Abstract:
A looped belt member is stretched by a drive roller and a driven roller and circulated therearound. Each of image formers forms a toner image associated with each single color constituting a color image. A plurality of transferring members are disposed on a circulating path of the belt member. Each transferring member is associated with each image former for transferring the toner image onto either the belt member or a recording medium held on the belt member. A cleaning blade is abutted against a part of the belt member wound on the drive roller for removing toner remained on the belt member after the toner image transfer is performed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a belt stretcher that can be downsized, and a color image formation apparatus incorporating the same, in particular to a so-called tandem color image formation apparatus for transferring toner images formed by a plurality of single-color toner image formers in sequence to a belt or a recording medium held on the belt, thereby forming a color image.  
           [0002]    Generally, each toner image former has a photoconductor having a photosensitive layer on the outer peripheral surface, a charger for uniformly charging the outer peripheral surface of the photoconductor, an exposer for selectively exposing the outer peripheral surface charged uniformly by the charger for forming an electrostatic latent image, and a developer for giving toner to the electrostatic latent image formed by the exposer for providing a visible image (toner image).  
           [0003]    The known tandem image formation apparatus for forming a color image is of the type wherein a plurality of (for example, four) toner image formers as mentioned above are placed for an intermediate transfer belt and toner images on photoconductors provided by the single-color toner image formers are transferred to the intermediate transfer belt in sequence for superposing the toner images of a plurality of colors (for example, yellow, cyan, magenta, and black) on the intermediate transfer belt, thereby providing a color image on the intermediate transfer belt. Another known tandem image formation apparatus is of the type wherein a recording medium, such as paper, is held on a recording medium holding belt and toner images provided by a plurality of single-color toner image formers are transferred to the recording medium in sequence for superposing the toner images of a plurality of colors on the recording medium, thereby providing a color image on the recording medium.  
           [0004]    The tandem color image formation apparatus of each type comprises a meander regulation (meander prevention) mechanism because a color shift occurs if the belt meanders.  
           [0005]    An apparatus as shown in FIG. 10 is a related tandem image formation apparatus for forming a color image (disclosed in Japanese Patent Publication No. 3-191368A).  
           [0006]    In this image formation apparatus, a plurality of (in this case, four) single-color toner image formers  101  (A, B, C, and D) are placed for an intermediate transfer belt  100 .  
           [0007]    Each single-color toner image former  101  has a photoconductor  102  having a photosensitive layer on the outer peripheral surface, a charger  103  for uniformly charging the outer peripheral surface of the photoconductor  102 , an exposer  104  for selectively exposing the outer peripheral surface charged uniformly by the charger  103  for forming an electrostatic latent image, and a developer  105  for giving toner to the electrostatic latent image formed by the exposer  104  for providing a visible image (toner image).  
           [0008]    The toner images on the photoconductors  102  provided by the single-color toner image formers  101  are primarily transferred to the intermediate transfer belt  100  in sequence by corona transfer devices  106  for superposing the toner images of a plurality of colors (for example, yellow, cyan, magenta, and black) in sequence on the intermediate transfer belt  100 , thereby providing a color image on the intermediate transfer belt  100 , and the color image is secondarily transferred from the intermediate transfer belt  100  to a recording medium P by a corona transfer device  110  in a secondary transfer section T 2 .  
           [0009]    The recording medium P is supplied one at a time from a paper feeding cassette  107  by a paper feeding belt  108  and passes through gate rollers  109 . In the secondary transfer section T 2 , a color toner image is transferred to the recording medium P. After then, the recording medium P is transported on an attractive transport belt  111 . The color toner image is fixed by a fixing section  112  and the recording medium P is ejected onto a paper ejection tray  114  by ejection rollers  113 .  
           [0010]    Each single-color toner image former  101  is provided with a cleaning blade  115  for removing toner remaining on the surface of the photoconductor  102  after the toner image is transferred to the intermediate transfer belt  100 , and the intermediate transfer belt  100  is provided with a cleaning blade  120  for removing toner remaining thereon after the secondary transfer.  
           [0011]    The intermediate transfer belt  100  is placed on a drive roller  130  and a driven roller  131 , and a tension roller  132  is placed on a slack side  100   a  of the intermediate transfer belt  100 . A press roller  133  for pressing the intermediate transfer belt  100  against the photoconductor  102  is placed on a tense side  100   b  of the intermediate transfer belt  100 .  
           [0012]    The cleaning blade  120  abuts the intermediate transfer belt  100  in a winding part of the intermediate transfer belt  100  around the driven roller  131 . Numeral  116  denotes a sensor for detecting a reference position of the belt.  
           [0013]    The cleaning blade  120  acts on circulation of the intermediate transfer belt  100  as resistance. It acts as large resistance particularly when the drive roller  130  is activated (when circulating the belt is started).  
           [0014]    In the related image formation apparatus, the cleaning blade  120  abuts the intermediate transfer belt  100  in the winding part of the intermediate transfer belt  100  around the driven roller  131  and thus the resistance of the cleaning blade  120  in the abutment part acts on the intermediate transfer belt  100  as a tensile force between the winding part and the winding part of the belt around the drive roller  130 .  
           [0015]    The abutment state of the cleaning blade  120  on the intermediate transfer belt  100  is not always stable because of the friction therebetween and becomes unstable particularly at the initial stage of driving. Therefore, in this stage, the tension acting on the intermediate transfer belt  100  between the drive roller  130  and the cleaning blade  120  also becomes unstable.  
           [0016]    Therefore, the expansion and contraction state of the intermediate transfer belt  100  becomes unstable so that a shift between the transfer positions of color toner images onto the intermediate transfer belt  100  easily occurs. Consequently, the quality of a color image is easily degraded.  
           [0017]    In this kind of color image formation apparatus, if the intermediate transfer belt meanders, a shift between colors occurs and thus it is desirable that a meander regulation (meander prevention) mechanism should be provided. In the apparatus as described above, the secondary transfer section T 2  is formed in the winding part of the intermediate transfer belt  100  around the driven roller  131 . Thus, it is undesirable that the driven roller  131  is used as a meander regulation roller. Since meander of the intermediate transfer belt is mainly regulated in the process from the winding start position of the intermediate transfer belt around the meander regulation roller to the winding end part, the intermediate transfer belt is easily displaced in the width direction thereof in the winding part and therefore the state of the secondary transfer easily becomes unstable.  
           [0018]    Thus, in the apparatus as described above, the tension roller  132  or the press roller  133  needs to be used as a meander regulation roller.  
           [0019]    However, in such a configuration, it is impossible to eliminate the tension roller  132  or the press roller  133  contrary to the requirement of simplification or downsizing of the apparatus. In addition, as seen in FIG. 10, the winding angle of the intermediate transfer belt  100  around the tension roller  132 , the press roller  133  is small and thus it is hard to provide a sufficient meander regulation effect.  
           [0020]    As a belt stretcher having a meander regulation mechanism, a mechanism as shown in FIG. 11 is known (disclosed in Japanese Patent Publication No. 5-52244A).  
           [0021]    In the figure, a belt  3  is placed on a drive roller  1  and a driven roller  2  and circulated in the arrow A direction, with a driving force provided from a drive motor  4 .  
           [0022]    The driven roller  2  is provided as a regulation roller for regulating meander and at least one end  2   c  of the driven roller  2  is supported so that it can be moved in the arrow C direction for regulating meander.  
           [0023]    The driven roller (regulation roller)  2  is provided at the end  2   c  with a tapered detection roller  5  (shaped like a truncated cone) that can be rotated independently of a roller main body  2   b  with respect to a shaft  2   d , and a string member  7  wound around a boss part  5   b  of the detection roller  5  for joint is fixed to a frame S at an opposite end thereof.  
           [0024]    Initially, the driven roller (regulation roller)  2  is placed so that as the axis-to-axis distance between the driven roller  2  and the drive roller  1 , L 2  on the side of a move end  2   c  is a little shorter than L 1  on the side of a fixed end  2   a  and accordingly initially the belt  3  moves in the arrow B direction. However, when the belt  3  moves in the arrow B direction and an edge part  3   a  of the belt  3  is wound around a taper face of the detection roller  5 , the detection roller  5  is rotated following the belt  3 , whereby the string member  7  is wound around the boss part  5   b  of the detection roller  5  and is pulled and the free end  2   c  is pulled in the arrow C direction by reaction force and the above-mentioned axis-to-axis distance L 2  on the move end  2   c  side becomes larger than the axis-to-axis distance L 1  on the fixed end  2   a  side. Accordingly, the belt  3  moves in an opposite direction to the arrow B and consequently meander of the belt  3  is regulated.  
           [0025]    According to such a belt stretcher, it is made possible to downsize the belt stretcher (and therefore downsize an image formation apparatus). That is, to regulate meander of a belt in a general belt stretcher, it is common practice to provide a third roller and implement the third roller as a meander regulation roller and thus at least three rollers are required, but the belt stretcher described with reference to FIG. 11 makes the third roller unnecessary and requires only two rollers, so that it is made possible to downsize the belt stretcher (and therefore downsize an image formation apparatus).  
           [0026]    A belt less stretched to prevent a position shift of an image is used as a belt used with an image formation apparatus (for example, an intermediate transfer belt).  
           [0027]    Thus, in fact, the belt stretcher in the related art shown in FIG. 11 is hard to provide the desired motion described above.  
           [0028]    For example, to provide the desired motion described above, it is considered that the one end  2   a  of the regulation roller  2  is supported immovably and that only the opposite end  2   c  is supported movably in the arrow C direction. In doing so, however, it becomes difficult to give a predetermined tension to the belt  3 , because the rollers  1  and  2 , the belt  3 , and the support members of the rollers  1  and  2  contain their respective dimension errors and an error also occurs in the axis-to-axis distance on the fixed end  2   a  side.  
           [0029]    To make it possible to give a predetermined tension to the belt  3 , for example, in FIG. 11, both the ends  2   a  and  2   c  of the regulation roller  2  may be urged (F 1  and F 2 ) initially by springs or the like, in the stretching direction of the belt  3  and the urging force F 2  on the free end  2   c  side may be set a little smaller than the urging force F 1  on the fixed end  2   a  side. However, to provide the desired motion described above, urging force F 3  is required for urging the free end  2   c  of the regulation roller  2  in the former position direction (opposite to the arrow C direction) after the free end  2   c  moves in the arrow C direction.  
           [0030]    However, in the belt stretcher shown in FIG. 11, the free end  2   c  of the regulation roller  2  moves in the initial stretching direction of the belt  3  (arrow C direction) and thus the urging force F 3  cannot be set, because the urging force F 3  and the urging force F 2  cancel each other out. Therefore, the belt stretcher shown in FIG. 11 is hard substantially to provide the desired motion described above.  
           [0031]    The problem described above can be solved by setting the moving direction of the regulation roller for meander regulation to any other direction than the initial stretching direction of the belt by the regulation roller.  
           [0032]    In doing so, however, another problem arises as described below:  
           [0033]    In a tandem color image formation apparatus, a plurality of single-color toner image formers are placed for a belt and toner images are transferred to the belt or a recording medium held on the belt. If the moving direction of the regulation roller for meander regulation is set to any other direction than the initial stretching direction of the belt by the regulation roller, the belt is displaced in a direction away from or close to the single-color toner image formers.  
           [0034]    If the belt is displaced in the direction away from the single-color toner image formers, it is feared that the contact state between the belt and each toner image former (photoconductor thereof, for example) may become unstable in the transfer section, causing a transfer failure to occur.  
           [0035]    If the belt is displaced toward the image formers, the winding angle of the belt with respect to the photoconductor, for example, in the toner image former closest to the regulation roller grows and the transfer bias fluctuates and thus it is still feared that a transfer failure may occur.  
         SUMMARY OF THE INVENTION  
         [0036]    It is therefore a first object of the invention to provide a color image formation apparatus that can decrease a shift between the transfer positions of color toner images onto an intermediate transfer belt for enhancing the quality of a color image.  
           [0037]    A second object of the invention to provide a color image formation apparatus that can lessen the number of rollers for downsizing the apparatus, and moreover can provide a sufficient meander regulation effect for consequently improving the image quality.  
           [0038]    A third object of the invention to provide a color image formation apparatus that can provide a good transfer state although the moving direction of the regulation roller-for meander regulation is set to any other direction than the initial stretching direction of the belt by the regulation roller.  
           [0039]    In order to achieve the above objects, according to the present invention, there is provided a color image formation apparatus, comprising:  
           [0040]    a drive roller;  
           [0041]    a driven roller;  
           [0042]    a looped belt member, which is stretched by the drive roller and the driven roller and circulated therearound;  
           [0043]    a plurality of image formers, each image former forms a toner image associated with each single color constituting a color image;  
           [0044]    a plurality of transferring members, disposed on a circulating path of the belt member, each transferring member being associated with each image former for transferring the toner image onto either the belt member or a recording medium held on the belt member; and  
           [0045]    a cleaning blade, abutted against a part of the belt member wound on the drive roller for removing toner remained on the belt member after the toner image transfer is performed.  
           [0046]    In this configuration, since the cleaning blade abuts against the part of the belt member wound on the drive roller, the expansion and contraction state of the intermediate transfer belt becomes stable even at the initial stage of driving.  
           [0047]    That is, as described above, the cleaning blade acts on circulation of the belt member as resistance and becomes large resistance particularly when driving is started. However, in the above configuration, the resistance of the cleaning blade in the abutment part cannot act as a tensile force of the belt member. It acts only on the winding part of the belt member around-the drive roller as resistance.  
           [0048]    Thus, if the abutment state of the cleaning blade on the belt member is not stable because of the friction therebetween, the state does not affect any tension acting on the belt member.  
           [0049]    Therefore, a shift between the transfer positions of color toner images onto the belt member or the recording medium is remarkably decreased even at the initial stage of image formation, and consequently the quality of a color image is enhanced.  
           [0050]    Preferably, a friction coefficient of an outer peripheral surface of the drive roller is larger than a friction coefficient of an inner surface of the looped belt member.  
           [0051]    In this configuration, the circulation of the belt member becomes further stable and therefore the expansion and contraction state of the belt member also becomes further stable.  
           [0052]    Here, it is preferable that the drive roller serves as a secondary transfer member together with a secondary transfer roller in a case where the toner images are primarily transferred onto the belt member. A hardness of the outer peripheral surface of the drive roller is smaller than a hardness of the secondary transfer roller.  
           [0053]    In this configuration, since a nip face in the secondary transfer section becomes a curved surface recessed to the side of the drive roller, a good strip property of the recording medium from the belt member in the secondary transfer section can be provided, and winding the recording medium around the belt member can be prevented.  
           [0054]    Preferably, the driven roller serves as a regulation roller, which moves in a direction other than a direction in which the driven roller initially stretches the belt member to regulate meander action of the belt member.  
           [0055]    In this configuration, since it is not necessary to provide any roller member other than the drive roller and the driven roller to regulate the meander action of the belt member, the apparatus can be downsized.  
           [0056]    Here, it is preferable that the driven roller moves in a direction perpendicular to the initial stretching direction.  
           [0057]    In this configuration, the meander action of the belt member can be regulated more efficiently. Moreover, it is made possible to fine regulation.  
           [0058]    Alternatively, it is preferable that the driven roller moves in a direction such that a part of the belt member at which a transferring member closest to the driven roller is separated from the associated image former.  
           [0059]    In this configuration, the winding angle of the belt member with respect to the image former does not grow and the fear of occurrence of a transfer failure is eliminated.  
           [0060]    Therefore, a good transfer condition can be attained although the moving direction of the driven roller for meander regulation is set to any other direction than the initial stretching direction.  
           [0061]    Here, it is preferable that the transferring member closest to the driven roller is provided as a transfer roller for urging the belt member toward the associated image former with an urging force greater than a force produced when the driven roller regulates the meander action of the belt member.  
           [0062]    In this configuration, the contact state between the belt member and the image former becomes stable, causing no transfer failure to occur. Even if the urging force of the transfer roller is increased, the frictional force with the belt member does not grow and consequently a smooth running state of the belt member can be provided.  
           [0063]    Further, it is preferable that the transferring members other than the transfer roller are provided as either transfer blades.  
           [0064]    In this configuration, it is made possible to provide an inexpensive image formation apparatus with a simple structure as compared with the case where all transferring members are implemented as transfer rollers.  
           [0065]    Alternatively, it is preferable that the color image formation apparatus further comprises a positioning member disposed between the driven roller and the transferring member closest to the driven roller, the positioning member being abutted against an inner surface of the looped belt member. Here, a friction coefficient of an abutting surface of the positioning member is less than a friction coefficient of the inner surface of the looped belt member.  
           [0066]    In this configuration, even if the driven roller is displaced, the contact between the belt member and the toner image former becomes stable, causing no transfer failure to occur.  
           [0067]    Since the positioning member has a good sliding proper relative to the belt member, the frictional force with the belt member does not much grow and consequently a smooth running state of the belt member can be provided.  
           [0068]    Here, it is preferable that all of the transferring members are provided as either transfer blades or corona transfer devices.  
           [0069]    In this configuration, since the need for implementing transfer rollers as the transferring members is eliminated, it is made possible to provide an inexpensive image formation apparatus with a simple structure.  
           [0070]    Preferably, the drive roller serves as a secondary transfer member together with a secondary transfer roller in a case where the toner images are primarily transferred onto the belt member. Here, a diameter of the driven roller is larger than a diameter of the drive roller.  
           [0071]    In this configuration, since the drive roller, which is a main factor member of causing meander to occur, has a smaller diameter than the driven roller, the meander distance itself of the intermediate transfer belt is decreased.  
           [0072]    On the other hand, since the driven roller implemented as the meander regulation roller of the intermediate transfer belt has a larger diameter than the drive roller, and thus the winding length of the belt member is enlarged so that the meander of the belt member is regulated reliably and smoothly.  
           [0073]    Therefore, the color image formation apparatus can provide a sufficient meander regulation effect and consequently the image quality is improved.  
           [0074]    Further, meander regulation rollers other than the driven roller become unnecessary, so that it is made possible to downsize the apparatus.  
           [0075]    Still further, the secondary transfer section is formed in the winding part of the belt member around the drive roller which is not the meander regulation roller, so that a stable secondary transfer state can be provided.  
           [0076]    Moreover, the drive roller forming the secondary transfer section has a small diameter, so that the strip property of the recording medium from the secondary transfer section is improved and winding trouble of the-recording medium becomes hard to occur.  
           [0077]    According to the present invention, there is also provided a color image formation apparatus, comprising:  
           [0078]    a drive roller;  
           [0079]    a driven roller;  
           [0080]    a looped belt member, which is stretched by the drive roller and the driven roller and circulated therearound;  
           [0081]    a plurality of image formers, each image former forms a toner image associated with each single color constituting a color image; and  
           [0082]    a plurality of transferring members, disposed on a circulating path of the belt member, each transferring member being associated with each image former for transferring the toner image onto either the belt member or a recording medium held on the belt member, wherein:  
           [0083]    the driven roller serves as a regulation roller, which moves in a direction other than a direction in which the driven roller initially stretches the belt member such that a part of the belt member at which a transferring member closest to the driven roller is separated from the associated image former, in order to regulate meander action of the belt member; and  
           [0084]    the transferring member closest to the driven roller is provided as a transfer roller for urging the belt member toward the associated image former with an urging force greater than a force produced when the driven roller regulates the meander action of the belt member.  
           [0085]    According to the present invention, there is also provided a color image formation apparatus, comprising:  
           [0086]    a drive roller;  
           [0087]    a driven roller;  
           [0088]    a looped belt member, which is stretched by the drive roller and the driven roller and circulated therearound;  
           [0089]    a plurality of image formers, each image former forms a toner image associated with each single color constituting a color image;  
           [0090]    a plurality of transferring members, disposed on a circulating path of the belt member, each transferring member being associated with each image former for transferring the toner image onto either the belt member or a recording medium held on the belt member; and  
           [0091]    a positioning member disposed between the driven roller and a transferring member closest to the driven roller, the positioning member being abutted against an inner surface of the looped belt member, wherein:  
           [0092]    the driven roller serves as a regulation roller, which moves in a direction other than a direction in which the driven roller initially stretches the belt member such that a part of the belt member at which the transferring member closest to the driven roller is separated from the associated image former, in order to regulate meander action of the belt member; and  
           [0093]    a friction coefficient of an abutting surface of the positioning member is less than a friction coefficient of the inner surface of the looped belt member.  
           [0094]    According to the present invention, there is also provided a color image formation apparatus, comprising:  
           [0095]    a drive roller having a first diameter;  
           [0096]    a driven roller having a second diameter larger than the first diameter;  
           [0097]    a looped belt member, which is stretched by the drive roller and the driven roller and circulated therearound;  
           [0098]    a plurality of image formers, each image former forms a toner image associated with each single color constituting a color image;  
           [0099]    a plurality of transferring members, disposed on a circulating path of the belt member, each transferring member being associated with each image former for primarily transferring the toner image onto the belt member; and  
           [0100]    a secondary transfer roller, which urges the belt member toward the drive member to transfer the toner image on the belt member onto a recording medium placed therebetween.  
           [0101]    According to the present invention, there is also provided a belt stretcher, comprising:  
           [0102]    a drive roller;  
           [0103]    a driven roller; and  
           [0104]    a looped belt member, which is stretched by the drive roller and the driven roller and circulated therearound,  
           [0105]    wherein the driven roller serves as a regulation roller, which moves in a direction other than a direction in which the driven roller initially stretches the belt member.  
           [0106]    In this configuration, the urging force for urging the driven roller to the initial position after the meander regulation is not offset by the initial urging force for stretching the belt member. Since it is not necessary to provide any roller member other than the drive roller and the driven roller to regulate the meander action of the belt member, the apparatus can be downsized. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0107]    The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0108]    [0108]FIG. 1 is a schematic front view to show a color image formation apparatus according to a first embodiment of the invention;  
         [0109]    [0109]FIG. 2 is an enlarged view of a secondary transfer section of the apparatus in FIG. 1;  
         [0110]    [0110]FIG. 3A is a schematic perspective view to show a belt stretcher incorporated in a color image formation apparatus according to a second embodiment of the invention;  
         [0111]    [0111]FIG. 3B is a schematic representation of the function of the belt stretcher;  
         [0112]    [0112]FIG. 4 is a schematic front view to show a color image formation apparatus according to a third embodiment of the invention;  
         [0113]    [0113]FIG. 5 is a schematic front view to show a color image formation apparatus according to a fourth embodiment of the invention;  
         [0114]    [0114]FIG. 6 is a schematic front view to show a color image formation apparatus according to a fifth embodiment of the invention;  
         [0115]    [0115]FIG. 7 is a schematic front view to show a color image formation apparatus according to a sixth embodiment of the invention;  
         [0116]    [0116]FIG. 8 is a schematic front view to show a color image formation apparatus according to a seventh embodiment of the invention;  
         [0117]    [0117]FIG. 9 is a sectional view taken on a line IX-IX of FIG. 8;  
         [0118]    [0118]FIG. 10 is a schematic front view to show a related color image formation apparatus; and  
         [0119]    [0119]FIG. 11 is a schematic plan view to show a related belt stretcher. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0120]    Referring now to the accompanying drawings, there are shown preferred embodiments of the invention.  
         [0121]    [0121]FIG. 1 is a schematic front view to show a color image formation apparatus according to a first embodiment of the invention.  
         [0122]    As shown in the figure, the color image formation apparatus comprises an intermediate transfer belt  30  placed on two rollers of a drive roller  10  and a driven roller  20  and circulated in the arrow direction shown (counterclockwise) and a plurality of (four) single-color toner image formers  40  (Y, C, M, and K) placed for the intermediate transfer belt  30 , and transfers toner images provided by the plurality of single-color toner image formers  40  to the intermediate transfer belt  30  in sequence by separate transferring members  51 ,  52 ,  53 , and  54 . Primary transfer sections are denoted by T 1 Y, T 1 C, T 1 M, and T 1 K.  
         [0123]    The single-color toner image formers  40 (Y) for yellow,  40 (M) for magenta,  40 (C) for cyan, and  40 (K) for black are placed. Each of these single-color toner image formers  40  (Y, C, M, and K) has a photoconductor  41  having a photosensitive layer on the outer peripheral surface, a charging roller  42  for uniformly charging the outer peripheral surface of the photoconductor  41 , an exposer  43  for selectively exposing the outer peripheral surface charged uniformly by the charging roller  42  with light (L) for forming an electrostatic latent image thereon, a developing roller  44  for giving toner to the electrostatic latent image formed by the exposer  43  for providing a visible image (toner image), and a first cleaning blade  45  for removing toner remaining on the surface of the photoconductor  41  after the toner image developed by the developing roller  44  is transferred to the intermediate transfer belt  30 .  
         [0124]    The single-color toner image formers  40  (Y, C, M, and K) are placed on the slack side of the intermediate transfer belt  30 .  
         [0125]    The full-color toner image provided by primarily transferring toner images in sequence to the intermediate transfer belt  30  and superposing the toner images on each other on the intermediate transfer belt  30  is secondarily transferred to a recording medium P such as paper in a secondary transfer section T 2  and is fixed on the recording medium P as it passes through a fixing roller pair  61 , then is ejected to a predetermined place (onto a paper ejection tray or the like, not shown) by a paper ejection roller pair  62 .  
         [0126]    Numeral  63  denotes a paper feeding cassette where recording media P are stacked, numeral  64  denotes a pickup roller for feeding the recording media P one at a time from the paper feeding cassette  63 , and numeral  65  denotes a gate roller pair for defining the supply timing of the recording medium P to the secondary transfer section T 2 .  
         [0127]    Numeral  66  denotes a secondary transfer roller for forming the secondary transfer section T 2  with the intermediate transfer belt  30 , and numeral  67  denotes a second cleaning blade for removing toner remaining on the surface of the intermediate transfer belt  30  after the secondary transfer.  
         [0128]    The second cleaning blade  67  abuts the intermediate transfer belt  30  in the winding part of the intermediate transfer belt  30  around the drive roller rather than around the driven roller  20 .  
         [0129]    As shown in FIG. 2, the drive roller  10  has a core part  12  and a surface layer  13 , which is made of a material having large friction against the back face of the intermediate transfer belt  30  (for example, rubber or the like).  
         [0130]    The secondary transfer section T 2  is implemented as the above-mentioned secondary transfer roller  66  pressed against the drive roller  10  via the intermediate transfer belt  30 , and the surface layer  13  of the drive roller  10  is made of an elastic layer having a lower hardness than the secondary transfer roller  66 .  
         [0131]    As described above, the second cleaning blade  67  acts on circulation of the intermediate transfer belt  30  as resistance and becomes large resistance particularly when the drive roller  10  is activated. However, according to the above configuration, since a second cleaning blade  67  for removing toner remaining on the intermediate transfer belt  30  after the secondary transfer abuts the intermediate transfer belt  30  in the winding part of the intermediate transfer belt  30  around the drive roller  10 , the resistance of second cleaning blade  67  in the abutment part cannot act as a tensile force of the intermediate transfer belt  30 . It acts only on the winding part of the intermediate transfer belt  30  around the drive roller  10  as resistance.  
         [0132]    Thus, if the abutment state of the second cleaning-blade  67  on the intermediate transfer belt  30  is not stable because of the friction therebetween, the state does not affect any tension acting on the intermediate transfer belt  30 .  
         [0133]    Therefore, the expansion and contraction state of the intermediate transfer belt  30  becomes stable even at the initial stage of driving.  
         [0134]    Thus, a shift between the transfer positions of color toner images onto the intermediate transfer belt  30  is remarkably decreased. It becomes hard to occur even at the initial stage of image formation so that the quality of a color image is enhanced.  
         [0135]    In addition, since the surface layer  13  of the drive roller  10  is made of a material having large friction against the back of the intermediate transfer belt  30 , the drive state of the intermediate transfer belt  30  becomes further stable. Therefore, the expansion and contraction state of the intermediate transfer belt  30  also becomes further stable.  
         [0136]    Therefore, the shift between the transfer positions of color toner images onto the intermediate transfer belt  30  is further decreased and the quality of a color image is further enhanced.  
         [0137]    Furthermore, since the secondary transfer section T 2  is implemented as the secondary transfer roller  66  pressed against the drive roller  10  via the intermediate transfer belt  30  and the surface layer  13  of the drive roller  10  is made of an elastic layer having a lower hardness than the secondary transfer roller  66 , a nip face N in the secondary transfer section T 2  becomes a surface recessed to the side of the drive roller  10 .  
         [0138]    Therefore, a good strip state of the recording medium P from the intermediate transfer belt  30  in the secondary transfer section T 2  can be provided, and winding the recording medium P around the intermediate transfer belt  30  can be prevented.  
         [0139]    [0139]FIG. 3A is a schematic perspective view to show a belt stretcher incorporated in a color image formation apparatus according to a second embodiment of the invention. Parts identical with or similar to those previously described with reference to FIG. 1 in the first embodiment are denoted by the same reference numerals in FIG. 3A.  
         [0140]    The second embodiment differs from the first embodiment mainly in that a driven roller  20  is used as a regulation roller moved for meander regulation in a direction perpendicular to the axial direction of the roller  20  (arrow X 1 , X 2  direction) to regulate meander of an intermediate transfer belt  30 , and that the moving direction of the regulation roller  20  for meander regulation is set to any other direction (see the arrow Y 1 , Y 2  direction) than the initial stretching direction of the belt  30  by the regulation roller  20  (see the arrow F 2 ).  
         [0141]    Although the moving direction of the regulation roller  20  for meander regulation may be set to any other direction than the initial stretching direction of the belt  30  by the regulation roller  20 (see the arrow F 2 ), in the embodiment, it is set to a direction (arrow Y 1 , Y 2  direction) roughly perpendicular to the initial stretching direction of the belt  30  by the regulation roller  20  (see the arrow F 2 ).  
         [0142]    More particularly, a drive roller  10  is rotatively supported at both ends by bearing members (not shown) and is rotated by a drive motor  11 . The driven roller  20  serves as the regulation roller as described later. The intermediate transfer belt  30  is circulated in the arrow A direction.  
         [0143]    Therefore, single-color toner image formers  40  (Y, C, M, and K) are placed on the slack side of the intermediate transfer belt  30 .  
         [0144]    A secondary transfer section T 2  is formed by the drive roller  10  unchanged in position and a secondary transfer roller  66 .  
         [0145]    As shown in FIG. 3A, the driven roller (regulation roller)  20  is urged at both ends  21  and  22  initially by urging members such as springs in the stretching direction of the belt  3  (see urging forces F 1  and F 2 ) and the urging force F 2  on the side of the free end  22  is set a little smaller than the urging force F 1  on the side of the fixed end  21 .  
         [0146]    A movable member  70  is placed at the free end  22  of the regulation roller  20 . A slot  71  is formed with the movable member  70  and the free end  22  of the regulation roller  20  is inserted into the slot  71 . Therefore, the free end  22  can be moved in the length direction of the slot  71  (arrow Y 1 , Y 2  direction) as it is guided by the slot  71 . The movable member  70  can be moved only in the stretching direction of the intermediate transfer belt  30  (arrow F 2  direction and opposite direction thereto) and the urging force F 2  acts on the movable member  70 , whereby the free end  22  of the regulation roller  20  is initially urged in the stretching direction of the intermediate transfer belt  30  by the urging force F 2  via the movable member  70 . Therefore, the free end  22  can be moved in the arrow Y 1 , Y 2  direction and the arrow F 2  direction (and opposite direction thereto). The fixed end  21  of the regulation roller  20  is urged in the F 1  direction via the bearing member (not shown) can be moved only in the arrow F 1  direction (and opposite direction thereto).  
         [0147]    The regulation roller  20  is formed at the free end  22  with a tapered detection roller  25  (shaped like a truncated cone) that can be rotated independently of a roller main body  24  with respect to a shaft  23 , and a string member  27  wound around a boss part  25   b  of the detection roller  25  for joint is fixed to a fixation frame  80  at an opposite end  27   a  thereof.  
         [0148]    A spring  81  is placed between the free end  22  of the regulation roller  20  and the fixation frame  80  and the free end  22  is urged in the arrow Y 2  direction by an urging force F 3  of the spring  81 . When the intermediate transfer belt  30  stops, a move in the arrow Y 2  direction is regulated by the action of the string member  27 .  
         [0149]    According to the described belt stretcher, as for the urging forces F 1  and F 2  initially urging both the ends  21  and  22  of the regulation roller  20  in the stretching direction of the belt  30 , the urging force F 2  on the free end  22  side is set a little smaller than the urging force F 1  on the fixed end  21  side, and thus initially as the axis-to-axis distance L 2  between the regulation roller  20  and the drive roller  10  on the free end  22  side is a little shorter than the axis-to-axis distance L 1  on the fixed end  21  side. Therefore, when the intermediate transfer belt  30  is circulated, initially the belt  30  moves in the arrow X 1  direction. However, when an edge part  31  of the belt  30  is wound around a taper face  25   a  of the detection roller  25 , the detection roller  25  is rotated following the belt  30 , whereby the string member  27  is wound around the boss part  25   b  of the detection roller  25  and is pulled. Incidentally, the free end  22  is pulled in the arrow Y 1  direction so that the meander is regulated by reaction force and the above-mentioned axis-to-axis distance L 2  on the free end  22  side becomes a distance L 2 ′ larger than the axis-to-axis distance L 1  on the fixed end  21  side, as shown in FIG. 3B. Accordingly, the belt  30  moves in the arrow X 2  direction and consequently meander of the belt  30  is regulated.  
         [0150]    According to the configuration as discussed in this embodiment, since the intermediate transfer belt  30  is placed only on the two rollers of the drive roller  10  and the driven roller (regulation roller)  20 , it is made possible to downsize the belt stretcher (and therefore downsize the image formation apparatus). That is, to regulate meander of a belt in a-general belt stretcher, it is common practice to provide a third roller and implement the third roller-as a meander regulation roller and thus at least three rollers are required, but the belt stretcher makes the third roller unnecessary and requires only two rollers, so that it is made possible to downsize the belt stretcher (and therefore downsize the image formation apparatus).  
         [0151]    Since the moving direction of the regulation roller  20  for meander regulation is set to any other direction than the initial stretching direction (F 2 ) of the belt  30  by the regulation roller  20 , the desired meander regulation motion described above can be provided.  
         [0152]    In addition, since the moving direction of the regulation roller  20  for meander regulation is set to the direction (Y 1 ) roughly perpendicular to the initial stretching direction (F 2 ) of the belt  30  by the regulation roller  20 , meander of the belt  30  can be regulated more efficiently. Moreover, the moving direction (Y 1 ) of the regulation roller  20  for meander regulation is set to the direction roughly perpendicular to the initial stretching direction (F 2 ) of the belt  30  by the regulation roller  20 , whereby as seen in FIG. 3B, an increment of the axis-to-axis distance (L 2  to L 2 ′) becomes small so that it is made possible to finely regulate meander of the belt.  
         [0153]    Still further, since the meander regulation move direction (Y 1 ) of the regulation roller  20  is the direction bringing the part of the belt  30  in a primary transfer section T 1 K (see FIG. 1) closest to the regulation roller  20  away from the single-color toner image former  40 (K), the winding angle of the belt  30  with respect to a photoconductor  41  in the primary transfer section T 1 K does not grow, so that the fear of occurrence of a transfer failure is eliminated.  
         [0154]    Displacing of the intermediate transfer belt  30  in the direction bringing the belt  30  away from the single-color toner image formers  40  can be circumvented by implementing at least the primary transferring member closest to the regulation roller  20  as contact-type member (for example, the transfer blade  54  shown in FIG. 1 or the like) for urging the belt  30  toward the single-color toner image former  40  (K) by a larger force than the force produced by the regulation roller  20  for bringing the belt  30  away from the single-color toner image formers  40 .  
         [0155]    Therefore, the color image formation apparatus can provide a good transfer state although the moving direction of the regulation roller  20  for meander regulation is set to any other direction than the initial stretching direction of the belt  30  by the regulation roller  20 .  
         [0156]    [0156]FIG. 4 is a schematic front view to show a color image formation apparatus according to a third embodiment of the invention. Parts identical with or similar to those previously described with reference to FIG. 3A in the second embodiment are denoted by the same reference numerals.  
         [0157]    The third embodiment differs from the second embodiment in that primary transferring member  54 ′ closest to a regulation roller  20  is implemented as a transfer roller for urging a belt  30  toward a single-color toner image former  40  (K) by a larger force F 4  than the force produced by the regulation roller  20  for bringing the belt  30  away from the single-color toner image former.  
         [0158]    In a case where the transfer blade is used to circumvent the displacement of the intermediate transfer belt  30  in the direction (Y 1 ) bringing the belt  30  away from the single-color toner image former  40 , the frictional force between the transfer blade and the intermediate transfer belt  30  grows with an increase in the urging force and therefore the configuration is not preferred.  
         [0159]    In contrast, according to the color image formation apparatus of this embodiment, since the primary transferring member closest to the regulation roller  20  is implemented as the transfer roller  54 ′ for urging the belt  30  toward the single-color toner image former  40  (K) by the larger force F 4  than the force produced by the regulation roller  20  for bringing the belt  30  away from the single-color toner image former, the contact state between the intermediate transfer belt  30  and the photoconductor  41  of the toner image former in a transfer section T 1 K becomes stable. If the urging force F 4  is increased, the frictional force with the intermediate transfer belt  30  does not grow and consequently a smooth running state of the intermediate transfer belt  30  can be provided.  
         [0160]    [0160]FIG. 5 is a schematic front view to show a color image formation apparatus according to a fourth embodiment of the invention. Parts identical with or similar to those previously described with the third embodiment are denoted by the same reference numerals.  
         [0161]    The fourth embodiment differs from the third embodiment only in that a holding belt  32  for holding a recording medium P is used as a belt in place of the intermediate transfer belt  30  and toner images provided by a plurality of single-color toner image formers  40  (Y, C, M, and K) are transferred to the recording medium P held on the belt  32  in sequence by separate transferring members  51 ,  52 ,  53  and  54 ′.  
         [0162]    In FIG. 5, numeral  68  denotes a transport roller pair for transporting the-recording medium P to a gate roller pair  65 , and numeral  69  denotes a roller for holding the recording medium P on the holding belt  32 .  
         [0163]    According to the embodiment, similar advantages to those of the third embodiment described above can also be provided.  
         [0164]    Since the transfer roller  54  of a last transfer section T 1 K transports the recording medium P reliably, the recording medium P enters a fixing roller pair  61  smoothly and becomes hard to wrinkle.  
         [0165]    Each of the transferring members  51 ,  52 , and  53  other than the transfer roller  54  may be implemented as a discharge-type transfer device (corona transfer device).  
         [0166]    [0166]FIG. 6 is a schematic front view to show a color image formation apparatus according to a fifth embodiment of the invention. Parts identical with or similar to those previously described with reference to FIG. 4 in the third embodiment are denoted by the same reference numerals.  
         [0167]    The fifth embodiment differs from the third embodiment in that a positioning member  28  for abutting the back face of a belt  30  for positioning the belt  30  is placed between a regulation roller  20  and a transfer section T 1 K closest to the regulation roller  20 .  
         [0168]    The positioning member  28  is formed of a rod-like body which is semicircular in cross section, extending over the full length of the belt  30  in the width direction thereof, and is fixed at both ends to a fixation frame (not shown). At least the contact part of the positioning member  28  with the back face of the belt  30  is made of a material having a good sliding proper relative to the back of the belt  30 .  
         [0169]    According to this configuration, even if the regulation roller  20  is displaced in the arrow Y 1  direction, the contact between the intermediate transfer belt  30  and the photoconductor  41  of the toner image former in the primary transfer section T 1 K becomes stable, causing no transfer failure to occur.  
         [0170]    Since the positioning member  28  has a good sliding proper relative to the back of the belt  30 , the frictional force with the intermediate transfer belt  30  does not much grow and consequently a smooth running state of the intermediate transfer belt  30  can be provided.  
         [0171]    Each of the transferring members  51 ,  52 ,  53 , and  54  of this embodiment is implemented as a transfer blade, but may be implemented as a corona transfer device.  
         [0172]    Therefore, this embodiment eliminates the need for implementing the transferring member as transfer rollers, and makes it possible to implement the transferring member as transfer blades or corona transfer devices, so that it is made possible to provide an inexpensive image formation apparatus with a simple structure.  
         [0173]    [0173]FIG. 7 is a schematic front view to show a color image formation apparatus according to a sixth embodiment of the invention. Parts identical with or similar to those previously described with reference to the fifth embodiment are denoted by the same reference numerals.  
         [0174]    The sixth embodiment differs from the fifth embodiment only in that a holding belt  32  for holding a recording medium P is used as a belt in place of the intermediate transfer belt  30  and toner images provided by a plurality of single-color toner image formers  40  (Y, C, M, and K) are transferred to the recording medium P held on the belt  32  in sequence by separate transferring members  51  to  54 .  
         [0175]    According to this embodiment, similar advantages to those of the fifth embodiment described above can also be provided.  
         [0176]    [0176]FIG. 8 is a schematic front view to show a color image formation apparatus according to a seventh embodiment of the invention. Parts identical with or similar to those previously described with the first embodiment are denoted by the same reference numerals.  
         [0177]    The seventh embodiment differs from the first embodiment only in that a driven roller  20 ′ has a larger diameter than a drive roller  10 .  
         [0178]    The secondary transfer section T 2  is implemented as a secondary transfer roller  66  pressed against the drive roller  10  via an intermediate transfer belt  30  in the winding part of the intermediate transfer belt  30  around the drive roller  10 .  
         [0179]    The driven roller  20 ′ is implemented as a meander regulation roller of the intermediate transfer belt  30 .  
         [0180]    An appropriate configuration can be adopted; in the embodiment, as shown in FIG. 9, beads  32 R and  32 L are placed along both sides of the back face of the intermediate transfer belt  30  and the driven roller  20 ′ is provided with regulation rings  28 R and  28 L that can rotate independently of a roller main body  24  relative to a shaft  23 , whereby the driven roller  20 ′ is implemented as the meander regulation roller.  
         [0181]    In FIG. 9, if the intermediate transfer belt  30  attempts to move (meander) in the arrow X 1  direction, the bead  32 R abuts a slope  28 Ra of the regulation ring  28 R in the process from a winding start part a around the regulation roller  20 ′ to a winding end part b (see FIG. 8), regulating the move of the intermediate transfer belt  30 . To the contrary, if the intermediate transfer belt  30  attempts to move in the arrow X 2  direction, the bead  32 L abuts a slope  28 La of the regulation ring  28 L, regulating the move of the intermediate transfer belt  30 . Accordingly, meander of the intermediate transfer belt  30  is regulated.  
         [0182]    According to the above configuration, since the drive roller  10  which is a main factor member of causing meander to occur, has a smaller diameter than the driven roller  20 ′, the meander distance itself of the intermediate transfer belt  30  is decreased.  
         [0183]    On the other hand, since the driven roller  20 ′ implemented as the meander regulation roller of the intermediate transfer belt  30  has a larger diameter than the drive roller  10 , and thus the winding length defined between parts a and b of the intermediate transfer belt  30  around the driven roller (meander regulation roller)  20  (see FIG. 8) is enlarged, meander of the intermediate transfer belt  30  is regulated reliably and smoothly.  
         [0184]    Therefore, the color image formation apparatus can provide a sufficient meander regulation effect and consequently the image quality is improved.  
         [0185]    Further, meander regulation rollers (tension roller  132 , press roller  133 , etc., previously described with reference to FIG. 10) other than the driven roller  20 ′ become unnecessary, so that it is made possible to downsize the apparatus.  
         [0186]    Still further, the secondary transfer section T 2  is formed in the winding part of the intermediate transfer belt  30  around the drive roller  10  which is not the meander regulation roller, so that a stable secondary transfer state can be provided.  
         [0187]    Moreover, the drive roller  10  forming the secondary transfer section T 2  has a small diameter, so that the strip property of the recording medium P from the secondary transfer section T 2  is improved and winding trouble of the recording medium P becomes hard to occur.  
         [0188]    As described above, the color image formation apparatus can provide the advantages that the number of rollers can be lessened for downsizing the apparatus, that a sufficient meander regulation effect can be provided for improving the image quality, and that winding trouble of the recording medium becomes hard to occur.  
         [0189]    The meander regulation mechanism described with reference to FIGS. 3A and 3B is applicable to the drive roller  20 ′ of the seventh embodiment. In such a configuration, in addition to the advantages described in the seventh embodiment, there is also obtained the same advantages attained by the meander regulation mechanism shown in FIG. 3A  
         [0190]    Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.