Patent Publication Number: US-10787332-B2

Title: Sheet conveyance apparatus and image forming apparatus including the same

Description:
The entire disclosure of Japanese patent Application No. 2017-073753, filed on Apr. 3, 2017, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to a sheet conveyance technique and particularly to skew correction. 
     Description of the Related Art 
     The sheet conveyance apparatus is mounted in a system for processing sheets such as printing paper, documents, etc., and conveys a sheet among processing sections in the system. This system includes, for example, an image forming apparatus such as a printer or a copying machine, a post-processing apparatus (finisher), or an automatic document feeder (ADF), and performs processes such as printing, imaging, sorting, binding, folding, etc. For the purpose of properly executing these processes, it is required for the sheet conveyance apparatus to feed a sheet to each element of the processing sections at a correct timing and in a correct posture. 
     One of functions of the sheet conveyance apparatus that keeps the sheet being conveyed in a correct posture is correcting the inclination of the leading end of the sheet with respect to a conveyance direction, that is, correction of the skew of the sheet. As conventional skew correction, for example, a roller registration method is known (for example, see JP 2016-078977 A). “Roller registration method” refers to skew correction that uses a registration roller (also referred to as a timing roller) whose main purpose is to temporarily stop the sheet and then send out the sheet at a proper timing. Specifically, while the leading end of a certain sheet is stopped by the registration roller, the sheet conveyance apparatus continues to feed the rear half portion of the same sheet toward the registration roller. As a result, a slack (loop) is formed in the sheet. Due to the elasticity of the sheet, a force to restore to the original flat shape is generated in this loop. This restoring force (the firmness) pushes the leading end of the sheet into a nip of the registration roller, and thus skew correction of the sheet is achieved. 
     In recent years, image forming apparatuses such as printers and copiers have been widely used in small offices/home offices (SOHO) and general households. Along with this, it is required that further miniaturization of image forming apparatuses is realized at low cost. In order to satisfy this requirement, it is necessary to further reduce the size of the sheet conveyance apparatus. Development of skew correction based on a gate registration method has been progressed as one of measures to meet this requirement for miniaturization (see, for example, JP 2016-160077 A). The “Gate registration method” refers skew correction utilizing a gate disposed in a sheet conveyance path. The “Gate” is a movable member with the ability to return to an original position, and when the gate is pushed in the conveyance direction with a force of a certain strength, the gate is retracted from the conveyance path, and when the force is weakened, the gate returns to the conveyance path. If the leading end of the sheet abuts only a part of the gate due to the skew, the sheet rotates about a normal passing through the leading end of the sheet by a reaction force from the gate before pushing the gate away. As a result, if the skew is corrected and the leading end abuts the entirety of the gate, the sheet can push the gate away and move forward. In this manner, in the skew correction by the gate registration method, it is not necessary to form a loop in the sheet. Therefore, it is not necessary to secure a space for a loop in the conveyance path, thus the conveyance path can be designed to be narrow, and miniaturization of the sheet conveyance apparatus can be achieved. 
     In order to further increase demand for image forming apparatuses such as printers and copiers for particularly SOHO and general households, it is also important to further improve the function by, for example, implementation of a duplex printing function. In a system having a duplex printing function, in general, the sheet conveyance apparatus performs skew correction in a path for reversing a sheet on one surface of which a printing process has been completed and returning the sheet to the printing process, that is, at a point (meeting point) where a reversing path has joined a feed path. Before this meeting point, the reversing path is often curved steeply in a U shape, and it can be said that curving of the reversing path is indispensable especially in miniaturizing the system. When a curved path exists before the point where skew correction is to be performed as described above, it is difficult to adopt the gate registration method for the skew correction. This is due to the following reason. 
     The sheet is fed into the curved path by a conveyance roller positioned at the starting point of the curved path and is pressed against the surface of a guide positioned outside the curve of the curved path by stress caused by the force of the conveyance roller, and is deformed along the surface. Meanwhile, in the gate registration method, the leading end of the sheet abuts a gate positioned at the terminal end of the curved path, and receives reaction force from the gate. Since this reaction force acts on the sheet in a direction to push back the sheet to the curved path, there is a region which is pressed against the surface of the guide by the stress due to this reaction force in the sheet. In this way, in the gate registration method, since the sheet is strongly pressed against the surface of the guide compared with the roller registration method, the sheet is likely to receive excessive frictional force from the surface. As a result, the sheet hardly rotates about a normal passing through the leading end, and thus the risk of insufficient skew correction is high. 
     SUMMARY 
     An object of the present invention is to solve the problem described above, and particularly, an object of the present invention to provide a sheet conveyance apparatus whose size can be further reduced by maintaining high reliability of skew correction by a gate registration method regardless of curvature of a sheet conveyance path. 
     To achieve the abovementioned object, according to an aspect of the present invention, a sheet conveyance apparatus that conveys a sheet along a curved path shorter than the sheet and corrects skew of the sheet at a terminal end of the curved path, reflecting one aspect of the present invention comprises: a conveyance roller that is disposed at a starting end of the curved path and sends the sheet into the curved path; a guide that is disposed outside a curve of the curved path and guides the sheet along the curved path; and a gate that is disposed at the terminal end of the curved path such that a leading end of the sheet abuts the gate, hinders advance of the leading end, generates, at the leading end, a moment about a normal of the sheet passing through the leading end, and allows advance of the leading end by being pushed away by the leading end, wherein the guide includes a projection projecting toward a movement space for the sheet on a surface facing the movement space, the projection being provided in a region further on an inside than both ends of the sheet in a width direction of the movement space, and wherein, when the leading end of the sheet abuts the gate, the projection abuts a center portion of the sheet in a longitudinal direction and generates a moment about an axis parallel to an advance direction of a portion that has come into contact with the projection at the center portion in accordance with the moment generated at the leading end by the leading end abutting the gate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1A  is a perspective view of an image forming apparatus according to an embodiment of the present invention showing an appearance thereof; 
         FIG. 1B  is a front view of this apparatus schematically showing an internal structure of a printer incorporated in this apparatus; 
         FIG. 2A  is an enlarged front view of a skew correction portion and a curved path circled by an ellipse in  FIG. 1B ; 
         FIG. 2B  is a perspective view of the skew correction portion and the curved path from an upper front viewpoint; 
         FIG. 3A  is an enlarged side view from a viewpoint obliquely above the skew correction portion shown in  FIGS. 2A and 2B ; 
         FIG. 3B  is an exploded view of the skew correction portion; 
         FIG. 3C  is an enlarged perspective view of one of swing members included in the skew correction portion; 
         FIG. 4  is a perspective view of the skew correction portion shown in  FIGS. 2A and 2B  showing movement of a gate when the leading end of a sheet abuts the gate; 
         FIGS. 5A and 5B  are respectively a front view and a perspective view of the guide shown in  FIGS. 2A and 2B ; 
         FIG. 5C  is a perspective view of a projection member to be fitted in the guide; 
         FIG. 5D  is a perspective view of the projection member fitted in the guide; 
         FIGS. 6A and 6B  are each a schematic section view of a curved path and the vicinity of the terminal end thereof taken along a straight line VI-VI shown in  FIG. 4 ; 
         FIG. 7A  is a perspective view of an outer guide in which a first modification embodiment of the projection member is fitted; 
         FIGS. 7B and 7C  are respectively a front view and a perspective view of the outer guide in which a second modification embodiment of the projection member is fitted; 
         FIGS. 8A and 8B  are respectively a front view and a perspective view of the outer guide in which a third modification embodiment of the projection member is fitted; and 
         FIGS. 8C and 8D  are respectively a front view and a perspective view of the outer guide in which a fourth modification embodiment of the projection member is fitted. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     [Appearance of Image Forming Apparatus] 
       FIG. 1A  is a perspective view of an image forming apparatus according to an embodiment of the present invention showing an appearance thereof. This image forming apparatus  100  is a multi-function peripheral (MFP), and has functions of a scanner, a color copier, and a color printer. An automatic document feeder (ADF)  110  is mounted on the upper surface of the housing of the MFP  100  so as to be openable and closable. A scanner  120  is incorporated in an upper portion of the housing positioned right under the ADF  110 , and a printer  130  is incorporated in a lower portion of the housing. A plurality of tiers of sheet feed cassettes  133  are removably attached to a bottom portion of the printer  130 . 
     The MFP  100  is an in-body discharge type. That is, a gap DSP is provided between the scanner  120  and the printer  130 , and a discharge tray  44  is disposed therein. A sheet discharge port (not visible in the drawing) is disposed at an end of the gap DSP, and a sheet is discharged therefrom to the discharge tray  44 . A reverse tray  47  is disposed above the discharge tray  44 . At the time of duplex printing, the sheet whose front surface has been subjected to printing is switched back on the reverse tray  47 . That is, the sheet is once conveyed from a reverse port (not visible in the drawing) opened above the sheet discharge port to such a position as to stick out above the reverse tray  47 , and thereafter, the conveyance direction thereof is reversed and the sheet is again retracted into the reverse port. An operation panel  51  is attached to a front portion of the housing positioned beside the gap DSP. A touch panel is embedded in the front surface of the operation panel  51  and surrounded by various mechanical push buttons. The touch panel displays a graphics user interface (GUI) screen such as an operation screen and an input screen for various information, and receives a user&#39;s input operation through a gadget such as an icon, a virtual button, a menu, a tool bar or the like included in the screen. 
     [Structure of Printer] 
       FIG. 1B  is a front view of the printer  130  schematically showing an internal structure thereof. In this figure, the elements of the printer  130  are illustrated as though the elements are seen through the front side of the housing. The printer  130  is a color printer of an electrophotographic system and includes a feeding unit  10 , an image forming section  20 , a fixing unit  30 , and a sheet discharging unit  40 . The feeding unit  10  and the sheet discharging unit  40  are part of a sheet conveyance apparatus incorporated in the MFP  100 , and convey a sheet in the housing of the printer  130 . The image forming section  20  and the fixing unit  30  cooperate to function as an image forming part, and draw an image with toner on a sheet conveyed by the feeding unit  10  and the sheet discharging unit  40 . 
     The feeding unit  10  feeds one sheet at a time from a sheet feed cassette  11  or a manual feed tray  16  to the image forming section  20  by using a plurality of conveyance rollers  12 P,  12 F,  12 R,  13 ,  14 , and  15 . Examples of a material of sheets that can be accommodated in the sheet feed cassette  11  and the manual feed tray  16  include paper and resin, and examples of the type of paper include plain paper, high quality paper, color paper, and coated paper. Examples of the size of the sheet include A3 to A7, B4 to B7, business card, bookmark, ticket, postcard, envelope, and photograph (L size). The posture of the sheet can be set both in a vertical position and in a horizontal position. 
     The image forming section  20  forms a toner image on a sheet SH 2  fed from the feeding unit  10 . Specifically, four image forming units  21 Y,  21 M,  21 C, and  21 K first respectively charge the surfaces of photosensitive drums  25 Y,  25 M,  25 C, and  25 K, and the surfaces of the photosensitive drums  25 Y to  25 K are exposed in patterns based on image data by using laser light irradiated from an optical scanning unit  26 . As a result, electrostatic latent images are formed on the surfaces of the photosensitive drums  25 Y to  25 K. The image forming units  21 Y to  21 K then respectively develop the electrostatic latent images with toner of different colors of yellow (Y), magenta (M), cyan (C), and black (K). The four color toner images are sequentially transferred onto the same position on the surface of an intermediate transfer belt  23  from the surfaces of the photosensitive drums  25 Y to  25 K by an electric field between primary transfer rollers  22 Y,  22 M,  22 C and  22 K and the photosensitive drums  25 Y to  25 K. Thus, one color toner image is formed at that position. When this color toner image passes through a nip between a driving roller  23 R for the intermediate transfer belt  23  and a secondary transfer roller  24 , the color toner image is further transferred, by an electric field between the rollers  23 R and  24 , onto the surface of the sheet SH 2  that is passing through the nip at the same time. The sheet SH 2  is further sent from the secondary transfer roller  24  to the fixing unit  30 . 
     The fixing unit  30  thermally fixes the toner image on the sheet SH 2  fed from the image forming section  20 . Specifically, when the sheet SH 2  passes through a nip between a fixing roller  31  and a pressure roller  32 , the fixing roller  31  applies heat of a built-in heater to the surface of the sheet SH 2 , and the pressure roller  32  presses the heated portion of the sheet SH 2  against the fixing roller  31  by applying pressure. The toner image is fixed on the surface of the sheet SH 2  by the heat from the fixing roller  31  and the pressure from the pressure roller  32 . Thereafter, the fixing unit  30  sends out the sheet SH 2  from an upper portion thereof. 
     The sheet discharging unit  40  firstly assigns a sheet SH 3  or SH 4  sent out from the fixing unit  30  to either a sheet discharge roller  43  or a reverse roller  46  by a switching claw  41 . The sheet discharge roller  43  sends out a sheet SH 3  that has moved along the switching claw  41  through the discharge port  42  to the discharge tray  44 . The reverse roller  46  first places a sheet SH 4  that has moved along the switching claw  41  on the reverse tray  47  through a reverse port  45  by normal rotation. Just before the trailing end of the sheet SH 4  passes, the reverse roller  46  reversely rotates to pull the sheet SH 4  from the reverse tray  47  into the reverse port  45 , that is, switches back the sheet SH 4  to send the sheet SH 4  to a circulation path  48 . In the circulation path  48 , a plurality of conveyance rollers return a sheet SH 5  delivered by the reverse roller  46  to a conveyance path in the feeding unit  10  in a reversed posture. Thereafter, the feeding unit  10  sends the sheet SH 5  to the image forming section  20  again, and the image forming section  20  forms a toner image on the back surface of the sheet SH 5 . The fixing unit  30  heats the sheet SH 5  again, and the sheet discharging unit  40  discharges the sheet SH 5  to the discharge tray  44  this time. 
     [Structure of Sheet Conveyance Apparatus] 
     In addition to the conveyance rollers  12 P,  12 F,  12 R,  13 ,  14 , and  15  of the feeding unit  10 , conveyance rollers  43  and  46  of the sheet discharging unit  40 , and the circulation path  48 , the sheet conveyance apparatus incorporated in the MFP  100  uses the rollers  23 R and  24  of the image forming section  20  and the rollers  31  and  32  of the fixing unit  30  for conveying the sheet. The sheet conveyance apparatus in particular includes a timing roller  14 , a skew correction portion  200 , and a curved path  300 . 
     —Timing Roller— 
     The timing roller  14  passes the sheet through the nip between the intermediate transfer belt  23  and the secondary transfer roller  24  at a proper timing. More specifically, the timing roller  14  first stops each time a sheet arrives from the upstream side of the conveyance path. As a result, the leading end of the sheet SH 1 , SH 5 , or SH 6 , which have been moved from any one of the sheet feed cassettes  11 , the manual feed tray  16 , and the circulation path  48 , also temporarily stops at the nip formed by the timing roller  14 . Thereafter, the timing roller  14  starts to rotate in response to a command from a main control unit, and sends the stopped sheet to the image forming section  20 . The main control unit is an electronic circuit (not shown) incorporated in the printer  130 , and by causing a microprocessor such as a central processing unit (CPU) or a micro-processing unit (MPU) to execute firmware, various commands are given to the elements  10  to  40  of the printer  130 . The main control unit particularly determines the timing of starting the rotation of the timing roller  14  on the basis of a timing at which that the toner image formed on the surface of the intermediate transfer belt  23  by the image forming units  21 Y to  21 K passes through the nip between the intermediate transfer belt  23  and the secondary transfer roller  24 . As a result, the sheet SH 2  delivered from the timing roller  14  passes through the nip between the intermediate transfer belt  23  and the secondary transfer roller  24  simultaneously with the toner image. As a result, the toner image is correctly transferred onto the sheet SH 2 . 
     —Skew Correction Portion— 
     As shown in  FIG. 1B , three sheet feed paths from the sheet feed cassette  11  and the manual feed tray  16  are merged into one path downstream of a vertical feed roller  13 , and the path further joins the circulation path  48  at a meeting point MP that is further downstream. A skew correction portion  200  is disposed between the meeting point MP and the timing roller  14 . The skew correction portion  200  performs skew correction by the gate registration method on the sheet SH 1 , SH 5 , or SH 6  that has moved from any one of the sheet feed cassettes  11 , the circulation path  48 , and the manual feed tray  16 . 
       FIG. 2A  is an enlarged front view of the skew correction portion  200  and the curved path  300  circled by an ellipse CVP in  FIG. 1B , and  FIG. 2B  is a perspective view of the skew correction portion  200  and the curved path  300  from an upper front viewpoint;  FIG. 3A  is an enlarged side view of the skew correction portion  200  from an obliquely upper viewpoint, and  FIG. 3B  is an exploded view of the skew correction portion  200 . In these figures, members unnecessary for description of the skew correction portion  200  are illustrated as though the members are removed or transparent. As shown in these figures, the skew correction portion  200  includes a driving roller  210 , a driven roller  220 , and a gate  230 . 
     The driving roller  210  includes a shaft  211 , sleeves  212 ,  213 ,  214 , and  215 , and a gear  216 . As shown in  FIG. 3A , both ends of the shaft  211  are supported by a chassis  131  of the printer  130  so as to be rotatable about the axis thereof. As shown in  FIG. 3B , the sleeves  212  to  215  are cylindrical members of the same size and are made of a soft resin and fixed coaxially to and at equal intervals along the shaft  211 . The gear  216  is coaxially fixed to one end of the shaft  211 , receives rotational force from an external motor (not shown), and rotates the shaft  211  about the axis thereof. Along with this rotation, the sleeves  212  to  215  also rotate. 
     The driven roller  220  includes four cylindrical members  222 ,  223 ,  224 , and  225  as shown in  FIGS. 2B and 3B . All of these cylindrical members are made of a soft resin and have the same size as the sleeves  212  and  215  of the driving roller  210 , and both ends of each cylindrical member are supported so as to be rotatable about the central axis in a coaxially aligned state. The direction of a common axis (X-axis direction in  FIGS. 2A, 2B, and 3A to 3C ) of the cylindrical members  222  to  225  are parallel to the shaft  211  of the driving roller  210 , and nips are formed by bringing the outer peripheral surfaces of the cylindrical members  222  to  225  to the sleeves  212  to  215  in one-to-one correspondence. The sheet that has passed through the meeting point MP of a sheet feed path and the circulation path  48  enters these nips. When the sleeves  212  to  215  rotate along with the rotation of the shaft  211 , the cylindrical members  222  to  225  are driven to rotate, and the sheet entering the nips therebetween is sent to the timing roller  14 . 
     As shown in  FIGS. 2B and 3B , the gate  230  includes four swing members  231 ,  232 ,  233 , and  234 , a connection plate  235 , and an elastic member  236 .  FIG. 3C  is an enlarged perspective view of one of the swing member  231  among the swing members  231  to  234 . Each of the swing members  231  to  234  is a molded resin product of the same size, and includes a hook portion  237 , a claw portion  238 , and a holding portion  239 . The hook portion  237  is a C-shaped portion, and the inner peripheral surface thereof is in contact with the outer peripheral surface of the shaft  211 . As a result, the swing members  231  to  234  are coaxially supported by the shaft  211  so as to be slidably rotatable about the shaft  211 . Among the sleeves  212  to  215 , the sleeves  212  and  215  are disposed on the outside and the sleeves  213  and  214  are disposed on the inside in the axial direction of the shaft  211 . The swing members  231  to  234  are particularly arranged such that one swing member is disposed further on the outside than each of the sleeves  212  and  215  in the axial direction of the shaft  211 , and one swing member is disposed at each position between the sleeves  212  and  213  and between the sleeves  214  and  215 . The claw portion  238  is a claw-like portion projecting in the radial direction from one end of the hook portion  237  in the circumferential direction. The holding portion  239  is positioned on the side opposite to the claw portion  238  on the outer peripheral surface of the hook portion  237 , and is a flat plane portion spreading along the tangential plane of the outer peripheral surface. The connection plate  235  is an elongated rectangular metal plate or a rigid resin plate, is disposed in parallel to the shaft  211 , and is held by the holding portion  239  of the swing members  231  to  234 . As a result, when the swing members  231  to  234  rotate about the shaft  211 , the swing members  231  to  234  always slide together. The elastic member  236  is, for example, a coil spring, one end of which is connected to the chassis  131  of the printer  130 , and the other end of which is connected to the center of the connection plate  235  in the longitudinal direction. Therefore, when the swing members  231  to  234  rotate about the shaft  211  altogether, the elastic member  236  expands and contracts in accordance with the displacement of the connection plate  235 . At this time, the restoring force of the elastic member  236  acts on the swing members  231  to  234  in such a direction as to keep the angle about the shaft  211  constant. At this constant angle, as shown in  FIGS. 2A and 2B , the claw portions  238  of the swing members  231  to  234  are positioned upstream of the nip between the driving roller  210  and the driven roller  220 , and the leading end of the sheet that has passed the meeting point MP of the sheet feed path and the circulation path  48  abuts the gate  230 . 
       FIG. 4  is a perspective view of the gate  230  showing the movement of the gate  230  when the leading end of the sheet abuts the gate  230 . As shown in  FIG. 1B , the sheet SH 1  passing through the meeting point MP from the sheet feed path is pushed by the force of the sheet feed roller  12 F or the vertical feed roller  13  which feeds the rear half portion thereof, the sheet SH 5  passing through the meeting point MP from the circulation path  48  is pushed by the force of a conveyance roller  481  for sending out the rear half portion thereof. Thus, the leading end of the sheet is caused to advance toward the nip between the driving roller  210  and the driven roller  220 . By causing the claw portions  238  to abut the leading portion, the swing members  231  to  234  temporarily prevent progress of the leading end. However, since the force of the conveyance rollers  12 F,  13 , and  481  received by the claw portions  238  from the leading end of the sheet is stronger than the restoring force received by the connection plate  235  from the elastic member  236 , the swing members  231  to  234  are rotated about the shaft  211 , in such a direction that the claw portions  238  are pushed away by the leading end of the sheet. In this way, the swing members  231  to  234  allow the leading end of the sheet to enter the nip between the driving roller  210  and the driven roller  220 . The driving roller  210  sends this sheet to the timing roller  14 . When the driving roller  210  finishes feeding the trailing end of this sheet, the claw portions  238  are released from the sheet, and thus the swing members  231  to  234  are returned to the original angle by the restoring force of the elastic member  236 , and the claw portions  238  are moved back to the positions upstream of the nip. 
     As shown in  FIG. 1B , the curvature of the sheet conveyance path is small from the conveyance rollers  12 F and  13  positioned at the most downstream portion of the sheet feed path to the nip between the driving roller  210  and the driven roller  220  compared with the curved path  300  positioned at the most downstream portion of the circulation path  48 . In particular, stress generated in the sheet due to reaction force that the claw portions  238  of the swing members  231  to  234  apply to the leading end of the sheet acts on the entire sheet, and a component that presses the sheet to the surface of a guide disposed outside the curve of the curved path is small. Therefore, frictional force that the sheet receives from the surface of the guide due to this component is weak. As a result, the skew correction portion  200  can reliably achieve skew correction on the sheet that has moved from the sheet feed path. Actually, when skew occurs in the sheet, the leading end of the sheet abuts one of the claw portions  238  of the swing members  231  to  234  before the other claw portions  238 . In this case, the sheet rotates around a normal passing through the leading end thereof by the reaction force from the claw portion  238  before pushing away the claw portion  238  that the leading end abuts. Since the frictional force from the guide due to this reaction force is weak, the sheet smoothly rotates to a position where the leading end abuts all the claw portions  238  of the swing members  231  to  234 . In this way, skew is reliably removed from the sheet. 
     —Curved Path— 
     As shown in  FIG. 1B , the curved path  300  is the most downstream portion of the circulation path  48 , and the terminal end thereof is positioned at the meeting point MP with the sheet feed path. As shown in  FIG. 2A , the curved path  300  is shorter than the sheet SH 5 , and there is no other conveyance roller between the conveyance roller  481  positioned at the starting end and the driving roller  210  of the skew correction portion  200  positioned at the terminal end. An inner guide  310  is disposed inside the curve of the curved path  300 , and an outer guide  320  is disposed outside of the curve. Each of the guides  310  and  320  is a plate-like member made of a metal or a hard resin whose plate surface is substantially curved in a J-shape, partitions the movement space for the sheet SH 5  spreading along the curved path  300  from the outside, and guides the sheet SH 5  along the curved path  300 . 
     The curved path  300  is more curved than the conveyance path from the sheet feed path to the skew correction portion  200 . Specifically, force FR 1  applied by the conveyance roller  481  positioned at the starting end of the curved path  300  to the sheet SH 5  and reaction force FR 2  applied to the leading end of the same sheet SH 5  by the claw portions  238  of the swing members  231  to  234  only form an angle θ smaller than 90°. In this case, stress generated inside the sheet SH 5  due to either of the forces FR 1  and FR 2  not only includes components ST 1  and ST 2  for pressing the sheet SH 5  against the surface of the outer guide  320 , but also both of the components ST 1  and ST 2  are remarkably large in the same range RNG of the curved path  300 . Therefore, frictional force that the sheet SH 5  receives from the outer guide  320  is generally stronger than frictional force that the sheet SH 1  sent from the sheet feed path to the skew correction portion  200  receives from the guide. 
     Although the sheet SH 5  passing through the curved path  300  receives the strong frictional force from the outer guide  320 , the skew correction portion  200  can reliably achieves skew correction on this sheet SH 5 . This is because the guides  310  and  320  respectively include projection members  330  and  340  in curved portions thereof as shown in  FIGS. 2A and 2B . 
       FIGS. 5A and 5B  are respectively a front view and a perspective view of the guides  310  and  320 . In  FIG. 5B , the inner guide  310  is illustrated as if the inner guide  310  is transparent, making the structure of the outer guide  320  easy to see. Downstream ends  311  and  321  of the guides  310  and  320  in the conveyance direction along the curved path  300  respectively include slits  312  and  322  of shapes elongated in the conveyance direction at a center portion in the width direction. The slit  322  of the outer guide  320  is longer than the slit  312  of the inner guide  310  and extends over almost the entire curved portion of the outer guide  320  as shown in  FIG. 5A . Further, upstream ends  313  and  323  of the guides  310  and  320  in the conveyance direction are parallel to the width direction, and ranges  314  and  324  from the upstream ends  313  and  323  to the slits  312  and  322  include smooth curved surfaces. Projection members  330  and  340  are respectively fitted in the slits  312  and  322 , and are opposed to each other with a movement space for the sheet SH 5  interposed therebetween. 
     The projection member  330  fitted in the inner guide  310  is, for example, a plate made of a soft resin and has a thickness equal to the width of the slit  312  of the inner guide  310 . As shown in  FIG. 5A , about a half  331  of the plate surface of the projection member  330  projects to the movement space for the sheet SH 5  than the inner guide  310 . A portion  332  projecting to the movement space among the side surface of the projection member  330  spreading along the circumference of the plate surface has a band shape that is smoothly curved along the conveyance direction, and forms a smooth J-shaped curved surface together with the range  314  from the upstream end  313  to the slit  312  in the surface of the inner guide  310 . Since the surface facing the movement space for the sheet SH 5  is smooth as described above, both the inner guide  310  and the projection member  330  hardly scratch the surface of the sheet SH 5  due to friction. 
       FIG. 5C  is a perspective view of the projection member  340  to be fitted in the outer guide  320 , and  FIG. 5D  is a perspective view of the projection member  340  fitted in the outer guide  320 . In  FIG. 5D , the projection member  340  and the outer guide  320  are illustrated as if these are transparent. The projection member  340  is, for example, a soft resin rod, and both ends thereof in the longitudinal direction are steeply curved in the same direction, and the width in a direction (X axis direction in the drawing) perpendicular to both of the longitudinal direction and the curve direction is equal to the width of the slit  322  of the outer guide  320 . As shown in  FIG. 5A , in a state in which the projection member  340  is fitted in the slit  322  of the outer guide  320 , substantially the entirety of the projection member  340  projects to the movement space of the sheet SH 5  as compared with the outer guide  320 . A surface  342  of the projection member  340  facing the movement space is positioned inside the curve of the projection member  340  and has a band shape smoothly curved along the conveyance direction, and forms a smooth J-shaped curved surface together with the range  324  from the upper end  323  to the slit  322  in the surface of the outer guide  320 . Furthermore, as shown in  FIG. 5D , the surface  342  of the projection member  340  facing the movement space of the sheet SH 5  has an arcuate outline in the width direction (X axis direction) of the movement space. Since the surface facing the movement space for the sheet SH 5  is smooth as described above, both the outer guide  320  and the projection member  340  hardly scratch the surface of the sheet SH 5  due to friction. 
       FIG. 6A  is a schematic section view of the curved path  300  and the vicinity of the terminal end thereof taken along a line VI-VI shown in  FIG. 4 , and in particular, shows a state in which a leading end LDE of the sheet SH 5  moving through the curved path  300  has reached the positions of the swing members  231  to  234 . When the sheet SH 5  is skewed, the leading end LDE thereof abuts one of the swing members  231  to  234 , for example, the claw portion  238  of the swing member  231  that is on the outside, ahead of the claw portions  238  of the other three swing members  232  to  234 . Due to reaction force FRE received by the leading end LDE of the sheet SH 5  from the claw portion  238 , a moment TRE about a normal (the normal to the page surface in  FIG. 6A ) passing through the leading end LDE is generated in the sheet SH 5 . Meanwhile, in the range where the projection member  340  fitted in the outer guide  320  is positioned in the curved path  300 , a component STE that presses a center portion CTR in the longitudinal direction of the sheet SH 5  against the surface  342  of the projection member  340  is large in the stress caused in the sheet SH 5  by the reaction force FRE. However, this component STE is acts greatly only in a region LHS positioned closer to the swing member  231  that the leading end LDE of the sheet SH 5  first abuts compared with a portion CNP in contact with the projection member  340  in the center portion CTR of the sheet SH 5 . Therefore, a moment TRC about the advance direction of the portion CNP in contact with the projection member  340  (the normal to the sheet surface in  FIG. 6A ) is generated in the center portion CTR of the sheet SH 5 . Since the sheet SH 5  is wider than the projection member  340 , the center portion CTR thereof rotates about the portion CNP in contact with the projection member  340  and moves obliquely with respect to the width direction (left and right direction in  FIG. 6A ) of the movement space for the sheet SH 5 . 
     The projection member  340  opposes the projection member  330  fitted in the inner guide  310  with the movement space for the sheet SH 5  therebetween. The interval between the projection members  330  and  340  is narrower than the interval between the guides  310  and  320  positioned further on the outside than the projection members  330  and  340  in the width direction of the movement space for the sheet SH 5 . Therefore, even when the center portion CTR of the sheet SH 5  is accidentally lifted off the surface  342  of the projection member  340  while rotating about the contact portion CNP with the projection member  340  of the outer guide  320 , the center portion CTR collides with the opposing projection member  330  and falls down. In this way, the center portion CTR of the sheet SH 5  is reliably inclined with respect to the width direction. 
       FIG. 6B  is a schematic section view of the curved path  300  and the vicinity of the terminal end thereof taken along the line VI-VI shown in  FIG. 4 , and in particular, shows a state in which the center portion CTR of the sheet SH 5  moving through the curved path  300  is inclined with respect to the width direction of the movement space. Since the center portion CTR is inclined in this manner in accordance with the reaction force FRE received by the leading end LDE of the sheet SH 5  from the claw portion  238  of the swing member  231 , even if the contact portion CNP receives strong frictional force from the projection member  340 , the leading end LDE smoothly rotates to a position where the leading end LDE abuts all the claw portions  238  of the swing members  231  to  234  (a position to be parallel to the left and right direction in  FIG. 6B ) without being prevented by the frictional force. The skew is reliably removed from the sheet SH 5  in this way, and thus the sheet SH 5  can advance by pushing away the claw portions  238  of the swing members  231  to  234 . 
     Advantages of Embodiment 
     In the sheet conveyance apparatus according to the above-described embodiment of the present invention, the outer guide  320  disposed along the curved path  300  includes the projection member  340  projecting to the movement space for the sheet SH 5 . This projection member  340  generates the moment TRC about the advance direction of the contact portion CNP with the projection member  340  at the center portion CTR of the sheet SH 5  in accordance with the moment TRE generated as a result of leading end LDE of the sheet SH 5  abutting the swing member  231  of the gate  230 . Accordingly, even in the case where the sheet SH 5  is strongly pressed against the outer guide  320  by the reaction force FR 2  from the gate  230  as a result of the abutment of the leading end LDE in addition to by the force FR 1  from the conveyance roller  481  that sends the sheet SH 5  to the curved path  300 , the sheet SH 5  can rotate about the contact portion CNP with the projection member  340  by the same reaction force FR 2 . In this manner, this sheet conveyance apparatus maintains the reliability of the skew correction by the gate registration method high irrespective of the steepness of the curve of the curved path  300 . As a result, this sheet conveyance apparatus can be further miniaturized. 
     Modification Embodiment 
     (A) The image forming apparatus  100  shown in  FIGS. 1A and 1B  is an MFP. The sheet conveyance apparatus according to the embodiment of the present invention may be incorporated in a single-function image forming apparatus such as a printer, a copier, a facsimile machine, or the like. In addition, the printing function of the apparatus may be of an inkjet system instead of the electrophotographic system. The sheet conveyance apparatus according to the embodiment of the present invention can be incorporated in any system as long the system is a sheet processing system such as a finisher or an ADF. 
     (B) The elastic member  236  of the gate  230  shown in  FIGS. 2A and 2B  is a coil spring. The elastic member may be a spring of a different shape such as a leaf spring or a torsion spring, or an elastomer instead of the coil spring. 
     (C) When all the claw portions  238  of the swing members  231  to  234  of the gate  230  shown in  FIG. 4  abut the leading end of the sheet SH 5 , the swing members  231  to  234  rotate about the shaft  211  to swing the claw portions  238  and thus allows the advance of the leading end of the sheet SH 5 . The movable member to be included in the gate may have a different structure as long as the movable member is disposed such that the sheet abuts the movable member in the movement space for the sheet and the movable member allows the advance of the leading end of the sheet by being pushed away by the leading end when the leading end of the sheet abuts the movable member. In particular, a plurality of different members need not abut the leading end of the sheet. For example, a mechanism in which a single plate member is disposed to lies. over the entire width of the movement space for the sheet, the inclination of the plate member to the conveyance direction of the sheet is variable, and the plate member is pushed down by the leading end of the sheet abutting the plate member may be employed. 
     (D) When all the claw portions  238  of the swing members  231  to  234  of the gate  230  shown in  FIG. 4  abut the leading end of the sheet SH 5 , force of the conveyance rollers  12 F,  13 , and  481  received from the leading end is stronger than the restoring force of the elastic member  236 , and thus the swing members  231  to  234  rotate in such a direction that the claw portions  238  are pushed away by the leading end of the sheet SH 5 . As a result, the timing at which the gate  230  allows the advance of the leading end of the sheet SH 5  is before the loop is formed a stagnated portion of the sheet SH 5 , and thus there is no need to secure a space for forming a loop of the sheet on the upstream side of the skew correction portion  200 . 
     In this manner, in the skew correction by the gate registration method, the size of the movement space for the sheet to be secured upstream of the gate can be reduced if the timing at which the gate allows the advance of the leading end of the sheet is before forming a loop in the stagnated portion. Therefore, the gate may be a mechanism in which, for example, the gate detects abutment of the leading end of the sheet on the movable member by a sensor and the movable member is retracted from the movement space for the sheet by an actuator such as a solenoid before forming a loop in the stagnated portion of the sheet instead of the mechanism in which the sheet moves the movable member such as the swing members  231  to  234 . 
     (E) The projection member  340  fitted in the outer guide  320  shown in  FIGS. 5A to 5D  is positioned inside the both ends in the width direction of the sheet SH 5  moving in the curved path  300 , and the surface thereof projects to the movement space for the sheet SH 5  compared with the surface of the outer guide  320  therearound. Due to this placement and shape of the projection member  340 , as shown in  FIG. 6A , when the leading end LDE of the skewed sheet SH 5  abuts the swing member  231  that is one of the swing members  231  to  234 , the reaction force FR 2  from the swing member  231  generates the moment TRC about the contact portion CNP with the projection member  340  in the center portion CTR of the sheet SH 5  and inclines the leading end LDE. As a result, the leading end LDE rotates smoothly to the position where the leading end LDE abuts all of the swing members  231  to  234 . 
     Therefore, it suffices as long as the projection to be included on the surface of the outer guide  320  facing the movement space for the sheet SH 5  has the following features. This projecting portion is positioned inside the both ends of the sheet SH 5  in the width direction of the movement space for the sheet SH 5  and projects to the movement space. Further, this projection comes into contact with the center portion CTR of the leading end LDE of the sheet SH 5  in the longitudinal direction of the sheet SH 5  when the leading end LDE abuts the gate  230 , and generates the moment TRC about the advance direction of the contact portion CNP with the projection at the center portion CTR of the sheet SH 5  in accordance with the moment TRE generated as a result of the leading end LDE abutting the gate  230 . The member capable of forming the projection having these features is not limited to the above-described projection member  340 , and various modifications as listed below are possible. 
       FIG. 7A  is a perspective view of the outer guide  320  in which a first modification embodiment  440  of the projection member is fitted. In this figure, as in  FIG. 5B , the inner guide  310  is illustrated as if the inner guide  310  is transparent. The projection member  440  according to the first modification embodiment differs from the projection member  340  shown in  FIGS. 5A to 5D  only in the following points. A surface  442  facing the movement space for the sheet is a flat surface parallel to both the width direction of the movement space and the advance direction of the portion of the sheet in contact with the surface  442 . That is, unlike the projection member  340  shown in  FIGS. 5A to 5D , the edge in the width direction of the projection member  440  is angular. An angular member like the projection member  440  may be utilized to form the projection of the outer guide  320  in the case where the member can give a negligible degree of damage to the surface of the sheet, in particular to the image thereon. The angular projection member  440  is easier to process than the arcuate projection member  340 , and is thus advantageous for reducing the cost. 
       FIGS. 7B and 7C  are respectively a front view and a perspective view of the outer guide  320  in which a second modification embodiment  540  of the projection member is fitted. In  FIG. 7C , as in  FIG. 7A , the inner guide  310  is illustrated as if the inner guide  310  is transparent. The projection member  540  according to the second modification embodiment differs from the projection member  440  of the first modification embodiment only in the following points. The size of the projection member  540  along the sheet conveyance direction is shorter than that of the projection member  440 , and a downstream end  541  of the projection member  540  in the conveyance direction is positioned upstream of the downstream end  321  of the outer guide  320 . Accordingly, the outer guide  320  includes a hole  522  of the same size as the projection member  540  in a center portion in the conveyance direction instead of the slit  322 . A range  325  from the downstream end  321  to the hole  522  in the surface of the outer guide  320  is parallel to the width direction and is smoothly curved along the conveyance direction. A surface  542  of the projection member  540  according to the second modification embodiment facing the movement space for the sheet is shorter than that of the projection member  440  of the first modification embodiment in the conveyance direction, and therefore the contact portion at the center portion of the sheet in the conveyance direction is shortened. Even in the case where the projection member  540  is short as described above, the leading end of the sheet is smoothly rotatable to the position at which the leading end abuts all the swing members  231  to  234  of the skew correction portion  200  as long as the center portion of the sheet is surely inclined due to the moment about the contact portion. Meanwhile, since the downstream end  321  of the outer guide  320  is parallel to the width direction, the leading end of the sheet can be more stably guided to the skew correction portion  200 . 
       FIGS. 8A and 8B  are respectively a front view and a perspective view of the outer guide  320  in which a third modification embodiment  640  of the projection member is fitted. In  FIG. 8B , as in  FIG. 5B , the inner guide  310  is drawn as if the inner guide  310  is transparent. The projection member  640  according to the third modification embodiment differs from the projection member  440  of the first modification embodiment only in the following points. An overhanging portion  643  projects from a downstream end portion  641  in the conveyance direction to the both side in the width direction and covers the entire width of the downstream end  321  of the outer guide  320 . The surface of the overhanging portion  643  facing the movement space for the sheet is parallel to the width direction and is smoothly curved along the conveyance direction. Since a surface  642  of the projection member  640  according to the third modification embodiment facing the movement space for the sheet has the same shape and the same size as the surface  442  of the projection member  440  of the first modification embodiment, the effect of giving a moment about the contact portion to the center portion of the sheet and inclining the sheet is similar to that of the projection member  440  of the first modification embodiment. Meanwhile, the projection member  640  according to the third modification embodiment is more advantageous than the projection member  440  according to the first modification embodiment in that the overhanging portion  643  more stably guides the leading end of the sheet to the skew correction portion  200 . 
       FIGS. 8C and 8D  are respectively a front view and a perspective view of the outer guide  320  in which a fourth modification embodiment  740  of the projection member is fitted. In  FIG. 8D , as in  FIG. 8B , the inner guide  310  is drawn as if the inner guide  310  is transparent. The projection member  740  according to the fourth modification embodiment is a similar member to the projection member  540  according to the second modification embodiment as a single body, and is different from the projection member  540  according to the second modification embodiment only in that the projection member  740  is disposed in combination with a floor member  750 . The floor member  750  covers the entire width of the downstream end  321  of the outer guide  320 , and the surface facing the movement space for the sheet is parallel to the width direction and is curved smoothly along the conveyance direction. Since a surface  742  of the projection member  740  according to the fourth modification embodiment facing the movement space for the sheet has the same shape and the same size as the surface  542  of the projection member  540  of the second modification embodiment, the effect of giving a moment about the contact portion to the center portion of the sheet and inclining the sheet is similar to that of the projection member  540  of the second modification embodiment. Meanwhile, the projection member  740  according to the fourth modification embodiment is more advantageous than the projection member  540  according to the second modification embodiment in that the floor member  750  more stably guides the leading end of the sheet to the skew correction portion  200 . 
     (F) The projection member  330  is fitted in the inner guide  310  shown in  FIGS. 5A to 5D  and opposes to the projection member  340  of the outer guide  320  with the movement space for the sheet therebetween. Since the interval between the projection members  330  and  340  is narrow, the center portion of the sheet reliably rotates about the contact portion of the outer guide  320  with the projection member  340 , and is inclined with respect to the width direction. However, in the case where the center portion of the sheet is reliably inclined even if the projection member  330  of the inner guide  310  is not provided because, for example, the projection amount of the projection member  340  of the outer guide  320  is sufficiently large, the projection member  330  may be omitted from the inner guide  310 . 
     (G) In the curved path  300  shown in  FIG. 2A , the force FR 1  that the sheet SH 5  receives from the conveyance roller  481  positioned at the starting end of the curved path  300  and the reaction force FR 2  that the sheet SH 5  receives from the claw portions  238  of the swing members  231  to  234  only form an angle θ smaller than 90°. In this case, since the stress components ST 1  and ST 2  that press the sheet SH 5  against the surface of the outer guide  320  are remarkably large in the same range RNG in the curved path  300 , there is a high risk that the rotation of the leading end of the sheet SH 5  is hindered by the frictional force from the outer guide  320  without the projection member  340 . However, even if the curve of the curved path is not steep such that the force from the conveyance rollers and the reaction force from the claw portions  238  of the swing members  231  to  234  form an angle of 90° or more, a projection member similar to the projection member  340  may be disposed on a guide disposed outside the curve. Also in this case, the rotation of the leading end of the sheet in accordance with the abutment on the claw portions  238  is promoted by the center portion thereof being rotated and inclined about the contact portion with the projection member. As a result, the reliability of the skew correction portion  200  can be maintained high. 
     The present invention relates to a sheet conveyance technique in which a projection is provided on a guide disposed outside the curve of a curved path, and when the leading end of a sheet having passed through the curved path abuts the gate, the sheet is inclined by rotating the center portion of the sheet about a contact portion with the projection. Thus, the present invention is clearly industrially applicable. 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims.