Patent Publication Number: US-7722030-B2

Title: Sheet feeder

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2006-350680, which was filed in the Japanese Patent Office on Dec. 26, 2006, the disclosure of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     Apparatuses and devices consistent with the present invention relate to sheet feeders and, in particular, to sheet feeders for conveying sheets by using a plurality of rotating bodies. 
     BACKGROUND 
     A related art sheet feeder is provided in an image-recording device such as a printer. The related art sheet feeder includes a sheet tray and feed rollers. Sheets placed on the related art sheet tray are separated one by one by the feed rollers, and the separated sheets are fed in the conveying direction. 
     The above-described related art sheet feeder includes a device which comprises two feed rollers apart in the conveying direction of sheets, as, for example, disclosed in Patent Document 1. 
     [Patent Document 1] Japanese Published Unexamined Patent Application No. 2003-146455 
     SUMMARY 
     Moreover, in related art sheet feeders, a greater conveyance resistance (conveyance friction) is applied to a sheet on conveyance of a wide sheet than on conveyance of a narrow sheet. As a result, there are concerns in the related art that a feed roller may slip on a sheet. Thus, in an attempt to address this problem, Patent Document 1 proposes providing two feed rollers. However, this approach has a disadvantage in that in a mechanism for conveying sheets with two feed rollers, it is impossible to completely synchronize the rotating speed of the two feed rollers. This causes a problem in the case where a sheet is deflected between the two feed rollers or pulled by them. In such a case, the deflected sheet may suffer damage such as creases or cuts. 
     Exemplary embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. However, the present invention is not required to overcome the disadvantages described above, and thus, an exemplary embodiment of the present invention may not overcome any of the problems described above. Accordingly, it is an aspect of the present invention to provide a sheet feeder capable of conveying a sheet reliably without causing damage where a conveyance resistance is imparted to the sheet. 
     A sheet feeder according to first aspect of the present invention is a sheet feeder comprising: a sheet tray which retains a sheet; at least one driving source; a first conveying mechanism including a first rotating body which is in contact with a sheet on the sheet tray and a first driving-force transfer mechanism which transfers a driving force of the driving source to the first rotating body; and a second conveying mechanism including a second rotating body which is in contact with the sheet on the sheet tray and a second driving-force transfer mechanism which transfers the driving force of the driving source to the second rotating body; wherein the first rotating body is rotated in a conveying direction of the sheet by the driving force transferred by the first driving-force transfer mechanism, so that the sheet positioned on the sheet tray is conveyed in a predetermined direction at a first speed; the second rotating body is rotated by a friction force between the sheet and the second rotating body, when a conveying speed of the sheet on the sheet tray is greater than a second speed which is smaller than the first speed; and the second rotating body is rotated in the conveying direction of the sheet by the driving force transferred by the second driving-force transfer mechanism, when the conveying speed of the sheet on the sheet tray is smaller than the second speed. 
     Also, a sheet feeder of second aspect of the present invention is a sheet feeder according to first aspect, wherein the second driving-force transfer mechanism includes a one-way clutch. 
     Also, a sheet feeder of third aspect of the present invention is a sheet feeder according to second aspect, wherein a peripheral speed of the first rotating body which is driven by the driving source is greater than a peripheral speed of the second rotating body which is driven by the driving source. 
     Also, a sheet feeder of fourth aspect of the present invention is a sheet feeder according to first aspect, wherein the first conveying mechanism further comprises a first arm member on which the first rotating body is provided so that the first rotating body may rotate; the second conveying mechanism further comprises a second arm member on which the second rotating body is provided so that the second rotating body may rotate. 
     Also, a sheet feeder of fifth aspect of the present invention is a sheet feeder according to fourth aspect, wherein an angle of an extending direction of the second arm member with respect to a surface of the sheet positioned on the sheet tray is greater than an angle of an extending direction of the first arm member with respect to the surface of the sheet positioned on the sheet tray. 
     Also, a sheet feeder of sixth aspect of the present invention is a sheet feeder according to first aspect, wherein the second rotating body is disposed separate from and upstream of the first rotating body in the conveying direction of the sheet. 
     Also, a sheet feeder of seventh aspect of the present invention is a sheet feeder according to first aspect, wherein the second conveying mechanism further comprises a speed detecting mechanism which detects a conveying speed of the sheet positioned on the sheet tray. 
     Also, a sheet feeder of eighth aspect of the present invention is a sheet feeder according to seventh aspect, wherein the second conveying mechanism further comprises a solenoid which switches whether the driving force of the driving source is transferred to the second rotating body. 
     Also, a sheet feeder of ninth aspect of the present invention is a sheet feeder according to seventh aspect, wherein the speed detecting mechanism detects a rotation speed of the second rotating body. 
     Also, a sheet feeder of tenth aspect of the present invention is a sheet feeder according to ninth aspect, wherein the speed detecting mechanism detects a rotation speed of the first rotating body and compares the rotation speed of the first rotating body with the rotation speed of the second rotating body. 
     According to an exemplary embodiment of the present invention, a sheet feeder is provided wherein if the first rotating body slips on a sheet, a conveying force is imparted to the sheet from the second rotating body and where the first rotating body does not slip on a sheet, a conveying force is imparted to the sheet only by the first rotating body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects of the present invention will become more apparent and more readily appreciated from the following description of exemplary embodiments of the present invention taken in conjunction with the attached drawings, in which: 
         FIG. 1  is a perspective view showing a multi function device according to an exemplary embodiment of the present invention; 
         FIG. 2  is a longitudinal sectional view showing a printer portion of the multi function device of  FIG. 1  according to an exemplary embodiment of the present invention; 
         FIG. 3  is a partially enlarged sectional view showing a sheet feeding device of the printer portion of  FIG. 2  according to an exemplary embodiment of the present invention; 
         FIG. 4  is an enlarged pattern diagram showing a sheet feeding mechanism according to an exemplary embodiment of the present invention; 
         FIG. 5  is a plan view of the sheet feeding mechanism shown in  FIG. 4 ; 
         FIG. 6  is a block diagram showing a controller of the sheet feeding device according to an exemplary embodiment of the present invention; and 
         FIG. 7  is a flow chart showing a process for switching and controlling a driving-force transfer mechanism according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION 
     Hereinafter, a description will be given of exemplary embodiments of the present invention with reference to the drawings. It is noted that the embodiments to be described below are only an example of embodiments of the present invention and, as a matter of course, these exemplary embodiments can be modified whenever necessary, within a scope not departing from the spirit of the present invention as defined in the following claims. 
     Embodiment 1 
       FIG. 1  shows a perspective view of a multi function device according to an exemplary embodiment of the present invention. A multi function device  1  (MFD) comprises a printer portion  2  and a scanner portion  3 , and the multi function device  1  has a print-function, a scan-function, a copy-function, and a facsimile-function. However, the multi-function device  1  is not limited to these functions, and one of ordinary skill in the art will appreciate that the multi-function device  1  may have additional portions and/or may provide additional related functions. 
     The multi function device  1  has a substantially rectangular shape which is greater in width and depth than height, with an upper part of the multi function device  1  being a scanner portion  3 . The scanner portion  3  is constituted as a so-called flat bed scanner. As shown in  FIG. 1 , a document cover  5  is provided so as to open and close freely as a top plate of the multi function device  1 . A platen glass and an image sensor are provided below the document cover  5 . A document placed on the platen glass is read for images by the image sensor. 
     The lower part of the multi function device  1  is a printer portion  2 . The printer portion  2  records images and text on recording sheets on the basis of print data including image data and/or text data input externally. An opening  10  is formed on the front face of the printer portion  2 . An upper sheet feed tray  12 , a lower sheet feed tray  13  and a sheet discharging tray  14  are vertically provided in a stage-like manner inside the opening  10 . In this exemplary embodiment, the upper sheet feed tray  12 , the lower sheet feed tray  13  and the sheet discharging tray  14  are respectively assembled into an integrated sheet feed cassette. 
     Recording sheets are stacked and accommodated in the upper sheet feed tray  12  and the lower sheet feed tray  13 . The respective trays may accommodate recording sheets with different dimensions. For example, A4 size sheets may be accommodated in one of the trays and sheets smaller than A4 size such as B5 size or postcard size can be accommodated in the lower sheet feed tray  13 . As an alternative example, recording sheets with small dimensions such as postcard size or photo L size can be accommodated in the upper sheet feed tray  12 . The tray face of the lower sheet feed tray  13  may be extended by pulling out a slide tray  15 , thereby making it possible to accommodate, for example, legal-size recording sheets. Recording sheets accommodated in the upper sheet feed tray  12  and the lower sheet feed tray  13  are fed inside the printer portion  2  to record desired images, and sheets are discharged to the sheet discharging tray  14 . It is noted that the printer portion  2  and the scanner portion  3  are activated on the basis of operational instructions given from an operation panel  4  provided on an upper part of the front face of the multi function device  1  and/or instructions sent from a computer via a printer driver or a scanner driver. 
     Hereinafter, with reference to  FIG. 2  and  FIG. 3 , a description will be given of the printer portion  2  of the multi-function device.  FIG. 2  is a longitudinal sectional view showing the printer portion  2  of the multi function device  1  according to an exemplary embodiment of the present invention.  FIG. 3  is a partially-enlarged sectional view showing the sheet feeding device  6  of the printer portion  2  of  FIG. 2  according to an exemplary embodiment of the present invention. 
     The printer portion  2  comprises the sheet feeding device  6 . The sheet feeding device  6  comprises the upper sheet feed tray  12 , the lower sheet feed tray  13  and a sheet feeding mechanism  33 . As seen best in  FIG. 3 , the sheet feeding mechanism  33  comprises a first feed roller  25 , a second feed roller  26 , first and second driving-force transfer mechanisms  30 ,  31 , a first swing arm  27  for supporting the first feed roller  25  and the second feed roller  26  and a second swing arm  28   
     Returning to  FIG. 2 , the lower sheet feed tray  13  is provided at the bottom of the printer portion  2 . The upper sheet feed tray  12  is provided at the upper side thereof. In other words, the upper sheet feed tray  12  and the lower sheet feed tray  13  are a two-stage vertical structure. The first feed roller  25  and the second feed roller  26  are attached within the printer portion  2  above the upper sheet feed tray  12  and the lower sheet feed tray  13 .  FIG. 2  shows a state that the first feed roller  25  and the second feed roller  26  are in contact with the lower sheet feed tray  13 . In a state in which the upper sheet feed tray  12  and the lower sheet feed tray  13  are both installed, the upper sheet feed tray  12  is drawn out relatively with respect to the lower sheet feed tray  13 , by which the first feed roller  25  and the second feed roller  26  are brought into contact with the lower sheet feed tray  13 . By contrast, in the case in which the upper sheet feed tray  12  is pushed into the device in the depth direction relatively with respect to the lower sheet feed tray  13 , a leading edge of the upper sheet feed tray  12  flips up the first feed roller  25  and the second feed roller  26 , and the roller faces thereof are brought into contact with the upper sheet feed tray  12 . Hereinafter, a description will be given for the sheet feeding device  6  in the case in which the state of the upper sheet feed tray  12  and the lower sheet feed tray  12  are positioned as shown in  FIG. 2 . In other words, for descriptive purposes of this exemplary embodiment, a two-stage sheet feed tray structure is adopted such as that shown in  FIG. 2 . However, as a matter of course, a one-stage sheet feed tray structure may also be used in accordance with the present invention. 
     As shown in  FIG. 2  and  FIG. 3 , an inclined separation plate  22  is provided at the back of the lower sheet feed tray  13 . The inclined separation plate  22  separates recording sheets sent in a superimposed manner from the lower sheet feed tray  13 , thus guiding upward the outermost (i.e., the topmost) recording sheet of the lower sheet feed tray  13 . After extending upward from the inclined separation plate  22 , the sheet conveying path  23  turns to the front face via a curved portion  17  formed in a curved manner. Thus, the sheet conveying path  23  extends from the back side of the multi function device  1  to the front face, and leads to the sheet discharging tray  14  via the image recording unit  24 . Therefore, recording sheets accommodated in the lower sheet feed tray  13  are guided by the sheet conveying path  23  so as to make a U turn from below to above into the image recording unit  24 . After images pass through the image recording unit  24 , the recording sheet is discharged to the sheet discharging tray  14 . 
     As shown in  FIG. 3 , the roller faces of the first feed roller  25  and the second feed roller  26  are in contact with the upper face of the lower sheet feed tray  13 . When a recording sheet is present in the lower sheet feed tray  13 , the roller faces of the first feed roller  25  and the second feed roller  26  are in contact with the surface of the recording sheet. In this instance, the recording sheet in the lower sheet feed tray  13  is supplied to the sheet conveying path  23  by the first feed roller  25  and the second feed roller  26 . As shown in the drawing, the first feed roller  25  and the second feed roller  26  are connected to a driving shaft  29  by first and second swing arms  27 ,  28  respectively so as to sway freely, and a driving force input from the driving shaft  29  is transferred by the first and second driving-force transfer mechanisms  30 ,  31  to the first feed roller  25  and the second feed roller  26 , respectively. Thereby, both the first feed roller  25  and the second feed roller  26  communicate with the recording sheet, thereby reducing a transfer loss of a conveying force imparted to the recording sheet. It is noted that a more detailed description will be given later for the supporting mechanism and the first and second driving-force transfer mechanisms  30 ,  31  of the first feed roller  25 , the second feed roller  26 . 
     The sheet conveying path  23  is formed by an outer guide face and an inner guide face opposing each other at an interval at portions other than those where the image recording unit  24  and others are disposed. The interval may be predetermined. For example, a curved portion  17  of the sheet conveying path  23  at the back of the multi function device  1  is constituted with an outer guide member  18  and an inner guide member  19  fixed to a frame of the multi-function device  1 . When the recording sheet is conveyed to the curved portion  17 , the recording sheet is bent in a curved form by the curved portion  17 . Therefore, when the recording sheet passes over the curved portion  17 , the recording sheet moves in contact with the guide face of the outer guide member  18 , thereby receiving a conveyance friction (i.e., a frictional resistance force) from the guide face in a direction reverse to the conveying direction. There is a tendency that the conveyance friction is greater particularly in a case where the recording sheet to be conveyed is thick paper which is elastically strong or glossy paper the recording surface of which is processed with a coating processing. 
     As shown in  FIG. 3 , an image recording unit  24  is provided in the sheet conveying path  23 . The image recording unit  24  comprises a carriage  38 . The carriage  38  comprises an inkjet recording head  39 . Guide rails  43 ,  44  are extended in a main scanning direction (a direction perpendicular to the sheet space of  FIG. 3 ) which is orthogonal with the conveying direction of the recording sheet. The carriage  38  is supported by the guide rails  43 ,  44  so as to move reciprocally in the main scanning direction. Various colors of ink are supplied to the inkjet recording head  39  through ink tubes  41  from ink cartridges arranged inside the multi function device  1  independently of the inkjet recording head  39 . Various colors of ink are ejected selectively as minute ink droplets from the inkjet recording head  39 , while the carriage  38  moves reciprocally, by which images are recorded on the recording sheet conveyed on a platen  42 . 
     As shown in  FIG. 3 , a conveying roller  60  and a pinch roller  61  are disposed at an upstream side of the image recording unit  24 . The pinch roller  61  is disposed below the conveying roller  60  in pressure contact with the conveying roller  60 . A recording sheet which is conveyed on the sheet conveying path  23  is caught between the conveying roller  60  and the pinch roller  61  and conveyed onto the platen  42 . A discharge roller  62  and a spur roller  63  are disposed at a downstream side of the image recording unit  24 , and the discharge roller  62  and the spur roller  63  are in pressure contact. Recording sheets on which images have been recorded are caught between the discharge roller  62  and the spur roller  63  and conveyed to the sheet discharging tray  14 . The conveying roller  60  and the discharge roller  62  are driven intermittently at a linefeed width, which may be predetermined, when a driving force is transferred from a motor  71  (shown in  FIG. 5 ). 
     Hereinafter, with reference to  FIG. 4  and  FIG. 5 , a description will be given of the sheet feeding mechanism  33 , the supporting mechanism, and the first and second driving-force transfer mechanisms  30 ,  31  of the first feed roller  25  and second feed roller  26 , respectively, according to an exemplary embodiment of the present invention. In this instance,  FIG. 4  is an enlarged pattern diagram for showing the sheet feeding mechanism  33 . Further,  FIG. 5  is a plan view of the sheet feeding mechanism  33  shown in  FIG. 4 . 
     The sheet feeding mechanism  33  comprises the first feed roller  25 , the second feed roller  26 , the first driving-force transfer mechanism  30 , the second driving-force transfer mechanism  31 , the first swing arm  27  and the second swing arm  28 . The first driving-force transfer mechanism  30  further comprises a plurality of gears and the second driving-force transfer mechanism  31  also comprises a plurality of gears. The first feed roller  25  and the plurality of gears constituting the first driving-force transfer mechanism  30  are connected to the first swing arm  27 , and the second feed roller  26  and the plurality of gears constituting the second driving-force transfer mechanism  31  are connected to the second swing arm  28 . 
     A driving shaft  29  is provided above the upper sheet feed tray  12 . The driving shaft  29  runs the width of the multi-function device  1 . A main body frame of the multi function device  1  supports the driving shaft  29  so that the driving shaft  29  may rotate freely. As shown in  FIG. 5 , a transfer gear  70  is connected to one end of the driving shaft  29 . The transfer gear  70  is meshed with a transfer gear  72  directly connected to the motor  71 . Therefore, when a motor  71  is driven and rotated, a rotational driving force (i.e., a rotational torque) in a predetermined direction is transferred to the driving shaft  29  via the transfer gear  72  and the transfer gear  70 . In this exemplary embodiment, as shown in  FIG. 4 , a counter-clockwise rotational driving force is transferred to the driving shaft  29  by the motor  71 . However, the rotational driving force may be in the clockwise rotational direction, depending on the arrangement of the plurality of the gears of the first and second driving-force transfer mechanisms  30 ,  31 . 
     The first swing arm  27  is formed by two plate members  75 . The plate members  75  are made of synthetic-resin. However, any material that provides adequate structural support may be used. The two plate members  75  are disposed facing each other at an interval, which may be predetermined, and are connected together at each end by a rib  76  between both ends of the respective long plate members  75 . As shown in  FIG. 4 , one end of the first swing arm  27  is connected to the driving shaft  29  so that the other end of the first swing arm  27  may move rotationally about the driving shaft  29 . Specifically, the driving shaft  29  is inserted through a shaft hole  74  formed in the one end of the first swing arm  27 . 
     The first feed roller  25  is connected to the other end of the first swing arm  27 . Specifically, two first feed rollers  25  are connected respectively to each end of a first supporting shaft  78  connected to the other end (i.e., the leading end) of the first swing arm  27  so that the first swing arm  27  may move rotationally. For example, as shown in  FIG. 5 , in this exemplary embodiment, each of the first feed rollers  25  is connected outside of a respective plate member  75 . The roller face of the first feed roller  25  is covered with a rubber member so as to easily supply a friction force to a recording sheet when the first feed roller  25  is in communication with the recording sheet. 
     As shown in  FIG. 4 , the first driving-force transfer mechanism  30  comprises a plurality of gears, the plurality of gears including a first transfer gear  77 , first and second intermediate gears  80 ,  81 , and a second transfer gear  79 . The first transfer gear  77  is fitted on the driving shaft  29  inside the plate member  75 . The first transfer gear  77  is firmly fixed to the driving shaft  29  and thus rotates in a same direction as the driving shaft  29 . The second transfer gear  79  is fitted to the supporting shaft  78  inside the plate members  75 . The second transfer gear  79  is firmly fixed to the first supporting shaft  78  and thus rotates in a same direction as the first supporting shaft  78 . The first and second intermediate gears  80 , 81  are installed serially between the first transfer gear  77  and the second transfer gear  79 . Thereby, a rotational driving force input into the driving shaft  29  is transferred to first feed rollers  25  via the first transfer gear  77 , the intermediate gears  80 , 81  and the second transfer gear  79 . 
     The first swing arm  27  is supported by the driving shaft  29  so as to move rotationally. Therefore, the first swing arm  27  is allowed to move rotationally downward by the weight of the first swing arm  27 , the first feed roller  25  and the first driving-force transfer mechanism  30  as well as a spring force resulting from a spring and halted at a position at which the face of the first feed roller  25  is in contact with the recording sheets. As shown in  FIG. 4 , the first swing arm  27  is inclined in a direction which makes an angle θ 1  with respect to the surface of the recording sheets. In other words, a direction of the first swing arm  27  extended from the driving shaft  29  is related to the surface of the recording sheet so as to make the angle θ 1 . When a rotational driving force (rotational torque) is transferred to the first feed roller  25 , with the first swing arm  27  in this state, a friction force is generated between the roller face and the recording sheet, by which the recording sheet is conveyed so as to be picked up in the direction indicated by the arrow  68 . In other words, the above described friction force is imparted to the recording sheet, and the recording sheet is conveyed by the friction force. Thus, the friction force denotes a conveying force. 
     As shown in  FIG. 4  and  FIG. 5 , the second swing arm  28  comprises two plate members  85  which are shorter than the two plate members  75  of the first swing arm  27 . The two plate members  85  of the second swing arm  28  are arranged to face each other at an interval. The interval may be predetermined. A first end of the second swing arm  28  is supported to the driving shaft  29  so that the second swing arm  28  may move rotationally. Specifically, the driving shaft  29  is inserted through a shaft hole (not shown) formed in the first end of the second swing arm  28 . The two plate members  85  are disposed between the plate members  75  of the first swing arm  27 . 
     The second feed roller  26  is provided at a second end (i.e., the leading end) of the second swing arm  28 . Specifically, the second feed roller  26  is connected to a second supporting shaft  88  provided at the leading end of the second swing arm  28  so that the second swing arm  28  may move rotationally. In this exemplary embodiment, the second feed roller  26  is connected inside of the two plate members  85 , as shown in  FIG. 5 . A rubber member is provided on a roller face of the second feed roller  26  so as to give a friction force easily when in sliding contact with a recording sheet. 
     The second driving-force transfer mechanism  31  comprises a plurality of gears. The plurality of gears comprises a first transfer gear  87 , first and second intermediate gears  90 ,  91  and a second transfer gear  89 . The first transfer gear  87  of the second driving-force transfer mechanism  31  has a same diameter as the first transfer gear  77  of the first driving-force transfer mechanism  30 . The first transfer gear  87  of the second driving-force transfer mechanism  31  has a same number of teeth as the first transfer gear  87  of the second driving-force transfer mechanism  31 . The first transfer gear  87  is connected to the driving shaft  29  inside the two plate members  85 . The first transfer gear  87  is firmly fixed to the driving shaft  29  and rotates in a same direction as a rotation of the driving shaft  29 . The second transfer gear  89  is connected to the second supporting shaft  88  inside the two plate members  75 . The second transfer gear  89  is firmly fixed to the second supporting shaft  88  and rotates in a same direction as the rotation of the second supporting shaft  88 . The first and second intermediate gears  90 ,  91  are provided serially between the first transfer gear  87  and the second transfer gear  89 . Thereby, a rotational driving force input into the driving shaft  29  is transferred to the second feed roller  26  via the first transfer gear  87 , the first and second intermediate gears  90 ,  91  and the second transfer gear  89  of the second driving-force transfer mechanism  31 . 
     The second driving-force transfer mechanism  31  also comprises a one-way clutch  92 . As shown in  FIG. 5 , the one-way clutch  92  is installed so as to be accommodated into an inner hole of the second feed roller  26 . The one-way clutch  92  is a clutch which transfers a rotational driving force (i.e., a rotational torque) to the second feed roller  26  when the rotational driving force, which conveys recording sheets in the direction indicted by the arrow  68 , is transferred to the second supporting shaft  88  and does not transfer the rotational driving force to the second feed roller when a rotational driving force in a direction reverse to that indicated by the arrow  68  is transferred to the second supporting shaft  88 . Instead, when the rotational driving force in the direction reverse to that indicated by the arrow  68  is transferred to the second supporting shaft  88 , the one-way clutch  92  slips. The effects and actions of the one-way clutch  92  will be described in more detail later. 
     The second swing arm  28  is connected to the driving shaft  29  so that the second swing arm  28  may move rotationally. Therefore, the second swing arm  28  is allowed to move rotationally downward by the weight of the second swing arm  28 , the second feed roller  26  and the second driving-force transfer mechanism  31  as well as a spring force resulting from a spring (not shown) and halts at a position at which the face of the second feed roller  26  is in contact with the recording sheets. The second swing arm  28  is shorter in the direction extended from the driving shaft  29  than the first swing arm  27 . Therefore, as shown in  FIG. 4 , the second swing arm  28  extended from the driving shaft  29  is inclined in a direction which forms an angle θ 2  with respect to the surface of the recording sheets. The angle θ 2  is greater than the angle θ 1 , which is formed by the first swing arm  27  described above. In other words, a direction of the second swing arm  28  extended from the driving shaft  29  is related to the surface of the recording sheets so as to form the angle θ 2 . When a rotational driving force (i.e., a rotational torque) is transferred to the second feed roller  26 , with this state kept, a friction force is generated between the roller face and the recording sheets, and the friction force is imparted to the recording sheets as a conveying force to convey the recording sheet in the direction indicated by the arrow  68 . In this instance, since the angle θ 2  is greater than the angle θ 1 , a conveying force imparted from the second feed roller  26  to the recording sheets is greater than a conveying force imparted from the first feed roller  25  to the recording sheets. 
     In this exemplary embodiment, the transfer gears of the respective first and second driving-force transfer mechanisms  30 ,  31  are appropriately designed in diameter and number of teeth, or the first feed roller  25  and the second feed roller  26  are appropriately designed in outer peripheral length, in order to produce a peripheral speed V 1  of the first feed roller  25  that is greater than a peripheral speed V 2  of the second feed roller  26 . For example, where each of the transfer gears comprising the first and second driving-force transfer mechanisms  30  and  31  is the same in structure, an outer diameter of the first feed roller  25  is made greater than an outer diameter of the second feed roller  26 , thus making it possible to set the peripheral speed V 1  greater than peripheral speed V 2 . Alternatively, where the outer diameter of the first feed roller  25  is the same as the outer diameter of the second feed roller  26 , the diameter and number of gears of the transfer gears of the first driving-force transfer mechanism  30  may be increased relative to the diameter and number of gears of the second driving-force transfer mechanism  31  in order to set the peripheral speed V 1  greater than the peripheral speed V 2 . 
     Since the sheet feeding mechanism  33  is constituted as described above, a rotational driving force is transferred from the driving shaft  29  via the first driving-force transfer mechanism  30  to the first feed roller  25 . Thereby, the first feed roller  25  is rotated at a peripheral speed V 1 . Further, a rotational driving force which rotates the second feed roller  26  at the peripheral speed V 2  slower than the peripheral speed V 1  is transferred to the second supporting shaft  88  via the second driving-force transfer mechanism  31 . In this instance, a recording sheet is conveyed in the conveying direction indicated by the arrow  68  (refer to  FIG. 4 ) by a conveying force imparted from the first feed roller  25 . On the other hand, a rotational driving force which rotates the second feed roller  26  at the peripheral speed V 2  is transferred also to the second supporting shaft  88  from the second driving-force transfer mechanism  31 . However, a rotational force imparted to the second feed roller  26  from the recording sheet conveyed by the first feed roller  25  is greater than the rotational driving force transferred to the second supporting shaft  88 , thereby a rotational force which rotates the second feed roller  26  at the peripheral speed V 1  the same as the first feed roller  25  is imparted from the recording sheet to the roller face of the second feed roller  26 . Specifically, the second feed roller  26  is rotated not at the peripheral speed V 2  but at the peripheral speed V 1  upon receipt of the rotational force from the recording sheet. In this instance, the second supporting shaft  88  is to be rotated reversely and relatively with respect to the second feed roller  26 . In other words, by appearance, a rotational driving force in a direction reverse to the direction conveying the recording sheet (the direction given by the arrow  68 ) is imparted to the second supporting shaft  88  of the second feed roller  26 . In this instance, the one-way clutch  92  slips and the rotational driving force transferred to the second supporting shaft  88  is not transferred to the second feed roller  26 . As described above, where a recording sheet is conveyed by the first feed roller  25  (i.e., where the first feed roller  25  does not slip), the second feed roller  26  which is to be rotated at the peripheral speed V 2  is in contact with the recording sheet conveyed by the first feed roller  25  and rotated together at the peripheral speed V 1  which is the same as the first feed roller  25 . That is, when a sheet on the sheet tray is conveyed by the first feed roller  25  at a speed which is greater than the peripheral speed V 2 , the second feed roller  26  is rotated by a friction force between the sheet and the second feed roller  26 . Therefore, there is no chance that a load in a direction reverse to the conveying direction is applied to the recording sheet from the second feed roller  26 . As a result, the recording sheet is free from creases or cuts. 
     Moreover, upon conveyance of a recording sheet which may be wide or thick, there is a case where a conveyance resistance (i.e., a conveyance friction) applied to the recording sheet at the curved portion  17  is greater than a conveying force of the first feed roller  25 . In this instance, the conveying force of the first feed roller  25  is insufficient and the first feed roller  25  slips on the surface of the recording sheet. In this instance, a conveying speed of the recording sheet decreases from V 1  to less than V 2 . Then, the second supporting shaft  88  of the second feed roller  26  is rotated in a direction which rotates the second feed roller  26  in the conveying direction indicated by the arrow  68  (refer to  FIG. 4 ) together with the second feed roller  26 . In this instance, a rotational driving force transferred by the one-way clutch  92  to the second supporting shaft  88  is transferred to the second feed roller  26 . Thereby, the second feed roller  26  which is rotated together with the recording sheet is positively rotated at the peripheral speed V 2  by the rotational driving force transferred from the shaft  88 , and the conveying speed of the recording sheet maintained at least substantially V 2 . As described above, the conveying force imparted by the second feed roller  26  to the recording sheet is greater than the conveying force of the first feed roller  25 , thereby the recording sheet may not stopped by the conveyance resistance but can be conveyed without fail. Therefore, since the multi function device  1  is provided with the sheet feeding mechanism  33 , where a recording sheet such as thick paper and gloss paper is conveyed, it is possible to constantly convey the recording sheet without fail, irrespective of the conveyance resistance applied to the recording sheet. To reduce the change of the conveying speed of the recording sheet, the speed V 2  may be slightly (eg. between 1 percent and 5 percent) smaller than V 1 . 
     Embodiment 2 
     Hereinafter, a description will be given for another exemplary embodiment of the present invention with reference to  FIG. 6  and  FIG. 7 .  FIG. 6  is a block diagram showing a controller  100  of a sheet feeding device according to an exemplary embodiment of the present invention.  FIG. 7  is a flow chart showing a processing procedure for switching and controlling a driving-force transfer mechanism according to an exemplary embodiment of the present invention. In this exemplary embodiment of the present invention, a controller  100  of the sheet feeding device  6  is independent of a main controller for controlling comprehensively a whole part of the multi function device  1 . However, the controller  100  may alternatively be incorporated within the main controller. Further, in the description that follows, parts of this exemplary embodiment which are in common with the exemplary embodiment described above will be given the same reference numbers and a detailed description thereof will be omitted. 
     As shown in  FIG. 6 , a controller  100  comprehensively controls an entire motion of the multi function device  1  including not only a printer portion  3  but also a scanner portion  2  (as shown, for example, in  FIG. 1 . As shown in  FIG. 6 , the controller  100  is comprises a micro computer comprising a CPU  101 , a ROM  102 , a RAM  103  and an EEPROM  104 , and the controller  100  is coupled to an Application Specific Integrated Circuit (ASIC)  106  via a bus  105 . 
     The ROM  102  accommodates programs and other information for controlling various motions of the multi function device  1 . The RAM  103  is used as a storage area and a work area which temporarily stores various data used for execution of the programs by the CPU  101 . Further, the EEPROM  104  accommodates various settings and flags to be retained after a power source is turned off. 
     The ASIC  106  generates a phase excitation signal for energizing a motor  71  according to a command from the CPU  101 , and provides the signal to a driving circuit  110  of the motor  71 , energizing a driving signal via the driving circuit  110  to the motor  71 , thereby performing the rotation control of the motor  71 . 
     The driving circuit  110  drives the motor  71  which is connected to the first feed roller  25  and the second feed roller  26 . The drive circuit  110  generates an electric signal for rotating the motor  71  upon receipt of an output signal from the ASIC  106 . The motor  71  rotates upon receipt of the electric signal, and a rotational driving force of the motor  71  is transferred to the first feed roller  25  and the second feed roller  26  via the driving shaft  29 , the first driving-force transfer mechanism  30  and a driving-force transfer mechanism  121 . The motor  71  is also connected to a conveying roller  60  and a discharge roller  62  (refer to  FIG. 3 ) via a driving mechanism made up of gears, driving shafts and others. Therefore, the rotational driving force of the motor  71  is transferred to the conveying roller  60  and the discharge roller  62 . In this exemplary embodiment, the driving-force transfer mechanism  121  replaces the driving-force transfer mechanism  31  of the above-described exemplary embodiment and transfers a rotational driving force of the driving shaft  29  to the second feed roller  26 . The driving-force transfer mechanism  121  is similar to the driving-force transfer mechanism  31  except that a solenoid  113  is used in place of the one-way clutch  92 . 
     The ASIC  106  is coupled to a first rotary encoder  115  and a second rotary encoder  116 . The first rotary encoder  115  detects a rotational quantity of the first feed roller  25  and the second rotary encoder  116  detects a rotational quantity of the second feed roller  26 . The CPU  101  calculates a rotating speed V 1 , of the first feed roller  25  and a rotating speed V 2 , of the second feed roller  26  on the basis of the rotational quantity of each of the first and second rotary encoders  115 ,  116 , respectively. 
     The ASIC  106  is coupled to the solenoid  113 . The CPU  101  controls the ASIC  106  to output an output signal at a timing on the basis of control programs accommodated in the ROM  102 , thereby activating the solenoid  113 . The timing may be predetermined. The solenoid  113  is assembled into the driving-force transfer mechanism  121 . The solenoid  113  connects or separates an intermediate gear  90  and a transfer gear  89  connected to the second feed roller  26  or detaches the intermediate gear  90  therefrom, and a solenoid shaft is connected via a known link mechanism either to the transfer gear  89  or the intermediate gear  90 . When a signal is input into the solenoid  113 , the solenoid  113  moves in a direction at which the transfer gear  89  is meshed with the intermediate gear  90 . Thereby, a rotational driving force is transferred to the second feed roller  26 . Further, in a state that no signal is output to the solenoid  113 , the transfer gear  79  is disengaged from the intermediate gear  90 . Therefore, in this state, no rotational driving force is transferred to the second feed roller  26 . 
     Turning now to  FIG. 7 , a description will be given of a processing procedure for switching and controlling a driving force transmission. This processing procedure may be executed after a print command is input and the motor  71  is driven and rotated. 
     In operation S 1 , a rotating speed V 1  of the first feed roller  25  and a rotating speed V 2  of the second feed roller  26  are detected. These rotating speeds V 1  and V 2  are detected on the basis of rotational quantities of the rotary encoders  115  and  116 . 
     In operation S 2 , it is determined whether the rotating speed V 1  is greater than the rotating speed V 2 . Where the first feed roller  25  does not slip, the second feed roller  26  is also rotated together with the recording sheet at a speed V 1  which is the same as the speed of the first feed roller  25 . Therefore, the rotating speed V 1  will not exceed the rotating speed V 2 . On the contrary, where the first feed roller  25  slips, recording sheet is conveyed at a speed slower than the rotating speed V 1 . Alternatively, the recording sheet is not conveyed but stopped, by which the second feed roller  26  is decreased in rotating speed or the recording sheet is stopped. On the other hand, the first feed roller  25  is rotated at the rotating speed V 1  in the midst of slippage. Therefore, in this instance, the rotating speed V 1  is greater than the rotating speed V 2 . In other words, in operation S 2 , the rotating speed V 1  is compared with the rotating speed V 2 , thereby making it possible to determine whether the first feed roller  25  slips. 
     In operation S 2 , if it is determined that the rotating speed V 1  is greater than the rotating speed V 2 , the CPU  101  of the controller  100  turns the solenoid  113  on (operation S 3 ). Thereby, the transfer gear  89  is meshed with the intermediate gear  90 , and a rotational driving force of the motor  71  is transferred to the second feed roller  26  via the second driving-force transfer mechanism  121 . As described above, even if the first feed roller  25  slips, a rotational driving force is instantly transferred to the second feed roller  26 , thereby the recording sheet is always conveyed smoothly and stably. It is noted that where the rotating speed V 1  is judged to be less than or equal to the rotating speed V 2  in operation S 2 , the solenoid  113  is kept off, and the recording sheet is continuously conveyed only by the first feed roller  25  (operation S 4 ). 
     In operation S 5 , it is determined whether the leading end of the recording sheet has arrived at a threshold position. The threshold position may be predetermined. This determination is made based on detection results of sheet sensors including an optical sensor installed in the sheet conveying path  23 . In this instance, the threshold position is, for example, such a position that the recording sheet can be conveyed from the first feed roller  25  by a conveying roller  60  installed downstream in the conveying direction. When the leading end of the recording sheet has arrived at this threshold position, the recording sheet is conveyed by the conveying roller  60 , and no switching of the transfer of a driving force by the solenoid  113  is performed. Therefore, if it is determined that the leading end of the recording sheet has arrived at the threshold position in operation S 5 , the process is complete and processing ends. If it is determined that the leading end of the recording sheet has not arrived at the threshold position (“No” in operation S 5 ), processing returns to operation S 2 . 
     It is noted that in this exemplary embodiment, the rotating speed V 1  is compared with the rotating speed V 2 , and it is determined whether the first feed roller  25  slips. However, whether the first feed roller  25  is slipping may also be determined based on whether the rotating speed V 2 , which is calculated on the basis of a detection signal of the rotary encoder  116 , is lower than a threshold 
     (1) The embodiment is a sheet feeder for conveying sheets in the conveying direction. The sheet feeder is provided with a sheet tray, a first rotating body, a second rotating body and a conveying-force transfer means. The first rotating body and the second rotating body both impart a conveying force to sheets retained on the sheet tray. The conveying-force transfer means imparts a conveying force from the second rotating body to a sheet where the first rotating body slips on the sheet but does not impart a conveying force from the second rotating body to the sheet where the first rotating body does not slip on the sheet. 
     (2) The conveying-force transfer means is provided with a one-way clutch which transfers a rotational driving force in the conveying direction to the second rotating body and does not transfer a rotational driving force in a direction reverse to the conveying direction to the second rotating body but slips on a sheet. 
     Thereby, it is possible to mechanically constitute a mechanism which imparts a conveying force from the second rotating body to a sheet upon slippage of the first rotating body. 
     (3) A peripheral speed on driving the first rotating body is set to be greater than a peripheral speed on driving the second rotating body. 
     Thereby, where the first rotating body does not slip on a sheet, no rotational driving force is transferred to the second rotating body by the one-way clutch. Therefore, no deflection or pulling of the sheet between the first rotating body and the second rotating body is caused. 
     (4) The sheet feeder of the present invention is provided with a first arm member for supporting the first rotating body so as to rotate and additionally provided with a second arm member for supporting the second rotating body so as to rotate. 
     Thereby, it is possible to realize favorably the support of the first rotating body and the second rotating body. 
     (5) In the sheet feeder, a conveying force imparted from the second rotating body to a sheet is greater than a conveying force imparted from the first rotating body to a sheet. 
     Thereby, the sheet feeder is able to convey even a sheet, which cannot be conveyed by the first rotating body alone, by the second rotating body alone without a conveying force of the first rotating body. For example, where the first rotating body slips completely on a sheet, the second rotating body is able to convey the sheet reliably. 
     (6) A second angle of the second arm member in an extended direction with respect to the surface of a sheet retained on the sheet tray is greater than a first angle of the first arm member in an extended direction with respect to the surface of a sheet retained on the sheet tray. 
     Thereby, it is possible to concretely realize a mechanism in which a conveying force imparted to a sheet from the second rotating body is set to be greater than a conveying force imparted to a sheet from the first rotating body. 
     (7) It is preferable in realizing the present invention that the second rotating body is disposed apart upstream of the first rotating body in the conveying direction. 
     (8) The sheet feeder is additionally provided with a curved sheet conveying path through which sheets conveyed from the sheet tray are allowed to pass. 
     Where a sheet is conveyed by a device provided with the above-described sheet conveying path, a relatively great conveyance resistance is applied to the sheet. Therefore, the present invention is favorably applicable to such a sheet feeder.