Patent Publication Number: US-2021181662-A1

Title: Device including rotator and belt, such as a fixing device for an image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/729,586, filed Dec. 30, 2019, which claims priority from Japanese Patent Application No. 2019-062907, and Japanese Patent Application No. 2019-062903, both of which were filed on Mar. 28, 2019. The contents of the aforementioned applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Aspects of the disclosure relate to a fixing device including a rotator and a belt, and an image forming apparatus including the fixing device. 
     BACKGROUND 
     A known belt-type fixing device includes a belt, a heat roller and a pad that sandwich therebetween the belt, a holder that supports the pad, and a U-shaped stay that supports the holder. The stay includes two walls and a bend portion connecting the two walls. Each of the two walls is in contact with the holder at its end opposite to the bend portion. Another known belt-type fixing device includes a belt, a heat roller and a pad that sandwich therebetween the belt, an upstream guide disposed upstream from the pad to guide the belt, a downstream guide disposed downstream from the pad to guide the belt, and a stay holding the upstream and downstream guides. The downstream guide is fixed to the stay with screws while the upstream guide is only supported by the stay. 
     SUMMARY 
     According to one aspect of the disclosure, a device includes a rotator having a rotation axis, a belt, a nip forming member, a holder, a first stay, an urging member, a second stay, and a connector. The nip forming member is surrounded by the belt. The nip forming member is configured to, with the rotator, pinch the belt to form a nip. The holder holds the nip forming member. The first stay supports the holder. The first stay extends in a width direction parallel to the rotation axis. The urging member urges the first stay toward the rotator. The second stay is positioned upstream of the first stay in a moving direction of the belt at the nip. The moving direction is perpendicular to the width direction. The connector extends through at least one of the first stay and the second stay to connect the first stay to the second stay. 
     According to another aspect of the disclosure, a device includes a rotator having a rotation axis, a belt, a nip forming member, a holder, a first stay, an urging member, an upstream guide, a downstream guide, and a connector. The nip forming member is surrounded by the belt. The nip forming member is configured to, with the rotator, pinch the belt to form a nip. The holder holds the nip forming member. The first stay supports the holder. The first stay extends in a width direction parallel to the rotation axis. The urging member urges the first stay toward the rotator. The upstream guide is configured to guide an inner peripheral surface of the belt at a position upstream of the nip forming member in a moving direction of the belt at the nip. The moving direction is perpendicular to the width direction. The downstream guide is configured to guide an inner peripheral surface of the belt at a position downstream of the nip forming member in the moving direction. The connector extends in the moving direction between the upstream guide and the downstream guide to connect the upstream guide and the downstream guide to the first stay. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a laser printer according to an illustrative embodiment of the disclosure. 
         FIG. 2  is a cross sectional view of a fixing device of the image forming apparatus. 
         FIG. 3  is an exploded perspective view of components to be disposed inside a belt of the fixing device. 
         FIG. 4A  is an enlarged, exploded perspective view of a nip forming member, a holder, and springs of the fixing device. 
         FIG. 4B  is a cross sectional view illustrating a structure around a boss of the holder. 
         FIG. 5  is a top view of the holder having the nip forming member and the springs attached thereto, viewed from a rotator of the fixing device. 
         FIG. 6A  is a perspective view illustrating a structure around an engaging portion of the holder. 
         FIG. 6B  is a top view illustrating the structure around the engaging portion of the holder. 
         FIG. 6C  is a side sectional view illustrating the structure around the engaging portion of the holder. 
         FIG. 7  is an exploded perspective view of the nip forming member, the holder, a first stay, a second stay, and a downstream guide, viewed toward the rotator. 
         FIG. 8A  is a perspective view of a side of a holder body opposite to the rotator. 
         FIG. 8B  is a cross sectional view illustrating the relationship between extension walls and the first stay. 
         FIG. 9A  is a perspective view of an upstream guide viewed from a downstream side in a moving direction, wherein an upstream end portion of a sliding sheet is engaged with the upstream guide. 
         FIG. 9B  is a perspective view of the upstream guide viewed from the downstream side in the moving direction, wherein the upstream end portion of the sliding sheet is sandwiched between the upstream guide and the second stay. 
         FIG. 10A  is a cross sectional view illustrating the structure around a connector of the stay. 
         FIG. 10B  is a cross sectional view illustrating the structure fastening the upstream guide, the first guide, and the downstream guide. 
         FIG. 10C  is a cross sectional view illustrating the structure fastening the upstream guide and a second stay. 
         FIG. 11  is a cross sectional view of a pressure unit viewed in a direction orthogonal to a particular direction, illustrating the positional relationship between screws. 
         FIG. 12  is a side sectional view of the holder and the first stay viewed from the downstream side in the moving direction. 
         FIG. 13  is an exploded perspective view of a pressure mechanism of the fixing device. 
         FIG. 14  is a perspective view of the holder, the first stay, a movement restriction member, and a bracket that are assembled. 
         FIG. 15  is a side sectional view of an inner side of the pressure mechanism viewed in the width direction. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative embodiment will be described with reference to the accompany drawings. 
     As illustrated in  FIG. 1 , an image forming apparatus  1  (e.g., a laser printer) includes a casing  2 , a sheet supply unit  3 , an exposure device  4 , an image forming unit  5 , and a fixing device  8 . 
     The sheet supply unit  3  includes a sheet tray  31  for accommodating sheets S (e.g., sheets of paper), and a sheet supply mechanism  32 . The sheet supply mechanism  32  supplies a sheet S from the sheet tray  31  toward the image forming unit  5 . 
     The exposure device  4  includes a laser emitter, a polygon mirror, lenses, and reflecting mirrors. The exposure device  4  is configured to expose a surface of a photosensitive drum  61  by scanning thereon at high speed a laser beam (indicated by a dot-and-dash line) emitted from the laser emitter based on image data. 
     The image forming unit  5  is disposed below the exposure device  4 . The image forming unit  5  is constituted as a process cartridge. The image forming unit  5  is removable from the casing  2  through an opening formed when a front cover  21  disposed at a front of the casing  2  is open. The image forming unit  5  includes a photosensitive drum  61 , a charger  62 , a transfer roller  63 , a developing roller  64 , a supply roller  65 , and a developer chamber  66  configured to store therein developer, for example, dry toner. 
     In the image forming unit  5 , the charger  62  uniformly charges the surface of the photosensitive drum  61 . Thereafter, the exposure device  4  exposes the surface of the photosensitive drum  61  to a laser beam, and the surface of the photosensitive drum  61  carries an electrostatic latent image corresponding to image data. The supply roller  65  supplies developer in the developer chamber  66  to the developing roller  64 . 
     The developing roller  64  supplies developer to the electrostatic latent image formed on the surface of the photosensitive drum  61 . The electrostatic latent image on the surface of the photosensitive drum  61  is thus visually developed as a developer image. Thereafter, when a sheet S supplied from the sheet supply unit  3  passes through between the photosensitive drum  61  and the transfer roller  63 , the developer image is transferred from the photosensitive drum  61  onto the sheet S. 
     The fixing device  8  is disposed at the rear of the image forming unit  5 . An overall structure of the fixing device  8  will be described in detail later. The fixing device  8  thermally fixes the developer image transferred onto a sheet S passing through the fixing device  8 . The image forming apparatus  1  uses conveying rollers  23  and discharge rollers  24  to discharge the sheet S having the developer image fixed thereto onto a discharge tray  22 . 
     As illustrated in  FIG. 2 , the fixing device  8  includes a heating unit  81  and a pressure unit  82 . The pressure unit  82  is urged toward the heating unit  81  by a pressure mechanism  300  ( FIG. 15 ). In the following description, a direction in which the pressure mechanism  300  urges the pressure unit  82  toward the heating unit  81  is referred to as “a particular direction”. The particular direction is a direction which is orthogonal to a width direction and a moving direction which will be described later, and in which the heating unit  81  and the pressure unit  82  face to each other. 
     The heating unit  81  includes a heater  110  and a rotator  120 . The pressure unit  82  includes a belt  130 , a nip forming member N, a holder  140 , a stay  200 , a belt guide G, a sliding sheet  150 , two springs SP, two buffers BF, five first screw SC 1 , two second screws SC 2 , and two third screws SC 3 . In the following description, a width direction of the belt  130  is referred to as just “a width direction”. The width direction extends in an axial direction of the rotator  120 . The width direction is orthogonal to the particular direction. 
     The heater  110  is a halogen lamp and, when turned on, produces light for radiant heat to heat the rotator  120 . The heater  110  is disposed within an interior space of the rotator  120  along a rotation axis of the rotator  120 . 
     The rotator  120  is a cylindrical roller extending in the width direction to receive heat from the heater  110 . The rotator  120  includes a metal-made tube  121  and an elastic layer  122  covering an outer peripheral surface of the tube  121 . The elastic layer  122  is made of rubber such as silicone rubber. The rotator  120  has an outside diameter greater at its both ends in the width direction than its central portion. In other words, the rotator  120  has a concave shape with its outside diameter gradually greater from its central portion toward its both ends. The rotator may have a different shape. For example, the rotator may be a cylindrical roller having a uniform outside diameter in the width direction. Alternatively, the rotator may be a crown-shaped roller having its outside diameter smaller from its central portion toward its both ends in the width direction. 
     The rotator  120  is rotatably supported by side frames  83  (one of which is illustrated in  FIG. 15 ), which will be described later. The rotator  120  receives a driving force from a motor disposed in the casing  2  to rotate counterclockwise in  FIG. 2 . 
     The belt  130  is a flexible, long tubular member. The belt  130  has a base made of, for example, metal and resin, and a releasable layer covering an outer peripheral surface of the base. The belt  130  is in frictional contact with the rotator  120  or a sheet S and rotates clockwise in  FIG. 2  with the rotation of the rotator  120 . A lubricant, such as grease, is applied to an inner peripheral surface of the belt  130 . The nip forming member N, the holder  140 , the stay  200 , the belt guide G, and the sliding sheet  150  are disposed within an interior space of the belt  130 . 
     In other words, the nip forming member N, the holder  140 , the stay  200 , the belt guide G, and the sliding sheet  150  are covered by the belt  130 . The holder  140  and the stay  200  function as a supporting member that supports the nip forming member N. As illustrated in  FIG. 3 , the nip forming member N, the holder  140 , the stay  200 , the belt guide G, and the sliding sheet  150  each have a greater dimension in the width direction than in directions orthogonal to the width direction. 
     As illustrated in  FIGS. 2 and 3 , the nip forming member N pinches the belt  130  with the rotator  120 , for forming a nip NP between the rotator  120  and the belt  130 . The nip forming member N includes an upstream nip forming member N 1  and a downstream nip forming member N 2 . 
     The upstream nip forming member N 1  has an upstream pad P 1  and an upstream fixing plate B  1 . 
     The upstream pad P 1  has a box shape. The upstream pad P 1  is made of rubber, such as silicone rubber. The upstream pad P 1  and the rotator  120  pinch the belt  130  therebetween, forming an upstream nip NP 1 . 
     In the following description, a moving direction of the belt  130  at the upstream nip NP 1  and the nip NP is referred to as just “a moving direction”. The moving direction is a direction where the belt  130  moves along an outer peripheral surface of the rotator  120 . This direction is, however, along a direction substantially orthogonal to the particular direction and the width direction, and thus illustrated as the direction orthogonal to the particular direction and the width direction. The moving direction is substantially the same as a direction directed from an entrance to the nip NP toward an exit therefrom. 
     The upstream pad P 1  is fixed to a surface of the upstream fixing plate B 1  facing the rotator  120 . The upstream pad P 1  slightly protrudes upstream in the moving direction relative to an upstream end of the upstream fixing plate B  1 . 
     The upstream fixing plate B  1  is made of a material harder than that of the upstream pad P 1 , for example, metal. The upstream fixing plate B 1  is longer in the width direction than the upstream pad P 1 . The upstream fixing plate B 1  has both end portions B 11 , B 12  in the width direction, each of which is located at an outer position relative to a corresponding one of both ends of the upstream pad P 1 . 
     The downstream nip forming member N 2  is disposed downstream apart from the upstream nip forming member N 1  in the moving direction. The downstream nip forming member N 2  has a downstream pad P 2  and a downstream fixing plate B 2 . 
     The downstream pad P 2  has a box shape. The downstream pad P 2  is made of rubber, such as silicone rubber. The downstream pad P 2  and the rotator  120  pinch the belt  130  therebetween, forming a downstream nip NP 2 . The downstream pad P 2  is spaced from the upstream pad P 1  in the moving direction. 
     This structure provides, between the upstream nip NP 1  and the downstream nip NP 2 , a middle nip NP 3  where no pressure from the pressure unit  82  directly acts. At the middle nip NP 3 , the belt  130  still contacts the rotator  120  but hardly receives pressure because there is nothing to pinch the belt  130  with the rotator  120 . Thus, the sheet S is heated by the rotator  120  under almost no pressure while passing the middle nip NP 3 . In this embodiment, the nip NP refers to a range from the upstream end of the upstream nip NP 1  to the downstream end of the downstream nip NP 2 , that is, the entire range where the outer peripheral surface of the belt  130  and the rotator  120  contact each other. In other words, the nip NP includes a portion not subjected to pressure from the upstream pad P 1  and the downstream pad P 2 . 
     The downstream pad P 2  is fixed to a surface of the downstream fixing plate B 2  facing the rotator  120 . The downstream pad P 2  slightly protrudes downstream in the moving direction relative to a downstream end of the downstream fixing plate B 2 . 
     The downstream fixing plate B 2  is made of a material harder than that of the downstream pad P 2 , for example, metal. The downstream fixing plate B 2  is longer in the width direction than the downstream pad P 2 . The downstream fixing plate B 2  has both end portions B 21 , B 22  in the width direction, each of which is located at an outer position relative to a corresponding one of both ends of the downstream pad P 2 . 
     The upstream pad P 1  has a higher hardness than the elastic layer  122  of the rotator  120 . The downstream pad P 2  has a higher hardness than the upstream pad P 1 . 
     The above hardness refers to a durometer hardness specified in ISO7619-1. The durometer hardness is a value that may be obtained from an amount of the penetration of a pin into a specimen under specified conditions. For example, when the durometer hardness of the elastic layer  122  is 5, that of the upstream pad P 1  is preferably 6 to 10, and that of the downstream pad P 2  is preferably 70 to 90. 
     The hardness of silicone rubber may be adjusted by changing the ratio of an additive (e.g., a silica filler and a carbon filler) to be added at the time of manufacture. Specifically, the hardness of silicone rubber increases with a higher ratio of an additive. The hardness decreases with the addition of silicone-based oil. As a rubber processing method, injection molding and extrusion may be adopted. Generally, injection molding is suitable for low hardness rubber and extrusion is suitable for high hardness rubber. 
     The holder  140  holds the nip forming member N. The holder  140  is made of a heat-resistant resin. The holder  140  includes a holder body  141  and two engaging portions  142 ,  143 . 
     The holder body  141  holds the nip forming member N. The holder body  141  is mainly located within a range of the belt  130 . More specifically, as illustrated in  FIG. 5 , the holder body  141  includes a pair of side walls W 5 , one at each of its both ends in the width direction. Each of the side walls W 5  includes protrusions W 10 , W 11 . A main portion of the holder body  141  except for the side walls W 5  is located within a width BB of the belt  130 . The springs SP are disposed within the width BB of the belt  130 . As illustrated in  FIGS. 2 and 3 , the holder body  141  is supported by the stay  200  (i.e., a first stay  210  and a second stay  220  which will be described later). 
     The engaging portions  142 ,  143  protrude from ends of the holder body  141  in the width direction. The engaging portions  142 ,  143  are located at different positions from the belt  130  in the width direction. As illustrated in  FIGS. 5 and 12 , the engaging portions  142 ,  143  are located outside of the width BB of the belt  130 . As illustrated in  FIGS. 2 and 3 , the engaging portions  142 ,  143  engage with respective ends of the first stay  210  in the width direction. 
     The stay  200  is located opposite to the nip forming member N relative to the holder  140  and supports the holder  140 . The stay  200  includes a first stay  210  and a second stay  220 . 
     The first stay  210  supports the holder body  141  of the holder  140 . The first stay  210  is made of metal. The first stay  210  includes a base portion  211  and a bend portion HB by hemming. 
     The base portion  211  has, at its first end in the particular direction, a contact surface Ft to contact the holder body  141  of the holder  140 . The contact surface Ft is a flat surface orthogonal to the particular direction. The base portion  211  is constituted as a downstream wall located downstream relative to the bend portion HB in the moving direction. The base portion  211  has a downstream surface Fa and an upstream surface Fb in the moving direction. 
     The bend portion HB is a portion bent by hemming. The bend portion HB is L-shaped and extends from a second end of the base portion  211  in the particular direction toward the holder body  141 . The bend portion HB has a bottom wall  212  extending from the base portion  211  upstream in the moving direction, and an upstream wall  213  extending from the bottom wall  212  toward the holder body  141  along the particular direction. The upstream wall  213  is disposed upstream of the base portion  211  that is a downstream wall in the moving direction. The upstream wall  213  is disposed parallel to the base portion  211 . The upstream wall  213  and the base portion  211  face each other in the moving direction with a space smaller than a thickness of the first stay  210 . 
     The bend portion HB is shorter in the width direction than the base portion  211 . The base portion  211  has both ends in the width direction, each of which is located at an outer position relative to a corresponding one of both ends of the bend portion HB. 
     The base portion  211  has, at each of its both end portions in the width direction, one load receiver  211 A to receive a load from the pressure mechanism  300  (refer to  FIG. 15 ). The load receivers  211 A are recesses that are open opposite the nip forming member N in the particular direction and formed at an end, in the particular direction, of the base portion  211  opposite to the nip forming member N. 
     The load receivers  211 A receive respective buffers BF made of, for example, resin. The buffers BF prevent the metal base portion  211  and metal pressure arms  310  (only one of which is illustrated in  FIG. 15 ) from rubbing against each other. Each of the buffers BF includes an engagement portion BF 1  to engage with a corresponding one of the load receivers  211 A, and a pair of legs BF 2  disposed upstream and downstream in the moving direction relative to each end, in the width direction, of the base portion  211 . 
     The second stay  220  supports the holder body  141  of the holder  140 . The second stay  220  is made of metal. The second stay  220  is disposed upstream of the first stay  210  in the moving direction. The second stay  220  includes a base portion  221  located parallel to the upstream wall  213  of the first stay  210 , and an extension portion  222  extending from an end of the base portion  221  opposite to the nip forming member N toward the first stay  210 . 
     The base portion  221  is longer in the width direction than the extension portion  222  and the bend portion HB of the first stay  210 . The base portion  221  has both ends in the width direction, each of which is located at an outer position relative to a corresponding one of both ends of the extension portion  222  and the bend portion HB. The first stay  210  and the second stay  220  are connected with two connectors CM. More specifically, each of the connectors CM connects a corresponding one of both ends of the base portion  211  of the first stay  210  and a corresponding one of both ends of the base portion  221  of the second stay  220  in the width direction. Each of the connectors CM connects the base portion  211  and the base portion  221  at a different position from the bend portion HB. 
     As illustrated in  FIG. 10A , each connector CM includes a crimped member SW crimped to the second stay  220  and a second screw SC 2  with which the crimped member SW is fastened to the first stay  210 . The crimped member SW includes a base SW 1 , a first protrusion SW 2 , and a second protrusion SW 3 . The base SW 1  is sandwiched between the first stay  210  and the second stay  220 . The first protrusion SW 2  extends from one end of the base SW 1  downstream in the moving direction. The second protrusion SW 3  extends from the other end of the base SW 1  upstream in the moving direction. 
     The second stay  220  has two holes Hf. Each of the holes Hf receives therein the second protrusion SW 3  of a corresponding one of the connectors CM. The second protrusion SW 3  protrudes upstream from the hole Hf in the moving direction, and its protruding end is crimped. The second stay  220  is thus pinched between the crimped end of the second protrusion SW 3  and an end of the base SW 1 . 
     The first stay  210  has two holes H 11 . Each of the holes H 11  receives therein the first protrusion SW 2  of a corresponding one of the connectors CM. The first protrusion SW 2  has a hole Ha in which the second screw SC 2  is screwed. The hole Ha has a closed end or is recessed with an opening on one side. The second screw SC 2  is screwed in the hole Ha and thus the first stay  210  is pinched between a head SC 21  of the second screw SC 2  and the base SW 1 . 
     As illustrated in  FIG. 3 , the holes H 11  are formed to be aligned with respective connectors CM. One of the holes H 11  is a round hole and the other one is a long hole which is long in the width direction. 
     As illustrated in  FIGS. 2 and 3 , the belt guide G guides the inner peripheral surface of the belt  130 . The belt guide G is made of a heat-resistant resin. The belt guide G includes an upstream guide G 1  and a downstream guide G 2 . 
     The upstream guide G 1  has an upstream guide surface Fu to guide the inner peripheral surface of the belt  130  at a position upstream from the nip forming member N in the rotation direction of the belt  130 , that is, in the moving direction at the nip NP. More specifically, the upstream guide surface Fu guides the inner peripheral surface of the belt  130  via the sliding sheet  150 . The upstream guide G 1  is spaced from the upstream pad P 1  in the moving direction. 
     The downstream guide G 2  has a downstream guide surface Fd to guide the belt  130  at a position downstream from the nip forming member N in the rotation direction of the belt  130 , that is, in the moving direction at the nip NP. More specifically, the downstream guide surface Fd guides the inner peripheral surface of the belt  130  via the sliding sheet  150 . The downstream guide G 2  is spaced from the downstream pad P 2  in the moving direction. The downstream guide G 2  is spaced in the particular direction from a rotation center X 1  of the rotator  120  further than the downstream pad P 2 . 
     The sliding sheet  150  is rectangular and reduces frictional resistance between each pad P 1 , P 2  and the belt  130 . The sliding sheet  150  is pinched at the nip NP between the inner peripheral surface of the belt  130  and each pad P 1 , P 2 . The sliding sheet  150  is made of an elastically deformable material. The sliding sheet  150  may be made of any material. In this embodiment, a polyimide-containing resin sheet is used. 
     The sliding sheet  150  has a base  151  and six hooks  152 . The base  151  is rectangular. The base  151  has a sliding surface Fs ( FIG. 2 ) on which the inner peripheral surface  131  of the belt  130  slides. The base  151  has an upstream end portion  151 A and a downstream end portion  151 B in the moving direction of the belt  130 . 
     The upstream end portion  151 A of the base  151  is fixed to the upstream guide G 1 . The base  151  is located covering the upstream guide surface Fu, the nip forming member N, and the downstream guide surface Fd. 
     The hooks  152  are located at the downstream end portion  151 B of the base  151 . The hooks  152  are part of the sliding sheet  150 . The hooks  152  are thus elastically deformable. Each of the hooks  152  has an end portion  152 A and a neck portion  152 B. 
     The end portion  152 A has a width (i.e., a dimension in the width direction) narrower the farther the end portion  152 A is from the base  151 . The end portion  152 A protrudes relative to both ends of the neck portion  152 B in the width direction. The neck portion  152 B connects the end portion  152 A and the base  151 . The neck portion  152 B has a width (i.e., a dimension in the width direction) narrower than the maximum width of the end portion  152 A. 
     The downstream guide G 2  has six hook engaging portions G 21  in association with the six hooks  152 . The hooks  152  and the hook engaging portions G 21  are respectively spaced apart from one another in the width direction. The hooks  152  engage in the hook engaging portions G 21 . 
     Each of the hook engaging portions G 21  has an aperture Hg in which a corresponding hook  152  engages. The end portion  152 A of the hook  152  has a minimum width smaller than a width of the aperture Hg. The neck portion  152 B has a width smaller than the width of the aperture Hg. The end portion  152 A has a maximum width greater than the width of the aperture Hg. 
     As illustrated in  FIG. 2 , the hook engaging portion G 21  is located at a position downstream from the downstream guide surface Fd in the rotation direction of the belt  130  and apart from the belt  130 . The hook engaging portion G 21  is spaced downstream from the base portion  211  of the first stay  210  in the moving direction. 
     The hook engaging portion G 21  faces the base portion  211  of the first stay  210  in the moving direction. More specifically, the aperture Hg of the hook engaging portion G 21  faces the base portion  211  in the moving direction. The hook  152  of the sliding sheet  150  is inserted into and engages with the aperture Hg from a downstream side in the moving direction. 
     The hook engaging portion G 21  is spaced apart from the base portion  211  by a distance greater than a length of the end portion  152 A of the hook  152  in the moving direction. The neck portion  152 B of the hook  152  has a length greater than a thickness of the hook engaging portion G 21 . 
     As illustrated in  FIG. 4A , the holder body  141  includes a support wall W 1 , an upstream wall W 2 , a middle wall W 3 , a downstream wall W 4 , and a pair of side walls W 5 . The holder body  141  has substantially a symmetric structure in the width direction. The following description about a structure around an end of the holder body  141  in the width direction will be made based on one end of the holder body  141  (i.e., a right end thereof in the drawings), and a description about the other end of the holder body  141  will be omitted. 
     The support wall W 1  supports the nip forming member N and is located opposite to the rotator  120  relative to the nip forming member N. The support wall W 1  has an upstream support surface F 1  for supporting the upstream fixing plate B 1  and a downstream support surface F 2  for supporting the downstream fixing plate B 2 . When viewed in cross section orthogonal to the width direction, the upstream support surface F 1  and the downstream support surface F 2  are orthogonal to the particular direction. The upstream support surface F 1  and the downstream support surface F 2  are at the same positions in the particular direction. When viewed in cross section orthogonal to the moving direction, the upstream support surface F 1  and the downstream support surface F 2  are curved such that their central portions are closer to the rotation center X 1  of the rotator than their both ends in the width direction. In other words, the central portions of the upstream support surface F 1  and the downstream support surface F 2  in the width direction are convex toward the rotator  120 . The upstream support surface F 1  and the downstream support surface F 2  protrude toward the rotator  120  by substantially the same amount. 
     The support wall W 1  has one boss W 6  ( FIG. 6A ) located at each of its both ends in the width direction. Each boss W 6  receives a spring SP. As illustrated in  FIG. 4B , the boss W 6  is located at a position farther from the rotator  120  than the upstream fixing plate B 1  and the downstream fixing plate B 2  in the particular direction. As illustrated in  FIGS. 4A and 5 , the bosses W 6  protrude away from each other from the respective ends of the support wall W 1  in the width direction. One of the bosses W 6  is located between a first end portion B 11  of the upstream fixing plate B 1  and a first end portion B 21  of the downstream fixing plate B 2  and the other is located between a second end portion B 12  of the upstream fixing plate B 1  and a second end portion B 22  of the downstream fixing plate B 2  in the moving direction. 
     The springs SP urge the upstream nip forming member N 1  and the downstream nip forming member N 2  away from each other. More specifically, the springs SP urge, in the moving direction, the upstream nip forming member N 1  toward the upstream wall W 2  and the downstream nip forming member N 2  toward the downstream wall W 4 . The springs SP urge, in the particular direction, the upstream nip forming member N 1  toward the upstream support surface F 1  of the support wall W 1  and the downstream nip forming member N 2  toward the downstream support surface F 2  of the support wall W 1 . 
     Each of the springs SP includes a coil portion S 1 , a first arm S 2 , and a second arm S 3 . The coil portion S 1  has one or more turns of wire. Each boss W 6  enters the coil portion S 1  of a corresponding spring SP, thereby supporting the spring SP. 
     The first arm S 2  diagonally extends from one end of the coil portion S 1  upstream in the moving direction and toward the rotator  120  to contact the first end portion B 11  of the upstream fixing plate B 1 . More specifically, the first end portion B 11  of the upstream fixing plate B 1  has a downstream end defining a recess B 13  recessed upstream. The first arm S 2  enters the recess B 13  and contacts the most recessed portion of the recess B 13 . 
     The second arm S 3  diagonally extends from the other end of the coil portion S 1  downstream in the moving direction and toward the rotator  120  to contact the first end portion B 21  of the downstream fixing plate B 2 . More specifically, the first end portion B 21  of the downstream fixing plate B 2  has a narrower width (i.e., a shorter length in the moving direction) than a central portion of the downstream fixing plate B 2  in the width direction. The first end portion B 21  of the downstream fixing plate B 2  has an upstream end located downstream further than an upstream end of the central portion of the downstream fixing plate B 2 . A distance between the most recessed portion of the recess B 13  at the first end portion B 11  of the upstream fixing plate B 1  and the first end portion B 21  of the downstream fixing plate B 2  is greater than an outside diameter of the coil portion S 1 . 
     In this embodiment, one spring SP disposed at a first end (i.e., a right end in the drawings) of the holder  140  in the width direction is identical in shape with the other spring SP disposed at a second end, opposite to the first end, of the holder  140 . As illustrated in  FIG. 5 , for the spring SP disposed at the first end of the holder  140  in the width direction, the first arm S 2  that urges the upstream fixing plate B 1  is located at an inner position relative to the second arm S 3  in the width direction. For the spring SP disposed at the second end of the holder  140  in the width direction, the second arm S 3  is located at an inner position relative to the first arm S 2  in the width direction. 
     The second end portion B 12  of the upstream fixing plate B 1  has a width narrower than the center portion of the upstream fixing plate B 1  in the width direction. A downstream end of the second end portion B 12  is located at the same position, in the moving direction, as the most recessed portion of the recess B 13  in the first end portion B 11 . For the spring SP disposed at the second end of the holder  140 , its first arm S 2  contacts the second end portion B 12  of the upstream fixing plate B 1 . 
     The second end portion B 22  of the downstream fixing plate B 2  has an upstream end defining a recess B 23  recessed downstream. The most recessed portion of the recess B 23  is located at the same position, in the moving direction, as the upstream end of the first end portion B 21  of the downstream fixing plate B 2 . For the spring SP disposed at the second end of the holder  140 , its second arm S 3  enters the recess B 23  and contacts the most recessed portion of the recess B 23 . 
     In other words, each of the recesses B 13 , B 23  of the fixing plates B 1 , B 2  is located at a position to engage with a corresponding arm S 2 , S 3  located at an inner position relative to the coil portion S 1  in the width direction. Unlike this embodiment, if a fixing plate has a recess to engage with a corresponding arm located at an outer position relative to the coil portion in the width direction, the fixing plate may have, in the width direction, its end spaced from the recess by a specified distance to ensure adequate strength at the end, which may lead to the need to increase the size of the fixing plate in the width direction. In this embodiment, however, each of the recesses B 13 , B 23  is formed at a position to engage with a corresponding arm S 2 , S 3  located at an inner position relative to the coil portion S 1  in the width direction, thus reducing the need to increase the size of the fixing plates B 1 , B 2  in the width direction. 
     Returning to  FIG. 4A , the first arm S 2  and the second arm S 3  have bend portions S 4  at their ends. The bend portions S 4  are ring-shaped. The bend portion S 4  of the first arm S 2  protrudes from the first arm S 2  toward the second arm S 3 . The bend portion S 4  of the second arm S 3  protrudes from the second arm S 3  toward the first arm S 2 . 
     The springs SP are sized not to interfere with the sliding sheet  150  in the fixing device  8  forming a nip between the rotator  120  and the belt  130  as illustrated in  FIG. 2 . When each spring SP is attached to the holder  140 , its end closest to the rotator  120  is located at substantially the same position as an end of the upstream wall W 2  or the downstream wall W 4  closest to the rotator  120  (or at a position away from the rotator  120  further than the end of the upstream wall W 2  or the downstream wall W 4 ). 
     The upstream wall W 2 , the middle wall W 3 , and the downstream wall W 4  extend from the support wall W 1  toward the rotator  120 . The upstream wall W 2  functions as a first restricting member that restricts upward movement of the upstream nip forming member N 1  in the moving direction by contacting the upstream pad P 1  of the upstream nip forming member N 1 . The upstream wall W 2  is disposed at an upstream end of the support wall W 1 . In the width direction, the upstream wall W 2  extends outwardly relative to each end of the support wall W 1  and extends in a direction away from each end of the nip forming member N. 
     The downstream wall W 4  functions as a second restricting member that restricts downward movement of the downstream nip forming member N 2  in the moving direction by contacting the downstream pad P 2  of the downstream nip forming member N 2 . The downstream wall W 4  is disposed at a downstream end of the support wall W 1 . In the width direction, the downstream wall W 4  extends outwardly relative to each end of the support wall W 1  and extends in the direction away from each end of the nip forming member N. 
     The middle wall W 3  is disposed between and spaced from the upstream wall W 2  and the downstream wall W 4 . 
     The upstream support surface F  1  is located between the upstream wall W 2  and the middle wall W 3 . The downstream support surface F 2  is located between the middle wall W 3  and the downstream wall W 4 . The upstream pad P 1  is spaced from the middle wall W 3  (refer to  FIG. 5 ). The downstream pad P 2  is spaced from the middle wall W 3  (refer to  FIG. 5 ). 
     Each of the side walls W 5  is located between the support wall W 1  and a respective one of the engaging portions  142 ,  143  in the width direction. The side walls W 5  extend in a direction crossing the width direction, more specifically, in a direction orthogonal to the width direction. The side walls W 5  connect both ends, in the width direction, of both of the upstream wall W 2  and the downstream wall W 4 . The side walls W 5  are spaced from the support wall W 1  in the width direction. 
     Each of the side walls W 5  has, at its end facing the rotator  120 , a recess W 7  that is recessed away from the rotator  120 . The recess W 7  is located at a position corresponding to the boss W 6  in the moving direction. In other words, the boss W 6  is located within a range of the recess W 7  in the moving direction. The recess W 7  faces the boss W 6  in the width direction. 
     The side wall W 5  includes a first portion W 8  and a second portion W 9 . The first portion W 8  is located upstream of the recess W 7  in the moving direction. The second portion W 9  is located downstream of the recess W 7  in the moving direction. The second portion W 9  is spaced downstream from the first portion W 8  in the moving direction. 
     The boss W 6  is located between the first portion W 8  and the second portion W 9  in the moving direction. A distance between the first portion W 8  and the second portion W 9  in the moving direction, that is, a dimension for the recess W 7  in the moving direction, is greater than an outside diameter of the coil portion S 1  of the spring SP. 
     The side wall W 5  further includes a first protrusion W 10  and a second protrusion W 11 . The first protrusion W 10  extends from an end of the first portion W 8  facing the rotator  120  toward the upstream pad P 1  in the width direction. The first protrusion W 10  restricts the movement of the upstream fixing plate B 1  toward the rotator  120 . The second protrusion W 11  extends from an end of the second portion W 9  facing the rotator  120  toward the downstream pad P 2  in the width direction. The second protrusion W 11  restricts the movement of the downstream fixing plate B 2  toward the rotator  120 . 
     As illustrated in  FIG. 5 , the first protrusion W 10  has a portion located at the same position as the first arm S 2  in the moving direction. In other words, the first arm S 2  has a portion located within a range of the first protrusion W 10  in the moving direction. In still other words, when projected in the width direction, the portion of the first arm S 2  overlaps the first protrusion W 10 . The first protrusion W 10  is configured to contact the first arm S 2  to restrict inclination and movement of the first arm S 2 , which may result from slight inclination and movement of the spring SP in the width direction. 
     The second protrusion W 11  has a portion located at the same position as the second arm S 3  in the moving direction. In other words, the second arm S 3  has a portion located within a range of the second protrusion W 11  in the moving direction. In still other words, when projected in the width direction, the portion of the second arm S 3  overlaps the second protrusion W 11 . The second protrusion W 11  is configured to contact the second arm S 3  to restrict inclination and movement of the second arm S 3 , which may result from slight inclination and movement of the spring SP in the width direction. 
     The distance between the first protrusion W 10  and the first arm S 2  in the width direction and the distance between the second protrusion W 11  and the second arm S 3  are preferably smaller than larger. For example, those distances are preferably smaller than three times the diameter of the wire of the spring SP. 
     The boss W 6  extends in the width direction to a position where the boss W 6  overlaps the first protrusion W 10  and the second protrusion W 11 . In other words, the boss W 6  protrudes, in the width direction, outward relative to an end of each protrusion W 10 , W 11  facing the bend portion S 4  of the spring SP. 
     As illustrated in  FIGS. 4A and 5 , the second end portion B 12  of the upstream fixing plate B 1  has a restriction recess B 14  recessed away from the upstream wall W 2  in the moving direction. The second end portion B 22  of the downstream fixing plate B 2  has a restriction recess B 24  recessed away from the downstream wall W 4  in the moving direction. 
     The upstream wall W 2  has a restriction protrusion W 21  to engage in the restriction recess B 14  and restrict movement of the upstream fixing plate B 1  in the width direction. The downstream wall W 4  has a restriction protrusion W 41  to engage in the restriction recess B 24  and restrict movement of the downstream fixing plate B 2  in the width direction. 
     The restriction recesses B 14 , B 24  and the restriction protrusions W 21 , W 41  are located, in the width direction, between each end of the upstream pad P 1  and the downstream pad P 2  and the boss W 6 . 
     As illustrated in  FIGS. 6A and 6B , the restriction protrusions W 21 , W 41  extend along the particular direction. The support wall W 1  has a through hole Hj to allow the restriction protrusion W 21  to pass therethrough. The support wall W 1  has a through hole Hk to allow the restriction protrusion W 41  to pass therethrough. For example, if a holder  140  is to be molded such that the support wall W 1  has such a restriction protrusion protruding from its surface facing the rotator  120 , the molded holder  140  may have burrs, in the form of curves and slopes, at corners between the restriction protrusion and the surface of the support wall W 1 . This may cause separation of the fixing plates B 1 , B 2  from the support wall W 1 . If the restriction recesses are enlarged to prevent the separation, the fixing plates B 1 , B 2  may rattle in the width direction. 
     In this embodiment, however, the restriction protrusions W 21 , W 41  are formed at the upstream wall W 2  and the downstream wall W 4  to pass through the respective through holes Hj, Hk, thus avoiding the above problem. This embodiment shows but is not limited to the through holes Hj, Hk. The support wall W 1  may have, at its surface facing the rotator  120 , a recess recessed away from the rotator  120  to allow the restriction protrusion to protrude from the most recessed portion of the recess. In other words, the surface, facing the rotator  120 , of the support wall W 1  may have a portion around the restriction protrusion that is farther from the rotator  120  than a remaining portion thereof. 
     As illustrated in  FIGS. 6A to 6C , the engaging portion  143  at the second end in the width direction includes a pair of pinching walls W 12  and a first connecting wall W 13  connecting the pinching walls W 12 . The pinching walls W 12  face each other in the moving direction and pinch therebetween an end, in the width direction, of the base portion  211  of the first stay  210 . Each of the pinching walls W 12  extends outward from the side wall W 5  in the width direction. 
     The first connecting wall W 13  is located opposite to the rotator  120  relative to an end of the base portion  211  in the width direction and in contact with the end of the base portion  211  in the width direction. The first connecting wall W 13  connects respective outer ends of the pinching walls W 12  in the width direction. The first connecting wall W 13  is apart from the side wall W 5  in the width direction. This provides, between the first connecting wall W 13  and the side wall W 5 , a space for exposing the load receiver  211 A ( FIG. 7 ) of the first stay  210  downward. The buffer BF ( FIG. 7 ) can be easily attached to the load receiver  211 A exposed downward. 
     The holder  140  further includes a second connecting wall W 14  and reinforcing portions WA. The second connecting wall W 14  connects the pinching walls W 12  to each other. The reinforcing portions WA connect the pinching walls W 12  and the side wall W 5 . The second connecting wall W 14  is located opposite to the first connecting wall W 13  relative to an end of the base portion  211  in the width direction. The second connecting wall W 14  is apart from the base portion  211  in the particular direction. The second connecting wall W 14  is apart from the first connecting wall W 13  in the width direction and is connected to the side wall W 5 . 
     The reinforcing portions WA reinforce the pinching walls  12  and each is provided to a corresponding one of the pinching wall W 12 . The reinforcing portions WA are symmetric in structure in the moving direction. 
     The reinforcing portions WA each have a first wall W 15  and a second wall W 16 . The first wall W 15  is disposed parallel to a corresponding pinching wall W 12  and is connected to the side wall W 5 . The second wall W 16  is disposed parallel to the side wall W 5  and connects the first wall W 15  and the pinching wall W 12 . The first wall W 15 , the second wall W 16 , the pinching wall W 12 , and the side wall W 5  define a hole W 17 . One of the legs BF 2  ( FIG. 7 ) of the buffer BF engages in the hole W 17 . 
     As illustrated in  FIG. 6C , a distance D 1  between the first portion W 8  and the boss W 6  in the moving direction is greater than the diameter of the wire of the spring SP ( FIG. 4 ). A distance D 2  between the second portion W 9  and the boss W 6  in the moving direction is greater than the diameter of the wire of the spring SP. 
     As illustrated in  FIG. 6A , each pinching wall W 12  has a through hole W 18  and a recess W 19 . The through hole W 18  is formed through the pinching wall W 12  in the moving direction. The recess W 19  is formed at an end of the pinching wall W 12  facing the rotator  120 . The through hole W 18  and the recess W 19  are opposite to the side wall W 5  relative to the second wall W 16 . The through hole W 18  and the recess W 19  are at the same positions in the width direction. The through hole W 18  and the recess W 19  receive a movement restriction member R illustrated in  FIGS. 13 and 14 . 
     The movement restriction member R restricts movement of the first stay relative to the holder  140  in the width direction. The movement restriction member R is a torsion spring made of a metal wire. As illustrated in  FIG. 13 , the movement restriction member R has a coil R 1 , a first arm R 2  extending from one end of the coil R 1 , and a second arm R 3  extending from the other end of the coil R 1 . 
     The base portion  211  of the first stay  210  has, at each end in the width direction, a through hole Hi. The through hole Hi is formed at an outer position relative to the load receiver  211 A in the width direction. 
     As illustrated in  FIG. 14 , the first arm R 2  of the movement restriction member R is inserted into and engages with the through hole W 18  in each pinching wall W 12  and the through hole Hi in the first stay  210 . The second arm R 3  of the movement restriction member R engages in the recess W 19  of each pinching wall W 12 . 
     The engaging portion  142  located at the first end in the width direction is identical in structure to the engaging portion  143  located at the second end except that the engaging portion  142  is devoid of the through hole W 18  and the recess W 19 . 
     As illustrated in  FIG. 7 , the holder body  141  further includes  16  ribs W 30 , two first extension walls W 31 , and two second extension walls W 32 . The ribs W 30  protrudes from a surface of the support wall W 1  opposite to the nip forming member N. 
     The ribs W 30  extend in the moving direction and are spaced from one another in the width direction. A distance between adjacent two of the ribs W 30  is smaller than a distance between the two first extension walls W 31 . The ribs W 30  are located symmetrically about a center C 2  of the holder  140  in the width direction. The ribs W 30  each contact at least the first stay  210 . 
     The base portion  211  of the first stay  210  contacts all of the ribs W 30 . The second stay  220  contacts some of the ribs W 30 . The second stay  220  has four protrusions CV to contact four of the ribs W 30 . 
     The protrusions CV protrude from an end, facing the holder  140 , of the base portion  221  of the second stay  220  along the particular direction. The protrusions CV are located symmetrically about a center C 1  of the second stay  220  in the width direction. A distance D 3  from the center C 1  of the second stay  220  to the farthest protrusion CV from the center C 1  in the width direction is smaller than a distance D 4  from the farthest protrusion CV to an end of the second stay  220  in the width direction. In  FIG. 7 , a correlation between the distances is represented relative to the farthest protrusion CV from the center C 1 . The correlation between the distances is satisfied for the closest protrusion CV to the center C 1 . 
     The base portion  221  of the second stay  220  has a plurality of holes Hc 2 , Hd 2 , He 2 , which will be described later. The protrusions CV are located at positions different from the holes Hc 2 , Hd 2 , He 2 . 
     The two first extension walls W 31  are located symmetrically about the center C 2  of the holder  140  in the width direction. The second extension walls W 32  are spaced upstream from the respective first extension walls W 31  in the moving direction. The first extension walls W 31  and the second extension walls W 32  are located closer to the center C 2  of the holder  140  (i.e., the holder body  141 ) in the width direction than the engaging portion  142 . A distance D 5  from the center C 2  of the holder  140  to a first extension wall W 31  or a second extension wall W 32  in the width direction is smaller than a distance D 6  from the first extension wall W 31  or the second extension wall W 32  to the engaging portion  142 . 
     In  FIG. 7 , a correlation between the distances is represented by the extension walls W 31 , W 32  and the engaging portion  142  disposed on a left half of the holder  140  relative to the center C 2 . The correlation between the distances is satisfied for the extension walls W 31 , W 32  and the engaging portion  143  that are disposed on a right half of the holder  140  relative to the center C 2  in the drawing. 
     As illustrated in  FIGS. 8A and 8B , the first extension walls W 31  are located at the downstream end of the support wall W 1  and extend from the support wall W 1  toward a side opposite to the nip forming member N. The first extension walls W 31  extend toward the side opposite to the nip forming member N further than the second extension walls W 32 . The first extension walls W 31  contact the downstream surface Fa of the base portion  211  of the first stay  210 . 
     The second extension walls W 32  extends from the support wall W 1  toward the side opposite to the nip forming member N. The second extension walls W 32  extend toward the side opposite to the nip forming member N further than the ribs W 30 . The second extension walls W 32  contact the upstream surface Fb of the base portion  211  of the first stay  210 . The first extension walls W 31  and the second extension walls W 32  sandwich the base portion  211  therebetween in the moving direction. 
     As illustrated in  FIG. 8B , the base portion  211  of the first stay  210  is located to the downstream nip forming member N 2  in the moving direction. More specifically, in the moving direction, a distance D 7  from a center C 3  of the base portion  211  in the width direction to an upstream end of the downstream pad P 2  is smaller than a distance D 8  from the center C 3  of the base portion  211  to a downstream end of the upstream pad P 1 . 
     As illustrated in  FIG. 9A , the upstream guide G 1  includes a peripheral wall G 11 , a plurality of ribs G 12 , five bosses G 13 , two fastenings G 14 , and two protrusions G 15 . The peripheral wall G 11  is arc-shaped in cross section and its outer surface is the upstream guide surface Fu. 
     The ribs G 12  protrudes from a surface of the peripheral wall G 11  opposite to the upstream guide surface Fu. Each of the ribs G 12  has an end surface to contact the upstream end portion  151 A of the sliding sheet  150 . The upstream end portion  151 A is sandwiched between the end surface of each of the ribs G 12  and the second stay  220  ( FIG. 9B ). 
     The bosses G 13 , the fastenings G 14 , and the protrusions G 15  protrude downstream in the moving direction from the surface of the peripheral wall G 11  opposite to the upstream guide surface Fu. The bosses G 13 , the fastenings G 14 , and the protrusions G 15  are spaced from one another in the width direction. The bosses G 13 , the fastenings G 14 , and the protrusions G 15  are cylindrical. The bosses G 13 , the fastenings G 14 , and the protrusions G 15  are at the same positions as the ribs G 12  in the width direction. 
     The protrusions G 15  protrudes downstream in the moving direction further than the fastenings G 14 . The bosses G 13  protrudes downstream in the moving direction further than the protrusions G 15 . 
     The bosses G 13  fix the upstream guide G 1  to the first stay  210  together with the downstream guide G 2  (refer to  FIG. 10B ). The bosses G 13  are spaced from one another in the width direction. The bosses G 13  are disposed at different positions from the upstream guide surface Fu. More specifically, the bosses G 13  are disposed on the surface of the peripheral wall G 11  opposite to the upstream guide surface Fu. The bosses G 13  are disposed at an end of the upstream guide G 1  opposite to the rotator  120  in the particular direction. 
     The fastenings G 14  fix the upstream guide G 1  to the second stay  220  (refer to  FIG. 10C ). One fastening G 14  is disposed between the outermost boss G 13 , which is disposed to one end of the upstream guide G 1 , of the five bosses G 13  and its adjacent boss G 13  in the width direction. The other fastening G 14  is disposed between the outermost boss G 13 , which is disposed to the other end of the upstream guide G 1 , of the five bosses G 13 , and its adjacent boss G 13  in the width direction. 
     The protrusions G 15  position the upstream guide G 1  to the second stay  220 . Each of the protrusions G 15  is located at a corresponding one of both end portions of the upstream guide G 1 . More specifically, the five bosses G 13  are disposed between the two protrusions G 15  in the width direction. 
     The upstream end portion  151 A of the sliding sheet  150  has five engagement holes Hc 1  formed in a one-to-one correspondence with the five bosses G 13 , two holes Hd 1  formed in a one-to-one correspondence with the two fastenings G 14 , and two holes He 1  formed in a one-to-one correspondence with the two protrusions G 15 . The holes Hc 1 , Hd 1 , He 1  are long in the width direction. 
     Each of the engagement holes Hc 1  is where a corresponding one of the bosses G 13  engages. After the holes Hc 1  and the bosses G 13  engage each other, the upstream end portion  151 A of the sliding sheet  150  is sandwiched and fixed between the upstream guide G 1  and the second stay  220  as illustrated in  FIG. 9B . 
     The base portion  221  of the second stay  220  has five holes Hc 2  formed in a one-to-one correspondence with the five bosses G 13 , two holes Hd 2  formed in a one-to-one correspondence with the two fastenings G 14 , and two holes He 2  formed in a one-to-one correspondence with the two protrusions G 15 . Each of the holes Hc 2  is larger than the outside diameter of a corresponding one of the bosses  13 . 
     Each of the holes Hd 2  is through which a shank SC 32  of a third screw SC 3  (refer to  FIG. 10C ) passes. Each of the holes Hd 2  is smaller than the outside diameter of each of the fastenings  14  and larger than the shank SC 32  of the third screw SC 3 . 
     One of the holes He 2  is a round hole and the other one is a long hole which is long in the width direction. This reduces distortion of the upstream guide G 1  in the width direction, which may result from thermal expansion of resin for the upstream guide G 1  with heat from the metal-made second stay  220 . 
     The base portion  221  further has two holes Hf for fixing the crimped members SW ( FIG. 3 ), one at each of its both ends. The holes Hc 2 , Hd 2 , He 2  are located between the two holes Hf in the width direction. 
     As illustrated in  FIG. 3 , the upstream wall  213  of the first stay  210  has five first holes Hc 3  formed in a one-to-one correspondence with the five bosses G 13 . As illustrated in  FIG. 10B , each boss G 13  passes through a corresponding one of the first holes Hc 3 . Each of the first holes Hc 3  is larger than the outside diameter of a corresponding one of the bosses  13 . The first holes Hc 3  are long in the width direction. 
     As illustrated in  FIG. 12 , the base portion  211  of the first stay  210  has five second holes Hc 4  formed in a one-to-one correspondence with the five bosses G 13 . The second holes Hc 4  are located at positions different from the ribs W 30  in the width direction. As illustrated in  FIG. 10B , a second hole Hc 4  is through which a shank SC 12  of the first screw SC 1  passes to fix the downstream guide G 2  to the base portion  211  of the first stay  210 . The second hole Hc 4  is larger than the outside diameter of the shank SC 12  of the first screw SC 1 . 
     As illustrated in  FIG. 7 , the downstream guide G 2  has five holes Hc 5  formed in a one-to-one correspondence with the five bosses G 13 . As illustrated in  FIG. 10B , a hole Hc 5  is through which the shank SC 12  of the first screw SC 1  passes. The hole Hc 5  is larger than the outside diameter of the shank SC 12  of the first screw SC 1 . 
     The downstream guide G 2  has five fixing portions G 22 . Each of the fixing portions G 22  has a hole Hc 5 . The fixing portions G 22  fix the downstream guide G 2  to the base portion  211  of the first stay  210 . The fixing portions G 22  are located upstream from the six hook engaging portions G 21  in the moving direction. The fixing portions G 22  are spaced from one another in the width direction and are each located between adjacent two of the hook engaging portions G 21 . 
     As illustrated in  FIG. 10B , a boss G 13  has, at its downstream end in the moving direction, a screw hole G 16  in which the first screw SC 1  is screwed. The screw hole G 16  has a closed end or is recessed with an opening on one side. The boss G 13  and screw hole G 16  together with the screw SC 1  is an example of a connector that connects the upstream guide G 1  and the downstream guide G 2  to the first stay  210 . 
     The screw hole G 16  may be defined by a grooved inner surface of each cylindrical boss G 13 . Alternatively, the screw hole G 16  may be defined by an inner surface of each cylindrical boss G 13  to be grooved by a first screw SC 1  screwed into each cylindrical boss G 13 . The same is applied to a screw hole G 17  ( FIG. 10C ), which will be described later. 
     Each boss G 13  passes through the holes Hc 1 , Hc 2 , Hc 3  and contacts the base portion  211  of the first stay  210 . Each boss G 13  is disposed in the holes Hc 2 , Hc 3  with a spacing from their edges in a state where the fixing device  8  is assembled. 
     Each first screw SC 1  is screwed, through the holes Hc 5 , Hc 4 , into the screw hole G 16  of a boss G 13 . The downstream guide G 2  and the base portion  211  of the first stay  210  are thus pinched between the end of each boss G 13  and a head SC 11  of each first screw SC 1 . In other words, the upstream guide G 1  and the downstream guide G 2  are fixed to the base portion  211  by tightening each first screw SC 1  in a state where the end of each boss G 13  and each fixing portion G 22  of the downstream guide G 2  sandwich the base portion  211  of the first stay  210 . In short, the upstream guide G 1 , the first stay  210 , and the downstream guide G 2  are fastened together with the five first screws SC 1 . Each of the first screws SC 1  screwed at the end of a corresponding boss G 13  is disposed in the holes Hc 5 , Hc 4  with a spacing from their edges. 
     As illustrated in  FIG. 10C , each fastening G 14  has, at its downstream end in the moving direction, a screw hole G 17  in which a third screw SC 3  is screwed. The screw hole G 17  has a closed end or is recessed with an opening on one side. 
     Each fastening G 14  passes through a hole Hd 1  in the sliding sheet  150  and contacts the base portion  211  of the second stay  220 . Each third screw SC 3  is screwed, through the hole Hd 2  in the base portion  221  of the second stay  220 , into the screw hole G 17  of a fastening G 14 . The base portion  221  of the second stay  220  is pinched between an end of each of the two fastenings G 14  and a head SC 31  of a corresponding one of the two third screws SC 3 , and the upstream guide G 1  is fixed to the second stay  220  with the two third screws SC 3 . 
     As illustrated in  FIG. 11 , heads SC 11  of the first screws SC 1 , heads SC 21  of the second screws SC 2 , and heads SC 31  of the third screws SC 3  face downstream in the moving direction. The protrusions G 15  are located farther from the center C 1  of the second stay  220  in the width direction than the first screws SC 1 . 
     The connectors CM are located closer to the load receivers  211 A than to the center C 1  of the first stay  210  in the width direction. The center of the second stay  220  in the width direction and the center of the first stay  210  in the width direction are at the same positions in the width direction, and thus indicated with the same reference number “C 1 ”. 
     More specifically, each of the connectors CM is located between the center C 1  of the first stay  210  and one of the load receivers  211 A in the width direction. The two connectors CM are located symmetrically about the center C 1  of the first stay  210  in the width direction. A distance D 9  from one connector CM to its adjacent load receiver  211 A in the width direction is smaller than a distance D 10  from the connector CM to the center C 1  of the first stay  210  in the width direction. 
     As illustrated in  FIG. 13 , the fixing device  8  includes a side frame  83 , a bracket  84 , and a pressure mechanism  300  at each of its both ends in the width direction. 
     The side frame  83  supports the heating unit  81  and the pressure unit  82 . The side frame  83  is made of metal. The side frame  83  has a spring engaging portion  83 A and a recess  83 B. The spring engaging portion  83 A engages one end of an urging member  320 , which will be described later. The recess  83 B allows an end of the base portion  211  of the first stay  210  in the width direction to pass. 
     The side frame  83  further has two protrusions  83 C and two holes  83 D. The protrusions  83 C position the bracket  84 . The protrusions  83 C are located at opposite positions relative to the recess  83 B in the moving direction. The holes  83 D are formed at opposite positions relative to the recess  83 B in the moving direction. 
     The bracket  84  has a first long hole  84 A, two second long holes  84 C, and two third long holes  84 D. The first long hole  84 A supports the first stay  210  movably in the particular direction. The first long hole  84 A is long in the particular direction. The engaging portion  143  of the holder  140  engages with the first long hole  84 A (refer to  FIG. 14 ). 
     The second long holes  84 C and the third long holes  84 D are long in the moving direction. The second long holes  84 C are formed at opposite positions relative to the first long hole  84 A in the moving direction. The third long holes  84 D are formed at opposite positions relative to the first long hole  84 A in the moving direction. 
     Each of the protrusions  83 C is engageable with a corresponding one of the second long holes  84 C. In a state where the protrusions  83 C engage in the second long holes  84 C, the bracket  84  is movable relative to the side frame  83  in the moving direction. The bracket  84  is positioned to the side frame  83  by aligning the first long hole  84 A with a specified mark, for example, on the side frame  83 , and the pressure unit  82  is thus appropriately positioned to the side frame  83 . 
     Thereafter, the positioned bracket  84  is fixed to the side frame  83  by tightening screws in the third long holes  84 D and the holes  83 D. The movement restriction member R contacts an outer surface of the bracket  84  in the width direction (refer to  FIG. 14 ). The holder  140  and the first stay  210  are thus positioned to the side frame  83  in the width direction. 
     The pressure mechanism  300  includes a pressure arm  310  and an urging member  320 . The pressure arm  310  presses the first stay  210  via a buffer BF. The pressure arm  310  is a L-shaped plate-like member made of metal. The pressure arm  310  has a hole  311 , a spring engaging portion  312 , and an engagement hole  313 . 
     The hole  311  is formed at one end of the pressure arm  310 . The pressure arm  310  is supported at the side frame  83  rotatably about the hole  311 . The spring engaging portion  312  is located at the other end of the pressure arm  310  and engages with an end of the urging member  320 . The engagement hole  313  is formed near a bend portion of the pressure arm  310  and engages the buffer BF. 
     The urging member  320  urges the first stay  210  toward the rotator  120 . In this embodiment, the urging member  320  is a helical tension spring. 
     As illustrated in  FIG. 15 , a cam  85  is disposed rotatably on the side frame  83 . The cam  85  is rotatable to switch the state of the fixing device  8  between a nip state and a nip release state. 
     In the nip state ( FIG. 2 ), a specified nip pressure is applied to between the heating unit  81  and the pressure unit  82 . In the nip release state, no nip pressure or a nip pressure smaller than the specified nip pressure is applied to between the heating unit  81  and the pressure unit  82 . 
     While the cam  85  is separated from the pressure arm  310 , the fixing device  8  is in the nip state. When the cam  85  rotates counterclockwise by substantially 90 degrees from the position illustrated in  FIG. 15 , the pressure arm  310  also rotates counterclockwise against an urging force from the urging member  320 , and thus the fixing device  8  enters the nip release state. 
     Technical advantages of the fixing device  8  according to the illustrative embodiment will now be described. 
     As illustrated in  FIGS. 2 and 4B , in the nip state, the two springs SP urge the fixing plates B 1 , B 2  toward the walls W 2 , W 4 , and the pads P 1 , P 2  contact the walls W 2 , W 4  to restrict movements of the nip forming members N 1 , N 2 . Similarly, in the nip release state, the pads P 1 , P 2  contact the walls W 2 , W 4  to restrict movements of the nip forming members N 1 , N 2 . This may stabilize the positions of the nip forming members N 1 , N 2  relative to the holder  140  while the nip state and the nip release state are repeatedly switched. This may also stabilize the position of the nip NP including the upstream nip NP 1  and the downstream nip NP 2 . 
     The nip forming members N 1 , N 2  may have manufacturing deviations, such as positional deviations of the pads P 1 , P 2  caused when attached to the fixing plates B 1 , B 2 . Even in this case, however, the urging forces of the two springs SP allow the pads P 1 , P 2  to contact the walls W 2 , W 4 , thus holding the pads P 1 , P 2  in position relative to the holder  140  and stabilizing the positions of the nips NP 1 , NP 2 . 
     Both ends of each fixing plate B 1 , B 2  in the width direction are urged toward the support wall W 1  by the respective springs SP. In this embodiment, the support surfaces F 1 , F 2  of the support wall W 1  protrude toward the rotator  120 , and the nip forming members N 1 , N 2  become deformed along the shapes of the support surfaces F 1 , F 2 . After assembly of the fixing device  8 , the surfaces of the pads P 1 , P 2  facing the rotator  120  becomes curved. This eliminates the need to manufacture the pads P 1 , P 2  to have curved surfaces facing the rotator  120 . The holder  140  made of resin is less subject to manufacturing deviations than the pads P 1 , P 2  made of rubber, thus reducing fluctuations on the pressure distribution at the nip NP in the width direction efficiently, unlike the case where the pads P 1 , P 2  are manufactured to have curved surfaces facing the rotator  120 . 
     From the above description, the illustrative embodiment may have the following advantages. 
     The nip forming members N 1 , N 2  are urged in contact with the respective walls W 2 , W 4 . This may stabilize the positions of the nips NP 1 , NP 2  regardless of manufacturing deviations of the nip forming members N 1 , N 2  and repeated switching between the nip state and the nip release state. Each spring SP has a coil portion S 1  of one or more turns of wire, which may prevent or reduce the spring SP, when compressed into between the nip forming members N 1 , N 2 , from undergoing plastic deformation, as compared to a differently shaped spring, for example, a V-shaped leaf spring. 
     The springs SP contact the fixing plates B 1 , B 2 , not the pads P 1 , P 2  located thereon. This may prevent the springs SP from deforming the pads P 1 , P 2  and thus stabilize the positions of the nips NP 1 , NP 2 . 
     The holder  140  includes the bosses W 6  to be inserted into the coil portions S 1  of the respective springs SP. The spring SP are attachable to the holder  140  simply by attaching the coil portions S 1  to the bosses W 6 , which facilitates installation of the springs SP. 
     Each of the bosses W 6  is located at a position farther from the rotator  120  than the fixing plates B 1 , B 2  in the particular direction. This positional relationship may enable each spring SP to urge the nip forming members N 1 , N 2  against the holder  140  and thus prevent or reduce the nip forming members N 1 , N 2  from falling out of the holder  140  at the installation. 
     In the above embodiment, the boss W 6  is located, in the moving direction, between the end portion B 11  of the upstream fixing plate B 1  and the end portion B 21  of the downstream fixing plate B 2 . A distance in the moving direction between the end portion B 11  of the upstream fixing plate B 1  and the end portion B 21  of the downstream fixing plate B 2  is greater than the outside diameter of a coil portion S 1 . The coil portion S 1  of each spring SP is attachable to a corresponding boss W 6  between the upstream fixing plate B 1  and the downstream fixing plate B 2 , which improves the installation of the springs SP. The springs SP are used to press the fixing plates B 1 , B 2  against the holder  140 . This structure prevents or reduces the nip forming members N 1 , N 2  from falling out of the holder  140  and reduces fluctuations on the nip pressure distribution. 
     In the above embodiment, the dimension for the recess W 7  in the moving direction is greater than the outside diameter of the coil portion S 1 . The coil portion S 1  of each spring SP is attachable to a corresponding boss W 6  through the recess W 7 , which improves the installation of the springs SP. 
     Each of the protrusions W 10 , W 11  has a portion located at the same position as the arm S 2 , S 3  in the moving direction. Each of the bosses W 6  extends to a position overlapping the protrusions W 10 , W 11  in the width direction. The protrusions W 10 , W 11  may prevent the springs SP from being inclined or falling out of the bosses W 6  at the installation. 
     The restriction protrusions W 21 , W 24  engage in the restriction recesses B 14 , B 24  of the fixing plates B 1 , B 2  to restrict movements of the fixing plates B 1 , B 2  in the width direction. The restriction recesses B 14 , B 24  and the restriction protrusions W 21 , W 41  are located between each end of the pads P 1 , P 2  and a corresponding one of the bosses W 6  in the width direction. This prevents the fixing device  8  from upsizing, unlike, for example, the structure including the restriction recesses and the restriction protrusions that are located at outer positions relative to the bosses in the width direction. 
     Each spring SP has the bend portions S 4  at the ends of the arms S 2 , S 3 . In a case where the spring SP is held in compression with tweezers, for example, the bend portions S 4  are used to allow engaging of the ends of tweezers so that the spring SP may be prevented from falling out of tweezers. 
     The bend portions S 4  are ring-shaped. In a case where the spring SP is held in compression with tweezers, the bend portions S 4  allow passing of the ends of tweezers through the respective rings so that the spring SP may be prevented from falling out of tweezers more reliably. 
     The upstream guide G 1 , the first stay  210 , and the downstream guide G 2  are fastened together with the first screws SC 1 . This reduces the number of screws required, unlike, for example, the structure where the upstream guide is fastened to the first stay with screws and then the downstream guide is fastened to the first stay with other screws. 
     Each boss G 13  is disposed in a corresponding first hole Hc 3  formed in the first stay  210  with a spacing left from the edges of the first hole Hc 3 . This prevents the first stay  210  from contacting the bosses G 13  even when the first stay  210  becomes deformed, and thus prevents the upstream guide G 1  from becoming deformed. 
     Each of the screw holes G 16  has a closed end or is recessed with an opening on one side. The screw holes G 16  may hold therein chips or shavings left after the first screws SC 1  are screwed into the screw holes G 16 . 
     The load receivers  211 A are located one at each end of the first stay  210  in the width direction, and the first stay  210  may have a greater likelihood of deformation at its center in the width direction than at its each end. The connectors CM are located closer to the load receivers  211 A than to the center of the first stay  210  in the width direction. This prevents deformation of the second stay  220 , unlike, for example, the structure including the connectors located closer to the center of the first stay in the width direction. 
     Each of the connectors CM is located between the center C 1  of the first stay  210  and one of the load receivers  211 A in the width direction. This reduces the length of the second stay  220  in the width direction and the weight of the fixing device  8 , unlike, for example, the structure including the connectors located at the same positions of the load-receivers. 
     The crimped members SW are crimped to the second stay  220 . This maintains a flatness of the first stay  210  where loads are applied, unlike, for example, the structure including the crimped members crimped to the first stay. 
     The upstream guide G 1  is fixed to the first stay  210  with the first screws SC 1  and to the second stay  220  with the third screws SC 3 . The upstream guide G 1  is thus securely supported by the stays  210 ,  220 . 
     The screwed screws SC 1 , SC 2 , SC 3  have their heads SC 11 , SC 21 , SC 31  all facing downstream in the moving direction. In other words, the screws SC 1 , SC 2 , SC 3  are screwed in the same direction, thus facilitating assembling of components using the screws. Unlike this embodiment, for example, if at least one first screw is screwed with its head facing upstream in the moving direction, the upstream guide should have a through hole formed therein to recess the head of the first screw. In this case, a perimeter of the through hole in the upstream guide surface of the upstream guide may become an edge that may impart a resistance to the circulation of the belt. In this embodiment, however, all of the first screws SC 1  are screwed with their heads SC 11  facing downstream in the moving direction. This eliminates the need to form through holes in the upstream guide G 1  to recess the heads SC 11  of the first screws SC 1 , and prevents the formation of edges on the upstream guide surface Fu. 
     The upstream guide G 1  includes the positioning protrusions G 15  at outer positions relative to any of the first screws SC 1  in the width direction. This prevents or reduces the upstream guide G 1  from being obliquely assembled to the second stay  220 , unlike, for example, the structure including each positioning protrusion sandwiched between the first screws in the width direction. 
     The first stay  210  and the second stay  220  are separate from each other and contact the holder  140  independently of each other. This allows accurate positioning of contact surfaces of the respective stays  210 ,  220  to contact the holder  140  and reduces fluctuations on the nip pressure, unlike, for example, a structure including a U-shaped stay with its ends to contact the holder. The first stay  210  includes the bend portion HB. This structure improves stiffness of the first stay  210  and allows the holder  140  to appropriately receive the force of the urging member  320 . The two connectors CM are located at positions different from the bend portion HB to prevent a loss of strength in a portion of the base portion  211  having stiffness increased by the bend portion HB. 
     The second stay  220  includes the protrusions CV located at positions different from the holes Hc 2 , Hd 2 , He 2 . This structure reduces deformation of the second stay  220  due to pressure applied from the holder  140  to the protrusions CV, and thus reduces fluctuations on the nip pressure distribution. 
     The first stay  210  has both ends where loads are applied. The both ends of the first stay  210  engage with the engaging portions  142 ,  143  and the first stay  210  is thus directly positioned to the holder  140 . This structure stabilizes the positioning accuracy of the holder  140  in the moving direction relative to the first stay  210  subjected to loads and thus reduces uneven nip pressure distribution. 
     The first connecting wall W 13  is located opposite to the rotator  120  relative to an end of the first stay  210  in the width direction and in contact with the first stay  210 . The first stay  210  is sandwiched between the holder body  141  and the first connecting wall W 13  in a direction in which loads are applied (i.e., the particular direction). This structure stabilizes the positioning accuracy of the holder  140  relative to the first stay  210 . This structure also allows temporary assembly of the holder  140  and the first stay  210 , which reduces the need to increase the number of assembly processes. 
     The holder  140  includes the second connecting wall W 14  that connects a pair of pinching walls W 12 , thus increasing stiffness of each of the engaging portions  142 ,  143 . 
     In this embodiment, the second connecting wall W 14  is spaced from the first stay  210 . This structure reduces the nip pressure distribution from varying in the width direction, unlike, for example, a structure where the second connecting wall contacts the first stay. 
     The pinching walls W 12  are reinforced with the reinforcing portions WA to increase stiffness of the engaging portions  142 ,  143 . 
     The first extension walls W 31  contact the downstream surface Fa of the first stay  210  to prevent the holder  140  from being inclined downstream in the moving direction. 
     The second extension walls W 32  contacts the upstream surface Fb of the first stay  210  to thereby sandwich the first stay  210  between the first extension walls W 31  and the second extension walls W 32 . This structure prevents deformation and distortion of the holder  140  in the moving direction. 
     The first extension walls W 31  and the second extension walls W 32  are located closer to the center C 2  of the holder body  141  in the width direction than to the engaging portions  142 ,  143 , thus reducing deformation at the center of the holder  140  in the moving direction. 
     The movement restriction member R is inserted into the through holes Hi in the first stay  210  and the through holes W 18  of the pair of pinching walls W 12  to position the first stay  210  relative to the holder  140  in the width direction. 
     The ribs W 30  are placed in contact with the first stay  210 . This improves accuracy of a contact between the holder  140  and the first stay  210  and distributes the nip pressure uniformly in the width direction, unlike, for example, the structure where the holder has a flat surface long in the width direction to be placed in contact with the entire contact surface of the first stay. Each of the ribs W 30  extends in the moving direction. This facilitates deformation of the support wall W 1  along the first stay  210 , unlike, for example, the structure where the ribs are long in the width direction, and thus distributes the nip pressure uniformly in the width direction. The contact surface Ft of the first stay  210  may be arcuate when viewed in the moving direction, with its center in the width direction protruding toward the holder  140  further than its ends. This case may achieve the above described advantages. 
     The first stay  210  receiving a force from the urging member  320  is disposed to the downstream nip forming member N 2 , thus maintaining the nip pressure of the downstream nip NP 2  appropriately. To remove a sheet S from the rotator  120 , the downstream nip forming member N 2  has a maximum pressure higher than the upstream nip forming member N 1 . As the first stay  210  is disposed to the downstream nip forming member N 2 , such a maximum pressure may be obtained reliably. 
     The second stay  220  includes the protrusions CV to contact some of the ribs W 30 . The first stay  210  and the second stay  220  thus support the support wall W 1  reliably. 
     The protrusions CV are located to the center C 1  of the second stay  220  in the width direction, thus preventing the center of the support wall W 1  in the width direction from becoming deformed toward the second stay  220 . 
     The first stay  210  has the second holes Hc 4  located at positions different from the ribs W 30  in the width direction. In other words, the second holes Hc 4  are absent at portions of the first stay  210  where the first stay  210  receives reaction forces from the ribs W 30 . This structure thus reduces deformation of the first stay  210  and keeps the nip pressure stably. 
     The sliding sheet  150  has the elastically deformable hooks  152 , which are easily engageable in the apertures Hg in the hook engaging portions G 21 . This facilitates attaching the sliding sheet  150 . 
     The end portion  152 A of each hook  152  has a minimum width smaller than a width of a corresponding aperture Hg and a maximum width greater than the width of the aperture Hg. This allows easy insertion of each hook  152  into the aperture Hg and reduces the tendency of each hook  152  to come out of the aperture Hg. 
     The neck portion  152 B of each hook  152  has a length greater than a thickness of a corresponding hook engaging portion G 21 , thus allowing fixing of the downstream end portion  151 B of the sliding sheet  150  to the downstream guide G 2  with sufficient allowance. 
     Each hook engaging portion G 21  is spaced from the first stay  210  by a dimension greater than the length of the end portion  152 A. When inserted into the aperture Hg, the end portion  152 A of each hook  152  does not contact the first stay  210 . This facilitates insertion of the end portion  152 A into the aperture Hg. 
     Each of the fixing portions G 22  of the downstream guide G 2  is located between adjacent two of the hook engaging portions G 21 . The hook engaging portions G 21  are thus non-obstructive while the downstream guide G 2  is fixed to the first stay  210 . This facilitates fixing the downstream guide G 2  to the first stay  210 . 
     The upstream end of the sliding sheet  150  is subjected to tension, because the belt  130  and the sliding sheet  150  at the nip NP are pulled downstream. However, the downstream end of the sliding sheet  150  is less susceptible to tension. In this embodiment, the sliding sheet  150  has the hooks  152  at the downstream end portion  151 B, which is less susceptible to tension. The downstream end portion  151 B of the sliding sheet  150  is fixed to the downstream guide G 2  by simply engaging the hooks  152  in the apertures Hg, without the need to use fasteners, for example, screws. This structure reduces the need to increase the number of parts and facilitates fixing the downstream end portion  151 B of the sliding sheet  150 , unlike, for example, the structure using screws to fix the downstream end portion of the sliding sheet. 
     The holes Hc 1  in the upstream end portion  151 A of the sliding sheet  150  engage with the bosses G 13  on the upstream guide G 1 , and the upstream end portion  151 A of the sliding sheet  150  is sandwiched between the upstream guide G 1  and the second stay  220 , thereby fixing the upstream end portion  151 A of the sliding sheet  150  to the upstream guide G 1 . This facilitates fixing the upstream end portion  151 A of the sliding sheet  150 . 
     The sliding sheet  150  is located covering the upstream guide surface Fu, thus reducing sliding friction between the upstream guide G 1  and the belt  130 . 
     While the disclosure has been described in detail with reference to the specific embodiment thereof, various changes, arrangements and modifications may be applied therein as will be described below. 
     In the illustrative embodiment, the halogen lamp is illustrated as a heater. Examples of the heater include a carbon heater. 
     In the illustrative embodiment, a cylindrical roller having the heater  110  therein is illustrated as a rotator. Examples of the rotator may include a belt whose inner peripheral surface may be heated by a heater. An outer peripheral surface of the rotator may be heated by a heater disposed outside of the rotator or using an induction heating (“IH”) element. A heater may be disposed within an interior space of a belt to indirectly heat the rotator contacting an outer peripheral surface of the belt. A heater may be disposed within an interior space of each of the rotator and the belt. 
     The above embodiment shows but is not limited to that the sliding sheet  150  is disposed between the belt  130  and the nip forming member N. The sliding member may be omitted. In this case, the nip forming member may be placed in contact with an inner peripheral surface of the belt. A sliding sheet with no hooks may be disposed between the belt and the nip forming member. A sliding sheet may have a downstream end portion as a free end portion fixed by no members. 
     The above embodiment shows but is not limited to two nip forming members N 1 , N 2 . Instead, one nip forming member may be provided. 
     The above embodiment shows but is not limited to the nip forming member including pads and fixing plates. The nip forming member may eliminate fixing plates or include pads only. The pads may be made of a hard material, which is resistant to deformation under pressure, such as resin or metal. 
     The above embodiment shows but is not limited to the restricting members (walls W 2 , W 4 ) integral with the holder  140 . The restricting members may be individual members separate from the holder. 
     The above embodiment shows but is not limited to two springs SP, each having the bend portions S 4  at the ends of the arms S 2 , S 3 . Each of the springs may have no bend portions or have a bend portion at one of the arms. 
     The above embodiment shows but is not limited to the ring-shaped bend portions S 4 . The bend portions may be arcuate or V-shaped. 
     The above embodiment shows but is not limited to the connectors CM, each including a crimped member SW and a second screw SC 2 . The connectors may be components fastened to the stays with screws. 
     The above embodiment shows but is not limited to that the urging member  320  is a helical tension spring. Examples of the urging member include a helical compression spring, a torsion spring, and a leaf spring. 
     The above embodiment shows but is not limited to that the movement restriction member R is a torsion spring. Examples of the movement restriction member include a U-shaped wire or plate, and a bolt and a nut. 
     The above embodiment shows but is not limited to that the second stay  220  has four protrusions CV. The second stay may have at least one protrusion. 
     The above embodiment shows but is not limited to that holder  140  and the stay  200  function as a supporting member. The support member may be only one of the holder and the stay. The holder and the stay may be integral with each other. 
     The above embodiment shows but is not limited to the that the belt guide G includes two guides G 1 , G 2 . The belt guide may include only one of the upstream guide and the downstream guide. The upstream guide and the downstream guide may be integral with each other. 
     The above embodiment shows but is not limited to the that the stay  200  includes two stays  210 ,  220 . The stay may include three or more stays. 
     The above embodiment shows but is not limited to that the sliding sheet  150  has the hooks  152  at the downstream end portion  151 B. The sliding sheet may have at least one hook at at least one of the upstream end portion and the downstream end portion. 
     The above embodiment shows but is not limited to that the downstream guide G 2  includes the hook engaging portions G 21  engageable with the hooks  152 . One of the upstream guide, the holder, the first stay and the second stay may include at least one hook engaging portion. 
     The above embodiment shows but is not limited to that the end portion  152 A of each hook  152  protrudes relative to both ends of the neck portion  152 B in the width direction. At least one hook may have an end portion protruding relative to one end of the neck portion  152 B in the width direction. 
     The above embodiment shows but is not limited to that the upstream end portion  151 A of the sliding sheet  150  is fixed to the upstream guide G 1 . The upstream end portion of the sliding sheet may be fixed to one of the holder, the downstream guide, the first stay, and the second stay. 
     The above embodiment shows but is not limited to that the sliding sheet  150  is located covering the upstream guide surface Fu, the nip forming member N, and the downstream guide surface Fd. The sliding sheet may cover at least the nip forming member. In other words, the belt guide may be placed in contact with an inner peripheral surface of the belt. In other words, the belt guide may be placed in contact with an inner peripheral surface of the belt. 
     Each of the elements or components which have been described in the illustrative embodiment and modifications may be used in any combination.