Patent Publication Number: US-7899385-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Japanese Patent Application No. 2005-370246 filed Dec. 22, 2005 in the Japan Patent Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     This invention relates to an image forming apparatus that forms an image on a sheet-like recording medium. 
     A conventional image forming apparatus (such as a printer) includes a medium storage unit (i.e. a sheet feed cassette or a sheet feed tray) that stores a recording medium such as a sheet and the like. This apparatus is designed to convey the recording medium stored in the medium storage unit to an image forming position (a position where an image is formed on the recording medium). Specifically, the conventional image forming apparatus includes a supply roller and a conveyance roller. The supply roller supplies the recording medium stored in the medium storage unit into a conveyance path. The conveyance roller conveys the recording medium after performing skew correction by temporarily restricting passage of the recoding medium conveyed by the supply roller in the conveyance path. 
     There is also a known printer as another type of image forming apparatus. The printer comprises a common motor to drive both a supply roller and a conveyance roller in order to achieve downsizing of the apparatus and cost reduction. In the printer, rotary driving force is transmitted such that while the supply roller is rotated in a forward direction (in a direction where a recording medium is conveyed toward an image forming position), the conveyance roller is rotated in a reverse direction. This configuration allows skew correction by the conveyance roller. 
     Specifically, the printer includes a first roller and a second roller. The first roller supplies a sheet stored in a sheet feed cassette to a conveyance path, and the second roller passes and further conveys the sheet conveyed by the first roller. 
     Also, the printer includes a feed roller, as a conveyance roller, that passes and further conveys the sheet conveyed by the second roller after performing skew correction. The feed roller is rotated in the reverse direction while both the first roller and the second roller are rotated in the forward direction to convey the sheet. When a leading end of the sheet conveyed by the second roller is pressed by the feed roller, a rotating direction of the motor is reversed. Thereby, the sheet conveyed by the second roller is conveyed after the feed roller performs skew correction. 
     When the sheet is conveyed to the second roller, a rotating shaft of the first roller is moved upward separating away from a surface of a sheet stored in the sheet feed cassette. Moreover, the second roller is configured so as to be stopped rotating and remain in a free state when the rotating direction of the motor is reversed. Accordingly, the first roller and the second roller do not interrupt conveyance of the sheet by the feed roller even when the rotating direction of the motor is reversed. 
     SUMMARY 
     In the above-described printer, however, each of the first roller and the second roller does not operate at least when a sheet is being conveyed by the feed roller, and thus a subsequent sheet cannot be supplied from the sheet feed cassette to the conveyance path. Accordingly, image printing on a plurality of sheets by the printer requires a long time. The printer, therefore, cannot satisfy a need to perform a large amount of printing in a short time. 
     As a solution to this problem, both the supply roller and the conveyance roller may be configured to be rotated in the forward direction thereby to allow the supply roller to supply a subsequent recording medium even when a preceding recording medium is being conveyed by the conveyance roller. Then, consecutive conveyance of a plurality of recording mediums may be achieved. In this case, the supply roller need not be separated or brought into a free state from the recording medium. 
     In the above-described consecutive conveyance, however, once the preceding recording medium being conveyed is skewed, the subsequent recording medium is likely to be affected. When a rear end of the recording medium being conveyed by the conveyance roller leaves the supply roller, the supply roller abuts the subsequent recording medium, and the subsequent recording medium is supplied to the conveying path. When the preceding recording medium being conveyed is skewed, a timing when the subsequent recording medium leaves the supply roller (in other words, a timing when the supply roller abuts the subsequent recording medium) is shifted in the rotating shaft direction. As a result, the subsequent recording medium is likely to be conveyed in a skewed state. 
     One aspect of the present invention may provide an image forming apparatus capable of inhibiting continuous occurrence of skew of a recording medium due to a consecutive conveyance of the recording medium by rotating both a supply roller and a conveyance roller in a forward direction. 
     In the one aspect of the present invention, there is provided an image forming apparatus including a medium storage portion, a supply roller, a conveyance roller, a drive device and a transmission device. 
     The medium storage portion is capable of storing a plurality of recording mediums in a stacked manner. The supply roller is disposed so as to abut an uppermost recording medium of the plurality of recording mediums stored in the medium storage portion. The supply roller is rotary driven in a forward direction as a rotating direction to convey the recording mediums to a position for image formation, thereby to supply the abutting uppermost recording medium to a conveyance path. The conveyance roller is disposed in the conveyance path. The conveyance roller is rotary driven in a forward direction as a rotating direction to convey the recording mediums to a position for image formation, thereby to allow passage of the uppermost recording medium conveyed by the supply roller. The drive device generates a rotary driving force. The transmission device transmits the rotary driving force generated by the drive device to the supply roller and the conveyance roller, thereby to cause both of the supply roller and the conveyance roller to rotate in the forward direction. 
     In the image forming apparatus, a conveyance speed of the recording medium by the conveyance roller is adapted to be faster than a conveyance speed of the recording medium by the supply roller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described below, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic perspective view showing an appearance of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic sectional side view of components provided in a body casing (excluding part of the components located in an upper area (e.g., a scanner unit)); 
         FIG. 3  is a diagrammatic perspective view of a supply tray and a supply unit without a second tray being disposed; 
         FIG. 4  is a diagrammatic perspective view of the supply tray and the supply unit with the second tray being disposed; 
         FIG. 5  is a plan view of the supply tray and the supply unit without the second tray being disposed; 
         FIGS. 6A and 6B  are cross-sectional views taken along line VI-VI of  FIG. 5 ; 
         FIGS. 7A and 7B  are cross-sectional views taken along line VII-VII of  FIG. 5 ; 
         FIG. 8  is a cross-sectional view along line VIII-VIII of  FIG. 5 ; 
         FIG. 9  is a plan view of the supply tray, the supply unit and a frame without the second tray being disposed; 
         FIGS. 10A ,  10 B and  10 C are cross-sectional views taken along line X-X of  FIG. 9 ; 
         FIG. 11  is a diagrammatic perspective view in a state of  FIG. 10C  seen from below the frame; 
         FIG. 12  is a diagrammatic perspective view of an image recording unit; 
         FIG. 13  is a schematic cross-sectional side view of the image recording unit; 
         FIG. 14  is a diagrammatic side view of the image recording unit; 
         FIG. 15  is a schematic plan view of a power transmission switching mechanism; 
         FIG. 16A  is a schematic plan view of a guide block in the power transmission switching mechanism; 
         FIG. 16B  is schematic front view of the power transmission switching mechanism; 
         FIGS. 17A and 17B  are schematic views explaining a rotary driving force transmission path in an intermittent supply mode; 
         FIGS. 18A ,  18 B and  18 C are schematic views explaining a rotary driving force transmission path in a consecutive supply mode; 
         FIGS. 19A and 19B  are schematic views explaining a rotary driving force transmission path in a subsequent medium processing process; 
         FIG. 20  is a block diagram showing a schematic configuration of a control system of the image forming apparatus; 
         FIG. 21  is a flowchart of an image recording process; and 
         FIG. 22  is a flowchart of the subsequent medium processing process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     1. Description of Structure 
     An image forming apparatus  1  of a present embodiment is a so-called multifunction apparatus including a printer function, a scanner function, a color copying function, a facsimile function. As shown in  FIG. 1 , the image forming apparatus  1  includes a body casing  2 , which is made of resin and has a substantially rectangular box-shaped configuration. 
     An operation panel  10  is disposed in a front upper portion of the body casing  2 . The operation panel  10  includes an operation portion  11  and a display portion (e.g., a liquid crystal display)  12 . The operation portion  11  includes various operation buttons for input operation. The display portion  12  displays an image of a message and the like. Moreover, a scanner unit  20  that reads an image from a medium is disposed at a rear position of the operation panel  10 . The scanner unit  20  is used for the scanner function, the color copying function and the facsimile function. 
     At the bottom part inside the body casing  2 , as shown in  FIG. 2 , a supply tray  30  is disposed. A plurality of sheets of a sheet-like recording medium, including, but not limited to paper sheets, plastic sheets and the like, can be stored substantially horizontally in the supply tray  30  in a stacked manner (accumulated manner). The supply tray  30  can be removed from the body casing  2  by horizontally pulling the supply tray  30  in a forward direction through an opening  2   a , which is formed at a front face of the body casing  2  (see  FIG. 1 ). In contrast, the supply tray  30  can be attached to the body casing  2  by horizontally inserting the supply tray  30  through the opening  2   a  of the body casing  2 . 
     A metallic box-like frame  4  (see  FIGS. 9 and 11 ) is disposed in a rear portion inside the body casing  2  and above the supply tray  30 . The metallic box-like frame  4  has an elongated shape in a right and left direction. 
     A supply unit  50  is supported by the frame  4  so as to be located above a rear end of the supply tray  30 . The supply unit  50  includes a supply roller  60  that supplies (conveys) the recording medium stored in the supply tray  30  sheet by sheet to a conveyance path  5  provided at a rear end inside the body casing  2 . The conveyance path  5  is formed such that the recording medium conveyed rearward from the supply tray  30  is turned upward and then is guided forward. 
     An image recording unit  70  that records (prints) an image on the recording medium, which is guided and conveyed by the conveyance path  5 , is disposed above the supply unit  50 . The recording medium after image recording thereon at the image recording unit  70  is discharged on a front portion of a top surface of the supply tray  30 . 
     A detailed explanation of each component in the image forming apparatus  1  will now be provided. 
     [1-1. Structure of Supply Tray] 
     As shown in  FIGS. 3 and 5 , the supply tray  30  can be a A4-sized, rectangular (when seen from above) thin dish-like component made of resin and is capable of storing the plurality of sheets of the recording medium in a stacked manner. The supply tray  30  is provided with a pair of side end guides  31 ,  32  in both sides. The side end guides  31 ,  32  allow the recording medium to be positioned such that a center line of the recording medium in the left and right direction (width direction) is at a fixed position, regardless of a size of the recording medium to be stored. 
     The side end guides  31 ,  32  include carrying plates  31   a ,  32   a  and side plates  31   b ,  32   b , respectively. The recording medium is placed on upper surfaces of the carrying plates  31   a ,  32   a . The side plates  31   b ,  32   b  are erected vertically upward from outside ends of the respective carrying plates  31   a ,  32   a  in the left to right direction. 
     A linear guide bar  31   c  extends from a bottom surface of the carrying plates  31   a  toward the side end guide  32 , while a linear guide bar  32   c  extends from a bottom surface of the carrying plates  32   a  toward the side end guide  31 . The linear guide bars  31   c ,  32   c  are disposed in parallel with and separate from each other by a predetermined distance in a front and back direction. The linear guide bars  31   c ,  32   c  are fitted in grooves  33   a ,  33   b , which are disposed in a base plate  33  of the supply tray  30  along the left and right direction. The side end guides  31 ,  32  are displaceable in the left and right direction by sliding the linear guide bars  31   c ,  32   c  along the grooves  33   a ,  33   b.    
     Rack gears are formed on opposite sides of the linear guide bars  31   c ,  32   c . Each of the rack gears engages with a pinion gear rotatably disposed in a center of the base plate  33  in the width direction of the plate  33 . 
     Both of the side end guides  31 ,  32  are connected to each other via the rack gears and the pinion gear and operated simultaneously so as to maintain a distance from each of the side plates  31   b ,  32   b  to the center line in the left and right direction of the supply tray  30  (so as to be symmetrical to each other). Consequently, the recording medium can be positioned such that the center line of the recording medium in the left and right direction is at a fixed position. 
     Portions of the side plates  31   b ,  32   b  to abut ends of the recording medium in the left and right direction have flat surfaces substantially parallel along the front and back direction (the direction to convey the recording medium). Thus, the recording medium positioned by the side end guides  31 ,  32  and stored in the supply tray  30  is prevented from being transported in the left and right direction (in a direction of a rotation axis of the supply roller  60 ). This can substantially reduce or prevent the recording medium from being skewed and allow the recording medium to be conveyed in a predetermined direction. 
     The supply tray  30  also includes a guide plate  34  in a rear end portion thereof and a metal separation member  34   a  disposed at a center of the guide plate  34  in the left and right direction. The separation member  34   a  has a plurality of tooth members arranged with a constant distance in an upper and lower direction, and a leading end of each tooth member is slightly protruded from a front surface of the guide plate  34 . Consequently, the plurality sheets of the recording medium pushed out by the supply roller  60  of the supply unit  50  abut the leading ends of the tooth members, and an uppermost sheet of the recording medium is separated from the other sheets of the recording medium. 
     Moreover, as shown in  FIG. 4 , the supply tray  30  is configured to allow attachment and removal of a second tray  40  on the supply tray  30 . 
     The second tray  40  is capable of storing a thick and small-sized recording medium such as a postcard or an envelope in a central portion in the left and right direction. The second tray  40  can be a rectangular, thin dish-like component, which is made of resin, and has substantially a same dimension in the left and right direction and a slightly shorter dimension in the front and back direction as compared with the supply tray  30 . The second tray  40  is capable of storing the plurality of sheets of the recording medium in a substantially horizontally stacked manner. 
     Moreover, the second tray  40  is provided with a pair of side end guides  41 ,  42  that allow the recording medium to be positioned such that a center line of the recording medium in the left and right direction (width direction) is at a fixed position, regardless of a size of the recording medium to be stored, in a same manner as in the supply tray  30 . 
     When the second tray  40  is placed at a predetermined position in a rear portion above the supply tray  30  (described in  FIG. 4 ), the recording medium stored in the second tray  40  is disposed at a position so as to prevent the supply roller  60  from moving toward the supply tray  30  (downward). Therefore, the supply roller  60  of the supply unit  50  abuts not on the recording medium stored in the supply tray  30 , but on the recording medium stored in the second tray  40 , and thereby the recording medium stored in the second tray  40  can be moved to the conveyance path  5 . 
     [1-2. Structure of Supply Unit] 
     A supply unit  50 , as shown in  FIGS. 3 to 5 ,  9  and  11 , includes a support shaft  51  supported by the frame  4 . The support shaft  51  is arranged along the left and right direction, over a range from a central portion to a right-side end of the supply tray  30  in the left and right direction. A big gear  53  is secured to a right-side end of the support shaft  51  whereas a small gear  54 , which has substantially a same diameter as the support shaft  61 , is secured near a left-side end of the support shaft  51 . 
     Moreover, the supply unit  50  includes an arm member  52  supported by the support shaft  51 . The arm member  52  is pivotable around the support shaft  51  with a distal end (a pivoting end) of the arm member  52  located in a rear lower position. A supply roller  60  is supported by the distal end of the arm member  52  so as to be rotated around a rotating shaft along a left and right direction. In other words, the arm member  52  is disposed so as to be pivoted around a pivot shaft (the support shaft  51 ), which is parallel to the rotating shaft of the supply roller  60  and is positioned above the recording medium stored in the supply tray  30  and on an opposite side of (upstream from) the rotating shaft of the supply roller  60  in the conveyance direction of the recording medium by the supply roller  60 . 
     As shown in  FIGS. 7A and 7B , the supply roller  60  is provided with a main body member  61 , which can be made of resin, and two roller members  62 ,  62 , which can be made of rubber, secured in the both side ends of the main body member  61 . The main body member  61  includes cylinder roller support members  63 ,  64 , in both left and right side ends of the main body member  61  in the rotating shaft direction. The roller members  62 ,  62  are secured to circumferences of the roller support members  63 ,  64 , respectively. The main body member  61  also includes a rod-like shaft  65 , which couples the cylinder roller support members  63 ,  64 , in a central portion of the main body member  61  in the rotating shaft direction. The rod-like shaft  65  includes a gear abutment portion  65   a , which is formed in a center of the shaft  65  ill the rotating shaft direction, and arm abutment portions  65   b ,  65   b , which are formed at both sides of the gear abutment portion  65   a  in the rotating shaft direction. The shaft  65  has a cruciform cross section except in the gear abutment portion  65   a  and the arm abutment portions  65   b ,  65   b  (see  FIG. 8 ). As shown in  FIGS. 6A and 6B , a cross section in the gear abutment portion  65   a  is configured to include a circle  65   a   1  sized to include the cruciform cross section and a pair of protrusions  65   a   2 ,  65   a   2  positioned opposite to each other on a circumference of the circle  65   a   1 . A cross section in each of the arm abutment portions  65   b ,  65   b  is configured to include a circle sized to include the cruciform cross section. 
     In the supply roller  60 , the shaft  65  of the main body member  61  is axially supported at the distal end of the arm member  52 . At the distal end of the arm member  52 , two axial support members  55 ,  55  are disposed so as to sandwich a driving gear  66  which transmits a rotary driving force to the supply roller  60 . 
     As shown in  FIGS. 7A and 7B , each of the axial support members  55  includes a through-hole  55   a  having a circular cross section formed along the left and right direction. The supply roller  60  is rotatably supported with the shaft  65  of the main body member  61  inserted through the through-hole  55   a  of each axial support member  55 . Each arm abutment portion  65   b  of the shaft  65  is positioned opposedly to a center side end of the each axial support member  55  of the arm member  52  in the left and right direction of the through-hole  55   a.    
     In other words, a clearance between the shaft  65  and the through-hole  55   a  is narrowest in a center side portion of a rotating shaft direction of the shaft  65 , and the supply roller  60  is axially supported by the distal end of the arm member  52  in the center side portion of the rotating shaft direction. 
     The above described configuration can reduce a degree of positional freedom of the shaft  65  in the center portion (i.e., jolt of the driving gear  66 ) in the left and right direction of the shaft  65  to effectively transmit the rotary driving force from an LF motor  6  to the shaft  65 , while allowing a large degree of positional freedom of the shaft  65  in both side-ends (the degree of freedom in an angle of the rotating shaft) in the left and right direction of the shaft  65 . Thus, the arm member  52  supports the supply roller  60  with a certain degree of freedom in the angle of the rotating shaft (the angle between a standard state of the shaft  65  as shown in  FIG. 7A  and a maximum tilting state of the shaft  65  as shown in  FIG. 7B  such as 3 degrees). 
     In the supply roller  60 , the shaft  65  of the main body member  61  is also inserted through the through-hole  66   a  formed in the driving gear  66 . The through-hole  66   a , as shown in  FIGS. 6A and 6B , has a cross-sectional configuration including a circle  66   a   1  corresponding to a size of the circle  65   a   1  of the gear abutment member  65   a  of the shaft  65  and a pair of sector cutouts  66   a   2  formed opposedly each other on the circumference of the circle  66   a   1 . A circumferential length of each of the sector cutouts  66   a   2  of the through-hole  66   a  is larger than a circumferential length of each of the protrusions  65   a   2  of the gear abutment member  65   a . Thus, the supply roller  60  has a certain amount (e.g., 60 degrees) of play in the rotating direction against the driving gear  66 . 
     In the arm member  52 , as shown in  FIGS. 6A ,  6 B and  8 , four power transmission gears  56 ,  56 ,  56 ,  56  are aligned along an extending direction of the arm member  52 . The power transmission gears  56 ,  56 ,  56 ,  56  connect the small gear  54  secured to the support shaft  51  and the driving gear  66 , through which the shaft  65  of the main body member  61  of the supply roller  60  is inserted. 
     The arm member  52  is pivotable around the support shaft  51  from a downward tilt position, in which the rotating shaft of the supply roller  60  is lower than the support shaft  51 , to a horizontal position, in which the rotating shaft of the supply roller  60  is substantially as high as a center of the support shaft  51 . As shown in  FIG. 3 , a first twist spring  57  is provided to a proximal end (a pivot shaft side) of the arm member  52 . The first twist spring  57  is designed to apply a downward bias force (i.e., a bias force in a direction to force the supply roller  60  to abut the recording medium stored in the supply tray  30 ) to the arm member  52  in an entire pivotable area of the arm member  52 . Thus, the supply roller  60  abuts an uppermost sheet of the recording medium stored in the supply tray  30  (see  FIG. 10A ). 
     Moreover, a second twist spring  58  is provided to the distal end of the arm member  52 , as shown in  FIGS. 3 and 5 . The second twist spring  58  is designed to apply a downward bias force (i.e., a bias force in a direction to increase the bias force of the first twist spring  57 ) to the arm member  52  only when the arm member  52  is raised close to the horizontal position (in other words, an angle between a surface including the rotating shaft and the pivot shaft of the supply roller  60  and a surface of the recording medium is smaller than a predetermined angle). When the second twist spring  58  abuts an abutment piece  4   a  (a part of the frame  4 ), which is disposed within a path of pivot movement of the arm member  52  around the pivot shaft, the second twist spring  58  is elastically deformed thereby to apply the bias force to the arm member  52 . 
     Specifically, a free end of the second twist spring  68  abuts the abutment piece  4   a  formed in the frame  4  to apply a downward bias force to the arm member  52 , when the arm member  52  is arranged so that the supply roller  60  abuts the recording medium stored in the second tray  40  (see  FIGS. 10B ,  10 C and  FIG. 11 ). The dashed dotted line in  FIG. 10B  represents a position of a bottom surface of the second tray  40  (in other words, a vertical position of the recording medium when only a sheet of the recording medium is stored in the second tray  40 ). The dashed dotted line in  FIG. 10C  represents a position of an uppermost sheet of the recording medium when the recording medium is fully loaded in the second tray  40 . 
     [1-3. Structure of Image Recording Unit] 
     Next, an exemplary structure of an image recording unit  70  will be described. 
     As shown in  FIGS. 2 ,  12  and  13 , the image recording unit  70  is provided with a conveyance roller  71  at a position where the recording medium is conveyed in a U-turn manner from the supply tray  30  in the conveyance path  5 . The conveyance roller  71  is supported by side plates of the frame  4  so as to be rotatable around a rotating shaft along the left and right direction. 
     The image recording unit  70  is also provided with a driven roller  72  which is rotatable around a rotating shaft parallel with the rotating shaft of the conveyance roller  71  and is rotated following the conveyance roller  71 . That is, the conveyance roller  71  and the driven roller  72  are formed as a set of rollers. 
     Moreover, a regist sensor  73  is disposed on a rear side of the conveyance roller  71  (an upstream side of the conveyance path of the recording medium) (see  FIG. 13 ). The regist sensor  73  can detect a leading end position and a rear end position of the recording medium conveyed from the supply tray  30 . 
     The image recording unit  70  further includes a platen  74  and a carriage  75  on a frond side of the driven roller  72  (a downstream side of the conveyance path of the recording medium). The platen  74  supports the recording medium from thereunder, and the carriage  75  is movable along the left and right direction (a main scanning direction) above the platen  74 . A recording head  76 , which can discharge inks of a plurality of colors to record a color image, is mounted on the carriage  75 . 
     An image is recorded when the inks are discharged from the recording head  76  toward the recording medium on the platen  74  while the carriage  75  is moved along the main scanning direction. 
     The image recording unit  70  further includes a discharge roller  77  on the front-side of the platen  74  (on a downstream side of the conveyance path of the recording medium). The discharge roller  77  is supported by the side plates of the frame  4  so as to be rotatable around a rotating shaft along the left and right direction. 
     As shown in  FIG. 12 , the image recording unit  70  is provided with an ink receiver  78  on a left side and a maintenance unit  79  in a right side outside a width of the recording medium to be conveyed. The recording head  76  periodically discharges ink at a flashing position disposed in the ink receiver  78  during recording operation to inhibit nozzle clogging. 
     2. Explanation of Driving Mechanism 
     Next, an exemplary driving mechanism of the image forming apparatus  1  of the present embodiment will be discussed. 
     As shown in  FIGS. 12 and 14 , the image forming apparatus  1  is provided with an LF motor  6  which can generate a rotary driving force in a forward direction and a reverse direction. The rotary driving force generated by the LF motor  6  is transmitted to the conveyance roller  71  and the discharge roller  77  via a gear driving mechanism  80 . 
     Specifically, the gear driving mechanism  80  includes a pinion  81 , a transmission gear  82 , an intermediate gear  83  and a transmission gear  84 . The pinion  81  is positioned in a driving shaft of the LF motor  6 . The transmission gear  82  and the intermediate gear  83  engage with the pinion  81  from both sides of the pinion  81 . The transmission gear  84  engages with the intermediate gear  83 . The transmission gear  82  is secured to a left end of the conveyance roller  71 . The transmission gear  84  is secured to a left end of the discharge roller  77 . A rotary encoder  86  that detects a conveyance amount of a recording medium is disposed in a part of the gear driving mechanism  80 . 
     As shown in  FIG. 15 , the rotary driving force generated by the LF motor  6  is transmitted from a right end of the conveyance roller  71  to the supply roller  60  of the supply unit  50  or a maintenance mechanism (details are not shown) of the maintenance unit  79  through a power transmission switching mechanism  90  which is disposed above the maintenance unit  79 . 
     The power transmission switching mechanism  90  switches a transmission state of the rotary driving force transmitted from the LF motor  6  via the conveyance roller  71  between a maintenance mode and a conveyance mode. In the maintenance mode, the rotary driving force is transmitted only to the maintenance unit  79 . In the conveyance mode, the rotary driving force is transmitted only to the supply roller  60  of the supply unit  50 . 
     In the conveyance mode, the transmission state can be switched between an intermittent supply mode and a consecutive supply mode. In the intermittent supply mode, while one of the conveyance roller  71  and the supply roller  60  is rotated in a forward direction, the rotary driving force is transmitted so that the other roller is rotated in a reverse direction. In the consecutive supply mode, on the other hand, the rotary driving force is transmitted so that both the conveyance roller  71  and the supply roller  60  are rotated in the forward direction. 
     The image forming apparatus  1  is designed such that a conveyance speed of the recording medium by the conveyance roller  71  is faster than a conveyance speed of the recording medium by the supply roller  60 . The forward direction for each of the rollers  60 ,  71  and  77  is a rotating direction to convey the recording medium from a supply side to a discharge side. Specifically, the forward directions for the supply roller  60  and the conveyance roller  71  are the rotating directions to convey the recording medium to an image forming position by the image recording unit  70 . The forward direction for the discharge roller  77  is the rotating direction to convey the recording medium from the image forming position by the image recording unit  70  to a discharge position. 
     Now, a specific structure of a power transmission switching mechanism  90  will be discussed. 
     As shown in  FIG. 15 , the power transmission switching mechanism  90  is provided with a drive gear  91  and a switching gear  93 . The drive gear  91  elongated in a shaft direction of the conveyance roller  71  is secured in the right end of the conveyance roller  71 . The switching gear  93  is disposed slidably against a slide shaft  92  arranged in parallel with the rotating shaft of the conveyance roller  71  and constantly engages with the driving gear  91 . 
     The power transmission switching mechanism  90  is provided with a first block  94  and a second block  95 . The first block  94  having an upwardly extending abutment piece  94   a  is disposed slidably against and rotatably around the slide shaft  92 . The second block  95  is disposed slidably against the slide shaft  92  and adjacent to the first block  94 . The first block  94  is disengageable from the switching gear  93 . 
     The power transmission switching mechanism  90  is provided with a first bias spring  96  and a second bias spring  97 . The first bias spring  96  is fitted around the slide shaft  92  and applies a bias force to the second block  95  in an arrow C direction in  FIG. 15 . The second bias spring  97  is fitted around the slide shaft  92  and applies a bias force to the switching gear  93  in an arrow E direction in  FIG. 15 . 
     In addition, the power transmission switching mechanism  90  is provided with an intermittent supply transmission gear  111 , a consecutive supply transmission gear  112  and a maintenance transmission gear  113 . Each of these transmission gears  111 ,  112  and  113  may engage with the switching gear  93  depending on a sliding position of the switching gear  93 . 
     The abutment piece  94   a  of the first block  94  is positioned to abut either a first engagement shoulder  75   a  or a second engagement shoulder  75   b  (see  FIG. 16A ) provided to the carriage  75 . Therefore, the switching gear  93 , the first block  94  and the second block  95  can be moved along the slide shaft  92  in either the arrow C direction or the arrow E direction in accordance with a movement of the carriage  75  in the arrow C direction or the arrow E direction. In a position where the first block  94  and the second block  95  face each other, an end cam portion (not shown) tilted against the rotating shaft of the slide shaft  92  is provided. The end cam portion is designed such that the abutment piece  94   a  is rotated in an arrow D direction in  FIG. 15  when the first block  94  is pushed by the second block  95  in the arrow C direction. 
     As shown in  FIGS. 16A and 16B , a plate-like guide block  100  is disposed above the first block  94 . The guide block  100  includes a guide groove  101  in which an end portion of the abutment piece  94   a  is slidable while passing through the guide groove  101  in an upward and downward direction. 
     As shown in the plan view of  FIG. 16A , the guide groove  101  includes a linear groove portion  101   a  extending in the arrow C and E directions and a clockwise circular groove portion  101   b  which communicates with a left end of the linear groove portion  101   a . A restriction piece  102 , which extends in a downward direction from above the guide block  100 , is provided in a central portion of the circular groove portion  101   b . The restriction piece  102  is formed along the linear groove portion  101   a . Moreover, a first set portion  101   c  and second set portion  110   d  are formed in one side of the circular groove portion  101   b.    
     Therefore, as shown in  FIG. 16A , when the carriage  76  is largely moved from the maintenance unit  79  in the arrow C direction and in a recording region for the recording medium, the first block  94  and the switching gear  93  are moved along the slide shaft  92  through the second block  95  being pushed by the first bias spring  96  in the arrow C direction. Then, the abutment piece  94   a  of the first block  94  is located at a position in the first set portion  101   c  (hereafter, this position will be referred to as “the first position (PO 1 )”). At the first position (PO 1 ), the switching gear  93  engages with the intermittent supply transmission gear  111 . 
     When the carriage  75  is moved from the first position (PO 1 ) in the arrow E direction at the maintenance unit  79 , the abutment piece  94   a  of the first block  94  is pushed by the first engagement shoulder  75   a  of the carriage  75  to a position in the second set portion  101   d  (hereafter, this position will referred to as “the second position (PO 2 )”). At the second position (PO 2 ), the switching gear  93  engages with the consecutive supply transmission gear  112 . 
     When the carriage  75  is moved from the second position (PO 2 ) in the arrow E direction, the abutment piece  94   a  of the first block  94  is pushed by the first engagement shoulder  75   a  of the carriage  75  along an oblique connecting surface  101   e  connecting the circular groove portion  101   b  to the linear groove portion  101   a . When the abutment piece  94   a  is located at a position at an entrance of the linear groove portion  101   a  (hereafter, this position will be referred to as “the third position (PO 3 )”), the switching gear  93  engages with the maintenance transmission gear  113 . In this state, the abutment piece  94   a  abuts the second engagement shoulder  75   b.    
     When the carriage  76  is moved from the third position (PO 3 ) further in the arrow E direction, the abutment piece  94   a  of the first block  94  is pushed by the second engagement shoulder  75   b  of the carriage  75  to a position at a rear end  101   a   1  (i.e., a right end in  FIG. 16A ) of the linear groove portion  101   a  (hereafter, this position will be referred to as “the fourth position (PO 4 )”). The fourth position (PO 4 ) is usually a home position (an original position). In this state, a side surface of the switching gear  93  abuts a bevel gear portion  113   a  of the maintenance transmission gear  113  and thereby movement of the switching gear  93  in the arrow E direction is prevented. As a result, the switching gear  93  is detached from the first block  94 , and remains in an engaged state with the maintenance transmission gear  113 . 
     In contrast to an operation described above, when the carriage  75  is moved from the fourth position (PO 4 ) in the arrow C direction, the abutment piece  94   a  is moved from the linear groove portion  101   a  to the circular groove portion  101   b . In this case, the abutment piece  94   a  is received by the first engagement shoulder  75   a , and thus is not led to the above-mentioned oblique connecting surface  101   e . Therefore, the abutment piece  94   a  slidably contacts with the restriction piece  102  and led to a left end of the circular groove portion  101   b  along a left oblique surface  101   f  of the circular groove portion  101   b , as shown in  FIG. 16A . Then, the abutment piece  94   a  engages with the first set portion  101   c.    
     Among the four positions explained above, the third position (PO 3 ) is a maintenance position which is also used as a waiting position. At this position, a cap portion  79   a  of the maintenance unit  79  covers a nozzle surface of the recording head  76  from underneath (see  FIG. 12 ). During maintenance operation, the LF motor  6  may be driven to selectively suctioning ink from a nozzle by actuating a suction pump (not shown). A recovery process of removing air bubbles from a buffer tank (not shown) disposed above the recording head  76  may also be performed. 
     When the carriage  75  is moved in the left direction from the maintenance unit  79  toward the image forming region, the nozzle surface is wiped with a cleaner (a wiper blade)  79   b . Consequently, ink attached to the nozzle surface may be removed. When the image forming apparatus  1  is not on, the carriage  75  is stopped right above the maintenance unit  79  (at the third position (PO 3 )) and the nozzle surface of the recording head  76  is covered with the cap  79   a  above the maintenance unit  79  in a close contact manner. 
     When the switching gear  93  is at the first position (PO 1 ) where the switching gear  93  engages with the intermittent supply transmission gear  111 , power is transmitted to the support shaft  51 , which is provided at the proximal end of the arm member  52 , through two intermediate gears  129   a ,  129   b , as shown in  FIGS. 17A ,  17 B and  19 B. Then, the rotary driving force is transmitted to the driving gear  66  through the power transmission gears  66 . 
     When the switching gear  93  is at the second position (PO 2 ) where the switching gear  93  engages with the consecutive supply transmission gear  112 , power is transmitted to the support shaft  51 , which is provided at the proximal end of the arm member  52 , through one intermediate gear  130 , as shown in  FIGS. 18A to 18C  and  19 A. Then, the rotary driving force is transmitted to the driving gear  66  through the power transmission gear  56 . 
     3. Explanation of Control System 
     Next, an exemplary control system of the image forming apparatus  1  of the present embodiment will be described with reference to  FIG. 20 . 
     As shown in  FIG. 20 , the image forming apparatus  1  is provided with a CPU  201 , a ROM  202 , a RAM  203  and an EEPROM  204 , which are all connected to an ASIC (Application Specific Integrated Circuit)  206  through a bus  205 . 
     The ROM  202  stores, for example, control programs to control various operations of the image forming apparatus  1 . The RAM  203  is used as a storage area (a work area) that temporarily stores various data to be used when the CPU  201  executes a program. 
     An NCU (Network Control Unit)  207  is connected to the ASIC  206 . A communication signal inputted from a public line through the NCU  207  is first demodulated by a MODEM  208  and then inputted to the ASIC  206 . When an image data is transmitted by the ASIC  206  to an outside via facsimile or the like, the image data is first modulated to a communication signal by the MODEM  208  and then the communication signal is outputted to the public line through the NCU  207 . 
     The ASIC  206 , in accordance with a command from the CPU  201 , generates signals, including a phase excitation signal to turn on electricity to the LF motor  6 , and provides the signals to a drive circuit  209  for the LF motor  6  or a drive circuit  211  for a CR motor  210  (a motor for driving the carriage  75 ). Thus, the ASIC  206  provides a drive signal to the LF motor  6  or the CR motor  210  via the drive circuit  209  or the drive circuit  211 , and thereby controls forward and reverse rotations, a stop operation and the like of the LF motor  6  or the CR motor. 
     A CIS (Contact Image Sensor)  212 , an operation panel  10 , a parallel interface  213  and an USB interface  214  are connected to the ASIC  206 . The CIS  212  operates as an image reading device in the scanner unit  20 . The operation panel  10  includes the operation portion  11  and the display portion  12 . The parallel interface  213  and the USB interface  214  are used to perform transmission and reception of data through a parallel cable or a USB cable with an external information processing apparatus, such as a personal computer. 
     The regist sensor  73 , the rotary encoder  85  and a linear encoder  215  to detect a position of the carriage  75  in the main scanning direction are connected to the ASIC  206 . 
     A drive circuit  216  is designed to make the recording head  76  selectively discharge ink toward the recording medium at a predetermined timing. Specifically, the drive circuit  216  receives the signals, which are generated and outputted by the ASIC  206 , and controls driving of the recording head  76  according to a drive control signal outputted from the CPU  201 . 
     Next, an image recording process performed by the CPU  201  will be discussed with reference to a flowchart in  FIG. 21 . The image recording process is started when an image recording command is inputted from an external information processing apparatus (e.g. a personal computer). 
     When the image recording process is started, a currently set supply mode is determined in S 101 . The image forming apparatus  1  of the present embodiment allows a user to select a supply mode for consecutively recording images on a plurality of sheets of the recording medium from the intermittent supply mode and the consecutive supply mode. The intermittent supply mode is a supply mode in which the recording medium conveyed from the supply tray  30  undergoes skew correction by the conveyance roller  71 , and is further conveyed to the image recording unit  70  (a mode to prioritize accuracy of image recording). The consecutive supply mode is a supply mode in which the recording medium conveyed from the supply tray  30  is further conveyed to the image recording unit  70  without skew correction being performed by the conveyance roller  71  (a mode to prioritize reduction of time required for image recording). 
     When it is determined in S 101  that the currently set supply mode is the intermittent supply mode, the process proceeds to S 102 . 
     In S 102 , the power transmission switching mechanism  90  is set at a transmission state for the intermittent supply mode. Specifically, the carriage  75  stopped at the waiting position (the third position (PO 3 )) is largely moved to the image recording region in the arrow C direction in  FIG. 16A . Then, the first block  94  pushed by the first bias spring  96  is moved in the arrow C direction along with the restriction piece  102  in the circular groove portion  101   b . When the carriage  75  come out of the circular groove portion  101   b , the first block  94  is received by the first set portion  101   c , and is retained at the first position (PO 1 ). At the first position, the switching gear  93  engages with the intermittent supply transmission gear  111 , and power is transmitted to the support shaft  51  of the supply unit  50  through the two intermediate gears  129   a ,  129   b  shown in  FIG. 17A . 
     Subsequently in S 103 , the recording medium is supplied from the supply tray  30  to the image recording unit  70 . Specifically, the LF motor  6  is rotated in a reverse direction, and the conveyance roller  71  is rotary driven in the reverse direction (in a counterclockwise direction in  FIG. 17A ) and the supply roller  60  is rotary driven in the forward direction (in the counterclockwise direction shown in  FIG. 17A ), as shown in  FIG. 17A . 
     This makes the plurality of sheets of the recording medium stored in the supply tray  30  strike the guide plate  34  disposed on the rear end of the supply tray  30 . An uppermost sheet of the recording medium, which is abutting the supply roller  60 , is separated from the other sheets and is supplied (conveyed) to the conveyance path  5 . Since the conveyance roller  71  is rotary driven in the reverse direction at this time, a leading end of the uppermost sheet of the recording medium enters a nip portion between the conveyance roller  71  and the driven roller  72  (and thus is prevented from passing therethrough). Thus, skew correction of the recording medium is performed. 
     Subsequently, in S 104 , the direction of the rotary driving force generated by the LF motor  6  is switched from reverse to forward. Specifically, the direction is switched when the recording medium has been conveyed by a predetermined amount since the leading end of the recording medium is detected by the regist sensor  73  (when the leading end of the recording medium has been conveyed to reach the conveyance roller  71 ). 
     Then, as shown in  FIG. 17B , the conveyance roller  71  is rotary driven in the forward direction (in the clockwise direction in  FIG. 17B ), and the recording medium is pinched at the nip portion between the conveyance roller  71  and the driven roller  72 . In this case, the supply roller  60  is made to be rotary driven in the reverse direction (in the clockwise direction in  FIG. 17B ). However, the supply roller  60  has the certain amount of play in the rotating direction, and thus is not rotary driven in the reverse direction immediately after the rotation of the LF motor  6  is switched from the reverse direction to the forward direction (a state shown in  FIG. 6B ). The supply roller  60  starts to be rotary driven in the reverse direction after a delay by the play (a state shown in  FIG. 6A ). This inhibits the supply roller  60  from hindering pinching of the recording medium by the conveyance roller  71  and the driven roller  72 . 
     After the delay by the play, the supply roller  60  starts to be rotary driven in the reverse direction and attempts to convey the recording medium in a direction different from a conveyance direction by the conveyance roller  71  (see  FIG. 6A ). However, a conveyance force generated by the rotation of the conveyance roller  71  in the forward direction is larger than that of the supply roller  60  in the reverse direction. Accordingly, the conveyance of the recording medium by the conveyance roller  71  is not hindered. 
     In the image forming apparatus  1 , rotary driving of the supply roller  60  results in application of a force, which urges the supply roller  60  to roll on the recording medium, to the arm member  52 . Specifically, when the supply roller  60  is rotary driven in the forward direction, a component force of the force applied to the arm member  52  acts to press the supply roller  60  toward the recording medium. Accordingly, a pressing force becomes larger, and thus the conveyance force becomes larger. 
     In contrast, when the supply roller  60  is rotary driven in the reverse direction, a component force applied to the arm member  52  acts to separate the supply roller  60  from the recording medium. Accordingly, a pressing force becomes smaller, and thus the conveyance force becomes smaller. As a result, the conveyance of the recording medium by the conveyance roller  71  is not hindered even when the supply roller  60  is rotated in the reverse direction. 
     In S 105 , image recording on the recording medium is started. Specifically, an image is recorded by discharging ink from the nozzle of the recording head  76  onto a surface of the recording medium while forwarding the recording medium intermittently and reciprocating the carriage  76  in the main scanning direction. 
     In S 106 , it is determined whether or not recording for one page (for a sheet of the recording medium) has been completed. When it is determined that recording for one page been completed, the present process proceeds to S 107 . 
     In S 107 , the recording medium after the image recording is discharged to a front upper position of the supply tray  30 . Specifically, the LF motor  6  is rotated in the forward direction by an appropriate step number, and the conveyance roller  71  and the discharge roller  77  are rotary driven in the forward direction by a predetermined amount. 
     In S 108 , it is determined whether or not image recording data for a subsequent page (for a following sheet of the recording medium) is present. When it is determined that image recording data for the subsequent page is present, the present process returns to S 103 , and the above-mentioned processings (S 103  to S 107 ) are performed. 
     When it is determined that image recording data for the subsequent page is not present in S 108 , the present image recording process is terminated. 
     On the other hand, when it is determined in S 101  that the currently set supply mode is not the intermittent supply mode (i.e., the currently set supply mode is the consecutive supply mode), the present process proceeds to S 109 . In S 109 , the power transmission switching mechanism  90  is set at the transmission state for the consecutive supply mode. 
     Specifically, the carriage  75  stopped at the first position (PO 1 ) is moved in the arrow E direction in  FIG. 16A  by a predetermined amount, and thereby the abutment piece  94   a  is pushed by the first engagement shoulder  75   a  of the carriage  75 . When the abutment piece  94   a  is positioned at the second set portion  101   d  (the second position PO 2 ), the switching gear  93  engages with the consecutive supply transmission gear  112 . Then, power is transmitted to the support shaft  51  of the supply unit  50  through one intermediate gear  130  as shown in  FIG. 18A . After that, even if the carriage  76  is moved in the arrow C direction (to the image recording region), the abutment piece  94  biased by the first bias spring  96  is retained at a low shoulder, i.e., the second set portion  101   d.    
     Then, in S 110 , the recording medium is supplied from the supply tray  30  to the image recording unit  70 . Specifically, the LF motor  6  is rotated in the forward direction, and the conveyance roller  71  is rotary driven in the forward direction (in the clockwise direction in  FIG. 18A ), as shown in  FIG. 18A , and the supply roller  60  is rotary driven in the forward direction. Consequently, an uppermost sheet of the plurality of sheets of the recording medium stored in the supply tray  30  is separated from the other sheets and conveyed to the conveyance path  5 . 
     Since the conveyance roller  71  is rotary driven in the forward direction, the recording medium passes between the conveyance roller  71  and the driven roller  72  without being affected by a regist operation, and is pinched by the conveyance roller  71  and the driven roller  72  when the leading end of the recording medium reaches the nip portion between the conveyance roller  71  and the driven roller  72 . 
     Even when one sheet of the recording medium is pinched by the conveyance roller  71  and the driven roller  72  and also abuts the supply roller  60 , i.e., located over both of the conveyance roller  71  and the supply roller  60  (as shown in  FIG. 18B ), conveyance of the recording medium by the conveyance roller  71  is not hindered. Specifically, as discussed above, the conveyance speed of the recording medium by the conveyance roller  71  is designed to be faster than the conveyance speed of the recording medium by the supply roller  60 . 
     Accordingly, the supply roller  60  is in a state of being pulled by the recording medium. In this state, a force in an opposite direction is applied to the arm member  62  contrary to a force due to the rotation of the supply roller  60  in the forward direction and causing the supply roller  60  to roll on the recording medium. The component force of the force in the opposite direction acts to separate the supply roller  60  from the recording medium. Consequently, the pressing force becomes smaller, and thus the conveyance force also becomes smaller. Therefore, even when the conveyance speed of the recording medium by the supply roller  60  is slow, the conveyance of the recording medium by the conveyance roller  71  is not hindered. 
     According to the image forming apparatus  1 , the recording medium is also inhibited from being skewed continuously by such consecutive conveyance. When the recording medium being conveyed by the conveyance roller  71  abuts the supply roller  60  (i.e., the recording medium is located over both of the rollers  60 ,  71 ), the conveyance speed of the recording medium by the conveyance roller  71  is faster than that of the supply roller  60 . Accordingly, the supply roller  60  is rotated ahead of the driving gear  66  by the play in the rotating direction since the supply roller  60  is pulled by the recording medium. 
     When a rear end of the recording medium being conveyed by the conveyance roller  71  leaves the supply roller  60 , the supply roller  60  abuts a subsequent (a currently uppermost) recording medium. Since the supply roller  60  is rotated ahead of the driving gear  66  by the play in the rotating direction, the supply roller  60  is not immediately rotary driven in the forward direction. The supply roller  60  starts to be rotary driven after the delay of the play. It may, therefore, be possible to inhibit continuous occurrence of skew of the recording medium due to the consecutive conveyance of the recording medium by rotating both the supply roller  60  and the conveyance roller  71  in the forward direction. 
     In S 111 , image recording on the recording medium is started. Specifically, an image is recorded by discharging ink from the nozzle of the recording head  76  onto a surface of the recording medium while forwarding the recording medium intermittently and reciprocating the carriage  75  in the main scanning direction. 
     In S 112 , it is determined whether or not image recording data for a subsequent page (for a following sheet of the recording medium) is present. When it is determined that image recording data for the subsequent page is not present, the present process proceeds to S 113 . 
     In S 113 , the power transmission switching mechanism  90  is set at the transmission state for the intermittent supply mode, and the present process proceeds to S 114 . 
     On the other hand, when it is determined in S 112  that image recording data for the subsequent page is present, the present process proceeds to S 114 . 
     In S 114 , it is determined whether or not recording for one page (for a sheet of the recording medium) has been completed. When it is determined that recording for one page been completed, the present process proceeds to S 115 . 
     In S 115 , it is determined whether or not the power transmission switching mechanism  90  is in the transmission state for the consecutive supply mode. When it is determined in S 115  that the power transmission switching mechanism  90  is not in the transmission state for the consecutive supply mode (i.e., in the transmission state for the intermittent supply mode), the present process proceeds to S 116 . 
     In S 116 , a subsequent medium processing is performed and then the image recording process is terminated. The details of the subsequent medium processing will be discussed later ( FIG. 22 ). 
     On the other hand, when it is determined in S 115  that the power transmission switching mechanism  90  is in the transmission state for the consecutive supply mode (i.e., when image recording data for the subsequent page is present), the present process proceeds to S 117 . 
     In S 117 , the recording medium after image recording is discharged and the subsequent recording medium is conveyed. Then, the present process is returned to S 111 . Specifically, the LF motor  6  is rotated in the forward direction continuously and a preceding sheet of the recording medium (the preceding page) is discharged while the subsequent sheet of the recording medium is conveyed to a recording start position (see  FIG. 18C ) 
     In the above described manner, the plurality of sheets of the recording medium may be conveyed continuously in the consecutive supply mode without temporarily stopping the recording medium by the conveyance roller  71 . Thus, a high speed recording operation can be achieved. 
     Next, the subsequent medium processing performed in S 116  of the image recording process ( FIG. 21 ) will be discussed with reference to a flowchart of  FIG. 22 . 
     When the subsequent medium processing is started, it is determined in S 201  whether or not the regist sensor  73  is on. Specifically, it is determined whether or not a leading end of a subsequent recording medium following the recording medium after the image recording has passed a position of the regist sensor  73 . 
     When it is determined in S 201  that the regist sensor  73  is not on (i.e., is off), the present process proceeds to S 202 . In S 202 , the LF motor  6  is rotated in the forward direction by an appropriate step number, and the supply roller  60  is rotary driven in the reverse direction by a predetermined amount. Then, the subsequent medium processing process is terminated. 
     In other words, as shown in  FIG. 19A , when the leading end of the subsequent recording medium has not reached the position of the regist sensor  73 , the subsequent recording medium is returned to the supply tray  30 . The recording medium after the image recording is discharged by the rotations of the conveyance roller  71  and the discharge roller  77  in the forward direction. 
     On the other hand, when it is determined in S 201  that the regist sensor  73  is on, the present process proceeds to S 203 . In S 203 , the LF motor  6  is rotated in the reverse direction by an appropriate step number, and the supply roller  60  is rotary driven in the forward direction by a predetermined amount. When the leading end of the subsequent recording medium has exceeded the position of the regist sensor  73 , the leading end of the subsequent recording medium is struck against the conveyance roller  71 , and thereby skew correction is performed. 
     Subsequently, in S 204 , the LF motor  6  is rotated in the forward direction by an appropriate step number, and the conveyance roller  71  and the discharge roller  77  are rotary driven in the forward direction by a predetermined amount. Then, the supply roller  60  is rotary driven in the reverse direction by a predetermined amount. As a result, as shown in  FIG. 19B , the subsequent recording medium after skew correction is discharged, while a further subsequent recording medium is returned to the supply tray  30 . Then, the present subsequent medium processing process is terminated. 
     As described above, when the leading end of the subsequent recording medium exceeds the position of the regist sensor  73  and is positioned downstream from the regist sensor  73  in the conveyance direction, the subsequent recording medium is conveyed in a discharge direction. In contrast, when the leading end of the subsequent recording medium has not reached the position of the regist sensor  73 , the subsequent recording medium is returned toward the supply tray  30 . 
     4. Advantage 
     In the image forming apparatus  1  of the present embodiment, as discussed above, the recording medium conveyed by the rotation of the supply roller  60  in the forward direction is conveyed through the conveyance roller  71 , which is rotated in the forward direction, in the consecutive supply mode. When the rear end of the recording medium conveyed by the conveyance roller  71  leaves the supply roller  60 , the supply roller  60  abuts the subsequent recording medium, and the subsequent recording medium is conveyed continuously. 
     That is, the present image forming apparatus  1  is capable of supplying (or conveying) the subsequent recording medium by the supply roller  60  while the recording medium is being conveyed by the conveyance roller  71 . It may, therefore, be possible to perform image recording on a large amount of recording medium in a short time. 
     Moreover, the present image forming apparatus  1  is configured such that the conveyance speed of the recording medium by the conveyance roller  71  is faster than the conveyance speed of the recording medium by the supply roller  60 . Accordingly, even when a preceding recording medium is delivered from the supply tray  30  with a rear end of the preceding recording medium overlapping a leading end of a subsequent recording medium, such an overlap may be eliminated before each of the recording mediums has been conveyed to the image recording position. 
     Furthermore, the supply roller  60  has the certain amount of play in the rotating direction in the image forming apparatus  1 . It may, therefore, be possible to effectively suppress continuous occurrence of skew of the recording medium due to the consecutive conveyance of the recording medium as above. 
     When the recording medium being conveyed by the conveyance roller  71  abuts the supply roller  60 , the conveyance speed of the recording medium by the conveyance roller  71  is faster than the conveyance speed of the recording medium by the supply roller  60 . Accordingly, the supply roller  60  is pulled by the recording medium by an amount corresponding to the amount of the play in the rotating direction. 
     When a rear end of the recording medium being conveyed by the conveyance roller  71  leaves the supply roller  60 , the supply roller  60  abuts a subsequent recording medium. Since the supply roller  60  is pulled by the recording medium by the amount corresponding to the amount of the play in the rotating direction, the supply roller  60  is not immediately rotated in the forward direction but starts to be rotated after the delay of the play. 
     It may, therefore, be possible to significantly reduce or inhibit the subsequent recording medium from being skewed. Thus, it may be possible, according to the image forming apparatus  1 , to effectively suppress continuous occurrence of skew of the recording medium due to the consecutive conveyance of the recording medium by rotating both of the supply roller  60  and the conveyance roller  71  in the forward direction. 
     In the image forming apparatus  1 , when the supply roller  60  is rotated in the forward direction, a force to urge the supply roller  60  to roll on the recording medium is applied to the arm member  52 . Then, a component force of the force applied to the arm member  52  acts to press the supply roller  60  toward the recording medium, and thereby the conveyance force becomes larger. 
     In contrast, when the supply roller  60  is pulled by the recording medium, a force in an opposite direction is applied to the arm member  52 . Then, a component force of the force in the opposite direction acts to separate the supply roller  60  from the recording medium, and thereby the conveyance force becomes smaller. 
     According to the present image forming apparatus  1 , therefore, it may be possible to ensure a conveyance force required to supply (convey) the recording medium stored in the supply tray  30  when the supply roller  60  is rotary driven in the forward direction, and also may be possible not to hinder the conveyance of the recording medium by the conveyance roller  71  when the supply roller  60  is pulled by the recording medium being conveyed by the conveyance roller  71 . 
     Further, according to the present image forming apparatus  1 , the recording medium conveyed by the rotation of the supply roller  60  in the forward direction is interrupted to pass through by the conveyance roller  71  rotating in the reverse direction and undergoes skew correction, in the intermittent supply mode. 
     At a timing when the recording medium conveyed by the supply roller  60  reaches the conveyance roller  71 , the direction of the rotary driving force generated by the LF motor  6  is switched from reverse to forward (the CPU  201  executes the processing in S 104 ). Then, the conveyance roller  71  is rotated in the forward direction, and the recording medium after skew correction is conveyed through the conveyance roller  71 . 
     The supply roller  60  having a certain amount of play in the rotating direction is not rotated in the reverse direction immediately after the rotation of the LF motor  6  is switched from the reverse direction to the forward direction. The supply roller  60  starts to be rotated in the reverse direction after a delay by the play. It may, therefore, be possible to prevent the recording medium from being pulled back due to the rotation of the supply roller  60  in the reverse direction before the conveyance roller  71  becomes ready to convey the recording medium. 
     According to the present image forming apparatus  1 , as described above, the supply mode of the recording medium may be switched between the consecutive supply mode to convey the recording medium continuously and the intermittent supply mode to convey the recording medium while skew correction is being performed by the conveyance roller  71 . This may be achieved simply by switching the power transmission switching mechanism  90  into the transmission state for the consecutive supply mode and the transmission state for the intermittent supply mode. Thus, it may be possible to perform image recording in an appropriate supply mode in accordance with a status of use. 
     5. Modification 
     Although one embodiment of the present invention has been described as above, it is to be understood that the present invention may be embodied in various forms. 
     For example, in the image forming apparatus  1  of the present embodiment, a certain degree of freedom is given to the angle of the rotating shaft of the supply roller  60  by forming a clearance between the shaft  65  of the supply roller  60  and the axial support member  55  of the arm member  52 . However, the certain degree of freedom may be given to the angle of the rotating shaft of the supply roller  60  by allowing flexible movement of the distal end of the arm member  52 . 
     Moreover, the certain degree of freedom may be given to angles in all directions as in the image forming apparatus  1  of the present embodiment, or to an angle in a specific direction of the rotating shaft of the supply roller  60 . The angle in a specific direction may be, for example, an angle along a plane parallel with the recording medium (an angle along the front and back direction) or an angle along a plane perpendicular to the recording medium (an angle along the up and down direction). 
     In the image forming apparatus  1 , the arm member  52  is biased by the second twist spring  58 , which is provided at the distal end of the arm member  52 , abutting the frame  4  and being elastically deformed. However, the arm member  52  may be biased by a spring  58 , which is provided to the frame  4 , abutting the arm member  52  and being elastically deformed. 
     The present invention is applied to an image forming apparatus that performs image recording in an ink-jet method in the present embodiment. However, the present invention may be applied to, for example, an image forming apparatus that performs image recording in a laser method.