Patent Publication Number: US-6215970-B1

Title: Duplex unit with recess for retracting manual sheet feeding tray and image forming apparatus using the same duplex unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This document claims priority and contains subject matter related to Japanese Patent Application No. 11-006378 filed in the Japanese Patent Office on Jan. 13, 1999, and on Japanese Patent Application No. 11-049745 filed in the Japanese Patent Office on Feb. 26, 1999, and the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a duplex unit that reverses a sheet to form images at both sides of the sheet, for use in an image forming apparatus such as a copying machine, a facsimile, a printer, or the like. 
     2. Discussion of the Background 
     As illustrated in FIG. 12, in a background image forming apparatus for forming images on both sides of a sheet, a reverse unit  101  for reversing a sheet is provided at a side (right side as viewed in FIG. 12) of a main body case  102 . The reverse unit  101  is rotatably provided around a first supporting shaft (not shown) which is provided at a lower part of the reverse unit  101  in the direction separating from the main body case  102 . Below the reverse unit  101 , a manual sheet feeding tray  103  that manually feeds a sheet is rotatably provided around a second supporting shaft  104  which is provided in a manual sheet feeding unit  117  in the direction separating from the main body case  102 . 
     The main body case  102  houses sheet feeding trays  105 , a sheet transfer path  109 , an image forming unit  107 , a fixing unit  108 , and a sheet stacker unit  106 . The sheet transfer path  109  is formed in the main body case  102  such that a sheet is conveyed from the sheet feeding tray  105  or the manual sheet feeding tray  103  to the sheet stacker unit  106  via the image forming unit  107  and the fixing unit  108 . 
     In a case  110  of the reverse unit  101 , a reverse path  111  and a refeed path  112  that is separated from the reverse path  111  are formed. At a separation point between the reverse path  111  and the refeed path  112 , a reverse roller  113  is provided. The reverse roller  113  is reversible and conveys the sheet that is guided in the reverse path  111  to the refeed path  112  in a switchback direction. A one end of the reverse path  111  connects to the sheet transfer path  109  in the main body case  102  and the other end connects to an opening  114  that is provided at the right-hand side of the case  110  in FIG.  12  and opens outward. The opening  114  is closed by a sheet reverse guide plate  115 . The sheet reverse guide plate  115  is rotatably supported around a supporting shaft  116  provided at the reverse unit  101  in the direction separating from the reverse unit  101 . 
     In the above-described image forming apparatus with the reverse unit  101  and the manual sheet feeding unit  117 , an image is formed on a sheet fed from the sheet feeding tray  105  or the manual sheet feeding tray  103  at the image forming unit  107  during the sheet passes through the sheet transfer path  109 . The sheet having the image is discharged to the sheet stacker unit  106 . 
     When images are formed on both sides of a sheet, the sheet having an image on one side is guided to the reverse path  111 . When the trailing edge of the sheet passes the separation point between the reverse  111  and the refeed path  112 , the reverse roller  113  is switched to rotate in the switchback direction (i.e., in the direction toward the refeed path  112 ), and the sheet is conveyed to the refeed path  112 . The sheet is then conveyed to the sheet transfer path  109  again from the sheet refeed path  112  so as to have an image on the backside of the sheet in the image forming unit  107 , and is discharged to the sheet stacker unit  106 . 
     In the background image forming apparatus with the above-described configuration in FIG. 12, because the manual sheet feeding tray  103  is installed outside of the case  110  of the reverse unit  101  and is protruded from the case  110 , people passing by the image forming apparatus may accidentally contact and break the manual sheet feeding tray  103 . In addition, when an operator moves the image forming apparatus, the manual sheet feeding tray  103  may be opened due to vibration. As a result, the manual sheet feeding tray  103  may be broken by striking against surrounding items. 
     When a sheet is jammed in the sheet transfer path  109 , an operator makes an operation space for removing the jammed sheet by rotating the reverse unit  101  around the first supporting shaft in the direction separating from the main body case  102 . The operator reaches a hand into the space with the jammed sheet. 
     When the operator opens the reverse unit  101  for removing the jammed sheet in the sheet transfer path  109 , the manual sheet feeding tray  103  needs to be opened first, and then the reverse unit  101  can be opened. On the other hand, when the operator closes the reverse unit  101  after removing the jammed sheet, the reverse unit  101  is closed first, and then the manual sheet feeding tray  103  needs to be closed. In other words, the operator needs to open and close not only the reverse unit  101  but also the manual sheet feeding tray  103  to remove the jammed sheet (i.e., the operator needs to perform two steps). The operator is therefore unnecessarily inconvenienced by opening and closing the reverse unit  101  and the manual sheet feeding tray  103  when the operator removes a jammed sheet. 
     Further, when a sheet having a length in the sheet conveying direction longer than the length of the reverse path  111  between the reverse roller  113  and the opening  114  (hereinafter referred to as a large size sheet) is reversed in the reverse unit  101 , an operator needs to open the opening  114  by rotating the sheet reverse guide plate  115  around the supporting shaft  116  to protrude the leading edge of the large size sheet from the case  110 . The open condition of the sheet reverse guide plate  115  is indicated by a dotted line in FIG.  12 . In addition, when the manual sheet feeding tray  103  is closed, the operator needs to open the manual sheet feeding tray  103  first, and then rotate the sheet reverse guide plate  115  to protrude the leading edge of the large size sheet from the case  110 . 
     Furthermore, when the operator uses an image forming apparatus installed at a place apart from an operator by a remote operation and when a large size sheet is reversed in the reverse unit  101 , the operator needs to go to the image forming apparatus to rotate the sheet reverse guide plate  115  around the supporting shaft  116  and open the opening  114 . Opening the sheet reverse guide plate  115  every time a large sized sheet is reversed in the reverse unit  101  becomes a burden on the operator. If an additional device for opening the sheet reverse guide plate  115  by a remote operation is installed to the image forming apparatus, an installation space and a number of construction parts increase in the image forming apparatus. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these problems. 
     The preferred embodiments of the present invention provide a novel duplex unit and image forming apparatus, in which a breakdown of manual sheet feeding tray caused by a protruded manual sheet feeding tray is prevented. 
     The preferred embodiments of the present invention provide a novel duplex unit and image forming apparatus that improve operability in opening and closing a duplex unit. 
     The preferred embodiments of the present invention provide a novel duplex unit and image forming apparatus that reverse various sizes of sheets efficiently without additional operations. 
     In order to achieve the above and other objectives, the present invention provides a novel duplex unit including a case housing a sheet reverse path that connects to a sheet discharging outlet and a sheet refeed inlet provided on a side of a main body case housing an image forming unit and that is configured to reverse the sheet supplied from the sheet discharging outlet and refeed the sheet to the sheet refeed inlet. The case is rotatable around a first supporting shaft so as to be opened and closed. The duplex unit further includes a manual sheet feeding tray that is rotatably supported by a horizontal second supporting shaft so as to be opened to manually feed a sheet to a manual sheet feeding inlet provided on the side of the main body case and to be closed otherwise. A recess is provided on the case so as to retract the manual sheet feeding tray into the recess without protruding from a side surface of the case when the tray is closed. 
     According to the present invention, the case and the manual sheet feeding tray may be engaged by engaging a concave portion of the case and a convex portion of the manual sheet feeding tray. 
     The first supporting shaft may be horizontal, and the first supporting shaft and the second supporting shaft may be located at different positions, and the concave portion may be shaped like an oblong such that a movement orbit of the convex portion resulting from a rotation of the manual sheet feeding tray always exists within an area of a movement orbit of the concave portion resulting from a rotation of the case. 
     According to a present invention, a depth of the concave portion may change in the longitudinal direction such that a strength of engagement of the concave portion and the convex portion when the case is opened is greater than when the case is closed. 
     A side wall of a portion of the concave portion where the convex portion is positioned when the manual sheet feeding tray is closed may be slanted in a direction along the movement orbit of the convex portion resulting from the rotation of the manual sheet feeding tray around the second supporting shaft. 
     The first supporting shaft may be arranged below the case. 
     According to another preferred embodiment of the present invention, an image forming apparatus includes a main body case housing an image forming unit. The image forming apparatus includes a reverse unit that has a case housing a sheet reverse path to reverse a sheet having an image formed at the image forming unit at one side of the sheet and to refeed the sheet toward the main body case. The case is rotatably supported such that the case rotates in the directions approaching and separating from a side surface of the main body case. The image forming apparatus includes a manual sheet feeding tray that is rotatably supported at one end thereof below the reverse unit and that is rotated from its closed position which is substantially vertical and parallel to a side surface of the case of the reverse unit to an open position to manually feed a sheet. A recess is formed at the side surface of the case of the reverse unit so as to retract the manual sheet feeding tray into the recess without protruding from the side surface of the case of the reverse unit when the manual sheet feeding tray is in the closed position. 
     According to another preferred embodiment of the present invention, an image forming apparatus includes a reverse unit including a reverse path connecting to a sheet discharging outlet that is provided on a side of the main body case and an opening opened downward at a bottom part of the reverse unit and a refeed path connecting to the reverse path and a sheet refeed inlet that is provided on the side of the main body case so as to reverse a sheet conveyed to the reverse path by feeding the sheet to the sheet refeed inlet with the trailing edge of the sheet switched to the leading edge of the sheet. The image forming apparatus includes a guide member having a guide surface, that is arranged facing the opening to guide the sheet protruded from the opening. The image forming apparatus includes a manual sheet feeding unit including a sheet feeding section having a guide surface, that is arranged facing the opening to guide the sheet protruded from the opening, and a manual sheet feeding tray that is rotatably provided at the sheet feeding section and that holds stacked sheets. 
    
    
     Other objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention 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 longitudinal side view illustrating a part of an image forming apparatus with a duplex unit according to a first embodiment of the present invention; 
     FIG. 2 is a side view illustrating rotations of a reverse unit and a manual sheet feeding tray of FIG. 1; 
     FIG. 3A is a perspective view illustrating a part of the image forming apparatus with the duplex unit of FIG. 1 in which the manual sheet feeding tray is closed, and FIG. 3B is a perspective view illustrating a part of the image forming apparatus with the duplex unit of FIG. 1 in which the manual sheet feeding tray is opened; 
     FIG. 4A is an enlarged perspective view of a concave portion, and FIG. 4B is an enlarged perspective view of a convex portion; 
     FIG. 5 is another enlarged perspective view of the concave portion; 
     FIGS. 6A and 6B are horizontal sectional views illustrating conditions when the concave portion and the convex portion are engaged; 
     FIG. 7 is an enlarged perspective view of a concave portion according to a second embodiment of the present invention; 
     FIGS. 8A and 8B are horizontal sectional views illustrating conditions when the concave portion and the convex portion are engaged in the second embodiment of the present invention; 
     FIG. 9 is a longitudinal side view illustrating a part of an image forming apparatus with a duplex unit according to a third embodiment of the present invention; 
     FIG. 10 is a longitudinal side view illustrating a part of an image forming apparatus with a duplex unit according to a fourth embodiment of the present invention; 
     FIG. 11 is an exploded perspective view illustrating a bracket and a guide member according to the fourth embodiment of the present invention; and 
     FIG. 12 is a longitudinal side view illustrating a part of a background image forming apparatus with a duplex unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present invention employed in an electrophotographic image forming apparatus are now described. The image forming apparatus includes a main body case housing an image forming unit and a duplex unit having both a reverse unit and a manual sheet feeding unit that are installed to the main body case as an option. 
     A first embodiment of the present invention is described referring to FIGS. 1 through 6. FIG. 1 is a longitudinal side view illustrating a part of an image forming apparatus with a duplex unit. Referring to FIG. 1, in a main body case  1 , a sheet transfer path  5  is provided such that a sheet is conveyed from a sheet feeding tray  2  to a sheet stacker unit (not shown) via an image forming unit  3  and a fixing unit  4 . 
     On the periphery of the main body case  1 , a sheet discharging outlet  6 , a sheet refeed inlet  7 , and a manual sheet feeding inlet  8  are provided to open toward a duplex unit  70 . The sheet discharging outlet  6  is open downstream of the image forming unit  3  in the sheet conveying direction, and is located on the separated sheet transfer path  5 . The sheet transfer path  5  is separated into each path toward the sheet discharging outlet  6  and toward the sheet stacker unit by a separation pick  9 . The sheet refeed inlet  7  connects to the sheet transfer path  5  upstream of the image forming unit  3  in the sheet conveying direction. 
     Around a photoconductive element  10  located at substantially a center part of the image forming unit  3 , there leave provided a charging device  11  that uniformly charges the photoconductive element  10 , a developing device  12  that develops latent images, which are formed on the photoconductive element  10  by exposing an original image scanned at a scanning unit (not shown) with an exposing device (not shown), with toner to form toner images, a transfer device  13  that transfers toner images to a sheet conveyed with an appropriate timing, a discharging device  14  that discharges a surface of the photoconductive element  10 , and a cleaning device  15  that removes residual toner on the photoconductive element  10 . 
     A pressure roller  16  and a fusing roller  17  are provided in the fixing unit  4  that fixes a transferred toner image on a sheet. 
     Next, a reverse unit  18 A is described. A case  18  of the reverse unit  18 A is installed next to the main body case  1  (i.e., at the right side as viewed in FIG.  1 ). 
     On the periphery of the case  18 , there are provided a sheet supply inlet  20   a  connecting to the sheet discharging outlet  6 , a sheet discharging outlet  21   a  connecting to the sheet refeed inlet  7 , and an opening  39  opening downward. 
     In the case  18 , a sheet reverse path  19  is provided to reverse a sheet having an image on one side that is supplied from the sheet discharging outlet  6  and then to refeed the sheet to the sheet refeed inlet  7 . The sheet reverse path  19  includes a reverse path  20  and a refeed path  21  that is separated from the reverse path  20 . One end of the reverse path  20  connects to the sheet discharging outlet  6  via the sheet supply inlet  20   a , and another end connects to the opening  39 . One end of the refeed path  21  connects to a separation point between the reverse path  20  and the refeed path  21  and another end connects to the sheet refeed inlet  7  via the sheet discharging outlet  21   a . At the separation point between the reverse path  20  and the refeed path  21 , a reverse roller  22  and a separation pick  23  are provided. The reverse roller  22  is reversible and conveys a sheet that is guided to the reverse path  20  from the sheet discharging outlet  6  to the refeed path  21  in a switchback direction. The separation pick  23  switches between the reverse path  20  and the refeed path  21 . On the refeed path  21 , plural sets of sheet conveying rollers  24  are provided to convey a reversed sheet to the sheet refeed inlet  7 . 
     A manual sheet feeding unit  34  is also installed next to the main body case  1  (i.e., at the right side as viewed in FIG.  1 ). The manual sheet feeding unit  34  includes a manual sheet feeding tray  28  that stacks sheets thereon and a sheet feeding section  26  that feeds sheets to the manual sheet feeding inlet  8 . In the sheet feeding section  26 , a manual sheet feeding path  31  is formed through which a sheet is conveyed from the manual sheet feeding tray  28  to the manual sheet feeding inlet  8 . Further, in the vicinity of a sheet feeding exit of the manual sheet feeding tray  28 , a sheet feeding roller  32  and a friction pad  33  are arranged. The sheet feeding roller  32  press-contacts a top sheet of the stacked sheets on the manual sheet feeding tray  28  to feed the sheet, and the friction pad  33  press-contacts the sheet feeding roller  32 . The friction pad  33  is provided with one end of a spring  33   a  whose another end is provided with the sheet feeding section  26 , and is biased in the direction of the sheet feed roller  32  by the spring  33   a.    
     FIG. 2 is a side view illustrating a part of the image forming apparatus with the duplex unit  70 . As illustrated by an imaginary line in FIG. 2, a pair of hinges  25  are fixed on the case  18  in a direction perpendicular to the sheet of FIG. 2 which protrudes downward from the both sides of the case  18 . The hinges  25  are rotatably provided around a first supporting shaft  27  which is horizontally provided in the sheet feeding section  26 . 
     As illustrated in FIGS. 1 and 2, on the case  18  of the reverse unit  18 A, a recess  29  is provided to retract the manual sheet feeding tray  28  into the recess  29  without protruding from the side surface of the case  18 . Owing to the recess  29 , a breakdown of manual sheet feeding tray  28  resulting from striking against people or surrounding items is avoided. The manual sheet feeding tray  28  is rotatably provided around a second supporting shaft  30  which is horizontally provided in the sheet feeding section  26 . 
     FIGS. 3A and 3B are perspective views illustrating a part of the image forming apparatus with the duplex unit  70 . In FIG. 3A, the manual sheet feeding tray  28  is closed. In FIG. 3B, the manual sheet feeding tray  28  is open. 
     The manual sheet feeding tray  28  and the reverse unit  18 A are detachably engaged with a convex portion  35  provided on each side end of the manual sheet feeding tray  28  detachably engaged with a concave portion  36  provided in each inner side surface of the recess  29 . An engaging device  37  includes the convex portion  35  and the concave portion  36 . 
     Referring to FIGS. 4A through 6, the engaging device  37  is described. FIG. 4A is an enlarged perspective view of the concave portion  36 , and FIG. 4B is an enlarged perspective view of the convex portion  35 . FIG. 5 is another enlarged perspective view of the concave portion  36 . FIGS. 6A and 6B are horizontal sectional views illustrating conditions when the concave portion  36  and the convex portion  35  are engaged. The concave portion  36  is shaped like an oblong. The convex portion  35  engages a part of the concave portion  36 . At a lower side of the concave portion  36 , a sloping surface  36   a  is formed extending from a bottom surface of the concave portion  36  in the direction of opening the manual sheet feeding tray  28 . Owing to the sloping surface  36   a , when the manual sheet feeding tray  28  is opened, the convex portion  35  is easily disengaged from the concave portion  36  by sliding on the sloping surface  36   a . When the convex portion  35  slides on the sloping surface  36   a , the sloping surface  36   a  and each inner side surface of the recess  29  against which the convex portion  35  abuts are pressed by the convex portion  35  and change their shapes to allow the convex portion  35  to move by the rotation of the manual sheet feeding tray  28 . 
     In the above-described image forming apparatus with the duplex unit  70 , a controller (not shown) is provided to control an image forming operation, switching operations between sheet transfer paths  5  toward the sheet stacker unit and toward the sheet discharging outlet  6 , and between the reverse path  20  and the refeed path  21 . 
     Next, an image forming operation in the image forming apparatus with the above-described configuration are described. 
     In the image forming unit  3 , the exposing device exposes the photoconductive element  10  that is uniformly charged by the charging device  11  to form a latent image corresponding to an image of an original document scanned by the scanner unit. The latent image is developed with charged toner to form a toner image on the photoconductive element  10 . The toner image is transferred to a sheet that is conveyed from the sheet feeding tray  2  or the manual sheet feeding tray  28  with an appropriate timing, when the sheet is biased during passing the transfer device  13 . Then, the toner image transferred to the sheet is fixed thereon when the sheet passes through the fixing unit  4 , i.e., between the pressure roller  16  and the fusing roller  17 . 
     When an image is formed on only one side of the sheet, the sheet passed through the fixing unit  4  is conveyed to the sheet stacker unit. 
     When images are formed on both sides of the sheet, the sheet having an image on one side is conveyed from the fixing unit  4  to the sheet discharging outlet  6  by switching the separation pick  9 . Then, the sheet is conveyed to the reverse path  20  from the sheet supplying inlet  20   a  while rotating the reverse roller  22  in the forward direction (i.e., in the direction toward the reverse path  20 ). When the trailing edge of the sheet passes the separation pick  23 , the separation pick  23  switches to the position in which the refeed path  21  connects to the reverse path  20 . When the trailing edge of the sheet in the reverse path  20  abuts on the reverse roller  22 , the reverse roller  22  is switched to rotate in the switchback direction (i.e., in the direction toward the refeed path  21 ). As a result of switching the rotation of the reverse roller  22 , the sheet is conveyed from the reverse path  20  to the refeed path  21 . The reversed sheet is refeed to the sheet transfer path  5  via the sheet refeed inlet  7  by the transfer rollers  24 . 
     The sheet conveyed to the sheet transfer path  5  is further conveyed to the image forming unit  3  with an appropriate timing. In the image forming unit  3 , a toner image, which is formed on the photoconductive element  10  by the above-described image forming operations for the backside of the sheet, is transferred to the backside of the sheet and fixed on the sheet in the fixing unit  4 . 
     The sheet having images on both sides is conveyed to the sheet stacker unit by switching the separation pick  9  to the position in which the sheet transfer path  5  connects to the sheet stacker unit. 
     Next, an image is formed on a sheet fed from the manual sheet feeding tray  28  as follows. First, the manual sheet feeding tray  28  is rotated around the second supporting shaft  30  in the direction separating from the case  18 , from the closed position indicated by a two-dots-and-dash line to the opened position indicated by a solid line in FIG.  1 . By this rotation of the manual sheet feeding tray  28 , the convex portion  35  slides on the sloping surface  36   a  in the direction indicated by an arrow C in FIG. 5, and then the case  18  and the manual sheet feeding tray  28  become disengaged. When the manual sheet feeding tray  28  is rotated to the predetermined position, the manual sheet feeding path  31  connects the manual sheet feeding tray  28  to the sheet transfer path  5 . As a result of rotating the sheet feeding roller  32 , friction is produced between the sheet feeding roller  32  and a sheet, and is used to convey the sheet toward the sheet transfer path  5 . A single sheet or plural sheets are conveyed from the manual sheet feeding tray  28  and abut on the friction pad  33 . When plural sheets abut on the friction pad  33 , the sheets other than the top sheet are caused to stop proceeding due to friction caused by the friction pad  33 . Therefore, only the top sheet is conveyed by the sheet feeding roller  32  to the sheet transfer path  5  through the manual sheet feeding path  31 . After the sheet is conveyed to the sheet transfer path  5  from the manual sheet feeding tray  28 , images are formed by the above-described image forming operations on one side or two sides of the sheet, and the sheet is discharged to the sheet stacker unit. 
     Next, referring to FIG. 2, a sheet removal operation when a sheet is jammed in the sheet transfer path  5  is described. First, the case  18  of the reverse unit  18 A is rotated around the first supporting shaft  27  in the direction separating from the main body case  1 . Subsequently, the manual sheet feeding tray  28  is pushed by the case  18  and starts rotating around the second supporting shaft  30  in the direction separating from the main body case  1  together with the case  18 . 
     By the above-described rotations of the case  18  and the manual sheet feeding tray  28 , the relative positions of the convex portion  35  and the concave portion  36  change because each rotation center of the case  18  and the manual sheet feeding tray  28  is different. Specifically, as illustrated in FIG. 2, as a result of rotating the case  18  around the first supporting shaft  27 , the concave portion  36  moves along an arcuate line having a radius equal to the distance between the first supporting shaft  27  and the concave portion  36 . A reference character A designates a movement orbit of an upper edge of the concave portion  36  (hereinafter referred to as a movement orbit A), and a reference character A′ designates a movement orbit of a lower edge of the concave portion  36  (hereinafter referred to as a movement orbit A′). Further, the convex portion  35  of the manual sheet feeding tray  28  that is engaged with the concave portion  36  moves along an arcuate line having a radius equal to the distance between the second supporting shaft  30  and the convex portion  35 . A reference character B designates a movement orbit of the convex portion  35  (hereinafter referred to as a movement orbit B). When the case  18  is closed, the convex portion  35  positions at a lower end of the concave portion  36 . As the case  18  is rotated to open, the convex portion  35  slides upward along the concave portion  36  from the lower end to the upper end of the concave portion  36 . While the case  18  rotates from the closed position to the open position, the movement orbit of the convex portion  35  (movement orbit B) always exists within an area of the movement orbit of the concave portion  36  (area between movement orbit A and movement orbit A′). Owing to the above-described configuration of the engaging device  37  including the convex portion  35  and the concave portion  36 , even though the reverse unit  18 A and the manual sheet feeding tray  28  rotate and the relative position of the reverse unit  18 A and the manual sheet feeding tray  28  is displaced, the reverse unit  18 A and the manual sheet feeding tray  28  can be maintained to be engaged. Thereby, the sheet transfer path  5  can be opened by rotating the case  18  of the reverse unit  18 A in the direction separating from the main body case  1  with the reverse unit  18 A and the manual sheet feeding tray  28  engaged. Owing to the engaging device  37 , an operator can open the reverse unit  18 A efficiently without additional operation of moving the manual sheet feeding tray  28  beforehand. Therefore, operability is increased. 
     After rotating the case  18  to the open position, an operator removes a jammed sheet in the sheet transfer path  5  by reaching into the open space with a hand. 
     Next, an operation for closing the case  18  after removing a jammed sheet is described. First, the case  18  is rotated around the first supporting shaft  27  towards the main body case  1  from the open position. At this time, the manual sheet feeding tray  28  moves together with the case  18  because the manual sheet feeding tray  28  is engaged with the case  18  by the engaging device  37 , and rotates around the second supporting shaft  30  towards the main body case  1 . Because each rotation center of the case  18  and the manual sheet feeding tray  28  is different, the relative positions of the concave portion  36  and the convex portion  35  change by rotating the case  18  and the manual sheet feeding tray  28 . Specifically, in the engaging device  37 , the convex portion  35  slides downward in the concave portion  36  from the upper end to the lower end. Owing to the above-described configuration of the engaging device  37 , the case  18  and the manual sheet feeding tray  28  rotate together with the case  18  and the manual sheet feeding tray  28  engaged. 
     When the case  18  is closed, the concave portion  36  and the convex portion  35  are engaged between one side wall of the concave portion  36  and the sloping surface  36   a  as illustrated in FIG.  6 A. Even if the image forming apparatus is moved by an operator, the manual sheet feeding tray  28  is prevented from being disengaged from the case  18  by vibration. On the other hand, when the manual sheet feeding tray  28  is drawn from the recess  29  and is opened in the direction indicated by the arrow C in FIG. 5, the convex portion  35  and the concave portion  36  are disengaged easily by sliding on the sloping surface  36   a,  so that the manual sheet feeding tray  28  is allowed to rotate smoothly. 
     When the case  18  is opened with the manual sheet feeding tray  28  engaged with the case  18 , the convex portion  35  of the manual sheet feeding tray  28  moves to the upper end of the concave portion  36 . As illustrated in FIG. 6B, the convex portion  35  is engaged between the side walls of the concave portion  36  at the upper end of the concave portion  36 . Because the convex portion  35  is engaged with the concave portion  36  securely at the upper end of the concave portion  36 , the manual sheet feeding tray  28  does not happen to be opened while an operator removes the jammed sheets. 
     When the reverse unit  18 A is rotated outward around the first supporting shaft  27  for removing a jammed sheet in the sheet transfer path  5 , because the first supporting shaft  27  is arranged below the reverse unit  18 A and at the position separating from the main body case  1 , a lower part of an inner surface (i.e., a surface facing the main body case  1 ) of the case  18  is caused to be much separated from the main body case  1 . Therefore, enough space for removing the jammed sheet is provided, so that the jammed sheet is smoothly removed. Further, in the above-described arrangement of the first supporting shaft  27 , because the center of gravity of the reverse unit  18 A is positioned closer to the main body case  1  relative to the center of the first supporting shaft  27 , the reverse unit  18 A can lean toward the main body case  1  with the assistance of the mass of the reverse unit  18 A. As a result, when a lock mechanism (not shown) for locking the reverse unit  18 A in the vertical position is installed in the reverse unit  18 A, a simple and low-cost lock mechanism can be employed. 
     In the above-described image forming apparatus according to the first embodiment of the present invention, the convex portion  35  and the concave portion  36  may be interchangeable between the case  18  and the manual sheet feeding tray  28 . 
     Alternatively, the first supporting shaft  27  and the second supporting shaft  30  may position at the same place, if the common shaft has enough strength for supporting both the reverse unit  18 A and the manual sheet feeding unit  34 . 
     Next, a second embodiment of the present invention is described referring to FIG.  7  and FIGS. 8A and 8B. FIG. 7 is an enlarged perspective view of a concave portion. FIGS. 8A and 8B are horizontal sectional views illustrating conditions when the concave portion and the convex portion are engaged. 
     For the sake of simplification of the description, the members having substantially the same functions as the ones used in the first embodiment are designated with the same reference numeral and their descriptions are omitted. 
     The elements of the image forming apparatus of the second embodiment are substantially the same as those of the first embodiment except a concave portion  38  formed in each inner side surface of the recess  29 . 
     The concave portion  38  is shaped like an oblong, and includes a shallow part  38   a  located at a lower end of the concave portion  38  and a deep part  38   b  located at an upper end of the concave portion  38 . 
     When the reverse unit  18 A is closed, the convex portion  35  is located at the lower end of the concave portion  38  and at the shallow part  38   a.  As illustrated in FIG. 8A, at the lower end of the concave portion  38 , a depth H of the side walls of the concave portion  38  is small, so a strength of engagement between the concave portion  38  and the convex portion  35  is small. Therefore, the manual sheet feeding tray  28  can be drawn from and retracted in the recess  29  easily by hand operation by an operator. 
     On the other hand, when the reverse unit  18 A is opened in the direction separating from the main body case  1 , the convex portion  35  is located at the upper end of the concave portion  38  and at the deep part  38   b . As illustrated in FIG. 8B, at the upper end of the concave portion  38 , a depth H′ is greater than the depth H, so a strength of engagement between the concave portion  38  and the convex portion  35  is great. Therefore, the manual sheet feeding tray  28  can be prevented from being opened accidentally. 
     Also in the image forming apparatus of the second embodiment, the convex portion  35  and the concave portion  38  may be interchangeable between the case  18  and the manual sheet feeding tray  28 . 
     Next, the third embodiment of the present invention is described referring to FIG.  9 . FIG. 9 is a longitudinal side view illustrating a part of an image forming apparatus with a duplex unit. 
     For the sake of simplification of the description, the members having substantially the same functions as the ones used in the first and second embodiments are designated with the same reference numeral and their descriptions are omitted. 
     The image forming apparatus in the third embodiment also includes the main body case  1  housing the image forming unit  3  and the duplex unit  70  having both the reverse unit  18 A and the manual sheet feeding unit  34 , that is installed to the main body case  1  as an option. The case  18  of the reverse unit  18 A houses the reverse path  20  in which the opening  39  is formed at the lower edge of the reverse path  20 . The length of the reverse path  20  from the reverse roller  22  to the opening  39  is set to the length of the sheet in the sheet conveying direction which is most frequently used, and is set to be shorter than the length of a large size sheet in the sheet conveying direction which is not frequently used (the sheet whose length in the sheet conveying direction is longer than the length of the reverse path  20  from the reverse roller  22  to the opening  39  is referred to as a large size sheet hereinafter). This is because the height of the reverse unit  18 A is limited to provide a space for installation of the manual sheet feeding unit  34  below the reverse unit  18 A. 
     A guide member  40  facing the opening  39  is integrally formed with the sheet feeding section  26  of the manual sheet feeding unit  34 . The guide member  40  includes a cutaway portion  41  having a curved guide surface  42  which descends toward the main body case  1  below the opening  39  and guides the sheet protruded from the opening  39 . 
     In the above-described image forming apparatus with the duplex unit  70  of the third embodiment, the large size sheet is reversed as follows. 
     When the large size sheet is conveyed to the reverse path  20 , a leading edge of the sheet is protruded from the opening  39 . Because the opening  39  opens downward at the bottom part of the reverse unit  18 A, the sheet protrudes downward from the opening  39  without protruding from the side of the reverse unit  18 A like the background image forming apparatus illustrated in FIG.  12 . The leading edge of the sheet protruded from the opening  39  enters the cutaway portion  41  and is guided toward the main body case  1  along the curved guide surface  42 . 
     In the image forming apparatus with the duplex unit  70  of the above-described configuration, various sizes of sheets can be reversed without enlarging the installation space of the image forming apparatus and without additional operations for opening the opening  39  and making space for the protruded sheet. Moreover, because the guide member  40  is integrally provided with the sheet feeding section  26  of the manual sheet feeding unit  34 , various sizes of sheets can be reversed in a simple configuration without increasing a number of construction parts. 
     Further, because the sheet stacked on the manual sheet feeding tray  28  is separated and fed one by one by the sheet feeding roller  32  and the friction pad  33  toward the sheet transfer path  5 , a sheet separation/feeding mechanism can be made compact in the sheet feeding section  26 . In addition, because the sheet feeding roller  32  and the friction pad  33  are provided in the vicinity of a sheet feeding exit of the manual sheet feeding tray  28 , a space downstream of the sheet feeding roller  32  and the friction pad  33  and above the manual sheet feeding path  31  can be provided to form the guide member  40  for the large size sheet. 
     Next, the fourth embodiment of the present invention is described referring to FIGS. 10 and 11. FIG. 10 is a longitudinal side view illustrating a part of an image forming apparatus with a duplex unit. FIG. 11 is an exploded perspective view illustrating a bracket and a guide member. For the sake of simplification of the description, the members having substantially the same functions as the ones used in the first through third embodiments are designated with the same reference numeral and their descriptions are omitted. 
     The image forming apparatus in the fourth embodiment does not include a manual sheet feeding unit. However, a guide member  43  facing the opening  39  of the reverse path  20  is provided below a reverse unit  18 B. In the guide member  43 , a curved guide surface  44  is formed to guide the large size sheet protruded from the opening  39  to reverse. As illustrated in FIG. 11, a plurality of curved guide surfaces  44  are made of entire upper curved surfaces of plural ribs  45  which are integrally formed with the guide member  43 . A reference numeral  46  in FIG. 11 indicates a bracket that is mounted on the side of the main body case  1  with screws  47 . On the outside of bent portions  48  which are formed on both sides of the bracket  46 , the pair of first supporting shafts  27  are installed so as to rotatably support the hinges  25  mounted on both sides of the case  18 . In addition, installation pieces  49  are fixed on both inner side walls of the bracket  46 . Further, on both upper side of the guide member  43 , respective slits  50  are formed on the center of the both side portions of the guide member  43  to protrude the hinges  25  inside the guide member  43 . The guide member  43  is installed to the installation pieces  49  of the bracket  46  with the screws  51 . 
     The reverse unit  18 B in the fourth embodiment includes the case  18  that houses the reverse path  20 , the refeed path  21 , the reverse roller  22 , the separation pick  23 , and the transfer rollers  24 , the guide member  43 , and the bracket  46  having the pair of first supporting shafts  27 . 
     In the image forming apparatus with the above-described configuration, when the large size sheet is conveyed into the reverse path  20 , the leading edge of the sheet is protruded from the opening  39 , and is guided along the descending curved guide surface  44  in the direction opposite to the main body case  1 . 
     The guide member  43  also serves as a covering member that covers the bracket  46  that rotatably supports the case  18 . 
     Alternatively, the curved guide surface  44  can be arranged such that the curve of the guide surface  44  descends toward the main body case  1 , if the opening  39  opens downward at the bottom part of the reverse unit  18 B and at a position apart from the main body case  1 . 
     Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.