Patent Publication Number: US-7593664-B2

Title: Image-forming device and belt unit having belt tension-adjusting mechanism

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2005-089137 filed Mar. 25, 2005. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The disclosure relates to an image-forming device and a belt unit employed in the image-forming device. 
     BACKGROUND 
     Some image-forming devices well known in the art are configured of image-carrying members for carrying developer images, and a belt disposed in confrontation with the image-carrying members so that the image-carrying members transfer developer onto the belt or onto a recording medium conveyed along the belt. In this type of image-forming device, the belt is adjusted to an appropriate tension for performing the transfers. 
     Japanese Patent Publication No. 2000-305372 proposes a technique for setting the tension state of the belt in a halted state lower than the tension state during a transfer operation. With this construction, a tension suitable for a transfer operation is generated in the belt when the belt is driven to perform a transfer operation, thereby ensuring stable movement of the belt. On the other hand, when the belt is halted for a long period of time, the tension in the belt is reduced from that during a transfer operation to prevent the belt from becoming misshapen (stretching that lingers in the belt due to pressure being applied in specific areas of the belt over a long period of time). 
     SUMMARY 
     While this construction can reduce the tension in the belt for certain situations, there are some cases in which a greater belt tension than that during a transfer operation is desired. For example, when a mechanism is provided for cleaning the belt, a cleaning member in this mechanism is placed in contact with the belt for removing foreign matter from the surface of the belt by applying a load thereto. However, since this cleaning member applies a large load to the belt during cleaning, it is preferable to produce a larger tension in the belt to ensure proper cleaning. However, generating a constant, large tension in the belt may cause the belt to wear faster or lead to other problems. Therefore, it is desirable to set the belt at a greater tension only when necessary. 
     In view of the foregoing, it is an object of the present invention to provide an image-forming device and a belt unit capable of setting the tension state in the belt greater than the state during a transfer operation. 
     This and other objects of the invention will be attained by an image-forming device including an image-carrying member, a plurality of support members, an endless belt, a transferring member, and a tension-adjusting mechanism. 
     The image-carrying member carries a developer image. The endless belt is disposed in confrontation with the image-carrying member and is supported on the plurality of support members to circularly move there around. The transferring member transfers the developer image in a direction from the image-carrying member to the endless belt. The tension-adjusting mechanism adjusts a tension generated in the endless belt between a first tension at which the developer image is transferred in the direction from the image-carrying member to the endless belt and a second tension greater than the first tension. 
     In another aspect of the invention, there is provided a belt unit including a plurality of support members, an endless belt, and a tension-adjusting mechanism. 
     The endless belt is supported on the plurality of support members to circularly move there around. The tension-adjusting mechanism adjusts a tension generated in the endless belt between a first tension at which a developer image is transferred in a direction from an image-carrying member to the endless belt and a second tension greater than the first tension. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a cross-sectional view of an image-forming device according to a first embodiment of the present invention; 
         FIG. 2  is a perspective view showing the structure of the image-forming device in  FIG. 1  around a paper-conveying belt; 
         FIG. 3  is a perspective view of the paper-conveying belt structure in  FIG. 2  in which a frame and the like have been dismantled; 
         FIG. 4A  is explanatory diagram illustrating the state of the belt during a transfer operation according to a first embodiment of the present invention; 
         FIG. 4B  is explanatory diagrams illustrating the state of the belt during a cleaning operation according to a first embodiment of the present invention; 
         FIG. 5  is an explanatory diagram illustrating a belt driving structure during a transfer operation according to a first embodiment of the present invention; 
         FIG. 6  is an explanatory diagram illustrating the belt driving structure during cleaning according to a first embodiment of the present invention; 
         FIG. 7  is an explanatory diagram illustrating the belt in a halted state according to a first embodiment of the present invention; 
         FIG. 8A  is an explanatory diagram illustrating the structure of a belt unit during a transfer operation according to a second embodiment of the present invention; 
         FIG. 8B  is an explanatory diagram illustrating the structure of a belt during a cleaning operation according to a second embodiment of the present invention; and 
         FIG. 9  is a cross-sectional view of an image-forming device according to a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Next, an image-forming device according to a first embodiment of the present invention will be described while referring to the accompanying drawings. 
     As shown in  FIG. 1 , the printer  1  includes a toner-image-forming unit  4 , a paper-conveying belt  6 , a fixing unit  8 , a paper supply unit  9 , a stacker  12 , and a controller  10  having a CPU, ROM, RAM, and the like. The printer  1  forms four-color images on a paper P based on image data inputted from an external source. 
     The toner-image-forming unit  4  includes four developing units  51 M,  51 C,  51 Y, and  51 BK accommodating toner in the colors magenta (M), cyan (C), yellow (Y), and black (BK), respectively. Each of the four developing units  51 M,  51 C,  51 Y, and  51 BK includes photosensitive drums  3 , chargers  31 , and an exposure device  41 . 
     Next, each of these components will be described in greater detail. 
     The photosensitive drums  3  are configured of members substantially cylindrical in shape. The photosensitive drums  3  are rotatably supported on shafts  3   a  and are juxtaposed at regular intervals along the horizontal. The cylindrical member configuring each photosensitive drum  3  is formed of an aluminum base material that is coated with a positive charging photosensitive layer, for example. The aluminum base material is grounded on a ground line of the printer  1 . 
     Each charger  31  is a Scorotron charger including a charging wire  32  extending along the width of the photosensitive drum  3 , and a shielding case  33  open on the photosensitive drum  3  side for accommodating the charging wire  32 . When a high voltage is applied to the charging wire  32 , the charging wire  32  charges the surface of the photosensitive drum  3  with a positive polarity. The shielding case  33  includes a grid provided in the open area on the photosensitive drum  3  side. When a prescribed voltage is applied to the grid, the surface of the photosensitive drum  3  is charged to the same potential as the grid voltage. 
     The exposure device  41  is configured of an LED array and is disposed at positions over each photosensitive drum  3  downstream of the charger  31  in the rotational direction of the photosensitive drum  3 . The exposure device  41  generates light from an LED in the LED array corresponding to one color component in image data inputted from an external source to irradiate light on the surface of the corresponding photosensitive drum  3 . The exposure device  41  may also be configured of a scanner including a light source that emits laser light, and a polygon mirror that is driven to rotate by a motor for reflecting and scanning the laser light over the surface of the photosensitive drum  3 . 
     When the exposure device  41  irradiates laser light onto the surface of the photosensitive drum  3  based on image data, the light reduces the surface potential of the photosensitive drum  3  at the exposed regions. In this way, an electrostatic latent image is formed on the surface of the photosensitive drum  3 . 
     Each of the developing units  51  is configured of a developer case  55  for accommodating toner, and a developing roller  52  disposed in the developer case  55 . The developing roller  52  is disposed in contact with the respective photosensitive drum  3  at a position downstream of the exposure device  41  in the rotational direction of the photosensitive drum  3 . Each of the developing units  51  charges toner to a positive polarity and supplies a uniform thin layer of this toner to the photosensitive drum  3 . At the point of contact between the developing roller  52  and the photosensitive drum  3 , the positively charged toner carried on the surface of the developing roller  52  is attracted to the positive electrostatic latent image formed on the photosensitive drum  3 , thereby forming a toner image on the surface of the photosensitive drum  3  through a reverse developing method. 
     The developing roller  52  is configured of a base material formed of an electrically conductive silicon rubber or the like in a cylindrical shape, the surface of which is coated with a synthetic resin containing fluorine or a rubber material. Toner accommodated in each developer case  55  is a nonmagnetic, single-component toner having a positive polarity. The developer cases  55  in the developing units  51 M,  51 C,  51 Y, and  51 BK accommodate toner in the colors magenta, cyan, yellow, and black, respectively. 
     The paper supply unit  9  is disposed in the lowest section of the printer  1  and includes a paper tray  91  for accommodating the paper P, and a pickup roller  92  for picking up and feeding the paper P. After the pickup roller  92  picks up and feeds the paper P from the paper tray  91  one sheet at a time, conveying rollers  99  receive each sheet of paper P and convey the sheet onto the paper-conveying belt  6 . 
     The paper-conveying belt  6  has an endless loop shape that moves seamlessly while supporting the paper P on the top surface thereof. The paper-conveying belt  6  is looped around a drive roller  62  and a follower roller  63 . Transfer rollers  61  are disposed at positions opposing each of the photosensitive drums  3 . 
     As shown in  FIG. 2 , a frame  130  has side walls  130   a  and  130   b  for supporting the drive roller  62  and follower roller  63 . The frame  130  is fixed in a main casing  1   a  (see  FIG. 1 ) of the printer  1 . More specifically, holes formed in the frame  130 , as shown in  FIG. 3 , function as stationary bearings  131 . The drive roller  62  is rotatably supported in the frame  130  by inserting a shaft  62   a  of the drive roller  62  into the bearings  131 . A gear  62   b  is fixed to an end of the shaft  62   a  and transfers a driving force to a motor M to the shaft  62   a . More specifically, the motor M has a drive shaft  141 . A driving force produced by the drive shaft  141  is transferred to the gear  62   b  via a gear  142  for driving the drive roller  62  to rotate. 
     As shown in  FIG. 3 , the follower roller  63  includes a shaft  63   a , and a roller portion  63   b . The ends of the shaft  63   a  are supported in bearings  133  provided on the frame  130 . Specifically, the shaft  63   a  is inserted into a hole formed in each of the bearings  133  and is rotatably supported therein. The bearings  133  are slidably disposed in through-holes  135  formed in the bearings  133  so as to be capable sliding along the inner surfaces of the through-holes  135 . By sliding the bearings  133  in the through-holes  135 , it is possible to increase and decrease the distance between the follower roller  63  and the drive roller  62 . As shown in  FIG. 2 , retaining members  124  are rotatably supported on the ends of the shaft  63   a . Spring members  121  (described later) urge the retaining members  124  in a direction away from the drive roller  62 . By urging the follower roller  63  away from the drive roller  62  in this way, a degree of tension can be produced in the paper-conveying belt  6 . 
     As shown in  FIG. 1 , the paper-conveying belt  6  moves in a circular manner when the drive roller  62  is driven to rotate so that an outer surface  6   a  moves in a left-to-right direction in  FIG. 1  when opposite the photosensitive drums  3 . Hence, a sheet of paper P conveyed by the conveying rollers  99  is conveyed on the outer surface  6   a  sequentially past each of the photosensitive drums  3  toward the fixing unit  8 . 
     A constant current source (not shown) can be connected to the transfer rollers  61  for applying a transfer bias of reverse polarity to the positively charged toner between the transfer rollers  61  and the photosensitive drum  3  in order to transfer toner onto the paper P. With this construction, the transfer rollers  61  can transfer toner images formed on the photosensitive drums  3  onto the paper P conveyed by the paper-conveying belt  6 . 
     The fixing unit  8  includes a heating roller  81  and a pressure roller  82 . After toner images in four colors have been transferred onto the paper P, the heating roller  81  and pressure roller  82  pinch and convey the paper P while fixing the toner images to the paper P by heat and pressure. 
     As described above, the stacker  12  is formed on the top surface of the printer  1 . The stacker  12  is disposed on the discharge end of the fixing unit  8  for receiving and accommodating the paper P discharged from the fixing unit  8 . 
     The controller  10  is configured of a controlling device having a CPU well known in the art for controlling the overall operations of the printer  1 . The controller  10  is connected to a ROM, RAM, or other type of storage device. 
     Next, an image-forming operation performed by the printer  1  on the paper P will be described. First, the pickup roller  92  picks up one sheet of paper P from the paper supply unit  9  and conveys the sheet to the paper-conveying belt  6  via the conveying rollers  99 . 
     The charger  31  opposing the leftmost photosensitive drum  3  in  FIG. 1  (the photosensitive drum  3  corresponding to the developing unit  51 M for magenta) charges the surface of the photosensitive drum  3  uniformly with a prescribed voltage. The exposure device  41  exposes the surface of the photosensitive drum  3  based on image data for the color magenta inputted from an external source. The potential on the surface of the photosensitive drum  3  drops only in areas that were exposed, forming an electrostatic latent image on the surface of the photosensitive drum  3 . Next, a positive developing bias is applied to the developing roller  52  in the developing unit  51 M so that positively charged magenta toner carried on the surface of the developing roller  52  is supplied to the surface of the photosensitive drum  3 . 
     The magenta toner is deposited only in areas on the surface of the photosensitive drum  3  having a lower potential than the developing bias, thereby developing the electrostatic latent image. Next, a negative transfer bias is applied to the transfer rollers  61  so that the positively charged toner image formed on the photosensitive drum  3  is transferred onto the surface of the paper P conveyed by the paper-conveying belt  6 . Through this transfer process, a positively charged magenta toner image is deposited on the surface of the paper P. 
     Subsequently, the paper P is sequentially conveyed through positions opposing the photosensitive drums  3  for cyan toner, yellow toner, and black toner, at which time toner images in these colors are formed on the surface of the respective photosensitive drum  3  and are superimposed on the paper P according to the same process described for magenta toner, resulting in a four-color toner image being formed on the paper P. The four-color toner image is fixed to the paper P in the fixing unit  8 , and the paper P is subsequently discharged onto the stacker  12 . 
     As shown in  FIG. 1 , the printer  1  according to the preferred embodiment also includes a cleaning device  100  disposed adjacent to the surface of the paper-conveying belt  6  where the paper-conveying belt  6  has been inverted after passing around the drive roller  62 . The cleaning device  100  includes a cleaning roller  101  that is rotatably placed in contact with the paper-conveying belt  6 . The cleaning roller  101  is configured of a roller member formed of synthetic resin or metal and covered with silicon foam, urethane foam, or the like. A backup roller  110  formed of a metal roller member, for example, is disposed on the opposite surface of the paper-conveying belt  6  from the cleaning roller  101  so that the paper-conveying belt  6  is interposed between the cleaning roller  101  and backup roller  110 . Hence, the backup roller  110  is disposed in contact with the paper-conveying belt  6  on the surface opposite the outer surface  6   a  (an inner surface  6   b ) and functions to pinch the belt together with the cleaning roller  101 . 
     The cleaning device  100  also includes a cleaning blade  107  that contacts the cleaning roller  101  for scraping off toner that has become deposited on the cleaning roller  101 , and a collecting box  103  for collecting toner removed by the cleaning blade  107 . 
     The printer  1  according to the preferred embodiment is capable of forming toner images directly on the paper-conveying belt  6 . For example, the printer  1  can form density patches and the like on the paper-conveying belt  6  to determine the density of the toner image. By providing the cleaning roller  101  adjacent to the paper-conveying belt  6 , as described above, the cleaning roller  101  can remove toner that has been deposited on the paper-conveying belt  6 . As shown in  FIG. 2 , the cleaning roller  101  is configured of a shaft  101   a , and a gear  101   b  fixed to an end of the shaft  101   a . A drive motor or other drive source (not shown) produces a driving force for rotating a gear  102   b . The gear  102   b  transfers this driving force to the gear  101   b , driving the gear  101   b  to rotate in a direction opposite the moving direction of the belt at the point of contact therewith, as shown in  FIGS. 4A and 4B . Hence, the surface of the cleaning roller  101  moves opposite the direction in which the belt moves at the point of contact between the two. 
     During a process to remove toner from the paper-conveying belt  6 , a constant voltage source (not shown) applies a bias of opposite polarity to the toner to the cleaning roller  101 . With this construction, toner remaining on the paper-conveying belt  6  becomes attracted to the cleaning roller  101  when the toner comes into contact with the cleaning roller  101  in a cleaning operation, thereby effectively removing the toner from the belt. Although the cleaning device  100  functions to remove toner in this description, the cleaning device  100  may also function to remove paper dust deposited on the belt, thereby achieving two functions with the same member. 
     As shown conceptually in  FIGS. 4A and 4B , the cleaning roller  101  in the printer  1  according to the preferred embodiment is capable of contacting and separating from the paper-conveying belt  6 . Specifically, when not performing a cleaning operation, the cleaning roller  101  is in a separated position shown in  FIG. 4A . When performing a cleaning operation, the cleaning roller  101  is moved next to and in contact with the paper-conveying belt  6 , as shown in  FIG. 4B . When the paper-conveying belt  6  is being cleaned, the paper-conveying belt  6  is pinched between the cleaning roller  101  and the backup roller  110 . 
     More specifically, as shown in  FIGS. 1 ,  5 , and  6 , the entire cleaning device  100  including the cleaning roller  101 , cleaning blade  107 , and collecting box  103  is capable of rotating about a shaft  105  extending parallel to the shaft  62   a  of the drive roller  62 . By rotating the cleaning device  100  about the shaft  105 , the cleaning device  100  can be moved between a separated position in which the cleaning roller  101  is separated from the paper-conveying belt  6  and a contact position in which the cleaning roller  101  abuts the paper-conveying belt  6  (indicated by  101  in  FIG. 1 ). 
     Further, spring members  109  urge the entire cleaning device  100  toward the contact position. When moved to the separated position, the entire cleaning device  100  is displaced downward against the urging force of the spring members  109 , as shown in  FIG. 5 . When moved to the contact position, the urging force of the spring members  109  places the cleaning roller  101  in contact with the paper-conveying belt  6 , as shown in  FIG. 6 . Movement of the cleaning device  100  is accomplished with cams  108  that are supported on a rotatable camshaft  108   a  (see  FIGS. 5 and 6 ). 
     The cams  108  displace the cleaning device  100  through contact with operating parts  104  disposed on the collecting box  103  of the cleaning device  100 . When the rotational position of the cams  108  is as shown in  FIG. 5 , the cams  108  apply a force to the cleaning device  100  via the operating parts  104 , pushing the cleaning roller  101  downward against the urging force of the spring members  109 . When the cams  108  are in the rotational position shown in  FIG. 6 , the cams  108  no longer transfer a force to the cleaning device  100  and, hence, the urging force of the spring members  109  lifts the cleaning device  100  upward so that the cleaning roller  101  is pressed against the paper-conveying belt  6 . Here, the spring members  109  are supported on the frame (not shown in  FIGS. 5 and 6 ) fixed to the main casing  1   a.    
     Next, a tension-adjusting mechanism  120  will be described. In the preferred embodiment, the paper-conveying belt  6  is cleaned by pinching the paper-conveying belt  6  between the cleaning roller  101  and the backup-roller  110 . However, this configuration applies a large load to the paper-conveying belt  6  due to the pinching of the cleaning roller  101  and backup roller  110 , producing slack in the paper-conveying belt  6 , as shown in  FIG. 4B , so that the movement of the paper-conveying belt  6  tends to become unstable. Hence, the printer  1  of the preferred embodiment includes the tension-adjusting mechanism  120  that is capable of adjusting the tension-state of the paper-conveying belt  6  in order to effectively reduce slack in the paper-conveying belt  6 , even when the conditions are conducive to generating slack. 
     The tension-adjusting mechanism  120  is configured to change the tension state of the paper-conveying belt  6  between a first tension state used when transferring toner images from the photosensitive drums  3 , and a second tension state for generating a greater tension than that in the first tension state. In other words, the tension-adjusting mechanism  120  can produce a tension state in the paper-conveying belt  6  that is greater than the first tension state used for transfer operations. 
     As described above, the paper-conveying belt  6  is supported in the printer  1  by the drive roller  62  and follower roller  63 . The spring members  121  urge the follower roller  63  in a direction away from the drive roller  62 . However, by adjusting the load applied to the follower roller  63  by the spring members  121 , the tension-adjusting mechanism  120  can adjust the tension state generated in the paper-conveying belt  6 . 
     The tension-adjusting mechanism  120  includes rotating members  122  that are rotatably supported on a shaft  123 . The shaft  123  is arranged parallel to the shaft  63   a  of the follower roller  63 . One end of each spring member  121  is fixed to an end of the corresponding rotating member  122 , while the other end is attached to the retaining member  124 , which is mounted on the shaft  63   a  of the follower roller  63 , as described above. Accordingly, when the rotating members  122  rotate on the shaft  123 , the ends of the spring members  121  attached to the rotating members  122  move closer to or away from the drive roller  62 , thereby adjusting the force applied to the retaining members  124  (a force in a direction away from the drive roller  62 ). More specifically, the rotating members  122  are positioned so as to contact cams  125 . The cams  125  are supported on a shaft  125   a . The shaft  125   a  is rotatably supported in the frame fixed to the main casing  1   a  and arranged parallel to the shaft  123  of the rotating members  122  and the shaft  63   a  of the follower roller  63 . Displacement of the cams  125  determines the rotational position of the rotating members  122  and, consequently, sets the amount of load applied by the spring members  121 . 
     In the preferred embodiment, displacement of the cams  125  corresponds with displacement of the cams  108 . Specifically, while the cleaning roller  101  is configured to approach and separate from the paper-conveying belt  6  in the preferred embodiment, the tension-adjusting mechanism  120  sets the paper-conveying belt  6  to the first tension state in association with the cleaning roller  101  separating from the paper-conveying belt  6 , and sets the paper-conveying belt  6  to the second tension state in association with the cleaning roller  101  abutting the paper-conveying belt  6 . 
     In other words, during a transfer operation, the displacement of the cams  125  is set so that the paper-conveying belt  6  is in the first tension state, as shown in  FIG. 5 . At this time, the spring members  121  generate an appropriate load to produce suitable tension in the paper-conveying belt  6  for performing transfer operations. At the same time, the displacement of the cams  108  is set so that the cleaning device  100  is pushed downward. 
     When cleaning the paper-conveying belt  6  with the cleaning device  100 , the cams  108  separate from the operating parts  104 , as shown in  FIG. 6 , so that the cleaning roller  101  is in contact with the paper-conveying belt  6 . At the same time, displacement of the cams  125  is set to produce the second tension state in the paper-conveying belt  6 . Specifically, the cams  125  are set as shown in  FIG. 6 , producing a greater load in the spring members  121  than in the state shown in  FIG. 5 . Hence, the spring members  121  pull stronger on the follower roller  63  than during the transfer operation shown in  FIG. 5 . 
     With this construction, the tension in the paper-conveying belt  6  is set higher in a cleaning operation than in a transfer operation, since a larger load is applied to the paper-conveying belt  6  when cleaning. Accordingly, this construction can effectively prevent sagging or other problems in the paper-conveying belt  6  caused by the load applied thereto. Hence, this construction ensures that the paper-conveying belt  6  moves with stability, thereby achieving stable cleaning. 
     When the paper-conveying belt  6  is at rest, the tension-adjusting mechanism  120  can set the tension produced in the paper-conveying belt  6  to a third tension state that produces a smaller tension than that in the first tension state. This third tension state can be employed as a measure to prevent the paper-conveying belt  6  from becoming misshapen. If the paper-conveying belt  6  is in a halted state for a long period of time while the tension in the paper-conveying belt  6  is kept high, for example, portions of the paper-conveying belt  6  that contact the drive roller  62  and follower roller  63  may begin to take on the shape of the drive roller  62  and follower roller  63  at the contact surface, which can lead to irregular transfers and other problems. 
     More specifically, the displacement of the cams  125  is set as shown in  FIG. 7 , so that the rotating members  122  rotate downward, setting the load in the spring members  121  less than that during a transfer operation (the state shown in  FIG. 5 ). At this time, the cams  108  are positioned according to the displacement of the cams  125  and hold the cleaning device  100  in a state in which the cleaning roller  101  does not contact the paper-conveying belt  6 . 
     One method of setting the displacement in the cams  125  is through use of a stepping motor SM, as shown in  FIG. 5 . More specifically, the stepping motor SM in the preferred embodiment is configured to drive the cams  108  under the control of the controller  10  (see  FIG. 1 ) described above. The controller  10  sets the displacement of the stepping motor SM depending on whether the paper-conveying belt  6  is performing a transfer, being cleaned, or at rest, thereby setting the cams  108  in the states shown in  FIGS. 5 ,  6 , and  7 . This configuration is only one example, and it should be apparent that the present invention is not limited to any structure, provided that the structure can set the cams  108  to a position corresponding to the states used for a transfer operation, a cleaning operation, and when the paper-conveying belt  6  is at rest. 
     The associated rotating operations of the cams  125  and cams  108  can be configured via gears or other components. Alternatively, a stepping motor may be provided for each of the cams  125  and cams  108 , and the controller  10  can control both stepping motors to move in association with each other so that displacement in the cams  125  and cams  108  is varied associatively. 
     Next, a belt unit according to a second embodiment of the present invention will be described with reference to  FIG. 8 . 
     While the cleaning member of the first embodiment described above is configured of a cleaning roller, the cleaning member according to the second embodiment is configured of a blade member  111  that is incapable of rotating and that contacts and scrapes the outer surface of the belt. Since the remaining structure of the belt unit according to the second embodiment is identical to that described in the first embodiment, like parts and components are designated with the same reference numerals to avoid duplicating description. As in the first embodiment, the cleaning device  200  according to the second embodiment is configured to rotate in response to displacement of the cams  108 . As the cleaning device  200  rotates, the blade member  111  moves into contact with the paper-conveying belt  6  and separates therefrom. 
     As in the first embodiment, the tension-adjusting mechanism  120  according to the second embodiment can switch the tension state of the paper-conveying belt  6  between the first tension state (shown in  FIG. 8A ) for transferring toner images from the photosensitive drums  3  to a second tension ( FIG. 8B ) for producing a tension greater than that in the first tension state required for cleaning. While not shown in  FIGS. 8A and 8B , the tension-adjusting mechanism  120  can also set the tension of the paper-conveying belt  6  when the paper-conveying belt  6  is at rest to a third tension state generating a smaller tension than that during a transfer operation. 
     Next, an image-forming device according to a third embodiment of the present invention will be described with reference to  FIG. 9 . While the preferred embodiments described above cover a structure for changing tension in a paper-conveying belt, the third embodiment describes a structure for changing the tension state of an intermediate transfer belt. Other than the fact that an intermediate transfer belt is the subject of tension adjustments, the remaining structure of the image-forming device (such as the drive roller  62 , follower roller  63 , and cleaning device  100 ) is identical to that described in  FIG. 1  and, therefore, like parts and components are designated with the same reference numerals to avoid duplicating description. 
     The printer  101  in  FIG. 9  is configured as an intermediate transfer type printer having an intermediate transfer belt  68  functioning to relay toner images carried on the photosensitive drum  3  to a transfer position for transferring the images onto paper. In this construction, toner images formed on the four photosensitive drums  3  in four different colors are transferred onto the intermediate transfer belt  68  temporarily. Subsequently, the toner images carried on the intermediate transfer belt  68  are transferred onto the paper P as the paper P passes through a contact position between the intermediate transfer belt  68  and a secondary roller  67 . 
     Since toner images are directly transferred onto the belt in a system using an intermediate transfer belt, cleaning is vital to achieve high precision image formation. Hence, as in the first embodiment, the printer  1  according to the third embodiment includes the cleaning device  100  disposed adjacent to the intermediate transfer belt  68 . Further, in order to improve the cleaning performance, the tension state of the intermediate transfer belt  68  is switched between a first tension state for transferring toner images from the photosensitive drum  3  onto the intermediate transfer belt  68 , and a second tension state for generating a tension greater than that in the first tension state in order to perform cleaning. 
     The mechanism for adjusting the tension state in the belt is identical to that described in the first embodiment. By adjusting the urging force applied to the follower roller  63  (an urging force provided by a spring member (not shown) in a direction away from the drive roller  62 ), the tension-adjusting mechanism switches between the first tension state and the second tension state. As in the first embodiment, the tension-adjusting mechanisms can adjust the tension state to a third tension state for producing a tension smaller than that in the first tension state. The associated structures of the follower roller  63  and cleaning device  100  are also identical to that described in the first embodiment. 
     While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
     While the image-forming device in the preferred embodiments described above is a color laser printer, the image-forming device may be another type of printer, such as a monochrome laser printer; or a device other than a printer, such as a facsimile device or a multifunction device having a printer function, scanner function, and the like. 
     While the belt in the preferred embodiments described above is a paper-conveying belt or an intermediate transfer belt, the belt according to the present invention may be a photosensitive belt that is exposed by light to form electrostatic latent images thereon. This type of belt is used when the image-forming device is configured to perform image-on image development (either the single-pass or multi-pass method). 
     In the preferred embodiments described above, the belt is set to a higher tension during cleaning (second tension state) than during a transfer operation. However, the timing at which the belt is set to the second tension state is not limited to this construction. For example, the belt may be put in the second tension state prior to a transfer operation to remove any misshapenness in the belt.