Patent Publication Number: US-9405269-B2

Title: Image forming apparatus with a guide member for a transfer belt

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority pursuant to 35 U.S.C. §119(a) from Japanese patent application numbers 2013-172372, and 2014-131946, filed on Aug. 22, 2013, and Jun. 26, 2014, respectively, the entire disclosures of which are incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Exemplary embodiments of the present invention relate to an image forming apparatus including a chassis that houses a transfer unit, the transfer unit including a plurality of rollers that support an endless belt. 
     2. Background Art 
     Conventional image forming apparatuses are constructed of a chassis that houses a transfer unit, which includes an endless intermediate transfer belt, and a sheet feed unit including a sheet conveyance belt. 
     In general, these image forming apparatus include four photoreceptors, each forming a toner image of one of the colors yellow (Y), magenta (M), cyan (C), and black (K), respectively, and a transfer unit configured as a belt unit. The Y-, M-, C-, and K-toner images formed on respective photoreceptors with a known electrophotographic process are then transferred in a superimposed manner onto the intermediate transfer belt of the transfer unit, thereby forming a full color toner image. In operation, the intermediate transfer belt rotates continuously while contacting all four photoreceptors. 
     The transfer unit switches the endless intermediate transfer belt between a posture in which the intermediate transfer belt is being conveyed during operation and a posture during replacement, or attachment/detachment of the transfer unit. During replacement, the intermediate transfer belt is separated from all four photoreceptors and the transfer unit can be removed from the image forming apparatus. 
     In the posture in which the transfer unit can be detached (i.e., during maintenance), the transfer unit is drawn to a proximal side from a relatively wide opening formed in a front wall of the chassis of the image forming apparatus, so that necessary maintenance can be performed. After maintenance, the intermediate transfer belt is attached in the unit attachment posture to the chassis from the front opening. 
     Although various approaches have been attempted, conventional belt units and image forming apparatuses do not disclose a structure capable of preventing the endless belt from contacting neighboring parts when the endless belt is replaced or attached/detached. 
     SUMMARY 
     In one embodiment of the disclosure, there is provided an improved image forming apparatus, including a plurality of rollers; an endless belt mountable to the plurality of rollers from a proximal side of the image forming apparatus in an axial direction of the plurality of rollers; a first side plate to support one end of the plurality of rollers at the proximal side of the image forming apparatus in the axial direction; and a plurality of guide members, disposed at a more proximal side than the first side plate in the axial direction, to guide the belt to be mounted to the plurality of rollers with the guide members contacting the belt. 
     These and other objects, features, and advantages of the present invention will become apparent upon consideration of the following description of preferred embodiments of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic view of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2A  shows a toner shape factor SF1 and  FIG. 2B  shows a toner shape factor SF2; 
         FIG. 3  illustrates a front view of an outer frame of a chassis of the image forming apparatus of  FIG. 1 ; 
         FIG. 4  illustrates a perspective front view of the outer frame of the chassis of  FIG. 3 ; 
         FIG. 5  illustrates an exploded perspective front view of the outer frame; 
         FIG. 6  illustrates a perspective rear side view of the outer frame of the chassis of  FIG. 3 ; 
         FIG. 7  illustrates an enlarged partial front view illustrating a mounting structure of a right side of a transfer unit inside the chassis of  FIG. 3 ; 
         FIG. 8  illustrates a perspective partial view illustrating a mounting structure of the right side of the transfer unit inside the chassis of  FIG. 3 ; 
         FIG. 9  illustrates a partial plan view excluding a transfer belt from the right side mounting structure of the transfer unit in the chassis of  FIG. 3 ; 
         FIG. 10  illustrates a partial plan view before attaching the transfer belt from the right side mounting structure of the transfer unit in the chassis of  FIG. 3 ; 
         FIG. 11  illustrates a perspective partial view before attaching the transfer belt from the right side mounting structure of the transfer unit in the chassis of  FIG. 3 ; 
         FIG. 12  illustrates an enlarged partial front view illustrating a left side mounting structure of the transfer unit in the chassis of  FIG. 3 ; 
         FIG. 13A  illustrates a perspective partial view of the left side mounting structure of the transfer unit in the chassis of  FIG. 3  and  FIG. 13B  is an explanatory view of an inserting status of a guide member; 
         FIGS. 14A and 14B  each illustrate a modified example of the left side mounting structure of the transfer unit in the chassis of  FIG. 3 ,  FIG. 14A  shows a perspective partial view, and  FIG. 14B  illustrates a displacement of a movable guide member; 
         FIG. 15  illustrates a sliding mechanism of the guide member supported by an unit joint frame as a modified example of  FIG. 14B ; 
         FIG. 16  illustrates a position controlling function of a planar guide member supported by the unit joint frame of the transfer unit of  FIG. 13 ; 
         FIG. 17  illustrates a perspective partial view when the transfer unit is drawn along the movable guide member supported by the unit joint frame of the transfer unit of  FIG. 14 ; 
         FIGS. 18A and 18B  illustrate a positioning plate and guide member as a right side mounting structure of  FIG. 3 , and  FIG. 18A  shows a first embodiment and  FIG. 18B  is a second embodiment; 
         FIG. 19  illustrates a perspective view illustrating a guide surface used in the right side mounting structure of the guide member according to the second embodiment of the present invention; 
         FIG. 20  illustrates a perspective view illustrating the guide member used in the image forming apparatus when a belt support frame is drawn according to another embodiment of the present invention; 
         FIG. 21  illustrates a partial front view illustrating a support structure for a right portion of the transfer unit and the guide member used in the image forming apparatus according to yet another embodiment of the present invention; 
         FIG. 22  illustrates a partial front view illustrating a status before winding the belt of the support structure for the guide member of  FIG. 21 ; 
         FIG. 23  illustrates a perspective partial view illustrating the support structure for the guide member of  FIG. 21 ; 
         FIG. 24  illustrates a perspective plan view illustrating relative positions of the positioning plate and the guide member used in the image forming apparatus according to the embodiment of the present invention; and 
         FIGS. 25A and 25B  illustrate schematic structures of the guide member used in yet another embodiment of the present invention, in which  FIG. 25A  shows a first modified embodiment, and  FIG. 25B  shows a second modified embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, preferred embodiments of an image forming apparatus will be described. 
     According to the present invention, when attaching a belt from a proximal side in a roller shaft direction relative to a plurality of rollers supported by side plates, the belt is moved such that an upper surface of the belt moves along a guide member. This movement of the guide member prevents the belt from deviating and interfering with surrounding members, so that the belt is prevented from being damaged. 
     In each of the following drawings, parts or components having the same function or shape are given the same reference numerals. 
       FIG. 1  illustrates an image forming apparatus according to a first embodiment of the present invention. 
     In  FIG. 1 , the image forming apparatus includes a body  100  of a copier, a sheet feed table  200  disposed below the body  100 , a scanner  300  disposed above the body  100 , and an automatic document feeder (ADF)  400  disposed above the scanner  300 . 
     The body  100  includes a chassis A 1 , an endless belt as a transfer belt  10  (hereinafter, to be referred to simply as a transfer belt  10 ), and a transfer unit  5  as a belt unit to mount the transfer belt  10  in a posture in which the belt is conveyed. The transfer unit  5  as a belt unit will be described in detail later. 
     As illustrated in  FIG. 1 , the transfer belt  10  is wound around first to third rollers  14 ,  15 , and  16 , respectively, and further, an outer tension roller  17  to apply tension to the belt is disposed at a portion where an outer surface of the belt is directed downward. With such a structure, the transfer belt  10  is wound in a posture at a time of belt conveyance rotatable in clockwise direction. 
     In the illustrated example, an intermediate transfer member cleaner  19  to remove residual toner remaining on the transfer belt  10  after image transfer is disposed between the third roller  16  and the outer tension roller  17 . 
     In addition, four image forming units  18  for the colors yellow (Y), magenta (M), cyan (C), and black (Bk) are disposed in a horizontal row along the conveyance direction of the transfer belt  10  between the first roller  14  and the second roller  15 . These image forming units  18  form a tandem image forming device  20 . 
     As illustrated in  FIG. 1 , an exposure unit  21  is disposed above the tandem image forming device  20 . 
     Further, between the first and second rollers  14  and  15 , and primary transfer rollers  62  to transfer toner images from photoreceptors  40 Y to  40 Bk to the transfer belt  10  are disposed. Each primary transfer roller  62  is disposed opposite each photoreceptor  40 Y to  40 Bk with the transfer belt  10  sandwiched in between. 
     A secondary transfer device  22  is disposed at a position opposite the tandem image forming apparatus  20  with the transfer belt  10  sandwiched in between. In an illustrated example, the secondary transfer device  22  is formed such that a secondary transfer belt  24  being an endless belt is stretched around two rollers  23 . The secondary transfer device  22  is formed such that one of the rollers  23  presses the third roller  16  via the transfer belt  10  so that the toner image formed on the transfer belt  10  is transferred to a sheet arriving at the secondary transfer device  22 . 
     A fixing device  70  configured to fix the transferred image onto the sheet is disposed adjacent to the secondary transfer device  22 . The fixing device  70  includes a fixing roller  251 , a heat roller  252 , a fixing belt  26  stretched around the fixing roller  251  and the heat roller  252 , and a pressure roller  27 . The fixing device  70  is configured such that the pressure roller presses the fixing belt  26 . 
     The secondary transfer device  22  further includes a sheet conveyance function to convey the sheet on which image has been transferred, to the fixing device  70 . A non-contact type charger may be used as the secondary transfer device  22 ; however, in such a case, the sheet conveyance function is lost. 
     Further, in the illustrated example, a sheet reverse unit  28  to reverse the sheet to print both sides thereof is disposed in parallel to the tandem image forming device  20  and below the secondary transfer device  22  and the fixing device  70 . 
     When a copy is created using the present color copier, a document is set on a document platen  30  of the ADF  400 . Alternatively, the document is set on a contact glass  321  on the scanner  300  after opening the ADF  400  and is pressed by the ADF  400  by closing it. 
     When a start button is pressed with the document placed on the ADF  400 , the document moves to the contact glass  321 . On the other hand, when the document is placed directly on the contact glass  321 , the scanner  300  is driven immediately so that a first carrier  33  and a second carrier  34  are activated. Then, the first carrier  33  emits light from its light source, receives light reflected from a surface of the document, and reflects the received light to the second carrier  34 . The second carrier  34  reflects the received light via the mirror toward a focusing lens  35  to be incident to a reading sensor  36  which reads a content of the document. 
     Pressing the start button on the control panel also causes a driving motor to be driven so that at least one of the rollers  14 ,  15 , and  16 , is driven to rotate and other two rollers are driven to rotate accompanied by the rotation of the at least one roller. As a result, the transfer belt  10  is rotated. At the same time, each image forming unit  18  rotates its photoreceptor  40  to form a monochrome image of yellow, magenta, cyan, and black along the conveyance direction on each photoreceptor  40 . Then, those monochrome images are sequentially transferred onto the transfer belt  10  while the transfer belt  10  is moving, so that a synthesized color image is formed on the transfer belt  10 . 
     The image forming units  18  each have the same structure except that the color of the developing agent (toner) is different, and therefore, affixes of Y, M, C, and Bk are appropriately omitted in the description hereinafter. 
     On the other hand, when the start button is pressed, one of sheet feed rollers  42  of the sheet feed table  200  is selectively rotated, so that a sheet is fed out from one of multiple paper trays  44  stored in a paper bank  43 . Next, the sheet is separated one by one by a separation roller  45  and inserted into a first conveyance path  46 , is conveyed by the conveyance roller  47  to a second conveyance path  48  inside the copier body  100 , and stops by contacting a registration roller pair  49 . 
     Otherwise, a sheet feed roller  50  is rotated to feed the sheet on a manual tray  51 , the sheet is separated one by one by a manual side separation roller pair  52  and is introduced into a third manual sheet conveyance path  53 , and stops by contacting the registration roller pair  49 . 
     Next, the registration roller pair  49  is rotated in synch with a synthesized color image formed on the transfer belt  10  so that the sheet is sent between the transfer belt  10  and the secondary transfer device  22 , in which the color image on the transfer belt  10  is transferred by the secondary transfer device  22  and the color image is transferred onto the sheet. 
     The secondary transfer device  22  includes two support rollers  23  and a secondary transfer belt  24  that can be driven by being stretched around the two support rollers  23 . 
     The secondary transfer device  22  conveys the sheet on which the toner image is transferred, toward the fixing device  70 , which applies heat and pressure to the sheet, so that the transferred image is fixed onto the sheet. Then, a pawl  55  switches the direction of the sheet and an ejection roller pair  56  discharges the sheet onto a sheet ejection tray  57  where the sheet is stacked. Alternatively, after being redirected by the pawl  55  to be introduced into the sheet reverse unit  28 , in which the sheet is reversed, the sheet is reintroduced to the transfer device and an image is recorded on its backside. Then, the sheet is ejected on the sheet ejection tray  57  via the ejection roller pair  56 . 
     After image transfer, the transfer belt  10  is cleaned by an intermediate transfer member cleaner  19  so that the toner remaining on the transfer belt  10  is removed after the image transfer and readied for a next image formation by the tandem image forming device  20 . 
     The transfer belt  10  is formed of one or multiple layers using polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE) copolymer, polyimide (PI), polycarbonate (PC), and the like. Herein, conductive materials, such as carbon black and the like, are dispersed therein, with a volume resistivity adjusted to be in a range from 10 8  to 10 12  Ωcm and a surface resistivity in a range from 10 9  to 10 13  Ωcm. If necessary, a release layer may be additionally coated on top of the transfer belt  10 . Examples of material used for coating include ethylene tetrafluoroethylene (ETFE) copolymer, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), perfluoroalkoxy (PEA) resins. In addition, fluorine resins of tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyvinylfluoride (PVF), and the like, may be used. 
     The transfer belt  10  may be manufactured by molding, centrifugal molding, and the like. The surface of the belt may be polished, if necessary. 
     If the volume resistivity of the transfer belt  10  exceeds the above range, a higher bias voltage needs to be applied to the belt for image transfer, resulting in a drastic cost increase for the power supply and is thus undesirable. Further, the charged potential of the transfer belt  10  in the transfer process and the transfer sheet releasing process becomes high and the self-discharge of electricity becomes difficult, necessitating some discharging means. Furthermore, if the volume resistivity and the surface resistivity are below the above range, the charged potential quickly attenuates, which is favorable for the discharging by self-discharge of electricity but is unfavorable because the current flows in the surface direction and toner dispersion occurs. 
     The volume and surface resistivities are measured as follows: Herein, a high resistivity meter (Hiresta IP from Mitsubishi Chemical) is connected with HRS probe (with an inner electrode diameter of 5.9 mm and inner diameter of ring electrode of 11 mm), and voltage of 100 volts to the surface and backside of the transfer belt  10  is applied (with a surface resistivity of 500 volts). Readings taken 10 seconds after voltage has been applied is used. 
     In addition, preferred materials for the transfer belt  10  include rubber or the like having an elastic layer. 
     The third roller  16  is disposed opposite the secondary transfer device  22  via the transfer belt  10  and includes a metal core formed of iron or stainless steel (SUS (A−1)) with foamed resins coated thereon. In the present embodiment, the foamed resins have a thickness of from 2 mm to 10 mm, but the thickness is not limited thereto. 
     Next, polymerized toner used in the present embodiment will be described with reference to  FIGS. 2A and 2B . 
       FIGS. 2A and 2B  each schematically show a toner shape to explain the shape factor SF1 and the shape factor SF2. The shape factors SF1 of the toner used for the present embodiment preferably range from 100 to 180 and SF2 from 100 to 180. Specifically, the shape factor SF1 shows a degree of circularity of a toner particle, and is represented by the following equation (1):
 
SF1={(MXLNG) 2 /AREA}×(100π/4)  (1).
 
     Here, a value produced by dividing the square of the maximum length MXLNG of an oval figure, which is the projection of the toner particle in a two-dimensional plane, by the area of the figure AREA and then multiplying the resulting quotient by 100π/4. 
     If the value of SF1 becomes 100, the toner shape is a true sphere. The greater the value of SF1, the more amorphous the toner shape becomes. 
     The shape factor SF2 is a value representative of the ratio of irregularity in the shape of toner and is represented by the following equation (2):
 
SF2={(PERI) 2 /AREA}×(100/4π)  (2).
 
     Here, a value produced by dividing the square of the peripheral length PERI of a figure, which is the projection of the toner in the two-dimensional plane, by the area AREA of the figure and then multiplying the resulting quotient by 100/4π. 
     If the value of SF2 becomes 100, no irregularity on the toner surface exists. The greater the value of SF2 is, the more remarkable the irregularity on the toner surface becomes. 
     The shape factors may be measured using a photograph of toner taken using a scanning electroscope (S-800, trade name, manufactured by Hitachi, Ltd.) and the image analyzed with an image analyzer (LUSEX3, trade name, manufactured by NIRECO Corp.). 
     As the toner shape approaches a sphere, toner-to-toner or toner-to-photoreceptor contact is made only by point-to-point contact, and the toner-to-toner attracting power decreases, thereby increasing fluidity. The attracting power between the toner and the photoreceptor also decreases, thereby increasing transferability. If either of the SF1 and SF2 exceeds 180, transferability decreases and the cleanability of the toner attached to the transfer device also decreases, which is undesirable. 
     The favorable toner particle diameter is ranging from 4 to 10 μm. If the toner particle diameter is less than 4 μm, background contamination is caused during the development, fluidity is degraded and agglomeration tends to occur, and then the white omission tends to occur. By contrast, if the toner particle diameter is greater than 10 μm, toner dispersion occurs and a high quality image cannot be obtained due to the degraded resolution. 
     In the present embodiment, toner with the volume average particle diameter of 6.5 μm is used. 
     A structure of the copier body  100  will be described in detail referring to  FIG. 1 . 
     The copier body  100  includes a transfer unit  5  as a belt unit, a chassis A 1  incorporating the transfer unit  5 , and a front opening  82 , disposed at a proximal side of the body  100 , through which the transfer unit  5  can be pulled out in the proximal direction. 
     Herein, as illustrated in  FIGS. 3, 8, and 13 , the transfer unit  5  is configured such that the endless transfer belt  10  is wound around the first and second rollers  14  and  15 , which are separated from each other maximally. Herein, the first and second rollers  14 ,  15  are disposed in a distal side than the proximal side and a front side plate  87  extending in a lateral side (see  FIGS. 3 and 5 ) supports the proximal side, thereby forming a belt unit frame  80 . 
     Further, the chassis A 1  of the copier body  100  supports such that the belt unit frame  80  can be pulled out to the proximal side as illustrated in  FIG. 5 . In particular, the chassis A 1  includes guide members  85   a ,  85   b  (collectively, guide member  85 ) ( FIG. 3  shows a right guide member  85   a  alone of the front side plate  87 ) disposed at lateral sides and projected so that the transfer belt  10  is guided toward the proximal side. 
     The belt unit frame  80  includes the front side plate  87  and a rear side plate  88 . The front side plate  87  is a first side plate to support the proximal side in an axial direction of the first and second rollers  14 ,  15  and the rear side plate  88  is a second side plate supporting the other rear end in the axial direction of the first and second rollers  14 ,  15 . Further, as illustrated in  FIGS. 19 and 20 , a stay  801  integrates the front side plate  87  and the rear side plate  88 , and connecting side plates  86 ,  89  that extend with a curve toward the front from the rear side plate  88 . 
     The belt unit frame  80  is supported by the chassis A 1  such that the connecting side plates  86 ,  89  disposed at the lateral ends of the belt unit frame  80  are supported by the chassis A 1  via the slide rail  90 . 
     Herein, the right and left connecting side plates  86 ,  89  cover an outer area of the first and second rollers  14 ,  15  disposed at lateral ends of the belt unit frame  80  with a predetermined interval from the belt unit frame  80  as illustrated in  FIG. 24 . 
     Guide members  85   a  and  85   b  to guide the transfer belt  10  are disposed at proximal sides of the right and left connecting side plates  86 ,  89 . The guide member  85  includes a projection projecting from a surface of the belt in the longitudinal direction. 
     A pair of right and left connecting side plates  86 ,  89  disposed at distal sides of the guide member  85  each includes a vertical wall and forms an inner side guide surface f 0  that continues from guide surfaces f 1 , f 2 . As illustrated in  FIG. 13 , a projection at an end of a guide member  85   d  fits a slit of a left joint plate  92  and regulates a position of the guide member  85   d.    
     Thus, each guide member  85   a ,  85   b  positions proximally at an outside of the belt unit frame  80  or the transfer unit  5 ; however, each guide member  85   a ,  85   b  may be supported at an edge of the opening of the front opening  82  of the outer frame  83  of the copier body  100  according to a second embodiment, which will be described later. 
     In brief, the transfer belt  10  is taken out as a whole unit and is kept in a unit detachment posture as shown by a broken line in  FIG. 3 , is pulled in a proximal direction along the roller axis direction relative to the transfer unit  5 , and is mounted from the proximal side. In this operation, what is needed is to prevent the outer circumferential surface of the transfer belt  10  in the unit detachment posture from interfering with the surrounding members around the edge of the opening by the guide surfaces f 1 , f 2  of the respective guide members  85   a ,  85   b , which will be described later, so that the outer circumferential surface of the transfer belt  10  is prevented from being damaged. 
     Further, a pair of guide members  85   a ,  85   b  positions at a proximal end of the right and left connecting side plates  86 ,  89  and is disposed opposite the pair of first and second rollers  14 ,  15 , respectively. 
     For example, the guide surfaces f 0 , f 1  of the right connecting side plate  86  and the guide member  85   d  disposed proximally of the connecting side plate move together, so that the outer circumferential surface of the transfer belt  10  does not interfere with the other peripheral parts at the edge of the opening, which is preferred. 
     Alternatively, as in another embodiment which will be described later (see  85   c  in  FIG. 19 ), a support pillar  831  forming an opening edge of the front opening  82  supports the guide member  85  and functions to prevent the outer circumferential surface of the transfer belt  10  from interfering with the surrounding members. 
     Next, referring to  FIGS. 3 to 6 , the chassis A 1  included in the image forming apparatus will be described. 
     The image forming apparatus includes the transfer unit  5 , the outer frame  83 , the front opening  82  formed on the outer frame  83 , and a cover to open and close the front opening  82 . 
     The front opening  82  retains a shape capable of allowing withdrawal of the transfer unit  5  in a proximal direction in  FIG. 3  in the axial direction of the first and second rollers  14 ,  15  and winding it around the rollers  14 ,  15  again. The front opening  82  is closed by a cover. The chassis A 1  includes herein the outer frame  83 , the front opening  82 , and an opening for a cover. 
     Herein, a belt unit mounting portion (as a regular position, which will be described later) is secured in a space inside the outer frame  83  forming a main part of the chassis A 1 . The transfer unit  5  as a belt unit is mounted in the regular position P 1  in a state of belt conveying posture (as illustrated by a solid line in  FIG. 1 ). 
     As illustrated in  FIGS. 4 and 24 , the outer frame  83  is formed as a three-dimensional rectangular frame by integrally coupling a plurality of support pillars  831 , front and rear stays  832 , lateral stays  833 , and a rear face plate  834 . 
     The transfer unit  5  is installed in the inner upper space of the thus-constructed outer frame  83 . The belt unit frame  80  of the transfer unit  5  is supported by the lateral stays  833  via the slide rail  90 . 
     As illustrated in  FIGS. 1, and 3 to 6 , the belt unit frame  80  includes the right and left connecting side plates  86  which are supported by and joined with the lateral stays  833  (see, for example,  FIG. 5 ) via the slide rail  90  (see  FIGS. 7 and 12 ). 
     With the structure above, bottom sides (see  FIG. 7 ) of the right connecting side plate  86  of the transfer unit  5  connect to a movable rail  901  of the slide rail  90  relative to the lateral stays  833  so as to be movably supported in a proximal-distal direction Y of the chassis A 1 . With this structure, the transfer unit  5  can be easily pulled out to the proximal side in the proximal-distal direction Y during maintenance and wound again after the maintenance is complete, through the front opening  82  sectioned by the front and rear, and right and left support pillars  831  of the outer frame  83  and upper and lower, front and rear stays  832  (of which an upper and front stay is not shown). 
       FIG. 4  shows a retracted state of the transfer unit  5 . At the time of maintenance, the transfer unit  5  once moves to a position suitable for removal and mounting which is lower than the regular position P 1 . Then, as illustrated in  FIG. 5 , the transfer unit  5  is held at a pull-out position P 2  in the proximal side in the proximal-distal direction Y than the front opening  82 . 
     In the retracted state in which the transfer unit  5  is held in the regular position P 1 , as illustrated in  FIGS. 7 and 8 , the transfer unit  5  is positioned such that a right end portion of the front side plate  87  is fastened to the front side right support pillar  831  via a right joint plate  91  as a positioning member. Herein, a positioning plate  836  ( 837  denotes a positioning plate on the left, see  FIG. 3 ) and a front edge of a right stay  833  as parts around the edge portion of the opening of the front opening  82  are integrally formed with the front, right support pillar  831 . The right end portion of the front side plate  87  of the transfer unit  5  is fastened at several portions to the positioning plate  836  integrally formed with the right support pillar  831  via the right joint plate  91  as a positioning member. 
     Herein, the transfer unit  5  (or the belt unit) employs the guide member  85   a  as a first positioning pin. The chassis A 1  includes a positioning pin  838   a  as a second positioning pin and a guide pin  838   b  as an engagement pin disposed in the vicinity of the positioning pin  838   a , both pins protrude toward a proximal side from the positioning plate  836  (see  FIGS. 7 and 18A ). 
     As shown in  FIG. 7 , the right joint plate  91  as a positioning member includes a first positioning slit h 1  into which the guide member  85   a  as a first positioning pin is inserted and a second positioning slit h 2  into which the positioning pin  838   a  as a second positioning pin is inserted. At the same time, a bottom end j 1  of the right joint plate  91  is stopped by an upper surface of the guide pin  838   b  as an engagement pin (see  FIGS. 18A and 18B ). With this configuration, the right joint plate  91  integrally coupled with the transfer unit  5  is securely positioned from all sides relative to the positioning plate  836  of the side of the chassis A 1 . With such an uncomplicated structure using pins and slits, positioning can be secured. 
     Further, as illustrated in  FIG. 7 , the guide pin  838   b  serves as a positioning member for the transfer unit  5  and the right joint plate  91  and supports the right joint plate  91  by contacting a bottom end of the right joint plate  91  when mounting the right joint plate  91  moving it from the proximate side to the distal side in the figure. Thus, the guide pin  838   b  performs a positional control to support the positioning pin  838   a  and the guide member  85   a  to enter into the positioning slits h 1 , h 2 . 
     Namely, the transfer unit  5  is installed in the chassis A 1  and is held at the regular position P 1 . In this state, the right joint plate  91  includes the positioning slits h 1 , h 2 . The guide member  85   a  and the positioning pin  838   a  protruding from the positioning plate  836  are inserted into the slits h 1 , h 2 , so that the transfer unit  5  as the belt unit can be positioned in the chassis A 1  easily. In addition, because the guide member  85   a  that protrudes from the front side plate  87  is configured to insert into the slit h 1 , the positioning member does not protrude largely from the front side plate toward front. Thus, the apparatus is prevented from becoming large. 
     In addition, because the bottom portion of the right joint plate  91  contacts the guide pin  838   h , positioning of a vertical direction Z is performed. As a result, when an inner distal side of the transfer unit and the positioning member supported via the outer frame  83  sinks due to its own weight, the guide pin  838   b  serves to support them. Here, ‘sb’ denotes a stopper bis (see  FIG. 7 ). 
     As such, when pulling out toward front the transfer unit  5  and the right joint plate  91  and mounting them thereafter, the positioning pin  838   a  and the guide member  85   a  of the right connecting side plate  86  engage with positioning slits h 1 , h 2  of the right joint plate  91  (see  FIGS. 8 and 9 ). With this structure, the transfer unit  5  can be securely positioned relative to the outer frame  83  of the chassis A 1 , so that the belt detachment can be done at ease. 
     Further, as illustrated in  FIG. 18A , the proximal upper side of the connecting side plate  86  (right side in the figure) is formed as the guide member  85   a , and a bottom end  86   d  has a same height of projection as that of the guide member  85   a . A recessed portion  86   e  positions between the guide member  85   a  and the bottom end  86   d . The recessed portion  86   e  is retracted from front relative to the guide member  85  and the bottom end  86   d  by a distance U 1 , using which the right joint plate  91  is retracted backward from the guide member  85  for replacement. Further, the distance U 1  enables the guide member  85   a  and the positioning pin  838   a  to be inserted into the positioning slits h 1 , h 2  of the right joint plate  91 . 
     Further, the guide pin  838   b  is used for positioning the openably closable cover. A handle  871  may be used to pull out the right joint plate of the transfer unit  5  from the regular position P 1  inside the chassis A 1  to the pull-out position P 2 . 
     Next, how to mount a left end portion of the front side plate  87  of the transfer unit  5  to a left end portion of the chassis A 1  will be described. 
     As illustrated in  FIGS. 12 and 13 , the left end portion of the front side plate  87  of the transfer unit  5  is fastened at several portions to the positioning plate  836  integrally formed with the front left support pillar  831  via the left joint plate  92 . As illustrated in  FIGS. 13A and 13B , the positioning plate  836  includes the positioning pin  838   a  and the guide pin  838   b  both protruding forward. A vertical planar guide member  85   b  extending forward is formed at a front end of the left connecting side plate  89  of the transfer unit  5 . The guide member  85   b  is formed to be inserted into the positioning slit h 2  of the left joint plate  92 . 
     Note that the left connecting side plate  89  is curved to prevent interfering with the positioning plate  836  and the vertical planar guide member  85   b  that extends forward is inserted into the positioning slit h 2  of the left joint plate  92 . 
     As illustrated in  FIG. 13A , when the front side plate  87  of the transfer unit  5  engages by being inserted into the positioning slit h 2  of the left joint plate  92 , positional error of the side of the front side plate  87  of the transfer unit  5  can be prevented. Further, when the left joint plate  92  is removed from the front side plate  87  during maintenance, the guide member  85   b  makes a detachment/attachment of the belt and rewinding of the belt easier. 
     Herein, the vertical planar guide member  85   b  is integrally formed at a front portion of the left connecting side plate  89 . Alternatively, a movable guide plate  85   dp  can be slidably supported to the front portion of the left connecting side plate  89  via a pair of engagement pins  892 . In this case, as illustrated in  FIGS. 14A and 14B , the movable guide plate  85   dp  is held at a front end of the left connecting side plate  89  via the engagement pins  892 . In particular, the movable guide plate  85   dp  is retractably disposed. As illustrated in  FIG. 15 , a pair of long slits  891  is disposed with an interval each other and on a line at a front end of the left connecting side plate  89 . The engagement pins  892  of the movable guide plate  85   dp  are slidably and fixedly engaged with the pair of long slits  891  within a slidable range of E. Further, the movable guide plate  85   dp  is pressed by a spring to protrude at a guided position (as illustrated by a solid line in  FIGS. 14B and 15 ), and is movably held at a holding position (as shown by a broken line in  FIG. 15 ) in response to elastic displacement of the spring. 
     When the movable guide plate  85   dp  is held at the guided position, a protruding-forward amount of the vertical guide surface f 2  becomes relatively large, and the outer circumferential surface of the transfer belt  10  is proximally pulled and is slid in the mounting direction. Further, when the left joint plate  92  is fastened to the front side plate  87 , the left joint plate  92  displaces by a retraction length E to a retracted position (as illustrated by a broken line in  FIG. 15 ), thereby preventing mutual interference. 
     Thus, except in the attachment/detachment of the transfer belt  10 , the movable guide plate  85   dp  can be held at a retracted position, thereby saving a space. 
     The vertical guide surface f 2  (belt contact surface) of the planar guide member  85   b  and the movable guide plate  85   dp  is a planar metal surface. Instead, a main part E 1  of a planar guide member  85   bp  or the left connecting side plate  89  as illustrated in  FIG. 25A , and the main part E 1  of the left connecting side plate  89  and a main part E 2  of a movable guide member  85   ds  as illustrated in  FIG. 25B  may be implanted with short fiber. 
     In such a case, because the material of resinous fiber is implanted, slidability is improved, thereby securing the durability. In particular, the guide member can be guided to move smoothly back and forth without its surface being damaged. Further, abrasion-resistant seal and mylar (a registered trademark of E. I. du Pont de Nemours and Company (“Dupont”)) can be attached to a planar guide member  85   p  and the transfer belt  10 . 
     As illustrated in  FIGS. 12, 13A, and 13B , a bottom end of the left joint plate  92  contacts the guide pin  838   b , so that the positioning in the vertical direction Z is performed, and this guide pin  838   b  is used for positioning an openably closable cover. Further, each handle  871  (see  FIG. 7 ) can be used to pull out the transfer unit  5  from the regular position P 1  in the chassis A 1  to the pull-out position P 2  as illustrated in  FIG. 5 . 
     In the description above, the planar guide member  85   p  is supported by a front end of the left connecting side plate  89 ; however, as another embodiment, the planar guide member  85   p  can be attached to the frontal side, left support pillar  831  (on the side of the outer frame  83 ). Details of another embodiment will be described later. Further, the planar guide member  85   p  is not curved, but may be formed to have a curve if required from the layout. The planar guide member  85   p  is a sheet metal but may be formed of resins or die-cast alloy. 
     Next, how to pull out the transfer unit  5  as the belt unit support unit will be described. 
     A front cover is opened during maintenance and the front opening  82  is exposed to outside. 
     The transfer unit  5  is held in a posture of unit attachment/detachment at the regular position P 1  at the time of maintenance. In that posture, the transfer unit  5  is switched to a posture of unit detachment. The right and left handles  871  are used to pull out the transfer unit  5  from the regular position P 1  in the outer frame  83  as illustrated in  FIG. 4  to the pull-out position P 2  (see  FIG. 5 ). 
     The transfer unit  5  is held in a posture of unit attachment/detachment at the time of maintenance. 
     After that, the outer tension roller  17  is removed from the transfer belt  10  on the transfer unit  5 , so that the transfer belt  10  is loosened. 
     Then, three rollers  14 ,  15 , and  16  are shifted to a waiting position downstream in the shift direction from the regular position P 1 . In this case, the transfer belt  10  is loosely stretched as illustrated by the broken line in  FIG. 3 , and the posture of unit attachment/detachment is held. 
     Herein, in removing the transfer unit  5 , the transfer belt  10  is loosely stretched around the rollers  14  and  15  farthest apart and a loose tension is applied to the belt  10 . In this case, the tension regulates a position of the belt  10 , thereby preventing contact with members other than the belt  10 , and the unit  5  can be taken out relatively easily. 
     Thereafter, the loosened transfer belt  10  is wound at an end portion of the pair of rollers  14  and  15 . 
     In winding operation toward the winding direction, the loosened transfer belt  10  expands laterally so that it is troublesome to wind the belt at the ends of the rollers  14  and  15  while preventing the transfer belt  10  from expanding. Then, in the present embodiment, guide members  85   a ,  85   b ,  85   dp ,  85   c , and  85   p  already described or to be described later are used to make the mounting operation of the belt  10  easier. 
     Specifically, the guide surface f 0  of the right and left connecting side plates  86 ,  89 , the guide surface f 1  and the vertical guide surface  12  of the frontal right side guide member  85   a  at the proximal side and the left side planar guide members  85   b ,  89   dp  prevent the transfer belt  10  from bending excessively toward the lateral direction. The vertical guide surface  12  are illustrated in  FIGS. 14B and 17 , and the transfer belt  10  is illustrated in  FIG. 16  by a solid line. The vertical guide surface f 2  displaces the transfer belt  10  toward the roller  15  (refer to a broken line  10   d  in  FIG. 16 ) to prevent a belt surface from displacing excessively. Thus, the movement of the belt  10  in the mounting direction is made easier. 
     Accordingly, when the transfer belt  10 , if deteriorated, is replaced with a new one, the new transfer belt  10  can be mounted easily on the transfer unit  5 . 
     In this case, the guide surface f 1  and the vertical guide surface f 2  are each formed as a surface extending backward as a mounting direction in winding the loosened transfer belt  10 , and the guide surface f 0  of the connecting side plate  86  is formed at a distal side of the connecting side plate  86  (see  FIGS. 10 and 17 ). In particular, when a curved portion of the crank-angled cross section of the connecting side plate  86  is formed as the guide surface f 1  having a curved R shape, the guide surface f 1  functions to slidably contact the outer surface of the transfer belt and restricts shifting of the belt while suppressing abrasion. 
     Herein, as illustrated in  FIG. 14 , the vertical guide surface f 2  of the left connecting side plate  89  and the planar guide member  85   b  can restrict shifting of the transfer belt  10  in the lateral direction X perpendicular to the pull-out direction Y. On the other hand, as illustrated in  FIG. 7  by a broken line, the guide member  85   a  having a crank-angled cross section can prevent interference of the belt  10  with other members both in removing and mounting the transfer belt  10  while preventing shifting of the transfer belt  10  in the lateral direction X and the vertical direction Z. 
     Thus, portions of the transfer belt  10  opposed to the rollers  14 ,  15  are prevented from interfering with the surrounding members in either case of unit attachment or detachment due to the functions of guide members  85  and the connecting side plate  86 . Specifically, because a pair of guide members  85   a ,  85   b  (or  85   dp ) is disposed opposite the pair or rollers  14 ,  15 , excess shifting of the belt until the belt is wound around the rollers is prevented, so that the mounting operation becomes easier. 
     Thus, in either case of unit attachment or detachment, a pair of connecting side plates  86 ,  89  and the guide members  85   a ,  85   b  (or  85   dp ) at the frontal side can guide the belt  10  so that the outer circumferential surface of the belt  10  does not interfere with the surrounding members and the outer circumferential surface thereof is securely prevented from being damaged. 
     Namely, as illustrated in  FIGS. 16, 18A, and 18B , a pair of guide members  85   a ,  85   b  (or  85   dp ) prevents the outer circumferential surface of the belt  10  from expanding in the horizontal direction when the transfer belt  10  is mounted. As a result, the belt mounting operation until a border of the belt transcends the front side plate  87  and is wound around the first and second rollers  14 ,  15  becomes easier. 
       FIGS. 10 and 11  show that the once pulled-out transfer belt  10  is thereafter wound around the roller  14 . Herein, as a positional relation, at around the roller  14  at one end, because the guide member  85   a  is projected to the most proximal direction in the transfer unit  5 , the belt can be put on a leading end of the guide member  85   a , so that the interference with the surrounding members can be securely prevented. 
     On the other hand,  FIG. 17  shows a case in which the once pulled-out transfer belt  10  is wound on the left roller  15  using the movable guide plate  85   dp . The transfer belt  10  moves along the vertical guide surface f 2  of the movable guide plate  85   dp . In this case, the transfer belt  10  is inserted to approach outward so that the transfer belt  10  does not interfere with the belt unit frame  80  at an inner side of the belt. Further, to prevent interference with the other parts locating outside, the transfer belt  10  is moved along the guide surface  12  and the inner guide surface f 0  continuing from the movable guide plate  85   dp  and the left connecting side plate  89  in the distal side. 
     Thus, according to the first embodiment of the image forming apparatus, when a unit body of the transfer unit  5  is pulled out of the chassis A 1  through the front opening  82  and then attached to the chassis A 1 , the outer circumferential surface of the belt is prevented from interfering with the other parts and components due to the guide members  85   a ,  85   b , and the like. Moreover, attachment/detachment of the belt to the regular position P 1  inside the chassis A 1  can be performed easily. 
     More specifically, when the guide members  85   a ,  85   b  are pulled out to the position P 2  together with the transfer unit  5 , and thereafter, are returned to mount at the regular position P 1 , each guide member  85   a ,  85   b  guides the transfer belt  10  toward the pair of rollers  14 ,  15 , and the winding of the transfer belt  10  is simplified. 
     Further, the guide members  85  are integrally formed with the right and left connecting side plates  86 ,  89  (positioning members) each serving to position the transfer unit  5  relative to the chassis A 1 . Thus, the guide members  85  are supported by the positioning members of the side of the chassis A 1 , the structure is not complicated compared to a case in which the guide members are disposed additionally. 
     Next, a description will be given of a second embodiment of the present invention. 
     As described above, the guide members  85   a ,  85   b  employed in the image forming apparatus are integrally mounted and supported to the proximal ends of the right and left connecting side plates  86 ,  89  that approach the vicinity of the front opening  82  of the chassis A 1  (see  FIGS. 3 and 24 ). A support structure of the second embodiment that is different from that of the first embodiment will be described referring to  FIGS. 18B and 19 . 
     The second embodiment according to the second embodiment includes a chassis A 2  and a guide member  85   c  used as a support structure in the chassis A 2  is different from the support structure in the first embodiment and other structures are similar. Therefore, a redundant description will be omitted. 
     The copier body  100  of the image forming apparatus includes the chassis A 2  and a transfer unit  5  is disposed inside the chassis A 2 . A bottom end of the right connecting side plate  86  is connected to and supported by the right stay  833  (see for example  FIG. 6 ) via the slide rail  90 . The belt unit frame  80  of the transfer unit  5  joined together with the right and left connecting side plates  86  and  89  is therefore supported by the right and left stays  833 . The transfer belt  10  disposed inside the chassis A 2  is held in a posture of the unit attachment/detachment at a time of maintenance. Further, the transfer unit  5  is pulled out and moved proximally in the longitudinal direction of the first and second rollers  14 ,  15 , and thereafter, the transfer belt  10  is held in a posture of the unit attachment/detachment at a time of mounting. 
     As illustrated in  FIG. 19 , an upper part of the connecting side plate  86  is disposed opposite the first roller  14 , serving as a tension member, with a predetermined interval, a bottom part of the connecting side plate  86  is connected to the slide rail  90 , and a middle part of the connecting side plate  86  is formed as a curved wall. The middle curved wall is formed to avoid interference with a positioning plate  836   a  that is formed integrally with the support pillar  831 . As a result, when the transfer unit  5  is taken out and moved, and thereafter, moved to be mounted again, the connecting side plate  86  can be moved without interfering with the positioning plate  836   a  fixed to the support pillar  831 . 
     The middle curved wall is projected forward and the bottom end  86   d  of the projected wall alone is projected by a distance U 1 . The middle curved wall is connected to and supported by the lateral stays  833  via the slide rail  90  (see  FIGS. 7 and 12 ). 
     The positioning plate  836   a  protrudes toward the opening  82  from the support pillar  831  and includes the positioning pin  838   a , the guide pin  838   b , and a guide member  85   c  with the guide surface f 1  having a crank-angled cross section, which protrudes forward. Herein, the guide member  85   c  is integrally formed with the positioning plate  836   a  that is integrally formed with the right support pillar  831  positioned outside the right connecting side plate  86 , not with the right connecting side plate  86 . 
     As such, the guide surface f 1  having the crank-angled cross section of the guide member  85   c  is disposed at an upper middle side of the positioning plate  836   a , so that the positioning plate  836   a  is prevented from contacting the outer circumference of the transfer belt  10 . 
     Positioning slits to engage the guide member  85   c  and the positioning pin  838   a  are disposed on the right joint plate  91  covering the positioning plate  836   a  similarly to the first embodiment. 
     Further, as illustrated in  FIG. 19 , the guide pin  838   b  serves as a positioning member for the transfer unit  5  and the right joint plate  91  and supports the right joint plate  91  by contacting the bottom end of the right joint plate  91  when mounting the right joint plate  91  by moving it from the proximal side to the distal side in the figure. In this case, the guide pin  838   b  serving as a positioning regulator allows the positioning pin  838   a  and the guide member  85   c  to be inserted into the positioning slits. “sb” means a stopper bis. 
     Parts and components such as the connecting side plate, the positioning plate integrally formed with the left support pillar  831 , and the positioning pins  838   a ,  838   b  disposed on the left side of the image forming apparatus are similarly, symmetrically formed as equivalents on the right side of the apparatus. 
     The guide member  85   c  according to the second embodiment is a sheet metal but may be formed of resins or die-cast alloy. The guide member  85   c  may be applied with fiber or a film. 
     Further, in the present embodiment, the guide member  85   c  is integrally formed with the positioning plate  836  that is a part of the body. Alternatively, however, the guide member  85   c  may be a separate member as long as it is a member disposed at an edge portion of the opening of the body. The guide member  85   c  may not be a member disposed on the body, but can be a member disposed at an edge of the front opening  82 . For example, the guide member  85   c  may be formed as a frame member of an upper part of the tandem image forming device  20  such as a photoreceptor unit frame or a frame of the feeder of the sheet conveyance section near the bottom of the apparatus. 
     Next, a description will be given of a third embodiment of the present invention. 
     As described in the first embodiment, the guide members  85   a ,  85   b  disposed in the chassis A 1  are each supported at a leading end of the connecting side plates  86 ,  89 , respectively. The third embodiment is different from the first embodiment in the support structure of the guide member. Because structures other than the support structure embodied by the guide member  85   d  that a chassis A 3  according to the third embodiment includes are the same as those in the first embodiment, redundant description will be omitted. 
     Herein, as illustrated in  FIGS. 20, 21, and 22 , the upper side wall of the connecting side plates  86 ,  89  disposed at the lateral ends of the transfer unit  5  inside the chassis A 3  is connected to and supported by the lateral stays  833  via a slide rail  90   a . Herein, the slide rail  90   a  includes a movable inner rail  901   a  that is integrally disposed on the upper side wall of the right connecting side plate  86 . Further, the slide rail  90   a  includes a fixed outer rail  902   a  that is integrally mounted on the stay  833 . Namely, the transfer unit  5  is movably supported in a proximal-distal direction Y relative to the fixed outer rail  902   a  of the outer frame  83 . The guide member  85   d  integrated with a common front end p 01  (see  FIG. 23 ) of the movable inner rail  901   a  and the right connecting side plate  86  that are joined together such that a trailing end of the guide member  85   d  contacts the front end p 01  and the proximal end of the guide member  85   d  protrudes forward. Thus, the guide member  85   d  is movably supported in the proximal direction of the roller axis. 
     With such a structure, as illustrated in  FIG. 20 , when the belt unit frame  80  of the transfer unit  5  is pulled out to the position P 2 , the movable inner rail  901   a  of the side of the belt unit frame  80  is pulled out to the pull-out position P 2  simultaneously. In such a state, the guide member  85   d  moves along the belt moving direction being a roller mounting axis direction from the proximal side of the roller axis direction. In this position, the guide member  85   d  at the leading end of the movable inner rail  901   a  prevents an excessive shilling of the outer surface of the belt and guides mounting of the belt. With this structure, the outer surface of the belt is prevented from interfering with other parts around the transfer unit. 
     In each of the embodiments described above, a structure in which the belt unit frame  80  of the transfer unit  5  is pulled out to the pull-out position P 2  is described. Alternatively to those examples, a case in which the belt unit frame  80  of the transfer unit  5  is not pulled out to the pull-out position P 2  and is held directly below the regular position P 1  will be described. 
     In this case, the transfer unit  5  positions at the same position as the regular position P 1  in the posture of the belt conveyance and directly below the regular position P 1  with a predetermined distance in the posture of the unit removal. In this case, the transfer unit  5  is held at the regular position P 1  in the moving direction only by switching from the posture of the belt conveyance to the posture of the unit removal. 
     Now, the transfer belt  10  that is stretched on the belt unit frame  80  is disposed at the regular position P 1 , is switched to the posture of the unit removal, and is wound around three rollers  14 ,  15 , and  16  (see  FIG. 1 ). Then, the outer tension roller  17  alone contacting the outer surface of the belt (see  FIG. 1 ) is removed and is pulled out along the roller axis direction. 
     Further, when pulling out the transfer belt  10  in the unit removal direction or returning to mount the transfer belt  10  from the proximal side in the roller axis direction, the guide surface f 1  of the guide member  85   a  can guide the outer circumferential surface of the transfer belt  10  in the moving direction thereof. 
     Substantially similarly to the first embodiment, the pair of guide members  85  and the connecting side plate  86  can guide the transfer belt  10  or an endless belt not to interfere with other parts and components and prevent the outer circumferential surface thereof from being damaged, so that the attachment/detachment of the transfer belt  10  can be performed easily. 
     In the above description, the transfer belt  10  can be applied to a belt unit support device of the secondary transfer belt  24 , which is also an endless belt (see  FIG. 1 ), that sends a recording sheet to a sheet discharging unit. 
     In addition, the present invention may be applied to the image forming apparatus that conveys the sheet between the photoreceptor and the conveyance belt and the toner image on the photoreceptor to the sheet. Thus, the present embodiment includes at least a following aspect: 
     A conveyance belt, similar to the transfer belt  10 , that is supported by a plurality of rollers such as the rollers  14 ,  15 , and  16  and can be mounted from the proximal side in the roller axis direction, as can side plates (such as the front side plate  87  and the rear side plate  88 ) that support an end of the plurality of rollers. Further, the present embodiment can be applied to an image forming apparatus including guide members (such as guide members  85   a ,  85   b ) to guide the conveyance belt to mount to the plurality of rollers with the guide members contacting an outer surface of the conveyance belt. 
     As the image forming apparatus that includes belt unit support chassis A 1  to A 3  can obtain supportive effects of the chassis. 
     As to the image forming apparatus, a color copier has been described heretofore, but the present embodiment can be applied to printers, facsimile machines, and other types of image forming apparatuses. 
     As described above, the right and left connecting side plates  86 ,  89 , and the guide members  85   a ,  85   b  may be separate members, but are formed integrally as a single member, such that the right connecting side plate  86  and the guide member  85   a  can be integrated. 
     Disposition of the rollers  14 ,  15  that support the belt  10  is not limited to the examples of embodiments. Without providing the outer tension roller  17  (see  FIG. 1 ) that contacts the outer surface of the belt, the belt can be supported by rollers that contact an inner surface of the belt alone. 
     Further, the side plates  87 ,  88  to support one end of the plurality of rollers  14 ,  15  may be separately formed or integrally formed in combination with other members. 
     Because the guide members  85   a ,  85   b  each include a guide surface to contact an outer surface of the belt, the planar shape of the guide member serves to guide the belt in mounting, and the belt and the guide member contact via a planar surface. Then, compared to the point contact or line contact, it is sure that the belt can be prevented from being damaged. 
     The image forming apparatus further includes an image carrier on which a toner image is formed, and a belt serving as a transfer belt on which the toner image on the image carrier is transferred. Thus, when the present embodiment is applied to an apparatus including a transfer belt that can be mounted on the plurality of rollers, the belt can be prevented from damaged by that the outer surface of the belt contacts other parts or components when the transfer belt is to be mounted. 
     According to the present invention, when attaching the belt from the proximal side in the roller shaft direction relative to the plurality of rollers supported by the side plates, the belt is moved along the guide member for attachment, because the guide member prevents excess shifting of the upper surface of the belt and properly guides the movement of the belt. Thus, the upper surface of the belt is prevented from being interfered with by other parts around the transfer unit. 
     Preferred embodiments of the present invention have been described heretofore; however, the present invention is not limited to the described embodiments and various modifications are possible within the scope of claims unless explicitly limited in the description. 
     For example, the image forming apparatus to which the present embodiments of the invention may be applied is not limited to the types of the apparatuses as described above, but may be applied to any other types of image forming apparatuses. The present invention may be applied to an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction apparatus having one or more capabilities of the above devices. The image forming apparatuses to which the present embodiment of the present invention is to be applied may be image forming apparatuses used for forming a predetermined image in biotechnology field. Effects described in the present embodiments are examples of preferred results of embodiments of the present invention and are not limited to what has been already described herein. 
     Additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.