Patent Publication Number: US-10775717-B2

Title: Image forming apparatus having a support below a transfer belt and a lever for raising the support, and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. P2018-237768, filed on Dec. 19, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments relate to an image forming apparatus and method of replacing image process units in an image forming apparatus. 
     BACKGROUND 
     Conventionally, an image forming apparatus (for example, a multifunction peripheral or MFP) includes a transfer belt and an image process unit including a photosensitive drum. The image process unit including the photosensitive drum is a consumable item, that the user replaces as needed. In order to prevent the transfer belt from being damaged at the time of replacing the image process unit, it is necessary to separate the transfer belt from the photosensitive drum. In order to bring the transfer belt into a non-contact state with respect to the photosensitive drum, the transfer belt is moved by a position switching roller arranged inside the transfer belt. 
     However, when the transfer belt is deteriorated and stretched by use over time, a sufficient tension cannot be applied to the transfer belt. Therefore, it becomes difficult to sufficiently separate the transfer belt from the photosensitive drum. As a result, a part of the image process unit is rubbed or hooked to the transfer belt to damage the transfer belt when the image process unit is replaced. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an image forming apparatus according to an embodiment. 
         FIG. 2  is a block diagram showing a color printing position of a transfer belt unit according to an embodiment. 
         FIG. 3  is a block diagram showing a monochrome printing position of the transfer belt unit according to the embodiment. 
         FIG. 4  is a block diagram showing a maintenance position of the transfer belt unit according to the embodiment. 
         FIG. 5A  illustrate perspective views of the transfer belt unit according to the embodiment. 
         FIG. 5B  is a cross sectional view of one side of the transfer belt unit. 
         FIG. 6A  is a front view of a control mechanism of the transfer belt unit in a printing position. 
         FIG. 6B  is a perspective view of the control mechanism of the transfer belt unit in the printing position. 
         FIG. 7A  is a front view of the control mechanism of the transfer belt unit in a maintenance position. 
         FIG. 7B  is a perspective view of the control mechanism of the transfer belt unit in the maintenance position. 
         FIG. 8  is a perspective view showing a transfer belt and a holding arm of the transfer belt unit. 
         FIG. 9  is an enlarged view of a support portion and a first interlocking mechanism of  FIG. 8 . 
         FIG. 10  an enlarged view that shows the position of the transfer belt and the support portion during the color printing process. 
         FIG. 11  is an enlarged view showing the position of the transfer belt and the support member during the monochrome printing process. 
         FIG. 12  an enlarged view showing the maintenance position of the transfer belt and the support portion. 
         FIG. 13  is a perspective view of a supporting shaft and a second tension switching roller of the transfer belt unit. 
         FIG. 14A  is a perspective view showing a state in which a waste toner box is locked. 
         FIG. 14B  is a diagram showing a state in which the waste toner box is unlocked. 
         FIG. 15  is a diagram showing a modification example of the support member. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming apparatus according to an embodiment includes a plurality of photosensitive drums, a transfer belt above the photosensitive drums and having an outer surface that faces the photosensitive drums, a tension roller in contact with an inner surface of the transfer belt, a cam that is in contact with the tension roller and is movable to a first position at which the cam applies a first tension to the transfer belt through the tension roller and a second position at which the cam applies a second tension less than the first tension to the transfer belt through the tension roller, a support below the transfer belt and having a supporting surface that faces the outer surface of the transfer belt, and a lever that is mechanically linked to the cam and the support and is rotatable from a third position at which the cam is in the first position to a fourth position at which the cam is in the second position. As the lever is moved from the third position to the fourth position, the supporting surface is raised against the outer surface of the transfer belt. 
     Hereinafter, an image forming apparatus according to an embodiment will be described with reference to the drawings. 
     The transfer belt unit  6  in the image forming apparatus  10  according to one embodiment will be described with reference to  FIGS. 1 to 14 .  FIG. 1  is a schematic diagram showing an example of a schematic diagram of the image forming apparatus  10 . In  FIG. 1 , the dimensions and shape of each member are exaggerated or simplified for ease of viewing (as in other figures). As shown in  FIG. 1 , the image forming apparatus  10  of the embodiment is, for example, an MFP (Multi-Function Peripheral), a printer, a copying machine, or the like. 
     The image forming apparatus  10  includes an apparatus main body  11  and a printer unit  17  in a central portion in the height direction. The apparatus main body  11  includes paper feed cassettes  18  at its lower portion and a manual feed tray  26 . The paper feed cassettes  18  are disposed inside the apparatus main body  11 . The manual feed tray  26  can be opened and closed. The manual feed tray  26  is provided on an automatic duplex device  40  which is provided on one side portion of the apparatus main body  11 . The automatic duplex device  40  can also be opened and closed. 
     A transport mechanism  29  (transport unit) is disposed in the vicinity of each paper feed cassette  18  in the apparatus main body  11 . The transport mechanism  29  feeds the sheet P fed from the paper feed cassette  18  to the main body transport path. 
     The printer unit  17  forms an image on the sheet P based on image data generated from an image read by the scanner unit or image data generated by a personal computer or the like. The printer unit  17  is a color printer using, for example, a tandem system. The printer unit  17  includes image process units  20 Y,  20 M,  20 C, and  20 K for yellow (Y), magenta (M), cyan (C), and black (K), respectively, which are attachable to and detachable from the apparatus main body  1 . The apparatus main body  1  further includes an exposure unit  19  and a transfer belt unit  6 . 
     The image process units  20 Y,  20 M,  20 C, and  20 K are disposed under the transfer belt  21 . The image process units  20 Y,  20 M,  20 C, and  20 K are arranged from the upstream side to the downstream side in the moving direction (the direction from the left side to the right side in the figure) of the transfer belt  21  and in parallel to the moving direction. 
     The exposure device  19  irradiates exposure light to the image process units  20 Y,  20 M,  20 C, and  20 K, respectively. The exposure device  19  may be configured to generate a laser scanning beam as an exposure light. The exposure unit  19  may include a solid-state scanning element such as an LED that generates exposure light. The configurations of the image process units  20 Y,  20 M,  20 C, and  20 K are the same except that the color of the toner is different. 
     Each of the image process units  20 Y,  20 M,  20 C, and  20 K has a well-known electrophotographic system. For example, each of the image process units  20 Y,  20 M,  20 C and  20 K includes a photosensitive drum (which is more generally a photoreceptor or a photosensitive member)  2 Y,  7 M,  7 C and  7 K shown in  FIG. 4  to  FIG. 7 . Around the photosensitive drums  7 Y,  7 M,  7 C and  7 K, the charger  22  and the developing device  23  are arranged along the rotational direction of the photosensitive drums  7 Y,  7 M,  7 C and  7 K to form an image process unit. The image process unit may further comprise a cleaning device. The primary transfer rollers  8 Y,  8 M,  8 C, and  8 K are disposed so as to face the photosensitive drums  7 Y,  7 M,  7 C, and  7 K via the transfer belt  21 . 
     The charger  22  uniformly charges the surfaces of the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. The exposure unit  19  generates exposure light modulated based on the image data of the respective colors. The exposure light exposes the surfaces of the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. The exposure unit  19  forms an electrostatic latent image on the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. The developing device  23  supplies toner to the photosensitive drums  7 Y,  7 M,  7 C, and  7 K by a developing roller to which a developing bias is applied. The developing device  23  develops the electrostatic latent image on the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. The cleaner has blades that contact the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. The blade removes residual toner on the surface of the photosensitive drum. 
     As shown in  FIG. 1 , a toner cartridge  28  is disposed above the transfer belt unit  6 . The toner cartridge  28  supplies toner to each of the developing units  23  of the image process units  20 Y,  20 M,  20 C, and  20 K. The toner cartridge  28  includes toner cartridges  28 Y,  28 M,  28 C, and  28 K. The toner cartridges  28 Y,  28 M,  28 C and  28 K contain toners of yellow, magenta, cyan and black, respectively. 
     The transfer belt  21  is formed in an endless shape, and moves in a circular manner. The transfer belt  21  is stretched around a drive roller  31 , a driven roller  32  and a tension roller  43 . The transfer belt  21  is in contact with an upper side of the photosensitive drums  7 Y,  7 M,  7 C and  7 K of the image process units  20 Y,  20 M,  20 C and  20 K. At positions opposite the photosensitive drums  7 Y,  7 M,  7 C and  7 K with respect to the transfer belt  21 , the primary transfer rollers  8 Y,  8 M,  8 C and  8 K of the image process units  20 Y,  20 M,  20 C and  20 K are arranged. 
     When the primary transfer voltage is applied to the primary transfer rollers  8 Y,  8 M,  8 C and  8 K, the toner images on the photosensitive drums  7 Y,  7 M,  7 C and  7 K are transferred to the transfer belt  21 . The photosensitive drums  7 Y,  7 M,  7 C, and  7 K constitute an image carrying member which carries a toner image from a developing position to a primary transfer position. 
     A secondary transfer roller  33  is opposed to the driving roller  31  with the transfer belt  21  interposed therebetween. The contact portion between the transfer belt  21  and the secondary transfer roller  33  constitutes a secondary transfer position. The driving roller  31  rotates and drives the transfer belt  21 . The rotationally driven transfer belt  21  forms an image carrying member which carries the toner image from the primary transfer position to the secondary transfer position. 
     When the sheet P passes through the secondary transfer position, a secondary transfer voltage is applied to the sheet P. When the secondary transfer voltage is applied to the secondary transfer roller  33 , the secondary transfer roller  33  transfers the toner image on the transfer belt  21  to the sheet P. 
     As shown in  FIG. 1 , a belt cleaner  34  is disposed in the vicinity of a tension roller  43 . The belt cleaner  34  removes the residual transfer toner on the transfer belt  21  from the transfer belt  21 . 
     A conveying roller  35 A, a conveying roller  35 B, and a registration roller  41  are provided along a main conveying path from the paper cassettes  18  to the secondary transfer roller  33 . The registration roller  41  adjusts the position of the leading end of the sheet P conveyed by the conveying roller  35 A. The registration roller  41  conveys the sheet P and causes the leading end of the transfer region for the toner image on the sheet P to reach the secondary transfer position. The transfer region for the toner image on the sheet P is a region excluding a region where white void is formed on the end portion of the sheet P. A fixing device  36  is disposed downstream of the secondary transfer roller  33 . A conveying roller  37  is disposed downstream of the fixing device  36 . The conveying roller  37  discharges the sheet P. 
     The fixing device  36  fixes the toner image by heating and applying pressure to the toner image on the sheet P. A reverse conveyance path  39  is disposed downstream of the fixing device  36 . The reverse conveyance path  39  inverts the sheet P and guides the sheet P to the upstream side of the registration roller  41 . The reverse conveyance path  39  is used to perform double-sided printing. The reverse conveyance path  39  is installed in the automatic duplex device  40 . 
     Next, the transfer belt unit  6  will be described with reference to  FIG. 2  to  FIG. 14 . 
     As shown in  FIG. 2  to  FIG. 14 , the drive roller  31  is disposed inside the transfer belt  21  on one end side of the transfer belt unit  6  and a tension roller  43  is disposed inside the transfer belt  21  on the other end side of the transfer belt unit  6 . An axial end of the tension roller  43  is connected to a roller support portion  44 . As shown in  FIG. 5B , the roller support portion  44  is connected to a tension spring  45  that is provided on a side portion of the transfer belt unit  6  in a compressed state. The roller supporting portion  44  is pushed to the outside by the elastic force of the tension spring  45 . As a result, the tension roller  43  supported by the roller support portion  44  pushes the transfer belt  21  outward. Tension is applied to the transfer belt  21  by the elastic force of the tension spring  45 . The lower side of the transfer belt  21  is stretched by a first position switching roller  42  and a driven roller  32  which are provided on the left and right end portions, respectively. 
     The transfer belt  21  has a control mechanism  46 A for the color photosensitive member and a control mechanism  46 B for the monochromatic photosensitive member, for controlling the pressing and releasing of the transfer belt  21  to the photosensitive member. 
     The control mechanism  46 A of the color photosensitive member will now be described. A slider  47  is disposed along the longitudinal direction of the transfer belt  21  on both sides (or alternatively, just one side) in the width direction of the transfer belt unit  6 . Recessed portions  48  having pressing surfaces  48   a  and  48   b  on both sides thereof are provided in the middle portion of the slider  47  in the longitudinal direction. An eccentric cam  49  rotatably supported by a base portion  49   a  is provided in the concave portion  49 . 
     For example, the cam portion  50  of the eccentric cam  49  can be rotated by 180 degrees. 
     When the cam portion  50  is positioned on the left side in the figure, pressing surface  48   a  is pressed to move slider  47  to the left side (refer to  FIG. 2 ). When the cam portion  50  is positioned on the right side in the figure, the pressing surface  48   b  is pressed to move the slider  47  to the right side (see  FIGS. 3 and 4 ). 
     Photosensitive drums  7 Y,  7 M,  7 C and  7 K of the image process units  20 Y,  20 M,  20 C and  20 K are disposed on the lower side of the transfer belt  21  with a predetermined interval therebetween. Photosensitive drums  7 Y,  7 M, and  7 C are color photosensitive members, and photosensitive drum  7 K is a monochrome photosensitive member. On the inside of the transfer belt  21 , primary transfer rollers  8 Y,  8 M, and  8 C are disposed at predetermined intervals to press the transfer belt  21  to the photosensitive drums  7 Y,  7 M, and  7 C, respectively. The primary transfer rollers  8 Y,  8 M, and  8 C are respectively fixed to one end of an L-shaped arm portion  52 . An interlocking part  52   a  is provided at the other end part of each arm portion  52 , and a shaft part  53  is provided at the elbow portion. 
     Each arm portion  52  is rotatable about the shaft portion  53  by a predetermined angle. 
     Three notches  51   a ,  51   b , and  51   c  are formed in the slider  47 . The interlocking portion  52   a  of each arm portion  52  is inserted into the corresponding notch portion  51   a ,  51   b , or  51   c  so as to be interlocked with each other. In  FIG. 2 , the slider  47  is biased to the left side in the figure by a spring member (not shown). In this state, the interlocking portion  52   a  of each arm portion  52  is held in position within the notch portions  51   a ,  51   b  and  51   c  in a non-contact manner. The arm portions  52  may be held in this state by a spring member (not shown). 
     When the slider  47  is moved to the left side in the figure by the eccentric cam  49 , the arm portions  52  are held at positions where the interlocking portion  52   a  is not in contact with the notch portions  51   a ,  51   b  and  51   c  (see  FIG. 2 ). When the slider  47  is moved to the right side in the figure, the arm portions  52  are pushed by the interlocking portions  52   a ,  51   b , and  51   c  to rotate the arm portions  51 . As a result, the primary transfer rollers  8 Y,  8 M, and  8 C are separated from the photosensitive drums  7 Y,  7 M, and  7 C with the transfer belt  21  interposed therebetween (see  FIG. 3 ,  FIG. 4 ). 
     A concave portion  47   a  is formed at an end portion of the slider  47  on the tension roller  43  side of the slider  47 . One end portion  47   a  of an approximately L-shaped operating arm  54  supported rotatably by a shaft portion  55  is engaged with the slider  47  through the recess portion  54   a . The actuating portion  54   b  of the actuating arm  54  presses against the first position switching roller  42 . The first position switching roller  42  is fixed to the other end of a pivot arm  57  which is rotatable about a support shaft  57   a . The first position switching roller  42  is pressed by the operating portion  54   b  of the operating arm  54  to press the transfer belt  21  against the photosensitive drums  7 Y,  7 M and  7 C of the color photosensitive member. The pivot arm  57  may bias the first position switching roller  42  toward the slider  47  by a spring member (not shown). 
     By moving the slider  47  to the right side in the figure, as shown in  FIG. 3 , the actuating arm  54  is rotated in a clockwise direction about the shaft portion  55 . The actuating portion  54   b  of the actuating arm  54  is disengaged from the first position switching roller  42 . Thus, the first position switching roller  42  is rotated in the counterclockwise direction by the tension of the transfer belt  21  to move towards the slider  47 . The primary transfer rollers BY,  8 M, and  8 C are also retracted at the same time as the first position switching roller  42 . Therefore, the transfer belt  21  is displaced in a direction away from the photosensitive drums  7 Y,  7 M, and  7 C. As a result of this displacement, the transfer belt  21  shifts from the color printing position shown in  FIG. 2  to the monochrome printing position shown in  FIG. 3 . 
     Next, the control mechanism  46 B of the monochrome photosensitive member will be described mainly with reference to  FIGS. 4, 6 and 7 . In the transfer belt unit  6 , a control mechanism  46   b  for a monochrome photoreceptor is provided between a slider  47  and a drive roller  31 . 
     In the control mechanism  46 B of the monochrome photosensitive member, the primary transfer roller  8 K is connected to a rotary shaft  61  via a connecting arm  60 . An actuating arm  62  is connected to the rotary shaft  61 . The connecting arm  60  and the actuating arm  62  are rotatable about the rotary shaft  61 . A first coil spring  64  is disposed between a hook  61   a  provided on the rotary shaft  61  and a hook  62   a  provided on the actuating arm  62 . The first coil spring  64  is held in a tensioned state. In  FIG. 6 , the rotary shaft  61  is biased counterclockwise by the urging force of the first coil spring  64 . The primary transfer roller  8 K fixed to the connecting arm  60  presses the transfer belt  21  against the monochrome photosensitive drum  7 K. 
     A second position switching roller  66  is disposed in the vicinity of the primary transfer roller  8 K. In  FIGS. 4 and 6A , the second position switching roller  66  presses the transfer belt  21  to the outside. The second position switching roller  66  is integrally fixed to an operating lever  68 . The operating lever  68  can swing in a predetermined range around the second position switching roller  66 . A pin  68   a  is provided on the operating lever  68  and is movably inserted into a long hole  62   b  formed in the actuating arm  62 . The operating lever  68  is swingable in accordance with the movement range of the pin  68   a  within the long hole  62   b.    
     A locking groove  68   b  is formed on the other end portion of the actuating lever  68 . A locking portion  69   a  is formed on the support portion  69  fixed to the base. The second coil spring  70  in a tensioned state is engaged with the locking groove  68   b  of the actuating lever  68  and the locking portion  69   a  of the supporting portion  69 . Further, a cam  73  rotatable about a support shaft  72  is provided above the second position switching roller  66 . The end portion of the cam  73  abuts against the second position switching roller  66  to press it downward. The second position switching roller  66  is pressed downward by the cam  73  against the urging force of the second coil spring  70 . 
     The second position switching roller  66  presses the transfer belt  21  to the outside at a position of the transfer belt between the photosensitive drums  7 C and  7 K. As a result, the transfer belt  21  is pressed against the photosensitive drums  7 C and  7 K. As described above, even when the transfer belt  21  is separated from the color photosensitive drums  7 Y,  7 M, and  7 C, the transfer belt  21  is pressed against the photosensitive drum  7 K by the second position switching roller  66  (see  FIG. 3 ). At this time, the transfer belt  21  assumes a monochrome printing position. 
     When the support shaft  72  shown in  FIGS. 6A and 6B  is rotated counterclockwise, the cam  73  is disengaged from the second position switching roller  66 , as shown in  FIGS. 7 and 7B . The operating lever  68  is pulled up toward the supporting shaft  72  by the urging force of the second coil spring  70 . 
       FIGS. 7A and 7B  show a lifting mechanism for the second position switching roller  66 . When the cam  73  is disengaged from the second position switching roller  66 , the second position switching roller  66 , together with the operating lever  68 , is lifted upward by the urging force of the second coil spring  70 . The operating lever  68  and the second position switching roller  66  are vertically swingable in accordance with the movement range of the pin  68   a  within the long hole  62   b.    
     The cam  73  also pushes the end of the actuating arm  62  when it rotates approximately 90 degrees. As a result, the actuating arm  62  rotates clockwise about the rotary shaft  61 . Since the connecting arm  60  is also rotated clockwise in conjunction with the rotation of the actuating arm  62 , the primary transfer roller  8 K is separated from the photosensitive drum  7 K. As a result, the second position switching roller  66  and the primary transfer roller  8 K move together in a direction in which they are separated from the photosensitive drum  7 K. Thus, the transfer belt  21  is displaced upward and separated from the photosensitive drum  7 K (see  FIG. 4 ). At this time, the transfer belt  21  assumes a maintenance position. The maintenance position is a position where the transfer belt  21  and the photosensitive drum  7 K are separated from each other, and the image process units  20 Y,  20 M,  20 C, and  20 K can be exchanged. 
       FIG. 8  shows a first interlocking mechanism  74  for operating a holding arm  75  serving as a supporting member for supporting the transfer belt  21  in a state in which the transfer belt  21  is separated from the monochrome photosensitive drum  7 K. The first interlocking mechanism  74  is disposed on a side wall  76  of the apparatus main body  11  which faces one end portion of the transfer belt unit  6  along the longitudinal direction of the photosensitive drum. The side wall  76  is provided with an operating lever  77  which is rotatable about a shaft portion  77   a . The operating lever  77  is formed in a substantially doglegged shape, for example, but may be formed in other shapes. 
     A first sector gear  78  is fixed to the shaft portion  77   a , for example. A second sector gear  80  coaxially fixed to a disk  80   a  meshes with a disk-shaped gear  79  that is meshed with the first sector gear  78 . A pin  80   b  is fixed to the second sector gear  80 . The holding arm  75  is supported so as to be swingable about an axis  75   a  fixed to the side wall  76 . The holding arm  75  is formed with a recess  75   b  that engages the pin  80   b  at its free end. The holding arm  75  has a support portion  75   c  which projects outward from a middle portion thereof in the longitudinal direction and is bent in a substantially doglegged shape. The support portion  75   c  extends in the width direction of the transfer belt  21  to a position below the lower surface of the transfer belt. 
     By rotating the operating lever  77  in a counterclockwise direction by a predetermined angle, the second sector gear  80  and the disk  80   a  are rotated counterclockwise via the first sector gear  78  and the gear  79 . The pin  80   b  rotates the holding arm  75  counterclockwise about the axis  75   a  by a predetermined angle. Then, the transfer belt  21  can be lifted upward by the support portion  75   c  of the holding arm  75 . The holding arm  75  holds and lifts the one end portion of the transfer belt  21  in the width direction by the support portion  75   c.    
     In the color printing position (see  FIG. 10 ) and the monochrome printing position (see  FIG. 11 ), the operating lever  77  is in a position to separate the transfer belt  21  from the support portion  75   c  of the holding arm  75 . The position of the transfer belt  21  shown in  FIG. 10  is assumed to be an initial position (see  FIG. 10 ). The operating lever  77  rotates by a predetermined angle, and the operating position in which the transfer belt  21  is held by the support portion  75   c  becomes the maintenance position (see  FIG. 12 ). 
     In the maintenance position, when the image process unit  20 K is replaced, the transfer belt  21  can be separated from the photosensitive drum  7 K to prevent damage. When viewed in the moving direction of the transfer belt  21 , the holding arm  75  is provided on the upstream side of the photosensitive drum  7 K. In this embodiment, the first interlocking mechanism  74  is provided only at one end portion in the width direction of the transfer belt  21 , but may be provided at both end portions in other embodiments. 
     Although the operating lever  77 , the first interlocking mechanism  74 , and the holding arm  75  are preferably disposed on the front side of the image forming apparatus  10 , i.e., on the front side of the electrophotographic process unit (EPU) (not shown), the position of the operating lever  77 , the first interlocking mechanism  74 , and the holding arm  75  can be appropriately selected. 
       FIG. 5A  is a view showing the side wall  76  having the operating lever  77 , the first interlocking mechanism  74 , and the holding arm  75  separated from the side surface  6   a  of the transfer belt unit  6 . The disk  80   a  in the first interlocking mechanism  74  is rotatably supported by the side wall  76 . On the back surface of the disc  80   a , there is formed a fitting receiving portion  82  which protrudes substantially in the shape of a substantially inverted letter J or a substantially U-shaped letter. As shown in  FIG. 5A  and  FIG. 13 , the side surface  6   a  of the transfer belt unit  6  opposed thereto is formed in such a manner that one end portion  72   a  of the support shaft  72  protrudes, for example. The end portion  72   a  is disposed such that a circular portion of the support shaft  72  is coaxial with the disk  80   a . Further, for example, a fitting projection  83  protrudes from the head portion of the head portion  72   a , which has a small diameter. The fitting projection  83  is fitted to the fitting receiving portion  82  of the disc  80   a . The fitting projection  83 , the fitting receiving portion  82  of the disc  80   a  and the end portion  72   a  of the supporting shaft  72  constitutes a link fitting portion. 
     Therefore, when the operating lever  77  is rotated, the supporting shaft  72  rotates by a predetermined angle in conjunction with the rotation of the disk  80   a  of the first interlocking mechanism  74 , so that the cam  73  of the supporting shaft  72  is disengaged from the second position switching roller  66 . Then, the second position switching roller  66  and the primary transfer roller  8 K are separated from the photosensitive drum  7 K by the urging force of the second coil spring  70 . When the operating lever  77  is returned to the original initial position, the first interlocking mechanism  74  and the second interlocking mechanism  85  are rotated in reverse, so that the holding arm  75  and the second position switching roller  66  are returned to their initial positions. 
     The first sector gear  78 , the gear  79 , the disk  80   a , and the support shaft  72  are interlocked with the first interlocking mechanism  74  to separate the second position switching roller  66  from the photosensitive drum  7 K in the second interlocking mechanism  85 . 
     As shown in  FIGS. 14A and 14B , a waste toner box  87  is disposed on the lower side of the toner cartridge  28 , for example, on the front side of the apparatus main body  11  of the image forming apparatus  10 . The waste toner box  87  is removably mounted. The operating lever  77  of the first interlocking mechanism  74  is detachably attached to the engaging end portion  87   a  of one end of the waste toner box  87  when the waste toner box  87  is attached to the apparatus main body  11 . The operating lever  77  is supported so as to be rotatable at a position that is projecting forward by the shaft portion  77   a  from the side wall  76  on which the first interlocking mechanism  74  is installed. 
     As shown in  FIG. 14A , a part of the operating lever  77  is engaged with the engaging end portion  87   a  of the waste toner box  87  at an initial position. In this state, the waste toner box  87  is locked so as not to be pulled out, so that it is prevented from being detached. As shown in  FIG. 14B , the operating lever  77  is disengaged from the engaging end portion  87   a  of the waste toner box  87  at an operating position which is rotated from the initial position. In this state, the waste toner box  87  can be pulled out from the apparatus main body  11 . Therefore, the operating lever  77  also serves to prevent the waste toner box  87  from being detached. 
     That is, the waste toner box  87  is disposed so as to face the end portion of the image process unit  20  in the longitudinal direction of the photosensitive drum  6 . Therefore, at the time of exchanging the image process unit  20 , it may be necessary to remove the waste toner box  87 . Therefore, when the image process unit  20  is exchanged, the transfer belt  21  is always lifted by the holding arm  75 , and then the lock of the waste toner box  87  is released. As a result, the transfer belt  21  is separated from the photosensitive drum  7 K. Therefore, the image process unit can be replaced without damaging the transfer belt  21 . 
     Although the second interlocking mechanism  85  is operated in conjunction with the operation of the first interlocking mechanism  74  by the operating lever  77  in the above description, both of them are operated substantially simultaneously. The first interlocking mechanism  74  may be operated in conjunction with the operation of the second interlocking mechanism  85  by the operating lever  77 . 
     The image forming apparatus  10  and the transfer belt unit  6  according to the present embodiment have the structure described above. Next, an operation method of the transfer belt unit  6  will be described with reference to  FIG. 2  to  FIG. 4  and  FIG. 10  to  FIG. 12 . In the color printing position shown in  FIG. 2  and  FIG. 10 , the slider  47  is positioned on the left side. The first position switching roller  42  presses the lower transfer belt  21  by being pressed by the operating portion  54   b  of the operating arm  54 . The second position switching roller  66  also presses the lower transfer belt  21  by the cam  73  of the support shaft  72 . The transfer belt  21  is pressed to the photosensitive drums  7 Y,  7 M,  7 C and  7 K by the primary transfer rollers BY,  8 M,  8 C and  8 K. The support portion  75   c  of the holding arm  75  is held in a non-contact state with the transfer belt  21 . 
     In this state, when the primary transfer voltage is applied to the primary transfer rollers BY,  8 M,  8 C and  8 K, the toner images on the photosensitive drums  7 Y,  7 M,  7 C and  7 K are transferred to the transfer belt  21 . When the sheet P passes through the second transfer position between the drive roller  31  and the secondary transfer roller  33 , a secondary transfer voltage is applied to the secondary transfer roller  33 . When the secondary transfer voltage is applied to the secondary transfer roller  33 , the secondary transfer roller  33  transfers the toner image on the transfer belt  21  to the sheet P. Then, the toner image on the sheet P is fixed by the fixing device  36 . By the image forming apparatus  10  of the embodiment, full-color printing is possible. 
     Next, when shifting to the monochrome printing position shown in  FIGS. 3 and 11  from the color printing position, the eccentric cam  49  is rotated by 180 degrees by the control mechanism  46 A of the color photosensitive member. Then, the slider  47  is moved to the right side. The operating arm  54  is rotated clockwise in conjunction with the movement of the slider  47 , and is disengaged from the first position switching roller  42 . The first position switching roller  42  is rotated counterclockwise by the tension of the transfer belt  21 . In conjunction with the movement of the slider  47 , the three arm portions  52  are also each rotated about its respective shaft portion  53 . Thus, the primary transfer rollers  8 Y,  8 M, and  8 C are rotated in a direction away from the photosensitive drums  7 Y,  7 M, and  7 C. The transfer belt  21  is thus separated from the color photosensitive drums  7 Y,  7 M, and  7 C for color. 
     The primary transfer roller  8 K remains on the side of the transfer belt  21 . The transfer belt  21  is set in a monochrome mode in which it is brought into contact only with the photosensitive drum  7 K for monochrome. The second position switching roller  66  presses the transfer belt  21  against the photosensitive drum  7 K (see  FIG. 6 ). 
     The holding arm  75  is kept in a state in which the support portion  75   c  is separated from the transfer belt  21 . 
     Therefore, monochrome printing using only the photosensitive drum  7 K is possible. 
     Next, when shifting to the maintenance position shown in  FIGS. 4 and 12  from the monochrome printing position, the operating lever  77  is rotated counterclockwise by a predetermined angle from the initial position to the operating position in the counterclockwise direction (see  FIG. 9 ). Then, in the second interlocking mechanism  85 , the first sector gear  78  is rotated by the same angle via the shaft portion  77   a , and the second sector gear  80  and the disk  80   a  are rotated by a predetermined angle via the gear  79 . The support shaft  72  is rotated by a predetermined angle from the fitting receiving portion  82  through the fitting projection  83  of the supporting shaft  72  in conjunction with the rotation of the disk  80   a . Then, as shown in  FIG. 7 , the cam  73  of the support shaft  72  is disengaged from the second position switching roller  66 . The second position switching roller  66  is separated from the photosensitive drum  7 K by the urging force of the second coil spring  70 . 
     At the same time, in the first interlocking mechanism  74 , the holding arm  75  is rotated counterclockwise around the axis  75   a  by the rotation of the second sector gear  80  via the pin  80   b.    
     Further, in  FIGS. 4 and 7 , the actuating arm  62  is rotated clockwise about the rotary shaft  61  by being pressed by the cam  73  of the rotating support shaft  72 . Then, since the connecting arm  60  also rotates around the rotary shaft  61 , the primary transfer roller  8 K is separated from the photosensitive drum  7 K. When the second position switching roller  66  and the primary transfer roller  8 K are moved, the transfer belt  21  is separated from the photosensitive drum  7 K by the tension thereof. Since the holding arm  75  is rotated by a predetermined angle, the support portion  75   c  lifts the one end portion of the transfer belt  21  so as to be held in a position separated from the photosensitive drum  7 K. 
     In this manner, by operating the operating lever  77 , the backward movement of the second position switching roller  66  from the photosensitive drum  7 K and the lifting of the transfer belt  21  by the support portion  75   c  of the holding arm  75  are performed in conjunction with each other. 
     In this way, the transfer belt  21  is moved to a position away from the  4  photosensitive drums  7 Y,  7 M,  7 C and  7 K. Sufficient space can be secured between the transfer belt  21  and the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. Even when both ends in the width direction of the transfer belt  21  elongate due to aging, they can be held in a position separated from the photosensitive drums  7 Y,  7 M,  7 C and  7 K by the support portion  75   c . Thus, the transfer belt  21  does not sag toward the photosensitive drums  7 Y,  7 M,  7 C, and  7 K due to its own weight. Therefore, the photosensitive drums  7 Y,  7 M,  7 C and  7 K and the components thereof can be removed without being brought into contact with the transfer belt  21 , and can be replaced. 
     In  FIG. 14 , in the initial position of the operating lever  77 , where copying is possible, the waste toner box  87  is locked to the engagement end portion  87   a . By turning the operating lever  77  to the operating position, the operating lever  77  is disengaged from the engaging end portion  87   a . In this state, the waste toner box  87  can be pulled out from the apparatus main body  11 , so that the waste toner can be discarded. 
     As described above, the image forming apparatus  10  according to the present embodiment and the transfer belt unit  6  thereof are provided. 
     By operating the operating lever  77 , the separation of the second position switching roller  66  from the photosensitive drum  7 K and the lifting of the transfer belt  21  by the support portion  75   c  of the holding arm  75  can be carried out in conjunction with each other. Even if both end portions of the transfer belt  21  elongate due to aging, a sufficient space can be secured at a position where the transfer belt  21  is separated from the photosensitive drums  7 Y,  7 M,  7 C and  7 K. Therefore, the transfer belt  21  can be prevented from being damaged when the photosensitive drums  7 Y,  7 M,  7 C, and  7 K are replaced. 
     Moreover, since the tension of the transfer belt  21  is not increased, the elongation of the transfer belt  21  is not increased. In particular, since both end portions in the width direction of the transfer belt  21  are easily elongated with time, the one end portion of the transfer belt  21  is held by the support portion  75   c . As a result, it is possible to prevent the transfer belt  21  from drooping due to the elongation of the transfer belt  21 . 
     Further, the waste toner box  87  can be pulled out only in the operating position where the transfer belt  21  is held by the support portion  75   c  of the holding arm  75  by the operation of the operating lever  77 . Therefore, it is possible to prevent the transfer belt  21  from being damaged at this point. 
     In the following description of the modification of the present embodiment, the same reference numerals are used for the same or similar parts and elements as those of the embodiment described above. 
     In the embodiment described above, the holding arm  75  is provided at one end portion in the width direction of the transfer belt  21 , and only the end portion of the transfer belt  21  is supported. Alternatively, the holding arms  75  may be provided at both ends in the width direction of the transfer belt  21 . Further, the holding arm  75  may be provided at one end portion or both end portions in the width direction of the transfer belt  21 , and the support portion  75   c  may be extended to the entire length in the width direction. 
     Further, as shown in  FIG. 15 , the support portion  75   c  of the holding arm  75  may be formed to have a convex surface on a surface  75   d  which abuts against the transfer belt  21 . As a result, it is possible to decrease the possibility that the holding arm  75  will damage the transfer belt  21 . As another example, the support portion  75   c  of the holding arm  75  may be formed in a round bar shape. 
     In the embodiment described above, the holding arm  75  is provided on the upstream side of the photosensitive drum  7 K in the moving direction of the transfer belt  21 . Alternatively, the holding arm  75  may be provided on the downstream side of the photosensitive drum  7 K. 
     In addition, the holding arm  75  may be attached to the second position switching roller  66  instead of the structure in which the holding arm  75  is attached to the side wall  76  and the pin  75   b  of the second sector gear  80  is attached to the side wall  76 . In this case, the position of the support portion  75   c  of the holding arm  75  can be switched in conjunction with the switching of the position of the second position switching roller  66 . In addition, the first interlocking mechanism  74  and the second interlocking mechanism  85  may each have a substantially identical configuration. Therefore, the holding arm  75  can hold the transfer belt  21  by the support portion  75   c  in conjunction with the second position switching roller  66 , and can be separated from the photosensitive drum  7 K. 
     In the embodiment described above, the transfer belt unit  6  provided in the image forming apparatus  10  has been described. The transfer belt unit  6  according to the embodiment has been described with reference to a unit including the transfer belt  21 . However, the transfer belt unit  6  of the present embodiment is not limited to the image forming apparatus  10 . The present invention is applicable to various types of transfer belt units  6  having an endless transfer belt  21 . 
     In the embodiment, the second position switching roller  66  is not limited to a roller. It may also be a bar, a plate or the like. These members are included in the switching member. The operating lever  77  may have a disc shape, a handle shape, or the like instead of the lever shape. These are included in the operating member. The second position switching roller  66  is included in the position switching member. The first interlocking mechanism  74  and the first interlocking mechanism  85  are included in the control unit. 
     According to at least one embodiment described above, the first interlocking mechanism  74  and the second interlocking mechanism  85  are interlocked with each other by the operating lever  77 , and the holding arm  75  for holding the end portion of the transfer belt  21  and the second position switching roller  66  for separating the transfer belt  21  from the photosensitive drum  7 K are provided. Therefore, even if both end portions of the transfer belt  21  are elongated due to aging, a sufficient space can be secured at a position where the transfer belt  21  is separated from the photosensitive drums  7 Y,  7 M,  7 C, and  7 K. The transfer belt  21  can be prevented from being damaged when the photosensitive drums  7 Y,  7 M,  7 C and  7 K are replaced. Moreover, there is no need to increase the tension of the transfer belt  21 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.