Patent Publication Number: US-7715756-B2

Title: Image forming apparatus with linear movement member

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2005-376117, filed Dec. 27, 2005, the entire contents of which are hereby incorporated by reference into the present application. 
   TECHNICAL FIELD 
   Aspects of the present invention relate to an image forming apparatus such as a laser printer. 
   BACKGROUND 
   An image forming apparatus of electro-photographic system such as a laser printer is generally provided with a photosensitive drum on which an electrostatic latent image is formed. A developing cartridge for developing the electrostatic latent image into a toner image is detachably mounted to the image forming apparatus. 
   A cover is provided on a body of the apparatus so as to be opened/closed. By opening this cover, the developing cartridge can be mounted to and detached from the apparatus body. 
   The developing cartridge is provided with a developing roller for supplying toner to the photosensitive drum. This developing roller can be pressed against or separated from the photosensitive drum in a state where the developing cartridge has been mounted to the apparatus body. 
   As a structure for allowing the developing roller to be pressed against or separated from the photosensitive drum, a structure including a lever for pressing a developing unit which holds the developing roller, a guide member for operating the lever, and a guide cam for rendering this guide member to reciprocally and linearly move has been proposed (see JP-A-2002-6716, for example). When an image is to be formed, the guide member is moved in one direction following rotation of the guide cam, and the lever is separated from the developing unit, whereby the developing roller is pressed against the photosensitive drum. On the other hand, when an image is not formed, the guide member is moved in the other direction following the rotation of the guide cam, so that the lever presses the developing unit, whereby the developing roller is separated from the photosensitive drum. 
   SUMMARY 
   In the conventional image forming apparatus, when the developing cartridge is to be detached from the apparatus body by opening the cover, the developing roller is not necessarily separated from the photosensitive drum. In some cases, the developing roller is kept pressed against the photosensitive drum. For example, in case where electric power is suddenly cut off during image forming operation or immediately after the image forming operation has finished, the developing roller is kept in the state pressed against the photosensitive drum. When the developing cartridge is detached from the apparatus body, smooth detaching operation of the developing cartridge may not be attained, in case where the developing roller is kept pressed against the photosensitive drum. 
   Aspects of the invention provide an image forming apparatus in which pressure of a developing agent carrier against an image carrier is removed when a cover is opened. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional side view showing a color laser printer according to an aspect of the invention; 
       FIG. 2  is a sectional side view showing a developing cartridge and a drum sub-unit shown in  FIG. 1 ; 
       FIG. 3  is a perspective view of a drum unit (in a state where four developing cartridges are mounted thereto) shown in  FIG. 1 , as seen from above at a left rear side; 
       FIG. 4  is a perspective view of the drum unit (in a state where one of the developing cartridges is being mounted or detached, while the other developing cartridges are detached) shown in  FIG. 1 , as seen from above at a left front side; 
       FIG. 5  is a right side view of the drum unit shown in  FIG. 1 ; 
       FIG. 6  is a perspective view of the developing cartridge shown in  FIG. 1 , as seen from a left rear side, showing a tilted state of a handle; 
       FIG. 7  is a perspective view of the developing cartridge shown in  FIG. 1 , as seen from a left rear side, showing an erected state of the handle; 
       FIG. 8  is a perspective view of the developing cartridge shown in  FIG. 1 , as seen from a left front side, showing the tilted state of the handle; 
       FIG. 9  is a perspective view of the developing cartridge shown in  FIG. 1 , as seen from a left front side, showing the erected state of the handle; 
       FIG. 10  is a plan view of the developing cartridge shown in  FIG. 1 ; 
       FIG. 11  is a right side view of the developing cartridge shown in  FIG. 1 ; 
       FIG. 12  is a sectional view taken along a line A-A in  FIG. 11 ; 
       FIG. 13  is a right side sectional view of the developing cartridge shown in  FIG. 1 , showing the tilted state of the handle; 
       FIG. 14  is a right side sectional view of the developing cartridge shown in  FIG. 1 , showing the pressurized state of the handle; 
       FIG. 15  is a perspective view of a body casing and a drum unit shown in  FIG. 1 , as seen from above at a right front side, in a state where an exterior board and a front cover of the body casing are removed and the drum unit is mounted to the body casing; 
       FIG. 16  is a perspective view of the drum unit, right and left rails, and a separating and pressing mechanism shown in  FIG. 15 , as seen from above at a right front side; 
       FIG. 17  is a perspective view of the rails, and the separating and pressing mechanism shown in  FIG. 15 , as seen from above at a right front side; 
       FIG. 18  is a perspective view of linear movement cam members, intermediate members, and synchronous movement mechanism shown in  FIG. 17 , as seen from above at a right front side; 
       FIGS. 19A to 19E  are perspective views for explaining movements of the linear movement cam member and the intermediate members, shown in  FIG. 18 ; 
       FIG. 20  is a right side view of the linear movement cam member and the intermediate members in a state of  FIG. 19A ; 
       FIG. 21  is a right side view of the linear movement cam member and the intermediate members in a state of  FIG. 19C ; 
       FIG. 22  is a right side view of the linear movement cam member and the intermediate members in a state of  FIG. 19E ; 
       FIG. 23  is a schematic left side view of a driving mechanism for moving the linear movement cam member between a pressing position and a releasing position, in a state where a front cover is closed; 
       FIG. 24  is a schematic left side view of the driving mechanism for moving the linear movement cam member between the pressing position and the releasing position, in a state where the front cover is being opened; 
       FIG. 25  is a schematic left side view of the driving mechanism for moving the linear movement cam member between the pressing position and the releasing position, in a state where the front cover is opened; and 
       FIG. 26  is a sectional view of a first clutch mechanism shown in  FIG. 23 . 
   

   DETAILED DESCRIPTION 
   General Overview 
   According to a first aspect of the invention, there is provided an image forming apparatus comprising: an image carrier on which an electrostatic latent image is formed; a developing agent carrier which supplies developing agent to the image carrier; a linear movement member which reciprocally and substantially linearly moves between a pressing position for pressing the developing agent carrier against the image carrier and a releasing position for releasing pressure; a drive source; a cover which is provided on a body of the apparatus so as to be opened and closed; a first input member which inputs a driving force from the drive source to the linear movement member as a force for moving the linear movement member; and a second input member which inputs a force for moving the linear movement member in conjunction with an opening motion of the cover. 
   According to the above described structure, while the cover is closed, it is possible to press the developing agent carrier against the image carrier or to release the pressure, by inputting the driving force from the drive source to the linear movement member by way of the first input member thereby to move the linear movement member between the pressing position and the releasing position. Moreover, because the linear movement member is moved to the releasing position by the force inputted to the linear movement member by way of the second input member in association with the opening motion of the cover, the pressure of the developing agent carrier against the image carrier can be released, in the state where the cover is opened. For this reason, the developing cartridge can be smoothly detached from the apparatus body, even in a structure where the developing agent carrier is provided in a developing cartridge which is detachably mounted to the apparatus body. 
   According to a second aspect of the invention, the linear movement member is integrally provided with a first rack gear and a second rack gear, the first input member includes a first input gear which inputs the driving force from the drive source to the first rack gear, and the second input member includes a second input gear which inputs the force for moving the linear movement member to the second rack gear. 
   According to the above described structure, because the first rack gear and the second rack gear are integrally provided on the linear movement member, it is possible to reliably move the linear movement member by the force inputted to the first rack gear by way of the first input gear, and also possible to reliably move the linear movement member by the force inputted to the second rack gear by way of the second input gear. In this manner, pressure of the developing agent carrier against the image carrier can be reliably exerted and released. 
   According to a third aspect of the invention, the image forming apparatus further comprises a first clutch mechanism capable of being switched to a transmission state in which the driving force from the drive source is transmitted to the first input gear and to an interruption state in which transmission of the driving force to the first input gear is interrupted. 
   According to the above described structure, the transmission and interruption of the driving force from the drive source to the first input gear can be switched by the first clutch mechanism. Therefore, it is possible to transmit the driving force from the drive source to the first input gear, and to move the linear movement member by the driving force. It is also possible to interrupt the driving force from the drive source to the first input gear, and to move the linear movement member by the force which is inputted from the second input gear to the second rack gear, irrespective of the drive source. In other words, in case where the linear movement member is moved by the driving force of the drive source, the driving force is transmitted to the first input gear, and in other cases, connection between the drive source and the first input gear is interrupted, whereby it is possible to prevent the drive source from hindering the movement of the linear movement member. 
   According to a fourth aspect of the invention, the image forming apparatus further comprises a clutch switching lever which switches the first clutch mechanism from the interruption state to the transmission state in conjunction with a closing motion of the cover and switches the first clutch mechanism from the transmission state to the interruption state in conjunction with the opening motion of the cover. 
   According to the above described structure, because the first clutch mechanism can be switched to the transmission state in association with the closing motion of the cover, it is possible to move the linear movement member thereafter, by the driving force from the drive source. Moreover, because the first clutch mechanism can be switched to the interruption state in association with the opening motion of the cover, it is possible to move the linear movement member thereafter, by the driving force inputted to the second rack gear by way of the second input gear, irrespective of the drive source. 
   According to a fifth aspect of the invention, the first clutch mechanism includes: a gear support shaft; a drive input gear capable of rotating about the gear support shaft, the drive input gear including an input gear portion and a sun gear portion which are formed on a coaxial cylindrical face around the gear support shaft, the driving force from the drive source being inputted to the input gear portion; a drive output gear capable of rotating about the gear support shaft, the drive output gear including an output gear portion which is formed on a cylindrical face around the gear support shaft and an inner gear portion which is formed on a cylindrical face opposed to the sun gear portion leaving a space, the first input gear being meshed with the output gear portion; a planetary gear which is interposed between the sun gear portion and the inner gear portion and is meshed with the sun gear portion and the inner gear portion; and a planetary gear base member including a planetary gear support portion which is arranged between the drive input gear and the drive output gear and is rotatable about the gear support shaft, the planetary gear support portion rotatably supporting the planetary gear, wherein the clutch switching lever is engaged with the planetary gear base member in conjunction with the closing motion of the cover and is separated from the planetary gear base member in conjunction with the opening motion of the cover. 
   According to the above described structure, in a state where the clutch switching lever is locked to the planetary gear base member, the rotation of the planetary gear base member is restricted. Therefore, when the drive input gear is rotated, the planetary gear will rotate on its own axis without changing its position in the circumferential direction around the gear support shaft. Due to the rotation of the planetary gear, the drive output gear having the inner gear portion which is meshed with the planetary gear is rotated around the gear support shaft. On the other hand, in a state where the clutch switching lever is separated from the planetary gear base member, the planetary gear base member can be rotated around the gear support shaft. Therefore, even though the driving force from the drive source is inputted to the drive input gear, the rotation force of the drive input gear will not be transmitted to the drive output gear, because the planetary gear will revolve around the gear support shaft (the planetary gear base member rotates around the gear support shaft), while the planetary gear rotates on its own axis. As the results, in the state where the cover is closed, it is possible to reliably transmit the driving force from the drive source to the first input gear which is meshed with the output gear part of the drive output gear. On the other hand, in the state where the cover is opened, it is possible to reliably interrupt the transmission of the driving force from the drive source to the first input gear. 
   According to a sixth aspect of the invention, the second input gear rotates in conjunction with the opening motion and the closing motion of the cover. 
   According to the above described structure, it is possible to generate the rotation force of the second input gear in association of the opening motion and the closing motion of the cover. 
   According to a seventh aspect of the invention, the image forming apparatus further comprises a second clutch mechanism capable of being switched to a transmission state, in which rotation force of the second input gear is transmitted to the second rack gear, and to an interruption state, in which transmission of the rotation force of the second input gear to the second rack gear is interrupted. 
   According to the above described structure, it is possible to switch the transmission and interruption of the rotation force of the second input gear to the second rack gear by the second clutch mechanism. In this manner, it is possible to transmit the rotation force of the second input gear to the second rack gear thereby to move the linear movement member. On the other hand, it is possible to input the driving force from the drive source to the first rack gear, interrupting the transmission of the rotation force of the second input gear to the second rack gear, thereby to move the linear movement member. 
   According to an eighth aspect of the invention, the second clutch mechanism is capable of being switched from the interruption state to the transmission state during the opening motion of the cover and being switched from the interruption state to the transmission state during the closing motion of the cover. 
   According to the above described structure, it is possible to switch the second clutch mechanism to the transmission state during the opening motion of the cover, and to transmit the rotation force of the second input gear to the second rack gear thereby to move the linear movement member. By rendering the linear movement member to move from the pressure exerting position to the releasing position by the rotation force of the second input gear at this time, the pressure of the developing agent carrier against the image carrier can be reliably removed. On the other hand, because the second clutch mechanism can be switched to the interruption state during the closing motion of the cover, it is possible to move the linear movement member to the pressure exerting position and the releasing position after the closing motion, by inputting the driving force from the drive source to the first rack gear. 
   According to a ninth aspect of the invention, the second clutch mechanism includes: a rotary gear which is meshed with the second input gear and is rotated in one direction in conjunction with the opening motion of the cover, the rotary gear being rotated in the other direction opposite to the one direction in conjunction with the closing motion of the cover; and a swinging arm which rotatably supports the second input gear and is moved, following the rotation of the rotary gear in the one direction, from a separated position in which the second input gear is separated from the second rack gear to a meshed position in which the second input gear is meshed with the second rack gear, the swinging arm being moved from the meshed position to the separated position, following the rotation of the rotary gear in the other direction. 
   According to the above described structure, the rotary gear is rotated in one direction in association with the opening motion of the cover, and following this rotation, the second input gear is meshed with the second rack gear while rotating. Accordingly, it is possible to reliably transmit the rotation force of the second input gear to the second rack gear during the opening motion of the cover. Moreover, the rotary gear is rotated in the other direction in association with the closing motion of the cover, and following this rotation, the second input gear is separated from the second rack gear. Accordingly, it is possible to reliably interrupt the transmission of the rotation force of the second input gear to the second rack gear during the closing motion of the cover. 
   According to a tenth aspect of the invention, the second input gear is meshed with the second rack gear in conjunction with the opening motion of the cover, thereby to input the force for moving the linear movement member to the releasing position to the second rack gear. 
   According to the above described structure, it is possible to move the linear movement member to the releasing position by the force which is inputted from the second input gear to the second rack gear in association with the opening motion of the cover. Accordingly, in the state where the cover is opened, it is possible to reliably remove the pressure of the developing agent carrier against the image carrier. 
   According to an eleventh aspect of the invention, when the linear movement member is moved to the releasing position, engagement between the second input gear and the second rack gear is released. 
   According to the above described structure, after the linear movement member has been moved to the releasing position, it is possible to prevent the force for moving the linear movement member from being inputted from the second input gear to the second rack gear. Therefore, it is possible to prevent the linear movement member from moving beyond the releasing position, and to prevent occurrence of troubles due to such excessive movement of the linear movement member. 
   Illustrative Aspects 
   1. General Structure of a Color Laser Printer 
     FIG. 1  is a sectional side view showing a color laser printer as an image forming apparatus according to an aspect of the invention. 
   This color laser printer  1  is of a laterally disposed tandem type in which a plurality of drum subunits  28  which will be described below are arranged in parallel in a horizontal direction. The color laser printer  1  includes, in a body casing  2 , a sheet supply section  4  for supplying a sheet  3 , an image forming section  5  for forming an image on the sheet  3  which has been supplied, and a sheet discharge section  6  for discharging the sheet  3  on which the image has been formed. 
   (1) Body Casing 
   The body casing  2  has a shape of a substantially rectangular box in a side view. A drum containing space  7  for containing a drum unit  26 , which will be described below, is defined inside the body casing  2 . 
   A mounting hole  8  communicated with the drum containing space  7  is formed on one side face of the body casing  2 . A front cover  9  as a cover for opening or closing the mounting hole  8  is provided on the side face where the mounting hole  8  is formed. This front cover  9  is tilted sideward from the body casing  2  thereby to open the mounting hole  8  and is erected along the one side face of the body casing  2  thereby to close the mounting hole  8 . The drum unit  26  can be mounted to and detached from the drum containing space  7  through the mounting hole  8 , in a state where the mounting hole  8  is opened. 
   In the following description, the side where the front cover  9  is provided (the right side in  FIG. 1 ) is referred to as a front side, and the opposite side (the left side in  FIG. 1 ) is referred to as a rear side. Moreover, right and left are so defined, when the color laser printer  1  is seen from the front side. Further, the front and rear, the right and left, and the upper and lower sides of the drum unit  26  and the developing cartridge  27  are so defined, in a state where they are contained in the body casing  2 , unless particularly described. 
   (2) Sheet Supply Section 
   The sheet supply section  4  is provided in a bottom part of the body casing  2 . This sheet supply section  4  includes a sheet supply tray  10  which stores the sheet  3 , a separating roller  11  and a separating pad  12  which are provided above a front end of the sheet supply tray  10  so as to be opposed to each other, a sheet supply roller  13  which is provided behind the separating roller  11 , and a sheet supply path  14  through which the sheet  3  passes. 
   The sheet supply path  14  is formed in a substantially U-shape in a side view, in such a manner that its upstream end is adjacent to the separating roller  11  and its downstream end is adjacent to a conveying belt  58  from a front side of the conveying belt  58 . The conveying belt  58  will be described below. 
   In the middle of the sheet conveying path  14 , a paper dust removing roller  15  and a pinch roller  16  are provided above a front side of the separating roller  11  so as to be opposed to each other. A pair of register rollers  17  are provided above the paper dust removing roller  15  and the pinch roller  16 . 
   A sheet press plate  18  on which the sheets  3  are mounted in a stacked manner is provided inside the sheet supply tray  10 . This sheet press plate  18  is tiltably held at its rearward end so as to move between a sheet mount position in which the sheet press plate  18  lies along a bottom plate of the sheet supply tray  10  with its forward end tilted downward, and a sheet supply position in which the sheet press plate  18  is inclined with its forward end tilted upward. 
   In addition, a lever  19  for lifting the forward end of the sheet press plate  18  upward is provided below a forward end part of the sheet supply tray  10 . This lever  19  is held below the forward end of the sheet press plate  18  so as to be tilted in a vertical direction. 
   By tilting the lever  19 , the forward end of the sheet press plate  18  is lifted upward, whereby the sheet press plate  18  is positioned in the sheet supply position. 
   When the sheet press plate  18  is positioned in the sheet supply position, the uppermost sheet of the sheet  3  on the sheet press plate  18  is pressed against the sheet supply roller  13 , and the uppermost sheet is supplied, with rotation of the sheet supply roller  13 , to a position between the separating roller  11  and the separating pad  12 . 
   When the sheet supply tray  10  is withdrawn from the body casing  2 , the sheet press plate  18  is positioned in the sheet mount position. When the sheet press plate  18  is positioned in the sheet mount position, it is possible to mount the sheets  3  on the sheet press plate  18  in a stacked manner. 
   The sheet  3  which has been supplied is caught between the separating roller  11  and the separating pad  12  and is separated one by one to be conveyed. The sheet  3  which has been conveyed passes between the paper dust removing roller  15  and the pinch roller  16 , and paper dust is removed. Thereafter, the sheet  3  is conveyed to the register rollers  17  along the sheet conveying path  14 . 
   The sheet  3  is conveyed to the conveying belt  58  after it has been registered by the register rollers  17 . 
   (3) Image Forming Section 
   The image forming section  5  includes a scanner part  20 , a process part  21 , a transfer part  22 , and a fixing part  23 . 
   (3-1) Scanner Part 
   The scanner part  20  is arranged in an upper part of the body casing  2 . This scanner part  20  has a support plate  24  which extends longitudinally and laterally, and a scanner unit  25  which is fixed on an upper face of the support plate  24 . In the scanner unit  25 , optical members such as four light sources, a polygon mirror, an fθ lens, a reflection mirror, a plane inclination correcting lens are arranged. Laser beams based on image data which have been emitted from the respective light sources are deflected and scanned by the polygon mirror, pass the fθ lens and the plane inclination correcting lens, and then are reflected by the reflection mirror. Thereafter, the beams are irradiated by rapid scanning onto surfaces of photosensitive drums  29  for respective colors, which will be described below. 
   (3-2) Process Part 
   The process part  21  is arranged below the scanner part  20  and above the sheet supply section  4 . The process part  21  includes the single drum unit  26 , and the four developing cartridges  27  corresponding to the respective colors. 
   (3-2-1) Drum Unit 
   The drum unit  26  has the four drum sub-units  28  corresponding to the respective colors, namely, a black drum sub-unit  28 K, an yellow drum sub-unit  28 Y, a magenta drum sub-unit  28 M, and a cyan drum sub-unit  28 C. 
   The drum sub-units  28  are arranged in parallel at intervals in a front-rear direction. More specifically, the black drum sub-unit  28 K, the yellow drum sub-unit  28 Y, the magenta drum sub-unit  28 M, and the cyan drum sub-unit  28 C are arranged in this order from the front side toward the rear side. 
   As described below, each of the drum sub-units  28  includes a pair of side frames  104  and a center frame  105  which is bridged between them (See  FIG. 4 ). 
     FIG. 2  is a sectional side view showing the developing cartridge  27  and the drum sub-unit  28 . It should be noted that a handle  214 , which will be described below, is omitted in  FIGS. 1 and 2 . 
   As shown in  FIG. 2 , each of the drum sub-units  28  holds a photosensitive drum  29  as an image carrier, and a scorotron type charger  30 , and a cleaning brush  31 . 
   The photosensitive drum  29  which is in a cylindrical shape and extends in a width direction includes a drum body  32  whose uppermost surface layer is a positively-chargeable photoconductive layer formed of polycarbonate, and a drum shaft  33  which is arranged along an axial direction of the drum body  32 . The drum body  32  is provided so as to rotate with respect to the drum shaft  33 . Both ends of the drum shaft  33  in the axial direction are inserted through the pair of side frames  104  (See  FIG. 4 ) and held by side plates  103  which will be described below (See  FIG. 4 ) so as not to be rotated. At the time of image forming, the photosensitive drum  29  is rotated by driving force from a motor (not shown) which is provided in the body casing  2 . 
   The scorotron type charger  30  is arranged diagonally above and behind the photosensitive drum  29  so as to be opposed to the photosensitive drum  29  leaving a space. The scorotron type charger  30  is held by the center frame  105 . This scorotron type charger  30  includes a discharge wire  34  which is opposed to the photosensitive drum  29  leaving a space, and a grid  35  which is provided between the discharge wire  34  and the photosensitive drum  29 . At the time of image forming, high voltage is applied to the discharge wire  34 , and the discharge wire  34  generates a corona discharge. At the same time, an electric voltage is applied to the grid  35 , whereby the surface of the photosensitive drum  29  is uniformly charged to positive polarity, while an amount of electric charge supplied to the photosensitive drum  29  is controlled. 
   The cleaning brush  31  is held by the center frame  105  and arranged behind the photo sensitive drum  29  so as to be opposed and to come into contact with the photosensitive drum  29 . At the time of image forming, cleaning bias is applied to the cleaning brush  31 . 
   (3-2-2) Developing Cartridge 
   As shown in  FIG. 1 , the developing cartridges  27  are respectively detachably provided in the drum sub-units  28  corresponding to the respective colors. Specifically, there are the four developing cartridges  27 , namely, a black developing cartridge  27 K detachably mounted to the black drum sub-unit  28 K, a yellow developing cartridge  27 Y detachably mounted to the yellow drum sub-unit  28 Y, a magenta developing cartridge  27 M detachably mounted to the magenta drum sub-unit  28 M, and a cyan developing cartridge  27 C detachably mounted to the cyan drum sub-unit  28 C. 
   Each developing cartridge includes, as shown in  FIG. 2 , a developing frame  36 ; an agitator  37  provided in the developing frame  36  and having an agitating member  48 ; a supply roller  38 ; a developing roller  39  as a developing agent carrier; and a layer thickness regulating blade  40  having a blade  53  formed of a metal plate spring member, a pressing portion  54  formed of an insulating silicone rubber and a fixing member  55 . 
   The developing frame  36  is formed into a box shape having an opening  41  at its lower end portion. A partition wall  42  divides the developing frame  36  into a toner containing room  43  and a developing room  44 . An communicating hole  45  that communicates the toner containing room  43  with the developing room  44  is formed in the partition wall  42 . 
   In each of the drum sub-units  28  corresponding to the developing cartridges  27 , the scorotron type charger  30  generates the corona discharge and uniformly charges the surface of the photosensitive drum  29  to the positive polarity. 
   The surface of the photosensitive drum  29  is exposed to the laser beam from the scanner part  20  by rapid scanning, after it has been positively charged by the scorotron charger  30 , along with the rotation of the photosensitive drum  29 , thereby to form an electrostatic latent image corresponding to an image to be formed on the sheet  3 . 
   As the photosensitive drum  29  further rotates, the toner which is carried on a surface of the developing roller  39  and has been positively charged is brought into contact with the photosensitive drum  29  along with rotation of the developing roller  39 . The toner is supplied to the electrostatic latent image which has been formed on the surface of the photosensitive drum  29 , that is, an exposed area which has been exposed to the laser beam and has a lowered electric potential, out of the surface of the photosensitive drum  29  which has been uniformly and positively charged. In this manner, the electrostatic latent image on the photosensitive drum  29  is developed into a visual image, and a toner image corresponding to each color by reversal development is carried on the surface of the photosensitive drum  29 . 
   The toner which remains on the photosensitive drum  29  after the transfer is recovered by the developing roller  39 . Moreover, paper dust from the sheet  3  which adheres to the surface of the photosensitive drum  29  after the transfer is recovered by the cleaning brush  31 . 
   (3-3) Transfer Part 
   As shown in  FIG. 1 , the transfer part  22  is arranged above the sheet supply section  4  and below the process part  21  in the body casing  2  along the front-rear direction. This transfer part  22  includes a driving roller  56 , a driven roller  57 , a conveying belt  58 , a transfer roller  59 , and a cleaning part  60 . 
   The driving roller  56  and the driven roller  57  are opposed to each other leaving a space in the front-rear direction. The driving roller  56  is arranged more rearward than the cyan drum sub-unit  28 C, and the driven roller  57  is arranged more forward than the black drum sub-unit  28 K. 
   The conveying belt  58  is an endless belt formed of a resin film of electrically conductive polycarbonate, polyimide or the like dispersed with electrically conductive particles such as carbon. This conveying belt  58  is wound around the driving roller  56  and the driven roller  57 . 
   At the time of image forming, a driving force from a motor (not shown) which is provided in the body casing  2  is transmitted to the driving roller  56  thereby to rotate the driving roller  56 . Then, the conveying belt  58  is cyclically moved between the driving roller  56  and the driven roller  57  so as to move in the opposite direction to the rotation direction of the photosensitive drum  29 , at respective transfer positions opposed to and in contact with the photosensitive drums  29  in the drum sub-units  28 . At the same time, the driven roller  57  is driven accordingly. 
   Within the conveying belt  58  which is wound around the driving roller  56  and the driven roller  57 , transfer rollers  59  are respectively disposed so as to be opposed to the photosensitive drums  29  interposing the conveying belt  58 . Each of the transfer rollers  59  has a roller shaft made of metal which is coated with a rubber roller formed of electrically conductive rubber. At the transfer positions where the transfer rollers  59  are opposed to and brought into contact with the conveying belt  58 , the transfer rollers  59  are driven to rotate in the same direction as the cyclical moving direction of the conveying belt  58 . At the time of image forming, transfer bias from a high voltage board, which is not shown, provided in the body casing  2  is applied to the transfer rollers  59 . 
   The cleaning part  60  is arranged below the conveying belt  58  which is wound around the driving roller  56  and the driven roller  57 . The cleaning part  60  includes a primary cleaning roller  61 , a secondary cleaning roller  62 , a scraping blade  63 , and a toner reservoir  64 . 
   The primary cleaning roller  61  is so arranged as to come into contact with the conveying belt  58  at a lower side, that is, the opposite side to the conveying belt  58  at an upper side which is in contact with the photosensitive drums  29  and the transfer rollers  59 . The primary cleaning roller  61  is adapted to be driven to rotate in the same direction as the cyclical moving direction of the conveying belt  58 , at the contact positions. At the time of image forming, primary cleaning bias is applied to the primary cleaning roller  61 . 
   The secondary cleaning roller  62  is so arranged as to come into contact with the primary cleaning roller  61  from below, and adapted to rotate in the opposite direction to the direction of rotation of the primary cleaning roller  61 , at the contact position. At the time of image forming, secondary cleaning bias is applied to the secondary cleaning roller  62 . 
   The scraping blade  63  is so arranged as to come into contact with the secondary cleaning roller  62  from below. 
   The toner reservoir  64  is provided below the primary roller  61  and the secondary roller  62  so that the toner falling down from the secondary cleaning roller  62  can be stored therein. 
   The sheet  3  which has been supplied from the sheet supply section  4  is conveyed from the front side toward the rear side by the conveying belt  58  which is cyclically moved by the driving roller  56  and the driven roller  57  so as to successively pass the transfer positions corresponding to the respective drum sub-units  28 . During the conveyance, the toner images of the respective colors which are carried on the photosensitive drums  29  in the drum sub-units  28  are sequentially transferred to the sheet  3 , whereby a color image is formed on the sheet  3 . 
   Specifically, after a black toner image carried on the surface of the photosensitive drum  29  of the black drum sub-unit  28 K has been transferred to the sheet  3 , a yellow toner image carried on the surface of the photosensitive drum  29  of the yellow drum sub-unit  28 Y is transferred in a superposing manner to the sheet  3  to which the black toner image has been already transferred. In succession, with the same operation, a magenta toner image carried on the surface of the photosensitive drum  29  of the magenta drum sub-unit  28 M and a cyan toner image carried on the surface of the photosensitive drum  29  of the cyan drum sub-unit  28 C are transferred to the sheet  3  in a superposing manner, whereby the color image is formed on the sheet  3 . 
   On the other hand, the toner adhered to the surface of the conveying belt  58  during the transfer operation is transferred from the surface of the conveying belt  58  to the primary cleaning roller  61  by the primary cleaning bias, and then, to the secondary cleaning roller  62  by the secondary cleaning bias. Thereafter, the toner which has been transferred to the secondary cleaning roller  62  is scraped off by the scraping blade  63 , and falls from the secondary cleaning roller  62  to be stored in the toner reservoir  64 . 
   (3-4) Fixing Part 
   The fixing part  23  is provided more rearward than the cyan drum sub-unit  28 C in the body casing  2  and arranged so as to be opposed in the front-rear direction to the transfer position where the photosensitive drum  29  is contacted with the conveying belt  58 . This fixing part  23  includes a heating roller  65  and a pressurizing roller  66 . 
   The heating roller  65  is a metal tube having a releasing layer formed on its surface. The heating roller  65  has a halogen lamp which is incorporated therein along an axial direction thereof. The heating roller  65  is heated by the halogen lamp so that the surface of the roller is heated to a fixing temperature. 
   The pressurizing roller  66  is disposed below the heating roller  65  so as to be opposed to the heating roller  65 . This pressurizing roller  66  presses the heating roller  65  from below. 
   The color image which has been transferred to the sheet  3  is conveyed to the fixing part  23 , and fixed by heating on the sheet  3 , while the sheet  3  passes between the heating roller  65  and the pressurizing roller  66 . In this manner, formation of the image on the sheet  3  is achieved. 
   (4) Sheet Discharge Section 
   In the sheet discharge section  6 , a conveying path  67  at a discharging side of the sheet  3  is provided in such a manner that its upstream end is adjacent to the fixing part  23  in a lower region, and its downstream end is adjacent to a sheet discharge tray  68  in an upper region. The conveying path  67  is formed in a substantially U-shape in a side view so that the sheet  3  is supplied rearward and thereafter discharged to the front side after the sheet  3  has been reversed. 
   In the middle of the conveying path  67  at the sheet discharge side, a conveying roller  69  and a pinch roller  70  are provided so as to be opposed to each other. Moreover, a pair of sheet discharge rollers  71  are provided at the downstream end of the conveying path  67  at the sheet discharge side. 
   The sheet discharge section  6  is further provided with the sheet discharge tray  68 . The sheet discharge tray  68  is formed by gradually denting an upper wall of the body casing  2  from the front side toward the rear side so that sheets  3  which have been discharged can be placed thereon in a stacked manner. 
   The sheet conveyed from the fixing part  23  is conveyed along the conveying path  67  at the sheet discharge side by means of the conveying roller  69  and the pinch roller  70 , and then discharged onto the sheet discharge tray  68  by the sheet discharge roller  71 . 
   2. Drum Unit 
     FIG. 3  is a perspective view of the drum unit  26  (in a state where the four developing cartridges  27  are mounted thereto) as seen from above at a left rear side.  FIG. 4  is a perspective view of the drum unit  26  (in a state where one of the developing cartridges  27  is being mounted or detached, while the other developing cartridges  27  have been detached) as seen from above at a left front side.  FIG. 5  is a left side view of the drum unit  26 . 
   The drum unit  26  contains the four drum sub-units  28  corresponding to the four colors. The drum unit  6  includes a front beam  101  and a rear beam  102  which are provided at both sides in the front-rear direction of the four drum sub-units  28  which are arranged in parallel along the front-rear direction, and a pair of side plates  103  which enclose the front beam  101 , the four drum sub-units  28 , and the rear beam  102  from both sides in the width direction (in the direction from left to right). 
   The drum unit  26  together with the four drum sub-units  28 , the front beam  101 , the rear beam  102 , and the pair of side plates  103  which are integrally assembled are slidably mounted to and detached from the drum containing space  7  (See  FIG. 1 ) in the body casing  2 . 
   (1) Drum Sub-unit 
   As shown in  FIG. 4 , the drum sub-unit  28  has the pair of side frames  104  which are opposed to each other at a distance in the width direction and the center frame  5  (See  FIG. 2 ) which is bridged between the two side frames  104  along the width direction. 
   Each of the side frames  104  is formed of resin material into a shape of a flat plate. The drum shaft  33  of the photosensitive drum  29  is inserted into the two side frames  104 . 
   The side frame  104  is provided with a guide groove  106  for guiding mounting and detaching movement of the developing cartridge  27  with respect to the drum sub-unit  28 . The guide groove  106  is formed along a substantially vertical direction from an upper end edge at the rear side of the side frame  104  to a position near a lower end at the front side of the side frame  104 . The lowermost end part (the deepest part) of the guide groove  106  is so arranged as to correspond to the position of a developing roller shaft  51  in a state where the developing roller  39  is in contact with the photosensitive drum  29 . The guide groove  106  slidably receives a collar member  205  which will be described below. 
   Moreover, a boss  107  is formed in each of the side frames  104 . This boss  107  is formed in a cylindrical shape and projected outward in the width direction from the side frame  104 . This boss  107  is arranged so as to be opposed in the width direction to a window  206  in the developing cartridge  27 , which will be described below, in a state where the developing cartridge  27  has been mounted to the drum sub-unit  28 . 
   Further, the side frame  104  at the left side is provided with a coupling inside insertion hole  109  which is opposed in the width direction to a coupling receiving gear  208  of the developing cartridge  27  which will be described below. This coupling inside insertion hole  109  is formed as a round hole passing through the side frame  104  at the left side, in the direction of its thickness. 
   The center frame  105  is formed of resin material. Support rollers  110  for supporting the developing cartridge  27  are provided on both ends in the width direction of an upper end part of the center frame  105 . The support rollers  110  are rotatably held by a rotation shaft (not shown) which extends in the width direction along the upper end part of the center frame  105 . 
   (2) Front Beam 
   The front beam  101  is integrally formed of resin material. The front beam  101  is disposed in front of the four sub-units  28  which are arranged in parallel along the front-rear direction so as to be bridged between the pair of side plates  103 . 
   The front beam  101  has a front side gripping part  111  attached to its center part in the width direction, and a support shaft  112  for rotatably supporting the front side gripping part  111 . 
   The front side gripping part  111  has a substantially U-shape, and both ends thereof are rotatably supported by the support shaft  112  at the center part in the width direction in such a manner that the front side gripping part  111  can be tilted between a stored position where the front side gripping part  111  is erected along the front beam  101  (See  FIG. 3 ) and an operative position where it is inclined frontward from the front beam  101  (See  FIG. 4 ). 
   The support shaft  112  is supported by the front beam  101  so as to pass through the front beam  101  in the width direction. Opposite ends in the width direction of the support shaft  112  project outward from the front beam  101 , and further, pass through the side plates  103  to project outward in the width direction. 
   (3) Rear Beam 
   The rear beam  102  is integrally formed of resin material. The rear beam  102  is disposed in rear of the four sub-units  28  which are arranged in parallel along the front-rear direction, so as to be bridged between the pair of side plates  103 . 
   As shown in  FIG. 3 , this rear beam  102  is formed in a substantially C-shape which is open rearward in a plan view. A rear side gripping part  113  is integrally provided in a center part in the width direction of the rear beam  102 . The rear side gripping part  113  has a substantially U-shape in a rear view, and both ends thereof are connected to the rear beam  102  and inclined from a lower part at the rear side upwardly toward the front side so as to project diagonally upwardly from the rear beam  102 . 
   (4) Side Plates 
   The side plates  103  are formed of material which is more rigid than the resin material for forming the front beam  101  and the rear beam  102 , such as metal or fiber reinforced resin. Preferably, the side plates  103  are formed of steel plate. 
   Each of the side plates  103  has a shape of a rectangular plate elongated in the front-rear direction in a side view. The side plates  103  are provided in such an arrangement, with respect to the front beam  101 , the four drum sub-units  28 , and the rear beam  102  which are arranged in parallel in the front-rear direction, that their front ends are opposed to the front beam  101  while their rear ends are opposed to the rear beam  102 . The side plates  103  are fixed to both the front and rear beams. 
   Respective upper ends of the side plates  103  are bent outward in the width direction in an L-shape in cross section thereby to form flange portions  114  projecting outward in the width direction. These flange portions  114  extend on rectilinear lines along the front-rear direction (in a horizontal direction). 
   Each of the side plates  103  is formed in a substantially L-shape extending rearward in a side view, at an upper end of its rearward end part. Two roll members  118  are rotatably provided in this rearward extending portion. These two roll members  118  are disposed interposing a spacer  119  in the front-rear direction. The roll member  118  at a front side is arranged below the flange portion  114 , and the roll member  118  at a rear side is arranged behind a rear end of the flange portion  114 . 
   The side plate  103  is further provided, in its rearward end area, with a cut-away portion  120  which is cut away from the rearward end in a U-shape in a side view. In a state where the drum unit  26  has been mounted in the body casing  2 , a positioning shaft (not shown) which is arranged in the body casing  2  is engaged with this cut-away portion  120 , whereby the drum unit  26  is positioned with respect to the body casing  2 . 
   Still further, the side plate  103  is provided, in its upper end area, with four light penetrating holes  115  for respectively receiving the bosses  107  of the drum sub-units  28 . The light penetrating holes  115  are formed at intervals in the front-rear, in the upper end area of the side plate  103 . These light penetrating holes  115  are round holes passing through the side plate  103  in a direction of its thickness at respective positions opposed to the bosses  107  of the drum sub-units  28 . The bosses  107  of the drum sub-units  28  are engaged with the light penetrating holes  115  so as to be exposed outward in the width direction, and thus, the rotations of the drum sub-units  28  around the drum shafts  33  with respect to the side plates  103  are restrained. 
   Still further, the side plate  103  is provided, at its lower end area, with shaft holes  116  through which axially end portions of the drum shafts  33  are passed through. The side plate  103  at the left side is provided with four coupling outside insertion holes  117  to which the coupling receiving gears  208  of the developing cartridges  27  are opposed in the width direction. These coupling outside insertion holes  117  are formed in the middle in the vertical direction of the side plate  103  at intervals in the front-rear direction. These coupling outside insertion holes  117  are formed as round holes passing through the side plate  103  in the direction of its thickness, at positions opposed to the coupling inside insertion holes  109  of the drum sub-units  28 . 
   3. Developing Cartridge 
     FIGS. 6 and 7  are perspective views of the developing cartridge  27  as seen from a left rear side, and  FIGS. 8  and  9  are perspective views of the developing cartridge  27  as seen from a left front side.  FIG. 10  is a plan view of the developing cartridge  27 ,  FIG. 11  is a right side view of the developing cartridge  27 , and  FIG. 12  is a sectional view taken along a line A-A in  FIG. 11 .  FIGS. 13 and 14  are right side sectional views of the developing cartridge  27 . It should be noted that the supply roller  38  and the developing roller  39  are shown in a simplified form in  FIGS. 13 and 14 . 
   (1) Developing Cartridge 
   The developing frame  36  of the developing cartridge  27  is integrally provided with a pair of side walls  201  which are opposed to each other in the width direction, an upper wall  202  which is bridged between upper end edges of the two side walls  201 , a front wall  203  which is bridged between front end edges of the two side walls  201 , and a rear wall  204  which is bridged between rear end edges of the two side walls  201 . The opening  41  is defined by lower end edges of the two side walls  201 , the front wall  203  and the rear wall  204  so that the developing roller  39  is exposed. 
   Windows  206  for detecting remaining amount of the toner which is contained in the toner containing room  43  are embedded in the two side walls  201 . These windows  206  are arranged so as to be opposed to each other interposing the toner containing room  43  so that a light for detecting the remaining amount of the toner can pass them through in the width direction. 
   The side wall  201  at the left side is provided with a gear mechanism which is covered with a gear cover  207 , as shown in  FIGS. 6 to 9 . This gear mechanism includes the coupling receiving gear  208  which is exposed from the gear cover  207 , and a gear train  230  (See  FIG. 12 ) which is meshed with the coupling receiving gear  208  inside the gear cover  207 . 
   Gear disposing part  209  in a cylindrical shape is formed at a lower end area of the gear cover  207  so as to project outward in the width direction. The coupling receiving gear  208  is disposed in the gear disposing part  209  and exposed from a distal end face of the gear disposing part  209 . 
   A coupling shaft (not shown) provided in the body casing  2  is coupled to the coupling receiving gear  208  so as to move forward and rearward, and so as not to be relatively rotated. A driving power of a motor (not shown) provided in the body casing  2  is inputted from this coupling shaft. 
   The gear train  230  includes an agitator driving gear fixed to a rotation shaft  47  of the agitator  37 , a supply roller driving gear fixed to a supply roller shaft  49  of the supply roller  38 , a developing roller driving gear fixed to the developing roller shaft  51  of the developing roller  39 , and so on, which are meshed with the coupling receiving gear  208  by way of intermediate gears. Therefore, the driving power inputted to the coupling receiving gear  208  is transmitted to the agitator  37 , the supply roller  38  and the developing roller  39  by way of the gear train  230 . 
   As shown in  FIG. 11 , a cap  210  which closes a toner filling port (not shown) for filling the toner into the toner containing room  43  is provided on the side wall  201  at the right side, at a position above the window  206 . Moreover, a bearing member  211  which rotatably supports a right end portion of the developing roller shaft  51  is provided in a lower end area of the side wall  201  at the right side. As shown in  FIG. 12 , the right end portion of the developing roller shaft  51  is rotatably inserted into the bearing member  211 , and the left end portion of the developing roller shaft  51  is rotatably inserted into the sidewall  201  at the left side, where by the developing roller shaft  51  is rotatably supported by the developing frame  36 . Both the right end portion and the left end portion of the developing roller shaft  51  are respectively projected from the bearing members  211  and the gear covers  207  outward in the width direction, and the projected portions are respectively covered with collar members  205 . 
   As shown in  FIGS. 6 to 9 , separating projections  212  in a substantially cylindrical shape are provided in the upper end parts of the two side walls  201  so as to project outward in the width direction from the parts connected to an upper end part of the rear wall  204 . 
   The upper wall  202  is provided with a handle  214  to be gripped when the developing cartridge is carried, which also serves as a pressure member. This handle  214  is formed in a shape of thin plate which is elongated in the width direction, and provided so as to be swung into an erected state in which it has been erected at a substantially right angle with respect to the upper wall  202  (See  FIGS. 7 and 9 ), a tilted state in which it has been tilted frontward than in the erected state to come close to the upper wall  202  (See  FIGS. 6 ,  8  and  13 ), and a pressurized state in which it has come closer to the upper wall  202  than in the tilted state (See  FIG. 14 ). 
   More specifically, as shown in  FIGS. 13 and 14 , handle support portions  215  in a substantially semicircular shape in a side view which are projected upward are integrally formed in a rearward end part of the upper wall  202 , at both ends in the width direction thereof. The handle support portions  215  are respectively formed with through holes  229  which pass them through in the width direction. On the other hand, as shown in  FIGS. 6 and 7 , the handle  214  is formed with cut-outs  231  at both ends in the width direction in the rearward end part thereof, with which the handle support portions  215  can be engaged. In each of the cut-outs  231 , an elastically deformable portion  232  in a substantially L-shape in a plan view is provided having its base end connected to a left side face of the cut-out  231 . An end of the elastically deformable portion  232  is opposed to and spaced from a right side face of the cut-out  231  in the width direction, and the handle support portion  215  is fitted between the end of the elastically deformable portion  232  and the right side face of the cut-out  231 . The end of the elastically deformable portion  232  and the right side face of the cut-out  231  are respectively provided with support shafts  233  so as to project in a direction of coming close to each other. In a state where the support shafts  233  are spaced apart by deforming the elastically deformable portion  232 , the handle support portion  215  is fitted into the cut-out  231 , and thereafter, by releasing the elastically deformable portion  232  from the deformed state, the support shafts  33  are allowed to be inserted into the through hole  229  of the handle support portion  215 . In this manner, the handle  214  can be fitted to the handle support portion  215  so as to be oscillated. 
   As shown in  FIGS. 9 and 12 , a pair of spring guide members  216  in a cylindrical shape are formed in a forward end part of the upper wall  202  at both ends thereof in the width direction (an axial direction of the developing roller  39 ) at an interval substantially equal to a length in the width direction (a length in the axial direction) of a sponge roller  50  of the developing roller  39 . The spring guide members  216  are respectively opposed to the handle support portions  215  spaced in the front-rear, and at the same time, respectively opposed to both ends of a rubber roller  52  of the developing roller  39  in the width direction. Each of the spring guide members  216  contains therein a contact member  217  which can move up and down, and a coil spring  218  which always urges the contact member  217  upward, as shown in  FIGS. 13 and 14 . 
   The contact member  217  integrally has a body portion  219  in a substantially round shape in a plan view whose upper face is convex-curved, a boss  220  projecting downward from a center of a lower face of the body portion  219 , and an extended portion  221  in a cylindrical shape which extends from a circumference of the lower face of the body portion  219  along an inner face of the spring guide member  216 . The extended portion  221  is provided with a plurality of locking hooks  222 . The locking hooks  222  are fitted into grooves  223  which are formed in the spring guide member  216 , and distal ends of the locking hooks  222  are locked to upper ends of the grooves  223 , so that the contact member  217  is not be detached from the spring guide member  216 . 
   The coil spring  218  is provided in a compressed state between the contact member  217  and the upper wall  202 , by inserting a spring fitting boss  224  formed in the upper wall  202  into a lower end of the coil spring  218 , and by inserting the boss  220  of the contact member  217  into an upper end of the coil spring  218 . 
   As shown in  FIG. 9 , recesses  225  capable of receiving the corresponding contact members  217  are formed on a lower face of the handle  214  (the face opposed to the upper wall  202 ) at respective positions corresponding to the contact members  217 . In a state where the handle  214  is tilted into the tilted state, the contact members  217  are respectively received in the recesses  225 , and top ends of the contact members  217  are butted against bottom faces of the recesses  225  (the lower face of the handle  214 ). 
   Moreover, as shown in  FIG. 10 , the handle  214  is formed with a gripping hole  226  in a substantially rectangular shape in a plan view which is elongated in the width direction in a center part of the handle  214 . Therefore, it is possible to grip the handle  214  by inserting fingers in the gripping hole  226 . 
   The handle  214  is further provided with pressing projections  227  in a substantially columnar shape in a side view projecting outward in the width direction, at both ends in the width direction in a front end area thereof. As shown in  FIG. 10 , the pressing projections  227  are formed having such a length that their distal end faces are positioned on a same plane S which includes distal end faces of the separating projections  212  projecting to the same sides. In other words, the distal end faces of the pressing projections  227  are arranged at the same position in the width direction as the distal end faces of the separating projections  212  which project to the same sides. Moreover, in a state where the developing cartridges  27  have been mounted to the drum sub-units  28  and the handle  214  has been tilted into the tilted state, the pressing projections  227  are positioned lower than the separating projections  212 , as shown in  FIG. 11 . 
   As shown in  FIGS. 8 and 9 , the front wall  203  is provided with projections to be supported  228  in a substantially trapezoidal shape in a side view which protrude frontward, at both ends thereof in the width direction. 
   (2) Mounting and Dismounting of the Developing Cartridge to and from the Drum Unit 
   The handle  214  is gripped by inserting fingers in the gripping hole  226  of the handle  214 , whereby the developing cartridge  27  corresponding to the particular color is mounted to the drum sub-unit  28  corresponding to the developing cartridge  27  from above, as shown in  FIG. 4 . 
   More specifically, the collar members  205  at both sides in the axial direction of the developing roller shaft  51  in each of the developing cartridges  27  are inserted into the guide grooves  106  in the side frames  104  of the corresponding drum sub-unit  28 , and then, the developing cartridge  27  is pushed downward along the guide grooves  106  into the drum sub-unit  28 . When the developing roller  39  has come into contact with the photosensitive drum  29 , further push of the developing cartridge  27  is restrained. Then, the developing cartridge  27  is tilted around the developing roller shaft  51  under its own weight in such a manner that the upper end part of the developing cartridge  27  is inclined toward the center frame  105  at the front side, and the projections to be supported  228  formed on the front wall  203  of the developing frame  36  is butted against the support roller  110  of the center frame  105  thereby to be supported. In this manner, the developing cartridge  27  is positioned with respect to the drum sub-unit  28 , and thus, mounting of the developing cartridge  27  to the drum sub-unit  28  is achieved. 
   After the developing cartridge  27  has been mounted in this manner, the fingers are released from the handle  214  in the erected state, and the handle  214  is tilted around the support shaft  233  under its own weight from the erected state to the tilted state. 
   When all the developing cartridges  27  have been mounted to the drum sub-units  28 , the front side gripping part  111  of the front beam  101 , the handles  214  of the respective developing cartridges  27 , and the rear side gripping part  113  of the rear beam  102  is arranged so as to be superposed in the front-rear direction, as shown in  FIG. 3 . 
   On the other hand, by gripping the handle  214  in the state where the developing cartridge  27  has been mounted to the drum unit  26  (the drum sub-unit  28 ), the handle  214  is pulled up from the tilted state to the erected state, and is lifted upward, whereby the developing cartridge  27  can be detached from the drum unit  26 . 
   4. Rail, Separating and Pressing Mechanism 
     FIG. 15  is a perspective view of the body casing  2  and the drum unit  26  as seen from above at a right front side, showing a state where an exterior board and the front cover  9  of the body casing  2  have been removed, and the drum unit  26  has been mounted to the body casing  2 . 
   The body casing  2  has a pair of body frames  301  which are opposed to each other in the width direction interposing the drum unit  26 . Rails  302  for guiding mounting and dismounting of the drum unit  26 , and a separating and pressing mechanism  303  are respectively provided on inner side faces of the body frames  301 . The separating and pressing mechanism  303  is provided for the purpose of separating and pressing the developing rollers  39  of the developing cartridges  27  which have been mounted to the drum unit  26  with respect to the photosensitive drums  29 , (for the purpose of moving the developing cartridges  27  between a contact position and a separated position, and further pressing the developing cartridges  27  against the photosensitive drums  29  in the contact position). 
   It should be noted that the separating and pressing mechanism  303  at the left side only is shown in  FIG. 15 . 
     FIG. 16  is a perspective view of the drum unit  26 , the right and left rails  302 , and the right and left separating and pressing mechanisms  303 , as seen from above at the right front side.  FIG. 17  is a perspective view of the right and left rails  302  and the right and left separating and pressing mechanisms  303 , as seen from above at the right front side. 
   (1) Rails 
   The right and left rails  302  are arranged so as to be opposed to each other in the width direction, interposing the drum unit  26 . Each of the rails  302  integrally has a rail securing part  304  opposed to a front end face of the body frame  301 , a rail body part  305  extending in the front-rear (a horizontal direction) along the inner side face of the body frame  301 , and a connecting part  306  which connects the rail securing part  304  to the rail body part  305 . 
   The rail securing part  304  is fixed to the front end face of the body frame  301  with a screw  307 . The rail body part  305  is formed in a substantially L-shape in cross section having its lower end portion folded inward in the width direction. In a state where the drum unit  26  has been mounted to the casing  2 , the flange portion  114  of the side plate  103  of the drum unit  26  is placed on a horizontally extending portion of the rail body part  305 . 
   The connecting part  306  is so formed as to connect an end edge of the rail securing part  304  at an inner side in the width direction to a front end edge of the rail body part  305 . The connecting part  306  is provided with a roll support shaft  308  which passes it through in the width direction. A rail roll  309  which is rotatably held by the roll support shaft  308  is provided so as to be opposed to an inner face of the connecting part  306  in the width direction. The uppermost portion of a peripheral face of the rail roll  309  is positioned higher than the lower end portion (the horizontally extending portion) of the rail body part  305 . 
   (2) Mounting of the Drum Unit  26  to the Body Casing 
   In order to mount the drum unit  26  to the body casing  2 , the front side gripping part  111  and the rear side gripping part  113  (See  FIG. 3 ) of the drum unit  26  is respectively gripped with both hands, as a first step, thereby to lift the drum unit  26 . Then, the front cover  9  is tilted to open the mounting hole  8  as shown in  FIG. 1 , and the drum unit  26  is inserted into the drum containing space  7  from the mounting hole  8 . 
   On this occasion, the roll members  118  provided in the rear end part of the drum unit  26  is rolled along the rail body parts  305  of the rails  302 . Then, releasing the hand from the rear side gripping part  113 , the two flange portions  114  of the drum unit  26  are respectively placed on the right and left rail rolls  309 . In this state, the drum unit  26  is pushed rearward. Consequently, the roll members  118  will roll along the rail body parts  305 , and the flange portions  114  will slide along the rail rolls  309 , whereby the drum unit  26  is smoothly moved. At the same time, the separating projections  212  and the pressing projections  227  of the developing cartridges  27  will slide along cam receiving parts  323  of holder securing parts  322 , which will be described below. 
   Then, the roll members  118  will fall rearward from the rails  302 , and the flange portions  114  will fall rearward from the rail rolls  309  to be placed on the horizontally extending portions of the rail body parts  305 . Consequently, the pressing projections  227  and the separating projections  212  of the developing cartridges  27  are respectively received in pressing projection receiving parts  325  and separating projection receiving parts  326  which will be described below, whereby mounting of the drum unit  26  to the body casing  2  is completed. 
   Thereafter, releasing the hand from the front side gripping part  111 , the front cover  9  is closed, and the mounting hole  8  is closed by the front cover  9 . In association with the closing motion of the front cover  9 , the front side gripping part  111  is rotated around the support shaft  112  from the erected position (See  FIG. 4 ) to the stored position (See  FIG. 3 ). 
   (3) Separating and Pressing Mechanism 
   As shown in  FIG. 17 , the separating and pressing mechanism  303  includes a pair of linear movement cam members  310 , intermediate members  311  respectively provided relative to the linear movement cam members  310 , cam holders  312  which hold the linear movement cam members  310  so as to rectilinearly move in the front-rear, and a synchronous movement mechanism  313  for rectilinearly moving the pair of linear movement cam members  310  synchronously. 
     FIG. 18  is a perspective view of the linear movement cam members  310 , the intermediate members  311 , and the synchronous movement mechanism  313  as seen from above at a right front side. Specifically, in  FIG. 18 , the cam holders  312  are omitted to show the perspective view of the separating and pressing mechanism  303  as seen from the above at the right front side.  FIGS. 19A to 19E  are perspective views for illustrating movements of one of the linear movement cam members  310  and the intermediate members  311 .  FIG. 20  is a right side view of the linear movement cam member  310  and the intermediate members  311  in a state of  FIG. 19A ,  FIG. 21  is a right side view of the linear movement cam member  310  and the intermediate members  311  in a state of  FIG. 19C , and  FIG. 22  is a right side view of the linear movement cam member  310  and the intermediate members  311  in a state of  FIG. 19E . 
   Each of the linear movement cam members  310  includes a cam body plate  314  extending along the inner side face of the body frame  301  (See  FIG. 15 ) in the front-rear, and four operating members  315  which are provided on the inner side faces in the width direction of the cam body plate  314 . 
   Four openings  316  in a substantially rectangular shape elongated in the front-rear are formed in the cam body plate  314  at equal intervals in the front-rear. 
   The four operating members  315  are respectively arranged forward of the four rectangular openings  316 . Each of the operating members  315  is formed in a shape of crank in a side view, and integrally has a pressing portion  317  extending along an upper end edge of the cam body plate  314  for pressing the pressing projection  227  of the developing cartridge  27  downward, a separating portion  318  extending along a lower end edge of the cam body plate  314  for rotating the intermediate member  311 , as described below, and a connecting portion  319  which connects a rearward end of the pressing portion  317  to a forward end of the separating portion  318 . 
   As shown in  FIGS. 20 to 22 , a projected part  320  as an operating part projected upward is formed at the rearward end of the separating portion  318 . 
   The most forward operating member  315  has a different shape from the other three operating members  315  (hereinafter referred to as the three rearward operating members  315 ). Specifically, the pressing portion  317  of the most forward operating member  315  is formed longer in the front-rear, as compared with those of the three rearward operating members  315 . On the other hand, the separating portion  318  of the most forward operating member  315  is formed shorter in the front-rear, as compared with those of the three rearward operating members  315 . According to such difference in shape (size), it is possible to press the developing rollers  39  of all the developing cartridges  27  against all the photosensitive drums  29 , or to press the developing roller  39  of only the black developing cartridge  27 K against the photosensitive drum  29 , and further, to separate the developing rollers  39  of all the developing cartridges  27  from the photosensitive drums  29 , as described below. 
   The four intermediate members  311  are respectively disposed in rear of the four operating members  315  and opposed to the four rectangular openings  316  in the width direction. As shown in  FIGS. 20 to 22 , each of the intermediate members  311  are formed in a substantially L-shape in a side view like a block having a thickness in the width direction. An intermediate member support shaft  321  passes one end part of the intermediate member  311  in the width direction, so that the intermediate member  311  is rotatably supported by this intermediate support shaft  321 . A lower end part of the intermediate member  311  is opposed to the projected part  320  of the separating portion  318  leaving a space in the front-rear, in a state where the intermediate member  311  is not in contact with the separating portion  318  (See  FIG. 20 ). 
   The four intermediate member support shafts  321  are arranged at equal intervals in the front-rear (the same intervals as the intervals of the separating projections  212  in a state where the four developing cartridges  27  have been mounted to the drum unit  26 ), as shown in  FIG. 17 . When the intermediate members  311  which are supported by the intermediate member support shafts  321  have been inserted into the corresponding rectangular openings  316 , the intermediate member support shafts  321  extend in the width direction of the cam body plate  314 , and inward ends in the width direction of the intermediate member support shafts  321  are supported by the cam holder  312  so as not to rotate. 
   As shown in  FIG. 17 , the cam holder  312  integrally has the holder securing part  322  in a shape of thin plate extending in the front-rear along the inner side face of the body frame  301 , and the cam receiving part  323  continued from a lower end edge of the holder securing part  322 . 
   The holder securing part  322  is fixed to the inner side face of the body frame  301  with screws  324 . 
   The cam receiving part  323  is formed in a substantially C-shape in cross section extending inwardly in the width direction from the lower end edge of the holder securing part  322  along an entire length thereof, then, bent downwardly, and further bent outwardly in the width direction. By cutting away the cam receiving part  323  from its upper face to its inner side face continuously, four pressing projection receiving parts  325  which can receive the pressing projections  227  of the developing cartridges  27 , and four separating projection receiving parts  326  which can receive the separating projections  212  of the developing cartridges  27  are alternately formed. Specifically, the four pressing projection receiving parts  325  are formed in the cam receiving part  323  at the same intervals in the front-rear as the intervals of the pressing projections  227  in the state where the developing cartridges  27  have been mounted to the drum unit  26 . In the same manner, the four separating projection receiving parts  325  are formed in the cam receiving part  323  at the same intervals in the front-rear as the intervals of the separating projections  227  in the state where the developing cartridges  27  have been mounted to the drum unit  26 . The separating projection receiving parts  326  are respectively arranged in rear of the pressing projection receiving parts  325 . When the separating projections  212  have been respectively received in the separating projection receiving parts  326 , the separating projections  212  are respectively opposed to the intermediate members  311  from above. 
   The synchronous movement mechanism  313  is so constructed that a driving force for linear movement can be transmitted from the left side linear movement cam member  310  to the right side linear movement cam member  310 , following the linear movement of the left side linear movement cam member  310 . Specifically, as shown in  FIG. 18 , the synchronous movement mechanism  313  includes a left side rack gear  327  which is formed on an upper face of a rearward end of the left side linear movement cam member  310 , a left side pinion gear  328  which is meshed with the left side rack gear  327 , a right side rack gear  329  which is formed on an upper face of a rearward end of the right side linear movement cam member  310 , a right side pinion gear  330  which is meshed with the right side rack gear  329 , and a connecting shaft  331  to which the left side pinion gear  328  and the right side pinion gear  330  are fixed so as not to relatively rotate. 
   (4) Separating and Pressing Operation 
   Referring now to  FIGS. 19A to 22 , operation of the separating and pressing mechanism  303  will be described. 
   As shown in  FIGS. 19A and 20 , in a state where the linear movement cam member  310  has been moved to the most forward pressing position, the separating portions  318  of the operating members  315  are opposed to the intermediate members  311  which are arranged in rear of the separating portions  318 , in a non-contact manner spaced in the front-rear. A larger interval is formed between the separating portion  318  of the most forward operating member  315  and the intermediate member  311  arranged behind it than between the separating portions  318  of the three rearward operating members  315  and the intermediate members  311  respectively arranged behind them. 
   In this state, the developing cartridges  27  are in contact positions in which the developing rollers  39  are respectively in contact with the photosensitive drums  29 . The pressing portions  317  of the operating members  315  are butted against the pressing projections  227  of the developing cartridges  27  from above thereby to press the pressing projections  227  downward. In each of the developing cartridges  27 , by pressing the pressing projections  227  downward, the handle  214  is rotated around the support shaft  233  into the pressed state, as shown in  FIG. 14 , and the contact members  217  are pushed down by the handle  214  (the recesses  225 ) thereby to contract the coil springs  218 . Urging forces of the coil springs  218  due to this contraction are applied to the upper wall  202  of the developing frame  36 , and the developing frame  36  is urged downwardly, whereby the developing roller  39  is pressed against the photosensitive drum  29 . At this moment, the coil springs  218  will generate the urging forces from 1N to 20N. 
   When the left side linear movement cam member  310  is moved rearward from this state, the left side pinion gear  328  is rotated with this movement of the left side linear movement cam member  310 . This rotation of the left side pinion gear  328  is transmitted to the right side pinion gear  330  by way of the connecting shaft  331 , and the right side pinion gear  330  is rotated in the same direction as the left side pinion gear  328 , whereby the right side linear movement cam member  310  is moved rearward. 
   As the rearward movement of the linear movement cam member  310  proceeds, the pressing portions  317  of the three rearward operating members  315  are released from engagement with the pressing projections  227  of the developing cartridges  27 , and therefore, pressure on the pressing projections  227  by the pressing portions  317  are removed. Then, as shown in  FIG. 19B , the separating portions  318  of the three rearward operating members  315  are butted against the lower end parts of the intermediate members  311  which are respectively arranged in rear of them, and press the lower end parts of the intermediate members  311  rearward, whereby the intermediate members  311  are rotated around the intermediate member support shafts  321  to be lifted upward. During this rotation, the intermediate members  311  are butted against the separating projections  212  which are respectively positioned above them, from below. Because the forces in an upward direction are applied from the intermediate members  311  to the separating projections  212 , the yellow developing cartridge  27 Y, the magenta developing cartridge  27 M and the cyan developing cartridge  27 C is lifted upward. 
   As the rearward movement of the linear movement cam member  310  further proceeds, the one end parts of the intermediate members  311  (the end parts through which the intermediate member support shafts  321  are passed) are butted against the upper faces of the separating portions  318  of the three rearward operating members  315 , as shown in  FIGS. 19C and 21 . Then, the yellow developing cartridge  27 Y, the magenta developing cartridge  27 M and the cyan developing cartridge  27 C is positioned at the separated positions, whereby the developing rollers  39  of the yellow developing cartridge  27 Y, the magenta developing cartridge  27 M and the cyan developing cartridge  27 C is separated from the photosensitive drums  29 . On this occasion, the pressing projections  227  of the black developing cartridge  27 K are pressed by the pressing portions  317  of the operating members  315 . In this manner, the developing roller  39  of only the black developing cartridge  27 K is kept pressed against the photosensitive drum  29 . 
   Thereafter, as the rearward movement of the linear movement cam member  310  further proceeds, the pressing portion  317  of the most forward operating member  315  is released from engagement with the pressing projection  227  of the black developing cartridge  27 K, and pressure against the pressing projection  227  by the pressing portion  317  is removed. Then, as shown in  FIG. 19D , the separating portion  318  of the most forward operating member  315  is butted against the lower end part of the intermediate member  311  which is arranged behind it, and press the lower end part of the intermediate member  311  rearward, whereby the intermediate member  311  is rotated around the intermediate member support shaft  321  to be lifted upward. During this rotation of the intermediate member  311 , the intermediate member  311  is butted against the separating projection  212  which is positioned above it, from below. Because the force in an upward direction is applied from the intermediate member  311  to the separating projection  212 , the black developing cartridge  27 K is lifted upward. 
   As the rearward movement of the linear movement cam member  310  further proceeds, the one end part of the intermediate member  311  (the end part through which the intermediate member support shaft  321  is passed) is butted against the upper face of the separating portion  318  of the most forward operating member  315 , as shown in  FIGS. 19E and 22 . Then, the black developing cartridge  27 K is moved to the separated position, whereby the developing roller  39  of the black developing cartridge  27 K is separated from the photosensitive drum  29 . In this manner, the developing rollers  39  of all the developing cartridges  27  are in the state separated from the photosensitive drums  29 . 
   It should be noted that by moving the linear movement cam member  310  forward from the state in  FIG. 19E , the states respectively shown in  FIGS. 19A to 19D  can be recovered. On this occasion, the projected parts  320  of the respective separating portions  318  are engaged with the intermediate members  311 , thereby to rotate the intermediate members  311  in a direction of moving apart from the separating projections  212  (downward). 
   5. Drive Mechanism of the Linear Movement Cam Member 
     FIGS. 23 ,  24  and  25  are left side views schematically showing a drive mechanism for moving the linear movement cam member  310  between the pressing position and the releasing position. 
   The drive mechanism for moving the linear cam member  310  between the pressing position (See  FIGS. 19A and 20 ) and the releasing position (See  FIGS. 19E and 22 ) includes a drive input member  401  which is integrally provided on an outer side face in the width direction of the left side linear movement cam member  310 , a motor driving mechanism  402  which is disposed above the drive input member  401 , and a cover driving mechanism  403  which is disposed below the drive input member  401 . 
   The drive input member  401  is formed in a substantially rectangular shape in a plan view extending in the front-rear. A first rack gear  404  to which a driving force from the motor driving mechanism  402  is inputted is formed on an upper face of the drive input member  401 . The first rack gear  404  is formed along the entire length of the drive input member  401  in the front-rear. On the other hand, a second rack gear  405  to which a driving force from the cover driving mechanism  403  is inputted is formed on a lower face of the drive input member  401 . The second rack gear  405  is formed on the lower face of the drive input member  401  except a forward end area of the lower face, and the forward end area is defined as an untoothed part  471  where the second rack gear  405  is not formed. 
   The motor driving mechanism  402  is supported by the body frame  301  at the left side. The motor driving mechanism  402  includes a motor  406  as a drive source, having its output shaft  407  arranged so as to extend in the width direction, a motor gear  408  fixed to the output shaft  407  of the motor  406 , an intermediate gear  409  which is meshed with the motor gear  408 , a first input smaller gear  410  which is meshed with the first rack gear  404 , a first input larger gear  411  which has a larger diameter than the first input smaller gear  410  and rotates integrally with the first input smaller gear  410 , a first clutch mechanism  412  capable of switching rotation of the intermediate gear  409  between a transmission state in which the rotation of the intermediate gear  409  is transmitted to the first input larger gear  411  and an interruption state in which the transmission is interrupted, and a clutch switching mechanism  413  for switching the first clutch mechanism  412  between the transmission state and the interruption state. 
     FIG. 26  is a sectional view of the first clutch mechanism  412 . 
   The first clutch mechanism  412  has a structure of a so-called planetary differential clutch. Specifically, as shown in  FIG. 26 , the first clutch mechanism  412  includes a gear support shaft  421  extending in the width direction, and a drive input gear  422 , a drive output gear  423  and a planetary gear base member  424  which are rotatably supported by the gear support shaft  421 . 
   The drive input gear  422  is rotatably held on a right end part of the gear support shaft  421 . The drive input gear  422  integrally has a cylindrical gear boss portion  425  inserted over the gear support shaft  421 , and a protruded portion  426  having a round shape in a side view which is protruded in a radial direction from a right end part of the gear boss portion  425 . 
   A sun gear portion  427  which is meshed with planetary gears  435 , which will be described below, is formed on an outer peripheral face of the gear boss portion  425  at a left end part thereof. 
   A circumferential edge of the protruded portion  426  has a thickness in the width direction, and an input gear portion  428  to be meshed with the intermediate gear  409  (See  FIG. 23 ) is formed on an outer peripheral face of the protruded portion  426 . 
   The drive output gear  423  is rotatably held on a left end part of the gear support shaft  421 , and spaced from the drive input gear  422  in the width direction. The drive output gear  423  integrally has a cylindrical gear boss portion  429  inserted over the gear support shaft  421 , and a protruded portion  430  having a round shape in a side view which is protruded in a radial direction from a right end of the gear boss portion  429 . 
   An output gear portion  431  which is meshed with the first input larger gear  411  is formed on an outer peripheral face of the gear boss portion  429  at a left end part thereof. 
   A cylindrical portion  432  projecting toward the drive input gear  422  is formed on a right side face of the protruded portion  430  in the middle in a radial direction thereof. The cylindrical portion  432  is formed in a cylindrical shape surrounding the gear support shaft  421 , and opposed to the sun gear portion  427  of the drive input gear  422 . An inner gear portion  433  which is meshed with the planetary gears  435 , which will be described below, is formed on an inner peripheral face (a face opposed to the sun gear portion  427 ) of the cylindrical portion  432 . 
   The planetary gear base member  424  is arranged between the drive input gear  422  and the drive output gear  423  so as to rotate around the gear support shaft  421 . This planetary gear base member  424  has a round shape in a side view. 
   A plurality of planetary gear support portions  434  are integrally formed on the planetary gear base member  424  circumferentially around the gear support shaft  421 . Each of the planetary gear support portions  434  is formed as a protuberance having a substantially C-shape in cross section which is open to the right side and protruded to the left side (toward the drive output gear  423 ). The planetary gears  435  are respectively supported by the planetary gear support portions  434  so as to rotate (rotate on their own axes), and are meshed with the sun gear portion  427  of the drive input gear  422  and the inner gear portion  433  of the drive output gear  423 . 
   At a circumferential edge of the planetary gear base member  424 , a gear forming portion  437  in a cylindrical shape surrounding the gear support shaft  421  is formed so as to project to the left side. A securing gear  436  to be meshed with a lock gear  447  of a clutch switching lever  441 , which will be described below is formed on an outer peripheral face of the gear forming portion  437 . 
   In a state where the lock gear  447  of the clutch switching lever  441  is locked to the securing gear  436  of the planetary gear base member  424 , the rotation of the planetary gear base member  424  is restricted. Then, the rotation force is inputted from the intermediate gear  409  to the input gear portion  428  of the drive input gear  422  thereby to rotate the drive input gear  422 . When the drive input gear  422  is rotated, the planetary gears  435  will rotate on their own axes without changing their positions in the circumferential direction around the gear support shaft  421 . Due to the rotations of the planetary gears  435 , the drive output gear  423  having the inner gear portion  433  which is meshed with the planetary gears  435  is rotated around the gear support shaft  421 . Then, the rotation force of the drive output gear  423  is transmitted to the first input larger gear  411  thereby to rotate the first input larger gear  411 . By the rotation of the first input larger gear  411 , transmission of the rotation force of the intermediate gear  409  to the first input larger gear  411  is achieved. 
   On the other hand, in a state where the clutch switching lever  441  is separated from the planetary gear base member  424 , the planetary gear base member  424  can be rotated around the gear support shaft  421 . Accordingly, even though the rotation force of the intermediate gear  409  is inputted to the input gear portion  428  of the drive input gear  422 , the drive output gear  423  will not rotate, because the planetary gears  435  will revolve around the gear support shaft  421  (the planetary gear base member  424  rotates around the gear support shaft  421 ), while they rotate on their own axes. To the contrary, even though the drive output gear  423  is rotated in a state where the drive input gear  422  is stopped (in a state where a brake force is exerted on the drive input gear  422  due to stop of the motor  406 ), the planetary gears  435  will revolve around the gear support shaft  421  (the planetary gear base member  424  rotates around the gear support shaft  422 ) while they rotate on their own axes, following the rotation of the drive output gear  423 , provided that the clutch switching lever  441  is separated from the planetary gear base member  424 . For this reason, in the state where the clutch switching lever  441  is separated from the planetary gear base member  424 , there is no connection between the drive input gear  422  and the drive output gear  423 , and the transmission of the rotation force of the intermediate gear  409  to the first input larger gear  411  is interrupted. 
   Specifically, when the lock gear  447  of the clutch switching lever  441  is locked to or separated from the securing gear  436  of the planetary gear base member  424 , the first clutch mechanism  412  is switched between a transmission state in which the rotation force of the intermediate gear  409  is transmitted to the first input larger gear  411  and an interruption state in which the transmission is interrupted. 
   As shown in  FIGS. 23 to 25 , a clutch switching mechanism  413  has the clutch switching lever  441  provided in front of the first clutch mechanism  412 , a lever moving mechanism  442  for moving the clutch switching lever  441  rear and forth in the front-rear, and an operating piece  443  for operating the lever moving mechanism  442  in association with opening and closing motions of the front cover  9 . 
   The clutch switching lever  441  integrally has an arm portion  444  extending in the front-rear, and a lock portion  446  connected to a rearward end of the arm portion  444 . 
   The arm portion  444  is formed in a substantially L-shape in a side view having its front end part bent upward. Moreover, an elongated hole  445  is formed in a center part in the front-rear of the arm portion  444  along the front-rear thereof. 
   The lock portion  446  is formed in a substantially C-shape in a side view which is open to the rear side, and the lock gear  447  which is meshed with the securing gear  436  of the planetary gear base member  424  is formed on a circular face of the C-shape. 
   The lever moving mechanism  442  has a pair of opposed pieces  448  which are opposed to each other interposing the arm portion  444  of the clutch switching lever  441 , an opposed piece support shaft  449  extending in the width direction to rotataly support the pair of opposed pieces  448 , a lever operating shaft  450  extending in the width direction at the middle in the front-rear of the opposed pieces  448  and inserted through the elongated hole  445  of the clutch switching lever  441 , a lock boss  451  extending in the width direction between rearward ends of the opposed pieces  448  to which the operating pieces  443  is locked, and a coil spring  452  which is suspended between front ends of the opposed pieces  448  and a rearward end of the clutch switching lever  441 . 
   It is to be noted that the opposed piece  448  at the right side only is shown in  FIGS. 23 to 25 , and the opposed piece  448  at the left side is omitted to simplify the drawings. 
   The operating piece  443  is formed in a substantially trapezoidal shape in a side view extending in the front-rear from the inner face of the front cover  9  toward a position below the lock boss  451  of the lever moving mechanism  442 , in a state where the front cover  9  is closed. 
   The cover driving mechanism  403  is opposed to the linear movement cam member  310  from below in the vertical direction, and provided with a cover associated moving member  461  which extends in the front-rear. This cover associated moving member  461  is connected to the front cover  9  by means of a link mechanism which is not shown, and adapted to linearly move forward in association with the opening motion of the front cover  9  and to linearly move rearward in association with the closing motion of the front cover  9 . 
   A rack gear member  462  is integrally provided on an outer side face in the width direction of the cover associated moving member  461 , and a rack gear  466  is formed on an upper face of the rack gear member  462 . The rack gear  466  is formed on the upper face of the rack gear member  462  except a forward end area of the upper face, and the forward end area is defined as an untoothed part  467  where the rack gear  466  is not formed. 
   Further, the cover drive mechanism  403  includes a pinion gear  463  which can be meshed with the rack gear  466 , a second input gear  464  which can be meshed with the second rack gear  405 , and a second clutch mechanism  465  which can be switched between a transmission state where rotation force of the pinion gear  463  is transmitted to the second input gear  464  and an interruption state where the transmission is interrupted. 
   The pinion gear  463  is in contact with the untoothed part  467  of the rack gear member  462 , in the state where the front cover  9  is closed. 
   The second input gear  464  is provided above the pinion gear  463 . In the closed state of the front cover  9 , the second input gear  464  is opposed to the second rack gear  405  on the lower face of the drive input member  401  leaving a space in the vertical direction. 
   The second clutch mechanism  465  is arranged in front of the pinion gear  463  and the second input gear  464 , and includes a rotary gear  468  which is meshed with them, and a swinging arm  469 . One end of the swinging arm  469  is held on a rotation shaft of the rotary gear  468  so as to rotate with an adequate friction resistance, and the other end thereof rotatably supports the second input gear  464 . 
   In the state where the front cover  9  is closed, the upper end edge of the operating piece  443  is butted against the lock boss  451  of the lever moving mechanism  442  from below, as shown in  FIG. 23 , whereby the locking boss  451  is lifted by the operating piece  443  to a position opposed to the lever operating shaft  450  in the front-rear. At this moment, the clutch switching lever  441  is positioned in the most rearward position, and the lock gear  447  is locked to the securing gear  436  of the planetary gear base member  424  in the first clutch mechanism  412 . In this manner, the first clutch mechanism  412  is in the transmission state where the rotation force of the intermediate gear  409  can be transmitted to the first input larger gear  411 . 
   Meanwhile, the second input gear  464  is opposed to the second rack gear  405  on the lower face of the drive input member  401  leaving a space in the vertical direction, and not related with the movement of the second rack gear  405  (the linear movement cam member  310 ). 
   When drive of the motor  406  is controlled in this state, and the output shaft  407  of the motor  406  is rotated in one direction (a direction as shown by an arrow mark in  FIG. 23 ), the rotation force (driving force) is transmitted to the intermediate gear  409  by way of the motor gear  408 , and further transmitted to the first input larger gear  411  by way of the first clutch mechanism  412 . Then, the first input smaller gear  410  is rotated integrally with the first input larger gear  411 , and the rotation force inputted from the first input smaller gear  410  to the first rack gear  404  will move the linear movement cam member  310  linearly rearward. On the other hand, when the output shaft  407  of the motor  406  is rotated in the other direction (a direction opposite to the direction as shown by the arrow mark in  FIG. 23 ), the rotation force is inputted to the first rack gear  404  by way of the motor gear  408 , the intermediate gear  409 , the first clutch mechanism  412 , the first input larger gear  411 , and the first input smaller gear  410 , whereby the linear movement cam member  310  will linearly move forward. 
   Due to such reciprocating linear movements of the linear movement cam member  310 , the linear movement cam member  310  is moved between the pressing position and the releasing position. Accordingly, it is possible to displace the linear movement cam member  310  to the state in which all the developing rollers  39  are pressed against the photosensitive drums  29 , the state in which the developing roller  39  of only the black developing cartridge  27 K is pressed against the photosensitive drum  29 , and the state in which all the developing rollers  39  are separated from the photosensitive drums  29 . 
   As the front cover  9  is opened, the operating piece  443  is separated from the lock boss  451 , as shown in  FIG. 24 . When the operating piece  443  has been separated from the lock boss  451 , the pair of opposed pieces  448  are tilted around the opposed piece support shaft  449  under their own weights in a direction of lowering the lock boss  451 . Consequently, the lever operating shaft  450  of the lever moving mechanism  442  is locked to a front end part of the elongated hole  445  in the clutch switching lever  441 , and a force directed forward is applied to the front end part of the elongated hole  445  from the lever operating shaft  450  thereby to move the clutch switching lever  441  forward. As the results, the locking gear  447  of the clutch switching lever  441  is separated from the securing gear  436 , and the first clutch mechanism  412  is in the interruption state in which the rotation force of the intermediate gear  409  to the larger first input gear  411  is interrupted. 
   Meanwhile, in association with the opening motion of the front cover  9 , the cover associated moving member  461  will linearly move forward. Then, the pinion gear  463  is meshed with the rack gear  466  of the rack gear member  462 , and is rotated counterclockwise, as shown in  FIG. 24 , by the movement of the rack gear  466 . Due to this rotation of the pinion gear  463 , the rotary gear  468  is rotated clockwise, as shown in  FIG. 24 . When the rotary gear  468  is rotated, the second input gear  464  meshed with the rotary gear  468  is rotated counterclockwise in  FIG. 24 , and the swinging arm  469  is rotated counterclockwise about a rotation shaft of the rotary gear  468 , as shown in  FIG. 24 . Consequently, the second input gear  464  is meshed with the second rack gear  405  on the lower face of the drive input member  401 , whereby the second clutch mechanism  465  is switched to the transmission state in which the rotation force of the pinion gear  463  is transmitted to the second input gear  464 . Then, the rotation force of the second input gear  464  is inputted to the second rack gear  405 , and the linear movement cam member  310  is moved rearward by the rotation force. 
   Thereafter, the front cover  9  is further opened. When the linear movement cam member  310  has been further moved rearward, in association with the opening motion, to be positioned at the releasing position, the second input gear  464  is opposed to the untoothed part  471 , as shown in  FIG. 25 . Therefore, even though the second input gear  464  is further rotated, the rotation force will not be inputted to the second rack  405 . 
   When the front cover  9  is closed thereafter, the cover associated moving member  461  is linearly moved rearward in association with the closing motion. As the rack gear  466  moves, the pinion gear  463  is rotated clockwise in  FIG. 25 . Then, the rotary gear  468  is rotated counterclockwise in  FIG. 25  by the rotation force of the pinion gear  463 . As the rotary gear  468  is rotated, the swinging arm  469  is rotated by the rotation force counterclockwise in  FIG. 24  around the rotation shaft of the rotary gear  468 , whereby the second input gear  464  is separated from the second rack gear  405  on the lower face of the drive input member  401 . In this manner, the second clutch mechanism  465  is switched to the interruption state in which transmission of the rotation force of the pinion gear  463  to the second input gear  464  is interrupted. 
   6. Operation and Effects 
   According to the above described structure, while the front cover  9  is closed, it is possible to press the developing roller  39  against the photosensitive drum  29  or to release the pressure, by inputting the driving force of the motor  406  to the first rack gear  404  by way of the first input smaller gear  410  to move the linear movement cam member  310  between the pressing position and the releasing position. Moreover, because the linear movement cam member  310  is moved to the releasing position by the force inputted to the second rack gear  405  by way of the second input gear  464  in association with the opening motion of the front cover  9 , pressures of all the developing rollers  39  against all the photosensitive drums  29  can be released, in the state where the front cover  9  is opened. For this reason, the developing cartridges  27  can be smoothly detached from the body casing  2 . 
   Moreover, because the drive input member  401  having the first rack gear  404  and the second rack gear  405  is integrally provided with the linear movement cam member  310 , it is possible to reliably move the linear movement cam member  310  by the force inputted to the first rack gear  404  by way of the first input smaller gear  410 , and also possible to reliably move the linear movement cam member  310  by the force inputted to the second rack gear  405  by way of the second input gear  464 . In this manner, pressure of the developing rollers  39  against the photosensitive drums  29  can be reliably exerted and released. 
   Further, transmission and interruption of the driving force from the motor  406  to the first input smaller gear  410  (the first input larger gear  411 ) can be switched by the first clutch mechanism  412 . Therefore, it is possible to transmit the driving force from the motor  406  to the first input smaller gear  410 , and to move the linear movement cam member  310  by the driving force. It is also possible to interrupt the driving force from the motor  406  to the first input gear, and to move the linear movement cam member  310  by the force which is inputted from the second input gear  464  to the second rack gear  405 , irrespective of the motor  406 . In other words, in case where the linear movement cam member  310  is moved by the driving force of the motor  406 , the driving force is transmitted to the first input gear, and in other cases, connection between the motor  406  and the first input smaller gear  410  is interrupted, whereby it is possible to prevent the motor  406  from hindering the movement of the linear movement cam member  310 . 
   Still further, because the first clutch mechanism  412  can be switched to the transmission state in association with the closing motion of the front cover  9 , it is possible to move the linear movement cam member  310  thereafter, by the driving force of the motor  406 . Moreover, because the first clutch mechanism  412  can be switched to the interruption state in association with the opening motion of the front cover  9 , it is possible to move the linear movement cam member  310  thereafter, by the driving force inputted to the second rack gear  405 , irrespective of the motor  406 . 
   Further, in the first clutch mechanism  412 , it is possible to reliably transmit the driving force from the motor  406  to the first input larger gear  411  which is meshed with the output gear part  431  of the drive output gear  423 , in the state where the front cover  9  is closed. On the other hand, in the state where the front cover  9  is opened, it is possible to reliably interrupt the transmission of the driving force from the motor  406  to the first input larger gear  411 . 
   Because the second input gear  464  is rotated in association with the opening motion and the closing motion of the front cover  9 , it is possible to generate the rotation force of the second input gear  464  in association with the opening motion and the closing motion of the front cover  9 . 
   Then, it is possible to switch the transmission and interruption of the rotation force of the second input gear  464  to the second rack gear  405  by the second clutch mechanism  465 . In this manner, it is possible to transmit the rotation force of the second input gear  464  to the second rack gear  405  thereby to move the linear movement cam member  310 . On the other hand, it is possible to input the driving force from the motor  406  to the first rack gear  404 , by interrupting the transmission of the rotation force of the second input gear  464  to the second rack gear  405 , thereby to move the linear movement cam member  310 . 
   Further, during the opening motion of the front cover  9 , it is possible to switch the second clutch mechanism  465  to the transmission state, and to transmit the rotation force of the second input gear  464  to the second rack gear  405  thereby to move the linear movement cam member  310 . Because the linear movement cam member  310  is moved from the pressing position to the releasing position by the rotation force of the second input gear  464  at this time, the pressure of the developing rollers  39  against the photosensitive drums  29  can be reliably removed. On the other hand, because the second clutch mechanism  465  can be switched to the interruption state during the closing motion of the front cover  9 , it is possible to move the linear movement cam member  310  to the pressing position and the releasing position after the closing motion of the front cover, by inputting the driving force from the motor  406  to the first rack gear  404 . 
   Further, in the second clutch mechanism  465 , the rotary gear  468  is rotated in one direction in association with the opening motion of the front cover  9 , and following this rotation, the second input gear  464  is meshed with the second rack gear  405  while rotating. In this manner, it is possible to reliably transmit the rotation force of the second input gear  464  to the second rack gear  405  during the opening motion of the front cover  9 . Moreover, the rotary gear  468  is rotated in the other direction in association with the closing motion of the front cover  9 , and following this rotation, the second input gear  464  is separated from the second rack gear  405 . In this manner, it is possible to reliably interrupt the transmission of the rotation force of the second input gear  464  to the second rack gear  405  during the closing motion of the front cover  9 . 
   Further, it is possible to move the linear movement cam member  310  to the releasing position by the force which is inputted from the second input gear  464  to the second rack gear  405  in association with the opening motion of the front cover  9 . Accordingly, in the state where the front cover  9  is opened, it is possible to reliably remove the pressure of the developing rollers  39  against the photosensitive drums  29 . 
   Further, when the linear movement cam member  310  has been moved to the releasing position, the second input gear  464  is opposed to the untoothed part  471 . Therefore, even though the second input gear  464  is rotated further, the rotation will not be inputted to the second rack gear  405 . Accordingly, it is possible to prevent the linear movement cam member  310  from moving beyond the releasing position, and to prevent occurrence of troubles due to such excessive movement of the linear movement cam member  310 .