Abstract:
A sheet feeder device includes: a sheet feed tray configured to store a sheet stack formed with a plurality of paper sheets; an elastic member capable of expanding and contracting; a contact mechanism configured to bring the elastic member into contact with an edge of the sheet stack; an expansion mechanism configured to expand the elastic member in a direction of stacking of the paper sheets; and a control unit configured to cause the expansion mechanism to expand the elastic member while the elastic member is kept in contact with the edge of the sheet stack by the contact mechanism.

Description:
[0001]    The entire disclosure of Japanese Patent Application No. 2015-220830 filed on Nov. 11, 2015 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The present invention relates to a sheet feeder device and an image forming apparatus. 
         [0004]    Description of the Related Art 
         [0005]    A commercial image forming apparatus such as a production printer is normally expected to cope with various kinds of paper sheets, and there is a demand for image forming apparatuses that are not easily affected by environmental disturbances such as humidity, regardless of thicknesses and types of paper sheets. Particularly, coated paper has a low air permeability and a high moisture absorption rate. Therefore, the sticking force between paper sheets is strong in a high-humidity environment, and multiple feeding or non-feeding often occurs in a sheet feed process. 
         [0006]    In a conventional sheet feeder device, to quickly dehumidify the inside of the sheet feed tray, dehumidified/dried air is stored in a storage tank in advance. Immediately after the sheet feed tray is set in the housing, or when a command signal for a start of a sheet feeding operation is received from a control unit, an air blow mechanism blows the dehumidified/dried air from the storage tank into the housing (see JP 2012-150217 A). 
         [0007]    However, in the sheet feed tray having paper sheets stacked therein, the sticking force between paper sheets is stronger at lower portions of the sheet stack due to the weight of the paper sheets, and air does not easily enter the sheet stack. As a result, the dryness factor of paper sheets differs between an upper portion and a lower portion of the sheet stack in the sheet feed tray. 
         [0008]      FIG. 13  shows changes in weight observed in a case where the temperature and humidity environments for coated paper sheets were changed. The first sheet and the third sheet from the top of a sheet stack, and the sheet in the middle position of the sheet stack (approximately the 250th sheet) were subjected to measurement. First, in the “initial stage”, the weights of the paper sheets were measured in a normal environment. The weights of the paper sheets measured after “moisture absorption” are the results of measurement carried out after the paper sheets were left in an 80% RH environment at 30 degrees centigrade for about 20 hours. The weights of the paper sheets measured “after about 5 minutes” are the results of measurement carried out after the paper sheets having absorbed moisture were moved to a 50% RH environment at 20 degrees centigrade, and were left there for about five minutes. 
         [0009]      FIG. 14  shows the rates of change in the weights of the “1st sheet”, the “3rd sheet”, and the “middle sheet” of  FIG. 13  between “after moisture absorption” and “after about 5 minutes”. The “middle sheet” in the middle position of the sheet stack has a lower rate of change in the weight of paper and a lower dehumidification rate (dryness factor) than the “1st sheet” and the “3rd sheet”, which are closer to the upper surface of the sheet stack. In this manner, it was confirmed that there was a difference in dehumidification performance between an upper portion and the middle position of the sheet stack. Therefore, when a paper sheet in the middle position is separated from the other paper sheets while the sticking force between the paper sheets having absorbed moisture is still strong, multiple feeding or non-feeding easily occurs. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention has been made in view of the above described problems in conventional technologies, and an object thereof is to reduce the influence of environmental disturbances, and prevent multiple feeding and non-feeding. 
         [0011]    To achieve the abovementioned object, according to an aspect, a sheet feeder device reflecting one aspect of the present invention comprises: a sheet feed tray configured to store a sheet stack formed with a plurality of paper sheets; an elastic member capable of expanding and contracting; a contact mechanism configured to bring the elastic member into contact with an edge of the sheet stack; an expansion mechanism configured to expand the elastic member in a direction of stacking of the paper sheets; and a control unit configured to cause the expansion mechanism to expand the elastic member while the elastic member is kept in contact with the edge of the sheet stack by the contact mechanism. 
         [0012]    According to an invention of Item. 2, in the sheet feeder device of Item. 1, the sheet feeder device preferably further comprises a suction conveyance unit configured to suck and convey the uppermost paper sheet of the sheet stack. 
         [0013]    According to an invention of Item. 3, in the sheet feeder device of Item. 1 or 2, the sheet feeder device preferably further comprises an air blow mechanism configured to blow air against the sheet stack, wherein the control unit the sheet feeder device causes the air blow mechanism to blow air into a space at the edge of the sheet stack, while the elastic member is kept in contact with the edge of the sheet stack by the contact mechanism, and the elastic member remains expanded by the expansion mechanism. 
         [0014]    According to an invention of Item. 4, in the sheet feeder device of Item. 2, the sheet feeder device preferably further comprises an air blow mechanism configured to blow air against the sheet stack, wherein before causing the suction conveyance unit to suck the uppermost paper sheet, the control unit preferably causes the contact mechanism to bring the elastic member into contact with the edge of the sheet stack, causes the expansion mechanism to expand the elastic member, and causes the air blow mechanism to blow air into a space at the edge of the sheet stack. 
         [0015]    According to an invention of Item. 5, in the sheet feeder device of any one of Items. 1 to 4, the sheet feeder device preferably further comprises a curvature adding unit configured to bend the sheet stack and cause the sheet stack to protrude on the side of the uppermost paper sheet of the sheet stack while the elastic member is kept in contact with the edge of the sheet stack by the contact mechanism. 
         [0016]    According to an invention of Item. 6, in the sheet feeder device of any one of Items. 1 to 5, the elastic member is preferably disposed in one of a position facing the top edge of the paper sheet in a direction of sheet conveyance in the sheet feed tray, and a corner position of the paper sheet. 
         [0017]    According to an invention of Item. 7, in the sheet feeder device of any one of Items. 1 to 5, the elastic member is preferably one of a plurality of elastic members capable of expanding and contracting, and one of the elastic members is preferably disposed in a position on the opposite side of the sheet feed tray from another one of the elastic members. 
         [0018]    According to an invention of Item. 8, in the sheet feeder device of any one of Items. 1 to 7, the elastic member is preferably formed with a plurality of materials having different degrees of expansion and contraction from one another. 
         [0019]    According to an invention of Item. 9, in the sheet feeder device of any one of Items. 1 to 7, the surface of the elastic member to be brought into the edge of the sheet stack preferably has one of a corrugated form and a viscous element attached thereto. 
         [0020]    According to an invention of Item. 10, in the sheet feeder device of any one of Items. 1 to 9, the elastic member is preferably formed with a rotatable endless belt. 
         [0021]    According to an invention of Item. 11, in the sheet feeder device of any one of Items. 1 to 10, during a standby period before sheet feeding, the control unit preferably repeats a contact operation for the elastic member with the contact mechanism, and an expanding operation for the elastic member with the expansion mechanism. 
         [0022]    To achieve the abovementioned object, according to an aspect, an image forming apparatus reflecting one aspect of the present invention comprises the sheet feeder device of any one of Items. 1 to 11. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
           [0024]      FIG. 1  is a schematic cross-sectional view of an image forming apparatus according to a first embodiment of the present invention; 
           [0025]      FIG. 2A  is a schematic cross-sectional view of a sheet feeding unit; 
           [0026]      FIG. 2B  is a diagram showing a situation where elastic members are expanded; 
           [0027]      FIG. 3  is a perspective diagram showing the structure of an elastic member; 
           [0028]      FIG. 4  is a plan view of the sheet feeding unit; 
           [0029]      FIG. 5  is a block diagram showing the functional configuration of the image forming apparatus; 
           [0030]      FIG. 6A  is a diagram showing the arrangement of elastic members in an image forming apparatus according to a first modification of the first embodiment; 
           [0031]      FIG. 6B  is a cross-sectional view taken along the line A-A of  FIG. 6A ; 
           [0032]      FIG. 7  is a diagram showing the arrangement of elastic members in an image forming apparatus according to a second modification of the first embodiment; 
           [0033]      FIG. 8A  is a diagram for explaining a curvature adding member of an image forming apparatus according to a second embodiment of the present invention; 
           [0034]      FIG. 8B  is a diagram showing a situation where paper sheets are curved; 
           [0035]      FIG. 8C  is a diagram showing a situation where elastic members are expanded; 
           [0036]      FIG. 9  is a perspective diagram showing the structure of an elastic member of an image forming apparatus according to a third embodiment of the present invention; 
           [0037]      FIG. 10A  is a cross-sectional view of an elastic member of an image forming apparatus according to a fourth embodiment of the present invention; 
           [0038]      FIG. 10B  is a diagram showing a situation where the elastic members are in contact with the top edge of a sheet stack; 
           [0039]      FIG. 11  is a diagram showing the structures of an elastic member, an upper roller, a lower roller, and a cam member of an image forming apparatus according to a fifth embodiment of the present invention; 
           [0040]      FIG. 12  is a diagram showing the arrangement of suction pads of an image forming apparatus according to a sixth embodiment of the present invention; 
           [0041]      FIG. 13  is a graph showing changes in weight of paper in various environments; and 
           [0042]      FIG. 14  is a graph showing rates of change in weight of paper in various positions in a sheet stack. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples. 
       First Embodiment 
       [0044]    First, a first embodiment of an image forming apparatus according to the present invention will be described. However, the present invention is not limited to the illustrated examples. 
         [0045]      FIG. 1  is a schematic cross-sectional view of an image forming apparatus  100  according to a first embodiment. The image forming apparatus  100  includes an image forming unit  10  and a sheet feeding unit  20 . The image forming apparatus  100  forms an image on a paper sheet P in accordance with image data transmitted from an external device such as a computer connected to the image forming apparatus  100  directly or via a network. 
         [0046]    The image forming unit  10  forms an image on a paper sheet P supplied from the sheet feeding unit  20 . The image forming unit  10  includes photosensitive members  11 Y,  11 M,  11 C, and  11 K for the respective colors of yellow (Y), magenta (M), cyan (C), and black (K), an intermediate transfer belt  12 , a secondary transfer roller  13 , a fixing unit  14 , and resist rollers  15 . 
         [0047]    After uniformly charged, the photosensitive members  11 Y,  11 M,  11 C, and  11 K are scanned and exposed by laser beams, and electrostatic latent images are formed in accordance with image data of the respective colors. Toners of the respective colors are then attached to the electrostatic latent images on the photosensitive members  11 Y,  11 M,  11 C, and  11 K, and thus, development is carried out. 
         [0048]    The toner images formed in the respective colors on the photosensitive members  11 Y,  11 M,  11 C, and  11 K are sequentially transferred onto the intermediate transfer belt  12 . 
         [0049]    With the secondary transfer roller  13 , the color toner image on the intermediate transfer belt  12  is transferred onto one surface of a paper sheet P supplied from the sheet feeding unit  20 . 
         [0050]    The fixing unit  14  heats and presses the toner image transferred onto the paper sheet P, to fix the toner image to the paper sheet P. 
         [0051]    The resist rollers  15  adjust the timing to send the paper sheet P to the secondary transfer roller  13  by temporarily stopping the conveyance of the paper sheet P supplied from the sheet feeding unit  20 , so that the image is appropriately positioned relative to the paper sheet P. 
         [0052]      FIG. 2A  is a schematic cross-sectional view of the sheet feeding unit  20 . The sheet feeding unit  20  includes a sheet feed tray  21 , elastic members  22 , a contact mechanism  23 , an expansion mechanism  24 , and a conveyance belt  25 . 
         [0053]    A stack of paper sheets P on which images are to be formed are stored in the sheet feed tray  21 . 
         [0054]    Each elastic member  22  is formed with an elastic material, such as sponge or rubber. Each elastic member  22  is disposed in a position in contact with the top edges of the paper sheets P in the direction of sheet conveyance in the sheet feed tray  21 . That is, the top edges of the paper sheets P to be conveyed are located on the side of the elastic members  22 . 
         [0055]      FIG. 3  shows the structure of each elastic member  22 . Each elastic member  22  is formed with a rubber member  221  and rubber members  222 . The rubber member  221  and the rubber members  222  are materials with different degrees of expansion and contraction. 
         [0056]    The elastic members  22  preferably have anisotropic characteristics, such as being weak in the direction of contact with the paper sheets P (the X-direction shown in  FIG. 3  and  FIG. 2A ) while being strong in the direction of expansion/contraction of the elastic members  22  (the Z-direction shown in  FIG. 3  and  FIG. 2A ). 
         [0057]    The rubber member  221  is formed with a stack of sheet-like soft rubber layers. The rubber members  222  are formed with relatively hard rubber members. In the sheet width direction (the Y-direction shown in  FIG. 3  and  FIG. 2A ) perpendicular to the sheet conveyance direction, the rubber member  221  is interposed between the rubber members  222 , and the edges of the respective layers in the rubber member  221  in the Y-direction are bonded to the rubber members  222 . The respective layers in the rubber member  221  are not bonded to one another. When the elastic members  22  are brought into contact with the paper sheets P, the respective layers in the rubber member  221  catch the paper sheets P, so that spaces are easily formed between the paper sheets P. 
         [0058]    The contact mechanism  23  brings the elastic members  22  into contact with the top edge of the sheet stack. The contact mechanism  23  includes a resilient member  231  and a cam member  232 . The resilient member  231  pushes the contact mechanism  23  in the X-direction shown in  FIG. 2A . As the cam member  232  rotates about a shaft, the position of the contact mechanism  23  in the X-direction changes, and the elastic members  22  are brought into contact with the top edge of the sheet stack or are moved away from the top edge.  FIG. 2A  shows a situation where the elastic members  22  are in contact with the top edge of the sheet stack. 
         [0059]    The expansion mechanism  24  expands the elastic members  22  in the stacking direction of the paper sheets P (the Z-direction shown in  FIG. 2A ). The expansion mechanism  24  includes a support point  241 , an arm  242 , and a pull-up unit  243 . The arm  242  can rotate about the support point  241 . The pull-up unit  243  is connected to the end of the arm  242  on the opposite side from the support point  241 . 
         [0060]    The lower surface of each elastic member  22  is bonded to the contact mechanism  23 , and the upper surface of each elastic member  22  is bonded to the lower surface of the pull-up unit  243 . The arm  242  is made to rotate about the support point  241 , to pull up the pull-up unit  243  in the Z-direction. In this manner, the elastic members  22  can be expanded. 
         [0061]    As shown in  FIG. 2B , spaces are formed between the paper sheets P at the top edges, as the elastic members  22  are expanded by the expansion mechanism  24  while the elastic members  22  are kept in contact with the top edge of the sheet stack by the contact mechanism  23 . These spaces are formed, because a force acting in the direction to separate the paper sheets from one another is generated by the friction between the top edges of the paper sheets and the elastic members  22 , and by the expansion of the elastic members  22 . 
         [0062]    The conveyance belt  25  is a suction conveyance means that sucks the uppermost paper sheet P of the sheet stack stored in the sheet feed tray  21 , and then conveys the paper sheet P. The conveyance belt  25  sends the uppermost paper sheet P of the sheet stack to the image forming unit  10 . A large number of through holes with small diameters are formed in the conveyance belt  25 , and a suction means provided on the inner side of the conveyance belt  25  sucks air through the conveyance belt  25 , to attract the paper sheet P. The conveyance belt  25  is stretched around rollers. As the rollers rotate, the conveyance belt  25  rotates. As the conveyance belt  25  rotates, the paper sheet P sticking to the conveyance belt  25  is conveyed. 
         [0063]      FIG. 4  is a plan view of the sheet feeding unit  20 . The sheet feeding unit  20  includes an air blow mechanism  26 . 
         [0064]    The air blow mechanism  26  blows air against the sheet stack. The air blow mechanism  26  is disposed on the sheet top-edge side in the direction of sheet conveyance in the sheet feed tray  21 , and is located between two elastic members  22 . 
         [0065]      FIG. 5  shows the functional configuration of the image forming apparatus  100 . The image forming apparatus  100  includes the image forming unit  10 , the sheet feeding unit  20 , a control unit  31 , an operating unit  32 , a display unit  33 , a storage unit  34 , and a communication unit  35 . These components are connected to one another by a bus. Of these components, the functional units already explained above will not be described below. 
         [0066]    The control unit  31  includes a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM), and collectively controls processing operations of the respective components of the image forming apparatus  100 . The CPU reads various processing programs stored in the ROM, and loads the programs into the RAM. In accordance with the loaded programs, the CPU performs various kinds of processing. 
         [0067]    The operating unit  32  includes a touch panel designed to cover the display screen of the display unit  33 , and various operation keys such as numeric keys and a start key. The operating unit  32  outputs operation signals based on user operations to the control unit  31 . 
         [0068]    The display unit  33  is formed with a liquid crystal display (LCD), and displays various screens in accordance with instructions indicated by display signals input from the control unit  31 . 
         [0069]    The storage unit  34  is formed with a storage device, such as a nonvolatile semiconductor memory or a hard disk, and stores data related to various kinds of processing. 
         [0070]    The communication unit  35  transfers data to and from an external device connected to a network, such as a local area network (LAN). 
         [0071]    The control to be performed in a sheet feeding operation is now described. 
         [0072]    First, the control unit  31  controls the contact mechanism  23  to bring the elastic members  22  into contact with the top edge of the sheet stack. During this process, the elastic members  22  remain compressed by the expansion mechanism  24  or are maintained at the natural length. 
         [0073]    After that, the control unit  31  controls the expansion mechanism  24  to expand the elastic members  22  while the elastic members  22  are kept in contact with the top edge of the sheet stack by the contact mechanism  23 . As a result, spaces are formed between the paper sheets P at the top edges. The control unit  31  and the sheet feeding unit  20  forma sheet feeder device according to an embodiment of the present invention. 
         [0074]    While the elastic members  22  are kept in contact with the top edge of the sheet stack by the contact mechanism  23 , and the elastic members  22  remain expanded by the expansion mechanism  24 , the control unit  31  controls the air blow mechanism  26  to blow air into the spaces at the top edge of the sheet stack. 
         [0075]    As the top edges of the paper sheets are separated from one another by the expansion mechanism  24 , the air blow mechanism  26  can easily blow air into the spaces between the paper sheets. Thus, the effect to dehumidify the paper sheets P becomes greater, and separation performance in the sheet separation process is improved. 
         [0076]    The control unit  31  then controls the conveyance belt  25  to suck the uppermost paper sheet P. Specifically, before controlling the conveyance belt  25  to suck the uppermost paper sheet P, the control unit  31  controls the contact mechanism  23  to bring the elastic members  22  into contact with the top edge of the sheet stack, controls the expansion mechanism  24  to expand the elastic members  22 , and controls the air blow mechanism  26  to blow air into the spaces at the top edge of the sheet stack. 
         [0077]    During a standby period before sheet feeding, the control unit  31  sequentially repeats the contact operation for the elastic members  22  with the contact mechanism  23 , the expanding operation for the elastic members  22  with the expansion mechanism  24 , and the air blowing operation with the air blow mechanism  26 . 
         [0078]    As described above, according to the first embodiment, spaces are formed between paper sheets as the elastic members  22  are expanded. As a result, the sheet drying efficiency (dehumidification performance) is increased, and the sticking force between the paper sheets is weakened. Thus, the influence of environmental disturbances can be reduced, and multiple feeding and non-feeding can be prevented. 
         [0079]    Furthermore, as air is blown into the spaces at the top edge of the sheet stack, the sheet drying efficiency can be further increased, and the sticking force between the paper sheets can be further weakened. 
         [0080]    After spaces are formed between the paper sheets, the uppermost paper sheet P of the sheet stack is sucked and conveyed. Thus, sheet feeding can be carried out, without multiple feeding and non-feeding. 
         [0081]    Furthermore, as the elastic members  22  are disposed in positions facing the top edges of the paper sheets P in the direction of sheet conveyance in the sheet feed tray  21 , the paper sheets P can be separated and then conveyed with high efficiency. 
         [0082]    Furthermore, the control unit  31  sequentially repeats the contact operation for the elastic members  22  with the contact mechanism  23 , the expanding operation for the elastic members  22  with the expansion mechanism  24 , and the air blowing operation with the air blow mechanism  26 . Thus, the paper sheets P can be more easily separated from one another. 
         [0083]    Furthermore, as the rubber member  221  and the rubber members  222  of each elastic member  22  have different degrees of expansion and contraction, a difference can be generated between the force acting in the direction of contact with the paper sheets P and the force acting in the direction of expansion of the elastic members  22 . 
         [0084]    [First Modification] 
         [0085]    Next, a first modification of the first embodiment will be described. 
         [0086]      FIG. 6A  shows the positions of elastic members  22 A and  22 B in an image forming apparatus of the first modification. 
         [0087]    In the above described case of the first embodiment, the elastic members  22  are disposed in positions facing the top edges of the paper sheets in the direction of sheet conveyance in the sheet feed tray  21 . In the first modification, on the other hand, the elastic member  22 A is disposed in a position on the opposite side of the sheet feed tray  21  from the elastic member  22 B. That is, paper sheets P are interposed between the elastic members  22 A and  22 B. 
         [0088]      FIG. 6B  is a cross-sectional view taken along the line A-A of  FIG. 6A . Where the elastic members  22 A and  22 B are disposed in positions on the opposite sides from each other, if a paper sheet P is pulled toward one side (the side of the elastic member  22 A, for example) of the sheet stack, the paper sheet P protrudes on the opposite side (the side of the elastic member  22 B, for example). Therefore, in the first modification, the paper sheets P are firmly supported by the elastic members  22 A and  22 B located on the opposite sides from each other, and spaces can be formed between the paper sheets P. Thus, higher sheet separation performance can be achieved, compared with the sheet separation performance in a case where the surface with which one of the edges of the sheet stack comes into contact is a rigid surface. 
         [0089]    [Second Modification] 
         [0090]    Next, a second modification of the first embodiment will be described. 
         [0091]      FIG. 7  shows the positions of elastic members in an image forming apparatus of the second modification. 
         [0092]    In the second modification, elastic members  22 C through  22 F are arranged in the corner positions (at the four corners) of the paper sheets P stored in the sheet feed tray  21 . Furthermore, the elastic member  22 C and the elastic member  22 E are located on the opposite sides from each other, and the elastic member  22 D and the elastic member  22 F are located on the opposite sides from each other. 
         [0093]    According to the second modification, the elastic members  22 C through  22 F are disposed in the corner positions of the paper sheets P, to facilitate separation of the paper sheets P from one another. 
         [0094]    In the example case of the second modification, the elastic members  22 C through  22 F are disposed in all the corner positions of the paper sheets P. However, elastic members may be provided in one or more of the corner positions of the paper sheets P. 
       Second Embodiment 
       [0095]    Next, a second embodiment of the present invention will be described. 
         [0096]    An image forming apparatus according to the second embodiment has substantially the same structure as that of the image forming apparatus  100  of the first embodiment. Therefore, like components are denoted by like reference numerals, and explanation of such components will not be made below. Instead, only the structures and processes characteristic of the second embodiment will be described below. 
         [0097]    In the image forming apparatus of the second embodiment, the sheet feeding unit  20  includes a curvature adding member  27 . 
         [0098]    Referring now to  FIGS. 8A through 8C , the curvature adding member  27  will be described. 
         [0099]    The curvature adding member  27  bends the sheet stack so that the sheet stack protrudes on the side of the uppermost paper sheet of the sheet stack while the elastic members  22  are kept in contact with the top edge of the sheet stack by the contact mechanism  23 . Specifically, the curvature adding member  27  includes a support point  271 , an arm  272 , and a support point  273 . The curvature adding member  27  rotates the elastic members  22 , the contact mechanism  23 , and the expansion mechanism  24  integrally about the support point  271 . As for the expansion mechanism  24 , only the pull-up unit  243  is shown in  FIGS. 8A  through  8 C. 
         [0100]      FIG. 8A  shows a situation where the elastic members  22  are brought into contact with the top edge of the sheet stack by the contact mechanism  23 . In this situation, the control unit  31  controls the curvature adding member  27  to rotate the elastic members  22  about the support point  271 . As a result, the paper sheets P protrude on the side of the uppermost paper sheet of the sheet stack, as shown in  FIG. 8B . That is, the paper sheets P are provided with a curvature. Since the elastic members  22  are still kept in contact with the paper sheets P by the contact mechanism  23  at this point, the elastic members  22  do not move away from the top edges of the paper sheets P. The support point  273  is rotatably connected to the contact mechanism  23 . 
         [0101]    In this situation, the control unit  31  controls the expansion mechanism  24  to expand the elastic members  22 . As a result, the paper sheets P separate from one another, and spaces are formed, as shown in  FIG. 8C . 
         [0102]    According to the second embodiment, spaces are easily formed between the paper sheets not only by the expansion of the elastic members  22  but also by the force (restoring force) of the paper sheets P recovering from the curved state to the original state. As a result, sheet separation performance is further improved. Thus, the influence of environmental disturbances can be reduced, and multiple feeding and non-feeding can be prevented. 
       Third Embodiment 
       [0103]    Next, a third embodiment of the present invention will be described. 
         [0104]    An image forming apparatus according to the third embodiment has substantially the same structure as that of the image forming apparatus  100  of the first embodiment. Therefore, like components are denoted by like reference numerals, and explanation of such components will not be made below. Instead, only the structures and processes characteristic of the third embodiment will be described below. 
         [0105]    In the image forming apparatus of the third embodiment, the sheet feeding unit  20  includes elastic members  41 , instead of the elastic members  22 . 
         [0106]      FIG. 9  shows the structure of each elastic member  41 . The X-direction, the Y-direction, and the Z-direction shown in  FIG. 9  correspond to those shown in  FIG. 2A . 
         [0107]    Each elastic member  41  is designed so that the surface to be brought into contact with the top edge of the sheet stack (the surface parallel to the Y-direction and the Z-direction) is corrugated in the X-direction. 
         [0108]    According to the third embodiment, the influence of environmental disturbances can be reduced, and multiple feeding and non-feeding can be prevented, as in the first embodiment. Furthermore, as the contact surface of each elastic member  41  is a corrugated surface, the top edges of the paper sheets P can be easily caught by the elastic members  41  brought into contact with the paper sheets P, and thus, spaces can be easily formed between the paper sheets P. 
       Fourth Embodiment 
       [0109]    Next, a fourth embodiment of the present invention will be described. 
         [0110]    An image forming apparatus according to the fourth embodiment has substantially the same structure as that of the image forming apparatus  100  of the first embodiment. Therefore, like components are denoted by like reference numerals, and explanation of such components will not be made below. Instead, only the structures and processes characteristic of the fourth embodiment will be described below. 
         [0111]    In the image forming apparatus of the fourth embodiment, the sheet feeding unit  20  includes elastic members  42 , instead of the elastic members  22 . 
         [0112]      FIG. 10A  shows the structure of each elastic member  42 . The X-direction, the Y-direction, and the Z-direction shown in  FIG. 10A  correspond to those shown in  FIG. 2A . 
         [0113]    Each elastic member  42  includes a main portion  421  and a viscous element  422  attached to the surface of the main portion  421  to be brought into contact with the top edge of the sheet stack (the surface parallel to the Y-direction and the Z-direction). 
         [0114]    As shown in  FIG. 10B , when the elastic members  42  are brought into contact with the top edge of the sheet stack, the viscous element  422  is deformed to form spaces between the paper sheets and enter the spaces. 
         [0115]    According to the fourth embodiment, the influence of environmental disturbances can be reduced, and multiple feeding and non-feeding can be prevented, as in the first embodiment. Furthermore, as each elastic member  42  includes the viscous element  422 , the top edges of the paper sheets P can be easily caught by the elastic members  42  brought into contact with the paper sheets P, and thus, spaces can be easily formed between the paper sheets P. 
       Fifth Embodiment 
       [0116]    Next, a fifth embodiment of the present invention will be described. 
         [0117]    An image forming apparatus according to the fifth embodiment has substantially the same structure as that of the image forming apparatus  100  of the first embodiment. Therefore, like components are denoted by like reference numerals, and explanation of such components will not be made below. Instead, only the structures and processes characteristic of the fifth embodiment will be described below. 
         [0118]    In the image forming apparatus of the fifth embodiment, the sheet feeding unit  20  includes an elastic member  43 , an upper roller  44 , a lower roller  45 , and a cam member  46 , instead of the elastic members  22  and the expansion mechanism  24 . 
         [0119]      FIG. 11  shows the structures of the elastic member  43 , the upper roller  44 , the lower roller  45 , and the cam member  46 . The X-direction, the Y-direction, and the Z-direction shown in  FIG. 11  correspond to those shown in  FIG. 2A . 
         [0120]    The elastic member  43  is formed with an endless belt, and is stretched around the upper roller  44  and the lower roller  45 . As the upper roller  44  and the lower roller  45  rotate, the elastic member  43  rotates. 
         [0121]    Furthermore, as the cam member  46  rotates about a shaft, the shaft of the upper roller  44  moves up and down. In the fifth embodiment, the cam member  46  functions as the expansion mechanism. 
         [0122]    The control unit  31  fixes the lower roller  45 , and controls the cam member  46  to rotate, to move the shaft position of the upper roller  44  further away from the lower roller  45 . In this manner, the elastic member  43  is expanded in the direction of stacking of the paper sheet P (the Z-direction). As a result, spaces are formed between the paper sheets P at the top edges.  FIG. 11  shows a situation where the elastic member  43  is expanded. 
         [0123]    After the top edges of the paper sheets are separated from one another, the control unit  31  controls the upper roller  44  or the lower roller  45  to rotate, so that the paper sheets P can be conveyed toward the conveyance belt  25 . 
         [0124]    According to the fifth embodiment, the influence of environmental disturbances can be reduced, and multiple feeding and non-feeding can be prevented, as in the first embodiment. Furthermore, as the elastic member  43  formed with an endless belt is used, the paper sheets P can be conveyed upward while the paper sheets P are separated. Thus, conveyance performance is improved. 
         [0125]    In the fifth embodiment, the shaft position of the upper roller  44  is moved further away from the lower roller  45  so that the elastic member  43  is expanded. However, the rotation of the lower roller  45  may be stopped, and only the upper roller  44  may be rotated. In this manner, the surface of the elastic member  43  in contact with the top edges of the paper sheets P can be expanded in the direction of stacking of the paper sheets P (Z-direction). 
       Sixth Embodiment 
       [0126]    Next, a sixth embodiment of the present invention will be described. 
         [0127]    An image forming apparatus according to the sixth embodiment has substantially the same structure as that of the image forming apparatus  100  of the first embodiment. Therefore, like components are denoted by like reference numerals, and explanation of such components will not be made below. Instead, only the structures and processes characteristic of the sixth embodiment will be described below. 
         [0128]    In the image forming apparatus of the sixth embodiment, the sheet feeding unit  20  includes suction pads  51 A,  51 B, and  51 C, instead of the conveyance belt  25 . 
         [0129]      FIG. 12  shows an example of arrangement of the suction pads  51 A,  51 B, and  51 C. The suction pads  51 A,  51 B, and  51 C are designed to suck the uppermost paper sheet P, and are arranged in the X-direction in which the elastic members  22  are brought into contact with the paper sheets P. The suction pads  51 A,  51 B, and  51 C can move in the Z-direction perpendicular to the sheet surfaces. 
         [0130]    While the paper sheets P are separated from one another by the expansion of the elastic members  22 , the control unit  31  controls the suction pads  51 A,  51 B, and  51 C to suck the uppermost paper sheet P in the Z-direction, in the order of proximity to the top edge of the paper sheet P. As a result, the uppermost paper sheet P is lifted up, and is then conveyed. 
         [0131]    According to the sixth embodiment, after spaces are formed between the paper sheets P, the suction pads  51 A,  51 B, and  51 C suck the uppermost paper sheet P of the sheet stack, and then convey the uppermost paper sheet P. Thus, the influence of environmental disturbances can be reduced, and multiple feeding and non-feeding can be prevented. 
         [0132]    The above descriptions of the respective embodiments are about examples of sheet feeder devices and image forming apparatuses according to embodiments of the present invention, and do not limit the present invention. 
         [0133]    Modifications may be made to the specific structures and the specific operations of the respective components of each device, without departing from the scope of the invention. 
         [0134]    For example, the characteristic structures and operations of the above described embodiments may be combined in any appropriate manner. 
         [0135]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustrated and example only and is not to be taken byway of limitation, the scope of the present invention being interpreted by terms of the appended claims.