Abstract:
A photosensitive member cartridge for use with an image-forming apparatus includes a photosensitive drum and a cartridge case comprising an upper wall, a bottom wall, a right side wall, and a left side wall. An outer surface of the upper wall faces upwards when the photosensitive member cartridge is installed in the image-forming apparatus. The right side wall extends upwardly from a right side of the bottom wall, and the left side wall extends upwardly from a left side of the bottom wall. The upper wall connects upper edges of the right side wall and the left side wall. A light-entrance portion is formed in the upper wall and is configured to allow a laser beam to pass through the upper wall and reach the photosensitive drum when the photosensitive member cartridge is installed in the image-forming apparatus.

Description:
This is a Continuation Application of application Ser. No. 12/801,688, filed Jun. 21, 2010, which in turn is a Continuation Application of application Ser. No. 12/289,686, filed Oct. 31, 2008, now U.S. Pat. No. 7,742,720, issued Jun. 22, 2010, which in turn is a Continuation Application of application Ser. No. 11/802,215, filed May 21, 2007, now U.S. Pat. No. 7,457,563, issued Nov. 25, 2008, which in turn is a Continuation Application of application Ser. No. 11/269,666, filed Nov. 9, 2005, now U.S. Pat. No. 7,245,850, issued Jul. 17, 2007, which in turn is a Divisional Application of application Ser. No. 11/053,846, filed Feb. 10, 2005, now U.S. Pat. No. 7,231,167, issued Jun. 12, 2007, which in turn is a Divisional of application Ser. No. 10/733,390, filed Dec. 12, 2003, now U.S. Pat. No. 6,873,810, issued Mar. 29, 2005, which in turn is a Divisional Application of application Ser. No. 10/364,416, filed Feb. 12, 2003, now U.S. Pat. No. 6,690,903, issued Feb. 10, 2004, which is in turn a Divisional of application Ser. No. 10/153,643, filed May 24, 2002, now U.S. Pat. No. 6,546,217, issued Apr. 8, 2003, which is a Divisional of application Ser. No. 09/928,514, filed Aug. 14, 2001, now U.S. Pat. No. 6,411,789, issued Jun. 25, 2002, which is a Divisional of application Ser. No. 09/501,659, filed Feb. 10, 2000, now U.S. Pat. No. 6,330,410, issued Dec. 11, 2001, which is a Continuation-in-Part of application Ser. No. 09/281,948, filed Mar. 31, 1999, now U.S. Pat. No. 6,041,203, issued Mar. 31, 1999, the entire disclosures of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a photosensitive member cartridge and a process cartridge for use in an electrostatographic image forming apparatus. 
     2. Description of Related Art 
     Some electrostatographic image forming apparatus in which a visible image is obtained by supplying toner to an electrostatic latent image formed on a photosensitive drum, are disclosed in, for example, Japanese Patent Publication Nos. 8-54786 and 9-319285, which correspond to U.S. Pat. Nos. 5,845,176 and 5,805,959, respectively. 
     The former publication discloses such structures that a photosensitive member cartridge including a photosensitive drum, and a developer cartridge including a developing roller, are separately set in a housing. 
     The latter publication discloses such structures that a photosensitive member cartridge and a developer cartridge are movably connected at one portion by a pin, so that a photosensitive member and a developing roller press against each other. 
     Generally, the surface of the photosensitive drum is coated with a light-sensitive material. To prevent the surface of the photosensitive drum from being scratched or contaminated, some measures need to be taken. Otherwise, when a portion of the photosensitive surface is scratched or contaminated with, for example, dust, such as would be caused by touching the surface of the photosensitive drum by hand, the photosensitive characteristics of the photosensitive surface are changed. Consequently, the image quality is adversely affected. 
     Accordingly, a well-known structure, in which the photosensitive drum is covered with a shutter, is employed for apparatus, such as those disclosed in the above-described publications. 
     However, the shutter needs to be constructed so as to open when the photosensitive member cartridge is set in a body (housing), and so as to close when the photosensitive member cartridge is removed from the body (housing). This makes the structure of the photosensitive member cartridge complicated. In addition, the possibility arises that a user may accidentally open the shutter and touch the surface of the photosensitive member cartridge. 
     Further, when the photosensitive member cartridge is placed on a table, the shutter may accidentally open, so that the photosensitive drum may be exposed. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is to provide a photosensitive member cartridge that obviates the danger of contamination or scratches on a photosensitive member. To achieve this aspect, the photosensitive member cartridge of the invention includes a case, and a photosensitive member and a transfer roller which are rotatably disposed in the case. The case covers the photosensitive member and the transfer roller. Specially, a bottom wall of the case covers a lower portion of the transfer roller, and upper and rear walls cover the photosensitive member. 
     The bottom wall extends below the transfer roller. On the extending portion of the bottom wall, a developer cartridge, including a developing roller, is set. Such structures prevent the photosensitive member and the transfer roller from being accidentally touched, which enhances image quality. 
     Also, it is unnecessary to provide complicated components such as a shutter, so that the structures of the photosensitive member cartridge are simplified. 
     Further, foot portions provided on the underside of the bottom wall stabilize the photosensitive member cartridge when it is placed on a table. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a schematic side sectional view of a printer embodying the image forming apparatus of the invention; 
         FIG. 2  is a side view of the printer, illustrating a situation where the insertion of a process unit into a body housing is started; 
         FIG. 3  is a plan view of a photosensitive member cartridge; 
         FIG. 4  is a side sectional view of the photosensitive member cartridge taken on plane IV-IV in  FIG. 3 ; 
         FIG. 5  is a right side view of the photosensitive member cartridge; 
         FIG. 6  is a left side view of the photosensitive member cartridge; 
         FIG. 7  is a front view of the photosensitive member cartridge; 
         FIG. 8  is a partially cut-away sectional view illustrating a pressing portion of a transfer roller; 
         FIG. 9  is a sectional view taken on plane IX-IX in  FIG. 8 ; 
         FIG. 10  is a perspective view of an urging device and an action-receiving portion; 
         FIG. 11(   a ) is a plan view of a lock device; 
         FIG. 11(   b ) is a sectional view taken on line XIb-XIb in  FIG. 11(   a ); 
         FIG. 12(   a ) is a left side view of a developer cartridge; 
         FIG. 12(   b ) is a right side view of the developer cartridge; 
         FIG. 13  is a plan view of the developer cartridge; 
         FIG. 14  is a view of the developer cartridge taken in the direction indicated by arrows XIV in  FIG. 13 ; 
         FIG. 15  is a sectional view of the developer cartridge, illustrating the structure of shaft bearings disposed at the right and left sides of the developer cartridge; 
         FIG. 16  is a plan view of a process unit; 
         FIG. 17  is a right side view of the process unit; 
         FIG. 18  is a left side view of the process unit; 
         FIG. 19  is a view of the process unit taken in the direction indicated by arrows XIX in  FIG. 17 ; 
         FIG. 20(   a ) illustrates a situation in which the process unit is being inserted into the body housing; 
         FIG. 20(   b ) illustrates a situation in which the process unit is further inserted; 
         FIG. 21  illustrates a situation in which the process unit has been set in the body housing; 
         FIG. 22  is an illustration of a drive system of the printer; 
         FIG. 23  is a plan view of the process unit showing the upper register roller  12   a  provided so as to expose its surface from the bottom opening  300   a  of the case  30 ; 
         FIG. 24  is an enlarged view of the circled portion B of  FIG. 23 ; 
         FIGS. 25A-25I  are views showing the register roller  12   a  and bearings  700 ,  710 ; 
         FIGS. 26A-26D  are views showing the bearings  700 ,  710  and the supporting portion  300   b  of the case  30 ; 
         FIG. 27A-27F  are views showing the bearing  700 ; 
         FIGS. 28A-28E  are views showing the bearing  710 ; 
         FIG. 29  is a view showing the bearing  700  with a bearing chamfered portion  700   f  and side wall  300   c  with a gap  300   e;    
         FIG. 30  is a view showing the register roller  12   a , and bearings  700 ,  710 ; 
         FIG. 31  is a view showing the register rollers  12   a ,  12   b , bearing  700  and spring  45 ; and 
         FIG. 32  is an illustration of a force that presses a developing roller against a photosensitive drum and other forces concerned. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment in which the invention is embodied in a laser beam-type printer will be described in detail hereinafter with reference to the accompanying drawings.  FIG. 1  is a schematic side sectional view of a printer embodying the image forming apparatus of the invention.  FIG. 2  is a partly cut-away side view of the printer, illustrating a situation where the insertion of a process unit  2  into a body housing  1  is started. 
     Referring to  FIG. 1 , the process unit  2  having a photosensitive member cartridge  3  and a developer cartridge  4  is removably disposed in a substantially central portion of the body housing  1  of the printer. As shown in a left portion of  FIG. 1 , a fixer  5  is disposed adjacent to the process unit  2 . A sheet feeder  6  is disposed below the process unit  2 . A sheet cassette  8  is attachable to a lower portion of the body housing  1  by moving the sheet cassette  8  from a front face of the body housing  1  (as indicated by arrow A). 
     A laser scanning unit  7  is mounted to a lower surface of a synthetic resin-made discharge sheet tray  1   a , via a frame. The discharge sheet tray  1   a  also serves as a cover. 
     When print data is transmitted to the printer from an external apparatus, for example, a personal computer, upon a print instruction, a sheet P (recording medium) is separated from a stack of sheets P on a support plate  9  of the sheet cassette  8  by a separator pad  11 , in a manner of one sheet at a time, as a sheet feed roller  10  of the sheet feeder  6  rotates. The separated sheet P is conveyed to a contact portion between a photosensitive drum  13  (photosensitive member) in the process unit  2  and a transfer roller  14  (transfer device) pressed against a lower face of the photosensitive drum  13 , via a pair of register rollers  12   a ,  12   b.    
     A laser beam is emitted from the laser scanning unit  7  having a laser light-emitting portion, a polygon mirror  18 , a lens  19 , a plurality of reflecting mirrors  20  and the like, through a light-emitting hole formed in a lower portion of the frame supporting the laser scanning unit  7 . The laser beam travels to an upper peripheral surface portion of the photosensitive drum  13 , via a light entrance portion  31  formed in a case  30  of the photosensitive member cartridge  3 , that is, a case of the process unit  2 . The peripheral surface of the photosensitive drum  13  is thereby exposed corresponding to the print data, so as to form an electrostatic latent image. 
     Developer (toner) supplied from a developing roller  22  of the developer cartridge  4  becomes deposited on the electrostatic latent image on the photosensitive drum  13 , thereby making the image visible. After the visible image formed by developer (toner) is transferred from the photosensitive drum  13  to the sheet P, the sheet P is conveyed between a heat roller  15  and a presser roller  16  in the fixer  5 , in which the sheet P is subjected to a heat-fixing process. The sheet P is then discharged onto the discharge sheet tray  1   a  via a sheet discharge passage  17 . 
     In this embodiment, the process unit  2  includes the photosensitive member cartridge  3  having at least the photosensitive drum  13 , and the developer cartridge  4  having at least the developing roller  22  (developing device) that is disposed in a case  21 . The developer cartridge  4  is designed so that the developer cartridge  4  is detachably attachable to the photosensitive member cartridge  3 , and so that the developing roller  22  is prevented from detaching by a lock device  46  described later. 
     The structures of the photosensitive member cartridge  3  and the developer cartridge  4  will be described in detail. As shown in  FIGS. 3 through 7 , the synthetic resin-made case  30  in the photosensitive member cartridge  3  is integrally formed with a bottom wall  30   a , a pair of side walls  30   c  extending upwardly from the right and left sides of the bottom wall  30   a , an upper wall  30   b  connecting the upper edges of the pair of side walls  30   c , and a rear wall  30   e  connected to one side of the upper wall  30   b , an end of each of the right and left side walls  30   c , and one side of the bottom wall  30   a . The photosensitive drum  13  is set in the case  30  near one side thereof, that is, in an area that is surrounded with the upper wall  30   b , the rear wall  30   e , the right and left side walls  30   c , and the bottom wall  30   a . The photosensitive drum  13  is rotatably journaled in the right and left side walls  30   c . The right and left side walls  30   c  and the bottom wall  30   a  extend forward of the photosensitive drum  13 . 
     As shown in  FIG. 4 , a lower portion of the transfer roller  14  disposed below the photosensitive drum  13  is covered with the bottom wall  30   a  of the case  30 . The transfer roller  14  is vertically movably journaled in such a manner that the transfer roller  14  can separate from a lower surface of the photosensitive drum  13  due to the weight of the transfer roller  14 . When the process unit  2  is set into the body housing  1 , the transfer roller  14  contacts the lower surface of the photosensitive drum  13 . More specifically, upwardly open “U”-shaped bearings  35  fitted to both end portions of the shaft  14   a  of the transfer roller  14  are raised by shaft bearing raisers  34  (see  FIGS. 8 and 9 ) that are urged upward by springs  33  disposed at the right and left side inner faces of the body housing  1 , so that the transfer roller  14  is moved upward to press the lower surface (transfer region) of the photosensitive drum  13 . On the other hand, when the process unit  2  is removed from the body housing  1 , the transfer roller  14  separates from the photosensitive drum  13  without receiving the action of the springs  33 . 
     The upper wall  30   b  of the case  30  covers an upper portion of the photosensitive drum  13 . The light entrance portion  31 , allowing irradiation of an upper surface of the photosensitive drum  13  with laser light emitted from the laser scanning unit  7 , is formed in the upper wall  30   b , and is elongated in the directions of an axis of the photosensitive drum  13 . Disposed adjacent to the light entrance portion  31  is a charger  36 , such as a scorotron or the like, that charges a photosensitive surface of the photosensitive drum  13 . An electrostatic latent image is formed on the surface of the photosensitive drum  13  by scanning laser light over the surface of the photosensitive drum  13  uniformly charged by the charger  36 . After the electrostatic latent image is made visible (developed) by deposition thereon of a thin layer of toner supplied via the developing roller  22 , the toner image is transferred to the sheet P fed in between the photosensitive drum  13  and the transfer roller  14 , which press against, and contact with, each other. 
     A portion of the case  30 , other than the upper wall  30   b , is open upward. The portion is defined by the right and left side walls  30   c , and the bottom wall  30   a  having a curved surface with a substantially quarter of a circle. The upwardly open portion is an accommodating portion  32  into which the developer cartridge  4  can be set at an angle from above. 
     With the above-described structures, almost all of the outer surfaces of the photosensitive drum  13  are covered with the rigidly and integrally formed case  30 , so that stability, when an operator handles the photosensitive drum  13  by hand, is increased. 
     Since the bottom wall  30   a  of the case  30  extends toward the direction away from the photosensitive drum  13 , the circumference of the photosensitive drum  13  is not touched, even when the operator holds the extending portion of the bottom wall  30   a . Therefore, when the operator handles the photosensitive member cartridge  3  by hand, the photosensitive surface of the photosensitive drum  13  remains untouched, so that the surface of the photosensitive drum  13  will not be contaminated and instead will always be kept clean. 
     In particular, when the developer cartridge  4  is connected to the photosensitive member cartridge  3 , the bearings  23   a ,  23   b  contact and move along a first section of the guide grooves  37  in a direction that includes a vertical component. The bearings  23   a ,  23   b  then contact and move along a second section of the guide grooves  37  in a direction that is substantially horizontal. The difference of direction of movement of the bearings  23   a ,  23   b  is due to the arcuate shape of the guide grooves  37 . Also, because of this arcuate shape, the bearings  23   a ,  23   b  move faster along the first section of the guide grooves then along the second section. 
     An urging device  42  that presses the developing roller  22  against the photosensitive drum  13  via the developer cartridge  4  is pivotably and expandably mounted to an inner surface of each of the right and left side walls  30   c . As shown in  FIGS. 4 and 10 , each urging device  42  has a pivot fulcrum member  39  provided with pivots  39   a ,  39   b  protruding integrally from the right and left sides thereof, a frame-like slide support member  40  that connects with and supports the pivot fulcrum member  39  that facilitates sliding movements therein, and an urging spring device  41 , such as a coil spring or the like, that is disposed in the frame of the slide support member  40  so as to urge the pivot fulcrum member  39  toward one end. The slide support member  40  has a cylindrical motion acting portion  43  extending laterally. The motion acting portion  43  of each urging device  42  is disposed so that the portion  43  protrudes outward from a guide hole  44  formed in the corresponding one of the right and left side walls  30   c.    
     The lock device  46  that prevents the developer cartridge  4 , which is fitted into the accommodating portion  32 , from moving upward out of the accommodating portion  32 , is disposed at an inner side of one of the right and left side walls  30   c  (the right side wall in the embodiment) of the photosensitive member cartridge  3 . As shown in  FIGS. 4 ,  5 ,  11 ( a ) and  11 ( b ), the lock device  46  is designed so that a rotating shaft  48  extending through the side wall  30   c  axially supports a lock lever  47  in such a manner that the lock lever  47  is pivotable relative to a side surface of the lock lever  47 . 
     A resin-made spring  49  extending downward from a lower end of the lock lever  47  is disposed so that a lower portion of the resin-made spring  49  contacts a restriction piece  30   d  protruding upward from the bottom wall  30   a  of the case  30 . A lower surface of the lock lever  47  has an arched contact portion  47   a  that restricts upward motion of one of action-receiving portions  61  (right-side one) protruding outward from the right and left side surfaces of the case  21  of the developer cartridge  4 , by contacting an upper surface of the action-receiving portion  61 . The action-receiving portions  61  have a generally inverted triangular shape in side view. 
     The action-receiving portions  61  are disposed so that they communicate with the lock device  46  and, furthermore, urging devices  42  that press the developing roller  22  against the photosensitive drum  13 . 
     Rollers  50  are disposed, as receiving members, at a plurality of positions (two positions in the embodiment, that is, at right and left end portions) in the bottom wall  30   a  of the case  30 , in the accommodating portion  32  of the photosensitive member cartridge  3  (see  FIGS. 3 and 4 ). The rollers  50  protrude from the inner surface of the bottom wall  30   a . When the developer cartridge  4  is inserted down into the accommodating portion  32 , the rollers  50  (receiving device) receive a portion of the weight of the developer cartridge  4 , at a side across the developing roller shaft  22   a  of the developing roller  22  from the photosensitive drum  13 , that is, a side of the developing roller shaft  22   a  of the developing roller  22  opposite from the photosensitive drum  13 . More specifically, the rollers  50  contact lower portions of a downward-convex curved surface of a toner containing chamber  24  (see  FIGS. 1 ,  12 ( a ) and  12 ( b )) formed in the case  21  of the developer cartridge  4 , and the rollers  50  reduce the rattling of the case  21  during the setting or removing operation. 
     The upper roller  12   a  of the pair of register rollers is mounted to the bottom wall  30   a  of the case  30  of the photosensitive member cartridge  3  in such a manner that the upper roller  12   a  is prevented from detaching. The bottom wall  30   a  has a laterally elongated introduction hole  51  adjacent to the upper roller  12   a , to introduce the sheet P from the pair of register rollers  12   a ,  12   b  into a transfer portion  52  between the photosensitive drum  13  and the transfer roller  14 . The upper surface of a portion of the bottom wall  30   a  extending between the introduction hole  51  and the transfer portion  52  has many ribs  53  extending in a direction from the introduction hole  51  to the transfer portion  52 . The ribs  53  are designed so that the sheet P can be smoothly conveyed with a reduced contact resistance on the lower surface of the sheet P. 
     A discharge hole  51   b  is formed between the bottom wall  30   a  and the rear wall  30   e  of the case  30 , to convey the sheet P between a heat roller  15  and a presser roller  16 , through the transfer portion  52  (see  FIG. 4 ). 
     The structure of the developer cartridge  4  will be described with reference to  FIGS. 1 ,  12   a ,  12   b  and  13  through  15 . After toner in the downward-convex toner containing chamber  24  in the case  21  is stirred by a rotationally driven stirrer  27  and discharged therefrom, toner is carried onto the outer peripheral surface of the developing roller  22  via a supply roller  25 . A blade  26  is provided for restricting the layer thickness of toner on the developing roller  22  (see  FIG. 1 ). The generally inverted triangular-shaped action-receiving portions  61  protrude from the right and left side outer ends of the toner containing chamber  24  in the case  21 . 
     The shaft bearings  23   a ,  23   b  rotatably fitted to the right and left end portions of the developing roller shaft  22   a  are formed from a material whose friction coefficient is small, such as an acetal resin or the like. Each of the bearings  23   a ,  23   b  has an engaging nail  62  that engages with an annular groove  63  so that the bearing will not detach from the shaft end. Each of the shaft bearings  23   a ,  23   b  has, at its base end side, an umbrella-shaped (conical) shaft diameter adjusting portion  64  whose diameter gradually increases. 
     At least one of the shaft bearings  23   a ,  23   b  (the right-side bearing  23   b  in the embodiment) is slidingly urged laterally outward by a spring device  65  (see  FIG. 15 ). Therefore, when the developer cartridge  4  is set at a predetermined position with respect to the photosensitive member cartridge  3 , the developing roller shaft  22   a  of the developing roller  22  are supported, without rattling, on the guide grooves  37  formed in the right and left side walls  30   c  of the photosensitive member cartridge  3 . 
     As shown in  FIGS. 16 and 19 , the case  21  of the developer cartridge  4  has, in an upper surface portion and a lower surface portion thereof, grip portions  70 ,  66 , respectively, that facilitate the handling of the developer cartridge  4  by an operator. 
     Further, as shown in  FIGS. 17 and 18 , the case  30  of the photosensitive member cartridge  3  has foot portions  69   a ,  69   b  at the lower portions thereof, so that the photosensitive member cartridge  3  can be stably placed on the table  67  regardless of whether the photosensitive member cartridge  3  stands alone or is combined with the developer cartridge  4 . The foot portions  69   a ,  69   b  need to be provided diagonally at least two positions and, preferably, four positions at each corner of the case  30 , as in the embodiment. More foot portions  69   a ,  69   b  may be provided if the space is available on the case  30 . 
     In the embodiment, as shown in  FIGS. 5-7 ,  17 , and  18 , when the photosensitive member cartridge  3  is viewed from the side, the foot portions  69   a ,  69   b  are provided so as to extend downwardly, at the lower portions of the case  30  near both ends of the forward and rearward sides thereof. The foot portions  69   a ,  69   b  contact the table  67  with the center of gravity of the photosensitive member cartridge  3  placed between the foot portions  69   a ,  69   b , so that the photosensitive member cartridge  3  is stabilized when it is placed on the table  67 . 
     The case  30  is formed with the upwardly open accommodating portion  32  near one side thereof. The developer cartridge  4  can be removably set into the accommodating portion  32  from above, while the photosensitive member cartridge  3  is placed on the flat surface of the table  67  in a stable posture via the foot portions  69   a ,  69   b . Therefore, replacement of the developer cartridge  4  can be easily performed. 
     Preferably, the foot portions  69   a ,  69   b  are provided at the forward and rearward sides of the case  30 , so as to place the centers of gravity of both the developer cartridge  4  and the photosensitive member cartridge  3  between the foot portions  69   a ,  69   b . With such arrangements, the developer cartridge  4  can be set in the photosensitive member cartridge  3 , which is placed on the table  67 , without causing unstable actions of the photosensitive member cartridge  3 , such as swinging. Further,  FIG. 7  shows the foot portions  69   a ,  69   b  provided at the right and left side end portions of the lower surface of the case  30 . The foot portions  69   a ,  69   b  may be provided at the lowermost portions of the case  30  so as to protrude toward the left and right sides. 
     The operation of setting the developer cartridge  4  with respect to the photosensitive member cartridge  3  will be described. To set the developer cartridge  4  with respect to the photosensitive member cartridge  3 , an operator holds with one hand, the grip portions  70 ,  66  formed on an upper surface portion and a lower surface portion of the case  21  of the developer cartridge  4 . When the developer cartridge  4  is inserted, with the developing roller  22  side being a leading side, into the accommodating portion  32  of the case  30  (inserting operation), one of the action-receiving portions  61  of the developer cartridge  4  contacts the lock lever  47 , and turns the lock lever  47  to a position indicated by a two-dot line in  FIG. 4 , against the force of the resin-made spring  49 . When the developer cartridge  4  is thus inserted (the inserting operation is completed), the action-receiving portion  61  is lowered and, therefore, the contact between the action-receiving portion  61  and the lock lever  47  discontinues. Therefore, the lock lever  47  is returned to the position indicated by a solid line in  FIG. 4 , by the restoration force of the resin-made spring  49 , so that the contact portion  47   a  of the lock lever  47  faces an upper surface of the action-receiving portion  61 , thereby establishing a locked state. The process unit  2  integrated with the developer cartridge  4  and the photosensitive member cartridge  3 , can be raised with one hand by holding the grip portions  70 ,  66 . That is, the operator sequentially can set the process unit  2  into a predetermined portion of the body housing  1  while holding the grip portions  70 ,  66  after the developer cartridge  4  is set with respect to the photosensitive member cartridge  3 . 
     During the insertion of the developer cartridge  4  into the accommodating portion  32 , the shaft bearings  23   a ,  23   b  disposed at the right and left side ends of the developing roller  22  slide down along the guide grooves  37  formed along the upper edges of the right and left side walls  30   c  of the case  30 , so as to approach the shaft  13   a  of the photosensitive drum  13 . The case  21  of the developer cartridge  4  is pivotable about the bearings  23   a ,  23   b  of the developing roller shaft  22   a  of the developing roller  22  when the bearings  23   a ,  23   b  are supported by the guide grooves  37 . Therefore, when the bearings  23   a ,  23   b  come to a position on the guide grooves  37  close to the shaft  13   a  of the photosensitive drum  13  (“U”-shaped portions of the guide grooves  37 ), a toner containing chamber  24  side portion of the case  21  of the developer cartridge  4  comes into the accommodating portion  32  of the photosensitive member cartridge  3  by a pivoting motion about the developing roller shaft  22   a  of the developing roller  22 . Thus, the developer cartridge  4  is fittingly set in the accommodating portion  32 . 
     In this state, the rollers  50  provided on the inner surface of the accommodating portion  32  slidingly contact a lower surface portion of the case  21  on the side of the toner containing chamber  24 , and the bearings  23   a ,  23   b  move along the guide grooves  37  to such a position that the developing roller  22  is located substantially closest to the photosensitive drum  13 . When the lock lever  47  pivots clockwise to return from the position indicated by the two-dot line to the position indicated by the solid line in  FIG. 4 , the contact portion  47   a  of the lock lever  47  faces the upper surface (protrusion  61   a ) of the action-receiving portion  61 , so that the developer cartridge  4  is prevented from moving out of the photosensitive member cartridge  3 . 
       FIGS. 16 through 19  are a plan view, a right side view, a left side view, and a rear view (a view of the side of the sheet discharge opening) of the process unit  2  with the developer cartridge  4  set with respect to the photosensitive member cartridge  3 . Since the urging devices  42  are pressed downward by the action-receiving portions  61  as the developer cartridge  4  is pushed downward into the process unit  2 , each urging device  42  normally assumes such a orientation that the motion acting portion  43  is relatively lowered, as shown in  FIGS. 2 and 4 . 
     The process unit  2  is designed so that the process unit  2  can be set into, and removed from, the body housing  1  when a lid  1   b  provided in the right-side end in  FIG. 1  (a front face) of the body housing  1  is downwardly turned to form a large opening (see  FIG. 2 ). 
     That is, as shown in  FIGS. 2 ,  20 ( a ),  20 ( b ) and  21 , a pair of right and left guide devices  55 , made of a resin or the like, are fixed to the inner surfaces of the right and left side portions of the body housing  1  (in the drawings, only the right-side guide device  55  is shown). Each guide device  55  has a rising slope surface that is open upward and extends upwardly inward from the right-side end of the body housing  1 , and an upper-side guide surface  55   a  extending downwardly inward from a summit  55   d  of the rising slope surface, and a lower-side guide surface  55   b  that is disposed below the upper-side guide surface  55   a  and that extends downwardly inward from the right-side end of the body housing  1 , and ends at a position near the lower roller  12 ( b ) of the pair of register rollers. 
       FIG. 2  indicates a position at which the insertion of the process unit  2  into the body housing  1  is started. The process unit  2  is inserted into the body housing  1  so that the shaft  13   a  of the photosensitive drum  13  approaches an upper inward end portion  55   c  of the upper-side guide surface  55   a . Subsequently, as shown in  FIG. 20(   a ), the motion acting portion  43  of each urging device  42  contacts the rising slope surface of the upper-side guide surface  55   a , and each of guiding members  56 , protruding laterally from lower portions of the right and left sides of the process unit  2  (photosensitive member cartridge  3 ), fits into the lower-side guide surface  55   b  (only one of the guiding members  56  is shown in the drawings), so that the process unit  2  is prevented from moving upward relative to the body housing  1 , and is allowed only to be moved further inward along the lower-side guide surface  55   b.    
     In this state, as the process unit  2  is pushed inward, the motion acting portion  43  of each urging device  42  is raised by the corresponding upper-side guide surface  55   a , so that the slide support member  40  of each urging device  42  pivots upward about the pivots  39   a ,  39   b . Therefore, the slide support member  40  of each urging device  42  restrains the corresponding one of the action-receiving portions  61  of the developer cartridge  4  in such a direction that a distal end portion of the slide support member  40  pushes the action-receiving portion  61 . When the motion acting portion  43  of each urging device  42  comes to the summit  55   d  of the upper-side guide surface  55   a , the slide support member  40  of each urging device  42  is pivoted to a most upward orientation. In this orientation, the distal end portion of the slide support member  40  of each urging device  42  keeps restraining the corresponding action-receiving portion  61  (see  FIG. 20(   b )). 
     As the process unit  2  is pushed further inward, the shaft  13   a  of the photosensitive drum  13  is set to a predetermined position in the inward end portion  55   c  of each upper-side guide surface  55   a . At this position, an operator releases the process unit  2  from the hands, so that the developer cartridge  4  side portion of the process unit  2  is lowered and set by the weight the process unit  2  into a state such that the upper register roller  12   a  disposed at a lower surface side of the case  30  of the photosensitive member cartridge  3  is placed on the lower register roller  12   b  disposed in the body housing  1 , and is pressed by a spring  45  shown in  FIG. 2  and, simultaneously, the guiding members  56  are supported at appropriate positions on the guide devices  55  (see  FIG. 21 ). 
     When the process unit  2  is set as described above, the distal end of the slide support member  40  of each urging device  42  presses the corresponding action-receiving portion  61 . Therefore, the developing roller  22  is pressed against the photosensitive drum  13  by the urging devices  42  and the action-receiving portions  61  via the developer cartridge  4 . 
     As shown in  FIG. 21 , it is preferable that a contact portion  72   a  between the photosensitive drum  13  and the developing roller  22 , that is, pressed portions thereof at the closest positions, (the contact portion  72   a  is on an inter-axis straight line  72  passing through the axis of the shaft  13   a  of the photosensitive drum  13  and the axis of the developing roller shaft  22   a  of the developing roller  22 ) be above or on a pressing action line  71  of the action of either one of the urging devices  42  onto the corresponding action-receiving portion  61  (that is, a straight line passing through the pivot  39   a  ( 39   b ) and a pressing point at which the distal end of the slide support member  40  contacts the action-receiving portion  61 ), or that the inter-axis straight line  72  substantially coincide with or extend substantially parallel to the pressing action line  71 . 
     As shown in  FIG. 22 , a gear mechanism  74  that transmits force from a drive motor  73  is disposed on the inner surface of one side (the left side in the embodiment) of the body housing  1 . The gear mechanism  74  rotates the sheet feed roller  10  of the sheet feeder  6 , the lower register roller  12   b , the developing roller  22 , the photosensitive drum  13 , the heat roller  15 , and conveying rollers in the sheet discharge passage. The developing roller  22  and the photosensitive drum  13  are rotated in opposite directions, as indicated in  FIGS. 1 and 21 , that is, the developing roller  22  is rotated counterclockwise and the photosensitive drum  13  is rotated clockwise. Furthermore, the mechanism is designed so that the circumferential velocity of the developing roller  22  is greater than that of the photosensitive drum  13 . 
     Therefore, as shown in  FIG. 32 , during image forming operation, the direction of a pressing force F 1  of each urging device  42  acting on the contact portion  72   a  is parallel to the pressing action line  71 , and the pressing force F 1  is split into a component F 1 V in a direction of the tangent of the circumferences of the developing roller  22  and the photosensitive drum  13  and a component F 1 H in a direction of the inter-axis straight line  72 . 
     The difference in circumferential velocity between the developing roller  22  and the photosensitive drum  13  creates a friction resistance force F 2  in an upward direction in  FIG. 32  with respect to the developing roller  22  and in a downward direction with respect to the photosensitive drum  13 , the friction resistance force F 2  having a value equal to a multiplication product of the friction coefficient and the component F 1 H of the pressing force in the direction of the inter-axis straight line  72  of the photosensitive drum  13  and the developing roller  22 . Therefore, the rotation moment caused by the friction resistance force F 2  acting on the developer cartridge  4  acts about the developing roller shaft  22   a  in a clockwise direction in  FIG. 32 . 
     If the pressing action line  71  extends above the developing roller shaft  22   a , a rotation moment acts on the developer cartridge  4  about the developing roller shaft  22   a  counter clockwise in  FIG. 32 , and reduces or offsets the rotation moment caused by the friction resistance force F 2 . In such a case, the developer cartridge  4  tends to rise, which is unfavorable. In this invention, however, the position of the developing roller shaft  22   a  is above or on the pressing action line  71 , or the inter-axis straight line  72  substantially coincides with, or extend substantially parallel to, the pressing action line  71 . Therefore, during image formation, the developing roller  22  is substantially prevented from rising, and stable pressing action can be achieved. 
     Furthermore, the pressing structure is formed by the urging devices  42  for pressing the developing roller  22  against the photosensitive drum  13 , and the action-receiving portions  61  provided on the developer cartridge  4 . The action-receiving portions  61  are disposed at a side of the developing roller  22 , the side being remote from the photosensitive drum  13 . Therefore, the photosensitive drum  13 , the developing roller  22  and the drive mechanisms will not become impediments, and the pressing action line  71  and the inter-axis straight line  72  can easily be set substantially parallel to each other and adjacent to each other. 
     If the developer cartridge  4  is supported pivotably about the developing roller shaft  22   a  of the developing roller  22  while image forming operation is being performed, with the developer cartridge  4  set with respect to the photosensitive member cartridge  3 , that is, if the arrangement is set such that during image forming operation, the bearings  23   a ,  23   b  on both ends of the developing roller shaft  22   a  of the developing roller  22  are stopped at the inward sides of the guide grooves  37  of the photosensitive member cartridge  3  and therefore prevented from moving further inward, and the weight Wo of the developer cartridge  4  is dispersedly supported at two positions in a side view, that is, the position of the shaft bearing  23   a  ( 23   b ), and a position at a side of the developing roller shaft  22   a  remote from the photosensitive drum  13 , for example, a position at which the bottom of the toner containing chamber  24  contacts the rollers  50 , or the like, so that split loads W 1 , W 2  of the weight Wo are supported at the two positions (see  FIG. 32 ), then the direction of the rotation moment about the axis (the developing roller shaft  22   a ) of the developing roller  22  caused by the friction resistance force F 2  received by the developing roller  22  from the photosensitive drum  13  becomes the same as the direction of the rotation moment about the axis (the developing roller shaft  22   a ) of the developing roller  22  caused by the weight (load Wo at the center of gravity G) of the developer cartridge  4 , that is, the clockwise direction in  FIG. 32 . Therefore, during image forming operation, the aforementioned friction resistance force F 2  does not act as a moment in such a direction as to raise the developer cartridge  4  against the weight (load Wo at the center of gravity G) of the developer cartridge  4 , so that the behavior of the developer cartridge  4  stabilizes. 
     In the above-described structure, the urging devices  42  are disposed at positions that are within the accommodating portion  32  in the photosensitive member cartridge  3 , and that become remote from the developing roller  22  of the developer cartridge  4  when the developer cartridge  4  is set in the accommodating portion  32 . The action-receiving portions  61  protrude outward from outer walls of the developer (toner) containing chamber  24 , which is disposed at a side remote from the developing roller  22 . Therefore, these component parts are unlikely to interfere with the operations of inserting or removing the developer cartridge  4 . 
     In the embodiment of the invention, the urging devices  42 , having a relatively complicated structure are disposed in the photosensitive member cartridge  3 , which requires less frequent replacement. The action-receiving portions  61  having a relatively simple structure, are disposed in the developer cartridge  4 , which requires frequent replacement. Therefore, the production costs of the process unit  2  and the developer cartridge  4  can be reduced, and the running cost can be reduced. Furthermore, since the protruded action-receiving portions  61  provided on the developer cartridge  4  are integral with the side surfaces of the case  21 , the action-receiving portion  61  can be formed together with the case  21 , thereby reducing the production cost and, further, making it easier for the action-receiving portions  61  to be pressed by the slide support members  40  of the urging devices  42  provided on the photosensitive member cartridge  3 . 
     The right and left urging devices  42  are disposed on inner surfaces of the right and left sides of the photosensitive member cartridge  3 , and are connected thereto in such a manner that the urging devices  42  are capable of changing the orientation between a direction of the urging force thereof and a non-urging direction. Therefore, if the developer cartridge  4  is simply placed over or adjacent to the photosensitive member cartridge  3 , the developing roller  22  is not pressed against the photosensitive drum  13 . Hence, the photosensitive member cartridge  3  and the developer cartridge  4  can be combined as a process unit  2  for packing and shipping, while obviating the danger of permanent deformation of the outer peripheral surface of the process unit  2 , or the danger of contamination of the photosensitive drum  13  with material components from the developing roller  22 . 
     Still further, since each urging device  42  is substantially made up of the pivot fulcrum member  39 , the slide support member  40  slidable relative to the pivot fulcrum member  39 , and the urging spring device  41  disposed between the two members, the urging devices  42  gain an increased degree of freedom in changing the orientation between the urging direction and the non-urging direction, compared with a conventional device that employs an urging spring to directly press an action-receiving portion and discontinue the pressing. Another advantage that the operations of pressing the action-receiving portions  61 , and discontinuing the pressing, can be reliably performed by the slide support members  40  can also be achieved. 
     Further, since each urging device  42  is rotatably connected at its pivot fulcrum member  39  to the inner surface of the right or left side of the case  30  of the photosensitive member cartridge  3 , most of the component parts of the urging devices  42  are unexposed outside the case  30 , regardless of whether the photosensitive member cartridge  3  stands alone or is combined with the developer cartridge  4  into the process unit  2 . Therefore, the danger of accidentally hitting and breaking any component part of the urging devices  42  is considerably reduced, and the ease of handling improves. 
     The slide support member  40  of each urging device  42  is provided integrally with the pin-like motion acting portion  43  protruding laterally to guide the slide support member  40  into the urging direction and the non-urging direction. The motion acting portions  43  of the urging devices  42  protrude outward from guide holes  44  in the right and left sides of the case  30  of the photosensitive member cartridge  3 . Thus, most of the component parts of the urging devices  42  are unexposed outside the case  30 . Therefore, the danger of accidentally hitting and breaking any component part of the urging devices  42  is considerably reduced, and the ease of handling improves. 
     The action-receiving portions  61  protruding outwardly from the right and left sides of the case  21  of the developer cartridge  4  perform the function to be pressed by the urging devices  42  and the function to cooperate with the lock lever  47  of the lock device  46  to prevent the developer cartridge  4  from rising relative to the photosensitive member cartridge  3 . Since the action-receiving portions  61  thus perform the two functions, the predetermined cost of the developer cartridge  4  can be considerably reduced. 
     The process unit  2  is designed so as to be removably set into the body housing  1  of the image forming apparatus. The body housing  1  is provided with the guide devices  55  that guide the urging devices  42  and switch the urging devices  42  between the urging state and the non-urging state. Therefore, the orientation of the urging devices  42  and the operation thereof will be changed or switched simply by setting the process unit  2  into the body housing  1  or removing the process unit  2  from the body housing  1 . Thus, the ease of operation considerably improves. 
     The upper wall  30   b  of the case  30  covers an upper portion of the photosensitive drum  13 . The rear wall  30   e , extending downwardly from the upper wall  30   b , covers a rear portion of the photosensitive drum  13 . The bottom wall  30   a  covers a lower portion of the transfer roller  14  disposed below the photosensitive drum  13 . Therefore, exposed portions on the photosensitive drum  13  are very small. The right and left side walls  30   c  cover each end of the transfer roller  14  and the photosensitive drum  13 . The case  30  is rigidly and integrally formed of synthetic resin, so that an operator will not touch the surface of the photosensitive drum  13 . The case  30  also prevents dust from adhering to the photosensitive drum  13 . The operator can securely handle the photosensitive drum  13 . 
     Further, the bottom wall  30   a  of the case  30  extends toward the direction away from the photosensitive drum  13 . Holding such an extending portion of the bottom wall  30   a  and the rear wall  30   e  by both hands, an operator will not touch the photosensitive drum  13 , and can stably handle the photosensitive member cartridge  3 . While the operator is holding the extending portion of the bottom wall  30   a  and the rear wall  30   e , the bottom wall  30   a  does not flex since the case  30  is a rigid body having a cross-sectional profile which is substantially concave, with the bottom wall  30   a  connected to the right and left side walls  30   c  of the case  30 . Therefore, the case  30  can be securely handled. 
     Also, the light entrance portion  31 , allowing irradiation of an upper surface of the photosensitive drum  13  with laser light emitted from the laser scanning unit  7 , and the charger  36  that charges a photosensitive surface of the photosensitive drum  13  provided in the upper wall  30   b , make the case  30  sturdy. 
     Since the upwardly open accommodating portion  32  is formed by extending the bottom wall  30   a  and the right and left side walls  30   c  to accommodate the developer cartridge  4  therein, the handling of the photosensitive member cartridge  3  with the developer cartridge  4  set therein can be easily performed. 
     The accommodating portion  32  covers the lower surface and the peripheral surfaces of the developer cartridge  4  when the developer cartridge  4  is set therein. Also, the accommodating portion  32  is open upwardly, so that the case  21  of the developer cartridge  4  set in the accommodating portion  32  is detached therefrom in such a direction that the developing roller  22  travels away from the photosensitive drum  13 . Thus, the operation of setting the developer cartridge  4  into the accommodating portion  32 , or removing the developer cartridge  4  from the accommodating portion  32 , can be easily performed. 
     The bottom wall  30   a  has an introduction hole  51   a  to introduce the sheet P. The upper surface of a portion of the bottom wall  30   a  has many extending ribs  53  to smoothly convey the sheet P from the introduction hole  51   a  to the contact portion, between the lower surface of the photosensitive drum  13  and the upper surface of the transfer roller  14 . The ribs  53  guide the sheet P and increase the rigidity to the bottom wall  30   a . When the developer cartridge  4  is set into the accommodating portion  32 , the ribs  53  face the lower surface of the developer cartridge  4 . The space between the ribs  53  and the lower surface of the developer cartridge  4 , becomes the path of the sheet P. Thus, the sheet conveying structures are simplified. 
     If the area of the bottom wall  30   a  covering the lower portion of the transfer roller  14  is increased, the wider portions of the exposed surfaces of the transfer roller  14  and the photosensitive drum  13  are covered, without increasing resistance when the sheet P is conveyed, since the introduction hole  51   a  and the discharge hole  51   b  are provided. 
     Even if the photosensitive member cartridge  3  remains removed from the body hosing  1  for a long time, the danger of permanent deformation of the outer peripheral surface of the transfer roller  14 , or the danger of contamination of the photosensitive drum  13  with chemicals from the transfer roller  14 , is obviated, since the photosensitive member cartridge  3  has the rigid case  30  on the peripheral surface thereof. 
     When the case  30  is set in a predetermined position with respect to the body housing  1 , the bearings  35  fitted to both end portions of the transfer roller  14  are raised by the shaft bearing raisers  34  and the springs  33 , so that the transfer roller  14  is moved upward to press the photosensitive drum  13 . By the simple operation of setting the case  30  into the body housing  1 , or removing the case  30  from the body housing  1 , pressures can be applied to, or removed from, the transfer portion  52 . 
     When the photosensitive member cartridge  3  is removed from the body housing  1 , the transfer roller  14  separates from the lower surface of the photosensitive drum  13  due to the weight of the transfer roller  14 . Therefore, even when a sheet P becomes jammed at the transfer portion  52 , the sheet P restrained at the transfer portion  52  can be released by only removing the photosensitive member cartridge  3  from the body housing  1 . Then, by removing the sheet P from the transfer portion  52  through the introduction hole  51   a , or the discharge hole  51   b , the paper jam can be easily cleared. 
     Register rollers  12   a ,  12   b  are a pair of rollers to convey the sheet P supplied from the sheet feeder  6  to the contact portion between the photosensitive drum  13  and the transfer roller  14 , while timing is being provided. The upper register roller  12   a  is provided on the side of the process unit  2 . The lower register roller  12   b  is provided on the side of the body housing  1 . In this embodiment, the process unit  2  is designed so that the process unit  2  can be removably set in the body housing  1  from the front face thereof When both register rollers  12   a  and  12   b  are provided to the body housing  1  as in a conventional apparatus, a portion of the case  30  covering the transfer roller  14  comes into contact with the upper register roller  12   a  when the process unit  2  is removed from the front face of the body housing  1 , as can be seen from  FIG. 1 . This makes the removal of the process unit  2  from the body housing  1  difficult. 
     By providing the upper register roller  12   a  on the side of the process unit  2 , the removal of the process unit  2  from the front face of the body housing  1  can be facilitated. The process unit  2  is set in the body housing  1  while moving inside the body housing  1  in a direction substantially parallel to the feeding direction of the sheet P conveyed by the pair of register rollers  12   a ,  12   b.    
     However, when the upper register roller  12   a  and the lower register roller  12   b  are provided on the sides of the process unit  2  and the body housing  1 , respectively, to facilitate the setting of the process unit  2  into the body housing  1  or the removal of the process unit  2  from the body housing  1 , it is important to maintain the appropriate positions, angles, and pressing pressures of the upper and the lower register rollers  12   a  and  12   b  with respect to each other to stably convey the sheet P. 
     Even when the process unit  2  is replaced with a new one, the new process unit  2  needs to be set in the body housing  1  while the appropriate positions, angles, and the pressing pressures of the upper and the lower register rollers  12   a  and  12   b  are being maintained with respect to with each other. 
     In the embodiment, the positioning mechanism of the pair of register rollers  12   a ,  12   b  is constructed as described below. 
     As shown in  FIG. 23 , the upper register roller  12   a  is provided so as to expose its surface from the bottom opening  300   a  of the case  30 . Unlike a general roller journaled in the side surfaces of an apparatus body or case, the register roller  12   a  is provided in the following manner. As shown in  FIG. 24 , which is a enlarged view of the circled portion B in  FIG. 23 , protrusions  700   a  of a bearing  700  fitted to the register roller  12 , contact supporting portions  300   b  integrally formed with the case  30 . The register roller  12   a  is disposed so as to vertically move in the directions indicated by an arrow in  FIG. 24  (the direction such that the register roller  12   a  is pressed against the register roller  12   b  and the opposite direction). 
     As shown in  FIG. 25B , the register roller  12   a  is a shaft formed by plated iron or stainless steel. The length of the register roller  12   a  is longer than the width of the sheet P. Bearings  700 ,  710  made of resin are disposed to each end of the register roller  12   a . As shown in  FIGS. 25A and 27A  through  27 F, the bearing  700  is provided with a integrally-formed cylindrical portion  700   b  with an insertion hole  700   c  for inserting an end of the register roller  12   a  and a supporting portion  700   d  formed with protrusions  700   a.    
     To the other end of the register roller  12   a , the bearing  710  is disposed. As shown in  FIGS. 25C and 28A  through  28 E, the bearing  710  has substantially the same shape as the bearing  700 , except for a cavity  710   e . That is, the bearing  710  is provided with a integrally-formed cylindrical portion  710   b  with an insertion hole  710   c  for inserting an end of the register roller  12   a  and a supporting portion  710   d  formed with protrusions  710   a  and the cavity  710   e , as shown in  FIG. 28D . Due to the cavity  710   e , the supporting portion  710   d  is easy to flex inward (toward the central portion thereof), as shown by arrows in  FIG. 28D . 
     After the above-described bearings  700 ,  710  are fitted to each end of the register roller  12   a , as shown in  FIG. 25B , the register roller  12   a  is mounted to the case  30  as described below. 
     As shown in  FIG. 29 , the bearing  700  is first inserted into a gap  300   e  defined between a side wall  300 C of the case  30  and a bottom wall  300   d  that covers an end portion of the register roller  12   a , at such an angle indicated in  FIG. 29 . Then, the bearing  700  is pushed to the direction indicated by the arrow G. As shown in  FIG. 29 , a chamfered portion  700   f  having a slanted surface is formed on the upper end portion of the supporting portion  700   d  above the protrusions  700   a . The chamfered portion  700   f  smoothly makes contact with the corner of the bottom wall  300   d , so that the bearing  700  can be smoothly fitted into the gap  300   e.    
     As shown in  FIGS. 24 and 27D , an inclined surface  720  is formed on the protrusion  700   a . The inclined surface  720  smoothly makes contact with the supporting portions  300   b , so that the bearing  700  can be easily fitted into the gap  300   e . The thus fitted bearing  700  into the gap  300   e  is shown in  FIG. 30 . 
     To fit the bearing  710  into the gap  300   e , the supporting portion  710   d  is flexed inward (toward the central portion thereof) as indicated by the arrows in  FIG. 28D . While flexing the supporting portion  710   d , so as to narrow the width thereof, the bearing  710  is pushed into the gap  300   e . The flexible supporting portion  710   d  makes the insertion of the bearing  710  into the gap  300   e  easy. An inclined surface  730  is also formed on the protrusions  710   a  of the bearing  710 , as shown in  FIG. 28D . The inclined surface  730  smoothly makes contact with the supporting portions  300   b , so that the bearing  710  can be easily fitted into the gap  300   e.    
       FIGS. 25D through 25I  show the bearings  700 ,  710  fitted into the gaps  300   e  in such a manner as described above. As shown in  FIGS. 25D through 25I , when the register roller  12   a  is moved up to the highest position where the cylindrical portions  700   b ,  710   b  of the bearings  700 ,  710 , respectively, contact the lower ends of the side walls  300   c , the further upward movement of the register roller  12   a  is restricted due to the bearings  700 ,  710  outwardly extending from the side wall  300   c . As shown in  FIGS. 26A through 26D , which are enlarged views of the bearings  700 ,  710  and their periphery shown in  FIGS. 25G-25I , when the register roller  12   a  is moved down to the lowest position due to its own weight, the further downward movement of the register roller  12   a  is restricted by contacting the protrusions  700   a ,  710   a  to the supporting portions  300   b.    
     The above-described structure enables the register roller  12   a  to move freely up and down in the direction such that the register roller  12   a  separates from and presses against the register roller  12   b.    
     As shown in  FIG. 1 , the register roller  12   b  has a shaft  12   c  formed of plated iron or stainless steel and an elastic layer  12   d  formed of urethane rubber and which is provided on the shaft  12   c . The shaft  12   c  is rotatably supported in the body housing  1 . The shaft  12   c  is connected to a motor via a gear (not shown). The timing of starting the rotation of the register roller  12   b  and the speed thereof are controlled by a controller (not shown). 
     As described above, the register roller  12   a  is provided so as to vertically move. The register roller  12   a  is positioned so as to maintain the appropriate position, angle, and the pressing pressure with respect to the register roller  12   b , when the process unit  2  is set in the body housing  1 . 
     With reference to  FIGS. 2 ,  9 ,  20 A,  20 B,  21 , and  31 , the operation of setting the process unit  2  into the body housing  1  and the mechanism of positioning the register roller  12   a  will be described below. 
     The process unit  2  is set into, and removed from, the body housing  1  when a lid  1   b , provided in the right-side end in  FIG. 1  (a front face) of the body housing  1 , is downwardly turned to form a large opening. As shown in  FIG. 2 , a pair of right and left guide devices  55 , made of resin, are fixed to the inner surfaces of the right and left side portions of the body housing  1 . Each guide device  55  has a rising slope surface that is open upward and extends upwardly inward from the right-side end of the body housing  1 , and an upper-side guide surface  55   a  extending downwardly inward from a summit  55   d  of the rising slope surface, and a lower-side guide surface  55   b  that is disposed below the upper-side guide surface  55   a  and that extends downwardly inward from the right-side end of the body housing  1 , and ends at a position near the register roller  12   b.    
       FIG. 2  indicates a position at which the insertion of the process unit  2  into the body housing  1  is started. When the process unit  2  is inserted into the body housing  1  from a position shown in  FIG. 2  to a position shown in  FIG. 20A , the process unit  2  is pushed inward so that the shaft  13   a  of the photosensitive drum  13  approaches an inward end portion  55   c  disposed inward from the upper-side guide surface  55   a . Subsequently, as shown in  FIG. 20A , the motion acting portion  43  of each urging device  42 , mounted to the process unit  2 , contacts the rising slope surface of the guide device  55 , and each of guiding members  56  (only one of the guiding members  56  is shown in the drawings), protruding laterally from lower portions of the right and left sides of the process unit  2  fits into the lower-side guide surface  55   b , so that the process unit  2  is prevented from moving upward relative to the body housing  1 , and is allowed only to be moved further inward along the lower-side guide surface  55   b.    
     At this time, the register roller  12   a  moves downwardly due to its own weight as shown in  FIG. 20A , and approaches the register roller  12   b  while slidingly contacting with the lower-side guide surface  55   b.    
     When the process unit  2  is thus pushed inward, the motion acting portion  43  of each urging device  42  is raised by the corresponding upper-side guide surface  55   a , so that the slide support member  40  of each urging device  42  pivots upward about the pivots  39   a ,  39   b . Therefore, the slide support member  40  of each urging device  42  restrains the corresponding one of the action-receiving portions  61  in such a direction that a distal end portion of the slide support member  40  pushes the action-receiving portion  61 , as shown in  FIG. 20B . When the motion acting portion  43  of each urging device  42  comes to the summit  55   d  of the rising slope surface, the slide support member  40  of each urging device  42  is pivoted to a most upward orientation. In this orientation, the distal end portion of the slide support member  40  of each urging device  42  keeps restraining the corresponding action-receiving portion  61 . 
     At this time, the register roller  12   a  contacts the register roller  12   b . The register rollers  12   a ,  12   b  move together while contacting each other as the process unit  2  is pushed further inward. 
     As the process unit  2  is pushed further inward, the shaft  13   a  of the photosensitive drum  13  is set to a predetermined position in the inward end portion  55   c . At this position, an operator releases the process unit  2  from the operator&#39;s hands, so that the process unit  2  is set by its own weight into a state such that the register roller  12   a  is placed on the register roller  12   b , as shown in  FIG. 21 . The bearings  700 ,  710  fitted at each end of the register roller  12   a  are pressed by springs  45 , as shown in  FIG. 31 . Consequently, the register roller  12   a  presses against the register roller  12   b  at the appropriate position, angle, and pressing pressure. 
     When the process unit  2  is set into the body housing  1 , as shown in  FIG. 21 , the distal end of the slide support member  40  of each urging device  42  presses the corresponding action-receiving portion  61 . Therefore, the developing roller  22  is pressed against the photosensitive drum  13  by the urging devices  42  and the action-receiving portions  61 . 
     In the state such as shown in  FIG. 21 , and as further shown in  FIG. 9 , the bearings  35 , vertically movably supporting the shaft  14   a  of the transfer roller  14 , are raised by the shaft bearing raisers  34  that are urged by springs  33 , so that the transfer roller  14  is moved upward to press the photosensitive drum  13  at a predetermined pressure. 
     As described above, as the process unit  2  is removed from the body housing  1 , the register roller  12   a  is not fixed and instead freely moves up and down. The register roller  12   a  is positioned so as to press against the register roller  12   b  at the appropriate position, angle, and pressure only after the process unit  2  is set in the body housing  1 . The accuracy of the position, angle, and pressing pressure of the register roller  12   a  with respect to the register roller  12   b  is determined by the springs  45  disposed on the body housing  1 . Therefore, even when the process unit  2  is replaced, the relationship between the register rollers  12   a ,  12   b  can be appropriately maintained. Consequently, the sheet P can be constantly and stably conveyed. 
     The positioning of the register rollers  12   a ,  12   b  is performed with respect to the bearings  700 ,  710 . The bearings  700 ,  710  are fitted to the register roller  12   a  so as to be spaced from each other at a distance that is longer than the width of the process unit  2 , and are provided so as to extend outwardly from the side surface of the process unit  2 . Therefore, the positioning of the register roller  12   a  is performed at each side surface of the body housing  1 . When the process unit  2  is attached to or detached from the body housing  1  from the front face thereof, the springs  45  for positioning the register roller  12   a  are unlikely to interface with the path of the process unit  2  during the setting or removal thereof, so that the process unit  2  can easily be set into, and removed from, the body housing  1 . 
     The register roller  12   a  is a shaft made of metal. While the rigidity of the register roller  12   a  is adequately maintained, the upper register roller  12   a , which is longer than the width of the sheet P, is mounted to the process unit  2  via a simple structure. Therefore, cost reduction can be achieved. 
     As shown in  FIG. 1 , a sheet guiding opening  80 , to guide the sheet P to the contact portion between the photosensitive drum  13  and the transfer roller  14 , is provided on the case  30  adjacent to the register roller  12   a . With the sheet guiding opening  80 , the sheet P is smoothly conveyed to the contact portion between the photosensitive drum  13  and the transfer roller  14 . 
     It is to be understood that the invention is not restricted to the particular forms shown in the foregoing embodiment. Various modifications and alternations can be made thereto without departing from the scope of the invention.