Abstract:
An image forming apparatus includes a transfer material carrying belt for carrying a transfer material; a support for supporting the transfer material carrying belt at a side opposite from a side carrying the transfer material; an image forming unit for forming an image on a transfer material carried on the transfer material carrying belt; wherein the support is disposed at a position out of a portion where the carrying belt constitutes a carrying surface for the transfer material, and includes a driving roller for driving the transfer material carrying belt and a follower roller which is disposed at a position where the transfer material is separated from the transfer material carrying belt after completion of image formation of the image forming unit.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image forming apparatus, for example, a copying machine, a printer, a facsimile machine, or the like. In particular, it relates to an image forming apparatus which forms an image on a piece of transfer medium borne on a belt for conveying the transfer medium. 
     Generally, an image forming apparatus which employs an electrophotographic process comprises an image forming stations, a conveying means, and a fixing means. In the image forming station, a latent image is formed with the use of light, magnetism, and electrical charge, and the latent image is developed into a visible image. The conveying means conveys the transfer medium to the image forming station so that the visible image formed in the image forming station is transferred onto the transfer medium. The fixing means fixes the visible image onto the transfer medium after the image is transferred to the transfer medium. 
     The image forming station comprises a medium, for example, an electrophotographic photosensitive member, on which an image is formed. Such a medium varies in characteristics, for example, shape. The image forming station comprises a means for forming a latent image, and a means for developing a latent image. These means also vary in characteristics so that they match with an image forming means. 
     A substantial number of image forming apparatuses, in particular, full-color image forming apparatuses, employ an electrostatic conveying means, that is, a conveying means which uses electrostatic force to hold a transfer medium on its surface while conveying the transfer medium from the transfer means to the fixing means. This is due to the fact that an electrostatic conveying means is superior to other conveying means in many respects. A color image forming apparatus is an apparatus which forms a full-color image by laying in layers multiple images on a transfer medium with the use of multiple image forming stations. 
     One of the well known patent documents which disclose image forming apparatuses which employ the above described system or the like is Japanese Patent Laid-Open Application No. 13,976/1990. FIG. 7 depicts, in general structure, one of the image forming apparatuses disclosed in that document. 
     The image forming apparatus in FIG. 7 comprises three image forming stations: I, II, and III, a conveying means  139 , and a fixing means  56 . The conveying means  139  has a conveyer belt  126  for conveying transfer medium, and extends below the three image forming stations I, II, and III. The fixing means is equipped with thermal rollers  56   a  and  57   b  for fixing the images on the transfer medium to the transfer medium, and is disposed at the exit portion of the conveying means  139 . The image forming stations I, II, and III are equipped with photosensitive drums  111 ,  112 , and  113 , charging devices  114 ,  115 , and  116 , developing devices  117 ,  118 , and  119 , charging devices  120 ,  121 , and  122  for image transfer, and cleaners  123 ,  124 , and  125 , correspondingly. 
     The conveyer belt  126  is formed of resin. In an image forming operation, its surface is electrically charged by a charging device  133  for adhesion, to adhere a transfer medium to the surface of the conveyer belt  126 , and charge it so that the transfer medium is reliably conveyed. 
     The conveyer belt  126  is supported by a driver roller  131  and a follower roller  134 , and is stretched between them with the application of a predetermined amount of tension. As the driver roller  131  is rotationally driven, the transfer belt  126  moves at a predetermined velocity. 
     After being released into the apparatus by a registration roller  49 , a transfer medium is pinched by the follower roller  134  of the transfer medium conveying means  139 , and a pressing roller  52  pressed upon the follower roller  134  through the conveyer belt  126 , and while the transfer medium  46  is passed between the two rollers, it is pressed upon the electrically charged conveyer belt  126 , so that the transfer medium  46  is electrostatically adhered to the conveyer belt  126  in a desirable manner, that is, without becoming wavy. 
     The rotational velocity of the registration roller  49  is set to be slightly higher than the velocity, or conveying velocity, of the conveyer belt  126 , to cause the transfer medium  46  to elastically bend slightly so that the transfer medium  46  is not affected by the rotational velocity of the registration roller  49 . 
     Under a high humidity-high temperature environment, the conveyer belt  126  is not sufficiently charged, which sometimes allows portions or the entirety of the transfer medium to behave as if “floating” on the surface of the transfer medium. If the floating of the transfer medium occurs, transfer failures may occur. For example, a portion or portions of an image may become misaligned, or the entirety of an image may become misaligned relative to the transfer medium. Further, a portion or portions of an image may drop out. In order to prevent such problems, an auxiliary roller  200  and an idler roller  203  are provided, which are enabled to freely rotate while pinching the conveyer belt  126 . 
     With the provision of the auxiliary roller  200  described above, the transfer medium is prevented from floating from the conveyer belt  126  when an image formed in the image forming station is transferred onto the transfer medium which is being moved forward by the conveyer belt  126 . In other words, it is assured that the transfer medium remains adhered to the conveyer belt  126 . Therefore, an excellent image, that is, an image which does not suffer from the above described partial misalignment, registration failure, or partial image drop-out, can be formed. 
     On the other hand, if the conveyer belt  126  meanders or deviates while running, an image transferred onto the transfer medium in one of the image forming stations may not align with an image from another image forming station (hereinafter, “registration misalignment”). In order to prevent this problem, an image forming apparatus is usually provided with one of several means for correcting the meandering or deviating. For example, an image forming apparatus may be provided with guiding ribs, which may be arranged along one edge, or both edges, of the conveyer belt  126 , along the entire length of the belt, or some of the aforementioned rollers which support and/or stretch the conveyer roller  126  may be provided with guiding grooves. Further, the rollers may be provided with “shoulder ribs” for guiding the conveyer belt  126 . 
     However, in the case of the above described conventional means for preventing the meandering or deviating of the conveyer belt, the driver roller  131  is positioned at the most downstream end, in terms of the transfer medium movement, of the range in which the conveyer belt conveys the transfer medium. Therefore, there are the following problems. 
     That is, one of the methods for making it difficult for the conveyer belt  126  to slip on the driver roller  131  is to increase the diameter of the driver roller  131  (standard method is to increase it to 20 mm or larger). However, if increasing the driver roller diameter becomes the priority, a problem occurs. More specifically, the larger the diameter of the driver roller  131 , the smaller the curvature of the conveyer belt  126  at the predetermined point at which the transfer medium  46  becomes separated from the conveyer belt  126  to be delivered to a fixing means  56 , and the smaller the curvature of the conveyer belt  126  at the separation point, the more difficult it is for the transfer medium  46  to be separated from the conveyer belt  126  by the curvature of the conveyer belt  126  and the resiliency of the transfer medium  46 . Thus, if the transfer medium  46  is weak in resiliency, or temperature and/or humidity are very high, the transfer medium  46  is liable to fail to separate from the conveyer belt  126 , and jam the apparatus. 
     On the contrary, if the driver roller diameter is decreased to prioritize the transfer medium separation from the conveyer belt  126  at the predetermined separation point (standard method is to reduce the diameter to 20 mm or less), the size of the contact area between the driver roller  131  and the conveyer belt  126  decreases, increasing the possibility that slipping occurs between the conveyer belt  126  and the driver roller  131 . Thus, it is not assured that the transfer medium  46  is reliably conveyed. 
     There are two commonly used methods for preventing slipping from occurring between the driver roller  131  and the conveyer belt  126 : a method in which the coefficient of friction across the contact surface, or interface, between the conveyer belt  126  and driver roller  131  is kept above a predetermined level, and a method in which the tension of the conveyer belt  126  is increased to increase the contact pressure across the interface. However, it is possible that these methods cause the creep of the conveyer belt  126  to deteriorate, or cause the conveyer belt  126  to fatigue, as the belt is repeatedly bent and/or stretched as it goes around the driver roller  131  and the follower roller  134 . Therefore, the tension of the conveyer roller  126  must not be limitlessly increased. 
     Further, the driver roller  131  is disposed adjacent to a thermal fixing means  56 , being therefore liable to be affected by the heat from the pair of heat rollers  56   a  and  56   b  of the thermal fixing means  56 . In other words, there is a possibility that the temperature of the driver roller  131  substantially increases while the apparatus is in operation. As the driver roller  131  increases in temperature, it increases in diameter, which in turn increases the velocity of the conveyer belt  126 . As a result, an image may be stretched or shrunk. This problem is of grave concern in the case of a color image forming apparatus in which a full-color image is formed by laying in layers multiple toner images, because the fluctuation in the conveying speed of the conveyer belt  126  exactly manifests as color deviation in the final image, or the full-color image, which is a large problem in terms of image quality. 
     In order to provide a large enough coefficient of friction between the driver belt  131  and the conveyer belt  126 , it is necessary to provide the surface of the driver roller  131  with a layer of material, such as rubber, which has a large coefficient of friction. Whereas, material high in frictional coefficient such as rubber is greater in thermal expansion by approximately an order of magnitude than the material (usually, metallic material such as iron, aluminum, and the like) for the core of the driver roller  131 , and the heat expansion caused by temperature increase has grave effects upon the performance of the conveying means. 
     In recent years, it has become increasingly necessary to reduce the size of an image forming apparatus, even in a full-color image forming apparatus. However, if a plural number of image forming stations are aligned in a series in an image forming apparatus as in the above described color image forming apparatus, the distance the transfer medium is conveyed becomes long. As a result, the main assembly of an image forming apparatus must be increased in dimension, in particular, in terms of the direction in which the transfer medium is conveyed. 
     For the purpose of reducing the size of the main assembly of an image forming apparatus, it is effective to reduce as much as possible the distance between the transfer conveying means  139  and fixing apparatus  56 , and the distance between the transfer conveying means  139  and the registration roller  49 . 
     However, in the case of the above described conventional apparatus, it is necessary to provide a certain amount of distance between the driver roller  131  and the fixing apparatus  56  in order to avoid the occurrence of color deviation for which the heat from the fixing apparatus  56  is responsible as described above. 
     There have been various methods for preventing the driver roller from being affected by the heat from the fixing apparatus  56 . According to one of the methods, the driver roller is disposed at a point  134  in FIG. 7, that is, the upstream end, in terms of the transfer medium conveyance direction, of the range in which the transfer medium is conveyed by the conveyer roller. 
     However, in the case of a conventional image forming apparatus such as the one described above, the diameter of the driver roller ( 134 ) must be greater than a certain range, in order for the driver roller ( 134 ) not to slip. This means that the distance between the registration roller  49  and the driver roller ( 134 ) exceeds a certain range. 
     There must be disposed unillustrated driving mechanisms for the registration roller  49  and driver roller  134  behind or in front of them, respectively. Therefore, the rollers must be arranged to provide a space large enough for the two driving mechanisms, which means more distance between the registration roller  49  and the driver roller ( 134 ). 
     In other words, in the case of the above described conventional structure for a transfer medium conveying means, whether the driver roller is disposed on the most downstream end ( 131 ) or most upstream end ( 134 ), in terms of the transfer medium conveyance direction, of the range in which the transfer medium is conveyed by the conveyer belt, the size of the apparatus main assembly must be greater in width in terms of the transfer medium conveyance direction than a certain range. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide an image forming apparatus in which a transfer medium is reliably conveyed by the conveying means, and reliably separates from the conveying means. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic vertical sectional view of an example of an image forming apparatus in accordance with the present invention, as seen from the front side of the apparatus. 
     FIG. 2 is a schematic vertical sectional view of the belt based conveying apparatus in the first embodiment of the present invention. 
     FIG. 3 is a schematic vertical sectional view of the belt based conveying apparatus in the modification of the first embodiment of the present invention. 
     FIG. 4 is a schematic vertical sectional view of the belt based conveying apparatus in the second embodiment of the present invention. 
     FIG. 5 is a schematic vertical sectional drawing for comparatively depicting a belt based conveying apparatus in the second embodiment of the present invention and a conventional belt based conveying apparatus, (a) and (b) depicting the former and the latter, respectively. 
     FIG. 6 a schematic vertical sectional view of the belt based conveying apparatus in the modification of the second embodiment of the present invention. 
     FIG. 7 is a schematic vertical sectional view of a conventional image forming apparatus as seen from the front side of the apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an image forming apparatus in accordance with the present invention will be described in detail with reference to the appended drawings. 
     Referring to FIG. 1, the electrophotographic color image forming apparatus as an example of an image forming apparatus in accordance with the present invention comprises four stations Pa, Pb, Pc, and Pd for forming images, in which photosensitive drums  1   a,    1   b,    1   c,  and  1   d  as a rotational image bearing member are correspondingly disposed. Each photosensitive drum is dedicated to a specific color. Along the peripheral surface of each photosensitive drum ( 1   a,    1   b,    1   c,  and  1   d ), a charging device for primary charge ( 2   a,    2   b,    2   c,  and  2   d ), an exposing means ( 3   a,    3   b,    3   c,  and  3   d ), a developing device ( 4   a,    4   b,    4   c,  and  4   d ), a charging device for transfer ( 5   a,    5   b,    5   c,  and  5   d ), a cleaning means ( 6   a,    6   b,    6   c,  and  6   d ) are disposed in this order in terms of the rotational direction of the photosensitive drum. 
     The image forming apparatus also comprises a conveying apparatus  7  which employs an endless belt. The belt based conveying apparatus  7  is below the four image forming stations, which are aligned in a straight line. Its endless belt horizontally runs along the bottom side of the photosensitive drum of each image forming station. It conveys a transfer medium  9 . More specifically, as the transfer medium  9  is fed into the image forming apparatus by a pair of registration rollers  8  disposed at one end of the conveying apparatus  7 , the conveying apparatus  7  conveys the transfer medium  9  between the charging device for image transfer ( 5   a - 5   b ) and the image forming station (Pa-Pd). 
     In this type of an electrophotographic color image forming apparatus, a full-color image is formed in the following manner. 
     First, a latent image correspondent to the yellow component of the image on an original copy is formed on the photosensitive drum  1   a  by the charging device  2   a  for primary charge and exposing means  3   a  in the first image forming station Pa, based on a known electrophotographic process. Next, this latent image is developed into a visible image (yellow image) by the developing device  3   a  which contains the developer (toner) with yellow color. Then, this visible image, an image composed of yellow toner, is transferred by the charging device  4   a  for image transfer, onto the transfer medium  9  delivered by the belt based conveying apparatus  7 . 
     While this yellow toner image is transferred onto the transfer medium  9 , a latent image correspondent to the magenta component of the image on the original copy is formed on the photosensitive drum  1   b  in the same manner as the latent image for the yellow component was formed. Then, the magenta toner image is composed of the magenta toner in the developing station  4   b.  Then, as the transfer medium  9 , onto which the yellow toner image has been completely transferred in the first image forming station Pa, is delivered to the charging device  5   b  for image transfer in the image forming station Pb, the magenta toner image is transferred onto a predetermined portion of the transfer medium  9  with the yellow toner image. 
     Then, the same image formation process as the one described above is carried out also for cyan and black colors. As a result, four toner images of different color are placed in layers on the transfer medium  9 . Next, the transfer medium  9  is conveyed to the fixing station  10  located at the downstream end of the belt based conveying apparatus  7 . In the fixing station  10 , the four images are fixed to the transfer medium  9  to yield a permanent multicolor (full-color) copy of the original image. 
     Meanwhile, the toner which is remaining on the photosensitive drum ( 1   a - 1   d ) after the image transfer is removed by the cleaning means ( 6   a - 6   d ) to prepared the photosensitive drum for the following step in which a latent image is formed. 
     The image forming apparatus in this embodiment is configured so that the distance in the transfer medium conveyance direction between the registration roller pair  8  and the point (adhesion point, or locations of roller  15  or  16 ) at which the transfer medium begins to be adhered to the conveyer belt, becomes shorter than the length of a transfer medium of the smallest size, for example a post card (100 mm×148 mm), usable in the apparatus, in terms of the transfer medium conveyance direction. Also, the distance in the transfer medium conveyance direction between the point (separation point) at which a transfer medium separates form the conveyer belt  100 , and the fixing nip which the fixing rollers  10   c  and  10   e  form is shorter than the length of a transfer medium of the smallest size usable in the apparatus, in terms of the transfer medium conveyance direction. With this structure, there is no need for placing any kind of apparatus for relaying the transfer medium across the aforementioned distances, in the intervals between the aforementioned registration roller pair  8  and the adhesion point, and between the separation point to the fixing portion. Therefore, it becomes possible to reduce the image forming apparatus size. 
     Next, referring to FIGS. 1 and 2, a belt based conveying apparatus  7  which is employed in the above described color image forming apparatus will be described. 
     Referring to FIG. 2, the conveyer belt  100  of a conveying apparatus  7  is stretched around a driver roller  11 , and three follower rollers  12 ,  13 , and  14  (first, second, and third follower rollers, correspondingly). As the driver roller  11  is rotated by a motor M, the conveyer belt  100  runs in the direction indicated by an arrow mark in the drawing. 
     The positions of the first follower roller  12  and the driver roller  11  relative to the conveying apparatus  7  are fixed. 
     The second follower roller  13  gives the conveyer belt  100  tension with the help of an elastic member  13   a  (spring or the like). In other words, it doubles as a tension roller. 
     The third follower roller  14  is configured to that the angle of the axial line of the follower roller  14  relative to the axial line of the driver roller  11  can be adjusted within a range in which the transfer process is successfully carried out. In other words, this roller doubles as an alignment roller. More specifically, the deviation of the conveyer belt  100  in the primary scan direction (direction perpendicular to transfer medium conveyance direction) can be controlled by adjusting the alignment of this third follower roller  14  relative to the driver roller  11 , so that the conveyer belt  100  remains substantially centered, that is, without deviating too far in the primary scan direction. 
     Referring to FIG. 2, the belt based conveying apparatus  7  in this embodiment comprises another follower roller, the fourth follower roller  15  to which voltage with a predetermined level is applied to electrostatically adhere the transfer medium to the conveyer belt  100 . This roller is located adjacent to the third follower roller  14 , and is paired with a pressing roller  16 , which opposes the fourth follower roller  15  through the conveyer belt  100 , and generates an overall compressive force of approximately 25 N. 
     Since the primary function of the pressing roller  16  is to press a transfer medium onto the conveyer belt  100  so that the transfer medium is better adhered to the conveyer belt  100 , the magnitude of the coefficient of friction of the peripheral surface of the pressing roller  16  is irrelevant. Therefore, there is no restriction regarding the material for the pressing roller  16 . 
     A range A in which the conveyer belt  100  bears a transfer medium on its outwardly facing surface is the range between the first and third follower rollers  12  and  14  of the conveying apparatus  7 . Moreover, the first follower roller  12  doubles as a separation roller which gives the conveyer belt  100  a curvature large enough to cause a transfer medium to separate from the conveyer belt  100  (separation based on curvature difference). 
     The driver roller  11  is disposed on the downstream side of the first follower roller  12  in terms of the running direction of the conveyer belt  100 , more specifically, the downstream end of the conveyer belt  100  and below the plane of the top portion (portion in range A) of the conveyer belt loop. Further, the fixing nip is on the top side of the top portion of the loop, that is, on the photosensitive drum side. In other words, the driver roller  11  is disposed at a level below the fixing apparatus  10 , substantially away from the opening  10   a,  a slit, of the fixing apparatus  10 , as shown in FIG.  1 . The temperature of the bottom portion of the fixing apparatus  10  remains relatively low due to the convection by natural air current (or forced air current created by an unillustrated fan or the like) adjacent to the fixing apparatus  10 . Therefore, the temperature of the bottom casing  10   b  increases very little. Consequently, the amount of the thermal energy which the driver roller  11  receives from the bottom casing  10   b  of the fixing apparatus  7  is very small. Further, as the temperature of the heater  10   d  contained in the heating roller  10   c  of the fixing apparatus  7  is increased for image fixation, a substantial amount of radiant heat is generated from the heater  10   d , and radiates from the peripheral surface, or heat applying surface, of the elastic layer which covers the heater  10   d . However, this radiant heat does not directly reach the driver roller  11 , which also helps minimize the increase in the temperature of the driver roller  11 . Thus, the configuration of the belt based conveying apparatus  7 , in accordance with the present invention can keep at the minimum level the fluctuation in the velocity of the conveyor belt  100 , so that color deviation can be kept at the minimum level. In other words, high quality images can be produced. 
     The driver roller  11  comprises a metallic core formed of iron, aluminum, or the like, and a layer of elastic material, such as rubber or urethane, coated on the peripheral surface of the metallic core. 
     The external diameter (30.5 mm) of the driver roller  11  is approximately twice as large as the external diameter (15 mm) of the first follower roller  12 . This arrangement provides a larger contact area between the driver roller  11  and conveyor belt  100 , helping to keep at the minimum level the fluctuation in the velocity of the conveyer belt  100  caused by the slipping which occurs between the driver roller  11  and the conveyer belt  100 . Further, in this embodiment, the first follower roller  12 , or the separation roller, is smaller in the external diameter, and therefore, it is greater in the curvature of its peripheral surface. Evidently, the greater the curvature of the peripheral surface of the separation roller, the greater the difference in curvature between the conveyer belt  100  and the transfer medium, which causes the transfer medium to separate from the conveyer belt  100 . Therefore, the conveyer apparatus structure in this embodiment more efficiently separates a transfer medium from the conveyer belt  100  than the conventional one, assuring that even a transfer medium weak in resiliency cleanly separates from the conveyer belt  100 . In other words, the present invention makes it possible to provide an image forming apparatus which does not suffer from paper jams. 
     Further, the conveyer belt  100  is wrapped around the driver roller  11  in such a manner that the contact angle θ of the conveyer belt  100  and the driver roller  11 , that is, the angle between the two lines formed by connecting the center of the driver roller  11  with one end of the contact area between the conveyer belt  100  and the driver roller  11  and the other end, is approximately 120° (θ=120°), being greater than 90° (FIG.  2 ). This arrangement reduces the fluctuation in the velocity of the conveyer belt  100  which is caused by the slipping between the conveyer belt  100  and the driver roller  11 . Therefore, even after the coefficient of the surface friction of the driver roller  11  reduces due to continuous extended usage, the slipping does not occur, and therefore, the conveyer belt  100  conveys a transfer medium at a constant velocity, making it possible to form high quality images. 
     Further, since the first follower roller  12 , the separation roller, is smaller in diameter than the driver roller  11 , and is formed of electrically conductive metallic material, it is unnecessary to coat the peripheral surface of the first follower roller  12  with rubber or the like. Therefore, there is no concern that the first follower roller  12 , or the separation roller, is electrically affected by the corona effected by Corotron to facilitate the separation. Thus, the first follower effectively functions as a counter electrode. 
     Further, Corotron  17  is disposed directly above the first follower roller  12  to increase the amount of ions in the limited ambience immediately adjacent to the point at which a transfer medium separates from the conveyer belt  100 . This setup prevents the electrical discharge from occurring between a transfer medium and the conveyer belt  100  and/or between a transfer medium and the first follower roller  12 . Therefore, it is possible to produce high quality images (images with no anomaly). 
     Itemized below are the effects peculiar to this embodiment of the present invention. 
     Referring to FIG. 2, a referential character A designates the portion of the peripheral surface of the conveyer belt, which is running through the top portion of the conveyer belt loop, and a referential character B designates the portion of the conveyer belt, which opposes the portion A from below. The relationship between the tensional forces which work on the portions A and B when the force applied to the tension roller  13  is F, and the conveyer belt  100  is running in the direction indicated by the arrow mark in the drawing is: Ta&gt;Tb. The magnitude of the Ta and Tb are determined by the magnitude of F. 
     If the flatness of a transfer medium is disturbed by the shock or the like which occurs at the time of the transfer medium adhesion, color deviation or the like occurs, which results in images of poor quality. Therefore, the portion A must be kept virtually flat. In order to keep the portion A virtually flat, the magnitude of the tension applied to the portion A must be higher than a certain level. 
     In the case of a conventional transfer medium conveying apparatus, the driver roller is disposed on the upstream end of the aforementioned portion A in terms of the transfer medium conveyance direction, for the purpose of minimizing the effect of the heat from the fixing apparatus upon the driver roller, and also for defining the upstream end of the above defined portion A. In this case, the relationship between the tensional forces Ta′ and Tb′, which work on the aforementioned portions A and B, respectively, when the conveyer belt is running in the direction indicated by the arrow in the drawing is: Ta′&lt;Tb′ and the magnitude of the Ta′ and Tb′ are determined by the magnitude of the force F′ applied to the tension roller. 
     In order to keep the portion A as flat as the conveyer apparatus structure in accordance with the present invention does, Ta′ must be equal to Ta (Ta′=Ta). As stated before, Ta&gt;Tb. Therefore, Tb′&gt;Ta′=Ta&gt;Tb. Thus, in order to satisfy this formula, the force to be applied to the tension roller in the conventional structure must be larger than the force applied to the tension roller in the structure in accordance with the present invention: F′&gt;F. In other words, if a transfer medium conveying means is structured as a conventional transfer medium conveying means in which the driver roller is disposed on the upstream end of the aforementioned portion A in terms of the transfer medium conveyance direction, and in which the driver roller defines the upstream end of the portion A, the force applied to the tension roller must be greater than that in the transfer medium conveying apparatus in accordance with the present invention in which the driver roller is disposed on the downstream end of the above defined portion A, and does not define the downstream end of the portion A. As described above, the greater the force applied to the tension roller, the worse the creep and fatigue of the conveyor belt, and therefore, the shorter the service life of the conveyer belt, and therefore, the higher the running cost for the main assembly of an image forming apparatus. 
     In other words, according to this embodiment of the present invention, the driver roller  11  can be placed on the downstream end of the conveyer belt in terms of the transfer medium conveyance direction without causing grave side effects, and the above defined portion A can be sufficiently tensioned by a force much smaller than the force required for the conventional transfer medium conveying apparatus. Therefore, the conveyer belt  100  lasts longer. Thus, it is possible to provide an image forming apparatus inexpensive in running cost. 
     Also according to this embodiment, four rollers are employed to suspend the conveyer belt  100 . However, the number of rollers may be three as depicted in FIG. 3, which is self-explanatory. 
     Embodiment 2 
     Another embodiment of the present invention is depicted in FIG.  4 . As is evident from the drawing, in this embodiment, the driver roller  11  is disposed on the upstream side of the follower roller  14  in terms of the transfer medium conveying direction of the conveyer belt  100 , substantially below the transfer medium conveying portion A. In other words, the driver roller is disposed as far away as possible from the fixing apparatus  10 . Therefore, the increase in the temperature of the driver roller  11  is minimum. Thus, the fluctuation in the velocity of the conveyer belt  100  is minimum. Therefore, color deviation is minimum. In other words, this embodiment also makes it possible to produce high quality images. 
     The comparison, in structure and positioning, between a belt based transfer medium conveying apparatus, a registration roller, and their adjacencies, in accordance with the present invention, and those of a conventional type is given in FIG.  5 . FIG. 5, (a) represents the present invention, and FIG. 5, (b) represents a conventional type. 
     In the conventional type, the driver roller  11  is disposed on the transfer medium bearing portion A side, that is, adjacent to the registration roller pair  8 , whereas in the type in accordance with the present invention, the follower roller  14  is disposed on the top loop side, that is, the transfer medium conveying side, of the conveyer belt, that is, adjacent to the registration roller pair  8 . Since the external diameter of the follower roller  14  may be substantially smaller than that of the driver roller  11 , the distance l between the belt based conveying apparatus  7  and the registration roller pair  8  defined in FIG. 5, (a), which represents the present invention, is substantially smaller than the corresponding distance (distance l) defined in FIG. 5, (b) which represents the conventional type. 
     The difference in the external diameter of the roller disposed adjacent to the registration roller  8  to support the conveyer belt is not the only factor which made this arrangement possible. 
     More specifically, referring to FIG. 5, the registration roller  8  and driver roller  11  both require a driving mechanism, which generally must be disposed in their adjacencies, for example, on their front or back sides as seen from the direction perpendicular to FIG.  5 . The spaces which the driving mechanisms for the driving roller  11  and registration roller pair  8  require are as large as the circular areas designated by referential characters  11   a  and  8   a  in the drawing, and the two rollers must be arranged so that the areas  11   a  and  8   a  do not mutually overlap. Therefore, this embodiment which makes it possible to dispose the registration roller pair  8  and driver roller  11  reasonably apart from each other without changing the internal space of the apparatus main assembly is more advantageous for the size reduction of the apparatus main assembly. 
     Even though four rollers are employed in this embodiment to suspend the conveyer belt, the number of rollers may be three as depicted in FIG. 6, which is self-explanatory. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or change as may come within the purposes of the improvements or the scope of the following claims. For example, in the preceding embodiments, the present invention was described with reference to a photosensitive drum in the form of a drum, but the photosensitive drum may be in the form of a belt, which is needless to say. Further, the present invention is also applicable to an image forming apparatus in which a plurality of image forming station are vertically aligned, and a transfer medium is vertically conveyed (for example, upward) by a conveyer belt ( 100 ).