Patent Publication Number: US-8540234-B2

Title: Image forming apparatus

Description:
Cross-Reference to Related Application 
     This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2011-256501 filed on Nov. 24, 2011. The entire subject matter of the application is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The following description relates to one or more techniques for an image forming apparatus having a function to sequentially feed a plurality of sheets placed on a loading unit such as a sheet tray. 
     2. Related Art 
     An image forming apparatus has been known that is provided with a pickup roller disposed above a feed tray and configured to prevent a lowered contact surface pressure between the pickup roller and sheets left on the feed tray by lifting a pressing plate in response to reduction in the number of the sheets left on the feed tray. 
     Further, the known image forming apparatus is configured to switch a power transmission state between a state where a force for lifting the pressing plate is transmitted to the side of the pressing plate and another state where the transmission of the force is interrupted, by swinging a swing member such as a stop arm. At this time, a known latchet mechanism, which includes a latchet gear and a latching member, prevents the pressing plate from moving down when the transmission of the force is interrupted. 
     Hereinafter, the position of the swing member in a situation where the force is transmitted to the side of the pressing plate will be referred to as a first position. In addition, the position of the swing member in a situation where the transmission of the force is interrupted will be referred to as a second position. 
     Nonetheless, when the force is transmitted to the side of the pressing plate and the pressing plate is lifted in a state where the latchet gear is engaged with the latching member, the latchet gear rotates in conjunction with the upward motion of the pressing plate. Then, when a tooth portion of the latchet gear collides against the latching member, an abnormal clattery sound is generated. 
     Thus, to prevent the abnormal clattery sound, the known image forming apparatus is configured (a) to cause an elastic member to always press the latching member against the latchet gear and (b) to render the latching member separate from the latchet gear by making a contact portion provided to the swing member collide against the latching member when the swing member swings from the second position to the first position. 
     SUMMARY 
     In the meantime, when the latchet gear rotates more than a width of a groove between adjacent teeth thereof (hereinafter referred to as a “tooth groove width”) since the swing member has swung to the first position, the latching member climbs over the tooth portion of the latchet gear so as to cause a collision between the tooth portion of the latchet gear and the latching member. 
     Therefore, a separation moment when the latching member is rendered separate from the latchet gear is required to be (A) the same moment as when the swing member swings to the first position or (B) a moment before the latchet gear rotates as much as the tooth groove width since the swing member has swung to the first position. 
     Meanwhile, in the aforementioned known image forming apparatus, the separation moment is determined based on dimensions such as the external dimensions and positional dimensions of the contact portion and the latching member and the tooth groove width of the latchet gear. Therefore, to satisfy the aforementioned requirements concerning the separation moment, manufacturing processes for components such as the contact portion, the latching member, and the latchet gear need to be strictly managed to reduce variations in the external dimensions and positional dimensions of the components. 
     However, the strictly-managed manufacturing processes for the components such as the contact portion, the latching member, and the latchet gear might lead to increased manufacturing costs of the components. Consequently, it might result in an increased manufacturing cost of the image forming apparatus. 
     Aspects of the present invention are advantageous to provide one or more improved techniques for an image forming apparatus that make it possible to prevent occurrence of an abnormal sound by separating a latching member from a latchet gear at an appropriate moment and avoid a rise in the manufacturing cost of the image forming apparatus. 
     According to aspects of the present invention, an image forming apparatus is provided that includes an image forming unit configured to form an image on a sheet, a loading unit configured to be loaded with a plurality of sheets stacked thereon, a feed roller disposed above the loading unit, the feed roller being configured to contact the sheets stacked on the loading unit and feed the sheets toward the image forming unit, a pressing plate configured to move upward the sheets stacked on the loading unit, a swing member configured to swing between a first position where a force for moving the pressing plate upward is transmitted to the pressing plate and a second position where transmission of the force to the pressing plate is interrupted, a latchet mechanism configured to forbid the pressing plate to move downward, the latchet mechanism including a latchet gear configured to rotate in conjunction with a motion of the pressing plate, and a latching member movably attached to the swing member, the latching member being configured to forbid rotation of the latchet gear by engaging with the latchet gear, and a pressing member configured to apply to the latching member a pressing force to press the latching member against the latchet gear when the swing member is in the second position, and not to apply the pressing force to the latching member, and render the latching member movable relative to the swing member when the swing member is in the first position. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a cross-sectional side view showing a configuration of an image forming apparatus in an embodiment according to one or more aspects of the present invention. 
         FIG. 2  shows a configuration of a part of the image forming apparatus around a pickup roller and a separation roller in the embodiment according to one or more aspects of the present invention. 
         FIG. 3  is a perspective view showing a configuration of a lifting mechanism for lifting a pressing plate in the embodiment according to one or more aspects of the present invention. 
         FIGS. 4A and 4B  are exploded perspective views showing a configuration of a clutch mechanism in the embodiment according to one or more aspects of the present invention. 
         FIG. 5  is another perspective view showing the configuration of the lifting mechanism in the embodiment according to one or more aspects of the present invention. 
         FIG. 6A  is a front view showing a configuration of a latching member in the embodiment according to one or more aspects of the present invention. 
         FIG. 6B  is a front view showing a configuration of a swing member in the embodiment according to one or more aspects of the present invention. 
         FIG. 6C  is a front view showing a configuration of an engagement arm in the embodiment according to one or more aspects of the present invention. 
         FIGS. 7 ,  8 ,  9 , and  10  illustrate operations of the lifting mechanism in the embodiment according to one or more aspects of the present invention. 
         FIGS. 11A and 11B  illustrate operations of the latching member and an elastic member attached to the swing member in the embodiment according to one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. 
     Hereinafter, an embodiment according to aspects of the present invention will be described with reference to the accompanying drawings. It is noted that, in the embodiment, aspects of the present invention are applied to an electrophotographic image forming apparatus. 
     1. Overall Configuration of Image Forming Apparatus 
     As shown in  FIG. 1 , an image forming apparatus  1  includes, in a housing  3  thereof, a monochrome image forming unit  5  configured to form an image on a sheet such as a recording sheet and a transparency by transferring a developer image onto the sheet. 
     The image forming unit  5  includes a process cartridge  7  that forms a development unit, an exposure unit  9  configured to expose a photoconductive drum  7 A, a transfer roller  11  configured to transfer a developer image formed on the photoconductive drum  7 A onto the sheet, and a fuser  13  configured to heat and fix the developer image transferred onto the sheet. 
     Further, a feed tray  15  includes a loading unit  15 A configured such that sheets to be fed to the image forming unit  5  are stacked thereon. The feed tray  15  is detachably attached to an apparatus main body, that is, the housing  3 . Specifically, in the embodiment, the feed tray  15  is attached to or detached from the apparatus main body when moved along a front-to-rear direction. 
     The sheets placed on the loading unit  15 A are fed toward the image forming unit  5  by a pickup roller  17 , separated on a sheet-by-sheet basis by a separation roller  19  and a separation pad  21 , and then conveyed to the image forming unit  5 . 
     The pickup roller  17  is disposed above the feed tray  15  in the apparatus main body. The pickup roller  17  is configured to contact a top one of sheets placed on the loading unit  15 A from above and feed the sheets toward the image forming unit  5 . 
     The separation pad  21  is disposed in a position opposed to the separation roller  19  and configured to apply a feeding resistance to the sheets. Meanwhile, the separation roller  19  is configured to rotate in contact with the top one of the sheets and apply a feeding force to the top sheet. Therefore, even when a plurality of sheets are fed from the pickup roller  17  toward the image forming unit  5 , the plurality of sheets are separated and fed to the image forming unit  5  on a sheet-by-sheet basis. 
     Further, the sheet fed out from the separation roller  19  is fed by a feeding roller  23 , and the feeding direction of the sheet is turned up and around by a feeding chute  23 A. After that, a skew correction is performed for the sheet by two registration rollers  25 , and then, the sheet is fed to the image forming apparatus  5  at a predetermined moment. 
     As shown in  FIG. 2 , the pickup roller  17  and the separation roller  19  are integrated via a roller holder  27 A to form a roller unit  27 . A driving gear  19 B configured to rotate integrally with the separation roller  19  is provided at an end of the separation roller  19  in an axial direction of the separation roller  19 , which end is on the same side as a driving shaft  19 A. 
     Further, a driven gear configured to rotate integrally with the pickup roller  17  is provided at an end of the pickup roller  17  in an axial direction of the pickup roller  17 , which end is on the same side as the driving gear  19 B. It is noted that  FIG. 2  does not show the driven gear or a driving source (such as an electric motor) configured to provide a driving force to the driven shaft  19 A. 
     An intermediate gear  27 B configured to engage with the driving gear  19 B and the driven gear is rotatably attached to the roller holder  27 A. Therefore, when the driving shaft  19 A rotates, and the separation roller  19  rotates, a rotational force is transmitted from the driving gear  19 B to the driven gear via the intermediate gear  27 B, and the pickup roller  17  is rotated. 
     The roller holder  27 A is rotatable around an axis line of the separation roller  19 , and the intermediate gear  27 B is rotatably supported by the roller holder  27 A. Therefore, when the separation roller  19  rotates in a state where the pickup roller  17  is not in contact with the sheet, that is, in a state where the roller holder  27 A is freely rotatable, the pickup roller  17  revolves around the separation roller  19  together with the roller holder  27 A, without rotating. 
     When the pickup roller  17  comes into contact with the sheet such that the rotation of the roller holder  27 A is regulated, the intermediate gear  27 B begins to rotate relative to the roller holder  27 A, and the pickup roller  17  begins to rotate. Thus, in the embodiment, when the rotational force is transmitted to the separation roller  19 , the pickup roller  17  begins to rotate after revolving to such a position as to contact an upper surface of the sheets placed on the loading unit  15 A. 
     Further, a roller swing arm  27 C, which extends in a direction substantially parallel to the driving shaft  19 A, is swingably linked with a side of the roller holder  27 A that is close to the pickup roller  17 . A middle portion in a longitudinal direction of the roller swing arm  27 C is supported swingably relative to the apparatus main body. A first end (a left end) in the longitudinal direction of the roller swing arm  27 C is linked with the roller holder  27 A. A second end (a right end) in the longitudinal direction of the roller swing arm  27 C extends up to the outside of the feed tray  15  in a width direction of the feed tray  15 . 
     Therefore, when the pickup roller  17  (or the roller holder  27 A) revolves to the side of the loading unit  15 A, the roller swing arm  27 C is swung such that the first end in the longitudinal direction thereof moves downward and the second end in the longitudinal direction thereof moves upward. 
     It is noted that the width direction is a direction perpendicular to a direction in which the sheet is fed by the pickup roller  17  and a thickness direction of the sheets placed on the loading unit  15 A. In the embodiment, a left-to-right direction is defined as the width direction. 
     As shown in  FIG. 1 , at a bottom portion of the feed tray  15 , there is provided a pressing plate  15 B configured to move the sheets placed on the loading unit  15 A upward, that is, toward the pickup roller  17 . The pressing plate  15 B is configured to swing upward in response to reduction in the number of the sheets placed on the loading unit  15 A. Thereby, it is possible to maintain a contact surface pressure between the pickup roller  17  and the sheet within a predetermined range and to prevent a feeding failure of the pickup roller  17 . 
     2. Lifting Mechanism of Pressing Plate 
     2. 1. Clutch Mechanism 
     As shown in  FIG. 3 , a lifting mechanism  30  for lifting the pressing plate  15 B includes a plurality of gears. The lifting mechanism  30  is configured to move the pressing plate  15 B upward by rotating a sector gear for lifting the pressing plate  15 B by a previously-set angle when the second end in the longitudinal direction of the roller swing arm  27 C moves upward. It is noted that the sector gear is not shown in  FIG. 3 . 
     An input gear  31  is configured to be supplied with a driving force from the aforementioned electric motor and rotate in synchronization with rotation of the electric motor. Therefore, the input gear  31  rotates when the electric motor rotates, regardless of the position of the pickup roller  17  (i.e., regardless of whether it is time to lift the pressing plate  15 B). 
     An output gear  32  is configured to output and transmit the driving force to the sector gear. Hence, when the output gear  32  rotates, the pressing plate  15 B moves upward. A latchet gear  33 , together with a latching member  34 , forms a latching mechanism  35 . 
     The latchet mechanism is a known mechanism configured to allow the latchet gear  33  to rotate in one direction and forbid the latchet gear  33  to rotate in the other direction by engagement between the latchet gear  33  and the latching member  34 . 
     Since the latchet gear  33  and the output gear  32  rotate integrally, the latchet gear  33  rotates in mechanical conjunction with movement of the pressing plate  15 B. Accordingly, in the embodiment, by the latchet mechanism  35 , the downward movement of the pressing plate  15 B is restricted, and the upward movement of the pressing plate  15 B is permitted. When the engagement between the latchet gear  33  and the latching member  34  is released, the pressing plate  15 B is put into a state movable downward. 
     Further, in the embodiment, transmission/interruption of the driving force from the input gear  31  to the output gear  32  is carried out by a clutch mechanism  36  using a planetary gear mechanism. It is noted that the clutch mechanism  36  and the latchet gear  33  are rotatably attached to the apparatus main body. 
     As shown in  FIG. 4A , the clutch mechanism  36  includes a sun gear  36 A, an inner gear  36 B, planetary gears  36 C, and a holder  36 D. The sun gear  36 A is rotated by the driving force transmitted via the input gear  31 . 
     In the embodiment, the sun gear  36 A and the input gear  31  are integrally formed of resin in a coaxially-arranged state. Therefore, the sun gear  36 A rotates in synchronization with the aforementioned electric motor. 
     The inner gear  36 B has a center line of rotation that is positionally coincident with a center line of rotation of the sun gear  36 A. Further, the inner gear  36 B includes teeth that are formed on a cylindrical inner circumferential surface and configured to engage with the planetary gears  36 C. The driving force is transmitted from the inner gear  36 B to the output gear  32 . 
     In the embodiment, the inner gear  36 B, the latchet gear  33 , and the output gear  32  are integrally formed of resin in a coaxially-arranged state. Therefore, when the inner gear  36 B rotates, the output gear  32  rotates, and the pressing plate  15 B moves. 
     The planetary gears  36 C are provided at an inner side of the inner gear  36 B and configured to engage with the sun gear  36 A and the inner gear  36 B. In the embodiment, the two planetary gears  36 C are disposed point-symmetrically with respect to the sun gear  36 A. 
     As shown in  FIG. 4B , the holder  36 D is configured to rotatably hold the planetary gears  36 C such that each planetary gear  36 C revolves around a center of rotation of the sun gear  36 A. Specifically, the holder  36 D includes holding shafts  36 E each of which is configured to rotatably hold a corresponding one of the planetary gears  36 C, and a holding plate  36 F configured to support the holding shafts  36 E such that each planetary gear  36 C revolves around the center of rotation of the sun gear  36 A. 
     Accordingly, when the holder  36 D is permitted to rotate in a state where the sun gear  36 A rotates, since a rotational resistance of the output gear  32  or the inner gear  36 B is larger than a rotational resistance of the holder  36 D, the two planetary gears  36 C revolve around the sun gear  36 A, but do not transmit the driving force to the inner gear  36 B. Therefore, the transmission of the driving force from the inner gear  31  to the output gear  32  is interrupted. 
     Meanwhile, when the holder  36 D is forbidden to rotate in a state where the sun gear  36 A rotates, since the rotational resistance of the inner gear  36 B is smaller than the rotational resistance of the holder  36 D, the two planetary gears  36 C rotate in their respective positions without revolving around the sun gear  36 A. Therefore, the driving force is transmitted to the inner gear  36 B. Thus, the driving force is transmitted from the inner gear  31  to the output gear  32 . 
     2. 2. Control of Clutch Mechanism 
     &lt;Details of Clutch Mechanism&gt; 
     As shown in  FIGS. 4A and 4B , the holding plate  36 F of the holder  36 D includes an engagement portion G provided with a plurality of projections formed on a cylindrical outer circumferential surface of the holding plate  36 F. As shown in  FIG. 5 , an engagement arm  37  configured to engage with the engagement portion  36 G is attached to the apparatus main body. The engagement arm  37  is configured to move between an engagement position to engage with the engagement portion  36 G and a separation position separated from the engagement portion  36 G. 
     As shown in  FIG. 6C , the engagement arm  37  includes a claw portion  37 A configured to engage with the projections of the engagement portion  36 G, and a bearing configured such that a swing shaft  3 A provided to the apparatus main body is inserted thereinto. Thus, the engagement arm  37  is allowed to swing around the bearing  37 B between the engagement position and the separation position. 
     As shown in  FIG. 5 , the swing motion of the engagement arm  37  is mechanically controlled by a cam tube  38  that has a cam surface  38 A on an outer circumferential surface thereof. Specifically, the engagement arm  37  includes a sliding-contact portion  37 C configured to slide in contact with the cam surface  38 A, and a spring hook  37 E configured to be linked with one end of a spring  37 D. The spring  37 D is configured to apply a force to press the sliding-contact portion  37 C against the cam tube  38  and a force to press the claw portion  37 A against the engagement portion  36 G in a state where the other end of the spring  37 D is fixed to the apparatus main body. 
     Therefore, in response to rotation of the cam tube  38 , as shown in  FIG. 7 , the claw portion  37 A is separated from the engagement portion  36 G when the sliding-contact portion  37 C is in contact with a cylindrical portion A of the cam surface  38 A. Meanwhile, as shown in  FIG. 8 , the claw portion  37 A is engaged with the engagement portion  36 G. 
     Namely, when the sliding-contact portion  37 C contacts the cylindrical portion A of the cam surface  38 A, the holder  36 D is allowed to rotate such that the transmission of the driving force from the input gear  31  to the output gear  32  is interrupted. Meanwhile, when the sliding-contact portion  37 C is within a region of a cutout portion B of the cam surface  38 A, the holder  36 D is forbidden to rotate such that the driving force is transmitted from the input gear  31  to the output gear  32 . 
     Further, as shown in  FIG. 3 , at one end in the axial direction of the cam tube  38 , there is provided a cam gear  38 B configured to rotate integrally with the cam tube  38 . As shown in  FIG. 7 , the cam gear  38 B is a tooth-lacking gear that includes a tooth portion  38 C having teeth configured to engage with the input gear  31 , and a tooth-lacking portion  38 D having no tooth. 
     As indicated by an alternate long and two short dashes arrow in  FIG. 3 , a spring  38 E such as a torsion coil spring is configured to always apply a force directed to rotate the cam gear  38 B. As shown in  FIG. 3 , on an outer circumferential surface of the cam tube  38 , there is provided a projection  38 F configured to engage with a cam stopper arm  39 . 
     The cam stopper arm  39  is configured to engage with the projection  38 F and restrict the cam gear  38 B from rotating, against an elastic force of the spring  38 E. The cam stopper arm  39  is attached to the apparatus main body so as to move between a position separate from the projection  38 F, that is, the outer circumferential surface of the cam tube  38  and a position where the cam stopper arm  39  is close to or in contact with the outer circumferential surface of the cam tube  38  to engage with the projection  38 F. 
     As shown in  FIG. 5 , a spring  39 A is configured to apply to the cam stopper arm  39  a force to swing the cam stopper arm  39  toward the projection  38 F. Meanwhile, the roller swing arm  27 C is configured to apply to the cam stopper arm  39  a force directed to separate the cam stopper arm  39  from the outer circumferential surface of the cam tube  38  when the second end (the right end) in the longitudinal direction of the roller swing arm  27 C moves higher than a previously-set position therefor. 
     &lt;Operations of Clutch Mechanism&gt; 
     When the second end (the right end) in the longitudinal direction of the roller swing arm  27 C moves higher than the previously-set position (i.e., when the pickup roller  17  moves lower than a previously-set position therefor), as shown in  FIG. 8 , the engagement between the cam stopper arm  39  and the projection  38 F is released. Therefore, the cam tube  38  and the cam gear  38 B begin to be rotated by the elastic force of the spring  38 E. 
     Then, as the sliding-contact portion  37 C moves into the region of the cutout portion B, the holder  36 D is forbidden to rotate, such that the driving force is transmitted from the input gear  31  to the output gear  32 . Accordingly, the pressing plate  15 B begins to move upward. 
     Then, when the cam tube  38  and the cam gear  38 B further rotate, as shown in  FIG. 9 , the engagement between the tooth portion  38 C of the cam gear  38 B and the input gear  31  is established. Thereby, the cam tube  38  and the cam gear  38 B are rotated by the driving force from the input gear  31 . It is noted that, at this time, as the sliding-contact portion  37 C is within the cutout portion B, the pressing plate  15 B continues to move upward. 
     When the cam tube  38  and the cam gear  38 B further rotate from the state shown in  FIG. 9 , as shown in  FIG. 10 , the sliding-contact portion  37 C begins to move into the region of the cylindrical portion A, and the claw portion  37 A is separated from the engagement portion  36 G. Thereby, the holder  36 D is allowed to rotate such that the transmission of the driving force from the input gear  31  to the output gear  32  is interrupted. Accordingly, the upward motion of the pressing plate  15 B is stopped. 
     Then, when the cam tube  38  and the cam gear  38 B further rotate, the tooth-lacking portion  38 D of the cam gear  38 B faces to the input gear  31 , and the cam stopper arm  39  engages with the projection  38 F. Hence, as shown in  FIG. 7 , the rotations of the cam tube  38  and the cam gear  38 B are stopped. 
     As described above, when the pickup roller  17  moves lower than the previously-set position therefor, the cam tube  38  and the cam gear  38 B rotate while changing their states as shown in the order of  FIGS. 8 ,  9 ,  10 , and  7 . Thereby, the pressing plate  15 B is moved upward by a predetermined distance. 
     2. 3. Latchet Mechanism 
     &lt;Overall Operations of Latchet Mechanism&gt; 
     The latchet mechanism  35  is configured to restrict the pressing plate  15 B from moving downward by the engagement between the latchet gear  33  and the latching member  34 , when the transmission of the driving force from the input gear  31  to the output gear  32  is interrupted. 
     Then, when the driving force is transmitted from the input gear  31  to the output gear  32 , and the pressing plate  15 B moves upward, the engagement between the latchet gear  33  and the latching member  34  is released, and the latching member  34  is placed in a position separate from (in non-contact with) the latchet gear  33 . 
     &lt;Configuration of Latchet Mechanism&gt; 
     The latchet gear  33  is integrated with the inner gear  36 B of the clutch mechanism  36 . As shown in  FIG. 6A , the latching member  34  includes a claw section  34 A, a bearing section  34 B, and an arm section  34 C. 
     The bearing section  34 B is configured such that the swing shaft  3 A provided to the apparatus main body is inserted thereinto. The arm section  34 C is an arm extending toward the latchet gear  33  from the bearing section  34 B (or the swing shaft  3 A). The claw section  34 A is provided at a distal end in the extending direction of the arm section. The claw section  34 A is configured to engage with the latchet gear  33  when the transmission of the driving force from the input gear  31  to the output gear  32  is interrupted. The claw section  34 A, the bearing section  34 B, and the arm section  34 C are integrally formed of resin. 
     In a state where the latching member  34  engages with the latchet gear  33  (i.e., the claw portion  34 A engages with the teeth of the latchet gear  33 ), as indicated by an alternate long and short dash line L 1  in  FIG. 7 , the longitudinal direction of the arm section  34 C extends horizontally. 
     Meanwhile, the axis line direction of the swing shaft  3 A (or the bearing section  34 B) is coincident with the horizontal direction perpendicularly intersecting the alternate long and short dash line L 1 . Further, the swing shaft  3 A is disposed lower than the latchet gear  33 . Therefore, the latching member  34  is allowed to swing up and down around the swing shaft  3 A. 
     As shown in  FIG. 6A , the claw section  34 A of the latching member  34  is formed substantially in a right triangle shape with a slanted section  34 D thereof facing the swing shaft  3 A, when viewed along a direction parallel to the axis line direction of the swing shaft  3 A. 
     Additionally, as shown in  FIG. 7 , the engagement arm  37  and a swing member  40  are swingably attached to the swing shaft  3 A. Therefore, the latching member  34  is attached to be swingable up and down relative to the swing member  40 . 
     As shown in  FIG. 6B , the swing member  40  includes a sliding-contact section  40 A configured to slide in contact with the cam surface  38 A, and a bearing section  40 B configured to bear the swing shaft  3 A inserted thereinto. As shown in  FIGS. 11A and 11B , the sliding-contact section  40 A is disposed on an opposite side of the claw section  34 A with respect to the swing shaft  3 A. 
     Therefore, when the sliding-contact section  40 A moves in such a direction as to be closer to the latchet gear  33 , as shown in  FIG. 8 , a right end portion of the swing member  40  that is an end portion close to the claw section  34 A (hereinafter referred to as a “claw-section-side portion”) moves in such a direction as to be farther away from the latchet gear  33 . Meanwhile, when the sliding-contact section  40 A moves in such a direction as to be farther away from the latchet gear  33 , as shown in  FIG. 7 , the claw-section-side portion of the swing member  40  moves in such a direction as to be closer to the latchet gear  33 . 
     In addition, the swing member  40  is configured such that the gravity force applied thereto causes a moment that urges the sliding-contact section  40 A to move in such a direction as to be closer to the latchet gear  33 . Further, the swing member  40  is connected with a spring (not shown) that applies to the swing member  40  a force for pressing the sliding-contact section  40 A against the cam surface  38 A. 
     Therefore, when the cam tube  38  and the cam gear  38 B rotate, the sliding-contact portion  37 C of the engagement arm  37  and the sliding-contact section  40 A of the swing member  40  move while tracing the cam surface  38 A. Thus, the engagement arm  37  and the swing member  40  swing in mechanical synchronization with each other. 
     Hereinafter, a position of the swing member  40  where the driving force is allowed to be transmitted from the input gear  31  to the output gear  32  such that the pressing plate  15 B moves upward will be referred to as a first position. In addition, a position of the swing member  40  where the transmission of the driving force from the input gear  31  to the output gear  32  is interrupted such that the pressing plate  15 B is forbidden to move upward will be referred to as a second position. 
     Therefore, when the swing member  40  is in the second position, as shown in  FIG. 7 , the claw-section-side portion of the swing member  40  is rendered closer to the latchet gear  33  than when the swing member  40  is in the first position. Meanwhile, when the swing member  40  is in the first position, as shown in  FIG. 8 , the claw-section-side portion of the swing member  40  is rendered farther away from the latchet gear  33  than when the swing member  40  is in the second position. 
     As shown in  FIGS. 11A and 11B , an elastic member  41  such as a torsion coil spring is attached to the swing member  40 . The elastic member  41  includes a rod-shaped acting portion  41 A that extends from the side of the swing shaft  3 A to the side of the claw section  34 A, a coil portion  41 B provided at one end in the longitudinal direction of the acting portion  41 A, and a held portion  41 C that extends from the coil portion  41 B to the same side as the acting portion  41 A and is held by the swing member  40 . 
     Into the coil portion  41 B, inserted is a cylindrical boss  40 C provided to the swing member  40 . Meanwhile, a distal end in the extending direction of the acting portion  41 A is held by a restricting portion  40 D. The restricting portion  40 D is configured to contact the distal end of the acting portion  41 A from the side of the latching member  34  and restrict the acting portion  41 A from moving toward the latching member  34 . 
     The latching member  34  includes a projection  40 E that protrudes toward the acting portion  41 A. As shown in  FIG. 7 , when the swing member  40  is in the second position, the projection  40 E applies, to the latching member  34 , an elastic force (a pressing force) for pressing the latching member  34  against the latchet gear  33  (more specifically, the projection  40 E transmits the pressing force from the acting portion  41 A to the latching member  34  therethrough). 
     &lt;Detailed Operations of Latchet Mechanism&gt; 
     When the swing member  40  is in the second position, the sliding-contact section  40 A contacts the cam surface  38 A. Therefore, as shown in  FIG. 7 , a side of the swing member  40  that is close to the restricting portion  40 D is placed in a position close to the latchet gear  33 . Hence, the acting portion  41 A of the elastic member  41  contacts the projection  40 E of the latching member  34  and applies the pressing force to the latching member  34 . 
     When the swing member  40  swings from the second position to the first position, as shown in  FIG. 8 , the acting portion  41 A (the elastic member  41 ) swings downward integrally with the swing member  40  so as to be farther away from the latchet gear  33 . Thus, the acting portion  41 A is separated from the projection  40 E. 
     Then, when the swing member  40  is placed into the first position, the acting portion  41 A of the elastic member  41  is not allowed to apply the pressing force. Thereby, the latching member  34  is put into a state swingable relative to the swing member  40 , and the claw section  34 A is separated from the latchet gear  33 . Thus, the engagement between the claw section  34 A and the latchet gear  33  is released. 
     Namely, when the swing member  40  is placed into the first position, the driving force is transmitted from the input gear  31  to the output gear  32 , such that the latchet gear  33  rotates in such a direction as to move the pressing plate  15 B upward. 
     Thereby, the claw section  34 A of the latching member  34  moves downward along a slanted surface of a tooth of the latchet gear  33 . Further, owing to the gravity force applied to the latching member  34 , the moment for separating the latching member  34  from the latchet gear  33  is applied to the latching member  34 . Thus, the claw section  34 A is separated from the latchet gear  33 . 
     3. Features of Image Forming Apparatus 
     In the embodiment, as described above, when the swing member  40  is in the second position, the pressing force is applied to the latching member  34 . Therefore, it is possible to certainly hold the engagement between the latchet gear  33  and the latching member  34 . 
     Further, in the embodiment, when the swing member  40  is in the first position, the pressing force is not applied to the latching member  34 , and the latching member  34  is movable relative to the swing member  40 . 
     Therefore, at the same time as the latchet gear  33  rotates, the latching member  34  is rendered separate from the latchet gear  33  upon receipt of a rotational force of the latchet gear  33 . Namely, in the embodiment, nearly at the same time as the swing member  40  is placed into the first position, the latching member  34  is separated from the latchet gear  33 . 
     Accordingly, in the embodiment, it is possible to separate the latching member  34  from the latchet gear  33  at an appropriate separation moment without having to provide any contact portion to the swing member  40 . Thus, it is possible to prevent an abnormal sound from being caused by collision between a tooth of the latchet gear  33  and the latching member  34  and to avoid a rise in the manufacturing cost of the image forming apparatus. 
     Further, in the embodiment, the elastic member  41  is attached to the swing member  40  and configured to swing integrally with the swing member  40 . Thereby, in the embodiment, the elastic member  41  moves in conjunction with the swing motion of the swing member  40 . Therefore, it is possible to easily eliminate the pressing force applied to the latching member  34 , at an appropriate moment. 
     Further, in the embodiment, the latching member  34  is attached to be swingable up and down relative to the swing member  40 . Moreover, the swing axis (the swing shaft  3 A) of the latching member  34  is disposed lower than the latchet gear  33 . 
     Thereby, in the embodiment, as described above, the gravity force applied to the latching member  34  acts as a force to separate the latching member  34  from the latchet gear  33 . Therefore, it is possible to certainly separate the latching member  34  from the latchet gear  33  and to certainly maintain a separation state where the latching member  34  is separated from the latchet gear  33  after the latching member  34  is put into the separation state. Accordingly, it is possible to certainly prevent occurrence of an abnormal sound. 
     Further, in the embodiment, in a situation where the latching member  34  engages with the latchet gear  33 , the arm section  34 C extends horizontally. Thereby, in the embodiment, it is possible to make the gravity force applied to the latching member  34  effectively function as a force to separate the latching member  34  from the latchet gear  33 . 
     It is noted that the expression “the arm section  34 C extends horizontally” may represent a situation where the arm section  34 C extends in a substantially horizontal direction (such as a direction different from the horizontal direction by an angle of  10  degrees) as well as a situation where the arm section  34 C extends in the definitely horizontal direction. Further, in the embodiment, the claw section is formed in a right triangle shape when viewed along the direction parallel to the axis line direction of the swing shaft  3 A. 
     Thereby, in the embodiment, as shown in  FIG. 7 , since a vertical section  34 E of the claw section  34 A is substantially parallel to the vertical direction, it is possible to easily separate the latching member  34  from the latchet gear  33 . Accordingly, when the swing member  40  is placed into the first position, and a rotational force is applied to the latchet gear  33 , nearly at the same time, the latching member  34  is separated from the latchet gear  33 . Thus, it is possible to certainly prevent occurrence of an abnormal sound. 
     It is noted that the expression “the claw section is formed in a right triangle shape when viewed along a direction parallel to the axis line direction of the swing shaft  3 A” may represent a situation where the claw section is formed substantially in a right triangle shape when viewed through an eye observation along a direction parallel to the axis line direction of the swing shaft  3 A. The term “a right triangle shape” does not necessarily have to represent a definitely right triangle shape. 
     Further, in the embodiment, the latching member  34  is provided with the projection  40 E that protrudes toward the acting portion  41 A and configured to apply the pressing force to the latching member  34  when the swing member  40  is in the second position. 
     Thereby, in the embodiment, the pressing force is applied to the latching member  34  via the projection  40 E. Therefore, it is possible to reduce the variation in the position of the latching member  34  to which the pressing force is applied. Thus, it is possible to stably operate the latching member  34 . 
     Further, in the embodiment, the swing member  40  is provided with the restricting portion  40 D configured to contact the acting portion  41 A from the side of the latching member  34  and restrict the acting portion  41 A from moving toward the latching member  34 . 
     Thereby, in the embodiment, it is possible to certainly restrict the pressing force from being applied to the latching member  34  when the swing member  40  is in the first position. 
     Further, in the embodiment, the slanted section  34 D of the claw section  34 A faces the swing shaft  3 A when viewed along a direction parallel to the axis line direction of the swing shaft  3 A. 
     Thereby, in the embodiment, when the swing member  40  is in the second position, the force applied to the vertical section  34 E of the claw section  34 A is directed toward the swing shaft  3 A. Meanwhile, the force applied to the vertical section  34 E is based upon a force in such a direction as to move the pressing plate  15 B downward. 
     Accordingly, when the swing member  40  is placed into the first position such that the rotational force is applied to the latchet gear  33 , as shown in  FIG. 8 , the force applied to the vertical section  34 E of the claw section  34 A disappears. Further, at the same time, a frictional force generated at the vertical section  34 E disappears. Therefore, nearly at the same time as the swing member  40  is placed into the first position, the latching member  34  is rendered separate from the latchet gear  33 . Thereby, it is possible to certainly prevent occurrence of an abnormal sound. 
     Hereinabove, the embodiment according to aspects of the present invention has been described. The present invention can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the present invention. However, it should be recognized that the present invention can be practiced without reapportioning to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present invention. 
     Only an exemplary embodiment of the present invention and but a few examples of their versatility are shown and described in the present invention. It is to be understood that the present invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, the following modifications are possible. 
     (Modifications) 
     In the aforementioned embodiment, the restricting portion  40 D is provided. However, in order to restrict the elastic member  41  from applying the pressing force to the latching member  34  when the swing member  40  is in the first position, for example, the elastic member  41  may be configured such that the acting portion  41 A is separated from the projection  40 E in a natural state of the elastic member  41 , without the restricting portion  40 D. 
     In the aforementioned embodiment, a torsion coil spring is employed as the elastic member  41 . However, the elastic member  41  may be a plate spring having only an element corresponding to the acting portion  41 A. 
     In the aforementioned embodiment, the elastic member  41  is attached to the swing member  40 . However, for instance, the elastic member  41  may be attached to the latching member  34 . Further, in the aforementioned embodiment, the latching member  34  and the swing member  40  are disposed on a side lower than the latchet gear  33 . However, the latching member  34  and the swing member  40  may be disposed on a side higher than the latchet gear  33 . 
     In the aforementioned embodiment, aspects of the present invention are applied to a monochrome laser printer. However, aspects of the present invention may be applied to other types of image forming apparatuses such as a color laser printer and an inkjet printer.