Sheet-fed printing machine and printing unit in a sheet-fed printing machine
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.
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.