Patent Publication Number: US-9405216-B2

Title: Tandem type image forming apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of prior U.S. application Ser. No. 14/270,161, filed May 5, 2014, which is a continuation of prior U.S. application Ser. No. 14/052,457, filed Oct. 11, 2013, which is a continuation of prior U.S. application Ser. No. 12/489,722, filed Jun. 23, 2009, which claims priority from Japanese Patent Application No. 2008-164973 filed Jun. 24, 2008. The entire content of the priority applications is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a tandem type image forming apparatus including a plurality of photosensitive drums. 
     2. Description of the Related Art 
     Image forming apparatuses such as printer, copying machine, include a photosensitive drum and a developing device. In view of the fact that endurance limit or service life is not the same in the photosensitive drum and the developing device, the photosensitive drum and developing unit are individually detachably mounted on the image forming apparatus to enable a user to replace the used photosensitive drum or developing unit with a new one. 
     Japanese Patent Application Publication No. 2004-94151 discloses a color printer including a plurality of photosensitive drums, a plurality of developing devices, and a transfer belt unit. The photosensitive drums are mounted from the upper portion of the printer and placed on the frame of the transfer belt so as to confront the corresponding developing devices. When one or more developing devices are replaced with new ones, the transfer belt unit is pivotally moved upward to thereby separate the photosensitive drums from the developing devices, and then the developing device is removed. 
     However, with the configuration disclosed in Japanese Patent Application Publication No. 2004-94151, after separation of the photosensitive drums from the developing devices, almost all part of the photosensitive drum is exposed. Accordingly, attention has to be paid in exchanging the drum so as not to touch the photosensitive drum. Further, the photosensitive drum separated from the developing device still remains in contact with the transfer belt unit. Accordingly, the drum replacing operation has to be performed so as not to damage the photosensitive drum and the transfer belt unit. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide an image forming apparatus with an improved operability in replacing photosensitive drums and/or developing devices. 
     In order to achieve the above and other objects, there is provided an image forming apparatus that includes a main casing, a belt unit, a photosensitive member unit and a developing device unit. The belt unit is supported by the main casing and includes an endless belt and a plurality of transfer members. The photosensitive member unit includes a plurality of photosensitive members disposed in one-to-one correspondence with the plurality of transfer members. Electrostatic latent images are formable on each photosensitive member. The plurality of photosensitive members is arranged along a line extending in a direction orthogonal to the axial direction of the photosensitive member. The photosensitive member unit is loaded in the main casing. The developing device unit includes a plurality of developing devices disposed in one-to-one correspondence with the plurality of photosensitive members. Each developing device includes a developing member that supplies toner to corresponding photosensitive member to develop the electrostatic latent images formed thereon and provide toner images which are transferred onto the endless belt by corresponding transfer member. The photosensitive member unit is held in contact with the endless belt when the toner images are transferred onto the endless belt. The developing device unit is loaded in the main casing. Both the belt unit and the photosensitive member unit are configured to be movable toward and away from the developing device unit. When the photosensitive member unit is separated from the developing device unit, the belt unit and the plurality of photosensitive members are also separated from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view showing a color multi-function peripheral as one example of image forming apparatuses; 
         FIG. 2A  shows a belt support frame as viewed from an internal side of the color multi-function peripheral; 
         FIG. 2B  shows the belt support frame supported on a body case of the color multi-function peripheral; 
         FIG. 3A  is an enlarged diagram showing a structure of a photosensitive drum unit; 
         FIG. 3B  is an explanatory diagram illustrating the relationship between a support protrusion and a guide groove when the photosensitive drum and an intermediate transfer belt are brought into contact with each other; 
         FIG. 4  is an enlarged diagram showing an urging member for urging the photosensitive drum unit; 
         FIG. 5A  shows an internal arrangement of the developing device unit when the photosensitive drums and the corresponding developing rollers are in contact with each other; 
         FIG. 5B  shows an external arrangement of the developing device unit when the photosensitive drums and the corresponding developing rollers are in contact with each other; 
         FIG. 6A  shows an internal arrangement of the developing device unit when the photosensitive drums and the corresponding developing rollers are separated; 
         FIG. 6B  shows an external arrangement of the developing device unit when the photosensitive drums and the corresponding developing rollers are separated; 
         FIG. 7  is an external arrangement of the developing device when the frontmost photosensitive drum and the corresponding developer roller are in contact with each other; 
         FIG. 8A  is a cross-sectional view showing an upper cover pivotally moved upward; 
         FIG. 8B  shows the support protrusion moving along a vertically extending groove; 
         FIG. 9  is a cross-sectional view of the color multi-function peripheral showing a state in which the photosensitive unit is drawn outward; and 
         FIG. 10  is a cross-sectional view of the color multi-function peripheral showing a state in which the developing device unit is drawn outward. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A color multifunctional peripheral (hereinafter simply referred to as “color MFP”) as one of image forming apparatuses will be described with reference to  FIGS. 1 through 10 . In the following description, the terms “upward”, “downward”, “frontward”, “rearward”, “rightward”, “leftward” and the like will be used throughout the description assuming that the color MFP is disposed in an orientation in which it is intended to be used. More specifically, the term “frontward” designates the right side of the color MFP in  FIG. 1 , while the term “rearward” designates the left side of the color MFP in  FIG. 1 . Also, the far side in  FIG. 1  will be referred to as “rightward” while the near side in  FIG. 1  as “leftward”. Further, the upside in  FIG. 1  will be referred to as “upward” and the downside in  FIG. 1  as “downward”. 
     As shown in  FIG. 1 , the color MFP  1  includes a main casing  2 , a flat head scanner  3  disposed above the main casing  2 , a sheet feed section  4 , and an image forming section  5 . The sheet feed section  4  and the image forming section  5  are disposed inside the main casing  2 . The sheet feed section  4  feeds sheets of paper (hereinafter simply referred to as “sheet P”), and the image forming section  5  forms images on the sheet P supplied from the sheet feed section  4 . 
     A front cover  21  is provided in the front side of the main casing  2 , while an upper cover  22  is provided in the upper portion of the main casing  2 . The front cover  21  is pivotally movable in front-to-rear direction about the bottom end thereof. The upper cover  22  has a pivot shaft  22 A at the rear end thereof so that the upper cover  22  is pivotally movable about the pivot shaft  22 A in the up-to-down direction. A discharge tray  22 B is formed in the upper surface of the upper cover  22 A. The sheet P discharged out of the main casing  2  is piled in the discharge tray  22 B. 
     The flat head scanner  3  is a well-known conventional image scanning device. The flat head scanner  3  irradiates light onto a sheet P and scans images on the sheet P, and generates image data according to the scanned images. The flat head scanner  3  is provided above the upper cover  22  (the main casing  2 ) with a space S interposed therebetween. The sheet P is first discharged into the space S from the main casing  2 , and is then stacked on the discharge tray  22 B. 
     The sheet feed section  4  is disposed in the bottom portion of the main casing  2 . The sheet feed section  4  includes a sheet feed cassette  41 , a pick-up roller  42 , a separation roller  43 , sheet feed rollers  44  and  45 . The sheet feed cassette  41  is detachably mounted in the main casing  2  and accommodates sheets P in a stacked state. The pick-up roller  42  is disposed rearward and upward of the sheet feed cassette  41  in the main casing  2 . The pick-up roller  42  conveys the sheets P stacked in the sheet feed cassette  41  to the separation roller  43 . The separation roller  43  separates the conveyed sheets P one by one. The sheet feed rollers  44  and  45  then convey each sheet P upward to pass between an intermediate transfer belt  91  and a secondary transfer roller  93 . 
     An image forming section  5  includes an exposing unit  6 , a photosensitive drum unit  7 , a developing device unit  8 , a belt unit  9  and a fixing unit  10 . 
     The exposing unit  6  is disposed above the sheet feed section  4 . The exposing unit  6  includes a conventional laser generating section, a polygon mirror, a plurality of lenses and a plurality of reflection mirrors, all of which are not shown in the drawings. In the exposing unit  6 , the laser generating section generates laser beams corresponding to yellow, cyan, magenta, and black. The laser beam is reflected upon the reflective mirrors and scanned by the polygon mirror, and finally irradiated onto the surface of the relevant photosensitive drum  71  (to be described later) provided in the photosensitive drum unit  7 . 
     The photosensitive drum unit  7  is disposed above the exposing unit  6  and between the developing device unit  8  and the belt unit  9 . The photosensitive drum unit  7  includes a drum supporting frame  70 , four photosensitive drums  71 , and four chargers  72 . The drum supporting frame  70  defines an outline of the photosensitive drum unit  7 , and the photosensitive drums  71  are juxtaposed within the drum supporting frame  70 . The photosensitive drums  71  are juxtaposed in the front-to-rear direction, i.e., along a line extending in a direction orthogonal to the longitudinal axis of the photosensitive drum  71 . Each of the chargers is disposed in correspondence with each of the photosensitive drums  71 . Detailed configuration of the drum supporting frame  70  will be described later. 
     The developing device unit  8  is disposed between the exposing unit  6  and the photosensitive drum unit  7 . The developing device unit  8  includes a developing device supporting frame  80 , four developing devices  81 , and four developing device holders  82 . The developing device supporting frame  80  constitutes an outer frame of the developing device unit  8 . Each of the developing devices  81  is disposed in correspondence with each of the photosensitive drums  71 , and detachably mounted in each of the developing device holders  82 . Details of the developing device unit  8  will be described later. 
     Each developing device  81  includes a developing roller  81 A, a supply roller  81 B, a toner accommodating chamber  81 C, and an agitator  81 D. The developing roller  81 A is disposed in opposition to the corresponding photosensitive drum  71 . The supply roller  81 B supplies toner (developer) to the developing roller  81 A. The toner accommodating chamber  81 C accommodates toner therein, and the agitator  81 D is rotatably provided within the toner accommodating chamber  81 C for agitating toner. In each developing device  81 , the agitator  81 D is driven to supply toner accommodated in the toner accommodating chamber  81 C to the supply roller  81 B. Toner is then supplied to the developing roller  81 A and thus borne on the surface of the developing roller  81 A. 
     The belt unit  9  is disposed above the photosensitive drum unit  7 . The belt unit  9  includes the intermediate transfer belt  91 , four primary transfer rollers  92 , the secondary transfer roller  93 , a drive roller  94 , a follow roller  95 , and a cleaning section  96 . 
     The intermediate transfer belt  91  is in the form of an endless belt. The drive roller  94  and the follow roller  95  are disposed in spaced-apart relation to each other in the front-to-rear direction. The intermediate transfer belt  91  is stretched around the drive roller  94  and the follow roller  95  and has an outer circumferential surface. Each of the photosensitive drums  71  opposes and contacts the outer circumferential surface of the intermediate transfer belt  91  at the bottom thereof. The secondary transfer roller  93  is disposed in opposition to and in contact with the outer circumferential surface of the intermediate transfer belt  91  at the rear side thereof. 
     Each of the primary transfer rollers  92  is provided inside the loop of the intermediate transfer belt so as to be in contact with the inner circumferential surface of the intermediate transfer belt  91 . Each primary transfer roller  92  is also disposed in opposition to corresponding photosensitive drum  71  so that primary transfer roller  92  and the corresponding photosensitive drum  71  can nip the intermediate transfer belt  91  therebetween. The secondary transfer roller  93  is disposed in opposition to the drive roller  94  so that the intermediate transfer belt  91  can be nipped therebetween. A transfer bias is applied to the primary transfer rollers  92  and the secondary transfer roller  93  during transfer operations. 
     The cleaning unit  96  is disposed above the intermediate transfer belt  91  at the front side thereof. The cleaning unit  96  includes a cleaning roller  96 A and a toner storage section  96 B. The cleaning roller  96 A cleans the toner remaining on the intermediate transfer belt  91 , and the toner storage section  96 B stores the removed toner. Note that the cleaning unit  96  is detachably mounted in the color MFP  1  through an opening (not shown) formed when a second front cover  22 C provided in the upper cover  22  is opened. 
     The fixing unit  10  is disposed above the secondary transfer roller  93  and the drive roller  94 . The fixing unit  10  includes a heat roller  11  and a pressure roller  12  disposed in opposition to the heat roller  11 . The pressure roller  12  applies pressure to the heat roller  11 . 
     In the image forming section  5  with the above-described configuration, each surface of the photosensitive drums  71  is first uniformly charged by the corresponding charger  72 , and then exposed to a laser beam emitted from the exposing unit  6 . Areas exposed to the laser beam have a lower potential, thereby forming an electrostatic latent image on the surface of each photosensitive drum  71  based on image data. 
     Toner borne on the surface of the developing roller  81 A is supplied to the surface of the photosensitive drum  71  on which the electrostatic latent image is formed. In this way, toner is selectively carried on the surface of the photosensitive drum  71 , and the latent image formed thereon is developed into a visible toner image. 
     The toner images borne on the surfaces of the photosensitive drums  71  are then transferred onto the surface of the intermediate transfer belt  91  so as to be sequentially superimposed one on the other by virtue of the transfer bias applied to the primary transfer rollers  92 . The full-color toner images on the surface of the intermediate transfer roller  91  are then transferred onto the sheet P when the sheet P passes between the intermediate transfer belt  91  and the secondary transfer roller  93  to which a transfer bias is applied. 
     The sheet P on which the full-color toner images are transferred is then conveyed to the fixing unit  10 . The toner images are then thermally fixed to the sheet P when the sheet P passes between the heat roller  11  and the pressure roller  12 . The sheet P is then discharged into the space S out of the main casing  2  by the discharge rollers  23 , and is finally stacked on the discharge tray  22 B. 
     Next, detailed configurations of the belt unit  9 , the photosensitive unit  7 , and the developing device unit  8  will be described. 
     &lt;Configuration of Belt Unit&gt; 
     As shown in  FIG. 2A , the belt unit  9  includes a pair of belt support frames  90  in addition to the intermediate transfer belt  91 , primary transfer rollers  92 , secondary transfer roller  93 , drive roller  94 , follow roller  95 . It should be noted that  FIG. 2A  shows only a counterpart of the belt support frame  90 . 
     The pair of belt support frames  90  is disposed in confrontation with each other at both widthwise ends of the intermediate transfer belt  91 . The direction in which the belt support frames  90  confront is in coincidence with left-to-right direction or the axial direction of the photosensitive drum  71 . The pair of belt support frames  90  supports the primary transfer rollers  92 , drive roller  94 , follow roller  95  and the cleaning section  96  (not shown in  FIGS. 2A and 2B ). As can be seen from  FIG. 2B , both ends of the rotation shaft  94 A of the drive roller  94  project outward from the belt support frames  90 . Specifically, the right end of the rotation shaft  94 A projects rightward from the right side belt support frame  90  and the left end of the rotation shaft  94 A projects leftward from the left side belt support frame  90 . Similar to the rotation shaft  94 A, both ends of the rotation shaft  95 A of the follow roller  95  project outward from the belt support frames  90 . 
     The inner surface of each belt support frame  90  is formed with first and second guide grooves  97  and  98  both extending rearward, i.e., in the direction in which the photosensitive drums  71  are arranged, from the frontmost end of the belt support frame  90 . Stated differently, the first and second guide grooves  97 ,  98  extend in the lengthwise direction of the intermediate transfer belt  91 . The first guide groove  97  has a U-shaped cross-section and is formed at a position lower than the intermediate transfer belt  91 . The first guide groove  97  is made up of a horizontally extending portion  97 A and a vertically extending portion  97 B vertically extending upward from the rearmost end of the horizontally extending portion  97 A, thereby forming an L-shape. The horizontally extending portion  97 A extends horizontally or in the front-to-rear direction from the frontmost end of the belt support frame  90  to the portion near the rearmost end thereof. The vertically extending portion  97 B is formed at the rear end of the horizontally extending portion  97 A and bent upward to face the intermediate transfer belt  91 . 
     Similar to the first guide groove  97 , the second guide groove  98  has a U-shaped cross-section. The second guide groove  98  is formed at the front portion of the belt support frame  90  and below the first guide groove  97 . The second guide groove  98  is also substantially of an L-shaped having a horizontally extending portion  98 A and a vertically extending portion  98 B. The horizontally extending portion  98 A extends horizontally or in the front-to-rear direction from the frontmost end of the belt support frame  90 . The horizontally extending portion  98 A is not as long as the corresponding portion  97 A of the first guide groove  97 . The vertically extending portion  98 B is formed at the rear end of the horizontally extending portion  98 A and bent upward to face the intermediate transfer belt  91 . 
     As shown in  FIG. 2B , the belt support frames  90  are supported on the main casing  2  via the rotation shafts  94 A,  95 A. More specifically, an elongated hole  22 E is formed in a support panel  22  attached to the front portion of the upper cover  22 , and the rotation shaft  95 A is penetrated into the elongated hole  22 E. The belt support frames  90  (belt unit  9 ) is thus pivotally movable about the rear-side rotation shaft  94 A in interlocking relation with the vertical movements of the upper cover  22  (see  FIG. 8A ). 
     &lt;Configuration of Photosensitive Drum Unit&gt; 
     As shown in  FIG. 3A , the photosensitive drum unit  7  includes four photosensitive drums  71  arranged in front-to-rear direction, four charging devices  72  arranged in association with the respective ones of the photosensitive drums  71  individually, and a drum support frame  70  for supporting the photosensitive drums  71  and the charging devices  72 . The drum support frame  70  is configured from a pair of side plates (only a counterpart thereof is shown in  FIG. 3A ) disposed in confrontation with each other. The direction in which the side plates confront is in coincidence with the right-to-left direction. First support protrusion  73  projects outward from each side plate of the drum support frame  70 . More specifically, one first support protrusion  73  projects leftward from the left side plate and another first support protrusion  73  projects rightward from the right side plate. Similar to the first support protrusions  73 , second support protrusions  74  project outward from the side plates of the drum support frame  70 , one projecting leftward from the left side plate and another projecting rightward from the right side plate. 
     The first support protrusions  73  are provided at the rear upper portions of the drum support frame  70  and engage the first guide grooves  97  formed in the belt support frame  90 . As shown in  FIG. 3B , the first protrusion  73  is disposed in the upper end of the vertically extending portion  97 B of the first guide groove  97  when the photosensitive drums  71  are in contact with the intermediate transfer belt  91 . 
     The second support protrusions  74  are provided vertically centrally of the drum support frame  70  at the front portions thereof and engage the second guide grooves  98  formed in the belt support frame  90 . More specifically, one second support protrusion  74  projects leftward from the left side belt support frame  90  through the second guide groove  98  and the other second support protrusion  74  project rightward from the right side belt support frame  90  through the second guide groove  98 . The projected length of the second support protrusions  74  is longer than that of the first support protrusions  73 . Similar to the first protrusions  73 , the second protrusions  74  are disposed in the upper end of the vertically extending portions  98 B of the second guide grooves  98  when the photosensitive drums  71  are in contact with the intermediate transfer belt  91 . 
     When the first and second protrusions  73 ,  74  engage the first and second guide grooves  97 ,  98 , respectively, vertical movements of the support frame  70  (photosensitive drum unit  7 ) relative to the belt support frame  90  (belt unit  9 ) are only possible in a limited range but are basically restricted. As shown in  FIG. 8A , the drum support frame  70  (photosensitive drum unit  7 ) is vertically swingable about the rotation shaft  94 A together with the belt unit  9  in interlocking relation with the vertical movements of the upper cover  22 . 
     According to the above-described configuration, operations for moving the photosensitive drum unit  7  and the belt unit  9  can be easily performed in comparison with the case in which the photosensitive drum unit and the belt unit are individually and independently moved. Further, an arrangement for vertically swinging the photosensitive drum unit  7  and the belt unit  9  is advantageous in that the structure can be simplified and operation can be easily performed in comparison with the arrangement in which the photosensitive drum unit and the belt unit are vertically slidably moved. 
     Inner surface of each side plate of the drum support frame  70  is formed with four grooves  75  for guiding the movements of the photosensitive drum unit  7  and for determining the positions of the developing devices  81  relative to the corresponding photosensitive drums  71 . In the rear lower portion of the drum support frame  70 , an engagement concave portion  70 A is formed. 
     Next, description will be made with respect to an arrangement for determining the position of the photosensitive drum unit  7  with respect to the main casing  2 . Operation of such an arrangement will be described later. 
     As shown in  FIG. 2B , a pair of side frames  24  (only a counterpart thereof is shown) is disposed within the main casing  2  in confronting relation with each other. The direction in which the side frames  24  confront is in coincidence with the right-to-left direction. A rod-shaped member  25  is bridged between the pair of side frames  24  at the rear lower portion of the belt support frame  90 . A photosensitive drum unit urging unit  26  is provided in the front portion of each side frame  24  for urging the photosensitive drum unit  7  to maintain the same in the loaded condition. 
     As shown in  FIG. 4  the urging unit  26  is disposed between the image forming section  5  (photosensitive drum unit  7  and belt unit  9 ) and the side frames  24 . The urging unit  26  includes an urging arm  26 A and coil spring  26 B. The urging arm  26 A is pivotally movable in front-to-rear direction relative to the main casing  2 . A supporting shaft  26 C extends in the left-to-right direction (toward forehand side in the sheet of  FIG. 4 ) is rotatably supported on the side frames  24  (not shown in  FIG. 4 ). Accordingly, the urging arm  26 A with the lower end attached to the supporting shaft  26 C is pivotally movable in the front-to-rear direction relative to the main casing  2 . The coil spring  26 B has one end attached to the front-side lower portion of the urging arm  26 A and another end attached to the rear-side of a spring support portion  24 A projecting inward in the left-to-right direction from the side frames  24 . 
     &lt;Configuration of Developing Device Unit&gt; 
     As shown in  FIGS. 5A and 5B , the developing device unit  8  includes a developing unit support frame  80 , developing devices  81 , developing device holders  82 , coil springs  83  and operation bars  84 . 
     The developing device supporting frame  80  is formed in a substantially box shape with an upper open surface. The developing device supporting frame  80  has side walls opposing to each other in the front-to-rear direction. That is, the side walls are disposed in a direction orthogonal to the axial direction of the developing rollers  81 A. A plurality of through-holes  80 A is formed on each side wall and each through-hole  80 A has an elongated shape. A plurality of openings is formed on bottom surface of the developing device supporting frame  80  so that laser beams emitted from the exposing unit  6  can pass through the openings. 
     A pair of guide sections  80 B and  80 C is formed on outer surface of each side wall, and the guide sections  80 B and  80 C extend in the front-to-rear direction at the bottom portion of the side walls. A roller  80 D is rotatably provided on the rear end of the guides  80 B and  80 C. A guide  27 , a stopper  28  and a roller  29  are provided in the main casing  2 . The guide  27  is provided to be positioned above the guide section  80 C and extends in the front-to-rear direction along the guide section  80 C. The stopper  28  is formed on the rear end of the guide  27  so as to prevent the roller  80 D from moving further rearward. The roller  29  is rotatably disposed above the front end of the guide  27 . 
     Each of the developing device holders  82  has a supporting shaft  82 A. The developing device holders  82  are pivotally movably supported to the side walls of the developing device supporting frame  80  so that the developing device holders  82  are pivotally movable about the supporting shafts  82 A relative to the developing device supporting frame  80 . Each of the developing device holders  82  is formed with an operation portion  82 B. The operation portion  82 B protrudes outward in the left-to-right direction from the developing device holder  82  and penetrates into the through-hole  80 A formed on the side walls of the developing device supporting frame  80 . Each of the operation portions  82 B is slidably movable within the through-holes  80 A substantially vertically in conjunction with the pivotal movements of the developing device holder  82 . A coil spring  83  is provided between the bottom surface of the developing device supporting frame  80  and a bottom end of each developing device holder  82  so that the developing device holders  82  are urged toward the respective photosensitive drums  71 . 
     The operation bar  84  is movably provided on the outer surface of each side wall of the developing device supporting frame  80 . The operation bar  84  is movable relative to the developing device supporting frame  80  in the front-to-rear direction, i.e. in a direction in which the developing rollers  81 A are arranged. The operation bar  84  is formed with four first recesses  84 A and a second recess  84 B. The first recesses  84 A are engageable with the operation portions  82 B. The first recesses  84 A are equally spaced apart in the front-to-rear direction to correspond to the arrangement of the operation portions  82 B. The second recess  84 B is designed to be engageable only with the frontmost operation portion  82 B. 
     A rack gear section  84 G is formed in the operation bar  84  at the rear bottom end thereof. A transmission gear  80 G is provided on the outer surface of each of the side walls at the rear end thereof and is meshingly engaged with the rack gear section  84 G. The operation bar  84  is moved in the front-to-rear direction with a driving force transmitted from the transmission gear  80 G. The transmission gear  80 G is also meshingly engaged with a drive gear  2 G provided in the main casing  2 . The drive gear  2 G is driven by a control section (not shown). The transmission gear  80 G follows the rotations of the drive gear  2 G and transmits the driving force to the rack gear section  84 G. 
     Here, description will be made with respect to operations for bringing the developing rollers  81 A in contact with or away from the photosensitive drums. 
     As shown in  FIGS. 5A and 5B , during color image forming operations, the operation portions  82 B engage the corresponding first recesses  84 A, and all of the developing device holders  82  (developing devices  81 ) are urged toward the corresponding photosensitive drums  71  by the coil springs  83 . In this way, all developing rollers  81 A are brought into contact with the corresponding photosensitive drums  71 . 
     When the MFP  1  is in a standby condition, or when cleaning of the photosensitive drums  71  and/or the intermediate transfer belt  91  is performed, or when replacement of the photosensitive drum unit  7  and/or developing device unit  8  is performed, the operation bar  84  is driven to move backward from the position shown in  FIG. 5B  by the rotations of the drive gear  2 G and the transmission gear  80 G. As a result, as shown in  FIGS. 6A and 6B , engagement between the operation portions  82 B and the corresponding first recesses  84 A is released, and all the operation portions  82 B are pushed downward by the operation bar  84 . In this way, all developing device holders  82  (developing device  81 ) are also pushed downward against the urging force of the coil springs  83 , and all of the developing rollers  81 A are separated from the corresponding photosensitive drums  71 . 
     All the developing rollers  81 A and the photosensitive drums  71  are brought into contact with each other again if the operation portions  82 B and the corresponding first recesses  84 A are brought in engagement with each other with the operation bar  84  moving forward from the position shown in  FIG. 6B  by the rotations of the drive gear  2 G and the transmission gear  80 G. 
     When the drive gear  2 G and the transmission gear  80 G rotate and drive the operation bar  84  to move forward from the position shown in  FIG. 5B , all the operation portions  82 B are disengaged from the corresponding first recesses  84 A as shown in  FIG. 7 , but only the frontmost operation portion  82 B engages the second recess  84 B. At this time, only the frontmost developing roller  81 A is brought in contact with the corresponding photosensitive drum  71  thereby enabling to perform image forming operations with the combination of the frontmost developing device  81  and the frontmost photosensitive drum  71 . The remaining three developing rollers  81 A are pushed downward by the operation bar  84  against the urging fore of the coil springs  83  and thus separated from the corresponding photosensitive drums  71 . In this way, the MFP  1  can perform monochromic printing in this state. 
     In the present embodiment, positions of the developing devices  81  can be easily changed by the movements of the operation bar  84 . The developing rollers  81 A and the photosensitive drums  71  are thus easily brought into contact with or away from each other. 
     Positions of the developing devices  81  are changed via respective developing device holders  82 , so that rigidity of the developing device supporting frame  80  is improved. More specifically, if the developing devices themselves are formed with the operation portions protruding outward from the side walls of the developing device supporting frame, grooves are necessary to be formed on each side wall from the upper end thereof to extend substantially downward for detachment of the developing devices. The above-described configuration according to the present embodiment achieves higher rigidity than the latter case because each side wall of the developing device supporting frame  80  is formed with the though-holes  80 A rather than the grooves. 
     Note that, in the color MFP  1  according to the present embodiment, the developing rollers  81 A and the photosensitive drums  71  are controlled to be separated from each other except when the developing rollers  81 A supplies toner to the photosensitive drums  71  during image forming operations, for example. Also note that, the operations described above are controlled to be executed when the photosensitive drum unit  7  and the developing device unit  7  are mounted in the main casing  2  and the upper cover  22  is closed. 
     &lt;Operation of Color MFP&gt; 
     Replacing operation of the photosensitive drum unit  7  and/or the developer device unit  8  in the color MFP  1  as arranged above will be described with reference to  FIGS. 8A, 8B, 9 and 10 . 
     As shown in  FIG. 8A , moving the upper cover  22  upward causes both the belt unit  9  and the photosensitive drum unit  7  to pivotally move upward about the rotation shaft  94 A. As a result, the belt unit  9  and the photosensitive drum unit  7  are separated from the developing device unit  8 . 
     At this time, the support protrusions  73 ,  74  move downward as shown in  FIG. 8B  along the vertically extending portions  97 B,  98 B of the first and second guide grooves  97 ,  98  and reach the rearmost ends of the horizontally extending portions  97 A,  98 A. By this downward movements of the support protrusions  73 ,  74 , the overall photosensitive drum unit  7  moves downward, thereby separating the photosensitive drums  71  from the intermediate transfer belt  91  as shown in  FIG. 8A . 
     When the belt unit  9  is pivotally moved upward to be separated from the developing device unit  8 , the front portion of the belt unit  9  moves into space S. Stated differently, the belt unit  9  is configured so that its movement is stopped in the space S. As such, moving distances of the belt unit  9  and the photosensitive drum unit  7  are small, so that operations for pivotally moving the belt unit  9  and the photosensitive drum unit  7  can readily be performed. Besides, the belt unit  9  and the photosensitive drum unit  7  can be made compact. Additionally, there is no need to move or retract the flat head scanner  3  when the belt unit  9  and the photosensitive drum unit  7  are pivotally moved, so that the structures of the main casing  2  and the flat head scanner  3  can be made simple. 
     As shown in  FIG. 9 , when the photosensitive drum unit  7  is replaced with a new one, the handle  70 H provided at the frontmost end of the drum supporting frame is drawn frontward to move the respective support protrusions  73 ,  74  along the corresponding horizontally extending portions  97 A,  98 A of the first and second guide grooves  97 ,  98 . Then, the photosensitive drum unit  7  can be easily removed or drawn out from the main casing  2  along the direction in which the photosensitive drums  71  are arranged. Afterwards, a new photosensitive drum unit  7  can be mounted in the main casing  2  through the reverse operation. 
     As described above, the color MFP  1  according to the embodiment of the invention, separation of the intermediate transfer belt  91  and the photosensitive drums  71  can be achieved by separating both the belt unit  9  and the photosensitive drum unit  7  from the developing device unit  8  by pivotally moving the belt unit  9  and the photosensitive drum unit  7  upward, therefore, the photosensitive drum unit  9  including a plurality of photosensitive drums  71  can be extracted from the main casing  2 . Replacement of the photosensitive drums  71  can be achieved only by upwardly pivotally moving the belt unit  9  and the photosensitive drum unit  7 . Hence, operability in the case of replacement of the photosensitive drum is greatly improved. 
     In order to replace any one or all of the developing devices mounted on the developing device unit  8  with a new one, the front cover  21  is firstly moved frontward, and subsequently the handle  80 H provided at the frontmost end of the developing device supporting frame  80  is drawn frontward, as shown in  FIG. 10 . Drawing the handle  80 H frontward causes the roller  80 D to move along the guide section  27 . While the roller  29  is being moved by the guide section  80 B, the developing device supporting frame  80  moves in the direction in which the developing rollers  81 A are arranged, thus the developing device unit  8  can be retracted from the main casing  2 . The developing device  81  subject to replacement is dismounted from the developing device holder and a new developing device  81  is mounted thereon. Afterwards, the developing device support holder  80  is pushed rearward to mount the developing device unit  8  in the main casing  2 . 
     Next, description will be made with respect to positioning of the photosensitive drums  71  with the corresponding developing devices  81 . After the photosensitive drum  71  and/or the developing device  81  is replaced with a new one and both the photosensitive drum unit  7  and the developing device unit  8  are mounted in the main casing  2 , the front cover  21  is closed and then the upper cover  22  is moved downward. Through those operations, both the belt unit  9  and the photosensitive drum unit  7  pivotally move downward about the rotation shaft  94 A to move to positions close to the developing device unit  8  (see  FIG. 6 ). 
     At this time, as shown in  FIG. 6B , the rotation shaft  81 E of the developing roller  81 A extending outward (leftward and rightward) from the frame of the developing device  81  is brought into engagement with the groove section  75  of the photosensitive drum unit  7 . This engagement can determine the position of the developing device  81  with respect to the corresponding photosensitive drum  71 . It should be noted that the groove section  75  also serves as a guide for guiding the movements of the photosensitive drum unit  7  per se. 
     As shown in  FIG. 2B , the drum supporting frame  70  is brought into abutment with the bar-shaped member  25 . More specifically, the concave portion  70 A formed in the drum supporting frame  70  engages the bar-shaped member  25 , enabling positioning of the drum supporting frame  70  with respect to the main casing  2  which in turn enables the positioning of the photosensitive drum  71  supported on the drum supporting frame  70 . 
     Further, as shown in  FIG. 4 , the second support protrusions  74 , which are formed in the drum supporting frame  70 , and penetrate into the second guide groove  98  formed in the belt supporting frame  90  and project outward (rightward and leftward) from the belt supporting frame  90 , are urged rearward by the urging arm  26 A and the coil spring  26 B. In this manner, the drum supporting frame  70  is urged against the bar-shaped member  25 , so that the photosensitive drum  71  (drum supporting frame  70 ) can be accurately positioned with respect to the main casing  2 . 
     Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. 
     For example, the above-described embodiment shows the intermediate transfer belt  91  as one example of belts, the belt to which the present invention is applied is not limited to the intermediate transfer belt but the present invention is applicable to any other types of belts such as a paper conveying belt. 
     In the above-described embodiment, an arrangement for moving the photosensitive drum  71  toward and away from the intermediate transfer belt  91  is such that the vertically extending portions  97 B,  98 B are continuously formed to the ends of the guide grooves  97 ,  98  for moving the support protrusions  73 ,  74  therealong. However, the present invention is not limited to such an arrangement. Under the condition in which the belt unit and the photosensitive drum unit are separated from the developing device unit, separation of the photosensitive drum from the belt unit can be achieved by changing the moving distances or pivotal angles of the belt unit and the photosensitive drum unit. 
     The above-described embodiment shows an example in which the bar-shaped member  25  is provided at the rear side of the photosensitive drum unit  7 . However, the present invention is not limited to such an example. The bar-shaped member  25  may be provided in the front side or centrally of the photosensitive drum unit  7 . Further, various types of well-known arrangements can be employed in lieu of the bar-shaped member for positioning the photosensitive drum  71 . 
     When the bar-shaped member is provided in the front side of the photosensitive drum unit  7 , it is desirable that the urging member for urging the photosensitive drum unit be provided at the rear side relative to the bar-shaped member. For the arrangement in which the bar-shaped member is provided centrally of the photosensitive drum unit and the bar-shaped members engage concave portions formed in the lower part of the drum supporting frame, it is desirable that the direction in which the photosensitive drum unit is urged be appropriately determined depending upon the position in which the urging member for urging the photosensitive drum unit is provided. 
     The above-described embodiment shows an example in which the two guide grooves  97 ,  98  are formed in the two belt supporting frames  90 , respectively. However, the present invention is not limited to such an example. For example, the number of guide grooves may either be one or three. In such cases, the number of support protrusions formed in the drum supporting frames has to be determined depending upon the number of guide grooves in due course. 
     The above-described embodiment shows an example in which the guide grooves  97 ,  98  are formed in the belt supporting frames  90  and support protrusions  73 ,  74  are formed in the drum supporting frames  70 . However, the present invention is not limited to such example. For example, an arrangement for drawing the photosensitive drum unit from the main casing  2  may be such that the support protrusions are formed in the supporting frames, and the guide grooves in the drum supporting frames. 
     In the above embodiment, the first recesses  84 A and the second recess  84 B are formed on the operation bar  84  and each is designed to be engageable with respective ones of the operation portions  82 B. In accordance with movements of the operation bar  84 , the developing rollers  81 A and the photosensitive drums  71  are brought in close to each other when the operation portions  82 B are engaged with the first recesses  84 A and the second recess  84 B, while the developing rollers  81 A and the photosensitive drums  71  are in separation to each other when the operation portions  82 B are disengaged from the first recesses  84 A and the second  84 B. However, the present invention is not limited to the above described embodiment. For example, protrusions may be formed on the bottom surface of the operation bar. In this case, in accordance with the movements of the operation bar, the developing rollers and the photosensitive drums are designed to be distanced from each other when the protrusions push the operation portions downward, while the developing rollers and the photosensitive drums are designed to be close to each other when the operation portions are disengaged from the protrusions in conjunction with the urging force of the urging members. Note that, in this case, as long as the color MFP can perform printing, the developing rollers may not necessarily be in direct contact with the photosensitive drums, but may be disposed in a position slightly distanced from the photosensitive drums. 
     The developing device holder  82  is pivotally supported to the developing device supporting frame  80  in the above-described embodiment. However, as a variation, the developing devices may be configured to be slidably movable relative to the developing device supporting frame  80 . Alternatively, the developing device unit may have such a configuration that the developing devices are directly supported to the supporting frame without employing developing device holders. 
     The coil springs  83  are employed as an urging member to urge the developing device holders  82  in the above-described embodiment, but leaf springs or extension springs may be used instead of the coil springs  83 . The photosensitive drum unit  7  and the developing device unit  8  are designed to be pulled out from the front side of the main body  2  in the front-to-rear direction in the above-described embodiment. However, the photosensitive drum unit  7  and the developing device unit  8  may be configured to be taken out from either left or right side of the main body  2  of the MFP  1 . 
     The color MFP is described as an example of image forming devices according to an embodiment of the present invention. This invention can be applied to a copier, and also to a printer that is not provided with a flat head scanner (an image scanning unit). Although the exposing unit  6  employs a laser as a light source in the above-described embodiment, a light emitted from LEDs, electroluminescence elements and florescent materials may also be employed to expose the photosensitive drums  71  to light. 
     In the above described embodiment, a contact development type image forming device is described in which toner borne on the surfaces of the developing rollers  81 A is supplied to the surfaces of the photosensitive drums  71  when the developing rollers  81 A and the photosensitive drums  71  are in contact with each other. However, the present invention is not limited to the above embodiment. The present invention may be applied to a non-contact type image forming device in which developer borne on developing rollers is supplied to photosensitive drums when the photosensitive drums and the developing rollers are close to, but not in direct contact with each other.