The present invention relates to a printing unit of a printing press having at least one press unit with a forme cylinder, a transfer cylinder, and an inking unit, at least one printing plate arrangeable on the forme cylinder, a drive for rotating the forme cylinder into a position or angular position which is suitable for effecting a printing-plate change, and a regulating or control device for supplying the drive with an actuating signal for rotating the forme cylinder into the position or angular position which is suitable for the printing-plate change. Furthermore, the invention relates to a method for carrying out the printing-plate change on the forme cylinder of the printing unit.
Printing units of web-fed rotary presses, in particular of newspaper presses, have a plurality of press units, every press unit comprising a transfer cylinder, a forme cylinder and an inking unit, as well as, optionally, a damping unit. Oil-less press units without a damping unit are also known. Furthermore, printing units of this type can have impression cylinders, it being possible for an impression cylinder to interact with one or more transfer cylinders of different press units. In addition to printing units which have impression cylinders of this type, printing units which do not have impression cylinders are also known, the transfer cylinders of two press units rolling on one another in printing units of this type without impression cylinders. As a rule, rubber blankets are clamped on the transfer cylinders and printing plates are clamped on the forme cylinders, for which reason the transfer cylinders are also designated blanket cylinders and the forme cylinders are also designated plate cylinders.
In order to carry out a printing-plate change on a forme cylinder of a printing unit, the respective forme cylinder can be rotated by way of a drive of the printing unit into a position or angular position which is suitable for carrying out the printing-plate change and in which the printing plate which is to be changed or exchanged can be grasped at its trailing edge or leading edge. The exact or precise movement to this position or angular position which is suitable for carrying out the printing-plate change or for grasping or gripping the printing plate which is to be changed is of decisive significance for carrying out the printing-plate change, as the trailing edge or leading edge of the printing plate can be grasped only in a narrowly defined angular range or positional range of the forme cylinder.
Furthermore, what are known as register values or register offset values have an influence on the position of the respective forme cylinder and thus on the moving to a position of the latter which is suitable for carrying out a printing-plate change, the positional accuracy of the colour separations on the printing material in relation to the outer edges of the printing material being ensured via register values or register offset values of this type. What is known as the circumferential register value or circumferential register offset plays a decisive role for moving to the position or angular position of the forme cylinder which is suitable for the printing-plate change.
In order to compensate for the circumferential register offset of a forme cylinder during the printing-plate change on the latter, the circumferential register value is set to zero before the actual printing-plate change, according to the prior art, and the printing unit is driven or displaced until the circumferential register offset for the forme cylinder assumes the value zero. Only subsequently is the forme cylinder rotated into the position or angular position which is suitable for the printing-plate change by way of the drive of the printing unit. After the printing-plate change has been carried out, the original circumferential register offset or circumferential register value is set again according to the prior art and the printing unit is displaced until the circumferential register offset for the forme cylinder has been moved to again. This zero setting and resetting of the circumferential register value or circumferential register offset before and after the actual printing-plate change are time-consuming. The circumferential register zero setting and circumferential register resetting can thus take up to two minutes. This is disadvantageous for reasons of productivity.