Abstract:
A sheet feeding apparatus includes stacker for stacking a sheet material; a feed roller for separating the sheet material stacked on the stacker; a rotatable separation roller rotatable by being driven by the feed roller; and a torque limiter for controlling rotation of the separation roller; wherein a second or subsequent sheet material staying in a nip between the feed roller and the separation roller is separated from the separation roller by a rotational direction of the feed roller being changed from a forward rotational direction to a backward direction and by a rotational direction of the separation roller being changed to a backward direction by the feed roller during a first sheet material which is in contact to the feed roller being nipped by the nip; and a first sheet material is fed toward downstream of the nip by the rotational direction of the feed roller being changed from the backward direction to the forward direction.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to a sheet feeding apparatus. More specifically, it relates to a sheet conveying apparatus provided with a mechanism for preventing the so-called double-feeding or multi-feeding (which hereafter will be referred to as multi-feeding), that is, the problem that two or more sheets are fed out of a recording sheet holder at the same time by a single sheet feeding action. 
         [0002]    As examples of the mechanism with which a sheet feeding apparatus is provided to prevent the problem that two or more sheets are fed into the main assembly of an apparatus at the same time, there are the mechanisms which employ a retard roller, a simplified version thereof, etc. 
         [0003]    A sheet feeding apparatus employing a retard roller, such as the one disclosed in Japanese Laid-open Patent Application 2000-177874, has a separation roller which is provided with a torque limiter, in order to prevent the so-called multi-feeding. More specifically, the sheet feeding apparatus is provided with a feed roller and a separation roller. The feed roller is rotated in the direction to convey a sheet into the main assembly of an apparatus (rotating feed roller in this direction will be referred to as “forward rotation”). As a sheet (single sheet of medium) is made to enter the area of contact (separation nip) between the feed roller and separation roller, by the feed roller, the separation roller is rotated by the rotation of the feed roller, with the presence of the sheet between the feed roller and separation roller. However, if two or more sheets enter the separation nip, the torque limiter reacts, and affects the rotation of the separation roller. That is, the separation roller is prevented by the torque limiter from being rotated by the forward rotation of the feed roller, and instead, is rotated in the direction opposite to the direction in which it is rotated to convey a sheet downstream, that is, the direction in which a sheet is to be conveyed to be fed into the main assembly of the apparatus (this direction in which separation roller is rotated will be referred to as “backward direction”). The peripheral surface of the separation roller is higher in coefficient of friction than the surface of a sheet. Therefore, the second sheet and sheets thereafter, which are in contact with the separation roller, are conveyed by the backward rotation of the separation roller in the backward direction, that is, the direction opposite from the direction in which the first sheet is being conveyed by the feed roller. Thus, only the sheet which is in contact with the feed roller is conveyed beyond the separation nip. This is how a sheet feeding apparatus employing an ordinary retard roller prevents two or more sheets from being fend and conveyed at the same time beyond the separation nip. 
         [0004]    A sheet feeding apparatus employing a simplified version of retard roller also has a separation roller with a torque limiter to prevent two or more sheets from being fed at the same time, as does a sheet feeding apparatus employing a retard roller. A sheet feeding apparatus employing a simplified version of retard roller is different from a sheet feeding apparatus employing an ordinary version of retard roller in that it is not provided with a power source dedicated to the backward rotation of the separation roller. More specifically, as the torque limiter is activated by the entrance of two or more sheets into the separation nip, the separation roller is made to simply stop, instead of being rotated backward. As the separation roller is stopped, the second sheet and the sheets thereafter, which are in contact with the separation roller, are not allowed to move downstream in terms of the sheet conveyance direction, beyond the separation nip. Thus, only the first sheet, that is, the sheet which is in contact with the feed roller is conveyed beyond the separation sheet by the feed roller. This is how the sheet feeding apparatus employing a simplified version of retard roller prevents two or more sheets from being fed and conveyed at the same time beyond the separation nip. 
         [0005]    However, the above-described conventional technologies suffer from various problems which are related to the working of the mechanism for preventing the multi-feeding. 
         [0006]    First, in the case of a sheet feeding apparatus employing an ordinary retard roller, the driving force for backwardly rotating the separation roller has to be transmitted to the separation roller during a sheet feeding operation. Thus, the sheet feeding apparatus of this type is mechanically more complication, and also, is lager in size, being therefore higher in manufacture cost. Further, the second sheet and the sheet thereafter are forcefully conveyed in the opposite direction from the direction in which they are to be conveyed if they were entered one by one into the separation nip. In other words, the second sheet and the rest in the separation nip are made to rub against the top sheet, that is, the first sheet. Therefore, the sheets sometimes sustain scratch marks or the like. 
         [0007]    In the case of a sheet feeding apparatus employing a simplified version of the retard roller, it has to be provided with a mechanism for removing the second sheet and the rest, the leading edges of which are in the separation nip or its immediate adjacencies, away from the separation nip and its immediate adjacencies. One of such mechanisms is provided with a return lever, which is to be operated to remove the sheets, the leading edge portion of which is in the separation nip or immediate adjacencies, from the separation nip or its immediate adjacencies. 
         [0008]    In the case of a conventional sheet feeding apparatus employing a simplified version of retard roller and a return lever, the return lever had to be operated, while keeping the feed roller and separation roller separated from each other, in order to prevent the sheets in the separation nip, from being damaged. Thus, the operation of the return lever has to wait until the sheet, which is being conveyed by the feed roller, goes through the separation nip, and reaches the point, beyond which it can be conveyed by a means other than the feed roller. 
         [0009]    Thus, while the first sheet is conveyed through the separation nip by the forward rotation of the feed roller, the second sheet and the rest in the separation nip remain therein, and therefore, the back surface of the first sheet, that is, the sheet being conveyed, rubs against the recording surface of each of the sheets remaining in the separation nip, leaving sometimes scratch marks on the recording surfaces of the second sheet and the rest in the separation nip. These scratch marks on the recording surface are rather conspicuous, in particular, when the glossy sheets, such as sheets of photographic paper, are used. 
       SUMMARY OF THE INVENTION 
       [0010]    The primary object of the present invention is to provide a sheet feeding apparatus which has a multi-feeding preventing mechanism, and is capable of preventing sheets from rubbing each other while the sheets are separated and/or conveyed, without having a mechanism dedicated to the prevention of the rubbing. 
         [0011]    According to an aspect of the present invention, there is provided a sheet feeding apparatus comprising stacker for stacking a sheet material; a feed roller for separating the sheet material stacked on said stacker; a rotatable separation roller rotatable by being driven by said feed roller; and a torque limiter for controlling rotation of said separation roller; wherein a second or subsequent sheet material staying in a nip between said feed roller and said separation roller is separated from said separation roller by a rotational direction of said feed roller being changed from a forward rotational direction to a backward direction and by a rotational direction of said separation roller being changed to a backward direction by said feed roller during a first sheet material which is in contact to said feed roller being nipped by said nip; and a first sheet material is fed toward downstream of said nip by the rotational direction of said feed roller being changed from the backward direction to the forward direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of a sheet feeding apparatus in one of the preferred embodiments of the present invention. 
           [0013]      FIG. 2  is a front view of the sheet feeding apparatus shown in  FIG. 1 . 
           [0014]      FIG. 3  is a sectional view of the feeding-and-separating portion of the sheet feeding apparatus shown in  FIG. 1 . 
           [0015]      FIG. 4  is a front view of the feeding-and-separating portion of the sheet feeding apparatus shown in  FIG. 1 . 
           [0016]      FIGS. 5(   a ),  5 ( b ), and  5 ( c ) are sectional views of the separation nip portion of the sheet feeding apparatus shown in  FIG. 1 . 
           [0017]      FIG. 6  is a flowchart of the sheet feeding operation of the sheet feeding apparatus shown in  FIG. 1 . 
           [0018]      FIGS. 7(   a ) and  7 ( b ) are sectional views of the separation roller of the sheet feeding apparatus in the first preferred embodiment of the present invention. 
           [0019]      FIGS. 8(   a ) and  8 ( b ) are sectional views of the separation roller of the sheet feeding apparatus in the second preferred embodiment of the present invention. 
           [0020]      FIGS. 9(   a ) and  9 ( b ) are sectional views of the separation roller of the sheet feeding apparatus shown in  FIG. 8 . 
           [0021]      FIGS. 10(   a ),  10 ( b ), and  10 ( c ) are perspective and sectional views, respectively, of the separation roller of the sheet feeding apparatus in the third preferred embodiment of the present invention. 
           [0022]      FIG. 11  is a sectional view of the sheet feeding apparatus in the fourth preferred embodiment of the present invention. 
           [0023]      FIG. 12  is also a sectional view of the sheet feeding apparatus in the fourth preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Next, some of the preferred embodiments of the present invention will be described with reference to the appended drawings. 
       Embodiment 1  
       [0025]      FIG. 1  is a perspective view of the sheet feeding apparatus in the first preferred embodiment of the present invention.  FIG. 2  is a front view of the sheet feeding apparatus shown in  FIG. 1 , as seen in the direction indicated by an arrow mark A in  FIG. 1 . 
         [0026]    The sheet conveying apparatus in this embodiment has a sheet storage portion, a sheet feeding-and-separating portion, and a portion for preventing two or more sheet from being fed at the same time. Thus, each of these portions will be roughly described in the listed order, with reference to  FIGS. 1 and 2 , etc. 
       (A) Sheet Storage Portion 
       [0027]    The sheet storage portion has a base  15  and a pressure plate  16 . The base  15  is a part of the frame of the sheet feeding apparatus (ASF (automatic sheet feeder)). The pressure plate  16  is for pressing the sheets layered on the base  15 , upon a feed roller  11 , which will be described later. The sheet storage portion has also a side guide  18  for precisely positioning the sheets in the sheet storage portion, in terms of the direction parallel to the width direction of the sheets. The force (pressure) with which the pressure plate  16  presses the sheets upon the feed roller  11  is provided by a leaf (plate) spring (unshown). Further, the sheet storage portion has a pair of sheet conveying wheels  19  for preventing the fed sheet from coming into contact with the feed roller  11 . 
         [0028]    The base  15  is provided with positional reference portions  15   a  and  15   b.  The positional reference portion  15   a  is for precisely positioning the sheets in the sheet storage portion, in terms of the direction parallel to the widthwise direction of the sheets. The positional reference portion  15   b  is for precisely positioning the sheets in the sheet storage portion, in terms of the direction in which the sheets are to be conveyed. Thus, the sheet storage portion can smoothly feed the sheets layered on the base  15 , into the feeding-and-separating portion. 
         [0029]    The pressure plate  16  is rotatably supported by the base  15 ; the rotational axle of the pressure plate  16 , which is attached to the top edge portion of the pressure plate  16 , is supported by the base  15 . The rotational movement of the pressure plate  16  is regulated by a combination of springs (unshown) and cams (unshown), etc. The pressure plate  16  is kept pressured toward the feed roller  11 , by the resiliency of the plate springs (leaf springs). The sheet storage portion is structured so that the pressure plate  16  can be pushed back by the cams with which the feeder shaft gear  23  (which will be described later) is provided, in the direction to move the pressure plate  16  away from the feed roller  11 . The timing, with which the sheets on the base  15  are fed into the main assembly of an apparatus to which the sheet feeding apparatus belongs, is controlled by the operation for pressing the pressure roller  16  toward the feed roller  11 , or moving the pressure plate  16  away from the feed roller  11 . 
         [0030]    The sheet conveyance reference portion  15   a  projects from a part of the base  15 , and functions as the positional reference for precisely positioning one of the lengthwise edges of each sheet in the sheet storage portion, in terms of the widthwise direction of the sheet. The side guide  18  precisely positions the other lengthwise edge of the sheet, in terms of the widthwise direction of the sheet. 
         [0031]    The sheet feeding apparatus is structured so that when it is not being used for feeding the sheets, that is, when it is on standby, the pressure plate  16  remains in a preset position (standby position) which provides a preset amount of distance (gap) between the pressure plate  16  and feed roller  11 , allowing a preset amount of sheets to be inserted in layers between the pressure plate  16  and feed roller  11 . 
         [0032]    The method for storing sheets in layers in the sheet storage portion is as follows: First, sheets are to be inserted into the gap between the feed roller  11  and pressure plate  16  along the sheet conveyance reference portion  15   a.  Then, the side guide  18  is to be moved in the direction indicated by an arrow mark C in  FIG. 1  until the gap between the side guide  18  and sheet conveyance reference guide  15  becomes equal to the width of the sheets in the sheet storage portion. Since the side guide  18  is movable in the direction parallel to the widthwise direction of the sheet feeding apparatus, sheets can be smoothly conveyed regardless of the width of the sheets as long as the width of the sheets falls within a preset range. 
         [0033]    The side guide  18  is attached so that it is movable relative to the pressure plate  16  in the widthwise direction of the sheets in the sheet storage portion. Further, in order to prevent the side guide  18  from being accidentally moved, the side guide  18  is provided with a pawl (unshown), whereas the pressure plate  16  is provided with the grooves, one of which catches the pawl of the side guide  18  to prevent the side guide  18  from accidentally moving. The side guide  18  is also provided with a lever (unshown), which is to be operated to unlatch the pawl of the side guide  18  from the grooves of the pressure plate  16  to enable the side guide  18  to move. 
         [0034]    The sheets layered in the sheet storage portion are pressured downward by gravity. Thus, the leading edge of each sheet, in terms of the sheet conveyance direction, comes into, and remains in contact with, the positional reference portion  15   b  for leading edge. The leading edge position reference portion  15   b  is made up of multiple ribs, minimizing thereby the amount of the friction which occurs between the leading edge position reference portion  15   b  and sheet when the sheet is fed. 
       (B) Feeding-and-Separating Portion 
       [0035]    The feeding-and-separating portion has a roller portion made up of the feeder roller  11  and separation roller  12 , and a driving mechanism portion for driving the roller portion. The feed roller  11  of the sheet feeding apparatus in this embodiment plays the role of an auxiliary separation roller, which works in coordination with the separation roller, to separate a sheet to be fed, from the stack of sheets in the sheet storage portion, in addition to the role of feeding the sheets into the main assembly of the apparatus to which the sheet separating apparatus belongs. 
         [0036]    The roller portion is made up of: the feed roller  11  for feeding a sheet; an auxiliary roller  11   a  which assists the feed roller  11  in conveying the sheet; the separation roller  12  for helping the sheet in contact with the feed roller  11  separating from the rest of the sheets in the sheet storage portion; and a torque limiter  12   b  which provides the separation roller  12  with a preset amount of torque. The feed roller  11  is rotatably supported by a feed roller shaft  10 . 
         [0037]    The feed roller  11  and separation roller  12  form the separation nip, which separates the sheet to be conveyed further, from the rest of the sheets in the sheet storage portion. The separation roller  12  is rotatably held by a separation roller holder  28 ; the axle of the separation roller  12  is rotationally supported by the separation roller holder  28 . Further, the separation roller  12  is kept pressured toward the feed roller  11  by a spring  27 . 
         [0038]    The pressure plate  16  presses the stack of sheets on the base  15 , upon the feed roller  11  with a preset timing. The feed roller  11  is rotationally driven while remaining in contact with the sheet kept pressed upon the feed roller  11  by the pressure plate  16 . Thus, the top sheet, that is, the sheet in contact with the feed roller  11 , is convey by the friction between the peripheral surface of the feed roller  11  and the surface of the top sheet in contact with the feed roller  11 . The feed roller  11  conveys the sheet in contact therewith by utilizing the friction between its peripheral surface and the surface of the sheet. Thus, the feed roller  11  is desired to be formed of a rubber, such as EPDM (copolymer of ethylene-propylene-diene), or a resin, such as foamed urethane, which is relatively high in coefficient of friction. 
         [0039]      FIG. 3  is a sectional view of the driving mechanism portion of the feeding-and-separating portion of the sheet feeding apparatus in this embodiment, at a plane perpendicular to the direction indicated by an arrow mark B in  FIG. 1 .  FIG. 4  is a front view of the driving mechanism portion of the sheet feeding apparatus in this embodiment. It should be noted here that the structural components, such as the feed roller shaft  10 , feed roller  11 , etc, are not shown in  FIG. 4 . 
         [0040]    The driving mechanism portion has: a return levers  13  for preventing two or more sheets from being fed at the same time; and a return lever control cam  14  for transmitting driving force to the return lever  13 . Further, the driving mechanism portion has: an input gear  21  for transmitting the driving force transmitted from the driving gear (unshown) of the main assembly; and a step gear  22  which is in mesh with the input gear  21  to transmit the driving force. The driving mechanism portion also has: a feeding shaft gear  23  which transmits the driving force transmitted thereto from the double gear  22  solidly attached to the feeding shaft  10 , to the feed roller  11 ; a control gear  24  which controls the driving of the return lever and separation roller  12 . 
         [0041]    Referring to  FIGS. 2-4 , the driving mechanism portion also has: a return lever pressing spring  26  which keeps the return lever  13  pressed so that the return lever  13  are allowed to rotate only in the preset direction; and a return lever spring  25  which keeps the return lever  13  and return lever control cam  14  pressed in the preset direction so that the positional relationship between the return lever  13  and return lever control cam  14  remain stable. Further, the driving mechanism portion has: a separation roller pressing spring  27  which keeps the separation roller  12  pressed toward the feed roller  11 ; and the separation roller holder  28  which rotatably supports the separation roller  12 . 
         [0042]    As driving force is transmitted from the driving gear of the main assembly to the input gear  21 , the input gear  21  rotates in the direction indicated by an arrow mark F in  FIG. 3 . As a result, the driving force is transmitted to the feed roller shaft gear  23 , while being reduced in speed by the step gear  22 . The feed roller shaft gear  23  is rotated in the direction indicated by an arrow mark E in  FIG. 3 . 
         [0043]    The control gear  24  is provided with a cam  24   a,  which is on one of the primary surfaces of the control gear  24 . The cam  24   a  is in contact with the cam follower portion  14   a  of the return lever control cam  14 , which is kept pressed by the return lever spring  26  in a preset direction. Thus, the cam follower portion  14   a  of the return lever control cam  14  is changed in position by the rotation of the cam  24   a  while remaining in contact with the peripheral surface of the cam  24   a.  That is, the return lever  13  is driven in synchronism with the driving of the feed roller shaft  10 , through the feed roller shaft gear  23 , control gear  24 , and return lever control cam  14 , in accordance with the cam profile (shape of peripheral surface of cam  24   a ). 
         [0044]    The control gear  24  is also provided with another cam (unshown), which is on the opposite surface from the surface having the cam  24   a.  The driving force from the feed roller shaft  10  is transmitted to the separation roller control cam (unshown) by way of this cam on the other side of the control gear  24 . The separation roller  12  is pushed back by the movement of the separation roller control cam in the direction opposite to the direction in which the separation roller  12  is kept pressed toward the separation roller pressing spring  27 , so that the separation roller  12  separates from the feed roller  11  with which it is in contact. In other words, the operation for placing the feed roller  11  and separation roller  12  in contact with each other, and the operation for separating them from each other, are controlled in synchronism with the driving of the feed roller shaft  10 . 
         [0045]    Further, the above described operation for moving the pressure plate  16  toward the feed roller  11 , and operation for moving the pressure plate  16  away from the feed roller  11 , are controlled by the cams (unshown) with which the lengthwise end portions of the feed roller shaft  10  are provided, one for one. More specifically, as the feed roller shaft  10  is rotated, these cams come into contact with the widthwise end portions of the pressure plate  16 , one for one, and press the pressure plate  16 , at the same time. As a result, the pressure plate  16  is pushed back in the direction opposite from the direction in which the pressure plate  16  is kept pressed toward the feed roller  11 . Thus, the pressure plate  16  is moved away from the feed roller  11 . 
         [0046]    When the sheets in the sheet storage portion are fed into the main assembly, the friction which occurs between the feed roller  11  and the top sheet of the stack of sheets in the sheet storage portion is greater than the friction between the top sheet and the sheet immediately under the top sheet. Ordinarily, therefore, only the top sheet is fed into the main assembly. However, in a case where sheets of recording medium, which are relatively high in coefficient of surface friction, are used, it is possible that two or more sheets will be fed into the main assembly at the same time. It is also possible, in a case where sheets having the burrs having resulted during the cutting of the sheets are used, or sheets are made to stick to each other by static electricity, that two or more sheets will be fed by the feed roller  11  at the same time. 
         [0047]    At this time, referring to  FIG. 7 , the mechanism in the sheet feeding apparatus in this embodiment, which is capable of separating the top sheet from the rest of the sheets in the sheet storage portion so that only the top sheet is fed into the main assembly, will be described. 
         [0048]      FIG. 7(   a ) is a sectional view of the separation roller  12  of the sheet feeding apparatus in this embodiment, at a plane which is perpendicular to the axial line of the separation roller  12 .  FIG. 7(   b ) is a sectional view of the separation roller shown in  FIG. 7(   a ), at a plane which coincides with the rotational axis of the separation roller  12 . 
         [0049]    Designated by a referential number  32  is the separation roller shaft, which rotates with the separation roller  12 . The separation roller shaft  32  is provided with a torque limiter  12   b,  which has: a coil spring  33  formed of a metallic or resinous substance; and a torque limiter shaft  31  which is solidly attached to the separation roller holder  28  (so that it does not rotate). The torque limiter shaft  31  is fitted in the center hole of the separation roller shaft  32 . The coil spring  33  is in the hollow of the torque limiter  12   b,  and is fitted around the torque limiter shaft  31  as if it was tightly wound around the torque limiter shaft  31 . 
         [0050]    The torque limiter  12   b  is structured so that as the separation roller shaft  32  is subjected to the torque which acts in the direction to rotate the separation roller  12  forward, the grip which the coil spring  33  has on the torque limiter shaft  32  reduces. 
         [0051]    If the amount of the abovementioned torque is no less than a preset value, the separation roller  12  is allowed to rotate by a preset angle, loosening thereby the grip which the coil spring  33  has on the torque limiter shaft  31 . As long as the torque is greater than the static friction between the coil spring  33  and torque limiter shaft  31 , the coil spring is made to slide around the torque limiter shaft  31 , allowing therefore, the separation roller  12  to continuously rotate forward. 
         [0052]    On the other hand, if the amount of the torque which the separation roller  12  receives is no more than the preset value, the coil spring  33  does not slide around the torque limiter shaft  31 , preventing therefore the separation roller  12  from rotating forward; the separation roller  12  remains stationary. That is, the torque limiter  12   b  has the function of preventing the separation roller  12  from rotating forward when the amount of the torque which the separation roller  12  receives is no more than a preset value. In other words, the torque limiter  12   b  sets the bottom limit for the amount of torque necessary to rotate the separation roller  12  forward. 
         [0053]    The surface layer of the separation roller  12  is formed of such a substance as rubber or foamed urethane, which is roughly the same in coefficient of friction as that of the feed roller  11 . Therefore, when the sheet is not in the separation nip between the feed roller  11  and separation roller  12 , the separation roller  12  is rotated by the rotation of the feed roller  11 . 
         [0054]    In terms of the sheet conveyance direction, the separation nip between the feed roller  11  and separation roller  12  is on the downstream side of the point at which the sheet comes into contact with the feed roller  11  for the first time. The sheet which is in contact with the feed roller  11  is conveyed into the separation nip by the forward rotation of the feed roller  11 . 
         [0055]    When it is only one sheet that has entered the separation nip, the rotational force (torque) is transmitted from the feed roller  11  to the separation roller  12  through the sheet between the feed roller  11  and separation roller  12 , because of the presence of friction between the feed roller  11  and sheet. The amount of the force (grip) which the torque limiter  12   b  is required to stop the separation roller  12  is set to a value which is smaller than the amount of the torque which the separation roller  12  receives from the feed roller  11  through the single sheet between the two rollers  11  and  12 , by being assisted by the friction between the feed roller  11  and sheet, and the friction between the sheet and separation roller  12 . Therefore, when it is only one sheet that has entered the separation nip, the torque limiter  12   b  does not respond, allowing the rotation (torque) from the feed roller  11  to rotate the separation roller  12  forward. Thus, the sheet is conveyed by the feed roller  11  while remaining pinched between the feed roller  11  and separation roller  12 . 
         [0056]    When it is two sheets that have entered the separation nip, the rotation (torque) of the feed roller  11  is transmitted to the first sheet by the friction between the feed roller  11  and first sheet, and then, is transmitted to the second sheet by the friction between the first and second sheets. Then, it is transmitted from the second sheet to the separation roller  12  by the friction between the second sheet and separation roller  12 . The amount of the rotational resistance which is to be provided by the torque limiter  12   b  to prevent the separation roller  12  from rotating is set to a value which is greater than the amount of the torque which is transmitted to the separation roller  12  from the feed roller  11  through the two sheets, that is, by the friction between the feed roller  11  and first sheet, the friction between the first and second sheets, and friction between the second sheet and separation roller  12 . Therefore, the torque limiter  12   b  responds; it stops the separation roller  12 . Further, the coefficient of friction between the first and second sheets is smaller than the coefficient of friction between the feed roller  11  and the first sheet. Therefore, only the first sheet is conveyed by the feed roller  11 , sliding on the second sheet, while the second sheet remains on the upstream side of the separation nip in terms of the sheet conveyance direction. 
       (C) Multi-feeding Preventing Portion 
       [0057]    As described above, if two sheets enter the separation nip of the sheet feeding apparatus in this embodiment, the sheet feeding apparatus separates the first sheet from the second sheet by the separation nip to convey only the first sheet further. However, if three or more sheets enter the separation nip, it is possible that the separation nip will fail to separate the first sheet from the rest, and therefore, two or more sheets will be conveyed beyond the separation nip. That is, if three or more sheets enter the separation nip, it is possible that the so-called multi-feeding will occur. Further, even if it is only two sheets that enter the separation nip, it is possible that the third sheet will enter the separation nip while the second sheet is still remaining in the separation nip after the first sheet is conveyed forward out of the separation nip. In order to prevent this type of multi-feeding, the sheet feeding apparatus in this embodiment is provided with a portion for preventing the multi-feeding (multi-feeding preventing portion). 
         [0058]    The multi-feeding preventing portion is made up of the return lever  13 , return lever control cam  14 , etc., which were described above. Referring to  FIG. 2 , the return lever  13  is made up of a supporting shaft, and three levers proper which are integral with the supporting shaft. The two of the three levers proper are at the lengthwise ends of the return lever  13 , one for one. Next, referring to  FIG. 3 , the return lever control cam  14  is supported by the supporting shaft of the return lever  13 . The force transmitted to the return lever control cam  14  is transmitted to the return lever  13  through the supporting shaft. 
         [0059]    Next, referring to  FIGS. 5(   a )- 5 ( c ) and  FIG. 6 , the operation of the sheet feeding apparatus, which is triggered by the command inputted into a recording apparatus to cause the recording apparatus to start a recording operation, will be described. 
         [0060]      FIGS. 5(   a )- 5 ( c ) are sectional views of the separation nip and its adjacencies of the sheet feeding apparatus, at a plane perpendicular to the feed roller  11  of the apparatus.  FIG. 6  is a flowchart of the operation of the sheet feeding apparatus. 
         [0061]    As the recording apparatus receives a recording start command, first, it issues a command for conveying a sheet to the recording portion of the recording apparatus (S 1 ). As the recording apparatus issues the command for conveying a recording sheet to the recording portion, it determines what kind of image is recorded on what kind of sheet (S 2 ). If the recording sheets to be used for recording are recording media, such as sheets of photographic paper, which very easily sustains scratch marks, the sheet feeding operation which characterizes the sheet feeding apparatus in this embodiment is carried out. When the recording sheets are not sensitive to scratching, like sheets of ordinary paper, the ordinary sheet feeding operation is carried out (S 3 ). 
         [0062]    In a case where the command issued by the recording apparatus was for recording on sheets of photographic medium, the sheet feeding operation which characterizes the sheet feeding apparatus in this embodiment is initiated. First, the pressure plate  16  is pressed by the plate spring (leaf spring) toward the feed roller  11  so that the top sheet in the sheet storage portion is placed in contact with the feed roller  11 . Then, the feed roller  11  is rotated in the forward direction (S 4 ). It should be noted here that in the case of the rotation of the feed roller  11 , the forward direction is the direction indicated by an arrow mark D 1  in  FIG. 5(   a ), whereas in the case of the rotation of the separation roller  12 , the forward direction is the direction indicated by an arrow mark H 1  in the same drawing. 
         [0063]    Referring to  FIG. 5(   a ), as two or more sheets in the sheet storage portion enter the separation nip, the top sheet  20 , that is, the sheet in contact with the feed roller  11 , is conveyed into the main assembly of the recording apparatus through the separation nip, whereas the second sheet and thereafter, that is, the sheets which are not in contact with the feed roller  11 , remain on the downstream side of the separation nip, because the separation roller  12  is stopped by the action of the torque limiter  12   b.    
         [0064]    Further, referring to  FIGS. 7(   a ) and  7 ( b ), in terms of the rotational direction of the separation roller  12  (coil spring  33 ), there is an angular range G in which the coil spring  33  does not act on the separation roller shaft  32 . Therefore, the separation roller  12  is allowed to freely rotate by an angle equivalent to the range G, until the coil spring  33  comes into contact with the separation roller  12 . 
         [0065]    The feed roller  11  is controlled in such a manner that it rotates by at least an angle equivalent to the range G before the sheet enters the separation nip. Therefore, it does not occur that the torque limiter  12   b  of the separation roller  12  does not function as soon as a sheet enters the separation nip. That is, the torque limiter  12   b  is ready to immediately respond by the time the sheet enters the separation nip. Therefore, as soon as two or more sheets enter the separation nip, the torque limiter  12   b  immediately separates the top sheet  20  from the rest, allowing only the top sheet  20  to be conveyed beyond the separation nip by the feed roller  11 . 
         [0066]    As for the second and subsequent sheets  20   a  which are remaining on the upstream side of the separation nip, they have to be moved out of the adjacencies of the separation nip to prevent the problem that the sheets become scarred as they rub against each other. Therefore, the sheet feeding apparatus in this embodiment is provided with a mechanism for rotating the separation roller  12  in the backward direction to move the sheets remaining in the adjacencies of the separation nip, out of the adjacencies, as is a conventional sheet feeding apparatus employing a retard roller. 
         [0067]    Whether or not the top sheet  20  has been conveyed beyond the separation nip by the forward rotation of the feed roller  11  far enough for the leading edge of the top sheet  20  to arrive at a point, which is a preset distance away from the separation nip, is detected by a detecting device P. As the arrival is detected (S 5 ), the forward rotation of the feed roller  11  is temporarily stopped (S 6 ). Then, the rotational direction of the feed roller  11  is changed to the backward direction (S 7 ). Incidentally, the backward rotational direction of the feed roller  11  is the direction indicated by an arrow mark D 2  in  FIG. 5(   b ), whereas the backward rotational direction of the separation roller  12  is the direction indicated by an arrow mark H 2  in the same drawing. 
         [0068]    As the feed roller  11  is rotated backward, the separation roller  12  is rotated backward by the rotation of the feed roller  11 , by an angle equivalent to the range G, without being interfered by the coil spring  33 . As the separation roller  12  is rotated backward by the rotation of the feed roller  11 , the second and subsequent sheets remaining in the adjacencies of the separation nip are moved out of the adjacencies by the friction between the separation roller  12  and the sheets. 
         [0069]    Referring to  FIGS. 5(   a )- 5 ( c ), the sheet feeding apparatus in this embodiment is provided with a sheet regulating member  30 , which is structured so that its tip can be moved next to the separation nip, or away from the separation nip. Immediately after the starting of the sheet feeding operation, the sheet regulating member  30  is kept in the attitude in which its tip is out of the sheet path. However, as the sheet feeding operation is started, the sheet regulating member  30  is rotated by a cam (unshown) or the like, in the direction indicated by an arrow mark J, in synchronism of the backward rotation of the feed roller  11 , so that the portion of the sheet path between the separation nip and sheet storage portion is blocked by the tip of the sheet regulating member  30 . Thus, even if the operation for conveying the top sheet  20  is restarted, the second and subsequent sheets are prevented from enter the separation nip, remaining in the area where they do not contact the top sheet  20  while the top sheet  20  is conveyed further. 
         [0070]    Meanwhile, the pressure plate  16 , which has been pressing the sheets upon the feed roller  11  from the beginning of the sheet feeding operation, is moved away from the sheets, by a cam (unshown) or the like, in synchronism with the backward rotation of the feed roller  11 , ensuring that the second and subsequent sheets  20   a  are smoothly moved out of the adjacencies of the separation nip by the backward rotation of the feed roller  11 . 
         [0071]    The amount by which the feed roller  11  needs to be rotated backward to move the second and subsequent sheets out of the adjacencies of the separation nip is affected by the type, size, thickness, etc., of the sheets. Thus, the sheet feeding apparatus in this embodiment is structured so that the amount by which the feed roller  11  can be rotated backward can be set according to one or more of the abovementioned factors in order to ensure that the amount by which the feed roller  11  is rotated backward matches the sheets being used for recording. Further, the apparatus may be structured to make the rotational range G substantially larger than the angle of the backward rotation of the feed roller  11  so that the apparatus can deal with more types of sheet. The backward rotation of the feed roller  11  is stopped as soon as the leading edge of the top sheet  20  is made to reach the point set in the sheet path according to the sheet being used for recording (S 8 , S 9 ). 
         [0072]    Thereafter, the feed roller  11  is rotated forward (S 10 ). Thus, the top sheet  20  is conveyed to the recording start point, without coming into contact with the second and subsequent sheets  20   a  after their removal from the adjacencies of the separation nip (S 11 ). Then, a preparatory operation for starting recording on the delivered sheet  20  is carried out (S 12 ), and the sheet feeding apparatus in this embodiment ends its operation for feeding the top sheet  20  in the sheet storage portion (S 13 ). 
         [0073]    The above described structure for the sheet feeding apparatus in this embodiment makes it possible to move the second and subsequent sheets out of the adjacencies of the separation nip by the backward rotation of the separation roller  12 . Thus, the sheet feeding apparatus in this embodiment can separate the top sheet in its sheet storage portion from the second and subsequent sheets in the sheet storage portion, and conveys the top sheet, while preventing the sheets from rubbing each other, without being provided with a mechanism dedicated to the prevention that sheets rub each other in the adjacencies of the separation nip. 
         [0074]    Further, after the second and subsequent sheets are moved out of the adjacencies of the separation nip, they are moved further away from the separation nip by the backward rotation of the separation roller  12 , which is caused by the rotation of the feed roller  11 . Therefore, it does not occur that as the feed roller  11  begins to be rotated forward again to convey the top sheet  20 , the top sheet  20  comes into contact with the second and subsequent sheets. 
       Embodiment 2  
       [0075]      FIGS. 8(   a ) and  8 ( b ) and  FIGS. 9(   a ) and  9 ( b ) are sectional views of the separation roller of the sheet feeding apparatus in the second preferred embodiment of the present invention.  FIG. 8(   a ) and  FIGS. 9(   a ) and  9 ( b ) are sectional views of the separation roller, at a plane perpendicular to the rotational axis of the separation roller.  FIG. 8(   b ) is a sectional view of the separation roller, at a plane which coincides with the rotational axis of the separation roller. The sheet feeding apparatus in this embodiment is the same as that in the first embodiment, except for the separation roller structure. 
         [0076]    Referring to  FIG. 8 , designated by a referential number  32   a  is a separation roller shaft, which rotates with the separation roller  12 . The coil spring  33   a  of the torque limiter  12   c  is fitted around the torque limiter shaft  31   a,  as if it were wound around the torque limiter shaft  31   a,  which is supported by the separation roller holder  28 . The torque limiter  12   c  is structured so that the grip which the coil spring  33   a  has on the separation roller shaft  32   a  functions as a torque limiting means. Further, the torque limiter shaft  31   a  is provided with a catch portions  34 . 
         [0077]    Referring to  FIGS. 9(   a ) and  9 ( b ), the sheet feeding apparatus in this embodiment is provided with a stopper  35 , which is in the adjacencies of the separation roller  12 . The stopper  35  is kept pressed by a spring (unshown) in the direction indicated by an arrow mark K 1  in the drawing, and latches with the catch portion  34  of the torque limiter shaft  31   a.    
         [0078]    Next, referring to  FIG. 9(   a ), the torque transmitted to the separation roller  12  in the direction to rotate the torque limiter shaft  31   a  in the direction indicated by the arrow mark H 1  adds to the force which the catch portion  34  of the torque limiter shaft  31   a  receives from the stopper  35  in the direction indicated by the arrow mark K 1 . Therefore, the torque limiter shaft  31   a  remains locked by the stopper  35 , being prevented from rotating in the direction indicated by the arrow mark H 1 . 
         [0079]    On the other hand, referring to  FIG. 9(   b ), the torque transmitted to the separation roller  12  in the direction to rotate the torque limiter  31   a  in the direction indicated by the arrow mark H 2  acts in the direction to push back the force applied to the catch portion  34  of the torque limiter shaft  31   a  by the stopper  35  in the direction indicated by the arrow mark K 2 . Thus, the stopper  35  disengages from the catch portion  34  of the torque limiter shaft  31   a.  That is, as the torque limiter shaft  31   a  is rotated in the direction indicated by an arrow mark H 2 , the stopper  35  disengages from the catch portion  34  of the torque limiter shaft  31   a,  allowing thereby the toque limiter shaft  31   a  (separation roller  12 ) to rotate in the direction indicated by the arrow mark H 2 . 
         [0080]    As the forward rotation of the feed roller  11  is continued after the entrance of the sheet(s) into the separation nip, the torque transmitted to the separation roller  12  from the feed roller  11  acts in the direction to rotate the separation roller  12  in its forward direction, that is, the direction indicated by the arrow mark H 1 . At this point in time, however, the torque limiter shaft  31   a  remains locked by the stopper  35 , as shown in  FIG. 9(   a ), and therefore, the torque limiter shaft  31   a  cannot rotate in the direction indicated by the arrow mark H 1 . Thus, the coil spring  33   a  fitted around the torque limiter shaft  31   a  acts on the separation roller shaft  32   a,  preventing thereby the separation roller  12  from rotating. That is, as two or more sheets enter the separation nip, the torque limiter  12   c  functions to prevent the separation roller  12  from rotating, so that only the top sheet  20  is conveyed beyond the separation nip by the feed roller  11 . 
         [0081]    Further, as the feed roller  11  is rotated backward in order to move the second and subsequent sheets, which are in the adjacencies of the separation nip, out of the adjacencies, the torque transmitted from the feed roller  11  to the separation roller  12  acts to rotate the separation roller  12  backward, that is, the direction indicated by the arrow mark H 2 . However, this torque acts in the direction to disengage the stopper  35  from the catch portion  34 , as shown in  FIG. 9(   b ). Thus, the torque limiter shaft  31   a  is freed from the stopper  35 , and rotates with the coil spring  33   a  fitted around the torque limiter shaft  31   a,  in the direction indicated by the arrow mark H 2 . That is, the coil spring  33   a  rotates with the separation roller shaft  32   a.  In other words, the torque limiter  12   c  does not interfere with the backward rotation of the separation roller  12 , allowing the separation roller  12  to move the second and subsequent sheets, which are in the adjacencies of the separation nip, out of the adjacencies of the separation nip. 
         [0082]    That is, the above described structural arrangement of the sheet feeding apparatus in this embodiment makes it possible to remove the second and subsequent sheets from the adjacencies of the separation nip by the backward rotation of the separation roller  12 . Therefore, the sheet feeding apparatus in this embodiment can prevent the sheets from rubbing each other while they are separated or conveyed, without requiring a mechanism dedicated to the prevention of the problem that sheets rub each other while they are separated or conveyed. 
       Embodiment 3  
       [0083]      FIG. 10(   a ) is a perspective view of the separation roller of the sheet feeding apparatus in the third preferred embodiment of the present invention, and  FIGS. 10(   b ) and  10 ( c ) are sectional views of the separation roller, at a plane perpendicular to the rotational axis of the separation roller, and a plane which coincides with the rotational axis of the separation roller, respectively. The sheet feeding apparatus in this embodiment is the same as the sheet feeding apparatus in the first embodiment, except for the separation roller structure. 
         [0084]    Referring to  FIGS. 10(   a )- 10 ( c ), designated by a referential number  32   c  is the shaft of the separation roller  12 , which rotates with the separation roller  12 . The torque limiter  12   d  is provided with a torque limiter shafts  31   c  and  36 , and coil springs  33   c  and  33   d.  The coil springs  33   c  and  33   d  are fitted around the shafts  31   c  and  36 , as if they were tightly wound around the shafts  31   c  and  36 , respectively. One of the lengthwise ends of the shaft  36  is D-shaped in cross section, ensuring that the shaft  36  remains solidly attached to the separation roller holder  28 . 
         [0085]    The torque limiter  12   d  is structured so that the forward rotation of the separation roller  12  is controlled by the amount of the grip which the coil spring  33   c  has on the separation roller shaft  31   c.  Further, the torque limiter  12   d  is structured so that the backward rotation of the separation roller  12  is controlled by the amount of the grip which the coil spring  33   d  has on the shaft  36 . 
         [0086]    As the forward rotation of the feed roller  11  is continued after the entrance of the sheet(s) into the separation nip, the torque transmitted to the separation roller  12  acts in the direction to rotate the separation roller  12  forward. However, the coil spring  33   d  is fitted around the shaft  36  in such a manner that as the separation roller  12  is rotated forward, the coil spring  33   d  increases in the amount of the grip it has on the shaft  36 , preventing thereby shaft  36  from rotating. On the other hand, the coil spring  33   c  is fitted around the shaft  31   c  in such a manner that as the separation roller  12  is rotated forward, the coil spring  33   c  reduces in the amount of the grip it has on the shaft  31   c,  affecting the rotation of the separation roller shaft  32   c  of the separation roller  12 . Therefore, when the separation roller  12  is rotated forward, the coil spring  33   c  and shaft  31   c  of the torque limiter  13   d  function as the torque limiter. Therefore, as two or more sheets enter the separation nip, the separation roller is not allowed to rotate forward, allowing thereby only the top sheet  20  to be conveyed by the feed roller  11 . 
         [0087]    Further, as the feed roller  11  is rotated backward to remove the second and subsequent sheets in the adjacencies of the separation nip, from the adjacencies, the torque transmitted to the separation roller  12  from the feed roller  11  acts on the separation roller  12  in a manner to rotate the separation roller  12  backward. However, the separation roller shaft  32   c  cannot rotate, because the coil spring  33   c  is fitted around the shaft  31   c  in such a manner that as the separation roller  12  is rotated backward, it increases in the amount of the grip it has on the shaft  31   c.  On the other hand, the coil spring  33   d  is fitted around the shaft  36  in such a manner that as the separation roller  12  is rotated backward, the coil spring  33   d  reduces in the amount of the grip it has on the shaft  36 , affecting the rotation of the torque limiter shaft  31 . In other words, when the separation roller  12  is rotated backward, the coil spring  33   d  and shaft  36  of the torque limiter  12   d  function as the torque limiter. Therefore, when the feed roller  11  is rotated backward, the separation roller  12  is allowed to be rotated by the rotation of the feed roller  11 . Therefore, as two or more sheets enter the separation nip, the separation roller is allowed to be rotated backward to remove the second and subsequent sheets in the adjacencies of the separation nip, from the adjacencies. 
         [0088]    That is, the above described structure of the sheet feeding apparatus in this embodiment makes it possible to move the second and subsequent sheets out of the adjacencies of the separation nip by the backward rotation of the separation roller  12 . In other words, the sheet feeding apparatus in this embodiment separates the top sheet in its sheet storage portion from the second and subsequent sheets in the sheet storage portion, and conveys only the top sheet, while preventing the sheets from rubbing each other during the separation and conveyance of the sheets, without being provided with a mechanism dedicated to the prevention of the problem that the sheets rub each other during their separation and/or conveyance. 
       Embodiment 4  
       [0089]      FIGS. 11 and 12  are the sectional views of the sheet feeding apparatus in the fourth preferred embodiment of the present invention. The sheet feeding apparatus in this embodiment is different from that in the first embodiment in that the former is provided with a feed roller dedicated to the separation of two or more sheets, in addition to the feed roller  11  for feeding the sheet(s) into the separation nip. Other structural features of the sheet feeding apparatus in this embodiment are the same as those of the sheet feeding apparatus in the first embodiment. 
         [0090]      FIG. 11  shows the sheet feeding apparatus in this embodiment. The feed roller  37  is rotatably supported by a feed roller supporting-and-regulating member  38 . The sheets  20   b  are stored in layers on the base  15  of the sheet feeding apparatus (automatic sheet feeder), and are fed by the feed roller  37  into the separation nip between the feed roller  39  and separation roller  12 , which are for separating the top sheet  20  from the rest  20   b.  The sheet feeding apparatus in this embodiment is structured so that the feed roller  39  begins to rotate forward in synchronism with the rotation of the feed roller  37 . As the feed roller  37  is rotated, the sheet(s)  20   b  are fed into the separation nip, in which the top sheet is separated from the rest by the forward rotation of the feeding roller  39 , and the function of the torque limiter of the separation roller  12 , so that only the top sheet will be conveyed beyond the separation nip by the feed roller  39 . 
         [0091]    After the top sheet is conveyed past the separation nip, the feed roller  39  is rotated backward to remove the second and subsequent sheets in the adjacencies of the separation nip, therefrom. As the feed roller  39  is rotated backward, the feed roller supporting-and-regulating member  38  is rotated by a cam (unshown) or the like in the direction to separate the feed roller  37  from the sheets stacked in the sheet storage portion, in synchronism with the starting of the backward rotation of the feed roller  39 , ensuring that the second and subsequent sheets in the adjacencies of the separation nip are smoothly removed therefrom. 
         [0092]    As described above, the structural arrangement for the sheet feeding apparatus in this embodiment makes it possible for the second and subsequent sheets in the adjacencies of the separation nip to be removed therefrom by the backward rotation of the separation roller  12 , even if the sheet feeding apparatus is provided with both a sheet feed roller dedicated to the feeding of the sheet(s) into the separation nip, and a feed roller dedicated to the separation of the top sheet from the rest. In other words, the present invention can make it possible to prevent the problem that the sheets rub each other while being fed into the main assembly of a recording apparatus by the sheet feeding apparatus, without providing the sheet feeding apparatus with a mechanism dedicated to the prevention of this problem. 
         [0093]    According to the preferred embodiments described above, it is possible to provide a sheet feeding apparatus, which has a mechanism for preventing the multi-feeding, that is, the problem that two or more sheets are fed into the main assembly of a recording apparatus, and therefore, is capable of preventing the problem that the sheets rub each other while they are separated and/or conveyed past the separation nip, without providing the sheet feeding apparatus with a mechanism dedicated to the prevention of the problem. 
         [0094]    While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
         [0095]    This application claims priority from Japanese Patent Application No. 323195/2007 filed Dec. 14, 2007 which is hereby incorporated by reference.