Patent Publication Number: US-8538307-B2

Title: Fixing device

Description:
TECHNICAL FIELD 
     The present invention relates to a fixing device (fixing apparatus) mounted in an image forming apparatus, using an electrophotographic type or an electrostatic recording type, such as a copying machine, a laser beam printer or a facsimile machine. 
     BACKGROUND ART 
     The image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus forms a toner image on a recording material and fixes thus toner image by heating and pressing the toner image to form an image. As a type of the fixing device used in such an image forming apparatus, a roller fixing type in which a pressing roller is press-contacted to a fixing roller including a heater therein to form a fixing nip and effects fixing has been conventionally employed. An example of such an image forming apparatus including the fixing roller and the pressing roller is described in Japanese Laid-Open Patent Application (JP-A) Hei 7-129018. 
     The image forming apparatus described in JP-A Hei 7-129018 includes a motor for driving the fixing device or the like. When this motor is reversely rotated, a pendulum gear and an inner gear having partly omitted teeth are engaged with each other by gears and a pendulum arm, so that the gears are rotated. Then, a gear set so as to rotate together with the gear having partly omitted teeth with a predetermined reduction ratio and thereby a mangle gear is rotationally moved and a nip is released by a torsion bar. Contrary to this, in the case where removal of the recording material is detected by a sensor arm, the motor is rotated in a normal direction to return the torsion bar in a reverse procedure, so that the nip is returned to an original state. According to such a constitution, release and restoration of the nip between the fixing roller and the pressing roller can be effected. 
     However, in JP-A Hei 7-129018, also after the fixing pendulum is swung in the reverse direction, an external gear  15   i , a gear  15   k  and a mangle gear  30  are continuously rotated. In this case, in a no-load state in which these gears do not receive a driving force of the fixing pendulum, the external gear  15   i , the gear  15   k  and the mangle gear  30  are continuously rotated rapidly and then are abruptly stopped when a balance is achieved. In a period from this rapid rotation to the abrupt stop, impact noise is generated. That is, in the case where a gear engagement state is transferred from a nip-released state by the pressing roller to a nip-functioning state by the pressing roller, the gears free from the engagement are rotated rapidly, so that the impact noise is generated. 
     A problem of the present invention is to provide a fixing device capable of suppressing a phenomenon such that the gears free from the engagement are rotated rapidly, during the transition between the gear engagement state in the nip-released state by the pressing roller and the gear engagement state in the nip-functioning state by the pressing roller, to generate the impact noise. 
     DISCLOSURE OF THE INVENTION 
     The present invention for solving the above-described problem is a fixing device comprising: a rotatable member for conveying a recording material for carrying an image; a back-up member for forming a fixing nip, together with the rotatable member, in which the image is to be fixed on the recording material; a pressure-applying mechanism for applying pressure to the fixing nip; a cam for releasing the pressure applied to the fixing nip by acting on the pressure-applying mechanism; a motor, capable of rotating normally and reversely, for driving the cam; a first power transmitting path for permitting transmission of power of the motor; a second power transmitting path for permitting transmission of the power of the motor; a swingable gear for transmitting the power of the motor to the first power transmitting path or the second power transmitting path; a swingable arm for holding the swingable gear; and a preventing member for preventing movement of the swingable arm by being engaged with the swingable arm, wherein of the first power transmitting path and the second power transmitting path, at least the first power transmitting path is a path for permitting transmission of the power of the motor to the cam, wherein when the motor rotates in one direction, the swingable arm is tilted so that the swingable gear is moved to a first position for permitting transmission of the power of the motor to the first power transmitting path, and when the motor rotates in the other direction, the swingable arm is tilted so that the swingable gear is moved to a second position for permitting transmission of the power of the motor to the second power transmitting path, and wherein when the swingable gear is located at the first position, the preventing member engages with the swingable arm so that the swingable gear is prevented from moving from the first position, and when the swingable gear is located at the second position, the preventing member is spaced from the swingable arm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing a structure of an image forming apparatus according to Embodiment 1 of the present invention. 
       Parts (a) and (b) of  FIG. 2  are partly enlarged perspective views showing a structure of a fixing device. 
         FIG. 3  is an enlarged side view showing a structure of a pressing and (pressure-)releasing mechanism. 
         FIG. 4  is an enlarged side view showing the structure of the pressing and releasing mechanism. 
         FIG. 5  is an enlarged side view showing the structure of the pressing and releasing mechanism. 
       Parts (a) and (b) of  FIG. 6  are side views showing a pressing and releasing mechanism provided in a fixing device according to Embodiment 2 of the present invention. 
       Parts (a) and (b) of  FIG. 7  are side views showing a structure in which a part of mechanisms such as a cam is removed from a pressing and releasing mechanism provided in a fixing device according to Embodiment 3 of the present invention. 
       Parts (a) and (b) of  FIG. 8  are side views showing the structure. 
       Parts (a) and (b) of  FIG. 9  are side views showing a structure of a pressing and releasing mechanism provided in a fixing device according to a comparative embodiment. 
       Parts (a) and (b) of  FIG. 10  are side views showing the structure of the pressing and releasing mechanism provided in the fixing device according to the comparative embodiment. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinbelow, with reference to the drawings, preferred embodiments of the present invention will be exemplarily described specifically. However, dimensions, materials, shapes and relative positions of constituent elements described in the embodiments are appropriately changed depending on structures and various conditions of apparatuses to which the present invention is applied and therefore the scope of the present invention is not intended to be limited thereto unless otherwise particularly specified. 
     Embodiment 1 
       FIG. 1  is a sectional view showing a structure of an image forming apparatus  1  according to Embodiment 1 of the present invention. The image forming apparatus  1  is a laser beam printer using an electrophotographic image forming process. As shown in  FIG. 1 , the image forming apparatus  1  includes an image forming apparatus main assembly (hereinafter simply referred to as an “apparatus main assembly”)  1 A and inside this apparatus main assembly, an image forming portion  51  which is an “image forming means” for forming an image on a sheet which is a “recording material (medium)” is provided. The image forming portion  51  includes a photosensitive drum  2  which is an “image bearing member” and a transfer roller  6  which is a “transfer device”, and the like. 
     At least the photosensitive drum  2  is included in a process cartridge  3  and has a constitution to be incorporated into the apparatus main assembly as the process cartridge  3 . Inside the apparatus main assembly  1 A, a laser scanner scanning optical system  4  is incorporated and an electrostatic image is formed on a surface of the photosensitive drum  2  by this laser scanner scanning optical system  4 . In the case of the “image forming means” or the “image forming portion”, this is a concept at least including the photosensitive drum  2  and may also be a concept integrally encompassing other members such as the transfer roller  6 , an unshown developing device for forming a toner image on the photosensitive drum  2 , and an unshown cleaner, and the like. 
     Next, with respect to the constitution (structure) of the image forming apparatus  1 , a schematic structure will be described along a sheet P. The sheet P separated and fed one by one from a sheet-feeding tray  52  is conveyed by a registration roller pair  5 . Onto the sheet P, the toner image on the photosensitive drum  2  is transferred by the transfer roller  6 . Thereafter, the sheet P is heated and pressed by a fixing device  7 . On the sheet P, the toner image is fixed. The sheet P is discharged on a discharge tray  9  by an upper sheet-discharging roller  8 . Inside the apparatus main assembly  1 A, a controller  63  which is a “controller means” for controlling drive of respective devices is provided. 
     Parts (a) and (b) of  FIG. 2  are partly enlarged perspective view showing a structure of the fixing device  7 . Part (a) of  FIG. 2  shows a state in which a fixing film  33  is urged against a pressing roller  31 . Part (b) of  FIG. 2  shows a state in which the fixing film  33  is not urged against the pressing roller  31 . The fixing film  33  is rotatably supported by a frame  35 . For convenience of illustration, in (a) of  FIG. 2  and (b) of  FIG. 2 , with respect to the frame  35 , only a part thereof is illustrated and other portions thereof are illustrated in a cut state. 
     First, as shown in (a) of  FIG. 2 , the fixing device  7  includes the fixing film  33 . Further, the fixing device  7  includes the pressing roller  31  which is a “rotatable member”, which forms a fixing nip in contact with the fixing film  33 , for conveying the sheet P which is the “recording material” in the nip while nipping the sheet P. Inside the fixing film  33 , a heater  60  which is a “heating means” is provided. However, inside at least one of the fixing film  33  and the pressing roller  31 , the heater  60  which is the “heating means” for heating at least one of the fixing film  33  and the pressing roller  31  may only be required to be provided. At each of both end portions of the fixing film  33 , a holder  32  for holding the fixing film  33  is disposed. With respect to the holder  32 , the fixing film  33  is slidable. With respect to these fixing film  33  and pressing roller  31 , these members are referred to as a fixing film unit  34  in the following description. 
     Further, the fixing device  7  includes an urging member  36  for urging one of the fixing film  33  and the pressing roller  31  toward the other. On the urging member  36 , a pressing spring  37  is fixed. The urging member  36  and the pressing spring  37  constitute a pressure-applying mechanism. By an urging force of the urging member  36 , the urging member  36  urges the fixing film unit  34  in a direction toward the pressing roller  31 . Thus, the fixing nip is formed between the fixing film  33  and the pressing roller  31 , and the sheet P on which an unfixed toner image is transferred passes through the nip. During the passing, the sheet P is heated and pressed, so that the unfixed toner image is fixed on the surface of the sheet P. 
     At the surface of the fixing film  33 , in order to obtain a god fixability of the unfixed toner image, an elastic layer of a rubber or the like is provided. Here, when the fixing film  33  and the pressing roller  31  are left standing in a state in which they are press-contacted as they are, there was a possibility that elastic layers at the surfaces of the fixing film  33  and the pressing roller  31  are deformed. 
     Further, the sheet P was clogged during the passing thereof through the nip between the fixing film  33  and the pressing roller  31  to cause a jam in some cases. In the case where the jammed sheet position is pulled out, when the fixing film  33  and the pressing roller  31  are kept in the press-contact state, a frictional load is large and therefore the sheet P is not easily removed, so that there was also a possibility that the sheet P is torn and remain at the inside of the apparatus main assembly ( FIG. 1 ). 
     In order to solve such problems, a cam  38  is rotated so that the surface state of the cam  38  can be transferred from a state in which a flattened surface  38   a  of the cam  38  is disposed in parallel to the urging member  36  ((a) of  FIG. 2 ) to a state in which a projection-like portion  38   b  of the cam  38  abuts against the urging member  36  ((b) of  FIG. 2 ). By such a constitution for releasing the pressure application (urging), in the fixing device  7 , the above-described deformation of the elastic layer  4   s  of the fixing film  33  and the pressing roller  31  is prevented, so that the jammed sheet P is easily cleared. An inner pressing and releasing mechanism  61  of such a fixing device  7  will be described specifically below. 
     The pressing and releasing mechanism  61  presses, in the case of the state shown in (a) of  FIG. 2 , the fixing film  33  against the pressing roller  31 . That is, in the case of the state in which the flattened surface  38   a  of the cam  38  is parallel to the urging member  36 , the urging member  36  which receives an urging force of the pressing spring  37  urges a projected portion  32   a  formed on the holder  32  in a direction toward the pressing roller  31 . The holder  32  is supported by the frame  35  so that it is movable in the direction toward the pressing roller  31  along an inducing hole  35   a  formed in the frame  35 . 
     Further, the pressing and releasing mechanism  61  retracts, in the case of the state shown in (b) of  FIG. 2 , the fixing film  33  from the pressing roller  31 . That is, in the case where the cam  38  is rotated to direct the projection-like portion  38   b  of the cam  38  toward the urging member  36  side thereby to push back the urging member  36 , the urging member  36  is movable against the urging force of the pressing spring  37 . Further, a pressure-functioning state and a pressure-released state of the fixing film  33  by the fixing film unit  34  are configured to be switched. Next, a characteristic constitution (structure) of the pressing and releasing mechanism  61  will be described. 
       FIGS. 3 to 5  are enlarged side views showing the structure of the pressing and releasing mechanism  61 . In  FIGS. 3 to 5 , in order to facilitate understanding of the description of a driving method, the fixing film unit  34  and the pressing roller  31  are omitted. As shown in  FIG. 3 , the pressing and releasing mechanism  61  includes a motor  62  which is a “driving means”. Further, the pressing and releasing mechanism  51  includes the cam  38 , which is rotationally driven by the motor  62 , for moving the urging member  36  in a direction in which the urging force by the urging member  36  is released or in a direction in which the urging force by the urging member  36  is restored. Further, the pressing and releasing mechanism  61  includes a swingable gear  44  which is swung depending on normal or reverse rotation of the motor  61 . Further, the pressing and releasing mechanism  61  includes a swingable arm stopper  103  which is a “preventing (regulating) member” capable of transmitting a driving force of the swingable gear  44  to the cam  38  by preventing the swing of the swingable gear  44 . These gears perform a characteristic operation in the pressing and releasing mechanism  61 . 
     Although will be described later, the motor  62  which is the “driving means” and a pressing roller gear  42  which is a “fixed gear” are connected by a first gear train (third power transmitting path). To this first gear train, a shaft gear  39  and gears  40  and  41  correspond. Further, although described later, by including the swingable gear  44 , the motor  62  which is the “driving means” and the cam  38  are connected by a surface gear train. The shaft gear  39 , gears  40  and  43 , the swingable gear  44 , a gear having partly omitted teeth  45  and a gear  46  correspond to this second gear train. 
     Specific description will be made by including the above-described principal constituent elements. First, to the motor  62 , the shaft gear  39  is attached. Further, with this shaft gear  39 , a gear  101  is engaged. With the gear  102 , a gear  102  is coaxially provided. Further, between the gears  101  and  102 , a torque limiter function is provided. Further, with the gear  102 , the swingable arm stopper  103  partly provided with gear teeth  103   a  is engaged. 
     On the other hand, the gear  40  is engaged with the shaft gear  39  of the motor  62 . The gear  41  is engaged with the gear  40 . The pressing roller gear  42  which is the “fixed gear” fixed to the pressing roller  31  is engaged with the gear  41 . The pressing roller gear  42  is attached to the pressing roller  31 . By such a constitution, the driving force of the motor  62  is transmitted to the pressing roller  31 . The fixing film  33  can be moved by the contact with the pressing roller  31 . Incidentally, here, the pressing roller gear  42  fixed to the pressing roller  31  will be described but can also be replaced with a gear attached to the fixing film  33 . 
     On the other hand, a gear  43  is engaged with the gear  40  engaging with the shaft gear  39  of the motor  62 . The swingable gear  44  is engaged with the gear  43 . A swingable arm  149  is attached to a shaft of the gear  43  and a shaft of the gear  44 . At an opposing position of teeth of the swingable gear  44 , the gear having partly omitted teeth  45  is disposed. A gear  48  is engaged with the gear having partly omitted teeth  45 . At a front surface side in  FIG. 3  more than the gear  46 , the gear  46  and the cam  38  are rotatably mounted. The gear  46  and the cam  38  are separate members from the gear  48  and are individually rotatable. In the case where the swingable gear  44  and the gear having partly omitted teeth  45  are engaged with each other, i.e., when the swingable gear is located at a first position, the driving force of the swingable gear  44  is transmitted to the gear having partly omitted teeth  45  and the gear  46  (first power transmitting path) and is finally transmitted to the cam  38 . 
     For the reason described above, the drive of the pressing roller  31  and the drive of the cam  38  are effected by the same motor  62 . However, the transmission of the driving force from the motor to the cam  38  is performed via a part or all of the gears  40  and  43 , the swingable gear  44 , the gear having partly omitted teeth  45 , and the gears  46 ,  47  and  48  which are a “driver transmitting gear train”. 
     With respect to the operation of the pressing and releasing mechanism  61 , there are three modes described below. A “first mode” is a mode during the pressing (pressure application) between the pressing roller  31  and the fixing film  33  and is a mode during the conveyance of the sheet P. In the case of this first mode, the pressing roller gear  42  is driven and the cam is in a rest (stopped) state. A “second mode” is a mode during pressure release between the pressing roller  31  and the fixing film  33 . In the case of this second mode, the pressing roller gear  42  is stopped and the cam  38  is rotated to place the pressing roller  31  and the fixing film  33  in a pressure-released state. A “third mode” is a mode during pressure restoration between the pressing roller  31  and the fixing film  33 . In the case of this third mode, the pressing roller gear  42  is rotated and at the same time, the cam  38  is rotationally moved to transfer the pressing roller  31  and the fixing film  33  into a pressure-restored state. These first to third modes will be described specifically while making reference to  FIGS. 3 to 5  below. 
     First, the first mode will be described with reference to  FIG. 3 . As shown in  FIG. 3 , the motor  62  is driven, so that the shaft gear  39  is rotated clockwise. By the clockwise rotation of the shaft gear  39 , the gear  40  is rotated counterclockwise, so that the gear  43  is rotated clockwise and the swingable gear  44  is rotated counterclockwise. At the opposing position of the teeth of the swingable gear  44 , the gear having partly omitted teeth  45  provided with a teeth-omitted surface  45   a  is disposed. The gear having partly omitted teeth  45  is provided with the teeth-omitted surface  45   a  where the teeth are not formed and a gear surface  45   b  where the teeth are formed. The gear having partly omitted teeth  45  is, in the case where the teeth-omitted surface  45   a  opposes the swingable gear  44 , stopped since the teeth-omitted surface  45   a  cannot receive the driving force of the swingable gear  44 . Thus, the driving force is blocked and therefore the cam  38  is stopped. 
     On the other hand, the shaft gear  39  is rotated clockwise, so that the gears  101  and  102  are rotated counterclockwise and the swingable arm stopper  103  is rotated clockwise. When the swingable arm stopper  103  is rotated in a direction of an arrow C, an end portion  103   b  of the swingable arm stopper  103  abuts against an abutment portion  149   a  formed on the swingable arm  149 . The abutment portion  149   a  is formed in a recessed portion, i.e., a recess, for receiving and stopping the end portion  103   b  of the swingable arm stopper  103 . A position P of the recessed portion of the swingable arm  149  is spaced from a position of a rotational movement shaft R of the swingable arm  149 . (=a rotation shaft of the gear  43 ) more than a position of a rotation shaft Q of the swingable gear  44 . By this, the swingable gear  44  can be retained at the first position with a small force. Further, a force received from the swingable arm  149  by the swingable arm stopper  103  is, as shown by an arrow in  FIG. 3 , designed so as to be directed toward a rotational movement shaft S of the swingable arm stopper  103 . By this, deformation of the stopper  103  when the swingable arm stopper  103  receives the force from the swingable arm  149  can be suppressed. Thus, when the swingable arm stopper  103  which is the “preventing means” is rotationally moved, the swingable arm stopper  103  is moved to a prevention position J where it prevents the operation of the swingable gear  44 . The prevention position J is a position taken by the swingable arm stopper  103  so that the end portion  103   b  of the swingable arm stopper  103  abuts against the abutment portion  149   a  of the swingable arm  149  to prevent the swingable gear  44  from being contacted to the gear  47 . Further, by the torque limiter function provided between the gears  101  and  102 , the gear  102  and the swingable arm stopper  103  are stopped. 
     Next, the second mode will be specifically described with reference to  FIG. 4 . As shown in  FIG. 4 , in the case where the mode is transferred from the first mode to the second mode, the motor  62  is driven, so that the shaft gear  39  is rotated counterclockwise. Even when the shaft gear  39  is rotated controller, a one-way function is incorporated into the gear  41  of the gears  40  and  41  and the pressing roller gear  42  which constitute the drive transfer gear train connected to the pressing roller  31  and therefore the pressing roller gear  42  is stopped and the fixing film  33  is stopped. 
     On the other hand, by the counterclockwise rotation of the shaft gear  39 , the swingable gear  44  of the gears  40  and  43 , the swingable gear  44 , the gear having partly omitted teeth  45  and the gear  46  which constitute the drive transfer gear train is swingable. The swingable arm  149  is mounted to the swingable gear  44  so as to slightly rub the swingable gear  44  and is swung about a center shaft of the gear  43  in a direction of an arrow B. By the swing of this swingable arm  149 , the swingable gear  44  and the gear  47  are engaged with each other (second position), so that the gear  47  is rotated counterclockwise and the gear  48  is rotated clockwise. When the gear  48  is rotated clockwise, the gear having partly omitted teeth  45  is rotated counterclockwise, the gear  46  is rotated clockwise, and the cam  38  is rotated counterclockwise. In this case, the gears  47  and  48  correspond to a second power transmitting path. As a result, as shown in  FIG. 4 , the cam  38  is rotated until an apex portion of the projection-like portion  38   b  of the cam  38  reaches a position where it contacts the urging member  36 , so that the urging member  36  is moved in a right direction to result in a state in which the pressure of the fixing film  33  to the pressing roller  31  is released. 
     On the other hand, by the counterclockwise rotation of the shaft gear  39  of the motor  62 , the gears  101  and  102  are rotated clockwise and thus the gear  102  is engaged with the teeth  103   a  and therefore the swingable arm stopper  103  is rotated in a direction of an arrow D until it abuts against the abutment portion  104 . When the swingable arm stopper  103  abuts against the abutment portion  104 , the gear  102  and the swingable arm stopper  103  are stopped by the torque limiter function provided between the gears  101  and  102 . 
     Next, the third mode will be specifically described with reference to  FIG. 5 . As shown in  FIG. 5 , in the case where the mode is transferred from the second mode to the third mode, the motor  62  is driven, so that the shaft gear  39  is rotated clockwise again. When the shaft gear  39  is rotated clockwise, the gear  40  is rotated counterclockwise, the gear  43  is rotated clockwise, and the swingable gear  44  is rotated counterclockwise. As a result, the swingable gear  44  engaged with the gear  47  is swung while rotating counterclockwise, so that the swingable arm  149  is swung about the center shaft of the gear  43  is a direction of an arrow A. The swingable gear  44  approaches the gear having partly omitted teeth  45  and engages with the gear having partly omitted teeth  45  first position). The teeth-omitted surface  45   a  of the gear having partly omitted teeth  45  is, in the second mode, rotated until it is directed in an opposite direction from the swingable gear  44  and therefore the gear surface  45   b  of the gear having partly omitted teeth  45  can be engaged with the swingable gear  44 . The gear having partly omitted teeth  45  is rotated clockwise, so that the gear  46  is rotated counterclockwise and the cam  38  is rotated clockwise. 
     On the other hand, by the clockwise rotation of the shaft gear  39 , the gears  101  and  102  are rotated counterclockwise, so that the swingable arm stopper  103  is rotated in a direction of an arrow C. When the end portion  103   b  of the swingable arm stopper  103  abuts against the abutment portion  149   a , the gear  102  and the swingable arm stopper  103  are stopped by the torque limiter function provided between the gears  101  and  102 . 
     Based on such constitution and function in Embodiment 1, the rotation of the cam  38  progresses and when the cam  38  is rotated and moved to a certain position, the cam  38  rotates on its axis by moment thereof generated by the urging member  36 . At that time, a speed of the rotation of the cam  38  on its axis by load of the moment exerted on the cam  38  by the urging member  36  becomes faster than a rotational speed at which the cam  38  is rotationally driven. 
     As a result, the gear  46  and the gear having partly omitted teeth  45  are also rotated by the rotation of the cam  38  on its axis, thus being rotated quickly. The swingable gear  44  receives the rotational force from the gear having partly omitted teeth  45 , so that a force for moving the swingable arm  149  in the direction of the arrow B ( FIG. 4 ) opposite from an arrow A is generated. However, the swingable arm  149  abut against the swingable arm stopper  103  and therefore the gear having partly omitted teeth  45  and the swingable gear  44  are kept in the connected state. That is, the force of the cam  38  is transmitted through the gear  46 , the gear having partly omitted teeth  45 , the swingable gear  44 , and the gears  43  and  44  which constitute the driving train and finally retains a state in which the shaft gear  39  and the motor  62  are also connected with the driving train. As a result, even when the cam  38  will rotate quickly, by a brake of the motor  62 , the motor  62  will rotate the cam  38  at a constant speed, so that it is possible to avoid the impact noise. 
     Embodiment 2 
     Parts (a) and (b) of  FIG. 6  are side views showing a structure of a pressing and releasing mechanism  261  provided in an image forming apparatus according to Embodiment 2 of the present invention. Part (a) of  FIG. 6  shows a state in which the pressing and releasing mechanism  261  does not urge the cam  38  by the urging member  36 . Part (b) of  FIG. 6  shows a state in which the pressing and releasing mechanism  261  urges the cam  38  by the urging member  36 . Of constituent elements of the pressing and releasing mechanism  261  provided in the image forming apparatus in Embodiment 2, those having the same structures and effects of those of the pressing and releasing mechanism  61  in Embodiment 1 will be appropriately omitted from description by using the same reference numerals or symbols. Also in Embodiment 2, its constitution is applicable to an image forming apparatus similar to that in Embodiment 1 and therefore the description of the image forming apparatus will be omitted. 
     Differences of the pressing and releasing mechanism between Embodiment 2 and Embodiment 1 are that a swingable arm stopper  203  in the pressing and releasing mechanism  261  operates and functions with respect to a direction opposite from the direction in Embodiment 1 and that gears  205 ,  206 ,  207  and  245  and an abutment portion  204  are provided. Further, Embodiment 2 is also different from Embodiment 1 in that a swingable arm  249  operates and functions with respect to a direction opposite from that in Embodiment 1. However, the swingable arm stopper  202  is similar to the case of the swingable arm stopper  103  in Embodiment 1 in that when the swingable arm stopper  203  which is a “preventing member” in Embodiment 2 is rotationally moved, the swingable arm stopper  203  is moved to a prevention position J ((b) of  FIG. 6 ) where it prevents the operation of the swingable gear  44 . The prevention position J is a position taken by the swingable arm stopper  203  so that an end portion  203   b  of the swingable arm stopper  203  abuts against an abutment portion  249   a  of the swingable arm  249  to prevent the swingable gear  44  from being contacted to the gear  47 . In Embodiment 1, both of the first power transmitting path and the second power transmitting path were the path through which the power of the motor was transmitted to the cam. On the other hand, in the case of Embodiment 2, the first power transmitting path (the gear  245 , the gear  46  and the cam  38 ) is a path through which the power is transmitted to the cam, and the second power transmitting path (the gear  47 , the gear  205 , the gear  206  and the gear  207 ) is a path through which the pressing roller is driven. Part (a) of  FIG. 6  is the side view in the case where the swingable gear  44  is located at the second position. Part (b) of  FIG. 6  is the side view in the case where the swingable gear  44  is located at the first position. 
     By including the swingable gear  44 , the motor  62  which is the “driving means” and the pressing roller gear  42  which is the “fixed gear” are connectable by a first gear train. The shaft gear  39 , the gears  40  and  43 , the swingable gear  44 , and the gears  47 ,  205 ,  206  and  207  correspond to the first gear train. By including the swingable gear  44 , the motor  62  which is the “driving means” and the cam  38  is connectable by a second gear train. The shaft gear  39 , the gears  40  and  43 , the swingable gear  44 , the gear  245  and the gear  46  correspond to the second gear train. 
     A characteristic constitution in Embodiment 2 will be described. The gear  101  is connected with the shaft gear  39  of the motor  62 . The gear  102  is the gear coaxial with the gear  101 , and the torque limiter function is provided between the gears  101  and  102 . To the gear  102 , the swingable arm stopper  203  partly provided with a gear tooth  203   a  is connected. 
     Next, an operation will be described. In Embodiment 2, there are two operation modes. A first mode is a mode during pressure application of the fixing film  33  to the pressing roller  31  and is a mode during conveyance of the sheet P, and is a state in which the pressing roller gear  42  is driven and the cam  38  is stopped. A second mode is a mode during release of the pressure application of the fixing film  33  to the pressing roller  31  and is a state in which the pressing roller gear  42  is stopped and the cam  38  is rotationally moved. 
     Details of the first mode will be described by using (a) of  FIG. 6 . In (a) of  FIG. 6 , the shaft gear  39  of the motor  62  is rotated counterclockwise. The pressing roller gear  42  is rotated and driven by the gears  40  and  43 , the swingable gear  44  and the gears  47 ,  205 ,  206  and  207  which are the drive transmitting gear train connected from the shaft gear  39  of the motor  62  to the fixing film  33 . Further, from the shaft gear  39  of the motor  62  to the cam  38 , the drive is transmitted through the gears  40  and  43 , the swingable gear  44  and the gears  245  and  46  which are the drive transfer gear train, but the swingable gear  44  and the gear  245  are spaced from each other, so that the cam  38  is stopped. 
     To the swingable arm stopper  203 , the drive is transmitted through the gears  101  and  102 , the swingable arm stopper  203  is rotated in a direction of an arrow D until it abuts against the abutment portion  204 . When the swingable arm stopper  203  abuts against the abutment portion  204 , the gear  102  and the swingable arm stopper  203  are stopped by the torque limiter function provided between the gears  101  and  102 . 
     Next, details of the second mode will be described by using (b) of  FIG. 6 . When the mode is transferred from the first mode to the second mode, the shaft gear  39  of the motor  62  is rotated reversely, i.e., is rotated clockwise in (b) of  FIG. 6 . The gear  40  is rotated counterclockwise, and the gear  43  is rotated clockwise. The swingable arm  249  is mounted to the swingable gear  44  so as to slightly rub the swingable gear  44 , so that the swingable arm  249  is swung about the center shaft of the gear  43  in the arrow A direction. As a result, the swingable gear  44  and the gear  47  are spaced from each other and therefore the gears  47 ,  205 ,  206  and  207  and the pressing roller gear  42  are stopped, so that the pressing roller  31  is stopped. With respect to the cam  38 , the drive is transmitted through the gears  40  and  43 , the swingable gear  44  and the gears  245  and  46  which are the drive transfer gear train, and finally the cam  38  is rotated clockwise to the position shown in (b) of  FIG. 6 , so that the urging member  36  is moved to result in the pressure-released state. 
     The driving force is transmitted to the swingable arm stopper  203  through the gears  101  and  102 , so that the swingable arm stopper  203  is rotated in the arrow C direction until the end portion  203   b  abuts against the abutment portion  249   a  provided to the swingable arm  249 . When the end portion  203   b  of the swingable arm stopper  203  abuts against the abutment portion  249   a , the gear  102  and the swingable arm stopper  203  are stopped by the torque limiter function provided between the gears  101  and  102 . In order to restore the pressing and releasing mechanism  261  to the pressure application state, the operation in the second mode is continued, so that the shaft gear  39  of the motor  62  is rotationally driven to the position shown in (a) of  FIG. 6 . 
     Based on such constitution and function in Embodiment 2, the rotation of the cam  38  progresses and when the cam  38  is rotated and moved to a certain position, the cam  38  rotates on its axis by moment thereof generated by the urging member  36 . At that time, a speed of the rotation of the cam  38  on its axis by load of the moment exerted on the cam  38  by the urging member  36  becomes faster than a rotational speed at which the cam  38  is rotationally driven. 
     As a result, the gear  46  and the gear  245  are also rotated by the rotation of the cam  38  on its axis, thus being rotated quickly. The swingable gear  44  receives the rotational force from the gear  245 , so that a force for moving the swingable arm  249  in the direction of the arrow B ((b) of  FIG. 6 ) opposite from an arrow A is generated. However, the swingable arm  249  abuts against the swingable arm stopper  203  and therefore the gear  245  and the swingable gear  44  are kept in the connected state. That is, the force of the cam  38  is transmitted through the gear  46 , the gear  245 , the swingable gear  44 , and the gears  43  and  44  which constitute the driving train and finally retains a state in which the shaft gear  39  and the motor  62  are also connected with the driving train. As a result, even when the cam  38  will rotate quickly, by a brake of the motor  62 , the motor  62  will rotate the cam  38  at a constant speed, so that it is possible to avoid the impact noise. 
     Embodiment 3 
     Parts (a) and (b) of  FIG. 7  are side views showing a structure of a pressing and releasing mechanism  361  provided in an image forming apparatus according to Embodiment 3 of the present invention. Part (a) of  FIG. 7  shows a state in which a swingable arm stopper  303  abuts against a swingable arm  349 . Part (b) of  FIG. 7  shows a state in which the swingable arm stopper  303  does not abut against the swingable arm  349 . Of constituent elements of the pressing and releasing mechanism  361  provided in the image forming apparatus in Embodiment 3, those having the same structures and effects of those of the pressing and releasing mechanism  61  in Embodiment 1 will be appropriately omitted from description by using the same reference numerals or symbols. Also in Embodiment 3, its constitution is applicable to an image forming apparatus similar to that in Embodiment 1 and therefore the description of the image forming apparatus will be omitted. 
     A differences of the pressing and releasing mechanism  361  in Embodiment 3 from the pressing and releasing mechanism  61  in Embodiment 1 is that a swingable arm stopper  303  is operated by a solenoid  308  in the pressing and releasing mechanism  361  to create an engaged state and an unengaged state with the swingable arm  349 . Further, Embodiment 3 is also different from Embodiment 1 in that when the swingable arm stopper  303  which is a “preventing means” in the pressing and releasing mechanism  351  is linearly operated, the swingable arm stopper  303  is moved to a prevention position K where it prevents the operation of the swingable gear  44 . The prevention position K is a position taken by the swingable arm stopper  303  so that an end portion  303   b  of the swingable arm stopper  303  abuts against an abutment portion  349   a  of the swingable arm  349  to prevent the swingable gear  44  from being contacted to the gear  47 . 
     Although will be described later, the motor  62  and a pressing roller gear  42  are connected by a first gear train. To this first gear train, a shaft gear  39  and gears  40  and  41  correspond. Further, although described later, by including the swingable gear  44 , the motor  62  and the cam  38  are connected by a surface gear train. The shaft gear  39 , gears  40  and  43 , the swingable gear  44 , a gear having partly omitted teeth  45  and a gear  46  correspond to this second gear train. 
       FIG. 3  will be described with reference to (a) and (b) of  FIG. 7 . Further, the same constitution as those in comparative embodiments and already-described Embodiment 1 or 2 quotes those constitutions in support thereof and will be omitted from description. The controller of the drive of the fixing film  33  and the drive of the cam  38  is similar to that in Embodiment 1 or 2. Embodiment 3 is characterized in the constitution of the swingable arm stopper  303  and therefore the description will be made by being limited to the operation of the swingable arm stopper  303 . 
     The swingable arm stopper  303  is constituted so as to produce linear motion. The swingable arm stopper  303  is provided with an elongated hole  303   a  and is supported linearly movably by two fixed shafts  306  and  307 . To one end of the swingable arm stopper  303 , an electromagnetic solenoid  308  is connected, so that the swingable arm stopper  303  effects the linear motion in interrelation with the operation of the electromagnetic solenoid  308 . As shown in (a) of  FIG. 7 , when the electromagnetic solenoid  308  is pressed, the swingable arm stopper  303  abuts against the abutment portion  349   a  of the swingable arm  349  to prevent the operation of the swingable arm  349 . As shown in (b) of  FIG. 7 , when the electromagnetic solenoid  308  is pulled, the swingable arm  349  is not prevented and therefore is movable to the position shown in (b) of  FIG. 7 . 
     Parts (a) and (b) of  FIG. 8  are side views showing the structure of the pressing and releasing mechanism  361 . These (a) and (b) of  FIG. 8  show a state in which the structure of the cam  38  in Embodiments 1 and 2 is added to the structure specifically described with reference to (a) and (b) of  FIG. 7 . The cam  38  is disposed at the front side in the drawing sheet surface of (a) and (b) of  FIG. 8  more than the swingable arm stopper  303 . 
     In the image forming apparatuses in Embodiments 1 to 3 described above, the operations of the pressure release and pressure restoration of the fixing device are performed by the cam  38 , and the rotational movement controller of the cam  38  is effected by the normal and reverse rotation of the motor  62  and by the swing of the swingable gear  44  in synchronism with the normal and reverse rotation of the motor  62 . Further, when the cam  38  will rotate quickly, the swingable arm stopper  303  which is the “preventing means” functions so as not to move the swingable gear  44 , so that the cam  38  and the motor are always connected and thus the cam  38  can be rotationally moved at a constant speed. Further, by rotationally moving the cam  38  at the constant speed, the impact noise is reduced. 
     That is, a phenomenon that during the transition between the gear engagement state in the nip-released state by the fixing film  33  and the gear engagement state during the nip function by the fixing film  33 , the gears free from the engagement are rotated quickly to generate the impact noise is suppressed. 
     Part (a) of  FIG. 9  is a side view showing a structure of a pressing and releasing mechanism  461  provided in an image forming apparatus according to the comparative embodiment. Part (a) of  FIG. 9  shows a state of a first mode in which the pressing roller gear  42  is rotated and the cam  38  is stopped. While making reference to this (a) of  FIG. 9  and (b) of  FIG. 9  which is described later, the case where the swingable arm stopper which is the “preventing means” is not present will be specifically described below. First, the first mode will be described. In (a) of  FIG. 9 , the shaft gear  39  rotates clockwise. The pressing roller gear  42  is rotationally driven by the gears  40  and  41  which are the drive transmitting gear train connected from the shaft gear  39  to the fixing film  33 . Further, from the shaft gear  39  to the cam  38 , the path is connected by the gears  40  and  43 , the swingable gear  44 , a gear having partly omitted teeth  445  and the gear  46  which are the drive transfer gear train. Of these, the gear having partly omitted teeth  445  is provided with a teeth-omitted surface  445   a.    
     The driving force to the gear having partly omitted teeth  445  is blocked since the teeth-omitted surface  445   a  of the gear having partly omitted teeth  445  is stopped at a phase where it does not receive the driving force of the swingable gear  44 , so that the cam  38  is stopped. 
     Part (b) of  FIG. 9  is a side view showing the structure of the pressing and releasing mechanism  461  provided in the image forming apparatus according to the comparative embodiment. Part (b) of  FIG. 9  shows a state of a second mode in which the pressing roller gear  42  is stopped and the cam  38  is rotated. The second mode will be specifically described while making reference to (b) of  FIG. 9 . When the mode is transferred from the first mode to the second mode, the shaft gear  39  is reversely rotated, thus being rotated counterclockwise in (b) of  FIG. 9 . The one-way function is incorporated into the gear  41  of the gears  40 ,  41  and  42  which are the drive transfer gear train connected from the shaft gear  39  to the pressing roller  31 . For that reason, the pressing roller gear  42  is stopped, i.e., the pressing roller  31  is also stopped. In the path from the shaft gear  39  to the cam  38 , the swingable gear  44  of the gears  40  and  43 , the swingable gear  44 , the gears  47  and  48 , the gear having partly omitted teeth  445  and the gear  46  which are the drive transfer gear train is swingable. A swingable arm  49  is mounted to this swingable gear  44  so as to slightly rub the swingable gear  44 , so that the swingable arm  49  is swung in the arrow B direction. As a result, the swingable gear  44  and the gear  47  are engaged with each other to rotate the gear  48  clockwise. As a result, the gear having partly omitted teeth  445  and the gear  46  are rotationally driven, and finally the cam  38  is rotated counterclockwise to the position shown in (b) of  FIG. 9 , so that the urging member  36  is moved to result in the pressure-released state. 
     Part (a) of  FIG. 10  is a side view showing a structure of a pressing and releasing mechanism  461  provided in an image forming apparatus according to the comparative embodiment. Part (a) of  FIG. 10  shows a state of a third mode in which both of the pressing roller gear  42  and the cam  38  are rotated. First, the third mode will be specifically described while making reference to (a) of  FIG. 10 . When the mode is transferred from the second mode to the third mode, the shaft gear  39  is reversely rotated again, i.e., is rotated clockwise in (a) of  FIG. 10 . As a result, the swingable gear  44  previously engaged with the gear  47  is engaged with the gear having partly omitted teeth  445  by the swing of the swingable arm  49  about the center shaft of the gear  43  in the arrow A direction. The teeth-omitted surface  445   a  of the gear having partly omitted teeth  445  is, during the operation in the second mode, moved to the position shown in (b) of  FIG. 9 , so that the drive transmission to the cam  38  is effected. Then, the cam  38  is rotationally moved to a position (state shown in (a) of  FIG. 9 ) where the swingable gear  44  and the teeth-omitted surface  445   a  coincide with each other, thus being returned to an initial position, so that the fixing film  33  and the fixing film unit  34  are restored to the pressure-applied state. 
     Part (b) of  FIG. 10  is a side view showing the structure of the pressing and releasing mechanism  461  provided in the image forming apparatus according to the comparative embodiment. Part (b) of  FIG. 10  shows a state of a third mode in which both of the pressing roller gear  42  and the cam  38  are rotated. The third mode will be specifically described while making reference to (b) of  FIG. 10 . In the above-described third mode, the cam  38  and the parts operating in interrelation with the cam  38  are rotated faster than those during normal drive by the force of the urging member  36  in some cases, and as a result, the impact noise is generated. With respect to the cause thereof, the following situation would be considered. Part (b) of  FIG. 10  shows a state in which the rotational movement of the cam  38  progresses from the state of (a) of  FIG. 10 , and when the states of (a) of  FIG. 10  and (b) of  FIG. 10  are compared, “moment of cam  38  in state of (a) of FIG.  10 =F 1 ×r 1 ”&lt;“moment of cam  38  in state of (b) of FIG.  10 =F 2 ×r 2 ” is satisfied. That is, in the mode in which the state is restored from the pressure-released state to the pressure-applied state, the cam  38  is rotationally moved in a direction in which the movement of the cam  38  is gradually increased. 
     Further, when the cam  38  is rotationally moved to a certain position, the cam  38  rotates on its axis by moment thereof generated by the urging member  36 . Assuming that this condition is the position shown in (b) of  FIG. 10 , a rotational speed v 2  (state of (b) of  FIG. 10 ) of the cam  38  on its axis by the moment of the cam  38  generated by the urging member  36  becomes faster than a rotational speed v 1  (state of (a) of  FIG. 10 ) of the driving gear train for rotationally driving the cam  38 . As a result, the gear  46  and the gear having partly omitted teeth  445  are also rotated by the rotation of the cam  38  on its axis, thus being rotated quickly. The swingable gear  44  receives the rotational force from the gear having partly omitted teeth  445 , with the result that the  49  is moved in the direction of the arrow B and thus the gear having partly omitted teeth  445  and the swingable gear  44  are spaced from each other. 
     In the state in which the gear having partly omitted teeth  445  and the swingable gear  44  are spaced from each other, the gear  46  and the gear having partly omitted teeth  445  which are the drive gear train connected to the cam  38  is in a state close to a no-load state and therefore the cam  38  is rotated to the initial position (state of (b) of  FIG. 9 ) where a balance of the forces is achieved. By this series of the operations, the pressing spring  37 , the urging member  36 , the cam  38 , the gear  46  and the gear having partly omitted teeth  445  which are the parts which operate or rotates more quickly than those during the normal drive are abruptly stopped when the balance is achieved, and at that time, there is a possibility that the impact noise is generated. In recent years, noise reduction is further required but in the apparatus in the comparative embodiment, it would be considered that the suppression of the generation of the impact noise is not realized. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, in the fixing device, the pressing and releasing mechanism can be utilized for superposing the phenomenon that the impact noise is generated, by the quick rotation of the gears free from the engagement, in the transition process between the gear engagement state in the nip-released state by the pressing member and the gear engagement state during the nip function by the pressing member.