Patent Publication Number: US-2015084267-A1

Title: Sheet feeding apparatus and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeding apparatus, and an image forming apparatus including the sheet feeding apparatus. 
     2. Description of the Related Art 
     Conventionally, an image forming apparatus, which forms an image on a sheet, has been provided with a sheet feeding apparatus that feeds stacked sheets by bringing the uppermost sheet and a feed roller separated therefrom into contact with each other every time a single feeding operation is performed. In such a configuration, in order to make an interval between the preceding sheet and the subsequent sheet (hereinafter referred to as a sheet feed interval) as small as possible during continuous printing to improve productivity, Japanese Patent No. 4249050 discusses a configuration in which the stacked uppermost sheet and the feed roller are brought into contact with each other before a trailing edge of the preceding sheet passes through the feed roller. 
     However, in a sheet feeding apparatus discussed in Japanese Patent No. 4249050, while an image is formed on or transferred onto sheets, the trailing edge of the preceding sheet is sandwiched between the feed roller and the stacked uppermost sheet, thereby causing a side effect of distorting the image. Further, a contact/separation operation between the uppermost sheet and the feed roller, and the timing of when driving of the feed roller is started need to be controlled independently of each other by different electronic components (a solenoid, a clutch, etc.), causing a cost increase. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a sheet feeding apparatus that is low in cost and high in productivity. 
     According to an aspect of the present invention, a sheet feeding apparatus includes a stacking portion on which a sheet is stacked, a drive source configured to generate a driving force, a pick-up member provided to be rotatable with the driving force of the drive source and configured to rotate in contact with the sheet stacked on the stacking portion to feed the sheet, a feed member configured to feed the sheet fed by the pick-up member, an elevating unit configured to raise and lower the stacking portion or the pick-up member with the driving force from the drive source to bring the sheet stacked on the stacking portion and the pick-up member into contact with each other, and a control unit configured to control the elevating unit. At least in a state where a predetermined number or more of sheets are stacked on the stacking portion, the control unit controls the elevating unit, at a timing of when a trailing edge in a feeding direction of a first sheet fed to the pick-up member is upstream of the feed member and downstream of the pick-up member, to bring a second sheet stacked on the stacking portion and to be fed subsequently to the first sheet and the pick-up member into contact with each other. 
     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 
         FIGS. 1A and 1B  illustrate an overall configuration of an image forming apparatus according to an exemplary embodiment of the present invention. 
         FIG. 2  illustrates a sheet feeding apparatus according to the exemplary embodiment of the present invention. 
         FIG. 3  is a schematic perspective view of a feeding cassette. 
         FIGS. 4A and 4B  illustrate an elevating unit that raises and lowers a stacking plate. 
         FIG. 5  illustrates a drive transmission path from a drive source. 
         FIG. 6  is a schematic perspective view of a clutch mechanism. 
         FIGS. 7A ,  7 B, and  7 C respectively illustrate operations to engage and disengage the clutch mechanism. 
         FIG. 8  is a timing chart of a feeding operation. 
         FIGS. 9A ,  9 B, and  9 C respectively illustrate continuous feeding operations when sheets are fully stacked. 
         FIG. 10  is a flowchart of the continuous feeding operation according to a first exemplary embodiment. 
         FIG. 11  is a block diagram of a control unit according to the first exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
       FIGS. 1A and 1B  illustrate a color digital printer as an example of an image forming apparatus to which a sheet feeding apparatus according to an exemplary embodiment of the present invention is applied.  FIG. 1A  is a perspective view of an external appearance of an image forming apparatus  100 , and  FIG. 1B  is a schematic sectional view of the image forming apparatus  100 . The image forming apparatus  100  is a full four-color laser printer using an electrophotographic process. More specifically, an image is formed on a sheet (a recording medium) S based on an image signal input to a controller unit (control unit) from an external host apparatus such as a personal computer, an image reader, or a counterpart facsimile apparatus. 
     An operation for an image forming unit  101  to form an image will be described below. A drum  1  of each of first to fourth cartridges PY, PM, PC, and PK is rotationally driven at a predetermined control speed in a counterclockwise direction indicated by an arrow illustrated in  FIG. 1B . A belt  4  is also rotationally driven at a speed corresponding to the speed of the drum  1  in a clockwise direction (a forward direction in drum rotation) indicated by an arrow illustrated in  FIG. 1B . A scanner unit  5  is also driven. 
     In synchronization with the above driving, a charging roller  2  in each of the cartridges uniformly charges a surface of the drum  1  to a predetermined polarity/potential at each predetermined control timing. The scanner unit  5  scans and exposes the surface of each of the drums  1  with a laser beam modulated according to an image signal for each of the colors. 
     Thus, an area on the surface of each of the drums  1 , which has been scanned and exposed with the laser beam, becomes an electrostatic latent image corresponding to the image signal. A development unit  3  develops the electrostatic latent image formed on the surface of each of the drums  1  as a toner image. By an electrophotographic image formation process operation, as described above, a toner image is formed on the drum  1 , and the toner image formed thereon is primarily transferred onto the belt  4 . 
     A feeding cassette  9  is detachably attached to the image forming apparatus  100  on the front side thereof (on the side on which an operator operates the apparatus and on the right side of the apparatus illustrated in FIG.  1 B), and is configured to allow a user to easily stack sheets and perform jam handling processing. 
     A pick-up roller  6  serving as a sheet feeding unit comes into contact with the sheets stacked on a stacking plate (stacking unit)  16  (see  FIG. 2 ) in the feeding cassette  9  to feed the sheets. The sheets fed by the pick-up roller  6  are fed one by one while being separated from one another by a feed roller  7  and a separation roller  8 , and are conveyed to a secondary transfer nip portion between a secondary transfer roller  12  and the belt  4  via a registration roller pair  11 . The separation roller  8  is attached to a main body of the image forming apparatus  100  via a torque limiter (not illustrated), and is brought into pressure contact with the feed roller  7  by an urging unit such as a spring (not illustrated). An exemplary embodiment of the present invention is not limited to the separation roller  8 . Alternatively, a separation pad may be used. In an exemplary embodiment of the present invention, any separation unit may be used as long as it can separate, when one or more sheets are fed together, the sheets from one another with a frictional force. 
     The sheet on which a toner image has been transferred in the secondary transfer nip portion is heated and pressurized by a fixing unit  13  so that the toner image is fixed thereto. The sheet to which the toner image has been fixed is discharged onto a sheet discharge tray  15  by a sheet discharge roller pair  14 . 
     Next, the sheet feeding apparatus will be described below.  FIG. 2  is a schematic perspective view of a sheet feeding apparatus  10 . The stacking plate  16  can be raised and lowered with the sheets stacked thereon. An elevating operation of the stacking plate  16  will be described with reference to  FIGS. 3 ,  4 A, and  4 B.  FIG. 3  is a schematic perspective view of the feeding cassette  9 ,  FIG. 4A  is a schematic perspective view illustrating a lowered state of the stacking plate  16  in the present exemplary embodiment, and  FIG. 4B  is a schematic perspective view illustrating a raised state of the stacking plate  16  in the present exemplary embodiment. 
     As illustrated in  FIG. 3 , the stacking plate  16  is rotatably positioned using a stacking plate rotation support portion  36  as a rotation center. The stacking plate  16  is raised and lowered by an elevating unit (stacking unit elevating unit)  50 . The elevating unit  50  urges the stacked sheets against the pick-up roller  6  by raising the stacking plate  16 , and separates the stacked sheets from the pick-up roller  6  by lowering the stacking plate  16 . 
     Even when the sheets are slightly stacked on the stacking plate  16 , the elevating unit  50  raises the stacking plate  16  until the sheets are sufficiently urged against the pick-up roller  6 . 
     The elevating unit  50  includes elevating levers  18 , elevating lever rotation support portions  37 , a pair of elevating cams  19 , and a connecting shaft  20  for connecting the elevating cams  19  to each other. 
     The elevating levers  18  are provided on both sides of the feeding cassette  9 , and are rotatably fixed to a casing of the image forming apparatus  100  using the elevating lever rotation support portion  37  as the rotation center. The elevating levers  18  are urged in a direction (upward direction) closer to the pick-up roller  6  by an urging member such as a spring (not illustrated). Engaging portions  17  with the elevating levers  18  are provided at both ends of the stacking plate  16 . While the feeding cassette  9  is mounted and aligned on the image forming apparatus  100 , the engaging portions  17  and the elevating levers  18  engage with each other, and the stacking plate  16  is raised in conjunction with rotation of the elevating levers  18 . The rotation of the elevating levers  18 , which are urged in the direction closer to the pick-up roller  6 , is restricted by the elevating cams  19  arranged above the elevating levers  18 . As illustrated in  FIGS. 4A and 4B , when the connecting shaft  20  rotates upon receiving a driving force from a drive unit (to be described below), the elevating cams  19  rotate so that the elevating levers  18  rotate up and down, and the stacking plate  16  is raised and lowered via the engaging portions  17 . 
     A drive unit  80  will be described below with reference to  FIG. 5 . The drive unit  80  transmits a driving force to the elevating unit  50  to raise and lower the stacking plate  16 . The drive unit  80  rotates the pick-up roller  6  via a driving transmission unit. 
     A drive source  21  is, for example, a motor of the drive unit  80  provided in the main body of the image forming apparatus  100 . A driving force generated by the drive source  21  is transmitted from a first drive gear  22  to a second drive gear  23  and from the second drive gear  23  to a chipped tooth gear  24 . A solenoid (see  FIG. 11 ) restricts the chipped tooth gear  24  and releases the restriction. Thus, the chipped tooth gear  24  selectively engages with the second drive gear  23 . When the solenoid releases the restriction of the chipped tooth gear  24 , the chipped tooth gear  24  engages with the second drive gear  23  so that the driving force is transmitted. Thus, the chipped tooth gear  24  starts to rotate. When the solenoid restricts the chipped tooth gear  24  at a position where the chipped tooth gear  24  rotates once and a chipped tooth portion of the chipped tooth gear  24  opposes the second drive gear  23 , the driving force is not transmitted. The chipped tooth gear  24  and the elevating cams  19  are fixed to the connecting shaft  20 , which is rotatably supported on the main body of the image forming apparatus  100 , and rotate integrally with the connecting shaft  20 . When the solenoid operates to release the restriction of the chipped tooth gear  24  based on an electric signal from a control unit (not illustrated), the chipped tooth gear  24  engages with the second drive gear  23 , the driving force generated by the drive source  21  is transmitted to the connecting shaft  20  via the chipped tooth gear  24 , and the connecting shaft  20 , together with the elevating cams  19 , rotates once. 
     An idler gear  31  serving as a drive transmission unit transmits a driving force to the pick-up roller  6  and the feed roller  7  via a clutch mechanism  60 . Each of the pick-up roller  6  and the feed roller  7  has a tooth surface formed therein, which engages with the idler gear  31 , and is rotationally driven in response to the rotation of the idler gear  31 . 
     A clutch input gear  26  serving as a clutch input portion rotates when the driving force of the drive unit  80  is input thereto, and a clutch output gear  27  serving as a clutch output portion transmits the driving force from the drive unit  80  to the pick-up roller  6  by engaging with the clutch input gear  26 . The idler gear  31  is arranged to engage with the clutch output gear  27 . Thus, while the clutch input gear  26  engages with the clutch output gear  27 , the rotation of the connecting shaft  20  is transmitted to the idler gear  31  so that the pick-up roller  6  and the feed roller  7  are driven. While the clutch input gear  26  does not engage with the clutch output gear  27 , the rotation of the connecting shaft  20  is not transmitted to the idler gear  31 . 
     When the connecting shaft  20  rotates once, the pick-up roller  6  and the feed roller  7  rotate. A conveyance distance of the sheet by the rotation is set to a distance that allows the sheet to be conveyed to the registration roller pair  11  on a downstream side. 
     Next, the clutch mechanism  60  will be described in detail below. The clutch input gear  26  in the clutch mechanism  60  engages with the clutch output gear  27  after the elevating unit  50  raises the stacking plate  16  to bring the stacked sheets into pressure contact with the pick-up roller  6 . Thus, the pick-up roller  6  starts to rotate after the sheets stacked on the stacking plate  16  come into pressure contact with the pick-up roller  6 . Therefore, the sheet feed interval does not vary. Even if the number of the sheets stacked on the stacking plate  16  changes and the timing of when the sheets and the pick-up roller  6  come into contact with each other deviates, the timing of when the pick-up roller  6  starts to feed the sheets is constant regardless of the amount of stacked sheets. 
       FIG. 6  is a schematic perspective view of the clutch mechanism  60  according to the present exemplary embodiment,  FIG. 7A  is a schematic view illustrating a state where the clutch mechanism  60  is disengaged,  FIG. 7B  is a schematic view illustrating a state where the clutch mechanism  60  is engaged, and  FIG. 7C  is a schematic view illustrating switching of the clutch mechanism  60  from an engaged state to a disengaged state. 
     As illustrated in  FIG. 6 , a clutch bearing  25  is fixed to the connecting shaft  20 , and rotates integrally with the connecting shaft  20 . The clutch bearing  25  includes keys  30 . A clutch input gear  26  includes key grooves  29 , a cam surface  32 , and an input-side gear tooth surface  35 . The clutch input gear  26  is retained in the clutch bearing  25  when the keys  30  in the clutch bearing  25  engage with the key grooves  29 , and is fixed in a rotational direction of the clutch bearing  25  and is movable in a longitudinal direction (rotational axis direction) of the connecting shaft  20 . The clutch output gear  27  includes a tooth surface  39  that engages with the idler gear  31  and an output-side gear tooth surface  38 , and is rotatably retained in the clutch bearing  25 . The clutch output gear  27  in the longitudinal direction of the cam connecting shaft  20  is fixed to the main body of the image forming apparatus  100 . As illustrated in  FIG. 7A , a clutch pressing spring  28  serving as an elastic member urges the clutch input gear  26  toward the clutch output gear  27 . 
     An operation to engage and disengage the clutch mechanism  60  will be described below with reference to  FIGS. 7A to 7C . 
     As illustrated in  FIG. 7A , while the cam surface  32  provided in the clutch input gear  26  is locked by a clutch restriction rib  33  provided in the main body of the image forming apparatus  100 , the input-side gear tooth surface  35  of the clutch input gear  26  separates from the output-side gear tooth surface  38  of the clutch output gear  27 . With the clutch mechanism  60  thus disengaged, the driving force is not transmitted. 
     As illustrated in  FIG. 7B , the input-side gear tooth surface  35  of the clutch input gear  26  engages with the output-side gear tooth surface  38  of the clutch output gear  27  so that the clutch mechanism  60  is engaged. With the clutch mechanism  60  thus engaged, the driving force is transmitted to the pick-up roller  6  and the feed roller  7  from the connecting shaft  20  via the idler gear  31 . Thus, the engagement and the disengagement of the clutch mechanism  60  is switched by the cam surface  32 , which rotates integrally with the connecting shaft  20 , and the clutch restriction rib  33 . 
     When the connecting shaft  20  rotates in the disengaged state of the clutch mechanism  60  illustrated in  FIG. 7A , the clutch bearing  25  fixed to the connecting shaft  20  rotates, and the clutch input gear  26  also rotates via the key grooves  29  and the keys  30 . The clutch restriction rib  33  is fixed to the main body of the image forming apparatus  100 , and when the clutch input gear  26  rotates, a relative position between the clutch restriction rib  33  and the cam surface  32  is shifted. 
     When the clutch input gear  26  rotates by a predetermined amount, the cam surface  32  is released from the restriction by the clutch restriction rib  33 . An urging force of the clutch pressing spring  28  brings the input-side gear tooth surface  35  of the clutch input gear  26  into contact with the output-side gear tooth surface  38  of the clutch output gear  27 . Thus, the clutch mechanism  60  enters the engaged state illustrated in  FIG. 7B . 
     A slope surface  40  is formed in the cam surface  32 . When the clutch input gear  26  further rotates, the clutch restriction rib  33  runs onto the slope surface  40 , as illustrated in  FIG. 7C . The cam surface  32  is locked by the clutch restriction rib  33  again. Thus, the input-side gear tooth surface  35  and the output-side gear tooth surface  38  separate from each other. 
     When the connecting shaft  20  further rotates in the state illustrated in  FIG. 7C , the clutch mechanism  60  enters the disengaged state illustrated in  FIG. 7A . As described above, a movement mechanism  70  for moving the clutch input gear  26  including the clutch pressing spring  28 , the cam surface  32 , and the clutch restriction rib  33  moves the clutch input gear  26  between an engaged position where it engages with the clutch output gear  27  and a disengaged position where it disengages therefrom. More specifically, the movement mechanism  70  moves, by using the cam surface  32  and the clutch restriction rib  33 , the clutch input gear  26  to a position along an axial direction of the cam connecting shaft  20  according to a rotational angle of the clutch input gear  26 . 
     The elevating cams  19  provided in the connecting shaft  20  and the clutch input gear  26  rotate in synchronization with each other. The cam surface  32  in the movement mechanism  70  is formed so that the movement mechanism  70  moves the clutch input gear  26  to the engaged position after the stacking plate  16  is raised by the elevating cams  19  to urge the sheets stacked thereon against the pick-up roller  6 . 
     The timing of a feeding operation of the sheet feeding apparatus  10  will be described below. 
       FIG. 8  is a timing chart of the feeding operation according to the present exemplary embodiment, where a rising edge and a falling edge of a line represent the start and the end of each of operations, respectively. 
     When a sheet feed signal is input to a control unit in response to an instruction from a user, the control unit starts to drive the drive source  21 . When a predetermined timing is reached based on a count value of a timer, the above-described solenoid is sucked in based on an electric signal from the control unit so that the chipped tooth gear  24  and the second drive gear  23  engage with each other. Thus, the driving force generated by the drive source  21  is transmitted to the connecting shaft  20  via the chipped tooth gear  24 , and the connecting shaft  20  starts to rotate together with the elevating cams  19  and the clutch bearing  25 . 
     When the elevating cams  19  rotate, the elevating levers  18  rotate, and the stacking plate  16  also starts to be raised and lowered via the engaging portions  17  with the elevating levers  18 . As illustrated in  FIG. 8 , the timing of when a sheet S and the pick-up roller  6  come into pressure contact with each other deviates depending on the amount of sheets S stacked on the stacking plate  16  (hereinafter, cases where the sheet stacking amount is large and small are referred to as “fully stacked” and “slightly stacked”, respectively). 
     In the present exemplary embodiment, the clutch mechanism  60  is engaged by the cam surface  32  and the clutch restriction rib  33  after the timing of when the sheet S on the stacking plate  16  and the pick-up roller  6  come into contact with each other even when the sheets S are fully stacked. Therefore, even if the sheet S and the pick-up roller  6  come into contact with each other, the feeding of the sheet S is not immediately started. As illustrated in  FIG. 8 , the feeding of the sheet S is not started until the clutch mechanism  60  is engaged. More specifically, the timing of when the clutch mechanism  60  is engaged is in a predetermined position during one rotation of the connecting shaft  20 . Thus, the timing of when the pick-up roller  6  feeds the sheet S is constant. 
     Thus, even if the timing of when the sheet S and the pick-up roller  6  come into contact with each other differs, the timing of when the pick-up roller  6  feeds the sheet S is constant regardless of the amount of the stacked sheets S. After the connecting shaft  20  rotates once, the cam surface  32  and the clutch restriction rib  33  disengage the clutch mechanism  60  so that the clutch mechanism  60  enters the disengaged state, as illustrated in  FIG. 7A . With the clutch mechanism  60  disengaged, the pick-up roller  6  and the feed roller  7  can be driven to rotate. Thus, the registration roller pair  11  on a downstream side does not have a conveyance resistance without back tension being applied to the sheet S. A conveyance distance of the sheet S by the pick-up roller  6  and the feed roller  7  can be freely set depending on a speed reduction ratio between the gears  27  and  31  corresponding to the one rotation of the connecting shaft  20  and a speed reduction ratio based on the diameters of the rollers. Therefore, the outer diameters of the pick-up roller  6  and the feed roller  7  do not need to be increased even if a configuration according to the present exemplary embodiment is used to eliminate a variation in the sheet feed interval. 
     A feeding operation performed when feeding is continuously performed will be described below with reference to  FIGS. 9A to 9C .  FIGS. 9A to 9C  illustrate a continuous feeding operation performed when the sheets S are fully stacked on the stacking plate  16 .  FIG. 9A  illustrates a state where the preceding sheet (first sheet) S 1  is fed by the above-mentioned feeding operation, reaches the registration roller pair  11  on a downstream side, and is conveyed by the registration roller pair  11 . 
     In the first exemplary embodiment, as illustrated in  FIG. 9B , before a trailing edge of the preceding sheet S 1  passes through the feed roller  7  after passing through the pick-up roller  6 , the subsequent sheet (second sheet) S 2  to be fed subsequently to the preceding sheet S 1  and the pick-up roller  6  are brought into contact with each other. In a state illustrated in  FIG. 9B  where the subsequent sheet S 2  and the pick-up roller  6  come in contact with each other, the clutch mechanism  60  has not yet been engaged. Thus, the feeding of the subsequent sheet S 2  is not started. 
     As illustrated in  FIG. 9C , after the trailing edge of the preceding sheet S 1  conveyed by the registration roller pair  11  passes through the feed roller  7 , the clutch mechanism  60  is engaged. Thus, the pick-up roller  6  feeds the subsequent sheet S 2 . 
     The above-mentioned feeding operation of the preceding sheet S 1  and the subsequent sheet S 2  will be described below with reference to  FIG. 10 .  FIG. 11  is a block diagram of a control unit according to the first exemplary embodiment. As illustrated in  FIG. 11 , a central processing unit (CPU)  100  is connected to a drive source (motor)  21 , a registration sensor  90 , a solenoid  91 , and a size acquisition unit  92 . The CPU  100  is also connected to a read-only memory (ROM) and a random access memory (RAM) and uses the RAM as a work memory to execute a program that is stored in the ROM and corresponds to a procedure illustrated in  FIG. 10 . In the first exemplary embodiment, the CPU  100 , the ROM, and the RAM constitute the control unit. 
     First, in step S 101 , a print job is executed from an operation unit  93  in the image forming apparatus  100  or from a computer  94  connected to the image forming apparatus  100  directly or via a network. When the print job has been executed, then in step S 102 , the control unit drives (turns on) the drive source  21 . In step S 103 , the control unit sucks in (turns on) the solenoid  91  at a predetermined timing. 
     When the solenoid  91  is sucked in, the driving force from the drive source  21  is transmitted to the connecting shaft  20 . In step S 104 , the control unit starts to raise the stacking plate  16 . In step S 105 , the control unit brings the uppermost sheet S (the preceding sheet S 1 ) stacked on the stacking plate  16  into contact with the pick-up roller  6 . At this time, the clutch mechanism  60  has not been engaged. Thus, the pick-up roller  6  and the feed roller  7  have not rotated. 
     In step S 106 , when the connecting shaft  20  further rotates with the driving force from the drive source  21 , the clutch mechanism  60  becomes engaged and the pick-up roller  6  and the feed roller  7  start to rotate, allowing the preceding sheet S 1  to be fed. A profile of the elevating cams  19  is configured so that the stacking plate  16  starts to move down after a leading edge of the preceding sheet S 1 , which the pick-up roller  6  has started to feed, reaches the feed roller  7 . Thus, the uppermost sheet S stacked on the stacking plate  16  and the pick-up roller  6  separate from each other. 
     In step S 107 , the registration sensor  90  detects the preceding sheet S 1  that has been fed by the pick-up roller  6  and the feed roller  7 . The control unit determines a timing of when the solenoid  91  for starting an operation to feed the subsequent sheet S 2  is turned on based on a timing of when the registration sensor  90  has detected the preceding sheet S 1  (a detection result) and on length information of the preceding sheet S 1  in a conveyance direction (acquired by the size acquisition unit  92 ). More specifically, the control unit determines the timing of when the solenoid  91  is turned on so that a timing of when the subsequent sheet S 2  and the pick-up roller  6  come into contact with each other is a timing of when the trailing edge of the preceding sheet S 1  is downstream of the pick-up roller  6  and upstream of the feed roller  7 . It is not desirable to bring the subsequent sheet S 2  and the pick-up roller  6  into contact with each other before the trailing edge of the preceding sheet S 1  passes through the pick-up roller  6 . Since the feed roller  7  and the pick-up roller  6  are connected to the same gear train, the pick-up roller  6  may get driven to rotate by the feed roller  7  that is driven to convey the preceding sheet S 1 . 
     In step S 108 , the control unit sucks in (turns on) the solenoid  91  based on the above-mentioned timing to start the operation to feed the subsequent sheet S 2 . In step S 109 , the control unit thus starts to raise the stacking plate  16 , similarly to the above. In step S 110 , the control unit brings the uppermost sheet S (the subsequent sheet S 2 ) on the stacking plate  16  being raised and the pick-up roller  6  into contact with each other at the timing of when the trailing edge of the preceding sheet S 1  is downstream of the pick-up roller  6  and upstream of the feed roller  7 . At this time, the clutch mechanism  60  has not been engaged. Thus, the pick-up roller  6  and the feed roller  7  have not rotated. 
     In step S 111 , when the connecting shaft  20  further rotates with the driving force from the drive source  21 , the clutch mechanism  60  is engaged and the pick-up roller  6  and the feed roller  7  start to rotate, allowing the subsequent sheet S 2  to be fed. When the pick-up roller  6  and the feed roller  7  are to start to rotate, the trailing edge of the preceding sheet S 1  has passed through the feed roller  7 . The reason for this is that in the first exemplary embodiment, the separation roller  8  separates from the feed roller  7 . Even in a configuration in which the separation roller  8  does not separate from the feed roller  7 , to prevent the leading edges of the sheets S from being turned over or prevent the sheets S from being doubly fed, it is desirable to start rotating the pick-up roller  6  after the trailing edge of the preceding sheet S 1  passes through the feed roller  7 . 
     As described above, in the first exemplary embodiment, in a state where the trailing edge of the preceding sheet S 1  is downstream of the pick-up roller  6  and upstream of the feed roller  7 , the pick-up roller  6  and the subsequent sheet S 2  are brought into contact with each other. Further, in the first exemplary embodiment, after the trailing edge of the preceding sheet S 1  passes through the feed roller  7 , the pick-up roller  6  starts to rotate. In the first exemplary embodiment, a profile of the elevating cams  19 , a timing of when the clutch mechanism  60  is engaged, and an external size of the pick-up roller  6  are designed to satisfy the above two conditions. 
     In the above-mentioned exemplary embodiment of the present invention, the timing of when the subsequent sheet S 2  and the pick-up roller  6  come into contact each other may be controlled only when the number of sheets S stacked on the stacking plate  16  is large, i.e., a predetermined number or more. Conversely, if the number of sheets S stacked on the stacking plate  16  is less than the predetermined number, the subsequent sheet S 2  and the pick-up roller  6  may be brought into contact with each other at a timing of when the trailing edge of the preceding sheet S 1  is downstream of the feed roller  7 . More specifically, in the exemplary embodiment of the present invention, at least when the predetermined number or more of sheets S are stacked on the stacking plate  16 , the subsequent sheet S 2  and the pick-up roller  6  are brought into contact with each other at the timing of when the trailing edge of the preceding sheet S 1  is upstream of the feed roller  7 . 
     An interval L between the trailing edge of the preceding sheet S 1  and the leading edge of the subsequent sheet S 2  is made constant by the action of the clutch mechanism  60  regardless of the stacked amount of the sheets S by making the timing of starting the feeding operation of the subsequent sheet S 2  relative to the preceding sheet S 1  (timing of sucking in the solenoid  91  to start rotating the connecting shaft  20 ) constant. When the subsequent sheet S 2  is conveyed to the registration roller pair  11 , the clutch mechanism  60  enters the state illustrated in  FIG. 9A . The foregoing operation is repeated so that the sheets S are continuously fed. 
     As described above, according to the first exemplary embodiment, one electronic component (the solenoid  91 ) can control the timing of when the pick-up roller  6  and the uppermost sheet S on the stacking plate  16  come into contact with each other and the timing of when the pick-up roller  6  starts to rotate. Further, when the trailing edge of the preceding sheet S 1  is positioned between the pick-up roller  6  and the feed roller  7 , the pick-up roller  6  can be brought into contact with the subsequent sheet S 2 . Thus, a sheet feed interval (interval L between the trailing edge of the preceding sheet S 1  and the leading edge of the subsequent sheet S 2 ) can be kept small. Therefore, a sheet feeding apparatus, which is low in cost and high in productivity, can be provided. 
     While the solenoid  91  controls an engagement between the chipped tooth gear  24  and the second drive gear  23  in the above-mentioned first exemplary embodiment, an electromagnetic clutch may be used to control the engagement. 
     While the configuration in which the clutch input gear  26  and the clutch output gear  27  engage with each other in a tooth surface shape has been described above in the first exemplary embodiment, any configuration that can transmit a driving force may be used. For example, the clutch input gear  26  and the clutch output gear  27  may come into contact with each other using a friction member having a high sliding resistance. 
     While the configuration in which one cam surface  32  and one clutch restriction rib  33  are provided has been described above in the first exemplary embodiment, a plurality of cam surfaces  32  and a plurality of clutch restriction ribs  33  may be provided in a rotational direction of the clutch input gear  26  so that the clutch mechanism  60  is engaged and disengaged a plurality of times with the rotation of the clutch input gear  26 . Further, a plurality of cam surfaces  32  and a plurality of clutch restriction ribs  33  may be provided in a diameter direction of the clutch input gear  26  so that the clutch mechanism  60  is engaged and disengaged simultaneously by the plurality of lock portions. 
     While a configuration in which the pick-up roller  6  is fixed and the stacking plate  16  is raised and lowered to bring the sheet S stacked on the stacking plate  16  and the pick-up roller  6  into contact with each other has been described above in the first exemplary embodiment, the exemplary embodiment of the present invention is not limited to this. The exemplary embodiment of the present invention may have a configuration in which the stacking plate  16  is fixed and the pick-up roller  6  is raised and lowered. 
     Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2013-198162 filed Sep. 25, 2013, which is hereby incorporated by reference herein in its entirety.