Abstract:
According to an aspect of the present invention, there is provided a belt unit including: a belt that is formed in an endless shape; a first roller that supports the belt from an inner side of the belt; a second roller that supports the belt from the inner side of the belt; regulation walls that are disposed on both sides of the second roller and that each includes a boss protruding outwardly, the boss having a tapered portion; and plate frames that are disposed on both sides of the belt and that each includes: a first groove portion that supports the first roller; and a second groove portion that supports the second roller, the second groove portion having a rounded edge formed to be run on by the boss through the tapered portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from Japanese Patent Application No. 2007-238615 filed on Sep. 14, 2007, and from Japanese Patent Application No. 2008-040911 filed on Feb. 22, 2008, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    An aspect of the present invention relates to a belt unit and an image forming apparatus using the same. 
         [0004]    2. Description of the Related Art 
         [0005]    An image forming apparatus is generally classified into a tandem image forming apparatus and an image forming apparatus of four-rotation type. In the tandem image forming apparatus, an intermediate transfer element or a transfer belt for sheet conveyance purpose is used. The transfer belt usually rotates upon receipt of rotational force from at least one drive roller supporting a belt. Meanwhile, in the four-rotation type image forming apparatus, a transfer belt to which tensile force is applied and in which a drive roller is equipped is used as an intermediate transfer element. 
         [0006]    When one of the rollers equipped in the transfer belt are twisted with respect to the other one during the transferring operation, the transfer belt moves in a direction orthogonal to the rotational direction; that is, so-called deviation to one side occurs. A method for bringing a flange into contact with an end face of the transfer belt, to thus regulate deviation of the belt, is generally used frequently to prevent deviation of the transfer belt. 
         [0007]    As another method for preventing deviation of a belt, a steering roller is used as one of rollers among which a transfer belt is suspended (see; for example, JP-2005-292480-A and JP-2006-65056-A). According to the method, the steering roller is provided, and hence, when the transfer belt deviates to one side, one roller shaft is tilted by a motor, thereby making a correction to movement of the transfer roller in an opposite direction. 
         [0008]    All of the above-described related-art methods present a problem in terms of the durability of the belt and the cost of components. According to the method that brings the flange into contact with the end face of the belt, the end of the belt keeps performing movement while contacting the flange, so that the end of the belt will wear out and encounter a decrease in durability. When force for moving the belt in one direction becomes greater, abrasion or cracking (rupture) takes place in the end of the transfer belt, which in turn induces variations in rotation of the transfer belt or sometimes cause defects in an image. Therefore, to reduce the deviation force on the belt, it is generally required to manage, with high accuracy, a difference between right and left belts in terms of a peripheral length, the thickness of the belts, and alignment among rollers. The necessary entails an increase in the price of components and presents an obstacle in cost reduction. 
         [0009]    In the meantime, according to the method providing the steering roller, a sensor for detecting the position of the end of the belt, a motor for tilting the roller shaft, a controller for controlling the motor in accordance with information detected by the sensor, and the like, are required. Therefore, the number of components is increased, which in turn entails an increase in the size and cost of the apparatus. 
       SUMMARY OF THE INVENTION 
       [0010]    According to an aspect of the present invention, there is provided a belt unit including: a belt that is formed in an endless shape; a first roller that supports the belt from an inner side of the belt; a second roller that supports the belt from the inner side of the belt; regulation walls that are disposed on both sides of the second roller and that each includes a boss protruding outwardly, the boss having a tapered portion; and plate frames that are disposed on both sides of the belt and that each includes: a first groove portion that supports the first roller; and a second groove portion that supports the second roller, the second groove portion having a rounded edge formed to be run on by the boss through the tapered portion. 
         [0011]    According to another aspect of the present invention, there is provided a belt unit including: regulation walls that are formed to regulate a side deviation of a belt in a direction perpendicular to a rotational direction of the belt and that each includes a boss protruding outwardly, the boss having a tapered portion; and plate frames that each includes groove portion having a rounded edge formed to be run on by the boss through the tapered portion in accordance the side deviation of the belt. 
         [0012]    According to still another aspect of the present invention, there is provided an image forming apparatus including the belt unit as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0014]      FIG. 1  is a diagrammatic cross-sectional view of an image forming apparatus of the embodiment; 
           [0015]      FIG. 2  is a perspective view of an intermediate transfer unit of the embodiment; 
           [0016]      FIG. 3  is a perspective view of a regulation wall of the embodiment; 
           [0017]      FIG. 4  is a cross-sectional view of a small roller of the embodiment; 
           [0018]      FIG. 5  is a descriptive view showing the arrangement of respective rollers and fitting grooves of side plate frames of the embodiment; 
           [0019]      FIG. 6  is a view showing the arrangement of a boss of the regulation wall and the fitting groove of the side plate frame; 
           [0020]      FIG. 7  is a view showing that the boss of the regulation wall remains struck on the fitting groove of the side plate frame; 
           [0021]      FIG. 8  is a view showing a relationship between the twisting amount in a small roller shaft and belt end stress; 
           [0022]      FIG. 9  is a perspective view of a fixing unit of the embodiment; 
           [0023]      FIG. 10  is a view showing the layout of a pressure belt, a deviation regulation wall, and a pressure frame of the embodiment; 
           [0024]      FIG. 11  is a view showing the arrangement of a boss of the deviation regulation wall, the pressure belt, the pressure frame, and the plate; and 
           [0025]      FIG. 12  is a view showing that the boss of the deviation regulation wall struck on the fitting groove of the pressure frame. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    An embodiment of the present invention will be described hereunder by reference to  FIGS. 1 through 8 . First, the summary of an electrophotographic apparatus of the present invention will be described by use of  FIG. 1  showing a diagrammatic cross section of the apparatus. 
         [0027]    As shown in  FIG. 1 , an intermediate transfer unit  17  is placed at the center of the apparatus, and a photosensitive member  1 , a transfer roller  10 , and a cleaner  12  are arranged around the intermediate transfer unit  17 . An electric charger  2  is also disposed around the photosensitive member  1 . Developing devices  4 K,  4 Y,  4 M, and  4 C filled with toner that is different four colors of colored fine powders are placed sequentially along the photosensitive member  1 . An exposing device  3  is disposed beneath the developing devices, and sheet holder  8  that holds a sheet and a sheet feeding device  9  are also positioned below the exposing device  3 . A fixing unit  11  and a sheet discharging device  18  are positioned in an upper portion of the electrophotographic apparatus. 
         [0028]    In such a configuration, the electric charger  2  uniformly charges the surface of the photosensitive member  1 . Next, based on information of an image or letters acquired by a personal computer, an image scanner, and the like, the exposing device  3  performs exposure on a per-dot basis, to thus form an electrostatic latent image on the surface of the photosensitive member  1 . Subsequently, toner is applied by the respective one of the developing devices  4 K,  4 Y,  4 M, and  4 C to develop an electrostatic latent image, whereby a toner image is visualized and the thus-visualized image is conveyed to the intermediate transfer unit  17 . 
         [0029]    By repeating the above-mentioned process for each developing devices  4 K,  4 Y,  4 M, and  4 C, a toner image conforming to the information of the image or the letters is formed on a surface of the intermediate transfer unit  17 . Subsequently, the toner image is transferred by a transfer roller  10  onto a sheet fed from the sheet holder  8  by the sheet feeding device  9 . The toner left on the intermediate transfer unit  17  is cleaned by the cleaner  12 . The sheet with the toner image transferred thereon is conveyed to the fixing unit  11 , where the toner image is fixed on the sheet and discharged by the sheet discharging device  18 . 
         [0030]    The intermediate transfer unit as a belt unit in the electrophotographic apparatus of the embodiment will now be described by reference to  FIGS. 2 ,  3  and  4  showing general views of the apparatus. 
         [0031]      FIG. 2  is a perspective view of the intermediate transfer unit  17 . The intermediate transfer unit  17  is made up of a large roller  5 , a tension roller  6 , a small roller  7 , a large roller shaft  30  on which the large roller  5  is supported, a small roller shaft  31  on which the small roller  7  is supported, a transfer belt  22 , regulation walls  23 , tension arms  24 , and tension springs  25 . The transfer belt  22  rotates in a direction of arrow C in the drawings. 
         [0032]      FIG. 3  is a detailed perspective view of the regulation wall  23 , wherein reference symbol γ denotes a cone angle of a boss  23   a  and “r” denotes a curvature of the tip end of the boss. When the transfer belt  22  shown in  FIG. 2  deviates to one side, the boss  23   a  contacts a small-roller-supporting fitting groove  41  (herein after simply called “fitting groove”) of a side plate frame  32  shown in  FIG. 5 , thereby running upon the fitting groove  41 . 
         [0033]      FIG. 4  is a cross-sectional view of the small roller  7 . The small roller  7  is supported by the small roller shaft  31  through ball bearings  26 . The large roller  5  shown in  FIG. 2  is supported by the large roller shaft  30  shown in  FIG. 2  through the ball bearings  26 . 
         [0034]    As shown in  FIG. 2 , in this embodiment, the transfer belt  22  is made from a PC alloy and has a thickness of 0.15 mm, a width of 250 mm, and a peripheral length of 380 mm. The large roller  5  is made from aluminum and has a diameter of 104.8 mm. The small roller  7  is made from aluminum and has a diameter of 30 mm. The large roller shaft  30 , the small roller shaft  31 , and the tension arm  24  are supported on side plate frames  32  as shown in  FIG. 5 . The large roller shaft  30  and the small roller shaft  31  support the large roller  5  and the small roller  7  through the ball bearings  26  as shown in  FIG. 4  and hence does not rotate with respect to the side plate frames  32  shown in  FIG. 5 . The tension roller  6  is made from aluminum and has a diameter of 12 mm. The regulation walls  23  are formed from polyacetal. A deviation preventing flange (herein after simply called “flange”) of the transfer belt  22  has an outer diameter of 33 mm; the base diameter of the boss is 9.0 mm; the cone angle γ of the boss is 10°; and a tip end of the boss has a curvature of 1.0 mm. The tension spring  25  is formed from SWPB and has a spring constant of 0.7 N/mm. 
         [0035]    The intermediate transfer unit  17  does not have drive source and is configured so as to rotate in a following manner upon receipt of rotational force from the photosensitive member  1 . To this end, a nip width of a first transfer section that contacts the photosensitive member  1  is widely ensured. In order to achieve optimum operation, it is better to ensure a value of 10 mm or more for the nip width of the first transfer section. In this embodiment, a nip width of about 24 mm is adopted. The transfer belt  22  is suspended and supported by at least two of rollers including the large roller  5  and the small roller  7 . The large roller  5  that is closer to the photosensitive member  1  is configured to have large diameter to provide a wide nip width. Since the transfer belt  22  serving as an intermediate transfer member is configured to contact with and to be rotated by the photosensitive member  1 , there is arranged a tension roller  6  that applies a moderate tensile force enough to remove slack to the transfer belt  22 . The tension roller  6  is supported by a tension arm  24 . A tension spring  25  supporting the tension arm  24  is configured so as to exert load on the tension roller  6 . A detailed configuration will be described later. 
         [0036]    A method for reducing the deviation of the transfer belt of the embodiment will now be described by reference to  FIGS. 5 ,  6 , and  7  that show an overview of the method. 
         [0037]    First, a twist in the small roller shaft (herein after called a “small roller shaft twist”) with respect to the large roller shaft and deviation of the belt will be described by reference to  FIG. 5 . Large-roller-supporting Fitting grooves  40  (herein after called “fitting grooves  40 ”) of the side plate frames  32  support the large roller shaft  30 . Small-roller-supporting fitting grooves  41  (herein after called “fitting grooves  41 ”) of the side plate frames  32  support the small roller shaft  31 . Arrow C designates the rotational direction of the transfer belt  22 . Reference symbol HL designates a distance between the fitting groove  40  of a side plate frame  32 L and the fitting groove  41  of the side plate frame  32 L. Reference symbol HL designates a distance between the fitting groove  40  and the fitting groove  41  at the L-side plate frame  32 L. Reference symbol HR designates a distance between the fitting groove  40  and the fitting groove  41  at the R-side plate frame  32 R. ΔH (=HR−HL) designates a difference between HL and HR. In this embodiment, ΔH&lt;0.3 mm is achieved. 
         [0038]    ΔT designates an twisting amount in the small roller shaft. ΔT=ΔH is achieved in a state where the large roller shaft  30  and the small roller shaft  31  are supported by the fitting grooves of the side plate frames  32 . 
         [0039]    In the case of ΔT=ΔH&gt;0 (HR&gt;HL), when the transfer belt  22  is rotated so as to follow the photosensitive member  1 , the transfer belt  22  rotates while causing out-of-plane deformation. At this time, deviation force toward the L side is generated in the transfer belt  22 , whereupon the transfer belt  22  moves to the L side. Conversely, in the case of ΔT=ΔH&lt;0, (HR&lt;HL) deviation force to the R side is generated in the transfer belt  22 , whereupon the transfer belt  22  moves to the R side. 
         [0040]      FIG. 6  is an enlarged view of a section A in  FIG. 5 , showing that the transfer belt  22  remains deviated to the L side and in contact with the regulation wall  23 . Reference symbol “l” designates a tangential line at a contact point between the boss  23   a  of the regulation wall  23  and the fitting groove  41 . Reference symbol β designates an angle formed by the tangential line “l” and the surface of the fitting groove  41  of the side plate frame  32 . Reference symbol “r” designates a curvature of the tip end of the boss of the regulation wall  23 . Reference symbol “e” designates a curvature of a section B in the fitting groove  41 . 
         [0041]      FIG. 7  is a view showing that the boss  23   a  of the regulation wall  23  runs upon the fitting groove  41 . 
         [0042]    Reference symbol Δε designates an amount of the running-on of the boss  23   b  upon the fitting groove  41  (herein after called a “running-on amount”). When the boss of the regulation wall  23  runs upon the fitting groove  41 , the striking action can be expressed as ΔT=ΔH−Δε. 
         [0043]    In this embodiment, in the case of ΔH&gt;0, the transfer belt  22  moves to the L side, to thus contact the regulation wall  23 . Subsequently, the regulation wall  23  moves to the L side along with the transfer belt  22 , thereby contacting the fitting groove  41 . On condition that deviation force of the transfer belt  22  (herein after called “belt deviation force”) is F and that frictional force developing between the boss of the regulation wall  23  and the fitting groove  41  is M, when F cos β&gt;M is achieved, the regulation wall  23  runs upon the fitting groove  41 . ΔT decreases with an increase in the running-on amount Δε of the boss, and the belt deviation force F decreases. Deviation of the transfer belt  22  toward the L side starts at a point in time when F cos β=M is satisfied. 
         [0044]      FIG. 8  shows a relationship between the twisting amount ΔT in the small roller shaft and stress σ that is generated in the end of the transfer belt  22  when the end of the transfer belt  22  collides with the regulation wall  23  (herein after called “belt end stress”). 
         [0045]    Reference symbol σk designates allowable stress at which the transfer belt  22  can fulfill its specification life. Reference symbol σmax designates belt end stress σ achieved when the twisting amount ΔT in the small roller shaft is maximum. 
         [0046]    In this embodiment, |ΔT|&lt;0.3 mm is satisfied at a non-operating state. Reference symbol σmin designates stress σ of the transfer belt  22  arising when F cos β=M is achieved. 
         [0047]    According to the configuration, even when a height difference ΔH arises in the right and left fitting grooves  41  and when the belt end stress σ is σk or greater, the boss of the regulation wall  23  runs onto the fitting groove  41  along the tapered boss  23   a  of the regulation wall  23 . When the transfer belt  22  starts deviating as a result of F cos β=M being achieved, σ&lt;σk is attained. Consequently, occurrence of abrasion or cracking (rupture) in the transfer belt  22 , variations in rotation of the transfer belt, and defects in an image can be prevented. A relationship between the end stress σ of the transfer belt  22  and the twisting amount ΔT in the small roller shaft, the shape of the boss  23   a  of the regulation wall  23 , and the frictional force M generated between the boss  23   a  and the fitting groove  41  will be described in detail later. 
       &lt;Relationship Between the Twisting Amount in the Small Roller and the Transfer Belt End Stress&gt; 
       [0048]    A relationship between the twisting amount ΔT in the small roller shaft and the belt end stress σ will be described by reference to  FIG. 8 . 
         [0049]      FIG. 8  is a view showing a relationship between the twisting amount ΔT in the small roller shaft and the belt end stress σ. 
         [0050]    A horizontal axis represents the twisting amount ΔT in the small roller shaft. When the R side of the small roller shaft is higher than the large roller shaft, the amount is designated with a positive (plus) sign. When the L side of the same is higher, the amount is designated with a negative (minus) sign. A vertical axis represents stress σ in the end of the transfer belt  22 . 
         [0051]    In the case of ΔT=0 [mm], belt deviation force F=0 [N] and belt end stress σ=0 [MPa] are achieved. However, in the case where ΔT≠0 [mm], belt deviation force is generated. For instance in the case of ΔT=0.06 [mm], belt end stress σ=2.1 [MPa] is achieved. In the case of ΔT=0.1 mm or more, belt end stress σ&gt; allowable belt stress σk=2.5 [MPa] is occurred, and abrasion or cracking (rupture) takes place in the end of the transfer belt  22 . Further, variations arise in the rotation of the transfer belt  22 , and defects may arise in an image. 
       &lt;About a Cone Angle of the Boss of the Regulation Wall&gt; 
       [0052]    A relationship between the cone angle γ of the boss of the regulation wall  23  and the deviation force F on the transfer belt  22  will be described by reference to  FIG. 6 . 
         [0053]    When the deviation force of the transfer belt  22  is F [N], force of F cos β [N] acts in the direction of the tangential line “l” when the boss  23   s  of the regulation wall  23  collides with the fitting groove  41 . In the meantime, when the boss of the regulation wall  23  runs upon the fitting groove  41 , frictional force M is generated between the boss  23   a  and the fitting groove  41 . In the case of F cos β&gt;M, the boss  23   a  runs upon the fitting groove  41 . 
         [0054]    In the embodiment, frictional force M generated between the boss  23   a  and the fitting groove  41  assumes a value of 9 [N]. In the regulation wall  23 , the cone angle γ of the boss  23   a  is 10°, and the curvature “r” of the tip end of the boss  23   a  is 1 [mm]. A curvature “k” of the section B in the fitting groove is 3 [mm]. In this case, an angle β formed by the tangential line “l” at a contacting point between the tip end of the boss  23   a  and the section B of the fitting groove  41  and the surface of the fitting groove  41  assumes a value of 40°. In the case of belt deviation force F&gt;12.7 [N], the boss of the regulation wall  23  runs upon the fitting groove  41 . Further, as a result of the boss of the regulation wall  23  running upon the fitting groove  41 , the belt deviation force F decreases. At the time when the belt deviation force F assumes a value of 12.7 [N], the belt deviation stops and ΔT assumes a value of 0.06 mm. 
         [0055]    When the cone angle γ of the boss  23   a  assumes a value of 6° or less, the value of the angle β becomes great, and the force F cos β at which the boss of the regulation wall  23  attempts to run upon the fitting groove  41  becomes small. In this case, F cos β&lt;M is attained, and the boss of the regulation wall  23  cannot run upon the fitting groove  41 . In the case of ΔT&gt;0.1 mm, belt end stress σ&gt; allowable belt stress σk is attained. Hence, it becomes impossible to prevent occurrence of abrasion or cracking (rupture) in the transfer belt, variations in the rotation of the transfer belt, and defects in an image. 
         [0056]    By setting the cone angle γ of the boss  23   a  to be equal to or greater than 6°, the boss  23   a  becomes easy to run upon the groove, and the belt deviation force can be diminished. 
         [0057]    From the above descriptions, in this embodiment, as a result of achievement of a relationship of angle γ&gt;6°, the deviation of the transfer belt  22  can be reduced, and occurrence of abrasion or cracking (rupture) in the transfer belt  22 , variations in the rotation of the transfer belt  22 , and defects in an image can be prevented. 
         [0000]    &lt;In the Case where the Intermediate Transfer Unit is Driven&gt; 
         [0058]    The intermediate transfer unit  17  is taken as being driven in this embodiment. Even when the large roller  5  or the small roller  7  of the intermediate transfer unit  17  is driven, the deviation force acting on the transfer belt  22  can be reduced by adoption of a similar configuration, and occurrence of abrasion or cracking (rupture) in the transfer belt  22 , variations in the rotation of the transfer belt  22 , and defects in an image can be prevented. 
       &lt;In the Case of a Tandem Type&gt; 
       [0059]    The color imaging forming apparatus of this embodiment is of a four rotation type, and the transfer belt  22  is used as an intermediate transfer element. However, even when the transfer belt  22  is used as an intermediate transfer element or for the purpose of conveying a sheet in an image forming apparatus of a tandem type, adoption of a similar configuration results in a reduction in the deviation of the transfer belt  22  and enables prevention of occurrence of abrasion or cracking (rupture) in the transfer belt  22 , variations in the rotation of the transfer belt  22 , and defects in an image. 
         [0060]    A case where the belt unit is used for a fixing unit will now be described as another embodiment of the present invention by reference to  FIGS. 9 through 12 . Even when the regulation walls  23  of the above embodiment are used in the intermediate transfer unit  17 , even a fixing unit using a belt can also prevent occurrence of abrasion or cracking (rupture) in a belt and defects in an image by adapting a similar configuration. 
         [0061]      FIG. 9  shows a perspective view of the fixing unit. The fixing unit is made up of a heating roller  51 , a pressure belt  52 , deviation regulation walls  53 , pressure frames  54 , and plates  55 . The drawing shows the arrangement of these elements. The heating roller  51  is rotatably supported by the plates  55  and rotated by, for example, an unillustrated gear. The pressure belt  52  is rotated by the rotational drive force of the heating roller  51 . 
         [0062]    The pressure belt  52  is suspended by the pressure unit and is brought into contact with the heating roller  51  by the pressure springs, or the like, while a shaft provided on the plates  55  is taken as a base point. 
         [0063]      FIG. 10  shows the layout of the pressure belt  52 , the deviation regulation walls  53 , and the pressure frames  54 . Each of the deviation regulation walls  53  that end faces of the pressure belt  52  contact has at one end face an essentially-cylindrical guide section for guiding an interior surface of the pressure belt  52  and at the other end face a boss K. The boss K is configured so as to be supported by a fitting groove J of the pressure frame  54 . 
         [0064]      FIG. 11  shows the arrangement of the boss K of the deviation regulation wall  53 , the pressure belt  52 , the pressure frame  54 , and the plate  55 , as well as showing that the pressure belt  52  deviates to in the direction of arrow G for reasons of distortion of the plate  55 . 
         [0065]      FIG. 12  shows that the boss K of the deviation regulation wall  53  runs upon the fitting groove J of the pressure frame  54  by the belt deviation force acting on the pressure belt  52 . Thus, the belt deviation force acting on the pressure belt  52  attributable to the distortion of the plate  55  can be diminished. 
         [0066]    As mentioned above, the pressure frame is embodied in the form of a side plate frame even in the fixing unit, whereby the belt deviation force acting on the pressure belt  52  is lessened, and abrasion or cracking (rupture) in the pressure belt  52 , wrinkles in paper incident to movement of the pressure belt  52  in the thrust direction, a paper jam which becomes likely to arise as a result of occurrence of abrasion or cracking (rupture) in the pressure belt  52 , and the like, can be prevented. 
         [0067]    According to an aspect of the present invention, deviation of a belt is regulated, while suppressing an increase in the number of components and accomplishing low-cost configuration of an intermediate transfer unit and a fixing unit, thereby providing an image forming apparatus having high image quality without variations in an image.