Patent Publication Number: US-8540230-B2

Title: Sheet feeding apparatus and image forming apparatus with curvature formation portion and reversely rotatable feeding roller

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeding apparatus and an image forming apparatus and, in particular, to a configuration for separating sheets one by one. 
     2. Description of the Related Art 
     Conventionally, an image forming apparatus such as a facsimile machine, a copying machine, and a laser-beam printer includes a sheet feeding apparatus for feeding sheets such as plain paper, coated paper, plastic sheet and cloth to an image forming unit in the image forming apparatus. 
     It is very important for the sheet feeding apparatus to separate and feed sheets one by one to the image forming unit. In order to prevent the sheet feeding apparatus from feeding a plurality of sheets at the same time (double feeding), various types of feeding methods have been proposed. 
     In recent years, importance has been attached not only to stable feeding by preventing the double feeding, but also to downsizing of the sheet feeding apparatus. Furthermore, the image forming unit requires to be further downsized as a printer and a facsimile machine are used in a family. 
     Some image forming apparatuses including a sheet feeding cassette or a sheet feeding tray for storing sheets are provided with a feeding roller capable of forwardly or reversely rotating at the upper part of the sheet feeding tray, for example, to downsize the image forming apparatus. 
     When a sheet is fed, the sheet feeding apparatus reversely rotates the feeding roller to reversely transport the sheet in the sheet feeding tray and causes the trailing edge of the sheet to abut on the rear wall of the sheet feeding tray, thereby temporarily curving the uppermost sheet to be separated from other sheets. 
     After the uppermost sheet is temporarily curved, the feeding roller is forwardly rotated to send the uppermost sheet along the upper face of a separation claw provided downstream in the sheet feeding direction in the sheet feeding tray, thereby separating and feeding sheets one by one. Such a configuration allows the function of separating and feeding sheets to fall within the range where the sheet feeding tray is attached as discussed in Japanese Patent Application Laid-Open No. 05-147752. 
     In recent years, it has become important to stably feed various types of paper different in thickness. It often becomes problematic to feed a thin sheet which is low in stiffness in particular. A method for increasing apparent stiffness of a sheet by curving a sheet in a sheet feeding cassette, for example, to feed such a thin sheet low in stiffness. 
     As such a method, there is a method in which a sheet stored in a sheet support portion for stacking sheets, for example, is curved in the direction orthogonal to the feed direction of the sheet to increase apparent stiffness with respect to the feed direction of the sheet. 
     If the sheet is loosened by a sloping surface, increasing the apparent stiffness allows the sheet to be surely loosened even if the sheet is thin and the double feeding to be prevented. The technique is discussed in Japanese Patent Application Laid-Open No. 2000-143002. 
     In such a conventional sheet feeding apparatus, in a method for separating sheets in such a manner that the sheet is caused to climb up the separation claw, however, for a thin sheet low in stiffness in particular, when the sheet climbs up the separation claw, the leading end of the sheet collides with the edge face of the separation claw and may be broken. For a sheet high in stiffness, the leading end of the sheet catches the separation claw, resultantly, cannot climb up the separation claw, and may be jammed. 
     Then, as described above, the apparent stiffness of the sheet is increased in such a manner that the sheet is curved in the direction orthogonal to the feed direction of the sheet to allow preventing the leading end of the sheet from being broken, and allow the sheet to easily pass on the separation claw. 
     When the sheet high in apparent stiffness is curved by rotating the feeding roller in the direction opposite to the feed direction, the feeding roller may slip when the curve produced by reversely transporting the sheet is formed not to allow the sheet to be reversely transported. 
     When the uppermost sheet is curved by rotating the feeding roller in the direction opposite to the feed direction to cause the trailing edge of the sheet to abut on the rear wall of the sheet feeding tray, a buckle may occur at the trailing edge of the sheet because the sheet is high in apparent stiffness, which may not form warping in the sheet. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a sheet feeding apparatus and an image forming apparatus capable of stably separating and feeding a sheet. 
     According to an aspect of the present invention, a sheet feeding apparatus includes a sheet support portion configured to support a plurality of sheets, a forwardly or reversely rotatable feeding roller configured to send out an uppermost sheet among the sheets, a separation claw provided with pressing portions configured to press both ends of a sheet supported by the sheet support portion in the width direction thereof orthogonal to a direction in which the sheet is fed, and configured to separate the uppermost sheet by forwardly or reversely rotating of the feeding roller, a curvature formation portion configured to form curvature so that both ends in a width direction of the sheet supported by the sheet support portion become lower than a center portion thereof and a control unit configured to control the feeding roller and the curvature formation portion to rotate the feeding roller in a direction opposite to the direction in which the sheet is fed, to draw out the uppermost sheet from the separation claw, move the separation claw so that the pressing portions lie at a position separated from and lower than the uppermost sheet, and rotate the feeding roller in the direction in which the sheet is fed. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a schematic diagram illustrating an entire configuration of a full color laser printer as an example of an image forming apparatus including a sheet feeding apparatus according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a configuration of the sheet feeding apparatus. 
         FIGS. 3A and 3B  are diagrams illustrating a configuration of the sheet feeding apparatus. 
         FIG. 4  is a control block diagram of the sheet feeding apparatus. 
         FIG. 5  is a flow chart illustrating a sheet feeding operation of the sheet feeding apparatus. 
         FIGS. 6A and 6B  are diagrams illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIG. 7  is a diagram illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIG. 8  is a diagram illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIGS. 9A ,  9 B, and  9 C are timing charts illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIG. 10  is a perspective view illustrating a configuration of a sheet feeding apparatus according to a second exemplary embodiment of the present invention. 
         FIG. 11  is a flow chart illustrating a sheet feeding operation of the sheet feeding apparatus. 
         FIGS. 12A and 12B  are diagrams illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIGS. 13A and 13B  are diagrams illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIG. 14  is a diagram illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIG. 15  is a diagram illustrating the sheet feeding operation of the sheet feeding apparatus. 
         FIGS. 16A-16C  are timing charts illustrating the sheet feeding operation of the sheet feeding apparatus. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
       FIG. 1  is a schematic diagram illustrating an entire configuration of a full color laser printer as an example of an image forming apparatus including a sheet feeding apparatus according to a first exemplary embodiment of the present invention. 
       FIG. 1  includes a full color laser printer  100  and a full color laser printer main body  100 A (hereinafter referred to as printer main body). The printer main body  100 A being the image forming apparatus main body includes an image forming unit  100 B for forming an image on a sheet such as recording paper, plastic sheet, and a cloth, and a sheet feeding apparatus  200  for feeding the sheets. 
     The image forming unit  100 B includes a process cartridge  7  ( 7 Y,  7 M,  7 C, and  7 K) for forming toner images in four colors, yellow, magenta, cyan, and black. The process cartridge  7  includes a photosensitive drum  1  ( 1 Y,  1 M,  1 Y, and  1 K) serving as an image bearing member rotationally driven in the direction indicated by an arrow (counterclockwise direction) by a driving unit (driving source) (not illustrated) and is detachably attached to the printer main body  100 A. 
     The image forming unit  100 B also includes a scanner unit  3  arranged at an upper portion perpendicular to the process cartridge  7  and irradiates the photosensitive drum  1  with a laser beam based on image information to form an electrostatic latent image on the photosensitive drum  1 . The process cartridge  7  includes a development unit  4  ( 4 Y,  4 M,  4 C, and  4 K) for adhering toner on the electrostatic latent image to visualize the electrostatic latent image as a toner image and a charging roller  2  ( 2 Y,  2 M,  2 C, and  2 K) for uniformly charging the surface of the photosensitive drum  1  as well as the photosensitive drum  1 . 
     In  FIG. 1 , an intermediate transfer belt unit  100 C includes an endless intermediate transfer belt  5  and a primary transfer roller  8  ( 8 Y,  8 M,  8 C, and  8 K) arranged inside the intermediate transfer belt  5  facing the photosensitive drum  1 . The intermediate transfer belt  5  is stretched around a drive roller  53 , a secondary transfer opposing roller  52 , and a driven roller  51 , and rotated in the direction indicated by an arrow B while abutting on all photosensitive drums  1 . 
     The primary transfer roller  8  presses the intermediate transfer belt  5  against the photosensitive drum  1  to form a primary transfer portion N 1  where the intermediate transfer belt  5  abuts on the photosensitive drum  1 . A bias application unit (not illustrated) applies transfer bias to the intermediate transfer belt  5 . 
     A primary transfer bias is applied to the intermediate transfer belt  5  by the primary transfer roller  8  and a toner image in each color on the photosensitive drum  1  is sequentially transferred to the intermediate transfer belt  5  to form a full color image on the intermediate transfer belt  5 . 
     A secondary transfer roller  9  which presses the secondary transfer opposing roller  52  via the intermediate transfer belt  5  to form a secondary transfer portion N 2  is arranged in a position opposing the secondary transfer opposing roller  52  on the outer peripheral surface side of the intermediate transfer belt  5 . A secondary transfer bias power supply (a high voltage power supply) serving as a secondary transfer bias application unit (not illustrated) applies a bias having a polarity opposite to the normal charge polarity of the toner to the secondary transfer roller  9 . Thereby, the toner image on the intermediate transfer belt  5  is transferred (secondary transferred) to the sheet P. 
     The sheet feeding apparatus  200  includes a sheet feeding cassette  21  which is detachably attached to the printer main body  100 A and a feeding roller  22  for feeding a plurality of sheets P stored in the sheet feeding cassette  21 . When the sheet P stored in the sheet feeding cassette  21  is fed, the feeding roller  22  pressed by the sheet P is rotated to send out the sheet P. 
     The image formation operation of the thus configured full color laser printer  100  is described below. 
     When an image signal is input to the scanner unit  3  from an image reading apparatus (not illustrated) connected to the printer main body  100 A or a host apparatus such as a personal computer, the scanner unit  3  irradiates the photosensitive drum  1  with a laser beam corresponding to the image signal. 
     The photosensitive drum  1  whose surface is uniformly and previously charged with a predetermined polarity and potential by the charging roller  2  is irradiated with the laser beam by the scanner unit  3  to form the electrostatic latent image on the surface thereof. Thereafter, the electrostatic latent image is developed by the development unit  4  to visualize the image. 
     For example, the photosensitive drum  1 Y is irradiated with the laser beam corresponding to the image signal in yellow component color by the scanner unit  3  to form a yellow electrostatic latent image on the photosensitive drum  1 Y. The yellow electrostatic latent image is developed by a yellow tonner from the development unit  4 Y to visualize the image as a yellow tonner image. 
     After that, the toner image arrives at the primary transfer portion N 1  where the photosensitive drum  1 Y abuts on the intermediate transfer belt  5  as the photosensitive drum  1 Y is rotated. In the primary transfer portion N 1 , the yellow tonner image on the photosensitive drum is transferred to the intermediate transfer belt  5  by the primary transfer bias applied to the primary transfer roller  8 Y. 
     When a portion bearing the yellow toner image on the intermediate transfer belt  5  is moved, a magenta toner image formed on the photosensitive drum  1 M in the method similar to the above one until then is transferred onto the yellow toner image on the intermediate transfer belt  5 . 
     Similarly, as the intermediate transfer belt  5  is moved, in the primary transfer portion, cyan and black toner images are superimposed on and transferred onto the yellow and the magenta toner images. Thereby, the full color toner image is formed on the intermediate transfer belt  5 . 
     Along with the toner image formation operation, the sheet P stored in the sheet feeding cassette  21  is sent out by the feeding roller  22  and conveyed to a registration roller  15 . 
     The sheet P conveyed to the registration roller  15  is subjected to timing adjustment by the registration roller  15  and conveyed to a secondary transfer portion N 2 . 
     A bias having a positive polarity is applied to the secondary transfer roller  9  in the secondary transfer portion N 2  to secondary transfer four-color toner images on the intermediate transfer belt  5  to the conveyed sheet P. The toner remained on the intermediate transfer belt  5  after the secondary transfer of the toner images is removed by a belt cleaner  11 . 
     The sheet P to which the toner images are transferred is conveyed to a fixing portion  10  and heated and pressed, thereby fixing the full color toner image as a permanent image. After that, the image is discharged outside the printer main body  100 A. 
     A sheet feeding apparatus  200  according to the present exemplary embodiment is described with reference to  FIGS. 2 ,  3 A, and  3 B. The feeding roller  22  for feeding the sheet P is arranged in a central portion in the widthwise direction orthogonal to the sheet feeding direction of sheet support portions  25  and  26  for supporting a plurality of sheets. The feeding roller  22  is fixed to a drive shaft  31  connected to a sheet feeding motor PM capable of forwardly or reversely rotating, described below and illustrated in  FIG. 4 . 
     The feeding roller  22  is capable of rotating in the direction, in which the sheet is fed, indicated by an arrow R and in the direction opposite to the direction, in which the sheet is fed, indicated by an arrow L in  FIGS. 2 and 3A , by receiving driving force from the sheet feeding motor PM. 
     The feeding roller  22  rotates in the direction indicated by the arrow R to feed the sheet to the upper left direction illustrated in  FIG. 1 . The direction in which the sheet is fed is the sheet feeding direction. The direction in which the sheet is moved to the right direction in  FIG. 1  by the rotation of the feeding roller  22  in the direction indicated by the arrow L is the sheet reverse feeding direction. 
     A separation claw  24  is rotatably provided on a separation claw base  30  disposed downstream in the sheet feed direction of the feeding roller  22  in the sheet feeding cassette  21  stacked with sheets with a rotary axis  24   b  as a fulcrum. 
     The separation claw  24  is provided with pressing portions  24   a  which are provided at both side ends of the widthwise direction orthogonal to the sheet feeding direction and presses both ends in the widthwise direction of the sheet P and urged by a compression spring  29 , so that the pressing portions  24   a  abut on the sheet. 
     In the sheet feeding apparatus  200  with this configuration, when the sheet P is fed, the feeding roller  22  is rotated in the reverse feeding direction to reversely transport the uppermost sheet P 1  of the sheets P stacked on the sheet feeding cassette  21 . 
     A curvature is formed by reversely transporting the uppermost sheet P 1  to separate the uppermost sheet P 1  from the second and subsequent sheets under the uppermost sheet P 1 , and then the feeding roller  22  is rotated in the sheet feed direction to feed the uppermost sheet P 1 . The uppermost sheet P 1  passes on the upper surface of the separation claw  24  and fed in a predetermined feeding direction positioned in the upper portion of the separation claw  24 . 
     In the present exemplary embodiment, the separation claw  24  is made of a low friction material such as polyoxymethylene (POM), so that the end of the uppermost sheet P 1  in the sheet feed direction (hereinafter referred to as leading end) can be smoothly drawn from the separation claw  24  when the uppermost sheet P 1  is reversely transported. 
     As illustrated in  FIG. 3B , the sheet feeding cassette  21  is provided with a center base plate  25  serving as a center supporting portion for supporting the center portion in the widthwise direction of the sheet P. The sheet feeding cassette  21  is provided with a side base plate  26  serving as a both-end supporting portion which is located on both sides in the widthwise direction of the center base plate  25 , separated from the center base plate  25 , and is relatively elevatable. The center base plate  25  and the side base plate  26  form a sheet supporting portion for supporting the sheet. 
     The center base plate  25  and the side base plate  26  are urged respectively by compression springs  27  and  28  against the feeding roller  22 . This causes the sheet P stacked on the sheet feeding cassette  21  to abut on the feeding roller  22 . The compression springs  27  and  28  are set to a suitable spring constant so that the sheet P is stably urged against the feeding roller  22  according to the stacked sheet P. 
     The configuration of a curvature formation portion  24 X characterized by the present invention is described below. On both sides of the sheet feeding cassette  21  there are provided depression levers  23   a  and  23   b  for depressing the side base plate  26  by depressing the side ends of the side base plate  26  for the sheet downward. The depression levers  23   a  and  23   b  are coupled with a coupling bar  35 . 
     The depression levers  23   a  and  23   b  are rotatably supported by shafts  34 . The depression levers  23   a  and  23   b  and the coupling bar  35  form a link mechanism. 
     The depression lever  23   a  is rotated with the rotation of a gear  32  fixed to a gear shaft  33  rotated by the driving force of a drive motor WM illustrated in  FIG. 4  described below, and depresses the side base plate  26  along with the depression lever  23   b  via the coupling bar  35 . 
     When the side base plate  26  is depressed by the depression levers  23   a  and  23   b,  the center base plate  25  is maintained at a position where the sheet P is urged against the feeding roller  22  by the compression spring  27 , so that only the side base plate  26  is depressed. 
       FIG. 4  is a control block diagram for a drive system of the sheet feeding apparatus  200 . In  FIG. 4 , a central processing unit (CPU)  60  serving as a control unit controls the sheet feeding motor PM and the drive motor WM. A timer  61  is incorporated in the CPU  60 . 
     The side base plate  26  is depressed by the depression levers  23   a  and  23   b  to produce a step between the side base plate  26  and the center base plate  25 . The step depresses both the left and the right ends of the sheet to curve the sheet. The pressing portions  24   a  of the separation claw  24  arranged at both ends of the sheet follow the upper surface at both ends of the sheet due to the elastic force of the compression spring  29  while pressing the upper surface, and move. 
     As illustrated in  FIG. 7 , in this state, all of the second and subsequent sheets P 2  under the uppermost sheet P 1  are high in their center portion in the direction orthogonal to the sheet feeding direction and are curved to lower both the left and right ends. In the present exemplary embodiment, the curvature formation portion  24 X is formed of the depression levers  23   a  and  23   b  and the drive motor WM. 
     The sheet feeding operation of the sheet feeding apparatus  200  performed by the CPU (control unit)  60  is described below with reference to a flow chart illustrated in  FIG. 5 , an operation chart illustrated in  FIGS. 6A ,  6 B,  7 , and  8 , and a timing chart illustrated in  FIGS. 9A to 9C . 
       FIG. 3A  already described above is a schematic diagram illustrating an initial state of the sheet feeding operation of the sheet feeding apparatus  200 . At this point, the sheet P is stacked in the sheet feeding cassette  21  with the upper surface thereof flat. The sheet feeding operation starts with this state as an initial state. 
     In step S 1 , the CPU  60  reversely rotates the feeding roller  22  in the reverse feeding direction (in the direction indicated by the arrow L) by a predetermined amount. This operation reversely transports the leading end side of the uppermost sheet P 1  and the leading end of the uppermost sheet P 1  passes by the end of the pressing portion  24   a  of the separation claw  24  abutting on the sheet (hereinafter referred to as the front end of the pressing portion  24   a ). 
     In the present exemplary embodiment, the number of rotations of the feeding roller  22  (or the amount of conveyance of the sheet by which the leading end of the sheet can pass by the separation claw  24 ) is regulated by the time control of the timer  61  incorporated in the CPU  60 . Other than that, the number of rotations (or the amount of conveyance of the sheet) may be controlled using a sensor. 
     The feeding roller  22  is reversely rotated to cause the uppermost sheet P 1  fed in the reverse feeding direction to abut on a back end wall  21   a  of the sheet feeding cassette  21  and movement in the reverse feeding direction is regulated. 
     For this reason, as illustrated in  FIG. 6A , the uppermost sheet P 1  is curved between the feeding roller  22  and the back end wall  21   a  of the sheet feeding cassette  21 . When the uppermost sheet P 1  is thus conveyed, a second sheet underlying the uppermost sheet P 1  may be conveyed by the force of friction with the uppermost sheet P 1  and curved. 
     In the present exemplary embodiment, however, the spring constants of the compression springs  27  and  28  are adjusted so that the conveyance force applied to the second and subsequent sheets P 2  via the uppermost sheet P 1  becomes smaller than a force by which the second and subsequent sheets P 2  are curved. 
     For this reason, when the uppermost sheet P 1  is conveyed, the leading end of the second and subsequent sheets P 2  will not pass by the front end of the pressing portion  24   a  of the separation claw  24 . Therefore, only the uppermost sheet P 1  is conveyed in the reverse feeding direction along the second and subsequent stacked sheets P 2  and separated from the second and subsequent sheets P 2 . 
     When the leading end of the uppermost sheet P 1  passes by the front end of the pressing portion  24   a  of the separation claw  24 , in step S 2 , as illustrated in  FIG. 6B , the drive motor WM is driven to rotate the depression levers  23   a  and  23   b  in the direction indicated by the arrow D and lower the depression levers  23   a  and  23   b.  As illustrated in  FIG. 7 , the depression levers  23   a  and  23   b  are lowered to depress the side base plate  26 . 
     When the side base plate  26  is depressed by the depression levers  23   a  and  23   b,  the height of the center base plate  25  remains unchanged, so that a step is produced between the center base plate  25  and the depressed side base plate  26 . When the step is produced, the second and subsequent sheets P 2  stacked in the sheet feeding cassette  21  are pressed by the separation claw  24  urged by the compression spring  29  against the top surface of the side base plate  26  in the direction in which the sheet P is depressed. 
     As a result, the second and subsequent sheets P 2  are curved along the step produced between the center base plate  25  and the side base plate  26 . In other words, the sheets are curved so that the center portion thereof is elevated and both left and right ends thereof are lowered in the direction orthogonal to the sheet feeding direction. 
     At this point, the uppermost sheet P 1  is curved along the sheet feeding direction on its trailing edge side, so that the uppermost sheet P 1  is hardly curved in the same direction as the direction in which the second and subsequent sheets P 2  are curved as illustrated in  FIG. 7 , which is a diagram viewed from the direction indicated by an arrow A in  FIG. 6B . Thus, the uppermost sheet P 1  is curved along the sheet feeding direction and the second and subsequent sheets P 2  are curved in the direction orthogonal to the sheet feeding direction to produce a gap G between the uppermost sheet P 1  and the top surface of the second and subsequent sheets P 2 . 
     The front end of the pressing portion  24   a  of the separation claw  24  abuts on the top surface of the sheet P 2 , so that the gap G is also substantially produced between the uppermost sheet P 1  and the front end of the pressing portion  24   a.  In other words, the uppermost sheet P 1  is separated from the pressing portion  24   a  of the separation claw  24 . 
     As illustrated in  FIG. 8 , in step S 3 , the feeding roller  22  is forwardly rotated in the sheet feeding direction (in the direction indicated by the arrow R) by a predetermined amount to feed the upper sheet P 1 . As described above, the front end of the pressing portion  24   a  of the separation claw  24  is urged in the direction of the surface of the second and subsequent curved sheets P 2 , so that the leading end is lower in position than the uppermost sheet P 1  and separated therefrom. 
     Therefore, the leading end of the reversely fed uppermost sheet P 1  will not abut on the front end of the pressing portion  24   a  of the separation claw  24 . Only the uppermost sheet P 1  climbs up the top surface of the separation claw  24  to be sent out and is separated from the second and subsequent curved sheets P 2  and fed. 
     When the uppermost sheet P 1  climbs up the top surface of the separation claw  24  to be sent out, the drive motor WM is driven to rotate the depression levers  23   a  and  23   b  in the direction in which the depression of the side base plate  26  is released, that is, in the direction indicated by an arrow U in which the depression levers  23   a  and  23   b  are elevated. 
     Thereby, in step S 4 , the depression levers  23   a  and  23   b  are elevated, the depression of the side base plate  26  performed by the depression levers  23   a  and  23   b  is released, the side base plate  26  is also elevated, and the sheet P stacked in the sheet feeding cassette  21  returns to a flat state. 
     After that, in step S 5 , the separated uppermost sheet P 1  is fed, and when the uppermost sheet P 1  passes by the feeding roller  22 , the drive of the feeding roller  22  is stopped. Thus, only the uppermost sheet P 1  among the sheets P stacked in the sheet feeding cassette  21  is fed. 
     In this step, the sheet feeding apparatus  200  returns to the initial state illustrated in  FIG. 3 . By repeating steps S 1  to S 5 , the second and subsequent curved sheets P 2  are equentially fed. 
     As described above, in the present exemplary embodiment, both ends of the sheet in the direction orthogonal to the sheet feeding direction are curved downward when the sheet is fed and the separation claw  24  is moved so that the front end of the pressing portion  24   a  of the separation claw  24  becomes lower than the uppermost sheet P 1 . 
     This allows the uppermost sheet P 1  to be surely fed along the top surface of the separation claw  24  without catching the front end of the pressing portion  24   a  of the separation claw  24  because the front end of the pressing portion  24   a  of the separation claw  24  is lower in position than the uppermost sheet P 1 , when the sheet is fed in the sheet feeding direction after the sheet is reversely fed. This allows the sheet to be stably separated and fed, and the sheet feeding apparatus to be downsized, and also allows the sheet to be stably fed without being broken and failing to be fed. 
     A second exemplary embodiment of the present invention is described below.  FIG. 10  is a perspective view illustrating a configuration of a sheet feeding apparatus according to the present exemplary embodiment. In  FIG. 10 , the same reference numerals and characters as those in  FIG. 2  represent the same or corresponding portions. 
     In  FIG. 10 , depression arms  36  are provided at both ends of the drive shaft  31  of the feeding roller  22  rotated by the sheet feeding motor PM illustrated in  FIG. 4  via a bidirectional torque limiter  37 . 
     The depression arms  36  are provided at both ends of the drive shaft  31  via the bidirectional torque limiter  37  to be rotated in the directions indicated by arrows L and R along with the drive of the feeding roller  22 . 
     Rollers  38  are rotatably fixed to the tip ends of the depression arms  36  and arranged at positions where the rollers  38  abut on the top surface of both ends of the sheet P stacked in the sheet feeding cassette  21  in the direction orthogonal to the sheet feeding direction. 
     When the depression arm  36  is rotated in the direction indicated by the arrow L in  FIG. 10 , the roller  38  depresses downward the end of the sheet P stacked in the sheet feeding cassette  21  to curve the sheet P while depressing the side base plate  26 . At this point, the center base plate  25  is not depressed and kept at a position where the sheet P is urged against the feeding roller  22 . 
     Thus, in the present exemplary embodiment, the sheet is curved so that the central portion thereof is elevated and both ends thereof are lowered in the direction orthogonal to the sheet feeding direction. The front end of the pressing portion  24   a  of the separation claw  24  is moved to a position lower than the uppermost sheet along with the side end portion of the curved sheet. 
     The curvature formation portion in the present exemplary embodiment is formed of the depression arms  36 , the drive shaft  31 , and the sheet feeding motor PM. An upper stopper  39  and a lower stopper  40  regulate the rotation range of the depression arms  36 , are arranged in a sheet feeding apparatus main body (not illustrated) formed of the printer main body  100 A, and extended to the cassette side. 
     The sheet feeding operation of the thus configured sheet feeding apparatus  200  is described below with reference to a flow chart illustrated in  FIG. 11 , operation diagrams illustrated in  FIGS. 12A ,  12 B,  13 A,  13 B,  14 , and  15 , and a timing chart illustrated in  FIG. 16 . 
       FIGS. 12A and 12B  are diagrams illustrating an initial state of the sheet feeding operation of the sheet feeding apparatus  200 . At this point, the sheet P is stacked in the sheet feeding cassette  21  with the upper surface thereof flat. The sheet feeding operation starts with this state as an initial state. 
     In step S 11 , the CPU  60  reversely rotates the feeding roller  22  in the reverse feeding direction (in the direction indicated by the arrow L) by a predetermined amount. In step S 11 , the feeding roller  22  continues rotating until the leading end of the uppermost sheet P 1  passes by the front end of the pressing portion  24   a  of the separation claw  24 . In the present exemplary embodiment, the number of rotations of the feeding roller  22  (or the amount of conveyance of the sheet) is regulated by time control of the timer  61  in the CPU  60 . 
     The feeding roller  22  is reversely rotated to cause the uppermost sheet P 1  to be fed in the reverse feeding direction to abut on the back end wall  21   a  of the sheet feeding cassette  21 , and movement in the reverse feeding direction is regulated. For this reason, as illustrated in  FIG. 13A , the uppermost sheet P 1  is curved between the feeding roller  22  and the back end wall  21   a  of the sheet feeding cassette  21  to separate the uppermost sheet P 1  from the second and subsequent sheets P 2 . 
     The feeding roller  22  still continues rotating. When the leading end of the uppermost sheet P 1  passes by the front end of the pressing portion  24   a  of the separation claw  24 , in step S 12 , the depression arm  36  rotated along with the feeding roller  22  presses downward the second and subsequent sheets P 2 . This depresses the second and subsequent sheets P 2  and the side base plate  26  to curve all of the second and subsequent sheets P 2 . 
     After that, as illustrated in  FIG. 13B , the depression arm  36  abuts on the lower stopper  40  and then stops rotating. 
     As described above, the depression arm  36  is attached to the drive shaft  31  via the bidirectional torque limiter  37 , so that, even if the feeding roller  22  continues rotating, the transmission of rotation to the depression arm  36  is shut off to stop only the depression arm  36 . The feeding roller  22  continues rotating by a predetermined amount even after the leading end of the uppermost sheet P 1  passes by the front end of the pressing portion  24   a  of the separation claw  24 . 
     When the side base plate  26  is depressed, the height of the center base plate  25  remains unchanged, so that a step is produced between the side base plate  26  depressed by the depression arm  36  via the sheet P and the center base plate  25 . Thereby, the second and subsequent sheets P 2  stacked in the sheet feeding cassette  21  are curved along the step produced between the center base plate  25  and the side base plate  26 . 
     At this point, as illustrated in  FIG. 13B , the uppermost sheet P 1  is curved along the sheet feeding direction on its trailing edge side, so that the uppermost sheet P 1  is hardly curved in the direction orthogonal to the direction in which the second and subsequent sheets P 2  are fed as illustrated in  FIG. 14 , which is a diagram viewed from the direction indicated by an arrow B in  FIG. 13B . 
     Thus, the uppermost sheet P 1  is curved along the sheet feeding direction and the second and subsequent sheets P 2  are curved along the step to produce the gap G between the uppermost sheet P 1  and the top surface of the second and subsequent sheets P 2 . 
     The front end of the pressing portion  24   a  abuts on the top surface of the sheet P 2 , so that the gap G is also substantially produced between the uppermost sheet P 1  and the front end of the pressing portion  24   a.  In other words, the uppermost sheet P 1  is separated from the pressing portion  24   a  of the separation claw  24 . 
     As illustrated in  FIG. 15 , in step S 13 , the feeding roller  22  is forwardly rotated in the sheet feeding direction (in the direction indicated by the arrow R) by a predetermined amount to feed the upper sheet P 1 . The pressing portion  24   a  of the separation claw  24  is lower in position than the uppermost sheet P 1  and separated therefrom. Therefore, the uppermost sheet P 1  climbs up the separation claw  24  to be sent out without abutting on the front end of the pressing portion  24   a  of the separation claw  24 . 
     In step S 13 , the depression arm  36  is rotated in the direction in which the feeding roller  22  is rotated, i.e., in the direction in which a curvature amount is decreased to abut on the upper stopper  39 . Thereafter, even if the feeding roller  22  continues rotating, only the depression arm  36  is stopped by the effect of the bidirectional torque limiter  37 . 
     The length of the depression arm  36  and the diameter of the feeding roller  22  are controlled so that the uppermost sheet P 1  climbs up the separation claw  24  at a stage where the curvature amount of the sheet is zero or before the sheet feeding operation retunes to the initial state illustrated in  FIG. 12A . 
     In other words, the length of the depression arm  36  and the diameter of the feeding roller  22  are controlled so that the uppermost sheet P 1  climbs up the separation claw  24  at a stage where the curvature amount of the sheet is zero or the gap G lies between the uppermost sheet P 1  and the second and subsequent sheets P 2 . 
     After that, in step S 14 , the separated uppermost sheet P 1  is sent out, and when the uppermost sheet P 1  passes by the feeding roller  22 , the drive of the feeding roller  22  is stopped. Thus, only the uppermost sheet P 1  among the sheets P stacked in the sheet feeding cassette  21  is fed. In this step, the sheet feeding apparatus  200  returns to the initial state illustrated in  FIG. 12A , and thereafter, by repeating steps  11  to  14 , the second and subsequent curved sheets P 2  are fed. 
     As described above, in the present exemplary embodiment, when the sheet is fed along the upper surface of the separation claw  24 , both ends of the sheet are curved by the depression arms  36  and the pressing portion  24   a  of the separation claw  24  is moved to a position lower than the uppermost sheet P 1 . This allows the sheet to be stably separated and fed, and the sheet feeding apparatus to be downsized, and also allows the sheet to be stably fed without being broken and failing to be fed. 
     In the second exemplary embodiment, although the torque limiter is used to operate the feeding roller  22  along with the depression arm  36 , the present invention is not limited thereto. The diameter of the feeding roller is adjusted to optimize the conveyance amount of the feeding roller and the movement angle of the depression arm, for example, allows eliminating the need for the torque limiter. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2011-283366 filed Dec. 26, 2011, which is hereby incorporated by reference herein in its entirety.