Patent Publication Number: US-10308465-B2

Title: Sheet transport device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2017-028311 filed Feb. 17, 2017. 
     BACKGROUND 
     The present invention relates to sheet transport devices and image forming apparatuses. 
     SUMMARY 
     A sheet transport device according to an exemplary embodiment of the present invention includes pre-transfer sheet-transport rollers that transport a sheet to a transfer position at which an unfixed image is transferred, the pre-transfer sheet-transport rollers including a first roller in which three or more separate rollers attached to a first shaft rotate, and a second roller in which three or more separate rollers attached to a second shaft are in contact with the separate rollers of the first roller and rotate. The hardness of an inner separate roller of the first roller is lower than the hardness of end separate rollers of the first roller. The outside diameter of an inner separate roller of the second roller is greater than the outside diameter of end separate rollers of the second roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  shows the configuration of an image forming apparatus including a sheet transport device according to a first exemplary embodiment or the like; 
         FIG. 2  shows, in an enlarged manner, the relevant part (the sheet transport device and portions constituting a second-transfer position) of the image forming apparatus in  FIG. 1 ; 
         FIG. 3  is a schematic perspective view of pre-transfer sheet-transport rollers in the sheet transport device in  FIG. 1 ; 
         FIG. 4A  shows the configuration of the sheet transport device,  FIG. 4B  shows a pressure-contact state between end separate rollers of the pre-transfer sheet-transport rollers, and  FIG. 4C  shows a pressure-contact state between inner separate rollers of the pre-transfer sheet-transport rollers; 
         FIG. 5A  shows a pressure state in the pre-transfer sheet-transport rollers, and a state of pressure-contact load between each pair of separate rollers while a sheet is not transported,  FIG. 5B  shows a state of pressure-contact load between each pair of separate rollers when a wide sheet is transported with the sheet-transport rollers in  FIG. 5A , and  FIG. 5C  shows a state of pressure-contact load between each pair of separate rollers when a narrow sheet is transported with the sheet-transport rollers in  FIG. 5A ; 
         FIGS. 6A and 6B  are a front view and a partial perspective view showing a sheet in a curved state when the sheet is fed out from the pre-transfer sheet-transport rollers; 
         FIGS. 7A to 7C  show, in a chronological order, contact states between an intermediate transfer belt and a sheet when the sheet is transported from the sheet transport device; 
         FIG. 8A  shows Comparison Example 1 of pre-transfer sheet-transport rollers; and  FIG. 8B  shows Comparison Example 2 of pre-transfer sheet-transport rollers; and 
         FIG. 9A  shows a state of a sheet when the sheet with wave-like deformation is fed out by the pre-transfer sheet-transport rollers, and a contact state between the intermediate transfer belt and the sheet in that state, and  FIG. 9B  shows a state of a sheet when the sheet that is curved so as to project toward a surface opposite to a transfer target surface is fed out by the pre-transfer sheet-transport rollers, and a contact state between the intermediate transfer belt and the sheet in that state. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
     First Exemplary Embodiment 
       FIGS. 1 and 2  show an image forming apparatus including a sheet transport device according to a first exemplary embodiment.  FIG. 1  shows the configuration of the image forming apparatus, and  FIG. 2  shows the relevant part (i.e., the sheet transport device and structural parts therearound) of the image forming apparatus. The arrows with reference signs X, Y, and Z in  FIGS. 1 and 2  represent the directions of the Cartesian-coordinate axes indicating width, height, and depth in three-dimensional spaces assumed in the drawings. 
     Configuration of Image Forming Apparatus 
     An image forming apparatus  1  according to the first exemplary embodiment includes, in the inner space of a housing  10 : multiple image forming parts  2  that form unfixed images (toner images), which are formed of developer, according to image information; an intermediate transfer part  3  that transports the toner images formed by the image forming parts  2 ; a paper feed part  4  that stores and feeds sheets  9  to which the toner images on the intermediate transfer part  3  are second-transferred; a fixing part  5  that fixes the toner images to the sheet  9 , to which the unfixed toner images are second-transferred at the second-transfer position of the intermediate transfer part  3 ; and the like. This image forming apparatus  1  also includes a sheet transport device  6  that transports the sheet  9  to the second-transfer position of the intermediate transfer part  3 . 
     Examples of the image information include text, figures, pictures, and colors. The housing  10  has, in the top surface thereof, an output-sheet storing part  12  that stores, in a stacked manner, the sheets  9  discharged after images are formed thereon. The one-dot chain line in  FIG. 1  indicates a transport path along which the sheets  9  are transported in the inner space of the housing  10 . 
     The multiple image forming parts  2  include four image forming devices  20 Y,  20 M,  20 C, and  20 K that form yellow (Y), magenta (M), cyan (C), and black (K) toner images, respectively. 
     As shown in  FIG. 1 , these four image forming devices  20  (Y, M, C, and K) each include: a photoconductive drum  21  that is rotationally driven in the direction indicated by an arrow A; a charging device  22  that charges an image carrying surface of the photoconductive drum  21 ; an exposure device  23  that forms, by radiating light, an electrostatic latent image on the charged image carrying surface of the photoconductive drum  21 ; a developing device  24  that develops the electrostatic latent image with developer to form a toner image; a first-transfer device  25  that first-transfers the toner image to the intermediate transfer part  3 ; and a drum cleaning device  26  that cleans the photoconductive drum  21  by removing undesired substances deposited on the image carrying surface thereof. In  FIG. 1 , all the components of the image forming device  20 K are denoted by reference signs ( 21  to  26 ), and some, but not all, of the components of the other image forming devices,  20 Y,  20 M, and  20 C, are denoted by reference signs. 
     The intermediate transfer part  3  is located above the image forming devices  20  (Y, M, C, and K), serving as the image forming parts  2 . 
     The intermediate transfer part  3  includes: the intermediate transfer belt  31  that revolves in the direction indicated by an arrow B so as to pass, in a contact manner, through first-transfer positions facing the first-transfer devices  25  of the photoconductive drums  21  of the image forming devices  20  (Y, M, C, and K); multiple support rollers  32   a  to  32   e  that are in contact with the intermediate transfer belt  31  from the inner circumferential surface so as to support the intermediate transfer belt  31  in a desired state, in a rotatable manner; a second transfer device  35  that presses a sheet  9  against the intermediate transfer belt  31  supported by the support roller  32   a  so that the toner images on the intermediate transfer belt  31  are second-transferred to the sheet  9 , and a belt cleaning device  36  that cleans the intermediate transfer belt  31  by removing undesired substances deposited thereon. 
     The intermediate transfer belt  31  serves as an image carrier that carries unfixed toner images to be transferred to the sheet  9 . 
     The support roller  32   a  serves as a driving roller that makes the intermediate transfer belt  31  revolve, as well as a backup roller used in the second transfer, the support roller  32   b  serves as a backup roller for the belt cleaning device  36 , the support roller  32   c  serves as a tension-applying roller that applies a certain tension to the intermediate transfer belt  31 , and the support rollers  32   d  and  32   e  serve as surface-forming rollers that support the intermediate transfer belt  31  so as to form a first transfer surface. 
     The paper feed part  4  is located below the image forming devices  20  (Y, M, C, and K), serving as the image forming parts  2 . 
     This paper feed part  4  includes a container  41  that stores, on the top surface of a loading plate  42 , a stack of sheets  9  of desired size and type, and a feeder  43  that feeds the sheets  9  from the container  41  on a one-by-one basis. The container  41  can be drawn toward the front side of the housing  10  (i.e., the side a user faces when operating the apparatus). More than one pair of the container  41  and the feeder  43  may be provided, if necessary. 
     The sheets  9  are recording media that can be transported along the transport path in the housing  10 , and to which toner images can be transferred and fixed. The sheets  9  are preliminarily cut in predetermined sizes. The sheets  9  other than those having a sheet shape, such as those of an envelope type, may also be used. 
     The fixing part  5  is located above the second-transfer position (i.e., the position between the intermediate transfer belt  31  and the second transfer device  35 ) TP 2  of the intermediate transfer part  3 . 
     The fixing part  5  includes a fixing device  50 . The fixing device  50  includes, inside a housing  51  having an introduction port and a discharge port for sheets  9 , a heating rotary body  52  of a roller, belt, or other type, which rotates in the direction indicated by the arrow and is heated by a heating unit (not shown) such that the surface temperature thereof is maintained at a predetermined temperature, and a pressure-applying rotary body  53  of a roller, belt, or other type, which is in contact with the heating rotary body  52  at a predetermined pressure so as to be substantially parallel to the axial direction of the heating rotary body  52  and is rotated in a driven manner. In the fixing device  50 , a portion at which the heating rotary body  52  and the pressure-applying rotary body  53  are in contact with each other constitutes a fixing processing part at which heat and pressure are applied. 
     The image forming apparatus  1  includes, in the inner space of the housing  10 , at a position between the paper feed part  4  and the intermediate transfer part  3 , a feed-and-transport path  44  that transports and feeds a sheet  9  fed from the paper feed part  4  to the second-transfer position TP 2  of the intermediate transfer part  3 . 
     The feed-and-transport path  44  includes multiple sheet-transport rollers  45  and  61 , multiple sheet guide members (not shown), and the like. The sheet-transport rollers  61  are transport rollers through which a sheet passes immediately before transferring (hereinbelow, pre-transfer sheet-transport rollers  61 ). The pre-transfer sheet-transport rollers  61  transport the sheet  9  toward the second-transfer position TP 2  of the intermediate transfer part  3 . The pre-transfer sheet-transport rollers  61  serve as registration rollers having a function of adjusting the timing of transporting (feeding) the sheet  9  to the second-transfer position TP 2  and a function of adjusting the transport orientation (i.e., correcting oblique feeding). The sheet-transport rollers  61  in the feed-and-transport path  44  constitute a part of the sheet transport device  6  described below. 
     Furthermore, a relay transport path  46  along which a sheet  9  after second transfer is transported to the fixing part  5  is provided in the inner space of the housing  10 , between the second-transfer position TP 2  of the intermediate transfer part  3  and the fixing part  5 . The relay transport path  46  includes a sheet guide member  47 . 
     Furthermore, a discharge transport path  48  along which a sheet  9  having an image formed and fixed thereon is transported so as to be discharged on the output-sheet storing part  12  is provided in the inner space of the housing  10 , between the fixing part  5  and a sheet discharge port in the housing  10 . The discharge transport path  48  includes discharging rollers  49  and a sheet guide member (not shown). 
     Image Forming Operation of Image Forming Apparatus 
     A basic image forming operation performed by the image forming apparatus  1  will be described. An operation in an example case where a full-color image formed of toner images of four colors (Y, M, C, and K) is formed will be described. 
     First, when an image-forming-operation start instruction is issued, as shown in  FIG. 1 , the photoconductive drums  21  of the four image forming devices  20  (Y, M, C, and K), serving as the image forming parts  2 , are rotated in the direction indicated by the arrows, and the charging devices  22  charge the image carrying surfaces of the photoconductive drums  21  to a certain (for example, negative) polarity and electric potential. Then, the exposure devices  23  perform exposure according to image signals decomposed into respective color components (Y, M, C, and K) on the respective charged photoconductive drums  21  to form electrostatic latent images of the respective color components, having certain electric potentials, on the image carrying surfaces of the photoconductive drums  21 . 
     Then, the developing devices  24  (Y, M, C, and K) of the image forming devices  20  (Y, M, C, and K) develop images by supplying color (Y, M, C, and K) toners charged to a certain (negative) polarity to the electrostatic latent images of the respective color components (Y, M, C, and K) formed on the photoconductive drums  21 , allowing the toners to electrostatically attach. As a result, the toner images of the four colors (Y, M, C, and K) are formed on the image carrying surfaces of the photoconductive drums  21  of the image forming devices  20  (Y, M, C, and K), respectively. 
     Then, the toner images of four colors formed on the respective photoconductive drums  21  of the image forming devices  20  (Y, M, C, and K) are sequentially (in order of Y, M, C, and K) first-transferred to the outer circumferential surface of the intermediate transfer belt  31  of the intermediate transfer part  3  by receiving transfer effects of the first-transfer devices  25 . The photoconductive drums  21  are cleaned by the drum cleaning devices  26 . 
     Then, the unfixed toner images first-transferred to the outer circumferential surface of the intermediate transfer belt  31  at the intermediate transfer part  3  and held thereon are transported to the second-transfer position TP 2  by the intermediate transfer belt  31 , which revolves in the direction indicated by an arrow B. Meanwhile, in the paper feed part  4 , a sheet  9  is transported such that it is fed out of the container  41  by the feeder  43  and is fed to the second-transfer position TP 2  via the feed-and-transport path  44 . Then, at the second-transfer position TP 2  of the intermediate transfer part  3 , the toner images on the intermediate transfer belt  31  are simultaneously second-transferred to one side of the sheet  9  by receiving the transfer effect from the second transfer device  35 . 
     Next, the sheet  9  to which the unfixed toner image is second-transferred is transported such that it is separated from the intermediate transfer belt  31  and is fed to the fixing part  5  via the relay transport path  46 . In the fixing device  50  of the fixing part  5 , the sheet  9  is introduced to the fixing processing part, at which the heating rotary body  52  and the pressure-applying rotary body  53  are in contact, and is subjected to heat and pressure as it passes therethrough. This way, the toner images are fused and fixed to the sheet  9 . 
     Then, the sheet  9  to which the toner images have been fixed in the fixing part  5  is discharged from the fixing device  50  of the fixing part  5 , is transported via the discharge transport path  48 , is discharged to the outside of the housing  10  by the discharging rollers  49 , and is then stored in the output-sheet storing part  12 . 
     Through the above-described operation, the sheet  9  having a full-color image formed on one side is output. 
     Configuration of Sheet Transport Device 
     Next, the sheet transport device  6  will be described. 
     As shown in  FIGS. 1 to 4 , etc., the sheet transport device  6  includes, at least, the pre-transfer sheet-transport rollers  61  that transport a sheet  9  to the second-transfer position TP 2  and that include a first roller  62  and a second roller  63 . 
     The first roller  62  includes a first shaft  64 , and four separate rollers  65 A,  65 B,  65 C, and  65 D that are fixed to and rotate with the first shaft  64 . The first shaft  64  is rotatably attached at the ends to a support frame  70  via bearings  71 . 
     The first roller  62  also serves as a driving roller that drivingly rotates in a rotation direction C by receiving a rotational force from a rotational driving device  74 , which includes a stepping motor, a rotation transmitting mechanism, etc. 
     The second roller  63  includes a second shaft  66 , and four separate rollers  67 A,  67 B,  67 C, and  67 D that are fixed to and rotate with the second shaft  66 . The second shaft  66  is rotatably attached at the ends to the support frame  70  via bearings  72  that are movable, in elongated holes (not shown), toward and away from the first roller  62 . 
     The second roller  63  serves as a driven roller in which the separate rollers  67 A,  67 B,  67 C, and  67 D are in contact with the separate rollers  65 A,  65 B,  65 C, and  65 D of the first roller  62 , respectively, and are rotated in a driven manner. 
     The ends of the second shaft  66  of the second roller  63  (or the bearings  72 ) are pressed toward the first shaft  64  of the first roller  62  at a certain pressure P by pressure-applying parts  75 , such as pressure-applying springs. Thus, the separate rollers  67 A,  67 B,  67 C, and  67 D are pressed against the separate rollers  65 A,  65 B,  65 C, and  65 D of the first roller  62  at a certain pressing force. 
     In the pre-transfer sheet-transport rollers  61  of the sheet transport device  6 , the hardness J 1  of the inner separate rollers  65 B and  65 C of the first roller  62  is lower than the hardness J 2  of the end separate rollers  65 A and  65 D thereof (J 1 &lt;J 2 ), and the outside diameter K 1  of the inner separate rollers  67 B and  67 C of the second roller  63  is greater than the outside diameter K 2  of the end separate rollers  67 A and  67 D thereof (K 1 &gt;K 2 ). 
     The hardness J 1  of the inner separate rollers  65 B and  65 C of the first roller  62  and the hardness J 2  of the end separate rollers  65 A and  65 D thereof are the hardness of elastic members measured with an Asker C hardness tester, when all the separate rollers  65 B,  65 C,  65 A, and  65 D are formed of an elastic member, such as rubber. 
     The hardness J 1  is, for example, about 0.5 to 0.9 times the hardness J 2 . How much the hardness J 1  is lower than the hardness J 2  may be set by taking into consideration, for example, the difference between the outside diameter K 1  of the inner separate rollers  67 B and  67 C and the outside diameter K 2  of the end separate rollers  67 A and  67 D of the second roller  63 . 
     The separate rollers  65 A,  65 B,  65 C, and  65 D according to the first exemplary embodiment are formed of a rubber material, such as an ethylene rubber or a nitrile rubber, and the hardness J 1  and the hardness J 2  of the separate rollers are set by adjusting the composition or the like of rubber material. The separate rollers  67 A,  67 B,  67 C, and  67 D of the second roller  63  have the same hardness (J 3 ), which is higher than the hardness J 1  and the hardness J 2  of the separate rollers  65  of the first roller  62 . 
     The outside diameter K 1  of the inner separate rollers  67 B and  67 C of the second roller  63  is, for example, about 1.1 to 1.2 times the outside diameter K 2  of the end separate rollers  67 A and  67 D thereof. How much the outside diameter K 1  is greater than the outside diameter K 2  may be set by taking into consideration, for example, the difference between the hardness J 1  of the inner separate rollers  65 B and  65 C and the hardness J 2  of the end separate rollers  65 A and  65 D of the first roller  62 . 
     The separate rollers  67 A,  67 B,  67 C, and  67 D in the first exemplary embodiment are formed of, for example, a synthetic resin material, such as acrylonitrile-butadiene-styrene (ABS) copolymer resin or polyacetal (POM) resin. The separate rollers  65 A,  65 B,  65 C, and  65 D of the first roller  62  have the same outside diameter K 3 , which equals the outside diameter K 2  of the end separate rollers  67 A and  67 D of the second roller  63 . 
     Furthermore, in the sheet transport device  6 , the first roller  62  of the pre-transfer sheet-transport rollers  61  is disposed on the side to be in contact with the surface of a sheet  9  to which an unfixed image is transferred. 
     In the image forming apparatus  1 , as shown in  FIG. 2 , etc., this configuration is achieved by disposing the first roller  62  closer to the intermediate transfer belt  31  of the intermediate transfer part  3 , which carries an unfixed toner image, than the second roller  63  is. 
     Furthermore, while the pre-transfer sheet-transport rollers  61  of the sheet transport device  6  are not transporting a sheet  9 , the ends of the second shaft  66  of the second roller  63  (or the bearings  72 ) are pressed by the pressure-applying parts  75  at substantially the same pressure P. Thus, the first shaft  64  of the first roller  62  and the second shaft  66  of the second roller  63  are maintained substantially parallel, at a certain distance L from each other. 
     As a result, as shown in  FIG. 4B , the end separate rollers  67 A and  67 D of the second roller  63  are pressed against the end separate rollers  65 A and  65 D of the first roller  62  to an extent that the end separate rollers  65 A and  65 D are slightly depressed. 
     Meanwhile, as shown in  FIG. 4C , the inner separate rollers  67 B and  67 C of the second roller  63  are pressed against the end separate rollers  65 A and  65 D of the first roller  62  to an extent that the end separate rollers  65 A and  65 D are depressed by a predetermined depression amount a, because the outside diameter K 1  of the inner separate rollers  67 B and  67 C of the second roller  63  is relatively large, and the hardness J 1  of the end separate rollers  65 A and  65 D of the first roller  62  is relatively low. 
     The first shaft  64  of the first roller  62  is disposed substantially parallel to the axial direction of the second-transfer position TP 2  of the intermediate transfer part  3  (more specifically, the axial direction of the support roller  32   a  and the axial direction of the second transfer roller of the second transfer device  35 ). 
     Operation of Sheet Transport Device 
     In the sheet transport device  6 , during the above-described image forming operation or the like, the pre-transfer sheet-transport rollers  61  (the first roller  62  and the second roller  63 ) start rotating at predetermined timing after temporarily stop rotating. The predetermined timing is, for example, timing not late for starting of transferring of toner images at the second-transfer position TP 2 . 
     Thus, a leading-end portion  9   a  of the sheet  9  in the transport direction D comes into contact with press-contact portions between the separate rollers  65 A to  65 D of the first roller  62  and the separate rollers  67 A to  67 D of the second roller  63 , which are not rotating, and the sheet  9  that is transported from the paper feed part  4  toward the second-transfer position TP 2  of the intermediate transfer part  3  via the feed-and-transport path  44  is temporarily stopped. 
     As a result, even if the leading-end portion  9   a  of the sheet  9  in the transport direction D is transported to the pre-transfer sheet-transport rollers  61  so as to be oblique to the transport direction D, the leading-end portion  9   a  of the sheet  9  becomes parallel to the press-contact portion between the first roller  62  and the second roller  63 , and is corrected so as to be substantially parallel to the axial direction of the first shaft  64  of the first roller  62 . 
     Subsequently, when the first roller  62  and the second roller  63  in the pre-transfer sheet-transport rollers  61  start rotating at predetermined timing, the leading-end portion  9   a  of the sheet  9  in the transport direction D starts to be transported while being nipped between the first roller  62  and the second roller  63 . 
     This way, the sheet  9  is transported by the pre-transfer sheet-transport rollers  61  toward the second-transfer position TP 2  of the intermediate transfer part  3 . 
     At this time, in the sheet transport device  6 , as shown in  FIG. 5A , when the ends of the second shaft  66  of the second roller  63  (or the bearings  72 ) are pressed with the pressure-applying parts  75  at a pressure P of 10 N (newton), the pressure-contact loads between the end separate rollers  65 A and  67 A, and  65 D and  67 D of the first roller  62  and the second roller  63  are both substantially 5 N, and the pressure-contact loads between the inner separate rollers  65 B and  67 B, and  65 C and  67 C of the first roller  62  and the second roller  63  are both substantially 5 N. 
     Note that, the hardness J 1  of the inner separate rollers  65 B and  65 C of the first roller  62  at this time is set to about 50 degrees, and the hardness J 2  of the end separate rollers  65 A and  65 D is set to about 80 degrees. The outside diameter K 1  of the inner separate rollers  67 B and  67 C of the second roller  63  is set to about 15 mm, and the outside diameter K 2  of the end separate rollers  67 A and  67 D is set to about 14 mm. 
     In particular, in the sheet transport device  6 , the outside diameter K 1  of the inner separate rollers  67 B and  67 C of the second roller  63  is greater than the outside diameter K 2  of the end separate rollers  67 A and  67 D. Hence, normally (if the distance L between the first shaft  64  and the second shaft  66  is constant), the pressure-contact load with respect to the inner separate rollers  65 B and  65 C of the first roller  62  is greater than the pressure-contact load with respect to the end separate rollers  65 A and  65 D. 
     However, in the sheet transport device  6 , because the hardness J 1  of the inner separate rollers  65 B and  65 C of the first roller  62  is lower than the hardness J 2  of the end separate rollers  65 A and  65 D, the inner separate rollers  65 B and  65 C elastically deform and absorb the pressure exerted by the inner separate rollers  67 B and  67 C of the second roller  63 , which have a greater outside diameter K 1 . 
     Accordingly, in the sheet transport device  6 , the pressure-contact loads between the inner separate rollers  65 B and  67 B, and  65 C and  67 C are substantially equal to the pressure-contact loads between the end separate rollers  65 A and  67 A, and  65 D and  67 D. 
     As a result, as shown in, for example,  FIG. 5B , when a wide sheet  9 A, which has a relatively large length in the width direction E and is transported by being nipped between both the inner separate rollers,  65 B,  65 C,  67 B, and  67 C, and the end separate rollers,  65 A,  65 D,  67 A, and  67 D, of the first roller  62  and the second roller  63 , is transported, the pressure-contact loads are as follows. 
     Because the wide sheet  9 A evenly extends between all pairs of the separate rollers, the pressure-contact loads between the end separate rollers  65 A and  67 A, and  65 D and  67 D, and the pressure-contact loads between the inner separate rollers  65 B and  67 B, and  65 C and  67 C are all substantially 5 N. 
     As shown in, for, example,  FIG. 5C , when a narrow sheet  9 B, which has a relatively small length in the width direction E and is transported by being nipped between only the inner separate rollers  65 B,  65 C of the first roller  62  and the inner separate rollers  67 B,  67 C of the second roller  63 , is transported, the pressure-contact loads are as follows. 
     Because the narrow sheet  9 B extends only between the inner separate rollers, the pressure-contact loads between the inner separate rollers  65 B and  67 B, and  65 C and  67 C are both about substantially 6 N, which are slightly higher than those in the case of the wide sheet  9 A, though they may slightly vary with the thickness of the narrow sheet  9 B. At this time, the pressure-contact loads between the end separate rollers  65 A and  67 A, and  65 D and  67 D are about 4 N, which are slightly lower than those in the case of the wide sheet  9 A, because the narrow sheet  9 B does not exist between the end separate rollers  65 A and  67 A, and  65 D and  67 D, whereas it exists between the inner separate rollers. The difference between the pressure-contact load (about 4 N) applied to the narrow sheet  9 B and the pressure-contact load (5 N) applied to the wide sheet  9 A is subtle. 
     Thus, in the sheet transport device  6 , because variations in the pressure-contact load applied from the respective separate roller pairs are small, it is possible to stably feed the sheet  9  ( 9 A,  9 B), transported by the pre-transfer sheet-transport rollers  61 , to the second-transfer position TP 2 , serving as the transport destination, regardless of the length of the sheet  9  in the width direction E, which is a direction intersecting the transport direction D. 
     More specifically, neither the wide sheet  9 A nor the narrow sheet  9 B is subjected to pressure-contact loads significantly varying among the multiple separate roller pairs of the pre-transfer sheet-transport rollers  61  when transported. Hence, whether the wide sheet  9 A or the narrow sheet  9 B is transported, there is no risk of the sheet being damaged due to excessively large pressure-contact loads applied from some separate roller pairs during transportation or risk of a transport defect due to lack of transportation force, which is caused by excessively small pressure-contact loads applied by some separate roller pairs. 
     In the sheet transport device  6 , as shown in  FIGS. 6A and 6B , when a wide sheet  9 A is transported, the pre-transfer sheet-transport rollers  61  can feed the wide sheet  9 A such that the middle portion thereof in the width direction E projects toward one surface ( 9   c ). Two-dot chain straight lines in  FIGS. 6A and 6B  show, for reference, a sheet  9  that is not curved in the width direction E, but is in a flat state. 
     Specifically, in the pre-transfer sheet-transport rollers  61  of the sheet transport device  6 , the inner separate rollers  67 B and  67 C of the second roller  63 , which have a relatively large outside diameter (K 1 ), press the middle portion of the wide sheet  9 A in the width direction E toward the inner separate rollers  65 B and  65 C of the first roller  62  with a large force corresponding to the large outside diameter thereof. At the same time, the inner separate rollers  65 B and  65 C of the first roller  62 , which have a relatively low hardness (J 1 ) and thus are likely to be elastically deformed because of their lower hardness, accept the pressed state. As a result, when the wide sheet  9 A is transported by the pre-transfer sheet-transport rollers  61 , the middle portion thereof in the width direction E is curved so as to project toward one surface ( 9   c ) side. 
     In the image forming apparatus  1  including the sheet transport device  6 , the first roller  62  of the pre-transfer sheet-transport rollers  61  is disposed on the side to be in contact with the surface  9   c  of a sheet  9  to which an unfixed toner image is to be transferred (transfer target surface). Hence, the wide sheet  9 A is transported from the sheet-transport rollers  61  as follows. 
     Specifically, as shown in  FIGS. 7A to 7C  in a chronological order, when the wide sheet  9 A is fed, first, the middle portion of the transfer target surface  9   c  in the width direction E comes into contact with a substantially flat outer surface  31   a  of the intermediate transfer belt  31  of the intermediate transfer part  3 , then, the ends of the transfer target surface  9   c  in the width direction E gradually approach and come into contact with the substantially flat outer surface  31   a  of the intermediate transfer belt  31 , and finally, the entire transfer target surface  9   c  becomes in flat contact with the outer surface  31   a  of the intermediate transfer belt  31 , and is fed to a portion serving as the second-transfer position TP 2 , at which the intermediate transfer belt  31  and (the second transfer roller of) the second transfer device  35  are in contact with each other. 
     As a result, in the sheet transport device  6 , it is possible to prevent the wide sheet  9 A transported by the pre-transfer sheet-transport rollers  61  from being creased or causing a transfer defect at the second-transfer position TP 2 , serving as the transport destination, as a result of, for example, the sheet  9 A being fed with wave-like deformation in the width direction E. Hence, the image forming apparatus  1  can properly perform image formation, without causing creases or a transfer defect at the second-transfer position TP 2 . 
     Comparison Example 
     In Comparison Example 1, as shown in  FIG. 8A , instead of the pre-transfer sheet-transport rollers  61 , for example, pre-transfer sheet-transport rollers  610 A in which all four separate rollers  65 A,  65 B,  65 C, and  65 D of the first roller  62  have the same hardness, J 2 , and the outside diameter K 1  of the inner separate rollers  67 B and  67 C of the second roller  63  is greater than the outside diameter K 2  of the end separate rollers  67 A and  67 D (K 1 &gt;K 2 ) are used. 
     In the pre-transfer sheet-transport rollers  610 A, when a pressure P of 10 N is applied to the ends of the second shaft  66  of the second roller  63  (or the bearings  72 ) with the pressure-applying parts  75 , the pressure-contact loads between the end separate rollers  65 A and  67 A, and  65 D and  67 D of the first roller  62  and the second roller  63  are both substantially 3 N, and the pressure-contact loads between the inner separate rollers  65 B and  67 B, and  65 C and  67 C are both substantially 7 N. 
     In Comparison Example 1, the pressure-contact loads between the end separate rollers and the pressure-contact loads between the inner separate rollers of the sheet-transport rollers  610 A significantly differ, which may cause failure to stably feed sheets  9  to the second-transfer position TP 2 , serving as the transport destination. 
     For example, when a wide sheet  9 A is transported, because the middle portion thereof in the width direction E is subjected to a higher pressure-contact load than the ends, the middle portion is damaged, which potentially causes creases or a transfer defect at the second-transfer position TP 2 , serving as the transport destination. When a narrow sheet  9 B is transported, because the sheet is subjected to an excessive pressure-contact loads from the inner separate rollers  65 B and  67 B, and  65 C and  67 C, the entire sheet is likely to be damaged, which also potentially causes creases or a transfer defect at the second-transfer position TP 2 , serving as the transport destination. 
     In Comparison Example 2, as shown in  FIG. 8B , instead of the pre-transfer sheet-transport rollers  61 , for example, pre-transfer sheet-transport rollers  610 B in which all four separate rollers  67 A,  67 B,  67 C, and  67 D of the second roller  63  have the same outside diameter, K2, and the hardness J1 of the inner separate rollers  65 B and  65 C of the first roller  62  is lower than the hardness J2 of the end separate rollers  65 A and  65 D (J1&lt;J2) are used. 
     In the pre-transfer sheet-transport rollers  610 B, when a pressure P of 10 N is applied to the ends of the second shaft  66  of the second roller  63  (or the bearings  72 ) with the pressure-applying parts  75 , the pressure-contact loads between the inner separate rollers  65 B and  67 B, and  65 C and  67 C of the first roller  62  and the second roller  63  are both substantially 4 N, and the pressure-contact loads between the end separate rollers  65 A and  67 A, and  65 D and  67 D are both substantially 6 N. 
     In Comparison Example 2, the pressure-contact loads between the inner separate rollers and the pressure-contact loads between the end separate rollers of the sheet-transport rollers  610 B significantly differ, which may cause failure to stably feed sheets  9  to the second-transfer position TP 2 , serving as the transport destination. 
     For example, when a wide sheet  9 A is transported, because the middle portion thereof in the width direction E is subjected to a lower pressure-contact load than the ends, the transportation force (transport speed) with respect to the middle portion thereof is lower than that with respect to the ends, which potentially causes creases or a transfer defect at the second-transfer position TP 2 , serving as the transport destination. When a narrow sheet  9 B is transported, although the sheet is not subjected to an excessive pressure-contact load from the inner separate rollers  65 B and  67 B, and  65 C and  67 C, low pressure-contact loads may result in insufficient transportation force. 
     In Comparison Example 3, instead of the pre-transfer sheet-transport rollers  61 , for example, pre-transfer sheet-transport rollers in which all four separate rollers  65 A,  65 B,  65 C, and  65 D of the first roller  62  have the same hardness, J 2 , and all four separate rollers  67 A,  67 B,  67 C, and  67 D of the second roller  63  have the same outside diameter, K 2 , are used. 
     As shown in  FIG. 9A , when a wide sheet  9 A is transported with the aforementioned pre-transfer sheet-transport rollers, the wide sheet  9 A may be fed such that the leading-end portion  9   a  thereof in the transport direction D is deformed in a wave-like shape in the width direction E. In this case, if the leading-end portion  9   a  of the wide sheet  9 A or the ends at the trailing end thereof first come into contact with the substantially flat outer surface  31   a  of the intermediate transfer belt  31  of the intermediate transfer part  3 , the middle portion thereof, which is deformed in a wave-like shape, may come into contact with the outer surface  31   a  of the intermediate transfer belt  31 . This causes creases or a transfer defect at the second-transfer position TP 2 , serving as the transport destination. 
     In Comparison Example 4, pre-transfer sheet-transport rollers, serving as the pre-transfer sheet-transport rollers  61 , in which the second roller  63  is disposed on the side to be in contact with the surface (transfer target surface)  9   c  of the sheet  9  to which an unfixed toner image is transferred are used. 
     As shown in  FIG. 9B , when a wide sheet  9 A is transported with the aforementioned pre-transfer sheet-transport rollers, the wide sheet  9 A is fed such that the middle portion thereof in the width direction E is curved so as to project toward the surface ( 9   d ) side, which is opposite to the transfer target surface  9   c . As a result, the leading-end portion  9   a  of the wide sheet  9 A or the ends at the trailing end thereof first come into contact with the substantially flat outer circumferential surface  31   a  of the intermediate transfer belt  31  of the intermediate transfer part  3 , and then, the middle portion thereof gradually approaches and comes into contact with the outer surface  31   a  of the intermediate transfer belt  31 . Hence, in particular, a portion of the middle portion thereof remain away from the outer surface  31   a  of the intermediate transfer belt  31  (that is, the portion remains as strain). This causes creases or a transfer defect at the second-transfer position TP 2 , serving as the transport destination. 
     Other Exemplary Embodiments 
     In the first exemplary embodiment, the pre-transfer sheet-transport rollers  61  include the first roller  62  having the four separate rollers  65 A to  65 D, and the second roller  63  having the four separate rollers  67 A to  67 D. However, the sheet-transport rollers  61  may include a first roller  62  having three or five or more separate rollers  65  and a second roller  63  having three or five or more separate rollers  67 . 
     Furthermore, although the first roller  62  and the second roller  63  each have two end separate rollers disposed at the ends of the shaft  64  or  66 , depending on the necessity, the first roller  62  and the second roller  63  may each have four or more separate rollers disposed at the ends of the shaft  64  of  66 . 
     Although the three or more separate rollers of the first roller  62  and the second roller  63  have the same width, i.e., the length in the axial direction, depending on the necessity, the width of a part of separate roller may differ from those of the others. 
     In the first exemplary embodiment, a configuration example in which the sheet transport device  6  is applied to the image forming apparatus  1  that uses the intermediate transfer part  3  (intermediate-transfer method) has been shown. However, the sheet transport device  6  may also be applied to an image forming apparatus that does not use the intermediate transfer part  3  (intermediate-transfer method). In that case, a photoconductor, such as the photoconductive drum  21 , that carries an unfixed toner image serves as an image carrier. In that case, the sheet transport device  6  transports a sheet  9  to a transfer position between the photoconductor, such as the photoconductive drum  21 , and a transfer device. 
     Other examples of the image forming apparatus to which the sheet transport device  6  is applied include, besides image forming apparatuses that employ an image recording method in which toner images are formed of developer, image forming apparatuses that use other image recording methods in which, for example, images are formed of other materials, such as ink. In that case, the sheet transport device  6  transports a sheet  9  to a print position at which ink droplets are discharged from an image-forming part (print head) to print an image. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.