Patent Publication Number: US-11042115-B1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-051924, filed on Mar. 23, 2020, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus and an image processing mechanism for use in an image forming apparatus. 
     BACKGROUND 
     As one of the configurations of an electro-photographic image forming apparatus, there is known a so-called tandem method in which toner images of four colors are drawn on one transfer belt. 
     The number of configurations in which an exposure unit including a rod-shaped so-called LED array in which light emitting diodes are aligned in the longitudinal direction is used for an exposure device for drawing an electrostatic latent image on a photoreceptor drum of each of four color image forming units has also increased. 
     There is also a configuration in which four exposure units facing each other for exposing each of four color image forming units are mounted on one chassis and the chassis is fixed to a frame of the image forming apparatus. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an outer appearance of an image forming apparatus according to an embodiment as viewed from the upper left of the front; 
         FIG. 2  is a configuration diagram of the image forming apparatus as viewed from the front side of the image forming unit; 
         FIG. 3  is a perspective view of an exposure unit; 
         FIG. 4  is a view of the exposure unit as viewed from the rear side; 
         FIG. 5  is a view of the exposure unit as viewed from the left side; 
         FIG. 6  is a perspective view of a frame and an exposure unit around an image forming unit as viewed from the upper left of the front; and 
         FIG. 7  is a perspective view of an exposure unit and a part of the image forming unit in a posture tilted 90 degrees as viewed from the front side. 
     
    
    
     DETAILED DESCRIPTION 
     An aspect of the present disclosure is to perform positioning four exposure units mounted on one chassis and image forming units of respective four colors with excellent accuracy. 
     In general, according to one embodiment, an image forming apparatus includes an image carrier that conveys a toner image from upstream to downstream in a conveyance direction; a first image forming unit that forms a toner image on the image carrier; a second image forming unit that forms a toner image on the image carrier downstream of the first image forming unit; a third image forming unit that forms a toner image on the image carrier downstream of the second image forming unit; a fourth image forming unit that forms a toner image on the image carrier downstream of the third image forming unit; a chassis that supports a first exposure unit for exposing the first image forming unit, a second exposure unit for exposing the second image forming unit, a third exposure unit for exposing the third image forming unit, and a fourth exposure unit for exposing the fourth image forming unit, and that includes a first fitting portion, and a second fitting portion between the second exposure unit and the third exposure unit in a direction parallel to the conveyance direction; and a frame that includes a third fitting portion to be fitted with the first fitting portion and a fourth fitting portion to be fitted with the second fitting portion, and supports the chassis whose relative position is positioned at the fourth fitting portion, the first image forming unit, the second image forming unit, the third image forming unit, the fourth image forming unit, and the image carrier, wherein one of the first fitting portion and the third fitting portion is an elongated hole having a long axis in a direction parallel to the conveyance direction, and the other is inserted into the elongated hole. 
     Hereinafter, an image forming apparatus and a control method of the image forming apparatus according to an embodiment will be described with reference to the drawings.  FIG. 1  is a perspective view of an outer appearance of an image forming apparatus  1  according to the embodiment as viewed from the upper left of the front. At the lower left of  FIG. 1 , an arrow x directing from the left side to the right side of the image forming apparatus  1 , an arrow y directing from the floor side to the top, and an arrow z directing from the front side to the rear side are illustrated. 
     The image forming apparatus  1  includes a display  110 , a control panel  120 , an image forming unit  130 , a sheet accommodating unit  140 , and an image reading unit  200 . 
     A toner cartridge is mounted on the image forming apparatus  1 . The toner cartridge may be filled with a decolorable toner that can be decolorized later or may be filled with a toner that cannot be decolorized later. 
     The image forming apparatus  1  forms an image on a sheet by using a toner. 
     The display  110  is an image display device such as a liquid crystal display or an organic EL (Electro Luminescence) display. The display  110  displays various pieces of information on the image forming apparatus  1 . 
     The control panel  120  includes a plurality of buttons for receiving user operations. The display  110  and the control panel  120  may be configured as an integrated touch panel. 
     The image forming unit  130  forms an image on a sheet based on image information generated by the image reading unit  200  and image information received via a communication line. 
     The image reading unit  200  reads the image information to be read based on brightness and darkness of light. The image reading unit  200  records the read image information. The recorded image information may be transmitted to another information processing apparatus via a network. The recorded image information may be formed as an image on the sheet by the image forming unit  130 . 
       FIG. 2  is a configuration diagram of the image forming apparatus  1  viewed from the front side. The image forming apparatus  1  includes the image reading unit  200 , the image forming unit  130 , the sheet accommodating unit  140 , and a conveyance unit  5 . 
     The conveyance unit  5  includes a registration roller  24  and a transfer roller  28 . The registration roller  24  adjusts the posture of the leading end of the sheet and conveys the sheet when the image forming unit  130  transfers the toner image to the sheet. The registration roller  24  conveys the sheet to the transfer roller  28 . 
     The image forming unit  130  includes an image forming unit  25 Y as a first image forming unit, an image forming unit  25 M as a second image forming unit, an image forming unit  25 C as a third image forming unit, an image forming unit  25 K as a fourth image forming unit, an exposure unit  26 , a transfer belt  27  as an image carrier, a fixing unit  29 , toner cartridges  33 Y,  33 M,  33 C, and  33 K, and a transfer belt cleaner  35 . 
     The transfer belt  27  has an image forming surface, which is a surface facing the image forming units  25 Y,  25 M,  25 C, and  25 K. The image forming surface is inclined with respect to the floor at an inclination angle D so as to be lowered from the left side to the right side as viewed from the front side in order to secure a sufficient length. The transfer belt  27  includes rollers  271 L and  271 R on the back surface of the image forming surface, which define the inclination angle D of the image forming surface. The transfer belt  27  is provided with a transfer roller  9  for attracting the toner image from each of the image forming units  25 Y,  25 M,  25 C, and  25 K, on the back surface of the surface facing each of the image forming units  25 Y,  25 M,  25 C, and  25 K between the roller  271 L and the roller  271 R. The transfer belt  27  conveys the toner images received in order from the image forming units  25 Y,  25 M,  25 C, and  25 K to the transfer roller  28 . The transfer roller  28  transfers the toner images to the sheet. 
     The image forming units  25 Y,  25 M,  25 C, and  25 K are aligned in order in the traveling direction of the facing surfaces of the transfer belt  27 . The image forming units  25 Y,  25 M,  25 C, and  25 K are installed in order near the floor along the inclination angle D so as to be along the transfer belt  27  which is inclined with respect to the floor from the left side to the right side as viewed from the front side. The image forming unit  25 Y forms a yellow toner image. The image forming unit  25 M forms a magenta toner image. The image forming unit  25 C forms a cyan toner image. The image forming unit  25 K forms a black toner image. Each of the image forming units  25 Y,  25 M,  25 C, and  25 K includes a photoreceptor drum  7  and performs charging, developing, cleaning, removing electricity, and the like on the photoreceptor drum  7 . 
     The fixing unit  29  fixes the toner images on the sheet on which the toner images are transferred by the transfer roller  28 . 
     The toner cartridges  33 Y,  33 M,  33 C, and  33 K are provided above the transfer belt  27 . The toner cartridge  33 Y contains a yellow toner for being supplied to the image forming unit  25 Y via a supply pipe. The toner cartridge  33 M contains a magenta toner for being supplied to the image forming unit  25 M via a supply pipe. The toner cartridge  33 C contains a cyan toner for being supplied to the image forming unit  25 C via a supply pipe. The toner cartridge  33 K contains a black toner for being supplied to the image forming unit  25 K via a supply pipe. 
     After the toner image is transferred to the sheet by the transfer roller  28 , the transfer belt cleaner  35  removes the toner remaining on the transfer belt  27 . 
     The exposure unit  26  exposes the four photoreceptor drums  7  to form an electrostatic latent image. 
       FIG. 3  is a perspective view of the exposure unit  26 . The exposure unit  26  includes an exposure unit  26 Y as a first exposure unit, an exposure unit  26 M as a second exposure unit, an exposure unit  26 C as a third exposure unit, an exposure unit  26 K as a fourth exposure unit, and a chassis  26 S. At the lower left of  FIG. 3 , an arrow x directing from the left side to the right side of the image forming apparatus  1 , an arrow y directing from the floor side to the top, and an arrow z directing from the front side to the rear side are illustrated. 
     The chassis  26 S holds the exposure units  26 Y,  26 M,  26 C, and  26 K so as to be sequentially lowered in accordance with the image forming units  25 Y,  25 M,  25 C, and  25 K which are provided in lower positions in order from the left side to the right side as viewed from the front side. 
     Each of the exposure units  26 Y,  26 M,  26 C, and  26 K takes a high posture or a low posture with respect to the chassis  26 S. The exposure units  26 Y,  26 M,  26 C, and  26 K can independently change the postures with respect to the chassis  26 S within the same stroke length range. 
     A bracket  261  of the chassis  26 S is a plate-shaped portion extending downward on the front side from a bucket  262  that holds the exposure units  26 Y,  26 M,  26 C, and  26 K. 
     At the bottom of the bracket  261 , there are fastening holes  263 Y,  263 M,  263 C, and  263 K that penetrate the front side and the rear side. The straight line connecting the fastening holes  263 Y,  263 M,  263 C, and  263 K to each other is inclined with respect to the floor at an inclination angle D. 
     At the top of the bracket  261 , slots  264 Y,  264 M,  264 C, and  264 K into which the exposure units  26 Y,  26 M,  26 C, and  26 K are inserted from the above are provided in a form of penetrating the bucket  262 . 
       FIG. 4  is a view of the exposure unit  26  as viewed from the viewpoint of the arrow A illustrated above  FIG. 3 , that is, as viewed from the rear side.  FIG. 5  is a view of the exposure unit  26  as viewed from the viewpoint of the arrow B illustrated on the left side of  FIG. 3 , that is, as viewed from the left side. 
     A front dowel  265 L as a first fitting portion, a front dowel  265 C as a second fitting portion, and a front dowel  265 R project from the lower rear surface of the bracket  261 . 
     The straight line connecting the front dowels  265 L,  265 C, and  265 R to each other is inclined with respect to the floor at an inclination angle D. 
     A rear dowel  266 L as a first fitting portion, a rear dowel  266 C as a second fitting portion, and a rear dowel  266 R project from the rear surface of the bucket  262 . 
     The straight line connecting the rear dowels  266 L,  266 C, and  266 R is inclined with respect to the floor at an inclination angle D. 
     The front dowel  265 L is located farther from the floor than the rear dowel  266 L. The front dowel  265 C is located farther from the floor than the rear dowel  266 C. The front dowel  265 R is located farther from the floor than the rear dowel  266 R. 
     The front dowel  265 C is located between the slot  264 M and the slot  264 C among the four slots  264 Y,  264 M,  264 C, and  264 K aligned in order from the left in the left-right direction. That is, the front dowel  265 C is located between the exposure unit  26 M and the exposure unit  26 C among the four exposure units  26 Y,  26 M,  26 C, and  26 K aligned in order from the left in the left-right direction. 
     The rear dowel  266 C is located between the slot  264 M and the slot  264 C among the four slots  264 Y,  264 M,  264 C, and  264 K aligned in order from the left in the left-right direction. That is, the rear dowel  266 C is located between the exposure unit  26 M and the exposure unit  26 C among the four exposure units  26 Y,  26 M,  26 C, and  26 K aligned in order from the left in the left-right direction. 
     The rear dowel  266 L of the left side is on the left side of the front dowel  265 L of the left side. The rear dowel  266 R of the right side is on the right side of the front dowel  265 R of the right side. That is, the distance between the left rear dowel  266 L and the right rear dowel  266 R in the left-right direction is larger than the distance between the left front dowel  265 L and the right front dowel  265 R. Since the left rear dowel  266 L and the right rear dowel  266 R are aligned in an inclined direction, the distance therebetween in the vertical direction is larger than the distance between the left front dowel  265 L and the right front dowel  265 R. 
       FIG. 6  is a perspective view of a frame  131  and the exposure unit  26  around the image forming unit  130  of the image forming apparatus  1  as viewed from the upper left of the front. The frame  131  supports the image forming units  25 Y,  25 M,  25 C, and  25 K, the exposure unit  26 , and the transfer belt  27 . The frame  131  serves as a reference for positioning the image forming units  25 Y,  25 M,  25 C, and  25 K, the exposure unit  26 , and the transfer belt  27 . The frame  131  includes a slot  132  on the front side. The image forming units  25 Y,  25 M,  25 C, and  25 K and the exposure unit  26  are inserted into the slot  132  from the front side. 
     Below the lower side of the slot  132 , there are a front dowel hole  135 L as a third fitting portion, a front dowel hole  135 C as a fourth fitting portion, and a front dowel hole  135 R as a third fitting portion. The front dowel holes  135 L,  135 C, and  135 R are aligned on a straight line parallel to the image forming surface of the transfer belt  27 . The front dowel holes  135 L and  135 R are elongated holes having a long axis parallel to the image forming surface of the transfer belt  27 . The front dowel hole  135 C between the front dowel hole  135 L and the front dowel hole  135 R is a round hole. 
     When the exposure unit  26  is inserted into the slot  132 , the front dowel  265 C is fitted to the front dowel hole  135 C. When the exposure unit  26  is inserted into the slot  132 , the front dowel  265 L is inserted into the front dowel hole  135 L with play in the long axis direction of the front dowel hole  135 L. When the exposure unit  26  is inserted into the slot  132 , the front dowel  265 R is inserted into the front dowel hole  135 R with play in the long axis direction of the front dowel hole  135 R. That is, the front dowel hole  135 C serves as a positioning hole for the exposure unit  26 . 
     On the rear wall of the frame  131  at the back of the slot  132 , there are a rear dowel hole  136 L as a third fitting portion, a rear dowel hole  136 C as a fourth fitting portion, and a rear dowel hole  136 R as a third fitting portion. The rear dowel holes  136 L,  136 C, and  136 R are aligned on a straight line parallel to the image forming surface of the transfer belt  27 . The rear dowel holes  136 L and  136 R are elongated holes having a long axis parallel to the image forming surface of the transfer belt  27 . The rear dowel hole  136 C between the rear dowel hole  136 L and the rear dowel hole  136 R is a round hole. 
     When the exposure unit  26  is inserted into the slot  132 , the rear dowel  265 C is fitted to the rear dowel hole  136 C. When the exposure unit  26  is inserted into the slot  132 , the rear dowel  265 L is inserted into the rear dowel hole  136 L with play in the long axis direction of the rear dowel hole  136 L. When the exposure unit  26  is inserted into the slot  132 , the rear dowel  265 R is inserted into the rear dowel hole  136 R with play in the long axis direction of the rear dowel hole  136 R. That is, the rear dowel hole  136 C serves as a positioning hole for the exposure unit  26 . 
     The front dowel holes  135 C and the rear dowel holes  136 C serve as positioning holes for the exposure unit  26  with respect to the frame  131 , and the front dowel holes  135 L and  135 R and the rear dowel holes  136 L and  136 R which are elongated holes serve to absorb the dimensional error of the exposure unit  26 . 
     Since the front dowel  265 C fitted to the front dowel hole  135 C is located between the exposure unit  26 M and the exposure unit  26 C in the left-right direction, the position errors of the exposure unit  26 Y and the exposure unit  26 K with respect to the positioning hole can be reduced. In addition, since the rear dowel  265 C fitted to the rear dowel hole  136 C is located between the exposure unit  26 M and the exposure unit  26 C in the left-right direction, the position errors of the exposure unit  26 Y and the exposure unit  26 K with respect to the positioning hole can be reduced. 
     Below the lower side of slot  132 , there are also fastening holes  133 Y,  133 M,  133 C, and  133 K. The fastening hole  263 Y of the exposure unit  26  inserted into the slot  132  and the fastening hole  133 Y of the frame  131  are fastened with one screw. The fastening hole  263 M of the exposure unit  26  inserted into the slot  132  and the fastening hole  133 M of the frame  131  are fastened with one screw. The fastening hole  263 C of the exposure unit  26  inserted into the slot  132  and the fastening hole  133 C of the frame  131  are fastened with one screw. The fastening hole  263 K of the exposure unit  26  inserted into the slot  132  and the fastening hole  133 K of the frame  131  are fastened with one screw. The fastening holes  133 Y,  133 M,  133 C, and  133 K are aligned on a straight line parallel to the image forming surface of the transfer belt  27 . 
     Above the upper side of the slot  132 , there are holes  137 Y,  138 Y,  139 Y,  137 M,  138 M,  139 M,  137 C,  138 C,  139 C,  137 K,  138 K, and  139 K aligned on a straight line parallel to the image forming surface of the transfer belt  27 . 
     The holes  137 Y,  138 Y, and  139 Y are used to mount the image forming unit  25 Y on the frame  131 . The holes  137 M,  138 M, and  139 M are used to mount the image forming unit  25 M on the frame  131 . The holes  137 C,  138 C, and  139 C are used to mount the image forming unit  25 C on the frame  131 . The holes  137 K,  138 K, and  139 K are used to mount the image forming unit  25 K on the frame  131 . 
     The holes  137 Y,  137 M,  137 C, and  137 K are round holes that serve as positioning holes. The holes  138 Y,  138 M,  138 C, and  138 K are elongated holes having a long axis parallel to the image forming surface of the transfer belt  27 . The holes  139 Y,  139 M,  139 C, and  139 K are holes through which screws for fastening them to the frame  131  are passed. 
       FIG. 7  is a perspective view of the exposure unit  26 Y. The photoreceptor drum  7  and a part of the image forming unit  25 Y are also drawn in a posture tilted 90 degrees as viewed from the front side. In addition, the exposure units  26 M,  26 C, and  26 K have the same configuration as the exposure unit  26 Y. The image forming units  25 M,  25 C, and  25 K have the same configuration as the image forming unit  25 Y. The exposure unit  26 Y includes a head  361 , a front hole  363 F, and a rear hole  363 R as a sixth fitting portion, springs  364 F and  364 R, a stage  365 , linkages  366 F and  366 R, a rail  367 , and a handle  368 . 
     The head  361  is an exposure device having an LED array of which longitudinal direction is along the front-rear direction. The head  361  emits light emitted from the LED array from an exposure window  362  of the upper surface. 
     The upper surface of the head  361  includes the front hole  363 F on the front side of the exposure window  362 . The front hole  363 F is an elongated hole having a long axis along the front-rear direction. The upper surface of the head  361  is a flat front seat surface between the exposure window  362  and the front hole  363 F. The upper surface of the head  361  includes the rear hole  363 R on the rear side of the exposure window  362 . The rear hole  363 R is a round hole that serves as a positioning hole. The upper surface of the head  361  is a flat rear seat surface between the exposure window  362  and the rear hole  363 R. 
     The lower surface of the head  361  is supported by the springs  364 F and  364 R. The upper portion of the spring  364 F comes into contact with the lower facing surface of the front seat surface of the head  361 . The upper portion of the spring  364 R comes into contact with the lower facing surface of the rear seat surface of the head  361 . The lower portions of the springs  364 F and  364 R are supported by the upper portion of the stage  365 . The stage  365  restricts the relative displacement of the springs  364 F and  364 R in the front-rear direction. 
     The lower portion of the stage  365  is supported by linkages  366 F and  366 R. The upper portion of the linkage  366 F is connected to the lower facing surface of the support portion of the spring  364 F of the stage  365 . The upper portion of the linkage  366 R is connected to the lower facing surface of the support portion of the spring  364 R of the stage  365 . The stage  365  restricts the relative displacement of the linkages  366 F and  366 R in the front-rear direction. 
     The lower portions of the linkages  366 F and  366 R are rotationally supported by the rail  367  and the handle  368 , respectively. By moving the handle  368  with respect to the rail  367  along the front-rear direction, the linkages  366 F and  366 R convert the shape change of the lower portion in the front-rear direction into the displacement of the upper portion in the vertical direction. By displacing the upper portions of the linkages  366 F and  366 R in the vertical direction, the head  361  is displaced in the vertical direction via the stage  365  and the springs  364 F and  364 R. The rail  367  is fixed to the slot  264 Y of the chassis  26 S of the exposure unit  26 . The rail  367  is fixed to the bucket  262 , and the head  361  moves up and down with respect to the bucket  262  as the handle  368  moves in the front-rear direction. 
     The image forming unit  25 Y supporting the photoreceptor drum  7  includes a chassis  251 , a front dowel  253 F, and a rear dowel  253 R as a fifth fitting portion, a front port  254 F, rear ports  254 RL and  254 RR, a bracket  257 , a dowel  258 , and a fastening hole  259 . 
     The chassis  251  rotatably supports the photoreceptor drum  7  around an axis parallel to the front-rear direction. The chassis  251  includes the bracket  257  on the front side. The bracket  257  includes the dowel  258  on the rear surface side. 
     The bracket  257  includes the fastening hole  259  that penetrates the front side and the rear surface side. The dowel  258  and the fastening hole  259  are aligned on a straight line parallel to the image forming surface of the transfer belt  27 . The rear surface of the bracket  257  includes another dowel on the straight line where the dowel  258  and the fastening hole  259  are aligned so as to interpose the fastening hole  259  between the rear surface of the bracket  257  and the dowel  258 . 
     When the image forming unit  25 Y is inserted into the slot  132 , the dowel  258  is fitted to the hole  137 Y of the frame  131 . When the image forming unit  25 Y is inserted into the slot  132 , another dowel is inserted into the hole  138 Y of the frame  131  with play in the long axis direction of the hole  138 Y. That is, the hole  137 Y serves as a positioning hole for the image forming unit  25 Y. The fastening hole  259  of the image forming unit  25 Y inserted in the slot  132 , and the hole  139 Y of the frame  131  are fastened with one screw. In addition, needless to say, the rear side of the chassis  251  also has a structure for positioning and fixing to the frame  131 . 
     The chassis  251  includes a window  252  at a position facing the lower surface of the photoreceptor drum  7 . The light emitted from the exposure window  362  on the upper surface of the head  361  of the exposure unit  26 Y reaches the photoreceptor drum  7  through the window  252 . 
     The lower surface of the chassis  251  includes the front dowel  253 F on the front side of the window  252 . The lower surface of the chassis  251  includes the front port  254 F between the front dowel  253 F and the window  252 . The lower surface of the front port  254 F is flat. 
     The lower surface of the chassis  251  includes the rear dowel  253 R on the rear side of the window  252 . The lower surface of the chassis  251  includes the rear ports  254 RL and  254 RR between the rear dowel  253 R and the window  252 . The lower surfaces of the rear ports  254 RL and  254 RR are flat. The rear ports  254 RL and  254 RR are aligned with each other in the left-right direction. 
     When the head  361  rises with respect to the bucket  262 , the front seat surface of the head  361  presses against the lower surface of the front port  254 F of the chassis  251 . The front seat surface and the lower surface of the front port  254 F, which press against each other, serve as positioning surfaces that determine the relative position of the chassis  251  of the head  361  in the vertical direction. When the head  361  rises with respect to the bucket  262 , the rear seat surface of the head  361  presses against the lower surfaces of the rear ports  254 RL and  254 RR of the chassis  251 . The rear seat surface and the lower surfaces of the rear ports  254 RL and  254 RR, which press against each other, serve as positioning surfaces that determine the relative position of the chassis  251  of the head  361  in the vertical direction. Since the head  361  presses against three portions, that is, the front port  254 F on the front side, the rear port  254 RL of the left on the rear side, and the rear port  254 RR of the right on the rear side, the relative posture between the head  361  and the chassis  251  is stable. 
     When the head  361  rises with respect to the bucket  262 , the rear dowel  253 R of the chassis  251  is fitted to the rear hole  363 R of the head  361 . When the head  361  rises with respect to the bucket  262 , the front dowel  253 F of the chassis  251  is inserted into the front hole  363 F of the head  361  with play in the long axis direction of the front hole  363 F. That is, the rear hole  363 R serves as a positioning hole that determines the relative positions of the head  361  and the chassis  251  in the front-rear direction and the left-right direction, and the front hole  363 F that is an elongated hole serves as a positioning hole that determines the relative positions of the head  361  and the chassis  251  in the left-right direction. 
     Since the relative positions of the frame  131  and the exposure unit  26  in the left-right direction are positioned between the exposure unit  26 M and the exposure unit  26 C, the position errors of the exposure unit  26 Y and the exposure unit  26 K in the left-right direction can be small, and thus, even in a structure which is performed by inserting the front dowel  253 F into the front hole  363 F and fitting the rear dowel  253 R into the rear hole  363 R, the relative positions of the head  361  and the chassis  251  in the left-right direction can be determined with a small stress without great distortion of the linkages  366 F and  366 R and other structures. 
     In addition, the front dowels  265 L,  265 C, and  265 R may not be aligned on a straight line, and any one of the front dowel  265 L and the front dowel  265 R may not be present. The rear dowels  266 L,  266 C, and  266 R may not be aligned on a straight line, and any one of the rear dowel  266 L and the rear dowel  266 R may not be present. The front dowel holes  135 L,  135 C, and  135 R may not be aligned on a straight line, and any one of the front dowel hole  135 L and the front dowel hole  135 R may not be present. The rear dowel holes  136 L,  136 C, and  136 R may not be aligned on a straight line, and any one of the rear dowel hole  136 L and the rear dowel hole  136 R may not be present. The front dowel hole  135 C and the rear dowel hole  136 C may be elongated holes having a long axis perpendicular to the image forming surface of the transfer belt  27 . The fastening holes  133 Y,  133 M,  133 C, and  133 K may not be aligned on a straight line, and the number of the fastening holes may not be four. The fastening holes  263 Y,  263 M,  263 C, and  263 K may not be aligned on a straight line, and the number of fastening holes may not be four. The holes  137 Y,  138 Y,  139 Y,  137 M,  138 M,  139 M,  137 C,  138 C,  139 C,  137 K,  138 K, and  139 K may not be aligned on a straight line. The dowel  258 , the fastening hole  259 , and one more dowel on the rear surface of the bracket  257  may not be aligned on a straight line. As an example of the structure of positioning the chassis  251  of the image forming unit  25 Y and the head  361  of the exposure unit  26 Y, the structure where the front dowel  253 F and the rear dowel  253 R which are convex structures are present in the chassis  251  of the image forming unit  25 Y, and the front hole  363 F and the rear hole  363 R having concave structures are present in the head  361  of the exposure unit  26 Y is described, but as long as the convex structure and the concave structure are paired, the convex structure and the concave structure may be present at any one of the chassis  251  and the head  361 . As an example of the structure of the portion where the chassis  251  of the image forming unit  25 Y and the head  361  of the exposure unit  26 Y press against each other, the structures having the front port  254 F, the rear ports  254 RL and  254 RR, the front seat surface, and the rear seat surface is described, but as long as the port and the seat surface are paired, the port and the seat surface may be present at any one of the chassis  251  and the head  361 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.