Abstract:
A sheet feeding apparatus has a plurality of sheet containers, a plurality of sheet feeding units and a plurality of curving conveying routes for guiding the sheets to a common conveying route. One of the sheet containers is adapted to hold envelopes and other sheets of high rigidity, and the sheet feeding apparatus is constructed such that a radius of curvature of the curving conveying route for the envelope and other sheets of high rigidity is made larger than a radius of curvature of a second curving portion of another conveying route.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a sheet feeding apparatus mounted in an image forming apparatus such as a photocopier, a printer, and the like. 
     2. Description of Related Art 
     As an image forming apparatus such as a photocopier, a printer, etc., a mechanism has been proposed in which a sheet feeding means is set in containing multiple sheets to automatically feed the sheets and after a pickup roller takes up the sheets, a feed roller and a retard roller cooperate to convey the sheets one by one to form images. Another known apparatus conveys sheets fed manually one by one to form images, in order for feeding differently sized sheets and special sheets such as thick papers and envelops. 
     To render such an image forming apparatus smaller, a conveying means for conveying sheets that are fed from a sheet containing means is also made smaller, and thereby, the radius of curvature of a curving portion in a conveying route is made relatively small as to be capable of conveying merely the regular cut sheets that are ordinarily used. With such a conventional art, however, when automatically feeding a special sheet such as a rigid thick sheet or envelop, the apparatus cannot feed the sheet along a sheet guide forming a curving portion due to a small radius of curvature at the curving portion in the conveying route, thereby raising a problem that the apparatus fails to stably feed sheets upon occurrence of paper jams or conveyance defects. 
     SUMMARY OF THE INVENTION 
     This invention solves the above problem. It is an object of the invention to provide a sheet feeding apparatus capable of sufficiently feeding rigid sheets such as thick papers and special sheets such as envelops and to provide an image forming apparatus in which this sheet feeding apparatus is mounted. 
     A representative constitution of the invention to achieve the foregoing object is a sheet feeding apparatus including a plurality of sheet containing means for containing sheets, a plurality of sheet feeding means for feeding respective sheets contained at the sheet containing means, and a plurality of conveying routes for merging respective sheets fed by the respective sheet feeding means into a common conveying route, wherein a radius of curvature of a first curving portion where at least one conveying route among the plural conveying routes merges into the common conveying route is larger than a radius of curvature of a second curving portion where other conveying routes merge into the common conveying route. 
     The invention, since thus constructed, allows rigid sheets such as thick papers or special sheets such as envelops to pass the first curving portion at which the radius of curvature is relatively large and to be introduced into the common conveying route, thereby being capable of adequately feeding rigid sheets such as thick papers or special sheets such as envelops. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional illustration showing a structure of an image forming apparatus having a sheet feeding apparatus according to the invention; 
     FIG. 2 is a perspective view showing a structure of a feeding unit; 
     FIGS.  3 ( a ),  3 ( b ) are cross-sectional illustrations showing a rocking operation of a lifter and a feeding rotary body; 
     FIGS.  4 ( a ),  4 ( b ) are cross-sectional illustrations showing a release operation of the lifter when a cassette serving as a sheet containing means is attached; and 
     FIG. 5 is a cross-sectional illustration showing routes of different radiuses of curvature in the sheet feeding apparatus according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, an embodiment in which the invention applies to a photocopier as an example of an image forming apparatus having a sheet feeding apparatus according to the invention will be described. 
     First, referring to FIG. 1, the entire constitution of an image forming apparatus having a sheet feeding apparatus according to the invention is described. In FIG. 1, an image forming apparatus body A includes a scanner unit B mounted at an upper portion of the body serving as an image reading means for reading image information on image surfaces on originals in a form of books or the like, an image forming unit C placed below the scanner unit B, and a sheet deck D built-in below the image forming unit C for containing sheets S made of paper, synthetic resin, or the like. 
     The scanner unit B includes a scanning light source  201 , a platen glass  202 , an original pressure plate  203  capable of opening and shutting to the apparatus body A, a mirror  204 , lens  205 , a photo receiver (photo-electric conversion element)  206 , an image processing unit or the like. 
     Originals such as originals in a form of books including thick paper, curling paper, etc., and originals in a form of sheets are put on the platen glass  202  in a manner that the original surface faces down. A back of the original is pushed by the original pressure plate  203  to make the original immovable. Upon pushing a start key, the scanning light source  201  scans in a direction of arrow a in FIG. 1 at a bottom of the platen glass  202  to read image information on the original surface. The image information of the original read by the scanning light source  201  is processed at the image processing unit and transmitted to a laser scanner  111  upon converted to an electrical signal. 
     The image forming apparatus A here functions as a photocopier when the laser scanner  111  inputs the processed signal from the image processing unit and as a printer when the scanner inputs an output signal from a personal computer. The apparatus A also functions as a facsimile machine when inputting a transmission signal from another facsimile machine or when transmitting the processed signal at the image processing unit to another facsimile machine. 
     A sheet cassette  1  serving as a sheet containing means is attached below the image forming unit C. In this sheet cassette  1 , a lower cassette  1   a  and an upper cassette  1   b  constitute a feeding unit. In this embodiment, as shown in FIG. 1, three feeding units U 1 , U 2 , U 3  are attached to render six cassettes attached. 
     The feeding unit U 1  placed at an upper position is detachably attached to the apparatus body A while other two lower feeding units U 2 , U 3  are detachably attached to the sheet deck D. Each upper cassette  1   b  of the feeding units U 1 , U 2 , U 3  is capable of containing and feeding relatively inflexible sheets such as thick paper or special sheets such as envelops. A sheet S fed from the upper cassette  1   b  is conveyed to be introduced into a common conveying route Z vertically extending in the image forming apparatus body A by way of a conveying path X having a relatively large radius of curvature. 
     The sheets S made of paper or synthetic resin contained in the upper and lower cassettes  1   a ,  1   b  are taken up by a pickup roller  3  serving as a sheet feeding means as described in detail below. After the sheets S are separated and fed one by one by cooperation of a feed roller  4  and a retard roller the sheets S are then carried by a conveying roller  104  to pass a plurality of conveying paths, namely, the conveying path X and a conveying path Y, which are merged into the common conveying route Z. The sheets S are then conveyed by a conveying roller  105  and introduced to a register roller  106 , which feeds the sheets S to the image forming unit C in synchronized with image forming operation. 
     The image forming unit C includes an electrophotographic photosensitive drum  112  serving as an image forming means, an optical system  113  for writing images, a developer  114 , a transfer charger  115 , and the like. A laser beam corresponding to image information emitted from a laser scanner  111  is scanned by the optical system  113  for writing images to form a latent image on a surface of the photosensitive drum  112  that has uniformly charged by a charger. The developer  114  forms a toner image on the latent image. The toner image is transferred by the transfer charger  115  onto a first side of the sheet S, which is conveyed by the register roller  106  in synchronized with rotation of the photosensitive drum  112 . 
     Numeral  117  denotes a conveyance belt for conveying a sheet S on which a toner image is formed; numeral  118  denotes a fixing apparatus; numeral  119  denotes a delivery roller. The sheet S on which the toner image is formed is conveyed to the fixing apparatus  118  by the conveyance belt  117 . After the toner image is fixed on the surface of the sheet S by application of heat and pressure, the delivery roller  119  delivers the sheet S to a sorter  120  mounted outside the apparatus body, and the sorter  120  holds the sheet S thereon. 
     When the apparatus records images on both sides of the sheets S, the delivery roller  119  cramps the sheet S delivered from the fixing apparatus  118  and rotates in a reverse direction to feed the sheet S at a time that a rear end of the sheet S passes a separation point  207 . After put on a double side tray  121  for a moment, the sheet S is conveyed again by the conveying rollers  104 ,  105  and reaches the register roller  106 . The reversed sheet S is delivered and fed to the sorter  120  after images are formed on the second side of the sheet S in the same manner as described above. 
     Referring to FIGS. 2,  3 ( a ),  3 ( b ), mechanisms for the rocking operation of the pickup roller  3  and the moving up and down operation of the lifter are described. FIG. 2 is a perspective view showing a structure of the feeding unit. A sheet mounting plate  2  formed at a bottom of and attached pivotably to the sheet cassette  1  serving as a sheet containing means, holds a sheet S made of a paper, thin synthetic resin plate, or the like on the plate. The sheet cassette  1  is attached to the image forming apparatus body A by pushed in a direction of arrow d in FIG.  2 . 
     The pickup roller  3  serving as a feeding rotary body takes, from the topmost sheet, the sheets S placed on the sheet mounting plate  2  of the sheet cassette  1 . The feed roller  4  arranged on a downstream side of the sheet feeding direction (hereinafter simply referred to as “downstream side”) of the pickup roller  3 , is rotated in the normal direction with respect to the sheet feeding direction, while the retard roller  5  is rotated in the reverse direction with respect to the sheet feeding direction, thereby separating and feeding, sheet by sheet, the sheets S picked up by the pickup roller  3 . 
     As shown in FIG. 2, a pickup roller shaft  3   a  of the pickup roller  3  and a feed roller shaft  4   a  of the feed roller  4  are supported and connected by a roller holder  6  constituting a rocking means for rocking the feeding rotary body. The roller holder  6  is pivotal around the feed roller shaft  4   a  in a range that the pickup roller shaft  3   a  is movable inside a long hole  7   a  formed in a feeding frame  7 . The roller holder  6  is normally urged by forcing means such as a spring or the like not shown to push the pickup roller  3  as to give a pressure on the sheets S mounted on the sheet mounting plate  2 . 
     One end of the pickup roller shaft  3   a  penetrates the long hole  7   a  in the feeding frame  7  and extends on a right side of a lifter mechanism shown in FIG. 2. A lift-up lever  9  is attached pivotably with respect to a shaft  8  as center, which is unitedly formed with the feeding frame  7 . The lift-up lever  9  is normally urged in a clockwise direction (arrow e direction in FIG. 2) around the shaft  8  by a spring  9   a  engaged to the feeding frame  7  via the end of the spring, thereby rendering a shaft contact piece  9   b , or one end of the lift-up lever  9 , contact with the pickup roller shaft  3   a  from the bottom side of the shaft  3   a.    
     A cam contact member  9   c , the other end of the lift-up lever  9 , extends near a pickup cam  10 . The cam contact member  9   c  of the lift-up lever  9  is in contact and engagement with the pickup cam  10  by pulling force of the spring  9   a . The lift-up lever  9  rocks around the shaft  8  upon revolution of the pickup cam  10 . 
     The pickup cam  10  is arranged as opposed to a gear  12  and is formed coaxially and in a united body with a partially toothless pickup gear  11 , which lacks tooth in part, on a side face of the gear  11 . The pickup gear  11  is normally urged in a counterclockwise direction (arrow f direction in FIG. 2) by an urging means not shown. In this embodiment, rotation of the pickup cam  10  is restricted by a flapper solenoid  13  in opposition to force of the urging means. 
     Numeral  14  denotes a main motor of the image forming apparatus body A; drive force from the motor  14  is transmitted to the gear  12  by a gear  15 , thereby rotating the gear  12  normally in arrow g direction in FIG.  2 . In FIG. 2, numeral  30  is a sheet level detector for detecting a surface level of the sheets S mounted on the sheet mounting plate  2 . The sheet level detector  30  includes a sheet level detection lever  31  arranged at a position corresponding to the sheet level of sheets S mounted on the sheet mounting plate  2 , and a sheet level detection sensor  32  arranged corresponding to a sensor side end  31 a of the sheet level detection lever  31 . 
     The sheet level detection lever  31  is supported pivotably with respect to the feeding frame  7 . When no sheet S is mounted on the sheet mounting plate  2 , or at an initial stage in which the sheet cassette  1  is just attached to the image forming apparatus body A upon putting the sheets S on the sheet mounting plate  2  while the sheet cassette is pulled out of the image forming apparatus body A, the sheet level detection lever  31  is in a down state, and the sensor side end  31   a  of the sheet level detection lever  31  renders the sheet level detection sensor  32  turned off. 
     If feeding starts while the sheets S are mounted on the sheet mounting plate  2 , the sheet mounting plate  2  moves up by operation of a lifter mechanism as described below, and thereby, the sheet surface of the sheets S mounted on the sheet mounting plate  2  pushes up the sheet level detection lever  31  to pivotally move the lever  31 . 
     When the apparatus recognizes that the sheet level reaches a position suitable to start a feeding operation, or that the topmost sheet level of the sheets S reaches a prescribed height, by transition from turned-off to turned-on of the sheet level detection sensor  32  according to a movement of the sensor side end  31   a  of the sheet level detection lever  31 , the lifter mechanism stops operation to move up the plate  2 , and feeding operation for sheets S starts. 
     As shown in FIGS.  3 ( a ),  3 ( b ), a partially toothless lift-up gear  18  is so arranged as to partly mesh the gear  12  and is formed coaxially with a lift-up cam  19 , a first cam  19   a , and a second cam  19   b  juxtaposedly. The first cam  19   a  is rotated by the gear  12  and the lift-up gear (partially toothless gear)  18 , thereby rocking a forwarding piece  20 , which is arranged near the lift-up gear  18 , around a shaft  21  by way of a first cam contact portion  20   a . A pawl  22  is held pivotably around a shaft  22   b  and normally urged toward a ratchet gear  17  by a spring  26  whose one end engages the feeding frame  7 . The forwarding piece  20  makes the ratchet gear  17  forward tooth by tooth in association with a preventive function of the pawl  22  against gear&#39;s backward rotation, thereby moving up a lifter  16  formed in a united body with the ratchet gear  17 . 
     The lifter  16  contacts a bottom of the sheet mounting plate  2  of the sheet cassette  1  to lift the sheet mounting plate  2 , thereby moving up the sheets S mounted on the sheet mounting plate  2 , and setting the topmost surface of the sheets at a constant level as to render the top surface of the sheets S in contact with the pickup roller  3 . 
     Numeral  23  denotes a lifter trigger lever, which is supported as to be capable of rocking around the shaft  8  formed in a united body with the feeding frame  7 . One end of the lifter trigger lever  23  is normally urged in a clockwise direction (arrow h direction in FIG. 2) with center of the shaft  8  by a spring  24  held at one end of a pickup lock lever  25  serving as a rocking restricting means as described below. 
     A restriction piece  23   a  as one end of the lifter trigger lever  23  is in contact with the lift-up cam  19  by pulling force of the spring  24 . When contacted and engaged with an engagement portion  19   c  of the lift-up cam  19 , as shown in FIG.  3 ( a ), the restriction piece  23   a  of the lifter trigger lever  23  restricts rotation of the lift-up gear  18  formed in a united body with the lift-up cam  19 . A contact piece  23   b , the other end of the lifter trigger lever  23 , is so placed as to contact with the pickup roller shaft  3   a  when the pickup roller  3  moves down to a prescribed level. 
     That is, when the pickup roller shaft  3   a  is lowered to the prescribed level, the contact piece  23   b  of the lifter trigger lever  23  is pulled down in opposition to the pulling force of the spring  24  to render the lifter trigger lever  23  move pivotally around the shaft  8  in the counterclockwise direction (arrow b direction in FIG.  3 ( b )), thereby disengaging the restriction piece  23   a  of the lifter trigger lever  23  from the engagement portion  19   c  of the lift-up cam  19  to release the restriction on rotation of the lift-up cam  19 . The second cam  19   b  rotating together as a united body with the lift-up cam  19  contacts to a second cam contact portion  20   c  of the forwarding piece  20 , thereby pushing the forwarding piece  20  around a shaft  21  in a clockwise direction (arrow c direction in FIG.  3 ( b )) to rock the forwarding piece  20 . The ratchet gear  17  is moved up by this rocking motion of the forwarding piece  20  and elevates the lifter  16  as described below. 
     Elevation of the lifter  16  increases the sheet surface level of the topmost sheet S mounted on the sheet mounting plate  2 , thereby moving up the pickup roller  3  that has contacted with the sheet surface. The contact piece  23   b  of the lifter trigger lever  23  moves up according to elevation of the pickup roller shaft  3   a , thereby rotationally returning the lifter trigger lever  23  around the shaft  8  as a center in a clockwise direction (arrow h direction in FIG.  3 ( a )). The lifter trigger lever  23  thereby makes the restriction piece  23   a  of the lifter trigger lever  23  engaged and contacted with the engagement portion  19   c  of the lift-up cam  19  and returns to a position to restrict the rotation of the lift-up cam  19 . This operation is repeated to render substantially the same the sheet surface level of the topmost sheet S mounted on the sheet mounting plate  2  of the sheet cassette  1 . 
     As a result, the sheet feeding apparatus enters a standby mode that the sheets S can be conveyed. Subsequently, the pickup cam  10  rotates upon exciting the flapper solenoid  13  in association with a feeding start signal, and the lift-up lever  9  rotates in a counterclockwise direction (arrow m direction in FIG.  3 ( b )) in opposition to pulling force of the spring  9   a  around the shaft  8  as a center. As the contact piece  9   b  of the lift-up lever  9  moves down, the pickup roller shaft  3   a  goes down, thereby rendering the pickup roller  3  contact with the topmost sheet S on the sheet mounting plate  2  to feed out the topmost sheet S. 
     Though the sheet S located at the pickup roller  3  can receive an adequate conveyance force when the sheet surface level of the topmost sheet S is substantially the same, the conveyance force may be lowered and cause impairments of conveyance if the sheet surface level is lower than a suitable level. 
     The pickup roller  3  contacts the sheet surface of the sheets S for a time determined by the shape and angular velocity of the pickup cam  10 , then, separates from the sheet surface, and moves up. A drive source not shown drives, for this period, the pickup roller  3 , the feed roller  4 , and the retard roller  5 , respectively, as described above, thereby separating one by one the sheets S mounted on the sheet mounting plate  2  and feeding the separated sheet S. 
     In a meantime, when the sheet surface level of the sheets S is lowered, the contact piece  23   b  of the lifter trigger lever  23  is pulled down by the pickup roller shaft  3   a , thereby moving pivotally the lifter trigger lever  23  in a counterclockwise direction (arrow b direction in FIG.  3 ( b )) in opposition to the pulling force of the spring  24  around the shaft  8  as a center. The restriction piece  23   a  of the lifter trigger lever  23  is disengaged from the engagement portion  19   c  of the lift-up cam  19  to release the restriction on rotation of the lift-up cam  19 . The second cam  19   b  rotating together in a united body with the lift-up cam  19 , therefore, comes to contact to the second cam contact portion  20   c  of the forwarding piece  20 , thereby rotationally moving the forwarding piece  20  in arrow c direction shown in FIG.  3 ( b ) around the shaft  21  as a center, and rendering a gear meshing portion  20   c  push up the ratchet gear  17  in arrow  1  direction in FIG.  3 ( b ). Upon this pushing-up movement, the ratchet gear  17  moves up for one tooth. The lifter  16  moves up unitedly with the ratchet gear  17  and resumes the sheet surface level. 
     This operation is repeated to keep the sheet surface level substantially the same in the sheet cassette  1 . The pickup roller  3 , according to series of those operations, contacts to the sheet surface only when taking the sheets S out of the sheet cassette  1 , and keeps away from the sheet surface for the remaining period. This motion of the pickup roller  3  maximizes prevention of double feeding of the sheets S by means of a pair of the feed roller  4  and the retard roller  5 , thereby ensuring prevention of double feeding. 
     Lift-up operation of the lifter  16  will be described in detail. When the pickup cam  10  is rotated for one turn under a state shown in FIG.  3 ( a ), a nail shaped portion  25   a  of the pickup lock lever  25  is engaged with a tip of the shaft contact piece  9   b  of the lift-up lever  9  as shown in FIG.  3 ( b ). 
     When the pickup roller  3 , or the pickup roller shaft  3   a , moves down to the prescribed level, the lifter trigger lever  23  rotationally moves in arrow b direction around the shaft  8  as a center as shown in FIG.  3 ( b ) to disengage the restriction piece  23   a  of the lifter trigger lever  23  from the engagement portion  19   c  of the lift-up cam  19 , thereby releasing the restriction on rotation of the lift-up cam  19 . 
     The lift-up cam  19  makes one turn unitedly with the lift-up gear  18  by meshing between the lift-up gear  18  and the gear  12 . When the lift-up gear  19  rotates, the lifter mechanism starts lift-up operation as follows. 
     As described above, the first and second cams  19   a ,  19   b  are disposed coaxially with the lift-up cam  19 . The first cam  19   a  is an eccentric cam and arranged as to be capable of contacting to the first cam contact portion  20   a  formed at the forwarding piece  20 . The forwarding piece  20  is attached rotatably around the shaft  21  as a center; an attachment hole for the shaft  21  is a long hole. The forwarding piece  20  is attached as to be properly movable in arrow i direction in FIG.  3 ( b ) and arrow j direction in FIG.  3 ( a ). 
     Upon rotation of the lift-up cam  19 , the first cam  19   a  rotates, thereby engaging and disengaging the ratchet gear  17  with and from the gear meshing portion  20   b  of the forwarding piece  20 . The ratchet gear  17  does not normally mesh the gear meshing portion  20   b  of the forwarding piece  20  as shown in FIG.  3 ( a ) when the lift-up operation is not needed. As shown in FIG.  3 ( b ), upon rotation of the lift-up gear  18 , the ratchet gear  17  comes to mesh the gear meshing portion  20   b  of the forwarding piece  20  in association with motion of the first cam  19   a.    
     The forwarding piece  20  rocks in arrows c, k directions in FIG.  3 ( b ) where the second cam  19   b  shown by a broken line in FIG.  3 ( b ) and the second cam contact portion  20   c  of the forwarding piece  20  contact to each other, thereby lifting up the ratchet gear  17  in arrow  1  direction in FIG.  3 ( b ). The pawl  22  disengages momentarily from the ratchet gear  17  since the ratchet gear  17  travels in arrow  1  direction in FIG.  3 ( b ), and then, the pawl  22  meshes one tooth below of the ratchet gear by pulling force of the spring  26  whose one end is held to the feeding frame  7 , thereby preventing the ratchet gear  17  from rotating further. 
     Finally, as shown in FIG.  3 ( a ), when the nail shaped portion  25   a  of the pickup lock lever  25  is disengaged from the tip of the shaft contact piece  9   b  of the lift-up lever  9 , the restriction piece  23   a  of the lifter trigger lever  23  restricts the rotation of the lift-up cam  19 , disengaging the first cam  19   a  from the first cam contact portion  20   a , thereby disengaging the gear meshing portion  20   b  of the forwarding piece  20  from the ratchet gear  17 . Repeating this operation moves the lifter  16  up to a normal position. 
     A brief summary of series of this lift-up operation is as follows. First, the lift-up gear  18  rotates to engage the forwarding piece  20  with the ratchet gear  17  (see, arrow i in FIG.  3 ( b )) and then to elevate the ratchet gear  17  (see, arrows c, k,  1  in FIG.  3 ( b )). The pawl  22  meshes a tooth located at one tooth below, thereby disengaging the forwarding piece  20  from the ratchet gear  17  (see, arrow j in FIG.  3 ( a )). 
     Now, referring to FIGS.  4 ( a ),  4 ( b ), a lifter release operation will be described. As described above, the ratchet gear  17  normally engages only with the pawl  22 , and if the sheet cassette  1  is pulled out of the image forming apparatus body A, a projection  40  formed at a rear portion of the sheet cassette  1  pushes a projection  22   a  of the pawl  22 . 
     The pawl  22  engages a cassette lock spring  27  and made immovable in a state shown in FIGS.  4 ( a ),  4 ( b ). Since no pawl engages the ratchet gear  17 , the lifter  16  falls down according to weight of the lifter  16 . 
     Referring to FIG. 5, constitution of conveying routes of sheets S from the feeding section to the image forming section, as the essential structure of the invention, will be described. In FIG. 5, the sheets S contained in the upper cassette  1   b  of the sheet cassette  1  serving as a sheet containing means are separated one by one and fed by respective operations of the pickup roller  3 , the feed roller  4 , and the retard roller  5 , which serve as the sheet feeding means, as described above. The separated sheet S is fed by the conveying roller  104  to pass the conveying path X as a conveying route, and then transferred to the common conveying route Z arranged linearly and vertically in the image forming apparatus A. The conveying roller  105  pulls up the sheet S and sends the sheet S to the register roller  106 . 
     Similarly, the sheets S contained in the lower cassette  1   a  of the sheet cassette  1  are separated one by one and fed by respective operations of the pickup roller  3 , the feed roller  4 , and the retard roller  5 , which serve as the sheet feeding means, as described above. The separated sheet S is fed by the conveying roller  104  to pass the conveying path Y as a conveying route, and then transferred to the common conveying route Z. The conveying roller  105  sends the sheet S to the register roller  106 . 
     As shown in FIG. 5, the conveying path X has a radius of curvature R of 30 millimeters whereas the conveying path Y has a radius of curvature R′ of 10 millimeters. The radius of curvature R of the conveying path X is designed to be relatively large as to be capable of conveying rigid sheets such as thick papers and special sheets such as envelops, whereas the radius of curvature R′ of the conveying path Y is designed to be small as to be capable of conveying regular cut sheets that are ordinarily used. 
     The radius of curvature R (=30 millimeters) of the conveying path X is set twice larger or more of the radius of curvature R′ (=10 millimeters) of the conveying path Y. In this embodiment, the radiuses of curvature R, R′ are suitable respective amounts given through experiments. 
     It is to be noted that the conveying path X having the larger radius of curvature is located above the conveying path Y having the smaller radius of curvature. This is because the upper conveying path X, though having the larger radius of curvature, ensures a clearance where no other conveying path exists over the conveying path. In addition, if rigid sheets are conveyed through a longer route, paper jam possibility may increase, and therefore, it is beneficial to shorten the route from the tray or cassette to the image forming section. Moreover, the upper conveying path X is located closer to the conveying roller  105  in the common conveying route Z, thereby transferring the rigid sheets S at an early stage to the conveying roller  105 . This apparatus has an advantage that such rigid sheets are further smoothly transferred through the curving portions upon using this conveying force in an ancillary fashion. 
     If inflexible sheets such as thick papers or special sheets such as envelops are to be subject to feeding operation where contained in a cassette connected to a conveying route having a curving portion of a small radius of curvature, those sheets S, though introduced into the conveying path Y by the pickup roller  3 , the feed roller  4 , and the conveying roller  5 , may be jammed or delayed due to conveyance resistance incurred from the curving portion of the conveying path Y where the sheets S fail to follow the curving portion of the conveying path Y due to rigidity of the sheets S, and the sheets S may suffer from impairments of conveyance. 
     However, if such sheets S are transferred where contained in the upper cassette  1   b , the sheets S reach the common conveying route Z by way of the conveying path X having the curving portion of the larger radius of curvature. Therefore, even though rigid the sheets S can pass the conveying path X easily, and the sheet feeding apparatus can transfer the sheets to the common conveying route Z and then to the register roller  106 . 
     As described above, because the radius of curvature R of the curving portion of the conveying path X is designed to be larger than the radius of curvature R′ of the curving portion of the conveying path Y, the apparatus can convey the sheets easily along the conveying routes forming the conveying paths even if the sheets are rigid sheets such as thick papers or special sheets such as envelops, and can convey the sheets without jam or delay of the sheets S under a low resistance at a time of the conveyance. 
     The apparatus having plural sheet containing means inside the image forming apparatus A, as described above, includes plural conveying paths generally constituted of U-turn paths as sheet conveying routes from the respective sheet feeding portions to the image forming section, in consideration of the structure of the apparatus. 
     With this embodiment, the radius of curvature of the curving portion of at least one conveying path among the plural conveying paths is arranged to be so larger as to be capable of conveying rigid sheets such as thick papers and special sheets such as envelops. 
     Thus, when images are to be formed on rigid sheets such as thick papers or special sheets such as envelops, users do not have to laboriously insert sheets one by one in a manual feeding slot as a conventional manner to implement a manual feeding, and the apparatus can automatically feed inflexible sheets such as thick papers or special sheets such as envelops upon containing the sheets in the regular sheet containing means, so that a stable feeding operation can be performed without jam or delay in a midway of the conveying routes. 
     In other words, when users perform image forming works on rigid sheets such as thick papers or special sheets such as envelops, the users do not have to laboriously insert sheets one by one in a manual feeding slot as a conventional manner, and the apparatus can automatically feed sheets upon containing the sheets in the regular sheet containing means, so that a stable feeding operation can be performed without jam or delay of sheets, thereby improving controllability of the apparatus. 
     It is to be noted that the upper cassette  1   b  described above can be a special cassette exclusive for containing special sheets such as envelops, or a cassette (adapter) for special sheets, not shown, may be optionally attached to feed sheets such as envelops, when such envelops are set in a regular cassette for regular papers. 
     Although in this embodiment, the image forming apparatus incorporating three sets of the conveying paths, in which each set has two conveying paths having different radiuses of curvature of the curving portions, is exemplified, various combinations of conveying paths are possible for the image forming apparatus, and, for example, the apparatus may have two sets, each set having conveying paths of three different radiuses of curvature.