Patent Publication Number: US-11036157-B2

Title: Image forming apparatus incorporating writing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos. 2018-223222, filed on Nov. 29, 2018, and 2019-129316, filed on Jul. 11, 2019, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     This disclosure relates to an image forming apparatus including a writing device. 
     Discussion of the Background Art 
     Various types of image forming apparatuses are known to include a latent image bearer, a writing device to write a latent image onto the latent image bearer, and a partition to divide the latent image bearer and the writing device. 
     SUMMARY 
     At least one aspect of this disclosure provides an image forming apparatus including an image bearer, a writing device, a partition, and a plurality of side members. The image bearer is configured to bear an image. The writing device is configured to write the image on the image bearer. The partition is disposed between the image bearer and the writing device. The plurality of side members is disposed facing and spaced apart from each other at an interval in an axial direction of the image bearer. The writing device is fixed to the plurality of side members and is spaced apart from the partition. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       An exemplary embodiment of this disclosure will be described in detail based on the following figured, wherein: 
         FIG. 1  is a schematic diagram illustrating an image forming apparatus according to an embodiment of this disclosure; 
         FIG. 2  is a schematic diagram illustrating a layout of an incident optical system of a second writing unit; 
         FIG. 3  is a schematic diagram illustrating a layout of a scanning optical system of a writing unit; 
         FIG. 4  is a perspective view illustrating a schematic configuration of a main part of a bypass sheet feeding device; 
         FIG. 5A  is a schematic plan view of the bypass sheet feeding device; 
         FIG. 5B  is a schematic side view of the bypass sheet feeding device; 
         FIG. 6  is a diagram illustrating a recording sheet separating operation of the bypass sheet feeding device; 
         FIG. 7  is a schematic perspective view illustrating an image forming device; 
         FIG. 8  is a diagram illustrating a schematic configuration of a housing frame of the image forming device, viewed from a side cover; 
         FIG. 9  is an enlarged perspective view illustrating the housing frame in the vicinity of a cover support pin, viewed from the inside of the image forming apparatus; 
         FIG. 10A  is an enlarged perspective view illustrating the housing frame in the vicinity of a positioning pin, viewed from a side cover; 
         FIG. 10B  is an enlarged perspective view illustrating the vicinity of the positioning pin of the housing frame, viewed from a front side of the housing frame; 
         FIG. 11  is a diagram illustrating transmission of vibration generated when feeding a recording sheet set on a bypass tray; 
         FIG. 12  is a diagram illustrating an example of a horizontal stripe image generated by vibration of the writing unit; 
         FIG. 13  is a schematic plan view illustrating the housing frame according to the present embodiment; 
         FIGS. 14A, 14B, and 14C  are diagrams illustrating a schematic configuration of the housing frame with the writing units removed; 
         FIG. 15A  is an enlarged perspective view of the first writing stay in the vicinity of the rear side panel; 
         FIG. 15B  is an enlarged perspective view of the first writing stay in the vicinity of the front side panel; 
         FIG. 16  is a graph illustrating a moment of inertia of the front and rear stay and a moment of inertia of the writing stay; 
         FIG. 17  is a diagram illustrating a layout of the writing stay; 
         FIGS. 18A, 18B, and 18C  are diagrams illustrating a schematic configuration of a main part of a comparative configuration; 
         FIG. 19  is a diagram illustrating measurement points of vibration in the vertical direction of the bypass sheet feeding device; 
         FIG. 20  is a diagram illustrating measurement points of vibration in the vertical direction of the second writing unit; 
         FIG. 21A  is a graph illustrating a waveform of measured vibration in the vertical direction of a comparative bypass sheet feeding device; 
         FIG. 21B  is a graph illustrating a waveform of measured vibration in the vertical direction of a comparative second writing unit; 
         FIG. 22A  is a graph illustrating a waveform of measured vibration in the vertical direction of the bypass sheet feeding device according to the present embodiment; 
         FIG. 22B  is a graph illustrating a waveform of measured vibration in the vertical direction of the second writing unit according to the present embodiment; and 
         FIGS. 23A, 23B, and 23C  are diagrams illustrating a main part of a variation. 
     
    
    
     DETAILED DESCRIPTION 
     It will be understood that if an element or layer is referred to as being “on”, “against”, “connected to” or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on”, “directly connected to” or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers referred to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper” and the like may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements describes as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors herein interpreted accordingly. 
     Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layer and/or sections should not be limited by these terms. These terms are used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present disclosure. 
     The terminology used herein is for describing particular embodiments and examples and is not intended to be limiting of exemplary embodiments of this disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Descriptions are given, with reference to the accompanying drawings, of examples, exemplary embodiments, modification of exemplary embodiments, etc., of an image forming apparatus according to exemplary embodiments of this disclosure. Elements having the same functions and shapes are denoted by the same reference numerals throughout the specification and redundant descriptions are omitted. Elements that do not demand descriptions may be omitted from the drawings as a matter of convenience. Reference numerals of elements extracted from the patent publications are in parentheses so as to be distinguished from those of exemplary embodiments of this disclosure. 
     This disclosure is applicable to any image forming apparatus, and is implemented in the most effective manner in an electrophotographic image forming apparatus. 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this disclosure is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes any and all technical equivalents that have the same function, operate in a similar manner, and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, preferred embodiments of this disclosure are described. 
     Hereinafter, a detailed description is given of an embodiment of this disclosure with reference to the drawings. 
     It is to be noted that elements (for example, mechanical parts and components) having the same functions and shapes are denoted by the same reference numerals throughout the specification and redundant descriptions are omitted. 
     A description is given hereinafter of embodiments where this disclosure is applied to an image forming apparatus  1000 , for example a copier in the following embodiments. 
     First, a description is given of the outline of the image forming apparatus  1000 , with reference to  FIG. 1 . 
     The image forming apparatus  1000  has the function as what is called a digital color copier that digitizes image information obtained by scanning and reading an original document, and uses the image information to form an image. Further, the image forming apparatus  1000 , that is, the copier, also has the function of a facsimile machine that sends/receives image data of an original document to/from a remote place, and the function of what is called a printer that prints, on a recording sheet, image information handled by a computer. 
     It is to be noted in the following examples that: the term “image forming apparatus” indicates an apparatus in which an image is formed on a recording medium such as paper, OHP (overhead projector) transparencies, OHP film sheet, thread, fiber, fabric, leather, metal, plastic, glass, wood, and/or ceramic by attracting developer or ink thereto; the term “image formation” indicates an action for providing (i.e., printing) not only an image having meanings such as texts and figures on a recording medium but also an image having no meaning such as patterns on a recording medium; and the term “sheet” is not limited to indicate a paper material but also includes the above-described plastic material (e.g., an OHP sheet), a fabric sheet and so forth, and is used to which the developer or ink is attracted. In addition, the “sheet” is not limited to a flexible sheet but is applicable to a rigid plate-shaped sheet and a relatively thick sheet. 
     Further, size (dimension), material, shape, and relative positions used to describe each of the components and units are examples, and the scope of this disclosure is not limited thereto unless otherwise specified. 
     Further, it is to be noted in the following examples that: the term “sheet conveying direction” indicates a direction in which a recording medium travels from an upstream side of a sheet conveyance passage to a downstream side thereof; the term “width direction” indicates a direction basically perpendicular to the sheet conveyance direction. 
     In  FIG. 1 , the image forming apparatus  1000  forms an image on a recording sheet by performing an intermediate transfer operation using an intermediate transfer belt  11 , and is a tandem-type electrophotographic apparatus that forms a toner image of each color with a corresponding dedicated process cartridge. A multistage sheet feeding device  2  is provided in the lowermost part of the image forming apparatus  1000  in the vertical direction. Moreover, an image forming device  1  is provided above the sheet feeding device  2 , and a scanner  3  is provided above the image forming device  1 . At each stage, the sheet feeding device  2  includes a sheet feed tray  21  and a sheet feeding unit  21   a . The sheet feed tray  21  contains a sheet bundle including recording sheets such as plain paper, OHP film sheet, and traced drawings, as recording sheets. The sheet feeding unit  21   a  feeds a recording sheet of the sheet bundle contained in the sheet feed tray  21 . The sheet feeding unit  21   a  includes a pickup roller  121  and a sheet separation and conveyance unit  122 . The pickup roller  121  picks up and feeds a recording sheet from the sheet feed tray  21 . The sheet separation and conveyance unit  122  separates and feeds the recording sheet fed by the pickup roller  121 . 
     A transfer device  10  is arranged at the substantially center of the image forming device  1 . In the transfer device  10 , multiple rollers are arranged inside an endless loop of the intermediate transfer belt  11  so that the intermediate transfer belt  11  is stretched around the multiple rollers. 
     The intermediate transfer belt  11  rotates in a clockwise direction in  FIG. 1 . In other words, the surface of the intermediate transfer belt  11  moves in the clockwise direction in  FIG. 1 . 
     Four process cartridges  40 Y,  40 M,  40 C, and  40 K for forming toner images in yellow, magenta, cyan, and black are arranged above the intermediate transfer belt  11 , along a direction of movement of the surface of the intermediate transfer belt  11 . Since the configurations of the four process cartridges  40 Y,  40 M,  40 C, and  40 K, each functioning as an image forming device, are identical to each other except for the color of toner, the suffixes “Y”, “M”, “C”, and “K” indicating respective colors are omitted below as appropriate. 
     Moreover, two optical writing units are provided above the four process cartridges  40 Y,  40 M,  40 C, and  40 K. The two optical writing units are a first writing unit  20   a  and a second writing unit  20   b , each functioning as a latent image writing unit, or simply, a writing unit. 
     The process cartridge  40  (i.e., the process cartridges  40 Y,  40 M,  40 C, and  40 K) includes a drum-shaped photoconductor  41  (i.e., photoconductors  41 Y,  41 M,  41 C, and  41 K) that functions as a latent image bear, or simply, an image bearer. Each photoconductor  41  is rotatable in a counterclockwise direction in  FIG. 2 . A charging device, a developing device, a photoconductor cleaning device, and a lubricant application device are provided around the photoconductor  41 . 
     In  FIG. 1 , the transfer device  10  includes the intermediate transfer belt  11 , a belt cleaning device  17 , and four primary transfer rollers  46  (i.e., primary transfer rollers  46 Y,  46 M,  46 C, and  46 K). 
     The intermediate transfer belt  11  is stretched (tensioned) by a plurality of rollers including a tension roller  14 , a drive roller  15 , and a secondary transfer counter roller  16 . The intermediate transfer belt  11  is endlessly moved in the clockwise direction in  FIG. 1  along with rotation of the drive roller  15  driven by a belt drive motor. 
     The four primary transfer rollers  46  (i.e., the primary transfer rollers  46 Y,  46 M,  46 C, and  46 K) are arranged to respectively contact an inner circumferential surface side of the intermediate transfer belt  11 . A primary transfer bias is applied to the primary transfer rollers  46  by a power supply. Moreover, the intermediate transfer belt  11  is pressed by the primary transfer rollers  46  from the inner circumferential surface toward the photoconductors  41  to form respective primary transfer nip regions. A primary transfer electric field is formed between the photoconductor  41  and the primary transfer roller  46  in each primary transfer nip region due to the influence of the primary transfer bias. The toner image formed on the surface of the photoconductor  41  is primarily transferred onto the intermediate transfer belt  11  under the influence of the primary transfer electric field and the nip pressure. 
     Moreover, the transfer device  10  includes a secondary transfer roller  22 . The secondary transfer roller  22  is disposed below the intermediate transfer belt  11  and functions as a secondary transfer body. The secondary transfer roller  22  includes a secondary transfer roller  22   a  that contacts and presses the secondary transfer counter roller  16  via the intermediate transfer belt  11 . Then, the secondary transfer roller  22   a  secondarily transfers the toner images on the intermediate transfer belt  11  collectively onto a recording sheet conveyed to the secondary transfer nip region formed between the secondary transfer roller  22   a  and the intermediate transfer belt  11 . 
     The belt cleaning device  17  is disposed downstream from the secondary transfer counter roller  16  in the direction of movement of the surface of the intermediate transfer belt  11 . The belt cleaning device  17  removes residual toner remaining on the surface of the intermediate transfer belt  11  after the images have been transferred onto the recording sheet. The belt cleaning device  17  further includes a lubricant applying mechanism. The lubricant applying mechanism applies lubricant to the surface of the intermediate transfer belt  11 . 
     A fixing device  25  is provided downstream from the secondary transfer roller  22   a  in a sheet conveyance direction. The fixing device  25  fixes the toner image formed on the recording sheet, to the surface of the recording sheet. A fixing pressure roller  27  is pressed against an endless fixing belt  26 . The recording sheet on which the transferred image is formed is conveyed to the fixing device  25  by an endless conveyance belt  24  bridged across a pair of rollers. 
     Moreover, a sheet reversing device  28  is provided below the secondary transfer roller  22   a . The sheet reversing device  28  reverses a recording sheet upon formation of an image on both the front and back sides of the recording sheet. 
     A bypass sheet feeding device  4  is disposed on the right side of the image forming device  1  in  FIG. 1 . The bypass sheet feeding device  4  includes a bypass tray  51  and a bypass sheet feeding device  150 . The bypass tray  51  loads a recording sheet to be fed by a bypass sheet feeding operation. The bypass sheet feeding device  150  feeds the recording sheet loaded on the bypass tray  51 . The bypass sheet feeding device  150  includes a bypass pickup roller  52  and a bypass separation and conveyance unit  53 . The bypass pickup roller  52  picks up and feeds a recording sheet from the bypass tray  51 . The bypass pickup roller  52  functions as a contact and separation member to contact and separate with respect to the recording sheet. The bypass separation and conveyance unit  53  separates and conveys the recording sheet fed from the bypass tray  51 . 
     When a color original document is copied with the image forming apparatus  1000  including the above-described configurations, the scanner  3  reads an image of the color original document placed on an exposure glass. Moreover, the intermediate transfer belt  11  is rotated to form a toner image on each photoconductor  41  by known image forming processes employed to the image forming apparatus  1000 . Then, the toner images formed on the photoconductors  41 Y,  41 M,  41 C, and  41 K are sequentially overlaid to be primarily transferred onto the intermediate transfer belt  11 . Accordingly, a four-color composite toner image is formed on the intermediate transfer belt  11 . 
     In synchronization with the image forming operations of the four single-color toner images being transferred onto the intermediate transfer belt  11 , the sheet feeding unit  21   a  separates and feeds recording sheets, one by one, from a selected one of the sheet feed trays  21  of the sheet feeding device  2 , and the separated recording sheet is conveyed toward a pair of registration rollers  29 . 
     Instead of feeding recording sheets from the sheet feed tray  21 , a recording sheet may be fed and conveyed by the bypass tray  51 . In this case, the recording sheets on the bypass tray  51  are separated and fed one by one by the bypass sheet feeding device  150 , toward the pair of registration rollers  29 . 
     Then, the separated recording sheet is conveyed to the pair of registration rollers  29  to contact a nip region of the pair of registration rollers  29 . By causing the recording sheet to contact the nip region of the pair of registration rollers  29 , the conveyance of the recording sheet is temporarily stopped, and the recording sheet is being held for standby. The pair of registration rollers  29  resumes the rotation at a proper timing in such a manner as to set the positional relationship between the four-color toner image overlaid on the intermediate transfer belt  11  and a leading end of the recording sheet, to a given position. As the pair of registration rollers  29  starts rotating, the standby recording sheet is conveyed again. Consequently, the secondary transfer roller  22   a  secondarily transfers the four-color composite toner image on the intermediate transfer belt  11 , to the given position of the recording sheet. Thus, a full color toner image is formed on the recording sheet. 
     Thus, the recording sheet with the full color toner image is conveyed to the fixing device  25  that is disposed downstream from the secondary transfer roller  22   a  along the sheet conveyance passage. The fixing device  25  fixes the full color toner image that has been secondarily transferred by the secondary transfer roller  22   a , to the recording sheet. The recording sheet with the fixed full color image is ejected by a pair of sheet ejection rollers  30  to the outside of the image forming apparatus  1000 . 
     In a case in which a duplex printing mode is selected to form images on both sides of a recording sheet, when the recording sheet having the full color toner image fixed on the first face is ejected from the fixing device  25 , the recording sheet is conveyed to the sheet reversing device  28  instead of being conveyed to the pair of sheet ejection rollers  30 . After the front and back sides of the recording sheet are reversed by the sheet reversing device  28 , the recording sheet is conveyed again to the pair of registration rollers  29 . The recording sheet passes through the secondary transfer nip region formed between the secondary transfer roller  22   a  and the intermediate transfer belt  11  and then through the fixing device  25 , so that a full color image is formed on a second face (the back side) of the recording sheet P. 
     Next, a description is given of the configuration and operations of the first writing unit  20   a  and the second writing unit  20   b.    
     The first writing unit  20   a  writes a magenta (M) color latent image on the photoconductor  41 M and a yellow (Y) color image on the photoconductor  41 Y. The second writing unit  20   b  writes a black (K) color image on the photoconductor  41 K and a cyan (C) color image on the photoconductor  41 C. The first writing unit  20   a  and the second writing unit  20   b  have the basic configuration identical to each other. Therefore, for convenience, the configuration and operations of one writing unit, that is, the second writing unit  20   b , are explained in the following description. 
     It is to be noted that the suffixes “Y”, “M”, “C”, and “K” indicating respective colors are omitted below as appropriate. 
       FIG. 2  is a schematic diagram illustrating a layout of an incident light optical system of the second writing unit  20   b.    
     A light source unit  70  that functions as a light source device includes a light source  71  and a ¼ (quarter) wavelength plate  72 . The light source  71  emits laser light with linearly polarized light. The ¼ wavelength plate  72  converts the laser light emitted from the light source  71  into circularly polarized light. The light source unit  70  further includes a collimator lens  73  and an aperture  74 . The collimator lens  73  converts the laser light converted into circularly polarized light by the ¼ wavelength plate  72 , into parallel light. The aperture  74  cuts out the laser light parallelized by the collimator lens  73 . The laser light emitted from the light source unit  70  is incident on a deflector  202  that functions as a light deflector, via an incident optical system. 
     The incident optical system includes a deflecting beam splitter (PBS)  203 , a ¼ wavelength plate  204 , and a cylindrical lens  205 . 
     The PBS  203  divides the laser light emitted from the light source unit  70 , into two in the sub-scanning direction (the front-rear direction in the drawing sheet of  FIG. 2 ). 
     The ¼ wavelength plate  204  converts the polarization characteristics of the laser beams L 1  and L 2  (see  FIG. 3 ) divided into two by the PBS  203 , from linear polarization to circular polarization. 
     The cylindrical lens  205  performs image formation of the laser lights L 1  and L 2  converted to circular polarization, on the mirror faces of the rotary polygon mirrors  202   a  and  202   b , respectively, provided in the deflector  202 . The cylindrical lens  205  has a light focusing function of the laser light converted into circular polarization, in the sub-scanning direction alone. 
     The laser beams L 1  and L 2  formed in a given laser profile by the above-described incident optical system are imaged on the mirror faces of the rotary polygon mirrors  202   a  and  202   b  of the deflector  202 , respectively. The deflector  202  stably drives the rotary polygon mirrors  202   a  and  202   b  integrally as a single unit, at a given rotational speed, about a rotary axis parallel to the sub-scanning direction. As described above, as the laser light is incident onto the mirror faces of the rotating polygon mirrors  202   a  and  202   b  in rotation, the laser light is scanned in the main scanning direction, as illustrated in  FIG. 2 . 
       FIG. 3  is a schematic diagram illustrating a layout of a scanning optical system of the second writing unit  20   b . It is to be noted that, as described above, the first writing unit  20   a  and the second writing unit  20   b  have the basic configuration identical to each other, and it is to be understood that  FIG. 3  also explains the layout of the scanning optical system of the first writing unit  20   a.    
     Of the laser lights scanned by the deflector  202 , the laser light L 1  (in other words, the laser light scanned on the mirror face of the upper rotary polygon mirror  202   a ) passes through the scanning lens  301  and the long lens  302 , and then penetrates (passes through) a dust-proof glass  305 . Then, the laser light L 1  is scanned on the surface of the photoconductor  41 K at a constant speed. On this optical path, a first mirror  303   a , a second mirror  303   b , and a third mirror  303   c  are disposed to reflect the laser light L 1 . 
     On the other hand, the laser light L 2  (in other words, the laser light scanned on the mirror face of the lower rotary polygon mirror  202   b ) passes through the scanning lens  301  and the long lens  302 , and penetrates (passes through) the dust-proof glass  305 . Then, the laser light L 2  is scanned on the surface of the photoconductor  41 C at a constant speed. On this optical path, a mirror  304  is disposed to reflect the laser light L 2 . These parts and components of the second writing unit  20   b  are encased in a casing  400 . 
       FIG. 4  is a perspective view illustrating a schematic configuration of the main part of the bypass sheet feeding device  150 . 
     The bypass sheet feeding device  150  includes a bypass pickup roller  52  and a bypass separation and conveyance unit  53 . The bypass pickup roller  52  contacts an uppermost recording sheet placed on top of the sheet bundle loaded on the bypass tray  51 . The bypass separation and conveyance unit  53  includes a bypass feed roller  153  and a bypass reverse roller  253 , to separate the recording sheet or sheets conveyed by the bypass pickup roller  52 , one by one to a single sheet, and convey the separated single sheet. 
     The bypass feed roller  153  includes a rotary shaft  153   b  that is coupled to a drive motor. Further, an arm  55  is rotatably attached to the rotary shaft  153   b  of the bypass feed roller  153 . The arm  55  is provided with a roller mounting portion  55   a  and a spring mounting portion  55   b.    
     An idler gear  54  and the bypass pickup roller  52  are rotatably attached to the roller mounting portion  55   a . The idler gear  54  meshes with a gear  153   a  mounted on the bypass feed roller  153  and a gear  52   a  mounted on the bypass pickup roller  52 . Thus, a rotation driving force of the drive motor is transmitted from the gear  153   a  mounted on the bypass feed roller  153  to a bypass pickup roller  63  via the idler gear  54 , so as to rotate the bypass pickup roller  52 . 
     A spring  56  is attached to the spring mounting portion  55   b  to bias the spring mounting portion  55   b  in an upward direction in  FIG. 4 . According to this configuration, the arm  55  is biased by the spring  56  in a counterclockwise direction in  FIG. 4 , and the bypass pickup roller  52  is biased toward a direction to contact an uppermost recording sheet P of the sheet bundle. 
       FIG. 5A  is a schematic plan view of the bypass sheet feeding device  150 .  FIG. 5B  is a schematic side view of the bypass sheet feeding device  150 . 
     The arm  55  includes a contact projection  55   c  that extends in an upward direction in  FIGS. 5A and 5B . The contact projection  55   c  is in contact with a movable member  57 . The movable member  57  is rotatably supported by a support shaft  57   a  that is mounted on the frame of the image forming apparatus  1000 . A solenoid  58  is attached to the movable member  57  to cause the bypass pickup roller  52  to contact with and separate from a recording sheet P. The movable member  57  is biased by a spring  59  toward a clockwise direction in  FIG. 5A . 
     With this configuration, when the solenoid  58  is not in operation, the contact projection  55   c  is pushed by the biasing force of the spring  59  toward a sheet conveyance direction. Accordingly, the arm  55  rotates in the clockwise direction in  FIG. 5B  against the biasing force of the spring  56 , and therefore the bypass pickup roller  52  is located at a standby position at which the bypass pickup roller  52  is separated away from the recording sheet P. 
     As the solenoid  58  is driven to pull a plunger  58   a  in a direction indicated by arrow J in  FIG. 5A , the movable member  57  rotates about the support shaft  57   a  in a direction to separate from the contact projection  55   c  in  FIG. 5A . Then, the contact projection  55   c  moves so as to follow the movable member  57  by the biasing force of the spring  56 , and the arm  55  rotates in the counterclockwise direction in  FIG. 5B . Accordingly, the bypass pickup roller  52  comes into contact with the recording sheet P loaded on the bypass tray  51 . 
       FIG. 6  is a diagram illustrating a recording sheet separating operation performed by the bypass sheet feeding device  150 . 
     First, the solenoid  58  is driven to move the bypass pickup roller  52  at the standby position at which the bypass pickup roller  52  is separated from the recording sheet P loaded on the bypass tray  51 , to a sheet feeding position at which the bypass pickup roller  52  contacts the uppermost recording sheet P of the sheet bundle loaded on the bypass tray  51 . Next, the drive motor is driven to rotate the bypass feed roller  153  and the bypass pickup roller  52 . Consequently, the recording sheet P of the sheet bundle on the bypass tray  51  is conveyed by a frictional force of the bypass pickup roller  52 , to a bypass separation nip region at which the bypass feed roller  153  and the bypass reverse roller  253  contact each other. 
     As illustrated in  FIG. 6 , a torque limiter  60  and an input gear  62   a  are mounted on a shaft of the bypass reverse roller  253 . The input gear  62   a  meshes with a drive gear  62   b  that is mounted on a drive input shaft to which a driving force of the drive motor is transmitted. The driving force, which drives the bypass reverse roller  253  in an opposite direction opposite a rotational direction of the bypass feed roller  153 , is input to the bypass reverse roller  253  via the drive gear  62   b , the input gear  62   a , and the torque limiter  60 . The torque limiter  60  is configured to cause the bypass reverse roller  253  to spin relative to the torque limiter  60  when a running torque (a rotational torque) of the bypass reverse roller  253  in an opposite direction opposite the rotational direction of the bypass feed roller  153  exceeds a given value. 
     The bypass reverse roller  253  is in contact with and pressed against the bypass feed roller  153  by a tooth surface pressure between the drive gear  62   b  and the input gear  62   a  and the biasing force of the spring  61 . 
     While the bypass reverse roller  253  is in contact with the bypass feed roller  153  or when a single sheet (one sheet) is conveyed to the sheet separation nip region, the bypass reverse roller  253  receives a rotation driving force of the bypass feed roller  153 . Therefore, the running torque (the rotational torque) of the bypass reverse roller  253 , which is applied in the opposite direction to the rotational direction of the bypass feed roller  153 , exceeds the given value. Hence, in this case, the bypass reverse roller  253  spins relative to the torque limiter  60  and rotates together with the bypass feed roller  153 , thereby conveying the recording sheet P in a direction indicated by arrow G. 
     By contrast, when a plurality of recording sheets P is conveyed to the sheet separation nip region, a running torque of the bypass reverse roller  253  applied in the opposite direction to the rotational direction of the bypass feed roller  153  becomes smaller due to slippage generated between recording sheets of the plurality of recording sheets P. As a result, a driving force that rotates in the opposite direction to the rotational direction of the bypass feed roller  153  is transmitted to the bypass reverse roller  253  via the torque limiter  60 , and the bypass reverse roller  253  rotates in the opposite direction to the rotational direction of the bypass feed roller  153 . Accordingly, of the plurality of recording sheets P nipped in the sheet separation nip region, second and subsequent recording sheets P except the uppermost recording sheet P are conveyed in a direction to return to the bypass tray  51  along with rotation of the bypass reverse roller  253 . Accordingly, the second and subsequent recording sheets P of the plurality of recording sheets P are returned to the bypass tray  51 . According to this configuration, the recording sheet P (i.e., the uppermost recording sheet P) on the bypass tray  51  is separated and conveyed by the bypass separation and conveyance unit  53 . 
       FIG. 7  is a schematic perspective view illustrating an image forming device  1  of the image forming apparatus  1000 . 
     As illustrated in  FIG. 7 , the bypass sheet feeding device  4  is attached to (mounted on) a side cover  1   a . The side cover  1   a  moves about the far side (the rear side) of the image forming apparatus  1000  to open and close with respect to a housing frame  100 . 
       FIG. 8  is a diagram illustrating a schematic configuration of the housing frame  100  of the image forming device  1 , viewed from the side cover  1   a.    
     As illustrated in  FIG. 8 , the housing frame  100  is a framework of the image forming apparatus  1000  and includes four supports, which are a first front-side support  101 , a first rear-side support  102 , a second front-side support  103 , and a second rear-side support  104 . The first rear-side support  102  is disposed at a position close to the side cover  1   a  on the rear side of the image forming apparatus  1000 . The first rear-side support  102  includes two cover support pins  102   a  at a given interval in a vertical direction. The cover support pins  102   a  rotatably support the side cover  1   a . The first front-side support  101  is disposed at a position close to the side cover  1   a  on the front side of the image forming apparatus  1000 . The first front-side support  101  includes two positioning pins  101   a  at a given interval in the vertical direction. The positioning pins  101   a  are used to position the side cover  1   a.    
     It is to be noted that, in  FIG. 8 , a partition  110  is disposed to separate or partition the writing units (i.e., the first writing unit  20   a  and the second writing unit  20   b ) and the photoconductors (i.e., the photoconductors  41 Y,  41 M,  41 C, and  41 K). Further, a front and rear stay  111   a  functions as a beam member provided in the housing frame  100 . 
       FIG. 9  is an enlarged perspective view illustrating the housing frame  100  in the vicinity of the cover support pin  102   a , viewed from the inside of the image forming apparatus  1000 . It is to be noted that  FIG. 9  illustrates one of the cover support pins  102   a  but the same setting is made to both of the cover support pins  102   a.    
     As illustrated in  FIG. 9 , the cover support pin  102   a  is inserted into an opening (a hole) of a cylindrical support portion  1   a   1  provided at an end portion on the rear side of the side cover  1   a , and the retaining ring  1   b  is fitted into a groove at the tip of the cover support pin  102   a . By so doing, the side cover  1   a  is rotatably supported by the cover support pin  102   a.    
       FIG. 10A  is an enlarged perspective view illustrating the housing frame  100  in the vicinity of the positioning pins  101   a , viewed from the side cover  1   a .  FIG. 10B  is an enlarged perspective view illustrating the vicinity of the positioning pins  101   a  of the housing frame  100 , viewed from the front side of the housing frame  100 . 
     As illustrated in  FIGS. 10A and 10B , two claws  130   b  are disposed one end on the front side of the side cover  1   a . The claws  130   b  are spaced apart at an interval in the vertical direction to be fitted to the respective positioning pins  101   a . As illustrated in  FIG. 10B , the claws  130   b  are mounted on a slide  130  that is attached to the side cover  1   a  to be movable within a given range in the vertical direction. The slide  130  is supported by a slide shaft  131  extending in the vertical direction. A coil spring  132  is inserted into the lower end of the slide shaft  131 . 
     As illustrated in  FIG. 10A , the slide  130  includes a lever  130   a  that is exposed from the side cover  1   a . The lever  130   a  is pushed down by a user. A downward arrow mark is provided on the lever  130   a.    
     When opening the side cover  1   a , a user pushes the lever  130   a  downward according to the downward arrow mark on the lever  130   a.    
     Then, the slide  130  moves downward against the biasing force of the coil spring  132 . This movement of the slide  130  disengages the claws  130   b  from the positioning pins  101   a , thereby enabling the side cover  1   a  to open and close relative to the housing frame  100 . In this state, as the front side of the side cover  1   a  is pulled in a direction to separate from the housing frame  100 , the side cover  1   a  rotates about the cover support pins  102   a  to open the side cover  1   a.    
     When closing the side cover  1   a , as a user rotates the side cover  1   a  in a direction to close relative to the housing frame  100 , respective sloped faces K at the tips of the claws  130   b  come into contact the positioning pins  101   a . In this state, as the side cover  1   a  is further closed, the slide  130  is pressed downward by the positioning pins  101   a , so that the slide  130  moves downward against the biasing force of the coil spring  132 . When the positioning pins  101   a  climb over the sloped faces K, the slide  130  moves upward due to the biasing force of the coil spring  132 . According to the movement of the slide  130 , the positioning pins  101   a  are engaged with the claws  130   b , and therefore the side cover  1   a  is positioned to the housing frame  100 . 
     As described above, when the side cover  1   a  is closed, the side cover  1   a  is positioned by the positioning pins  101   a . Therefore, the bypass sheet feeding device  4  to which the side cover  1   a  is attached is positioned to a target position on the image forming device  1 . 
       FIG. 11  is a diagram illustrating transmission of vibration generated when feeding a recording sheet P set on the bypass tray  51 . 
     When the solenoid  58  is activated to cause the bypass pickup roller  52  to contact the recording sheet P loaded on the bypass tray  51 , vibrations are generated in each direction. Of the vibrations generated in each direction, the vibration in the vertical direction adversely affects image formation, for example, producing images with horizontal stripes. The vibration generated when the bypass pickup roller  52  contacts a recording sheet P loaded on the bypass tray  51  is transmitted from the bypass sheet feeding device  4  to the side cover  1   a , and then to the first front-side support  101  and the first rear-side support  102  of the housing frame  100  via the positioning pins  101   a  and the cover support pins  102   a . Then, as indicated by arrows in  FIG. 11 , these vibrations are transmitted from the first front-side support  101  and the first rear-side support  102  to the partition  110  that divides or partitions the photoconductors  41  (i.e., the photoconductors  41 Y,  41 M,  41 C, and  41 K) and the writing units  20  (i.e., the first writing unit  20   a  and the second writing unit  20   b ). 
     The partition  110  is attached to the housing frame  100  in a direction perpendicular to the vertical direction. In other words, the partition  110  is attached to the housing frame  100  in a horizontal direction. The partition  110  is a thin plate across the vertical direction, and is therefore deformable in the vertical direction. Therefore, among the vibrations generated when feeding the recording sheet P, when the vibration in the vertical direction is transmitted to the partition  110 , the partition  110  significantly vibrates in the vertical direction. 
       FIG. 12  is a diagram illustrating an example of a horizontal stripe image generated by vibration of the first writing unit  20   a  and the second writing unit  20   b.    
     There is a case in which the first writing unit  20   a  and the second writing unit  20   b  are supported directly to the partition  110 . There is another case in which a writing stay functioning as an attachment member, to which the writing units  20  (i.e., the first writing unit  20   a  and the second writing unit  20   b ) are attached, is fastened to the partition  110 , and therefore are supported by the partition  110 . In these cases, the first writing unit  20   a  and the second writing unit  20   b  significantly vibrate together with the partition  110 . In a case in which a latent image is written to the photoconductor  41  (i.e., the photoconductors  41 Y,  41 M,  41 C, and  41 K) while the first writing unit  20   a  and the second writing unit  20   b  are vibrating, the writing position of laser light to be emitted onto the photoconductor  41  (in other words, an emission position of laser light on the photoconductor  41 ) changes to the sub-scanning direction. As a result, as illustrated in  FIG. 12 , it is likely that a horizontal streak image Q, which extends in a direction perpendicular to the sheet conveyance direction, is generated on an image formed on the recording sheet P. 
       FIG. 13  is a schematic plan view illustrating the housing frame  100  according to the present embodiment.  FIG. 14A  is a plan view illustrating a schematic configuration of the housing frame  100  with the first writing unit  20   a  and the second writing unit  20   b  removed.  FIG. 14B  is a cross sectional view of the housing frame  100 , along a dotted line of α-α in  FIG. 14A .  FIG. 14C  is a cross sectional view of the housing frame  100 , along a dotted line of β-β in  FIG. 14A . 
     As illustrated in  FIGS. 13, 14A, 14B, and 14C , the housing frame  100  includes the first front-side support  101 , the first rear-side support  102 , the second front-side support  103 , the second rear-side support  104 , a front side panel  113   a , and a rear side panel  113   b . The front side panel  113   a  and the rear side panel  113   b  function as side members and are disposed facing each other and spaced apart at a given interval in the axial direction of the photoconductors  41 . The front side panel  113   a  is fastened to the first front-side support  101  and the second front-side support  103 , both are disposed on the front side of the housing frame  100 . The rear side panel  113   b  is fastened to the first rear-side support  102  and the second rear-side support  104 , both are disposed on the rear side of the housing frame  100 . The housing frame  100  further includes three front and rear stays, which are a first front and rear stay  111   a , a second front and rear stay  111   b , and a third front and rear stay  111   c . These front and rear stays (i.e., the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ) function as beam members and are fastened to the front side panel  113   a , the rear side panel  113   b , and the partition  110 . In other words, the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c  are aligned in a direction parallel to the partition  110  (i.e., the horizontal direction), at given intervals. To be more specific, the first front and rear stay  111   a  is disposed on the housing frame  100 , at one end on a side closer to the bypass sheet feeding device  4 , and the third front and rear stay  111   c  is disposed on the housing frame  100 , at the other end on an opposite side to the bypass sheet feeding device  4 . The second front and rear stay  111   b  is disposed between the first front and rear stay  111   a  and the third front and rear stay  111   c , substantially at the center of the housing frame  100 . 
     A first writing stay  112 - 1  and a second writing stay  112 - 2  are disposed (spaced apart) at a given interval between the first front and rear stay  111   a  and the second front and rear stay  111   b . The first writing stay  112 - 1  and the second writing stay  112 - 2  function as attachment members. The second writing unit  20   b  is attached to the first writing stay  112 - 1  and the second writing stay  112 - 2  to write latent images to the photoconductor  41 K for a black color image and the photoconductor  41 C for a cyan color image. It is to be noted that the second writing unit  20   b , the first writing stay  112 - 1 , and the second writing stay  112 - 2  compose a writing device. Similarly, a third writing stay  112 - 3  and a fourth writing stay  112 - 4  are disposed (spaced apart) at a given interval between the second front and rear stay  111   b  and the third front and rear stay  111   c . The third writing stay  112 - 3  and the fourth writing stay  112 - 4  function as attachment members. The first writing unit  20   a  is attached to the third writing stay  112 - 3  and the fourth writing stay  112 - 4  to write latent images to the photoconductor  41 M for a magenta color image and the photoconductor  41 Y for a yellow color image. It is to be noted that the first writing unit  20   a , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4  compose a writing device. 
     To be more specific, as illustrated in  FIG. 14A , two through holes  110   a  are provided on the partition  110 , between the first front and rear stay  111   a  and the second front and rear stay  111   b . The laser light of the second writing unit  20   b  passes through the through holes  110   a  to emit the photoconductors  41  (i.e., the photoconductors  41 C and  41 K). The first writing stay  112 - 1  is disposed between the first front and rear stay  111   a  and the through hole  110   a  on the near side of the bypass sheet feeding device  4 . The second writing stay  112 - 2  is disposed between the second front and rear stay  111   b  and the through hole  110   a  on the far side of the bypass sheet feeding device  4 . 
     Further, two through holes  110   a  are provided on the partition  110 , between the second front and rear stay  111   b  and the third front and rear stay  111   c . The third writing stay  112 - 3  is disposed between the second front and rear stay  111   b  and the through hole  110   a  on the near side of the bypass sheet feeding device  4 . The fourth writing stay  112 - 4  is disposed between the third front and rear stay  111   c  and the through hole  110   a  on the far side to the bypass sheet feeding device  4 . 
     Hereinafter, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4  are occasionally referred to as a writing stay  112  in a singular form when there is no need to particularly distinguish each writing stay. At both ends in the front-and-back direction (i.e., the axial direction of the photoconductor  41 ) of the writing stay  112 , two fastening portions  112   c  that fasten the front side panel  113   a  and the rear side panel  113   b  are provided along a direction of alignment of the front and rear stays  111  (i.e., the first front and rear stay  111   a , the second front and front and rear stay  111   b , and the third front and rear stay  111   c ). Each writing stay  112  is fastened to the front side panel  113   a  and the rear side panel  113   b  by screws  114 . 
     Thus, in the present embodiment, each writing stay  112  is fastened (fixed) to the front side panel  113   a  and the rear side panel  113   b  but is not fastened (fixed) to the partition  110 . In other words, each writing stay  112  is fixed to the front side panel  113   a  and the rear side panel  113   b  and is spaced apart from the partition  110 . With this configuration, the writing stay  112  does not vibrate together with the partition  110 , thereby restraining vibration of the writing stay  112 . Accordingly, vibrations of the first writing unit  20   a  and the second writing unit  20   b  supported by the writing stay  112  are restrained. 
     Further, in the present embodiment, each writing stay  112  is fastened to the front side panel  113   a  and the rear side panel  113   b  and not to the partition  110 . With this configuration, vibration of the bypass sheet feeding device  4  is transmitted to the writing stay  112  via the front side panel  113   a  and the rear side panel  113   b . The front side panel  113   a  and the rear side panel  113   b  are disposed in parallel in the vertical direction. Therefore, vibrations of the front side panel  113   a  and the rear side panel  113   b  are hardly transmitted in the vertical direction in comparison to the partition  110  that is disposed in a direction perpendicular to the vertical direction. Therefore, the vibration in the vertical direction from the bypass sheet feeding device  150  is significantly reduced even when the vibration is transmitted to each writing stay  112  via the front side panel  113   a  and the rear side panel  113   b . As a result, vibration of each writing stay  112  is restrained, and vibrations of the first writing unit  20   a  and the second writing unit  20   b  supported by each writing stay  112  are also restrained. Consequently, production of images with horizontal streaks as illustrated in  FIG. 12  is prevented. 
     Further, as illustrated in  FIG. 14C , each writing stay  112  is fastened to the front side panel  113   a  and the rear side panel  113   b , at a position spaced away from the partition  110 , and therefore each writing stay  112  is not in contact with the partition  110 . As described above, by disposing each writing stay  112  to be spaced away from the partition  110 , vibration of the partition  110  is not transmitted directly. Accordingly, vibrations of the first writing unit  20   a  and the second writing unit  20   b  supported by the writing stay  112  are restrained. 
     Screw fastening portions  201  to be fastened to the writing stay  112  are provided at the four corners of the first writing unit  20   a  and the second writing unit  20   b . Positioning holes are formed at the screw fastening portions  201  at both ends in the front-and-back direction of the first writing unit  20   a  and the second writing unit  20   b , on the near side of the bypass sheet feeding device  4 . A positioning projection  112   a  is provided on the rear side of the first writing stay  112 - 1  and another positioning projection  112   a  is provided on the rear side of the third writing stay  112 - 3 . Similarly, a positioning projection  112   b  is provided on the front side of the first writing stay  112 - 1  and another positioning projection  112   b  is provided on the front side of the third writing stay  112 - 3 . In addition, screw holes  112   d  are provided on the front side and the rear side of each writing stay  112 . A screw groove is formed on an inner circumferential surface of each of the screw holes  112   d . In addition, a base  112   f  is disposed around the screw hole  112   d  of each writing stay  112  (see  FIG. 15 ) to increase the positional accuracy in each writing stay  112  in the vertical direction. The flatness of the bearing surface of the base  112   f  is set higher than the flatness of the surface of each writing stay  112 . 
     The second writing unit  20   b  is positioned by the positioning projection  112   a  and the positioning projection  112   b , provided on the first writing stay  112 - 1 . Then, by screwing screws  115  into the respective screw holes  112   d  provided in the first writing stay  112 - 1  and the second writing stay  112 - 2 , the second writing unit  20   b  contacts the base  112   f  (see  FIG. 15 ). Therefore, the first writing stay  112 - 1  and the second writing stay  112 - 2  are fastened with the screws  115 . The first writing unit  20   a  is positioned by the positioning projection  112   a  provided on the third writing stay  112 - 3 . Then, by screwing the screws  115  into the respective screw holes  112   d  provided in the third writing stay  112 - 3  and the fourth writing stay  112 - 4 , the first writing unit  20   a  contacts the base  112   f  (see  FIG. 15 ). Therefore, the third writing stay  112 - 3  and the fourth writing stay  112 - 4  are fastened with the screws  115 . 
     In the present embodiment, the writing units  20  are fastened to each writing stay  112 , thereby fixing the writing units  20  to each writing stay  112  reliably and firmly. Consequently, the rattling of the writing units  20  is prevented, and therefore vibrations of the writing units  20  are restrained. 
     Each writing stay  112  is formed by bending a sheet metal. As illustrated in FIG.  14 C, each writing stay  112  has a cross-sectional arch shape including a mounting face and a pair of reinforcement faces. The mounting face extends in the vertical direction, and the writing units  20  are mounted on the mounting face. The pair of reinforcement faces extends downward from both ends in the left and right directions (the horizontal direction) of the mounting face and perpendicular to the mounting face. As described above, each writing stay  112  has a polygonal cross-sectional shape with three faces. Therefore, the writing stay  112  has higher rigidity and is less affected by vibration, than the partition  110  having a planar shape. Accordingly, even when the vibration of the bypass sheet feeding device  150  is transmitted to the writing stay  112 , the vibration of the writing stay  112  is restrained, and the vibration of the writing units  20  supported by the writing stay  112  is therefore restrained. 
     Further, in the present embodiment, the contact face of the writing stay  112  on which the writing stay  112  contacts at least the writing units  20  (in other words, the bearing surface of the base  112   f ) is flat. Therefore, the writing units  20  are attached (mounted) stably, and the vibration of the writing units  20  are restrained. 
     Furthermore, in the present embodiment, the first writing unit  20   a  and the second writing unit  20   b  are fastened to the writing stays  112  alone, vibration of any other member is not transmitted to the first writing unit  20   a  and the second writing unit  20   b , and therefore the first writing unit  20   a  and the second writing unit  20   b  are restrained from receiving such vibration. Accordingly, the vibrations to the first writing unit  20   a  and the second writing unit  20   b  are restrained or prevented. 
     Next, a detailed description is given of attachment of the writing stay  112  to the front side panel  113   a  and the rear side panel  113   b . Each writing stay  112  functions as an attachment member to which the writing unit is attached and as a positioner to position the writing units along the vertical direction. Therefore, each writing stay  112  is positioned and attached to the front side panel  113   a  and the rear side panel  113   b  in the vertical direction. In the following description, attachment of the first writing stay  112 - 1  is described. However, it is to be noted that the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4  are attached to corresponding side panel in the same steps as the first writing stay  112 - 1 . 
       FIG. 15A  is an enlarged perspective view of the first writing stay  112 - 1  in the vicinity of the rear side panel  113   b .  FIG. 15B  is an enlarged perspective view of the first writing stay  112 - 1  in the vicinity of the front side panel  113   a.    
     As illustrated in  FIG. 15A , the rear side panel  113   b  has a rear-end positioning hole  113   b   1  to position the rear end of the first writing stay  112 - 1 . A rear-end positioning projection  112   e   1  is provided at the rear end of the first writing stay  112 - 1 . The rear-end positioning projection  112   e   1  of the first writing stay  112 - 1  is fitted into the rear-end positioning hole  113   b   1 , so that the rear side of the first writing stay  112 - 1  is positioned in the vertical direction. 
     Further, as illustrated in  FIG. 15B , the front side panel  113   a  has a front-end positioning hole  113   a   1  to position the front end of the first writing stay  112 - 1 . A front-end positioning projection  112   e   2  is provided at the front end of the first writing stay  112 - 1 . The front-end positioning projection  112   e   2  of the first writing stay  112 - 1  is fitted into the front-end positioning hole  113   a   1 , so that the front side of the first writing stay  112 - 1  is positioned in the vertical direction. 
     As described above, the first writing stay  112 - 1  is fixed (fastened) to the front side panel  113   a  and the rear side panel  113   b  while being positioned to the front side panel  113   a  and the rear side panel  113   b  in the vertical direction. According to this configuration, the first writing stay  112 - 1  is accurately mounted on the target position in the vertical direction on the housing frame  100 . Similarly, other writing stays, which are the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 , are mounted on the front side panel  113   a  and the rear side panel  113   b . Accordingly, the first writing unit  20   a  and the second writing unit  20   b  to be fastened to each writing stay  112  are attached to (mounted on) the target position in the vertical direction. 
       FIG. 16  is a graph illustrating moments of inertia of the first front and rear stay  111   a , the second front and rear stay  111   b , and the writing stay  112 .  FIG. 17  is a diagram illustrating a layout of the writing stays  112 . 
     As illustrated in  FIG. 16 , the moment of inertia of the first front and rear stay  111   a  and the moment of inertia of the second front and rear stay  111   b  are far greater than the moment of inertia of the writing stays  112 . In the present embodiment, the first front and rear stay  111   a , the second front and rear stay  111   b , and the writing stay  112  share the same material and the same lengths in the front-and-back directions. That is, the first front and rear stay  111   a  and the second front and rear stay  111   b  have flexural rigidity far stronger than the writing stays  112 . It is to be noted that the flexural rigidity is expressed in the following equation: Flexural Rigidity=EI/L, where “E” represents Young&#39;s modulus, “I” represents the moment of inertia, and “L” represents the length in the front-and-back direction. 
     Further, the material of the front and rear stays and the material of the writing stay may be different from each other, the Young&#39;s modulus of the front and rear stays may be greater than the Young&#39;s modulus of the writing stay  112 , and the flexural rigidity of the front and rear stays is greater than the flexural rigidity of the writing stay  112 . 
     As described above, the front and rear stays having greater flexural rigidity are fastened to the front side panel  113   a  and the rear side panel  113   b . By so doing, areas on the front side panel  113   a  and the rear side panel  113   b , near the contact portions at which the front and rear stays contact the front side panel  113   a  and the rear side panel  113   b , are less affected by vibration than areas other than the areas near the contact portions. Therefore, a region (i.e., a region C 1  indicated by a broken line in  FIG. 17 ) on the front side panel  113   a  and the rear side panel  113   b  between two front and rear stays (i.e., the first front and rear stay  111   a  and the second front and rear stay  111   b ) is an area to which vibration is hardly transmitted because the vibration is terminated (cut) near the contact portions at which the front and rear stays contact the front side panel  113   a  and the rear side panel  113   b . Accordingly, by fastening the writing stay  112  at any position in the region C 1  illustrated in  FIG. 17  on the front side panel  113   a  and the rear side panel  113   b , the vibration of the writing stay  112  is restrained preferably. 
     In the present embodiment, the second front and rear stay  111   b  is disposed at the center in the left-right direction of the image forming device  1 . With this configuration, the writing stays  112  (i.e., the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) are disposed near the front and rear stays (i.e., the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ). As a result, the writing stays  112  (i.e., the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) are fastened to the front side panel  113   a  and the rear side panel  113   b , in the areas near the contact portions with the front and rear stay, which are the areas hardly affected by vibration (i.e., a region indicated by a broken line C 2  in  FIG. 17 ). Accordingly, vibration of the writing stays  112  are more restrained. 
     Next, a description is given of a verification experiment for verifying vertical vibration for a comparative example and the present embodiment. In the comparative example, each writing stay  112  is fastened to the partition  110 . 
       FIGS. 18A, 18B, and 18C  are diagrams illustrating a schematic configuration of a main part of the comparative example. 
     To be more specific,  FIG. 18A  is a plan view illustrating a schematic configuration of a housing frame of the comparative example.  FIG. 18B  is a cross sectional view of the housing frame of the comparative example, along a dotted line of α-α in  FIG. 18A .  FIG. 18C  is a cross sectional view of the housing frame of the comparative example, along a dotted line of β-β in  FIG. 18A . As indicated by broken lines Z illustrated in  FIGS. 18A, 18B, and 18C , in the comparative example, each writing stay  112  is fastened to the front side panel  113   a  and the rear side panel  113   b , and two front portions and two rear portions of each writing stay  112  are fastened to the partition  110 . 
     With respect to the comparative example and the present embodiment, the measurement was performed on vibration in the vertical direction (vertical vibration) of the bypass sheet feeding device  150  and vibration in the vertical direction (vertical vibration) of the first writing unit  20   a.    
       FIG. 19  is a diagram illustrating measurement points of vibration in the vertical direction of the bypass sheet feeding device  150 . 
     As indicated by a portion S 1  illustrated in  FIG. 19 , vibration in the vertical direction of the bypass sheet feeding device  150  was measured at the approximate center in the front-and-back direction (the axial direction) of a bracket  150   a  that holds the solenoid  58  of the bypass sheet feeding device  150 . 
       FIG. 20  is a diagram illustrating measurement points of vibration in the vertical direction of the second writing unit  20   b.    
     As indicated by a portion S 2  illustrated in  FIG. 20 , the vertical vibration of the second writing unit  20   b  was measured at the portion S 2  between the first mirror  303   a  disposed on the optical path of laser light to be emitted to the photoconductor  41 K (for forming a black color image) provided in the housing of the second writing unit  20   b  and the long lens  302  disposed on the optical path of laser light to be emitted to the photoconductor  41 C (for forming a cyan color image) provided in the housing of the second writing unit  20   b.    
       FIG. 21A  is a graph illustrating a waveform of measured vibration in the vertical direction of a comparative bypass sheet feeding device.  FIG. 21B  is a graph illustrating a waveform of measured vibration in the vertical direction of a comparative second writing unit.  FIG. 22A  is a graph illustrating a waveform of measured vibration in the vertical direction of the bypass sheet feeding device  150  of the present embodiment.  FIG. 22B  is a graph illustrating a waveform of measured vibration in the vertical direction of the second writing unit  20   b.    
     For convenience, the comparative example employs the same reference numerals as the present embodiment. As can be clearly seen from an area X 1  in  FIG. 21B , the vertical vibration of the second writing unit  20   b  increases at a timing slightly delayed from a timing at which the bypass sheet feeding device  150  vibrates upward and downward. In the configuration of the comparative example, the writing stays  112  are fastened to the front side panel  113   a , the rear side panel  113   b , and the partition  110 . Therefore, the vibration in the vertical direction of the bypass sheet feeding device  150  is transmitted to the partition  110 , which vibrated the writing stays  112  together with the partition  110 . 
     By contrast, in the present embodiment, as indicated by an area X 2  illustrated in FIG.  22 B, even at the timing slightly delayed from the timing at which the bypass sheet feeding device  150  vibrates upward and downward, the second writing unit  20   b  hardly vibrates in the vertical direction. As described above, it was confirmed that, by employing the configuration of the present embodiment, the vertical vibration of the bypass sheet feeding device  150  to be transmitted to the writing stays  112  is reduced, and that the vibration of the writing unit supported by the writing stays  112  is restrained. 
     Next, a description is given of a schematic configuration of a variation of the image forming apparatus  1000 . 
       FIGS. 23A, 23B, and 23C  are diagrams illustrating a schematic configuration of the main part of the variation. To be more specific,  FIG. 23A  is a plan view illustrating a schematic configuration of the housing frame  100  of the variation.  FIG. 23B  is a cross sectional view of the housing frame  100  of the variation, along a dotted line of α-α in  FIG. 23A .  FIG. 23C  is a cross sectional view of the housing frame  100  of the variation, along a dotted line of β-β in  FIG. 23A . 
     For convenience, the variation employs the same reference numerals as the present embodiment. As illustrated in  FIGS. 23A, 23B, and 23C , the second writing unit  20   b  in the variation is disposed on the side close to the bypass sheet feeding device  150  that is a vibration source and is attached to the first writing stay  112 - 1  and the second writing stay  112 - 2 . The first writing stay  112 - 1  and the second writing stay  112 - 2  in the variation are fastened to the front side panel  113   a  and the rear side panel  113   b  and not to the partition  110 . By contrast, the first writing unit  20   a  in the variation is disposed on the side far from the bypass sheet feeding device  150  and is attached to the third writing stay  112 - 3  and the fourth writing stay  112 - 4 . The third writing stay  112 - 3  and the fourth writing stay  112 - 4  in the variation are fastened to the front side panel  113   a , the rear side panel  113   b , and the partition  110 . 
     In the comparative example illustrated in  FIGS. 18A through 18C , the mechanical rigidity of the entire housing frame  100  may be increased by fastening the writing stay  112  to the partition  110  in addition to the front side panel  113   a  and the rear side panel  113   b . However, as described above, if the writing stay  112  is fastened to the partition  110  in addition to the front side panel  113   a  and the rear side panel  113   b , the writing units are likely to greatly vibrate in the vertical direction. 
     However, as the writing unit moves away from the bypass sheet feeding device  150  that is a vibration source, the vertical vibration attenuates. Further, the vibration in the vertical direction of the bypass sheet feeding device  150  is transmitted from the partition  110 , on the near side of the bypass sheet feeding device  150 . The vertical vibration of the bypass sheet feeding device  150  is transmitted from the partition  110 , on the near side of the bypass sheet feeding device  150 . The vertical vibration of the bypass sheet feeding device  150  is reduced at the contact portion on the second front and rear stay  111   b  that has the large flexural rigidity. As a result, vibration on the partition  110  generating on the far side of the bypass sheet feeding device  150  from the second front and rear stay  111   b  is more reduced (smaller) than vibration on the partition  110  generating on the near side of the bypass sheet feeding device  150  from the second front and rear stay  111   b . Therefore, even though the third writing stay  112 - 3  and the fourth writing stay  112 - 4  are fastened to the partition  110 , the writing units do not vibrate largely together with the partition  110 . Accordingly, the first writing unit  20   a  that is attached to the third writing stay  112 - 3  and the fourth writing stay  112 - 4  does not vibrate greatly. In other words, the second writing unit  20   b  that is disposed closer to the vibration source vibrates largely. Therefore, a writing unit disposed closest to a vibration source vibrates largely and is not fixed to the partition  110 . 
     Further, the third writing stay  112 - 3  and the fourth writing stay  112 - 4  are fastened to the partition  110 , so that mechanical rigidity of the entire housing frame  100  is enhanced (increased) when compared with the configuration in which not any writing stay  112  (i.e., the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) is fastened with the partition  110 . 
     The configurations according to the above-descried embodiments are not limited thereto. This disclosure can achieve the following aspects effectively. 
     Aspect 1. 
     In Aspect 1, an image forming apparatus (for example, the image forming apparatus  1000 ) includes an image bearer (for example, the photoconductors  41 Y,  41 M,  41 C, and  41 K), a writing device (for example, the writing units  20   a  and  20   b  and the writing stays  112 - 1 ,  112 - 2 ,  112 - 3 , and  112 - 4 ), a partition (for example, the partition  110 ), and a plurality of side members (for example, the front side panel  113   a  and the rear side panel  113   b ). The image bearer is configured to bear an image. The writing device is configured to write the image on the image bearer. The partition is disposed between the image bearer and the writing device. The plurality of side members is disposed facing and spaced apart from each other at an interval in an axial direction of the image bearer. The writing device is fixed to the plurality of side members and being spaced apart from the partition. 
     As described in the embodiments above, the partition has a planar shape and described in the embodiment, the partition plate is plate-shaped, and is likely to vibrate easily in a direction perpendicular to the surface of the partition. The image forming apparatus is provided with a contact and separation member (for example, the bypass pickup roller  52 ) to contact and separate with respect to the recording sheet (for example, the recording sheet P). Due to impact generated when the contact and separation member contacts a counter member, vibration may occur in the direction perpendicular to the surface of the partition in the image forming apparatus. When this vibration is transmitted to the partition, the partition significantly vibrates in the direction perpendicular to the surface of the partition in the image forming apparatus. 
     In a known image forming apparatus, a writing device is fastened (fixed) to the partition. Therefore, as the partition vibrates in the direction perpendicular to the surface of the partition as described above, the writing device vibrates together with the partition, and therefore it was likely to generate an image unevenness in a specific cycle. 
     By contrast, in Aspect 1, the writing device is fastened (fixed) to the plurality of side members (for example, the front side panel  113   a  and the rear side panel  113   b ) which are separate members from the partition. As a result, the writing device is prevented from vibrating together with the partition. Further, since the writing device does not contact the partition, the vibration of the partition is restrained from transmitting to the writing device directly, and therefore the vibration of the writing device is prevented. Accordingly, the vibration of the writing device is preferably prevented, and therefore the image unevenness generated in the specific cycle is restrained reliably. 
     Aspect 2. 
     In Aspect 1, the plurality of side members are side panels (for example, the front side panel  113   a  and the rear side panel  113   b ). 
     According to this configuration, the front side panel  113   a  and the rear side panel  113   b  hardly vibrate in the direction perpendicular to the surface of the partition, and therefore the side panels are prevented from vibrating together with the partition. Since the writing device is fixed to the side plates, vibration of the writing device is restrained. 
     Aspect 3. 
     In Aspect 1 or Aspect 2, the writing device includes a writing unit (for example, the first writing unit  20   a  and the second writing unit  20   b ) and an attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ). The writing unit is configured to write the image on the image bearer. The attachment member is configured to attach the writing unit. 
     According to this configuration, the writing unit is less likely to vibrate by attaching the writing unit to the attachment member. Accordingly, the vibration of the writing device is preferably restrained, and therefore the image unevenness generated in the specific cycle is restrained reliably. 
     Aspect 4. 
     In Aspect 3, the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) is fixed to the plurality of side members (for example, the front side panel  113   a  and the rear side panel  113   b ) and be spaced apart from the partition (for example, the partition  110 ). 
     According to this configuration, the attachment member is prevented from vibrating together with the partition. Further, the attachment member does not contact the partition, the vibration of the partition  110  is restrained from directly transmitting to the attachment member, and therefore the vibration of the attachment member is prevented. Accordingly, the vibration of the writing unit (for example, the first writing unit  20   a  and the second writing unit  20   b ) attached to the attachment member is preferably restrained, and therefore the image unevenness generated in the specific cycle is restrained reliably. 
     Aspect 5. 
     In Aspect 3 or Aspect 4, the writing unit (for example, the first writing unit  20   a  and the second writing unit  20   b ) is disposed in contact with only the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third 
     According to this configuration, vibration is transmitted from the attachment member whose vibration is restrained, and therefore the vibration of the writing unit is prevented reliably. 
     Aspect 6. 
     In any one of Aspects 3 to 5, the flexural rigidity of the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) is greater than the flexural rigidity of the partition (for example, the partition  110 ). 
     According to this configuration, the attachment member is less likely to vibrate than the partition  110 , and therefore the vibration of the writing unit (for example, the first writing unit  20   a  and the second writing unit  20   b ) is restrained. 
     Aspect 7. 
     In Aspect 6, the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) has a polygonal cross-sectional shape. (In the present embodiment, the attachment member has an arch shape.) 
     According to this configuration, as described in the embodiments above, the rigidity of the attachment member is greater than the rigidity of the partition (for example, the partition  110 ) having a planar shape. 
     Aspect 8. 
     In any one of Aspects 3 to 7, the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) has a contact portion (for example, the base  112   f ) to contact the writing unit (for example, the first writing unit  20   a  and the second writing unit  20   b ), and a contact face of the contact portion to contact the writing unit is flat. 
     According to this configuration, as described in the embodiments above, the writing unit is attached stably, and the vibration of the writing unit is restrained. 
     Aspect 9. 
     In any one of Aspects 3 to 8, the image forming apparatus further includes a beam member (for example, the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ) fixed to the plurality of side members (for example, the front side panel  113   a  and the rear side panel  113   b ). The flexural rigidity of the beam member is greater than the flexural rigidity of the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ). 
     According to this configuration, as described in the embodiments above, the vibration of the side members (in other words, the pair of side panels) is restrained. Accordingly, the vibration of the attachment member that is fixed to the side members is restrained, and therefore the vibration of the writing device is restrained. 
     Aspect 10. 
     In Aspect 9, the moment of inertia of the beam member (for example, the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ) is greater than the moment of inertia of the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ). 
     According to this configuration, as described in the embodiments above, the rigidity of the beam member is greater than the rigidity of the attachment member. 
     Aspect 11. 
     In Aspect 9 or Aspect 10, a Young&#39;s modulus of the beam member (for example, the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ) is greater than a Young&#39;s modulus of the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ). 
     According to this configuration, as described in the embodiments above, the rigidity of the beam member is greater than the rigidity of the attachment member. 
     Aspect 12. 
     In any one of Aspects 9 to 11, the image forming apparatus (for example, the image forming apparatus  1000 ) further includes a plurality of beam members (for example, the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ) including the beam member and being disposed in a direction parallel to the partition (for example, the partition  110 ) at given intervals. 
     According to this configuration, as described with reference to  FIG. 17 , the attachment member (for example, the first writing stay  112 - 1 , the second writing stay  112 - 2 , the third writing stay  112 - 3 , and the fourth writing stay  112 - 4 ) is fastened (fixed) to the region where the vibration of the plurality of side members (for example, the front side panel  113   a  and the rear side panel  113   b ) is hardly transmitted, and the vibration of the attachment member is reduced. Thus, the vibration of a writing device (for example, the writing units  20   a  and  20   b  and the writing stays  112 - 1  to  112 - 4 ) supported by the attachment member is restrained. 
     Aspect 13. 
     In any one of Aspects 9 to 12, the beam member (for example, the first front and rear stay  111   a , the second front and rear stay  111   b , and the third front and rear stay  111   c ) is fixed to the partition (for example, the partition  110 ). 
     According to this configuration, the mechanical rigidity of the entire housing frame (for example, the housing frame  100 ) is increased. 
     Aspect 14. 
     In any one of Aspects 1 to 13, the image forming apparatus (for example, the image forming apparatus  1000 ) further includes a plurality of writing devices (for example, the writing units  20   a  and  20   b  and the writing stays  112 - 1 ,  112 - 2 ,  112 - 3 , and  112 - 4 ) including the writing device. At least one writing device of the plurality of writing devices is fixed to the plurality of side members (for example, the front side panel  113   a  and the rear side panel  113   b ) and is spaced apart from the partition (for example, the partition  110 ). The plurality of writing devices except the at least one writing device is fixed to the plurality of side members and disposed in contact with the partition. 
     According to this configuration, the vibration of a specific writing device (that is, the at least one writing device) is restrained. 
     Aspect 15. 
     In Aspect 14, the at least one writing device of the plurality of writing devices (for example, the writing units  20   a  and  20   b  and the writing stays  112 - 1 ,  112 - 2 ,  112 - 3 , and  112 - 4 ) is a writing device (for example, the second writing unit  20   b  in the present embodiment) disposed closest to a vibration source (for example, the bypass sheet feeding device  150  in the present embodiment). 
     According to this configuration, as described in the embodiments above, the vibration of the specific writing device (the second writing unit  20   b  in the present embodiment) is restrained. 
     The effects described in the embodiments of this disclosure are listed as most preferable effects derived from this disclosure, and therefore are not intended to limit to the embodiments of this disclosure. 
     The embodiments described above are presented as an example to implement this disclosure. The embodiments described above are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, or changes can be made without departing from the gist of the invention. These embodiments and their variations are included in the scope and gist of the invention, and are included in the scope of the invention recited in the claims and its equivalent. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.