Patent Publication Number: US-10768556-B2

Title: Image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2019-003626 filed on Jan. 11, 2019, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus. 
     As an example of electro-photographic image forming apparatuses such as a copier and a printer, there has been known an intermediate-transfer type image forming apparatus which primarily transfers toner images of different colors, respectively formed by a plurality of image forming portions, onto an intermediate transfer belt to be sequentially superimposed one on another on the intermediate transfer belt, which is arranged along the plurality of image forming portions, and then secondarily transfers the toner images from the intermediate transfer belt onto a sheet. About the image forming apparatus of this type, it is feared that there is a risk of the intermediate transfer belt meandering or twisting. To cope with this risk, there have been proposed image forming apparatuses capable of suppressing occurrence of meandering and twisting of the intermediate transfer belt. 
     In an example of such image forming apparatuses, of an outer circumferential surface of a roller around which a belt is wound, a central part in a rotation-axis direction is a high friction-coefficient surface, and opposite side parts in the rotation-axis direction are low friction-coefficient surface. Thus, the roller exerts a stronger belt conveying force in its central part than in its opposite side parts, and as a result, the belt is constantly being drawn toward the central part of the roller. This function helps prevent meandering of the belt. 
     SUMMARY 
     According to one aspect of the present disclosure, an image forming apparatus includes a plurality of image forming portions, an intermediate transfer belt, a belt displacement detector, an attraction force changer, and a controller. The plurality of image forming portions form toner images of different colors. The intermediate transfer belt is an endless belt wound around a drive roller and a driven roller, and is arranged along the plurality of image forming portions, and onto the intermediate transfer belt. The toner images respectively formed by the plurality of image forming portions are primarily transferred onto the intermediate transfer belt to be sequentially superimposed one on another. The belt displacement detector detects displacement of the intermediate transfer belt in a rotation axis direction of the driven roller. The attraction force changer is operable to change electrostatic attraction force of the driven roller with respect to the intermediate transfer belt exerted on each of opposite end sides of the driven roller in the rotation axis direction thereof. The controller controls operation of the attraction force changer. When the belt displacement detector detects the displacement of the intermediate transfer belt, the controller makes the attraction force changer increase the electrostatic attraction force of the driven roller with respect to the intermediate transfer belt exerted on a side of the driven roller opposite to a side toward which the displacement has taken place. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing a configuration of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a block diagram showing a configuration of the image forming apparatus according to the embodiment of the present disclosure; 
         FIG. 3  is a sectional view showing an area around a transfer portion of the image forming apparatus according to the embodiment of the present disclosure; 
         FIG. 4  is a top view showing an area around a driven roller of the image forming apparatus according to the embodiment of the present disclosure; 
         FIG. 5  is a graph showing a relationship between the electric resistance value and the grip force of the driven roller in each of image forming apparatuses according to an example and a comparative example; 
         FIG. 6  is a sectional view showing a portion around a transfer portion of an image forming apparatus according to a modified example of the embodiment of the present disclosure; and 
         FIG. 7  is a top view showing an area around a driven roller of the image forming apparatus according to the modified example of the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present disclosure will be described below with reference to the accompanying drawings. It should be noted that the present disclosure is not limited to what is specifically mentioned below. 
       FIG. 1  is a schematic sectional view showing a configuration of an image forming apparatus  1 .  FIG. 2  is a block diagram showing a configuration of the image forming apparatus  1 .  FIG. 3  is a sectional view showing an area around a transfer portion  30  of the image forming apparatus  1 . An example of the image forming apparatus  1  according to the present embodiment is a tandem-type color printer which uses an intermediate transfer belt  31  to transfer a toner image onto a sheet P. The image forming apparatus  1  may instead be what is called a multifunction peripheral which has, for example, a print function (printing), a scan function (in age reading), a facsimile function, etc. 
     As shown in  FIG. 1 ,  FIG. 2 , and  FIG. 3 , the image forming apparatus  1  includes the following components arranged in a main body  2  thereof a sheet feeder  3 , a sheet conveyer  4 , an exposure portion  5 , an image forming portion  20 , the transfer portion  30 , a fixing portion  7 , a sheet discharge portion  8 , a controller  9 , and a storage  10 . 
     The sheet feeder  3  holds a plurality of sheets F, and feeds them out separately one by one in printing. The sheet conveyer  4  conveys the sheets P fed out from the sheet feeder  3  to a secondary transfer portion  33  and the fixing portion  7 , and further delivers the sheets P, having undergone image fixing, to the sheet discharge portion  8  via a sheet discharge port  4   a . When double-side printing is to be performed, the sheet conveyer  4  guides the sheets P, each having undergone image fixing on a first side thereof, to a reverse sheet conveyer  4   c  via a branch portion  4   b , and conveys the sheets P back to the secondary transfer portion  33  and the fixing portion  7 . The exposure portion  5  radiates, toward the image forming portion  20 , laser light L controlled based on image data. 
     The image forming portion  20  includes a yellow image forming portion  20 Y, a magenta image forming portion  20 M, a cyan image forming portion  20 C, and a black image forming portion  20 B. These four image forming portions  20  are identical in basic configuration. Thus, in the following description, the color identification symbols “Y”, “M”, “C”, and “B” may be omitted unless necessary for specific identification. 
     The image forming portions  20  each include a photosensitive drum  21 , which is an image carrier supported to be rotatable in a predetermined direction (a clockwise direction in  FIG. 1  and  FIG. 3 ). The image forming portions  20  each further include a charging portion  22 , a developing portion  23 , and a drum cleaning portion  24 , which are arranged around the photosensitive drum  21  along the rotation direction thereof. Between the developing portion  23  and the drum cleaning portion  24 , a primary transfer portion  32  is arranged. 
     The charging portion  22  charges the outer circumferential surface of the photosensitive drum  21  at a predetermined potential by means of a charging roller, for example. Then, a laser beam radiated from the exposure portion  5  forms an electrostatic latent image of a document image on the outer circumferential surface of the photosensitive drum  21 . That is, the photosensitive drum  21 , as an image carrier, carries an electrostatic latent image. The developing portion  23  supplies toner to the electrostatic latent image to forrn a toner image. The four image forming portions  20  form toner images of different colors. 
     The transfer portion  30  includes the intermediate transfer belt  31 , primary transfer portions  32 Y,  32 M,  32 C, and  32 B, the secondary transfer portion  33 , and a belt cleaning portion  34 . The intermediate transfer belt  31  is an intermediate transfer body which is supported to be rotatable in a predetermined direction (a counterclockwise direction in  FIG. 1  and  FIG. 3 ), and onto which toner images formed by the four image forming portions  20  are primarily transferred to be sequentially superimposed one on another. The four image forming portions  20  are arranged in what is called a tandem alignment, being aligned from upstream to downstream sides of the intermediate transfer belt  31  in a rotation direction thereof. 
     The primary transfer portions  32 Y,  32 M,  32 C, and  32 B are arranged above the image forming portions  20 Y,  20 M,  20 C, and  20 B corresponding to different colors. The secondary transfer portion  33  is arranged at a position that is on an upstream side of the fixing portion  7  in a sheet-conveyance direction in the sheet conveyer  4  and that is on a downstream side of the image forming portions  20 Y,  20 M,  20 C, and  20 B, corresponding to the different colors, in a rotation direction of the intermediate transfer belt  31  in the transfer portion  30 . The belt cleaning portion  34  is arranged on an upstream side of the image forming portions  20 Y,  20 M,  20 C, and  20 B, corresponding to the different colors, in the rotation direction of the intermediate transfer belt  31 . 
     Toner images are primarily transferred onto the outer circumferential surface of the intermediate transfer belt  31  at the primary transfer portions  32 Y,  32 M,  32 C, and  32 B, corresponding to the different colors. Then, along with the rotation of the intermediate transfer belt  31 , the toner images on the image forming portions  20  are transferred one after another onto the intermediate transfer belt  31  with a predetermined timing to be superimposed one on another. In this manner, the yellow, magenta, cyan, and black toner images are superimposed one on another on the outer circumferential surface of the intermediate transfer belt  31 , forming a color toner image. The drum cleaning portion  24  performs cleaning by removing toner and the like remaining on the outer circumferential surface of the photosensitive drum  21  after the primary transfer. 
     The color toner image on the outer circumferential surface of the intermediate transfer belt  31  is transferred onto a sheet P, which has been synchronously conveyed by the sheet conveyer  4 , at a secondary transfer nip portion formed in the secondary transfer portion  33 . The belt cleaning portion  34  performs cleaning by removing toner and the like remaining on the outer circumferential surface of the intermediate transfer belt  31  after the secondary transfer. 
     The fixing portion  7  applies heat and pressure to the sheet P having the toner image transferred thereon, and thereby fixes the toner image on the sheet P. 
     The controller  9  includes a CPU, an image processor, and other electronic circuits and electronic components, of which none is illustrated. The CPU performs processing related to the functions of the image forming apparatus  1  by controlling the operation of each component provided in the image forming apparatus  1  based on programs and data for control stored in the storage  10 . The sheet feeder  3 , the sheet conveyer  4 , the exposure portion  5 , the image forming portion  20 , the transfer portion  30 , and the fixing portion  7  each individually receive instructions from the controller  9  and operate together to perform printing with respect to the sheet P. Furthermore, the controller  9  is also capable of obtaining an output value from a belt displacement detector  41 , which will be described later, to control the operation of an attraction force changer  50 , which will also be described later. 
     The storage  10  is configured with, for example, a combination of a non-volatile storage device, such as a program ROM (Read Only Memory) and a data ROM, and a volatile storage device, such as a RAM (Random Access Memory), of which none is illustrated. 
     Next, a configuration of and around the transfer portion  30  will be described by using  FIG. 3  and  FIG. 4 .  FIG. 4  is a top view showing an area around a driven roller  36  of the image forming apparatus  1 . 
     The intermediate transfer belt  31  is an endless belt wound around a drive roller  35  and the driven roller  36 . The intermediate transfer belt  31  is made to rotate in the counterclockwise direction in  FIG. 3  by the drive roller  35  powered by an unillustrated drive motor. The driven roller  36  rotates in the counterclockwise direction in  FIG. 3  following the rotation of the intermediate transfer belt  31 . In the secondary transfer portion  33 , a secondary transfer roller  33   r  is arranged. The secondary transfer roller  33   r  is opposed to the drive roller  35  with the intermediate transfer belt  31  interposed between them, and is in contact with the outer circumferential surface of the intermediate transfer belt  31 . 
     The intermediate transfer belt  31  is arranged along the four image forming portions  20 . Above each of the four image forming portions  20 , a primary transfer roller  32   r  is arranged, with the intermediate transfer belt  31  interposed between them. The primary transfer roller  32   r  is opposed to the photosensitive drum  21  with the intermediate transfer belt  31  interposed between them, and is in contact with an inner circumferential surface of the intermediate transfer belt  31 . 
     The intermediate transfer belt  31  is a conductive belt having a multilayer structure that includes, in order from the inner circumferential side, a base layer, an elastic layer, and a coat layer, for example. The base layer is made of, for example, a PVDF (polyvinylidene difluoride) resin, a polyimide resin, and the like, and has a predetermined rigidity as a base member. The elastic layer is made of, for example, a hydrin rubber, a polychioroprene rubber, a polyurethane rubber, and the like, and has an elasticity to prevent a dropout phenomenon in an image caused by stress concentration. The coat layer is made of, for example, an acrylic resin, a silicone resin, a fluororesin, and the like, and protects the elastic layer. The coat layer, constituting the outer circumference surface, contacts the photosensitive drum  21  and the secondary transfer roller  33   r.    
     Here, the intermediate transfer belt  31  may instead have a structure without a base layer, a structure including another layer in addition to a base layer, an elastic layer, and a coat layer, or a monolayer structure having only an elastic layer. 
     The belt cleaning portion  34  is opposed to the driven roller  36  with the intermediate transfer belt  31  interposed between them, and is in contact with the outer circumferential surface of the intermediate transfer belt  31 . The belt cleaning portion  34  includes, for example, a scraper  34   a  and a spiral conveyer  34   b . The scraper  34   a  is in contact with the outer circumferential surface of the intermediate transfer belt  31 , and scrapes off toner and the like remaining on the outer circumferential surface of the intermediate transfer belt  31  after secondary transfer. The spiral conveyer  34   b  conveys the toner and the like having been scraped off by the scraper  34   a  from the outer circumferential surface of the intermediate transfer belt  31  into an unillustrated exhaust toner collection container provided outside the belt cleaning portion  34 . 
     Further, the image forming apparatus  1  includes the belt displacement detector  41  and the attraction force changer  50 . The belt displacement detector  41  and the attraction force changer  50  are arranged near the driven roller  36 . 
     The belt displacement detector  41  is arranged, for example, as shown in  FIG. 3  and  FIG. 4 , on an upstream side of the driven roller  36  in the rotation direction of the intermediate transfer belt  31 . The belt displacement detector  41  is arranged at each of opposite end portions of the intermediate transfer belt  31  in the rotation axis direction. The belt displacement detector  41  has, for example, an actuator and a transmissive optical sensor (of which neither is illustrated). The actuator swings when it comes into contact with the intermediate transfer belt  31 . The optical sensor detects blockage of the optical path caused by the swinging actuator. The belt displacement detector  41  detects displacement of the intermediate transfer belt  31  in the rotation axis direction of the driven roller  36 . A detection signal from the belt displacement detector  41  is fed to the controller  9 . 
     The attraction force changer  50  includes a rotation body  51 , a contact member  52 , a voltage applier  53 , and a switch  54 . 
     The rotation body  51  is arranged, as shown in  FIG. 4 , adjacent to the driven roller  36 . The rotation body  51  has a rotation axis that is parallel to the rotation axis of the driven roller  36 . The rotation body  51  rotates in contact with the outer circumferential surface of the driven roller  36  substantially over the entire area in the rotation axis direction of the driven roller  36 . The rotation body  51  is constituted by a foam or solid conductive rubber roller, a brush roller using conductive threads, or the like. 
     The rotation body  51  is divided into a plurality of regions in the rotation axis direction thereof. The rotation body  51  is divided into, for example, five regions  51   a ,  51   b ,  51   c ,  51   d , and  51   e  in its rotation axis direction. The five regions  51   a ,  51   b ,  51   c ,  51   d ,  51   e  of the rotation body  51  are fixed to an axis portion  51   x  at predetermined intervals in the rotation axis direction of the rotation body  51 . Here, the number of regions into which the rotation body  51  is divided is not restricted to five. An outer circumferential surface of the rotation body  51  is preferably divided into at least three regions including a central part (the region  51   c ) and opposite end parts (regions  51   a  and  51   e ) in the rotation axis direction of the rotation body  51 . 
     The contact member  52  is arranged adjacent to the rotation body  51 . The contact member  52  is constituted by, for example, a conductive plate spring, a bar-shaped brush using conductive threads, or the like. The contact member  52  is in contact with an outer circumferential portion of the rotation body  51 , and leads electricity outputted from the voltage applier  53  to the rotation body  51 . The contact member  52  includes five contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e  corresponding to the five regions  51   a ,  51   b ,  51   c ,  51   d , and  51   e , respectively, of the rotation body  51 . The contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e  are individually in contact with the five regions  51   a ,  51   b ,  51   c ,  51   d , and  51   e , respectively, of the rotation body  51 . 
     The voltage applier  53  includes a power supply and a control circuit (of which neither is illustrated). The voltage applier  53  is electrically connected with the contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e  via switches  54   a ,  54   b ,  54   c ,  54   d , and  54   e . The voltage applier  53  applies voltage to the five regions  51   a ,  51   b ,  51   c ,  51   d , and  51   e  of the rotation body  51  via the contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e . The five separate regions  51   a ,  51   b ,  51   c ,  51   d , and  51   e  of the rotation body  51  are in contact with substantially an entire area of an outer circumferential surface of the driven roller  36  in its rotation axis direction. This enables the attraction force changer  50  to change the electrostatic attraction force of the driven roller  36  with respect to the intermediate transfer belt  31  exerted on each of the opposite end sides of the driven roller  36  in its rotation axis direction. 
     In a case where the belt displacement detector  41  has detected the displacement of the intermediate transfer belt  31 , the controller  9  makes the attraction force changer  50  increase the electrostatic attraction force of the driven roller  36  with respect to the intermediate transfer belt  31  exerted on the side of the driven roller  36  opposite to the side toward which the intermediate transfer belt  31  has been displaced. 
     Specifically, for example, when the belt displacement detector  41  detects a rightward displacement of the intermediate transfer belt  31  in  FIG. 4 , the controller  9  makes the voltage applier  53  raise the voltage applied to the left side in  FIG. 4 , that is, the voltage applied to at least one of the regions  51   a  and  51   b  of the rotation body  51 . That is, in a case where the intermediate transfer belt  31  has been displaced rightward, the controller  9  increases the electrostatic attraction force of the driven roller  36  with respect to the intermediate transfer belt  31  exerted on the left-side part of the driven roller  36 . This forces the intermediate transfer belt  31  to move leftward, back into its appropriate position. 
     In contrast, for example, when the belt displacement detector  41  detects a leftward displacement of the intermediate transfer belt  31  in  FIG. 4 , the controller  9  makes the voltage applier  53  raise the voltage applied to the right side in  FIG. 4 , that is, the voltage applied to at least one of the regions  51   e  and  51   d  of the rotation body  51 . That is, in a case where the intermediate transfer belt  31  has been displaced leftward, the controller  9  increases the electrostatic attraction force of the driven roller  36  with respect to the intermediate transfer belt  31  exerted on the right-side part of the driven roller  36 . This forces the intermediate transfer belt  31  to move rightward, back into its appropriate position. 
     Here, the voltage applied by the voltage applier  53  is preferably raised or lowered in accordance with the level of displacement of the intermediate transfer belt  31 . For example, the larger the displacement of the intermediate transfer belt  31  is, the more the voltage applied by the voltage applier  53  is raised. In a case where the displacement of the intermediate transfer belt  31  is of such a level as will cause no problem in image forming operation, the voltage applied by the voltage applier  53  may be turned off. A voltage may be applied to the region  51   c  in the central part in the axial direction, depending on the level of displacement of the intermediate transfer belt  31 . 
     With the configuration described above, when the intermediate transfer belt  31  has been displaced, the electrostatic attraction force of the driven roller  36  with respect to the intermediate transfer belt  31  can be increased on the side opposite to the side toward which the displacement has taken place. This makes it possible to move the intermediate transfer belt  31  in a direction opposite to the direction in which the intermediate transfer belt  31  has been displaced. Thus, it is possible to keep suppressing meandering and twisting of the intermediate transfer belt  31  over a long period of time, without any component being worn out. Further, it is also possible to suppress complication of the structure, and thus to achieve a compact and simple image forming apparatus  1 . 
     The attraction force changer  50  includes the rotation body  51  configured as described above, the contact member  52 , and the voltage applier  53 . With this configuration, it is possible to apply voltage indirectly to the driven roller  36  via the rotation body  51 . For example, in a case where the belt cleaning portion  34  is opposed to the driven roller  36  with the intermediate transfer belt  31  interposed between them, and is in contact with the outer circumferential surface of the intermediate transfer belt  31 , the outer circumferential surface of the driven roller  36  needs to have a shape that does not have any stepped portion over the entire area in the rotation axis direction. This can be achieved with the attraction force changer  50  configured as described above by changing the electrostatic attraction force of the driven roller  36  with respect to the intermediate transfer belt  31  exerted on each of the opposite end sides of the driven roller  36  in its rotation axis direction. 
       FIG. 5  is a graph showing a relationship between the electric resistance value and the grip force of the driven roller  36 . Specifically,  FIG. 5  is a diagram showing the change of grip force of an outer circumferential surface of each of five driven rollers that was observed when the voltage applied to the outer circumferential surface was gradually raised in each of the five driven rollers, the five driven rollers having different electric resistance values under application of the voltage of 100 V thereto. 
     From  FIG. 5 , it is clear that, in the driven roller of which the electric resistance value when the voltage of 100 V is applied thereto is equal to or smaller than 1.0×10 5 Ω, the grip force does not greatly rise even when the applied voltage is raised. In contrast, it is clear that, in the driven roller of which the electric resistance value when the voltage of 100 V is applied thereto is equal to or larger than 1.0×10 6 Ω, the grip force rises as the applied voltage is raised. Thus, it is preferable that the electric resistance value of the driven roller when the voltage of 100 V is applied thereto be equal to or larger than 1.0×10 6 Ω. 
     Further, it is preferable that the friction coefficient of the outer circumferential surface of the driven roller  36  with respect to the intermediate transfer belt  31  be smaller than that of the outer circumferential surface of the drive roller  35  with respect to the intermediate transfer belt  31 . With this configuration, the intermediate transfer belt  31  can move smoothly on the outer circumferential surface of the driven roller  36 . This makes it easy to draw the displaced intermediate transfer belt  31  back into its appropriate position. 
     Further, it is preferable that the surface hardness of the driven roller  36  be higher than that of the drive roller  35 . With this configuration, the intermediate transfer belt  31  can move smoothly on the outer circumferential surface of the driven roller  36 . This makes it easy to draw the displaced intermediate transfer belt  31  back into its appropriate position. 
     Further, it is preferable that a lubricant be applied to the outer circumferential surface of the driven roller  36 . With this configuration, the intermediate transfer belt  31  can move smoothly on the outer circumferential surface of the driven roller  36 . This makes it easy to draw the displaced intermediate transfer belt  31  back into its appropriate position. 
     Next, a description will be given of an image forming apparatus  1  of a modified example of the embodiment of the present disclosure, with reference to  FIG. 6  and  FIG. 7 .  FIG. 6  is a sectional view showing an area around a transfer portion of the image forming apparatus  1  of the modified example.  FIG. 7  is a top view showing an area around a driven roller  38  of the image forming apparatus  1  according to the modified example. The image forming apparatus  1  according to the modified example includes a belt cleaning portion  37 , a driven roller  38 , and an attraction force changer  50  shown in  FIG. 6  and  FIG. 7 . 
     The belt cleaning portion  37  is arranged away from the driven roller  38 , at a position that is on a downstream side of a secondary transfer portion  33  but is on an upstream side of the driven roller  38  in the rotation direction of an intermediate transfer belt  31 . The belt cleaning portion  37  includes, for example, a scraper  37   a , a spiral conveyer  37   b , and a counter roller  37   c , The scraper  37   a  is opposed to the counter roller  37   c  with the intermediate transfer belt  31  interposed between them, is in contact with an outer circumferential surface of the intermediate transfer belt  31 , and scrapes off toner and the like remaining on the outer circumferential surface of the intermediate transfer belt  31  after secondary transfer. The spiral conveyer  37   b  conveys the toner and the like having been scraped off by the scraper  37   a  from the outer circumferential surface of the intermediate transfer belt  31  into an unillustrated exhaust toner collection container provided outside the belt cleaning portion  37 . 
     The driven roller  38  is divided into a plurality of regions in its rotation axis direction. The driven roller  38  is divided into, for example, five regions  38   a ,  38   b ,  38   c ,  38   d , and  38   e  in its rotation axis direction. The five regions  38   a ,  38   b ,  38   c ,  38   d , and  38   e  of the driven roller  38  are fixed to an axis portion  38   x  of the driven roller  38  at predetermined intervals in the rotation axis direction of the driven roller  38 . Here, the number of regions into which the driven roller  38  is divided is not restricted to five. An outer circumferential surface of the driven roller  38  is preferably divided into at least three regions including a central part (the region  38   c ) and opposite end parts (regions  38   a  and  38   e ) in the rotation axis direction of the driven roller  38 . 
     The attraction force changer  50  includes a contact member  52 , a voltage applier  53 , and a switch  54 . 
     The contact member  52  is arranged adjacent to the driven roller  38 . The contact member  52  is constituted by, for example, a conductive plate spring, a bar-shaped brush using conductive threads, or the like. The contact member  52  is in contact with an outer circumferential portion of the driven roller  38 , and leads electricity outputted from the voltage applier  53  to the driven roller  38 . The contact member  52  includes five contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e  corresponding to the five regions  38   a ,  38   b ,  38   c ,  38   d , and  38   e , respectively, of the driven roller  38 . The contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e  are individually in contact with the five regions  38   a ,  38   b ,  38   c ,  38   d , and  38   e , respectively, of the driven roller  38 . 
     The voltage applier  53  includes a power supply and a control circuit (of which neither is illustrated). The voltage applier  53  is electrically connected with the contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e  via switches  54   a ,  54   b ,  54   c ,  54   d , and  54   e . The voltage applier  53  applies voltage to the five regions  38   a ,  38   b ,  38   c ,  38   d , and  38   e  of the driven roller  38  via the contact members  52   a ,  52   b ,  52   c ,  52   d , and  52   e . This enables the attraction force changer  50  to change the electrostatic attraction force of the driven roller  38  with respect to the intermediate transfer belt  31  exerted on each of the opposite end sides of the driven roller  38  in its rotation axis direction. 
     In a case where a belt displacement detector  41  has detected the displacement of the intermediate transfer belt  31 , the controller  9  makes the attraction force changer  50  increase the electrostatic attraction force of the driven roller  38  with respect to the intermediate transfer belt  31  exerted on the side of the driven roller  38  opposite to the side toward which the intermediate transfer belt  31  has been displaced. 
     With the configuration of the modified example described above as well as with the embodiment described previously, when the intermediate transfer belt  31  has been displaced, the electrostatic attraction force of the driven roller  38  with respect to the intermediate transfer belt  31  can be increased on the side opposite to the side toward which the displacement has taken place. This makes it possible to move the intermediate transfer belt  31  in a direction opposite to the direction in which the intermediate transfer belt  31  has been displaced. Thus, it is possible to keep suppressing meandering and twisting of the intermediate transfer belt  31  over a long period of time, without any component being worn out. 
     Further, in the image forming apparatus  1  of the modified example, the outer circumferential surface of the driven roller  38  is divided into at least three regions including a central part and opposite end parts in its rotation axis direction, and the attraction force changer  50  includes the contact member  52  and the voltage applier  53 . With this configuration, it is possible to apply voltage directly to the driven roller  38 . Thus, it is possible, with a simple configuration including a smaller number of components, to change the electrostatic attraction force of the driven roller  38  with respect to the intermediate transfer belt  31  exerted on each of the opposite end sides of the driven roller  38  in its rotation axis direction. 
     It should be understood that the embodiments of the present disclosure described above are in no way meant to limit its scope; the present disclosure can be implemented with any modifications made without departing from its spirit. 
     For example, although, in the embodiments described above, the image forming apparatus  1  is a tandem-type image forming apparatus for color printing, but the image forming apparatus  1  is not restricted to this type. The image forming apparatus  1  can also be a color-printing image forming apparatus of any type other than the tandem type, as long as it is provided with an intermediate transfer belt.