Patent Publication Number: US-8126361-B2

Title: Image forming apparatus with combined belt unit and sheet guide member

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2005-244499 filed Aug. 25, 2005. The entire content of this priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image forming apparatus. 
     BACKGROUND 
     A known image forming apparatus such as a laser printer employs an endless belt for the purpose of carrying a sheet of paper, performing an intermediate transfer and the like. The aforementioned belt is detachably installed as a unit in the device for maintenance, replacement and the like. 
     It is preferable to form the aforementioned belt unit as compact as possible for performing efficient and easy maintenance. If the belt unit as a whole is enlarged, there may be difficulties in attachment or detachment of the unit with respect to the device, and the attachment/detachment mechanism (for example, slide mechanism) has to be enlarged accordingly. A mere simplification of the belt unit structure, however, may cause difficulties in finding the portion to be gripped other than the belt. The possibility to cause the operator to come into contact with the belt becomes substantially high, which is likely to give the damage to the belt. 
     Thus, there is a need in the art for an image forming apparatus that allows easy attachment or detachment of a belt unit as well as excellent maintenance performance. 
     SUMMARY 
     The image forming apparatus as one aspect of the invention is provided with a storage portion that stores a recording medium, an ejection mechanism that ejects the recording medium stored in the storage portion, and an image forming portion that forms an image on the recording medium within a body casing. The recording medium ejected by the ejection mechanism is guided by a guide member toward a predetermined direction, and further carried toward the image forming portion by a belt unit. The belt unit includes a frame portion, a plurality of support rollers rotatably provided to the frame, and a carrier belt extended between the support rollers for carrying the recording medium guided by the guide member. The guide member and the belt unit are combined into a single unit so as to be attachable to and detachable from the body casing. 
     In the aforementioned structure, since the belt unit and the guide member have been combined into the single unit, the guide member, which functions as a portion for guiding the recording medium, can also function as the grip portion during attachment or detachment of the belt unit. The belt unit, thus, may be easily attached or detached, and the maintenance is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects in accordance with the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a sectional view schematically showing an example structure of a laser printer according to the present invention; 
         FIG. 2  is a sectional view of the laser printer shown in  FIG. 1  having a front cover opened; 
         FIG. 3  is a front sectional view showing a support mechanism of a transfer roller and a photosensitive drum; 
         FIG. 4  is a side view showing a belt unit and a guide member of the laser printer shown in  FIG. 1 ; 
         FIG. 5A  is a sectional view showing a state where the belt unit and the guide member are installed in a body of the device; 
         FIG. 5B  is a sectional view showing a state where the belt unit and the guide member are detached from the body; 
         FIG. 6  is a sectional view of the laser printer shown in  FIG. 1  in the state where an image forming unit is pull out; 
         FIG. 7  is a sectional view of the laser printer shown in  FIG. 1  in the state where the belt unit is displaced from its set position; 
         FIG. 8  is a sectional view of the laser printer shown in  FIG. 1  in a state where the belt unit is completely detached; 
         FIG. 9  is a sectional view schematically showing another example structure of the laser printer according to the present invention; 
         FIG. 10  is a sectional view of the laser printer shown in  FIG. 9  in the state where the front cover is opened, and the image forming unit is detached; 
         FIG. 11  is a sectional view showing the belt unit and the guide member of the laser printer in  FIG. 9 ; and 
         FIG. 12  is a sectional view of the laser printer shown in  FIG. 9  in the state where the belt unit is completely detached. 
     
    
    
     DETAILED DESCRIPTION 
     A first example structure of the image forming apparatus as an aspect of this invention will be described referring to  FIGS. 1 to 8 . 
     A laser printer  1 , which is a color laser printer of direct transfer tandem type, includes a substantially box-like body casing  2  as shown in  FIG. 1 . A front cover  3  that can be opened and closed is attached to a front surface (right side of the drawing) of the body casing  2 . Process cartridges  26  or a belt unit  15  within the body casing  2  may be replaced by opening the front cover  3  as shown in  FIG. 2 . An ejected paper tray  5  on which sheets of paper  4  that have been subjected to the image forming process are stacked is formed on the upper surface of the body casing  2 . 
     A paper feed tray  7  on which the sheets of paper  4  subjected to the image forming process are stacked is set so as to be withdrawn forward. A platen  9  that is tiltably operated under the urging force of a spring  8  so as to lift up the front edge of the paper  4  is provided within the paper feed tray  7 . A pick-up roller  10  and a separation pad  11  in pressure contact therewith under the urging force of a spring (not shown) are provided above the front end of the paper feed tray  7 . A pair of paper feed rollers  12  are provided diagonally forward above the pick-up roller  10 , and a pair of resist rollers  13  are further provided above the pair of paper feed rollers  12 . 
     The upper most paper  4  among those stacked on the paper feed tray  7  is pressed toward the pick-up roller  10  by the platen  9 . As the pick-up roller  10  rotates, the paper  4  is sandwiched between the pick-up roller  10  and the separation pad  11  so as to be separated and fed one by one. The paper  4  that has been fed from the sandwiched state between the pick-up roller  10  and the separation pad  11  is further carried to the resist rollers  13  by the paper feed rollers  12 . The paper  4  is fed by the resist rollers  13  to the belt unit  15  backward thereof at a predetermined timing. 
     The belt unit  15  is structured to be attachable to and detachable from the body casing  2 , and provided with a carrier belt  18  that horizontally extends between a pair of support rollers  16 ,  17 , which are apart from each other in the front-rear direction. The support roller  17  at the rear is a drive roller that is rotated to be driven by power of a motor (not shown), and the support roller  16  at the front is a tension (driven) roller for applying a tensile force to the carrier belt  18 , which will be described later. The carrier belt  18  is an endless belt formed of a resin material, for example, polycarbonate and the like. It is operated to circulate counterclockwise as shown in  FIG. 1  when the support roller  17  as the drive roller is rotated to be driven such that the paper  4  placed on the carrier belt  18  is carried backward. Photosensitive drums  31  and four transfer rollers  19  each oppositely provided in the respective process cartridges  26  are arranged at constant intervals in a longitudinal direction at the inside of the carrier belt  18 . The carrier belts  18  are, thus, interposed between the respective photosensitive drums  31  and the corresponding transfer rollers  19 . During the transfer process, the transfer bias is applied between the transfer rollers  19  and the photosensitive drums  31 , respectively. The structure of the belt unit  15  will be described later. 
     A cleaning roller  21  is provided below the belt unit  15  for removing toner or paper dust adhered to the carrier belt  18 . The cleaning roller  21  is formed by applying a foaming material, for example, silicon around a metal shaft member, opposite to the metal back-up roller  22  provided on the belt unit  15  with respect to the carrier belt  18 . A predetermined bias voltage is applied between the cleaning roller  21  and the back-up roller  22  such that the toner on the carrier belt  18  may be electrically introduced to the cleaning roller  21 . The cleaning roller  21  abuts against a metal recovery roller  23  for removing the toner adhered thereon. The recovery roller  23  further abuts against a blade  24  that scratches to remove the toner adhered on the recovery roller  23 . 
     Four process cartridges  26  corresponding to such colors as Magenta, Yellow, Cyan and Black, respectively are detachably arranged in the longitudinal direction above the belt unit  15 . A scanner portion  27  provided above those process cartridges serves to irradiate a laser beam L corresponding to the respective colors onto a surface of the corresponding photosensitive drum  31  on the basis of the image data through rapid scanning. 
     The process cartridge  26  includes a cartridge frame  30 , a photosensitive drum  31  and a scorotron type electrifier  32  provided below the cartridge frame  30 , and a development cartridge  34  detachably set to the cartridge frame  30 . 
     The photosensitive drum  31  is formed by coating a positively charged photosensitive layer formed of a polycarbonate, for example around a surface layer of a metal drum body that has been grounded. 
     The scorotron type electrifier  32  is provided diagonally backward above the photosensitive drum  31  such that they face with each other at a predetermined interval so as not to contact with each other. The scorotron electrifier  32  serves to positively electrify over a whole surface of the photosensitive drum  31  uniformly by causing the wire for charging, for example, tungsten, to generate a corona discharge. 
     The development cartridge  34  has a substantially box-like shape having a toner containing chamber  38  at the upper portion, and a supply roller  39 , a development roller  40 , and a layer thickness control blade  41  provided at the lower portion. Each of the toner containing chambers  38  contains nonmagnetic single component toner that has been positively charged as the development agent for such colors as Yellow, Magenta, Cyan, and Black, respectively. Each of the respective toner containing chambers  38  is provided with an agitator  42  that agitates the toner. 
     The supply roller  39  is formed by coating a metal roller shaft with a conductive foaming material. The development roller  40  is formed by coating a metal roller shaft with a conductive rubber material. The toner discharged from the toner containing chamber  38  is supplied to the development roller  40  accompanied with the rotation of the supply roller  39 , and positively friction charged between the supply roller  39  and the development roller  40 . The toner supplied onto the development roller  40  proceeds between the layer thickness control blade  41  and the development roller  40  in conjunction with the rotation of the development roller  40  and is further friction charged sufficiently so as to be carried on the development roller  40  to form a thin layer with a uniform thickness. 
     The surface of the photosensitive drum  31  during rotation is uniformly charged positively by the scorotron electrifier  32 . Thereafter it is exposed to rapid scanning of the laser beam from the scanner portion  27  such that the electrostatic latent image corresponding to the image to be formed on the paper  4  is generated. 
     Upon rotation of the development roller  40 , the toner carried thereon that has been positively charged is brought into contact with the opposite photosensitive drum  31  such that the toner is supplied to the electrostatic latent image generated on the surface of the photosensitive drum  31 . Accordingly the electrostatic latent image on the photosensitive drum  31  may be visualized, that is, the toner image is carried on the surface of the photosensitive drum  31 . 
     The toner image carried on the surface of the respective photosensitive drums  31  is sequentially transferred on the paper  4  under the negative transfer bias voltage applied to the transfer rollers  19  while the paper  4  carried by the carrier belt  18  is fed through the respective transfer positions between the photosensitive drums  31  and the transfer rollers  19 . The paper  4  on which the toner image has been transferred is fed to a fixation unit  43 . 
     The fixation unit  43  is disposed to the rear of the carrier belt  18  within the body casing  2 , and provided with a heat roller  44  including a heat source, for example, a halogen lamp, which is rotated to be driven, and a press roller  45  oppositely positioned to press the heat roller  44  and driven through rotation. The fixation unit  43  heats the paper  4  that carries the four-colored toner image while being sandwiched and carried between the heat roller  44  and the press roller  45  so as to fix the toner image on the paper  4 . The thermally fixed paper  4  is carried to ejection rollers  47  at the upper portion of the body casing  2  by a carrier roller  46  diagonally backward above the fixation unit  43 . The paper  4  is then discharged onto the ejected paper tray  5  by the ejection rollers  47 . 
     The belt unit and peripheral structure thereof will be described in detail. 
     Referring to  FIGS. 3 and 4 , the belt unit  15  is provided with the carrier belt  18  for carrying the paper, the support rollers  16  and  17  for supporting the carrier belt  18 , and a belt frame  50  that rotatably supports the support rollers  16  and  17 . 
     The belt frame  50  formed of an insulated synthetic resin material includes side walls  50 A and  50 B at left and right sides, which are connected at the lower side. The support rollers  16  and  17  are supported at the front and rear ends of the belt frame  50 , respectively. The support roller  17  is connected with a gear mechanism (not shown) provided in the body casing  2  upon installation of the belt unit  15  in the body casing  2 , and driven by power of a motor (not shown) provided in the body casing  2 . 
     The belt frame  50  is provided with four groove-like transfer roller fit portions  52  arranged in the longitudinal direction each extending in the lateral direction, and having the upper portion opened for accommodating the transfer roller  19 . The transfer roller  19  is formed by coating a metal roller shaft  19 A with a conductive rubber material  19 B. Both ends of the roller shaft  19 A extend from the left and right ends of the rubber material  19 B. Referring to  FIG. 4 , roller shaft insertion holes  53  are formed through which the roller shaft  19 A of the transfer roller  19  is inserted at the left and right ends of the transfer roller fit portion  52  in the left and right side walls  50 A and  50 B of the belt frame  50  as shown in  FIG. 4 . Each of the roller insertion holes  53  has a substantially rectangular shape, and dimensions in the vertical and longitudinal directions larger than the outer diameter of the roller shaft  19 A. In the state where the belt unit  15  is detached from the body casing  2 , the roller shaft  19 A is allowed to be displaced in the radial direction (vertical and longitudinal directions) within the range of the roller shaft insertion hole  53 . 
     A restriction member  55  is attached to the left end of the roller shaft  19 A of the transfer roller  19  as shown in  FIG. 3 . The restriction member  55  is attached such that the roller shaft  19 A is allowed to be inserted through the center cylindrical portion (not shown in detail). It is attached to be relatively rotatable around the axis of the roller shaft  19 A, and not to be relatively shifted along the axial direction. The restriction member  55  is provided with a pair of plate-like restriction portions extending from the cylindrical portion thereof in the longitudinal direction (radial direction of the roller shaft  19 A). The transfer roller fit portion  52  of the belt frame  50  has a groove portion  56  that extends in the longitudinal direction and opens upward, with which the plate-like restriction portion of the restriction member  55  is fit. The plate-like restriction portion of the restriction member  55  has a convex surface  55 A with arc-like cross section of both side surfaces. The left and right convex surfaces  55 A abut against the inner wall of the groove portion  56  such that the transfer roller  19  is positioned in the axial direction, and the sliding displacement of the roller shaft  19 A in the radial direction (vertical and longitudinal directions) is allowed as well as the tilting thereof in the radial direction. 
     Referring to  FIG. 4 , the belt unit  15  of the example structure is combined with a guide member  101  for guiding the paper. Referring to  FIGS. 1 and 2 , the guide member  101  serves to guide the paper ejected by the pick-up roller  10  corresponding to the ejection mechanism installed in the body casing  2  onto the carrier belt  18 . The guide member  101  and the belt unit  15  are combined into a single unit so as to be moveable with respect to the pick-up roller  10  and attachable to and detachable from the body casing  2 . 
     Referring to  FIG. 4 , the guide member  101  has a portion (guide portion  101 B) serving as a cover member that covers an upper portion of a carrier surface  18 A of the carrier belt as well as a grip portion held by the operator for attachment or detachment of the belt unit  15 . 
     The guide member  101  integrally formed with the side walls  50 A and  50 B of the belt frame  50  into a single unit to extend upstream of the carrier belt  18  in the carrier direction (front side of  FIG. 1 ). The guide member  101  includes a guide surface  101 C for guiding the paper, a curved portion  101 A and the guide portion  101 B. The curved portion  101 A forms a portion of a carrier path that turns back the fed paper, which serves as a curved wall to form the upper convex portion. 
     The curved portion  101 A is at the level that exceeds the height of the carrier belt  18 . The guide portion  101 B extends diagonally downward from the position higher than the carrier belt  18 , and has an end portion opposite to the carrier surface  18 A of the carrier belt  18 . The guide member  101  and the belt frame  50  do not have to be formed as an integrally molded product so long as they are combined. For example, the guide member  101  and the belt frame  50  which are independently molded may be fixed together with a screw, for example. 
     The guide member  101  has an opening  101 D that opens downward so as to allow fingers of the operator to be inserted from the lower portion thereinto. The space upward of the opening  101 D is defined by the curved portion  101 A, the guide portion  101 B, and left and right side walls (In  FIG. 4 , one of the side walls,  101 E is only shown). The guide portion  101 B, and the left and right side walls  101 E and  101 F are continued with an rising wall portion  101 G which is connected to the left and right side walls  50 A and SOB of the belt frame  50  via a bottom wall  101 H. 
     Referring to  FIGS. 1 and 2 , a tension mechanism  70  including a coil spring  54  is provided at the front end of the belt unit  15  at the side of the body casing  2 . More specifically, the tension mechanism  70  is provided with a pair of left and right levers  71  having the center portion rotatably supported at a rotary shaft  71 A along the lateral direction (one side is only shown in  FIG. 1 ), and a pair of coil springs  54  each urging the respective levers  71  (one side is only shown in  FIG. 1 ). Each of the coil springs  54  has the rear end fixed to the body casing  2 , and the front end as a free end connected to the lower end portion of the respective levers  71 . Each of the levers  71  has the upper end portion swingable against the elastic force applied by the coil spring  54 . Two pairs of the levers  71  and the coil springs  54  are arranged to interpose the front end of the accommodated belt unit  15  between the pair of the levers  71 . 
     The portion of the device for accommodating the belt unit  15  is provided with unit support portions  59  and  60  for positioning the belt unit  15 . The unit support portion  59  supports a pair of left and right bearings  16 A each of which supports the left end and right end of the roller shaft of the support roller  16  that extends from the frame  51  of the belt unit  15 . A guide groove  60 A is formed in the unit support portion  60  (see  FIG. 1 ), through which a pair of bearings  17 A each supporting the left end and right end of the roller shaft of the support roller  17  in the belt unit  15  is inserted. The aforementioned state represents the state where the belt unit  15  is normally installed in the body casing  2 . 
     In the normally installed state as described above, the upper end of each of the levers  71  abuts against the bearing  16 A of the belt support roller  16  from backside. The support roller  16  is urged in the direction (forward) where the belt support roller  16  is apart from the support roller  17  under the elastic force of the coil spring  54  so as to apply the tensile force to the carrier belt  18 . 
     A belt tension release portion (not shown) is installed in the body casing  2 , which retracts the upper portion of the lever  71  upon activation while expanding the coil spring  54  elastically so as to allow passage of the bearing  16 A. 
     Meanwhile, a metal frame  58  is provided within the body casing  2  so as to cover left and right sides of the process cartridges  26  and the belt unit  15 , respectively. Pairs of the unit support portions  59  and  60  are provided at the front and rear portions of the frame  58 , respectively as shown in  FIG. 1 . The unit support portion  60  at the rear side has the open upper portion with which a bearing  17 A that rotatably supports the end of the belt support roller  17  at the rear side is fit as shown in  FIG. 5A . As the bearing  17 A is retained with the tension mechanism, the support roller  17  is held in position. The unit support portion  59  at the front side has a portion of the upper surface as a horizontal surface  59 A at a constant height. The bearing  16 A is disposed on the horizontal surface  59 A such that the belt support roller  16  is held in position in the height direction. The horizontal surface  59 A of the unit support portion  59  at the front side allows the longitudinal displacement of the belt support roller  16 . 
     The frame  58  is provided with pairs of left and right bearing members  63 L and  63 R corresponding to the respective transfer rollers  19  for supporting the roller shafts  19 A thereof at the left and right sides as shown in  FIGS. 1 to 3 . Each of the bearing members  63 L and  63 R has a bearing groove  64  with the upper portion opened, with which an end of the roller shaft  19 A is fit from upward such that the roller shaft  19 A is rotatably supported. A guide surface  65  is formed at the open edge of the bearing groove  64  into which the roller shaft  19 A is introduced. The bearing member at the left side of the bearing members  63 L and  63 R shown in  FIG. 3 , that is,  63 L is formed of an insulated synthetic resin material, and attached to the frame  58  so as to be allowed to have a vertical displacement. Meanwhile, it is held in position with respect to the longitudinal and lateral directions of the frame  58 . A spring  66  is attached to the lower portion of the bearing member  63 L so as to be urged upward by the spring  66 . 
     The bearing member  63 R at the right side of the drawing is formed of an electrically conductive synthetic resin material, and is attached to the insulated member  67  formed of the insulated synthetic resin material fixed to the frame  58  so as to be allowed to have a vertical displacement. Meanwhile, it is held in position with respect to the insulated member  67  in the longitudinal and lateral directions. The spring  66  having conductivity is attached to the lower portion of the right bearing member  63 R at the right side so as to be urged upward thereby. The lower end of the spring  66  is connected to one end of an electrode plate  68 . The other end of the electrode plate  68  extends outward from the frame  58 , which is connected to the transfer bias application unit (not shown) provided within the body casing  2 . The transfer bias application unit is electrically connected to the transfer roller  19  via the electrode plate  68 , the spring  66 , and the bearing member  63 R such that the transfer bias is applied between the transfer roller  19  and the photosensitive drum  31  upon the image forming process. 
     The photosensitive drum  31  includes a drum shaft  31 A extending from both ends of the drum body as shown in  FIG. 3 . Bearing bodies  80  each formed of the synthetic resin material are rotatably provided around the outer peripheries of the respective ends of the drum shaft  31 A. The frame  58  includes drum positioning grooves  81  at left and right sides corresponding to the respective photosensitive drums  31 . As the bearing body  80  is fit with each of the left and right drum positioning grooves  81 , the drum shaft  31 A is rotatably supported in position with respect to the frame  58 . The drum shaft  31 A is connected to a gear mechanism (not shown) within the body casing  2 , and the photosensitive drum  31  is driven by a power of a motor (not shown). 
     The example structure has a frame  111  that holds four process cartridges  26  within the body casing  2 . Referring to  FIG. 6 , an image forming unit  110  formed of the frame  111  and those four process cartridges  26  may be detached from an opening  2 A formed in the body casing  2 . In order to detach the carrier belt  18 , the front cover  3  is opened as shown in  FIG. 2 , and the image forming unit  110  is pulled out as shown in  FIG. 6  so as to have access to the belt unit  15  via the opening  2 A. 
     The belt unit  15  is detachable with respect to the body casing  2  via the opening  2 A in the state where the image forming unit  110  is detached from the body casing  2  as shown in  FIGS. 7 and 8 . In the example structure, as the guide member  101  combined with the belt unit  15  is at the position closer to the opening  2 A than the carrier belt  18 . Pulling out the image forming unit  110  allows easy access to the guide member  101  via the opening  2 A. Accordingly, the belt unit  15  may be easily detached from the body casing  2  while gripping the guide member  101 . 
     Referring to  FIG. 5A , a pivot shaft  105  is supported by one of a pair of support portions  103  at the left and right sides of the body of the apparatus at a position different from that of the rotary shaft of the support roller  17 , opposite to the side where the guide member  101  combined with the belt unit  15  is provided. The pivot shaft  105  extends in parallel with the rotary shaft of the support roller  17 , having the rear lower portion of both left and right ends supported at the pair of support portions  103 . In the example structure, when the belt unit  15  is pivotally turned with respect to the pivot shaft  105  while lifting up the guide member  101  as shown in  FIG. 5B , the rotary shaft (roller shaft) of the support roller  17  and the bearing  17 A displace upward to go through the guide groove  60 A of the unit support portion  60  such that the belt unit  15  is allowed to be detached from the installed position. 
     Meanwhile, the belt unit  15  is inserted through the opening  2 A to allow the bearings  17 A at both ends of the rear support roller  17  to be engaged with the guide groove  60 A of the unit support portion  60 , and the bearings  16 A at both ends of the front support roller  16  to be placed on the unit support portion  59  for the purpose of installing the belt unit  15  into the body casing  2 . This makes it possible to hold the belt unit  15  in a horizontal state by the unit support portions  59  and  60  arranged in the longitudinal direction. In the aforementioned process, both end portions of the roller shaft  19 A of each transfer roller  19  are guided into the bearing grooves  64  formed in the bearing members  63 L and  63 R so as to be fit therewith. This makes it possible to have the respective transfer rollers  19  positioned in the longitudinal direction. 
     Then the image forming unit  110  is slidably placed inside to install the respective process cartridges  26  upward of the belt unit  15 . The bearing bodies  80  at both ends of the drum shaft  31 A are fit with the respective drum positioning grooves  81  of the frame  58  so as to have the photosensitive drum  31  to be positioned with respect to the frame  58 . As the transfer roller  19  is pressed downward by the photosensitive drum  31  against the urging force applied by the spring  66 , the transfer roller  19  may be positioned in the vertical direction. 
     In the above-described structure, the belt unit  15  is combined with the guide member  101  that guides the paper, which are moveable with respect to the pick-up roller  10 , and attachable and/or detachable with respect to the body casing. As the guide member  101  that guides the paper to the carrier belt  18 , and the belt unit  15  are combined, the accuracy in positioning of the guide member  101  and the carrier belt  18  may be effectively improved. When the belt unit  15  is attached or detached, the guide member  101  functions as the grip portion. Accordingly the guide member  101  and the belt unit  15  in combination may be attached to or detached from the body casing as the moveable unit with respect to an ejection mechanism. This makes it possible to allow easy attachment or detachment of the belt unit  15  as well as improvement of the maintenance operation. 
     A portion of the guide member  101  serves as a cover member that covers the carrier surface  18 A of the carrier belt  18  so as to prevent the contact between the operator&#39;s hand and the carrier belt  18 . As the guide member  101  extends to the position closer to the opening  2 A than the carrier belt  18 , it becomes closer to the opening  2 A than the carrier belt  18 . This makes it possible to allow easy access to the guide member  101  during detachment of the belt unit  15 . The belt unit  15  may be installed with no need of deep insertion of the operator&#39;s hand. The attachment or detachment of the belt unit, thus, may be easily performed. 
     The pivot shaft  105  is supported by the support portion  103  of the apparatus at a position different from that of the rotary shaft of the support roller  17 , opposite to the side where the guide member  101  combined with the belt unit  15  is provided. The belt unit  15  is pivotally operated around the pivot shaft  105  so as to be detached from the device. Accordingly, the belt unit  15  may be easily detached by operating the guide member  101 . 
     In the body casing  2 , the image forming unit  110  formed on frame  111  that supports a plurality of cartridges is detachably held at the frame  111  and is allowed to be pulled out from the body casing  2 . The aforementioned image forming unit  110  may be detached from the opening  2 A formed in the body casing  2 . The belt unit  15  may be attached or detached with respect to the body casing  2  via the opening  2 A in the state where the image forming unit  110  is detached from the body casing  2 . This makes it possible to perform the maintenance of the belt unit  15  independently from the image forming unit  110  while using the opening  2 A for attachment or detachment of both units. 
     In the example structure, the paper feed tray  7  is allowed to be pulled out from the body casing  2  at one side surface (that is, front surface side). The front cover  3  that is capable of opening and closing the opening  2 A formed in the body casing  2  is provided on the side surface (front surface) from where the paper feed tray  7  is pulled out. The belt unit  15 , thus, is allowed to be attached or detached via the opening  2 A. The above-described structure allows both detachment of the belt unit  15  and withdrawal of the paper feed tray  7  to be performed from the same side, resulting in considerably convenient structure. 
     Another example structure of the present invention will be described referring to  FIGS. 9 to 12 . 
     The example structure is a color laser printer  80  of intermediate transfer tandem type using an intermediate transfer belt  86 . The components of the structure which are the same as those described in the aforementioned example will be designated with the same reference numerals, and the explanation thereof, thus, will be omitted. 
     The laser printer  80  includes a belt unit  81  that is attachable to and detachable from the body casing  2 . The belt unit  81  has a belt frame  82  formed of an insulated synthetic resin material with a substantially triangle side view. Belt support rollers  83 ,  84 , and  85  are provided at the front end, rear end and lower end of the belt frame  82 , respectively, by which an intermediate transfer belt  86  extends thereamong. Four transfer rollers  19  are arranged on the upper surface of the belt frame  82 . A secondary transfer roller  87  opposite to the belt support roller  85  at the lower end of the belt frame  82  with respect to the intermediate transfer belt  86  is provided at the lower portion of the belt unit  81 . The secondary transfer bias is applied between the secondary transfer roller  87  and the belt support roller  85 . In the laser printer  80 , the toner image generated by four photosensitive drums  31  is transferred onto the intermediate transfer belt  86  in four colors, and thereafter, the toner image transferred onto the intermediate transfer belt  86  is further transferred onto the paper  4  while passing through the press contact position between the secondary transfer roller  87  and the intermediate transfer belt  86 . The intermediate transfer belt  86  functions as the carrier belt that grips the paper  4  with the secondary transfer roller  87  so as to be carried in the belt running direction and to transfer the toner image thereon. 
     A frame (not shown) within the body casing  2  is provided with a pair of unit support portions at the front and rear sides for supporting the belt support rollers  83  and  84  at the front and rear sides, respectively. In the example shown in  FIG. 9 , the unit support portion  59  that supports the support roller  84  is only shown, and the unit support portion that supports the support roller  83  is not shown. However, the example may have the same structure as that of the unit support portion  60  shown in  FIG. 1 . The frame is also provided with a pair of bearing members corresponding to the transfer roller  19  at left and right sides for supporting the roller shaft  19 A of the transfer roller  19 . The example of  FIG. 9  shows one of those bearing members, that is, the bearing member  63 L only. 
     In the example structure shown in  FIG. 11 , the belt unit  81  is combined with a guide member  121  that guides the paper. Referring to  FIG. 12 , the guide member  121  and the belt unit  81  can move in combination with respect to the pick-up roller  10 , and are attachable and/or detachable with respect to the body casing. As the guide member  121  that guides the paper into the intermediate transfer belt  86  is combined with the belt unit  81 , the accuracy in positioning of the guide member  121  and the intermediate transfer belt  86  is improved to higher degrees. Moreover, upon attachment or detachment of the belt unit  81 , a portion of the guide member  121  is allowed to function as the grip portion. This makes it possible to perform attachment or detachment of the guide member  121  and the belt unit  81  in combination with respect to the body casing  2 . 
     The guide member  121  extends upstream of the intermediate transfer belt  86  in the carrier direction with respect to the secondary transfer roller  87 . This may eliminate the need of accessing the area around the carrier surface of the intermediate transfer belt  86  for locating a position to grip during attachment or detachment. Thus, interaction on the carrier surface is unlikely, thus providing more protection to the carrier belt. 
     Referring to  FIG. 10 , the opening  2 A is formed in a portion of the body casing  2 . Referring to  FIG. 12 , the belt unit  81  is detachably provided with respect to the body casing via the opening  2 A. The guide member  121  is arranged at the position closer to the opening  2 A than the intermediate transfer belt  86 . As the guide member  121  is closer to the opening  2 A than the intermediate transfer belt  86 , access to the guide member  121  is improved, thus making it easier for attaching and/or detaching the belt unit  81 . The belt unit  81  may be easily attached with no need of inserting the operator&#39;s hand so deeply. This makes it possible to perform the attachment or detachment of the belt unit easily. 
     An image forming unit  110  formed of a frame  111  that supports a plurality of pairs of process cartridges  26  is detachably held by frame  111  and is allowed to be pulled out from the body casing  2 . The image forming unit  110  is allowed to be attached to and detached from the opening  2 A of the body casing  2  as shown in  FIG. 10 . The belt unit  81  is detachably provided with respect to the body casing  2  via the opening  2 A as shown in  FIG. 12  in the state where the image forming unit  110  is detached from the body casing  2  as shown in  FIG. 10 . The belt unit  81  may be maintained independent from the image forming unit  110 , thus using the opening  2 A for attachment or detachment of both the image forming unit and the belt unit, respectively. 
     In the example structure, the paper feed tray  7  may be pulled out from one side surface (that is, front surface side) of the body casing  2 . The front cover  3  serving to open and close the opening  2 A formed in the body casing  2  is provided at the one side surface (front surface) from where the paper feed tray  7  is pulled out. The belt unit  81  may be detachably provided at the one side surface via the opening  2 A. The detachment of the belt unit  81  and withdrawal of the paper feed tray  7  may be performed from the same side, resulting in the structure with improved convenience. 
     the combination of the belt unit and the guide member described herein may be formed as an integrally molded product of the part of the belt unit (for example, frame) and the guide member. Alternatively, it may be structured by fixing an independent guide member to the part of the belt unit (for example, frame).