Patent Publication Number: US-9896291-B2

Title: Sheet conveying apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sheet conveying apparatus and an image forming apparatus, and more specifically to a configuration for correcting a skew of a sheet. 
     Description of the Related Art 
     Conventionally, an image forming apparatus such as a printer, a copier, and a facsimile includes a skew correcting portion conveying a sheet to an image forming portion and correcting a skew of the sheet. As such skew correcting portion, Japanese Patent No. 2893540 proposes an apparatus using a registration roller pair correcting a skew of a sheet and aligning an image formed in the image forming portion with a front end of the sheet. In correcting a skew of the sheet, firstly, the front end of the sheet being conveyed by an upstream conveying roller pair is caused to abut against a nip portion of the registration roller pair being at a stop. 
     Thereby, the sheet deflects and forms a loop. The front end of the sheet then follows a nip line of the registration roller pair due to stiffness of the sheet, so that the skew of the sheet is corrected. It is noted that after the correction, the image formed in the image forming portion is aligned with the front end of the sheet by adjusting timing for starting to rotate the registration roller pair. 
     Here, conventionally, there is an apparatus in which a registration roller pair is disposed along a curved conveying path and a loop space is provided so that a loop of the sheet can be formed upstream in the sheet conveying direction of the registration roller pair. Normally, in a case where a conveying path composed of a pair of conveyance guides is curved, the sheet abuts against the registration roller pair in a state partially separated from an outer conveyance guide in the curve direction by stiffness of the sheet itself and then forms a loop in the loop space. 
     However, because size of the loop formed by the sheet is different depending on the sheet being conveyed, there is a case where an enough loop space cannot be obtained and a skew cannot be corrected. 
     Then, conventionally, Japanese patent Application Laid-open No. Hei. 8-113389 proposes a sheet conveying apparatus in which a part of the conveyance guide guiding a sheet to the registration roller pair is configured to be swingable. Then, a loop forming area is assured by temporarily receding the conveyance guide, swingable by the loop of the sheet (referred to a ‘swingable guide’ hereinafter), from the conveying path. The sheet conveying apparatus corrects a skew of the sheet by forming a loop corresponding to a loop amount of the sheet in that area. 
     However, the sheet conveying apparatus described in Japanese patent Application Laid-open No. Hei. 8-113389 is unable to set a swing force of the swingable guide that enables to form similar loops enabling to correct a skew for both of a non-stiff sheet and a stiff sheet such as a thick sheet and a coated sheet. That is, if the swing force of the swingable guide is set so as to be able to form a loop by the non-stiff sheet, the swingable guide considerably swings by being pressed by a front end part of the sheet before the sheet abuts against the registration roller pair. In such a case, there is a possibility that the sheet deviates out of the conveying path, causing conveyance failures such as jamming and troubles such as scratch and fold. 
     Still further, if the swing force of the swingable guide is set to be higher, while it allows the stiff sheet to form an adequate loop and to correct a skew, the non-stiff sheet is unable to swing the swingable guide. In this case, because the sheet cannot form an adequate loop, a skew correcting ability drops. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a sheet conveying apparatus includes a first conveying portion conveying a sheet, a second conveying portion including an abutment portion against which a front end of the sheet conveyed by the first conveying portion abuts and conveying the sheet, a curved guide including a curved portion and forming a curved sheet conveying path between the first and second conveying portions, and a moving portion disposed to overlap with the curved portion in a width direction orthogonal to a sheet conveying path and movable between a projecting position where the moving portion projects to the sheet conveying path and a recede position where the moving portion recedes in a direction approaching to the curved guide from the projecting position. The moving portion is moved from the projecting position to the recede position by being pressed by the sheet as the sheet is conveyed by the first conveying portion in the state in which the front end of the sheet abuts against the abutment 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  is a schematic diagram illustrating an entire configuration of a printer, i.e., an exemplary image forming apparatus, including a sheet conveying apparatus of an embodiment. 
         FIG. 2  is a perspective view illustrating a configuration of a registration unit provided in the sheet conveying apparatus. 
         FIG. 3  is a sectional side view illustrating the registration unit in a state in which a turnable guide is located at a projecting position. 
         FIG. 4  is a sectional side view illustrating the registration unit in a state in which the turnable guide is located at a recede position. 
         FIG. 5A  is a sectional side view illustrating a state in which a front end of a stiff sheet presses the turnable guide. 
         FIG. 5B  is a sectional side view illustrating a state in which the front end of the stiff sheet passes through the turnable guide and arrives at a curved outer guide. 
         FIG. 5C  is a sectional side view illustrating a state in which the stiff sheet forms a loop. 
         FIG. 6A  is a sectional side view illustrating a state in which a front end of a non-stiff sheet arrives at the turnable guide. 
         FIG. 6B  is a sectional side view illustrating a state in which the front end of the non-stiff sheet arrives at a bent portion of the turnable guide. 
         FIG. 6C  is a sectional side view illustrating a state in which the non-stiff sheet forms a loop. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the invention will be described in detail below with reference to the drawings.  FIG. 1  is a schematic diagram illustrating a configuration of a printer, i.e., an exemplary image forming apparatus, including a sheet conveying apparatus of the embodiment of the invention. 
     As shown in  FIG. 1 , the printer  100  includes a printer body  101  including an image forming portion  102  and others and an image reading apparatus  103  provided at an upper part of the printer body  101 . Formed between the image reading apparatus  103  and the printer body  101  is a sheet discharging space S. The printer body  101  includes the image forming portion  102  and a sheet conveying apparatus  105  conveying the sheet fed by a sheet feeding apparatus  104  to the image forming portion  102 . The printer body  101  also includes a control portion  120  controlling an image forming operation of the image forming portion  102  and sheet conveying and skew correcting operations and others of the sheet conveying apparatus  105 . 
     The image forming portion  102  is of a four-drum full-color type and includes a laser scanner  13  and four process cartridges  10 Y,  10 M,  10 C, and  10 K forming four color toner images of yellow (Y), magenta (M), cyan (C), and black (K). The image forming portion  102  also includes toner cartridges  19 Y,  19 M,  19 C, and  19 K storing toner of the respective colors of yellow (Y), magenta (M), cyan (C), and black (K), and an intermediate transfer unit  60 . 
     Here, because the configuration of the respective process cartridges is the same except of the colors of the toners, only the yellow (Y) process cartridge  10 Y will be described below, and an explanation of the process cartridges  10 M,  10 C, and  10 K will be omitted here. The process cartridge  10 Y includes a photosensitive drum  11 , a charging roller  12 , i.e., an electrifying portion, a developing unit  14 , and a cleaner  15 . 
     The intermediate transfer unit  60  includes an intermediate transfer belt  61  wrapped around a transfer driving roller  62 , a driven roller  63 , and a tension roller  64  and rotating in a direction of an arrow in  FIG. 1 . The intermediate transfer unit  60  also includes primary transfer rollers  17 Y,  17 M,  17 C, and  17 K provided inside of the intermediate transfer belt  61  and in contact with the intermediate transfer belt  61  at positions facing the respective photosensitive drums. 
     Then, the respective negative color toner images on the respective photosensitive drums are sequentially superimposed and transferred onto the intermediate transfer belt  61  by positive transfer bias applied to the intermediate transfer belt  61  through the primary transfer rollers  17 Y,  17 M,  17 C, and  17 K. Still further, a secondary transfer outer roller  35  is provided at a position facing the transfer driving roller  62  of the intermediate transfer unit  60 . The color image formed on the intermediate transfer belt  61  is transferred to the sheet P by a transfer nip  36  composed of the secondary transfer outer roller  35  and the intermediate transfer belt  61 . A fixing portion  40  is disposed above the transfer nip  36 , and a sheet discharging roller pair  41  is disposed above the fixing portion  40 . 
     Next, an image forming operation of the printer  100  constructed as described above will be described. At first, image information of the document read by the image reading apparatus  103  is transmitted to the laser scanner  13 . It is noted that there is a case when image information is inputted to the image forming portion  102  from an external device such as a personal computer not shown. Then, in the image forming portion  102 , the surface of the photosensitive drum  11  of the process cartridge  10 Y is scanned by laser light corresponding to the image information of yellow emitted from the laser scanner  13 . Thereby, the surface of the photosensitive drum  11  homogeneously electrified with predetermined polarity and potential by the charging roller  12  is sequentially exposed, and a yellow electrostatic latent image is formed on the photosensitive drum  11  of the process cartridges  10 Y. 
     After that, the image forming portion  102  visualizes the electrostatic latent image by developing by the developing unit  14  by using yellow toner and applies primary transfer bias to the primary transfer rollers  17 . Thereby, the yellow toner image on the photosensitive drum  11  is transferred onto the intermediate transfer belt  61 . In the same manner, the magenta, cyan, and black toner images formed on the respective photosensitive drums of the process cartridges  10 M,  10 C, and  10 K are sequentially superimposed and transferred onto the intermediate transfer belt  61 , and a full-color toner image is formed on the intermediate transfer belt  61 . 
     It is noted that the toner left on the photosensitive drum  11  after transferring the toner images on the intermediate transfer belt  61  is removed and recovered by the cleaner  15 . Still further, if amounts of the toner of the respective color developing units decrease, toner is replenished from toner cartridges  19 Y,  19 M,  19 C, and  19 K. 
     Still further, in parallel with this toner image forming operation, the sheet P stored in the sheet feeding cassette  20  is fed one by one by a pickup roller  20   a  of the sheet feeding apparatus  104 . The sheet P is conveyed by a pre-registration roller pair  25 , i.e., a first conveying portion, to a registration unit  22  by passing through a curved conveying path  24 , i.e., a curved sheet conveying path, between the pre-registration roller pair  25  and the registration unit  22 . The registration unit  22  includes a registration roller pair  23 , i.e., an exemplary second conveying portion, conveying the sheet. 
     The sheet P is subjected to skew correction by abutting the front end thereof against and forming a loop following a nip portion N (abutment portion, see  FIG. 3 ) of a registration roller pair  23 . After that, the sheet P whose skew has been corrected is conveyed to the transfer nip  36  by the registration roller pair  23  whose rotation is synchronizing with the toner image on the intermediate transfer belt  61 . Then, the color toner image on the intermediate transfer belt  61  is transferred onto the sheet P under predetermined pressure and electrostatic load bias applied at the transfer nip  36 . 
     It is noted that in the present embodiment, a registration detecting unit  110  is disposed between the registration unit  22  and the transfer nip  36 . The registration detecting unit  110  reads an image pattern of the toner image on the intermediate transfer belt  61  and corrects drawing of the laser scanner  13  corresponding to inclination of the image. Then, based on a signal from the registration detecting unit  110 , the control portion  120  controls the laser scanner  13  such that the toner image on the intermediate transfer belt  61  is straightened up. 
     After transferring the toner image onto the sheet P, slight residual toner left on the intermediate transfer belt  61  is removed and recovered by a cleaning unit  70 . The sheet P onto which the toner image has been transferred is conveyed to the fixing portion  40  and is subjected to heat and pressure in passing through the fixing portion  40  to fix the toner image. After that, the sheet P is discharged on a discharged sheet tray  50  by a sheet discharging roller pair  41 . 
     Next, the registration unit  22  provided in the sheet conveying apparatus  105  of the present embodiment will be described.  FIG. 2  is a perspective view illustrating a configuration from the pre-registration roller pair  25  to the registration roller pair  23  provided downstream of the pre-registration roller pair  25  and correcting a skew of the sheet P. 
     As shown in  FIG. 2 , the pre-registration roller pair  25  includes a first roller  25   a  whose circumferential surface is formed of rubber and a second roller  25   b  formed of polyacetal (POM). Then, the second roller  25   b  is disposed to face the first roller  25   a , is supported so as to be movable with respect to the first roller  25   a , and is in pressure contact with the first roller  25   a  by resilient force of a spring  26 . It is noted that while the first and second rollers  25   a  and  25   b  are provided by two each in the present embodiment, the number of the respective rollers may be only one or three or more. 
     The registration roller pair  23  includes a driven roller  23   a  whose circumferential surface is formed of rubber and a driving roller  23   b  formed of polyacetal (POM). Then, the driven roller  23   a  is disposed to face the driving roller  23   b , is supported so as to be movable with respect to the driving roller  23   b , and is in pressure contact with the driving roller  23   b  by resilient force of a spring not shown. Still further, a gear  27  is attached to one end of a roller shaft  23   c  of the driving roller  23   b , and the driving roller  23   b  and the driven roller  23   a  rotate as the roller shaft  23   c  rotates by receiving driving force from a driving motor not shown through the gear  27 . 
     Here, a sensor  28  detecting position of the front end of the sheet conveyed by the pre-registration roller pair  25  is provided between the pre-registration roller pair  25  and the registration roller pair  23 . Then, based on detection information of the sensor  28 , the control portion  120  stops the registration roller pair  23  and abuts the sheet P conveyed by the pre-registration roller pair  25  against the registration roller pair  23  being stopped. After that, the control portion  120  conveys the sheet P by the pre-registration roller pair  25  by a predetermined distance to form a loop of a predetermined length and to abut the front end of the sheet against the nip portion of the registration roller pair  23  to correct a skew of the sheet P. 
     As shown in  FIGS. 2 and 3 , the curved conveying path is formed by a curved outer guide  24   a  (curved guide) including a curved portion  71 , a curved inner guide  24   b  and an upper guide  24   c  (leading guide) facing the curved outer guide  24   a . The curved portion  71  is provided with two opening portions  24   d  arrayed in the width direction and with an opening  24   e.    
     Turnable guides  29  and the sensor  28  are turnably supported by the curved outer guide  24   a . The two turnable guides  29  project out to the curved conveying path  24  through the opening portions  24   d . Still further, the sensor  28  is biased by a spring not shown so that the sensor  28  projects out to the curved conveying path  24  through the opening  24   e  to be able to detect the front end of the sheet. The turnable guides  29  are disposed in the curved portion  71  so as to overlap in the sheet width direction and are configured to be turnable between a projecting position where the turnable guide  29  projects to the curved conveying path  24  and a recede position where the turnable guide  29  recedes in a direction of approaching from the projecting position to the curved outer guide  24   a  in  FIG. 3 . The two turnable guides  29  constitute a moving portion  30 . 
     The turnable guides  29  are biased to the projecting position by springs  31  (biasing portions), respectively. While two of the springs  31  are provided to two turnable guide  29  in the present embodiment, it is also possible to arrange such that the two turnable guides  29  are biased to the projecting position by one spring  31 . It is noted that spring force of the spring  31  is arranged such that the spring  31  exhibits a bias force that turns the turnable guide  29  even for a sheet having relatively less stiffness such as a thin sheet and a recycled sheet. 
     By the way, in a case when a sheet is conveyed askew, one end in the width direction orthogonal to the sheet conveying direction of the sheet arrives early at the registration roller pair  23  and another widthwise end of the sheet arrives lately at the registration roller pair  23 . In such a case, if the sheet is continuously conveyed, one end side of the sheet makes a large loop and approaches to the curved outer guide  24   a  and the other end side separates away from the curved outer guide  24   a . As a result, because the other end side of the sheet is conveyed while floating in the air within the curved conveying path  24 , a shape of the sheet becomes unstable, and in a case of conveying a non-stiff sheet in particular, there is a case when the sheet does not stably enter the nip portion N of the registration roller pair  23 . In such a case, wrinkles and folds may be generated. 
     Then, according to the present embodiment, the curved outer guide  24   a  is provided with the two turnable guides  29 , i.e., movable members, with a predetermined widthwise interval. It is noted that the two turnable guides  29  are provided at positions enabling to deal with a sheet whose widthwise length is narrow and are turnable respectively independently with respect to the curved conveying path  24 . This arrangement makes it possible for the two turnable guides  29  to turn respectively independently corresponding to a skew direction and a skew amount of the sheet P. 
     Then, the sheet P is conveyed along the turnable guides  29  to the registration roller pair  23  while stabilizing its shape regardless of the skew direction and the skew amount by turning the two turnable guides  29  independently. It is noted that the present embodiment is arranged such that the sheet P arrives at the turnable guide  29  after being detected by the sensor  28  so that the turn of the turnable guide  29  does not affect the detection timing of the sensor  28 . 
     The turnable guide  29  includes an upstream slope  29   b  and a downstream slope  29   c  (inclined planes). Then, the curved outer guide  24   a  extends downstream in the sheet conveying direction of the turnable guide  29 . The sheet that has passed through the curved conveying path  24  is guided to the nip portion N of the registration roller pair  23  by the curved outer guide  24   a  and the upper guide  24   c , i.e., the leading guide. 
     It is noted that in the present embodiment, the upper guide  24   c  extends to an upper part of the turnable guide  29 . This arrangement makes it possible to guide the sheet to a position between the upper guide  24   c  and the curved outer guide  24   a  and to the nip portion N of the registration roller pair  23  after passing through a part between the upper guide  24   c  and the turnable guide  29 . 
     Here, according to the present embodiment, the upstream slope  29   b  of the turnable guide  29  is inclined such that the further downstream in the sheet conveying direction, the narrower a distance between the turnable guide  29  and the upper guide  24   c  becomes. That is, the further downstream in the sheet conveying direction, the more the upstream slope  29   b  is inclined in a direction of approaching the upper guide  24   c . The downstream slope  29   c  of the turnable guide  29  is inclined such that the further downstream in the sheet conveying direction, the wider the distance between the upper guide  24   c  and the turnable guide  29  becomes. That is, the further downstream in the sheet conveying direction, the more the downstream slope  29   c  inclines in a direction of separating from the upper guide  24   c . In other words, the turnable guide  29  bends at a project portion  29   a  (bent portion), i.e., a boundary between the upstream slope  29   b  and the downstream slope  29   c.    
     The project portion  29   a  of the turnable guide  29  is located near a nip line A, i.e., a tangential line of the nip portion N of the registration roller pair  23 . It is noted that the project portion  29   a  of the turnable guide  29  is also located in a vicinity of a sheet conveying path B formed by the curved outer guide  24   a  and the curved inner guide  24   b . This arrangement makes it possible to cause the sheet P to enter the nip portion N of the registration roller pair  23  straightly even in a case when the sheet P is conveyed by passing through the project portion  29   a  without turning the turnable guide  29  as described later. 
     Here, the curved outer guide  24   a  is provided with a guide plane  72  (indicated by a dot line in  FIG. 3 ) extending along the sheet conveying direction on sides of the turnable guide  29  in the width direction orthogonal to the sheet conveying direction as shown and described in  FIG. 2 . Still further, as shown in  FIGS. 3 and 5 , the curved outer guide  24   a  includes an upstream guide plane  33  formed upstream in the sheet conveying direction of the turnable guide  29  and guiding the sheet to the turnable guide  29  and a downstream guide plane  34  formed downstream of the turnable guide  29  and guiding the sheet to the nip portion N. 
     Still further, as shown in  FIG. 4 , the turnable guide  29  is configured to be turnable to a position where the project portion  29   a  is substantially coincident with the guide plane (outline)  72  of the curved outer guide  24   a . That is, the project portion  29   a  and the upstream slope  29   b  are formed to run along the guide plane  72  when the turnable guide  29  is located at the recede position. Then, because the turnable guide  29  turns to such position, it is possible to assure a loop space necessary for correcting a skew at upstream of the nip portion N of the registration roller pair  23 . 
     Next, a sheet skew correcting operation of the registration unit  22  including the turnable guide  29  and others constructed as described above will be described. It is noted that the explanation will be made below focusing only on an operation of one turnable guide among the two turnable guides  29  and an explanation of the other turnable guide will be omitted here. 
     Firstly, a skew correcting operation of a relatively stiff sheet P 1 , such as a thick sheet and a coated sheet, whose stiffness is greater than a predetermined stiffness will be described with reference to  FIGS. 5A through 5C . The operation is applied to a thick sheet of 80 g/mm 2  or more in the present embodiment. Here, because the bias force of the spring  31  is set as described above, the stiffness of the sheet overcomes the bias force of the spring  31  in conveying the stiff sheet P 1 . Therefore, the sheet P 1  is conveyed while turning the turnable guide  29  from the projecting position to the recede position by the front end thereof as shown in  FIG. 5A . 
     Then, as shown also in  FIG. 5A , the guide plane  72  of the curved outer guide  24   a  guides the front end of the sheet P 1  when the sheet P 1  turns the turnable guide  29  to the recede position by the front end thereof. Accordingly, it is possible to guide the sheet stably by the guide plane  72  even if the turnable guide  29  is turned by the relatively stiff sheet P 1 . Therefore, in the present embodiment, it is possible to restrain such conveyance failure that the sheet deviates out of the conveying path and jams otherwise from occurring due to the turn of the turnable guide  29  being pressed by the stiff sheet P 1 . 
     After that, the front end of the sheet P 1  passes through the turnable guide  29  and reaches the downstream guide plane  34  of the curved outer guide  24   a . At this time, the sheet P 1  becomes straight by the stiffness thereof on a basis of the front end of the sheet as shown in  FIG. 5B . It is because the sheet P 1  is supported by the upstream and downstream guide planes  33  and  34  with the turnable guide  29  between them. Thereby, the sheet P is conveyed without running along the turnable guide  29 , so that the turnable guide  29  returns in a direction of the projecting (standby) position. 
     Next, even if the sheet P 1  abuts against the registration roller pair  23 , the pre-registration roller pair  25  continuously conveys the sheet P 1  in this state, so that the sheet P 1  forms a loop. At this time, the sheet P 1  forms the loop so as to run along the guide plane  72  of the curved outer guide  24   a  while turning the turnable guide  29  downward as shown in  FIG. 5C , so that a skew of the sheet is corrected. 
     It is noted that a mode in which the turnable guide  29  returns toward the projecting position is exemplified in  FIGS. 5A and 5B . However, it is also possible to arrange such that the front end of the sheet arrives at the registration roller pair  23  while keeping the turnable guide  29  at the position (recede position) shown in  FIG. 5A . In such a case, the deflection of the sheet increases at the upstream side of the turnable guide  29  and the front end of the sheet follows the nip line after when the front end of the sheet has arrived at the registration roller pair  23 . 
     Next, a skew correcting operation of a sheet P 2  whose stiffness is relatively weak such as a thin sheet and a recycled sheet whose stiffness is less than a predetermined stiffness will be described with reference to  FIGS. 6A through 6C . Here, in the case of conveying the non-stiff sheet P 2 , the sheet P 2  is conveyed along the turnable guide  29  as shown in  FIG. 6A  without turning the turnable guide  29  from the projecting position because the sheet P 2  is not conveyed while overcoming the bias force of the turnable guide  29 . That is, the bias force of the spring  31  is set such that the turnable guide  29  does not move from the projecting position even if the turnable guide  29  is pressed by the front end of the sheet P 2 . Because the sheet P 2  is conveyed without turning the turnable guide  29  even after that as shown in  FIG. 6B  until when the sheet P 2  abuts against the registration roller pair  23 , the turnable guide  29  is kept being located at the projecting position. 
     Here, the project portion  29   a  of the turnable guide  29  is located near the nip line A, i.e., the tangential line of the nip portion N of the registration roller pair  23  and the conveying path B of the curved conveying path  24  (see  FIG. 3 ) as described above. Therefore, the sheet P 2  can enter the nip portion N of the registration roller pair  23  straightly even if the turnable guide  29  is located at the projecting position and hardly buckles. 
     Then, the sheet P 2  abuts against the registration roller pair  23  and the pre-registration roller pair  25  continuously conveys the sheet P 2  in this state, so that the sheet P 2  gradually forms a loop. Soon after that, the turnable guide  29  is pressed from the projecting position to the recede position by being pressed by the looped sheet P 2  as shown in  FIG. 6C , and the loop is formed along the curved outer guide  24   a , so that it becomes possible to correct a skew of the sheet. 
     It is noted that the downstream slope  29   c  of the turnable guide  29  is inclined such that the further the curved conveying path  24  advances in the sheet conveying direction, the wider the distance between the turnable guide  29  and the upper guide  24   c  becomes as described above. Therefore, in the case of conveying the non-stiff sheet P 2 , the sheet P 2  moves along the downstream slope  29   c  of the turnable guide  29  after passing over the project portion  29   a  of the turnable guide  29 . After that, the sheet P arrives smoothly the downstream guide plane  34  of the curved outer guide  24   a  and is conveyed along the downstream guide plane  34 . As a result, the sheet P 2  can be conveyed stably to the nip portion N without receiving any stress. 
     Here, when  FIG. 5C  is compared with  FIG. 6C , it can be seen that the sheets assume similar shapes after abutting against the registration roller pair  23 . That is, the loops by which a skew of the sheets can be corrected are formed. As a result, it becomes possible to stably correct a skew. 
     As described above, according to the present embodiment, the turnable guide  29  is provided so as to project to the curved conveying path  24  from the curved portion  71  of the curved outer guide  24   a  and guides the sheet that has been conveyed by the pre-registration roller pair  25  to the nip portion N of the registration roller pair  23 . Then, the turnable guide  29  turns from the projecting position to the recede position by being pressed by the sheet when the sheet whose stiffness is greater than the predetermined stiffness heads toward the registration roller pair  23  and when the sheet abuts against the registration roller pair  23  and bends. 
     Still further, even if the turnable guide  29  moves from the recede position by being pressed by the front end of the sheet, the turnable guide  29  returns to the projecting position by the stiffness of the sheet and the bias force of the spring  31  before the front end of the sheet arrives at the nip portion N of the registration roller pair  23 . 
     Still further, while the turnable guide  29  does not turn even if it is pressed by the sheet whose stiffness is less than the predetermined stiffness heads toward the registration roller pair  23 , the turnable guide  29  turns by being pressed by the sheet when the sheet bends by abutting against the registration roller pair  23 . Then, it becomes possible to conduct the sheet skew correcting operation by arranging such that the turnable guide  29  does not turn even if it is pressed by the sheet whose stiffness is less than the predetermined stiffness heads toward the registration roller pair  23  and turns by being pressed by the sheet when the sheet bends. 
     That is, the sheet skew correcting operation can be stably conducted because the sheet can form the loop regardless of its stiffness by arranging as described in the present invention. 
     Still further, the turnable guide  29  is disposed so as to overlap with the guide plane  72  of the fixed curved outer guide  24   a  in the sheet width direction in the present embodiment. Therefore, even if the turnable guide  29  turns by being pressed by the front end of the sheet, the guide plane  72  can guide the front end of the sheet. Accordingly, it is possible to restrain the conveyance failure such as jamming otherwise caused by the deviation of the sheet out of the conveying path and damage to the sheet from occurring. 
     It is noted that the case where the two turnable guides  29  are turnably provided in the width direction has been described in the present embodiment, the number of the turnable guides may be three or more, or the same advantageous effects can be obtained even if the number is only one. It is noted that while the turnable guide  29  has been exemplified as the moving member in the above description, the turnable guide  29  may be modified such that it is reciprocable (slidable) in a thickness direction of the sheet to be conveyed. 
     Still further, while a skew of the sheet is corrected by abutting the sheet against the nip portion N of the registration roller pair  23  in the present embodiment, the present invention is not limited to such arrangement. That is, any configuration may be adopted as long as the abutment portion abutting with the sheet and forming the loop is provided. For instance, it is possible to arrange such that a skew of the sheet is corrected as the sheet abuts against a shutter member biased by a spring. 
     Still further, while the pre-registration roller pair  25 , i.e., the first conveying portion, and the registration roller pair  23 , i.e., the second conveying portion, are constructed by the roller pairs, the present invention is not limited to such arrangement. That is, one of both of the rollers of the pre-registration roller pair  25  and the registration roller pair  23  may be composed of another rotating body such as a belt. 
     It is also noted that while the mode of biasing the turnable guide  29  to the projecting position by means of the spring  31  has been exemplified in the present embodiment, the present invention is not limited to such configuration. For instance, it is also possible to configure such that the turnable guide  29  is biased to the projecting position by own weight of the turnable guide  29 . 
     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. 2014-174821, filed on Aug. 29, 2014, and Japanese Patent Application No. 2015-149424, filed on Jul. 29, 2015, which are hereby incorporated by reference herein in its entirety.