Patent Publication Number: US-8543042-B2

Title: Image forming apparatus and image forming method

Description:
BACKGROUND OF THE INVENTION 
     The entire disclosure of Japanese Patent Application No. 2009-073913, filed Mar. 25, 2009 is expressly incorporated herein by reference. 
     1. Technical Field 
     The present invention relates to an image forming apparatus and an image forming method. More specifically, the present invention relates to an apparatus and method for forming an image by developing a latent image using a liquid developer including toner and carrier, transferring the developer onto a medium such as recording paper, fusing and fixing the toner image transferred onto the medium. 
     2. Related Art 
     Various wet-type image forming apparatuses are currently known in the art which develop a latent image to form a electrostatic latent image using a high viscosity liquid developer using a toner comprising solid particles distributed in a liquid solvent. The developer used in the wet-type image forming apparatus obtained by suspending solid particles or toner particles in a high viscosity insulating organic solvent or carrier liquid comprising silicon oil, mineral oil, edible oil, and the like. The toner particles have a very small diameter of approximately 1 μm. Because such fine toner particles are used, the wet-type image forming apparatus can form an image having a higher quality than that of a dry-type image forming apparatus which uses powder toner particles having a particle diameter of about 7 μm. One example of a liquid developer currently known in the art is disclosed in Japanese Patent Document JP-A-2002-156839, which discloses an image forming apparatus including an image forming means for forming an electrostatic latent image on an image carrying member, a developing means for developing the electrostatic latent image on the image carrying member by using a developing solution in which developer particles are distributed in a solvent, an intermediate transfer medium which comes into contact with the image carrying member to transfer the visible image from the image carrying member, a backup member which comes into contact with the intermediate transfer medium, a transfer means for transferring the visible image from the intermediate transfer medium to a transferred body, a determining means for determining a type of the transferred body to which the visible image is transferred, a control means for variably controlling a pressing force against the transferred body by the backup member in accordance with the type of the transferred body determined by the determining means. 
     One problem with the image forming apparatus described in JP-A-2002-156839, however, is that the pressing means and backup member of the structure do not allow a concave portion to be provided in the backup member for housing a transfer material holding mechanism that holds the transfer material. More specifically, there is a problem that such a structure does not operate smoothly when the concave portion of backup member faces the intermediate transfer medium. 
     BRIEF SUMMARY OF THE INVENTION 
     A first aspect of the invention is an image forming apparatus according including a transfer belt on which an image is transferred, a belt suspension roller that suspends the transfer belt, a transfer roller that is arranged to face the belt suspension roller with the transfer belt in between and includes a shaft portion and a concave portion formed in a shaft direction, and a support member that is arranged on the shaft portion of the transfer roller. The support member includes a first circumferential portion arranged in the concave portion of the transfer roller as seen from a shaft direction of the transfer roller, a second circumferential portion arranged at one side of the concave portion in a rotation direction of the transfer roller as seen from the shaft direction of the transfer roller, and a third circumferential portion arranged at the other side of the concave portion from the one side in the rotation direction of the transfer roller as seen from the shaft direction of the transfer roller. 
     An image forming method according to a second aspect of the invention includes carrying an image on a transfer belt suspended by a belt suspension roller, transferring the image to a transfer material by passing the transfer material through a transfer nipping area formed by causing a transfer roller having a concave portion formed in a shaft direction to come into contact with the belt suspension roller with the transfer belt in between, transferring the image to the transfer material, and thereafter causing a support member that is arranged on a shaft portion of the transfer roller and includes a first circumferential portion arranged in the concave portion of the transfer roller as seen from a shaft direction of the transfer roller, a second circumferential portion arranged at one side of the concave portion in a rotation direction of the transfer roller as seen from the shaft direction of the transfer roller, and a third circumferential portion arranged at the other side of the concave portion from the one side in the rotation direction of the transfer roller as seen from the shaft direction of the transfer roller to come into contact with a second support member arranged at a shaft end portion of the belt suspension roller. 
     According to the image forming apparatus and the image forming method of an aspect of the invention, although the transfer roller is urged to the belt suspension roller, there is a contact member on the shaft portion of the transfer roller and a contacted member on the shaft portion of the belt suspension roller, so that, when the concave portion is not in contact with the transfer belt, the transfer roller can apply a predetermined pressure to the transfer nip, and when the concave portion faces the transfer belt, a positional relationship between the transfer roller and the belt suspension roller can be maintained. 
     According to the image forming apparatus and the image forming method of an aspect of the invention, even when the transfer roller having the concave portion is used, it is possible to seamlessly move between a constant load state in which a certain amount of pressure is applied to the transfer nip and a constant positional state in which the positional relationship between the transfer roller and the belt suspension roller is constant without generating vibration or the like. As such, there are no adverse effects in the image forming process and image quality deterioration can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a diagram showing main constituent elements constituting an image forming apparatus according to an embodiment of the invention; 
         FIG. 2  is a perspective view of a secondary transfer roller used in the image forming apparatus according to the embodiment of the invention; 
         FIGS. 3A and 3B  are a diagram for explaining an operation of a secondary transfer unit in the image forming apparatus according to the embodiment of the invention; 
         FIGS. 4A and 4B  are a diagram for explaining the operation of the secondary transfer unit in the image forming apparatus according to the embodiment of the invention; 
         FIGS. 5A and 5B  are a diagram for explaining the operation of the secondary transfer unit in the image forming apparatus according to the embodiment of the invention; 
         FIGS. 6A and 6B  are a diagram for explaining the operation of the secondary transfer unit in the image forming apparatus according to the embodiment of the invention; 
         FIG. 7  is a diagram for explaining a relationship between a contact member and a contacted member; 
         FIG. 8  is a diagram for explaining a shape of a contact member of an image forming apparatus according to another embodiment of the invention; 
         FIG. 9  is a diagram for explaining a contact member and a contacted member of an image forming apparatus according to another embodiment of the invention; 
         FIGS. 10A and 10B  are a diagram for explaining an operation of a secondary transfer unit in an image forming apparatus according to another embodiment of the invention; and 
         FIG. 11  is a diagram for explaining a contact member and a contacted member of an image forming apparatus according to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described with reference to the drawings.  FIG. 1  is a diagram showing the main constituent elements constituting an image forming apparatus according to the embodiment of the invention. Each color section of the image forming sections are arranged in the center of the image forming apparatus. The developing devices  30 Y,  30 M,  30 C, and  30 K are arranged in a lower part of the image forming apparatus, and components such as a transfer belt  40 , a secondary transfer section or secondary transfer unit  60 , a fixing unit  90 , and the like are arranged in an upper part of the image forming apparatus. 
     More particularly, the fixing unit  90  is laid out above the transfer belt  40 , so that it is possible to reduce the footprint of the entire image forming apparatus. Such a layout can be realized because the embodiment has a configuration in which a transfer material, such as paper, which is passed through a secondary transfer process in the secondary transfer unit  60  is then sucked by a transfer material transportation device  230 , suction devices  210  and  270 , and the like, and then transported to the fixing unit  90 . 
     The developing devices  30 Y,  30 M,  30 C, and  30 K include photoreceptors  10 Y,  10 M,  10 C, and  10 K, corona charging devices  11 Y,  11 M,  11 C, and  11 K, exposure units such as LED arrays  12 Y,  12 M,  12 C, and  12 K, and the like for forming an image of toner. The photoreceptors  10 Y,  10 M,  10 C, and  10 K are uniformly charged by the corona charging devices  11 Y,  11 M,  11 C, and  11 K, while exposures are performed by the exposure units  12 Y,  12 M,  12 C, and  12 K on the basis of an inputted image signal, and latent images are formed on the charged photoreceptors  10 Y,  10 M,  10 C, and  10 K. 
     The developing devices  30 Y,  30 M,  30 C, and  30 K approximately includes developing rollers  20 Y,  20 M,  20 C, and  20 K, developer containers or reservoirs  31 Y,  31 M,  31 C, and  31 K containing a liquid developer of each color of yellow (Y), magenta (M), cyan (C), and black (K), and anilox rollers  32 Y,  32 M,  32 C, and  32 K which are coating rollers for coating the developing rollers  20 Y,  20 M,  20 C, and  20 K with the liquid developers of each color from the developer containers  31 Y,  31 M,  31 C, and  31 K, which develop electrostatic latent images formed on the photoreceptors  10 Y,  10 M,  10 C, and  10 K. 
     The transfer belt  40  is an endless belt which is suspended by a drive roller  41  and tension rollers  42 ,  52 , and  53 , and is driven to rotate by the drive roller  41  while being in contact with the photoreceptors  10 Y,  10 M,  10 C, and  10 K in a primary transfer sections  50 Y,  50 M,  50 C, and  50 K. In the primary transfer sections  50 Y,  50 M,  50 C, and  50 K, the photoreceptors  10 Y,  10 M,  10 C, and  10 K and primary transfer rollers  51 Y,  51 M,  51 C, and  51 K are arranged to face each other with the transfer belt  40  in-between. Toner images of each color on the developed photoreceptors  10 Y,  10 M,  10 C, and  10 K are sequentially transferred to overlap on the transfer belt  40  by using contact positions between the photoreceptors  10 Y,  10 M,  10 C, and  10 K and the transfer belt  40  as transfer positions, so that a full color toner image is formed. 
     In the secondary transfer unit  60 , a secondary transfer roller  61  is arranged so as to face the belt drive roller  41  with the transfer belt  40  being disposed in-between the secondary transfer roller  61  and the belt drive roller  41 . A cleaning device including a secondary transfer roller cleaning blade  62  is also arranged in the secondary transfer unit  60 . At the transfer position where the secondary transfer roller  61  is arranged, a single color toner image or a full color toner image formed on the transfer belt  40  is transferred onto a transfer material such as paper, film, cloth, and the like transported through a transfer material transportation path L. 
     Further downstream in the transfer material transportation path L, an air blower  400  is disposed for blowing air to a space between the transfer belt  40  and the secondary transfer roller  61 , along with the first suction device  210 , the transfer material transportation device  230 , and the second suction device  270 , which are arranged in this order, and which cause the transfer material to be transported to the fixing unit  90 . In the fixing unit  90 , the single color toner image or the full color toner image transferred onto the transfer material is fused and fixed onto the transfer material. 
     The tension roller  42 , the belt drive roller  41 , and the like suspend the transfer belt  40 . A cleaning device including a transfer roller cleaning blade  49  is arranged so as to be in contact with the transfer belt  40  at a position where the transfer belt  40  is suspended by the tension roller  42 . The cleaning device cleans residual toner and carrier on the transfer belt  40 . The tension roller  42  may have a driving force to drive the transfer belt  40 , and the belt drive roller  41  may be used as a mere belt suspension roller. 
     A paper feeder (not shown in  FIG. 1 ) supplies the transfer material to the image forming apparatus. The transfer materials set in the paper feeder are fed into the transfer material transportation path L one by one at a predetermined timing. In the transfer material transportation path L, the transfer material is transported to the secondary transfer position by gate rollers  101 ,  101 ′ and a transfer material guide  102 , and the single color developed toner image or the full color developed toner image formed on the transfer belt  40  is transferred onto the transfer material. 
     As described above, the transfer material on which the toner image is secondary transferred is further transported to the fixing unit  90  by a transfer material transportation means based on the transfer material transportation device  230  and the like. The fixing unit  90  includes a heating roller  91  and a pressure roller  92  urged against the heating roller by a predetermined pressure. The transfer material is passed through a nipping point between the heating roller  91  and the pressure roller  92 , so that the single color toner image or the full color toner image previously transferred onto the transfer material is fused and fixed to the transfer material such as paper. 
     Here, the developing devices will be described in more detail. Since the configurations of image forming sections and developing devices of each color are the same, hereinafter, the description is based on the image forming section and the developing device of yellow (Y). 
     In the image forming section, a photoreceptor cleaning roller  16 Y, a photoreceptor cleaning blade  18 Y, a corona charging device  11 Y, an exposure unit  12 Y, a developing roller  20 Y of the developing device  30 Y, a first photoreceptor squeeze roller  13 Y, and a second photoreceptor squeeze roller  13 Y′ are arranged along the rotation direction of the outer circumference of the photoreceptor  10 Y. 
     The photoreceptor cleaning roller  16 Y comes into contact with the photoreceptor  10 Y and rotates counterclockwise, so that the photoreceptor cleaning roller  16 Y cleans any residual liquid developer remaining after the transfer process such that no liquid developer is transferred onto the photoreceptor  10 Y. A bias voltage which attracts toner particles in the liquid developer is applied to the photoreceptor cleaning roller  16 Y, so that the material collected by the photoreceptor cleaning roller  16 Y is a solid-rich liquid developer including a large number of toner particles. 
     On the downstream side of the photoreceptor cleaning roller  16 Y, the photoreceptor cleaning blade  18 Y which is in contact with the photoreceptor  10 Y cleans a carrier-rich liquid developer on the photoreceptor  10 Y. 
     On the outer circumference of the developing roller  20 Y in the developing device  30 Y, a cleaning blade  21 Y, an anilox roller  32 Y, and a compaction corona generator  22 Y are arranged. A regulating blade  33 Y which adjusts an amount of liquid developer supplied to the developing roller  20 Y is in contact with the anilox roller  32 Y. An auger  34 Y is accommodated in a liquid developer container  31 Y. The primary transfer roller  51 Y of the primary transfer section is arranged in a position so as to face the photoreceptor  10 Y with the transfer belt  40  being disposed in-between. 
     The photoreceptor  10 Y is a photoreceptor drum comprising a cylindrical member with a photosensitive layer such as an amorphous silicon photoreceptor formed on the outer circumference thereof, which rotates clockwise. 
     The corona charging device  11 Y is arranged on the upstream side of the nipping point between the photoreceptor  10 Y and the developing roller  20 Y in the rotation direction of the photoreceptor  10 Y. A voltage is applied from a power supply unit (not shown in  FIG. 1 ) to the corona charging device  11 Y to charge the photoreceptor  10 Y. The exposure unit  12 Y emits light onto the photoreceptor  10 Y charged by the corona charging device  11 Y on the downstream side of the corona charging device  11 Y, and a latent image is formed on the photoreceptor  10 Y. In the image forming process from the beginning to the ending, a roller or the like arranged in an earlier stage is defined to be more upstream than a roller or the like arranged in a later stage. 
     The developing device  30 Y includes the compaction corona generator  22 Y for performing a compaction operation and the developer container  31 Y for containing a liquid developer in which a toner is distributed in a carrier at a weight ratio of approximately 20%. 
     The developing device  30 Y includes the developing roller  20 Y for carrying the liquid developer, the anilox roller  32 Y which is a coating roller for coating the developing roller  20 Y with the liquid developer, the regulating blade  33 Y for regulating the amount of the liquid developer coating on the developing roller  20 Y, the auger  34 Y for agitating and transporting the liquid developer to supply the liquid developer to the anilox roller  32 Y, the compaction corona generator  22 Y for bringing the liquid developer carried on the developing roller  20 Y into a compaction state, and the developing roller cleaning blade  21 Y for cleaning the developing roller  20 Y. 
     The liquid developer contained in the developer container  31 Y is not a generally-used volatile liquid developer having low density (about 1 to 3 wt %), low viscosity, and volatility at room temperature and using Isopar (trademark: Exxon) as a carrier, but instead is a non-volatile liquid developer having high density, high viscosity, and non-volatility at room temperature. In other words, the liquid developer according to the embodiment of the invention is high-viscosity (viscoelasticity is about 30 to 300 mPa·s when a shear velocity is 1000 (1/s) at 25° C. by using HAAKE RheoStress RS600) liquid developer that is prepared by adding solid particles having average diameter of 1 μm, in which colorants (such as pigments) are dispersed in a thermoplastic resin, into a liquid solvent such as an organic solvent, silicon oil, mineral oil, or edible oil with a dispersant so as to have a toner solid content of about 25%. 
     The anilox roller  32 Y functions as a coating roller for supplying the liquid developer to the developing roller  20 Y and coating the developing roller  20 Y with the liquid developer. The anilox roller  32 Y is a cylindrical member and with a surface comprising concavities and convexities which are formed by a finely, uniformly, and spirally carved groove so that the developer is easily carried on the surface. The liquid developer is supplied from the developer container  31 Y to the developing roller  20 Y by the anilox roller  32 Y. When the apparatus is in operation, as shown in  FIG. 1 , the auger  34 Y rotates counterclockwise and supplies the liquid developer to the anilox roller  32 Y, and the anilox roller  32 Y rotates counterclockwise and coats the developing roller  20 Y with the liquid developer. 
     The regulating blade  33 Y is an elastic blade with an elastic material coated on its surface, and comprises a rubber material or the like including a urethane rubber which comes into contact with the surface of the anilox roller  32 Y. The regulating blade  33 Y regulates and adjusts the film thickness and amount of the liquid developer carried and transported by the anilox roller  32 Y, and adjusts the amount of the liquid developer to be supplied to the developing roller  20 Y. 
     The developing roller cleaning blade  21 Y is constituted by a rubber material or the like which comes into contact with the surface of the developing roller  20 Y. The developing roller cleaning blade  21 Y is arranged downstream from a developing nipping area where the developing roller  20 Y comes into contact with the photoreceptor  10 Y in the rotation direction of the developing roller  20 Y, so that the developing roller cleaning blade  21 Y scrapes off and removes any residual liquid developer on the developing roller  20 Y. 
     The compaction corona generator  22 Y is an electric field application means for increasing the charge bias on the surface of the developing roller  20 Y, and an electric field is applied from the compaction corona generator  22 Y to the developing roller  20 Y at a compaction position by the compaction corona generator  22 Y. Alternatively, a compaction roller or the like may be used as the electric field application means for the compaction instead of the corona charging device shown in  FIG. 1 . 
     The developer which is carried by the developing roller  20 Y and subsequently compacted is then developed in accordance with the latent image on the photoreceptor  10 Y by applying a predetermined electric field to the developing nipping are where the developing roller  20 Y comes into contact with the photoreceptor  10 Y. 
     Any residual developer after the development process is then scraped off and removed by the developing roller cleaning blade  21 Y, and dropped into a collection section in the developer container  31 Y to be reused. The carrier and the toner reused in this way are not mixed with a carrier and a toner of any other color. 
     A photoreceptor squeeze device arranged upstream to the primary transfer section is arranged so as to face the photoreceptor  10 Y downstream to the developing roller  20 Y. The photoreceptor squeeze device collects redundant toner of the toner image developed on the photoreceptor  10 Y. The photoreceptor squeeze device includes the first photoreceptor squeeze roller  13 Y and the second photoreceptor squeeze roller  13 Y′ which comprise elastic roller members rotating in sliding contact with the photoreceptor  10 Y, and which collect redundant carrier and unnecessary fogging toner from the toner image developed on the photoreceptor  10 Y in order to increase the toner particle ratio in the visible toner image. A predetermined bias voltage is applied to the photoreceptor squeeze rollers  13 Y and  13 Y′. 
     The surface of the photoreceptor  10 Y which has passed through the photoreceptor squeeze device including the first photoreceptor squeeze roller  13 Y and the second photoreceptor squeeze roller  13 Y′ proceeds to the primary transfer section  50 Y. 
     In the primary transfer section  50 Y, the developer image developed on the photoreceptor  10 Y is transferred onto the transfer belt  40  by the primary transfer roller  51 Y. In the primary transfer section, the toner image on the photoreceptor  10 Y is transferred onto the transfer belt  40  by an operation of the transfer bias applied to the primary transfer backup roller  51 Y. Here, the photoreceptor  10 Y and the transfer belt  40  move at the same speed, so that the driving load for rotation and movement is reduced and disturbance to the visible toner image on the photoreceptor  10 Y is suppressed. 
     Toner images of magenta (M), cyan (C), and black (K) are respectively formed on the photoreceptors  10 M,  10 C, and  10 K of the developing devices  30 M,  30 C, and  30 K by the same process as that of the developing device  30 Y. The transfer belt  40  passes through the nipping areas of the primary transfer sections  50  of yellow (Y), magenta (M), cyan (C), and black (K), and the developers (developed images) on the photoreceptors of each color are transferred and overlapped onto the transfer belt  40 . Then the transfer belt  40  proceeds into the nip portion of the secondary transfer unit  60 . 
     The transfer belt  40  which has passed through the secondary transfer unit  60  rotates so as to receive another set of images to be transferred in the primary transfer section  50 , and the transfer belt  40  is cleaned on the upstream side of the primary transfer section  50  by the transfer belt cleaning blade  49  or the like. 
     The transfer belt  40  has a three-layer structure in which an elastic intermediate layer of polyurethane is provided on a polyimide base layer and a PFA surface layer is provided on the elastic intermediate layer. The transfer belt  40  is used such that the side of the polyimide base layer is suspended by the belt drive roller  41  and the tension rollers  42 ,  52 , and  53 , and the toner image is transferred onto the side of the PFA surface layer. As such, the transfer belt  40  has elasticity which is effective in sending and transferring toner particles having a small diameter onto concave portions of the transfer material in the secondary transfer process because the transfer belt  41  has good following capability and good adhesion to the surface of the transfer material. 
     Next, the secondary transfer roller  61  used in the image forming apparatus according to the embodiment will be described in more detail.  FIG. 2  is a perspective view of the secondary transfer roller used in the image forming apparatus according to the embodiment of the invention. As shown in  FIG. 2 , the secondary transfer roller comprises a roller body  601  portion, a roller shaft portion  602 , a concave portion  605 , an elastic member  607 , a transfer material holding mechanism  610 , a transfer material holding section  611 , a transfer material holding section receiving section  612 , a transfer material peeling section  640 , and a contact member  650 . 
     The roller shaft portion  602  is provided at both ends of the roller body portion  601  of the secondary transfer roller  61 , and the secondary transfer roller  61  is mounted in the apparatus main body rotatably around the roller shaft portion  602 . The concave portion  605  extending in the shaft direction is provided in the roller body portion  601 . The transfer material holding mechanism  610  is provided in the concave portion  605 , and the elastic member  607  is provided along the entire exterior of the roller body portion  601  except in the area where the concave portion  605  is formed. The transfer material holding mechanism  610  is a mechanism for holding and releasing the transfer material. The elastic member  607  comprises a semi-conductive rubber layer having an electrical resistance. When the transfer material is passed through the secondary transfer nipping area of the secondary transfer unit while the transfer material is wound around the elastic member  607 , the toner image is transferred from the transfer belt  40  onto the transfer material. 
     The transfer material holding mechanism  610  is includes a plurality pairs of the transfer material holding section  611  and the transfer material holding section receiving section  612  which are discretely provided in the roller shaft direction and a plurality of the transfer material peeling sections  640  which are appropriately provided between the above pairs in the roller shaft direction. All the transfer material holding sections  611  are constituted to be movable. The transfer material holding sections  611  can hold the transfer material by holding the transfer member between the transfer material holding sections  611  and the transfer material holding section receiving sections  612 , and then releasing the transfer material by widening a gap between the transfer material holding sections  611  and the transfer material holding section receiving sections  612 . All the transfer material peeling sections  640  operate so as to push out the transfer member held by the transfer material holding sections  611  and the transfer material holding section receiving sections  612  in a direction away from the secondary transfer roller  61 . 
     Two contact members  650  are provided at both ends of the roller shaft potion  602  of the secondary transfer roller  61 . The contact member  650  has a structure in which a contact surface and the like are provided in an opening area where the concave portion  605  is provided in the secondary transfer roller  61  as seen from the roller shaft direction, and the contact surface comes into contact with a contacted member described more fully below, so that the position between the secondary transfer roller  61  and the belt drive roller  41  is regulated. 
     Next, a structure for regulating the position between the secondary transfer roller  61  and the belt drive roller  41  while applying a predetermined pressure to the secondary transfer nip of the secondary transfer section  60  comprising the secondary transfer roller  61  in which the concave portion  605  for accommodating the transfer material holding mechanism  610  is provided.  FIGS. 3A to 6B  are diagrams for explaining the operation of the secondary transfer unit  60  in the image forming apparatus according to the embodiment of the invention.  FIGS. 3A ,  4 A,  5 A, and  6 A are diagrams of the secondary transfer unit  60  as seen from the side of the apparatus, while  FIGS. 3B ,  4 B,  5 B, and  6 B are schematic cross-sectional views of the secondary transfer unit  60 .  FIGS. 3A to 6B  show the contact member  650 , a rotation support shaft  670 , a frame member  671 , an urging member  672 , a roller shaft portion  689  of the belt drive roller  41 , and the contacted member  690 .  FIG. 7  is a diagram selectively showing a configuration related to the secondary transfer roller  61 , the belt drive roller  41 , and the like, where the diagram explains a relationship between the contact member  650  and the contacted member  690 .  FIG. 7  illustrates a first handover surface  661  of the contact member  650 , a contact surface  663 , and a second handover surface  662 . 
     In the secondary transfer unit  60 , the roller shaft potion  602  of the secondary transfer roller  61  is rotatably attached to the frame member  671  at both ends of the roller shaft portion  602 . The frame member  671  is rotatable around the rotation support shaft  670 , and is urged in the direction of the arrow in  FIGS. 3 to 6  by the urging member  672 . By such a configuration, the secondary transfer roller  61  is urged toward the belt drive roller  41 , and a predetermined pressure can be applied to the secondary transfer nipping area between the secondary transfer roller  61  and the belt drive roller  41 . The toner particles on the transfer belt  40  are efficiently transferred onto the transfer material at the secondary transfer nipping area by the transfer pressure and a transfer bias at the secondary transfer nipping area. 
     Two contact members  650  are provided at both ends of the roller shaft potion  602  of the secondary transfer roller  61 . Two contacted members  690  are provided at both ends of the roller shaft potion  689  of the belt drive roller  41  so as to correspond with the two contact members  650 . As shown in  FIGS. 3B ,  4 B,  5 B, and  6 B, the contact member  650  and the contacted member  690  are arranged so that the positions of the contact member  650  and the contacted member  690  in the shaft direction are the same. 
     The contact member  650  has a shape as shown in  FIG. 7 , and includes the contact surface  663  which is a distance R 3  from the rotation center O of the secondary transfer roller  61 , with the first handover surface  661  and the second handover surface  662  provided on both sides of the contact surface  663 . The contact surface  663  is provided in an contact area C 3  comprising the opening area in which the concave portion  605  is provided in the secondary transfer roller  61  as seen from the roller shaft direction. When the concave portion  605  comes to a position facing the belt drive roller  41  or the transfer belt  40  as the apparatus operates, the contact member  650  comes into contact with the contacted member  690  of the belt drive roller  41  on the contact area C 3  of the contact surface  663 , so that the urging pressure of the secondary transfer roller  61  is received by the contacted member  690 , and the distance and positional relationship between the secondary transfer roller  61  and the belt drive roller  41  are maintained. 
     As the secondary transfer roller  61  and the belt drive roller  41  rotate, although a constant load state in which a constant load is applied to the secondary transfer nipping area and a constant positional state in which the positional relationship between the secondary transfer roller  61  and the belt drive roller  41  is alternately repeated, the both states can be seamlessly shifted to each other without generating vibration or the like by the area C 1  of the first handover surface  661  and the area C 2  of the second handover surface  662  provided on both sides of the contact surface  663 . As such, there are no adverse effects in the image forming process and image quality deterioration can be prevented. In this embodiment, although the first handover surface  661  (area C 1 ) and the second handover surface  662  (area C 2 ) comprise a tapered surface, a curved surface having a predetermined curvature may also be employed. 
     The contact member  650  shown in  FIG. 7  is a member provided on the shaft portion of the secondary transfer roller  61 , and the distance between the circumference of the contact member  650  and the rotation center of the secondary transfer roller  61  varies (the distances from the contact area C 3 , the area C 1 , the area C 2 , and other areas to the rotation center are different from one another), so that the contact member  650  functions as a kind of cam. 
     The contacted member  690  is a member such as a bearing, and the distance from the rotation center O′ of the belt drive roller  41  to the circumference of the contacted member  690  is r. As each roller rotates, the contacted member  690  comes into contact with the contact surface  663  of the contact member  650 , and receives the load of the secondary roller  61  urged by the urging member  672 , so that the distance and the positional relationship between the secondary transfer roller  61  and the belt drive roller  41  are maintained. 
     As each roller rotates, the secondary transfer roller  60  operates in sequence from the state shown in  FIGS. 3A and 3B , to the state shown in  FIGS. 4A and 4B , to the state shown in  FIGS. 5A and 5B , to the state shown in  FIGS. 6A and 6B , to the state shown in  FIGS. 3A and 3B , and so forth.  FIG. 3  shows a state in which the concave portion  605  does not face the belt drive roller  41  or the transfer belt  40 . At this time, the urging force from the urging member  672  is applied to the second transfer nip, and a predetermined transfer pressure is secured. An appropriate transfer bias is further applied between the secondary transfer roller  61  and the belt drive roller  41 , and the toner particles on the transfer belt  40  are transferred onto the transfer material at the secondary transfer nip. In the state shown in  FIGS. 3A and 3B , the contact surface  650  and the contacted member  690  are completely separated from each other. 
       FIGS. 4A and 4B  shows a state in which each roller rotates a certain amount and the concave portion  605  approaches but does not reach the belt drive roller  41  or the transfer belt  40 . At this time, as each roller rotates, the area C 1  of the first handover surface  661  of the contact member  650  gradually approaches the contacted member  690 . In other words, the distance between the first handover surface  661  and the contacted member  690  gradually shortens. When each roller further rotates, the contact member  650  and the contacted member  690  come into contact with each other at the boundary between the first handover surface  661  (area C 1 ) and the contact surface  663  (contact area C 3 ), and the load from the secondary transfer roller  61  is received by the contacted member  690 , so that the distance and positional relationship between the secondary transfer roller  61  and the belt drive roller  41  are maintained. At the time when the contact member  650  and the contacted member  690  come into contact with each other, the concave portion  605  faces the belt drive roller  41  or the transfer belt  40 . 
       FIGS. 5A and 5B  show a state in which each roller further rotate, such that the concave portion  605  completely faces the belt drive roller  41  or the transfer belt  40 , and each roller rotates while the contact surface  663  (contact area C 3 ) of the contact member  650  and the contacted member  690  are in contact with each other. At this time, the urging pressure of the secondary transfer roller  61  urged by the urging member  672  is received by the contacted member  690 , so that the distance and positional relationship between the secondary transfer roller  61  and the belt drive roller  41  are maintained. 
     As each roller further rotates, the contact member  650  and the contacted member  690  separate from each other at the boundary between the contact surface  663  (contact area C 3 ) of the contact member  650  and the second handover surface  662  (area C 2 ), and the second handover surface  662  (area C 2 ) of the contact member  650  and the contacted member  690  gradually move further apart, as shown in  FIGS. 6A and 6B . At this time, the concave portion  605  is separated from the belt drive roller  41  or the transfer belt  40 , the elastic member  607  of the secondary transfer roller  61  comes into contact with the transfer belt  40 , and the urging force from the urging member  672  is applied to the secondary transfer nip. The contact member  650  and the contacted member  690  are separated from each other, and the load from the contact member  650  to the contacted member  690  is also released. 
     According to the above embodiment, although the secondary transfer roller  61  is urged toward the belt drive roller  41 , the contact member  650  is on the shaft portion of the secondary transfer roller  61  and the contacted member  690  is on the shaft portion of the belt drive roller  41 , so that, when the concave portion  605  is not in contact with the transfer belt, the secondary transfer roller  61  can apply a predetermined pressure on the transfer nipping area. As such, when the concave portion faces the transfer belt, the positional relationship between the secondary transfer roller  61  and the belt drive roller  41  can be maintained. 
     According to the above embodiment, even when the secondary transfer roller  61  having the concave portion  605  is used, it is possible to seamlessly move between the constant load state in which a certain amount of pressure is applied to the secondary transfer nip and the constant positional state in which the positional relationship between the secondary transfer roller  61  and the belt drive roller  41  is constant without generating vibration or the like, so that there are no adverse effects in the image forming process and image quality deterioration can be prevented. 
     Next, another embodiment of the invention will be described.  FIG. 8  is a diagram for explaining a shape of the contact member of the image forming apparatus according to another embodiment of the invention. In this embodiment, the shapes of the first handover surface  661  and the second handover surface  662  are different from those of the above embodiment, and while other aspects of the configuration are the same as that of the above embodiment. 
     While in the above embodiment, the first handover surface  661  and the second handover surface  662  are formed by a tapered surface, in this embodiment, the first handover surface  661  (area C 1 ) and the second handover surface  662  (area C 2 ) comprise a curved surface having a predetermined radius R. Here, the first handover surface  661  (area C 1 ) and the second handover surface  662  (area C 2 ) are formed by different curved surfaces having different curvatures. More specifically, the first handover surface  661  (area C 1 ) is formed by a curved surface having a curvature of R 1  (centered at the cross shown in  FIG. 8 ) and the second handover surface  662  (area C 2 ) is formed by a curved surface having a curvature of R 2  (centered at the other cross shown in  FIG. 8 ). The curvature R 1  and the curvature R 2  are set to be R 1 &gt;R 2 . 
     The second handover surface  662  (area C 2 ) having the curvature of R 2  is formed on the contact member  650  so that the second handover surface  662  corresponds to the side of the opening edges of the concave portion  605  where the transfer material holding mechanism  610  is provided, and the top margin of the transfer material can be reduced by setting the curvature R 2  to be smaller than the curvature R 1 . Also, transport banding can be reduced by setting the curvature R 1  to be larger than the curvature R 2 . 
     The contact plane of the first handover surface  661  (area C 1 ) and a contact plane of the contact surface  663  (contact area C 3 ) are configured to share the boundary portion α between the first handover surface  661  (area C 1 ) and the contact surface  663  (contact area C 3 ). Further, a contact plane of the second handover surface  662  (area C 2 ) and the contact plane of the contact surface  663  (contact area C 3 ) are configured to share the boundary portion β between the second handover surface  662  (area C 2 ) and the contact surface  663  (contact area C 3 ). By employing this configuration, it is possible to seamlessly move between the constant load state in which a certain amount of pressure is applied to the secondary transfer nip and the constant positional state in which the positional relationship between the secondary transfer roller  61  and the belt drive roller  41  is constant without generating vibration or the like, so that influence on the image forming process can be suppressed and image quality deterioration can be prevented. In this embodiment, the same effects as those of the previously described embodiment can be obtained. 
     Next, another embodiment of the invention will be described.  FIGS. 9 and 10A  and  10 B are diagrams related to an alternative embodiment of the invention.  FIG. 9  is a diagram explaining the contact member  650  and the contacted member  690  of the image forming apparatus according to another embodiment of the invention, and  FIGS. 10A and 10B  is a diagram explaining an operation of the secondary transfer unit  60  of the image forming apparatus according to another embodiment of the invention.  FIG. 10A  is a diagram of the secondary transfer unit  60  as seen from the side of the apparatus, and  FIG. 10B  is a schematic cross-sectional view of the secondary transfer unit  60 . 
     The difference between this embodiment and the previously described embodiment is that this embodiment uses a contact member such as a bearing having a circumference with a constant radius from to the rotation center O of the roller to form the contact member  650  provided on the roller shaft portion  602  of the secondary transfer roller  61 . In this embodiment, the same effects as those of the previously described embodiment can be obtained, and at the same time, a general purpose component such as a bearing can be used as the contact member  650 , so that an effect that the apparatus can be manufactured at a lower cost can be obtained. 
     Next, another embodiment of the invention will be described.  FIG. 11  is a diagram for explaining the contact member  650  and the contacted member  690  of the image forming apparatus according to still another embodiment of the invention.  FIG. 11  is a diagram selectively showing a configuration related to the secondary transfer roller  61 , the belt drive roller  41 , and the like. In the embodiment shown in  FIG. 11 , a member such as a bearing having a circumference with a constant radius to the rotation center O of the secondary transfer roller  61  is used as the contact member  650 , while a member in which the contact surface, the first handover surface, and the second handover surface are formed is used as the contacted member  690 . By using such a contact member  650  and a contacted member  690 , the same effects as those of the previously described embodiment can be obtained. 
     As may be understood by one of ordinary skill in the art, the embodiments described herein are exemplary only and other embodiments may be used without departing from the meaning and scope of the invention.