Patent Publication Number: US-6908081-B2

Title: Sheet feeder and image forming apparatus having the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a sheet feeder that feeds cut sheets and an image forming apparatus including the sheet feeder, and more particularly, to a device that separates and feeds sheets, one by one, by abutting leading edges of the sheets in a sheet feed direction against an inclined separating surface formed by separating plates. 
     2. Description of Related Art 
     Conventional image forming apparatuses, such as laser-beam printers, color ink-jet printers, facsimile machines and copying machines, include a sheet feeder that feeds cut sheets, one by one, to an image forming unit provided therein. As disclosed in U.S. Pat. No. 6,158,733 and Japanese Laid-Open Patent Publication No. 2001-278507, the sheet feeder includes an inclined sheet holder, which holds a stack of sheets thereon, a separating plate, which is disposed below the sheet holder and has an inclined separating surface, and a sheet feed roller that is attached to an end of a rotatable arm having a predetermined length so as to face the sheet holder. The inclined separating surface extends such that the inclined separating surface and an upper surface of the sheet holder form an obtuse angle therebetween. Thus, the sheets stacked on the sheet holder are held by the inclined separating surface of the separating plate with the leading edges of the sheets contacting the inclined separating surface. 
     As the sheet feed roller rotates while pressing a topmost sheet in the stack on the sheet holder, the leading edge of the topmost sheet, which is being downwardly fed, abuts against the inclined separating surface. At that time, with a reaction force from the inclined separating surface, the topmost sheet is conveyed in a state where the lower part of the sheet is bent in a direction orthogonal to a direction that the rest of the sheets extend (the topmost sheet is convexedly warped such that the surface of the topmost sheet comes away from the stack of the sheets). Before long, the trailing edge of the sheet comes away from the inclined separating surface and thus only the topmost sheet is separated from the stack of the sheets. After that, the separated sheet is fed to the image forming unit by a conveying roller provided in a sheet feed path, to form an image thereon. Then, the sheet having the image is ejected from the image forming apparatus. 
     Japanese Laid-Open Patent Publication No. 2000-168980 discloses a sheet feeder that includes a sheet holder disposed in a horizontal position, a fixed guide plate, and movable guide plates. The fixed guide plate is inclined such that an inclined angle (exterior angle), which is formed between an extension of a surface of the sheet holder and a surface of the fixed guide plate, becomes β (smaller than 90 degrees). The movable guide plates are provided at one end of the sheet holder and at regular intervals in a sheet width direction. The movable guide plates pivot about the one end of the sheet holder. The fixed guide plate is also provided at the one end of the sheet holder. The movable guide plates are maintained by urging springs so that an inclined angle, which is formed between the extension of the surface of the sheet holder and surfaces of the movable guide plates, becomes α (also smaller than 90 degrees but larger than the inclined angle β, in an initial state). A frictional coefficient of the inclined surface of the fixed guide plate is smaller than that of the inclined surfaces of the movable guide plates. 
     In the sheet feeder described above, when a sheet feeding operation is performed by the sheet feed roller in a state where sheets are fully loaded on the sheet holder, the leading edges of the sheets press the movable guide plates and thus the movable guide plates pivot. When the inclined angles of the inclined surfaces (inclined separating surfaces) of the movable guide plates become the same degree as the inclined angle β of the fixed guide plate, which is a gentler slope than the inclined angle α, the leading edges of the sheets abut against the inclined surface (inclined separating surface) of the fixed guide plate having the smaller coefficient and a distance between the leading edges of the sheets and the contacting portion of the sheet feed roller and the sheet becomes longer. Thus, the leading edge of the topmost sheet is bent and the topmost sheet is smoothly separated from the stack. 
     On the other hand, when a small number of sheets are loaded on the sheet holder, the leading edges of the sheets are located near the center of the rotation of the movable guide plates. In addition, the arm length of the rotation moment for rotating the movable guide plates (a distance between a support point of the movable guide plate to a point of action of the force) is short, so that the leading edges of the sheets cannot rotate the movable guide plates. However, the distance between the leading edges of the sheets and the contacting portion of the sheet feed roller and the topmost sheet is longer than the distance therebetween when the full of sheets are loaded on the sheet holder. Further, an angle between the rotatable arm of the sheet feed roller and the surface of the sheet is large. Therefore, a sheet conveying force becomes large. Thus, even when the movable guide plates do not pivot, the leading edge of the topmost sheet is bent along the inclined surfaces of the movable guide plates and thus the topmost sheet can be smoothly separated from the stack one by one. The inclined separating surface, which is formed by the movable guide plates and/or the fixed guide plate, includes a flat surface extending along the sheet width direction. 
     A sheet feed roller to be provided in the sheet feeder may not have a length across the entire width of the sheet. Recently, the length of the sheet feed roller is designed as short as possible in order to achieve miniaturization and low power consumption of the sheet feeder. Therefore, a small sheet feed roller is provided at substantially the middle of the sheet width direction to contact a portion of a middle area of the sheet with the sheet feed roller. Further, most sheet feeders feed cut sheets of different types and sizes by using the same mechanism. When large-sized sheets are fed, the sheet feed roller contacts only a part of the sheets in the sheet width direction to feed the sheets. 
     When soft or week sheets are fed by the sheet feeder that includes the above-described small sheet feed roller that contacts only a part of the sheets, the sheets are conveyed one by one while being warped such that the lower edges (leading edges in a sheet feed direction) of the middle areas of the sheets are abutting against a portion on an extension of the acting portion of the sheet feed roller, in the inclined separating surface. At that time, side areas of the sheets, which are a distance from the acting portion of the sheet feed roller in the sheet width direction, become free ends, which are free from the sheet feeding force by the rotation of the sheet feed roller. Therefore, during the sheet feeding operation by the sheet feed roller, both side areas of the sheets are kept substantially flat while the sheets are being conveyed. 
     When the above-described situation is viewed from a direction facing the sheets stacked on the sheet holder, the inclined separating surface includes a flat surface along the sheet width direction. With this structure, the lower edges (leading edges in the sheet feed direction) of the side areas of the sheets contact the inclined separating surface before the leading edges of the middle areas of the sheets contact the inclined separating surface. Because of this, the middle areas of the sheets will be further warped. Accordingly, the lower edges (leading edges in the sheet feed direction) of the middle areas of the sheets easily come off from the corresponding portion of the inclined separating surface. 
     On the other hand, however, the lower edges (leading edges in the sheet feed direction) of the sheets hardly contact a separating member, which is provided to the inclined separating surface, according to a width and/or a position where the separating member is provided. Thus, it becomes difficult to contact the leading edges of the sheets with the separating member, and the separating action by the separating member cannot be surely exerted on the sheets. This may cause a multi-feed problem (feeding two or more sheets at a time). Such a problem remarkably occurs when sheet to be fed are wide in width (the sheet is long in the sheet width direction) and when the sheet feed roller is short in length. 
     SUMMARY OF THE INVENTION 
     The invention thus provides a sheet feeder, which can surely separate and feed sheets, one by one, from a stack of sheets, while a drive system for feeding sheets is compacted, and an image forming apparatus including the above-described sheet feeder. 
     The sheet feeder of the invention feeds sheets. The sheet feeder includes a sheet holder that holds a plurality of sheets, a sheet feed roller that holds the sheets in cooperation with the sheet holder and feeds a topmost sheet from the sheets held by the sheet holder, and an inclined separating surface that holds leading edges of the sheets in a sheet feed direction. The inclined separating surface includes a fixed member and a movable member. The fixed member has a fixed surface that has a separating unit having a high coefficient of friction. The separating unit is provided to a first portion which corresponds to a second portion of the sheets where the sheet feed roller contacts, at the leading edges of the sheets, in a sheet width direction. The movable member is provided adjacent to the fixed member and has a movable surface that can descend in the sheet feed direction when contacted with the sheets held by the sheet holder when the sheet feed roller feeds the sheets. 
     Therefore, when sheets held by the sheet holder are weak or soft, the movable member surely receives the leading edges of the side areas of the sheets, on both sides of the fixed member. When sheets are rigid or strong, the movable member descends against an urging force from an urging member when the pressing force from the leading edges of the sheets increases. Thus, the movable member descends in a direction to move away from the leading edges of the sheets in order to make a clearance between the movable member and the leading edges of the sheets. As a result of this, interference between the leading edges of the sheets and the movable member, which is caused by the support of the movable member, can be avoided, so that the separating action by the separating unit can be surely exerted on the leading edges of the middle areas of the sheets. Further, the separating action is further improved and an occurrence of a paper jam due to, such as a multi-sheet feeding, can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a perspective view of an image forming apparatus according to a first embodiment of the invention; 
         FIG. 2  is a perspective view of a sheet feeder according to the first embodiment of the invention; 
         FIG. 3  is a front view showing essential parts of the sheet feeder; 
         FIG. 4  is a sectional view taken along a line IV—IV of  FIG. 3 ; 
         FIG. 5  is a perspective view of a split-type inclined separating surface, which includes a fixed separating plate and movable separating plates, according to the first embodiment; 
         FIG. 6  is an enlarged sectional view of one of the movable separating plates; 
         FIG. 7A  is a front view showing a split-type inclined separating surface according to a second embodiment; 
         FIG. 7B  is a sectional view of one of the movable separating plates of the second embodiment, while the movable plate is at the up position; 
         FIG. 7C  is a sectional view of one of the movable separating plates of the second embodiment, while the movable plate is at the down position; 
         FIG. 8A  is a plan view of the fixed separating plate including a sheet separating unit; 
         FIG. 8B  is a sectional view taken along a line VIIIb—VIIIb of  FIG. 8A ; 
         FIG. 8C  is a sectional view taken along a line VIIIc—VIIIc of  FIG. 8A ; 
         FIG. 9  is a sectional view taken along a line IX—IX of  8 A; 
         FIG. 10A  is a plan view of the sheet separating unit; and 
         FIG. 10B  is a sectional view taken along the line Xb—Xb in FIG.  10 A. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described with reference to the accompanying drawings. A first embodiment of the invention will be described below. 
     A multifunctional image forming apparatus  1  of  FIG. 1  has a facsimile function, a printing function, a copying function, and a scanning function. As shown in  FIG. 1 , the image forming apparatus  1  includes a substantially box-shaped body  2 . The body  2  has an operating panel  3  on its upper surface. The operating panel  3  is provided with various buttons and/or keys, such as a start button, numeric (0 to 9) buttons, and function keys. By pressing the buttons and keys, various operations are performed. A liquid crystal display (LCD)  4  is provided at an upper portion of the operating panel  3  to display setting conditions of the image forming apparatus  1  and various messages as needed. A side, on which the operating panel  3  is provided, is defined as a front of the image forming apparatus  1 , and an opposite side is defined as a rear of the image forming apparatus. The right and left sides of the image forming apparatus  1  are defined as right and left, respectively, when viewed from the front of the image forming apparatus. 
     A document holding portion  5  is provided at the rear of the LCD  4 . The document holding portion  5  holds original documents, which are to be copied and transmitted to another facsimile machine in the facsimile mode or which are to be copied in the copy mode. The original documents placed on the document holding portion  5  are conveyed to a scanning unit (not shown) provided in the body  2  and surfaces of the original documents are scanned by the scanning unit. Then, the scanned documents are ejected onto a document discharge portion  7  provided at the front of the body  2  (under the operating panel  3 ). 
     A sheet feeder  10 , on which a stack of recording sheets P are loaded, is provided at the rear of the document holding portion  5 . The sheets P placed on the sheet feeder  10  are conveyed, one by one, to a color ink-jet type image forming unit (not shown) provided in the body  2 . At the image forming unit, predetermined images are printed onto the sheets P, and then, the sheets P are ejected onto a sheet discharge portion  6 . The image forming unit is not limited to the ink-jet type, but can be other types, for example, a laser printing type using toner or a thermal transfer type using an ink ribbon. 
     As shown in  FIGS. 2  to  4 , the sheet feeder  10  includes a frame  11 . The frame  11  includes an inclined sheet holder  12 , a pair of side walls  14  and a guide  13 , i.e., pair of guide members  13   a ,  13   b . The sheet holder  12  is downwardly inclined in a direction toward the front of the image forming apparatus  1 . The side walls  14  integrally stand from right and left edges of the sheet holder  12 . The guide members  13   a ,  13   b  are provided to the sheet holder  12  and inside with respect to the side walls  14 . The guide members  13   a ,  13   b  are slidable in right and left directions with respect to the sheet holder  12 . The sheet holder  12  and the side walls  14  are made of synthetic resin and are integral to form a monolithic structure. The sheet holder  12  can hold a stack of sheets P thereon. The frame  11  has a lower frame portion  11   a . The lower frame portion  11   a  is provided with a plurality of separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  to receive leading edges of the sheets P and to guide and send the sheets P, one by one, to the image forming unit. Upper surfaces of the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  form an inclined separating surface  8 , which is an upwardly convex surface. The separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  protrude from the lower frame portion  11   a  in a sheet advance direction indicated by an arrow A in  FIGS. 2 and 4  to hold the leading edges of the sheets P stacked on the sheet holder  12  (FIGS.  2  and  4 ). Explanations of the inclined separating surface  8  and the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  will be provided later. 
     The guide members  13   a ,  13   b  are coupled to racks  16 , which are disposed on the back of the sheet holder  12  and extend in a horizontal direction. A pinion  17  is also provided on the back of the sheet holder  12  so as to engage the racks  16 . In synchronization with the racks  16  and the pinion  17 , the pair of the guide members  13   a ,  13   b  slide in a width direction of the sheet holder  12  (in the right and left directions) (FIGS.  2  and  3 ). Thus, the guide members  13   a ,  13   b  can get closer to and get away from each other to guide side edges of the sheets P in accordance with the width of the sheets P stacked between the guide members  13   a  and  13   b . Consequently, the sheets P can be placed in the middle of the sheet holder  12  in the width direction of the sheets P. 
     A drive shaft  20  is rotatably supported between the side walls  14 , at an appropriate distance upward from the upper surface of the separating plate  15 . A sheet feed roller unit  22  is provided substantially at the middle of the drive shaft  20 , that is, at the middle of the sheet P in the width direction (in the right and left directions). The sheet feed roller unit  22  includes a case  24  having a sheet feed roller  21 . The drive shaft  20  is inserted into the case  24  of the sheet feed roller unit  22  so that only the case  24  can freely rotate. A gear train  23 , including gears  23   a ,  23   b ,  23   c ,  23   d , is provided to an outer surface of one of the side walls  14  in order to transmit power to the sheet feed roller  21  from a drive motor (not shown) provided in the body  2  (FIG.  2 ). 
     As shown in  FIG. 4 , the case  24  of the sheet feed roller unit  22  contains a drive gear  25  that integrally rotates with the drive shaft  20 , a planet gear  27  that engages the drive gear  25 , and an intermediate gear  28 . A gear  29  is provided so as to engage the intermediate gear  28  and integrally rotate with the sheet feed roller  21 . A part of the gear  29  is covered with a lower portion of the case  24 . An arm  26  is rotatably fitted to the drive shaft  20 . The planet gear  27  is rotatably supported by the arm  26 . A torsion spring  30  ( FIG. 3 ) is fitted into the drive shaft  20 . The case  24  is urged by the torsion spring  30  so that the sheet feed roller  21  is pressed against a surface of a topmost sheet P in the stack. 
     As the gear  23   d , which is fixed to an end of the drive shaft  20 , rotates in a normal direction (in a counterclockwise direction in  FIG. 2 ) at a sheet feeding operation, the drive gear  25  also rotates in the normal direction. At that time, a force for rotating in a reverse direction (in a clockwise direction in  FIG. 2 ) is applied to the planet gear  27  engaging the drive gear  25 , so that the arm  26  swings in the normal direction and thus the planet gear  27  engages the intermediate gear  28 . Therefore, the intermediate gear  28  rotates in the normal (counterclockwise) direction and the gear  29  and the sheet feed roller  21  rotate in the reverse (clockwise) direction. Thus, the topmost sheet P, which is in contact with the sheet feed roller  21 , is conveyed downward in FIG.  4 . Also, when the drive shaft  20  rotates in a normal direction, the planet gear  27  also presses the case  24  through the intermediate gear  28  with a sheet feed force (rotation force) against the surface of the topmost sheet P. 
     When the gear  23   d  rotates in the reverse (clockwise) direction, the planet gear  27  is applied with a force for rotating in the normal direction from the drive gear  25  rotating in the reverse direction. Therefore, the arm  26  swings in the reverse direction, so that the planet gear  27  disengages from the intermediate gear  28  and thus the power to the sheet feed roller  21  is disconnected. Consequently, the sheet feed roller  21  stops rotating, thereby stopping the sheet feeding operation (the sheets P are not fed). 
     Next, the structure of the inclined separating surface  8  of the first embodiment of the invention will be described in detail. The inclined separating surface  8  is provided with a sheet separating unit  31  having a high coefficient of friction. The sheet separating unit  31  contacts the leading (lower) edges of the middle areas of the sheets P in the sheet width direction to separate the sheets P, one by one, from the sheets P stacked on the sheet holder  12 . The sheet separating unit  31 , projecting from the inclined separating surface  8 , is disposed on an extension of a line of sheet feeding action Q of the sheet feed roller  21  (FIGS.  5  and  7 ). The inclined separating surface  8  has a upwardly curved surface such that a portion near the sheet separating unit  31  is uplifted and the surface is gradually lowered near the ends of the inclined separating surface  8 , in the sheet width direction. 
       FIGS. 2  to  6  show details of the shape and structure of the inclined separating surface  8  of the first embodiment. In the first embodiment, the inclined separating surface  8  is defined by upper surfaces of a fixed separating plate  15 , first movable separating plates  32   a ,  32   b  and second movable separating plates  33   a ,  33   b , which are made of synthetic resin. As shown in  FIG. 3 , the fixed separating plate  15  is disposed at a portion corresponding to the middle area of the sheet holder  12  in the width direction of the sheet holder  12  (at a portion on the extension of the line of sheet feeding action Q of the sheet feed roller  21 ). The first movable separating plates  32   a ,  32   b  are disposed on the left and right sides of the fixed separating plate  15 , respectively. The second movable separating plate  33   a  is disposed on the left of the first movable separating plate  32   a , and the second movable separating plate  33   b  is disposed on the right of the first separating plate  32   b . Although the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  are separated from each other, the upper surfaces of the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  form the continuous inclined separating surface  8 . That is, the middle of upper surface of the fixed separating plate  15  in the right and left direction is in the highest level and outer sides of the upper surfaces of the separating plates  33   a ,  33   b  are in the lowest level. 
     As shown in  FIG. 6 , a rotation support shaft  41  integrally protrudes from a base to an end of each of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b . The lower frame portion  11   a  of the frame  11  has recessed portions  40 , in which the rotation support shafts  41  of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  are rotatably fitted. Torsion springs  42  are fitted to the rotation support shafts  41  while both ends are caught at predetermined positions, in order to upwardly urge the separating plates  32   a ,  32   b ,  33   a ,  33   b , independently. With this structure, appropriate movable separating plates  32   a ,  32   b ,  33   a ,  33   b  can be rotated by a pressing force from the stack of sheets P so as to make a clearance between the leading edges of the sheets P to be fed and the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  according to the size of the sheets P. Therefore, a resistance (the urging force from the torsion springs  42 ) to the pressing force from the sheets P can become an appropriate strength, which is neither too strong nor too weak. The movable separating plates  32   a ,  32   b ,  33   a ,  33   b  are designed such that back surfaces  43  of the base end portions of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  contact inner walls of the recessed portions  40  to restrict excessive upward rotation of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b . With this restriction, the upper surfaces of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  are not lifted to the level higher than the upper surface of the fixed separating plate  15  and the second movable separating plates  33   a ,  33   b  are not lifted to the level higher than the upper surfaces of the first separating plates  32   a ,  32   b . Alternatively, different ways can be adopted to restrict the excessive upward rotation of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b.    
     As the sheet feed roller  21  rotates, the leading edges of the sheets P to be supplied abut against the upper surfaces of the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  and thus a pressing force from the sheets P downwardly acts on the upper surfaces of the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b . By the pressing force from the sheets P, the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  downwardly rotate (descend) against the urging forces from the torsion springs  42  (i.e. the free ends of the separating plates  32   a ,  32   b ,  33   a ,  33   b  point down by rotating downward about their base ends) to release the force from the sheets P. 
     As shown in  FIG. 3 , an engaging projection  44  laterally projects from the first movable separating plates  32   a ,  32   b  so as to engage the undersides of the second movable separating plates  33   a ,  33   b , respectively, from below. With this structure, even when a downward load acts on the second movable separating plates  33   a ,  33   b  only, the second movable separating plates  33   a ,  33   b  rotate downward and thus engage the respective engaging projections  44  of the first movable separating plates  32   a ,  32   b  to press the engaging projections  44  downwardly. Thus, the first movable separating plates  32   a ,  32   b  also rotate downward with the second movable separating plates  33   a ,  33   b.    
     In the first embodiment, a distance between outer edges of the movable separating plates  33   a ,  33   b  is approximately 210 mm and a difference of elevation in the convexedly curved surface of the inclined separating surface  8  is approximately between 2 and 3 mm (a radius of curvature of the order of 1500 mm) (FIG.  3 ). It is designed such that the upper surfaces of the fixed separating plate  15  and the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  are upwardly inclined approximately 3 degrees with respect to the horizontal plane so that their free ends (the direction indicated by the arrow A in  FIGS. 2 ,  4  and  8 A) are lifted with respect to the horizontal plane, when the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  are free from a load (in a initial state). An inclined angle between the sheet holder  12  and each upper surface of the separating plates  15 ,  32   a ,  32   b ,  33   a ,  33   b  is an obtuse angle of approximately 112.5 degrees, when no load is applied to the movable separating plates  32   a ,  32   b ,  33   a ,  33   b.    
     The sheet separating unit  31  includes a separating strip  34 , extending in the front to rear direction, so that the separating strip  34  is disposed at the upper surface of the fixed separating plate  15 , at substantially the middle in the right and left direction.  FIGS. 8  to  10 B show the structure of the fixed separating plate  15  and the sheet separating unit  31  having the separating strip  34  in detail. The fixed separating plate  15  has a slit  36 , which penetrates the fixed separating plate  15  and extends along the sheet advance direction (the direction A in FIGS.  4  and  8 A). A mounting block  37 , made of synthetic resin, is detachably attached to the underside of fixed separating plate  15  by screws  38 . The separating strip  34  is made of a material having a high coefficient of friction, such as polyester urethane resin. A bridge plate  39  includes a leaf spring made of phosphor bronze and has a plurality of cantilever supporting portions  39   a  and an outer frame portion  39   b . The plurality of cantilever supporting portions  39   a  inwardly protrude from the outer frame portion  39   b , like comb teeth, as shown in FIG.  10 A. The separating strip  34  is resiliently supported by the cantilever supporting portions  39   a  of the bridge plate  39  such that the cantilever supporting portions  39   a  are inserted into the separating strip  34  (FIGS.  10 A and  10 B). 
     Only the outer frame portion  39   b , which has a substantially rectangular shape when viewed from above (FIG.  10 A), is pinched between the mounting block  37  and the fixed separating plate  15 , so that the separating strip  34  and the cantilever supporting portions  39   a  are held in midair in a recessed portion  37   a  of the mounting block  37  ( FIGS. 8B ,  8 C and  9 ). Therefore, when a downward pressing force is applied to the separating strip  34  from above by the leading edges of the sheets P due to the sheet feeding operation by the sheet feed roller  21 , the separating strip  34  is pressed downward and thus the plurality of the cantilever supporting portions  39   a  warp downwardly. Consequently, the upper surface of the separating strip  34  becomes the same level as the upper surface of the fixed separating plate  15 . 
     As shown in  FIG. 10B , the upper surface of the separating strip  34  is saw-toothed (uneven) so that a high frictional resistance can be applied to the sheets P when the leading edges of the sheets P contact and slide over the separating strip  34 . The shape of the upper surface of the separating strip  34  further increases the coefficient of friction, in addition to the frictional coefficient of the material forming the separating strip  34 . 
     Referring to  FIGS. 4 and 5 , the sheet separating action performed by the sheet feeder  10  in the above-described structure when the sheet feed roller  21  rotates to feed sheets P will be described below. First, a plurality of sheets P are loaded onto the sheet holder  12  of the sheet feeder  10  in advance. The guide members  13   a ,  13   b  are slid to guide and regulate side edges of the stacked sheets P. Therefore, the sheets P are disposed at the middle of the sheet holder  12  while the center line of the sheets P is disposed along the center line of the sheet holder  12  in the sheet width direction. In this state, the leading edges of the sheets P stacked on the sheet holder  12  are in contact with the upper surface of the fixed separating plate  15  and/or the separating strip  34 , but not in contact with the first movable separating plates  32   a ,  32   b  nor the second movable separating plates  33   a ,  33   b.    
     When a print command is executed in response to a signal transmitted from an external control device, such as a personal computer and a facsimile machine, the drive motor (not shown) drives to rotate the sheet feed roller  21 . Therefore, the drive shaft  20  rotates in the reverse direction via the gear train  23  including the gears  23   a  to  23   d , and then the sheet feed roller  21  rotates in the clockwise direction in FIG.  5 . Accordingly, only a topmost sheet P pressed by the sheet feed roller  21  is conveyed in the direction indicated by the arrow A in FIG.  5 . 
     Sheets P having different rigidity will be placed on the sheet holder  12 . When weak or soft sheets P (e.g. sheets are thin in thickness) are stacked on the sheet holder  12 , the leading edges of the middle areas of the sheets P abut against the sheet separating unit  31  (the separating strip  34 ), which is disposed on the extension of the line of the sheet feeding action Q of the sheet feed roller  21 . When a sheet feeding operation is performed in this condition, a topmost sheet P, which is subjected to the sheet feeding action, is fed in the direction indicated by the arrow A while deforming such that a middle area of the topmost sheet P is uplifted in a direction to come away from the stack of the sheets P, between the sheet feed roller  21  and the sheet separating unit  31 . 
     On the other hand, when the strong or rigid sheets P, such as cardboard, post cards, envelops, and overhead transparency films, are fed from the sheet holder  12 , a topmost sheet P, which is subjected to the sheet feeding action, deforms such that the middle area of the sheet P is uplifted in a direction toward the stack of the sheets P, between the sheet feed roller  21  and the sheet separating unit  31 . However, the sheet P is conveyed while its side areas, which are other than the middle area of the sheet P and are not subjected to the sheet feeding action, are flat. When the above-described situation happens, a distance (in straight line) between a point on a contact line (nip line)  45  of the sheet feed roller  21  and the sheet P subjected to the sheet feeding action and a point of the leading edge of the middle area of the topmost sheet P becomes shorter than a distance (in straight line) between a point on an extension  46  of the contact line  45  and a point on the side area of the leading edge of the topmost sheet P. 
     Even when the above case happens, in the first embodiment, the position of the fixed separating plate  15 , which corresponds to the middle areas of the sheets P in the sheet width direction, is not changed and the first separating plates  32   a ,  32   b  and/or the second separating plates  33   a ,  33   b  can surely receive the leading edges of the side areas of the weak or soft sheets P. On the other hand, for the strong or rigid sheets P, when the downward pressing force from the sheets P increases, the free ends of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  downwardly rotate against the urging forces from the torsion springs  42 . By doing so, the upper surfaces of the separating plates  32   a ,  32   b ,  33   a ,  33   b  (the inclined separating surface  8 ) descend in a direction to get away from the leading edges of the sheets P to make a clearance between the leading edges of the sheets P and the separating plates  32   a ,  32   b ,  33   a ,  33   b . Therefore, the leading edge of the topmost sheet P can be prevented from being interfered with the first movable separating plates  32   a ,  32   b  and/or the second movable separating plates  33   a ,  33   b  during the sheet feeding operation. Thus, the leading edge of the topmost sheet P is not pressed or supported by the first movable separating plates  32   a ,  32   b  and/or the second movable separating plates  33   a ,  33   b . Consequently, the sheet separating action by the sheet separating unit  31  can surely be applied to the leading edges of the middle areas of the sheets P. In addition, the occurrence of a paper jam due to, such as the multi-feed problem (feeding two or more sheets at a time), can be restricted. 
     When the width (size) of the sheets P stacked on the sheet holder  12  is small (when the width of the sheets P is shorter than a distance between the outer edges of the first movable separating plates  32   a ,  32   b  adjacent to the fixed separating plate  15 ), the leading edges of the side areas of the sheets P contact and downwardly press the first movable separating plates  32   a ,  32   b , which correspond to the side areas of the sheets P, and thus the first movable separating plates  32   a ,  32   b  rotate downward. Therefore, a clearance is created between the leading edges of the sheets P and the first movable separating plates  32   a ,  32   b . Because the second movable separating plates  33   a ,  33   b  do not interfere with the sheets P, the second movable separating plates  33   a ,  33   b  stay as they are (do not rotate downward). When the width (size) of the sheets P is large (when the width of the sheets P is longer than a distance between the inner edges of the second movable separating plates  33   a ,  33   b ), the leading edges of the side areas of the sheets P abut against the upper surfaces of the second movable separating plates  33   a ,  33   b  (the inclined separating surface  8 ) to rotate the second movable separating plates  33   a ,  33   b  downward. At that time, the first movable separating plates  33   a ,  33   b  can downwardly rotate with the second movable separating plates  32   a ,  32   b  via the engaging protrusions  44  even though the leading edges of the sheets P does not abut against the upper surfaces of the first movable separating plates  33   a ,  33   b  (the inclined separating surface  8 ). Accordingly, the interference of the leading edges of the sheets P and the separating plates  32   a ,  32   b ,  33   a ,  33   b  during the sheet feeding operation can be further minimized. 
     The sheet separating unit  31  having a high coefficient of friction protrudes from the inclined separating surface  8 , in the extension of the line of the sheet feeding action Q of the sheet feed roller  21 , in order to abut against the leading edges of the middle areas of the sheets P to separate the sheets P one by one from the stack. In addition, because the inclined separating surface  8  has a curved surface such that the portion near the sheet separating unit  31  is uplifted and the surface is gradually lowered near the ends, in the sheet width direction, the leading edges of the middle areas of the sheets P can be sufficiently subjected to the sheet separating action by contacting the sheet separating unit  31  while the leading edges of the side areas of the sheets P do not interfere with the inclined separating surface  8 . Accordingly, the sheet feeding problem, such as the multi-feed problem, can be prevented. 
       FIGS. 7A-7C  show a structure of an inclined separating surface  8  according to a second embodiment of the invention. In the second embodiment, an urging member  142 , such as a coil spring, is provided at the underside of each of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  to urge the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  upward. Thus, the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  can move up and down while their upper surfaces are maintained parallel to themselves. Other structures of a sheet feeder  10  of the second embodiment are similar to the structure of the sheet feeder  10  of the first embodiment. In the second embodiment, also, movable separating members  32   a ,  32   b ,  33   a ,  33   b  are pressed by the leading edges of the sheets P stacked on the sheet holder  12 , and thus, descend to form an upwardly convex surface appropriate to the sheet feeding, in accordance with the rigidity of the sheets P stacked on the sheet holder  12 . Accordingly, with this structure, the second embodiment can also provide the same effects as those obtained by the first embodiment. 
     As described above, in the sheet feeder  10  shown in each of the above-described embodiments, the inclined separating surface  8  is formed by the fixed separating plate  15  and the movable separating plates  32   a ,  32   b ,  33   a ,  33   b , are disposed on both sides of the fixed separating plate  15  and can descend when contacting the sheets P. Therefore, when the sheets P stacked on the sheet holder  12  are weak or soft, the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  can surely receive the leading edges of side areas of the sheets P. On the other hand, when the sheets P are strong or rigid, the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  descend against the urging forces from the urging members, such as the torsion springs  42 , when the pressing force from the leading edges of the sheets P increase. Thus, the inclined separating surface  8 , are moved in a direction to get away from the leading edges of the sheets P to make a clearance between the inclined separating surface  8  and the leading edges of the sheets P. Therefore, the leading edges of the sheets P can be prevented from being interfered with the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  when the sheets P are fed. Accordingly, the separating action by the separating unit  31  can be surely applied to the leading edges of the middle areas of the sheets P. Further, the separating action can be further improved and a paper jam due to, for example, the multi-feed problem, can be prevented. 
     The first and second movable separating plates  32   a ,  33   a  are disposed on the left of the fixed separating plate  15  and the first and second movable separating plates  32   b ,  33   b  are disposed on the right of the fixed separating plate  15 . Each of the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  can descend individually. Therefore, an appropriate resistance can be applied to the sheets P via the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  in accordance with the width (size) of the sheets P. The first movable separating plates  32   a ,  32   b , which are disposed adjacent to the fixed separating plate  15 , can descend before the second movable separating plates  33   a ,  33   b  descend. Therefore, an appropriate separating action can be applied to the sheets P when the width (size) of the sheets P is small. 
     In the above-described sheet feeders  10 , the first and second movable separating plates  32   a ,  33   a  are disposed on the left of the fixed separating plate  15  and the first and second movable separating plates  32   b ,  33   b  are disposed on the right of the fixed separating plate  15 . The engaging projection  44  is provided to the first movable separating members  32   a ,  32   b  so that the first movable separating members  32   a ,  32   b  can descend together with the second movable separating plates  33   a ,  33   b  when the second movable separating plates  33   a ,  33   b  descend. Accordingly, when the width (size) of the sheets P is large, the interference between the leading edges of the side areas of the sheets P and the inclined separating surface  8  can be minimized. Thus, an appropriate separating action can be applied to the leading edges of the sheets P. 
     The inclined separating surface  8  includes an upwardly curved surface such that a portion near the separating unit  31  is uplifted and the surface is gradually lowered near the ends, in the sheet width direction, when viewed from a direction perpendicular to the sheet feed direction (the force acting direction of the sheet feed roller  21 ). 
     Accordingly, the inclined separating surface  8  extends substantially along the curved line formed by the leading edges of the sheets P, so that the leading edges of the middle areas of the sheets P can be sufficiently applied with the separating action by contacting the separating unit  31  while the leading edges of the side areas of the sheets P are free from the interference with the inclined separating surface  8 . As a result, the sheet feeding problem, such as the multi-feed problem, can be further effectively prevented. 
     While the invention has been described in detail with reference to the specific embodiments thereof, it would be apparent to those skilled in the art that various changes, arrangements and modifications may be applied therein without departing from the spirit and scope of the invention. For example, the sheet separating unit  31  (the separating strip  34 ) is not limited to that shown in the above-described embodiments. A sheet separating unit with a different structure can be used as long as the sheet separating unit causes friction. 
     Further, in the above-described embodiments, the invention has been applied to the sheet feeder  10  that includes the inclined sheet holder  12  with the fixed separating plate  15 , the first movable separating plates  32   a ,  32   b , and the second movable separating plates  33   a ,  33   b  inclinatorily provided at the lower portion of the sheet holder  12 . However, the invention can be applied to a sheet feeder wherein the sheet holder  12  is provided substantially in the horizontal position and the fixed separating plate  15  and the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  are disposed such that the sheet advance direction of the sheet P stacked on the sheet holder  12  extends toward the upwardly slanting direction. 
     Further, in each embodiment, the inclined separating surface  8  forms a surface that is gently upwardly curved in the initial state. However, the inclined separating surface  8  may be a flat surface in the initial state. Further, the inclined separating surface  8  is not limited to the inclined separating surface  8  that includes the upwardly curved surface in a state where the movable separating plates  32   a ,  32   b ,  33   a ,  33   b  descend. Various inclined separating surfaces can be adopted if the inclined separating surfaces form a surface that is gradually lowered near the ends in the sheet width direction, from the separating strip  34  of the fixed separating plate  15  as the top, by the upper surfaces of the first and second separating members  32   a ,  32   b ,  33   a ,  33   b , extending in the right and left direction. 
     In the above-described embodiments, the description has been made by applying the invention to the multifunctional image forming apparatus  1 . However, if an image forming apparatus includes the sheet feeder  10  of either of the above-described embodiments, the invention can be applied thereto. For example, the invention can be applied to a printer that does not have a facsimile function if the printer includes the sheet feeder  10  of either of the embodiments. According to the image forming apparatus including the sheet feeder  10  of either of the first or second embodiment, the multi-feed problem can be surely prevented. Therefore, predetermined images can be surely formed on sheets P, which are supplied one by one, by the image forming unit. 
     In the above-described embodiments, the sheets P are guided by the pair of the guide members  13   a ,  13   b  to contact the middle points of the leading edges of the sheets P with the sheet separating unit  31  (the separating strip  34 ), regardless of the size (width) of the sheets P. However, it is unnecessary to place the sheets P at the middle of the sheet holder  12 . If the leading edges of the sheets P near the middle points of the sheets P contact the sheet separating unit  31  (the separating strip  34 ) (the sheets P are deviated to some extent in the right and left direction), substantially the same effects are promised by the invention. 
     There is no problem even when the sheet separating unit  31  (the separating strip  34 ) is shifted to some extent from the extension of the line of sheet feeding action Q as long as the sheet separating unit  31  (the separating strip  34 ) is provided near the extension.