Abstract:
A spacing mechanism includes a first member; a second member; a rotation shaft; an urged member supported by the second member; an urging member for urging the urged member against an urging force toward the first member to space the second member from the first member, a first gear and a load receiving portion rotatable integrally with the urging member; a second gear engaged with the first gear; an elastic member having one end fixed to the second member and another end contacted to the load receiving portion. When the shaft rotates from a predetermined rotational phase and the urged member is released from the urging member, the elastic member applies a load to the second member in a direction in which the second member is rotated.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to a separation mechanism, a fixing apparatus (device), a sheet feeding-conveying apparatus (device), and an image forming apparatus. 
         [0002]    An image forming apparatus, such as a copying machine, a printer, and the like, which uses an electrophotographic image forming method, has various sections in which a combination of a cam and a cam follower is used to place a pair of components in contact with each other, or separate the pair of components from each other. For example, in the sheet feeding-conveying device of an image forming apparatus, a combination of a cam and a cam follower is used to place a feed roller for conveying a sheet of recording paper, in contact with a sheet of recording paper in the sheet holding portion of the image forming apparatus, in which sheets of recording paper are held in layers, or separating the feed roller from the sheet in the sheet holding portion. Further, in the fixing device in an image forming apparatus, another combination of a cam and a cam follower is used to place a heat roller and a pressure roller in contact with each other, or separate the heat roller and pressure roller from each other. 
         [0003]    A mechanism, such as those described above, which employs a combination of a cam and a cam follower suffers from a phenomenon referred to as “unintended cam acceleration”, which results in the following problem. Unintended cam acceleration is such a phenomenon that when the relationship, in terms of rotational phase, between a cam and a cam follower is specific, such a rotational force that pushes the gear (cam) in the rotational direction of the gear (cam) is generated. Thus, the gear (cam) which is being rotated by a preset amount of rotational force generated by a motor, temporarily rotates faster than the preset speed at which it is to be rotated by the motor. 
         [0004]    This unintended cam acceleration sometimes results in the generation of collisional noises. More concretely, in the case of some sheet feeding devices, the peripheral surface of their cam is provided with a recess to cause their cam follower to fit into the recess to control the cam in rotational phase. Thus, if the above described unintended cam acceleration occurs at the same time as the cam follower fits into the recess of the cam, collisional noises occur. Further, in the case of a fixing device, the unintended cam acceleration occurs when the heat roller, and the pressure roller which remained separated from the heat roller, are placed in contact with each other. Thus, the heat roller and pressure roller are suddenly pressed upon each other. Consequently, collisional noises sometime occur. 
         [0005]    One of the solutions to the above described problem is disclosed in Japanese Laid-open Patent Application 2007-3599. According to this patent application, a load braking force is applied to the cam with the use of a damper to reduce the speed of the unintended cam acceleration, in order to minimize the collisional noises. Referring to  FIG. 10 , which shows the structure of the conventional fixing device disclosed in the abovementioned patent application document, the fixing device is provided with a mechanism for placing its fixation roller  2  and pressure roller  4  in contact with, or separate from, each other. The pressure roller  4  is supported by a pressure roller supporting member  5 , and remains pressed upon the fixation roller  2  by a pair of compression springs  6 . The pressure roller supporting member  5  is provided with a slave roller  36  (cam follower), which is in contact with a cam  34 . Further, there is a slave roller  35 , which is attached to the same shaft as the cam  34 . The slave roller  35  transmits the driving force from a motor  24  to the cam  34 . 
         [0006]      FIG. 11(   a ) shows the state of the fixing device structured as described above, in which fixation roller  2  and pressure roller  4  are in contact with each other, and  FIG. 11(   b ) shows the state of the fixing device, in which the fixation roller  2  and pressure roller  4  remain separated from each other.  FIG. 11(   c ) shows the state of the fixing device, during a period in which the pressure roller  4  which was remaining separated from the fixation roller  2  comes into contact with the fixation roller  2 . During this period, a damper  26  is in action. When the fixing device is in the state shown in  FIG. 11(   a ), the toothless portion of a partially toothless gear  25  faces a slave gear  27  which is coaxially attached to the shaft of the damper  26 . That is, the partially toothless gear  25  is not in mesh with the slave gear  27 . Therefore, the damper  26  is inactive. However, as the state of the fixing device changes from the one shown in  FIG. 11(   b ) to the one shown in  FIG. 11(   c ), the toothed portion of the partially toothless gear  25  meshes with the slave gear  27 , putting thereby the damper  26  into action. Because the fixing device is structured as described above, the damper  26  comes into action to minimize the collisional noises attributable to the collision caused between the pressure roller  4  and fixation roller  2  by the unintended cam acceleration. 
         [0007]    However, the above described conventional mechanism for placing the pressure roller  4  and fixation roller  2  in contact with, or separate them from each other, suffers from the following problem. Unintended cam acceleration is a phenomenon that as a cam is rotated, such a torque that works in the direction to accelerate the cam is generated at the point of contact between the cam and cam follower, whereby the gear which is in connection to the cam is increased in rotational speed. In the past, a damper is used to prevent the cam from being accelerated. The conventional damper-based mechanism cannot completely eliminate the unintended cam acceleration. That is, the cam is briefly accelerated sometimes. Thus, in a case where the angle by which the cam rotates from when the cam begins to be unintentionally accelerated to when the cam is stopped is small, the cam sometimes fails to be reduced in speed. Moreover, the damper used to prevent the cam from being unintentionally accelerated is changed in the amount of braking (damping) force, by the change in ambience and/or its cumulative usage. Further, a frictional damper is changed in braking (damping) force (torque), by the cumulative usage of a product which employs. 
       SUMMARY OF THE INVENTION 
       [0008]    The primary object of the present invention made in consideration of the above described problem is to minimize the collisional noises which occur when a cam and a cam follower are abruptly placed in contact with each other, or separated from each other. 
         [0009]    According to an aspect of the present invention, there is provided a spacing mechanism comprising a first member; a second member urged toward said first member; a rotation shaft rotatable by a driving force from a driving source; an urged member supported by said second member; an urging member for receiving the driving force said rotation shaft to rotate, said urging member urging said urged member against an urging force toward said first member to space said second member from said first member, in a state that said rotation shaft is at a predetermined rotational phase; a first gear and a load receiving portion which are rotatable integrally with said urging member; a second gear engaged with said first gear to be driven by said first gear; and an elastic member having one end portion fixed to a fixed portion of said second member and another end portion contacted to said load receiving portion, wherein when said rotation shaft rotates from the predetermined rotational phase and said urged member is released from said urging member, said elastic member applies a load to said second member in a direction in which said second member rotates by receiving the driving force. 
         [0010]    According to another aspect of the present invention, there is provided a fixing device for pressing a recording material to fix an image on a recording material, said fixing device comprising a first rotatable member; a second rotatable member for cooperating with said first rotatable member to form a nip for nipping the recording material, said second rotatable member being urged to said first rotatable member to be rotated by said first rotatable member; a rotation shaft rotatable by a driving force from a driving source; an urged member supported by said second rotatable member; an urging member for receiving the driving force said rotation shaft to rotate, said urging member urging said urged member against an urging force toward said first rotatable member to space said second rotatable member from said first rotatable member, in a state that said rotation shaft is at a predetermined rotational phase; a first gear and a load receiving portion which are rotatable integrally with said urging member; a second gear engaged with said first gear to be driven by said first gear; and an elastic member having one end portion fixed to a fixed portion of said second rotatable member and another end portion contacted to said load receiving portion, wherein when said rotation shaft rotates from the predetermined rotational phase and said urged member is released from said urging member, said elastic member applies a load to said second rotatable member in a direction in which said second rotatable member rotates by receiving the driving force. 
         [0011]    According to a further aspect of the present invention, there is provided a feeding device for feeding a recording material, said device comprising a rotatable feeding member for feeding the recording material; a supporting member for supporting the recording material, said supporting member being urged toward said feeding member to feed the recording material by rotation of said feeding member in a state that the recording material is nipped between said supporting member and said feeding member; a rotation shaft rotatable by a driving force from a driving source; an urged member supported by said second member; an urging member for receiving the driving force said rotation shaft to rotate, said urging member urging said urged member against an urging force toward said feeding member to move said supporting member away from said feeding member, in a state that said rotation shaft is at a predetermined rotational phase; a first gear and a load receiving portion which are rotatable integrally with said urging member; a second gear engaged with said first gear to be driven by said first gear; and an elastic member having one end portion fixed to a fixed portion of said supporting member and another end portion contacted to said load receiving portion, wherein when said rotation shaft rotates from the predetermined rotational phase and said urged member is released from said urging member, said elastic member applies a load to said feeding member in a direction in which said feeding member rotates by receiving the driving force. 
         [0012]    Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, and shows the general structure of the apparatus. 
           [0014]      FIG. 2  is a perspective view of the fixing device in the first embodiment, in which a sheet of recording medium is being heated and pressed. 
           [0015]      FIG. 3  is a drawing for describing the separation mechanism with which the driving force transmitting mechanism in this embodiment is provided. 
           [0016]      FIG. 4  is a drawing for showing the structure of the load applying gear, load bearing gear, and pressure application spring. 
           [0017]      FIG. 5  is a drawing for describing the relationship between the lead applying gear, and separation cam. 
           [0018]      FIG. 6  is a drawing for describing the braking (load) torque generated by the separation mechanism in this embodiment. 
           [0019]      FIG. 7  is a drawing for describing the change in the amount of the unintended acceleration (leading) torque and the amount of braking torque. 
           [0020]      FIG. 8  is a perspective view of the sheet feeding-conveying device in the second embodiment of the present invention. 
           [0021]      FIG. 9  is a drawing for describing the separation mechanism in the second embodiment. 
           [0022]      FIG. 10  is a perspective view of an example of conventional fixing device. 
           [0023]      FIG. 11  is a drawing for describing the separation mechanism of the conventional fixing device. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Hereinafter, the present invention is concretely described with reference to a couple of embodiments of the present invention, and appended drawings. The measurements, materials, and shapes of the structural components, and also, the positional relationship among the components, disclosed in the following embodiments of the present invention, are not intended to limit the present invention in scope. That is, they are to be altered as necessary according to the structure of an apparatus to which the present invention is applied, and various conditions under which the apparatus is operated. 
       EMBODIMENTS 
       [0025]    The separation mechanism in this embodiment is employable by an image forming apparatus such as a copying machine, a printer, etc., which uses an electrophotographic image forming method. To begin with, referring to  FIG. 1 , an image forming apparatus by which the separation mechanism in this embodiment is employed is described about its general structure.  FIG. 1  is a schematic sectional view of the image forming apparatus in these embodiments, and shows the general structure of the apparatus. 
         [0026]    Referring to  FIG. 1 , the image forming apparatus in this embodiment is a laser printer  100 . It is made up of a main assembly  101  (which hereafter may be referred to as “printer main assembly”), an image forming section  102 , and a sheet feeding-conveying device  103  which feeds and conveys a sheet S of recording medium. 
         [0027]    The image forming section  102  has a photosensitive drum  10 , a rotary developing device  12 , a laser-based optical exposing system  20 , etc. The rotary developing device  12  has four monochromatic developing devices, more specifically, a black color developing device, a yellow color developing device, a magenta color developing device, and a cyan color developing device. Each developing device employs its own toner cartridge. The rotary developing device  12  can be rotated in the clockwise direction indicated by an arrow mark in  FIG. 1 , so that any of the four developing devices can be moved into a development position in which it opposes the photosensitive drum  10 . 
         [0028]    Further, the image forming section  102  is provided with: an endless transfer belt  11 , onto which four monochromatic toner images, different in color, are transferred in layers after their formation on the photosensitive drum  10 ; and a secondary transfer roller  13   a  which forms the secondary transferring section for transferring the toner image (made up of layered four monochromatic toner images) from the transfer belt  11  onto a sheet S of recording medium. There is also provided on the downstream side of the secondary transferring section  13 , a fixing device  14  for fixing the unfixed image on the sheet S (recording medium), a pair of discharge rollers  15  for discharging the sheet S out of the printer main assembly  101  after the fixation of the image, etc. 
         [0029]    As image formation signals are outputted from un unshown controlling device with which the printer main assembly  101  is provided, an optical image of the first color, which is created by converting the information of the image to be formed into optical signals, is projected upon the peripheral surface of the photosensitive drum  10 , from the laser-based optical exposing system  20 . By the way, the peripheral surface of the photosensitive drum  10  is charged before it is exposed to the optical image. Thus, as the optical image is projected upon the peripheral surface of the photosensitive drum  10 , an electrostatic latent image is formed on the peripheral surface of the photosensitive drum  10 . 
         [0030]    Then, the electrostatic latent image on the photosensitive drum  10  is developed by one of the four developing devices in the rotary developing device  12 . As a result, a toner image of the first color is formed on the photosensitive drum  10 . Then, the toner image on the peripheral surface of the photosensitive drum  10  is transferred onto the transfer belt  11 . In a case where the laser printer  100  is in the multicolor mode, the transfer belt  11  is rotated further so that a toner image of the next color is formed thereon after the transfer of the toner image of the first color. While the toner image of the second color is formed, the rotary developing device  12  is rotated by 90° so that the developing device which corresponds to the second color opposes the photosensitive drum  10 , to prepare for the development of the electrostatic latent image for the second color. 
         [0031]    After the transfer of the toner image of the first color, the photosensitive drum  10  is repeatedly subjected to the process of forming a latent image, process of developing the latent image, and process of transferring the developed image, to form the second, third, and fourth monochromatic toner images. Consequently, four monochromatic toner images, different in color, are sequentially layered on the transfer belt  11 . While the first to third monochromatic toner images are layered upon the transfer belt  11 , the transfer roller  13   a  is kept separated from the transfer belt  11 . Then, the transfer roller  13   a  is placed in contact with the transfer belt  11  before the electrostatic image for the fourth color begins to be written. 
         [0032]    Meanwhile, one of the sheets S in a sheet feeding-conveying device  103  is moved out of the sheet feeding-conveying device  103  in synchronism with the progression of the above described image forming operation, so that the sheet S arrives at the secondary transferring section at the same time as the arrival of the toner image on the photosensitive drum  10  at the secondary transferring section  13 . Then, the toner image on the transfer belt  11  is transferred onto the sheet S by the secondary transfer roller  13   a  in the secondary transferring section  13 . Then, the sheet S is conveyed to the fixing device  14 . Then, the sheet S is subjected to heat and pressure by the fixing device  14 . Consequently, the unfixed tone image on the sheet S is permanently fixed to the sheet S. After the fixation of the toner image to the sheet S as described above, the sheet S is discharged by the pair of discharge rollers  15  into the delivery tray  16 , which is an integral part of the top wall of the printer main assembly  101 . 
       Embodiment 1 
       [0033]    Next, referring to  FIGS. 2 and 3 , the fixing device  14 , and a driving force transmission mechanism used by the fixing device  14 , are described.  FIG. 2  is a perspective view of the fixing device  14  in the first embodiment, which is applying heat and pressure to a sheet S of recording medium.  FIG. 3  is a drawing for describing the separation mechanism with which the driving force transmission mechanism in the first embodiment is provided. More specifically,  FIG. 3(   a ) is a perspective view of the fixing device  14 , when the pressure roller of the fixing device  14  is in contact with the heat roller of the fixing device  14 .  FIG. 3(   b ) is a perspective view of the fixing device  14  when the pressure roller has just begun to be separated from the heat roller.  FIG. 3(   c ) is a perspective view of the fixing device  14  after the separation of the pressure roller from the heat roller. 
         [0034]    Referring to  FIG. 2 , in the first embodiment, the driving force transmission mechanism employed by the fixing device  14  is made up of multiple gears which are rotatable by the driving force from a fixation motor  203 . More concretely, the driving force transmission mechanism has: a rotational shaft  203   a  which is rotated by the driving force from the fixation motor  203  as a drive force source; a one-way gear  204  which is rotated by the rotation of the rotational shaft  203 ; and a slave gear  205  which is attached to the rotational shaft  203   a  in such a manner that it becomes coaxial with the one-way gear  204 . Further, the driving force transmission mechanism has: a slave gear  206  which is in mesh with the slave gear  205 ; and a slave gear  207  which is in mesh with the slave gear  206  and is attached to the rotational shaft of the heat roller  201  in such a manner that it becomes coaxial with the heat roller  201 . Further, it has: a slave gear  208  which is in mesh with the one-way gear  204 ; a load bearing gear  213 , as the first gear, which is coaxially attached to the shaft of the slave gear  208 ; and a load application gear  214 , as the second gear, which is in mesh with the load bearing gear  213 . Thus, the lead bearing gear  213  rotates by receiving the driving force from the fixation motor  203  through the rotational shaft  203   a  and one-way gear  204 . 
         [0035]    As the sheet S of recording medium, on which the toner image is present, is conveyed to the fixing device  14 , it is moved into the nip which the heat roller  201  and pressure roller  202  of the fixing device  14  form. In the fixation nip, the sheet S is subjected to heat and pressure, whereby the unfixed toner image on the sheet S is permanently fixed to the sheet S. The fixing device  14  uses the driving force from the fixation motor  203  to rotationally drive the heat roller  201  through the rotational shaft  203 , slave gears  205 ,  206 , and  207 . The pressure roller  202  is a slave roller to the heat roller  201 . That is, it is rotated by the rotation of the heat roller  201 . The temperature of the heat roller  201  is controlled by an unshown heater. 
         [0036]    From the standpoint of usability, and also, the longevity of the printer main assembly  101 , it is necessary for the pressure roller  202  to remain separated from the heat roller  201  while a jammed sheet of recording medium is removed and/or the electric power source of the printer main assembly  101  is off. Next, the separation mechanism, which keeps the pressure roller  202  separated from the heat roller  201 , is described. 
         [0037]    The separation mechanism in this embodiment is provided with: a separation cam  210  as a pressure applying member; a separation cam follower  211 , as a member to be pressed by the separation cam  210 ; a fixing device housing  21  which is integral with the separation cam follower  211 ; and a pressure application springs  209 , as a pressure applying member, which applies pressure to the fixing device housing  212 . The fixing device housing  212 , which is a supporting member, rotatably supports the pressure roller  202 . 
         [0038]    The separation mechanism rotationally drives the separation cam  210  by receiving driving force from the fixation motor  203  through the rotational shaft  203   a , one-way gear  204 , and slave gear  208 . The separation mechanism is structured so that it presses the separation cam follower  211  when the load bearing gear  213  (rotational shaft  203   a ) is in a preset position in terms of its rotational phase. 
         [0039]    Both the separation cam follower  211  and pressure roller  202  (as second member) are held by the fixing device housing  212 . The fixing device housing  212  is kept under the pressure from the pair of pressure application springs  209 , and is movable toward the rotational axis of the heat roller  201  (as first member). 
         [0040]    The distance between the rotational axis of the pressure roller  202  and the rotational axis of the heat roller  201  is changed by the rotational phase of the separation cam  210 . That is, whether the pressure roller  202  is kept in contact with the heat roller  201  or not is determined by the rotational phase of the separation cam  210 . More concretely, while the separation cam  210  is pressing the separation cam follower  211 , the pressure roller  202  remains separated from the heat roller  201 , whereas while the separation cam  210  is not pressing the separation cam follower  211 , the pressure roller  202  presses on the heat roller  201 . 
         [0041]    In this embodiment, the separation cam  210  and load bearing gear  213  (as first gear) are coaxially attached to the same shaft. The load application gear  214  (as second gear) is in mesh with the load bearing gear  213 . Further, a load application lever  215  and a load generation spring  216  are attached to the load bearing gear  214  (second gear). In this embodiment, a combination of the load application lever  215  and load generation spring  216  makes up the load applying member (braking member) in accordance with the present invention. 
         [0042]    Referring to  FIG. 3(   a ), as the rotational shaft  203   a  is rotated by the driving force from the fixation motor  203  in the direction by an arrow mark A 1 , the heat roller  201  is rotated in the direction indicated by an arrow mark B by the rotational force transmitted thereto through the slave gears  205 ,  206 , and  207 . Thus, a sheet S of recording medium (unshown) is conveyed through the nip which the rotating heat roller  201  and pressure roller  202  form. By the way, the separation mechanism in this embodiment is structured so that when the fixation motor  203  rotates in the direction A 1 , the driving force from the fixation motor  203  is not transmitted to the slave gear  208 , and therefore, it does not occur that the separation cams  210  are rotationally driven. 
         [0043]    When it is necessary for the pressure roller  202 , which is in contact with the heat roller  201  as shown in  FIG. 3(   a ), to be separated from the heat roller  201 , the rotational shaft  203   a  is rotated in the direction indicated by an arrow mark A 2  in  FIG. 3(   b ), by the driving force from the fixation motor  203 . As the rotational shaft  203   a  is rotated in the direction A 2 , the separation cam  210  rotates in the direction indicated by an arrow mark C. As the separation cam  210  rotates by a certain angle, it begins to press on the separation cam follower  211  against the pressure generated by the pressure application spring  209 , causing thereby the pressure roller  202  to be separated by a preset distance from the heat roller  201  as shown in  FIG. 3(   c ). 
         [0044]    As the rotational shaft  203   a , which is in the state shown in  FIG. 3(   c ), is rotated further to rotate the separation cam  210  in the direction C, the pressure roller  202  comes back into contact with the heat roller  201  as shown in  FIG. 3(   a ). During this portion of the rotational movement of the separation cam  210 , which allows the pressure roller  202  to come back into contact with the heat roller  201 , it sometimes occurs that the separation cam  210  is accelerated in its rotational movement, and therefore, the pressure roller  202  is allowed to abruptly come into contact with the heat roller  201 , as if it collides with the heat roller  201 , generating therefore a substantial amount of noises (collisional noises). In this embodiment, therefore, in order to minimize the collisional noises attributable to the abrupt contact (collision) between the pressure roller  202  and heat roller  201 , the separation mechanism is structured so that braking torque, which is opposite in direction from the unintended acceleration of the separation cam  210 , is applied to the load bearing gear  213  by the load generation spring  216  to cancel the torque which gives the separation cam  210  the unintended acceleration. This structural arrangement is the characteristic feature of the separation mechanism in this embodiment. 
         [0045]    Next, referring to  FIG. 4 , the characteristic feature of the separation mechanism in this embodiment is described.  FIG. 4  is a drawing for showing the structure of the load application gear, load bearing gear, and load generation spring. More specifically,  FIG. 4(   a ) is a perspective view of the separation mechanism as seen from the load generation spring  216  side of the load application gear  214 .  FIG. 4(   b ) is a perspective view of the separation mechanism as seen from the opposite side of the load application gear  214  from the load generation spring  216 .  FIG. 4(   c ) is a plan view of the separation mechanism as seen from the load generation spring  216  side of the load application gear  214 . 
         [0046]    In the first embodiment, the load application lever  215  is coaxially attached to the same shaft as the load application gear  214 . It internally holds the load generation spring  216  which is a torsion coil spring. The load application lever  215  is independent from the load application gear  214 . That is, it is rotatable about the rotational axis of the load application gear  214 , independently from the load application gear  214 . 
         [0047]    One end of the load generation spring  216  is fixed to the boss  214   b  (load generation spring anchoring portion) of the load application gear  214  (second gear), whereas the other end is rested on the load application lever  215  in a manner to cause the load application lever  215  to rotationally move about the rotational axis of the load application gear  214 . The load application gear  214  and load bearing gear  213  are the same in the number of teeth. When the load application gear  214  and load bearing gear  213  are in their positions, in terms of rotational phase, shown in  FIG. 4 , the opposite edge of the load application lever  215  from the load generation spring  216  is in contact with the boss  214   a  of the load application gear  214 . 
         [0048]    When the separation mechanism is in the state shown in  FIG. 4 , the pressure generated by the load generation spring  216  remains within the confines of the load application gear  214 . Therefore, the load (braking torque) generated by the load generation spring  216  is not applied to the load bearing gear  213 . As the load application lever  215  is rotated to the position, in terms of rotational phase, at which it comes into contact with the boss  213   a  (load bearing portion) of the load bearing gear  213 , the braking torque begins to be applied to the load bearing gear  213 . The position of the boss  213   a  of the load bearing gear  213  relative to the load bearing gear  213  is such that the distance from the center of the load bearing gear  213  to the outward edge of the boss  213   a  is equal to the radius of the pitch circle of the load bearing gear  213 . 
         [0049]    Next, referring to  FIGS. 5 and 6 , the braking torque generated in this embodiment is described.  FIG. 5  is a drawing for describing the relationship between the load bearing gear  213  and separation cam  210 . It is a plan view of the separation mechanism as seen from the load application lever side of the mechanism.  FIG. 5(   a ) shows the state of the separation mechanism immediately before the pressure roller  202  is made to separate from the heat roller  201 .  FIG. 5(   b ) shows the state of the separation mechanism when the pressure roller  202  is remaining separated from the heat roller  201 .  FIG. 5(   c ) shows the state of the separation mechanism immediately after the pressure roller  202 , which was remaining separated from the heat roller  201 , begins to be placed in contact with the heat roller  201 . The separation mechanism is structured so that the separation cam  210  and load bearing gear  213  move together at the same speed. When the pressure roller  202  is placed in contact with, or separated from, the heat roller  201 , both the separation cam follower  211  and load bearing gear  213  rotate in the direction indicated by an arrow mark X. 
         [0050]    When the separation mechanism is in the state shown in  FIG. 5(   a ), that is, immediately before the pressure roller  202  begins to be separated from the heat roller  201 , the load application lever  215  is not in contact with the boss  213   a  of the load bearing gear  213 . Thus, the load bearing gear  213  is yet to be affected by the braking torque. That is, while the pressure roller  202 , which remained in contact with the heat roller  201 , begins to be separated from the heat roller  201 , the load bearing gear  213  is not subjected to the braking torque by the load application gear  214 . 
         [0051]    Referring to  FIG. 5(   b ), when the pressure roller  202  is separated by the largest distance from the heat roller  201 , the load application lever  215  is in contact with both the boss  214   a  of the load application gear  214 , and the boss  213   a  of the load bearing gear  213 . Thus, the load bearing gear  213  is yet to be subjected to the braking torque, because the load application lever  215  is still in contact with the boss  214   a  of the load application gear  214 . 
         [0052]    Next, referring to  FIG. 5(   c ), immediately after the pressure roller  202 , which was kept separated from the heat roller  201 , begins to be moved toward the heat roller  201 , the opposite end edge of the load application lever  215  from the load generation spring  216 , is in contact with only the boss  213   a  of the load bearing gear  213 . Thus, the torsion coil spring  216  is wound by the boss  214   b  of the load application gear  214  and the boss  213   a  of the load bearing gear  213 . As a result, the load bearing gear  213  is subjected to the force (braking torque) generated by the resiliency of the torsion coil spring  216 . Next, referring to  FIG. 6 , the relationship among the force applied to the boss  214   b  of the load application gear  214 , force applied to the boss  213   a  of the load bearing gear, and braking torque is described. 
         [0053]    Referring to  FIG. 6 , there are generated torques T 1  and T 2  at the point of contact between the load application gear  214  and load bearing gear  213 . The amount of the torque T 1  is the product of multiplication between a force F 1 , which is generated at the point of contact between the load generation spring  216 , and the boss  214   b  of the load application gear  214 , and the length L 1  of the arm portion of the load generation spring  216 . As for the amount of torque T 2 , it is the product of the multiplication between (F 2 ×cos θ), which is the circumferential component of the force by which the load application lever  215  presses on the boss  213   a  of the load bearing gear  213 , and the distance L 2  between the center of the load bearing gear  213  and the point of contact between the boss  213   a  of the load bearing gear  213 , and the load application lever  215 . 
         [0054]    The effective amount of the braking torque to which the load bearing gear  213  is subjected is the difference between the torques T 1  and T 2 , because the torques T 1  and T 2  are opposite in direction from each other. When the separation mechanism is in the state shown in  FIGS. 5(   c ) and  6 , the load application lever  215  is in contact with the boss  213   a  of the load bearing gear  213 , and applies a force F 2  to the load bearing gear  213 . The angle between the direction of the force F 2  and the rotational direction of the load bearing gear  213  is θ. Therefore, the torque T 2  is reduced by an amount equivalent to the angle θ. Thus, the torque T 1  remains greater than the torque T 2 . Therefore, the load bearing gear  213  remains subjected to the braking torque, which is generated in the direction indicated by an arrow mark Y, which is opposite from the direction of the unintended cam acceleration. 
         [0055]    When the separation mechanism is in the state shown in  FIG. 5(   c ), torque is generated in the direction indicated by the arrow mark X, and therefore, the unintended cam acceleration does not occur. Thus, the unintended cam acceleration can be cancelled by the actual amount of braking torque, which is the difference between above described torques T 1  and T 2 , which applies in the direction indicated by the arrow mark Y in  FIG. 6 . 
         [0056]    However, the amount of the unintended cam acceleration torque to which the separation cam  210  is subjected changes according to the rotational phase (angle) of the separation cam  210 . Thus, the amount of the braking torque has also to be changed according to the rotational phase of the separation cam  210 .  FIG. 7  shows the change in the amount of the unintended cam acceleration torque to which the separation cam  210  is subjected, and the change in the necessary amount of the braking torque.  FIG. 7  is a drawing for describing the change in the unintended cam acceleration torque and the change in the necessary amount of braking torque. More specifically,  FIG. 7(   a ) shows the positional relationship, in terms of rotational phase, among the separation cam  210 , boss  213   a  of the load bearing gear  213 , boss  214   a  of the load application gear  214 , and load application lever  215 , right after the pressure roller  202  began to separate from the heat roller  201 .  FIG. 7(   b ) shows the relationship, while the pressure roller  202  is moving away from the heat roller  201 .  FIG. 7(   c ) shows the relationship when the pressure roller  202  is back in contact with the heat roller  201  after being separated from the heat roller  201 . A graph  1 , which is in the right side of  FIG. 7 , shows the amount of the unintended cam acceleration torque, to which the separation cam  210  is subjected. 
         [0057]    When the separation mechanism is in the state shown in  FIG. 7(   a ), the amount of the pressure generated by the compression springs  209  is high. Therefore, the unintended cam acceleration torque is large. However, it gradually reduces as the distance between the pressure roller  202  and heat roller  201  reduces. In this embodiment, the separation mechanism is structured so that as the distance between the pressure roller  202  and heat roller  201  reduces, the angle θ reduces. An angle θ 1  shown in  FIG. 7(   b ) is smaller in value than an angel θ 1  shown in  FIG. 7(   a ). The smaller the angle θ, the smaller the braking torque. That is, the separation mechanism is structured so that as the distance between the pressure roller  202  and heat roller  201  reduces, the braking torque reduces, and also, so that as the pressure roller  202  comes into contact with the heat roller  201  as shown in  FIG. 7(   c ), the angle θ becomes zero (θ=0 deg). Therefore, the torques T 1  and T 2  shown in  FIG. 6  become equal, and therefore, there is generated no braking torque to which the load bearing gear  213  is subjected. 
         [0058]    As described above, in the case of the separation mechanism in the first embodiment, the unintended cam acceleration torque is cancelled by providing the separation cam  210  (load bearing gear  213 ) with such braking torque that is opposite in direction from the unintended cam acceleration torque and equal in value. With the separation mechanism being structured as described above, it is possible to minimize the noises attributable to collisional contact between the pressure roller  202  and heat roller  201 , which is caused by the unintended acceleration of separation cam rotation. Further, the separation mechanism is structured so that whether or not the braking torque is applied is dependent upon the rotational phase of the separation cam  210  (load bearing gear  213 ). Therefore, the braking torque always begins to be applied at a preset rotational phase of the separation cam  210 , and also, the amount of braking torque is dependent upon the rotational phase of the separation cam  210 . Therefore, it is ensured that the braking torque is always applied by a proper amount. Further, because the braking torque is generated by the force of the torsion coil spring  216  through the gears, the separation mechanism in this embodiment has a merit of being small in the amount by which the braking torque is affected by the change in ambience and/or cumulative usage of the fixing device  14 . 
         [0059]    By the way, in the first embodiment, the load bearing gear  213  and load application gear  214  are the same in tooth count. However, they do not need to be the same in tooth count. That is, the tooth count of the former may be multiple of the tooth count of the latter. By making the tooth count of the former a multiple of the tooth count of the latter, it is possible to generate a large amount of braking torque with the use of a braking torque generation spring (torsion coil spring) which is relatively small in the amount of resiliency. 
       Embodiment 2 
       [0060]    Next, referring to  FIGS. 8 and 9 , the second embodiment of the present invention is described.  FIG. 8  is a perspective view of the sheet feeding-conveying device in the second embodiment.  FIG. 9  is a drawing for describing the separation mechanism in the second embodiment. More specifically,  FIG. 9(   a ) shows the state of the sheet feeding-conveying device in the second embodiment immediately before the feeding-conveying roller  1  moves back into the state shown in  FIG. 8  after one full rotation.  FIG. 9(   b ) is a plan view of the separation mechanism in this embodiment, shown in  FIG. 9(   a ), minus the sheet feeding-conveying cam.  FIG. 9(   c ) is a plan view the separation mechanism in this embodiment, shown in  FIG. 9(   a ), minus the sheet feeding-conveying cam, which is in the same state as the state shown in  FIG. 8 . 
         [0061]    In the second embodiment, a sheet feeding-conveying cam  301  is used as a pressing member for positioning the partially toothless gear of the sheet feeding-conveying device  103 . It is used to prevent the partially toothless gear  311  (first gear) from being unintentionally accelerated in rotational speed. 
         [0062]    Referring to  FIG. 8 , the sheet feeding-conveying device in the second embodiment has: a feeding-conveying roller  1  as the first member; a center plate (second member) which supports the sheets (of recording medium) in the sheet feeding-conveying device  103 ; and a pair of center plate springs  303  which keep the sheets in the sheet feeding-conveying device  103  pressed against the sheet feeding-conveying roller  1 . Next, referring to  FIG. 9 , the driving force transmitting mechanism employed by the sheet feeding-conveying device  103  has: a slave gear  310 ; a partially toothless gear  311  as the first gear; and a load bearing gear  304  as the second gear. The slave gear  310  which receives the driving force from the sheet feeding motor (unshown driving force source) transmits the driving force to the partially toothless gear  311 , whereby the partially toothless gear  311  is rotated. 
         [0063]    The sheet feeding-conveying roller  1  is rotated by the unshown sheet feeding-conveying motor in the direction indicated by an arrow mark Z in  FIG. 9  while being controlled in its rotation by the partially toothless gear  311 . The sheet feeding cam  301  is on the same shaft as the sheet feeding-conveying roller  1 . The center plate  3  is integral with a sheet feeding cam follower  302 , which remains pressured toward the sheet feeding-conveying roller  1  by the sheet feeding cam  301 . The position of the center plate  3  is controlled by the sheet feeding-conveying cam follower  302  fixed to the sheet feeding cam  301  and center plate  3 . 
         [0064]    Thus, as the sheet feeding roller  1  rotates, the sheet feeding cam  301  also rotates with the sheet feeding roller  1 , controlling thereby the center plate  3 . When the rotational shaft (partially toothless gear  311 ) is in a preset position in terms of its rotational phase, the sheet feeding cam  301  presses on the sheet feeding cam follower  302 . However, the sheets on the center plate  3  are yet to come into contact with sheet feeding roller  1 . 
         [0065]    The second embodiment can prevent the sheet feeding-conveying device  103  from generating the collisional noises when the sheet feeding-conveying roller  1  moves back into the position shown in  FIG. 8 . Next, referring to  FIG. 9 , how the sheet feeding-conveying device  103  can be prevented from generating the above described collisional noises is described. 
         [0066]    The rotation of the partially toothless gear  311  and the rotation of the sheet feeding roller  1  are controlled by reception of the driving force from the unshown sheet feeding motor by the slave gear  310 , and the transmission of the driving force from the slave gear  310  to the partially toothless gear  311 . The partially toothless gear  311  rotates in the direction indicated by the arrow mark Z like the sheet feeding-conveying roller  1 . At this point in operation, the sheet feeding cam follower  302  is kept pressed by the resiliency of the center plate springs  303  in the direction W. Therefore, when the sheet feeding-conveying device  103  is in the state shown in  FIG. 9(   a ), torque is generated in the direction Z, that is, the direction to unintentionally accelerate the sheet feeding cam  301 . 
         [0067]    When the state of the sheet feeding cam  301  changes from the one shown in  FIG. 9(   a ) to the one shown in  FIG. 9(   c ), that is, when the state of the sheet feeding cam  301  changes from the one in which its sheet feeding cam  301  presses on the sheet feeding cam follower  302 , to the one in which the sheet feeding cam  301  is in the recess of the sheet feeding cam follower  302 , the driving force is not transmitted to the partially toothless gear  311  from the slave gear  310 . By the way, in the second embodiment, as the sheet feeding cam  301  fits into the recess of the sheet feeding cam follower  302 , it stops pressing on the sheet feeding cam follower  302 . 
         [0068]    Thus, it sometimes occurs that when the sheet feeding cam  301  is in the state shown in  FIG. 9(   a ), it abruptly begins to rotate at a high speed, and is abruptly stopped as it rotates to the position shown in  FIG. 9(   c ). Therefore, the sheet feeding-conveying device  103  sometimes generates collisional noises. Thus, the collisional noises are minimized by the generating braking torque in the opposite direction from the direction Z with the use of the load generation spring  306  as in the first embodiment. The load application gear  306  is positioned so that it can mesh with the partially toothless gear  311 . The braking torque is generated by a combination of the load application lever  305  and load generation spring  306 . The structural arrangement for generating the braking torque which is opposite in direction from the unintended cam acceleration is the same as the one in the first embodiment. Therefore, its detailed description is not given here. The section of the sheet feeding-conveying device  103  in the second embodiment, which is made up of the load application lever  305  and load generation spring  306  is the load applying member (braking torque applying member) in accordance with the present invention. 
         [0069]    Unlike the first embodiment, in the second embodiment, a torque which is slightly weaker than the unintended cam acceleration torque is applied in the opposite direction from the direction Z. While the state of the sheet feeding-conveying device  103  changes from the one shown in  FIG. 9(   a ) to the one shown in  FIG. 9(   c ), the driving force from the sheet feeding motor, cannot be received by the slave gear  310  until the sheet feeding cam  301  rotates to the position shown in  FIG. 8 . Thus, by applying braking torque which is slightly weaker than the unintended sheet feeding cam acceleration torque, in the opposite direction from the direction Z, not only is it possible to properly position the partially toothless gear  311 , but also, to minimize the collisional noises attributable to the positioning of the partially toothless gear  311 . As described above, the present invention makes it possible to reliably control the unintended cam acceleration which occurs to the mechanism which employs a partially toothless gear, to minimize the collisional noises, as in the first embodiment. 
         [0070]    By the way, in the preceding embodiments of the present invention, the separation mechanism in accordance with the present invention was employed by the fixing device  14  and sheet feeding-conveying device  103 . However, the preceding embodiments are not intended to limit the present invention in scope. That is, the present invention is applicable to other sections of the image forming apparatus  100  than those described above. For example, the separation mechanism in accordance with the present invention may be employed to separate the two components which form the transfer nip, from each other. That is, it may be employed to separate a transferring member such as the secondary transfer roller  13   a  (secondary member) from the image bearing member such as the transfer belt  11  (first member). Moreover, it may be employed to separate the two components which make up the developing section. That is, it may be employed to separate a developing member such as the rotary developing device  12  (second member) from the photosensitive drum  10  (first member). 
         [0071]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
         [0072]    This application claims priority from Japanese Patent Application No. 016893/2014 filed Jan. 31, 2014, which is hereby incorporated by reference.