BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeding apparatus for feeding a sheet to an image forming apparatus, etc. 
     2. Related Background Art 
     FIG. 13 schematically shows the sectional construction of a sheet feeding apparatus  300  arranged in an image forming apparatus as a prior art. In FIG. 13, reference numerals  301 ,  302  and  303  respectively designate a sheet feeding tray receiving (storing) sheets, a middle plate, and a coil spring as a biasing means for biasing the middle plate  302  upward. The middle plate  302  is axially supported by the sheet feeding tray  301  and can stack paper sheets of all sizes which is available by the image forming apparatus. 
     Reference numerals  304  and  305  respectively designate a separating pad for preventing double feeding of sheets, and a sheet feeding roller. The sheet feeding roller  305  is formed in a fan shape for feeding (sending) out one sheet on the uppermost face of a sheet bundle stacked on the middle plate  302  by frictional force rotated by an unillustrated controlling means. 
     When a small diameter portion of the fan shape of the sheet feeding roller  305  is opposed to the separating pad  304 , a sheet feeding roller  306  separates the separating pad  304  and the sheet feeding roller  305  from each other and is rotatably held so that the sheet feeding roller  306  is rotated in accordance with a movement of the sheet. 
     The sheet fed from the sheet feeding roller  305  is conveyed by conveying roller pairs  307 ,  308  arranged in a sheet conveying path  309 . These conveying roller pairs  307 ,  308  further apply conveying force to the fed sheet and convey the sheet even when the rotation of the sheet feeding roller  305  is stopped. The conveying rollers  307  and  308  are respectively a driving roller rotated by driving force, and a conveying roller biased against the conveying roller  307  by an unillustrated biasing means and rotatable around a predetermined axis as a center. Reference numeral  310  designates a feeder frame for holding the sheet feeding roller  305 , etc., and attachably and detachably supporting the sheet feeding tray  301 . 
     Reference numeral  311  designates a presence/absence sensor flag for detecting the presence/absence of sheets on the middle plate  302 . Presence/absence information of the sheet can be inputted by this presence/absence sensor flag  311  to the image forming apparatus by switching operating states of an unillustrated detecting means. Reference numerals  312  and  313  respectively designate a rear end regulating plate and a side end regulating plate for determining a stacking position of the sheets stacked on the middle plate  302 . 
     However, in the above conventional example, the middle plate  302  has a rotatable one-plate structure supported by a supporting shaft so that the following problems exist. 
     (1) A weight of the sheets stacked onto the middle plate  302  is greatly changed in accordance with a sheet size. Therefore, force (=sheet feeding pressure) for pressing a sheet by the biasing means for biasing the middle plate  302  against the sheet feeding roller  305  varies, so that the sheet feeding pressure is changed in accordance with the sheet size. Accordingly, it was difficult to stably feed the sheet in accordance with various sheet sizes. 
     (2) It was also difficult to stably feed the sheet in accordance with various specific gravities since the sheet feeding pressure varies depending on the specific gravities of sheets even when the sheets have the same size. 
     (3) The above problems (1) and (2) become further notable in the sheet feeding tray of a large capacity in which the number of stackable sheets is large. 
     There is a case in which it is necessary for a user to adjust or switch the sheet feeding pressure to obtain a required sheet feeding pressure. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems inherent in the prior art, and therefore, an object of the present invention is to provide an apparatus for restraining a change in sheet feeding pressure in accordance with the size of a stacked sheet and a specific gravity of the sheet so that the paper can be stably fed. 
     The present invention is characterized by 
     sheet stacking means rotatably supported by an apparatus body and supporting a sheet; 
     sheet feeding means arranged on a leading end side in a sheet feeding-out direction of the sheet stacking means and feeding out the sheet supported by the sheet stacking means; 
     biasing means for pressing the sheet supported by the sheet stacking means against the sheet feeding means; and 
     load transmitting means for converting a load of the sheet applied to a rear end side in the sheet feeding-out direction of the sheet stacking means to a biasing force for biasing the leading end side of the sheet stacking means toward the sheet feeding means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view for explaining the sectional construction of a sheet feeding apparatus in accordance with a first embodiment of the present invention; 
     FIG. 2 is a view for explaining an operation of the sheet feeding apparatus in accordance with the first embodiment of the present invention; 
     FIG. 3 is a view for explaining the sectional construction of the sheet feeding apparatus in accordance with the first embodiment of the present invention (at a stacking time of the sheet of a small size); 
     FIG. 4 is a view for explaining the operation of the sheet feeding apparatus in accordance with the first embodiment of the present invention (when stacking sheets of a small size); 
     FIG. 5 is a view for explaining the sectional construction of the sheet feeding apparatus in accordance with the first embodiment of the present invention (without sheets stacked thereon); 
     FIG. 6 is a view for explaining the sectional construction of a sheet feeding apparatus in accordance with a second embodiment of the present invention; 
     FIG. 7 is a view for explaining the sectional construction of a sheet feeding apparatus in accordance with a third embodiment of the present invention; 
     FIG. 8 is a view for explaining an operation of the sheet feeding apparatus in accordance with the third embodiment of the present invention; 
     FIG. 9 is a view for explaining the sectional construction of an image forming apparatus having the sheet feeding apparatus in accordance with the first to third embodiments of the present invention; 
     FIG. 10 is a view for explaining the sectional construction of a sheet feeding apparatus in accordance with a fourth embodiment of the present invention; 
     FIG. 11 is a plan view of the sheet feeding apparatus in accordance with the fourth embodiment of the present invention; 
     FIG. 12 is a view for explaining the sectional construction of an image forming apparatus having the sheet feeding apparatus in accordance with the fourth embodiment of the present invention; and 
     FIG. 13 is a view for explaining the sectional construction of a conventional sheet feeding apparatus in a state in which sheets are stacked in the conventional sheet feeding apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will next be explained. 
     FIG. 1 is a view for explaining the sectional construction of a sheet feeding apparatus SF 1  to which the present invention is applied. This sheet feeding apparatus SF 1  is arranged in a lower portion of an image forming apparatus described later and supplies sheets to an image forming means. 
     FIG. 1 is a view for explaining the sectional construction of the sheet feeding apparatus SF 1  in a state in which a sheet bundle PL of a sheet size (size A4 in Japan and Europe, and letter size in USA) used in most cases is stacked in the sheet feeding apparatus SF 1 . 
     In FIG. 1, reference numeral  1  designates a sheet feeding tray (a sheet feeding cassette) detachably attachable to the sheet feeding apparatus SF 1  and holding the bundle of sheets to be fed. Reference numeral  2  designates a first middle plate as a first supporting member for supporting one portion of the held sheets. Reference numeral  3  designates a rotary supporting shaft of the first middle plate  2 , arranged in a main body  1   a  of the sheet feeding tray  1 . 
     Reference numeral  4  designates a second middle plate as a second supporting member for supporting a predetermined range of the sheets unable to be supported by the first middle plate  2 . A connecting joint  5  rotatably connects the first middle plate  2  and the second middle plate  4  to each other. A link arm  6  as a link means functions as a moving means of the second middle plate  4 . A round hole  6   a  on one end side of the link arm  6  is rotatably connected to a link shaft  7  arranged in the main body  1   a  of the sheet feeding tray  1  as a fixedly supporting portion. A boss portion  6   b  arranged at the other end of the link arm  6  is rotatably connected to a rotating hole  8  as a part of the second middle plate  4  (the link shaft  7  (the round hole  6   a ) and the boss portion  6   b  (the rotating hole  8 ) serve as rotating fulcrums of the link arm  6 ). 
     In the above construction, a quadric link mechanism is formed by the first middle plate  2 , the second middle plate  4  and the link arm  6 . 
     No load of sheets applied to the second middle plate  4  can be transmitted to the first middle plate  2  in a state in which the link arm  6  is set to be vertical. Accordingly, in this embodiment, the link shaft  7  is arranged on a downstream side from the rotating hole  8  in a sheet feeding direction when sheets are fully stacked. 
     When the link shaft  7  of the link arm  6  is located on a lower side of the second middle plate  4 , the link shaft  7  is arranged on an upstream side from the rotating hole  8  in the sheet feeding direction. 
     Reference numeral  9  designates a rear end regulating member suitably fixed onto the second middle plate  4 . The rear end regulating member  9  can be moved by a user to a position in conformity with a paper size and positions rear ends of the sheets. Reference numerals  10 ,  11  and  12  respectively designate a separating pad for separating overlapped sheets from each other to avoid double feeding, a pad spring for biasing the separating pad, and a sheet feeding roller formed in a fan shape and feeding an uppermost sheet by applying frictional force to this uppermost sheet. 
     A sheet feeding roller  13  separates the sheet feeding roller  12  and the separating pad  10  from each other and gives only minimum resistance to the sheet being fed by follow movement without any large resistance when a small diameter portion of the fan shape of the sheet feeding roller  12  is opposed to the separating pad  10 . 
     Reference numerals  14 ,  15  and  16  respectively designate a conveying roller A for further applying conveying force to the fed sheet, a conveying roller A biased by an unillustrated biasing means in a conveying direction of the conveying roller A and pivotally supported so as to be freely rotated, and a sheet feeding conveying path for passing the fed sheet therethrough. 
     The above members are respectively arranged in a feeder frame  17 . Reference numeral  18  designates a presence/absence sensor flag for detecting presence/absence of the sheet on the first middle plate  2 . Presence/absence information of the sheet can be inputted by this presence/absence sensor flag  18  to an image forming apparatus by switching operating states of an unillustrated detecting means. Reference numeral  19  designates a middle plate spring for biasing the first middle plate in a feeding direction of the sheet feeding roller  12 . 
     The first middle plate  2  and the second middle plate  4  constitutes a sheet stacking portion in cooperation with each other. The first middle plate  2  supports a predetermined range including a sheet portion pressed against the sheet feeding roller  12  from a leading end side of the sheet in the sheet feeding direction. The first middle plate  2  is also rotated around the rotary supporting shaft  3  as an axis so that the sheet is pressed against the sheet feeding roller  12 . The second middle plate  4  supports a predetermined range on a rear end side of the sheet in the sheet feeding direction from the first middle plate  2 . 
     An operation of the sheet feeding apparatus SF 1  shown in FIG. 1 will be explained by using FIGS. 2 to  5 . 
     Reference numeral Ml in FIG. 2 designates the mass of a portion of a sheet bundle seated on the second middle plate  4  in FIG.  1 . Further, reference numerals g, θ and GP respectively designate a gravitational acceleration, an angle of a main shaft of the link arm  6  formed with respect to the horizontal plane, and a center of gravity of sheets seated on the second middle plate  4 . 
     A load is originally also distributed to a portion of the connecting joint  5 , and strictly speaking, the load on that portion should be included in calculation. However, this load is minute as compared to the action of force shown in FIG. 2, and does not have any considerable influence on effects of the present invention if this load is not taken into account. 
     FIG. 3 shows a situation when sheets PS of a size smaller than the sheet size shown in FIG. 1 are stacked. Reference numerals in FIG. 3 are the same as in FIG.  1 . FIG. 4 is a view showing an operation of the present invention in FIG.  3 . In FIG. 4, reference numerals M 2  Ψ and respectively designate the mass of a portion of a sheet bundle seated on the second middle plate  4  in FIG. 3, and an angle of a straight line connecting a center of the rotary supporting shaft  3  of the first middle plate  2  and a center of the connecting joint  5  formed with respect to the horizontal plane. 
     FIG. 5 shows the sheet feeding apparatus of this embodiment when no sheet is stacked. Reference numerals in FIG. 5 are identical with those in FIG.  1 . 
     With the above construction, a sheet feeding operation is performed as follows. 
     When it is detected by a posture of the presence/absence sensor flag  18  that a sheet P is put on the first middle plate  2 , the sheet P is fed and an image writing operation can be started. 
     First, the sheet feeding roller  12  begins to be rotated by an unillustrated driver means and a control means. Then, the sheet P (PL or PS) biased upward together with the first middle plate  2  by the middle plate spring  19 , etc. comes in contact with the sheet feeding roller  12  and receives feeding force by friction. 
     A sheet Pt arranged on the uppermost face (uppermost position) begins to be moved by this feeding force in a rightward direction in FIG.  3  and is inserted to the nip between the separating pad  10  and the sheet feeding roller  12 . 
     The separating pad  10  is biased by the pad spring  11  in a feeding direction of the sheet feeding roller  12 . Accordingly, advance of sheets except for one sheet on the uppermost face is stopped by frictional force of the separating pad  10 , or abutting force at a leading end of the separating pad  10 . 
     The sheet Pt on the uppermost face is further advanced by the frictional force of the sheet feeding roller  12  having a frictional coefficient higher than that of the separating pad  10 . A leading end of this sheet Pt is guided by the sheet feeding conveying path  16  and is inserted into a nipping portion of the conveying roller pairs  14 ,  15  so that the sheet Pt further receives conveying force. 
     In the meantime, a period for making an outer circumference of the fan shape of the sheet feeding roller  12  come in contact with the sheet Pt is terminated, and the sheet Pt attains a state in which the sheet Pt is nipped in a nipping portion of the sheet feeding roller  13  and the separating pad  10 . However, the conveying force of the conveying roller A 14  is set to be stronger so that the conveyance of the sheet Pt is continued. Thus, the sheet Pt is conveyed to an unillustrated image forming apparatus. 
     Next, in the above explanation, force for biasing the sheet PL or PS in the feeding direction of the sheet feeding roller  12  is given by the middle spring  19 . However, as shown in FIGS. 2 and 4, the sheet PL or PS is pushed up in the feeding direction of the sheet feeding roller  12  by the dead weight of sheets stacked on the second middle plate  4 . This pushing-up operation will next be explained. 
     First, when long sheets PL as shown in FIG. 1 are stacked, a center of gravity GP of the sheets riding on the second middle plate  4  is located near the link arm  6 . In this case, all of a load of the sheets PL applied onto the second middle plate  4  is approximately applied to the link arm  6  from balance of moment of a force. 
     However, in this case, since the link arm  6  is inclined by the angle θ, tensile force of M 1 ·g/sin θ in FIG. 2 is applied to the link arm. 
     Force in a horizontal direction given as M 1 ·g/tan θ as resultant force of this load and this tensile force is applied to the first middle plate  2  through the connecting joint  5 , so that the posture of the second middle plate  4  is stabilized. 
     Reaction force to this force in the horizontal direction becomes moment T 1  in the counterclockwise direction with the rotary supporting shaft  3  of the first middle plate  2  as a center. Accordingly, the reaction force is applied in a direction in which the first middle plate  2  is rotated in the counterclockwise direction, i.e., the bundle of sheets P is pressed against the sheet feeding roller  12 . 
     A sheet feeding pressure for making the sheet Pt located on the uppermost face come in contact with the sheet feeding roller  12  is determined by a sum of the biasing force of the middle plate spring  19  and reaction force for canceling the above moment T 1  (=sheet feeding pressure assistant force). 
     Next, when short sheets PS as shown in FIG. 3 are stacked, a center of gravity GP of the sheets seated on the second middle plate  4  is located near the connecting joint  5 . However, a load of the sheets stacked on the second middle plate  4  at this time is very small in comparison with the case of FIG.  2 . In this case, similar to the above case, moment T 2  in the counterclockwise direction around the rotary supporting shaft  3  of the first middle plate  2  is generated, but a value of this moment T 2  is very small in comparison with the moment T 1 . 
     Accordingly, in this case, it may be considered that a pressure for making the sheet Pt located on the uppermost face come in contact with the sheet feeding roller  12  is determined almost by only the biasing force of the middle plate spring  19 . 
     In the above explanation, a state fully laden with the sheets P is explained. However, when the sheets P have the same size, position of the center of gravity of the sheets P stacked on the second middle plate  4  does not depend on a stacked amount of the sheets P, so that the above operation takes place also when the apparatus is not fully lade with the sheets. 
     However, as the stacked amount of the sheets P is reduced, the angle θ of the link arm  6  is increased. Therefore, a value of 1/tan θ is changed and effects of the action are gradually reduced as the stacked amount of the sheets is changed from a full stacking to a less stacking. 
     As explained above, the following effects are obtained in the above embodiment. 
     (1) The sheet feeding pressure assistant force according to the dead weight of a portion of a sheet bundle stacked on the second middle plate  4  is applied to a long sheet PL extending in the sheet feeding direction. The sheet feeding pressure assistant force is reduced as the sheet length in the sheet feeding direction is shortened as in a sheet PS. 
     Thus, a sheet feeding pressure automatic adjusting function for automatically correcting the sheet feeding pressure can be realized in accordance with the sheet length, so that stable sheet feeding performance with respect to various sheet sizes can be realized. 
     (2) Even when sheets have the same length, the values of specific gravities of the sheets fluctuate greatly depending on their kinds. When such sheet bundles of various kinds are used, the sheet feeding pressure assistant force reflecting the difference in specific gravity of the sheets is generated by the sheet feeding pressure automatic adjusting function, so that stable sheet feeding performance can be realized. 
     (3) The sheet feeding pressure automatic adjusting function in the present invention can be applied also to a separating mechanism, as in pad separation or claw separation, in which the sheet feeding pressure effects considerable influence on problems of the sheet feeding such as double feeding and a sheet feeding defect. Accordingly, stable sheet feeding performance can be realized without any sheet feeding pressure adjusting work on the part of a user. 
     A sheet feeding apparatus SF 2  in a second embodiment of the present invention will next be explained with reference to FIG.  6 . FIG. 6 is a view for explaining a sectional construction of the sheet feeding apparatus SF 2 . This embodiment is a modified example of the first embodiment. 
     In FIG. 6, the constructions of reference numerals  9  to  17  and a sheet PL are identical with those in FIG.  1 . Therefore, an explanation of these constructions is omitted here. 
     Reference numerals  20  and  21  respectively designate a sheet feeding tray and a first middle plate rotatably supported by a rotary supporting shaft  3  integrated with the sheet feeding tray  20 . 
     Reference numerals  22  and  23  respectively designate a second middle plate and a dashing (hitting) block as an engaging means coupled to the second middle plate  22 . Reference numeral  24  designates a link arm F. A hole on one end side of the link arm F is rotatably fitted to a shaft F 25  integrated with the sheet feeding tray  20 . A boss on the other end side of the link arm F is rotatably fitted to a side wall hole F 26  of the second middle plate  22 . 
     Reference numeral  27  designates a link arm R. A hole on one end side of the link arm R is rotatably fitted to a shaft R 28  integrated with the sheet feeding tray  20 . A boss on the other end side of the link arm R is rotatably fitted to a side wall hole R 29  of the second middle plate  22 . Accordingly, the link arms F 24  and R 27  function as a moving means of the second middle plate  22 . 
     The differences between the first and second embodiments are as follows. 
     (1) The second middle plate  22  is held by a link mechanism (quadric parallel link) independently of the first middle plate  21 . 
     (2) A sheet feeding pressure assistant force is transmitted by contact of the hitting block  23  and a dashing (hitting) face  21   a  of the first middle plate  21  instead of a shaft coupling portion. 
     In this embodiment, the following effects can be obtained from the above matters in addition to the effects of the first embodiment. 
     (1) Since the sheet feeding pressure assistant force is transmitted by the contact of the hitting block  23  and the hitting face  21   a  of the first middle plate  21 , an applying direction of the sheet feeding pressure assistant force is directed to a vertical direction on a contact face, so that moment with the rotary supporting shaft  3  as a center is increased even when the sheet feeding pressure assistant force is equal. 
     (2) Since the second middle plate  22  becomes a quadric parallel link, it is possible to apply the sheet feeding pressure assistant force according to the load of sheets arranged on the second middle plate even when the sheet size is an intermediate length between the lengths of sheets PL and PS. 
     FIG. 7 is a view for explaining the sectional construction of a sheet feeding apparatus SF 3  showing a third embodiment of the present invention. In this figure, the section of the sheet feeding apparatus SF 3  is taken near its side wall on this side in a sheet feeding tray  30 . The constructions of a middle plate, etc. near the center of a sheet width are similar to those in FIG.  6 . In FIG. 7, the constructions of reference numerals  10  to  19  are equal to those in the first embodiment. 
     Reference numerals  30 ,  31  and  32  respectively designate a sheet feeding tray, a first middle plate, and a rotary supporting shaft for rotatably holding the first middle plate  31 , formed integrally with the sheet feeding tray  30 . 
     Reference numeral  33  designates a second middle plate. In FIG. 7, a vertical face is formed by bending and rising this second middle plate  33  on this side thereof. An equivalent vertical face is formed in a symmetric position deeper than this vertical face in FIG. 7 although this equivalent vertical face is not illustrated. 
     Reference numeral  34  designates a dashing (hitting) roller rotatably attached to the second middle plate  33  and is hit against a hitting face  35  projected from the first middle plate  31 . Reference numerals  36 ,  37 ,  38  and  39  respectively designate a shaft F projected from the second middle plate  33 , a position projecting roller F, a shaft R projected from the second middle plate  33 , and a position projecting roller R. 
     Reference numerals  40  and  41  respectively designate a slanting face F positioned and fixed integrally with or separately from the sheet feeding tray  30 , and a slanting face R similar to the slanting face F 40  (the slanting faces F 40  and R 41  are arranged on both sides in the width direction of a sheet). 
     In this embodiment, inclination angles of the slanting faces F 40  and R 41  are set to be equal to each other, but it is clear that similar effects are obtained even when the inclination angles are different from each other, and characteristics are different but. 
     Accordingly, the second middle plate  33  is held by a sliding means functioning as a moving means having the above construction. 
     FIG. 8 is a view showing an operation of the sheet feeding apparatus in this embodiment. In this figure, reference numerals M 3 , M 4  and M 5  respectively designate the mass of a sheet bundle stacked on the second middle plate  33 , a mass component of the sheet bundle applied to the position projecting roller R, and a mass component of the sheet bundle applied to the position projecting roller F. 
     Reference numerals δ and ε respectively designate an inclination angle of each of the slanting faces F 40  and R 41  with respect to the horizontal plane, and an inclination angle of the hitting face  35  with respect to the horizontal plane. The other reference numerals are similar to those in FIG.  2 . 
     A sheet feeding operation in the above construction is similar to that in the first embodiment. This embodiment is characterized in a pressurizing method of a sheet feeding assistant pressure. 
     As shown in FIG. 8, a load distributed in accordance with a distance from a center of gravity GP of paper is generated in each of the position projecting rollers F 37  and R 39 . This load is applied to each of the slanting faces F 40  and R 41 . Therefore, it is necessary from component force shown in FIG. 8 to receive external force of (M 4 +M 5 )·g·sin δ=M 3 ·g·sin δ in a slanting face direction so as to maintain a posture of the second middle plate  33 . 
     This external force is transmitted by contact of the hitting roller  34  and the hitting face  35 . When the difference between the inclination angle δ of the slanting faces F 40  and R 41  and an angle (π/2−ε) formed by the horizontal plane and a virtual line of an inclination face of the hitting face  35  in a vertical direction is set to δ−(π/2−ε))=Δ, contact force FC at a contact point of the hitting roller  34  and the hitting face  35  is provided as follows. 
     
       
           FC=M   3 · g ·sin δ/cos Δ 
       
     
     Thus, moment T 3  in the counterclockwise direction with the rotating fulcrum  32  as a center is generated and a sheet feeding pressure assistant force according to the load of a sheet bundle stacked on the second middle plate is generated. 
     Characteristic effects in this embodiment are as follows. 
     (1) The load of stacked sheets and the sheet feeding pressure assistant force have a proportional relation except for a factor of 1/cos Δ irrespective of an amount of the sheets stacked on the second middle plate  33 . 
     When the above factor 1/cos Δ is calculated in this embodiment mode, this factor becomes 303 at the time of full stacking and 1.41 at less stacking. Accordingly, a change in this factor is very small in comparison with changes in the first and second embodiments (change factor: in 1/tan θ, 1 at the time of full stacking and 0 at less stacking in FIG. 1 of the first embodiment). Accordingly, it is possible to apply stable sheet feeding pressure assistant force irrespective of the stacked amount. 
     (2) Since shapes of the slanting faces F and R can be freely determined, the inclination angle can be selected and a curved slanting face, etc. can be also adopted, so that a degree of freedom in design of the sheet feeding pressure assistant force is high. 
     (3) Since the second middle plate  33  is arranged on the slanting faces F and R, an assembly property is preferable. 
     One example of an image forming apparatus having the sheet feeding apparatus of the above embodiment mounted thereto will next be explained by using FIG.  9 . 
     The sheet feeding apparatus of the present invention is mounted to a lower side of an image forming apparatus  220 . Reference numerals  221 ,  222 ,  223  and  224  respectively designate a conveying roller pair B for conveying paper, a drum-transfer roller pair for transferring an image onto a sheet, a laser scanner unit for writing a latent image onto a drum, and a fixing unit for fixing the transferred latent image onto the sheet. Reference numerals  225  and  226  respectively designate discharging roller pairs A and B for discharging the sheet on which the image has been formed to the exterior of the image forming apparatus. 
     In the above construction, a sheet feeding operation and image formation are performed as follows. When it is detected by the posture (rotation position) of an existence sensor flag  18  that sheets P are stacked on a middle plate  2 , the sheet feeding operation and subsequent image writing can be started. 
     First, a sheet feeding roller  12  begins to be rotated by an unillustrated driving means and a control means. Then, the sheets biased upward together with the middle plate  2  by a biasing means  19  receive frictional force by the sheet feeding roller  12 . 
     A sheet Pt located on the uppermost face begins to be moved by this frictional force in a rightward direction in this figure and is inserted into a nipping portion of a separating pad  10  and the sheet feeding roller  12 . The separating pad  10  is biased by the biasing means in a feeding direction of the sheet feeding roller  12 . Accordingly, the advance of sheets except for one sheet on the uppermost face is stopped by the frictional force of the separating pad  10  or abutting force at a leading end of the separating pad  10 . 
     The sheet Pt on the uppermost face is further advanced by the frictional force of the sheet feeding roller  12  having a frictional coefficient higher than that of the separating pad  10 , and a leading end of this sheet is guided by a sheet feeding conveying path  16 . Thus, the sheet Pt is inserted into a nipping portion of a conveying roller A 14  and a conveying roller A 15  and further receives conveying force. 
     In the meantime, a period for making an outer circumference of a fan shape of the sheet feeding roller  12  come in contact with the sheet Pt is terminated, and the sheet Pt is nipped in a nipping portion of a sheet feeding roller  13  and the separating pad  10 . However, conveying force of the conveying roller A 14  is set to be stronger, so that the conveyance of the sheet Pt is continued. 
     The sheet being conveyed is next inserted into a nipping portion of the conveying roller pair  221  and further receives conveying force. A latent image written onto a drum by the laser scanner unit  223  is developed by an unillustrated developing means and is then transferred to the sheet Pt inserted into a nipping portion of the drum-transfer roller pair  222 . 
     The image is fixed to the sheet Pt by the fixing unit  224 . Thereafter, this sheet Pt is discharged to the exterior of the image forming apparatus by rotating the discharging roller pairs A 225 , B 226 . Thus, the image is formed on the sheet. 
     A fourth embodiment of the present invention will next be explained with reference to FIGS. 10 to  12 . 
     An image forming apparatus having a sheet feeding apparatus of this fourth embodiment will first be explained with reference to FIG.  12 . In this explanation, a laser beam printer is used as an example of this image forming apparatus. 
     In FIG. 12, reference numeral  120  designates a laser beam printer as the image forming apparatus. A sheet feeding tray  130  as a cassette body is mounted to his laser beam printer  120  and plural sheets P are stacked on this sheet feeding tray  130 . A sheet feeding roller  103  rotated only at a sheet feeding time is arranged in a sheet feeding port of the laser beam printer  120 . The sheets P are fed from the sheet feeding tray  130  when this sheet feeding roller  103  is rotated in the direction of an arrow R 3  in FIG. 12 (in the counterclockwise direction). The sheets P fed by the sheet feeding roller  103  are separated one by one by a separating means  102  and are conveyed to an image forming section by a conveying roller pair  104  and a registration roller pair  105 . 
     The laser beam printer  120  has the conveying roller pair  104  for conveying the sheets P, the registration roller pair  105 , a toner image transfer section  108  as the image forming section, a developing unit  110 , a transfer roller  106 , and a fixing device  115 . The toner image transfer section  108  transfers a toner image to the sheets P guided by this registration roller pair  105 . The developing unit  110  visualizes an electrostatic latent image on a photosensitive drum  107  constituting this toner image transfer section  108 . The transfer roller  106  transfers the toner image visualized on the photosensitive drum  107  to the sheet P. The fixing device  115  fixes the toner image onto the sheets P. 
     The sheets P is fed from the sheet feeding tray  130  by the sheet feeding roller  103  and is separated one by one by the separating means  102  and is guided to the toner image transfer section  108  by the conveying roller pair  104  and the registration roller pair  105 . 
     The photosensitive drum  107  is rotated in the direction of an arrow R 7  in FIG. 12 (in the clockwise direction), so that the photosensitive drum  107  is uniformly charged by a charger  109 . Thereafter, the photosensitive drum  107  is exposed to a selective laser beam based on an image signal and emitted from a laser scanner  122  so that an electrostatic latent image is formed. This electrostatic latent image on the photosensitive drum  107  is visualized (as a toner image) by the developing unit  110 . 
     Next, the toner image formed on the photosensitive drum  107  is electrically attracted by the transfer roller  106 , so that the toner image is sequentially transferred to the printing face (an upper face in FIG. 12) of the sheets P passing through the toner image transfer section  108 . Thus, the toner image is formed on the sheets P. 
     Thereafter, the sheets P are guided to a nipping portion of a heating means  113  of the fixing device  115  and a pressurizing roller  114  coming in press contact with this heating means  113 . The toner image transferred onto the sheet face in a process in which the sheet P passes through the nipping portion is heated and pressurized. Thus, the toner image is fixed onto the sheet face. 
     The sheet P passing through the fixing device  115  is discharged by discharging rollers  118 ,  119  onto a paper discharging tray  121  through a paper discharging path  116 . 
     A sheet feeding apparatus SF 4  in this embodiment will next be explained with reference to FIGS. 10 and 11. 
     In FIG. 10, a load transmitting member  132  and an arm member  133  are arranged on both left-hand and right-hand sides of the sheet feeding tray  130  with respect to a middle plate  131 . An intermediate portion  132   a  of the load transmitting member  132  is pivotally supported with a shaft  134   a  at a side wall  130   b  of a main body  130   a  of the sheet feeding tray  130  on its leading end side. Similarly, a leading end portion  133   a  of the arm member  133  in its paper passing direction is rotatably supported at the side wall  130   b  with a shaft  134   b.    
     A load receiving member  135  is arranged within the sheet feeding tray  130  and is integrally formed by continuously arranging a rising portion  135   a  arranged along left-hand and right-hand side walls of the sheet feeding tray  130  by a plane portion  135   b.  A leading end portion of the load receiving portion  135  and a rear end portion of the load transmitting member  132  are pivotally supported by a connecting shaft  136   a.  A rear end portion of the load receiving member  135  and a rear end portion of the arm member  133  are pivotally supported by a connecting shaft  136   b.  Further, a rear end side supporting portion  131   a  of the middle plate  131  is axially supported by a connecting shaft  136   c  in an intermediate portion of the load receiving member  135 . 
     For example, the middle plate  131  has a sheet stacking face having a length close to a standard sheet size such as size A 4  and the letter size, in a paper passing direction. A leading end side supporting portion  131   b  is arranged at a leading end of the middle plate  131  in the paper passing direction. An elongated hole  131   c  is formed in this leading end side supporting portion  131   b,  and a shaft  136   d  arranged in a leading end portion of the load transmitting member  132  is slidably inserted with play into this elongated hole  131   c.  Springs  137 ,  137  are arranged as a biasing means for pushing the middle plate  131  upward on a leading end side of the sheet feeding tray  130  in the paper passing direction. When sheets P of a standard size are stacked on the middle plate  131 , the weight of the sheets P is approximately uniformly distributed to the leading end side supporting portion  131   b  and the rear end side supporting portion  131   a.    
     A rear end regulating member  138  is movably arranged on the plane portion  135   b  along the paper passing direction. This rear end regulating member  138  can be moved in the paper passing direction in accordance with a sheet size. A width regulating member  139  regulates a width direction position of the sheets P stacked on the middle plate  131 . This width regulating member  139  is movably supported in a width direction of the main body  130   a  of the sheet feeding tray  130 . 
     In this embodiment, a distance L 1  from the shaft  134   a  to the shaft  134   b  is equal to a distance L 2  from the connecting shaft  136   a  to the connecting shaft  136   b.  A distance L 3  from the shaft  134   a  to the connecting shaft  136   a  is equal to a distance L 4  from the shaft  134   b  to the connecting shaft  136   b . The connecting shaft  136   a  is located backward from the shaft  134   a  in the paper passing direction and is rotated in the range of a lower side. The connecting shaft  136   b  is located backward from the shaft  134   b  in the paper passing direction and is rotated in the range of a lower side. The connecting shafts  136   a  and  136   b  constitute a parallel link mechanism. Thus, the load receiving member  135  maintains a horizontal state and is moved in parallel. Further, the shaft  136   d  is moved in the range of a leading end side from the shaft  134   a  in the paper passing direction. 
     Namely, the connecting shaft  136   a  and the shaft  136   d  are separated by predetermined distances from the shaft  134   a.  An angle θ formed by a line connecting the connecting shaft  136   a  and the shaft  134   a  and a line connecting the shafts  134   a  and  136   d  is set to a predetermined angle (in a range of from 90 to 180 degrees). Thus, a leading end side of the middle plate  131  can be rotated in a vertical direction with the connecting shaft  136   c  as a center as the load receiving member  135  constituting the link mechanism is moved in parallel. Accordingly, as the number of stacked sheets P is reduced, the middle plate  131  is rotated by the springs  137 ,  137  to reach the state indicated by a two-dotted chain line shown in FIG.  1  and the sheets P come in press contact with the sheet feeding roller  103 . 
     A movement of the load receiving member  135  can be adjusted by slightly changing the relation among values of the above distances L 1 , L 2 , L 3  and L 4 . Namely, for example, a vertical moving amount of a rear end portion of the load receiving member  135  is larger than that of a leading end portion of the load receiving member  135  when setting the relation to L 1 &lt;L 2  or L 3 &lt;L 4 . Accordingly, a locus of the load receiving member  135  can be set in conformity with an individual design condition such as a restriction of space. 
     An operation of the middle plate will next be explained in a situation in which sheets of respective sizes are stacked. 
     When sheets P of a standard size are fully stacked onto the middle plate  131 , the weight of the sheets P is approximately uniformly distributed in the leading end side supporting portion  131   b  and the rear end side supporting portion  131   a  of the middle plate  131 . Namely, a load applied to the rear end side supporting portion  131   a  of the middle plate  131  is applied to the connecting shaft  136   c  of the load receiving member  135  and the middle plate  131 , so that the load receiving member  135  begins to be moved downward in parallel. Thus, moment for rotating the load transmitting member  132  in the counterclockwise direction is generated. This moment gives force in a pushing-up direction of a leading end portion of the middle plate  131 . 
     In contrast to this, the weight of the sheets P applied to the leading end side supporting portion  131   b  of the middle plate  131  acts as moment for rotating a leading end of the middle plate  131  in a pushing-down direction. These two moments are applied in a mutual canceling direction, thereby reducing the difference in force for pushing down the leading end of the middle plate  131  by a sheet bundle due to a difference in density of the sheets P. Accordingly, variation in sheet feeding pressure based on the weight of the sheets P stacked on the middle plate  131  can be suppressed to a small range. 
     When the sheets P of a small size are fully stacked, force for pushing down the leading end side supporting portion  131   b  by the sheets P is reduced by reducing the weight of the sheets P. However, the center of gravity of the sheet bundle is simultaneously moved onto a leading end side in the paper passing direction. Accordingly, a load distributed to the leading end side supporting portion  131   b  and the rear end side supporting portion  131   a  begins to be largely applied to the leading end side supporting portion  131   b.  Therefore, force for rotating the leading end side supporting portion  131   b  of the middle plate  131  in a pushing-up direction is reduced, so that variation in sheet feeding pressure is reduced. In this case, a ratio of loads applied to the leading end side supporting portion  131   b  and the rear end side supporting portion  131   a  is inversely proportional to a distance from a position of the center of gravity of the sheet bundle to the shaft  136   d  inserted into the leading end side supporting portion  131   b,  and a distance from that position to the connecting shaft  136   c  for pivotally supporting the rear end side supporting portion  131   a.  Accordingly, it is sufficient to set optimum positions of the leading end side supporting portion  131   b  and the rear end side supporting portion  131   a  by moving the rear end regulating member  138  in accordance with a sheet size for guaranteeing paper passage. 
     Conversely, since the elongated sheets P of such as legal size etc. have a length longer than the length of a sheet stacking face of the middle plate  131 , the rear end portion of the sheets is placed on the plane portion  135   b  of the load receiving member  135 . In such a construction, the weight of a portion of the elongated sheets P which sticks out from the sheet stacking face is applied in a pushing-up direction of the leading end side supporting portion  131   b,  and the sheet feeding pressure is increased in comparison with the sheet P of a standard size. However, since no thin paper is generally used in the elongated sheets P, there is no fear of double feeding and no serious problem is caused. 
     The weight of sheets P is mutually canceled by the leading end side supporting portion  131   b  and the rear end side supporting portion  131   a  of the middle plate  131  by constructing the load transmitting member  132 , the load receiving member  135 , the arm member  133  and the middle plate  131  as mentioned above. Accordingly, variation in the sheet feeding pressure due to sizes and densities of the sheets P can be restrained. 
     Further, the middle plate  131  and the weight of a sheet bundle are supported in two highly rigid portions by arranging the shaft  134   a  of the load transmitting member  132  and the shaft  134   b  of the arm member  133  in relatively highly rigid portion of the side wall  130   b.  Accordingly, a movement of the middle plate  131  can be stabilized. 
     Further, since positions of the shafts  134   a,    134   b  can be located downward, a height of the side wall  130   b  of the sheet feeding tray  130  can be lowered so that a large amount of sheets P can be easily put in and out. 
     Since no middle plate  131  is directly supported by the main body  130   a  of the sheet feeding tray  130 , the width regulating member  139  having a sufficient length in the paper passing direction can be arranged. Accordingly, a slanting movement of the sheets P is restrained, so that printing accuracy can be improved. 
     Further, the length of the middle plate  131  is set to a length close to a standard sheet size in the paper passing direction, and a rear end of the elongated sheets is placed onto the load receiving member  135 . Accordingly, it is not necessary to deepen a bottom portion of the main body  130   a  of the sheet feeding tray  130  for the elongated sheets P. The rear end regulating member  138  is also arranged on the load receiving member  135 . Accordingly, vertical moving range of the rear end regulating member  138  does not change even when the rear end regulating member  138  is set in conformity with the sheets P of any size. Therefore, it is not necessary to save a space for avoiding abuttal between the upper end of the rear end regulating member  138  and a laser beam printer, so that the space can be effectively utilized.
The present invention relates to a sheet feeding apparatus which has load transmitter for converting a load of a sheet applied to a rear end side in a sheet feeding-out direction of sheet stacker to a biasing force for biasing a leading end side of the sheet stacker toward sheet feeder.