Patent Publication Number: US-9422126-B2

Title: Sheet conveyance device

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a sheet conveyance device which conveys a sheet. 
     Conventionally, as a sheet conveyance device which conveys a sheet, a sheet conveyance device which includes a sheet reversing unit and is applied to a sheet-fed offset rotary printing press equipped with a reversing mechanism and capable of printing on one or both of the two surfaces of a sheet, has been proposed, as described in Japanese Patent Laid-Open No. 58-219058 (literature 1). In the printing press proposed in literature 1, a sheet conveyance device including a reversing unit is interposed between first and second, adjacent printing units, and performs a selective reversing operation for a sheet conveyed by the sheet conveyance device to allow single-sided printing and double-sided printing on the sheet. 
     In the printing press described in literature 1, the reversing unit includes a transfer cylinder (reference numeral 17) and impression cylinder (reference numeral 16). In double-sided printing, the trailing edge of a sheet conveyed while the leading edge of the sheet is gripped by the transfer cylinder is gripped by the impression cylinder to convey the sheet with its trailing edge leading, and turn it. 
     However, in the printing press described in literature 1, when the sheet size is changed, engagement of a gear which drives the impression cylinder is canceled before activation of the printing press, the phase of the impression cylinder relative to the transfer cylinder is changed so that a gripper device of the impression cylinder is opposed to the trailing edge of the sheet held on the transfer cylinder, and then the gear must be engaged again. This increases the operator&#39;s burden, and its preparation takes a considerable time. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to propose a sheet conveyance device which can easily cope with a change in size of a sheet. 
     In order to achieve the above-mentioned object, according to the present invention, there is provided a sheet conveyance device comprising a first conveyance unit which includes a first holder that holds one edge of a sheet, and conveys the sheet held by the first holder, a second conveyance unit which includes a second holder that holds the one edge of the sheet, and conveys the sheet held by the second holder, a third conveyance unit which is supported to be swingable between a reception position at which the third conveyance unit receives the sheet from the first conveyance unit, and a transfer position at which the third conveyance unit transfers the sheet to the second conveyance unit, the third conveyance unit including a third holder that holds the other edge of the sheet conveyed by the first conveyance unit, and conveying the sheet held by the third holder, an independent driving unit which independently drives the first conveyance unit, a device driving unit which drives an entire device including the second conveyance unit and the third conveyance unit, and a control unit which controls the independent driving unit to adjust a speed at which the third conveyance unit conveys the sheet, based on a dimension of the sheet in a conveyance direction. 
     According to an aspect of the present invention, even if the sheet size is changed, driving of the first conveyance unit ( 39 ) is controlled through the independent driving unit ( 254 ) based on the changed dimension of the sheet in the conveyance direction. With this operation, the trailing edge of the sheet with its size changed can be held by the third holder ( 31   bt ) of the third conveyance unit ( 31   b ) which swings at a predetermined period. 
     According to another aspect of the present invention, when the trailing edge of the sheet conveyed by the transport cylinder ( 39 ) is held by the third holder ( 31   bt ) of the third conveyance unit ( 31   b ), the speed of the transport cylinder ( 39 ) is adjusted so that the third holder ( 31   bt ) of the third conveyance unit ( 31   b ) which swings at a predetermined period is opposed to the trailing edge of the sheet at the reception position. With this operation, even if the sheet size is changed, the trailing edge of the sheet can reliably be transferred from the transport cylinder ( 39 ) to the third conveyance unit ( 31   b ). Also, the speed of the transport cylinder ( 39 ) is adjusted so that the fourth holder ( 37   a ) of the fourth conveyance unit ( 37 ) is opposed to the first holder ( 39   a ) of the transport cylinder ( 39 ) after the sheet is held by the third holder ( 31   bt ). With this operation, the leading edge of the next new sheet can reliably be transferred from the fourth conveyance unit ( 37 ) to the first conveyance unit ( 39 ). 
     According to still another aspect of the present invention, if the dimension of the sheet in the conveyance direction is larger than a standard size (reference size), the rotation speed of the transport cylinder ( 39 ) is controlled to be higher than the reference speed after the sheet is received from the fourth conveyance unit ( 37 ), and lower than the reference speed after the sheet is transferred from the transport cylinder ( 39 ) to the third conveyance unit ( 31   b ). With this operation, even if the dimension of the sheet in the conveyance direction is large, the leading edge of the next new sheet conveyed from the fourth conveyance unit ( 37 ) can reliably be held by the transport cylinder ( 39 ) after the trailing edge of the sheet is reliably held while the transport cylinder follows a motion of the third conveyance unit ( 31   b ). However, if the dimension of the sheet in the conveyance direction is smaller than the standard size, the rotation speed of the transport cylinder ( 39 ) is controlled to be lower than the reference speed after the sheet is received from the fourth conveyance unit ( 37 ), and higher than the reference speed after the sheet is transferred from the transport cylinder ( 39 ) to the third conveyance unit ( 31   b ). With this operation, even if the dimension of the sheet in the conveyance direction is small, the leading edge of the next new sheet conveyed from the fourth conveyance unit ( 37 ) can reliably be held by the transport cylinder ( 39 ) after the trailing edge of the sheet is reliably held while the transport cylinder follows a motion of the third conveyance unit ( 31   b ). 
     According to still another aspect of the present invention, when the sheet is transferred from the transport cylinder ( 39 ) to the third conveyance unit ( 31   b ), and when the sheet is received from the fourth conveyance unit ( 37 ), the transport cylinder ( 39 ) is rotated at the reference speed by the independent driving motor ( 254 ). This allows reliable reception and transfer of the sheet, regardless of the dimension of the sheet in the conveyance direction. 
     According to still another aspect of the present invention, if the dimension of the sheet in the conveyance direction is the standard size, the transport cylinder ( 39 ) is always rotated at a constant reference speed by the independent driving motor ( 254 ). This allows reliable reception and transfer of the sheet with the standard size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing the schematic arrangement of a digital printing apparatus according to the first embodiment of the present invention; 
         FIG. 2  is a side view showing a reversing mechanism portion shown in  FIG. 1 ; 
         FIG. 3  is a top view showing the circumferential surface structure of a pre-reversal double-diameter cylinder shown in  FIG. 2 ; 
         FIG. 4  is a control block diagram of the digital printing apparatus shown in  FIG. 1 ; 
         FIGS. 5A to 5E  are side views showing double-sided printing processes ( 1 ) to ( 5 ) in the digital printing apparatus shown in  FIG. 1 ; 
         FIG. 6  is a timing chart showing the speed control sequence of the pre-reversal double-diameter cylinder shown in  FIG. 2 ; 
         FIG. 7  is a side view for explaining a sheet gripping change operation from the pre-reversal double-diameter cylinder to a reversing swing arm shaft pregripper if the sheet size is a standard size; 
         FIG. 8  is a side view for explaining a sheet gripping change operation from the pre-reversal double-diameter cylinder to the reversing swing arm shaft pregripper if the sheet size is larger than the standard size; 
         FIG. 9  is a side view for explaining a sheet gripping change operation from the pre-reversal double-diameter cylinder to the reversing swing arm shaft pregripper if the sheet size is smaller than the standard size; 
         FIG. 10  is a circuit block diagram according to the second embodiment of the present invention, in which the speed of a pre-reversal double-diameter cylinder is controlled in consideration of an error of the sheet size; and 
         FIG. 11  is a circuit block diagram according to the third embodiment of the present invention, in which the speed of a pre-reversal double-diameter cylinder is controlled in consideration of the actual measurement value of the sheet size. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail below with reference to the accompanying drawings. 
     (1) First Embodiment 
     Arrangement of Digital Printing Apparatus 
     A digital printing apparatus  1  (sheet processing apparatus) according to this embodiment includes a sheet feed device  2  (sheet supply device), a digital printing unit  3  (processing unit), and a sheet delivery device  4  (sheet discharge device), as shown in  FIG. 1 . 
     The sheet feed device  2  includes a pile board  21  on which a plurality of sheets S 1  are stacked, and a sucker device  23  which conveys the top sheet S 1  on the pile board  21  onto a feeder board FB. The sucker device  23  includes a pair of suction ports  23   a  and  23   b , which are connected to a negative pressure source  25  via a continuous supply valve  26  and an intermittent supply valve  27 . 
     The continuous supply valve  26  and intermittent supply valve  27  enable/disable, at different timings, the suction operation of the suction ports  23   a  and  23   b  using a negative pressure from the negative pressure source  25 . 
     A swing arm shaft pregripper  31   f  is disposed on the distal end side of the feeder board FB in the sheet conveyance direction. The swing arm shaft pregripper  31   f  is swingably supported on a frame  3   a  of the digital printing unit  3 , and includes a gripper device (not shown) which grips and holds the leading edge (front edge) of the sheet S 1  as its one edge. A feed-side transfer cylinder  32  is opposed to the swing arm shaft pregripper  31   f , and rotatably supported on the frame  3   a . A gripper device  32   a  which holds the leading edge of the sheet S 1 , transferred by a gripper device of the swing arm shaft pregripper  31   f , in a gripped state is provided on the feed-side transfer cylinder  32 . The swing arm shaft pregripper  31   f  and feed-side transfer cylinder  32  constitute an upstream sheet conveyance device. Note that in the following description, the gripper device is formed by a plurality of grippers aligned in the cylinder axis direction with predetermined gaps between them. 
     A printing cylinder  33  (second conveyance unit) serving as a downstream transport cylinder is disposed on the downstream side of the swing arm shaft pregripper  31   f  in the sheet conveyance direction to be in contact with the feed-side transfer cylinder  32 . The printing cylinder  33  is rotatably supported on the frame  3   a , and has a diameter three times that of the feed-side transfer cylinder  32 . The printing cylinder  33  includes printing cylinder gripper devices  33   a ,  33   b , and  33   c  (second holders) which hold the leading edge of the sheet S 1  upon receiving it from the gripper device  32   a  of the feed-side transfer cylinder  32 , and support surfaces  33   d ,  33   e , and  33   f  which are provided in correspondence with the printing cylinder gripper devices  33   a ,  33   b , and  33   c , and support the sheet S 1 . The printing cylinder  33  is implemented by a triple-diameter cylinder provided with three pairs of printing cylinder gripper devices  33   a ,  33   b , and  33   c  and support surfaces  33   d ,  33   e , and  33   f . The printing cylinder gripper devices  33   a ,  33   b , and  33   c  are provided at positions 120° out of phase with each other in the circumferential direction. 
     An inkjet nozzle portion  34  is opposed to the circumferential surface of the printing cylinder  33  on the downstream side of the contact portion of the printing cylinder  33  with the feed-side transfer cylinder  32  in the sheet conveyance direction. 
     The inkjet nozzle portion  34  includes a plurality of inkjet nozzle heads  34   a  to  34   d  (to be referred to as ink heads hereinafter) which are juxtaposed in the sheet conveyance direction along the circumferential surface of the printing cylinder  33 , and store inks of different colors. Each of the ink heads  34   a  to  34   d  is oriented in a direction perpendicular to the circumferential surface of the printing cylinder  33 . The ink heads  34   a  to  34   d  are arranged in proximity to the printing cylinder  33  to have small gaps with the sheet S 1  having its entire surface sucked by the support surfaces  33   d ,  33   e , and  33   f . The printing cylinder  33  and inkjet nozzle portion  34  constitute a sheet printing device. 
     An ink drying lamp  35  is opposed to the printing cylinder  33  on the downstream side of a printing region  33 K, printed by the inkjet nozzle portion  34  of the printing cylinder  33 , in the sheet conveyance direction, and serves as a drying device which irradiates the sheet S 1  with light such as infrared or ultraviolet rays to dry ink printed on the sheet S 1 . Note that drying includes applying thermal energy to the ink to evaporate the moisture of the ink, and curing the ink. 
     The printing cylinder  33  is arranged on the downstream side of the inkjet nozzle portion  34  in the sheet conveyance direction to be in contact with a delivery-side transfer cylinder  36  rotatably supported on the frame  3   a . The delivery-side transfer cylinder  36  has a gripper device  36   a  which holds the leading edge of the sheet S 1 , conveyed by the printing cylinder  33 , upon receiving it from the printing cylinder gripper devices  33   a ,  33   b , and  33   c.    
     A delivery-side transfer cylinder  37  (fourth conveyance unit) serving as an upstream transport cylinder is arranged on the downstream side of the contact portion of the delivery-side transfer cylinder  36  with the printing cylinder  33  in the sheet conveyance direction to be in contact with the delivery-side transfer cylinder  36 . The delivery-side transfer cylinder  37  is rotatably supported on the frame  3   a . The delivery-side transfer cylinder  37  has a gripper device  37   a  (upstream gripper device) which receives and holds the leading edge of the sheet S 1  conveyed by the delivery-side transfer cylinder  36 . 
     A delivery cylinder  38  is arranged on the downstream side of the contact portion of the delivery-side transfer cylinder  37  with the delivery-side transfer cylinder  36  in the sheet conveyance direction to be in contact with the delivery-side transfer cylinder  37 . The delivery cylinder  38  is rotatably supported on the frame  3   a . The delivery cylinder  38  has a gripper device  38   a  (downstream gripper device) which receives and holds the leading edge of the sheet S 1  conveyed by the delivery-side transfer cylinder  37 . 
     A belt conveyor-shaped delivery belt  40  which conveys the sheet S 1  is disposed below the delivery cylinder  38 . A pile board  41  which stacks sheets S 1  having undergone a digital printing process by the digital printing unit  3  is provided on the leading edge side of the delivery belt  40  in the sheet conveyance direction. The delivery cylinder  38 , delivery belt  40 , and pile board  41  constitute the sheet delivery device  4 . Also, the path of the sheet S 1  conveyed by the delivery cylinder  38  and delivery belt  40  constitutes a sheet discharge path. 
     A pre-reversal double-diameter cylinder  39  (first conveyance unit) serving as a transport cylinder is arranged on the downstream side of the contact portion of the delivery-side transfer cylinder  37  with the delivery cylinder  38  in the sheet conveyance direction to be in contact with the delivery-side transfer cylinder  37 . The pre-reversal double-diameter cylinder  39  is rotatably supported on the frame  3   a . The pre-reversal double-diameter cylinder  39  includes a gripper device  39   a  (first holder) which is implemented by a double-diameter cylinder with a diameter twice that of the delivery-side transfer cylinder  37 , and receives and holds the leading edge of the sheet S 1  conveyed by the delivery-side transfer cylinder  37 . The pre-reversal double-diameter cylinder  39  also includes a circumferential surface  39   c  (support surface) which supports the entire surface of the sheet S 1  with its leading edge held by the gripper device  39   a.    
     A reversing swing arm shaft pregripper  31   b  (third conveyance unit) having a reversing gripper device  31   bt  (third holder) which receives and holds the trailing edge (rear edge) of the sheet S 1  as its other edge is opposed to the pre-reversal double-diameter cylinder  39  on the downstream side of the contact portion of the pre-reversal double-diameter cylinder  39  with the delivery-side transfer cylinder  37  in the sheet conveyance direction, as shown in  FIG. 2 . 
     A plurality of swing arms  202  are fixed to a reversing swing arm shaft  201  with predetermined gaps between them in the cylinder axis direction. The reversing swing arm shaft  201  is pivotally supported on the frame  3   a . A swing arm gripper  203  is pivotally attached to the distal end of each of the plurality of swing arms  202  through a gripper shaft  203   a.    
     A gripper pad  205  is provided at a position at which it is opposed to each swing arm gripper  203 , and is attached to a gripper pad holding portion  204  fixed to the distal ends of the swing arms  202 . A plurality of sets of swing arm grippers  203  and gripper pads  205  constitute the reversing gripper device  31   bt  which grips and holds the trailing edge of the sheet S 1 . The reversing gripper device  31   bt , swing arms  202 , reversing swing arm shaft  201 , and gripper pad holding portion  204  constitute the reversing swing arm shaft pregripper  31   b.    
     The reversing swing arm shaft pregripper  31   b  is supported to be swingable between a reception position (a broken line in  FIG. 1 ), at which it receives the sheet S 1  from the pre-reversal double-diameter cylinder  39 , and a transfer position (a solid line in  FIG. 1 ), at which it transfers by a gripping change the sheet S 1  onto the printing cylinder  33 , by pivoting the pivotal reversing swing arm shaft  201 . 
     The reversing swing arm shaft pregripper  31   b  is opposed to the printing cylinder  33  on the downstream side of the contact portion of the printing cylinder  33  with the delivery-side transfer cylinder  36  in the rotation direction of the printing cylinder  33 , and on the upstream side of the contact portion of the printing cylinder  33  with the feed-side transfer cylinder  32  in the rotation direction of the printing cylinder  33 . 
     A plurality of groove-shaped recessed portions  39   b  are formed in the circumferential surface  39   c  of the pre-reversal double-diameter cylinder  39 , pivotally supported on the frame  3   a , with gaps between them in the axial direction to extend circumferentially, as shown in  FIG. 3 . The recessed portions  39   b  are opposed to the gripper device  37   a  of the delivery-side transfer cylinder  37 , and the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b . The pre-reversal double-diameter cylinder  39  has a driving system independent of those of, for example, the printing cylinder  33 , delivery-side transfer cylinder  37 , and reversing swing arm shaft pregripper  31   b , and is driven independently of the remaining cylinders by an independent driving motor  254  which independently drives it. Note that the pre-reversal double-diameter cylinder  39 , reversing swing arm shaft pregripper  31   b , and printing cylinder  33  constitute the sheet conveyance device. 
     The operation of the gripper device  37   a  of the delivery-side transfer cylinder  37  is controlled so as to selectively transfer the sheet S 1  to the gripper device  38   a  of the delivery cylinder  38 , and the gripper device  39   a  of the pre-reversal double-diameter cylinder  39 . Also, the operation of the gripper device  38   a  of the delivery cylinder  38  is controlled so as to selectively receive the leading edge of the sheet S 1  conveyed by the delivery-side transfer cylinder  37 . 
     The delivery-side transfer cylinders  36  and  37 , pre-reversal double-diameter cylinder  39 , and reversing swing arm shaft pregripper  31   b  constitute a sheet reversing path used to turn and convey the sheet S 1 . The sheet reversing path is used to receive the sheet S 1  from the printing cylinder  33 , and turn and transfer the sheet S 1  onto the printing cylinder  33 . 
     The gripper device  37   a  of the delivery-side transfer cylinder  37 , and the gripper device  38   a  of the delivery cylinder  38  constitute a sheet conveyance path switching device which selectively switches the path of the sheet S 1  between the sheet reversing path and the sheet discharge path. 
     &lt;Configuration of Control System for Digital Printing Apparatus&gt; 
     The digital printing apparatus  1  includes a control unit  251  having a CPU (Central Processing Unit) configuration which controls the overall printing operation, as shown in  FIG. 4 . The control unit  251  is connected to a sheet size input unit  252  which receives the size of the sheet S 1 , a single-/double-sided printing mode input unit  253  (printing condition input unit) which selects a single- or double-sided printing mode, the independent driving motor  254  (independent driving unit) which independently drives the pre-reversal double-diameter cylinder  39 , and a prime motor  255  (device driving unit) which drives the entire printing press. The prime motor  255  interlocks and drives the driving system for the printing press other than the pre-reversal double-diameter cylinder  39 . 
     &lt;Printing Operation of Digital Printing Apparatus&gt; 
     The printing operation of the digital printing apparatus  1  configured as mentioned above will be described separately for the case wherein the single-sided printing mode is selected and that wherein the double-sided printing mode is selected. 
     When the single-sided printing mode is selected by operating a printing mode selection switch  80  by the operator, the continuous supply valve  26  is actuated. With this operation, the suction ports  23   a  and  23   b  suck the sheet S 1  on the pile board  21 , and convey it onto the feeder board FB, as shown in  FIG. 1 . When the single-sided printing mode is selected, the independent driving motor is controlled by the control unit  251  to stop the rotation of the pre-reversal double-diameter cylinder  39 . This suppresses wasteful power consumption to allow energy saving. 
     The continuous supply valve  26  opens every time the same number of sheets S 1  as the numbers of printing cylinder gripper devices  33   a ,  33   b , and  33   c  of the printing cylinder  33  are supplied during 360° rotation of the printing cylinder  33 , that is, at each timing (period) at which the printing cylinder gripper devices  33   a ,  33   b , and  33   c  in the printing cylinder  33 , and the gripper device  32   a  of the feed-side transfer cylinder  32  are opposed to each other. As the continuous supply valve  26  opens, a negative pressure is supplied from the negative pressure source  25  to the suction ports  23   a  and  23   b  to perform suction. Supply of the sheets S 1  so that all the printing cylinder gripper devices  33   a ,  33   b , and  33   c  of the printing cylinder  33  grip the sheets S 1  will be referred to as continuous sheet feed hereinafter. Also, the period at which the continuous supply valve  26  opens/closes in continuous sheet feed will be referred to as a first period hereinafter. With this operation, the sucker device  23  conveys the sheets S 1  onto the feeder board FB at the first period. 
     The leading edge of the sheet S 1  conveyed by the feeder board FB is held by the gripper device of the swing arm shaft pregripper  31   f , and the sheet S 1  is conveyed onto the feed-side transfer cylinder  32  upon a swing of the swing arm shaft pregripper  31   f . The leading edge of the sheet S 1  conveyed onto the feed-side transfer cylinder  32  is transferred by a gripping change to the gripper device  32   a  of the feed-side transfer cylinder  32 . 
     The leading edge of the sheet S 1  conveyed with rotation of the feed-side transfer cylinder  32  is transferred by a gripping change from the gripper device  32   a  of the feed-side transfer cylinder  32  to one of the printing cylinder gripper devices  33   a ,  33   b , and  33   c  of the printing cylinder  33 , and the sheet S 1  is conveyed with rotation of the printing cylinder  33 . In the printing cylinder  33 , a suction force acts on suction holes  33   g  on the downstream side in the rotation direction from a suction start position  33   i , so the entire surface of the sheet S 1  is sucked to and brought into tight contact with the support surfaces  33   d ,  33   e , and  33   f  as the sheet S 1  passes through the suction start position  33   i.    
     A digital printing process is performed on the obverse surface of the sheet S 1  conveyed by the printing cylinder  33  by discharging minute drops of ink from the ink heads  34   a  to  34   d  of the inkjet nozzle portion  34 . The sheet S 1  is in tight contact with the support surface of the printing cylinder  33 , and is therefore conveyed while minute intervals with the ink heads  34   a  to  34   d  are maintained. Ink discharged while these minute intervals are maintained can be adhered to the sheet S 1  with high accuracy, thereby allowing high-quality printing. 
     The ink on the sheet S 1  printed by the inkjet nozzle portion  34  dries with light emitted by the ink drying lamp  35  when the sheet S 1  passes between the printing cylinder  33  and the ink drying lamp  35 . The sheet S 1  is then conveyed onto the delivery-side transfer cylinder  36 . 
     In the contact portion between the printing cylinder  33  and the delivery-side transfer cylinder  36 , the leading edge of the sheet S 1  is transferred by a gripping change from the printing cylinder gripper devices  33   a  to  33   c  of the printing cylinder  33  to the gripper device  36   a  of the delivery-side transfer cylinder  36 , as shown in  FIG. 5A . At this time, the leading edge of the sheet S 1  passes through a suction end position  33   j , so no suction force acts from the suction holes  33   g . This makes it possible to easily peel the sheet S 1  off the support surfaces  33   d ,  33   e , and  33   f  to allow a smooth gripping change. Then, the leading edge of the sheet S 1  held by the gripper device  36   a  of the delivery-side transfer cylinder  36  is transferred by a gripping change from the gripper device  36   a  of the delivery-side transfer cylinder  36  to the gripper device  37   a  of the delivery-side transfer cylinder  37  in the contact portion between the delivery-side transfer cylinders  36  and  37 , as shown in  FIG. 5B . 
     In the single-sided printing mode, in the phase in which the leading edge of the sheet S 1  is positioned in the contact portion between the delivery-side transfer cylinders  37  and  38 , the gripper device  37   a  of the delivery-side transfer cylinder  37  cancels holding of the leading edge of the sheet S 1 , and the gripper device  38   a  of the delivery cylinder  38  grips and holds the leading edge of the sheet S 1  at the same time. With this operation, the sheet S 1  printed on its one surface is transferred from the delivery-side transfer cylinder  37  onto the delivery cylinder  38 , and conveyed. 
     Holding, by the gripper device  38   a , of the sheet S 1  transferred onto the delivery cylinder  38  is canceled at the timing at which the gripper device  38   a  of the delivery cylinder  38  is positioned above the delivery belt  40 , and is placed on the delivery belt  40 . 
     The sheet S 1  placed on the delivery belt  40  is conveyed as the delivery belt  40  travels, and the sheet S 1  having undergone a digital printing process on its obverse surface is discharged onto the pile board  41  of the sheet delivery device  4 . 
     In the single-sided printing mode, all sheets S 1  are switched to the sheet discharge path, so no sheet S 1  is conveyed to either the pre-reversal double-diameter cylinder  39  or reversing swing arm shaft pregripper  31   b . Further, in the single-sided printing mode, the pre-reversal double-diameter cylinder  39  is kept stopped without rotation, and the delivery-side transfer cylinder  37  and reversing swing arm shaft pregripper  31   b  provided on the upstream and downstream sides of the pre-reversal double-diameter cylinder  39  operate, but the recessed portions  39   b  in the pre-reversal double-diameter cylinder  39  are opposed to the gripper device  37   a  of the delivery-side transfer cylinder  37 , and the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b , so the gripper devices  37   a  and  31   bt  do not interfere with the pre-reversal double-diameter cylinder  39 . 
     On the other hand, when the double-sided printing mode is selected by an operation input to the single-/double-sided printing mode input unit  253 , the operator inputs the dimension of the sheet S 1  in the conveyance direction to the sheet size input unit  252 . When a printing operation starts, the control unit  251  actuates the intermittent supply valve  27  to make the suction ports  23   a  and  23   b  suck and convey the sheet S 1  on the pile board  21  onto the feeder board FB. 
     The intermittent supply valve  27  is controlled at the timing at which the sheets S 1  are alternately supplied so as to open, close, open, close, . . . , at the timing of continuous supply, that is, the timing at which the printing cylinder gripper devices  33   a ,  33   b , and  33   c  of the printing cylinder  33 , and the gripper device  32   a  of the feed-side transfer cylinder  32  are opposed to each other. This period is twice that of continuous supply. In this manner, supply of the sheets S 1  so that the printing cylinder gripper devices  33   a ,  33   b , and  33   c  of the printing cylinder  33  alternately grip the sheets S 1  will be referred to as intermittent sheet feed hereinafter, and the period at which the intermittent supply valve  27  opens/closes in intermittent sheet feed will be referred to as a second period hereinafter. With this operation, the sucker device  23  conveys the sheets S 1  onto the feeder board FB at the second period. 
     The sheet S 1  fed onto the feeder board FB by the sucker device  23  is transferred onto the printing cylinder  33  through the swing arm shaft pregripper  31   f  and feed-side transfer cylinder  32  in the same way as in the single-sided printing mode. At this time, since the sheet S 1  is fed at the timing of intermittent sheet feed, the printing cylinder gripper devices  33   a  to  33   c  of the printing cylinder  33  receive the sheet S 1  alternately conveyed from the feed-side transfer cylinder  32 . 
     The sheet S 1  transferred onto the printing cylinder  33  is conveyed to the inkjet nozzle portion  34 , and obverse surface printing is performed on one surface (obverse surface). The control unit  251  prints on the sheet S 1  alternately held by the printing cylinder gripper devices  33   a  to  33   c  of the printing cylinder  33 , based on a phase signal from the rotary encoder  84 . On the other hand, the ink heads  34   a  to  34   d  of the inkjet nozzle portion  34  are controlled so as not to print on the support surfaces  33   d  to  33   f  corresponding to the printing cylinder gripper devices  33   a  to  33   c  which do not hold the sheet S 1 . 
     For double-sided printing, the control unit  251  controls the conveyance path switching device  82  so that the sheet S 1  printed on its obverse surface by the inkjet nozzle portion  34  is transferred onto the pre-reversal double-diameter cylinder  39  without transferring it from the delivery-side transfer cylinder  37  onto the delivery cylinder  38 . 
     More specifically, in conveyance path switching control, in the phase in which the sheet S 1  which is printed on its obverse surface and has undergone no digital print process on its other surface (reverse surface) is positioned in the contact portion between the delivery-side transfer cylinder  37  and the delivery cylinder  38 , the grippers of the gripper device  37   a  of the delivery-side transfer cylinder  37  are kept closed without opening to maintain the state in which the gripper device  37   a  holds the leading edge of the sheet S 1 . At this time, the grippers of the gripper device  38   a  of the delivery cylinder  38  are kept open without closing. With this operation, the sheet S 1  printed only on its obverse surface continues to be conveyed by the delivery-side transfer cylinder  37  without a gripping change from the delivery-side transfer cylinder  37  to the delivery cylinder  38 . 
     The leading edge of the sheet S 1  conveyed by the delivery-side transfer cylinder  37  is held by closing the grippers of the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  in the contact portion between the delivery-side transfer cylinder  37  and the pre-reversal double-diameter cylinder  39 . At the same time, holding of the leading edge of the sheet S 1  is canceled by opening the grippers of the gripper device  37   a  of the delivery-side transfer cylinder  37 . With this operation, the leading edge of the sheet S 1  is transferred by a gripping change from the gripper device  37   a  of the delivery-side transfer cylinder  37  to the gripper device  39   a  of the pre-reversal double-diameter cylinder  39 , as shown in  FIG. 5C . 
     At this time, since the groove-shaped recessed portions  39   b  ( FIG. 3 ) are formed in the circumferential surface  39   c  of the pre-reversal double-diameter cylinder  39  to be opposed to the gripper device  37   a  of the delivery-side transfer cylinder  37 , the gripper device  37   a  of the delivery-side transfer cylinder  37  passes through the grooves of the recessed portions  39   b  to prevent the circumferential surface of the pre-reversal double-diameter cylinder  39  from suffering damage. 
     The sheet S 1  conveyed with rotation of the pre-reversal double-diameter cylinder  39  is conveyed with rotation of the pre-reversal double-diameter cylinder  39 , as shown in  FIG. 5D . The reversing swing arm shaft pregripper  31   b  swings from a transfer position (solid line) to a reception position (broken line) to make the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  hold the trailing edge of the sheet S 1 , and holding of the leading edge of the sheet S 1  by the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  is canceled at the same time. With this operation, the sheet S 1  is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b.    
     At this time, since the groove-shaped recessed portions  39   b  ( FIG. 3 ) are formed in the circumferential surface of the pre-reversal double-diameter cylinder  39  to be opposed to the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b , the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  passes through the grooves of the recessed portions  39   b  to prevent the circumferential surface of the pre-reversal double-diameter cylinder  39  from suffering damage. 
     An operation in which the pre-reversal double-diameter cylinder  39  receives the sheet S 1  from the delivery-side transfer cylinder  37 , and transfers it to the reversing swing arm shaft pregripper  31   b  (driving control of the pre-reversal double-diameter cylinder  39 ) will be described in detail. If the dimension in the conveyance direction, which is input to the sheet size input unit  252 , is a standard size (middle-sized paper), the control unit  251  controls the independent driving motor  254  to rotate the pre-reversal double-diameter cylinder  39  at a reference speed. The reference speed means the rotation speed at which the pre-reversal double-diameter cylinder  39  rotates at a peripheral speed equal to those of the printing cylinder  33  and delivery-side transfer cylinder  37 . The pre-reversal double-diameter cylinder  39  rotates at the reference speed with no difference in peripheral speed between the printing cylinder  33  and the delivery-side transfer cylinder  37 . 
     The control operation of the rotation speed of the pre-reversal double-diameter cylinder  39  by the control unit  251  will be described with reference to  FIG. 6 .  FIG. 6  shows the rotation speed of the pre-reversal double-diameter cylinder  39  when the digital printing apparatus  1  operates at a steady speed, that is, the printing cylinder  33  and delivery-side transfer cylinder  37  rotate at a constant speed.  FIG. 6  shows the time or the phase of the digital printing apparatus  1  on the abscissa, and the rotation speed of the pre-reversal double-diameter cylinder  39  on the ordinate. Note that t 0  is the reception timing at which the leading edge of the sheet S 1  is transferred by a gripping change from the delivery-side transfer cylinder  37  to the pre-reversal double-diameter cylinder  39 , t 1  is the first adjustment start timing of the rotation speed of the pre-reversal double-diameter cylinder  39 , t 2  is the first adjustment end timing of the rotation speed of the pre-reversal double-diameter cylinder  39 , and t 3  is the transfer timing at which the trailing edge of the sheet S 1  is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b . Also, t 4  is the second adjustment start timing of the rotation speed of the pre-reversal double-diameter cylinder  39 , t 5  is the second adjustment end timing of the rotation speed of the pre-reversal double-diameter cylinder  39 , and t 6  is the reception timing at which the leading edge of the sheet S 1  is transferred by a gripping change from the delivery-side transfer cylinder  37  to the pre-reversal double-diameter cylinder  39  again. 
     Note that the above-mentioned timings t 0  to t 6  indicate the times or the phases of the digital printing apparatus  1 , and reception timings t 6  and t 0  are identical when the timing is represented as a phase. Also, the interval from first adjustment start timing t 1  to first adjustment end timing t 2  is defined as a first speed adjustment region, and that from second adjustment start timing t 4  to second adjustment end timing t 5  is defined as a second speed adjustment region. 
     If the dimension of the sheet S 1  in the conveyance direction is a standard size (middle-sized paper), when the digital printing apparatus  1  operates at a steady speed, the pre-reversal double-diameter cylinder  39  is rotated by the independent driving motor  254  at a constant speed v 0  (reference speed) with no change in speed from reception timing t 0  to reception timing t 6 , as indicated by bold lines in  FIG. 6 . The pre-reversal double-diameter cylinder  39  must be rotated at a peripheral speed equal to those of the printing cylinder  33  and delivery-side transfer cylinder  37 . Hence, when the digital printing apparatus  1  operates at a steady speed, the printing cylinder  33  and delivery-side transfer cylinder  37  are driven by the prime motor  255 , while the pre-reversal double-diameter cylinder  39  is rotated at a constant speed v 0  by the independent driving motor  254 . However, when the digital printing apparatus  1  does not operate at a steady speed, the pre-reversal double-diameter cylinder  39  is rotated by the independent driving motor  254  at a peripheral speed which is equal to those of the printing cylinder  33  and delivery-side transfer cylinder  37  and different from the reference speed. 
     Upon this operation, at reception timing t 0 , the gripper device  37   a  of the delivery-side transfer cylinder  37 , and the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  are opposed to each other, so the leading edge of a sheet S 1  with the standard size is transferred by a gripping change, and the sheet S 1  is wound around the circumferential surface  39   c  of the pre-reversal double-diameter cylinder  39  and conveyed, as shown in  FIG. 2 . 
     At transfer timing t 3 , the trailing edge of the sheet S 1  which has the standard size and is conveyed by the pre-reversal double-diameter cylinder  39  rotated at the constant speed v 0  is opposed to the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  at a predetermined period at which the reversing swing arm shaft pregripper  31   b  is set at the reception position, as shown in  FIG. 7 . 
     As the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  grips the trailing edge of the sheet S 1 , and the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  cancels holding of the leading edge of the sheet S 1 , the sheet S 1  is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b . The reversing swing arm shaft pregripper  31   b  then swings from the reception position to the transfer position, and transfers the turned sheet S 1  onto the printing cylinder  33 , as shown in  FIG. 8 . 
     As described above, if the sheet S 1  has the standard size, the control unit  251  controls the pre-reversal double-diameter cylinder  39  to simply rotate at the reference speed through the independent driving motor  254 , so no change in speed with respect to the reference speed occurs. 
     Control if the dimension of the sheet S 1  in the conveyance direction is larger than the standard size, as shown in  FIG. 8 , will be described next. The case of a sheet S 1   a  (maximum-sized paper) with a maximum dimension in the conveyance direction, that the digital printing apparatus  1  can print, will be explained. At reception timing t 0 , transfer timing t 3 , and reception timing t 6 , the control unit  251  rotates the pre-reversal double-diameter cylinder  39  at a reference speed (speed v 0 ) equal to that in the case of the sheet S 1  with the standard size (middle-sized paper), as indicated by solid lines in  FIG. 6 . On the other hand, in the first speed adjustment region, the speed of the pre-reversal double-diameter cylinder  39  is controlled to gradually increase with respect to the reference speed from first adjustment start timing t 1 , and return to the reference speed at first adjustment end timing t 2 . Then, in the second speed adjustment region, the speed of the pre-reversal double-diameter cylinder  39  is controlled to gradually decrease with respect to the reference speed from second adjustment start timing t 4 , and return to the reference speed at second adjustment end timing t 5 . Note that the control unit  251  rotates the pre-reversal double-diameter cylinder  39  at the reference speed (speed v 0 ) in the interval from reception timing t 0  to first adjustment start timing t 1 , that from first adjustment end timing t 2  to second adjustment start timing t 4 , and that from second adjustment end timing t 5  to reception timing t 6 . 
     In this case, at reception timing t 0  and transfer timing t 3 , the pre-reversal double-diameter cylinder  39  receives the sheet S 1   a  from the delivery-side transfer cylinder  37  and transfers it to the reversing swing arm shaft pregripper  31   b  while rotating at the reference speed. This allows a reliable gripping change of the sheet S 1   a.    
     Normally, when maximum-sized paper with a large size is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b , the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  is set at a position, indicated by a broken line in  FIG. 8 , at transfer timing t 3  as the pre-reversal double-diameter cylinder  39  rotates while its rotation speed is kept at the constant speed v 0  (reference speed). In this case, the trailing edge of the sheet S 1   a  has not yet reached the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  set at the reception position, and therefore cannot be gripped by the reversing gripper device  31   bt.    
     However, in the arrangement of this embodiment, in the first speed adjustment region, the pre-reversal double-diameter cylinder  39  is accelerated from the reference speed to advance the phase of the pre-reversal double-diameter cylinder  39  more than that of the digital printing apparatus  1 , thereby setting the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  at a position, indicated by a solid line in  FIG. 8 , at transfer timing t 3 . Upon this operation, the trailing edge of the sheet S 1   a  is opposed to the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  set at the reception position. 
     By controlling the speed of the pre-reversal double-diameter cylinder  39  in this way, the trailing edge of the sheet S 1   a  is gripped by the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b , and holding of the leading edge of the sheet S 1   a  is canceled by the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  at the same time. With this operation, the sheet S 1   a  is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b . The reversing swing arm shaft pregripper  31   b  then swings from the reception position to the transfer position, and transfers the turned sheet S 1   a  onto the printing cylinder  33 , as shown in  FIG. 5E . 
     After the trailing edge of the sheet S 1   a  (maximum-sized paper) is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b  at transfer timing t 3 , the control unit  251  rotates the pre-reversal double-diameter cylinder  39  at the speed v 0  (reference speed). Then, in the second speed adjustment region, the pre-reversal double-diameter cylinder  39  is decelerated from the reference speed to retard the phase of the pre-reversal double-diameter cylinder  39 , which has advanced more than that of the digital printing apparatus  1 . Upon such phase control, at reception timing t 6 , the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  is opposed to the gripper device  37   a  of the delivery-side transfer cylinder  37 , as shown in  FIG. 2 . With this operation, the leading edge of the sheet S 1   a  is transferred by a gripping change from the gripper device  37   a  of the delivery-side transfer cylinder  37  to the gripper device  39   a  of the pre-reversal double-diameter cylinder  39 . 
     With this arrangement, the control unit  251  increases/decreases the rotation speed of the pre-reversal double-diameter cylinder  39  to control (adjust) the phase of the pre-reversal double-diameter cylinder  39  relative to that of the digital printing apparatus  1  in the first and second speed adjustment regions, that do not influence reception timing t 0 , transfer timing t 3 , and reception timing t 6  at which a gripping change of the sheet S 1   a  (maximum-sized paper) is done. 
     As described above, even if a sheet S 1   a  with a dimension in the conveyance direction, which is larger than the standard size, is used, the leading edge of the sheet S 1   a  can reliably be transferred by a gripping change from the delivery-side transfer cylinder  37  to the pre-reversal double-diameter cylinder  39  by increasing/decreasing the rotation speed of the pre-reversal double-diameter cylinder  39 . Also, the trailing edge of the sheet S 1   a  can reliably be transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b.    
     The case wherein the dimension of the sheet S 1  in the conveyance direction is smaller than the standard size, as shown in  FIG. 9 , will be described next. The case of a sheet S 1   b  (minimum-sized paper) with a minimum dimension in the conveyance direction, that the digital printing apparatus  1  can print, will be explained. At reception timing t 0 , transfer timing t 3 , and reception timing t 6 , the control unit  251  rotates the pre-reversal double-diameter cylinder  39  at a reference speed (speed v 0 ) equal to that in the case of the sheet S 1  with the standard size (middle-sized paper), as indicated by broken lines in  FIG. 6 . On the other hand, in the first speed adjustment region, the speed of the pre-reversal double-diameter cylinder  39  is controlled to gradually decrease with respect to the reference speed from first adjustment start timing t 1 , and return to the reference speed at first adjustment end timing t 2 . Then, in the second speed adjustment region, the speed of the pre-reversal double-diameter cylinder  39  is controlled to gradually increase with respect to the reference speed from second adjustment start timing t 4 , and return to the reference speed at second adjustment end timing t 5 . Note that the control unit  251  rotates the pre-reversal double-diameter cylinder  39  at the reference speed (speed v 0 ) in the interval from reception timing t 0  to first adjustment start timing t 1 , that from first adjustment end timing t 2  to second adjustment start timing t 4 , and that from second adjustment end timing t 5  to reception timing t 6 . 
     In this case, at reception timing t 0  and transfer timing t 3 , the pre-reversal double-diameter cylinder  39  performs reception and transfer operations while rotating at the reference speed, thus allowing a reliable gripping change of the sheet S 1   b.    
     Normally, when minimum-sized paper with a small size is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b , the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  is set at a position, indicated by a broken line in  FIG. 9 , at transfer timing t 3  as the pre-reversal double-diameter cylinder  39  rotates while its rotation speed is kept at the constant speed v 0  (reference speed). In this case, the trailing edge of the sheet S 1   b  has already passed through the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  set at the reception position, and therefore cannot be gripped by the swing arm gripper  203 . 
     However, in the arrangement of this embodiment, in the first speed adjustment region, the pre-reversal double-diameter cylinder  39  is decelerated from the reference speed to retard the phase of the pre-reversal double-diameter cylinder  39  more than that of the digital printing apparatus  1 , thereby setting the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  at a position, indicated by a solid line in  FIG. 9 , at transfer timing t 3 . Upon this operation, the trailing edge of the sheet S 1   b  is opposed to the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  set at the reception position. 
     By controlling the speed of the pre-reversal double-diameter cylinder  39  in this way, the trailing edge of the sheet S 1   b  is gripped by the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b , and holding of the leading edge of the sheet S 1   b  is canceled by the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  at the same time. With this operation, the sheet S 1   b  is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b . The reversing swing arm shaft pregripper  31   b  then swings from the reception position to the transfer position, and transfers the turned sheet S 1   b  onto the printing cylinder  33 , as shown in  FIG. 5E . 
     After the trailing edge of the sheet S 1   a  (minimum-sized paper) is transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b  at transfer timing t 3 , the control unit  251  rotates the pre-reversal double-diameter cylinder  39  at the speed v 0  (reference speed). Then, in the second speed adjustment region, the pre-reversal double-diameter cylinder  39  is accelerated from the reference speed to advance the phase of the pre-reversal double-diameter cylinder  39 , which has retarded more than that of the digital printing apparatus  1 . Upon such phase control, at reception timing t 6 , the gripper device  39   a  of the pre-reversal double-diameter cylinder  39  is opposed to the gripper device  37   a  of the delivery-side transfer cylinder  37 , as shown in  FIG. 2 . With this operation, the leading edge of the sheet S 1   b  is transferred by a gripping change from the gripper device  37   a  of the delivery-side transfer cylinder  37  to the gripper device  39   a  of the pre-reversal double-diameter cylinder  39 . 
     With this arrangement, the control unit  251  increases/decreases the rotation speed of the pre-reversal double-diameter cylinder  39  to control (adjust) the phase of the pre-reversal double-diameter cylinder  39  relative to that of the digital printing apparatus  1  in the first and second speed adjustment regions, that do not influence reception timing t 0 , transfer timing t 3 , and reception timing t 6  at which a gripping change of the sheet S 1   b  (minimum-sized paper) is done. 
     As described above, even if a sheet S 1   b  with a dimension in the conveyance direction, which is smaller than the standard size, is used, the leading edge of the sheet S 1   b  can reliably be transferred by a gripping change from the delivery-side transfer cylinder  37  to the pre-reversal double-diameter cylinder  39  by increasing/decreasing the rotation speed of the pre-reversal double-diameter cylinder  39 . Also, the trailing edge of the sheet S 1   b  can reliably be transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b.    
     Then, as shown in  FIG. 10 , as the reversing swing arm shaft pregripper  31   b  swings from a reception position indicated by a broken line to a transfer position indicated by a solid line, the sheet S 1  (sheet S 1 , S 1   a , or S 1   b ) with its trailing edge leading is conveyed onto the printing cylinder  33 . At this time, the trailing edge of the turned sheet S 1  is transferred by a gripping change from the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  to one of the gripper devices  33   a  to  33   c.    
     The gripper devices  33   a  to  33   c  of the printing cylinder  33  alternately hold a new sheet S 1  conveyed from the feed-side transfer cylinder  32 . The reversing swing arm shaft pregripper  31   b  is positioned at the transfer position at the timing at which it is opposed to the printing cylinder gripper devices  33   a  to  33   c  which hold no new sheet S 1 , and the trailing edge of the sheet S 1  is transferred from the reversing gripper device  31   bt  to the printing cylinder gripper devices  33   a  to  33   c . With this operation, a new sheet S 1  transferred from the feed-side transfer cylinder  32 , and a turned sheet S 1  transferred from the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  are alternately held by the printing cylinder gripper devices  33   a  to  33   c  of the printing cylinder  33 , and are conveyed to the inkjet nozzle portion  34 . 
     The trailing edge of the turned sheet S 1  transferred from the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b  is held and conveyed by the gripper devices  33   a  to  33   c  of the printing cylinder  33  while the surface (the obverse surface having undergone a digital printing process) of the sheet S 1 , which has already undergone a digital printing process by the inkjet nozzle portion  34 , is in contact with the support surfaces  33   d ,  33   e , and  33   f  of the printing cylinder  33 , and the surface (the reverse surface having undergone no digital printing process) of the sheet S 1 , which has not yet undergone a digital printing process, is exposed. The inkjet nozzle portion  34  performs a digital printing process on the reverse surface of the sheet S 1  conveyed in tight contact with the circumferential surface of the printing cylinder  33  in a turned state. 
     The control unit  251  controls the inkjet nozzle heads  34   a  to  34   d  of the inkjet nozzle portion  34  to perform reverse printing on the turned sheet S 1  transferred from the reversing gripper device  31   bt  of the reversing swing arm shaft pregripper  31   b , and perform obverse printing on the new sheet S 1  alternately held by the printing cylinder gripper devices  33   a  to  33   c  of the printing cylinder  33 . With this operation, the inkjet nozzle heads  34   a  to  34   d  alternately perform obverse printing and reverse printing in correspondence with the new sheet S 1  and turned sheet S 1  alternately held by the printing cylinder  33 . 
     The sheet S 1  having undergone reverse printing on its reverse surface is discharged from the delivery belt  40  onto the pile board  41  sequentially through the delivery-side transfer cylinders  36  and  37 , and delivery cylinder  38 , as in the single-sided printing mode. 
     According to this embodiment, even if a sheet S 1   a  or S 1   b  with a dimension in the sheet conveyance direction, which is larger or smaller than that of the standard size (middle-sized paper), is used, the independent driving motor  254  is controlled to increase/decrease (adjust) the rotation speed of the pre-reversal double-diameter cylinder  39  based on the dimension in the sheet conveyance direction. It is therefore possible to reliably receive the leading edge of the sheet S 1  from the delivery-side transfer cylinder  37  to the pre-reversal double-diameter cylinder  39 , and transfer the trailing edge of the sheet S 1  from the pre-reversal double-diameter cylinder  39  to the reversing swing arm shaft pregripper  31   b . This obviates the need for mechanical adjustment that accompanies a change in sheet size to relieve the operator&#39;s burden. This also obviates the need for a preparatory operation to improve the productivity. 
     Also, the sheet S 1  is sequentially transferred to the feed-side transfer cylinder  32 , printing cylinder  33 , delivery-side transfer cylinders  36  and  37 , pre-reversal double-diameter cylinder  39 , and reversing swing arm shaft pregripper  31   b  by a gripping change by the gripper devices. This makes it possible to obtain high registration accuracy and high obverse/reverse registration accuracy of the obverse and reverse surfaces of the sheet S 1  in the conveyance direction or widthwise direction of the sheet S 1 , thus improving the printing quality of the sheet S 1 . 
     (2) Second Embodiment 
     The second embodiment is the same as the first embodiment except for the configuration of the control block of the digital printing apparatus  1 . Only a control block of a digital printing apparatus  200  according to the second embodiment will be described below. 
     &lt;Configuration of Control System for Digital Printing Apparatus&gt; 
     The digital printing apparatus  200  includes a control unit  351  having a CPU configuration which controls the overall printing operation, as shown in  FIG. 10 . The control unit  351  is connected to a sheet size input unit  252  which receives the sheet size as standard information, a sheet size error detection unit  255  which includes a photoelectric sensor arranged near a printing cylinder  33 , a single-/double-sided printing mode input unit  253  which selects a single- or double-sided printing mode, an independent driving motor  254 , and a prime motor  255 . The sheet size error detection unit  255  detects an error of the sheet size, that is, the dimension in the conveyance direction, which is actually printed for standard data input via the sheet size input unit  252 . 
     The control unit  351  receives signals output from the sheet size input unit  252 , sheet size error detection unit  255 , and single-/double-sided printing mode input unit  253  to control the independent driving motor  254 . Differences from the first embodiment lie in that the sheet size input unit  252  receives the sheet size as standard information, and the sheet size error detection unit  255  is provided. 
     &lt;Operation of Adjusting Rotation Speed of Pre-Reversal Double-Diameter Cylinder&gt; 
     The control unit  351  recognizes the sheet S 1  as one of a sheet S 1  with a standard size (middle-sized paper), a sheet S 1   a  (maximum-sized paper) with a large dimension in the conveyance direction, and a sheet S 1   b  (minimum-sized paper) with a small dimension in the conveyance direction, based on the standard information (middle-sized paper, maximum-sized paper, or minimum-sized paper) of the sheet S 1  input to the sheet size input unit  252 . 
     The sheet size error detection unit  255  detects errors of the sheet sizes (sheet conveyance direction) for three types of standard information for the first sheet S 1  (middle-sized paper), sheet S 1   a  (maximum-sized paper), or sheet S 1   b  (minimum-sized paper) supplied for each lot, and sends these errors to the control unit  351 . The control unit  351  adds/subtracts one (error data corresponding to input standard information) of three types of error data input from the sheet size error detection unit  255  to/from standard information (one of middle-sized paper, maximum-sized paper, and minimum-sized paper), and determines the actual size of the sheet. The control unit  351  controls driving of the independent driving motor  254  to increase/decrease the rotation speed of a pre-reversal double-diameter cylinder  39  based on the obtained actual size of the sheet. 
     With this operation, the leading edge of the sheet S 1   a  from a delivery-side transfer cylinder  37  can reliably be received by a gripping change by the pre-reversal double-diameter cylinder  39 , regardless of the sheet size. Also, the trailing edge of the sheet S 1   a  can reliably be transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to a reversing swing arm shaft pregripper  31   b.    
     Note that by sending, in advance, standard information input from the control unit  351  to the sheet size input unit  252 , the sheet size error detection unit  255  may detect only error information for the sent standard information and output it to the control unit  351 . 
     (3) Third Embodiment 
     Only a control block of a digital printing apparatus  300  according to the third embodiment will be described below. 
     &lt;Configuration of Control System for Digital Printing Apparatus&gt; 
     The digital printing apparatus  300  includes a control unit  451  having a CPU configuration which controls the overall printing operation, as shown in  FIG. 11 . The control unit  451  is connected to a sheet size detection unit  257  arranged near a printing cylinder  33 , a single-/double-sided printing mode input unit  253  which selects a single- or double-sided printing mode, an independent driving motor  254 , and a prime motor  255 . The sheet size detection unit  257  detects the dimension in the conveyance direction (size). A difference from the first embodiment lies in that the sheet size detection unit  257  is provided in place of the sheet size input unit  257 . 
     &lt;Operation of Adjusting Rotation Speed of Pre-Reversal Double-diameter Cylinder&gt; 
     The sheet size detection unit  257  detects the dimension, in the conveyance direction, of a sheet S 1  conveyed by a pre-reversal double-diameter cylinder  39 , and outputs it to the control unit  451 . The control unit  451  recognizes the dimension of each sheet S 1  in the conveyance direction based on the output from the sheet size detection unit  257 . The control unit  451  controls the independent driving motor  254  to increase/decrease the rotation speed of the pre-reversal double-diameter cylinder  39  based on the measurement data of the sheet S 1  detected by the sheet size detection unit  257 , that is, the actual size of the sheet S 1 . With this operation, the leading edge of the sheet S 1  from a delivery-side transfer cylinder  37  can reliably be received by a gripping change by the pre-reversal double-diameter cylinder  39 , regardless of the sheet size. Also, the trailing edge of the sheet S 1  can reliably be transferred by a gripping change from the pre-reversal double-diameter cylinder  39  to a reversing swing arm shaft pregripper  31   b.    
     (4) Other Embodiments 
     Although a sheet conveyance device is applied to the digital printing apparatus  1  (sheet processing apparatus) in the above-mentioned embodiment, the present invention is not limited to this. The sheet conveyance device according to the present invention may also be applied to, for example, an offset print process apparatus, inspection process apparatus, foil transfer process apparatus, and embossing process apparatus as other sheet processing apparatuses. 
     Also, assuming that a sheet S 1  (middle-sized paper) has a standard size, the rotation speed of the pre-reversal double-diameter cylinder  39  is increased/decreased when sheets S 1   a  and S 1   b  with sizes in the sheet conveyance direction, which are larger and smaller than the standard size, are conveyed. The present invention is not limited to this, and assuming that a sheet S 1   a  with a maximum dimension in the conveyance direction has a standard size, the rotation speed of the pre-reversal double-diameter cylinder  39  may be adjusted when a sheet with a dimension in the sheet conveyance direction, which is smaller than the standard size, is conveyed. Also, assuming that a sheet S 1   b  with a minimum dimension in the conveyance direction has a standard size, the rotation speed of the pre-reversal double-diameter cylinder  39  may be adjusted when a sheet with a dimension in the sheet conveyance direction, which is larger than the standard size, is conveyed. 
     Moreover, although the printing cylinder  33  implemented by a triple-diameter cylinder is used in the above-mentioned embodiments, the present invention is not limited to this, and a printing cylinder implemented by a double-, quadrupole- or sextuple-diameter cylinder may be used.