Patent Publication Number: US-8532557-B2

Title: Attachment assist device and image forming apparatus including same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent specification is based on and claims priority from Japanese Patent Application Nos. 2009-167733, filed on Jul. 16, 2009, and 2010-064242 filed on Mar. 19, 2010 in the Japan Patent Office, the contents of which are hereby incorporated by reference herein in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an attachment assist device to assist attachment of a retractable unit to a main body, a sheet feeder including the same, and an image forming apparatus including the same. 
     2. Discussion of the Background Art 
     In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, include a reading unit to read image data of documents, an image forming unit to form images on sheets of recording media according to the image data ready by the reading unit, and a sheet feeder to feed the sheets to the image forming unit. The image forming unit includes an optical writing device to direct a writing light (i.e., a laser beam) onto a surface of an image carrier (i.e., a photoconductor), thus forming an electrostatic latent image thereon, and a development device to develop the latent image with toner. In multicolor image formation, the image forming unit forms cyan, magenta, yellow, and black toner images on a single photoconductor or respective photoconductors, which are transferred therefrom and superimposed one on another on the sheet, thus forming a multicolor image on the sheet. 
     Image forming apparatuses further include a sheet tray that can contain multiple sheets and be retracted into and pulled out from a main body. For example, the sheet feeder includes a pickup roller to feed the sheets contained in the sheet tray to the image forming unit sequentially from the top. When the sheet tray is empty or when users desire to change a sheet size or the like, the user pulls the sheet tray out of the main body, fills the sheet tray with sheets, and then pushes the sheet tray back into the main body. 
     In such image forming apparatuses, if the sheet tray is not properly positioned in the sheet feeder or main body in a direction in which the sheet tray is properly inserted into the main body (hereinafter “insertion direction”), the image forming apparatus can form a substandard image on the sheet, with the image deviating from the center of the sheet in a width direction of the sheet. However, when the sheet tray is too heavy, the user has to press the sheet tray with greater force to insert the sheet tray into the sheet feeder. Yet, if the user pushes the heavy sheet tray with excessive force, the sheet tray might hit the sheet feeder, impacting the sheet tray as well as the sheet feeder. The impact to the sheet tray can disturb the sheets stacked in the sheet tray and damage the sheet tray. 
     In view of the foregoing, several approaches described below have been tried to assist attachment of the sheet tray into the apparatus, in particular, insertion of the sheet tray into the main body of the apparatus. 
     For example, JP-2006-151687-A discloses an image forming apparatus that includes an engagement pin provided in the sheet tray, a drawing mechanism to draw the sheet tray into the main body, and a guide groove formed in the main body to guide the engagement pin, thus guiding insertion of the sheet tray into the main body. The drawing mechanism includes a toggle spring and a pivotable arm to engage the engagement pin provided in the sheet tray. Additionally, the guide groove includes a linear portion in parallel to the insertion direction of the sheet tray and a bent portion bent toward a pivot point of the pivotable arm. 
     With this configuration, when the pivotable arm pivots to a predetermined position with the engagement pin engaged with the pivotable arm and guided by the guide groove, the drawing mechanism pulls the sheet tray via the engagement pin with the bias force exerted by the toggle spring. Additionally, when the engagement pin is guided by the bent portion of the guide groove bent toward the pivot point of the pivotable arm, the pivotable arm can pivot easily with the bias force of the toggle spring. 
     Additionally, for example, JP-2007-70068-A discloses a velocity-dependent damper unit to adjust a load for decelerating movement of the sheet tray according to a velocity with which the sheet tray is drawn by the bias force of the toggle spring. Thus, fluctuations in the velocity of the movement of the sheet tray can be reduced. 
     Although such an arrangement has advantages, it also has several drawbacks. 
     For example, because the sheet tray should be pulled into the main body against various resistive forces acting on the sheet tray, using the damper unit can increase the force required for pulling out the sheet tray from the main body. Examples of such resistive forces include sliding resistance due to the weight of sheets contained in the sheet tray, frictional resistance in positioning the sheet tray, and resistance of a sheet feed mechanism being engaged or disengaged from the sheet tray. 
     At present, barrier-free facilities have been promoted to provide easier access for elderly people and people with disabilities, and, in December 2000, the U.S. government released Section 508 of the Rehabilitation Act, specifying standards for accessibility. More specifically, Section 508 specifies that the maximum force required to activate controls and keys operated mechanically shall be 5 lbs. (22.2 N). Therefore, the increase in the force to pull the sheet tray should be limited, and thus it is important to reduce the resistance in insertion of the sheet tray while increasing efficiency in insertion of the sheet tray. 
     The above-described problem is not limited to drawing the sheet tray but is also present in drawing any retractable unit, such as a duplex unit for forming images on both sides of sheets, that is closably openable relative to the main body of the image forming apparatus. 
     Therefore, the inventors of the present invention recognize that there is a need for an attachment assist device capable of positioning the retractable unit properly at a predetermined position in the main body as well as drawing the retractable unit into the main body efficiently. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, one illustrative embodiment of the present invention provides an attachment assist device to assist attachment of a retractable unit to a main body. 
     The attachment assist device includes an engagement member provided in one of the main body and the retractable unit, a catch portion provided in the other of the retractable unit and the main body, to engage the engagement member, and a drawing unit provided in the main body, connected to the retractable unit via the engagement member and the catch portion. Multiple resistive forces act on the retractable unit while the retractable unit moves from a drawing start position in an attachment direction to a drawing completion position in the main body. When the engagement member engages the catch portion, the drawing unit draws the retractable unit from the drawing start position to the drawing completion position against the multiple resistive force. The catch portion engages the engagement member when the retractable unit is set at the drawing start position. An engagement position where the engagement member engages the catch portion is disposed inside a rectangular area defined by two opposing corners respectively positioned at points of action of two of the multiple resistive forces acting on the retractable unit on a projection plane on which the retractable unit is projected in the attachment direction. 
     In another illustrative embodiment, the engagement position where the engagement member engages the catch portion is disposed beneath the rectangular area defined by the points of action of the two of the multiple resistive forces acting on the retractable unit on the projection plane of the retractable unit. 
     Yet in another illustrative embodiment, an image forming apparatus includes an image forming unit to form images on sheets of recording media, a retractable unit removably attachable to a main body of the image forming apparatus, and the attachment assist device described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view illustrating an image forming apparatus according to an illustrative embodiment of the present invention; 
         FIG. 2  is a schematic view illustrating an image forming unit for yellow included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  illustrates a configuration around a side frame of the image forming apparatus when the side frame is opened with respect to the image forming apparatus shown in  FIG. 1 ; 
         FIG. 4  is a perspective view illustrating a configuration of a sheet tray included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 5A  is a top view illustrating the sheet tray attached to a sheet feeder; 
         FIG. 5B  is a top view illustrating a sheet tray including a pinion and a rotary damper as a velocity-dependent damper; 
         FIG. 5C  is an enlarged view illustrating the pinion and the rotary damper shown in  FIG. 5B ; 
         FIG. 6  is a projection in an insertion direction of the sheet tray in proportion pattern  1  of resistive forces; 
         FIG. 7  is a projection in the insertion direction of the sheet tray in proportion pattern  2  of the resistive forces; 
         FIG. 8  is a projection in the insertion direction of the sheet tray in proportion pattern  1  of the resistive forces, and the engagement position is disposed at any given vertical position therein; 
         FIG. 9  is a top view illustrating the sheet tray shown in  FIG. 8 , attached to the sheet feeder; and 
         FIG. 10  is a projection in the insertion direction of the sheet tray, in which the engagement position is disposed at a position identical or similar to a point of action of a resultant of the respective resistive forces. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that 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 thereof, and particularly to  FIG. 1 , an image forming apparatus according to an illustrative embodiment of the present invention is described. 
     It is to be noted that, in the description below, reference characters Y, M, C, and BK attached to the end of each reference numeral indicate only that components indicated thereby are used for forming yellow, magenta, cyan, and black images, respectively, and hereinafter may be omitted when color discrimination is not necessary. 
       FIG. 1  illustrates a configuration of an image forming apparatus that in the present embodiment is a tandem-type multicolor laser printer (hereinafter simply “printer”) including multiple photoconductors arranged in parallel to each other. 
     As shown in  FIG. 1 , a printer  500  includes an image forming section  200 , a sheet feeder  300  to feed sheets S of recording media to the image forming section  200 . The image forming section  200  includes four image forming units  1 Y,  1 C,  1 M, and  1 BK for forming yellow (Y), cyan (C), magenta (M), and black (BK) images, respectively. The image forming units  1 Y,  1 C,  1 M, and  1 BK respectively include drum-shaped photoconductors  2 Y,  2 M,  2 C, and  2 BK arranged at constant intervals in a lateral direction in  FIG. 1 . Each photoconductor  2  rotates clockwise in  FIG. 1  as indicated by an arrow shown in  FIG. 1 , driven by a driving source, not shown, when the printer  500  is activated. Each image forming unit  1  further includes image forming components, such as a development device, necessary for electrophotographic image formation, provided around the photoconductor  2 . The image forming units  1 Y,  1 C,  1 M, and  1 BK have similar configurations except for the color of toner used therein, and thus only the image forming unit  1 Y is described in further detail below with reference to  FIG. 2 . 
       FIG. 2  is a schematic view illustrating the image forming unit  1 Y for yellow included in the printer  500  shown in  FIG. 1 . 
     As shown in  FIG. 2 , in the yellow image forming unit  1 Y, as the image forming components, a charger  4 Y, a development device  5 , a cleaning unit  3 Y are disposed around the photoconductor  2 Y clockwise in that order according to the sequence of electrostatic image forming processes. The charger  4 Y includes a charging roller  4   a Y and charges the photoconductor  2 Y according to data of an image to be formed on the photoconductor  2 Y. The development device  5 Y includes a development roller  5   a Y, a development blade  5   b Y, and screws  5   c Y and supplies toner to the charged photoconductor  2 Y. The cleaning unit  3 Y includes a cleaning brush  3   a Y, a cleaning blade  3   b Y, and a collection screw  3   c Y and removes toner from the photoconductor  2 Y. 
     The photoconductor  2 Y includes a cylindrical aluminum base having a diameter of within a range from 30 mm to 120 mm and a photosensitive organic semiconductor layer overlying a surface of the aluminum base, for example. It is to be noted that the photoconductor  2 Y is not necessarily cylindrical but can be belt-shaped. 
     Referring to  FIG. 1 , an exposure unit  80  is provided beneath the photoconductors  2 . The exposure unit  80  serves as a latent image forming member and scans uniformly-charged surfaces of the respective photoconductors  2  with respective laser beams  8  according to image data of respective colors, thus forming electrostatic latent images thereon. In each image forming unit  1 , a slit extending in a direction parallel to an axis of rotation of the photoconductor  2  is formed between the charger  4  and the development device  5  so that the laser beam  8  emitted from the exposure unit  80  can reach the photoconductor  2 . 
     The exposure unit  80  shown in  FIG. 1  is a laser scanning type and includes four semiconductor laser light sources, a polygon mirror, and the like although not shown in  FIG. 1 . The exposure unit  80  directs the laser beams  8 , modulated according to the image data, emitted from the four semiconductor light sources, not shown, to the photoconductors  2 , respectively. The exposure unit  80  includes a metal or resin frame housing optical components and control-related components, and a translucent dustproof member is provided on its upper surface as a light emission port. It is to be noted that, although the printer  500  shown in  FIG. 1  includes a single exposure unit  80 , alternatively, multiple individual exposure units may be provided for the respective image forming units  1 . Additionally, the exposure unit  80  can employ known light emitting diode (LED) arrays and an imaging element in combination instead of the semiconductor laser light sources. In this case, several thousands to several tens of thousands of micromachined LED are lined, and thus optical writing is performed with the LEDs corresponding to respective dots forming the latent image on the photoconductor  2 . That is, the optical system requires only a simple converging lens, eliminating the need of a mechanical driving system such as a polygon mirror, and thus reliability can be enhanced while reducing the cost as well as the size of the exposure unit  80 . 
     Yellow, cyan, magenta, and black toners are consumed in image development performed by the respective development devices  5 , and toner detectors, not shown, detect the amount or concentration of toner in the respective development devices  5 . Four toner cartridges  40 Y,  40 C,  40 M, and  40 BK are provided in an upper portion of the printer  500 , and the respective color toners are supplied from the toner cartridges  40 Y,  40 C,  40 M, and  40 BK by toner supply devices, not shown, to the development devices  5 . An exterior of each toner cartridge  40  is formed as a container made of resin or paper, for example, and a discharge port is formed therein. Each toner cartridge  40  is configured to facilitate attachment and removal of the toner cartridge  40  from an attachment portion  400  provided in the upper portion of the printer  500 . When the toner cartridge  40  is attached to the attachment portion  400 , the discharge port formed in the toner cartridge  40  is connected to a toner supply member provided in a main body of the printer  500 . Additionally, the printer  500  includes a preventive of errors in attachment of the toner cartridges  40  because wrong color toner is supplied to the development device  5  if the toner cartridge  40  is attached at a wrong position. For example, the toner cartridges  40  may be different in shape so that each toner cartridge  40  can match only the position of corresponding color in the attachment portion  400 . 
     Each development device  5  includes two screws  5   c  for agitating toner and carrier and transporting developer including the toner and the carrier. When the development device  5  is installed in the printer  500 , one end of the toner supply member is connected to an upper portion of the screw  5   c  on the left in  FIG. 2 . Referring to  FIG. 2 , the toner is supplied by the screws  5   c  to the development roller  5   a  rotating counterclockwise in  FIG. 2 , and the development blade  5   b  adjusts the thickness of a toner layer on a circumferential surface of the development roller  5   a  to a predetermined or given thickness. The development roller  5   a  includes a stainless steel or aluminum sleeve rotatably fixed to a frame (not shown) of the development device  5  so that a constant distance is maintained between the photoconductor  2  and the development roller  5   a , and the sleeve contains magnets that form predetermined magnetic force lines. The electrostatic latent image formed on each photoconductor  2  by the corresponding laser beam  8  is developed by the development device  5  using the corresponding color toner. 
     Referring to  FIG. 1 , the printer  500  further includes an intermediate transfer unit  6  disposed above the photoconductors  2 . The intermediate transfer unit  6  includes an intermediate transfer belt  6   a , serving as an image carrier, stretched around a secondary-transfer facing roller  6   b , rollers  6   c  and  6   d , and a cleaning facing roller  6   e . As the roller  6   b  rotates, driven by a driving source (not shown), the intermediate transfer belt  6   a  rotates counterclockwise in  FIG. 1  as indicated by an arrow shown in  FIG. 1 . The intermediate transfer belt  6   a  is an endless belt and positioned so that the surface of each photoconductor  2  can contact the intermediate transfer belt  6   a  after passing an area facing the development device  5 . Four primary-transfer rollers  7  are provided on an inner circumferential side of the intermediate transfer belt  6   a  at positions facing the respective photoconductors  2 . 
     A belt cleaning unit  6   h  is provided on an outer circumferential side of the intermediate transfer belt  6   a  at a position facing the cleaning facing roller  6   e . The belt cleaning unit  6   h  removes any toner remaining on a surface of the intermediate transfer belt  6   a , paper dust, and the like from the intermediate transfer belt  6   a . The cleaning facing roller  6   e  disposed facing the belt cleaning unit  6   h  is movable and includes a mechanism to press against the intermediate transfer belt  6   a  at an appropriate pressure to keep the intermediate transfer belt  6   a  taut constantly. Additionally, the belt cleaning unit  6   h  moves in conjunction with the cleaning facing roller  6   e.    
     For example, the intermediate transfer belt  6   a  includes a resin film or rubber base having a thickness within a range of from 50 μm to 600 μm and has a resistivity at which the toner image formed on each photoconductor  2  can be transferred onto the surface of the intermediate transfer belt  6   a  electrostatically with a bias applied to the corresponding primary-transfer roller  7 . It is to be noted that the intermediate transfer belt  6   a  and the related components are supported by a common unit casing and together form the intermediate transfer unit  6  removably attachable to the printer  500 . For example, the intermediate transfer belt  6   a  may be a polyamide belt in which carbon is dispersed and have a volume resistivity within a range of about 10 6  Ω·cm to 10 12  Ω·cm. Additionally, a rib is formed in at least one end portion in a width direction of the intermediate transfer belt  6   a , perpendicular to the direction in which the intermediate transfer belt  6   a  rotates, to inhibit the intermediate transfer belt  6   a  from moving in the width direction, thus maintaining reliable rotation of the intermediate transfer belt  6   a.    
     For example, each primary-transfer roller  7  includes a metal core (metal roller) and an electrically conductive rubber material overlying the metal roller, and a driving source, not shown, applies a transfer bias to the metal roller. Examples of the electrically conductive rubber material include urethane rubber in which carbon is dispersed to adjust its volume resistivity to about 10 5  Ω·cm. Alternatively, the primary-transfer roller  7  may be a metal roller without an electrically conductive rubber surface layer. 
     A secondary-transfer unit  14  including a secondary-transfer roller  14   a  is positioned on the right of the intermediate transfer unit  6  in  FIG. 1 , and a power source  14   b  is provided in the secondary-transfer unit  14 . The secondary-transfer roller  14   a  is disposed on the outer circumferential side of the intermediate transfer belt  6   a  at a position facing, via the intermediate transfer belt  6   a , the secondary-transfer facing roller  6   b  that supports the intermediate transfer belt  6   a . For example, the secondary-transfer roller  14   a  includes a metal core (metal roller) and an electrically conductive rubber material overlying the metal roller, and a driving source  14   b  applies a transfer bias to the metal roller. Carbon is dispersed in the electrically conductive rubber material to adjust its volume resistivity to about 10 7  Ω·cm. The secondary-transfer roller  14   a  contacts the intermediate transfer belt  6   a  at the position facing the secondary-transfer facing roller  6   b , and thus a secondary-transfer nip (a secondary transfer position) is formed therebetween. In the secondary-transfer nip, which is the contact portion between the secondary-transfer roller  14   a  and the intermediate transfer belt  6   a , the toner image formed on the intermediate transfer belt  6   a  is electrostatically transferred onto the sheet S passing therethrough by applying the transfer bias to the secondary-transfer roller  14   a.    
     The sheet feeder  300  disposed beneath the exposure unit  80  includes multiple retractable sheet trays  9 A and  9 B that can be pulled out to a front side of the printer  500 . For example, the number of the sheet trays may be two. The sheet feeder  300  further includes feed rollers  10 A and  10 B, two pairs of separation rollers  11 A and  11 B, and two pairs of conveyance rollers  12 A and  12 B for the sheet trays  9 A and  9 B, respectively. The sheets S contained in the sheet trays  9 A and  9 B are selectively sent out as the corresponding one of the feed rollers  10 A and  10 B rotates. Subsequently, the corresponding one of the pairs of separation rollers  11 A and  11 B separates the sheets S one by one by, and then the corresponding one of the pairs of conveyance rollers  12 A and  12 B feeds the sheet S to a feed path P 1 . 
     A pair of registration rollers  13  is provided along the feed path P 1  to adjust a timing at which the sheet S is sent to the secondary-transfer nip. 
     The printer  500  further includes a manual bypass tray  25 , a feed roller  26 , a pair of reverse rollers  27  serving as a separator, a reverse roller  22 , and a roller  24  disposed facing the reverse roller  22 . When not used, the manual bypass tray  25  can be housed in a side frame F that is a part of the main body of the printer  500  and disposed on a side. The sheet S placed on the top on the manual bypass tray  25  is fed by the feed roller  26  to the pair of reverse rollers  27 , which separates the top sheet S from the rest, and then the reverse roller  22  and the roller  24  transport the sheet S through the feed path P 1  to the pair of registration rollers  13 . 
     A fixing device  15  including a heater is provided above the secondary-transfer nip in  FIG. 1 . The fixing device  15  includes a fixing roller  15   a  containing the heater and a pressure roller  15   b  pressing against the fixing roller  15   a . It is to be noted that, alternatively, a fixing device using a belt or employing an induction heating (IH) mechanism may be used. When a pivotable switchable guide  63  is at the position shown in  FIG. 1 , the sheet S on which the toner image is fixed is guided by a guide member  61   a  forming a discharge path and then discharged by a pair of discharge rollers  62  in a direction indicated by arrow D shown in  FIG. 1  onto a discharge tray  60  formed on an upper side of the printer  500 . 
     The printer  500  further includes a duplex unit  30  including sheet paths and rollers to reverse the sheet S and feed sheet S again to the secondary-transfer nip for forming image on both sides of the sheet in duplex printing. 
     More specifically, the duplex unit  30  is housed in the side frame F and includes a switchback path P 5 , a resupply path P 6 , the switchable guide  63 , a second switchable guide G 2 , and a third switchable guide G 3  to transport the sheet S to the feed path P 1  after the toner image is formed on a first surface (e.g., a front side) of the sheet S. The side frame F further contains reverse rollers  18   a  and  18   b  and the reverse roller  22  connected to a driving source, not shown, that can be rotated in reverse by controlling the driving source. The reverse rollers  18   a  and  18   b  face each other and hereinafter also referred to as a pair of reverse rollers  18 . The reverse roller  22  is in contact with the rollers  23  and  24 , and, when the reverse roller  22  rotates clockwise in  FIG. 1 , the reverse roller  22  and the roller  24  rotating in combination send out the sheet S from the manual bypass tray  25 . Further, when the reverse roller  22  rotates counterclockwise in  FIG. 1 , the reverse roller  22  and the roller  23  rotating in combination transport the sheet S through the resupply path P 6  again toward the pair of registration rollers  13 . 
     When the switchable guide  63  pivots clockwise from the position shown in  FIG. 1 , the sheet S on which the toner image is fixed is guided by a pair of rollers  17  to a reverse path P 4 , guided by the second switchable guide G 2  to the pair of reverse rollers  18 , and then transported to the switchback path P 5 . After the sheet S is forwarded to the switchback path P 5 , the reverse roller  18   a  as well as the second switchable guide G 2  rotate counterclockwise in  FIG. 1 , thereby transporting the sheet S from the switchback path P 5  to the resupply path P 6 . Subsequently, a pair of rollers  15   c  and  20  and a pair of rollers  14   c  and  21  transport the sheet S through the resupply path P 6 , after which the reverse roller  22  and the roller  23  transport the sheet S to the pair of registration rollers  13 . 
     The printer  500  further includes a sheet feeder  50  disposed beneath the sheet feeder  300  as an additional sheet feed unit. The sheet trays  9 C and  9 D are respectively provided with feed rollers  10 C and  10 D, and the sheet feeder  50  further includes separation rollers  11 C and  11 D for the sheet trays  9 A and  9 B and two pairs of conveyance rollers  12 C and  12 C. Although the sheet feeder  50  shown in  FIG. 1  includes two sheet trays  9 C and  9 D, the total sheet containing capacity of the sheet feeder  50  may be increased by increasing the number of sheet trays or the capacity of each sheet tray. 
     In the printer  500 , when the third switchable guide G 3 , positioned above the fixing device  15  and downstream from the pair of rollers  17  in a direction in which the sheet S is transported (hereinafter “sheet conveyance direction), pivots counterclockwise from the position shown in  FIG. 1 , the sheet S on which the toner image is fixed is guided to a discharge path P 8 , and thus the sheet S can be discharged to a discharge unit (not shown) or a post-processing apparatus. Examples of the discharge unit include bin trays including multiple discharge trays arranged vertically. It is to be noted that, in  FIG. 1 , reference characters Fa and  70  represent an attachment assist device and a shaft with which the side frame F is hinged to the main body of the printer  500 . 
     Next, operations performed in single-sided printing are described below with reference to  FIGS. 1 and 2 . 
     The exposure unit  80  directs the laser beam  8 Y emitted from the semiconductor laser source (not shown) according to image data of yellow onto the surface of the photoconductor  2 Y charged uniformly by the charging roller  4   a Y, thus forming an electrostatic latent image on the photoconductor  2 Y. Then, the development roller  5   a Y supplies toner to the latent image, thereby developing it into a visible yellow toner image. Subsequently, the primary-transfer roller  7 Y primarily transfers the yellow toner image onto the surface of the intermediate transfer belt  6   a  rotating in synchronization with the photoconductor  2 Y. The above-described latent image formation, image development, and primary transfer of the image are also performed on the photoconductors  2 C,  2 M, and  2 BK sequentially. 
     Consequently, the yellow, cyan, magenta, and black toner images are superimposed one on another on the intermediate transfer belt  6   a , forming a four-color toner image, and the intermediate transfer belt  6   a  transports the four-color image in the direction (counterclockwise) indicated by the arrow shown in  FIG. 1 . Meanwhile, the cleaning unit  3  removes any remaining toner, paper dust, and the like, from the photoconductor  2  that has passed the position where the primary-transfer roller  7  faces the photoconductor  2  via the intermediate transfer belt  6   a.    
     The four-color toner image formed on the intermediate transfer belt  6   a  is transferred by the secondary-transfer roller  14   a  onto the sheet S transported in synchronization with the intermediate transfer belt  6   a . Subsequently, the belt cleaning unit  6   h  cleans the surface of the intermediate transfer belt  6   a  in preparation for subsequent image formation and image transfer. 
     Then, the sheet S is transported through a post-transfer path P 2  to the fixing device  15 , which fixes the toner image on the sheet S, and then the discharge rollers  62  discharge the sheet S onto the discharge tray  60  with the image surface faced down. 
     Next, operations performed in duplex printing are described below with reference to  FIGS. 1 and 2 . 
     After a first toner image is transferred onto the first surface of the sheet S in the above-described transfer process, the sheet S passes through the fixing device  15  and then is guided to the pair of rollers  17  by the switchable guide  63 . Then, the sheet S is transported through the reverse path P 4 , guided by the third guide G 3  disposed downstream from the pair of rollers  17  in the sheet conveyance direction, to the position above the second switchable guide G 2  at the position shown in  FIG. 1  and then further transported to the switchback path P 5  by the pair of reverse rollers  18 . At that time, the reverse roller  18   a  rotates clockwise in  FIG. 1 . A pair of rollers  19  rotatable in both normal and reverse directions is provided in the switchback path P 5 . The pair of rollers  19  rotates in the normal direction until the sheet S enters fully in the switchback path P 5  and then rotates in reverse, thus reversing the sheet S. When the pair of rollers  19  and the pair of reverse rollers  18  rotate in reverse, the second switchable guide G 2  pivots counterclockwise from the position shown in  FIG. 1 . With an end of the sheet S that is on the trailing side before the sheet S enters the switchback path P 5  now forming a leading end, the sheet S is transported through the resupply path P 6  by the rollers  15   c ,  20 ,  14   c , and  21  to the feed path P 1 . Thus, the sheet S reaches the pair of registration rollers  13 . Subsequently, the pair of registration rollers  13  transports the sheet P with its first surface carrying the first toner image to the secondary-transfer nip where the secondary-transfer roller  14   a  faces the intermediate transfer belt  6   a , timed to coincide with image formation, and thus a second toner image formed on the intermediate transfer belt  6   a  is transferred onto a second surface (e.g., back side) of the sheet S. 
     When the sheet S reaches a predetermined position, formation of respective single-color toner images constituting the second toner image transferred onto the second surface of the sheet S are sequentially started. The second four-color toner image is formed in image forming processes similar to those in single-sided printing and then transferred onto the intermediate transfer belt  6   a . It is to be noted that the sheet S is turned upside down at that time, and accordingly emission of laser beams  8  from the exposure unit  80  is controlled so that the latent images are formed from the opposite side in the sheet conveyance direction relative to those of the first toner image. 
     Then, the fixing device  15  fixes the second toner image on the sheet S, and then the discharge rollers  62  discharge the sheet S carrying the images on both sides thereof onto the discharge tray  60 . 
     It is to be noted that, in the printer  500 , sheet conveyance is controlled so that multiple sheets S can be transported through the sheet conveyance paths simultaneously to reduce time required for duplex printing. Additionally, a controller, not shown, of the printer  500  controls timings of formation of images formed on both sides of the sheet S. 
     Additionally, the polarity of toner images formed on the photoconductors  2  is negative, and thus the primary-transfer rollers  7  are given positive electrical charges so that the toner images can be transferred from the respective photoconductors  2  onto the intermediate transfer belt  6   a . Similarly, the secondary-transfer roller  14   a  is given positive electrical charges so that the toner image can be transferred from the intermediate transfer belt  6   a  onto the sheet S. 
     It is to be noted that, although the description above concerns a configuration in which multicolor image formation is performed in both single-sided printing and duplex printing, the photoconductors  2 Y,  2 M, and  2 C for yellow, magenta, and cyan, respectively, are not used in monochrome printing using only black toner. Therefore, in monochrome printing using only black toner, the photoconductors  2 Y,  2 M, and  2 C are not activated. Further, the printer  500  includes a disengagement mechanism to disengage the photoconductors  2 Y,  2 M, and  2 C from the intermediate transfer belt  6   a . More specifically, in the printer  500 , an inner frame  6   f  supporting the roller  6   d  and the primary-transfer rollers  7  is pivotable around a frame shaft  6   g . In monochrome printing, the inner frame  6   f  is pivoted away from the photoconductors  2 Y,  2 M, and  2 C (in  FIG. 1 , clockwise) with only the photoconductor  2 BK in contact with the intermediate transfer belt  6   a , and black image formation is performed in this state. Thus, operational lives of the image forming units  1 Y,  1 M, and  1 C can be extended by disengaging the photoconductors  2 Y,  2 M, and  2 C from the intermediate transfer belt  6   a  as well as inactivating the photoconductors  2 Y,  2 M, and  2 C and the development devices  5 Y,  5 M, and  5 C in monochrome printing. 
     An outer cover, not shown, of the printer  500  is openably closable for maintenance such as replacement of components. The components (image forming components) of each image forming unit  1  shown in  FIG. 2  are held in a common unit casing, that is, the image forming unit  1  is configured as a removably insertable or retractable process cartridge into the printer  500 . Thus, the components of the image forming unit  1  can be replaced at once by replacing the process cartridge, and thus handling of the components in maintenance can be easier. 
     Additionally, when each image forming unit  1  is configured as a process cartridge, insertion and removal of the process cartridge can be facilitate by providing a guide or handle in the process cartridge. Further, providing the process cartridge with a storage device, such as an integrated circuit (IC) tag, storing characteristics and operational conditions of the process cartridge can facilitate management of the process cartridge. 
     Additionally, when the intermediate transfer unit  6  is removable form the printer  500  with the intermediate transfer belt  6   a  disengaged from the photoconductors  2 , handling of the intermediate transfer unit  6  in maintenance work can be easier. 
       FIG. 3  illustrates a configuration around the side frame F when the side frame F is opened with respect to the printer  500 . 
     It is to be noted that, in  FIG. 3 , reference characters  12 Aa and  12 Ab respectively represent the rollers  12 A on the side of the main body (hereinafter “main body side”) and the side of the side frame F (hereinafter “side-frame side”), and reference characters  12 Ba and  12 Bb respectively represent the rollers  12 B on the main body side and the side-frame side. 
     Referring to  FIG. 3 , the side frame F is pivotable around the shaft Fa with respect to the printer  500 , and the duplex unit  30  and the secondary-transfer unit  14  are housed in the side frame F. When the side frame F is pivoted clockwise from the position shown in  FIG. 1 , the secondary-transfer unit  14  and an interior of the duplex unit  30  are exposed as shown in  FIG. 3 . The side frame F further includes a stopper  31 , and, when users operate a lock lever (not shown), the stopper  31  is disengaged from an engagement member  32  provided in the main body of the printer  500 , thus pivoting the side frame F. With this configuration, the multiple sheet paths, that is, the feed path P 1 , the post-transfer path P 2 , and the resupply path P 6 , can be exposed by pivoting the side frame F, thus facilitating removal of sheets from these sheet paths when the sheets jammed therein. 
     The secondary-transfer unit  14  is positioned between the post-transfer path P 2  and the switchback path P 5  and rotatable around the roller  23 . When the side frame F is opened with respect to the main body of the printer  500  as shown in  FIG. 3 , the secondary-transfer roller  14   a  is disengaged from the intermediate transfer belt  6   a . Additionally, the secondary transfer unit  14  is given rotational behavior to disengage the roller  14   c  from the roller  21 . The exterior of the secondary-transfer unit  14  includes the secondary-transfer roller  14   a  and the rollers  14   c  and  23 , and thus the secondary-transfer unit  14  has a function to transport the sheet S. 
     The fixing device  15  includes the roller  15   c  for transporting the sheet S and a guide surface for guiding the sheet S, and a right side surface of the fixing device  15  in  FIG. 3  forms the resupply path P 6 . The fixing device  15  is supported by a housing of the printer  500  so that the fixing device  15  can be pulled out to the right in the state shown in  FIG. 3 . This configuration facilitates removal of sheets when the sheets jammed inside the fixing device  15 . 
     The roller  15   c  for transporting the sheet S is urged toward the roller  20  by a spring, not shown, and the roller  14   c  is urged toward the roller  21  by a spring, not shown. Additionally, the rollers  12 Ab and  12 Bb on the main body side are urged to the rollers  12 Aa and  12 Ba on the side-frame side by springs (not shown), respectively. 
     With this configuration, the rollers  14   c ,  15   c ,  12 Ab, and  12 Bb biased by the respective springs (not shown) urge the side frame F at the position shown in  FIG. 1  (e.g., closed position) in a direction in which the side frame F pivots down in  FIG. 1  and thus opens with respect to the main body of the printer  500 . Consequently, a stopper surface  31   b  of the stopper  31  is in contact with the engagement member  32 , thus setting the side frame F in position. In other words, the rollers  14   c ,  15   c ,  12 Ab, and  12 Bb on the main body side together form a bias unit when the side frame F is set in position relative to the main body of the printer  500  with the stopper  31  on the side-frame side and the engagement member  32  on the main body side. 
     Next, a configuration of the sheet tray  9 C among the sheet trays  9 A,  9 B,  9 C, and  9 D is described below with reference to  FIG. 4 . It is to be noted that the sheet trays  9 A,  9 B, and  9 D have configurations similar to that of the sheet tray  9 C, and thus descriptions thereof are omitted. 
       FIG. 4  is a perspective view illustrating the sheet tray  9 C. 
     As shown in  FIG. 4 , projections  92   a  and  92   b  are respectively provided on both sides of the sheet tray  9 C. The sheet feeder  50  (shown in  FIG. 1 ) serving as a main body includes guide rails  93   a  and  93   b  that support the projections  92   a  and  92   b , respectively. With the projections  92   a  and  92   b  supported by the guide rails  93   a  and  93   b , respectively, the sheet tray  9 C can be pulled out from the sheet feeder  50  to the front side (front side of the printer  500 ) in a direction (e.g., sheet width direction) perpendicular to the sheet conveyance direction and inserted in the sheet feeder  50  slidingly. The sheet trays  9 A,  9 B, and  9 D having the similar configuration to that of the sheet tray  9 C are configured as removably insertable or retractable units to the sheet feeder  300  or  50  serving as a main body. 
     The sheet tray  9 C includes a bottom plate  99  swingable upward to lift the sheets S contained in the sheet tray  9 C, an end fence  91  to guide trailing end portions of the sheets S, a pair of side guides  94 L and  94 R to guide the sheets S on both sides in the sheet width direction. 
     The sheet tray  9 C further includes a handle supporter  96  disposed in a center portion on the front side of the sheet tray  9 C, and a handle  120  is attached to the handle supporter  96 . The handle  120  supported by the handle supporter  96  is movable in a direction of insertion and removal of the sheet tray  9 C, whereas the handle supporter  96  limits movement of the handle  120  in the width direction as well as an upward direction. 
       FIG. 5A  is a top view illustrating the sheet tray  9 C attached to the sheet feeder  50 . 
     Referring to  FIG. 5A , the sheet feeder  50  further includes the attachment assist device  70  to pull the sheet tray  9 C attached to the sheet feeder  50  in the direction of insertion of the sheet tray  9 C (hereinafter “insertion direction” or “attachment direction”) to a predetermined position at which the sheet tray  9 C presses against a contact member  750 . Thus, the position of the sheet tray  9 C is defined in an anteroposterior direction, that is, the direction of insertion and removal of the sheet tray  9 C and the sheet tray  9 C is kept in the sheet tray  9 C. 
     When the handle  120  of the sheet tray  9 C attached to sheet feeder  50  is pulled to the front side of the sheet feeder  50 , the sheet tray  9 C moves to the front side and then is pulled out from the sheet feeder  50 . 
     As shown in  FIG. 5A , the sheet feeder  50  further includes an elevation motor  51 , a spring  52  winding around an output shaft  51   a  of the elevation motor  51 , and a coupling  53  to transmit a driving force of the motor  50  to the sheet tray  9 C. The spring  52  presses the coupling  53  toward the sheet tray  9 C. 
     The output shaft  51   a  of the elevation motor  51  is movable in an axial direction thereof, and thus the coupling  53  attached to an edge of the output shaft  51   a  is movable in the direction of removal of the sheet tray  9 C (hereinafter “removal direction”). 
     Corresponding to this configuration, a rotary shaft  101  is provided in the sheet tray  9 C, and an engagement projection  101   a  is provided at a back end (in  FIG. 5A , an upper end) of the rotary shaft  101  to engage the coupling  53 . 
     The sheet tray  9 C further includes a pressing member  102  that is fixed to the other end (in  FIG. 5A , a lower end) of the rotary shaft  101  and lifts the bottom plate  99  to press the sheet S against the feed roller  10 C depicted in  FIG. 1  so that the feed roller  10 C can draw the sheet S out from the sheet tray  9 C. 
     When the sheet tray  9 C is attached to the sheet feeder  50 , the engagement projection  101   a  of the rotary shaft  101  presses the coupling  53  in the axial direction of the output shaft  51   a  (insertion direction). As the elevation motor  51  rotates, an engagement groove  53   a  of the coupling  53  is aligned with the engagement projection  101   a  of the rotary shaft  101 , and then the spring  52  presses the coupling  53 , thus engaging the engagement groove  53   a  of the coupling  53  with the engagement projection  101   a  of the rotary shaft  101 . As a result, the driving force is transmitted through the coupling  53  to the rotary shaft  101 . 
     Additionally, a positioning hole  600  is formed in the sheet feeder  50 , and the sheet tray  9 C further includes a positioning boss  601 . When the sheet tray  9 C is inserted into the sheet feeder  50 , the positioning boss  601  engages the positioning hole  600 , thereby defining the position of the sheet tray  9 C relative to the sheet feeder  50 . 
     In the above-described configuration, when attached to the sheet feeder  50 , the sheet tray  9 C receives resistive forces b, c, d, and e. The resistive force c is a reactive force of the spring  52  pressed by the coupling  53 . The resistive force b is caused by friction between the positioning hole  600  and the positioning boss  601 . Moreover, the resistive forces d and e are caused by sliding contact between the projections  92   a  and  92   b  of the sheet tray  9 C and the guide rails  93   a  and  93   b  of the sheet feeder  50 , respectively, when the sheet tray  9   c  is inserted into the sheet feeder  50 . 
     The sheet feeder  50  includes the attachment assist device  70  to pull the sheet tray  9 C against the resistive forces b, c, d, and e to a predetermined position (e.g., an insertion completion position) at which the sheet tray  9 C is fully inserted into the sheet feeder  50 . In the configuration shown in  FIG. 5A , the attachment assist device  70  is disposed on the back of the sheet tray  9 C attached to the sheet feeder  50 . 
     In the present embodiment, the attachment assist device  70  includes a drawing arm  710  pivotably supported by a fixed shaft  700  and a toggle spring  730  serving as an elastic drawing member. The drawing arm  710  and the toggle spring  730  together form a toggle mechanism. A first end of the toggle spring  730  is fixed to a fixed portion  720  and a second end of the toggle spring  730  is connected to the drawing arm  710 . 
     It is to be noted that, the fixed shaft  700 , the drawing arm  710 , the fixed portion  720 , and the toggle spring  730  are provided in the sheet feeder  50  serving as the main device. 
     The attachment assist device  70  further includes an engagement pin  740  positioned in the sheet tray  9 C, serving as an engagement member to engage the drawing arm  710 . The engagement pin  740  engages a catch groove  710   a , serving as a catch portion, provided in an edge portion of the drawing arm  710 . In the configuration shown in  FIG. 5A , the engagement pin  740  projects from the back side of the sheet tray  9 C to engage the drawing arm  710  disposed on the back of the sheet tray  9 C when the sheet tray  9 C is attached to the sheet feeder  50 . 
     It is to be noted that a reference number  800  shown in  FIG. 5  represents an engagement position where the engagement pin  740  of the sheet tray  9 C enagages the drawing arm  710  of the sheet feeder  50  or the engagement state between the engagement pin  740  and the drawing arm  710 . 
     With this configuration, when the sheet tray  9 C is inserted into the sheet feeder  50 , the engagement pin  740 , serving as the engagement member, provided in the sheet tray  9 C is pressed in the catch groove  710   a , serving as the catch portion, formed in the edge portion of the drawing arm  710 , and then the drawing arm  710  pivots. At that time, the toggle spring  730  causes rotational moment acting on the drawing arm  710 . 
     Therefore, the engagement pin  740  of the sheet tray  9 C caught in the catch groove  710   a  formed in the edge portion of the drawing arm  710  is pulled with a drawing force a in the insertion direction, which is an upward direction in  FIG. 5A , and then is pressed against the contact member  750 . Thus, the position of the sheet tray  9 C is defined in the anteroposterior direction, which a vertical direction in  FIG. 5A . 
     The attachment assist device  70  further includes a velocity-dependent damper  735  to cause the drawing arm  710  to rotate slowly. The velocity-dependent damper  735  includes a first end fixed to a fixed portion and a second end connected to the second end of the toggle spring  730 . The velocity-dependent damper  735  has a typical configuration and contains a fluid. When the drawing arm  710  draws the sheet tray  9 C rapidly, the velocity-dependent damper  735  decelerates rotation of the drawing arm  710  because the resistance of the fluid increases. The force required to pull out the sheet tray  9 C can be reduced when the velocity-dependent damper  735  is a unidirectional damper that does not exert attenuation in the direction in which the sheet tray  9 C is pulled out from the sheet feeder  50 . 
     Alternatively, the drawing arm  710  may be supported by the fixed shaft  700  via a rotary damper as shown in  FIGS. 5B and 5C . 
     Referring to  FIGS. 5B and 5C , an attachment assist device  70 A includes a pinion  736  and a rotary damper  737  as another configuration of the velocity-dependent damper  735  shown in  FIG. 5A . The pinion  736  is disposed coaxial to the drawing arm  710  and engages a gear attached to the rotary shaft of the rotary damper  737  as shown in  FIG. 5C , thus exerting damper effects on the drawing arm  710 . Except the velocity-dependent damper the attachment assist device  70 A has a similar configuration to that of the attachment assist device  70  shown in  FIG. 5A . 
     It is to be noted that the attachment assist device  70  is not limited to the above-described configuration using a toggle mechanism, as long as the main unit and the sheet tray  9 C can engage with each other. 
     Next, referring to  FIGS. 6 through 9 , descriptions are given below of the position of the engagement position  800  between the sheet tray  9 C and the sheet feeder  50  or the position of the engagement pin  740  in two cases in which proportions of the resistive forces b, c, d, and e in the sum thereof are different. The proportions of the resistive forces b, c, d, and e are respectively 50%, 30%, 10%, and 10% in proportion pattern  1  and 72%, 0%, 14%, and 14% in proportion pattern  2 , for example. 
     In  FIGS. 6 through 8 , reference character A represents a point of action on which the drawing force a of the attachment assist device  70  acts, that is, the position of the engagement position  800 .  FIGS. 6 and 7  illustrate the sheet tray  9 C projected in the insertion direction in proportion patterns  1  and  2  of the resistive forces, respectively. 
     In proportion pattern  1 , the resistive force b caused by the friction between the positioning hole  600  and the positioning boss  601  rivals the resistive force c, which is the reactive force of the spring  52  pressed by the coupling  53 , and the sum of the resistive forces b and c accounts for 70% or greater of the total resistive force received by the sheet tray  9 C. 
     In this case, it is preferable that the point of action A on which the drawing force a exerted by the attachment assist device  70  acts, that is, the position of the engagement position  800  or the engagement pin  740 , be inside a hatched rectangular area  901  shown in  FIG. 6 . The rectangular area  901  shown in  FIG. 6  is defined by points of action B and C of the resistive forces b and c, which together account for 70% or greater of the total resistive force. 
     When the engagement position  800  is disposed inside the rectangular area  901  as described above, the drawing force a is generated in the area defined by the points of action B and C of the two dominant resistive forces b and c in the total resistive force, which reduces the distance between a point of action of the resultant of the four resistive forces b, c, d, and e and the engagement position  800  to which the drawing force a is exerted. As a result, a moment to rotate the sheet tray  9 C resulting from differences between the drawing force a and the resistive forces b, c, d, and e can be prevented. 
     It is to be noted that, to prevent the sheet tray  9 C from receiving the resistive force c, the rotational position of the coupling  53  may be adjusted when the sheet tray  9 C is pulled out so that the engagement projection  101   a  of the rotary shaft  101  provided in the sheet tray  9 C can engage the engagement groove  53   a  of the coupling  53  when the sheet tray  9 C is inserted into the sheet feeder  50 . In this configuration, proportions of the resistive forces are classified as proportion pattern  2 . In proportion pattern  2 , because the resistive force b by itself accounts for more than 70% of the total resistive force, the rectangular area  901  defined by multiple resistive forces together accounting for a proportion greater than 70% is not formed. 
     Instead, in proportion pattern  2 , as shown in  FIG. 7 , it is preferable that the engagement pin  740  be disposed at a position identical or similar to the point of action of the greatest resistive force among the four resistive forces b, c, d, and e (hereinafter “greatest resistive point”) on the projection plane of the sheet tray  9 C in the insertion direction. In the present embodiment, the position identical or similar to the greatest resistive point is considered to be inside an area  902  defined by lines at a distance of 10% of a maximum width X of the sheet tray  9 C from the greatest resistive point in a horizontal direction as well as a vertical direction. 
       FIG. 8  illustrates the sheet tray  9 C projected in the insertion direction in a configuration in which the proportions of the resistive forces are proportion pattern  1  and the engagement pin  740  engages the drawing arm  710  at a given position in the vertical direction. 
     Herein, the sheet tray  9 C is typically longer in the horizontal direction, whereas shorter in the vertical direction, and thus a vertical positional deviation of the point of action A of the drawing force a relative to the points of action B, C, D, and E of the resistive forces b, c, d, and e is not likely to generate a vertical moment. Therefore, as shown in  FIG. 8 , the vertical position of the engagement position  800 , that is, the engagement pin  740  on which the drawing force a acts, can be any given position. More specifically, a hatched area  903  shown in  FIG. 8  is not defined in the vertical direction although defined in the horizontal direction by the points of action B and C of the resistive forces b and c, which together account for 70% or greater of the total resistive force, similarly to the rectangular area  901  shown in  FIG. 6 . 
     In the above-described configuration in which the engagement position  800  of the attachment assist device  70  is disposed inside the area  903  having no defined borders in the vertical direction, that is, the engagement pin  740  engages the drawing arm  710  at any given vertical position on the projection plane in the insertion direction, the position of the attachment assist device  70  is not limited to the back of the sheet tray  9 C but can be beneath the sheet tray  9 C as shown in  FIG. 9 . That is, the engagement position  800  can be beneath the rectangular area  901  shown in  FIG. 6 . Therefore, the sheet feeder  50  has an increased flexibility in deciding the relative positions of the sheet tray  9 C and the attachment assist device  70 , thus attaining a relatively compact sheet feeder. It is to be noted that, in  FIG. 9 , the velocity-dependent damper  735  is omitted for simplicity. 
     Although the description above concerns the position of the engagement pin  740  when the attachment assist device  70  pulls the sheet tray  9 C, embodiments according to the present invention are not limited thereto. For example, the components of each image forming unit  1  shown in  FIG. 2  can be housed in a common unit casing as a removably insertable process cartridge and a similar attachment assist device may be used to assist attachment, in particular, insertion of the process cartridge into the printer  500 . More specifically, a guide for insertion, a handle, and the like are provided in the process cartridge and, in replacement, the used process cartridge can be pulled out from the printer  500  serving as a main body to the front side after the exterior cover of the printer  500  is opened. Then, an unused process cartridge can be slid into the printer  500  from the front side to be attached to the printer  500 . This configuration can facilitate insertion and removal of the process cartridge from the printer  500 , and thus maintenance work can be easier. Further, damage to the process cartridge during insertion and removal thereof can be eliminated or reduced. 
     Additionally, each toner cartridge  40  is configured to be slidably attached to and removed from the attachment portion  400  in the present embodiment, and a similar attachment assist device may be used to pull the toner cartridge  40  into the attachment portion  400 . In this case, handling of the toner cartridges  40  can be facilitated and damage thereto can be eliminated or reduced. 
     As described above, it is preferred to eliminate or reduce rotational moment exerted to the sheet tray  9 C caused by imbalance between the drawing force for drawing the sheet tray  9 C and the resistive forces b, c, d, and e and an increase in the resistivity resulting from such rotational moment. Therefore, as shown in  FIG. 10 , it is preferable that, on the projection plane of the sheet tray  9 C in the insertion direction, the engagement position  800  be positioned at a position identical or similar to a point of action of the resultant F of the four resistive forces b, c, d, and e exerted to the sheet tray  9 C while the sheet tray  9 C moves from an drawing start position to an drawing completion position. In other words, rotational moment can be prevented by disposing the engagement position  800  on a line of action of the resultant of the four resistive forces b, c, d, and e. 
     Herein, the drawing start position of the sheet tray  9 C means a position at which the engagement pin  740  of the sheet tray  9 C engages the drawing arm  710  and thus the attachment assist device  70  starts drawing the sheet tray  9 C. The drawing completion position means a position at which drawing of the sheet tray  9 C ends and thus the sheet tray  9 C is fully attached the predetermined position in the back portion of the sheet feeder  50 . 
     It is to be noted that, although  FIG. 6  illustrates a preferable position of the engagement position  800  in a case in which the sum of the resistive forces b and c accounts for 70% or greater of the total resistive force, the threshold is not limited to 70% but may be adjusted depending on the number of positions where a resistive force is generated, the strength of each resistive force, distribution of the resistive forces, or the like. In other words, the engagement position  800  is preferably disposed inside the rectangular area  901  defined by the points of action B and C of the two greater resistive forces b and c among the multiple resistive forces b, c, d, and e serving as opposing corners. 
     Similarly, although  FIG. 7  illustrates a preferable position of the engagement position  800  in the case in which only a single greatest resistive force b accounts for more than 70% of the total resistive force, the threshold is not limited to 70% but may be adjusted depending on the number of positions where a resistive force is generated, the strength of each resistive force, distribution of the resistive forces, or the like. In other words, the engagement position  800  is preferably disposed at the position identical or similar to the point of action B of the greatest resistive forces b among the multiple resistive forces b, c, d, and e. Further, although  FIG. 7  illustrates the area  902  defined by the lines at a distance of 10% of the maximum width X of the sheet tray  9 C from the greatest resistive point in the horizontal direction as well as the vertical direction as an example of the position identical or similar to the greatest resistive point, the threshold is not limited to 10%. 
     It is to be noted that, in this specification, the points of action B, C, D, and E of the resistive forces b, c, d, and e are not defined in three dimensions but are two-dimensional positions on the projection plane in the insertion direction of the sheet tray  9 C. 
     As described above, in the attachment assist device according to the present embodiment, the engagement position  800  is preferably disposed, on the projection plane of the sheet tray  9 C in the insertion direction, inside the rectangular area  901  (shown in  FIG. 6 ) whose opposing corners are the points of action B and C of the two resistive forces selected from the multiple resistive forces b, c, d, and e acting on the sheet tray  9 C while the sheet tray  9 C moves from the drawing start position to the drawing completion position. 
     With this configuration, the sheet tray  9 C can be pulled with the engagement pin  740  engaged with the drawing arm  710  inside the rectangular area  901 , which can prevent or inhibit generation of the moment resulting from the positional difference between the point of action of the drawing force a and those of the resistive forces b, c, d, and e on the projection plane of the sheet tray  9 C in the insertion direction, and thus the sheet tray  9 C can be inserted into the sheet feeder  50  reliably and set in position properly. Consequently, users can insert the sheet tray  9 C into the sheet feeder  50  with reduced force. 
     Additionally, because the sheet tray  9 C can be prevented from tilting or being caught in the sheet feeder  50  with the moment prevented or reduced, the user does not need to insert the sheet tray  9 C mightily into the sheet feeder  50 . Moreover, because the sheet tray  9 C can be inserted into the sheet feeder  50  slowly and smoothly, impact to the sheet feeder  50  and the sheet tray  9 C in the insertion of the sheet tray  9 C can be reduced, thus eliminating or reducing misalignment of sheets contained in the sheet tray  9 C as well as damage to the sheet tray  9 C. Damage to corners of sheets such as curled corners can be prevented or reduced. 
     As described above, in the attachment assist device according to the present embodiment, the engagement position  800  is disposed, on the projection plane of the sheet tray  9 C in the insertion direction, at the position identical or similar to the point of action of the resultant F of the multiple resistive forces b, c, d, and e acting on the sheet tray  9 C while the sheet tray  9 C moves from the drawing start position to the drawing completion position. 
     Thus, the sheet tray  9 C can be pulled in the sheet feeder  50  with the engagement pin  740  engaged with the drawing arm  710  at the position identical or similar to of resultant F of the multiple resistive forces b, c, d, and e. This configuration can prevent or inhibit generation of the moment resulting from the positional difference between the point of action of the drawing force a and those of the resistive forces b, c, d, and e on the projection plane of the sheet tray  9 C in the insertion direction. Accordingly, although the sheet tray  9 C is pulled against the resistive forces b, c, d, and e as well as attenuation of the movement velocity of the sheet tray  9 C by the damper  735  from the sheet feeder  50 , the force necessary to pull the sheet tray  9 C into the main body can be reduced, which can reduce the force required to pull out the sheet tray  9 C from the sheet feeder  50  as well. 
     As described above, in the attachment assist device according to the present embodiment, the engagement position  800  is disposed, on the projection plane of the sheet tray  9 C in the insertion direction, at the position identical or similar to the point of action B of the greatest force b among the multiple resistive forces b, c, d, and e acting on the sheet tray  9 C while the sheet tray  9 C moves from the drawing start position to the drawing completion position. 
     With this configuration, the sheet tray  9 C can be pulled into the sheet feeder  50  with the engagement pin  740  engaged with the drawing arm  710  at the position identical or similar to the greatest resistive force b among the multiple resistive forces b, c, d, and e. This configuration can prevent or reduce the moment resulting from the positional difference between the point of action of the drawing force a and those of the resistive forces b, c, d, and e on the projection plane of the sheet tray  9 C in the insertion direction, and thus the sheet tray  9 C can be inserted into the sheet feeder  50  reliably and set in position properly. 
     Additionally, the attachment assist device according to the present embodiment includes the toggle spring  730 , serving as the elastic member to generate drawing force, and the velocity-dependent damper  735  that decelerates the velocity with which the sheet tray  9 C is pulled to the drawing completion position. 
     With this configuration, the sheet tray  9 C can be inserted into the sheet feeder  50  slowly and smoothly, and impact to the sheet tray  9 C or the sheet feeder  50  can be reduced. This configuration enables good alignment of the sheet tray  9 C relative to the sheet feeder  50  in the insertion direction, eliminating or reducing misalignment of sheets, jamming of sheets, and curl of the sheets fed by the feed roller  10 C from the sheet tray  9 C. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.