Patent Publication Number: US-8126358-B2

Title: Image forming apparatus having member for moving developer carrier in and out of contact with photosensitive element included in drum unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of co-pending U.S. application Ser. No. 12/394,580, filed Feb. 27, 2009, which claims priority to Japanese Patent Application No. 2008-050665 filed on Feb. 29, 2008, the disclosure of which is hereby incorporated into the present application by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus such as a color laser printer. 
     BACKGROUND 
     The so-called tandem type color printer is known as an electrophotographic color printer formed by parallelly arranging photosensitive drums corresponding to yellow, magenta, cyan and black respectively. 
     The tandem type color printer includes a developing roller opposed to each photosensitive drum. An electrostatic latent image is formed on the surface of the photosensitive drum. When the electrostatic latent image is opposed to the developing roller following rotation of the photosensitive drum, the developing roller feeds a toner to the electrostatic latent image. Thus, a toner image is formed on the surface of the photosensitive drum. A sheet is transported by a belt to be successively opposed to the photosensitive drums respectively. Toner images of the corresponding colors are formed on the photosensitive drums respectively and transferred to the sheet in a superposed manner, to form a color image on the sheet. On the other hand, a black toner image is formed only on the black photosensitive drum and transferred to the sheet, to form a monochromatic image on the sheet. 
     When the monochromatic image is formed, no toner images are formed on the remaining photosensitive drums of yellow, magenta and cyan except black, and hence the corresponding developing rollers are desirably separated from the yellow, magenta and cyan photosensitive drums respectively, to be prevented from consumption. 
     Therefore, an image forming apparatus including a translation cam member linearly movable in the direction of arrangement of photosensitive drums and intermediate members displaced by linear movement of the translation cam member for separating the photosensitive drums from developing rollers by pressing developing apparatuses upward is proposed as a tandem type image forming apparatus. The translation cam member so linearly moves as to switch the image forming apparatus to an all-color separation state where the developing rollers are separated from all photosensitive drums, a black contact state where the corresponding developing roller is in contact with a black photosensitive drum while the remaining developing rollers are separated from yellow, magenta and cyan photosensitive drums, and an all-color contact state where the developing rollers are in contact with all photosensitive drums. 
     In the image forming apparatus according to this proposal, the photosensitive drums of the four colors are integrally retained on a drum unit, which is mountable on/detachable from a main body. The developing apparatuses of the respective colors are detachably mounted on the drum unit. 
     In the conventional structure according to this proposal, however, the drum unit may positionally deviate from the main body due to force applied from the intermediate members to the developing apparatuses when the former presses the latter. When the drum unit positionally deviates from the main body, an image formed on a sheet also positionally deviates following this deviation. 
     SUMMARY 
     One aspect of the present invention may provide an image forming apparatus capable of preventing a tandem type photosensitive unit from positionally deviating from a main body when a developing roller is separated from a photosensitive drum. 
     The same or different aspect of the present invention may provide an image forming apparatus including: a main body; a tandem type photosensitive unit, integrally retaining a plurality of photosensitive drums in a parallelly arranged state, mounted in the main body to be movable along the direction of the arrangement of the photosensitive drums; a developer cartridge provided correspondingly to each photosensitive drum, retaining a developing roller for feeding a developer to the corresponding photosensitive drum and detachably mounted on the tandem type photosensitive unit; a translation member provided in the main body to be linearly movable in the direction of the arrangement; and a link member provided correspondingly to each developer cartridge, swingably supported on a first support shaft projectingly provided on the tandem type photosensitive unit and displaced to a pressing position where a first end portion thereof is opposed to a prescribed portion of each developer cartridge while a second end portion opposite to the first end portion is pressed by the translation member following linear movement of the translation member so that the first end portion presses the prescribed portion in a direction for separating the developing roller from the photosensitive drum and a releasing position where the translation member is released from pressing the second end portion so that the first end portion is released from pressing the prescribed portion, while each developer cartridge is mounted on the tandem type photosensitive unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side sectional view showing a color laser printer according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of a drum unit shown in  FIG. 1  as viewed from above the left front side, in a state where one of developer cartridges is mounted while the remaining developer cartridges are detached. 
         FIG. 3  is a perspective view of the drum unit as viewed from above the left front side, in a state where one of the developer cartridges is in the process of mounting/detachment while the remaining developer cartridges are detached. 
         FIG. 4  is a front elevational view of the drum unit. 
         FIG. 5  is a sectional view of the drum unit taken along the line A-A in  FIG. 4 . 
         FIG. 6  is a right side elevational view of a first side plate shown in  FIG. 4 . 
         FIG. 7A  is a left side elevational view of a left second side plate shown in  FIG. 2 . 
         FIG. 7B  is a right side elevational view of the left second side plate. 
         FIG. 8A  is a right side elevational view of a right second side plate shown in  FIG. 2 . 
         FIG. 8B  is a left side elevational view of the right second side plate. 
         FIG. 9  is a perspective view of a left shaft receiving member shown in  FIG. 5 . 
         FIG. 10  is a perspective view of a right shaft receiving member shown in  FIG. 5 . 
         FIG. 11A  is a perspective view of a front beam, four drum subunits, a rear beam and a pair of first side plates shown in  FIG. 3  as viewed from above the left front side, in a state to be combined with one another. 
         FIG. 11B  is a perspective view as viewed from above the left front side for illustrating a state of combining photosensitive drums with the pair of first side plates. 
         FIG. 11C  is a perspective view as viewed from above the left front side for illustrating a state, subsequent to the state shown in  FIG. 11B , of combining the photosensitive drums with the pair of first side plates. 
         FIG. 11D  is a perspective view as viewed from above the left front side for illustrating a state, subsequent to the state shown in  FIG. 11C , of combining the photosensitive drums with the pair of first side plates. 
         FIG. 11E  is a perspective view showing a state where the right shaft receiving member is engaged with the corresponding first side plate. 
         FIG. 11F  is a perspective view as viewed from above the left front side showing a state of combining the right and left second side plates with the pair of first side plates. 
         FIG. 12  is a sectional view of principal parts showing an earthing structure of each photosensitive drum. 
         FIG. 13  is a partial sectional view of the drum unit as viewed from the left side. 
         FIG. 14  is a perspective view of principal parts of each pressing member and each link lever shown in  FIG. 13 . 
         FIG. 15  is a side elevational view of a translation cam provided on a main body casing shown in  FIG. 1 . 
         FIG. 16  is a perspective view of principal parts showing a state where all developer cartridges are in pressure contact with the photosensitive drums. 
         FIG. 17  is a perspective view of principal parts showing a state where only a black developer cartridge is in pressure contact with the corresponding photosensitive drum. 
         FIG. 18  is a perspective view of principal parts showing a state where all developer cartridges are separated from the photosensitive drums. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is now described with reference to the drawings. 
     1. Overall Structure of Color Laser Printer 
       FIG. 1  is a side sectional view showing a color laser printer according to an embodiment as an example of an image forming apparatus according to the present invention. 
     A color laser printer  1  is a tandem type color laser printer including a main body casing  2  as an example of a main body. A front cover  9  is provided on one side surface of the main body casing  2  in an openable/closable manner. 
     In the following description, it is assumed that the side (right side in  FIG. 1 ) provided with the front cover  9  is the front side, and the opposite side (left side in  FIG. 1 ) is the rear side. The right and left sides are decided with reference to the color laser printer  1  as viewed from the front side. The right-and-left direction may hereinafter be referred to as the width direction. A drum unit  26  is described with reference to directions in a state mounted on the main body casing  2 , unless otherwise stated. 
     The drum unit  26  as an example of a tandem type photosensitive unit is provided in the main body casing  2 . A mounting port  8  is formed on the front surface of the main body casing  2 . The drum unit  26  is mountable in and detachable from the main body casing  2  through the mounting port  8  in a state where the front cover  9  is opened. 
     The drum unit  26  includes four photosensitive drums  29 , four drum subunits  28  and four developer cartridges  27 . The developer cartridges  27  are mountable on and detachable from the photosensitive drums  29  and the drum subunits  28 . 
     The photosensitive drums  29  are provided correspondingly to black, yellow, magenta and cyan respectively. The photosensitive drums  29  are parallelly arranged along the anteroposterior direction in the order of black, yellow, magenta and cyan at regular intervals from one another in the anteroposterior direction. 
     The drum subunits  28  are provided in one-to-one correspondence to the photosensitive drums  29 . The drum subunits  28  are arranged at the back of the photosensitive drums  29  corresponding thereto respectively. The drum subunits  28  retain scorotron chargers  30  and cleaning brushes  31  respectively. 
     The developer cartridges  27  are provided in one-to-one correspondence to the photosensitive drums  29 . The developer cartridges  27  are arranged in front of the photosensitive drums  29  corresponding thereto respectively. The developer cartridges  27  include boxy casings  36  whose lower end portions are opened rearward. Agitators  37 , feed rollers  38 , developing rollers  39  and layer-thickness regulating blades  40  are retained in the casings  36 . The developing rollers  39  are arranged to be exposed rearward from the casings  36 , and rotatably supported on both sidewalls of the casings  36  in the right-and-left direction. Both end portions of roller shafts  51  of the developing rollers  39  protrude outward from both sidewalls of the casings  36  in the right-and-left direction. The casings  36  accommodate toners of the respective colors. Detection windows  46  are formed on both sidewalls of the casings  36  in the right-and-left direction. The detection windows  46  transmit detection light for detecting the quantities of the toners remaining in the casings  36 . According to this embodiment, the developing rollers  39  and the photosensitive drums  29  are in contact with one another. One-component developers are employed as the toners. In other words, the color laser printer  1  according to this embodiment employs a contact one-component developing system. 
     The surfaces of the photosensitive drums  29  are uniformly charged by the scorotron chargers  30 . Thereafter the surfaces of the photosensitive drums  29  are selectively exposed by light from an exposure unit  20  arranged above the drum unit  26 . Electrostatic latent images based on image data are formed on the surfaces of the photosensitive drums  29  due to this exposure. In the developer cartridges  27 , on the other hand, the toners accommodated in the casings  36  are agitated and transported by the agitators  37 , and fed onto the developing rollers  39  through the feed rollers  38 . The toners fed onto the developing rollers  39  are regulated to constant thicknesses by the layer-thickness regulating blades  40 , and carried on the developing rollers  39  as thin layers. When the electrostatic latent images are opposed to the developing rollers  39  following rotation of the photosensitive drums  29 , the developing rollers  39  feed the toners to the electrostatic latent images, for visualizing the electrostatic latent images with the toners. Thus, toner images are formed on the surfaces of the photosensitive drums  29 . 
     A sheet feeding cassette  10  accommodating sheets  3  is arranged on a bottom portion of the main body casing  2 . The sheets  3  accommodated in the sheet feeding cassette  10  are transported onto a transport belt  58  by various rollers. The transport belt  58  is opposed to the four photosensitive drums  29  from below. Transfer rollers  59  are arranged on positions opposed to the photosensitive drums  29  through an upper portion of the transfer belt  58  respectively. The sheets  3  transported onto the transport belt  58  successively pass through the spaces between the transport belt  58  and the photosensitive drums  29  due to traveling of the transport belt  58 . The toner images formed on the surfaces of the photosensitive drums  29  are transferred to the sheets  3  by transfer biases applied to the transfer rollers  59 , when the same are opposed to the sheets  3 . 
     A fuser  23  is provided on a downstream side of the transport belt  58  in the direction for transporting the sheets  3 . The sheets  3  having the toner images transferred thereto are transported to the fuser  23 . The fuser  23  fixes the toner images to the sheets  3  by heating and pressing. The sheets  3  having the toner images fixed thereto are ejected onto a sheet ejection tray  68  provided on the upper surface of the main body casing  2  by various rollers. 
     2. Drum Unit 
       FIG. 2  is a perspective view of the drum unit as viewed from above the left front side, in a state where one of the developer cartridges is mounted while the remaining developer cartridges are detached.  FIG. 3  is a perspective view of the drum unit as viewed from above the left front side, in a state where one of the developer cartridges is in the process of mounting/detachment while the remaining developer cartridges are detached.  FIG. 4  is a front elevational view of the drum unit.  FIG. 5  is a sectional view of the drum unit taken along the line A-A in  FIG. 4 .  FIG. 6  is a left side elevational view of a first side plate.  FIG. 7A  is a left side elevational view of a left second side plate.  FIG. 7B  is a right side elevational view of the left second side plate.  FIG. 8A  is a right side elevational view of a right second side plate.  FIG. 8B  is a left side elevational view of the right second side plate.  FIG. 9  is a perspective view of a left shaft receiving member.  FIG. 10  is a perspective view of a right shaft receiving member.  FIG. 11A  is a perspective view of a front beam, the four drum subunits, a rear beam and a pair of first side plates as viewed from above the left front side, in a state to be combined with one another.  FIGS. 11B to 11D  are perspective views as viewed from above the left front side for illustrating a state of combining the photosensitive drums with the pair of first side plates,  FIG. 11E  is a perspective view showing a state where the right shaft receiving member is engaged with the corresponding first side plate, and  FIG. 11F  is a perspective view as viewed from above the left front side showing a state of combining the right and left second side plates with the pair of first side plates. 
     The drum unit  26  includes a front beam  75 , a rear beam  76 , a pair of right and left first side plates  77  (see  FIG. 5 ) and a pair of right and left second side plates  78  in addition to the four photosensitive drums  29 , the four drum subunits  28  and the four developer cartridges  27 , as shown in  FIG. 3 . 
     In the drum unit  26 , the four photosensitive drums  29 , the four drum subunits  28 , the four developer cartridges  27 , the front beam  75 , the rear beam  76 , the pair of first side plates  77  and the pair of second side plates  78  are slidably mounted in/detached from the main body casing  2  (see  FIG. 1 ). 
     (1) Drum Subunit 
     Each drum subunit  28  includes a pair of side frames  83  opposed to each other at an interval in the width direction and a center frame  84  extended between the side frames  83 , as shown in  FIG. 11A . 
     Each side frame  83  is made of resin, and in the form of a generally triangular plate in side elevational view. A threaded portion  85  is formed on the rear lower end portion of the side frame  83 . The side frame  83  is provided with cylindrical bosses  86  formed on a position above the threaded portion  85  and a position in front of this position to protrude outward in the width direction respectively. 
     The center frame  84  is made of resin, and in the form of a rectangle elongated in the width direction in plan view. The center frame  84  retains the scorotron charger  30  and the cleaning brush  31  for cleaning the surface of the corresponding photosensitive drum  5 , as shown in  FIG. 1 . The pair of side frames  83  and the center frame  84  are integrally molded. 
     (2) Front Beam 
     The front beam  75  is made of resin. As shown in  FIGS. 3 and 5 , the front beam  75  is arranged in front of the four drum subunits  28  and the four photosensitive drums  29 , and extended between the pair of first side plates  77 . 
     The front beam  75  retains a support shaft  90 . The support shaft  90  is so arranged as to pass through the front beam  75  along the width direction. Both ends of the support shaft  90  protrude outward from the front beam  75  in the width direction, and further pass through the first and second side plates  77  and  78  to protrude outward in the width direction. 
     The support shaft  90  rotatably supports a front-side grasp portion  89 , which is arranged at a central portion of the front beam  75  in the width direction. The front-side grasp portion  89  is generally U-shaped. Free end portions of the front-side grasp portion  89  are rotatably supported by the support shaft  90 , so that the front-side grasp portion  89  is swingable between an accommodated position upright along the front beam  75  and an operating position inclined frontward from the front beam  75 . 
     On each side surface of the front beam  75  in the width direction, a first threaded portion  93  is formed on a rear lower end portion, as shown in  FIG. 11A . On each side surface of the front beam  75  in the width direction, further, second threaded portions  95  are formed on a central portion and a front upper position respectively. 
     (3) Rear Beam 
     The rear beam  76  is made of resin. The rear beam  76  is arranged at the back of the four drum subunits  28  and the four photosensitive drums  29  and extended between the pair of first side plates  77 , as shown in  FIGS. 3 and 5 . 
     A back-side grasp portion  91  is integrally formed on a central portion of the rear beam  76  in the width direction. The back-side grasp portion  91  is generally U-shaped in rear elevational view. Free end portions of the back-side grasp portion  91  are coupled to the rear beam  76 , so that the back-side grasp portion  91  is inclined from a rear lower side toward a front upper side, to protrude obliquely upward from the rear beam  76 . 
     On each side surface of the rear beam  76  in the width direction, a first threaded portion  97  is formed on a front upper end portion, as shown in  FIG. 11A . On each side surface of the rear beam  76  in the width direction, further, a second threaded portion  99  is formed on a rear end portion. 
     (4) First Side Plate 
     The right and left first side plates  77  are prepared by press-working metallic plates with the same press die, to have the same shapes. 
     Each first side plate  77  is generally in the form of an elongated rectangular plate extending in the anteroposterior direction, as shown in  FIG. 6 . The front and rear end portions of the first side plates  77  are opposed to the front beam  75  and the rear beam  76  respectively, as shown in  FIG. 11B . 
     The front end portion of each first side plate  77  extends from a rear lower side toward a front upper side. In the front end portion of each first side plate  77 , a first penetration hole  100  is formed on a position opposed to the corresponding first threaded portion  93  of the front beam  75 . In the front end portion of each first side plate  77 , further, a second penetration hole  101  is formed on a position opposed to the corresponding second threaded portion  95  of the front beam  75 . In addition, a support shaft insertion hole  113  receiving the support shaft  90  is formed on the front end portion of each first side plate  77 . 
     The rear end portion of each first side plate  77  is generally L-shaped in side elevational view. More specifically, the rear end portion of each first side plate  77  is inclined toward a rear upper side, to further extend upward. In the rear end portion of each first side plate  77 , a third penetration hole  102  is formed on a position opposed to the corresponding first threaded portion  97  of the rear beam  76 . In the rear end portion of each first side plate  77 , further, a fourth penetration hole  103  is formed on a position opposed to the corresponding second threaded portion  99  of the rear beam  76 . 
     Four circular drum retaining holes  104  are formed on each first side plate  77 . The drum retaining holes  104  are formed between the front and rear end portions of each first side plate  77  at regular intervals from one another in the anteroposterior direction. 
     Developer receiving grooves  110  generally V-shaped in side elevational view are formed in each first side plate  77  on positions above the front upper sides of the drum retaining holes  104  by notching upper edge portions of the first side plate  77  generally triangularly in side elevational view respectively. The developer receiving grooves  110  are so formed as not to interfere with the lower end portions of the casings  36  of the developer cartridges  27 . 
     An engaging groove  105 A is formed on the rear end portion of each developer receiving groove  110 . Ends of clip members  106  (see  FIG. 11E ) described later are engaged with the engaging grooves  105 A respectively. On the lower end portion of each first side plate  77 , other engaging grooves  105 B are formed under each engaging grooves  105 A respectively. The other end of the clip member  106  is engaged with the corresponding engaging groove  105 B. 
     In each first side plate  77 , a fifth penetration hole  108  is formed at the back of each drum retaining hole  104  on a position opposed to the threaded portion  85  of each side frame  83 . Each first side plate  77  is further provided with boss holes  109  receiving the bosses  86  of each side frame  83  and erasing light passing holes  80  for passing erasing light. Each erasing light passing hole  80  is arranged between the drum retaining hole  104  and the fifth penetration hole  108  respectively. 
     (5) Second Side Plate 
     Each second side plate  78  is made of fiber-reinforced resin, for example. Each second side plate  78  is generally in the form of an elongated rectangular plate in side elevational view, vertically wider than and anteroposteriorly generally identical in length to the first side plate  77  (see  FIG. 6 ), as shown in  FIGS. 7A ,  7 B,  8 A and  8 B. The front and rear end portions of each second side plate  78  are opposed to the front and rear beams  75  and  76  respectively, as shown in  FIG. 2 . 
     A flange portion  116  as an example of a guided portion extending outward in the width direction is formed on the upper end portion of each second side plate  78  over the anteroposterior direction. This flange portion  116  slidably comes into contact with a roller member  118 , provided on each main body side plate  117  described later as an example of a unit guide portion, from above. An inclined surface  173  inclined from the lower surface toward a front upper side is formed on the front end portion of the flange portion  116 . 
     Each second side plate  78  is further provided with detection light passing holes  119  opposed to the detection windows  46  (see  FIG. 1 ) of the developer cartridges  27  respectively when the developer cartridges  27  are mounted between the right and left second side plates  78 . 
     Each second side plate  78  is further provided with first screw insertion holes  121  for receiving screws  107  on positions opposed to the threaded portions  85  of the drum subunits  28  respectively. 
     The front end portion of each second side plate  78  is formed narrower in the vertical direction than an intermediate portion thereof. The lower edge of the front end portion of each second side plate  78  is inclined toward a front upper side. In the front end portion of each second side plate  78 , second screw insertion holes  111  are formed on positions opposed to the second threaded portions  95  of the front beam  75  respectively. A support shaft insertion hole  114  receiving the support shaft  90  is further formed on the front end portion of each second side plate  78 . 
     The rear end portion of each second side plate  78  is also formed narrower in the vertical direction than the intermediate portion thereof. The lower edge of the rear end portion of each second side plate  78  is inclined toward a rear upper side. In the rear end portion of each second side plate  78 , a third screw insertion hole  112  is formed on a position opposed to the corresponding second threaded portion  99  of the rear beam  76 . 
     A notched portion  74  is formed on the rear end portion of each second side plate  78  by notching the lower edge of the notched portion  74  in a generally V-shaped manner. More specifically, the notched portion  74  has an upper edge extending in the anteroposterior direction in side elevational view, a lower edge inclined toward a front upper side with a constant gradient and a front edge coupling the front ends of the upper and lower edges with each other. Also in the rear end portion of each first side plate  77 , a notched portion  172  (see  FIG. 6 ) is formed on a position overlapping with the corresponding notched portion  74  of the second side plate  78  when the drum unit  26  is assembled. The notched portion  172  is generally identical in shape to the notched portion  74 , while the front and lower edges thereof are positioned rearward beyond those of the notched portion  74 . When the drum unit  26  is mounted in the main body casing  2 , the notched portions  172  receive a main body reference shaft  115  (described later) provided on the main body casing  2 , and come into contact with the main body reference shaft  115  from upper and front sides. When the drum unit  26  is mounted in the main body casing  2 , the notched portions  74  do not interfere with the main body reference shaft  115 . 
     On the inner surface (the right side surface of the left second side plate  78  or the left side surface of the right second side plate  78 ) of each second side plate  78  in the width direction, four cartridge guide portions  124  for guiding mounting/detachment of the developer cartridges  27  with respect to the right and left second side plates  78  are formed at regular intervals in the anteroposterior direction. Each cartridge guide portion  124  is formed by two protrusions protruding inward in the width direction from the inner side surface of the second side plate  78  at an interval from each other. The cartridge guide portions  124  are inclined from the upper end portion of the second side plate  78  toward a rear lower side with a constant gradient, and coupled with cartridge retaining portions  127  respectively. Each cartridge retaining portion  127  is formed parallelly to a line connecting the center of each photosensitive drum  29  and the corresponding developing roller  39  with each other, while the lower end portion thereof is opened toward the mounting position of the photosensitive drum  29 . The lower edge of the cartridge guide portion  124  is opposed to the corresponding developer receiving groove  110  of the first side plate  77  in the width direction. 
     On the upper end portion of the inner side surface of each second side plate  78 , four first support shafts  140  are projectingly provided at regular intervals from one another in the anteroposterior direction. On the upper end portion of the inner side surface of each second side plate  78 , further, four second support shafts  141  as examples of four support shafts are projectingly provided at regular intervals from one another in the anteroposterior direction. Each second support shaft  141  is arranged on a position separated frontward from each first support shaft  140 . Each first support shafts  140  swingably support link levers  142  (see  FIG. 13 ) described later, although not shown in  FIGS. 7A to 8B . Each second support shafts  141  swingably support pressing members  143  (see  FIG. 13 ) as examples of cartridge pressing members described later, although not shown in  FIGS. 7A to 8B . 
     (5-1) Left Second Side Plate 
     The left second side plate  78  is provided with drum coupling insertion holes  120  exposing the left axial end portions of the photosensitive drums  29  respectively, as shown in  FIG. 2 . 
     Four such drum insertion holes  120  are formed on the lower end portion of the second side plate  78  at intervals from one another in the anteroposterior direction. Each drum coupling insertion hole  120  is formed as a round hole penetrating in the thickness direction on a position opposed to the axial left end portion of each photosensitive drum  29  and the corresponding drum retaining hole  104  provided on the first side plate  77  in the thickness direction. The drum coupling insertion holes  120  are opposed to four coupling gear (not shown) on the main body side respectively. The drum coupling insertion hole  120  has a diameter larger than the outer diameter of each flange member  79  and smaller than the outer diameter of the left shaft receiving member  82  described later. Thus, the drum coupling insertion hole  120  can prevent the corresponding left shaft receiving member  82  from dropping when the drum unit  26  is assembled. 
     In the left second side plate  78 , a developer coupling insertion hole  122  is formed on an intermediate portion of each cartridge guide portion  124  in the vertical direction. When each developer cartridges  27  are mounted between the right and left second side plates  78 , a coupling passive gear  123  (see  FIG. 3 ) provided on the left side surface of the developer cartridge  27  is opposed to each developer coupling insertion hole  122 . 
     (5-2) Right Second Side Plate 
     The right second side plate  78  is provided with four charge grid electrode openings  129 , four charge wire electrode openings  125 , four erasing light passing holes  132  and four air intakes  133  as examples of air passages as shown in  FIGS. 8A and 8B . The right second side plate  78  is also provided with four developing electrode openings (not shown). 
     (5-2-1) Charge Grid Electrode Opening 
     Each charge grid electrode opening  129  is arranged at the back of the lower end portion of each cartridge guide portion  124 . The charge grid electrode opening  129  is formed as a rectangular hole in side elevational view penetrating in the thickness direction on a position opposed to a charge grid electrode  61  as an example of a unit-side feeding member for feeding power to a grid electrode of the corresponding scorotron charger  30  (see  FIG. 1 ), as shown by broken lines in  FIG. 8A . Thus, the charge grid electrode  61  is exposed from each charge grid electrode opening  129 . 
     A charge grid connecting guide portion  130  as an example of a frame-shaped connecting guide portion surrounding each charge grid electrode opening  129  in side elevational view is provided on the right side surface of the right second side plate  78 . The charge grid connecting guide portion  130  is in the form of a rib protruding rightward from the right side surface of the right second side plate  78 . When the drum unit  26  is mounted in the main body casing  2 , a main body-side charge grid contact (not shown) provided in the main body casing  2  is arranged in the charge grid electrode opening  129  over each charge grid connecting guide portion  130  in the process of this mounting. In other words, the main body-side charge grid contacts are guided into the charge grid electrode openings  129  by the charge grid connecting guide portions  130 , to be connected to the charge grid electrode  61  respectively. 
     (5-2-2) Charge Wire Electrode Opening 
     Each charge wire electrode opening  125  is arranged above the upper end surface of the first side plate  77  at the back of the upper end portion of the corresponding cartridge guide portion  124  when the drum unit  26  is assembled. This charge wire electrode opening  125  is formed as a rectangular hole in side elevational view penetrating in the thickness direction on a position opposed to a charge wire electrode  60  as an example of a unit-side feeding member for feeding power to a wire electrode of the corresponding scorotron charger  30  (see  FIG. 1 ), as shown by broken lines in  FIG. 8A . Thus, the charge wire electrode  60  is exposed from each charge wire electrode opening  125 . 
     A charge wire connecting guide portion  126  as an example of a frame-shaped connecting guide portion surrounding each charge wire electrode opening  125  in side elevational view is provided on the right side surface of the right second side plate  78 . The charge wire connecting guide portion  126  is so formed as to protrude rightward from the right side surface of the right second side plate  78 , and so inclined that the quantity of protrusion thereof is reduced as separated from the charge wire electrode opening  125  with reference to the apices facing the charge wire electrode opening  125 . When the drum unit  26  is mounted in the main body casing  2 , a main body-side charge wire contact (not shown) provided in the main body casing  2  is arranged in the charge wire electrode opening  125  over each charge wire connecting guide portion  126  in the process of this mounting. In other words, the main body-side charge wire contacts are guided into the charge wire electrode openings  125  by the charge wire connecting guide portions  126 , to be connected to the charge wire electrode  60  respectively. 
     (5-2-3) Erasing Light Passing Hole 
     Each erasing light passing hole  132  is arranged obliquely under the rear side of each charge grid electrode opening  129 . The erasing light passing hole  132  is formed as a round hole penetrating in the thickness direction on a position opposed to the corresponding erasing light passing hole  80  provided on the first side plate  77 . Erasing lamps (not shown) are provided outside the drum unit  26 . Erasing light emitted from each erasing lamp is so fed to the peripheral surface of each photosensitive drum  29  (drum body  32 ) through the corresponding erasing light passing holes  132  and  80  as to expose the peripheral surface of the photosensitive drum  29  and erase positive charge remaining on the peripheral surface of the photosensitive drum  29 . 
     (5-2-4) Air Intake 
     Each air intake  133  is formed between each charge wire electrode opening  125  and each charge grid electrode opening  129 . This air intake  133  is formed as a hole penetrating in the thickness direction on a position opposed to the corresponding scorotron charger  30  retained by the drum subunit  28 . Air flowing into each air intake  133  is fed to each scorotron charger  30 . Ozone generated from the scorotron chargers  30  may be discharged from the drum unit  26  through the air intakes  133 . 
     (6) Photosensitive Drum 
     Each photosensitive drum  29  includes a cylindrical drum body  32  and two flange members  79  engaged with both end portions of the drum body  32  to be not relatively rotatable respectively, as shown in  FIG. 5 . 
     The outermost layer of the drum body  32  is formed by a positively chargeable photosensitive layer. 
     Each flange member  79  is made of resin. The flange members  79  are partially inserted into both end portions of the drum body  32 . In the left flange member  79 , passive grooves  33  are provided on the outer (left) end surface of the photosensitive drum  29  in the axial direction, so that driving force from a motor (not shown) provided in the main body casing  2  is transmitted thereto. 
     The right and left flange members  79  are supported by the right and left shaft receiving members  81  and  82  respectively, to be rotatable with respect to the first side plate  77 . 
     (6-1) Left Shaft Receiving Member 
     Each left shaft receiving member  82  is made of resin. The left shaft receiving member  82  integrally includes a cylindrical portion  134  in the form of a cylinder and a flange portion  135  in the form of an annular plate spreading outward from one (left) peripheral edge of the cylindrical portion  134 , as shown in  FIG. 9 . 
     The cylindrical portion  134  has an outer diameter generally identical to the inner peripheral surface of the corresponding drum retaining hole  104  formed on the first side plate  77  and an inner diameter generally identical to the outer peripheral surface of an end portion (flange member  79 ) of the corresponding photosensitive drum  29 . 
     (6-2) Right Shaft Receiving Member 
     Each right shaft receiving member  81  is made of conductive resin. The right shaft receiving member  81  includes a cylindrical portion  136  in the form of a cylinder, a flange portion  137  in the form of a disc so formed as to block one side of the cylindrical portion  136  and an earth shaft  138  passing through the flange portion  137  in the thickness direction, as shown in  FIG. 10 . 
     The cylindrical portion  136  has an outer diameter generally identical to the inner peripheral surface of the corresponding drum retaining hole  104  formed on the first side plate  77  and an inner diameter generally identical to the outer peripheral surface of the end portion (flange member  79 ) of the corresponding photosensitive drum  29 . 
     The flange portion  137  has an outer diameter larger than that of the cylindrical portion  136 . 
     The earth shaft  138  extends along the central axis of the cylindrical portion  136 . An end of the earth shaft  138  slightly protrudes from the surface of the flange portion  137  opposite to the side provided with the cylindrical portion  136 , as shown in  FIG. 11E . An engaging hole  139  passing through the earth shaft  138  in the diametral direction is formed on the protruding portion. The forward end portion of the other end of the earth shaft  138  is in the form of a circular cone. 
     (7) Assembling of Drum Unit 
     First, the four drum subunits  28  are arranged at the regular intervals in the anteroposterior direction, as shown in  FIG. 11A . 
     Then, the front beam  75  is arranged in front of the headmost drum subunit  28  at an interval therefrom. Further, the rear beam  76  is arranged at the back of the rearmost drum subunit  28  at a slight interval therefrom. 
     Thereafter the first side plates  77  are arranged on both sides of the front beam  75 , the four drum subunits  28  and the rear beam  76  in the width direction respectively. Then, the first side plates  77  are combined with the front beam  75 , the four drum subunits  28  and the rear beam  76  through screws  92  and  96 . 
     More specifically, each first side plate  77  is so arranged that each pair of boss holes  109  are opposed to the bosses  86  of each side frame  83  in the width direction. Then, each first side plate  77  is brought into contact with the outer side surface of each side frame  83  in the width direction. Thus, the bosses  86  are engaged with the corresponding boss holes  109  respectively. Further, the first penetration hole  100  is opposed to the first threaded portion  93  of the front beam  75 , the third penetration hole  102  is opposed to the first threaded portion  97  of the rear beam  76 , and each fifth penetration hole  108  is opposed to the threaded portion  85  of the side frame  83  of each drum subunit  28 . The screw  92  is fitted into the first threaded portion  93  through the first penetration hole  100 . Further, the screw  96  is fitted into the first threaded portion  97  through the third penetration hole  102 . Thus, the first side plates  77  are combined with both sides of the front beam  75 , the four drum subunits  28  and the rear beam  76  in the width direction, as shown in  FIG. 11B . 
     Then, the photosensitive drums  29  are mounted on the right and left first side plates  77 . While  FIGS. 11B to 11E  show a state where the photosensitive drums  29  excluding the headmost photosensitive drum  29  are already mounted, these photosensitive drums  29  can be mounted on the first side plates  77  by a method similar to that for mounting the headmost photosensitive drum  29 . 
     First, the headmost photosensitive drum  29  is arranged on a position where both axial end portions thereof are opposed to the drum retaining holes  104  of the first side plates  77  in the width direction through the space between the corresponding drum subunit  28  and the front beam  75 , as shown by broken lines in  FIG. 11B . Then, one axial end portion (flange member  79 ) of this photosensitive drum  29  is inserted into the drum retaining hole  104  formed in one of the first side plates  77 . Thereafter, another axial end portion (flange member  79 ) of the photosensitive drum  29  is inserted into the drum retaining hole  104  formed in the other first side plate  77 . Thus, both axial end portions of the photosensitive drum  29  are exposed from the drum retaining holes  104  respectively, as shown in  FIG. 11C . 
     Thereafter the left shaft receiving member  82  is combined with the left first side plate  77  and the flange member  79  provided on the left end portion of the photosensitive drum  29 , as shown by broken lines in  FIG. 11C . More specifically, the cylindrical portion  134  of the left shaft receiving member  82  is press-fitted into the space between the outer peripheral surface of the flange member  79  exposed from the drum retaining hole  104  of the left first side plate  77  and the drum retaining hole  104 . Then, the left shaft receiving member  82  is press-fitted into the drum retaining hole  104  up to a position where the flange portion  135  thereof comes into contact with the outer side surface of the left first side plate  77 . Thus, the left shaft receiving member  82  is so combined that the same is supported to be not rotatable with respect to the left first side plate  77  while the left flange member  79  is rotatably supported with respect to the left shaft receiving member  82 . 
     Then, the right shaft receiving member  81  is combined with the right first side plate  77  and the flange member  79  provided on the right end portion of the photosensitive drum  29 , as shown by broken lines in  FIG. 11D . More specifically, the cylindrical portion  136  of the right shaft receiving member  81  is press-fitted into the space between the outer peripheral surface of the flange member  79  exposed from the drum retaining hole  104  of the right first side plate  77  and the drum retaining hole  104 . Then, the right shaft receiving member  81  is press-fitted into the drum retaining hole  104  up to a position where the flange portion  137  thereof comes into contact with the outer side surface of the right first side plate  77 . Thus, the right shaft receiving member  81  is so combined that the same is supported to be not rotatable with respect to the right first side plate  77  and the right flange member  79  is rotatably supported with respect to the right shaft receiving member  81 . 
     Then, the clip member  106  is inserted into the engaging hole  139  formed on the earth shaft  138  of the right shaft receiving member  81 , as shown in FIG.  11 E. An end portion of the clip member  106  is bent and engaged with the engaging groove  105 A of the right first side plate  77 . The other end of the clip member  106  extends from the earth shaft  138  in the diametral direction of the flange portion  137  and is bent along the outer peripheral surface of the flange portion  137 , to be engaged with the engaging groove  105 B of the right first side plate  77 . Thus, the right shaft receiving member  81  is pressed against the right first side plate  77  due to the elasticity of the clip member  106 . 
     Then, each second side plate  78  is arranged on the outer side of each first side plate  77  in the width direction, as shown in  FIG. 11F . Then, the second side plate  78  is combined with each first side plate  77 , the front beam  75 , the four drum subunits  28  and the rear beam  76  with screws  94 ,  98  and  107 . 
     More specifically, each second side plate  78  is so arranged that one of the second screw insertion holes  111  is opposed to the corresponding second threaded portion  95  of the front beam  75 , the other second screw insertion hole  111  is opposed to the second penetration hole  101  of the first side plate  77 , the third screw insertion hole  112  is opposed to the fourth penetration hole  103  of the first side plate  77  and the first screw insertion holes  121  are opposed to the fifth penetration holes  108  of the first side plate  77 . Then, the screws  94  are fitted into the second threaded portions  95  through the screw insertion hole  111  and the second penetration hole  101 . Further, the screw  98  is fitted into the second threaded portion  99  through the third screw insertion hole  112  and the fourth penetration hole  103 . In addition, the screw  107  is fitted into the threaded portion  85  of each side frame  83  through the corresponding first screw insertion hole  121  and the corresponding fifth penetration hole  108 . Thus, each second side plate  78  is combined with each first side plate  77 , the front beam  75 , the four drum subunits  28  and the rear beam  76 , as shown in  FIG. 11F . 
     3. Earthing of Photosensitive Drum 
       FIG. 12  is a sectional view of principal parts showing an earthing structure of each photosensitive drum. 
     Each photosensitive drum  29  includes an electrode  144  for connecting the drum body  32  and the earth shaft  138  with each other in an electrically conductable manner. 
     The electrode  144  is formed by a metallic plate made of phosphor bronze, for example. This electrode plate includes an electrode plate body  145  generally in the form of a disc in side elevational view, contact portions  146  coming into contact with the inner peripheral surface of the drum body  32  and an arm portion  147  coming into contact with the earth shaft  138 . 
     Four contact portions  146  are arranged at regular angular intervals (90 degrees) on the peripheral edge of the electrode plate body  145 , for example. Each contact portion  146  protrudes outward in the diametral direction of the electrode plate body  145  from the peripheral edge of the electrode plate body  145 . 
     One longitudinal end portion of the arm portion  147  is connected to the electrode plate body  145 . Thus, the arm portion  147  is supported on the electrode plate body  145  in a cantilever state with a fixed end formed by the portion connected with the electrode plate body  145  and a free end formed by the end opposite to the fixed end in the longitudinal direction. The free end portion of the arm portion  147  is in contact with the earth shaft  138 . Thus, the drum body  32  of the photosensitive drum  29  and the earth shaft  138  of the right shaft receiving member  81  are connected with each other in an electrically conductable manner through the electrode  144 . 
     The earth shaft  138  of the right shaft receiving member  81  is pressed by the clip member  106  engaged with the engaging grooves  105 A and  105 B of the right first side plate  77 , as shown in  FIG. 11E . The first side plate  77  and the clip member  106  are made of metallic materials. Therefore, the drum body  32  of the photosensitive drum  29  and the earth shaft  138  of the right shaft receiving member  81  are electrically connected with each other, and the drum body  32  of the photosensitive drum  29  is connected to the first side plate  77  in an electrically conductable manner through the electrode  144 , the earth shaft  138  and the clip member  106 . 
     When the drum unit  26  is mounted in the main body casing  2 , the main body reference shaft  115  described later enters the notched portion  172 , so that the right first side plate  77  can be earthed through the main body reference shaft  115 . Further, the right first side plate  77  and the photosensitive drum  29  are electrically connected with each other through the right shaft receiving member  81 , whereby the photosensitive drum  29  can be earthed through the right shaft receiving member  81 , the right first side plate  77  and the main body reference shaft  115 . 
     The right shaft receiving member  81  is made of a conductive resin material. Therefore, the photosensitive drum  29 , electrically conductable to the first side plate  77  through the cylindrical portion  134  (see  FIG. 9 ) of the right shaft receiving member  81  press-fitted into the first side plate  77 , can be earthed without the aforementioned clip member  106 . The photosensitive drum  29  can be earthed through the aforementioned clip member  106  if the aforementioned right shaft receiving member  81  is not made of a conductive material, while the photosensitive drum  29  is desirably earthed through both of the right shaft receiving member  81  and the clip member  106 . 
     4. Developer Cartridge 
     The coupling passive gear  123  is arranged on a left sidewall  149  of the casing  36  of each developer cartridge  27 , as shown in  FIG. 3 . When each developer cartridge  27  is mounted between the right and left second side plates  78 , each coupling passive gear  123  is opposed to each developer coupling insertion hole  122 . A drive input shaft (not shown) is inserted into each developer coupling insertion hole  122  and coupled to each coupling passive gear  123 , so that the driving force from the motor (not shown) provided in the main body casing  2  can be transmitted to each coupling passive gear  123  through each drive input shaft. 
     In each developer cartridge  27 , developer pressing bosses  150  are provided on front upper portions of both sidewalls  149  to protrude outward from the sidewalls  149  in the width direction. When the developer cartridges  27  are mounted on the drum unit  26 , the developer pressing bosses  150  come into contact with the link levers  142  described later from above, and are pressed by the pressing members  143  described later from above. 
     5. Pressing Member 
       FIG. 13  is a partial sectional view of the drum unit as viewed from the left side.  FIG. 14  is a perspective view of principal parts of each pressing member and each link lever shown in  FIG. 13 . 
     The drum unit  26  includes four pressing members  143 , as shown in  FIG. 13 . 
     Each pressing member  143  is in the form of a generally triangular plate. The corresponding second support shaft  141  of the second side plate  78  is inserted into one corner portion of the pressing member  143 , to swingably support the pressing member  143 . The pressing member  143  is so provided as to extend from the second support shaft  141  obliquely toward a rear upper side. 
     The forward end portion of the pressing member  143  is urged downward by a coil spring  151  wound on the corresponding second support shaft  141 , as shown in  FIG. 14 . An end of the coil spring  151  is engaged on the flange portion  116  of the second side plate  78 , while the other end thereof is wound on the second support shaft  141  and thereafter engaged with the pressing member  143 . 
     6. Link Lever 
     As shown in  FIG. 13 , the drum unit  26  includes four link levers  142 . 
     Each link lever  142  is in the form of an elongated rectangle in side elevational view. An intermediate portion of the link lever  142  in the longitudinal direction is swingably supported on the corresponding first support shaft  140  of the second side plate  78 . An end portion of the link lever  142  is in the form of a plate extending in the right-and-left direction, to form an acting portion  153  receiving force from a cam portion  158  described later, as shown in  FIG. 14 . The other end portion of the link lever  142  forms a spacing portion  154  whose width in the right-and-left direction is rendered larger than the width of the end portion for pressing the developer pressing boss  150  (see  FIG. 13 ) of the corresponding developer cartridge  27 . 
     7. Mounting of Developer Cartridge 
     The developer cartridge  27  of each color is mounted between the right and left second side plates  78  from above, as shown in  FIG. 3 . At this time, both end portions of the roller shafts  51  protruding from both side surfaces of the casing  36  of the developer cartridge  27  in the width direction are introduced into the corresponding cartridge guide portions  124  from above. Then, the developer cartridge  27  is moved downward while both end portions of the roller shafts  51  are guided by the cartridge guide portions  124 . When the developer cartridge  27  is guided into the corresponding cartridge retaining portion  127  (see  FIG. 7B ) and the developing roller  39  comes into contact with the photosensitive drum  29 , the developer cartridge  27  is inhibited from further movement, and the developing roller  39  is positioned with respect to the photosensitive drum  29 . Thereafter the developer cartridge  27  is slightly inclined frontward. 
     Thus, the developer pressing boss  150  of each developer cartridge  27  slips into the space under the pressing member  143  through the space between the pressing member  143  and the spacing portion  154  of the link lever  142  to lift the pressing member  143  from below against the urging force of the coil spring  151 , as shown in  FIG. 13 . Consequently, the developer pressing boss  150  is urged downward by the pressing member  143 , to press the developer cartridge  27 . In this state, the developer pressing boss  150  is in contact with the spacing portion  154  of the link lever  142  from the front side. 
     According to this embodiment, the color laser printer  1  employs the contact one-component developing system, to require no positional accuracy of the developing roller  39 . Therefore, the first side plate  77  may not be employed for positioning the developing roller  39 . 
     8. Internal Structure of Main Body Casing 
       FIG. 15  is a side elevational view of a translation cam provided on the main body casing  2 .  FIG. 16  is a perspective view of principal parts showing a state where all developer cartridges are in pressure contact with the photosensitive drums.  FIG. 17  is a perspective view of principal parts showing a state where only the black developer cartridge is in pressure contact with the corresponding photosensitive drum.  FIG. 18  is a perspective view of principal parts showing a state where all developer cartridges are separated from the photosensitive drums. 
     The main body casing  2  is provided therein with a pair of main body side plates  117  ( FIGS. 16 to 18  show only the right main body side plate  117 ) opposed to each other at an interval in the width direction. A roller shaft  171  extending inward in the width direction is supported on the front end portion of each main body side plate  117 . A roller member  118  for coming into contact with the flange portion  116  of each second side plate  78  of the drum unit  26  from below and slidably guiding the drum unit  26  into a drum receiving space  7  is supposed by each roller shaft  171 . 
     The main body reference shaft  115  is extended between the rear end portions of the right and left main body side plates  117 . 
     In order to mount the drum unit  26  in the main body casing  2 , the front cover  9  of the main body casing  2  is first opened. Then, the rear edge of the flange portion  116  of each second side plate  78  of the drum unit  26  is brought into contact with the roller member  118  from above. Thereafter the drum unit  26  is so moved rearward that the flange portion  116  of each second side plate  78  slides on the roller member  118  and the drum unit  26  is guided into the main body casing  2 . When the roller member  118  comes into contact with the inclined surface  173  provided on the front side of each flange portion  116  so that the drum unit  26  entirely moves downward and the notched portion  172  of each first side plate  77  comes into contact with the main body reference shaft  115 , the drum unit  26  is inhibited from further movement. Thus, the drum unit  26  is completely mounted in the main body casing  2 . 
     The drum unit  26  is detached from the main body casing  2  by an operation reverse to the aforementioned operation. 
     9. Spacing Mechanism 
     The main body casing  2  is provided therein with a spacing mechanism  155  for displacing each developer cartridge  27  between a contact position coming into contact with each photosensitive drum  29  and a separating position separating from each photosensitive drum  29 . 
     The spacing mechanism  155  includes a pair of translation cams  152 , rails  156  retaining the translation cams  152  to be linearly movable in the anteroposterior direction and a synchronized movement mechanism  166  for linearly moving the pair of translation cams  152  in a synchronized manner. 
     (1) Translation Cam 
     The translation cams  152  are provided on the right and left sides in the main body casing  2  respectively.  FIGS. 16 to 18  show only the right translation cam  152 . The right and left translation cams  152  are identical in structure to each other, and hence the right translation cam  152  is described below. 
     The translation cam  152  integrally includes a translation cam body plate  157  and four cam portions  158  provided on the inner side surface of the translation cam body plate  157 . 
     The translation cam body plate  157  is generally in the form of an elongated rectangle in side elevational view extending in the anteroposterior direction, and the rear end portion thereof is rectangularly notched in side elevational view from the upper surface, as shown in  FIG. 15 . Further, the translation cam body plate  157  is generally U-shaped in section, and the upper and lower edges thereof are bent inward in the width direction. 
     The four cam portions  158  are provided correspondingly to the developer cartridges  27  (see  FIG. 3 ) of the respective colors. The cam portions  158  protrude inward in the width direction at intervals from one another on the inner side surface of the translation cam body plate  157 , and are generally rectangular in side elevational view. The rear end portion of each cam portion  158  form a first inclined surface  159  inclined from the lower edge thereof toward rear upper side. The lower end portion of each cam portion  158  form a second inclined surface  170  inclined toward inner upper side in the width direction from the translation cam body plate  157 . 
     The rear three cam portions  158  (three cam portions  158  other than the headmost cam portion  158 ) are so formed that the intervals between the adjacent cam portions  158  are equal to each other. The headmost cam portion  158  is so formed that the interval between the same and the cam portion  158  adjacent thereto is a second interval larger than the interval between the rear three cam portions  158 . 
     (2) Rail 
     Rails  156  are provided on the right and left sides in the main body casing  2  respectively. The right and left rails  156  are identical in structure to each other, and hence the right rail  156  is described below. 
     The rail  156  includes a main portion  160  generally rectangular in side elevational view fixed to the main body side plate  117  to extend in the anteroposterior direction, a first flange portion  161  extending inward in the width direction from the upper edge of the main portion  160  and a second flange portion  162  extending inward in the width direction from the lower edge of the main portion  160 , as shown in  FIG. 16 . 
     The second flange portion  162  has a stopper  163  further extending upward from the inner edge thereof in the width direction. Four recesses  164  notched from the upper end of the stopper  163  are formed on an intermediate portion of the stopper  163  in the anteroposterior direction. 
     A notched portion  165  rectangularly notched from the upper surface in side elevational view is formed on the rear end portions of the main portion  160  and the first flange portion  161 . The corresponding translation cam  152  is so arranged on the second flange portion  161  that the cam portions  158  protrude inward in the width direction. The translation cam  152  is slidable along the rail  156 , and the rear end portion thereof is regularly exposed upward from the notched portion  165 , regardless of the position of the translation cam  152 . 
     (3) Synchronized Movement Mechanism 
     The synchronized movement mechanism  166  is so structured as to transmit driving force for linear movement from the left translation cam  152  to the right translation cam  152  following linear movement of the left translation cam  152 , for example. 
     In other words, the synchronized movement mechanism  166  includes a left rack gear (not shown) formed on the upper surface of the rear end portion of the left translation cam  152 , a left pinion gear  167  meshing with the left rack gear, a right rack gear (not shown) formed on the upper surface of the rear end portion of the right translation cam  152 , a right pinion gear  168  meshing with the right rack gear and a connecting shaft  169  on which the left and right pinion gears  167  and  168  are mounted to be not relatively rotatable. 
     The driving force from the motor (not shown) is input in the left translation cam  152 . 
     (4) Spacing/Pressing Operation 
     The operation of the spacing mechanism  155  is described mainly with reference to  FIGS. 16 to 18 . 
     When the drum unit  26  is mounted in the main body casing  2  and each translation cam  152  is moved to the headmost position as shown in  FIG. 16 , the first inclined surface  159  of each cam portion  158  and the acting portion  153  of the link lever  142  arranged at the back thereof are opposed to each other in a non-contact state at an interval in the anteroposterior direction. An interval larger than those between the first inclined surfaces  159  of the rear three cam portions  158  and the acting portions  153  of the link levers  142  arranged at the back thereof is formed between the first inclined surface  159  of the headmost cam portion  158  and the acting portion  153  of the link lever  142  arranged at the back thereof. 
     In this state, each developer cartridge  27  is arranged on the contact position where the developing roller  39  and the photosensitive drum  29  are in contact with each other. Each pressing member  143  comes into contact with the developer pressing boss  150  of each developer cartridge  27  from above, to press each developer pressing boss  150  downward. 
     When the driving force of the motor (not shown) is input in an input rack gear of the left translation cam  152  to move the left translation cam  152  rearward from this state, the left pinion gear  167  rotates following this movement of the left translation cam  152 , and this rotation of the left pinion gear  167  is transmitted to the right pinion gear  168  through the connecting shaft  169  so that the right pinion gear  168  rotates in the same direction as the left pinion gear  167 , thereby moving the right translation cam  152  rearward. 
     When the rearward movement of each translation cam  152  progresses, the first inclined surfaces  159  of the rear three cam portions  158  come into contact with the acting portions  153  of the link levers  142  arranged at the back thereof, to press end portions of the rear three link levers  142  rearward. Thus, each link lever  142  so pivots on the first support shaft  140  as to lift the other end portion (spacing portion  154 ) upward. In the process of this pivoting of each link lever  142 , the spacing portion  154  of the link lever  142  comes into contact with the developer pressing boss  150  positioned above the same from below, to push up the developer pressing boss  150 . Thus, yellow, magenta and cyan developer cartridges  27 Y,  27 M and  27 C are lifted upward against the pressing force of the corresponding pressing members  143 . 
     When the rearward movement of the translation cam  152  further progresses and the acting portions  153  of the link levers  142  come into contact with the second inclined surfaces  170  of the rear three cam portions  158  as shown in  FIG. 17 , the yellow, magenta and cyan developer cartridges  27 Y,  27 M and  27 C are arranged on the separating positions, and the developing rollers  39  of the yellow, magenta and cyan developer cartridges  27 Y,  27 M and  27 C separate from the photosensitive drums  29 . At this time, the developer pressing boss  150  of a black developer cartridge  27 K is pressed by the corresponding pressing member  143 . Thus, only the developing roller  39  of the black developer cartridge  27 K is pressed against the corresponding photosensitive drum  29 . At this time, the acting portion  153  of each link lever  142  enters the corresponding recess  164  of the rail  156 . Each second inclined surface  170  is inclined toward an inner upper side in the width direction from the translation cam body plate  157 . Therefore, the pressing force applied to the acting portion  153  of the link lever  142  from the second inclined surface  170  of the cam portion  158  includes an inward force component in the width direction. The link lever  142  is fixed to the drum unit  26  through the first support shaft  140 . Consequently, the drum unit  26  can be positioned in the width direction. 
     When the rearward movement of the translation cam  152  thereafter further progresses, the first inclined surface  159  of the headmost cam portion  158  comes into contact with the acting portion  153  of the link lever  142  arranged at the back thereof, to press an end portion of the headmost link lever  142  toward a rear lower side. Thus, the link lever  142  so pivots on the first support shaft  140  as to lift the other portion (spacing portion  154 ) upward. In the process of this pivoting of the link lever  142 , the spacing portion  154  of the link lever  142  comes into contact with the developer pressing boss  150  positioned above the same from below, to push up the developer pressing boss  150 . Thus, the black developer cartridge  27 K is lifted upward against the pressing force applied by the pressing member  143 . 
     When the rearward movement of the translation cam  152  further progresses and the acting portion  153  of the link lever  142  comes into contact with the second inclined surface  170  of the headmost cam portion  158  as shown in  FIG. 18 , the black developer cartridge  27 K is arranged on the separating position, and the developing roller  39  of the black developer cartridge  27 K separates from the photosensitive drum  29 . Thus, the developing rollers  39  of all developer cartridges  27  separate from the photosensitive drums  29 . 
     At this time, the acting portion  153  of each link lever  142  enters the corresponding recess  164  formed on the stopper  163  of the second flange portion  162  of the rail  156 . The link lever  142  is in contact with the second inclined surface  170 , whereby the drum unit  26  is positioned in the width direction by the second inclined surface  170 . The second inclined surface  170  is inclined toward an inner upper side in the width direction from the translation cam body plate  157 . Therefore, the pressing force applied to the acting portion  153  of the link lever  142  from the second inclined surface  170  of the cam portion  158  includes an inward force component in the width direction. The link lever  142  is fixed to the drum unit  26  through the first support shaft  140 . Consequently, the drum unit  26  can be positioned in the width direction. 
     The color laser printer  1  can be returned from the state shown in  FIG. 18  to the states shown in  FIGS. 16 and 17  respectively by moving the translation cam  152  frontward. When the first inclined surface  159  of each cam portion  158  and the acting portion  153  of the link lever  142  positioned at the back thereof are separated from each other at this time, the upward force applied from the link lever  142  to the developer pressing boss  150  is canceled. Then, the developer cartridge  27  is pressed downward by the pressing member  143 , to be arranged on the contact position where the developing roller  39  and the photosensitive drum  29  are in contact with each other. 
     10. Functions/Effects 
     According to the aforementioned embodiment, as hereinabove described, the tandem type photosensitive unit is constituted of the pair of first side plates  77 , the pair of second side plates  78 , the front beam  75 , the rear beam  76  and the four drum subunits  28 . 
     The drum unit  26  retains the four photosensitive drums  29  in the parallelly arranged state. The developer cartridges  27  retaining the developing rollers  39  are detachably mounted on the drum unit  26 . The link lever  142  is provided correspondingly to each developer cartridge  27 . Each link lever  142  is swingably supported on the corresponding first support shaft  140  projectingly provided on the drum unit  26 . The drum unit  26  is mounted in the main body casing  2  to be movable along the direction (hereinafter referred to simply as “arrangement direction” in this section) of the arrangement of the photosensitive drums  29 . The translation cams  152  are provided in the main body casing  2  to be linearly movable in the arrangement direction. 
     When each developer cartridge  27  is mounted on the drum unit  26 , the end portion of the link lever  142  is opposed to a prescribed portion of each developer cartridge  27 . The other end portion of the link lever  142  is pressed by the translation cam  152  following linear movement of the translation cam  152 , so that the end portion of the link lever  142  presses the prescribed portion of the developer cartridge  27  in the direction for separating the developing roller  39  from the photosensitive drum  29 . Thus, the developing roller  39  can be separated from the photosensitive drum  29 . The translation cam  152  is released from pressing the other end portion of the link lever  142  by further linear movement from this state, so that the end portion of the link lever  142  is released from pressing the prescribed portion of the developer cartridge  27 . 
     Thus, the first support shaft  140  provided on the drum unit  26  supports the link lever  142 , which in turn swings on the first support shaft  140  so that the developing roller  39  approaches to/separates from the photosensitive drum  29 . Therefore, the pressing force applied from the translation cam  152  to the link lever  142  does not act on the drum unit  26 . Consequently, the drum unit  26  can be prevented from positionally deviating from the main body casing  2  when the developing roller  39  is separated from the photosensitive drum  29 . 
     The main body casing  2  has the main body reference shaft  115  extending in the axial direction of the photosensitive drums  29 . The drum unit  26  comes into contact with the main body reference shaft  115  in the state mounted in the main body casing  2 . Thus, the drum unit  26  is positioned with respect to the main body. The direction of the pressing force applied from the translation cam  152  to the other end portion of each link lever  142  is generally identical to the direction where the drum unit  26  comes into contact with the main body reference shaft  115 . Even if force acts from the translation cam  152  to the drum unit  26  through the link lever  142 , therefore, the direction of this force is generally identical to the direction where the drum unit  26  comes into contact with the main body reference shaft  115 . When each developing roller  39  is separated from the corresponding photosensitive drum  29 , therefore, the drum unit  26  can be prevented from positionally deviating from the main body casing  2 . 
     The translation cam  152  is provided with the cam portion  158  for pressing the other end portion of the link lever  142  correspondingly to the link lever  142 . When coming into contact with the other end portion of the link lever  142 , the cam portion  158  presses the other end portion of the link lever  142 . When separating from the other end portion of the link lever  142 , on the other hand, the cam portion  158  is released from pressing the other end portion of the link lever  142 . Therefore, the link lever  142  can so swing as to separate the developing roller  39  from the photosensitive drum  29  due to the simple structure of the cam portion  158  provided on the translation cam  152 . 
     Further, the cam portion  158  can be switched to a first state where every cam portion  158  comes into contact with the other end portion of the corresponding link lever  142 , a second state where at least one cam portion  158  comes into contact with the other end portion of the corresponding link lever  142  while the remaining cam portions  158  separate from the other end portions of the corresponding link levers  142  and a third state where every cam portion  158  separates from the other end portion of the corresponding link lever  142 , due to the linear movement of the translation cam  152 . 
     The developer cartridges  27  are provided correspondingly to black, yellow, magenta and cyan respectively. In the first state, the cam portions  158  come into contact with the other end portions of the link levers  142  of all colors, so that the developing rollers  39  of all colors are separated from the photosensitive drums  29 . In the second state, the corresponding cam portions  158  come into contact with the other end portions of the link levers  142  corresponding to yellow, magenta and cyan respectively, so that the developing rollers  39  of yellow, magenta and cyan are separated from the photosensitive drums  29  while the black developing roller  39  comes into contact with the photosensitive drum  29 . In the third state, the cam portions  158  separate from the other end portions of the link levers  142  of all colors, so that the developing rollers  39  of all colors come into contact with the photosensitive drums  29 . Therefore, the yellow, magenta and cyan developing rollers  39  can be prevented from consumption by bringing the cam portions  158  into the second state in formation of a monochromatic image. 
     Further, the surface of each cam portion  158  coming into contact with the other end portion of the corresponding link lever  142  is formed into the second inclined surface  170  inclined with respect to the axial direction of the corresponding photosensitive drum  29 . Therefore, the pressing force applied from the cam portion  158  to the other end portion of the link lever  142  includes a force component in the axial direction of the photosensitive drum  29 . Consequently, force in the axial direction of the photosensitive drum  29  acts on the drum unit  26 , whereby the drum unit  26  can be positioned in this axial direction. 
     The pressing member  143  is provided correspondingly to each developer cartridge  27 , for pressing the developer cartridge  27  in the direction where the corresponding developing roller  39  and the corresponding photosensitive drum  29  are opposed to each other. This pressing member  143  can press the developing roller  39  against the photosensitive drum  29 . Consequently, the developing roller  39  can excellently feed the corresponding toner to the photosensitive drum  29 . 
     Each second support shaft  141  is projectingly provided on the drum unit  26 . The second support shaft  141  swingably supports the pressing member  143 , which in turn is elastically pressed against the prescribed portion of the corresponding developer cartridge  27  by the coil spring  151 . Thus, the developing roller  39  can be pressed against the photosensitive drum  29 . 
     11. Other Embodiment 
     While the single main body reference shaft  115  extended between the rear end portions of the right and left main body side plates  117  is employed in the aforementioned embodiment as an example of the main body reference shaft in the aforementioned embodiment, main body reference shafts  115  may alternatively be formed by two shafts protruding from the right and left main body side plates  117  in directions opposed to each other to be arranged on the same straight line. In this case, the forward end portion of each main body reference shaft  115  is arranged inward beyond the notched portion  74  formed on each first side plate  77  in the right-and-left direction when the drum unit  26  is mounted in the main body casing  2 . 
     The embodiments described above are illustrative and explanatory of the invention. The foregoing disclosure is not intended to be precisely followed to limit the present invention. In light of the foregoing description, various modifications and alterations may be made by embodying the invention. The embodiments are selected and described for explaining the essentials and practical application schemes of the present invention which allow those skilled in the art to utilize the present invention in various embodiments and various alterations suitable for anticipated specific use. The scope of the present invention is to be defined by the appended claims and their equivalents.