Patent Publication Number: US-7581729-B2

Title: Sheet conveying apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sheet conveying apparatus and an image forming apparatus, and particularly to a skew feed correcting portion for correcting the skew feed of a sheet. 
   2. Description of the Related Art 
   In an image forming apparatus such as a printer, a copying machine or a printing machine, a toner image or an ink image is formed on a conveyed sheet by an electrophotographic printing method, an offset printing method, an ink jet printing method or the like. 
   A color image forming apparatus using, for example, the electrophotographic printing method is classified, from its construction, chiefly into a tandem type in which a plurality of image forming portions are juxtaposed, and a rotary type in which a plurality of image forming portions are disposed in a cylindrical shape. Also, from its transferring method, it is classified into a direct transfer type in which a toner image is directly transferred from a photosensitive drum which is an image bearing member to a sheet, and an intermediate transfer type in which a toner image is once transferred to an intermediate transfer member, and thereafter is transferred to a sheet. 
   Here, in an intermediate transfer tandem type wherein image forming portions of four colors are juxtaposed on an intermediate transfer belt, it is not necessary to hold a sheet on a transfer drum or a transfer belt as in the direct transfer type. Therefore, a color image forming apparatus adopting the intermediate transfer tandem type can cope with a variety of materials such as super-thick paper and coat paper and moreover, is suitable for the realization of high productivity because of its features such as the parallel processing in the plurality of image forming portions and the collective transfer of a full-color image. 
   Now, in recent years, in the field of such an electrophotographic printing apparatus, there have been provided apparatuses which make the most of the merit of making no print, and aims, for example, at a print-on-demand (POD) market for effecting a small number of prints. However, in order to be accepted by such a quick printing market, it is necessary to achieve a high quality of image, and as an important factor for achieving the high quality of image, there is an image position to a sheet. This image position accuracy includes the deviation of the front side and back side when two-side images have been formed. 
   When the image position accuracy to the sheet is to be improved, it is necessary to improve the accuracy of the registration in the conveyance direction of the sheet, the registration in a direction of the sheet, an image magnification, skew feed correction, etc. 
   Here, the registration in the conveyance direction, the registration in the direction orthogonal to the conveyance direction and the magnification can be corrected by electrical control, but the skew feed in difficult to correct by the electrical control. 
   For example, it is possible to detect the skew feed of a sheet, and form an image inclined in accordance therewith to thereby correct the image position to the sheet. However, particularly in the case of a color image in which three or four colors are superposed one upon another, when an image is inclined on each sheet, the hue is changed for each sheet by the deviation of the dot formation of each color. Also, much time is required for the calculation for inclining the image, thus resulting a remarkable reduction in productivity. Consequently, the correction of the skew feed is determined by the performance of the conveyance accuracy of the sheet. 
   So, heretofore, a skew feed correcting portion for correcting the skew feed of a sheet has been provided in a sheet conveying apparatus for conveying the sheet to a transferring portions, so as to correct the skew feed of the sheet. Here, a method of correcting the skew feed of the sheet is divided broadly into the following four methods. 
   1. A pair of registration rollers are disposed upstream of the transferring portions, and the leading edge of the conveyed sheet is rammed against the registration rollers being at a halt to thereby push in the sheet from the trailing edge side thereof, and form a loop, thereby effecting skew feed correction. Thereafter, the registration rollers are re-started in synchronism with the image, to thereby effect the skew feed correction and the image adjustment in the conveyance direction of the sheet. 
   2. By the use of a retractable shutter instead of the registration rollers, the leading edge of the conveyed sheet is rammed against it is thereby form a loop, thus effecting skew feed correction. Thereafter, on the basis of the detection of the leading edge of the sheet by a sensor, the position adjustment with the sheet is effected, and skew feed correction and the image adjustment in the conveyance direction of the sheet are effected. 
   3. The sheet is rammed against a conveyance reference wall provided in the conveyance direction of the sheet by a obliquely feed roller and is conveyed at the same time to thereby correct the skew feed of the sheet, and the image position adjustment in the conveyance direction is effected by detecting the leading edge of the sheet by a sensor and controlling the conveyance speed of the sheet (disclosed, for example, in Japanese Patent Application Laid-open No. H11-189355). 
   4. Provision is made of means for detecting the skew feed of the sheet and two drive rollers capable of driving independently in a direction perpendicular to the conveyance direction of the sheet, and the speeds of the respective drive rollers are changed in accordance with the skew feed amount of the sheet to thereby correct the skew feed (disclosed, for example, in Japanese Patent Application Laid-open No. H05-201587). 
   Here, the third method, when viewed regarding the deviation of the front side and back side of the images during two-side printing, has the merit that although the sheet is switched back, whereby the leading edge and the trailing edge change places with each other between a first side and a second side, the sides do not change places with each other and therefore, skew feed can be corrected on the same reference for both of the first side and the second side. In the other methods, skew feed correction is effected at the leading edge and therefore, the reference for effecting correction differs between the first side and the second side and thus, when the parallelism between the leading and trailing edges of the sheet is not sufficient, the deviation between the front side and the back side will occur even if the correction capability is high. 
     FIG. 16  of the accompanying drawings is a plan view showing the construction of a skew feed correcting portion which corrects skew feed by such a third method. This skew feed correcting portion is provided in a sheet conveying apparatus for conveying a sheet on the so-called center reference. 
   In  FIG. 16 , a obliquely feed roller guide portion  100  is provided with a side regulating plate  105  and obliquely feed rollers  253 ,  254  and  255 . These obliquely feed rollers  253 ,  254  and  255  are inclined and provided on a lower guide  102  side so as to convey the sheet toward the side regulating plate  105 . When the sheet S is conveyed by the obliquely feed rollers  253 ,  254  and  255 , the sheet S is rammed against a reference wall  105   a  provided on the side regulating plate  105  shown in  FIG. 18  of the accompanying drawings which will be described later, and has its skew feed corrected thereby. 
   The reference numeral  115  designates a fixed lower guide constituting a sheet conveying path together with the obliquely feed roller guide portion  100 , which is movable relative to the fixed lower guide  115  in a width direction orthogonal to the conveyance direction of the sheet. 
   Here, the obliquely feed roller guide portion  100  is movable in accordance with the size of the sheet S, and is moved in advance to a position corresponding to the size before the sheet S is conveyed thereto, thereby contriving the higher efficiency of the correction time. When the skew feed of a sheet Smax of a maximum size is to be corrected, as shown in  FIG. 16 , the gap between the obliquely feed roller guide portion  100  and the fixed lower guide  115  becomes a maximum Gmax. Also, when the skew feed of a sheet 5 min of a minimum size is to be corrected, as shown in  FIG. 17  of the accompanying drawings, the gap between the obliquely feed roller guide portion  100  and the fixed lower guide  115  becomes a minimum Gmin. The wider is the range of the passable sheet size, the greater becomes the gap between the obliquely feed roller guide portion  100  and the fixed lower guide  115  when the sheet of the maximum size is passed. 
   Now, the sheet conveying apparatus provided with such a conventional skew feed correcting portion suffers from the following problems. The greater is the difference between the maximum size and minimum size of conveyable sheets, the wider becomes the gap between the obliquely feed roller guide portion  100  and the fixed lower guide  115  when the skew feed of the sheet of the maximum size is corrected. When the gap between the guides thus becomes wide, downward flexure indicated by arrow P at the central portion as shown in  FIG. 18  is caused to occur to the sheet S by its gravity. 
   When the sheet S is conveyed with the downward flexure thus caused to occur to the sheet, as shown a side view of in the apparatus of  FIG. 19  of the accompanying drawings, the central portion of the sheet S is sometimes caught by the entrance lower guide  300   b  of a pair of registration rollers  131  and  132  provided downstream of a lower guide  102  and the fixed lower guide  115 . When the sheet S is thus caught, jam is caused or bending occurs to the central portion of the sheet. This tendency is more remarkable in the case of thin sheets of weak stiffness. 
   SUMMARY OF THE INVENTION 
   So, the present invention has been made in view of such present situation, and has as its object to provide a sheet conveying apparatus and an image forming apparatus which can correct skew feed without causing jam or bending and can convey a sheet. 
   The present invention provides a sheet conveying apparatus provided with a skew feed correcting portion for correcting the skew feed of a conveyed sheet, the skew feed correcting portion comprising: a first guide portion movable in a width direction orthogonal to a conveyance direction of the sheet to regulate a position of one side edge of the conveyed sheet; a second guide portion provided opposite to the first guide portion to guide the other side edge of the sheet; and an auxiliary guide provided between the first guide portion and the second guide portion, and movable in the width direction of the sheet, wherein the first guide portion is moved in advance to a position corresponding to a size of the conveyed sheet, and the auxiliary guide is moved to a guide position corresponding to the size of the sheet in operative association with the first guide portion. 
   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  schematically shows the construction of a color image forming apparatus which is an example of an image forming apparatus according to a first embodiment of the present invention. 
       FIG. 2  is a perspective view of a registration unit provided in the color image forming apparatus. 
       FIG. 3  is a perspective view of a registration roller portion provided in the registration unit. 
       FIG. 4  is a perspective view of a obliquely feed roller guide portion provided in the registration unit. 
       FIG. 5  is a side view of a obliquely feed guide moving driving portion provided in the registration unit. 
       FIG. 6  is a perspective view of a fixed guide portion provided in the registration unit. 
       FIG. 7  is a perspective view of the back side of the fixed guide portion. 
       FIG. 8  is a side view of the registration unit showing a state when it conveys a sheet of a maximum size. 
       FIG. 9  is a side view of the registration unit showing a state when it conveys a sheet of a minimum size. 
       FIG. 10A  is a cross-sectional view of the registration unit. 
       FIG. 10B  is an enlarged view of a portion  10 B encircled by the circle of  FIG. 10A . 
       FIG. 11A  shows the guide interval between the fixed guide portion and obliquely feed roller guide portion of the registration unit and the pendent amount of the sheet. 
       FIG. 11B  represents the relation between the guide interval and the pendent amount of the sheet. 
       FIG. 12  is a top plan view illustrating another construction of an auxiliary guide provided in the registration unit. 
       FIG. 13  is a top plan view of a registration unit provided in an image forming apparatus according to a second embodiment of the present invention. 
       FIG. 14  is a top plan view of a registration unit provided in an image forming apparatus according to a third embodiment of the present invention. 
       FIG. 15  illustrates the construction of an auxiliary guide provided in the registration unit. 
       FIG. 16  is a top plan view of a conventional skew feed correcting portion showing a state when it conveys a sheet of a maximum size. 
       FIG. 17  is a top plan view of the conventional skew feed correcting portion showing a state when it conveys a sheet of a minimum size. 
       FIG. 18  illustrates an inconvenience peculiar to the conventional skew feed correcting portion. 
       FIG. 19  is another view illustrating the inconvenience peculiar to the conventional skew feed correcting portion. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   An aspect of carrying out the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
     FIG. 1  schematically shows the instruction of a color image forming apparatus which is an example of an image forming apparatus according to a first embodiment of the present invention. 
   In  FIG. 1 , the color image forming apparatus main body (hereinafter referred to as the apparatus main body)  1 A of the color image forming apparatus  1  is provided with a sheet feeding portion  1 B for feeding a sheet S, an image writing-in portion  90  for forming a toner image, and a transferring portion  1 C for transferring the toner image formed by the image writing-in portion  90  to the sheet S fed by the sheet feeding portion  1 B. The image forming portion of the color image forming apparatus  1  is constituted by the image writing-in portion  90  and the transferring portion  1 C. 
   In  FIG. 1 , a sheet conveying apparatus  20  conveys the sheet S fed by the sheet feeding portion  1 B to the transferring portion  1 C. This sheet conveying apparatus  20  is provided with a registration unit  30  which is a skew feed correcting portion for effecting the skew feed correction and timing correction of the sheet S, and a conveying roller portion  20 A for conveying the sheet to the registration unit  30 . 
   Here, the image writing-in portion  90  is constituted by four image forming units  90   a ,  96 ,  97  and  98  of yellow (Y), magenta (M), cyan (C) and black (Bk). Each of these image forming units  90   a ,  96 ,  97  and  98  is comprised of a photosensitive member  91 , an exposing device  93 , a developing device  92 , a primary transfer device  45  and a photosensitive member cleaner  95 . 
   The color image forming apparatus  1  according to the present embodiment is of an intermediate transfer tandem type in which the image forming units  90   a ,  96 ,  97  and  98  of the four colors as image forming portions are juxtaposed on an intermediate transfer belt, which will be described later. The colors formed by the image forming units  90   a ,  96 ,  97  and  98  are not restricted to these four colors, nor the arrangement order of the colors is restricted to the named order. 
   Also, the sheet feeding portion  1 B is provided with a sheet supplying cassette  10  which is a sheet contained in a form stacked on a lift-up device  11  and from which the sheets can be drawn out, and sheet feeding means  12  for feeding out the sheets S contained in the sheet supplying cassette  10 . Here, as this sheet feeding means  12 , mention may be made of a type utilizing the frictional separation by a sheet feeding roller or the like, or a type utilizing the separation suction by air, but in the present embodiment, the sheet feeding method by air is mentioned as an example. 
   Also, the transferring portion  10 C is provided with an intermediate transfer belt  40  passed over such rollers as a drive roller  42 , a tension roller  41  and a secondary transfer inner roller  43 , and driven to convey in the direction indicated by the arrow in  FIG. 1 . 
   Here, this intermediate transfer belt  40  has transferred thereto a toner image formed on the photosensitive member, by a predetermined pressure force and an electrostatic load bias given by a primary transfer device  45 . Also, in a secondary transferring portion formed by the secondary transfer inner roller  43  and a secondary transfer outer roller  44  substantially opposed to each other, the predetermined pressure force and the electrostatic load bias are given to thereby cause an unfixed image to be attracted to the sheet S. 
   A patch detecting sensor  47  is provided upstream of the secondary transfer inner roller  43 , and detects the color misregister of multi-transferred toner images and the leading edge position of the toner images. Also, a cleaner  46  is provided downstream of the secondary transfer inner roller  43 , and collects any toners residual on the intermediate transfer belt  40 . 
   When an image is to be formed in the color image forming apparatus  1  of such a construction, the surface of the photosensitive member  91  is first uniformly charged in advance by charging means (not shown). Thereafter, the exposing device  93  emits light to the photosensitive member  91  rotated in the direction indicated by the arrow, on the basis of the signal of image information sent thereto, and applies this light suitably via reflecting means or the like, whereby a latent image is formed on the surface of the photosensitive member. Any untransferred toner residual on the photosensitive member  91  is collected by the photosensitive member cleaner  95 . 
   Next, toner development is effected on the electrostatic latent image formed on the photosensitive member  91  in this manner by the developing device  92 , and a toner image is formed on the photosensitive member. Thereafter, a predetermined pressure force and an electrostatic load bias are given to the intermediate transfer belt  40  by the primary transfer device  45 , and the toner image is transferred onto the intermediate transfer belt  40 . 
   The image formation by the Y, M, C and Bk image forming units  90   a ,  96 ,  97  and  98  of the image writing-in portion  90  is effected at the timing whereat the images are superposed on the upstream toner image primary-transferred onto the intermediate transfer belt. As the result, finally, a full-color toner image is formed on the intermediate transfer belt  40 . 
   Also, the sheet S is fed out of the sheet supplying cassette  10  by the sheet feeding means  12  in timed relationship with the image formation of the image writing-in portion  90 , whereafter the sheet S passes through a conveying path  20   a  and is conveyed to the registration unit  30 . In this registration unit  30 , the skewfeed correction and timing correction of the sheet are effected, whereafter the sheet S is conveyed to a secondary transferring portion formed by the secondary transfer inner roller  43  and the secondary transfer outer roller  44  opposed to each other. Thereafter, in the secondary transferring portion, a predetermined pressure force and an electrostatic load bias are given to the sheet S, whereby a full-color toner image is secondary-transferred onto the sheet S. 
   The sheet S onto which the toner image has been thus secondary-transferred is conveyed to a fixing device  50  by an ante-fixing conveying portion  51 . Then, in this fixing device  50 , a predetermined pressure force by rollers substantially opposed to each other or a belt, and generally a heating effect by a heat source such as a heater are applied to the sheet S to thereby fuse and fix the toner on the sheet S. 
   Next, the sheet S having thereon a fixed image obtained in this manner is intactly discharged onto a sheet discharge tray  61  by a branching-off conveying device  60 . When images are to be formed on the two sides of the sheet S, the sheet S is conveyed to a reversal conveying device  70  by the changeover of a flapper (not shown). 
   Here, when the sheet S is thus conveyed to the reversal conveying device  70 , the sheet S has its leading and trailing edges changed in places by a switchback operation and is conveyed to a re-conveying path R provided in a duplex conveying device  80 . Thereafter, the sheet S joins from the sheet re-feeding path  20   b  of the sheet conveying apparatus  20  is timed relationship with the sheet of the succeeding job conveyed from the sheet feeding portion  1 B, and is likewise conveyed to the secondary transferring portion. The image forming process is similar to that for the first side and therefore need not be described. 
   Now, the registration unit  30  is provided with a sheet conveying path R 1  and skew feed correcting means provided with a reference guide  105   a  provided on one side edge of the sheet conveying path R 1  and extending in the conveyance direction of the sheet as shown in  FIG. 4 , and obliquely feed rollers  253 ,  254  and  255  for pushing a side edge of the sheet S against the reference guide  105   a . The skew feed of the sheet S is corrected by this skew feed correcting means so as to correct the position of the sheet S in the width direction thereof (main scanning direction). 
   Here, the positioning of the sheet is effected by the end portion of the sheet being rammed against the reference guide  105   a  by the obliquely feed rollers  253 ,  254  and  255 . The position of this reference guide  105   a  in a direction orthogonal to the conveyance direction of the sheet is offset by a predetermined amount outwardly from the position of a regular sheet conveyance reference (the reference of the side edge portion of the sheet when the sheet is set as being regularly conveyed with the center reference). This is because if the reference guide  105   a  is positioned at the sheet conveyance reference, the sheet and the reference guide  105   a  may interfere with each other due to the irregularity of the sheet actually conveyed with the center reference. 
   Therefore, in the registration unit  30 , one side edge of the sheet is regulated by the reference guide  105   a  at a position offset by a predetermined amount outwardly from the sheet conveyance reference, whereafter this offset amount is returned. Thereby, the sheet being conveyed can be positioned at the sheet conveyance reference. Accordingly, in the registration unit  30 , the skew feed of the sheet being conveyed can be corrected and also, the sheet being conveyed can be made coincident with a sheet conveyance reference position in the direction orthogonal to the conveyance direction, and an image forming process of high accuracy can be carried out. 
     FIG. 2  is a perspective view of such a registration unit  30 . This registration unit  30  is constituted by an obliquely feed roller guide portion  100 , a fixed guide portion  110 , an obliquely feed guide moving driving portion  120 , a registration roller portion  130 , a registration roller slide portion  150 , a registration roller pressure releasing portion  160 , etc. The reference numeral  200  designates the frame of the registration unit  30 . 
   The registration roller portion  130  serves to convey the sheet having had its skew feed corrected to the secondary transferring portion at predetermined timing, and is comprised of an upper registration roller  131  and a lower registration roller  132 , as shown in  FIG. 3 . 
   The lower registration roller  132  is supported for sliding movement in the axial direction thereof and rotation by a slide bearing  133  fixed to the frame  200  (see  FIG. 2 ). Also, the upper registration roller  131  is slidably and rotatably supported by a slide bearing  133  fixed onto a pressure arm  134 . The pressure arm  134  is rotatably fixed to a shaft  200   a  formed on the frame  200 , and is biased in the direction indicated by the arrow in  FIG. 3  by a tension spring  136  to thereby bring the upper registration roller  131  into pressure contact with the lower registration roller  132 . 
   A registration roller gear  137  is fixed to the lower registration roller  132 , and the drive from a registration roller driving portion  140  is transmitted to the lower registration roller  132  through this registration roller gear  137 . 
   This registration roller driving portion  140  is provided with a motor  141  fixed to the frame  200 , and drive gears  142  and  143  for transmitting the drive of the motor  141  to the registration roller gear  137 . Also, the drive gear  143  meshing with the registration roller gear  137  has the length of its tooth surface so that the meshing engagement thereof may not be released even if the registration roller gear  137  performs its sliding operation with the axial sliding of the lower registration roller  132  which will be described later. Further, the motor  141  rotates in a counter clockwise direction as viewed from a plane to which the motor  141  is mounted. 
   Also, one end portion of the lower registration roller  132  is rotatably held by a holder  138 , which is supported for movement in the axial direction thereof by a bearing. This holder  138  has mounted thereon a sensor flag  213  for detecting the home position (HP) of the pair of registration rollers  131  and  132  in the main scanning direction. 
   This holder  138  is fixed to a timing belt  151  by a stopper  212  and a screw. Thereby, when the timing belt  151  is moved in a direction orthogonal to the conveyance direction of the sheet by the drive from the registration roller driving portion  140 , the lower registration roller  132  is adapted to be moved integrally with the holder  138 . 
   A runner receiver  210  is fixed to the other end portion of the lower registration roller  132 , and a runner  211  fixed to the upper registration roller  131  is engaged with this runner receiver  210 . Thereby, when the lower registration roller  132  is moved following the operation of the timing belt  151 , the upper registration roller  131  and the lower registration roller  132  can be reciprocally moved in the direction orthogonal to the conveyance direction of the sheet in synchronism with each other. 
   Upstream of the pair of registration rollers  131  and  132  with respect to the conveyance direction of the sheet, there are provided an entrance upper guide  300   a  and an entrance lower guide  300   b  for receiving the sheet having had its skew feed corrected by a obliquely feed roller guide portion  100  which will be described later. Also, downstream of the pair of registration rollers  131  and  132  with respect to the conveyance direction of the sheet, an exit guide  301  is fixed to the frame  200 . Further, the entrance upper guide  300   a  and the exit guide  301  are provided with sheet detecting sensors  302  and  303 , respectively. 
   The registration roller slide portion  150  is comprised of a slide motor  159 , a timing belt  155   a  and pulleys  155   b  and  155   c , and is provided with a driving train  155  for transmitting the drive of the slide motor  159  to the timing belt  151 . The slide motor  159  is fixed to a motor stand  152  shown in  FIG. 2  and is screwed to a motor supporting plate  153 . 
   When the slide motor  159  is, for example, clockwise rotated, this rotation is transmitted to the timing belt  151  through the driving train  155 . Thereby, together with the holder  138 , the pair of registration rollers  131  and  132  slide in the direction indicated by the arrow A in  FIG. 3 , and the sheet offset to correct the aforedescribed skew feed is returned to the central conveyance reference position. When the slide motor  159  has been counterclockwise rotated, the pair of registration rollers  131  and  132 , together with the holder  138 , are moved in the direction indicated by the arrow B in  FIG. 3 . 
   The registration roller pressure releasing portion  160  is for releasing the pressure contact between the pair of registration rollers  131  and  132 , and is constituted by a pressure releasing shaft  161  supported by a bearing positioned on the frame  200 , and a releasing cam  162  fixed to the pressure releasing shaft  161 . The pressure releasing shaft  161  is driven by a registration releasing motor  165 . 
   Here, a deep groove ball bearing  164  is forced into the releasing cam  162  at a position eccentric from the rotation center thereof, and when the releasing shaft  161  is caused to make one full rotation, pressurization and release can be effected each one time by the action of the contact and separation of the deep groove ball bearing  164 . 
   Also, a sensor flag (not shown) is mounted on the pressure releasing shaft  161 , and the phase of the pressure releasing shaft  161  is detected by a detecting sensor  167  positioned on and fixed to a sensor supporting plate fixed to the frame  200  to thereby control the rotation of the registration releasing motor  165 . The phase of the releasing cam  162  is determined so as to shield the detecting sensor  167  when a sensor flag pressurizing operation is being performed. 
   The obliquely feed roller guide portion  100  which is a first guide portion (movement guide portion) in the present embodiment is constituted by a base portion  101 , a lower guide portion  102 , an upper guide portion  103  and a hinge portion  104 , as shown in  FIG. 4 .  FIG. 4  shows a state in which the upper guide portion  103  has been separated from the base portion  101 . This upper guide portion  103  is adapted to fit the rotation center shaft  106  of the hinge portion  104  usually fixed onto the base portion  101  into an aperture  107   a  in an arm  107 , to thereby be held on the base portion  101  for pivotal movement in a vertical direction. 
   On the lower guide  102 , obliquely feed rollers  253 ,  254  and  255  made of rubber which are obliquely feed means driven by a motor (not shown) and obliquely feeding the sheet are disposed at an angle inclined at θ (deg) with respect to the conveyance direction of the sheet indicated by the arrow C. Also, this lower guide  102  is provided with a side regulating plate  105  provided with a reference wall  105   a  which is a reference guide extending in the conveyance direction of the sheet which constitutes skew feed correcting mans together with the obliquely feed rollers  253 ,  254  and  255 . The sheet S, when conveyed by the obliquely feed rollers  253 ,  254  and  255 , strikes against the reference wall  105   a  provided on the side regulating plate  105 , whereby it is conveyed while its skew feed is corrected. 
   Also, runner pressure units  260 ,  261  and  262  provided with pressure runners  271 ,  272  and  273  which are rotatable idler runners, and releasing motors  263 ,  264  and  265 , respectively, are fixed onto the upper guide  103 . 
   Here, these releasing motors  263 ,  264  and  265  are for effecting the changeover of the contact (pressurized) state and the non-contact state of the pressure runners  271 ,  272  and  273  on the respective obliquely feed rollers  253 ,  254  and  255 . In the present embodiment, the release of the contact of the pressure runners  271 ,  272  and  273  takes place at a point of time whereat the leading edge of the sheet S has been nipped between the registration rollers, and the contact thereof takes place before the trailing edge of the sheet leaves the pressure runners  271 ,  272  and  273  and the next sheet arrives. 
   The obliquely feed guide moving driving portion  120  is for effecting the positioning of the obliquely feed roller guide portion  100 . This obliquely feed guide moving driving portion  120 , as shown in  FIG. 5 , is provided with a base portion  121 , a first bearing stand  122  and a second bearing stand fixed to the base portion  121 , and a lead screw  124  rotatably supported by the first and second bearing stands  122  and  123 . The base portion  121  is fixed to the frame  200 . 
   Also, a nut  125  sliding in accordance with the rotation of the lead screw  124  is provided on the lead screw  124 , and further a bracket  126  is mounted on this nut  125 . The nut  125  and the obliquely feed roller guide portion  100  are connected together by this bracket  126 . 
   Also, a drive motor  127  which is a stepping motor is connected to one end of the lead screw  124 , and changes its driving pulse number in accordance with the size of the sheet, and effects the positioning of the obliquely feed roller guide portion  100 . 
   A fixed guide portion  110  which is a second guide portion in the present embodiment, as shown in  FIG. 6 , is constituted by a fixed lower guide  115  fixed to the frame  200 , a hinge stand  114  fixed to the frame  200 , and a fixed upper guide portion  111 . Here, this fixed upper guide portion  111  is adapted to be held on the hinge stand  114  for pivotal movement in a vertical direction.  FIG. 6  shows a state in which the fixed upper guide portion  111  has been separated. 
   Description will now be made of the operation of the thus constructed registration unit  30 . 
   Before the sheet is conveyed, the position of the obliquely feed roller guide portion  100  is adjusted by the obliquely feed guide moving driving portion  120  so that the reference wall  105   a  may come to a position offset in advance by a predetermined amount (F mm) from a sheet conveyance reference position in accordance with the width of the sheet S being conveyed. 
   Next, the sheet fed in by the sheet conveying apparatus  20  is nipped and conveyed by the obliquely feed rollers  253 ,  254 ,  255  and the pressure runners  271 ,  272 ,  273  shown in  FIG. 4 , and is obliquely fed toward the reference wall  105   a  at an angle θ (deg), and one side edge of the sheet abuts against the reference wall  105   a . When the sheet has been nipped by the obliquely feed rollers  253 ,  254 ,  255  and the pressure runners  271 ,  272 ,  273 , the roller of the conveying roller portion  20 A of the sheet conveying apparatus  20  is pressure-released by a pressure releasing mechanism (not shown). 
   Next, the sheet abutting against the reference wall  105   a  is fed to the pair of registration rollers  131  and  132 . Thereafter, the sheet is fed by a predetermined amount, whereupon a pressure releasing operation of the pressure runners  271 ,  272  and  273  opposed to the obliquely feed rollers  253 ,  254  and  255 , respectively, to the obliquely feed roller  253  is performed by runner pressure units  260 ,  261  and  262 . 
   Further, at a stage whereat such a pressure releasing operation of the pressure runners  271 ,  272  and  273  has been completed, the slide motor  159  of the registration roller slide portion  150  is rotated in the clockwise direction to thereby move the pair of registration rollers  131  and  132  in the direction indicated by the arrow A in  FIG. 3  already described (the amount of movement in F mm). Thereby, the sheet having had its skew feed corrected by the reference guide  105   a  at a position deviating outwardly from the sheet conveyance reference can be made coincident with a regular sheet conveyance reference position. 
   Next, when the sheet S is fed to the secondary transfer outer roller  44 , the pair of registration rollers  132  and  133  are pressure-released by a registration roller pressure releasing portion  160 . Also, when the trailing edge of the sheet passes between the pair of registration rollers  131  and  132 , the slide motor  151  is rotated in the clockwise direction to prepare for the next sheet, and the pair of registration rollers  131  and  132  are moved in the direction indicated by the arrow B in  FIG. 3  already described (the amount of movement is F mm). Further, after the completion of the movement of the pair of registration rollers  131  and  132 , a pressure releasing operation is performed by the registration roller pressure releasing portion  160 . 
   As shown in  FIG. 6 , an auxiliary guide  117  which is an essential construction of the present invention is provided on the fixed lower guide  115  for sliding to the obliquely feed roller guide portion  100  side. Here, this auxiliary guide  117  has two shafts  118   a  and  118   b  fixed thereto as shown in  FIG. 7  which is a view of the fixed lower guide  115  as it is seen from its back. 
   These two shafts  118   a  and  118   b  are slidably supported by bearings  119   a  to  119   d  fixed onto the fixed lower guide  115 . Also, they are biased toward the center by springs  112   a  and  112   b  through E rings  113   a  and  113   b , and the E rings  113   a  and  113   b  are adapted to be protruded to and stopped at a predetermined position whereat they contact with a side of the fixed lower guide  115 . 
   This stop position is substantially the center of the guide gap Gmax between the fixed guide portion  110  and the obliquely feed roller guide portion  100  in the width direction during the conveyance of a sheet of a maximum size, as shown in  FIG. 8 . Here, the auxiliary guide  117  is thus protruded to substantially the center of the guide gap Gmax, whereby the sheet can be guided while the central portion of the sheet being conveyed is supported from below it by the auxiliary guide  117 , and the central portion of the sheet can be prevented from being downwardly flexed. 
     FIG. 9  represents the state during the conveyance of a sheet of a minimum size, and at this time, the obliquely feed roller guide portion  100  has been moved from its position shown in  FIG. 8  to the fixed guide portion  110  side. When the obliquely feed roller guide portion  100  is thus moved, the obliquely feed roller guide portion  100  is moved while pushing the auxiliary guide  117  toward the fixed lower guide  115  against the forces of the springs  112   a  and  112   b . Thereby, the obliquely feed roller guide portion  100  can be moved without being hindered by the auxiliary guide  117 . 
   Also, when the obliquely feed roller guide portion  100  is moved from the position during the conveyance of the sheet of the minimum size shown in  FIG. 9  to the position during the conveyance of the sheet of the maximum size shown in  FIG. 8 , this auxiliary guide  117  is returned to substantially the center of the guide gap Gmax shown in  FIG. 8  by the springs  112   a  and  112   b.    
   Now, the height of the auxiliary guide  117  on the entrance side with respect to the conveyance direction of the sheet is lower than the fixed lower guide  115  of the fixed guide portion  110  (and the lower guide portion  102  of the obliquely feed roller guide portion  100 ), as shown in  FIGS. 10A and 10B . Thereby, the leading edge portion of the sheet can be prevented from being caught by the auxiliary guide  117  when the sheet is conveyed from the sheet conveying apparatus  20  to the registration unit  30 . 
     FIG. 11B  shows the relation between the guide gap G and the flexure amount H of the central portion of the sheet when the sheet S is placed on the fixed lower guide  115  of the fixed guide portion  110  and the lower guide portion  102  of the obliquely feed roller guide portion  100 , in accordance with the kind of the sheet. 
   Here, generally a sheet having a great basis weight is thicker and greater in stiffness and therefore, becomes small in the flexure amount H even if the guide gap is great, but coat paper of even the same basis weight is weak in stiffness and great in the flexure amount H. Also, even a considerably thin sheet of plain paper having a basis weight of 35 g/m 2  is of the order of 1 mm in the flexure amount H for a guide gap of 60 mm, and becomes very little different from the other sheets. 
   In the present embodiment, the heights of the obliquely feed roller guide  102  and the fixed lower guide  115  are the same, and the difference F thereof in the height direction from a registration roller entrance lower guide  300   b  shown in  FIG. 10B  is 5 mm. Therefore, if the guide gap is set to 80 mm or less, the sheet will not be caught by the registration roller lower guide  300   b  even if the sheet is downwardly flexed, and jam will not occur. Also, the sheet of the minimum size of 139 mm, and the guide gap G in that case is 7.5 mm. 
   On the other hand, the sheet of the maximum size is 364 mm, and the guide gap G at this time is 120 mm, and in this case, as shown in  FIG. 11A , the sheet comes to be downwardly flexed. So, when in the present embodiment, the sheet of the maximum size is to be conveyed, the auxiliary guide  117  is adapted to be most protruded in accordance with the size of the sheet. Specifically, the protruded position of this auxiliary guide  117  is at 56 mm from the fixed lower guide  115  which is substantially the center of the guide gap G which is the setting with a surplus relative to 80 mm. The width of the auxiliary guide  117  is 5 mm. 
   As described above, when the sheet passes through the sheet conveying path R 1  of the registration unit  30 , the obliquely feed roller guide portion  100  regulating the position of one side edge of the sheet by the skew feed correcting portion is moved in advance to a position corresponding to the size of the sheet, and in operative association with the movement of the obliquely feed roller guide portion  100 , the auxiliary guide  117  is moved to a guide position corresponding to the size of the sheet, whereby the sheet can be prevented from being downwardly flexed in the guide gap G. Thereby, the sheet can be conveyed with its skew feed corrected without jam or bending occurring to the sheet. 
   In a case where the difference between the sheet of the maximum size and the sheet of the minimum size is great and the flexure of the sheet cannot be prevented against the guide gap G by the single auxiliary guide  117 , provision may be made of a plurality of auxiliary guides, e.g. two auxiliary guides  117   a  and  117   b  as shown in  FIG. 12 . 
   Here, these two auxiliary guides  117   a  and  117   b  are set so as to assume a substantially uniform interval Gd during the conveyance of the sheet of the maximum size. Also, one auxiliary guide  117   b  is biased toward the center by a spring  112   c  other than a spring for the other auxiliary guide  117   a , whereby the auxiliary guides  117   a  and  117   b  are pushed into the fixed lower guide  115  side by the movement of the obliquely feed roller guide portion  100 . 
   A second embodiment of the present invention will now be described. 
     FIG. 13  is a to plan view of a registration unit provided in an image forming apparatus according to the present embodiment. In the present embodiment, two (plural) auxiliary guides  117   a  and  117   b  are provided between a obliquely feed roller guide portion  100  and a fixed guide portion  110 , and are adapted to be moved by a link mechanism  217  provided with links  217   a  to  217   f.    
   Here, the links  217   a  and  217   b  each having one end thereof pivotally supported by the fixed guide portion  110  have their other end connected to one end of the links  217   d  and  217   c  while being connected together at a central portion  218   a . Also, the links  217   e  and  217   f  each having one end thereof pivotally supported by the obliquely feed roller guide portion  100  have their other ends connected to the other ends of the links  217   d  and  217   c  while being connected together at a central portion  218   c.    
   The auxiliary guides  117   a  and  117   b  are placed on the connected portions of the links  217   a  to  217   f . Thereby, in operative association with the obliquely feed roller guide portion  100  being moved in accordance with the size of the sheet, the auxiliary guides  117   a  and  117   b  are moved by the link mechanism with the interval Gd therebetween kept uniform, but with the distance therebetween changed. Of course, in a case where a plurality of auxiliary guides are unnecessary, the number of the links may be decreased and a single auxiliary guide may be provided in the central portion. 
   A third embodiment of the present invention will now be described. 
     FIG. 14  is a top plan view of a registration unit provided in an image forming apparatus according to the present embodiment. In the present embodiment, the auxiliary guide is formed by an elastically deformable member, e.g. a coil spring  117   d.    
   Here, this coil spring  117   d , as shown in  FIG. 15 , is passed over shafts  118   d  and  118   e  and also, is tied from a obliquely feed roller guide portion  100  to a fixed guide portion  110 , and the upper portion of the coil spring  117   d  is used as a guide. The shafts  118   d  and  118   e  are fixed to the obliquely feed roller guide portion  100  side, and are slidably held by the fixed guide portion  110 . 
   Thereby, when the obliquely feed roller guide portion  100  is moved to a position corresponding to a sheet of a small size, the shafts  118   d  and  118   e  come to be contained in the fixed guide portion side. Then, along therewith, the coil spring  117   d  is compressed by being sandwiched between the obliquely feed roller guide portion  100  and the fixed guide portion  110 . 
   While in the foregoing, description has been provided of an example in which the present invention is applied to a registration unit provided with a pair of registration rollers  131  and  132 , the present invention is not restricted thereto. For example, even in a conveying portion such as a duplex conveying portion, in a case where it is provided with a fixed guide portion and a movable guide portion constituting a sheet conveying path, and the movable guide portion is moved in the width direction in accordance with the size of the sheet, it is possible to apply the present invention thereto. 
   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. 2005-259933, filed Sep. 7, 2005, which is hereby incorporated by reference herein in its entirety.