Abstract:
At least one of a plurality of rollers supporting an endless belt includes a flange arranged on an end portion of the roller. The flange includes a cylindrical portion that is inserted into an opening portion of the roller at its end. Outer circumference of the cylindrical portion makes contact with inner circumference of the roller, and inner circumference of the flange makes contact with a rotation shaft of the roller. The inner edge of a surface where the flange is in contact with the rotation shaft is located on the inner side of the outer edge of a surface where the cylindrical portion is in contact with the roller in the longitudinal direction of the rotation shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2007-174743 filed in Japan on Jul. 03, 2007. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a belt unit and an image forming apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    A belt used in a belt unit of image forming apparatuses sometimes drifts. For controlling and preventing such belt drifting, various technologies are developed. For example, Japanese Patent No. 3402896 discloses a conventional technology for preventing a drifting of a conveyor belt in a thrust direction by using flanges. Specifically, the flanges having outer diameters larger than that of a driven roller around which the conveyor belt is extended are arranged on both ends of the driven roller such that the conveyor belt is sandwiched by the flanges. Therefore, the drifting of the conveyor belt in the thrust direction can be prevented by the flanges. Furthermore, Japanese Patent Application Laid-Open No. 2000-155480 discloses another conventional technology for preventing the belt drifting by arranging flanges with springs. Specifically, the flanges are arranged on both ends of a shaft of a driven roller and connected to side plates via springs. The springs apply spring pressure to the flanges in the thrust direction, and thereby the belt drifting can be prevented. 
         [0006]    However, the flanges in the former conventional technology are formed in general circular shapes although they are formed to have the outer diameters larger than that of the driven roller. Therefore, if the conveyor belt drifts during its rotation and side edges of the conveyor belt hit the flanges, the flanges may be removed from the roller. The flange is generally made of resin by injection molding instead of metal for realizing cost reduction and weight reduction. Such a flange can be easily fitted into the roller; however, the flange often causes moment toward a direction away from the conveyor belt, causing the flange to be removed from the roller. Furthermore, with the latter conventional technology, it is necessary to have a space for arranging the springs in the thrust direction. Therefore, it is difficult to reduce a size of the belt unit. Moreover, necessary components increase because of the springs, increasing necessary costs. 
         [0007]    Still another conventional technology is known as shown in  FIG. 12 , which is a cross sectional top view of flanges  23 . Specifically, the flanges  23  are arranged on both ends of a roller  31  and each of the flanges  23  includes a pair of cylindrical portions  23   a  arranged opposite to each other across a rotation shaft  32  (hereinafter, “shaft  32 ”). An outer periphery of the cylindrical portion  23   a  is in contact with an inner periphery of the roller  31  thereby the cylindrical portion  23   a  supports the roller  31 . Similarly, an inner periphery of the flange  23  is in contact with the shaft  32  thereby the flange  23  supports the shaft  32 . Thus, the flanges  23  can prevent a drifting of a belt  8 . However, although the flanges  23  have outer diameters larger than that of the roller  31 , their thickness is relatively thin and uniform in a radial direction. Therefore, if the roller  31  is subjected to external force from the belt  8  extended around the roller  31 , the flanges  23  may be removed from the roller  31  and thereby side edges of the belt  8  cannot be controlled at a predetermined position. 
         [0008]    More specifically, the flanges  23  are provided by impaction (injection) of the cylindrical portions  23   a  into the roller  31 . As shown in  FIG. 13 , the roller  31  with the flanges  23  is mounted on a belt unit in an image forming apparatus such that both ends of the shaft  32  of the roller  31  are mounted on side plates  15  via springs  16 . The springs  16  apply spring pressure to the shaft  32  to apply tension to the belt  8 . With this configuration, a line of action of force Fa does not match a line of action of force Fb. Therefore, moment toward a direction indicated by an arrow in  FIG. 13  occurs on the flange  23  and the cylindrical portions  23   a . The moment about a point of an outer edge Bout causes force toward a direction along which the flange  23  is removed from the roller  31 . When the roller  31  rotates, the force is repeatedly applied as load onto the flange  23 , resulting in causing the flange  23  to be removed from the roller  31  at an endurance time. For preventing such removal of the flange  23 , it is possible to attach an E ring to the shaft  32  at a position farther from a center position of the shaft  32  than the flange  23  in the thrust direction. However, operations for attaching the E ring are necessary, increasing an operation procedure, and the E ring causes a problem such as friction of a sliding surface between the flange  23  and the E ring. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to at least partially solve the problems in the conventional technology. 
         [0010]    According to an aspect of the present invention, there is provided a belt unit including an endless belt and a plurality of rollers supporting the endless belt. At least one of the rollers includes a flange arranged on an end portion of the roller. The flange includes a cylindrical portion that is inserted into an opening portion of the roller at its end. The outer circumference of the cylindrical portion makes contact with inner circumference of the roller, and the inner circumference of the flange makes contact with a rotation shaft of the roller. The inner edge of a surface where the flange is in contact with the rotation shaft is located on inner side of the outer edge of a surface where the cylindrical portion is in contact with the roller in the longitudinal direction of the rotation shaft. 
         [0011]    Furthermore, according to another aspect of the present invention, there is provided an image forming apparatus comprising a belt unit that includes an endless belt and a plurality of rollers supporting the endless belt. At least one of the rollers includes a flange arranged on an end portion of the roller. The flange includes a cylindrical portion that is inserted into an opening portion of the roller at its end. The outer circumference of the cylindrical portion makes contact with inner circumference of the roller, and the inner circumference of the flange makes contact with a rotation shaft of the roller. The inner edge of a surface where the flange is in contact with the rotation shaft is located on inner side of the outer edge of a surface where the cylindrical portion is in contact with the roller in the longitudinal direction of the rotation shaft. 
         [0012]    The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of an image forming apparatus according to an embodiment of the present invention; 
           [0014]      FIG. 2  is a perspective view of a roller in a belt unit shown in  FIG. 1 ; 
           [0015]      FIG. 3  is a perspective view of a flange to be attached to the roller shown in  FIG. 2 ; 
           [0016]      FIG. 4  is a schematic diagram of the flange in a belt unit according to the first embodiment; 
           [0017]      FIG. 5  is a schematic diagram of a flange in a belt unit according to a second embodiment of the present invention; 
           [0018]      FIG. 6  is a schematic diagram of a flange in a belt unit according to a third embodiment of the present invention; 
           [0019]      FIG. 7  is a schematic diagram for explaining a state where the belt unit shown in  FIG. 6  is mounted on side plates; 
           [0020]      FIGS. 8A and 8B  are schematic diagrams for explaining a level of noncontact between side edges of a belt and a roller when the roller is deformed by a force applied from the flange shown in  FIG. 6 ; 
           [0021]      FIGS. 9A and 9B  are schematic diagrams for explaining a level of deflection of a roller deformed by a force applied from a belt shown in  FIG. 6 ; 
           [0022]      FIG. 10  is a schematic diagram of a flange in a belt unit according to a fourth embodiment of the present invention; 
           [0023]      FIG. 11  is a schematic diagram of a flange in a belt unit according to a fifth embodiment of the present invention; 
           [0024]      FIG. 12  is a schematic diagram of a conventional belt unit; and 
           [0025]      FIG. 13  is a schematic diagram for explaining a state where the belt unit shown in  FIG. 12  is mounted on side plates. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. The same reference numerals are assigned to the same components in both the above conventional technologies and the below embodiments. 
         [0027]      FIG. 1  is a schematic diagram of an image forming apparatus according to a first embodiment of the present invention. An exposing unit  3  irradiates image forming units  4 ,  5 ,  6 , and  7  with a light, so that the image forming units  4 ,  5 ,  6 , and  7  form color toner images, respectively. The color toner images are sequentially superimposed onto the belt  8  that is an endless belt. A transfer sheet stacked in a sheet cassette  1  is fed by a feed roller  2  and conveyed to a registration roller  9 . Then, the toner image is transferred onto the transfer sheet by a secondary transfer roller  10 . A fixing unit  11  fixes a transferred image on the transfer sheet, and the transfer sheet is discharged by a discharge roller  12 . Thus, an output image is obtained from the image forming apparatus. A residual toner on the belt  8  is removed by a cleaning unit  13 . Waste toners removed by the cleaning unit  13  and discharged from the image forming units  4 ,  5 ,  6 , and  7  are accumulated in a waste toner container  14 . 
         [0028]    The belt  8  serving as a main component in a belt unit is extended between rollers  30  and  31 .  FIG. 2  is a perspective view of the roller  31  with the shaft  32  and a flange  33 .  FIG. 3  is a schematic diagram of the flange  33 , in which a reference letter “I” indicates an inner periphery of the flange  33  with which the shaft  32  is supported and a reference letter “O” indicates an outer periphery of the flange  33  with which the roller  31  is supported. 
         [0029]    The roller  30  is rotated by a driving force from a driving motor (not shown). The shaft  32  supports the roller  31  and a predetermined spring pressure is applied to the shaft  32  by a spring (not shown). The belt  8  is extended between the rollers  30  and  31  with a predetermined tension so that the belt  8  rotates along with a rotation of the roller  30 . The flange  33  is arranged on each of end portions of the roller  31 .  FIG. 4  is a schematic diagram of the flange  33  cut in half along the shaft  32 . The flange  33  includes a cylindrical portion  33   a  in an integrated manner. The cylindrical portion  33   a  is also arranged on other half of the flange  33  (not shown) such that the cylindrical portions  33   a  face each other across the shaft  32 . An outer periphery of the cylindrical portion  33   a  is in contact with an inner periphery of the roller  31  to support the roller  31 . Similarly, the inner periphery of the flange  33  is in contact with the shaft  32  to support the shaft  32 . 
         [0030]    The flange  33  is configured in such a manner that an outer diameter is larger than an outer diameter of the roller  31  and a radial thickness of a portion on a side of an outer periphery from the cylindrical portion  33   a  is substantially two-thirds of a radial thickness of a portion on a side of an inner periphery from the cylindrical portion  33   a . A radial thickness of the cylindrical portion  33   a  is set narrower than that of the flange  33  and slightly narrower than that of the roller  31 . An inner edge of the cylindrical portion  33   a  is bent toward the shaft  32  at a predetermined position. In the example shown in  FIG. 4 , Ain indicates an inner edge of a surface where the inner periphery of the flange  33  is in contact with the shaft  32 , Aout indicates an outer edge of the surface where the inner periphery of the flange  33  is in contact with the shaft  32 , Bin indicates an inner edge of a surface where the outer periphery of the cylindrical portion  33   a  is in contact with the roller  31  (a base point from which the cylindrical portion  33   a  is bent), and Bout indicates an outer edge of the surface where the outer periphery of the cylindrical portion  33   a  is in contact with the roller  31 . As shown in  FIG. 4 , the Ain is closer to a center position of the shaft  32  in a longitudinal direction than the Bout. 
         [0031]    The roller  31  having the above configuration is arranged in the same manner as shown in  FIG. 13 , in which end portions of the shaft  32  are mounted on side plates via springs. At this state, the cylindrical portion  33   a  is subjected to force Fb equivalent to a tension of the belt  8  from the outer periphery being in contact with the roller  31  in a direction indicated by an arrow shown in  FIG. 4 . Furthermore, the flange  33  is subjected to force Fa equivalent to a spring pressure from the inner periphery being in contact with the shaft  32  in a direction opposite to that of Fb. However, because the Ain of the flange  33  is closer to the center position of the shaft  32  in the longitudinal direction than the Bout of the cylindrical portion  33   a , a point of action of the Fa from the shaft  32  is also shifted to a position closer to the same center position. As a result, moment caused by the above forces can be reduced. Therefore, the flange  33  is not easily removed from the roller  31 . The roller  31  is consistently stabilized at a position where the force Fa and the force Fb are balanced with each other. Thus, it is possible to prevent the flange  33  from being removed from the roller  31  without increasing an operation procedure. 
         [0032]    The flange  33  serves as an intermediary between the rotatable roller  31  and the non-rotatable shaft  32 , and rotates along with the roller  31 . The flange  33  controls drifting of the belt  8  in an axial direction of the roller  31  in the belt unit. Specifically, when the belt  8  drifts in the axial direction of the roller  31 , an edge of the belt  8  hits an edge of the flange  33  because the flange  33  has an outer diameter larger than that of the roller  31 . Therefore, further drifting of the belt  8  in the axial direction of the roller  31  can be prevented. 
         [0033]      FIG. 5  is a schematic diagram of a flange  43  in a belt unit according to a second embodiment of the present invention. The flange  43  includes a cylindrical portion  43   a  and is arranged such that a portion on a side of an outer periphery from the cylindrical portion  43   a  is further from the center position of the shaft  32  in the longitudinal direction than a portion on a side of an inner periphery from the cylindrical portion  43   a . Thus, an inner edge Ain and an outer edge Aout of a surface where the flange  43  is in contact with the shaft  32  are arranged closer to the center position of the shaft  32  in the longitudinal direction than an outer edge Bout of a surface where the cylindrical portion  43   a  is in contact with the roller  31 . Furthermore, the flange  43  and the cylindrical portion  43   a  are formed with the same thicknesses. 
         [0034]    As described above, because the flange  43  is arranged in such a manner that the inner edge Ain and the outer edge Aout are arranged on positions closer to the center position of the shaft  32  in the longitudinal direction than the outer edge Bout, the point of action of the force Fa applied from the shaft  32  is also shifted toward the same center position. Therefore, it is possible to suppress moment caused by the above force. Thus, the flange  43  is not easily removed from the roller  31 . Furthermore, it is possible to form the flange  43  with an even thickness. 
         [0035]      FIG. 6  is a schematic diagram of a flange  53  in a belt unit according to a third embodiment of the present invention. The flange  53  includes a cylindrical portion  53   a  and is arranged such that a portion on a side of an outer periphery from the cylindrical portion  53   a  is much farther from the center position of the shaft  32  in the longitudinal direction than the portion on the side of the outer periphery from the cylindrical portion  43   a  shown in  FIG. 5 . Therefore, an outer edge Bout of a surface where an outer periphery of the cylindrical portion  53   a  is in contact with the roller  31  is farther from the center position of the shaft  32  in the longitudinal direction than an outer edge Aout of a surface where an inner periphery of the flange  53  is in contact with the shaft  32 . Furthermore, a center position between an inner edge Ain and the outer edge Aout of a surface where the inner periphery of the flange  53  is in contact with the shaft  32  matches a center position between an inner edge Bin and the outer edge Bout of a surface where the outer periphery of the cylindrical portion  53   a  is in contact with the roller  31 . 
         [0036]    As described above, because the center position between the inner edge Ain and the outer edge Aout matches the center position between the inner edge Bin and the outer edge Bout, the point of action of the force Fa applied from the shaft  32  is shifted to a position corresponding to a point of action of the force Fb applied from the roller  31 . Therefore, it is possible to eliminate a chance of occurrence of moment. Thus, it is possible to prevent the flange  53  from being removed from the roller  31  at an endurance time. It is also applicable to bond the roller  31  and the cylindrical portion  53   a . At this state, it is more assuredly prevent the flange  53  from being removed from the roller  31 . 
         [0037]      FIG. 7  is a schematic diagram for explaining an arrangement of the flange  53  in the belt unit with which moment does not occur. A position of the force Fa applied from the shaft  32  to the flange  53  is adjusted to support the force Fb applied from the belt  8  so that moment in a direction along which the flange  53  is removed from the roller  31  does not occur. 
         [0038]      FIGS. 8A and 8B  are schematic diagrams of the rollers  31  deformed by a force applied from the flange  53 . In the example shown in  FIG. 8B , the flange  53  is arranged such that a center position of a surface where the flange  53  is in contact with the shaft  32  corresponds to each of side edges of the belt  8 . Therefore, the level of noncontact between the side edges of the belt  8  and the roller  31  can be smaller than that in the example shown in  FIG. 8A . Thus, the belt unit shown in  FIG. 8B  can attain a stable belt rotation. 
         [0039]      FIGS. 9A and 9B  are schematic diagrams of the rollers  31  deformed by a force applied from the belt  8 . In the example shown in  FIG. 9B , the flange  53  is supported at a position closer to a center position of the shaft  32  in the longitudinal direction than the side edge of the belt  8 . Therefore, the belt unit shown in  FIG. 9B  can suppress the level of deflection of the roller  31  than that in the example shown in  FIG. 9A . As a result, the belt unit shown in  FIG. 9B  can attain a stable belt rotation. 
         [0040]      FIG. 10  is a schematic diagram of a flange  63  and a belt unit according to a fourth embodiment of the present invention. A reinforcing tape  57  is attached to a back surface of each of side edges of the belt  8 . The flange  63  is arranged in substantially the same manner as those in the first and the second embodiments. Specifically, a center position of an inner edge Ain and an outer edge Aout of a surface where an inner periphery of the flange  63  is in contact with the shaft  32  is within a width of the reinforcing tape  57  in the longitudinal direction of the shaft  32 . In the example shown in  FIG. 10 , the center position matches a center position of the width of the reinforcing tape  57 . 
         [0041]    As described above, the reinforcing tape  57  is attached to the back surface of each of the side edges of the belt  8 , and the flange  63  is arranged such that the center position of the inner edge Ain and the outer edge Aout is within a width of the reinforcing tape  57  in the longitudinal direction of the shaft  32 . Therefore, the roller  31  is in contact with the reinforcing tape  57  and the flange  63  can support the force Fb applied from the belt  8  to the roller  31  at a position corresponding to the reinforcing tape  57 . Thus, it is possible to suppress the level of deflection of the roller  31  and suppress occurrence of moment. Furthermore, as shown in  FIG. 10 , if the center position between the inner edge Ain and the outer edge Aout is set to a position corresponding to the center position of the width of the reinforcing tape  57 , a line of action of the force Fa matches a line of action of the force Fb. As a result, it is possible to eliminate a chance of occurrence of moment. Thus, it is possible to prevent the flange  63  from being removed from the roller  31  by suppressing and eliminating the occurrence of moment. 
         [0042]      FIG. 11  is a schematic diagram of a flange  73  and a belt unit according to a fifth embodiment of the present invention. The fifth embodiment is based on the fourth embodiment. That is, a clearance groove  58  of the reinforcing tape  57  is formed on an outer periphery of each of end portions of the roller  31 . The flange  73  is arranged in substantially the same manner as that in the third embodiment. The flange  73  is arranged such that an outer portion on a side of an outer periphery of the flange  73  from a cylindrical portion  73   a  is farther from the center position of the shaft  32  in the longitudinal direction than an inner portion on a side of an inner periphery of the flange  73  from the cylindrical portion  73   a . Because the clearance groove  58  is arranged, a radial thickness of the outer portion is set to be substantially twice a radial thickness of the flange  73 . Therefore, a center position between the inner edge Ain and the outer edge Aout is closer to the center position of the shaft  32  in the longitudinal direction than an inner edge  58   a  of the clearance groove  58 . In the example shown in  FIG. 11 , the center position between the Ain and the Aout corresponds to a position of the inner edge  58   a.    
         [0043]    As described above, the reinforcing tape  57  is attached to the back surface of each of the side edges of the belt  8 , the clearance groove  58  with a depth deeper than the thickness of the reinforcing tape  57  and with a width wider than the width of the reinforcing tape  57  is arranged on the outer periphery of each of the end portions of the roller  31 , and the center position of the Ain and the A out is closer to the center position of the shaft  32  in the longitudinal direction than the inner edge  58   a  of the clearance groove  58 . Therefore, the flange  73  can support the force Fb applied from the inner edge  58   a  of the clearance groove  58  to the roller  31  at a position of the inner edge  58   a . Thus, it is possible to suppress the level of deflection of the roller  31  and suppress the occurrence of moment. Furthermore, as shown in  FIG. 11 , if the position of the inner edge Ain corresponds to the position of the inner edge  58   a , the line of action of the force Fa matches the line of action of the force Fb. Therefore, it is possible to eliminate the chance of occurrence of moment. Thus, it is possible to prevent the flange  73  from being removed from the roller  31  by suppressing and eliminating the occurrence of moment. According to the fifth embodiment, the clearance groove  58  is formed on the outer periphery of each of the end portions of the roller  31  by removing end portions of the roller  31 . A groove can be formed easily by the above method; however, it is also applicable to directly form a concave groove and grooves in other shapes. 
         [0044]    The above described image forming apparatuses are only examples and the present invention in its broader aspects is not limited to the specific details described in the embodiments. Thickness, shapes, and the like of the flange and the cylindrical portion can be changed and modified as appropriate within the spirit and the scope of the general inventive concept as defined by the appended claims and their equivalents. Furthermore, the endless belt can be a conveyor belt (direct transfer belt) that conveys a transfer sheet, a photosensitive element belt, and the like instead of the intermediate transfer belt described as an example in the embodiments. Moreover, the belt unit can be employed in other devices instead of the image forming apparatus. 
         [0045]    As described above, according to an aspect of the present invention, the flange is configured to support the roller and the shaft with the outer periphery of the cylindrical portion and the inner periphery of the flange. Therefore, it is possible to suppress moment that is caused by a force applied from a supporting portion of the roller and the force applied from a supporting portion of the shaft and rotates the flange in a direction perpendicular to a direction of rotating the roller. Thus, it is possible to prevent the flange from being removed from the roller. 
         [0046]    Furthermore, according to another aspect of the present invention, it is possible to form the flange with an even thickness. 
         [0047]    Moreover, according to still another aspect of the present invention, it is possible to eliminate a chance of occurrence of moment. Thus, removal of the flange can be more effectively prevented. 
         [0048]    Furthermore, according to still another aspect of the present invention, it is possible to suppress a deflection of the belt by using the flange to be rotated with the roller in an integrated manner. Moreover, even when the roller is deformed by the force applied from the flange, the level of noncontact between the side portions of the endless belt and the roller can be suppressed. Thus, it is possible to maintain the stable belt rotation. Furthermore, the amount of deformation by a force applied from the endless belt to the roller due to tension of the endless belt can be suppressed. 
         [0049]    Moreover, according to still another aspect of the present invention, the roller is configured to contact with the reinforcing tape attached to the endless belt, and the flange can support the force applied from the endless belt to the roller at a position of the reinforcing belt. Therefore, it is possible to suppress the level of deflection of the roller and reduce the level of moment. Furthermore, the flange can support the force applied from the inner edge of the clearance groove to the roller at a position of the inner edge. Therefore, the level of deflection of the roller can be suppressed, the level of moment can be reduced, and the removal of the flange can be prevented. Moreover, it is possible to provide the image forming apparatus including the above belt unit. 
         [0050]    Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.