Patent Publication Number: US-2015061219-A1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, particularly to a configuration lifting a sheet stacking portion on which a sheet is stacked. 
     2. Description of the Related Art 
     A conventional image forming apparatus such as a printer, a copying machine, and a facsimile machine includes a sheet feeding device in which a sheet stored in a sheet storage unit drawably provided in an image forming apparatus body is delivered and fed to an image forming portion by a sheet feeding unit. In some sheet feeding devices, a sheet stacking unit is provided in the sheet storage unit so as to be able to be lifted and lowered, and the sheet is fed by rotating a pickup roller while the sheet stacking unit is lifted to press the sheet against the pickup roller. 
     In the sheet feeding device, it is necessary to hold a level of the uppermost sheet stacked on sheet stacking unit at a predetermined level at which the sheet can be fed. For this reason, the sheet feeding device includes a sensing unit that senses an upper-surface position of the sheet stacked on the sheet stacking unit and a lifter mechanism that lifts the sheet stacking unit. The sheet stacking unit is lifted by driving the lifter mechanism based on a signal from the sensing unit to hold the level of the uppermost sheet at the predetermined level at which the sheet can be fed by the pickup roller. 
     A driving portion (driving unit) that drives the lifter mechanism is provided in the image forming apparatus body, and the lifter mechanism is coupled to the driving portion when the sheet storage unit is mounted. This enables the lifter mechanism to lift the sheet stacking unit based on the sensing of the sensing unit. The driving portion includes a dedicated lifter motor that drives the lifter mechanism (see Japanese Patent Laid-Open No. 9-86680 and 5-193761). 
     However, in the conventional image forming apparatus, cost increases because the dedicated lifter motor is used to drive the lifter mechanism, and the number of components increases for the use of the dedicated lifter motor. When component accuracy varies, a lifting amount of the sheet stacking unit fluctuates to generate a variation in level of the uppermost sheet, which degrades sheet feeding performance. 
     It is desirable to provide an image forming apparatus that can prevent the degradation of the sheet feeding performance at low cost. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an image forming apparatus includes: an image forming portion that forms an image on a sheet; a sheet storage portion that is drawably mounted on an apparatus body, the sheet storage portion including a sheet stacking portion that stacks a sheet and is able to be lifted and lowered and a lifting portion that lifts the sheet stacking portion; a sheet feeding portion that feeds a sheet while abutting the sheet on a sheet stacked on the lifted sheet stacking portion; a driving portion that drives a driven portion; a body-side lifting portion that is provided in the apparatus body to engage with the lifting portion of the sheet storage portion mounted on the apparatus body; a drive transmission portion that is provided between the driving portion and the body-side lifting portion, the drive transmission portion selectively transmitting drive of the driving portion to the lifting portion of the sheet storage portion through the body-side lifting portion to lift the sheet stacking portion, the drive of the driving portion being transmitted to the driven portion; and a controller that controls the drive transmission portion. 
     According to the invention, the drive transmission portion selectively transmits the drive of the driving portion driving the driven portion to the lifting portion of the sheet storage portion through the body-side lifting portion to lift the sheet stacking portion. Therefore, the degradation of the sheet feeding performance can be prevented at low cost. 
     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 
         FIG. 1  is an entire configuration diagram of a full-color laser beam printer that is of an example of an image forming apparatus according to a first embodiment of the invention; 
         FIG. 2  is a view illustrating a configuration of a sheet feeding device of the full-color laser beam printer; 
         FIGS. 3A and 3B  are views illustrating a configuration of a sheet feeding cassette of the sheet feeding device; 
         FIGS. 4A and 4B  are views illustrating a configuration of a drive transmission portion provided on a main body side of the full-color laser beam printer; 
         FIG. 5  is a control block diagram of the sheet feeding device; 
         FIGS. 6A and 6B  are first views illustrating stacking plate lifting operation of the sheet feeding device; 
         FIG. 7  is a view illustrating operation in lifting a stacking plate of the drive transmission portion; 
         FIGS. 8A and 8B  are second views illustrating the stacking plate lifting operation of the sheet feeding device; 
         FIG. 9  is a view illustrating a configuration of the sheet feeding device provided in an image forming apparatus according to a second embodiment of the invention; 
         FIGS. 10A and 10B  are views illustrating a missing tooth gear constituting the drive transmission portion lifting the stacking plate of the sheet feeding cassette of the sheet feeding device; 
         FIG. 11  is a view illustrating the operation in lifting the stacking plate of the drive transmission portion; 
         FIG. 12  is a view illustrating a configuration of the sheet feeding device provided in an image forming apparatus according to a third embodiment of the invention; 
         FIGS. 13A and 13B  are views illustrating a planetary gear unit constituting the drive transmission portion lifting the stacking plate of the sheet feeding cassette of the sheet feeding device; and 
         FIGS. 14A and 14B  are views illustrating the operation in lifting the stacking plate of the drive transmission portion. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, exemplary embodiments of the invention will be described in detail with reference to the drawings.  FIG. 1  is an entire configuration diagram of a full-color laser beam printer  101  that is of an example of an image forming apparatus according to a first embodiment of the invention. An image forming portion  101 B forming an image on a sheet S such as recording paper, a plastic sheet, and a cloth and a sheet feeding device  200  feeding the sheet S are provided in a full-color laser beam printer body  101 A (hereinafter referred to as a printer body) that is of an apparatus body. 
     The image forming portion  101 B includes process cartridges  111 C,  112 C,  113 C, and  114 C that form four color toner images of yellow, magenta, cyan, and black. The process cartridges  111 C,  112 C,  113 C, and  114 C include photosensitive drums  111 ,  112 ,  113 , and  114 , and are drawably mounted on the printer body  101 A. The image forming portion  101 B also includes a laser scanner unit  120 . The laser scanner unit  120  is arranged immediately above the process cartridges  111 C,  112 C,  113 C, and  114 C, and irradiates the photosensitive drums  111 ,  112 ,  113 , and  114  with a laser beam based on image information. 
     An intermediate transfer belt unit  101 C in  FIG. 1  includes an endless intermediate transfer belt  130  and primary transfer rollers  130   a  each of which is provided inside the intermediate transfer belt  130  while opposed to the photosensitive drums  111 ,  112 ,  113 , and  114 . The intermediate transfer belt  130  is entrained about a driving roller  121 , a secondary transfer counter roller  105 , and the like, and rotates in an arrow direction while abutting on all the photosensitive drums  111 ,  112 ,  113 , and  114 . 
     The primary transfer roller  130   a  presses the intermediate transfer belt  130  to form a primary transfer portion in which the intermediate transfer belt  130  abuts on the photosensitive drums  111 ,  112 ,  113 , and  114 , and the primary transfer roller  130   a  applies a transfer bias to the intermediate transfer belt  130  using a bias applying unit (not illustrated). When the primary transfer roller  130   a  applies the primary transfer bias to the intermediate transfer belt  130 , the color toner images on the photosensitive drums  111 ,  112 ,  113 , and  114  are sequentially transferred to the intermediate transfer belt  130  to form a full color image on the intermediate transfer belt  130 . 
     On an outer circumferential surface side of the intermediate transfer belt  130 , a secondary transfer roller  122  is arranged at a position opposed to the secondary transfer counter roller  105 . The secondary transfer roller  122  comes into pressure contact with the secondary transfer counter roller  105  with the intermediate transfer belt  130  interposed therebetween, thereby forming a secondary transfer portion. The toner image on the intermediate transfer belt  130  is transferred to the sheet S (secondary transfer) by applying a bias having an opposite-polarity to a normal charging polarity of toner to the secondary transfer roller  122  from a secondary transfer bias power supply (not illustrated) that is of a secondary transfer bias applying unit. 
     The sheet feeding device  200  includes a sheet feeding cassette  201  and a pickup roller  202 . The sheet feeding cassette  201  is the sheet storage portion that is drawably mounted on the printer body  101 A. The pickup roller  202  is the sheet feeding portion that delivers the sheet S stored in the sheet feeding cassette  201 . A stacking plate (sheet supporting plate)  206  is provided in the sheet feeding cassette  201  so as to be able to be lifted and lowered. The stacking plate  206  is the sheet stacking portion that presses the stored sheet S against the pickup roller  202  while supporting the sheet S. In feeding the sheet S stored in the sheet feeding cassette  201 , the stacking plate  206  is lifted by a lifting mechanism (to be described later), and the sheet S supported by the stacking plate  206  is pressed against the pickup roller  202 . 
     The pickup roller  202  is rotated while the sheet S is pressed against the pickup roller  202 , thereby delivering the sheet S. In  FIG. 1 , a controller  8  controls image forming operation and sheet feeding operation. For example, the controller  8  controls drive of a sheet feeding motor M (to be described later) in  FIG. 5  that drives the pickup roller  202  and drive of a constraining mechanism (to be described later) during the feed of the sheet S. 
     The image forming operation of the full-color laser beam printer  101  having the above configuration will be described below. In the case that the color image is formed on the sheet, the photosensitive drums  111 ,  112 ,  113 , and  114  of the process cartridges  111 C,  112 C,  113 C, and  114 C are rotated counterclockwise at a predetermined control speed. According to the drum rotations, the intermediate transfer belt  130  is also rotated at a speed corresponding to the speeds of the photosensitive drums  111 ,  112 ,  113 , and  114  in a direction indicated by an arrow, and the laser scanner unit  120  is also driven at the substantially same time. 
     In synchronization with the drive, photosensitive drum surfaces are evenly charged in the process cartridges  111 C,  112 C,  113 C, and  114 C. Then, the laser scanner unit  120  performs exposure to the photosensitive drums  111 ,  112 ,  113 , and  114  based on each color component image signal sent from the controller  8 . Therefore, electrostatic latent images are formed on the photosensitive drum surfaces. 
     Then, the process cartridges  111 C,  112 C,  113 C, and  114 C develop the electrostatic latent images of magenta, yellow, cyan, and black using color toners to form the toner images on the photosensitive drums  111 ,  112 ,  113 , and  114 . When the toner images reach transfer regions where the photosensitive drums  111 ,  112 ,  113 , and  114  abut on the intermediate transfer belt  130  according to the rotations of the photosensitive drums  111 ,  112 ,  113 , and  114 , the primary transfer roller  130   a  applies the primary transfer bias. Therefore, the toner images on the photosensitive drums  111 ,  112 ,  113 , and  114  are transferred onto the intermediate transfer belt  130  in the order of magenta, yellow, cyan, and black according to the rotation of the intermediate transfer belt  130 , and the color toner images are formed on the intermediate transfer belt  130 . 
     On the other hand, in parallel with the color toner image forming operation, the controller  8  drives the sheet feeding motor M to rotate the pickup roller  202  in predetermined sequence control timing, whereby the sheet S is delivered from the sheet feeding cassette  201  to reach a nip portion between a feed roller  203  and a retard roller  204 . At this point, when only one sheet S is delivered, a large rotational torque is applied through the sheet S to the retard roller  204  that forms a separation nip portion together with the feed roller  203 . For this reason, by action of a torque limiter (not illustrated) arranged in a driving shaft  204   a  of the retard roller  204 , the retard roller  204  rotates while dragging the conveyed sheet S. 
     In the case that at least two sheets S are delivered from the pickup roller  202 , a driving force is transmitted to the retard roller  204  through the torque limiter because only a frictional force between the sheets S is transmitted to the retard roller  204 . Therefore, the retard roller  204  rotates reversely, one sheet S on the side of the feed roller  203  is left, and all the remaining sheets S are returned in a direction opposite to a sheet feeding direction, which allows the sheet S to be surely separated and delivered one by one. 
     The sheet S separated one by one by the separation nip portion is delivered to a nip portion of a pair of conveying rollers  250   a  including a conveying roller  250  that is of a driving roller and a driven roller  252 . Then the sheet S is delivered to the secondary transfer portion including the secondary transfer counter roller  105  and the secondary transfer roller  122 . In the secondary transfer portion, the color toner images on the intermediate transfer belt are collectively transferred onto the sheet S by the transfer bias applied to the secondary transfer roller  122 . 
     Then, the sheet S to which the color toner images are transferred is separated from the intermediate transfer belt  130 , and delivered to a nip portion between a fixing film  107  and a pressure roller  108 . In the nip portion, the sheet S is subjected to heating and pressurization, thereby fixing the color toner images onto the sheet S. Then, a pair of discharge rollers  109  and  110  discharges the sheet S to which the color toner images are fixed onto a discharge tray  115  provided on a top surface of the printer body  101 A. 
       FIG. 2  is a view illustrating a configuration of the sheet feeding device  200  of the first embodiment. In  FIG. 2 , a support member  205  is vertically swingable with a shaft  203   a  of the feed roller  203  as a support point, and the pickup roller  202  is turnably supported by the support member  205 . In the first embodiment, the pickup roller  202 , the feed roller  203 , and the retard roller  204  are rotated by the drive of sheet feeding motor M. 
     A support shaft  213  is provided in a cassette body  207  of the sheet feeding cassette  201 , and a downstream end side of the stacking plate  206  turns vertically about the support shaft  213  as illustrated in  FIGS. 3A and 3B . In  FIGS. 2 and 3A  and  3 B, a lifter plate  208  is a turning member that lifts the stacking plate  206 , and the lifter plate  208  turns vertically while a lifter shaft  208   a  that is of a turning shaft is supported by a bearing  211  provided in the cassette body  207 . 
     A lifter gear  209  that is of a lifting gear is fixed to the lifter shaft  208   a  of the lifter plate  208 . When the lifter gear  209  rotates in an arrow direction, the lifter plate  208  turns about the lifter shaft  208   a  to lift the stacking plate  206 . A cassette interface gear  210  engaging with the lifter gear  209  is provided in the sheet feeding cassette  201 . In the first embodiment, a lifter mechanism  200 A that is of the lifting portion lifting the stacking plate  206  includes the lifter plate  208 , the lifter gear  209 , and the cassette interface gear  210 . 
     In the configuration of the first embodiment, a dedicated driving source such as a motor driving the lifter mechanism  200 A is not provided, but the lifter mechanism  200 A is driven by a driving source used in another mechanism of the full-color laser beam printer  101 . In  FIG. 2 , a driving motor  227  is the driving portion as another mechanism. The driving motor  227  drives the conveying roller  250  that is of the sheet conveying portion as an example of the driven portion. The driving motor  227  drives the lifter mechanism.  200 A. The drive of the driving motor  227  is transmitted to a stage gear  228  and a lifter mechanism driving gear  221  attached to a roller shaft  251  of conveying roller  250 , whereby the conveying roller  250  and the driven roller  252  rotate. 
     A drive transmission portion  200 B transmits the drive of the driving motor  227  to the lifter mechanism  200 A through a body-side lifting unit  200 C when the sheet feeding cassette  201  is mounted on the printer body  101 A. The body-side lifting unit  200 C engages with the lifter mechanism  200 A when sheet feeding cassette  201  is mounted. The body-side lifting unit  200 C includes lift driving stage gears  255   a  and  255   b  and an interface gear  217  engaging with the cassette interface gear  210 . 
     The drive transmission portion  200 B is provided between the driving motor  227  and the body-side lifting unit  200 C, and includes an idler gear  220  engaging with the lifter mechanism driving gear  221 . The drive transmission portion  200 B also includes a missing tooth gear  216 . The missing tooth gear  216  is provided between the idler gear  220  and the lift driving stage gear  255   b  to selectively transmit the rotation of the idler gear  220  to the interface gear  217  through the lift driving stage gears  255   a  and  255   b . A solenoid  219  is the switching unit that switches the missing tooth gear  216  between a state in which the drive of the driving motor  227  is not transmitted to the body-side lifting unit  200 C and a state in which the drive of the driving motor  227  is transmitted to the body-side lifting unit  200 C. 
     A cassette handle  212  is provided in the cassette body  207 . A user holds the cassette handle  212  to draw the sheet feeding cassette  201  in the direction of arrow A, which allows the user to stack the sheet S. The engagement between the cassette interface gear  210  and the interface gear  217  is uncoupled when the sheet feeding cassette  201  is drawn. 
     A position sensor  224  constitutes a sensing unit that senses whether a position of the sheet S on the stacking plate  206  reaches a position (predetermined range) in a level direction in which the pickup roller  202  can feed the sheet S. The position sensor  224  is a photosensor. The position sensor  224  outputs an on/off signal according to the position of the support member  205 , and the controller  8  (to be described later) senses whether the position of the sheet S on the stacking plate  206  reaches a sheet feedable range of the pickup roller  202  based on the on/off signal. In the first embodiment, the sheet feedable position is set to an optimum range where the sheet S delivered by the pickup roller  202  can proceed into the nip portion between the feed roller  203  and the retard roller  204 . 
     As illustrated in  FIG. 4A , a first missing tooth gear portion  226 A, a trigger cam portion  223 , and a missing tooth gear constraining cam portion  225  are provided in the missing tooth gear  216  constituting the drive transmission portion  200 B. A second missing tooth gear portion  226 B is also provided in the missing tooth gear  216  as illustrated in  FIG. 4B  that is of a view when the missing tooth gear  216  is viewed from the side opposite to  FIG. 4A . That is, the first missing tooth gear portion  226 A and the second missing tooth gear portion  226 B, which differ from each other in a thickness direction, are provided in the missing tooth gear  216 . 
     When the solenoid  219  constrains the rotation, the missing tooth gear  216  stops at the position where a missing tooth  226   a  of the first missing tooth gear portion  226 A faces the idler gear  220  in  FIG. 2 . At this point, the missing tooth gear  216  also stops at the position where a missing tooth  226   b  of the second missing tooth gear portion  226 B faces the lift driving stage gear  255   b  in  FIG. 2 . 
     As illustrated in  FIG. 4A , a lever  214  is in pressure contact with the trigger cam portion  223  of the missing tooth gear  216 . The lever  214  is turnably provided in the printer body  101 A with a shaft  214   a  as the support point, and the lever  214  is biased toward an arrow direction by a lever pressing spring  215 . A hook portion  225   a  is provided in a circumferential surface of the missing tooth gear constraining cam portion  225  of the missing tooth gear  216  in order to latch a constraining member  218  of the solenoid  219 . 
     For example,  FIGS. 4A and 4B  illustrate the state before starting of the sheet feeding operation. At this point, the constraining member  218  of the solenoid  219  is latched in the hook portion  225   a , and therefore the rotation of the missing tooth gear  216  is constrained even if the trigger cam portion  223  is pressed by the lever  214 . As described later, when the solenoid  219  is turned on, the latch of the constraining member  218  is released, and the missing tooth gear  216  is pressed by the lever  214  that is in pressure contact with the trigger cam portion  223 , whereby the missing tooth gear  216  rotates. 
       FIG. 5  is a control block diagram of the sheet feeding device  200 . The controller  8  is connected to the sheet feeding motor M, the driving motor  227 , the solenoid  219 , the position sensor  224 , and a mounting sensor  300  sensing that the sheet feeding cassette  201  is mounted on the printer body  101 A. 
     The stacking plate lifting operation of the sheet feeding device  200  having the above configuration will be described below.  FIGS. 6A and 6B  illustrate an initial state when the sheet feeding cassette  201  is mounted on the printer body  101 A. At this point, a top surface of the sheets S stored in the sheet feeding cassette  201  is separated from the pickup roller  202 . Therefore, in order to feed the sheet S, it is necessary to lift the stacking plate  206  to the sheet feedable position of the pickup roller  202  to press the sheet S against the pickup roller  202 . 
     When a signal is input to the controller  8  from the mounting sensor  300  sensing that the sheet feeding cassette  201  is mounted on the printer body  101 A, the controller  8  rotates the driving motor  227  in order to rotate the stage gear  228  and the lifter mechanism driving gear  221  engaging with the stage gear  228 , whereby the conveying roller  250  and the driven roller  252  rotate. When the lifter mechanism driving gear  221  rotates, the idler gear  220  engaging with the lifter mechanism driving gear  221  rotates as illustrated in  FIG. 7 . However, at this point, the drive of the idler gear  220  is not transmitted to the missing tooth gear  216 , because the idler gear  220  rotates at the position facing the missing tooth  226   a  of the first missing tooth gear portion  226 A of the missing tooth gear  216 . 
     At the time the feed of the sheet S is started, the controller  8  applies a voltage to the solenoid  219 . Therefore, the constraining member  218  of the solenoid  219  moves in an arrow direction in  FIG. 7 , and trips from the hook portion  225   a  of the missing tooth gear constraining cam portion  225  to release the constraint of the rotation of the missing tooth gear  216 . At this point, because the missing tooth gear  216  is pressed through the trigger cam portion  223  by the lever  214  biased by the lever pressing spring  215 , the missing tooth gear  216  rotates in the direction of an arrow a in  FIG. 7  when the constraint of the rotation is released. 
     When the missing tooth gear  216  rotates, the idler gear  220  engages with the missing tooth gear  216 , whereby the missing tooth gear  216  is rotated by the idler gear  220 . When the missing tooth gear  216  rotates, (the second missing tooth gear portion  226 B of) the missing tooth gear  216  engages with the lift driving stage gear  255   b , and the lift driving stage gear  255   b  starts to rotate. The rotation of the lift driving stage gear  255   b  is transmitted to the lifter gear  209  through the lift driving stage gear  255   a , the interface gear  217 , and the cassette interface gear  210 , and the lifter gear  209  rotates in the direction of an arrow b. 
     Therefore, the lifter plate  208  turns upward with the bearing  211  as the support point, and the stacking plate  206  is pushed up about the support shaft  213  to lift the sheet S. Immediately after the sheet feeding cassette  201  is mounted, it is necessary that the sheet S stacked on the stacking plate  206  move over a long distance to the sheet feedable position. For this reason, the controller  8  continuously applies the voltage to the solenoid  219  to rotate the missing tooth gear  216 , and the stacking plate  206  is continuously lifted. 
     Then, as illustrated in  FIGS. 8A and 8B , when the sheet S on the stacking plate  206  abuts on the pickup roller  202 , the pickup roller  202  and the support member  205  turn upward with the shaft  203   a  of the feed roller  203  as the support point. When the position sensor  224  senses a flag portion  205   a  provided in the upwardly-turned support member  205 , the controller  8  stops the voltage application to the solenoid  219 . 
     When the voltage application to the solenoid  219  is stopped, as illustrated in  FIGS. 4A and 4B , the constraining member  218  is latched in the hook portion  225   a  of the missing tooth gear constraining cam portion  225  to constrain the rotation of the missing tooth gear  216 , which stops the stacking plate  206  at the sheet feedable position of the pickup roller  202 . When the rotation of the missing tooth gear  216  is constrained, a torque acts on the lifter gear  209  in the direction opposite to the arrow b in  FIG. 7  by loads of the stacking plate  206  and the sheet S. However, because a one-way clutch CL is provided in the interface gear  217  in order to prevent the reverse rotation, the one-way clutch CL does not rotate the lifter gear  209 , but the stacking plate  206  is held at the stopping position. 
     After the stacking plate  206  is lifted to the sheet feedable position, the controller  8  drives the sheet feeding motor M to rotate the pickup roller  202 , thereby feeding the sheet S. At this point, the level of the top surface of the sheets S decreases gradually when the sheet S is repeatedly fed. With decreasing level of the top surface, the flag portion  205   a  of the support member  205  turns downward together with the pickup roller  202 , and the position sensor  224  becomes a transmissible state. At this point, the controller  8  applies the voltage to the solenoid  219  based on the signal from the position sensor  224 . 
     A time for which the voltage is applied to the solenoid  219  is set within a time for one revolution of the missing tooth gear  216 . Accordingly, a rotation amount of the lifter gear  209  is kept constant because a constant amount of drive is transmitted to the lifter gear  209 . As described above, in the first embodiment, in the case that the sheets S are continuously fed, the constraint of the missing tooth gear  216  is released for a constant time with the solenoid  219  based on the signal from the position sensor  224 , whereby the rotation amount of the lifter gear  209  is kept constant. As a result, the stacking plate  206  can be lifted by a predetermined amount, and the top surface of the sheets S can be held at the substantially constant level. 
     At this point, in the first embodiment, a reduction ratio of a gear train from the missing tooth gear  216  to the lifter gear  209  is set to 0.0308, the number of teeth of the first missing tooth gear portion  226 A of the missing tooth gear  216  is set to 36, the number of teeth of the second missing tooth gear portion  226 B is set to 3, and the drive is transmitted onto the downstream side. Because of the settings of the numbers of teeth, the lifter gear  209  rotates by 0.833° every time the constraining member  218  is released, and the stacking plate  206  is lifted by a constant amount (about 1 mm). When a predetermined number of sheets S are fed, the constraining member  218  is released to lift the stacking plate  206  by about 1 mm (predetermined amount) at the abutment part between the uppermost surface of the stacked sheets S and the feed roller  203 . 
     As described above, in the first embodiment, when the sheet feeding cassette  201  is mounted, the missing tooth gear  216  is continuously rotated until the sheet S on the stacking plate  206  reaches the sheet feedable position based on the sensing of the position sensor  224 . When a predetermined number of sheets S are fed during the continuous feed of the sheets S, the missing tooth gear  216  rotates once, and the stacking plate  206  is lifted by the predetermined amount. Thus, the drive of the driving motor  227  is selectively transmitted to the lifter gear  209  in order to drive the driven portion, which allows the stacking plate  206  to be lifted by the predetermined amount without use of the dedicated lifter motor. That is, in the first embodiment, the drive transmission portion  200 B selectively transmits the drive of the driving motor  227  driving the driven portion to the body-side lifting unit  200 C in order to lift the stacking plate  206 . Therefore, the degradation of the sheet feeding performance can be prevented at low cost. 
     In the first embodiment, the missing tooth gear  216  rotates once to lift the stacking plate  206  in the case that the sheets S are continuously fed. Alternatively, for example, the time for which the voltage is applied to the solenoid  219  is increased, and the missing tooth gear  216  may rotate twice or three times to increase a lifting amount of the stacking plate  206 . The lifting amount of the stacking plate  206  per rotation of the missing tooth gear  216  can be set to a desired amount except 1 mm by changing the reduction ratio of the gear train or the number of missing teeth of the missing tooth gear  216 . The motor used as the driving portion driving the lifter mechanism  200 A is not limited to the driving motor  227  driving the conveying roller  250  that is of the driven portion. Alternatively, for example, a motor driving the intermediate transfer belt  130  that is of the driven portion in the image forming portion may be used. 
     A second embodiment of the invention will be described below.  FIG. 9  is a view illustrating a configuration of the sheet feeding device provided in an image forming apparatus according to a second embodiment of the invention. In  FIG. 9 , the component identical or equivalent to that in  FIG. 2  is designated by the identical numeral. Referring to  FIG. 9 , a missing tooth gear  240  constitutes the drive transmission portion  200 B. As illustrated in  FIG. 10A , a trigger cam portion  241  and a constraining cam portion  242  are provided in the missing tooth gear  240 . 
     In the second embodiment, in the trigger cam portion  241 , three pressing places  241   a  are provided in the rotation direction. In the constraining cam portion  242 , three hook portions  242   a  for the constraining member  218  are provided, and a first missing tooth gear portion  240 A is provided in order to engage intermittently with the idler gear  220 . The first missing tooth gear portion  240 A includes three missing teeth  243   a ,  243   b , and  243   c.    
     As illustrated in  FIG. 10B  in which the missing tooth gear  240  is viewed from the side opposite to  FIG. 10A , a second missing tooth gear portion  240 B is provided in the missing tooth gear  240  in order to engage intermittently with the missing tooth gear lift driving stage gear  255   b . The second missing tooth gear portion  240 B includes three missing teeth  243   d ,  243   e , and  243   f . That is, in the missing tooth gear  240 , the first missing tooth gear portion  240 A and the second missing tooth gear portion  240 B are provided in the thickness direction. 
     The lifting operation of the stacking plate  206  of the sheet feeding device  200  having the above configuration will be described below. When the signal is input to the controller  8  from the mounting sensor  300  sensing that the sheet feeding cassette  201  is mounted on the printer body  101 A, the controller  8  rotates the driving motor  227  to rotate the lifter mechanism driving gear  221 . 
     Therefore, the conveying roller  250  and the driven roller  252  rotate. When the lifter mechanism driving gear  221  rotates, the idler gear  220  engaging with the lifter mechanism driving gear  221  rotates in the arrow direction in  FIG. 11 . However, at this point, the drive is not transmitted to the missing tooth gear  240  because the idler gear  220  rotates at the position facing one of the three missing teeth  243   a ,  243   b , and  243   c  of the first missing tooth gear portion  240 A, for example, at the position facing the missing tooth  243   a.    
     Then, the controller  8  applies the voltage to the solenoid  219  at the time the feed of the sheet S is started, whereby the constraining member  218  of the solenoid  219  disengages from the hook portion  242   a  of the constraining cam portion  242  to release the constraint of the rotation of the missing tooth gear  240 . At this point, one of the three pressing place  241   a  provided in the trigger cam portion  241  of the missing tooth gear  240  is pushed by the lever  214  biased by the lever pressing spring  215 . Therefore, the missing tooth gear  240  rotates in the direction of the arrow a when the constraint of the rotation is released. 
     When the missing tooth gear  240  rotates, the idler gear  220  engages with the missing tooth gear  240 , and therefore the missing tooth gear  240  is rotated by the idler gear  220 . Thus, when the missing tooth gear  240  rotates, the missing tooth gear  240  engages with the lift driving stage gear  255   b  to start the rotation of the lift driving stage gear  255   b . The rotation of the lift driving stage gear  255   b  is transmitted to the lifter gear  209  through the lift driving stage gear  255   a , the interface gear  217 , and the cassette interface gear  210  to rotate the lifter gear  209  in the direction of the arrow b. 
     Therefore, the lifter plate  208  turns upward with the bearing  211  as the support point, and the stacking plate  206  is pushed up about the support shaft  213  to lift the sheet S. Immediately after the sheet feeding cassette  201  is mounted, it is necessary that the stacking plate  206  move over a long distance to the sheet feedable position. For this reason, the controller  8  continuously applies the voltage to the solenoid  219  to lift the stacking plate  206 . 
     After the stacking plate  206  is lifted to the sheet feedable position, the controller  8  drives the sheet feeding motor M to rotate the pickup roller  202 , thereby feeding the sheet S. When the sheet S is repeatedly fed, the level of the top surface of the sheets S decreases gradually. With decreasing level of the top surface, the flag portion  205   a  of the support member  205  turns downward together with the pickup roller  202 , and the position sensor  224  eventually becomes the transmissible state. At this point, the controller  8  applies the voltage to the solenoid  219  based on the signal from the position sensor  224 . 
     At this point, in the second embodiment, the time for which the voltage is applied to the solenoid  219  is set within the time for one third of the one revolution of the missing tooth gear  240 . Even if the voltage application time is set in the above manner, because the hook portions  242   a  are provided at three places in the constraining cam portion  242 , the missing tooth gear  240  stops when rotating one third of one revolution. Thus, in the second embodiment, in the case that the sheets S are continuously fed, the missing tooth gear  240  rotates one third to lift the stacking plate  206 , which allows the lifting amount of the stacking plate  206  to be decreased during the one-time lifting control of the stacking plate  206 . 
     In the second embodiment, the reduction ratio of the gear train from the missing tooth gear  240  to the lifter gear  209  is set to 0.0308, each second missing tooth gear portion  240 B is arranged at an angle of 120°, and three teeth are provided in each second missing tooth gear portion  240 B. The time for which the voltage is applied to the solenoid  219  is set within the time for one third of one revolution of the missing tooth gear  240  to release the constraining member  218 , whereby the lifter gear  209  rotates by 0.833°. Therefore, the stacking plate  206  is lifted by about 1 mm in the abutment part between the uppermost surface of the stacked sheets S and the feed roller  203 . 
     As described above, in the second embodiment, the stacking plate  206  is lifted by rotating the missing tooth gear  240  one third of one revolution, namely before the missing tooth gear  240  rotates once. Accordingly, the stacking plate  206  can be lifted without rotating the missing tooth gear  240  once, and the time for which the voltage is applied to the solenoid  219  can be shortened. 
     A third embodiment of the invention will be described below.  FIG. 12  is a view illustrating a configuration of a sheet feeding device according to a third embodiment of the invention. In  FIG. 12 , the component identical or equivalent to that in  FIG. 2  is designated by the identical numeral. Referring to  FIG. 12 , a planetary gear unit (planetary gear mechanism)  230  constitutes the drive transmission portion  200 B. A switching lever  236  that is of the switching unit is swingably provided in the printer body  101 A with a shaft  236   a  as the support point, and the switching lever  236  switches a drive transmission state of the planetary gear unit  230 . The solenoid  219  performs the switching of the switching lever  236 . 
     As illustrated in  FIGS. 13A and 13B , the planetary gear unit  230  includes planetary gears  231   a  and  231   b , an internal gear  232 , a carrier gear  233 , and a sun gear  234 . In the third embodiment, the carrier gear  233  is the input side of the planetary gear unit  230 , and the internal gear  232  is the output side. That is, the carrier gear  233  engages with the idler gear  220 , and the internal gear  232  engages with the interface gear  217 . 
     The sun gear  234  includes a gear portion  234   a  and a cam portion  234   b . A hook portion  234   c  is provided in a circumferential surface of the cam portion  234   b , and shaft portions  233   a  are provided at two places in the carrier gear  233  in order to turnably support the planetary gears  231   a  and  231   b . The internal gear  232  includes an internal gear portion  232   a  with which the planetary gears  231   a  and  231   b  engage and an output-side gear portion  232   b  that engages with the interface gear  217 . 
     In the planetary gear unit  230 , when the switching lever  236  is latched in the hook portion  234   c  of the sun gear  234  to constrain the rotation of the sun gear  234 , the rotation of the carrier gear  233  is transmitted to the internal gear  232  on the output side. In the case that the switching lever  236  does not constrain the rotation of the sun gear  234 , the rotation of the carrier gear  233  is not transmitted to the internal gear  232 . 
     The lifting operation of the stacking plate  206  of the sheet feeding device  20  including the planetary gear unit  230  will be described below.  FIG. 14A  illustrates the initial state of the drive transmission portion  200 B when the sheet feeding cassette  201  is mounted on the printer body  101 A. Because the top surface of the sheet is separated from the pickup roller in the initial state, it is necessary to lift the stacking plate to the sheet feedable position of the pickup roller to press the sheet against the pickup roller. 
     When the signal is input to the controller  8  from the mounting sensor  300  sensing that the sheet feeding cassette  201  is mounted on the printer body  101 A, the controller  8  rotates the driving motor  227  to rotate the lifter mechanism driving gear  221 , thereby rotating the conveying roller  250  and the driven roller  252 . When the lifter mechanism driving gear  221  rotates, the idler gear  220  engaging with the lifter mechanism driving gear  221  rotates, and therefore the carrier gear  233  engaging with the idler gear  220  rotates in the planetary gear unit  230 . 
     However, at this point, the rotation of the sun gear  234  is not constrained because the switching lever  236  is not latched in the hook portion  234   c  provided in the cam portion  234   b  of the sun gear  234 . In this case, although the drive of the idler gear  220  is transmitted to the carrier gear  233 , the rotation of the carrier gear  233  is not transmitted to the internal gear  232 , and the interface gear  217  does not rotate. 
     Then, the controller  8  applies the voltage to the solenoid  219  at the time the feed of the sheet S is started. Therefore, as illustrated in  FIG. 14B , the constraining member  235  presses the switching lever  236  to swing the switching lever  236 , and the switching lever  236  is latched in the hook portion  234   c  of the cam portion  234   b  of the sun gear  234  to constrain the rotation of the sun gear  234 . 
     When the rotation of the sun gear  234  is constrained, the drive is transmitted to the internal gear  232  to rotate the internal gear  232 , and the interface gear  217  engaging with the output-side gear portion  232   b  of the internal gear  232  rotates. When the interface gear  217  rotates, the rotation of the interface gear  217  is transmitted to the lifter gear  209  through the cassette interface gear  210 , and the lifter gear  209  rotates in the arrow direction. 
     Therefore, the lifter plate  208  turns upward with the bearing  211  as the support point, and the stacking plate  206  is pushed up about the support shaft  213  to lift the sheet S. Immediately after the sheet feeding cassette  201  is mounted, it is necessary that the stacking plate  206  move over a long distance to the sheet feedable position. For this reason, the controller  8  continuously applies the voltage to the solenoid  219  to lift the stacking plate  206 . 
     After the stacking plate  206  is lifted to the sheet feedable position, the controller  8  drives the sheet feeding motor M to rotate the pickup roller  202 , thereby feeding the sheet S. When the sheet S is repeatedly fed, the level of the top surface of the sheets S decreases gradually. With decreasing level of the top surface, the support member  205  turns downward together with the pickup roller  202 , and the position sensor  224  eventually becomes the transmissible state. At this point, the controller  8  applies the voltage to the solenoid  219 . 
     The time for which the voltage is applied to the solenoid  219  is set within the time for one revolution of the sun gear  234 . Therefore, the rotation amount of the lifter gear  209  is kept constant because the constant amount of drive is transmitted to the lifter gear  209 . In the case that the sheets S are continuously fed, the rotation of the sun gear  234  is constrained for a constant time based on the signal from the position sensor  224 , whereby the rotation amount of the lifter gear  209  is kept constant. As a result, the stacking plate  206  can be lifted by a constant amount, and the top surface of the sheets S can be held at the substantially constant level. 
     As described above, in the third embodiment, when the sheet feeding cassette  201  is mounted, or when a predetermined number of sheets S are fed, the rotation of the sun gear  234  is constrained to selectively transmit the drive of the driving motor  227  to the lifter gear  209 . Therefore, the stacking plate  206  can be lifted by the predetermined amount without use of the dedicated lifter motor. 
     In the configuration of the third embodiment, stacking plate  206  is lifted by rotating the sun gear  234  once. Alternatively, the number of hook portions of the sun gear  234  is increased, and the time for which the voltage is applied to the solenoid  219  may be decreased to finely control the S lifting amount of the stacking plate  206 . The lifting amount of the stacking plate  206  per one revolution can be changed by changing the reduction ratio of the gear train. 
     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, equivalent structures, and functions. 
     This application claims the benefit of Japanese Patent Application No. 2013-178990, filed Aug. 30, 2013, which is hereby incorporated by reference herein in its entirety.