Patent Publication Number: US-10759618-B2

Title: Sheet conveying apparatus and image forming apparatus

Description:
This application is a Divisional Application of U.S. patent application Ser. No. 15/409,681, filed Jan. 19, 2017, which is a Divisional Application of U.S. patent application Ser. No. 15/153,904, filed May 13, 2016, now U.S. Pat. No. 9,586,778, which is a Divisional Application of U.S. patent application Ser. No. 14/365,746, filed on Jun. 16, 2014, now U.S. Pat. No. 9,388,005, which is a National Stage Entry of International Application No. PCT/JP2013/052779, filed on Jan. 31, 2013, which claims the benefit of Japanese Patent Application No. 2012-025190, filed Feb. 8, 2012, which are all hereby incorporated by reference herein in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a sheet conveying apparatus configured to correct a skew feed of a sheet being conveyed, and an image forming apparatus including the sheet conveying apparatus. 
     BACKGROUND ART 
     In general, in an image forming apparatus, accuracy of a recording position (hereinafter referred to as “recording accuracy”) of an image with respect to a sheet is one of the important factors from the viewpoint of keeping the image quality. Therefore, for example, when a sheet being conveyed is skewed in an image formation, it is necessary to correct the skewed sheet to form an image in an appropriate position on the sheet. As described above, various sheet conveying apparatus having a skew feed correction function are proposed to improve the recording accuracy in the conventional image forming apparatus. 
     For example, the sheet conveying apparatus described in Patent Literature 1 includes a plurality of conveying roller pairs configured to convey a sheet to an image forming portion, which are arranged on an upstream side in a sheet conveying direction so as to be closest to the image forming portion (hereinafter simply referred to as “upstream side”), and a locking member rotatable about a rotary shaft for one roller of the conveying roller pairs. The locking member has an abutment surface against which the sheet abuts, and is urged by an urging spring so that the abutment surface is located at a home position (position at which a leading edge of a skewed sheet is brought into abutment with the abutment surface so that skew feed is corrected). When a leading edge of the conveyed sheet abuts against the abutment surface of the locking member, the sheet is locked by an urging force applied to the locking member, and the locked sheet is deflected to be curved to form a loop. The loop thus formed causes the leading edge portion of the sheet to be aligned with a width direction orthogonal to the sheet conveying direction, to thereby correct the skew feed. After that, when the locking member is pushed and rotated by stiffness of the sheet, the sheet is nipped by nips of the conveying roller pairs with the leading edge of the sheet being aligned in the width direction, and conveyed to a downstream side by the conveying roller pairs. 
     By the way, in recent years, much higher throughput (number of sheets subjected to image formation per unit time) of an image forming apparatus has been demanded. In order to meet this demand, it has been required to convey sheets at a higher speed, and reduce a distance between a trailing edge of a preceding sheet and a leading edge of a succeeding sheet (hereinafter referred to as “sheet-to-sheet distance”). In accordance therewith, it is necessary to return the locking member to its home position within a short sheet-to-sheet distance. 
     In this context,  FIGS. 23A and 23B  illustrate a locking member  170  provided in a conventional sheet conveying apparatus. As illustrated in  FIGS. 23A and 23B , the conventional locking member  170  is supported in a manner of being reciprocally rotatable about a rotary shaft  131   a  of a conveying roller  131 , which forms a nip N with a conveying rotatable member  132 . After correcting skew feed of a sheet S by bringing a leading edge of the sheet S into abutment against an abutment surface  170   a  at a home position HP, the locking member  170  is rotated to guide the sheet S to the nip N. Then, after the sheet S passes through the nip N, the locking member  170  is reversely rotated to return to the home position HP. 
     In this case, a requisite minimum sheet-to-sheet distance is equal to a total distance of a distance D 1  from a position at which a trailing edge of a preceding sheet S passes the abutment surface  170   a  of the locking member  170  to the home position HP at which the skew feed of the sheet S is corrected, and a distance D 2  required for conveying a succeeding sheet S to the home position HP while the preceding sheet S is moved by the distance D 1 . The locking member  170  performs reciprocating rotation, and hence the distance D 1  is generated so as to return the locking member  170  to the home position HP after the sheet S passes through the nip N, and the locking member  170  takes a time ΔT for moving the distance D 1 . On the other hand, the distance D 2  is a distance (ΔT×V) obtained by multiplying the time ΔT during which the locking member  170  moves the distance D 1  by a conveying speed V of the sheet S. As the conveying speed V of the sheet S becomes higher, the distance becomes longer. Therefore, in the conventional sheet conveying apparatus, when the conveying speed V of the sheet S is increased, the sheet-to-sheet distance needs to be set longer, and hence further enhancement of the throughput is practically impossible. 
     Thus, in the sheet conveying apparatus in which skew feed of sheets is corrected by using the locking member, enhancement of throughput of the sheet conveyance has been limited due to a time period for returning the locking member. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Laid-Open No. H09-183539 
     SUMMARY OF INVENTION 
     In view of the circumstances, the present invention provides a sheet conveying apparatus configured to enhance throughput, and an image forming apparatus including the sheet conveying apparatus. 
     According to the present invention, there is provided a sheet conveying apparatus, including: a sheet conveying path through which a sheet is conveyed; a locking member having an abutment surface against which a leading edge of the sheet conveyed through the sheet conveying path abuts so that a skew feed of the sheet is corrected; a holding member configured to hold the locking member, the holding member being rotated integrally with the locking member from a waiting position when the locking member is pushed by the leading edge of the sheet being conveyed; a roller pair configured to nip the sheet of which the leading edge is in abutment against the abutment surface of the locking member; a first urging portion configured to urge the holding member so as to move the holding member to the waiting position after the sheet is nipped between the roller pair; a movable support portion configured to support the locking member in a manner that the locking member is movable with respect to the holding member so that the locking member is located in a retracted position in which the locking member is brought into contact with a surface of the sheet being conveyed by the roller pair and allows the sheet to pass in a state in which the holding member is located in the waiting position; and a second urging portion configured to urge the locking member in a manner that the locking member is moved from the retracted position to a position in which the abutment surface abuts against a leading edge of a succeeding sheet as a trailing edge of the sheet being conveyed by the roller pair passes the locking member. 
     According to the present invention, the throughput can be enhanced. 
     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 DRAWINGS 
         FIG. 1  is a schematic sectional view of an overall structure of an image forming apparatus according to an embodiment of the present invention. 
         FIG. 2A  is a perspective view of a sheet conveying portion according to a first embodiment. 
         FIG. 2B  is a perspective view of the sheet conveying portion illustrated in  FIG. 2A  as viewed from the opposite side. 
         FIG. 3  is a schematic sectional view of the sheet conveying portion according to the first embodiment. 
         FIG. 4  is a plan view illustrating a state in which a sheet enters the sheet conveying portion in a skewed state. 
         FIG. 5  is a sectional view illustrating a state in which a leading edge of the sheet abuts against an abutment surface of a locking member located in a protruding position. 
         FIG. 6  is a sectional view illustrating a state in which the leading edge of the sheet strikes against the abutment surface of the locking member and the sheet is curved. 
         FIG. 7  is a sectional view illustrating a state in which the abutment surface is pushed by the curved sheet so that a holding member configured to hold the locking member is rotated. 
         FIG. 8  is a sectional view illustrating a state in which the leading edge of the sheet is disengaged from the abutment surface and the locking member is rotated by a repulsive force received from the sheet. 
         FIG. 9  is a sectional view illustrating a state in which the locking member is moved to a retracted position while being held in contact with a surface of the sheet disengaged from the abutment surface. 
         FIG. 10  is a sectional view illustrating a state in which the sheet passes above the locking member retracted to the retracted position (the sheet passage position). 
         FIG. 11  is a sectional view of the locking member immediately before the sheet passes through a nip. 
         FIG. 12  is a sectional view illustrating a state in which the locking member is returned to the protruding position after the sheet passes through the nip. 
         FIG. 13  is a view illustrating a state in which a sheet having a different sheet width is conveyed. 
         FIG. 14  is a perspective view of a sheet conveying portion according to a modification of the first embodiment. 
         FIG. 15  is a perspective view of a sheet conveying portion according to a second embodiment of the present invention. 
         FIG. 16  is a sectional view illustrating a state in which the leading edge of the sheet abuts against the abutment surface of the locking member and the sheet is curved. 
         FIG. 17  is a sectional view illustrating a state in which the abutment surface is pushed by the curved sheet and the holding member configured to hold the locking member is rotated so that the pressed portion abuts against a pressing portion. 
         FIG. 18  is a view illustrating a state in which the leading edge of the sheet is disengaged from the abutment surface and the locking member is rotated by a pressing force received from the pressing portion. 
         FIG. 19  is a perspective view of a sheet conveying portion according to a third embodiment. 
         FIG. 20A  is a view illustrating a state in which a sheet enters the sheet conveying portion according to the third embodiment. 
         FIG. 20B  is a view illustrating a blocked state in which an optical path of a detecting sensor is blocked by a detecting lever. 
         FIG. 21A  is a view illustrating a state in which the holding member is moved to a second position and the leading edge of the sheet is disengaged from the abutment surface. 
         FIG. 21B  is a view illustrating a state in which the detecting lever is separated from the optical path of the detecting sensor. 
         FIG. 22A  is a sectional view illustrating a state in which the sheet passes the locking member retracted to the retracted position. 
         FIG. 22B  is a view illustrating the state in which the detecting lever is separated from the optical path of the detecting sensor. 
         FIG. 23A  is a sectional view illustrating a state in which the sheet passes through a nip by rotation of a locking member in a conventional sheet conveying apparatus. 
         FIG. 23B  is a sectional view illustrating a state in which the locking member is reversely rotated to return to a home position after the sheet has gone past the nip. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. The image forming apparatus according to the embodiment is an image forming apparatus, such as a copier, a printer, a facsimile machine, and a multifunction peripheral combining those machines, including a sheet conveying portion serving as a sheet conveying apparatus, which includes a skew feed correcting portion configured to correct a skew feed of a sheet being conveyed. In the following embodiments, the image forming apparatus will be described, taking an electrophotographic color image forming apparatus (hereinafter simply referred to as “image forming apparatus”)  1  configured to form toner images of four colors as an example. 
     First Embodiment 
     The image forming apparatus  1  according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 13 . First, an overall structure of the image forming apparatus  1  according to the first embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a schematic sectional view of the overall structure of the image forming apparatus  1  according to the embodiment of the present invention. 
     As illustrated in  FIG. 1 , the image forming apparatus  1  according to the first embodiment includes a sheet feeding portion  2  configured to feed a sheet S, a sheet conveying portion  3  configured to convey the sheet S while correcting skew feed of the sheet S, and an image forming portion  4  configured to form an image on the sheet S conveyed from the sheet conveying portion  3 . Further, the image forming apparatus  1  includes a fixing portion  5  configured to fix an unfixed image, which is formed by the image forming portion  4 , onto the sheet S, and a delivery portion  6  configured to deliver the sheet S on which the image is fixed. 
     The sheet feeding portion  2  is arranged in a lower portion of the image forming apparatus  1 , and includes a detachably mountable sheet containing portion  21  configured to contain the sheets S and a pick-up roller  22  configured to send the sheets S contained in the sheet containing portion  21 . The sheet feeding portion  2  includes a separating portion  23  configured to separate one by one the sheets S sent by the pick-up roller  22 . The sheet conveying portion  3  is arranged on a downstream side of the sheet feeding portion  2  in a sheet feeding direction (hereinafter simply referred to as “downstream side”), and includes a skew feed correcting portion  7  configured to correct a skew feed of the sheet S. Note that, the sheet conveying portion  3  will be described in detail later. 
     The image forming portion  4  is arranged on the downstream side with respect to the sheet conveying portion  3 , and includes photosensitive drums  41   a ,  41   b ,  41   c , and  41   d  on which toner images of yellow, magenta, cyan, and black are respectively formed, and exposure devices  43   a ,  43   b ,  43   c , and  43   d  configured to form electrostatic latent images respectively on surfaces of the photosensitive drums  41   a  to  41   d . Further, the image forming portion  4  includes process cartridges  42   a ,  42   b ,  42   c , and  42   d  configured to respectively develop the electrostatic latent images formed by the exposure devices  43   a  to  43   d  into toner images, transfer portions  44   a ,  44   b ,  44   c , and  44   d  configured to respectively transfer the toner images onto the sheet S, and a transfer belt  45  configured to convey the sheet S. The photosensitive drums  41   a  to  41   d  are arranged to be rotatable by motors (not shown). The process cartridges  42   a  to  42   d  each is made into a unit integrally including a charger, a developing device, and a cleaner, which are arranged around a corresponding one of the photosensitive drums  41   a  to  41   d . The transfer portions  44   a  to  44   d  are arranged on an inside of the transfer belt  45  while opposing to the photosensitive drums  41   a  to  41   d , respectively. The transfer belt  45  is arranged so that the rotation of the transfer belt  45  moves the sheets S sequentially to the transfer portions  44   a  to  44   d.    
     The fixing portion  5  is arranged downstream of the image forming portion  4 , and includes a fixing roller  51  provided with a built-in heater and a pressure roller  52  which is pressed against the fixing roller  51 . The delivery portion  6  is arranged downstream of the fixing portion  5 , and includes a delivery roller pair  61  configured to deliver the sheets S to an outside of the image forming apparatus, and a delivery tray  62  configured to stack the sheets S delivered to the outside of the image forming apparatus. 
     Next, an image forming process of the image forming apparatus  1  according to the first embodiment structured as described above will be described. When an image forming operation is started, based on image information input from a personal computer (not shown), first, the exposure device  43   a  emits a laser beam according to an image signal of a yellow color component of an original to the photosensitive drum  41   a  uniformly charged by the charger of the process cartridge  42   a . In this way, a yellow electrostatic latent image is formed on the photosensitive drum  41   a . Next, the yellow electrostatic latent image is developed with a yellow toner contained in the developing device of the process cartridge  42   a . In this way, the yellow electrostatic latent image is visualized as a yellow toner image. In the same manner as described above, the electrostatic latent images of magenta, cyan, and black are formed respectively on the surfaces of the photosensitive drums  41   b  to  41   d , and those electrostatic latent images are visualized respectively as a magenta toner image, a cyan toner image, and a black toner image. 
     Along with the above-mentioned toner image forming operation, the sheets S contained in the sheet containing portion  21  are sent to the sheet conveying portion  3  located on the downstream side while being separated one by one by the pick-up roller  22  and the separating portion  23 . The sheet S sent to the sheet conveying portion  3  is subjected to skew feed correction by the skew feed correcting portion  7  in the sheet conveying portion  3 , and then conveyed to the transfer portion  44   a  of the image forming portion  4  at a predetermined timing by a registration roller pair  11  located downstream of to the sheet conveying portion  3 . 
     When the sheet S is conveyed to the transfer portion  44   a , the yellow toner image formed on the surface of the photosensitive drum  41   a  is transferred onto the sheet S by transfer bias applied to a transfer charger (not shown). After the yellow toner image is transferred onto the sheet S, the magenta toner image, the cyan toner image, and the black toner image are sequentially transferred in a superimposed manner onto the yellow toner image while the sheet S is conveyed by the transfer belt  45  in the same manner as described above. In this way, a full-color toner image is formed on the sheet S. 
     The sheet S on which the full-color toner image is transferred is conveyed to the fixing portion  5 , and the toners are fused and mixed while being heated and pressurized by the fixing roller  51  and the pressure roller  52 . In this way, the full-color toner image is fixed as a full-color image. After that, the sheet S on which the full-color image is fixed is delivered by the delivery roller pair  61  provided downstream of the fixing portion  5  onto the delivery tray  62  arranged in an upper portion of the image forming apparatus  1 . With this, the image forming process is completed. 
     Note that, duplex printing is performed as follows. After an image is fixed to a first side of the sheet S in the fixing portion  5 , the delivery roller pair  61  is reversely rotated so that the sheet S is not delivered onto the delivery tray  62  by the delivery roller pair  61  but conveyed into a duplex conveying path  12  with the front side and the back side of the sheet S being reversed. The sheet S conveyed into the duplex conveying path  12  is re-conveyed to the sheet conveying portion  3  by oblique-feed roller pairs  13  and a U-turn roller pair  14  so that the sheet S is subjected to the skew feed correction at the skew feed correcting portion  7  of the sheet conveying portion  3 . Then, the sheet S is conveyed to the image forming portion  4  so that an image is fixed to a second side of the sheet S by the image forming portion  4 . 
     Next, the sheet conveying portion  3  of the image forming apparatus  1  according to the first embodiment will be described with reference to  FIGS. 2A to 13  in addition to  FIG. 1 . First, an overall structure of the sheet conveying portion  3  will be described with reference to  FIGS. 2A, 2B, and 3 .  FIG. 2A  is a perspective view of the sheet conveying portion  3  according to the first embodiment.  FIG. 2B  is a perspective view of the sheet conveying portion  3  illustrated in  FIG. 2A  as viewed from the opposite side.  FIG. 3  is a schematic sectional view of the sheet conveying portion  3  according to the first embodiment. 
     As illustrated in  FIGS. 2A, 2B, and 3 , the sheet conveying portion  3  includes a plurality of conveying rollers  31  as a sheet conveying unit, a plurality of conveying rotatable members  32  as a sheet conveying unit, a feeding frame  33 , and the skew feed correcting portion  7 . The plurality of conveying rollers  31  are firmly fixed to a roller shaft  31   a , and the roller shaft  31   a  is rotatably supported by the feeding frame  33  in parallel to a sheet width direction Y orthogonal to a sheet conveying direction X. The plurality of conveying rotatable members  32  are supported by a rotatable member shaft  32   a  to be rotatable about the rotatable member shaft  32   a  so as to respectively oppose to the plurality of conveying rollers  31 . The rotatable member shaft  32   a  is supported by the feeding frame  33  in parallel to the roller shaft  31   a  so that the plurality of conveying rotatable members  32  and the plurality of conveying rollers  31  form respective nips N. Note that, the sheet width direction Y is parallel to a direction of a rotary shaft of each of the photosensitive drums  41   a  to  41   d.    
     The feeding frame  33  includes a guide portion  33   a  configured to guide the sheet S together with a guide frame  15  to the nips N, the guide portion  33   a  being provided upstream of the nips N formed by the plurality of conveying rotatable members  32  and the plurality of conveying rollers  31 . In cooperation with the guide frame  15 , the guide portion  33   a  regulates both sides in a thickness direction of the sheet S on the upstream side of the nips N, and guides the sheet S to the nips N. Further, the guide portion  33   a  and the guide frame  15  form a loop forming portion  34  in which the sheet S is curved by abutting against abutment surfaces  70   a  of locking members  70  described below so as to form a loop. Skew feed of the sheet S conveyed to the sheet conveying portion  3  is corrected through formation of the loop in the loop forming portion  34 . Note that, in this embodiment, although the guide frame  15  configured to guide the sheet S to the nips N together with the guide portion  33   a  is additionally provided, a guide portion configured to guide the sheet S to the nips N together with the guide portion  33   a  may be provided on the feeding frame  33 . 
     The skew feed correcting portion  7  includes a holding member  72  supported by to the feeding frame  33  so as to be rotatable, a first urging spring  73  as a first urging portion configured to urge the holding member  72 , and the plurality of locking members  70  supported by the holding member  72  so as to be rotatable. Further, the skew feed correcting portion  7  includes a plurality of second urging springs  71  as second urging portions configured to respectively urge the plurality of locking members  70 . 
     The holding member  72  is supported by the feeding frame  33  so as to be rotatable, about a rotary shaft  72   b  parallel to the roller shaft  31   a , between a first position illustrated in  FIG. 3  and a second position illustrated in  FIG. 8  as described below. The holding member  72  is urged by the first urging spring  73  in a direction indicated by an arrow Z 2  in  FIG. 3  so that the holding member  72  is located in the first position. In other words, a stopper  89  regulates the holding member  72  from being rotated in the direction indicated by the arrow Z 2  by the first urging spring  73 . Further, the holding member  72  includes a regulating portion  72   a  configured to regulate rotation of the plurality of locking members  70 , and the regulating portion  72   a  is provided on a rear surface side of the holding member (downstream side in the sheet conveying direction). 
     The locking member  70  is formed into a substantially L-shape, and includes the abutment surface  70   a  configured to abut against a downstream leading edge (hereinafter simply referred to as “leading edge”) of the sheet S on the sheet conveying path, a striking surface  70   b  configured to strike against the regulating portion  72   a , and a rotary shaft  70   c  parallel to the rotary shaft  72   b . The locking member  70  has one end provided with the abutment surface  70   a  and the other end provided with the striking surface  70   b . The locking member  70  is held by the holding member  72  so as to be movable (rotatable) so that the abutment surface  70   a  and the striking surface  70   b  are rotated about the rotary shaft  70   c . A movable support portion provided on the holding member  72  rotatably supports the rotary shaft  70   c , and hence the locking member  70  is movably held by the holding member  72 . The holding member  72  and the locking member  70  are movable (rotatable) integrally with each other. When the holding member  72  is located in the first position, the locking member  70  is movable to a protruding position in which the abutment surface  70   a  protrudes on the sheet conveying path upstream of the nip N and to a retracted position in which the abutment surface  70   a  is retracted toward the holding member  72 . The striking surface  70   b  strikes against the regulating portion  72   a  so as to regulate the rotation of the locking member  70  urged by the second urging spring  71  toward the protruding position (in the direction indicated by the arrow K 1  in  FIG. 3 ). In this way, the locking member  70  is maintained in the protruding position. 
     Next, how the skew feed correction of the sheet S is performed by the skew feed correcting portion  7  according to the first embodiment will be described with reference to  FIGS. 4 to 13  in addition to  FIG. 3 .  FIG. 4  is a plan view illustrating a state in which the sheet S enters the sheet conveying portion  3  in a skewed state of the sheet S.  FIG. 5  is a sectional view illustrating a state in which the leading edge of the sheet S abuts against the abutment surface  70   a  of the locking member  70  located at the protruding position.  FIG. 6  is a sectional view illustrating a state in which the leading edge of the sheet S strikes against the abutment surface  70   a  of the locking member  70  and the sheet S is curved.  FIG. 7  is a sectional view illustrating a state in which the abutment surface  70   a  is pushed by the curved sheet S so that the holding member  72  configured to hold the locking member  70  is rotated.  FIG. 8  is a sectional view illustrating a state in which the leading edge of the sheet S is disengaged from the abutment surface  70   a  and the locking member  70  is rotated by a repulsive force received from the sheet S.  FIG. 9  is a sectional view illustrating a state in which the locking member  70  is moved to the retracted position while being held in contact with the surface of the sheet S disengaged from the abutment surface  70   a .  FIG. 10  is a sectional view illustrating a state in which the sheet S passes above the locking member  70  retracted to the retracted position (sheet passage position).  FIG. 11  is a sectional view of the locking member  70  immediately before the sheet S passes through the nip N.  FIG. 12  is a sectional view illustrating a state in which the locking member  70  is returned to the protruding position after the sheet S passes through the nip N.  FIG. 13  is a view illustrating a state in which a sheet S having a different sheet width is conveyed. 
     As illustrated in  FIG. 4 , for example, when the sheet S fed from the sheet feeding portion  2  enters the sheet conveying portion  3  in a skewed state with respect to the sheet conveying direction X, without provision of the skew feed correcting portion  7 , the sheet S is conveyed to the image forming portion  4  on the downstream side in the skewed posture of the sheet S as it is. When the sheet S is conveyed to the image forming portion  4  in the skewed posture as it is, the image to be transferred onto the sheet S is recorded in a tilted state with respect to the sheet S. As a result, recording accuracy is deteriorated. However, in this embodiment, the skew feed correcting portion  7  is arranged in the sheet conveying portion  3 , and hence the sheet S can be conveyed while being subjected to skew feed correction. As a result, deterioration in recording accuracy can be prevented. In the following, an operation of the skew feed correcting portion  7  will be described in detail. 
     First, as illustrated in  FIG. 3 , before the sheet S enters the sheet conveying portion  3 , the holding member  72  is located at the first position by an urging force of the first urging spring  73 . Then, the locking member  70 , which is held by the holding member  72  located at the first position, is urged by the second urging spring  71  so that the striking surface  70   b  strikes against the regulating portion  72   a . With this, the locking member  70  comes to the protruding position. In this way, the abutment surface  70   a  of the locking member  70  is located on the sheet conveying path. In the following, this position in a state in which the holding member  72  is located at the first position while the locking member  70  is located at the protruding position is referred to as “home position” as a waiting position. 
     When the sheet S enters the sheet conveying portion  3  in the skewed state of the sheet S with respect to the sheet conveying direction X, first, a preceding leading edge of the sheet S abuts against the abutment surface  70   a  of one of the plurality of locking members  70  (for example, locking member  70 H illustrated in  FIG. 4 ). In this state, the holding member  72  and the locking members  70  remain at the home position without being rotated, and the sheet S is locked. This is because the urging force of the first urging spring  73  is set to be larger than a conveying force for conveying the sheet S (for example, feeding force of the sheet feeding portion  2 ). Then, when the sheet feeding portion  2  further feeds the sheet S, as illustrated in  FIG. 5 , the portions of the leading edge of the sheet S sequentially abut against the abutment surfaces  70   a  of the plurality of locking members  70  (in the order of the locking members  70 G,  70 F, and  70 E illustrated in  FIG. 4 ) while the preceding portion of the leading edge of the sheet S being locked to the abutment surface  70   a  of the locking member  70 H. 
     In this process, as illustrated in  FIGS. 6 and 7 , inside the loop forming portion  34  formed by the guide portion  33   a  and the guide frame  15 , the sheet S forms a loop curved in an arrow direction illustrated in  FIG. 6 . Note that, the curved loop of the sheet S at this time is larger on the right side than on the left side illustrated in  FIG. 4 . A series of those movements causes the leading edge of the sheet S to be aligned with the abutment surfaces  70   a  of the plurality of locking members  70  so that the leading edge of the sheet S becomes parallel to the sheet width direction Y orthogonal to the sheet conveying direction X. In this way, skew feed of the sheet S is corrected. 
     When the sheet S forms a predetermined loop, a pressing force of moving the holding member  72  and the locking members  70  in a direction (rotational direction) indicated by the arrow Z 1  in  FIG. 6  against the urging force of the first urging spring  73  is generated by stiffness of the sheet S. With this, as illustrated in  FIG. 7 , the holding member  72  is rotated in the direction Z 1  together with the locking members  70 , and the leading edge of the sheet S is nipped by the nips N between the conveying rollers  31  and the conveying rotatable members  32  in the process of rotation of the holding member  72 . Note that, when the locking members  70  are rotated together with the holding member  72  by the sheet S which presses the abutment surfaces  70   a , the locking members  70  are rotated while being located at the protruding position. This is because the striking surface  70   b  is struck against the regulating portion  72   a  of the holding member  72 , and hence the protruding position is maintained. Meanwhile, at this time, the locking members  70  are not rotated in a direction indicated by the arrow K 2  (refer to  FIG. 9 ). 
     In this context, skew feed correction performance of the skew feed correcting portion  7  becomes greater in proportion to a size of the loop formed inside the loop forming portion  34  formed by the guide portion  33   a  and the guide frame  15 . In other words, as illustrated in  FIG. 7 , it is desired that a larger space be secured for the loop forming portion  34 . Further, the predetermined loop refers to a loop which is formed in the sheet S inside the loop forming portion  34  and increases apparent stiffness of the sheet S by the part of the loop abutting against the guide frame  15  so that the holding member  72  and the locking members  70  are rotated. When the sheet S forms such a loop inside the loop forming portion  34 , the loop partially abuts against the guide frame  15 . As a result, the apparent stiffness of the sheet S becomes higher, and hence the holding member  72  and the locking members  70  can be rotated. 
     As illustrated in  FIG. 8 , when the holding member  72  pushed by the leading edge of the sheet S is further rotated together with the locking members  70  and reaches the second position, the abutment surfaces  70   a  of the locking members  70  held by the holding member  72  are retracted from the sheet conveying path. When the abutment surfaces  70   a  of the locking members  70  are retracted from the sheet conveying path, the leading edge of the sheet S passes beyond top points of the abutment surfaces  70   a . In other words, the leading edge of the sheet S is disengaged from the abutment surfaces  70   a . When the leading edge of the sheet S is disengaged from the abutment surfaces  70   a , the locking members  70  receive the repulsive force from the sheet S nipped by the nips N between the conveying rollers  31  and the conveying rotatable members  32  in a direction (direction to the retracted position) indicated by the arrow M in  FIG. 8 . After the locking members  70  receive the repulsive force from the sheet S in the direction indicated by the arrow M, as illustrated in  FIG. 9 , the locking members  70  start to be rotated in the direction indicated by the arrow K 2  against an urging force of the second urging springs  71 , in other words, start to move to the retracted position. Note that, the second urging springs  71  are configured to urge the locking members  70  in the direction indicated by the arrow K 1  with a force smaller than a moment of the repulsive force of the sheet S. By receiving the repulsive force of the sheet S, the locking members  70  are rotated in the direction indicated by the arrow K 2 . 
     Further, in accordance with elimination of the pressing force from the sheet S, the holding member  72  starts to be rotated in the direction indicated by the arrow Z 2  in  FIG. 9  toward the first position by the urging force of the first urging spring  73 . In accordance with the movement of the holding member  72  in the direction indicated by the arrow Z 2 , the locking members  70  are moved further to the retracted position while contacting the surface of the sheet S. Then, as illustrated in  FIG. 10 , when the holding member  72  returns to the first position at which the holding member  72  abuts against the stopper  89 , the locking members  70  are regulated from moving to the protruding position (moving to the surface side) by the sheet S being in the process of passing through the sheet conveying path, and wait at the retracted position while being held in contact with the surface of the sheet S. In  FIG. 10  in which the holding member  72  is located at the first position, the abutment surfaces  70   a  of the locking members  70 , which are in contact with the surface of the sheet S, are located on the upstream side with respect to a nip of the registration roller pair  11 . After the sheet S passes through the sheet feeding portion  2 , the stiffness of the sheet S decreases. In accordance therewith, as illustrated in  FIG. 11 , the locking members  70  gradually return to the protruding position (home position). Further, after a trailing edge of the sheet S passes through the sheet conveying path (has gone past the nips N between the conveying rollers  31  and the conveying rotatable members  32 ), as illustrated in  FIG. 12 , the locking members  70  are returned to the protruding position by the urging force of the second urging springs  71 , and the abutment surfaces  70   a  return to the positions on the sheet conveying path. In other words, the abutment surfaces  70   a  enter a state of waiting at the home position for correcting skew feed of a succeeding sheet. In this way, by repeating the series of operations described above with reference to  FIGS. 5 to 12 , skew feed of the sheets S sequentially fed from the sheet feeding portion  2  can be sequentially corrected. 
     As described above, in the image forming apparatus  1  according to the first embodiment, the sheet S is brought into abutment against the abutment surfaces  70   a  of the locking members  70  so that skew feed of the sheet S is corrected. Then, the holding member  72  moves to the second position together with the locking members  70 . After that, when the leading edge of the sheet S is disengaged from the abutment surfaces  70   a  at the second position, the holding member  72  returns to the first position and the locking members  70  wait at the retracted position until the sheet S has gone past the nips N. Thus, immediately after the sheet S passes through (has gone past) the nips N, the locking members  70  can be returned to the protruding position at which the leading edge of the succeeding sheet S can be brought into abutment against the abutment surfaces  70   a , and the skew feed correcting portion  7  can be returned to the home position. With this, a time period between a time when the sheet S passes through (has gone past) the nips N and a time when the locking members  70  return to the home position can be reduced. As a result, a sheet-to-sheet distance is prevented from increasing even at a higher sheet conveying speed, and hence higher throughput can be obtained. 
     Further, when the width of the sheet S is relatively large (sheet S indicated by the solid line in  FIG. 13 ), two locking members  70 E and  70 H arranged to correspond mainly to vicinities of both end portions of the sheet S act on the leading edge of the sheet S to correct a skew feed of the sheet S. Meanwhile, when the width of the sheet S to be used is relatively small to an extent of not reaching the locking member  70 E or  70 H (sheet S indicated by the double-dotted line in  FIG. 13 ), locking members  70 F and  70 G arranged closer to a central portion than the locking members  70 E and  70 H correct a skew feed of the sheet S. The image forming apparatus  1  includes the locking members  70 F and  70 G so that contact pressure exerted on the abutment surfaces  70   a  of the locking members  70 , which abut against the leading edge of the sheet S, can be reduced and that local imprints to be generated by contact of the leading edge of the sheet S having a relatively large width with the locking members  70  can be prevented. 
     Further, in order to obtain skew feed correction performance which enables more accurate correction of the skew feed of the sheet S, it is preferred that the plurality of locking members  70  corresponding to various widths of the sheets S be arranged at as large intervals as possible and substantially symmetrically with respect to a center of the width of the sheet S. This is because the purpose of reducing correction angle errors of the leading edge of the sheet S with respect to a direction of the rotary shafts of the conveying rollers  31  and the conveying rotatable members  32 . Thus, the locking members  70  are arranged in the vicinities of both the end portions of the sheet S to be conveyed. However, it is preferred to arrange the locking member  70  also in a vicinity of a conveying central portion C of the sheet S so that the sheet S having a relatively small width is also subjected to skew feed correction. 
     Further, at this time, it is preferred to set an interval between the two locking members  70 F and  70 G on both sides near the conveying central portion C of the sheet conveying path of the sheet S to be smaller than a minimum width of the sheet S. In that case, it is preferred to arrange the abutment surfaces  70   a  of the locking members  70 F and  70 G, which abut against the leading edge of the sheet S, slightly more downstream in the sheet conveying direction than the abutment surfaces  70   a  of the locking members  70 E and  70 H. With this, even when correcting skew feed of the sheet S having a large width, none of the locking members  70 F and  70 G abuts against the leading edge of the sheet S. Thus, the correction angle errors can be reduced. 
     Further, a distance between the abutment surfaces  70   a  and the nips N between the conveying rollers  31  and the conveying rotatable members  32  is reduced. With this, the sheet S is conveyed while being nipped by the nips N between the conveying rollers  31  and the conveying rotatable members  32  immediately after the locking members  70  correct a skew feed of the sheet S. Thus, an effect of the skew feed correction of the sheet S can be maintained. Further, the plurality of abutment surfaces  70   a  of the locking members  70 , which abut against the leading edge of the sheet S, are provided substantially symmetrically with respect to the center of the sheet width in the sheet width direction Y orthogonal to the sheet conveying direction X. With this, it is possible to obtain higher skew feed correction performance which enables more accurate skew feed correction of the sheet S. Further, local imprints can be prevented from being formed even through abutment of the sheet S against the locking members  70 . 
     Note that, as illustrated in  FIG. 14 , for example, respective rear surface sides of the plurality of locking members  70  of the sheet conveying portion  3  may be connected to each other with a connecting portion  75 . 
     Second Embodiment 
     Next, an image forming apparatus  1 A according to a second embodiment of the present invention will be described with reference to  FIGS. 15 to 18  as well as  FIG. 1 . The image forming apparatus  1 A according to the second embodiment is different from the image forming apparatus  1  according to the first embodiment in that the locking members  70  are moved by being pressed against a pressing portion  16  as a pressing member when the holding member  72  is rotated to the second position. Thus, in the second embodiment, differences from the first embodiment, specifically, the structure configured to rotate the locking members  70  will be mainly described. Thus, the same components as those of the image forming apparatus  1  according to the first embodiment are denoted by the same reference symbols, and the descriptions thereof are omitted herein. 
     First, an overall structure of the image forming apparatus  1 A according to the second embodiment will be described with reference to  FIG. 15  as well as  FIG. 1 .  FIG. 15  is a perspective view of a sheet conveying portion  3 A according to the second embodiment. 
     As illustrated in  FIG. 1 , the image forming apparatus  1 A according to the second embodiment includes the sheet feeding portion  2 , the sheet conveying portion  3 A, the image forming portion  4 , the fixing portion  5 , the delivery portion  6 , and the pressing portion  16  provided on a main body of the image forming apparatus  1 A. As illustrated in  FIG. 15 , the sheet conveying portion  3 A includes the plurality of conveying rollers  31 , the plurality of conveying rotatable members  32 , the feeding frame  33 , and a skew feed correcting portion  7 A. The skew feed correcting portion  7 A includes the holding member  72 , the first urging spring  73 , the plurality of locking members  70 , the plurality of second urging springs  71 , a pressed portion  74  configured to abut against the pressing portion  16 , and the connecting portion  75 . The connecting portion  75  connects the plurality of locking members  70  to each other on the rear surfaces sides of the plurality of locking members  70 . The pressed portion  74  is connected to the connecting portion  75 . 
     Next, how the skew feed correcting portion  7 A of the sheet conveying portion  3 A of the image forming apparatus  1 A according to the second embodiment corrects skew feed of the sheet S will be described with reference to  FIGS. 16 to 18 .  FIG. 16  is a sectional view illustrating a state in which the leading edge of the sheet S abuts against the abutment surfaces  70   a  of the locking members  70  and the sheet S is curved.  FIG. 17  is a sectional view illustrating a state in which the abutment surfaces  70   a  are pushed by the curved sheet S and the holding member  72  configured to hold the locking members  70  is rotated so that the pressed portion  74  abuts against the pressing portion  16 .  FIG. 18  is a view illustrating a state in which the leading edge of the sheet S is disengaged from the abutment surfaces  70   a  and the locking members  70  are rotated by a pressing force received from the pressing portion  16 . 
     As illustrated in  FIG. 16 , when the holding member  72  and the locking members  70  are located at the home position, the pressed portion  74  is located out of contact with the pressing portion  16 . In this state, when the sheet S enters the sheet conveying portion  3 A and the leading edge of the sheet S abuts against the abutment surfaces  70   a  of the locking members  70 , the sheet S is curved to form a loop. With this, the holding member  72  is pushed by the sheet S so as to be rotated to the second position together with the locking members  70 . 
     As illustrated in  FIG. 17 , when the holding member  72  configured to hold the locking members  70  located at the protruding position is rotated to the second position, the pressed portion  74  abuts against the pressing portion  16 . In accordance therewith, the locking members  70  located at the protruding position are pressed toward the retracted position. At this time, the holding member  72  reaches the second position, and the abutment surfaces  70   a  of the locking members  70  held by the holding member  72  are retracted from the sheet conveying path. As a result, the leading edge of the sheet S is disengaged from the abutment surfaces  70   a . Thus, when the locking members  70  are pushed toward the retracted position, as illustrated in  FIG. 18 , the locking members  70  can be moved without hindering conveyance of the sheet S. Further, at this time, even in a case where the leading edge of the sheet S remains in abutment against the abutment surfaces  70   a , when the locking members  70  are rotated toward the retracted position, the leading edge of the sheet S can be disengaged from the abutment surfaces  70   a  by the stiffness of the sheet S. 
     As described above, the image forming apparatus  1 A according to the second embodiment includes the pressing portion  16  and the pressed portion  74 . With this, when the locking members  70  are moved to the second position together with the holding member  72 , the locking members  70  can be reliably rotated toward the retracted position. 
     Third Embodiment 
     Next, an image forming apparatus  1 B according to a third embodiment of the present invention will be described with reference to  FIGS. 19 to 22B  as well as  FIG. 1 . The image forming apparatus  1 B according to the third embodiment is different from the image forming apparatus  1  according to the first embodiment in that a leading edge detecting portion configured to detect the leading edge of the sheet S is provided to the skew feed correcting portion  7 B. Thus, in the third embodiment, differences from the first embodiment, specifically, the leading edge detecting portion will be mainly described. Thus, the same components as those of the image forming apparatus  1  according to the first embodiment are denoted by the same reference symbols, and the descriptions thereof are omitted herein. 
     First, an overall structure of the image forming apparatus  1 B according to the third embodiment will be described with reference to  FIG. 19  as well as  FIG. 1 .  FIG. 19  is a perspective view of a sheet conveying portion  3 B according to the third embodiment. 
     As illustrated in  FIG. 1 , the image forming apparatus  1 B according to the third embodiment includes the sheet feeding portion  2 , the sheet conveying portion  3 B, the image forming portion  4 , the fixing portion  5 , and the delivery portion  6 . As illustrated in  FIG. 19 , the sheet conveying portion  3 B includes the plurality of conveying rollers  31 , the plurality of conveying rotatable members  32 , the feeding frame  33 , and a skew feed correcting portion  7 B. The skew feed correcting portion  7 B includes the holding member  72 , the first urging spring  73 , the plurality of locking members  70 , the plurality of second urging springs  71 , a detection lever  17  as a sheet detecting member, and a detecting sensor  18 . The detection lever  17  and the detecting sensor  18  form the leading edge detecting portion. 
     The detection lever  17  is connected coaxially to the rotary shaft  70   c  of the locking members  70 , and moved in conjunction with the movement of the locking members  70 . The detecting sensor  18  is an optical sensor (for example, photo sensor) forming an optical path with a light emitting element and a light receiving element, and is fixed to the feeding frame  33 . The optical path of the detecting sensor  18  is blocked with the detection lever  17  when the holding member  72  and the locking members  70  are in the home position. The detecting sensor  18  produces a detection signal when blocking of the optical path with the detection lever  17  is cancelled. In other words, when the holding member  72  is moved to the second position together with the locking members  70 , the leading edge of the sheet S is detected, and in response thereto, the detecting sensor  18  produces a detection signal. 
     Next, how the skew feed correcting portion  7 B of the sheet conveying portion  3 B of the image forming apparatus  1 B according to the third embodiment corrects skew feed of the sheet S will be described with reference to  FIGS. 20A, 20B, 21A, 21B, 22A, and 22B .  FIG. 20A  is a view illustrating a state in which the sheet S enters the sheet conveying portion  3 B according to the third embodiment.  FIG. 21A  is a view illustrating a state in which the holding member  72  is moved to the second position and the leading edge of the sheet S is disengaged from the abutment surfaces  70   a .  FIG. 22A  is a sectional view illustrating a state in which the sheet S passes over the locking members  70  retracted to the retracted position. Note that,  FIGS. 20A, 21A, and 22A  are views each illustrating a state of the locking members  70  and the holding member  72 .  FIGS. 20B, 21B, and 22B  are views each illustrating a state of the leading edge detecting portion. 
     When the holding member  72  and the locking members  70  are located in the home position as illustrated in  FIG. 20A , the leading edge detecting portion is in a blocked state in which the optical path of the detecting sensor  18  is blocked by the detection lever  17  as illustrated in  FIG. 20B . Next, when the holding member  72  is moved to the second position together with the locking members  70  as illustrated in  FIG. 21A , the detection lever  17  interlocks with the locking members  70  as illustrated in  FIG. 21B  and is separated from the optical path of the detecting sensor  18 . With this, the blocking of the optical path of the detecting sensor  18  with the detection lever  17  is cancelled, and in response thereto, the sheet S is detected to have reached a desired position. Then, the detecting sensor  18  produces a detection signal. 
     When the detection signal produced by the detecting sensor  18  is received, the image forming portion  4  starts an operation of forming an image to be transferred onto the conveyed sheet S. After that, the same operation as that in the first embodiment is performed, and as illustrated in  FIG. 22A , the locking members  70  are retracted to the retracted position while the holding member  72  is located in the first position, and wait until the sheet S passes through the nips N. As illustrated in  FIG. 22B , during the waiting, the detection lever  17  is separated from the optical path of the detecting sensor  18 . Then, the locking members  70  return to the protruding position in accordance with completion of the passage of the sheet S through the nips N. As a result, the detection lever  17  blocks the optical path of the detecting sensor  18 , and the detecting sensor  18  stops producing the detection signal. 
     As described above, the image forming apparatus  1 B according to the third embodiment includes the detection lever  17  which interlocks with the locking members  70 , and the detecting sensor  18 . With this, in addition to skew feed correction of the sheet S with the locking members  70  and the holding member  72 , the position of the leading edge of the sheet S can be detected. Specifically, the leading edge of the sheet S subjected to skew feed correction at the sheet conveying portion  3 B is detected, and hence image formation can be performed by the image forming portion  4  at an appropriate timing based on the position of the sheet S. As a result, for example, it is no longer necessary to additionally provide a leading edge detecting portion configured to detect the position of the leading edge of the sheet S, and hence manufacturing cost can be suppressed. 
     Further, the detection lever  17  moves in the same way as that of the locking members  70 . Thus, substantially simultaneously with completion of the passage of the trailing edge of the sheet S through the sheet conveying path, the detection lever  17  can come to and wait at the home position at which the leading edge of the succeeding sheet S is detected (position at which the optical path of the detecting sensor  18  is blocked). With this, even when a sheet-to-sheet distance is short under a high sheet conveying speed condition, the detection lever  17  can return to the home position so as to detect the leading edge of the succeeding sheet S. In this way, user&#39;s demands for higher throughput of the image forming apparatus can be met. 
     The embodiments of the present invention are described above, but the present invention is not limited to the embodiments described above. Further, the advantages described in the embodiments of the present invention are merely described as most preferred advantages to be achieved by the present invention. Thus, the advantages of the present invention are not limited to those described in the embodiments of the present invention. 
     For example, in the embodiments of the present invention, the locking members  70  are held by the holding member  72  so as to be rotatable between the protruding position and the retracting position. However, the present invention is not limited thereto. For example, the locking members  70  may be supported by the holding member  72  so as to pop up and down (slidably move) between the protruding position and the retracted position. 
     Further, as described in the embodiments of the present invention, the urging springs are used as a first urging unit and a second urging unit, but the present invention is not limited thereto. The first urging unit and the second urging unit may include elastic members configured to urge a holding member and a locking member, respectively. Further, as described in the embodiments of the present invention, the conveying rollers  31  and the conveying rotatable members  32  are used as a sheet conveying unit, but the present invention is not limited thereto. For example, the sheet conveying unit is not particularly limited as long as the sheet S can be nipped and conveyed. 
     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 is a Divisional Application of U.S. patent application Ser. No. 15/153,904, filed May 13, 2016, and allowed on Nov. 1, 2016, which is a Divisional Application of U.S. patent application Ser. No. 14/365,746, filed on Jun. 16, 2014, and allowed on Feb. 16, 2016, which is a National Stage Entry of International Application No. PCT/JP2013/052779, filed on Jan. 31, 2013, which claims the benefit of Japanese Patent Application No. 2012-025190, filed Feb. 8, 2012, which are all hereby incorporated by reference herein in their entireties. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1 A,  1 B image forming apparatus 
           3 ,  3 A,  3 B sheet conveying portion (sheet conveying apparatus) 
           4  image forming portion 
           7 ,  7 A,  7 B skew feed correcting portion 
           16  pressing portion (pressing member) 
           17  detection lever (sheet detecting member) 
           18  detecting sensor 
           31  conveying roller (sheet conveying unit) 
           32  conveying rotatable member (sheet conveying unit) 
           70  locking member 
           70   a  abutment surface 
           71  second urging spring (second urging unit) 
           72  holding member 
           73  first urging spring (first urging unit) 
           74  pressed portion 
         S sheet