Patent Publication Number: US-7917072-B2

Title: Image forming apparatus and belt transporting apparatus with moving member

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2007-255286 filed Sep. 28, 2007. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an image forming apparatus and a belt transporting apparatus. 
     2. Related Art 
     For example, in an image forming apparatus employing an electrophotographic method, a transfer apparatus that transfers an image held by an image carrier such as a photoconductor and an intermediate transfer body to a recording medium such as a sheet is used. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including: an image carrier that holds an image; an endless transfer belt that is rotated while a recording medium is sandwiched between the transfer belt and the image carrier, and transfers the image held by the image carrier to the recording medium; a first roll member that holds the transfer belt and brings the transfer belt into contact with the image carrier; a second roll member that holds the transfer belt together with the first roll member; a third roll member that holds the transfer belt together with the first roll member and the second roll member; and a moving member that makes the second roll member movable so as to make a center distance between the first roll member and the second roll member larger on one end side of the transfer belt than the center distance on the other end side, and makes the third roll member movable so as to make a center distance between the first roll member and the third roll member smaller on the one end side than the center distance on the other end side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiment (s) of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a diagram that illustrates an image forming apparatus to which the exemplary embodiment is applied; 
         FIG. 2  is a diagram that illustrates the whole secondary transfer unit to which the present exemplary embodiment is applied; 
         FIGS. 3A and 3B  are side views of the secondary transfer unit shown in  FIG. 2 ; 
         FIGS. 4A and 4B  are diagrams that illustrate a shape of the outer side cam and the inner side cam; 
         FIG. 5  is a diagram that illustrates a surface velocity of the secondary transfer belt in the reference state; 
         FIGS. 6A and 6B  are diagrams for explaining motions in the secondary transfer unit when the image on the outer side is elongated; 
         FIG. 7  is a diagram for explaining the surface velocity of the secondary transfer belt when the image on the outer side is elongated; 
         FIGS. 8A and 8B  are diagrams for explaining motions in the secondary transfer unit when the image on the inner side is elongated; and 
         FIG. 9  is a diagram for explaining the surface velocity of the secondary transfer belt when the image on the inner side is elongated. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings. 
       FIG. 1  is a diagram that illustrates an image forming apparatus  1  to which the exemplary embodiment is applied. Incidentally, in the  FIG. 1 , a X direction (a right-and-left direction on paper), a Y direction (a front-and-back direction on the paper) and a Z direction (an up-and-down direction on the paper) relative to the image forming apparatus  1  are shown as arrows. These directions are shown in subsequent drawings as necessary. 
     The image forming apparatus  1  shown in  FIG. 1  is what is termed a tandem-type, or an intermediate-transfer-type image forming apparatus. The image forming apparatus  1  includes plural image forming units  10  ( 10 Y,  10 M,  10 C, and  10 K) in which toner images of respective color components are formed by electrophotographic manner, an intermediate transfer belt  15  that sequentially transfers (primarily transfers) the toner images of respective color components formed in the respective image forming units  10  and makes them held, a secondary transfer unit  20  in which the overlapped toner images having transferred to the intermediate transfer belt  15  are collectively transferred (secondarily transferred) to a paper sheet P, a fixing apparatus  60  that makes the image having been secondarily transferred to the paper sheet P fixed thereon, and a controller  40  that controls the operation of respective apparatus (respective units). 
     Note that, in the following, the image forming apparatus  1  will be described by providing the front side of the paper (front side) of the image forming apparatus  1  shown in  FIG. 1  as an outer side and the back side of the space (rear side) as an inner side. Further, these outer and inner sides are tentatively expressed to specify one end of a driving roll  22  or the like in the secondary transfer unit  20  which will be described later. 
     Each of the image forming units  10  ( 10 Y,  10 M,  10 C, and  10 K) includes a photoconductor  11  that is rotated in a direction of an arrow A. Around the photoconductor  11 , a charging unit  12  that electrically charges the photoconductor  11 , a laser exposure unit  13  that makes an electrostatic image formed on the photoconductor  11  (an exposure beam is shown as a symbol Bm in  FIG. 1 ), a developing unit  14  that stores toner of each color component and visualizes the electrostatic image on the photoconductor  11  by using the toner, a primary transfer roll  16  that transfers the toner image of each color component formed on the photoconductor  11  to the intermediate transfer belt  15 , and a photoconductor cleaner  17  that removes toner remaining on the photoconductor  11  are sequentially arranged. These image forming units  10  is arranged in a substantially linear fashion, and the image forming units of yellow ( 10 Y), magenta ( 10 M), cyan ( 10 C) and black ( 10 K) are arranged in this order from the upstream side of the intermediate transfer belt  15 . 
     The intermediate transfer belt  15  as one example of a image carrier is made of a resin, such as a polyimide, a polyamide or the like to which an appropriate dose of an antistatic agent such as carbon black is added, and is configured by a film-formed endless belt. The intermediate transfer belt  15  is driven to circulate at a predetermined velocity in a direction as indicated by an arrow B in  FIG. 1  by a variety of rolls. These rolls include a driving roll  31  that is rotated by a motor (not shown in the figure) being excellent in running with a constant speed and drives intermediate transfer belt  15  to circulate, an idle roll  32  that supports the intermediate transfer belt  15  extending in a substantially linear shape along the direction in which the photoconductors  11  are arranged, a tension applying roll  33  that applies certain tensile force to the intermediate transfer belt  15 , and prevents the intermediate transfer belt  15  from meandering, a backup roll  34  that is arranged so as to be opposed to a secondary transfer unit  20  while the intermediate transfer belt is sandwiched therebetween. 
     A voltage that has a polarity contrary to the charged toner is to be applied to each of the primary transfer rolls  16  opposed to the corresponding one of the photoconductors  11  and arranged inside the intermediate transfer belt  15  that extends in a substantially linear shape. Each of the toner images on the respective photoconductors  11  is thus adhered electrostatically to the intermediate transfer belt  15  one after another to form the overlapped toner images on the intermediate transfer belt  15 . 
     The secondary transfer unit  20  is to transport a sheet P as a recording medium and to transfer secondarily a superimposed toner image formed on the intermediate transfer belt  15  to the sheet P. The secondary transfer unit  20  includes a secondary transfer belt  21  which functions as a transfer belt, the driving roll  22  which is as one of a first roll member and that is looped with the secondary transfer belt  21 , an image adjustment roll  23  which is as one of the second roll member, and a belt adjustment roll  24  which is as one of roll members of the third roll member. The secondary transfer belt  21  is a so-called rubber belt whose material is an elastomer such as chloroprene. The driving roll  22  is to rotationally drive the secondary transfer belt  21 . Further, the secondary transfer belt  21  is circularly driven in a direction of an arrow C shown in the figure at a predetermined speed by the driving roll  22 . At this time, the image adjustment roll  23  and the belt adjustment roll  24  are driven and rotated by being transmitted a driving force from the driving roll  22  via the secondary transfer belt  21 . Note that the respective driving roll  22 , image adjustment roll  23  and belt adjustment roll  24  are positioned and held in a position by setting a virtual belt inner circumference formed by respective outer circumferences and tangents of the outer circumference longer than an inner circumference in a free state of the secondary transfer belt  21 , by a self contractive force of the secondary transfer belt  21 , and by being brought into contact with a side plate, a cam and a bearing portion in the secondary transfer unit  20  which will be described later. 
     Further, the backup roll  34  and the driving roll  22  in the secondary transfer unit  20  sandwich the intermediate transfer belt  15  to form a transfer nip portion (secondary transfer position). 
     Further, the driving roll  22  to which the present exemplary embodiment is applied is a roll member having a multilayered structure in which a semi-conductive (for example, volume resistivity is around the  105  to the  108 ) rubber foam layer (e.g. epichlorohydrin rubber or the like) is wrapped around a mandrel of a metal (e.g. SUS material). The driving roll  22  is a roll member which also serves as a function of a transfer roll (that applies transfer voltage or grounds on the earth). The material of the image adjustment roll  23  and the belt adjustment roll  24  is not necessary to be particularly specified, however, a roll of a metal having a small bending by the tension of a belt (e.g. SUS material) is preferable. Note that the secondary transfer unit  20  is provided with a cleaning mechanism (not shown in the figure) to clean the surface of the secondary transfer belt  21 . Note that the above-described image adjustment roll  23  and belt adjustment roll  24  will be described in detail later. 
     Then, a belt cleaner  35  that is arranged so that the belt cleaner  35  and the driving roll  31  sandwiches the intermediate transfer belt  15  and the belt cleaner  35  is opposed to the driving roll  31 , and removes residual toner and paper powder on the intermediate transfer belt  15  after secondary transfer and cleans the surface of the intermediate transfer belt  15  is attached on the downstream side relative to a secondary transfer position of the intermediate transfer belt  15 . On the other hand, inside the intermediate transfer belt  15  on the upstream side relative to a yellow image forming unit  10 Y, there is arranged a reference sensor  42  that generates a reference signal which is referenced to obtain image forming timing in each image forming unit  10 . The reference sensor  42  recognizes a mark provided inside the intermediate transfer belt  15  (back side of an image holding face) to generate the reference signal. Each image forming unit  10  is configured so as to start image forming by a command from the controller  40  based on the recognition of the reference signal. 
     In the present exemplary embodiment, a paper sheet transportation system includes a paper sheet tray  50  that stores paper sheets P, and a feed roll  51  that takes one of the paper sheets P piled on the paper sheet tray  50  from the paper sheet tray  50  at certain timing and transports it to a transporting route  55 . Further, the paper sheet transportation system includes transporting rolls  52  that transports a paper sheet P fed by the feed roll  51 , a transportation guiding member  53  that feeds the paper sheet P transported by the transporting roll  52  into the secondary transfer position, transporting belts  54  that transport the paper sheet P transported by the secondary transfer belt  21  after the secondary transfer, to the fixing apparatus  60 . 
     Further, the fixing apparatus  60  includes a heating roll  61  that incorporates a heating source (not shown in the figure) and is arranged rotatably, and a pressure belt  62  that is rotatably arranged by pressing against this heating roll  61 . A high-releasable fluororubber layer is formed on the surfaces of the heating roll  61  and the pressure belt  62 . 
     Subsequently, descriptions will be given of an image formation process in the image forming apparatus  1  of the present exemplary embodiment. Firstly, image data is outputted from an image-reading apparatus (not shown in the figure), a personal computer (PC) (not shown in the figure) or the like, and then is inputted into the image forming apparatus  1 . In the image forming apparatus  1 , the image processing is carried out on the data by an image processing apparatus (not shown in the figure), and then image forming operation is executed using the image forming units  10  and the like. In the image processing apparatus, the image processings are carried out on the inputted reflectance data. The image processings include various predetermined image editings such as shading correction, displacement correction, brightness/color-space conversion, gamma correction, border erasing, color editing, editing by moving and the like. The image data having been subjected to the image processings is converted into gradation data of four color materials of yellow (Y), magenta (M), cyan (C) and black (K), and the converted data is outputted to the laser exposure unit  13 . 
     The laser exposure unit  13  irradiates the exposure beam Bm outputted from, for example, a semi-conductor laser, onto each of the photoconductors  11  of the image forming units  10 . The surface of each of the photoconductors  11  of the image forming units  10  is electrically charged by the charging unit  12 , and then scanned and exposed by the laser exposure unit  13  so as to form an electrostatic latent image. The formed electrostatic latent image is then developed as respective toner images of yellow (Y), magenta (M), cyan (C) and black (K) by the respective developing units  14  of the image forming units  10 . 
     The toner images formed on the respective photoconductors  11  of the image forming units  10  are transferred onto the intermediate transfer belt  15  at respective primary transfer positions where the respective photoconductors  11  are brought into contact with the intermediate transfer belt  15 . The images that have been thus primarily transferred and are then transported to the secondary transfer unit  20  as the intermediate transfer belt  15  rotates. Incidentally, residual toner remaining on the photoconductors  11  after the transfer to the intermediate transfer belt  15  is removed by the photoconductor cleaner  17 . 
     On the other hand, in the paper sheet transportation system, the feed roll  51  rotates in accordance with the timing for image formation, and thus the paper sheet P of a predetermined size is supplied from the paper sheet tray  50 . The paper sheet P supplied by the feed roll  51  is then transported along the transporting route  55  by the transporting rolls  52 , and reaches the secondary transfer unit  20  via the transportation guiding member  53 . Before the paper sheet P reaches the secondary transfer unit  20 , the paper sheet P is stopped once, and an alignment of the positions of the paper sheet P and the toner image is achieved by rotating a resist roll (not shown in the figure) that adjusts the position of the paper sheet P, in accordance with the timing of the moving of the intermediate transfer belt  15  that holds the toner image as described above. 
     Then, the sheet P fed from a resist roll (not shown in the figure) is transported to the transfer nip portion to be formed between the driving roll  22  of the secondary transfer belt  21  and the backup roll  34  of the intermediate transfer belt  15  to be sandwiched between the intermediate transfer belt  15  and the secondary transfer belt  21 . At this time, a transfer electric field is formed between the backup roll  34  and the driving roll  22  by applying a transfer voltage taking one as a transfer voltage applying electrode and the other as a counter electrode (earth). An unfixed toner image carried on the intermediate transfer belt  15  is electrostatically transferred on the sheet P. Note that an exemplary embodiment in which the driving roll  22  is served also as a power supply roll may be employed. 
     Thereafter, the sheet P on which the toner image is electrostatically transferred is transported as such in a state peeled off the intermediate transfer belt  15  by the secondary transfer belt  21 , and the sheet P is transported to the transporting belt  54  provided on the downstream side in a sheet transporting direction of the secondary transfer belt  21 . Here, since the secondary transfer belt  21  is used in the present exemplary embodiment, the sheet P which has passed through the secondary transfer position is hardly attached on the intermediate transfer belt  15  side and is apt to be transported in a state adsorbed on the secondary transfer belt  21  side. Further, the sheet P that is transported on the secondary transfer belt  21  is peeled off the secondary transfer belt  21  by a curvature of the image adjustment roll  23  holds the secondary transfer belt  21  near the image adjustment roll  23 , and transported toward the further downstream side. The transporting belt  54  executes speed control so as to match it with a most suitable transporting speed of the fixing apparatus  60  to transport the sheet P to the fixing apparatus  60 . The unfixed toner image on the sheet P transported to the fixing apparatus  60  is subjected to fixing processing with heat and pressure by the fixing apparatus  60  to be fixed on the sheet P. Then, the sheet P holding the fixed image is discharged outside the image forming apparatus  1  by a discharge roll (not shown in the figure). Further, after completion of transfer to the sheet P, the residual toner remaining on the intermediate transfer belt  15  is removed by the belt cleaner  35 . 
       FIG. 2  is a diagram that illustrates the whole secondary transfer unit  20  to which the present exemplary embodiment is applied. Note that  FIG. 2  is a diagram viewing the secondary transfer unit  20  shown in  FIG. 1  from the top, and there is shown a sheet transporting face side on the frontward side of the space.  FIGS. 3A and 3B  are side views of the secondary transfer unit  20  shown in  FIG. 2 . 
       FIG. 3A  illustrates a side view of the outer side of the secondary transfer unit  20 .  FIG. 3B  illustrates a side view of the inner side of the secondary transfer unit  20 . Note that  FIGS. 3A and 3B  show a reference state (reference position of respective rolls) of the secondary transfer unit  20  which will be described later. 
     The secondary transfer unit  20  that functions as a belt transporting apparatus includes a function of adjusting a length in a sub-scanning direction on the outer and inner sides of the image on the sheet P when being secondarily transferred, and a function of suppressing meanders of the secondary transfer belt  21  which may occur associated with the adjustment, as described later. 
     As shown in  FIG. 2 , in the secondary transfer unit  20 , the driving roll  22  includes a driving roll axis  22   a , the image adjustment roll  23  includes an image adjustment roll axis  23   a , and the belt adjustment roll  24  includes a belt adjustment roll axis  24   a  respectively. Further, there are provided a side plate  210  of the outer side on the outer side and a side plate  220  of the inner side on the inner side relative to these three rolls. Furthermore, there are provided a connection body  230  which connects the side plate  210  of the outer side and the side plate  220  of the inner side. 
     In addition, the rotation axes of these three rolls are held by the side plate  210  of the outer side and the side plate  220  of the inner side that are provided on both end sides. 
     A driving motor (not shown in the figure) is connected to the driving roll axis  22   a  in the outside of the side plate  220  of the inner side, and the driving roll  22  receives a power from the driving motor to rotate the secondary transfer belt  21 . 
     As shown in  FIG. 3 , the image adjustment roll  23  is attached on the downstream side of the driving roll  22  in a direction of rotation C of the secondary transfer belt  21 . Further, in the present exemplary embodiment, a diameter of the image adjustment roll  23  is set smaller than a radius of the driving roll  22 . In addition, a sheet transporting face, which is a face to adsorb and transport the sheet P after being secondarily transferred to the secondary transfer belt  21  is formed by the driving roll  22  and the image adjustment roll  23 . 
     Further, a bearing portion  73 O is provided on the outer side of the image adjustment roll axis  23   a  and a bearing portion  73 I on the inner side thereof. Furthermore, the bearing portion  73 O on the outer side is inserted into an elongate hole  83 O provided on the side plate  210  of the outer side and the bearing portion  73 I on the inner side is inserted into an elongate hole  83 I provided on the side plate  220  of the inner side, respectively. These elongate holes  83 O and  83 I have a longitudinal shape in a direction along the sheet transporting face. Accordingly, the image adjustment roll  23  is movable in a direction along the sheet transporting face. 
     The belt adjustment roll  24  is provided on the downstream side of the image adjustment roll  23  and on the upstream side of the driving roll  22  in the direction of rotation C of the secondary transfer belt  21 . Further, as shown in  FIG. 3A , the belt adjustment roll  24  is provided on the left side of the image adjustment roll  23  (bottom side of the image adjustment roll  23  in a state attached to the image forming apparatus  1  shown in  FIG. 1 ) when viewing from the outer side. Furthermore, in the present exemplary embodiment, a diameter of the belt adjustment roll  24  is set substantially the same as a diameter of the image adjustment roll  23 . A sheet peeling off face is formed on the secondary transfer belt  21  by the image adjustment roll  23  and the belt adjustment roll  24 , and a non-sheet transporting face is formed on the secondary transfer belt  21  by the belt adjustment roll  24  and the driving roll  22 . 
     Further, a bearing portion  74 O is provided on the outer side of the belt adjustment roll axis  24   a  and a bearing portion  74 I on the inner side thereof. Furthermore, the bearing portion  74 O on the outer side is inserted into an elongate hole  84 O provided on the side plate  210  of the outer side and the bearing portion  74 I on the inner side is inserted into an elongate hole  84 I provided on the side plate  220  of the inner side, respectively. These elongate holes  84 O and  84 I have a longitudinal shape in a direction along the non-sheet transporting face. Accordingly, the belt adjustment roll  24  is movable in a direction along the non-sheet transporting face. 
     Note that in the present exemplary embodiment, as shown in  FIGS. 3A and 3B , the image adjustment roll  23  and the belt adjustment roll  24  are arranged relative to the driving roll  22  so that the sheet transporting face formed on the secondary transfer belt  21  by the driving roll  22  and the image adjustment roll  23 , and the non-sheet transporting face formed on the secondary transfer belt  21  by the belt adjustment roll  24  and the driving roll  22  are substantially parallel. Further, as described above, the directions of two elongate holes are provided along the sheet transporting face and the non-sheet transporting face respectively. Thus, even if the image adjustment roll  23  and the belt adjustment roll  24  are moved along the elongate holes respectively, a parallel state between the sheet transporting face and the non-sheet transporting face is maintained. 
     Next, a cam mechanism which is used to move the image adjustment roll  23  and the belt adjustment roll  24 , and functions as a moving member and a setting member will be described. 
     As shown in  FIG. 2 , the secondary transfer unit  20  further includes a cam axis  25   a  that configures the cam mechanism, and an outer side cam  25  and an inner side cam  26  attached to the outer side and the inner side of the cam axis  25   a , respectively. 
     The cam axis  25   a  is provided substantially parallel to the driving roll axis  22   a , and is provided between the driving roll axis  22   a  and the image adjustment roll axis  23   a  as well as the belt adjustment roll axis  24   a . The cam axis  25   a  in the present exemplary embodiment is arranged substantially on a line of a vertical bisector (reference line S described later) formed by the image adjustment roll axis  23   a  and the belt adjustment roll axis  24   a  as apparent from  FIGS. 3A and 3B . Further, as shown in  FIG. 2 , the cam axis  25   a  is held by penetrating through the side plate  210  of the outer side and the side plate  220  of the inner side. Furthermore, in the secondary transfer unit  20 , the outer side cam  25  is attached outside relative to the side plate  210  of the outer side and the inner side cam  26  is attached outside relative to the side plate  220  of the inner side. 
     As described above, one end side of the outer side cam  25  is fixed on one end portion of the outer side of the cam axis  25   a , and an outer side cam face  25 F (an outer circumference face) having a curved surface of a fan shape is provided on the other end side. Further, the outer side cam face  25 F is configured so as to be brought into contact with the bearing portion  73 O on the outer side of the image adjustment roll  23  and the bearing portion  74 O on the outer side of the belt adjustment roll  24 , respectively. 
     On the other hand, one end side of the inner side cam  26  is fixed on the inner side of the cam axis  25   a , and an inner side cam face  26 F (another outer circumference face) having a curved surface of a fan shape is provided on the other end side. Further, the inner side cam face  26 F is configured so as to be brought into contact with the bearing portion  73 I on the inner side of the image adjustment roll  23  and the bearing portion  74 I on the inner side of the belt adjustment roll  24 , respectively. 
     A driving mechanism (not shown in the figure) is connected to the end portion on the inner side of the cam axis  25   a . The cam axis  25   a  is subjected to rotation force by this driving mechanism, so that the outer side cam  25  and the inner side cam  26  fixed on the cam axis  25   a  are rotated in the same direction. 
       FIGS. 4A and 4B  are diagrams that illustrate a shape of the outer side cam  25  and the inner side cam  26 . Note that in  FIGS. 4A and 4E , in order to illustrate the shape of the outer side cam face  25 F and the inner side cam face  26 F, reference lines S are respectively shown. 
     First, referring to  FIG. 4A , the shape of the outer side cam  25  will be described. The outer side cam  25  has an axisymmetric shape relative to the reference line S as an axis. In the outer side cam  25 , a distance between the center of the cam axis  25   a  and the outer side cam face  25 F (hereinafter, referred to as a cam radius) becomes longer, as the angle relative to the reference line S about the cam axis  25   a  becomes larger. 
     Specifically, as shown in  FIG. 4A , the cam radius is a first radius R 1  that is a minimum cam radius of the outer side cam  25  on the reference line S. As the angle relative to the reference line S is increased clockwise or counterclockwise, the cam radius becomes a third radius R 3  that is a maximum cam radius of the outer side cam  25 . Further, between a position where the cam radius is the first radius R 1  and a position where the cam radius is the third radius R 3 , a position where the cam radius is a second radius R 2  to be utilized in the reference state which will be described later is axisymmetrically provided on both the left side and the right side of the outer side cam  25  relative to the reference line S as an axis. 
     The bearing portion  73 O on the outer side of the image adjustment roll  23  is provided so as to be brought into contact with an area on the right side of the reference line S on the outer side cam face  25 F shown in  FIG. 4A . On the other hand, the bearing portion  74 O on the outer side of the belt adjustment roll  24  is provided so as to be brought into contact with an area on the left side of the reference line S on the outer side cam face  25 F. Thus, in response to a rotation angle of the cam axis  25   a , the bearing portion  73 O of the image adjustment roll  23  may be brought into contact with any position in the area on the right side of the outer side cam face  25 F and the bearing portion  74 O of the belt adjustment roll  24  may be brought into contact with any position in the area on the left side. Further, since the bearing portions on the outer side of the image adjustment roll  23  and the belt adjustment roll  24  are brought into contact with the outer side cam face  25 F respectively, these two rolls are simultaneously adjusted in association with the rotation of the cam axis  25   a.    
     Next, referring to  FIG. 4B , a shape of the inner side cam  26  will be described. 
     Similarly to the outer side cam  25 , the inner side cam  26  has a symmetric shape relative to the reference line S as an axis. However, the inner side cam  26  is different in shape as compared with the outer side cam  25 . Described specifically, a cam radius of the inner side cam  26  becomes shorter, as the angle relative to the reference line S about the cam axis  25   a  becomes larger. 
     Specifically, as shown in  FIG. 4B , the cam radius is the third radius R 3  that is a maximum cam radius of the inner side cam  26  on the reference line S. As an angle relative to the reference line S is increased clockwise or counterclockwise, the cam radius becomes the first radius R 1  that is a minimum cam radius of the inner side cam  26 . Further, similarly to the outer side cam  25 , between a position where the cam radius is the third radius R 3  and a position where the cam radius is the first radius R 1 , a position where the cam radius is the second radius R 2  to be utilized in the reference state which will be described later is axisymmetrically provided on both the left side and the right side of the inner side cam  26  relative to the reference line S as an axis. 
     The bearing portion  73 I on the inner side of the image adjustment roll  23  is provided so as to be brought into contact with an area on the left side of the reference line S on the inner side cam face  26 F shown in  FIG. 4B . On the other hand, the bearing portion  74 I on the inner side of the belt adjustment roll  24  is provided so as to be brought into contact with an area on the right side of the reference line S on the inner side cam face  26 F. Thus, in response to a rotation angle of the cam axis  25   a , the bearing portion  73 I of the image adjustment roll  23  may be brought into contact with any position in the area on the left side of the inner side cam face  26 F and the bearing portion  74 I of the belt adjustment roll  24  may be brought into contact with any position in the area on the left side. Further, since the bearing portions on the inner side of the image adjustment roll  23  and the belt adjustment roll  24  are brought into contact with the inner side cam face  26 F respectively, these two rolls are simultaneously adjusted in association with the rotation of the cam axis  25   a.    
     The outer side cam  25  and the inner side cam  26  are fixed on the outer side and the inner side of the same cam axis  25   a  respectively as described above. At this time, the outer side cam  25  and the inner side cam  26  are oppositely attached so that each reference line S is aligned. Accordingly, the position where the cam radius is the first radius R 1  in the outer side cam  25  and the position where the cam radius is the third radius R 3  in the inner side cam  26  are opposed. Further, the position where the cam radius is the third radius R 3  in the outer side cam  25  and the position where the cam radius is the first radius R 1  in the inner side cam  26  are opposed. Furthermore, the position where the cam radius is the second radius R 2  in the outer side cam  25  and the position where the cam radius is the second radius R 2  in the inner side cam  26  are opposed. 
     In this way, the outer side cam  25  and the inner side cam  26  are opposed in the position where the cam radius is the second radius R 2 . However, the outer side cam  25  and the inner side cam  26  are oppositely arranged so that, as one cam radius is increased, another cam radius is decreased. 
     Note that a cam radius between a rotation center of the outer side cam  25  and a contact region of the image adjustment roll  23  on the outer side cam face  25 F configures a first distance, and a cam radius between a rotation center of the outer side cam  25  and a contact region of the belt adjustment roll  24  on the outer side cam face  25 F configures a second distance. Further, a cam radius between a rotation center of the inner side cam  26  and a contact region of the image adjustment roll  23  on the inner side cam face  26 F configures a third distance, and a cam radius between a rotation center of the inner side cam  26  and a contact region of the belt adjustment roll  24  on the inner side cam face  26 F configures a fourth distance. 
     Next, the reference state of the secondary transfer unit  20  will be described. 
     Note that, in the following description, as shown in  FIG. 3A , on the outer side, a center distance between the driving roll axis  22   a  and the image adjustment roll axis  23   a  along the sheet transporting face is referred to as a first outer side center distance LO 1 , a center distance between the driving roll axis  22   a  and the belt adjustment roll axis  24   a  along the non-sheet transporting face is referred to as a second outer side center distance LO 2 , and further a center distance between the image adjustment roll axis  23   a  and the belt adjustment roll axis  24   a  along the sheet peeling off face is referred to as a third outer side center distance LO 3 . 
     Furthermore, as shown in  FIG. 3B , on the inner side, a center distance between the driving roll axis  22   a  and the image adjustment roll axis  23   a  along the sheet transporting face is referred to as a first inner side center distance LI 1 , and a center distance between the driving roll axis  22   a  and the belt adjustment roll axis  24   a  along the non-sheet transporting face is referred to as a second inner side center distance LI 2 , and further a center distance between the image adjustment roll axis  23   a  and the belt adjustment roll axis  24   a  along the sheet peeling off face is referred to as a third inner side center distance LI 3 . 
     In the reference state, as shown in  FIG. 3A , in the outer side cam  25 , the bearing portion  73 O of the image adjustment roll  23  and the bearing portion  74 O of the belt adjustment roll  24  are respectively brought into contact with the position where the cam radius is the second radius R 2 . On the other hand, as shown in  FIG. 3B , in the inner side cam  26 , the bearing portion  73 I of the image adjustment roll  23  and the bearing portion  74 I of the belt adjustment roll  24  are respectively brought into contact with the position where the cam radius is the second radius R 2 . 
     Accordingly, in the reference state, all bearing portions on the inner and the outer sides of the image adjustment roll  23 , and all bearing portions on the inner and the outer sides of the belt adjustment roll  24  are brought into contact with a position where the cam radius on the outer side cam face  25 F or the inner side cam face  26 F is the second radius R 2 . 
     Thus, all the first outer side center distance LO 1 , the second outer side center distance LO 2 , the first inner side center distance LI 1 , and the second inner side center distance LI 2  are equal (LO 1 =LO 2 =LI 1 =LI 2 ). Further, since the image adjustment roll  23  and the belt adjustment roll  24  are parallel, the third outer side center distance LO 3  and the third inner side center distance LI 3  are also equal (LO 3 =LI 3 ). 
     Here, the sum of the respective center distances on the outer side of the secondary transfer belt  21  (LO 1 +LO 2 +LO 3 ) is substantially equal to the sum of the respective center distances on the inner side thereof (LI 1 +LI 2 +LI 3 ) according to the above-described relation. Therefore, in the secondary transfer belt  21  that is wrapped in a state of the above-described center distances, a circumference of the secondary transfer unit  20  (hereinafter, referred to as the circumference) on the inner side is equal to that on the outer side. 
     In this way, since the circumference of the secondary transfer belt  21  on the outer side is equal to that on the inner side, and the driving roll  22 , the image adjustment roll  23  and the belt adjustment roll  24  are reciprocally in a sate of a substantially parallel, the meanders of the secondary transfer belt  21  hardly occur. 
       FIG. 5  is a diagram that Illustrates a surface velocity of the secondary transfer belt  21  in the reference state. 
     As described above, in the reference state, the first outer side center distance LO 1  is equal to the first inner side center distance LI 1 , on the sheet transporting face. Thus, as shown in  FIG. 5 , since a distance where the secondary transfer belt  21  proceeds per unit time between the axes of the driving roll  22  and the image adjustment roll  23  is the same amount on the outer side and the inner side, the surface velocity (moving speed) of the secondary transfer belt  21  is substantially equal on the outer side V out  to that on the inner side V in . 
     Accordingly, if position accuracy of the components of each unit such as the secondary transfer unit  20  that configures an image forming apparatus is equal on the inner side to that on the outer side, when secondary transfer is performed in the reference state, an image on the outer side has a substantially similar length to an image on the inner side. 
       FIGS. 6A and 6B  are diagrams for explaining motions in the secondary transfer unit  20  when the image on the outer side is elongated. Elongation of the image on the outer side is executed in the case where the outer side of the image is reduced, in comparison with the inner side, when secondary transfer is performed in the above-described reference state. 
     Note that, as described above, in response to the rotation angle of the cam axis  25   a , the bearing portions of the image adjustment roll  23  and the belt adjustment roll  24  may be brought into contact with any position relative to the outer side cam face  25 F and the inner side cam face  26 F. In the following, a description will be given of an adjustment of the image adjustment roll  23  and the belt adjustment roll  24  by a cam mechanism when the amount of movement of these rolls is maximized, as an example. 
     When the image on the outer side is elongated, the cam axis  25   a  in the reference state is rotated to a direction same as the direction of rotation C of the secondary transfer belt  21 . At this time, the outer side cam  25  is rotated clockwise as shown in  FIG. 6A  and the inner side cam  26  is rotated counterclockwise as shown in  FIG. 6B . 
     Then, on the outer side cam face  25 F, the bearing portion  73 O of the image adjustment roll  23  is brought into contact with a position where the cam radius is the third radius R 3  and the bearing portion  74 O of the belt adjustment roll  24  is brought into contact with a position where the cam radius is the first radius R 1 . 
     As a result of this, since the bearing portion  73 O on the outer side of the image adjustment roll  23  is brought into contact with a position on the outer side cam face  25 F where the cam radius is increased from that in the reference state from R 2  to R 3 ), the distance from the cam axis  25   a  is also increased and the bearing portion  73 O is moved in a direction leaving from the driving roll  22 . 
     On the other hand, since the bearing portion  74 O on the outer side of the belt adjustment roll  24  is brought into contact with a position on the outer side cam face  25 F where the cam radius is decreased from that in the reference state (from R 2  to R 1 ), the distance from the cam axis  25   a  is also decreased and the bearing portion  74 O is moved in a direction approaching to the driving roll  22  by tensile force of the secondary transfer belt  21 . 
     Interlocking with the outer side cam  25 , in the inner side cam  26 , the bearing portion  73 I of the image adjustment roll  23  is brought into contact with a position on the inner side cam face  26 F where the cam radius is the first radius R 1  and the bearing portion  74 I of the belt adjustment roll  24  is brought into contact with a position on the inner side cam face  26 F where the cam radius is the third radius R 3 . 
     As a result of this, since the bearing portion  73 I on the inner side of the image adjustment roll  23  is brought into contact with a position on the inner side cam face  26 F where the cam radius is decreased from that in the reference state (from R 2  to R 1 ), the distance from the cam axis  25   a  is decreased and the bearing portion  73 I is moved in a direction approaching to the driving roll  22  by the tensile force of the secondary transfer belt  21 . 
     On the other hand, since the bearing portion  741  on the inner side of the belt adjustment roll  24  is brought into contact with a position on the inner side cam face  26 F where the cam radius is increased from that in the reference state (from R 2  to R 3 ), the bearing portion  74 I is moved in a direction leaving from the driving roll  22 . 
     In this way, since the outer side cam  25  has an axisymmetric shape as described above, an end portion on the outer side of the image adjustment roll  23  and an end portion on the outer side of the belt adjustment roll  24  are oppositely moved forward and backward each other, relative to an end portion on the outer side of the driving roll  22 . Similarly, since the inner side cam  26  has an axisymmetric shape as described above, an end portion on the inner side of the image adjustment roll  23  and an end portion on the inner side of the belt adjustment roll  24  are oppositely moved forward and backward each other, relative to an end portion on the inner side of the driving roll  22 . 
     Further, in the outer side cam  25  and the inner side cam  26 , since the relation between an increase and a decrease of the cam radius is opposite, if one end side of the image adjustment roll  23  or the belt adjustment roll  24  approaches the driving roll  22 , the other end side is moved so as to leave from the driving roll  22 . 
     As a result of this, the first outer side center distance LO 1  is increased in comparison with that in the reference state and the second outer side center distance LO 2  is decreased in comparison with that in the reference state. Further, the first inner side center distance LI 1  is decreased in comparison with that in the reference state and the second inner side center distance LI 2  is increased in comparison with that in the reference state. 
       FIG. 7  is a diagram for explaining the surface velocity of the secondary transfer belt  21  when the image on the outer side is elongated. 
     Here, in the secondary transfer belt  21 , if the center distances of the rolls over which the secondary transfer belt  21  is wrapped are different between the outer side and the inner side, since the distance when the secondary transfer belt  21  proceeds per unit time becomes longer for the longer center distance, the surface velocity of the secondary transfer belt  21  becomes fast. On the contrary, since the distance when the secondary transfer belt  21  proceeds per unit time becomes shorter for the shorter center distance, the surface velocity of the secondary transfer belt  21  becomes slow. 
     Accordingly, in a state shown in  FIG. 7 , the surface velocity V out  on the outer side on the sheet transporting face of the secondary transfer belt  21  is increased in comparison with the surface velocity V in  on the inner side. Thus, when a toner image is secondarily transferred on the sheet transporting face of the secondary transfer belt  21 , the image on the outer side is elongated and the image on the inner side is shortened. 
     Note that on the non-sheet transporting face, since the relation between center distances is opposite to that on the sheet transporting face, the surface velocity V out  on the outer side is decreased as compared with the surface velocity V in  on the inner side (not shown in the figure). Thus, an average speed (time spent on one round) of the secondary transfer belt  21  on the outer side is equal to that on the inner side. 
     Further, as shown in  FIGS. 6A and 6B , on the outer side cam face  25 F or the inner side cam face  26 F, the bearing portion  73 O of the image adjustment roll  23  on the outer side and the bearing portion  74 I of the belt adjustment roll  24  on the inner side are brought into contact with a position where the cam radius is the third radius R 3 . Thus, the first outer side center distance LO 1  is equal to the second inner side center distance LI 2 . (LO 1 =LI 2 ) 
     On the other hand, as shown in  FIG. 6B , on the outer side cam face  25 F or the inner side cam face  26 F, the bearing portion  74 O of the belt adjustment roll  24  on the outer side and the bearing portion  73 I of the image adjustment roll  23  on the inner side are brought into contact with a position where the cam radius is the first radius R 1 . Thus, the second outer side center distance LO 2  is equal to the first inner side center distance LI 1 . (LO 2 =LI 1 ) 
     Further, the third outer side center distance LO 3  is equal to the third inner side center distance LI 3 , since the image adjustment roll  23  and the belt adjustment roll  24  are symmetrically moved. (LO 3 =LI 3 ) 
     The sum of the respective center distances on the outer side of the secondary transfer belt  21  (LO 1 +LO 2 +LO 3 ) is equal to the sum of the respective center distances on the inner side (LI 1 +LI 2 +LI 3 ) according to the above-described relation. Accordingly, in a state when the respective rolls have the above-described center distances, the circumference of the secondary transfer belt  21  that is wrapped over these rolls on the inner side is equal to that on the outer side. 
     Further, as described above, in order to elongate the image on the outer side, the image adjustment roll  23  is made oblique relative to the driving roll  22 . Since the image adjustment roll  23  is thus oblique relative to the driving roll  22 , force to cause meander in the secondary transfer belt  21  is generated. However, interlocking with the image adjustment roll  23 , since the belt adjustment roll  24  is made oblique in the opposite direction to that of the image adjustment roll  23 , relative to the driving roll  22 , the force in the secondary transfer belt  21  which causes meanders described above is offset. 
     As described above, the circumference of the secondary transfer belt  21  on the outer side is equal to that on the inner side and the force which causes the secondary transfer belt  21  to attempt meander is offset. Accordingly, occurrence of meanders in the secondary transfer belt  21  is suppressed. 
       FIGS. 8A and 8B  are diagrams for explaining motions in the secondary transfer unit  20  when the image on the inner side is elongated. Elongation of the image on the inner side is executed in the case where the inner side of the image is reduced, in comparison with the outer side, when the secondary transfer is performed in the reference state. 
     Note that, as described above, in response to the rotation angle of the cam axis  25   a , the bearing portions of the image adjustment roll  23  and the belt adjustment roll  24  may be brought into contact with any position relative to the outer side cam face  25 F and the inner side cam face  26 F. In the following, a description will be given of an adjustment of the image adjustment roll  23  and the belt adjustment roll  24  by a cam mechanism when the amount of movement of these rolls is maximized, as an example. 
     When the image on the inner side is elongated, the cam axis  25   a  in the reference state is rotated to an opposite direction to the direction of rotation C of the secondary transfer belt  21 . At this time, the outer side cam  25  is rotated counterclockwise as shown in  FIG. 8A  and the inner side cam  26  is rotated clockwise as shown in  FIG. 8B . 
     Then, on the outer side cam face  25 F, the bearing portion  73 O of the image adjustment roll  23  is brought into contact with a position where the cam radius is the first radius R 1  and the bearing portion  74 O of the belt adjustment roll  24  is brought into contact with a position where the cam radius is the third radius R 3 . 
     As a result of this, since the bearing portion  73 O on the outer side of the image adjustment roll  23  is brought into contact with a position on the outer side cam face  25 F where the cam radius is decreased from that in the reference state (from R 2  to R 1 ), the distance from the cam axis  25   a  is also decreased and the bearing portion  73 O is moved in a direction approaching to the driving roll  22  by tensile force of the secondary transfer belt  21 . 
     On the other hand, since the bearing portion  74 O on the outer side of the belt adjustment roll  24  is brought into contact with a position on the outer side cam face  25 F where the cam radius is increased from that in the reference state (from R 2  to R 3 ), the distance from the cam axis  25   a  is also increased and the bearing portion  74 O is moved in a direction leaving from the driving roll  22 . 
     Interlocking with the outer side cam  25 , in the inner side cam  26 , the bearing portion  73 I of the image adjustment roll  23  is brought into contact with a position on the inner side cam face  26 F where the cam radius is the third radius R 3  and the bearing portion  74 I of the belt adjustment roll  24  is brought into contact with a position on the inner side cam face  26 F where the cam radius is the first radius R 1 . 
     As a result of this, since the bearing portion  73 I on the inner side of the image adjustment roll  23  is brought into contact with a position on the inner side cam face  26 F where the cam radius is increased from that in the reference state (from R 2  to R 3 ), the distance from the cam axis  25   a  is increased and the bearing portion  731  is moved in a direction leaving from the secondary transfer belt  21 . 
     On the other hand, since the bearing portion  74 I on the inner side of the belt adjustment roll  24  is brought into contact with a position on the inner side cam face  26 F where the cam radius is decreased from that in the reference state (from R 2  to R 1 ), the bearing portion  74 I is moved in a direction approaching to the driving roll  22  by tensile force of the secondary transfer belt  21 . 
     In this way, in the case where the inner side is elongated, since the outer side cam  25  has an axisymmetric shape as described above, an end portion on the outer side of the image adjustment roll  23  and an end portion on the outer side of the belt adjustment roll  24  are oppositely moved forward and backward each other, relative to an end portion on the outer side of the driving roll  22 , similarly to the case where the outer side is elongated. Similarly, since the inner side cam  26  has an axisymmetric shape as described above, an end portion on the inner side of the image adjustment roll  23  and an end portion on the inner side of the belt adjustment roll  24  are oppositely moved forward and backward each other, relative to an end portion on the inner side of the driving roll  22 . 
     Further, in the outer side cam  25  and the inner side cam  26 , since the relation between an increase and a decrease of the cam radius is opposite, if one end side of the image adjustment roll  23  or the belt adjustment roll  24  approaches the driving roll  22 , the other end side is moved so as to leave from the driving roll  22 . 
     As a result of this, the first outer side center distance LO 1  is decreased in comparison with that in the reference state and the second outer side center distance LO 2  is increased in comparison with that in the reference state. Further, the first inner side center distance LI 1  is increased in comparison with that in the reference state and the second inner side center distance LI 2  is decreased in comparison with that in the reference state. 
       FIG. 9  is a diagram for explaining the surface velocity of the secondary transfer belt  21  when the image on the inner side is elongated. 
     In a state shown in  FIG. 9 , the surface velocity V in  on the inner side on the sheet transporting face of the secondary transfer belt  21  is increased in comparison with the surface velocity V out  on the outer side. Thus, when a toner image is secondarily transferred on the sheet transporting face of the secondary transfer belt  21 , the image on the inner side is elongated and the image on the outer side is shortened. 
     Note that, on the non-sheet transporting face, since the relation between center distances is opposite to that on the sheet transporting face, the surface velocity V in  on the inner side is decreased in comparison with the surface velocity V out  on the outer side (not shown in the figure). Thus, an average speed (time spent on one round) of the secondary transfer belt  21  on the outer side is equal to that on the inner side. 
     Further, as shown in  FIGS. 8A and 8B , on the outer side cam face  25 F or the inner side cam face  26 F, the bearing portion  73 O of the image adjustment roll  23  on the outer side and the bearing portion  74 I of the belt adjustment roll  24  on the inner side are brought into contact with a position where the cam radius is the first radius R 1 . Thus, the first outer side center distance LO 1  is equal to the second inner side center distance LI 2 . (LO 1 =LI 2 ) 
     On the other hand, as shown in  FIG. 8B , on the outer side cam face  25 F or the inner side cam face  26 F, the bearing portion  74 O of the belt adjustment roll  24  on the outer side and the bearing portion  73 I of the image adjustment roll  23  on the inner side are brought into contact with a position where the cam radius is the third radius R 3 . Thus, the second outer side center distance LO 2  is equal to the first inner side center distance LI 1 . (LO 2 =LI 1 ) 
     Further, the third outer side center distance LO 3  is equal to the third inner side center distance LI 3 , since the image adjustment roll  23  and the belt adjustment roll  24  are symmetrically moved. (LO 3 =LI 3 ) 
     Similarly, in the case where the image of the inner side is elongated, the sum of the respective center distances on the outer side of the secondary transfer belt  21  (LO 1 +LO 2 +LO 3 ) is equal to the sum of the respective center distances on the inner side (LI 1 +LI 2 +LI 3 ) according to the above-described relation. Accordingly, in a state when the respective rolls have the above-described center distances, the circumference of the secondary transfer belt  21  that is wrapped over these rolls on the inner side is equal to that on the outer side. 
     Further, as described above, in order to elongate the image on the inner side, the image adjustment roll  23  is made oblique relative to the driving roll  22 . Since the image adjustment roll  23  is thus oblique relative to the driving roll  22 , force to cause meander in the secondary transfer belt  21  is generated. However, interlocking with the image adjustment roll  23 , since the belt adjustment roll  24  is made oblique in the opposite direction to that of the image adjustment roll  23 , relative to the driving roll  22 , the force in the secondary transfer belt  21  which causes meanders is offset. 
     As described above, the circumference of the secondary transfer belt  21  on the outer side is equal to that on the inner side and the force which causes the secondary transfer belt  21  to attempt meander is offset. Accordingly, occurrence of meanders in the secondary transfer belt  21  is suppressed. 
     Note that, by using the secondary transfer belt  21  in the present exemplary embodiment, the image adjustment may be carried out even when, for example, an elongation or reduction occurs on the inner side and the outer side of an image by factors of the secondary transfer unit  20  itself, or when a toner image is transported in a different state from an originally intended image between the outer side and the inner side in the intermediate transfer belt  15 . 
     Further, shapes of the outer side cam  25  and the inner side cam  26  are not limited to the shape illustrated in the present exemplary embodiment. The shapes of the cam may be changed suitably by the relation of positions between the image adjustment roll  23  and the driving roll  22  and between the belt adjustment roll  24  and the driving roll  22 . 
     Furthermore, the number of roll members is not limited to three. Meanders of belt that occur when a difference in elongation and reduction of an image is adjusted by changing a center distance between the driving roll and the image adjustment roll may be suppressed using plural belt adjustment rolls. 
     In addition, in the present exemplary embodiment, a transfer position is formed by opposing the secondary transfer unit  20  to the intermediate transfer belt  15 , but it is not limited to this. For example, there is also considered a case where the transfer position is formed by opposing a drum-shaped photoconductor or a belt-shaped photoconductor on a transfer belt (secondary transfer unit  20 ). Even in such a case, by using the secondary transfer unit  20  described in the present exemplary embodiment, the image adjustment may be carried out as well as the meanders of the belt may be suppressed. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.