Abstract:
An image forming apparatus, such as a laser printer, may use high levels of heat to form images, and some elements may expand and contract with changing temperatures. An apparatus may include a heat roller disposed against a pressure roller, with an endless belt member enclosing the pressure roller. The belt member may also have different frictional characteristics on an inside and outside of the roller, such that when peripheral speeds change due to expansion of the pressure roller, the belt member may be configured to slip against the pressure roller, and maintain a frictional relationship with the heat roller (or a printing medium, such as paper, that is pinched between the rollers). Additionally, the rotational speed of an axis of the pressure roller may be adjusted to compensate for the thermal expansion.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2007-252263, filed on Sep. 27, 2007, the entire subject matter of which is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    Aspects described herein relate to a fixing unit having a pressure roller and a heat roller, and an image forming apparatus including such a fixing unit. 
       BACKGROUND 
       [0003]    A known image forming apparatus, e.g., a laser printer, includes a fixing unit that is configured to fix a developer image transferred from a photosensitive member onto a recording sheet by heat. The fixing unit includes a heat roller that is subjected to heat from a heat source and receives power, and a pressure roller that is pressed against the heat roller. 
         [0004]    To enhance the speed of image formation processing of the image forming apparatus, the fixing unit is used to enhance the speed of heat fixing. However, it is difficult to increase a surface temperature of the heat roller further in view of, for example, the melting point of a fluorine resin coated on the surface of the pressure roller. Thus, a nip width between the pressure roller and the heat roller is increased to extend a contact area between a recording sheet and the heat roller, to cope with high-speed heat fixing. 
         [0005]    However, when the pressure roller is pressed against the heat roller, a part of the pressure roller that the heat roller contacts is deformed, e.g. dented, because the pressure roller is covered with an elastic layer. Such deformation of the pressure roller may increase a force of the pressure roller hindering sheet conveyance more than a force of the heat roller facilitating sheet conveyance, which may lead to a slippage between the heat roller and a recording sheet. 
         [0006]    To solve this problem, the pressure roller may be forced to rotate while the heat roller is rotated. In this case, the outside diameter of the pressure roller changes because a surface (elastic layer) of the pressure roller expands with heat, and thus a peripheral speed of the pressure roller changes, which causes a difference in peripheral speed between the pressure roller and the heat roller, and results in a slippage between the pressure roller and the heat roller. Accordingly, a difference in sheet conveyance speed between both sides (a side facing the heat roller and a side facing the pressure roller) of a recording sheet occurs, which also causes a slippage between the heat roller and a sheet. 
         [0007]    When the heat roller and the sheet slip, a trace of the heat roller may be left on the sheet, and a developer image on the sheet may be scraped against the heat roller, which may impair image quality. 
       SUMMARY 
       [0008]    Aspects described herein provide a fixing unit configured to enhance image quality and an image forming apparatus including such a fixing unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Illustrative aspects will be described in detail with reference to the following figures in which like elements are labeled with like numbers and in which: 
           [0010]      FIG. 1  is a side sectional view of an internal structure of a laser printer as an illustrative example of an image forming apparatus using features described herein; 
           [0011]      FIG. 2  is a side view of a fixing unit of  FIG. 1 ; 
           [0012]      FIG. 3  schematically shows a heat roller, a pressure roller, a belt member, and a regulating member; 
           [0013]      FIG. 4A  is a side view of the heat roller, the pressure roller, the belt member, and the regulating member; 
           [0014]      FIG. 4B  is a perspective view of the pressure roller and the regulating member; and 
           [0015]      FIG. 5  schematically shows a heat roller, a pressure roller, a belt member, and a regulating member according to an alternate embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    An illustrative embodiment will be described in detail with reference to the accompanying drawings. An image forming apparatus according to aspects described herein applies to a laser printer  1  as shown in  FIG. 1 . It will be appreciated that these aspects also apply to other types of image forming apparatuses, such as a copier and a multifunction apparatus as well. 
         [0017]    For ease of discussion, in the following description, the top or upper side, the bottom or lower side, the left or left side, the right or right side, the front or front side, and the rear or rear side are used to define the various parts when the laser printer  1  is disposed in an orientation in which it is intended to be used. In  FIG. 1 , the right side is referred to as the front or front side, the left side is referred to as the rear or the rear side, the up side is referred to as the top or upper side, and the down side is referred to as the bottom or lower side. 
         [0018]    As shown in  FIG. 1 , the laser printer  1  may include, in a main body  2 , a sheet supply section  3 , a light exposure unit  4 , a process cartridge  5 , and a fixing unit  6 . The sheet supply section  3  is configured to supply a recording sheet, e.g., a sheet P. The process cartridge  5  is configured to transfer an image of developer, e.g., toner, onto the sheet P. The fixing unit  6  may be configured to fix the toner image onto the sheet by heat. 
         [0019]    The sheet supply section  3  may include a sheet supply tray  31 , a sheet pressing plate  32 , a pickup roller  33 , a separation pad  34 , dust removing rollers  35 ,  36 , and registration rollers  37 . The sheet supply tray  31  may be disposed in a lower portion of the main body  2  and configured to be attached to and removed from the main body  2 . The pickup roller  33  and the separation roller  34  are disposed in a front upper portion of the sheet supply tray  31 . The dust removing rollers  35 ,  36  are disposed at a downstream side from the pickup roller  33  in a direction where the sheet P is conveyed (hereinafter referred to as the sheet conveyance direction). The registration rollers  37  are disposed at the downstream side from the dust removing rollers  35 ,  36  in the sheet conveyance direction. 
         [0020]    In the sheet supply section  3 , a sheet P in a stack of sheets in the sheet supply tray  31  is moved to the pickup roller  33  by the sheet pressing plate  32 , singly conveyed by the pickup roller  33  and the separation pad  34 , passed through the dust removing rollers  35 ,  36  and the registration rollers  37 , and conveyed to the process cartridge  5 . 
         [0021]    The light exposure unit  4  may be disposed in an upper portion of the main body  2 . The exposure unit  4  may include a light emitting portion (not shown), a polygon mirror  41  configured to be driven to rotate, lenses  42 ,  43 , and reflecting mirrors  44 ,  45 ,  46 . In the exposure unit  4 , as shown in a broken line, a laser beam emitted from the light emitting portion, based on image data, may be deflected by the polygon mirror  41 , pass through the lens  42 , be folded by the reflecting mirrors  44 ,  45 , pass through the lens  43 , and be bent downward by the reflecting mirror  46 , to be directed to a surface of the photosensitive drum  52  in the process cartridge  5  at high speed scanning. 
         [0022]    The process cartridge  5  may be disposed under the exposure unit  4 , and configured to be attached to and removed from the main body  2 . The process cartridge  5  includes a cartridge frame  51  that is hollow and serves as an outer frame. The process cartridge  5  further includes a photosensitive member, e.g., a photosensitive drum  52 , a scorotron charger  53 , a transfer member, e.g., a transfer roller  54 , and a developer cartridge  55  in the cartridge frame  51 . 
         [0023]    The developer cartridge  55  may be mounted in the cartridge frame  51  in a detachable manner. The developer cartridge  55  includes a developing roller  56 , a layer-thickness regulating blade  57 , a supply roller  58 , and a toner chamber  59 . Developer, e.g., toner, stored in the toner chamber  59 , is supplied to the developing roller  56  along with the rotation of the supply roller  58 . At this time, toner is electrically charged between the supply roller  58  and the developing roller  56  by friction. The toner supplied to the developing roller  56  goes in between the layer-thickness regulating blade  57  and the developing roller  56  along with the rotation of the developing roller  56 , and is carried on the developing roller  56  as a thin layer having a constant thickness. 
         [0024]    The photosensitive drum  52  may be rotatably supported by the cartridge frame  51 . The photosensitive drum  52  includes a drum body that is grounded and an outer surface thereof that is formed of a photosensitive layer. 
         [0025]    The transfer roller  54  may be disposed below the photosensitive drum  52 , contacting the photosensitive drum  52  from below, and rotatably supported by the cartridge frame  51 . During image transfer, a bias is applied to the transfer roller  54 . 
         [0026]    In the process cartridge  5 , the surface of the photosensitive drum  52  may be uniformly and positively charged by the scorotron charger  53 , and exposed to a laser beam emitted from the exposure unit  4  by high-speed scanning. An electric potential in the exposed area of the surface of the photosensitive drum  52  becomes low, and an electrostatic latent image is formed based on the image data. 
         [0027]    When the developing roller  56  is rotated, toner carried on the developing roller  56  is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  52 . As toner is selectively carried on the surface of the photosensitive drum  52 , the latent image on the photosensitive drum  52  becomes visible, and a toner image is formed by reversal. 
         [0028]    The photosensitive drum  52  and the transfer roller  54  are rotated to convey the sheet P therebetween. While the sheet P is conveyed between the photosensitive drum  52  and the transfer roller  54 , the toner image carried on the photosensitive drum  52  is transferred onto the sheet P. 
         [0029]    The fixing unit  6  may be disposed at the rear of the process cartridge  5  or at a downstream side of the process cartridge  5  in the sheet conveyance direction. The fixing unit  6  may include a heat roller  61 , a pressure roller  62  configured to be pressed against the heat roller  61 , a belt member  63  disposed around the pressure roller  62 , and a frame member  64 . Ejection rollers  71 ,  72 , and a sheet ejection path  73  are provided at the downstream side from the fixing unit  6  in the sheet conveyance direction, so as to eject the sheet P conveyed from the fixing unit  6  out of the main body  2 . 
         [0030]    In the fixing unit  6 , the toner image transferred onto the sheet P is fixed by heat while the sheet P passes between the heat roller  61  and the belt member  63  disposed around the pressure roller  62 . After passing through the fixing unit  6 , the sheet P is conveyed to the sheet ejection path  73  by the ejection rollers  71 , and ejected to a sheet ejection tray  74  by the ejection rollers  72 . 
         [0031]    As shown in  FIGS. 2 and 3 , the fixing unit  6  may further include arm members  65  (only one shown) and extension springs  66  (only one shown), and belt supporting members  67  (only one shown). The pressure roller  62  is supported by the arm members  65  and the extension springs  66 . 
         [0032]    The heat roller  61  may be formed in a generally cylindrical shape, and may have a heat source  61 B, such as a halogen heater, therein. The heat roller  61  may be configured such that a surface thereof becomes heated to a temperature for fixing toner by the heat source  61 B. Both ends  61 A of the heat roller  61  protruding axially therefrom may be supported by the frame member  64 , so that the heat roller  61  is rotatable. A transmission gear  61 G may be fixed to one end  61 A, and rotated along with the heat roller  61 . Power from a drive source (not shown) disposed in the main body  2  may be supplied via gears (not shown) to the transmission gear  61 G, which causes the heat roller  61  to rotate. 
         [0033]    The pressure roller  62  may include a cylindrically-shaped roller portion  62 R made of an elastic layer, e.g., a silicone rubber, and a rotation shaft  62 A extending through the roller portion  62 R and protruding outward from both ends of the roller portion  62 R. The rotation shaft  62 A may be rotatably supported by the arm members  65 . A transmission gear  62 G may be fixed to an end of the rotation shaft  62 A and configured to rotate along with the pressure roller  62 . The transmission gear  62 G is engaged with the transmission gear  61 G. Thus, power from the heat roller  61  may be transmitted to the pressure roller  62  via the transmission gears  61 G,  62 G, and the pressure roller  62  may be forced to rotate along with the heat roller  61 . 
         [0034]    The arm members  65  may be disposed on both ends of the rotation shaft  62 A of the pressure roller  62 . A front side (right side in  FIG. 2 ) of each arm member  65  may rotatably support a support shaft  64 A disposed in the frame member  64 , and an upper rear side of each arm member  65  may be attached to one end of the extension spring  66 . The other end of the extension spring  66  may be attached to the frame member  64 . Thus, the pressure roller  62  rotatably supported by the arm members  65  is capable of moving in a direction of an arrow of  FIG. 2 . As each arm member  65  is urged toward the heat roller  61  under a force applied from the extension spring  66 , the pressure roller  62  is also urged or pressed toward the heat roller  61 . 
         [0035]    The belt member  63  may be an endless member (e.g., circular, such as a conveyor belt) having a perimeter greater than a perimeter of the pressure roller  62  (that is, specifically, a maximum perimeter of the pressure roller  62  thermally expanding). In other words, the endless member may loosely enclose the pressure roller to accommodate thermal expansion of the roller  62 . The belt member  63  may be disposed around the pressure roller  62  and partially sandwiched between the heat roller  61  and the pressure roller  62 . While the toner image is thermally fixed, the sheet P passes between the heat roller  61  and the belt member  63 . Thus, during heat fixing, the belt member  63  is sandwiched between the pressure roller  62  and the sheet P and is slidable on the pressure roller  62 . The coefficient of friction between the belt member  63  and the pressure roller  62  is smaller than that between the belt member  63  and the sheet P. The belt member  63  is pressed against the heat roller  61  because the pressure roller  62  is pressed against the heat roller  61 . 
         [0036]    The belt member  63  may be an endless film formed of a heat-resistant resin such as polyimide (PI) or an endless electroformed film formed of nickel or stainless steel. 
         [0037]    When the belt member  63  is formed of a conductive material, e.g., an electroformed film of nickel or stainless steel, the belt member  63  may be electrically grounded. Specifically, as shown in  FIG. 4A , the belt member  63  formed of a conductive material is stretched around the belt supporting member  67 , which is formed of a conductive resin and is electrically grounded, so that the belt member  63  can be electrically grounded. 
         [0038]    A surface of the belt member  63  that contacts the heat roller  61  (or sheet P during heat fixing) may be coated with fluorine resin. Further, fluorine resin can be coated on a surface of the belt member  63  that contacts the pressure roller  62 . 
         [0039]    As shown in  FIG. 3 , the belt member  63  has a width W 2  extending in an axial direction of the pressure roller  62 , which is greater than a width WI of the roller portion  62 R of the pressure roller  62  extending in the axial direction. The width W 2  of the belt member  63  is smaller than a width W 3  of the heat roller  61  extending in an axial direction of the heat roller  61 . Thus, the relationship W 1 &lt;W 2 &lt;W 3  is established. 
         [0040]    The belt supporting members  67  may be attached to the rotation shaft  62 A of the pressure roller  62  disposed at each end of the roller portion  62 R. As shown in  FIGS. 4A and 4B , the belt supporting member  67  includes a bearing  67 A, belt supporting portions  67 B disposed symmetrically with respect to the bearing  67 A, and belt regulating portion  67 C disposed on the belt supporting portions  67 B. The bearing  67 A is fitted around the rotation shaft  67 A. The belt supporting portions  67 B are configured to support the belt member  63  from within. 
         [0041]    The bearing  67 A may be rotatably fitted around the rotation shaft  62 A, so that the belt supporting member  67  will not rotate along with the rotation shaft  62 A. As shown in  FIG. 4B , when the bearing  67 A is attached to the rotation shaft  62 A, a protrusion  67 D disposed on an inner surface of the bearing  67 A is engaged in a groove  62 B formed in the rotation shaft  62 A. This engagement prevents the belt supporting member  67  from moving in an axial direction of the rotation shaft  62 A. 
         [0042]    The belt supporting portions  67 B may be generally arc-shaped symmetrically from the bearing  67 A. A peripheral surface of each belt supporting portion  67 B serves as a support surface for supporting the belt member  63 . 
         [0043]    The peripheral surface of each belt supporting portion  67 B may be arcuately recessed so as to match a peripheral surface of the heat roller  61 . With this shape, the arcuately recessed portion of the peripheral surface of each belt supporting portion  67 B is regulated by the heat roller  61 . Thus, the belt supporting member  67  is configured not to rotate even when the belt member  63  is slidingly rotated. 
         [0044]    As shown in  FIG. 4B , the belt regulating portions  67 C are formed on the support surfaces of the corresponding belt supporting portions  67 B, and protrude outward further than the belt supporting portions  67 B with respect to a radial direction of the bearing  67 A. The belt regulating portions  67 C are configured to be located outward with respect to the axial direction of the rotation shaft  62 A when the belt supporting member  67  is attached to the rotation shaft  62 A. 
         [0045]    In  FIG. 4A , the belt supporting member  67  at its original state is indicated by dashed lines. When the belt supporting member  67  is attached to the rotation shaft  62 A, and the belt member  63  is disposed around the pressure roller  62  and the belt supporting member  67 , the belt supporting portions  67 B are bent toward the bearing  67 A as shown in a solid line of  FIG. 4A . As the belt supporting portions  67 B have the property of returning to their original state, the bent belt supporting portions  67 B exert force in arrowed directions of  FIG. 4A  to apply tension to the belt member  63 . 
         [0046]    The operation of the fixing unit  6  configured above will be described in the portion appearing below. 
         [0047]    As shown in  FIG. 3 , when the heat roller  61  rotates, its power is transmitted from the transmission gears  61 G,  62 G to the pressure roller  62 , which causes the pressure roller  62  to rotate. The belt member  63  is slidingly rotated along the rotation of the heat roller  61  and the pressure roller  62  in such a manner that the belt member  63  is conveyed between the heat roller  61  and the pressure roller  62 . As shown in  FIG. 1 , while the sheet P having an image thereon is conveyed between the heat roller  61  and the belt member  63 , it is sandwiched between the heat roller  61  and the pressure roller  62  via the belt member  63 , so that the toner is fixed by heat onto the sheet P. At this time, the peripheral speeds of the heat roller  61  and the pressure roller  62  become substantially equal to the peripheral speed of the belt member  63 . 
         [0048]    When heat fixing continues, the outside diameter of the pressure roller  62  changes due to thermal expansion, which causes a difference in peripheral speed between the heat roller  61  and the pressure roller  62 . At this time, the peripheral speed of the heat roller  61  and the peripheral speed of the belt member  63  are maintained approximated to each other because friction from the heat roller  61  is transmitted to the belt member  63  via the sheet P. However, a difference in peripheral speed between the pressure roller  62  and the belt member  63  occurs. As described above, the coefficient of friction between the belt member  63  and the pressure roller  62  is smaller than that between the belt member  63  and the sheet P. Thus, the inner surface of the belt member  63  and the pressure roller  62  slip, and slippage between the outer surface of the belt member  63  and the sheet P is reduced. 
         [0049]    According to the above description, some or all of the following advantages can be obtained. 
         [0050]    Even if a difference in peripheral speed between the heat roller  61  and the pressure roller  62  occurs, friction from the heat roller  61  is transmitted to the belt member  63  via the sheet P, so that the inner surface of the belt member  63  and the pressure roller  62  slip. When the pressure roller  62  and the belt member  63  slip, the difference in peripheral speed between the heat roller  61  and the pressure roller  62  is absorbed, so that occurrence of the peripheral speed difference between the heat roller  61  and the belt member  63  is reduced. As the occurrence of the peripheral speed difference between the heat roller  61  and the belt member  63  is reduced, slippage between the heat roller  61  and the sheet P can be reduced, which can offer enhanced image quality. 
         [0051]    As the slippage between the heat roller  61  and the sheet P can be reduced, an elastic layer having a lower hardness can be applied to the pressure roller  61  to increase a nip width between the heat roller  61  and the pressure roller  62 . This enables enhanced speed of fixing. 
         [0052]    The slippage between the pressure roller  62  and the belt member  63  is relatively small in comparison with a structure using a stationary pressure member instead of the pressure roller  62 . Thus, adverse effects, such as the wearing away of the belt member  63 , abrasion of fluorine resin coated on the surface of the belt member  63 , and electrostatic buildup on the belt member  63  due to sliding friction between the pressure roller  62  and the belt member  63  can be reduced. 
         [0053]    Power of the heat roller  61  may be transmitted via the transmission gears  61 G,  62 G to the pressure roller  62 , and the pressure roller  62  is forced to rotate. Thus, places on a surface of the pressure roller  62  where the pressure roller  62  is pressed by the heat roller  61  (or where a nip width is formed) can be changed evenly, and thus permanent deformation due to stress imposed in one place by the heat roller  61  can be reduced from the pressure roller  62 . Accordingly, fluctuations of the nip pressure due to permanent deformation of the pressure roller  62  can be reduced. 
         [0054]    Power of the heat roller  61  may be transmitted via the transmission gears  61 G,  62 G to the pressure roller  62 . Comparing with a structure where power is transmitted from a drive source (not shown) disposed in the main body  2  via a plurality of gears (not shown) to the transmission gear  62 G, the embodiment of the invention can reduce the number of parts. 
         [0055]    The width W 2  of the belt member  63  is greater than the width WI of the pressure roller  62 , which is greater than the width of a recording sheet P. Thus, the width of the sheet P can fit within the width W 2  of the belt member  63 . As the sheet P can be reliably pressed by the heat roller  61 , the toner image can be fixed onto the sheet P by heat, which can offer enhanced image quality. 
         [0056]    The belt member  63  may move in the axial direction of the rotation shaft  62 A in response to rotation of the heat roller  61  and the pressure roller  62 . However, as the belt member  63  is disposed around the belt supporting member  67  that has the belt regulating portions  67 C and that does not move in the axial direction of the rotation shaft  62 A, the movement of the belt member  63  in the axial direction can be regulated. Thus, the belt member  63  can be prevented from moving extremely to one end of the pressure roller  62  and coming off from the pressure roller  62  or shifting greatly from the pressure roller  62 . As a result, the sheet P can be prevented from shifting in the axial direction. As the sheet P can be prevented from shifting in the axial direction, it can be prevented from rubbing against the heat roller  61 , which can offer enhanced image quality. 
         [0057]    The belt supporting member  67  applies tension to the belt member  63 , which can keep the belt member  63  from becoming wrinkled. Thus, the heat roller  61 , the sheet P, and the belt member  67  can be brought into intimate contact with each other, which can enhance transmission of heat from the heat roller  61  to the sheet P. As the transmission of heat to the sheet P is enhanced, the toner image can be efficiently fixed onto the sheet P by heat, which can offer enhanced speed of fixing. In addition, as the belt member  63  and the sheet P are brought into contact with each other, the slippage between the belt member  63  and the sheet P can be reduced more reliably. Thus, the toner image formed on the sheet P can be fixed by heat without being disturbed, so that image quality can be enhanced reliably. 
         [0058]    As the width W 2  of the belt member  63  is greater than the width W 1  of the pressure roller  62 , the belt supporting members  67  disposed on both ends of the roller portion  62 R of the pressure roller  62  can apply tension to the belt member  63  and regulate the movement of the belt member  63  in the axial direction. In addition, as the width W 3  of the heat roller  61  is greater than the width W 2  of the belt member  63 , the belt member  63  is pressed against the heat roller  61  across the full width of the belt member  63  in the axial direction. Thus, friction from the heat roller  61  can be transmitted to the belt member  63 . As friction from the heat roller  61  is transmitted to the belt member  63 , the difference in peripheral speed between the heat roller  61  and the pressure roller  62  can be absorbed. Thus, the slippage between the sheet P and the heat roller  61  can be reduced more reliably, so that image quality can be enhanced reliably. 
         [0059]    The belt member  63  formed of a heat-resistant resin, nickel, or stainless steel, can be slidingly rotated between the heat roller  61  and the pressure roller  62  with stability because it will not soften or deform even by contact with the heat roller  61  whose surface is heated to high temperatures. As the belt member  63  is slidingly rotated between the heat roller  61  and the pressure roller  62  with stability, the toner image can be stably fixed onto the sheet P by heat, which can offer enhanced image quality reliably. 
         [0060]    When the belt member  63  is electrically grounded, electrostatic buildup on the belt member  63  due to sliding friction between the pressure roller  62  and the belt member  63  can be prevented. As electrostatic buildup on the belt member  63  is prevented, disturbance of electrically charged toner (image) that is not fixed by heat on the sheet P can be reduced, so that image quality can be enhanced reliably. 
         [0061]    When a surface of the belt member  63  that contacts the heat roller  61  (or sheet P during heat fixing) is coated with fluorine resin, adhesion of toner onto the belt member  63  can be reduced. Thus, adhesion of toner from the belt member  63  to the backside of the sheet P can be reduced, and dirt on the sheet P can be reduced. When a surface of the belt member  63  that contacts the pressure roller  62  is coated with fluorine resin, the pressure roller  62  and the belt member  63  can smoothly slide. Thus, the slippage between the heat roller  61  and the sheet P can be reduced, and image quality can be enhanced reliably. 
         [0062]    This illustrative embodiment shows, but is not limited to, the structure where the transmission gears  61 G,  62 G are used as an example of a power transmission member. Instead, a toothed belt may be used. Alternatively, a combination of two or more different types of power transmission members, such as a transmission gear and a toothed belt, may be used. 
         [0063]    With reference to  FIG. 3 , a rotational speed of the rotation shaft  62 A of the pressure roller  62  per unit time when the pressure roller  62  is forced to rotate will be described. 
         [0064]    Power of the heat roller  61  may be transmitted via the transmission gears  61 G,  62 G to the pressure roller  62 . During heat fixing, the outside diameter of the heat roller  61  does not change, and thus it can be estimated that the peripheral speed of the heat roller  61  does not change (or is constant). Under this estimation, a rotational speed of the end  61 A of the heat roller  61  and a rotational speed of the transmission gear  61 G do not change, and thus the rotational speed of the transmission gear  62 G of the pressure roller  62  connected to the transmission gear  61 G and the rotational speed of the rotation shaft  62 A do not change. 
         [0065]    During heat fixing, the outside diameter of the pressure roller  62  changes due to thermal expansion. When the outside diameter of the pressure roller  62  changes but the rotational speed of the rotation shaft  62 A does not change, the peripheral speed of the pressure roller  62  changes. Thus, a difference in peripheral speed between the heat roller  61  and the pressure roller  62  occurs, and a difference between the peripheral speed of the belt member  63  to which friction of the heat roller  61  is applied via the sheet P and the peripheral speed of the pressure roller  62  occurs. As a result, the belt member  63  and the pressure roller  62  slip. 
         [0066]    To reduce the slippage between the belt member  63  and the pressure roller  62 , the difference in peripheral speed between the heat roller  61  and the pressure roller  62  needs to be reduced. To address this, it is possible to determine the rotational speed N of the rotation shaft  62 A per unit time at which, even if the outside diameter of the pressure roller  62  changes, the peripheral speed of the pressure roller  62  approximates to the peripheral speed V of the heat roller  61 . In other words, the rotational speed of the shaft  62 A may be adjusted to compensate for changes in peripheral speed caused by this thermal expansion. 
         [0067]    The rotational speed N of the rotation shaft  62 A may be determined in a range between the rotational speed N 1  of the rotation shaft  62 A at high temperature and the rotational speed N 2  of the rotation shaft  62 A at room temperature during heat fixing (N 1 ≦N≦N 2 ). The rotational speed N 1  of the rotation shaft  62 A is a value that approximates the peripheral speed of the pressure roller  62  to the peripheral speed of the heat roller  61  when the outside diameter of the pressure roller  61  is maximum D MAX  (when thermally expanded during heat fixing). The rotational speed N 2  of the rotation shaft  62 A is a value that approximates the peripheral speed of the pressure roller  62  to the peripheral speed of the heat roller  61  when the outside diameter of the pressure roller  61  is minimum D MIN  (at room temperature during heat fixing). 
         [0068]    Because N 1 =V/πD MAX , and N 2 =V/πD MIN , the rotational speed N of the rotation shaft  62 A of the pressure roller  62  per unit time can satisfy the following condition: 
         [0000]    
       
      
       V/πD 
       MAX 
       ≦N≦V/πD 
       MIN  
      
     
         [0069]    where 
         [0070]    V is peripheral speed of the heat roller  61  (conveyance speed of a recording sheet P), 
         [0071]    D MAX  is maximum outside diameter of the pressure roller  62  during heat fixing (outside diameter of the pressure roller  62  at thermal expansion), and 
         [0072]    D MIN  is minimum outside diameter of the pressure roller  62  during heat fixing (outside diameter of the pressure roller  62  at room temperature). 
         [0073]    This condition can be achieved by setting a gear ratio of the transmission gears  61 G,  62 G to satisfy V/πD MAX ≦N≦V/πD MIN . 
         [0074]    When the rotational speed N of the rotation shaft  62 A satisfies the above condition, the difference in peripheral speed between the heat roller  61  and the pressure roller  62  can be reduced. Thus, the difference between the peripheral speed of the pressure roller  62  and the peripheral speed of the belt member  63 , which approximates to the peripheral speed of the heat roller  61 , can be also reduced. When the difference in peripheral speed between the pressure roller  62  and the belt member  63  is reduced, the slippage between the pressure roller  62  and the belt member  63  can be reduced. This reduction can reduce a force that may hinder rotation of the belt member  63  (conveyance of a sheet P), which is generated when the pressure roller  62  and the belt member  63  slip. When the force that may hinder sheet conveyance is reduced, the slippage between the heat roller  61  and the sheet P can be reduced, and image quality can be enhanced reliably. In addition, when the slippage between the pressure roller  62  and the belt member  63  is reduced, the pressure roller  62  and the belt member  63  can be made less prone to wear. 
         [0075]    This illustrative embodiment shows, but is not limited to, the structure where power of the heat roller  61  is transmitted via the transmission gears  61 G,  62 G to the pressure roller  62 . Alternatively, power from a drive source, e.g. a motor, disposed in the main body  2 , may be supplied to the transmission gear  62 G connected to the pressure roller  62  via gears and the power of the pressure roller  62  may be transmitted via the transmission gears  61 G,  62 G to the heat roller  61 . Alternatively, power from the drive source may be transmitted to the pressure roller  62  via driving power transmission member, e.g., a gear, but not via the heat roller  61 . 
         [0076]    This illustrative embodiment shows, but is not limited to, the structure where power of the heat roller  61  is transmitted via the transmission gears  61 G,  62 G to the pressure roller  62  and the pressure roller  62  is forced to rotate. For example, as shown in  FIG. 5 , the transmission gear  62 G may be omitted and the power of the heat roller  61  may be transmitted via the belt member  63  to the pressure roller  62 , so that the pressure roller  62  may be rotated. In other words, the pressure roller  62  may be rotated upon receipt of friction between the inner surface of the belt member  63 , which is slidingly rotated by the heat roller  61 , and the roller portion  62 R (or a sheet P during heat fixing). 
         [0077]    When the pressure roller  62  is caused to rotate in this manner, the difference in peripheral speed between the heat roller  61  and the pressure roller  62  do not occur, and thus the difference in peripheral speed between the heat roller  61  and the belt member  63  does not occur. Because the difference in peripheral speed between the heat roller  61  and the belt member  63  does not occur, the slippage between the heat roller  61  and the sheet P can be reduced, so that image quality can be enhanced reliably. Because the difference in peripheral speed between the heat roller  61  (or the belt member  63 ) and the pressure roller  62  does not occur, the slippage between the pressure roller  62  and the belt member  63  can be reduced. Thus, adverse effects, such as the wearing away of the belt member  63 , abrasion of fluorine resin coated on the surface of the belt member  63 , and electrostatic buildup on the belt member  63  due to sliding friction between the pressure roller  62  and the belt member  63  can be reduced. Further, as the pressure roller  62  is caused to rotate, torque of the drive source (not shown) disposed in the main body  2  can be reduced. 
         [0078]    The illustrative embodiment shows, but is not limited to, the example where the relationship among the width W 1  of the pressure roller  62 , the width W 2  of the belt member  63 , and the width W 3  of the heat roller  61  is W 1 &lt;W 2 &lt;W 3 . The relationship may be W 1 =W 2 =W 3 , W 1 &lt;W 2 =W 3 , or W 1 =W 2 &lt;W 3 . 
         [0079]    The illustrative embodiment shows, but is not limited to, the structure including the belt supporting member  67  that is configured to apply tension to the belt member  63  and regulate the axial movement of the belt member  63 . The structure may include a tension-applying member that is configured to apply tension to the belt member and a regulating member that is configured to regulate the axial movement of the belt member individually. Alternatively, the structure may include one of the tension-applying member and the regulating member. The tension-applying member may be a roller that is configured to slide on the inner surface of the belt member and apply tension to the belt member between the tension-applying member and the pressure roller. The regulating member may be a disk-shaped member having the outside diameter greater than the roller portion of the pressure roller or a cylindrical-shaped member having a belt regulating portion that is rotatably attachable to the rotation shaft of the pressure roller. 
         [0080]    The illustrative embodiment shows, but is not limited to, the structure where the belt member  63  is electrically grounded, as shown in  FIG. 4A , by electrically grounding the belt supporting member  67  formed of a conductive resin and disposing the belt member  63  around the belt supporting member  67 . The belt member  63  may be electrically grounded by bringing a conductive rod-shaped member that is electrically grounded in contact with the inner surface of the belt member. 
         [0081]    The illustrative embodiment shows, but is not limited to, the structure where the pressure roller  62  is pressed against the heat roller  61  by a pressing mechanism made up of the arm members  65  and extension springs  66 . A known pressing mechanism may be used for the pressure roller  62 . The illustrative embodiment shows, but is not limited to, the structure where the pressure roller  61  is pressed against the heat roller  62 . The heat roller may be pressed against the pressure roller. 
         [0082]    A sheet P may include plain paper, cardboards, postcards, and transparency sheets. 
         [0083]    The illustrative embodiment shows, but is not limited to, the structure that uses the light exposure unit  4  configured to scan laser light onto the photosensitive drum  52 , the photosensitive drum  52  as a photosensitive member, the developing cartridge  55  as a developing device, and the transfer roller  54  as a transfer member. These parts may be modified in material and structure without departing from the scope of the invention. 
         [0084]    While the features herein have been described in connection with various example structures and illustrative aspects, it will be understood by those skilled in the art that other variations and modifications of the structures and aspects described above may be made without departing from the scope of the inventions described herein. Other structures and aspects will be apparent to those skilled in the art from a consideration of the specification or practice of the features disclosed herein. It is intended that the specification and the described examples only are illustrative with the true scope of the inventions being defined by the following claims.