Patent Publication Number: US-8118299-B2

Title: Manual sheet feeder and image forming apparatus including same

Description:
BACKGROUND 
     1. Technical Field 
     This disclosure relates to a manual sheet feeder employed in an image forming apparatus such as a copier, a printer, a facsimile machine, and so forth, and more particularly to a manual sheet feeder including a sheet stopper, and an image forming apparatus including the manual sheet feeder. 
     2. Description of the Background 
     Recently, a type of sheets usable in image forming apparatuses such as copiers and so forth has been increased with market expansion, increasing demand for user-friendly manual sheet feeders for use in the image forming apparatuses. 
     Manual sheet feeders employed in such image forming apparatuses generally include a sheet stopper for stopping and aligning a leading edge of a stack of sheets manually inserted into the manual sheet feeder. The sheet stopper prevents the leading edge of the stack of the sheets from entering between sheet feed rollers even when the stack of the sheets is set haphazardly in a sheet tray, thereby preventing misfeeds in the form of multiple sheets from being fed at one time or sheets being diagonally fed. Further, proper alignment of the leading edge of the manually fed stack of sheets is of increasing importance in order to satisfy increasing demand for higher printing speed. 
     Published Unexamined Japanese Patent Application No. 2002-96935 (hereinafter referred to as JP-2002-96935-A) discloses a manual sheet feeder including a sheet stopper. The sheet stopper is caused to contact a leading edge of a sheet in conjunction with rotation of a pickup arm that moves a pickup roller upward and downward. 
       FIG. 1  is a vertical cross-sectional view illustrating a configuration of the manual sheet feeder of the related art disclosed in JP-2002-96935-A. Referring to  FIG. 1 , a manual sheet feeder  10  includes a pickup roller  12  rotatably provided to a pickup arm  16  rotating around a shaft  14  of a sheet feed roller  14   a . When the sheet feed roller  14   a  is rotated, the pickup roller  12  is rotated by gears, not shown. A separation roller  14   b  is provided to contact the sheet feed roller  14   a . Further, a stopper release member  18  is integrally formed with the pickup arm  16 . 
     The manual sheet feeder  10  further includes a sheet stopper  20  rotatably provided thereto. The sheet stopper  20  includes a first arm  20   a  and a second arm  20   b  each extending from a rotary shaft  22  in a direction opposite to each other. A linear portion L is provided at a leading edge of the second arm  20   b.    
     A regulation member  24  for regulating rotation of the sheet stopper  20  is rotatably provided to the manual sheet feeder  10 . The regulation member  24  includes a first arm  24   a  and a second arm  24   b  each extending from a rotary shaft  26  in a direction substantially perpendicular to each other. A protrusion M to engage with the linear portion L provided to the second arm  20   b  of the sheet stopper  20  is provided at a leading edge of the second arm  24   b  of the regulation member  24 . 
       FIG. 1  illustrates a state in which the pickup roller  12  is moved upward so that the protrusion M provided to the second arm  24   b  of the regulation member  24  engages with the linear portion L provided to the second arm  20   b  of the sheet stopper  20 . Accordingly, rotation of the sheet stopper  20  is regulated, and the sheet stopper  20  is positioned at a standby position. As a result, when a stack of sheets S is manually inserted into a sheet tray  28 , a leading edge of the stack of the sheets S is stopped and aligned by the sheet stopper  20 . 
     When sheet feeding is started, the sheet feed roller  14   a  is rotated, and the pickup roller  12  is also rotated in conjunction with rotation of the sheet feed roller  14   a . The pickup arm  16  is rotated downward so that the pickup roller  12  is moved downward to contact a top surface of the stack of the sheets S in the sheet tray  28 . 
     The downward rotation of the pickup arm  16  causes the stopper release member  18  integrally formed with the pickup arm  16  to press the first arm  24   a  of the regulation member  24  so that the regulation member  24  is rotated in a counterclockwise direction in  FIG. 1 . Accordingly, the protrusion M provided to the second arm  24   b  of the regulation member  24  is released from the linear portion L of the sheet stopper  20 . As a result, regulation of rotation of the sheet stopper  20  is released, and the sheet stopper  20  is now rotatable. 
     The sheet S fed from the sheet tray  28  by the pickup roller  12  pushes the first arm  20   a  of the sheet stopper  20  now rotatable, so that the sheet S is conveyed between the sheet feed roller  14   a  and the separation roller  14   b  while rotating the sheet stopper  20  in a clockwise direction in  FIG. 1 . 
     When sheet feeding is completed, the sheet stopper  20  is rotated in a counterclockwise direction in  FIG. 1  by moment of inertia to return to the standby position. Meanwhile, the regulation member  24  is rotated in a clockwise direction in  FIG. 1  by moment of inertia, so that the protrusion M provided to the second arm  24   b  of the regulation member  24  engages with the linear portion L provided to the second arm  20   b  of the sheet stopper  20 . 
     In the manual sheet feeder  10  disclosed in JP-2002-96935-A described above, when a larger number of the sheets S is placed on the sheet tray  28 , the pickup roller  12  contacts a top surface of the stack of the sheets S immediately after being moved downward at the start of sheet feeding. As a result, an amount of downward movement of the pickup roller  12  is reduced. In such a case, an amount of downward rotation of the pickup arm  16  is reduced as well, so that a distance in which the stopper release member  18  integrally formed with the pickup arm  16  pushes the first arm  24   a  of the regulation member  24  is also reduced. Consequently, an amount of rotation of the regulation member  24  in a counterclockwise direction in  FIG. 1  is reduced, and that makes it difficult to release the second arm  20   b  of the sheet stopper  20  from the second arm  24   b  of the regulation member  24 . 
     Therefore, when the larger number of the sheets S is placed on the sheet tray  28 , it is difficult to release regulation of rotation of the sheet stopper  20  in the manual sheet feeder  10  of JP-2002-96935-A. 
     Further, when sheet feeding is completed, the regulation member  24  and the sheet stopper  20  are caused to engage with each other by rotation of each of the sheet stopper  20  and the regulation member  24  caused by moment of inertia as described above. Consequently, each of the sheet stopper  20  and the regulation member  24  may not be reliably rotated, preventing engagement of the regulation member  24  with the sheet stopper  20 . 
     Therefore, rotation of the sheet stopper  20  at the completion of sheet feeding may not be reliably regulated, ultimately causing paper misfeeds or the like. 
     BRIEF SUMMARY 
     In an aspect of this disclosure, a manual sheet feeder is provided to stop and align a leading edge of a stack of sheets in a sheet tray at a certain position using a sheet stopper even when the stack of sheets is swiftly inserted into the sheet tray. In addition, even when a larger number of sheets is placed on the sheet tray, regulation of rotation of the sheet stopper is reliably released. 
     Further, rotation of the sheet stopper is reliably regulated at completion of sheet feeding. 
     In another aspect of this disclosure, an image forming apparatus including the manual sheet feeder described above is provided. 
     In an illustrative embodiment, a manual sheet feeder includes a pickup roller provided to contact a sheet placed on a sheet tray to convey the sheet to a sheet feed roller; a pickup arm rotatably provided to a shaft of the sheet feed roller to rotatably support the pickup roller; a sheet stopper rotatably provided to stop and align a leading edge of the sheet placed on the sheet tray; a rotation regulation member rotatably provided to regulate rotation of the sheet stopper in a direction of sheet feed by engaging with the sheet stopper positioned at a standby position; a solenoid including a movable core having a straight line motion; a solenoid link rotatively coupled to the movable core; and a first spring to bias the pickup arm in a direction in which the pickup arm is rotated downward. At start of sheet feeding operation, the pickup arm is rotated by the first spring in a direction in which the pickup roller is moved downward in conjunction with rotation of the solenoid link caused by the straight line motion of the movable core in one direction, and the rotation regulation member is rotated in conjunction with the rotation of the solenoid link caused by the straight line motion of the movable core in the one direction to release regulation of rotation of the sheet stopper by releasing engagement with the sheet stopper. 
     In another illustrative embodiment, an image forming apparatus includes the manual sheet feeder described above. 
     The aforementioned and other aspects, features and advantages will be more fully apparent from the following detailed description of illustrative embodiments, the accompanying drawings, and the associated claims. 
    
    
     
       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 like reference numerals designate identical or corresponding parts throughout the several views and wherein: 
         FIG. 1  is a vertical cross-sectional view illustrating a configuration of a manual sheet feeder of the related art; 
         FIG. 2  is a plan view illustrating a configuration of a manual sheet feeder according to illustrative embodiments; 
         FIG. 3A  is a vertical cross-sectional view illustrating the configuration of the manual sheet feeder illustrated in  FIG. 2 ; 
         FIG. 3B  is an enlarged cross-sectional view illustrating how a solenoid link is connected to a movable core of a solenoid; 
         FIG. 4  is a vertical cross-sectional view illustrating the configuration of the manual sheet feeder when sheet feeding is not performed; 
         FIG. 5  is a vertical cross-sectional view illustrating the configuration of the manual sheet feeder when sheet feeding is started; 
         FIG. 6  is a vertical cross-sectional view illustrating the configuration of the manual sheet feeder when sheet feeding is performed; 
         FIG. 7  is a vertical cross-sectional view illustrating the configuration of the manual sheet feeder when sheet feeding is performed; 
         FIG. 8  is a vertical cross-sectional view illustrating the configuration of the manual sheet feeder when sheet feeding is completed; and 
         FIG. 9  is a vertical cross-sectional view illustrating a configuration of an image forming apparatus according to illustrative embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     In describing illustrative 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. 
     A description is now given of a configuration of a manual sheet feeder  30  according to illustrative embodiments. 
       FIG. 2  is a plan view, and  FIG. 3A  is a vertical cross-sectional view, respectively illustrating the manual sheet feeder  30  according to illustrative embodiments. 
     The manual sheet feeder  30  includes a sheet feed roller  32 , a separation roller  34 , and a pickup arm  38 . An end of the pickup arm  38  is rotatably supported by a shaft  36  of the sheet feed roller  32  so that the pickup arm  38  is rotated around the shaft  36 . At the other end of the pickup arm  38 , a pickup roller  40  is rotatably supported by a shaft  42 . The pickup roller  40  is rotated in conjunction with rotation of the sheet feed roller  32  via gears, not shown. The pickup arm  38  includes a base  44  extending in a horizontal direction at an end thereof. A contact arm  46  is fixed to an upper portion of the base  44 . 
     It is to be noted that, in  FIG. 3A , the pickup arm  38  is illustrated by broken lines for ease of understanding. The pickup arm  38  is biased by a spring such as a coil spring  48  to be rotated downward. 
     A sheet tray  50  is provided below the pickup roller  40 . For simplification, the sheet tray  50  is not shown in  FIG. 2 . A conveyance guide  52  to guide a sheet fed from the sheet tray  50  to a position between the sheet feed roller  32  and the separation roller  34  is provided on a downstream side from the sheet tray  50  relative to a direction of conveyance of the sheet, that is, a direction of sheet feed. 
     A sheet stopper  56  rotatably supported by a shaft  54  fixed to a housing of the manual sheet feeder  30 , not shown, is provided above a portion between the sheet tray  50  and the conveyance guide  52 . The sheet stopper  56  includes a first arm  56   a  and a second arm  56   b  each extending in a direction perpendicular to a longitudinal direction of the shaft  54 . 
     A rotation regulation member  60  rotatably supported by a shaft  58  fixed to the housing of the manual sheet feeder  30 , not shown, is provided above the sheet stopper  56 . The rotation regulation member  60  includes a first arm  60   a  and a second arm  60   b  each extending in a direction perpendicular to a longitudinal direction of the shaft  58 . A protrusion  61  is provided at a leading edge of the second arm  60   b  to engage with the second arm  56   b  of the sheet stopper  56 . An outer surface of the protrusion  61  is formed with a curved surface R. 
     The sheet stopper  56  is biased by a spring such as a torsion spring  57  to be rotated in a direction indicated by an arrow B in  FIG. 3A , that is, a counterclockwise direction in  FIG. 3A . A lock member, not shown, is provided to the pickup arm  38  such that rotation of the sheet stopper  56  in the counterclockwise direction is stopped at a position of the sheet stopper  56  as illustrated in  FIG. 3A , that is, a position where the first arm  56   a  of the sheet stopper  56  is positioned to stop a leading edge of a stack of sheets placed on the sheet tray  50  (hereinafter referred to as a standby position). 
     The rotation regulation member  60  is biased by a spring such as a torsion spring  63  to be rotated in a direction indicated by an arrow C in  FIG. 3A , that is, a clockwise direction in  FIG. 3A . 
     The manual sheet feeder  30  further includes a solenoid  62  and a solenoid link  66  rotatably supported by a shaft  64  fixed to the housing of the manual sheet feeder  30 , not shown. 
     The solenoid link  66  includes a plate-shaped connection part  70  extending in a horizontal direction and connected to a movable core  68  of the solenoid  62 . The solenoid link  66  further includes a first arm  66   a  extending downward in a direction perpendicular to the connection part  70 , and a second arm  66   b  and a third arm  66   c  each fixed to a bottom end of the first arm  66   a  and extending in opposite directions horizontally from both sides of the first arm  66   a . A leading edge of the second arm  66   b  is configured to contact the contact arm  46  of the pickup arm  38 . A leading edge of the third arm  66   c  is configured to contact the first arm  60   a  of the rotation regulation member  60 . 
     The connection part  70  of the solenoid link  66  is connected to the movable core  68  of the solenoid  62  as follows. Specifically, as illustrated in  FIG. 3B , which is an enlarged vertical cross-sectional view illustrating the connection part  70  of the solenoid link  66  and the movable core  68  of the solenoid  62  cut along a line A-A in  FIG. 2 , a hole  72  is provided at an end of the connection part  70  in a direction perpendicular to the connection part  70 . The end of the connection part  70  engages with a notch  74  cut at a leading edge of the movable core  68  in a horizontal direction. When a pin  78  inserted into a hole  76  provided to the movable core  68  in a direction perpendicular to the movable core  68  passes through the hole  72  of the connection part  70 , the connection part  70  and the movable core  68  are connected to each other. It is to be noted that an internal diameter of each of the holes  72  and  76  and an outer diameter of the pin  78  are determined to provide a space between the holes  72  and  76  and the pin  78  such that the solenoid link  66  can be rotated when the movable core  68  has straight line motion. 
     A flange-shaped stopper  80  is provided around an outer circumference of the movable core  68  to stop the straight line motion of the movable core  68  when the solenoid  62  is turned on to withdraw the movable core  68 . In addition, a second stopper, not shown, is provided to stop the straight line motion of the movable core  68  when the solenoid  62  is turned off to push the movable core  68 . 
     A description is now given of a series of sheet feeding operations performed by the manual sheet feeder  30  having the above-described configuration, with reference to  FIGS. 2 to 8 . It is to be noted that the torsion springs  57  and  63  are not Illustrated in  FIGS. 4 to 8  for simplification. 
     In a state of rest when sheet feeding is not being performed, the solenoid  62  is turned off and the movable core  68  is pushed as illustrated in  FIG. 2 . At this time, the solenoid link  66  is rotated around the shaft  64  in a clockwise direction in  FIG. 2 . 
     In such a state, the second arm  66   b  of the solenoid link  66  presses the contact arm  46  of the pickup arm  38  as illustrated in  FIG. 4  to rotate the pickup arm  38  around the shaft  36  in a counterclockwise direction. As a result, the pickup roller  40  is moved upward. 
     At this time, the sheet stopper  56  is positioned at the standby position, and the second arm  56   b  engages with the second arm  60   b  of the rotation regulation member  60 . In other words, rotation of the sheet stopper  56  in the direction of sheet feed is regulated. When a stack of sheets S is placed on the sheet tray  50  in such a state, a leading edge of the stack of the sheets S is stopped by the first arm  56   a  of the sheet stopper  56 . Accordingly, the sheets S are reliably prevented from entering between the sheet feed roller  32  and the separation roller  34 . 
     Prior to the start of sheet feeding, the sheet feed roller  32  is rotated in a clockwise direction, and the pickup roller  40  is rotated in a clockwise direction via gears, not shown. In addition, the separation roller  34  is rotated in a counterclockwise direction. 
     When sheet feeding is started, the solenoid  62  is turned on to withdraw the movable core  68 . At this time, the solenoid link  66  is rotated around the shaft  64  in a counterclockwise direction in  FIG. 2 . 
     When the solenoid link  66  is rotated, the third arm  66   c  of the solenoid link  66  pushes the first arm  60   a  of the rotation regulation member  60  as illustrated in  FIG. 5 , so that the rotation regulation member  60  is rotated around the shaft  58  in a counterclockwise direction in  FIG. 5 . Accordingly, the second arm  60   b  of the rotation regulation member  60  is released from the second arm  56   b  of the sheet stopper  56 , and regulation of rotation of the sheet stopper  56  is also released. 
     At the same time, the second arm  66   b  of the solenoid link  66  is separated from the contact arm  46  of the pickup arm  38  as illustrated in  FIG. 5  due to rotation of the solenoid link  66 , and the pickup arm  38  is rotated around the shaft  36  in a clockwise direction by the spring  48 . As a result, the pickup roller  40  is moved downward to contact a top sheet of the stack of the sheets S (hereinafter referred to as a top sheet S) placed on the sheet tray  50 . 
     As illustrated in  FIG. 6 , the top sheet S is fed from the sheet tray  50  by the pickup roller  40  and is conveyed between the sheet feed roller  32  and the separation roller  34 . A leading edge of the top sheet S contacts the first arm  56   a  of the sheet stopper  56  while the top sheet S is conveyed. However, because rotation of the sheet stopper  56  is not regulated, the top sheet S rotates the sheet stopper  56  in a clockwise direction in  FIG. 6  against a rotary force in the counterclockwise direction biased by the torsion spring  57 , so that the top sheet S passes through the sheet stopper  56  and is conveyed between the sheet feed roller  32  and the separation roller  34 . 
     When a rear edge of the top sheet S passes through the pickup roller  40 , the solenoid  62  is turned off to push the movable core  68 . Accordingly, the solenoid link  66  is rotated around the shaft  64  in a clockwise direction in  FIG. 2 . 
     When the solenoid link  66  is rotated, the second arm  66   b  of the solenoid link  66  pushes the contact arm  46  of the pickup arm  38  as illustrated in  FIG. 7  to rotate the pickup arm  38  around the shaft  36  in a counterclockwise direction in  FIG. 7 . As a result, the pickup roller  40  is moved upward. Thereafter, the top sheet S is conveyed only by the sheet feed roller  32  and the separation roller  34 . At this time, the rotation regulation member  60  is biased by the torsion spring  63  to rotate in a clockwise direction in  FIG. 7 , so that the first arm  60   a  of the rotation regulation member  60  contacts the third arm  66   c  of the solenoid link  66 . 
     When the rear edge of the top sheet S passes between the sheet feed roller  32  and the separation roller  34 , the solenoid  62  is turned on, and the second arm  66   b  of the solenoid link  66  is separated from the contact arm  46  of the pickup arm  38  to return to the state illustrated in  FIG. 6 . Thereafter, the next sheet is fed from the sheet tray  50 . 
     The series of operations illustrated in  FIGS. 6 and 7  is repeatedly performed to sequentially feed the sheets S from the sheet tray  50  and convey the sheets S between the sheet feed roller  32  and the separation roller  34 . Sheet feeding is stopped when copy of a document is completed or all the sheets S placed on the sheet tray  50  are fed. 
       FIG. 8  illustrates a state in which the sheet tray  50  is empty after the last sheet of the stack of the sheets S (hereinafter referred to as a last sheet S) is fed from the sheet feed roller  32  and the separation roller  34 . When the last sheet S is fed from the sheet feed roller  32  and the separation roller  34 , rotation of each of the sheet feed roller  32  and the separation roller  34  is stopped. At this time, the sheet stopper  56  is returned to the standby position by a force applied from the torsion spring  57  so that the sheet stopper  56  is rotated around the shaft  54  in a counterclockwise direction in  FIG. 8 . Simultaneously, the second arm  56   b  of the sheet stopper  56  contacts the curved surface R of the protrusion  61  provided at the leading edge of the second arm  60   b  of the rotation regulation member  60 . 
     The second arm  56   b  of the sheet stopper  56  pushes the protrusion  61  provided at the leading edge of the second arm  60   b  of the rotation regulation member  60  upward. Because the outer surface of the protrusion  61  is formed with the curved surface R as described above, the rotation regulation member  60  is rotated in a counterclockwise direction against a force in the clockwise direction applied from the torsion spring  63 . Thereafter, the leading edge of the second arm  56   b  of the sheet stopper  56  engages with the protrusion  61  provided at the leading edge of the second arm  60   b  of the rotation regulation member  60  to return to the state illustrated in  FIG. 3A . 
     A description is now given of an image forming apparatus  100  employing the manual sheet feeder  30  described above. 
     The image forming apparatus  100  according to illustrative embodiments is a tandem type full-color printer.  FIG. 9  is a vertical cross-sectional view illustrating a configuration of the image forming apparatus  100 . 
     In the image forming apparatus  100 , four image forming units  101 Y,  101 C,  101 M, and  101 K (hereinafter collectively referred to as image forming units  101 ) are arranged parallel to one another in a vertical direction in  FIG. 9  at equal intervals to form images of a specific color of yellow (Y), cyan (C), magenta (M), or black (K). Each of the four image forming units  101  has the same basic configuration, differing only in the color of toner used. It is to be noted that suffixes Y, C, M, and K are added to reference numerals for those components provided in each of the image forming units  101  to correspond to a color of a toner image formed by the corresponding image forming units  101 . 
     The image forming units  101  include drum-type photoconductors  102 Y,  102 C,  102 M, and  102 K, respectively (hereinafter collectively referred to as photoconductors  102 ). When the image forming apparatus  100  is operated, the photoconductors  102  are rotated by a drive source, not shown. It is to be noted that, alternatively, a belt-type photoconductor may be used as the photoconductors  102 . 
     As illustrated in  FIG. 9 , a charger  104 Y, a developing device  105 Y, a cleaning device  103 Y, and so forth are provided around the photoconductors  102 Y. Although not denoted by reference numerals in  FIG. 9 , chargers  104 C,  104 M, and  104 K; developing devices  105 C,  105 M, and  105 K; and cleaning devices  103 C,  103 W, and  103 K are provided respectively around the photoconductors  102 C,  102 M, and  102 K. It is to be noted that the chargers  104 Y,  104 C,  104 W, and  104 K; the developing devices  105 Y,  105 C,  105 M, and  105 K; and the cleaning devices  103 Y,  103 C,  103 M, and  103 K are hereinafter collectively referred to as the chargers  104 ; the developing devices  105 ; and the cleaning devices  103 , respectively. 
     An irradiating device  108  to scan laser beams corresponding to image data of the respective colors on surfaces of the photoconductors  102  evenly charged by the chargers  104  to form electrostatic latent images of the respective colors is provided below the photoconductors  102 . A narrow space extending in a direction of a rotary axis of each of the photoconductors  102  is provided between each of the chargers  104  and the developing devices  105 , such that the laser beams emitted from the irradiating device  108  are directed to the surfaces of the photoconductors  102 . 
     The irradiating device  108  employs a laser scanning method using a laser light source, a polygon mirror, and so forth. The irradiating device  108  emits laser beams  108 Y,  108 C,  108 M, and  108 K (hereinafter collectively referred to as laser beams  108 ), each modulated based on image data to be formed, from four semiconductor lasers, not shown. The irradiating device  108  includes a housing formed of metal or resin to store optical components and control members. A translucent dust-proof member is provided at each of output openings on an upper surface of the irradiating device  108 . 
     Although the irradiating device  108  includes a single housing according to illustrative embodiments, alternatively, multiple irradiating devices may be provided respectively for the image forming units  101 . Further, in place of the irradiating device  108  using the laser beams as described above, alternatively, an irradiating device using a combination of the well-known LED arrays and the imaging means may be used. 
     When toner of each color of yellow, cyan, magenta, or black is consumed by the corresponding developing devices  105 , a toner detector, not shown, detects shortage of the toner, so that replenishing toner is supplied by supply means, not shown, to the corresponding developing devices  105  from toner cartridges  140 Y,  140 C,  140 M, or  140 K each storing toner of a specific color of yellow, cyan, magenta, or black and provided in an upper portion of the image forming apparatus  100 . In order to prevent toner of different color from being supplied to the developing devices  105  because the toner cartridges  140 Y,  140 C,  140 M, or  140 K is set at a position not corresponding to the specific color, erroneous attachment prevention means is provided. For example, a storage TS is shaped corresponding to a shape of each of the toner cartridges  140 Y,  140 C,  140 M, and  140 K. 
     An intermediate transfer unit  106  is provided above the photoconductors  102 . A roller  106   b  is rotated such that an intermediate transfer belt  106   a  wound around multiple rollers  106   b ,  106   c ,  106   d , and  106   e  is rotated in a direction indicated by an arrow D in  FIG. 9 . 
     The intermediate transfer belt  106   a  is a seamless belt, and is arranged to contact to a part of each of the photoconductors  102  after development. Primary transfer rollers  107 Y,  107 C,  107 M, and  107 K are provided opposite the photoconductors  102  in an inner circumferential portion of the intermediate transfer belt  106   a.    
     A cleaning device  106   h  is provided opposite the roller  106   e  outside the intermediate transfer belt  106   a . The cleaning device  106   h  removes any foreign substance such as residual toner and paper dust adhering to a surface of the intermediate transfer belt  106   a . The roller  106   e  provided opposite the cleaning device  106   h  includes a mechanism to apply tension to the intermediate transfer belt  106   a . The roller  106   e  is moved to constantly provide appropriate tension to the intermediate transfer belt  106   a , and the cleaning device  106   h  is moved in conjunction with movement of the roller  106   e.    
     A secondary transfer roller  114   a  is provided near the roller  106   b  outside the intermediate transfer belt  106   a . A bias is applied to the secondary transfer roller  114   a  so that a full-color toner image borne by the intermediate transfer belt  106   a  is electrostatically transferred onto the sheet S passing between the intermediate transfer belt  106   a  and the secondary transfer roller  114   a.    
     Two sheet feed cassettes  109 A and  109 B positioned one above the other are drawably provided below the irradiating device  108 . The sheet S stored in the sheet feed cassette  109 A or  109 B is selectively fed by rotation of a pickup unit  110 A or  110 B corresponding to the sheet feed cassette  109 A or  109 B, and is conveyed to a conveyance path P 1  by a separation unit  111 A or  111 B and a pair of conveyance rollers  112 A or  112 B. 
     A pair of registration rollers  113  is provided along the conveyance path P 1  to convey the sheet S to a secondary transfer position in synchronization with the full-color toner image formed on the intermediate transfer belt  106   a . The sheet S is conveyed from the pair of the registration rollers  113  to the secondary transfer position formed between the intermediate transfer belt  106   a  and the secondary transfer roller  114   a.    
     The manual sheet feeder  30  according to illustrative embodiments provided on a right lateral surface of the image forming apparatus  100  illustrated in  FIG. 9  is rotated when not in use to be stored in a frame F which is a part of the image forming apparatus  100 . The top sheet S stored in the manual sheet feeder  30  is fed between the sheet feed roller  32  and the separation roller  34  by the pickup roller  40 , and is conveyed to the pair of the registration rollers  113  by a pair of conveyance rollers  122  and  124  through the conveyance path P 1 . The configuration and operations of the manual sheet feeder  30  are like that described above. 
     A fixing device  115  including heating means is provided above the intermediate transfer unit  106 . Although the fixing device  115  includes rollers  115   a  and  115   b  each having a heater therein, alternatively, a belt may be used in place of the rollers  115   a  and  115   b . Further alternatively, an induction heater may be used as the heating means. 
     A switching guide G 1  is rotatable. When the switching guide G 1  is in a state as illustrated in  FIG. 9 , the sheet S having a fixed full-color toner image thereon is guided to a discharge path P 3 , and is discharged to a discharge stack T provided at the top of the image forming apparatus  100  by a pair of discharge rollers  116 . 
     The image forming apparatus  100  further includes a conveyance path and rollers for reversing and re-feeding the sheet S so that images are automatically formed on both sides of the sheet S. Specifically, a switchback path P 5  is provided above the discharge stack T. 
     The sheet S fed from the manual sheet feeder  30  is conveyed to the secondary transfer position and the fixing device  115  so that an image is formed on a front side of the sheet S. 
     The switching guide G 1  is rotated in a clockwise direction in  FIG. 9  so that the sheet S having the image on the front side thereof is conveyed to a pair of reversely rotatable conveyance rollers  118   a  and  118   b  through a conveyance path P 4 , a part of which is formed by a left lateral surface of a sheet guide  130 . Thereafter, the sheet S is guided by the pair of the conveyance rollers  118   a  and  118   b  to the switchback path P 5  formed by an inner tray  131 . 
     After a rear edge of the sheet P passes through a leading edge of the sheet guide  130  positioned on a downstream side relative to the direction of conveyance of the sheet S, the conveyance roller  118   a  is rotated in a counterclockwise direction in  FIG. 9  to guide the sheet S to a re-feeding path P 6 . 
     The sheet S passes through the pair of the conveyance rollers  120  and  114   d  and the pair of the conveyance rollers  121  and  114   c  respectively provided along the re-feeding path P 6  and is conveyed to a position where the reversely rotatable drive roller  122  and a roller  123  contact each other. The sheet S sandwiched between the drive roller  122  and the roller  123  is then conveyed to the pair of the registration rollers  113  again through the conveyance path P 1 . 
     The image forming apparatus  100  illustrated in  FIG. 9  further includes a sheet feeder  150  at the bottom thereof. According to illustrative embodiments, two sheet feed cassettes  109 C and  109 D are provided within the sheet feeder  150 . Alternatively, a number of the sheet feed cassettes may be increased, or a large-capacity sheet feed cassette capable of storing a larger number of sheets than normal may be built into the sheet feeder  150 . 
     A description is now given of operations of the image forming apparatus  100  with the above-described configuration in a case in which an image is formed on only one side of the sheet S, that is, the front side of the sheet S. 
     The laser beam  108 Y corresponding to image data of yellow emitted from the semiconductor laser by the operation of the irradiating device  108  is directed onto the surface of the photoconductor  102 Y evenly charged by the charger  104 Y to form an electrostatic latent image of yellow on the surface of the photoconductor  102 Y. 
     The electrostatic latent image of yellow thus formed on the surface of the photoconductor  102 Y is then developed by the developing device  105 Y with toner of yellow to form a toner image of yellow. The toner image of yellow thus formed is primarily transferred by a primary transfer roller  107 Y onto the surface of the intermediate transfer belt  106   a  rotated in synchronization with the photoconductor  102 Y. The above-described series of electrostatic latent image formation, development, and primary transfer is also sequentially performed by the photoconductors  102 C,  102 M, and  102 K with an appropriate timing. 
     As a result, the toner images of yellow, cyan, magenta, and black are sequentially superimposed on one another on the surface of the intermediate transfer belt  106   a  so that a full-color toner image is formed on the surface of the intermediate transfer belt  106   a . The full-color toner image thus formed is borne on the surface of the intermediate transfer belt  106   a  and is moved in the direction indicated by the arrow D in  FIG. 9  along with rotation of the intermediate transfer belt  106   a . Meanwhile, the surface of each of the photoconductors  102  is cleaned by the cleaning devices  103 , respectively, to remove any foreign substance such as residual toner adhering to the surfaces of each of the photoconductors  102 . 
     The full-color toner image formed on the surface of the intermediate transfer belt  106   a  is secondarily transferred onto the front side of the sheet S conveyed in synchronization with rotation of the intermediate transfer belt  106   a  by the secondary transfer roller  114   a . Thereafter, the surface of the intermediate transfer belt  106   a  is cleaned by the cleaning device  106   h  to be ready for the next series of image formation and transfer operations. 
     The full-color toner image transferred onto the front side of the sheet S is fixed to the sheet S by the fixing device  115 . Thereafter, the sheet S having a fixed image on the front side thereof is discharged by the discharge roller  116  to the discharge stack T with the side of the sheet S having the fixed image, that is, the front side of the sheet S, facing down. 
     A description is now given of operations of the image forming apparatus  100  when images are formed on both sides of the sheet S, that is, the front and back sides of the sheet S. 
     As described above, first, the full-color toner image is transferred onto the front side of the sheet S from the intermediate transfer belt  106   a , and the sheet S having the transferred full-color toner image on the front side thereof is conveyed to the fixing device  115 . After the full-color toner image is fixed to the front side of the sheet S by the fixing device  115 , the sheet S is guided by the switching guide G 1  to the pair of the conveyance rollers  118   a  and  118   b . Subsequently, the sheet S is conveyed to the conveyance path P 5  formed by the inner tray  131  by the pair of the conveyance rollers  118   a  and  118   b . When the rear edge of the sheet S passes through the leading edge of the sheet guide  130 , the reversely-rotatable drive roller  118   a  is rotated in a counterclockwise direction in  FIG. 9  so that the rear edge of the sheet S is now turned into a leading edge of the sheet S, and the sheet S is conveyed between the pair of the rollers  122  and  123  by the pairs of the rollers  114   d  and  120 , and  114   c  and  121 . Thereafter, the sheet S is conveyed to the pair of the registration rollers  113  as described above. The sheet S having the image on the front side thereof is again conveyed by the pair of the registration rollers  113  with an appropriate timing to the secondary transfer position where the secondary transfer roller  114   a  is provided. Accordingly, a full-color toner image formed on the surface of the intermediate transfer belt  106   a  is transferred onto the back side of the sheet S. 
     The full-color toner image to be transferred onto the back side of the sheet S is formed on the intermediate transfer belt  106   a  in a way similar to formation of the full-color toner image for the front side of the sheet S as described above when the sheet S is conveyed to a predetermined position. However, because the leading and rear edges of the sheet S are reversed as described above, the laser beams  108  respectively directed to the surfaces of the photoconductors  102  are controlled such that image data is formed from a reverse side thereof in the direction of conveyance of the sheet S compared to a side of the image data first formed for the front side of the sheet S. 
     The sheet S having the transferred full-color toner image on the back side thereof is again conveyed to the fixing device  115  so that the full-color toner image is fixed to the back side of the sheet S. Thereafter, the sheet S having images on both the front and back sides thereof is discharged to the discharge stack T by the discharge roller  116 . 
     In order to efficiently form the images on both the front and back sides of the sheet S, multiple sheets S can be consecutively conveyed through the conveyance paths within the image forming apparatus  100  at the same time. It is to be noted that a timing to form the images for the front and back sides of the sheet S is controlled by control means, not shown. 
     Although formation of the full-color images on the single side or the both sides of the sheet S is described above, the image forming units  101 Y,  101 C, and  101 M are not operated when monochrome images are formed by the image forming apparatus  100 . The image forming apparatus  100  includes a mechanism to separate the photoconductors  102 Y,  102 C, and  102 M from the intermediate transfer belt  106   a  during monochrome image formation. Specifically, the roller  106   d  and an internal frame  106   f  supporting the primary transfer rollers  107 Y,  107 C, and  107 M are rotatably supported with a shaft  106   g  acting as a pivot. Accordingly, when the roller  106   d  and the internal frame  106   f  are rotated in a direction of separating from the photoconductors  102 Y,  102 C, and  102 M, that is, a clockwise direction in  FIG. 9 , only the photoconductor  102 K contacts the intermediate transfer belt  106   a  to form a monochrome image with toner of black. 
     When a paper jam occurs in the conveyance paths of the image forming apparatus  100 , the frame F is rotated around a rotary shaft Fa provided at the bottom of the frame F so that an upper portion of the frame F can be opened. A lock lever, not shown, is operated to open the frame F, so that almost all the conveyance paths within the image forming apparatus  100  are exposed. As a result, the sheet S jammed in the conveyance paths can be easily removed, and further, maintenance such as cleaning can be easily performed. 
     A secondary transfer unit  114  provided between the conveyance paths P 2  and P 6  is rotatable around a shaft of the roller  123 , such that the secondary transfer roller  114   a  is separated from the intermediate transfer belt  106   a , and the rollers  114   c  and  114   d  are separated from the rollers  121  and  120 , respectively, when the frame F is opened. 
     The secondary transfer unit  114  includes a power source  114   b  therein, and external surfaces of a casing of the secondary transfer unit  114  function to convey the sheet S. 
     The manual sheet feeder  30  according to the foregoing illustrative embodiments is applicable not only to image forming apparatuses such as full-color printers, but also to copiers, facsimile machines, printers, and so forth. 
     As can be appreciated by those skilled in the art, 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. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 
     This patent specification is based on Japanese Patent Application No. 2008-308076 filed on Dec. 3, 2008 in the Japan Patent Office, the entire contents of which are hereby incorporated herein by reference.