Abstract
A printing unit in a sheet-fed printing machine includes a printing cylinder, an upstream transfer cylinder and a downstream transfer cylinder. The transfer cylinders each include a device for decelerating/accelerating a sheet. The device, in particular, includes at least one gripper bar being movable in a substantially radial direction to change the tangential speed of the gripper bar and thus of the sheet. A printing unit of this construction allows the sheets to be transported at a lower speed and at a shorter distance from one another during the printing process, resulting in a better print quality and a higher throughput of the machine. A sheet-fed printing machine including such a printing unit is also provided.
Claims
1. A printing unit in a sheet-fed printing machine, the printing unit comprising: a printing cylinder; a transfer cylinder disposed upstream of said printing cylinder in a sheet transport direction, said upstream transfer cylinder having a device for decelerating a sheet from a higher machine sheet speed to a lower printing sheet speed; and a transfer cylinder disposed downstream of said printing cylinder in the sheet transport direction, said downstream transfer cylinder having a device for accelerating a sheet from the lower printing sheet speed to the higher machine sheet speed.
2. The printing unit according to claim 1, wherein said device for decelerating a sheet and said device for accelerating a sheet each include at least one respective gripper bar being movable in a substantially radial direction.
3. The printing unit according to claim 2, wherein said device for decelerating a sheet and said device for accelerating a sheet each include a respective cam mechanism for actuating said respective gripper bar.
4. The printing unit according to claim 2, which further comprises at least one pivot joint supporting each respective gripper bar.
5. The printing unit according to claim 4, wherein said at least one pivot joint is a four-bar linkage.
6. The printing unit according to claim 1, wherein said device for decelerating a sheet and said device for accelerating a sheet each include a respective rotary drive for changing rotational speeds of a respective one of said transfer cylinders.
7. The printing unit according to claim 1, wherein said device for decelerating a sheet and said device for accelerating a sheet each include a respective rocker with a respective gripper bar fixed thereto, said rockers each carrying out a movement relative to a respective one of said transfer cylinders.
8. The printing unit according to claim 1, wherein said printing cylinder has a plurality of sheet-holding areas.
9. The printing unit according to claim 8, wherein said printing cylinder is a 1/5-revolution cylinder with five of said sheet-holding areas.
10. The printing unit according to claim 8, wherein each two respective sheet-holding areas have a respective gap there between.
11. A sheet-fed printing machine, comprising a printing unit according to claim 1.
12. The sheet-fed printing machine according to claim 11, wherein the printing machine is a digital printing machine.
13. The sheet-fed printing machine according to claim 12, which further comprises inkjet heads spaced apart from said printing cylinder.
14. The printing unit according to claim 1, wherein: said device for decelerating a sheet decelerates a sheet already disposed on said upstream transfer cylinder; and said device for accelerating a sheet accelerates a sheet already disposed on said downstream transfer cylinder.
15. The printing unit according to claim 1, wherein: said device for decelerating a sheet decelerates a tangential speed of a sheet while maintaining an angular speed of said upstream transfer cylinder; and said device for accelerating a sheet accelerates a tangential speed of a sheet while maintaining an angular speed of said downstream transfer cylinder.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
(1) FIG. 1 is a diagrammatic, longitudinal-sectional view of a digital printing press;
(2) FIG. 2 is an end-elevational view of a printing unit of the invention in a digital printing press;
(3) FIG. 3 is a perspective view of a transfer cylinder of the printing unit of FIG. 2;
(4) FIG. 4 is an end-elevational view of a portion of the transfer cylinder of FIG. 3; and
(5) FIG. 5 is an end-elevational view of an alternative embodiment of the transfer cylinder.
DETAILED DESCRIPTION OF THE INVENTION
(6) Referring now in detail to the figures of the drawings, which are not drawn to scale and in which corresponding elements and components bear the same reference symbols, and first, particularly, to FIG. 1 thereof, there is seen a sheet-fed printing machine 100, which is embodied as a digital printing machine. A respective sheet 1000 coming from a feeder 1 is transported through a printing unit 2 to a delivery 3 in a direction of transport T. The transporting of a respective sheet 1000 is mainly carried out by using cylinders, namely a transfer cylinder 5 and a printing cylinder 10. Inkjet print heads 4 are disposed above the printing cylinder 10 to print on a sheet 1000 that is moved past at a short distance by using the printing cylinder 10. The printing cylinder 10 is thus also referred to as a jetting cylinder.
(7) In the illustrated embodiment, the printing cylinder 10 includes three sheet-holding areas 11, which are separated from one another by a gap 12.
(8) FIG. 2 illustrates a printing unit 2 of the invention. The printing unit 2 includes an upstream transfer cylinder 5, a printing cylinder 10 and a downstream transfer cylinder 5. Each one of the transfer cylinders 5 has a device 50 for decelerating/accelerating a sheet 1000. These devices 50, which are not illustrated in FIG. 2, will be shown in further figures and will be described below. Before it is transferred to the upstream transfer cylinder 5, a sheet 1000 is transported in the direction of transport T at a machine speed v.sub.M. At this speed, the sheet 1000 is transferred to the transfer cylinder 5, which decelerates the sheet 1000 to a lower printing speed v.sub.D. At this lower printing speed v.sub.D, a respective sheet 1000 is then transferred to the printing cylinder 10 and moved underneath the inkjet heads 4 at the printing speed v.sub.D, receiving the print. Then a respective sheet 1000 is transferred to the downstream transfer cylinder 5 at the printing speed v.sub.D. The downstream transfer cylinder 5 reaccelerates the sheet 1000 to the machine speed v.sub.M and transports the sheet 1000 to potential downstream drying, varnishing and processing units at the high machine speed v.sub.M.
(9) The speed change of a respective sheet 1000 by using the devices 50 for accelerating and decelerating is achieved by a radial displacement of a gripper bar 51 having grippers 52 (shown in FIG. 3) while the transfer cylinder 5 rotates and the grippers 52 hold a sheet 1000. Thus the gripper bar 51 of an upstream transfer cylinder 5 is moved in a radial direction from a larger radius r.sub.M of the gripper bar to a smaller radius r.sub.D. Since the device 50 for decelerating and the gripper bar 51 are fixed to the transfer cylinder 5 and since the latter continues to rotate at a constant rotational speed, this results in a reduction of the tangential speed of a respective sheet 1000, allowing a respective sheet 1000 to be transferred to the printing cylinder 10 at the lower speed v.sub.D. Thus a greater number of sheets 1000 disposed at a smaller distance from one another may be transported and printed on the printing cylinder 10, for instance on 4 instead of 3 or on 5 instead of 4 sheet-holding areas 11. In an analogous way, a sheet 1000 taken over from the printing cylinder by the downstream transfer cylinder 5 is reaccelerated by the radial displacement of the gripper bar 51 from the smaller radius r.sub.D to the larger radius r.sub.M. Due to the fact that the sheets 1000 are printed on at the lower printing speed v.sub.D, a better resolution and a higher print quality are possible. Due to the reduced spacing between the sheets 1000 on the printing cylinder 10, the inkjet heads 4 operate more efficiently and have shorter dead times.
(10) The construction of the transfer cylinders 5 and of the devices 50 for decelerating and accelerating is shown in more detail in FIG. 3. A part of the device 50 for decelerating/accelerating is the gripper bar 51 extending over the width of the transfer cylinder 5 and having the grippers 52 fixed thereto. A respective sheet 1000 may be held by the grippers 52 and rests on a sheet-supporting area 13 of the transfer cylinder 5. The transfer cylinder 5 is driven by a drive 6, which may be embodied as a gearwheel integrated into the gear train of the drive of a sheet-fed printing machine 100. The radial displacement of the gripper bar 51 described with reference to FIG. 2 occurs with the aid of a four-bar linkage 53, namely by deflecting a connecting rod 55 of the four-bar linkage 53. The connecting rod 55 is supported in hinge points 56 on a base body of the transfer cylinder 5 by using rockers 54. The deflection of the connecting rod 55 to initiate the substantially radial displacement of the gripper bar 51 is achieved by a cam mechanism. A cam follower 57, which is fixed to the connecting rod 55, is moved along a stationary control cam 58 while the transfer cylinder 5 rotates. In this process, compression springs 59 ensure that the cam follower 57 is in continuous contact with the control cam 58. The construction of the four-bar linkage 53 for implementing a radial actuating movement s is shown more clearly in the detail view of FIG. 4. As is apparent from FIG. 3, the embodiment shown therein includes a divided four-bar linkage 53: first rockers 54 of the four-bar linkage are fixed to both the drive-side and the operator-side ends of the gripper bar 51. A second rocker 54 of the four-bar linkage 53 is disposed centrally.
(11) FIG. 5 illustrates an alternative embodiment of the device 50 for decelerating/accelerating sheets 1000. In this case, the gripper bar 51 including grippers 52 for holding a respective sheet 1000 is not constructed to be radially displaceable. Instead, the gripper bar 51 is disposed on a rocker 60. The rocker 60 is supported to pivot on the base body of the transfer cylinder 5. While the transfer cylinder 5 rotates at a constant rotational speed, the rocker 60 carries out a pivoting movement in a pivoting area 62, causing a speed component v to be superposed on the constant rotational speed φ of the transfer cylinder 5 and the gripper bar 51 to be accelerated or decelerated. The pivoting movement of the rocker 60 is initiated by a cam mechanism 57, 58. A cam follower 57 rolling off on a stationary control cam 58 during the rotation of the transfer cylinder is disposed on the rocker 60.
