Patent Publication Number: US-11048195-B2

Title: Fuser including rotatable member and endless belt

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of prior U.S. application Ser. No. 16/255,182, filed Jan. 23, 2019, which claims priority from Japanese Patent Application No. 2018-009303 filed on Jan. 24, 2018 and Japanese Patent Application No. 2018-184418 filed on Sep. 28, 2018, the content of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF DISCLOSURE 
     The disclosure relates to a fuser that fuses a toner image onto a recording medium. 
     BACKGROUND 
     A known fuser, for example, as disclosed in JP2010-231008A, includes a heat roller, a pad member that nips an endless belt in cooperation with the heat roller between the pad member and the heat roller and serves to form a nip portion between the heat roller and the endless belt, and a holding portion that holds the pad member. The pad member includes a pressurizing pad that contacts the endless belt. The pressurizing pad is attached to a supporting plate. The pressurizing pad attached to the supporting plate is mounted in a recess in the holding portion, thereby holding the pad member in position relative to a moving direction of the endless belt at the nip portion. 
     SUMMARY 
     The following summary presents a simplified summary of certain features. The summary is not an extensive overview and is not intended to identify key or critical elements. 
     According to one or more aspects of the disclosure, a fuser is described including a heater, a belt, a rotating member, and a pad. A nip portion is formed between the belt and the rotating member as the pad presses the belt toward the rotating member. The pad is biased toward a restricting member. The biasing may be performed by one or more springs. The pad may be adhered to a plate that receives a biasing force from the spring. The spring may be a compression spring, a tension spring, and/or a plate spring among other types of springs. 
     These and other features and advantages are described in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a laser printer including a fuser in an illustrative embodiment according to one or more aspects of the disclosure. 
         FIG. 2  is a cross-sectional view of the fuser. 
         FIG. 3  is an exploded perspective view of a pressure unit of the fuser. 
         FIG. 4  is a top plan view of the pressure unit. 
         FIG. 5  is a perspective view of the pressure unit and a side guide of the fuser. 
         FIG. 6  is a cross-sectional view of the pressure unit and the side guide. 
         FIG. 7  is a cross-sectional view of the fuser in a nip released state. 
         FIG. 8A  is a perspective view of a pressure unit of a fuser according to a first modification. 
         FIG. 8B  is a cross-sectional view of the pressure unit of the fuser according to the first modification. 
         FIG. 9  is a perspective view of a pressure unit of a fuser according to a second modification. 
         FIG. 10A  is a perspective view of a nip forming member of a fuser according to a third modification. 
         FIG. 10B  is a perspective view of the nip forming member attached to a holder of the fuser according to the third modification. 
         FIG. 10C  is a partially-cutaway top plan view of a pressure unit of the fuser according to the third modification. 
         FIG. 11  is a cross-sectional view of a pressure unit of a fuser according to a fourth modification. 
         FIG. 12  is a perspective view of the pressure unit of the fuser according to the fourth modification. 
         FIG. 13  is a perspective view of the pressure unit of the fuser according to the fourth modification. 
         FIG. 14  is a cross-sectional view of a pressure unit of a fuser according to a fifth modification. 
         FIG. 15  is a perspective view of the pressure unit of the fuser according to the fifth modification. 
         FIG. 16  is a cross-sectional view of a pressure unit of a fuser according to a sixth modification. 
         FIG. 17  is a perspective view of the pressure unit of the fuser according to the sixth modification. 
         FIG. 18A  is a cross-sectional view of a pressure unit of a fuser according to a seventh modification. 
         FIG. 18B  is a top plan view of the pressure unit of the fuser according to the seventh modification. 
         FIG. 19A  is a cross-sectional view of a pressure unit of a fuser according to an eighth modification. 
         FIG. 19B  is a perspective view of a portion of the pressure unit of the fuser according to the eighth modification, illustrating a helical compression spring and its surrounding components. 
         FIG. 20  is a cross-sectional view of a pressure unit of a fuser according to a ninth modification. 
         FIG. 21A  is a cross-sectional view of a pressure unit of a fuser according to a tenth modification. 
         FIG. 21B  is a perspective view of a portion of the pressure unit of the fuser according to the tenth modification, illustrating a flat spring and its surrounding components. 
         FIG. 22A  is a cross-sectional view of a pressure unit of a fuser according to an eleventh modification. 
         FIG. 22B  is a perspective view of a portion of the pressure unit of the fuser according to the eleventh modification, illustrating a flat spring and its surrounding components. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative embodiment and modifications according to one or more aspects of the disclosure will be described with reference to the accompanying drawings. In the following description, directional terminology such as “top/upper,” “bottom/lower,” “front,” “rear,” “left,” “right” etc., as labelled in the drawings, may be used. With respect to the page of  FIG. 1 , the left side may be defined as the front; the right side may be defined as the rear; the facing or near side may be defined as the right; the opposite side or far side may be defined as the left; the upper side may be defined as the top, and the lower side may be defined as the bottom. Because the disclosed components can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. 
     As depicted in  FIG. 1 , a laser printer  1  includes a casing  2 , a sheet feeder  3 , an exposure device  4 , a process cartridge  5 , a fuser  8 , conveying rollers  23  and  24 , and a discharge tray  22 . 
     The casing  2  has an opening defined therein. The casing  2  includes a front cover  21  configured to move between an open position providing access to an interior space of the casing  2  through the opening, and a closed position (as depicted in  FIG. 1 ) preventing access to the interior space. 
     The sheet feeder  3  is disposed in the casing  2  at its lower portion. The sheet feeder  3  includes a feed tray  31 , a lifter plate  32 , and a feed mechanism  33 . The feed tray  31  is configured to hold a stack of one or more sheets S. The lifter plate  32  is configured to lift a front end portion of the sheet stack. The feed mechanism  33  is configured to feed each of the one or more sheets S to the process cartridge  5 . 
     The exposure device  4  is disposed in the casing  2  at its upper portion. The exposure device  4  includes a light source (not depicted), and components, such as a polygon mirror, lenses, and reflecting mirrors, that are illustrated without reference numerals. 
     The exposure device  4  is configured to emit a laser beam from the light source based on image data to a surface of a photosensitive drum  61  (described below) of the process cartridge  5 . The laser beam scans across the surface of the photosensitive drum  61  at high speed. The surface of the photosensitive drum  61  is thus exposed to light. 
     The process cartridge  5  is configured to be inserted into and removed from the casing  2  through the opening when the front cover  21  is in the open position. The process cartridge  5  is disposed below the exposure device  4  in the casing  2 . The process cartridge  5  includes a drum unit  6  and a developing unit  7 . The drum unit  6  includes the photosensitive drum  61 , a charger  62 , and a transfer roller  63 . The developing unit  7  is configured to be attached to and separated from the drum unit  6 . The developing unit  7  includes a developer roller  71 , a supply roller  72 , a blade  73 , and a reservoir  74  configured to hold or store toner. 
     In the process cartridge  5 , the surface of the photosensitive drum  61  is uniformly charged by the charger  62 . The surface of the photosensitive drum  61  is then exposed to the laser beam from the exposure device  4  to form an electrostatic latent image based on image data on the photosensitive drum  61 . The toner in the reservoir  74  is supplied to the developer roller  71  via the supply roller  72 . The toner entered between the developer roller  71  and the blade  73  is carried on the developer roller  71  as a thin layer whose thickness is constant. The toner on the developer roller  71  is supplied to the electrostatic latent image on the photosensitive drum  61 , thereby developing the electrostatic latent image into a visible toner image. The toner image is thus formed on the photosensitive drum  61 . The toner image on the photosensitive drum  61  is then transferred onto a sheet S fed between photosensitive drum  61  and the transfer roller  63 . 
     The fuser  8  is disposed to the rear of the process cartridge  5 . The sheet S having the toner image transferred thereon is conveyed to the fuser  8  where the toner image is fused or fixed on the sheet S. The sheet S is then discharged by the conveying rollers  23  and  24  onto the discharge tray  22 . 
     As depicted in  FIG. 2 , the fuser  8  includes a rotatable member, e.g., a heat roller  81 , a heater  82 , an endless belt  83 , and a pressure unit  84 . One of the heat roller  81  and the pressure unit  84  is biased toward the other, thereby forming a nip portion NP between the heat roller  81  and the endless belt  83 . The toner image is fused onto the sheet S when the sheet S passes through the nip portion NP. A state in which the nip portion NP is formed as depicted in  FIG. 2  may be referred to as a “nipped state” while a state in which the nip portion NP is not formed as depicted in  FIG. 7  may be referred to as a “nip released state”. 
     The fuser  8  may be described in conjunction with a width direction of the endless belt  83 , a moving direction of the endless belt  83  at the nip portion NP, and an opposing direction in which the heat roller  81  is opposed to the pressure unit  84  (e.g., a nip forming member  85  to be described below). The width direction of the endless belt  83  may correspond to a right-left direction. The moving direction of the endless belt  83  at the nip portion NP, which may be simply referred to as the “belt moving direction” hereinafter, may correspond to a front-to-rear direction. The opposing direction may correspond to the top-bottom direction. 
     The heat roller  81  has a cylindrical body. The heat roller  81  includes a tubular member and a release layer formed over an outer peripheral surface of the tubular member. The tubular member may include metal, e.g., aluminum. The release layer may include fluoro-resin. The heat roller  81  is configured to receive a drive force from a motor (not depicted) and rotate counterclockwise in  FIG. 2 . The heat roller  81  is in contact with an outer peripheral surface of the endless belt  83 . 
     The heater  82  is configured to heat the heat roller  81  and disposed within the heat roller  81  or in an interior space of the heat roller  81 . The heater  82  may be, for example, a halogen lamp, that may emit light upon energization to heat the heat roller  81  through radiant heat. 
     The endless belt  83  is a flexible tubular-shaped member. The endless belt  83  may include a base layer and a release layer formed over an outer peripheral surface of the base layer. The base layer may include, for example, metal such as stainless steel, or resin such as polyimide resin. The release layer may include fluoro-resin. The rotation of the heat roller  81  may cause the endless belt  83  to rotate or circularly move in a clockwise direction in  FIG. 2 . 
     The endless belt  83  has an inner peripheral surface  83 A to which lubricant such as grease, is applied. The lubricant helps to enhance slidability between the inner peripheral surface  83 A and the pressure unit  84 , so that the endless belt  83  may move smoothly. 
     The pressure unit  84  includes the nip forming member  85 , a holder  86  that supports the nip forming member  85 , and a stay  87  that supports the holder  86 . When the nip forming member  85  is supported by the holder  86 , a portion of the nip forming member  85  (e.g., a pad  88 ) protrudes upward toward the heat roller  81  relative to a surface of the holder  86  closer to the heat roller  81 . The surface of the holder  86  closer to the heat roller  81  corresponds to an upper surface the holder  86  in the illustrative embodiment. 
     The nip forming member  85  is configured to nip the endless belt  83  in cooperation with the heat roller  81  such that the nip portion NP is formed between the heat roller  81  and the endless belt  83 . The nip forming member  85  is located within a loop or an internal space of the endless belt  83 . The pressure unit  84  serves to form the nip portion NP where heat and pressure are applied to the sheet S to fuse the toner image on the sheet S. In the illustrative embodiment, the nip portion NP is a portion where the outer peripheral surface of the endless belt  83  contacts the heat roller  81 . A portion of the nip portion NP may not receive pressures from the pad  88 . 
     The nip forming member  85  includes the pad  88  and a plate member  89 . The pad  88  is configured to nip the endless belt  83  in cooperation with the heat roller  81  between the pad  88  and the heat roller  81 , and presses the endless belt  83  against the heat roller  81 . The pad  88  is fixed or attached to the plate member  89 . 
     As depicted in  FIG. 3 , the pad  88  has a rectangular parallelepiped shape and is elongated in the right-left direction. The pad  88  includes elastic material, such as rubber, and is elastically deformable. Each of the pad  88 , the plate member  89 , the holder  86 , and the stay  87  is generally symmetric with respect to a respective center thereof in the right-left direction. In other words, a right portion and a left portion of the fuser  8  including the pad  88 , the plate member  89 , the holder  86 , and the stay  87  are similar to each other, so that the fuser  8  will be described in detail below, in conjunction with the right portion of the fuser  8 , and detailed description with respect to the left portion will be omitted herein. 
     Referring to  FIG. 3 , the plate member  89  is a metal plate member having rigidity higher than rigidity of the pad  88 . The plate member  89  includes a base portion  89 A to which the pad  88  is attached, an extended portion  89 B that extends rightward from a right end of the base portion  89 A, and a first boss C 1  extending frontward from a front end of the extended portion  89 B, e.g., from an upstream end of the extended portion  89 B in the belt moving direction. 
     The base portion  89 A includes an attachment region Ab to which the pad  88  is attached. The base portion  89 A has a width (e.g., distance in the front-rear direction) greater than a width of the pad  88 , so that a space is provided between the pad  88  and a respective one of the front end and the rear end of the base portion  89 A. The space between the pad  88  and the rear end (the downstream end) of the base portion  89 A serves as a projecting portion C (indicated by hatching with parallel diagonal lines) that projects rearward relative to the pad  88  that has been attached to the base portion  89 A. 
     The base portion  89 A has a length (e.g., distance in the right-left direction) greater than the length of the pad  88 , so that a space is provided between the attachment region Ab and a respective right and left end of the base portion  89 A (the left end not depicted). 
     The extended portion  89 B has a width (e.g., a distance in the front-rear direction) less than the width of the base portion  89 A. The extended portion  89 B is located at a rear end portion of the base portion  89 A. As depicted in  FIG. 4 , the extended portion  89 B of the plate member  89 , when mounted to the holder  86 , has a first portion P 1  protruding rightward relative to the holder  86 . The first portion P 1  includes the first boss C 1 . 
     The first boss C 1  is sized to engage in an internal space of a biasing member (e.g., a helical compression spring S 1  to be described below) in its diametrical direction. The first boss C 1  is spaced from the right end of the extended portion  89 B. When the first boss C 1  is engaged in the helical compression spring S 1 , the helical compression spring S 1  may contact particular portions of a front end surface of the extended portion  89 B. The particular portions are located to the right and left of the first boss C 1 . 
     The holder  86  may include resin or metal. The holder  86  includes a base portion  86 A, an upstream wall  86 B, a restricting member, e.g., a downstream wall  86 C, a restriction wall  86 D, two first engaging walls  86 E, and two second engaging walls  86 F. The base portion  86 A is a plate-like portion and has a support surface FS extending in a direction orthogonal to the opposing direction or the top-bottom direction. The base portion  86 A is elongated in the right-left direction. The support surface FS supports the plate member  89  to allow the plate member  89  to slidably move in the belt moving direction or the front-rear direction. 
     As depicted in  FIG. 2 , the upstream wall  86 B protrudes upward toward the heat roller  81  from a front end portion of the base portion  86 A. The upstream wall  86 B has a curved surface that guides the inner peripheral surface  83 A of the endless belt  83 . 
     The downstream wall  86 C protrudes upward toward the heat roller  81  from a rear end portion of the base portion  86 A. The downstream wall  86 C also has a curved surface that guides the inner peripheral surface  83 A of the endless belt  83 . When the nip forming member  85  has been mounted to the holder  86 , the downstream wall  86 C is disposed downstream of the pad  88  in the belt moving direction. 
     Referring back to  FIG. 3 , the downstream wall  86 C includes a contact surface FT and a recess portion G. The contact surface FT is disposed at a front surface of the downstream wall  86 C facing frontward and contacts the pad  88 . The contact surface FT contacts the pad  88  in the belt moving direction, and is orthogonal to the belt moving direction. The contact surface FT faces upstream in the belt moving direction. The recess portion G is recessed into the contact surface FT toward the rear. 
     The distance from the contact surface FT to the upstream wall  86 B in the front-rear direction is greater than the width (e.g., distance in the front-rear direction) of the base portion  89 A of the plate member  89 . This configuration may allow the base portion  89 A to be readily placed onto the support surface FS through a space between the upstream wall  86 B and the downstream wall  86 C. 
     The recess portion G is grooved to allow the projecting portion C of the plate member  89  to engage therein. The recess portion G extends through the downstream wall  86 C in the right-left direction. As depicted in  FIGS. 2 and 4 , a distance L 1  of the recess portion G in the front-rear direction is greater than a distance L 2  of the projecting portion C in the front-rear direction. In other words, the recess portion G has a depth (e.g., a distance in the front-rear direction) that is greater than a projecting amount of the projecting portion C relative to the pad  88  in the front-rear direction. 
     The recess portion G has an upper surface and a lower surface located farther from the heat roller  81  than the upper surface. The lower surface is flush with the support surface FS of the base portion  86 A. The lower surface of the recess portion G may be located farther from the heat roller  81  than the support surface FS in the top-bottom direction. 
     The restriction wall  86 D restricts the movement of the base portion  89 A of the plate member  89  in the right-left direction by contacting an end (e.g., the right end) of the base portion  89 A. The restriction wall  86 D is disposed at a respective right and left end portion of the support surface FS of the base portion  86 A (left restriction wall  86 D not depicted), so that the base portion  89 A may be located between the right and left restriction walls  86 D. The restriction wall  86 D extends from the support surface FS upward toward the heat roller  81  and is spaced from the downstream wall  86 C in the front-rear direction. 
     The distance in the front-rear direction from the rear end of the restriction wall  86 D to the contact surface FT is greater than the width (e.g., distance in the front-rear direction) of the extended portion  89 B of the plate member  89 . This configuration may allow the extended portion  89 B to be readily placed onto the support surface FS through a space between the restriction wall  86 D and the downstream wall  86 C. In the illustrative embodiment, the restriction wall  86 D is integral with the upstream wall  86 B and the height of the restriction wall  86 D (e.g., distance in the top-bottom direction from the support surface FS) is equal to the height of the upstream wall  86 B (e.g., distance in the top-bottom direction from the support surface FS). In another embodiment, the restriction wall  86 D may not necessarily be integral with the upstream wall  86 B but may be separated from the upstream wall  86 B. In yet another embodiment, the height of the restriction wall  86 D may be less than the height of the upstream wall  86 B. 
     The first engaging walls  86 E engage with an upper end portion of an upstream wall  87 B (described below) of the stay  87 . The first engaging walls  86 E sandwich the upstream wall  87 B in the front-rear direction. Each of the first engaging walls  86 E extends downward from the base portion  86 A toward the stay  87 . 
     The second engaging walls  86 F engage with an upper end portion of a downstream wall  87 C (described below) of the stay  87 . The second engaging walls  86 F sandwich the downstream wall  87 C in the front-rear direction. Each of the second engaging walls  86 F extends downward from the base portion  86 A toward the stay  87 . 
     The stay  87  may include resin or metal. The stay  87  has a U-shaped cross section, and includes a base wall  87 A, the upstream wall  87 B, and the downstream wall  87 C. The base wall  87 A has a plate shape and includes a surface orthogonal to the top-bottom direction. The base wall  87 A is elongated in the right-left direction. 
     The upstream wall  87 B extends upward toward the holder  86  from a front end portion of the base wall  87 A. The downstream wall  87 C extends upward toward the holder  86  from a rear end portion of the base wall  87 A. The stay  87  includes an upstream extended portion  87 D extending rightward from a right end of the upstream wall  87 B. The upstream extended portion  87 D has a height in the top-bottom direction less than a height of the upstream wall  87 B in the top-bottom direction. The upstream extended portion  87 D is located on an upper portion of the upstream wall  87 B. Similarly, the downstream wall  87 C includes a downstream extended portion  87 F extending rightward from a right end of the downstream wall  87 C. The downstream extended portion  87 F has the same size as the upstream extended portion  87 D and is located at the same position or level as the upstream extended portion  87 D in the top-bottom direction. 
     The upstream extended portion  87 D has a protruding portion  87 E protruding upward toward the holder  86  from an upper end of the upstream extended portion  87 D. The upstream extended portion  87 D and the protruding portion  87 E serve as a second portion. As depicted in  FIGS. 4 and 5 , when the stay  87  has been attached to the holder  86 , the second portion, e.g., the upstream extended portion  87 D and the protruding portion  87 E, is located to the right of the holder  86 . 
     The protruding portion  87 E includes a second boss C 2  protruding rearward from a rear surface of the protruding portion  87 E. The second boss C 2  is sized to engage in an internal space of the helical compression spring S 1  in its diametrical direction. When the nip forming member  85  and the stay  87  has been attached to the holder  86 , the second boss C 2  is opposite to the first boss C 1  of the plate member  89  in the front-rear direction, so that an axis of the helical compression spring S 1  extends along the front-rear direction. 
     The helical compression spring S 1  biases the nip forming member  85  in the front-rear direction toward the contact surface FT of the holder  86 . The helical compression spring S 1  is disposed to the right of the holder  86 . The helical compression spring S 1  has one end contacting the extended portion  89 B of the plate member  89  and the other end contacting the protruding portion  87 E of the stay  87 . The helical compression spring S 1  is disposed at a right end portion of the plate member  89 . The helical compression spring S 1  is compressed between the plate member  89  and the stay  87  to bias the plate member  89  toward the rear. 
     The fuser  8  further includes left and right side guides  90  (the left side guide  90  not depicted in  FIG. 5 ) that guide the inner peripheral surface  83 A of the endless belt  83 . Since the left and right side guides  90  have similar configuration, the right side guide  90  is described in detail below. The side guide  90  is disposed at a right end portion of the stay  87 . The side guide  90  includes a disk-shaped base portion  91  having a restriction surface  91 A, a tubular-shaped belt guide portion  92  extending from the restriction surface  91 A toward the left (as depicted in  FIG. 6 ), and two stay support portions  93  and  94  that respectively support the extended portions  87 D and  87 F of the stay  87 . 
     The restriction surface  91 A of the base portion  91  restricts the movement of the endless belt  83  in the right-left direction by contacting the end (e.g., the right or left end) of the endless belt  83 . The belt guide portion  92  includes a curved guide surface  92 A that guides the inner peripheral surface  83 A of the endless belt  83 . Each of the stay support portions  93  and  94  has a rectangular tube shape and is located within an internal space defined by the belt guide portion  92 . Each of the stay support portions  93  and  94  protrudes leftward from the base portion  91 . 
     Each of the stay support portions  93  and  94  protrudes from the base portion  91  by a first amount. The belt guide portion  92  protrudes from the base portion  91  by a second amount. The first amount is less than the second amount. The first amount and the second amount are set or determined such that, when the extended portions  87 D and  87 F of the stay  87  are respectively engaged in the stay support portions  93  and  94 , the belt guide portion  92  surrounds the helical compression spring S 1  (refer to  FIG. 6 ). In other words, as depicted in  FIG. 6 , when the side guide  90  has been attached to the stay  87 , the helical compression spring S 1  is located within the internal space defined by the belt guide portion  92  and overlaps with the side guide  90  in the right-left direction. 
     Technical advantages of the fuser  8  according to the illustrative embodiment will now be described. In the nipped state as depicted in  FIG. 2 , the helical compression spring S 1  biases the nip forming member  85  toward the downstream wall  86 C, so that the nip forming member  85  may contact or abut against the contact surface FT. This configuration may restrict the rearward movement of the nip forming member  85 . In the nip released state as depicted in  FIG. 7 , the helical compression spring S 1  also biases the nip forming member  85  toward the downstream wall  86 C, similar to the nipped state, so that the nip forming member  85  may contact or abut against the contact surface FT. The rearward movement of the nip forming member  85  may thus be restricted. If the endless belt  83  is repeatedly nipped or released, the nip forming member  85  may be held in position relative to the holder  86 . This may stabilize the position of the nip portion NP. The pad  88  is pressed against the contact surface FT due to the biasing force of the helical compression spring S 1 . This configuration may hold the pad  88  in position relative to the holder  86 , and may stabilize the position of the nip portion NP if the nip forming member  85  should have manufacturing deviations, such as a positional deviation of the pad  88  relative to the plate member  89  (e.g., positional deviation caused when the pad  88  is attached to the plate member  89 ). 
     In addition to the advantages described above, the illustrative embodiment may have the following advantages. The helical compression spring S 1  biases the nip forming member  85  toward the downstream wall  86 C, which is disposed downstream of the nip forming member  85  in the belt moving direction. This configuration may prevent or reduce, in the nipped state, the nip forming member  85  from being moved by friction with the endless belt  83  against the biasing force of the helical compression spring S 1 . 
     The helical compression spring S 1  biases the plate member  89  that is more rigid than the pad  88 . This configuration may further stabilize the positions of the pad  88  and the nip portion NP. 
     The recess portion G of the downstream wall  86 C receives the projecting portion C of the plate member  89 , thereby preventing or reducing the nip forming member  85  from coming out of the holder  86 . 
     The recess portion G has a dimension in the front-rear direction that is longer than the dimension of the projecting portion C in the front-rear direction, so that an end of the projecting portion C may not contact an interior end (e.g., a most recessed portion) of the recessed portion G. This configuration may allow the biasing force of the helical compression spring S 1  to be effectively used as a force for pressing the pad  88  against the contact surface FT. 
     The helical compression spring S 1  is supported by the stay  87 , which is separate from the holder  86 . This configuration may favorably bias the nip forming member  85  toward the downstream wall  86 C of the holder  86 . 
     The helical compression spring S 1  is located to one side of the holder  86  in the right-left direction, so that the helical compression spring S 1  may be attached readily. 
     The first boss C 1  and the second boss C 2  engage in an internal space of the helical compression spring S 1  in its radial direction, thereby preventing or reducing the helical compression spring S 1  from coming off from the plate member  89  or the stay  87 . This configuration may hold the helical compression spring S 1  securely with the bosses C 1  and C 2 . 
     The helical compression spring S 1  is disposed under a portion of the side guide  90  and is surrounded by the belt guide portion  92  of the side guide  90 . This configuration may protect the helical compression spring S 1  with the side guide  90 . 
     One helical compression spring S 1  is disposed at a respective left and right end portion of the plate member  89 . This configuration may balance biasing forces applied by the helical compression springs S 1  to the plate member  89 . 
     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 Like numerals in the drawings denote like components and the detailed description of those components described above is omitted, with respect to  FIGS. 8A-22B . 
     In the illustrative embodiment, the helical compression spring S 1  serves as a biasing member. Examples of the biasing member may include heat resistant rubber and springs other than a helical compression spring. For example, a flat spring S 2  as depicted in  FIG. 8A , may serve as a biasing member. To use the flat spring S 2  as a biasing member, the plate member  89  and the stay  87  in the illustrative embodiment may be modified into a plate member  289  and a stay  287  in a first modification, as depicted in  FIGS. 8A and 8B . 
     The plate member  289  includes a base portion  89 A similar to that in the illustrative embodiment, and an extended portion  89 C, which is slightly different from the extended portion  89 B in the illustrative embodiment. The extended portion  89 C has a first engagement opening H 1 , e.g., a slot, instead of having the first boss C 1  in the illustrative embodiment. The first engagement opening H 1  receives or engages an end portion of the flat spring S 2 . 
     The stay  287  is different from the stay  87  of the illustrative embodiment in that the stay  287  does not include the protruding portion  87 E as in the illustrative embodiment, and includes a downstream extended portion  87 H, which is different from the downstream extended portion  87 F in the illustrative embodiment. The downstream extended portion  87 H has a second engagement opening H 2  that receives or engages another end portion of the flat spring S 2 . 
     The flat spring S 2  includes a base portion S 23  extending in the top-bottom direction, a first spring leg portion S 21  extending from an upper end of the base portion S 23  toward the front, and a second spring leg portion S 22  extending from a lower end of the base portion S 23  toward the front. The spring leg portions S 21  and S 22  have bends so that their respective distal end portions (e.g., front end portions) extend away from each other in the top-bottom direction. 
     As depicted in  FIG. 8B , the distal end portion of the first spring leg portion S 21  engages in the first engagement opening H 1  of the plate member  289  while the distal end portion of the second spring leg portion S 22  engages in the second engagement opening H 2  of the stay  287 . The distal end portion of the first spring leg portion S 21  engages the rear edge of the first engagement opening H 1 , to bias the plate member  289  toward the rear. This modification may also have advantages similar to those of the illustrative embodiment. 
     Examples of the biasing member may include a tension spring, e.g., a helical tension spring S 3 , as depicted in  FIG. 9 . To use the helical tension spring S 3  as a biasing member, the plate member  89  and the stay  87  in the illustrative embodiment may be modified into a plate member  389  and a stay  387  in a second modification, as depicted in  FIG. 9 . 
     The plate member  389  includes a base portion  89 A, similar to that in the illustrative embodiment, and an extended portion  89 D which is slightly different from the extended portion  89 B in the illustrative embodiment. The extended portion  89 D differs from the extended portion  89 B of the illustrative embodiment, in that the extended portion  89 D is disposed at a front end portion of the base portion  89 A and includes a first engagement opening H 11 , e.g., a circular hole, which receives one end portion of the helical tension spring S 3 . In  FIG. 9 , although the holder  86  is omitted for clarity of illustration, the restriction wall  86 D of the holder  86  may be, for example, spaced from the upstream wall  86 B, in association with the position of the extended portion  89 D. The restriction wall of the second modification may thus be located further to the rear than the restriction wall  86 D of the illustrative embodiment. 
     The stay  387  is different from the stay  87  of the illustrative embodiment in that the stay  387  does not include the protruding portion  87 E disposed at the upstream extended portion  87 D as in the illustrative embodiment but includes a protruding portion  87 J disposed at the downstream extended portion  87 F. The protruding portion  87 J includes a second engagement opening H 12  that receives or engages another end portion of the helical tension spring S 3 . In the second modification, the helical tension spring S 3  biases the plate member  389  toward the rear. This modification may also have advantages similar to those of the illustrative embodiment. 
     As depicted in  FIGS. 10A and 10B , examples of the biasing member may include a spring portion S 4  integrally formed with a plate member  489 . To use the spring portion S 4  integral with the plate member  489  as a biasing member, the holder  86  in the illustrative embodiment may be modified into a holder  486  in a third modification as depicted in  FIGS. 10A-10C . 
     The plate member  489  includes a base portion  89 A similar to that in the illustrative embodiment, and a spring portion S 4  disposed at a respective right and left end portions of the base portion  89 A (the left spring portion S 4  not depicted in  FIGS. 10A-10C ). 
     The base portion  89 A has an attachment surface FF to which the pad  88  is attached. 
     The attachment surface FF is a surface of the rectangular-shaped base portion  89 A closer to the heat roller  81  or an upper surface of the base portion  89 A. 
     The spring portion S 4  includes an elastically deformable portion S 41 , a connected portion S 42  located to the rear of the deformable portion S 41 , and a contact portion S 43  located to the front of the deformable portion S 41 . The deformable portion S 41  is a flat spring having a “V” shape in cross section, and deformable in the front-rear direction. 
     The deformable portion S 41  is located farther from the heat roller  81  than the pad  88  in the top-bottom direction. In other words, the deformable portion S 41  protrudes from the connected portion S 42  downward in a direction away from the pad  88 . 
     The connected portion S 42  extends rearward from the deformable portion S 41  and then leftward and connects to the base portion  89 A having the attachment surface FF. A rear end portion of the connected portion S 42  and a rear end portion of the base portion  89 A engage in the recess portion G of the holder  486 . The contact portion S 43  extends frontward from the deformable portion S 41  and contacts the holder  486 . 
     The holder  486  is different from the holder  86  of the illustrative embodiment in that the holder  486  does not include the restriction wall  86 D, and includes a base portion  486 A and an upstream wall  486 B, which are slightly different from the base portion  86 A and the upstream wall  86 B of the illustrative embodiment, respectively. The base portion  486 A includes an opening  86 G configured to receive the deformable portion S 41 . The opening  86 G extends through the base portion  486 A in the top-bottom direction and have an open right end. 
     The upstream wall  486 B includes an engagement recess portion  86 H that engages the contact portion S 43  of the spring portion S 4 . The engagement recess portion  86 H has an open rear end and an open right end. The engagement recess portion  86 H includes a second restriction surface F 2  that restricts the upward movement of the spring portion S 4  (e.g., movement in a direction from the nip forming member  85  toward the heat roller  81 ). 
     The recess portion G also has a second restriction surface F 2  that restricts the upward movement of the spring portion S 4 . As depicted in  FIG. 10C , the engagement recess portion  86 H has a first restriction surface F 1  that restricts the sideways movement (e.g., leftward movement) of the spring portion S 4 . In the third modification, the engagement recess portion  86 H has the first restriction surface F 1 . Alternatively, for example, the opening  86 G, may have a first restriction surface. 
     The third modification may have the following advantages. The deformable portion S 41  is located farther from the heat roller  81  than the pad  88  in the top-bottom direction. This configuration may prevent or reduce the deformable portion S 41  from contacting the endless belt  83 , for example, as compared with a configuration in which a deformable portion protrudes in a direction toward the pad  88 . 
     The first restriction surface F 1  restricts the sideways movement (e.g., leftward movement) of the spring portion S 4  in the right-left direction. This configuration may hold the plate member  489  in position relative to the holder  486  in the right-left direction. 
     The holder  486  includes the engagement recess portion  86 H that engages the contact portion S 43  of the spring portion S 4 . The contact portion S 43  may be inserted into the recess portion  86 H while the spring portion S 4  is being compressed. The plate member  489  may thus be attached or mounted to the holder  486  readily. 
     The second restriction surface F 2  may restrict the movement of the spring portion S 4  in the direction from the nip forming member  85  toward the heat roller  81 . This configuration may prevent or reduce the plate member  489  from coming out of the holder  486 . 
     Although the fuser  8  includes one nip forming member  85  in the illustrative embodiment, the fuser  8  may include, for example, two, nip forming members. 
     A fourth modification in which a fuser  8  includes two nip forming members will be described referring to  FIG. 11 . The fuser  8  may include a nip forming member  85 , and another nip forming member X separate from the nip forming member  85 . The nip forming member X may have configuration similar to that of the nip forming member  85 . 
     As depicted in  FIG. 11 , the fuser  8  further includes a flat spring S 5 , as an example of a biasing member and a second biasing member, and a holder  186  that is slightly different from the holder  86  of the illustrative embodiment. 
     The nip forming member X is configured to nip the endless belt  83  in cooperation with the heat roller  81  such that an upstream nip portion NPu is formed between the heat roller  81  and the endless belt  83 . The nip forming member X is located within a loop or an internal space of the endless belt  83 . The nip forming member X is disposed upstream of the nip forming member  85  in the belt moving direction. The nip forming member  85  is configured to nip the endless belt  83  in cooperation with the heat roller  81  such that a downstream nip portion NPd is formed between the heat roller  81  and the endless belt  83 . In the fourth modification, the nip forming member X is spaced from the nip forming member  85  in the belt moving direction. This configuration may create an intermediate nip portion NPi between the upstream nip portion NPu and the downstream nip portion NPd. A pressure unit  84  according to the fourth modification does not include, at the intermediate nip portion NPi, members or components that nip the endless belt  83  in cooperation with the heat roller  81 . Accordingly, less pressure may be applied by the pressure unit  84  to the intermediate nip portion NPi. This configuration may allow the sheet S passing through the intermediate nip portion NPi to receive heat from the heat roller  81  without receiving much pressure from the pressure unit  84 . The nip portion NP in the fourth modification is a portion where an outer peripheral surface of the endless belt  83  contacts the heat roller  81 . The nip portion NP may range from an upstream end of the upstream nip portion NPu to a downstream end of the downstream nip portion NPd. In the fourth modification, a state in which the nip portion NP is formed, as depicted in  FIG. 11 , is referred to as a “nipped state”, and a state in which the nip portion NP is not formed is referred to as a “nip released state”. 
     The nip forming member X includes a pad Y and a plate member Z. The pad Y is configured to nip the endless belt  83  in cooperation with the heat roller  81  between the pad Y and the heat roller  81  and to press the endless belt  83  against the heat roller  81 . The pad Y is fixed to the plate member Z. The pad Y is similar to the pad  88  in the illustrative embodiment. 
     The plate member Z is similar to the plate member  89  in the illustrative embodiment. 
     In one example, as depicted in  FIG. 12 , the plate member Z includes a projecting portion CA, a base portion ZA, and an extended portion ZB. The projecting portion CA is similar to the projecting portion C of the plate member  89 . The base portion ZA is similar to the base portion  89 A. The extended portion ZB is similar to the extended portion  89 B. The extended portion ZB of the plate member Z is located at a front end portion of the base portion ZA. 
     The holder  186  is slightly different from the holder  86  of the illustrative embodiment. 
     The holder  186  includes an upstream wall  186 B and a base portion  186 A, which are slightly different from the upstream wall  86 B and the base portion  86 A, respectively. The upstream wall  186 B is an example of a restricting member. 
     The upstream wall  186 B includes a contact surface FTA and a recess portion GA that are disposed at a rear (e.g., downstream) portion of the upstream wall  186 B. The contact surface FTA may contact the pad Y in the belt moving direction. The contact surface FTA is orthogonal to the belt moving direction. The contact surface FTA faces rearward or downstream in the belt moving direction. The recess portion GA is recessed into the contact surface FTA toward the front. 
     The recess portion GA is grooved to allow the projecting portion CA of the plate member Z to engage therein. The recess portion GA extends through the upstream wall  186 B in the right-left direction. The recess portion GA has a depth (e.g., a distance in the front-rear direction) that is greater than a projecting amount of the projecting portion CA relative to the pad Y in the front-rear direction. In other words, the relation between the depth of the recess portion GA and the projecting amount of the projecting portion CA in the belt moving direction relative to the pad Y is the same as the relation between the depth of the recess portion G and the projecting amount of the projecting portion C relative to the pad  88  in the belt moving direction. 
     The base portion  186 A includes a support surface FS that supports the plate members  89  and Z to allow the plate members  89  and Z to slidably move in the belt moving direction or the front-rear direction. The base portion  186 A further includes a recess portion CP and a projection PP. The recess portion CP is recessed into the base portion  186 A from a right end of the base portion  186 A. The projection PP is located in the recess portion CP, and projects rightward from a most recessed portion of the recess portion CP. 
     The flat spring S 5  is received in a space in the recess portion CP while the upward movement of the flat spring S 5  is restricted by the projection PP. 
     The flat spring S 5  may include resin or metal. The flat spring S 5  includes a base portion S 51 , an arm portion S 52 , and another arm portion S 53 . The base portion S 51  connects the two arm portions S 52  and S 53  to each other. The base portion S 51  is located below the projection PP. 
     The arm portion S 52  engages with the extended portion  89 B of the plate member  89 . 
     The arm portion S 52  extends upward from a rear end of the base portion S 51 , such that a top portion of the arm portion S 52  extends further toward the rear than a bottom portion of the arm portion S 52 . 
     The arm portion S 53  engages with the extended portion ZB of the plate member Z. The arm portion S 53  extends upward from a front end of the base portion S 51 , such that a top portion of the arm portion S 53  extends further toward the front than a bottom portion of the arm portion S 53 . 
     As depicted in  FIG. 13 , the flat spring S 5  is compressed between the extended portion  89 B of the plate member  89  and the extended portion ZB of the plate member Z, thereby biasing, in the belt moving direction, the plate member  89  toward the downstream wall  86 C and the plate member Z toward the upstream wall  186 B. In the fourth modification, the biasing member that biases the plate member  89  and the second biasing member that biases the plate member Z are integrated into one flat spring S 5 . 
     The flat spring S 5  compressed between the extended portion  89 B of the plate member  89  and the extended portion ZB of the plate member Z tends to move upward due to its restoring force. Since the base portion S 51  of the flat spring S 5  contacts the projection PP, the upward movement of the flat spring S 5  may be restricted. This configuration may prevent or reduce the flat spring S 5  from coming off from the plate members  89  and Z. 
     In the nipped state as depicted in  FIG. 11 , the plate members  89  and Z are biased by the flat spring S 5  toward the respective walls  86 C and  186 B. Accordingly, the pads  88  and Y may contact or abut against the respective walls  86 C and  186 B, thereby restricting the movements of the nip forming members  85  and X. In the nip released state, the pads  88  and Y may also contact or abut against the respective walls  86 C and  186 B and movements of the nip forming members  85  and X may be restricted. If the endless belt  83  is repeatedly nipped or released, the nip forming members  85  and X may be held in position relative to the holder  186 . This may stabilize the positions of the upstream nip portion NPu and the downstream nip portion NPd, as well as the nip portion NP. The pads  88  and Y may contact or abut against the respective walls  86 C and  186 B due to the biasing force of the flat spring S 5 . This configuration may hold the pads  88  and Y in position relative to the holder  186  and may stabilize the position of the nip portion NP if the nip forming members  85  and X should have manufacturing deviations, such as a positional deviation of the pads  88  and Y relative to the respective plate members  89  and Z (e.g., positional deviation caused when the pads  88  and Y are attached to the plate members  89  and Z). 
     In the fourth modification, the biasing member and the second biasing member are integrated into one flat spring S 5 . This configuration may reduce the number of components and costs of the fuser  8 . 
     A fifth modification will now be described referring to  FIGS. 14 and 15 . In the fifth modification, the biasing member and the second biasing member may be integrated into one flat spring S 6 , which is different from the flat spring S 5  of the fourth modification as depicted in  FIG. 11 . 
     A holder  186  according to the fifth modification is different from that of the fourth embodiment, in that the holder  186  according to the fifth modification has a downstream wall  86 C having a recess portion CP 1 , as depicted in  FIG. 15 . 
     The flat spring S 6  may include resin or metal. The flat spring S 6  includes a base portion S 61 , a spring portion S 62 , another spring portion S 63 , and an engaging portion S 64 . The base portion S 61  connects two spring portions S 62  and S 63  to each other. The base portion S 61  is located below the projection PP. 
     The spring portion S 62  biases the plate member  89  toward the downstream wall  86 C. The spring portion S 62  has a U-shaped cross section with an open end facing downward. The spring portion S 62  extends upward from a rear end of the base portion S 61  and then extends downward while making a U-turn. A rear portion of the spring portion S 62  extends downward below the base portion S 61 . The spring portion S 62  is disposed between the plate member  89  and the projection PP while being compressed in the belt moving direction. 
     The spring portion S 63  biases the plate member Z toward the upstream wall  186 B. The spring portion S 63  has a U-shaped cross section with an open end facing downward. The spring portion S 63  extends upward from a front end of the base portion S 61  and then extends downward while making a U-turn. The spring portion S 63  is disposed between the plate member Z and the projection PP while being compressed in the belt moving direction. 
     The engaging portion S 64  engages the holder  186 . The engaging portion S 64  extends rearward from a lower end of the spring portion S 62 . The downstream wall  86 C has the recess portion CP 1  that receives the engaging portion S 64 . The flat spring S 6  of the fifth modification also biases the nip forming members  85  and X toward the respective walls  86 C and  186 B. 
     A sixth modification will now be described referring to  FIGS. 16 and 17 . In the sixth modification, a helical compression spring S 7  may be disposed between plate members  89  and Z in a compressed state. The helical compression spring S 7  biases the nip forming member  85  toward the downstream wall  86 C and biases the nip forming member X toward the upstream wall  186 B. The sixth modification does not require the projection PP, so that a holder  186  of the sixth modification does not have the projection PP. To hold the helical compression spring S 7  between the plate members  89  and Z, the plate members  89  and Z have bosses C 3  and C 4 , respectively. The bosses C 3  and C 4  are sized to engage in an internal space of the helical compression spring S 7  in its diametrical direction. 
     A seventh modification will now be described referring to  FIGS. 18A and 18B . In the seventh modification, two nip forming members  585  and X 1  may be biased in a direction toward each other, unlike the fourth to sixth modifications, as depicted in  FIGS. 11-17 , in which two nip forming members  85  and X are biased in a direction away from each other. 
     The nip forming member  585  includes a pad  88 , which is similar to that of the fourth modification as depicted in  FIG. 11 , and a plate member  589 , which is slightly different from the plate member  89  of the fourth modification. The plate member  589  of this seventh modification includes components similar to those of the plate member  89  of the fourth modification. However, arrangements of the components are different between the fourth modification and the seventh modification. More specifically, the plate member  589  includes a base portion  89 A and an extended portion  89 B, which are similar to those of the fourth modification. The extended portion  89 B is located at a front end portion of the base portion  89 A, unlike the fourth modification. 
     The nip forming member X 1  includes a pad Y, which is similar to that of the fourth modification, and a plate member Z 1 , which is slightly different from the plate member Z of the fourth modification. The plate member Z 1  of this seventh modification has components similar to those of the plate member Z of the fourth embodiment. However, arrangements of the components are different between the fourth modification and the seventh modification. More specifically, the plate member Z 1  includes a base portion ZA and an extended portion ZB, which are similar to those of the fourth modification. The extended portion ZB is located at a rear end portion of the base portion ZA, unlike the fourth modification. 
     The holder  186  includes a projection PP 1  that extends upward from the support surface FS of the holder  186 . The projection PP 1  extends in the right-left direction from an end (e.g., right end) of the holder  186  to an opposite end (e.g., left end) of the holder  186 . The projection PP 1  has recess portions GB and GC. The recess portion GB receives a front (e.g., upstream) end of the plate member  589 . The recess portion GC receives a rear (e.g., downstream) end of the plate member Z 1 . The pad  88  is disposed relative to the plate member  589  such that, when the pad  88  is in contact with the projection PP 1 , the front end of the plate member  589  does not contact an interior end (e.g., a most recessed portion) of the recessed portion GB. The pad Y is disposed relative to the plate member Z 1  such that, when the pad Y is in contact with the projection PP 1 , the rear end of the plate member Z 1  does not contact an interior end (e.g., a most recessed portion) of the recess portion GC. The projection PP 1  is an example of a restricting member. 
     The nip forming members  585  and X 1  are biased by a flat spring S 8  toward the projection PP 1 . The flat spring S 8  includes a base portion S 81 , a spring portion S 82 , and another spring portion S 83 . 
     The base portion S 81  connects the two spring portions S 82  and S 83  to each other. The base portion S 81  includes a flat portion extending in the front-rear direction or the belt moving direction, a slanting portion extending upward and rearward from a rear end of the flat portion, and another slanting portion extending upward and frontward from a front end of the flat portion. At least a portion of the base portion S 81  is disposed below the projection PP 1 . 
     The spring portion S 82  biases the plate member  589  toward the projection PP 1 . The spring portion S 82  has a U-shaped cross section with an open end facing downward. The spring portion S 82  extends upward from a rear end of the base portion S 81  and then extends downward while making a U-turn. The spring portion S 82  is disposed between the plate member  589  and the downstream wall  86 C while being compressed in the belt moving direction. 
     The spring portion S 83  biases the plate member Z 1  toward the projection PP 1 . The spring portion S 83  has a U-shaped cross section with an open end facing downward. The spring portion S 83  extends upward from a front end of the base portion S 81  and then extends downward while making a U-turn. The spring portion S 83  is disposed between the plate member Z 1  and the upstream wall  186 B while being compressed in the belt moving direction. In the seventh modification, the nip forming members  585  and X 1  are biased toward the projection PP 1 , so that the pads  88  and Y may contact or abut against the projection PP 1 . This may achieve effects similar to those of the illustrative embodiment. 
     An eighth modification will now be described referring to  FIGS. 19A and 19B . In the eighth modification, two nip forming members  685  and X 2  may be biased toward a projection PP 1  by a helical tension spring S 9 , which is different from the flat spring S 8 , of the seventh modification, that biases the two nip forming members  585  and X 1  toward the projection PP 1 . 
     In the eighth modification, the nip forming member  685  includes a pad  88 , which is similar to that of the seventh modification as depicted in  FIGS. 18A and 18B , and a plate member  689 , which is slightly different from the plate member  585  of the seventh modification. The plate member  689  includes a base portion  89 A, which is similar to that of the seventh modification, an extended portion  689 B, and another extended portion  689 C. 
     The extended portion  689 B engages with an end of the helical tension spring S 9 . The extended portion  689 B extends rightward from a right end of the base portion  89 A. 
     The extended portion  689 C serves to prevent the end of the helical tension spring S 9  from coming out of the extended portion  689 B. The extended portion  689 C extends in the front-rear direction from a right end of the extended portion  689 B. 
     The nip forming member X 2  includes a pad Y, which is similar to that of the seventh modification, and a plate member Z 2 , which is slightly different from the plate member Z 1  of the seventh modification. The plate member Z 2  includes a base portion ZA, which is similar to that of the fourth modification as depicted in  FIG. 12 , an extended portion Z 21 , and another extended portion Z 22 . The extended portions Z 21  and Z 22  are similar to the extended portion  689 B and the extended portion  689 C, respectively. The extended portion Z 21  engages with an opposite end of the helical tension spring S 9 . The extended portion Z 22  serves to prevent the opposite end of the helical tension spring S 9  from coming out of the extended portion Z 21 . The eighth modification may also enable the two nip forming members  685  and X 2  to be biased toward the projection PP 1. 
     A ninth modification will now be described referring to  FIG. 20 . In the ninth modification, two nip forming member  785  and X 3  may be biased toward the projection PP 1  by a flat spring S 10 . 
     The nip forming member  785  includes a pad  88 , which is similar to that of the eighth modification as depicted in  FIGS. 19A and 19B , and a plate member  789 , which is slightly different from the plate member  689  of the eighth modification. The plate member  789  includes a base portion  89 A, which is similar to that of the eighth modification, an extended portion  689 B and another extended portion  689 C (not depicted). The extended portion  689 B extends rightward from a right end of the base portion  89 A, similar to the extended portion  689 B of the eighth modification. The extended portion  689 B is located at a front end portion of the base portion  89 A, unlike the eighth modification. 
     The nip forming member X 3  includes a pad Y, which is similar to that of the eighth modification, and a plate member Z 3 , which is slightly different from the plate member Z 2  of the eighth modification. The plate member Z 3  includes a base portion ZA, which is similar to that of the eighth modification, an extended portion Z 21  and another extended portion Z 22  (not depicted). The extended portion Z 21  extends rightward from a right end of the base portion ZA, similar to the extended portion Z 21  of the eighth modification. The extended portion Z 21  is located at a rear end portion of the base portion ZA, unlike the eighth modification. 
     The flat spring S 10  has a U-shaped cross section. The flat spring S 10  has a rear end engaging with the extended portion  689 B of the plate member  789  and a front end engaging with the extended portion Z 21  of the plate member Z 3 . The ninth modification may also enable the nip forming members  785  and X 3  to be biased toward the projection PP 1 . 
     A tenth modification will now be described referring to  FIGS. 21A and 21B . In the tenth modification, two nip forming members  885  and X 1  may be biased by a flat spring S 11  toward the rear or a downstream side in the belt moving direction. The nip forming member  885  is slightly different from the nip forming member  85  of the illustrative embodiment as depicted in  FIG. 3 . The nip forming member X 1  is similar to the nip forming member X 1  of the seventh modification as depicted in  FIGS. 18A and 18B . 
     The nip forming member  885  includes a pad  88 , which is similar to that of the illustrative embodiment (in  FIG. 3 ) and a plate member  889 , which is slightly different from the plate member  89  of the illustrative embodiment. The plate member  889  includes a base portion  89 A and an extended portion  89 B, which are similar to those of the illustrative embodiment, but does not include the first boss C 1 , which the plate member  89  of the illustrative embodiment includes. The nip forming member X 1  of the tenth modification includes a plate member Z 1  including an extended portion ZB. The extended portion ZB is located at a position different from the extended portion  89 B of the plate member  889  in the right-left direction. 
     The holder  186  includes a projection PP 2  that is elongated in the right-left direction, similar to the projection PP 1  of the seventh modification as depicted in  FIGS. 18A and 18B . Unlike the projection PP 1  of the seventh modification, the projection PP 2  has a stepped portion PP 21  at an end thereof in the right-left direction (e.g., a right end), so that a base portion S 111  (described below) of the flat spring S 11  may not interfere with the stepped portion PP 21 . The projection PP 2  is an example of a restricting member. 
     The projection PP 2  includes a recess portion GC, which is similar to that of the seventh modification, but does not include a recess portion GB. The flat spring S 11  includes the base portion S 111 , a spring portion S 112 , and another spring portion S 113 . 
     The spring portion S 112  biases the plate member  889  toward the downstream wall  86 C. The spring portion S 112  has a U-shaped cross section with an open end facing upward. The spring portion S 112  is disposed between the projection PP 2  and the extended portion  89 B of the plate member  889  while being compressed in the belt moving direction. 
     The spring portion S 113  biases the plate member Z 1  toward the projection PP 2 . The spring portion S 113  has a U-shaped cross section with an open end facing upward. The spring portion S 113  is disposed between the upstream wall  186 B and the extended portion ZB of the plate member Z 1  while being compressed in the belt moving direction. 
     The base portion S 111  connects the spring portions S 112  and S 113  to each other. The base portion S 111  has a portion extending rearward from the spring portion S 113 , another portion extending rightward from the rear end of the portion, and still another portion extending frontward from the right end of the other portion and connecting to the spring portion S 112 . 
     In the tenth modification, the two nip forming members  885  and X 1  are biased toward the downstream wall  86 C and the projection PP 2 , respectively, so that the pads  88  and Y may contact or abut against the respective downstream wall  86 C and the projection PP 2 . This configuration may also achieve effects similar to those of the illustrative embodiment. In the tenth modification, the nip forming members  885  and X 1  are both biased toward the rear or a downstream side in the belt moving direction. This configuration may prevent or reduce the nip forming members  885  and X 1  from being moved by friction with the endless belt  83  against the biasing force of the flat spring S 11 . 
     An eleventh modification will now be described referring to  FIGS. 22A and 22B . In the eleventh modification, two nip forming members  985  and X 4  may be biased by a flat spring S 12  toward the rear or a downstream side in the belt moving direction. 
     The nip forming member  985  includes a pad  88 , which is similar to that of the tenth modification as depicted in  FIGS. 21A and 21B , and a plate member  989 , which is slightly different from the plate member  889  of the tenth modification. The plate member  989  includes a base portion  89 A, which is similar to that of the tenth modification, and an extended portion  989 B that extends frontward from a right end of the base portion  89 . The extended portion  989 B has a recess  989 C at a front end thereof. 
     The nip forming member X 4  includes a pad Y, which is similar to that of the tenth modification, and a plate member Z 4 , which is slightly different from the plate member Z 1  of the tenth modification. The plate member Z 4  includes a base portion ZA, which is similar to that of the tenth modification. The base portion ZA has a recess Z 41  at a front right end portion thereof. The recess Z 41  of the plate member Z 4  is located between the recess  989 C of the plate member  989  and the pad Y in the right-left direction. 
     The plate member Z 4  is located below the plate member  989 . In one example, a holder  186  of the eleventh modification includes a support surface FS 1  that supports the plate member  989  such that the plate member  989  is movable in the belt moving direction, and a support surface FS 2  that supports the plate member Z 4  such that the plate member Z 4  is movable in the belt moving direction. The support surface FS 1  is located above the support surface FS 2 . 
     The flat spring S 12  includes a base portion S 121 , a spring portion S 122 , and another spring portion S 123 . 
     The spring portion S 122  biases the plate member  989  toward the downstream wall  86 C. The spring portion S 122  has a U-shaped cross section with an open end facing upward. The spring portion S 122  is disposed between the upstream wall  186 B and the extended portion  989 B (e.g., the recess  989 C) of the plate member  989  while being compressed in the belt moving direction. 
     The spring portion S 123  biases the plate member Z 4  toward a projection PP 2 , which is similar to that of the tenth modification. The projection PP 2  has a stepped portion PP 21 , so that the extended portion  989 B of the plate member  989  may not interfere with the stepped portion PP 21 . The spring portion S 123  has a U-shaped cross section with an open end facing upward. The spring portion S 123  is disposed between the upstream wall  186 B and the plate member Z 4  (e.g., the recess Z 41 ) while being compressed in the belt moving direction. 
     The base portion S 121  connects the spring portions S 122  and S 123  to each other. The base portion S 121  is connected to upper ends of the spring portions S 122  and S 123 . 
     The eleventh modification may also enable the two nip forming members  985  and X 4  to be biased toward the downstream wall  86 C and the projection PP 2 , respectively. 
     In the illustrative embodiment, each of the two biasing members biases a respective one of the right and left end portions of the plate member. In another embodiment, for example, one, biasing member may bias a central portion of the plate member in its longitudinal direction (e.g., the right-left direction). The biasing member and the second biasing member may be separate members. 
     In the illustrative embodiment, the pad  88  is pressed against the inner peripheral surface  83 A of the endless belt  83 . In another embodiment, for example, a slide sheet may be disposed between the inner peripheral surface of the endless belt and the pad for smooth rotation of the endless belt. 
     In the illustrative embodiment, the restricting member is integral with the holder  86 . In another embodiment, a restricting member may not be integral with the holder but may be a member separate from the holder. 
     In the illustrative embodiment, the restricting member, e.g., the downstream wall  86 C, is disposed downstream of the nip forming member  85  in the belt moving direction. In another embodiment, a restricting member may be disposed upstream of a nip forming member in the belt moving direction. 
     In the illustrative embodiment, the plate member  89  is a relatively thin plate. In another embodiment, a plate member may be a relatively thick member having a thickness greater than the plate member  89 . 
     In the illustrative embodiment, configuration, according to one or aspects of the disclosure, that serves to form a nip portion NP is applied to the fuser  8 . In another embodiment, configuration according to one or aspects of the disclosure may be applied to a sheet conveying device other than the fuser. For example, in a sheet conveying device including a conveying roller and a conveying belt configured to convey a sheet by holding the sheet between the conveying roller and the conveying belt, configuration according to one or more aspects of the disclosure may be applied to the conveying belt. 
     In the illustrative embodiment, the pad  88  has a rectangular parallelepiped shape. In another embodiment, a pad may have a shape different from the rectangular parallelepiped shape. 
     In the illustrative embodiment, the halogen lamp is used as the heater  82 . In another embodiment, a carbon heater may be used as the heater  82 . 
     In the illustrative embodiment, the heat roller  81  having the heater  82  therein is illustrated as a rotatable member. Examples of the rotatable member may include an endless heating belt whose inner peripheral surface may be heated by a heater. 
     A fuser may include an external heater that heats an outer peripheral surface of a rotatable member, or an induction heating (“IH”) element. A rotatable member contacting an endless belt may be indirectly heated by a heater disposed within an interior space of the endless belt. A heater may be disposed within an interior space of each of the rotatable member and the endless belt. 
     Configuration, according to one or more aspects of the disclosure, that serves to form a nip portion NP may be applied to various types of fusers. For example, in a fuser including a fuser roller, a pressure roller that forms a nip portion NP between the fuser roller and the pressure roller, and a heater unit that contacts the fuser roller at a predetermined pressure and heats the fuser roller, the fuser being configured to fuse a toner image onto a sheet at the nip portion NP, configuration according to one or aspects of the disclosure may be applied to the heater unit. For example, if the heater unit includes an endless belt and a heating member that nips the endless belt in cooperation with the fuser roller between the heating member and the fuser roller, the heating member may be biased by a biasing member. 
     In the illustrative embodiment, aspects of the disclosure are applied to the laser printer  1 . In another embodiment, aspects of the disclosure may be applied to other types of image forming apparatuses, such as copiers and multi-functional devices. 
     Each of the elements or components which have been described in the illustrative embodiment and modifications may be used in any combination.