Printing apparatus

A printing apparatus includes a first plate cylinder and a second plate cylinder. Each plate cylinder supports a plate mounted peripherally thereof having two image areas. Four ink feeders contact and feed inks to the image areas on the plate when mounted on the first and second plate cylinders. A first blanket cylinder has a diameter equal to a diameter of the first plate cylinder and is contactable therewith. A second blanket cylinder has a diameter equal to a diameter of the second plate cylinder and is contactable therewith. An impression cylinder having a diameter half the diameter of either the first and second plate cylinders as well as the first and second blanket cylinders, is contactable with the first and second blanket cylinders. A pair of grippers that each hold an end of the printing paper maintain two sheets of printing paper peripherally of the impression cylinder. A paper feed cylinder feeds the printing paper to the impression cylinder. A paper discharge cylinder discharges the printing paper from the impression cylinder after a required printing process. A gripper control mechanism controls the grippers to selectively carry out either one of a first opening and closing operation once for every rotation of the impression cylinder and a second opening and closing operation once for every two rotations of the impression cylinder.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a printing apparatus for feeding ink to
 plates mounted peripherally of a plate cylinder, and transferring the ink
 through a blanket cylinder to printing paper wrapped around an impression
 cylinder.
 2. Description of the Related Art
 A known printing apparatus of the type noted above includes a plate
 cylinder for supporting a plurality of plates mounted peripherally
 thereof, a plurality of ink feeders for feeding different color inks to
 image areas on the plurality of plates mounted on the plate cylinder,
 respectively, and a single impression cylinder. Such a printing apparatus
 is capable of color printing by using the single plate cylinder and the
 single impression cylinder. Thus, the entire apparatus requires a reduced
 area for its installation and operation.
 Japanese Patent Publication (Unexamined) H3-143634 (1991), for example,
 describes a printing apparatus for printing images on printing paper with
 two color inks. This apparatus includes a plate cylinder for supporting
 two plates mounted peripherally thereof, two ink feeders for feeding the
 different inks to the respective plates on the plate cylinder, a single
 dampening water feeder for feeding dampening water to each plate on the
 plate cylinder, and an impression cylinder having half a diameter of the
 plate cylinder. A sheet of printing paper is fed for every two rotations
 of the impression cylinder.
 However, the printing apparatus described in the above patent publication
 always prints with two color inks, and is incapable of monochromatic
 printing.
 SUMMARY OF THE INVENTION
 An object of the present invention, therefore, is to provide a printing
 apparatus with a plate cylinder for supporting a plate mounted
 peripherally thereof, the plate having two image areas, for allowing
 images to be printed selectively with a single color ink and with two
 color inks.
 The above object is fulfilled, according to the present invention, by a
 printing apparatus comprises a plate cylinder for supporting a plate as
 mounted peripherally thereof, the plate having two image areas, two ink
 feeders for contacting and feeding inks to the image areas on the plate
 mounted on the plate cylinder; a blanket cylinder equal in diameter to and
 contactable with the plate cylinder; an impression cylinder having a
 diameter n/2 (n being an odd number) of the diameter of the plate cylinder
 and the blanket cylinder, and contactable with the blanket cylinder; n
 printing paper holder(s) (each) having a gripper for holding an end of the
 printing paper to maintain n sheet(s) of the printing paper peripherally
 of the impression cylinder; a printing paper feeder for feeding the
 printing paper to the impression cylinder; a printing paper discharger for
 discharging the printing paper from the impression cylinder after a
 required printing process; and a printing paper holder control mechanism
 for controlling the printing paper holders to carry out selectively a
 first opening and closing operation once for every rotation of the
 impression cylinder, and a second opening and closing operation once for
 every two rotations of the impression cylinder.
 In a preferred embodiment of the invention, the printing paper holder
 control mechanism comprises a fixed cam disposed laterally of the
 impression cylinder, the fixed cam being contactable with a cam follower
 attached to the gripper of the printing paper holder, to open the gripper,
 thereby opening and closing the printing paper holder once for every
 rotation of the impression cylinder; a movable cam disposed coaxially with
 the fixed cam, the movable cam being movable between an open/close
 prohibiting position for prohibiting the printing paper holder from
 opening and closing by action of the fixed cam, and an open/close
 permitting position for permitting the printing paper holder to open and
 close by action of the fixed cam; a movable cam drive mechanism for
 reciprocating the movable cam between the open/close prohibiting position
 and the open/close permitting position with rotation of the impression
 cylinder; and a movable cam stop mechanism for stopping the movable cam in
 the open/close permitting position regardless of the rotation of the
 impression cylinder.
 Preferably, the movable cam drive mechanism comprises a cam rotatable
 synchronously with the blanket cylinder; and a link mechanism coupled to
 the movable cam, the link mechanism including a cam follower contactable
 with the cam rotating synchronously with the blanket cylinder, to be
 pivotable with the rotation of the blanket cylinder to reciprocate the
 movable cam between the open/close prohibiting position and the open/close
 permitting position.
 The movable cam stop mechanism may comprise a stopper for prohibiting
 pivotal movement of the link mechanism.
 In another aspect of the invention, a printing apparatus comprises a first
 and a second plate cylinders each for supporting a plate as mounted
 peripherally thereof, the plate having two image areas, two ink feeders
 for contacting and feeding inks to the image areas on the plate mounted on
 the first plate cylinder; two ink feeders for contacting and feeding inks
 to the image areas on the plate mounted on the second plate cylinder; a
 first blanket cylinder equal in diameter to and contactable with the first
 plate cylinder; a second blanket cylinder equal in diameter to and
 contactable with the second plate cylinder; an impression cylinder having
 a diameter n/2 (n being an odd number) of the diameter of the first and
 second plate cylinders and the first and second blanket cylinders, and
 contactable with the first and second blanket cylinders; n printing paper
 holder(s) (each) having a gripper for holding an end of the printing paper
 to maintain n sheet(s) of the printing paper peripherally of the
 impression cylinder; a printing paper feeder for feeding the printing
 paper to the impression cylinder; a printing paper discharger for
 discharging the printing paper from the impression cylinder after a
 required printing process; and a printing paper holder control mechanism
 for controlling the printing paper holders to carry out selectively a
 first opening and closing operation once for every rotation of the
 impression cylinder, and a second opening and closing operation once for
 every two rotations of the impression cylinder.
 The printing apparatus according to the present invention provides a first
 and a second plate cylinders each for supporting a plate as mounted
 peripherally thereof, the plate having two image areas. This construction
 allows a selection between printing with one or two color inks and
 printing with four color inks.
 In a further aspect of the invention, a printing apparatus comprises a
 plate cylinder for supporting a plate as mounted peripherally thereof, the
 plate having two image areas; two ink feeders for selectively contacting
 and feeding inks to the image areas on the plate mounted on the plate
 cylinder; a blanket cylinder equal in diameter to and contactable with the
 plate cylinder; an impression cylinder having n/2 (n being an odd number)
 of the diameter of the plate cylinder and the blanket cylinder, and
 contactable with the blanket cylinder; n gripper(s) each for holding an
 end of the printing paper on the impression cylinder; a printing paper
 feeder and discharger for feeding the printing paper to the impression
 cylinder and discharging the printing paper from the impression cylinder;
 and a control mechanism for controlling the gripper(s) and the printing
 paper feeder and discharger to carry out selectively a first operation
 once for every rotation of the impression cylinder when both images of the
 two image areas on the plate are the same image, and a second operation
 once for every two rotations of the impression cylinder when both images
 of the images areas on the plate are different images.
 Other features and advantages of the present invention will be apparent
 from the following detailed description of the embodiments of the
 invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the present invention will be described hereinafter with
 reference to the drawings.
 The construction and operation of a printing apparatus embodying the
 invention will be described first. FIG. 1 is a schematic side view of the
 printing apparatus according to the present invention.
 This printing apparatus records images on blank plates mounted on first and
 second plate cylinders 11 and 12, feeds inks to the plates having the
 images recorded thereon, and transfers the inks from the plates through
 first and second blanket cylinders 13 and 14 to printing paper held on an
 impression cylinder 15, thereby printing the images on the printing paper.
 The first plate cylinder 11 is movable between a first printing position
 shown in a solid line and an image recording position shown in a two-dot
 chain line in FIG. 1. The second plate cylinder 12 is movable between a
 second printing position shown in a solid line in FIG. 1 and the same
 image recording position.
 Around the first plate cylinder 11 in the first printing position are an
 ink feeder 20a for feeding an ink of black (K), for example, to the plate,
 an ink feeder 20b for feeding an ink of magenta (M), for example, to the
 plate, and dampening water feeders 21a and 21b for feeding dampening water
 to the plate. Around the second plate cylinder 12 in the second printing
 position are an ink feeder 20c for feeding an ink of cyan (C), for
 example, to the plate, an ink feeder 20d for feeding an ink of yellow (Y),
 for example, to the plate, and dampening water feeders 21c and 21d for
 feeding dampening water to the plate. Further, around the first or second
 plate cylinder 11 or 12 in the image recording position are a plate feeder
 23, a plate remover 24, an image recorder 25 and a developing device 26.
 The first blanket cylinder 13 is contactable with the first plate cylinder
 11, while the second blanket cylinder 14 is contactable with the second
 plate cylinder 12. The impression cylinder 15 is contactable with the
 first and second blanket cylinders 13 and 14 in different positions. The
 apparatus further includes a paper feed cylinder 16 for transferring
 printing paper supplied from a paper storage 27 to the impression cylinder
 15, a paper discharge cylinder 17 with chains 19 wound thereon for
 discharging printed paper from the impression cylinder 15 to a paper
 discharge station 28, and a blanket cleaning unit 29.
 Each of the first and second plate cylinders 11 and 12 is coupled to a
 plate cylinder moving mechanism not shown, and driven by this moving
 mechanism to reciprocate between the first or second printing position and
 the image recording position. In the first printing position, the first
 plate cylinder 11 is rotatable synchronously with the first blanket
 cylinder 13. In the second printing position, the second plate cylinder 12
 is rotatable synchronously with the second blanket cylinder 14. Adjacent
 the image recording position is a plate cylinder rotating mechanism, not
 shown, for rotating the first or second plate cylinder 11 or 12 whichever
 is in the image recording position.
 The plate feeder 23 and plate remover 24 are arranged around the first or
 second plate cylinder 11 or 12 in the image recording position.
 The plate feeder 23 includes a supply cassette 63 storing a roll of
 elongate blank plate in light-shielded state, a guide member 64 and guide
 rollers 65 for guiding a forward end of the plate drawn from the cassette
 63 to the surface of the first or second plate cylinder 11 or 12, and a
 cutter 66 for cutting the elongate plate into sheet plates. Each of the
 first and second plate cylinders 11 and 12 has a pair of grippers, not
 shown, for gripping the forward and rear ends of the plate fed from the
 plate feeder 23.
 The plate remover 24 has a pawl mechanism 73 for separating a plate from
 the first or second plate cylinder 11 or 12 after a printing operation,
 and a conveyor mechanism 69 for transporting the plate separated by the
 pawl mechanism 73 to a discharge cassette 68.
 The forward end of the plate drawn from the feeder cassette 63 is guided by
 the guide rollers 65 and guide member 64, and gripped by one of the
 grippers on the first or second plate cylinder 11 or 12. Then, the first
 or second plate cylinder 11 or 12 is rotated by the plate cylinder
 rotating mechanism not shown, whereby the plate is wrapped around the
 first or second plate cylinder 11 or 12. The rear end of the plate cut by
 the cutter 66 is gripped by the other gripper. While, in this state, the
 first or second plate cylinder 11 or 12 is rotated at low speed, the image
 recorder 25 irradiates the surface of the plate mounted peripherally of
 the first or second plate cylinder 11 or 12 with a modulated laser beam
 for recording images thereon.
 On the plate P mounted peripherally of the first plate cylinder 11, the
 image recorder 25, records an image area 67a to be printed with black ink,
 and an image area 67b to be printed with magenta ink, as shown in FIG. 2A.
 On the plate P mounted peripherally of the second plate cylinder 12, the
 image recorder 25 records an image area 67c to be printed with cyan ink,
 and an image area 67d to be printed with yellow ink, as shown in FIG. 2B.
 The image areas 67a and 67b are recorded in evenly separated positions,
 i.e. in positions separated from each other by 180 degrees, on the plate P
 mounted peripherally of the first plate cylinder 11. Similarly, the image
 areas 67c and 67d are recorded in evenly separated positions, i.e. in
 positions separated from each other by 180 degrees, on the plate P mounted
 peripherally of the second plate cylinder 12.
 When this apparatus is used in monochromatic or two-color printing, as
 described hereinafter, an image is formed in the image areas 67a and 67b
 or image areas 67c and 67d to be printed with the same ink.
 In the foregoing embodiment, two image areas 67a and 67b or 67c and 67d are
 provided on the single plate P mounted peripherally of the first or second
 plate cylinder 11 or 12, in order to simplify the structure of the first
 or second plate cylinder 11 or 12. Alternatively, the first or second
 plate cylinder 11 or 12 may include two sets of grippers, each set for
 holding the forward and rear ends of one plate P. Then, as shown in FIG.
 3, each of the first and second plate cylinders 11 and 12 may support two
 plates P each having a single image area. As used herein, the term
 "holding a plate having two image areas" should be interpreted to include
 the case of holding a single plate having two image areas and the case of
 holding two plates each having a single image area. In the latter case
 also, the two plates P should be held as evenly separated on the first or
 second plate cylinder 11 or 12, so that the image areas recorded on the
 respective plates are in evenly separated positions, i.e. in positions
 separated from each other by 180 degrees.
 Referring again to FIG. 1, the ink feeders 20a and 20b are arranged around
 the first plate cylinder 11 in the first printing position, while the ink
 feeders 20c and 20d are arranged around the second plate cylinder 12 in
 the second printing position, as described hereinbefore. Each of these ink
 feeders 20a, 20b, 20c and 20d (which may be referred to collectively as
 "ink feeders 20") includes a plurality of ink rollers 71 and an inkwell
 72.
 The ink rollers 71 of the ink feeders 20a and 20b are swingable by action
 of cams or the like not shown. With the swinging movement, the ink rollers
 71 of the ink feeder 20a or 20b come into contact with one of the two
 image areas 67a and 67b formed on the plate P mounted peripherally of the
 first plate cylinder 11. Thus, the ink is fed only to an intended one of
 the image areas 67a and 67b. Similarly, the ink rollers 71 of the ink
 feeders 20c and 20d are swingable by action of cams or the like not shown.
 With the swinging movement, the ink rollers 71 of the ink feeder 20c or
 20d come into contact with one of the two image areas 67c and 67d formed
 on the plate P mounted peripherally of the second plate cylinder 12. Thus,
 the ink is fed only to an intended one of the image areas 67c and 67d.
 The dampening water feeders 21a, 21b, 21c and 21d (which may be referred to
 collectively as "dampening water feeders 21") feed dampening water to the
 plates P before the ink feeders 20 feed the inks thereto. Of these
 dampening water feeders 21, the water feeder 21a feeds dampening water to
 the image area 67a on the plate P, the water feeder 21b feeds dampening
 water to the image area 67b on the plate P, the water feeder 21c feeds
 dampening water to the image area 67c on the plate P, and the water feeder
 21d feeds dampening water to the image area 67d on the plate P.
 The developing device 26 is disposed under the first or second plate
 cylinder 11 or 12 in the image recording position. This developing device
 26 includes a developing unit, a fixing unit and a squeezing unit, which
 are vertically movable between a standby position shown in two-dot chain
 lines and a developing position shown in solid lines in FIG. 1.
 In developing the images recorded on the plate P by the image recorder 25,
 the developing unit, fixing unit and squeezing unit are successively
 brought into contact with the plate P rotated with the first or second
 plate cylinder 11 or 12.
 The first and second blanket cylinders 13 and 14 movable into contact with
 the first and second plate cylinders 11 and 12 have the same diameter as
 the first and second plate cylinders 11 and 12, and have ink transfer
 blankets mounted peripherally thereof. Each of the first and second
 blanket cylinders 13 and 14 is movable into and out of contact with the
 first or second plate cylinder 11 or 12 and the impression cylinder 15 by
 a contact mechanism described hereinafter.
 The first and second blanket cylinders 13 and 14 with the soft blankets
 mounted peripherally thereof are slightly reduced in diameter when in
 contact with the first and second plate cylinders 11 and 12. As used
 herein, the term "the same diameter" allows for slight errors due to such
 changes in diameter. Further, as described hereinafter, the impression
 cylinder 15, paper feed cylinder 16 and paper discharge cylinder 17 have
 half the diameter of the first and second plate cylinders 11 and 12 and
 the first and second blanket cylinders 13 and 14. This also allows for
 slight errors due to changes in diameter.
 FIG. 4 is a schematic view of the contact mechanism for acting on the first
 blanket cylinder 13. The contact mechanism for the second blanket cylinder
 14 is similar in structure to the contact mechanism shown in FIG. 4.
 The first blanket cylinder 13 is rotatably supported by a shaft 101. An
 eccentric shaft 102 decentered from the shaft 101 is formed on a side
 thereof. The eccentric shaft 102 is surrounded by an eccentric bearing 103
 decentered from the shafts 101 and 102. Thus, as shown in FIG. 4, the axis
 104 of the shaft 101, i.e. the axis of the first blanket cylinder 13, the
 axis 105 of the eccentric shaft 102 and the axis 106 of the eccentric
 bearing 103 are offset from one another.
 The eccentric shaft 102 has a plate 107 fixed thereto, while the eccentric
 bearing 103 has a plate 108 fixed thereto. The two fixed plates 107 and
 108 are interconnected by two coupling plates 111 and 112 forming a link
 mechanism. The forward end of a cylinder rod 114 of an air cylinder 113 is
 connected to a connection between the two coupling plates 111 and 112. The
 air cylinder 113 has a cylinder body coupled to an end of a rotary plate
 116 rotatable about a shaft 115. The other end of the rotary plate 116 is
 coupled through a rod 117 to a plate 118 fixed to the eccentric bearing
 103.
 The rotary plate 116 is coupled to a shaft 120 of an eccentric member 119
 through two coupling plates 121 and 122 forming a link mechanism. The
 forward end of a cylinder rod 124 of an air cylinder 123 fixed to a main
 body of the apparatus is connected to a connection between the two
 coupling plates 121 and 122. The eccentric member 119 has a worm wheel 125
 connected thereto and meshed with a worm gear 127 rotatable by a motor
 126.
 With the cylinder rods 114 and 124 of the air cylinders 113 and 123
 extended as shown in FIG. 4, the surface of the first blanket cylinder 13
 is spaced slightly from the surfaces of the first plate cylinder 11 and
 the impression cylinder 15.
 When the air cylinder 113 is driven to retract the cylinder rod 114, the
 first blanket cylinder 13 is moved toward and into contact with the first
 plate cylinder 11 by action of the link mechanism formed of the two
 coupling plates 111 and 112.
 When the air cylinder 123 is driven to retract the cylinder rod 124, the
 first blanket cylinder 13 is moved toward and into contact with the
 impression cylinder 15 by action of the link mechanism formed of the two
 coupling plates 121 and 122. At this time, the rotary plate 116 also
 rotates clockwise about the shaft 115, whereby the first blanket cylinder
 13 moves not only toward the impression cylinder 15 but also toward the
 first plate cylinder 11. Consequently, the first blanket cylinder 13 is
 maintained in contact with the first plate cylinder 11.
 Rotation of the eccentric member 119 results in a slight movement of its
 shaft 120. Thus, the contact pressure of the first blanket cylinder 13 for
 contacting the impression cylinder 15 and the first plate cylinder 11 may
 be adjusted by driving the motor 126 to rotate the worm wheel 125
 connected to the eccentric member 119 thereby to move the shaft 120
 slightly. This enables adjustment of a printing pressure in time of
 printing with the first blanket cylinder 13.
 Referring again to FIG. 1, the blanket cleaning unit 29 disposed between
 the first and second blanket cylinders 13 and 14 cleans the surfaces of
 the first and second blanket cylinders 13 and 14 by feeding a cleaning
 solution to an elongate cleaning cloth extending from a delivery roll to a
 take-up roll through a plurality of pressure rollers, and sliding the
 cleaning cloth in contact with the first and second blanket cylinders 13
 and 14. The cleaning cloth may further be brought into contact with the
 surface of the impression cylinder 15 for cleaning the same.
 The impression cylinder 15 contactable by the first and second blanket
 cylinders 13 and 14 has half the diameter of the first and second plate
 cylinders 11 and 12 and the first and second blanket cylinders 13 and 14,
 as noted hereinbefore. Further, the impression cylinder 15 has a gripper
 573, which will be described later in regards to FIGS. 17 and 21-32 for
 holding and transporting the forward end of printing paper.
 The paper feed cylinder 16 disposed adjacent the impression cylinder 15 has
 the same diameter as the impression cylinder 15. The paper feed cylinder
 16 has a gripper 574, which will also be described later in regards to
 FIGS. 17 and 21-32, for holding and transporting the forward end of each
 sheet of printing paper fed from the paper storage 27 by a reciprocating
 suction board 74. When the printing paper is transferred from the feed
 cylinder 16 to the impression cylinder 15, the gripper 573 of the
 impression cylinder 15 holds the forward end of the printing paper which
 has been held by the gripper 574 of the feed cylinder 16.
 The paper discharge cylinder 17 disposed adjacent the impression cylinder
 15 has the same diameter as the impression cylinder 15. The discharge
 cylinder 17 has a pair of chains 19 wound around opposite ends thereof.
 The chains 19 are interconnected by coupling members, not shown, having a
 plurality of grippers 575 which will also be described later in regards to
 FIGS. 17 and 21-32 arranged thereon. When the impression cylinder 15
 transfers the printing paper to the discharge cylinder 17, one of the
 grippers 575 of the discharge cylinder 17 holds the forward end of the
 printing paper having been held by the gripper 573 of the impression
 cylinder 15.
 With movement of the chains 19, the printing paper is transorted to the
 paper discharge station 28 to be discharged thereon.
 The paper feed cylinder 16 is connected to a drive motor through a belt not
 shown. The paper feed cylinder 16, impression cylinder 15, paper discharge
 cylinder 17 and the first and second blanket cylinders 13 and 14 are
 coupled to one another by gears mounted on end portions thereof,
 respectively. Further, the first and second blanket cylinders 13 and 14
 are coupled to the first and second plate cylinders 11 and 12 in the first
 and second printing positions, respectively, by gears mounted on end
 portions thereof. Thus, a motor, not shown, is operable to rotate the
 paper feed cylinder 16, impression cylinder 15, paper discharge cylinder
 17, the first and second blanket cylinders 13 and 14 and the first and
 second plate cylinders 11 and 12 synchronously with one another.
 FIG. 5 is a block diagram showing a principal electrical structure of the
 printing apparatus. This printing apparatus includes a control unit 140
 having a ROM 141 for storing operating programs necessary for controlling
 the apparatus, a RAM 142 for temporarily storing data and the like during
 a control operation, and a CPU 143 for performing logic operations. The
 control unit 140 has a driving circuit 145 connected thereto through an
 interface 144, for generating driving signals for driving the ink feeders
 20, image recorder 25, developing device 26, blanket cleaning unit 29, the
 moving mechanisms for moving the first and second plate cylinders 11 and
 12, the contact mechanisms for the first and second blanket cylinders 13
 and 14, and so on. The printing apparatus is controlled by the control
 unit 140 to execute prepress and printing operations as described
 hereinafter.
 The prepress and printing operations of the printing apparatus will be
 described next. FIG. 6 is a flow chart showing an outline of the prepress
 and printing operations of the printing apparatus.
 First, the printing apparatus executes a prepress process for recording and
 developing images on the plates P mounted on the first and second plate
 cylinders 11 and 12 (step S1). This prepress process follows the steps
 constituting a subroutine as shown in the flow chart of FIG. 7.
 The first plate cylinder 11 is first moved to a prepress position, i.e. the
 image recording position shown in a two-dot chain line in FIG. 1. (step
 S11).
 Next, a plate P is fed to the outer periphery of the first plate cylinder
 11 (step S12). To achieve the feeding of the plate P, the pair of
 grippers, not shown, grip the forward end of plate P drawn from the supply
 cassette 63, and the rear end of plate P cut by the cutter 66.
 Then, an image is recorded on the plate P mounted peripherally of the first
 plate cylinder 11 (step S13). For recording the image, the image recorder
 25 irradiates the plate P mounted peripherally of the first plate cylinder
 11 with a modulated laser beam while the first plate cylinder 11 is
 rotated at low speed by the cylinder rotating mechanism not shown.
 Next, the image recorded on the plate P is developed (step S14). The
 developing step is executed by raising the developing device 26 from the
 standby position shown in two-dot chain lines to the developing position
 shown in solid lines in FIG. 1 and thereafter successively moving the
 developing unit, fixing unit and squeezing unit into contact with the
 plate P rotating with the first plate cylinder 11.
 Upon completion of the developing step, the first plate cylinder 11 is
 moved to the first printing position shown in the solid line in FIG. 1
 (step S15).
 Subsequently, the printing apparatus carries out an operation similar to
 steps S11 to S15 by way of a prepress process for the plate P mounted
 peripherally of the second plate cylinder 12 (steps S16 to S20).
 Completion of the prepress steps for the plates P mounted peripherally of
 the first and second plate cylinders 11 and 12 brings the prepress process
 to an end.
 Referring again to FIG. 6, the prepress process is followed by a printing
 process for printing the printing paper with the plates P mounted on the
 first and second plate cylinders 11 and 12 (step S2). The operation of the
 printing apparatus in this printing process will be described in detail
 hereinafter.
 Upon completion of the printing process, the plates P used in the printing
 are removed (step S3). To remove the plates P, the first plate cylinder 11
 is first moved to the prepress position shown in the two-dot chain line in
 FIG. 1. Then, while the first plate cylinder 11 is rotated
 counterclockwise, the pawl mechanism 73 separates an end of the plate P
 from the first plate cylinder 11. The plate P separated is guided by the
 conveyor mechanism 69 into the discharge cassette 68. After returning the
 first plate cylinder 11 to the first printing position, the second plate
 cylinder 12 is moved from the second printing position to the prepress
 position to undergo an operation similar to the above, thereby having the
 plate P removed from the second plate cylinder 12 for discharge into the
 discharge cassette 68.
 Upon completion of the plate removing step, the first and second blanket
 cylinders 13 and 14 are cleaned (step S4). For cleaning the first and
 second blanket cylinders 13 and 14, the contact mechanisms as shown in
 FIG. 4 separate the first and second blanket cylinders 13 and 14 from the
 first and second plate cylinders 11 and 12 and the impression cylinder 15.
 The first and second blanket cylinders 13 and 14 are thereafter rotated.
 In this state, the cleaning cloth of blanket cleaning unit 29 supplied
 with the cleaning solution is placed in contact with and slid on the
 surfaces of the first and second blanket cylinders 13 and 14, thereby
 cleaning the first and second blanket cylinders 13 and 14.
 After completing the cleaning of the first and second blanket cylinders 13
 and 14, the printing apparatus determines whether or not a further image
 is to be printed (step S5). If a further printing operation is required,
 the apparatus repeats steps S1 to S4.
 If the printing operation is ended, the printing apparatus cleans the inks
 (step S6). For cleaning the inks, an ink cleaning device, not shown,
 provided for each ink feeder 20 removes the ink adhering to the ink
 rollers 71 and inkwell 72 of each ink feeder 20.
 With completion of the ink cleaning step, the printing apparatus ends the
 entire process.
 The operation of the printing apparatus in the above-mentioned printing
 process will be described next. In order to selectively perform four-color
 printing and monochromatic printing or two-color printing, as described
 hereinafter, this printing apparatus must control opening and closing of
 the gripper 573 on the impression cylinder 15, the gripper 574 on the
 paper feed cylinder 16, and the gripper 575 on the paper discharge
 cylinder 17. The construction of a gripper control mechanism according to
 the present invention for controlling the opening and closing of these
 grippers 573, 574 and 575 will be described in detail hereinafter.
 A four-color printing operation using the four color inks of yellow,
 magenta, cyan and black will be described first.
 FIGS. 8 through 12 are explanatory views showing a four-color printing
 operation of this printing apparatus. For expediency of illustration,
 these figures depict printing paper S having a length corresponding to the
 circumference of each of the impression cylinder 15, paper feed cylinder
 16 and paper discharge cylinder 17. In practice, the length of printing
 paper S is greater than the circumference of each of the impression
 cylinder 15, paper feed cylinder 16 and paper discharge cylinder 17.
 Further, in FIGS. 8 through 12, for expediency of illustration again,
 reference K is affixed to regions of the first plate cylinder 11 and first
 blanket cylinder 13 used for printing with black ink, reference M to
 regions of the first plate cylinder 11 and first blanket cylinder 13 used
 for printing with magenta ink, reference C to regions of the second plate
 cylinder 12 and second blanket cylinder 14 used for printing with cyan
 ink, and reference Y to regions of the second plate cylinder 12 and second
 blanket cylinder 14 used for printing with yellow ink.
 It is assumed that, in the prepress process preceding the printing process,
 as described hereinbefore, and as shown in FIG. 2A, the image areas 67a
 and 67b for printing with the black and magenta inks respectively are
 recorded on the plate P mounted peripherally of the first plate cylinder
 11. Similarly, as shown in FIG. 2B, the image areas 67c and 67d for
 printing with the cyan and yellow inks respectively are assumed to be on
 the plate P mounted peripherally of the second plate cylinder 12.
 As described hereinbefore, the ink feeders 20a, 20b, 20c and 20d are
 supplied with the black, magenta, cyan and yellow inks, respectively.
 Then, the ink feeders 20a, 20b, 20c and 20d are ready to feed the black,
 magenta, cyan and yellow inks to the image areas 67a, 67b, 67c and 67d on
 the plates P shown in FIGS. 2A and 2B, respectively.
 First, each dampening water feeder 21 and each ink feeder 20 are placed in
 contact with only a corresponding one of the image areas on the plates P
 mounted on the first and second plate cylinders 11 and 12. Consequently,
 dampening water is fed to the image areas 67a, 67b, 67c and 67d from the
 corresponding water feeders 21a, 21b, 21c and 21c, and so are the black,
 magenta, cyan and yellow inks from the ink feeders 20a, 20b, 20c and 20d,
 respectively. These inks are transferred to the corresponding regions of
 the first and second blanket cylinders 13 and 14, respectively.
 By repeating this operation, the inks are fed to the plates P mounted on
 the first and second plate cylinders 11 and 12 and to the first and second
 blanket cylinders 13 and 14. This ink feeding operation is repeated until
 the printing process is completed.
 When the inks have been fed to the plates P mounted on the first and second
 plate cylinders 11 and 12 and to the first and second blanket cylinders 13
 and 14, the printing paper S is fed to the paper feed cylinder 16 as shown
 in FIG. 8. The printing paper S is subsequently passed from the paper feed
 cylinder 16 to the impression cylinder 15.
 With a further rotation of the impression cylinder 15, the forward end of
 printing paper S mounted on the impression cylinder 15 reaches a position
 opposed to the first blanket cylinder 13. Then, the contact mechanism
 shown in FIG. 4 moves the first blanket cylinder 13 into contact with the
 impression cylinder 15. In this state, the forward end of printing paper S
 comes into contact with an end of the region of the first blanket cylinder
 13 used for printing with the black ink, as shown in FIG. 9. The black ink
 has been transferred from the image area 67a of the plate P mounted on the
 first plate cylinder 11 to the region of the first blanket cylinder 13
 used for printing with the black ink. Thus, the black ink is transferred
 to the printing paper S with a further rotation of the first blanket
 cylinder 13 and impression cylinder 15.
 With a still further rotation of the impression cylinder 15, the forward
 end of printing paper S mounted on the impression cylinder 15 reaches a
 position opposed to the second blanket cylinder 14. Then, the contact
 mechanism as shown in FIG. 4 moves the second blanket cylinder 14 into
 contact with the impression cylinder 15. In this state, the forward end of
 printing paper S comes into contact with an end of the region of the
 second blanket cylinder 14 used for printing with the cyan ink, as shown
 in FIG. 10. The cyan ink has been transferred from the image area 67c of
 the plate P mounted on the second plate cylinder 12 to the region of the
 second blanket cylinder 13 used for printing with the cyan ink. Thus, with
 a further rotation of the second blanket cylinder 14 and impression
 cylinder 15, the cyan ink is transferred to the printing paper S to which
 the black ink has already been transferred.
 As the impression cylinder 15 continues to rotate with the first and second
 blanket cylinders 13 and 14 in this state, the printing paper S becomes
 completely wrapped around the impression cylinder 15 as shown in FIG. 11.
 Since the impression cylinder 15 has half the diameter of the first and
 second plate cylinders 11 and 12 and the first and second blanket
 cylinders 13 and 14, the printing paper S wrapped around the impression
 cylinder 15 comes, in its second rotation, into contact with the region of
 the first blanket cylinder 13 used for printing with the magenta ink. The
 magenta ink has been transferred from the image area 67b of the plate P
 mounted on the first plate cylinder 11 to the region of the first blanket
 cylinder 13 used for printing with the magenta ink. Thus, with a further
 rotation of the first blanket cylinder 13 and impression cylinder 15, the
 magenta ink is transferred to the printing paper S to which the black and
 cyan inks have already been transferred.
 With a still further rotation of the impression cylinder 15, the printing
 paper S comes into contact with an end of the region of the second blanket
 cylinder 14 used for printing with the yellow ink. The yellow ink has been
 transferred from the image area 67d of the plate P mounted on the second
 plate cylinder 12 to the region of the second blanket cylinder 14 used for
 printing with the yellow ink. With a further rotation of the second
 blanket cylinder 14 and impression cylinder 15, therefore, the yellow ink
 is transferred to the printing paper S to which the black, cyan and
 magenta inks have already been transferred. This completes the four-color
 printing.
 As shown in FIG. 12, the forward end of printing paper S printed in the
 four colors is passed from the impression cylinder 15 to the paper
 discharge cylinder 17. Meanwhile, printing paper S to be printed next is
 fed to the paper feed cylinder 16, and then transferred from the feed
 cylinder 16 to the impression cylinder 15.
 The printing paper S printed in the four colors is transported by the pair
 of chains 19 to the paper discharge station 28 along with one of the
 grippers 575 of the paper discharge cylinder 17, as shown in FIG. 13.
 As described above, the impression cylinder 15 has half the diameter of the
 first and second plate cylinders 11 and 12 and the first and second
 blanket cylinders 13 and 14. Consequently, the impression cylinder 15
 rotates twice as the first and second plate cylinders 11 and 12 and the
 first and second blanket cylinders 13 and 14 rotate once. The printing
 paper S mounted peripherally of the impression cylinder 15 is printed in
 the four colors of yellow, magenta, cyan and black while the impression
 cylinder 15 rotates twice. Thus, the printing apparatus can continuously
 execute the four-color printing by feeding a new sheet of printing paper S
 from the paper feed cylinder 16 for every two rotations of the impression
 cylinder 15.
 A monochromatic printing operation using one of the four color inks of
 yellow, magenta, cyan and black will be described next.
 When this printing apparatus performs printing with only the black ink, for
 example, the same image is formed in the image areas 67a and 67b on the
 plate P mounted on the first plate cylinder 11. The ink feeder 20a is
 arranged to feed the black ink to both of the image areas 67a and 67b. The
 other ink feeders 20b, 20c and 20d are kept away from the first and second
 plate cylinders 11 and 12. Further, the contact mechanisms as shown in
 FIG. 4 places the first blanket cylinder 13 in contact with the impression
 cylinder 15, and the second blanket cylinder 14 out of contact with the
 impression cylinder 15.
 In this state, a new sheet of printing paper S is fed from the paper feed
 cylinder 16 for every rotation of the impression cylinder 15. As a result,
 with every rotation of the impression cylinder 15, the image in the black
 ink is transferred from the image area 67a or 67b in which the same image
 is formed to the printing paper S mounted peripherally of the impression
 cylinder 15, to achieve monochromatic printing.
 A two-color printing operation using one of the magenta and black inks and
 one of the yellow and cyan inks will be described next.
 When this printing apparatus performs printing with the black and cyan
 inks, for example, the same image is formed in the image areas 67a and 67b
 on the plate P mounted on the first plate cylinder 11, and the same image
 in the image areas 67c and 67d on the plate P mounted on the second plate
 cylinder 12. The ink feeder 20a is arranged to feed the black ink to both
 of the image areas 67a and 67b, while the ink feeder 20c is arranged to
 feed the cyan ink to both of the image areas 67c and 67d. The other ink
 feeders 20b and 20d are kept away from the first and second plate
 cylinders 11 and 12. Further, the contact mechanisms as shown in FIG. 4
 place the first and second blanket cylinder 13 and 14 in contact with the
 impression cylinder 15.
 In this state, a new sheet of printing paper S is fed from the paper feed
 cylinder 16 for every rotation of the impression cylinder 15. As a result,
 with every rotation of the impression cylinder 15, the image in the black
 ink is transferred from the image area 67a or 67b in which the same image
 is formed to the printing paper S mounted peripherally of the impression
 cylinder 15, and thereafter the image in the cyan ink is transferred from
 the image area 67c or 67d in which the same image is formed to the
 printing paper S. Thus, a two-colors print is made on the printing paper
 S.
 As noted above, when executing four-color printing, monochromatic printing
 or two-color printing, this apparatus must control opening and closing of
 the gripper 573 on the impression cylinder 15, the gripper 574 on the
 paper feed cylinder 16 and the grippers 575 on the paper discharge
 cylinder 17.
 The gripper control mechanism of the present invention for controlling
 opening and closing of these grippers 573, 574 and 575 will be described
 hereinafter. FIGS. 14 through 16 are schematic views of the gripper
 control mechanism for opening and closing the grippers 573, 574 and 575.
 The gripper control mechanism for opening and closing the grippers 573, 574
 and 575 includes a link mechanism having a main link 592 pivotable about a
 main link axis 593. The main link 592 has a cam follower 594 in contact
 with a cam 591 mounted coaxially with the second blanket cylinder 14. The
 main link 592 is biased by a spring 596 in a direction to press the cam
 follower 594 upon the cam 591. Thus, the main link 592 is pivotable about
 the main link axis 593 as the cam 591 rotates synchronously with the
 second blanket cylinder 14.
 A movable cam 584 is disposed laterally of the impression cylinder 15 to be
 rotatable about the axis of the impression cylinder 15. A movable cam 585
 is disposed laterally of the paper feed cylinder 16 to be rotatable about
 the axis of the paper feed cylinder 16. Further, a movable cam 586 is
 disposed laterally of the paper discharge cylinder 17 to be rotatable
 about the axis of the paper discharge cylinder 17. The movable cam 584
 disposed laterally of the impression cylinder 15 is connected to the main
 link 592 through a link 601. The movable cam 585 disposed laterally of the
 paper feed cylinder 16 is connected to the movable cam 585 through a link
 602 and a coupling plate 604. The movable cam 586 disposed laterally of
 the paper discharge cylinder 17 is connected to the main link 592 through
 a link 603.
 With the above construction, when the cam follower 594 of the main link 592
 contacts a recess of the cam 591, the main link 592 and the movable cams
 584, 585 and 586 are arranged in the position shown in FIG. 14. When, in
 this state, the second blanket cylinder 14 rotates with the first blanket
 cylinder 13 and impression cylinder 15, the cam 591 pushes the cam
 follower 594 of the main link 592, whereby the main link 592 pivots about
 the main link axis 593 clockwise in FIG. 14.
 With the pivotal movement of the main link 592, the movable cam 584
 connected to the main link 592 through the link 601 rotates about the axis
 of the impression cylinder 15 counterclockwise in FIG. 14. The rotation of
 the movable cam 584 causes the movable cam 585 connected to the movable
 cam 584 through the link 602 and coupling plate 604 to rotate about the
 axis of the paper feed cylinder 16 clockwise in FIG. 14. The pivotal
 movement of the main link 592 also causes the movable cam 586 connected to
 the main link 592 through the link 603 to rotate about the axis of the
 paper discharge cylinder 17 counterclockwise in FIG. 14.
 Consequently, when the cam follower 594 of the main link 592 contacts a
 bulge of the cam 591, the main link 592 and the movable cams 584, 585 and
 586 are arranged in the position shown in FIG. 15. The main link 592 and
 movable cams 584, 585 and 586 reciprocate between the position shown in
 FIG. 14 and the position shown in FIG. 15 as the cam 591 makes one
 rotation with the second blanket cylinder 14, i.e. as the impression
 cylinder 15 makes two rotations.
 A stopper 597 is disposed below the main link 592 to be swingable by a
 solenoid 598 about an axis 605. The stopper 597 is engageable with a pin
 595 projecting from the main link 592 to hold the main link 592 against
 pivotal movement.
 To prohibit pivotal movement of the main link 592, the solenoid 598 is
 operated to swing the stopper 597 counter-clockwise about the axis 605.
 When, in this state, the cam 591 causes the main link 592 to pivot to the
 position shown in FIG. 15, the distal end of stopper 597 engages the pin
 595 projecting from the main link 592 as shown in FIG. 16. Then, the main
 link 592 is locked to the position shown in FIG. 16 regardless of the
 rotating angle of cam 591. Each of the movable cams 584, 585 and 586 is
 locked to the same state as where the cam follower 594 of the main link
 592 is in contact with the bulge of the cam 591.
 Constructions of fixed cams 587, 588 and 589 and grippers 573, 574 and 575
 arranged in a corresponding relationship to the above movable cams 584,
 585 and 586 will be described next.
 FIG. 17 is a schematic view showing the gripper control mechanism along
 with the fixed cams and grippers. FIG. 17 shows the movable cams 584, 585
 and 586 in solid lines, and the fixed cams 587, 588 and 589 in two-dot
 chain lines. Further, the links 592, 601, 602 and 603 and coupling plate
 604 shown in FIGS. 14 through 16 are depicted in a simplified form in FIG.
 17.
 FIGS. 18 and 19 are explanatory views of a positional relationship among
 the movable cams 584, 585 and 586 and fixed cams 587, 588 and 589. FIG. 18
 shows a state where the main link 592 is disposed in the position shown in
 FIG. 14. FIG. 19 shows a state where the main link 592 is disposed in the
 position shown in FIG. 15. FIGS. 18 and 19 show the movable cams 584, 585
 and 586 two-dot chain lines, and the fixed cams 587, 588 and 589 in solid
 lines.
 As shown in these figures, the fixed cam 587 is disposed laterally of and
 coaxially with the movable cam 584 disposed laterally of the impression
 cylinder 15. The fixed cam 588 is disposed laterally of and coaxially with
 the movable cam 585 disposed laterally of the paper feed cylinder 16.
 Further, the fixed cam 589 is disposed laterally of and coaxially with the
 movable cam 586 disposed laterally of the paper discharge cylinder 17.
 The impression cylinder 15 has the gripper 573 including a blade 576 and a
 cam follower 581 attached to the blade 576. The gripper 573 is rotatable
 with the impression cylinder 15 to move the cam follower 581 into contact
 with the bulge of the movable cam 584 or fixed cam 587, thereby to close
 the blade 576.
 The paper feed cylinder 16 has the gripper 574 including a blade 577 and a
 cam follower 582 attached to the blade 577. The gripper 574 is rotatable
 with the paper feed cylinder 16 to move the cam follower 582 into contact
 with the bulge of the movable cam 585 or fixed cam 588, thereby to open
 the blade 576.
 Further, the chains 19 wound around the opposite ends of the paper
 discharge cylinder 17 have the plurality of grippers 575 each including a
 blade 578 and a cam follower 583 attached to the blade 578. With rotation
 of the paper feed cylinder 16, each gripper 575 is movable with the chains
 19 to move the cam follower 583 into contact with the bulge of the movable
 cam 586 or fixed cam 589, thereby to open the blade 578. Each gripper 575
 opens the blade 578 when the cam follower 583 contacts a contact piece 599
 disposed above the paper discharge station 28 shown in FIG. 1.
 Operations of the above printing apparatus for transferring the printing
 paper S will be described next. FIGS. 20 through 32 are explanatory views
 showing the printing paper transfer operations. The links 592, 601, 602
 and 603 and coupling plate 604 shown in FIGS. 14 through 16 are depicted
 in a simplified form in FIGS. 20 through 32 also.
 First, a printing paper transfer operation in printing the printing paper S
 with the four color inks of yellow, magenta, cyan and black will be
 described. In this case, as noted hereinbefore, the printing paper S must
 be fed from the paper feed cylinder 16 to the impression cylinder 15 for
 every two rotations of the impression cylinder 15.
 In this case, the solenoid 598 is operated to hold the stopper 597 in the
 position shown in FIGS. 14 and 15, to render the main link 592 pivotable.
 Then, printing paper S is drawn from the paper storage 27 by action of the
 suction board 74 shown in FIG. 1. As shown in FIG. 20, the forward end of
 printing paper S is transported toward the gripper 574 on the paper feed
 cylinder 16. At this time, the cam follower 582 of the gripper 574 is in
 contact with the bulge of the fixed cam 588 to maintain the gripper 574
 open.
 In this state, the paper feed cylinder 16 rotates to move the cam follower
 582 of the gripper 574 past the bulge of the fixed cam 588. Then, as shown
 in FIG. 21, the gripper 574 closes to grip the forward end of printing
 paper S.
 With a further rotation of the paper feed cylinder 16, the gripper 574 on
 the paper feed cylinder 16 moves to a position opposed to the gripper 573
 on the impression cylinder 15 as shown in FIG. 22. Then, the cam follower
 582 of the gripper 574 starts opening the gripper 574 by the action of
 fixed cam 588, while the cam follower 581 of the gripper 573 contacts the
 bulge of fixed cam 587 to start closing the gripper 573. As a result, the
 forward end of printing paper S is transferred from the gripper 574 to the
 gripper 573.
 With rotation of the impression cylinder 15 in this state, the first
 printing process is started for the printing paper S mounted on the
 impression cylinder 15. In this state, as shown in FIG. 23, the cam
 follower 581 of the gripper 573 remains in contact with the bulge of the
 fixed cam 587, whereby the forward end of printing paper S remains
 gripped.
 FIG. 24 shows a state where, with a further rotation of the impression
 cylinder 15, the gripper 573 on the impression cylinder 15 has moved to a
 position opposed to one of the grippers 575 on the paper discharge
 cylinder 17. In the illustrated state, the cam follower 594 of the main
 link 592 is in contact with the recess of the cam 591. Thus, the movable
 cams 584, 585 and 586 and fixed cams 587, 588 and 589 are in the
 positional relationship shown in FIG. 18. Consequently, the cam follower
 581 of the gripper 573 on the impression cylinder 15 is in contact with
 the movable cam 584 to maintain the gripper 573 closed. The cam follower
 583 of the gripper 575 on the paper discharge cylinder 17 is in contact
 with the movable cam 586 to maintain the gripper 575 open. Therefore, the
 printing paper S is not transferred from the gripper 573 to the gripper
 575.
 FIG. 25 shows a state where, with a further rotation of the impression
 cylinder 15, the gripper 573 on the impression cylinder 15 has moved to a
 position opposed to the gripper 574 on the paper feed cylinder 16. In this
 state also, the cam follower 594 of the main link 592 is in contact with
 the recess of the cam 591 to maintain the movable cams 584, 585 and 586
 and fixed cams 587, 588 and 589 in the positional relationship shown in
 FIG. 18. Consequently, the cam follower 581 of the gripper 573 on the
 impression cylinder 15 is in contact with the movable cam 584 to maintain
 the gripper 573 closed. The cam follower 582 of the gripper 574 on the
 paper feed cylinder 16 is in contact with the movable cam 585 to maintain
 the gripper 574 closed. Therefore, the two grippers 573 and 574 remain
 closed when opposed to each other.
 With a further rotation of the impression cylinder 15 in this state, the
 second printing process is started for the printing paper S mounted on the
 impression cylinder 15. In this state, as shown in FIG. 26, the cam
 follower 581 of the gripper 573 remains in contact with the bulge of the
 fixed cam 587, whereby the forward end of printing paper S remains
 gripped. In this state, a second sheet of printing paper S is fed to the
 paper feed cylinder 16. At this time, the cam follower 582 of the gripper
 574 on the paper feed cylinder 16 is in contact with the bulge of the
 fixed cam 588 to maintain the gripper 574 open.
 FIG. 27 shows a state where, with a further rotation of the impression
 cylinder 15, the gripper 573 on the impression cylinder 15 has moved to
 the position opposed to the gripper 575 on the paper discharge cylinder
 17. In the illustrated state, as distinct from the state shown in FIG. 24,
 the cam follower 594 of the main link 592 is in contact with the bulge of
 the cam 591. Thus, the movable cams 584, 585 and 586 and fixed cams 587,
 588 and 589 are in the positional relationship shown in FIG. 19.
 Consequently, the cam follower 581 of the gripper 573 on the impression
 cylinder 15 contacts the recess of the movable cam 584 to start opening
 the gripper 573. The cam follower 583 of the gripper 575 on the paper
 discharge cylinder 17 contacts the recess of the movable cam 586 to start
 closing the gripper 575. As a result, the printing paper S is transferred
 from the gripper 573 to the gripper 575.
 With a further rotation of the paper discharge cylinder 17 in this state,
 the printing paper S is transported with the forward end thereof gripped
 by the gripper 575 as shown in FIG. 28. As in the case shown in FIG. 22,
 the forward end of the second sheet of printing paper S is transferred
 from the gripper 574 to the gripper 573.
 With a still further rotation of the paper discharge cylinder 17, as shown
 in FIG. 29, the printing paper S is transported along the chains 19 to the
 position above the paper discharge station 28. In the position above the
 paper discharge station 28, the cam follower 583 of the gripper 575
 contacts the contact element 599, whereby the gripper 575 is opened to
 release the printing paper S on the discharge station 28.
 A printing paper transfer operation will be described next, which operation
 is carried out in monochromatic printing of the printing paper S with one
 of the yellow, magenta, cyan and black inks, or in two-color printing with
 one of the magenta and black inks and one of the yellow and cyan inks. In
 this case, as noted hereinbefore, the printing paper S must be fed from
 the paper feed cylinder 16 to the impression cylinder 15 for every
 rotation of the impression cylinder 15.
 In this case, the solenoid 598 is operated to lock the stopper 597 to the
 position shown in FIG. 16, to place the distal end of the stopper 597 in
 engagement with the pin 595 projecting from the main link 592.
 Consequently, the main link 592 is maintained in the same state as where
 the cam follower 594 is in contact with the bulge of the cam 591.
 Then, printing paper S is drawn from the paper storage 27 by action of the
 suction board 74 shown in FIG. 1. As in the case shown in FIG. 20, the
 forward end of printing paper S is transported toward the gripper 574 on
 the paper feed cylinder 16. At this time, the cam follower 582 of the
 gripper 574 is in contact with the bulge of the fixed cam 588 to maintain
 the gripper 574 open.
 In this state, the paper feed cylinder 16 rotates to move the cam follower
 582 of the gripper 574 past the bulge of the fixed cam 588. Then, as in
 the case shown in FIG. 21, the gripper 574 closes to grip the forward end
 of printing paper S.
 With a further rotation of the paper feed cylinder 16, the gripper 574 on
 the paper feed cylinder 16 moves to a position opposed to the gripper 573
 on the impression cylinder 15 as in the case shown in FIG. 22. Then, the
 cam follower 582 of the gripper 574 starts opening the gripper 574 by the
 action of fixed cam 588, while the cam follower 581 of the gripper 573
 contacts the bulge of fixed cam 587 to start closing the gripper 573. As a
 result, the forward end of printing paper S is transferred from the
 gripper 574 to the gripper 573.
 With rotation of the impression cylinder 15 in this state, a printing
 process is started for the printing paper S mounted on the impression
 cylinder 15. In this state, as in the case shown in FIG. 23, the cam
 follower 581 of the gripper 573 remains in contact with the bulge of the
 fixed cam 587, whereby the forward end of printing paper S remains
 gripped.
 FIG. 30 shows a state where, with a further rotation of the impression
 cylinder 15, the gripper 573 on the impression cylinder 15 has moved to a
 position opposed to one of the grippers 575 of the paper discharge
 cylinder 17. In the illustrated state, the stopper 597 maintains the main
 link 592 in the same state as where the cam follower 594 is in contact
 with the bulge of the cam 591. Thus, the movable cams 584, 585 and 586 and
 fixed cams 587, 588 and 589 are in the positional relationship shown in
 FIG. 19.
 Consequently, as in the case shown in FIG. 27, the cam follower 581 of the
 gripper 573 on the impression cylinder 15 contacts the recess of the
 movable cam 584 to start opening the gripper 573. The cam follower 583 of
 the gripper 575 on the paper discharge cylinder 17 contacts the recess of
 the movable cam 586 to start closing the gripper 575. As a result, the
 printing paper S is transferred from the gripper 573 to the gripper 575.
 In this state, a second sheet of printing paper S is transported as gripped
 by the gripper 574 on the paper feed cylinder 16.
 FIG. 31 shows a state where, with a further rotation of the impression
 cylinder 15, the gripper 573 on the impression cylinder 15, after passing
 the printing paper S over to the gripper 575 on the paper discharge
 cylinder 17, has moved to the position opposed to the gripper 574 on the
 paper feed cylinder 16 holding the second sheet of printing paper S. In
 the state also, the stopper 597 maintains the main link 592 in the same
 state as where the cam follower 594 is in contact with the bulge of the
 cam 591. Thus, the movable cams 584, 585 and 586 and fixed cams 587, 588
 and 589 are in the positional relationship shown in FIG. 19.
 Consequently, as in the state shown in FIG. 22, the cam follower 582 of the
 gripper 574 on the paper feed cylinder 16 starts opening the gripper 574
 by the action of fixed cam 588, while the cam follower 581 of the gripper
 573 contacts the bulge of fixed cam 587 to start closing the gripper 573.
 As a result, the forward end of printing paper S is transferred from the
 gripper 574 to the gripper 573.
 With a further rotation of the impression cylinder 15, paper feed cylinder
 16 and paper discharge cylinder 17, as shown in FIG. 32, a printing
 operation is started for the second sheet of printing paper S mounted on
 the impression cylinder 15. The printing paper S held by the paper
 discharge cylinder 17 is transported along the chains 19 to the position
 above the paper discharge station 28, and allowed to fall on the discharge
 station 28.
 As described above, the gripper control mechanism is operable for
 controlling the stopper 597 to release the main link 592 and allow the
 pivotal movement of the main link 592. Then, the printing paper S may be
 transferred among the impression cylinder 15, paper feed cylinder 16 and
 paper discharge cylinder 17 for every two rotations of the impression
 cylinder 15. When the stopper 579 is actuated to prohibit the pivotal
 movement of the main link 592, the printing paper S may be transferred
 among the impression cylinder 15, paper feed cylinder 16 and paper
 discharge cylinder 17 for every single rotation of the impression cylinder
 15. Thus, the apparatus is capable of selectively executing four-color
 printing and monochromatic printing or two-color printing.
 In the above printing apparatus, the impression cylinder 15 has half the
 diameter of the first and second plate cylinders 11 and 12 and the first
 and second blanket cylinders 13 and 14. The diameter of the impression
 cylinder may be n/2, where n is an odd number, of the diameter of the
 first and second plate cylinders 11 and 12 and the first and second
 blanket cylinders 13 and 14.
 FIG. 33 is an explanatory view illustrating the construction of a printing
 apparatus in such an embodiment. FIG. 33 shows, among the components of
 the entire printing apparatus, an impression cylinder 215 along with the
 first and second plate cylinders 11 and 12 and the first and second
 blanket cylinders 13 and 14 as in the apparatus shown in FIG. 1. The other
 aspects of the construction are the same as in the printing apparatus
 described hereinbefore.
 In this printing apparatus, the impression cylinder 215 has a diameter
 which is 3/2 of the diameter of the first and second plate cylinders 11
 and 12 and the first and second blanket cylinders 13 and 14. In this
 printing apparatus, therefore, the first and second plate cylinders 11 and
 12 and the first and second blanket cylinders 13 and 14 make three
 rotations for every two rotations of the impression cylinder 215. The
 impression cylinder 215 has three grippers 83 arranged at equal intervals
 peripherally thereof for holding three sheets of printing paper S.
 When this printing apparatus is used to perform four-color printing with
 the yellow, magenta, cyan and black inks, three sheets of printing paper S
 wrapped around the impression cylinder 215 are printed in the four colors
 of yellow, magenta, cyan and black with two rotations of the impression
 cylinder 215. Thus, for every two rotations of the impression cylinder
 215, three new sheets of printing paper S are fed from the paper feed
 cylinder 16 to perform the four-color printing continuously.
 This printing apparatus may also be used for mono-chromatic printing of the
 printing paper with one of the yellow, magenta, cyan and black inks, or
 two-color printing with one of the magenta and black inks and one of the
 yellow and cyan inks. In this case, three new sheets of printing paper S
 may be fed from the paper feed cylinder 16 for every rotation of the
 impression cylinder 215.
 The foregoing embodiments have been described where the present invention
 is applied to the printing apparatus invariably employing the first and
 second plate cylinders 11 and 12 and the first and second blanket
 cylinders 13 and 14 to be capable of four-color printing. The invention is
 applicable also to a printing apparatus employing a single plate cylinder
 and a single blanket cylinder for printing in two colors or less.
 The present invention may be embodied in other specific forms without
 departing from the spirit or essential attributes thereof and,
 accordingly, reference should be made to the appended claims, rather than
 to the foregoing specification, as indicating the scope of the invention.
 The present application claims priority benefit under 35 U.S.C. Section 119
 of Japanese Patent Application No.9-293603 filed in the Japanese Patent
 Office on Oct. 9, 1997, the entire disclosure of which is incorporated
 herein by reference.