Abstract:
A fixing device has a frame, a heating roller, a press roller, a peeler, a guide member, and a wheel. The heating roller includes a roller rotation axis and a cylindrical surface. The press roller is pressed against the cylindrical surface to feed a recording medium together with the heating roller along a feeding path. The peeler has a peeling surface having a leading edge, where the peeling surface faces the feeding path. The guide member faces the peeling surface to guide the recording medium to downstream of the feeding path. The guide member is movable between a near position and a far position relative to the peeling surface. The wheel is positioned in a manner that a part of the circumferential edge protrudes from the peeling surface to the guiding member in a virtual plane perpendicular to the wheel rotation axis.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2006-323809 filed on Nov. 30, 2006. The entire content of this priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fixing device for thermally fixing an image onto a recording sheet, and an image forming device provided with such a fixing device. 
     BACKGROUND 
     Generally, a well-known fixing device used for an image forming device includes a heating roller heated by an internal heat source, a press roller for holding a recording sheet together with the heating roller, and a peeling claw facing the heating roller to peel off the recording sheet from the heating roller. 
     The fixing device feeds the recording sheet onto which a toner image has been transferred, holding the recording sheet between the heating roller and the press roller, to thermally fix the toner image onto the recording sheet. When the recording sheet having the toner image thereon is stuck to the cylindrical surface of the heating roller, the peeling claw is used to peel off the recording sheet from the surface of the heating roller. 
     When the recording sheet is jammed near the peeling claw, the jammed recording sheet frequently presses the peeling claw to the heating roller, which may cause damage on the cylindrical surface of the heating roller. In order to avoid the damage, Japanese Patent Application Publication S61-200564 discloses a peeling device in which the leading edge of the peeling claw is displaceably supported to the frame to remove away from the heating roller when the peeling claw is pressed by the jammed recording sheet. According to this configuration, even if the peeling claw is pressed by the jammed recording sheet, the leading edge of the peeling claw is moved away from the heating roller, thereby preventing any damage of the heating roller. 
     However, even if the above configuration is employed, the leading edge of the peeling claw is not always moved away from the heating roller, depending on the posture and/or the number of the recording sheets jammed near the peeling claw. In this case, the leading edge of the peeling claw may be forced to strongly press the heating roller, thereby causing damage thereto. 
     Therefore, an object of the present invention is to provide a fixing device which reliably prevents damage of the heating roller which may caused by the jammed recording paper jam. 
     SUMMARY 
     The present invention provides a fixing device having a frame, a heating roller, a press roller, a peeler, a guide member, a first bias unit, and a wheel. The heating roller is provided to the frame to produce heat, the heating roller having a roller rotation axis and a cylindrical surface. The press roller is pressed against the cylindrical surface to feed a recording medium together with the heating roller along a feeding path. The peeler is provided to the frame having a peeling surface having a leading edge. The leading edge faces the cylindrical surface to peel off the recording medium on the cylindrical surface. The peeling surface faces the feeding path. The guide member is positioned facing the peeling surface to guide the recording medium which has passed through between the heating roller and the press roller to downstream of the feeding path. The guide member is movable between a near position and a far position, the near position being closer to the peeling surface than the far position. The first bias unit biases the guide member to the near position. The wheel is supported to the frame and having a wheel rotation axis and a circumferential edge. The wheel rotation axis is parallel to the roller rotation axis. The wheel is positioned in a manner that a part of the circumferential edge protrudes from the peeling surface to the guiding member in a virtual plane perpendicular to the wheel rotation axis. 
     The present invention provides a fixing device having a frame, a heating roller, a press roller, a peeler, a guide member, a first bias unit, and a rotation member. The heating roller is provided to the frame to produce heat. The heating roller has a roller rotation axis and a cylindrical surface. The press roller is pressed against the cylindrical surface to feed a recording medium together with the heating roller along a feeding path. The peeler is provided to the frame and having a peeling surface having a leading edge, the leading edge facing the cylindrical surface to peel off the recording medium on the cylindrical surface. The peeling surface faces the feeding path. The guide member is positioned facing the peeling surface to guide the recording medium which has passed through between the heating roller and the press roller to downstream of the feeding path. The guide member is movable between a near position and a far position. The near position is closer to the peeling surface than the far position. The first bias unit biases the guide member to the near position. The rotation member is supported to the frame and has a rotation axis and an outer circumference. The rotation axis is parallel to the roller rotation axis. The rotation member is positioned in a manner that a part of the outer circumference protrudes from the peeling surface to the guiding member in a virtual plane perpendicular to the wheel rotation axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a side cross-sectional view showing a laser printer having a fixing device according to the present invention; 
         FIG. 2  is a side view showing the fixing device; 
         FIG. 3  is a perspective view of the fixing device; 
         FIG. 4A  is a block diagram of a controller; 
         FIG. 4B  is a block diagram of a determination unit; 
         FIG. 5  is a flowchart showing a processing performed by the determination unit; 
         FIG. 6  is a side view showing the fixing device when a paper jam occurs; 
         FIG. 7  is a side view showing the fixing device when the different type of the paper jam from that of  FIG. 6  occurs; 
         FIG. 8  is a perspective view showing a relationship between an optical sensor, a guide member, and a detection lever; and 
         FIG. 9  is a side view showing another embodiment of the fixing device. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment according to the present invention will be described referring to the accompanying drawings. In the following description, the expressions “front”, “rear”, “above” and “below” are used to define the various parts when a fixing device is disposed in an orientation in which it is intended to be used. Directional arrows pointing four directions (front, rear, up, and down) shown in each drawing are used as the directions referred to in the following description. 
       FIG. 1  shows a laser printer  1  having a sheet supply section  4  and an image forming section  5  in a main body casing  2 . The sheet supply section  4  functions to supply a sheet  3 , and has a sheet supply tray  11  detachably attached to the bottom portion inside the main body casing  2  and a sheet pressing plate  12  provided in the sheet supply tray  11 . The sheet supply section  4  further has a sheet supply roller  13  and a sheet supply pad  14  which are provided in the upper portion of one end side of the sheet supply tray  11 , and paper powder removing rollers  15 ,  16  provided on the downstream side relative to the sheet supply roller  13  in the feeding direction of the sheet  3 . The sheet supply section  4  further has resist rollers  17 ,  17  provided on the downstream side relative to the paper powder removing rollers  15  and  16  in the sheet feeding direction. 
     In the sheet supply section  4  having the above configuration, the recording sheets  3  stacked in the sheet supply tray  11  are pressed to the sheet supply roller  13  by the sheet pressing plate  12 , fed one by one by the sheet supply roller  13  and sheet supply pad  14 , and finally supplied to the image forming section  5  by the rollers  13 - 16 . 
     The image forming section  5  has a scanning unit  20 , a process cartridge  30 , and a fixing device  40  for forming an image onto the supplied sheet  3 . 
     The scanner section  20  is provided at the upper portion inside the main body casing  2  and has a laser beam source (not shown), a rotatable polygon mirror  21 , lenses  22 ,  23 , and reflection mirrors  24 ,  25 , and  26 . A laser beam which is emitted from the laser beam source based on image data passes through the polygon mirror  21 , the lens  22 , the reflection mirrors  24 ,  25 , the lens  23 , and the reflection mirror  26  in the order mentioned and impinges on the surface of a photoconductive drum  33  in the process cartridge  30  through a high speed scanning process. 
     The process cartridge  30  is provided below the scanner section  20  and detachably attached to the main body casing  2 . The process cartridge  30  includes a developer cartridge  32 , the photoconductive drum  33 , a scorotron charger  34 , and a transfer roller  35  within a hollow casing  31 . 
     The developer cartridge  32  is detachably attached to the casing  31  and has a developing roller  36 , a layer thickness regulation blade  37 , a supply roller  38 , and a toner hopper  39 . Toner in the toner hopper  39  is supplied to the developing roller  36  by the rotation of the supply roller  38  in the direction of an arrow (counterclockwise direction). The toner is frictionally charged positively between the supply roller  38  and the developing roller  36 . The toner supplied on the developing roller  36  enters between the layer thickness regulation blade  37  and the developing roller  36  by the rotation of the developing roller  36  in the direction of an arrow (counterclockwise direction). The toner is then carried on the developing roller  36  to form a thin layer having a constant thickness. 
     The photoconductive drum  33  is supported to the casing  31  to rotate in the direction of an arrow (clockwise direction). The main body of the photoconductive drum  33  is grounded. The surface of the photoconductive drum  33  is formed from a positively charged photosensitive layer. 
     The scorotron charger  34  is disposed above the photoconductive drum  33  to face the photoconductive drum  33  separated therefrom by a given distance. The scorotron charger  34  is a charger for generating corona discharge from a charging wire of tungsten for positive charge and positively charging the surface of the photoconductive drum  33  uniformly. 
     The transfer roller  35  is disposed below the photoconductive drum  33  so as to face the photoconductive drum  33  in a contact manner with the photoconductive drum  33  and supported by the casing  31  so as to rotate in the direction of an arrow (counterclockwise direction). The transfer roller  35  has a roller shaft made of metal and a roller surface formed of electrically conductive rubber material that covers around the roller shaft. A transfer bias voltage is applied to the transfer roller  35  during a transfer process. 
     The surface of the photoconductive drum  33  is positively charged uniformly by the scorotron charger  34 , and then is exposed to the laser beam by the high-speed scanning process from the scanner section  20 . As a result, the potential of the exposed portion is decreased to form an electrostatic latent image based on image data. The “electrostatic latent image” refers to the area exposed by the laser beam and having a lowered electric potential in the surface of the photoconductive drum  33  that has been positively charged uniformly. Thereafter, when the toner carried on the developing roller  36  comes into contact with the photoconductive drum  33  by the rotation of the developing roller  36 , the toner is supplied to the electrostatic latent image on the surface of the photoconductive drum  33 . Then, the toner is carried on the electrostatic latent image on the photoconductive drum  33 , so that the electrostatic latent image is visualized. In this manner, the toner image is formed by a reverse development method. 
     Thereafter, the photoconductive drum  33  and transfer roller  35  are rotatably driven so as to hold the sheet  3  for feeding. The toner image carried on the surface of the photoconductive drum  33  is transferred onto the sheet  3  while the sheet  3  passes between the photoconductive drum  33  and the transfer roller  35 . 
     As shown in  FIG. 2 , the fixing device  40  is disposed on the sheet feeding direction on the downstream side relative to the process cartridge  30 . The fixing device  40  has a heating roller  41 , a press roller  42 , a peeling claw  43 , and a guide member  44  in a frame  45 . The press roller  42  is disposed contacting with the heating roller  41  to hold the sheet  3  between the heating roller  41  and the press roller  42 . The peeling claw  43  peels off the sheet  3  on the heating roller  41 . The guide member  44  is provided on the downstream side relative to the heating roller  41  and press roller  42  in the sheet feeding direction. 
     The heating roller  41  is formed of a metal tube having a cylindrical surface  41   b  coated with a fluorine resin. The heating roller  41  includes a heater  41   a  made of a halogen lamp. The heating roller  41  is heated by the heater  41   a . The heating roller  41  is rotated about a roller axis which is co-axial with the heater  41   a . The heating roller  41  provides heat to the sheet  3  though the cylindrical surface  41   b.    
     The press roller  42  is disposed below the heating roller  41  so as to face the heating roller  41  for pressing the cylindrical surface of the heating roller  41  using a biasing unit (not shown). The press roller  42  has a roller shaft  42   a  made of metal and a roller formed of a rubber material covering around the roller shaft. The press roller  42  is rotated, following the rotation of the heating roller  41 . 
     The peeling claw  43  is movably supported to the frame  45 . The frame  45  supports the heating roller  41 , the press roller  42 , the peeling claw  43 , and the guide member  44 . The frame  45  is assembled and fixed in the main body casing  2 . The peeling claw  43  is provided for peeling off the sheet  3  from the heating roller  41  that has passed between the heating roller  41  and press roller  42 , thereby preventing the sheet  3  from being caught by the heating roller  41  due to the sticking of the sheet  3  to the heating roller  41 . 
     As shown in  FIG. 3 , a plurality of peeling claws  43  are provided to the frame  45  and arranged in the axial direction of the heating roller  41 . As shown in  FIG. 2 , the peeling claw  43  has a main body  43   a  and a pivotal shaft  43   b  provided near the upper end of the main body  43   a . The main body  43   a  has a substantially triangular platy shape vertically. The main body  43   a  has a leading edge  43   c  tapered narrowly and directed to the heating roller  41 . The leading edge  43   c  is configured to touch the cylindrical surface  41   b  of the heating roller  41  when a paper jam by the sheet  3  has not occurred near the peeling claw  43 . The main body  43   a  has a lower surface which serves as a peeling surface  43   d  for guiding the sheet  3  that has been peeled from the heating roller  41  to the downstream of the sheet feeding direction. The pivotal shaft  43   b  is movably received in a groove  45   a  formed in the frame  45 . The groove  45   a  extends in the vertical direction so as to be able to move the pivotal shaft  43   b  in the vertical direction. Thus, the peeling claw  43  is movably supported along the groove  45   a  to the frame  45 . In a normal operation, the peeling claw  43  is supported and positioned at the lower end of the groove  45   a . Accordingly, when an excessive load is applied to the peeling claw  43  from below, the peeling claw  43  can be moved upward along the groove  45   a.    
     A torsion spring  43   s  is wound around the pivotal shaft  43   b . One end of the torsion spring  43   s  is engaged to the frame  45 , and the other end of the torsion spring  43   s  is engaged to the main body  43   a . As a result, the peeling claw  43  is urged to the heating roller  41  in the counterclockwise direction in  FIG. 2 , i.e., in particular, the leading edge  43   c  is pressed against the cylindrical surface  41   b  of the heating roller  41  with a predetermined force. When the leading edge  43   c  is pressed to the cylindrical surface  41   b  with the predetermined force, the peeling claw  43  does not cause any damage to the cylindrical surface  41   b  of the heating roller  41 . 
     A remove assistance member  45   b  is formed integrally with the frame  45  above the peeling claw  43 . As a result, the remove assistance member  45   b  is fixedly assembled in the main body casing  2  of the laser printer  1 . Accordingly, when a paper jam occurs between the peeling claw  43  and the guide member  44 , the peeling claw  43  is moved upward along the groove  45   a  due to the jammed sheet. At the same time, a portion adjacent to the leading edge  43   c  of the main body  43   a  is brought into contact with the remove assistance member  45   b  to partially rotate the main body  43   a  about the pivotal shaft  43   b  away from the heating roller  41 , i.e., in the clockwise direction in  FIG. 2 . Accordingly, the leading edge  43   c  is moved away from the cylindrical surface  41   b  of the heating roller  41 . 
     Referring to  FIG. 3 , the guide member  44  has a plurality of guide plates  44   a  arranged in the axial direction of the heating roller  41 . Each of the guide plates  44   a  is formed integrally with the guide member  44 , and has a guide surface  44   b  ( FIG. 2 ) facing the peeling surface  43   d  of the peeling claw  43  by a predetermined distance. As shown in  FIG. 2 , the guide surface  44   b  is so configured that the sheet  3  which has passed through between the heating roller  41  and press roller  42  is brought into contact with the guide surface  44   b  to be guided upward in the sheet feeding direction. 
     The guide member  44  has a pivotal shaft  44   c  at a lower portion thereof. The pivotal shaft  44   c  protrudes in the axial direction of the heating roller  41 . The pivotal shaft  44   c  is pivotally supported by the frame  45  to allow the guide member  44  to rotate about the pivotal shaft  44   c . Accordingly, the guide surface  44   b  of the guide member  44  can be displaced between a near position and a far position with respect to the peeling surface  43   d  of the peeling claw  43 . When the guide member  44  is at the near position, the distance between the guide surface  44   b  and the peeling surface  43   d  is shorter. On the other hand, when the guide member  44  is at the far position, the distance between the guide surface  44   b  and the peeling surface  43   d  is farther. 
     A torsion spring  44   s  is wound around the pivotal shaft  44   c . One end of the torsion spring  44   s  is engaged to the frame  45 , and the other end of the torsion spring  44   s  is engaged to the guide member  44 . Accordingly, the guide member  44  is biased about the pivotal shaft  44   c  in the counterclockwise direction in  FIG. 2 , i.e., in the direction that the guide surface  44   b  comes closer to the peeling surface  43   d . In other words, the guide member  44  is urged to remain at the near position due to an elastic force of the torsion spring  44   s.    
     As shown in  FIG. 3 , a plurality of gears  46  are provided near the peeling claws  43  to the frame  45 , with one gear  46  being positioned near one peeling claw  43 . The gear  46  has a rotary shaft  46   a  extending in the same direction as the axial direction of the heating roller  41 . The gear  46  is provided downstream with respect to the peeling claw  43  in the sheet feeding direction, and rotatably supported to the frame  45  about the rotary shaft  46   a . The gear  46  has a plurality of bumps and dips arranged alternately on the outer circumference  46   b . Accordingly, the gear  46  is easily and reliably rotated by friction between the outer circumference  46   b  of the gear  46  and a surface of the sheet  3  when the sheet  3  is passed through the gear  46 . As shown in  FIG. 2 , the outer circumference  46   b  of the gear  46  protrudes from the peeling surface  43   d  of the peeling claw  43  to the guide member  44  in a vertical plane to the rotary shaft  46   a . With the above configuration, the gear  46  functions to separate the sheet  3 , that has been guided by the peeling surface  43   d  of the peeling claw  43 , from the peeling surface  43   d.    
     As shown in  FIG. 3 , a plurality of ribs  47  are provided to the frame  45  in the axial direction of the heating roller  41 . Two ribs  47  which are adjacent to each other are paired to interpose the peeling claw  43  and the gear  46  therebetween. The rib  47  protrudes from the peeling surface  43   d  to the guide member  44  in the vertical plane, as shown in  FIG. 2 . Similarly to the gear  46 , the rib  47  functions to separate the sheet  3  which has been fed by the peeling surface  43   d  of the peeling claw  43  from the peeling surface  43   d.    
     The guide member  44  is provided with a detection lever  48 . The detection lever  48  is positioned on the downstream side of the heating roller  41  and press roller  42  in the sheet feeding direction. As shown in  FIG. 3 , the detection lever  48  has a lever portion  48   a , a pivotal shaft  48   b , and a shield plate  48   c , which are formed integrally together. The lever portion  48   a  is positioned at the center of the width of the guide member  44 . The lever portion  48   a  has a bar shape. The pivotal shaft  48   b  passes through one end of the lever portion  48   a  so as to attach the detection lever  48  to the frame  45 . 
     The detection lever  48  is movably supported to the guide member  44  as to be pivotable between a falling position and a standing position about the pivotal shaft  48   b . The lever portion  48   a  is fell down rearwards in the falling position when the lever portion  48   a  contacts with the sheet  3  therewith. On the other hands, the lever portion  48   a  remains upright vertically in the standing position when the lever portion  48   a  is not contact with the sheet  3 . 
     The detection lever  48  is urged by a torsion spring  48   d  so as to return to the standing position. Referring to  FIG. 2 , one end of the torsion spring  48   d  is engaged to a boss  48   e  formed on one side of the lever portion  48   a . The other end of the torsion spring  48   d  is engaged to a boss  44   d  formed on the guide member  44 . Accordingly, the leading edge of the sheet  3  that has passed between the heating roller  41  and press roller  42  can be brought into contact with the lever portion  48   a.    
     The lower end of the lever portion  48   a  is connected to the pivotal shaft  48   b  extending in parallel with the axial direction of the heating roller  41 . The pivotal shaft  48   b  is pivotally supported to the guide member  44 . One end of the pivotal shaft  48   b  protrudes from one end of the guide member  44  in a width direction thereof (a left-right direction in  FIG. 3 ), as shown in  FIG. 3 . The shield plate  48   c  is formed integrally at the one end of the pivotal shaft  48   b . The shield plate  48   c  lies in a plane orthogonal to the axial direction of the pivotal shaft  48   b  and extends from the pivotal shaft  48   b  to the front direction of the laser printer. 
     An optical sensor  49  is provided at the right end portion of the frame  45 . The optical sensor  49  has a light-emission portion  49   a  and a light reception portion  49   b . The light emission portion  49   a  and the light reception portion  49   b  are disposed opposite to each other, so that a light beam emitted from the light emission portion  49   a  is received by the light reception portion  49   b . In other words, the light beam travels from the light emission portion  49   a  to the light reception portion  49   b . The optical sensor  49  is connected to a controller  60  for controlling the operation of the laser printer  1 . The controller  60  is assembled in the main body casing  2 . 
     When the sheet  3  is not passing or when a paper jam does not occur, the shield plate  48   c  is positioned between the light emission portion  49   a  and the light reception portion  49   b . Specifically, when the guide surface  44   b  of the guide member  44  is located near the peeling surface  43   d  and the detection lever  48  is located at the standing position, the shield plate  48   c  is positioned between the light emission portion  49   a  and the light reception portion  49   b.    
     That is, the passage of the sheet  3  and occurrence of a paper jam can be detected based on a light detection signal generated by the optical sensor  49  which receives the light beam. 
     The optical sensor  49  can detect the movement of the guide member  44  as well as the passage of the sheet  3 . When the sheet  3  comes to the guide member  44 , the detection lever  48  is pivoted to move the shield plate  48   c  upward from the position between the light emission portion  49   a  and the light reception portion  49   b . On the other hand, when the guide member  44  is pivoted about the pivotal shaft  44   c , the shield plate  48   c  is simultaneously displaced rearward since the detection lever  48  is supported to the guide member  44 . As a result, the shield plate  48   c  is moved away from the position between the light emission portion  49   a  and the light reception portion  49   b.    
     Next, the operational relationship between the shield plate  48   c  and the optical sensor  49  will be explained together with the operations of the guide member  44  and the detection lever  48 . 
     When the guide surface  44   b  of the guide member  44  is located near the peeling surface  43   d  and the detection lever  48  is located in the falling position, the shield plate  48   c  is moved upward and out of the position between the light emission portion  49   a  and the light reception portion  49   b.    
     When the guide surface  44   b  is located away from the peeling surface  43   d  and the detection lever  48  is located in the falling position, the shield plate  48   c  is moved upward and rearward out of the position between the light emission portion  49   a  and the light reception portion  49   b.    
     When the guide surface  44   b  is located away from the peeling surface  43   d  and the detection lever  48  is located in the standing position, the shield plate  48   c  is moved rearward out of the position between the light emission portion  49   a  and the light reception portion  49   b.    
     As described above, if the detection lever  48  remains at the falling position for at least a predetermined time period due to a force applied from the sheet  3 , or if the guide surface  44   b  is located far from the peeling surface  43   d  for the predetermined time period due to the force applied from the sheet  3 , it is considered that a paper jam occurs near the guide member  44  and the peeling claw  43 , which is detected by the optical sensor  49 . It is noted that the predetermined time period is a standard for determining whether the paper jam has occurred in the fixing device  40 . 
     The light detection signal of the optical sensor  49  is sent to the controller  60  as shown in  FIG. 4A . The controller  60  includes a CPU  61 , a RAM  62 , a nonvolatile random access memory (NVRAM)  63 , and a ROM  64 . The CPU  61  reads out and executes a program stored in the ROM  64  and a setting value stored in the NVRAM  63  according to the usage of the laser printer  1  to control the operation of the laser printer  1 . The controller  60  implements the program as a determination unit  70  for detecting the passage and jam of the sheet  3  based on the light detection signal from the optical sensor  49 . The determination unit  70  includes a timer  71 , a counter  72 , and comparison section  73 , as shown in  FIG. 4B . 
     The timer  71  generates an oscillation signal in order to measure the light detection time at the light reception portion  49   b  of the optical sensor  49 . The counter  72  receives the oscillation signal from the timer  71  and the light detection signal of the optical sensor  49 . The counter  72  sends the light reception time period to the comparison section  73  based on the received oscillation signal. The comparison section  73  compares a predetermined time period Tth and the light reception time period to output a state signal of the sheet  3 . The predetermined time period Tth has been stored in the controller  60 . Specifically, when the comparison section  73  receives the light reception time period which is shorter than the predetermined time Tth, the comparison section  73  outputs a signal indicating passage of the sheet  3 . When the comparison section  73  receives the light reception time period which is longer than the predetermined time Tth, the comparison section  73  outputs another signal indicating occurrence of a paper jam, which means that at least one of the detection lever  48  and the guide member  44  is felt down rearward. 
     With the above configuration, the determination unit  70  performs a determination processing, as shown in  FIG. 5 . The determination unit  70  first determines whether the light beam is detected at the light reception portion  49   b  (S 1 ). If the light beam is not detected (S 1 ;No), the determination unit  70  determines that a paper jam does not occur, and finishes the processing. If the light beam is detected (S 1 ;Yes), the determination unit  70  starts the counter  72  and determines whether the light beam has been received for the predetermined time period (S 2 ). If the light beam has been received for the predetermined time period (S 2 ;Yes), the determination unit  70  determines that the paper jam has occurred (S 3 ). If the time period for receiving the light beam is less than the predetermined time period (S 2 ;No), the determination unit  70  determines that the sheet  3  has passed without any trouble (S 4 ). 
     As shown in  FIG. 1 , the fixing device  40  thermally fixes toner on the sheet  3  while passing the sheet  3  between the heating roller  41  and press roller  42 . The sheet  3  is then transferred along a sheet discharge path  51  by means of the guide member  44 . The sheet  3  that has been fed to the sheet discharge path  51  is discharged onto a sheet discharge tray  53  by means of a sheet discharge roller  52 . Alternatively, the sheet  3  is fed back to the inside of the main body casing  2  by the a reverse rotation of the sheet discharge roller  52  or switching of a flapper  54 , and re-supplied to the upstream of the image forming section  5  by means of a plurality of reverse feeding rollers  55  for double-sided printing. 
     The operation of the laser printer  1  will be described as follows. 
     As shown in  FIG. 1 , when the laser printer  1  starts a printing operation, the recording sheet  3  is pushed up from the sheet supply tray  11  by the sheet pressing plate  12  and fed to the image forming section  5  through the rollers  13  to  16 . Subsequently, a toner image formed on the photoconductive drum  33  is transferred onto the sheet  3  by the process cartridge  30 . 
     Then, the sheet  3  onto which the toner image has been transferred is thermally-fixed by the fixing device  40 , while being held between the heating roller  41  and the press roller  42 . 
     The sheet  3  onto which the toner image has thermally been fixed passes through the heating roller  41 . At this time, since the leading edge  43   c  of the peeling claw  43  is biased by the torsion spring  43   s  to the cylindrical surface  41   b  of the heating roller  41 , as shown in  FIG. 2 , the leading edge  43   c  peels off the end portion of the sheet  3  from the heating roller  41 . Subsequently, the sheet  3  is fed to the rear side along the peeling surface  43   d  of the peeling claw  43 . 
     Then, the sheet  3  is brought into contact with the outer circumference  46   b  of the gear  46 , because the outer circumference  46   b  protrudes from the peeling surface  43   d  to the guide member  44  in the vertical plane. The sheet  3  is then separated from the peeling surface  43   d  while the gear  46  is rotated by a friction with the outer circumference  46   b  of the gear  46 . Since the gear  46  is configured to be rotatable, the gear  46  does not act as a resistance against the feeding of the sheet  3 . Accordingly, the sheet  3  is smoothly fed rearward. Further, the sheet  3  is then brought into contact with the rib  47  provided near the gear  46  to be separated from the peeling surface  43   d . The separated sheet  3  is guided upward by the guide surface  44   b  of the guide member  44 , passed through the sheet discharge path  51 , and discharged onto the sheet discharge tray  53 . When the sheet  3  is stuck to the heating roller  41 , the sheet  3  may directly be fed to the sheet discharging path  51  without contacting with the guide member  44 . 
     The sheet  3  is accidentally jammed on the rear side of the heating roller  41  and the press roller  42 . In this case, as shown in  FIG. 6 , the sheet  3  is pushed into between the peeling surface  43   d  of the peeling claw  43  and the guide surface  44   b  of the guide member  44  in a crumpled state. Then, a pressure is applied from the sheet  3  to the guide surface  44   b , and the guide member  44  is then pivoted about the pivotal shaft  44   c  to the rear side against the biasing force of the torsion spring  44   s . Therefore, an excessive load is not applied to the peeling surface  43   d  of the peeling claw  43 , thereby preventing the surface of the heating roller  41  from being scratched by the leading edge  43   c  of the peeling claw  43 . 
     If the peeling surface  43   d  of the peeling claw  43  is kept to be strongly pressed to the heating roller  41  by the sheet  3  in spite of the rearward movement of the guide member  44 , e.g., if the recording sheet  3  is sequentially pushed into between the guide member  44  and peeling claw  43  and a crumpled mass of the sheet  3  is becoming bigger, the peeling claw  43  is pushed up. In this case, the pivotal shaft  43   b  is moved upward along the groove  45   a . Then, the leading edge  43   c  of the peeling claw  43  is brought into contact with the remove assistance member  45   b  disposed above the peeling claw  43  and then pushed out to the rear side about the pivotal shaft  43   b . Thus, the peeling claw  43  is pushed up while being rotated in the clockwise direction about the pivotal shaft  43   b . As a result, the leading edge  43   c  is removed away from the cylindrical surface  41   b  of the heating roller  41 . 
     Thus, even if a large load generated by the crumpled sheet  3  is applied to the peeling surface  43   d , the leading edge  43   c  of the peeling claw  43  is moved away from the cylindrical surface  41   b  of the heating roller  41 , preventing the cylindrical surface  41   b  of the heating roller  41  from being scratched. 
     When the sheet  3  is normally fed without an occurrence of a paper jam, the sheet  3  is brought into contact with the detection lever  48  to pivot the detection lever  48  to the falling position periodically. Therefore, the state of the optical sensor  49  is periodically switched between a state in which the light beam is blocked by the shield plate  48   c  and another state in which the light beam is received by the light reception portion  49   b . In this case, the determination unit  70  determines that the sheet  3  is fed normally without the occurrence of a paper jam. 
     On the other hand, when the sheet  3  is jammed on the rear side of the heating roller  41 , e.g., when the sheet  3  is jammed so as to lay down the detection lever  48  to the rear side as shown in  FIG. 6 , the reception portion  49   b  continues receiving the light beam from the light emission portion  49   a  in spite of the rearward movements of the detection lever  48  and/or the guide member  44 . Therefore, the determination unit  70  determines occurrence of a paper jam when determining that the light reception portion  49   b  has continuously received the light beam for the predetermined time period. 
     On the other hand, when the paper jam occurs, as shown in  FIG. 6 , the detection lever  48  is not laid down, but the guide member  44  is laid down to the rear side, the detection lever  48  supported to the guide member  44  is displaced to the rear side, as shown in  FIG. 8 . Accordingly, the shield plate  48   c  is moved out of the position between the light emission portion  49   a  and the light reception portion  49   b . The light reception portion  49   b  then continues receiving the light beam from the light emission portion  49   a , and the determination unit  70  determines the occurrence of a paper jam when determining that the light reception portion  49   b  has continuously received the light beam for the predetermined time period. 
     When the paper jam happens, the jammed sheet  3  presses the guide member  44  to the rear side to fall down the guide member  44  about the pivotal shaft  44   c , the damage of the heating roller  41  caused by the peeling claw  43  can be prevented. 
     The lever portion  48   a  is provided at the center of the width of the guide member  44 , so that the condition of the sheet  3  can be reliably detected. 
     According to the fixing device  40  of the present embodiment, even if the paper jam occurs on the rear side of the heating roller  41 , the guide member  44  is laid down to the rear side, thereby preventing the peeling claw  43  from scratching the cylindrical surface  41   b  of the heating roller  41 . Further, even if the sheet  3  is going to strongly press the peeling claw  43  to the heating roller  41 , the leading edge  43   c  is removed away from the cylindrical surface  41   b  of the heating roller  41 , while the peeling claw  43  is moved upward along the groove  45   a . This structure prevents the peeling claw  43  from scratching the cylindrical surface  41   b  of the heating roller  41  when the paper jam occurs in the fixing device  40 . 
     Further, when the guide member  44  is laid down, the paper jam can be detected by the displacement of the detection lever  48 . And the determination unit  70  then determines that the paper jam has occurred. Accordingly, the paper jam can be detected without providing an additional new sensor in the main body casing  2 , so that a user of the laser printer  1  can notice the occurrence of the paper jam. 
     Further, the gear  46  and the rib  47  separate the sheet  3  from the peeling surface  43   d  of the peeling claw  43  readily. Accordingly, the sheet  3  can smoothly be fed. Even if the sheet  3  is guided by the gear  46  and then coming into contact with the peeling claw  43  again, the rib  47  separates the sheet  3  away from the peeling claw  43 . This structure enhances preventing the sheet  3  to be jammed near the peeling surface  43   d . Accordingly, an occurrence of the paper jam near the peeling claw  43  can be reliably prevented. Further, the damage of the heating roller  41  by the peeling claw  43  can be reliably prevented. 
     The gear  46  protrudes from the rib  47  to the guide member  44  in the vertical plane. This structure prevents toner on the sheet  3  from being transferred to the rib  47 . Accordingly, the occurrence of the paper jam near the peeling claw  43  caused by the larger friction of the rib  47  can be prevented. 
     The plurality of ribs  47  are provided in the width direction of the frame  45  at intervals, so that the sheet  3  can be reliably transferred. 
     The plurality of guide plates  44   a  assist guiding the sheet  3  having the thermally-fixed image reliably. Accordingly, the paper jam of the sheet  3  near the peeling claw  43  is reliably prevented. 
     The present invention has been described with reference to the above embodiment. However, the present invention is not limited to the above embodiment, but modifications and changes are within the scope of the claims. 
     The present invention is applicable to any other types of image forming devices such as a copier and/or a multi function printer. 
     Instead of the gear  46  described above, a roller with a smooth outer circumference can be employed. 
     Instead of the shield plate  48   c , a bar-like shield member can be employed. In another embodiment, the shield plate  48   c  is positioned out of the position between the light emission portion  49   a  and the light reception portion  49   b  when the sheet  3  does not pass between the peeling claw  43  and the guide member  44 . And, the detection lever  48  is pivoted to block the light beam when the sheet  3  is passing. In this case, the shape and position of the shield plate  48   c  are determined such that the shield plate  48   c  blocks the light beam from the light emission portion  49   a  when the guide plate  44  is laid down to the rear side. 
     In another embodiment, the light emission portion  49   a  does not face the light reception portion  49   b , but a light beam emitted from the light emission portion  49   a  can be guided to the light reception portion  49   b  by means of any type of a reflecting member. 
     In another embodiment, another gear  46 A can be provided to the frame  45  on the downstream side of the rib  47  in the sheet feeding direction, as shown in  FIG. 9 . The gear  46 A protrudes from the rib  47  to the guide member  44  in the vertical plane. The above structure enhances the feed of the sheet  3 . The above structure prevents a part of toner image fixed to the sheet  3  from being deposited to the rib  47 .