Abstract:
An image forming apparatus assembly includes a main body, and a belt unit mounted to the main body and including an endless belt, a plurality of rollers including a first roller configured to apply a driving force to the belt and to stretch the belt, and a second roller around which the belt is wound with a wrapping angle not less than 90° and configured to stretch the belt, a frame configured to integrally support the rollers, a first engaging portion disposed in the frame and configured to position the first roller to the main body, and a second engaging portion disposed in the frame and configured to position the second roller to the main body. In addition, a guide member is configured to guide the belt unit and disposed in the main body, and including a first engaged portion engaged with the first engaging portion and a second engaged portion engaged with the second engaging portion.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     There are various types of image forming apparatuses which use an electrophotographic image forming method, an electrostatic recording method, or the like, to form an image. Some of them are widely in use as a printer, or a copying machine. They use a photosensitive drum (or photosensitive drums) as their image bearing member(s). They are also provided with such processing means as a charging means, an exposing means, and a developing means, which are disposed in the adjacencies of the peripheral surface of the photosensitive drum. Further, they are provided with an image transfer mechanism, a recording medium conveyance mechanism, an image fixing mechanism, etc. 
     Further, some of them employ an intermediary transfer member for transferring a toner image on a photosensitive drum onto recording medium. Among them, some have an endless intermediary transfer belt and/or an endless recording medium conveyance belt. The endless intermediary transfer belt temporarily holds a toner image after the transfer of a toner image onto the intermediary transfer belt from a photosensitive drum. The recording medium conveyance belt is an endless belt which conveys recording medium onto which a toner image is transferred from a photosensitive drum. 
     An image forming apparatus which is equipped with an endless belt such as those described above has to be provided with a means for preventing a problem peculiar to a system which uses an endless belt, more specifically, the problem that as the endless belt is circularly moved, it tends to laterally shift. As for the method for preventing an endless belt from laterally shifting, there have been used the following three methods (1)-(3), for example. 
     (1) Ordinarily, a system which employs an endless belt is structured so that the endless belt is suspended by multiple rollers, which include a belt driving roller (which hereafter may be referred to simply as driver roller). Thus, as the endless belt is circularly moved, the belt tends to shift toward one of the lengthwise ends of the driver roller, for example, which is smaller in diameter than the other lengthwise end. This property of the system is utilized to control the lateral shifting of the endless belt. That is, one (or more) of the rollers by which the endless belt is suspended is shaped so that its center portion is largest in diameter and the roller gradually reduces in diameter toward its lengthwise ends. With the provision of this setup, the endless belt is likely to remain centered relative to the roller in terms of the direction (widthwise direction of endless belt) which is roughly perpendicular to the direction in which the endless belt is circularly moved.
 
(2) The process for manufacturing a system which employs an endless belt is strictly controlled in terms of the preciseness of each roller of the system, and the level of parallelism among the rollers. Further, the endless belt is provided with a rib (or ribs) which extends in the circumferential direction of the endless belt, and also, the system is provided with a member with which the rib comes into contact as the belt shifts in the direction which is roughly perpendicular to the direction in which the belt is circularly moved.
 
(3) The process for manufacturing the endless driving belt driving system is strictly controlled in terms of the preciseness of each roller, and the endless belt is provided with the rib for controlling the lateral shifting of the endless belt. Further, the amount by which the endless belt laterally shifts is measured during the process of assembling the endless belt driving system, and the rollers are adjusted in terms of the level of parallelism at which they are arranged.
 
     However, the three methods described above have the following problems. That is, in the case of a method such as the method (1), which tries to keep an endless belt centered by shaping an endless belt supporting roller so that the center portion of the roller is largest in diameter, the portions of the belt which are supported by the portions of the roller, which are smaller in diameter than the center portion, slacken, and/or the belt fails to be properly centered, unless the endless belt is elastic enough to conform to the shape of the belt supporting roller. Further, if the endless belt is extremely thin, the endless belt is likely to be overcome by the force which tends to cause the endless belt to converge toward its center. Consequently, the center portion of the belt wrinkles. 
     Further, in the case of the method such as the method (2) which strictly controls the process of manufacturing the belt supporting rollers, in terms of the preciseness of each roller, and regulating the lateral movement of the endless belt, by providing the endless belt with a rib or ribs, the following problems sometimes occurred. That is, the force which acts in the direction to cause the endless belt to laterally shift cannot be ignored without having a negative effect. More specifically, as the endless belt is circularly moved while being controlled in its lateral shift, stress accumulates in the rib of the endless belt, which sometimes caused the rib to jump the rib regulating member, which sometimes caused the endless belt itself and/or its rib to break. Further, the high level of precision of which the process of manufacturing the endless belt driving system is required increases in cost the components of the endless belt supporting system. 
     Further, even after the amount by which the endless belt tends to laterally shift is measured after the assembly of the belt supporting system (device), and the belt supporting rollers are aligned to cancel the tendency of the endless belt to laterally shift, as in the case of the method (3) described above, the following problem sometimes occurred. That is, if an endless belt supporting system (mechanism) is not exactly in the same condition as that in which it was when it was assembled, that is, if it becomes distorted, its endless belt laterally shifts, causing eventually the belt to shift far enough for the rib to creep onto the rib regulating member. As the belt creeps onto the rib regulating member, it is possible for the endless belt and/or its rib (ribs) to become damaged. Thus, even if the endless belt supporting device is adjusted after it is installed in the main assembly of an image forming apparatus, for example, the endless belt begins to laterally shift because of the frictional wear of the rollers, changes in the amount of friction between the endless belt and rollers attributable to the surface deterioration of the belt and rollers, stretching of the belt, and/or the like causes. Consequently, the rib creeps onto the rib regulating member, which will result in damage to the rib itself and/or the belt. 
     Thus, various inventions have been made to deal with this problem. One of such inventions is disclosed in Japanese Laid-open Patent Application 2005-92153. According to this patent application, an image forming apparatus is provided with a platen roller as one of the rollers by which an endless belt is suspended, a belt driving roller which can be changed in its angle relative to the platen roller, and another roller which can be placed in contact with, or separated from, the endless belt. The apparatus is changed in the state of contact between its belt contacting member and endless belt so that the angle of the belt driving roller relative to the platen roller changes in response to the change in the state of contact between the belt contacting roller and endless belt. 
     The art disclosed in Japanese Laid-open Patent Application 2005-92153, however, is possibly problematic in that as the transferring means frame, by which the belt supporting rollers are supported, progressively distorts, the rollers become progressively misaligned among themselves. Eventually, the misalignment becomes too much to be dealt with. Further, in the case of an image forming apparatus, the belt cleaning member of which is positioned in such a manner that it is pressed against the belt driving roller (belt steering roller), with the presence of the belt between the belt cleaning member and the belt driving roller, the belt cleaning member becomes unstable in the state of contact between itself and the belt, as the belt driving roller is tilted to steer the endless belt. Thus, it is possible that the belt cleaning member will reduce in the belt cleaning performance (CLN performance). Thus, it is desired to realize a structural arrangement for an endless belt driving system that does not require an endless belt driving roller to be changed in angle in order to reduce the force which causes the endless belt to laterally shift. 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide an image forming apparatus, which is structured neither to steer its belt driving roller nor change the belt driving roller in angle, and yet, is significantly smaller in the amount of the force which acts in the direction to cause the belt to laterally shift than any image forming apparatus in accordance with the prior art. 
     According to an aspect of the present invention, there is provided an image forming apparatus comprising a main assembly; an intermediary transfer unit including an endless intermediary transfer belt, a plurality of rollers including a driving roller for applying a driving force to said intermediary transfer belt and stretching said intermediary transfer belt, and an intermediary transfer frame integrally supporting said rollers; a main assembly rail member, fixed to said main assembly, for guiding said intermediary transfer unit, said main assembly rail member regulating and positioning at least said driving roller and a roller around which said intermediary transfer belt is wound with a wrapping angle not less than 90° in a state that intermediary transfer unit is mounted in said main assembly. 
     According to the present invention, among the multiple rollers by which an endless belt is supported, the rollers (at least belt driving roller) which are greater in the angle of contact (wrapping angle) between themselves and belt are supported by the rails attached to the main assembly of an image forming apparatus so that they are precisely positioned relative to each other by the rails. Thus, the belt supporting rollers, which have significant effect upon the lateral shifting of the endless belt are kept properly and firmly positioned relative to the main assembly of an image forming apparatus. Therefore, the present invention can prevent the problem that because of the distortion of the intermediary transfer belt unit frame, a belt driving roller becomes misaligned. Therefore, it can substantially reduce an image forming apparatus in the amount of the force which acts in the direction to cause the endless belt to laterally shift. 
     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 
         FIG. 1  is a schematic sectional view of a typical image forming apparatus to which the present invention is applicable. It shows the general structure of the apparatus. 
         FIG. 2(   a ) is a schematic sectional view of the intermediary transfer unit in one of the embodiments of the present invention, when the image forming apparatus is being used for image formation, and  FIG. 2(   b ) is a schematic sectional view of the tension roller portion of the intermediary transfer unit, shown in  FIG. 2(   a ).  FIGS. 2  ( c ) and  2  ( d ) are external views, one for one, of one of the primary transfer roller holders in this embodiment. 
         FIGS. 3(   a ) and  3 ( b ) are schematic sectional views of the intermediary transfer unit of the image forming apparatus shown in  FIG. 1 , when the intermediary transfer belt is not in contact with the photosensitive drums, and when the intermediary transfer belt is in contact with the photosensitive drums, respectively.  FIG. 3(   c ) is a perspective view of the cam, in this embodiment, for allowing intermediary transfer belt from separating from the photosensitive drums. 
         FIGS. 4(   a )- 4 ( c ) are drawings for describing the operation for separating the intermediary transfer rollers from the corresponding photosensitive drums. 
         FIGS. 5(   a )- 5 ( c ) also are drawings for describing the operation for separating the primary transfer rollers from the corresponding photosensitive drums. 
         FIG. 6  is an external perspective view of the intermediary transfer unit in this embodiment. 
         FIG. 7(   a ) is a plan view of the intermediary transfer unit in this embodiment, and  FIG. 7(   b ) is a detailed view of one of the rails of the main assembly of the image forming apparatus in this embodiment, which is for supporting and positioning the belt supporting rollers. 
         FIG. 8(   a ) is a schematic sectional view of the primary transfer roller positioning portion of the image forming apparatus in this embodiment, and  FIG. 8(   b ) also is a schematic sectional view of the primary transfer roller positioning portion of the image forming apparatus. 
         FIG. 9(   a ) is a schematic sectional view of the driver roller positioning portion of the image forming apparatus in this embodiment, and  FIG. 9(   b ) also is a schematic sectional view of the driver roller positioning portion of the image forming apparatus. 
         FIGS. 10(   a ) and  10 ( b ) are schematic sectional views of the intermediary transfer belt unit, in this embodiment, and its adjacencies when the unit is being inserted into, or removed from, the main assembly of the image forming apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, one of the preferred embodiments of the present invention is described with reference to the appended drawings. The dimension, material, and shape of the structural components of the image forming apparatus in this embodiment, and the positional relationship among the structural components, are not intended to limit the present invention in scope, unless specifically noted. Further, if two or more components in the appended drawings are the same in referential code, they are the same in structure and/or function. Therefore, if one of them is described, the others are not described. 
       FIG. 1  is a schematic sectional view of the image forming apparatus in this embodiment of the present invention, and shows the general structure of the apparatus. Referring to  FIG. 1 , an image forming apparatus  200  is an example of a full-color image forming apparatus (which has copying function, printing function, and facsimile function). The image forming apparatus  200  has a main assembly  200   a  which has four image formation stations Sa, Sb, Sc and Sd, which are aligned in the listed order, in the direction parallel to the rotational direction (indicated by arrow mark R 7  in  FIG. 1 ) of the intermediary transfer belt  7  of the apparatus, with the image forming stations Sa and Sd positioned most upstream and downstream, respectively, in terms of the moving direction of the belt  7 . 
     The image forming stations Sa, Sb, Sc and Sd are structured so that they form yellow, magenta, cyan, and black toner images, respectively. They are provided with electrophotographic photosensitive members (which hereafter are referred to as “photosensitive drums”) which are in the form of a drum. Each photosensitive drum is a latent image bearing member of the apparatus. 
     The image forming apparatus  200  is structured so that its photosensitive drums  1   a ,  1   b ,  1   c  and  1   d  are rotationally driven in the directions indicated by arrow marks Ra, Rb, Rc and Rd, respectively (clockwise direction in  FIG. 1 ). The four image formation stations Sa, Sb, Sc and Sd are provided with primary charging devices  2   a ,  2   b ,  2   c  and  2   d , respectively, which are charging means, and exposing devices  3   a ,  3   b ,  3   c  and  3   d , respectively, which are latent image forming means. The charging devices  2   a ,  2   b ,  2   c  and  2   d , and exposing devices  3   a ,  3   b ,  3   c  and  3   d , are positioned in the adjacencies of the peripheral surface of the photosensitive drums  1   a ,  1   b ,  1   c  and  1   d , respectively, with the charging devices  2   a ,  2   b ,  2   c  and  2   d  positioned upstream of the exposing devices  3   a ,  3   b ,  3   c  and  3   d , in terms of the rotational direction of the corresponding photosensitive drums  1   a ,  1   b ,  1   c  and  1   d , respectively. 
     Further, the image formation stations Sa, Sb, Sc and Sd are provided with developing apparatuses  100   a ,  100   b ,  100   c  and  100   d  which are developing means, and primary transfer rollers  5   a ,  5   b ,  5   c  and  5   d  which are the primary transferring means. Each developing device  100  and each primary transfer roller  5  are positioned in the adjacencies of their corresponding photosensitive drum  1 . Further, the image forming stations Sa, Sb, Sc and Sd are provided with cleaning blades  6   a ,  6   b ,  6   c  and  6   d , which are the means for cleaning the photosensitive drums  1   a ,  1   b ,  1   c  and  1   d , respectively. The cleaning blades  6   a ,  6   b ,  6   c  and  6   d  are positioned in the adjacencies of the peripheral surface of the photosensitive drums  1   a ,  1   b ,  1   c  and  1   d , respectively. 
     Further, the image forming apparatus  200  is provided with an endless intermediary transfer belt  7 , which is suspended and kept tensioned by the four primary transfer rollers  5   a - 5   d , idler roller  18 , driving roller  8 , and tension roller  17 , which are positioned in a predesignated positional relationship. While an image is formed by the image forming apparatus  200 , the intermediary transfer belt  7  is kept pressed by the primary transfer rollers  5   a - 5   d  from the inward side of the belt  7  in terms of the loop which the belt u forms, in such a manner that the outward surface of the belt  7  remains in contact with each of the photosensitive drums  1   a ,  1   b ,  1   c  and  1   d.    
     As the primary transfer rollers  5   a - 5   d  are pressed against the photosensitive drums  1   a - 1   d , primary transfer nips T 1   a , T 1   b , T 1   c  and T 1   d  are formed between the intermediary transfer belt  7  and the photosensitive drums  1   a - 1   d , respectively. The intermediary transfer belt unit  40  is structured so that as the belt driving roller  8  which doubles as the roller for backing up the intermediary transfer belt  7 , is rotated in the direction indicated by an arrow mark R 8 , the intermediary transfer belt  7  rotates in the direction indicated by an arrow mark R 7 . The rotational speed of the intermediary transfer belt  7  is set to be roughly the same as the rotational speed (process speed) of each of the above described photosensitive drums  1   a - 1   d.    
     Further, the image forming apparatus  200  is provided with a secondary transfer roller  9 , as an image transferring second means, which is positioned so that it remains in contact with the outward surface of the intermediary transfer belt  7  and opposes the belt driving roller  8  with the presence of the intermediary transfer belt  7  between itself and the belt driving roller  8 , forming thereby a secondary transfer nip T 2 , as the secondary transfer station, between itself and the intermediary transfer belt  7 . Further, there is a belt cleaner  11 , as a cleaner for cleaning the intermediary transfer belt  7 , which is positioned in such a manner that it remains in contact with the outward surface of the intermediary transfer belt  7  and opposes the secondary transfer roller  9 , with the presence of the intermediary transfer belt  7  between itself and tension roller  17 . 
     Sheets P of recording medium, which are to be used for image formation are stored in layers in a sheet feeder cassette  10 , which is in the bottom portion of the main assembly  200   a  of the image forming apparatus  200 . Each sheet P of recording medium is conveyed to the above described secondary transfer nip T 2  by a recording medium conveying device, which is made up of a sheet feeder roller, a sheet conveyance roller, a pair of registration rollers, etc (none of which are illustrated in drawings). There is a fixing device  13  on the downstream side of the secondary transfer nip T 2  in terms of the direction in which the sheet P of recording medium is conveyed. The fixing device  13  has a fixation roller  14 , and a pressure roller  15  which is kept pressed upon the fixation roller  14 . Further, there is a delivery tray (unshown) on the downstream side of the fixing device  13 . 
     The process through which a full-color toner image is formed on a sheet P of recording medium by the image forming apparatus  200  which is structured as described above, is as follows: To begin with, an original is read, and image formation signals which reflect magenta, cyan, yellow and black color components of the original are created. Meanwhile the photosensitive drums  1   a - 1   d  are rotationally driven by a photosensitive drum driving motor (unshown) in the direction indicated by the arrow marks Ra, Rb, Rc and Rd, respectively, at a preset process speed, while the photosensitive drums  1   a - 1   d  are uniformly charged across their peripheral surface, by the primary charging devices  2   a ,  2   b ,  2   c  and  2   d , respectively. Then, the uniformly charged portion of the peripheral surface of each of the photosensitive drums  1   a - 1   d  is exposed by the corresponding exposing device  3   a ,  3   b ,  3   c  or  3   d , based on the information of the image to be formed. Thus, electrical charge is removed from the exposed points of the uniformly charged portion of each of the photosensitive drums  1   a - 1   d . Consequently, four electrostatic latent images, which correspond in pattern to the monochromatic magenta, cyan, yellow, and black images, into which the image of the original has been separated, are effected on the peripheral surfaces of the photosensitive drums  1   a - 1   d , respectively. 
     The electrostatic latent images on the photosensitive drums  1   a - 1   d  are developed into visible images, more specifically, yellow, magenta, cyan and black images formed of yellow, magenta, cyan and black toners, by the development sleeves  102   a ,  102   b ,  102   c  and  102   d  of the developing devices  100   a - 100   d , respectively. These four monochromatic toner image, different in color, are sequentially transferred (primary transfer) onto the outward surface of the intermediary transfer belt  7  by the primary transfer rollers  5   a ,  5   b ,  5   c  and  5   d , in the primary transfer nips T 1   a , T 1   b , T 1   c  and T 1   d , respectively. Consequently, the four monochromatic toner images, different in color, are placed in layers on the intermediary transfer belt  7 . The toner remaining on the peripheral surface of each of the photosensitive drums  1   a - 1   d  is recovered into a recovery toner box (unshown) by a photosensitive drum cleaning means. 
     The four toner images, different in color, layered on the intermediary transfer belt  7  as described above are transferred (secondary transfer) onto a sheet P of recording medium. More concretely, as one of the sheets P of recording medium in the sheet feeder cassette  10  is moved out of the cassette  10  by a recording medium feeding/conveying device, it is conveyed further by a pair of registration rollers (unshown) to the secondary transfer nip T 2  with such a timing that the sheet P arrives to the secondary transfer nip T 2  at the same time as the toner images on the intermediary transfer belt  7 . Then, the toner images on the intermediary transfer belt  7  are transferred together (secondary transfer) onto the sheet P by the secondary transfer roller  9 . 
     After the secondary transfer of the four toner images, different in color, onto the sheet P of recording medium, the sheet P is conveyed to the fixing device  13 , in which the sheet P and the toner images thereon are subjected to heat and pressure. Consequently, the toner images become fixed to the surface of the sheet P. After the fixation of the toner images to the sheet P, the sheet P is discharged into the delivery tray (unshown), ending the operation for forming a full-color image on one side (surface) of the sheet P. 
     [General Description of Intermediary Transfer Unit] 
     Next, referring to  FIGS. 2(   a )-( d ), the intermediary transfer unit  40  is described.  FIG. 2(   a ) is a schematic sectional view of the intermediary transfer unit  40  in this embodiment of the present invention, when the image forming apparatus is being used for image formation, and  FIG. 2(   b ) is a schematic sectional view of the tension roller portion of the intermediary transfer unit  40 , shown in  FIG. 2(   a ).  FIGS. 2(   c ) and  2 ( d ) are external views, one for one, of one of the primary transfer roller holders in this embodiment. 
     First, referring to  FIG. 2(   a ), the overall structure of the intermediary transfer unit  40  is described. The intermediary transfer unit  40  has the intermediary transfer belt  7 , which is endless and is supported and kept tensioned by multiple rollers, including the belt driving roller  8 . 
     Referring also to  FIG. 2(   a ), more specifically, in the case of the intermediary transfer unit  40  in this embodiment, the intermediary transfer belt  7  is supported and kept tensioned by three belt supporting/tensioning rollers, more specifically, the driver roller  8  for driving the intermediary transfer belt  7 , the idler roller  18  which is rotated by the movement of the intermediary transfer belt  7 , and the tension roller  17 . 
     The driver roller  8  is provided with a thin surface layer, which is formed of rubber. It is rotatably supported by a frame  20  ( FIG. 6 ) of the intermediary transfer unit  40 , with the placement of a pair of bearings between the lengthwise ends portion of the driver roller  8  and frame  20 . The driver roller  8  and the other rollers are all supported by this frame  20  of the intermediary transfer unit  40 . 
     As the intermediary transfer unit  40  is inserted into the apparatus main assembly  200   a , the driver roller  8 , which is supported by the intermediary transfer unit frame  20 , is not only supported by the main assembly rails  32 , but also, is precisely positioned relative to the apparatus main assembly  200   a  by the main assembly rails  32 . The rollers, such as the driver roller  8 , which are supported by the main assembly rails  32  and precisely positioned relative to the apparatus main assembly  200   a  by the main assembly rails  32 , are such rollers of the intermediary transfer unit  40  that are not less than 90° in the angle of contact between themselves and intermediary transfer belt  7 . Here, the “angle of contact” or “wrapping angle” means the angle between the line which connects the center of a given roller and the downstream end of the area of contact between the roller and intermediary transfer belt  7 , in terms of the rotational direction of the roller, and the line which connects the center of the given roller and the upstream end of the area of contact between the roller and intermediary transfer belt  7 . The rollers which support and keep tensioned the intermediary transfer belt  7  and are greater than 90° in the angle of contact between themselves and intermediary transfer belt  7  include not only the abovementioned driver roller  8 , but also, the tension roller  17  for providing the intermediary transfer belt  7  with a preset amount of tension. 
     The above-mentioned idler roller  18  is rotatably supported by a pair of idler roller bearings  21  ( FIG. 3(   b )) which are supported by the intermediary transfer unit frame  20  in such a manner that they can be pivotally moved about the pivots by which they are attached to the intermediary transfer unit frame  20 . Further, the intermediary transfer unit  40  is provided with a registration patch sensor  28  for adjusting the image forming apparatus  200  in color and/or density, which is positioned so that it opposes the idler roller  18 . 
     Next, referring to  FIG. 2(   b ), the tension roller  17  is rotatably supported by a pair of tension roller bearings  23 , by its lengthwise ends, with the placement of a pair of belt tension springs  24  between each tension bearing  23  and intermediary transfer unit frame  20  ( FIG. 6 , as well as  FIG. 2(   b )), in the state of being compressed. The tension belt springs  24  are compression springs such as coil springs. 
     The tension roller bearings  23  are held to the intermediary transfer unit frame  20  in such a manner that they are allowed to slide relative to the intermediary transfer unit frame  20  in the direction parallel to the direction in which the belt tension springs are compressed or allowed to expand, providing thereby the intermediary transfer belt  7  with the preset amount of tension. Further, the tension roller bearings  23  are held to the intermediary transfer unit frame  20  in such a manner that they are allowed to move in the vertical direction by a small distance. Thus, the tension roller  17  is allowed to vertically moved relative to the intermediary transfer unit frame  20 , even though they are held to the intermediary transfer unit frame  20 . 
     The primary transfer rollers  5   a - 5   d  are positioned so that they oppose the photosensitive drums  1   a - 1   d , respectively. More specifically, referring to  FIGS. 2(   c ) and  2 ( d ), the primary transfer rollers  5   a - 5   d  are supported by the primary transfer roller holders  25   a - 25   d  ( FIG. 3(   b ), as well as  FIGS. 2(   c ) and  2 ( d )) in such a manner that they are allowed to pivotally move or move in a straight line to be pressed against, or to be moved away from, the photosensitive drums  1   a - 1   d , respectively. 
     The intermediary transfer belt  7  in this embodiment is formed of PEEK (Polyether-ether-Ketone), and is 91.9 mm in circumference, 346.0 mm in width, and 48 μm in thickness, for example. The material for the intermediary transfer belt  7  does not need to be limited to PEEK. For example, polyimide, polycarbonate, PVDF, ETFE, PTFE, and the like also can be listed as desirable substances as the material for the intermediary transfer belt  7 . 
     The intermediary transfer belt  7  is provided with a pair of ribs, which are on the inward surface of the intermediary transfer belt  7 , being position in the adjacencies of the lateral edges of the intermediary transfer belt  7 , and therefore, being parallel to the recording medium conveyance direction. Each rib in this embodiment is perpendicular to the inward surface of the intermediary transfer belt  7 , and extends in the direction parallel to the moving direction of the intermediary transfer belt  7  across the entirety of the intermediary transfer belt  7 . It is made of urethane, and is 3 mm in width, and 1.2 mm in height; it is a protrusion which looks like a long, narrow, and endless belt. 
     First, referring to  FIGS. 3(   a ) and  3 ( b ), the mechanical structure for moving the primary transfer rollers  5   a - 5   d  from the photosensitive drums  1   a - 1   d , respectively, is described.  FIGS. 3(   a ) and  3 ( b ) are schematic sectional views of the intermediary transfer unit  40  in this embodiment, when the intermediary transfer belt is not in contact with the photosensitive drums, and when the intermediary transfer belt is in contact with the photosensitive drums, respectively. 
     Referring to  FIG. 3(   a ), the intermediary transfer unit  40  is structured so that when the image forming apparatus  200  is used for outputting a black monochromatic image, or when the intermediary transfer unit  40  needs to be installed or uninstalled, the primary transfer rollers  5   a - 5   d  can be moved away from the intermediary transfer belt  7  so that the intermediary transfer belt  7  is allowed to separate from the photosensitive drums  1   a - 1   d . When the primary transfer rollers  5   a - 5   d  move away from the photosensitive drums  1   a ,  1   b ,  1   c  and  1   d , the idler roller  18  also moves in the same direction as the primary transfer rollers  5   a - 5   d . This structural arrangement is also effective to extend the lives of the primary transfer rollers  5   a - 5   d , and to prevent the intermediary transfer belt  7  from being scarred during the installation or uninstallation of the intermediary transfer unit  40 . 
     Next, referring to  FIGS. 3(   a ) and  3 ( b ), the primary transfer rollers  5   a - 5   d  are held to the intermediary transfer unit frame  20  ( FIG. 6) , with the placement of the primary transfer roller holders  25   a ˜ 25   d  between the primary transfer rollers  5   a - 5   d  and intermediary transfer unit frame  20 , in such a manner that the primary transfer rollers  5   a - 5   d  can be pressed against, or moved away from, the photosensitive drums  1   a - 1   d , respectively. As for the idler roller  18 , it is held to the intermediary transfer unit frame  20 , with the placement of the idler roller bearings  21  between itself and the intermediary transfer unit frame  20 , in such a manner that it can be moved toward, or away from, the photosensitive drum  1   d.    
     Next, referring to  FIGS. 3(   b ) and  3 ( c ),  FIGS. 4(   a )- 4 ( c ), and  FIGS. 5(   a )- 5 ( c ), the operation for moving the primary transfer rollers  5   a - 5   d  away from the intermediary transfer belt  7  is described.  FIG. 3(   c ) is an external perspective view of a cam  27  for moving the primary transfer rollers  5   a - 5   d  away from the intermediary transfer belt  7 .  FIGS. 4(   a )- 4 ( c ) are side views of the intermediary transfer unit  40 , which are for describing the operation for moving the primary transfer rollers  5   a - 5   d  away from the intermediary transfer belt  7 .  FIGS. 5(   a )- 5 ( c ) are plan views of the intermediary transfer unit  40 , which is for describing the operation for moving the primary transfer rollers  5   a - 5   d  away from the intermediary transfer belt  7 . 
     Referring to  FIG. 3(   b ), the shaft  26  for the cams  27  is rotatably supported by the intermediary transfer unit frame  20  ( FIG. 6) , being positioned in parallel to the photosensitive drums  1   a - 1   d  and rollers  5   a - 5   d ,  8 ,  17  and  18 . The pair of the aforementioned cams  27  are attached to the lengthwise portions of the shaft  26 , one for one, which are on the inward side of the intermediary transfer unit frame  20 . As driving force is inputted into the shaft  26  for the cams  27  from an unshown driving force source, the cams  27  rotate with the shaft  26 . The intermediary transfer unit  40  is also provided with a pair of Bk sliders  29  and a pair of CL sliders  30 , which are positioned so that they come into contact with the cams  27 . Thus, as the cams  27  rotate, the pair of Bk sliders  29  and the pair of CL sliders  30  are moved by the cams  27  in the left and right directions, respectively, in  FIG. 3 . 
     Referring again to  FIGS. 2(   c ) and  2 ( d ) (along with  FIGS. 3 and 4) , the intermediary transfer roller holders  25   a - 25   d  are provided with protrusions  25   e - 25   h , respectively. Incidentally,  FIGS. 2(   c ) and  2 ( d ) show only the protrusions  25   f  and  25   g , which correspond to the intermediary transfer roller holders  25   b  and  25   c , respectively. 
     Next, referring to  FIGS. 4(   a )- 4 ( c ), the Bk slider  29  and Cl slider  30  are moved in the left and right directions, respectively, while the protrusions  25   e - 25   h  are kept in contact with the slanted edges  29   a  and  30   a  of the sliders  29  and  30 , respectively. Thus, the primary transfer rollers  5   a - 5   d  are moved away from the portions of the intermediary transfer belt  7 , which are in contact with the photosensitive drums  1   a - 1   d , respectively. 
     As described above, the idler roller  18  is rotatably supported by the pair of idler roller bearings  21  which are supported by the intermediary transfer unit frame  20  so that the bearings  21  are allowed to pivotally move about the axles attached to the intermediary transfer unit frame  20 . Each idler bearing  21  is kept pressed toward the intermediary transfer belt  7  by an idler roller spring  22  ( FIG. 3(   b )). Thus, it is kept in contact with the main assembly rail  32  ( FIG. 7(   b )) solidly attached to the apparatus main assembly  200   a  shown in  FIG. 1 ; it is accurately positioned relative to the apparatus main assembly  200   a  by being under the pressure from the spring  22 . The main assembly rail  32  is solidly attached to the apparatus main assembly  200   a , and guides the intermediary transfer unit  40  when the intermediary transfer unit  40  is installed into, or uninstalled from, the apparatus main assembly  200   a.    
     As the Bk slider  29  is moved, its portion  29   b  ( FIG. 5 ) pushes the idler roller bearings  21  in the opposite direction from the intermediary transfer belt  7 . Thus, the idler roller  18  is moved in the opposite direction from the intermediary transfer belt  7 . 
     The operation for separating the primary transfer rollers  5   a - 5   d  from the intermediary transfer belt  7  can be divided into three stages, which puts the intermediary transfer unit  40  (image forming apparatus  200 ) in three modes, one for one.  FIGS. 4(   a )- 4 ( c ) show the intermediary transfer unit  40  when the intermediary transfer unit  40  is in the three modes, respectively. 
       FIG. 4(   a ) shows the intermediary transfer unit  40  when the intermediary transfer unit  40  is in the CL mode, in which all the primary transfer rollers  5   a - 5   d , the movement of which relative to the photosensitive drums  1   a - 1   d  (intermediary transfer belt  7 ), respectively, is controlled by the protrusions  25   e - 25   h , are kept pressed against the photosensitive drums  1   a - 1   d  (intermediary transfer belt  7 ).  FIG. 4(   b ) shows the intermediary transfer unit  40  when the intermediary transfer unit  40  is in the Bk mode, in which only the primary transfer roller  5   d  (“Bk” stands for “black”), the movement of which relative to the photosensitive drum  1   d  (intermediary transfer belt  7 ) is controlled by the protrusion  25   h , is kept pressed against the corresponding photosensitive drum (photosensitive drum  1   a ).  FIG. 4(   c ) shows the intermediary transfer unit  40  when the intermediary transfer unit  40  is in the full separation mode, in which all the primary transfer rollers  5   a - 5   d  are separated from the photosensitive drums  1   a - 1   d  (none of the primary transfer rollers  5   a - 5   d  is kept pressed against the photosensitive drums  1   a - 1   d ), respectively. 
       FIGS. 5(   a ),  5 ( b ) and  5 ( c ) show the positional relationships among the cam  27 , Bk slider  29 , and CL slider  30 , which correspond to the CL mode, Bk mode, and full-separation mode, respectively.  FIGS. 4(   a )- 4 ( c ) and  FIGS. 5(   a )- 5 ( c ) correspond to the CL mode, Bk mode, and full-separation mode, which are different by 120° in the angle of the cam  27 . 
     The cam  27  (referring to  FIG. 3(   c ) as well) has a surface which corresponds to the Bk slider, and a surface which corresponds to the CL slider. It is structured so that each time it is rotated by 120°, it causes the Bk slider and CL slider to differently move. 
     In  FIGS. 4(   a ) and  5 ( a ), the Bk slider  29  is in its leftmost position, and the CL slider is in its leftmost position. In  FIGS. 4(   b ) and  5 ( b ), the Bk slider is in its leftmost position, whereas the CL slider is in its rightmost position. In  FIGS. 4(   c ) and  5 ( c ), the Bk slider is in this rightmost position, and the CL slider is in its rightmost position. 
     In this embodiment, the operation for separating the primary transfer rollers  5   a - 5   d  from the intermediary transfer belt  7  is carried out by coordinating the moving direction of the Bk slider, moving direction of the CL slider, the shapes of the slanted surfaces  29   a  and  30   a  of the Bk and CL sliders  29  and  30 , respectively, and shape of the portions  29   b  and  30   b  of the Bk and CL sliders  29  and  30 , respectively. 
     The primary transfer rollers  5   a - 5   d  and idler roller  18  are precisely positioned relative to the units (unshown unit for supporting photosensitive drums  1   a ˜ 1   d , for example) other than the intermediary transfer unit  40 , registration patch detection sensor  28  ( FIG. 2(   a )), etc. 
     [Structural Arrangement for Positioning Rollers] 
     Next, the structural arrangement for precisely positioning the rollers of the intermediary transfer unit  40  is described.  FIG. 6  is an external perspective view of the intermediary transfer unit  40  in this embodiment.  FIG. 7(   a ) is a plan view of the intermediary transfer unit  40  in this embodiment, and  FIG. 7(   b ) is a detailed plan view of one of the pair of main assembly rails  32  in this embodiment.  FIGS. 8(   a ) and  8 ( b ) are sectional views of the roller positioning portions of the main assembly rail  32  in this embodiment.  FIGS. 9(   a ) and  9 ( b ) are sectional views of the roller positioning portion of the main assembly rail  32  in this embodiment.  FIGS. 10(   a ) and  10 ( b ) are sectional views of the combination of the main assembly rail  32  and intermediary transfer unit  40  in this embodiment, when the intermediary transfer unit  40  is being inserted into, or removed from, the apparatus main assembly  200   a.    
     Referring to  FIG. 6 , the frame  20  of the intermediary transfer unit  40  is precisely positioned relative to the main assembly rail  32  ( FIG. 7(   b )), by the driver roller positioning portion  8   a  of the driver roller  8 , and rotation control bosses  20   a  and  20   b  of the intermediary transfer unit frame  20 . The intermediary transfer unit frame  20  is provided with a pair of the control bosses, which are at the lengthwise ends of the intermediary transfer unit frame  20 , one for one. The driver roller positioning portions  8   a  and  8   b  of the driver roller  8  are supported by the pair of main assembly rails  32  and  32  (unshown in  FIG. 6) , one for one, solidly attached to the apparatus main assembly  200   a.    
     The tension roller  17  is precisely positioned relative to the apparatus main assembly  200   a  by the pair of tension roller bearings  23 , the vertical movement of which is regulated by the protrusions  33   a  and  33   c  ( FIG. 8(   a )) of the main assembly rail  32 . Next, referring to  FIG. 6 , the intermediary transfer unit  40  is provided with the pair of the tension roller bearings  23 , which are located at the lengthwise ends of the intermediary transfer unit  40 , and the tension roller  17  is supported by these tension roller bearings  23  and  23 , by its lengthwise end portions. 
     The idler roller  18  is one of the intermediary transfer belt supporting rollers, which the registration patch detection sensor  28  ( FIG. 3(   a )) opposes. In order for the registration patch detection sensor  28  to accurately detect the registration patch, the sensor  28  is required to be highly precisely positioned in terms of the distance between itself and intermediary transfer belt  7 . In this embodiment, therefore, each idler roller bearing  21  ( FIGS. 3 and 4)  is placed in contact with the corresponding main assembly rail  32  ( FIG. 7(   b )) to precisely position the registration patch detection sensor  28  relative to the main assembly rail  32 . 
     The belt cleaner  11  is regulated in the angle of its blade relative to the intermediary transfer belt  7  and its distance, while remaining supported by the pair of tension rollers  23  and  23  located at the lengthwise ends of the intermediary transfer unit  40 . Further, the belt cleaner  11  is positioned relative to the intermediary transfer belt  7  in such a manner that the vertical movement of its belt cleaner positioning portions  11   a  and  11   b  is regulated by the protrusions  33   a  and  33   b  ( FIG. 8(   a )), with which each of the main assembly rails  32  is provided, being thereby kept precisely positioned relative to the intermediary transfer belt  7 . 
     The intermediary transfer unit frame  20  and main assembly rail  32  are long and narrow members. Their lengthwise ends are shaped so that one of their lengthwise ends can support the driver roller  8 , and the other end can support the tension roller  17 . The intermediary transfer unit frame  20  is supported by the main assembly rail  32 . More specifically, it is inserted into the apparatus main assembly  200   a  so that it fits into the groove of each of the main assembly rails  32  from one of the lengthwise ends of the main assembly rail  32  while supporting the driver roller  8  and tension roller  17  by its lengthwise ends, one for one. Then, it is inserted into the apparatus main assembly  200   a  while being guided by the main assembly rails  32 . 
     It is from the right side of the apparatus main assembly  200   a  in  FIG. 1  that the intermediary transfer unit  40  is inserted into, or removed from, the apparatus main assembly  200   a . Therefore, when the intermediary transfer unit  40  is installed into, or removed from the apparatus main assembly  200   a , it is from its belt cleaner side that it is inserted into the apparatus main assembly  200   a . Thus, the relationship among the portions of the various components, by which the various portions are precisely positioned, is as shown in  FIG. 7(   a ): positioning portion  11   a =tension roller bearing  23 &lt;rotation control portion  20   a &lt;positioning portion  8   a.    
     By varying various components of the apparatus main assembly  200   a  and intermediary transfer unit  40  in position, in terms of the direction parallel to the lengthwise direction of the main assembly rail  32 , the various components can all be precisely positioned by the main assembly rail  32 . 
     That is, the main assembly rail  32  shown in  FIG. 7(   b ) is one of the pair of the main assembly rails  32 , which are solidly attached to the front and rear plates, one for one, of the main frame of the apparatus main assembly  200   a  ( FIG. 1) , and which correspond one for one to the lengthwise ends of the intermediary transfer unit  40  which is inserted into the apparatus main assembly  200   a . The positional relationship among the above-described positioning portions of the various components are as follows: 
     That is, as the intermediary transfer unit  40  is inserted into the apparatus main assembly  200   a  in such an attitude that the belt cleaner positioning portion  11   a  of the belt cleaner  11  becomes the first portion of the intermediary transfer unit  40  to enter the apparatus main assembly  200   a  through the entrance  34  for the belt cleaner positioning portion  11   a , the belt cleaner positioning portion  11   a  is precisely positioned by the protrusions  33   a  and  33   b  ( FIG. 8(   a )), with which the deepest end portion A of the main assembly rail  32  is provided. Further, the bearing  23  for the tension roller  17  is introduced into the tension roller positioning slot through the entrance  35 , and precisely positioned relative to the main assembly rail  32  by the protrusions  33   a  and  33   b , with which the portion B of the main assembly rail  32  is provided ( FIG. 8(   a )). 
     Further, the bosses  20   a  and  20   b  of the intermediary transfer unit frame  20 , which are for preventing the intermediary transfer unit  40  from rotationally moving are guided into the slanted portions of the grooves  36   a  and  36   b , with which the portion C of the main assembly rail  32  is provided, along the edges of the slanted portions, and are precisely positioned relative to the main assembly rail  32  (that is, apparatus main assembly  200   a ) ( FIG. 8(   b )). The portion of the main assembly rail  32 , which makes up the top wall of the groove  36   b , functions as the protrusion  36   c  which regulates the boss  20   b  in coordination with the groove  36   b . Further, the main assembly rail  32  is provided with a guiding portion  36   d  for guiding the rotation control bosses  20   a  and  20   b  into the grooves  36   a  and  36   b , respectively. The guiding portion  36   d  is on the right-hand side of the protrusion  36   c  (right-hand side in  FIG. 8(   b )). 
     Further, the trailing end portion D of the main assembly rail  32 , in terms of the direction in which the intermediary transfer unit  40  is inserted into the apparatus main assembly  200   a , is provided with a recess  37 , with which the driver roller positioning portion  8   a  of the driver roller  8  is placed in contact to precisely position the driver roller  8 . As the driver roller positioning portion  8   a  is placed in contact with the edge of the recess  37  of the main assembly rail  32 , it is pressed by an unshown pressing mechanism in the diagonally upward direction, being thereby precisely positioned by being placed in contact with the main assembly rail  32  at three points. 
     Further, the portion E of the main assembly rail  32  is provided with a protrusion  38 , which is below the recess  37 . Thus, as the intermediary transfer unit  40  is inserted into the apparatus main assembly  200   a , the bearing  21  of the idler roller  18  fits into the gap between the protrusion  38  and its opposing wall, whereby it is precisely positioned relative to main assembly rail  32 . 
     Further, the main assembly rail  32  is provided with a groove  36 , which extends in the lengthwise direction of the main assembly rail  32 . The right-hand end portion ( FIG. 7   b )) of the groove  36  is provided with a portion  36   f  through which the cleaner positioning portion  11   a , rotation control bosses  20   a  and  20   b , etc., are put in the listed order, when the intermediary transfer unit  40  is inserted into the apparatus main assembly  200   a . The left end portion ( FIG. 7(   a )) of the groove  36  has the above described branches  36   a  and  36   b , protrusion  36   c , and guiding portion  36   d  of the groove  36 . 
     Further, the main assembly rail  32  is provided with a through hole  32   a , through which the shaft  26  ( FIG. 3(   b )), into which the force for rotating the cam  27  to separate the primary transfer rollers  5   a - 5   d  from the intermediary transfer belt  7  is inputted, is put. The hole  32   a  is parallel to the front-to-rear direction of  FIG. 7(   a ). It is near the entrance portion  36   f  of the guiding groove  36 . The above-described boss  20   b  is for preventing the rotation control boss  20   a  from falling into the hole  32   a  when the intermediary transfer unit  40  is inserted into the apparatus main assembly  200   a  along the main assembly rails  32 . 
       FIG. 9(   b ) shows how the idler roller positioning portion of the idler roller  18  is precisely positioned at the portion E of the main assembly rail  32  shown in  FIG. 7(   b ). Referring to  FIG. 9(   b ), the bearing  21  of the idler roller  18  is precisely positioned relative to the main assembly rail  32  (hence, apparatus main assembly  200   a ) by being pressed upon the main assembly rail  32  by the idler roller spring  22  ( FIG. 3(   b )). Thus, it is ensured that the idler roller  18  is properly aligned, and remains aligned, with the other rollers, and also, that the idler roller  18  is precisely positioned relative to the registration patch sensor  28  which is precisely positioned relative to the main assembly rail  32 . 
     Next, referring to  FIG. 10(   a ), the positioning of the abovementioned various rollers, which occurs while the intermediary transfer unit  40  is removed from the apparatus main assembly  200   a , is described. 
     If an attempt is made to pull the intermediary transfer unit  40  straight out of the apparatus main assembly  200   a  while the intermediary transfer unit  40  is in the state shown in  FIG. 3(   a ), the photosensitive drums  1   a - 1   d , developing device  100   a - 100   d , etc. interfere with the belt cleaner  11 . Therefore, the intermediary transfer unit  40  has to be pulled out in such an attitude that can prevent the interferences.  FIG. 10(   b ) shows the attitude in which the intermediary transfer unit  40  should be kept in order to prevent the interference during the extraction of the intermediary transfer unit  40 . That is, as long as the intermediary transfer unit  40  is kept in the attitude shown in  FIG. 10(   b ), it can be pulled straight out of the apparatus main assembly  200   a  in the rightward direction of  FIG. 10(   b ). 
     As described above, in this embodiment, at least the driver roller  8  is supported by the main assembly rail  32  solidly attached to the apparatus main assembly  200   a , and also, is precisely positioned relative to the main assembly  32 . That is, the driver roller  8  which is greater in the angle of contact with the intermediary transfer belt  7  than other belt suspending rollers, having therefore greater influence upon the lateral shifting of the intermediary transfer belt  7 , is firmly held to the apparatus main assembly  200   a . Therefore, it is possible to prevent the driver roller  8  from becoming misaligned by the distortion of the intermediary transfer unit frame  20 . In other words, this embodiment of the present invention can substantially reduce the amount of force which acts in the direction to laterally shift the intermediary transfer belt  7 . 
     Also in this embodiment, the driver roller  8 , tension roller  17 , and idler roller, by which the intermediary transfer belt  7  is suspended and kept tensioned, are all precisely positioned relative to the apparatus main assembly  200   a  by the pair of main assembly rails  32  and  32  which are solidly attached to the left and right walls of the apparatus main assembly  200   a  in terms of the direction in which the intermediary transfer unit  40  is inserted into, or pulled out of the apparatus main assembly  200   a . Therefore, the alignment among the rollers by which the intermediary transfer belt  7  is suspended are not affected by the distortion of the intermediary transfer unit frame  20 . Thus, the present invention can reduce the intermediary transfer unit  40  in the amount of misalignment among the belt suspending rollers of the intermediary transfer unit  40 . 
     The greater a given roller among the rollers by which the intermediary transfer roller  7  is suspended, in its angle of contact with the intermediary transfer belt  7 , the greater it is in the effect upon the lateral shift of the intermediary transfer belt  7 . Therefore, any of the belt suspending rollers, which is greater in its angle of contact with the intermediary transfer belt  7  than 90° is desired to be precisely positioned relative to the main assembly rail  32 , even if the roller is not the driver roller  8 . In this embodiment, therefore, not only the driver roller  8 , but also, the tension roller which also is greater in its angle of contact with the intermediary transfer belt  7  is precisely positioned relative to the main assembly rail  32 . 
     Moreover, in this embodiment, all the rollers by which the intermediary transfer belt  7  is suspended and kept tensioned are precisely positioned by the main assembly rails  32 . Therefore, it does not occur that the belt cleaner  11  is reduced in its cleaning performance by the steering of the intermediary transfer belt  7 ; the performance of the belt cleaner  11  remains as high as possible. 
     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. 
     This application claims priority from Japanese Patent Application No. 125678/2012 filed Jun. 1, 2012 which is hereby incorporated by reference.