In sheet-fed offset printing machines, a printing plate is attached to a plate cylinder of the print units by first clamping the leading edge of the printing plates in a corresponding attachment device on the plate cylinder. Next the printing plate is mounted around the outside circumference of the plate cylinder. The trailing edge of the printing plate is then introduced into a corresponding tensioning device, whereupon the printing plate is tensioned around the outside circumference of the plate cylinder via the trailing edge of the printing plate. Presently, a large number of automated devices are known for the clamping and tensioning of printing plates onto a plate cylinder which enable these steps to be carried out simply by actuating corresponding operating elements. For example, an operator only has to introduce the leading edge of the printing plate into the corresponding clamping bar and then actuate a corresponding operating element which triggers the clamping. Similarly, introducing the trailing edge of the printing plate into the corresponding tensioning device of the plate cylinder can also be carried out in an automated manner such that the attachment of the printing plate to the plate cylinder is totally automatic.
A device for mounting printing plates on the plate cylinder of sheet-fed offset printing machines is known from DE 4,214,168 C2, in which the leading edge of the printing plate is first attached to the plate cylinder by an attachment device and the trailing edge of the printing plate is then attached to a clamping bar arranged in a groove provided in the cylinder. In this device, the clamping bar comprises a plurality of sections arranged axially relative to the plate cylinder. Each section of the clamping bar is provided with a spring element which exerts a corresponding tensile force on the trailing edge of the printing plate. The individual sections of the clamping bar are moveable about two different pivot axes, such movement of the individual clamping bar sections is possible approximately in the circumferential direction of the plate cylinder. The spring elements assigned to the individual clamping bar sections, pivot the individual clamping bar sections from the front wall of the groove in the plate cylinder towards the middle of the groove. In order to introduce the edge of a printing plate into the clamping devices, all the of the clamping bar sections are pivoted counter to the spring force in the direction of the front wall of the groove and the end of the printing plate is inserted into the clamping gap. The clamping device is then actuated. The clamping of the trailing edge of the printing plate in conjunction with the application of a tensile force via the corresponding spring elements results in an automatically increasing clamping force being applied to the printing plate.
The rolling between the blanket cylinder and the back pressure cylinder, however, can result in the stretching of the printing material. In addition, the stretching of the printing material is particularly influenced by the effects of the damping solution. If not corrected for this stretching will adversely effect the print quality, particularly at the trailing edge of the printing image. Accordingly, it is desirable to able to vary the setting or exertion of tensile forces on the printing plate across the format width of the printing machine. For example, it is desirable to allow for correction of a change in the printing length (length of the printing image on the printing material) caused by the effects of the damping solution on the printing material. This is a particular problem at the trailing edge of the printing image.
In the device disclosed in the above mentioned publication DE 4,214,168 C2, these corrections can be accomplished by changing the corresponding pre-tensioning forces for the spring elements acting on the individual clamping-bar sections. Specifically, the individual adjustment of the tensioning forces exerted on the trailing edge of the printing plate must be carried out after mounting and tensioning a printing plate on the plate cylinder and after producing the first few printed copies (a few machine rotations).
A disadvantage of this procedure is that the individual setting of the pre-tensioning forces of the spring elements assigned to the individual clamping-bar sections is a very time consuming operation. Furthermore, once the pre-tensioning of the spring elements are set, they keep that particular setting even after the used printing plate has been removed. Thus, when a new printing plate is mounted, it will be tensioned around the outside circumference of the plate cylinder with a force which varies across the format width of the printing machine. In particular, the tensile force which is applied to the new plate will be based on the settings of the individual spring elements which were used to correct the print quality of the print order which was run with the previous printing plate. It is possible that the settings which were used to correct the print quality of the previous print order may have a negative impact on the print quality of the new print order, particularly with respect to the first few copies. Accordingly, the entire tensioning device has to be returned to a starting position after a used printing plate has been removed. If this resetting or return of the tensioning device to the starting position is forgotten, it is also possible that a new printing plate will be mounted with such widely different tensile forces over the format width that the printing plate may be permanently damaged.
A device for the in-register mounting of printing plates on the plate cylinder of printing machines is known from DE 4,321,751 C. With this device, a hand tool with an eccentric cam surface is provided which interacts with a clamping bar in order to allow for the individual correction of the tensile forces exerted on the trailing edge of the printing plate. Specifically, the eccentric cam surface mounted on the hand tool acts on a portion of the clamping in such a way that the tensile forces exerted on the trailing edge of the printing plate can be increased. The printing plate can therefore be stretched in a defined manner in specific sections with the stretching force being applied solely during the actuation of the hand tool. Once this stretching of the printing plate over the format width of the printing machine has been accomplished, only the spring forces act on the printing plate. With this device, after a used printing plate which has been stretched in individual sections has been removed, a new printing plate can be mounted around the outside circumference of the plate cylinder with a constant tensioning force across the format width of the machine. However, the hand tool can only be used to stretch portions of the printing plate and no allowance is made for reducing the tensile forces which act on the trailing edge of the printing plate.