Patent Publication Number: US-8985571-B2

Title: Sheet feeding apparatus and image forming apparatus

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 of a side end regulating portion which regulates positions of side ends of sheets accommodated in a sheet cassette detachably mounted on the sheet feeding apparatus. 
     2. Description of the Related Art 
     Today, in widely-used image forming apparatuses including a copying machine, a printer, and a facsimile, sheets are fed to an image forming portion by a sheet feeding apparatus to form images. According to such a sheet feeding apparatus, a sheet cassette is usually detachably mounted on an apparatus main body and sheets accommodated in the sheet cassette are automatically fed to the image forming portion. 
     According to a sheet cassette used for such a sheet feeding apparatus, for example, a middle plate on which sheets are stacked and which presses the sheets against a feed roller is provided so as to be movable upward and downward. Further, in a sheet cassette with such a middle plate, a rear end regulating portion which regulates positions of side ends of sheets on upstream (hereinafter, rear ends) in a sheet feeding direction accommodated on the middle plate in a stacked manner is slidably provided so that sheets of different sizes can be accommodated. Further, a pair of side end regulating portions which regulates positions of side ends of sheets in a direction perpendicular to a sheet feeding direction (hereinafter, a width direction) is also provided. 
     The pair of side end regulating portions regulates the side end of a sheet and the rear end regulating member regulates the rear end of the sheet, so that the position of the sheet is always at a predetermined position. Consequently, when the sheet cassette is accommodated in the apparatus main body, sheets are stably fed regardless of a sheet size. 
     According to a sheet cassette with such a side end regulating portion, one of side end regulating portions is fixed and the other can slide in the width direction. The one fixed side end regulating portion (hereinafter, fixed side end regulating portion) serves as a reference surface in the width direction of a sheet. Sheets are fed along the fixed side end regulating portion, so that predetermined printing precision is accomplished. 
     According to such a sheet cassette, when a direction of mounting/detaching the sheet cassette is the width direction, at the time when the sheet cassette is set, sheets are sometimes shifted because of an inertial force to be separated from the fixed side end regulating portion. Conventionally, among a pair of side end regulating portions, a slidable side end regulating portion (hereinafter, movable side end regulating portion) is provided with a sheet pressing portion to which a force is applied by a biasing unit such as a spring for pressing sheets. When sheets are shifted, this sheet pressing portion presses the shifted sheets back to the fixed side end regulating portion. 
     External dimensions of sheets vary. For example, when the tolerance of a width direction dimension of a sheet is ±2 mm, the position where the sheet pressing portion protrudes must be in the range of ±2 mm at minimum with respect to the position of a width direction end of a normal sheet. Accordingly, the position of the movable side end regulating portion at the time of regulating the side end of a sheet must be determined in view of variations in external dimensions of sheets. When the position of the movable side end regulating portion is set as described above, when sheets are not stacked, the position where the sheet pressing portion protrudes enters the center of sheets at least by 2 mm. 
     To ensure a sheet regulating force by the side end regulating portion, a biasing force to be applied to the sheet pressing portion is sometimes increased. As the stacking amount of sheets is reduced, the rigidity of a sheet bundle is also reduced. Accordingly, when the biasing force to be applied to the sheet pressing portion is increased as mentioned above, as the stacking amount of sheets is reduced, the sheets are deformed, so that the sheets cannot be fed stably. When the biasing force to be applied to the sheet pressing portion is reduced so that the sheets are not deformed even if the stacking amount of sheets is reduced, a force of regulating positions of sheets when sheets are fully stacked is insufficient. 
     Devices for preventing deformation of sheets when fewer sheets are stacked while increasing a pressing force upon the fully stacked sheets are conventionally suggested. For example, a configuration which uses a link to positively vary the pressing force by the sheet pressing portion and reduces the pressing force when the stacking amount of sheets is reduced is suggested (see Japanese Patent Laid-Open No. 2000-118730). However, this configuration increases components, resulting in an increase in cost. A configuration that a relief is formed in a sheet pressing portion so that components are not increased and this relief reduces a pressing force by the sheet pressing portion or the relief does not contact the sheets so that the sheets are not pressed when the stacking amount of sheets is reduced is suggested (see Japanese Patent Laid-Open No. 2000-219330). 
     According to these conventional sheet feeding apparatuses and image forming apparatuses, a plurality of sheet pressing portions is sometimes provided on the movable side end regulating portion along the sheet feeding direction. As illustrated in  FIGS. 8A ,  8 B,  9 A, and  9 B, sheets having different sheet feeding direction lengths are sometimes accommodated in a sheet cassette.  FIGS. 8A and 8B  illustrate a state that A4 size (297 mm×210 mm/ISO 216) sheets are accommodated in the sheet cassette.  FIGS. 9A and 9B  illustrate a state that A5 size (210 mm×148 mm/ISO 216) sheets are accommodated in the sheet cassette. 
     As illustrated in  FIG. 8A , A4 size sheets Sa are stacked along a side regulating plate  714  serving as the fixed side end regulating portion that forms a reference surface which is a sheet conveyance reference in the width direction. A force is applied to the A4 size sheets Sa toward the side regulating plate  714  by sheet pressing portions  716  and  717  mounted on a side regulating plate  715  serving as an opposing movable side end regulating portion. 
     As illustrated in  FIG. 8B , a biasing member  801  is placed between the side regulating plate  715  and the sheet pressing portion  716  and a biasing member  802  is placed between the side regulating plate  715  and the sheet pressing portion  717 . These biasing members  801  and  802  enable the sheet pressing portions  716  and  717  to apply biasing forces P 81  and P 82  to the A4 size sheets Sa. 
     The biasing force P 81  by the sheet pressing portion  716  on upstream in a sheet feeding direction is a biasing force required to be applied to the stacked sheets Sa with respect to the side regulating plate  714  so that the sheets are in a predetermined attitude, and is, e.g., 1.0 N to 3.0 N. The biasing force P 82  by the sheet pressing portion  717  on downstream in a sheet feeding direction with respect to the sheet pressing portion  716  is applied to a fed sheet Sa at the time of sheet feeding to maintain the attitude of the sheet Sa to be in the predetermined attitude. This biasing force P 82  is such that sheets are not deformed and is required to be 0.1 N to 0.5 N. The position where the biasing force P 81  acts is preferably near the center of gravity of a sheet, i.e., a sheet feeding direction center of a side surface of the sheet (so the center, in the sheet feeding direction, of a side surface of the sheet) so that the sheet is not rotated about the sheet pressing portion  716  because of an impact when a sheet cassette is inserted. 
     The biasing force P 81  by the sheet pressing portion  716  is applied to the A4 size sheet Sa. In this case, the attitude of the sheet Sa is determined by a triangular area connecting ends M 81  and M 82  of the side regulating plate  714  and a point of application of the biasing force P 81 . Assume that the sheet pressing portion  716  is the center of rotation of the sheet Sa and rotation of the sheet Sa is prevented by the side regulating plate  714 . 
     As illustrated in  FIG. 8B , when the area connecting the ends M 81  and M 82  of the side regulating plate  714  and the point of application of the biasing force P 81  is an acute triangle, an effect that the side regulating plate  714  prevents the rotation of the sheet Sa with respect to a moment of rotation of the sheet Sa can be expected. The position where the biasing force P 81  is applied by the sheet pressing portion  716  is generally in the range of ¼ to ½ of the length in the sheet feeding direction of the A4 size sheet Sa. Referring to  FIG. 8B , the position where the biasing force P 81  acts is set to ½ of the length in the sheet feeding direction of the A4 size sheet Sa, i.e., the sheet feeding direction center of the A4 size sheet Sa. 
     As illustrated in  FIG. 9A , A5 size sheets Sb are stacked along the side regulating plate  714  like the A4 size sheets Sa. A force is applied to the A5 size sheets Sb toward the side regulating plate  714  by the sheet pressing portions  716  and  717 . As illustrated in  FIG. 9B , the sheet pressing portions  716  and  717  apply biasing forces P 81  and P 82  to the A5 size sheets Sb, respectively. 
     As described above, the position of the sheet pressing portion  716  is determined in view of the attitude and position of the A4 size sheet Sa. When the position of the sheet pressing portion  716  is set to such a position, as illustrated in  FIG. 9B , the biasing force P 81  is applied to a sheet feeding direction rear end of the A5 size sheet Sb and the A5 size sheet Sb is supported by an area from M 83  to M 84  with respect to the side regulating plate  714 . M 83  indicates a corner of the A5 size sheet Sb at a rear end in the sheet feeding direction. In this case, an area connecting M 83 , M 84 , and the point of application of the biasing force P 81  is substantially a right-angled triangle. 
     Assume that the sheet pressing portion  716  is the center of rotation of the A5 size sheet Sb and the rotation of the sheet Sb is prevented by the side regulating plate  714 . In this case, when the area connecting M 83 , M 84 , and the point of application of the biasing force P 81  is a right-angled triangle, a unit which resists to a moment of counterclockwise rotation of the A5 size sheet Sb is not provided. Accordingly, the attitude of the sheet Sb cannot be maintained. As described above, because the biasing force P 82  by the sheet pressing portion  717  is significantly reduced, the moment of rotation cannot be supported by the biasing force P 2  by the sheet pressing portion  717 . 
     When the stacking amount of the A5 size sheets Sb is reduced and thus the rigidity of a sheet bundle is reduced, the sheet feeding direction rear end of a sheet Sb is deformed by the biasing force P 81  by the sheet pressing portion  716 . Consequently, as illustrated by a line Sb′ in  FIG. 9B , a front end of the sheet Sb is pressed out and the attitude of the sheet Sb cannot be maintained. 
     As described above, according to the conventional configuration, when the size (length in the sheet feeding direction) of accommodated sheets is different, positional shifts caused by an impact at the time of mounting/detaching a sheet cassette and deformation of sheets and changes in attitude of the sheets caused by a biasing force cannot be prevented for sheets of all corresponding sizes. For example, the sheet pressing portion can be provided for each sheet size. However, in this case, the configuration is complicated and an increase in cost is problematic. 
     The present invention has been achieved in view of the above problems, and provides a sheet feeding apparatus and an image forming apparatus that can prevent changes in attitude of sheets when a sheet cassette is mounted and deformation of the sheets and changes in attitude of the sheets after the sheet cassette is mounted regardless of a sheet size. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet feeding apparatus that feeds sheets, accommodated in a sheet cassette which is detachably mounted on an apparatus main body, in a sheet feeding direction by a sheet feeding portion, the sheet cassette including: a cassette main body arranged to accommodate a stack of sheets stacked in a sheet stacking direction; a pair of opposing side end regulating portions which are provided in the cassette main body and are arranged to regulate the position of the side ends of the sheets in a width direction perpendicular to the sheet feeding direction, wherein at least one of the side end regulating portions is movable in the width direction; a sheet pressing portion provided on one of the side end regulating portions and arranged to press a side end of the stack of sheets to press the sheets against a reference surface provided on the other side end regulating portion serving as a sheet conveyance reference; wherein the sheet pressing portion comprises an indented portion extending upwardly in the sheet stacking direction from a lower end of the sheet pressing portion. 
     According to the present invention, the sheet pressing portion is provided with the indented portion against which an upstream end of a sheet in the sheet feeding direction does not abut. Accordingly, changes in attitude of sheets when the sheet cassette is mounted and deformation of the sheets and changes in attitude of the sheets after the sheet cassette is mounted can be prevented regardless of the sheet size. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of a laser printer which is an example of an image forming apparatus which includes a sheet feeding apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a diagram for describing the configuration of a sheet cassette which is detachably provided in a printer main body of the laser printer; 
         FIGS. 3A and 3B  are diagrams illustrating a state when A4 size sheets are stacked in the sheet cassette; 
         FIGS. 4A and 4B  are diagrams illustrating a state when A5 size sheets are stacked in the sheet cassette; 
         FIG. 5  is a perspective view of a sheet cassette mounted on a sheet feeding apparatus according to a second embodiment of the present invention; 
         FIG. 6  is a perspective view of a sheet cassette mounted on a sheet feeding apparatus according to a third embodiment of the present invention; 
         FIG. 7  is a plan view of a sheet cassette mounted on a sheet feeding apparatus according to a fourth embodiment of the present invention; 
         FIGS. 8A and 8B  are diagrams illustrating a state when A4 size sheets are stacked in a conventional sheet cassette; and 
         FIGS. 9A and 9B  are diagrams illustrating a state when A5 size sheets are stacked in the conventional sheet cassette. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments for carrying out the present invention are described below in detail with reference to the drawings.  FIG. 1  is a diagram illustrating the configuration of a laser printer which is an example of an image forming apparatus which includes a sheet feeding apparatus according to a first embodiment of the present invention. The laser printer (hereinafter, printer) is indicated by reference numeral  300 . A laser printer main body (hereinafter, printer main body) is indicated by reference numeral  300 A. This printer main body  300 A includes an image forming portion  301  which forms images on sheets S and a sheet feeding apparatus  300 B which feeds the sheets S from a sheet cassette  307 . 
     The image forming portion  301  includes scanner units  303  and four process cartridges  302  which form toner images of four colors, i.e., yellow (Y), magenta (M), cyan (C), and black (Bk). The image forming portion  301  also includes an intermediate transfer unit  300 C placed above the process cartridges  302 . Each process cartridge  302  includes a photosensitive drum  3021 . 
     The intermediate transfer unit  300 C includes an intermediate transfer belt  304  and primary transfer rollers  305  that are provided inside the intermediate transfer belt  304  and abut the intermediate transfer belt  304  at positions opposing the respective photosensitive drums  3021 . By applying a positive transfer bias to the intermediate transfer belt  304  by the primary transfer rollers  305 , negative toner images of the respective colors on the photosensitive drums are successively multi-layer transferred to the intermediate transfer belt  304 . A full color image is thus formed on the intermediate transfer belt. 
     An image forming operation of the printer  300  with such a configuration will now be described. When the image forming operation starts, the scanner unit  303  irradiates laser light based on image information from a personal computer (not illustrated) and successively exposes a surface of the photosensitive drum  3021  uniformly charged to a predetermined polarity/potential with the laser light, so that an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image is then developed by a toner and thus visualized. 
     A yellow toner image, a magenta toner image, a cyan toner image, and a black toner image formed on the photosensitive drums  3021  of the respective process cartridges  302  are then transferred to the intermediate transfer belt  304  by the primary transfer rollers  305  in the respective primary transfer portions. A full color toner image is thus formed on the intermediate transfer belt  304 . 
     In parallel with the toner image forming operation, a sheet S accommodated in the sheet cassette  307  is fed by a feed roller  308  serving as a sheet feeding portion placed near a front end of the sheet S. Skew feeding of this sheet S is corrected by a pair of registration rollers  309  and then the pair of registration rollers  309  is driven so that the full color toner image on the intermediate transfer belt is registered with respect to the sheet S in a secondary transfer portion  310 . 
     The sheet S is conveyed to the secondary transfer portion  310  by the pair of registration rollers  309  so that the full color toner image on the intermediate transfer belt  304  is registered with respect to the sheet S in the secondary transfer portion  310 . The full color toner image is thus transferred to the sheet S at a time in the secondary transfer portion  310 . The sheet S to which the full color toner image is transferred as described above is then conveyed to a fixing portion  311 . Application of heat and pressure to the sheet S in this fixing portion  311  makes toners of the respect colors melt and mix with each other, so that the toners are fixed on the sheet S as a full color image. The sheet S which passes through the fixing portion  311  is then discharged to a discharge tray  320  provided on a top surface of the printer main body with its image surface facing downward. 
     The sheet cassette  307  is detachably provided in the printer main body  300 A which is also a main body of the sheet feeding apparatus  300 B. When sheets are supplemented (added), as illustrated in  FIG. 2 , the sheet cassette  307  is drawn from the printer main body  300 A in a width direction perpendicular to a sheet feeding direction. 
     As illustrated in  FIG. 2 , a sheet stacking plate  313  serving as a sheet stacking portion is placed in a frame  312  which forms a main body of the sheet cassette  307  so as to be rotatable about support portions  313   a  and  313   b  in a vertical direction. Every time a stacked sheet is fed, a side end of the sheet stacking plate  313  on downstream in the sheet feeding direction is rotated upward by a biasing unit (not illustrated) between the frame  312  and the sheet stacking plate  313 . 
     A pair of side regulating plates  314  and  315  which is a pair of side end regulating portions for regulating a position of a width direction side end of the sheet S is further placed in the frame  312  so that the plates  314  and  315  oppose with each other. The side regulating plate  314  is fixed to the frame  312 . The fixed side regulating plate  314  serving as a fixed side end regulating portion forms a reference surface of the sheet S in the width direction. When the sheet S is fed, it is fed along the fixed side regulating plate  314 , so that predetermined printing precision is accomplished. 
     The side regulating plate  315  can slide in the width direction and is configured to slide in the width direction according to the size of accommodated sheets. While the side regulating plate  315  which is at least one of the side regulating plates  314  and  315  and serves as the side end regulating portion is movable in the first embodiment, the side regulating plate  314  which is the other side end regulating portion can be also movable. 
     Sheet pressing portions  316  and  317  that press sheets are provided on an inner wall surface of the movable side regulating plate  315  serving as the movable side end regulating portion. These sheet pressing portions  316  and  317  press the sheet S stacked on the sheet stacking plate  313  against the opposing fixed side regulating plate  314 . By these sheet pressing portions  316  and  317 , when the sheet S is shifted in the width direction to be separated from the fixed side regulating plate  314  because of an impact at the time of accommodating (inserting) the sheet cassette  307 , the sheet S can be pressed back to the fixed side regulating plate  314 . 
     Sheets having different lengths in the sheet feeding direction, i.e., A4 size and A5 size sheets in the first embodiment can be accommodated in the sheet cassette  307 .  FIGS. 3A and 3B  illustrate a state that A4 size sheets Sa are accommodated in the sheet cassette  307 . As illustrated in  FIG. 3A , the A4 size sheets Sa are stacked along the side regulating plate  314  which forms the reference surface serving as a conveyance reference in the width direction at the time of conveying sheets. A force is applied to the A4 size sheets Sa toward side regulating plate  314  by the sheet pressing portions  316  and  317  mounted on the side regulating plate  315 . 
     As illustrated in  FIG. 3B , biasing members  101  and  102  such as springs are placed between the side regulating plate  315  and the sheet pressing portion  316  and between the side regulating plate  315  and the sheet pressing portion  317 , respectively. By these biasing members  101  and  102 , the sheet pressing portion  316  applies a biasing force P 11  and the sheet pressing portion  317  applies a biasing force P 12  to an A4 size sheet Sa. 
     According to the first embodiment, a support hole  104  into which a support shaft  103  provided on the sheet pressing portion  316  on a upstream in the sheet feeding direction is inserted is formed in the side regulating plate  315 . The inner diameter of the support hole  104  is larger than the outline of the support shaft  103  by a predetermined quantity. The sheet pressing portion  316  is thus supported by the side regulating plate  315  so as to be movable in the sheet width direction and swingable in a horizontal direction indicated by an arrow by a difference between the diameter of the support shaft  103  and the diameter of the support hole  104 . 
     The biasing force P 11  by the sheet pressing portion  316  is a biasing force required to be applied to the stacked sheet Sa to obtain a predetermined attitude with respect to the side regulating plate  314 , e.g., 1.0 N to 3.0 N. A position where the biasing force P 11  acts is preferably near the center of gravity of the sheet Sa, i.e., a sheet feeding direction center of a side surface of the sheet so that the sheet Sa is not rotated about the sheet pressing portion  316  because of an impact at the time of inserting the sheet cassette. 
     The biasing force P 12  by the sheet pressing portion  317  positioned on downstream in a sheet feeding direction with respect to the sheet pressing portion  316  is a force, at the time of feeding a sheet, applied to the fed sheet Sa for maintaining a predetermined attitude of the sheet Sa. The biasing force P 12  is such that the sheet Sa is not deformed and about 0.1 N to 0.5 N is required. 
     The biasing force P 11  by the sheet pressing portion  316  is applied to the A4 size sheet Sa toward the side regulating plate  314 . In this case, the attitude of the sheet Sa is determined by a triangular area connecting ends M 11  and M 12  of the side regulating plate  314  and a point of application of the biasing force P 11  on the axis of the support shaft  103 . Assume that the sheet pressing portion  316  is the center of rotation of the A4 size sheet Sa and the rotation of the A4 size sheet Sa is prevented by the side regulating plate  314 . 
     When the area connecting the ends M 11  and M 12  of the side regulating plate  314  and the point of application of the biasing force P 11  is an acute triangle, the effect that the side regulating plate  314  prevents the rotation of the A4 size sheet Sa can be expected. The position where the biasing force P 11  by the sheet pressing portion  316  acts is thus set to ½ of the length in the sheet feeding direction of the A4 size sheet Sa, i.e., the sheet feeding direction center. 
       FIGS. 4A and 4B  illustrate a state that short A5 size sheets Sb whose upstream ends in the sheet feeding direction are pressed by the sheet pressing portion  316  are accommodated in the sheet cassette  307 . As illustrated in  FIG. 4A , the A5 size sheets Sb are stacked along the side regulating plate  314  and a force is applied to the A5 size sheets Sb toward the side regulating plate  314  by the sheet pressing portions  316  and  317  mounted on the side regulating plate  315 . 
     As illustrated in  FIGS. 3A ,  3 B,  4 A and  4 B, an indented or depressed (concave) configuration  316   c  which extends in the vertical direction and against which, when the A5 size sheets Sb are stacked, corners of the sheets Sb at rear ends in sheet feeding direction do not abut is provided at the sheet conveying direction center of a sheet side of the sheet pressing portion  316 . Sheet biasing portions  316   a  and  316   b  that abut the side ends of the sheets to apply a force to the sheets are provided on sides of the indented configuration  316   c . The indented configuration  316   c  which is an indentation or depression is formed from a bottom end to a top end of the sheet pressing portion  316  at a position opposing the corners of the A5 size sheets Sb at the rear ends in the sheet feeding direction. 
     Among the sheet biasing portions  316   a  and  316   b  serving as a side end pressing portion, the sheet biasing portion  316   a  on upstream in the sheet feeding direction does not apply a force to an A5 size sheet Sb. By forming the indented configuration  316   c  in the sheet pressing portion  316 , a force is applied to the downstream of the A5 size sheet Sb with respect to the corner of the sheet Sb at the rear end in the sheet feeding direction by the sheet biasing portion  316   b  of the sheet pressing portion  316  on downstream in the sheet feeding direction. 
     As illustrated in  FIG. 4B , the positional relationship between the support shaft  103  of the sheet pressing portion  316  to which the biasing member  101  applies a force and the sheet biasing portion  316   b  is such that the sheet biasing portion  316   b  is offset in the sheet feeding direction by a length L. As described above, the sheet pressing portion  316  can rotate about the support shaft  103  in parallel with the sheet feeding direction indicated by the arrow, i.e., in the horizontal direction. Accordingly, even when the outline dimension of the sheets Sb varies, a force can be uniformly applied to the side ends of the sheets Sb. 
     A biasing force P 11 ′ only by the sheet biasing portion  316   b  is smaller than the biasing force P 11  by the two sheet biasing portions  316   a  and  316   b . When the A5 size sheets Sb are accommodated, a force is applied to the sheets Sb only by the sheet biasing portion  316   b , so that the biasing force P 11 ′ can be set to be small. The biasing force P 11 ′ by the sheet biasing portion  316   b  can correspond to various conditions according to the length L, the positional relationship between the sheet pressing portion  316  and a sheet, and the biasing force by the biasing member  101 . 
     An operation of pressing the A5 size sheet Sb by the sheet pressing portion  316  with such a configuration will now be described. When the sheet cassette  307  is mounted, the biasing force P 11 ′ is applied to the rear end of the A5 size sheet Sb. Further, the A5 size sheet Sb is supported with respect to the opposing side regulating plate  314  by an area from ends M 13  to M 14  of the side regulating plate  314  illustrated in  FIG. 4B . The end M 13  of the side regulating plate  314  is a corner of the A5 size sheet Sb at the rear end in the sheet feeding direction. In this case, an area connecting the ends M 13  and M 14  and the point of application of the biasing force P 11 ′ is an acute triangle. 
     Assume that the sheet biasing portion  316   b  of the sheet pressing portion  316  is the center of rotation of the A5 size sheet Sb and the rotation of the A5 size sheet Sb is prevented by the side regulating plate  314 . In this case, when the area connecting the ends M 13  and M 14  and the point of application of the biasing force P 11 ′ is an acute triangle, the effect that the side regulating plate  314  prevents the rotation of the A5 size sheet Sa can be expected. 
     Even when the stacking amount of the A5 size sheets Sb is reduced and thus the rigidity of a sheet bundle is reduced, the sheet bundle is not deformed and the attitude of the sheet bundle can be maintained because the sheet biasing portion  316   b  does not press the easily deformable rear ends. Because the biasing force P 11 ′ by the sheet biasing portion  316   b  is smaller than the biasing force P 11 , it is advantageous to the deformation of the sheet bundle. 
     As described above, according to the first embodiment, the indented configuration  316   c  is provided in the sheet pressing portion  316  so that the corner at the rear end of the A5 size sheet Sa whose length in the sheet feeding direction is shorter among two types of sheets, i.e., A4 and A5 size sheets is not pressed (a force is not applied to the corner). The attitude of the A5 size sheet Sb can be stabilized at the time of accommodating and feeding the sheet Sb, as well as the A4 size sheet Sa. 
     Namely, a force is not applied to a corner at the rear end of a sheet whose length in the sheet feeding direction is shorter among at least two types of sheets. Changes in attitude of the sheet caused by an impact at the time of mounting/detaching the sheet cassette  307 , the deformation of the sheet, and changes in attitude of the sheet during feeding can be prevented. While the case of providing one indented configuration  316   c  in the sheet pressing portion  316  is described, when the other shorter sheet is accommodated, the other indented configuration can be provided at the position where a force is not applied to a corner at the rear end of the sheet. 
     A second embodiment of the present invention will now be described.  FIG. 5  is a perspective view of a sheet cassette mounted on a sheet feeding apparatus of the second embodiment. The same reference numerals in  FIG. 5  as those of the above-described  FIGS. 3A and 3B  indicate the same or corresponding parts. 
     Referring to  FIG. 5 , a sheet pressing portion  401  is placed on upstream in the sheet feeding direction of the side regulating plate  315 . An indented configuration  401   c  which is an indentation or depression formed at a sheet feeding direction center and extending in a vertical direction and sheet biasing portions  401   a  and  401   b  that are placed on sides of the indented configuration  401   c  and abut a side end of a sheet to apply a force to the sheet are provided on a sheet side of the sheet pressing portion  401 . The indented configuration  401   c  is formed at a position opposing a corner of the A5 size sheet Sb at the rear end in the sheet feeding direction. 
     According to the second embodiment, a lower portion of the sheet biasing portion  401   a  on upstream in the sheet feeding direction, e.g., a portion from the top surface of the sheet stacking plate  313  to a predetermined height h 1  is cut out. This cutout portion is continuous with the indented configuration  401   c . Namely, the sheet biasing portion  401   a  on upstream in the sheet feeding direction is placed at a position higher than the top surface of the sheet stacking plate  313  by the predetermined height h 1 . 
     In the case of stacking the A4 size sheets Sa, when the stacking amount of sheets is equal to or lower than the height h 1  from the top surface of the sheet stacking plate  313 , the sheet biasing portion  401   a  does not apply a force to the sheets Sa. As a result, a force to be applied to the A4 size sheets Sa is reduced. Even in the case of reduced stacking amount of sheets that the rigidity of a sheet bundle is relatively reduced, the sheets are not deformed, the sheet bundle is stabilized, and its attitude can be maintained. 
     As described above, by providing the sheet biasing portion  401   a  on upstream in the sheet feeding direction at a position higher than the top surface of the sheet stacking plate  313  by the predetermined height h 1 , the attitude of the A4 size sheet Sa can be also stabilized. Regardless of the sheet size, desired printing precision and feed performance can be realized. 
     A third embodiment of the present invention will now be described.  FIG. 6  is a perspective view of a sheet cassette mounted on a sheet feeding apparatus of the third embodiment. The same reference numerals in  FIG. 6  as those of above-described  FIGS. 3A and 3B  indicate the same or corresponding parts. 
     Referring to  FIG. 6 , a sheet pressing portion  501  is placed on upstream in the sheet feeding direction of the side regulating plate  315 . An indented configuration  501   c  which is a depression or indentation starting from the bottom end of the sheet pressing portion  501  and extending upward from the top surface of the sheet stacking plate  313  in a horizontal state to a predetermined height h 2  is formed at the sheet feeding direction center of a sheet side of the sheet pressing portion  501 . The indented configuration  501   c  formed upward from the bottom end of the sheet pressing portion  501  to the predetermined dimension is provided at a position opposing a corner of the A5 size sheet Sb at a rear end in the sheet feeding direction. 
     By forming such an indented configuration  501   c , a sheet biasing portion  501   a  of the sheet pressing portion  501  can apply a force to a corner of the A5 size sheet Sb at the rear end in the sheet feeding direction before the stacking amount of sheets is equal to or lower than the height h 2  from the top surface of the sheet stacking plate  313 . Even in the case of the A5 size sheet Sb, before the stacking amount of sheets reaches the height h 2  from the top surface of the sheet stacking plate  313 , the biasing force P 11  by the sheet pressing portion  501  can apply to the sheet Sb. 
     Even in the case of the A5 size sheet Sb, when the biasing force P 11  of the sheet pressing portion  501  is insufficient because of a large density of sheets, width direction movement of the sheet Sb can be regulated in the range of the stacking amount of sheets that the rigidity of a sheet bundle is high. When the stacking amount of sheets is reduced and thus the rigidity of the sheet bundle is also reduced, the indented configuration  501   c  opposes a corner of the sheet Sb at a rear end in the sheet feeding direction. Accordingly, the same effects as in the above-described first embodiment can be obtained. 
     As described above, according to the third embodiment, before the stacking height of sheets on the sheet stacking plate  313  is a predetermined one, the biasing force P 11  by the sheet pressing portion  501  can be applied to the sheet Sb and the width direction movement of the sheet Sb can be regulated. 
     A fourth embodiment of the present invention will now be described.  FIG. 7  is a plan view of a sheet cassette mounted on a sheet feeding apparatus of the fourth embodiment. The same reference numerals in  FIG. 7  as those of above-described  FIGS. 3A and 3B  indicate the same or corresponding portions. 
     Referring to  FIG. 7 , two (a plurality of) biasing members  601   a  and  601   b  are provided between the sheet pressing portion  316  and the side regulating plate  315 . When A5 size sheets are stacked on the sheet stacking plate  313 , by providing an indented configuration  316   c  which is a depression or indentation in the sheet pressing portion  316 , a biasing force to be applied to the sheet Sb only by the sheet biasing portion  316   b  can be a biasing force only by the biasing member  601   b.    
     A biasing force which is applied to the rear end of an A5 size sheet whose length in the sheet feeding direction is short can thus be optimized and a force can be applied to the rear end of the A5 size sheet with higher precision. A biasing force to be applied to the A4 size sheet Sa is set so that the sum of the forces by the two biasing members  601   a  and  601   b  is equal to that of P 11  illustrated in  FIG. 1 . 
     As described above, according to the fourth embodiment, the biasing force applied to the sheet Sb only by the sheet biasing portion  316   b  can be the biasing force only by the biasing member  601   b . The biasing force applied to the rear end of the A5 size sheet is thus optimized. Changes in attitude of sheets caused by an impact at the time of mounting the sheet cassette  307  and deformation of the sheets and changes in attitude of the sheets during feeding can be prevented in an optimized manner. 
     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 such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-254483, filed Nov. 15, 2010, which is hereby incorporated by reference herein in its entirety.