Patent Publication Number: US-2009226223-A1

Title: Transfer device, conveying device and image forming apparatus including these devices

Description:
This Nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2008-57216 filed in Japan on 7 Mar. 2008, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a transfer device and conveying device including a drive roller, a driven roller and a belt tensioned and supported between, and circulated by, these rollers and an image forming apparatus including these devices. 
     (2) Description of the Prior Art 
     There is a known image forming apparatus in which a photoreceptor drum is arranged on the paper feed path that is defined by a transfer belt which holds and conveys a sheet of paper so that the toner image formed on the photoreceptor drum is transferred to the sheet to form an image thereon. In this image forming apparatus, as the transfer belt is tensioned and supported between, and circulated by, a plurality of tensioning rollers, there has been the problem that the belt tends to deviate sidewards of these tensioning rollers due to dimensional distortion of the transfer belt, misalignment between the multiple tensioning rollers and the like. As a result of this, there has occurred image mis-registration in color image printing. 
     To deal with this, in the image forming apparatus disclosed in patent document 1 (Japanese Patent Application Laid-open 2002-333779), a pair of guide ribs  152  are formed at the side edges along the circumferential direction of a transfer belt  151  as shown in  FIG. 1  so that the inner circumferential surfaces of these guide ribs  152  will abut the both sides ends of a driven roller  154  to thereby regulate the sideward movement of transfer belt  151 . In addition, driven roller  154  is designed to have a greater length (in the width direction of transfer belt  151 ) than the length of a drive roller  153 . In this way, drive roller  153  will not get involved in regulation of the transfer belt with guide ribs  152 , hence the frictional force between transfer belt  151  and drive roller  153  becomes stable and image mis-registration can be made small. 
     However, in the image forming apparatus of patent document 1, when drive roller  153  is adapted so as not to get involved in regulating the transfer belt with guide ribs  152  by making the length of drive roller  153  shorter than the length of driven roller  154 , there is the risk that the movement of transfer belt  151  conversely becomes unstable. 
     In particular, when drive roller  153  is being driven, flexure  155  arises as shown in  FIG. 2  (as indicated by arrows  156 ) and transfer belt  151  tends to travel, approaching the center of drive roller  153  due to its elasticity. Further, when the drive gear for driving drive roller  153  is disposed on one end side of drive roller  153 , transfer belt  151  is distorted due to reactive force to the driving force since drive roller  153  is rotationally driven from one side. As a result, a skewing force  157  acting on transfer belt  151  arises, causing a skew. 
     On the other hand, when the drive roller and the driven roller are designed to have approximately the same length so that drive roller  153  can take a role of regulating the transfer belt in cooperation with the guide ribs, if the end face of the driven roller comes to be positioned lower than that of the drive roller due to the tolerance and the design looseness in the thrust direction, the guide rib of the transfer belt for preventing a skew abuts the end face of the drive roller and is positioned at the more interior side, and may climb over the drive roller unable to create steady engagement, in the worst case. As a result, the guide ribs will not produce any preventive effect against skewing and the transfer belt starts to skew. If the belt skews extremely, the transfer belt is wrinkled while continuing to skew. If this condition continues, the transfer belt may rupture or may be locked due to its rotational failure. 
     SUMMARY OF THE INVENTION 
     In view of what has been discussed above, it is an object of the present invention to provide a transfer device or a conveying device including drive and driven rollers and a transfer or conveying belt with guide ribs, which can prevent the guide ribs from climbing over the drive roller and prevent the belt from skewing, by shifting one roller to the other with respect to the width direction, as well as to provide an image forming apparatus including these devices. 
     The first aspect of the present invention resides in a transfer device comprising: a transfer belt to which an image formed on an image bearer is transferred; a drive roller that receives a drive force via a drive gear arranged at one end thereof; and a driven roller that receives no drive force, characterized in that the transfer belt is tensioned and supported between, and conveyed by, the two rollers, the transfer belt has first and second guide ribs formed on the undersurface thereof, the drive roller and the driven roller are specified to have a length approximately equal to each other and shorter than the spacing between the first and second inner circumferential surfaces of the first and second guide ribs, and the drive roller and the driven roller are arranged such that the first spacing between a first end of the drive roller on the drive gear side and the first guide rib is wider than the second spacing between a second end of the driven roller on the drive gear side and the first guide rib. 
     The second aspect of the present invention resides in a conveying device comprising: a conveying belt for conveying sheets; a drive roller that receives a drive force via a drive gear arranged at one end thereof; and a driven roller that receives no drive force, characterized in that the conveying belt is tensioned and supported between, and conveyed by, the two rollers, the conveying belt has first and second guide ribs formed on the undersurface thereof, the drive roller and the driven roller are specified to have a length approximately equal to each other and shorter than the spacing between the first and second inner circumferential surfaces of the first and second guide ribs, and the drive roller and the driven roller are arranged such that the first spacing between a first end of the drive roller on the drive gear side and the first guide rib is wider than the second spacing between a second end of the driven roller on the drive gear side and the first guide rib. 
     Herein, the belt may have an endless belt configuration. 
     Further, the third aspect of the present invention resides in an image forming apparatus which, at least, include either the transfer device or the conveying device. 
     According to the present invention, the transfer belt or the conveying belt first and second guide ribs formed on the undersurface thereof, the drive roller and the driven roller are specified to have a length approximately equal to each other and shorter than the spacing between the first and second inner circumferential surfaces of the first and second guide ribs, and the drive roller and the driven roller are arranged such that the first spacing between a first end of the drive roller on the drive gear side and the first guide rib is wider than the second spacing between a second end of the driven roller on the drive gear side and the first guide rib. As a result, when the belt is started to convey, it is possible to prevent the guide rib of the belt from climbing over the drive roller for conveying the belt on its drive gear side, hence prevent the transfer belt from skewing 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a transfer device in patent document 1; 
         FIG. 2  is an illustrative view showing the principle of the skewing of a transfer belt; 
         FIG. 3  is a schematic view showing the overall configuration of an image forming apparatus to which one embodiment of a transfer device of the present invention is applied; 
         FIG. 4  is a configurational view showing a transfer device; 
         FIG. 5  A is a schematic plan view showing a transfer device; 
         FIG. 5B  is a sectional view showing a transfer belt  31  and guide ribs  61 ; and 
         FIG. 6  is a view showing an experiment on the skewing of a transfer belt in a transfer device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, one embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 3  is a schematic view showing the overall configuration of an image forming apparatus to which one embodiment of the transfer device of the present invention is applied.  FIG. 4  is a configurational view showing the transfer device. 
     An image forming apparatus  100  is to form a monochrome image represented by the image data that is obtained by scanning a document or that is received from without, and is essentially comprised of a document sheet feeder (ADF)  101 , an image reader  102 , a printing portion  103 , a recording sheet conveyor  104  and a paper feeder  105 . 
     As will be detailed layer, the paper conveying device of the present embodiment is applied to document sheet feeder (ADF)  101  and recording sheet feeder  104 . In document sheet feeder  101 , the fluctuation of the conveyed position of document sheets is inhibited by controlling the front end feed operation of the document sheets in accordance with the rigidity of the document sheets. In recording sheet feeder  104 , the fluctuation of the conveyed position of recording sheets is inhibited by controlling the front end feed operation of the recording sheets in accordance with the rigidity of the recording sheets. 
     In document feeder  101 , when, at least, one document sheet is set on a document set tray  11  and the document sheets are pulled out from document set tray  11 , sheet by sheet, the document is conducted to and passed over a document reading window  102   a  of image reader  102  and discharged to a document output tray  12 . 
     A CIS (contact image sensor)  13  is arranged over document reading window  102   a.  This CIS  13  repeatedly reads the image on the underside of the document in the main scan direction while the document is passing over document reading window  102   a,  to thereby output the image data that represents the image on the underside of the document. 
     Further, image reader  102  illuminates the document surface with light from the lamp of a first scan unit  15  when the document sheet passes over document reading window  102   a  and the reflected light from the document sheet surface is lead to an image focusing lens  17  by way of the mirrors of first and second scan units  15  and  16 , so that the image on the document sheet surface is focused by image focusing lens  17  onto a CCD (charge coupled device)  18 . CCD  18  repeatedly reads the image of the document sheet surface in the main scan direction to thereby output image data that represents the image on the document sheet surface. 
     On the other hand, when the document sheet is placed on the platen glass on the top of image reader  102 , first and second scan units  15  and  16  are moved keeping a predetermined speed relationship relative to each other while the document sheet surface on the platen glass is illuminated by first scan unit  15 , and the light reflected off the document sheet surface is lead to image focusing lens  17  by means of first and second scan units  15  and  16  so that the image on the document sheet surface is focused by image focusing lens  17  onto CCD  18 . 
     The image data output from CIS  13  or CCD  18  is subjected to various kinds of image processes by a control circuit such as a microcomputer etc. and then output to printing portion  103 . 
     Printing portion  103  is to record the document images represented by image data on sheets of paper and includes a photoreceptor drum  21 , a charger  22 , an optical writing unit  23 , a developing device  24 , a transfer device  25 , a cleaning unit  26 , a fusing unit  27  and the like. 
     While photoreceptor drum  21  rotates in one direction, its surface is cleaned by cleaning unit  26  and then charged uniformly by charger  22 . Charger  22  may be either a corona discharge type or a roller or brush type that contacts with photoreceptor drum  21 . 
     Optical writing unit  23  is a laser scanning unit (LSU) including two laser emitters  28   a  and  28   b  and two mirror groups  29   a  and  29   b.  This optical writing unit  23  receives image data and emits laser beams from laser emitters  28   a  and  28   b  in accordance with the image data. These laser beams are radiated on photoreceptor drum  21  by way of respective mirror groups  29   a  and  29   b  to thereby illuminate the photoreceptor drum  21  surface that has been uniformly electrified, forming an electrostatic latent image on the photoreceptor drum  21  surface. 
     In order to achieve a high-speed printing operation, this optical writing unit  23  employs a two-beam system including two laser emitters  28   a  and  28   b  to thereby reduce the burden entailing the high frequency of irradiation. 
     Here, as the optical writing unit  23 , an array of light emitting elements, e.g., an EL writing head or LED writing head may be used instead of the laser scanning unit. 
     Developing device  24  supplies toner to the photoreceptor drum  21  surface to develop the electrostatic latent image into a toner image T ( FIG. 4 ) on the photoreceptor drum  21  surface. Transfer unit  25  transfers the toner image T on the photoreceptor drum  21  surface to the sheet P that is conveyed by sheet conveyor  104 . Fusing unit  27  heats and presses the recording sheet P to fix the toner image T onto the recording sheet P. There after, the recording sheet P is further conveyed by sheet conveyor  104  and discharged to a paper output tray  47 . In this while, cleaning unit  26  removes and collects the toner left over on the photoreceptor drum  21  surface after development and transfer. 
     Here, as shown in  FIG. 4 , transfer unit  25  includes a transfer belt  31 , drive roller  32 , driven roller  33 , elastic conductive roller  34  and the like, and rotates transfer belt  31  by supporting and tensioning the belt on the aforementioned rollers  32  to  34  and other rollers. Transfer belt  31  has a predetermined resistivity (e.g., 1×10 9  to 1×10 13  Ω/cm) and conveys the recording sheet P placed on its surface  31   a.  Elastic conductive roller  34  is pressed against the photoreceptor drum  21  surface with transfer belt  31  in between, so as to press the recording sheet P on the surface  31   a  of transfer belt  31  against the photoreceptor drum  21  surface. Applied to this elastic conductive roller  34  is an electric field that has a polarity opposite to the charge of the toner image T on the photoreceptor drum  21  surface. This electric field of the opposite polarity causes the toner image T on the photoreceptor drum  21  surface to transfer to the recording sheet P on transfer belt  31 . For example, when the toner image T bears negative (−) charge, the polarity of the electric field applied to elastic conductive roller  34  is set to be positive (+). The toner left over on transfer belt  31  is removed by a transfer cleaning unit  60 . 
     Fusing unit  27  includes a heat roller  35  and pressing roller  36 . A heater is arranged inside heat roller  35  in order to set the heat roller  35  surface at a predetermined temperature (fusing temperature: approximately 160 to 200 deg. C.). A pair of unillustrated pressing members are arranged at both ends of pressing roller  36  so that pressing roller  36  comes into pressing contact with heat roller  35  with a predetermined pressure. As the recording sheet P reaches the pressing contact portion (called as the fusing nip portion) between heat roller  35  and pressing roller  36 , the unfixed toner image T on the recording sheet P is fused and pressed while it is being conveyed by the rollers  35  and  36 , so that the toner image T is fixed to the recording sheet P. 
     Sheet conveyor  104  includes a plurality of conveying rollers  41  for conveying recording sheet P, a pair of registration rollers  42 , a conveyance path  43 , an inversion/conveyance path  44 , a plurality of branch claws  45 , a pair of paper discharge rollers  46  and the like. 
     Conveyance path  43  receives the recording sheet delivered from paper feeder  105  and conveys the recording sheet P until its leading end reaches registration rollers  42 . Since registration rollers  42  are temporarily suspended at that timing, the leading end of the recording sheet P reaches and abuts registration rollers  42  and the sheet bends. The resiliency of this bent recording sheet P makes the front edge of the recording sheet P substantially parallel to registration rollers  42 . Thereafter, registration rollers  42  start rotating so as to convey the recording sheet P to transfer unit  25  of printing portion  103  and then the recording sheet P is further conveyed by paper discharge rollers  46  to paper output tray  47 . 
     Suspension and rotation of registration rollers  42  can be controlled by switching on or off the clutch between registration roller  42  and its drive shaft or by turning on or off the motor as the drive source of registration rollers  42 . 
     When another image is recorded on the rear side of the recording sheet P, a plurality of branch claws  45  are turned to switch the paper path from conveyance path  43  to inversion/conveyance path  44  so that the recording sheet P is turned upside down and returned through inversion/conveyance path  44  to registration rollers  42  in conveyance path  43 . In this way, another image will be recorded on the rear side of the recording sheet. 
     Arranged at the necessary positions along conveyance path  43  and inversion/conveyance path  44  are several sensors for detecting the position of the recording sheet etc., and based on the position of the recording sheet detected at each sensor, the drives of the conveying rollers and registration rollers are controlled so as to convey and position the recording sheet. 
     Paper feeder  105  includes a plurality of paper feed trays  51 . Each paper feed tray  51  is a tray for holding a stack of recording sheets P and is arranged under image forming apparatus  100 . Also, each paper feed tray  51  includes a pickup roller or the like for pulling out recording sheets, one by one, so as to deliver the picked up recording sheet to conveyance path  43  of sheet conveyor  104 . 
     Since image forming apparatus  100  of the present embodiment is aimed at high speed printing processing, each paper feed tray  51  has a volume capable of stacking 500 to 1500 sheets of recording paper P of a regular size. 
     Arranged on the flank of image forming apparatus  100  are a large capacity paper cassette (LCC)  52  for accommodating large amounts of a plurality of types of recording sheets P and a manual feed tray  53  for essentially supplying recording sheets P of irregular sizes. 
     Paper output tray  47  is arranged on the side opposite from manual feed tray  53 . It is also possible to optionally provide an output paper finisher (for stapling, punching, etc.) or a multi-bin paper output tray, in place of the paper output tray  47 . 
     As transfer belt  31 , a film-formed material made of a flexible and unstretchable resin so as not to impede smooth rotation as a transfer belt is usually used. As the examples of such resin, publicly known thermoplastic resin, thermoplastic elastomer, thermosetting resin and the like can be listed. 
       FIG. 5A  is a schematic plan view showing a transfer device.  FIG. 5B  is a sectional view of transfer belt  31  and guide ribs  61 . 
     Transfer belt  31  has an outer surface  31  and an undersurface  31   b,  and a pair of endless guide ribs  61 A and  61 B (first and second guide ribs) formed circumferentially along both the side edges, with respect to the width direction of transfer belt  31 . That is, guide ribs  61 A and  61 B are formed projectively on undersurface  31   b  of transfer belt  31 . 
     These guide ribs  61 A and  61 B abut both ends of drive roller  32  and driven roller  33  on their inner circumferential surfaces  61 Aa and  6 Ba (first and second inner circumferential surfaces) so as to limit sideward movement of transfer belt  31 . 
     The drive roller and driven roller are specified to have approximately the same length R (with respect to the width direction of transfer belt  31 , designate at X) and to be shorter than the distance, designated at X 0 , between inner circumferential surfaces  61 Aa and  61 Ba. Transfer belt  31  tensioned and supported between drive roller  32  and driven roller  33  is conveyed in the direction of arrow A in  FIG. 5A . 
     A drive gear  62  is arranged at the left side of drive roller  32 , so that the drive power from a driving motor is transmitted to drive roller  32  via drive gear  62 . In this arrangement, drive roller  32  and driven roller  33  are arranged such that the spacing, designated at X 1  (the first spacing) between the end, designated at  32   a  (the first end) of drive roller  32  on the drive gear  62  side and the inner circumferential surface  61 Aa of guide rib  61 A (the first guide rib) on the drive gear  62  side is wider than the spacing, designated at X 2  (the second spacing) between the end, designated at  33   a  (the second end) of driven roller  33  on the drive gear  62  side and the inner circumferential surface  61 Aa (X 1 &gt;X 2 ). This arrangement of drive roller  32  and driven roller  33  makes it possible to prevent guide rib  61  from climbing over drive roller  32 , hence prevent transfer belt  31  from skewing. This will be detailed hereinbelow. 
       FIG. 6  is an illustration when the experiment for examining the skew of transfer belt  31 . The arrangement is the same as that shown in  FIGS. 5A and 5B , but the experiment was carried out by changing spacings X 1  and X 2 . 
     In this case, the projected amount X 3  of drive roller  32  to the left from the left side end  33   a  of driven roller  33  was varied to observe the skew of the transfer belt. The result is shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Projected Amount (X3) 
                 Skew 
               
               
                   
                   
               
             
            
               
                   
                 1.05 mm 
                 XX 
               
               
                   
                 0.80 mm 
                 X 
               
               
                   
                  0.4 mm 
                 X 
               
               
                   
                 0.05 mm 
                 ◯ 
               
               
                   
                 0.00 mm 
                 ◯ 
               
               
                   
                 −0.05 mm   
                 ◯ 
               
               
                   
                 −0.04 mm   
                 ◯ 
               
               
                   
                 −0.80 mm   
                 ◯ 
               
               
                   
                 −1.05 mm   
                 ◯ 
               
               
                   
                   
               
            
           
         
       
     
     Here, when the projected amount X 3  is negative, this implies that the left side end  32   a  of drive roller  32  is positioned to the right from the left side end  33   a  of driven roller  33 . 
     From the observation result, when the projected amount X 3  was 0.05 mm or lower, or when the left side end  32   a  of drive roller  32  was not projected a distance greater than 0.05 mm leftwards from left side end  33   a  of driven roller  33 , no skew of transfer belt  31  was observed. 
     When flexure  71  arises while drive roller  32  is being driven, transfer belt  31  tends to travel moving toward the center of drive roller  32  due to its elasticity (indicated by arrows  72 ). Further, since drive gear  62  for driving drive roller  32  is disposed on the left side end  32   a  of drive roller  32 , transfer belt  31  is distorted due to the reactive force to the drive force etc., causing a skewing force  73  in the direction opposing drive gear  62 . Thus, transfer belt  31  skews to the right in  FIG. 6 . 
     Accordingly, in the experiment, when the projected amount X 3  was 0.05 mm or below even though it was positive, no skew was observed. That is, as shown in  FIG. 5A , when drive roller  32  and driven roller  33  are arranged such that spacing X 1  between the end  32   a  of drive roller  32  on the drive gear  62  side and the inner circumferential surface  61 Aa of guide rib  61 A is wider than spacing X 2  between the end  33   a  of driven roller  33  on the drive gear  62  side and the inner circumferential surface  61 Aa, it is possible to positively prevent transfer belt  31  from skewing. 
     Though the above embodiment was described by taking an example of transfer belt  31  for conveying recording paper as the belt having guide ribs  61 A and  61 B provided thereon, the present invention should not be limited to this. That is, guide ribs  61 A and  61 B may be formed in an intermediate transfer belt  31 B ( FIG. 4 ) to which the toner image T formed on the photoreceptor drum  21  is directly transferred to the belt surface. Also in this configuration, it is possible to positively prevent intermediate transfer belt  31 B from skewing. 
     Accordingly, examples of the transfer belt for transferring the toner image formed on photoreceptor drum  21  may and should include both transfer belt  31  for transferring the toner image to recording paper and intermediate transfer belt  31 B to which the toner image is directly transferred. 
     Further, though the above embodiment was described by exemplifying transfer belt  31  for conveying recording sheets to which toner images are transferred, the invention should not be limited to this configuration or the invention can be applied to a belt for conveying sheets to which no toner image is transferred. That is, the belt formed with guide ribs  61  and  61 B may be applied not only to a transfer device but also can be applied to a mere sheet conveying device.