Patent Publication Number: US-7708276-B2

Title: Sheet conveying path switching device used in image forming apparatus, and sheet conveying device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, such as a copying machine, a printer and a facsimile device. More particularly, the present invention relates to a sheet conveying path switching device for switching a sheet conveyance direction in such an image forming apparatus, and to a sheet conveying device that uses this sheet conveying path switching device. 
     2. Description of the Related Art 
     In order to downsize the whole image forming apparatus, it is desired to reduce the size of a sheet conveying device that conveys a recording sheet as a recording medium from sheet storage means such as a paper cassette or a paper feed tray. Also, in recent years, image forming apparatuses that support various sizes and types of sheets have become popular. In such image forming apparatuses, sheets of several sizes and types are previously stored in sheet storage means, and a type or size of sheet that is selected arbitrarily by a user or a sheet that is selected automatically by the image forming apparatus can be fed. For this reason, the sheet storage means fills a larger space in the image forming apparatus, thus there is a strong need of reducing the size of the sheet conveying device. From the standpoint of conveyance of sheets, the same thing is required in ADF documents as well. 
     Furthermore, recently, image forming apparatuses that are capable of selecting one-side printing or both-side printing of a sheet to form an image thereon have become popular. In an image forming apparatus capable of forming an image on both sides of a sheet, a sheet, one side of which has an image fixed, is sent out to a recording body loading section (sheet discharging tray) provided in the apparatus main body, or conveyed to a recording body reversing section (both-side conveying means) in order to form the image on the reverse side. Therefore, separating means for selectively switching the sheet conveyance direction is provided within the sheet conveying path. Even in the case of forming an image on one side, the conveying path needs to be switched when sheets onto which images have been fixed are sorted and discharged. 
     As described in, for example, Japanese Unexamined Patent Publication No. 2005-178954, there is adopted a sheet conveying device in which a simple and inexpensive separating arm is used as the separating means. In a conveying path within the sheet conveying device where a separating section is provided, the size of the conveyance direction is changed significantly, depending on the positional relation of the apparatus, and the space occupied by the conveying path is reduced in order to downsize the sheet conveying device. As a result, the conveying path is provided with a curvature section that has a predetermined curvature in order to change the conveyance direction of the conveying path continuously and smoothly, whereby the curvature radius of the curvature section can be set relatively small enough to convey a sheet of a fixed size, which is normally used in an image forming apparatus. 
     In the sheet conveying device having such a configuration as above, when conveying a highly rigid, firm sheet such as a cardboard, or a special type of sheet such as an envelope, the leading edge of the sheet P comes into contact with the wall surface of the curvature section due to the small curvature radius of the curvature section, whereby the friction resistance between the sheet and the wall surface increases. For this reason, there is a problem that the highly rigid sheet or special type of sheet cannot go forward along the conveying path formed in the curvature section, causing a paper jam or conveyance failure that inhibits stable conveyance operation. It is conceivable to use suction-type conveying means using air, in order to prevent such a problem described above. However, such conveying means requires a suction device, thus there is a disadvantage that the configuration of the sheet conveying device becomes complex and the size of the same increase, causing a cost increase. 
     In, for example, Japanese Patent Application No. 3257712, there is proposed a sheet conveying device that can securely feed sheets to each of the conveying paths intended to convey the sheets at small sheet intervals in response to the high speed of the sheet conveying device, the sheet conveying device being realized at low cost. In this sheet conveying device, as described hereinafter, by using a special switching belt, the conveying force can be applied to a sheet at a sheet-conveyance reversing section so that, even when conveying a firm sheet, smoother conveyance can be performed, compared to a fixed curving guide path having a small curvature radius. However, there is a problem in this device that a risk of a paper jam increases, as described hereinafter. 
     Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Patent Publication No. H10-129883 and Japanese Patent Application No. 3723067. 
     SUMMARY OF THE INVENTION 
     The present invention is contrived in view of the above-described background, and an object of the present invention is to provide an easy-to-use, inexpensive, and small sheet conveying path switching device and a sheet conveying device using the sheet conveying path switching device, which can stably and separately convey not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets, and can further separately convey a bent sheet. 
     Another object of the present invention is to provide an image forming apparatus having the abovementioned sheet conveying device. 
     In an aspect of the present invention, a sheet conveying path switching device comprises a sheet carrying-in path and a switching separating arm. Outlines of two separating/conveying paths out of a plurality of separating/conveying paths positioned on a downstream side of the switching separating arm are configured by a movable belt surface, the two separating/conveying paths being located closer to the separating arm than the other separating/conveying paths. 
     In another aspect of the present invention, a sheet conveying path switching device comprises a sheet carrying-in path; a switching separating arm; and a plurality of separating/conveying paths disposed on a downstream side of the switching separating arm. The switching separating-arm is configured by a fixed guiding section and an end switching arm piece that is movable. 
     In another aspect of the present invention, a sheet conveying device comprises a sheet conveying path switching device having a sheet carrying-in path, and a switching separating arm. Outlines of two separating/conveying paths out of a plurality of separating/conveying paths positioned on a downstream side of the switching separating arm are configured by a movable belt surface, the two separating/conveying paths being located closer to the separating arm than the other separating/conveying paths. 
     In another aspect of the present invention, a sheet conveying device comprises a sheet conveying path switching device having a sheet carrying-in path, a switching separating arm, and a plurality of separating/conveying paths disposed on a downstream side of the switching separating arm. The switching separating arm is configured by a fixed guiding section and an end switching arm piece that is movable. 
     In another aspect of the present invention, an image forming apparatus comprises a sheet conveying path switching device having a sheet carrying-in path, and a switching separating arm. Outlines of two separating/conveying paths out of a plurality of separating/conveying paths positioned on a downstream side of the switching separating arm are configured by a movable belt surface, the two separating/conveying paths being located closer to the separating arm than the other separating/conveying paths. 
     In another aspect of the present invention, an image forming apparatus comprises a sheet conveying path switching device having a sheet carrying-in path, a switching separating arm, and a plurality of separating/conveying paths disposed on a downstream side of the switching separating arm. The switching separating arm is configured by a fixed guiding section and an end switching arm piece that is movable. 
     In another aspect of the present invention, a recording medium conveying device comprises an introducing path to which a recording medium is introduced; two conveying paths that are separated from the introducing path to form the shape of a letter “Y”; and a conveying path switching device that guides the recording medium from the introducing path to either one of the two conveying paths by switching a separating arm that is disposed so as to face the introducing path. Wall surface sections of the two conveying paths with which the leading edge of the recording medium introduced from the introducing path contacts are configured by a surface of a rotatable single belt that is extended to form the shape of a triangle. 
     In another aspect of the present invention, an image forming apparatus comprises an image forming device for forming an image on a recording medium; and a recording medium conveying device for conveying the recording medium. The recording medium conveying device comprises an introducing path to which a recording medium is introduced; two conveying paths that are separated from the introducing path to form the shape of a letter “Y”; and a conveying path switching device that guides the recording medium from the introducing path to either one of the two conveying paths by switching a separating arm that is disposed so as to face the introducing path. Wall surface sections of the two conveying paths with which the leading edge of the recording medium introduced from the introducing path contacts are configured by a surface of a rotatable single belt that is extended to form the shape of a triangle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
         FIGS. 1A and 1B  are figures for explaining switching of a separating arm of a conventionally known separating device; 
         FIG. 2  is an external perspective view showing a schematic configuration of the same separating device; 
         FIGS. 3A through 3C  are figures each showing a state in which a sheet is sent upward in another conventionally known separating device; 
         FIGS. 4A through 4C  are figures each showing a state in which a sheet is sent downward in the same separating device; 
         FIG. 5  is a figure showing a schematic configuration of an image forming apparatus according to a first embodiment of the present invention; 
         FIG. 6  is a figure showing a configuration of a sheet conveying path switching device according to the first embodiment; 
         FIG. 7  is a perspective view showing a state in which an endless belt is divided in a direction perpendicular to a sheet conveyance direction; 
         FIG. 8  is a flowchart for explaining the timing to drive a separating arm and belt; 
         FIG. 9  is a figure showing a modification of the same sheet conveying path switching device; 
         FIG. 10  is a figure showing a schematic configuration of the image forming apparatus according to a second embodiment of the present invention; 
         FIG. 11  is a figure showing a schematic configuration of the sheet conveying path switching device according to the second embodiment; 
         FIG. 12  is an external perspective view showing a schematic configuration of the conveying path switching device; 
         FIG. 13  is an external perspective view showing a schematic configuration of the separating arm; 
         FIGS. 14A and 14B  are figures each showing a schematic configuration of the separating arm according to a modification; 
         FIG. 15  is a perspective view showing a schematic configuration of a modification of the conveying path switching device; and 
         FIG. 16  is a cross-sectional view showing a schematic configuration of another modification of the conveying path switching device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the present invention hereinafter, the prior art of the present invention and the problems thereof are described with reference to the drawings. 
     As already described above,  FIGS. 1A and 1B  each shows a schematic configuration of a sheet conveying device in which a separating arm is used as the separating means and the curvature radius of a curvature section on a separating/conveying path is small.  FIG. 1A  shows a first state in which the arm member of the separating arm  22  guides a sheet S to a sheet discharging tray side, while  FIG. 1B  shows a second state in which a sheet S is guided to a switchback conveying path and a re-conveying path side in order to perform printing on both sides of the sheet. Also,  FIG. 2  shows a perspective view of the separating device. It should be noted in this figure that a guide plate  23  located on the upper side for forming a conveying path after separation and an upper roller of a pair of conveying rollers are omitted for simplification of the illustration. 
     As shown, the separating arm  22  is configured such that a plurality of arm members  22   a  are disposed coaxially on an axis  22   b . Leading ends of the arm members  22   a  can fitted, respectively, into halls (may be formed into a concave shape)  18   a  and  19   a  that are provided on a lower left guide plate  18  and an lower right guide plate  19  respectively as shown in  FIG. 2 , whereby a conveyance direction of a sheet is separated. A shutter section  22   c  that detects a rotational position is fixed to the axis  22   b , and an edge of the shutter section  22   c  is detected by a photosensor  21 , whereby the first state in which the separating arm  22  guides a sheet to a sheet receiving section can be detected. Also, on the side opposite to the shutter section  22   c , a pulley  22   d  is fixed to the axis  22   b , and connected to a stepping motor  20  by means of a timing belt  24  so as to be driven by the stepping motor  20 . The separating arm  22  is operated by the driving force of the stepping motor  20 . 
     As shown in  FIGS. 1A and 1B , there is provided a carrying-in path  26  to which a sheet is conveyed from a fixing device (only a pair of fixing rollers  25   a  are shown) positioned on an upstream side of the sheet conveyance direction, and there is also provided two conveying paths  27  and  28  that extend beyond the separating arm  22  and are positioned on a downstream side of the sheet conveyance direction. These conveying paths  26 ,  27  and  28  form a separating/conveying path. The first separating/conveying path  27  guides a sheet to the sheet receiving section side, and the second separating/conveying path  28  guides a sheet to the switchback conveying path side. 
     In the conventional sheet conveying device having such a configuration, as described above, when conveying a highly rigid, firm sheet such as a cardboard, or a special type of sheet such as an envelope, the leading edge of the sheet S comes into contact with the wall surface of the curvature section due to the small curvature radius of the curvature section, whereby the friction resistance between the sheet and the wall surface increases. For this reason, there is a problem that the highly rigid sheet or special type of sheet cannot go forward along the conveying path formed in the curvature section, causing a paper jam or conveyance failure that inhibits stable conveyance operation. 
     In order to solve such a problem,  FIGS. 3A through 3C  each shows a configuration of the separating device of the sheet conveying device disclosed in the abovementioned Japanese Patent Application No. 3257712. As shown in these figures, this separating device is provided with a switching belt  41  that is located at an extension of a sheet conveyance direction of a sheet S passing through a nip between a pair of rollers  31 , and is capable of rotating normally and reversely. By changing the direction of rotation of the switching belt  41 , the conveyance direction of the sheet S conveyed by the pair of rollers  31  is changed. 
       FIG. 3A  shows a state in which the leading edge of the sheet S is slightly pushed by the pair of rollers  31  toward the extension of the conveyance direction of the sheet that has passed through the nit between the rollers. At this moment, the switching belt  41  already rotates in a counterclockwise direction as shown by the arrows, in order to send the sheet S to the conveying path  27  side. When the leading edge Sa of the sheet S abuts on the switching belt  41 , the leading edge Sa of the sheet S is guided to the conveying path  27  side by the switching belt  41  rotating in the direction shown by the arrows, as shown in  FIG. 3B . Thereafter, the sheet S is guided to guide plates  36  and  37  as shown in  FIG. 3C , sandwiched between a pair of conveying rollers  33 , and then travels upward within the conveying path  27 . It should be noted that a reference numeral Sb represents the rear edge of the sheet S. 
     In a conveyance of a sheet toward the other separating/conveying path  28 , as shown in  FIG. 4A , the leading edge of the sheet S is sandwiched between the pair of rollers  31 , while the switching belt  41  rotates in a clockwise direction as shown by the arrows, in order to send the sheet S to the conveying path  28  side. When the sheet S reaches the switching belt  41 , and the leading edge Sa abuts on the same belt, the leading edge Sa of the sheet is guided in the direction of rotation of the switching belt  41  shown by the arrows, as shown in  FIG. 4B . Therefore, the sheet S is moved forward by a guide of the guide plates  36 ,  38 , and conveyed to the lower side shown in the figure through the conveying path  28  while being sandwiched between a pair of conveying roller  35 , as shown in  FIG. 4C . 
     In this conventional sheet conveying device that has a separating device using such a switching belt described above, as already described above, the conveying force can be applied to the sheet at a sheet-conveyance reversing section so that, even when conveying a firm sheet, smoother conveyance can be performed, compared to a fixed curving guide path having a small curvature radius. However, in the configurations shown in  FIGS. 3A through 3C  and  4 A through  4 C, when sending the sheet to a desired separating/conveying path, it is not guaranteed that the leading edge of the sheet abuts on the desired separating/conveying path of the switching belt  41 , thus there may occur that the sheet abuts on a section of the switching belt on a different separating/conveying path side, and then the leading edge of the sheet is pushed back to the desired separating/conveying path. In other words, since a sheet is bent or tends to bend especially after an image is fixed thereon, in the case in which the sheet S is bent downward in, for example,  FIGS. 3A through 3C , the leading edge of the sheet abuts first on the lower-right diagonal side of the switching belt  41 . Thereafter, the leading edge is raised as the sheet S is conveyed by the pair of rollers  31 , and the sheet is pushed back to the conveying path  27  side. Therefore, the risk of a paper jam increases. 
     Hereinafter, each embodiment of the present invention in which the abovementioned problems of the prior art are resolved is described in detail with reference to the drawings. 
     First Embodiment 
     Hereinafter, the present embodiment is described with reference to the drawings. 
       FIG. 5  shows a schematic configuration of a full-color printer (referred to as “printer” hereinafter)  100 , which is an example of the image forming apparatus according to the present embodiment. This printer  100  has a belt-like photoreceptor  1  as a latent image supporting body. An unshown charging device, a writing optical unit  2  as an exposure device, a multi-stage developing unit  3  as a development device, an intermediate transfer belt  11  as a transfer device, an unshown cleaning device and the like are disposed around the photoreceptor  1 . 
     The multi-stage developing unit  3  has, starting from the bottom, development devices  3 K,  3 C,  3 M and  3 Y storing black, cyan, magenta and yellow developers respectively, the development devices being disposed detachably in a multi-stage manner. Here, references numerals K, C, M and Y represent members for black, cyan, magenta and yellow colors respectively. Also, a primary transfer roller  12 , which is primary transfer means, is provided on the inside of the intermediate transfer belt  11  so as to face the photoreceptor  1  (right side in the figure). 
     Furthermore, three rows of paper cassettes  45   a ,  45   b  and  45   c  each storing sheets are provided in a lower section of the printer  100 . Separate feeding means  46   a ,  46   b  and  46   c  are provided so as to correspond to the paper cassettes  45   a ,  45   b  and  45   c  respectively. Moreover, a manual feed tray  63  that can be pulled out and pushed in is provided on the right-side surface of the printer  100 .  FIG. 5  shows a state in which the manual feed tray  63  is pulled out. 
     Separate feeding means  46   d  is provided in order to feed a sheet from the manual feed tray  63 . A pair of grip rollers  47  are provided in order to convey sheets fed by these separate feeding means  46   a ,  46   b ,  46   c  and  46   d.    
     A pair of resist rollers  48  are provided above the pair of grip rollers  47  (downstream side of the sheet conveyance direction). A secondary transfer roller  49 , which is secondary transfer means, is provided above the pair of resist rollers  48  so as to face a transfer counter roller  13 , which is one of the rollers around which the intermediate transfer belt  11  is wound. 
     It should be noted that the separate feeding means  46   a ,  46   b  and  46   c , the pair of grip rollers  47 , the pair of resist rollers  48  and the like are configured to be driven independently by a clutch, stepping motor, or the like. 
     A fixing device  25  is provided above the secondary transfer roller  49 , and a conveying path switching device  51  that selectively separates the conveyance direction is provided above the fixing device  25 . Reference numerals  64  through  70  each represents a sheet sensor disposed appropriately within each sheet conveying path. It should be noted that a sheet is appropriately guided by a guide member (no reference numeral is applied), such as a guide plate provided in each sheet conveying path. The upper surface of the printer  100  main body is configured as a sheet discharging tray  62 , and a pair of discharging rollers  61  for discharging a sheet to the sheet discharging tray  62  are provided in an upper left section of the fixing device  25 . 
     Moreover, a reversing device  80  that reverses a sheet is provided on the right side of the printer  100  main body. The reversing device  80  has a reverse forward path  81  and a reverse backward path  82  as switchback paths. There are also provided a pair of switchback rollers  83  that sandwich a sheet sent into the reverse forward path  81 , an entry sensor  84  provided on an upstream side of the pair of switchback rollers  83 , and a reversing unit switching arm  85 . 
     Next, the operation of the printer is described. 
     When the printer receives data items of full-color images, the photoreceptor  1  rotates in a clockwise direction (right-handed direction) in  FIG. 5 , and the surface of the photoreceptor  1  is charged uniformly by the charging device. Next, the photoreceptor  1  is irradiated with a laser beam corresponding to the image of each color by the writing optical unit  2 , whereby latent images corresponding to the respective image data items are formed on the surface of the photoreceptor. As the latent images reach the position of the multi-stage developing unit  3  by means of the rotation of the photoreceptor  1 , the development devices  3 K,  3 C,  3 M and  3 Y corresponding to the respective formed colors are selectively caused to face the photoreceptor  1 . Then, toner is supplied from any of the facing development devices  3 K,  3 C,  3 M and  3 Y onto the photoreceptor  1 , whereby a toner image is formed. The toner image on the photoreceptor  1  is transferred once onto the intermediate transfer belt  11  by the primary intermediate transfer roller  12 . On the other hand, after the toner image is transferred, the surface of the photoreceptor  1  is cleaned by the unshown cleaning device. 
     This cycle is repeated for all colors so that toner images yellow, magenta, cyan and black are sequentially transferred onto the intermediate transfer belt, whereby a full-color toner image having a combination of the four colors is formed. 
     On the other hand, a feed command is issued in accordance with timing of toner image formation, sheets are selectively fed one by one from the paper cassettes  45   a ,  45   b  and  45   c  or the manual feed tray  63  by any of the separate feeding means  46   a ,  46   b ,  46   c  and  46   d , and each sheet reaches the pair of resist rollers  48  via the pair of grip rollers  47  and stops temporarily. The sheet is sent to a secondary transfer position by the pair of resist rollers  48  in accordance with the timing of the toner image supported on the intermediate transfer belt  11 . In the secondary transfer position, the toner image supported on the intermediate transfer belt  11  is transferred onto the sheet at once by the secondary transfer roller  49 . 
     When the sheet on which the toner image is transferred passes through the fixing device  25 , the toner image is fused onto the sheet by heat and pressure. On the other hand, after the toner image is transferred, the intermediate transfer belt  11  is cleaned by an intermediate transfer belt cleaning device, which is not shown, to prepare for formation of the next electrostatic latent image. 
     The conveying path switching device  51  switches the conveyance direction of the sheet on which the toner image is fixed by the fixing device  25 , so as to direct the sheet to the sheet discharging tray  62  or reversing device  80 .  FIG. 6  shows a schematic configuration of the enlarged conveying path switching device  51 . 
     The conveying path switching device  51  has the separating arm  22  as sheet conveyance direction separating means, and the separating arm  22  is switched to the right or left in the figure by an actuator (not shown) such as a solenoid. The conveying path  27  conveys the sheet to the sheet discharging tray  62  side, while the conveying path  28  conveys the sheet to the reversing device  80  side. 
     The sheet, which is obtained after the toner image is fixed thereon by the fixing device  25 , is introduced to the position of the separating arm  22  by the conveying path  26 , and conveyed toward either one of the conveying paths  27  and  28  by the separating arm  22 . The belt surface of an endless belt  53  serves as a part of an outline of each of the separating/conveying paths  27  and  28 . Incidentally, the endless belt  53  can be divided into two parts, and these parts can also serve as an endless belt for the separating/conveying path  27  and an endless belt for the separating/conveying path  28 . 
     The conveying path  26  and the separating/conveying paths  27  and  28  form a conveying path that is in roughly a shape of a letter “Y”. For example, when one-side printing is selected, the sheet is conveyed to the conveying path  27  in the direction of the sheet discharging tray  62  by switching the separating arm  22  using the conveying path switching device  51 , and the pair of discharging rollers  61  discharge the sheet to the sheet discharging tray  62  configured on the upper surface of the printer  100  main body. The heavy slid line shown in  FIG. 5  each indicates a sheet conveyance passage at the time of one-side printing (when feeding from the paper cassettes  45   a ,  45   b  and  45   c  or from the manual feed tray  63 ). 
     When both-side printing is selected, the sheet is conveyed to the conveying path  28  in the direction of the reversing device  80  side by switching the separating arm  22  using the conveying path switching device  51 . The sheet that is sent into the reverse forward path  81  of the reversing device  80  by the switching performed by the conveying path switching device  51  is sandwiched between the pair of switchback rollers  83 , and sent to the reverse backward path  82  by a reversing operation of the switchback rollers. In the reverse forward path  81 , when the entry sensor  84  provided on the upstream side of the pair of switchback rollers  83  detects the rear edge of the sheet, the directions of rotation of the switchback rollers are reversed in response to the detection, and the pair of switchback rollers  83  are driven in the reverse direction, whereby the rear edge and the front edge of the sheet are inverted, and thus obtained sheet is conveyed in a direction opposite to the entering direction. The sheet that is sandwiched between the pair of switchback rollers  83  is conveyed by the conveying operation thereof, and the reversing unit switching arm  85  is driven by the unshown solenoid. Accordingly, the sheet can enter the reverse backward path  82 . The sheet that passes through the reverse backward path  82  and is inverted is conveyed to the pair of resist rollers  48  by a reverse outlet  86  in order to perform back-side printing. 
     The dashed lines shown in  FIG. 5  represent sheet conveyance passages within the reversing device  80  at the time of both-side printing. The sheet is re-conveyed to the transfer position of the intermediate transfer belt  11  by the pair of resist rollers  48  in synchronization with the timing of toner image formation, and then the toner image is transferred. The sheet on which the toner image is transferred is conveyed to the fixing device  25 , wherein the toner image is fixed onto the sheet. The sheet, the second side of which is formed with the image, is conveyed to the discharging rollers  61  by the conveying path switching device  51  through the passage shown by the heavy solid line, and then discharged onto the sheet discharging tray  62  provided outside the machine. 
     Further detailed configuration and operation of the conveying path switching device  51  are described. 
     In the printer  100  of the present embodiment, there is provided the endless belt  53  extended around two rollers  29  and  30  such that after the sheet is introduced to either one of the two conveying paths  27  and  28  by the separating arm  22 , the leading edge of the sheet comes into contact with the surface of the driven belt  53  when the sheet is conveyed to the sheet discharging tray  62  or reversing device  80 , and the sheet is sent in the corresponding conveyance direction. 
     More specifically, the leading edge of the sheet is gripped by the surface of the endless belt  53 , and the conveying path switching device  51  guides the leading edge of the sheet to a sandwiching section located on a downstream side. This belt  53  is configured such that the direction of normal and reverse rotation thereof is freely switched, wherein the belt  53  is rotated in accordance with a predetermined conveyance direction in which the sheet is conveyed to the sheet discharging tray  62  or reversing device  80 , thus the resistance obtained when the leading edge of the sheet comes into contact with this belt can be reduced. Therefore, not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed without causing a paper jam or conveyance failure. 
     Furthermore, by providing such an extended belt, the space required for changing the direction of conveying sheets (curvature radius) can be reduced, whereby a compact printer can be obtained, inhibiting the increase of the cost. 
     In the illustrated example, there is configured rotating/conveying means for conveying a sheet in both directions of the sheet discharging tray  62  and reversing device  80  by means of one belt  53  whose direction of normal and reverse rotation can be freely switched, but a small endless belt can be disposed in both areas of the rollers  29  and  30 , of course, so as to take charge of conveying a sheet in the direction of the sheet discharging tray  62  and the direction of the reversing device  80 . 
     Also, a belt extending rotating member  29  can be configured as one of the pair of discharging rollers  61 . In this case, an opposing rotating member  54  can be downsized, and the pair of discharging rollers  61  can be shifted to the right side in the figure, whereby a large dimension for mounting the sheet discharging tray  62  can be ensured. 
     As the endless belt  53 , a known electrically conductive material, such as a silicone rubber with carbon black dispersed therein, is used. By using an electrically conductive material, the occurrence of frictional electrification at the abutment surface between the sheet and the belt can be prevented, and particularly an abnormal image that may be caused when performing printing on the second side can be prevented from occurring. 
     Furthermore, when mounting the endless belt  53  on the belt extending rotating members  29  and  30 , it may be wrapped around the belt extending rotating members  29  and  30 . In this case, it is preferred to obtain an extension percentage at which the leading edge of the sheet can be securely gripped by the belt surface, an extension percentage at which the linear velocity of the belt is same as that of the opposing rotating members  54  and  55  (when the belt is driven), or an extension percentage at which the belt does not interfere with the base section of the separating arm. 
     Moreover, by applying a driving force of a belt driving section (not shown) that can be rotary driven in both normal and reverse directions to at least one of the belt extending rotating members  29  and  30 , the direction of rotation of the belt  53  can be switched between the normal direction and reverse direction. 
     Specifically, when conveying the sheet to the conveying path  27  on the sheet discharging tray  62  side, the direction of rotation of the belt  53  is a right-handed direction (CW) in the figure, and when the sheet is conveyed to the conveying path  28  on the reversing device  80  side, the direction of rotation of the belt  53  is a left-handed direction (CCW) in the figure. Alternatively, by applying the driving force of the driving section to the rotating members  54  and  55  facing the belt extending rotating members  29  and  30  via the belt  53 , the belt  53  may be driven and rotated. 
     Torque limiters  56  and  57  may be provided on such rotating member driving section or the opposing rotating members  54  and  55 . That way, the belt  53  can be securely switched so that the sheet is conveyed to different conveying paths, and the belt  53  can be prevented from being damaged even when a trouble such as a paper jam occurs. Furthermore, the conveying paths can be easily opened so as to release the sandwiching state between the belt  53  and the opposing rotating members  54  and  55 . 
     It should be noted that, by disposing the rotating members  54  and  55  in positions facing the rotation axis of the belt  53 , the sheet is sandwiched and conveyed regardless of which one of the belt extending rollers  29  and  30  and opposing rotating members  54  and  55  is applied with the driving force, thus the friction resistance between the belt  53  and the sheet increases, improving the sheet conveying performance. 
     The separating arm  22  is constituted by a movable end switching arm piece  22   a  and a fixed guiding section  22   b . By providing the movable section and a fixed section, even when the bottom section of the fixed guiding section  22   b  is brought extremely close to the belt  53 , the base section of the separating arm does not come into contact with the belt surface when the sheet conveyance passage is switched, thus the sheet can be guided securely. By bending a side of the fixed guiding section  22   b  to form a curvature section of each separating/conveying path, the sheet can be conveyed smoothly. 
     As shown in  FIG. 7 , the belt  51  may be divided into a plurality of parts in a direction perpendicular to the sheet conveyance direction. By dividing the belt, fixed guiding sections  22   b ′,  22   b ″ can be disposed between partial belts  51   a ,  51   b  and  51   c  (the figure shows only three belt pieces, but the number of the partial belts can be two, four or more). By disposing these sections, the bottom section of each fixed guiding section (the flat section of each fixed guiding section shown in  FIG. 7 ) can be positioned above a lower side surface of each of the partial belts  51   a ,  51   b  and  51   c , and the separating arm  22  can be inserted into the area of the endless belt. Such configuration allows the sheet to be guided smoothly from the side of the fixed guiding section to the belt surface. 
     The operation of switching the separating arm  22  (movable end switching arm piece  22   a ) is performed prior to the operation of switching the direction of rotation of the belt  53 . By doing so, the sheet can be securely guided to different conveying paths without increasing a sheet conveying interval. 
       FIG. 8  is a flowchart of an example of the operations of the separating arm  22  and the endless belt  53 . 
     In a controller (not shown) within the printer  100  main body, when a both-side printing command is inputted (step S 1 ), the detection state of the photosensor (not shown but corresponding to the reference numeral  21  shown in  FIG. 2 ) of the separating arm is checked (step S 2 ). When the detection state is OFF, the end switching arm opens the separating/conveying path  28  through which the sheet is conveyed to the reversing device  80 , and thus rotates the endless belt  53  in the left-handed direction in  FIG. 2  (step S 3 ) while keeping the position of the end switching arm (e.g., stepping motor excitation), and thereafter printing operation is started (step S 4 ). 
     If, on the other hand, the detection state of the photosensor is ON in the step S 2 , the end switching arm opens the separating/conveying path  27  through which the sheet is conveyed to the sheet discharging tray  62 , and closes the separating/conveying path  28  through which the sheet is conveyed to the reversing device  80 , thus the stepping motor (not shown but corresponding to the reference numeral  20  in  FIG. 2 ) is rotated normally until the detection state becomes OFF (steps S 5  and S 6 ). Then, a timer or the like is used to weigh the time (step S 7 ). When the detection state is not switched to OFF within predetermined time period (step S 8 ), there is a possibility that a part is broken, thus the operation of the image forming apparatus is stopped, and such trouble is displayed by an operating section or the like, which is not shown (step S 9 ). 
     As described above, according to the present embodiment, by configuring the rotatable belt  53  with which the leading edge of the sheet comes into contact at a section between the separating arm  22  and the two conveying paths  27  and  28 , a load that is caused when the leading edge of the sheet and the belt abut on each other can b reduced significantly. Therefore, not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed without causing a paper jam or conveyance failure. 
     In a modification of the present embodiment, as shown in  FIG. 9 , pairs of sandwiching rollers  72  and  74  are disposed in the vicinity of both ends of the base section of the fixed guiding section of the separating arm  22  configured by the movable end switching arm piece  22   a  and the fixed guiding section  22   b . Moreover, base vertex angle sections  22   c  and  22   d  of the fixed guiding section  22   b  are formed into a shape such that the leading edge of the sheet is guided to the sandwiching sections of the pairs of sandwiching rollers  72  and  74 . By providing such a configuration, the sheet can be guided smoothly. 
     Moreover, by configuring the separating arm  22  with the fixed guiding section  22   b  and the movable end switching arm piece  22   a , the base vertex angle sections  22   c  and  22   d  can be brought close to the sandwiching rollers  72  and  73  on the outline side of the fixed guiding section  22   b  as much as possible without considering the rotation area of the separating arm (specifically, in the case of a conventional separating arm that has a guide surface extending to the vicinity of a pair of sandwiching rollers  72  and  74  in order to easily guide a sheet to the pair of sandwiching rollers  72  and  74 , an end on a downstream side of the arm interferes with the inner guide when the arm rotates, thus there is a risk that the sheet conveying paths may be closed), whereby the sheet can be guided smoothly. In other words, in the case of the conventional configuration, there is a difference in level between the base section of the switching arm and the outline guide that generates a load, but such a level difference can be eliminated in the above-mentioned configuration, as a result of which the load of conveyance can be reduced. 
     It should be noted in this case that a plurality of the fixed guiding sections  22   b  and movable end switching arm pieces  22   a  may be disposed in the direction perpendicular to the sheet conveyance direction, as with the embodiment shown in  FIG. 2 . With this configuration, the pair of sandwiching rollers  72  and  74  and the pair of base vertex angle sections  22   c  and  22   d  can be disposed alternately in a width direction. With this configuration, a difference in level between the pair of sandwiching rollers  72  and  74  and the pair of base vertex angle sections  22   c  and  22   d  in the conveyance direction can be eliminated, preventing the leading edge of the sheet from colliding with the sides of the pair of sandwiching rollers  72  and  74 . As a result, the sheet can be guided smoothly to the sandwiching section between the pair of sandwiching rollers  72  and  74 . 
     In the present embodiment, the conveying path switching device  51  is disposed in the vicinity of the base section of the separating arm, thus the conventional separating arm interferes with the belt  53  when the separating arm rotates. Therefore, by disposing the conventional separating arm between the belts in the same manner in place of the fixed guiding sections  22   b ′,  22   b ″ described with reference to  FIG. 7 , interference between the separating arm and the belts can be prevented. 
     It should be noted that the present embodiment describes the sheet switching position (the sheet discharging tray and the both-side reversing path) obtained after fixing the toner image, but the present invention is not limited to this embodiment. For example, the image forming apparatus can be applied to a draft conveying path switching position of an automatic draft feeder (ADF) for conveying and reading a draft, a switching position when discharging a sheet having an image formed thereon to a plurality of loading sections, and a separating position at which the conveying paths are divided into two or more. Moreover, the image forming apparatus may have a configuration in which a loading space is formed between an image reading section and an image forming section, and the image forming section can be applied not only to an apparatus of electrophotographic type or various other types. 
     According to the present embodiment, the following effects can be obtained. 
     (1) In the sheet conveying path switching device having a sheet carrying-in path and a switching separating arm, the outlines of the two separating/conveying paths out of a plurality of separating/conveying paths positioned on the downstream side of the switching separating arm are configured by a movable belt surface, the two separating/conveying paths being located closer to the separating arm than the other separating/conveying paths. Therefore, not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed, and even bent sheets can be conveyed securely. 
     (2) Since the outlines of the two separating/conveying paths are configured with the belt surface of a single endless belt, a simple configuration can be obtained. 
     (3) The rotating member that faces the belt extending rotating member is disposed on an inner rim of each separating/conveying path, and the sheet is sandwiched between the opposing rotating member and the belt surface and then conveyed, whereby the applied pressure onto the belt can be adjusted without relaying on the tension of the belt, or particularly the friction resistance force can be adjusted, whereby the sheet can be conveyed securely. 
     (4) A drive may be applied directly to the belt extending rotating member to stably drive the belt. If the belt that is capable of rotating normally and reversely is drive by driving the opposing rotating member, it is not necessary to dispose a driving member on a belt rotation axis located on an outer rim of the sheet conveying path, which is an advantage in terms of the layout, whereby the number of parts can be reduced. 
     (5) Since one of the belt extending rotating members serves as one of the pair of discharging rollers, the sheet conveyance characteristics of the sheet discharge/conveyance can be securely improved simply by increasing the length of the belt. 
     (6) The opposing rotating members are provided with the torque limiters. Therefore, in the case in which weight that exceeds the weight tolerated by the conveying members is applied to each conveying member provided in the conveying path switching device due to a paper jam or the like, the drive of the opposing rotating member is stopped. Accordingly, the weight applied to the belt can be kept within the range of the tolerated weight to prevent the belt from being damaged, whereby a sheet conveying device that is hardly broken can be provided 
     (7) Since the belt is electrically conductive, frictional electrification can be prevented from occurring at the abutment surface, whereby an abnormal image that may be caused when performing printing on the second side can be prevented from occurring. 
     (8) Since the switching separating arm is constituted by the fixed guiding section and the movable end switching arm piece, the function of switching the conveying paths and the function of reversing the conveyance direction can be provided for each section. Even if the curvature of each side of the fixed guiding section is increased, and the sheet conveying paths are switched compactly, the leading edge of the sheet slides on each side of the fixed guiding section, preventing the occurrence of a conveyance failure. 
     (9) The base section of the fixed guiding section is brought as close as possible to the belt surface configuring the outlines of the separating/conveying paths, thus the sheet to be conveyed from each side of the fixed guiding section to the belt surface can be conveyed smoothly. 
     (10) The endless belt is constituted by a plurality of belt sections that are obtained in the direction perpendicular to the sheet conveyance direction, thus the cost of these members can be cut. 
     (11) The base section of the fixed guiding section can be disposed in the area of the endless belt, closer than the belt surface configuring the outlines of the separating/conveying paths, whereby the sheet to be conveyed from each side of the fixed guiding section to the belt surface can be conveyed more smoothly. 
     (12) The operation of switching the separating arm is performed prior to the operation of switching the direction of rotation of the belt, thus, in a state in which sheets are continuously enter the carrying-in path, the sheets can be securely guided from the carrying-in path to either one of the separating/conveying paths even in a case in which the sheet intervals are small. Thus a sheet conveying device having an excellent productivity can be provided. 
     (13) In the sheet conveying path switching device that has the sheet carrying-in path, switching separating arm, and a plurality of separating/conveying paths located on the downstream side of the switching separating arm, the switching separating arm is constituted by the fixed guiding section and the movable end switching arm piece, thus not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed, and even bent sheets can be conveyed securely. 
     (14) By disposing the pair of sandwiching rollers in the vicinity of both ends of the base of the fixed guiding section, and by causing the base vertex angle sections to face a roller of the pair of sandwiching rollers that configures the outline of the separating/conveying paths, the risk of occurrence of a paper jam can be further prevented. 
     Second Embodiment 
     Next, the present embodiment is described with reference to the drawings. 
       FIG. 10  shows a schematic configuration of a full-color printer  100 A, which is an example of the image forming apparatus according to the present embodiment. As shown, the printer  100 A is substantially the same as the printer  100  according to the above-described first embodiment shown in  FIG. 5 , except that the conveying path switching device  51  is configured differently. Therefore, repeated descriptions are omitted hereinafter, and mainly the configuration and operation of a conveying path switching device  91  of the present embodiment are described with reference to the drawings. 
     The conveying path switching device  91  of the present embodiment switches the conveyance direction of a recording sheet obtained after a toner image is fixed thereon by the fixing device  25 , to guide the recording sheet to the sheet discharging tray  62  or the reversing device  80 .  FIG. 11  shows a schematic configuration of the conveying path switching device  91 . 
     The conveying path switching device  91  has the separating arm  92  as recording medium conveyance direction separating means, and the separating arm  92  is switched to the right or left in the figure by an actuator (not shown) such as a solenoid. The conveying path  27  conveys the recording sheet to the sheet discharging tray  62  side, while the conveying path  28  conveys the recording sheet to the reversing device  80  side. 
     The recording sheet, which is obtained after the toner image is fixed thereon by the fixing device  25 , is introduced to the position of the separating arm  92  by the introducing path  26 , and conveyed toward either one of the conveying paths  27  and  28  in a shape of a letter “Y” by the separating arm  92 . For example, when one-side printing is selected, the recording body is conveyed to the conveying path  27  in the direction of the sheet discharging tray  62  by switching the separating arm  92  using the conveying path switching device  91 , and the pair of discharging rollers  61  discharge the recording body to the sheet discharging tray  62  configured on the upper surface of the printer  100 A main body. The heavy slid line shown in  FIG. 10  each indicates a sheet conveyance passage at the time of one-side printing (when feeding from the paper cassettes  45   a ,  45   b  and  45   c  or from the manual feed tray  63 ). 
     When both-side printing is selected, the recording sheet is conveyed to the conveying path  28  in the direction of the reversing device  80  side by switching the separating arm  92  using the conveying path switching device  91 . The recording sheet that is sent into the reverse forward path  81  of the reversing device  80  by the switching performed by the conveying path switching device  91  is sandwiched between the pair of switchback rollers  83 , and sent to the reverse backward path  82  by a reversing operation of the switchback rollers. In the reverse forward path  81 , when the entry sensor  84  provided on the upstream side of the pair of switchback rollers  83  detects the rear edge of the recording sheet, the directions of rotation of the switchback rollers are reversed in response to the detection, and the pair of switchback rollers  83  are driven in the reverse direction, whereby the rear edge and the front edge of the recording sheet are inverted, and thus obtained recording sheet is conveyed in a direction opposite to the entering direction. The recording sheet that is sandwiched between the pair of switchback rollers  83  is conveyed by the conveying operation thereof, and the reversing unit switching arm  85  is driven by the unshown solenoid. Accordingly, the recording sheet can enter the reverse backward path  82 . The recording sheet that passes through the reverse backward path  82  and is inverted is conveyed to a pair of resist rollers  49  by a reverse outlet  86  in order to perform back-side printing. 
     The dashed lines shown in  FIG. 10  represent recording body conveyance passages within the reversing device  80  at the time of both-side printing. The sheet is re-conveyed to the transfer position of the intermediate transfer belt  11  by the pair of resist rollers  49  in synchronization with the timing of toner image formation, and then the toner image is transferred. The sheet on which the toner image is transferred is conveyed to the fixing device  25 , wherein the toner image is fixed onto the sheet. The sheet, the second side of which is formed with the image, is conveyed to the discharging rollers  61  by the conveying path switching device  91  through the passage shown by the solid line, and then discharged onto the sheet discharging tray  62  provided outside the machine. 
     Moreover, in the printer  100 A of the present embodiment, a wall surface, with which the leading edge of the recording sheet comes into contact at a section between the separating arm  92  and the two conveying paths  27  and  28 , is extended to form a triangular shape by three rollers  96 ,  97  and  98  to form a rotatable belt  93 . This belt  93  is configured rotatably, thus even when the leading edge of the recording sheet comes into contact with this belt, the surface of the belt  93  can be moved in the recording body conveyance direction by the rotation of the belt  93 . Accordingly, the conveyance load that is caused when the leading edge of the recording sheet comes into contact with the wall surface of one of the two conveying paths  27  and  28  can be reduced. Therefore, not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed without causing a paper jam or conveyance failure. By simply providing the belt that is extended to form a triangle, an easy-to-use sheet conveying path switching device can be provided in a small space, inhibiting the increase of the cost. 
     As the material of the belt  93 , an electrically conductive member, such as a silicone rubber with carbon black dispersed therein, is preferably used. By using an electrically conductive member, the occurrence of frictional electrification at the abutment surface between the recording sheet and the belt can be prevented. Particularly an abnormal image that may be caused by the frictional electrification when performing printing on the second side can be prevented from being generated. 
     Also, the belt  93  is provided with a belt driving section M that can be rotary drive in both normal and reverse directions, and this belt driving section M is rotary driven when the recording sheet is conveyed in the different directions of the two conveying paths  27  and  28 . Specifically, when conveying the recording sheet to the conveying path  27  on the sheet discharging tray  62  side, the direction of rotation of the belt  93  is a right-handed direction (CW) in the figure, and when the recording sheet is conveyed to the conveying path  28  on the reversing device  80  side, the direction of rotation of the belt  93  is a left-handed direction (CCW) in the figure. By rotary driving the belt  93  in this manner, the recording sheet conveying performance can be improved, whereby the recording sheet can be conveyed more stably. Moreover, the operation of switching the separating arm  92  is performed in synchronization with the operation of switching the direction of rotation of the belt  93 , whereby the recording sheet can be securely guided to different conveying paths. In addition, by providing a torque limiter in the belt driving section M, the direction of rotation of the belt  93  can be securely switched so that the recording sheet is conveyed to different conveying paths. 
     A pair of turn rollers  94  that are driven in order to sandwich and convey the recording sheet may be disposed in a position facing the belt  93 . By providing such a turn roller  94 , the friction resistance between the belt  93  and the recording sheet increases, and the recording sheet conveying performance is improved. Furthermore, the turn roller  94  is made of a material that slides easily, such as a polyacetal, whereby even if there is generated the difference between a frictional coefficient between the turn roller  94  and the surface of the recording sheet and a frictional coefficient between the belt  93  and the back of the recording sheet, or even if there is generated the speed difference between turn roller  94  and the belt  93  due to a tolerance caused by assembling the components, the risk of the occurrence of rubbing and the like between the turn roller and the recording sheet can be lowered. 
       FIG. 12  shows a schematic configuration of the conveying path switching device  91 . The operation of the conveying path switching device  91  is described in further detail with reference to  FIG. 12 . It should be noted in  FIG. 12  that the turn roller  94  is not provided. 
     The belt  93  is extended to form a triangle by the rollers  96 ,  97  and  98 , and this belt is divided into a plurality of sections in a direction perpendicular to the sheet conveyance direction. The roller  97  is connected to the belt driving section M capable of being rotary driven in both normal and reverse directions, and rotates in a direction corresponding to a change in the rotation direction of the belt driving section M to rotary drive the belt  93  in a horizontal direction. The rollers  96  and  98  are pivotally supported so as to follow the rotation of the belt  93  caused by the rotation of the roller  97 . In  FIG. 12 , the roller  97  is driven, but the roller  98  may be driven or both rollers  97  and  98  may be driven. Accordingly, the belt surface in the sheet entering direction is stretched by the driven rollers, whereby the leading edge of the recording sheet securely comes into contact with the belt surface, improving the conveying performance. Therefore, the recording sheet can be conveyed stably to either one of the two conveying paths  27  and  28 . 
     Also, as with the belt  93 , the separating arm  92  is divided into a plurality of parts in the direction perpendicular to the sheet conveyance direction. In order to prevent skew from occurring when the leading edge of the recording sheet reaches the plurality of separating arms  92 , the separating arms  92  are preferably disposed in the vicinity of the center of the recording sheet conveyance direction or in positions symmetrical with respect to the center of the recording sheet conveyance direction. Therefore, the separating arms  92  are disposed, respectively, in rotation axes  100  of the rollers  96  extending the belt  93 , so as to be alternate with the rollers  96 , whereby the separating arms are rotatably supported in the horizontal direction. 
     Here, the direction of rotation of each roller  96  is opposite to the direction of rotation of each separating arm  92  with respect to the rotation axis  100 . Specifically, as shown in  FIG. 11 , when the recording sheet is conveyed to the conveying path  27  on the sheet discharging tray  62  side, the roller  96  is rotated in the right-handed direction (direction of a in the figure) with respect to the rotation axis  100  in order to rotate the belt  93  in the right-handed direction (CW) by means of the belt driving section M, and the separating arm  92  is rotated in the left-handed direction (direction of b in the figure) with respect to the rotation axis  100  in order to shift the leading edge of the recording sheet to the right. 
     Hereinafter, a mechanism for rotating the roller  96  and the separating arm  92  in opposite directions is described. 
       FIG. 13  shows a schematic configuration of the separating arm  92  supported by the rotation axis  100 . As shown in  FIG. 13 , the separating arm  92  is supported so as to slide easily with respect to the rotation axis  100 , so that the rotation of the roller  96  is not transmitted to the separating arm  92  via the rotation axis  100  or the load applied to the rotation axis  100  is not increased by the operation of rotating the separating arm  92 . An upper section of the separating arm  92  is provided with a convex section  101  in which the near side and far side in the figure form flat surfaces  101   a  and  101   b  respectively. The near-side flat surface  101   a  of the convex section  101  is provided with an elastic member  102  that is biased from the near side toward the far side, and the far-side flat surface  101   b  is provided with a solenoid  103 . In such a configuration, the separating arm  92  can be rotated in the right-handed and left-handed directions with respect to the rotation axis  100  by an on/off operation of the solenoid  103  and a change in the biasing force of the elastic member  102  in response to the on/off operation. The on/off operation of the solenoid  103  is performed at the same time that the operation of switching the direction of rotation of the belt  93  is performed. Specifically, in response to a pulse obtained when the direction of rotation of the belt driving section M is changed, whether to turn on or off the solenoid  103  is determined. With this configuration, the separating arm  92  can be shifted in the direction opposite to the direction of rotation of the belt  93 , whereby the leading edge of the recording sheet can be stably guided to either the conveying path  27  or  28 . 
       FIGS. 14A and 14B  each shows a schematic configuration of another example of a mechanism that rotates the roller  98  and the separating arm  92  in opposite directions. As shown in  FIGS. 14A and 14B , a plurality of rotating members  104  are disposed between the separating arm  92  and the outer circumference of the rotation axis  100 . The upper part of the separating arm  92  is provided with a convex section  105 , and the horizontal direction of the convex section  105  is provided with regulating sections  106  that abut on the convex section  105  when the separating arm  92  is shifted in the right-handed and left-handed directions, and regulates the shifting. In such a configuration, when the rotation axis  100  rotates in the right-handed direction (direction of the arrow a) as shown in  FIG. 14A , the separating arm  92  is rotated by the rotating member  104  in the left-handed direction (direction of the arrow b), which is opposite to the direction of rotation of the rotation axis  100 . Then, the separating arm  92  rotates in the direction of the arrow b, the shifting of the separating arm  92  is regulated when the convex section  105  abuts on the regulating section  106  as shown in  FIG. 14B , and the separating arm  92  stops at this position. In this manner, the separating arm  92  rotates in the direction of the arrow b so that the leading end thereof is shifted, whereby the conveying path  27  side is opened, the belt  93  is rotated in the direction of the arrow a, and the recording sheet is conveyed in a direction of the arrow c. It should be noted that the position where the separating arm  92  is regulated is preferably located in a position where the leading edge of the recording sheet can enter at an acute angle with respect to the belt  93 . 
     Also, as shown in  FIG. 11 , the distance between the rotation axis  100  and the outer circumference of the separating arm  92  is made larger than the external diameter of the roller  96 , and the outer circumference of the separating arm  92  extends to the outside of the outer circumference of the roller  96  in the vicinity of the rotation axis. Accordingly, the outer circumference of the separating arm  92  prevents the leading edge of the recording sheet from entering an area of contact between the belt  93  and the roller  96 , whereby the leading edge of the recording sheet enters the belt surface other than the area of contact between the belt and the roller  96 . The belt surface other than the area of contact between the belt and the roller  96  can securely grip the leading edge of the recording sheet by means of elastic deformation of the belt  93 , thus the recording sheet can be conveyed to the conveying path  27  or  28  more stably. 
     It should be noted that  FIGS. 13 ,  14 A and  14 B describe that the position where the separating arm  92  rotates is regulated by the convex section provided in the upper section of the separating arm  92 , but the present invention is not limited to this embodiment. For example, when the separating arm  92  is rotated in the right-handed or left-handed direction without providing the convex section, the area below the rotation axis in the separating arm  92  may be caused to abut on a conveying guide to regulate the abovementioned position. 
     Furthermore, the relationship between the separating arm  92  and the rotation axis  100  is not limited to the above configuration, thus a mechanism that can rotate the belt  93  and the separating arm  92  in directions opposite to each other can be applied to various embodiments. 
       FIG. 15  shows a schematic configuration of a modification of the conveying path switching device  91  according to the present embodiment. It should be noted that the size of the turn roller  94  is larger in a conveying path switching device  91 A, and a turn roller  94   a  is in contact with the outer circumferential surface of the opposing belt  93 . Hereinafter, only the differences between the conveying path switching device  91 A and the conveying path switching device  91  shown in  FIG. 12  are described. 
     The driving force of the turn roller  94   a  positioned in the near side in the figure (on the conveying path  27  side) is transmitted from the belt driving section M to a gear  111  via a rotation axis  110 . The rotation axis  110  is rotated in the right-handed or left-handed direction (direction of the arrow b in the figure) by the belt driving section M by means of the gear  111 . Also, a turn roller  94   b  disposed on the far side in the figure (on the conveying path  28  side) is rotated in the same direction as the turn roller  94   a  via the rotation axis  110  of the turn roller  94   a  and a belt  115  extended around pulleys  112 ,  113  and  114 . 
     The pulley  114  extending the belt  115  is supported by the rotation axis  100  supporting the separating arm  92  and roller  96 . The separating arm  92  is rotated by the rotation of the belt  115  in the same direction as the rotation axis  110  via the pulley  114 . Therefore, the belt  113  and the separating arm  92  can be rotated in the directions opposite to each other, whereby the recording sheet can be stably conveyed toward the conveying path  27  or  28 . Furthermore, the separating arm  92  is provided with a torque limiter  117 . When the position of the separating arm  92  is regulated the torque limiter  117  operates, and the load applied to the belt driving section M can be reduced. It should be noted in  FIG. 15  that the position of the separating arm  92  is regulated by causing the area below the rotation axis of the separating arm  92  to abut on the conveying guide when the separating arm  92  is rotated in the right-handed or left-handed direction without providing the convex section in the upper section of the separating arm. 
     In such a configuration, the recording sheet that is introduced to the position of the separating arm  92  by the introducing path  26  is guided by a separating arm  112  to an abutment section between the turn roller  94  and the belt  93 , and is conveyed toward the conveying path  27  or  28  by the forces of friction between the surface of the recording sheet and the turn roller  94  and belt  93 . 
     Further,  FIG. 15  describes that the roller  97  and the turn roller  94   a  are driven using the same belt driving section M, but the present invention is not limited to this embodiment. For example, the roller  97  and the turn roller  94   a  may be provided with different driving sections. Not only the roller  97  but also the roller  98  may be provided with a different driving section. Moreover, the turn roller  94   b  located in the far side may be provided with a driving section that is different from the one provided in the turn roller  94   b  in the near side. 
       FIG. 16  shows a schematic configuration of another modification of the conveying path switching device  91 . A conveying path switching device  91 B shown in  FIG. 16  has turn rollers  94 A and  94 B that are larger than the one provided in the conveying path switching device  91 A shown in  FIG. 15 , has a deformed belt  93  abutting on the outer circumferential surface of the opposing belt  93 . Hereinafter, only the differences between the conveying path switching device  91 B and the conveying path switching device  91 A shown in  FIG. 15  are described. 
     In  FIG. 16 , the turn roller  94 A disposed on the conveying path  27  side is configured to have a size so as to sandwich the belt  93  between the turn roller  94 A and the roller  97  and between the turn roller  94 A and the roller  96 . The turn roller  94 B disposed on the conveying path  28  side is similarly configured to have a size so as to sandwich the belt  93  between the turn roller  94 B and the roller  98  and between the turn roller  94 B and the roller  96 . Therefore, as shown in  FIG. 16 , the belt  93  comes into contact with an outer circumference of the turn roller  94 A between the rollers  96  and  97 , and with an outer circumference of the turn roller  94 B between the rollers  96  and  98 , and two sides of the belt are deformed inward along the outer circumferences of the turn rollers  94 A and  94 B. In such a state, the dimension of the area of contact between the belt  93  and the recording sheet that has passed through the separating arm  92  increases, thus the recording sheet can be conveyed more stably toward the conveying path  27  or  28 . 
     As described above, in the present embodiment, when the recording medium is conveyed to either one of the two conveying paths that are introduced from the introducing path, the leading edge of the recording medium is guided by the surface of the rotatable belt. Accordingly, when the leading edge of the recording medium comes into contact with the belt surface configuring the wall surfaces of the two conveying paths, the belt surface can be moved in the recording medium conveyance direction by the rotation of the belt. As a result, the conveyance load that is applied when the leading edge of the recording medium comes into contact with the wall surfaces of the two conveying paths can be reduced. Therefore, not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed without causing a paper jam or conveyance failure. Moreover, by simply providing the belt that is extended to form a triangle, an easy-to-use sheet conveying path switching device can be provided in a small space, inhibiting the increase of the cost. 
     As described above, according to the present embodiment, the following effects can be obtained. 
     (1) The wall surfaces to which the leading edge of the recording sheet comes into contact at a section between the separating arm  92  and the two conveying paths  27  and  28  are configured to form the rotatable belt  93 , whereby the friction resistance between the leading edge of the recording sheet and the belt can be reduced. Therefore, not only sheets of a fixed size that are normally used, but also highly rigid sheets and special sheets can be stably conveyed without causing a paper jam or conveyance failure. 
     (2) The belt  93  can be rotary driven in both normal and reverse directions by the belt driving section M, whereby the recording sheet conveying performance improves, and the recording sheet can be conveyed more stably. 
     (3) Also, the operation of switching the separating arm  92  is performed at the same time that the operation of switching the direction of rotation of the belt  93  is performed, whereby the recording sheet can be securely guided to different conveying paths. 
     (4) The turn roller  94  that is driven so as to sandwich and convey the recording sheet is disposed in the position facing the belt  93 , whereby the friction resistance between the belt  93  and the recording sheet can be increased, the recording sheet conveying performance improves, and the recording sheet can be conveyed more stably. 
     (5) Furthermore, by providing the torque limiter in the belt driving section M, the direction of rotation of the belt  93  can be securely switched so that the recording sheet is conveyed to different conveying paths. 
     (6) Moreover, the turn roller  94  is composed of a sliding member, whereby the risk of the occurrence of rubbing and the like between the turn roller and the recording sheet can be lowered. 
     (7) In addition, by using an electrically conductive member as the material of the belt  93 , frictional electrification can be prevented from occurring at the abutment surface between the belt and the recording sheet. Particularly an abnormal image that may be caused by the frictional electrification when performing printing on the second side can be prevented from being generated. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof. 
     The present patent application claims priority under 35 U.S.C. sctn. 119 upon Japanese patent applications No. 2006-148818, filed on May 29, 2006, No. 2006-270578, filed on Oct. 2, 2006, and No. 2007-032869, filed on Feb. 14, 2007, the content of each of which is hereby incorporated by reference herein in its entirety.