Abstract:
An image forming apparatus include a door through which a tray carrying a cartridge passes, wherein in interrelation with the door closing operation, (i) movement of the tray from a movable position to an image forming position, (ii) movement of a drive transmission member, for transmitting a driving force to the cartridge from a main assembly, from a first spacing position to a drive-connecting position, (iii) movement of an urging member for urging the cartridge to the main assembly from a second spacing position to a contact position, and (iv) movement of an electric energy supply member, for supplying electric power to the cartridge from the main assembly, from a third spacing position to an electric energy supplying position, are carried out in the order named.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to an image forming apparatus which forms an image on recording medium. 
         [0002]    Here, an image forming apparatus is an apparatus which forms an image on recording medium with the use of one of various known image formation principles and methods such as an electrophotographic process, an electrostatic recording process, a magnetic recording process, etc. It includes a copying machine, a printer (laser printer, LED printer, etc.), a facsimile machine, a word processor, an image displaying apparatus (electronic blackboard, electronic whiteboard, etc.). Recording medium includes medium on which an image can be formed by an image forming apparatus. It includes a sheet of paper, an OHT sheet, an electronic display, and the like, for example. 
         [0003]    A cartridge is a cartridge in which an image bearing member on which an image is formed, and all or parts of means for processing the image bearing member, are integrally disposed. It is removably installable in the main assembly of an image forming apparatus, and contributes to the process for forming an image on recording medium. The main assembly of an image forming apparatus is what remains after the removal of a cartridge from an image forming apparatus which employs the so-called cartridge system. 
         [0004]    Some examples of an image bearing member are an electrophotographic photosensitive member used by an electrophotographic process, a dielectric member used by an electrostatic recording process, a magnetic member used by a magnetic recording process, etc. That is, they are various components on which an image can be formed with the use of one of various image formation principles and methods. A means for processing an image bearing member is a device which is used to process an image bearing member to form an image on the image bearing member. 
         [0005]    Hereinafter, for convenience&#39;s sake, the present invention is described with reference to an electrophotographic image forming apparatus which employs a cartridge system. Examples of a cartridge are a process cartridge, a development cartridge, and the like. 
         [0006]    A process cartridge is a cartridge in which an electrophotographic photosensitive member, and at least one among a charging means, a developing means, and a cleaning means, are integrally disposed, and which is removably installable in the main assembly of an electrophotographic image forming apparatus. 
         [0007]    Thus, a process cartridge includes: a cartridge in which a developing means as a processing means, and an electrophotographic photosensitive member, are integrally disposed, and which is removably installable in the main assembly of an electrophotographic image forming apparatus; and a cartridge in which an electrophotographic photosensitive member, a charging means, and a developing means are integrally disposed, and which is removably installable in the main assembly of an electrophotographic image forming apparatus; and a cartridge in which an electrophotographic photosensitive member, a charging means, and a cleaning means are integrally disposed, and which is removably installable in the main assembly of an electrophotographic image forming apparatus. 
         [0008]    A process cartridge in which an electrophotographic photosensitive member and a developing means are integrally disposed is referred to as a process cartridge of the so-called all-in-one type. In comparison, a process cartridge in which an electrophotographic photosensitive member and processing means other than a developing means are integrally disposed is referred to as a process cartridge of the so-called separation type. That is, a process cartridge of the separation type does not have a developing means. Therefore, it has to be used in combination with a development unit which has a developing means. 
         [0009]    A process cartridge can be installed into, or removed from, the main assembly of an image forming apparatus by a user by himself or herself. Thus, it can make an image forming apparatus easier to maintain. 
         [0010]    A development cartridge has a developer bearing member (which may be referred to as development roller, hereafter) which supplies an electrophotographic photosensitive member with developer. It stores powdery developer (toner) which is used by a development roller to develop an electrostatic latent image formed on an electrophotographic photosensitive member. It also is removably installable in the main assembly of an electrophotographic image forming apparatus. 
         [0011]    In the case of an electrophotographic image forming apparatus which employs a development cartridge, its electrophotographic photosensitive member is attached to the main assembly or cartridge supporting member of the main assembly of the apparatus, or is a part of the aforementioned process cartridge of the so-called separation type (which does not have developing means). A development cartridge also can be removably installable in the main assembly of an electrophotographic image forming apparatus. Thus, it can make an electrophotographic image forming apparatus easier to maintain. 
         [0012]    That is, a process cartridge includes both a process cartridge of the so-called all-in-one type, and a process cartridge of the so-called separation type. Further, it includes a development cartridge which is used in combination with a process cartridge of the so-called separation type. Further, it includes a development cartridge which is removably installable in the main assembly of an electrophotographic image forming apparatus, the electrophotographic photosensitive member of which is fixed to the main assembly or cartridge supporting member of the apparatus, so that it can process the electrophotographic photosensitive member. In other words, a cartridge includes a unit which can be removably installable in the main assembly of an electrophotographic image forming apparatus so that it can contribute to the image forming process carried out by the apparatus to form an image on recording medium. 
         [0013]    Some conventional image forming apparatuses are provided with a movable tray (movable member; cartridge supporting member) on which multiple process cartridges are horizontally aligned. They are structured so that the tray can be pulled out of their main assembly, and also, so that when the tray is outside their main assembly, the process cartridge are placed on, or removed from, the tray. 
         [0014]    Some of the above-described image forming apparatuses with a movable tray are provided with a cover which can cover or expose the opening with which their main assembly is provided to allow the tray to be pulled out of the main assembly. They are structured so that as the cover is opened or closed, their interfacing members are placed in contact, or separated from, the corresponding portions of each of the cartridges in the main assembly (Japanese Laid-open Patent Application 2009-128506). The interfacing members include a member which presses and keeps pressed a cartridge, a member which engages with a cartridge to transmit cartridge driving force to the cartridge, and a member which supplies a cartridge with electric power, for example. 
         [0015]    The conventional image forming apparatus disclosed in Japanese Laid-open Patent Application 2009-128506 is structured so that its member for supporting its tray is moved in the frontward or rearward, and also, in the upward or downward, directions of the main assembly, by the rotational movement of its front door. More specifically, as the door is closed, the tray holding member is moved rearward while being moved downward, and then, is moved further rearward, by the movement of the door. 
         [0016]    As the tray and the cartridges thereon are moved by the above described downward movement of the tray holding member, the electric power supplying members, with which the main assembly of the image forming apparatus is provided, come into contact with the intermediary contacts, with which the tray is provided, from the bottom side of the tray. Then, as the tray supporting member is moved further rearward, the cartridge pressing members come into contact with the cartridges from the top side of the tray (cartridges), and at the same time, the driving force transmitting couplings (driving force transmitting members) engage with the driving force input portions of the cartridges. 
         [0017]    As the door is opened, the interfacing members are separated from the counterparts of the corresponding cartridges, one for one, in the opposite order from the above-described one in which they are engaged. In other words, the conventional image forming apparatus disclosed in Japanese Laid-open Patent Application 2009-128506 is structured so that the cartridges in its main assembly can be easily replaced from the front side of the apparatus. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention is one of the results of further development of the above described prior art. 
         [0019]    Thus, the primary object of the present invention is to provide an image forming apparatus which is similar in structure to the above described one, and yet, is substantially smaller in the amount of the force of which an operator is required to open or close the door of the apparatus, and can ensure that as a cartridge or cartridges are installed into the main assembly of the apparatus, it is accurately placed in their preset image formation position in the main assembly. 
         [0020]    According to an aspect of the present invention, there is provided an image forming apparatus for forming an image on a recording material, to which apparatus a cartridge is detachably mountable, said image forming apparatus comprising a main assembly; an opening provided in said main assembly; a main assembly door movable between a closing position for closing said opening and an open position for opening said opening; a tray for carrying said cartridge, said tray is capable of taking (i) an image forming position in which said cartridge is in a position capable of image forming operation in said main assembly, when said main assembly door is in the closing position, (ii) a mounting and demounting position for permitting mounting and demounting of said cartridge outside said main assembly, when said main assembly door is in the open position, and (iii) a movable position which is between the mounting and demounting position and the image forming position and in which said tray is movable to the image forming position in interrelation with movement of main assembly door from the open position to the closing position; a supporting member movably supporting said tray and movable in interrelation with said main assembly door; a drive transmission member capable of taking a connecting position for transmitting a driving force to said cartridge when said main assembly door is in the closing position, and a first spacing position for spacing from said cartridge when said main assembly door is in the open position; an urging member capable of taking a contact position in which said urging member urges said cartridge to said main assembly when said main assembly door is in the closing position, and a second spacing position for spacing from said cartridge when said main assembly door is in the open position; and an electric energy supply member capable of taking an electric energy supplying position for supplying electric energy to an electric power supply contact portion of said cartridge when said main assembly door is in the closing position, and a third spacing position for spacing from said electric power supply contact portion of said cartridge when said main assembly door is in the open position, wherein in interrelation with the movement of said main assembly door from the open position to the closing position, (i) movement of said tray from the movable position to the image forming position, (ii) movement of said drive transmission member from the first spacing position to the connecting position, (iii) movement of said urging member from the second spacing position to the contact position, and (iv) movement of said electric energy supply member from the third spacing position to the electric energy supplying position, are carried out in the order named. 
         [0021]    According to another aspect of the present invention, there is provided an image forming apparatus for forming a on a recording material, to which apparatus a cartridge is detachably mountable, said image forming apparatus comprising a main assembly; an opening provided in said main assembly; a main assembly door movable between a closing position for closing said opening and an open position for opening said opening; a tray for carrying said cartridge, said tray is capable of taking (i) an image forming position in which said cartridge is in a position capable of image forming operation in said main assembly, when said main assembly door is in the closing position, (ii) a mounting and demounting position for permitting mounting and demounting of said cartridge outside said main assembly, when said main assembly door is in the open position; an urging member for urging said cartridge carried on said tray in a longitudinal direction of said cartridge which is from one longitudinal end to the other longitudinal end of said cartridge; and a fixed side plate opposed to the other end portion side of said cartridge carried on said tray, wherein said cartridge is provided at the other end portion with a positioning portion, and wherein when said tray carrying said cartridge is in the image forming position, said positioning portion is contacted to said side plate by said cartridge being urged by said urging member so that said cartridge is positioning in the longitudinal direction. 
         [0022]    These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is an external perspective view of a typical image forming apparatus (in first embodiment of present invention) to which the present invention is applicable. 
           [0024]      FIG. 2  is a schematic sectional view of the image forming apparatus in the first embodiment, at a vertical plane parallel to the recording medium conveyance direction of the apparatus, as seen from the left-hand side of the apparatus. 
           [0025]      FIG. 3  is an external perspective view of the image forming apparatus shown in  FIG. 1 , when the front door of the apparatus is wide open. 
           [0026]      FIG. 4  is a schematic sectional view of the image forming apparatus in the first embodiment, at a vertical plane parallel to the recording medium conveyance direction of the apparatus, as seen from the left-hand side of the apparatus when the front door of the apparatus is wide open. 
           [0027]      FIG. 5  is an external perspective view of the image forming apparatus shown in  FIG. 1 , when the cartridge tray of the apparatus is in its outermost position. 
           [0028]      FIG. 6  is a schematic sectional view of the image forming apparatus in the first embodiment, at a vertical plane parallel to the recording medium conveyance direction of the apparatus, as seen from the left-hand side of the apparatus when the cartridge tray of the apparatus is in its outermost position. 
           [0029]      FIG. 7  is a perspective view of the cartridge in the first embodiment, as seen from the side from which it is not driven (no-driving side, hereafter). 
           [0030]      FIG. 8  is a perspective view of the cartridge in the first embodiment, as seen from the side from which it is driven (driving side, hereafter). 
           [0031]      FIG. 9  is a perspective view of the cartridge tray. 
           [0032]      FIG. 10  is a perspective view of the front door and tray holding member of the apparatus, and the mechanism of the apparatus, which is made to moves the tray supporting member, by the movement of the front door, and the adjacencies of the mechanism. 
           [0033]      FIG. 11  is a drawing illustrating the movement of the tray holding member of the image forming apparatus, which is caused by the movement of the front door when the door is pivotally opened. 
           [0034]      FIG. 12  is an enlarged view of the tray guiding hole of the tray holding member. 
           [0035]      FIG. 13  is a perspective view (1) of the tray position regulating means. 
           [0036]      FIG. 14  is a perspective view (2) of the tray position regulating means. 
           [0037]      FIG. 15  is a sectional view (3) of the tray position regulating means. 
           [0038]      FIG. 16  is a perspective view (4) of the tray position regulating means. 
           [0039]      FIG. 17A  is a perspective view illustrating the means for supplying the cartridge with electric power. 
           [0040]      FIG. 17B  is an enlarged perspective view of a part of  FIG. 17A , illustrating the means for supplying the cartridge with electric power. 
           [0041]      FIG. 17C  is a perspective view of a part of  FIG. 17B , illustrating the means for supplying the cartridge with electric power. 
           [0042]      FIG. 18  is a drawing (2) illustrating the means for supplying the cartridge with electric power. 
           [0043]      FIG. 19  is a perspective view (1) of the means for pressing, and keeping pressed, the cartridge. 
           [0044]      FIG. 20  is a perspective view (2) of the means for pressing, and keeping pressed, the cartridge. 
           [0045]      FIG. 21  is an enlarged view of the cartridge tray guiding hole of the means for pressing, and keeping pressed, the cartridge. 
           [0046]      FIG. 22  is a drawing (1) illustrating the means for pressing, and keeping pressed, the cartridge. 
           [0047]      FIG. 23  is a drawing (2) illustrating the means for pressing, and keeping pressed, the cartridge. 
           [0048]      FIG. 24  is a drawing (1) illustrating the means for transmitting the cartridge driving force to the cartridge. 
           [0049]      FIG. 25  is a drawing (2) illustrating the means for transmitting the cartridge driving force to the cartridge. 
           [0050]      FIG. 26  is a perspective view of the latch of the cartridge, and the catch of the main assembly of the image forming apparatus. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       [0051]    &lt;&lt;Overall Structure of Image Forming Apparatus&gt;&gt; 
         [0052]      FIG. 1  is an external perspective view of the image forming apparatus  100  in the first embodiment of the present invention.  FIG. 2  is a schematic sectional view of the apparatus  100 , at a vertical plane parallel to the recording medium conveyance direction of the apparatus  100 , as seen from the left-hand side of the apparatus  100 . The image forming apparatus  100  is a full-color laser printer (electrophotographic image forming apparatus) based on four primary colors. It uses an electrophotographic process, and employs multiple (four) cartridges P, more specifically, the first to fourth cartridges PY, PM, PC and PK, respectively. 
         [0053]    That is, this image forming apparatus  100  can form a full-color image (based on four primary colors) or a monochromatic image on a sheet S of recording medium, in response to electrical image formation signals which are outputted from an external host apparatus  100  and are inputted into the control section  200  of the image forming apparatus  100  through an interface portion  300 . The external host apparatus  400  is a personal computer, an image reader, a facsimile apparatus from which electrical image formation signals are sent, etc. 
         [0054]    The control section  200  is a controlling means which controls the electrophotographic image formation process carried out by the image forming apparatus  100 . It exchanges various electrical information with the external host apparatus  400 . Further, it processes: the electrical information inputted from various processing devices and sensors; and command signals to be outputted to the various processing devices. Further, it controls the preset initial sequence, preset electrophotographic image formation sequence, etc. 
         [0055]    In the description of the following embodiments of the present invention, the front side (front surface side) of the image forming apparatus  100  is the side on which the door (main assembly door)  31  is present. The rear side (rear surface side) is the opposite side of the image forming apparatus  100  from the front side. The front-rear direction includes both the rear-to-front (frontward) direction and front-to-rear (rearward) direction. The left and right of the image forming apparatus  100  are the left and right of the apparatus  100  when the apparatus  100  is seen from the front side of the apparatus  100 . The left-right direction (rightward direction) includes both the left-to-right direction and the opposite direction from the left-to-right direction (rightward direction). Further, the upward and downward directions of the apparatus  100  are the upward and downward direction with reference to the gravity direction. The upward direction is the bottom-to-top direction of the apparatus  100 . The downward direction is the top-to-bottom direction. 
         [0056]    Further, the lengthwise direction is the direction which is parallel to the rotational axis of the electrophotographic photosensitive member, or an image bearing member, on which a latent image is formed. The widthwise direction is the direction which is perpendicular to the above described lengthwise direction (perpendicular direction). Moreover, one of the two ends in terms of the lengthwise direction is the driving end, whereas the other lengthwise end is the no-driving end. In this embodiment, the right side of the image forming apparatus  100  in terms of the lengthwise direction is the driving side, and the left side of the apparatus  100  is the no-driving side. 
         [0057]    The main assembly  100 A (main assembly frame) of the image forming apparatus  100  has an internal chamber  100 B (cartridge chamber  100 B, hereafter) in which cartridges are installed. This cartridge chamber  100 B can hold four cartridges, more specifically, first to fourth cartridges PY, PM, PC and PK, in the positions designated to the four cartridges, respectively, in such a manner that the four cartridge horizontally align in parallel (tandem) in the rear-to-front direction in the listed order (inline structure; tandem type). The designated cartridge position in the cartridge chamber  100 B, or the position designated to a given cartridge, in the cartridge chamber  100 B, is the position in which the cartridge can actually contribute to an image forming operation. 
         [0058]    The cartridge P contributes to the image formation process for forming an image on a sheet S of recording medium. It is removably installed in the apparatus main assembly  100 A of the image forming apparatus  100  to be used for image formation. In this embodiment, each of the four cartridges P has an electrophotographic photosensitive member (which will be referred to simply as drum, hereafter)  1 , as an image bearing member, on which a latent image is formed. Each cartridge P has also a charging means  2 , a developing means  3 , and a cleaning means  4 , which are for electrophotographically processing the drum  1 . In other words, the process cartridge in this embodiment is a process cartridge of the so-called all-in-one type. 
         [0059]    In this embodiment, the charging means  2  is a charge roller of the contact type. The developing means  3  is a developing device of the contact, or non-contact, type. It has a development roller  3   a , as a developer bearing member, which develops a latent image into a developer image (image formed of developer) by supplying the drum  1  with developer. It has also a developer storage  3   b  which stores developer. The cleaning means  4  is a cleaning device of the blade type. It has a cleaning blade  4   a  as a cleaning member, a waste toner storage  4   b , etc. Further, each cartridge P has its own mechanism for electrophotographically processing its own drum  1 ; it has its own devices, such as the above-described ones, for forming an image. That is, the four cartridges P are the same in configuration except for the color of the developer (which will be referred to as toner) they store. 
         [0060]    That is, the first cartridge PY stores yellow (Y) toner in its developing device  3 . It forms a yellow (Y) toner image on the peripheral surface of its drum  1 . The second cartridge PM stores magenta (M) toner in its developing device  3 . It forms a magenta (M) toner image on the peripheral surface of its drum  1 . The third cartridge PC stores cyan (C) toner in its developing device  3 . It forms a cyan (C) toner image on the peripheral surface of its drum  1 . The fourth cartridge PK stores black (K) toner in its developing device  3 . It forms a black (K) toner image on the peripheral surface of its drum  1 . 
         [0061]    The apparatus main assembly  100 A is provided with a laser scanner unit  11  as an exposing device unit (exposing means) for exposing the drum  1  of each cartridge P to form a latent image on the drum  1 . The laser scanner unit  11  is disposed so that it will be on the top side of the cartridges PY, PM, PC and PK after the installation of the cartridges into the cartridge chamber  100 B. This scanner unit  11  scans (exposes) the peripheral surface of the drum  1  of each cartridge P, through the exposure window  6 , with which the top wall of the cartridge frame  5  is provided, by outputting a beam L of laser light while modulating the beam L according to the information regarding the color of the monochromatic image, which is inputted into the control section  200  from the external host apparatus  400 . 
         [0062]    The apparatus main assembly  100 A is also provided with an intermediary transfer unit  12  as an image transferring unit (transferring member), onto which the toner image formed on the peripheral surface of the drum  1  of each cartridge P is transferred (primary transfer), and from which the transferred toner image is transferred (secondary transfer) onto a sheet S of recording medium. The intermediary transfer unit  12  is disposed so that it will be on the bottom side of the cartridges P after the installation of the cartridges P in the cartridge chamber  100 B. 
         [0063]    This unit  12  has an endless belt  13 , a driver roller  14 , a turn roller  15 , and a tension roller  16 . The endless belt  13  is the intermediary transferring member (second image bearing member, intermediary recording medium). It is made of a dielectric substance, and is flexible. It is wrapped around the combination of the rollers  14 ,  15  and  16 , and is circularly moved while being kept tensioned. The driver roller  14  and turn roller  15  are in the rear side of the apparatus main assembly  100 A, whereas the tension roller  16  is in the front side of the apparatus main assembly  100 A. 
         [0064]    When each cartridge P is in its preset position (image formation position) in the apparatus main assembly  100 A, the downwardly facing portion of its drum  1  is in contact with the upwardly facing surface of the portion of the belt  13 , which corresponds in position to the top portion of the loop (belt loop, hereafter) which the belt  13  forms. There are disposed four primary transfer rollers  17  on the inward side of the belt loop, being positioned so that they oppose the drums  1  in the cartridges P, one for one, with the presence of the belt  13  between themselves and drums  1 . 
         [0065]    The nip which the drum  1  in each cartridge P, and the belt  13 , form is the primary transfer nip T1. The apparatus main assembly  100 A is provided with a secondary transfer roller  22 , which is pressed against the driver roller  14 , with the presence of the belt  13  between the rollers  14  and  22 . The nip between the secondary transfer roller  22  and belt  13  is the secondary transfer nip T2. 
         [0066]    There is disposed a sheet feeder unit  18  below the intermediary transfer unit  12 . The sheet feeder unit  18  stores multiple sheets S of recording medium (which may be referred to simply as recording medium), onto which a toner image is transferred. Further, it feeds the sheets S of recording medium, one by one, into the apparatus main assembly  100 A, and conveys further to the intermediary transfer unit  12 . 
         [0067]    This sheet feeder unit  18  has: a sheet feeder tray  19  in which multiple sheets S of recording medium (which may be referred to simply as regarding medium) are stored in layers; a sheet feeder roller  20 , a separation pad  21 , a pair of registration rollers  20   a ; etc. The sheet feeder tray  19  is removable from the apparatus main assembly  100 A; it can be pulled out of, or inserted into, the apparatus main assembly  100 A, from the front side of the apparatus main assembly  100 A (front loading). A referential code  19   a  stands for a handhold with which the front plate of the sheet feeder tray  19  is provided. 
         [0068]    The apparatus main assembly  100 A is provided with a fixing device  23  and a pair of sheet discharging rollers  24 , which are in the top portion of the rear side of the apparatus main assembly  100 A. The fixing device  23  is a fixation unit (fixing means) which fixes an unfixed toner image on a sheet S of recording medium to the sheet S, by applying heat and pressure to the sheet S and the toner image thereon, and then, discharges the sheet S. A part of the top surface of the apparatus main assembly  100 A is shaped so that it functions as a delivery tray  25 . The fixing device  23  used in this embodiment is such a fixing device that has a fixation film assembly  23   a  and a pressure roller  23   b . The pair of sheet discharging rollers  24  are rollers  24   a  and  24   b.    
         [0069]    As the cartridges P are installed into the apparatus main assembly A, they are placed into the cartridge chamber  100 B in such a manner that they are placed in their designated positions in which they are usable for an image forming operation, remaining pressed by a cartridge pressing members  42  ( FIG. 22 ) one for one, as will be described later. Further, the driving force output portions (driving force transmitting members)  39  and  40  ( FIGS. 24 and 25 ) of the apparatus main assembly  100 A engage with the driving force input portions (coupling portions)  53  and  54  ( FIG. 8 ) of the cartridges P, as will be described later. Further, electrical connection is established between the electric power supplying system (electric power supplying member)  75  ( FIG. 18 ) of the apparatus main assembly  100 A and the electrical contacts (electric power supply contacts)  55  of the cartridges P ( FIG. 7 ), as will be described later. 
       &lt;&lt;Image Forming Operation&gt;&gt; 
       [0070]    The operation carried out by the image forming apparatus  100  in this embodiment to form a full-color image is as follows. First, the drum  1  in each of the first to fourth cartridges PY, PM, PC and PK begins to be rotationally driven in the counterclockwise direction indicated by an arrow mark  FIG. 2 , at a preset control speed. At the same time, the belt  13  begins to be circularly moved in the clockwise direction (the same direction as the direction in which the peripheral surface of the drum  1  moves in the area of contact between the belt  13  and drum  1 ), at a speed which corresponds to the peripheral velocity of the drum  1 . Further, the scanner unit  11  also begins to be driven. 
         [0071]    In synchronism with the starting of the driving of the abovementioned components, the charge roller  2  in each cartridge P begins to uniformly charge the peripheral surface of the drum  1  in the cartridge P to preset polarity and potential level. The scanner unit  11  begins to scan (expose) the uniformly charged portion of each drum  1 , with the beam L of laser light which it outputs while modulating the beam L according to the image formation signals, which correspond in color to one of the monochromatic primary color images, into which the image to be formed has been separated. Consequently, an electrostatic latent image, which reflects the image signals, is formed on the peripheral surface of each drum  1 . Then, the latent image is developed into a toner image (image formed of toner (developer)) by the corresponding developing device  3 . 
         [0072]    Through the electrophotographic image formation process described above, a yellow (Y) toner image, which corresponds in color to the yellow component of the full-color image to be formed, is formed on the drum  1  of the first cartridge PY. Then, the toner image is transferred (primary transfer) onto the belt  13 , in the primary transfer nip T1 of the cartridge PY. 
         [0073]    On the drum  1  of the second cartridge PM, a magenta (M) toner image, which corresponds in color to the magenta (M) component of the full-color image, is formed. Then, the magenta (M) toner image is transferred (primary transfer) onto the belt  13 , in the primary transfer nip T1 of the cartridge PM, in such a manner that it is layered onto the yellow (M) toner image which has already been transferred onto the belt  13 . 
         [0074]    On the drum  1  of the third cartridge PC, a cyan (C) toner image, which corresponds in color to the cyan (C) component of the full-color image, is formed. This toner image is transferred (primary transfer) onto the belt  13 , in the primary transfer nip T1 of the cartridge PM, in such a manner that it is layered upon the yellow (Y) and magenta (M) toner images, which have just been transferred onto the belt  13 . 
         [0075]    On the drum  1  of the fourth cartridge PK, a black (K) toner image, which corresponds in color to the black (K) component of the full-color image, is formed. This toner image is transferred (primary transfer) onto the belt  13 , in the primary transfer nip T1 of the cartridge PK, in such a manner that is it layered onto the yellow (Y), magenta (M) and cyan (C) toner images which have already been transferred onto the belt  13 . 
         [0076]    Consequently, an unfixed full-color toner image is synthetically effected on the belt  13  by the layered four monochromatic yellow (Y), magenta (M), cyan (C) and black (K) toner images on the belt  13 . The transfer residual toner, that is, the toner remaining on the peripheral surface of the drum  1  in each cartridge P after the primary transfer of the toner image onto the belt  13 , is removed by the cleaning device  4  of the cartridge P. 
         [0077]    Meanwhile, the feed roller  20  begins to be driven with a preset control timing, whereby the multiple sheets S of recording medium are fed into the apparatus main assembly  100 A one by one, while being separated from each other by the coordination between the feed roller  20  and separation roller  21 , and are conveyed to the pair of registration rollers  20   b . Then, each sheet S of recording medium is introduced by the registration rollers  20   b  into the secondary transfer nip T2 with a preset control timing. Then, the sheet S is conveyed through the secondary transfer nip T2 while remaining pinched between the secondary transfer roller  22  and driver roller  14 . While the sheet S is conveyed through the secondary transfer nip T2, the four monochromatic toner images, different in color, layered on the belt  13  are transferred together onto the surface of the sheet S as if they are peeled away from the belt  13 . 
         [0078]    Then, the sheet S of recording medium is separated from the surface of the belt  13 , and is introduced into the fixing device  23  through the recording medium conveyance passage  20   b . Then, it is subjected to heat and pressure in the fixation nip N of the fixing device  23 . Consequently, the layered four monochromatic toner images, different in color, on the sheet S are fixed to the sheet S. Thereafter, the sheet S is conveyed out of the fixing device  23 , and is discharged as a full-color print by a pair of discharge roller  24  into the delivery tray  25 . The secondary transfer residual toner, that is, the toner remaining on the surface of the belt  13  after the separation of the sheet S from the belt  13 , is removed by the cleaning means  26 . 
       &lt;&lt;Method for Replacing Cartridge in Apparatus Main Assembly  100 A&gt;&gt; 
       [0079]    As each of the first to fourth cartridges PY, PM, PC and PK is used for image formation, the developer in the developing device  3  of each cartridge P is consumed for the image formation. Thus, as the developer in any of the cartridges PY, PM, PC and PK is consumed so much for image formation that the amount of the developer remaining in the cartridge P is not large enough for the image forming apparatus  100  to yield an image which is high enough in quality to satisfy the user who purchased the cartridge P, the cartridge loses its commercial value. 
         [0080]    Thus, the image forming apparatus  100  is provided with a means (unshown) for detecting the amount of the toner remaining in each cartridge P so that the detected amount of the developer in the cartridge P is compared by the control section  200  with a threshold value which is preset to inform a user of the remaining length of the service life of the cartridge, and/or warning the user that the cartridge P is near the end of its service life. If the control section  200  determines that the detected amount of the developer in a given cartridge is less than the threshold value, it shows a message that indicates the remaining length of the service life of the cartridge P, or warns the user that the cartridge P is near the end of its service life, on the display section (unshown) of the apparatus main assembly  100 A, prompting thereby the user to prepare a replacement cartridge, and/or to replace the cartridge P in the apparatus main assembly  100 A with a replacement cartridge to keep the image forming apparatus  100  at a preset level in terms of the quality of the image the image forming apparatus  100  outputs. 
         [0081]    For the purpose of improving the image forming apparatus  100  in this embodiment in usability, the image forming apparatus  100  is provided with a cartridge tray (drawer, movable member)  35 , in which the cartridges P are placed. The image forming apparatus  100  is structured so that the cartridge tray  35  can be pulled out, or pushed into, the apparatus main assembly  100 A from the front side of the apparatus main assembly  100 A (front access type). 
         [0082]    More specifically, the front wall of the apparatus main assembly  100 A is provided with an opening  30  through which the cartridges P can be inserted into, or removed from, the cartridge chamber  100  B in the apparatus main assembly  100 A. 
         [0083]    Referring to  FIGS. 1 and 2 , the apparatus main assembly  100 A is provided with a main assembly door  31  (member which can be opened or closed), which is movable between the position A (closed position), in which it covers the opening  30 , and the position B (open position), in which it exposes the opening  30 . A referential code  31   a  stands for a recess in which a finger can be placed to open or close the door  31 . That is, the main assembly door  31  is enabled to take the closed position A, in which it covers the opening  30 , and the open position B, in which it keeps the opening  30  fully exposed. 
         [0084]    In this embodiment, the door  31  is pivotally opened or closed relative to the apparatus main assembly  100 A about a horizontal shaft (hinge shaft)  32  which is located at the bottom edge of the door  31 . That is, the door  31  can be pivotally moved about the hinge shaft  31 , into its upright position in which it is shut against the apparatus main assembly  100 A. As the door  31  is shut, the opening  30  is covered. 
         [0085]    Further, the door  31  can be pivotally moved frontward about the hinge shaft  32 , into a roughly horizontal position, in which it keeps fully exposes the opening  30  of the front wall of the apparatus main assembly  100 A. The recess  31   a  with which the door  31  is provided is where the finger(s) of an operator can be put to open or close the door  31 . 
         [0086]    Next, referring to  FIGS. 13-15 ,  17 ,  19 ,  23 , etc., the apparatus main assembly  100 A is provided with a main frame (main assembly frame), which has a pair (left and right) of sub-frames  80 L and  80 R. The apparatus main assembly  100 A is also provided with a pair (left and right) tray holding members (movable means)  34 L and  34 R, which are positioned on the inward side of the sub-frames  80 L and  80 R, respectively. The tray holding members  34 L and  34 R oppose each other, and extend in the front-rear direction. 
         [0087]    Further, the apparatus main assembly  100 A is provided with a cartridge tray (movable member)  35 , which is shaped like a skeletal frame, and is rested on the tray holding members tray holding members  34 L and  34 R, being thereby enabled to horizontally (in parallel to surface on which apparatus main assembly  100 A is resting) and linearly move in the front-rear direction, between the left and right sub-frames  80 L and  80 R. The cartridges PY, PM, PC and PK are supported in the apparatus main assembly  100 A by being mounted in this tray  35 . That is, the tray  35 , which is a movable member, is for supporting and moving the cartridges P. It is movable between the mutually opposing lateral sub-frames (plates)  80 L and  80 R, which function as the parts (sub-frames) of the main frame of the apparatus main assembly  100 A. 
         [0088]    Referring to  FIGS. 1 and 2 , when the door  31  is in its closed position A, the tray  35  is in its image formation position (inward position) C, in which it keeps each cartridge P in the tray  35  in the image formation position (inward position). In this embodiment, the image formation position C of the tray  35  is such a position for the tray  35  that keeps the drum  1  in each cartridge P in the tray in contact with the belt  13  of the intermediary transfer unit  12  ( FIG. 2 ). 
         [0089]    Thus, as the door  31  is opened (pivotally moved frontward), the tray holding members  34 L and  34 R are moved frontward by a preset amount, and then, are moved upward by a preset amount, by the movement of the door  31 . Consequently, the tray  35 , in which the cartridges PY, PM, PC and PK are held, is moved frontward, and then, upward, from its image formation position C, described above, as shown in  FIG. 4 , being thereby moved into a locking (unlocking) position D, in which the tray  35  can be unlocked from the tray holding members  34   l  and  34 R to be horizontally movable in the front-rear direction, or locked to the tray holding members  34 L and  34 R. As the tray  35  is moved as described above, the drum  1  in each cartridge P in the tray  35  separates from the belt  13  as shown in  FIG. 4 . The mechanism which causes the movement of the door  31  to move the tray holding members  34 L and  34 R will be described later. 
         [0090]    As will be described later, also as the door  31  is opened, the electrical connection between the electrical contact  55  of each cartridge P and the electric power supply system  75  of the apparatus main assembly  100 A is interrupted (electric power supply interruption) by the pivotal opening movement of the door  31 . Further, the driving force input portions  53  and  54  of each cartridge P are disengaged from the driving force output portions  39  and  40  of the apparatus main assembly  100 A (disabling of driving force transmission) by the movement of the door  31 . Further, each cartridge P is freed from the pressure applied by the corresponding pressing member  42  to keep the cartridge P properly positioned (pressure removal). 
         [0091]    Referring to  FIGS. 3 and 4 , next, an operator is to grasp the handhold (tray unlocking means)  35   a  with which the front portion  35   b  of the tray  35  is provided. Grasping the handhold  35   a  in a preset manner causes the tray locking latch (tray movement regulating means)  101  of the tray  35  to disengage from the catch portion (tray movement regulating means)  102  of the main assembly  100 A, as shown in  FIG. 15(   a )→ 15 ( b ). 
         [0092]    Consequently, it becomes possible for the tray  35 , which is in the tray locking (unlocking) position D in the apparatus main assembly  100 A, to be pulled out of the apparatus main assembly  100 A. More specifically, it becomes possible for the tray  35  to be horizontally slid frontward along the tray holding members  34 L and  34 R. Thus, the tray  35  can be pulled out of the apparatus main assembly  100 A, and made to protrude into its preset outward position, that is, the cartridge mounting (dismounting) position E which makes it possible for the cartridges P to be mounted into, or removed from, the tray  35 , in a preset manner, as shown in  FIGS. 5 and 6 . 
         [0093]    That is, the first to fourth cartridges PY, PM, PC and PK held by the tray  35  are moved out of the apparatus main assembly  100 A through the opening  30 , and become fully exposed from the apparatus main assembly  100 A; the top surface of each cartridge P becomes exposed. As the tray  35  is pulled out of the apparatus main assembly  100 A by a sufficient amount, that is, as the tray  35  is moved outward of the apparatus main assembly  100 A by a preset amount from the tray locking (unlocking) position D, it is prevented by an unshown stopper from being pulled out farther. Further, after the tray  35  is horizontally pulled out to the cartridge mounting (dismounting) position E, it is horizontally held in the cartridge mounting (dismounting) position E by the tray holding members  34 L and  34 R, and a pair of linkage arms  37 L and  37 R, which will be described later. 
         [0094]    When the tray  35  is in its cartridge mounting (dismounting) position E, it is supporting the cartridges P in such a manner that each cartridge P can be vertically (upwardly) moved out of the tray  35 . Incidentally, each cartridge P can be held by the tray  35  by being vertically (downwardly) moved into the tray  35 . Thus, if a given cartridge is used up, and therefore, needs to be replaced, it is to be vertically lifted from the tray  35 , as indicated by a broken line in  FIG. 6 , so that it can be extracted from the tray  35 . Then, a brand-new cartridge (or used cartridge which still contains usable amount of developer) is to be mounted (fitted into) on the tray  35  from the top side of the tray  35 . That is, it is after the tray  35  was pulled out of the apparatus main assembly  100 A that the cartridges P are mounted on, or extracted from, the tray  35  so that they can be moved into, or extracted from, the apparatus main assembly  100 A. 
         [0095]    In the case of the image forming apparatus  100  in this embodiment, the tray  35  supports the cartridges PY, PM, PC and PK in the listed order, in terms of the upstream side to the downstream side in terms of the direction in which the tray  35  is moved from the tray locking (unlocking) position D to the cartridge mounting (dismounting) position E. That is, as the cartridges PY, PM, PC and PK, in which yellow (Y), magenta (M), cyan (C) and black (K) developers, respectively, are stored, are mounted into the tray  35 , they align in the listed order. That is, among the multiple cartridges P, which are different in the color of the toner they contain, the cartridge PK which contains the black (K) developer is supported in the most downstream slot in the tray  35  in terms of the direction in which the tray  35  is moved from the tray locking (unlocking) position D to the cartridge mounting (dismounting) position E. 
         [0096]    The cartridge PK is greater in developer consumption than the other cartridges P. That is, it is highest in replacement frequency. In other words, the cartridge PK, which is likely to be most frequently replaced, is supported by the frontmost portion of the tray  35  (frontmost portion of apparatus main assembly  100 A). Therefore, all that is necessary to be done to expose the cartridge PK from the apparatus main assembly  100 A to replace the cartridge PK is to pull the tray  35  out of the apparatus main assembly  100 A by the amount large enough to expose only the cartridge PK. In other words, when it is only the cartridge PK that needs to be replaced, it is unnecessary for the tray  35  to be pulled out of the apparatus main assembly  100 A all the way until the outward movement of the tray  35  is regulated by the stopper. In other words, this structural arrangement improves an electrophotographic image forming apparatus which employs a cartridge system, in the efficiency with which the cartridge PK can be replaced. 
         [0097]    That is, when it is the cartridge PK, that is, the cartridge which is positioned most downstream in the tray  35  in terms of the direction in which the tray  35  is pulled out of the apparatus main assembly  100 A, that needs to be mounted into, or extracted from, the tray  35 , the tray  35  does not need to be pulled out of the apparatus main assembly  100 A all the way to the cartridge mounting (dismounting) position E. In other words, the multiple cartridges P in the tray  35  can be sequentially removed from the tray  35 , starting from the most downstream one, in terms of the direction in which the tray  35  is pulled out of the apparatus main assembly  100 A, as the tray  35  is moved from the tray locking (unlocking) position D to the cartridge mounting (dismounting) position E. That is, it is when the tray  35  is in the cartridge mounting (dismounting) position E that all the cartridges P can be removed from the tray  35 . 
         [0098]    As described above, the tray  35  is such a member that can be moved in the direction (vertical direction) which is perpendicular (intersectional) to the direction of the axial line of the drum  1  of each cartridge P in the tray  35  (lengthwise direction of cartridge P). That is, the direction in which the tray  35  is moved between the tray locking (unlocking) position D and position E is perpendicular to the lengthwise direction of each cartridge P in the tray  35 . In other words, the multiple cartridges P are mountable in the tray  35  in such a manner that they align in parallel (tandem) in the direction in which the tray  35  is moved between the tray locking (unlocking) position D and position E. 
         [0099]    Further, the tray  35  in which the cartridges P are placed to be moved into, or extracted from, the apparatus main assembly  100 A can be placed in the image formation position (most inward position) C, tray locking (unlocking) position D, and cartridge mounting (dismounting) position E. 
         [0100]    The image formation position (most inward position) C for the tray  35  is in the apparatus main assembly  100 A. It is such a position that as the tray  35  is moved into the image formation position C, it places each cartridge P in the tray  35  in the image formation position for the cartridge P ( FIG. 2 ). 
         [0101]    The cartridge mounting (dismounting) position E for the tray  35  is such a position for the tray  35  that when the tray  35  is in the cartridge mounting (dismounting) position E, it is protruding from the apparatus main assembly  100 A, through the opening  30 , as far as it is allowed to protrude, to enable the cartridges P to be mounted into, or removed from, the tray  35  ( FIGS. 5 and 6 ). 
         [0102]    The tray locking (unlocking) position D for the tray  35  is in the tray path between the cartridge mounting (dismounting) position E and image formation position C. It is the tray position from which the tray  35  is moved to the image formation position C by the pivotal movement of the door  31  from its open position B to the closed position A ( FIG. 4 ). 
         [0103]    The left and right tray holding members  34 L and  4 R make up the means for upwardly moving the tray  35  from the image formation position C to the tray locking (unlocking) position D before the tray  35  can be moved to the cartridge mounting (dismounting) position E which allows the cartridges P to be removed from, or placed in, the tray  35 . 
         [0104]    In other words, the tray holding members  34 L and  34 R are supporting members for supporting the tray  35 . They can be placed in the first position which allows the tray  35  to be moved between the cartridge mounting (dismounting) position E and tray locking (unlocking) position D, and the second position which places the tray  35  in the image formation position C. More specifically, as the door  31  is moved from its open position B to its closed position A, that is, as the door  31  is closed, the tray holding members  34 L and  34 R are moved from their first position to their second position by the movement of the door  31 . Further, as the door  31  is opened, that is, the door  31  is moved from its closed position A to the open position B, the tray holding members  34 L and  34 R are moved from their second position to their first position by the movement of the door  31 . 
       &lt;&lt;Cartridge&gt;&gt; 
       [0105]      FIGS. 7 and 8  are external perspective views of one of the cartridges P. More specifically,  FIG. 7  is an external perspective view of the cartridge P as seen from the no-driving side, whereas  FIG. 8  is an external perspective view of the cartridge P as seen from the driving side. 
         [0106]    Each cartridge P is an assembly of various components. Its left-right direction is the same as the direction of axial line of the drum  1  in the cartridge P. It is in the form of a rectangular parallelepiped, and its lengthwise direction also is the same as the lengthwise direction of the axial line of the drum  1 . The drum  1  is rotatably supported between the left and right walls of the cartridge frame  5 , by the bearings  51  and  52 , which are attached to the left and right walls. The right bearing  51  is provided with a coupling (driving force transmitting portion)  53  as the component through which drum driving force is inputted into the cartridge P. The right wall of the cartridge frame  5  is provided with a coupling  4  (driving force transmitting portion)  54  as the components through which the force for driving the development roller  3   a  is inputted into the cartridge P. 
         [0107]    The left wall of the cartridge frame  5  is provided with a protrusion  57  for positioning the cartridge P in terms of the left-right direction, and the electrical contact (electric power supplying contact)  55 . Further, the left and right walls of the cartridge frame  5  are provided with a pair of eaves-like protrusions  56 , which protrude in the left-right direction from the outward surface of the left and right wall of the cartridge frame  5 . The top wall of the cartridge frame  5  is provided with the exposure window  6 , which extends in the lengthwise direction of the cartridge P. 
         [0108]    Regarding the cartridge P, the lengthwise right end, which is provided with the couplings  53  and  54  is the driving side, and the lengthwise left end, or the opposite end of the cartridge P from the lengthwise right end, which is provided with the electrical contact  55 , is the no-driving end. That is, when the cartridge P is properly positioned in the tray  35 , its coupling  53  and  54  are at one (right end) of the lengthwise ends, in terms of the direction perpendicular to the direction in which the tray  35  is moved between the cartridge mounting (dismounting) position E and tray locking (unlocking) position D, and the electric power supply contact  55  is at the opposite end of the cartridge P from the couplings  53  and  54 . 
       &lt;&lt;Tray&gt;&gt; 
       [0109]      FIG. 9  is an external perspective view of the tray  35 . The tray  35  has a rectangular main frame, which has four compartments created by equally separating the internal space of the main frame with three partitioning plates  35   f , in terms of the front-rear direction. Hereafter, the four compartments will be referred to as the first to fourth compartment  35 ( 1 )- 35 ( 4 ), listing from the rear wall  35   c  of the main frame toward the front wall  35   b . It is in these first to fourth compartments  35 ( 1 )- 35 ( 4 ) of the main frame of the tray  35  that the first to fourth cartridges PY, PM, PC and PK are held, respectively. 
         [0110]    Each cartridge P is inserted into the corresponding compartment of the tray  35  from above. As the cartridge P is inserted into the tray  35 , the bottom surface of the left and right protrusions of the cartridge P is caught by the top surface of the left and right sub-frame  35   d  and  35   e . Consequently, the cartridge P is supported by the tray  35 . That is, the tray  35  supports each cartridge P in such a manner that the cartridge P can be moved out of the tray  35  by being moved straight upward, and also, that the cartridge P can be supported by the tray  35  by being moved straight downward into the tray  35  from above. Incidentally, the tray  35  loosely supports each cartridge P. Because the tray  35  and cartridge P are structured as described above, the cartridge P in the tray  35  can be easily replaced. 
         [0111]    The bottom surface of the left and right sub-frames  35   d  and  35   e  are rested on the top surface of the left and right tray holding members  34 L and  34 R, respectively, being thereby supported by the left and right tray holding members  34 L and  34 R. That is, the tray  35  is supported between the left and right sub-frames (plates)  80 L and  80 R, and is allowed to horizontally slide on the top surface of the tray holding members  34 L and  34 R in the front-rear direction; the tray  35  can move relative to the tray holding members  34 L and  34 R in the front-rear direction. 
         [0112]    Referring to  FIGS. 5 and 6 , if any of the cartridges P held in the tray  35  needs to be replaced, it can be replaced by pulling the tray  35  out of the apparatus main assembly  100 A to the cartridge mounting (dismounting) position E. After the replacement of the cartridge P, the tray  35  is to be pushed back all the way into the apparatus main assembly  100 A so that the tray  35  is placed where it was before it was pulled out, as shown in  FIGS. 3 and 4 . As the tray  35  is pushed back all the way into the apparatus main assembly  100 A by a user, it is pressed by a spring (pressing means)  103  ( FIG. 15 ) in the rear-to-front direction. Thus, the user is ensured that the tray  35  was pushed into the apparatus main assembly  100 A deep enough for the tray  35  to be locked into its preset position. This structural arrangement will be described later. 
         [0113]    As the tray  35  is pushed back into the apparatus main assembly  100 A deep enough to be locked into its preset position, the tray locking latch (movement regulating means)  101  of the tray  35  engages with the catch (latch engaging portion of apparatus main assembly  100 A)  102  ( FIG. 15(   b )→ 15 ( a )). That is, the tray  35  is pushed back from the cartridge mounting (dismounting) position E to the tray locking (unlocking) position D in the apparatus main assembly  100 A, and is held in the tray locking (unlocking) position D. 
         [0114]    Then, the user is to close the door  31  (which is open) as shown in  FIGS. 1 and 2 . As the door  31  is closed, the image forming apparatus  100  becomes ready for an image forming operation. More concretely, as the door  31  is pivotally moved from the exposing position B to the covering position A, the tray holding members  34 L and  34 R are moved rearward by the preset amount, and also, downward by the preset amount, by the movement of the door  31 , whereby the tray  35  is moved from the tray locking (unlocking) position D to the image formation position C. Consequently, each cartridge P held by the tray  35  is placed in its designated position in the cartridge chamber  100 B in the apparatus main assembly  100 A. 
         [0115]    Further, the driving force output portions  39  and  40  of the apparatus main assembly  100 A are made to engage with the driving force input portions  53  and  54  of the corresponding cartridge P, by the pivotal closing movement of the door  31 . During this engagement, each cartridge P comes under the pressure generated by the driving force outputting portions  39  and  40 . Thus, it is moved toward its no-driving side (leftward). Consequently, the protrusion  57  of the cartridge P, which is for positioning the cartridge P in terms of the left-right direction, comes into contact with the left sub-frame (plate)  80 L, precisely positioning thereby the cartridge P relative to the apparatus main assembly  100 A in terms of the left-right direction. 
         [0116]    Thus, each cartridge P, which is under the pressure from the pressing member  42 , is held in its designated position  41  in the apparatus main assembly  100 A. Further, the electric power supplying system  75  of the apparatus main assembly  100 A comes into contact with the electrical contact  55  of each cartridge P, establishing thereby electrical connection between the apparatus main assembly  100 A and cartridge P. The movement of various components of the image forming apparatus  100 , which is caused by the pivotal closing movement of the door  31 , will be described hereinafter. 
       &lt;&lt;Linkage Between Door and Tray Holding Member&gt;&gt; 
       [0117]    Next, referring to  FIGS. 10-12 ,  17  and  19 , the mechanical linkage between the door  31  and tray holding members  34 L and  34 R, which causes the movement of the door  31  to move the tray holding members  34 L and  34 R, is described. 
         [0118]      FIG. 10  is a perspective view of the mechanical linkage between the door  31  and tray holding members  34 L and  34 R. For convenience&#39;s sake, the left and right sub-frames  80 L of the apparatus main assembly  100 A are not shown in  FIG. 10 . The hinges  32 L and  32 R of the door  31  are attached to the apparatus main assembly  100 A so that they align with each other in the left-right direction. More concretely, the pin portion of each hinge  32  is rotatably supported by an unshown bearing, with which the corresponding sub-frame  80  of the apparatus main assembly  100 A is provided. The bearing may be a part of the frame  80  ( FIGS. 17A ,  17 B and  17 C). 
         [0119]    The door  31  is provided with a pair of arms  37 L and  37 R, which are at the ends of the door  31 , one for one, in terms of the left-right direction. The arms  37 L and  37 R are positioned so that their pivot pins  38 L and  38 R of the arms  37 L and  37 R, respectively, align in the left-right direction of the apparatus main assembly  100 A, and are rotatably supported by unshown bearings, with which the left and right sub-frame  80 L and  80 R of the apparatus main assembly  100 A are provided, one for one. The bearings may be parts of the left and right sub-frames  80 L and  80 R, one for one. 
         [0120]    There are provided a pair (left and right) of connective rods  201 L and  201 R on the outward side of the left and right sub-frames  80 L and  80 R ( FIGS. 17A ,  17 B and  17 C), respectively, which are positioned so that they extend in the front-rear direction of the apparatus main assembly  100 A. The connective rods  201 L and  201 R are supported by the unshown guiding members (with which the left and right sides of the apparatus main assembly  100 A are provided), in such a manner that they are movable only in the front-rear direction. 
         [0121]    The connective arms  37 L and  37 R are provided with shafts  37   a  and  37   b , which are perpendicular to the arms  37 L and  37 R, respectively. The shaft  37   a  of the left connective arm  37 L is fitted in a vertically elongated hole  201 A, with which the front end portion of the left connective rod  201 L is provided. The shaft  37   b  is fitted in the groove  31 B, with which the left lateral portion of the door  31  is provided. Further, the shaft  37   a  of the right connective arm  37 R is fitted in the vertically elongated hole  201 A, with which the front end portion of the right connective rode  201 R is provided. The shaft  37   b  is fitted in the groove  31 B, with which the left lateral portion of the door  31  is provided. 
         [0122]    Further, the left and right tray holding members  34 L and  34 R are provided with a pair of pins (shafts)  34   c  and  34   d , which are implanted into the tray holding members  34 L and  34 R, respectively. The pins (shafts)  34   c  and  34   d  are fitted in the guiding holes  36  ( FIG. 11 ) with which the left and right sub-frame  80 L and  80 R of the apparatus main assembly  100 A are provided, respectively. With these pins (shafts)  34   c  and  34   d  being fitted in the guiding holes  36 , one for one, the tray holding members  34 L and  34 R are supported by the left and right sub-frame  80 L and  80 R, respectively. 
         [0123]    The pin (shaft)  34   c  of the left tray holding member  34 L is put through the guiding hole  36 , and is fitted in the vertically elongated hole  201 B, with which the left connective rod  201 L is provided. Although the right tray holding member  34 R is not shown in  FIG. 10 , the pin  34   c  of the right tray holding member  34 R is put through the guiding hole  36 , and is fitted in the vertically elongated hole  201 B, with which the right connective rod  201 R is provided, connecting thereby the left tray supporting member  34 L to the left connective rod  201 L. 
         [0124]    As described above, the door  31  and tray holding members  34 L and  34 R are in connection to each other through the connective arms  37 L and  37 R, and connective rods  201 L and  201 R. Thus, as the door  31  is opened or closed, the left and right tray holding members  34 L and  34 R are subjected to such a force that acts in the direction to move them in the front-rear direction. Incidentally, the apparatus main assembly  100 A may be structured so that the axial line of the hinge portions  38 L and  38 R of the connective arms  37 L and  37 R align with the axial line of the hinge portions  32 L and  32 R of the door  31 . Further, instead of providing the connective arms  37 L and  37 R, the door  31  may be directly connected to the connective rods  201 L and  201 R. 
         [0125]      FIG. 11  shows the pair of pins (shafts)  34   c  and  34   d , with which the left tray holding member  34 L is provided, and the guiding holes  36  with which the left sub-frame  80 L is provided. Although the right tray holding member  34 R is not shown in  FIG. 10 , it is the same as the left tray holding member  34 R. That is, the apparatus main assembly  100 A is structured so that the pins (shafts)  34   c  and  34   d  of the tray holding right member  34 R, and the corresponding guiding holes  36 , are symmetrical in shape and position with those of the tray holding left member  34 L, and corresponding holes  36 . 
         [0126]    Therefore, the left and right tray holding members  34 L and  34 R are afforded a certain amount of latitude in terms of their movement relative to the left and right sub-frames  80 L and  80 R, within the guiding range of the guiding holes  36 . 
         [0127]      FIG. 12  is an enlarged view of the guiding holes  36  and their adjacencies. Each guiding hole  36  has the first, second and third guiding ranges  36   a ,  36   b  and  36   c , respectively. The first guiding range  36   a  extends in the front-rear direction. The second guiding range  36   b  is in connection to the first guiding range  36   a , and diagonally extends upward from the front end of the first guiding range  36   a , in the direction in which the pin (shaft)  34   c  moves relative to the left tray holding member  34 L as the door  31  is pivotally is moved. The third guiding range  36   c  is in connection to the top end of the second guiding range  36   b . It catches the pins (shafts)  34   c  and  34   d , and securely holds them. 
         [0128]    As the door  31  is pivotally opened, the pins (shafts)  34   c  and  34   d  of the tray holding left and right members  34 L and  34 R, respectively, are moved by the movement of the door  31  as will be described hereinafter. Therefore, the tray holding left and right members  34 L and  34 R are also moved by the movement of the door  31  as will be described later. That is, first, they are horizontally moved by the first guiding range  36   a , by a distance a1. Then, they are moved diagonally upward (by distance a2, in horizontal direction, and distance b1, in vertical direction) by the second guiding range  36   b . Finally, they are horizontally moved by a distance a3, by the third guiding range  36   c.    
         [0129]      FIG. 11(   a ) is a schematic side view of the image forming apparatus  100  when the door  31  is in its closed position A, that is, when the door  31  is completely shut against the apparatus main assembly  100 A (opening  30 ). When the image forming apparatus  100  is in the state shown in  FIG. 11(   a ), the tray holding left and right members  34 L and  34 R are in the rear end portion of the apparatus main assembly  100 A, into which they were moved by the combination of the connective arms  37 L and  37 R, and connective rods  201 L and  201 R. The pins (shafts)  34   c  and  34   d  are in the rear end portion of the first guiding range  36   a  of the corresponding guiding holes  36 , respectively. 
         [0130]    Therefore, the tray holding left and right members  34 L and  34 R are in the bottom position (second position), relative to the left and right sub-frames  80 L and  80 R. Therefore, the tray  35  held by the tray holding members  34 L and  34 R is also in the preset bottom position, that is, its image formation position C ( FIG. 2 ). 
         [0131]    When the tray  35  is in its image formation position C, each of the cartridges PY, PM, PC and PK is also in its designated position in the cartridge chamber  100 B, in which it can contribute to an image forming operation. Further, the top portion of the left end portion of each cartridge P, and the top portion of the right end portion of each cartridge P, are under the downward pressure generated by the pressing members  42 . Therefore, the bottom surface of the bearing  51  on the driving side, and the bottom surface of the bearing  52  on the no-driving side, are pressed upon the cartridge positioning portions  41  ( FIGS. 13 and 17 ) with which the cartridge positioning left and right members  81 L and  81 R of the apparatus main assembly  100 A, being thereby fixed to the cartridge positioning members  81 L and  81 R. Thus, each cartridge P is kept accurately positioned relative to the apparatus main assembly  100 A; it is kept in its image formation position in the apparatus main assembly  100 A. 
         [0132]    Thus, it is ensured that when the cartridge holding left and right members  34 L and  34 R (hence, cartridges P in tray  35  held by members  34 L and  34 R) are in the above-described state, the downwardly facing portion of the peripheral surface of the drum  1  in each cartridge P remains in contact with the top surface of the portion of the belt  13  of the belt unit  12 , which corresponds in position to the top portion of the loop (belt loop) which the belt  13  forms. 
         [0133]    Further, the coupling portions  53  and  54  of each cartridge P are in engagement with the drum driving coupling  39  and development roller driving coupling  40  of the apparatus main assembly  100 A, respectively. Further, the electrical contact  55  of each cartridge P can be supplied with electric power by the apparatus main assembly  100 A. 
         [0134]    The right rear side of the tray  35  is under the pressure generated by the spring  103  ( FIG. 15 ) in the front-to-rear direction, and the tray locking latch  101  is in the catch  102 , with which the right sub-frame  80 R of the apparatus main assembly  100 A is provided. Therefore, it is ensured that the tray  35  is kept in the preset position in the apparatus main assembly  100 A. 
         [0135]      FIG. 11(   b ) is a side view of the image forming apparatus  100  when the door  31  is half open. As the door  31  is opened when it is in the closed state as shown in  FIG. 11(   a ), the tray holding left and right members  34 L and  34 R are pulled frontward by the movement of the door  31 , and therefore, they are moved frontward in the apparatus main assembly  100 A. Thus, the pins (shafts)  34   c  and  34   d  of the tray holding members  34 L and  34 R move frontward by the distance a1 while being guided by the first guiding range  36   a  of the guiding hole  36 .  FIG. 11(   b ) shows the state of the image forming apparatus  100 , in which the door  31  has been opened halfway as described above. 
         [0136]    While the tray holding members  34 L and  34 R are moved by the distance a1 as described above, first, the electrical connection between the electrical contact  55  of each cartridge P and the apparatus main assembly  100 A is interrupted. Then, each cartridge P becomes disengaged from the corresponding pressing member  42 , being thereby freed from the pressure generated by the pressing member  42 . Further, the drum driving coupling  39  and development roller driving coupling  40  are disengaged from each cartridge P. Also during this movement of the tray supporting members  34 L and  34 R, in order to prevent the tray  35  from following the movement of the tray holding members  34 L and  34 R, the tray locking latch  101  remains in engagement with the catch of the apparatus main assembly  100 A, preventing thereby the tray  35  from moving. 
         [0137]    As the door  31  is pivotally opened wider, the tray holding members  34 L and  34 R are moved further frontward in the apparatus main assembly  100 A, by the movement of the door  31 , while the pins (shafts)  34   c  and  34   d  of the tray supporting members  34 L and  34 R are guided by the second guiding range  36   b  of the guiding holes  36 . Thus, the tray  35  is moved diagonally upward. At this point, the tray locking latch  101  of the tray  35  is in engagement with the catch of the apparatus main assembly  100 A. Therefore, it does not occur that the tray  35  is horizontally moved. In other words, the tray  35  follows only the vertical movement of the tray holding members  34 L and  34 R. Consequently, the drum  1  which each cartridge P has is separated from the belt  13  of the apparatus main assembly  100 A. 
         [0138]      FIG. 11(C)  shows the door  31 , when the door  31  is fully open. When the door  31  is in the state shown in  FIG. 11(C) , the tray holding members  34 L and  34 R have been moved diagonally upward by the second guiding range  36   b , and the pins (shafts)  34   c  and  34   d  are in the third guiding range  36   c , which is horizontal. That is, the tray holding members  34 L and  34 R are horizontally moved after they are moved diagonally upward. 
         [0139]    This structural arrangement is for keeping the cartridges P and tray holding members  34 L and  34 R stable in position in terms of their height direction, when the cartridge(s) P is replaced after the tray  35  is pulled out of the tray holding members  34 L and  34 R. It is also for preventing the tray holding members  34 L and  34 R from returning to where they were. 
         [0140]    As described above, the tray holding members  34 L and  34 R are horizontally moved after they are moved diagonally upward. In other words, they are held in their first position after being moved by a preset distance relative to the left and right sub-frames  80 L and  80 R. Thus, the tray  35 , which is being held by the tray holding members  34 L and  34 R, is held in its tray locking (unlocking) position D ( FIG. 4 ) after being moved upward by the preset distance relative to the left and right sub-frame  80 L and  80 R, from its image formation position C ( FIG. 2 ). 
         [0141]    As the tray  35  is unlocked when it is in its tray locking (unlocking) position D, it becomes unregulated in terms of its movement in the horizontal direction. Therefore, it can be slid relative to the tray holding members  34 L and  34 R in the front-rear direction. That is, it is possible for the tray  35  to be slid between the cartridge mounting (dismounting) position E, in which each cartridge P in the tray  35  can be replaced, and the tray locking (unlocking) position D. 
         [0142]    While the tray  35  is pulled out from within the apparatus main assembly  100 A, the protrusion  57  of each cartridge P, which is for keeping the cartridge P properly positioned in terms of the left-right direction when the cartridge P is in the tray  35 , horizontally moves along the inward surface of the left sub-frame  80 L. Therefore, the area of the inward surface of the left sub-frame  80 L, along which the cartridge positioning protrusion  57  moves, is desired to be flat, that is, free of peaks and valleys. That is, by not placing the holes, with which the left sub-frame  80 L needs to be provided, in the area of the sub-frame  80 L, along which the cartridge positioning protrusion  57  moves, it is possible to prevent the cartridge positioning protrusion  57  from being scarred and/or shaved, and therefore, it is possible for the tray  35  to be smoothly pulled out. Examples of the holes with which the left sub-frame  80 L needs to be provided are the holes through which the springs  75  for the electrical contacts of the apparatus main assembly  100 A, guiding holes  36  for the tray holding left member  34 L, etc. 
         [0143]      FIGS. 17 and 19  are perspective views of the combination of the sub-frames  80 L and  80 R, tray  35 , process cartridges P, and the components closely related thereto, when the tray  35  is in the cartridge mounting (dismounting) position E, in which all the cartridges P can be replaced. When the tray  35  is in the position shown in  FIGS. 17 and 19 , the tray  35 , which is protruding from the apparatus main assembly  100 A as far as it is allowed, is supported by the tray supporting portions  121 L and  121 R of the connective arms  37 L and  37 R, respectively. Thus, it is possible to prevent the problem that the portion of the tray  35 , which is next to the front of the apparatus main assembly  100 A, is bent downward by the weight of the tray  35  itself and the weight of the cartridges P, seriously enough to scar the peripheral surface of the drum  1 , and/or the weight of the tray  35  itself and the weight of the cartridges P unbalance the apparatus main assembly  100 A seriously enough to tip the apparatus main assembly  100 A frontward. 
         [0144]    The connective arms  37 L and  37 R are rotationally moved by the movement of the door  31 . Therefore, it is possible to place the supporting portions  121 L and  121 R of the connective arms  37 L and  37 R, respectively, in such a manner that when the door  31  is closed, they will be inside the apparatus main assembly  100 A, whereas when the door  31  is opened, they will be outside the apparatus main assembly  100 A. Thus, it is possible to improve the image forming apparatus in the usability related to the cartridge replacement operation, by being enabled to securely hold the tray  35  in its loading (unloading) position E, without increasing the apparatus main assembly  100 A in size. 
         [0145]    Regarding the shape and number of supporting points, the supporting portions  121 L and  121 R which support the tray  35  in the cartridge mounting (dismounting) position E, do not need to be as shown in  FIGS. 17 and 18 . Further, the tray supporting portions (portions which support tray  35  after tray  35  is pulled out) may be constructed as integral parts of the door  31 . 
       &lt;&lt;Tray Position Regulating Means&gt;&gt; 
       [0146]    Next, referring to  FIGS. 13-16 , and  26 , the means for regulating the tray  35  in position is described. 
         [0147]      FIG. 13  is a perspective view of the combination of the left sub-frame  80 L, door  31 , and tray  35 , when the tray  35  is entirely in the apparatus main assembly  100 A, more specifically, after it was moved from the cartridge mounting (dismounting) position E, which is outside the apparatus main assembly  100 A, to the tray locking (unlocking) position D ( FIG. 4 ), which is inside the apparatus main assembly  100 A.  FIG. 14  is perspective view of the combination of the left sub-frame  80 L, door  31 , and tray  35 , when the tray  35  is almost entirely in the apparatus main assembly  100 A; it has not reached the tray locking (unlocking) position D.  FIG. 15  is a drawing illustrating the movement of the tray movement regulating means for guiding the tray  35  to the tray locking (unlocking) position D toward the end of the inward movement of the tray  35  into the apparatus main assembly  100 A. 
         [0148]    When the tray  35  is in the state shown in  FIG. 14 , that is, when the tray  35  has been almost entirely pushed into the apparatus main assembly  100 A, the supporting portions  121 L and  121 R of the connective arms  37 L and  37 R, respectively, have just come into contact with the bottom surface of the tray  35 , as shown in  FIG. 16(   a ), while the connective arms  37 L and  37 R are rotationally moved by the pivotal closing movement of the door  31 . Thus, it is impossible to close the door  31  to move the tray holding members  34 L and  34 R rearward, and then, move them diagonally downward. 
         [0149]    In comparison, when the tray  35  is in the state shown in  FIG. 13 , that is, after the tray  35  has been entirely pushed into the apparatus main assembly  100 A, the supporting portions  121 L and  121 R of the connective arms  37 L and  37 R, respectively, do not interfere with the tray  35 , as shown in  FIG. 16(   b ), while the connective arms  37 L and  37 R are rotationally moved by the pivotal opening movement of the door  31 . Therefore, it is possible for the door  31  to be closed to lower the tray holding members  34 L and  34 R (hence, the tray  35 ). 
         [0150]    Referring to  FIG. 15 , the tray  35  is under the pressure applied to the tray  35  from the rear side of the tray  35  by the spring  103  through the pressing member  104 . Therefore, if a user fails to push the tray  35  all the way into the apparatus main assembly  100 A, the tray  35  is pushed back by the aforementioned pressure by a distance which is proportional to the length by which the spring  103  is compressed by the inward movement of the tray  35 , as shown in  FIG. 15(   b ). After the tray  35  is pushed back, the position of the tray  35  is as shown in  FIGS. 14 and 16(   a ). 
         [0151]    That is, if the user erroneously tries to close the door  31 , that is, without moving the tray  35  into the tray locking (unlocking) position D, the supporting portions  121 L and  121 R of the connective arms  37 L and  37 R surely interfere with the tray  35 , prompting the user to correct the error. In comparison, as the tray  35  is put in the position shown in  FIGS. 13 and 16(   a ), that is, as the tray  35  is pushed far enough into the apparatus main assembly  100 A so that it is put in the tray locking (unlocking) position D, the tray locking latch  101  engages with the latch catching means  102  of the apparatus main assembly  100 A. Thus, the tray  35  remains properly positioned relative to the apparatus main assembly  100 A as shown in  FIG. 15(   a ). 
         [0152]      FIG. 26  shows the shape of the tray locking latch  101 , and the shape of the latch catching means  102  of the apparatus main assembly  100 A. The tray locking latch  101  is such a member that is made to protrude or retract through the hole  35   b H, with which the right side  35   b R of the front piece  35   b  of the frame of the tray  35  is provided, by the movement of the movable handhold  35   a  of the tray  35 . The tray locking latch  101  is made to protrude into the hole  35   b H of the frame of the tray  35 , by an unshown pressing means, and is kept in the hole  35   b H. However, as the handhold  35   a  is pulled frontward, the tray locking latch  101  is pulled back into the tray  35  through the hole  35   b H by the movement of the handhold  35   a.    
         [0153]    The latch catching means  102  of the apparatus main assembly  100 A is a vertically elongated hole with which the right sub-frame  80 R of the apparatus main assembly  100 A is provided. As the tray locking latch  101  of the tray  35  is made to protrude through the hole  35   b H, and fits into the elongated hole  102  as the latch catching means of the apparatus main assembly  100 A, the tray  35  is kept in the tray locking (unlocking) position D, whereas as the tray locking latch  101  is retracted into the tray  35  through the hole  35   b H, it comes out of the elongated hole  101 ; it disengages from the right sub-frame  80 R of the apparatus main assembly  100 A. 
         [0154]    The latch catching means  102  of the apparatus main assembly  100 A is a vertically elongated hole. Therefore, the tray  35  is allowed to be moved from the tray locking (unlocking) position D to the image formation position C, or from the image formation position C to the tray locking (unlocking) position D, while the tray locking latch  101  is remaining in engagement with the latch catching means  101  of the apparatus main assembly  100 A. 
         [0155]    When the tray  35  is in the tray locking (unlocking) position D, the tray locking latch  101  is in engagement with the latch catching means  102  of the apparatus main assembly  100 A. Therefore, the tray  35  is locked in the tray locking (unlocking) position D, being thereby prevented from moving. 
         [0156]    As the handhold  35   a  is pulled frontward when the tray  35  is in the tray locking (unlocking) position D, the tray locking latch  101  is retracted into the tray  35  by the movement of the handhold  35   a , being thereby disengaged from the latch catching means of the apparatus main assembly  100 A. In other words, the tray  35  is freed (unlocked) from the apparatus main assembly  100 , in terms of its frontward movement. Consequently, the tray  35  is made to slightly protrude from the front side of the apparatus main assembly  100 A, by the pressure applied to the tray  35  by the spring  103  through the pressing means  104 . Thus, the user can slide the tray  35  into the cartridge mounting (dismounting) position E, by pulling the tray  35  by the handhold  35   a.    
         [0157]    On the other hand, as the tray  35  is pushed into the apparatus main assembly  100 A from the cartridge mounting (dismounting) position E, the rear piece  35   c  of the tray frame comes into contact with the pressing member  104  slightly before the tray  35  reaches the tray locking (unlocking) position D. Then, as the tray  35  is pushed further into the apparatus main assembly  100 A against the pressure applied to the tray  35  by the spring  103  through the pressing member  104 , the tray locking latch  101  is caught by the latch catching means  101  of the apparatus main assembly  100 A the moment the tray  35  reaches the tray locking (unlocking) position D. Thus, the tray  35  is locked in the tray locking (unlocking) position D, being thereby regulated in movement. 
         [0158]    Incidentally, the shape and number of the tray locking latch  101  of the tray  35 , and those of the latch catching means of the apparatus main assembly  100 A, do not need to be as shown in  FIGS. 15 and 26 . Further, the shape and number of the tray pressing means  103 , and those of the tray pressing member and  104 , do not need to be as shown in  FIG. 15 . 
       &lt;&lt;Interface&gt;&gt; 
       [0159]    Next, the structure of the interface between each cartridge P and image forming apparatus, more specifically, the structural arrangement for supplying each cartridge with electric power, structural arrangement for keeping each cartridge P pressed upon the apparatus main assembly A, and structure of the coupling through which driving force is transmitted from the apparatus main assembly  100 A to each cartridge P, are described. 
         [0000]    (Structural Arrangement for Supplying Cartridge with Electric Power) 
         [0160]      FIGS. 17 and 18  are drawings illustrating the method for supplying each cartridge P with the electric power from the apparatus main assembly  100 A. There are disposed springy electrical contacts as electric power supplying members (electrical contact unit)  75  ( 75   a ,  75   b ,  75   c  and  75   d ) for supplying the electrical contacts of each cartridge P with electric power, on the left side (no-driving side) of the apparatus main assembly  100 A, more specifically, on the inward surface of the left sub-frame  80 L of the apparatus main assembly  100 A. 
         [0161]      FIG. 17A  is a perspective view of the combination of the left and right sub-frames  80 L and  80 R, tray supporting left member  34 L, tray  35 , and various components of theirs, when the tray  35  is in the cartridge mounting (dismounting) position E, into which the tray  35  was moved by the opening of the door  31 . The cartridges P are supported by the tray  35 , being aligned in parallel in the horizontal direction. Each cartridge P is provided with the electrical contact  55 , which is on the outward surface of the left wall of the cartridge P, being at a preset position in terms of the vertical direction; the cartridges P are the same in the position of their electrical contact in terms of the vertical direction. 
         [0162]    The apparatus main assembly  100 A is provided with the springy electrical contacts  75   a - 75   d , which are electrically in contact with the power supplying portion  75  of the apparatus main assembly  100 A, which is on the outward side of the left sub-frame  80 L of the apparatus main assembly  100 A. The springy electrical contacts  75   a - 75   d  are aligned in the horizontal direction of the apparatus main assembly  100 A, and are the same in position in terms of the vertical direction of the apparatus main assembly  100 A.  FIG. 17B  is an enlarged view of one of the springy electrical contacts of the apparatus main assembly  100 A, and its adjacencies.  FIG. 17C  is a perspective view of the springy electrical contact shown in  FIG. 17B , as seen from the inward side of the left sub-frame  80 L. 
         [0163]    One end of each of the springy electrical contacts  75   a - 75   d  of the apparatus main assembly  100 A is electrically in contact with the corresponding electric power supplying portion  74  of the apparatus main assembly  100 A. The other end is held by the electrical contact holder  76  of the apparatus main assembly  100 A, in such a manner that its point  77  of contact protrudes toward the corresponding cartridge P through the hole  76   d , with which the electric contact holder  76  is provided. 
         [0164]    The top and bottom ends of the electrical contact holder  76  of the apparatus main assembly  100 A is rotatably held by a pair of unshown bearings, one for one, with which the apparatus main assembly  100 A is provided. The electrical contact holder  76  of the apparatus main assembly  100 A is under the pressure generated by the springy electrical contact  75  of the apparatus main assembly  100 A in a direction to cause the electrical contact holder  76  of the apparatus main assembly  100 A to rotate in such a direction that causes its point  77  of contact to come into contact with the corresponding cartridge P. However, the electrical contact holder  76  of the apparatus main assembly  100 A is regulated in rotational movement by the combination of the protrusion  76   c  of the electrical contact holder  76 , and the surface of the cam portion  201 D of the connective rod  201 L. 
         [0165]      FIGS. 18(   a ) and  18 ( b ) are drawings of the electrical contact  55  of one of the cartridges P, and the corresponding electrical contact  75  of the apparatus main assembly  100 A, when the two electrical contacts  55  and  75  are electrically in contact with each other, and when they are not in contact with each other, respectively. That is, each drawing is a part of the sectional view of a part of the image forming apparatus  100 , which includes the electrical contact  55  of the cartridge P, springy electrical contact  75  of the apparatus main assembly  100 A, electrical contact holder  75  of the apparatus main assembly  100 A, electric power supplying portion  74  of the apparatus main assembly  100 A, and connective rod  201 L, at a horizontal plane, as seen from above the apparatus main assembly  100 A. 
         [0166]      FIG. 18(   a ) shows the state of the abovementioned components after the cartridge P was placed in its preset position for image forming operation, by the descending of the tray  35  to its image formation position C. When these components are in the state shown in  FIG. 18(   a ), the point  77  of contact of the springy electrical contact  75  of the apparatus main assembly  100 A is protruding toward the cartridge P through the hole, with which the left sub-frame  80 L of the apparatus main assembly  100 A is provided, and is in contact with the electrical contact  55  of the cartridge P. 
         [0167]    That is, when the door  31  is in its closed position A, the springy electrical contact  75  of the apparatus main assembly  100 A is in its power supplying position J, that is, the position in which it can supply the electrical contact  55  of the cartridge P with electrical power. In other words, it is electrically in contact with the electrical contact  55  of the cartridge P. 
         [0168]      FIG. 18(   b ) shows the state of the abovementioned components after the cartridge P was moved upward from its preset position for image formation, by the ascending of the tray  35  from its image formation position C to its tray locking (unlocking) position D. As the door  31  is pivotally opened, the connective rod  201 L is moved frontward of the apparatus main assembly  100 A by the pivotal movement of the door  31 , whereby the electrical contact holder  76  of the apparatus main assembly  100 A is rotated by the surface of the cam portion  201 D in the direction to cause the springy electrical contact  75  of the apparatus main assembly  100 A to separate from the electrical contact  55  of the cartridge P. 
         [0169]    That is, as the door  31  is pivotally moved into its open position B, the springy electrical contact  75  of the apparatus main assembly  100 A is moved into its separation position K, in which it does not contact the electrical contact  55  of the cartridge P. That is, the springy electrical contact  75  of the apparatus main assembly  100 A is separated from the electrical contact  55  of the cartridge P in terms of electrical connection. 
         [0170]    Further, the cartridge electrical contact contacting portion  77  of the springy electrical contact  75  of the apparatus main assembly  100 A is retracted outward of the apparatus main assembly  100 A through the hole with which the left sub-frame  80 L of the apparatus main assembly  100 A is provided, enabling thereby the tray  35  to be pulled out of the apparatus main assembly  100 A without being placed in contact with the springy electrical contact  75  of the apparatus main assembly  100 A. 
         [0171]    The surface of the cam portion  201 D of the connective rod  201 L is made up of portions  201   e  and  201   g  which are parallel to the moving direction of the tray  35 , and a portion  201   f  which is slanted relative to the moving direction of the tray  35 . Therefore, when the door  31  is completely closed relative to the apparatus main assembly  100 A (closed position A), the springy electrical contact  75  of the apparatus main assembly  100 A remains in contact with electrical contact  55  of the cartridge P (power supplying position J). On the other hand, when the door is fully open (open position B), the springy electrical contact  75  of the apparatus main assembly  100 A remains separated from the electrical contact  55  of the cartridge P (separation position K). 
         [0172]    Regarding the amount of the stroke of the springy electrical contact  75  of the apparatus main assembly  100 A, which is caused by the rotational movement of each electrical contact holder  76  of the apparatus main assembly  100 A, that is, the amount by which the springy electrical contact  75  of the apparatus main assembly  100 A is compressed as the electrical contact holder  76  of the apparatus main assembly  100 A is rotationally moved from its power supplying position J, in which it is when the door  31  is closed, and contacts the electrical contact  55  of the cartridge P, to its separation position K, in which it is when the door  31  is open, by the rotational movement of the electrical contact holder  76 . 
         [0173]    That is, the amount of this stroke has to be large enough to ensure that the springy electrical contact  75  of the apparatus main assembly  100 A is prevented from coming into contact with the electrical contact  55  of the process cartridge P when the tray  35  is pulled out of the apparatus main assembly  100 A, and yet, small enough to ensure that springy electrical contact  75  of the apparatus main assembly  100 A remains in contact with the electrical contact  55  of the process cartridge P when the door  31  is closed, even if the dimensional defects of the various components related to the stroke of the springy electrical contact  75  of the apparatus main assembly  100 A are in the extreme range of the tolerance. The greater the amount of this stroke, the greater the margin for the dimensional defects of the components, but, the greater the amount of force which has to be applied by a user to compress the springy electrical contact  75  of the apparatus main assembly  100 A, that is, the amount of force required of the user to operate the image forming apparatus  100 . Therefore, it is important that the amount of the stroke of the springy electrical contact  75  of the apparatus main assembly  100 A is reduced as much as possible. In other words, it is important that the components related to the stroke of the springy electrical contact  75  of the apparatus main assembly  100 A are positioned relative to each other as accurately as possible. 
         [0174]    Regarding the positioning of each cartridge P relative to the apparatus main assembly  100 A in this embodiment, in terms of the left-right direction, each cartridge P is positioned by the placement of the positioning protrusion  57  of the cartridge P in contact with the left sub-frame  80 L of the apparatus main assembly  100 A as described above. Therefore, the shaft portion of the electrical contact spring holder  76  and the connective rod  201 L of the apparatus main assembly  100 A are also positioned relative to the apparatus main assembly  100 A, in terms of the left-right direction, by their placement in contact with the left frame  80 L. Therefore, the image forming apparatus  100  in this embodiment is smaller in the amount of nonuniformity in the distance between the electrical contact  55  of the cartridge P and the springy electrical contact  75  of the apparatus main assembly  100 A than any image forming apparatus in accordance with the prior art. In other words, the image forming apparatus  100  in this embodiment is less in the amount of the stroke of the springy electrical contact  75  of the apparatus main assembly  100 A, being therefore smaller in the amount of force of which a user is required to open or close the door  31 , than any image forming apparatus in accordance with the prior art. 
         [0175]      FIGS. 17 and 18 , which were mentioned above, show the method, in this embodiment, for supplying the cartridges P in the apparatus main assembly  100 A with electrical power. This method employs one electrical contact per electrical contact  55  of a process cartridge P. That is, the method requires one springy electrical contact  75  per electrical contact  35  of the process cartridge P, that is, the electrical contact which is to be supplied with electric power. However, even if each cartridge P is provided with multiple electrical contacts, the same method as the one employed in this embodiment is employable. In such a case, the image forming apparatus  100  may be structured so that the multiple springy electrical contacts  75  of the apparatus main assembly  100 A are held by the electrical contact holder  76  of the apparatus main assembly  100 A, or the apparatus main assembly  100 A may be provided with multiple electrical contact holders  75  per cartridge P. 
         [0176]    In the case of  FIGS. 17 and 18 , the image forming apparatus  100  is structured so that the top and bottom end portions  76   a  and  76   b  of the electrical contact holder  76  of the apparatus main assembly  100 A are rotatably held by the pair of bearings, one for one, and the springy electrical contact  75  of the apparatus main assembly  100 A is placed in contact with, or separated from, the electrical contact  55  of the cartridge P by the pivotal movement of the electrical contact holder  76  of the apparatus main assembly  100 A. However, the image forming apparatus  100  may be structured so that the springy electrical contact  75  of the apparatus main assembly  100 A is placed in contact with, or separated from, the electrical contact  55  of the cartridge P, by being horizontally moved in the left-right direction of the apparatus main assembly  100 A, like the driving force transmitting couplings, which will be described later. 
       (Structure of Pressing Mechanism) 
       [0177]    Referring to  FIGS. 13 ,  14 ,  17  and  19 , the apparatus main assembly  100 A is provided with a pair (left and right) of positioning members  81 L and  81 R, which are on the inward side of the left and right lateral wall of the apparatus main assembly  100 A. These left and right positioning members  81 L and  81 R are provided with cartridge positioning portions (recesses)  41 , which also are on the inward side of the left and right lateral wall of the apparatus main assembly  100 A. The cartridge positioning portions  41  catch and hold the driving-side bearing  51  and no-driving-side bearing  52  of the cartridges P, by the bottom surface of the bearings  51  and  52 . Further, the apparatus main assembly  100 A is provided with four pairs of cartridge pressing members  42 , which are for pressing on the left and right end portions of the top surface of the cartridges P, one for one, to cause the bearing  51  (on drive side) and bearing  52  (on no-driving side) of each cartridge P, to fit into the cartridge positioning portions (recesses)  41 , with which the apparatus main assembly  100 A is provided, and keeping the bearings  51  and  52  in the cartridge positioning portions  41 . 
         [0178]      FIG. 23  is a perspective view of the portion of the image forming apparatus  100 , which is related to the cartridge pressing mechanism of the apparatus  100 , as seen from the left side of the apparatus  100 , when the door  31  is closed.  FIG. 19  is a perspective view of the same portion of the image forming apparatus  100  as the one shown in  FIG. 23 , as seen from the right side of the apparatus  100 , when the tray  31  has been pulled out all the way from the apparatus main assembly  100 A as shown in  FIGS. 17A ,  17 B and  17 C.  FIG. 20  is a perspective view of a part of the cartridge pressing mechanism. It shows the structure of the part of the cartridge pressing mechanism. 
         [0179]    Referring to  FIGS. 17 ,  19  and  23 , there are disposed a pair of pressing rods  46 L and  46 R, on the outward side of the left and right sub-frames  80 L and  80 R, respectively. There are also disposed a pair of linkage rods  202 L and  202 R for causing the connective rods  201 L and  201 R to move with the pressing rods  46 L and  46 R, on the outward side of the left and right sub-frames  80 L and  80 R, respectively. 
         [0180]    The left and right pressing rods  46 L and  46 R are provided with a pair of elongated horizontal holes  46 A, which are positioned a preset distance apart from each other, and in which a pair of pins (shafts)  47 , with which each of the left and right frames  80 L and  80 R is provided, are fitted. With these pins (shafts)  47  fitted in the elongated horizontal holes  46 A, the pressing rods  46 L and  46 R are supported by the left and right sub-frames  80 L and  89 R, being enabled to be moved relative to the left and right sub-frames  80 L and  80 R, respectively, in the front-rear direction. 
         [0181]    The left and right linkage rods  202 L and  202 R are supported by a pair of shafts (pivots)  83 , with which the left and right sub-frame  80 L and  80 R are provided, one for one, being enabled to be rotationally moved relative to the left and right sub-frame  80 L and  80 R, respectively. Each of the linkage rods  202 L and  202 R is provided with a pair of horizontal shafts  202   a  and  202   b.    
         [0182]    The horizontal shaft  202   a  of the left linkage rod  202 L is fitted in the elongated vertical hole  202 B, with which the right connective rod  201 L is provided, whereas the horizontal shaft  202   b  of the left linkage rod  202 L is fitted in the elongated vertical holes  46 B, with which the left pressing rod  46 L is provided. 
         [0183]    Similarly, the horizontal shaft  202   a  of the right linkage rod  202 R is fitted in the elongated vertical hole  202 B, with which the right connective rod  201 R is provided, whereas the horizontal shaft  202   b  of the right linkage rod  202 R is fitted in the elongated vertical holes  46 B, with which the ride pressing rod  46 L is provided. 
         [0184]    Therefore, the pressing rods  46 L and  46 R are enabled to be moved in the front-rear direction by the front-rear movement of the connective rods  201 L and  201 R, through the linkage rods  202 L and  202 R, respectively. That is, the pressing rods  46 L and  46 R are moved in the front-rear direction by the pivotal opening, or closing, movement of the door  31 . 
         [0185]    As the door  31  is opened, that is, as the door  31  is moved from its closed position ( FIG. 13 ) to its open position ( FIGS. 17A ,  17 B and  17 C), the connective rods  201 L and  201 R are moved frontward of the apparatus main assembly  100 A, by the movement of the door  31 , whereas the pressing rods  46 L and  46 R are moved rearward of the apparatus main assembly  100 A by the movement of the door  31 , through the linkage rods  202 L and  202 R, respectively. On the other hand, as the door  31  is closed, that is, as the door  31  is moved from its open position ( FIGS. 17A ,  17 B and  17 C) to the closed position ( FIG. 23 ), the connective rods  201 L and  201 R are moved rearward of the apparatus main assembly  100 A by the movement of the door  31 , whereas the pressing rods  46 L and  46 R are moved frontward of the apparatus main assembly  100 A by the movement of the door  31 , through the linkage rods  202 L and  202 R. 
         [0186]    Further, the apparatus main assembly  100 A is structured so that the pair of pressing members  42  are movable relative to the pressing member holders  44 L and  44 R, which are disposed on the inward side of the left and right sub-frame  80 L and  80 R, respectively, in the vertical direction of the apparatus main assembly  100 A, and also, so that they are made by the pressing springs  43  to press on the left and right end portions of the top surface of each cartridge P. 
         [0187]    Referring to  FIG. 20 , each of the left and right pressing member holders  44 L and  44 R is provided with a pin (shaft)  44   a , which is at the center of the holder  44  in terms of the front-rear direction. These pins (shafts)  44   a  are fitted in the elongated vertical holes  45 , one for one, with which the left and right sub-frames  80 L and  80 R of the apparatus main assembly  100 A are provided. 
         [0188]    Further, each of the left and right pressing member holders  44 L and  44 R is provided with a pair of pins (shafts)  44   b , which are positioned a preset distance apart from each other in terms of the front-rear direction. These pins (shafts)  44   b  are put through the elongated vertical holes  82 , one for one, with which each of the left and right sub-frames  80 L and  80 R is provided, and are fitted in the guiding holes  48 , one for one, with which each of the left and right pressing rods  46 L and  46 R is provided. 
         [0189]    Therefore, the left and right pressing member holders  44 L and  44 R are regulated in position in terms of the vertical direction of the apparatus main assembly  100 A, by the left and right pressing rods  46 L and  46 R. Further, they are regulated in position in terms of the front-rear direction, by the elongated vertical holes  45  of the left and right sub-frames  80 L and  80 R, respectively. That is, when the left and right pressing rods  46 L and  46 R are moved in the front-rear direction, the left and right pressing member holders  44 L and  44 R are allowed to move only in the vertical direction. 
         [0190]      FIG. 21  is an enlarged view of the guiding hole  48 , and its adjacencies, of the pressing rod  46 L, as seen from the left side of the apparatus main assembly  100 A. The guiding holes  48  are made up of the first, second, and third guiding ranges  48   a ,  48   b  and  48   c , respectively. The first guiding range  48   a  horizontally extends in the front-rear direction. The second guiding range  48   b  extends diagonally upward from the front end of the first guiding range  48   a  in the rearward direction. The third guiding range  48   c  horizontally extends from the front (top) end of the second guiding range  48   b  in the rearward direction. It catches the pin (shaft)  44   b , and securely holds the pin (shaft)  44   b.    
         [0191]      FIG. 21(   a ) shows the positional relationship between the guiding hole  48  and pin (shaft)  44   b  when the door  31  is completely shut against the apparatus main assembly  100 A. That is, when the door  31  is completely closed, the pressing rod  46 L is on the frontward side of the apparatus main assembly  100 A. Therefore, the pin (shaft)  44   b  (hence, pressing member holder  44 L) is regulated in position in terms of the vertical direction, by the first guiding range  48   a . Further, each pressing member  42  is pressing on the left end portion of the top surface of the corresponding cartridge P. 
         [0192]      FIGS. 21(   b ) and  21 ( c ) show the positional relationship between the guiding hole  48  and pin (shaft)  44   b  when the door  31  is partially open. As the door  31  is pivotally moved (opened) from the closed position ( FIG. 21(   a )), the pressing rod  46 L is moved rearward of the apparatus main assembly  100 A by the movement of the door  31 . Consequently, the pressing member holder  44 L is moved upward by the pressing rod  46 L while being guided by the second guiding range  48   b  of the guiding hole  48 . 
         [0193]    During this upward movement of the pressing member holder  44 L, it does not occur that the pressing member holder  44 L horizontally moves, because the pin (shaft)  44   a  of the pressing member holder  44 L is in the vertically elongated hole  45 . In other words, the pressing member holder  44 L moves only in the vertical direction. Consequently, the pressing member  42  is separated from the top surface of each cartridge P. 
         [0194]      FIG. 21(   d ) shows the positional relationship between the guiding hole  48  and pin (shaft)  44   b  when the door  31  is fully open. When the door  31  is in the state shown in  FIG. 21(   d ), the pressing rod  46 L is its rearward position in the apparatus main assembly  100 A. Therefore, the pin (shaft)  4   b  (hence, pressing member holder  44 L) is prevented from vertically moving, by the third guiding range  48   c  of the guiding hole  48 . Therefore, the pressing member  42  is kept separated from the top surface of the left end portion of each cartridge P. 
         [0195]      FIG. 21  is illustrates the mechanism for causing the pressing member  42  to keep on pressing on the left end portion of each cartridge P, or allowing the pressing member  42  to separate from the cartridge P. As for the mechanism for causing the pressing member  42  to keep on pressing on the right end portion of each cartridge P, it is similar to the one shown in  FIG. 21 . 
         [0196]    Therefore, while the left and right pressing rods  46 L and  46 R are horizontally moved by the distance a4, by the pivotal opening movement of the door  31 , the pins (shaft)  44   b  (hence, left and right pressing member holders  44 L and  44 R) are regulated in position in terms of the vertical direction, by the first guiding range  48   a . Thereafter, the pins (shaft)  44   b  (hence, left and right pressing member holders  44 L and  44 R) are vertically moved by a distance b2, by the second guiding range  48   b , while the left and right pressing rods  46 L and  46 R are moved by a distance a5 by the movement of the door  31 . 
         [0197]    Lastly, while the left and right pressing rods  46 L and  46 R are horizontally moved by a distance a6, they are regulated in position in terms of the vertical direction, by the third guiding range  48   c . Incidentally, the vertically elongated holes  82 , with which the left and right sub-frames  80 L and  80 R are provided, one for one, are large enough not to interfere with the vertical movement of the pins  44   b , which occurs during the opening or closing of the door  31 . 
         [0198]      FIGS. 22(   a ) and  22 ( b ) illustrate the manner in which the left and right end portions of one of the cartridges P in the tray  35  are placed in contact with, or separated from, the corresponding pressing members  42 . That is, each of  FIGS. 22(   a ) and  22 ( b ) is a part of the sectional view of the apparatus main assembly  100 A, at a vertical plane parallel to the moving direction of the tray  35 , as seen from the left side of the apparatus main assembly  100 A. It shows the cartridge P, pressing member  42 , pressing spring  43 , and pressing member holder  44 . 
         [0199]      FIG. 22(   a ) shows the combination of the cartridge P, pressing member  42 , pressing spring  43 , and pressing member holder  44  when the door  31  is closed (closed position A), and each cartridge P is under the pressure applied thereto from the pressing spring  43  through the pressing member  42 , and in its preset image formation position. As the door  31  is pivotally moved into the closed position A, the pressing member holder  44  and pressing member  42  are pressed down by the first guiding range  48   a  of the pressing rod  46 . However, the pressing member  42  is stopped as it comes into contact with the left and right portions of the top surface of the cartridge P, being thereby prevented from moving downward further. Consequently, a gap (distance b3) is created between the pressing member engaging portion  44   c , with which the pressing member holder  44  is provided, and the pressing member holder engaging portion  42   a  of the pressing member  42 . 
         [0200]    That is, when the door  31  is in its closed position A, the pressing member  42  is in its cartridge contacting position H, in which it keeps the corresponding cartridge P fixed in position, by remaining in contact with the cartridge P. More concretely, the pressing member  42  is made by the pressure it receives from the pressing spring  43 , to press on the left and right end portions of the top surface of the cartridge P. Thus, the driving-side bearing  51  and no-driving-side bearing  52  of the cartridge P are kept in contact with the corresponding bearing positioning portions  41 , being thereby prevented from moving. 
         [0201]    The direction in which the pressing member  42  is moved to be placed in contact with the cartridge P is as follows. That is, it is perpendicular to the direction (engaging direction) in which the drum driving coupling  39  and development roller driving coupling  40 , which are driving force transmitting members (which will be described later), engage with the counterparts of the cartridge P, and also, the direction (contacting direction) in which the aforementioned power supplying member  75  comes into contact with the cartridge P (contacting direction). 
         [0202]      FIG. 22(   b ) shows the positional relationship among the abovementioned components when the door  31  is fully open (in open position B), and therefore, each cartridge P is free from the pressure from the pressing spring  43 , having therefore been moved upward from the image formation position. More concretely, as the door  31  is opened, the pressing rod  46  is moved rearward of the apparatus main assembly  100 A by the movement of the door  31 . Thus, the pressing member holder  44  is kept in its top position by the third guiding range  48   c  of the pressing rod  46 . As the pressing member holder  44  is moved upward by the distance b3, from its position shown in  FIG. 22(   a ), in which it causes the pressing member  42  (pressing spring  43 ) to press on the cartridge P, the pressing member contacting portion  44   c  of the pressing member holder  44  comes into contact with the pressing member holder contacting portion  42   a  of the pressing member  42 . 
         [0203]    Thereafter, therefore, the pressing member holder  44  and pressing member  42  are moved upward together. As described above, the amount (b2−b3) by which the pressing member  42  is moved upward is greater than the above described amount (b1) by which each cartridge P is moved upward. Therefore, each cartridge P is freed from the pressure applied by the pressing spring  43  (pressing member  42 ), and the pressing member  42  is retracted to its preset position in which it does not interfere with the horizontal movement of the tray  35 . 
         [0204]    That is, when the door  31  is in its open position B, the pressing member  42  is in its separation position I, in which it remains separated from the cartridge P. In other words, as the door  31  is opened, each cartridge P is freed by the pivotal opening movement of the door  31 , from the pressure applied to the cartridge P in the left-right direction. 
       (Structure of Driving Force Transmitting Coupling) 
       [0205]    Referring to  FIGS. 19 and 25 , the apparatus main assembly  100 A is provided with the drum driving force transmitting coupling  39  and development roller driving force transmitting coupling  40 , which are on the inward side of the sub-frame  80 R of the apparatus main assembly  100 A. The drum driving coupling  39  and development roller driving coupling  40  are the driving force output portions (driving force transmitting members) of the apparatus main assembly  100 A, which are engaged with the driving force input portions (driving force transmitting portion engaging portions)  53  and  54  of the cartridge P to transmit rotational driving force to the drum  1  and development roller  3   a  of each cartridge P. 
         [0206]      FIG. 24  is an enlarged view of the combination of the drum driving coupling  39  of the apparatus main assembly  100 A and the driving force input portion  53  of the cartridge P. It shows the manner in which the couplings  39  and  53  are engaged with each other, or disengaged from each other. For the sake of the simplification of description, the right sub-frame  80 R of the apparatus main assembly  100 A, and cartridge P, are not shown in  FIG. 24 .  FIG. 25  is a perspective view of the combination of the drum driving coupling  39  and development roller driving coupling  40 , and its adjacencies, as seen from the right side of the apparatus main assembly  100 A, when the door  31  is fully open (in open position B). It shows the structure of the combination, and its adjacencies. 
         [0207]    The drum driving coupling  39  and development roller driving coupling  40  are rotatably supported by the right sub-frame  80 R in such a manner that they are moveable in the direction parallel to their axial line, relative to the unshown driving unit shafts, which are outside the right sub-frame  80 R. They are under the pressure generated by the coupling pressing springs (pressing means: pressing members)  211 , in the direction to cause the coupling portions  39   a  and  40   a  to engage with the driving force input portions  53  and  54  of the cartridge P. 
         [0208]    Referring to  FIGS. 17A ,  17 B and  17 C, the right sub-frame  80 R is provided with four round holes  84 , the centerline of each of which coincides with the axial line of the corresponding drum driving coupling  39 . A drum driving coupling disengagement lever  213  is fitted in the round hole  84 , in such a manner that it can be pivotally moved ( FIG. 24 ). 
         [0209]    Next, referring to  FIGS. 24 and 25 , one end of the drum driving coupling disengagement lever  213  is provided with a pin (shaft)  313   a , which is fitted in the vertically elongated hole  201 C, with which the right connective rod  201 R is provided. Further, a drum driving coupling disengagement cam  212  is in engagement with the drum driving coupling disengagement lever  213 . It is movable in the direction parallel to the axial line of the drum driving coupling  39 . 
         [0210]    Further, the drum driving coupling disengagement cam  212  is provided with a rib  212   e , which is held by an unshown holding member, in such a manner that the drum driving coupling disengagement cam  212  is prevented from moving in the rotational direction of the drum driving coupling disengagement lever  213 . 
         [0211]      FIG. 24(   a ) is an enlarged view of the drum driving coupling  39  and its adjacencies, when the door  31  is completely shut against the apparatus main assembly  100 A (closed position A). When the drum driving coupling  39  is in the state shown in  FIG. 24(   a ), it is kept by a coupling pressing spring  211  in a preset position in which its coupling portion  39   a  is protruding inward of the apparatus main assembly  100 A far enough to remain in engagement with the driving force input portion  53  of the cartridge P in the apparatus main assembly  100 A. That is, when the door  31  is in its closed position A, the drum driving coupling  39  is in its engagement position F, in which it can transmit driving force to the driving force input portion  53  of the cartridge P. 
         [0212]      FIG. 24(   b ) shows the state of the drum driving coupling  39  when the door  31  is fully open (open position B). As the door  31  is pivotally moved (opened) from its closed position A to its open position B, the right connective rod  201 R is moved frontward of the apparatus main assembly  100 A by the movement of the door  31 . Thus, the drum driving coupling disengagement lever  213  is pivotally moved relative to the drum driving coupling disengagement cam  212 . Consequently, the drum driving coupling disengagement cam  212  is moved rightward of the apparatus main assembly  100 A in the direction parallel to the rotational axis of the drum driving coupling disengagement cam  212 , by the surface  212 A of the drum driving coupling disengagement cam  212 . 
         [0213]    Thus, the end surface  212   f  of the drum driving coupling disengagement cam  212  comes into contact with the drum driving coupling  39 , causing the drum driving coupling  39  to move in the direction to separate from the driving force input portion  53  of the cartridge P, against the pressure generated by the coupling pressing spring  211 . That is, when the door  31  is in its open position B, the drum driving coupling  39  is in its separation position G, in which it remains separated from the cartridge P. 
         [0214]    Similarly, the right sub-frame  80 R is provided with a round hole  85 , the axial line of which coincides with the axial line of the development roller driving coupling  40 . The development roller driving coupling disengagement lever  215  is rotatably fitted in the round hole  85  ( FIGS. 17 and 25 ). 
         [0215]    One end of the development roller driving coupling disengagement lever  215  is provided with a pin (shaft)  215   a , which is fitted in the vertically elongated hole  46 C, with which the right pressing rod  46 R is provided. 
         [0216]    Further, the development roller driving couple disengagement cam  214  is in engagement with the development roller driving coupling disengagement lever  215 , being enabled to be moved in the direction parallel to the rotational axis of the development roller driving coupling  40 . Further, the rib  214   e , with which the development roller driving coupling disengagement cam  214  is provided, is held by an unshown holding member. Therefore, the development roller driving coupling disengagement cam  214  is regulated in its movement in terms of the direction in which the development roller driving couple disengagement lever  215  is pivotally moved. 
         [0217]    Thus, as the door  31  is opened, that is, as it is pivotally moved from its closed position A to its open position B, the right connective rod  201 R is moved frontward of the apparatus main assembly  100 A by the movement of the door  31 , whereby the pressing rod  46 R is moved rearward of the apparatus main assembly  100 A by the frontward movement of the right connective rod  201 R, through the linkage rod  202 R. Thus, the development roller driving coupling disengagement lever  215  is pivotally moved relative to the development roller driving coupler disengagement cam  214 . 
         [0218]    Further, the development roller driving coupling disengagement cam  214  is moved rightward of the apparatus main assembly  100 A, along the rotational axle of the development roller driving coupling  40 , by the surface  214 A of the development roller driving couple disengagement cam  214 . Therefore, the end surface of the development roller driving couple disengagement cam  214  comes into contact with the development roller driving coupling  40 , causing the development roller driving coupling  40  to move in the direction to separate from the driving force input portion  54  of the cartridge P, against the pressure generated by the coupling pressing spring  211 . 
         [0219]    That is, as the door  31  is opened, the drum driving coupling  39  and development roller driving coupling  40  are vertically moved relative to the tray  35 , and also, are retracted into the area in which they do not regulate the tray  35  and each cartridge P in their horizontal movement. 
         [0220]    Referring to  FIG. 24 , the surface  212 A of the drum driving coupling disengagement cam  212  is made up of straight portions  212   b  and  212   d , and a slanted portion  212   c . Thus, when the door  31  is completely shut against the apparatus main assembly  100 A (closed position A), the drum driving coupling  39  remains in engagement with the corresponding cartridge P (engagement position F), whereas the door  31  is fully open (open position B), it remains separated from the corresponding cartridge P (separation position G). 
         [0221]    The surface  214 A of the development roller driving couple disengagement cam  214  is similarly structured to that of the drum driving coupling disengagement cam  212 . Thus, when the door  31  is completely shut against the apparatus main assembly  100 A (closed position A), the development roller driving coupling  40  also remains in engagement with the corresponding cartridge P (engagement position F), whereas when the door  31  is fully open (open position B), the development roller driving coupling  40  remains separated from the corresponding cartridge P (remains in separation position G). 
       (Operational Timing of Interface Portions) 
       [0222]    The image forming apparatus  100  is provided with the above described mechanism. Therefore, as the door  31  is moved from its closed position A to its open position B, the interface portions of the apparatus main assembly  100 A, which correspond one for one to the cartridges P, are made by the movement of the door  31  to disengage from the cartridges P. More concretely, the springy electrical contacts  75 , pressing members  42 , drum driving couplings  39 , and development roller driving couplings  40  of the apparatus main assembly  100 A are disengaged from the cartridges P. Further, as the tray  35  is moved by the tray holding member  34  from its image formation position C to its unlocking (locking) position D, it becomes possible for the tray  35  to be freely slid between the unlocking (locking) position D and cartridge mounting (dismounting) position E. 
         [0223]    Therefore, the tray  35  can be pulled out of the apparatus main assembly  100 A so that cartridges P can be mounted into the tray  35 , or the cartridges P in the tray  35  can be replaced, or the tray  35  can be pushed back into the apparatus main assembly  100 A. 
         [0224]    As described above, the springy electrical contacts  75 , pressing members  42 , drum driving couplings  39 , development roller driving couplings  40 , and tray  35  of the apparatus main assembly  100 A are moved by the movement of the door  31 . As for the timing with which each of the abovementioned components is moved, it can be adjusted by changing the above described mechanism for regulating the movement of these components, in the profile of the surface of the cams of the above described movement controlling mechanism. 
         [0225]    For example, the timing with which the tray  35  is vertically moved can be adjusted by adjusting the guiding range  36   b  of the guiding hole  36 , in position in terms of the horizontal direction, within the range of the guiding hole  36 . By configuring the guiding hole  36  so that the guiding range  36   a  is longer and the guiding range  36   c  is shorter, than they are in this embodiment, it is possible to cause the tray  35  to descent at a point closer to the starting of the closing of the door  31  (sooner after door  31  begins to be closed). 
         [0226]    Similarly, the timing with which the springy electrical contacts  75  of the apparatus main assembly  100 A are moved can be adjusted by modifying in profile the cam portion  201 D ( FIG. 18 ) of the connective rod  201 L. 
         [0227]    The timing which the pressing members  42  are moved can be adjusted by modifying in shape the guiding hole  48  ( FIG. 21 ) of the pressing rod  46 . 
         [0228]    The timing with which drum driving coupling  39  is moved can be adjusting in profile the surface of the cam portion  214  ( FIG. 25 ) of the development roller driving coupling  214 . 
         [0229]    In this embodiment, the timings with which the abovementioned components begin to be moved are set so that these components are sequentially moved in the following order, by the closing movement of the door  31 . 
         [0230]    Toward the end of the diagonally downward movement of the left and right tray holding members  34 L and  34 R caused by the second guiding range  36   b  of the guiding portion  36 , the pins (shafts)  34   c  and  34   d  are in the first guiding range  36   a  of the guiding portion  36 , that is, one of the horizontal guiding ranges of the guiding portion  36 . That is, as the left and right tray holding members  34 L and  34 R are moved in the diagonally downward direction, the driving-side bearing  51  and no-driving-side bearing  52  of each cartridge P are caught by their bottom surface, by the bearing positioning portions  41 , with which the left and right positioning members  81 L and  81 R of the apparatus main assembly  100 A, one for one, are provided. 
         [0231]    As the drum driving coupling disengagement lever  213  is rotationally moved, the drum driving coupling disengagement cam  212  is moved inward of the apparatus main assembly  100 A by the slanted surface portion  212   c  of the drum driving coupling disengagement cam  212 . That is, the drum driving coupling  39  is under the pressure generated by the coupling pressing spring  211 . Therefore, the drum driving coupling  39  engages with the driving force input portion  53  of the corresponding cartridge P. Similarly, as the development roller driving coupling disengagement lever  215  is pivotally moved, the development roller driving coupling  40  is moved inward of the apparatus main assembly  100 A by the slanted surface portion  214   c . Since the development roller driving coupling  40  is under the pressure generated by the coupling pressing spring  211 , the development roller driving coupling  40  engages with the driving force input portion  54  of the cartridge P. 
         [0232]    Thus, each cartridge P is subjected to the pressure generated by the coupling pressing spring  211 , through the drum driving coupling  39  and development roller driving coupling  40 , being thereby moved leftward of the apparatus main assembly  100 A. Thus, the protrusion  57  of the cartridge P, which is for positioning the cartridge P in terms of the left-right direction, comes into contact with the left sub-frame  80 L of the apparatus main assembly  100 A. Consequently, the cartridge P is accurately positioned relative to the apparatus main assembly  100 A in terms of the left-right direction. 
         [0233]    The left and right pressing rods  44  are made to move diagonally downward by the second guiding range  48   b  of the guiding hole  48  with which the pressing rod  46  is provided, causing thereby the left and right pressing members  42  to downwardly move. Thus, the left and right pressing members  42  press on the left and right end portion of the top surface of the cartridge P, causing thereby the cartridge P to move downward. Consequently, the driving-side bearing  51  and no-driving-side bearing  52  of the cartridge P are immovably fit into the bearing positioning portions  41 . 
         [0234]    The electrical contact holder  76  of the apparatus main assembly  100 A is moved inward of the apparatus main assembly by the slanted portion  201   f  of the connective rod  201 L. Thus, the cartridge electrical contact contacting portion  77  of springy electrical contact  75  of the apparatus main assembly  100 A is placed in contact with the electrical contact  55  of the cartridge P, establishing thereby electrical connection between the contacts  75  and  55 . 
         [0235]    The above-described timings with which these components begin to be moved may be summarized as follows: As the door  31  is moved from its open position B ( FIG. 3 ) to its closed position A (FIG. ( 2 ), the following operations (1)-(4) are made to sequentially occur in the numerical order, by the movement of the door  31 , which is one of the characteristic features of this embodiment. 
         [0000]    (1) Movement of the tray  35  from its tray locking (unlocking) position D to its image formation position C (FIG.  4 → FIG. 2 ).
 
(2) Movement of driving force transmitting members  39  and  40  from their separation position G to their engagement position F ( FIG. 24(   b )→ FIG. 24(   a )).
 
(3) Movement of pressing member  42  from its separation position I to its contact position H ( FIG. 22(   b )→ 22 ( a )).
 
(4) Movement of power supplying member  75  from its separation position K to its power supplying position J ( FIG. 18(   b )→( a )).
 
         [0236]    When the door  31  is opened, the above described movements are made to occur in the reverse order ((4)→(3)→(2)→(1)). That is, as the door  31  is opened, first, the power supplying member  75  separates. Next, the left and right pressing members  42  separate. Then, the driving couplings  39  and  40  separate. Lastly, the tray  35  moves upward (from image formation position C to tray locking (unlocking) position D). 
         [0237]    Next, the merits of setting, as described above, the order in which these movements are triggered are described. While the door  31  is opened, first, the driving force input portions  53  and  54  of each cartridge P are moved to where they oppose the drum driving coupling  39  and development roller driving coupling  40 , respectively, in Step (1). 
         [0238]    Then, the drum driving coupling  39  and development roller driving coupling  40  are placed in contact with the driving force input portions  53  and  54  of the cartridge P in Step (2), ensuring that the drum driving coupling  39  and development roller driving coupling  40  are engaged with the driving force input portions  53  and  54 , respectively, of the cartridge P. 
         [0239]    On the other hand, while the door  31  is closed, first, drum driving coupling  39  and development roller driving coupling  40  are separated from the driving force input portions  53  and  54  of the cartridge P, respectively. Then, each cartridge P is moved upward along with the tray  35 . If a cartridge P is moved upward along with the tray  35  while the driving couplings  39  and/or  40  are remaining in engagement with the driving force input portions  53  and/or  54 , respectively, the cartridge P is tilted so that its lengthwise end, by which the cartridge P is in engagement with the driving couplings of the apparatus main assembly  100 A, is positioned higher than its opposite end. Consequently, the cartridge is twisted across its portion between the lengthwise end portions at which the cartridge P is supported by the apparatus main assembly  100 A, making it possible for the driving couplings to be damaged. 
         [0240]    Further, if the tray  35  is moved upward while the driving coupling(s) are still protruding inward of the apparatus main assembly  100 A through the hole of the right sub-frame  80 R, the driving coupling(s) comes into contact with the right piece  35   e  of the tray  35 , making it possible for the driving coupling(s) to be damaged. 
         [0241]    That is, this is why Step (2) is carried out after the completion of Step (1) to prevent the drum driving coupling  39  and/or development roller driving coupling  40  from being damaged by being twisted at the point of engagement between the apparatus main assembly  100 A and cartridge P, and/or their contact with the right piece  35   e  of the tray  35 . 
         [0242]    Further, each cartridge P is moved leftward in Step (2), whereby the protrusion  57  of the cartridge P, which is for positioning the cartridge P relative to the apparatus main assembly  100 A in terms of the left-right direction, is placed in contact with the left sub-frame  80 L. 
         [0243]    More concretely, as described above, the apparatus main assembly  100 A has the coupling pressing springs (pressing means)  211  which keep the drum driving coupling  39  and development roller driving coupling  40  pressed on the driving force input portions  53  and  54  of the cartridge P. Thus, the cartridge P is pressed by these springs  21 , from the side where the driving force input portions  53  and  54  are, toward the side where the power supplying contacts  55  are. Consequently, the cartridge P is moved leftward, and therefore, the protrusion  57  of the cartridge, which is for positioning the cartridge P relative to the apparatus main assembly  100 A, is placed in contact with the left sub-frame  80 L. 
         [0244]    Thereafter, in Step (3), the left and right end portions of the top surface of each cartridge P are pressed, whereby the bearing  51  on the driving side, and bearing on the no-driving side, are fitted in the bearing positioning portions  41 , being thereby fixed in position. Thus, it is ensured that each cartridge P is precisely position relative to the main frame of the apparatus main assembly  100 A in terms of the left-right direction. 
         [0245]    Incidentally, in Step (2), the bearings  51  and  52  of each cartridge P are placed in contact with the bearing positioning portions  41 , one for one. However, the amount of resistance, which the bearings  51  and  52  are subjected as they are vertically lowered into the bearing positioning portions  41  is relatively small. Therefore, the friction which occurs as each cartridge P is moved leftward of the apparatus main assembly  100 A is also relatively small. Therefore, it does not occur that the bearings  51  and  52  are frictionally shaved by the bearing positioning portions  41 . Further, in Step (2), the springy electrical contact  75 , with which the no-driving side of the apparatus main assembly  100 A is provided, remains separated from the electrical contact  55  of each cartridge P. Therefore, it does not occur that when each cartridge P is moved leftward of the apparatus main assembly  100 A, it is subjected to such force that interferes with its leftward movement, by the no-driving side. 
         [0246]    That is, the movement of each cartridge is not interfered by the cartridge pressing mechanism and power supplying mechanism. Therefore, the resiliency of the coupling pressing spring  211  has only to be strong enough to move the corresponding cartridge P leftward of the apparatus main assembly  100 A. As described above, while the door  31  is opened, the drum driving coupling  39  and development roller driving coupling  40  are horizontally moved against the resiliency of the coupling pressing spring  211 . Thus, the amount of force necessary to open the door  31  can be reduced by reducing the coupling pressing spring  211  in resiliency. 
         [0247]    Also in Step (3), the left and right end portions of the top surface of each cartridge P are pressed, whereby the bearing  51  on the driving side, and bearing on the no-driving side, are fitted into the bearing positioning portions  41 , being thereby fixed in position. Thereafter, in Step (4), electrical connection is established between the springy electrical contact  75  of the apparatus main assembly  100 A and the electrical contact  55  of the cartridge P. Therefore, it does not occur that the contact pressure between the springy electrical contact  75  of the apparatus main assembly  100 A and the electrical contact  55  of the cartridge P pushes back (rightward of apparatus main assembly  100 A) the cartridge P. 
         [0248]    Further, each cartridge P is positioned so that its protrusion  57  for positioning the cartridge P in terms of the left-right direction remains in contact with the left sub-frame  80 L through Step (2). Then, it is ensured in Step (3) that each cartridge P remains fixed in position to the location into which the cartridge P has been moved. Therefore, it is ensured that the positional relationship between the springy electrical contact  75  of the apparatus main assembly  100 A and the electrical contact  55  of the cartridge P in terms of the left-right direction of the apparatus main assembly  100 A remains as it is preset. Thus, it is possible to reduce the image forming apparatus  100  in the amount of the stroke of the springy electrical contact  75  of the apparatus main assembly  100 A, and therefore, to reduce the amount of the force of which a user is required to open or close the door  31 . 
         [0249]    That is, in this embodiment, while the door  31  is opened, Steps (1)-(4) are sequentially carried out. Therefore, not only can this embodiment reduce the amount of force necessary to open or close the door  31 , but also, can ensure that each cartridge P is precisely positioned relative to the main frame of the apparatus main assembly  100 A, for the reasons given above. 
         [0250]    Incidentally, it is not mandatory that Steps (1)-(4) are carried out so that they do not overlap. For example, Step (2) may be started while Step (1) is carried out. That is, all that is necessary is that Step (1) is finished before the drum driving coupling  39  and development roller driving coupling  40  begin to be placed in contact with the driving force input portions  53  and  54  of the corresponding cartridge P. That is, all that is necessary is that the movement of the driving force input portions  53  and  54  of each cartridge P to where they oppose the drum driving coupling  39  and development roller driving coupling  40 , respectively, is completed before Step (2) is started. 
         [0251]    Further, from the standpoint of reducing the image forming apparatus  100  in the amount of force necessary to open the door  31 , it is desired that the cartridges P are made slightly different in the timing with which each of Steps (2)-(4) begins to be carried out. For example, it is desired that the cartridges P are made slightly different in the timing with which the drum driving coupling  39  is placed in contact with the driving force input portion  53  of the cartridge P (PY, PM, PC and PK), so that the amount of load to which the door  31  is subjected is chronologically distributed. 
         [0252]    Further, the timing with which the drum driving coupling  39  is placed in contact with the driving force input portion  53  of the cartridge P, may be slight different from the timing with which the development roller driving coupling  40  is placed in contact with the driving force input portion  54  of the cartridge P, so that the load to which the door  31  is subjected is chronologically distributed. With the employment of this arrangement, it is possible to reduce the image forming apparatus  100  in the amount of force to which the door  31  is subjected when the door  31  is opened or closed, and therefore, to reduce the amount of force required of a user to open the door  31 . 
         [0000]    (Merits of Positioning Cartridge relative to Lateral Wall of Apparatus Main Assembly  100 A) 
         [0253]    In this embodiment, as described above, the protrusion  57  of each cartridge P, which is for positioning the cartridge relative to the apparatus main assembly  100 A in terms of the left-right direction of the apparatus main assembly  100 A is placed in contact with the left sub-frame  80 L of the apparatus main assembly  100 A by the resiliency of the coupling pressing spring (pressing member)  211 , in order to position the cartridge P relative to the apparatus main assembly  100 A in terms of the left-right direction of the cartridge. 
         [0254]    That is, the apparatus main assembly  100 A has the pressing member  211 , which is for pressing a cartridge P from one side of the apparatus main assembly  100 A to the other in the apparatus main assembly  100 A, in terms of the direction perpendicular to the direction in which the tray  35  is moved inward or outward of the apparatus main assembly  100 A along the surface along which the tray  35  is moved. In this embodiment, the pressing members  211  are inside the driving force transmitting members  39  and  40 , one for one. 
         [0255]    Further, the cartridges P are positioned relative to the main frame of the apparatus main assembly  100 A, in terms of the abovementioned perpendicular direction, as the cartridge positioning protrusion  57  of the cartridge P is placed in contact with, that is, caught by, the left sub-frame  80 L of the apparatus main assembly  100 A, by being pressed by the pressing member  211 , when the tray  35  is in the image formation position C while the cartridges P are in the tray  35 . Pressing of the cartridge P by the pressing member  211 , and removal of the pressure applied by the pressing member  211 , from the cartridge P, are caused by the operation for opening or closing the door  31 . 
         [0256]    In this embodiment, the above described perpendicular direction is the same as the lengthwise direction (parallel to axial line of drum) in a cartridge P properly positioned in the apparatus main assembly  100 A, and the left-right direction of the apparatus main assembly  100 A. The left sub-frame  80 L is one of the lateral walls of the apparatus main assembly  100 A, which opposes the opposite end of a cartridge P in the tray  35  in terms of the above-described perpendicular direction. 
         [0257]    Therefore, the image forming apparatus  100  in this embodiment is higher in the accuracy with which the springy electrical contact  75  of the apparatus main assembly  100 A is positioned relative to the electrical contact  55  of the cartridge P than any conventional image forming apparatus. That is, it is less in the amount of stroke of the springy electrical contact  75  of the apparatus main assembly  100 A, being therefore, smaller in the amount of force required of a user to open or close than any image forming apparatus in accordance with the prior art. 
         [0258]    In addition, from the standpoint of improving the above described various units in terms of the positional accuracy relative to the cartridges P in terms of the left-right direction, it is desired to be the left sub-frame  80 L that is provided with the units. 
         [0259]    In this embodiment, it is to the left sub-frame  80 L, relative to which the cartridges P are positioned to be accurately positioned relative to the apparatus main assembly  100 A, that the springy electrical contacts  75  of the apparatus main assembly  100 A, which are the units for establishing electrical connection between the apparatus main assembly  100 A and the cartridges P in the apparatus main assembly  100 A, are attached. Further, the position of the springy electrical contacts  75  of the apparatus main assembly  100 A, relative to the apparatus main assembly  100 A, in terms of the direction parallel to the axial line of the drum, is set by the left sub-frame  80 L. That is, it is the left sub-frame  80 L that has the electrical contact unit  75  which establishes electrical connection between the apparatus main assembly  100 A and the cartridges P. Further, it is by the left sub-frame  80 L that the electrical contact units  75  are positioned in terms of the above-described perpendicular direction. 
         [0260]    The laser scanner unit (exposing unit)  11  is held by the exposing device holding member (unshown). Further, the exposing device holding member is fixed to the apparatus main assembly  100 A in such a manner that the portion of the exposing device holding member, which is for positioning the exposing device holding member, in terms of the lengthwise direction of the member (left-right direction of apparatus main assembly  100 A), is in contact with the left sub-frame  80 L. Therefore, the exposing device holding member is accurately positioned relative to the apparatus main assembly  100 A in terms of the abovementioned perpendicular direction, by the left sub-frame  80 L. Therefore, the image forming apparatus  100  in this embodiment is higher in the accuracy with which the drum  1  is exposed by the beam of laser light outputted by the exposing device than any conventional image forming apparatus. 
         [0261]    Further, the intermediary transfer belt unit  12  is fixed to the apparatus main assembly  100 A, with its positioning portion (unshown) being placed in contact with the left sub-frame  80 L, in terms of its lengthwise direction (left-right direction of apparatus main assembly  100 A). That is, the intermediary transfer unit  12 , which is disposed so that it opposes the drums  1 , and also, so that developer images, are transferred (primary transfer) from the drums  1 , and from which the developer images are transferred (secondary transfer) onto a sheet S of recording medium, is accurately positioned relative to the apparatus main assembly  100 A by the left sub-frame  80 L of the apparatus main assembly  100 A. 
         [0262]    Therefore, it is possible to improve an image forming apparatus in the accuracy with which the belt  13 , driver roller  14 , turn roller  15 , tension roller  16 , and primary transfer roller  17  are positioned relative to the drum  1 . Therefore, it is possible to reduce the drum  1 , belt  13 , driver roller  14 , turn roller  15 , tension roller  16 , and primary transfer roller  17  in their measurement in terms of the left-right direction of the apparatus main assembly  100 A. That is, it is possible to reduce the image forming apparatus and cartridge therefor in their size in terms of the left-right direction of the apparatus main assembly  100 A, and therefore, in their cost. 
         [0263]    Further, the fixing device (fixation unit)  23  is fixed to the apparatus main assembly  100 A, with its positioning portion (unshown) for positioning the fixing device  23  in terms of its lengthwise direction, being in contact with the left sub-frame  80 L of the apparatus main assembly  100 A. That is, the fixing unit  23 , which is for applying heat and pressure to a developer image, and the sheet S of recording medium on which the developer image is present, to fix the developer image to the sheet S, is accurately positioned relative to the apparatus main assembly  100 A by the left sub-frame  80 L. 
         [0264]    Therefore, it is possible to improve an image forming apparatus in the accuracy with which its fixing device  23  is positioned. That is, the fixation unit  23  is accurately position with reference to the left sub-frame  80 L. Therefore, the image forming apparatus is minimized in the positional deviation between the fixing members  23   a  and  23   b , in terms of the left-right direction of the apparatus main assembly  100 A, and the developer image on the belt  13 . Therefore, it is possible to reduce the fixing members  23   a  and  23   b  in their dimension in terms of the left-right direction of the apparatus main assembly  100 A. That is, it is possible to reduce the fixing members  23   a  and  23   b  in size in terms of the left-right direction of the apparatus main assembly  100 A, and therefore, in cost. 
         [0265]    Further, the sheet feeder unit  18 , in which multiple sheets S of recording medium, onto which a developer image is transferred, are stored, and from which the sheets S are conveyed one by one, are positioned by the left sub-frame  80 L in terms of the above described perpendicular direction. Therefore, the image forming apparatus  100  is minimized in the positional deviation between the developer image on the belt  13 , and the sheet S of recording medium conveyed out of the sheet feeder unit  18 , in terms of the left-right direction. Therefore, it is minimized in the positional deviation between the sheet S of recording medium and the image on the sheet S. 
       &lt;&lt;Miscellanies&gt;&gt; 
       [0266]    (a) The application of the present invention is not limited to such a process cartridge of the all-in-one type as the one in this embodiment that is provided with the image bearing member  1 , on which a latent image is formed, and the developing means  3 , which is for developing the latent image on the image bearing member  1 , with developer. 
         [0267]    That is, the present invention is also applicable to a process cartridge of the so-called separation type, which is equipped with the image bearing member  1  on which a latent image is formed, and processing means other than the developing means  3  which is for developing the latent image on the image bearing member  1 , with developer. 
         [0268]    Further, the present invention is also applicable to a development cartridge which has the developing means for developing the latent image on the image bearing member  1 , with the use of developer, and a developer storage in which the developer to be used for developing the latent image. 
         [0269]    Further, the present invention is applicable to an image forming apparatus structured so that a combination of a process cartridge of the so-called separation type, and a development cartridge, is supported by its tray  35 , and at least one of the process cartridge of the separation type and development cartridge is removably supported by the tray  35 . 
         [0270]    Moreover, a cartridge to which the present invention is applicable includes a unit in the form of a cartridge, which is removably mountable in the apparatus main assembly  100 A and contributes to the process for forming an image on recording medium. 
         [0271]    (b) The electrophotographic full-color image forming apparatus in this embodiment is an image forming apparatus in which four cartridges, which are different in the color of the developer they contain, are removably installable. However, this embodiment is not intended to limit the present invention in the number of cartridges installable in the main assembly of an image forming apparatus; the number of the cartridge installable in the image forming apparatus is optional. That is, the present invention is also applicable to an image forming apparatus which employs only one cartridge, and also, an image forming apparatus which employs two, three, or no less than five cartridges. Further, the present invention is applicable to a monochromatic image forming apparatus, that is, an image forming apparatus which employs only one cartridge. 
         [0272]    (c) The image forming apparatus  100  in this embodiment is structured so that its tray  35  is linearly moved in the direction parallel to the surface on which the apparatus main assembly  100 A is rested. However, this embodiment is not intended to limit the present invention in the direction in which the tray  35  (also, cartridge supporting member) is moved. For example, the present invention is also applicable to an image forming apparatus structured so that its tray  35  is movable in such a direction that is perpendicular to the lengthwise direction of the drum  1 , parallel to the surface on which the apparatus main assembly  100 A is rested, or diagonally upward or downward. Further, the present invention is also applicable to an image forming apparatus structured so that its tray stopper can be disengaged to remove the tray  35  from the apparatus main assembly  100 A. 
         [0273]    (d) The present invention is applicable to the image forming apparatus  100  in this embodiment, even if the intermediary transfer unit  12  of the apparatus  100  is replaced with a recording medium conveyance-transfer belt for bearing and conveying a sheet S of recording medium. That is, the present invention is applicable to an image forming apparatus having a transferring device (recording medium conveying-transferring means) which conveying recording medium so that the developer image formed on the drum  1  is directly transferred onto a sheet S of recording medium. 
         [0274]    (e) An image forming apparatus to which the present invention is applicable is not limited to an image forming apparatus (printer) such as the one in this embodiment. That is, the present invention is also applicable to an image forming apparatus other than the one in this embodiment. For example, the present invention is also applicable to a copying machine, a facsimile machine, etc., and a multifunction image forming apparatus capable of functioning as two or more of the preceding examples of image forming apparatus. 
         [0275]    (f) The application of the present invention is not limited to an image forming apparatus which uses an electrophotographic process. That is, the present invention is also applicable to an image forming apparatus which uses an electrostatic recording process which employs an electrostatically recordable dielectric member, a magnetic recording process which employs a magnetically recordable magnetic member, and the like image forming apparatus. 
         [0276]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0277]    This application claims priority from Japanese Patent Applications No. 259492/2012 filed Nov. 28, 2012 and 259493/2012 filed Nov. 28, 2012, which are hereby incorporated by reference.