Patent Publication Number: US-9417587-B2

Title: Image forming apparatus having belt unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Applications No. 2014-072767 filed Mar. 31, 2014 and No. 2014-072768 filed Mar. 31, 2014. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus having a mechanism for restraining meandering and deviated running. 
     BACKGROUND 
     Among various conventional image forming apparatuses known in the art, one technology discloses an image forming apparatus having a mechanism for restraining meandering and deviated running of a belt such as a sheet conveying belt or an intermediate transfer belt. Specifically, according to the image forming apparatus, an abutting rib provided on an inner surface of the belt is engaged with an annular groove formed at an end portion of a tension roller to which the belt is mounted. Further, the image forming apparatus is provided with a detachable tray for accommodating a process cartridge movably provided, and a roller member provided in the detachable tray. The roller member is adapted to press an outer surface of the belt when the abutting rib is about to be moved onto an outer peripheral surface of the tension roller after the rib is disengaged from the annular groove. 
     There is known an image forming apparatus having a configuration that prevents meandering and deviated running of a belt such as a sheet conveying belt or an intermediate transfer belt. Specifically, this image forming apparatus has a guide rib provided on an inner surface of the belt, and the guide rib is engaged in a groove formed at an end portion of a roller over which the belt is mounted. Further, this image forming apparatus has a cleaning blade for cleaning the belt applying a uniform pressing force to the entire width of the belt in an axial direction of a roller to prevent the guide rib from separating from the groove and running on an outer peripheral surface of the roller. 
     SUMMARY 
     As described above, in the above conventional image forming apparatus, the cleaning blade applies a uniform pressing force to the entire width of the belt in the roller axial direction. Thus, once the belt starts deviating from the roller due to meandering, the deviation may continue. Furthermore, according to the conventional image forming apparatus, when the guide rib disengages from the groove and runs on the outer peripheral surface of the roller, the belt is sandwiched between the cleaning blade and the roller and is continuously applied to a load. Consequently, the belt may be damaged. 
     In addition, according to the above-described conventional image forming apparatus, the detachable tray provided with the roller member is movable relative to the belt, so that a distance between the roller member and the belt is changed each time the detachable tray is moved. This may cause disengagement of the abutting rib from the annular groove, resulting in failure to prevent meandering and/or deviated running of the belt. 
     According to one aspect, present specification provides a belt unit and an image forming apparatus capable of restraining meandering and/or deviated running of the belt. 
     According to one aspect, the belt unit includes a roller, a first guide, a belt, a second guide, and a regulating portion. The roller extends in an axial direction and defines a radial direction perpendicular to the axial direction. The first guide is provided on a surface of the roller. The first guide is in a form of one of a convex shape and a concave shape. The belt is configured to be looped taut around plurality of the rollers. The second guide is provided on an inner belt surface. The second guide is in a form of one of a concave shape and a convex shape. The first guide and the second guide are engaged with each other with an engagement depth in the radial direction. The regulating portion is opposite to an outer belt surface and provides a gap smaller than the engagement depth. The gap is a length between the regulating portion and the outer belt surface. 
     According to another aspect, the belt unit includes a roller, a first guide, a belt, a second guide, and a regulating portion. The roller extends in an axial direction, defines a radial direction perpendicular to the axial direction, and has a center portion in the axial direction. The first guide is provided on a surface of the roller. The first guide is in a form of a concave shape. The belt is configured to be looped taut around plurality of the rollers. The second guide is in a form of a convex shape configured to engage with the first guide with an engagement depth in the radial direction. The belt has a center belt portion in the axial direction. The regulating portion is opposite to an outer belt surface and has a portion aligned with the first guide in the radial direction. The regulating portion is profiled to provide a gap between the regulating portion and the outer belt surface in the radial direction such that the gap is gradually decreased toward the center belt portion in the axial direction. The gap has a minimum gap length smaller than the engagement depth. 
     According to another aspect, the belt unit includes a roller, a first guide, a belt, a second guide, and a regulating portion. The roller extends in an axial direction and defines a radial direction perpendicular to the axial direction. The first guide is provided on a surface of the roller and has a concave shape. The belt is configured to be looped taut around plurality of the roller and has a center portion in the axial direction. The second guide is provided at an inner belt surface. The second guide is engaged with the first guide with an engagement depth in the radial direction. The first guide and the second guide provide a first gap therebetween in the radial direction. The first gap is increased toward the center portion in the axial direction. The regulating portion is opposite to an outer belt surface and has a portion aligned with the second guide in the radial direction. The regulating portion provides a second gap between the regulating portion and the outer belt surface. The second gap is smaller than the engagement depth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the disclosure will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a printer; 
         FIG. 2  is a lower view of a belt unit according to a first embodiment; 
         FIG. 3  is a cross-sectional view of the belt unit taken along line X-X of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of the belt unit taken along line Y-Y of  FIG. 2 ; 
         FIG. 5  is a lower view of a belt unit according to a second embodiment; 
         FIG. 6  is a cross-sectional view of a right portion of a belt unit according to a third embodiment; 
         FIG. 7  is a cross-sectional view of a right portion of a belt unit according to a fourth embodiment; 
         FIG. 8  is a cross-sectional view of a right portion of a belt unit according to a fifth embodiment; 
         FIG. 9  is a cross-sectional view of a right portion of a belt unit according to a sixth embodiment; 
         FIG. 10  is a cross-sectional view of a right portion of a belt unit according to a seventh embodiment; 
         FIG. 11  is a lower view of a belt unit according to an eighth embodiment; 
         FIG. 12  is a cross-sectional view of the belt unit taken along line X-X of  FIG. 11 ; 
         FIG. 13  is a cross-sectional view of the belt unit taken along line Y-Y of  FIG. 11 ; 
         FIG. 14  is an enlarged view of a cross-section of the belt unit; 
         FIG. 15  is a cross-sectional view of a right portion of a belt unit according to a ninth embodiment; 
         FIG. 16  is a cross-sectional view of a right portion of a belt unit according to a tenth embodiment; 
         FIG. 17  is a cross-sectional view of a right portion of a belt unit according to an eleventh embodiment; 
         FIG. 18  is a cross-sectional view of a right portion of a belt unit according to a twelfth embodiment; 
         FIG. 19  is a cross-sectional view of a right portion of a belt unit according to a thirteenth embodiment; 
         FIG. 20  is a cross-sectional view of a right portion of a belt unit according to a fourteenth embodiment; and 
         FIG. 21  is a cross-sectional view of a right portion of a belt unit according to a fifteenth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A printer according to a first embodiment will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description. 
     The terms “upward”, “downward”, “upper”, “lower”, “above”, “below”, “beneath”, “right”, “left”, “front”, “rear” and the like will be used throughout the description assuming that the printer is disposed in an orientation in which it is intended to be used. In use, the printer is disposed as shown in  FIG. 1 . 
     First Embodiment 
     A printer  1  as an example of an image forming apparatus according to a first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . In the following description, a left side of  FIG. 1  is defined as a front side F of the printer  1 , a front side of  FIG. 1  is defined as a right side R of the printer  1 , and an upper side of  FIG. 1  is defined as an upper side U of the printer  1 . 
     The printer  1  is a direct transfer tandem type printer that can form a color image using developer or coloring materials (e.g., black, yellow, magenta, and cyan). In the following description, in a case of distinguishing the components of the printer  1  on a basis of color, “K (Black)”, “Y (Yellow)”, “M (Magenta)”, and “C (Cyan)” those indicating the color are added to the reference numerals of the respective components. 
     The printer  1  includes, in a main casing  2 , a sheet delivery section  3 , a belt unit  4 , an image forming section  5 , a cleaner  6 , and a discharge tray  7 . The sheet delivery section  3  includes a tray  31 , a pickup roller  32 , and a registration roller  33 . The tray  31  is adapted to accommodate a plurality of printing sheets W. The pickup roller  32  is adapted to feed each printing sheet W from the tray  31 . The registration roller  33  is adapted to feed the sheet W onto the belt unit  4  while performing skew correction. 
     The belt unit  4  includes a drive roller  41 , a support roller  42 , and an endless belt  43  mounted over the drive roller  41  and the support roller  42 . An arrow A in each drawing indicates a moving direction of an endless belt  43 . When the drive roller  41  is driven by a drive motor (not shown), a belt  43  is moved in a clockwise direction in  FIG. 1 , causing the sheet W fed onto the belt  43  to be conveyed to a fixing unit  49  described later. The support roller  42  is an example of a roller. Hereinafter, a surface of the belt  43  facing the drive roller  41  and support roller  42  will be referred to as an inner surface, and a surface of the belt  43  opposite to the drive roller  41  and support roller  42  will be referred to as an outer surface. A detailed configuration of the belt unit  4  will be described later. 
     The image forming section  5  includes four process sections  48 K to  48 C corresponding to respective colors, and the fixing unit  49 . In each process section  48 , a photosensitive body (not shown) is subjected to charging and is exposed to light to form an electrostatic latent image. Then, toner is transferred onto the electrostatic latent image, and the toner image is transferred onto the sheet W. The sheet W on which the image has been formed is then thermally fixed by the fixing unit  49  and is discharged onto the discharge tray  7 . The cleaner  6  including a cleaning roller  6 A is positioned below the belt unit  4 . The cleaner is adapted to collects residual toner or paper dust deposited onto a surface of the belt  43 . The cleaner  6  is an example of a cleaning unit. 
     As illustrated in  FIG. 2 , the belt unit  4  further includes a pair of side frames  44 R and  44 L, a connecting portion  45 , and a bottom frame  46 . Each side frame  44 R,  44 L has a flat plate-like shape elongated in a sub-scanning direction and is disposed on each lateral side (left side and right side) of the belt  43 . The sub-scanning direction corresponds to a frontward/rearward direction of the printer  1 . 
     The support roller  42  extends in a main scanning direction and is supported at each front end portion of each side frame  44 R and  44 L so as to be rotatable about a rotation axis extending in the main scanning direction. Hereinafter, the rotation axis will be referred to as a roller rotation axis Z. The main scan direction corresponds to the extending direction of the rotation axis of the roller, i.e., leftward/rightward direction of the printer  1 . Each side frame  44 R and  44 L is an example of a bearing portion. 
     Specifically, as illustrated in  FIG. 3 , the support roller  42  includes a roller body  51 , a pair of flange sections  52 R and  52 L, and a roller shaft body  53 , and these members are integrally rotated about the roller rotation axis Z. Note that illustration of the left-side flange  52 L is omitted in  FIG. 3 . The roller body  51  has a hollow cylindrical shape whose axis is coincident with the roller rotation axis Z. Each of the flange sections  52 R and  52 L has an annular shape whose axis is coincident with the roller rotation axis Z, and is provided on each lateral end (right end and left end) of the roller body  51 . 
     The right flange section  52 R has a sleeve portion  52 - 1  disposed over the roller shaft body  53  and coaxial therewith, a flange  52 - 2  extending radially outwardly from the sleeve portion  52 - 1 , a large diameter portion  52 - 3  extending from a radially outer end portion of the flange  52 - 2  toward the roller body  51  and having an outer diameter equal to that of the roller body  51 , and a small diameter portion  52 - 4  extending from the flange  52 - 2  in a direction away from the roller body  51  and having an outer diameter smaller than that of the large diameter portion  52 - 3 . An annular stepped guide portion or a guide concave portion  52 A is defined by an entire outer peripheral surface of the small diameter portion  52 - 4  and the flange  52 - 2 . More specifically, a planar surface of the flange section  52 R, the surface being opposite to the roller body  51 , functions as a stepped surface  52 B. 
     As a modification, instead of the annular stepped guide portion  52 A, an annular guide groove is available. For providing the annular guide groove, an annular protrusion protruding radially outwardly from the small diameter portion  52 - 2  is further provided. The annular stepped guide portion  52 A and the annular guide groove are examples of a first guide. 
     The roller shaft body  53  extends through the roller body  51  and the pair of the flange sections  52 R and  52 L, and each axial end portion of the roller shaft body  53  protrudes outward from each flange section  52 R and  52 L. The protruding portions of the roller shaft body  53  are fitted with sleeve members  54 R and  54 L, respectively. Note that illustration of the left sleeve member  54 L is omitted in  FIG. 3 . An annular bearing groove  54 A is formed over an outer peripheral surface of each of the sleeve members  54 R and  54 L. On the other hand, a bearing hole  55  is formed in each of the side frames  44 R and  44 L, and an annular bearing protrusion  55 A protrudes radially inwardly from an inner peripheral surface of the bearing hole  55 . The annular bearing protrusion  55 A is engaged with the annular bearing groove  54 A, whereby the support roller  42  is rotatably supported by the pair of side frames  44 R and  44 L while movement of the support roller  42  in leftward/rightward direction is prevented. 
     A convex-shaped guide rib  60  is formed over an entire periphery of a right end portion of an inner peripheral surface of the belt  43 . The guide rib  60  is an example of a second guide. The guide rib  60  is engaged with the annular stepped guide portion  52 A of the flange section  52 R. With this configuration, even when the belt  43  starts to meander during movement, the guide rib  60  and stepped surface  52 B of the annular stepped guide portion  52 A abut against each other (in other words, the guide rib  60  and stepped surface of the annular stepped guide portion  52 A are engaged with each other) thereby preventing the belt  43  from being deviated in the leftward/rightward direction with respect to the support roller  42 . Incidentally, the drive roller  41  is supported at the rear end portions of the pair of the side frames  44 R and  44 L, respectively, so as to be rotatable about a rotation axis extending in the main scanning direction. 
     As shown in  FIG. 2 , the connecting portion  45  is longer than the belt  43  in the leftward/rightward direction and has each end connected to each front end of each side frames  44 R,  44 L. As shown in  FIGS. 3 and 4 , the bottom frame  46  has a flat plate-like shape and is fixed to the pair of the side frames  44 R and  44 L and the connecting portion  45  so as to cover a lower surface of a front half portion of the belt  43 . More specifically, fixing holes  44 A are formed in the pair of the side frames  44 R and  44 L, respectively, and fixing protrusions  46 A protrude outward from left and right end faces of the bottom frame  46 , respectively. The fixing protrusions  46 A are inserted into the corresponding fixing holes  44 A. 
     Further, as illustrated in  FIG. 4 , a flat plate-like abutting portion  46 B is provided at a front end of the bottom frame  46 . The abutting portion  46 B is in abutment with a rear surface of the connecting portion  45 . As a result, the bottom frame  46  is fixed to and integrated with the pair of the side frames  44 R and  44 L and the connecting portion  45 . The bottom frame  46  is provided for avoiding the user&#39;s direct access to the belt  43 . The bottom frame  46  is an example of a plate. 
     As illustrated in  FIGS. 2 and 3 , a rotary body  61  is provided in the bottom frame  46  so as to be rotatable about a rotation axis extending parallel to the roller rotation axis Z. More specifically, a through-hole  46 C is formed in the bottom frame  46  at a position below the flange section  52 R for positioning the rotary body  61 . Incidentally, instead of the through-hole  46 C, a recess can be formed at an upper surface of the bottom frame  46  for positioning the rotary body  61 . Further, bearings  46 D and  46 D are formed in left and right inner surfaces of the through-hole  46 C. 
     As illustrated in  FIG. 3 , the rotary body  61  includes a body  61 A and shaft portions  61 B and  61 B protruding leftward and rightward from the body  61 A. The body  61 A is a cylindrical roller body whose axis is coincident with the roller rotation axis Z and having a constant diameter over an entire length thereof. The roller body  61 A has a length in leftward/rightward direction substantially the same as a width of the guide rib  60 . The shaft portions  61 B and  61 B are rotatably supported by the bearings  46 D and  46 D, respectively. With this structure, the rotary body  61  is rotated when the belt  43  is circularly moved while contacting with the belt  43 . The rotary body  61  is an example of a rotating member. The bottom frame  46  is an example of a support portion. The rotary body  61  and the bottom frame  46  is an example of a regulating portion. 
     As illustrated in  FIG. 3 , a gap D 1  between an upper surface of the rotary body  61  and outer surface of the belt  43  is smaller than a length D 2  over which the guide rib  60  and annular stepped guide portion  52 A overlap each other in a radial direction of the support roller  42 , i.e., vertical direction in  FIG. 3 . Thus, even when the guide rib  60  is urged to be separated from the annular stepped guide portion  52 A (guide rib  60  comes off the annular stepped guide portion  52 A), the outer surface of the belt  43  abuts the rotary body  61 , thereby preventing the guide rib  60  from coming off from the annular stepped guide portion  52 A. 
     Further, the rotary body  61  is provided at the bottom frame  46  which is supported by the side frame  44  serving as a bearing portion for bearing a shaft of the support roller  42 . Thus, positional relationship and the gap between the outer surface of the belt  43  and the rotary body  61  are less likely to be changed even when the belt unit  4  or the image forming section  5  is moved for replacement, in comparison with a case where the rotary body  61  is provided at a component separated from the belt unit  4  such as the image forming section  5 . Thus, coming off of the guide rib  60  from the annular stepped guide portion  52 A can be prevented. 
     Further, the rotary body  61  is positioned so as to align, in the vertical direction, both the guide rib  60  and a portion  52 C of the large diameter portion  52 - 3 , the portion  52 C being closer to the annular stepped guide portion  52 A than a remaining portion of the large diameter portion  52 - 3  to the annular stepped guide portion  52 A in the direction of the roller rotation axis Z. In other words, the rotary body  61  faces the guide rib  60  and the adjacent portion  52 C through the belt  43 . With this configuration, coming off of the guide rib  60  from the annular stepped guide portion  52 A can be prevented more reliably in comparison with a case where the rotary body  61  is vertically aligned with only one of the guide rib  60  and the adjacent portion  52 C in the direction of the roller rotation axis Z. 
     Further, a component in abutment with the outer surface of the belt  43  is the rotary body  61 . Thus, damage to the belt  43  due to the abutment can be effectively restrained in comparison with a case where a stationary non-rotatable member abuts against the outer surface of the belt  43 . 
     Further, as described above, the rotary body  61  is rotated when being brought into contact with the belt  43  in a state where the rotary body  61  is positioned at the through-hole  46 C of the bottom frame  46 . Thus, in comparison with a configuration shown in  FIG. 7  in which the rotary body is not positioned in the through-hole or recess but is positioned outside of the bottom frame, the rotary body  61  having an enlarged diameter can be used under the condition that the gap D 1  between the bottom frame  46  and outer surface of the belt  43  in  FIG. 3  is equal to that in  FIG. 7 . Therefore, in the first embodiment, a contacting area of the rotary body  61  relative to the belt  43  can be increased to lower a pressing force of the rotary body  61  against the belt  43 , thereby preventing damage to the belt  43 . 
     Further, the rotary body  61  is provided on the outer surface side of a part of the belt  43 , the part not ranging from the image forming section  5  to the cleaner  6  but ranging from the cleaner  6  to the image forming section  5 . In other words, the roller body  51  contacts the belt  43  that has been cleaned by the cleaner  6 . This can prevent the rotary body  61  from being contaminated by coloring materials adhered to the belt  43  and prevent the coloring materials on the contaminated rotary body  61  from being transferred back to the belt  43 . 
     Second Embodiment 
     A belt conveying apparatus according to a second embodiment of the present invention is shown in  FIG. 5 . The second embodiment is the same as the first embodiment except a configuration of the regulation portion. Thus, like parts and components are designated by the same reference numerals as those shown in the first embodiment to avoid duplicating description. 
     As illustrated in  FIG. 5 , a bottom frame  70  has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 3  except for the followings. That is, through-holes  71 R and  71 L are formed at both left and right end portions of the bottom frame  70 . The through-holes  71 R and  71 L are arrayed with each other in the direction of the roller rotation axis Z, and located at substantially symmetrical positions with respect to a widthwise center (leftward/rightward center) position of the belt  43 . Further, each of the through-holes  71 R,  71 L is directed obliquely with respect to the direction of the roller rotation axis Z. 
     Rotary bodies  72 R and  72 L are positioned in the through-holes  71 R and  71 L, respectively. Each of the rotary bodies  72 R and  72 L is contactable with the belt  43 , and has substantially the same shape as that of the rotary body  61 . Thus, the rotary bodies  72 R and  72 L are rotated with movement of the belt  43  in a state where they are positioned in the through-holes  71 R and  71 L, respectively. 
     The rotary body  72 R and the rotary body  72 L have rotation axis ZR and rotation axis ZL, respectively. These rotation axes ZR and ZL are inclined with respect to the moving direction of the belt  43  or with respect to the direction of the movement of the roller rotation axis Z, such that a distance between the axes ZR and ZL is gradually decreased in a movement direction of a lower surface of the belt  43 , i.e., toward the front side of the belt unit  4 . 
     More specifically, each rotary body  72 R,  72 L has an inner axial end face and an outer axial end face in the leftward/rightward direction. Here, with respect to the right rotary body ZR, the rotation axis ZR at the inner axial end face is positioned forward of the roller rotation axis Z in the movement direction of the lower surface of the belt  43 . Thus, the rotary body  72 R applies to the belt  43  a feeding force that feeds the belt  43  in a diagonally rightward direction FR when the rotary body  72 R is rotated along with the movement of the belt  43 . 
     On the other hand, with respect to the left rotary body ZL, the rotation axis ZL at the inner axial end face is positioned forward of the roller rotation axis Z in the movement direction of the lower surface of the belt  43 . Thus, the rotary body  72 L applies to the belt  43  a feeding force that feeds the belt  43  in a diagonally leftward direction FL when the rotary body  72 L is rotated along with the movement of the belt  43 . Thus, a force is applied to the belt  43  by the two rotary bodies  72 R and  72 L, such that the belt  43  is urged toward lateral directions opposite to each other, i.e., toward right edge of the belt  43  and the left edge of the belt  43 . This force can restrain wrinkling of the belt  43  while preventing the guide rib  60  from coming off from the annular stepped guide portion  52 A. 
     Third Embodiment 
     A belt conveying apparatus according to a third embodiment of the present invention is shown in  FIG. 6 . The third embodiment is the same as the first embodiment except a configuration of the regulation portion. Thus, like parts and components are designated by the same reference numerals as those shown in the first embodiment to avoid duplicating description. 
     A bottom frame  80  includes fixing protrusions  80 A similar to the fixing protrusions  46 A, and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 3  except for the followings. That is, a through-hole  80 C is formed in the bottom frame  80 . The through-hole  80 C has a leftward/rightward length substantially the same as the width of the belt  43 . A single rotary body  81  is positioned in the through-hole  80 C, and includes a cylindrical body  81 A and shaft portions  81 B and  81 B each protruding from each lateral end of the cylindrical body  81 A. 
     The body  81 A has a width in leftward/rightward direction substantially the same as the width of the belt  43 . Thus, an end portion of the body  81 A is aligned with the guide rib  60  in leftward/rightward direction. In other words, the end portion of the body  81 A faces a part of the outer surface of the belt  43 , the part corresponding to the guide rib  60 . This can prevent the guide rib  60  from coming off from the annular stepped guide portion  52 A. Further, damage to the belt  43  can be eliminated because enlarged contact area of the body  81 A relative to the belt  43  can be provided. 
     Fourth Embodiment 
     A belt conveying apparatus according to a fourth embodiment of the present invention is shown in  FIG. 7 . The fourth embodiment is the same as the first embodiment except a configuration of the regulation portion. Thus, like parts and components are designated by the same reference numerals as those shown in the first embodiment to avoid duplicating description. 
     A bottom frame  90  has a fixing protrusion  90 A similar to the fixing protrusions  46 A of the first embodiment, and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 3  except for the followings. That is, a pair of support ribs  91  and  91  protrude upward from an upper surface of the bottom frame  90  at a position in alignment with the support roller  42 . Bearings  91 A are formed in the support ribs  91  and  91 , respectively. 
     A rotary body  92  is rotatably supported by the pair of support ribs  91 , 91 . The rotary body  92  includes a cylindrical body  92 A and shaft portions  92 B and  92 B each protruding from each lateral end of the cylindrical body  92 A. The body  92 A has a width (axial length) substantially the same as the width of the guide rib  60 . The shaft portions  92 B and  92 B are rotatably supported by the bearings  91 A and  91 A, respectively. With this configuration, the rotary body  92  is rotated when being brought into contact with the belt  43 . A gap D 1  between an upper surface of the rotary body  92  and outer surface of the belt  43  is smaller than the length D 2  over which the guide rib  60  and the annular stepped guide portion  52 A overlap each other in the radial direction of the support roller  42 . This configuration prevents the guide rib  60  from coming off from the annular stepped guide portion  52 A. 
     Fifth Embodiment 
     A belt conveying apparatus according to a fifth embodiment of the present invention is shown in  FIG. 8 . The fifth embodiment is the same as the first embodiment except for the regulation portion. Thus, like parts and components are designated by the same reference numerals as those shown in the first embodiment to avoid duplicating description. 
     A bottom frame  100  includes a fixing protrusion  100 A similar to the fixing protrusions  46 A and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 3  except for the followings. That is, a pair of support ribs  101  and  101  (only a right support rib  101  is illustrated in  FIG. 8 ) protrude upward from an upper surface of the bottom frame  100 . Bearings  101 A are formed in the support ribs  101  and  101 , respectively. 
     A rotary body  102  is rotatably supported by the pair of support ribs  101 , 101 . The rotary body  102  includes a cylindrical body  102 A and shaft portions  102 B and  102 B each protruding from each end of the body  102 A. The body  102 A has leftward/rightward width substantially the same as the width of the belt  43 . Specifically, an axial end portion of the body  102 A is aligned with the guide rib  60  in leftward/rightward direction. In other words, the axial end portion of the body  102 A faces a part of the outer surface of the belt  43 , the part corresponding to the guide rib  60 . This structure can prevent the guide rib  60  from coming off from the annular stepped guide portion  52 A. Further, damage to the belt  43  can be eliminated because enlarged contact area of the body  102 A relative to the belt  43  can be provided. 
     Sixth Embodiment 
     A belt conveying apparatus according to a sixth embodiment of the present invention is shown in  FIG. 9 . The sixth embodiment is the same as the first embodiment except for the regulation portion. Thus, like parts and components are designated by the same reference numerals as those shown in the first embodiment to avoid duplicating description. 
     A bottom frame  110  includes a fixing protrusion  110 A similar to the fixing protrusions  46 A, and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 3  except for the following. That is, a guide convex portion  111  is integrally protrudes upward from an upper surface of the bottom frame  110 . The guide convex portion  111  has leftward/rightward width substantially the same as that of the guide rib  60 . 
     A gap D 1  between an upper surface of the guide convex portion  111  and outer surface of the belt  43  is smaller than the length D 2  over which the guide rib  60  and annular stepped guide portion  52 A overlap each other in the radial direction of the support roller  42 . This structure can prevent the guide rib  60  from coming off from the annular stepped guide portion  52 A. The bottom frame  110  and guide convex portion  111  are an example of a regulating portion. 
     Seventh Embodiment 
     A belt conveying apparatus according to a seventh embodiment of the present invention is shown in  FIG. 10 . The seventh embodiment is the same as the first embodiment except for the regulation portion. Thus, like parts and components are designated by the same reference numerals as those shown in the first embodiment to avoid duplicating description. 
     A bottom frame  120  includes a fixing protrusion  120 A similar to the fixing protrusions  46 A, and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 3  except for the following point. That is, a guide convex portion  121  is provided on an upper surface of the bottom frame  120  and integrally therewith. The guide convex portion  121  has a leftward/rightward width substantially the same as the width of the belt  43 . More specifically, each end portion of the guide convex portion  121  is aligned with each guide rib  60  in leftward/rightward direction. In other words, the end portion of the guide convex portion  121  faces a part of the outer surface of the belt  43 , the part corresponding to the guide rib  60 . This structure can prevent the guide rib  60  from coming off from the annular stepped guide portion  52 A. 
     The guide rib  60 , the annular stepped guide portion  52 A, and the rotary body  61  can be provided only at one of the left and right end portion of the support roller  42 , or at both the left and right end portions thereof. Further, the guide rib  60  and the annular stepped guide portion  52 A can be provided at the drive roller  41  and a bearing portion thereof. Further, a convex-shaped regulating portion can be provided at the outer peripheral surface side of the roller, and a concave-shaped regulating portion can be provided at the belt side. 
     Eighth Embodiment 
     The drive roller  41  is supported at rear side portions of the pair of the side frames  44 R and  44 L, respectively, so as to be rotatable about a rotation axis extending in the main scanning direction. Although not illustrated in  FIG. 12 , the belt  43  has a left end portion whose inner peripheral surface, i.e. peripheral surface on the support roller  42  side, has another convex-shaped guide rib  60 . The convex-shaped guide rib  60  on the left end portion protrudes toward the inner side of the belt  43  as well as the convex-shaped guide rib  60  on the right end portion of the belt  43 . 
     As illustrated in  FIG. 11 , two rotating bodies  131 R and  131 L are provided in the bottom frame  46  so as to be rotatable about a rotation axis extending parallel to the roller rotation axis Z. Specifically, in the bottom frame  46 , two through-holes  46 C are formed at respective positions below the flange sections  52 R and  52 L so as to penetrate the bottom frame  46 . Each of the through-holes  46 C only needs to be a recessed portion; the through-holes  46 C may not necessarily be through-holes providing communication between the upper and lower sides of the bottom frame  46 . That is, each of the through-holes  46 C may be a recessed portion formed on an upper surface of the bottom frame  46 . Bearings  46 D and  46 D are formed in left and right inner walls of each through-hole  46 C, respectively (see  FIG. 12 ). 
     As illustrated in  FIG. 12  and  FIG. 13 , the right-side rotating body  131 R includes a body  131 A and shaft portions  131 B and  131 B protruding to the leftward and rightward from the body  131 A. The body  131 A has such a shape that a gap from the periphery thereof to the outer surface of the belt  43  decreases toward a widthwise center of the belt  43 . Specifically, the body  131 A has a conical shape whose diameter increases in the left direction. The body  131 A has a width in the leftward/rightward direction substantially the same as a width of the guide rib  60 . The shaft portions  131 B and  131 B are supported at the bearings  46 D and  46 D of the through-hole  46 C, respectively. 
     Although not illustrated in  FIG. 12 , the body  131 A of the left-side rotating body  131 L has such a shape that a gap from the periphery thereof to the outer surface of the belt  43  decreases toward the widthwise center of the belt  43 . Specifically, the body  131 A has a conical shape whose diameter increases in the right direction. The rotating bodies  131 R and  131 L are positioned respectively in the through-holes  46 C of the bottom frame  46 , and are rotated when being brought into contact with the belt  43 . The rotating bodies  131 R and  131 L are each an example of a rotating member. The bottom frame  46  is an example of a support portion. The rotating bodies  131 R and  131 L and the bottom frame  46  are an example of a regulating portion. 
     As illustrated in  FIG. 12 , a shortest gap D 1  between each of the rotating bodies  131 R and  131 L and outer surface of the belt  43  is smaller than a length D 2 , the thickness of the guide rib  60  as illustrated in  FIG. 3 . Note that the guide rib  60  has the constant length D 2  along a portion that overlaps with the annular stepped guide portion  52 A in the radial direction of the support roller  42 , i.e. the vertical direction as indicated in  FIG. 3 . Specifically, the shortest gap D 1  is a gap between an upper surface of a largest diameter portion of each of the rotating bodies  131 R and  131 L and the outer surface of the belt  43 . Thus, even when the guide rib  60  starts separating from the annular stepped guide portion  52 A, the outer surface of the belt  43  abuts the rotating bodies  131 R and  131 L. Accordingly, the separation or the disengagement of the guide rib  60  from the annular stepped guide portion  52 A can be prevented. 
     Further, as illustrated in  FIG. 14 , when the guide rib  60  starts to come off from the annular stepped guide portion  52 A, the guide rib  60  may be sandwiched between the rotating body  131 R and the annular stepped guide portion  52 A or a portion adjacent to the annular stepped guide portion  52 A. In this case, the rotating body  131 R receives a pressing force F 1  caused by the abutting against the belt  43 . 
     As described above, the body  131 A of the rotating body  131 R has such a shape that the gap therefrom to the outer surface of the belt  43  decreases toward the widthwise center of the belt  43 . Thus, the belt  43  receives, from the rotating body  131 R, a restoring force F 2  that pushes back the guide rib  60  to the right end side of the support roller  42  as a horizontal component force of a reaction force of the pressing force F 1 . The pressing force F 1  becomes larger as the belt  43  is pulled toward the widthwise center of the support roller  42 , and the restoring force F 2  correspondingly increases. As a result, the guide rib  60  is pushed back to the annular stepped guide portion  52 A. As described above, a state where the belt  43  is kept to deviate from the support roller  42  can be prevented. 
     Further, the two rotating bodies  131 R and  131 L are disposed at substantially symmetrical positions with respect to the center position of the support roller  42  (see  FIG. 11 ). Thus, each side of the restoring force F 2  that pulls the belt  43  in the leftward/rightward direction is applied to the belt  43  when they are rotated together with movement of the belt  43 . This can suppress wrinkling of the belt  43 . 
     Further, the outer peripheral surface of each of the rotating bodies  131 R and  131 L is linearly inclined with respect to the roller rotation axis Z. As compared to a configuration in which the outer peripheral surface forms a stepped shape brought close to the belt outer surface in a stepwise manner, damage of the belt  43  due to abutment of the rotating bodies  131 R and  131 L can be prevented more reliably. The outer peripheral surface is an example of a surface of the roller. 
     In addition, the rotating bodies  131 R and  131 L are provided in the bottom frame  46 , which is integrally disposed in the side frame  44  that bears a shaft of the support roller  42 . Thus, as compared to a case where the rotating bodies  131 R and  131 L are provided in a separated component from the belt unit  4 , such as the image forming section  5 , a positional relationship and a gap between the outer surface of the belt  43  and each of the rotating bodies  131 R and  131 L can be maintained even when the belt unit  4  or image forming section  5  is moved for replacement. As a result, disengagement of the guide rib  60  from the annular stepped guide portion  52 A can be prevented. 
     Further, the rotating bodies  131 R and  131 L are each disposed so as to overlap both the guide rib  60  and a portion  52 C of the flange section  52 R (or flange  52 L) that is adjacent to the annular stepped guide portion  52 A in the direction of the roller rotation axis Z. In other words, the rotating bodies  131 R and  131 L each opposes the guide rib  60  and the adjacent portion  52 C with respect to the belt  43 . As compared to a configuration in which each of the rotating bodies  131 R and  131 L vertically overlaps with only one of the guide ribs  60  and the adjacent portion in the direction of the roller rotation axis Z, disengagement of the guide rib  60  from annular stepped guide portion  52 A can be prevented more reliably. 
     Further, the regulating portion includes the rotating bodies  131 R and  131 L. Thus, as compared to a configuration in which the regulating portion is constituted by a fixed member that is not rotatable, damage of the belt  43  due to the contact with the regulating portion can be prevented more significantly. Further, as described above, the rotating bodies  131 R and  131 L are rotated when being brought into contact with the belt  43  in a state where they are positioned in the respective through-holes  46 C of the bottom frame  46 . Thus, in comparison with the configuration (see  FIG. 15 ) in which the rotating body is not installed in the concave portion and the gap between the bottom frame  46  and outer surface of the belt  43  is the same, the rotating body  131  can have the larger diameter. This correspondingly increases a contact area between each of the rotating bodies  131 R and  131 L and belt  43  to lower the pressing force of the rotating body  131  against the belt  43 , thereby preventing damage of the belt  43 . 
     Further, the rotating bodies  131 R and  131 L are each provided on the outer surface of a part of the belt  43  that moves from the cleaner  6  toward the image forming section  5 . This structure can prevent the rotating bodies  131 R and  131 L from being contaminated by coloring materials adhered to the belt  43 . Further, this can prevent a re-adhesion of the coloring materials on the rotating bodies  131 R and  131 L to the belt  43 . Further, adhesion of the coloring materials onto the rotating bodies  131 R and  131 L from the belt  43  can be prevented, and thus the force pushing back the guide rib  60  toward the annular stepped guide portion  52 A is maintained. 
     The rotating bodies  131 R and  131 L are disposed on the vicinity of the outer surface of a non-stretched portion  43 B of the belt  43 . Here, a stretched portion  43 A is a part of the belt  43  that is moved from the support roller  42  toward the drive roller  41  and is used for conveyance of the sheet W; the non-stretched portion  43 B is a part of the belt  43  that is moved from the drive roller  41  toward the support roller  42  and is not used for conveyance of the sheet W. As compared to a configuration in which the rotating bodies  131 R and  131 L are positioned on the outer surface side of the stretched portion  34 A, the sheet W and rotating bodies  131 R and  131 L can be prevented from interfering with each other more reliably. 
     Ninth Embodiment 
       FIG. 15  illustrates a ninth embodiment. This embodiment differs from the above embodiment in the configuration of the regulation portion, and other configurations are the same as those in the above embodiment. Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     A configuration illustrated in  FIG. 15  differs from that of  FIG. 12  in the structure of the regulating portion. Specifically, an underframe  140  includes a fixing convex portion  140 A similar to the fixing protrusions  46 A and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 12  except for the followings. That is, a pair of base portions  141  and  141  protrude upward on an upper surface of the underframe  140 . Bearings  141 A are formed in the base portions  141  and  141 , respectively. 
     A rotating body  142 R includes a body  142 A and shaft portions  142 B and  142 B protruding in the leftward/rightward direction from the body  142 A. The body  142 A has a gap from the periphery thereof to the outer surface of the belt  43  that decreases toward the widthwise center of the belt  43 . Specifically, the body  142 A has a conical shape whose diameter increases in the leftward direction. The body  142 A has a width in the leftward/rightward direction, which is substantially the same as that of the guide rib  60 . The shaft portions  142 B and  142 B are supported at the bearings  141 A and  141 A of the through-hole  46 C, respectively. With this configuration, the rotating body  142 R is rotated when being brought into contact with the belt  43 . 
     A shortest gap D 1  between the upper surface of the rotating body  142 R and the outer surface of the belt  43  is smaller than the length D 2  over which the guide rib  60  and the annular stepped guide portion  52 A overlap with each other in the radial direction of the support roller  42 . Thus, even when the guide rib  60  starts separating from the annular stepped guide portion  52 A, the guide rib  60  is pushed back to the annular stepped guide portion  52 A by the rotating body  142 R, resulting in the prevention of a state where the belt  43  is kept to deviate from the support roller  42 . 
     Tenth Embodiment 
       FIG. 16  illustrates a tenth embodiment. This embodiment differs from the above embodiments in the configuration of the regulation portion, and other configurations are the same as those in the above embodiments. Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     An underframe  150  includes a fixing convex portion  150 A similar to the fixing protrusions  46 A and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 12  except for the followings. That is, a pair of base portions  151  and  151  protrude upward on an upper surface of the underframe  150 . Bearings  151 A are formed in the base portions  151  and  151 , respectively. 
     A rotating body  152 R includes a cylindrical body  152 A and shaft portions  152 B and  152 B protruding in the leftward/rightward direction from the body  152 A. The body  152 A has a width in the leftward/rightward direction, which is substantially the same as that of the guide rib  60 . The rotating body  152 R is rotatably supported about a rotation axis inclined so that the body  152 A approaches the outer surface of the belt  43  toward the widthwise center of the belt  43 . That is, the rotating body  152 R has a gap from the periphery thereof to the outer surface of the belt  43  that decreases toward the center of the belt  43 . 
     A shortest gap D 1  between an upper surface of the rotating body  152 R and outer surface of the belt  43  is smaller than the length D 2  over which the guide rib  60  and the annular stepped guide portion  52 A overlap each other in the radial direction of the support roller  42 . Thus, even when the guide rib  60  starts separating from the annular stepped guide portion  52 A, the guide rib  60  is pushed back to the annular stepped guide portion  52 A by the rotating body  152 R, thereby preventing a state where the belt  43  is kept to deviate from the support roller  42 . 
     Eleventh Embodiment 
       FIG. 17  illustrates an eleventh embodiment. This embodiment differs from the above embodiments in the configuration of the regulation portion, and other configurations are the same as those in the above embodiments. Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     An underframe  160  includes a fixing convex portion  160 A similar to the fixing protrusions  46 A and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 12  except for the followings. That is, a guide convex portion  161 R integrally protrudes upward on an upper surface of the underframe  160 . The guide convex portion  161 R has a width in the leftward/rightward direction, which is substantially the same as that of the guide rib  60 . 
     An upper surface of the guide convex portion  161 R is inclined such that a gap from the periphery thereof to the outer surface of the belt  43  decreases toward the widthwise center of the belt  43 . Further, a shortest gap D 1  between an upper surface of the guide convex portion  161 R and outer surface of the belt  43  is smaller than the length D 2  over which the guide rib  60  and the annular stepped guide portion  52 A overlap with each other in the radial direction of the support roller  42 . Thus, even when the guide rib  60  starts separating from the annular stepped guide portion  52 A, the guide rib  60  is pushed back to the annular stepped guide portion  52 A by the guide convex portion  161 R, thereby preventing a state where the belt  43  is kept to deviate from the support roller  42 . 
     Twelfth Embodiment 
       FIG. 18  illustrates a twelfth embodiment. This embodiment differs from the above embodiments in the configuration of the regulation portion, and other configurations are the same as those in the above embodiments. Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     An underframe  170  includes a fixing convex portion  170 A similar to the fixing protrusions  46 A and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 12  except for the followings. That is, the regulating portion is not provided on an upper surface of the underframe  170 . 
     On the other hand, a guide convex portion  171 R integrally protrudes on a lower surface of a drawer  50 . The guide convex portion  171 R has a width in the leftward/rightward direction, which is substantially the same as that of the guide rib  60 . The lower surface of the guide convex portion  171 R is inclined such that a gap therefrom to the outer surface of the belt  43  decreases toward the widthwise center of the belt  43 . Further, a shortest gap D 3  between the lower surface of the guide convex portion  171 R and outer surface of the belt  43  is smaller than a length D 4  over which the guide rib  60  and the annular stepped guide portion  52 A overlap with each other in the radial direction of the support roller  42 . 
     With the above configuration, even when the guide rib  60  starts separating from the annular stepped guide portion  52 A, the guide rib  60  is pushed back to the annular stepped guide portion  52 A by the guide convex portion  171 R, thereby preventing a state where the belt  43  is kept to deviate from the support roller  42 . The guide convex portion  171 R is disposed widthwise outside from an area in which the sheet W to be conveyed to the belt  43  passes. 
     Thirteenth Embodiment 
       FIG. 19  illustrates a thirteenth embodiment. This embodiment differs from the embodiment illustrated in  FIG. 17  in the configuration of the guide rib, and other configurations are the same as those in the embodiment of  FIG. 17 . Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     In the configuration illustrated in  FIG. 19 , a guide rib  180  is provided at an outer surface of the belt  43 . The guide rib  180  is an example of a second guide. The guide rib  180  is provided at a position at which at least a part thereof overlaps with the guide convex portion  161 R in the direction of the roller rotation axis Z. In addition, at least a part of the guide rib  180  opposes the guide convex portion  161 R. Further, the shortest gap D 1  between the guide convex portion  161 R and outer surface of the belt  43  is smaller than a length D 5  over which the guide rib  180  protrudes from the belt  43  in the radial direction of the support roller  42 . 
     With the above configuration, when the belt  43  starts to deviate toward the widthwise center of the support roller  42 , the guide rib  180  is sandwiched between the guide convex portion  161 R and flange section  52 R. At this time, the belt  43  receives, from the guide convex portion  161 R, a restoring force that pushes back the guide rib  180  toward the right end of the support roller  42  as a horizontal component of a reaction force of the guide convex portion  161 R, thereby preventing a state where the belt  43  is kept to deviate from the support roller  42 . 
     Fourteenth Embodiment 
       FIG. 20  illustrates a fourteenth embodiment. This embodiment differs from the embodiment illustrated in  FIG. 18  in the configuration of the guide rib, and other configurations are the same as those in the embodiment of  FIG. 18 . Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     In the configuration illustrated in  FIG. 20 , a guide rib  190  is provided at an outer surface side of the belt  43 . The guide rib  190  is an example of a second guide. The guide rib  190  is provided at a position at which at least a part thereof overlaps with the guide convex portion  171 R when viewed in the direction of the roller rotation axis Z. In addition, at least a part of the guide rib  190  opposes the guide convex portion  171 R. Further, the shortest gap D 3  between the guide convex portion  171 R and outer surface of the belt  43  is smaller than a length D 6  over which the guide rib  190  protrudes from the belt  43  in the radial direction of the support roller  42 . 
     With the above configuration, when the belt  43  starts to deviate to the widthwise center of the support roller  42 , the guide rib  190  is sandwiched between the guide convex portion  171 R and the flange section  52 R. At this time, the belt  43  receives, from the guide convex portion  171 R, a restoring force that pushes back the guide rib  190  to the right end side of the support roller  42  as a horizontal component force of a reaction force of the guide convex portion  171 R, thereby preventing a state where the belt  43  is kept to deviate from the support roller  42 . 
     Fifteenth Embodiment 
       FIG. 21  illustrates a fifteenth embodiment. This embodiment differs from the above embodiments in the configurations of the guide rib and regulation portion, and other configurations are the same as those in the above embodiments. Thus, the same parts in the subsequent drawings are denoted by the same reference numerals, so detailed descriptions thereof are omitted and only different parts will be described. 
     In the configuration illustrated in  FIG. 21 , a convex-shaped guide rib  200  is formed on a right end portion of the inner peripheral surface of the belt  43 , i.e. on the support roller  42  side. The guide rib  200  is an example of a second guide. The guide rib  200  is engaged in the annular stepped guide portion  52 A of the flange section  52 R. A protruding length of the guide rib  200  from the belt  43  becomes smaller toward the center of the belt  43 . 
     An underframe  201  includes a fixing convex portion  201 A similar to the fixing protrusions  46 A and has substantially the same shape as that of the bottom frame  46  illustrated in  FIG. 12  except for the followings. That is, a guide convex portion  202 R is integrally formed on an upper surface of the underframe  201 . An upper surface of the guide convex portion  202 R is substantially parallel to the outer surface of the belt  43 , and the guide convex portion  202 R has a width in the leftward/rightward direction, which is substantially the same as that of the guide rib  200 . A gap D 7  between an upper surface of the guide convex portion  202 R and outer surface of the belt  43  is smaller than a length D 8  over which the guide rib  200  and annular stepped guide portion  52 A overlap with each other in the radial direction of the support roller  42 . 
     With the above configuration, even when the guide rib  200  starts separating from the annular stepped guide portion  52 A, the guide rib  200  is pushed back to the annular stepped guide portion  52 A by the guide convex portion  202 R, thereby preventing a state where the belt  43  is kept to deviate from the support roller  42 . 
     While the description has been made in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit and scope of the above described embodiments. 
     Various modifications are conceivable. For example, the image forming apparatus is not limited to a printer  1 , but can be applied to a copying machine, a facsimile machine, and a multifunction device provided with a plurality of functions such as a printing function and reading function. Further, image forming apparatus other than an electro-photographic type apparatus, such as an ink jet printer is also available. Further, a monochromatic printer is also available in addition to the color printer. 
     Further, the belt unit may have a configuration in which the belt is mounted on one roller or not less than three rollers. Further, the belt is not limited to the conveying belt but may be an intermediate transfer belt or a photosensitive belt. 
     Further, the shape of the rotating member is not limited to cylindrical, but conical shape or a spherical shape is available. Further, the regulating portion need not have the rotary body or convex portion but may be realized only by the bottom frame  46 . In the latter case, a gap between the upper surface of the bottom frame  46  and outer surface of the belt  43  is smaller than the length D 2 . 
     Further, the regulating portion such as the rotary body  61  can only be aligned with one of the guide rib  60  and the adjacent portion  52 C in the direction of the roller rotation axis Z.