Patent Publication Number: US-8528902-B2

Title: Sheet conveying apparatus and image forming apparatus with differential roller diameters

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying apparatus having a structure to convey a sheet from a first conveying path to a second conveying path which are mutually merged, and more specifically, relates to the sheet conveying apparatus enabling to reduce noise at the time of sheet conveyance from the first conveying path to the second conveying path and an image forming apparatus including the sheet conveying apparatus. 
     2. Description of the Related Art 
     Recently, with an image forming apparatus such as a copying machine, noise caused by a sheet itself occurring during sheet conveyance becomes striking according to gradual reduction of operation noise. For example, a top end of a sheet S to be conveyed from a conveying path  30  causes top end hitting noise when being abutted to a vertical conveying path  45  at a merging section (i.e., a curved portion) where the conveying path  30  and the conveying path  45  are at a right angle, as illustrated in  FIG. 12A . Further, as illustrated in  FIG. 12B , a rear end of the sheet S causes rear end bounce noise as being abutted to a guide face of the conveying path  45  when reaching the merging section after passing through the conveying path  30 . 
     In the related art, to solve such a noise problem, there has been known an image forming apparatus which reduces noise caused by abutment of a sheet conveyed through a sheet conveying path by arranging a flexible sheet such as mylar (registered trademark) at a merging section where two sheet conveying paths are merged (see Japanese Patent Laid-open No. 11-043238). 
     However, with the above structure in which sheets are abutted to the flexible sheet, since top ends and rear ends of sheets are abutted to the same part of the flexible sheet repeatedly for each sheet, there is a possibility of losing capability of sheet conveyance owing to scrape or breakage of the flexible sheet with use. 
     Further, there has been known an image forming apparatus having a structure to reduce noise occurring when a sheet conveyed from a first pair of rollers at the upstream side is abutted to a merging section on a midstream toward a second pair of rollers at the downstream side in a curved sheet conveying path (see U.S. Patent Application Publication No. 2009/0026691 A1). In the image forming apparatus, a guide plate having holes formed and having sound absorbing material fixed at a back face thereof is arranged at the merging section (i.e., a curved portion). 
     However, with the above structure in which holes are formed at the sheet conveying path and the sound absorbing material is arranged at the back face thereof, it is difficult to obtain a sufficient effect of noise reduction as a result of that a face to which a sheet is directly abutted is formed of a rigid member such as a metal plate. Here, a member having low rigidity for noise reduction can be utilized. However, since durability is decreased when a low rigidity member is utilized, it has been difficult to satisfy both of high noise reduction performance and durability. 
     Meanwhile, there has been proposed an apparatus including mutually-merged first and second sheet conveying paths and a belt conveying portion which is moved to guide sheets to the second sheet conveying path with a structure to enhance noise reduction performance by abutting top ends and rear ends of sheets to the belt conveying portion (see U.S. Patent Application Publication No. 2007/0057444 A1). 
     The above apparatus includes the first conveying path which conveys a sheet with a first conveying portion, the second conveying path which conveys a sheet with a second conveying portion as being merged to the first conveying path at the downstream side, and the belt conveying portion which cushions abutment of a sheet as being arranged at the second conveying path. 
     Incidentally, with the apparatus enhancing noise reduction performance by abutting top ends and rear ends of sheets to the belt conveying portion, the first conveying portion includes split-like rubber drive rollers formed of a roller body separated in plural in an axial direction as a drive roller. Here, since the split-like rubber drive rollers arranged at the belt conveying portion in an opposed manner are abutted to a rubber-made endless belt at the belt conveying portion, sheets are to be nipped by the rubber-made roller body and the rubber-made endless belt. Accordingly, frictional charging difference becomes large between a nipped part and an un-nipped part of a sheet owing to the roller body and the endless belt which are made of rubber. That is, charges are kept at the sheet as a result of frictional charging of the sheet at the nipped part between the roller body and the endless belt while few charges are kept at the un-nipped part. In an image forming apparatus of an electrophotographic system, a toner image is transferred to a sheet by applying transfer bias to a transfer portion. Therefore, when frictional charging difference occurs on the sheet, there is a possibility to cause imaging instability. Accordingly, it is not preferable to perform nipping and conveying with a conveying member causing contact of rubber to an image transfer face of a sheet. 
     To address the above issues, the present invention provides a sheet conveying apparatus having a structure not to cause imaging failure while enabling to reduce noise occurring at a curved portion of a conveying path of which conveying direction is varied and an image forming apparatus including the sheet conveying apparatus. 
     SUMMARY OF THE INVENTION 
     A sheet conveying apparatus includes a curve-shaped conveying path which conveys a sheet being conveyed in a first conveyance direction toward a second conveyance direction which is curved against the first conveyance direction, a drive conveying portion which is arranged at the curve-shaped conveying path and which conveys the sheet in the second conveyance direction with a nip of a pair of mutually-pressed rollers, and a belt conveying portion which is arranged at a curved portion of the curve-shaped conveying path and which conveys the sheet conveyed in the first conveyance direction toward the drive conveying portion with rotation of an endless belt member, wherein the belt conveying portion is configured to include a first belt support roller which is arranged coaxially with one of the pair of rollers and a second belt support roller which is arranged at an upstream side of the drive conveying portion from the first belt support roller, to wind the belt member around the first and second belt support members, and to set an outer diameter of the first belt support roller including the belt member to be smaller than an outer diameter of the one roller. 
     According to the present invention, the belt member is not contacted to the one roller when the sheet conveyed in the first conveyance direction is conveyed toward the second conveyance direction. Thus, sheets can be conveyed only by the nipping of the pair of rollers. Accordingly, it is possible to reliably prevent occurrence of inconvenience such as imaging instability due to frictional charging difference of the related art. In this manner, it is possible to avoid occurrence of imaging failure while reducing noise occurring at the vicinity of the curved portion of the curve-shaped conveying path. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view illustrating a printer being an example of an image forming apparatus including a sheet conveying apparatus according to the present invention; 
         FIG. 2  is a side view illustrating a structure of a main part of the first embodiment of the present invention; 
         FIG. 3  is a sectional view illustrating a tension state of an endless conveying belt in the first embodiment; 
         FIG. 4A  is a side view illustrating arrangement of the conveying belt in the first embodiment and  FIG. 4B  is a plane view illustrating a state of a nip of a conveying roller and a conveying wheel in the first embodiment as viewed from the direction of arrow A in  FIG. 2 ; 
         FIGS. 5A to 5C  are side views illustrating motion of a sheet in the first embodiment; 
         FIG. 6  is a perspective view illustrating a drive source in the first embodiment; 
         FIG. 7  is a block diagram illustrating a control system of the first embodiment; 
         FIG. 8  is a plane view illustrating a first modification in which the conveying wheel is modified in shape; 
         FIGS. 9A to 9D  are side views illustrating motion of a sheet in a second embodiment of the present invention; 
         FIG. 10  is a flowchart illustrating operation in the second embodiment; 
         FIG. 11  is a perspective view of a third embodiment having a structure in which driving force is directly applied to an endless conveying belt from a drive source; and 
         FIGS. 12A and 12B  are side views illustrating a structure in the related art. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     In the following, embodiments of the present invention will be exemplarily described in detail with reference to the drawings.  FIG. 1  is a sectional view illustrating an image forming apparatus  10  including a sheet conveying apparatus according to the present invention. 
     As illustrating in  FIG. 1 , the image forming apparatus  10  includes a reader portion  1  which reads an original and an image forming portion  2  which forms an image (i.e., a toner image) on a sheet conveyed from a sheet conveying apparatus with an electrophotographic system based on the read original. Further, the image forming apparatus  10  includes a sheet feeding portion  3  which feeds a sheet to the image forming portion  2  and a transfer charger  26  to which bias is applied for transferring a toner image formed at the image forming portion  2  to the sheet. Furthermore, the image forming apparatus  10  includes a fixing portion  4  which fixes the toner image transferred to the sheet, a pair of discharge rollers  5  which discharges the image-fixed sheet, and a reversing portion  6  which reverses faces of the image-fixed sheet. In the following, detailed structures are described in order from the reader portion  1 . 
     [Reader portion  1 ] An original (not illustrated) placed on an original base plate glass  11  is irradiated by a scanning optical system  12  having a light source and a reflection mirror set. The reflection light is imaged at a CCD  14  via a reducing lens  13 , and then, is photoelectrically converted and A/D-converted. 
     [Image forming portion  2 ] A laser light emitting portion  21  scans a photosensitive drum  23  with laser light owing to rotation of a polygon mirror  20  based on image information read by the reader portion  1  and forms a latent image on the photosensitive drum  23  which is previously charged by a charger  24 . A development device  25  develops the latent image and forms a toner image on the photosensitive drum  23 . The transfer charger  26  transfers the toner image formed on the photosensitive drum  23  to a sheet S. After the toner image is transferred, remaining toner on a drum face is removed by a cleaning portion  27 . 
     [Sheet feeding portion  3 ] A sheet cassette  31  in which sheets S are stacked and accommodated is detachably attached at a lower part of the image forming apparatus  10 . The sheets S fed by a pick-up roller  32  are separated and a sheet is conveyed by a conveying roller  33  and a retard roller  34 . Then, the sheet S is skew-feeding corrected by a registration portion  9  and is conveyed to the image forming portion  2 . 
     [Fixing portion  4 ] The sheet S to which the toner image transferred at the image forming portion  2  is conveyed to a fixing portion  4  by a conveying belt  8 . The fixing portion  4  is provided with a pair of rollers  42  which includes a heat roller having a halogen heater (not illustrated) at the inside thereof and a pressure roller which is pressed to the heat roller with predetermined pressing force by a spring (not illustrated). The sheet S on which non-fixed toner is placed is heated and pressurized when passing through a nip of the pair of rollers  42 . Accordingly, the toner image is melted and fixed. In a one-sided copy mode, the sheet S after the fixing process is discharged to the outside of an apparatus body by the pair of discharge rollers  5  and is stacked on a discharge tray  7 . 
     [Reversing portion  6 ] In a duplex copy mode, the sheet S is conveyed to the reversing portion  6  by a pair of switchback rollers  61  after passing through the fixing portion  4 . Subsequently, the sheet S is conveyed to a duplex conveying path  62  owing to reverse rotation of the pair of switchback rollers  61  and is conveyed to the registration portion  9  by a re-feeding roller  63  for newly forming an image. Then, the sheet S is discharged to the outside of the apparatus body through the same process as the one-sided copy mode. Here,  FIG. 1  illustrates a conveying wheel  35 , a conveying roller  36 , a second belt support roller  37 , a conveying belt  55  being an endless belt member, and conveying rollers  64 ,  65 . 
     [Structure of conveying portion] Next, the structure of a section where sheet conveying paths are merged after feeding will be described in detail with reference to  FIGS. 1 to 4 .  FIG. 2  is a side view illustrating a structure of a main part of the first embodiment of the present invention.  FIG. 3  is a sectional view illustrating a tension state of the endless conveying belt of the first embodiment.  FIG. 4A  is a sectional side view illustrating a winding support state of the conveying belt  55  formed of the endless belt member. 
     In the present embodiment, description is performed mainly on a structure in which the present invention is applied to a belt conveying portion  19  at a section having the conveying roller  33 , the retard roller  34 , the conveying wheel  35  and the conveying roller  36  which convey sheets S fed from each sheet cassette  31  at upper and lower stages toward the image forming portion  2 . Here, not limited to the above, it is natural that the present invention can be applied to a belt conveying portion at a section having the re-feeding roller  63 , the conveying wheel  35  and the conveying roller  36  which feed sheets S fed from the duplex conveying path  62  once again to the image forming portion  2 . 
     As illustrated in  FIGS. 1 and 2 , a first conveying path  101  extended in a lateral direction to convey a sheet S with the conveying roller  33  and the retard roller  34  is arranged at a merging section of the sheet conveying paths after feeding. Further, a second conveying path  104  and a third conveying path  102  extended in a vertical direction (i.e., straightly in an up-down direction) and intersecting at an end point of the first conveying path  101  are arranged at the merging section. The first conveying path  101 , the second conveying path  104  and the third conveying path  102  are mutually connected at a curved portion  103 . 
     That is, the sheet conveying apparatus includes a curve-shaped conveying path which conveys a sheet S being conveyed in a first conveyance direction (i.e., the direction of arrow B) toward a second conveyance direction (i.e., the direction of arrow C) which is curved against the first conveyance direction. The curve-shaped conveying path includes the first conveying path  101 , the curved portion  103  and the second conveying path  104 . 
     Further, the sheet conveying apparatus includes a second conveying portion  18  which conveys the sheet S in the second conveyance direction with the nip of the conveying roller  36  and the conveying wheel  35  being a pair of mutually-pressed rollers arranged at the downstream side of the curve-shaped conveying path from the curved portion (i.e., the downstream side from the curved portion  103 ). In addition, the sheet conveying apparatus includes the belt conveying portion  19  which is arranged at the curved portion  103  of the curve-shaped conveying paths ( 101 ,  103 ,  104 ) and which passes (i.e., conveys) the sheet S conveyed in the first conveyance direction toward the second conveying portion  18  owing to rotation of the conveying belt  55  being the endless belt member. The belt conveying portion  19  includes a first belt support roller  51 , a second belt support roller  37 , and the conveying belt  55 . The belt conveying portion  19  is arranged at a side where the conveying belt  55  is contacted to an image transfer face of the conveyed sheet S. 
     The second conveying portion (i.e., a drive conveying portion)  18  includes the conveying roller  36  being a drive roller which is rotated receiving driving force and the conveying wheel  35  which is driven and rotated as being pressed to the conveying roller  36  in the second conveying path  104 . Further, the conveying roller  33  and the retard roller  34  structure a first conveying portion (i.e., another drive conveying portion)  17  which is arranged at the first conveying path  101  and which conveys the sheet S in the first conveyance direction (i.e., the direction of arrow B). 
     Further, the belt conveying portion  19  includes the first belt support roller (i.e., a tension wheel)  51  which is arranged coaxially with the conveying wheel  35  being one of the pair of rollers ( 36 ,  35 ). In addition, the belt conveying portion  19  includes the second belt support roller (i.e., a tension wheel)  37  which is arranged at the upstream side of the second conveying path  18  from the first belt support roller  51 . The conveying belt  55  is wound around the first and second belt support rollers  51 ,  37  (i.e., the first and second belt support rollers). Here, the outer diameter R 1  (see  FIG. 3 ) of the first belt support roller  51  including the conveying belt  55  is set to be smaller than the outer diameter R 2  (see  FIG. 3 ) of the conveying wheel  35 . 
     In short, as illustrated in  FIGS. 3 and 4A , the conveying belt  55  is looped over and stretched between the first belt support roller  51  which is supported by a rotation shaft  35   a  being coaxial with the conveying wheel  35  and the second belt support roller  37  which is supported by a rotation shaft  29  at the upstream side thereof. As described above, the outer diameter R 1  where the conveying belt  55  is wound around the first belt support roller  51  is set to be smaller than the outer diameter R 2  of the conveying wheel  35 . 
     As illustrated in  FIG. 4B , the outer diameter of the first belt support member  51  is set to be a size at a degree so that the surface of the conveying belt  55  is not protruded from the conveying wheels  35 ,  35  at both sides to the surface side in a state that the conveying belt  55  is wound thereaound. Accordingly, the conveying belt  55  is not abutted to the conveying roller  36  in a state that the conveying wheels  35 ,  35  at both sides are abutted to the conveying roller  36 . Thus, the above structure prevents a sheet S from being nipped and conveyed by a conveying pair of rubber (i.e., the conveying roller  36 ) and rubber (i.e., the conveying belt  55 ). 
     As illustrated in  FIG. 4A , guides  38 ,  39 ,  40  are supported at the apparatus body side of the image forming apparatus  10  in a predetermined state. The second belt support roller  37  is rotatably supported by the rotation shaft  29  of which both ends in the axial direction is supported by a bracket  56  fixed to a guide  38 . Here, conveying guide faces of the above are indicated by numerals  38   a ,  39   a ,  40   a , respectively. 
     As illustrated in  FIG. 2 , an arm member  54  is supported at one end thereof by the rotation shaft  29  which is supported by the bracket  56  so as to be swingable in the direction of arrow D of  FIG. 2 . The conveying wheel  35  and the first belt support roller  51  are supported by the other end of the arm member  54  via the rotation shaft  35   a . The conveying wheel  35  supported as being swingable about the center O of the rotation shaft  29  in the direction of arrow D is pressed toward the conveying roller  36  by a spring (not illustrated). 
     That is, as illustrated in  FIG. 4B , four conveying wheels  35  and two first belt support rollers  51  axially supported as being sandwiched by two conveying wheels  35  respectively, are supported at both ends of the rotation shaft  35   a  which is supported by the arm members  54 ,  54 . With the above structure, the sheet S can be smoothly conveyed to the downstream side as being nipped by the conveying roller  36  and the conveying wheel  35 . 
     As illustrated in  FIG. 4A , the center O of the second belt support roller  37  which stretches the conveying belt  55  positions at the third conveying path  102  side from a tangent line L lined from a nip point between the conveying roller  33  and the retard roller  34  as passing on a top part of the conveying guide face  39   a  of the guide  39 . Accordingly, the top end of the sheet S conveyed from the first conveying path  101  can be appropriately abutted to a tension portion X 2  of the conveying belt  55 . 
     Here, a control system of the present embodiment is described with reference to  FIG. 7 . As illustrated in  FIG. 7 , a controller (CPU)  15  arranged in the apparatus body of the image forming apparatus  10  receives input of a detection signal from a sheet detection sensor  22  which detects the sheet S passing though the first conveying path  101 , the third conveying path  102 , the curved portion  103 , and the second conveying path  104 . A drive source M 1  such a motor which drives the first conveying portion (i.e., the other drive conveying portion)  17  as rotating the conveying roller  33  and the retard roller  34  is connected to the controller  15 . Further, a drive source M 2  such as a motor which drives the second conveying portion (i.e., the drive conveying portion)  18  and the belt conveying portion  19  as rotating the conveying roller  36  is connected to the controller  15 . The controller  15  controls the drive sources M 1 , M 2  based on the detection signal of the sheet detection sensor  22 . 
     As illustrated in  FIG. 6 , driving force of the drive source M 2  is transmitted from a rotation shaft  28  to a transmission gear G 1  and is further transmitted to a rotation shaft  36   a  via a shaft gear G 2  which is engaged with the transmission gear G 1 , so that the conveying roller  36  is rotated. Then, the driving force of the drive source M 2  is transmitted from the conveying roller  36  to the rotation shaft  35   a  supporting the conveying wheel  35  via the conveying wheel  35  which is abutted to the conveying roller  36  and is transmitted to the conveying belt  55  via the first belt support roller  51  which is supported by the rotation shaft  35   a . In the present embodiment, since the belt conveying portion  19  is configured to be capable of being driven by the drive source M 2  which is commonly used to the second conveying portion  18  as described above, the structure thereof is simplified. 
     In the present embodiment, the conveying belt (i.e., the belt member)  55  is formed of high slidability material at least at the outer circumferential face. That is, the conveying belt  55  is formed of EPDM-based rubber material and the outer circumferential face of the conveying belt  55  is coated with silicon-based (i.e., silicone-based) material having a low friction coefficient (i.e., high slidability material). Further, the conveying wheel  35  being a driven roller pressed to the conveying roller  36  is formed of synthetic resin material such as ABS resin and POM resin (i.e., polyacetal resin). 
     In the above sheet conveying apparatus, the conveying roller  36  and the conveying wheel  35  corresponding to the sheet cassette  31  at the upper stage convey a sheet S conveyed to the curved portion  103  via the first conveying path  101  and a sheet S conveyed via the first conveying path  101  respectively to the downstream side of the second conveying path  104 . Meanwhile, the conveying roller  36  and the conveying wheel  35  corresponding to the sheet cassette  31  at the lower stage convey a sheet S fed to the curved portion  103  via the third conveying path  102  to the downstream side of the second conveying path  104 . Further, the conveying roller  36  and the conveying wheel  35  corresponding to the duplex conveying path  62  convey a sheet S fed from the duplex conveying path  62  to the curved portion  103  via the first conveying path  101  and a sheet S conveyed via the third conveying path  102  respectively to the downstream side of the second conveying path  104 . 
     Next, operation of the present embodiment will be described with reference to  FIGS. 5A to 5C . First, when a sheet S is conveyed from the first conveying path  101  to the curved portion  103  in a state that the conveying roller  36  is rotated counterclockwise and the conveying wheel  35  and the conveying belt  55  are rotated clockwise as being controlled by the controller  15 , the following situation occurs. That is, the top end of the sheet S is abutted to the tension portion X 2  of the conveying belt  55  (see  FIG. 4A ), as illustrated in  FIG. 5A . In this case, since impact due to collision of the top end of the sheet S is softened by tension elasticity of the conveying belt  55 , noise occurrence at the time of collision is effectively suppressed. 
     Then, as illustrated in  FIG. 5B , the top end of the sheet S is passed and moved upward by the rotating conveying belt  55 . At that time, since the surface of the conveying belt  55  is coated with silicone-based material having a low friction coefficient (i.e., high slidability material), the sheet top end is prevented from being stuck to the surface of the conveying belt  55 . Accordingly, the top end of the sheet S enters toward the nip of the conveying roller  36  and the conveying wheel  35  as being guided with contacting to the conveying belt  55 . 
     Subsequently, when the rear end of the sheet S conveyed by the second conveying portion  18  passes through the first conveying path  101  as illustrated in  FIG. 5C , the rear end is about to collide with the guide  38  as being bounced in a restoration direction owing to elastic force of the sheet S. However, since the sheet rear end is abutted to the conveying belt  55  without colliding with the guide  38 , the impact thereof is softened by tension elasticity of the conveying belt  55 . Accordingly, occurring noise can be effectively suppressed. Subsequently, the sheet S is conveyed to the downstream side as being nipped by the nip of the conveying roller  36  and the conveying wheel  35 . Then, an image is transferred at the image forming portion  2  as the surface to which the conveying wheel  35  is contacted being a toner transfer face (i.e., an image transfer face). 
     According to the present embodiment, when the sheet S conveyed from the first conveying path  101  is conveyed to the second conveying path  104 , the sheet S can be conveyed only by nipping between the conveying roller  36  and the conveying wheel  35  without contact between the conveying belt  55  and the conveying roller  36 . Accordingly, it is possible to reliably prevent occurrence of inconvenience such that imaging instability becomes apparent as a result of occurrence of large frictional charging difference between a nipped part and an un-nipped part of the sheet S caused by sheet nipping with a rubber-made roller body and a rubber-made endless belt in the related art. In this manner, occurrence of imaging failure can be avoided while reducing noise occurring at the vicinity of the curved portion  103 . 
     &lt;First Modification&gt; In the present embodiment, the conveying wheel  35  and the first belt support roller  51  which stretches the conveying belt  55  are fixed and supported separately by the rotation shaft  35   a  as being prepared as separate members. However, the present invention is not limited to the above. That is, as the first modification illustrated in  FIG. 8 , conveying wheels  52 ,  52  having a larger diameter than the first belt support roller  51  are integrally formed at both sides of the first belt support roller  51 , and then, the first belt support roller  51  is rotated integrally with the conveying wheels  52 ,  52  which are rotated by the conveying roller  36 . 
     According to the first modification, the rotation shaft  35   a  is only required to be configured to simply support the conveying wheels  52 ,  52  and the first belt support roller  51  being rotatable in a state that both ends thereof are simply supported by arm members  54 ,  54  without being required to be rotatably supported. With the above structure, it is possible to obtain an effect of structural simplification. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. In the present embodiment, the mechanical structure is the same as the first embodiment only with different control. Accordingly, the same numeral is given to the same part and description thereof will not be repeated. Here, the present embodiment will be described mainly on operation thereof by utilizing operational views of  FIGS. 9A to 9D  and a flowchart of  FIG. 10 . 
     In the present embodiment, conveyance speeds V 1 , V 2 , and V 3  set to satisfy relation of “V 1 &gt;V 2 &gt;V 3 ” are utilized for description of conveyance speeds of the conveying roller  33 , the conveying roller  36 , and the conveying belt  55 . The difference among the above conveyance speeds are derived from relation among the conveying roller  36 , the conveying wheel  35  of which outer diameter is smaller than that of the conveying roller  36 , and the conveying belt  55  which is wound around the first belt support roller  51  having a small diameter and the second belt support roller  37 . 
     In the present embodiment, the controller  15  controls the driving of the drive sources M 1 , M 2  such as motors (see  FIG. 7 ) based on sheet detection by the sheet detection sensor  22 . Accordingly, control is performed as setting the conveying belt  55  at the conveyance speed V 2  by setting the conveying roller  36  at the conveyance speed V 1  or as setting the conveying belt  55  at the conveyance speed V 3  by setting the conveying roller  36  at the conveyance speed V 2 , while setting the conveying roller  33  constantly at the conveyance speed V 2 . That is, when there is not a sheet S at the nip of the second conveying portion  18  of the present embodiment, the second conveying portion  18  is controlled to be switched to the conveyance speed V 1  being faster than the conveyance speed V 2  of the first conveying portion (i.e., the other drive conveying portion)  17 . On the contrary, when there is a sheet S at the nip of the second conveying portion  18 , the second conveying portion  18  is controlled to be switched to the conveyance speed V 2  being equal to the conveyance speed V 2  of the first conveying portion  17 . 
     Here, in the sheet conveying apparatus, prior to conveying of the sheet S, control is performed as illustrated in  FIG. 9A . That is, the drive sources M 1 , M 2  are each controlled by the controller  15  so that the first conveying portion  17  is at the conveyance speed V 2 , the second conveying portion  18  is at the conveyance speed V 1  being faster than the conveyance speed V 2 , and the belt conveying portion  19  is at the conveyance speed V 2  obtained according to the conveyance speed V 1  of the conveying roller  36  (step S 1 ). 
     In the above state, when the sheet S is conveyed from the first conveying path  101  to the curved portion  103 , the top end of the sheet S is abutted to the conveying belt  55 . Then, based on the detection signal of the sheet detection sensor  22  (see  FIG. 7 ), the controller  15  determines whether there is a sheet S at the nip of the second conveying portion  18  (step S 2 ). As a result, when it is determined that there is not a sheet S at the nip of the second conveying portion  18  (“No” in step S 2 ), the control of step S 1  is repeated. 
     On the contrary, when it is determined that there is a sheet S at the nip of the second conveying portion  18  (“Yes” in step S 2 ), the controller  15  controls the drive sources M 1 , M 2  so that the first conveying portion  17  is at the conveyance speed V 2 , the second conveying portion  18  is at the conveyance speed V 2 , and the belt conveying portion  19  is at the conveyance speed V 3  (step S 3 ), as illustrated in  FIG. 9B . Since the conveyance speed of the second conveying portion  18  becomes equal to the conveyance speed V 2  of the first conveying portion  17 , it is possible to prevent occurrence of inconvenience such that the sheet S is pulled while conveying is performed by the second conveying portion  18  in a state that the sheet rear end side is nipped by the nip of the first conveying portion  17 . Here, similarly to the first embodiment, it is possible to obtain the effect of suppressing noise occurrence at the time of collision of the top end of the sheet S by softening impact due to the collision. 
     Subsequently, as illustrated in  FIG. 9C , the top end of the sheet S is guided while being moved upward as keeping contact with the conveying belt  55  and enters toward the nip of the second conveying portion  18 . While it is determined that there is a sheet S at the nip of the second conveying portion  18  (“Yes” in step S 4 ), the controller  15  repeats the control of step S 3 . 
     Further, the controller  15  determines whether there is a sheet S at the nip of the second conveying portion  18  based on the detection signal of the sheet detection sensor  22  (step S 4 ). Here, the sheet S is conveyed to the downstream side at the conveyance speed V 2  by the second conveying portion  18  and the rear end collides with the conveying belt  55  which is rotated at the conveyance speed V 3 . The impact at that time is appropriately softened by the conveying belt  55 . Accordingly, similarly to the first embodiment, the effect of suppressing occurring noise can be obtained. 
     When it is determined that there is not a sheet S at the nip of the second conveying portion  18  in step S 4  (“No” in step S 4 ), the processing proceeds to step S 5 . That is, the sheet S is conveyed to the downstream side as being nipped at the second conveying portion  18 . Then, when the sheet S passes through the nip of the second conveying portion  18 , the controller  15  increases the conveyance speed of the second conveying portion  18  from V 2  to V 1  while keeping the first conveying portion  17  at the conveyance speed V 2 . Accordingly, the conveyance speed of the belt conveying portion  19  is increased from V 3  to V 2  to return to the initial state of  FIG. 9A . 
     In the present embodiment, the outer circumferential face of the conveying belt  55  may be coated with material having a low friction coefficient as the first embodiment. However, the similar effect can be obtained even without the coating. Further, in the present embodiment, the conveyance speed of the belt conveying portion  19  at the initial state is controlled to be equal to the conveyance speed V 2  of the conveying roller  33 . However, it is not limited to the above. For example, the drive source M 2  is controlled so that the conveying belt  55  of the belt conveying portion  19  is at conveyance speed being faster than the conveyance speed V 2  when conveyance efficiency of the sheet top end due to the belt conveying portion  19  is not sufficient. In this case, the conveying capability of the sheet top end can be improved. 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described with reference to  FIG. 11 . In the present embodiment, similarly to the first embodiment, the conveying roller  36  is configured to be rotated by driving of the drive source M 2  to transmit rotation to the conveying wheel  35 . Here, the driving of the belt conveying portion  19  is configured as follows. 
     As illustrated in  FIG. 11 , driving is transmitted from a drive source M 4  such as a motor arranged at the apparatus body side to the rotation shaft  29  of the second belt support roller  37  via a transmission gear G 4  and a drive gear G 3  which is engaged with the transmission gear G 4 . Here, any of four conveying wheels  35  is not drive-connected to the rotation shaft  35   a  and the second belt support roller  37  is drive-connected to the rotation shaft  29 . Accordingly, conveyance speed of the conveying belt  55  at the belt conveying portion  19  can be set with control of the controller  15  without relation to the conveying roller  36  and the conveying wheel  35 . In this manner, the belt conveying portion  19  of the present embodiment is configured to be capable of being driven independently from the second conveying portion  18  by the drive source M 4  being different from the drive source M 2  of the second conveying portion  18 . 
     According to the above structure, rotation speed of the drive source M 2  is not required to be switched based on detection of the sheet detection sensor  22  not like the second embodiment. Therefore, conveyance speed is continuously coordinated among the conveying belt  55 , the conveying roller  36 , and the conveying wheel  35 . 
     Further, similarly to the second embodiment, it is also possible to control the drive source M 4  so that the conveyance speed of the conveying belt  55  is set to be faster than the conveyance speed V 2  when conveyance efficiency of the sheet top end due to the conveying belt  55  is not sufficient. In this case, the conveying capability of the sheet top end can be improved. The rest of the structure and effects are the same as those of the first embodiment. 
     Here, it is also possible that the conveying belt  55  and the first belt support roller  51  are configured to be rotatable without being drive-connected to the rotation shaft  35   a . In this case, inconvenience such that a sheet top end is folded can be reliably avoided by releasing force with rotation of the conveying belt  55  following the sheet S which receives conveying force from the conveying roller  33  when the sheet top end is abutted to the conveying belt  55  as illustrated in  FIG. 5A , for example. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-281486, filed Dec. 17, 2010, which is hereby incorporated by reference herein in its entirety.