (attached to) an unshown catch with which the drum supporting frame  44  is provided. 
         [0072]    The driving roller  11  is supported by the side plates  41   a  and  41   b;  the bearings  11   a  and  11   b  for the driving roller  11  are fitted in the unshown elongated slots of the side plates  41   a  and  41   b,  respectively. Therefore, the driving roller  11  is allowed to slide in the direction parallel to the top portion of the intermediary transfer belt  10 , in terms of the belt loop. Thus, the bearings  11   a  and  11   b  can be fitted into the elongated slots  44   c  by sliding the driving roller  11  to the top opening of the elongated slots  44   c  which function as the portions for positioning the driving roller  11  relative to the drum supporting frame  44 . The bearings  11   a  and  11   b  for the driving roller  11  are kept pressed by the pressing members  47  in the same manner as the above described positioning boss  40   a  of the bearing  11   a.    
         [0073]    The side plates  41   a  and  41   b,  which function as the transfer unit the supporting portions, support the belt suspending/backing member  40 , driving roller  11 , and tension roller  12 , which function as the belt suspending members, at their lengthwise ends, in terms of the widthwise direction of the intermediary transfer belt  10 , which is an endless belt. Further, the side plates  41   a  and  41   b  are attached to the drum supporting frame  44 , which is a part of the frame of the apparatus main assembly, in such a manner that the former is movable relative to the latter. Thus, the transfer unit  26  is properly positioned relative to the main assembly of the image forming apparatus  100 . 
         [0074]    The left and right drum supporting frames  44  are the same in structural component, and therefore, are minimum in difference. Thus, precisely positioning the belt suspending/backing member  40  and driving roller  11  relative to the left and right drum supporting frames  40  ensures that they are highly precisely aligned. 
         [0075]    The force for driving (rotating) the driving roller  11  is transmitted to the driving roller  11  by meshing the gear  45  attached to the driving roller  11 , with the gear with which the apparatus main frame is provided. Thus, it is ensured by precisely positioning the driving roller  11  relative to the main assembly of the image forming apparatus  100  that the two gears remain precisely engaged for the driving roller rotating force to be highly precisely transmitted to the driving roller  11 . 
         [0076]    As described above, the belt suspending/backing member  40  which opposes the secondary transfer roller  20  is used as the component for positioning the transfer unit  26 , and also, is given the role of being a part of the frame of the transfer unit  26 . Thus, the transfer unit  26  is simpler in structure and lower in cost than the transfer unit  25  in the first embodiment. 
         [0077]    Further, the belt suspending/backing member  40  and driving roller  11  are kept highly precisely positioned in terms of both the direction parallel to the top portion of the intermediary transfer belt  10  in terms of the belt loop, and the direction perpendicular to the top portion of the intermediary transfer belt  10  in terms of the belt loop. More specifically, they are kept highly precisely positioned by placing the positioning boss  40   a  of the belt suspending/backing member  40 , and the bearing  11   a  for the driving roller  11 , in the circular slot  44   b  and elongated slot  44   c,  respectively, of the drum supporting frame  44 , which function as the belt backing member positioning portion and driving roller positioning portion, respectively. Therefore, it is ensured that the intermediary transfer belt  10  is reliably moved. 
         [0078]    Incidentally, in each of the preceding two embodiments of the present invention, the present invention was applied to the image forming apparatus  100  of the intermediary transfer type, which employs an intermediary transfer belt. That is, in the case of the preceding embodiments, the toner images formed on the photosensitive drums  1 , which are image bearing members, were transferred onto the intermediary transfer belt  10 , and then, are transferred onto the sheet  6  of recording medium from the intermediary transfer belt  10 . However, the present invention is also applicable to an image forming apparatus which employs an endless electrostatic belt for conveying a sheet of recording medium, that is, an image forming apparatus which directly transfers the toner images formed on the photosensitive drums  1 , which are image bearing members, onto the sheet of recording medium being conveyed by the electrostatic endless belt while remaining electrostatically adhered to the belt. 
         [0079]    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. 
         [0080]    This application claims priority from Japanese Patent Application No. 262593/2011 filed Nov. 30, 2011, which is hereby incorporated by reference.