Patent Publication Number: US-9851669-B2

Title: Fixing device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fixing device included in the image forming apparatus using electrophotographic technology. 
     Description of the Related Art 
     In general, fixing devices mounted in an image forming apparatus, such as a copying machine and a laser printer, convey a recording medium through a nip portion formed by a first fixing member and a second fixing member that are in pressure contact with each other and heat-fix an unfixed toner image onto the recording medium. 
     Among such fixing devices, some fixing devices include a pair of pressure mechanisms that urge both ends of the first fixing member against the second fixing member using the elastic force of a helical compression spring so that the first fixing member and the second fixing member to are in pressure contact with each other. To improve the pressure balance between the two pressure mechanism, a configuration that aligns the winding end positions of the helical compression springs disposed at both ends has been developed (refer to Japanese Patent No 3501616). However, the fixing device described in Japanese Patent No. 3501616 has the following issues. That is, by aligning the positions of the winding ends of the helical compression springs, the pressures at both the ends of the fixing member are forced to be the same. In such a technology, since at the ends of the helical compression spring, the protrusion level of the spring winding end of the coil in the axial direction of the coil is the highest, the portions in the vicinity of the spring winding ends receive a large reaction force from spring supporting portions, as indicated by outlined arrows illustrated in  FIG. 11 . Each of arrows in  FIG. 11  indicates the magnitude of a reaction force received by a helical compression spring  87  from a spring support member (the length of the arrow) and the direction of the reaction force (the direction of the arrow). According to the fixing device described in Japanese Patent No. 3501616, the helical compression spring  87  receives reaction forces F 11  and F 12  in one of two spring support areas thereof in the cross section that passes through an axial line  87   s  of the helical compression spring  87  and reaction forces F 13  and F 14  in the other spring support area. The reaction force F 11  in the vicinity of the spring winding end is larger than the reaction force F 12 . The reaction force F 14  in the vicinity of the spring winding end is larger than the reaction force F 13 . Accordingly, the helical compression spring  87  does not receive a uniform reaction force from the supporting portion. Consequently, a force that rotates the helical compression spring  87  is easily generated. As a result, the direction of action of a force Fs of the helical compression spring  87  is inclined from the direction of a pressure Ft applied in the nip portion and, thus, loss of the pressure applied in the nip portion easily occurs. 
     In addition, a configuration that corrects the balance between the reaction forces exerted on a helical compression spring by cutting and grinding the spring terminals has been developed. However, if the helical compression spring having cut and ground spring ends is employed in fixing devices, the cost increases. In addition, the following issue arises. That is, the helical compression spring having cut and ground spring ends has a small thickness of the coil in the vicinity of the winding end and, thus, the rigidity easily decreases. If a high load is imposed on the thin coil portion, the spring deforms. As a result, the pressure in the nip portion decreases. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a fixing device for fixing a toner image onto a recording medium by conveying and heating the recording medium on which the toner image is formed at a nip portion is provided. The fixing device includes a first fixing member, a second fixing member configured to form the nip portion together with the first fixing member, a frame configured to support the second fixing member, and a pair of pressure mechanisms provided on either end of the first fax member in a longitudinal direction of the first fixing member. The pressure mechanisms urge the first fixing member against the second fixing member. Each of the pressure mechanisms includes a lever having one end supported by the frame in a rotatable manner in a pressure direction in which the first fixing member is urged and a helical compression spring disposed between a first spring support portion provided on the other end of the lever and a second spring support portion provided on the frame. The pressure mechanism urges the first fixing member against the second fixing member via the lever by an elastic force of the spring. At least one of the first spring supporting portion and the second spring supporting portion includes a first supporting area and a second supporting area closer to the spring in an axial direction of the spring than the first supporting area, the first supporting area is in contact with an area of the spring close to a winding end of the spring, and the second supporting area is in contact with an area of the spring farther away from the winding end in a winding direction of the spring than the first supporting area. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are transverse sectional views of a fixing device according to a first exemplary embodiment. 
         FIGS. 2A and 2B  are schematic illustrations of the fixing device according to the first exemplary embodiment. 
         FIG. 3  is a schematic perspective view of the fixing device according to the first exemplary embodiment. 
         FIG. 4  is a schematic side view of the fixing device according to the first exemplary embodiment. 
         FIGS. 5A to 5C  are schematic perspective views and cross sectional views of a spring support member of the fixing device according to the first exemplary embodiment. 
         FIG. 6  is a schematic side view of the fixing device according to the first exemplary embodiment. 
         FIGS. 7A to 7D  are a schematic perspective view of the fixing device and schematic illustrations of a spring and the spring support member according to the first exemplary embodiment. 
         FIG. 8  is a schematic side view of the fixing device according to the first exemplary embodiment. 
         FIGS. 9A to 9C  are schematic side views of the fixing device according to an exemplary embodiment using an open-end helical compression spring. 
         FIGS. 10A and 10B  are a perspective view and side views of a spring support member of an exemplary embodiment using an open-end helical compression spring. 
         FIG. 11  is a schematic side view of an existing fixing device. 
         FIGS. 12A and 12B  are schematic illustrations of a fixing device according to a second exemplary embodiment. 
         FIG. 13  is a perspective view of the fixing device according to the second exemplary embodiment. 
         FIGS. 14A and 14B  are side views of the fixing device according to the second exemplary embodiment. 
         FIGS. 15A and 15B  are side views of a helical compression spring according to the second exemplary embodiment. 
         FIGS. 16A and 16B  are schematic illustrations of the helical compression spring according to the second exemplary embodiment. 
         FIGS. 17A and 17B  are schematic illustrations of the fixing device according to the second exemplary embodiment. 
         FIGS. 18A and 18B  are schematic illustrations of a fixing device according to Comparative Example 1. 
         FIGS. 19A and 19B  are schematic illustrations of a fixing device according to Comparative Example 2. 
         FIG. 20  is a side view of a fixing device according to a third exemplary embodiment. 
         FIGS. 21A and 21B  are schematic illustrations of the fixing device according to the third exemplary embodiment. 
         FIGS. 22A and 22B  are schematic illustrations of a fixing device according to Comparative Example 3. 
         FIGS. 23A and 23B  are schematic illustrations of a fixing device according to Comparative Example 4. 
         FIG. 24  is a perspective view of a fixing device according to a fourth exemplary embodiment. 
         FIG. 25  is a side view of the fixing device according to the fourth exemplary embodiment. 
         FIG. 26  illustrates an intersect angle of a fixing device according to a comparative example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     First Exemplary Embodiment 
     A fixing device  72  according to the present exemplary embodiment is described below with reference to  FIGS. 1A and 1B  and  FIGS. 2A and 2B . Note that in the following description, the term “longitudinal direction” of a member that constitutes the fixing device refers to a direction perpendicular to the recording medium conveyance direction.  FIG. 1A  is a schematic cross-sectional view of the fixing device  72  viewed in the longitudinal direction.  FIG. 1B  is an enlarged view of a nip portion of the fixing device  72 .  FIG. 2A  is a schematic illustration of the fixing device  72  when viewed from a film  10  side in the fixing device.  FIG. 2B  is a schematic illustration of the fixing device  72  when viewed from the downstream side in a recording medium conveyance direction. 
     According to the present exemplary embodiment, the fixing device  72  includes a cylindrical film  10 , a heater  30  in contact with the inner peripheral surface of the film  10 , and a pressure roller  20 . The heater  30  forms a fixing nip portion N 2  together with the pressure roller  20  via the film  10 . The fixing device  72  conveys, in the fixing nip portion N 2 , a recording medium having a toner image formed thereon and, simultaneously, heats the toner image. Thus, the toner image is fixed onto the recording medium. The fixing device  72  further includes a heater holder  41  that supports the heater  30 , a pressure stay  42  that increases the bending rigidity, and a fixing flange  45  serving as a regulating member that regulates the movement of the film  10  in the longitudinal direction. 
     The film  10 , the heater  30 , the heater holder  41 , the pressure stay  42 , and the fixing flange  45  are integrated into a film unit (a first fixing member). According to the present exemplary embodiment, the fixing device  72  is configured to urge the film unit against the pressure roller  20  (a second fixing member). 
     The film  10  includes a base layer  11  and the release layer  12  provided on the outer surface of the base layer  11 . In addition, to increase fixability, an elastic layer  13  formed of, for example, silicone rubber may be disposed between the base layer  11  and the release layer  12 . If the elastic layer  13  is provided, an unfixed toner image T borne by a recording medium P can be encompassed and, thus, the heat can be uniformly provided to the toner image. It is desirable that the thickness of the elastic layer  13  be 50 μm and greater and 500 μm or less in order to reduce the warm-up time. The base layer  11  can be generated by forming a thin-wall metal having a high thermal conductivity, such as SUS or Ni, or a heat resistant resin, such as polyimide resin, a polyamide-imide resin, or PEEK, into a thin-wall flexible continuous belt. To form the release layer  12 , a fluorine contained resin, such as PFA, PTFE, FEP, or a mixture thereof, is coated on the outer surface of the base layer  11 . Alternatively, the outer surface of the base layer  11  is covered by a tube made of the above-described resin. To increase the durability of the release layer  12 , it is desirable that the thickness of the release layer  12  be 5 μm and greater. In addition, if the release layer  12  is too thick, the thermal conductivity decreases and, thus, the fixability decreases. Accordingly, it is desirable that the thickness of the release layer  12  be 50 μm and less. 
     The heater holder  41  is made of liquid crystal polymer, a phenol resin, PPS, or PEEK. The heater holder  41  is formed so as to have a transverse section in the shape of a half-moon gutter. The lower surface of the heater holder  41  (a surface adjacent to the pressure roller  20 ) has a groove  41   a  having a recess shape formed along the longitudinal direction of the heater holder  41 . The heater  30  is supported by the groove  41   a . The film  10  is loosely fitted onto the outer periphery of the heater holder  41 . Both ends of the heater holder  41  having the loosely fitting film  10  are supported by both ends of a frame  91  via the fixing flanges  45 . As illustrated in  FIG. 1B , the heater holder  41  includes a protrusion  41   b  on the downstream side in the recording medium conveyance direction. In the fixing nip portion N 2 , the protrusion  41   b  extends in the longitudinal direction along a portion of the heater holder  41  in contact with the inner peripheral surface of the film  10 . The protrusion  41   b  protrudes from a sliding surface of the heater  30  that slides or the film  10  toward the outer surface of the film  10  by a protrusion amount h. The protrusion  41   b  is disposed so as to be located at the same position in the recording medium conveyance direction throughout its length. According to the fixing device of the present exemplary embodiment, the protrusion amount h is set to 0.2 mm. As illustrated in  FIG. 1B , a contact portion is divided into two contact portions, that is, a contact portion of the film  10  and the heater  30  and a contact portion of the film  10  and the heater holder  41 . As used herein, the term “sliding surface” refers to the contact portion between the film  10  and the heater  30 . 
     As illustrated in  FIG. 1A , the pressure roller  20  includes a core shaft portion  21 , an elastic layer  22  disposed on the outer surface of the core shaft portion  21 , and a release layer  24  disposed on the outer surface of the elastic layer  22 . The elastic layer  22  can be formed of, for example, silicone rubber or fluorine-contained rubber. To form the release layer  24 , a fluorine contained resin, such as PEA, PTFE, FEP, or a mixture thereof, is coated. Alternatively, a tube made of the above-described resin is used as the release layer  24 . According to the present exemplary embodiment, the core shaft portion  21  is formed from an iron core shaft having φ22, and the elastic layer  22  is formed of the silicone rubber having a thickness of 4 mm. The release layer  24  is formed from a PFA tube having a thickness of 50 μm. 
     The heater  30  is in contact with the inner peripheral surface of the film  10  and heats the film  10 . The heater  30  includes an elongated substrate extending in the longitudinal direction. The substrate can be formed as a ceramic (e.g., alumina or aluminum nitride) substrate or a heat resistant resin (e.g., polyimide, PPS, or liquid crystal polymer) substrate. The substrate has a heating resistor layer on the back surface thereof (a surface remote from the pressure roller  20 ) along the longitudinal direction of the substrate. The beating resistor layer is applied to the substrate in a band-like shape. The heating resistor layer is formed of, for example, Ag/Pd (silver-palladium), RuO 2 , or Ta 2 N. In addition, the substrate has glass coat on the back surface thereof in order to protect the heating resistor layer and ensure electrical insulation. Furthermore, the substrate has a sliding layer on a surface thereof that is in contact with the inner peripheral surface of the film  10  in order to increase the slidability. The sliding layer is formed of, for example, a heat resistant resin (e.g., a polyimide or polyamide-imide resin) or glass coat. According to the present exemplary embodiment, the size of the substrate of the heater  30  is 350 mm in the longitudinal direction, 10 mm in the short direction, and 0.6 mm in the thickness direction. 
     The pressure stay  42  is formed into a U shape using a material having rigidity (e.g., a metal). The pressure stay  42  is disposed on the upper surface of the heater holder  41  (a surface distant from the pressure roller  20 ) inside the film  10 . The pressure stay  42  urges both ends of the pressure stay  42  in the longitudinal direction toward the axial line of the pressure roller  20  via the fixing flange  45  supported by the frame  91 . Thus, the heater  30  is urged against the surface of the pressure roller  20  via the film  10 , and an inner nip N 3  having a predetermined width is formed between the heater  30  and the film  10 . In addition, a fixing nip N 2  having a predetermined width is formed between the film  10  and the pressure roller  20 . Heat necessary for the heat fixing of the unfixed toner image T is transferred from the heater  30  to the film  10  in the inner nip N 3 , and the heat is transferred from the film  10  to the recording medium P in the fixing nip N 2 . At that time, the recording medium is conveyed. 
     Upon receiving a print instruction, a control unit  44  drives a motor serving as a driving source to rotate a drive gear disposed at an end of the core shaft portion  21  of the pressure roller  20  in the longitudinal direction. Thus, the pressure roller  20  rotates at a predetermined circumferential velocity in a direction of an arrow. At that time, a rotary force that attempts to rotate the film  10  in a direction opposite to the rotational direction of the pressure roller  20  is exerted on the film  10  due to a frictional force generated between the surface of the pressure roller  20  and the surface of the film  10  in the fixing nip N 2 . In this manner, the film  10  is driven to rotate in the direction of the arrow at a circumferential velocity that is substantially the same as that of the pressure roller  20  outside the heater holder  41  with the inner peripheral surface of the film  10  in contact with the sliding layer of the heater  30 . 
     A thermistor  35  serving as a temperature detecting unit detects the temperature of the film  10  and outputs a temperature detection signal to the control unit  44 . The thermistor  35  is disposed so as to be capable of detecting the temperature of an area through which the recording medium P having any of all the sizes allowable for the fixing device  72  passes. The control unit  44  receives the temperature detection signal from the thermistor  35  and controls the power supplied to the heating resistor layer on the basis of the temperature detection signal so that the film  10  has a predetermined target temperature. In this manner, the recording medium P having the unfixed toner image T thereon is led to the fixing nip N 2  along an entry guide  28  with the temperature of the film  10  maintained at the predetermined target temperature. Thereafter, the recording medium P is pinched by the film  10  and the pressure roller  20  and is conveyed. In the conveyance stage, the heat of the film  10  heated by the heater  30  and the pressure from the fax nip N 2  are applied to the recording medium P. Due to the heat and pressure, the unfixed toner image T is fixed onto the surface of the recording medium P. After passing through the fixing nip N 2 , the recording medium P is separated from the film  10  by self stripping and is elected by the conveyance roller  26 . The pressure mechanism according to the present exemplary embodiment is described below with reference to  FIGS. 3 and 4 .  FIG. 3  is a perspective view of the fixing device  72 .  FIG. 4  is a side view of the fixing device  72  viewed in a direction of an arrow R in  FIG. 3 . The pressure roller  20  is rotatably supported by a frame  91  disposed at both ends of the pressure roller  20  in the longitudinal direction via bearings  120 . A guide portion  91   a  that regulates the direction in which the film unit is pressed is disposed on the frame  91 . 
     Each of a pair of the pressure mechanisms includes a lever  84 , a turning center  91   b  of the lever  84  and a spring support portion  93  (a second spring support portion) provided in the frame  91 , and a helical compression spring  87 . The pressure mechanisms are provided at either end of the film  10  in the longitudinal direction. 
     The lever  84  is a member having one end supported by the turning center  91   b  in the frame  91  in a rotatable manner in a direction in which the film  10  is pressed. 
     The helical compression spring  87  is disposed and compressed between a spring support portion  840  (a first spring support portion) provided at the other end of the lever  84  and a spring support portion  93  of the frame  91 . The other end of the lever  84  supports a lower end  87   a  of the helical compression spring  87 . In contrast, the spring support portion  93  is provided in the frame  91  and supports an upper end  87   b  of the helical compression spring  87 . The spring support portion  93  has a function of regulating the height of the helical compression spring  87  so that the pressure of the helical compression spring  87  is maintained at a predetermined pressure (a specified load). According to the present exemplary embodiment, the helical compression spring  87  has a free height of 35 mm and a specified height of 27 mm upon pressurization. The lever  84  can rotate about the turning center  91   b  due to the elastic force of the helical compression spring  87  and exerts a pressure Ft on the fixing flange  45  via the lever  84 . Thus, the lever  84  can urge the film unit against the pressure roller  20 . Note that by moving the lever  84  in a direction in which the helical compression spring  87  is compressed using a cam member  95 , the pressure applied in the fixing nip N 2  can be released. 
     The spring supporting portion according to the present exemplary embodiment is described below with reference to  FIG. 4 . The helical compression spring  87  is wound in a right hand direction. The number of effective turns of the helical compression spring  87  is 10. The helical compression spring  87  is a closed-end spring. The helical compression spring  87  is compressed and supported between the spring support portion  93  to which the helical compression spring  87  is fixed and the spring support portion  840  of the lever  84 .  FIGS. 5A and 5B  are external perspective views of the spring support portion  93 . As illustrated in  FIG. 5B , a mounting surface of the spring support portion  93  that is mounted on the frame  91  has positioning pins  93   f  formed therein. Each of the positioning pins  93   f  is aligned with a mounting hole  91   c  of the frame  91 . Thus, the spring support portion  93  is mounted on the frame  91  in place. As illustrated in  FIG. 5A , a spring support surface of the spring support portion  93  has a groove  93   c  formed therein around a cylindrical portion  93   a . A front end portion of the groove  93   c  includes a flat portion  93   d  that receives a spring winding end  87   c  (refer to  FIG. 6 ) of the helical compression spring  87  mounted thereon. As illustrated in  FIG. 5C , the depth of the groove  93   c  gradually decreases in a direction away from the flat portion  93   d.    
       FIG. 6  is an enlarged side view of the supporting portion of the helical compression spring  87  of the fixing device  72 . The lever  84  is a plate member (a plate). An edge portion of the lever  84  has a convex portion  84   b  that is inserted into an inner-diameter portion of the helical compression spring  87  and a spring support area  84   c  (a second spring support area) and a spring support area  84   d  (a first spring support area) formed on either side of the convex portion  84   b . When viewed in the axial direction of the helical compression spring  87 , the spring support area  84   c  and the spring support area  84   d  are disposed so as to be symmetrical with respect to the axial center of the helical compression spring  87 . The spring support area  84   d  that is further away from the turning center  91   b  of the lever  84  is located at a height which is stepped down from the spring support area  84   c , in terms of a plane perpendicular to the axial line  87   s  of the mounted helical compression spring. That is, the spring support area  84   d  is offset from the spring support area  84   c  in the axial direction of the helical compression spring  87  away from the helical compression spring  87 . 
       FIG. 7A  is an external perspective view of the fixing device  72  when viewed from below at an angle.  FIGS. 7B, 7C, and 7D  illustrate only the helical compression spring  87  and the spring support portion  93  assembled together. As illustrated in  FIGS. 7B, 7C, and 7D , at the upper end  87   b  of the helical compression spring  87 , the spring winding end  87   c  is brought into contact with the flat portion  93   d  of the spring support portion  93  and, thus, the position of the upper end  87   b  of the helical compression spring  87  in the rotational direction about the axial line of the helical compression spring  87  is regulated. The coil portion extending from the spring winding end  87   c  is supported by the groove  93   c  of the spring support portion  93 . As illustrated in  FIG. 7C , since the number of effective turns of the spring is an integer, the other spring winding end  87   d  supported by the lever  84  is located beneath the spring winding end  87   c . At the lower end  87   a  of the helical compression spring  87 , a portion close to the spring winding end  87   d  in the winding direction of the helical compression spring  87  is in contact with the spring support area  84   d  of the lever  84  and, thus, is supported by the spring support area  84   d , and a portion distant from the spring winding end  87   d  is in contact with the spring support area  84   c  and, thus, is supported by the spring support area  84   c.    
       FIG. 8  is a side view of the fixing device in which the magnitude and direction of the reaction force received by the helical compression spring  87  from the spring support portion  93  and the spring support portion  840  when the helical compression spring  87  is compressed and supported is indicated by an arrow. In  FIG. 8 , the magnitude is indicated by the length of the arrow, and the direction is indicated by the direction of the arrow. The upper end  87   b  of the helical compression spring  87  substantially uniformly receives the reaction forces F 11  and F 12  from the groove  93   c  of the spring support portion  93 , and the lower end  87   a  substantially uniformly receives the reaction forces F 13  and F 14  from the spring support area  84   c  and the spring support area  84   d  of the lever  84 , respectively. The magnitudes of the reaction forces F 11  and F 12  at one end of the helical compression spring  87  are substantially the same, and the magnitudes of the reaction forces F 13  and F 14  at the other end are substantially the same. Accordingly, a rotary force is not exerted on the helical compression spring  87  which is compressed and supported. As a result, the acting force Fs of the spring can efficiently act on the pressure Ft in the nip portion. 
     While the present exemplary embodiment has been described with reference to the closed-end helical compression spring, the same effect can be provided even when an open-end helical compression spring is employed. 
       FIGS. 9A and 9B  illustrate an exemplary embodiment in which an open-end helical compression spring is supported.  FIG. 9A  illustrates an exemplary embodiment in which a spring support portion  910  is directly formed in the frame  91 . A spring winding end  87   c  of the helical compression spring  87  (one of the winding ends of the helical compression spring  87 ) is supported by the spring support portion  910 , which also serves as a rotation stopper for the helical compression spring  87 . The helical compression spring  87  is compressed and supported between the spring support portion  910  and the spring support portion  840  of the lever. The frame  91  has a convex portion  93  formed thereon. The convex portion  93   y  supports the inner diameter of the helical compression spring  87 . A spring support area  93   g  and the spring support area  93   h  that support the upper end  87   b  are formed on either side of the convex portion  93   y . The height level of the spring support area  93   g  that is close to the spring winding end  87   c  in the winding direction of the helical compression spring  87  (a second supporting area is lower than the height level of the spring support area  93   h  (a first supporting area). That is, the spring support area  93   g  is offset from the spring support area  93   h  in the axial direction of the helical compression spring  87  away from the helical compression spring  87 . Accordingly, the upper end  87   b  and the lower end  87   a  can be configured so that the spring support areas  93   g  and  93   h  receive the reaction forces having the same magnitude and, in addition, the spring support areas  84   c  and  84   d  receive the reaction forces having the same magnitude. As a result, a rotary force is not exerted on the compressed and supported helical compression spring  87  and, thus, the acting force Fs of the spring can efficiently act on the pressure Ft. 
       FIG. 9B  illustrates an exemplary embodiment in which the helical compression spring  87  is supported by the spring support portion  93  at four points.  FIG. 10A  is a perspective view of a spring support portion  93  according to the present exemplary embodiment.  FIG. 10B  is a top view and side views of a spring support member. A rotation stopper portion  93   d  allows the spring winding end  87   c  to be brought into contact therewith. A spring supporting area  93   i  (a first supporting area) that receives the side surface of the spring is formed next to the flat portion  93   d . In addition, three supporting areas  93   j ,  93   k , and  93   m  (second supporting areas) are formed so as to have the rotation center that is the same as the center of a cylindrical portion  93   a  that supports the inner diameter of the spring. The supporting areas  93   j ,  93   k , and  93   m  are arranged along the winding direction of the helical compression spring  87  that is used so as to have 90-degree phase difference from each other. 
     As illustrated in  FIG. 10B , the four spring support areas are formed so as to be closer to the helical compression spring  87  in the axial direction of the helical compression spring  87  as the helical compression spring  87  extends from the spring supporting area  93   i  to the supporting area  93   m . As a result, when the helical compression spring  87  is compressed and supported, the helical compression spring  87  received substantially the same reaction force from each of the spring supporting areas  93   i ,  93   j ,  93   k , and  93   m . Consequently, no rotary force is applied to the helical compression spring  87  that is compressed and supported between the spring support portion  840  of the lever  84  and the spring support portion  93  and, thus, the acting force Fs can efficiently act on the pressure Ft in the nip portion. 
       FIG. 9C  illustrates an exemplary embodiment in which the surface that is in contact with the helical compression spring  87  is changed to a sloped surface, unlike the configuration illustrated in  FIG. 9A . The spring supporting portion is formed so as to be integrated into the frame  91 . Each of the spring support area  93   g  and the spring support area  93   h  that support the upper end  87   b  and the spring support area  84   d  and the spring support area  84   c  of the lever  84  is sloped. The spring support areas are closer to the helical compression spring  87  in the axial direction of the helical compression spring  87  as the helical compression spring  87  extends from the spring support area  93   g  that is close to the spring winding end  87   c  to the spring support area  93   h  in the winding direction of the helical compression spring. The spring support areas are closer to the helical compression spring  87  in the axial direction of the helical compression spring  87  as the helical compression spring  87  extends from the spring support area  84   d  that is close to the spring winding end  87   d  to the spring support area  84   c  that is distant from the winding end  87   d  in the winding direction of the helical compression spring. In such a configuration, if the helical compression spring  87  is compressed and supported, one end of the helical compression spring  87  receives the reaction forces that are substantially the same from the spring support areas  93   g  and  93   h  and the other end of the helical compression spring  87  receives the reaction forces that are substantially the same from the spring support areas  84   c  and  84   d . Accordingly, no rotary force is exerted on the helical compression spring  87  that is compressed and supported and, thus, the acting force Fs can efficiently act on the pressure Ft in the nip portion. 
     As described above, according to the present invention, the fixing device having a pair of pressure mechanisms using a helical compression spring can reduce the inclination of the helical compression spring and can reduce a decrease in the pressure in the nip portion. While the above-described exemplary embodiment has been described with reference to a right-handed helical compression spring, the same effect can be provided even when a left-handed helical compression spring is employed. That is, it is only required that the spring support areas are formed so as to be closer to the spring in the axial direction of the spring as the spring extends away from the winding end of the spring in the winding direction. 
     Second Exemplary Embodiment 
     A fixing device  72  according to the present exemplary embodiment is described below with reference to  FIGS. 1A and 1B  and  FIGS. 12A and 12B .  FIGS. 1A and 1B  can be applied to the fixing device  72  according to the present exemplary embodiment as in the first exemplary embodiment. Note that in the following description, the term “longitudinal direction” of a member that constitutes the fixing device refers to a direction perpendicular to the recording medium conveyance direction. 
       FIG. 1A  is a schematic cross-sectional view of the fixing device  72  viewed in the longitudinal direction of the fixing device  72 .  FIG. 1B  is an enlarged view of a nip portion of the fixing device  72 .  FIG. 12A  is a schematic illustration of the fixing device  72  when viewed on a film  10  side of the fixing device.  FIG. 12B  is a schematic illustration of the fixing device  72  when viewed on the downstream side in a recording medium conveyance direction. For convenience of description,  FIGS. 12A and 12B  illustrate the configuration including an ideal helical compression spring. That is, the upper and lower spring end surfaces of the helical compression spring  87  are perpendicular to the central axis of the helical compression spring. 
     According to the present exemplary embodiment, the fixing device  72  includes a cylindrical film  10 , a heater  30  in contact with the inner peripheral surface of the film  10 , and a pressure roller  20 . The heater  30  forms a fixing nip portion N 2  together with the pressure roller  20  via the film  10 . The fixing device  72  conveys, in the fixing nip portion N 2 , a recording medium having a toner image formed thereon and, simultaneously, heats the toner image. Thus, the toner image is fixed onto the recording medium. The fixing device  72  further includes a heater holder  41  that supports the heater  30 , a pressure stay  42  that increases the bending rigidity, and a fixing flange  45  serving as a regulating member that regulates the movement of the film  10  in the longitudinal direction. The film  10 , the heater  30 , the heater holder  41 , the pressure stay  42 , and the fixing flange  45  are integrated into a film unit (a first fixing member). According to the present exemplary embodiment, the fixing device  72  is configured to urge the film unit against the pressure roller  20  (a second fixing member). The film  10  includes a base layer  11  and the release layer  12  provided on the outer surface of the base layer  11 . In addition, to increase fixability, an elastic layer  13  formed of, for example, silicone rubber may be disposed between the base layer  11  and the release layer  12 . If the elastic layer  13  is provided, an unfixed toner image T borne by a recording medium P can be encompassed and, thus, the heat can be uniformly applied to the toner image. It is desirable that the thickness of the elastic layer  13  be 50 μm and greater and 500 μm or less in order to reduce the warm-up time. The base layer  11  can be generated by forming a thin-wall metal having a high thermal conductivity, such as SUS or Ni, or a heat resistant resin, such as polyimide resin, a polyamide-imide resin, or PEEK, into a thin-wall flexible continuous belt. 
     To form the release layer  12 , a fluorine contained resin, such as PFA, PTFE, FEP, or a mixture thereof, is coated on the outer surface of the base layer  11 . Alternatively, the outer surface of the base layer  11  is covered by a tube made of the above-described resin. To increase the durability, it is desirable that the thickness of the release layer  12  be 5 μm and greater. In addition, if the release layer  12  is too thick, the thermal conductivity decreases and, thus, the fixability decreases. Accordingly, it is desirable that the thickness of the release layer  12  be 50 μm and less. 
     The heater holder  41  is made of liquid crystal polymer, a phenol resin, PPS, or PEEK. The heater holder  41  is formed so as to have a transverse section in the shape of a half-moon gutter. The lower surface of the heater holder  41  (a surface adjacent to the pressure roller  20 ) has a groove  41   a  having a recess shape formed along the longitudinal direction of the heater holder  41 . The heater  30  is supported by the groove  41   a . The film  10  is loosely fitted onto the outer periphery of the heater holder  41 . Both ends of the heater holder  41  (in the longitudinal direction) having the loosely fitting film  10  are supported by both the ends of a frame  91  (not illustrated) via the fixing flange  45 . 
     As illustrated in  FIG. 1B , the heater holder  41  includes a protrusion  41   b  provided in the fixing nip portion N 2  on the downstream side in the recording medium conveyance direction. The protrusion  41   b  extends along a portion of the heater holder  41  in contact with the inner peripheral surface of the film  10  in the longitudinal direction. The protrusion  41   b  protrudes from a sliding surface of the heater  30  that slides on the film  10  toward the outer surface of the film  10  by a protrusion amount h. The protrusion  41   b  is disposed so as to be located at the same position in the recording medium conveyance direction throughout its length. According to the fixing device of the present exemplary embodiment, the protrusion amount h is set to 0.2 mm. As illustrated in  FIG. 1B , a contact portion is divided into two contact portions, that is, a contact portion of the film  10  and the heater  30  and a contact portion of the film  10  and the heater holder  41 . As used herein, the term “sliding surface” refers to the contact portion between the film  10  and the heater  30 . 
     As illustrated in  FIG. 1A , the pressure roller  20  includes a core shaft portion  21 , an elastic layer  22  disposed on the outer surface of the core shaft portion  21 , and a release layer  24  disposed on the outer surface of the elastic layer  22 . The elastic layer  22  can be formed of, for example, silicone rubber or fluorine-contained rubber. 
     To form the release layer  24 , a fluorine contained resin, such as PFA, PTFE, FEP, or a mixture thereof, is coated. Alternatively, a tube made of the above-described resin is used as the release layer  24 . 
     According to the present exemplary embodiment, the core shaft portion  21  is formed from an iron core shaft having φ22, and the elastic layer  22  is formed of the silicone rubber having a thickness of 4 mm. The release layer  24  is formed from a PFA tube having a thickness of 50 μm. 
     The heater  30  is in contact with the inner peripheral surface of the film  10  and heats the film  10 . The heater  30  includes an elongated substrate extending in the longitudinal direction. The substrate can be formed as a ceramic (e.g., alumina or aluminum nitride) substrate or a heat resistant resin (e.g., polyimide, PPS, or liquid crystal polymer) substrate. The substrate has a heating resistor layer on the back surface thereof (a surface remote from the pressure roller  20 ) along the longitudinal direction of the substrate. The heating resistor layer is applied to the substrate in a band-like shape. The heating resistor layer is formed of, for example, Ag/Pd (silver-palladium), RuO 2 , or Ta 2 N. In addition, the substrate has glass coat on the back surface thereof in order to protect the heating resistor layer and ensure electrical insulation. Furthermore, the substrate has a sliding layer on a surface thereof that is in contact with the inner peripheral surface of the film  10  in order to increase the slidability. The sliding layer is formed of, for example, a heat resistant resin (e.g., a polyimide or polyamide-imide resin) or glass coat. According to the present exemplary embodiment, the size of the substrate of the heater  30  is 350 mm in the longitudinal direction, 10 mm in the short direction, and 0.6 mm in the thickness direction. 
     The pressure stay  42  is formed into a U shape using a material having rigidity (e.g., a metal). The pressure stay  42  is disposed on the upper surface of the heater holder  41  (a surface remote from the pressure roller  20 ) inside the film  10 . The pressure stay  42  urges both ends of the pressure stay  42  in the longitudinal direction toward the axial line of the pressure roller  20  via the fixing flange  45  supported by the frame  91 . Thus, the heater  30  is urged against the surface of the pressure roller  20  via the film  10 , and an inner nip N 3  having a predetermined width is formed between the heater  30  and the film  10 . In addition, a fixing nip N 2  having a predetermined width is formed between the film  10  and the pressure roller  20 . Heat necessary for the heat fixing of the unfixed toner image T is transferred from the heater  30  to the film  10  in the inner nip N 3 , and the heat is transferred from the film  10  to the recording medium P in the fixing nip N 2 . At that time, the recording medium is conveyed. 
     Upon receiving a print instruction, a control unit  44  drives a motor serving as a driving source to rotate a drive gear disposed at an end of the core shaft portion  21  of the pressure roller  20  in the longitudinal direction. Thus, the pressure roller  20  rotates at a predetermined circumferential velocity in a direction of an arrow. At that time, a rotary force that attempts to rotate the film  10  in a direction opposite to the rotational direction of the pressure roller  20  is exerted on the film  10  due to a frictional force generated between the surface of the pressure roller  20  and the surface of the film  10  in the fixing nip N 2 . In this manner, the film  10  is driven to rotate in the direction of the arrow at a circumferential velocity that is substantially the same as that of the pressure roller  20  outside the heater holder  41  with the inner peripheral surface of the film  10  in contact with the sliding layer of the heater  30 . 
     A thermistor  35  serving as a temperature detecting unit detects the temperature of the film  10  and outputs a temperature detection signal to the control unit  44 . The thermistor  35  is disposed so as to be capable of detecting the temperature of an area through which the recording medium P having any of all the sizes allowable for the fixing device  72  passes. The control unit  44  receives the temperature detection signal from the thermistor  35  and controls the power supplied to the heating resistor layer on the basis of the temperature detection signal so that the film  10  has a predetermined target temperature. 
     In this manner, the recording medium P having the unfixed toner image T thereon is led to the fixing nip N 2  along an entry guide  28  with the temperature of the film  10  maintained at the predetermined target temperature. Thereafter, the recording medium P is pinched by the film  10  and the pressure roller  20  and is conveyed. In the conveyance stage, the heat of the film  10  heated by the heater  30  and the pressure from the fixing nip N 2  are applied to the recording medium P. Due to the heat and pressure, the unfixed toner image T is fixed onto the surface of the recording medium P. After passing through the fixing nip N 2 , the recording medium P is separated from the film  10  by self stripping and is ejected by the conveyance roller  26 . 
     The pressure mechanism according to the present exemplary embodiment is described below with reference to  FIGS. 13 and 14A .  FIG. 13  is a perspective view of the fixing device  72 .  FIG. 14A  is a side view of the fixing device  72  viewed in a direction of an arrow R in  FIG. 13 . The pressure roller  20  is rotatably supported by a frame  91  disposed at both ends of the pressure roller  20  in the longitudinal direction via a bearing (not illustrated). A guide portion  91   a  that regulates the direction in which the film unit is pressed is disposed on the frame  91 . 
     Each of a pair of the pressure mechanisms includes a lever  84 , a turning center  91   b  and a spring support portion  93  provided in the frame  91 , and a helical compression spring  87 . The pressure mechanisms are provided at either end of the film  10  in the longitudinal direction. 
     The lever  84  is a member having one end supported by the turning center  91   b  in the frame  91  in a rotatable manner in a direction in which the film  10  is pressed. The helical compression spring  87  is disposed and compressed between the other end of the lever  84  and a spring support portion  93  of the frame  91 . The other end of the lever  84  supports a lower end  87   a  of the helical compression spring  87 . In contrast, the spring support portion  93  is formed in the frame  91  and supports the upper end  87   b  of the helical compression spring  87 . The spring support portion  93  has a function of regulating the height of the helical compression spring  87  so that the pressure of the helical compression spring  87  is maintained at a predetermined pressure (a specified load). According to the present exemplary embodiment, the helical compression spring  87  has a free height of 35 mm and a specified height of 27 mm upon pressurization. The lever  84  can rotate about the turning center  91   b  due to the elastic force of the helical compression spring  87  and exerts a pressure Ft on the fixing flange  45  via the lever  84 . Thus, the lever  84  can urge the film unit against the pressure roller  20 . Note that by moving the lever  84  in a direction in which the helical compression spring  87  is compressed using a cam member  95 , the pressure applied in the fixing nip N 2  can be released. 
     In the following description of the helical compression spring, the direction of an arrow R illustrated in  FIG. 13  is the right direction, and the direction opposite to the direction of the arrow R is the left direction. The helical compression spring  87  located on the right side of the fixing device in the longitudinal direction is referred to as “helical compression spring  87 R”, and the helical compression spring  87  located on the left side of the fixing device in the longitudinal direction is referred to as “helical compression spring  87 L”. 
     The pressure mechanism according to the present exemplary embodiment is characterized in that the winding direction of the helical compression spring  87 R is opposite to the winding direction of the helical compression spring  87 L. In the present exemplary embodiment illustrated in  FIGS. 14A and 14B , the helical compression spring  87 R is a right-handed helical compression spring, and the helical compression spring  87 L is a left-handed helical compression spring. In addition, the pressure mechanism according to the present exemplary embodiment is characterized in that the position of the winding end of the helical compression spring  87 R is substantially symmetrical to the position of the winding end of the helical compression spring  87 L with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. A technique to determine the phase of the winding ends of the helical compression spring  87  is described below with reference to  FIG. 14B .  FIG. 14B  illustrates a spring terminal of the helical compression spring  87  and the spring support portion  93  viewed in a direction of an arrow A of  FIG. 14A . The spring support portion  93  includes a cylindrical portion  93   a  having a diameter close to the inner diameter of the helical compression spring  87  and a convex portion  93   b  that determines the phase of the winding ends of the helical compression spring  87 . In the pressure mechanisms at either end of the film  10 , the convex portions  93   b  of the pressure mechanisms are disposed at positions so as to be substantially symmetrical with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. The pressure mechanism according to the present exemplary embodiment has a configuration so that the phase of the winding end is determined by lightly press-fitting the upper end  87   b  of the helical compression spring  87  to the cylindrical portion  93   a  with a phase of the winding end of the upper end  87   b  being in contact with the convex portion  93   b.    
     The behavior of the helical compression spring  87  when the helical compression spring  87  is compressed and the effect in the above-described configuration are described below.  FIG. 15A  is a transverse sectional view of the helical compression spring  87  placed on the lever  84  without being compressed.  FIG. 15B  is a side view of the helical compression spring  87  compressed so that the positions of the lower end  87   a  and the upper end  87   b  are aligned using the spring support portion  93  and the lever  84 . The coil central axis of the helical compression spring  87  is not perpendicular to the receiving surface of the spring end due to the step formed between the spring winding portion and the spring winding end at the spring terminal. That is, the coil central axis is inclined from the perpendicular line of the receiving surface. The direction in which the central axis is inclined is related to the phase of the spring winding ends. When the helical compression spring  87  is compressed so that the position of the lower end  87   a  of the helical compression spring  87  is aligned with the position of the upper end  87   b , the helical compression spring  87  bends, as illustrated in  FIG. 15B . A play is provided between the lever  84  having a receiving surface  84   a  that receives the lower end  87   a  at one end and the turning center  91   b  in the frame  91 . Accordingly, the lever  84  moves such that the bend of the helical compression spring is reduced.  FIG. 16A  is a side view of the helical compression spring  87  after the lever  84  has moved such that the bend of the helical compression spring is reduced. As illustrated in  FIG. 16A , the center  87   b C of the upper end  87   b  is deviated from the center  87   a C of the lower end  87   a . The direction of the deviation of the center  87   b C of the upper end  87   b  from the center  87   a C of the lower end  87   a  is determined by the phase of the spring winding ends.  FIG. 16B  illustrates the positional relationship between the center  87   b C of the upper end  87   b  and the center  87   a C of the lower end  87   a  of the helical compression spring  87 . 
       FIG. 17A  is a schematic illustration of the fixing device  72  as viewed from above in a direction perpendicular to a nip surface according to the present exemplary embodiment.  FIG. 17B  is a schematic illustration of the fixing device  72  as viewed in the downstream side in the recording medium conveyance direction. Let  87 Rb denote the upper end of the helical compression spring  87 R, and let  87 Ra denote the lower end of the helical compression spring  87 R. Let  87 Lb denote the upper end of the helical compression spring  87 L and let  87 La denote the lower end of the helical compression spring  87 L. According to the present exemplary embodiment, when the helical compression spring  87  is compressed and, thus, the pressure is generated, the lever  84  moves in a direction such that the bend of the helical compression spring  87  is released. Since the winding directions of the helical compression spring  87 R and the helical compression spring  87 L are opposite to each other and, in addition, the positions of the winding ends are located so as to be substantially symmetrical, the directions in which the levers  84  move to reduce the bends are symmetrical with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. Accordingly, the levers  84  are positioned as indicated in  FIG. 17A . The point  84   c  of application of the pressure Ft is shifted in the longitudinal direction of the film  10 , from the position in the configuration in which an ideal helical compression spring that generates no bend is employed illustrated in  FIGS. 12A and 12B . 
     In addition, the slope of the central axis of the helical compression spring  87 R is symmetrical to the slope of the central axis of the helical compression spring  87 L with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. Similarly, a pressure vector FsR of the helical compression spring  87 R is symmetrical to a pressure vector FsL of the helical compression spring  87 L with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. 
     According to the present exemplary embodiment, the point  84   c  of application of the pressure Ft is shifted toward the middle of the film in the longitudinal direction. However, a distance dL between a point  84   c L of application of the pressure Ft applied to a left film guide  45  and a left nip end is the same as a distance dR between a point  84   c R of application of the pressure Ft applied to a right film guide  45  and a right nip end. In addition, in the configuration according to the present exemplary embodiment, the center  87   a C of the lower end, which is the point of effort of the pressure Fs, is shifted in the recording medium conveyance direction. However, a distance ML between the center  87 LaC of the lower end, which is the point of effort of the left pressure Fs, and a turning center  91   b L is the same as a distance MR between the center  87 RaC of the lower end, which is the point of effort of the left pressure Fs, and a turning center  91   b R. Accordingly, the pressures Fs applied to the turning center  91   b  on the right and left sides are the same. As a result, a difference in surface pressure applied to the turning center  91   b  between right and left is less likely to occur and, thus, the difference in fixability level between right and left portions of the image can be reduced. 
     For ease of understanding of the effect of the present exemplary embodiment, the pressure configuration of a comparative example is described below. In the Comparative example, the helical compression spring  87 R and the helical compression spring  87 L are of the same type. Accordingly, the winding directions of the helical compression spring  87 R and the helical compression spring  87 L are the same. In addition, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L, or the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L if the helical compression spring  87 L is rotated about the axis by 180°. In Comparative example 1 described below, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the position of the winding end of the helical compression spring  87 L. In contrast, in Comparative example 2 described below, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L if the helical compression spring  87 L is rotated about the axis by 180°. 
       FIG. 18A  is a schematic illustration of the fixing device  72  as viewed from above in a direction perpendicular to a nip surface according to Comparative example 1.  FIG. 18B  is a schematic illustration of the fixing device  72  as viewed from the downstream side in the recording medium conveyance direction. When the helical compression spring  87  is compressed and, thus, a pressure is generated, the lever  84  moves from the position thereof in the configuration in which an ideal helical compression spring is employed illustrated in  FIGS. 12A and 12B  in order to reduce the bend of the helical compression spring  87 . Since the winding directions of the helical compression spring  87 R and the helical compression spring  87 L are the same and, in addition, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the positions of the winding ends have the same phase, the directions of the bends of the helical compression spring  87 R and the helical compression spring  87 L are the same. Accordingly, the directions in which the levers  84  move in order to reduce the bends are the same. Consequently, as illustrated in  FIG. 18A , the point  84   c  of application of the pressure Ft is shifted in the longitudinal direction of the film  10 . At that time, the distance dL between the point  84   c L of application of the pressure Ft applied to the left film guide  45  and the left nip end is shorter than the distance dR between the point  84   c R of application of the pressure Ft applied to the right film guide  45  and the right nip end. According to the principle of leverage, as the point  84   c  of application of the pressure moves toward the nip end, the surface pressure applied to the nip end increases. Thus, the surface pressure on the left side of the nip is low, and the surface pressure on the right side of the nip is high. That is, the surface pressures on the right side and the left side in the nip differ from each other. Accordingly, the fixability on the left side in the longitudinal direction tends to be worse than the fixability on the right side in the longitudinal direction. 
       FIG. 19A  is a schematic illustration of the fixing device  72  as viewed from above in a direction perpendicular to the nip surface according to Comparative example 2.  FIG. 19B  is a schematic illustration of the fixing device  72  as viewed from the downstream side in the recording medium conveyance direction. 
     When the helical compression spring  87  is compressed and, thus, a pressure is generated, the lever  84  moves from the position thereof in the configuration in which an ideal helical compression spring is employed illustrated in  FIGS. 12A and 12B  in order to reduce the bend of the helical compression spring  87 . Since the winding directions of the helical compression spring  87 R and the helical compression spring  87 L are the same and, in addition, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L if the helical compression spring  87 L is rotated about the axis by 180°, the directions of the bends of the helical compression spring  87 R and the helical compression spring  87 L are opposed 180° from each other. Accordingly, the directions in which the levers  84  move in order to reduce the bends are also opposed 180° from each other. Consequently, as illustrated in  FIG. 19A , the center  87   a C of the lower end, which is the point of application of the pressure Fs, is shifted from the point indicated in  FIGS. 12A and 12B . If the center  87   a C of the lower end, which is the point of application of the pressure Fs, is located at the point indicated in  FIGS. 19A and 19B , the distance ML between the center  87 LaC of the lower end, which is the point of effort of a left pressure FsL, and the turning center  91   b L is shorter than that indicated by  FIGS. 12A and 12B . In contrast, the distance MR between the center  87 RaC of the lower end, which is the point of effort of a right pressure FsR, and the turning center  91   b R is longer than that indicated by  FIGS. 12A and 12B . Accordingly, the moment arm of the pressure Fs with respect to the turning center  91   b  on the right side is longer than on the left side. Thus, the pressure Ft on the right side is greater than on the left side. Consequently, the surface pressure on the left side in the nip is low, and the surface pressure on the right side in the nip is high. Accordingly, the fixability on the left side tends to be worse than the fixability on the right side. 
     The result of comparison of Comparative example 1, Comparative example 2, and the present exemplary embodiment in terms of the difference in fixability in the longitudinal direction of an image is illustrated in Table 1. In addition, the result of comparison of Comparative example 1, Comparative example 2, and the present exemplary embodiment in terms of gloss level in the longitudinal direction is illustrated in Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Position 
                   
                 Position 
                 Difference 
               
               
                   
                 30 mm from 
                   
                 30 mm from 
                 in 
               
               
                   
                 Left Edge 
                   
                 Right Edge 
                 Fixability 
               
               
                   
                 of 
                 Middle of 
                 of 
                 between 
               
               
                 Fixability 
                 Recording 
                 Recording 
                 Recording 
                 Right and 
               
               
                 Evaluation 
                 Medium 
                 Medium 
                 Medium 
                 Left 
               
               
                   
               
             
            
               
                 Comparative 
                 Excellent 
                 Excellent 
                 poor 
                 YES 
               
               
                 example 1 
               
               
                 Comparative 
                 Excellent 
                 Excellent 
                 poor 
                 YES 
               
               
                 example 2 
               
               
                 Present 
                 Excellent 
                 Excellent 
                 Excellent 
                 NO 
               
               
                 Embodiment 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Position 
                   
                 Position 
                 Difference 
               
               
                   
                 30 mm from 
                   
                 30 mm from 
                 in 
               
               
                   
                 Left Edge 
                   
                 Right Edge 
                 Fixability 
               
               
                   
                 of 
                 Middle of 
                 of 
                 between 
               
               
                 Gloss 
                 Recording 
                 Recording 
                 Recording 
                 Right and 
               
               
                 Evaluation 
                 Medium 
                 Medium 
                 Medium 
                 Left 
               
               
                   
               
             
            
               
                 Comparative 
                 Excellent 
                 Excellent 
                 Fair 
                 YES 
               
               
                 example 1 
               
               
                 Comparative 
                 Excellent 
                 Excellent 
                 Poor 
                 YES 
               
               
                 example 2 
               
               
                 Present 
                 Excellent 
                 Excellent 
                 Excellent 
                 NO 
               
               
                 Embodiment 
               
               
                   
               
            
           
         
       
     
     In the evaluation, Xerox Business 4200 (75 g/m2) letter paper sheets were used as the recording media P. In addition, a uniform image that covered the entire page of the recording medium was printed as the toner image T, which was heat fixed to the recording media P using the fixing devices having the above-described configurations. 
     To evaluate the fixability performance, an adhesive cellophane tape was put on the toner image fixed onto the recording medium P by a surface pressure of 0.49 N/cm 2  (50 gf/cm 2 ) for one minute and, thereafter, the cellophane tape was removed. Then, evaluation was made on the basis of the level of the image failure of the toner image (caused by the removal of the cellophane tape). If the image failure exceeds 5% of the toner image, the fixability performance is evaluated as “poor”. In contrast, if the image failure is less than or equal to 5% of the toner image, the fixability performance is evaluated as “excellent”. 
     The gloss evaluation was made using a gloss meter available from Nippon Denshoku Industries Co., LTD. If the measured value is less than or equal to 10%, the gloss is evaluated as “poor”. If the measured value is between 10% and 13%, the gloss is evaluated as “fair”. If the measured value is greater than or equal to 13%, the gloss is evaluated as “excellent”. The fixability performance and the gloss were evaluated in the following manner. That is, three points were selected in the recording medium so as to be arranged in a direction perpendicular to the recording medium conveyance direction (hereinafter referred to as “three points in the longitudinal direction of the film  10 ”). The mean value of the measured values at each of the three points in the recording medium conveyance direction was calculated. The three mean values were used for evaluation in the longitudinal direction of the film. The three points in the recording medium conveyance direction are points 39.4 mm, 139.4 mm, and 239.4 mm from the leading edge of the recording medium in the recording medium conveyance direction. The three points in the longitudinal direction of the film  10  are two points 30 mm from the right edge and the left edge in a direction perpendicular to the recording medium conveyance direction and a point 107.95 mm from each of the edges (in the middle of the recording medium in the direction perpendicular to the recording medium conveyance direction). 
     As can be seen from the results indicated by Tables 1 and 2, according to the present exemplary embodiment, the difference in the fixability performance and the difference in the gross between right and left can be reduced more than in each of the Comparative examples 1 and 2. 
     The following configuration is discussed below. That is, as illustrated in  FIG. 1B , the protrusion  41   b  is provided downstream of the heater holder  41  in the recording medium conveyance direction so as to extend along the longitudinal direction of a portion of the heater holder  41  in contact with the inner peripheral surface of the film  10 . The protrusion  41   b  crushes toner particles on the recording medium that are melted in the inner nip N 3  so as to improve the fixability and the gloss. However, the function of the protrusion  41   b  to improve the fixability and the gloss is easily influenced by the surface pressure in the nip. For example, if there is a portion having a low surface pressure in the nip, the portion is more easily recognized as a gloss difference in the configuration having the protrusion  41   b  than in the configuration having no protrusion  41   b . In particular, when, as in the Comparative examples, the difference in pressure between right and left occurs, the difference in gloss and the difference in fixability easily occur. According to the configuration of the present exemplary embodiment, the difference in the surface pressure in the nip between right and left is reduced and, thus, the difference in gloss and the difference in fixability between right and left can be reduced. 
     In addition, as described above, for the configuration that rotates the lever  84  using the cam member  95  and pushes up the helical compression spring  87  to compress the helical compression spring  87  to release the pressure, the effect of improvement using the present exemplary embodiment is great. Since the helical compression spring  87  is more compressed when the pressure is released than under the pressurization condition, the helical compression spring  87  bends more than under the pressurization condition. Thus, the force to straighten out the bend increases. Consequently, the force to move the lever  84  increases, and the moving distance increases. Accordingly, the difference in the surface pressure in the nip between right and left caused by the movement of the lever in the Comparative examples 1 and 2 is increased. However, according to the configuration of the present exemplary embodiment, even when the moving distance is large, the right and left levers  84  move in the opposite directions. Accordingly, the difference in the surface pressure in the nip between right and left is less likely to occur. 
     As described above, according to the present exemplary embodiment, the difference in fixability and gloss (uneven fixability and uneven gloss) in an image can be reduced. In addition, since the difference in the surface pressure in the nip between right and left can be reduced, the difference in the conveyance force between both the ends of a recording medium is less likely to occur and, thus, the occurrence of wrinkling on a recording medium can be prevented. 
     In Comparative example 1 and Comparative example 2, wear of the rotary member is accelerated at a point at which the surface pressure in the nip is high. Thus, the lifetime of the fixing device is reduced. According to the configuration of the present exemplary embodiment, the difference in the surface pressure in the nip can be reduced. Thus, the lifetime of the fixing device can be increased from that of each of Comparative example 1 and Comparative example 2. 
     Note that the effect to increase the recording medium conveyance performance in the fixing nip N 2  of the pressure mechanism of the fixing device according to the present exemplary embodiment can be applied to recording medium conveyance devices that convey a recording medium using a nip portion formed by two rotary members (first and second rotary members) in tight contact with each other, in addition to fixing devices. 
     Third Exemplary Embodiment 
     A fixing device according to the third exemplary embodiment is described below with reference to  FIGS. 20 to 24 . Note that in the present exemplary embodiment, description of constituent elements that are similar to those of the second exemplary embodiment is not repeated. Only a pressure mechanism that applies the pressure Ft and the location of the helical compression spring  87  differ from those of the second exemplary embodiment. 
       FIG. 20  is a side view of the fixing device according to the present exemplary embodiment. The fixing device according to the present exemplary embodiment is described next with reference to  FIG. 20 . A fixing flange  45  and a pressure roller  20  are supported by a frame  111  disposed on either end of the pressure mechanism in the longitudinal direction. A guide portion  111   a  that regulates the direction in which a film unit is urged against the pressure roller  20  is disposed on the frame  111 . The pressure roller  20  is rotatably supported by the frame  111  via a bearing (not illustrated). The helical compression spring  87  applies a pressure Ft to the fixing flange  45  via a lever  114  and, thus, urges the film unit against the fixing flange  45 . The helical compression spring  87  has a lower end  87   a  fixed to one end of the lever  114  and an upper end  87   b  fixed to a spring support portion  93 . The helical compression spring  87  is disposed immediately above the fixing flange  45  and the pressure roller  20 . The spring support portion  93  is fixed to the frame  111  and causes the helical compression spring to have a specified height so that the pressure of the helical compression spring  87  is maintained at a predetermined pressure. According to the present exemplary embodiment, the helical compression spring  87  has a free height of 35 mm and a specified height of 27 mm. To release the pressure, the lever  114  is rotated about a turning center  111   b  provided in the frame  111  using a cam member  95 . In addition, according to the pressure configuration of the present exemplary embodiment, the pressure Fs of the helical compression spring  87  has substantially the same direction and magnitude as the pressure Ft. Accordingly, a force equivalent to the pressure Fs is applied to the heater holder  41  and the film  10  via the fixing flange  45 . 
     According to the present exemplary embodiment, the winding direction of a helical compression spring  87 R is opposite to the winding direction of a helical compression spring  87 L. In addition, the position of the winding end of the helical compression spring  87 R is substantially symmetrical to the position of the winding end of the helical compression spring  87 L with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. In  FIGS. 21A and 21B , the helical compression spring  87 R is a right-handed helical compression spring, and the helical compression spring  87 L is a left-handed helical compression spring. 
       FIG. 21A  is a schematic illustration of the fixing device  72  as viewed from above in a direction perpendicular to a nip surface.  FIG. 21B  is a schematic illustration of the fixing device  72  as viewed from the downstream side in the recording medium conveyance direction. In  FIG. 21A , one of the axes of the pressure roller  20  that defines the right direction as a positive direction is referred to as an “x-axis”, and an axis that is parallel to the nip surface and that extends in the recording medium conveyance direction is referred to as a “y-axis”. 
     According to the present exemplary embodiment, when the helical compression spring  87  is compressed and thus, the pressure is generated, the lever  114  moves in a direction so that the bend of the helical compression spring  87  is released. Since the winding directions of the helical compression spring  87 R and the helical compression spring  87 L are opposite to each other and, in addition, the positions of the winding ends are located so as to be substantially symmetrical, the directions in which the levers  114  move to reduce the bends are symmetrical with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. Accordingly, as illustrated in  FIG. 21A , the point  84   c  of application of the pressure Ft is shifted toward the middle of the film  10  in the longitudinal direction of the film  10 . 
     In addition, the slope of the central axis of the helical compression spring  87 R is symmetrical to the slope of the central axis of the helical compression spring  87 L with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. Similarly, a pressure vector FsR of the helical compression spring  87 R is symmetrical to a pressure vector FsL of the helical compression spring  87 L with respect to the transverse plane in the middle of the film  10  in the longitudinal direction. 
     For ease of understanding of the effect of the present exemplary embodiment, the pressure configuration of a comparative example is described below. In the comparative example, the helical compression spring  87 R and the helical compression spring  87 L are of the same type. Accordingly, the winding directions of the helical compression spring  87 R and the helical compression spring  87 L are the same. In addition, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L, or the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L if the helical compression spring  87 L is rotated about the axis by 180°. In Comparative example 3 described below, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the position of the winding end of the helical compression spring  87 L. In contrast, in Comparative example 4 described below, the helical compression spring  87 R and the helical compression spring  87 L are disposed so that the position of the winding end of the helical compression spring  87 R has the same phase as the winding end of the helical compression spring  87 L if the helical compression spring  87 L is rotated about the axis by 180°. 
     Comparative example 3 is described below.  FIG. 22A  is a schematic illustration of the fixing device  72  as viewed from above in a direction perpendicular to a nip surface according to Comparative example 3.  FIG. 22B  is a schematic illustration of the fixing device  72  as viewed from the downstream side in the recording medium conveyance direction. The directions in which the levers  84  move in order to straighten out the bends of the helical compression springs  87  are the same. Accordingly, as illustrated in  FIG. 22A , the point  84   c  of application of the pressure Ft is shifted in the longitudinal direction of the film  10 . At that time, a distance dL between the point  84   c L of application of the pressure Ft applied to the left film guide  45  and the left nip end is shorter than a distance dR between the point  84   c R of application of the pressure Ft applied to the right film guide  45  and the right nip end. Thus, the surface pressure on the left side of the nip is low, and the surface pressure on the right side of the nip is high. That is, the surface pressures on the right side and the left side in the nip differ from each other. Accordingly, the fixability on the left side in the longitudinal direction tends to be worse than the fixability on the right side in the longitudinal direction. 
       FIG. 23A  is a schematic illustration of the fixing device  72  as viewed from above in a direction perpendicular to the nip surface according to Comparative example 4.  FIG. 23B  is a schematic illustration of the fixing device  72  as viewed from the downstream side in the recording medium conveyance direction. 
     The directions in which the helical compression springs  87  move the levers  84  to straighten out the bends thereof are opposed 180° from each other. Accordingly, the positions are determined as illustrated in  FIG. 23A . According to the present comparative example, the problem is that an intersect angle is formed between the generatrix direction of the film  10  and the axial direction of the pressure roller  20 . The pressure Fs of the helical compression spring  87  has substantially the same direction and magnitude as the pressure Ft. Accordingly, a force equivalent to the pressure Fs is applied to the heater holder  41  and the film  10  via the fixing flange  45 . Since the y-axis component of the pressure vector FsR of the helical compression spring  87 R is positive and the v-axis component of the pressure vector FsL of the helical compression spring  87   l  is negative, a force to attempt to rotate in the clockwise direction in  FIG. 23A  is exerted on the heater holder  41  and the film  10 . Thus, the heater holder  41  and the film  10  rotate by a value equivalent to a play of the fixing flange  45 . Consequently, as illustrated in  FIG. 26 , an intersect angle is formed between the generatrix direction of the film  10  and the axial direction of the pressure roller  20 . As a result, a range XR where the protrusion  41   b  is not present in the fixing nip N 2  is formed on the right side in the longitudinal direction. In a range XL where the protrusion  41   b  of the heater holder  41  is present in the fixing nip N 2 , the protrusion  41   b  crushes the toner particles on the recording medium that are melted in the inner nip N 3  so as to improve the fixability and the gloss. However, in the range XR, the effect of the protrusion  41   b  to crush the toner particles on the recording medium that are melted in the inner nip N 3  and improve the fixability and the gloss cannot be obtained. Accordingly, the difference in fixability and the difference in gloss between the range XL and the range XR occur. Consequently, the difference in fixability and the difference in gloss between the middle portion and each of the right and left portions of the image easily occur. 
     According to the present exemplary embodiment, the point  84   c  of application of the pressure Ft is shifted so as to be close to the middle of the film  10  in the longitudinal direction of the film  10 . However, the distance dL between the point  84   c L of application of the pressure Ft applied to the left film guide  45  and the left nip end is the same as the distance dR between the point  84   c R of application of the pressure Ft applied to the right film guide  45  and the right nip end. In addition, the center  87   a C of the lower end, which is the point of effort of the pressure Fs, is shifted in the recording medium conveyance direction. However, the distance between the center  87 LaC of the lower end, which is the point of effort of the left pressure Fs and the turning center  91   b L is the same as the distance between the center  87 RaC of the lower end, which is the point of effort of the right pressure Fs, and the turning center  91   b R. Thus, the moment arms on the right and left sides are the same. As a result, according to the configuration of the present exemplary embodiment, the difference in pressure between right and left is less likely to occur. 
     According to the present exemplary embodiment, the y-axis component of the pressure vector FsR of the helical compression spring  87 R and the y-axis component of the pressure vector FsL of the helical compression spring  87 L have the same sign and, thus, the same direction. Accordingly, a force that rotates the heater holder  41  and the film  10  is negligibly generated. Consequently, an intersect angle is negligibly formed between the longitudinal axis of the film  10  and the longitudinal axis of the pressure roller  20 . As described above, according to the present exemplary embodiment, uneven fixability and uneven gloss are less likely to occur in the image. 
     The result of comparison of the above-described Comparative examples 3 and 4 and the present exemplary embodiment in terms of the difference in fixability along the length of an image is given in Table 3. In addition, the result of comparison of Comparative examples 3 and 4 and the present exemplary embodiment in terms of the difference in gloss along the length of an image is given in Table 4. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Position 
                   
                 Position 
                   
               
               
                   
                 30 mm from 
                   
                 30 mm from 
               
               
                   
                 Left Edge 
                   
                 Right Edge 
                 Difference 
               
               
                   
                 of 
                 Middle of 
                 of 
                 in 
               
               
                 Fixability 
                 Recording 
                 Recording 
                 Recording 
                 Fixability 
               
               
                 Evaluation 
                 Medium 
                 Medium 
                 Medium 
                 in Image 
               
               
                   
               
             
            
               
                 Comparative 
                 Excellent 
                 Excellent 
                 poor 
                 YES 
               
               
                 Example 3 
               
               
                 Comparative 
                 Excellent 
                 Excellent 
                 poor 
                 YES 
               
               
                 Example 4 
               
               
                 Present 
                 Excellent 
                 Excellent 
                 Excellent 
                 NO 
               
               
                 Embodiment 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Position 
                   
                 Position 
                   
               
               
                   
                 30 mm from 
                   
                 30 mm from 
               
               
                   
                 Left Edge 
                   
                 Right Edge 
               
               
                   
                 of 
                 Middle of 
                 of 
                 Difference 
               
               
                 Gloss 
                 Recording 
                 Recording 
                 Recording 
                 in Gloss 
               
               
                 Evaluation 
                 Medium 
                 Medium 
                 Medium 
                 in Image 
               
               
                   
               
             
            
               
                 Comparative 
                 Excellent 
                 Excellent 
                 Fair 
                 YES 
               
               
                 Example 3 
               
               
                 Comparative 
                 Fair 
                 Excellent 
                 Poor 
                 YES 
               
               
                 Example 4 
               
               
                 Present 
                 Excellent 
                 Excellent 
                 Excellent 
                 NO 
               
               
                 Embodiment 
               
               
                   
               
            
           
         
       
     
     The evaluation method is the same as that described in the second exemplary embodiment. The results in Tables 3 and 4 indicate that the configuration according to the present exemplary embodiment reduces the difference in fixability and the difference in gloss throughout an image more than the configurations of Comparative examples 3 and 4. 
     As described above, according to the present exemplary embodiment, the fixing device having a configuration that negligibly generates the difference in fixability and gloss throughout an image. In addition, since the difference in the surface pressure in the nip between right and left can be reduced, the difference in conveyance force between both the ends is less likely to occur and, thus, the rate of occurrence of paper wrinkling can be reduced. Furthermore, by employing the configuration according to the present exemplary embodiment, the difference in the surface pressure in the nip can be reduced and, thus, the lifetime of the fixing device can be increased. 
     Note that the effect of the pressure mechanism of the fixing device according to the present exemplary embodiment to improve the sheet transportability can be applied to recording medium conveyance devices that convey a recording medium using a nip portion formed by two rotary members (first and second rotary members) in tight contact with each other, in addition to fixing devices. 
     As another example of application, the lever  114  and the cam member  95  (i.e., a pressure release mechanism) may be removed from the configuration of the present exemplary embodiment. Even in such a case, like the present exemplary embodiment, the winding direction of the helical compression spring  87 R is set so as to be opposite to the winding direction of the helical compression spring  87 L. In addition, the position of the winding end of the helical compression spring  87 R is set so as to be substantially symmetrical to the position of the winding end of the helical compression spring  87 L. In this manner, the fixing device that negligibly generates the difference in fixability and the difference in gloss throughout an image can be provided. 
     In the configuration without the lever  114  and the cam member  95  (i.e., a pressure release mechanism), the helical compression spring  87  directly applies pressure on the fixing flange  45  without using the lever  114 . In such a case, in the configuration of Comparative example 3, the fixing flange  45  moves in the longitudinal direction of the film by a value equivalent to a play given by assembling of the fixing flange  45 . Accordingly, the point  84   c  of application of the pressure Ft is shifted in the longitudinal direction of the film and, thus, the difference between the distance dL and the distance dR is generated. Consequently, the difference in the surface pressure in the nip between right and left occurs and, thus, the difference in fixability and the difference in gloss between right and left occur. In the configuration of Comparative example 4, an intersect angle is formed between the longitudinal axis of the film  10  and the longitudinal axis of the pressure roller  20 . Accordingly, the difference in fixability and the difference in gloss between right and left occur. As described above, in the configurations of the comparative examples, the difference in fixability and the difference in gloss throughout an image (i.e., uneven fixability and uneven gloss) are generated. In contrast, in configurations similar to the configuration according to the present exemplary embodiment, since the fixing flanges  45  move in a right-left symmetrical manner. Thus, the difference in the surface pressure is less likely to occur. In this manner, the difference in fixability and the difference in gloss between right and left can be reduced. 
     Fourth Exemplary Embodiment 
     An image forming apparatus according to the fourth exemplary embodiment is described below. The image forming apparatus includes the recording medium conveyance device of the present invention. Note that according to the present exemplary embodiment, description of constituent elements that are similar to those of the second exemplary embodiment is not repeated. Unlike the second exemplary embodiment, the spring support portion  93  is replaced with an upper end supporting table  130 , and the position of the upper end supporting table  130  is fixed by a regulating member  94 . 
     According to the second exemplary embodiment, by regulating the height of the helical compression spring  87  to a specified height, a predetermined pressure is obtained. However, a variation of the spring constant and a variation of the free height occur among helical compression springs. Accordingly, even when the spring lengths are regulated so as to be specified heights, a difference in pressure between right and left occurs, in reality. According to the present exemplary embodiment, by addressing the above-described issue, a configuration capable of adjusting the pressure to a predetermined pressure can be provided. 
     The pressure mechanism that applies the pressure Ft is described below with reference to  FIGS. 24 and 25 .  FIG. 24  is a perspective view of a fixing device.  FIG. 25  is a side view of the fixing device as viewed in a direction of an arrow R in  FIG. 24 . 
     The upper end supporting table  130  is a spring terminal supporting member that fixedly supports the upper end  87   b  of the helical compression spring  87 . The upper end supporting table  130  is movable in a direction in which the helical compression spring  87  is compressed. In addition, the movement of the upper end supporting table  130  is regulated in a direction in which the helical compression spring  87  is compressed by the regulating member  94 . 
     Adjustment of the pressure is performed in the following manner. That is, the upper end supporting table  130  is moved by a jig (not illustrated) so that the pressure of the helical compression spring  87  is maintained at predetermined pressure. At that time, the pressure of the helical compression spring  87  is measured by a pressure meter attached to the jig via the upper end supporting table  130 . The upper end supporting table  130  is fixed at a position at which the measured value of the pressure meter is the predetermined pressure by the regulating member  94 . In this manner, the position of the upper end supporting table  130  in a pressure direction of the helical compression spring  87  is regulated relative to the frame  91  and, thus, the predetermined pressure is obtained. According to the present exemplary embodiment, the regulating member  94  is formed from a screw. The screw is screwed in the pressure direction of the helical compression spring  87 , and the top end of the screw supports the upper end supporting table  130 . In this manner, the position of the upper end supporting table  130  is regulated. 
     According to the present exemplary embodiment, the pressure mechanism is supported by the top end of the screw serving as the regulating member  94 , and the position of the upper end supporting table  130  is regulated. Accordingly, the upper end supporting table  130  is easily inclined. As described above in the second exemplary embodiment, to straighten out the bend of the helical compression spring  87  occurring when the helical compression spring  87  is compressed, the helical compression spring  87  moves the lever  84  and, thus, is inclined from the direction of the pressure Ft. Consequently, the spring end surface at the upper end  87   b  is inclined in a direction in which the amount of compression of the helical compression spring  87  decreases and the inclination of the helical compression spring  87  from the direction of the pressure Ft increases. In this manner, the lever  84  is moved. 
     Accordingly, as in the second exemplary embodiment, in the present exemplary embodiment, the helical compression spring  87 R having a winding direction that is opposite to the winding direction of the helical compression spring  87 L is employed. In addition, the position of the winding end of the helical compression spring  87 R is set so as to be substantially symmetrical to the position of the winding end of the helical compression spring  87 L. As a result, an effect that is the same as the effect of the second exemplary embodiment can be obtained. That is, the difference in the surface pressure in the nip between right and left is reduced, and the difference in fixability and the difference in gloss throughout an image can be reduced. 
     By employing the above-described configuration, a fixing device that negligibly generates the difference in fixability and the difference in gloss throughout an image can be provided. 
     In addition, by employing the configuration according to the present exemplary embodiment, the difference in the surface pressure in the nip between right and left can be reduced. Thus, the difference in conveyance force between both the ends is less likely to occur. As a result, the rate of occurrence of paper wrinkling can be reduced. 
     Furthermore, by employing the configuration according to the present exemplary embodiment, the difference in the surface pressure in the nip can be reduced and, thus, the lifetime of the fixing device can be increased. 
     Note that according to the present exemplary embodiment, even when like the third exemplary embodiment, the position of the helical compression spring is changed, the same effect can be obtained. 
     The effect to increase the recording medium conveyance performance in the fixing nip N 2  of the pressure mechanism of the fixing device according to the present exemplary embodiment can be applied to recording medium conveyance devices that convey a recording medium using a nip portion formed by two rotary members in tight contact with each other, in addition to fixing devices. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-015749 filed Jan. 29, 2015 and No. 2015-074301 filed Mar. 31, 2015, which are hereby incorporated by reference herein in their entirety.